Rotating a sphere with a stepper motor [help is needed]

Discussion in 'Replica Props' started by ThePropBox, Sep 14, 2015.

  1. ThePropBox

    ThePropBox Active Member

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    Hi all of you!

    Let me explain the problem I'm currently struggeling with.
    I'm trying to build up a gimbal-like system using stepper motors. Imagine a smaller hollow sphere inside a bigger hollow sphere.
    A stepper motor should be mounted between both spheres being attached on the outer one whereas the inner one should then rotate.

    As this system is using mostly hollow spheres whith diameters from 350mm down to 300 a space saving motor would be great.
    So I stumbled over this one here which got me curious: http://en.nanotec.com/products/564-st6318-ultraflat-stepper-motor/

    Now I'm wondering if this stepper motor would be strong enought to handle this amount of weight since the inner sphere itself comes down to around 2kg.
    The inner sphere should be able to be rotated to different positions quite fast so I'm not sure if the holding torque would match the weight. The sphere is balanced though with a bearing mounted on the other side to keep it steady.

    Do you guys think it actually works with the specs the motor provides? I'm slowly coming into motorizing objects and props so keep that in mind :)

    Rotor Inertia, 16gcm²
    Resistance per Winding, 3,8Ohm
    Inductance per Winding, 2mH
    Holding Torque unipolar, 6Ncm
    Holding Torque series, 0Ncm
    Holding Torque parallel, 0Ncm


    For explanation I did a quick sketch in 123D to give you a better understading of the whole concept. The red thingy is the stepper, the blue one on the other side a fixed mount with a bearing.

    THANK YOU so much for your time reading (and hopefully answering),
    Mario
     

    Attached Files:

  2. Fishbowl

    Fishbowl Well-Known Member

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    I don't have any idea how to do the math to solve this problem :wacko but based on the information you've given, if you're planning on building what I think you're planning on building.... then i'm very excited (and I hope someone solves this problem)!
     
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  3. ThePropBox

    ThePropBox Active Member

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    Me neither therefore I'm hoping there is some engineer I can trust on this forum to help me out with that :)


    You sneeky little... :D :lol :devil
     
  4. Duncanator

    Duncanator Sr Member

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    Guessing from the specs of the stepper motor, I'd say that it probably isn't strong enough to move such a large, heavy sphere as quickly as you described. Direct driving the sphere from the shaft without gear reduction would take more torque and holding power. I assume you are trying to keep the motor hidden from view.

    I do have an idea for a different option. I didn't look very deeply into the nanotec website, but I noticed that they sell hollow shaft motors. You could put a larger, hollow-shaft stepper motor inside the smaller sphere, and run the wiring to it through the motor shaft. This way you could use a more powerful motor and hide it inside the sphere. The wires should be fine so long as the sphere only has to do a few revolutions, and then could unwind. If you need continuous rotation, you would have to install some sliding contacts for the wiring.

    I hope that helps.
     
  5. ThePropBox

    ThePropBox Active Member

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    Thanks a lot for your reply!
    I guess I need to reveal a little bit more of what I'm trying to achieve. The inner sphere will have most of it's parts removed giving the viewer a look on the inside.
    They actually do sell those but for just mentioned reason that's definitely a no-go for me since I need the motor to be as hidden as possible. The movable weight should be around 800g but I've been oversizing to give the motor's ability to move certain weights a little more air to breath.

    I thought about taking this one since it looks a little bit more appealing looking at the specs: http://en.nanotec.com/products/451-st4209-high-resolution-stepper-motor-nema-17/
    Let's say the 22mm thick one got a holding torque of around 17Ncm, three times the value the first one I mentioned. That's one I maybe can just hide but I'm certainly not sure if it's even going to have enough torque...
     
  6. Duncanator

    Duncanator Sr Member

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    I have worked on models that used motors similar to that one, and they are pretty strong. It would definitely do a better job than the little one. The little one might be able to do it, but it will be working very hard. It may not be able to handle quick movements or rapid direction changes.

    I have no idea what you are making. Initially you said the sphere was 2kg, so 800g would make life easier on the motor. Mostly it is the starting and stopping of the inertia of the sphere that will strain your motor. Sudden direction changes will be hard, so you should try to ramp-up and ramp-down the speed when moving.
     
  7. ThePropBox

    ThePropBox Active Member

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    Maybe I can let go of the idea to have too quick movements and, like you already mentioned, slowly bringing the motor up to speed to get a more fluent movement.
    I'd love to try the little motors first but each one costs around 70 bucks. Maybe I'll head for the first one and see if it can handle the weight. Otherwise I'd have to make some sacrifices in looks and go with the stronger one.
    I'm just kinda worried that the inertia will kill the little guy over a longer run. But I guess I'd have to find out the hard way because there's not really a reliable way to say "That motor works definitely" unless I try it in this specific case...
     
  8. Duncanator

    Duncanator Sr Member

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    Yup. Sometimes the ole' "try it out" system is the way to go.

    I think the downside to the weaker motors is that when moving quickly and trying to stop suddenly, the inertial mass will drive the sphere past where you want it to stop, and then the motor will try to drive it back in the other direction to the stop point. This can make the sphere appear to hunt for position, or wobble into it's resting position.

    As far as damage is concerned: It doesn't look like either of these motors has a gearbox that might get trashed by this strain. Instead your motor may get hot from the mass overshooting it's stop point, and having to be driven back again.
     
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  9. ThePropBox

    ThePropBox Active Member

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    You know what... I'm going to build this thing as far as I can to really try two or three motors in order to see which one fits my needs.
    I guess there will be some compromises but I'm sure I can live with that.
    It's a shame that there aren't many of those circular and low profile steppers around, but I guess in the industry they don't care as much for the looks than I do haha.

    Thank you ever so much for your help! I'll be sure to get the worklog started as soon as I finished the project which is currently lying on my workbench :)
     

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