I am using Micrologix 1200 for controlling Electric modulating valve (at the inlet of Tank) based Ultrasonic Level sensor is giving 4-20mA (corresponding to 0-5 mts) signal to the PLC. Can anyone help me with how I can take the 4-20mA signal and give a 0-100% output to the valve i.e at 0 mts. valve will open 100% and at 5 mts. valve will be open 0%.

Will highly appreciate a reply at the soonest.

 

SK:
Try the next algorithm. You must be aware that it is not a final product, but just the one that fits what you described. You also have to change the addressess, and other values, according to your PLC configuration. You must have in mind the set-point value and the differential, and the proportion error-output variable vale.

SOR BST XIC I:1.0/0 NXB XIC B3:0/0 BND XIO I:1.0/2 BST OTE B3:0/0 NXB MOV I:5.2 N7:11 BND EOR

SOR XIC B3:0/0 OSR B3:31/15 MOV 13763 N7:10 EOR

SOR XIC B3:0/0 GRT N7:10 0 SUB N7:10 N7:11 N7:12 EOR

SOR XIC I:1.0/3 JSR 3 EOR

SOR END EOR

You will find that it is a simple, but powerfull algorithm for a proportional control.
It is the heart of a system that I named "Levelcon" .
There are several project documents that describe developed projects (in its earlier versions) at:
http://www.intelligentes.com/PLCProgramming/main.htm
Some of them are not translated yet, but those will be in the future.

Hope you find it useful, even, I like to know. Keep in touch!
VBravo
[email protected]
http://www.intelligentes.com/PLCProgramming/main.htm

 

I think you will have a control problem with this circuit. You should really be looking at using a PID control.
However to the original question. Work out the highest value that can be obtained from the A to D conversion, say 4095, subtract the value from the input off this and feed the result to the output.
At 0mts 4095-0=4095 hence valve open 100%
At 2.5mts 4095-2048=2047 or valve open 50%
At 5mts 4095-4095=0 so valve closes

But look at a PID control!!

 

Use Open Source Software. It will solve all of your problems!

Hope this helps!

Larry Lawver
Rexel / Central Florida

 

I have done the same thing in a different way. I have used the SCP function in the RSLOGIX 500 and used the following parameters:

SCP1
------
Input:F8:0
Input min:0
Input Max:5
Scaled min: 100
Scaled max: 0
Destination:F8:10

SCP2
------
Input:F8:10
Input min:0
Input Max:100
Scaled min: 6242
Scaled max: 31208
Destination:O:1.0

I tried it on RSEmulate500 and it worked!

Thanks for the tip!