An estimation based robust tracking controller design for uncertain nonlinear systems

We study the use of the equivalent control based disturbance estimation idea in robust control design problems. For matched disturbances, the estimated parts can directly be eliminated by a cancellation term in the control so that the system only needs to be robustified against residual estimation errors. For mismatched disturbances, the design method is presented on a tracking problem of a nonlinear system in strict feedback form by directly incorporating the estimated disturbances into the backstepping control design. A nonlinear damping technique is used to deal with the controllable estimation errors. A semi-global ultimate boundedness result is obtained for tracking error by a Lyapunov synthesis.