Design of a force feedback glove using magnetorheological fluid Scott Winter1
Mourad Bouzit2
Rutgers University
Rutgers University
ABSTRACT
2.1
A novel new type of force feedback system was designed as a glove or master format. At the 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. An exoskeleton was devised and built, utilizing a rapid prototyping machine to transmit the forces from the MRF actuators to the fingertips. The entire system is lightweight, low power and easily portable.
Magnetorheological Fluid (MRF or MR-Fluid) is a smart material that has the property of solidifying when exposed to a magnetic field. The type of MRF used for this project is a hydrocarbonbased medium with 22% solids by volume, mostly iron partials, commercially available product from Lord Corporation (MRF22ED[12]). In practice, MRF functions by aligning the suspended iron particles’ poles to create a more viscous substance. Lord Corporation provides two models for using MRF[2][3], the pressure model and the force model. Devices such as MRF clutches would use the direct shear mode, while flow control devices would use the pressure driven flow mode. For the purposes of this project, the pressure mode is more relevant; however, both can be applied.
Keywords: haptic, force feedback, magnetorheological fluid, rapid prototype
1
INTRODUCTION
Human interface with a computer is a non-trivial problem and has been the subject of much research. The standard interface method for most people is currently based on input from a keyboard and mouse. When we expand these methods to those common in the Virtual Reality (VR) field, we find several more options, such as 3D trackers, more complex graphic displays, motion sensors, as well as tactile and force feedback haptic displays. Numerous examples can be found in Virtual Reality Technology[1] and other publications. In order to further the field of haptics, a device based on Magnetorheological Fluid (MRF) was developed and named Magnetorheological Actuated Glove Electronic System or MRAGES. The design is intended to serve as a starting point for the use of MRF in haptics. While not suitable for every application, the system demonstrates that a MRF based haptic master can be compact, lightweight, low power and functional.
Figure 1. Completed glove
2
RELEVANT BACKGROUND
Several examples of force feedback gloves exist in academia and industry today, such as the RMII-ND[10] or Cybergrasp[11]. These gloves utilize pneumatics or electric motors to provide their power. The MRAGES is unique in that it uses a MRF actuator, passive in nature, to resist the users motion and dose not provide any active forces to the system. Unlike the other systems, the MRF actuators also allow resistive forces to be applied as the hand opens as well as closing. Traditional cable tendon systems are only able to provide forces opposing the closing of the hand. Additionally none of the other systems utilize rapid prototype parts for there construction. 1 2
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Magnetorheological Fluid (MRF)
Figure 2. Basic operational modes for controllable fluid devices: (a) pressure driven flow mode, and (b) direct shear mode
Some work has been done in making miniature actuators using active fluids such as the work by Shinichi Yokota[4]. Here Electro-rheological Fluid (ERF) is used to create miniature valves, motors, and machines ERF differs from MRF in that a current, instead of a magnetic field, is used to orient the active partials in the fluid Rapid Prototype (RP) System. A 3D systems Viper Stereolithography (SLA) machine was utilized in order to cut down fabrication time for various parts of the system. The Viper SLA uses a UV 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. Stereolithography has been around for several years now, and its ability to make complex parts is well proven. NASA developed a robotic arm utilizing SLA parts[5]. By choosing to use SLA parts, the NASA group was able to develop a prototype robotic arm in a short amount of time. Another example is a robotic hand[6][7] that has a number of small, fine, and complex parts with interior control channels.
3
MRF ACTUATORS
The MRF actuators are the heart of the power system. They represent a novel approach to applying forces to the human hand in that they use a controllable force dampener. They have been designed to be powerful, light, use low power, and have a relatively low static force level. 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. A total throw of 1.25” is achieved in this design. The internal wiring is designed to utilize a 0.33A at 5V, giving the actuator a high power to weight ratio.
3.2
Figure 3. Single actuator
The major drawback to the actuator design is that the actuator produces a large amount of force even in the off state. The ones used in the final version of the glove the static forces ranged from 1.4N to 1.9N. Minor modifications can greatly increase the maximum forces at the cost of increasing these static forces 3.1
Internal Components
A finished actuator is comprised of several pieces (see Figure 4), these being the external retaining shell, two O-ring holders retaining a PTFE (generic Teflon) O-ring, a 1.5” steel (3/8 OD, 11/32 ID) cylinder, a steel spindle and a center stainless steel shaft. The center shaft and steel spindle together form the actuators piston. The spindle is wrapped with magnet wire to create an electromagnet. 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 is sealed using liquid silicon.
Actuator Testing
During early stages of the actuator development, a number of rudimentary tests were carried out using a microcontroller I/O port and transistor for power amplification. A commercially available, load cell was used for force measurement. Displacement was measured by use of a linear potentiometer. The first goal of these initial tests was to determine if a force profile could be created for the actuator and whether forces were controllable. Testing showed that a profile was passable and the actuator could be used as an element of a haptic glove. The second goal of these initial tests was to determine if velocity and piston displacement affected the forces. From experiments showed that the position had no effect, and the velocity effect was quite low. These determinations showed that no elaborate profiling is necessary. 3.3
Actuator Control
Hardware PWM channels from an embedded PIC18F8720 microcontroller were used control of the actuators. An SGSThomson L293D Push-Pull channel driver boosts the signal and drives the actuators. In addition, a high wattage 10Ω resistor is in series with the actuators to restrict the current to within th maximum value of 0.33A for the 34-gauge wire. Embedded programming on the PIC, determined through a calibration routine (see section 6.2), and control signals from a host PC are used to determine the duty cycle (DC). Values anywhere between 0 and 255 can be set directly, or through the calibrated subroutine. In this manner, the actuators are controllable with a granularity of 28.
4
EXOSKELETON AND MECHANICAL STRUCTURE
The Exoskeleton structure of the haptic glove provides the mechanism for 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, machined, and commercial parts. Sensors, attached to the structure, measure forces and displacement.
Figure 4. CAD drawing of actuator
Simulations were made using the FEMM[8] software to determine magnetic field strength. Taking an unused spindle 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 steel cylinder was added in order to magnify the effects of the electromagnet. By adding this component the strength of the magnetic field is increased from approximately 0.06 Tesla to 0.25 Tesla according to the FEMM simulations. It should be noted that the FEMM software can not account for the magnetic properties of the MRF. The MRF used is approximately 22% iron, and it will have a positive effect on the magnetic field. The O-ring is an important part of the retaining components since it interacts with the center shaft as it moves up and down within it. PTFE (Teflon) -003 (specified as dash 003) O-rings were used for two reasons. First, they have a low friction coefficient and secondly, they have 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. 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.
Figure 5. Full glove
4.1
Exoskeleton
A 7x7, 1/16” PTFE-coated aircraft cable is used as the control cable for the system. It runs through the Exoskeleton finger spine and provides the force transmission. In this instance, the cable is being used as a push-pull control cable and therefore a thick cable, with less of a bend radius, is needed. The push-pull affect is required since this a passive actuator cannot reset its piston position independently. Figure 6 shows the CAD model used to create the rapid prototype parts of a single finger. Other hardware that can be seen in Figure 5 includes: 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.
mAh NiHM AA batteries could be used to power the glove for over an hour at max power. Table 1: Power Requirements
Figure 6. CAD representation of single finger.
Each individual piece of the finger spine is designed to spread as the finger flexes, as shown in Figure 7. During this flexion, the cable will push against the bottom of the pieces and draw out the actuator’s piston. 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 close in size to the cable. 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.
Part
#/glove
Actuator L293D Op-Amp Bridges 18F8720 Total
5 2 5 5 1
6
4.2
Sensors
The force gauge sits at the back of the actuator, serving as an intermediary, physically connecting it to the base of the glove. It consists of a strain gauge glued onto a sheet metal cutout of 0.025” thickness stainless steel. In this manner, all forces felt at the back of the actuator are transmitted through the force gauge. Sensor readings are then fed through a Wheatstone Bridge and op-amp into an A/D channel on the PIC. A linear potentiometer is used as the linear sensor in the glove. Currently the forces are read at the back of the fingers and are based on a prior calibration (Section 6.1). In future iterations a more complex approach is necessary to relate these force measurements to the fingertip position.
5
ELECTRIC HARDWARE
The design of the glove allows for all of the electrical components to be mounted on the glove itself or on a set of control boards on the upper arm. 5.1
Control Boards
A total of four Electric Control Boards were used to interface with the glove hardware. Two housed the sensor components not located on the glove itself and one power board with actuator control components and display LED’s. The fourth board is the microcontroller development board. It is a commercially available off-the-shelf (COS) part containing the PIC microcontroller and USB for interfacing with a host PC. 5.2
Power Requirements
The total power requirements for the glove can be found in Table 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 elevated values, four rechargeable 2500
Total (mA) 1600 120 6.5 145 300 2171.5
Calculation method Max value Datasheet Datasheet Typical Max value
SOFTWARE ARCHITECTURE
There are three basic models that are written to control the glove and interface with the programmer and/or user. First is the main 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 a pre-existing suite of exercise, developed for a separate project. By mimicking the commands used in a preexisting project, it allows for the new glove to have a fully developed host program, once the glove is stable enough for longterm use. 6.1
Figure 7. Bending of control cable holder
Current (mA) 320 60 1.3 29 300
Master Control Program
The main control program (MCP) is the basis for normal operation of the glove. Upon startup, the PIC’s embedded program will automatically enter the MCP, enters an infinite loop where it polls for commands sent over the USB, polls the sensors and filtering the sensor readings. Commands processed by the MCP include setting forces and sending sensor results back to the host PC. Both the setting of the forces and the sensor readings are dependent on proper calibration of the force sensors. Two types of calibrations 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. Each finger’s force sensor must be calibrated separately and requested by the user. Calibration is done using a set of known weights supported by the sensor. A number of measurements are taken and averaged. Results are then fed into a linear regression function on the PIC. Results are saved into the PIC’s EEPROM. During power-up the PIC will read the EEPROM for these values and use them for force calculations. Once all force sensors are calibrated, the force curve for the actuators may be found. This calibration task is accomplished by cycling through the DC levels on all fingers, while moving the pistons and reading the force sensors. The measurements for ten DC levels are sent to the Host PC for a polynomial curve fitting. Results are sent back over the USB and stored in the PIC’s EEPROM for future use. The linear sensors’ do not require any elaborate calibration. The absolute maximum and minimum distances are a product of the actuator’s design. The PIC will check for maximum and minimum readings during operation and use these reading to calculate the percentage that the actuator is extended. After calibration the forces may be set in two ways. First, by directly setting the duty cycle (DC) for each actuator individually. The second method involves using the force profile to find the appropriate DC value. If the second method is used, after each loop a new force reading is compared to what is required by the original request. DC values are then adjusted to attempt to produce the desired forces. A window is used to ensure values do not drift too far off the calibration curves.
6.2
Calibration and Testing Console
The calibration and testing console was created as a basic way to communicate with the glove. Since there is no direct way to interface with the microcontroller all communication was done using the host PC’s USB and the console program. 6.3
RMII-ND Mimic
An additional embedded module was written to interface with preexisting programs written for the RMII-ND. A hardware jumper is used to select this branch at power-up. The commands used for the MCP are then unavailable and replaced with the command set that the RMII-ND uses. The RMII measures this bending using two metrics per finger, these being the piston displacement and the flexion. The MRAGES artificially constructs some information for flexion, since only one metric, displacement is used. Flexion converted from the displacement as a percentage of the current displacement. Abduction is simply dropped.
7
HUMAN TESTING
A short preliminary human trial was conducted was to determine the viability of the glove for use as an interface with a computer. Testing was conducted to determine the degree as to which the glove is able to provide decisively different forces. The human testing program was divided into four sections: intensity perception, relative force perception, absolute force perception and the subjective questionnaire. A full test lasted approximately 30 minutes. It should be noted that hand size had a strong influence on the results. The glove was designed to fit on a large hand, making it difficult to find subjects with similar hand size. In the first part of the intensity perception, 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, giving a total of 6 levels. Upon completing this familiarization, the subject is to identify the levels that the glove is producing without reference to the actual values. The process is repeated 30 times in the same order for all subjects. As a result, 46.7% of the time subjects answered correctly and an additional 30.7% of the time subjects were off by one level. For the relative force perception, subjects are given two force levels. When the pair is complete, the subject is asked to identify which force level was higher or if they were of equal. DC levels varying by amounts of 0, 25 and 50 were used. Only a single finger is used for this test. The process is repeated 30 times in the same order for all subjects. 45.3% of responses for this test were correct. The third portion, absolute force perception asked subject to reference a force level and responded when they felt a force change. The forces were slowly increased until the subject responded. This test was dropped from further examination as subjects seamed to respond too subjectively. The subjective questionnaire asked subjects about comfort level, workspace, finger restriction and improvements they wanted. Most subjects felt that the comfort level was adequate for the term of use but would like to see some padding or other changes added in the next revision. The padding necessary as the exoskeleton can push into the fingers. All respondents indicated that they did not feel that the arm was constrained in any way by the wires or the weight. Most, of the participants indicated that the fingers felt constrained in some way; however, all felt that the workspace was adequate. Additionally, most felt that the max forces should be increased.
8
CONCLUSIONS
A fully functional haptic force feedback glove utilizing MRF was designed, produced, programmed, tested and documented in relation to this project. Human testing provided the proof of concept for the design. Through this testing it was shown that the actuator,
exoskeleton, electronics, programming and overall designs were able to function together to make a usable device. Interfacing with the device is easily accomplished by sending simple commands over the USB connection. While the human testing showed the feasibility of the glove, it also shows that improvements are needed. As a proof of concept device, the glove performed admirably; however, it must provide more distinct forces and be more comfortable if further developed. The MRF actuator’s limitations of high static forces can be minimized, but not eliminated, without a complete redesign of the mechanism, making all fluid completely contained without a seal. This makes the current actuator not readily suitable for fine point motor control of the hand. 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 a device such as the wearable haptic device proposed in [9] as an alternative to the pneumatic cylinders. One of the more unique features of the overall system is the exoskeleton and its use of a control cable for power transmission. This control cable 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 study, it can be concluded that the exoskeleton design is a usable alternative to more traditional cable or tendon systems for haptic devices.
REFERENCES [1] Burdea, G. C.; Coiffet, P. “Virtual Reality Technology” WileyInterscience, New Jersey, 2003 [2] Mark R. Jolly, Jonathan W. Bender, and J. David Carlson, “Properties and Applications of Commercial Magnetorheological Fluids,” SPIE 5th Annual Int Symposium on Smart Structures and Materials, San Diego, CA, March 15, 1998. [3] Lord Materials Division Engineering Note: Designing with MR Fluids. [4] Shinichi Yokota “Micro Actuators using functional Fluids,” FPTC03. The Fourth International Symposium on Fluid Power Transmission and Control, Wuhan, China April 8-10, 2003. [5] Robert Ambrose, “Interactive robot joint design, analysis and prototyping,” Proceedings. IEEE International Conference on Robotics and Automation, May 21-27 1995 Page(s):2119 - 2124 vol.2. [6] Jey Won, Kathryn J. DeLaurentis and Constantinos Mavroidis “Fabrication of a Robotic Hand Using Rapid Prototyping,” DETC’00. Proceedings of DETC00 26, Biennial Mechanisms and Robotics Conference. Baltimore, Maryland September 10-13 2000. [7] J. Won,, K. DeLaurentis, and C. Mavroidis, “Rapid Prototyping of Robotic Systems,” Proceedings of the 2000 IEEE International Conference on Robotics and Automation, April 24-28 2000, San Francisco, CA. [8] David Meeker, Finite Element Method Magnetics (FEMM) Verison 4.0, http://femm.foster-miller.net/ [9] Guilin Yang; Hui Leong Ho; Weihai Chen; Wei Lin; Song Huat Yeo; Kurbanhusen, M.S. “A haptic device wearable on a human arm” IEEE Conference on Robotics, Automation and Mechatronics, Volume 1, 1-3 Dec. 2004 Page(s):243 - 247 vol.1 [10] M Bouzit, G Burdea, G Popescu, and R Boian. “The rutgers master II-new design force-feedback glove,” IEEE/ASME Transactions on Mechatronics, 7(2):256–263, June 2002. [11] Immersion Corporation Cybergrasp manual Version 1.2 [12] LORD Rheonetic Magnetically Responsive Technology HydrocarbonBased MR Fluid MRF-122-2ED Product Bulletin.
