vb.net barcode reader source code BRAKING EFFECT in Software

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21.2.5 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 anymore. 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 torques exceeding 400 oz-in.
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60 50 Torque 40 (oz-in) 30 20 10 0 0 100 200 300 400 Speed (pulses per second) Continuous Brush DC Motor 500
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Stepper
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FIGURE 21-5 With a stepper motor, torque increases as the speed of the motor is reduced.
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WORKING WITH STEPPER MOTORS
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21.2.6 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-V 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 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 A, the power requirement at any one time is 0.50 A.
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21.3 Controlling a Stepper Motor
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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. 21-6.
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FIGURE 21-6 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).
FIGURE 21-7 The basic wiring diagram for the UCN5804.
WORKING WITH STEPPER MOTORS
21.3.1 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 UCN5804. This chip is designed expressly for use with the common unipolar stepper motor and provides a four-step actuation sequence. Stepper motor translator chips tend to be modestly priced, at about $5 to $10, depending on their features and where you buy them. Fig. 21-7 (refer to the parts list in Table 21-1) shows a typical schematic of the UCN5804. Heavier-duty motors (more than about 1 A per phase) can be driven by adding power transistors to the four outputs of the chips, as shown in the manufacturer s application notes. Note the direction pin. Pulling this pin high or low reverses the rotation of the motor.
21.3.2 USING LOGIC GATES TO CONTROL STEPPER MOTORS
Another approach to operating unipolar stepper motors is to use discrete gates and clock ICs. You can assemble a stepper motor translator circuit using just two IC packages. The circuit can be constructed using TTL or CMOS chips. The TTL version is shown in Fig. 21-8 (refer to the parts list in Table 21-2). Four exclusive OR gates from a single 7486 IC provide the steering logic. You set the direction by pulling pin 12 HIGH or LOW. The stepping actuation is controlled by a 7476, which con tains two JK flip-flops. The Q and Q outputs of the flip-flops control the phasing of the motor. Stepping is accomplished by triggering the clock inputs of both flip-flops. The 7476 can t directly power a stepper motor. You must use power transistors or MOSFETs to drive the windings of the motor. See the section titled Translator Enhancements for a complete power driving schematic as well as other options you can add to this circuit. The CMOS version, shown in Fig. 21-9 (refer to the parts list in Table 21-3), is identical to the TTL version, except that a 4070 chip is used for the exclusive OR gates and a 4027 is used for the flip-flops. The pinouts are slightly different, so follow the correct schematic for the type of chips you use. Note that another CMOS exclusive OR package, the 4030, is also available. Don t use this chip; it behaves erratically in this, as well as other pulsed, circuits.
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