0 Preface
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With the growth of China's national economy, the amount of road traffic has increased significantly, and the speed of driving has also increased significantly. It has been followed that traffic accidents are on the rise. Airbags are a very effective measure to reduce casualties.
1 Main features of MMA2204D
The most characteristic change in a car collision is acceleration. Therefore, airbag design generally detects acceleration based on acceleration changes generated during a car collision. Accelerometers are commonly used in testing. Accelerometers used in airbags should generally have the following properties:
â—‡ should have self-test function:
应 There should be a certain amplification and frequency selection capability for the induced electrical signal:
â—‡ should have good temperature stability, high entanglement, firm structure and small size. Easy to install;
â—‡ should be able to convert the acceleration generated by a car collision into an electrical signal proportional to it (such as current, voltage, etc.), and the sensitivity is high:
â—‡In order to reduce the cost of airbags, the price of accelerometers should be as low as possible.
2 Accelerometer chip selection
There are many types of devices that can be used as accelerometers. In recent years. Due to the rapid development of electronic technology and integrated technology, the performance of micromachined silicon piezoresistive accelerometers and silicon capacitor accelerometers has been improved day by day, and the price has been reduced to an acceptable level. According to the performance of the acceleration used by the airbag, this paper chooses Freescale's silicon capacitor micromechanical accelerometer MMA2204D. This is a silicon capacitive accelerometer with integrated signal amplification, signal processing, low pass filtering and compensation. The device's zero-acceleration bias, full-scale range, and filtering characteristics are all set by the manufacturing unit, eliminating the need for external passive components. Figure 1 shows the functional block diagram of its internal simplified structure.
3 MMA2204D's main functions
Freescale's MMA2204D accelerometer is a surface micromechanical integrated circuit accelerometer. The chip contains a surface micromechanical capacitance measuring unit (g-cell) and a CMOS single adjustment application type ASIC, wherein the g-cell is a semiconductor mechanical structure. The MMA2204D accelerometer has a polycrystalline silicon inertia block inside. It is suspended between two fixed polycrystalline poles. The acceleration acts to move the inertia block, causing the capacitance to change, and then converting the capacitance into a voltage output. To avoid mechanical shock, the accelerometer is also equipped with a damping and over-range braking mechanism. In addition, the MMA2204D features self-test, filtering and status outputs.
3.1
Self-test
The sensor in the MMA2204D can verify the mechanical and electrical integrity of the accelerometer at any time after installation by self-testing characteristics, which is critical for automated airbag systems. For the car, the integrity of its system must be ensured. MMA2204D uses the fourth board as the self-checking board in g-celI, and the s-out est pin is active high, so the electrostatic force generated by it can cause the offset of the intermediate board, and the accelerometer can control the ASIC to measure This deviation produces a relative proportional output voltage. This process ensures the function of the mechanical and electronic parts of the accelerometer.
3.2 Filtering
The MMA2204D automotive airbag accelerometer contains a four-pole switched-capacitor filter that uses the Bessel filter because it has a time-continuous response function. Therefore, a pulse-like integral can be saved. Because the filter uses switched-capacitor technology, there is no need for additional missing components (resistors and capacitors) to set the lower cutoff frequency.
3.3 Status
The MMA2204D accelerometer contains fault detection and fault latches whose status pins can output status signals from the fault latch and the self-test. Whenever one or more of the following conditions occur, its status pin will output a high level:
(1) The clock frequency is lower than the lowest frequency of the clock detector;
(2) The supply voltage is lower than the voltage threshold for low voltage detection;
(3) The parity of the EPROM is an odd number.
4 MMA2201 main performance parameters and application circuit
The main performance parameters of MMA2204D are listed in Table 1. The hardware design circuit based on MMA2204D automotive airbag system is shown in Figure 2.
5 Conclusion
The MMA2204D-based automotive airbag circuit is simple in design, large in measurement range, high in sensitivity, and low in temperature and frequency. In addition, the MMA2204D chip also comes with self-test circuit, and the price is moderate, which can fully meet the design requirements of the airbag accelerometer.
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