RE: [CAD_CAM_EDM_DRO] Re: stepper accuracy compared to servos...

I have a real problem with feedback on steppers. One of the fundamental
problems with steppers is that, once you have 1/2 step of error, the peak
torque is reached. Beyond that the torque is actually in the wrong
direction. As it comes over the top you have the step missing ring.

If you tried to create an error correction system with good response, you
would correct for a missed step as soon as possible. There is no reasonable
way to correct for a single step of error while it is ringing. Attempting
to do so would be a positive feedback loop for resonance. The only way to
correct is to try and step back after things have calmed down a bit.

You could, of course, treat the windings more like an induction motor and
try to do a vector sort of thing, leading and lagging the field relative to
the poles, but that sort of control gets complex very fast. I have never
heard of anyone putting feedback on a stepper for that sort of control. All
of the feedback steppers seem to be limited to step correction, which can
repair errors in position but cannot really improve dynamic performance.

We had a butcher term in the engineering group of "Whole hog or no pork".
The translation was: If you're going to do it, then do it, don't do a part
of it. If you're going to add the complexity of feedback then there is not
much point to retaining the basic torque delivery system of a stepper. If
you really need feedback, then use a servo.

> -----Original Message-----
> From: eewizard_1 [mailto:eewizard_1@...]
> Sent: Saturday, January 31, 2004 2:01 AM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: [CAD_CAM_EDM_DRO] Re: stepper accuracy compared to servos...
>
>
>
> Everyone can go back and forth on stepper vs. servo motors but the
> real fact is that neither motor, on it's own, is accurate. Several
> recent posts have talked of Z axis creep. A little Sherline with
> steppers will have ZERO accuracy the first time you try to cut a 1/4"
> x 1/2" slot in hardened tool steel. The steppers will just buzz and
> go nowhere with the computer oblivious to what's happening. A servo
> might grind away until it gets to where it should but if you take a
> servo and break the encoder feedback you might see the motor run and
> crash the limit switch.
>
> The "feedback" is what really gives accuracy on any system. With the
> right electronics a stepper with a 2000PPR encoder can top a servo
> with a 500PPR encoder. The same servo with the 2000PPR can top a
> stepper with the 500PPR encoder.
>
> Steppers are used a lot because the "feedback" usually is the FREE
> prayer that under good conditions it will "step" when you want and
> the stepper drive can be nothing but a few low cost transistors and
> resistors.
>
> Servos must have an encoder with relatively good electronics and thus
> cost more to put on a machine. So, they're not as common on lower
> cost (the smaller higher volume) machines.
>
> Another down side for steppers is lack of power. Steppers can have
> torque at a stall but it drops fast at high step rates. Servo motors
> can crank a lot at speed, but need more expensive electronics to do
> it.
>
> What do the "Big Boys" use? A great full size mill will probably
> have AC servo motors with Heidenein glass scales.
>
> Al
>
>
>
>
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