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Impact of Accelerometer Weight on Test

05/11/2010 6:47 AM

Hello everyone, I'm once more perched at your doorstep with a question.

My question is pretty much summed up in the title, but as always, I know you need more information to be able to provide more accurate answers :)

I am going to conduct vibration tests on a PCB board. The board is safely mounted into an aluminum tray. It is roughly the size of 330 x 330 mm with four "lobes" (The PCB is bolted onto the tray from horizontal and vertical lines in the middle of both X and Y axes). Thus I have four 160 x 160 mm PCB lobes on the board.

This PCB has various components with varying weights on it (I'm not an electronics engineer, all I know is some are DC/DC converters and some are controllers :) ). One weighs 52g, another weighs 15g. I want to place accelerometers on these components to measure the G load they face while the PCB is vibrated.

My question is; would the weight of the accelerometer affect the measurement for these components? I'm not sure of the weight of the accelerometer, but it's one of the cube-types that is 1cm on a side; pretty lightweight as far as I know. And if I should suspect, how can I measure it? And if it does affect, how can I compensate for it?

I hope the question is clear enough. If you need any more information just tell and I'll give it the best way I can.

Thanks in advance!

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Pathfinder Tags: accelerometer vibration testing
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#1

Re: Impact of accelerometer weight on test

05/11/2010 7:03 AM

Yes the weight of the accelerometer will affect your measurements, but it depends on the relative mass of it compared with that of the component. (if it is say 1 or 2% of the component's mass then I'd say it's probably ok, but if it's 10% then it would be rather significant I'd have thought)
It maybe better to mount the accelerometer immediately beneath the component on the other side of the board?
Del (this is all just a cat gut feeling)

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#2

Re: Impact of Accelerometer Weight on Test

05/12/2010 3:32 AM

The G loads on the components will be exactly what the vibration machine applies to the Unit Under Test. Unless the PCBs are mounted in some way designed to reduce shock.

In other words I don't think I understand the question.

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#3
In reply to #2

Re: Impact of accelerometer weight on test

05/12/2010 3:38 AM

The applied G will be the same, but the resultant on the component may not be.
The resonant frquency of the component will be changed by adding a mass onto it. Imagine a long slim component, add a weight to the top and a horizontal vibration (at the right frequency) will have a much worse affect. If adding a mass does nothing...how come the weights added to the wobbly bridge did anything?
Del

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#4
In reply to #2

Re: Impact of accelerometer weight on test

05/12/2010 3:40 AM

Yes, the G loads applied to the base of my component will be the same, but those G loads might have different effects on different parts of the unit under test. At some places it might be damped to a lower G, and at some points it might be amplified to show a higher G due to geometry and resonances.

By placing an accelerometer on my component, I am effectively increasing its weight, and thus, changing the resonant frequency it will respond to. So it might start resonating at, say, 350Hz instead of 370Hz. What I want to know is what would be the magnitude of this impact, and if it would change the results of my test significantly...

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#8
In reply to #4

Re: Impact of accelerometer weight on test

05/12/2010 1:00 PM

Ah thanks Del and ArchMagus,

Sounds as though your accelerometer will weigh about the same as one of the ICs on the board: so yes it will have a significant effect.

Aside: how come I'm not getting E-Mails to tell me that new comments have been added (to any threads)?

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#5

Re: Impact of accelerometer weight on test

05/12/2010 3:43 AM

Hi,

the problem is much more complicated.

Your board is able to go through very many resonances if you tune the exciter frequency up.

If the whole board is vibrating as a plate then you may have success with one of these accelerometers. If you need simgle components resolved in their vibration characteristics you will need an optical pickoff.

If you have a considerable budget you can work with a holografic system, giving information about the complete surface under test.

You may think about a line-projection system that is projecting parallel laser lines in visible light onto your board, these lines are shaped by your components and this shape of the lines are measured and stored by an optical camera with frame-grabber linked to a computer.

At vibration or other (thermally, load)-induced deformation some more deformation is induced, captured and the difference to the static situation calculated.

This will give much more information within much less time than the accelerometer measurements.

Single point vibration measurements are available too, these need an x-y-table to move the laser in order to scan the complete surface.

RHABE

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#6

Re: Impact of accelerometer weight on test

05/12/2010 8:09 AM

Without detailed vibration specifications and knowledge of your board, it is difficult to advise. However, you should be aware that it can make a huge difference if your vibration is swept or random. My advice is to do a survey with a strobe-light to get a feel for the vibration modes that will be present. You don't need to mount an accelerometer for this. Then mount one accelerometer on the shaker as a reference and place the lowest mass accelerometer you can find, opposite your most massive component on the board. From your optical survey you will have measured the frequency of the dominant mode of your most massive component. As long as the frequency hasn't shifted much with the accelerometer mounted, your measurements will be fairly accurate. If the frequency has shifted substantially, you will need to use a lower mass accelerometer.

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#7

Re: Impact of Accelerometer Weight on Test

05/12/2010 12:03 PM

I assisted with vibration testing of a replacement power supply board in a legacy system. A 3-axis reference accelerometer was placed on the fixture, and another was bonded to the underside of the aluminum heatsink to monitor the response of the UUT (Unit Under Test). The board mounted accelerometer permits us to determine the resonant frequencies so we know where to dwell in the sweeps. The point was to monitor the operational performance of the board while subject to varying amplitudes and frequencies according to various sections of MIL-STD-810 and MIL-STD-167. I didn't really care what forces each component sees, I just need to know what forces are input into the UUT. The shaker tables used the reference accelerometers for feedback to maintain the necessary input excitation as well as to record the forces input over time. If you want to visually monitor the parts while under test, I might recommend you synchronize a strobe light to the shaker table frequency as you sweep through the ranges. With that you should be able to observe the behavior of the parts while under test.

This power supply board uses two Vicor half-bricks and a smaller DC/DC with a custom aluminum heatsink. Each half-brick weighed just over 100 grams. It is fairly heavy for a STD-32 bus type of board. The accelerometers were very small, weighing in at about 1.5 grams similar to these. The weight was much less than 1% so we were able to ignore it's affect on the system.

http://www.omega.com/pptst/ACC104A.html

Here's a pic of the board.

We did manage to crack the PWB when the vibration levels were brought up to 50G. It pointed out areas where we either needed to stiffen the PWB or stiffen the mounting arrangement to reduce board flexure. You can see internal stresses in the FR-4 causing whiteish streaks along the vertical axis. We also cracked some traces where the increased resistance caused localized heating and discoloration of the board. The gold contacts also exhibited fretting where the connector contacts rubbed but otherwise the connectors held up well throughout the testing regimen.

A close up area of the area with the most damage

Good luck with your quest.

Cheers!!

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