4-20mA and disturbances

aand74

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Dec 2005
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Deinze
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We have a signal of 4-20mA that goes about 100m far.
In the cabinet there is the 4-20mA source, and 100m further in the field there is a proportional valve that is controlled by this signal.
Now we want to check back what the exact signal is at the valve (to detect disturbances or irregularities).
We decided to loop the signal inside the cabinet through a 4-20mA analog input of the plc. As this is a current signal, I suppose that any disturbance seen at the position 100m in the field would also be seen at the analog input in the cabinet? Or is this not always correct?
 
The 4-20mA 'source' is presumably the output of a PID controller because that's what usually drives a proportional modulating control valve. Is it?

Given that the 4-20mA signal is a current signal, the current will be the same signal everywhere in the loop unless there is a ground loop which adds current to the loop current between ground points.

Assuming you don't have a ground loop, the value that the 4-20mA represents (output 0-100%) should be 'available' as a control output tag or value from the controller. Can't you deal the output value directly? If the output spikes, the 4-20mA will spike. If the output oscillates because the tuning too agressive, the 4-20mA will osciallate in synch with the output.

Yes, you can feed the 4-20mA back into an analog input, but it just doesn't make sense to me to do so.

What makes more sense is taking a valve position feedback signal from the positioner back to an analog input to see how well the valve position tracks the controller's output value.
 
In theory, what you're describing should work. However, there are many variables which can impact the quality of your signal (going to the valve and coming back). That being said, is there absolutely no way for you to add a remote block with an analog output to control this valve? That would be the right way to do it... I've never seen reliable analog signals sent over such distance.
 
This may help with the question!

A 4-20 mA signal will read correctly up to the point you exceed the total load limit of the transmitter. This is usually 500 to 1,000 Ohms. Since a typical PLC input or indicator resistance is 250 Ohms, you can run a lot of wire. Back to basics - Ohms law says V = I x R. The resistance, in Ohms per thousand feet at 149 degrees F, of various sizes of wire is:


AWG = Ohms / 1,000 feet
22 = 19.0
20 = 11.9
18 = 7.51
16 = 4.73
14 = 2.97
12 = 1.87

I found this on another forum.
 
This may help with the question!

A 4-20 mA signal will read correctly up to the point you exceed the total load limit of the transmitter. This is usually 500 to 1,000 Ohms. Since a typical PLC input or indicator resistance is 250 Ohms, you can run a lot of wire. Back to basics - Ohms law says V = I x R. The resistance, in Ohms per thousand feet at 149 degrees F, of various sizes of wire is:


AWG = Ohms / 1,000 feet
22 = 19.0
20 = 11.9
18 = 7.51
16 = 4.73
14 = 2.97
12 = 1.87

I found this on another forum.

Looks like the right calculation if you run that cable in a perfect vacuum and zero interference.
 
I've never seen reliable analog signals sent over such distance.
I've seen passive 2 wire loop powered or active analog output 4-20mA signals run 300m, 400m, or 500m even 1000m in hundreds, if not thousands of situations. It's the beauty of 4-20mA, it's robust.

4-20mA is incredibly robust when wired shielded twisted pair and not run in the same cable tray or conduit with heavy current conductors.

In my experience, noise on 4-20mA signals is actually uncommon. Noise induced on the input side (flow and pressure in particular)of a field transmitter, which is then transmit as a 4-20mA is far more common, but that's not the fault of the current based 4-20mA; the 4-20mA is just 'following' the input, which is what it is supposed to do.

In theory, what you're describing should work.
It's theory and it works all the time in practice, it's basic Ohms law. A given voltage typically 24Vdc) will drive a given amount of current through the loop's resistance. Basic DC electronics.

run that cable in a perfect vacuum and zero interference.
No need for any of that, just size the cable properly and connect the shield at one end.
 
I've seen passive 2 wire loop powered or active analog output 4-20mA signals run 300m, 400m, or 500m even 1000m in hundreds, if not thousands of situations. It's the beauty of 4-20mA, it's robust.

4-20mA is incredibly robust when wired shielded twisted pair and not run in the same cable tray or conduit with heavy current conductors.

In my experience, noise on 4-20mA signals is actually uncommon. Noise induced on the input side (flow and pressure in particular)of a field transmitter, which is then transmit as a 4-20mA is far more common, but that's not the fault of the current based 4-20mA; the 4-20mA is just 'following' the input, which is what it is supposed to do.

It's theory and it works all the time in practice, it's basic Ohms law. A given voltage typically 24Vdc) will drive a given amount of current through the loop's resistance. Basic DC electronics.

No need for any of that, just size the cable properly and connect the shield at one end.

I stand corrected then. I have never seen such long distances for analog signals and would have always leaned toward a remote I/O design for such an application.

How sensitive is it to power circuits and what kind of cabling would you run for this?
 
100 meters should be no problem, as long as you use a proper cable being bigger as 0.5 mm.
another problem can be ground loop, so be sure the line is isolated and not grounded anywhere.
If possible use a loop back like a simple potmeter on the valve and check this in the controller cabinet.
A lot of valves have a feedback output (like the siemens valves.

and yes you are correct, you can see the current anywhere in the loop, however check the operation of the valve and if the source does have enough voltage for the valve. simple test is check if the current reaches 20 mA.
 
About the 'Ground loops' : are the source (f.i. plc analog output) and the actuator (in my case the proportional valve) normally completely isolated from ground, or do there exist components that have a connection to the local ground? So, are ground loops normally only caused by bad cabling by the user?
What is the best way to detect a ground loop, or how can I assure myself with certainty that a circuit has no ground loops?
 
I always try to avoid ground loops, so i put all instrumentation grounds on one separate rails, with 1 wire going to the central ground rails. (the cabinet itself)
then loose this wire and check if there is any other ground connection ( i have seen screws thru a cable)
On the valve side it should be isolated, and on the source it can be grounded.
 

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