Robust Control Law for Pneumatic Artificial Muscles

This paper presents a modified integral sliding surface, sliding mode control law for pneumatic artificial muscles. The cutoff frequency tuning parameter λ is squared to increase the gradient from absement (integral of position) to position and higher derivatives to reflect the more dominant terms in the actuator dynamics. The sliding mode controller is coupled with proportional and integral action compensation. The control system is sufficiently robust so that use of an observer and input-output feedback linearization are not required. Closed-loop control experiments are compared with traditional sliding mode controller designs presented in the literature for pneumatic artificial muscles. Experiments include the tracking of sinusoidal waves at 0.5 and 1 Hz, tracking of square-like waves with seventh-order trajectory transitions at a rate of 0.2 Hz without and with a steady-state period of 10 seconds, as well as a step input response. These experiments indicate that the control law provides similar bandwidth, tracking, and steady-state performance as approaches requiring nonlinear feedback and model observation for pneumatic artificial muscles. Experiments demonstrate an accuracy of 50 μm at steady-state with no overshoot and maximum tracking errors less than 0.4 mm for smooth square-like trajectories.