Arm Mounted Exoskeleton to Mechanically Assist Activities of Daily Living

Transcription

1 Arm Mounted Exoskeleton to Mechanically Assist Activities of Daily Living Katelyn S. Jobes, Marcus J. Bernier, Shawn L. Dryer, and Douglas E. Dow Program in Electromechanical Engineering College of Engineering Wentworth Institute of Technology, Boston, MA Loss of mobility due to muscle weakness negatively affects quality of life and ability to complete activities of daily living. Some arm exoskeleton systems have been developed as assistive or rehabilitative devices, but a need for a more modest system to assist with activities of daily living remains. The purpose of this project was to develop an assistive device to supplement strength and torque in individuals with muscle weakness. An electromyographic muscle sensor records the muscle activity of the user that is used as an input to a microcontroller. The microcontroller processes the signal and outputs a voltage to control the direction of a linear actuator. The system was tested for speed of extension and retraction, torque, and ability to lift and support loads of up to forty pounds. The development of such an arm exoskeleton system has the potential to lead toward future devices that could improve the quality of life for many individuals experiencing muscle weakness by enabling them to perform activities of daily living that require more arm strength than they would otherwise be able to generate. Corresponding Author: Katelyn S. Jobes, jobesk@wit.edu Introduction Muscle weakness in the arms can arise from stroke, muscular dystrophy, neuromuscular disease, trauma or old age 1, 2. Loss of arm strength can lead to loss of ability to complete activities of daily living or reduction in neuromuscular activity of a limb, which may lead to declines in tissue and joint health, and quality of life. This is particularly true for older individuals on prolonged bedrest 3. Physical therapy and rehabilitation programs help to restore some function, but muscle weakness remains in many cases, limiting maximum force and function of a limb and decreasing quality of life. Avoidance of using an impaired arm leads to learned non-use. Learned non-use contributes to inadequate levels of self-directed exercise and negatively impacts individuals recovering from strokes. A large number of people have reduced arm function that hinders activities of daily living. There are currently 5 million survivors of stroke living in the United States. About half of individuals who have survived a stroke have limited movement of at least one arm, but only 15% of those individuals will recover usable arm function 4. Neurological disorders such as multiple sclerosis and cerebral palsy, account for an additional 2 million or more patients in the United States with progressively reduced arm movement and strength. According to the U.S. Census Bureau, the number of seniors in the United States is projected to reach 53.4 million by 2020 and 69.4 million by Limb muscles from older individuals have significantly more fat, fewer muscle fibers, and 25-35% less mass than muscles from younger individuals 6. A study of over 3,000 individuals aged found that decreased muscle mass and increased muscle fat are associated with increased risk of mobility loss 7. Some exoskeletal apparatuses have been developed to supplement limb strength or to assist in rehabilitation. Such systems may even increase limb strength and torque well beyond the level usually obtained by able bodied humans. Such systems are typically expensive, bulky, complex, and require skill to operate, appropriate for many military and industrial applications 8. Robotic systems, such as ExoRob (2009 IEEE International Conference, Guilin, China), have been developed for assistance in regaining range of motion, but systems like ExoRob lack the ability to assist with

2 strength 9. Another system, the Massachusetts Institute of Technology developed Manus 10, is designed to assist with regaining motor control by allowing a user to practice reaching toward a computer generated object on a screen. A fixed system such as the MIT Manus is appropriate for physical therapy or even as an assistive technology for workstation based tasks. However, such a fixed system would not be useful for any mobile applications, such as assistance with activities of daily living 10. A need remains for a more modest system to assist with certain activities of daily living that would require more limb strength or torque than an individual with muscle weakness could otherwise accomplish. The purpose of this project was to develop and test an arm mounted apparatus that supplements arm strength and torque, enabling an individual to perform occasional activities that require more force, such as rising from a chair or lifting an object. Modules of this system were tested for mechanical properties, such as force and torque. The development of such an exoskeleton system has the potential to lead toward practical devices that could improve the quality of life for many individuals with muscle weakness by enabling them to perform certain activities of daily living that require more arm force and torque than they would otherwise be able to undertake. Design The basic mechanical aspects of the proposed system are demonstrated in Figures 1 and 2. A hinged brace will support the user s arm and provide a base onto which the actuator will be attached via mounting brackets at the shoulder and wrist. The system will attach to the user s arm through a series of adjustable straps. The electrical aspects of the system were accomplished through the use of a muscle sensor as an input to a microcontroller, which outputs a voltage to the linear actuator to control the direction of motion. Supplemental force to assist arm extension and movement was primarily relieved through the use of a linear actuator. A linear actuator was used in the prototype to supply supplemental torque that will be used to lower the force required by the user to complete activities of daily living. Linear actuators have previously been used for similar applications. Figure 1: Basic Diagram of System Prototype Figure 2: SolidWorks Assembly of System Prototype An example of such a development that has been reported is the RoboKnee 11, an exoskeleton leg apparatus designed to enhance strength through load relief and increase endurance 11. The linear actuator selected for our prototype, Windy Nation LIN-ACT , is rated for a 225 pound load, which surpasses loads necessary for activities of daily living that typically require the user to lift loads of up to 30 pounds. The torque associated with such activities is determined by multiplying the load by the length of the user s forearm. The distance associated with the torque of the actuator would be approximately the same as the length of the forearm due to the attachment point of the actuator being at the wrist. Additionally, the selected linear actuator has an 8 inch extension that has demonstrated a range of

3 motion from an approximately 90 o bend at the elbow to near full arm extension. On average, the distance from the elbow to wrist is approximately 25.4 cm, and the distance from shoulder to elbow is approximately 27.4 cm 12. Thus, the distance from wrist to shoulder at a 90 o elbow bend is approximately 37.4 cm. The minimum length of the actuator is 30.8 cm 12, which would allow the user to bend their arm at the elbow to an angle of less than 90 o. The full extension of the arm from wrist to shoulder is approximately 52.8 cm 13, which is slightly longer than the full extension of the actuator at 51.3 cm 12. This range of motion should be sufficient to allow the user to pick up and carry loads associated with activities of daily living. A muscle sensor, MyoWare Muscle Sensor AT (Raleigh, NC) 14, was used to determine the muscle activity of the arm. The MyoWare sensor uses electromyography (EMG), measurements of muscle activity through changes in electrical potential, to determine arm movement through electrodes placed on the skin 14. The sensor module amplified and filtered the EMG signal voltage to be used as an input for the proposed system. The controller of the prototype was of an Arduino UNO microcontroller. Arduino microcontrollers are used to process multiple input and output signals. Once a program is loaded onto the microprocessor, the Arduino UNO can run independently of the computer 15. The primary input of the system was the EMG signal, and the output was a voltage through a relay to control the direction of the linear actuator. Neoprene was selected as the material for the straps due to its use in orthopedic compression devices, such as knee and ankle braces, and its durability and elasticity 16. The straps have Velcro strips as fasteners, allowing the patient to easily adjust the tightness of the strap for comfort and arm size. The base strap size is based on average arm circumference with adjustments available to suit a range of arm sizes, spanning from a minimum of 9.4 in., average 11.1 in., on the forearm to a maximum of 14.7 in., average 13.1 in., on the upper arm 17. The brace will be supported by a rigid material, such as aluminum or an inflexible plastic, which will allow the actuator to be firmly attached. The neoprene straps will attach to the brace in order to allow the user to use the apparatus. Besides the system providing supplemental force to assist with arm motions for activities of daily living, the combined activity of the muscles and system may lead to an improvement in the health and function of the joints, muscles and tissue. An increase in muscle mass, and therefore strength, may be accomplished through a low-load highvelocity (LLHV) training routine. One study suggests that a LLHV routine is as least as effective as a high-load low-velocity (HLLV) routine 18. A LLHV routine would be accomplished through this project. By using a muscle sensor to control the speed of the apparatus, the user would be able to control the apparatus through their natural arm movements. The load taken off of the user through the assistance of the apparatus, in conjunction with the speed, would result in either a LLHV routine or a routine falling between LLHV and HLLV. Assembly and System Development The linear actuator will be mounted with brackets onto a rigid base at points near the wrist and shoulder to allow a range of motion from a 90 o bend at the elbow to a fully extended arm. The Arduino UNO will read the EMG signal from the muscle sensor as an input to determine the direction of the actuator s movement. The Arduino UNO will process the signal and use a relay to control the voltage and polarity going to the actuator. With a positive voltage output, the actuator will extend, and the actuator will retract with a negative voltage input. At a zero voltage, the actuator will hold its position. The neoprene straps were created by sewing Velcro strips onto strips of neoprene fabric. Due to the thickness and toughness of neoprene, heavy duty thread and leather needles were selected to attach the Velcro strips to the neoprene. Testing Method A testing assembly composed of two pieces of rectangular aluminum tubing attached via a hinge was used to determine the load supporting ability of the apparatus without the interference of human muscles (Figure 3). The linear actuator was mounted onto the aluminum tubing at points representing the locations of the shoulder and wrist. An eye hook was used to attach weights to the apparatus for testing purposes.

4 Where S is the speed of the actuator s extension or retraction in inches per second and V is the input voltage. Figure 3: Testing Apparatus Results The linear actuator was tested to evaluate suitability for the prototype. The speed of the linear actuator varied with the supplied voltage. The time for full extension and retraction was measured at increments of 2 volts ranging from 0 volts (turned off) to 12 volts (the recommended highest input voltage). The results are shown in Table 1. The speed of extension and retraction was calculated by dividing the length of extension, 8 inches, by the time measured for full extension or retraction. No significant difference was observed in either the time or the speed of extension and retraction. Voltage Input (V) Table 1: Actuator Speed Measurements Time Out (s) Time In (s) Speed Out (in/s) Speed In (in/s) N/A N/A 0 0 Plotting the voltage versus the speed of extension showed a linear relationship exists between the voltage and speed having an R 2 value of (Figure 4). Using the line of best fit function in Microsoft Excel, the following equation was produced: S = ( in/s*v) * V Figure 4: Influence of Voltage on Actuator Speed R 2 Value of The testing apparatus was also used to test the effects of loaded weight on the speed of extension and retraction. For this test, weights of up to 40 pounds were attached to the eye hook of the testing apparatus (Table 2). Table 2: Effects of Weight on Actuator Speed Weight (lb) Retraction Time (s) Extension Time (s) The time of extension was mostly unaffected by the addition of weight to the point of 34.3 lbs. This was possibly due to the assistance of gravity with the downward motion of the actuator extension. However, as weight was added to the testing apparatus, the time of retraction lifting the weight upwards increased.

5 Conclusions and Future Directions This project developed modules for a prototype of an assistive device that could be further refined to better suit the needs of users experiencing arm muscle weakness. The function of the linear actuator controlled by the Arduino appeared to provide the torque and speed necessary for the supplemental arm strength. The comfort of the user was taken into consideration with adjustable straps and a nonirritating support system. However, this project has the potential to be modified to be more ergonomic through altering the support system to better mimic the natural shape of the arm. Additionally, the system could be compacted to increase portability and user mobility. This project has presented a more affordable alternative to other arm mounted exoskeleton systems that could be accessible to more users. Further development and testing will be required toward clinical applications References 1. Bhakta, Bipin B., J. Alastair Cozens, M. Anne Chamberlain, and John M. Bamford. "Impact of Botulinum Toxin Type A on Disability and Carer Burden Due to Arm Spasticity after Stroke: A Randomised Double Blind Placebo Controlled Trial." Journal of Neurology, Neurosurgery, and Psychiatry. BMJ Publishing Group Ltd., 23 Mar Web. 02 Feb Miller, Freeman, MD, and Jan Koreska, P.Eng. "Height Measurement Of Patients With Neuromuscular Disease And Contractures." Developmental Medicine & Child Neurology 34.1 (1992): Wiley Online Library. Web. 2 Feb Pathania, Sangram, and Marie Curtice. "Hospital Care for Frail Older People." British Geriatrics Society. British Geriatrics Society, 11 Mar Web. 02 Feb Sunderland, A., D. J. Tinson, E. L. Bradley, D. Fletcher, R. Langton Hewer, and D. T. Wade. "Enhanced Physical Therapy Improves Recovery of Arm Function after Stroke. A Randomised Controlled Trial." Journal of Neurology, Neurosurgery & Psychiatry 55.7 (1992): Journal of Neurology, Neurosurgery & Psychiatry. Web. 2 Feb Krebs, H., L. Dipietro, S. Levy-Tzedek, S. Fasoli, A. Rykman-Berland, J. Zipse, J. Fawcett, J. Stein, H. Poizner, A. Lo, B. Volpe, and N. Hogan. "A Paradigm Shift for Rehabilitation Robotics." IEEE Engineering in Medicine and Biology Magazine 27.4 (2008): IEEE Xplore Digital Library. Web. 9 Feb Evans, William J., and Jan Lexell. "Human Aging, Muscle Mass, and Fiber Type Composition." The Journals of Gerontology Series A: Biological Sciences and Medical Sciences 50A.Special (1995): Oxford Journals. Web. 9 Feb Visser, M., B. H. Goodpaster, S. B. Kritchevsky, A. B. Newman, M. Nevitt, S. M. Rubin, E. M. Simonsick, and T. B. Harris. "Muscle Mass, Muscle Strength, and Muscle Fat Infiltration as Predictors of Incident Mobility Limitations in Well- Functioning Older Persons." The Journals of Gerontology Series A: Biological Sciences and Medical Sciences 60.3 (2005): Oxford Journals. Web. 9 Feb Cavallaro, Ettore E., PhD, Jacob Rosen, PhD, and Joel C. Perry, PhD. "Real-Time Myoprocessors for a Neural Controlled Powered Exoskeleton Arm." IEEE Transactions on Biomedical Engineering IEEE Trans. Biomed. Eng (2006): IEEE Xplore Digital Library. Web. 2 Feb Rahman, Mohammad H., Maarouf Saad, Jean P. Kenne, and Phil Archambault. "Modeling and Control of a 7DOF Exoskeleton Robot for Arm Movements." 2009 IEEE International Conference on Robotics and Biomimetics (ROBIO) (2009). IEEE Xplore Digital Library. Web. 4 Feb Bleakley, Scott. "The Effect of the Myomo Robotic Orthosis on Reach Performance After Stroke, Doctoral Dissertation." D- Scholarship at Pitt. University of Pittsburgh, 19 Apr Web. 4 Feb Pratt, J.E., B.T. Krupp, C.J. Morse, and S.H. Collins. "The RoboKnee: An Exoskeleton for Enhancing Strength and

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