Design of disturbance observer for non-minimum phase systems using PID controllers

Since the application of disturbance observer (DOB) approach has been limited to minimum phase systems (or systems having no zero dynamics), a new DOB structure has been proposed by the authors. The proposed DOB structure is a simple modification of the original DOB by adding a new filter in parallel with the plant. The filter, which is called as V-filter, is designed so that the parallel connection with the plant becomes of minimum phase. Thus, with the help of V-filter, the original DOB idea can be applied to such parallel connection. For the disturbance rejection performance of the proposed control algorithm, a gain condition of the V-filter has been proposed in low frequency range. In this paper, it is shown that if a given non-minimum phase system can be stabilized by a PID controller, we can easily construct the additional filter by inverting the controller's transfer function. The proposed scheme is applied to a simple non-minimum phase system, and simulation results show that substantial improvement in the performance has been achieved compared with the control system without DOB.