NONLINEAR SENSITIVITY ENHANCEMENT OF RESONANT MICROSENSORS AND ITS APPLICATION TO LOW POWER MAGNETIC SENSING
Seungkeun Choi, Yong-Kyu Yoon, Seong-Hyok Kim and Mark Allen
Nonlinearities in resonating structures can be used to increase the sensitivity of sensors based on these structures. An example system, a torsional resonant magnetic sensor, is analyzed to illustrate the effect. The system is composed of a disk-type silicon resonator combined with a permanent magnet supported by multiple micromachined silicon beams, excitation and sensing coils, and a magnetic feedback loop. The effects of nonlinearity on sensitivity have been characterized as a function of beam width and the number of beams using analytical models as well as numerical analysis. By increasing the number of beams while reducing the beam width (and thereby maintaining constant nominal linear resonant frequency), large nonlinearity has been obtained, resulting in an increased change in operating resonant frequency per unit applied magnetic field. The interaction between an external magnetic field surrounding the sensor and the permanent magnet generates a rotating torque on the silicon resonator disk, changing the effective stiffness of the beams and therefore the resonant frequency of the sensor. By monitoring shifts in the resonant frequency while changing the orientation of the sensor with respect to the external magnetic field, the direction of the external magnetic field can be determined. Self-resonance-based electromagnetic excitation of the mechanical resonator enables it to operate with very low power consumption and low excitation voltage. A total system power consumption of less than 140 ?W and a resonator actuation voltage of 1.4 mVrms from a ?1.2 V power supply have been demonstrated with a sensitivity of 0.28 Hz/rotational degree to the Earth's magnetic field.
Keywords: Electronics and devices, Instrumentation and measurement and Nanoscale science and low-D systems