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20kHz to 2MHz Laser Vibrometer

Discussion in 'Electronic Basics' started by ravide, Nov 23, 2005.

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  1. ravide

    ravide Guest

    Dear Friends,
    I am currently looking for a cheap Laser Vibrometer to measure
    ultrasonic vibrations upto ideally 2MHz. I know Polytech have a line of
    laser vibrometers but unfortunately its out of my budget. Currently i
    am using AE (acoustic emission sensor) to measure vibrations upto 2MHz
    but due to the size of the sensor i am unable to attach it directly to
    the test specimen which in my case is a steel rod embeded in concrete
    and hence i am using an attachment fixed to the rod and the sensor
    placed on this attachment.

    Does anyone have any suggestion on any type of noncontact measurement
    which operates in a wideband (upto 2MHz or more) such as noncontact
    ultrasonic transducers or magnetostrictive sensors or EMATs? My budget
    is approximately $2000.

    Is there any Laser measurement system or something that can be
    fabricated to measure these high and wideband frequencies using lasers?
    I read in previous posts where someone mentioned that its quite easy to
    fabricate a laser based vibration measurement system...but i am quite
    sceptical on the accuracy.

    Thanking you in advance for your time and any comments are much
    appreciated,

    best regards,
    Ravi
     
  2. Guest

    I use the Ono Sokki. I think it's a 1700 series. I am away from the
    clean area right now. But I don't think you can even buy a case for it
    at $2000
     
  3. Do you have a instrument capable of a fourier transform to say 4 Mhz
    and 10 bit resolution laying around? If so the optical part is
    difficult but maybe doable in a 2K budget, depending on your skills.

    Steve Roberts
     
  4. ravide

    ravide Guest

    Hi Steve,
    I have a card which is capable of fourier transform up to 20MHz with a
    12 bit resolution. Do you know if i could outsource the fabrication as
    i am not very skillful at this. Do you know where i can get info on
    fabricating such a device? Is it really doable?

    Thanking you in advance for your time,

    regards,
    Ravi
     
  5. take a look at the following for some examples, its not too much
    different from a laser doppler velocimeter

    Sun Yusheng and Jin Jundong
    Optics and Laser technology 20 (100) 1988

    High resolution self-mixing laser-Doppler vibrometry with a
    laser-diode-pumped microchip LiNdP4O12 laser.
    Otsuka, Kenju; Fukazawa, Takeki; Sekita, Nobuatsu; Higashihara,
    Shin-ichi.
    Japanese Journal of Applied Physics, Part 2: Letters (2000),
    39(7B), L763-L765. Publisher: Japan Society of Applied Physics, in
    English.

    Multipoint optical fiber vibrometer Raffaella Di Sante Lorenzo Scalise
    Rev. Sci. Instrum. 73, 1321 (2002)


    If you do it with fiber optics, it would not seem to be a problem,
    again depends on what you need.
    really take a look at:

    Applied Physics Letters Volume 76 , Number 23, 5 June 2000.

    minimum parts required:

    1 hene laser or stable 1520 nm laser (telcom surplus, very cheap)
    1 4 port fiber optic 50/50 coupler (~ 525$ for visible, much
    cheaper for IR)
    1 small length of fiber as needed maybe 15$
    1 beaker of index matching fluid again maybe 50$
    1 high speed photo detector depends a lot on specs.
    4 couplers for the fiber optics,
    1 microscope objective
    1 aluminum plate to mount it on.

    thats less then 1000$ in parts, leaving you 1000$ for a fast detector
    and some positioning.

    FFT board that you already have.

    adding direction sensitivity or scanning to that is complex but
    doable.


    Steve Roberts
     
  6. Tm

    Tm Guest

    Could the optics from a dvd or cd device be used? I guess it could launch
    into fiber with some effort. Would it be stable enough?

    Tom
     
  7. Robert Baer

    Robert Baer Guest

    There may be no need to attach anything to the rod, or at minimum a
    small reflective "patch".
    Then one might use simple interferometry and a "$10" laser..

    Or magnetize the rod or attach a small magnet (RS 64-1895 Rare Earth
    Magnet), and use a simple coil pickup to an appropiate receiver.

    Sound pickup to 2MHz i think would be a problem, but at least check
    out the possibility.
     
  8. ravide

    ravide Guest

    Thank you so much for all the input. Steve, thanks alot for the listed
    papers and your great suggestions, i will start looking at them.

    Robert, thanks for your suggestions. Would a simple interferometry and
    '$10' laser be accurate enough for high frequency measurement or would
    it be better looking at fibre optic hydrophone type sensors (i have
    included a website below)?

    Just to give you a bit of detail on my application, I am using a novel
    ultrasonic generator which is capable of vibrating any structure at
    different natural vibration modes simultaneously using a self adaptive
    feedback system. I will be using a pattern recognition software
    (wavelet based analysis) to discern differences in vibration pattern to
    infer the condition of the structure after some pre-processing of the
    time domain signal (de-noising and SNR improvements using another
    wavelet based analysis). In order to transform the time domain signal
    to frequency domain i will be using a novel Hilbert transform which is
    capable of frequency,phase and amplitude estimation with a relatively
    high degree of acuracy. I came across this website
    http://www.rp-acoustics.de/english/FOPH2000_e.htm
    The Fiber Optic Probe Hydrophone and PVDF hydrophone are used to
    measure liquid pressure but do you think a similar concept using
    fibre-optics can be adapted for my application where i am measuring
    vibrations in solids?

    Once again, thank you so much for all your input, i really appreciate
    it.

    best regards,
    Ravi
     
  9. Robert Baer

    Robert Baer Guest

    Almost sounds like you might want interference (2D/3D object) holograms.
     
  10. Look at thorlabs, they stock a 2X2 fiber coupler, this has 4 fibers
    coming out of it. One goes to the source laser, one goes to the
    detector , one goes to a batch of index matching liquid in a beaker at
    act as a dummy load for one port. The forth port is aimed at your
    vibrating rod. This builds a sensitive fiber optic interferometer.
    Adding a microscope objective increases the amount of light you could
    couple from the pump diode to the system. adding another one lets you
    look at a very tiny point on the rod.

    SEE Applied Physics Letters Volume 76 , Number 23, 5 June 2000. for
    a drawing.

    The hene or red diode (a pointer would do) 2X2 coupler is 525$ ,
    the IR coupler is a cheaper 190$ or so.

    Steve Roberts
     
  11. ravide

    ravide Guest

    Hi Steve,
    Thank you so much for your help. Based on what you mentioned i have the
    following items to purchase

    1. 2X2 fiber coupler
    http://www.thorlabs.com/ProductDetail.cfm?DID=6&ObjectGroup_ID=374&Product_ID=26499
    ($525)
    2. 1 or 2 microscope objective
    http://www.thorlabs.com/ProductDetail.cfm?DID=6&ObjectGroup_ID=1044&Product_ID=31752
    ($422)
    3. High speed Photodetector. Which would you reccommend?
    http://www.thorlabs.com/NewGroupPage9.cfm?Category_ID=77&ObjectGroup_ID=1295
    (DET210 - $129) or
    http://www.thorlabs.com/NewGroupPage9.cfm?Guide=10&Category_ID=77&ObjectGroup_ID=4
    (PDA 400 - $389)
    4. 1 small length of fiber as needed maybe 15$
    5. 1 beaker of index matching fluid again maybe 50$ (where can i
    purchase this?)
    6. 4 couplers for the fiber optics,
    7. 1 aluminum plate to mount it on.

    Lastly would it be possible to obtain a schematic drawing of such a
    system? I am unable to obtain the Applied Physics Letters Volume 76 ,
    Number 23, 5 June 2000. Any help on this matter is greatly
    appreciated. looking forward to your kind response.

    with best regards,
    Ravi
     

  12. A schematic is shown in the first picture at

    http://www.dipmec.unian.it/misure/projects/PhD/disante/disante_en.htm

    you only need about half the parts, his shows a dual vibrometer. This
    means he has built two of them and is running them both off another 2x2
    splitter

    Steve
     
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