1st order raisonance frequency about 0.001

[reno001]

Hello,
__ __
/ \
/ \ <- metal top (Cymbal)
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| //////////| < - PZT
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I am working on a piezo micro motor.
This works with High Frequency.
A disc is vibrating and other metal parts increase the displacement which occur the final roto-motion….
When I do a modal analysis of this structure Ansys is telling me the first resonance frequency is about 0.001.
Thinking than the error could be related to BC I have enhance the boundary condition.
But that does not change the results.
Is there anyone who came across these strange results?
Is it possible to figure out what is going on?

Many think for your attention.

Regards
Renaud / Cranfield University / Nanotechnology Group

 

[cbrn]

Hi,
are you absolutely sure that ALL the values you input are dimensionally consistent, i.e. that form a consistent system of units? Maybe you expected the results to be in Hz (i.e. 1/s) and for some reason you are calculating in "ms" implicit time unit, so you get results in mHz, for example.
In my experience, the most frequent source of mistake is with the mass unit.

Regards

 

[reno001]

Hi cbrn,

Thanks for you advice.
The system unit looks ok. The second resonance frequency correlates with experimental results. But I am no wondering if I am not just lucky? In the past, I also used the same material declaration for small cantilevers (multilayers structure) and the results correlate with analytical model.
I am still without answer.
Regards.

Renaud / Nanotechnology Group / Cranfield University

 

[AOktem]

Are you sure that you are not encountering a rigid body motion?The frequencies for rigid body motions sometimes come out as very close but not exactly equal to zero.

 

[cbrn]

Hi,
good point, AOktem!! In my experience, the cases when rbm comes out with "near-zero but not zero" frequency is when there is a wrong restraint, i.e. a very "weak" restraint in a general D.O.F.
Completely free general D.O.F. usually gives me a zero frequency (and null-modeshape).

Regards

 

[reno001]

Well done,
Finally I modified the clamping by increase of the area and I found some very different results. I can see clearer now.
Thanks