Workspace Nonlinear Disturbance Observer for Robust Position Control of Flexible Joint Robots

Control of tasks by Flexible Joint Robots (FJR) is highly susceptible to disturbances, coupling, and vibrations that result from joint elasticity. A possible solution to this problem is a Disturbance Observer (DOB), which can address these issues in the joint space. However, this approach has limitations for workspace motion control, as the effect of nonlinear link dynamics and workspace coupling force is not considered. For these reasons, a novel DOB design for FJR in the workspace is deemed necessary, yet most research has focused on the joint DOB design. In this letter, a novel approach to modeling and control of the FJR in the workspace is proposed. A robust workspace nonlinear DOB for FJR is developed, taking into consideration the coupled dynamics of FJR as well as various disturbances that deteriorate the workspace position control, such as motor/link side disturbance, inertia variation, and workspace coupling force. In addition, the singularity problem that arises from modeling FJR for workspace DOB is accounted for by the proposed method. The effectiveness of the proposed method is demonstrated through simulations and experiments, and its stability is also verified.