Lorentz Force Magnetic Sensors

This dissertation describes micromechanical Lorentz force magnetic sensors for electronic compass applications. Recent development in commercially available MEMS accelerometers and gyroscopes has been focused on the reduction of size, power consumption and cost, which has led to the integration of a 3axis accelerometer and 3axis gyroscope on the same chip, known as the 6axis combo sensor. The growing market of smartphone, tablet and wearable device drives the need fora 9axis combo sensor, which adds a 3axis magnetic sensor to the 6axis combo sensor. However, previous approaches to 9axis combo sensors have been complicated by two facts: (1) the commercially available magnetic sensors cannot be cofabricated with MEMS inertial sensors, therefore increasing the size, cost and difficulty of calibration. (2) magnetic material is required for commercially available magnetic sensors, which introduceproblems such as hysteresis and limited measurement range.
Singleaxis, dualaxis and triaxis Lorentz force magnetic sensors are designed and fabricated using microfabrication processes which are fullycompatible with MEMS accelerometers and gyroscopes. The magnetic sensors are based on micromachined resonators, which use no magnetic materials and can be easily integrated with CMOS electronics. The single structure triaxis magnetic sensor described in this workdemonstrates 0.1°heading accuracy with 1 mW power consumption, whichis estimated to be 10X better in SNR compared to existing commercially available compasses. The design trade-offs between size, performance, and cost are also explored.
In order to improve stability over temperature and eliminate the need for an external frequency source, both AM and FM readout closed-loop magnetic sensors are investigated. AM readout magnetic sensors can be realized by applying a Lorentz force in-phase with velocity to an electrostatically excited MEMS oscillator. FM magnetometers can be realized either by using the Lorentz force to create axial tension on a MEMS resonator, thereby changing its resonant frequency, or by applying a Lorentz force in quadrature with velocity to an electrostatically excited MEMS oscillator. Analytical and experiment results of all three types of closed-loop magnetic sensors are compared and discussed in detail in terms of their magnetic field sensitivity, bandwidth, resolution, offset, and temperature sensitivity.
To further reduce the offset and the drift induced by the electrostatic force, current chopping is investigated. The current chopping technique periodically switches the polarity of the sensitivityof the magnetic sensors, while the offset remains the same. The offset of the sensor is reduced from 25 mT to 31 μT, which is 10 times better than the existing Hall-effect magnetic sensors. The long-term drift is also reduced by 120X.
Stephen O’Driscoll
Rajeevan Amirtharajah
Publication date: 
January 31, 2015
Publication type: 
Ph.D. Dissertation
Li, M. (2015). Lorentz Force Magnetic Sensors. United States: University of California, Davis.

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