Design of Parallel Variable Stiffness Actuators

Direct-drive motors (DDMs) have been increasingly used for robot actuation because they provide high-fidelity torque control, but they typically have low torque density. Gearing can be used to increase the torque density of motors, but gearing decreases the power density of the actuator. Parallel elastic actuators (PEAs), composed of a spring attached in parallel to a motor, can increase both the torque and power density of the actuator without jeopardizing torque control fidelity. PEAs can also generate efficient oscillatory motion by applying the torque of the motor through resonant oscillations. However, conventional fixed stiffness springs used in PEAs only enable efficient oscillatory motion at a fixed resonant frequency defined by the stiffness of the spring. In this article, we present a parallel variable stiffness actuator (PVSA) consisting of a DDM connected in parallel to a variable stiffness spring. PVSAs retain the torque control bandwidth of DDMs and PEAs and can be used to amplify the torque and power of the motor over a range of oscillation frequencies. We present a compact design of a PVSA where a direct-drive motor, a high energy density composite spring, and a variable stiffness mechanism are arranged in a conventional cylindrical geometry, similar to a motor-gearbox assembly. We foresee the use of PVSAs in mobile robots and wearable devices, where energy efficient oscillatory motion at different frequencies, along with high torque and power density is indispensable.