Vibration Test Tolerances 2

[vibefreq]

We have been given a requirement for a 0.5g sine sweep 5-2000hz with a tolerance of +-10%. So far we have not been able to maintain the +-10% at the shaker table resonant frequency (about 1300hz).
Has anyone ever found a solution to holding +-10% at the shaker table resonance?
The controller manufacturer says that +-10% at high frequencies cannot be controlled.

Thank you for your reply

 

[GregLocock]

Can you post a plot of the ampitude you get at the moment?

Cheers

Greg Locock


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[IRstuff]

If your table has a resonance in the test band, then it's not the right tester for the test

TTFN
faq731-376

 

[btrueblood]

Ditto what IRstuff said.

 

[GregLocock]

So, either change the table, get a deviation to the test standard, or if you are feeling gung-ho, try and bring your current setup into spec. The latter may or may not be feasible, it certainly is not normal behaviour for a test laboratory.



Cheers

Greg Locock


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[vibefreq]

Yes IRstuff you are right; however, to test parts of the size and weight we have our table is large enough to have resonances below our max test frequency. In some cases we test to 2000hz on a 48 inch slip-plate. This particular slip-plate is 24"
If someone has a design for a 48 inch slip-plate with a resonance above 2Khz, I would be grateful.
I need to get the plots from another group.

 

[IRstuff]

Hydrostatic bearing tables should not have resonances in this range:

http://www.etssolution.com/slip-table-bt-600-800.php

TTFN
faq731-376

 

[vibefreq]

ETSSolution Slip-Tables claim they are good to 2000hz but read the quote from the brochure "Cross-axis motion is constrained and is typically less than 10% of input at frequencies up to 2000 Hz. except at resonance."
And that's the crutch, they all work good, "except at resonance", which beyond about 20" X 20" is going to be less than 2000hz
I'm sure I could find locations on our slip=plate where I can get the results needed, but not across the entire surface.

 

[amanuensis]

You have to keep in mind that 0.5g at 2000Hz corresponds to a displacement of 0,06 micron!
So it seems logical that your specifications are quite difficult to maintain.

 

[vibefreq]

Thank you all.
I'm going to experiment with the 1 g sweep, which seems to be the industry standard, and then start the process of changing the requirements. It has been rumored that it is a customer requirement, so it is going to be uphill task.

 

[BrianE22]

Not fun but maybe you could build a "band attenuate" analog filter to reduce the drive at the resonant frequencies.

 

[IRstuff]

That's something you can program into the controller, one would think. Seems like it would simply be a PSD with a fat notch taken out of it. I've seen relatively complex PSDs that seemed to imply an expectation that the controller could handle the ups and downs of the PSD.

TTFN
faq731-376

 

[GregLocock]

An example of that would be the excellent HP3562A, which is sine sweep mode uses feedback from an accelerometer to control the input signal. trouble is that the sine sweep rate may be so fast that it misses lightly damped resonances. So there's a couple of obvious cures for that.

Cheers

Greg Locock


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