Pneumatic Quasi-Passive Variable Stiffness Mechanism for Energy Storage Applications

State-of-the-art compliant actuators with variable stiffness meet the requirements for exoskeletons only to a limited extent, usually due to their higher mechanical complexity and large mass. In this letter, we present a quasi-passive lightweight pneumatic mechanism that emulates stiffness modulation in the pneumatic cylinder using fast-switching valves and without the need of an air supply. Depending on the state of the valves, the mechanism can operate in different modes. For example, timely control of three fast-switching solenoid valves can modulate the internal equilibrium pressure and adjust the stiffness of the cylinder, or it can be turned off to allow free movement, which is sometimes critical in wearable robotic applications. The novel approach to stiffness modulation is mathematically described. Furthermore, we discuss how changing the initial equilibrium pressure of the mechanism affects the stiffness and energy storage capacity, which has been studied in several experiments. The experiments show successful modulation of the stiffness without the need for an external pressurized air supply. The obtained measured findings are satisfactorily consistent with the derived theoretical model.