Development of a pneumatic soft actuator as a hand finger for a collaborative robot Michele Gabrio Antonelli
Walter D’Ambrogio
Francesco Durante
DIIIE University of L’Aquila Via G. Gronchi,18, L’Aquila Italy +39 0862 434329
DIIIE University of L’Aquila Via G. Gronchi, 18, L’Aquila Italy +39 0862 434352
DIIIE University of L’Aquila Via G. Gronchi, 18, L’Aquila Italy +39 0862 434327
[email protected]
[email protected]
[email protected]
confined in restricted areas and the human-robot interaction is absolutely forbidden. Nevertheless, there are some operations too complex to be performed by a robot (i.e. the recognition of objects in spaces with different lighting conditions) and too demanding or alienating for a human worker, in which the high accuracy of a robot is required (i.e. positioning of mechanical components). To combine the accuracy and the repetitive performances of the robots with the individual competency and human’s ability and to make possible that robots share the same workspace with humans [1], collaborative robots were conceived and introduced in the manufacturing industry. Several constructors produce collaborative robots that address safety and reliability, the two main requirements for coexistence and human-robot interaction. Robots have a light structure without edges and are often covered with soft materials to reduce the effects of accidental impacts. Moreover, robots are equipped with force control systems, vision systems to prevent impact damages and particular spaces can be inhibited by software. Finally, some of them, with an anthropomorphous structure, are provided with kinematic redundancy in order to perform bio-inspired movements. Specifications and guidelines defined in [1] include the workspace, the behavior of the human operator and the end-effectors. Regarding the latter, typical industrial end-effectors are vacuum grippers and grippers equipped with two or more rigid fingers pneumatically powered or by electrical motors: safety is assured by setting the maximum value of the developed force equal to about 20 N or by the implementation of a force sensor that stops fingers if the load exceeds a value preliminary fixed by the operator. Currently, industrial applications do not make an extensive use of soft actuators, intrinsically safe and already adopted in the field of medicine [2] and rehabilitation [3]. Nevertheless, several researchers developed soft actuators able to perform a bending motion and suitable to be used as fingers of a collaborative robot. These actuators are made of a soft rubberbased material with high compliance and pneumatically powered. Two types of solutions for bending can be distinguished: a tube or an element with a suitable shape made only of the soft material; the combination of a soft material tube with an external suitable rigid reinforcement. In the first case, the tube is divided into several chambers; the combination of different air pressure values inside them provides for the bending motion [4, 5]; otherwise, the tube should be not axisymmetric showing different values of thickness: the bending motion is carried out because the thinner edge increases its length while the thicker one remains at the same length [6]; moreover, the tube can show an asymmetric bellow profile with the same thickness: the working principle is equal to the previous asymmetric actuator but the presence of the bellows allow to a higher flexibility and greater rate of expansion [7-9]; finally, the tubular structure can be realized by two bonded
ABSTRACT Pneumatic soft actuators produce flexion and meet the new needs of collaborative robotics, which is rapidly emerging in the industry landscape 4.0. The soft actuators are not only aimed at industrial progress, but their application ranges in the field of medicine and rehabilitation. Safety and reliability are the main requirements for coexistence and human-robot interaction; such requirements, together with the versatility and lightness, are the precious advantages that is offered by this new category of actuators. The objective is to develop an actuator with high compliance, low cost, high versatility and easy to produce, aimed at the realization of the fingers of a robotic hand that can faithfully reproduce the motion of a real hand. The proposed actuator is equipped with an intrinsic compliance thanks to the hyper-elastic silicone rubber used for its realization; the bending is allowed by the high compliance of the silicone and by a square-meshed gauze which contains the expansion and guides the movement through appropriate cuts in correspondence of the joints. A numerical model of the actuator is developed and an optimal configuration of the five fingers of the hand is achieved; finally, the index finger is built, on which the experimental validation tests are carried out.
CCS Concepts • Computing methodologies➝Model development and analysis • Computer systems organization➝Robotic components • Computer systems organization➝Sensors and actuators.
Keywords Soft actuator; Pneumatics; Finite Element Model; Collaborative Robotics.
1. INTRODUCTION In the manufacturing industry, robots are typically used for performing operations that must be carried out repeatedly, quickly and with a high level of accuracy. Due to these topics, robots are Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from
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DOI: https://doi.org/10.1145/3185066.3185079
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and violet colored, respectively; for sake of clearness, the external gauze, the red one, is not represented in the overall FE model. Then, the construction of the model requires the geometrical parameters of the actuator. As shown in Figure 1, the variable geometric input parameters of the model are: F1 (length of the Proximal Phalanx), F2 (length of the Intermediate Phalanx), F3 (length of the Distal Phalanx), open_F1 (axial extension of the Metacarpophalangeal Joint), open_F2 (axial extension of the Proximal Interphalangeal Joint), open_F3 (axial extension of the Distal Interphalangeal Joint) and α (circumferential extension of the cut). The value of the parameter hand (length of the Metacarpal) was fixed equal to 10 mm: it does not affect the behavior of the actuator. The same value of α was adopted for all the joints in order to reduce the number of variable geometric input parameters. The extensions of the interphalangeal joints are constructed in order to be symmetrical with respect to the centers of them, placed in correspondence of the end of the previous phalanx. Constant geometric input parameters are the external diameter of the tube (equal to 20 mm) and the thickness of the tube (equal to 2 mm). The dimension of the elements, after preliminary simulations, was fixed equal to 2 mm along the axial direction (equal to the size of the mesh of the gauze) and equal to 2.24 mm in the circumferential direction, to have an integer number of nodes (equal to 28) along the circumference.
concentric films with different stiffness and balloon structures are designed between the two films: the swelling of the balloons structure, due to the compressed air, provides for the bending [10]. In the second case, the axisymmetric tube can be combined to an external rigid gauze, as in McKibben pneumatic muscles [11], divided into two sectors with different braided angles so that, under the pressure action, the sector with braid angle lower than 54.7° is contracted and the sector with braid angle greater than 54.7° is extended [12]; otherwise, the same braid angle can be adopted around a asymmetric tube with a not constant rectangular section in which the section of the proximal side is greater than the distal one [13]; finally, the same braid angle can be adopted around a tube with a constant rectangular section [14] or with a semicircular section [15] in which a surface is totally covered by a layer of an inextensible material. The external gauze has always the aim to avoid expansions of the tube. The present work is focused on the development of a pneumatic soft actuator with high compliance, low cost, high versatility and easy to produce. The actuator must act as a finger of a robotic hand that can faithfully reproduce the motion of a real hand. The actuator is made of a silicone rubber tube wrapped by a square-meshed gauze. The bending is allowed by the high compliance of the silicone rubber and by appropriate cuts of the gauze in correspondence of the joints. A numerical model of the actuator, implemented by a finite elements (FE) code, will be detailed and the achieved optimal configuration of the five fingers of the hand will be reported; the prototype of the index finger and its construction will be described. Finally, the experimental validation of the numerical model carried out with the experimental prototype will be discussed.
2. THE NUMERICAL MODEL The actuator is made of a silicone rubber tube closed at one end and wrapped by a polyamide square-meshed gauze (2 mm side dimension). When the inner tube is pressurized, the gauze has the aim to contain the axial and radial expansions of the tube and to guide the movement through appropriate cuts in correspondence of the joints. The placement, the axial and the circumferential extensions of the cuts allow to perform the bending of the actuator, as a real finger. The overall actuator shows a non-linear behavior caused by the non-linear constitutive law of the silicone-rubber and the geometrical non-linearity due to the large strains. Moreover, an interaction of two materials, silicon rubber and polyamide, occurs. Finally, the particular placement of the joints along the tube does not allow to treat the actuator as an axisymmetric structure. For these reasons, a 3D FE model was developed: at first, for the design of the index finger and then for the other fingers. The adopted commercial FE code was Ansys 18.2, Academic edition. The construction of the model is carried out by an algorithm developed in Matlab environment that produces a script file in the Ansys Parametric Design Language (APDL) to be read by the FE code. The construction of the model is firstly based on the definition of the types of material to be modeled. On the basis of a previous study [11], the silicone rubber of the tube was modeled as a hyper-elastic material with the firstorder Mooney-Rivlin formulation, based on the c10 e c01 coefficients equal to 0.0694 and 0.0628, respectively; Poisson ratio equal to 0.46. Polyamide material of the external gauze was modeled as linear isotropic by the Young modulus, 2000 MPa, and Poisson ratio, 0.1. The closed end of the tube was modeled as a plug made of aluminum whose behavior is linear isotropic (Young modulus equal to 70 GPa; Possion ratio equal to 0.33), in order to avoid strains of the distal surface of the actuator. In Figure 1, silicone rubber, polyamide and aluminum are cyan, red
Figure 1. The FE model of the actuator. The wall thickness of the tube counts three nodes. The algorithm constructs the nodes of the external surface; then, the nodes of the intermediate one; finally, the nodes of the inner surface. After the node construction, the first set of elements is for the tube: SOLID185 elements (hexahedral element with 8 nodes and three degrees of freedom for each node) of the Ansys library. The second set is for the plug with the same SOLID185 elements. Finally, the third set is for the gauze: LINK180 elements (beam element with 2 nodes and three degrees of freedom for each node) of the Ansys library. In correspondence of the joints, cuts were modeled without the construction of the axial elements of the gauze for the entire axial and circumferential extension of the joint. Since external gauze can be assumed rigidly locked with the tube, the numerical model does not simulate a possible sliding of the gauze on the tube. For this reason, LINK180 elements adopt the same nodes of the external surface of the tube belonging to SOLID180 elements. A fixed constraint was applied in correspondence of the open end of the tube. A pressure value was applied to the internal surfaces of the tube in order to act
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perpendicularly to each element. The pressure was applied according to a ramp, from zero to a pressure value equal to 0.30 MPa. Non-linear analysis were carried out and based on the Newton-Raphson method. Several analysis were carried out with the aim to find the optimal values of the variable geometric input parameters to obtain output parameters consistent with the real human finger. Starting from the measurements of the index finger [16], values of geometric input parameters were adjusted in order to obtain numerical output angles in the range of the physiological angles θ1, θ2 and θ3 of the real index finger, where θ1 is the angle around the Metacarpophalangeal joint (MCP), θ2 is the angle around the Proximal Interphalangeal joint (PIP) and θ3 is the angle around the Distal Interphalangeal joint (DIP). Each angle is measured as the external angle between the straight line of the previous phalanx and the straight line of the next one. Table 1 reports the physiological values of the index finger and the optimal values of the numerical model. Numerical values of the angles were achieved by an image analysis carried out on the screenshot of the Ansys displacement results; moreover, they refer to the maximum flexion of the finger for a pressure value equal to 0.20 MPa. α was achieved equal to 270°.
F1
Physiological Value 39.78±3.13 mm
Numerical Value 40 mm
F2
22.38±2.51 mm
22 mm
F3
15.82±2.26 mm
20 mm
open_F1
-
12 mm
open_F2
-
16 mm
open_F3
-
10 mm
θ1
0°≤ θ1≤75°
74°
θ2
0°≤ θ2≤100°
98°
θ3
0°≤ θ3≤65°
74°
Pinky
F1
32 mm
33 mm
F2
-
18 mm
F3
27 mm
19 mm
open_F2
-
16 mm
open_F3
16 mm
10 mm
Figure 2. The mold for the silicone rubber tube. During the casting operation, the mold is placed on a vibrating plate to facilitate the casting of the silicone rubber and the removal of air bubbles formed during the preparation of the silicone rubber and the casting. After the removal of the tube from the mold and the removal of burrs from the tube, as shown in Figure 2, one side of the gauze is glued along the entire axial length of the tube. Then, the gauze is rolled up to the tube; a strict adherence between the gauze and the tube must be assured. Two rolling turns are necessary. It is important to respect a good overlap of the mesh of the gauze. The opposite side of the gauze is welded to the underlying layers of it. Welding is carried out along the entire axial length of the tube, in correspondence of the glued side, and it is placed in the lower part of the actuator when cuts will not be carried out. Gauze is also welded in correspondence of the closed end of the tube to act like a plug. Then, cuts are carried out by scissors, according to the parameters reported in Table 1. The stacked layers corresponding to the cuts are welded to avoid sliding among them. The last operation is the assembly of the plug at the open end of the actuator. It is made of a steel tube, mounted inside a 3D printed cylindrical bushing, equipped with a pneumatic fitting for the air inlet/outlet. Pneumatic seal is assured by the no-leak device inside the tube; mechanical seal is assured
Table 2. Optimal configuration of the thumb and the pinky Thumb
12 mm
The prototype of the actuator was based on the numerical values of the index finger parameters. It is made of two components: a one closed end tube and an external gauze. The silicone rubber adopted for the tube is the XIAMETER RTC-4250-S, a bicomponent silicone rubber for molds produced by Dow Corning; the gauze, thickness equal to 0.25 mm, is made of Polyamide 66. The tube was made by casting the viscous silicone rubber into a 3D printed mold. As shown in Figure 2, the mold is made of two shells placed into a cylindrical basement. When the shells are assembled, they create a cylindrical hole. An internal core is placed between the shells: it shows a suitable shape in order to be coaxial with the cylindrical hole. Moreover, it shows an annular cavity to realize a no-leak device, such an O-ring, in the tube. The height of the core is lower than the height of the shells in order to realize the closed end of the tube. A set of 12 screws assures the assembly of the shells; adhesive tape is placed around the contact surfaces of the shells to avoid the leak of silicone rubber.
The value of θ3 does not satisfy the correspondence with the physiological parameters of a real finger. This inaccuracy is certainly due to the criterion according to which the α value is equal for all the joints. Nevertheless, it means that the finger will perform a more effective closing operation. Moreover, the physiological length of F3 does not include the length of the soft tissue at the tip of the distal phalanx. For this reason the physiological and numerical values are different. On the basis of this result, an optimal configuration of the five fingers was achieved. The same parameters of the index were adopted for the middle and the ring fingers. Table 2 reports the variable geometric input parameters for the thumb and the pinky, in accordance with the physiological parameters.
Parameter
12 mm
3. THE EXPERIMENTAL PROTOTYPE
Table 1. Comparison between the physiological and numerical values of the index finger parameters Parameter
open_F1
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by an external belt that pushes the tube against the plug. Figure 3 shows the welding operations and the performing of the cuts. The experimental prototype of the index finger shows the following dimensions: overall length, 100 mm; external diameter, 20.5 mm; thickness of the tube, 2 mm.
pressure regulator provided for the air inlet in the prototype; a manometer was used for the measurement of the current air pressure value. A digital camera was placed in front of the prototype, at a suitable distance to avoid image distortions. Tests were carried out in quasi-static conditions: starting from the zero value, pressure value was increased of 0.02 MPa until the maximum closure of the finger occurred. At each pressure step, a picture of the finger was taken, for a total amount of 14 pictures (until the pressure value equal to 0.26 MPa). The same tests were carried out with the numerical model: the maximum pressure value was set to 0.30 MPa. Results corresponding to the same pressure steps of the experimental tests were acquired in terms of screenshots of the output displacements results of the FE model. Then, pictures of the experimental prototype and of the numerical model were subjected to an image analysis in order to achieve the values of the joints angles. Finally, angles values were compared. Figure 5 shows three examples of image analysis.
a)
b)
c)
Figure 3. Phases of the assembly of the actuator: a) the welding of the gauze; b) the cuts; c) the welding of the stacked layers. The mass of the tube wrapped by the gauze is equal to 13 grams; the overall mass of the actuator (with the rear plug) is equal to about 80 grams. Figure 4 shows the index finger prototype at rest and close to the maximum flexion.
Figure 5. Examples of image analysis of the experimental and numerical results. Figures 6 shows the comparative graphs of the angles as function of the pressure value. A good correspondence can be noticed between the experimental and numerical results. Some significant differences occurs for θ3 in the pressure range 0 – 0.10 MPa. This behavior is probably due to a not suitable adherence between the tip of the tube and the external gauge: the pressure energy does not allow the flexion of the joint but provides for a short elongation of the Distal Phalanx. Moreover, the good correspondence of the curves allow to affirm that no sliding occurs between the gauze and the tube and that the hypothesis according to which the gauze is rigidly locked is acceptable. On the basis of the achieved results, the numerical model was experimentally validated. It can be used as a design tool for such kinds of soft actuators.
Figure 4. Prototype of the index finger in two different conditions.
4. THE EXPERIMENTAL TESTS The experimental activity aimed to test the functionality of the prototype and to validate the numerical model. As regards the functionality, the prototype was mounted on an aluminum frame and preliminary tests were carried out to check the presence of possible air leaks, air bubbles inclusions in the wall thickness of the tube and possible backslashes between the tube and the gauze. The integrity of the prototype and the suitable assembly were proved. Then, the behavior of the prototype, as function of the value of the air pressure, was achieved: the maximum closure of the actuator, when the tip of it touches the rear plug, occurred for a pressure value equal to 0.25 MPa. At this value, the silicone showed some bulges in correspondence of the joints. This phenomenon must be taken into account in future activities. The model validation was carried out by the comparison of the output angles θ1, θ2 and θ3 resulting from the numerical model and the same angles measured with the experimental prototype. A suitable testbed was assembled: it is made of an aluminum frame for the placement of the prototype, as shown in Figure 4. A precision
5. CONCLUSIONS A soft actuator to be adopted as a finger for a collaborative robot was developed. It is made of an inner tube in silicone rubber externally covered by a square-meshed gauze. The bending of the actuator is provided by appropriate cuts on the gauze, in correspondence of the joints. On the basis of a validated numerical model, implemented by a FE code, a prototype of the actuator, inspired to the index finger, was realized and experimentally tested. The technological process proved to be effective and easy to be carried out. The prototype shows
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[5] Suzumori, K., Endo, S., Kanda, T., Kato, N., and Suzuki, H. 2007. A bending pneumatic rubber actuator realizing softbodied manta swimming robot. In Proceedings of the IEEE International Conference on Robotics and Automation ICRA ’07 (Roma, Italy, April 2007). 4975-4980.
lightness and high compliance. The behavior of the prototype is quite similar to the a human finger. Some differences occur in terms of the angle around the Distal Interphalangeal joint.
[6] Udupa, G., Sreedharan, P., and Aditiya, K. 2010. Robotic gripper driven by plexible microactuator based on an innovative technique. In Proceedings of the 6th IEEE Workshop on Advanced Robotics and its Social Impacts (Seoul, Korea, October 2010). 1-6. [7] Ogura, K., Wakimoto, S., Suzumori, K. and Nishioka, Y. 2009. Micro Pneumatic Curling Actuator – Nematode Actuator. In Proceedings of the 2009 IEEE International Conference on Robotics and Biomimetics (Bangkok, Thailand, February 21-26, 2009). 462-467 [8] Wakimoto, S., Ogura, K., Suzumori, K. and Nishioka, Y. 2009. Miniature Soft Hand with Curling Rubber Pneumatic Actuators. In Proceedings of IEEE International Conference on Robotics and Automation (Kobe, Japan, May 12-17, 2009). 556-561. [9] Udupa, G., Sreedharan, P., Dinesh, P. S., and Kim, D. 2014. Asymmetric Bellow Flexible Pneumatic Actuator for Miniature Robotic Soft Gripper. Journal of Robotics. vol. 2014, Article ID 902625, 11 pages, 2014. doi:10.1155/2014/902625 [10] Konishi, S., Nokata, M., Jeong, O. C., Kusada, S., Sakakibara, T., Kumayama, M., and Tsutsumi, H. 2006. Pneumatic Micro Hand and Miniaturized Parallel Link Robot for Micro Manipulation Robot System. In Proceedings of the 2006 IEEE International Conference on Robotics and Automation (Orlando, Florida, May, 2006). 1036-1041.
Figure 6. Comparisons between the experimental and numerical angles.the experimental and Figure 6. Comparisons between numerical angles.
Nevertheless, current results are encouraging for future research activities. In particular, a kineto-static numerical model must be implemented to predict the developed forces as function of the pressure; then, the experimental measurements must be carried out about the real developed forces; bulges at the joints must be solved. Finally, after the full characterization of the actuator, the robotic hand must be realized and applied to a collaborative robotic to perform a real application.
[11] Antonelli, M. G., Beomonte Zobel, P., Durante, F., and Raparelli, T. 2017. Numerical modelling and experimental validation of a McKibben pneumatic muscle actuator. Journal of Intelligent Material Systems and Structures. 28, 1, DOI 10.1177/1045389X17698245 [12] Faudzi, A. A. M., Rusydi, M., Razif, M., Nordin, I. N. A. M., Suzumori, K., Wakimoto, S., and Hirooka, D. 2012. Development of Bending Soft Actuator with Different Braided Angles. In Proceedings of the 2012 IEEE/ASME International Conference on Advanced Intelligent Mechatronics (Kaohsiung, Taiwan, July 11-14, 2012). 10931098.
6. ACKNOWLEDGMENTS The authors want to thank Eng. Annabella Cretara and Eng. Patrizio Caramanico for their precious help in the development of the presented research activity.
7. REFERENCES
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