4-20mA output from PLC to Chart recorder

[Handcraftedsince87]

Greetings,
So this seems like it should be pretty simple, but I'm guessing there's something basic that I'm missing, and am hoping you guys could help me out with it. We have a flow meter. That flow meter used to control 2 chart recorders and a PLC input all on the same loop. We ended up needing a new steam flow meter and they recommended we seperate the loops. Now the steam meter goes to an analog input (works great) on a Logix5000 plc, and from there we have 2 analog outputs. One to a chart recorder, and one to a computer that monitors our opacity and Co2 and what not. I can't seem to get any output on the chart recorder channel when I measure, or on the chart itself. I've tried switching channels to no avail.
On the ESP cpu, it's outputing, but I don't thing it's working correctly. It only outputs whatever I set my low signal to in the properties for the card. Everything is set on 4-20ma, and with a 0-240 scale. To move the floating number I used a "move" instruction to move the data from the steam fow input to the correct output channel. It's an OF8 AB analog output card, and I did double check it's wired up to the current output and not voltage.
Like I said, Im sure I'm missing something simple and would appreciate any help I could get pointing me in the right direction.

Thanks in advance!

Cheers

 

[VAN]

Only two things I would think to check;
1) Make sure the output is on in the card, and limits set appropriately.
2) Measure your output using a voltmeter (for current).

Out of curiosity what is the output card?

 

[Handcraftedsince87]

the card is a 1756-OF8. What do you mean by making sure the card is "on"? The card was already established in the PLC, and has other outputs working on it, but the channels I picked were unused.

edit: Granted I'm fairly new to 4-20 control loops, I seem to have had good luck so far I suppose because everything has seemed to make sense, but now I find myself a little stumped. after re reading your post maybe I can offer a little bit more insight into the situation.
I'm using a milliamp clamp meter (Fluke 773) on the chart recorder channel with "1mA output". I get the same output wether measured with the clamp meter, or inline with test probes, and I show almost 20v when voltage is measured across the output in front of the chart recorder. On the other output that is only outputing whatever I set my low set point to in the PLC card properties (4mA, 10mA, etc) That reading shows up with my clamp meter as well as if I break the loop and measure inline (it changes just slightly which I attributed to the changing of the circuit resistance after adding the meter to the loop)

 

[VAN]

In some cards you can disable output channels but in this card it doesn't appear you can.

Anyway, you setup all the settings in the card?
(Double click the card/Right Click 'Properties' -> Configuration/Output State/Limits are all setup for the correct channel?)

If you have only the Fluke on the channel are you able to read 4-20mA? If you can't might be a different issue (like a missing comm).

 

[Handcraftedsince87]

I believe everything is setup correctly. 0-20ma with 4 being low and 20 being high, 0 to 250 engineering units which is the same as out steam flow meter. not sure about the "limits". they seem to be set to 10 and -10 as a default, but I believe I had tried spreading out those limits as a matter of troubleshooting with no results. with only the fluke meter on the channel I actually get the same 4mA reading (low signal setting in card properties) that I'm getting on the other channel. (It should be outputing more along the lines of 12-14mA right now) Is the difference maybe because of the resistance of the chart recorder? I thought it was 250 ohms.. the same resistance as the other ESP loop that isn't working

 

[Handcraftedsince87]

edit: thanks for the help so far VAN, much appreciated. Where at in WA are you?

 

[VAN]

Well sounds like you have a few routes to go and haven't really narrowed down the problem. You say both channels give you only 4mA with the same fluke? Might have a blown fuse in the fluke.

I don't know what the Ohms are in the Fluke or the chart reader, could try finding a small 4-20mA gauge. I have usually had better luck with a 4-20mA gauge.

This is an example of how the card might look when setup correctly.

I'm north Seattle area, ever go skiing at the Olymipic National Forest - Hurricaine Ridge?

 

[sparkie]

I'm curious (if you have not yet resolved this), you said that the output sends out a mA signal, but only at your lowest setting. I would first check within RSLogix and see if the number is increasing and decreasing as it should. The word from the output itself and not the the spot you copy it to.

If that word is changing like it should, then I would check the word you copy it to and see if it is being properly updated. I know very little about modern Allen Bradley PLCs (we use almost entirely SLC 500's at work), but it seems you may want to make sure the program is performing as it should first.

 

[Tom Jenkins]

If it's not software it could be wiring. Take the recorder out of the circuit, force the output, and measure with a meter. Then run the recorder ciruit with a simulator. Possibilities could be polarity, too much impedance in the circuit, power supply not adequate.

 

[Handcraftedsince87]

OK, I believe its been solved. I knew the words were moving correctly, And I knew my meter was working correctly, What I wasn't sure about was setting up the "high and low clamp limits". Turns out they needed to be scaled to my high and low engineering units.
They both are outputting correctly, however the only way I can measure the loop that goes to the chart recorder, is if I completely remove it from the loop and measure inline with my Fluke meter to the PLC output. If I use the clamp, or measure inline with the chart recorder still attached, I only get about 1 milliamp. (I found out this was true earlier as it would only output the low engineering (4ma) when measured w/o the chart recorder). Like I said, I'm pretty confident its outputting correctly, as it reads correct with only my meter in line on the loop. The other loop reads correctly with the clamp meter. If anyone has experience with this meter (773) and could help explain this it would be greatly appreciated. Also, I've ran into under powered loop power supply's before, but the wiring schematic from AB showed this PLC output supplies the loop power, unless I'm missing something?
Hurricane ridge is awesome. Before I came to to this Boiler, I was at a Mill out in Forks. We liked to spend alot of our free time in PA and the Olympics. I love the fact the I'm still only 25 minutes away from being able to enter Olympic National Park to hike camp and fish
This website and you guys are an amazing resource in helping me do my job. I can't thank everyone enough. I am going to upgrade my membership as soon as I finish typing this.

Thanks again, All of the advice is greatly appreciated

 

[Tom Jenkins]

I guess I don't understand why you couldn't measure "inline" wit the loop. Were you perhaps on the "-" side of the recorder, with the recorder "-" tied to ground? Making this kid of troubleshooting easy is why I always used disconnect terminals on all analog loops.

My mom used to live in Grapeview, and I've been through Shelton many times. Did you work at the lumber mill in the "middle" of town?

 

[Handcraftedsince87]

I had tried both sides of the loop, same results either side. I believe the chart recorder needs an internal jumper moved to allow 250ohms of resistance. I think right now it's a lot higher, which is why I'm only getting 1mA at about 20 volts when measured with the recorder in the loop. I just need to find that Jumper to get 250 and I think I'll be in business. Like I said, I know the PLC can output correctly when put across my meter at 250 ohms.

Grapeview is beautiful. We're looking at buying some property out by Harstine island which isn't too far south of there. I do work at the mill right downtown. There's about 10 of us that work in the boiler, and oddly enough, we're a different Union then the rest of the waterfront. Right now, it's Simpson Lumber, but that is about to change as we are currently "for sale". Kind of sad, Simpson's been in Shelton for 100 years. All we can do is wait and hope we're not bought by Interfor!

Thanks again!

edit: The chart recorder faults out if the polarity is reversed on the wiring.

 

[Handcraftedsince87]

Indeed that was it. I swapped the jumper on the pen 1 card and all is right in the world again. Thanks for all the help

Cheers!

 

[NetNathan]

Yes, some chart recorders....use a 250 OHM resistor for 4-20ma or 0-20ma inputs. The chart recorder only recognizes voltage signals.
The Honeywell chart recorders I use, actually supply a separate 250 OHM resistor (which you have to remember to add to order) for ma inputs.

 

[Handcraftedsince87]

Yeah, this was a Honeywell chart recorder. we used to have to supply a 250 ohm resistor on our old 353 PID controllers, but these just have an internal jumper on the pen card that needs to be switched when going from 0-5v to 4-20mA.