# Proportional Control Valve and PLC

D

#### DAM

In the ladder logic, how do I tell the PLC to tell the PCV to control the pressure to 80 PSI? Is there some kind of "set point = X.XmA" command to do this? I cannot seem to find the command in these wonderfully written books available.

J

#### J.Zafar

Hi,
Use the PID block in your library
Rgds

P

#### Peter Nachtwey

> In the ladder logic, how do I tell the PLC to tell the PCV to control the pressure to 80 PSI?

This depends on the PLC, the Valve and the feedback. Usually you need to set a setpoint for a PID controller to 80 psi.

> Is there some kind of "set point = X.XmA" command to do this?

I have not seen one. Proportional valves usually use a voltage control signal.

> I cannot seem to find the command in these wonderfully written books available.

J

#### Johan Bengtsson

That can only be done directly if the valve have an integrated preassure sensor and controller, they exist but I don't think you have got one if you didn't directly order that. If that is what you have you just tell it what pressure you want thru your analog output card with whatever instructions you use to put out an analog value.

If not (most probable) you need a pressure sensor, a PID block in your PLC, and an analog input card (to cennect your sensor to) as well as your analog output card.

Does this help?

/Johan Bengtsson

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E

#### Ed

You need to use a scaling function. All analog outputs have a digital value that corresponds to the analog value on the output. For instance, on an A/B SLC500, 4-20mA=3277-16384. Since most i/p controllers have a range of 3-60psi, 4ma=3277=3psi and 20ma=16384=60psi. If you use the SCP (Scale with parameters)instruction, you set 3 as Input Minimum, 60 as input maximum, 3277
as output minimum and 16384 as output maximum. Then the input to the SCP instruction is in PSI and the output would be the analog output address. This value will automatically be converted to 4-20ma! Hope this helps.

H

#### Hakan Ozevin

The author means a pnoeumatic valve. As you now, such valves usually have an I/P converter run by a 4-20 mA signal.

Must make an arithmetic operation and send (or MOVE) the result to the analog output.