Design of a structure-controlled variable stiffness actuator based on rotary flexure hinges

This paper presents a new variable stiffness actuator (VSA) based on a structure-controlled method: controlling the mechanical structure of the actuator by rotating four flexure hinges. The VSA possesses a property that the output position and stiffness are independently controlled. This is realized by a serial arrangement of a principle driving motor, a small stiffness variation motor and a novel variable stiffness mechanism (VSM). The VSM consists of four combined notch flexure hinge frames, each of which contains an inner rotary flexure shaft and outer stationary hinge housing. The stiffness is adjusted by rotating the inner flexure shaft to change the second moment of area. Its value is independent to the VSA output angle when the flexure rotational angle is fixed. The working principle of this VSA is elaborated and the optimization mechanical design of the VSM is presented. A prototype has been implemented and its primary characteristic test results validate the design features.