Constrained Nonlinear Disturbance Observer for Robotic Systems

Disturbance observer (DOB) is a well-known two-loop control structure that imparts robustness to a controller with a simple implementation. As a nonlinear DOB for the robotic systems, we proposed so-called nonlinear robust internal-loop compensator (NRIC) framework in our previous work. In this paper, we further extend the NRIC in such a way that an optimization scheme can be embedded in the control structure. The proposed method is called constrained NRIC (C-NRIC), because the optimization allows us to impose constraints, by which a controller acquires additional properties. As a particular use case of the C-NRIC framework, we design contact-responsive motion controllers that enables a robot to react to unknown interactions while accurately tracking the desired trajectory in free motion. The effectiveness of such designs is validated through the real-world experiments.