Tunable Diode Laser Absorption Spectrometer Coal Mine Methane Detection System Based on DSP

Renbao WANG College of Electrical and Information Engineering

Anhui University of Science and Technology Huainan, China

e-mail: rbwang@aust.edu.cn

Hongyan FENG College of Chemical Engineering

Nanjing University of Science and Technology Nanjing, China

e-mail: wrbfhy@hotmail.com

Abstract—Tunable diode laser absorption spectroscopy (TDLAS) technology is a new detective method developed with the combination of a tunable diode laser source and a long absorption path. It has the characteristic of high sensitivity, high precision and rapid response. Based on TDLAS and high performance microcontroller (DSP), the system is developed to monitor the coal mine methane. The second harmonic detection is used to lower the detection limit further that is below 0.086 mg/m3. The results show that the intensity of second harmonic signal is in proportion to the methane concentration. It has an obvious advantage not only in sensitivity but also in stability and can achieve the real-time measurement of methane concentration. It has been used abroad for detection of methane in some industrial fields.

Keywords-tunable diode laser absorption spectroscopy; DSP; second harmonic detection; methane

I. INTRODUCTION Methane is a major component of coal mine atmospheres.

Because of its flammable and explosive characteristic, the gas explosion is a big problem in the mining industry. Its explosion limit is 5.3%~15.0% in the air. Therefore, it needs the real-time, accurate monitoring of methane concentration, which is of great importance to the personal safety and the mine safety production.

Nowadays, the gas concentration is monitored by the hot catalytic component in our country, but the monitoring accuracy is not high and easy aging in the process of application[1]. Therefore, it is urgent that a new monitoring method is invented to overcome these shortcomings. Tunable diode laser absorption spectroscopy (TDLAS) technology is a gas detection method which provides high sensitivity, high precision and rapid responsibility, and the technology has the large development in recent years and detects gas qualitatively or quantitatively, based on the scan characteristic of diode laser, to obtain the absorption spectra in the characteristic absorption region[2-3]. In this paper, on the basis of the combination of high performance microcontroller (DSP) and TDLAS, there is a high detection sensitivity, a high detection stability and a real-time measurement of methane. Second harmonic detection which is usually used to detect low signals is to lower the detection limit further. At the same time, the results of variation of methane concentration in the long time are analyzed by the technology.

II. EXPERIMENTAL PRINCIPLE According to the Beer-Lambert law, when the light

intensity I0(v) passes through the gas, it will be attenuated. And the light intensity I(v) of transmission is given as follows:

])(exp[)()( 0 CLvIvI νΦ−= (1)

Where, )(vΦ is the gas absorption coefficient which is related to the pressure and temperature of gas and is in according with a linear function of Lorenz; L is the length of absorption path; C is the gas concentration. If the current that is injected to the light source is modulated by the sinusoidal wave, the laser frequency is given as below:

)sin()()( tatvtv ω+= (2)

Where, )(tv is the laser frequency that is not modulated by sinusoidal wave; a is the amplitude; ω is the modulation frequency of sinusoidal wave. In the case of the small

absorption, it meets CLvk )( 0−ν <<0.05. It is calculated with equation (1) and equation (2):

]})]sin()([1){()( 0 CLtatvvIvI ω+Φ−= (3)

Based on the equation (3), the harmonics of transmittance of gas can be analyzed. The bigger harmonic order is, the weaker intensity of harmonic signal is. The peak of second harmonic signal and the center of absorption spectra are overlapped approximately. So, the gas concentration can be detected by measuring the second harmonic signals generally. The second harmonic signal[4] is given as below:

ttdtavCLIS f ωωωπ

π

π2)sin(0

2 ×+Φ= ∫− (4)

Under the condition of low air pressure, Doppler broadening predominates in absorption lines of molecules. So,

Project Supported by the scientific research fund of Anhui University of Science and Technology (QN200826)

978-1-4244-4964-4/10/$25.00 ©2010 Crown

the absorption line )(vΦ can be described by the linear function of Lorenz[5]. And it is given as below:

2

0

)(1)(

v

gvvv

δ−

+

Φ=Φ (5)

Where, Φ0 is the absorption coefficient of pure gas on the

center of absorption lines; gv is the center frequency of absorption peak; δv is half-width of absorption line. When the center wavelength which is exported by the source is locked in

the gas absorption peak accurately, v and gv are equal. It is calculated with equation (4) and equation (5):

ttdtaCLIS

v

f ωω

δωπ

π

π2

)sin(1 2

002 ×

+

Φ= ∫− (6)

In the lock-in amplifier (including the multiplier and the integrator), the sinusoidal signal which frequency is 20Hz is to demodulate. Let the demodulated signal be given as below:

)2sin( taVr ω= (7)

Thus, the output signal V0 of multiplier is given as below:

])22cos[(

])22cos[(

00

002

tCLKI

tCLKIVSV rfO

ωωωω

+×Φ−

−×Φ=×= (8)

Where, the output signal V0 includes the difference frequency signal and the sum frequency signal. When the measured signal and the reference signal are the same frequency, the part of difference frequency signal turns into DC. The DC component can be obtained by the low-pass filter. The ratio of coefficient K can be obtained by the calibration. Thence, a linear relationship can be obtained between the gas concentration and the intensity of second harmonic signal through the lock-in amplifier.

III. EXPERIMENTAL DEVICE Experimental device is set up and is shown in Figure 1. It

mainly consists of the optical parts and the electronic parts.

Figure 1. Bloch diagram of the system.

3.1 Optical Parts It mainly consists of the tunable diode laser and the gas

absorption cell in the optical parts.

According to the HITRAN database, methane has the three absorption lines which the distance is very near in the scope of wavelength 1653.72 ± 0.01nm, which is seen as a line which intensity is sum of the three absorption lines. And in the scope of 0.50nm there is not the strong absorption lines of other gases (such as water vapor, nitrogen, carbon dioxide, etc.), The intensity of the absorption line is more 6500 times than the intensity of other gas absorption lines in the same scope. Such, the absorption line not only achieve a lower detection limit but also avoid interference of other gases. So, the tunable diode laser which central wavelength is 1653.72nm is selected in the system[6].

3.2 Electronic Parts

3.2.1 Hardware It mainly consists of the temperature control circuit of

tunable diode laser, the signal modulation circuit, the photoelectric detection circuit, the lock-in amplifier and the data processing part of DSP in the hardware.

The temperature of the tunable laser diode is controlled by the temperature control circuit[7], which can ensure that the output center wavelength is 1653.72nm. The output saw tooth current signal which frequency is 60Hz and is exported by DSP modulates the drive current of laser. So, the temperature control and current control of laser can been achieved. And the center wavelength of laser can scan the chosen absorption line. Laser beam is converted to electrical signal by the detector after it is absorbed by the gas in the absorption cell. In addition, DSP also exports two sinusoidal waves signal. One, which frequency is 20Hz, modulates the laser directly. Another, which frequency is 40Hz, is the demodulation signal of the lock-in amplifier, which achieves phase-sensitive detection with modulation signal. So, the intensity of second harmonic signal can been obtained. After the intensity of second harmonic signal is magnified and is been A/D sampling, low-pass filtering, back ground deduction and cumulative average by DSP, the concentration of gas is able to be calculated. TMS320LF5402 DSP (TI Company) which algorithm is simple and operation speed is fast is used for the core processor in the system.

3.2.2 Software Data collection is completed, synchronized with the

scanning cycle of laser wavelength, in the software. The collected signal is been A/D sampling, low-pass filtering, back ground deduction and cumulative average. The intensity of second harmonic signal and the concentration of gas are displayed.

On the Basis of the idea, the digital low-pass filter, which orders are 16 ranks in this system, is designed in the MATLAB/SIMULINK simulation environment. In accordance with the basic principles of correlation demodulation , the simulation module of SIMULINK that is shown in Figure 2 is built. The measured signal and the reference signal are all the sinusoidal waves and the same frequency. And the system was added with noise signal. The output signal of simulation is shown in Figure 3. Under the condition of adding noise signal, when the measured signal and the reference signal are the same frequency, the accepted signal is similar to DC, which can be concluded by the results of simulation. Therefore, after demodulated by the demodulation signal which frequency is 40Hz (2 times of frequency of modulation signal), the intensity of second harmonic signal is obtained.

Figure 2. Simulation structure diagram of basic correlation demodulation principle.

Figure 3. Simulation waveform of measured signal and reference signal at the same frequency.

IV. EXPERIMENTAL RESULTS The samples of methane, which volume fractions are 1.0%,

2.0%, 3.0%, 4.0%, 5.0% respectively, are prepared in our lab.

The experiments are measured based on Figure 1, which can obtain the intensity of 2f signal under the different volume fraction of gas. The good linear relationship is showed in Figure 4 between the intensity and the concentration. The linear fitting coefficient is more than 0.990. It is evidenced that the method of second harmonic detection can achieve a high detection sensitivity of gas by the experiments further.

Figure 4. The 2f signal intensity of different methane concentration.

Figure 5. Long time monitoring of the coal mine methane concentration.

The continuous monitoring of ten hours is carried out in the mine coal, and the curve of concentration variation is showed in Figure 5, in which the concentration of gas maintains 0.885mg/m3 basically under the normal working state of ventilator. Because of the deficiency of air volume, the concentration which maximum is 0.952mg/m3 increases at about 12:00. The difference of concentration between the maximum and minimum is 0.086mg/m3 in the whole experimental process. So, the whole system is very stable, and can achieve the accurate measurement of gas concentration.

V. CONCLUSION Based on tunable diode laser absorption spectroscopy

technology, this system combined with DSP and second harmonic detection has characteristics of high stability, high sensitivity and rapid responsibility, and the detection limit is below 0.086mg/m3. Changing the wavelength of tunable diode laser, other gases can also be monitored. Therefore, there is the very broad application prospect in the industrial control and environmental protection.

ACKNOWLEDGMENT The authors wish to thank for providing this template. This

work was supported by the Department of Electrical Engineering of Anhui University of Science and Technology. The authors would like to thank Liang Zhe for his appreciative help in performing the experiments in our laboratory.

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