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Measuring motor RPM with micro.

Discussion in 'Electronic Design' started by [email protected], Jun 14, 2006.

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  1. Guest

    Hi all.
    I am trying to measure the RPM of a motor and have a few questions.
    The motor is turning slowly(about 132RPM after gearing). There is an
    "optical encoder"
    on the shaft that gives me 8 pulses / rev.This gives me a pulse every
    56ms or so.
    How can I calculate the RPM of the motor WITHOUT using floating point
    routines and in
    a fairly fast time.
    I could wait 1 second , count the number of pulses / 8 * 60 to get RPM
    , but this takes far to long(1s) :0( I can measure fewer pulses , but
    the time is still to long.
    I could measure the period of 1 pulse , invert / 8 * 60 to give RPM.
    This is much quicker , but involves floating point routines. I can gate
    a timer in the micro with the pulses from the encoder to give me a
    count in the micro , but this will also involve floating point routines
    to get back to rpm.
    Is there a simple solution to this problem(probably) and I've missed
    the boat completely or am I stuck with the options I have described:0(
  2. Paul Burke

    Paul Burke Guest

    More information needed. What range of RPM, what resolution required?

    You might be able to use a lookup table for the inversion, perhaps with
    interpolation to increase the resolution.

    Paul Burke
  3. The 'key' is to choose your values carefully. If (for instance), you have
    a master clock at 2323200 Hz, and you count pulses on this between the
    edges of the optical encoder, then this directly gives RPM*1000. If you
    instead use a more readily available clock, like perhaps 8MHz, then the
    result needs division by 3.443, to give the RPM in the same '*1000' form.
    Though it sounds as if you would need FP for this, if you multiply by 74,
    and then right shift the result 8 times to give /256, you get division by
    3.46, which may well be acceptable. This would give 131350 for the 132RPM
    example given. Generally if you use integers, and select a scale like
    this, you can achieve the same results, without needing FP maths.

    Best Wishes
  4. Guest

    Hi there. Thanks for the reply.
    The rpm range is about 130 to say 700. Lookup table is a
    possibility.Not exactly sure how to impliment it yet.
    Something to look at though.
  5. Ian Bell

    Ian Bell Guest

    If you want a fast answer the best you can do is measure the period between
    pulses. Be aware though that the resolution with this method is better at
    low rpm values and worse at high values. You first step is therefore to
    decide what resolution you need.

    If you can wait one second and count pulses then the opposite is true -
    resolution is better at higher rpm and vive versa.

    Assuming speed is what you require the fastest method will be a look up
    table. You need to scale the pulse period count then check against
    boundaries (max and min allowed values) then subtract the minimum. Use this
    value as an offset into your look up table which contains the (previously
    calculated) rpm values.

    If you micro has a 16 bit divider there may be an alternative. RPM =
    60000/Tp where Tp is the period between pulses in mS. If you count is not
    in mS (highly likely) then some scaling will be necessary but with some
    thought you can usually limit it to simple shifts and a change of the

    By the way, this is just the sort of problem that typifies many embedded
    applications and demonstrates why it is important to know your micro
    architecture and not to blindly code in C. Well done for thinking about the
    problem first.

  6. Tim Wescott

    Tim Wescott Guest

    Oi. Kids these days.

    You need revolution/minute, and you have clock ticks/revolution, right?

    So do the math:

    revolution/minute = (clock ticks/minute)/(clock ticks/revolution)

    Let's say that you have 1us clock ticks. That means you have

    (60000000 clock tick/minute)/(clock ticks/rev)

    That'll fit into 32 bit integers, no problem.

    Want RPM in tenths? Use

    (clock ticks/(minute/10))/(clock ticks/revolution)

    That'll _still_ fit in 32 bit integers.

    Granted, going higher will require some fancy footwork, but you should
    see where I'm going.

    Dig through the archives of the Embedded Systems Programming magazine --
    the have had more than one on doing this kind of stuff with integer


    Tim Wescott
    Wescott Design Services

    Posting from Google? See

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at
  7. Rich Grise

    Rich Grise Guest

    I saw a guy do this in hardware with two or three banks of counters.
    He used the first to count, say, milliseconds in one period, then
    took that result and preloaded another counter which divided some
    fixed clock by it, giving 1/t * some factor, which he then displayed,
    or counted again, I don't remember exactly. but it _can_ be done. :)

    Good Luck!
  8. Guest

    Or for a different approach, let a pulse begin an exponential
    discharge, and at the next pulse read the v of the discharge: pulse v =
    rpm. A key question here is how accurate you need it, and not how
    accurate at all times but how accurate where. To explain this, you
    might for example need it accurate at one speed, but could tolerate
    increaseing inaccuracy at speeds further afield..

  9. John Fields

    John Fields Guest

    Build a little integrator?

    use the one-shot to make all the pulses one width and then use them
    to charge a cap:

    PULSES IN>----| ONE SHOT |--+--[DIODE>]--+-->OUT TO µC ADC
    +----------+ | |
  10. John Fields

    John Fields Guest


    PULSES IN>----| ONE SHOT |--+--[DIODE>]--[R]--+-->OUT TO µC ADC
    +----------+ | |
  11. Jim Thompson

    Jim Thompson Guest

    Oops! Why not just a single R/C to average the output of the

    ...Jim Thompson
  12. Mochuelo

    Mochuelo Guest

    If you drop the diode and one resistor, this nonlinear circuit turns
    out to be linear and cheaper.
  13. This old thread in s.e.d might be useful......

    From: Tony Williams <>
    Subject: PIC; Fast frequency measurement?
    Date: Tue, 20 Jun 2000 10:25:57 +0100
    Message-ID: <>
  14. John Fields

    John Fields Guest

  15. John Fields

    John Fields Guest

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