Stability and robustness properties of a simple adaptive controller

A low-order adaptive tracking controller is proposed for linear time-invariant plants with a relative degree not exceeding two. The order of the plant is not required to be known a priori. The algorithm is robustly stable with respect to small linear time-invariant plant perturbations and bounded disturbances. The robustness is achieved by using a projection of the parameter estimates in the control law. An additional a priori information needed to design the controller is bounds on the plant parameters. In the absence of plant perturbations and disturbances, perfect tracking is obtained