Electro-mechanical feedback for realization of a mechanical spectrum analyzer

An important feature of MEMS, still underutilized, is the tight coupling between the electrical and mechanical quantities in the micro domain. This paper demonstrates the benefits of such a functional coupling for a specific device: a real-time mechanical spectrum analyzer. A surface micromachined accelerometer was designed, with a voltage-controlled gain. By applying an harmonic common-mode voltage on the actuation capacitors, the intrinsic nonlinear coupling performs a correlation between the input acceleration and an electrically-controlled signal. To reduce the distortion, a differential, twin-accelerometer scheme is used. Time-multiplexed signal processing gives both the real and the imaginary components of the selected spectral line. A mechanical spectrum analyzer for low-frequency domains is obtained when the driving electrical frequency is swept over the range of interest. The operating principle has been confirmed by measurements, and is expected to yield new opportunities in the field of low-power, low-cost condition monitoring equipment.