Stepper motors and high speed lumia

[laserist]

Back in the bad old days we did some lumia effects using high speed dc motors with diffraction gratings, prismatic plastic, cylindrical lens mosaics etc. Sometimes even as scan through effects. They were "interesting", but I never really liked the flakiness and lack of fine control of the motor speeds. (Flakiness due in large part to the total lack of attention we brought to the task of balancing the effect wheels...) I just ran across these steppers:

http://www.ebay.com/itm/20-Escap-ste...Bjcr6gD6jJOg6w

I've seen a spec sheet claiming 9000 RPM - I'm wondering if these coupled with the various extremely low cost stepper drivers brought on by the 3D printer world would be useful for high speed lumia. I really can't believe there'd be much jitter at high RPMs, nor all that much current required to drive a diffraction grating. Thoughts?

brian

[mixedgas]

Steppers have a horrible load/torque curve..
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http://www.daycounter.com/Calculator...lculator.phtml
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This explains it better then I can:
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http://www.nmbtc.com/step-motors/eng...-relationship/
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Lumia wheels of decent size have huge inertias, so you end up needing lots of torque to get the wheel to start to spin.
You may need to ramp up the step speed very, very, slowly to maintain torque..
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Steve

[laserist]

Thanks Steve. I expected the ramp up would be significant. The effect I'm trying to emulate was rarely about making significant changes in RPM. I'd just like the motion to be a bit more stable without going to full fledged servos.

Brian

[laserist]

Actually it turns out that Escap describes these as "disk magnet motors" rather than steppers. Rather than the drum rotor of a conventional stepper these have a thin rotor made up of of a multi-polar rare earth disk magnet. They spec 171,000 R/s^2 acceleration and top speed of 6000 RPM (not the 9000 I saw elsewhere). Worst comes to worst they should be fun to play with...

[laserist]

I got to play with a high speed stepper motor last night and they are fast. I had it running at 8500 RPM at 24 volts (there rated up to 65 Volts) with a $2 Chinese stepper driver that really wasn't up to the task. On the minus side their low speed operation could only be described as "gritty". Apparently you can spec the detent torque anywhere from 0 to 20% of the holding torque, and these seem to be at the high end of the range. You can also spec them for ministepping these were not. Also they are loud. They sound like a itsy bitsy jet engine ramping up. It hit 59 db according to my cell phone sound level app - I'm willing to assume my cell phone wasn't really weighted for 8500 Hz. As it turns out it's an OEM part for a small bench top Mill I've been wanting to get running. For lumia it's going to take a completely enclosed box with a lot of effort put into sound abatement. That won't happen soon...