PLC and Proportional Control Valve


Thread Starter


The PCV sends a feedback signal (in volts) to the PLC telling the PLC what pressure it is at, the
PLC sends a control signal to the PCV (in amps) telling it what to do next. How does the PLC
know what a feedback signal of 5V means and how does the PCV know what a control signal of 5mA
means? Any books that explain what is going on would help...Thanks

James Fountas

The short answer is the PLC doesn't know.

Here is a slightly longer answer. A PLC is usually setup to use an algorithm such as proportional integral derivative (PID) control. The PID loop has tuning parameters assigned to it which dictate how much it will adjust the control signal based on the input signal. The
tuning parameters are just numerical values.

A pressure control valve (PCV) or flow control valve (FCV) do not generally send signals to the PLC. It is a pressure transmitter (PT) of
flow transmitter (FT) that send the signal to the PLC. The PLC executes the logic and then sends the signal to the PCV, FCV, or whatever
the final control element is.

The signal the PLC receives could be 1-5 VDC, 4-20 mA, or a bunch of other less common options. This signal could correspond to any range 0
- 0.324 gpm, 30 - 1000 lps, 0-1 psi, -14.7-1000psi, etc. These are just a few made up ranges just to show there is no set relationship.
The signal the PLC send could be 1-5 VDC, 4-20 mA, etc. This signal will general correspond to a valve being 0 -100% open or 100 - 0%
open. But the amount of flow through the valve can vary depending on the size of the valve, the valve characteristics, process pressure,
fluid characteristics, etc.

The algorithm the PLC uses to compute what the output/control signal is based on the input/process signal has to be based on all the
previous and other factors. These other factors can include response time of the valve, the system, etc.

I never read a book that explained it well. I learned it through experience and the effort of teaching it to others.
A normal closed loop for the job above is simpy Proportional in the algorithm.
If you see other things in there, they are for the birds.
Level and pressure having 1/s in their transfer function leave no offset to set point change.
And very little on load change.
Example: Coolant is to be sent around a process system using a proportinal controlled valve. The valve is callibrated 0-100% = 4-20ma. (it will fully open when it recieves a 20ma signal)

Fisrt the coolant must be cooled to 2 degrees centigrade utilising a temperature sensor. The sensor is callibrated 0-100 degrees = 0-10vdc.(when sensing 100 degrees it gives out 10vdc).

The PLC sees the 0-10v as value 0-4000 and a math equation must first be used to change the raw input to an actual input. Move the value to
a raw value register, divide by 40 and place into
an actual value register, so now the PLC sees the
0-10vdc input as 0-100 (degrees C.) A similar math is required for the PV output value

At 2 degrees C. the proportinal valve is enabled and a ladder diagram sets a value of 50 in th PV register to open the valve 50%. The PLC analogue output card would then send 12 ma to the proportional valve to open it 50%.