Passivity Based versus Disturbance Observer

Disturbance Observer based robot control algorithms provide good performance while giving a straightforward method of gain selection. Unfortunately, the stability of these algorithms has not been shown rigorously for nonlinear systems such as robots. This paper demonstrates that a disturbance observer based algorithm can be made equivalent to the well known passivity based approaches of Sadegh and Horowitz, and Slotine and Li. By doing so, the stability of the disturbance observer based approach can be proven rigorously. An additional beneet of the equivalence of the two approaches is the ability to combine an adaptive controller to the disturbance observer based approach. This addition yields a controller that is both easy to tune and robustly stable to parameter variations. 1. INTRODUCTION Sadegh and Horowitz (1990) and Slotine and Li (1987) concurrently, but separately, developed computed torque type algorithms that made use of the passivity of the robot dynamics to obtain globally, asymptotically stable tracking control laws for robotic manipulators. At the time, these algorithms were a substantial contribution to the theory of robot control and are still popular today. have been applying the disturbance observer theory to design tracking control algorithms for robotic manipu-lators. Instead of explicitly designing a nonlinear control law to compensate the nonlinearities in the robot's equations of motion, the nonlinearities are treated as