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

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
>
>