Steppers are NOT less ACCURATE was Re: seeking CNC conversion advice

Chuck, List,

Accuracy is the ability to repeatably attain a specific arbitrary
position. For us, it means being able to attain a SERIES of arbitrary
positions.

Servos are NOT more accurate than steppers, however implemented...

They ARE faster, more powerful, and provide feedback if something
goes wrong.

Both systems have a finite set of achievable positions. Being finite,
these positions may or may not be on the arbitrary desired point(s).
But for a given resolution, both systems are equivalent in this
regard. They will be equally OFF position, due to either step size,
or encoder counts.

Steppers have a smallest possible move of one step; which may be a
microstep. This creates its set of achievable positions. The stepper
will achieve all these positions within its step accuracy, which is
determined mostly by the accuracy of the stampings which make up the
rotor and stator. Even then, these positions are all repeatable, at
least as far as the stepper is concerned(since we're comparing the
motor types, not the machine they are hooked to). Some steppers are
incredibly precisely made. Typically, most steppers have a step error
of 3%.

Servos have a step size too. It is the distance rotated to make the
encoder move to the next state. There is an update rate with servos,
which is how often the encoder information is returned to the motors
power supply to effect a change in its speed or direction.
Historically, this was 1000 times per second. Many systems now update
faster, but there are still many using the 1000hz "standard". The
servo also suffers from following error, which is created due to the
time between the encoder pulse being generated, and when the
calculations (most often PID) are completed to effect the change in
its power supply. This means the servo is constantly playing a GAME
OF "CATCH UP". More expensive servo systems play this game very well,
and following error is reduced to the point that it does not matter
except during high speed moves. But it is well to remember that
servos are nearly always "heading" towards the correct position,
rather than sitting at it, as with a stepper.

A stepper will always "hit" all of its steps, which means it always
works to it's highest accuracy (until it stalls, at which point it
loses position). A servo, even on commercial cnc machines, will be
less accurate the faster it goes. Long time cnc programmers know this
and account for it; most cnc machine manuals mention this fact and
provide calc's to determine just how much accuracy you're giving up
at a given feedrate.

Hope this helps,

Ballendo




--- In CAD_CAM_EDM_DRO@yahoogroups.com, Chuck Knight <
<snip> As for servos and accuracy -- they're more accurate than
steppers, when implemented correctly.

> In practical terms, there is often a series of
> compromises which makes them *roughly* comparable.