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Vibration analysis???

Discussion in 'Electronic Design' started by mikem, Apr 26, 2004.

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  1. mikem

    mikem Guest

    I would like to construct a rudimentary vibration analysis system.
    The goal would be to find the angular postition of the "heavy
    side" of a shaft rotating at ~2400rpm (40 rev/sec). Ideally, the
    system should report where (and how much weight) needs to be
    added to the shaft opposite the heavy side. Think of how a tire
    shop "spin balances" your new tires...

    I know such systems can be purchased; however I have more time
    than money...

    I know about accelerometers, how to condition them, interface
    them to an ADC, and have a strong background in analog and digital
    electronics.

    One option that I see is a purely analog solution, which uses
    an optical pickup to provide a single pulse per shaft rotation as a
    reference, a PLL to get a clock locked to 50x or 100x the shaft
    rotation rate, a switched-capacitor synchronous clock-tuned bandpass
    filter to select only the fundamental of the accelerometer
    signal, and then to use an oscilloscope to display the phase of
    the filtered accelerometer signal relative to the optical pickup zero
    crossing. A refinement would be to use a scope with a polar display.

    I am also looking for some algorithms to do this in DSP software.
    I am familiar with writing code for the ADSP2189, TMS430, Cygnal8051,
    various PICs and VisualBasic on PCs. I dont have a strong DSP
    background, so I am looking for suggestions of basic computation
    based methods which would do this task.

    At 40hz, I think it may be possible to implement this on a laptop,
    using the laptop screen as a graphical or polar display. It is not
    even necessary to update the display in real time, so postprocessing
    a few revs worth of buffered ADCed accelerometer data may be
    acceptable.

    A stand-alone DSP chip driving an lcd or circle of leds display
    would be a cadillac solution.

    Looking for ideas.

    MikeM
     
  2. Me

    Me Guest

    I suggest the use of an optical phase quadrature shaft encoder (e.g. HP or
    sim.) to deduce shaft position. This avoids the need for PLLs to
    interpolate within a revolution. The PQ encoder should drive a
    parallel-readable up/down counter.

    You can place the encoder inline on the existing shaft if the shaft dia. is
    rite, or run a toothed rubber belt to an idler shaft with the encoder on
    it. Larger toothed wheels (1:1 ratio) will reduce the lumpiness of the
    drive transfer function.

    You can get resolutions up to several thousand points/revolution, but I
    would assume that you are only looking for a degree or so. I recommend a
    power of two (512, 1024 etc.) so that simple AND masking can give the
    position within the current revolution without tricky modulo 360 stuff.

    Some encoder models have an optional zero detect output, which can be very
    handy for setting accurate zero or other reference points.

    If the CPU is quick enough, you could poll the up/down counter for each
    count change and store the force sensor reading for that instant.

    You will need a fairly fast (>= 100Ksps) ADC. If it has a digital
    anti-alias filter, you will need to allow for the time delay it introduces.

    Due to the vagaries of bearing rattle etc., you may need to average the
    measurements with the corresponding points from previous revolutions to
    reduce noise. A variable averaging time constant is very handy here to let
    you get a stable reading or assess the bearings as desired.

    With a bit of effort, you could make the display indicate the part of the
    rotor which needs to be brought to the top for manual attention.

    Interesting job.
    Jim Adamthwaite
     
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