Stability-Guaranteed Assist-as-Needed Controller for Powered Orthoses

This brief describes the assistance regulation controller (ARC), a nonlinear admittance controller for powered orthoses (POs) and wearable robotics that simultaneously facilitates task completion and encourages user effort. This brief also introduces a novel acceleration-limited proportional derivative controller (ALPDC) that guarantees the stability of the ARC's inner position control loop. The stability analysis of the ALPDC shows that this simple and robust position controller promotes safer human-robot interactions in a large class of admittance-controlled haptic devices. Both the ARC and ALPDC are implemented on a one-degree-of-freedom PO designed to assist forearm flexion and extension. Experiments performed by a healthy male subject confirm that the ALPDC guarantees stable user-device interactions and bounded tracking errors during highly dynamic forearm motions that lead to instability with a conventional controller.