On 30 Jan 2005 01:50:24 GMT, Dan Murphy wrote:

Cliff wrote in news:1eaov0tta9pt4shgd5eqsa0lcnk1s5btuh@
4ax.com:

On Sat, 29 Jan 2005 08:39:21 -0800, Eric R Snow
wrote:

It is a stepper system. I
would avoid making a machine with steppers.

Why?

They are open loop for one.

Dan
The biggest problem with steppers is that the torque drops drastically
when the motors are close to their maximum rpm. And the max rpm is not
very high. In slow speed applications a properly sized stepper is a
good choice. They can be run open loop, which means no position or
speed feedback. And they have lots of torque at low speeds. Also,
because of the way they are made, the accuracy of positioning when
moving one complete step at a time is very good. All error is
corrected every 4th step. When microstepping more error is likely but
will still be corrected every 4th complete step. So if the motor is
being micro stepped at ten microsteps per full step then the error
will be corrected every 40th microstep. Microstepping is accomplished
by using varying voltage to the windings. Instead of energizing one
winding, or a pair of windings, all the windings are supplied with
varying voltage which will prevent the motor from locking at the
natural steps built mechanically into the motor. You can imagine this
by picturing an iron pendulum suspended between two electromagnets.
With the electromagnet on the right energized the pendulum will swing
over to it. But if the left hand electromagnet is energized the
pendulum will be attracted to it. by varying the energy supplied to
the electromagnets the pendulum can be held at any position between
the electromagnets. This is how the microstep drives work. A printer
is a good example of a stepper driven system. A stepper driven machine
tool the size of a bridgeport is a bad example. Even though a servo
driven system is much more complex electronically than a stepper
system the advantages of high speed and positioning accuracy of the
servo system outweigh the simpler stepper system.
ERS