CN104237135A - System and method for detecting CO gas based on quartz tuning fork enhanced photoacoustic spectrometry technology - Google Patents
System and method for detecting CO gas based on quartz tuning fork enhanced photoacoustic spectrometry technology Download PDFInfo
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- CN104237135A CN104237135A CN201410566870.5A CN201410566870A CN104237135A CN 104237135 A CN104237135 A CN 104237135A CN 201410566870 A CN201410566870 A CN 201410566870A CN 104237135 A CN104237135 A CN 104237135A
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Abstract
The invention provides a system and a method for detecting CO gas based on a quartz tuning fork enhanced photoacoustic spectrometry technology, relates to a system and a method for detecting the CO gas and aims at solving the problem of low detection accuracy of the existing CO gas photoacoustic spectrometry detection technology. A data processing module transmits a current control signal to a laser device controller via a function generator, and meanwhile, transmits a temperature control parameter to the laser device controller; the laser device controller drives a laser device to emit near infrared laser which is incident into a gas chamber after being collimated and focused; the gas absorbs optical energy so that the optical energy is converted into heat energy and further converted into an acoustic pressure signal; a quartz tuning fork mounted in the gas chamber converts an acoustic signal into an electric signal; the electric signal of the quartz tuning fork is converted and amplified and then input into the measurement channel input end of a phase-locked amplifier; the phase-locked amplifier inverts the concentration of the CO gas to be detected by performing secondary harmonic detection in combination with the reference signal of the function generator. The system and the method for detecting the CO gas based on the quartz tuning fork enhanced photoacoustic spectrometry technology are applicable to CO gas detection.
Description
Technical field
The present invention relates to a kind of CO gas detecting system and method.
Background technology
The CO gas of underground coal mine is inflammable and explosive is also toxic gas, can not get Timeliness coverage, not only easily produce blast, cause fire, and contaminated environment even causes pernicious poisoning if leak or assemble.CO gas effectively can reflect the overheated and ageing process running solid insulating material in power transformer, by the detection to CO gas, can forecast inside transformer Hidden fault and the development of paper oil insulation, realize early diagnosis.Initial Stage of Fire, CO gas can judge whether combustible combustion as a kind of characteristic gas, and then realizes fire alarm and monitoring.
CO gas detect is widely used in modern production life, and particularly a lot of occasion needs to detect trace CO gas, and conventional CO detection method can be divided into metal oxide method, electrochemical process and spectral absorption method etc.Catalyst combustion type CO gas-detecting device output signal is linear with gas volume fraction, is suitable for the monitoring of higher gas volume fraction, poor selectivity, large by air speed influence.Metal-oxide semiconductor (MOS) CO gas-detecting device is large to low volume fraction gas signal exporting change, and highly sensitive, high life, response speed block, selectivity is not good.Field effect transistor cast CO gas-detecting device good stability, high temperature resistant, resistance to strong acid, poor sensitivity, selectivity is poor.Comparatively speaking, the method based on the CO gas detect of spectroscopy theory has the following advantages: realize noncontact online auto monitoring; Instrumental sensitivity is high, as long as select suitable spectral band, and just can the precision of realize target gas ppm magnitude; Because measurement result only reflects the concentration average level in a region, its result is representative.
T unable filter (Tunable Diode Laser Absorption Spectroscopy, TDLAS) its essence is a kind of absorption spectroscopy techniques, utilize narrow linewidth and the wavelength tuning characteristic of semiconductor laser, laser output wavelength is changed by change temperature and electric current, scan gas absorption peak to be measured, determine gas concentration by the absorption of analytical gas to light.
Cavity ring down spectroscopy technology (Cavity ring down spectroscopy, CRDS) be the direct absorption spectroscopy techniques of a kind of high sensitivity grown up the eighties in last century, current cavity ring down spectroscopy technology has been widely used in the fields such as explosive gas detect, atmospheric surveillance.
FFIR technology (Fourier Transform infrared spectroscopy, TFIR) is the Main Means of atmosphere environment supervision.The infrared conversion spectrum technology of Fourier can identify according to the absorbing state of molecule under specific wavelength infrared radiation, quantize material composition.By adopting open light path design, the effective optical path lengthening of light in gas to be measured can be made to 1km.FTIR technology has that selectivity is good, highly sensitive, resolution is high, the response time soon, the not advantage such as consume carrier gas.Agriculturally, generally believe that the infrared conversion instrument of open light path Fourier is used to measure the effective tool of ammonia concentration in agricultural industry, be day by day used widely in air quality context of detection.
Differential optical absorption spectrum (Differential Optical Absorption Spectroscopy, DOAS) has been widely recognized and application as a kind of atmospheric surveillance technology within the scope of European Union.In the evolution of DOAS technology, external numerous scholar successively improves aspects such as DOAS system configuration, sniffer, inversion algorithms, perfect DOAS technology.At present, DOAS systematic study is produced and is mainly concentrated on Sweden, Germany, the U.S., France and the developed country such as Russian.
Direct absorption spectroscopy requires that light path is longer, and to optical path pool design, manufacture requirements strictly, the influence of fluctuations monitoring limit of electrical noise and light source power, the ground unrest that environment brings is difficult to eliminate.
Optoacoustic spectroscopy (photoacoustic spectroscopy, PAS) originates from 1880, and Bell first reported optoacoustic effect, and Viegerov completes spectrochemical analysis for gases first subsequently.But due to detection technique restriction, until 20 beginnings of the century, along with the development of laser technology and promoting the use of of high sensitivity microphone, optoacoustic spectroscopy is just able to high speed development.Optoacoustic spectroscopy is to object gas absorption region by modulation of source, gas absorption light modulated in photoacoustic cell is also excited to high-energy state, in the process of radiationless transition to low-energy state, Conversion of Energy is molecular kinetic energy, temperature is caused to produce cyclical variation, progressively form the change of one-period pressure, further generation voice signal, and this sound signal frequencies is identical with modulating light frequency.Voice signal can be caught by highly sensitive microphone, and its intensity is directly proportional to gas concentration in photoacoustic cell.In order to improve systems axiol-ogy sensitivity and antijamming capability, usually adopt wavelength-modulation technique and harmonic detecting technique.
But at present, the accuracy of detection of existing CO gas optoacoustic spectroscopy detection technique is not high.
Summary of the invention
The present invention is the problem that the accuracy of detection in order to solve existing CO gas optoacoustic spectroscopy detection technique is not high, thus provides a kind of CO gas detecting system based on quartz tuning fork strengthened optoacoustic spectroscopy and method.
Based on the CO gas detecting system of quartz tuning fork strengthened optoacoustic spectroscopy, comprise data processing module, laser light source module and photoacoustic signal detection module,
Data processing module comprises prime amplifier 9, first lock-in amplifier 10, second lock-in amplifier 13, data collecting card 1 and computing machine 14;
Laser light source module comprises function generator 2, laser controller 3, laser instrument 4, fiber optic collimator bag 5 and condenser lens 6;
Photoacoustic signal detection module comprises air chamber 7, quartz tuning-fork 8, reference gas chamber 11 and photodetector 12; Described quartz tuning-fork 8 is arranged in air chamber 7, is filled with CO gas in described air chamber 7; CO gas is filled with in reference gas chamber 11;
The current controling signal input end of current controling signal output terminal function generator 2 connects by data collecting card 1, and the first electrical signal of function generator 2 is connected with the electric signal input end of laser controller 3; The temperature control signals output terminal of data collecting card 1 is connected with the temperature control signals input end of laser controller 3;
Laser controller 3 drive laser 4 Output of laser, described laser is incident to condenser lens 6 after fiber optic collimator bag 5 collimates, and line focus lens 6 focus in air chamber 7, and excite CO gas to produce sound pressure signal, this laser is incident to reference gas chamber 11 through after air chamber 7;
Quartz tuning-fork 8 receives sound pressure signal and is converted to electric signal, and the electrical signal of described quartz tuning-fork 8 is connected with the electric signal input end of prime amplifier 9; The output terminal of described prime amplifier 9 is connected with the first electric signal input end of the first lock-in amplifier 10; Second electrical signal of the reference signal input end function generator 2 of described first lock-in amplifier 10 connects; The described output terminal of the first lock-in amplifier 10 is connected with the first data signal input of data collecting card 1;
Photodetector 12 is for the laser signal that detects in reference gas chamber 11 and be converted to electric signal, as measuring-signal; The measuring-signal output terminal of described photodetector 12 is connected with the first electric signal input end of the second lock-in amplifier 13; Second electrical signal of the reference signal input end function generator 2 of described second lock-in amplifier 13 connects; The described output terminal of the second lock-in amplifier 13 is connected with the second data signal input of data collecting card 1;
Data collecting card 1 communicates with computing machine 14.
Based on the CO gas detection method of the quartz tuning fork strengthened optoacoustic spectroscopy of said system, it is realized by following steps:
Control signal is sent to information acquisition card 1 by computing machine 14, current controling signal is sent to laser controller 3 through function generator 2 by information acquisition card 1, temperature control parameter is sent to laser controller 3 simultaneously, laser controller 3 drive laser 4 launches near-infrared laser, incides in air chamber 7 after fiber optic collimator bag 5 collimates and condenser lens 6 focuses on; The luminous energy of the CO gas absorption near-infrared laser in air chamber 7 is converted to heat energy, and then is converted to sound pressure signal, and the quartz tuning-fork 8 being positioned at air chamber 7 converts acoustical signal to electric signal;
By the electric signal of quartz tuning-fork 8 after conversion, amplifying, be input to the Measurement channel input end of a lock-in amplifier 10, the reference signal of lock-in amplifier 10 associative function generator 2 carries out second-harmonic detection, and then inverting obtains CO gas concentration to be measured.
It also comprises the steps:
Reference gas chamber 11 after being positioned at air chamber 7 is filled with the CO gas identical with air chamber CO gas concentration, and the photodetector 12 after employing is positioned at reference gas chamber 11 detects the laser signal through gas absorption, and described laser signal is converted to electric signal;
No. two lock-in amplifiers 13 receive this electric signal, access the high_frequency sine wave sent from function generator 2 simultaneously, and adopt harmonic signal as frequency discrimination signal, then carry out FEEDBACK CONTROL, the optical maser wavelength departed from is locked in Absorption Line center, realizes laser instrument frequency stabilization.
The invention has the beneficial effects as follows: one, the present invention adopts quartz tuning fork strengthened sound pressure detection device to substitute traditional microphone, improves the quality factor q of sound pressure detection device, and the sensitivity of optoacoustic spectroscopy CO gas detecting system is improved 2-3 magnitude;
Two, the present invention adopts quartz tuning fork strengthened sound pressure detection device to substitute traditional microphone, improves the frequency f of modulation signal, enhances system environment resistant low frequency noises ability;
Three, the present invention designs quartz tuning fork strengthened acoustic pressure sniffer, and the use of resonatron improves the detection sensitivity of sound pressure signal, inhibits environment with the interference of audio-frequency noise;
Four, the present invention designs laser instrument frequency stabilization backfeed loop, and laser center wavelength, as frequency discrimination signal, is locked in CO gas absorption peak position by harmonic signal, avoids the long-time laser wavelength drift detected because the factors such as environment temperature cause.
Accompanying drawing explanation
Fig. 1 the present invention is based on quartz tuning fork strengthened trace gas detection system structural representation;
Fig. 2 is quartz tuning fork strengthened sound pressure detection device structural representation;
Fig. 3 is the stable measurement result schematic diagram that the present invention measures for a long time;
Embodiment
Embodiment one, CO gas detecting system based on quartz tuning fork strengthened optoacoustic spectroscopy, comprise data processing module, laser light source module and photoacoustic signal detection module,
Data processing module comprises prime amplifier 9, first lock-in amplifier 10, second lock-in amplifier 13, data collecting card 1 and computing machine 14;
Laser light source module comprises function generator 2, laser controller 3, laser instrument 4, fiber optic collimator bag 5 and condenser lens 6;
Photoacoustic signal detection module comprises air chamber 7, quartz tuning-fork 8, reference gas chamber 11 and photodetector 12; Described quartz tuning-fork 8 is arranged in air chamber 7, is filled with CO gas in described air chamber 7; CO gas is filled with in reference gas chamber 11;
The current controling signal input end of current controling signal output terminal function generator 2 connects by data collecting card 1, and the first electrical signal of function generator 2 is connected with the electric signal input end of laser controller 3; The temperature control signals output terminal of data collecting card 1 is connected with the temperature control signals input end of laser controller 3;
Laser controller 3 drive laser 4 Output of laser, described laser is incident to condenser lens 6 after fiber optic collimator bag 5 collimates, and line focus lens 6 focus in air chamber 7, and excite CO gas to produce sound pressure signal, this laser is incident to reference gas chamber 11 through after air chamber 7;
Quartz tuning-fork 8 receives sound pressure signal and is converted to electric signal, and the electrical signal of described quartz tuning-fork 8 is connected with the electric signal input end of prime amplifier 9; The output terminal of described prime amplifier 9 is connected with the first electric signal input end of the first lock-in amplifier 10; Second electrical signal of the reference signal input end function generator 2 of described first lock-in amplifier 10 connects; The described output terminal of the first lock-in amplifier 10 is connected with the first data signal input of data collecting card 1;
Photodetector 12 is for the laser signal that detects in reference gas chamber 11 and be converted to electric signal, as measuring-signal; The measuring-signal output terminal of described photodetector 12 is connected with the first electric signal input end of the second lock-in amplifier 13; Second electrical signal of the reference signal input end function generator 2 of described second lock-in amplifier 13 connects; The described output terminal of the second lock-in amplifier 13 is connected with the second data signal input of data collecting card 1;
Data collecting card 1 communicates with computing machine 14.
Principle of work: in optoacoustic spectroscopy, the photoacoustic signal using quartz tuning fork strengthened optoacoustic spectroscopy to detect can be expressed as:
S（P）＝KIαQ(P)ε(P)
Wherein: K is system constants, I is laser power, and α is the absorption coefficient (with gas absorption cross section to be measured and concentration dependent amount) of gas to be measured, and ε (P) is acousto-optic conversion efficiency, the quality factor that Q (P) is tuning fork.The quality factor q of tuning fork reflects the size of the damping that the loss of vibrational energy or vibration are subject to, and generally speaking the quality factor of quartz tuning-fork can reach 10
4magnitude.Because quartz tuning-fork has very high Q value, therefore adapting to enhanced photo acoustic spectral technique can provide higher detection sensitivity.
The characteristic of quartz tuning-fork shows, the sound wave produced between raising one's arm two can make quartz tuning-fork two raise one's arm generation symmetric vibration, just can produce effective piezoelectric signal.In general environment, the frequency of noise is generally less than 32KHz, namely wavelength is greater than 3cm, and quartz tuning-fork two raise one's arm between distance be about 0.3mm, much smaller than the wavelength of noise, even if the frequency of noise is close to quartz tuning-fork natural frequency, two vibrations of raising one's arm of the quartz tuning-fork that noise causes from afar are equidirectional, can not produce effective piezoelectric signal, therefore quartz tuning-fork can realize the immunity to environment acoustic pressure noise.
The photoacoustic signal that optoacoustic effect produces has a feature, be exactly the size of acoustical signal and modulating frequency inversely, so traditional optoacoustic spectroscopy is generally located at 1-4KHz scope modulating frequency, and quartz tuning-fork frequency of operation is: 32.768KHz, and this has cut down photoacoustic signal.Therefore the present invention adopts project organization as shown in Figure 2, adds resonatron, effectively can improve photoacoustic signal at quartz tuning-fork two ends.Internal diameter is an important parameter of resonatron, works as resonatron, and when resonatron internal diameter is much smaller than wavelength, the transmission mode of sound wave can be similar to regards one dimension resonatron as.The scope of resonatron internal diameter D is:
Wherein: V is the velocity of sound, f is sound wave frequency, and η is Viscosity Factor of Gas, and ρ is the density of gas.Resonatron internal diameter range of choices is: 0.36mm < D < 6.1mm.
In resonatron, form standing wave, the length of resonatron equals the integral multiple of half-wavelength, according to wavelength and frequency relation:
Wherein: n is natural number, V is the velocity of sound, and L is resonatron length, and Δ l is the correction of resonatron length at openend, generally gets 0.6D.For the resonatron of the equal opening in two ends, when n gets strange, the center of resonatron is the antinode of pressure wave, quartz tuning-fork can be put into this position, can obtain stronger voice signal.Frequency when quartz tuning-fork: f
0during=32.768KHz, L=4.408mm.
Data processing module output current control signal is to function generator 2, function generator 2 produces the low frequency sawtooth of tens Hz and frequency is the high_frequency sine wave of f, the low frequency sawtooth wave output terminal of function generator 2 and high frequency sinusoidal wave output terminal all connect two current modulated input ends of laser controller 3, laser controller 3 inside is containing adding circuit and current driving circuit, and a drive current of laser controller 3 exports the laser diode input end of termination laser instrument 4; Data processing module also output temperature control signal connects a temperature modulation signal input part of laser controller 3, laser controller 3 inside is containing TEC driving circuit and thermistor interface circuit, the TEC input end of the output termination laser instrument 4 of laser controller 3, the input end of laser controller 3 also connects the thermistor output terminal of laser instrument 4, be used for the temperature of detection laser, the temperature of FEEDBACK CONTROL laser instrument; Laser light source module, by controlling temperature and the electric current of laser instrument 4, realizes the modulation of laser output wavelength and the modulation of laser emission frequency.Wavelength corresponding to CO gas absorption peak position can be selected to be 1565.98nm, and laser controller 3 temperature modulation and current-modulation acting in conjunction, regulate laser instrument 4 in the mode of length scanning, makes the centre wavelength of laser instrument 4 Output of laser be 1565.98nm.Regulate current controling signal, the set of frequency of high_frequency sine wave is f, and the transmission frequency f that modulation obtains laser instrument 4 Output of laser is quartz tuning-fork resonant frequency f
0half, that is: f=f
0/ 2.
The described gas detecting system based on quartz tuning fork strengthened optoacoustic spectroscopy, laser instrument 4 current modulation frequency is the high_frequency sine wave frequency f that function generator 2 exports, and the frequency of quartz tuning-fork 8 is f
0, be usually set as f=f
0/ 2.Therefore the frequency f to quartz tuning-fork 8 is needed
0calibrate, to ensure that the reference signal of lock-in amplifier 10 and lock-in amplifier 13 meets harmonic detecting requirement.In the calibration mode, a pin of quartz tuning-fork 8 is used by as measuring junction, and another pin is coupled with a sine wave.While the sinusoidal wave frequency of scanning, the excitation current of quartz crystal oscillator is also measured, carrys out determiner resonant frequency f by the sweep frequency finding maximum excitation electric current corresponding
0.
The laser modulated, through fiber optic collimator bag 5, condenser lens 6, incides in air chamber 7.Light signal is converted to sound pressure signal by gas to be measured, and sound pressure signal intensity is directly proportional to gas concentration to be measured.Sound pressure signal is converted to piezoelectric current signal by quartz tuning-fork 8, and piezoelectric current signal intensity is directly proportional to sound pressure signal intensity.Raise one's arm due to quartz tuning-fork two and be positioned at resonatron center, the interference from extraneous same frequency signal can be eliminated.During measurement, a pin ground connection of quartz tuning-fork 8, another pin connects the input end of prime amplifier 9, and prime amplifier is transimpedance prime amplifier, current signal can be converted to voltage signal, and is amplified by signal.The measuring-signal input end of the output termination lock-in amplifier 10 of prime amplifier, the high frequency sinusoidal wave output terminal of the reference signal input termination function generator of lock-in amplifier 10.Lock-in amplifier 10 carries out second-harmonic detection, is finally inversed by gas CO concentration to be measured.
Be positioned at the reference gas chamber after air chamber 7 11 gas to be measured containing identical CO concentration, be positioned at the photodetector 12 after reference gas chamber 11 and detect laser signal through gas absorption, and light signal is converted to electric signal.The measuring-signal input end of the output termination lock-in amplifier 13 of photodetector 12, the high frequency sinusoidal wave output terminal of the reference signal input termination function generator of lock-in amplifier 13.Lock-in amplifier 13 carries out third harmonic demodulation, the input end of the output termination data collecting card 1 of lock-in amplifier 13.Utilize the zero crossing characteristic of odd harmonic, adopt harmonic signal as frequency discrimination signal, according to certain feedback control algorithm, the optical maser wavelength departed from is locked in Absorption Line center, realizes laser instrument frequency stabilization.
The difference of embodiment two, the CO gas detecting system based on quartz tuning fork strengthened optoacoustic spectroscopy according to embodiment one is, fiber optic collimator bag 5, condenser lens 6 are positioned in same light path.
The difference of embodiment three, the CO gas detecting system based on quartz tuning fork strengthened optoacoustic spectroscopy according to embodiment two is, reference gas chamber 7 is rectangular structure;
Its left side wall and right side wall are respectively arranged with laser incidence window 71 and laser emitting window 72, the upper end of laser incidence window 71 and the upper end of laser emitting window 72 all to the tilted setting of air chamber, and are all 5 ° ~ 7 ° with vertical direction angulation; Incidence window 71 and exit window 72 are in light path according to claim 2;
The upper surface of reference gas chamber 7 and lower surface have air intake opening 73 and gas outlet 74.
The difference of embodiment four, the CO gas detecting system based on quartz tuning fork strengthened optoacoustic spectroscopy according to embodiment three is, reference gas chamber 7 is rectangular parallelepiped, the then long 15mm of reference gas chamber 7 optical path direction, the wide 15mm in vertical optical path direction, high 12mm.
The difference of embodiment five, the CO gas detecting system based on quartz tuning fork strengthened optoacoustic spectroscopy according to embodiment four is, laser instrument 4 is semiconductor laser with tunable, the inner integrated TEC refrigerator of this laser instrument and thermistor.
The difference of embodiment six, the CO gas detecting system based on quartz tuning fork strengthened optoacoustic spectroscopy according to embodiment one is, quartz tuning-fork 8 comprises quartz crystal oscillator 81 and double resonance pipe 82; Described quartz crystal oscillator 81 is frequencies is 32.768KHz, slough the pole structure of vacuum shell;
Double resonance pipe 82 is symmetrically distributed in quartz crystal oscillator both sides, and described double resonance pipe 82 is 100 microns near quartz crystal oscillator 81 1 lateral section and quartz crystal oscillator 81 liang of distances of raising one's arm between plane.
The difference of embodiment seven, the CO gas detecting system based on quartz tuning fork strengthened optoacoustic spectroscopy according to embodiment six is, quartz crystal oscillator 81 liang raise one's arm be centrally located at condenser lens 6 focus on.
The difference of embodiment eight, the CO gas detecting system based on quartz tuning fork strengthened optoacoustic spectroscopy according to embodiment seven is, prime amplifier 9 is transimpedance prime amplifier.
Embodiment nine, CO gas detection method based on the quartz tuning fork strengthened optoacoustic spectroscopy of embodiment one, it is realized by following steps:
Control signal is sent to information acquisition card 1 by computing machine 14, current controling signal is sent to laser controller 3 through function generator 2 by information acquisition card 1, temperature control parameter is sent to laser controller 3 simultaneously, laser controller 3 drive laser 4 launches near-infrared laser, incides in air chamber 7 after fiber optic collimator bag 5 collimates and condenser lens 6 focuses on; The luminous energy of the CO gas absorption near-infrared laser in air chamber 7 is converted to heat energy, and then is converted to sound pressure signal, and the quartz tuning-fork 8 being positioned at air chamber 7 converts acoustical signal to electric signal;
By the electric signal of quartz tuning-fork 8 after conversion, amplifying, be input to the Measurement channel input end of a lock-in amplifier 10, the reference signal of lock-in amplifier 10 associative function generator 2 carries out second-harmonic detection, and then inverting obtains CO gas concentration to be measured.
It also comprises the steps:
Reference gas chamber 11 after being positioned at air chamber 7 is filled with the CO gas identical with air chamber CO gas concentration, and the photodetector 12 after employing is positioned at reference gas chamber 11 detects the laser signal through gas absorption, and described laser signal is converted to electric signal;
No. two lock-in amplifiers 13 receive this electric signal, access the high_frequency sine wave sent from function generator 2 simultaneously, and adopt harmonic signal as frequency discrimination signal, then FEEDBACK CONTROL is carried out, the centre wavelength of laser instrument 4 is locked in an absorption peak position of CO gas to be measured, centre wavelength can be 1565.98nm.
The described gas detecting system based on quartz tuning fork strengthened optoacoustic spectroscopy, the laser incidence window 71 of laser instrument 4 Output of laser, fiber optic collimator bag 5, condenser lens 6, air chamber 7, laser emitting window 72, reference gas chamber 11, photodetection 12 are placed in same light path in turn.It is the pillar quartz crystal oscillator 81 that 32.768KHz sloughs vacuum shell that quartz tuning-fork 8 comprises frequency, and two of quartz crystal oscillator 81 is raised one's arm and is centrally located at the focal position of described condenser lens, obtains the ceiling capacity of laser excitation;
Also comprise double resonance pipe 82, one-sided resonance pipe range is 4mm, and resonatron internal diameter is 0.55mm, quartz crystal oscillator two raise one's arm be positioned at resonatron form the antinode of standing wave, improve sound pressure signal quality; Raise one's arm near quartz crystal oscillator one lateral section and quartz crystal oscillator 81 distance of plane of resonatron 82 is 100 microns, and distance crosses the formation that conference affects standing wave.
Incidence window 71 and the exit window 72 of air chamber 7, equal upper end, to the tilted setting of air chamber, is 5 ~ 7 ° with angle alleged by vertical direction, and such setting avoids laser to interfere phenomenon.
The described gas detecting system based on quartz tuning fork strengthened optoacoustic spectroscopy, laser diode current modulating frequency is the high_frequency sine wave frequency f that function generator exports, and the frequency of quartz tuning-fork 8 is f
0, be usually set as f=f
0/ 2.Before systematic survey, to the frequency f of quartz tuning-fork 8
0calibrate.In the calibration mode, a pin of quartz tuning-fork 8 is used by as measuring junction, and another pin is coupled with a sine wave.While the sinusoidal wave frequency of scanning, the excitation current of quartz crystal oscillator is also measured, carrys out determiner resonant frequency f by the sweep frequency finding maximum excitation electric current corresponding
0.
In measurement mode, the modulating frequency of laser is set to the half of quartz tuning-fork natural frequency, that is: f
0/ 2, quartz tuning-fork pin ground connection, another pin is as end of probe, and the signal of generation carries out second harmonic demodulation by data processing module.Data processing module output current control signal is to function generator 2, function generator 2 produces the low frequency sawtooth of tens Hz and frequency is the high_frequency sine wave of f, function generator 2 exports wherein two input ends of termination laser controller 3, laser controller 3 inside is containing adding circuit and current driving circuit, and an output of laser controller 3 connects the laser diode input end of laser instrument 4; Data processing module also exports one of them input end that temperature control signal connects laser controller 3, laser controller 3 inside is containing TEC driving circuit and thermistor interface circuit, the TEC input end of the output termination laser instrument 4 of laser controller 3, the input end of laser controller 3 also connects the thermistor output terminal of laser instrument 4, be used for the temperature of detection laser, the temperature of FEEDBACK CONTROL laser instrument; Laser light source module, by controlling temperature and the electric current of laser instrument 4, realizes the modulation of laser output wavelength and the modulation of laser emission frequency.Wavelength corresponding to CO gas absorption peak position can be selected to be 1565.98nm, and laser temperature modulation and laser diode current modulation acting in conjunction, regulate laser instrument in the mode of length scanning, make the centre wavelength of laser instrument Output of laser be 1565.98nm.Regulate current controling signal, the set of frequency of high_frequency sine wave is f, and the transmission frequency f that modulation obtains laser instrument Output of laser is quartz tuning-fork resonant frequency f
0half, that is: f=f
0/ 2.
The laser transmitted in optical fiber, to fiber optic collimator bag 5, is become directional light by the Laser output modulated, and focuses on, incide in air chamber 7, make laser energy converge to quartz crystal oscillator 81 liang of centers of raising one's arm of quartz tuning-fork 8 through condenser lens.Gas absorption luminous energy to be measured is also converted to heat energy, and absorb luminous energy and be directly proportional to gas concentration, when given volume, thermal power transfer is sound pressure signal, and therefore sound pressure signal intensity is directly proportional to gas concentration to be measured.Sound pressure signal is converted to electric signal by quartz tuning-fork 8, and raising one's arm due to quartz tuning-fork two is positioned at resonatron center, can eliminate the interference from extraneous same frequency signal.During measurement, a pin ground connection of quartz tuning-fork 8, another pin connects the input end of prime amplifier 9, and prime amplifier is transimpedance prime amplifier, quartz tuning-fork piezoelectric current signal is converted to voltage signal, and is amplified by signal.The measuring-signal input end of the output termination lock-in amplifier 10 of prime amplifier, the high frequency sinusoidal wave output terminal of the reference signal input termination function generator of lock-in amplifier 10.Lock-in amplifier 10 carries out second-harmonic detection, need demarcate system according to calibrating gas during primary detection, measurement multi-group data carries out curve fitting, and calibrated system can carry out gas detect, and second harmonic signal and the gas concentration of lock-in amplifier 10 output are proportional.
System also comprises the feedback control loop of laser instrument frequency stabilization, be positioned at the to be measured gas of the reference gas chamber after air chamber 7 11 containing same concentrations, be positioned at the photodetector 12 after reference gas chamber 11 and detect laser signal through gas absorption, and light signal is converted to electric signal.The measuring-signal input end of the output termination lock-in amplifier 13 of photodetector 12, the high frequency sinusoidal wave output terminal of the reference signal input termination function generator of lock-in amplifier 13.Lock-in amplifier 13 carries out third harmonic demodulation, the input end of the output termination data collecting card 1 of lock-in amplifier 13.Utilize the zero crossing characteristic of odd harmonic, adopt harmonic signal as frequency discrimination signal, according to certain feedback control algorithm, the optical maser wavelength departed from is locked in Absorption Line center, the centre wavelength selected can be 1565.98nm, realizes laser instrument frequency stabilization.
During embody rule, the present invention carries out the frequency calibration of quartz tuning-fork 8, when measuring first, demarcates system by calibrating gas; After system calibrating, in measurement mode, gas concentration measure loop and laser instrument frequency stabilization feedback control loop work simultaneously, good portability, and stability is high, and antijamming capability is strong, can on-line monitoring for a long time.
Claims (10)
1., based on the CO gas detecting system of quartz tuning fork strengthened optoacoustic spectroscopy, comprise data processing module, laser light source module and photoacoustic signal detection module, it is characterized in that:
Data processing module comprises prime amplifier (9), the first lock-in amplifier (10), the second lock-in amplifier (13), data collecting card (1) and computing machine (14);
Laser light source module comprises function generator (2), laser controller (3), laser instrument (4), fiber optic collimator bag (5) and condenser lens (6);
Photoacoustic signal detection module comprises air chamber (7), quartz tuning-fork (8), reference gas chamber (11) and photodetector (12); Described quartz tuning-fork (8) is arranged in air chamber (7), and described air chamber is filled with CO gas in (7); Reference gas chamber is filled with CO gas in (11);
The current controling signal input end of current controling signal output terminal function generator (2) connects by data collecting card (1), and the first electrical signal of function generator (2) is connected with the electric signal input end of laser controller (3); The temperature control signals output terminal of data collecting card (1) is connected with the temperature control signals input end of laser controller (3);
Laser controller (3) drive laser (4) Output of laser, described laser is incident to condenser lens (6) after fiber optic collimator bag (5) collimation, and line focus lens (6) focus in air chamber (7), and exciting CO gas to produce sound pressure signal, this laser is incident to reference gas chamber (11) through after air chamber (7);
Quartz tuning-fork (8) receives sound pressure signal and is converted to electric signal, and the electrical signal of described quartz tuning-fork (8) is connected with the electric signal input end of prime amplifier (9); The output terminal of described prime amplifier (9) is connected with the first electric signal input end of the first lock-in amplifier (10); Second electrical signal of reference signal input end function generator (2) of described first lock-in amplifier (10) connects; The output terminal of described first lock-in amplifier (10) is connected with the first data signal input of data collecting card (1);
Photodetector (12) is for the laser signal that detects in reference gas chamber (11) and be converted to electric signal, as measuring-signal; The measuring-signal output terminal of described photodetector (12) is connected with the first electric signal input end of the second lock-in amplifier (13); Second electrical signal of reference signal input end function generator (2) of described second lock-in amplifier (13) connects; The output terminal of described second lock-in amplifier (13) is connected with the second data signal input of data collecting card (1);
Data collecting card (1) communicates with computing machine (14).
2. the CO gas detecting system based on quartz tuning fork strengthened optoacoustic spectroscopy according to claim 1, is characterized in that fiber optic collimator bag (5), condenser lens (6) is positioned in same light path.
3. the CO gas detecting system based on quartz tuning fork strengthened optoacoustic spectroscopy according to claim 2, is characterized in that, reference gas chamber (7) is rectangular structure;
Its left side wall and right side wall are respectively arranged with laser incidence window (71) and laser emitting window (72), the upper end of laser incidence window (71) and the upper end of laser emitting window (72) all to the tilted setting of air chamber, and are all 5 ° ~ 7 ° with vertical direction angulation; Incidence window (71) and exit window (72) are positioned in same light path with fiber optic collimator bag (5), condenser lens (6);
The upper surface of reference gas chamber (7) and lower surface have air intake opening (73) and gas outlet (74).
4. the CO gas detecting system based on quartz tuning fork strengthened optoacoustic spectroscopy according to claim 3, it is characterized in that, reference gas chamber (7) is rectangular parallelepiped, then the long 15mm of reference gas chamber (7) optical path direction, the wide 15mm in vertical optical path direction, high 12mm.
5. the CO gas detecting system based on quartz tuning fork strengthened optoacoustic spectroscopy according to claim 4, is characterized in that laser instrument (4) is semiconductor laser with tunable, the inner integrated TEC refrigerator of this laser instrument and thermistor.
6. the CO gas detecting system based on quartz tuning fork strengthened optoacoustic spectroscopy according to claim 1, is characterized in that quartz tuning-fork (8) comprises quartz crystal oscillator (81) and double resonance pipe (82); Described quartz crystal oscillator (81) is frequency is 32.768KHz, slough the pole structure of vacuum shell;
Double resonance pipe (82) is symmetrically distributed in quartz crystal oscillator both sides, and the distance that described double resonance pipe (82) is raised one's arm between plane near quartz crystal oscillator (81) lateral section and quartz crystal oscillator (81) two is 100 microns.
7. the CO gas detecting system based on quartz tuning fork strengthened optoacoustic spectroscopy according to claim 6, it is characterized in that quartz crystal oscillator (81) two raise one's arm be centrally located at condenser lens (6) focus on.
8. the CO gas detecting system based on quartz tuning fork strengthened optoacoustic spectroscopy according to claim 7, is characterized in that prime amplifier (9) is for transimpedance prime amplifier.
9., based on the CO gas detection method of the quartz tuning fork strengthened optoacoustic spectroscopy of claim 1, it is characterized in that: it is realized by following steps:
Control signal is sent to information acquisition card (1) by computing machine (14), current controling signal is sent to laser controller (3) through function generator (2) by information acquisition card (1), temperature control parameter is sent to laser controller (3) simultaneously, laser controller (3) drive laser (4) launches near-infrared laser, incides in air chamber (7) after fiber optic collimator bag (5) collimation and condenser lens (6) focus on; The luminous energy of the CO gas absorption near-infrared laser in air chamber (7) is converted to heat energy, and then is converted to sound pressure signal, and the quartz tuning-fork (8) being positioned at air chamber (7) converts acoustical signal to electric signal;
By the electric signal of quartz tuning-fork (8) after conversion, amplifying, be input to the Measurement channel input end of a lock-in amplifier (10), the reference signal of lock-in amplifier (10) associative function generator (2) carries out second-harmonic detection, and then inverting obtains CO gas concentration to be measured.
10. the CO gas detecting system based on quartz tuning fork strengthened optoacoustic spectroscopy according to claim 9, is characterized in that it also comprises the steps:
Reference gas chamber (11) after being positioned at air chamber (7) is filled with the CO gas identical with air chamber CO gas concentration, employing is positioned at the laser signal of the detection of the photodetector (12) after reference gas chamber (11) through gas absorption, and described laser signal is converted to electric signal;
No. two lock-in amplifiers (13) receive this electric signal, access the high_frequency sine wave sent from function generator (2) simultaneously, and adopt harmonic signal as frequency discrimination signal, then FEEDBACK CONTROL is carried out, the optical maser wavelength departed from is locked in Absorption Line center, realizes laser instrument frequency stabilization.
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