CN103364346A - Fast-Fourier-transform-based open-circuit measurement method for smoke plume opacity - Google Patents

Abstract

The invention discloses a method for measuring open-circuit opacity based on fast Fourier transform, which includes: pulse modulation on a light source; The signal is subjected to fast Fourier transform to obtain a frequency domain signal. After subtracting the fundamental frequency from the frequency domain signal, an inverse Fourier transform is performed again to obtain a time domain signal after filtering out stray light. After the original signal is processed, the opacity is calculated according to the formula. The invention has the advantages of strong suppression of stray light, no need to measure the background value or adding components such as optical filters, simple operation, strong reliability and adaptability, and continuous automatic measurement.

Description

Open-circuit measurement method of plume opacity based on fast Fourier transform

technical field

The invention relates to the technical field of urban air quality monitoring and management, in particular to an open-circuit measurement method for smoke plume opacity based on fast Fourier transform. the

Background technique

Plume opacity refers to the degree of light attenuation when passing through the plume. The opacity directly reflects the content of particulate matter in the plume. It is an effective method to evaluate the pollution degree of the plume and has been approved by the US National Environmental Protection Agency. Department listed as a reference indicator ¹ . At present, the main methods for detecting opacity include naked eye observation, digital photography and transilluminator ^{observation1-3} . Among them, the light source used in the transmissometer observation method is generally an artificial light source such as LED, tungsten halogen lamp or laser. The degree of attenuation reflects the opacity of the plume. The transmissometer observation method can be measured online in real time, continuously and automatically, and the results are reliable. For the same plume, the measurement results obtained by the transmissometer are measured by naked eye observation and digital photography. ^A reference standard for the reliability of measurement results3. However, when the transmissometer is open-circuit observation, in addition to receiving the signal light reflecting the opacity of the plume, some ambient stray light also enters the detector. During open-circuit observation, after the light from the sun or artificial light source is scattered or diffusely reflected by the plume itself and the surrounding environment, some light will always enter the detector along the direction of the signal beam, resulting in stray light. The existence of stray light reduces the signal-to-noise ratio and affects the measurement accuracy, especially when observing plumes from fugitive emissions, the impact of stray light is even greater. At present, there are mainly two methods for suppressing stray light. One method is to regard the measured value when there is no smoke plume in the measurement area as the background value, and the difference between the measured value and the background value when there is smoke plume passing through is regarded as the measurement result. The method improves the signal-to-noise ratio to a certain extent, but it cannot overcome the influence of stray light caused by the light scattering of the plume itself, especially for the white plume, whose scattering effect on light is very obvious. Another method is to add a filter at the front end of the detector to filter out the light outside the wavelength range of the light source, so as to achieve the purpose of suppressing stray light. In particular, the spectral wavelength range of sunlight overlaps with the working wavelength range of the transmissometer, resulting in a considerable portion of stray light that cannot be filtered out.

references

1. Method 9-Visual Determination of the Opacity of Emissions from Stationary Sources; Fed.Regist.1971,36,13624-13628;

2. Reitze, A.W. Air Pollution Control Law: Compliance and Enforcement. Environmental Law Institute: Washington, D.C, 2001;

3. McFarland Michael J.; Olivas Arthur C.; Atkins Sally G. Fugitive emissions opacity determination using the digital opacity compliance system (DOCS); JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION. 2007,325-157,

Contents of the invention

Aiming at the problem of stray light interference in the open-circuit measurement of opacity, the present invention provides a method for measuring open-circuit opacity based on fast Fourier transform technology, which can strongly suppress stray light and does not need to measure background values or With the addition of optical filters and other components, it is easy to operate, reliable and adaptable, and can be continuously and automatically measured. the

The purpose of the present invention is achieved through the following technical solutions. the

Based on the open-circuit measurement method of the opacity of the fast Fourier transform technology, the light source is first modulated into pulsed light of a certain frequency. The modulation method is to design a square wave generator circuit. The square wave generator consists of resistor R1, resistor R2, and capacitor C. Composed of a NE555 integrated circuit block, the strobe frequency of the light source is controlled by a square wave generator. the

The emitted light is received by the detector after passing through the plume. The detector converts the optical signal into a voltage signal, and then the acquisition card samples the voltage signal, and the sampled time-domain signal is sent to the computer. The data acquisition frequency is between 3-20 times the modulation frequency, and the time-domain sampling signal is 500ms is a unit, and the number of sampling points per unit is N points. After each unit is processed by Fast Fourier Transform (FFT), the fundamental frequency of 0HZ is removed from the obtained frequency domain signal, because the stray light from the sun within 500ms The light intensity can be regarded as constant, so the fundamental frequency signal represents the stray light, and after the fundamental frequency is removed, the inverse Fourier transform (IFFT) of N points is performed to obtain the time-domain voltage signal after the stray light is filtered out, and the inverse Fourier transform The parameters of the transform (IFFT) are the same as those of the Fourier transform. This process (hereinafter referred to as FFT-IFFT) is completed by a program based on the Labview platform. Collect signals when there is no plume and plume in the measurement area, and then calculate according to formula 1 to obtain the opacity information of the plume. The schematic diagram of the measurement optical path is shown in Figure 1. the

    (1) 

(1)

In the above formula, O is the opacity, V ₁ is the signal measured by the detector and processed by FFT-IFFT when there is no smoke plume in the measurement area, and V ₀ is the signal measured by the detector and processed by the detector when there is smoke plume in the measurement area. After the signal processed by FFT-IFFT, the ratio of the two can eliminate the influence of background atmospheric opacity. The invention has the advantages of strong suppression of stray light, no need to measure the background value or adding components such as optical filters, simple operation, strong reliability and adaptability, and continuous automatic measurement.

Description of drawings

Figure 1 is a schematic diagram of the measurement optical path. the

Figure 2 is a schematic diagram of the square wave generator design. Among them, integrated circuit pin 2-trigger terminal 3-output terminal 4-reset terminal 6-threshold terminal 7-discharge terminal 8-power supply voltage input terminal R1-resistance R2-resistance C-capacitance. the

Fig. 3 is a flowchart of a method for measuring opacity based on fast Fourier transform technology. the

Figure 4 is a comparison of the measured signal of the light source directly measured without modulation and the measured signal of the modulated light source using Fourier transform technology. Among them, 1 is the signal obtained by direct measurement of the light source without modulation, and 2 is the measurement signal obtained by the modulation of the light source and then processed by FFT-IFFT. the

Detailed ways

The present invention is described below in conjunction with specific measurement methods and accompanying drawings, but it should not be construed as a limitation of the present invention. the

The pulse modulation light source used in the present invention is closely related to the sampling characteristics of the data acquisition card and the implementation of the fast Fourier transform method, so the realization method of the pulse modulation of the light source is different from other pulse light sources. In this implementation case, the light source is a small semiconductor laser with an output wavelength of 532nm, and the output pulse frequency is adjustable. The square wave generator is constructed based on the NE555 oscillator. The output end is welded together with the laser modulation signal input end. The square wave generator is composed of resistor R1, resistor R2, capacitor C and a piece of NE555 integrated circuit block. The design principle is shown in the attached picture shown in 2. In the figure, Vcc is the input power supply voltage, the amplitude is between 5-15v. In this implementation case, R1=1KΩ, R2=10 KΩ, C=0.2mF. At this time, the square wave generator outputs a square wave frequency of 400HZ. If the value of R1, R2 or C is changed, other frequency outputs can be obtained. According to R1 , R2 resistance selection is different, the output frequency of the square wave generator can be selected between 100~400HZ. the

After the light source is pulse-modulated, the outgoing light shoots to the plume, and the principle of the measurement optical path is shown in Figure 1. The data acquisition card samples the voltage signal from the detector with a sampling rate of 4000, and then transmits the time-domain sampling signal to the computer processing software program. The software processing process is shown in Figure 3. The program first reads the time-domain sampling signal, and then segments the time-domain sampling signal with a unit of 500 ms. Each unit has 100 sampling points, and each unit performs fast Fourier After leaf transform (FFT) processing, the frequency domain signal is obtained, and the fundamental frequency of 0HZ is removed from the obtained frequency domain signal, because the stray light intensity from the sun can be regarded as unchanged within 500ms, and the fundamental frequency signal reflects the stray light. After the fundamental frequency is removed, a 100-point inverse Fourier transform is performed to obtain a time-domain signal after removing stray light and then output it. the

Attached Figure 4 is a comparison of the measured signal obtained by direct measurement of the light source without modulation and the measured signal of the modulated light source using Fourier transform technology, where 1 is the signal obtained by direct measurement of the light source without modulation, and 2 is the signal obtained after the light source has been modulated and then processed by FFT-IFFT measurement signal. The measurement time is in a clear day, put a neutral filter with standard opacity (opacity 0.50) into the light path, and perform direct measurement without modulation and measurement after Fourier transform respectively, as shown in the attached image In 4, it can be seen that there are many obvious fluctuations in the direct measurement signal, which are caused by the fluctuation of stray light intensity from the sun. When the sun light intensity changes, the stray light intensity will also change, especially when the sun is blocked by clouds When the signal is modulated, this change is more obvious. The stray light directly affects the accuracy of the open-circuit opacity measurement. After the signal is modulated, it is relatively stable, and the signal fluctuation caused by the stray light is extremely inconspicuous. According to the above two methods, each neutral filter with standard opacity is measured 430 times, and the measurement results are shown in Table 1. Method 1 refers to the direct measurement of the voltage of the light source without modulation, and then calculates the opacity method; method 2 refers to the method in which the light source is modulated (in this example, the modulation frequency is 400HZ, but not limited to this frequency), and then the measurement signal is processed by FFT-IFFT, and finally the opacity is calculated. It can be seen that compared with the traditional method of directly measuring the light source without modulation, the open-circuit measurement method of plume opacity based on fast Fourier transform has a significant decrease in the average measurement error and standard deviation. the

Table 1 Comparison of the measurement effects of the two measurement methods. Measured average Mean absolute error (%) standard deviation method 1 0.54 7.28 0.04 Method 2 0.50 0.51 0.00

Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention and not to limit. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent replacements to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all of them should be included in the scope of the present invention. within the scope of the claims. the

Claims (4)

1. based on the light obscuration open circuit measuring method of Fast Fourier Transform (FFT), it is characterized in that light source is carried out pulsed modulation; The light signal that detector detects imports computing machine into after the data collecting card sampling; After program is read in the time-domain sampling signal, first signal is carried out segmentation, then each segment signal is carried out Fast Fourier Transform (FFT) and obtains frequency-region signal, frequency-region signal is cut the fundamental frequency amount after, carry out again the time-domain signal after an inverse Fourier transform obtains the filtering parasitic light.

2. the light obscuration open circuit measuring method based on Fast Fourier Transform (FFT) according to claim 1 is characterized in that the modulation of source circuit is comprised of two resistance, a chip capacitor and a slice NE555 integrated circuit block.

3. the light obscuration open circuit measuring method based on Fast Fourier Transform (FFT) according to claim 1 is characterized in that Fast Fourier Transform (FFT) is identical with the inverse fast fourier transform parameter.

4. the light obscuration open circuit measuring method based on Fast Fourier Transform (FFT) according to claim 1 is characterized in that:

Step 1: when measured zone is obtained the signal V that processes through Fast Fourier Transform (FFT) during without plume ₀

Step 2: when measured zone has plume, obtain the signal V that processes through Fast Fourier Transform (FFT) ₁

Step 3: calculate light obscuration: O=1-V according to following formula ₁/ V ₀

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