Cascade or not?

Hi Guys

I need your help, I have inherited a Siemens Plc program from a colleague one of the key features of this system is PID loop controlling an air flow, that Works ok, the problem is that we are also monitoring a gas concentraction, when this value exceeds the setpoint the flow needs to raise to get the concentration down.

Any suggestions?

Br
Brian

This is a situation where cascade control may be worth the extra complexity to implement and tune the loops. First, you likely have a much faster response (smaller time constant) on the inner, flow control loop. Furthermore, depending on the control valve, you may be able to linearize the response, or at least hide non-linear behavior to the concentration loop. Since I would expect the concentration response to be somewhat linear to flow rate, you may be able to more aggressively tune the cascaded outer loop; as opposed to directly controlling the possibly less-well-behaved valve directly from the concentration measurement. Plus, in your case, you have a history of controlling flow, and the cascaded arrangement allows reverting to flow control by putting the outer loop in manual. Sounds like a win-win.

If you're interested in controlling the concentration, then cascade might be the way to go.
If you're interested in merely avoiding a high concentration limit, then override control (aka constraint control) would be more suitable.
The latter is reasonably straightforward to achieve if your PID has an external reset feedback option.
I'm not familiar with Siemens...