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Using hobby servo to rotate 360 degrees?

Discussion in 'Electronic Basics' started by Eric R Snow, Feb 5, 2004.

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  1. Eric R Snow

    Eric R Snow Guest

    Greetings,
    I can buy a chip that will convert the signal from an HP encoder,
    rotary or linear, to step and direction which can be used with a
    stepper driver chip to turn a stepper motor. What I want to do is use
    the motor to spin a needle above a dial so the thing emulates a travel
    dial indicator. But I need 1000 steps, not 100. I could use a
    microstepping drive but these are expensive. I was wondering if an RC
    servo with the pot removed could be used for this. And done cheaper.
    Any ideas?
    Thanks,
    Eric R Snow
     
  2. Garrett Mace

    Garrett Mace Guest

    Cheaper, yes; meeting your specifications? No. You won't get 1000 repeatable
    steps out of a servo. And without position feedback, it would be very
    difficult to actually know how far the needle went; you couldn't say 1000
    steps would equal one revolution.

    Do you actually need 1000 positions of the needle around the dial? Or would
    100 or 200 individual positions be ok, as long as it still took 1000 steps
    to make one revolution? For the latter, you could simply use an up/down
    counter IC to divide the input steps by 5 or 10.
     
  3. Dbowey

    Dbowey Guest

    Erik posted, in part:
    << I was wondering if an RC servo with the pot removed could be used for this.
    And done cheaper. >>

    That won't work. Without the pot the motor will spin continuously.

    Don
     
  4. Eric R Snow

    Eric R Snow Guest

    Yeah, I actually need the needle to have 1000 positions. You see, this
    is to emulate a mechanical dial indicator. They can be read while
    moving while rapidly changing numbers can't ( at least by me) be read.
    So, I figured if I go to all the trouble of making some kind of
    electronic dial indicator it would be neat if it was also accurate
    then the digital display could be disposed of. Though I have 'em
    (digital displays) all over the shop I prefer analog to digital when
    just glancing at it. Gecko makes a small servo drive and US digital
    sells a small, 1000 line encoder that could be used for feedback.
     
  5. Eric R Snow

    Eric R Snow Guest

    What if a 1000 line encoder is connected to the servo? I forgot to put
    in my first post that's the plan.
    ERS
     
  6. Garrett Mace

    Garrett Mace Guest



    Allegro sells cheap microstepping drive chips with onboard translators, but
    I don't think they have a 5th or 10th microstep capability. So if you could
    find a motor with a weird number of steps, like 500 or 250 or 125, you could
    do it that way. But be warned that microstepping is used mainly to smooth
    operation, and the steps divisions are not usually linear. So to use a
    stepper motor, you really need to use something that cogs at each position.
    I highly doubt you could really make a mechanical linkage and be able to
    accurately eyeball 0.36 degree increments, but that's what you say you
    need....

    So, here's the plan: get your stepper chip and hook it up to the encoder.
    Fine, you spin the encoder once, and the motor spins 10 times if it's a 3.6
    degree motors, and 5 times if it's a 1.8 degree motor. Now, go to
    http://www.sdp-si.com and buy some nice spur or worm gears in a 10:1 or 5:1
    ratio. Their plastic gears are pretty good, but for maximum accuracy go for
    a couple of their metal ones. Now you have a dial that makes 1000 steps in a
    revolution. The plastic gears won't set you back more than a buck or two
    each, the metal ones...a bit more.
     
  7. Eric R Snow

    Eric R Snow Guest

    I forgot about the microsteps being non-linear. On a 3 inch diameter
    dial face 100 divisions gives a spacing of .090" or about 3/32. 200
    marks, so there is one mark for every .0005" and the spacing is down
    to about 3/64" which is close to 1mm. Dividing that space by 5 gives a
    spacing of about .009". No marks, just interpolating. The average
    human eye can easily see the difference between .007". I can see that
    with my glasses on. It's surprising how much detail we really can see.
    Makes me wonder how small the rods and cones are in our eyes.
    ERS
     
  8. Garrett Mace

    Garrett Mace Guest

    Yes, but what did you think of the gearing idea?

    You might also want to investigate how fast this stepper motor is going to
    have to spin and react, there is definitely an upper limit.
     
  9. Eric R Snow

    Eric R Snow Guest

    That's why I wanted to use a servo. Steppers can lose steps if
    commanded to step too fast. Gearing is fine but makes the stepper spin
    that much faster. I built a small X-Y stage for engraving using
    steppers and software from, well, I forget. Anyway, gearing down can
    get really fine motion and was used on the X-Y stage. So your idea is
    good but for the speed. Most the time it would work but really
    cranking might cause lost steps.
    ERS
     
  10. Garrett Mace

    Garrett Mace Guest

    That's why I wanted to use a servo. Steppers can lose steps if

    You could use a microcontroller to capture the encoder pulses and keep a
    running count of the encoder position verus the dial position, and if the
    speed of the incoming pulses is faster than what you discover the stepper
    motor can handle, the microcontroller will drive the motor only at the
    stepper's maximum speed until it catches up to the encoder. Of course this
    would only work with intermittent high speed moves, not continuous high
    speed motion. Maybe if a high speed is detected, you could kick in a
    divide-by-10 or so, and light an LED that lets you know the dial is
    indicating ten revolutions? And then when the speed goes down again, the
    needle can advance back to the true-angle representation.

    Overall it's looking to be just a bit more complicated than a digital
    readout....
     
  11. Eric R Snow

    Eric R Snow Guest

    Yeah.Oh well...
     
  12. Si Ballenger

    Si Ballenger Guest

    You might could use a servo controller like the mini ssc II and 4
    servos. I use the servo controller for 180 deg revolution, but I
    think it can be set for servo 90 deg rotation with 254 positions.
    Stack the 4 servos on top of each other with each turning 90 deg
    so the top servo horn will rotate a full 360 deg when each servo
    is rotated 90 deg. This would give ~1016 position resolution in
    one rotation. Probably not practical, but an interesting gizmo
    experiment. I'm looking at using 2 servos set at 180 deg rotation
    and stacked to pan my webcam 360 deg.
     
  13. Garrett Mace

    Garrett Mace Guest

    You might could use a servo controller like the mini ssc II and 4
    Why not use a 1:2 gear linkage in this case too; choose a more heavy-duty
    servo. On my servo-driven webcam I find that 256 steps across (nearly) 180
    degrees is more than fine-grained enough; usually, useful movements are in
    the 5 to 15 steps range. Moving one step you can barely even see a
    difference in the image. 256 steps across 360 degrees wouldn't be too bad.
     
  14. Si Ballenger

    Si Ballenger Guest

    Gearing (or pullies made from plastic bottle/jar caps/tops) can
    make the 360 from a single servo. The origional poster needs 1000
    discrete positions in the 360 rotation. The alternative for
    single servo operation would be to use the 2:1 gearing and
    make/buy a servo control circuit/chip with 10 bit resolution to
    get the 1000 steps.
     
  15. bj

    bj Guest

    Hi
    I'm sort of new to to this electronics thing but I have watched digital
    displays moving so fast that you can't make sense of them when they
    are changing rapidly.I know what you're getting at with the trusted
    needle
    that is visible all all the time. I don't know what you are trying to
    measure
    but most things are reduceable to a voltage and there is no shortage of
    analogue voltmeters.
    My other thought is to to use a three column LED display (using three
    different colours ) your eyes would soon learn to interpret that
    information
    and I dont't think it would take that many diodes to get down to the .36
    degrees in your original scenario
    cheers BJ
     
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