Linear and Nonlinear Low Level Control of a Soft Pneumatic Actuator

The research on soft material robots has been a highly discussed topic in recent years. Due to the nonlinear material behavior, the control of the pressure in the actuators chambers is difficult. In this paper a concept for low level control of soft pneumatic actuators is developed. For this purpose a test bed with a proportional directional valve is designed. The concept includes mathematical models of the control element. The actuator is modeled as a sphere. For the valve the flow through the opened area is modeled and the tubes friction and systems leakage are also considered. All needed and unknown parameters are identified with particle swarm identification. Due to the non-linearity of the valve and the actuator model, approaches for nonlinear control will be examined. A state space representation of the plant and control element will be assembled. First a PID controller is developed, which is tuned with the Ziegler-Nichols method. In contrast, a robust control theory, the sliding mode control, is investigated. It will be shown, that the PID controller as well as the sliding mode control are compatible with the low level control. It becomes apparent, that the PID controller reacts inferior compared to the sliding mode controller, if there is a system change like rising in leakage. The control strategies are also tested on the real plant and are appropriate strategies for tracking a trajectory.