Department of Chemical Engineering

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Department Process Engineering now has a new name, and will be known from March 2023, as Department of Chemical Engineering.

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Browsing Department of Chemical Engineering by Author "Albertus, Grant John"

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    Automating the initialisation of relay autotuning using control performance monitoring
    (Stellenbosch : Stellenbosch University., 2020-03) Albertus, Grant John; Auret, Lidia; Dorfling, Christie; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: Relay autotuning is an automated procedure that obtains accurate tuning parameters for the process controller when required. An automated controller tuning procedure would be desirable within industry due to the poor controller response resultant from incorrect manual tuning of control loops. However, it is not known in industry how relay autotuning should be implemented and present literature studies do not extensively address this problem. Therefore, the aim of the research was to determine a control performance monitoring (CPM) technique that initiates relay autotuning due to incorrect tuning parameters. Furthermore, key parameters and factors associated with relay autotuning were evaluated such that a robust procedure can be proposed. To simulate realistic process conditions a milling circuit simulation model with disturbances and sensor noise was used as a case study. Historical benchmarking was identified as a technique suitable to initiate the relay autotuning procedure. The product particle sizeโ€™s (PSE) variance was selected as a benchmark to which the current control performance was assessed. Defined as the PSEโ€™s variance at a period of good controller performance without the presence of faults, the 90th percentile of the PSE (๐œŽ๐‘กโ„Ž๐‘Ÿ๐‘’๐‘ โ„Ž๐‘œ๐‘™๐‘‘2) at normal operating conditions (NOC) was utilised as the historical benchmark. Using the benchmark, the current controller performance was assessed as poor if the variance of the PSE (๐œŽ๐‘ƒ๐‘†๐ธ2) was persistently larger than the ๐œŽ๐‘กโ„Ž๐‘Ÿ๐‘’๐‘ โ„Ž๐‘œ๐‘™๐‘‘2. As a result, the relay start time was defined as the allowable time ๐œŽ๐‘ƒ๐‘†๐ธ2 is above ๐œŽ๐‘กโ„Ž๐‘Ÿ๐‘’๐‘ โ„Ž๐‘œ๐‘™๐‘‘2 before relay autotuning is initiated. In addition to ๐œŽ๐‘กโ„Ž๐‘Ÿ๐‘’๐‘ โ„Ž๐‘œ๐‘™๐‘‘2, the historical benchmarking method required selection of a moving variance length. For the project, the choice of a 1 hour sliding window was assessed as suitable, as it was able to detect the oscillations that occur. The poor controller performance was detected as an increase in the moving variance over time. To simulate conditions of poor controller performance and incorrect tuning parameters, valve degradation was implemented on the PSE control loop. The valve was changed from linear to quick opening characteristics. Without retuning the controller, there was a persistent increase in variance due to the current tuning parameters being too aggressive. Therefore, oscillations within the milling circuit was produced. The relay autotuning procedure was evaluated as beneficial in the reduction of variance when valve degradation was implemented. Therefore, relay autotuning can attenuate faults which introduce oscillations into the process if the original tuning parameters are too aggressive. In addition, key parameters were assessed for industrial application. The relay amplitude is suggested to be the smallest value possible to overcome the hysteresis band. Furthermore, a smaller relay amplitude reduces the inflated tuning parameters observed at lower sensor noise levels. With respect to the historical benchmarking technique, earlier initialisation of the relay autotuner resulted in better controller performance. Lastly, varying the extent of valve wear showed that retuning is not necessary for small degrees of valve wear. Despite the improved controller performance, economic performance assessment of relay autotuning and key parameters were inconclusive.