Virtual impedance adjustment in unconstrained motion for an exoskeletal robot assisting the lower limb

The objective of this paper is to establish the criteria for adjusting the virtual impedance of an exoskeletal robot for the lower limb in order to minimize the operator's physical stress in unconstrained motion. The exoskeletal robot HAL (Hybrid Assistive Limb)-3 which we developed for assisting the motor function of the lower limb was used for experiments in this research. To accomplish the objective (i) the physical parameters around the joint of HAL-3 were identified and (ii) the relationships between the virtual impedance values and the physical stress of operators were examined through experiments for swinging motions of the lower leg. The physical stress was evaluated with myoelectricity, the musculoskeletal moment of the operator and the operator's feelings during the experimental motion. As a result, we found that the physical stress tended to decline with the decrease of virtual inertia and virtual Coulomb friction, and to increase slightly with the decrease of virtual gravitational moment. The decrease of virtual viscous friction made the physical stress increase gradually after it declined to a trough during the positive virtual viscous friction. Based on the results, we could establish the criteria for adjusting the virtual impedance for minimizing the operator's physical stress in unconstrained motion.