Tracking control with hysteresis compensation for manipulator segments driven by pneumatic artificial muscles

In this paper a feedforward control structure is introduced for a manipulator joint driven by a symmetrical arrangement of two antagonistic muscles. The setup allows the specification of the desired joint angle, as well as the single muscle forces. Based upon the manipulator kinematic and the inverse static relation in a single muscle, the pressure signal for the muscles´ supplying valves is calculated. Applying this control structure allows to compensate the nonlinear characteristic mean relation in a single muscle actuator, while the muscles´ hysteretic behavior introduces a hysteresis in the relation between the desired and the actual joint angles. It is shown, that if the muscles are subject to comparable forces the resulting hysteretic behavior can be described by a Preisach model. The inverse of this model can then be applied as a feedforward term to compensate the static hysteretic behavior. Experiments confirm the improvement in tracking control as compared to the system solely controlled by a feedback regulator.