pid with transfer function

[comerford1]

hi i have a simple program that deals with pids and a transfer function.
basically i want to control a transfer function with a pid and try and
tune
the response. i am having difficulty closing the loop. when i connect
the
output of the transfer function to to process variable it brakes the
wire.
is it hard to close the loop?? i hope the file will open for you. if it
does vcould you send back an ammended version. this would be great
help.

[LocalDSP]

When you simply close a loop in your diagram your wire becomes member of a cycle. The LabVIEW error you get if you try to run the VI says:
"These wires form a cycle, making two parts of the diagram interdependent, so each must wait for an input from the other and neither can execute."

You are dealing with a sample system and not (like an analog system) with a continuous system. So an output value fed back to the input can only be used in the following iteration.

The solution: Your system need to be included in either a While or a For loop (if it is not already the case) and then you need to create a shift register SR (right click on the edge of your structure and add shift register). Connect the output of your system to the right terminal of the SR and the f
eedback input to the left terminal of the SR.

Note again that you are then dealing with a sampled system (more like a z-domain model) so if you are trying to simulate an analog loop (s-domain) you'll see some variations from what you may expect the higher the system bandwidth is. Make sure to sample with s sufficient "time" resolution.

[Jeremy_L]

Hi Comerford1,

Here is a link to an example PID Controller program that may help.

Jeremy L.
National Instruments