Elastic Structure Preserving Impedance Control of Bidirectional Antagonistic Variable Stiffness Actuation

Abstract

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.

Simultaneous Motion Tracking and Joint Stiffness Control of Bidirectional Antagonistic Variable-Stiffness Actuators

“…However, we showed that the origin x = 0 is a uniformly asymptotically stable equilibrium point of the closed-loop system ( 24)-( 26). Regulation [18] Tracking Desired Regulation [18] Tracking…”

“…The modal damping factors were chosen as ξ q = 0.2 and ξ η,i = 0.1. To demonstrate the benefits of the proposed tracking controller, a comparison with the regulation controller [18] has been performed. For the sake of comparability the parameters K q , ξ q , ξ η,i and the scaling factor for the motor inertia shaping were always chosen equal for both the regulation and the tracking controller.…”

“…The upper diagram in Fig. 3 shows the desired trajectory together with the actual joint positions achieved with [18] and the proposed controller. For this experiment K q = 30 Nm/rad and τ 0,1 = τ 0,2 = 0 Nm have been chosen.…”

“…As expected, the performance in case of the regulation controller decreases with increasing frequency. This is due to the fact that [18] does not involve any feedforward terms to compensate for the dynamics necessary to realize the desired link-side trajectory. Interestingly, the maximum tracking errors with the proposed tracking controller are only marginally affected by the increasing frequency in the desired trajectory, confirming the practical applicability also for highly dynamic motions.…”

“…For this system a sliding mode control is designed and validated in simulation [16]. In [17] and [18] the elastic structure preserving (ESP) control approach of [7] was extended to a bidirectional antagonism with variable stiffness. However, only the regulation control problem was addressed.…”