Where am I? Sensors and Methods for Mobile Robot Positioning

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154 Part II Systems and Methods for Mobile Robot Positioning Figure 6.3: Simulation results showing the effect of outliers and the result of removing the outliers. (Reproduced and adapted with permission from [Betke and Gurvits, 1994].) (46 ft) and the angles are at most 45 degrees. The simulation results show that large errors due to misidentified landmarks and erroneous angle measurements can be found and discarded. Subsequently, the algorithm can be repeated without the outliers, yielding improved results. One example is shown in Figure 6.3, which depicts simulation results using the algorithm Position Estimator. The algorithm works on an input of 20 landmarks (not shown in Figure 6.3) that were randomly placed in a 10×10 meters (32×32 ft) workspace. The simulated robot is located at (0, 0). Eighteen of the landmarks were simulated to have a one-percent error in the angle measurement and two of the landmarks were simulated to have a large 10-percent angle measurement error. With the angle measurements from 20 landmarks the Position Estimator produces 19 position estimates p - p (shown as small blobs in 1 19 Figure 6.3). Averaging these 19 estimates yields the computed robot position. Because of the two landmarks with large angle measurement errors two position estimates are bad: p 5 at (79 cm, 72 cm) and p at (12.5 cm, 18.3 cm). Because of these poor position estimates, the resulting centroid 18 a (average) is at P = (17 cm, 24 cm). However, the Position Estimator can identify and exclude the b two outliers. The centroid calculated without the outliers p 5and p 18is at P = (12.5 cm, 18.3 cm). The final position estimate after the Position Estimator is applied again on the 18 “good” landmarks (i.e., c without the two outliers) is at P = (6.5 cm, 6.5 cm). 6.2 Ultrasonic Transponder Trilateration Ultrasonic trilateration schemes offer a medium- to high-accuracy, low-cost solution to the position location problem for mobile robots. Because of the relatively short range of ultrasound, these systems are suitable for operation in relatively small work areas and only if no significant obstructions are present to interfere with wave propagation. The advantages of a system of this type fall off rapidly, however, in large multi-room facilities due to the significant complexity associated with installing multiple networked beacons throughout the operating area. Two general implementations exist: 1) a single transducer transmitting from the robot, with multiple fixed-location receivers, and 2) a single receiver listening on the robot, with multiple fixed transmitters serving as beacons. The first of these categories is probably better suited to applications involving only one or at most a very small number of robots, whereas the latter case is basically unaffected by the number of passive receiver platforms involved (i.e., somewhat analogous to the Navstar GPS concept).
Chapter 6: Active Beacon Navigation Systems 155 6.2.1 IS Robotics 2-D Location System IS Robotics, Inc. [ISR], Somerville, MA, a spin-off company from MIT's renowned Mobile Robotics Lab, has introduced a beacon system based on an inexpensive ultrasonic trilateration system. This system allows their Genghis series robots to localize position to within 12.7 millimeters (0.5 in) over a 9.1×9.1 meter (30×30 ft) operating area [ISR, 1994]. The ISR system consists of a base station master hard-wired to two slave ultrasonic “pingers” positioned a known distance apart (typically 2.28 m — 90 in) along the edge of the operating area as shown in Figure 6.4. Each robot is equipped with a receiving ultrasonic transducer situated beneath a cone-shaped reflector for omnidirectional coverage. Communication between the base station and individual robots is accomplished using a Proxim spread-spectrum (902 to 928 MHz) RF link. The base station alternately Pinger side view "A" pinger Base station "B" pinger Figure 6.4: The ISR Genghis series of legged robots localize x-y position with a master/slave trilateration scheme using two 40 kHz ultrasonic “pingers.” (Adapted from [ISR, 1994].) fires the two 40-kHz ultrasonic pingers every half second, each time transmitting a two-byte radio packet in broadcast mode to advise all robots of pulse emission. Elapsed time between radio packet reception and detection of the ultrasonic wave front is used to calculate distance between the robot’s current position and the known location of the active beacon. Inter-robot communication is accomplished over the same spread-spectrum channel using a time-division-multiple-access scheme controlled by the base station. Principle sources of error include variations in the speed of sound, the finite size of the ultrasonic transducers, non-repetitive propagation delays in the electronics, and ambiguities associated with time-of-arrival detection. The cost for this system is $10,000. 6.2.2 Tulane University 3-D Location System Researchers at Tulane University in New Orleans, LA, have come up with some interesting methods for significantly improving the time-of-arrival measurement accuracy for ultrasonic transmitterreceiver configurations, as well as compensating for the varying effects of temperature and humidity. In the hybrid scheme illustrated in Figure 6.5, envelope peak detection is employed to establish the approximate time of signal arrival, and to consequently eliminate ambiguity interval problems for a more precise phase-measurement technique that provides final resolution [Figueroa and Lamancusa, 1992]. The desired 0.025 millimeters (0.001 in) range accuracy required a time unit discrimination of 75 nanoseconds at the receiver, which can easily be achieved using fairly simplistic phase measurement circuitry, but only within the interval of a single wavelength. The actual distance from transmitter to receiver is the summation of some integer number of wavelengths (determined by the
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INTRODUCTION Leonard and Durrant-Wh
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Part I Sensors for Mobile Robot Pos
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218 Appendices, References, Indexes
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220 Appendices, References, Indexes
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Systems-at-a-Glance Tables Odometry
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Systems-at-a-Glance Tables Global N
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Systems-at-a-Glance Tables Landmark
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REFERENCES 1. Abidi, M. and Chandra
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238 References 31. Boltinghouse, S.
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240 References MOBOT-IV.” Proceed
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242 References 90. Dahlin, T. and K
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244 References 120. Fisher, D., Hol
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246 References 156. Janet, J., Luo,
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252 References 249. Sanders, G.A.,
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254 References 278. Turpin, D.R., 1
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256 References 309. EATON - Eaton-K
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258 References 349. SPSi - Spatial
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260 References 380. MacKenzie, P. a
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262 Index AC ..............47, 59,
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264 Index Datums ..................
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268 Index lateral-post ............
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282 Index Paper 52 Paper 53 Paper 5
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