Position Control for a Soft Actuator With High Uncertainty and Hysteresis

A soft inflatable actuator composed of low shore hardness elastomer with thin chamber walls will exhibit high nonlinearity and hysteresis in its motion. The nonlinearity is in the relation between pressure and volume which contains some unstable working zones where there is no one-to-one mapping. Hysteresis causes the actuator to follow different trajectories between the loading (inflating) and unloading (deflating) motions. This makes precise control of its position difficult. To cope with the nonlinearity, we design an adaptive super-twisting controller and account for the uncertain relation between volume and pressure in the control design by fitting a nonlinear function. To address hysteresis, we propose a method to force the tip of the actuator to converge to any reference position on the global deflating path. This method helps to eliminate the uncertainty in the position of the actuator's tip. The controller presented moves the actuator to the desired position in the shortest time with minimal chattering in the control actions. This control method has good performance which is demonstrated through empirical results.