vb.net read barcode from camera Odometry: The Art of Dead Reckoning in Software

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33.4 Odometry: The Art of Dead Reckoning
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Hop into your car. Note the reading on the odometer. Now drive straight down the road for exactly one minute, paying no attention to the speedometer or anything else (of course, keep your eyes on the road!). Again note the reading on the odometer. The information on the odometer can be used to tell you where you are. Suppose it says one mile. You know that if you turn the car around exactly 180 and travel back a mile, at whatever speed, you ll reach home again. This is the essence of odometry; reading the motion of a robot s wheels to determine how far it s gone. Odometry is perhaps the most common method for determining where a robot is at any given time. It s cheap, easy to implement, and fairly accurate over short distances. Odometry is similar to the dead-reckoning navigation used by sea captains and pilots before the age of satellites, radar, and other electronic schemes. Hence, odometry is also referred to in robot literature as dead reckoning.
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33.4.1 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 20). The disc can be either the reflectance or the slotted type:
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With a reflectance disc, infrared light strikes the disc and is reflected back to a photodetector. With a slotted disc, infrared light is alternately blocked and passed and is picked up on the other side by a photodetector.
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With either method, a pulse is generated each time the photodetector senses the light.
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33.4.2 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
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33.4 ODOMETRY: THE ART OF DEAD RECKONING
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Bias Magnet Hall Effect Sensor
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FIGURE 33-9 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.
Ferrous Metal Gear
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. 33-9). 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.
33.4.3 THE FUNCTION OF ENCODERS IN ODOMETRY
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 in diameter (9.42 in in circumference), and the encoder wheel has 32 slots. Each pulse of the encoder represents 0.294 in of travel (9.42/32). If the robot senses 10 pulses, it can calculate the movement to 2.94 in. 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.)
33.4.4 ERRORS IN ODOMETRY
If the robot uses the traditional two-wheel-drive approach, you should attach optical encoders to both wheels. This is necessary because the drive wheels of a robot are bound to turn at 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. 33-10). 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 degrees or more. Again, the encoders will detect this change. 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 multiply the number of pulses by the dis-
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