The growing demand of including an inertial measurement unit (IMU) in smart-phones, tablets and wearable devices to facilitate navigation and other location based services is rapidly increasing the market for low cost inertial sensors. At present the most commonly used magnetic sensor is a hall-effect sensor and therefore typically an IMU contains two or more separate chips wire-bonded in a single package. Since a Lorentz force sensor can be designed in the same process as other inertial sensor, it increases the possibility of fabricating a 6 DOF (Degrees of Freedom) or a 9 DOF IMU in a single die. The goal of this dissertation is to design a complete magnetic sensor system consisting of a Lorentz force sensor and interfacing low power electronics.
A micro-electromechanical-systems (MEMS) three-axis Lorentz force magnetometer based on a 024∗04mm2 MEMS resonator is presented here. Magnetic field can be detected in two axes using a single MEMS structure. Placing two structures perpendicular to each other in a single die makes three-axis sensing possible. Sensing is performed by exciting the MEMS resonator at its in-plane and out-of-plane mechanical resonant frequencies of 40.5 kHz and 107.4 kHz respectively. A modest die-level vacuum packaging results in in-plane and out-of-plane mechanical quality factors of 110 and 310 respectively. The sensor has a bandwidth of 184 Hz for z-axis and 189 Hz for x/y-axis magnetic field.With an excitation power of 2 mW, the sensor resolution is 285nT/√Hz for z-axis magnetic field inputs and 344nT/√Hzfor x/y-axis magnetic field inputs. With an averaging time of 288 s the sensor shows an offset stability of 23 nT.
Two different sensing schemes were investigated for low power low noise sensing: 1) Continuous Time Current (CTC) using a trans-impedance amplifier (TIA) and 2) Continuous Time Voltage (CTV) sensing using a voltage buffer. Both the TIA and the buffer was designed and fabricated in TSMC’s 0.18 micron process. A magnetic sensor fabricated in Stanford’s epi-seal process was used to characterize the analog circuits. Although with similar power consumption (40μW) the CTV scheme achieves lower noise resolution (230nT/√Hz compared to 900nT/√Hz achieved in CTC), higher magnetic sensitivity of the CTC scheme makes it a more favorable candidate to use in a close loop system.
To drive a Lorentz current through the low resistance MEMS flexure, a power efficient system was also designed. A dc-dc converter was used to lower the supply voltage from1.8V to 542mV which was then chopped at the natural frequency of the MEMS sensor to generate the drive current. The dc-dc converter was designed to provide 1.16mA drive current to a MEMS flexure whose resistance can be as low as 450 Ω with a power efficiency over 70%. The total power consumption of the dc-dc converter is 0.844 mW.
To implement the complete close loop sensing system, a low power ASIC (with power consumption of 327.6μW) was also designed using TSMC’s 0.18 micron process. With slightly lower power consumption (1.17mW) than that of the hall-effect sensor, the complete system is estimated to achieve a lower magnetic noise resolution of 172nT/√Hz.
September 30, 2015
Rouf, V. T. (2015). System Design of a Lorentz Force MEMS Magnetic Sensor. United States: University of California, Davis.