Delivered by Ingenta to: Guest User IP: 178.57.68.109 On: Sun, 26 Jun 2016 01:07:36 Copyright: Aerospace Medical Association SHORT COMMUNICATION 696 Aviation, Space, and Environmental Medicine x Vol. 83, No. 7 x July 2012 H ALL KL, P HILLIPS CA, R EYNOLDS DB, M OHLER SR, N EIDHARD- D OLL AT. Pneumatic muscle actuator (PMA) task-specific resistance for potential use in microgravity exercise. Aviat Space Environ Med 2012; 83:696–701. Introduction: A pneumatic muscle actuator (PMA) is a device that mimics the behavior of skeletal muscle by contracting and generating force when activated. This type of actuator has a high power to weight ratio and unique characteristics which make it ideal for human interac- tion. PMAs, however, are difficult to control due to nonlinear dynamics. Our objective was to control a PMA as a source of task-specific resistance in simulated isokinetic strength training. Task-specific resistance will benefit those in need of strength training through a joint’s range of motion, including astronauts who need to counteract muscle atrophy during prolonged spaceflight. The lightweight, clean, and compact PMA driven by pressurized air is able to produce resistance in microgravity. Methods: An open-loop control method based on a three-element phenomeno- logical inverse model was developed to control the PMA. A motor was simultaneously controlled to act as simulated human quadriceps work- ing against the PMA-produced resistance. Results: For ankle weight replacement resistance profiles, the PMA control method produced resistance and PMA displacement tracking errors (RMSE) of 0.36–1.61 Nm and 0.55–1.59 mm, respectively. Motor position (simulated joint angle) tracking errors ranged from 0.47 to 2.82°. Discussion: Results in- dicate that the inverse model based control system produces task-specific PMA resistance and displacement. Closed-loop motor control was able to simulate isokinetic movement successfully. More complicated resis- tance profiles reveal the need for closed-loop control. Future work focuses on advancing both the PMA control strategies and the capabili- ties of the human simulator so that actual human operator applications can be realized. Keywords: aerospace exercise, microgravity resistive training, rehabilita- tion, biomimetic actuator, pneumatic muscle actuator. T HE PNEUMATIC MUSCLE actuator (PMA) is a mechanical device that mimics the behavior of skeletal muscle in that it contracts and generates force in a non- linear manner when activated (11). PMAs are constructed of a tubular shaped rubber bladder and an inextensible fiber mesh that either surrounds or is embedded in the rubber matrix. The fiber mesh provides support and enhances actuation. The rubber bladder is completely sealed except for an air valve that allows air to enter and exit. Once pressurized, the PMA expands in a radial di- rection, resulting in contraction and force production in the longitudinal direction. PMA operation is illustrated in prior work (14). The level of contraction and force production is dependent on whatever is attached to or working against the PMA. This type of actuator has several unique characteris- tics, some of which make it an ideal actuator for applica- tions involving human interaction. PMAs are capable of generating a high force output. They have higher power to weight and power to volume ratios than electric motors or hydraulic actuators (12). They have a higher force output than a pneumatic cylinder of equal volume (10). PMAs are cost effective, clean, compact, and can be used in harsh environments because they do not have moving parts such as pistons or guiding rods (6). PMAs can be used in microgravity environments because gravity is not necessary to produce contraction or force generation. They are also a safe alternative to other actuators. They provide “soft actuation,” mean- ing safety is enhanced through a low mass/inertia struc- ture that combines high strength with actuator and/or structural compliance (2). Their “soft actuation” allows them to be used around humans without posing safety risks associated with other actuators that are heavy and noncompliant. There is a potential safety mechanism built into the manner in which the PMA operates and transfers force in this research. Force is only produced if there is something working against the PMA. If, hypo- thetically speaking, a human working against PMA- produced force for strength training purposes does not want to continue the exercise, they can stop safely and without consequence. The main disadvantage in using PMAs is the difficulty involved with controlling them due to their nonlinear dynamics and time varying be- havior. Control is particularly difficult when it is neces- sary to control both PMA displacement and force using only one parameter (internal pressure of the PMA). Losses of muscle strength and bone mineral density along with aerobic deconditioning are major health con- cerns for astronauts during prolonged spaceflights (1,3). The lack of gravity-related loading in a microgravity environment leads to a loss of bone mineral density at a rate of 1 to 2% per month and a reduction of muscle strength in the lower limbs at a rate of 2 to 8% per week (7). In bed rest studies simulating weightlessness, knee From Wright State University, Dayton, OH. This manuscript was received for review in May 2011. It was accepted for publication in February 2012. Address correspondence and reprint requests to: Chandler Phillips, 3640 Colonel Glenn Hwy., 207 Russ Engineering Center, Dayton, OH 45435; chandler.phillips@wright.edu. Reprint & Copyright © by the Aerospace Medical Association, Alexandria, VA. DOI: 10.3357/ASEM.3105.2012 Pneumatic Muscle Actuator (PMA) Task-Specific Resistance for Potential Use in Microgravity Exercise Kara L. Hall, Chandler A. Phillips, David B. Reynolds, Stanley R. Mohler, and Amy T. Neidhard-Doll