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How to make speed of DC motor match speed of another?

Discussion in 'Electronic Basics' started by Eric R Snow, Jun 11, 2006.

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  1. Eric R Snow

    Eric R Snow Guest

    I have a situation where a dc motor drives a wheel which pushes an
    aluminum wire at variable speed through a 15 foot tube. I would like
    to be able to pull this wire also at the tube exit at the same time to
    avoid jams. When the speed of the pushing roller changes the speed of
    the pulling motor must change with it. I can think of several ways to
    sense the push motor speed but not how to use this to make the pull
    motor spin at the same rate. Is there a simple and cheap way to do
  2. purple_stars

    purple_stars Guest

    this is a non-trivial problem. i used to work with a printer
    manufacturer and some of their printer models had two motors, one at
    either side of the printer, to move the mechanisms across the page.
    they used stepper motors to do it, and spent a lot of time and energy
    tuning them to work together, working out their step tables, etc.
    yours is a slightly different problem because you just need your "front
    end motor" to pull up the slack, so probably it is easier than their
  3. Greg Neill

    Greg Neill Guest

    It might be easier to just have the motor provide a constant
    torque (within reason!) so that it maintains a constant force
    on the wire. This should be proportional to the power being
    drawn by the motor.
  4. Watched on in amusement while a similar project was delayed by 2 months
    while they tried to run 2 motors in synchronism. Told 'em it's not that
    simple and wouldn't be happening but they'd a bit between their teeth, 2
    months already down the tubes and "hey! we're precision engineers, we
    understand gearboxes and we've face to save". Eventually solved by a single
    motor, long PU toothed belt and timing pulleys.
    It's not straight forward.
    If you've just got to have the 2 motors, then maybe continuously overspeed
    the pull unit while feeding the power through a variable magnetic clutch. If
    wire is thin, could result in secondary stretch problems and extra work of
    needing to monitor tension and forming a local servo loop with the pull
    motor and mag clutch. Although I suspect what I know as "wire", is lots
    thinner than your stuff :)
  5. ehsjr

    ehsjr Guest

    The forces don't have to be equal. Have the pull motor
    run faster, and allow the pull mechanism to slip.
  6. Jamie

    Jamie Guest

    use ABS (absolute position using gray code) encoders on both motors
    of the same type and same ratio of swing etc..
    2 vector or alike drives the monitors each others "wait/syncro" signal.
    vector drives also work very nice for this.
    but all in all, if you want to hack it, a couple of
    gray code encoders properly aligned going into a
    PLC 8 bit data input cards you can force both motors
    to stay in perfect track of each other.
  7. Jasen Betts

    Jasen Betts Guest

    Gee, that sounds a whole lot like a MIG welder :)
    I won't suggest oil as that's no be real useful in a mig welder,

    can you get a teflon coated liners for your tube?
    one way would be use a stepper motor with the coils switched by an encoder
    on the pushing motor.
  8. Deefoo

    Deefoo Guest

    Didn't they invent synchro's for that? Search for synchro/resolver/LVDT.

    Example: in old marine radars they used to have two motors, one rotating the
    antenna and a second one rotating the deflection coil on the CRT to follow
    the antenna.

  9. Guest

    How about sensing torque on the puller motor, and using that to control
    the pusher motor proportionally? The higher the puller torque, the
    more push is needed at the other end. This might require some sort of
    PID control to keep from "hunting" due to the intertia of the system,
    but it seems a safer bet than trying to match the speeds.

    Just a thought...

    Bob Masta
  10. kell

    kell Guest

    Just get rid of the push motor.
  11. Eric R Snow

    Eric R Snow Guest

    Greetings Greg,
    Thanks for the suggestion. I had thought about that and it helps when
    others come up with suggestions that tend to validate my ideas. There
    are available adjustable slip clutches meant for constant duty. So
    maybe that's what I'll need to do. I'm trying to keep the drive as
    compact and as light as possible though.
  12. Eric R Snow

    Eric R Snow Guest

    Gee Jasen, it seems like you may be reading my mind. It's kind silly
    maybe but it's a small welder and the weld shop says that even with
    the teflon liner the wire tends to tangle. With larger MIG welders
    there are pull systems, like the Cobra, that work great. I could even
    adapt one to my small unit. But they are expensive and heavy. So I'm
    trying to think of ways to make a cheap one. Mostly just for my own
    pleasure. Stepper motors could work. US Digital sells a chip that
    accepts signals from an encoder and outputs step and direction. I have
    one of these, an encoder, and a chip that accepts step and direction
    and will drive steppers up to a couple amps at 24 volts. But stepper
    drives are sensitive to electrical noise. And the encoder is slated
    for a different project. It looks like a slip clutch just may be the
    way to go.
  13. bg

    bg Guest

    Eric R Snow wrote in message ...
    Think in terms of a reel to reel tape deck. A capstan and pinch roller can
    spool out tape at a constant speed with much less than 1 percent variation,
    while a torque motor takes up the slack on the output. The input to your
    tube could be set up like a capstan and pinch roller. The motor could be
    running at the desired speed and the pinch roller engages only when wire is
    needed. The output of the tube would again be a capstan and pinch roller but
    continuously energized to pull the wire. Depending on how much tension you
    put on the wire, a brake may be needed at the input.
    A simple AC motor running at less than full power should work at the output
  14. feebo

    feebo Guest

    similar problem to cassette tapes. In those, the speed of the tape is
    controlled by the capstan and pinch roller but the wind-on spool runs
    much fatser, the speed is maintained by tension on the tape and a
    slip-clutch on the wind-on spool drive. you can see this by "playing"
    the cassette mech with no tape in - the wind-on spool spins rapidly.

    I would go down a similar route with the pull wheel running faster but
    with a slip-clutch. This way it applies as much tension as you need
    but you have no sync problems.

    The slip-clutch is fairly easily constructed by using a belt drive
    onto the wheel axle (best onto another wheel mounted so as to get the
    gearing right). The amount of tension can then be easily adjusted by
    tensioning the drive belt, but if the wheel is stopped (i.e. by your
    hand) the belt slips on the axle.

    Get an old cassette mech and play - it'll give you a load of clues for
  15. Eric R Snow

    Eric R Snow Guest

    The slip clutch is the way to go. I'm gonna build one using a wave
    washer for a spring and aluminum bronze against hardened steel as the
    friction surfaces.
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