Relay Auto-tuners in Modelica

Sammanfattning: The PID controller is by far the most popular controller for process control in industry today [Åström and Murray, 2008]. The combination of its relatively simple usage and its satisfactory results within several areas has gained its popularity [Visioli, 2006]. However, the tuning of the controller is crucial. An automatic tuning method was developed by [Åström and Hägglund, 1984] in the mid-eighties and auto-tuners has since then been diligently used in the process industry. This thesis will focus on the implementation of an auto-tuner based on asymmetric relay feedback in the modeling language Modelica. Also, creating a good work flow and providing valuable information for the user. The auto-tuner provides an estimated low order model together with parameters for a PID controller, based on the AMIGO tuning rules. After an introduction the report begins with a chapter containing a theoretical background concerning the PID controller, tuning methods, asymmetric relay autotuners and Modelica/Dymola. This is followed by an explanation of the method used, including a step-by-step description. The report ends with chapters dealing with results, discussions and comments about future development. The result chapter is divided into different sections, each dealing with one of the processes used for testing. Every section presents the experimental results together with comparisons against other parameter set-ups. The last chapter contains a discussion regarding the overall results, both in terms of usage and performance. The results, in terms of responses to load disturbances and step changes, were satisfactory for all processes that were tested in the thesis, although some processes required more preparatory work than others. As long as the process is in steadystate when the experiment begins, the resulting PID parameters turns out to work very good in comparison to the ones being used in the models today. Despite the good performance there are smaller things that needs to be further developed. For instance, a feature that is supposed to generate two plots automatically needs to be improved.