4-20mA pressure transmitter programming problem

[Samson0225]

I am trying to use a Rockwell SLC 5/03 to program a pressure transmitter to open and close a valve with a 4-20mA signal. The PLC is reading the signal, but I need to reverse it so that a 4mA signal input creates a 20mA signal output. I am trying to fill and empty a tank of water without it overflowing. The a high signal is being sent to my pressure transmitter when the tank is full and I need my I/P converter to close my valve at this point. As it stands, the I/P converter is reading a 20mA signal when the tank is full and opens the valve. I need this to be reversed, which is why I am utilising the PLC. I have tried many things and have failed to reverse the signal. Any help or insight would be greatly appreciated.

 

[controlsdude]

Have you tried using the programming function Scale(SCP)? This might work for you. otherwise your looking at a lookup table or some compute statement to reverse your scaling.

 

[danw2]

I'm not convinced you'll get the performance that you're looking for because fill rates will vary so drastically with level, but to do what you want:

Convert the 4-20mA input to 0 - 100% or 0.00 - 1.00

Then do the math statement:
one minus [input in percent] = [output in percent].

example:
The input is at 8.0mA or 25% of span.
When input is at 25%, the output should be 75%

math: 1 - 0.25 = 0.75 or 75% output (16.0mA) to the valve.

 

[Nufy]

If you use the raw data count from the card you could simply do the following.

4095 - (Analog Value )

4 - 20 mA = 0 - 4095 counts so...

4095-20mA(4095) = 0(4 mA o/p)

4095 - 4 ma(0) - 4095 (20 mA)

Copy the result to your AO point.

 

[Samson0225]

To danw2: what instruction do you suggest for this operation?

 

[controlsdude]

compute statement with that formula inside it

 

[danw2]

I don't know. I work in another brand's function block:

 

[Samson0225]

Thank y'all very much. We got it operational using two SCP instructions and XIO's and XIC's like binary numbers on the ladder logic. Now in the process of tweaking the program and draining the tank using a solenoid valve with the program.

 

[roydm]

You can't expect to just reverse the signal and it will workthat will have a gain of 1 and no reset'
What you need is a PID loop so you can set the gain and integral action.
Roy