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, it can be output in the form of digital and analog signals, and the volume is only 5mm × 5mm × 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.
1 Working mechanism of acceleration sensor ADXL202
1.1 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 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 circuit of the two sensitive elements converts the respective capacitance changes into a square wave signal whose amplitude is proportional to the acceleration, and is 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.
1. 2 ADXL202 pin function introduction
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 output pulse width of the working cycle of ST is changed by about 10% under the action of electrostatic force, which is equivalent to an output signal with an acceleration of 800 mg. This pin can be used to test Determine whether the chip is working normally; 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; pin XFILT 1. YFILT plus filter capacitor is used to set the bandwidth range of the detection signal. The VDD pin is used as the power supply input terminal of the chip.
1.3 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.
2 Design scheme of car anti-theft system using ADXL202
Because the thief steals the car, it will cause the car to vibrate or tilt, and the ADXL202 can measure the dynamic or static acceleration in the range of 0-5kHz, ± 2g. The measurement of dynamic acceleration can be used for vibration detection; while using static gravity acceleration as an input vector, the spatial direction of the object can be determined.
2.1 Overall scheme design
ADXL202 is powered by a 3V DC power supply. In order to reduce the interference of the power supply to the sensor, O must be connected between the pin VDD and the common ground COM. 1μF decoupling capacitor. Since the chip and the microprocessor share a power supply, a 100Ω resistor needs to be added between the power supply and VDD to reduce the interference of the digital signal on the sensor output signal.
The T2 pin is connected with an RSET resistor of 625kΩ to determine the output digital signal period as 5ms, that is, the frequency is 200Hz. The digital signals output from the XOUT and YOUT pins are directly sent to the counter and timer port of the microprocessor, so as to measure the acceleration of gravity to obtain the change of the inclination of the vehicle body. The analog signals output from the XEILT and YFILT pins are passed through the voltage follower to increase the load capacity, and then sent to the A / D port of the microprocessor to measure the tiny vibration of the car body.
In car anti-theft, the measurement of the tilt angle of the car body requires that the noise of its signal must be very small, which limits the signal bandwidth. For vibration measurement, a wide signal range of 10 to 200 Hz is required. First, set the filter capacitors CX and CY of the XFILY and YFILT pins to 0.027μF, so that the bandwidth of the digital and analog signals output is limited to less than 200Hz, and then the digital and analog signals of the ADXl202 output bandwidth of 200Hz After low-pass filtering and high-pass filtering, you can obtain the bandwidth required for tilt and vibration monitoring. The schematic diagram of the ADXL202 as a vibration measurement and tilt measurement sensor is shown in Figure 4.
2.2 Measurement of tilt angle
For the theft method of trailer or vehicle transportation, if the angle of the car body changes by 5 ° relative to the initial state, it can be determined that a car theft has occurred.
The measured value of the tilt angle is obtained by the digital signal output from the XOUT and YOUT ports. Due to the low-pass filtering of the capacitors at the XFILT and YFILT ports, the signal bandwidth is 200 Hz. Then the noise calculation formula according to ADXl202 is:
Among them, Noise is the geometric mean value of noise, BW is the signal bandwidth, and f (Hz) is the frequency. Then the average noise level is:
If the sensitive axis of the ADXl202 changes from the horizontal position by 5 °, it is equivalent to the digital signal output by the ADXL202 changing by about 87mg, so the noise level of 14.4mg seems too high. Because the expected noise level cannot exceed one-tenth of the signal, which is 8.7mg. Sampling the 200 Hz digital signal output from the XOUT and YOUT ports 16 times, and averaging the collected signals, digital low-pass filtering can be achieved, and the signal bandwidth is reduced to 12.5 Hz. The noise level at this time is:
This noise level can fully meet the system requirements.
The temperature change coefficient of ADXl202 is 2mg / ° C, and the output changes by about 17mg for every 1 ° change within the monitored range of 0 ° to 5 ° inclination. Since the temperature difference between outdoor day and night is likely to reach 8.5 ℃, temperature drift is likely to cause false alarms, which must be limited. You can use the software differentiator for signal processing, that is, if the output change rate of the ADXL202 is greater than 87mg / min, it is considered to be a warning; if it is less than this value, it can be considered to be a temperature drift caused by temperature changes, and no alarm is given.
2.3 Measurement of tiny vibrations of automobiles
Through the measurement of the small vibration of the car body, the monitoring of the behavior of the car body can be realized. If the energy of the vibration exceeds the set threshold within a short period of time, it can be determined that there is a situation where the car is destroyed.
The signals needed to monitor the micro vibration of the car are obtained from the XFILT and YFILT ports. Although the analog signals output at the XFILT and YFITL ports have been 200 Hz low-pass filtered, the signal still needs to be subjected to 10 Hz high-pass filtering to meet the 10 to 200 Hz bandwidth required for vibration monitoring. In addition, the load capacity of the analog signal output from the XFILT and YFILT ports is very weak. You can send the band-pass filtered signal to the voltage follower to improve its load capacity. Then, send the signal to the A / D inside the microprocessor. The port performs analog-to-digital conversion. According to the sampling theorem (Nyquist Criteria), the sampling frequency of the emblem processor is set at 400 Hz, that is, it is sampled every 2.5 ms.
Using software integrator to measure the energy of vibration is the sum of the absolute values ​​of 16 amplitude samples recorded by the microprocessor, and the energy of vibration within 40ms can be obtained. If the sum of this absolute value exceeds the set digital threshold, the microprocessor can initiate an alarm.
The anti-theft monitoring device composed of the acceleration sensor ADXL202 not only has the sensitivity of the traditional anti-theft sensor devices, but also expands the monitoring range of the anti-theft car (can be used for early warning of theft methods of trailer and vehicle handling), and also simplifies the Circuit structure improves system reliability. ADXL202 can completely replace traditional sensitive components such as magnetic effect sensors. Become a reliable and sensitive device for automobile anti-theft monitoring signals. In addition, ADXL202 is also widely used in computer peripherals, information products, motion monitoring and other aspects.
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