vb.net barcode reader source code QUADRATURE ENCODING in Software

Making QR in Software QUADRATURE ENCODING

20.5.5 QUADRATURE ENCODING
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So far we ve investigated shaft encoders that have just one output. This output pulses as the shaft encoder turns. By using two LEDs and phototransistors, positioned 90 out of phase (see Fig. 20-27), you can construct a system that not only tells you the amount of travel, but the direction as well. This can be useful if the wheels of your robot may slip. You can determine if the wheels are moving when they aren t supposed to be, and you can determine the
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Phototransistor and Baffle Mounting Bracket Disc
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Circuit Board with LED and Phototransistor Soldered to It
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FIGURE 20-26 How to mount an infrared LED and phototransistor on a circuit board for use with an optical shaft encoder disc.
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WORKING WITH DC MOTORS
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direction of travel. This so-called two-channel system uses quadrature encoding the channels are out of phase by 90 degrees (one quarter of a circle). Use the flip-flop circuit in Fig. 20-28 to separate the distance pulses from the direction pulses. Note that this circuit will only work when you are using quadrature encoding, where the pulses are in the following format:
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off/off on/off on/on off/on ( and repeat.)
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Shaft
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Phototransistor Phototransistor
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LED 1 LED 2
LEDs 90 Degrees out of Phase
FIGURE 20-27 LEDs and phototransistors mounted on a twochannel optical disc. a. The LEDs and phototransistors can be placed anywhere about the circumference of the disc; b. the two LEDs and phototransistors must be 90 out of phase.
FIGURE 20-28 A two-channel shaft encoder circuit for use with a quadrature (also called two channel, 2-bit Gray code, or sine/cosine) encoder. One of the outputs of the flip-flop indicates distance (or relative speed) and the other the direction of rotation.
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20.6 From Here
To learn more about . . . Selecting the right motors for your robot Using stepper motors Interfacing motors to computers and microcontrollers More on odometry and measuring the distance of travel of a robot Read 19, Choosing the Right Motor for the Job 21, Working with Stepper Motors 14, Computer Peripherals 33, Navigation
CHAPTER
WORKING WITH STEPPER MOTORS
he past chapters have looked at powering robots using everyday continuous DC motors. DC motors are cheap, deliver a lot of torque for their size, and are easily adaptable to a variety of robot designs. By their nature, however, the common DC motors are rather imprecise. Without a servo feedback mechanism or tachometer, there s no telling how fast a DC motor is turning. Furthermore, it s difficult to command the motor to turn a specific number of revolutions, let alone a fraction of a revolution. Yet this is exactly the kind of precision robotics work, particularly arm designs, often requires. Enter the stepper motor. Stepper motors are, in effect, DC motors with a twist. Instead of being powered by a continuous flow of current, as with regular DC motors, they are driven by pulses of electricity. Each pulse drives the shaft of the motor a little bit. The more pulses that are fed to the motor; the more the shaft turns. As such, stepper motors are inherently digital devices, a fact that will come in handy when you want to control your robot by computer. By the way, there are AC stepper motors as well, but they aren t really suitable for robotics work and so won t be discussed here. Stepper motors aren t as easy to use as standard DC motors, however, and they re also harder to get and more expensive. But for the applications that require them, stepper motors can solve a lot of problems with a minimum of fuss.
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WORKING WITH STEPPER MOTORS
21.1 Inside a Stepper Motor
There are several designs of stepper motors. The most popular variety is the four-phase unipolar stepper, like the one in Fig. 21-1. A unipolar stepper motor is really two motors sandwiched together, as shown in Fig. 21-2. 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. 21-3).
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