| Summary: | 博士 === 國立臺灣大學 === 化學工程學研究所 === 95 === The multi-loop controller and multivariable controller have been used in the control of multi-input-multi-output chemical plants. The simple controller structure and the easiness to handle loop failure are the most attractive advantages of multi-loop control. Because of loop interactions, the design of such controllers is more difficult to meet specifications. Theoretically, loop interactions can be eliminated by making use of multivariable controller. However, most research works are focused on one controller (i.e., a multi-loop or a multivariable controller) and one design objective (i.e., set-point tracking or disturbance rejection). In this thesis, a systematic design method is proposed to design generalized controllers for multi-input-multi-output systems.
For disturbance rejection, a new design of controllers based on the synthesis approach is proposed for single-loop systems. The controllers are derived for several common types of process models including one that contains right-half-plane zeros. In multi-loop control systems, an effective disturbance is derived to consider the total effects of a load input on a process output. Then, a muli-loop control system can be treated as several equivalent single-loop systems with equivalent open-loop processes and effective disturbances. Similarly, in multivariable control system, an inverse-based decoupler is given to decouple the system into several individual single-loop systems. The design of this inverse-based controller emphasizes on a systematic procedure to obtain physically realizable controllers for practical implementation. By the equivalent single-loop systems in multi-loop control and decoupled open-loop process in multivariable control, the method of controller synthesis in single-loop systems can be applied to both control systems.
For set-point tracking, most methods have been proposed to design the multi-loop controller for stable processes. Actually, some chemical units are integrating processes. An extension is presented to design a multi-loop PI/PID controller for a transfer function matrix whose some elements having pure integrators. Moreover, a modified RGA is given to overcome the difficulty encountered in computing the RGA of an integrating process for loop pairing.
To achieve both control objectives, a three-element multivariable control system with two-degree-of-freedom is proposed. Among the three elements, one in the main loop is designed for rejecting disturbance, and the other two that serve as pre-filter and dynamic preset are devised for set-point tracking. For set-point tracking, the two elements are designed to have a dead-time compensated response as that of a Smith predictor. For disturbance rejection, the theoretically and practically achievable performances are assessed in both controls and indices are defined to compare their control performances. Then, these indices are extended to select the structure of controller in multivariable control systems. Finally, a systematic approach is proposed to design the generalized controller for multi-input-multi-output processes.
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