An Interpretation of Internal Disturbance

Internal disturbance is a key concept of active disturbance rejection control (ADRC) but what it is has not been rigorously answered for a long time. This paper proposes a interpretation of internal disturbance. It finds the controllable canonical form of the linear time-invariant (LTI) plant without zero has an integrator chain and a feedback term. It points out the output of the feedback term is the internal disturbance signal, while the integrator chain is the one in ADRC. This interpretation can be extended to nonlinear plants. Thus, a large class of plants indeed can be viewed as a disturbed integrator chain. Furthermore, state feedback stabilization is interpreted as to reject the internal disturbance and control the un-disturbed integrator chain. The rejecting-and-controlling approach can be used to simplify the controller design for some nonlinear plants.