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1 RPM Motor help.

  1. Feb 11, 2009 #1
    Hello all.

    This is my first post as I need some help.

    I'm building a "barn Door" project which I'm sure you all know is a device to track the stars.

    With the measurements I've built it to I need to drive the screw at 1 RPM.

    I'm finding it very hard to get a 1 RPM DC motor either 230v or 12v.

    Does anyone know of a source?

    If I went the route of a gearbox of say 6000:1 what speed motor would I
    need to attach to this gearbox to obtain 1 RPM.

    Problem is I would like ample torque as well.

    Hope you can help.

    Cremer
     
  2. jcsd
  3. Feb 11, 2009 #2

    Mech_Engineer

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    I would look at www.globalspec.com a search through the AC or DC motors. You'll never find a regular motor that runs at 1 rpm, you'll have to get a motor with a gearbox on it that reduces the shaft speed to your desired value.

    A 6000rpm motor of course.

    Torque is a more difficult issue, and will depend on the geometry and weight of the mechanism being moved.
     
  4. Feb 11, 2009 #3
    Search on Gear Motors. You may also try Synchronous Gear Motors.
     
  5. Feb 11, 2009 #4

    FredGarvin

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    A better choice for this kind of application would be a stepper motor. You can turn them at pretty much any speed and they will be very accurate positionally. I would look around at suppliers to see if they have anything that will fit your specs.
     
  6. Feb 11, 2009 #5

    russ_watters

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    Note that the problem of torque is largely self-solving due to the need for a large gear reduction for speed control, as long as the mount is going to be balanced.
     
  7. Feb 11, 2009 #6
    It would seem like a stepper motor might tend to excite vibrations in the structure, not a good thing for an optical system.
     
  8. Feb 12, 2009 #7
    Thanks guys and I'll look through those sites.

    I did consider stepper motors but someone said that the steps to incite slight movement just like dr D has mentioned.
     
  9. Feb 12, 2009 #8

    FredGarvin

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    Stepper motors are used in highly accurate machining centers and the like. I have never heard of an induced vibration duer to one either. I am not quite sure how that would happen.
     
  10. Feb 12, 2009 #9
    Telescope vibration, telescope motor vibration.

    http://www.bbastrodesigns.com/vibration.html" [Broken]

    "Stepper motors snap from step to step when operating in fullstep or halfstep mode. This causes a jittery motion in the eyepiece that can be quite objectionable..."
     
    Last edited by a moderator: May 4, 2017
  11. Feb 12, 2009 #10
  12. Feb 12, 2009 #11
    Machining centers are usually very compact and stiff. Telescopes and other optical systems are often quite extensive (long dimensions) and necessarily not very stiff (too much weight) so that they cannot enjoy the stiffness of a machining center.
     
  13. Feb 12, 2009 #12

    mgb_phys

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    Not by the time it's gone through a 600:1 gearbox!

    Have you considered just a falling weight - historically it's worked pretty well for smooth star tracking.
     
  14. Feb 12, 2009 #13
    The only thing the 600:1 gear box will add to the system is inertia. The frequency of the pulsations will go through the gear box unmodified, so that if it excites a structural resonance, you still have a problem.

    The falling weight idea is a good one, provided that friction does not consume to much energy and does not develop stiction induced vibration.
     
  15. Feb 12, 2009 #14
    So say I did use a stepper motor....

    I have a 1.8 degree 200 stepper motor which means that one step = 0.06 degree.

    How do I get it to turn exactly 1 RPM

    360 degree divide by 0.06 degree = 6000 steps.

    I think I need help
     
    Last edited: Feb 12, 2009
  16. Feb 12, 2009 #15

    mgb_phys

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    If you have a 200count stepper that bounces 1/2 step you are going to need a very tight gearbox for that frequency of movement to transmit through to the output shaft.

    With a hanging weight you can always overcome friction (theres always more power available!) stiction could be a problem depending on what materials and machining services you have available.

    OP - how big an instrument are you planning?
     
  17. Feb 12, 2009 #16

    mgb_phys

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    I think you have got a little confused.
    A 200count stepper moves 1/200 of a circle on each step, ie 200steps for a full circle or 1.8deg/step.

    You can't drive your telescope directly becuase on each step it would jump 1.8deg.
    Normally you would have a large arc with gear teeth on the edge and a worm gear so that one complete rotation of the worm gear moved the arc 1 tooth ( perhaps 0.05deg) then it's easy to run the motor at some reasonable number of rpm to give exactly the correct rotation speed and the whole thing is fairly smooth

    An easier arrangement is to use a threaded rod and a moving nut attached to your telescope

    tnt_alt.jpg

    Description of a small system here http://home.att.net/~jsstars/slomo/slomo.html [Broken]
     
    Last edited by a moderator: May 4, 2017
  18. Feb 12, 2009 #17
  19. Feb 12, 2009 #18

    mgb_phys

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    Don't use a curved rod - use a straight one and change the motor speed in software (trivial with a stepper)
    For a camera you don't need sub-arc second tracking accuracy you can probably just have one speed for each 5-10deg altitude intervals.

    In engineering the thing is also called a 'sine bar' because the change in the screw length is the sin of the angle.
     
  20. Feb 14, 2009 #19
    You may also consider antibacklash gearing.
     
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