Re: [CAD_CAM_EDM_DRO] General Question REgarding Stepper vs. Servos

Statman Designs, LLC wrote:

>I often hear a lot about the limitations of stepper motor systems being the
>potential for lost steps and the resonance issues at low speed. I have a
>question for those of you running stepper motor CNC machines: do any of you
>actually miss and steps? My systems have been up and running many, many
>hours and have never missed a step. With a micro-stepping drive like
>Xylotex (for small machines) or Gecko (for larger machines) isn't the
>resonance issue resolved? I have zero low-speed resonance on my sytems with
>the Xylotex drives.
>
>

I have some Gecko G201As on some 300 Oz-In size 34 motors. If there is
any binding in the machine, it will still lose steps. It is much more prone
to losing steps at higher speeds, no surprise there.

>I think the fact that steppers lose torque at high-speed is the only
>limitation that seems to matter. Therefore, a stepper system will be
>somewhat slower than a servo system. Or am I missing something else?
>
>

If money is no concern, a stepper system capable of any speed can be made.
Where the big difference is, is that physically small servo motors with
a belt
reduction can deliver more speed and torque per Dollar than big steppers
and direct drive. If you get your big stepper motors cheap through a
surplus
outlet, that changes the equation.

>Not trying to start a religious war about stepper and servo systems, just
>trying to understand the real differences.
>
>

Well, there is more. If you use a true servo system, where the CNC computer
has encoder feedback coming directly to it, then it always knows what the
following error is, and can command an emergency stop whenever it exceeds
a limit you set. Or, it can allow a large error in rapid traverse, but
a much
smaller error at cutting speeds.

With a step/direction servo, the driver has some fixed limit programmed into
it, and the CNC control only knows that (often large) error has not been
exceeded. This is not the same as knowing the instantaneous following error
at all times.

Jon