Comparison of Model-Based Approaches to the Compensation of Hysteresis in the Force Characteristic of Pneumatic Muscles

In this paper, a comparison of three different feedforward compensation strategies that counteract hysteresis effects in the nonlinear force characteristic of pneumatic muscles is presented: The generalized Bouc-Wen model is a dynamic hysteresis model and enables a description of the given highly asymmetric hysteresis, and as alternative hysteresis models for the comparison, the quasi-static Maxwell-slip model and the Prandtl-Ishlinskii model are considered. The parameters of all these hysteresis models have been experimentally identified using evolutionary optimization algorithms. Each of the identified hysteresis models is suitable for an additional feedforward control action in an existing nonlinear control structure for a high-speed linear axis that is actuated by pneumatic muscles to further reduce the tracking error. This cascaded nonlinear control structure consists of fast underlying control loops for the internal muscle pressures and an outer adaptive backstepping control loop for both the carriage position and the mean muscle pressure. Here, the adaptive control part counteracts nonlinear friction and the remaining model uncertainty. Comprehensive experimental results from an implementation of the proposed control approach on a test rig at the Chair of Mechatronics, University of Rostock, Rostock, Germany, point out both the benefits and efficiency of the corresponding feedforward hysteresis compensation strategies.