Design, modeling and control of a novel compact, energy-efficient, and rotational serial variable stiffness actuator (SVSA-II)

This paper presents a new compact, rotational serial variable stiffness actuator (SVSA-II) based on a symmetrical adjustable pivot lever mechanism that generates elastic torque and permits output stiffness adjustment. By introducing a symmetrical stiffness transmission mechanism consisting of two sets of lever arm mechanisms mounted on a rotational output shaft, the internal friction and energy loss can be reduced to improve the energy efficiency. The new mechanism permits stiffness adjustment from zero to infinity without any structure constraints. The displacement of the springs occurs in the direction vertical to the output arms based on the spring frame. This modification improves the conversion efficiency of stored energy to output toque. The symmetrical Archimedean Spiral cam introduced here reduces the complexity of stiffness control using linear transmission. Control experiments are performed to evaluate the physical performance of the SVSA-II. Our results demonstrate that the SVSA-II with a proportional derivative (PD) feedback and feedforward controller exhibits a good response and high accuracy for position regulation, stiffness regulation and tracking tasks. Moreover, low damping coefficients and relatively high energy storage efficiencies are observed under different stiffness conditions in oscillatory damping experiments.