RE: [CAD_CAM_EDM_DRO] Re: Why is the push to such high step rates (Grex and Arc100)?

Hi Les,

At first glance what you say about generating stepping rates appears true
but in fact isn't. There is some jitter on the MACH pulse output stream but
not nearly as extreme as you might think. I'm only using 25kHz maximum with
the Electronic Leadscrew and a PIC micro controller and I don't do it that
way.

You can see how it's done by checking out the article I wrote for the
November 2006 Circuit Cellar magazine which has the source code posted at
www.circuitcellar.com or on the E-Leadscrew group. The article explains how
to get all the frequencies and not just 25kHz, 12.5Khz etc. The price is
jitter but not as extreme as you suggest.

John

>
>
> Hi Chris,
>
> One reason for high step rates is harmonic purity. Take Mach3
> running at
> 45kHz. There are a limited range of frequencies it can produce:
> 45kHz
> 45kHz/2 = 22.5kHz
> 45kHz/3 = 15kHz
> 45kHz/4 = 11.25kHz
> and so on
>
> Now what happens if you need 30kHz? Mach will produce a burst
> of pulses
> at 22.5kHz then a burst of pulses at 45kHz then back to 22.5
> and so on.
> The average will be 30kHz. Your poor stepper motor is trying
> to rapidly
> switch between two speeds. Of course inertia means there is
> no chance of
> it accurately following the rapidly changing signal. What happens in
> practice is that a lot of torque is wasted trying to track these
> variations. If you are running close to the motor's max torque anyway
> you will end up losing steps. The really evil thing about
> this problem
> is that you cannot predict it. You can set up your machine so it runs
> great until one day you feed at just the wrong speed and the
> motor loses
> steps.
>
> Now try the same thing with a pulse generator that can
> produce 4MHz. The
> two frequencies are 30.075kHz and 29.851kHz. This variation
> is so tiny
> that it will have virtually no effect on the motor.
>
> Servos are much less affected by this problem but it will
> still affect
> performance at higher speeds.
>
> As well as the above problem, Mach has to contend with
> delays generated
> by PC hardware. These delays cause the pulse train to be even
> more ragged.
>
> Les
>
>
> lcdpublishing wrote:
> >
> > Both of the devices, the GREX and the ARC100 promote some very
> > intresting features and performance specs. At first glance
> I thought
> > I might be able to utilize these features but after looking at the
> > numbers a bit more closely, I couldn't see a practical
> application for
> > hobby use. So, that got me thinking about commercial applications
> > and, well, I just couldn't think of one so I had to ask.
> >
> > I know from the posts by people in the various groups that
> there are a
> > number of these units out there. What is it that they are building
> > that utilizes such amazing performance?
> >
> > Chris
> >
> >
>
>
>
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