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 steppers, as they are also known, 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 encoder feedback control systems of some type. Typically,
this involves an optical or magnetic encoder 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 steppers and servos, 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 encoder and other 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 stepper 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.
Stepping
motors can also be used in closed loop systems, much like servos,
with the addition of an encoder and feedback drive circuitry. Performance
is improved at the expense of additional cost.
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