Disturbance compensation strategies for a high-speed linear axis driven by pneumatic muscles

This paper presents a cascaded control scheme for a new 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. This innovative drive concept allows for an increased workspace as well as higher carriage velocities as compared to a direct actuation. 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 proposed control has a cascade structure. The internal pressure of each pneumatic muscle is controlled by a fast underlying control loop. Hence, the control design for the outer control loop can be simplified by considering these controlled muscle pressures as ideal control inputs. The control design of the outer control loop involves a decoupling of rocker angle as well as mean internal pressure of both pneumatic muscles. Remaining model uncertainties as well as nonlinear friction are counteracted by an additional ILC controller. The disturbance compensation by ILC is also compared to an adaptive control approach and a nonlinear disturbance observer. Experimental results from an implementation on a test rig show a high control performance.