Multiple PID Loops (PLC)

[506818]

I have an air flow into a system which is mixed with a separate gas. Each the air and gas has its own control valve. Once mixed, the flow then goes to 10 individual lines running in parallel each line has its own control valve to adjust flow down each line. There is then an excess line where remaining/not required air can be expelled. I'm a novice here - what's the best approach for controlling this? As I understand, the various PID loops will conflict and would be difficult to control.

 

[danw2]

What are you trying to control? Mix concentration? flow rate from each drop?

Whatever parameter you are trying to control, you need an instrument to measure that variable, because that's what PID uses as its input.

 

[506818]

Yes it's flow rate for each line that's being controlled and there will be a flow meter for each valve to PID control.

I want to understand how the multiple loops will interact and what the best practice would be via the PLC code.

 

[xnuke]

Sounds like you might want a ratio control loop using measured gas concentration downstream of the mixing point to control the air added with the gas as an uncontrolled variable to get the right mixture. There are multiple ways to do this, e.g., measure gas flow and ratio the measured air flow.

Trying to control load flow and source flow usually results in controllers fighting. You may want a pressure control loop to control the mixture pressure into the line manifold that is a master loop to the ratio loop - you may even want to add a mixture buffer tank that you maintain pressure in at a constant value unless you are willing to allow a really slow pressure control loop so it doesn't fight the flow control loops. This buffer tank would go between the mixer point and the point where the first line taps off. Hopefully you have a good process engineer to work this out with, because if the process isn't designed in a way that is controllable, nothing you do in software can control it.

Then you probably want separate flow control loops with measured flow on each of the ten individual lines.



xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.