Sliding Mode Control for a High-Speed Linear Axis Driven by Pneumatic Muscles

This paper presents a cascaded sliding mode control scheme for a new pneumatic linear axis. Its guided carriage is driven by a nonlinear mechanism consisting of a rocker with a pair of pneumatic muscle actuators arranged at both sides. Modelling leads to a system of four nonlinear differential equations including polynomial approximations of the volume characteristic as well as the force characteristic of the pneumatic muscles. The differential flatness of the system is exploited in combination with sliding mode techniques to stabilize the error dynamics. Furthermore, a proxy-based sliding mode controller was designed, which is a modified version of sliding mode control as well as an extension of PID control. It allows for accurate tracking during normal operation and smooth recovery from large position errors after unexpected incidents. The internal pressure of each pneumatic muscle is controlled by a fast underlying control loop, whereas in an outer control loop the carriage position and the mean internal pressure of the muscles are controlled. Remaining model uncertainties are compensated by a disturbance observer. Experimental results show an excellent control performance.