The anti-theft system of modern automobiles usually adopts the methods of impact and vibration monitoring on the car body for anti-theft warning, and the most commonly used devices are magnetic effect sensors. Although the sensitivity of the magnetic effect acceleration sensor is very good, due to the assembly and installation errors of the magnetic sensor, its frequency response is unstable, which will cause the subsequent signal processing circuit and the microcontroller interface circuit to be more complicated, resulting in lower reliability of the system alarm. The false positive rate is higher. In addition, the method of transporting trailers or vehicles is also a common method of stealing and stealing cars. The most effective way to deal with this type of theft is to monitor the tilt angle of the car body, and the magnetic effect sensor cannot measure static acceleration and cannot deal with the car. The inclination angle of the body is measured, and it is impossible to monitor and warn of this theft method. In short, the use of magnetic effect sensors can not complete the task of anti-theft monitoring well, which requires a more reasonable and reliable sensor device to replace. ADXL202 is a low-cost, low-power, single-chip integrated dual-axis acceleration sensor designed and manufactured by AD company using MEMs process. It is powered by a single power supply of 3 to 5.25V, and the operating current is less than 6mA. It can measure the dynamic range of O to 5kHz and ± 2g Or static acceleration, the resolution is 2mg at 60Hz, can be output in the form of digital and analog signals, the volume is only 5mm & TImes; 5mm & TImes; 2mm; It can monitor the small vibration of the car body and the inclination angle of the vehicle at the same time. Applying it to an automobile anti-theft system not only expands the monitoring range of the system, but also simplifies the system and improves the reliability of the anti-theft system's report management. Therefore, it is selected as the sensor device of the automobile anti-theft system. First, the working mechanism of the acceleration sensor ADXL202 (1) Basic principle of acceleration measurement using ADXL202 The module structure of ADXL202 is shown in Figure 1. It is a measurement module that integrates two independent sensitive elements with mutually perpendicular measurement directions on a single chip. It is a capacitive acceleration sensor made of polysilicon micro-machined surface technology; A mass plate supported by the elastic structure on the surface of the sheet is attached to one pole plate of the capacitor, and the other pole of the capacitor is fixed by pulling. When acceleration causes the relative position of the mass to change, the capacitance value also changes; the signal processing circuits of the two sensitive elements convert the respective capacitance changes into square wave signals whose amplitude is proportional to the acceleration, and are modulated by the duty cycle (DCM) After that, the analog signal can be converted into a digital signal and output through the pin XOUTYOUT. In addition, ADXL202 can also output analog signals through the XFILT and YFILT pins. (2) Introduction to ADXL202 pin functions The package form of ADXL202 is LCC-8, and the schematic diagram of its package and pins is shown in Figure 2. The pin ST is used for chip self-test. After VDD is powered on, the ST's output duty cycle pulse width changes by about 10% under the action of electrostatic force, which is equivalent to an output signal with 800mg acceleration. This pin can be used to test Determine whether the chip is working properly; pin T2 needs to be connected to an RSET resistor, and the working cycle period T2 is set within the range of 0.5ms to 10ms; pin COM is used as a common ground terminal; pins XOUT and YOUT are used to output digital signals; pins XFILT, YFILT plus filter capacitor is used to set the bandwidth range of the detection signal, and the VDD pin is used as the power supply input terminal of the chip. (3) Determination of acceleration value The acceleration sensor can use XOUT, YOUT pin and XFILT, YFILT pin to output digital signal or analog signal respectively. When using digital signals for measurement, as long as the counter / timer port of the microprocessor is used to collect the duty cycle pulse width T1 of the signal and the duration T2 of the duty cycle, the value of the measured acceleration can be obtained using formula (1) . The calculation formula is: The signal diagram is shown in Figure 3. When using analog signals for measurement, the analog signals output from the XFILT and YFILT pins can be directly used, or the digital signals output from the XOUT and YOUT pins can be RC filtered to be restored to analog signals. The first method outputs a wider signal bandwidth, but requires an external voltage follower to carry the load; the second method can amplify the output analog signal, but the frequency response characteristics are low. Which output method to use should be determined according to the specific conditions of use.
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