Friday, July 14, 2006

-3640 The Magical Mysterical number

Can somebody please tell me why on the PID loop that generates the pressure set point in the cascade for flow control seems to have an offset of -3640? If I put in that number for the setpoint, and then vary the flow about zero, then the output of the loop behaves as predicted. Like wise if i put the setpoint at -1820, and vary the flow about 1820 it also behaves as predicted.

Since I can vary the setpoint and the process variable with the same results, I must conclude that the problem is not one of them. And then I disconnected the PID from the cascade and sent the output to a dummy variable. Same thing! So the problem must be the PID, right? What could be wrong with the PID block? Its the same PID block that is working correctly in many other places ... except one, but that's a different story altogether.


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