A Feasibility Demonstration of a Sensing Control Unit for Measurement of Large Capacitance for Particulate Matter Sensor

On-board diagnostics (OBD) of diesel vehicles require various sensors to detect system malfunctions. The Particulate Matter (PM) sensor is one of OBD devices which gather information which could be critical in determining a crack in the diesel particulate filters (DPFs). The PM sensor detects PM which penetrates cracked DPFs and converts the amount of PM into electrical values. The PM sensor control unit (SCU) receives those analog signals and converts them to digital values through hardware and software solutions. A capacitive sensing method would be a stable solution because it detects not raw analog signals but electrical charges or a time constant going through the capacitive load. Therefore, amount of PM would be converted reasonable value of capacitance even though there is a little amount of PM. Naturally, it is beneficial to have a wide range of capacitive variances of sensors but, commercial capacitance-to-digital converter(CDC) integrated circuits (ICs) are focused on the touch-pad industries in which a few pF-order capacitances are enough.

This paper presents the feasibility of a large capacitive (generally having hundreds of picofarad (pF)) SCU. We have used hardware (H/W) systems with amplifying the CDC IC's capacitive input range to hundreds of pF-order capacitance input. Verification tests were performed to the capacitance limits by comparing input value, measured value, root mean square (RMS) noise and peak-to-peak noise of each case. After the verification, we demonstrated the feasibility of the control unit as a PM SCU with a capacitive measuring method. Experiments were conducted with the real engine test bed (ETB) generating PM which penetrated the cracked DPFs. PM SCU measured capacitive variance from the PM sensing electrodes and processed the capacitance digital value to the microcontroller (MCU) of the SCU.