Two-Degrees-of-Freedom Controller Synthesis to Robustify the Computed Torque Method

This paper studies a novel controller structure for nonlinear systems with a robotic application. In robotics, the computed torque control method, which is roughly equivalent to state feedback linearization, is known for its high-speed tracking capabilities if the parameters of the dynamic model are known with precision. For MIMO linear systems, the robust controller synthesis technique is available to ensure robust stability and performance in the presence of model uncertainty. Two-degrees-of-freedom (2DOF) controllers also show excellent disturbance rejection properties. We propose a controller structure and design method complementing the computed torque method where the feedback includes a nonlinear, linearizing component and a 2DOF robustifying component. The paper also studies two example systems to illustrate the controller synthesis workflow, including a two-degrees-of-freedom robotic manipulator. Simulations show that the stability and performance of the closed-loop system with the 2DOF design is better than its non-robust or serial compensator counterparts.