Product integrated control system
Project summary:
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A prepackaged domestic hot water system that uses a plate heat exchanger to heat up domestic water to a user selected temperature setpoint. One side of the heat exchanger is to connect to a hydronic loop while the other side connects to the cold supply of city water. The control loop of this prepackaged product consists of a controller, temperature transmitters (or RTDs), and a very accurate electro-hydraulic actuator. The controller is continuously in communication with the outlet temperature transmitter located on the cold side as well as the actuator located on the hot side. By receiving temperature measurements from the temperature transmitter, the controller sends correction signals to the actuator, which is continuously regulating the amount of heat entering the heat exchanger.​
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In light of rapid flow/load fluctuations, the domestic water outlet temperature had to stabilize to a maximum of +/-2°F of setpoint, which was unheard of in the industry. ​​
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​Challenges:
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The control loop needed to stabilize the water outlet temperature down to a very tight tolerance which was a challenge even if the volumetric flow of water was steady.
The control loop not only needed to keep the outlet temperature within a very tight tolerance in steady state, it also needed to keep it from overshooting/undershooting during sudden volumetric flow changes of water (domestic load).
Traditional feedback PID loops rely on providing feedback to the loop once disturbance has already taken place. A method which inherently leads to a delay in generating a response by the control system. The controller and the user interface also needed to have many user configurable features: Modbus communication, data logging, alarms, I/O scaling, and etc.
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​​​​End result:
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By use of a PLC equipped with an HMI, a hybrid PID control system was designed such that it incorporated both feed-forward and feed-back signals. Through lots of experimenting and analysis, PID parameters were fine tuned such that in steady state, the control system could keep the outlet water temperature within +/1°F of user selected setpoint which was even lower than the initial design criteria.
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The main challenge was to keep a tight tolerance when there was a sudden volumetric flow change (sudden domestic load drop), a situation that could arise in the field. A sudden drop in the domestic load could cause large temperature overshoots that could cause people using the domestic water getting burned. A feedback-only controller could not have the actuator close down fast enough before seeing a sudden temperature spike from the outlet temperature transmitter.
The above problem was solved by implementing a special feed-back feed-forward PID loop and inventing a very unique self-adjusting algorithm. After a few months of developing and testing this novel control system and algorithm, the results turned out impressive. With this invention, in case of a 50% load drop in less than 10 seconds, the domestic outlet temperature did not rise by more than 2°F from the setpoint. This invention is now patented.