control three interdependent things with pid control?

[Fibo]

I have 3 things (they are valves) that I need to control to set flows... but adjusting one of the three valves will affect how the others behave... basically they are all 4-20mA controls, but if one is moved the others will read something different.

My question, I'm having a hard time thinking about how to control these three things, I need to be able to tell any of the three to go to a certain flow regardless of where the others are... is PID the answer?  If so, I need to tune them one at a time, and I don't know what to set the other 2 vlaves while I tune 1... does it matter? cause, whatever PID constants I get from tuning will be dependent on what the other 2 valves are at... or is there another way?

much thanks!

[DavidMaidman]

You need to do a search on MIMO control systems (if you cant find anything useful on the internet try 'the art of control engineering, Dutton, Thompson, Barraclough) which has a whole chapter dedicated to MIMO systems.

Also, it is always a good idea to prototype/draw out your control system showing setpoints, control/correction elements and outputs etc so that you can see how loops and variables interact. Proptoype it on your labVIEW front panel and then you can start to build in the IO elements, gain settings etc.

In my expoerience drawing out your control system always makes it easier to understand.

[Fibo]

Oh really, would you say it's fairly complicated to pull this off?  I was hoping it would be a more straightforward PID control approach or something.  I looked up some MIMO stuff, looked complicated.

thanks!

[Tejinder_G.]

Hi Fibo,

I've linked an article you can read through for a quick intro to MIMO control.

Can I simulate MIMO models in LabVIEW?

However, I wonder if this necessarily needs a MIMO model.  Regardless of how one valve affects the other, if you've got a sensor on each of your valves and implement PID separately on each one, then the relationship between the 3 valves is eliminated, is it not?  We've now got 3 single-input single-output systems.

Message Edited by Tejinder G. on 03-29-2010 01:05 PM