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

>The one point I would disagree with is the
>"you are guaranteed to have a dynamic accuracy of much better than one full
>step" statement. Your accuracy is only going to be plus or minus one step,
>never "better than". Factor in the angular inaccuracies of a typical >stepper
>motor, say +/-5% (and even worse when micro stepping) along with dynamic
>errors, and steppers can not compete with TRUE servo systems.

The error is not +/- 1 step but +/- 1/2 step in a full step system.
Consider first the static case.
Say stepper set up to give .001 axis movement/step
The max error in positioning would be +/- .0005 not +/- .001
Since for any commanded position there is a step position that is within
.0005 of that location and the system simply chooses the closest step location. The step position accuracy comes on top of this but is tiny
+/- 5% angular error means the nominal 1.8deg step could vary between 1.71
and 1.89 degrees this adds an positioning error of +/- .00009 so the total worst case static error is +/- .00059.

In the dynamic case assuming that no steps are lost the additional error induced by motion/acceleration cannot be more than 1/2 step (if it ever got above 1/2 step you would lose a step at the next pulse) Thus the total dynamic error in a stepper system must always be less than the worst case
+/- 0.00059 (static Error) +/- .0005 (dynamic error) for a total of
+/- 0.00109
which is +/- 1.09 steps. Remember this is the WORST CASE on average the error will be slightly over half of this.

>True servo systems, either full analogue or digital SERCOS, will always >have better dynamic responses than any step/dir system. If steppers were >really that good, everyone would be using them.

This is simply wrong, for many applications it would be possible to design a stepper system with as good a dynamic response as a servo system, the big difference between stepper and servo systems as someone else has pointed out is that typical stepper systems have no feedback and so you don't know when they lose a step, the only real advantage of servo systems in smaller size machines is that they can tell you how inaccurate you are in real time, with stepper systems you can only find out if it went wrong after the fact by re-homing and comparing actual/expected position.

The reason everyone does not use steppers is simply good engineering practice.
One of the best definitions of an engineer I have seen is "An engineer is someone who can do for $5 what any fool can do for $500"

When you design a system you look at the requirements and normally chose the most cost effective solution to meet the requirements. For small machines that usually means steppers, for bigger machines it often means servo's.




Chris Cain

On Friday 30 January 2004 5:19 pm, cnczeus wrote:
> > (If you missed the whole post, it's worth reading IMO...)
>
> I believe you refer to the in motion accuracy of steppers to that of
> servos and I must agree that steppers are better, however once a
> single step is lost the remainder of the motion is far worse than the
> worst of the servo drive.

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