[Robotgroup] Stepper Motor Help..?

[Robo Al]

It is my understanding that the motors themselves will have mechanical
and electrical limits on how fast you can step them. 

If I remember correctly, when you speed them up really fast you 
should have the signal skip steps and figure out where the rotor 
is in relation to the stator.

You are also going to have back emf affecting the circuit, which 
should increase with increases in speed. Some people use this to
figure out position.

You may be better off with less steps per rev., something like
15 deg per step. Digikey sells motors up to 18 deg. per step.

What ever happened to the DC controller you mentioned that you
could just set it to a specific rpm?

Alberto


On Wed, 2006-12-06 at 10:37 -0600, Vern Graner wrote:
> Ok, so by now many (if not all) of you should be familiar with The
> RoboSpinArt machine :)
> 
> http://www.robospinart.com
> 
> As you may know, a key component is a rotating platter what holds a
> paper card. In the prototype machine, we used a DC motor and found that,
> though it spins quite fast enough, it is hard to control the speed
> precisely. I would prefer to have fine control of it in order to
> facilitate printing on the rotating paper using one of the Parallax ink
> jet printer kits:
> 
> http://www.parallax.com/detail.asp?product_id=27949
> 
> So, I read up on a bit on stepper motors and controllers, and bought
> this controller for testing:
> 
> http://www.parallax.com/detail.asp?product_id=27938
> 
> Based on the specs, in THEORY, we should be able to get a maximum of
> 1500 RPM out of this controller. Here's how we came to this conclusion
> (in case there's something we missed):
> 
> -The stepper motor is 1.8 degrees per step
> 
> -1.8 degrees per step divided by 360 degrees per
>   revolution = 200 steps per revolution
> 
> -The controller max is 5000 steps per second
> 
> -5000 steps per second divided by 200 steps
>   per revolution = 25 revolutions per second
> 
> -25 revolutions per second * 60 seconds = 1500 RPM
> 
> Ok, so 1500 RPM is acceptable as the DC motor we were using was spinning
> at ~1800 RPM. So, Paul built up a test jig with a BSII and a surplus
> stepper motor:
> 
> http://www.notepad.org/step/
> 
> With the demo code loaded, we tried to ramp up the speed to 1500 RPM...
> And then the trouble begins..  :)
> 
> The stepper motor spins up to about (visual observation guess) ~130 RPM
> and then stops spinning and just "buzzes".  :(  Some googling revealed
> that in order to keep the stator from "slipping", you can juice up the
> voltage. So I replaced the 12v 1000ma wall wart with a bench top supply
> that could deliver 35v at 3 amps. On testing again, I think it got up to
> about 300 rpm, but still no where *near* the theoretical maximum of 1500
> RPM.  :(
> 
> So, thinking maybe I had a bad stepper I tried a couple of other motors
> I had laying about. All performed substantially the same.  :(
> 
> So, anyone with some stepper motor experience have any idea as to what I
> can do to get a stepper to spin at 1500 RPM with this little controller?
> 
> ADVthanksANCE!  :)
> 
> Vern