Passivity of Series Elastic Actuation Under Model Reference Force Control During Null Impedance Rendering

Series elastic actuation (SEA) is an interaction control paradigm that relies on a compliant force sensing element and utilizes the model of this compliant dynamics in closed-loop force control. We present sufficient conditions for passivity of SEA under model reference force control (MRFC) during null impedance rendering. We prove that overestimation of robot inertia and underestimation of the stiffness of the series elastic element can ensure coupled stability of interaction for SEA under MRFC during null impedance rendering, as long as a lower limit on damping compensation is not violated. We experimentally verify the passivity bounds and demonstrate the null impedance rendering performance of MRFC.