Trajectory tracking of a pneumatically driven parallel robot using higher-order SMC

This paper presents a higher-order sliding mode control scheme for a fast two-degree-of-freedom parallel robot driven by two pairs of pneumatic muscle actuators. The control-oriented modelling leads to a system of nonlinear differential equations including polynomial approximations of the volume characteristic as well as the force characteristic of the pneumatic muscles. The robot consists of a light-weight closed-chain structure with four moving links connected by revolute joints. The two base joints are active and driven by pairs of pneumatic muscles by means of toothed belt and pulley. The proposed control has a cascade structure: The internal pressure of each pneumatic muscle is controlled by a fast underlying control loop. The central outer sliding mode control loop deals with decoupling control of the end-effector position in the xz-plane and the mean internal pressures of the muscles. Remaining model uncertainties as well as nonlinear friction are counteracted by an observer-based disturbance compensation. Experimental results from an implementation on a test rig show a high control performance.