Help with basic algorithm on LabVIEW

[Greg McKaskle]

It makes much more sense now. This is why we still program computers with code, and not descriptive paragraphs in a forum.

LV executes as a synchronous dataflow push language. Since it is synchronous, it means that nodes synchronize their execution and data production. It means a subVI will not produce its outputs and write to its caller's wires until it completes. It also will not begin until all of its inputs have arrived. In your case, this means that the IFT runs once each time your PID and model run. Its internal loops run to completion, and return the last value in the loop each time it is called.

What to do about this. You have two options. I'll describe both and let you decide which direction to go.

Option A is to merge the IFT loop and state with the loop in the caller, Main. Assuming your setpoint will change more slowly than your PID and other elements will execute, you will continue calling IFT each iteration, and occasionally it will change the setpoint, advancing it to the next value in the sequence. It will need something to trigger off of, and I think time is the appropriate choice here. You can use the coputer clock and have each of the subVIs read the clock and make changes as time advances. It will be somewhat more flexible if you move from computer time to an abstract system which you advance independent of the computer clock. This would be necessary if your system responds on a very different time scale than the computer -- simulation of an ice age or tokamak reactor for example.

The second choice is to move some of your dataflow from synchronized wires into unsynchronized storage such as a global variable. If you take the setpoint, and perhaps MV --since this doesn't change, I'm not sure what to do with it -- and make a global variable called setpoint, you can now have unsynchronized loops access the setpoint. Your Main program now has two loops, the one which calls PID and model using the current value of setpoint, and the independent loop that updates setpoint to perturb the system. You may also find it useful to grab the setpoint and change it by hand to see how the system responds. You'd do this by either opening the global panel or making your Main panel have a mode where you write a panel value to the global untimed.

I think that this less synchronized approach may work fine for you, and possibly be easier to program. It introduces globals, but LV makes parallel access to globals quite safe. If the loop time increases or you want more guaranteed synchronization, the merged loop is better in that regard.

Hope this helps.
Greg McKaskle