Non-Contact Sensing for High-Temperature, Capacitive MEMS Sensors in Active Fuel Emission Control

This paper describes a non-contact sensor interface for MEMS capacitive sensors used in active fuel emission control (AFEC). The sensor elements are used as passive capacitors in a LC resonant circuit, the resonant frequency being a function of the sensor capacitance. Detailed analysis of the inductive link reveals a desired operation region in which the normalized resonant frequency response is linear, independent of quality factor, and sensitivity is only weakly dependent on antenna coupling factor. High-temperature sensor interface electronics are proposed to lock to the resonant frequency of the inductive tank circuit and provide a digital readout that is communicated to the combustion node controller. These circuits require a gm-stabilized negative conductance to form an oscillator and a high-speed counter that acts as FM demodulator in a multiplexed architecture that accommodates an array of sensors.