Help for system vibration coupling problem

[KLATencor]

We have optical plate, in which one pair of scanning mirrors (8KHz and 13Hz each) and some optical lenses are mounted. The optical plate is mounted on an isolated stage. My question is, assuming the stage is isolated so well that no vibration is from ground, what kind of vibration (frequency) we should measure on the optical plate? Only 8KHz and 13Hz from scanner? Does it have anything to do with the way of mounting?

 

[GregLocock]

In a linear system the only vibration frequencies present are the excitation frequencies.It is physically impossible to generate other frequencies from them in a linear system.

OK, what is a linear system? All compliances must be linear. No clearances.So your mounting may be at fault if you are seeing (especially) 3rd harmonics of either of your 2 frequencies.

It is possible to generate phantom waves (not the correct terminology) for instance if you mix 12900 and 13000 Hz you will see a 100 Hz envelope on a scope, and be able to hear it as well. None the less, correct FFT analysis will show that there is no 100 Hz present in the system.

What other frequencies are present? Did you mean 8 kiloHertz and 13 Hertz in your post?

Cheers

Greg Locock

 

[ANTOINE]

Well you will mainly see the scanning frequencies as greg sais, but this is assuming the control of the mirror provides pure sine motions.
If the scanning motions are more like sawtooth, then you will see many harmonics on a FFT (decomposition in sines).
If the motion involves shocks, as if for example, there are brutal changes in the scanning direction (sawtooth) then broadband noise will appear too, maybe able to excite the suspension modes of the assembly..You will then see them on the responses.
Then, it has to deal with the mounting : all sx natural frequencies o the suspended body can appear...

Thomas.

 

[charliebPE]

I'll add that I have yet to find a transducer that did not introduce harmonics due to inherent non-linearities. Neither piezoelectric ceramics nor magnetostrictive materials, both of which will operate in your frequency regime, are free of this problem.

Be that as it may, a dynamic system with a high Q will tend to suppress the relative magnitude of any harmonics when operated around its resonant frequency. (It's acting as a narrowband filter.)

Charlie Bright, PE