Automobile speed detecting method based on battery ripple voltage measurement
Abstract With the continuous improvement of automobile manufacturing technology, the engine structure of the new automobile is compact, which makes the ignition coil part of the automobile engine not easy to be directly close. Therefore, the traditional pulse ignition method of the vehicle speed detection method can not meet the current measurement requirements. Through theoretical analysis, it is found that The speed of the automobile generator is related to the speed of the engine. The three-phase voltage output by the generator is bridge-rectified and applied to both ends of the vehicle battery. The method of taking out the voltage signal from the car cigarette lighter is quick and easy, and the operation is simple. Safety. In this paper, the multi-stage band-pass filter circuit is used to realize the noise filtering of the weak electrical signal, voltage amplification, and finally the fast discrete Fourier transform is used to complete the real-time spectrum analysis of the signal, so that the frequency information related to the vehicle speed can be obtained, and thus the calculation is performed. Get the car engine speed.
Key words automobile engine speed measurement; fast Fourier transform; active filter; harmonic analysis
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There are many ways to measure the engine speed. Commonly used methods include ignition coil pulse, automobile vibration analysis and measurement, and photoelectric sensor measurement. Because the engine of the new car is relatively tight and cannot be close to the ignition pulse coil, the ignition coil pulse measurement is not applicable. The vibration measurement method is inaccurate for the V6 engine with weak vibration, and the interference detection at high speed. The problem, at the same time, these two methods need to open the front end cover of the car, the operation is cumbersome, the safety factor is low; the car battery voltage is extracted by the car cigarette lighter, and the battery voltage contains the pulsating harmonic component, if the DC signal is isolated, Extract the AC signal, filter out the clutter and high-frequency interference, amplify the signal related to the generator speed, use TI's TMS320F2812 as the platform for signal acquisition, and use the fast Fourier transform to calculate the frequency components and analyze the harmonics. The wave frequency gives information about the car's speed, and the car's speed is obtained by indirect conversion.
1 Signal filtering and transformation
1.1 Signal coupling Firstly, the DC voltage component of the battery terminal voltage of the signal is filtered out by the DC-blocking circuit. As shown in Fig. 1, the DC component is filtered by the capacitor C67, and then the weak AC signal is coupled to the lower portion through the voltage follower circuit. The first stage op amp performs signal processing.
1.2 High-pass filtering FILterSolution software is used to design an infinite gain negative feedback active 2nd-order high-pass filter with a cutoff frequency of 600 Hz. The low frequency signal generated by the voltage regulator of the car charging system is filtered out and connected in series by two second-order high-pass filters. The form increases the attenuation gain as shown in Figure 2.
1.3 Low-pass filtering calculates the parameters of the infinite-gain multi-channel negative feedback active low-pass filter, and then uses software simulation to design a feasible filter, as shown in Figure 3, and a series of switched-capacitor filters. The signal is subjected to high-frequency attenuation. The clock frequency of the switched-capacitor filter is 1 mHz, which is provided by the DSP. According to the requirement of max291, the cut-off frequency is calculated to be 10 kHz.
1.4 Bandpass filtering combines the low-pass and high-pass filters to form a bandpass filter, which can obtain the speed-dependent frequency signal <1~10kHz), and finally through a single-source power supply op amp, as shown in Figure 4. The alternating signal is boosted to a DC change signal, and the output is limited to 3 V through D6, thereby entering the A/D measurement channel of the DSP for sampling analysis. The circuit is shown in Figure 4.
2 Signal acquisition and analysis Signal acquisition adopts TMS320F2812 platform, 2 812 comes with 12-bit high-precision A/D conversion, and the minimum resolution is 0.000 732 V, which satisfies the accuracy requirements of signal detection. The maximum allowable measurement voltage is 3. V, so the last stage output uses a +5 V single-supply op amp to raise the signal voltage above 0 V, the output is divided by a resistor, and the limiter circuit limits the voltage to within 3 V. Then connect to the A/D1 of the DSP. The DSP starts the A/D conversion in an event-triggered manner. According to Shannon's theorem, the sampling frequency is twice the highest measurable signal frequency, so that the acquired signal can be restored. The highest measurable frequency is 10 kHz, so the sampling frequency is set to At 20 kHz, the FFT transform uses 1,024 points and the minimum resolution frequency is 20 Hz. To improve measurement accuracy, higher FFT conversion points can be used.
By performing fast Fourier transform on the analog voltage signal, the spectrum of the analog signal can be calculated. TI provides a fast Fourier transform function library, and the frequency spectrum can be calculated by directly calling the library file fft. According to the calculated frequency spectrum, the position of the maximum point of the amplitude is analyzed, thereby calculating the fundamental frequency of the battery. If the spectrum has a fixed interference frequency and the amplitude of the interference frequency is larger than the amplitude of the fundamental frequency, A second large-value algorithm is used to obtain a frequency that varies with the speed of the vehicle.
3 Automobile engine speed calculation method Firstly, each parameter is named: n is the generator speed; N is the vehicle speed; f is the three-phase voltage frequency of the generator; P is the generator rotor pole pair; F is the rectified ripple frequency ;k is the ratio of generator to engine speed; K is the ratio of speed to fundamental frequency 
Among them, K=60/(k×p×i) is a constant, and the car rotation speed of the formula (4) is proportional to the fundamental frequency of the battery terminal voltage. Generally, the pole pair P4 of a typical 4-cylinder 4-stroke automobile generator is P=6, the speed ratio is k=1/2, i=6, so K=5/6, the car speed N=5/6 x F (5)
4 Experimental data analysis The experimental data obtained from the Changan Benben car test are shown in Table 1. The comparison between the oscilloscope and the DSP is made in the table. The speed N of the engine is observed by the car tachometer, the fundamental frequency F is measured, and the proportional coefficient and the rotational speed ratio are calculated by the formula (4). F = 1 000 Hz in Table 1, substituting into equation (5) to calculate N = 833 r / min and the actual measurement results 900 basically coincide, the error is derived from the speed ratio coefficient.
5 Error analysis The harmonics generated by automotive electrical equipment are the main source of interference, and this frequency is between 1~10 kHz of the fundamental frequency of the generator output, the amplitude is relatively large, and finally it is necessary to determine the fundamental frequency of the rotating speed. Distinguish by finding the maximum algorithm. Equation (4) shows that the main influencing factors of the error are the engine and generator speed ratio k, and the frequency F measured by the DSP. Because the speed ratio of different cars is not constant, if the general setting k is 2, it is obviously unreasonable, so this kind of The most fundamental error is the measurement method; the error of the measurement frequency is also the influencing factor, and the measurement frequency resolution determines the resolution of the vehicle speed measurement. The sampling frequency is 20 kHz, the measured frequency error is 20 Hz, assuming the coefficient k=2; substituting N=5/6×F=17 r/min, the resolution is 17 r/min, if the measurement is automatically reduced according to the engine speed The frequency can be used to obtain higher measurement accuracy. For example, the sampling frequency is set to 6 kHz, and the rotational speed error is (6000/1 024) × 5 / 6 ≈ 5 r / min.
6 Conclusion Although the measurement method is convenient and fast, the error and uncertainty of the speed ratio coefficient lead to the calculated error of the car speed. Therefore, other methods are needed to assist in calculating the speed ratio. For example, when the vehicle is idling, the acceleration sensor is used to measure the vibration. The frequency deduces the car speed. Because the range of the vehicle idle speed is generally between 700 and 1 500 r/min, the measurement of the idle speed is relatively simple. At the same time, the fundamental frequency of the battery voltage is measured, and the proportional coefficient is calculated by the rotational speed and the fundamental frequency. By using this coefficient multiplied by the measured fundamental frequency, the car speed can be measured. This method has high precision and convenient measurement. Secondly, by increasing the number of points in the FFT transform and using a higher-frequency DSP, the resolution can be improved and the measurement accuracy can be increased.
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