Re: stepper motors and ball screws

Ball screws are made such that the tread is a semi-circular track in both
the screw stock and the ball nut and it uses ball bearing that fit into the
tracks on both the nut and screw. This allows the friction to be rolling
friction instead of sliding friction that you have in a conventional
threaded rod and nut. The ball nut has 1 or more tubes that recirculate the
bearings so that as they reach one end of the nut they are channeled back to
the other end. Ball screws come in 2 flavors, rolled thread (low cost) or
precision ground (expensive, but low backlash and high accuracy). The main
advantage of ball screws is the tremendous durability and the reduction in
friction.

I took this from http://www.cs.uiowa.edu/~jones/step/

Stepping motors can be viewed as electric motors without commutators.
Typically, all windings in the motor are part of the stator, and the rotor
is either a permanent magnet or, in the case of variable reluctance motors,
a toothed block of some magnetically soft material. All of the commutation
must be handled externally by the motor controller, and typically, the
motors and controllers are designed so that the motor may be held in any
fixed position as well as being rotated one way or the other. Most stepping
motors can be stepped at audio frequencies, allowing them to spin quite
quickly, and with an appropriate controller, they may be started and stopped
"on a dime" at controlled orientations.

For some applications, there is a choice between using servomotors and
stepping motors. Both types of motors offer similar opportunities for
precise positioning, but they differ in a number of ways. Servomotors
require analog feedback control systems of some type. Typically, this
involves a potentiometer to provide feedback about the rotor position, and
some mix of circuitry to drive a current through the motor inversely
proportional to the difference between the desired position and the current
position.

In making a choice between stepping motors and servomotors, a number of
issues must be considered; which of these will matter depends on the
application. For example, the repeatability of positioning done with a
stepping motor depends on the geometry of the motor rotor, while the
repeatability of positioning done with a servomotor generally depends on the
stability of the potentiometer and other analog components in the feedback
circuit.

Stepping motors can be used in simple open-loop control systems; these are
generally adequate for systems that operate at low accelerations with static
loads, but closed loop control may be essential for high accelerations,
particularly if they involve variable loads. If a stepping motor in an
open-loop control system is overtorqued, all knowledge of rotor position is
lost and the system must be reinitialized; servomotors are not subject to
this problem.


Tim
[Denver, CO]

> -----Original Message-----
> From: twil62551@...
> Gentlemen: I am new to this. Woould some one explain to me what
> 'stepper motors' and 'ball screws' are and what they are used
> for? I have been tryin to follow the discussions, but am lost.
> I have got a small home shop (lathe, mill), , but it is a hobby,
> now a passion. Any explanations would be appreciated.