Subject:
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Re: r/c servos and firmware
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Newsgroups:
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lugnet.robotics
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Date:
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Mon, 23 Aug 1999 01:55:31 GMT
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Viewed:
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1445 times
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In lugnet.robotics, Ralph Hempel writes:
> Not quite. A typical stepper motor has 3 or four windings which must
> be energized in the right sequence to go in the desired direction.
That's not the type of stepper motor I'm talking about. I've seen the type
you're describing, and they have the advantage that you can drive them with
really slow pulses if you want. They're essentially just 3-phase AC motors.
The kind of stepper motor I'm talking about is nothing more than an ordinary DC
motor with two extra permanent magnets. One of the extra magnets is attached to
the shaft so it rotates, and the other magnet is attached to the housing so it
stays put. When there is no power to the motor these two permanent magnets
align themselves so their N and S poles are as close as possible. To make the
motor go, you apply power for a limited and specific amount of time -- long
enough to go more than 0.5 rotations but not long enough to go 1.5 rotations
(because of inertia the limits are each a little less but you get the idea.)
When the power goes off the magnets take over and stop the motor at exactly 1.0
rotations. Repeat the pulses to get however many rotations you want. Use
gearing to slow this down (depending on how many steps per rotation you want
and how much torque you need).
I've actually built this entirely with LEGO components, but I used a rubber
band in place of the magnets. It works perfectly. My pulses have to be between
0.07 and 0.11 seconds (inclusive) for it to work. It runs continuously at 11
pulses per second, but I think this can be a bit lower (I haven't done the
ranging on this yet). I'll put pictures and the NQC program up on my web site
tomorrow.
> They also have the nasty habit of losing count, under load...it's not as
> easy as "putting in pulses" and having the motor do what you want without
> some sophisticted motor driver circuitry.
You're right -- stepper motors can only be used with fixed loads, or with so
much gearing-down that the variance in load falls within the 0.5-to-1.5 range I
mentioned above. I recommend them for some applications but not others. An
ideal application would be rotating something that's mounted on top your robot
-- say, the camera on a roving photobot, or the firing apparatus on a robot
that fires (real or imaginary) projectiles at its opponents. I plan to use it
in my designs to drive a motorized multiplexor (a device to overcome the RCX's
3-output limit)
- Robert Munafo http://www.mrob.com/
LEGO: TC+++(8480) SW++ #+ S-- LS++ Hsp M+ A@ LM++ YB64m IC13
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Message has 2 Replies:
 | | all-LEGO stepper motor (was Re: r/c servos and firmware
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| (...) It looks like I might not be able to do this today. My web hosting service (hostsave.com) stopped providing my web pages sometime in the last 24 hours and I'm not sure how long it will take to get them to fix it. Also, a correction: I meant to (...) (25 years ago, 23-Aug-99, to lugnet.robotics)
| | | All-LEGO stepper motor
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| I have added a web page showing how to build a stepper motor entirely out of LEGO components: (URL) code and a description of how it works are included. By the way, yes, my web site was not responding for a while. My provider has fixed it and I hope (...) (25 years ago, 23-Aug-99, to lugnet.robotics)
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Message is in Reply To:
| | RE: r/c servos and firmware
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| (...) There are multiturn servos for winches in RC sailboats, but they are REALLY expensive. (...) Well don't give up too fast. They ARE useful for steering, or lifting, or grabbing because we can set the limits of motion. This makes things like (...) (25 years ago, 20-Aug-99, to lugnet.robotics)
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