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In this paper we present an occupant detection system based on a capacitive sensing technique. The capacitive detector is designed to distinguish between an empty and occupied vehicle seat. Additionally, the coupling between the seated passenger and the car-body can be investigated. This occupant detection system provides valuable additional information for airbag controlling system. By launching airbags well matched and controlled, the injuring risk of a passenger can be minimized in case of a car crash. In comparison to standard pressure sensors capacitive sensors are contactless and have no wearing parts [1], this leads to economic benefits. The detector operates in a frequency range between 50 kHz and 5 MHz with 5 mV RMS sine signals, by using a precision impedance analyzer. In our laboratory setup, the car seat measuring assembly is shielded by a copper chassis cage which can be connected either to the low or high-potential of the impedance analyzer or can be kept floating. This feature enables us to analyze the effect of the variable passenger to ground coupling. Each of the 16 sensor electrodes, located at the backrest and seating surface area, can be switched to low potential, high potential or can be left unconnected during a measurement. In order to determine the posture of the passenger body, we record coupling capacitances between single electrode pairs. Furthermore impedance characteristics over a wide frequency range can be analyzed based on appropriate impedance models in order to receive more information about the occupancy status of the car seat.

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