Noise on our SC-2345 board/pyrometers

[Patrick H.]

I will try to describe the situations the best I possibly can.

We set up a signal conditioning board (SC-2345) to be our controller for a durability test rig. We also set up another SC-2345 to only acquire data from 8 pyrometers reading the temperature during the tests.

The problem we have is with noise on the temp reading. Most of them work properly and we can live with about 5-7 degrees F of noise since it is not crucial to the exact accuracy. We have a few that are about 30-50 degrees F of noise when nothing is running. Then if we turn on our Hydraulic Control Manifold, that noise will go away, but it will switch to a seemingly random other 2-3 pyros. There isn't any consistency at all with which one will get the noise.

We spent countless hours investigating and we found that the shields on the cables have some potential in them. We can't figure out from where, but we grounded them on one end and that seems to have fixed that problem of the potential but not the noise. We also have a potential on our NI box itself of about .3V with no reason to have that either. We thought it was possibly a bad ground in the building outlet, but we tested those and they were grounded as well as all conduit the wire runs through. We even put everything on a UPS and it didn't do anything.

The pyros are on nylon brackets so they are not picking up interference from the machine that way. The wires are all running straight into the modules in the box, so there is no exposed BNC connections or anything like that.

With all of the tested we did, it seems that the problem is within the NI box somewhere but we can't figure out where.

Could it just be underpowered and it is struggling to run?
Any advice on how to locate the problem or ways to fix this noise would be great. Also, please post questions so I can answer them so other people can read them to clear more up.

Thanks

Patrick

[Patrick H.]

And our PC DAQ card is a 2 channel PCI 6229

[Patrick H.]

Another random thought,

Could it be a ground problem in the PCI slot? Is a 2 channel 6229 card grounded together (both channels into the same PCI ground)? Since the one box influences the other and this is the only point where they connect to the same piece of equipment.

[Drew_T]

Patrick,

What it sounds like is that you've got some grounding issues, like you pointed out before.  What might be happening is that your ground references may not be the same between your sensors, carrier, and pc.  This could cause ground loops through the hardware, resulting in strange noise like you are seeing.  I would recommend taking a look at the resource below, specifically section 5.

http://zone.ni.com/devzone/cda/tut/p/id/3344

Good Luck!

Drew

[Patrick H.]

Thanks you for the advice.I have read that document a few times and I have some updates since you posted.

1.) Our grounds have been vigorously checked and they are not coupled and they all are how they should be.

2.) We shorted the thermocouple module that we are using and there is no noise that resulted from that. So this means there is no noise in the sc-2345 board or PCI card.

3.)We bought a new pyrometer and will run new cabling through the conduit today to see if it is just a problem in the pyrometers we have.

4.) I have checked the new pyrometer with the power supply we will be using and a multimeter to check for any noise in the voltage (no noise).

Anyone have any other ideas to try I would appreciate it. Also, I will update this post after I have traveled up and installed the new pyrometer.

Patrick

[Patrick H. · Accepted solution]

After countless hours of trying to resolve this problem, we re-wired and put in the new pyrometers and it worked beautifully. We can fully blame the bad pyrometers from the original company we bought from.