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.