Elastic Structure Preserving Impedance Control of Bidirectional Antagonistic Variable Stiffness Actuation

We propose an impedance controller for articulated soft robots implemented by bidirectional antagonistic variable stiffness (BAVS) actuation, where two motors are connected via nonlinear elastic elements to a single link allowing for a joint stiffness modulation. Naturally, the highly elastic elements introduce undesired oscillatory dynamics into the plant. To address this problem, we present a controller that allows to impose a desired link-side stiffness and damping behavior while preserving the intrinsic inertial elastic properties of the system. This allows us to solve the global asymptotic regulation problem while simultaneously imposing a desired joint stiffness preset on the BAVS actuator. We provide a passivity and stability analysis based on a physically motivated storage and Lyapunov function. Experimental results on the underarm BAVS joint of DLR David validate our control law.