Analysis and Simulation of an Exoskeleton Controller that Accommodates Static and Reactive Loads

Exoskeletons are structures of rigid links mounted on the body that promise to restore, rehabilitate, or enhance the human motor function. A major challenge in the practical use of exoskeletons for daily activities relate to the coupled control of human-exoskeleton system. This paper provides a method to resolve the control problem by relegation of the human control and exoskeleton control to two control subsystems. The first subsystem represents the execution of voluntary control from commands generated from the central nervous system. This subsystem is responsible primarily for the kinetic or dynamic components of the motion, including motion generation. The second subsystem represents the exoskeleton controller, responsible for joint level accommodation of all gravitational, static, and certain reactive forces. If all such forces are static, the exoskeleton controller can be viewed as a compensator that maintains the body in static equilibrium. The proposed strategy provides a clear partition between natural voluntary control by the CNS, and artificial assist by the exoskeleton controller. Two methods are presented for implementation of the proposed control algorithm. The first method is based on the principal of virtual work. The second method is a recursive algorithm based on force and moment balance equations. Analytical results are presented to study feasibility regions of exoskeleton control strategies in terms of mechanical efficiency. Finally, simulation results are presented to demonstrate the efficacy of the algorithm for a powered ankle-foot orthosis application.