A micromachined magnetic field sensor for low power electronic compass applications

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Choi, Seungkeun
Allen, Mark G.
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A micromachined magnetic field sensing system capable of measuring the direction of the Earths magnetic field has been fabricated, measured, and characterized. The system is composed of a micromachined silicon resonator combined with a permanent magnet, excitation and sensing coils, and a magnetic feedback loop. Electromagnetic excitation of the mechanical resonator enables it to operate with very low power consumption and low excitation voltage. The interaction between an external magnetic field surrounding the sensor and the permanent magnet generates a rotating torque on the silicon resonator disc, changing the effective stiffness of the beams and therefore the resonant frequency of the sensor. MEMS-based mechanically-resonant sensors, in which the sensor resonant frequency shifts in response to the measurand, are widely utilized. Such sensors are typically operated in their linear resonant regime. However, substantial improvements in resonant sensor performance can be obtained by designing the sensors to operate far into their nonlinear regime. This effect is illustrated through the use of a magnetically-torqued, rotationally-resonant MEMS platform. Platform structural parameters such as beam width and number of beams are parametrically varied subject to the constraint of constant small-deflection resonant frequency. Nonlinear performance improvement characterization is performed both analytically as well as with Finite Element Method (FEM) simulation, and confirmed with measurement results. These nonlinearity based sensitivity enhancement mechanisms are utilized in the device design. The complete magnetic sensing system consumes less than 200 microwatts of power in continuous operation, and is capable of sensing the direction of the Earths magnetic field. Such low power consumption levels enable continuous magnetic field sensing for portable electronics and potentially wristwatch applications, thereby enabling personal navigation and motion sensing functionalities. A total system power consumption of 138W and a resonator actuation voltage of 4mVpp from the 1.2V power supply have been demonstrated with capability of measuring the direction of the Earths magnetic field. Sensitivities of 0.009, 0.086, and 0.196 [mHz/(Hz and #903;degree)] for the Earths magnetic field were measured for 3, 4, and 6 beam structures, respectively.
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