UBC Theses and Dissertations
Performance assessment of feedback controller output and process output Ye, Rebecca Yan
Performance assessment tools are very useful in providing information about the control loop performance. To maintain high process performance, effective methodologies to measure the actual performance of the control loops are urgently needed.. -Since the industry normally has a fixed set-point for each loop, most of the control action is to reject disturbance, i.e. regulatory control. The most popular performance index for regulatory control nowadays is the Harris index due to it's simplicity of usage. The Harris index requires only routine operating data and prior knowledge of the process delay. It compares the variance of the process output with the benchmark of minimum variance. An application of the Harris index to PID controller tuning is discussed in this thesis. A new input/output index (I/O) is developed in this thesis. It requires more knowledge about the process model than the Harris index, but provides more information about the performance of a closed-loop system. The key feature of this //0 index is that the user can specify a benchmark which considers the balance between process output variance and the controller output variance. However, in this case, the process model needs to be identified so that the upper bound of the controller output variance and the lower bound of the process output variance can be obtained to set the benchmark for thorough performance assessment of the control system. In industry, during closed-loop control, both controller output and process output variances are desired to be as little as possible. Though Minimum Variance Control (MVC) may minimize the process output variance, it is not desirable in many cases due to excessive control action and sensitivity to changes in process dynamics. Therefore, balancing the process output and process input variances becomes very important. This new performance index uses the upper bound of the controller output and lower bound of the process output from MVC. In fact, it can indicate graphically the optimal operating point for each loop. Hence, this I/O index is more practical and reliable. An extensive simulation study was performed to examine the operation, usefulness and limitations of the performance indices. These simulations confirm the benefit of using the new I/O index for assessing regulatory control.
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