Total Harmonic Distortion of vibration shaker system 1

[DMPPA]

Hello There,

I have question regarding our electrodynamic vibration shaker system.

Recently we have noticed some abnormal sound (like turning fork) from the system when it is used with head expander. It occurs at the frequencies,300 Hz, 600Hz, 900Hz, 1200 Hz, 1500Hz, 1800 Hz and so on. This sound appears even without the head expander and the situation get worsen with head expander.

We conducted, total harmonic distortion test at vertical axis with bare head expander as mentioned below.


Control acceleration = 1g

Test Frequency(Hz)
5
10
20
30
500
900
1200
1500

CH1 THD +N %
N/A (5Hz)
N/A (10Hz)
8.49286 (20Hz)
7.44264 (30Hz)
8.98874 (500Hz)
53.0711 (900Hz)
26.9406 (1200Hz)
9.44025 (1500Hz)

(CH2 THD + N %)
N/A (5Hz)
N/A (10Hz)
7.02918 (20Hz)
6.28316 (30Hz)
7.08433 (500Hz)
45.2265 (900Hz)
20.5199 (1200Hz)
9.21418 (1500Hz)





Control acceleration = 5g

Test Frequency(Hz)
500
900
1200
1500

(CH1 THD+ N %)
5.1278 (500Hz)
17.4902 (900Hz)
6.4243 (1200Hz)
2.0185 (1500Hz)

(CH2 THD +N %)
6.01514 (500Hz)
12.2896 (900Hz)
6.2920 (1200Hz)
2.7729 (1500Hz)




We think that above calculated THD values are very high and a clear distortion is visible in the graph of the head expander (pls see attached graphs).

Shaker manufacturer advised that system is working fine after reviewing these results.

If you can review this data and provide any feedback would be highly appreciated.

Thank you

BR,
Prasanna

 

[Strong]

Looks like there may be a shaker resonance near 900 - 1000 Hz. This is the most audible frequency for hearing, so it is most noticeable. Look for loose parts/fasteners outside and inside shaker. Try a different mounting location.

Walt

 

[DMPPA]

Hello Walt,

Thank you for the response and suggestions. It's true that sound gets most audible when it passes through ≈ 900Hz frequency .
I've tried with different control accelerometer locations for bare armature condition and still i can hear a clear sound when the frequency sweep through those frequencies. Sounds like the issue with the armature?

BR,
Prasanna

 

[Strong]

There could be an issue with armature support (looseness or crack) or with housing mount (gimble?) or housing base on support/foundation. Some detective work is required!

Walt

 

[GregLocock]

When I started out we used to have mechanics assigned at random to the modal lab, and I knew nothing. We often used to damage the voice coils in the small shakers due to inappropriate setups, basically lateral forces on the stinger. I'd expect that to cause 3rd order distortion, followed sooner or later by an expensive smell of burning insulation.

Cheers

Greg Locock


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[DMPPA]

To give you some more background, The shaker system capacity is 600kgf (random force).Typically we use the shaker for vibration ESS testing. Total moving mass is around 16kg including fixturing and all moving parts of the shaker. The Grms value for these tests does not exceed 8 Grms. Test profiles are created based on MIL-STD-810 and DO-160 standards.

In fact we are relatively new for vibration testing field as well as we intend to use this machine mostly for ESS tests and couple of DVTs.

The thing that concerns me is that shaker manufacturer's response on this inquiry. Even though we report this issue with all of this test data , they claim that the "system is working fine". They mentioned that as long as the response is between the 3dB tolerance limit we would get sufficient accuracy for our testing (regardless of the distortion).

I would investigate further and try to find some clue. Can anyone please suggest some steps for diagnosis?

Any suggestions would be highly appreciated.

BR,
Prasanna

 

[GregLocock]

Probably if you weren't keeping the identity of the shaker and amp a secret we could have look at the specs.

Cheers

Greg Locock


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[IRstuff]

I would look at the power/current consumption by frequency to see if there is a resonance that the shaker is overcoming. Even if the control response is flat; if the shaker is drawing 50% more power at the frequency where you hear the noise, that would certainly limit your ability to drive the shaker at its rated performance at that frequency, which would make it noncompliant to its specification. Moreover, if the shaker is indeed drawing excess power to drive at that frequency, it would be generating mechanical harmonics, by virtue of it clipping its signal, that would cause the controller to have to compensate elsewhere in the frequency space, which will likewise draw additional power, and reduce the life of the driving circuits.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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