Design and Control of a Compact Variable Stiffness Actuator Based on a Cam-Circular Beam Mechanism

This article presents a novel variable stiffness actuator (VSA) capable of wide-range stiffness regulation and has a compact structure. A cam-circular beam mechanism is proposed in the design of variable stiffness mechanisms. By adjusting the pivot position, the stiffness of the actuator can be regulated from a minimum value to near infinite. Different from the VSAs, which use rotation-to-translation converting mechanisms in the design of stiffness regulation mechanisms, the stiffness regulation of the proposed actuator is achieved by simply rotating the pivot shaft with the help of circular beams. This contributes to the improvement of the compactness and power density of the actuator. Moreover, the transmission ratio between the beam deformation and the joint deflection is determined by the cam mechanism, then the stiffness profile and range of the stiffness can be customized. The control of the actuator is realized by using a proxy-based control method, and an adaptive law is proposed to adjust the control parameter and the joint mechanical stiffness online. Then, significant force compliance is achieved at the control and mechanical level, which guarantees interaction safety when unexpected interruptions occur. The prototype of the proposed actuator is developed and experiments are carried out, which verify the stiffness regulation capability of the actuator and the control performance.