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Old 02-13-2013, 10:37 PM   #1
k2pilot
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Stepping motor design question (Engineers, GTFIH)

Engineers of e46! Hoping there's someone amongst the great diversity of knowledge this place has that can lend me a hand with a challenge.

A project I've been working on involves stabilizing a payload inside a slew ring, like so,


currently I'm using a BLDC motor to drive the inner ring via the teeth on the inside, this works fine, but thanks to the high ratio natural to slew ring gears, I need very high speed and subsequently high cost motors to achieve the speeds necessary to dampen appropriately. Since the goal of this project is to yield something somewhat low cost and scalable, I'm trying to find a way to get rid of the most expensive component, the drive motor for the ring.

It dawned on me it might be possible to make the entire ring like a reverse stepper motor.


So the magnets would be located on the inner drive gear, which is where the power source would also be.

I've come to learn most everything in this field has already been designed and created, I'm hoping one of you guys may know the name of a manufacture or supplier of such a contraption, or at the very least the name for this particular style stepper motor.
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Old 02-13-2013, 10:45 PM   #2
davidwarren
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K2- this is totally unrelated to your post, sorry.
Which fanatic was building some secret machine and kept asking where to find weird stuff? What ever happened with that?
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Old 02-13-2013, 11:04 PM   #3
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Forgive my ignorance, but wouldn't it be possible to gear a motor up so that you could use a lower-speed, higher-torque (and thus cheaper, I'm assuming) model?
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Old 02-13-2013, 11:25 PM   #4
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Forgive my ignorance, but wouldn't it be possible to gear a motor up so that you could use a lower-speed, higher-torque (and thus cheaper, I'm assuming) model?
Yeah but then you'd have a gearbox instead of a direct drive, uping parts count, cost, and possibility of failure.
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Old 02-13-2013, 11:34 PM   #5
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K2- this is totally unrelated to your post, sorry.
Which fanatic was building some secret machine and kept asking where to find weird stuff? What ever happened with that?
sneaky?
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Old 02-14-2013, 11:00 AM   #6
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why not a small cheap motor and a planetary gear box? both are plentiful and you aren't reinventing the wheel.
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Old 02-14-2013, 11:10 AM   #7
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I'm confused. So instead of using a planetary gear set or something of the sort, you want to use another gear with a bunch of magnets to drive the outer ring?

Also, to better understand the application...your payload is somehow mounted to the outer ring gear correct? And this machine you're making has to stabilize the rotation of that outer ring?
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Old 02-14-2013, 11:45 AM   #8
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I think we need a scale factor so we can suggest motor sizes and what not. Also, is your goal to only stabilize about the ring axis? I would also suggest looking up automation industry specific magazines for vendors.
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Old 02-14-2013, 08:47 PM   #9
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I don't think I did a good job explaining my intent. The goal here is to eliminate any sort of motor or gear drive in the traditional sense, and instead turn the entire ring in to one giant stepper motor.

Look at the roll axis in these gimbals,



Notice how they have servos, in this case via a belt drive, slewing the roll axis? The high ratio inherent to the ring design makes their performance sub par, so why not do away with the drive motor and gear assembly entirely, instead making the whole ring internally powered? I can't see any reason why the wouldn't be a practical solution, and it should yield some pretty high slew rates without much added mass. I'm thinking of it like an ABS wheel speed sensor, but powered.
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Old 02-15-2013, 10:35 AM   #10
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So if I'm understanding this correctly, you want to be able to rotate the interior with a direct drive motor, correct? In the picture, the red ring would be static and whatever is in the blue plane would rotate.


Honestly the first thing that comes to mind is an array of electromagnets embedded in the outer ring with the inner ring on ball bearings. I'll see if I can sketch it so you see what I'm talking about.

Edit:
Not the simplest solution but it would undoubtedly be smooth. You're looking at a cross section of one side of the setup and the gaps are exaggerated so I had room to draw. The inner ring is on bottom/inside. The outer ring sits on ball bearings and has electromagnets placed radially on an inner edge. The inner ring has permanent magnets placed similarly on the outer edge. I think you see where this is going, but turning the magnets on/off in sequence could drive the inner ring in either direction.
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Old 02-15-2013, 11:19 AM   #11
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^ That would do a great job of providing rotational force but would be complex in order to make it controllable as a stabilization solution.

Unfortunately OP, I think your best bet is a high torque stepper motor to accurately stabilize your payload unless you are willing to go deep into electronics work.
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Old 02-15-2013, 11:35 AM   #12
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Direct motor driven gimbal has already been done.

http://centuryheli.com/products/prod...5N7&pageid=204
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Old 02-15-2013, 12:33 PM   #13
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So if I'm understanding this correctly, you want to be able to rotate the interior with a direct drive motor, correct? In the picture, the red ring would be static and whatever is in the blue plane would rotate.


Honestly the first thing that comes to mind is an array of electromagnets embedded in the outer ring with the inner ring on ball bearings. I'll see if I can sketch it so you see what I'm talking about.

Edit:
Not the simplest solution but it would undoubtedly be smooth. You're looking at a cross section of one side of the setup and the gaps are exaggerated so I had room to draw. The inner ring is on bottom/inside. The outer ring sits on ball bearings and has electromagnets placed radially on an inner edge. The inner ring has permanent magnets placed similarly on the outer edge. I think you see where this is going, but turning the magnets on/off in sequence could drive the inner ring in either direction.
Thank you! This is what I was trying to say when I meant turn the entire ring in to a stepper motor, the outer ring will be acting as a stator in a stepper motor. I don't get where people are getting this direct drive thing, I want to eliminate it, and turn the whole ring in to a motor instead.

Any reason you opted for having the magnets perpendicular rather than parallel the rotational axis? I was thinking more along the lines of a regular stepper motor and having the bearings sit offset from the


but having the stator and magnet locations swapped. Should be easy to modify a regular stepper motor drive board to work.

the inside of the outer ring will have something like this sitting adjacent the bearing track,



So what I'm wondering, any reason that won't work?
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Old 02-20-2013, 09:50 PM   #14
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I honestly put about 3 minutes of thought into it between classes and didn't think about which way to put the magnets, should work either way. If I'm following what you have in mind then I think it should work.
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Old 02-20-2013, 10:01 PM   #15
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What's the contraption? Robotics? Drone?
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Old 02-20-2013, 10:15 PM   #16
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