Hybrid Pneumatic-Hydraulic Actuator with Adaptable Stiffness

This work presents a method to control the stiffness of a hybrid actuator. The resulting stiffness is required to meet the conditions of real life applications, such as human prosthetics, human-robot interaction, and delicate robot interaction. The hybrid actuator is basically a pneumatic-hydraulic muscle, which can operate simultaneously in both pneumatic and hydraulic modes. The main challenge in this work is to manage the switching between pneumatic and hydraulic modes. In pneumatic mode when a load is applied to the actuator, air in the tank is allowed to compress resulting in muscle extension. While in hydraulic mode, the fluid is pressurized and the resultant system stiffness is higher. In both cases, the McKibben muscle is full with hydraulic fluid. It has been shown that the performance of the actuator is mostly the same in terms of response and bandwidth in both modes of operation. The use of different types of controllers to improve the system performance is investigated. It is found that the parallel configuration combined with PID controller is the best solution for achieving the required muscle performance.