Where am I? Sensors and Methods for Mobile Robot Positioning

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56 Part I Sensors for Mobile Robot Positioning The Watson fluxgate/rate gyro combination balances the shortcomings of each type of device: the gyro serves to filter out the effects of magnetic anomalies in the surrounding environment, while the compass counters the long-term drift of the gyro. Furthermore, the toroidal ring-core fluxgate sensor is gimbal-mounted for improved accuracy. The Watson unit measures 6.3×4.4×7.6 centimeters (2.5×1.75×3.0 in) and weighs only 275 grams (10 oz). This integrated package is a much more expensive unit ($2,500) than the low-cost Zemco fluxgate compass, but is advertised to have higher accuracy (±2(). Power supply requirements are 12 VDC at 200 mA, and the unit provides an analog voltage output as well as a 12-bit digital output over a 2400-baud RS-232 serial link. 2.4.2.3 KVH Fluxgate Compasses KVH Industries, Inc., Middletown, RI, offers a complete line of fluxgate compasses and related accessories, ranging from inexpensive units targeted for the individual consumer up through sophisticated systems intended for military applications [KVH]. The C100 COMPASS ENGINE (see Figure 2.24) is a versatile low-cost (less than $700) developer's kit that includes a microprocessorcontrolled stand-alone fluxgate sensor subsystem based on a two-axis toroidal ring-core sensor. Figure 2.24: The C-100 fluxgate compass engine was tested at the University of Michigan in a flying robot prototype. (Courtesy of [KVH].) Two different sensor options are offered with the C-100: 1) the SE-25 sensor, recommended for applications with a tilt range of ±16 degrees and 2) the SE-10 sensor, for applications anticipating a tilt angle of up to ±45 degrees. The SE-25 sensor provides internal gimballing by floating the sensor coil in an inert fluid inside the lexan housing.The SE-10 sensor provides an additional 2-degree-offreedom pendulous gimbal in addition to the internal fluid suspension. The SE-25 sensor mounts on top of the sensor PC board, while the SE-10 is suspended beneath it. The sensor PC board can be
Chapter 2: Heading Sensors 57 separated as much as 122 centimeters (48 in) from the detachable electronics PC board with an optional cable if so desired. The resolution of the C100 is ±0.1 degrees, with an advertised accuracy of ±0.5 degrees (after compensation, with the sensor card level) and a repeatability of ±0.2 degrees. Separate ±180 degree adjustments are provided for declination as well as index offset (in the event the sensor unit cannot be mounted in perfect alignment with the vehicle’s axis of travel). System damping can be userselected, anywhere in the range of 0.1 to 24 seconds settling time to final value. An innovative automatic compensation algorithm employed in the C100 is largely responsible for the high accuracy obtained by such a relatively low-priced system. This software routine runs on the controlling microprocessor mounted on the electronics board and corrects for magnetic anomalies associated with the host vehicle. Three alternative user-selectable procedures are offered: & Eight-Point Auto-Compensation — starting from an arbitrary heading, the platform turns full circle, pausing momentarily at approximately 45-degree intervals. No known headings are required. & Circular Auto-Compensation — Starting from an arbitrary position, the platform turns slowly through a continuous 360-degree circle. No known headings are required. & Three-Point Auto-Compensation — Starting from an arbitrary heading, the platform turns and pauses on two additional known headings approximately 120 degrees apart. Correction values are stored in a look-up table in non-volatile EEPROM memory. The automatic compensation routine also provides a quantitative indicator of the estimated quality of the current compensation and the magnitude of any magnetic interference present [KVH Industries, 1993]. The C100 configured with an SE-25 coil assembly weighs just 62 grams (2.25 oz) and draws 40 mA at 8 to 18 VDC (or 18 to 28 VDC). The combined sensor and electronics boards measure 4.6×11 centimeters (1.8×4.5 in). RS-232 (300 to 9600 baud) and NMEA 0183 digital outputs are provided, as well as linear and sine/cosine analog voltage outputs. Display and housing options are also available. 2.4.3 Hall-Effect Compasses Hall-effect sensors are based on E. H. Hall's observation (in 1879) that a DC voltage develops across a conductor or semiconductor when in the presence of an external magnetic field. One advantage of this technology (i.e., relative to the fluxgate) is the inherent ability to directly sense a static flux, resulting in much simpler readout electronics. Early Hall magnetometers could not match the sensitivity and stability of the fluxgate [Primdahl, 1979], but the sensitivity of Hall devices has improved significantly. The more recent indium-antimonide devices have a lower sensitivity limit -3 of 10 Gauss [Lenz, 1990]. The U.S. Navy in the early 1960s showed considerable interest in a small solid-state Hall-effect compass for low-power extended operations in sonobuoys [Wiley, 1964]. A number of such prototypes were built and delivered by Motorola for evaluation. The Motorola Hall-effect compass employed two orthogonal Hall elements for temperature-nulled non-ambiguous resolution of the geomagnetic field vector. Each sensor element was fabricated from a 2×2×0.1 millimeter indiumarsenide-ferrite sandwich, and inserted between two wing-like mumetal flux concentrators as shown in Figure 2.25. It is estimated the 5 centimeter (2 in) magnetic concentrators increased the flux density through the sensing elements by two orders of magnitude [Wiley, 1964]. The output of the Motorola unit was a variable-width pulse train, the width of the pulse being proportional to the
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7KH8QLYHUVLW\RI0LFKLJDQ Where am I
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Acknowledgments This research was s
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Page 10 and 11: INTRODUCTION Leonard and Durrant-Wh
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CHAPTER 5 ODOMETRY AND OTHER DEAD-R
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R e t r o r e f l e c t i v e m a r
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CHAPTER 8 MAP-BASED POSITIONING Map
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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 Beacon 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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248 References 187. Larsson, U., Ze
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250 References 220. Nolan, D.A., Bl
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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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266 Index glass ...................
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268 Index lateral-post ............
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270 Index NPN .....................
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272 Index signal ..... 17, 31, 38,
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274 Index Abidi ...................
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278 Index Weiman ..................
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280 Index Video Index This CD-ROM c
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282 Index Paper 52 Paper 53 Paper 5
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