A Testable Robust Stability Framework for the Variable Impedance Control of 1-DOF Exoskeleton With Variable Stiffness Actuator

Abstract

We present a framework for achieving a robust stability test of the variable stiffness actuator (VSA) exoskeleton programmed with the gain scheduling-based variable impedance control (GSVIC). In this brief, the focused impedance control framework involves the cascaded position torque control loop, in which both control loops are closed, and the index of the GSVIC is related to the joint torque of the human. However, there is a lack of research on the variation rate of such a biofeedback signal. This lack of information is necessary when performing the robust stability test of the linear parameter-varying (LPV) system. To acquire a bounded variation rate, the mechanical stiffness variation component of the VSA is used to transfer the biofeedback information into the GSVIC. In this case, the LPV system has only one variable parameter with the known and bounded variation rate, i.e., the physical stiffness of the VSA. The operating range of the impedance controller, determined from the test of robust stability via parameter-dependent Lyapunov functions, can meet our application requirements. The whole control system follows the paradigm of the variable impedance task in accordance with human intention. The effectiveness of the control scheme is experimentally verified on a VSA prototype and two subjects wearing the VSA exoskeleton.