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optical rpm measuring w. frequency counter

Discussion in 'Electronic Basics' started by J.T., Sep 20, 2003.

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  1. J.T.

    J.T. Guest

    does anyone have easy and accurate way of measuring rpms of diesel motor (or
    any spinning device) optically.
    Idea is to glue little reflecting tape to shaft and optically read pulses,
    then convert pulses for frequency counter to display rpms
    any help, schematic ??
  2. Baphomet

    Baphomet Guest

    Not optical but....
  3. Dana Raymond

    Dana Raymond Guest

    Suck devices are off-the-shelf
  4. Gareth

    Gareth Guest

    One way to do this is to use a strobe with an adjustable frequency. You
    adjust the frequency of the strobe until the spinning device appears to
    be still. The rate of rotation should then be the strobe frequency,
    i.e. if the strobe frequency is 10 Hz, the rate of rotation is 10*60 =
    600 RPM.

    One problem with this method though is that the spinning thing will have
    rotated a whole number of times in one strobe period, not necessarily
    once, so in the above example the speed could be 1200, 1800 or... RPM so
    you need to check at a few other strobe frequencies to be sure, or start
    the strobe at the highest possible frequency and work down, the first
    frequency that appears to be still should then be the correct one.
  5. CFoley1064

    CFoley1064 Guest

    Hi, J.T. The problem with measuring this type of thing optically is that dust,
    dirt, grime, and the thousand slings and arrows of outrageous fortune are all
    conspiring to dirty your sight lines up.

    Many high speed inductive prox switches can detect a cam follower signal up to
    1KHz..Unless your motor or spiinning device goes faster than 60,000 RPM, all
    you have to do is power up the prox switch (usually requires 12 to 24VDC),
    point it at a piece of ferrous metal that's sticking out a little, and use a
    pullup resistor from the output to your + (for NPN or current-sinking type
    devices -- be sure to specify). You then just hook up your frequency counter
    to the output, and you're good to go. Inductive prox switches are available
    surplus, if you look around. View in fixed-font...

    | |
    | .-.
    .-----. | |
    | + | | | R
    | | '-'
    | |OUT |
    | - o--------o----------------o
    '-----' To Freq Counter
    === .------o
    GND |

    created by Andy´s ASCII-Circuit v1.22.310103 Beta

    Remember to choose R such that you don't exceed max current.

    Good luck.
  6. Sailor

    Sailor Guest

    Yes this is a way of doing it..
    But make also a mark on motor where spinning part passes. So when your
    making your measurements you have a reference point. It wil make it much
    easier to see if the picture is still or not.
    I've done this may times to measure fan speeds

    Greetings to all
  7. Spajky

    Spajky Guest

    if the shaft has a some outpointing part like this letter: (Q)

    spinning Q -magnetised coil-amplifier-frequency counter

    the principle is the same like for guitar pick up or original Hammond
    organ tone generator; just that outpointig shaft part has to be iron-
    can be mounted additionally to the shaft (& will function just like a
    guitar string close to its magnetic pick up (electrical guitar
    normally!) producing pulses while shaft rotates!
    cheap & effective way to do it!

    -- Regards, SPAJKY
    & visit -
    Celly-III OC-ed,"Tualatin on BX-Slot1-MoBo!"
    E-mail AntiSpam: remove ##
  8. Don Kelly

    Don Kelly Guest

    Stripes on the shaft and an optical pickup will work. Accuracy depends on
    the number of stripes.The more stripes, the better the accuracy and counting
    should be for a fixed interval of 1 to 10 seconds (experience with a counter
    set to its normal frequency measuring mode gives disappointing accuracy -).
    60 stripes on a shaft at 1000rpm will give 100 pulses per second. Counting
    for 1 second will get you to 1% accuracy on the basis of +/- 1 count. A 10
    second count gives 0.1% accuracy for the 10 second average. This longer term
    average also eliminates the effects of torque pulsations.
    There are other options- A disk attached to an end of the shaft(if
    available) with 60 black and 60 clear segments can be used with a light and
    photocell works well. One or two pulses/revolution will not give decent
    accuracy so the magnetic option is not good unless you can mount some form
    of toothed wheel.
  9. I bought a digital photo tachometer from TNC electronics in Woodstock NY for
    less than $50 USD. Just point and measure. These are used for model airplane
    tuneups measuring the speed of the prop. He made minor mods to get me the
    resolution I needed at no charge.
    ph 914-679-8549

    An alternative anyway.
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