Ball Bar Tests & accuracy

For the most part, stepper versus servo will not be much of a comparison in
accuracy. One of the big advantages of a servo is that, because of the
feedback, you can sustain large dynamic errors and still not fault the
machine.

With a stepper system it will become apparent very quickly if you lose
steps. For the most part, if you are not losing steps, you are guaranteed
to have a dynamic accuracy of much better than one full step. If you get
anywhere near a step in lag or lead, you will probably lose that step.
While running a complicated program, if your position reference is the same
after as before, you can know that the dynamic accuracy during the cutting
was also good and within better than one step. That would be one real step,
not a microstep, as the system cannot "slip" a microstep, only a real step.

With a servo system, your only absolute knowledge of dynamic accuracy will
come from the position fault error. If it does not fault, at least you know
that it stayed within that error level. If the servo drive does not have
integral position gain, you can get a ballpark idea of dynamic error by
plotting dynamic drive current. With a scope or other instrumentation on
drive current, physically displace the motor shaft and document the
relationship between motor shaft error and drive current. This is
approximately the same relationship that exists while it is running. You
can then run a part program and continue to watch drive current. As the
part is running, the dynamic position error of the axis is roughly shown by
drive current. With an intelligent drive you can monitor dynamic error, but
this technique works pretty well for simple dumb drives as well.

The bottom line is that, while servos have a number of advantages, accuracy
is not one of them. Steppers can provide a great deal of accuracy at low
cost. With a typical 2000 line encoder providing 8000 counts per
revolution, the 128 count fault level of a Gecko is 0.016 revolutions. A
200 step motor will lose a step at 0.005 revolutions (1/200). In operation,
a stepper will probably be well within 0.002 revolutions. In order to meet
that sort of level, the Gecko servo would have to maintain an error of less
than 16 counts. This can happen, but you cannot know that it has happened,
all you know is that it never got greater than 128.

G. Jackson

> From: "ballendo" <ballendo@...>

<s>

> P.S. I'd sure like to see a renishaw ballbar test of a Gecko (step-
> servo) machine... And compare it to a "real" servo setup, like Jon
> E's...

Hassan did balbar tests with his Gecko powered mill while a student at
Grand Valley State. No comparison with a servo setup but some
comparisons with other software.

Ray