Can someone clarify the meaning of the output from the NI PID vi's? From the documentation for the PID control toolkit, it appears that the output is an absolute quantity and not a delta (change) to the previous output. The problem I have is that if I am running proportional control only, and the controller is moving into a steady cooling air flow rate to achieve my setpoint temperature, the error goes to zero and the output from the controller drops to zero. This completely ruins the temperature control.
Simply: assume that the output to the control valve I am using needs 12mA (4-20mA controlled) constant value (flow rate) to maintain my setpoint. When the error goes to zero, the controller output goes to zero. This, of course, leads to terrible oscillation etc.
Other PID implementations I have used are all delta (change) based. In other words, you calculate a P, I, and D correction to the previous output.
What am I missing? All of the NI labview PID examples use the output directly to drive I/O, and never use it as a change (delta).
I read this newsgroup's archives and found a similar question, but the answer was not helpful.
thanks much in advance!
