Design and Control of a Passive Noise Rejecting Variable Stiffness Actuator

Inspired by the biomechanical and passive properties of human muscles, we present a novel actuator named passive noise rejecting Variable Stiffness Actuator (pnrVSA). For a single actuated joint, the proposed design adopts two motor-gear groups in an agonist-antagonist configuration coupled to the joint via serial nonlinear springs. From a mechanical standpoint, the introduced novelty resides in two parallel non-linear springs connecting the internal motor-gear groups to the actuator frame. These additional elastic elements create a closed force path that mechanically attenuates the effects of external noise. We further explore the properties of this novel actuator by modeling the effect of gears static frictions on the output joint equilibrium position during the co-contraction of the agonist and antagonist side of the actuator. As a result, we found an analytical condition on the spring potential energies to guarantee that co-activation reduces the effect of friction on the joint equilibrium position. The design of an optimized set of springs respecting this condition leads to the construction of a prototype of our actuator. To conclude the work, we also present two control solutions that exploit the mechanical design of the actuator allowing to control both the joint stiffness and the joint equilibrium position.