Robust output tracking controller design for a class of uncertain MIMO nonlinear systems

A new control design method based on signal compensation is proposed for a class of uncertain multi-input multioutput (MIMO) nonlinear systems in block-triangular form with nonlinear uncertainties, external disturbances, unknown virtual control coefficients and strongly coupled interconnections. By this method, the controller design is performed in a backstepping manner. At each step of backstepping procedure, a nominal virtual controller is first designed to get desired output tracking for the nominal disturbance-free subsystem, and then a robust virtual compensator is designed to restrain the effect of the uncertainty, disturbance involved in the subsystem and couplings among the subsystems. It is proved that global robust stability and global robust practical tracking property of the closed-loop system can be ensured and the tracking errors can be made as small as desired. The trait of our approach is that the designed controller is linear and time-invariant, so the explosion of terms in the control law is avoid.