A step motor
(or stepper motor as they are commonly referred) is a digital device, in that digital information is
processed to accomplish an end result, in this case, controlled motion. It is reasonable
to assume that a step motor will faithfully follow digital instructions just as a computer
is expected to. This is the distinguishing feature of a step motor.
In essence, step motors are electrical motors that are driven by
digital pulses rather than a continuously applied voltage. Inherent in this concept is
open-loop control, wherein a train of pulses translates into so many shaft revolutions,
with each revolution requiring a given number of pulses. Each pulse equals one rotary
increment, or step (hence, step motors), which is only a portion of one complete rotation.
Therefore, counting pulses can be applied to achieve a desired
amount of shaft rotation. The count automatically represents how much movement has been
achieved, without the need for feedback information, as would be the case in servo
systems.
Precision of
step motor controlled motion is determined primarily
by the number of steps per revolution; the more steps, the greater the precision. For even
higher precision, some step motor drivers divide normal steps into half-steps or
micro-steps. Accuracy of the step motor is a function of the mechanical precision of its
parts and assembly. Whatever the error that may be built into a step motor, it is
noncumulative. Consequently, it can be negligible.
A step motor is an electromagnetic, rotary actuator, that mechanically converts
digital pulse inputs to incremental shaft rotation. The rotation not only has a direct
relation to the number of input pulses, but its speed is related to the frequency of the
pulses.
Between steps, the motor holds its' position (and its' load) without
the aid of clutches or brakes. Thus a step motor can be precisely controlled so that it
rotates a certain number of steps, producing mechanical motion through a specific
distance, and then holds its load when it stops. Furthermore, it can repeat the operation
any prescribed number of times. Selecting a step motor and using it advantageously depends
on three criteria: desired mechanical motion, speed, and the load.
With the appropriate logic, step motors can be bi-directional,
synchronous, provide rapid acceleration, stopping, and reversal, and will interface easily
with other digital mechanisms. They are further characterized as having low rotor moment
of inertia, no drift, and a noncumulative positioning error. Generally step motors are operated without feedback in an open-loop
fashion and sometimes match the performance of more expensive DC Servo Systems. The only
inaccuracy associated with a step motor is a noncumulative positioning error measured in %
of step angle.
Variable Reluctance (VR) - VR motors are characterized as having a soft iron multiple
rotor and a wound stator. They generally operate with step angles from 5 degrees to 15
degrees at relatively high step rates, and have no detent torque (detent torque is the
holding torque when no current is flowing in the motor). In Figure 5, when phase A is
energized, four rotor teeth line up with the four stator teeth of phase A by magnetic
attraction. The next step is taken when A is turned off and phase B is energized, rotating
the rotor clockwise 15 degrees; Continuing the sequence, C is turned on next and then A
again. Counter clockwise rotation is achieved when the phase order is reversed.
Permanent Magnet (PM) - PM motors differ from VR's by having
permanent magnet rotors with no teeth, and are magnetized perpendicular to the axis. In
energizing the four phases in sequence, the rotor rotates as it is attracted to the
magnetic poles. The motor shown in Figure 6 will take 90 degree steps as the windings are
energized in sequence ABCD. PM's generally have step angles of 45 or 90 degrees and step
at relatively low rates, but they exhibit high torque and good damping characteristics.
Hybrid- Combining the
qualities of the VR and the PM, the hybrid motor has some of the desirable features of
each. They have high detent torque and excellent holding and dynamic torque, and they can
operate at high stepping speeds. Normally, they exhibit step angles of 0.9 to 5 degrees.
Bi-filar windings are generally supplied (as depicted in Figure 7), so that a
single-source power supply can be used . If the phases are energized one at a time, in the
order indicated, the rotor would rotate in increments of 1.8 degrees. This motor can also
be driven two phases at a time to yield more torque, or alternately one then two then one
phase, to produce half steps or 0.9 degree increments.
Although the step motor has been overshadowed in the past by servo systems for motion
control, it now is emerging as the preferred technology in more and more areas. The major
factor in this trend is the prevalence of digital control, and the emergence of the
microprocessor. Today we have many step motor applications all around us. They are
used in printers (paper feed, print wheel), disk drives, photo-typesetting, X-Y plotters,
clocks and watches, factory automation, aircraft controls, and many other applications.
Ingenuity and further advances in digital technology will continue to extend the list of
applications.
Amount, speed, and direction of rotation of a step motor are determined by appropriate
configurations of digital control devices. Major types of digital control devices are:
Motor Drivers, Control Links, and Controllers. These devices are employed as shown in
Figure 8. The Driver accepts clock pulses and direction signals and translates these
signals into appropriate phase currents in the motor. The Indexer creates the clock pulses
and direction signals. The computer or PLC (programmable logic controller) sends commands
to the indexer.
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