Force-Mode Control of Rotary Series Elastic Actuators in a Lower Extremity Exoskeleton Using Model-Inverse Time Delay Control

For a physical human-robot interaction (pHRI) system such as an exoskeleton, it has been an important issue to control the interaction force between the user and the actuators because it directly determines the accuracy of the realized impedance. However, the accurate force control of the exoskeleton system has not been fully achieved due to the difficulties caused by uncertainties from pHRI. To overcome this problem, in this study, the series elastic actuator (SEA) was operated by a time delay control (TDC), which directly compensated the uncertainties without an accurate model of them, realizing the designed model dynamics in the actuator. However, in spite of the successful application of TDC, the delays between the desired and model dynamics severely degraded the force control performance of the SEA. Thus, a new strategy named model-inverse time delay control was proposed by introducing a virtual reference predicted by the inverse of model dynamics in TDC. The proposed method significantly improved performance of the force control, and it was verified by demonstrating zero impedance control with pHRI, free oscillation of a pendulum, and force control in the exoskeleton.