Help Needed with PIDs

What could happen if a PID and a valve had an "x" configuration in order to control an "x" process, but a mistake while calibrating the valve you leave it working backwards?

Think about it a little.

You have PID with SP at 50%

It has to open the valve to get to PV of 50%

What happens when PID tries to open the valve to get PV to rise, when valve fuctions backwards?

Think about other scenarios too, maybe PID has to close the valve to get PV to rise.

etc.

I got my theory that the proportional part of the PID don´t let you close the valve an probably try to reach the setpoint anyway. For sure maybe it no reach the setpoint but probably lead the valve to a closer point.

Think more,

Lets assume that you have:
-SP of 1000 l/h
-Valve is linear
-When you open valve, PV rises and vice versa.
-You start from PV of 0 l/h

In what direction does PID try to move valve in real world when PV has not reached SP?

How does PV respond to PID moving valve in that direction?

Thanks, i see your point now. In my actual need probably this wont worse because is a protection that in case of high temperature send the valva to a fixed position, so you never start from zero and that´s probably the cause that the valve do a little control despite it was backwards. Probably that´s no happen if my process has to beging from zero. You clear my doubt.

regards and thanks again

OK then think about this:

Lets assume you control your temp by some liquid witch has some temperature x, its more than you wanted product temp y. Liquid goes to heat exchanger where you heat you product to desired temp. Now you measure the product temp from output side of that exchanger.

It heats up too much, over temp y. Now the PID controlling the valve controlling the flow of heating liquid does its trick and controls that valve, now in witch direction does that valve go?

As the process variable drops and the valve is asked to open, the controller will see no change in the process variable and hence the Error will increase causing the controller to further increase the control variable to the valve.

Also if you are using Integral Action (reset), the sum of error will increase and will cause integral windup!

Hope this helps

PID Controllers have a Reverse Action setting, which must be set to the opposite of the process action
http://www.jashaw.com/pid/tutorial/pid3.html
"PROCESS ACTION
Defines the relationship between changes in the valve and changes in the measurement.
DIRECT Increase in valve position causes an increase in the measurement.
REVERSE Increase in valve position causes a decrease in the measurement.

CONTROLLER ACTION
Defines the relationship between changes in the measured variable and changes in the controller output.
DIRECT Increase in measured variable causes an increase in the output.
REVERSE Increase in measured variable causes a decrease in the output.
The controller action must be the opposite of the process action. "

Of course the Valve action must be set according to the required fail state.

rmoreno17 said:

What could happen if a PID and a valve had an "x" configuration in order to control an "x" process, but a mistake while calibrating the valve you leave it working backwards?

Click to expand...

Basically when the signal calls to open the valve the valve shuts. IF it wants valve to shut it opens. This confirms the need to check that whatever you are controlling with this solid state stuff actually goes the right way. All too easy to make a mistake. Wire a 3 phase motor on a motor operated valve backwards and see what happens (operates in reverse) - same for a crane press down and hook goes up. Upper limit switch is in up relay circuit and is not effective so you two block the hook and tear the crane up.

MORAL OF STORY
test equipment to ensure correct operating direction.

Dan Bentler