Advanced Solution for Pressure Based Side Airbag Systems
Side airbags in the front and rear side doors are getting more and more standard in modern cars. Up to now mainly acceleration sensors are utilised to detect a side crash. In recent years a new concept for side crash detection has been used by some car ma
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Advanced Solution for Pressure Based Side Airbag Systems
M. Kandler, Infineon Technologies AG Abstract Side airbags in the front and rear side doors are getting more and more standard in modern cars. Up to now mainly acceleration sensors are utilised to detect a side crash. In recent years a new concept for side crash detection has been used by some car manufacturers: The detection of the pressure pulse which is generated within the side door during the side crash. Compared to acceleration based systems the pressure detection method has some advantages. The firing decision can be made very fast and reliable. Furthermore the differentiation between misuse cases, i.e. non fire crashes, and fire crashes is very good. Infineon produces pressure sensor ICs for this application since 1998.
1
Introduction
In the present paper specialised pressure sensor IC’s for side crash detection are described in detail. A partitioning for a side airbag system consisting of the door module, the transmission line and the airbag ECU is presented. The core of the pressure sensor IC’s are surface micromachined capacitive sensor cells, monolithically integrated with an A/D converter and digital signal conditioning circuitry. The pressure sensor IC‘s offer a digital interface which is compatible with standard µCs. Furthermore self diagnosis features are implemented which are essential for safety relevant applications. Two types of pressure sensors are available. A basic type providing an absolute pressure signal via SPI and an advanced type providing a Manchester encoded pre-processed relative pressure signal. The advanced type allows to reduce the number of devices and hence the construction of cost effective door modules. Both types have a small SMD package which is proven for automotive requirements.
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Safety
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Technology and Signal Processing
The automotive safety market demands cost effective and highly specialised semiconductor components complying with high quality standards. These requirements can only be met by using a standard VLSI process for high volume production. The fabrication process for the presently available pressure sensor IC‘s is a 0,8µm BICMOS process running on standard equipment for high process stability. The basic element of the sensor IC is the pressure sensor cell. In comparison to bulk micromachining, where a pressure sensing membrane is obtained by anisotropic etching of the wafer backside, the surface micromachined sensor cell is fabricated by using standard deposition and etching steps. Figure 1 shows a principle cross section of a sensor cell consisting of an elastic membrane of conductive polycrystalline silicon (poysilicon), a sealed cavity and a conductive electrode on the substrate surface (well).
Fig. 1.
Cross-section of the surface micromachined capacitive sensor and reference cells (not in scale).
The intermediate cavity is formed by a particular sacrificial layer etching technique. In this process step the previously deposited field oxide is locally removed through small holes in the po
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