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Copyright 2001 The McGraw-Hill Companies, Inc. Click Here for Terms of Use.
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280 WORKING WITH STEPPER MOTORS
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Inside a Stepper Motor
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There are several designs of stepper motors. For the time being, we ll concentrate on the most popular variety, the four-phase unipolar stepper, like the one in Fig. 19.1. A unipolar stepper motor is really two motors sandwiched together, as shown in Fig. 19.2. Each motor is composed of two windings. Wires connect to each of the four windings of the motor pair, so there are eight wires coming from the motor. The commons from the windings are often ganged together, which reduces the wire count to five or six instead of eight (see Fig. 19.3).
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In operation, the common wires of a unipolar stepper are attached to the positive (sometimes the negative) side of the power supply. Each winding is then energized in turn by grounding it to the power supply for a short time. The motor shaft turns a fraction of a revolution each time a winding is energized. For the shaft to turn properly, the windings must be energized in sequence. For example, energize wires 1, 2, 3, and 4 in sequence and the motor turns clockwise. Reverse the sequence, and the motor turns the other way.
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The wave step sequence is the basic actuation technique of unipolar stepper motors. Another, and far better, approach actuates two windings at once in an on-on/off-off fourstep sequence, as shown in Fig. 19.4. This enhanced actuation sequence increases the driving power of the motor and provides greater shaft rotation precision. There are other varieties of stepper motors, and they are actuated in different ways. One you may encounter is bipolar. It has four wires and is pulsed by reversing the polarity of the power supply for each of the four steps. We will discuss the actuation technique for these motors later in this chapter.
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Design Considerations of Stepper Motors
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Stepping motors differ in their design characteristics over continuous DC motors. The following section discusses the most important design specifications for stepper motors.
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STEPPER PHASING
A unipolar stepper requires that a sequence of four pulses be applied to its various windings for it to rotate properly. By their nature, all stepper motors are at least two-phase. Many are four-phase; some are six-phase. Usually, but not always, the more phases in a motor, the more accurate it is.
DESIGN CONSIDERATIONS OF STEPPER MOTORS 281
FIGURE 19.1 A typical unipolar stepper motor.
Rotor (shaft)
Coil
Stator cup
Stator cup 2
Coil FIGURE 19.2 Inside a unipolar stepper motor. Note the two sets of coils and stators. The unipolar stepper is really two motors sandwiched together.
STEP ANGLE
Stepper motors vary in the amount of rotation of the shaft each time a winding is energized. The amount or rotation is called the step angle and can vary from as small as 0.9 (1.8 is more common) to 90 . The step angle determines the number of steps per revolution. A stepper with a 1.8 step angle, for example, must be pulsed 200 times for the shaft to turn one complete revolution. A stepper with a 7.5 step angle must be pulsed 48 times for one revolution, and so on.
282 WORKING WITH STEPPER MOTORS
Phase 1
Ground Phase 2 Ground
Phase 3 Phase 4
FIGURE 19.3 The wiring diagram of the unipolar stepper. The common connections can be separate or combined.
STEP 1 Clockwise 2
PHASE 1
PHASE 2
PHASE 3
PHASE 4
Counterclockwise 3 4 OFF ON
FIGURE 19.4 The enhanced on-on/off-off four-step sequence of a unipolar stepper motor.
PULSE RATE
Obviously, the smaller the step angle is, the more accurate the motor. But the number of pulses stepper motors can accept per second has an upper limit. Heavy-duty steppers usually have a maximum pulse rate (or step rate) of 200 or 300 steps per second, so they have an effective top speed of one to three revolutions per second (60 to 180 rpm). Some smaller steppers can accept a thousand or more pulses per second, but they don t usually provide very much torque and aren t suitable as driving or steering motors. Note that stepper motors can t be motivated to run at their top speeds immediately from a dead stop. Applying too many pulses right off the bat simply causes the motor to freeze up. To achieve top speeds, you must gradually accelerate the motor. The acceleration can
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