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There are scores of ways to relay position information to a robot. You ve already seen two beacon-type systems: infrared and radio frequency. And there are plenty more. Sadly, there isn t enough space in this book to discuss them all, but the following sections outline some techniques you might want to consider. Many of these systems rely on line of sight between the beacon or lighthouse and the robot. If the line of sight is broken, the robot may very well get lost.
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Three-point triangulation
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Traditional three-point triangulation is possible using
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either of two methods:
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I Active beacon. A sensor array on the robot determines its location by integrating the rel-
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ative brightness of the light from three active light sources.
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I Active robot. The robot sends out a signal that is received by three sensors located
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around the room. The sensors integrate the robot s position, then relay this information back to the bot (via RF or an infrared radio link).
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Coupled sonar and IR light
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This technique calculates time of flight using sound, and it offers excellent accuracy. You equip three active beacons with sonar transmitters and high-output infrared light-emitting diodes. You then connect the three beacons electrically so they will fire in sequence. When fired, both the sonar transmitter and IR LEDs emit a short 40 kHz signal. Because light travels much faster than sound, the robot will detect the IR signal first followed by the sound signal. The difference in time between the reception of the IR and sound signals represents distance. Each beacon provides a circle path that the robot can be in. All three circles will intersect at only one spot in the room, and that will be the location of the robot. See Fig. 38.21 for a demonstration of how this works.
Exploring Other Position-referencing Systems
Over the years a number of worthwhile techniques have been developed to help robots know where they are. We ve covered many of the most common techniques here. If your
646 NAVIGATING THROUGH SPACE
Beacon C
Robot
Beacon B
Beacon A
FIGURE 38.21 Three active beacons, connected to fire in sequence, provide both infrared and ultrasonic sound signals. Using a burst of infrared light, the robot times how long it takes for the sonar ping to reach it. Repeated three times one for each beacon the robot is able to make an accurate fix within the room.
budget and construction skill allow for it, however, you might want to consider any or all of the following.
GLOBAL POSITIONING SATELLITE
Hovering over the earth are some two dozen satellites that provide accurate world-positioning data to vehicles, ships, and aircraft. The satellite network, referred to as global positioning system (GPS), works by triangulation: the signals from three or more satellites are received and their timings are compared. The difference in the timings indicates the relative distances between the satellites and the receiver. This provides a fix by which the receiver can determine not only the latitude and longitude most anywhere on the earth, but also elevation. GPS was primarily developed by the United States government for various defense systems, but it is also regularly used by private commerce and even consumers. Until recently, the signals received by a consumer-level GPS receiver have been intentionally fuzzied to decrease the accuracy of the device. (This is called selective availability, imposed by the U.S. government for national security reasons.) Instead of the accuracies of a few feet or less that are possible with military-grade GPS receivers, consumer GPS receivers have had a nominal resolution of 100 meters, or about 325 feet. In practical use, with selective availability activated in the GPS satellites, the actual error is typically 50 100 feet. Selective availability has since been deactivated (but could be re-activated in the event of
EXPLORING OTHER POSITION-REFERENCING SYSTEMS 647
hostilities between the United States and another country), and the resolution of consumer GPS receivers can be under 20 25 feet. Furthermore, a system called differential GPS, in which the satellite signals are correlated with a second known reference, demonstrably increases the resolution of GPS signals to less than five inches. When used outdoors (the signal from the satellites is too weak for indoor use) this can provide your robot with highly accurate positioning information, especially if your bot wanders hundreds of feet from its base station. Real-time differential GPS systems are still fairly costly, but their outputs can read into the robot s computer in real time. It takes from one to three minutes for the GSP system to lock onto the satellites overhead, however. Every time the lock is broken the satellite signals are blocked or otherwise lost it takes another one to three minutes to reestablish a fix. If you re interested in experimenting with GPS, look for a receiver that has a NMEA0183 or RS-232 compatible computer interface. A number of amateur radio sites on the Internet discuss how to use software to interpret the signals from a GPS receiver.
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