Where am I? Sensors and Methods for Mobile Robot Positioning

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60 Part I Sensors for Mobile Robot Positioning V o I Magnetizing current Time Offset H y M Magnetization Time a. b. V Sensor signal Figure 2.26: External current pulses set and reset the direction of magnetization, resulting in the “flipped” response characteristics shown by the dashed line. Note the DC offset of the device remains constant, while the signal output is inverted. (Adapted from [Petersen, 1989].) Offset Time synchronous detector driven by the same square-wave source. The rectified DC voltages V H1 and VH2 are thus proportional to the measured magnetic field components H 1 and H 2 . The applied field direction is dependant on the ratio of V to H, not their absolute values. This means that as long as the two channels are calibrated to the same sensitivity, no temperature correction is required [Fraden, 1993]. 2.4.5 Magnetoelastic Compasses A number of researchers have recently investigated the use of magnetoelastic (also known as magnetostrictive) materials as sensing elements for high-resolution magnetometers. The principle of operation is based on the changes in Young’s modulus experienced by magnetic alloys when exposed to an external magnetic field. The modulus of elasticity E of a given material is basically a measure of its stiffness, and directly relates stress to strain as follows: E ' F , (2.16) where E = Young’s modulus of elasticity F = applied stress , = resulting strain. Any ferromagnetic material will experience some finite amount of strain (expansion or shrinkage) in the direction of magnetization due to this magnetostriction phenomenon. It stands to reason that if the applied stress F remains the same, strain , will vary inversely with any change in Young’s modulus E. In certain amorphous metallic alloys, this effect is very pronounced. Barrett et al. [1973] proposed a qualitative explanation, wherein individual atoms in the crystal lattice are treated as tiny magnetic dipoles. The forces exerted by these dipoles on one another depend upon their mutual orientation within the lattice; if the dipoles are aligned end to end, the opposite
Chapter 2: Heading Sensors 61 R C V B H 1 Square-wave generator Sensors H 2 Coil L V(H) 1 L Sensor Amplifier Synchronous detector V(H) 2 Figure 2.27: Block diagram of a two-axis magnetic compass system based on a commercially available anisotropic magnetoresistive sensor from Philips [Petersen, 1989]. poles attract, and the material shrinks ever so slightly. The crystal is said to exhibit a negative magnetostriction constant in this direction. Conversely, if the dipoles are rotated into side-by-side alignment through the influence of some external field, like poles will repel, and the result is a small expansion. It follows that the strength of an unknown magnetic field can be accurately measured if a suitable means is employed to quantify the resulting change in length of some appropriate material displaying a high magnetostriction constant. There are currently at least two measurement technologies with the required resolution allowing the magnetoelastic magnetometer to be a realistic contender for highsensitivity low-cost performance: 1) interferometric displacement sensing, and 2) tunneling-tip displacement sensing. Lenz [1990] describes a magnetoelastic magnetometer which employs a Mach-Zender fiber-optic interferometer to measure the change in length of a magnetostrictive material when exposed to an external magnetic field. A laser source directs a beam of light along two optical fiber paths by way of a beam splitter as shown in Figure 2.28. One of the fibers is coated with a material (nickel iron was used) exhibiting a high magnetostrictive constant. The length of this fiber is stretched or compressed Laser diode Optical fiber Sensing leg Light coupler Photodetectors Reference leg Figure 2.28: Fiber-optic magnetometers, basically a Mach-Zender interferometer with one -7 fiber coated or attached to a magnetoelastic material, have a sensitivity range of 10 to 10 Gauss. (Adapted from [Lenz, 1990].)
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Acknowledgments This research was s
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2.4.2.2 Watson Gyrocompass ........
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6.4 Summary .......................
Page 10 and 11: INTRODUCTION Leonard and Durrant-Wh
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CHAPTER 5 ODOMETRY AND OTHER DEAD-R
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132 Part II Systems and Methods for
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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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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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280 Index Video Index This CD-ROM c
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282 Index Paper 52 Paper 53 Paper 5
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