Positive-Negative Stiffness Actuators

Compliant actuators are typically designed to possess a tunable positive stiffness characteristic in order to generate restoring force upon displacement. These actuators either require two independent motor units or closed-loop control to change both their equilibrium position and output stiffness. The introduction of negative stiffness, in combination with tunable positive stiffness, may reduce the complexity and extend the capability of these actuators in unexpected ways. In this paper, we present a compliant actuator that employs a passive negative stiffness mechanism in conjunction with an effectively tunable positive stiffness mechanism. We show that such actuator enables open-loop stiffness modulation and equilibrium position control using a single motor unit, as opposed to more conventional variable stiffness and series elastic actuators. The paper presents the theoretical foundation of positive-negative stiffness actuators and demonstrates low-power stiffness modulation and equilibrium position control achievable with a prototype positive-negative stiffness actuator.