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Design and implementation of process control strategies on the universal water system

Poonlua, Warunthon (2017) Design and implementation of process control strategies on the universal water system. Honours thesis, Murdoch University.

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This thesis outlines the design and implementation of several control strategies for controlling the water levels in the Murdoch University Universal Water System (UWS). The controllers were designed using four strategies: Proportional Integral controller (PI), Generic Model Control (GMC), Dynamic Matrix Control (DMC) and Fuzzy control. The UWS was able to operate either as three separate sub-systems or as a single integrated system. This thesis documents the first time that the entire plant was controlled as one system. A single-process input and single-process output was designed and implemented in the plant.

It is more difficult to run the UWS as a single integrated system compared to running the UWS in three separate system. The single integrated system had more inputs and outputs including the recycles stream. Each process cause disturbances for tanks downstream. In addition, there were system limitations such as the pump speed must be greater than 53%. The pumps and solenoid valves will trip when the water level in the tank was too high or too low. They will also trip if there is not enough water in the reservoir tank. However, with these difficulties come with many advantages. The controllers of each tank are able to be tested on disturbance rejection from inputs or outputs of different areas.

The main objectives of this thesis were to operate the plant as a single integrated system, commission the plant, implement different control strategies using LabVIEW, and test each controller on set-point tracking and disturbance rejections. The performance of each controller were evaluated and compared using Integrated Squared Error, Integrated Absolute value of the Error and Total Variation of the manipulated variable measurements. In addition, step change tests were used to confirm that the controllers track the new set points when the set point is changed and remains stable.
Commissioning was completed to confirm that the UWS was ready for the implementation of the controllers. The commissioning phase included restarting the Compact RIO controller, checking wiring connections, checking the code for errors and creating Student Templates to run the whole plant.

The pairings between manipulated variables and process variables were found by using the Relative Gain Array method to analyse which pairs were suitable and had no interaction between the processes.

The UWS contains five tanks. However, the level in three tanks were selected to be controlled in this thesis. The two of these tanks were pure capacity systems. Therefore, the PI controller was required to be tuned using a relay tuning method. The remaining tank was a first order system therefore; the PI parameters were tuned using the approximate First-Order plus Time Delay tuning rules. The GMC was implemented by creating a mathematical model of each process. All physical measurements must be in SI units, therefore; conversion equations were required. The DMC was implemented using a predictive model of the system. The Fuzzy Control was implemented by creating a large set of rules to make decisions at each event of the process.
The results of set-point tracking and disturbance rejection of each controller showed that the PI and GMC focused on minimising errors while the DMC and Fuzzy Control focused on minimising the MV’s movement.

The decision for choosing the best controller was dependant on what the highest priority for control was. The PI and GMC was selected when the set point was critical. The DMC and Fuzzy Control was recommended when the set point was not critical but still required the process to reach steady state. This strategy may make valves last longer and the maintenance cost of this controller will be reduced.

Finally, all thesis objectives were completed, and the intended purpose of the Universal Water System; to be able to implement controllers to control the UWS plant as a single integrated system, was also achieved.

Publication Type: Thesis (Honours)
Murdoch Affiliation: School of Engineering and Information Technology
Supervisor: Cole, Graeme
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