Internal model control structure using adaptive inverse control strategy Muhammad Shafiq* Systems Engineering Department, King Fahd University of Petroleum & Minerals, Dharan 31261, Saudi Arabia Received 5 March 2003; accepted 19 November 2004 Abstract In this paper, we propose a new adaptive internal model control scheme based on adaptive finite impulse response filters. This scheme provides the same design procedure for both minimum and nonminimum phase plants. The plants under consideration may contain time delay. The tracking objective is accomplished for both invertible and noninvert- ible stable plants. The internal model of the plant and its inverse are estimated by recursive least-squares and least- mean-squares algorithms, respectively. The closed loop is designed such that the system from the reference input to the plant output can be approximately represented by a pure delay. The effect of the process zeros on the output is compensated by using adaptive finite impulse response filters. This avoids the cancellation of noncancellable zeros of the plant and forces the plant output to track the reference input with a delay. The stability of the closed loop for both minimum and nonminimum phase systems is guaranteed. Computer simulation and laboratory scale experimental results are included in the paper to demonstrate the effectiveness of the proposed method. © 2005 ISA—The Instru- mentation, Systems, and Automation Society. Keywords: Adaptive control; Internal model control; Nonminimum phase systems; Real time 1. Introduction Internal model control IMC structure has long been successfully used for controlling open-loop stable plants. Most of the industrial processes are open-loop stable. The IMC structure is composed of an explicit model of the plant and a stable feed- forward controller. The IMC controller guarantees the internal stability of the closed loop and param- eters of the controller can be easily tuned online without disturbing the stability of the closed-loop system 1. Asymptotic tracking in the IMC structure is achieved by incorporating the inverse of the plant model in the feed-forward path. But, the inverse of the nonminimum phase plant is unstable. The use of this inverse in the IMC control loop gives rise to instability in the system. There are many plants with nonminimum phase behavior, such as dc mo- tors with field regulation, blast furnaces, hydraulic pumps, distillation columns, and so on. In this situation it becomes very important to obtain the stable inverse of the plant model to use in the IMC scheme to accomplish the tracking objective. When the plant parameters are not exactly known or changing slowly, then IMC controllers can be designed online using adaptive control strategies 2. The discrete-time model of the plant is identified online and then stable controllers are designed 3. The effect of numerator polynomial in the discrete-time model of the plant can be compensated by approximate inverse systems 4. The identified discrete-time transfer function usu- ally has zeros outside the unit circle and so the *E-mail address: mshafiq@ccse.kfupm.edu.sa ISA TRANSACTIONS ® ISA Transactions 44 2005 353–362 0019-0578/2005/$ - see front matter © 2005 ISA—The Instrumentation, Systems, and Automation Society.