ETF Approach To Analysis And Design For Multivariable Control Systems

As most of the industrial processes can be described as multivariable systems, the research of multivariable control systems is one of the hotspots in industrial process control area. The two commonly used strategies are decentralized control and decoupling control, which are easy to operate on the spot and easy to understand by the local engineers. As for the processes with slight coupling, decentralized control is competent to get satisfactory control results after the optimal paring solution is selected. Decoupling control mehod is more perferable to the decentralized control for the processes with serious coupling. How to handle the interactions among loops is always a headache question for the control engineers. In this thesis, several effective multivarible control strategies are proposed, which is based on the ETF therories and classical PID control technique. It includes:1) The concept of equivalent transfer function for multivarible processes is introduced. Furthermore, the important property of ETF and its parameterization methods are also presented. A brand-new approach for high-order ETF model parameterization is involved, which is based on the open-loop charateristic sequence. The relationship between the open-loop charateristic sequence and charateristic sequence of the ETF matrix is derived. The parameters of ETF models are uniquely determined by solving the equation sets about the charateristic sequence. In addition, the parameteriazation methods for some special multivariable processes are also involved.2) A ETF based centralized PI/PID controller design method for multivariable processes is presented, in which each element of the controller is designed individualy for corresponding ETF element. The control target of multivariable process can be attained simultaneously when the control targets of the single variable processes are achieved. Thus, centralized PI/PID controller can provide better overall control performance , particularly compared to the decentralized PI/PID controller.3) A systematic ETF based decoupling control scheme is brought up, which can be divided into one-piece type and split type according to the different control structure.The one-piece decoupling controller can be derived by the desired closed-loop transfer fucntion matrix and the ETF matrix, while the split one is composed of two parts: the decoupler and multiloop PI/PID controller. By using the approximation relationship between ETF matrix and the inverse matrix and selecting the desirable forward transfer function matrix after decoupled, the decoulper will be derived directly. After that, the decentralized PI/PID controller is easily designed for the desirable transfer function matrix by available single loop controller design methods.4) A novel ETF based partial decoupling control strategy is raised. The obvious difference from the former one is the structure of decoupler. By measuring the effect of each ETF element in decoupling control, a partial decoupling control structure selection criterion is proposed with the aim for the decoupler to have least complexity and satisfied overall system performance. Besides, the possible negative effect caused by the approximation between ETF matrix and the inverse matrix is also considered. To gurantee the reasonability, the ETF parameters are obtained under certain constraints.5) Two experimental tests are designed to verify the effectiveness of ETF based decoupling control methods. The controlled plants are respectively double-level control system and 4-room constant temperature control system. The results of both mathematical simulation and the experimental test demonstrate that the ETF based decoupling control strategies are simple and effective.