Spatial Robust Whole-Body Dynamic Trajectory Optimization of a Lower-Limb Exoskeleton

The goal of this paper is to design spatial (3D) robust reference trajectories for lower limb exoskeletons through optimization. Robustness is defined as the magnitude of the smallest force applied at the Center of Mass that cannot be rejected without violating joint torque or ground reaction force bounds. A larger robustness margin in the reference trajectory increases the capability of the controller to reject perturbations. A trajectory optimization problem was augmented by adding the maximization of this robustness metric to it. The augmented trajectory optimization is then employed to design dynamic trajectories for three different tasks. We demonstrated that our method increases the robustness metric value by comparing the results from the proposed method with a nominal trajectory optimization that does not take robustness into account. Moreover, the designed trajectories were implemented on a lower limb exoskeleton with a paraplegic user to show the feasibility of the proposed method. The user managed successfully to walk on level ground, perform side-stepping, and ascend stairs.