More importantly he is the man responsible for bringing ...... The CyberGrasp[3] from Immersion Corporation is, as of this writing, the only currently .... robotic hand[17][18], which has a number of small, fine parts designed and built for the ..... At the base of the mechanical components of this haptic system is a leather glove,.
© 2006 Scott H. Winter ALL RIGHTS RESERVED
A HAPTIC FORCE FEEDBACK GLOVE USING MAGNETORHEOLOGICAL FLUID
BY SCOTT H. WINTER
A thesis submitted to the Graduate School – New Brunswick Rutgers, The State University of New Jersey In partial fulfillment of the requirements For the degree of Masters of Science Graduate Program in Electrical and Computer Engineering Written under the direction of Mourad Bouzit, PhD and approved by ___________________________ ___________________________ ___________________________ ___________________________
New Brunswick, New Jersey May, 2006
ABSTRACT OF THE THESIS
A HAPTIC FORCE FEEDBACK GLOVE USING MAGNETORHEOLOGICAL FLUID
By Scott H. Winter Thesis Director: Mourad Bouzit, PhD
Haptic Force Feedback can be summarized as a system, which provides a sense of feeling and the application of forces by a computer to a user. This type of feedback system was designed as a glove or master format in order to provide forces at the fingertips.
As a core of the device, passive actuators were developed using
Magnetorheological Fluid (MRF). This fluid is a smart material that has the property of changing its viscosity when exposed to a magnetic field. By varying the intensity of electromagnets, embedded within the fluid based actuators, the resistive forces of the actuators can be dynamically set. The project’s nature requires the use of many tools, falling under several conventional disciplines. Various experiments were required to develop test equipment, sensors, a working actuator, its mechanical parts and power transition systems. An embedded microcontroller was programmed to control the system and circuitry designed to work with sensors and power distribution.
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Acknowledgements This project would not have been possible without the generous help that I received from selfless individuals. Dr. Mourad Bouzit, my advisor, a person contributed a great amount to this thesis. Whenever I had problems he was there to steer me in the right direction. His almost inexhaustible knowledge base was the greatest resource for this project. He financed this project through seed money that was granted to him for the start of the Bio-Robotics Laboratory. I am proud to have had him as my thesis advisor. Dr. Grigore Burdea served as a co-advisor for this project and was someone who helped to guide me in my thesis work. More importantly he is the man responsible for bringing me into the research environment and creating a culture of research, in which I was actively involved through the VR lab. Joseph Lippencott is someone who I consider myself lucky to have met. While not related in any way to the Department of Electrical and Computer engineering, or any lab I am associated with, he selflessly helped me in the construction of the metallic components of the glove. He taught me how to use a variety of machine tools and helped me in their use whenever he could. Brett Butterline, Kar Lun James Chun, Stephen M. Carter, William D. Kish and the rest of the CAIP staff were always there to help me with ideas, materials and general support for this project. Hristiyan Kortev is the original programmer for a set of exercises for the RMII glove. A sub-set of this project involves these exercises and Mr. Kortev proved very helpful in their use. iii
Dr. Rares Boian is an inspiration to anyone who has had the privilege to know him. During the early part of this project he was a person off whom I could discuss ideas and from whom I could expect good suggestions. Angela Peterson was there for me whenever I needed her. She was someone who was a great help in the final stages of this project. In particular she helped in the final construction of the glove and polishing of this document. John Petrowski taught me to use the Rapid Prototype machine and helped me to get started with the stereo lithography portion of the part construction. Abhijit A Tamba is a member of Dr. Bouzit’s lab, and I found him to be both helpful and knowledgeable.
He also helped me in the construction of a power
amplification board. I would also like to thank all the participants in the Human Study, the CAIP center for use of the facilities, and all members of the Bio-Robotic and Human Machine Interface Lab (VR lab).
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Table of Contents ABSTRACT OF THE THESIS .......................................................................................... ii Acknowledgements............................................................................................................ iii List of Figures .................................................................................................................... ix List of Tables .................................................................................................................... xii Chapter 1: Introduction ........................................................................................................1 1.1. Goal of the Thesis .....................................................................................................1 1.2. Project Overview ......................................................................................................2 1.3. Thesis Contributions .................................................................................................4 1.4. Thesis Scope .............................................................................................................5 Chapter 2: Relevant Background .........................................................................................6 2.1. Haptic Systems .........................................................................................................6 2.1.1. Haptic Interfaces ................................................................................................7 2.1.2. Force Feedback Gloves......................................................................................8 2.1.2.1. RMII-ND.....................................................................................................8 2.1.2.2. CyberGrasp ..............................................................................................10 2.1.2.3. L.R.P Force Feedback Data Glove ...........................................................11 2.1.2.4. Hand Force Feedback Glove.....................................................................12 2.2. Magnetorheological Fluid ......................................................................................13 2.3. Rapid Prototype System..........................................................................................15 2.4. Actuators.................................................................................................................17 Chapter 3: MRF Actuators.................................................................................................20 3.1. Final Actuator Construction....................................................................................20 v
3.1.1. Piston................................................................................................................22 3.1.1.1. Standard pistion ........................................................................................23 3.1.1.2. Hex-Pistion ...............................................................................................27 3.1.2. Actuator Cylinder.............................................................................................31 3.1.3. Sealing..............................................................................................................32 3.2. Actuator Design Considerations .............................................................................33 3.3. Actuator Evaluation ................................................................................................36 3.3.1. Setup ................................................................................................................36 3.3.2. Program............................................................................................................37 3.3.3. Results..............................................................................................................38 3.4. Actuator Evaluation ................................................................................................42 Chapter 4: Exoskeleton, Sensors, Mechanical and Physical structure...............................45 4.1. Exoskeleton Mechanics ..........................................................................................47 4.2. Exoskeleton Finger Spine .......................................................................................48 4.3. Actuator and Sensors Mounting .............................................................................54 4.3.1. Force Sensor.....................................................................................................54 4.3.2. Position Sensor.................................................................................................55 4.3.3. Actuator Mounting...........................................................................................56 Chapter 5: Electronic Architecture ....................................................................................57 5.1. Sensor Boards .........................................................................................................58 5.1.1. Force Sensor Electronics..................................................................................59 5.1.2. Linear Sensor Electronics ................................................................................61 5.2. Power Amplification Board ....................................................................................62
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5.3. Embedded Microcontroller Board ..........................................................................64 5.4. User Component Mounting ....................................................................................66 Chapter 6: Software Architecture ......................................................................................67 6.1. Embedded Control Program ...................................................................................67 6.1.1. Embedded Calibration Module ........................................................................69 6.1.2. Sensor and Data Acquisition Module ..............................................................71 6.1.3. Force Control Module......................................................................................72 6.2. Calibration and Testing program ............................................................................73 6.3. Embedded Rehabilitation Host Program ................................................................77 Chapter 7: Initial Human Testing ......................................................................................79 7.1. Evaluation Description ...........................................................................................79 7.2. Intensity Perception ................................................................................................80 7.3. Relative Force Perception.......................................................................................82 7.4. Absolute Force Perception......................................................................................83 7.5. Subjective Questionnaire ........................................................................................84 7.6. Analysis of Initial Human Testing..........................................................................85 Chapter 8: Conclusions and Future work...........................................................................87 8.1. Actuator ..................................................................................................................88 8.2. Exoskeleton.............................................................................................................88 8.3. Sensor and Power Electronics.................................................................................89 8.4. Embedded Microcontroller and Control Program ..................................................90 8.5. Overall Conclusion .................................................................................................90 Appendix A........................................................................................................................92
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Appendix B ........................................................................................................................97 References........................................................................................................................100
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List of Figures Figure 1.1. Block diagram of MRAGES ............................................................................ 3 Figure 1.2. Completed glove and control boards................................................................ 4 Figure 2.1. GiHapIn interface ............................................................................................. 8 Figure 2.2. RMII-ND .......................................................................................................... 9 Figure 2.3 CyberGrasp and CyberGlove .......................................................................... 11 Figure 2.4. L.R.P Force Feedback Data Glove ................................................................. 12 Figure 2.5. PERCRO HFF ................................................................................................ 13 Figure 2.6. Basic operational modes for controllable fluid devices ................................. 14 Figure 2.7. RD-1097-1 MR dampener.............................................................................. 14 Figure 2.8. Robotic RP parts............................................................................................. 16 Figure 3.1: CAD drawing of final actuator....................................................................... 21 Figure 3.2. Picture of completed actuator......................................................................... 21 Figure 3.3. Cad drawing of standard spindle .................................................................... 23 Figure 3.4. Piston close-up............................................................................................... 23 Figure 3.5. FEMM simulation of spindle with 350 wraps................................................ 26 Figure 3.6. Field strength plot from Figure 3.5 along red line, from top down................ 27 Figure 3.7. CAD drawing of hex-spindle.......................................................................... 27 Figure 3.8. Hex-piston ...................................................................................................... 28 Figure 3.9. MATLAB curve fitting for hex cuts............................................................... 30 Figure 3.10. FEMM simulation without steel cylinder..................................................... 31 Figure 3.11. Field Strength plot from Figure 3.10 along red line, from top down. .......... 32 Figure 3.12. Early piston supporting 900g mass............................................................... 34 ix
Figure 3.13. Test setup...................................................................................................... 37 Figure 3.14. Flow-model of initial testing program.......................................................... 38 Figure 3.15. 2D plot of force vs. DC for 0.320 diameter spindle ..................................... 39 Figure 3.16. 2D plot of force vs. position for 0.320 diameter spindle.............................. 40 Figure 3.17. 3D Plot of DC, force and position for 0.320 diameter spindle..................... 41 Figure 3.18. 2D plot of force vs. speed for 0.320 diameter spindle.................................. 42 Figure 3.19. 3D plot of DC, Speed and force for 0.320 diameter spindle ........................ 42 Figure 4.1. Full glove........................................................................................................ 45 Figure 4.2. Underside of glove ......................................................................................... 46 Figure 4.3. CAD representation of full finger................................................................... 47 Figure 4.4. Wire-frame of exoskeleton finger spine ......................................................... 49 Figure 4.5. Early Exoskeleton finger spine....................................................................... 50 Figure 4.6. Bending of exoskeleton finger spine .............................................................. 51 Figure 4.7 Cable bending example ................................................................................... 52 Figure 4.8. Bowing cable .................................................................................................. 53 Figure 4.9. Fingertip mount .............................................................................................. 53 Figure 4.10. Force sensor close-up ................................................................................... 54 Figure 4.11. Potentiometer close-up ................................................................................. 55 Figure 4.12. Rapid prototype parts for sensor attachment ................................................ 56 Figure 5.1. Sensor boards.................................................................................................. 58 Figure 5.2. Schematic of single finger force sensor.......................................................... 60 Figure 5.3. Power amplification board ............................................................................. 62 Figure 5.4. Power amplification........................................................................................ 63
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Figure 5.5. Microcontroller development board ............................................................... 64 Figure 5.6. PIC block diagram.......................................................................................... 65 Figure 5.7. Mounting diagram ......................................................................................... 66 Figure 6.1. Diagram of all embedded program modules .................................................. 68 Figure 6.2. Force gauge during calibration....................................................................... 69 Figure 6.3. Calibration and testing console ...................................................................... 74 Figure 6.4. Response time of actuators............................................................................. 75 Figure 6.5. Low pass filter response time of actuators ..................................................... 76 Figure 6.6. Rehabilitation system setups .......................................................................... 78
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List of Tables Table 3.1. Relevant magnet wire information .................................................................. 25 Table 3.2. Table of actuators............................................................................................. 43 Table 5.1. Power requirements ......................................................................................... 63 Table 7.1. Results of intensity perception tests................................................................. 81 Table 7.2. Results for relative force perception trials....................................................... 82
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Dedication
To my mom and dad: It is from your devotion that I have the strength to be who I am.
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1
Chapter 1: Introduction
1.1. Goal of the Thesis Human interface with a computer is a non-trivial problem and has been the subject of much research. The standard input method for most people is currently based on input from a keyboard and mouse. Output usually involves graphic and/or text displayed on a monitor or printout, and can involve auditory feedback as well. When we expand these methods to those common in the Virtual Reality (VR) field, we find that several more options for I/O exist. These include devices such as 3D trackers, which can give absolute position and rotation of an object, 3D navigation devices, more complex graphical displays, and motion sensors, as well as tactile and force feedback haptic displays. Numerous examples can be found in Virtual Reality Technology[29] and other publications. The Virtual reality devices mentioned have found some home within the gaming, medical, military and research communities. Regrettably, the mass consumption of such devices has not yet taken place, despite the efforts of many talented individuals in industry and academia. With the ever-increasing use of all types of virtual reality devices and feedback devices, a turning point is surly on the horizon, when enough devices are in use to support development and enrichment of the software.
As the software
2 development increases people will have a greater desire to use the associated hardware creating a snowball effect. If we focus on the haptic devices, in particular force feedback devices, we can see a clear advantage of their use. These devices are able to provide an additional sensation to the user that he cannot get through the standard interfaces. In fact, haptic feedback devices are now used in commercial and military fly-by-wire systems[36] since it was decided that feedback is required for proper piloting when using a fly-by-wire control system. Force feedback systems have also been successfully used in various medical applications such as those identified by Dr. Burdea[38]. In order to further the field of haptics I have developed a haptic device based on Magnetorheological Fluid (MRF) and have named it a Magnetorheological Actuated Glove Electronic System or MRAGES. My design is intended to serve as a starting point for the use of MRF in this type of haptic devices. Since no other MRF based glove was found during the research portion of this project, it can be assumed that there is no other glove of this type in existence. MRF is not suitable for every application; this system is designed to demonstrate that a MRF based haptic master can be compact, functional and a viable alternative to other methods of providing forces.
1.2. Project Overview The haptic master force feedback system, which I have developed, was designed to have the RMII-ND[45] functionality as a subset of its own. In this regard it was necessary to produce an adjustable rate of forces, measure these forces acting upon the user, and have some proper sense of the finger placement. Along with fulfilling the goal
3 of the thesis laid out in section 1.1, the glove needed to meet the same requirements of any haptic glove, that is be light, comfortable, responsive and easy to use.
Figure 1.1. Block diagram of MRAGES
To fulfill these goals, the design of the MRAGES was undertaken. This required considerable testing of the MRF to find the best way to use the fluid as a haptic device. The short timetable and nature of my expertise required the learning of new tools to develop the hard components of the actuator, mechanical transition systems and mounting of the system on the hand. For these hard components, parts were designed in AutoCAD Mechanical desktop[21], and machined out of steel and other materials or “built” using 3D Systems Stereo Lithograph rapid prototype tools[19]. A Microchip PIC18F8720[39] microcontroller development board was programmed to control the system and communicate with a PC over USB.
Power and sensor circuitry were
developed to provide the link between these systems and the microcontroller. These boards were constructed and some initial human testing was conducted.
4
Figure 1.2. Completed glove and control boards
The completed glove, shown in Figure 1.2, associated software/firmware, and the human testing are the results of this thesis project.
1.3. Thesis Contributions •
Designed, built, and tested a novel actuator using MRF
•
Designed and built a method of mechanical power transmission from actuator to human hand using an exoskeleton structure
•
Developed embedded sensors and control system for the glove
•
Integrated components to form a new type of haptic glove
•
Conducted preliminary usability human study
5
1.4. Thesis Scope The remaining sections of this thesis will cover the following topics: •
Related works
•
MRF based passive actuators
•
Exoskeleton and mechanical transmission systems
•
Mechanical mounting
•
Embedded control system and sensors
•
MRAGES pilot study
•
Conclusions and future work
6
Chapter 2: Relevant Background
In order to understand the scope of this project, we must look at what exists in the field and the tools that have been used for this project. Other devices that accomplish the same goals exist in the field as well as in academia. These devices, however, all differ in the way that they produce forces; none use MRF as the basic technology for providing forces. We can therefore look at them as competing technologies. We must also examine the MRF to understand its limitations and promise. MRF is used in other force applications, some similar to the ones in this project. The rapid prototype system was employed as much as possible in order to minimize part fabrication time. It therefore has become an integral portion of the project and deserves attention.
2.1. Haptic Systems When the first computers were created, all information was inputted by hand, at first by punch cards or keyboards and the computer would then print or display the results, usually as tabulated numbers and references. Later graphical user interfaces (GUI), the mouse and auditory feedback were introduced[26]. mouse and GUI remain the dominant form of computer interaction.
Today the keyboard,
7 Much research and progress has been made relating to the field of graphic and display systems and there are numerous methods of processing, programming and displaying the information.
Many examples can be found in Virtual Reality
Technology[29] and numerous other publications.
Less work has been done when
referring to haptic systems.
2.1.1. Haptic Interfaces There are a variety of methods that have been used to provide forces to a human user. Some of the most readily available and affordable are desk-mounted and relate to the gaming industry. These include items such as MOMO Force Feedback Racing Wheel or Force 3D Pro Joystick from Logitech[41]. These devices provide force feedback to the user through electric motors for resisting the user’s movement or to simulate road variations or air turbulence. Another type of desk-mounted system would include the Phantom[42] line from Sensible Technologies of stylus like devices provide a commercially available method for 3D interactions with a computer. This desk-mounted system is also powered by electric motors and provides a single point of haptics within the virtual environment. A more experimental single point force feedback system for larger workspaces might include the Scalable-SPIDAR[32]. Here the typical desk or ceiling attachment is replaced with a system of pulleys and the user is allowed to move about a larger scale environment. A novel device, the GiHapIn[43][44] was designed to provide forces to a number of fingers, three in the prototype. The GiHapIn has a robotic arm with three robotic
8 fingers and mounts for human fingers. The user places their finger into the mounts and the robot inversely mimics the user, while compensating for the addition of haptic forces.
Figure 2.1. GiHapIn interface
2.1.2. Force Feedback Gloves Several glove type haptic systems currently exist, both in the commercial and academic universes. The previous section gave a short overview of various types of force feedback haptic systems. This section will give a more in-depth view of several haptic force feedback gloves, as a representation of force feedback haptic gloves.
2.1.2.1. RMII-ND One type of haptic force feedback glove is the RMII-ND[1], developed and currently used by the Rutgers Human Machine Interface Lab.
This glove uses
pneumatics as its primary mode of providing force feedback to the user. By utilizing a
9 graphite-on-glass pneumatics piston for each finger, the static forces are quite low, only 0.014N of force when the glove is not powered. Pistons are directly attached to the fingers and are located between the palm and the fingertip. The working portion of the device (the part attached to the user’s hand) is light and comfortable to wear, weighing only about 100g. Each finger is equipped with three types of sensors for measuring finger position, two Hall-Effect sensors for a finger’s flexion and abduction and one IR sensor for displacement of the piston. Overall, the glove can produce a force of 16N per finger, opposing the closing of the hand. Most recently, this device was involved in a post carpal tunnel surgery orthopedic rehabilitation study[47] when it was successfully used to help patients who recently required surgery for their carpel tunnel. Other studies using this glove included work with post stroke victims[2] [46], which also showed promise.
Figure 2.2. RMII-ND
10 Several supporting devices are also required when using the RMII. A compressed air supply is required for its operation. The air supply can be a dedicated compressor as was the case in the most recent study, or a “house” air supply, as might be found in labs or hospitals. Compressed air is fed into a second part, the Multiplex Telarehab Interface (MTI), to which each glove attached while in use. The MTI contains the pneumatics valves for the operation of the Rutgers-Master Haptic glove and the sensor electronics to measure fingertip positions versus the palm. A pressure sensor is also located in the MTI to act as part of the force sensor. The Host PC also connects to the MTI, where all commands are processed.
2.1.2.2. CyberGrasp (Immersion) The CyberGrasp[3] from Immersion Corporation is, as of this writing, the only currently available force feedback glove. It is listed as having a working end weight of 350g and works using a tendon-based system to oppose the closing of the fingers with up to 12N of force. An exoskeleton is mounted on the back of the hand to guide the tendons over the back of the hand to the fingertips. A specialized PC and umbilical device are used to power and control the CyberGrasp.
Optionally, a backpack containing the
support components can be worn to make the setup more mobile. Since this is the only commercially available force feedback glove, there is little option for someone looking to design an interactive system using a force feedback glove, unless they are willing to undertake the design of a new glove or use a primarily academic device. While the glove has a working end weight of about one pound, the additional equipment makes transportation to a new location difficult since the functional control unit (FCU) has a weight of 44 pounds (20kg). The CyberGrasp also has no
11 position or flexion sensors and therefore requires an additional device, such as the recommended CyberGlove[4], to provide sensing. The size of the exoskeleton, can also be a problem, as it rides high on the user’s hand and can be cumbersome.
Figure 2.3 CyberGrasp and CyberGlove
2.1.2.3. L.R.P Force Feedback Data Glove The L.R.P Force Feedback Data Glove[53] is another exoskeleton-based glove. Three aluminum joints per finger (two for the thumb) are used to connect the phalanx’s and resist the closing of each finger segment. Utilizing a cable, tendon system it provides forces on each phalanx from an off hand system of 14 torque motors and provides 14.0N of force per finger segment. Additional this off hand power scheme allows for a working end weight of 350 grams.
Sensing is included in the system by optical encoders for
position and strain gauges for forces.
12
Figure 2.4. L.R.P Force Feedback Data Glove
2.1.2.4. Hand Force Feedback Glove Another haptic system providing forces to the fingers is the Hand Force Feedback (HFF)[35] from PERCRO. This system has a large exoskeleton and uses tendons to connect motors to each of the finger segments, creating a tension drive system. When all segments are mounted on the hand, a glove type system develops. The HFF also has a separate design for the thumb; as it functions slightly differently form other fingers. While this is an interesting glove type system, the researchers concluded that the implementation referenced has a low performance due to high friction of the motors and the coupling effect in the tension drive system.
13
(a)
(b)
Figure 2.5. PERCRO HFF[54]: (a) Fingers, (b) Thumb
2.2. Magnetorheological Fluid (MRF) Magnetorheological Fluid (MRF or MR-Fluid) is a smart material, which has the property of solidifying when exposed to a magnetic field. The type of MRF used for this project is a commercially available product from Lord Corporation. The type used here is MRF-22ED[5] and is a hydrocarbon-based medium with 72% of its weight in solids and 22% solids by volume. The exact composition of the solids are proprietary to Lord Corporation. However, it is made of suspended iron particles, used as elements to react with the magnetic field, so it is safe to assume that most of the 72% weight is the iron content. In practice, MRF functions by aligning the suspended iron particles’ poles to create a more viscous substance. The viscosity of the material is what is used to create the forces in the actuators as discussed in Chapter 3. Lord Corporation provides two models for using MRF[7][12], the pressure model and the force model as shown in Figure 2.6. Devices such as MRF Clutches would use the direct shear mode, while flow
14 control devices would use the pressure driven flow mode[14]. For the purposes of this project, the direct shear mode is more relevant; however, both can be applied.
Figure 2.6. Basic operational modes for controllable fluid devices: (a) pressure driven flow mode, and (b) direct shear mode
Haptics work using MRF has been done in which the MRF is used to replicate biological tissues[10][11]. The ability of MRF to replicate tissues is due to its surface tension when energized. Most work with MRF is limited to larger and heavier devices that are not applicable to haptics. Their concepts remain the same, however, in that the design of active force dampeners is similar.
Figure 2.7. RD-1097-1 MR dampener[55]
Some types of commercially available MRF devices include motion dampers, both large scale for buildings (seismic dampeners) and small scale for items like seats or
15 even a prosthetic knee, clutches and breaks. A smaller commercially available MRF dampener would be the RD-1097-1[55] controllable friction dampener. This dampener has a weight of 480 grams with a diameter of 1.63 inches. The length is 7.68 inches compressed and 9.96 inches extended. This size piston gives a force profile of 9N when inactive and an instantaneous force of 100N with 1 amp of power. Some work has been done in making very small actuators using active fluids such as the work described by Shinichi Yokota[13]. Here Electro-rheological Fluid (ERF) is used to create miniature valves, motors, and machines. ERF is similar to MRF, with the notable exception that in ERF a current is used to orient the active partials in the fluid, while in MRF a magnetic field is used. Since a high current is necessary, on the kilovolt order such as in Smart Technology’s ERF[15], this type of material is not well suited to be worn by humans.
2.3. Rapid Prototype (RP) System A 3D systems Viper Stereolithography (SLA) machine[19] is a device which was utilized for this project in order to cut down on the development and fabrication time for various parts of the systems as described in the later sections. The Viper SLA uses a laser to solidify a portion of a resin reservoir’s surface. The solidified portion is then lowered into the reservoir and the laser solidifies the next level of the part. By continuing this operation in succession, a complex three-dimensional part can be formed in a relatively short period of time. Designing the part is done using CAD software, in this case Mechanical Desktop[21], and then exporting the file into a STL (Stereolithography comparable) file. STL files contain the geometric information needed to now create a 3D part using
16 stereolithography. 3D System provides a software system, Lightyear[20], for interfacing with their SLA machines. Lightyear imports the STL files and creates a pallet for the stereolithography to take place. It also creates support structures to suspend the parts above the pallet when they are being built. Care must be taken when using Lightyear’s automatic support creation. Sometimes, the supports extend into interior features of the part, such as holes and ridges. This can affect the final performance of the part. Cleaning of the final part is also an important step. If uncured resin is left on the part, or inside any features, it will solidify over time and degrade the part’s performance. Robotic systems, a closely related field to haptic force feedback, have been previously designed using RP systems. NASA developed a robotic arm utilizing SLA parts in 1995[16]. By choosing to use SLA parts, the NASA group was able to design and develop a low cost prototype robotic arm in a short amount of time. Other, robotic devices have also been designed utilizing RP parts. An example of this would be a robotic hand[17][18], which has a number of small, fine parts designed and built for the purposes of making a robotic hand. Additionally, the hand has fairly complex parts with interior control channels for the purposes of moving the fingers of the robotic hand.
(a) (b) Figure 2.8. Robotic RP parts[56]: (a) robotic finger, (b) universal joint
17
2.4. Actuators Many types of linear actuators, motors, pneumatic or hydraulic cylinders, and other devices to produce controllable forces are currently commercially available. In order to limit the comparison, only possible choices for actuators in use with this project will be discussed. As shown in the later sections of this paper, the actuators developed for this project are mounted directly onto the glove, and therefore any replacement actuators would also need to fulfill this requirement. This implies a low weight and stroke length of approximately one inch or more, with a base size as small as possible. Pneumatics and hydraulics are not considered because of the “off glove” power requirements. Standard electric linear actuators are reliable and available; however, when size and weight restrictions are applied, the availability becomes more limited. Firgelli’s LS6[22] is an inline linear motion actuator with a 20mm (0.79 inch) stroke, 6.35mm (1/4 inch) diameter and overall length of 70mm (2.7 inch). It is rated for 4N force and draws 250mA at 3Vdc. With this peak force the speed is 5mm/sec. While the stroke length is approximately the correct size, the overall length makes it unsuitable for this application. No weight is specified. CK Design also makes a compact electric linear actuator[23] where the motor is parallel to the shaft and then enclosed in a housing. Using this design CK Design is able to achieve a 2-inch stroke with a 3.265-inch housing. The shaft is 0.5-inch and fits into a housing slightly less then an inch in girth and 1.56 inches in height. This motor has a weight of 6.8oz. A linear sensing potentiometer is included in the design and, with six pounds (25N) of load, the speed of the device is rated for 6in/sec. Unfortunately, the size
18 and weight of the motor is incorrect for on-hand mounting, but can be used as an example of electric motor functionality. An attractive alternative to the standard electric actuator could be a piezoelectric motor, such as the Squiggle motors SQL series[24]. This type of motor can be made very small with a 0.13-inch diameter housing and 0.094-inch shaft. The throw of this design is approximately 1.2 inches, a good size for this project. The controller/driver board is small (2”x3”) and can be mounted farther up the arm, to maintain portability. When active, this actuator has a recommended load of approximately 2N and a max load of over 4N. At the recommended load, with a 12V power source, the max speed of the device is about 4.5 mm/sec and draws 0.5W. While promising, the speed of the device makes its use less attractive. There is also no built-in turn off for overload, and therefore, possible damage could occur if overstressed. An attractive commercially available actuator is an electromagnetic type such as the LEU Micro Brushless model[25] from Rockwell Automation. Available with a throw of 1.378-inches with a length of 2.343-inches and thickness of 0.735-inch, it is small enough to fit on the human hand if carefully arranged. Able to provide continuous force of 7.5N and a peek force of 22N, its forces are well suited to the force feedback task. Due to its brushless, magnetic chamber there should be little ambient force, although this is not mentioned in the specifications. While the mechanical movements and forces are highly desired for the task, it has certain key deficiencies that make it undesirable for mounting on humans. Foremost is the weight of the device, 160g(5.6oz) per device or 800g (1.76 lbs) to implement all five
19 fingers. Additionally, the max temperature of the device is 125oC and a heat sink is recommended to achieve this max temperature.
20
Chapter 3: MRF Actuators
The MRF actuators are the heart of the MRAGES system. They represent a novel approach to applying forces to the human hand.
They have been designed to be
powerful, light, use low power, and have a relatively low static force level. Constructed out of both machined and rapid prototype parts, the actuators measure approximately 2”x0.5”x0.5”when fully compressed, and have a weight of roughly 16 grams each. The actuators used provided forces up to 6 Newtons; however, simple changes in design can create much higher forces (see Section 3.2 for details). A total throw of 1.25” is achieved in this design.
3.1. Final Actuator Construction A finished actuator is comprised of several pieces (see Figure 3.1 and Figure 3.2), these being the external retaining shell, 2 O-ring holders retaining a PTFE (generic Teflon) O-ring, a 1.5” steel (3/8 OD, 11/32 ID) cylinder, a steel spindle, a center stainless steel shaft, and a steel cylinder. The spindle is warped with magnet wire and a pair of wire washers is added to keep the leads and unshielded wire from shorting between each other as well as any of the metallic parts. The cylinder is filled with MRF fluid before final assembly and sealed using liquid silicon.
21
Figure 3.1: CAD drawing of final actuator
Figure 3.2. Picture of completed actuator
22 By making use of the rapid prototype machine, the actuator’s non-active, retaining parts, are a non-metallic, non-magnetic material. These non-active parts are designed to hold the active elements, mentioned in the subsequent subsection; however, they are not designed to provide any active forces directly. They are simply used to hold the fluid, O-ring and other elements in place.
3.1.1. Piston Two types of pistons were experimented with in order to finalize a design. The Standard Piston, the simpler of the two, was eventually used because the manufacture time was significantly less then the Hex-Piston and the resulting performance difference did not dictate making this tradeoff. In all sections, with the exception of section 3.1.1.2 and where the Hex-Piston is specifically mentioned, any reference to a piston or spindle refers to a standard piston and standard spindle. Attached to each piston, a stainless steel gun barrel straight 1/16” wire is used for the center shaft. In order to reduce friction with the O-ring, this part should also be highly polished. Care must therefore be taken if threading (0-80 thread) the wire at both ends for later attachment to the spindle, to from a completed piston. Micro-abrasions on the shaft caused by the threading of the shaft create additional friction at the ends of the shaft when moving within the O-ring. These micro-abrasions should be avoided as much as possible and therefore attachments were made by brazing one end of the center shaft to the spindle, and attaching a pre-threaded component to the other end using a coupling.
23
3.1.1.1. Standard piston
Figure 3.3. Cad drawing of standard spindle
Figure 3.4. Piston close-up
The piston is the active element of the actuator.
To produce a spindle, a low-
carbon steel rod is sized to the external diameter of 0.325” by manual use of a lathe. The spindle is then attached to the center shaft to form a piston. The actual size of the
24 external diameter of the spindles will vary since they are cut manually. Actual diameters of spindles used in the final version of the glove can be found in Table 3.2 on page 43. The nominal size was determined experimentally and subjectively and provided an appropriate compromise to the decrease in the active forces with smaller spindles and alternatively higher active forces with larger spindles. The center can be taped with a 080 thread, within a 1/8” trough internal diameter, for attachment to the center shaft later. Alternatively, the spindle can also be brazed together with the center shaft using a high strength solder. This brazing technique was used in the final version. A notch was added to allow the wires to pass safely around the spindle while it moves through the steel cylinder. To provide the magnetic field, and therefore control the forces in the actuator, the steel spindle is wrapped with magnet wire to create an electromagnet. Therefore, the spindle must have a large enough trough to saturate the MRF (section 2.2). A spindle length of ¼ inch was settled upon as an appropriate size. This allowed for almost a 0.18 x 0.2 inch trough with a 0.035 inch wall for the spindle to wrap magnet wire. The available tools used to cut the spindle determined this size. The smallest cutoff bit available is 0.09 inch and two cuts were used to create the spindle. It was theorized by Equation 1 (where a=height, b=width, da=wire diameter) that this would allow for approximately 700 wraps of 34-gauge magnet wire. Simulations were made using the FEMM[48] software to confirm its adequacy. Since the MRF saturates at 0.5 tesla (5000 Gauss) it is desirable to create a part, using these standard sizes, which would come as close to the saturation point as possible.
25
ab d a2
Equation (1)
R Rl Lm
Equation (2)
N≈
N≈
Table 3.1. Relevant magnet wire information[49] Gage
Wire Diameter
Ω/1000ft
Ω/inch
Max amp
34
0.0063in
261
0.02175
0.33
Taking an unused piston as an example, the spindle was wrapped, tested, and unwrapped. It was approximated that 350 wraps could be produced using the resistance level of the wire to calculate the total amount of wire. The number of wraps could be estimated by using Equation 2 (R=resistance, Rl=Resistance/length, Lm=Mean Length) with a total resistance of 5.4Ω. Relevant information about the wire used can be found in Table 3.1. It was later confirmed by direct measurement with a ruler that the total length of the wire is 228” and, using an average circumference of 0.225, the estimated the number of wraps is 323 for an error of about 8%. In experimentation, the lower wrap count proved to be enough for proper functionality and no further changes were made. When simulated, the magnetic field strength shows to be just over 0.20 Tesla (2000 gauss, Figure 3.5 and Figure 3.6). These gauss readings could not be verified due to the cylindrical outer shell preventing the gauss meter from direct readings of the magnet.
26
Figure 3.5. FEMM simulation of spindle with 350 wraps
It should be noted that the FEMM software cannot account for the magnetic properties of the MRF. Since the MRF that is used is 22% iron it will have a positive effect on the magnetic field, hence a stronger field is created. The FEMM does not have MRF as a material and the exact makeup of the MRF is proprietary and unavailable for consideration, hence air was used in place of the MRF. Air is a poorer conductor of the magnetic field and this simulation can therefore only be used to give a minimum field strength and for comparison between different coils.
27
Figure 3.6. Field strength plot from Figure 3.5 along red line, from top down.
3.1.1.2. Hex-Pistion
Figure 3.7. CAD drawing of hex-spindle
28
Figure 3.8. Hex-piston
As previously mentioned, the Hex-piston was dropped in favor of a Standard piston.
However, the Hex-piston was studied thoroughly for this project before its
discontinued use. To produce a Hex-Spindle, a low-carbon steel rod is sized to the external diameter of 0.340” by manual use of a lathe. The hex shape is then formed through grinding each of the six sides down by 0.0171”. The spindle is then attached to the center shaft to form a piston. The hex cuts were then added to allow the MRF fluid to flow between the top chamber and the lower chamber as the spindle moves up and down within the cylinder. Depths were decided through experimentation with circular spindles (section 3.2), used in the initial phases due to the ease of their production. A max and static force profile was established using circular spindles. A circular diameter of 0.320 was used to start testing
29 of the Hex-Spindle. This diameter gives a flow area of 0.0104 in2. The total area was transferred to the hexagonal form using Equation 3. In order to formulate Equation 3, essentially the inverse of Equation 4, a 5000 point solution for Equation 4 was inversely plotted in MATLAB and basic curve fitting functions were used to find Equation 3 as a result (see Figure 3.9). Equation 4 is based on Equation 5, the area of a Segment of Circle. Depth = - 1.1e4 * x 4 + 1.3e3 * x 3 - 60 * x 2 + 2 * x + .0015
a=6
r2 r -d r -d * (2 * acos - sin(2 * acos )) 2 r r
Area =
r 2 πθ ( − sin θ ) 2 180
Equation (3)
Equation (4)
Equation (5)
30
Figure 3.9. MATLAB curve fitting for hex cuts
Most beneficial to using the Hex-spindle is that it is essentially always touching the edges of the steel cylinder. Accordingly, this touching provides less chance of “catching” or a situation where the edge and wall stick due to them contacting at an angle. The hex shape of the spindle also gave another advantage when wrapping and creating the leads. When a circular spindle is used, a cut must be made in the edge to allow some of the wire to exit the spindle and create leads. The flat edge gives ample room for wires to circumnavigate the spindle. Unfortunately the benefits are not enough to justify the time requirements for creating a Hex-piston. Trials were discontinued after only a few samples were made.
31
3.1.2. Actuator Cylinder A steel cylinder was added in order to magnify the affects of the electromagnet on the piston. To create the steel cylinder a 3/8” OD stock tube was machined to have an 11/32 ID. The interior of the cylinder must be polished in order to reduce friction and prevent catching. The effect of the steel cylinder can be seen when comparing Figure 3.10 and Figure 3.11 to Figure 3.5 and Figure 3.6. Without this external steel cylinder, the field strengths are approximately 0.2 Tesla. When the cylinder is introduced the magnetic field in the simulation increased to approximately 0.6 Tesla, or 300%. Like the spindles, iron would produce a stronger magnetic field but availability of stock material and machinability prevents using iron.
Figure 3.10. FEMM simulation without steel cylinder
32
Figure 3.11. Field Strength plot from Figure 3.10 along red line, from top down.
3.1.3. Sealing The O-ring sealing the MRF fluid within the retaining shell and the center shaft sliding through it causes other static forces. PTFE (Teflon) -003 (specified as dash 003) O-rings were used for two reasons. First, they have a low friction coefficient, which by itself tends to produce lower friction forces. Second, it has a hard Durometer (hardness scale for rubber) rating. With a hard rating, the O-ring is less likely to break or distort when the center shaft moves within it. PTFE also has a low reactive nature, making it suitable to be used with most types of liquids. This did not appear to create any chemical advantages. However, the effects of long-term exposure to MRF are unknown.
PTFE is recommended for use with
hydrocarbon oils, the base fluid in MRF, but the exact makeup of the MRF is proprietary and unavailable for consideration in O-ring selection.
33 Several O-ring materials were tried during the early stages of the actuator design, including Buna-N, Viton, and Silicone, as well as PTFE, all round and Viton X-type. All O-rings with the exception of the PTFE, apparently required two O-rings to properly seal the actuator. Using two O-rings created more friction on materials with inherently higher friction forces.
3.2. Actuator Design Considerations The active strength of the actuator is based on two factors. These include the gap between the spindle and the cylinder and the strength of the magnetic field. A smaller gap will produce a stronger actuator. Conversely, it will also create more forces when the actuator is not energized. The limited path for the MRF to move from one chamber to the other creates this static force. The design constraints mentioned in section 3.1 are a compromise between this active and passive force profile. As can be seen in Figure 3.12 the actuators can be made strong enough to support 900 grams of mass in a static position. This setup has a tighter tolerance between the piston and the steel cylinder and is therefore not conducive for a haptic glove, although it could be practical for other applications.
34
Figure 3.12. Early piston supporting 900g mass
The strength of the magnetic field, when activated, is the other consideration when determining the maximum strength of the piston. Magnetic fields, when induced by a coil, are dependent on materials involved, number of turns and amperage through the wire. The material is set as steel; although soft iron would be better, it is not practical since it is not available in the sizes that are needed and is more difficult to machine. The number of turns and the amperage used are not mutually exclusive. Smaller wire will enable more wraps around the spindle, however the wire will handle less amperage. Since the wire wraps are apparently more important, as small a wire as feasible was used. It was decided that 34-gauge wire should be used because of its small size and recommended capacity of 0.33 Amps. Smaller wire is very easily broken and is therefore
35 hard to use. In practice, the 34-gauge wire has enough capacity for the purposes of this project.
When coupled with a spindle trough of approximately 0.18” the resulting
magnetic field is strong enough to create a distinctively controllable resistance in the actuator. When you increase the diameter of the steel cylinder and the spindle, you inherently have a larger control area. This control area could be defined as the area between the spindle and the steel cylinder.
When you have a larger spindle, you can
create a stronger magnetic field by increasing the depth and/or width of the trough. With the relaxed size constraints, an increase in spindle width can still represent a smaller percent of the overall size it occupies within the cylinder. Currently, the spindle is ¼ inch in a 1-¼ inch cylinder, or 20% of the overall size. This stronger magnetic field would allow for more space between the spindle and the cylinder without much loss of the overall strength, further reducing the static forces. Another option available with a longer cylinder would be to have two or more spindles trough. The actuator’s magnetic coils are hand wrapped at this time, producing imbalances and irregularities between the different actuators. A more automated process could produce a more standard and tighter coil. In return, this tighter, regular coil would produce a stronger, more uniform magnetic field, and therefore, a more uniform force profile amongst the actuators. Other testing, such as using different density MRF, could be done to examine changes to properties of the actuators. The addition of a diaphragm and air cavity at the base of the actuator would also provide some additional stability to the design by providing a volume to replace the volume lost by the shaft as it exits through the o-ring.
36
3.3. Actuator Evaluation During the early stages of the actuator development a number of rudimentary tests were carried out by the PIC microcontroller. These tests were used to determine the effectiveness of Pulse Width Modulation (PWM) on this type of actuator and maximum and minimum forces, as well as the effect of piston position and speed within the actuator.
3.3.1. Setup To measure forces, a commercially available, strain gauge based, load cell used. A actuation was suspended from this load cell. Since the more interesting portion of the force spectrum is at the low end, the minimum rated load cell of 8N was used. Overloading (non-linear portion) on this cell is accurate to 150% or 12N. A profile was created to take advantage of the normal and overloading ranges and coded into the test program. Displacement, and therefore velocity, was measured using a liner potentiometer. This type of potentiometer is linear to 0.5% and has low friction and is well suited to the project. Additional control circuitry for power isolation and a slider to insure that the piston moved directly up and down were added to the setup (A mount and slider not seen in Figure 3.13).
37
Figure 3.13. Test setup
3.3.2. Program A C++ host program was created to send commands be sent over the USB port to the PIC. The host program is responsible for timing and therefore sends commands to change the digital output port from high to low each time a change was required. The C++ timer has one millisecond precision, consequently, when 10 separate duty cycles are required, the period of the PWM can only be 10ms. This is too low for truly accurate force profiling but good enough for the initial phases.
38 NO
Time up?
Yes Set output on PIC
NO
PIC
Read Sensors through PIC
Output
A/D readings
Write Result
RAW File Processed Results
Done?
Yes
Process File
Figure 3.14. Flow-model of initial testing program
. Measurements on the load cell were made using the calibration weights and the calibration results were hard coded into the C++ program.
The PIC was used to read
analog signals from the sensors, digitize them, and report the signals to the C++ program. The program records the data and processes the results using calibrations that were done previously. MATLAB was then used to display the results.
3.3.3. Results Figure 3.15 to Figure 3.18 show examples of the results of the initial tests described within this section. All figures are the results of one test, however, several tests were conducted using diameters ranging from 0.340 to 0.318 inch by 0.002 to 0.003 inch increments. Each diameter was tested more than once for repeatability. Tests on 0.320 will be described as the example of the superset of all tests and individually as the
39 solution that was expanded on for later spindle sizing. It should be noted that the setup used produced more accurate force results in the positive (downward) direction. This is due to the nature of the load cell and the setup constructed to test the actuator.
Figure 3.15. 2D plot of force vs. DC for 0.320 diameter spindle
An important factor to be discerned from the tests is the maximum and minimum forces when the DC cycle is at 0 and 1, or no active magnetic field and max magnetic field. In Figure 3.15, the maximum forces at 0 and 1 DC are approximately 2N and 8.5N respectively. The force at 0 DC was decided as an acceptable static force for the purposes of a haptic glove. Tests conducted with larger spindle sizes produced higher maximum forces at 1 DC at a cost of increased static forces. Lower forces were decided to be more important at the 0 DC level. Continued reducing of the spindle size did lower
40 the static forces at a cost of the maximum forces, but this lower maximum force was deemed too low for this haptic application. When evaluating the effectiveness of using PWM signals to produce a smooth force curve, there must be a more conservative result. The PWM used during this test was done using a slow 10ms period. Even with this slow period we can see a “horn” shape to the forces vs. DC in Figure 3.15. This horn shape shows that the actuator has the ability to be controlled. The period of the PIC’s hardware PWM is much shorter and we would therefore expect a greater degree of control. Spindle position within the cylinder appears not to have any effect on the total force produced by the piston at any DC tested. As can be seen in Figure 3.17 and Figure 3.16, as the “top” and “bottom” of the curve for each DC being essentially flat. This indicates that force across most of the actuator’s stroke is approximately equal. The “sides” of the curves represent the transitional periods and are therefore ignored in this discussion. Figure 3.16 is a representation of a single DC value (90%) from Figure 3.17.
Figure 3.16. 2D plot of force vs. position for 0.320 diameter spindle
41
Figure 3.17. 3D Plot of DC, force and position for 0.320 diameter spindle
Another important question that this test was designed to answer is; what effect does the velocity of the spindle within the cylinder have on the forces that are produced? From the results of testing, there appears to be a peak force that can be produced by the piston. Evidence of this phenomenon is apparent in Figure 3.19 and Figure 3.18 (single loop at 90% DC from Figure 3.19). When viewing the top and bottom of each DC curve, there are portions where the forces are stable over a reasonable velocity. evidence is that there appears to be a flat curve at these force extremes.
Further
42
Figure 3.18. 2D plot of force vs. speed for 0.320 diameter spindle
Figure 3.19. 3D plot of DC, Speed and force for 0.320 diameter spindle
3.4. Actuator Evaluation Since each actuator is made by hand, they are unable to be constructed so that they have uniform characteristics.
Each one has differences relating to the number of
wire turns around the spindle, the spindle size, quality of the polished surfaces, quality of alignment, micro scratches, tightness of the wire coil and many other factors that affect
43 the performance and usability of the actuator. Table 3.2 contains relevant information as to the various actuators built for this project. Note that many factors cannot be measured with available equipment and therefore cannot be listed. Table 3.2. Table of actuators Name
Finger on Glove
Spindle Diameter
Resistance
# of wraps (estimated)
Initial Movement No power
Initial movement Full Power
Empty
N/A
N/A
N/A
N/A
1.078N
N/A
Hex
N/A
Not Measured
N/A
3.038N
5.49
A
N/A
Not Measured
N/A
1.47N
5.88
B
Middle
0.328”
6.3
407
1.519
4.90
C
Thumb
0.325”
5.8
377
1.764
5.12
D
Ring
0.326”
6.0
389
1.862
4.50
E
Index
0.327
6.4
414
1.372
5.00
F
Pinky
0.326
6.2
402
1.67
4.51
G
N/A
0.325
6.2
403
1.568
Leaked when tested
See section 3.1.1.2 .325” (Cut size Not actual)
When we look at the actuators that were used on the final glove we see that the maximum, minimum and average forces were 1.764,1.372 and 1.637 respectively when not active. When full power was applied the forces were 5.88, 4.5 and 4.8 for the maximum, minimum and average respectively. The correlation, or lack there of, with the estimated number of wraps is the important factor to be discerned from examining the table. With regard to the construction of the piston, this information shows that the other factors previously mentioned are the more important when producing an actuator. An example of how this lack of correlation would be produced could be from a wire coil that was not wrapped tightly around the piston’s spindle. In this example the less tightly wound coil protrudes from the trough of the spindle, effectively giving it a
44 larger size then the size shown in the table. This phenomenon would create a higher static force. Additionally since it is not as tightly wound, there could be a larger number of turns that do not directly follow the spindles natural path, coursing the magnetic field strength to be reduced. Reduced magnetic field produces a lower active force. Due to these conditions it is highly recommended that an automated process be used in the winding of the spindle.
45
Chapter 4: Exoskeleton, Sensors, Mechanical and Physical structure
Figure 4.1. Full glove
46
Figure 4.2. Underside of glove
The Exoskeleton structure of the MRAGES provides the mechanism for mounting the glove on the user’s hand, mounting the actuator, transferring force from the actuator to the fingertip, measuring forces, and measuring finger flexion. These hard parts are mounted onto a commercially available batting glove. The components represent a mix of rapid prototype parts, machined parts, and commercially available items. Sensors are attached to the structure to measure forces and displacement.
47
4.1. Exoskeleton Mechanics Figure 4.3 shows the CAD model used to create the rapid prototype parts of a single finger. Other hardware not included in Figure 4.3 but can be seen in Figure 4.1 and Figure 4.2 include: springs that allow the fingertip mount to spread for larger fingers, a control cable, coupling, and the linear potentiometer. A simple description of the design is that of a transmission system using a modified control cable.
Figure 4.3. CAD representation of full finger
The control cable is connected to the fingertip on one end and to the actuator on the other by way of a coupling. When the finger bends, the arch length of the finger’s outside increases. Therefore, the cable will pull out the piston as the finger bends and conversely, push it inward as the finger straightens. If the actuator is energized, the piston will resist these motions. At the base of the mechanical components of this haptic system is a leather glove, which was chosen and modified specifically for this project. The exoskeleton finger
48 spine, section 4.2, is designed to spread out as the user bends his fingers. This spreading is accomplished through a stretchy fabric on the glove, which covers the top of the fingers and where the exoskeleton finger spine is glued.
Fabric used for this purpose
must be both strong and flexible. Lycra was considered for a custom glove, however time constraints prevented its construction. Additional Velcro support straps were attached to the sheet metal support plate. Velcro allows for easy attachment and cinches the material around the hand. It is important that the glove be a close fit in order to function properly. The length of the fingers and their fit will directly impact sensitivity to the glove. Not shown in Figure 4.3 but seen in Figure 4.1, is the control cable and its coupling. This coupling has a post attached to it in order for it to be connected to the potentiometer, used for liner measurements. The springs seen on the fingertip mount are used to allow this mount to expand around the finger and therefore give a more comfortable feel. 0-80 bolts, which are used to attach the springs, are also used to secure the control cable in place.
The entire setup is mounted on a commercially available
batting glove. Slight modifications were made to the glove to provide a better platform for the mechanical components. A sheet metal cutout provided an intermediary gluing and stiffening platform for the stationary components of the mechanical system.
4.2. Exoskeleton Finger Spine The heart of the mechanical power transmission system is the exoskeleton finger spine, which acts as a control cable holder. This part is designed to function with all other hard parts and is a result of many iterations of experimental configurations. It is the largest (linear size) and, with the exception of the actuator itself, most complex hard
49 component of the system. Several iterations of the exoskeleton finger spine were tested before coming up with this final version. The exoskeleton finger spine must not only provide a path for the control cable but also it must be comfortable, provide a straight path for the actuator center shaft, be easy to work with and allow for low friction when parts move within it.
Figure 4.4. Wire-frame of exoskeleton finger spine
A 7x7, 1/16” PTFE-coated aircraft cable is used as the control cable for the system. This type of cable is made out of 7 bunches of 7-strand twisted stainless steel wire rope. Breaking strength is normally the factor used in determining the size of a cable (wire rope) to be used. 1/16” aircraft cable has a breaking strength of 480 lbs., far exceeding the requirements of this project. In this instance, the cable is being used as a push-pull control cable and therefore a thicker cable, with less of a bend radius, is needed. The PTFE coating provides a low friction surface when the cable moves through
50 the exoskeleton finger spine (Section 4.2).
Synthetic grease is also used to lessen the
friction between the cable and the exoskeleton finger spine
(a)
(b) Figure 4.5. Early Exoskeleton finger spine: (a) cad model, (b) RP part
As can be seen in Figure 4.4, there are three sections of the exoskeleton finger spine. The forward-most section (left) in Figure 4.4 is an extension that resizes the holder to the length of the finger by buttressing the exoskeleton finger spine to the fingertip mount. This section can be increased or decreased in length depending on the user’s finger size, which finger it is being used for, and glove size. The rear section has a larger tunnel through it, in order to accommodate the cable coupling. Due to the supports that are produced during the rapid prototype “build,” some surfaces become uneven and jagged.
51 This phenomenon would happen if a slit were present on top of the rear section (Figure 4.5). Therefore, the top of the rear section should be milled to provide a smooth slit. A slot of 0.040” (1 mm) is cut to accommodate the potentiometer post extending from the cable coupling. The rise in the middle section is required for a gentle transition to match the height of the actuator when it sits atop the slider. Careful attention must be made to this section when supports are created in preparation for a rapid prototype build; sometimes the software loads supports inside the cable path. These will affect the overall friction and reduce sensitivity of the device. In order to more easily produce and work with this part, all sections are joined together. By joining them, using a thin connection during RP build, the part can be glued to the glove in an easier fashion. These joining sections are cut after the glue dries and the cable is threaded through its path. Once the joints are cut, the cable and glove’s fabric keep the exoskeleton finger spine in approximately the same shape while the glove is not on the user’s hand or the fingers are not flexed.
Figure 4.6. Bending of exoskeleton finger spine
52 Each individual piece of the front and middle sections is designed to spread as the finger flexes. During this flexion, the cable will push against the bottom of the pieces and draw out the actuator’s piston (see Figure 4.7 as an example). Conversely, when the finger reduces flexion, the cable hits the top of the pieces and pushes the piston inward. It is therefore important that the cable path be as close in size to the cable as possible. Due to the nature of the rapid prototype machine and cleaning process, a larger than necessary cable path was made for the control cable to pass within. This path should be reduced if another process is used to make the exoskeleton finger spine.
Figure 4.7 Cable bending example
The control cable will bow if the space between the segments becomes too large. Maximum gap space is determined by the cable’s bend radius and therefore requires proper spacing between the segments. This spacing requirement particularly comes into play when the cable is bent and the user is opening their hand, causing the cable to push against the actuator. Earlier iterations of the holder led to this almost fully-enclosed cable attached through every piece.
53
Figure 4.8. Bowing cable
The control cable terminates, on one end, in a fingertip mount where it is attached in two places by a pair of screws compressing the cable between them. Springs are attached to the compression screws to grasp a finger when it is placed into the fingertip mount. The mount will expand to accommodate fingers up to 1/8th inch thicker than the nominal size of approximately ½ inch.
Figure 4.9. Fingertip mount
The other end of the control cable terminates in a tri-coupling where the raw cable is brazed into a machined brass piece. Two other connections are made in the tri-
54 coupling. One is a tapped hole to which the actuator is attached. The other is for a 1/32 inch brass rod to connect to the linear sensor (section 4.3.2).
4.3. Actuator and Sensors Mounting In order to provide proper sensing the two sensor components (force and position) for each finger must be mounted on the glove. The MRF actuator is also located directly on the back of the hand and must be mounted properly to achieve good functionality.
4.3.1. Force Sensor
Figure 4.10. Force sensor close-up
The sensitivity of the gauge is determined by the thickness of the stainless steel sheet. In this case, the gauges were made of 0.025” thickness stainless steel sheet metal. Thinner stock material will give a greater sensitivity. The saturation point of the strain gauge is quickly reached, and therefore, a relatively thick material is used. The force gauge sits at the back of the actuator, connecting it to the slider. It consists of a strain gauge glued onto a sheet metal cutout.
The slider is glued to the
55 glove via the intermediary sheet metal plate. In this manner, all forces felt at the back of the actuator are transmitted through the force gauge.
4.3.2. Position Sensor An Alps[50] linear potentiometer is used as the linear sensor in the glove. It is a commercially available off-the-shelf (COS) part. Linearity of the sensor is the most important feature of the potentiometer’s usage. Although the total resistance is listed to be 10kΩ, the linearity is listed at 0.5%. The same potentiometer was used in the initial testing and is further described in section 3.3. It is low friction and will not add much additional resistance to the forces felt by the user.
Figure 4.11. Potentiometer close-up
In order to attach the sensor to the glove, it is glued to the top of the actuator. Using hot glue, a small rapid prototype part, Figure 4.12, is connected to the sensor’s moving head. A 1/32 inch brass rod is used to connect the sensor head to the control cable tri-coupling via another rapid prototype part.
56
Figure 4.12. Rapid prototype parts for sensor attachment
4.3.3. Actuator Mounting One actuator per finger is mounted on the back of the glove. The Thumb and fingers have slightly different mounting techniques due to the nature of human anatomy. As seen in Figure 4.1, a metal plate is glued to the glove, as an intermediary for the slider mentioned in section 4.3.1. This slider must be in line with the hand’s metacarpals so that the finger bending motion will extend the actuator’s piston without causing the piston to rub or catch on to the steel cylinder. For the four regular fingers this intermediary metal plate is fixed in place and movement is not necessary since the finger’s metacarpals are unable to move. The thumb’s metacarpal bone is opposable and therefore special consideration must be taken when placing the slider above this bone.
A separate metal sheet is used as the
intermediary here and the actuator is allowed to move with the metacarpal of the thumb.
57
Chapter 5: Electronic Architecture
In addition to the PIC development board, three boards were made to provide power to the glove’s actuators and sensor reading circuitry. Boards were constructed and mounted in such a way that they would be able to be mounted on the user’s upper arm. Therefore there would be a minimum amount of wire tethering the user to the host PC and the external power supply. When viewed as a whole, the boards contain the power amplification and sensor circuitry for all five fingers and allow each one to be independent of all other fingers. All boards were soldered by hand, including leads required for the PIC development board. Circuit designs were tested on a breadboard before transferring them to the solder board. Designing and fabricating a PCB would mean an unacceptable time lag and therefore could not be completed in the time required for this project. Abhijit Tamba graciously constructed the power board from the given design. All boards are powered through a separate fused power cable constructed for this project.
Each board requires a +5V and GND for normal operation.
The PIC
development board is powered through the USB connection to the PC. If desired, the development board can be configured to run off of the same power cable as the other
58 boards. Ground is attached to the common line on the power cable to insure a proper common ground for all signals and A/D conversions.
5.1. Sensor Boards The two sensor boards are designed to connect to each other to allow them to function as a single unit and therefore need only one cable to send information back to the PIC for A/D conversions and one for power supply. All signals from the 3-finger sensor board are routed through the 2-finger board for this reason. As can be seen in Figure 5.1, the wire routing for this consolidation makes the underside quite complex and cluttered. A rapid prototype base is used to both protect the wiring and mount the boards on the users. Both the force sensor circuitry and the linear sensor circuitry are located on the same boards and in equal amounts, three pair on one board and two pair on the other.
Figure 5.1. Sensor boards
59 The sensors’ values read by the PIC are referenced against the internal +Vs and GND values. Values read by the PIC are referenced against these values and converted to a 10-bit number. If higher precision is required, then an external +Vs reference can be used. Reference voltages are limited by the highest value produced by any of the 10 sensors. Since the linear sensors can create a higher value, they should be used as the max reading. See section 5.1.2 for a detailed decision on setting a lower reference voltage.
5.1.1. Force Sensor Electronics Each force sensor consists of a strain gauge, a Wheatstone Bridge, an op-amp and a potentiometer for adjusting the gain on the op-amp. This circuit is fed into an A/D channel on the PIC and read when prompted by the embedded firmware. There is also a large amount of wiring that is used to connect the strain gauge to the sensor board and, to a lesser degree, to connect the boards to each other for the single sensor cable mentioned in the previous section. Resistance variations create an unbalanced bridge and lead to a shifting of the voltage measurements, toward the rails, when outputted from the op-amp. Reducing the gain set by the potentiometer can compensate for shifting; however, this limits the voltage swing that is seen through the A/D converter. All things being ideal, the voltage should have a nominal level of 2.55V, half way between the +5V and GND rails and provide a voltage swing to the maximum amount allowed by the op-amps. In this case, AD620[52] op-amps are used and are listed to have a swing of at least –Vs+1.1V to +Vs-1.2V, or >1.1V to >3.8V, for a total swing range of 2.7V minimum.
60
Figure 5.2. Schematic of single finger force sensor
The actual swing of the force sensors became lower because of these resistance variations. A swing of 1V to 1.5V is a more realistic approximation for the setup created on most fingers. Nominal values depend on the way in which the bridge is unbalanced due to the variations in resistance levels. In future designs, resistance wire or high accuracy, low value, potentiometers should be used to balance the bridge and more accurate resistors would be an appropriate choice for the bridge. The swing range would need to read the maximum forces that a finger would produce during an exercise. A force range of ± 16N was used since the range of the RMII-II was also 16N. This is the range that is calibrated for by the calibration program in section 6.1.1. Assuming a voltage swing of 1.25V, the 10 bit A/D on the PIC gives a resolution of 0.16N/bit. The accuracy and repeatability of the PIC’s A/D module is not listed in the datasheet however experience shows that there is some inherent error in the A/D module. If we assume a possible error of two LSD bits, then the reading can be off by up to three
61 levels or almost 0.5N. This error level made integration into some previously written programs difficult.
5.1.2. Linear Sensor Electronics Linear potentiometers are used as linear sensors. The circuitry for this type of sensor is a simple matter of adding some resistance inline with the potentiometer, in order to protect the PIC’s A/D model from being connected directly to +Vs, and connecting the A/D to the potentiometer through this inline resistance. 1kΩ resistors are added between the potentiometers and +Vs for this reason. As mentioned previously, all cabling is unified to reduce the number of cables that are required. Total resistance used is 10kΩ+1kΩ as specified in the datasheet, with a run of 32mm (1.26 inch) on the potentiometer. Since the full extension of the actuator is only 1 inch, only 80% of the potentiometer’s values will be seen. For the glove, the base position is a potentiometer value of 0Ω and therefore a value of 0V is seen by the A/D. The maximum voltage output from the potentiometer is of greater interest since it can be used to determine a reference voltage for the A/D conversions. The circuit mentioned here produces a maximum voltage seen at the A/D of 4.64V, assuming a true 5V power source is used. Therefore, in this case, a new reference voltage would give only a minimum improvement. Given the total throw of the actuator is 1.25 inches, a 10 bit A/D converter and a swing of 4.64V would give a resolution of 0.0013 inches/bit. Here the two bit error of the A/D would give a error of only 0.0039 inches. The potentiometer has 1% linearity and therefore 0.00125 error, making the A/D the limiting factor.
62
5.2. Power Amplification Board PWM signals coming from the PIC cannot be used to directly drive the actuators because of the limited amount of amperage the PIC can produce. Each channel is only able to source 25mA of current. An SGS-Thomson L293D Push-Pull channel driver is used to boost the signal and drive the actuators. In addition to the actuator, a high wattage 10Ω resistor is in series with the actuators to restrict the current to under the maximum value of 0.33A for the 34-gauge wire. When inspecting the actuator with the highest impedance (Table 3.2), a maximum current of 0.32A is produced. Additionally, an LED for each finger is visible on the power board. The intensity of this LED is linked to the PWM duty cycle.
Figure 5.3. Power amplification board
Embedded programming on the PIC and control signals set to it determine the duty cycle (DC) to a value between 0 and 255.
In this manner, the actuators are
controllable with a granularity of 28. Section 3.3 discussed the early method of controlling the PWM and Section 6.1 will discuss the embedded control method.
63
Figure 5.4. Power amplification
Table 5.1. Power requirements Part Actuator L293D Op-Amp Bridges 18F8720 Total
#/glove 5 2 5 5 1
Current (mA) 320 60 1.3 29 300
Total (mA) 1600 120 6.5 145 300 2171.5
Calculation method Max actuator on glove Typical / Datasheet Typical / Datasheet Resistor values Max value, Datasheet
64 The total power requirements for the glove can be found in Table 5.1.
The
PIC18F8720 power given is Table 1 is the absolute max so it can be assumed that the actual value will be far less. The actuator values given are as if the max forces were being produced 100% of the time. Even using these high values, as an example, four rechargeable 2500 mAh NiHM AA batteries could be used to power the glove and its supporting electronics for over an hour at max power.
5.3. Embedded Microcontroller Board
Figure 5.5. Microcontroller development board
A commercially available PIC microcontroller, USB interface board was used to for the embedded controls on this project. The key components of the board are the Microchip PIC 18LF8720 microcontroller and the FT245BM from FTDI. The PIC is used a MCU and the FT245BM is the USB controller. The on-chip components of the PIC, which are utilized for this project, are ten A/D channels, two timers, five PWM channels, data EEPROM, 10 general-purpose I/O lines, along with and the ALU and program memory. Any device that satisfies these requirements is sufficient to run the glove. Figure 5.6 shows the block diagram provided by Microchip Corporation. The pins and on-chip peripherals used for this project are highlighted in yellow. All system
65 architecture, such as the program memory and data path, ALU, timing generator and other critical system components are also used but not highlighted.
Figure 5.6. PIC block diagram[39]
66
5.4. User Component Mounting When the component boards are fully constructed, they must be mounted to the human user in such a way that maximizes the effect of the components while coursing the least amount of discomfort motion loss. Therefore the sensor components are the closest to the actuators and control systems are farthest away. All of the boards are fitted with a mounting plate and Velcro for attachment to the upper arm. Figure 5.7 shows the mounting diagram for the current glove. The section for battery and wireless is shown here but not implemented.
Figure 5.7. Mounting diagram
67
Chapter 6: Software Architecture
There are three basic programs that are written to control the glove and interface with the programmer and/or user. First is the MRAGES embedded control program. It is this embedded program that runs the PIC during its normal operation and during all calibrations. A separate PC console-based program is used for calibration and testing of the glove. The third program located on the PIC is used to interface with the suite of exercise programs[47] developed by Hristiyan Kortev for a separate project done through the Human-Machine Interface Laboratory. The total requirements of PIC memory are 15314 bytes of program memory and 649 bytes for data.
6.1. Embedded Control Program The Embedded control program (ECP) is the basis for normal operation of the MRAGES. Upon startup, the PIC’s embedded program will automatically enter the ECP (other options are described in section 6.3) and begin polling for commands sent over the USB. All commands and parameters are followed by an XOR checksum in order to assure that the command was received correctly.
Error and success responses are
transmitted following the receipt of commands to check the integrity of the system.
68 Appendix A lists the commands available to the user during normal operation and the format required for their usage. Figure 6.1 shows a diagram of all the program modules produced to form the embedded program. The PIC C is not an object-oriented language however; the program is broken down into these parts for a more detailed analysis of how different portions of the program interact with each other and the glove. Timer control module
EEPROM interface module
Calibration module
Force control module
MRAGES main control module
Embedded rehabilitation host main module
USB Data link module Host PC
PWM control module Senor acquisition and filtering module
Figure 6.1. Diagram of all embedded program modules
69
6.1.1. Embedded Calibration Module Calibration on the embedded side is also accomplished through the requests and responses from the console program (section 6.2). Requests must be initialized by an outside element since there is no way to directly enter or display data on the PIC development board. There are two types of calibrations that can be requested: first, the request to calibrate the force sensors, and second, to calibrate the force curves for future requests. The force curve calibration determines the DC level required to produce a particular force.
Figure 6.2. Force gauge during calibration
Force
sensor
calibration
is
initialized
when
the
command
START_SENSOR_CALABRATION is given by the console program. Other internal commands that are used are described at the end of Appendix A. Each finger’s sensor must be calibrated separately and requested by the user. Calibration is done using a set of known weights supported by the sensor (see Figure 6.2). Through the console program, the user is asked to place a distinct amount of weight on the sensor, and then respond that
70 the weight has settled. A number of measurements are taken and averaged. The process is repeated for seven different weights and then fed into a linear regression function on the PIC. Results are saved into the PIC’s EEPROM as a stable place to store the linear regression formula. During power-up the PIC will read the EEPROM for these values and use them for force calculations. Once all five fingers are calibrated, then the force curve may be found. A command of START_FORCE_CALABRATION initializes the calibration process. This calibration task is accomplished by asking the user to place the glove on his/her hand then cycling through the DC levels on all fingers while reading the force sensors. The user must open and close his/her fingers at a consistent rate during this process to have an accurate calibration. A digital low pass filter is used during the measurement process in order to minimize the noise. The highest measurement for each of the ten DC levels is then sent back to the Host PC for a fifth order polynomial curve fitting. Third order can also be used without much sacrifice in the results. Due to the memory limitations on the PIC, it was not possible to conduct the curve fitting on the PIC. Readings are sent over to the PC for processing and the results are sent back over the USB and stored in the PIC’s EEPROM for future use. The linear sensors’ calibrations are not stored in the EEPROM, nor do they require any elaborate calibration. The absolute maximum and minimum distances are a product of the actuator’s design as described in Chapter 3. The PIC will check for maximum and minimum readings during operation and use these reading to calculate the
71 percentage that the actuator is extended. This percentage is then multiplied by the absolute max of the actuator to form a reading.
6.1.2. Sensor and Data Acquisition Module Measurements of the force and linear sensors are accomplished through the PIC’s on-chip A/D converter. There are several reading types that are available to the user, along with several start and stop requests that relate to these reading types. Types include the raw values of the data, converted data, and a request for a timestamp with either type. Data can also be requested as a finger pair. The raw values from the A/D converter module are available to the programmer if required; however, for normal operation, a low pass filter is used to filter all measurements from the A/D. Continuously polling the A/D ports and then using a standard digital low pass filter shown in Equation 6 create these filtered results. In this equation, X(k) is the raw result. A λ of 0.90 is used and was found experimentally to work well for this setup.
A request to start continuously measuring results
(START_MESUREMENTS) is needed to start or reset the process. Measurements for each sensor are made individually.
Yk = Yk −1 * λ + X (k ) *(1 − λ )
Equation (6)
Measuring and filtering, after initialization, is done as a component of the main loop. The results are stored and are therefore available as soon as a request comes in. A request for newly converted data will require a small delay. While the values of the calibration equations are available, the conversion will not take place unless requested.
72 Time stamping is done using the internal counters that are available on the PIC. The counter uses the clock crystal on the development board and is not calibrated with any other process or value. While this is accurate for short periods of time, a drift will occur and is therefore not recommended for extended periods of time. When requested by the START_TIMER command, a timer is initialized and run so that an interrupt will occur every millisecond, based on clock cycles and clock frequency. Seconds and milliseconds are counted and sent back through the USB upon request. A raw request will read new values and send them across the link with the timestamp indicating when the transmission took place.
Filtered requests are time
stamped with the time the request is filled. Sensor readings are done much faster than one millisecond but it is possible that the value was computed in the previous millisecond.
6.1.3. Force Control Module Two methods are provided to set the forces of the actuators: by directly setting the DC values for each of the actuators and by requesting a specific force level as determined by the calibration described in section 6.1.1. A mechanism (START_FORCES and STOP_FORCES) is also available for turning on and off individual fingers or the glove as a whole.
No forces will be felt on any finger that is not activated using these
commands. Directly setting the DC values is accomplished by sending a SET_DC command, followed by DC values for each of the five fingers. The DC value can be between 0 and 255 inclusively. Changes using this command will keep the DC statically set until another command is received via the USB or a restart takes place.
73 When using the SET_FORCES_INTERNAL command, the internal calibration functions are used to calculate the initial DC value. Once again, the command should be followed by values for all five fingers. When this command is used to set forces, the DC will attempt to adapt itself based on a window to produce the forces desired. After each loop a new force reading is taken. This reading is compared to what is set through the SET_FORCES_INTERNAL command. DC values are then raised or lowered to attempt to produce the desired forces. A window is used to ensure that the values do not drift too far off of the calibration curves. Since the calibration curves are used for this command, it is important to take care in the calibration process if the user wishes to set forces internally.
6.2. Calibration and Testing program A console program was developed to provide a system for testing and calibrating the glove. USB communications are done using a virtual COM port selected by the user at the initiation of the program.
From here, the user may select to calibrate the glove
(discussed in section 6.1.1), run a number of testing programs, or administer the Human testing program, discussed in Chapter 7. In addition to the functions to conduct the calibration of the glove, as mentioned in section 6.1.1, there is a mechanism to save or load the calibration to the host PC’s hard drive. These functions are provided to view the calibration information, manually make changes, and back up the EEPROM data.
74
Figure 6.3. Calibration and testing console
Test programs are designed to interface with the glove using the commands discussed in section 6.1. Data retrieved during these programs are stored and later displayed using MATLAB for interpretation. The goal of this type of testing is two-fold: first, to ensure the functionality of the glove commands and proper representation of the forces, and secondly, to measure things such as response time of the MRF actuator or to compare the speed versus forces of the actuator. Insuring the proper functionality of the glove proved to be a much easier task than measuring responses of the actuators. Testing functionality consisted of simply using one of the calibrated force sensors, and then moving the piston up and down while applying various DC values to the actuator and recording the results. In this way, a proper calibration technique was developed. Figure 6.4 and Figure 6.5 show an example of a Matlab produced graph for a test to measure the response time of the force sensor to a change in the DC value. Figure 6.4
75 has the unfiltered readings while Figure 6.5 is the same plot with a low pass filter applied. This example will be used to demonstrate all tests that were made using the technique described here. For this test a motor was controlled using one PWM port and the actuator using another. The readings graphed are of a normalized DC for the actuator and show when PWM signals were applied to the actuator. This motor is a rotary motor and is converted to a linear motion using a worm drive. In this way, a constant speed could be applied to the actuator’s piston. Time is measured using the time stamp technique described in section 6.1.2. Forces shown are recorded as unprocessed, unfiltered, raw voltages and then processed in Matlab. In this case, the force sensor was the commercially available one, previously described in section 3.3. Unfortunately, no picture is available for the motor setup since it was destroyed during testing.
Figure 6.4. Response time of actuators
76
Figure 6.5. Low pass filter response time of actuators
When examining Figure 6.4 or Figure 6.5, we see can see three distinct points of interest. The first is when the motor is activated, happening in the 6th second. Up until this point the force level is fairly constant at approximately 0.8N. If the low pass filter was activated, or post data filtered, then the force reading would be smooth, but the force level would still not be 0N. The force level offset is superimposed on the graph due to voltage differences that occurred between the calibration of the force sensor and the running of this test. These could be filtered out as well; however, since this test was interested in response time of the actuator and MRF fluid, filtering is not necessary at this time.
77 After the 6th second we can see that the actuator will apply some force when moving without any active resistance from the actuator. This resistance is inherent in the actuator and the subject of much discussion throughout this paper. After approximately 7.5 seconds the DC of the actuator is set to full power. The result of the activation of the actuator is a spike in the force readings, as seen in Figure 6.4 and Figure 6.5. Additional tests produced similar results.
6.3. Embedded Rehabilitation Host Program An additional embedded program was written to interface with the program written by Hristiyan Kortev for an orthopedic rehabilitation exercise suit. During initialization and boot-up of the embedded PIC program, a branch can be selected through an external hardware jumper. If this branch is selected, the commands described in section 6.1 are replaced with those used by the RMII-ND in an effort to reuse these exercises on the host PC. Figure 6.6 shows the possible rehabilitation setups, using either the green path, RS-232 to the RMII-ND or the orange USB path to the MRAGES. The goal is a seamless use of either path depending on the equipment available. Large strides were made in this effort to integrate the MRAGES to the RMII systems; however, the quality of the results was not high enough to warrant using the RMII programs.
With regard to the finger bending within the WTK program, the
translation was of a high quality. The RMII measures this bending using two metrics per finger, these being the piston displacement and the flexion of the finger.
Some
information was artificially constricted by the MRAGES since only one metric, displacement, is used for the same measurement.
78
Figure 6.6. Rehabilitation system setups
With regard to the force readings, stability of the signals was a critical problem. As mentioned in section 5.1.1, the swing range of the sensors was small, creating a small digitized result. The range of the result was further reduced since the RMII sends single byte information for a force per finger and scales it by a factor of two. Furthermore, the sensors on the MRAGES are bi-directional and only the absolute value is used for the mimicking program. With the low swing range of 1V, the sensor produces very few levels for use in the WTK exercises. Here noise becomes a significant problem and makes the sensor reading and display on the WTK exercises almost useless. To compensate for these shortcomings, the force sensor range can be reduced in the short-term. Long-term solutions require increasing the swing range, as described in section 5.1.1, as well as having a more sensitive A/D converter.
Additionally, bi-
directional force readings are not required for this application and could also be eliminated to increase the sensitivity.
79
Chapter 7: Initial Human Testing
7.1. Evaluation Description The goal of the testing process was to determine the viability of the glove for use as an interface with a computer. Basic requirements for a usable interface require that a person be able to feel distinctive differences when different force levels are set on the glove. Testing was conducted to determine the degree as to which the MRAGES is able to provide these decisively different forces. Using commands described in section 6.1.3 and Appendix A, a set of tests was developed to determine the functionality of the glove when referring to the goal of human testing. The test program was integrated into the same console base program described in section 6.2. A total of ten subjects participated with seven subjects completing the study. Three participants dropped out of the study during or before the identifying levels stage of the tests. All dropped out because it was obvious that the glove did not properly fit their hand and, thus, will not be analyzed here as part of the study. All subjects were male due to the larger hand size found in males. The glove was constructed to fit the author’s hand (approximately 7 ¾ inches or slightly larger), which is larger than most found in the study. Hand size is measured from the tip of the index
80 finger to the first wrinkle in the skin on the wrist with the palm side up.
All
measurements are made to the nearest ¼ inch. For each subject a form (see Appendix B) was filled out. The test program consists of four parts and a subjective questionnaire (exit interview). Additionally, the subject is given a description of the glove, its working parts, how it was constructed and programmed, as well as time to ask any questions they might have relating to the glove. The glove is then fitted to the hand and a comfort level required to conduct the tests verified. Each session took approximately half an hour to complete.
7.2. Intensity Perception There are two parts to the Intensity Perception test. In the first part, the intensity familiarization, the subject is asked to open and close his hand while the force level of the glove is varied. In three seconds on, three seconds off intervals, the DC for all fingers is varied, from 0 DC (off) to 250 DC (full power), by 50 DC period values each cycle. This process is required to be repeated a minimum of three times; however, the subject may request additional runs. The purpose of the intensity familiarization is to familiarize the subject to the glove, its force levels and again ensure that the subject has a good comfort level. In order to perform the second part of the test, identifying levels, the subject is informed of the values of the forces felt during the intensity familiarization and is able to see when the values change. Additionally values are given orally. The second part of the intensity perception testing is known as the intensity evaluation. Upon completing the intensity familiarization, the subject is asked to identify the levels that the glove is producing without reference to the actual values. Identifying
81 the levels is accomplished by asking the subject when they are ready. When ready, the force is administered through the glove for three seconds and then shut off, completing a single trial. The trial is not repeated if the subject asks for additional time. A decision for the value is made and recorded. The process is repeated 30 times in the same order for all subjects. Table 7.1. Results of intensity perception tests Subject’s Initials AK GB JA JC MB SA SP Average
Hand Size 7 ¼” 7 ½” 7 ½” 7 ¾” 7 ½” 7 ¾” 7 ¼“ 7 ½”
% Correct 30% 47% 54% 43% 57% 73% 23% 46.7%
% Off by one level 40% 43% 23% 24% 21% 24% 40% 30.7%
The tabulated results for the intensity perception test can be found in Table 7.1. As can be seen in Table 7.1, the results were mixed but acceptable.
Subject SA
performed the best, having 73% correct and 97% within one level difference. Subject SP had the worst performance, getting only 23% correct. He was still, however, able to determine 63% of the trials within one level. Averaging the results shows that more than 75% of the time a decision was made to be within one level of the correct value. It is also worth noting that many subjects had a string of incorrect guesses at the start of the test and then were able to more easily determine the force level as the testing continued. This observation is not expressed in Table 7.1 results. The glove’s ability to produce forces recognizably different and distinctive was proved in the intensity perception test. In this section, subjects answered the correct level an average of 46.7% of the time. A six level system is used; therefore, if the subject was purely guessing, then the correct answer should be given only 17% of the time. Clearly,
82 this result shows that the glove is effective at the basic level of producing a distinctive range of forces.
7.3. Relative Force Perception A separate test consists of the subject identifying if there is a difference between a pair of forces. Subjects are given two force levels, each one active for three seconds and administered upon request. When the pair is complete, the subject is asked to identify which force level was higher or if they were of equal value. Only a single finger is used for this test. A decision for the pair is made and recorded. The process is repeated 30 times in the same order for all subjects. Table 7.2. Results for relative force perception trials Subject’s Initials AK GB JA JC MB SA SP Average
Hand Size
% Correct
7 ¼” 7 ½” 7 ½” 7 ¾” 7 ½” 7 ¾” 7 ¼“ 7 ½”
43% 47% 43% 38% 50% 43% 53% 45.3%
% of 50 DC level Correct 0% 67% 67% 20% 67% 83% 50% 50.6%
High-range correct 40% 20% 40% 40% 30% 10% 50% 44.3%
Mid-range correct 40% 70% 40% 40% 60% 60% 50% 51.4%
Low-range correct 50% 50% 50% 30% 60% 60% 60% 51.4%
Pair trials results can be found in Table 7.2 show mixed results. During these tests, granulations of 25 DC levels and 50 levels are used in the pair trials. Results are broken down into all sets, those that use 50 level differences and into the high (175-250] mid (75-175] and low [0-75] ranges. Six of the trials differed by 50 levels. The remaining had differences of 25 DC levels or were of equal values. Hand sizes are also mentioned for easy reference. Results of the relative force perception trials were less enthusiastic than those of the intensity perception. With three choices, a pure guess would produce a correct
83 answer 33% of the time. During the testing, a correct answer was given an average of 45% of the time, with only a slight variation, as shown in Table 7.2. Results proved to be better then pure guessing, showing that the glove was effective; however, they were not as good as expected. Also, the results show that the glove is less effective in the high range, while the low and mid ranges showed similar, slightly better, results. Originally, the mid-range was expected to show better results than the other two. Due to the nature of the RMF and previously conducted tests, it was expected that the mid-range should be the most sensitive to the subjects. It was also expected that the pair trials would produce more correct trials than the intensity perception. This proved not to be true, as seen in Table 7.2. It is believed that, since most of the trials were either 25 DC levels or at the same force, the decision of the subject was somewhat more subjective. If all the level differences were at 50 DC, it is felt that the results would be more dramatic in a subject’s ability to identify the stronger of the force pairs. This 50 DC test is somewhat apparent in Table 7.2 since a 50 DC breakdown is given in a column. Only six trials were used in this part of the study, and one of the subjects did poorly enough to skew the results.
7.4. Absolute Force Perception The fourth portion of the test is a more subjective portion than the others. Subjects are given a base DC value that serves as a reference value. Reference values are compared to a second trial value, and the subject is asked when there is a discrepancy between the reference and the trial value. Reference values are from 0 DC to 200 DC in 50 DC value increments. Trial values start at the reference value for the first trial and then increase by 5 DC until the subject declares that he is feeling a difference between the
84 two values. The idea is to subjectively measure when the subject feels a point of different forces between the two values. Only a single finger is used for this test. The absolute force perception test proved to be too subjective for meaningful results.
A more complex, objective test should be developed in its place for future
studies.
7.5. Subjective Questionnaire Following the tests, a questionnaire is filled out to gain a better understanding of how the subject felt about the glove and the tests preformed.
Other important
information such as the subject’s hand size, as measured from the wrist to the tip of the middle finger, is recorded. The subjective questionnaire was used to gain a basic understanding of how the participants viewed the glove.
All questions can be found in Appendix B.
The
discussion here should not be interpreted as complete but as a representation of the author’s opinion of the relevant information that has been interpreted from the results of the questionnaire. The two most frequent comments for the “Any improvements” question were that they would like to see lower static forces and that the top of the fingers should be padded. Both of these were known problems when starting the study. The first, lower static forces, has been extensively discussed throughout this document and is one of the glove’s most dramatic limitations. While all attempts were made to reduce this deficiency, it was still very noticeable at the time of testing. A need for additional padding is caused by the exoskeleton on the top of the fingers. All exoskeleton parts are made from rapid prototype material and are therefore a
85 hard plastic. When the hard plastic compresses through the glove material, it hits the finger and can cause some discomfort, particularly for people with sensitive hands or skin. Several changes to the physical glove can be made to minimize this discomfort. The most intuitive would be to add padding, as suggested by several subjects. A redesign of the point where the exoskeleton and glove meet or the use of a different material in the exoskeleton would also help to minimize this problem. When asked about the comfort level of the glove, all subjects rated the glove either a 3 or 4 out of 5, with 5 being very good. Since all participants wore the glove for at least 20 minutes and many for 30 minutes, this rating would be considered only acceptable and not good enough for a commercial product or for long-term use. Further refinements would be necessary for longer studies. All respondents indicated that they did not feel that the arm was constrained in any way by the wires or the weight.
Most, but not all, of the participants indicated that
the fingers felt constrained in some way; however, all felt that the workspace was adequate and useable. Additionally, most but not all, felt that the max forces should be increased. Force level decisions and tradeoffs are discussed primarily in Section 3.1.
7.6. Analysis of Initial Human Testing The human tests produced results that showed that this type of glove, its design, and the RMF actuators used are capable of producing an easy-to-use and functional human interface device. Results also show that this particular glove, while usable, lacks some of the delicacies that some other available gloves retain. While most respondents in the subjective questionnaire agreed that the glove produced enough force to be usable, it was the ability to discern between two closely spaced forces that was lacking.
86 The limitations mentioned here do show some deficiencies in the glove; however, the testing does show that the glove functions properly and provides a controllable level of feedback. Results are encouraging and, with the proper refinement, the tests could be developed to further prove the functionality of the glove. A larger testing group would be one of the key considerations for any new test. Additionally, some subjects appeared to have tired hands toward the end of the testing, and this could therefore impact the results. Results were also skewed due to improper fitting of the glove. Due to the nature of the project and logical sizing, the glove was designed to fit the author’s hand. Since this was a large hand and therefore a large glove, the ability to find subjects with similarly sized hands proved difficult and some subjects with less-than-perfect fits were accepted. The results in Table 7.1 and Table 7.2 do not show this to be a significant cause of error; however, several subjects dropped out of the study because of hand sizes that were too small, and intuitively, we can conclude that a better fitting glove will produce better results. Further studies should include only people with more closely matching hand sizes or the addition of different sized gloves.
87
Chapter 8: Conclusions and Future work
This project proved to be highly successful and produced a usable force feedback glove. A fully functional haptic force feedback glove utilizing MRF was designed, produced, programmed, tested and documented in relation to this thesis project. Human testing provided the proof of concept for the design. Through this testing it was shown that the actuator, exoskeleton, electronic, programming and overall designs were able to function together to make a usable device.
Interfacing with the device is easily
accomplished by way of sending simple commands over the USB connection from a host PC. Based on the initial research, there are no other haptic projects, which use MRF actuators or an exoskeleton, in the method done for this project. The actuator and exoskeleton, in combination, provided a novel way of providing and transmitting forces to the hand of the user. The passive nature of the glove makes it a device with a low risk of injury, suitable to some populations where active forces may present problems. Overall, the design concepts developed during this thesis show great promise for the field of haptics and could yield a novel new class of haptic devices. Because of the low power requirements of the glove it is feasible to include wireless and lightweight battery
88 connections. By removing the last two tethers to the PC and desk, the user would have a dramatic increase in the workspace.
8.1. Actuator One of the more novel features of the glove is the MRF actuators, discussed in Chapter 3. Since this device provides the power and control of the forces, its design was the most tested and experimented with during this project.
As a result of this
experimentation, a usable actuator was produced. The actuator also has relatively low power consumption and can utilize non-toxic, FDA approved MRF if desired. The actuator’s static force limitations can be minimized, but not eliminated, without a complete redesign of the mechanism, making all fluid completely contained without a seal. While utilizing the current design, it may be more productive to use this design on a less sensitive portion of the body. A leg, arm, ankle or other larger and less sensitive body part may be a better fit. By placing the actuator on these alternate, less sensitive body parts, the design’s deficiencies will be mitigated while the various strengths can be exploited. As an alternative to the exoskeleton used in this project, the actuators could be used in the wearable haptic device proposed in [33] as an alternative to the pneumatic cylinders currently proposed.
8.2. Exoskeleton One of the more unique features of the overall system is the exoskeleton and its use of a control cable for power transmission, as discussed in sections 4.1 and 4.2. The exoskeleton system functioned well and was able to provide force transition in both the push and pull direction. From this push/pull ability and the feedback from the human
89 study, we can conclude that the exoskeleton design is a usable alternative to more traditional cable or tendon systems for haptic devices. For future version of this glove, the deficiencies identified during the manufacturing and testing should be addressed. Despite all effort to remove the residue some would remain in the exoskeleton’s control cable pathway. To correct this problem, a different technique to form the exoskeleton must be used, possibly: 3D printing, injection molding, compression molding, casting, or some other method of forming plastics and resins. As mentioned in section 7.5, several people responded that the exoskeleton pressed against the top portion of their fingers. The base of the exoskeleton should be curved and padded to make it conform more easily to the shape of each finger. For this iteration of the glove the exoskeleton was manually glued to the glove. Movement of the hand within the glove and human error can cause problems during the attachment procedure. An automated procedure would clearly produce a better result
8.3. Sensor and Power Electronics Both sets of electronics are placed on solder boards and are wired to the embedded system and the hardware on the glove itself. These electronics are essentially the bridge between the controls and the glove. Each board functions properly and is able to provide usable results or control methods to the PIC for interpretation and later utilization by the host pc. Furthermore, the boards are fitted to a base plate to allow them to be mounted to the user’s forearm, creating a higher degree of mobility. In future versions using a printed circuit board (PCB) instead of the current solder board would reduce the time to produce the boards, make a more uniform product, reduce the chance for error, and create a more professional looking product. One general
90 improvement to the sensing system would be improving the A/D conversion to a higher bit resolution to improve the overall sensitivity. As mentioned in section 5.1.1 the Wheatstone Bridge is unbalanced and all efforts to balance that bridge should be made or use a higher accuracy Bridge. Since the actuator’s magnetic coil is hand wrapped, there is an imbalance between the actuators. This imbalance can be adjusted for in the ECP (section 6.1); however, it is preferable that all actuators be balanced on the board level. The Addition of an abduction sensor such as the one on the RMII-ND would also give more sensory points and therefore more controllability.
8.4. Embedded Microcontroller and Control Program Currently the embedded processor, a Microchip PIC18F8720 microcontroller, is located on a development board. Using this commercially available part, the system is able to perform all necessary functions. The ECP is able to control all aspect of the glove during normal operation and provides a large amount of flexibility for programmers to use the glove. A custom PCB would be advantageous in a future version of the glove. As already mentioned in section 8.3, a faster and more precise A/D module would be desirable to include. Other niceties, for wiring and size can be considered for a custom PCB. The ECP, could include a Kalman filter that can be used to smooth out more of the noise inherent in sensors.
8.5. Overall Conclusion By producing the MRAGES, it was the goal that this glove could be a starting point for a new type of haptic devices, those utilizing MRF actuators. Since this is a first
91 iteration prototype, a consumer ready device was not expected, instead a proof of concept device was created. Since this is the first glove of this type, and it was shown to function properly, I feel that this project reaches the goal set forth at the start of this project.
92
Appendix A Commands.h /* this file contains commands for the MRAD_HM use these commands to interface with the PIC embedded program Modified 8/11/05 commands.h Scott Winter */ //these commands use standard format // this is (size, command, par1, par2 ...., parN, xor checksum) //returns (size, command, val1 val2 ..... valN, xor checksum) or SUCCESS #define START_TIMER 0x90 /*starts or resets timer1& interupt example 0x1,0x90,0x91 */ #define STOP_TIMER 0x9F /*stops timer1interupt example 0x1,0x9F,0x9E */ #define A_D_CONVERTION_TIMESTAMP 0x91 /*reads and sends a single AD read par1: AD port return 4 byte time stamp and 2 bytes for AD read example: 0x2, 0x91, 0x2, 0x93 - reads port 2 */ #define A_D_CONVERTION_TIMESTAMP_PAIR 0x92 /*reads and sends a pair of AD read represented by the finger par1: AD port return 4 byte timestamp and 2 bytes AD read on finger*2 and finger*2+1 example: 0x2, 0x92, 0x2, 0x90 */ #define GET_TIME_STAMP 0x93 /*returns timestamp reading, 4 bytes (2byte sec, 2byte ms unsigned ints);
93 example: 0x1 0x93, 0x92 */ #define GET_SINGLE_FILTERED_PAIR 0x94 /*reads a low pass filtered pair of AD on a finger pair par:1finger returns; a A/D filtered pair (4byte psudo-dubble) on finger*2 and finger*2+1 example: 0x2, 0x94, 0x0,0x96 */ #define GET_SINGLE_FILTERED_PAIR_TIMESTAMP 0x95 /*reads a low pass filtered pair of AD on a finger pair par:1finger returns; timestamp + a A/D filtered pair (4byte psudo-dubble) on finger*2 and finger*2+1 example: 0x2,0x95,0x0,0x97 */ #define GET_CALABRATION 0x96 /*requests PIC to send calabrations for one finger par1: finger returns force and DC curves a,b, x0-x5 all psudo-doubles 32bytes total example: 0x2, 0x96, 0x1, 0x95 */ #define SET_CALABRATION 0x97 #define A_D_CONVERTION_TOKEN 0xA9 /*reads and sends a single AD read par1: AD port return 2 bytes for AD read example: 0x2, 0xA9, 0x2, 0xA8 */ #define LOOPBACK 0xAF /*echos byte par1: byte to echo return par1 Example 0x2, 0xAF, 0x12,0xBF */ #define START_SENSOR_CALABRATION 0xB0 /* Asks MRAD to start calabrate one forces sensor par1: finger 0-4 0=thumb 4=pinky return: intracalabration requests Example: 0x2 0xB0 0x1 0xB3 -
94 */ #define START_FORCE_CALABRATION 0xB1 /* Asks MRAD to start calabrate forses for DC on all fingers use Start_Sendor_Calabration first on all fingers for proper calabration par1: highest finger 0-5 0=thumb 1= thumb and index, 2 =thumb, index and middle, ..... 4=all retur: intra calabration requests Example: 0x1 0xB1 0xB0 */ #define GET_READINGS_RAW 0xB2 /* Asks MRAD to read sensors and return new raw values par1: NA returns: 10 raw values (thumb force,index force ....pinky force , thumb position, index position .....pinky position) as ints Example: 0x1 0xB2 0xB3 */ #define GET_READINGS_FILTER 0xB3 /* Asks MRAD to send low pass filtered values par1: NA returns: 10 raw values (thumb force,index force...pinky, thumb position .....pinky pos) as doubles Example: 0x1 0xB3 0xB2 */ #define GET_READINGS_ALL 0xB4 //returns filtered, processed /* Asks MRAD to send processed readings par1: NA returns: 10 values (thumb force,position, index force, position .....) force in N pos in inch as Example: 0x1 0xB4 0xB5 - calabrate index finger */ #define GET_READINGS_ALL_UNFILTERED 0xB5 //this command is unused and not tested use GET_READINGS_ALL /* Asks MRAD to sample data and send processed readings par1: NA Example: 0x1 0xB5 0xB4 returns: 10 values (thumb force,position, index force, position .....) force in N pos in inch */ #define START_MESUREMENTS 0xB6 /* Asks MRAD to start periodicaly messuring sensors par1: NA
95 Example: 0x1 0xB6 0xB7 */ #define START_FORCES 0xB7 /* Asks MRAD to start start forces on 1 or all fingers par1: finger 0-5 0=thumb 4=pinky 5=all Example: 0x2 0xB7 0x1 0xB4 - start forces on index finger */ #define STOP_FORCES 0xB8 /* Asks MRAD to stop forces all fingers par1: NA Example: 0x1 0xB8 0xB9 */ #define STOP_MESSUREMENTS 0xB9 /* Asks MRAD to stop periodic messurements par1: NA Example: 0x1 0xB9 0xB8 */ #define SET_FORCES_INTERNAL 0xBA /* Asks MRAD to set DC base on internal calculations for forces asked for NOTE: forces are sent as doubles*DOUBLE_FACTOR for 4 bytes per value par1: thump force par5: index finger force Example: force of 0x0003D090 = 250000 = 2.5N NOTE: xor still 8 bit 0x15 0xBA 0x0003D090 0x0003D090 0x0003D090 0x0003D090 0x0003D090 0xEC sets all to 2.5N */ #define SET_DC 0xBB /* Asks MRAD to set DC for all five fingers par1: thumb par2: index par5: pinky Example: 0x6 0xBB 0x64 0x64 0x64 0x64 0x64 0xD9 - sets all to DC=0x64 */ //These commands do not use standard format and are used intra-calabration // they should not be sent to the glove during normal operation or to start calabrations #define CALABRATE_REQUEST_WEIGHTS 0xC0 // {CALABRATE_REQUEST_WEIGNTS,finger ,direction(strain down?) ,weightx100g}; #define CALABRATE_REQUEST_FORCES 0xC2
96 #define CALABRATE_RESPONCE_FORCES 0xC3 //format 0xC3, m(poly value), n(number of readings), nx(X val, floatsx5), checksum #define CALABRATE_VALUE 0xC4 //format 0xC4, ( 6floats)x5fingers, checksum #define OK_RESPONCE_WEIGHTS 0xC1 //NOTE: cannot store doubles into eeprom or trasmitted as doubles so we factor by DOUBLE_FACTOR on store and // devide by DOUBLE_FACTOR on read //responce to error #define CHECKSUM_ERROR 0xAA #define CALABRATION_ERROR 0xFE #define SUCCESS 0x55 //these may be modified for optimal operations #define DOUBLE_FACTOR 100000 #define THROW 1.0 //max through for the piston #define LAMDA 0.90 #define READINGS 20
97
Appendix B This form is used for the human testing program with the correct answers for the objective part filled out.
THE REMINDER OF THIS PAGE IS INTENTIONALLY LEFT BLANK
98 Initials __________________
Date______________
Hand Size________________
Sex -- M / F
Time In_________ Out___________
Complete -- Y / N
Picture?_____________ Part 1 – Intensity familiarization Part 2 – Match the values to the ones felt in Part 1. 0 200
1 50
2 3 4 5 6 150 250 100 200 150
15 16 17 18 19 150 100 250 150 50
20 21 200 50
7 50
8 250
9 0
22 23 100 0
24 50
10 100
11 0
25 26 200 0
12 12 250 200
14 50
27 28 29 200 150 100
Part 3 – Mark which trial had the higher force or if they were equal (1,2,=) 0 1
1 1
2 2
3 2
4 1
5 =
6 2
7 2
8 1
9 1
10 2
11 2
12 2
13 =
14 2
15 2
16 =
17 1
18 1
19 =
20 1
21 1
22 1
23 1
24 =
25 2
26 2
27 2
28 2
29 =
Part 4 - Mark the amount when a difference is felt Reference Difference
0
50
100
150
200
99 Questionnaire: 1=very bad 3=no opinion 5= very good How would you rate the comfort level of the glove 1-5?_______________ Do you think that the glove fit was too big, too small, correct?__________ Do you think that your motion was constricted about the fingers?________ Do you think that the finger workspace was adequate?________________ Do you feal that the arm workspace was constrictive?_________________ How would you rate the force level of the glove 1-5?_________________ Would you like the forces higher, lower, same?______________________ How would you rate the usability of the glove 1-5?___________________ How would you rate the look of the glove 1-5?______________________ Which finger do you think preformed best?_________________________ Any improvement you would you would like to see?__________________ Additional thoughts on using the glove:
Comments:
100
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Feedback
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Glove
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