  This paper proposes a new Active Disturbance Rejection based robust
trajectory tracking controller design method in state space. It can compensate
not only matched but also mismatched disturbances. Robust state and control
input references are generated in terms of a fictitious design variable, namely
differentially flat output, and the estimations of disturbances by using
Differential Flatness and Disturbance Observer. Two different robust controller
design techniques are proposed by using Brunovsky canonical form and polynomial
matrix form approaches. The robust position control problem of a two
mass-spring-damper system is studied to verify the proposed robust controllers.
