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Dan Bookwalter

DAQ during a vibration test

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I have to run some tests on our vibration setup that require some basic data acquisition , however , once I turn the amplifier on (not even shaking) I get about 50mv of noise on my signal. normally this wouldnt be an issue for us since we are just looking for a DUT to open , but , in this case it is an issue. I have bonded all the equipment grounds together along with the vibration controller , amplifier , shaker and thermal chamber , used shielded cables , lots of averaging of the acquired signals , using differential inputs , but , nothing I do seems to make much difference. I have also tried doing a little digital filtering without much success , but , I am just starting to look into that.

How are other people doing this stuff ??

Any help would be appreciated...

Thanks

Dan

p.s. the DAQ card is a PCI-6071

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What usually works best is to use a card with isolated input channels. In the past I have used NI SCC signal conditioning accessories to isolate channels entering the DAQ card.

Works great, but can get expensive fairly quickly. In the end, if you have more than 5 or so channels requiring isolation, or measuring extremely low voltages, it is better to use SCXI systems.

Hopefully you are using a differential AI channel referenced to ground?

PS I have never used your card before.

Neville.

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QUOTE (Neville D @ Aug 6 2008, 03:25 PM)

Neville

"Hopefully you are using a differential AI channel referenced to ground? " ..... Yep....

I currently have 6 channels to monitor , but , in the near future it could be as many as 20 , at the current time I doubt i can purchase any more equipment , it seems that gas prices :angry: have had some financial impact on our business (auto industry) , imagine that ;) ..... so for the time being i will have to use what we have already...

Dan

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QUOTE (Dan Bookwalter @ Aug 6 2008, 12:34 PM)

Neville

"Hopefully you are using a differential AI channel referenced to ground? " ..... Yep....

I currently have 6 channels to monitor , but , in the near future it could be as many as 20

Dan

Have you tried tieing all other unused channels to ground?

Using shielded cables ?

Do you get the noise on all the channels?

Try plugging the amp into a different power line? There are devices that can isolate the power line for a couple of plugs (around $300 or so).

Maybe "measure" the noise on a AI channel and then subtract out the channel in software.

Noise issues are always difficult to fix and usually isolation is the only thing that always works.

I think there are some NI DAQ cards that have channel isolation built into them (for a future upgrade)..

Neville.

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QUOTE (Neville D @ Aug 6 2008, 04:11 PM)

Have you tried tieing all other unused channels to ground?

Using shielded cables ?

Do you get the noise on all the channels?

Try plugging the amp into a different power line? There are devices that can isolate the power line for a couple of plugs (around $300 or so).

Maybe "measure" the noise on a AI channel and then subtract out the channel in software.

Noise issues are always difficult to fix and usually isolation is the only thing that always works.

I think there are some NI DAQ cards that have channel isolation built into them (for a future upgrade)..

Neville.

Neville

Yep noise on all channels , but , i will ground all the others anyhow , using shielded cables , no chance of rewiring the amplifier as it is 480V 3 Phase... i had thoght about measuring the nosie and subtracting as you mention , i will have to look into that idea a little more... usually i have pretty good luck fixing noise issues , but , this one is not playing nice...

Thanks

Dan

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QUOTE (Dan Bookwalter @ Aug 6 2008, 01:42 PM)

How are other people doing this stuff ??

p.s. the DAQ card is a PCI-6071

We've used accelerometers, high-bandwidth torque transducers and (played with) laser vibrometers connected to a NI PCI-4472 card and an 18-bit M-series card (I don't recall the number). We were using a 6071E, but the NVH expert we hired laughed the board out of our NVH use; 16-bits is the bare minimum if you have everything perfect and 20 to 24-bits is prefered. The 4472 is nice in that it has anti-aliasing filters on-board.

You mentioned that you're connecting your grounds together... that may be causing problems if you're creating a ground loop. 50 mV of noise would be 10-bits for a 12-bit card... that's not a lot and could be induced by lots of things; what frequency is the noise at and what could be causing that? You mentioned that you are using shielded cable; are you using twisted pair? Are the shields connected at only one end? No ends? Both ends? Shields should be connectedat one end and carry through for low frequency signals, but the rules change when you get to mid and high frequencies. High frequency noise should have the shield connected at both ends. Mid frequency noise can require both, one, or neither end of the shield to be connected. Then you get into conncting caps between shield and ground and it gets to be a mess. I'm not sure how to help more without knowing more about the hardware.

Tim

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QUOTE (Dan Bookwalter @ Aug 6 2008, 10:34 PM)

Neville

Yep noise on all channels , but , i will ground all the others anyhow , using shielded cables , no chance of rewiring the amplifier as it is 480V 3 Phase... i had thoght about measuring the nosie and subtracting as you mention , i will have to look into that idea a little more... usually i have pretty good luck fixing noise issues , but , this one is not playing nice...

Thanks

Dan

EMI filter for three phase are not that expensive if I remember correctly and they can be connected in two different way, have I been toold from an electricant. He told also me anecodote that in one mill he worked in they had 400Vs in reinforcement of the concrete, it was pure luck that they noticed that wall was hot. This caused by by slopy 3 phase wiring.

It is an idea worth looking up I belive. I have also had these problems and using different power supply lines for the measurement equipment and amp maybe a good idea, althought it is against what you normally get recommended. Do you have any other machines that use inverters on the same prower line?

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QUOTE (Tim_S @ Aug 6 2008, 05:03 PM)

We've used accelerometers, high-bandwidth torque transducers and (played with) laser vibrometers connected to a NI PCI-4472 card and an 18-bit M-series card (I don't recall the number). We were using a 6071E, but the NVH expert we hired laughed the board out of our NVH use; 16-bits is the bare minimum if you have everything perfect and 20 to 24-bits is prefered. The 4472 is nice in that it has anti-aliasing filters on-board.

You mentioned that you're connecting your grounds together... that may be causing problems if you're creating a ground loop. 50 mV of noise would be 10-bits for a 12-bit card... that's not a lot and could be induced by lots of things; what frequency is the noise at and what could be causing that? You mentioned that you are using shielded cable; are you using twisted pair? Are the shields connected at only one end? No ends? Both ends? Shields should be connectedat one end and carry through for low frequency signals, but the rules change when you get to mid and high frequencies. High frequency noise should have the shield connected at both ends. Mid frequency noise can require both, one, or neither end of the shield to be connected. Then you get into conncting caps between shield and ground and it gets to be a mess. I'm not sure how to help more without knowing more about the hardware.

Tim

Tim

one thing i need to make clear is that i am not trying to control the shaker with the card , just take some data off of our devices on test... as far as the cables are concerned they are shielded twisted pair with the shileds connectd to ground at one end. I have always been a firm believer in single point grounds hence the reason i have tied everything together at one point. I can try connecting both ends to ground once to see what happens. also i am not sure i made it clear that the noise appears only after i enable the modules on the amplifier , just powering up the amp doesnt generate any noise.

Dan

QUOTE (AnalogKid2DigitalMan @ Aug 6 2008, 05:46 PM)

What technology is the amplifier based on (i.e VFD, servo, voice coil driver)?

Are the 480V lines in steel conduit?

Do you have access to an oscilloscope?

Sorry for more queries.

The amplifier is a Voice Coil Driver , the 480V is not in metal conduit , but , i dont think that is where the problem lies... the noise is only present after i enable the amp modules , just turning the amplifier on does not induce any noise...

yep have several scopes here....

Dan

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QUOTE (Dan Bookwalter @ Aug 7 2008, 06:08 AM)

Tim

one thing i need to make clear is that i am not trying to control the shaker with the card , just take some data off of our devices on test... as far as the cables are concerned they are shielded twisted pair with the shileds connectd to ground at one end. I have always been a firm believer in single point grounds hence the reason i have tied everything together at one point. I can try connecting both ends to ground once to see what happens. also i am not sure i made it clear that the noise appears only after i enable the modules on the amplifier , just powering up the amp doesnt generate any noise.

Dan

The amplifier is a Voice Coil Driver , the 480V is not in metal conduit , but , i dont think that is where the problem lies... the noise is only present after i enable the amp modules , just turning the amplifier on does not induce any noise...

yep have several scopes here....

Dan

Dan:

Can you scope out the frequency content of the noise near the DAQ and then trace higher amplitude back upstream towards the amp? Appears like amp enable makes the drive amps the culprit- what's the make/model. Sounds like you have a good handle on grounding and wiring practices, these things can be a bear since it could be conductive, capacitive coupled, or radiated induced noise. Have you tried moving the amp ground somewhere else and is it an option to safely run it ungrounded as a test? Even with a system schematic and pics it is hard to guess what the issue might be without being on-site.

-AK2DM

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QUOTE (AnalogKid2DigitalMan @ Aug 7 2008, 05:11 PM)

Dan:

Can you scope out the frequency content of the noise near the DAQ and then trace higher amplitude back upstream towards the amp? Appears like amp enable makes the drive amps the culprit- what's the make/model. Sounds like you have a good handle on grounding and wiring practices, these things can be a bear since it could be conductive, capacitive coupled, or radiated induced noise. Have you tried moving the amp ground somewhere else and is it an option to safely run it ungrounded as a test? Even with a system schematic and pics it is hard to guess what the issue might be without being on-site.

-AK2DM

If it get better ungrounded an EMI-filter maybe worth considering.

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QUOTE (AnalogKid2DigitalMan @ Aug 7 2008, 11:11 AM)

Dan:

Can you scope out the frequency content of the noise near the DAQ and then trace higher amplitude back upstream towards the amp? Appears like amp enable makes the drive amps the culprit- what's the make/model. Sounds like you have a good handle on grounding and wiring practices, these things can be a bear since it could be conductive, capacitive coupled, or radiated induced noise. Have you tried moving the amp ground somewhere else and is it an option to safely run it ungrounded as a test? Even with a system schematic and pics it is hard to guess what the issue might be without being on-site.

-AK2DM

well i did some quick FFT stuff on the data i took and it just shows up as random white noise , i still have to check and make sure i got the analysis stuff correct. I think its the strong electrical/magnetic fields around the shaker that is causing the issues , the amp is about 15 feet away , but the shaker is right there obviously since the DUT's have to be attached to it to shake ;) .... the Amp is an LDS SPAK20 and the shaker is an LDS V850-240.

Dan

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QUOTE (Dan Bookwalter @ Aug 7 2008, 11:39 AM)

When you wrote shaker I was thinking of something like http://www.bksv.com/doc/bp2101.pdf, which is quite a bit smaller. What you have seems like a guaranteed noise source, though I would think that it would be more at a fixed frequencies. You could get broad-spectrum noise if the shaker uses a variable frequency drive and the drive is attempting to hold position. (We have this problem with synchronous AC motors.)

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QUOTE (Tim_S @ Aug 7 2008, 01:27 PM)

When you wrote shaker I was thinking of something like http://www.bksv.com/doc/bp2101.pdf, which is quite a bit smaller. What you have seems like a guaranteed noise source, though I would think that it would be more at a fixed frequencies. You could get broad-spectrum noise if the shaker uses a variable frequency drive and the drive is attempting to hold position. (We have this problem with synchronous AC motors.)

Yep , these are a little larger ....they are 20 KVA amps and 4000 lbf shakers , although not that large in the vibration business... i am now wondering about the drive , since when you enable the modules it automatically centers the armature and holds it there , so they may be doing a VFD type drive...

Dan

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