A Unit Operations Laboratory Experiment Combined with a Computer Simulation to Teach PID Controller Tuning

Abstract

Controller tuning is important for chemical engineering practitioners but a laboratory exercise tolearn tuning methods can be time consuming and taxing on the equipment. Alternatively, apurely equation-based, virtual controller tuning exercise might not be interesting or seemrelevant to real world processes. We developed and implemented a laboratory experiment thatcombines a physical process with a computer model to teach the practical aspects of PIDcontroller tuning. The computer model allows students to run virtual experiments to discover theeffect of changing each control parameter and to test various controller tuning methods. Thevirtual experiments are tied to reality and learning is strengthened by applying the knowledgegained to control the physical experiment.The objective of the experiment is to control the process temperature inside a jacketed, well-stirred reactor vessel upon a change in set point. The temperature of the circulating fluid throughthe jacket around the reactor is adjusted using a PID controller with a temperature bathcontaining a heater and a refrigeration unit.A computer model that captures all the necessary physics was developed using COMSOLMultiphysics software. The model includes a circulation hose connecting a reaction vessel to atemperature bath. The circulation hose is made continuous by connecting the outlet to the inletthrough a periodic boundary condition. The fluid flow is modeled with the k- turbulent flowequations. The heat input to the process is simulated as a volume heat source inside thetemperature bath and is adjusted to drive the process temperature to the set point using thestandard proportional, integral, derivative control equation. The fluid flow equations are firstsolved in a stationary study using an average temperature. The resulting velocity field is thenassumed to be constant in a time dependent study of the heat transfer.The output for both the simulated and physical experiments includes temperature responsecurves for the circulating bath, jacket, and reactor and the integral absolute error providing ameasure of the deviation of the process temperature from the set point.Students studied the effect of changing control parameters and tried to minimize the integralabsolute error in the simulation and the physical process. Student satisfaction and learning wasevaluated using a multiple choice test before and after the lab exercise and collecting a survey atthe end of the course. There was marked improvement in the number of correct answers to thetest after the lab and the survey indicated that the students were pleased with a balanced use ofsimulation and physical experiments. Some students particularly liked the friendly competitionof trying to find the lowest integral absolute error and the use of the COMSOL model. Combingcomputer simulation with a physical experiment appears to be interesting, enjoyable, andeffective in teaching practical aspects of process control.