Multi-Tank leveling(flow control) with PLC5

[Tom Jenkins]

One thing to keep in mind is that flow through a ball valve is not linear with % open. This will affect your tuning, and you may have to live with slow response to avoid hunting.

 

[KNeland]

Here’s something you might consider.

Say you’ve tank A and tank B with respective levels and valves.
Make a split-range PID with output as follows:
0% (A 100% open, B 0% Open), 25% (A 100% open, B 50% Open), 50% (A 100% open, B 100% Open),
75% (A 50% open, B 100% Open), 100% (A 0% open, B 100% Open)
The input to the PID is (level B – level A) with a range of -100 to 100, action is direct acting. Tune this with mostly proportional action and a very small reset value.
Nominally, you’ll want a setpoint of 0.
Let us know what happens

This sounds good to me, however I am not sure of how to go about making a "split-range PID". I understand conceptually what your saying, and it actually is straightforward. But how to get one PID (PD) to output control to 2 different valves...I have an analog 4-20 output to control each valve. SO it is a matter of getting,...what...say 0 to -100 to equal the 4-20ma of one of the valves...hmmm

 

[jamesau]

B%Open = Min(2*PID_output, 100)
A%Open = Min(2*(100 - PID_output), 100)
...and you'll have a little more manipulation for fail-open...

Heed Mr Jenkins' warning... maybe can read up on what action positioner is used... characterizers are an option but a bother...

I do like the mechanical solution posted by others.

Goof luck and let us know what happens