Xiangrong Shen, Garrett Waycaster, and Sai-Kit Wu
Pneumatic artificial muscle, actuation, robust control, biologically inspired robotic systems
This paper describes the design and control of a unique variable-radius pulley-based pneumatic artificial muscle (PAM) actuation system. The introduction of variable-radius pulley is inspired by the mismatching between the desired torque curve and the available torque curve provided by a traditional PAM system. With the pulley radius as a function of the joint position, this new feature enables the designer to modulate the shape of the torque curve, and thus achieves a significantly higher flexibility. The new actuation system also incorporates a spring-return mechanism, inspired by the fact that a large number of bio-robotic systems require a significantly higher torque in one direction than the other. Utilizing the knee joint actuation system for lower-limb prostheses as a case study example, the design procedure for the entire system is presented, with the results showing that this actuation system is able to provide a higher performance at a smaller weight and volume compared with the traditional PAM actuation system. To enable the robotic application of the new PAM actuation system, this paper also presents a robust model-based controller design, which is developed based on a dynamic model that covers the major nonlinearities of the system. Based on this model, the standard sliding mode control approach is applied, which provides robust control of joint motion in the presence of model uncertainties and disturbances. This controller is implemented on an experimental demonstrator, and the effectiveness demonstrated with sinusoidal tracking performances.
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