Relay Feedback Method for Process Modelling

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From a practical and simplicity point of views, the relay feedback identification is extended to the various fields of process control industries. The proposed techniques yield significantly improved accuracy over the conventional relay feedback method by deriving the explicit expressions for unknown process model parameters in terms of the output limit cycle data. A relay with hysteresis instead of an ideal relay is used in all the identification procedures to make the resultant system less sensitive to measurement noise. Next, the applicability of the relay feedback is shown by identifying nonlinear processes with static nonlinearities. On-line identification and tuning are major design objectives to achieve for certain critical applications. The modification is done by adding a relay in parallel to the controller to induce a limit cycle oscillation without breaking the closed-loop control. Based on half limit cycle data, a systematic procedure is developed to estimate a low order transfer function model. To obtain the optimum control input variations, the controller tuning rules are derived from the time weighted integral performance criteria. In addition, the necessary condition for the existence of the limit cycle for unstable processes are obtained for first and seconder order process models. Finally, a simple control strategy is given to induce a sustained limit cycle oscillation where the conventional relay feedback fails for unstable processes with large time delay. All the above benefits are achieved at no significant incremental costs in terms of implementation resources and application complexities. Both simulation examples and laboratory tests are furnished to illustrate the effectiveness of the proposed approaches.Many research work on modifying the relay feedback auto-tuning method have been reported in recent years. Still there is much room for further improvement and extensions of such methods. In this thesis, several new results are obtained that improve the accuracy of identification, relay analysis, auto-tuning and cascade control. Brief descriptions of the seven chapters of the thesis are as follows...
Supervisor: S. Majhi