plc.ucd
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I was hoping you could shed some light on the PID instruction. I have done a lot of research but I still have some gaps in my understanding.
Here is the situation:
We have 2 lines that are mixing fluids of different viscosity at a junction. The liquids are then blended via a static mixer and sent to the filler stations. One of the liquids is in a large vat, and the other liquid is in totes that are pressurized. The totes are swapped out every couple hours. At the end of the day, the lines are CIPed.
Here is how the program works
Each flow is controlled by a PID loop. The operator sets the flow rate for the liquid coming for the large vat, and a ratio value that will calculate the addition of the second liquid.
The problem that is occurring is in the morning when both lines have been cleaned, and the CV has been zeroed (via MOV instruction). The liquid pumps are both primed to verify that there are satisfactory flowrates. When the PID instruction begins to execute, ~ 20 gallons of product must be dumped because the PID takes awhile before settling down. During tote exchanges the PID loop settles down extremely quickly. It is like it starts up where it left off.
I have tried forcibly changing the pump % to a value that is close to the operating condition only during startup, and then hoping that when the PID takes over it will need little adjustment. The flowrates get into the ballpark this way, however once the PID loop takes over, the flowrates vary pretty drastically until it is finally settled in.
I have the PID instructions in a periodic task that scans every 250 ms. I really appreciate the help, please let me know if you need more information. I have also played with the tuning constants, but I haven't had much luck. I can post the values as well.
I was hoping you could shed some light on the PID instruction. I have done a lot of research but I still have some gaps in my understanding.
Here is the situation:
We have 2 lines that are mixing fluids of different viscosity at a junction. The liquids are then blended via a static mixer and sent to the filler stations. One of the liquids is in a large vat, and the other liquid is in totes that are pressurized. The totes are swapped out every couple hours. At the end of the day, the lines are CIPed.
Here is how the program works
Each flow is controlled by a PID loop. The operator sets the flow rate for the liquid coming for the large vat, and a ratio value that will calculate the addition of the second liquid.
The problem that is occurring is in the morning when both lines have been cleaned, and the CV has been zeroed (via MOV instruction). The liquid pumps are both primed to verify that there are satisfactory flowrates. When the PID instruction begins to execute, ~ 20 gallons of product must be dumped because the PID takes awhile before settling down. During tote exchanges the PID loop settles down extremely quickly. It is like it starts up where it left off.
I have tried forcibly changing the pump % to a value that is close to the operating condition only during startup, and then hoping that when the PID takes over it will need little adjustment. The flowrates get into the ballpark this way, however once the PID loop takes over, the flowrates vary pretty drastically until it is finally settled in.
I have the PID instructions in a periodic task that scans every 250 ms. I really appreciate the help, please let me know if you need more information. I have also played with the tuning constants, but I haven't had much luck. I can post the values as well.
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