4-20mA Output scaling

Willlly

Member
Join Date
Feb 2018
Location
Florida
Posts
12
Hey guys, I have been having a bit of trouble with analog output scaling.
I’m programming out of a compactlogix, I’m trying to output 4-20mA to a valve only using a set point, would an SCP or PID be better for this test station?
It has been while since I actually programmed, trying to get back into it. Any info will help, thanks
 
You are just wanting to send a constant signal to the valve to hold it in a set position, or do you want to control the valve based on an input, such as flow rate?

If you just want to set the valve, you can use an SCP function (would be an add-on-instruction for RSLogix 5000) to scale the output from 0-100 to 4000-20000, or whatever units your output card is set up for. PID would be for controlling the output based on an input.

If you just want to get some signal to the valve, you don't need an SCP at all. Just write a value in the the Local:1.O.Data (Or whatever yours may be) register.
 
Yes, I wasn't sure if I needed a SCP. All I am trying to do is create a tag to simply put a set point to control the 4-20 from my PanelView. Lets say I put in a setpoint of 50 only to open half way. would that work? If my scaling is correct?
 
ok I have my SCP scaled but my valve is only staying open half way, could this be a module config or something? I have not dealt with analog output cards a lot.
 
Yes it could. What's the part number for your output card, and how is it configured? Post a screenshot of the setup screen and your SCP
 
Input min=0

Input max=100

Scaled min=4000

Scaled max=20000

I have my module configured to raw/proportional data with low limit at -29822 and high and +29085. I have also tried engineering units with a high low of 4000 and 20000. but still get the same outcome. 1769-OF8C is my analog output card
 
According to the Help file, the scaling for 4-20 mA "Raw/Proportional" is -29,822 to 29,085.

Because your SCP block never sets a value lower than 4000, you're always sending out at least 12.something mA, keeping your valve at at least 50%, and never more than roughly 80% open.

Either change the scaled min / max on your SCP block to -29,822 & 29,085, or change your Configuration to "Engineering Units" and see what happens.

AO.png
 
I know I am missing something simple that I have not checked or done. I tried both ways with the same outcome. Ill keep digging into it, but what else could it be?
 
I know I am missing something simple that I have not checked or done. I tried both ways with the same outcome. Ill keep digging into it, but what else could it be?

Could it be the valve or wiring?

Try using a4-20mA signal generator and see if the valve responds correctly.
 
In the controller tags, open up the Input file for the module / channel. There is diagnostic information there that may tell you what's going on.

AO.png
 

Similar Topics

Hi, I need to read three 4-20mA signals from a DCS(ABB) in a remote 6 channel analog input module with RS485 modbus port. When I connected...
Replies
2
Views
466
Hi all, Well, here I find myself at a former client after I left water treatment trying to help him get the plant back up and running after a...
Replies
2
Views
1,023
Gents I'm working on a small project here and the customer requested to control the drive with 4-20mA by using Micro820 Analog output 4-20mA...
Replies
10
Views
5,899
I'm using an AB 2085-OF4 module for outputting process current control to various VFDs. When using this device, I have the channels configured...
Replies
4
Views
2,955
If you are fusing a 4-20mA output from a signal isolator. What size fuse would you use?
Replies
3
Views
1,785
Back
Top Bottom