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DC motor control. auto reversing

Discussion in 'Electronic Basics' started by Nick Zentena, Dec 30, 2005.

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  1. Nick Zentena

    Nick Zentena Guest

    I'm interested in building something that can control a small DC
    motor. Basically it needs to turn a wheel 1.5 revolutions. Stop. Then turn
    back 1.5 revolutions. The speed needs to be constant. One obvious problem is
    I'm not sure of the speed and won't be until I've tested the whole thing.

    I've found plenty of websites talking about motor controllers. Some even
    have the ability to reverse. But none have the ability to reverse
    themselves. Can anybody point me to a website or book that might explain
    this sort of thing?

    A little help on motor sizing would be great to. I figure the load will
    vary from about 500 grams to say a max of 1000 grams.

  2. Jasen Betts

    Jasen Betts Guest

    yeah, but how many watts (or (fdractional)horsepower etc) do you need
    from the motor. (you need to describe the task in more detail)

    are you lifting the 1Kg (safest to design for the worst case) weight up
    a great height, or just moving it a few milimetres to the left?

    if the motor has to stop and start for some of the time it's not moving at a
    constant speed.

    how constant do you need the speed to be, cheap tape players typically
    manage 0.5% variation (IIRC) would that be sufficient?

    what sort of time frame is involved. do you want it to turn at a rate of 45
    degrees per month for half a year and then reverse and turn 45 degrees per
    month in the other direction for the other half of the year? or is this
    something that must be done in less than one second?

  3. Nick Zentena

    Nick Zentena Guest


    I need to spin this:

    It's 5" or maybe 6".

    My current setup can't handle something that short. The current roller is
    rated at 120V 0.15 amps and can easily handle a larger load then the drum in
    that picture. That's what 18 watts. The difference is the bigger setup turns
    two rubber wheels that the big drum sits on. The gap between the wheels is
    too big for a small drum. I'm likely going to try and drive the short drum
    using the magnet fitted to it's base.

    Just spinning it then reversing direction.
    I'd bet even a 10% variation wouldn't be a deal breaker. It might actually
    help by introducing some random action into the process. OTOH I'd like
    something that wasn't too random -)

    Worse case is 30 minutes of steady turning. It won't be used much just
    when I need the smaller drum.

  4. Nick Zentena

    Nick Zentena Guest

  5. ehsjr

    ehsjr Guest


    -------- --------- ----------
    | Sensor |===>| Counter |=====>| Reverser |=====>Motor
    -------- --------- ----------
    ^ ^ ^
    2 magnets 4017 DPDT
    + reed counter, relay,
    switch debouncer & driver,
    optional delay

    Glue magnets to the drum, 180 degrees apart.
    Each time a magnet rotates to the reed, the reed
    triggers the counter to increment by 1. Condition
    the signal from the reed with a debounce.
    Wire up the counter to count to 6, then reset.
    Invert the outputs of counts of the last 3 counts
    to cause the DPDT to de-energize; outputs from
    the first three counts cause it to energize.

    You mention that the motor must stop after 1.5
    revolutions, then reverse. Did you mean there
    must be a pause before it starts going in the
    opposite direction? If so, a delay function
    can be triggered each time the DPDT relay
    coil voltage changes state.

  6. Jasen Betts

    Jasen Betts Guest

    hmm a black cylindrical thing with a red band and a stepped end.

    looks a little like a land mine but probably isnt.
    it appears to be reasonably well balanced so there's not going to a lot of
    torque needed.
    most DC motors will meet that 10% figure quite easily if not loaded heavily.
    I'll assume that's typical... is that slowest or fastest?

    my first idea is to make some sort of adaptor to let you use your existing
    machine (or add another roller to it etc...)

    Otherwise it seems to be a matter of taking a small DC motor and gearing it
    down from the 10000+RPM they typically run at to about 0.04 RPM for your
    worst case.

    I'd suggest getting the largest motor you can power easily, the bigger the
    motor the less its speed will fluctuate if there's a rough spot on one of
    the gears etc.

    small gear sets boxes are available from hobby model making places.
    I've seen some that can be "stacked" to give any degree of speed reduction.

    I expect the motor will run smoothly enough so as to not need any regulation
    of its speed.

    For permanent magnet motors with brushes the speed can be reduced quite
    effectively by reducing the voltage to the motor.

    the next problem is the 520 degree turn and then the reversing at the end.
    and coming back (and then stopping?)

    It seems to me that a disk with two features that activate a sensor and a
    system to count the passing of these features would be the way to go.

    start at one of the features, count three new features on the way out,
    and three new features on the way back.

    a setup like that can easily get 5 degree accuracy, and with a bit of care
    could exceed 1 degree accuracy.

    a small circuit board with a few dollars worth of parts.

    another idea is that a small three phase AC motor could be used they give
    an lmost perfect constant speed (but only one speed) and are about as easy
    to reverse as DC motors. (they ca be driven off a single phae AC supply
    with a simple circuit)

    If you need an "industrial strength" solution you could spend a little
    more money and do it "properly" with a PLC (programmable logic controller
    - basically a small computer thingy designed for process controll I expect
    that even the smallest (cheapest) PLC would be capable of tasks many times
    more comple than this one) one of those motor controllers you saw earlier
    and sensors from the same (or a similar) supplier... this has the advantage
    that you don't need to solder it together (and once setup it'll probably be
    more reliable) and the disadvantage that you'll need to program the PLC to
    get the behavior you want. (this is done by hooking it up to a PC and
    running the manufacurers software and entering the program aftwer which
    the device is disconected and runs autonomously)

    Disclaimer: I don't know a whole lot about this sort of stuff. so research
    everything for yourself, consult real experts etc...

  7. Nick Zentena

    Nick Zentena Guest

    No-) Small film tank. Will hold a max of two rolls of film or various
    other combinations.
    I think I didn't explain this right. When I said 30 minutes I meant the
    setup would need to be on and running for 30 minutes at worst. I looked up
    the commerical product that spins the drum. It claims 75rpm. But they also
    require a range of 65rpm to 75rpm for the process. I'm guessing a fully
    loaded large drum slows down thier motor.

    No repeating until the external timer shuts off the power to the motor.


  8. Nick Zentena

    Nick Zentena Guest

    No it just needs to change direction. I'm not even sure if it needs to be
    exactly 1.5 turns but it sounds like a reasonable number.


  9. Jasen Betts

    Jasen Betts Guest

    At 70rpm the half-cycle of 1.5 turns would take approx 1.3 seconds
    so 1.5 turns and 1.5 back would be 2.6 seconds
    ahh... I'm no mechanical engineer (watch the flames fly) but,
    I'd be looking to do an all-mechanical solution

    reversing every 1.3 seconds will be hard on most motors, and/or on any
    mechanisms light enough to be reversed in that way.

    or could you fit the tank inside a tube that would fit on your larger machine?

    it sounds like a old washing machine agitator may have the function you want
    but on the wrong axis, the speed van be varied if needed by using different
    sized pulleys on the motor iirc 1.5 turns is fairly typical and is a little
    over 1 second before reversing...

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