A micromechanical silicon oscillating accelerometer

Abstract : This thesis describes the design and testing of a silicon oscillating accelerometer (SOA). The SOA is a silicon, micromechanical, oscillating beam accelerometer. The accelerometer is configured with two silicon tuning fork oscillators that are electrostatically driven and sensed in their out of phase vibrational resonance. The oscillators move on flexure beams which are configured to have one end anchored and the other end fixed to a shared seismic mass. When the seismic mass experiences an input acceleration, one oscillator's vibrating beams are loaded in tension while the other oscillator's beams are loaded in compression. This loading causes an increase and decrease in the respective oscillator's natural frequency. This change in frequency is proportional to the input acceleration. Automatic gain control electronics regulate the oscillation amplitude at resonance. The two oscillator's frequency outputs are differenced to exploit common mode error rejection. The SOA has been fabricated from single crystal silicon using a bulk dissolved wafer on glass micromachining process. It is vacuum packaged in a leadless ceramic chip carrier to achieve high quality factor and thus a sharp resonance. Results of both closed form and finite element analyses are described and are in good agreement Sensors were fabricated and experimental data including quality factor, input acceleration sensitivity or scale factor, and oscillator temperature sensitivity results are presented. An oscillator frequency of 27 kHz, a Q factor over 100,000, a scale factor of 4 Hz per g, and an oscillator temperature sensitivity of 0.4 Hz per degree Celsius have been achieved. The experimental data is shown to be in agreement with analysis results.