Re: [CAD_CAM_EDM_DRO] Re: stepper vs servo

cadcamcenter@... wrote:

>
> THE POSITIONING OF ANY STEP IS WITHIN 5% OF A FULL ROTATION. I do not
> understand this part. When the accuracy of a step is 5% does this
> mean 5% of a single step or 5% of a full revolution? I am still
> learning about steppers and servos, but I find it hard to visualise
> how a 1.8 degree stepper in moving 1 step (1.8 degree) can
> misposition itself 18 degrees. Again my feeble mind tells me that
> inaccuracy in steps over a full revolution are not cummalative
> (unlike missed steps). Any inaccuracy of a particular step should
> only affect the accuracy of the positioning where that particular
> step occured. The sum total of inaccuracy in 200 steps of a full
> revolution will zero themself out, meaning if for example step no. 30
> is off by +1 degree, then somewhere in the sum total of the other 199
> steps it must be off by -1 degree. I stand ready to be corrected.

No, I don't think you've stated that correctly. The error DOES NOT
'zero it self out', the error just stands forever as an error at that
position. And, it repeats, every rotation of the motor. it is not
cumulative,
I think is what you meant to say, but it is an error, and is not corrected,
except possibly in software.

> Anyway, what I was trying to point out was that (I think) a servo is
> always correcting itself (a servo does overcorrect itself doen't it)

No, absolutely NOT! When properly tuned, it does NOT overcorrect, but
it corrects any errors that may creep in, such as effects of friction,
cutting
tool forces, inertia, etc.

> leading to a wavy path, although the wave may be very microscopic.
> The jagged steps of a stepper in reality may be of larger magnitude
> than the waviness of a servo, but, theoretically anyway, if the steps
> of is sufficient small, the path of a stepper will not exhibit the
> waviness of a servo.

But, stepper steps are necessarily NOT very small, otherwise you will have
extremely slow movement of the machine. Conventional steppers have very
heavy
limits on speed, in the area of 1000 full steps a second with the best
drivers.
So, you can't gear them down to get the step size down to the infinitesimal.

Yes, there will be some wander in a servo system, too, but it will usually
be
an order of magnitude less than the stepper's steps. I have seen CNC
work done on stepper-based machines, and the stair-stepping on the
inside of arc cuts is usually clearly visible. My servo machine doesn't
do that. You absolutely can't see any evidence of stair stepping! I can
measure the effect of backlash in bores, as a slight deviation from a
perfect
circle, and I know where that comes from.

Jon