PID-Control (Tank Level)

[joooh]

Hi!

I want to realize a PID-control for a tank level. I've already looked at all related examples in LabVIEW, but I have a general question:

Where is the measured value brought into the process (where would I connect the DAQ to the control-loop)?? I can't figure out how it's done in the examples (there is always an "initial value" outside the while-loop)....

Would be great to get some answers!

[nathand]

Generally you take the DAQ value and feed it into the PID as the process variable.  The examples store the process variable in a shift register in order to simulate a system in which the current value depends on the previous value, but in a real system the physics of the system handle that for you.  For example in a tank control system, the tank itself holds some amount of liquid, which doesn't change by a large amount within one sample period.  To simulate that properly you need to store a value equivalent to the liquid volume in a shift register.