Shock-resistant low-power ferromagnetic MEMS magnetometers

A ferromagnetic micromechanical magnetometers has been designed, fabricated, and tested whose sensitivity, frequency response, shock resistance, and noise floor have been determined. The need for miniature high-sensitivity low-power magnetometers is driven by applications such as wireless sensor networks, which are useful for monitoring and controlling large areas, large facilities, and large numbers of mobile assets. Existing magnetometer technologies use too much power (fluxgate), are not sensitive enough (hall-effect), or require extensive support-systems (squids).;The magnetometer presented in this dissertation is a novel MEMS-based magnetometer that is capable of a high sensitivity that does not scale down with dimension and consumes no power other than that needed for the sense circuits. The concept behind the operation of this low-power magnetometer is straight-forward: a micromechanical compass. An ambient magnetic field will produce a torque (taumag) on the magnet. The magnetic torque is then transferred to a torsional microflexure and an angular deflection (&phis;) is generated that is proportional to the magnetic torque and inversely proportional to the angular stiffness of the torsional flexure. Despite the fact that the magnetic torque scales down with a cubic dependence, so does the angular mechanical stiffness of the torsion bar. The result is that the ratio of the two, and hence the angular deflection produced by the sensed magnetic field, has no dependence on dimensional scaling.;Preliminary shock tests were performed at high-G levels (∼5,000 to 10,000 G's) to investigate the relative robustness of the various magnetometer designs. Theory and results showed that the magnetometer can be designed to be resistant to shock, and still achieve a high sensitivity.;Sensitivity and frequency-response data has been taken using both a laser Doppler vibrometer (LDV), and capacitive sensing circuitry. The LDV has shown that this magnetometer technology is capable of sensing nT magnetic fields for a frequency range of 10 to 100 Hz. Analysis of the mechanical noise limits for the magnetometer technology were found to be in the pT/Hz range for the most sensitive magnetometers, but were unobservable due to the higher noise floor in the LDV. MEMS magnetometers incorporating capacitance-sensing electrodes and off-chip sensing circuitry were capable of detecting mT magnetic fields, with the system being limited by parasitic capacitances and circuit noise ( mT/Hz range).