A pneumatic muscle actuator (PMA) is a device that mimics behavior of skeletal muscle by contracting and generating force in a nonlinear manner when activated. PMAs have a high power to weight ratio and possess unique characteristics which make them ideal for human interaction. Due to their nonlinear dynamics, PMAs are difficult to control, presenting challenges in system implementation. Despite these challenges, PMAs have great potential as a source of resistance for strength training and rehabilitation. The main goal of this work was to control a PMA for use in isokinetic exercise, potentially benefiting anyone in need of optimal strength training through a joint's range of motion. This includes astronauts who need to counteract muscle atrophy and bone loss during prolonged spaceflight. The lightweight PMA driven by pressurized air does not need gravity to produce resistance, making it an attractive option for a microgravity exercise device.
The control system developed is based on an inverse three-element phenomenological model and adaptive nonlinear control. The system operates as a type of haptic controller, automatically adjusting resistance to assist a simulated neuromuscular actuator in maintaining the desired velocity. A human quadriceps dynamic simulator (HQDS) was developed so that control effectiveness and accommodation could be tested prior to human implementation. A motor, which produces torque analogous to quadriceps' torque production about the knee, is used in conjunction with the HQDS to simulate neuromuscular actuation. Tracking error results for motor shaft position (simulated joint angle), velocity (simulated lower leg angular velocity), and PMA displacement indicate that the control system is effective at producing PMA displacement and resistance necessary for a scaled, simulated neuromuscular actuator to maintain low-velocity isokinetic movement during simulated concentric and eccentric knee extension. This work is an important step towards human implementation of PMA produced resistance for isokinetic strength training and rehabilitation.