4-20 mA loop sheilding

[mgopalan]

I have an application where I have a 4-20 mA transducer at the end of a 250 ft long cable. Plus, this entire setup up is in a nasty environment electrically (lots of RF and generator noise, think oil rig with gigantic generators, motors and nasty power).

I am using a 4 conductor (2 twisted pairs, one pair not used) cable with a foil shield and a drain wire and I have connected the shield at one end only (it is floating at the sensor side) and have connected the other end of the shield to my Analog Ground. I am also using a 400 Ohm sense resistor and a unity gain follower before I get to the A/D.

This setup works great in the lab and fails miserably at the field location. The DAQ system goes wild with noisy (as in white noise up to the rails of the A/D input levels) data...

I have tried... Common mode chokes on the 4-20 mA lines, I have made sure that I am not connecting my Analog Ground to the Case Ground (Earth)..

so the question I have is this...

Do I connect the shield to the Analog Ground of the 12V DC power supply (and therefore the AGND of my sense resistor, Buffer Amp and the A/D as well) or to the Case Ground which is tied to the safety Earth of my AC power supply and the EARTH/GND pin of my 110VAC connector..

The shield itself only protects against Capacitively coupled noise right? The common mode chokes should clean up any common mode noise...so why am I getting crappy data? This is driving me nuts...

Any help?

MG

 

[mgopalan]

whoops wrong forum... sorry.

 

[cbarn24050]

Hi, your shield is probably bringing in all the interference, connect it to the earth at both ends and not your analoge ground. Powering your 4-20ma loop from a seperate isolated supply helps as well as low pass filtering onthe ad input circuit.

 

[seymours2571]

mgopalan,

What type of insturmentation are you using? I use NI SCXI chassis with the instrumentation modules. I experienced similar problems as you have and found out that the common mode drain line was connected to chassis ground, which in turn was connected to earth ground. The shield drain wire was attached to the earth ground at the A/D converter module and left to float at the transmitter.

The pressure transmitter was connected to a pipe that was isolated from the earth ground by rubber hose and pvc pipe. We were using a VFD drive to control the pump motor. Apparently, VFD drives produce a tremendous amount of electric noise into the environment. The isolated section of piping, with the pressure transmitter piped to it, was picking up the noise from the VFD drive and sinking it down the signal wires, through the common mode drain resistors, and onto earth ground.

The solution was to earth ground the isolated section of piping. Once I did that, the signals ran smooth.

On a side note..I read on of your other posts about your job position and all of your duties. I am in the same boat as you. I am the CFD, FEA, instrumetation, field support, and design guy for my company. Anyway, I have just started implementing our data acquisition equipment and plan to move to some system control within the next six months. I would like to share/receive any tips or tricks regarding the instrumentation, FEA, or CFD. I know I could use the help.

Thanks,

Steve
sseymour@idexcorp.com

 

[betkro]

Mgopalan, I think that you have a communications problem rather than a grounding problem.

Try using industrial grade instrumentation cable from Belden or similar. Don’t forget to terminate both sides of the line with the same EOL resistor, matching the characteristic impedance of the cable. Normally, it is 100 Ohm nominal, not the 400R you intend to use. If still doesn’t work, try an unbalanced connection with properly designed line drivers, or, use an off-the-shelf converter for RS-485 data transmission. That will certainly work.

 

[mgopalan]

for cbarn24050 : If I connect the shield to Earth on both sides do I not just create a 500 ft antenna?

for betkro : Analog 4-20 ma Loop....There is not digital data here.. how would RS485 apply?

seymours2571 : So you are saying that the isolated section of pipe attached to the transducer (which was not connected to your shield) was picking up noise form the VFD and coupling it to ground through the signal wires. Once you grounded the transducer case itself, the problem went away.

I know that our transducer's case is earth grounded at the the install point (because there a giant WECO hammer union attaching it to the rig itself... The recent mod that I did was to add a common mode line filter to AC power input line and add common mode protection to the transducer signal wires themselves. Theoretically, these should remove any RF or EM coupled common mode noise into the signal lines. My shield is still coneccted to analog ground at the A/D.

I think these have cleared up the data to the point where everything works reasonably well right now...I am still having some problems with the digital side of things but all that seems to be protocol layer stuff..nothing that a couple of months of recoding wouldnt fix..

On the system design level, all the DAQ is custom. I use a 12V power supply (Lambda Switching Power Supply) to drive the transducer and my 400 Ohm sense resistor. The resulatant signal is passed through a unity gain Buffer Amp (LTC1151) to a 16 bit A/D (AD976) to two processors. One which drives a thermal strip chart while the other converts the data into serisl data to be sent to a laptop if needed. I cannot use off the shelf stuff because people on drilling rigs have a tenbdency to wash down our equipment with a fire hose once in a while...so everything has to be sealed and protected very tightly..

I guess I am still pretty confused...If I start completely fresh and do the following...

1) Use an isolated AC-DC power supply to power my 4-20 mA transducer. Lets say that this is a 24V power supply.
2) connect a 400 Ohm sense resistor, to the return line so I can convert the loop current to a voltage between 1.6V and 8.0V. This means that even if I rail our the transducer, I still have 16V of complaiance left to power the transducer itself.
3) I use a unity gain follower to buffer the 1,6V to 8.0V signal to have a very low impedance output. I do this so I dont have worries about loading the input of the A/D converter.
4) Where do I connect the shield? I have basically three choices right?
a) The Nuetral LIne of the AC input
b) The Earth Line of the AC input
c) The Gnd of the DC Power Supply which is also the ground reference for Amp and A/D.
So Which one should I use?


Also...I hear people talking about driving the shield to prevent noise..Anyone have any suggestions on that?

Any books on this stuff?

seymour257 - 1 Am willing to help out all with what little I know..What do you work on?

 

[cbarn24050]

Hi, possibly but as it's grounded at both ends does that matter? At the moment the "antenna" is connected to your equipment. As to where you should connect it, the rig ground, ie. the metalwork and not your instrumentation.

 

[zeitghost]

It's just struck me that grounding the far end through a capacitor (1uF?ish) would reduce 50/60Hz ground loops but still get rid of higher frequency rubbish.

Just a thought.

rgds
Zeit.

 

[Highspeed]

Have you tried double shields...
In a similar situation (not as long though), Inner shield was connected as follow :
Signal >>> Instrumentation Amplifier >>>> DAQ
>>>> Inverting AMP >>> Shield
opened at the other end
Top layer shield was earthed both ends.
HS

 

[betkro]

All this sounds as pink noise.

 

[djs]

Hello;
You might be having some problems with an un-isolated instruemnt. If the 4-20 ma output of the instrument is not isolated, then you have a groung loop. Try placing a loop-power optical isolator between your instruemnt and the converter.
BTW we design controls system for harsh enviorments all the time using off the self parts. Think Type 4X.