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Stepper motors were developed in the early 1960's as a low cost
alternative to position servo systems in the emerging computer peripheral
industry. The main advantage of stepper motors is that they can
achieve accurate position control without the requirement for position
feedback. In other words, they can run "open-loop", which
significantly reduces the cost of a position control system.
Stepper motors utilize a doubly-salient topology, which means they
have "teeth" on both the rotor and stator. Torque is generated
by alternately magnetizing the stator teeth electrically, and the
permanent magnet rotor teeth try to line up with the stator teeth.
There are many different configurations of stepper motors, and even
more diverse ways to drive them. The most common stator configuration
consists of two coils (A and B). These coils are arranged around
the circumference of the stator in such a way that if they are driven
with square waves which have a quadrature phase relationship between
them, the motor will rotate. To make the motor rotate in the opposite
direction, simply reverse the phase relationship between the A and
B signals. A transition of either square wave causes the rotor to
move by a small amount, or a "STEP". Thus, the name "stepper
motor". The size of this step is dependent on the teeth arrangement
of the motor, but a common value is 1.8 degrees, or 200 steps per
revolution. Speed control is achieved by simply varying the frequency
of the squarewaves.
Because stepper motors can be driven with square waves, they are
easily controlled by inexpensive digital circuitry and do not even
require PWM. For this reason, stepper motors have often been inappropriately
referred to as "digital motors". However, by utilizing
power modulation techniques to change the quadrature squarewaves
into sine and cosine waveforms, even MORE step resolution is possible.
This is called "micro-stepping", where each discrete change
in the sine and cosine levels constitutes one microstep. Theoretically,
there is no limit to the position resolution achievable with micro-stepping,
but in reality, it is limited by the motor mechanical and electrical
tolerances. Some stepper motors are designed specifically for micro-stepping,
and consist of tightly matched impedances between the A and B coils,
and tighter machining tolerances on the teeth, at the expense of
higher cost. |
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