Automating the initialisation of relay autotuning using control performance monitoring

Albertus, Grant John (2020-03)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

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 size’s (PSE) variance was selected as a benchmark to which the current control performance was assessed. Defined as the PSE’s variance at a period of good controller performance without the presence of faults, the 90th percentile of the PSE (𝜎𝑡ℎ𝑟𝑒𝑠ℎ𝑜𝑙𝑑2) 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 (𝜎𝑃𝑆𝐸2) was persistently larger than the 𝜎𝑡ℎ𝑟𝑒𝑠ℎ𝑜𝑙𝑑2. As a result, the relay start time was defined as the allowable time 𝜎𝑃𝑆𝐸2 is above 𝜎𝑡ℎ𝑟𝑒𝑠ℎ𝑜𝑙𝑑2 before relay autotuning is initiated. In addition to 𝜎𝑡ℎ𝑟𝑒𝑠ℎ𝑜𝑙𝑑2, 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.

AFRIKAANSE OPSOMMING: Relê outo instemming is ’n geoutomatiseerde prosedure wat akkurate instemmingsparameters vir die proseskontroleerder verkry wanneer nodig. ’n Geoutomatiseerde prosedure vir beheerderinstemming sal verkieslik wees in industrie as gevolg van die swak beheerderrespons wat volg uit verkeerde handinstemming van beheerlusse. Dit is egter nie bekend in industrie hoe relê outo instemming geïmplimenteer moet word nie en huidige literatuurstudies adresseer nie hierdie probleem breedvoerig nie. Daarom is die doel van hierdie navorsing om ’n beheer doeltreffendheid monitering (CPM) -tegniek wat relê outo instemming inisieer as gevolg van verkeerde instemmingsparameters, te bepaal. Verder, sleutelparameters en faktore geassosieer met relê outo insteming is geëvalueer op so ’n manier dat ʼn robuuste prosedure voorgestel kan word. Om realistiese proseskondisies te simuleer, is ’n meulstroomsimulasiemodel met steuringe en sensorgeraas gebruik as ’n gevallestudie. Historiese normstelling is geïdentifiseer as ’n gepaste tegniek om die relê outo instemming prosedure te inisieer. Die produkpartikelgroote (PSE) se variansie is geselekteer as ’n norm waarvolgens die stroombeheerderdoeltreffendheid geassesseer is. Gedefinieer as die PSE se variansie by ’n periode van goeie beheerdoeltreffendheid sonder die teenwoordigheid van foute, is die 90ste persentiel van die PSE (𝜎𝑡ℎ𝑟𝑒𝑠ℎ𝑜𝑙𝑑2) by normale bedryfskondisies (NOC) gebruik as die historiese norm. Deur die norm te gebruik, is die stroombeheerder se doeltreffendheid geassesseer as swak as die variansie van die PSE (𝜎𝑃𝑆𝐸2) aanhoudend groter as die 𝜎𝑡ℎ𝑟𝑒𝑠ℎ𝑜𝑙𝑑2 was. As ’n gevolg is die relê begintyd gedefinieer as die toelaatbare tyd 𝜎𝑃𝑆𝐸2 oor 𝜎𝑡ℎ𝑟𝑒𝑠ℎ𝑜𝑙𝑑2 voor relê outo instemming geïnisieer is. Meer as 𝜎𝑡ℎ𝑟𝑒𝑠ℎ𝑜𝑙𝑑2, het die historiese normstellingtegniek ’n seleksie van beweging variansie lengte nodig gehad. Vir die projek is die keuse van ’n een uur skuifvenster geassesseer as gepas, omdat dit die ossillasies wat voorkom kon waarneem. Die swak beheerderdoeltreffendheid is waargeneem as ’n verhoging in die bewegende variansie oor tyd. Om die kondisies van die swak beheerderdoeltreffendheid en verkeerde intemmingsparameters te simuleer, is klepdegradasie op die PSE beheerlus geïmplementeer. Die klep is verander van liniêr na vinnige openingskarakteristieke. Sonder om die beheerder oor in te stem, was daar ’n aanhoundende verhoging in variansie as gevolg van die stroom instemmingsparameters wat te aggressief was. Daarom is ossillasie binne die meulstroom geproduseer. Die relê outo instemming prosedure is geëvalueer as voordelig in die verlaging van variansie as klepdegradasie geïmplementeer word. Daarom kan relê outo instemming foute verminder wat ossillasies in die proses veroorsaak as die oorspronklike instemmingparameters te aggressief is. Daarby is sleutelparameters geassesseer vir industriële toepassing. Die relê-amplitude is voorgestel om die kleinste waarde moontlik te wees om die histerese band te oorkom. Verder, ’n kleiner relê-amplitude verminder die uitgesitte instemmingsparameters waargeneem by laer sensorgeraasvlakke. Met respek tot die historiese normstellingtegniek het die vroeër inisiëring van die relê outo instemmer beter beheerderdoeltreffendheid tot gevolg gehad. Laastens, deur die omvang van klepverwering te varieer, is dit gewys dat herinstemming nie noodsaaklik is vir klein grade van klepverweer nie. Ten spyte van die verbeterde beheerderdoeltreffendheid, is die ekonomiese doeltreffendheidassesseering van relê outo instemming en sleutelparameters onbeslis.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/108236
This item appears in the following collections: