On active disturbance rejection for systems with input time-delays and unknown dynamics

This paper concerns with the typical problem of disturbance rejection in process control in the presence of time-delays and, possibly, the integrating or unstable dynamics. It is shown how unknown disturbance can be rejected actively in the absence of detailed mathematical model of the process. Disturbance rejection ability and stability are analyzed using the language of frequency response engineers are familiar with and, for the sake of simplicity, the discussion is limited to the first-order plus time delay (FOPTD) systems. The concept of delay margin is proposed to facilitate engineering design. A chemical reactor concentration control problem is used to provide the insight of this particular disturbance rejection strategy. Comparative simulation study shows a substantial improvement in the performance over a couple of existing methods seen recently in the literature.