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vibration sensor

Discussion in 'Electronic Design' started by colin, May 15, 2007.

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  1. colin

    colin Guest

    Hi, Im looking for a vibration sensor to help with shaft balancing,
    most seem to be on/off alarm types or very expensive accelerometers,
    however RS had an analogue piezo type but wich is discontinued.

    I played about with an old piezo sounder, ive found if I put a lump of metal
    on the center of the piezo disc it gives a nice .5vpp at 80hz when I hold
    onto the metal and the case is pressed against the bearing housing.

    Is there anywhere in the UK I can get something similar but ready made that
    will measure small vibrations ?
    would be nice if I could feed it into the ADC of a PIC without just a low
    pas filter.

    I can feel the vibrations with my finger, im not sure what level of
    acceleration that coresponds too, but I hope to balance it so I can no
    longer feel any vibration at all.

    thanks
    Colin =^.^=
     
  2. Rich Grise

    Rich Grise Guest

    Microphone?

    Good Luck!
    Rich
     
  3. In what manner is a $2 accellerometer expensive?


    --
    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: http://www.tinaja.com/whtnu.xml email:

    Please visit my GURU's LAIR web site at http://www.tinaja.com
     
  4. colin

    colin Guest

    $2 isnt expensive at all, what did you see and where at that price?
    the ones in farnell are more like £30

    Ive probably got a microphone I can try somewhere,
    but then the piezo acts like a microphone allbeit a very resonant one,
    I think the moving element needs a reasonable amount of mass,
    or it might be more sensitive to the noise as sound than the vibration I
    need.

    thanks
    Colin =^.^=
     
  5. Jim Thompson

    Jim Thompson Guest

    Surely they can be had even cheaper than that. My Radio Shack
    "universal" remote lights up at the slightest movement... so slight I
    can't trick it ;-)

    ...Jim Thompson
     
  6. colin

    colin Guest

  7. The extra 49,999 pieces you have to buy to get that price? ;-)


    Best regards,
    Spehro Pefhany
     
  8. linnix

    linnix Guest

    What you need is an inertia activated floating capacitor. It can be
    built with silicon on insulator on 6 inch wafer using 1 micron
    process. Projected cost is approximately $2000 for 1 or for 1000.
    Want to spit a wafer?
     
  9. colin

    colin Guest

    well ive just come accros ADXL311 at £9 is not too bad, also it will allow
    me to precisly level the thing too, resolves to 0.1deg. two birds with one
    stone.



    hmm i dont like those legless chips however, oh well.



    Colin =^.^=
     
  10. Check out the 3-axis accelerometers as well, some of them are not too
    expensive.


    Best regards,
    Spehro Pefhany
     

  11. Do a search on DigiKey for MSP1007-ND. Maybe that will get you started.

    Good luck.

    John
     
  12. whit3rd

    whit3rd Guest

    The possible meanings of 'vibration' include all three axes as well
    as multiple torsional modes. Probably you have some
    particular load-bearing forces in mind, and a single axis force
    sensor (like a load cell) under your rotating machinery might be
    enough.

    So that's my input: use a load cell. It can be as easy as cementing a
    strain gage to one of your struts. Or you can get calibrated
    load cells with preamps. Make a variable-frequency (digital pulses)
    output, and feed to a digital processor for detection.

    The important frequencies are going to be multiples of your rotation,
    and lots of averaging schemes can reject out-of-band information
    while building up a solid number from noisy input. Look into phase-
    locking
    detectors (not a phase-lock amp, which is a tricky analog gizmo, just
    a phase-lock-loop with multiple counters).

    So, in phase 0-90 degrees of frequency A you increment
    counter-1;
    in 90-180 degrees you increment counter-2; in 180-270 degrees you
    decrement counter-1; in 270-360 degrees you decrement counter-2.
    With the force sensor running into a voltage-controlled-oscillator,
    you
    can average over hours of operation and get a good measure of
    even a small signal. Counter-1 and counter-2 add in quadrature to
    give a vibration amplitude, and the relative values and signs will
    give
    a phase...

    It sounds harder than it is. There's a nifty seismology gizmo
    called
    a hammer seismometer; you pound with a hammer, and detect each blow
    with a microswitch on the hammer handle. A digital accumulator
    builds up a synchronized composite signal from a seismometer placed
    nearby,
    and after a few dozen strokes the summed signal isn't noisy any more.
     
  13. mpm

    mpm Guest

    I have a vibration meter specifically designed for motor maintenance.
    It is practically brand new with less than 30 minutes total use. Still
    in box, w/pkg, etc..
    Was several hundred dollars (US), & I would part with it for a lot
    less.
    Model: Entek IRD, model # 808 Trobleshooter.
    Email me offline if interested and we'll talk. -mpm
     
  14. colin

    colin Guest

    thanks, ive seen a reasonable signal from a piezo tranbsducer, but only when
    i held onto a metal slug glued to the center, the slug alone was not enuogh.

    The signal wil be fed to the dsPIC wich drives the BLDC motor, so the signal
    can be corelated to the motor phase. probably average the signal and display
    it on the pc, or do an fft.

    I think i just need the fundamental, particularly the phase relationship, so
    i can balance the shaft.

    I think out of balance forces should be detected by 1 axis on the bearing
    housing, of course il have to balance the shaft in several places so il move
    it from one houising to the next.

    Colin =^.^=
     
  15. Jasen

    Jasen Guest

    a phonograph pickup ? (used is much cheaper than new)

    Bye.
    Jasen
     
  16. SioL

    SioL Guest

    Circuir Cellar, last issue, last page, see advert for Parallax IO devices,
    note one of them is a vibration sensor.

    SioL
     
  17. linnix

    linnix Guest

    Piezo transducers need physical contacts for detection.

    Floating capacitor does not. It can be totally isolated.
    Most accelerometers are based on floating capacitors.
    You can mount several bare sensors on the motor, and plot the inertial
    vector.
    Depends on how many you need. $2 to $3 each is doable.
     
  18. colin

    colin Guest

    well ive put one of those piezo transducers with mounting lugs and mounted
    it onto the bottom of the rubber mounted chasis then a screw from the non
    isolated chasis section is adjusted to just make contact with the brass disc
    inside the transducer via the top hole. works quite well with a 2stage 200hz
    rc filter. get realy strong 83hz signal at 5krpm.

    mechanical resonance is at about 5300 rpm, and the phase of the vibration
    varies about 180' either side of the peak.

    all ive got to do now is work out how that relates to where ive got to put
    some weights on the shaft,
    any ideas ?

    I have a strobe wich will flash at the peak, gona try fixing some small
    weights with pvc tape on the shaft and see how it affects the position.

    its 1.2M long and I have a sensor at each end. interestingly it seems to
    differ depending on wich direction the shaft is rotatiing. i need to be able
    to reverse direction and still be in balance. I have a motor in the middle
    and an optical disc at each end all of wich probaly contribute to inbalance.
    The shaft itself is quite straight as much as I can tell with a 0.01mm dial
    guage, although at 12mm dia its quite heavy.

    thanks
    Colin =^.^=
     
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