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Tham khảo tài liệu 'where.am.i-sensors.and.methods.for.mobile.robot.positioning.-.borenstein(2001) part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Chapter 2 Heading Sensors 61 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 high-sensitivity 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 Figure 2.28 Fiber-optic magnetometers basically a Mach-Zender interferometer with one fiber coated or attached to a magnetoelastic material have a sensitivity range of 10 -7 to 10 Gauss. Adapted from Lenz 1990 . 62 Part I Sensors for Mobile Robot Positioning in conjunction with any magnetoelastic expansion or contraction of its coating. The output beam from this fiber-optic cable is combined in a light coupler with the output beam from the uncoated reference fiber and fed to a pair