Three-Axis Lorentz-Force Magnetic Sensor for Electronic Compass Applications

A low-power microelectromechanical-systems (MEMS) three-axis Lorentz-force magnetic sensor is presented. The sensor detects magnetic field in two axes with a single MEMS structure. Three-axis sensing is performed using two perpendicular structures on the same die. The MEMS device is a micromechanical resonator, and sensing is conducted using excitation currents at the device's in-plane and out-of-plane mechanical resonant frequencies which are 20.55 and 46.96 kHz, respectively. A die-level vacuum seal results in in-plane and out-of-plane mechanical quality factors of 1400 and 10 000, respectively. With 0.58 mW used to provide the two-axis excitation current, the sensor's noise is equivalent to 137 nT/root Hz for the z-axis magnetic field inputs and 444 nT/root Hz for the x-and y-axis fields. For the z-axis field measurements, Brownian noise is the dominant noise component, while the x- and y-axis field measurements are limited by the electronic noise in the discrete capacitive-sensing electronics. The major source of offset error is residual motion induced by electrostatic force. The offset is reduced to 14 mu T using a dc compensation voltage applied to the MEMS structure to null the electrostatic force. After compensation, the offset stability is 400 nT with a 0.7-s averaging time. [2011-0345]