* Correspondence to: Manuel A. Duarte-Mermoud, Department of Electrical Engineering, University of Chile, Av. Tupper 2007, Casilla 412-3, Santiago, Chile. E-mail: mduartem@tamarugo.cec.uchile.cl Contract/grant sponsor: CONICYT; contract/grant numbers: FONDECYT 1950502 and FONDECYT 1970351 Received 7 December 1999 Revised 8 March 2000 Copyright  2002 John Wiley & Sons, Ltd. Accepted 1 February 2001 INTERNATIONAL JOURNAL OF ADAPTIVE CONTROL AND SIGNAL PROCESSING Int. J. Adapt. Control Signal Process. 2002; 16:85}106 (DOI: 10.1002/acs.674) TECHNICAL NOTE Experimental evaluation of combined model reference adaptive controller in a pH regulation process Manuel A. Duarte-Mermoud * - , Franklin A. Rojo and Ricardo Pe H rez Department of Electrical Engineering, University of Chile, Av. Tupper 2007, Casilla 412-3, Santiago, Chile Department of Chemical and Bioprocess Engineering, P. Catholic University of Chile, Av. V. Mackenna 4860, Casilla 306-22, Santiago, Chile SUMMARY An experimental evaluation of the combined model reference adaptive control (CMRAC) is presented in this paper. This adaptive control scheme was used to control a relatively complex process like the pH of a solution in a tank reactor at laboratory level. For comparison purposes, some very well-known control strategies were also implemented, which include PID control and standard model reference adaptive control (MRAC). Tracking and regulation capabilities of the control strategies studied were analysed and compared. Experimental results indicate that CMRAC behaves as well as the standard MRAC and a very well-tuned PID for a speci"c and known operating point. Advantages of the adaptive controllers are shown when the operating point changes. Copyright  2002 John Wiley & Sons, Ltd. KEY WORDS: pH control; process control; PID control; model reference adaptive control; combined model reference adaptive control 1. INTRODUCTION The combined model reference adaptive control (CMRAC) is a relatively new control strategy that has been completely studied from the theoretical point of view [1}3] but only few applica- tions have been reported. In particular, the application of the CMRAC to a pH control process has not been reported. This study is important since it will show new directions in which theoretical analysis of the method should go in order to apply adaptive control at the industrial level in a more accurate and con"dent fashion.