Lower limb exoskeleton parasitic force modeling and minimizing with an adaptive trajectory controller

Parasitic force caused by joint misalignment is a common and challenging problem in the design and control of lower limb exoskeletons due to the complex human joint morphology. The force will generate high tangential force on the skin, which leads pain or at least discomfort for the wearer. This paper presents a model of the parasitic force in a lower limb exoskeleton, aiming to minimize this force with an adaptive trajectory controller (ATC). The controller uses parasitic force in the shank between the human and the exoskeleton as the control signal, and adjusts joint trajectories to minimize the parasitic force. In this paper, a lower limb exoskeleton with two-degrees of freedom (DOFs) in the knee joint is presented. Parasitic force relates to the joint misalignment is modeled and analyzed, upon which a trajectory controller is developed. Both simulations and experimental results are included, which showed that the proposed method was capable of effectively reducing the parasitic force in motion assistance.