.net upc-a reader DESIGN CONSIDERATIONS OF STEPPER MOTORS 283 in Software

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DESIGN CONSIDERATIONS OF STEPPER MOTORS 283
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be quite swift in human terms. The speed can be one-third for the first few milliseconds, two thirds for the next few milliseconds, then full blast after that.
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Steppers can t deliver as much running torque as standard DC motors of the same size and weight. A typical 12-volt, medium-sized stepper motor may have a running torque of only 25 oz-in. The same 12-volt, medium-sized standard DC motor may have a running torque that is three or four times more. However, steppers are at their best when they are turning slowly. With the typical stepper, the slower the motor revolves, the higher the torque. The reverse is usually true of continuous DC motors. Fig. 19.5 shows a graph of the running torque of a medium-duty, unipolar 12-volt stepper. This unit has a top running speed of 550 pulses per second. Since the motor has a step angle of 1.8 , that results in a top speed of 2.75 revolutions per second (165 rpm).
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BRAKING EFFECT
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Actuating one of the windings in a stepper motor advances the shaft. If you continue to apply current to the winding the motor won t turn any more. In fact, the shaft will be locked, as if you ve applied brakes. As a result of this interesting locking effect, you never need to add a braking circuit to a stepper motor because it has its own brakes built in. The amount of braking power a stepper motor has is expressed as holding torque. Small stepper motors have a holding torque of a few oz-in. Larger, heavier-duty models have holding torque exceeding 400 oz-in.
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VOLTAGE, CURRENT RATINGS
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Like DC motors, stepper motors vary in their voltage and current ratings. Steppers for 5-, 6-, and 12-volt operation are not uncommon. But unlike DC motors, if you use a higher voltage than specified for a stepper motor you don t gain faster operation, but more running
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60 50 Torque 40 (oz-inch) 30 20 10 0 0 100 200 300 400 Speed (pulses per second) Continuous Brush DC Motor 500
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FIGURE 19.5 With a stepper motor, torque increases as the speed of the motor is reduced.
284 WORKING WITH STEPPER MOTORS
and holding torque. Overpowering a stepper by more than 80 to 100 percent above the rated voltage may eventually burn up the motor. The current rating of a stepper is expressed in amps (or milliamps) per phase. The power supply driving the motor needs to deliver at least as much as the per-phase specification, preferably more if the motor is driving a heavy load. The four-step actuation sequence powers two phases at a time, which means the power supply must deliver at least twice as much current as the per-phase specification. If, for example, the current per phase is 0.25 amps, the power requirement at any one time is 0.50 amps.
Controlling a Stepper Motor
Steppers have been around for a long time. In the old days, stepper motors were actuated by a mechanical switch, a solenoid-driven device that pulsed each of the windings of the motor in the proper sequence. Now, stepper motors are invariably controlled by electronic means. Basic actuation can be accomplished via computer control by pulsing each of the four windings in turn. The computer can t directly power the motor, so transistors must be added to each winding, as shown in Fig. 19.6.
USING A STEPPER MOTOR CONTROLLER CHIP
In the absence of direct computer control, the easiest way to provide the proper sequence of actuation pulses is to use a custom stepper motor chip, such as the Allegro Microsystems
+12VDC R1-R4 1K Phase 4 Control Phase 3 Control Phase 2 Control Phase 1 Control c b e Q1-Q4 2N3055
FIGURE 19.6 The basic hookup connection to drive a stepper motor from a computer or other electronic interface. The phasing sequence is provided by software or other means through a port following a four-bit binary sequence: 1010, 0110, 0101, 1001 (reverse the sequence to reverse the motor).
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