Nonlinear Trajectory Control of a High-Speed Linear Axis Driven by Pneumatic Muscle Actuators

Abstract

This paper presents a flatness-based 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 of the muscle driven positioning system leads to a system of nonlinear differential equations including polynomial approximations of the volume characteristic as well as the force characteristic of pneumatic muscles. Taking advantage of the flatness property of the given system model, a flatness-based cascade control approach involving nonlinear feedforward and feedback control is employed. 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 flatness-based control design of the outer control loop involves a decoupling of carriage position as well as mean internal pressure of both pneumatic muscles as flat outputs. Steady-state accuracy in view of model uncertainties is achieved by an integral control part. Simulations show an excellent control performance with small control errors.