Tracking and disturbance rejection of MIMO nonlinear systems with a PI or PS controller

We study tracking and disturbance rejection for a class of MIMO nonlinear systems, with a Linear proportional plus integral (PI) compensator, in the continuous time Case, and a linear proportional plus sum (PS) compensator in the discrete time case. We show that if the nonlinear plant is exponentially stable and has a strictly increasing dc steady state I/O map then a simple PI or PS compensator can be used to yield a stable unity feedback closed loop system which asymptotically tracks reference inputs that tend to constant vectors and asymptotically rejects disturbances that tend to constant vectors. This extends earlier work of Desoer and Lin