Soft Robotics

Soft Robotics:

3D-printing air pressure sensors and actuators

Colophon Graduation Project // Master Thesis Integrated Product Design April 7, 2015 Delft University of Technology Faculty of Industrial Design Engineering Landbergstraat 15 2628 CE Delft The Netherlands www.io.tudelft.nl Graduation Committee Prof.dr.ir. J.M.P. Geraedts - Chair Prof.dr.ing. W.A. Poelman - Mentor Prof.dr.ir. P.P. Jonker - Mentor Graduation Student Rob Scharff [email protected]

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Preface This thesis discusses my graduation project on the 3D-printing of air pressure actuators and sensors. I really enjoyed working on the project and hope it will be followed up by more projects on soft robotics and air pressure technology in particular. I would like to thank my supervisory team for their efforts to make this project a success. More specifically, I would like to thank Prof.dr.ing. Wim Poelman for his high level of of involvement during the entire project, Prof.dr.ir. Pieter Jonker for the continuous supply of useful connections, ideas and information, and my chair Prof.dr.ir. Jo Geraedts for supervising the overall progress of the project. Next, I am thankful for the pleasant cooperation with Materialise, and in particular Willem Verleysen, without whose expertise and printing capacity this result would not have been possible. Last but not least, I would like to thank my parents for their (financial) support and Annelou Jansen. Rob Scharff, April 2015

Rob Scharff // Master Thesis

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Executive summary This master thesis discusses the 3Dprinting of air pressure sensors and actuators to improve human-robot interaction. By adding an air pressure sensor to a 3D-printed air chamber, manipulations of the air chamber can be detected. Designing 3D-printed air chambers in such a way that the reaction to an applied air pressure is different throughout the air chamber, actuators can be created. These actuators and sensors can interact with each other. This technology, referred to as air pressure technology, falls under the category of soft robot technologies. The compressibility, fast development process, underactuation, customizability, light weight and natural movements that can be realized using air pressure technology make it a very promising technology for applications as for example cooperative robots, protheses and orthoses. However, the technology also has hurdles to overcome as for example the difficulties in simulating the robot’s behaviour due to the large deformations and nonlinearity of the material. Another diffculty is in registering the position and shape of the robot’s body and actuators. During this Research by Design project, the possibilities of air pressure technology were explored. By selective laser sintering the flexible TPU 92A-1 material, multiple principles were tested. Besides exploring the interaction between air pressure technology sensors and actuators, prototypes were made IV

to show how this technology can be used to create bending actuators, rotational actuators, bidirectional actuators and stiffness actuators. Moreover, the influence of different design parameters on the performance of the actuators was explored. The knowledge gained during this prototyping process is presented in a robotic hand that is able to shake hands and create gestures using air pressure technology. The hand contains two air pressure sensors, five seperate bending actuators, an actuator to create stiffness and bidirectional movement and a torsion actuator. This hand functions as a benchmark product to show what potentialities can be realized using air pressure technology and inspire designers to find their own applications for the technology. A technology roadmap has been created to plan out future activities for further development and implementation of air pressure technology.

Table of content Introduction 1 Research approach 2 Part 1: Research phase 3 1. Soft robotics 5 Production 5 Materials 6 Actuators 7 Sensors 8 Simulation 8 2. Air pressure technology 10 Production 10 Actuators 14 Sensors 17 Power & control 19 3. Application areas 23 Protheses 23 Human-simulating robots 25 Exploratory robots 25 Industrial mechanisms 26 Cooperative robots 28 Telecommunication 29 Haptic communication in consumer product interfaces 30 Orthoses 31 Part 2: Concept phase 33 4. Idea generation 35 5. Idea selection 37 Stakeholder analysis 37 Choice of focus 38 6. Prototyping 42 Torsion models 43 Bellow designs 47 Octopus 53 Double bellows 56 Sensor finger 58 Part 3: Detailling phase 61 7. Interactive robot hand 63 Potentialities 64 Functionalities 66 8. Human-robot interaction 67 9. Technology Roadmap 71 Potentialities 71 Functionalities 75 References 77 Appendix 78 1. Conclusions first iteration of prototypes 78 2. Experiment FDM-printed flower muscle 80 3. Description of areas of potentialities 81 4. Design guidelines: 3D-printing for air pressure technology 86


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Introduction It is expected that in the future, robots will become omnipresent in our lives. Since the faculty of Industrial Design Engineering is about designing for our future, this is an interesting development that creates the need to rethink the way we look at product design. An interesting field within the field of robotics is that of soft robotics, that focuses on the applications of deformable materials in robotic applications. This creates the challenge to reexamine the materials and mechanisms that are used to make machines and robots to make them more versatile, lifelike and compatible for human interaction (Majidi, 2014). The Disney Research Lab (Slyper & Hodgins, 2012) initiated a specifically interesting soft robotics research project. This project was about the prototyping of robot appearance, movement and interactions, using flexible 3D printing and air pressure sensors. Flexible air chambers that can be integrated in the desired appearance of the robot are modelled in such a way that different types of manipulation result into noticeably different changes in air pressure. This graduation project aims at elaborating on this Disney research, by exploring the possibilities of combining these 3D-printed, integrated sensors with air pressure actuators that are also integrated in the 3D print model. This way, very direct haptic interactions can be created that can improve the interaction between human and robot. Throughout the report, this technology will be referred to as ‘air pressure technology’.

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The thesis has been subdivided into three parts. The first part discusses the research phase. It will provide an exploration of soft robotics and air pressure technology in particular. Based on this analysis and research on human-robot interaction, interesting application areas will be defined. The second part of the report will discuss the concept phase. During this phase, ideas for further exploration of air pressure technology were generated.The ideas that showed most potential for the stakeholders of this project were then selected, elaborated and prototyped.The outcomes of the testing of these prototypes were used as input for the detailling phase. This phase is discussed in part 3 of this thesis. This part discusses the embodiment of a benchmark product, which aims at showing how air pressure technology can improve humanrobot interaction.

Research approach This graduation project will be a Research by Design project. There has been chosen for a research project since the area of soft robotics, and air pressure technology in particular, is relatively new. Currently, there is too little information available about the possibilities for designers to be able to implement this technology as a possible solution in their problem solving process. This project aims at providing a framework of knowledge of- and practical experience with- air pressure technology that designers can use to implement the technology. The benchmark product is developed as a tool to transfer this knowledge. The benchmark product is accompanied by a technology roadmap. This roadmap can be used as a framework to plan and coordinate the technology development.

The design process will be approached as an association process between potentialities and functionalities, as proposed by Wim Poelman (Poelman, 2005). Figure 1 indicates how this process is divided over the phases discussed in the introduction. The benchmark product shows what potentialities can be realized using air pressure technology and the ways in which the technology can improve human-robot interaction. The benchmark should inspire people to further explore the potentialities of the technology and/or apply the technology in a new context.

Figure 1:

Design approach visualization


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Part 1: Research phase This part of the report will discuss the research phase of this graduation project. The first chapter of this part will explain the concept of soft robotics and explore the advantages and disadvantages of soft robotics as compared to hard robots. The second chapter will take a closer look at air pressure technology and explore its advantages and disadvantages as compared to other technologies used in soft robotics. The last chapter of this part of the thesis will use the information from the first chapters and research on human-robot interaction to define interesting application areas for air pressure technology

Since the term soft robotics is used a Rob Scharff // Master Thesis

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Soft robotics Figure 2:

lot throughout this report, it is imporClassification tant to define what is meant with ‘soft of robots by robots’. Majidi describes soft robots D.Trivedi et al. as robots that contain little or no rigid material and are primarily composed of fluids gels, soft polymers and other easily deformable matter (Majidi, 2014). The classification by D. Trivedi et al. (Trivedi et al., 2008) is more descriptive of the soft robot-class by including the degrees of freedom of the object (see figure 2). Based on the material (hard/soft) and degrees of freedom (continuum/ discrete and non-redundant/redundant/hyperredundant) four robot categories have been identified. The soft robot class has infinite degrees of freedom, continuous actuators and a large strain. The potentialities of soft robotics in general as compared to the three types of hard robots are presented in the table below. This chapter will describe the production, materials, actuators, sensors, control mechanisms and simulation of soft robots and explain how these differ from traditional robotics.

Figure 3:

Comparison of robot types by D.Trivedi et al.

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Production Whereas hard robots are usually composed of standard components as actuators, soft robots typically have actuators that are integrated into and distributed throughout the surface (Trivedi et al., 2008). This creates the need for more complex shapes. For this reason, the 3D printing of moulds for silicone casting or directly 3D printing the object in a flexible material are commonly used production techniques in soft robotics. The differences between these production techniques and those of hard robots mainly lie in: Amount of components Hard robots usually consist of many different components that need to be separately designed and/or ordered. As indicated above, soft robots usually have its functions integrated throughout the surface of the robot. The lower amount of components

makes the design process more clear and transparent. The designer has fewer, but often more complex, parts to design and saves time on ordering and assemblage.

product. This difference in development speed between hard robots and soft robots is even strengthened by the time savings on tooling, assemblage, design and ordering.

Design freedom The absence of standard components in soft robots gives the designer more design freedom, since they are not bound to the shape and size of standard components. This can make the product more effective, and saves modelling time.

Durability & repair Hard robots are usually more durable than soft robots. The used materials and components are more reliable and constant in quality than 3D printed ones. When a part of a hard robot fails, it can usually be fixed or replaced, whereas failure of one function of a soft robot usually means the entire robot needs to be replaced.

Batch size The prices of parts for hard robots are usually strongly related to the batch size. Standard components get a lot cheaper with bulk purchases. Specially developed parts are only feasible with large batches, due to the high tooling costs. 3D printing-costs are barely related to batch size. This makes it more feasible to develop a single robot for one specific purpose. Accessibility 3D printing service providers as for example Materialise make 3D printing a very accessible production method to a large public. This is strengthened by the fact that the products are directly created from a file, meaning everyone with a basic knowledge of CAD and 3D printing is able to create 3D-printed parts themselves. These developments, combined with the low amount of parts and absence of need for a high batch size makes the production of soft robots accessible to a large public. Development speed The fact that products can be directly printed from a file, makes 3D printing a fast process to get from design to Rob Scharff // Master Thesis

Materials Whereas most hard robots are composed of materials such as metals and hard plastics with a Young’s modulus greater than 109 Pa, materials in natural organisms usually have a Young’s modulus in the order of 102 – 106 Pa. This big difference in mechanical compliance is a big reason why these hard robots are not suitable for intimate human interaction and rarely exhibit the multi-functionality and elastic versatility that is found in natural organisms. The Young’s modulus of soft robot materials are usually in the order of 105 - 107 Pa (Majidi, 2014), which make it a much better fit to the Young’s modulus of natural organisms. Soft robot materials are often under large deformation during normal operation. This is a big difference with hard robots, which are designed to have small deformation under normal working loads (Trimmer, 2014). These large deformations lead to complications in standard engineering tools, as will be explained later in this chapter.

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Actuators Hard robots typically have an actuator for every joint. They measure the position of each joint using encoders. The tip position of the robot can be accurately determined using the encoder information as is shown in figure 4b. Inverse kinematics can be used to calculate the desired angles that would result into the desired tip position. These angles can be compared to the values that are measured by the encoders. The controller is than able to compensate for these values. When a loading is applied, the links remain straight and only the joints change position. The encoders measure this position change, and the controller can compensate for this loading (figure 4c) (Trivedi et al., 2008). Hard robots grab objects with a specialized end effector that is designed for a specific size and type of object. They use separate legs, track and wheels to contact the ground and enable locomotion (Trivedi et al., 2008). Figure 4:

Hard vs. soft actuators (Trivedi et al., 2008) Figure 5 (right):

Soft actuator correcting for gravity (Trivedi et al., 2008)

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As discussed in the production chapter, the actuators of soft robots are typically integrated into and throughout the surface. Soft robot actuators are ‘under-actuated’, which means there is not an actuator for every degree of freedom. The structure of a soft robot is a continuum. These factors make it very hard to create accurate movements. Whereas an applied load would only affect the joints in a hard robot, a soft robot is continuously deformed over its entire arm. The limited sensors and actuators are not able to correct for this effect. Figure 5 shows this effect with an actuator that tries to correct for gravity (Trivedi et al., 2008). Soft actuators are usually not focused on a specific object, but are capable of handling objects of varying size by manipulating their whole arm as is shown in Figure 4d (Trivedi et al., 2008). Another problem in soft robot actuators is elastic hysteresis. Materials as for example rubber show a high degree of hysteresis. This means it takes more energy to load the material than to unload the material. The difference in energy is dissipated as heat due to internal friction.

Sensors As already indicated at the actuator chapter, hard robots are very capable of sensing their own movements using encoders. Moreover, they are often equipped with a lot of sensors to capture their environment. Soft robots are less capable of sensing their shape, because they are under-actuated. This was already illustrated in figure 5. A sensor that measures the moment at the base of a soft robot arm, cannot differentiate between a point load and a distributed load (Trivedi et al., 2008). An array of sensors and a complex control system would be needed to be able to accurately control a soft robot. Since most of these components are rigid and would make the robot less soft, most designers choose not to equip the soft robots with any sensors at all.

the following methods might provide a solution to the problem: Abaqus Abaqus is a dynamic simulation software that is better capable of dealing with nonlinear material behavior. It is capable of dealing with large deformations and changes in the direction of the force that result from these deformations. The software is also able to take force transmission between self-intersecting parts into account. Hyperelastic materials can be simulated using a model as for example the Mooney-Rivlin- or Ogden- and Blatz-Ko- model. However, an essential condition for these simulations is that the material characteristics are well known.

Experimental verifiction A physically correct simulator alone is not enough to simulate reality. Many Simulation material characteristics need to be exMost of the current engineering tools perimentally calibrated (Lipson, 2014). are based on the assumption that Although datasheets with general deformations are small under the characteristics of the 3D printing manormal working loads. This assumpterials are available, these values vary tion is true for most materials used in hard robots, but not for soft, nonlinear, with build conditions as for example printing orientation, printing location anisotropic materials used in soft roand the amount of recycled material. bots. Many of these materials exhibit Therefore, software is needed that acnegative Poisson ratios, change their curately records these building condiapparent stiffness under rapid forces or creep at hardening under constant tions. This software is under development by Materialise. loads. Finite element analysis is too cumbersome and slow to be practical. Besides experimental verification on (Trimmer, 2014). material level, experimental verification could also be applied on a functionStatic and dynamic simulation of soft level. An example could be the measrobots is an essential design tool uring of the effect of different design to facilitate the development in the parameters (bellow height, bellow field of soft robotics. It would enable width, actuator height, printing oriendesigners to predict and design the tation, etc) on the performance (force, movement of the actuators and sensors in a real environment. This aspect movement, etc) of a bellow. This inforis currently underdeveloped, although mation can then be used to develop a Rob Scharff // Master Thesis

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design tool that can generate a bellow design for specific performance goals. Kinematic simulation Recently, progress in the field of dynamic simulation of soft robots has been made by using an approach based on nonlinear relaxation for kinematic simulation instead of using traditional finite element solvers. This approach represents a structure as a network of simple elements such as springs, beams and masses. An advantage of this approach is that it allows for easy incorporation of new nonlinear and active elements as for example actuators, contacts and arbitrary reactive materials. The elements can be separately controlled and have its own behavior. (Lipson, 2014)

Figure 6:

Dynamic simulation of nonlinear materials with nonlinear relaxation approach (Lipson, 2014)

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Conclusion Soft Robotics A big advantage of soft robots over traditional hard robots is the speed with which a complex robot can be developed in combination with the batch-independency of the costs. This makes it possible to create single robots for a very specific purpose at short notice. Most soft robot actuators are underactuated. An advantage of underactuated actuators is the automatic adaptation of these actuators to their environment. A downside is that it is very hard to accurately determine the position and shape of the actuator. This creates the need for an array of sensors over the entire product, or a different approach to robotics in which it is not necessary to know the exact position and shape of the actuator. Prediction and simulation of robot behaviour is an important challenge in the field of soft robotics. Designers need to be able to create an estimation of the behaviour of their designs to make soft robotics succeed as an alternative for hard robots. However, most of the current engineering tools are unable to cope with large deformations and nonlinear materials.

Air pressure technology As mentioned in the introduction, the term ‘air pressure technology’ is used to define the technology of 3D-printing air chambers in flexible material to use as air pressure actuators snd/or sensors.

3D printing Air pressure technology focuses on 3D printing to create the flexible air chambers. This production method is also commonly used in alternative soft robot technologies.

This chapter will explore the production & materials, actuators, sensors and power & control of air pressure technology. For each of these aspects, the technology will be compared to alternative technologies within soft robotics.

This chapter will explore the different ways of 3D printing flexible materials. The illustration below pictures the layered manufacturing processes as classified by Pham. (Gibson, Rosen, & Stucker, 2010) Flexible materials can be printed using liquid polymer, discrete particles and molten material. There has been chosen to take one flexible material of each printing technique to represent the full spectrum of materials. One additional material was added for the molten material-method, since there were two flexible materials that deviated a lot from each other. The chosen materials are NinjaFlex (FDM), PORO-LAY (FDM), Tango Plus (Objet) and TPU 92A-1 (SLS). These materials and their corresponding printing techniques will be discussed in this subchapter.

Production & materials There are several production processes used to create soft robots. This chapter will discuss the most commonly used and/or most promising ones. Since this graduation project is aimed at air pressure technology, there will be an emphasis on how these production methods can be used to create flexible air chambers. However, the mentioned production methods can also be used to create soft robots that make use of a different technology.

Figure 7:

Layered Manufacturing processes as classified by Pham


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Fused Deposition Modelling (FDM) Fused Deposition Modelling is the most common extrusion-based printing technique. The filament that is fed into the system is liquefied in a heating chamber. Extrusion pressure is generated by a tractor wheel arrangement that pushes the filament into the chamber. The extruded material is then plotted according to a predefined path, where is should solidify and bond with adjacent material. Support structures are created to enable the creation of complex geometrical Figure 8 features (Gibson et al., 2010).

lent abrasion resistance (FennerDrives, n.d.). The PORO-LAY material is developed by German inventor Kai Perthy and is very different from the NinjaFlex material. The material is rigid when it is printed, but becomes sponge-like when it is soaked in water. The elastic and fibrous material can even be made conductive, when the pores are filled with an electrolyte like salt water or ionic fluids.

(right):

FDM-printed actuator sealed with Plasti-Dip

Air chambers for air pressure technology are preferably printed with only one small opening to connect to a tube. The size of this hole is insufficient to remove support structures printed within the air chamber. Therefore, the design freedom is restricted to structures that can be printed without support structures. This means large overhangs should be avoided.

Figure 9 (right):

Moreover, untreated FDM-prints are not airtight.The FDM-printed objects Sponge-like can be sealed by dipping it in a mix of PORO-LAY Loctite and MEK. This was shown in material a recent Instructables-project in which an air muscle was created using a FDM printer (Mikey77, 2014). A small experiment with an FDM-prototype showed that FDM-prints can also be sealed using Plasti-Dip spray. This is a much easier, faster, cheaper and safer process. More information about this process can be found in appendix 2. The material characteristics of NinjaFlex vary with build conditions. No complete datasheet of this material could be found. However, the filament has a shore hardness of approximately 85A, high elasticity and excel11

Objet PolyJet This printing technique was used in the Disney research. PolyJet machines print several acrylic-based photopolymer materials in 0,0015mm layers from heads containing 1,536 individual nozzles, which results in rapid, line-wise deposition efficiency. Ultraviolet light is used to cure the photopo-

lymer immediately as it is printed. This results in fully cured models without post-curing. Only two photopolymers can be printed at a time. However, up to 25 different effective materials can be printed by varying the relative composition of the two photopolymers (Gibson et al., 2010).

The support structures are built in a gel-like material, which can be removed by hand and water jetting (Gibson et al., 2010). The Disney team used a minimal hole size of 8mm to remove the support material and indicated a hole of 3mm was too small to remove the support material (Slyper & Hodgins, 2012). Because of the composition of the photopolymer material, it is usually weaker than FDM- or SLS-printed objects. The Tango Plus material tears more easily than silicone, but the tearing can be minimized by rounding all corners and edges (Slyper & Hodgins, 2012). The characteristics of the material are listed below. The values may vary with build conditions.

Selective Laser Sintering Selective Laser Sintering (SLS) is part of the Powder Bed Fusion-based 3D printing technologies. It spreads thin layers of powder using a counter-rotating powder leveling roller. The process takes place is an enclosed chamber at an elevated temperature just below the melting point and/ or glass transition temperature of the powdered material. Once the powder layer is formed and preheated, a focused laser beam thermally fuses a slice of the cross-section. The laser is directed using a galvanometers with a mirror attached to them. The powder surrounding the fused slice remains loose and functions as a support structure for the next layer. Once a layer is completed, the building platform is lowered by one layer thickness and a new layer op powder is laid using the counter-rotating roller (Gibson et al., 2010).

PolyJet process (Gibson et al., 2010)

Figure 12:

SLS process (Gibson et al., 2010)

A hole of approximately 8mm is sufficient to remove the support material powder. This increases the possibilities for printing complex geometries. An additional advantage of the flexible material is that the objects can be manipulated to ease the removal of the powder. The flexible TPU 92A-1 material is a complicated material to work with, since the flexibility is strongly dependent on the structure and thickness of the printed object. However, the


Figure 10 (left):

Figure 11 (left):

Tango Plus material characteristics (Materialise, 2014a)

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material is very strong and durable. The material characteristics are listed below. As with the previous printing techniques, the values may vary with build conditions. Figure 13:

TPU 92A-1 material characteristics (Materialise, 2014b) Figure 15 (right):

Creating Oogoo air chambers (Instructables, n.d.)

Silicone casting Another production technique used in soft robotics is silicone casting. This technique can also be used to create air chambers. A mold can be 3D printed to be able to create a complex pneumatic network called a pneu-net (Mosadegh et al., 2014). Once the shape has been removed from the mold, the air chambers can be sealed by adding a layer of silicone on top of the object. This process is shown in the images below.

Silicone materials have a Young’s modulus in the order of 105 Pa. Stretchy silicons, as for example Elastosil M4601 with a high strain and low durometer will deform more when a certain pressure is applied to the air chamber than rigid silicons with a low strain and high durometer, as for Figure 14: example PDMS. Creating silicone air chambers (Instructables, n.d.) Figure 16 (right):

Irregular pattern of holes in state switching material 13

Oogoo This soft materiial is created by mixing silicone caulk and cornstarch.The material in this example from Instructables is called Oogoo, but materials like Sugru have similar qualities. Air chambers are created by placing two layers of Oogoo on top of each other, with a small layer of plastic in between. The Oogoo layers will stick to each other, but will not stick to the plastic layer, thereby creating air chambers.

State-switching material Recently, a material has been developed that is able to exhibit multiple material behaviors. The material is able to switch between normal elastic, abnormal elastic and plastically behavior by regulating the pressure on the material. The meta-material consists of rubber plates with a pattern of irregular holes. The technology was presented by Martin van Hecke at a conference and was reported by the university of Leiden.

Actuators This chapter will compare air pressure technology actuators with alternative soft robotic actuators. Air pressure technology actuators The development of air pressure technology actuators is one of the goals of this graduation project. This analysis will focus on existing technologies that make use of pressurization of flexible air chambers as actuation method. The same principles might be applied in air pressure technology as well.

Materialise gripper This actuator is used as a gripper to pick up vulnerable objects as for example fruits and vegetables. It is 3D-printed in one part using selective laser sintering. When air pressure is added, the gripper tentacles will bend towards each other due to the inner shape of the air chambers. Due to the underactuation, the gripper adapts to the shape of the object. Figure 19:

TPU 92A-1 Materialise gripper

Pneumatic artificial muscles (PAMs) PAMs are lightweight but powerful actuators that consist of a thin, flexible, tubular membrane with fibre reinforcement (Trivedi et al., 2008).

Two coupled PAMs are needed to create a bidirectional motion. This setup is usually referred to as an antagonistic set-up and is similar to how human muscles work. When one of the actuators moves the load, the other one acts as a break to stop the load at its desired position (Daerden & Lefeber, 2000). This is shown in the figure below.

Bending elastomer actuator The principle of a simple bending actuator is described by Onal et al. (Onal, Chen, Whitesides, & Rus, 2011). Using silicone casting, a pneunet is created inside a rectangular shape. An inextensible thin layer is placed on one side of the rectangle. When air pressure is applied, the material will deform and bend around this inextensible layer, as is shown below. The speed of the bending depends on the rate of inflation, the geometry of the internal channels and exterior walls and the materials used for fabrication (Mosadegh et al., 2014).

Figure 17 (left):

Robot-arm with PAMs by Festo

Figure 18 (left):

Antagonistic set-up (Daerden & Lefeber, 2000) Figure 20:

Working principle of bending elastomer actuator (Onal et al., 2011) Rob Scharff // Master Thesis

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Figure 22 (right):

Festo elephant trunk

Figure 21:

Pneumatic finger This principle for actuating a pneumatic finger was patented by J.C. Cool in 1971 (Cool & Hooreweder, 1971) It is constructed from 3 phalanxes that are connected by plastic joints. A tube is connected to the phalanxes using two clamps. When the actuator is pressurized, the areas between the phalanxes will be filled with air and the finger will bend around the joints. The movement is fast, looks natural, has sufficient power and can be well controlled. Because of the small air volumes, the actuator has a small power consumption.

Working principle of pneumatic finger (Cool & Hooreweder, 1971)

Figure 23 (right):

Working principle of jamming gripper (Brown et al., 2010)

Festo Elephant Trunk This actuator by Festo is inspired by an elephant trunk. To control the movement of the trunk, it has been 15

subdivided into a lower-, middle- and upper-part, each consisting of three 3D-printed bellows. Movement can be created by adding air pressure to one or more of these bellows, thereby expanding the length relative to the bellows without air. This create a bending movement.

Jamming gripper The jamming gripper is a balloon filled with granular material. When the balloon is pressed onto an object, it conforms to the object’s shape. A vacuum is then applied to harden and hold the object without requiring sensory feedback. This technique is very suitable to pick up unfamiliar objects or objects that widely vary in shape and surface properties (Brown et al., 2010).

Dielectric elastomer actuators Dielectric elastomer actuators consist of an elastomeric film with an electrode on both sides of the film. When a voltage is applied, the film will contract in its thickness and expand in the film plane direction. Ionic polymer metal composites (IPMCs) IPMCs are composed of an ionic polymer that are chemically plated or physically coated with conductors. When an electric field is imposed across the thickness of a strip of these composites, a large bending displacement can be realized. This technology can be applied as large motion actuators (Shahinpoor, Bar-Cohen, Xue, Simpson, & Smith, 1998). Shape-memory alloys and polymers An interesting way of powering soft robots is by using shape memory alloys or polymers. Shape memory materials are able to return from a deformed state to their original state by an external stimulus as for example heat. The meshworm robot by MIT shows how Nitinol can be used to actuate a soft robot (MIT News Office, 2012). Nitinol is a shape-memory material that returns to its original state when it is heated. This is done by applying an electrical current to a small thread of Nitinol. The Nitinol thread is wound around a mesh tube to deform the tube and create movement.

Liquid-crystal elastomers (LCEs) Liquid crystal elastomers are elastomer films that undergo reversible shape deformation on heating. This heat makes the material change from its nematic to isotropic state, causing a contraction, as is shown in the image below (Yu & Ikeda, 2006). This contraction can be used to create movements in elastomer films. Figure 25:

Phase shift from nematic to isotropic state (Yu & Ikeda, 2006)

Wax coating A relatively new soft robotic actuator is that of a lattice foam structure coated in wax. When the wax is locally heated, it becomes soft and the structure collapses around this point. The the material becomes soft. Figure 26:

Wax-coated foam in lattice structure

Figure 24 (left):

Shape memory alloy meshworm by MIT


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Sensors This chapter will compare air pressure technology sensors with alternative soft robotic sensors. Air pressure technology Air pressure technology measures changes in the air pressure of flexible air chambers to detect deformations. By smartly modeling the shape of the air chambers, the increase in air pressure might give an indication of the type of deformation (Slyper & Hodgins, 2012). An example of this principle is the accordion shape in figure 27. The volume in the air chamber changes significantly if the accordion Figure 27:

Manipulating air chamber shape to create manipulation detection (Slyper & Hodgins, 2012)

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is pushed or pulled, resulting in a big change in air pressure. Squeezing and bending of the accordion does not change the volume of the shape significantly. It should be noted that the technology might not be able measure a difference between a small push or pull and firm squeeze or bend. However, a very big increase or decrease in air pressure can only mean a firm push or pull. This principle of sensing the type of manipulation by cleverly modeling the shape of the air chambers can also be used to create air chambers that ‘recognize’ pull-, press-, twist-, bend- and

bidirectional bend- manipulations. An interesting aspect of the technology is that the sensors can be integrated in the product design. The pressure can be measured by connecting an air pressure sensor to the air chamber. This sensor can also be placed outside the product by routing the pressure using plastic tubing (Slyper & Hodgins, 2012). An advantage of this method is that the printed product itself is completely soft and has no electronics. This would make it capable of sensing while being underwater. An alternative to a pressure sensor is to measure air pressure by reading off the current supply to a micropump. In this case, the micropump could function as both actuator and sensor. However, this means the pump needs to be turned on before it is able to measure differences in air pressure. Therefore, this technology can only be used in systems were: a. The micropump needs to be turned off when a pressure difference is measured. A timer or other measurement could then be used to reactivate the pump. b. The micropump needs to adapt its power or another in series pump needs to be activated. This is difficult to control, since it would also influence the measured current. c. The measurement is used only for measurement purposes or to activate another actuator.

The tactile feel of the air chamber depends on the airtightness of the air chamber. If the chamber is leaky, the chamber feels squishy and requires recovery time to regain its shape. If the chamber is airtight, it feels like a firm balloon. (Slyper & Hodgins, 2012). When the sensor is airtight, the sensor function can be used to measure manipulations of the air chamber over a longer duration of time. When the sensor is leaky, only changes in manipulation can be measured. Besides the airtightness of the air chamber, the tactile feel also depends on the chosen printing process, printing material, wall thickness and geometry. Piezo-sensors Piezo sensors build up an electrical charge when bent. This change in electrical charge can be measured. Piezo sensors are even capable of picking up hovering movements due to the pyroelectrical effect. Because the capacitors slowly discharge after being bent, this sensor type is only capable of sensing changes in direction and is not capable of sensing an absolute value. Piezo sensors can be printed onto foil to create an array of measurements. Recently, a method has been developed to integrate piezoelectric crystals onto stretchable rubbers (Qi et al., 2010). An example of the use of polymeric piezo materials is the Light Touch Matterproject, in which the technology is combined with flexible OLEDs (LTM, n.d.). Figure 28:

Light Touch Matter project (LTM, n.d.)


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Figure 29:

Conductive rubber A conductive material can be added to a rubber. When the rubber is deformed, the resistance of the material changes. This change is resistance can be measured. An example is the flexible stretch sensor shown below.

Conductive rubber flexible stretch sensor 3D capturing It is difficult to deduce what a soft robot is doing from a small amount of sensors. Therefore, 3D capturing of the environment might be a useful method to capture the complex movements. 3D capturing can also be used to capture the robot’s environment. Depending on what to capture and the application of the robot, the 3D capturing device can be placed outside or inside the robot. However, placing the 3D capturing device in the soft robot would make the robot less soft. A commonly used way of 3D capturing is by using cameras with image editing software.

Power & control This chapter will compare air pressure technology power and control systems with alternative soft robotic power and control systems. Air pressure technology Air pressure technology uses air pressure to actuate. Advantages of using air pressure is that it provides rapid inflation because of the low viscosity, that it is easily controlled and measured, it is widely available, it is light in weight and it can be discarded to the atmosphere after use (Mosadegh et al., 2014). There are multiple power sources that can be used to create this pressure. The type of control elements that are needed strongly depend on the choice of power source. The different options will be discussed below.

Compressor A compressor uses electrical energy to compress air. This compressed air can be used to activate pneumatic Conductive ink actuators. The weight, rigidity, size, Conductive or semi-conductive inks noise and high power supply that are can be sandwiched between thin flexi- needed make it undesirable to inteble polyester sheets. These polyester grate the compressor in the robot. sheets have electrically conductive However, an external compressor that pathways imprinted on them. The is connected to the soft robot through conductive ink realizes an electrical tubes is a very common approach of connection between the upper and powering a soft robot. A big advanlower conductor. When a comprestage of the compressor is that it has a sive force is applied, the resistance of lot of power and can therefore power this connection changes. This way, stronger actuators. Moreover, a coman array of force sensing elements pressor is relatively cheap. (sensels) is created which are electrically isolated from each other. If the A compressor gives a constant presspatial dimensions and separation of sure. To control different air chambers the sensels are known, the measured with different pressures at different force data can be con- verted into a times, solenoid valves and pressure pressure profile (Morin, Bryant, Reid, & regulators are needed. These compoWhiteside, 2000). nents need to be electronically con-

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trolled and need power supplies as well. Electronically controlled pressure regulators are expensive and heavy. Therefore, the need for pressure regulation should be avoided if possible. An alternative solution to control the air pressure is to use pulse-width modulated solenoid valves. These valves are far less expensive. A disadvantage of this approach is the inefficiency due to the loss of air. Moreover, the speed of the pulse-width modulation needs to be calibrated to the volume of the air chamber to create accurate pressure control. More complex control systems can be created using elements as for example oneway valves.

improve the autonomy of the robot. An advantage is that it is able to work with a small supply voltage and can therefore be safely used. A disadvantage of the usage of micropumps is that the flow and pressure are very limited. Moreover, micropumps are noisy, relatively expensive and inefficient. Since micropumps are less powerful than a compressor, a separate micropump is needed for every actuator. This means each actuator can be easily controlled by electronically switching the pump on or off. The pressure that is used to activate the actuator can be controlled by pulse-width modulation. An additional advantage of using micropumps is that they can be used as a sensor by measuring changes in the current supply.

Figure 30 (left):

Compressor to pressurize pneumatic gripper Figure 31:

Micropump

Micropump Micropumps need a small supply voltage to create a small air pressure. This compressed air can be used to activate most pneumatic actuators. Micropumps are capable of pressurizing small actuators. Since the micropumps are very small, they can be placed inside the soft robot, to Rob Scharff // Master Thesis

Pressurized gas cartridge Another possibility is the use of a pressurized gas cartridge as for example air or CO2. A big advantage is that there is no need for electricity and the power source is therefore usable in wet environments. The cartridges are silent and can be small enough to fit inside the soft robot, to make it more autonomous. A disadvantage is the fact that it is a finite power source. The higher the amount of necessary pressurized air in between two replacement moments, 20

the bigger the cartridge need to be. This will make the robot heavier and more rigid. Figure 32:

Miniature pressurized gas cartridges

ample one-way valves can be placed between a manipulated air chamber and actuator to create more complex interactions. This principle is applied in for example manual blood pressure measurement sets.

Figure 33 (right):

The control of pressurized gas carManual blood tridges is similar to that of a compresmeasurement sor. Solenoid valves and pressure regset ulators are needed to control multiple air chambers with different pressures. However, some components can be replaced by using multiple small gas cartridges instead of one big one. Since gas cartridges are a finite power source, pressure regulation by pulsewidth modulation of a solenoid valve is undesirable. Manual pressurization An alternative method is to use the air that is compressed by manipulation in one air chamber directly to activate an actuator. The pressure in the manipulated air chamber and the actuator is always equal and the amount of air is constant. This principle is applied in for example squeezing dolls. Advantages of this method are that it is silent, completely soft, very direct and analogue. Moreover, there is no need for electricity. A disadvantage is the fact that the air pressure for the actuator needs to be created by an external force. If this force needs to be created by a person, this severely limits the actuation pressure. The actuator directly reacts to the squeezing, so a delay between the manipulation and actuation is not possible. The pressure and flow is controlled by the manipulation of the air chamber. Non-electrical components as for ex21

On-board chemical pressure generation A last alternative is to use on-board chemical pressure generation. Onal et al. presents a method of selfregulated chemical decomposition of hydrogen peroxide into oxygen to generate pressure (Onal et al., 2011). Since there is a direct conversion from chemical to mechanical energy, no electricity is needed. Since the reaction takes place in the soft robot, there is no pressure loss in tubes to route the pressure. The solution is also silent and portable. Moreover, the pneumatic battery is operational in any direction. A disadvantage is the fact that a supply of hydrogen peroxide is needed. Hydraulic An advantage of hydraulic systems is that they are more powerful than pneumatic systems. However, fluid is incompressible, which makes hydraulic systems more rigid. Moreover, hydraulic systems can leak, which makes it less suitable for close human interactions. The control components are similar to pneumatic components.

Electric An advantage of electrically powered actuators is that in most cases electricity is already present for the sensors and control system. This way, no additional power source or converter is needed. Another advantage of electric actuators is the possibility of pulse-width modulation. Moreover, many actuators can be used as sensor as well. A disadvantage of electric actuators is that their performance in terms of force and speed quickly drops when the actuator size needs to be smaller. Another disadvantage of electrical actuators is their vulnerability to water and dirt. Electronic control systems are very compact and allow a high level of complexity. Moreover, most sensors are also electrical components.


Conclusion air pressure technology The 3D printing of flexible air chambers is a very quick production process to create a product with integrated sensors and actuators. Selective laser sintering TPU 92A-1 seems the most promising due to its combination of durability and ease of support material removal. The bellowactuator as for example seen in the Materialise gripper seems to be one of the most promising actuators and optimally benefits from the design possibilities of 3D printing. New potentialities can be realized by creating variations of this bellow. An advantage of air pressure technology sensors is the ability to sense without the need for electronics inside the product. The ability to measure absolute pressures and the ability to sense the type of manipulation can create interesting potentialities. The characteristics of pneumatics, such as for example its compressibility are a good match with soft robotics. The choice of power supply strongly depends on the necessary complexity of the control system, the actuator strength needed, whether or not the product should be completely soft and whether or not the power supply needs to be integrated in the product.

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Application areas The next step was to explore the desired functionalities. As with the technology-analysis, a focus is needed to ensure enough depth in information. Therefore, the analysis part will specifically focus on the Human-Robot Interaction (HRI) in soft robotics with the potentialities of air pressure technology in mind.

Figure 34:

The desired HRI cannot be seen apart from the functionality of the robot. The task to be accomplished sets the tone for the system’s design and use (Yanco & Dury, 2004). Therefore, instead of giving a general overview on HRI, the information from the literature study on HRI, soft robotics and air pressure technology was clustered to identify interesting applications. For each application, more research was done to define a full set of desired HRIs and other functionalities.

Clustering sets of applications with similar desired functionalities

Underactuation Underactuation is desirable in prosthetic hands, since it more closely simulates the human hand. When gripping objects of varying size, it is desirable to have an actuator that automatically adapts to the objects. Fully actuated hands would need a lot of sensors to gain information about the object and then react to this information. This would create the need for a lot of sensors and more complex control system, which would make the hand a lot heavier. Aesthetics 3D printing could lead to an improved appearance. Using 3D printing, the actuators could be integrated in the final appearacce of the prosthesis.

Protheses An interesting field of application is the field of prosthetics. This subchapter specifically focuses on prosthetic hands, but most of the functionalities needed here are desired in different prosthetics as well.

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Lightweight A prosthetic hand is dead weight and should therefore be as light as possible. Electric-driven prostheses tend to be big and heavy due to the frame and actuators. An advantage of 3D printing is that it is possible to create very lightweight structures instead of using massive components. Moreover, air pressure technology has integrated sensors and actuators that are very lightweight due to their hollow shape.

Prosthesis wearers do not see their prosthesis as a replacement, but as an accessory to their body (Geurts, 2010). Overly realistic prostheses might lead to the uncanny valley. This term is used by Mori to describe the abrupt shift from empathy to revulsion when humanlike robots approach, but fail to attain, a lifelike appearance. The presence of movement even steepens the slopes of the uncanny valley, as illustrated in figure 35. (as cited by MacDorman and Kageki 2012).

prosthetic hand, makes it more comfortable and natural when for example shaking hands.

This theory was confirmed in research (Seyama & Nagayama, 2007). MacDorman recommends taking the first peak of the graph as a design goal. Although the second peak is higher, the risk of falling into the uncanny valley is higher (MacDorman, 2005). Marijn Geurts concluded that the natural appearance is an important issue and this is achieved by making the prosthesis look like ‘it is covering a healthy limb’. Current prosthetics make use of a cosmetic glove that is placed over the mechanical construction. The design freedom of 3D-printing allows the creation of complex shapes, although the design freedom is limited by the functional shapes as for example the actuators. Natural movements and tactile feel Besides the aesthetics of the hand, the movements and tactile feel of the hand are also important. A very realistic looking hand might still become uncanny when the movements are not natural or when it feels cold and hard. Movements created using air pressure technology look more natural than prosthetics with electric actuators. Besides the natural look of the movement, electric actuators are also slow due to the gear conversion needed to create enough torque. The compressibility of an air pressure technology Rob Scharff // Master Thesis

Efficiency Since the power supply needs to be carried, the energy-efficiency of the hand is an important factor to take into account. The pneumatic hand created by Dick Plettenburg (Personal communication) uses a small CO2 cartridge as a power supply. This cartridge needs to keep the hand running for at least a day. When cyclic loading is applied to 3D-printed flexible material, the viscoelasticity of the material causes hysteresis to occur, which leads to a dissipation of mechanical energy. A disadvantage of electric actuators is that their performance quickly drops when the actuator is made smaller.

Figure 35 (left):

Uncanny valley graph by Mori

Durability Prosthetic hands are used intensively and should therefore be very durable. An advantage of air pressure technology is that there is no need for electronics or gears inside the hand. This makes it less vulnerable to sand and water. A disadvantage is that the 3Dprinted material is less durable than for example an aluminium body. Customizability Customizability is an important issue in prosthetics. The prosthetic arm needs to fit the limb of the prosthesis wearer perfectly. Since every limb is different, a customized fit is needed. This customization costs a lot of time and money. 3D-printing could speed up this process, by 3D scanning a limb, digitally adapting the fit of the hand to the limb and then directly printing the hand from this file.

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Customizability might also be used to make the size and roughness of the hand fit the intact hand of the prosthesis wearer or could even be used to create a special designed futuristic limb that fits the taste of the prosthesis wearer. Costs 3D-printed prosthetics are costcompetitive since a lot of time can be saved on customization and assembling. Exploratory robots An interesting field of application of soft robots is in exploratory robots. Soft robots could also be used in rescue missions in collapsed buildings or as a human replacement in a radioactive- or otherwise dangerous environment. As soft robotics technology is further miniaturized, the technology might also be used in minimal invasive surgery. Figure 36:

Exploratory soft robot tentacle by MIT

Navigating & autonomy An important aspect of these robots is the ability to navigate through unknown environments. This means the robot needs to have sensors. For some applications, as for example minimal invasive surgery, the robot should be able to communicate its observations over distance to allow it to be controlled by teleoperation. 25

Other applications require the robot to react on its observations autonomously. When there is no possibility of powering the robot through air tubes or wires, the robot should be capable of carrying its own power source. Compliance matching Field exploration and rescue robots will often encounter easily deformable surfaces like sand, mud, and soil. Therefore, it is important that the forces that are transferred between the robot and surface are evenly distributed over a large contact area. This means that the contacting materials should share similar mechanical rigidity to minimize interfacial stress concentration, which is known as compliance matching (Majidi, 2014). This is also important in minimal invasive surgery. Conformability Soft robots generate little resistance to compressive forces. This means they have the ability to conform to obstacles and are able to squeeze through openings smaller than their nominal dimensions (Trivedi et al., 2008). Locomotion In order to explore, the robot should have actuators that allow it to move from one place to another. This means it should also have a control mechanism that creates a repetitive sequence of motions that creates this locomotion. Grasping The goal of the aforementioned robots is to collect data or manipulate their environment. In order to do so, actuators with for example grasping- and anchoring-functions are needed. An example of this type of application is

for example biopsy and angioplasty (Majidi, 2014). The actuators often need to be soft to be able to transport them to the desired location for actuation.

create these interactions. Figure 37:

Chinese dentist simulation robot

Human-Simulating robots Soft robots are composed of materials that match the compliance of biological matter and therefore are mechanically biocompatible and capable of lifelike functionalities (Majidi, 2014). The technology could be used to create training tools for medical purposes or to create bio-inspired solutions for robotic purposes. Natural movements and tactile feel As with the prostheses, natural movements and tactile feel are very important to create realistic approximations of human behaviour and appearance. An example could be the creation of natural eye movement in robots by mimicking the human eye, as proposed by Pieter Jonker (Personal Communication). Underactuated pneumatic actuators look more natural than discrete electric actuators. The compressibility of the air chambers could make the robot feel more realistic. Interactivity Direct interaction between manipulations of the robot and haptic feedback by the robot could be very useful. This principle could be applied in for example reanimation manikins or even crash test dummies that are able to simulate human reflexes. An example of this interactivity is shown in a Chinese dentist-training robot with a gag reflex. The ability to integrate sensors and actuators throughout the surface of the robot make air pressure technology a very interesting technology to Rob Scharff // Master Thesis

Customizability An interesting application for soft robotics is in trial operation for difficult operations. Pieter Jonker (Personal Communication) suggests the use of endoscopic data to create a 3D model of a placenta. This placenta can then be used for trial operation. The ease and speed of customisation using 3Dprinting make air pressure technology an very suitable technology for this application. Industrial mechanisms An interesting area of application is that of industrial mechanisms as for example pick and place-applications. An example is the air pressure technology gripper developed by Materialise. Flexibility Air pressure technology is not a competitive technology for very high precision and very high-speed pick and place applications. However, an advantage of air pressure technology is that the underactuation makes it 26

Figure 38 (right):

Materialise gripper

better capable of gripping unknown objects of varying sizes. Moreover, the compressibility of air pressure grippers as for example the jamming gripper and Materialise gripper make it better suitable for gripping fragile objects

rials as for example stainless steel and aluminium. Air pressure technology might therefore be better suitable for short-term production environments as for example proposed in the Factory-in-a-day project.

Cleanability In gripper applications as for example food processing, hygiene and cleanability is very important. It is a big advantage when the sensors and actuators can be integrated in an easyto-clean outer shape (Linde, Personal Communication)

Production costs Of course, the production costs are a very important aspect in production facilities. The production costs of 3Dprinted grippers are low compared to hard grippers that are assembled from many parts. This difference is especially big when the batch size is low, as for example a gripper design for one specific situation.

Speed Speed is very important in production facilities. This means it should always been taken into account. Because of the high speed, current grippers are feed forward controlled. This means the position of the object is captured first by using for example cameras and video analysis software. The gripper will then react to this information in a pre-defined way and does not respond to the load itself. This means sensors in the gripper itself are not necessary, although feedback from sensors inside the gripper might be an interesting addition to prevent slipping and increase precision. An advantage of air pressure technology is that the gripper itself is very light so it can be moved at a higher speed. Durability Besides efficiency, the durability of the gripper is also very important. A problem in current gripper technology is the fragility of many grippers. Integration of actuators and sensors could be an interesting way to improve the durability. However, the durability of 3D-printed materials is not as good as that of grippers assembled from mate-

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Energy efficiency Electric grippers mostly are more energy efficient than pneumatic grippers. Due to hysteresis, air pressure technology grippers use more energy than pneumatic actuators as for example pneumatic cylinders. An advantage is that air pressure is already widely available in production facilities, so big and more efficient compressors can be used.

Cooperative robots Soft robots are more capable of safely interacting with humans (Majidi, 2014). This makes it an interesting technology for cooperative robots as for example production robots and care robots. Care robots could help completing simple tasks for the elderly as for example cleaning and lifting. This could create a cheaper healthcare by replacing people and give elderly people more autonomy. Safety The unteractuation and compressibility of soft robotics make it a safer technology for interaction with humans. This makes it possible to closely interact and cooperate with robots, instead of putting them behind cages. In order to make soft robots completely safe, they need to be able to capture their environment and movements at all times. This way, the robot can prevent collisions with its environment. Communication When humans interact with robots more closely, communication is a very important issue. The robot’s appearance and movements influence the way people judge the robot’s personality and interact with the robot (Woods, 2006) (Walters, Koay, Syrdal, Dautenhahn, & Boekhorst, 2009) (Salem, Rohlfing, Kopp, & Joublin, 2011). Currently, the robot’s feedback towards the human is underdeveloped. While robot’s communication to humans is limited to screens and sounds, a large part of communication between humans is done through body language. By studying the way humans communicate and implying the same principles in robotics, the robot’s communication can be enriched. Rob Scharff // Master Thesis

For example, when a human walks into the working area of the robot, the robot could briefly point towards the human to show he has noted him and will not collide with him (Marco Rozendaal, Personal communication). The soft appearance, compressibility, interactivity and natural movements that can be realized with soft robots, can be used to create improved communication with robots. Elicit emotions Robots that cooperate with humans could also be used to elicit emotions or help humans expressing their emotions. Especially children are able to develop a symbolic relationship with a robot and regard it as a person if its appearance is human or animal like (Woods, 2006). Robots that have a more humanoid appearance tend to be perceived as more intelligent with richer personalities than robots with a mechanic appearance (Walters et al., 2009). The compressibility and natural looking movements of air pressure technology could make robots that are better capable of eliciting emotions. Examples of emotion-eliciting robots are robots in entertainment parks that interact with children with gestures and speech. An example of a robot that helps children deal with their emotions is the Shrekkie, which is a small squeezing robot for children, that distracts them from the pain when they get an injection. Ease of control Cooperative robots need to interact with people that are not trained programmers. Therefore, the control of the robot should be very intuitive. An example of more intuitive program28

Figure 39:

ming is the hands-on programming seen in for example the Baxter-robot. People can teach Baxter a task by moving its arm to show him what he should do. Another interesting development is that of robots with a learning capability by reflecting their actions.

ate technology for this application. An example of a product that could be enriched using air pressure technology is the Frebble. This product lets people squeeze their partner’s and over distance. However, the squeezing is unnatural and therefore fails to communicate the emotion.

Baxter robot Figure 40 (right):

Frebble

Communicating shapes over distance Another functionality desired in telecommunication is the communication of 3D information over distance. This could be done using dynamic shape displays, as for example the MIT-project pictured below (Brownlee, 2013). Figure 41 (right):

Dynamic shape display by MIT (Brownlee, 2013)

Telecommunication Soft robots could be used in telepresence applications to enrich long distance communication between humans with haptic communication. This social connectedness and remote collaboration is the focus point of the ‘Connecting Care’ research program (Ridder, n.d.). Simulating physical presence A desirable functionality in telecommunication is the simulation of physical presence over distance. In hard robots, it is very hard to realize the richness found in human contact. The compressibility, underactuation, natural looking movements and possibility of creating organic shapes make air pressure technology a more appropri-

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The hand movements of a person are three dimensionally captured and translated to a matrix. This allows the person to conduct physical actions over distance. Moreover, it is also possible to interact with the matrix by gestures and physical contact. This enables the user to create temporary shapes as shown in Figure 42. This concept can be abstracted to a very direct interaction between sensors and actuators to create temporary shapes. This seems an interesting application area for the air pressure technology. It would be interesting too

explore what could be achieved by combining direction-sensitive air pressure sensors and 3D capturing technology to create air pressure actuated shapes.

Haptic communication in consumer product interfaces Another interesting area of application is that of haptic communication in consumer product interfaces. Gesture-control Most consumer product interfaces are still controlled with traditional buttons and give feedback with LEDs or displays. This interaction could be more intuitive by allowing the user to operate the product using intuitive gestures. Air pressure technology sensors could be an interesting technology to recognize these gestures.

(Poelman, Personal Communication). An interesting example of providing haptic feedback using air pressure is the touch screen with pop-up buttons developed by researchers at the Carnegie Mellon University. (Green, 2009) Figure 42 This could provide the user with tactile (left): feedback to make sure he is pressing Dynamic the right button. shape display by MIT Production (Brownlee, Aspects as production speed, price 2013) and reliability are important aspects in consumer products. For this reason, additive manufacturing is not widely applied in the consumer market. Figure 43:

Touch screen with pop-up buttons from Mellon University

Interactivity The product could provide haptic feedback as a reaction to those gestures. The interactivity and lightweight of air pressure technology make it an interesting technology for this application. Examples could be a car seat that can be pressed to indicate where more support is needed, a gaming console that responds when pressure is applied to it and reacts by giving haptic feedback of the game, or a vacuum cleaner that uses the hose as interface for control and feedback Rob Scharff // Master Thesis

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Orthoses Another interesting application for soft robots is the field of orthotics. Examples could be a soft ankle foot orthotic to help prevent foot dragging, or grasping assistance and other fine motion assistance for patients who have suffered a stroke or traumatic brain injury. Sensors In orthoses, a compressible sensor could be very useful to enrich the functionality of the orthosis. For example, the air pressure orthosis that was developed by Polygerinos (Polygerinos et al., 2013) to assist finger motion for patients who suffered traumatic brain injury could be enriched with a sensor function to detect finger movement of the patient. This way, the actuator can strengthen the movements of the patient instead of working fully autonomous. Figure 44:

Rehabilitation orthosis by Polygerinos et al.

Interactivity Using the direct interaction between air pressure technology sensors and actuators could be used to create orthoses that directly correct the user when the sensor detects a wrong force. ‘Second skin’ Orthoses should function like a second skin that compensates for miss31

ing or impaired motor function by cooperating with the body’s healthy tissue. This would decrease the dependency on a physical therapist, and therefore give the patient greater physical independence and new opportunities to relearn or discover motor functions (Majidi, 2014) The compressibility of air pressure technology combined with the possibility of integrating sensors and actuators in a body customized to a patient makes it a very interesting technology to create the feeling of a second skin.


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Part 2: Concept phase This part of the report will discuss the creative process that followed the research phase. The first chapter of this part will discuss the idea generation process and methods. The result of this idea generation was a big list of ideas for potentialities. The second chapter will discuss the selection of potentialities for further exploration by prototyping. The following chapter will discuss the prototyping process and will present the conclusions from these prototypes.


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Idea generation This chapter will discuss the idea generation process. The starting point of the idea generation were the potentialities and functionalities that were found in the research phase. The application areas that were defined in the research phase were explored for desirable functionalities during a Make-session. Biomimicry and literature was used to create ideas on how to create these functionalities using air Figure 46 pressure technology. Another source (right): of inspiration were the observations Make session) from the first iteration of prototypes. A simplified model of the idea generation process is illustrated in figure 46. The steps that have been taken are shown in the elliptical shapes. These will be discussed below.

a Make-session, the participants are asked to depict interactions by creating quick models using a surplus of available materials. Make sessions are very effective in encouraging people to engage in associative, biosociative and creative thinking. Therefore, Make sessions are very suitable for supporting the jump to imagining the future on the basis of deeper interpretations of the past.

Make Session In the research phase, the potentiaties of air pressure technology were used to identify eight interesting areas for applications. The next step was to brainstorm about what functionalities would be desirable for these areas of applications, without thinking about the known potentialities of air pressure technology. This way, an extended list of desired functionalities is created. Since this brainstorm session is about future interactions, there has been chosen for a Make-session. In

Five participants took part in the Make session. During the Make-session, the eight application areas were briefly introduced on a screen, accompanied by a few interaction problems concerning this application area to get the idea generation started. For each application area there was a short brainstorm in which the participants tried to visualize their ideas using the available materials. After each brainstorm, each participant was asked to choose and build his best idea and write down what the prototype is about and what its functionalities are.

Figure 45:

Simplified model of idea generation process

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Biomimicry & Literature The next step was to explore which functionalities of this extended list could be realized using air pressure technology or a combination of air pressure technology with other technologies. The two methods used were biomimicry and conducting literature. Biomimicry Biomimicry is a design approach that aims at finding solutions to human challenges by emulating strategies and patterns that are found in nature. This idea generation method was chosen because air pressure technology, and soft robotics in general, have a lot in common with natural solutions. Many organisms also deal with underactuation and have a high amount of degrees of freedom. Since nature’s solutions are the result of billions of years of development and soft robotics is a relatively new, a lot can be learned by looking at how nature solves problems. This was done using a biomimicry methodology consisting of 5 steps (Goel, McAdams, & Stone, 2014): 1. Needs or curiosity The first step of this methodology is to choose to start from a set of needs or to explore a curiosity. These approaches are identified as the problem-driven and biology-driven design approach. Since the list of functionalities defines the needs, the problem-driven design approach was used. 2. Decompose The second step is to decompose the needs into a black box model and functional model.


3. Query The third step was to query a knowledge base to identify solutions to each function of the functional model. This was done by consulting sources as for example asknature.org (AskNature, 2015). This website provides a catalogue of nature’s solutions to human design challenges. When a specific query gave no results, an engineeringto-biology thesaurus was used to find biological search terms. This tool provides links between engineering and biological terminologies. (Goel et al., 2014) 4. Make connections The fourth step is to make connections between the problem and solutions through analogies and metaphors. 5. Create concepts The last step involves concept generation to create biologically inspired conceptual solutions. Literature Literature was searched to find existing solutions to realize the extended list of functionalities. A similar approach as in the biomimicry method was chosen, but instead of only consulting sources with biological solutions, all types of sources were accepted during the query phase. Observations first prototype iteration A third source of ideas came from the observations of the first prototype iteration. These observations led to ideas for improvement or even ideas for entirely new potentialities. The conclusions of the first iteration of prototypes can be found in appendix 1.

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Idea selection This chapter will discuss the selection of potentialities for further exploration by prototyping. To ensure enough depth in exploration a focus was chosen by eliminating some areas of potentialities. The choice for this focus is based on the interests of the different stakeholders. This chapter will start by providing an overview of the different stakeholders and their interests. Thereafter, the focus will be explained and substantiated based on these stakeholders’ interests. Since this is a research project, this focus does not mean that the eliminated areas become irrelevant. These areas might be interesting for future research. This chapter will finish with a description of the chosen area of potentialities. The eliminated areas of potetialities can be found in appendix 3. Stakeholder analysis The most important stakeholders are the TU Delft, the graduation student and Materialise. During this analysis, the other stakeholders will be ignored, since the importance of their interests are negligible in comparison to the importance of the interests of the aforementioned stakeholders. An overview of the stakeholders’ interests is presented in the interest matrix below. Each stakeholder’s interests are explained below. Materialise Materialise is interested in this graduation project because of its relevance to their current activities. Materialise is a 3D printing service provider that is Selective Laser Sintering (SLS) the flexible TPU 92A-1 material. They have already explored the potentiality of creating pneumatic actuators with this material and patented a 3D printed, pneumatic gripper. Materialise 37

is also involved in the Factory-in-aday project, that focuses on setting up a production environment in a very short time. For those reasons, Materialise is interested in gathering more knowledge about this technology with a focus on the TPU 92A-1 material. They are especially interested in ideas that might improve their current gripper. Materialise agreed on compensating the printing costs of 3D-prints that they find relevant to their interests. On a more general level, Materialise is a company and therefore more commercially oriented. Therefore, aspects such as marketability, patentability, profitability and exclusivity are important to them. Graduation student My main personal interest is to graduate at the faculty of Industrial Design Engineering (IDE). Therefore, it is important that the ideas chosen for prototyping help answering the problem statement from the graduation assignment. Another important issue is the feasibility of exploring the idea within the time frame of the graduation project. A better result can be expected when the chosen ideas more closely match my personal strengths and interests. As a student from the Integrated Product Design (IPD) Master Programme my main strengths lie in the design of innovative, technical solutions. My personal interest and motivation for choosing this graduation project was the prototyping and gathering of new knowledge that was involved in this project. TU Delft, IDE The faculty IDE at TU Delft is also interested in the graduation of the

student. The relevance to the field of industrial design and in particular the relevance to designing for our future is very important. These interests are represented in the graduation assignment. Therefore, it is important to them that the chosen ideas help answering the graduation assignment. Since this is an internal research project, the project should fit in the research portfolio of the faculty. This project is related to the ‘Factory in a day’-project the faculty is involved in. The faculty is especially interested in ideas that are focused on gathering new knowledge in this field.

Choice of focus Based on these stakeholders’ interests, the extended list of potentialities that resulted from the idea generation was narrowed down. This process is illustrated in figure 48. Choices that were made are illustrated in the elliptical shapes. Groups of potentialities are illustrated in the rectangular shapes. This subchapter will discuss the choices that were made, based on the stakeholders’ interests presented in the previous subchapter. The next subchapter will discuss the groups of potentialities.

Figure 47 (left):

Stakeholders’ interests Figure 48:

Choice process


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Focus on SLS and TPU 92A-1 As illustrated, the first choice was to focus on SLS printing with the TPU 92A-1 material. Materialise was interested to be a partner in this research project and was willing to compensate the printing costs for experiments that they find relevant to gaining knowledge on the field of 3D-printed air pressure sensors and actuators. Moreover, they were willing to share the knowledge they gained from the engineering of the Materialise gripper. Since Materialise considers the performance of the other printing techniques and materials besides SLSprinting TPU 92A-1 insufficient, they will not compensate those costs. Therefore, the choice was made to focus on TPU 92A-1. By benefitting from Materialise’s knowledge and resources, more progress could be made during the graduation project. This is also in the interest of the TU Delft and the graduation student. The potential of alternative 3D printing techniques and materials is discussed in appendix 3. Focus on technical aspects The next choice was to focus on the technical aspects instead of factors as for example appearance and user experience. This choice was made for multiple reasons. First of all, as an IPD-student, I am more interested and skilled in the technical aspects than the appearance and user experience. Therefore, more progress was expected when choosing to focus on the technical aspects. Moreover, a more thorough exploration of the technical possibilities of the technology is needed before the user experience of these technical possibilities can be explored. The potential of elaborating

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on appearance and user experience is discussed in appendix 3. Focus on exclusively using air pressure technology The next choice was to focus on exclusively using air pressure technology. An endless amount of combinations of air pressure technology with other technologies is possible. The danger of focusing too much on this big range of possibilities is the loss of depth in exploring air pressure technology itself. This loss of depth could prevent the gathering of new knowledge. The potential of combining this technology with other technologies and materials is discussed in appendix 3. Focus on actuators and sensors Finally, the choice was made to focus on the actuators and sensors and not the control mechanisms. Although the control mechanisms could be interesting for all parties, the available time frame was too small to investigate sensors, actuators and control systems. Since the sensors and actuators are explicitly mentioned in the graduation assignment and the interaction between them can also be controlled using electronic control mechanisms as for example solenoid valves, it was chosen to eliminate the printing of control mechanisms from the chosen focus. The potential of integrating control mechanisms in the 3D printed objects is discussed in appendix 3.

Chosen scope of potentialities for prototyping The chosen scope of potentialities aims at exploring possible solutions to the challenges that are listed below. This section will briefly explain how the stakeholders’ interests are represented in these challenges. Moreover, it will give an indication of how solutions to these challenges can improve human-robot interaction. Creating stiffness Humans and many animals contract their agonist muscle simultaneously with their antagonist muscle to create stiffness. This is an interesting principle to apply in air pressure technology as well. This ability to switch between a stiff and weak state is a useful potentiality that could be used in many applications as for example simulating the human muscle system to create more natural looking robots. Optimized efficiency The goal of optimizing the efficiency is to create actuators that need less air and pressure to create the same performance. This is especially useful in industrial mechanisms, where the operating costs need to be kept low to maximize the profitability. An example of a product in this context is the Materialise gripper. Due to this relevance to the Materialise gripper and Factory-in-a-day, this challenge represents the interests of the stakeholders. Besides industrial mechanisms, optimized efficiency is also important in portable products with a limited air pressure supply. An example of this type of application is a pneumatic prosthetic hand.

Shape control The goal of shape control is to control the shape of air pressure technology actuators when actuated. This could be used to more closely simulate human actuators as for example fingers. This could make robots’ movements more natural looking and improve the human-robot interaction, which is an important aspect of the graduation assignment. When there is more insight in which design parameters can be used to manipulate the shape of an actuator, the design of the actuators can be rapidly optimized for a specific task. This is interesting for applications in the Factory-in-aday context. Position control The goal of position control is to control the position of the air pressure technology actuators with- and without external forces acting on it. Controlling the position of actuators without external forces could be used to create gestures. This way, robots could communicate with humans more intuitively. When the position is controlled with external forces acting on it, this could be used in applications as for example locomotion. This could be applied in exploratory robots. Torsion The goal of torsion is to create the ability of rotating (parts of) the 3Dprinted object using air pressure. Rotation is an essential type of movement that is needed in many applications. An example is the controlling of the orientation of the Materialise gripper. Another example is the ability to create human movements as for example gestures. Direct sensor/actuator interaction


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The goal of direct sensor/actuator interaction is to create the ability of actuating air pressure technology actuators by dispersing air from another air chamber. This way, no air pressure supply is needed. This would be very useful in for example providing (haptic) feedback on the manipulation of the sensor. The exploration of sensor/actuator interactions is an important part of the graduation assignment. Static sensor/actuator interaction The goal of static sensor/actuator interaction is to measure the dispersing of air with an air pressure sensor to actuate an air pressure technology actuator. By adding a power source and a control system, more complex interactions can be realized as for example actuating with a high pressure as a reaction on a very small touch. This exploration of sensor/actuator interactions is an important part of the graduation assignment. Sensor on actuator The goal of a sensor on an actuator is to create the possibility of creating an air pressure technology sensor that can be moved by an air pressure technology actuator. This will create the possibility of two new types of interactions. The first interaction is that of the air pressure technology sensor giving feedback on external forces that are working on the moving actuator. The second one is to move the air pressure technology sensor once it is pressed, by actuating the air pressure technology actuator. As mentioned above, the exploration of sensor/actuator interactions is an important part of the graduation assignment. Bidirectional movement 41

The goal of bidirectional movement is to create an air pressure actuator that is able to create an identical movement in two directions. This could be used to improve the freedom of movement of an object. This could be useful in simulating human movements as well as in industrial mechanisms.

Prototyping This chapter will discuss the selected ideas for the second prototyping iteration.These prototypes will explore several potentialities to solve the chosen challenges. Figure 49 shows how these ideas to solve these challenges are divided over the several prototypes. This chapter will discuss the inspiration for- and result of- each of these prototypes.

Figure 49:

Overview of possible solutions for challenges divided over prototypes


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Torsion models

Figure 51 (right):

rotation of fingers in Materialise gripper

started slipping, thereby creating a rotating movement. While there is literaInspiration ture on creating bending or elongating The inspiration for torsion was a com- actuators, articles of the creation of bination of multiple findings during the torsion using air pressure technology ideation. One of the conclusions of the were not found at all. make-session was that torsion was of great importance in multiple application areas, as for example adapting the position of a gripper for gripping, simulating human movements and creating gestures. This functionality was then attempted to translate to a potentiality using biomimicry. This method led to an article in which the rotation of an elephant trunk was described (Kier & Smith, 1985). An elephant has helical muscles in its trunk that, when contracted, cause rotation of the trunk. The elephant has two helical muscles, which are each Design other’s opposites. One of them is Four different torsion-models have used for clockwise rotation, while the been created. Three of these models other is used for counter-clockwise are based on the helical muscle in rotation. By contracting both muscles an elephant trunk. The other one is simultaneously, a torsional stiffness based on the observation of the Matecan be created. rialise gripper in prototype iteration. The helical-inspired torsion models are pictured in figure 52. The concepts have been named Rigatoni, Fusili and Ruote, referencing to their similarity to these pasta types.

Figure 50:

Torsion in elephant trunk Figure 52 (right):

Torsion models Rigatoni, Fusilli and Ruote

Another idea for creating torsion was based on observations of the Materialise gripper during the first prototype test iteration (see appendix 1). When the pressure was increased while gripping an object that was unevenly divide over the fingers, the fingers 43

The Rigatoni model pictured on the left can be best explained by looking at its section. This is imaged in figure 53. When pressurized, the difference

in wall thickness around the section will cause more deformation at the side with the smaller thickness. Since the relative position of this smaller thickness is rotated over its height, the position of this relatively bigger expansion rotates as well, creating a rotational movement.

(Kier & Smith, 1985). Higher angles will result in elongation, whereas lower values will result into shortening. However, the calculation used to find this value assumes a constant volume. Since this is not the case, it is expected that the model will show some elongation or shortening. Nevertheless, this elongation or shortening is expected to be lower than the other models.

Results All prototypes show some torsion. This means the working principles are verified. However, the amount of rotation can be improved. The Triple bellow model showed an The Fuote model pictured in the middle is based on a relatively thick helical initial rotation of 42.9 degrees at a thread, with thin connections between pressure of 4 bar. However, due to inaccuracy in the printing process, the them to create a sealed-of air chamaxis was caked against the top and ber. bottom of the model and was unable to rotate freely. Therefore, the axis was The Ruote model pictured on the right is also based on a section that is removed and a steel axis was added, as can be seen in figure 54. rotated over the height of the object. The difference with the Rigatoni model is that only approximately one fourth of the internal volume is used for pressurization. The section is subdivided into four segments. Three of these segments should show minimal deformation due to a greater wall thickness and cross for stiffness. The fourth segment is designed as a bellow, aimed at maximizing deformation. An advantage of this method is the lower volume of air needed. A disadvantage could be lower performance. Another difference between this model and the previous ones is the pitch of the helix. Whereas the other have a relatively low pitch, the Rigatoni model has a pitch of 54.4 degrees. This value has been calculated as the angle for which there is no elongation or shortening Rob Scharff // Master Thesis

Figure 53 (left):

Section of the Rigatoni (left) and Ruote (right) model

Figure 54:

Triple bellow model with steel axis

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Figure 56 (right):

Graph showing the relation between the rotation and applied pressure for the Ruote and triple bellow model

This adaptation resulted into a rotation of 51.6 degrees at a pressure of 4 bar. The model is reasonably resistant to bending, but is very easily compressed. However, this can be prevented by adapting the thickness of the axis between the frames. This working principal shows potential and could be further improved. The rotation of the current model seems to be limited by the connection of the bellow to the frame. This could be improved by connecting the bellows to the frame by ball joints. The bellow could be linear instead of helical and the efficiency could be further improved by improving the design of the bellow. A downside of creating an axis and ball joints in the model is the chance of failure due to printing inaccuracy and vulnerability to wear. Moreover, even after optimalization, the maximum rotation is limited to the point where the 3 bellows start blocking each other

the helically rotated cross, the model maintained its resistance to axial and lateral external forces. The relation between the applied pressure and rotation has been plotted for the triple bellow and Ruote model in figure 56. This graph shows that the Ruote model is much more efficient in the beginning and quickly stagnates at around 1 bar.

Figure 55 (left):

Ruote, Fusili and Rigatoni model

The Ruote model initially showed a rotation of 12 degrees at a pressure of 4 bar as a result of the resistance created by the circular shape. Contrary to the Rigatoni model, most of the circular shape could be removed from the model as can be seen in figure 17. The remaining shape consists of a helically rotated cross with a bellow between two sides of the cross. This model showed a rotation of 47.6 degrees at a pressure of 4 bar. Due to 45

The rotation seems to be limited by the initial rotation of the cross over the length of the model. In other words, once the cross at the bottom of the model is parallel to the cross at the top, the maximum rotation has been reached. Therefore, decreasing the pitch of the helix could lead to a larger overall rotation. Other aspects that could be further improved are the design of the bellow, the connection of the structure to the frame and the ratio between bellow and open space.

The Fusili model showed a rotation of 45.9 degrees at a pressure of 1.5 bar. A higher pressure was not possible due to leakages as a result of elongation. The rotation was accompanied by an elongation of 35% and a significant increase in volume. Applying a vacuum caused the structure to buckle with a compression of 60%. This is mainly due to the small wall thickness and diameter. The structure is also easily bent or compressed by external forces. When compressed by an external force, the structure immediately buckles. This makes the structure incapable of carrying another structure. For some applications, as for example screwing, this elongation or compression that is related to the rotation might be desirable. The same is true for the proneness to bending and compression by external forces. However, for most applications, a structure that does not compress or elongate while rotating and does not easily bend or compress when applying external forces is desirable.

The Rigatoni model showed a rotation of 14.7 degrees at a pressure of 4 bar. This is mainly due to the rotational resistance created by the circular shape. Pressurization mainly results into lateral expansion instead of rotation. The structure is resistant to compression and bending, although some bending occurs due to collapsing of the parts with the smaller wall thickness. The amount of torsion of this model is insufficient and the working principle shows little potential for improvement. Conclusion As can be seen in figure 57,the Ruote and Triple bellow model show the most potential for further exploration. These models show a good amount of rotation without elongation and the potential for further improvement. They can be created in such a way that they are capable of holding a structure without bending or compressing due to external forces. The Ruote model seems the most promising of the two, since it requires a smaller volume of air, does not need integrated joints or axes, and shows more rotation at a lower pressure.

Figure 57:

Evaluation of torsion models


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Bellow designs

Figure 59 (right):

working principle of (silicone) bending actuator (Mosadegh et al., 2014)

Inspiration The inspiration for the optimization of the bellow was inspired by the observations of the first prototype iteration, literature on pneumatic actuators and biology. Design guidelines for bellows found in literature were implemented in the curved bellow to optimize the performance. The varying segment bellow was inspired by biology. In nature, actuators are getting thinner and shorter towards the tip of the actuator. A good example is the human arm, where the upper arm is the longest and thickest and the fingertip is the shortest and thinnest. The proportion between the length of the segments of the fingers are build up according to the golden ratio, meaning that the relative lengths of the segments can be calculated using the formula in figure 58. A copy of the current Materialise bellow was created to compare the results of the other bellow designs to.

air chamber as illustrated in figure 59(B). The inside walls of this bending actuator are thinner and have more surface area. Therefore, these inside walls are more prone to expansion than the other exterior walls. A similar principle is used in the current Materialise gripper. This principle functioned as a starting point for the optimization.

Figure 58:

Formula golden ratio

Figure 60 (right):

Indication of used parameters for geometrical analysis

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Design Curved bellow The working principle of pneumatic bending actuators is based on an extensible top layer and inextensible but flexible bottom layer. A basic (silicone) version by Mosadegh et al. is illustrated in figure 59(A) (Mosadegh et al., 2014). An improvement on this bending actuator was created by adding gaps between the inside walls of each

Polygerinos et al. (Polygerinos et al., 2013) have performed a geometrical analysis of a bending actuator. They take the parameters (l) number of bellows, (t) thickness of the walls, (b) height of the bellows and (d) distance between the bellows into account, as shown in figure 60.

This analysis is based on the hyperelastic Yeoh model and assumes (i) incompressibility of the material across the width of the actuator, (ii) the thickness of the bellow is much smaller than the air chamber height and (iii) each bellow is rigidly rotating around the interconnection point at the paper layer.

This led to the following model, where Lw is the width-justified lever of FE and Ma determines how an increase in air pressure affects the exerted force at a certain configuration as a function of the inflation distance a. As can be seen, a smaller value of a will achieve a larger value of Ma. It can also be concluded that a larger value of b will result into a large increase of Ma, since Ma is proportional to b2. Lastly, the model implies that d should be kept small. Therefore, it can be concluded that an actuator design with a larger height, thinner walls and higher number of bellows is desirable to improve the exerted force. However, this model does not take the volume of air, and thus efficiency, into account.

These principles also account for bending bellow actuators, with the only difference that the axial elongation should be (3.) maximized for the outer layer, but (4.) minimized for the inner layer to create a bending movement. Freyer et al. use a standard bellow as a starting point for optimization. This bellow is shown in figure 62 (a).

Figure 61 (left):

Model for geometrical analysis of bellow (Polygerinos et al., 2014) Figure 62:

Balancing bellow design between stiffness and The first goal can be reached by using elongation relatively high radii in the bellow actua- (Freyer et al., tor. The second goal is reached by 2014) maximizing the sectional wall lengths. This can be done by maximizing the relation of inner and outer diameter and can be further increased by creating indentations in the waves. This is shown in figure 62(b). However, the minimizing of the inner diameter decreases the lateral stiffness. Therefore, a balance needs to be found.

Another problem is the minimization of expansion in outer diameter. Adding Another approach to optimizing the stiffening rings to the bellows can debellow design was done by Freyer et crease this expansion. This is shown al. (Freyer et al., 2014). In this paper, in figure 62(c). Combining the optithree important property and permized models b and c led to a final formance goals of 3D printed linear model shown in figure 62(d). Figure pneumatic bellow actuators are given: 63 presents this bellow and its dimen1. minimum stress in the material sions in detail . during elongation 2. minimized expansion in outer diameter 3. maximum axial elongation Rob Scharff // Master Thesis

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Figure 63:

Bellow design and dimensions (Freyer et al., 2014)

Curved bellow By cutting the linear pneumatic actuator by Freyer et al. in half and adding an inextensible layer, the curved bending actuator was created. A section view of this model is pictured in figure 64. The fingertip of this bellow has also been improved. Instead of a solid fingertip, the fingertip is hollow. Therefore, it should be better capable of distributing the forces over the fingertip to prevent it from slipping. Figure 64:

Section view of curved bellow Figure 65 (right):

Materialise bellow and section view of varying segment bellow

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Varying segment bellow For the varying segment bending actuator, a bellow setup according to the golden ratio was created. However, The result was not satisfactory. This was mainly due to the amount of bellows. Whereas fingers only have three segments, the bellow was preferred to have more as indicated by the model presented in figure 61. When increasing the amount of segments, the golden ratio quickly becomes too strong, meaning the difference between the biggest and smallest segment quickly becomes too big. This made in impossible to create the smallest segment due to the minimal wall thickness. Therefore, a more reasonable ratio was chosen. A section view of the final model is presented in figure 65. Lastly, one of the fingers of the Materialise bellow was copied. This model is presented in figure 65. All bellows were designed at the same length. Therefore, the Materialise bellow can be used as a reference for evaluating the other bellows. It should be noted that small discrepancies between the Materiale bellow and the bellow of the Materialise gripper can arise, as a result of measurement inaccuracy in the .stl-files.

Movement The optimized bellow creates a larger bending than the Materialise bellow at Volume The internal volume of the curved bel- the same pressure. This is illustrated in figure 68, which plots the displacelow is 2.5 times less than the Matement in the X and Y direction for both rialise bellow. The volumes were not grippers. The beginnings of both measured during pressurization due graphs are similar, but in the end the to a small leakage in the Materialise optimized bellow showed much more gripper. However, it was noted that bending with an increase in pressure the Materialise bellow showed much more undesirable expansion.This was than the Materialise bellow. Another interesting conclusion from the graph especially visible in the expansion is that the optimized gripper shows of the inextensible layer as can be a very linear relationship between the seen in figure 66. The curved bellow showed a very minimal increase in vol- displacement and increase in pressure until a pressure of approximately ume to create the bending. Because 2 bar. The graph of the Materialise the distance between the edges on the inextensible layer is very small, the bellow is never linear and the slope already starts decreasing rapidly at inextensible layer showed no visible expansion at pressures up to 4 bar, as approximately 1 bar. can be seen in figure 67. However, the stiffness of the bellow also decreases when the indentations in the bellow shape are maximized Results

Figure 66 (left):

Increase in volume in Materialise bellow during pressurization of 4 bar Figure 67 (left):

Increase in volume in curved bellow during pressurization of 4 bar Figure 68:

Graph showing relation between X+Y displacement and applied pressure for the Materialise- and curved bellow


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Figure 70 (right):

Relation between horizontally applied force and applied pressure for three different bellows

Force The force of the bellows were measured using the setup shown in figure 69. As can be seen, the bellows are measured in their vertical position. It should be noted that the measured force is one-dimensional, while the bellow excerts force in two directions. The ratio between the verical and horizontal force strongly depends on the applied pressure and position of the bellow. The results of these measurements are shown in figure 70. It can be seen that the Materialise bellow applied significantly more force than the other bellows. An approach to understanding the excerted forces of a bellow is by looking at a pressurized bellow as a spring. The further it is pushed away from its equilibrium position, the higher the excerted force. The ‘spring constant’ is determined by the stiffness of the geometry. This stiffness is not a constant, but depends on the deformed shape of the bellow and the air pressure within the bellow.

Figure 69:

Test setup measurement force Figure 71 (right):

Shape of the varying segment gripper at 4 bar.

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Shape The design of the bellow is a good way of manipulating the shape of the bellow under pressurization. Due to a higher amount of segment length at the top than at the bottom of the varying segment bellow, the varying segment bellow shows less rotation in the lower part of the bellow. This can be seen in figure 71. This is a clear difference with the Materialise bellow pictured in figure 67.

Conclusion The curved bellow showed significantly more movement, with a smaller amount of air. This proves that the curved effect is working very well. It can be concluded that adding length at the outer side of the bending actuator improves the maximum movement that can be made. The force of the bellow is dependent on the stiffness of the geometry, the distance between the place it excerts force and its equilibrium position and the air pressure within the air chamber. The insight in the effect of different bellow designs can be used to optimize the bellow design for each specific application. Some applications might need a very strong actuator while other applications might need a very movement-efficient bellow, a very flexible bellow or a bellow with a specific shape. The adaptation of the bellow to the application’s need is a very good fit with 3D-printing.


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Octopus Inspiration The inspiration for this concept came from a concept called the ‘Shrekkie’. This concept was aimed at children that had to get an injection. The ‘Shrekkie’ is a product that these children could pinch as a natural reaction to pain. This pinch causes the ‘Shrekkie’ to move so the child is distracted from his/her pain. This is a very interesting interaction to realize using air pressure technology. The very direct interaction and the compressibility needed for pinching is a good match with the strengths of the technology. This inspired the concept of pressurizing the actuator directly from the air that is dispersed from the ‘sensor’. There has been chosen for an octopus as a metaphor for the technology. As discussed at the idea generation chapter, the infinite degrees of freedom and flexibility show many similarities with solutions found in nature, as for example octopuses.

Figure 72:

Section view of octopus

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As mentioned at the inspiration for the optimized bellow, actuators become shorter and thinner towards the tip in nature. This is also true for an octopus’ tentacles. Whereas, the narrowing bellow only changes the length of the bellow towards the top, this prototype’s segments decreases both in length and thickness towards the top of the actuator. Contrary to silicone bending actuators, where the inextensible layer is also the seal of the object, the inextensible layers of 3D printed bellows can be designed as well. The search for applications of this advantage led to the potentiality of shape control by creating adaptations to the inextensible layer. Each of the octopus’ tentacles has a different inextensible layer. Design The octopus has three seperated air chambers. The first air chamber is in the head of the octopus. This air chamber is meant to be used as a sensor. The other air chambers are used for actuation. Each air chamber contains four tentacles. A section view of the octopus can be seen in figure 72.

Each of the octopus tentacles has a different inextensible layer. The design of each of these inextensible layers will be briefly discussed below: 1. 2mm thickness This inextensible layer is thicker than the others. It is expected to be more inextensible, but also less flexible. 2. Inextensible layer with indentations This inextensible layer is 1.5mm thick with indentations between the bellows. 3. 1.5mm thickness This inextensible layer has a thickness of 1.5 mm, so the effect of the indentations can be explored. 4. 1.1mm thickness This inextensibe layer has a thickness of 1.1mm. It is expected to be less inextensible but more flexible. It might also be more prone to expansion due to the applied pressure. 5. Stiffening ribs This inextensible layer has ribs perpendicular to the length of the actuator to prevent lateral expansion. 6. Thickened middle This inflexible layer has an overall thickness of 1.5mm, with a thicker piece of 2.5mm in the middle. Since this part is less flexible, the tentacle is expected to bend less around this piece. This way, the shape of the actuator can be controlled.

ments have been made rigid by filling them with material. It is expected that there will only be bending at the unfilled segments, thereby creating the effect of rigid parts and hinges. This way the shape of the actuator can be made more like a human finger. 8. Thickness variation over width The last tentacle has a variation in thickness over the width of the inextensible layer. The left part of the inextensible layer is 2mm, whereas the right part is only 1.1mm. Besides the normal rotation, this tentacle is expected to move slightly sidewards. Results The octopus showed leakages in the tentacles as well as the sensor. A temporary fix was created by applying a Plasti-Dip coating to the octopus. This is a rubbery coating that is able to stretch a little bit without tearing. Moreover, it greatly enhanced the look and feel of the octopus. This made the octopus testable again at low pressures. The actuation through dispersing air from the sensor to the actuators worked a little bit. The tentacles showed some movement when the sensor was pushed. However, the force needed to realize this movement was very high and the movement of the tentacles only small. The force

Figure 73: Octopus model with Plasti-dip coating

7. Filled segments This tentacle was inspired by the pneumatic finger design of Cool & Hooreweder (Cool & Hooreweder, 1971). Some of the tentacles segRob Scharff // Master Thesis

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needed to create enough pressure for actuation is too high for most applications. As a second experiment, the FDM printed actuator (see appendix 2) was connected to the head of the octopus. This actuator is fully actuated at 2 bar, but could not be fully actuated using the sensor. The interaction between the sensor and actuators using an air pressure sensor and pump worked very well. The air pressure sensor with a range of 0-10 kPa responded very well to small manipulations of the sensor air chamber. This information was used to pressurize the tentacles. As indicated above, the octopus was a little leaky, so the actuators slowly depressurized when the sensor air chamber was no longer pressed. A solenoid valve was added to realize quick depressurization when the sensor was not pressed. When the sensor air chamber was pressed for a longer amount of time, the pressure slowly dropped as a result of the small leakages. The effect of the decreasing bellow height and width was clearly visible in the shape of the tentacles. The bellow shows more rotation at the bigger bellows, as can be seen in figure 74. The inflexible layers did not have much effect on the performance of the tentacles. The only tentacle that showed a clear difference was the tentacle with the filled segments. This tentacle showed a lot less movement. The rest of the tentacles showed alFigure 74: most identical movements. shape of octopus tentacle

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Conclusion The variations in the thickness of the inflexible layer had almost no influence on the performance of the actuator. Variations in the profile of the inextensible layer do influence the expansion of the inextensible layer. However, these variations also have consequences for the design of the bellow. Adaptations to the bellow design, as for example decreasing the height and width of the segments towards the top or varying the length of the segments are very effective tools to manipulate the shape of the bending actuator. Plasti-dip is a useful coating to improve the look and texture of TPU 92A-1 objects. The coating is flexible enough to endure the bending of the actuators. Pressurizing the actuators by dispersing air from another air chamber costs a lot of force and only creates a small movement. Preferably, an air pressure sensor and a pump are used to create the interaction between the air chambers.

Double bellow

Results

Inspiration The inspiration for this prototype came from multiple sources. One of these sources was the first iteration of prototyping. When a vacuum was applied to the materialise gripper, a movement in the opposite direction was created. This inspired to use a double bellow, of which one was pressurized and one was vacuumed.

Alternating pressure to both sides of the bellow to create bidirectional bending. It is possible to create a bidirectional bending by switching between the side to pressurize. However, the stiffness that is added by the double bellow, greatly decreases the effectiveness of the bellow.

Another interesting functionality was the desire to sense the position of an actuator. Disney research showed how sensors can be designed in such a way that it reacts stronger on a particular kind of manipulation (Slyper & Hodgins, 2012). This inspired to use a bellow actuator design as a bending air pressure sensor. Design The design of the double bellow is similar to the design of two curved bellows, which share an inflexible layer. The design of the curved bellow has been discussed earlier. A section view of the double bellow is shown in figure 75.

Pressure to one side, sensor to other When an air pressure sensor with a range of approximately 0-10kPa is connected to one side of the double bellow, it can be used as a position sensor. The change in shape of the bellow due to bending creates a large increase or decrease in volume. This results into clearly measurable differences in pressure. Small changes in shape can easily be detected by changes in air pressure. This change in shape could be a result of pressurizing the other bellow, but could be due to an external force as well. This means single bellows can also be used as a sensor for external forces instead of using them as an actuator. It has already been discussed that

Figure 75 (left):

Section view of double bellow model Figure 76:

Double bellow model


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the actuation pressure can be used to control the position of the bellow. However, this does not take external forces to the bellow into account. When an external force stops the bellow, it will not reach the expected position with a certain pressure. The sensor bellow could be used to detect these external forces. An actuation pressure could be linked to a certain sensor bellow value for when no external forces act on the bellow. When the measured sensor value deviates from this value at a certain actuation pressure, an external force acts on the bellow. The amount of deviation could even indicate the direction and magnitude of this force. The actuation pressure could then be increased to compensate for this external force. The double bellow is relatively stiff and the movement pattern is quite constant for different types of external forces. However, the bellows are under-actuated and in air pressure technology different shapes of the bellow could give the same sensor bellow value. A clever design of the sensor bellow could minimize this problem. Moreover, the way the sensor reacts on an the compensation force could provide information of the location and magnitude of the external force as well. This would be interesting for further research. Stiffness By adding pressure to both sides of the bellow, the stiffness of the structure can be improved. This effect was not clearly measurable due to the small size of the double bellow. However, the principle was shown with a scaled up version of the double bellow that was integrated in the final model. This will be discussed in part 3. 57

Pressure to one side, vacuum to other It is possible to create an alternating movement by switching the compression and vacuum side of the pump. However, using the same pump for the vacuum as well as the compression is not more efficient and only results into a very small bending. This is due to two main issues. The first issue is that the compression- and vacuum-power of a pump is about the difference between inlet and outlet. The second issue is that pumps are better capable of creating compression that a vacuum. For this experiment, a vacuum pump with a maximum compression of 250kPa and a maximum vacuum of -70kPa was used. When alternating between two equally sized air chambers, there is not enough volume at the suction side to create the maximum pressure. When a separate vacuum and compression pump are used, the double bellow is still less efficient than a single bellow with only a compression pump. This is due to the increased stiffness of the geometry. Conclusion By alternatingly pressurizing the bellows of the double bellow, a bidirectional movement can be created. By adding a sensor to one of the bellows, while pressurizing the other, the sensor can be used as a position control sensor. When both sides of the bellow are pressurized, stiffness can be created. However, the increased stiffness due to the geometry of the double bellow makes the double bellow much less effective.

Therefore, there has been chosen to direct the air pressure from the fingertip through the inside of the actuator Inspiration The inspiration for this idea was found towards a static connection using a in nature. Humans and animals usually flexible tube. The air pressure sensor have sensors integrated in their actua- can then be connected to the static tors, as for example the human finger. connection. It is important that the tube is as short as possible, since the People can actuate their finger to let their fingertip sense an object, but can extra volume decreases the increase in pressure caused by a manipulaalso use their actuator to react on a measurement from the fingertip (as for tion of the fingertip. Because this tube needs to be assembled into the 3D example pulling your finger out of hot print, the bottom of the frame of the water. These are important interactions that cannot be realized without a bellow needs to be open. This can be sealed with an O-ring against a botsensor on an actuator. tom that is 3D-printed in PA-material. The fingertip was also printed as a Design It is difficult to connect the air pressure separate part, since it would be imsensor to the fingertip. One possibility possible to remove the support material inside the fingertip through the is to connect the air pressure sensor hole for the tube. The fingertip can be directly into the fingertip. However, this would mean that the sensor itself glued on top of the finger. is moving when the actuator is presResults surized. This would cause difficulties As can be seen in figure 78, a piece of with the wiring of the sensor. the frame of the sensor bellow has not been printed. Since a complete frame is essential to seal it to the bottom with an O-ring, the actuator could not be used. However, it was possible to test the sensor functionality. A flexible tube was glued to the upper part and to the air tube connection. Then, the fingertip was glued on top of the bellow. Pinching the bellow resulted into a measurable difference in air pressure measured by the air pressure sensor. This means it would have been possible to let the actuator react on a manipulation of the sensor. In spite of the small wall thickness, the sensor fingertip was still quite stiff. It is doubtful whether the fingertip would deform when the actuator bumps it into an object. This stiffness was mainy caused by the spherical shape of the fingertip. Sensor finger


Figure 77 (left):

Sensor fingertip model

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Figure 78 :

Incomplete printed sensor finger

Conclusion The prototype could not be tested sufficiently due to a printing error. However, it can be concluded that the fingertip would preferably be easier deformable to ensure a measurable increase in air pressure when the actuator bumps the sensor into an object. This could be done by adapting the shape of the fingertip or by printing the fingerprint in a more flexible FDM material. Another improvement of the sensor finger could be made by easing the assembly of the tube towards the top of the finger.

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Part 3: Detailling phase This part of the report will discuss the detailling phase of this graduation project. The first chapter will discuss the choice of a benchmark product for demonstrating air pressure technology. This chapter will also describe the potentialities and functionalities of the hand. The second chapter will discuss how this benchmark product is illustrative for how air pressure technology can improve humanrobot interaction. The final chapter of this report will present a technology roadmap for air pressure technology.


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Interactive robotic hand The next step was to design a benchmark product that shows how air pressure sensors and actuators can improve human-robot interaction. This was done by an association process between the potentialities that were explored during the prototype iterations and the extended list of functionalities, as proposed by Poelman (Poelman, 2005). This process is illustrated in figure 79.

Figure 79 (right):

Association process between potentialities and functionalities

The main goal of this benchmark product is to show people how this technology can improve human-robot interaction and inspire people to find their own applications of the technology. In order to so, the benchmark product should have a strong context. Preferably, the product shows multiple functionalities that can be used to improve human-robot interaction. Besides the context, the benchmark should show as many of the prototyped potentialities as possible. In any case, the potentiality of sensor-actuator interaction should be shown in the prototype, since this is one of the main aspects of this project. During the association process multiple benchmark products were taken into consideration. The most interesting ones were the development of an orthopaedic hand for rehabilitation purposes and a robotic interactive hand. Based on the amount of functionalities and potentialities that could be presented in the benchmark, the choice was made for the interactive robotic hand that can shake hands. Shaking hands is a strong metaphor to show how the characteristics of this technology might improve humanrobot interaction.

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This chapter will discuss why this benchmark product is a good way of showing people how this technology can improve human-robot interaction. The chapter will start by explaining which potentialities are integrated in the hand. Next, the functionalities that can be realized with the hand will be explained. Finally, a vision of how the combination of all these factors will improve the human-robot interaction is presented.

Potentialities The goal of the benchmark is to show as many of the prototyped potentialities as possible, while maintaining a strong context of functionality. The pneumatic hand shows the potentialities of position control, shape control, torsion, optimized efficiency, static sensor actuator interaction and bidirectional movement. This paragraph will discuss how these potentialities are integrated in the hand.

The bottom thumb joints are fixed in the position that is desired for gripping and shaking hands. The other fingers move towards the thumb. Figure 81:

Different bellows used for the fingers and the thumb of the hand

Figure 80 (left):

Uncoated 3Dprinted interactive robot hand

Fingers The fingers are built from curved bellows. Each finger has a different length and a seperate airchamber to allow individual control of the fingers. The thumb is built from a stronger bellow to show how design differences result into different performance. The complex range of movements of the thumb are very hard to simulate. Therefore, the movement of the thumb has been simplified. This was inspired by the underactuated mechanic prosthesis hand by Jan Frankenhuyzen (Personal communication). Rob Scharff // Master Thesis

Palm The palm of the hand has two integrated air chambers that are connected to an air pressure sensor through a tube. These air chambers are located at the contact surfaces of the hand while shaking hands. This way, gripping the hand will result into an increase in air pressure. The magnitude of this increase depends on the gripping force. The air chambers are airtight, so absolute pressures can be measured Figure 82:

Sensor air chambers at both sides of the hand

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Figure 85 (right):

Creating stiffness by pressurizing both sides ot the double bellow

Figure 83:

Wrist The wrist of the robotic hand has an integrated torsion actuator. There has been chosen for the working principle of the Ruote model, since this principle is capable of creating a relatively large rotation while being resistant to bending and compression. This makes is suitable to carry the rest of the hand.

Torsion bellow Figure 86 (right):

Close-up of Plasti-dipped wrist of the hand

A double bellow has been placed on top of the torsion actuator. This bellow can move the hand forwards and backwards. By adding a sensor to one of the bellows while the other bellow is actuated, external forces on the hand can be detected, since this will result into a discrepancy between the applied pressure and expected sensor value. When both bellows are actuated simultaneously, the stiffness of the wrist can be increased. This is shown in figure 85. Figure 84:

Forward movement of the hand by double bellow

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Functionalities The hand can be used to realize two functionalities. These functionalities will be discussed below. Interactive handshake The first functionality of the hand is shaking hands. This functionality shows interaction between the sensor and the bending actuators. The air chambers in the hand palm are connected to an air pressure sensor with a range of 0-10kPa. This range is very suitable to pick up differences between a weak and firm handshake. The sensor is externally connected to the hand through a tube. This way, the hand itself is fully controlled by air and can be submerged under water. The sensor signal is used as input on an arduino. Based on the measured air pressure, the arduino sends a PWM signal to solenoid valve connected to a compressor and the actuator. The actuation pressure is determined by the PWM of the solenoid valve. A higher pressure results in a higher gripping force. A flow regulator has been placed between the solenoid valve and the actuator to gain more control. The pressure could also be controlled using a proportional pressure control valve. This solution is more efficient, but far more expensive.

Creating gestures The second functionality of the hand is the creation of gestures. The hand’s ability to rotate, bend forward and backward and individually moving the fingers and thumb allows the creation of many gestures. These gestures can be used to create communication between robots and humans through body language. As mentioned above, this functionality uses all bending actuators, the double bellow and the torsion bellow. Each air chamber is seperately controlled using PWM solenoid valves. Predefined sequences of pressurizing the actuators can can be programmed in arduino. The position of the actuator can be determined by the amount of pressure towards the hand.

Figure 88:

Raising-hand gesture to ask for attention

Figure 87 (left):

Interactive handshake


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Human robot Interaction freedom and continuous actuators, an The goal of this graduation project is air pressure technology robotic hand to investigate and show how air pres- more closely approaches the versatilsure sensors and actuators can be ity of the human hand. It is not only used to improve human-robot interac- capable of gripping objects of various tion. This section will discuss answers sizes, but is also able to shake hands to this question and will explain how and create gestures. Moreover, each the functionalities of the benchmark air chamber can be used as a sensor are illustrative for the improvement of as well as actuator. This makes it easy human-robot interaction by air presto adapt the hand to different tasks by sure technology. changing the control system, whereas mechanical solutions are often meVersatility chanically restricted to a specific One of the unique characteristics of air task. Although the control system of pressure technology that is illustrated this benchmark will still be manually in this hand is its versatility. The ability adapted, future robots could recogto cope with a richness of variation in nize tasks and automatically adapt its the environment is a typical character- actuator to this task. istic of organisms. Since a machine is often considered more robotic when Communication it more closely approaches human- or An important aspect of human-robot animal-like behaviour, this characterinteraction is the communication istic is considered a robotic characbetween the robot and the human. teristic. For example: a machine that When looking at the current state of grips a specific object when it is on a communication between robots and specific spot is mostly not considered humans, especially the communicaa robot, while a machine that moves tion from the robot to the human is around its environment in search for underdeveloped. This pneumatic hand objects to grip is considered more aims to show people how air pressure robotic. When we want robots to be technology might improve the combetter capable of coping with a richmunication of task relevant information ness of variation in its environment, and robot personality from the robot it is evident that the robot must have to the human. access to actuators and sensors that can deal with this richness in variation Personality as well. It is expected that robots will be omnipresent in our future. Each robot A good example of the versatilwill have a different function and level ity found in organisms is the human of intelligence. This means we need hand. The human hand can be used to be able to get an impression of a for numerous of different tasks. This robot at first glance. This first imprescontrasts sharply to mechanic solusion should answer questions like tions that are often focused on one what to expect from the robot and specific task as for example gripping whether the robot is dangerous. When a specific object and need an actualooking at human-human interaction, tor for every degree of freedom. Due this first impression is mostly based to the hyper-redundancy in degrees of on appearance and body language 67

instead of verbal communication. This way of sharing information could be very powerfully implied in robotics as well. For example: a very fragile robot that is not meant to be touched, might behave very skittish, while a social robot might immediately approach the human and introduce itself. The ability to create natural movements, the compressibility and the interactivity of air pressure technology are features of that make it a very suitable technology for realizing these indirect transfers of information.

Task relevant knowledge Besides the communication of personality, body language can also be used to communicate task relevant knowledge to the human. The meaning we attribute to certain body language as for example gestures could also be implied in robots. For example, a robot that needs help from a programmer might raise its hand. Another example is that of a robot briefly pointing its face towards a person when it enters the room to indicate it has seen the person and will not A pneumatic hand that is not only harm him. A self-learning robot that is capable of shaking hands, but can not completely sure of what objects also adapt the force of its grip to that to grip might start out with vibrating of the human, shows these features hands and slow movements, while it very well. Moreover, shaking hands increases the speed and stability of is an excellent example of a typically the hand when it is more certain of its human way of indirectly transferring task. This communication of task relinformation. A simple handshake gives evant knowledge through gestures is us a lot of information about the other also illustrated in the pneumatic hand. person. A firm handshake is linked to a strong and dominant male person, while a soft handshake might be linked to a children or elderly. These principles could be implied in robotics as well. For example, based on the gripping force in the sensor of the hand, the robot could make an estimation of whether it deals with a child or an adult. Instead of adapting the force of its grip to the human, the Safety & interactivity robot could also give a very firm hand- Another strength of air pressure techshake to indicate that he is a strong nology that is illustrated in the benchrobot. mark is its fitness to be used in close cooperation with humans. Due to the flexibility and compressibility, the chance of harming a human is minimal. This is shown by the very direct and close interactions that the human and robot have while shaking hands. The integrated sensors and direct interaction between this sensor and the actuators make the technology even Rob Scharff // Master Thesis

Figure 90:

Creating gestures to communicate task relevant knowledge

Figure 89 (left):

Communicating personality through handshake

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safer, since the hand can automatically react when it senses something is wrong. This is illustrated by the ability of the hand to adapt its force to that of the human. It will adapt the force of its grip to that of the human, so it will never crush the hand of a child. Besides making the interaction between humans and robots safer, the interactivity can also be used to make the interaction between the human and robot more direct and intuitive. An example of this could be very interactive handheld control systems that are controlled by pushing an air chamber and give feedback of the consequences of this action by pressurizing an air chamber. Development & production speed and availability Besides the functional characteristics, the development & production of air pressure technology products have consequences for the humanrobot interaction as well. Using widely available CAD-software and 3D printing service providers as for example Materialise, air pressure technology products can be created by anyone with a basic knowledge about air pressure technology and 3D printing. This is in contrast to most conventional production techniques that are far less accessible to the public. Another interesting contrast between 3D printing and conventional production techniques is the financial feasibility of producing single pieces or very small batches. The production is done directly from a digital file, so no initial investments as for example injection moulds are needed. Besides the lack of need for tooling, air pressure technology objects can be 3D printed in 69

one piece, thereby saving assembly time. These factors make 3D printing a very fast process to get from a concept to a product. These differences might create a shift in how robots are designed and deployed. A good example is in production robots. Conventional production robots are very big, rigid and expensive machines that are used to do one particular task as fast and as many times as possible. These machines take a long time to build and are meant to be used for many years. There might be a shift towards robots such as for example Baxter. If someone needs help in production, he hires a Baxter robot, designs and 3D prints the necessary tools, and starts producing the next day. This concept is currently developed in Factory-in-aday This radically changes the way people have to evaluate tools. Mechanic metal grippers might be more durable and precise than air pressure technology grippers, but the production speed and ease of use of the air pressure technology is far more important in the previously mentioned context.

Technology Roadmap This master thesis will finish with a technology roadmap to plan future research. This roadmap is presented in figure 91. The roadmap has been subdivided into two parts: the development of potentialities and the development of functionalities. It is important to notice that these parts are in constant interaction: A new application might create the need to develop a new potentiality, while a new potentiality might inspire towards a new application of the technology. The potentialities distinguish between actuators, sensors, control and simulation. The functionalities will discuss developments regarding application of the technology. When a certain development is related to a specific printing technique, this technique is mentioned in the box.

The content of each box will be briefly explained. More information can be found throughout this report. Potentialities Actuators Preliminary work The most important preliminary work on 3D printed pneumatic actuators is the SLS-printed bellow seen in the Materialise gripper, and the PolyJetprinted linear actuator by Freyer et al (2014). These bellow designs served as a starting point for this graduation project. Another interesting project was the FDM-printed pneumatic actuator posted on instructables (Mikey77, 2014).

Master Thesis During this graduation project, new actuating possibiities have been explored, resulting in a bidirectionaltorsion- and stiffness actuator. BeImportant external developments that sides the exploration of new actuating preceded this graduation project are possibilities, the bellow of the bending indicated in purple boxes, the imactuator has been optimized for differportant accomplishments that were ent goals. This provided insight in the made in this graduation project are influence of several design parameters indicated in green boxes and the most on the performance of the bellow. important challenges for the future are indicated in red boxes. Future challenges Although this graduation provided inThe time span for this roadmap is 7 sight in the influence of design paramyears. The length of the boxes is not eters on the performance of the belrelated to the amount of time needed low, there are still a lot of variables of for that development. However, read- which the influence is unknown. This ing a column from left to right does is needed to be able to quickly design indicate a sequence in developments. a bellow that matches the desired Sequential developments are indicatperformance. This would be a next ed with arrows.The developments that step that would strengthen projects as should be made within 7 years are fol- for example Factory-in-a-day. lowed by a future vision of air pressure technology.


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Preliminary work Figure 91; Technology Roadmap

SLS, bending actuator Materialise

Actuators PolyJet, linear actuator

Potentialities

Freyer et al.

Sensors

PolyJet, ‘intelligent’ air pressure sensors Disney Research Lab

2014 SLS, bidirectional-, torsion& stiffness actuators SLS, Exploration bellow optimalization FDM, actuator

Mikey, Instructables

PlastiDip sealing

SLS, sensor interaction with integrated air pressure actuators

Position & shape control of actuators through input static or actuated sensor

Control

Control using external valves and compressor Kinematic simulation Lipson

Simulation

Building conditions management Materialise

Functionalities

Abaqus

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Application

SLS, Gripper Materialise

SLS, Robotic hand: demonstrator of technology’s potentialities Design manual Exploration of Areas for application

2020

Vision

Exploring exact influence of design parameters on bellow performance Integration of sensors of different technologies 3D capturing soft robots

Control possibilities with new sensing capabilities Integration of control components and power sources in 3D-prints Experimental verification and evaluation environment for soft robots Parametric design tool on performance goals Development of simulation tool for air pressure technology

Evaluating appearance and user experience Design manual Case studies: Applications of air pressure technology


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Sensors Preliminary work The starting point for this project were the PolyJet printed robots with integrated air pressure technology sensors.

Future challenges An important next step is the integration of control components and power sources in the 3D-prints. This can be done in several ways, as for example 3D printing one way valves or printing housings and parts of micropumps. Once air pressure technology is Master Thesis enriched with other sensor technoloThis graduation project was the first project to combine integrated air pres- gies, this creates the need for more advanced control systems. sure sensors with air pressure actuators. Simulation Preliminary work Future challenges The preliminary work presented in the Due to the underactuation and matetechnology roadmap approaches the rial behaviour, it is very hard to know the position and shape of each part of simulation of soft robotics in a different way. Abaqus is a software package a soft robot when external forces act that is capable of creating dynamic on it. This could be enhanced using simulations with non linear matedifferent sensor technologies and/or rial behaviour. An essential condition 3D capturing. The way of integration to this software is that the material of these technologies is an important characteristics are well known. Part of aspect for future research, since a this problem is covered by Materialrobot’s awareness of its position is ise, who created software to register crucial to many robotic applications. the building conditions of each print, thereby minimizing the inconsistency Control in material characteristics between Preliminary work Since this is the first project that com- different 3D-prints. Another approach bines air pressure technology sensors to the simulation of soft robots is that of kinematic simulation. This approach with air pressure technology actuators, there was no preliminary work on represents a structure as a network of simple elements such as springs, control for such systems. beams and masses. An advantage of this approach is that it allows for easy Master Thesis incorporation of new nonlinear and An important step in controlling soft active elements as for example actuarobotics was the position control cretors, contacts and arbitrary reactive ated by adapting the pressure to the materials. actuator. Moreover, when one of the bellows of the double bellow was connected to a sensor, it could be used to Master Thesis determine the position of the actuator Although an overview of the most promising technologies were presentand correct for manipulations of the ed , no important accomplishments actuator from its environment. The pressure was controlled using a com- in the field of simulation were made during this project, pressor and PWM solenoid valves.

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Future challenges The development of a tool that allows designers to simulate the static and dynamic behaviour of their prototypes is a very important step to enable designers to adopt air pressure technology as a possible solution in their problem solving process.

Functionalities

Application Preliminary work The Materialise gripper is the only air pressure technology actuator sold in a commercial context. The combination of air pressure technology actuators and sensors has not yet been applied Another interesting development is the in a commercial context development of evaluation environments for soft robotics. This idea was Master Thesis recently presented in a presentation The goal of this graduation project is by Bret Victor (2014). to show people the potentialities and inspire them to find their own applications. This is done using the robotic hand presented in this part as a demonstrator. Furthermore, an exploration of application areas has been made to inspire people where and how this technology might have added value. When there is more insight in the exact influence of design parameters on the performance of the bellow, this information can be used to create a design tool that automatically generates a CAD file of a bellow, based on the parameters and performance goals that are required for each specific project. This design tool should also take 3D-printing design rules as for example minimal wall thicknesses into account. This development would further increase the development speed of air pressure technology applications. This is already one of the strengths of the technology.

Figure 92 (left):

Seeing Spaces by Bret Victor (Victor, 2014)

To enable designers without a lot of knowledge about air pressure technology to design air pressure technology products, a design manual has been written. This manual can be found in appendix 4. Future challenges Case studies in which air pressure technology is applied as a solution are very important for further development of the technology. Not only is this the final goal of developing the technology, these case studies will unveal the need for new potentialities as well. This connection to practice is very important during the development of the technology, since it will also provide information about the user experience of the technology. The design manual is never finished and should be supplemented with new and improved knowledge about designing for air pressure technology.


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Vision In the future, when robots are omnipresent in our lives, air pressure technology will be well known to designers. Its light weight, compressibility, interactivity, low costs, customizability, underactuation, natural looking movements and high development speed make it a popular solution for many applications. Soft robots can be developed within two days. Special CAD-tools are able to automatically generate actuators based on performance goals and outer dimensions. Control components and air chambers for sensors can be easily added to the model. Once the model is finished, a dynamic simulation can be made to verify the design. The model can be printed within a day. Depending on the application, it can be enriched with sensors to allow it to be fully aware of its body and actuators. Standard modules for controlling air pressure technology robots make it very easy to learn the robot a task and create complex sensor-actuator interactions.

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References AskNature. (2015). Ask Nature. from http://www.asknature.org Brown, E., Rodenberg, N., Amend, J., Mozeika, A., Steltz, E., Zakin, M. R., Jaeger, H. M. (2010). Universal robotic gripper based on the jamming of granular material. doi: 10.1073/pnas.1003250107/-/DCSupple mental Brownlee, J. (2013). MIT Invents A Shapeshifting Display You Can Reach Through And Touch. from http://www.fastcodesign.com/3021522/inno vation-by-design/mit-invents-a-shapeshifting-display-you-can-re ach- through-and-touch Cianchetti, M., Ranzani, T., Gerboni, G., Nanayakkara, T., Althoefer, K., Das gupta, P., & Menciassi, A. (2014). Soft Robotics Technologies to Ad dress Shortcomings in Today’s Minimally Invasive Surgery: The STIFF-FLOP Approach. Cool, J. C., & Hooreweder, G. J. O. v. (1971). Hand prosthesis with adaptive internally powered fingers. Daerden, F., & Lefeber, D. (2000). Pneumatic Artificial Muscles: actuators for robotics and automation FennerDrives. (n.d.). NinjaFlex 3D Printing Filament. from http://www.fennerdri ves.com/25c4272a-f5ab-404a-af98-a0ade180419c/_/3d/ Freyer, H., Breitfeld, A., Ulrich, S., Bruns, R., & Wulfsberg, J. (2014). 3D-print ed elastomeric bellow actuator for linear motion. Geurts, M. (2010). Armwear: Cool Prosthetics. Gibson, I., Rosen, D. W., & Stucker, B. (2010). Additive Manufacturing Tech nologies. Goel, A. K., McAdams, D. A., & Stone, R. B. (2014). Biologically Inspired De sign. Goetz, J., Kiesler, S., & Powers, A. (2012). Matching Robot Appearance and Behavior to Tasks to Improve Human-Robot Cooperation Green, K. (2009). Touch Screens with Pop-up Buttons. from http://www.tech nologyreview.com/news/413274/touch-screens-with-pop-up-buttons/ Kier, W. M., & Smith, K. K. (1985). Tongues, tentacles and trunks: the biome chanics of movement in muscular hydrostats. Instructables. (n.d.). Soft Robots: Make An Artificial Muscle Arm And Gripper. from http://www.instructables.com/id/Soft-Robots-Make-An-Artificial Muscle-Arm-And-Gri/ Lipson, H. (2014). Challenges and Opportunities for Design, Simulation, and Fabrication of Soft Robots. LTM. (n.d.). Light Touch Matters. from http://www.light-touch-matters-project. eu/ MacDorman, K. F. (2005). Androids as an Experimental Apparatus: Why Is The re an Uncanny Valley and Can We Exploit It? Majidi, C. (2014). Soft Robotics: A Perspective—Current Trends and Pros pects for the Future. Materialise. (2014a). PolyJet Digital Material Properties. Materialise. (2014b). Laser Sintering Material Properties. Mikey77. (2014). Soft robots: 3D printing artificial muscles. Retrieved from http://www.instructables.com


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MIT News Office. (2012). Soft autonomous robot inches along like an earth worm. from http://newsoffice.mit.edu/2012/autonomous-earthworm-ro bot-0810 Morin, E. L., Bryant, J. T., Reid, S. A., & Whiteside, R. A. (2000). Calibration Issues of Tekscan Systems For Human Pressure Assessment. Mosadegh, B., Polygerinos, P., Keplinger, C., Wennstedt, S., Shepherd, R. F., Gupta, U., . . . Whitesides, G. M. (2014). Pneumatic Networks for Soft Robotics that Actuate Rapidly. Onal, C. D., Chen, X., Whitesides, G. M., & Rus, D. (2011). Soft mobile robots with on-board chemical pressure generation. Poelman, W. (2005). Technology Diffusion in Product Design. Polygerinos, P., Lyne, S., Wang, Z., Fernando, L., Nicolini, Mosadegh, B., . . Walsh, C. J. (2013). Towards a Soft Pneumatic Glove for Hand Rehabilitation. Qi, Y., Jafferis, N. T., Lyons, K., Jr., Lee, C. M., Ahmad, H., & McAlpine, M. C. (2010). Piezoelectric ribbons printed onto rubber for flexible energy con version. Ridder, H. d. (n.d.). Design Innovation for Healthcare. Saerbeck, M., & Bartneck, C. (2010). Perception of Affect Elicited by Robot Motion. Salem, M., Rohlfing, K., Kopp, S., & Joublin, F. (2011). A Friendly Gesture: Investigating the Effect of Multimodal Robot Behavior in Human-Robot Interaction. Seyama, J. i., & Nagayama, R. S. (2007). The Uncanny Valley: Effect of Realism in the Impression of Artificial Human Faces. Shahinpoor, M., Bar-Cohen, Y., Xue, T., Simpson, J. O., & Smith, J. (1998). Ionic Polymer-Metal Composites (IPMC) As Biomimetic Sensors and Actuators-Artificial Muscles. Slyper, R., & Hodgins, J. (2012). Prototyping Robot Appearance, Movement, and Interactions using Flexible 3D Printing and Air Pressure Sensors. Solidworks. (n.d.). Understanding Nonlinear analysis. Trimmer, B. (2014). A Journal of Soft Robotics: Why Now? Trivedi, D., Rahn, C. D., Kier, W. M., & Walker, I. D. (2008). Soft robotics: Biological inspiration, state of the art, and future research. Victor, B. (2014). Seeing Spaces. Walters, M. L., Koay, K. L., Syrdal, D. S., Dautenhahn, K., & Boekhorst, R. t. (2009). Preferences and Perceptions of Robot Appearance and Embodiment in Human-Robot Interaction Trials. Walters, M. L., Syrdal, D. S., Dautenhahn, K., Boekhorst, R. t., & Koay, K. L. (2007). Avoiding the Uncanny Valley – Robot Appearance, Perso nality and Consistency of Behavior in an Attention-Seeking Home Scenario for a Robot Companion. Woods, S. (2006). Exploring the design space of robots: Children’s perspecti ves. doi: 10.1016/j.intcom.2006.05.001 Yanco, H. A., & Dury, J. (2004). Classifying Human-Robot Interaction: An Up dated Taxonomy Yu, Y., & Ikeda, T. (2006). Soft actuators based on liquid-crystalline elastomers. 77

Appendix 1.Conclusions first iteration of prototypes Jamming gripper The goal of this test was to verify whether the TPU 92A-1 material is flexible enough to be used as a vacuum gripper. The sphere with the smallest thickness (0.9mm) was chosen to be used as a vacuum gripper, since this is the most flexible one. Coffee grid was added to the globe and a small piece of cloth was taped to the air hole, to prevent the coffee grid from getting into the tubing and vacuum pump. An Arduino-controlled vacuum pump was used to create the vacuum. When gripping objects, the gripper was firmly pressed upon an object, upon which the air was sucked out of the globe. This approach was used to attempt gripping several objects, as for example a pen, Results The 3D printed vacuum gripper was able to grab a small range of objects. The objects needed to be cylindrical so the The reason that caused the inability to pick up most objects was the lack of flexibility. This prevents the globe from sufficiently adapting to the objects shape. This characteristic is best described by the flexural modulus. The flexural modulus of rubber used Conclusion The TPU 92A-1 material is not flexible enough to get grip on most objects. The flexibility can be increased by reducing the wall thickness. However, as indicated above, the globe with 0.9 mm was already leaking a little bit, so a further reduction in wall thickness is not possible. 3D print technology for Rob Scharff // Master Thesis

vacuum grippers could be used when the sizes of the grippers and graspable objects are scaled up. When the same wall thickness is used, the relative flexibility is bigger. Figure 93:

TPU 92A-1 jamming gripper

John Guest cartridges The tubes were connected to the spheres using John Guest cartridges. These cartridges consists of three parts that can be pushed into the product. This principle has already been applied to rigid 3D prints. Results Due to the flexibility of the TPU 92A-1 material, the O-ring was often pushed into a small hole that was supposed to stop the O-ring when the tube was inserted. When the tube was inserted carefully an airtight connection was created. Conclusion John guest cartridges can be used to create an airtight connection. However, due to the flexibility of the material, the O-ring is sometimes pushed into a wrong area when the tube is inserted. Since a hole of approximately 10 mm is needed to remove the support material, there always needs to be an additional hole besides the opening for the John Guest cartridge. Therefore, the added value of these cartridges over threaded fittings is low 78

Wall thicknesses The goal of this test was to explore the effect of the wall thicknesses. Three spheres with a wall thickness of 0.9, 1.2 and 1.5mm were printed. Results The sphere with the smallest wall thickness showed leakages at the top of the sphere. The spheres were clearly distinquishable by their stifness. Conclusion For most actuator and sensor applications, the wall thickness should be between 0.9 and 1.2mm. The minimal thickness depends on the geomery of the object Figure 94 (right):

Slipping of the fingers of the Materialise gripper

Gripper The goal of this test was to get familiar with the material and the actuators. The gripper was analyzed on several aspects, as for example the inflation, force, the actuation speed and the bellow efficiency. Results During the analysis of the gripper, the following aspects were onserved 1. Slipping of fingers When the applied pressure was increased to a certain level, the fingers started slipping. This effect can be seen in figure 44. This happened especially fast when the gripper was gripping an object that was unevenly divided over the grippers’ fingers. 2. Inflation When the applied pressure was increased, undesirable inflation appeared at the inflexible layer

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3. Ability to grip objects The gripper was capable of gripping a variety of objects. Due to the underactuation, the gripper adapts to the object. The right amount of applied pressure strongly depends on the type of objects. For vulnerable objects, the pressure is kept as low as possible to avoid damaging the object. This makes it very hard to grip small and vulnerable objects, since a larger amount of pressure is needed to create enough rotation, which means the applied forces will be higher 4. Connection of threaded fitting The flexibility of the material made it very hard to insert the threaded fitting. It is very hard to cut the material, since it will just deform and bounce back.

2. Experiment FDM-printed flower muscle There are multiple flexible materials available for FDM printers, as for example NinjaFlex and flexPLA. These materials are very flexible and strong. Untreated FDM-printed objects are not airtight. However, the FDM-printed objects can be sealed using a mix of Loctite and MEK. This was shown in a recent Instructables-project in which an air muscle was created using a FDM printer (Mikey77, 2014).

Figure 95:

Flower muscle with Plasti-dip coating unactuated and fully actuated

A small experiment with a FDM-prototype showed that flexible FDM-prints can also be sealed using Plasti-Dip spray. This is a much easier and cheaper process, since MEK is a dangerous substance and Loctite is expensive. Moreover, the dipping process needs a dipping reservoir which will eventually dry out. For this experiment, the flower muscle that was used in the instructables project was printed on an adapted Ultimaker and a Felix-printer. The next step was to pinch a small hole in the model and insert a tube. Pressurizing this object had almost no effect. The model and tube were then coated with 3 layers of Plasti-Dip. The result is shown in figure X. Applying a pressure of approximately 2 bar led to full actuation. Although the material is not as durable as for example TPU 92A-1, the process is very valuable as a quick way of testing certain principles before principles.


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3. Description of areas of potentialities This appendix will discuss the areas of potentialities that have been eliminated from the focus. Each eliminated area of potentialities will be described including some of the ideas that were generated within this area. For each area, a conclusion has been written in which the potential of the area of potentialities will be estimated. Alternative 3D printing techniques and materials Besides selective laser sintering, air chambers could also be created using PolyJet- or FDM-printing technology. Both printing techniques have already been used to create air pressure actuators. The flexible material Tango Plus can be printed using a PolyJet printer. This material has already been used to create air pressure sensors (Slyper & Hodgins, 2012) and actuators (Freyer, Breitfeld, Ulrich, Bruns, & Wulfsberg, 2014). The material is more flexible than TPU 92A-1 but tears far more easily with microcracks already showing at a pressure of 500mbar (Freyer et al., 2014). An interesting feature of the PolyJet printing technique is the ability to print multiple (hard and soft) materials simultaneously. However, the connections between the rigid and elastomeric material dissolve at 900mbar. (Freyer et al., 2014) Figure 96:

TPU 92A-1 Jamming gripper There are multiple flexible materials available for FDM printers, as for

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example NinjaFlex and flexPLA. These materials are very flexible and strong. Untreated FDM-printed objects are not airtight. However, the FDM-printed objects can be sealed using a mix of Loctite and MEK. This was shown in a recent Instructables-project in which an air muscle was created using a FDM printer (Mikey77, 2014). A small experiment with a FDM-prototype showed that flexible FDM-prints can also be sealed using Plasti-Dip spray. This is a much easier process. More information about this process can be found in appendix 2. A downside of FDM is the complexity of removing support material from air chambers. A last alternative to 3D-print flexible objects is the flexible resin for the Form1 printer. This is an affordable SLA printer. However, this material has not yet been tested. Some of the ideas for this area of potentialities that might be interesting for further research are: Bone structures The ability to print a hard and soft material simultaneously could be used to imitate bones and muscles. The soft material could be used to create pneumatic muscles that act on a rigid structure printed in hard material. Printing a hinge in the hard material, and placing a pneumatic muscle on both sides could imitate the functionality of the biceps and triceps. This could be used to create very natural looking movements. Bi-metal inspired bending It would be interesting to verify whether the ability to print different materials could be used to create a bi-metal inspired bending actuator. When two layers of materials with different heat expansion coefficients are printed

on top of each other, heating would cause one layer to expand more than the other layer, thus creating a bending movement. Jamming gripper The flexible materials for both PolyJet and FDM are more flexible than the TPU 92A-1 that is used in selective laser sintering. Since the lack of flexibility was the main factor limiting the functionality of the jamming gripper from the first iteration of prototypes (for results, see appendix 1), printing the jamming gripper using PolyJet or FDM might improve its functionality. Air permeable structures As mentioned above, FDM-printed objects are not airtight until sealing them. Since only the outside of the printed object is sealed, the inside of the product remains permeable. Although this might sound like a disadvantage, this characteristic can also be exploited. It could be used to create internal structures that manipulate the stiffness of the object without locking up air. Conclusion Some of the ideas within this area of potentialities are very interesting to explore. However, the range of applications of these ideas seems limited, due to the limitations in strength, durability, removability of support material and reliability of these printing techniques and materials. The uniqueness of the PolyJet-technique seems to be the ability to print multiple materials. The uniqueness of the FDMtechnique seems the approachability of this technique, and the permeability of the material.


Appearance and user experience Besides the technical possibilities, another topic for exploration could be the appearance and user experience of the material and air pressure technology. Some of the ideas from this area of potentialities that might be interesting for further exploration are: Material experience Since the goal is to use the printed TPU 92A-1 material in objects that directly interact with humans, the user experience of this material is very important. This should show how people experience characteristics as for example the appearance and texture of the material and how these characteristics can be manipulated. An example could be the use of coatings as for example flocking to give the object a softer appearance. Currently, there are difficulties in finding coatings that are flexible enough. Force experience Besides the material itself, the forces that the printed objects act on its environment are of importance to the experience as well. As discussed by Ronit Slyper (Slyper & Hodgins, 2012), the tactile feel of pressing the air pressure sensors can be adapted. A leaky seal creates a squishy feel, while an airtight seal makes the air chamber feel like a firm balloon. Research could show what is possible and what is desirable in which situation. Moreover, the perceived safety of the user in relation to the actuator forces is also very important. It would be interesting to compare these results to the perceived safety of mechanical robots with similar forces.

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Movement and shape experience The movements and shape of the object are of importance to the experience as well. More natural movements and shapes might be perceived as more pleasant than mechanical ones. However, when an object simulates human movements too accurately, this might give the user an uncanny feeling. Another interesting aspect could be the use of air pressure technology to create robot gestures, enabling the robot to give feedback to humans. In order to do so, an exploration of gestures is needed. This should show which gestures have a good communicative value and how exactly these gestures should look like. Conclusion This area of potentialities is imporManometer tant to successfully implement the working prin- technology in society. However, this ciple type of research would probably be more useful during a later stage in the development of the technology, when there is more knowledge about the technical possibilities of the technology.

Integration of technology with other technologies or materials Lots of new potentialities can be realized by combining air pressure technology with existing technologies and materials. Some of the ideas from this area of potentialities that might be interesting for further exploration are: Manometer By combining the air pressure actuated parts with mechanical components, new potentialities can be realized. One possibility would be to use the working principle from a manometer. The deformation of a hollow tube under pressure is transferred to a gear rotation as is illustrated below.

Figure 97 (right):

Paddlewheel It would be a possibility to integrate a paddlewheel in the 3D-print. When an air chamber needs to be depressurized, the air can be sent through this paddlewheel to create a rotation. Using filling material An interesting direction is the use of a filling material inside (some of) the air chambers. The air chambers could also be filled with water to create hydraulic actuators and sensors. The incompressibility of water might enable new potentialities. Other filling materi83

als could be for example coffee grid to create a jamming gripper (Appendix 1) or kinetic sand to create stiffer, yet deformable air chambers. Electronics Another interesting direction is the integration of electronics in the 3D printed objects. Actuators as for example LEDs could be integrated to create new interactions and possibilities. Another possibility would be the integration of sensors as for example piezo sensors to be able to monitor the movements of actuators more precisely. Last but not least, it would be interesting to explore the possibility of partly printing the actuators, as for example the housing of a micropump. This way, only the piezo-elements of the micropump have to be added. Conclusion The possibilities of combining this technology with other technologies are endless. This might lead to exciting new potentialities. Air control mechanisms Besides creating actuators and sensors, air pressure technology could also be used to create control mechanisms. These control elements can be integrated in the 3D prints to replace rigid external valves. 3D printed control mechanisms as for example oneway valves in 3D prints could prove itself very useful in creating more complex interactions between the different air chambers. Some of the ideas from this area of potentialities that might be interesting for further exploration are:

Tesla valve An interesting one-way valve design is that of Tesla. Whereas most valves have moving parts, this valve is based on the creation of different flow paths from the different inlets (see figure 98). Whereas the path from the right to the left has a low resistance, the path from the left to the right has a very high resistance. It would be interesting to integrate this valve in a 3D printed object. Marble blockade Another control mechanism could be created by using a marbles to block air chambers. One possibility would be to design the air chamber in such a way that the marble is only capable of blocking one of the inlets of the air chambers. This way, a one way valve could be created. This principle is shown in figure 98. Another possibility would be to create a cove inside the air chamber that locks up the ball. When the cove is on the bottom of the air chamber, the ball is locked up and air can freely pass the air chamber. When the cove is on the top of the air chamber, the ball will block of the air chamber. This way, a torsioncontrolled valve could be created. This principle is illustrated in figure 99.

Figure 99:

One way valve (above) and torsion controlled valve (below) using marble blockades

Figure 98:

Tesla one-way valve Rob Scharff // Master Thesis

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Sensor-actuator interactions The possibility to add valves between different air chambers, as discussed above, enables a lot of new sensoractuator interactions. An example is given below. When Air chamber A (sensor) is pressed, the pressure in air chamber B will build up. Air chamber B could be a bending actuator that is actuated stepwise with each pressing of air chamber A. When a pressure sensor is added to air chamber B, the air chamber could also be used as a counter of how many times air chamber A is pressed. This information could be linked to a valve that releases the air from air chamber B for every x time air chamber A is pressed. This way, a pneumatic capacitor could be created. Of course, this is just one of the many possibilities. Figure 100:

Using integrated oneway valves to create a pneumatic ‘capacitor’

Conclusion This area of potentialities is very promising for further research. More interesting and complex interactions between the sensors and actuators can be realized when air control components are used. The possibility to integrate these components into the 3D prints, instead of adding rigid components afterwards is very interesting. This might reduce the size, weight, rigidity and costs of soft robots.

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4. Design guidelines: 3D printing for air pressure technology This appendix will provide an overview of the practical design knowledge about designing for air pressure technology that was gained during this graduation project. Future research could supplement and improve on this manual. This design manual could be uploaded to the online soft robotics toolkit (www.softroboticstoolkit.com) Seperate design guidelines have been written for each 3D-printing technique. Selective laser sintering This subchapter aims to provide design guidelines for selective laser sintering air chambers in TPU 92A-1. The most important considerations during the design of 3D-printed air chambers are the wall thicknesses, the removal of support material and the way of connecting the object. These challenges will be explained below. Design actuators The optimal bellow design strongly depends on the desired performance of the bellow. The bellow design by Freyer et al. can be taken as a starting point. The dimensions of this bellow are pictured in figure 63. The indentations in the bellow can be made smaller to create a stronger but less effective bending actuator. When a torsion bellow is created, the wall thickness of the bellow should be made thicker to compensate for the helical rotation. Scaling the bellow up, reduces the required pressure to reach a certain rotation significantly. Pressures of up to 6 bar can be applied to the bellows.


Design sensors The design of the sensor also depends on the application of the sensor. Figure 27 indicates how variations in the printed shape can be used to create sensors that can be used to determine the type of manipulation. The wall thickness in combination with the geometry and airtightness of the air chamber determines how hard it is to deform the sensor. These parameters can be manipulated to fit the application. Leaky air chambers are easier to compress. However, an airtight air chamber is needed to measure absolute pressure over a longer period of time Assembling of the model 1. Gluing Preferably, the use of glue to assemble the model is avoided. When the use of glue is absolutely necessary, it is important to choose a type of glue that is suitable for flexible materials. A suitable glue is Loctite 5. 2. Fittings Fittings are needed to connect a tube to an air chamber. There are lots of different types and sizes of fittings. Considering the minimum hole size of 10mm needed to successfully remove support material (see chapter 5), 1/4’’ threaded push-in fittings are very convenient. The TPU 92A-1 material is very hard to cut. Therefore, inserting the threaded push-in fittings can be very difficult. The best way to insert the threaded fittings is by starting with a tap wrench. Thereafter, the threaded fitting can be screwed in. Instead of 86

cutting through the material, the fitting simply oppresses the material. This may result in unwanted bumps of material. When multiple threaded push-in fittings are screwed in close to each other, the printed object can start to bend due to the surplus of material. When modelling the object, it is recommended to add the threaded pushin fittings and tubing in an assembly as well. This way, mistakes can be detected in an early stage of the process. First check whether the hole can be reached by the tap wrench. Next, check whether there is enough room to place the fitting on top of the whole and reach it with a wrench. Last but not least, check whether the tube can be tucked away without going beyond the minimum bending-radius. Make sure the product is able to stand on itself when a tube is connected to it (so avoid fittings on the bottom of the product). Appearance of the model TPU 92A-1 can be coated using a Plasti-Dip coating to enhance the feel and look of the product. This coating is available in many different colours and can be bought as a spray or a dip.

tion into account. Round geometries should be printed parallel to the printing layering. Round geometries that are oriented perpendicular to the printing layers are very prone to warping and become oval. The printing orientation and location in the printer also influences the airtightness of the air chambers. 2. Wall thickness The minimum wall thickness used in a model should be no less than 1mm. Use of smaller thicknesses result into printing difficulties and models that leak air. The top of spherical 3D prints is extra vulnerable for leakages. Therefore, spheres are preferably printed with a wall thickness of 1.5mm or higher. The maximum wall thickness used in a model should be no more than 10mm. Use of thicker parts is usually not necessary and therefore a waste of material. Moreover, excessive use of material leads to warping of the model.

3. Removal of support material All air chambers must have an opening big enough to remove the support material. This hole should be approximately 10mm in diameter, so a brake cable can be inserted in the air chamber to remove the support material. Three layers of spray should be In most cases, the most convenient enough to create an opaque layer of way of doing so is by using the same Plasti-dip. However, some bellows opening that is needed for the threadmight be hard to reach and need extra ed push in fitting. layers. Unwanted Plasti-Dip can be easily pealed of. To ensure that all support material can be removed, the brake cable should 3D printing of the model be guided through all segments of the air chamber. The path towards 1. Orientation, location & Geometry the opening should be as smooth a It is important to take the geometry possible. Obstacles and unguided or and printing orientation and locasharp corners should be avoided. An 87

advantage of the flexible material is that it can be manipulated from the outside of the product to remove support materials from difficult spots. Fused deposition molding This subchapter aims to provide design guidelines for fused deposition molding of FilaFlex. Printer To print this flexible material, it is important to have a 3D-printer with an extruder in the print head. The following settings were used: Printing temperature: 230 ° C Flowrate: 107% Print speed: 20 mm/s Design Since it is not possible to remove support structures from the inside the air chambers, support structures should be avoided. This means no large flat overhangs can be created. Preferably, the gap between the two sides of the air chamber are kept smaller than 2mm. Bigger gaps can be filled by adding extra material on top and/ or building with an angle. The connection of the air chamber to the tube was created by adding a square hole with sides equal to the outer diameter of the tube.

connection of the tube to the printed object is sealed as well. Polyjet This subchapter aims to provide design guidelines for Polyjet printing Tango Black Plus material. This printer has not been used to create prototypes during this graduation project. The guidelines listed below are found in literature. -The minimum hole size for support removal should be 8mm. - All edges should be rounded to avoid tearing - Applied pressure should not exceed 500mbar to avoid tearing Recently, the faculty of Industrial Design Engineering has obtained a Objet Connex3 printer and a flexible material for the Form1 printer. Experimenting with these printers could provide new insights in their suitability for air pressure technology.

Sealing The FDM-printed objects are not airtight by itself. However, the objects can be sealed using Plasti-Dip spray. Three layers of Plasti-Dip spray are enough to seal the printed object. It is important to insert the tube in the printed object before spraying, so the Rob Scharff // Master Thesis

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