Design of an Optimal 4-bar Mechanism Based Gravity Balanced Leg Orthosis

Abstract

In this paper we propose the design and control of a 4-bar mechanism based gravity balanced orthosis for providing gait training to persons with disability. Human leg joints have a varying instantaneous centre of rotation and hence 4-bar mechanisms have been used to actuate the orthosis joints. Human gait is first recorded using a vision system and the hip and knee joint trajectories extracted from the data. Optimal 4-bar mechanisms are then designed using a genetic algorithm that gives the smallest mechanism that can replicate the hip and knee joint trajectories accurately. The orthosis joints are gravity balanced so that the potential energy of the system in any orientations is constant, and the wearer does not feel the weight of the system. Experimental and simulation results prove that the exoskeleton can effectively model the changing centre of rotation of the hip and knee joints and follow the desired human trajectories.