Decentralized PID Controller Design for a MIMO Evaporator Based on Colonial Competitive Algorithm Ramin Rajabioun, Farzad Hashemzadeh, Esmaeil Atashpaz-Gargari, Bahman Mesgari, Farzad R. Salmasi Control and Intelligent Processing Centre of Excellence (CIPCE), School of Electrical and Computer Engineering, University of Tehran, Tehran, Iran (r.rajabioun@ece.ut.ac.ir, f.hashemzadeh@ece.ut.ac.ir, e.atashpaz@ece.ut.ac.ir, b.mesgari@ece.ut.ac.ir, farzad_rs@ieee.org ) Abstract: Recently, Colonial Competitive Algorithm (CCA) has proven its superior capabilities, such as faster convergence and better global minimum achievement in optimization problems. In this paper, CCA is utilized to optimize the coefficients of a decentralized PID controller for a MIMO evaporator system. The optimization criterion is considered as the Integral Absolute Error (IAE) to minimize the tracking error. As the first step, the evaporator’s three input-three output transfer matrix is identified using measured dataset based on the prediction error model method. In order to design decentralize controllers, input-output pairing is performed based on the relative Gain Array method. Decentralized PID controllers are then designed using Ziegler-Nichols, Genetic Algorithm, and the proposed CCA techniques. The simulation results verify the superiority of CCA to the Ziegler-Nichols and Genetic Algorithm tuning techniques for decentralized PID controllers. 1. INTRODUCTION Proportional-integral-derivative (PID) controller has been the most popular control loop feedback mechanism since 1950s, and has been extensively used in controlling industrial plants. In addition to its capabilities, PID can be implemented easily in industrial control processes. PID controller tries to correct the error between the measured outputs and desired outputs of the process so that transient and steady state responses are improved as much as possible. Although it is used widely, PID tuning is still an area of research in realm of both academic and industrial control engineering, while different methods have been proposed (Bao et al., 1999 : Chidambaram & Sree, 2003 : Lee et al., 2004 : Wang et al., 1998). Among them, Ziegler-Nichols tuning method is considered to be a very popular approach (Ziegler and Nichols, 1942). One the other hand, controlling Multi Input Multi Output systems is not straight forward due to the coupling and interactions between channels. In last several decades, designing controllers for MIMO systems has attracted a lot of research interests (Christen et al., 1997 : Roffel et al., 2000 : Su et al., 2007). Among the proposed MIMO controllers, decentralized PID controllers have been deployed extensively due to their less complexity, high performance and easy implementation (Halevi et al., 1997 : Su et al., 2007 : Xiong et al., 2007). In this paper, Colonial Competitive Algorithm (CCA) is used to tune parameters of a decentralized PID controller for a MIMO evaporator. First, the model of the evaporator is identified based on measured data. The plant is a four-stage three input- three output industrial evaporator which is used to reduce the water content of products like milk. The plant inputs are feed flow, vapour flow to the first evaporator stage, and cooling water flow. The three outputs of the evaporator are the dry matter content, the flow, and the temperature of the out coming product. Measured Input/Output data used for identification of the plant parameters is taken from (URL1). Prediction error model (Nelles, 2001) is used to identify the transfer matrix parameters of the plant. In order to design decentralized controller, the appropriate pairings among inputs and outputs are chosen to weaken the interactions between channels. Then a decentralized PID controller is designed using Ziegler-Nichols method for the identified model. The determined PID coefficients are used as the initial estimate of two evolutionary optimization algorithms, Genetic Algorithm (GA) and Colonial Competitive Algorithm (CCA), (Atashpaz-Gargari & Lucas 2007 a, b : Biabangard-Oskouyi et al., 2007). CCA is a novel global search strategy that uses the socio-political competition among empires as a source of inspiration. Like other evolutionary ones that start with an initial population, CCA begins with initial empires. Any individual of an empire is a country. There are two types of countries; colony and imperialist state that collectively form empires. Imperialistic competition among these empires forms the basis of the CCA. During this competition, weak empires collapse and powerful ones take possession of their colonies. Imperialistic competition converge to a state in which there exist only one empire and its colonies are in the same position and have the same cost as the imperialist. The simulation results show the superiority of the optimized decentralized PID controller using CCA, to the Ziegler-Nichols and GA approaches. Proceedings of the 17th World Congress The International Federation of Automatic Control Seoul, Korea, July 6-11, 2008 978-1-1234-7890-2/08/$20.00 © 2008 IFAC 9952 10.3182/20080706-5-KR-1001.1096