Force and Stiffness Backstepping-Sliding Mode Controller for Pneumatic Cylinders

In most applications that involve human-robot interactions, such as prosthetics, orthotics, rehabilitation, and locomotion, compliant actuators with variable stiffness can be used to improve safety and comfort of the device. Another advantage of the stiffness control is minimizing the energy consumption by adjusting the stiffness of the actuator to the natural stiffness of the controlled system. This paper introduces a new backstepping-sliding mode force-stiffness controller for pneumatic cylinders. The global ultimate-bounded stability of the closed-loop system was proven by the Lyapunov method. Based on a detailed mathematical model of the pneumatic system that includes the dynamics of the valves, the algorithm was proven able to track the desired force and stiffness independently without chattering. Validating experiments using a real-time platform were performed for a pneumatic cylinder suitable for wearable robotics applications. The performance of the proposed algorithm was compared with the performance of a previously reported pneumatic force-stiffness sliding mode controller.