visual basic barcode scanner input ODOMETRY: THE ART OF DEAD RECKONING in Software

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ODOMETRY: THE ART OF DEAD RECKONING
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OPTICAL ENCODERS
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You can use a small disc fashioned around the hub of a drive wheel, or even the shaft of a drive motor, as an optical shaft encoder (described in Anatomy of a Shaft Encoder, in 18). The disc can be either the reflectance or the slotted type:
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I With a reflectance disc, infrared light strikes the disc and is reflected back to a pho-
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todetector.
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I With a slotted disc, infrared light is alternately blocked and passed and is picked up on
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the other side by a photodetector. With either method, a pulse is generated each time the photodetector senses the light.
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MAGNETIC ENCODERS
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You can construct a magnetic encoder using a Hall effect switch (a semiconductor sensitive to magnetic fields) and one or more magnets. A pulse is generated each time a magnet passes by the Hall effect switch. A variation on the theme uses a metal gear and a special Hall effect sensor that is sensitive to the variations in the magnetic influence produced by the gear (see Fig. 38.9). A bias magnet is placed behind the Hall effect sensor. A pulse is generated each time a tooth of the gear passes in front of the sensor. The technique provides more pulses on each revolution of the wheel or motor shaft, and without having to use separate magnets on the rim of the wheel or wheel shaft.
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THE FUNCTION OF ENCODERS IN ODOMETRY
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As the wheel or motor shaft turns, the encoder (optical or magnetic) produces a series of pulses relative to the distance the robot travels. Assume the wheel is 3 inches in diameter (9.42 inches in circumference), and the encoder wheel has 32 slots. Each pulse of the encoder represents 0.294 inches of travel (9.42/32). If the robot senses 10 pulses, it knows it has moved 2.94 inches. If the robot uses the traditional two-wheel drive approach, you attach optical encoders to both wheels. This is necessary because the drive wheels of a robot are bound to turn at
Bias magnet Hall effect sensor
Ferrous metal gear
FIGURE 38.9 A Hall effect sensor outfitted with a small bias magnet and sensitive to the changes in magnetic flux caused by a rotating ferrous metal gear.
630 NAVIGATING THROUGH SPACE
slightly different speeds over time. By integrating the results of both optical encoders, it s possible to determine where the robot really is as opposed to where it should be (see Fig. 38.10). As well, if one wheel rolls over a cord or other small lump, its rotation will be hindered. This can cause the robot to veer off course, possibly by as much as 3 to 5 or more. Again, the encoders will detect this change. It s best to make odometry measurements using a microcontroller that is outfitted with a pulse accumulator or counter input. These kinds of inputs independently count the number of pulses received since the last time they were reset. To take an odometry reading, you clear the accumulator or counter and then start the motors. Your software need not monitor the accumulator or counter. Stop the motors, and then read the value in the accumulator or counter. Multiply the number of pulses by the known distance of travel for each pulse. (This will vary depending on the construction of your robot; consider the diameter of the wheels and the number of pulses of the encoder per revolution.) If the number of pulses from both encoders is the same, you can assume that the robot traveled in a straight line, and you have only to multiple the number of pulses by the distance per pulse. For example, if there are 1055 pulses in the accumulator-counter, and if each pulse represents 0.294 inches of travel, then the robot has moved 310.17 inches straight forward.
ERRORS IN ODOMETRY
In a perfect world, robots would not need anything more than an odometer to determine exactly where they were at any given time. Unfortunately, robots live and work in a world that is far from perfect; as a result, their odometers are far from accurate. Over a 20- to 30foot range, for example, it s not uncommon for the average odometer to misrepresent the position of the robot by as much as half a foot or more! Why the discrepancy First and foremost: wheels slip. As a wheel turns, it is bound to slip, especially if the surface is hard and smooth, like a kitchen floor. Wheels slip even
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