Parallel Variable Stiffness Actuators

In this paper, we introduce a new type of compliant actuator named the Parallel Variable Stiffness Actuator (PVSA) which consists of a variable stiffness spring placed in parallel with a direct-drive motor. Parallel variable stiffness actuators provide (i) high-fidelity force control and (ii) controllable energy storage, as they inherit the benefits of direct-drive motors and variable stiffness springs. We present a compact design of the PVSA using a flat motor connected to an adjustable mechanical advantage torsional spring. We show that this PVSA is (1) not subject to the fundamental force control bandwidth limitation of series elastic and variable stiffness actuators, and most notably, (2) enables resonant energy accumulation despite the limited deformation of the spring and the constrained motion of the load attached to the actuator. The latter differentiates parallel variable stiffness actuators from fixed-stiffness parallel elastic actuators. PVSAs may be used with smaller direct-drive motors to match the peak power of larger motors without compromising force control fidelity. PVSAs may be used to implement resonant forcing under joint angle limitations in walking, jumping, running, swimming robots, or robotic exoskeletons used to augmented human motion in the aforementioned tasks.