CN113640249A - Formaldehyde detection system based on tunable laser spectrometry and detection method thereof - Google Patents
Formaldehyde detection system based on tunable laser spectrometry and detection method thereof Download PDFInfo
- Publication number
- CN113640249A CN113640249A CN202110770775.7A CN202110770775A CN113640249A CN 113640249 A CN113640249 A CN 113640249A CN 202110770775 A CN202110770775 A CN 202110770775A CN 113640249 A CN113640249 A CN 113640249A
- Authority
- CN
- China
- Prior art keywords
- resistor
- laser
- signal
- formaldehyde
- reflector
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Pending
Links
- WSFSSNUMVMOOMR-UHFFFAOYSA-N Formaldehyde Chemical compound O=C WSFSSNUMVMOOMR-UHFFFAOYSA-N 0.000 title claims abstract description 174
- 238000001514 detection method Methods 0.000 title claims abstract description 57
- 238000001307 laser spectroscopy Methods 0.000 title claims abstract description 23
- 238000010521 absorption reaction Methods 0.000 claims abstract description 25
- 230000003287 optical effect Effects 0.000 claims abstract description 21
- 238000006243 chemical reaction Methods 0.000 claims abstract description 20
- 238000012545 processing Methods 0.000 claims abstract description 17
- 238000005070 sampling Methods 0.000 claims abstract description 15
- 238000000034 method Methods 0.000 claims abstract description 13
- 239000003990 capacitor Substances 0.000 claims description 19
- 210000004027 cell Anatomy 0.000 claims description 9
- 210000005056 cell body Anatomy 0.000 claims description 5
- 239000003570 air Substances 0.000 abstract description 15
- 230000035945 sensitivity Effects 0.000 abstract description 7
- 239000012080 ambient air Substances 0.000 abstract description 5
- 239000007789 gas Substances 0.000 description 33
- 238000004891 communication Methods 0.000 description 16
- 238000005259 measurement Methods 0.000 description 12
- 230000001276 controlling effect Effects 0.000 description 6
- 238000010586 diagram Methods 0.000 description 6
- 238000013461 design Methods 0.000 description 5
- 238000002798 spectrophotometry method Methods 0.000 description 5
- 101100015484 Saccharomyces cerevisiae (strain ATCC 204508 / S288c) GPA1 gene Proteins 0.000 description 4
- 230000008859 change Effects 0.000 description 4
- 239000004065 semiconductor Substances 0.000 description 4
- 229910000661 Mercury cadmium telluride Inorganic materials 0.000 description 3
- 101100434411 Saccharomyces cerevisiae (strain ATCC 204508 / S288c) ADH1 gene Proteins 0.000 description 3
- 101150102866 adc1 gene Proteins 0.000 description 3
- 230000003321 amplification Effects 0.000 description 3
- 230000005540 biological transmission Effects 0.000 description 3
- 238000005516 engineering process Methods 0.000 description 3
- 230000036541 health Effects 0.000 description 3
- 238000003199 nucleic acid amplification method Methods 0.000 description 3
- 230000002093 peripheral effect Effects 0.000 description 3
- 230000001105 regulatory effect Effects 0.000 description 3
- 239000000243 solution Substances 0.000 description 3
- ISWSIDIOOBJBQZ-UHFFFAOYSA-N Phenol Chemical compound OC1=CC=CC=C1 ISWSIDIOOBJBQZ-UHFFFAOYSA-N 0.000 description 2
- YRKCREAYFQTBPV-UHFFFAOYSA-N acetylacetone Chemical compound CC(=O)CC(C)=O YRKCREAYFQTBPV-UHFFFAOYSA-N 0.000 description 2
- 230000033228 biological regulation Effects 0.000 description 2
- 230000000711 cancerogenic effect Effects 0.000 description 2
- 231100000357 carcinogen Toxicity 0.000 description 2
- 239000003183 carcinogenic agent Substances 0.000 description 2
- 239000003153 chemical reaction reagent Substances 0.000 description 2
- 150000001875 compounds Chemical class 0.000 description 2
- 239000013078 crystal Substances 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- 239000003344 environmental pollutant Substances 0.000 description 2
- 238000004817 gas chromatography Methods 0.000 description 2
- 238000004128 high performance liquid chromatography Methods 0.000 description 2
- 230000008520 organization Effects 0.000 description 2
- 230000036961 partial effect Effects 0.000 description 2
- 231100000719 pollutant Toxicity 0.000 description 2
- 230000008569 process Effects 0.000 description 2
- 238000000041 tunable diode laser absorption spectroscopy Methods 0.000 description 2
- 206010019233 Headaches Diseases 0.000 description 1
- 206010030113 Oedema Diseases 0.000 description 1
- 208000002193 Pain Diseases 0.000 description 1
- 238000002835 absorbance Methods 0.000 description 1
- 238000000862 absorption spectrum Methods 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- HSFWRNGVRCDJHI-UHFFFAOYSA-N alpha-acetylene Natural products C#C HSFWRNGVRCDJHI-UHFFFAOYSA-N 0.000 description 1
- 208000006673 asthma Diseases 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 238000004364 calculation method Methods 0.000 description 1
- 238000004587 chromatography analysis Methods 0.000 description 1
- 238000002848 electrochemical method Methods 0.000 description 1
- 125000002534 ethynyl group Chemical group [H]C#C* 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- 230000002349 favourable effect Effects 0.000 description 1
- 238000001914 filtration Methods 0.000 description 1
- 231100000869 headache Toxicity 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 208000030603 inherited susceptibility to asthma Diseases 0.000 description 1
- 230000005764 inhibitory process Effects 0.000 description 1
- 238000002347 injection Methods 0.000 description 1
- 239000007924 injection Substances 0.000 description 1
- 230000003993 interaction Effects 0.000 description 1
- 230000007794 irritation Effects 0.000 description 1
- 238000002955 isolation Methods 0.000 description 1
- 230000000670 limiting effect Effects 0.000 description 1
- 238000012886 linear function Methods 0.000 description 1
- 230000007774 longterm Effects 0.000 description 1
- WSFSSNUMVMOOMR-NJFSPNSNSA-N methanone Chemical compound O=[14CH2] WSFSSNUMVMOOMR-NJFSPNSNSA-N 0.000 description 1
- 239000000203 mixture Substances 0.000 description 1
- 230000001590 oxidative effect Effects 0.000 description 1
- 238000005192 partition Methods 0.000 description 1
- 230000010363 phase shift Effects 0.000 description 1
- 230000005622 photoelectricity Effects 0.000 description 1
- 238000005375 photometry Methods 0.000 description 1
- 108090000623 proteins and genes Proteins 0.000 description 1
- 102000004169 proteins and genes Human genes 0.000 description 1
- 238000004451 qualitative analysis Methods 0.000 description 1
- 238000004445 quantitative analysis Methods 0.000 description 1
- 238000006479 redox reaction Methods 0.000 description 1
- 230000002829 reductive effect Effects 0.000 description 1
- 230000007363 regulatory process Effects 0.000 description 1
- 210000002345 respiratory system Anatomy 0.000 description 1
- 230000002441 reversible effect Effects 0.000 description 1
- 230000003595 spectral effect Effects 0.000 description 1
- 238000004611 spectroscopical analysis Methods 0.000 description 1
- 238000001228 spectrum Methods 0.000 description 1
- 230000003068 static effect Effects 0.000 description 1
- 230000001360 synchronised effect Effects 0.000 description 1
- 230000007704 transition Effects 0.000 description 1
- 238000002834 transmittance Methods 0.000 description 1
Images
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
- G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
Landscapes
- Physics & Mathematics (AREA)
- Spectroscopy & Molecular Physics (AREA)
- Analytical Chemistry (AREA)
- Health & Medical Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Chemical & Material Sciences (AREA)
- Optics & Photonics (AREA)
- Biochemistry (AREA)
- General Health & Medical Sciences (AREA)
- General Physics & Mathematics (AREA)
- Immunology (AREA)
- Pathology (AREA)
- Investigating Or Analysing Materials By Optical Means (AREA)
Abstract
The invention belongs to the technical field of formaldehyde detection equipment, and particularly relates to a formaldehyde detection system based on tunable laser spectrometry and a detection method thereof. Wherein, formaldehyde detecting system includes: the sampling unit is used for realizing air sampling; the laser is used for emitting laser to the sampled air; the multi-reflection absorption cell is used for reflecting laser rays incident to the sampling air for multiple times; the photoelectric detector is used for receiving the optical signal, converting the optical signal into a current signal and carrying out current-voltage conversion on the current signal; and the signal processing unit is used for converting the voltage signal after the conversion of the convection voltage into the concentration of the formaldehyde. The method has the characteristics of high detection sensitivity, no need of complex pretreatment and capability of detecting the formaldehyde concentration of the ambient air in real time.
Description
Technical Field
The invention belongs to the technical field of formaldehyde detection equipment, and particularly relates to a formaldehyde detection system based on tunable laser spectrometry and a detection method thereof.
Background
Formaldehyde is colorless and has pungent smell, is identified as a carcinogen by the world health organization, has great harm to human body, can be combined with protein, and can cause severe irritation of respiratory tract, edema, eye pricking pain and headache after high-concentration formaldehyde is inhaled, and bronchial asthma can also occur; formaldehyde has been identified as a carcinogen by the world health organization, and long-term contact can bring about great negative effects on human health. Therefore, in order to ensure the safety of the living environment, the detection of the formaldehyde content in the indoor air is particularly important, and if the formaldehyde content in the detection exceeds the relevant regulations of national indoor air quality standard (GB/T18883-.
At present, numerous detection methods are proposed for formaldehyde in air at home and abroad, for example: GB/T15516-1995 specifies acetylacetone spectrophotometry; GBZ/T300.99-2017 specifies phenol reagent spectrophotometry; GB/T18204.2-2014 specifies AHMT spectrophotometry, phenol reagent spectrophotometry, gas chromatography, photoelectricity photometry and electrochemical sensor methods; ISO16000-3:2011 and HJ683-2014 specify high performance liquid chromatography. The main types of the method can be divided into the following 3 types:
1. the spectrophotometry method is characterized in that the concentration of formaldehyde can be measured by detecting the absorbance of a solution at a specific wavelength by utilizing the color change of the reaction between an organic molecule solution and the formaldehyde.
2. And (3) performing chromatography by directly detecting the content of formaldehyde in the gas through high performance gas chromatography, or detecting the solution of the concentrated formaldehyde gas through high performance liquid chromatography. The disadvantage is that the technical requirements for operators are high, and the detection instrument is difficult to be effectively used by common personnel.
3. The electrochemical method is used for judging the concentration of formaldehyde according to the potential and the electric quantity generated during the oxidation-reduction reaction of formaldehyde. The method is easily interfered by other oxidizing gases, and the electrochemical sensor is easily oxidized and needs to be replaced frequently.
The method has the advantages that the requirements of measurement accuracy and measurement range are met, the air sample is required to be subjected to complex treatment and conversion to realize the inversion of the concentration of the formaldehyde, the detection period is long, the steps are complex, online measurement cannot be realized, the concentration change of the formaldehyde pollutants in the air is difficult to determine in real time, and certain limitation exists in practical application.
Therefore, it is necessary to design a formaldehyde detection system and a formaldehyde detection method based on tunable laser spectrometry, which have high detection sensitivity, do not need complex pretreatment, and can detect the formaldehyde concentration in the ambient air in real time.
For example, chinese patent application No. CN201811593010.5 discloses a formaldehyde detection device and detection method based on a mid-infrared tunable laser, in which a fast modulation signal is superimposed on a slow scanning signal generated by a control system and is applied to a laser driver, and the laser driver realizes tuning of laser output frequency and power by changing control temperature and injection current of an ICL laser diode; modulating light signals pass through the gas to be detected, receiving emergent light by a corresponding photoelectric detector to obtain voltage signals related to light intensity, inputting the voltage signals into a digital quadrature phase-locked amplifier for demodulation, and then extracting second harmonic signals; finally, calculating the gas concentration value at the moment according to the obtained second harmonic signal amplitude; the TDLAS wavelength modulation-remote sensing detection technology is combined with the optical remote sensing detection technology to realize open light path detection on the basis of the TDLAS wavelength modulation technology, so that gas sampling is not needed in the whole measurement process, laser emitted by a laser is reflected back through an indoor wall and received by a detector, and a formaldehyde concentration value is finally obtained by analyzing the received signal. However, the method has the disadvantages that the interference is more introduced by a remote sensing detection mode adopted by the system, the single reflection optical path is about 10m, the measurement accuracy is insufficient, and the obtained data is the concentration (ppm × m) in unit length and cannot accurately reflect the concentration level of formaldehyde.
Disclosure of Invention
The invention aims to solve the problems that the existing formaldehyde detection method in the prior art has low detection sensitivity and complex operation steps, and the concentration change of formaldehyde pollutants in the air is difficult to determine in real time, and provides a formaldehyde detection system based on tunable laser spectroscopy and a formaldehyde detection method thereof, which have high detection sensitivity, do not need complex pretreatment and can detect the formaldehyde concentration of ambient air in real time.
In order to achieve the purpose, the invention adopts the following technical scheme:
formaldehyde detection system based on tunable laser spectroscopy, including:
the sampling unit is used for realizing air sampling;
the laser is used for emitting laser to the sampled air;
the multi-reflection absorption cell is used for reflecting laser rays incident to the sampling air for multiple times;
the photoelectric detector is used for receiving the optical signal, converting the optical signal into a current signal and carrying out current-voltage conversion on the current signal;
and the signal processing unit is used for converting the voltage signal after the conversion of the convection voltage into the concentration of the formaldehyde.
Preferably, the multi-reaction absorption pool comprises a pool body, a small reflector, a large reflector and a retro-reflector which are arranged in the pool body; the small reflector and the large reflector are both concave reflectors with the same curvature radius; the large reflector and the retro-reflector are positioned on the same side of the pool body, and the small reflector is positioned on the opposite side of the large reflector and the retro-reflector.
Preferably, a plane reflector is further arranged in the tank body of the multi-reflection absorption tank; the plane reflector and the large reflector are positioned on the same side.
Preferably, the small mirrors are arranged in parallel.
Preferably, the laser driving module comprises a laser constant current driving circuit, and the laser constant current driving circuit comprises an operational amplifier U1A, an operational amplifier U1B, a triode Q1, a resistor R1, a resistor R2, a resistor R3, a resistor R4, a resistor R5, a resistor R6, a resistor R7, a resistor R8, a resistor R9, a capacitor C1 and a capacitor C2; the non-inverting input end of the operational amplifier U1A is grounded, and the inverting input end of the operational amplifier U1A is electrically connected with a resistor R4, a resistor R5, a resistor R1 and a capacitor C1 respectively; the output end of the operational amplifier U1A is electrically connected with a resistor R6; the non-inverting input end of the operational amplifier U1B is electrically connected with a resistor R6 and a resistor R2 respectively; the inverting input end of the operational amplifier U1B is electrically connected with the resistor R8, the resistor R9 and the capacitor C2 respectively; the output end of the operational amplifier U1B is electrically connected with a resistor R7; one end of the resistor R3 is electrically connected with the resistor R2, and the other end of the resistor R3 is electrically connected with the capacitor C2, the resistor R9 and the emitter of the triode Q1 respectively; the base electrode of the triode Q1 is electrically connected with the resistor R7; the collector of the transistor Q1 is connected to ground.
Preferably, the system also comprises a laser temperature control module and a detector temperature control module; the laser temperature control module is used for controlling the temperature of the laser; the detector temperature control module is used for controlling the temperature of the photoelectric detector.
Preferably, the laser device is further provided with a collimator, and the collimator is used for converging laser beams emitted by the laser device.
Preferably, the signal processing unit further comprises a lock-in amplifier.
The invention also provides a detection method of the formaldehyde detection system based on the tunable laser spectrometry, which comprises the following steps:
s1, modulating the laser driving current by superposing a high-frequency sine wave and a low-frequency triangular wave;
s2, the laser emitted by the laser reaches the photoelectric detector through the gas to be detected for photoelectric conversion, and the harmonic signal of the gas absorption signal is extracted through the phase-locked amplifier;
and S3, obtaining a relative value of the gas concentration according to the relation between the gas concentration and the harmonic signal, calibrating the relative value, and finally obtaining the absolute value of the formaldehyde concentration.
Compared with the prior art, the invention has the beneficial effects that: (1) the method does not need complex pretreatment, and can detect the concentration of formaldehyde in the ambient air in real time; (2) the invention can reach the measuring optical path of more than 10m, even more than 40m, and can effectively reduce the 1/f noise of the laser in the low-frequency area and the thermal noise of the detector, thereby improving the detection sensitivity.
Drawings
FIG. 1 is an overall signal chain diagram of a driving part in a signal processing unit according to the present invention;
fig. 2 is a constant current driving circuit diagram of a semiconductor laser in a signal processing unit according to the present invention;
FIG. 3 is an overall signal link diagram of the laser temperature control module and the detector temperature control module in the signal processing unit of the present invention;
FIG. 4 is a schematic structural view of a multiple reverse absorption cell according to the present invention;
fig. 5 is an overall signal link diagram of the interface unit of the present invention.
Detailed Description
In order to more clearly illustrate the embodiments of the present invention, the following description will explain the embodiments of the present invention with reference to the accompanying drawings. It is to be understood that the drawings in the following description are merely exemplary of the invention and that other drawings and embodiments can be derived by those skilled in the art without inventive step.
Example 1:
the invention provides a formaldehyde detection system based on tunable laser spectrometry, which comprises:
the laser is used for emitting laser to the sampled air;
the multi-reflection absorption cell is used for reflecting laser rays incident to sampling air for multiple times so as to enable the measuring optical path to be longer;
the photoelectric detector is used for receiving the optical signal, converting the optical signal into a current signal and carrying out current-voltage conversion on the current signal;
and the signal processing unit is used for converting the voltage signal after the conversion of the convection voltage into the concentration of the formaldehyde.
In addition, the formaldehyde detection system based on tunable laser spectroscopy can further comprise:
the interface unit is used for realizing external information transmission;
and the power supply unit is used for supplying power to the system circuit board.
The power supply unit supplies power to the signal processing unit, the laser, the photoelectric detector, the interface unit and the sampling unit. The photoelectric detector is connected with the signal processing unit in a communication mode. The laser is in communication with the photodetector. And the signal processing unit is used for realizing the functions of circuit driving and temperature control of the light source, harmonic signal processing and human-computer interface interaction.
Further, the formaldehyde detection system based on the tunable laser spectrometry further comprises a laser temperature control module and a detector temperature control module; the laser temperature control module is used for controlling the temperature of the laser; the detector temperature control module is used for controlling the temperature of the photoelectric detector; the laser temperature control module and the detector temperature control module both adopt an MAX1968 chip and a peripheral PID circuit to control the stability of temperature.
As shown in fig. 1, the laser driving module uses the DDS chip AD9958 to generate a basic signal, wherein 1F of the DDS output is used for laser driving modulation, and 2F is converted into a square wave through a comparator for the phase-locked circuit to demodulate the frequency reference. In fig. 1, the main control MCU is electrically connected to two digital-to-analog converters DAC1, and the two digital-to-analog converters DAC1 respectively transmit the converted analog signals to the temperature control module (the laser temperature control module and the detector temperature control module) and the laser driving module; the laser driving module is electrically connected with the laser module (semiconductor laser) and is used for driving the laser module to emit laser; the temperature control module feeds back information to the MCU through a signal channel (AD _ NTC). The MCU is also connected with a CAN bus and has a SYNC communication mode.
The design of a driving circuit for a laser driving module adopts 2 synchronous DDS output channels, and the output frequency calculation formula of each DDS channel is shown as the following formula:
wherein f issThe system clock frequency is adopted, the FTW is a frequency tuning word, and the range is that the FTW is more than or equal to 0 and less than or equal to 231，232Representing the phase accumulator capacity.
The phase precision offset adjustment formula is as follows:
in the formula (I), the compound is shown in the specification,
POW represents a phase offset word as a phase offset amount. The DDS output has a high frequency tuning resolution, a 14-bit phase shift resolution, and a 10-bit output amplitude scaling resolution. The circuit module is designed to adopt a 25M passive clock crystal oscillator as an external reference clock crystal oscillator, the output resolution of the DDS is 10bits, the gain error is-10% F.S. to + 10% F.S., the maximum output current bias is 25uA, and the precise driving of the laser is realized.
As shown in fig. 2, the laser driving module includes a laser constant current driving circuit, and the laser constant current driving circuit includes an operational amplifier U1A, an operational amplifier U1B, a transistor Q1, a resistor R1, a resistor R2, a resistor R3, a resistor R4, a resistor R5, a resistor R6, a resistor R7, a resistor R8, a resistor R9, a capacitor C1, and a capacitor C2; the non-inverting input end of the operational amplifier U1A is grounded, and the inverting input end of the operational amplifier U1A is electrically connected with the resistor R4, the resistor R5, the resistor R1 and the capacitor C1 respectively; the output end of the operational amplifier U1A is electrically connected with a resistor R6; the non-inverting input end of the operational amplifier U1B is electrically connected with a resistor R6 and a resistor R2 respectively; the inverting input end of the operational amplifier U1B is electrically connected with the resistor R8, the resistor R9 and the capacitor C2 respectively; the output end of the operational amplifier U1B is electrically connected with a resistor R7; one end of the resistor R3 is electrically connected with the resistor R2, and the other end of the resistor R3 is electrically connected with the capacitor C2, the resistor R9 and the emitter of the triode Q1 respectively; the base electrode of the triode Q1 is electrically connected with the resistor R7; the collector of the transistor Q1 is connected to ground.
Further, the operational amplifier U1A and the operational amplifier U1B both use a chip AD 8620; the model of the triode Q1 is FZT 1049; the resistances of the resistor R1, the resistor R2, the resistor R3, the resistor R4, the resistor R5, the resistor R6, the resistor R7, the resistor R8 and the resistor R9 are all 10K omega; the capacitance values of the capacitor C1 and the capacitor C2 are both 100 nF.
The resistor R1, the resistor R4, the resistor R5 and the capacitor C1 form an RC filter circuit, the resistor R8, the resistor R9 and the capacitor C2 form the RC filter circuit, the operational amplifier U1A, the operational amplifier U1B and the RC filter circuit form an active filter, and low-frequency signal filtering is performed on a current signal output by the laser device to reduce noise interference. The transistor Q1 is used for further amplifying the current signal and outputting a current ILD。
The laser output current is shown as follows:
in the formula ILDFor driving an output current, V, to the lasermodThe voltage value across R6. Under the condition that the common-mode voltage of the circuit is between-10V and +10V, the common-mode rejection ratio can reach 110dB, the input bias current is only 130pA, and the input offset current is about 40 pA.
The laser temperature control module and the detector temperature control module both adopt a Peltier TEC module power driver and a peripheral PID circuit carried on the Peltier TEC module power driver to realize the temperature accurate control of the laser and the detector, the circuit can realize the automatic temperature regulation function, and meanwhile, the detector temperature control module comprises a current-voltage conversion circuit, and an integral signal link of the laser temperature control module and the detector temperature control module is shown in figure 3.
In fig. 3, the upper half is a laser temperature control module, the laser is in communication connection with a driving module through a channel AD _ NTC, the driving module is in communication connection with the laser through a channel MOD, the driving module is connected with a peripherally-mounted PID circuit through a TEC _ CTL, the laser is connected with the PID circuit through the channel NTC, the PID circuit is in communication connection with a temperature control module, and the temperature control module is in communication connection with the laser.
In fig. 3, the lower half is a detector temperature control module, a sensor (temperature) is connected to an I/V (current/voltage converter), the sensor is connected to a receiver module through a channel AD _ NTC in communication, the I/V is connected to the receiver module, the receiver module is connected to a peripherally-mounted PID circuit through a TEC _ CTL, the sensor is connected to a PID circuit through the channel NTC, the PID circuit is connected to a temperature control module in communication, and a temperature control command is connected to the sensor in communication.
The temperature control module of the laser and the temperature control module of the detector adopt MAX1968 chips and peripheral PID circuits to control the stability of the temperature, and the temperature can be controlled within the range of +/-0.1 ℃ of a set temperature value. MAX1968 adopts a direct current control mode so as to better eliminate surge interference signals in driving current, and simultaneously designs a heating current and voltage protection limiting interface which can be independently adjusted and set, so that a thermoelectric refrigerator module is fully protected. The external temperature regulating circuit selects an analog PID circuit, can realize timely judgment and effective inhibition on interference in the regulating process, can eliminate static deviation and is favorable for overcoming dynamic deviation. The output voltage of the circuit regulated by PID is as follows:
wherein, UtFor a stable output voltage after a change by PID adjustment, etFor differences between the target value to be stabilized and the actual output value of the circuit, KpFor controlling the proportionality coefficient of the circuit, TiFor controlling the integral coefficient of the circuit, TdIs the differential coefficient of the control circuit. Adjusting K in a real circuit by altering the RC parameterp、 Ti、TdThe magnitude of the three coefficients.
Furthermore, the laser is also provided with a collimator, and the collimator is used for converging laser beams emitted by the laser.
Further, as shown in fig. 4, a schematic structural diagram of a multiple reflection absorption cell is shown, where the multiple reflection absorption cell includes a cell body, a small-sized mirror, a large-sized mirror, and a retro-reflector installed in the cell body; the small reflector and the large reflector are both concave reflectors with the same curvature radius; the large reflector and the retro-reflector are positioned on the same side of the tank body, and the small reflector is positioned on the opposite side of the large reflector and the retro-reflector.
Furthermore, a plane reflector is also arranged in the tank body of the multi-reflection absorption tank; the plane reflector and the large reflector are positioned on the same side.
Further, the small-sized reflecting mirror can be provided in a plurality.
In fig. 4, two pieces are used for the small mirror. In the actual experimental operation process, the small reflector, the large reflector, the retro-reflector and the plane reflector are all placed in a pool body prepared in advance for experiment. After laser light enters the absorption cell, because the small-sized reflector and the large-sized reflector are both concave reflectors with the same curvature radius, the laser light can be reflected for multiple times between the small-sized reflector and the large-sized reflector, the reflected laser light is reflected back to the small-sized reflector after being reflected by the retro-reflector, the increase of reflection optical distance is realized, and further, longer measurement optical distance is realized, even the measurement optical distance is more than 40m (as shown in figure 4), thereby meeting the measurement precision. The plane reflector is used for changing the emitting direction of the emitted laser, so that the emitted laser can be conveniently obtained.
Further, the photoelectric detector specifically adopts an HgCdTe photodiode; the HgCdTe photodiode acts as a detector. The photoelectric detector adopts an HgCdTe photoelectric diode to convert an optical signal into a current signal, and the current signal is transmitted to the signal processing unit after current-voltage conversion to extract a harmonic signal. And an integrated lock-in amplifier is adopted to extract a target signal in the output signal of the detector, and signal synchronization is carried out through the MCU and the laser driving module, so that the acquired signal is a real-time concentration detection signal. Meanwhile, the CAN bus is adopted for data intercommunication, the CAN bus adopts a two-wire serial communication mode, the error detection capability and the real-time performance are strong, and the device CAN work in a high-noise interference environment. The photoelectric detector receives the light signal, converts the light signal to output a weak current signal, builds a current-voltage conversion circuit through an AD8620 chip, and converts the weak current signal into a voltage signal to be supplied to a post-stage circuit for signal processing. Because the output current signal of the photoelectric detector is weak, in order to ensure the signal-to-noise ratio of the signal, an AD8620 double-operational amplifier is adopted to form a current-voltage conversion circuit and a voltage follower circuit. The voltage follower circuit is added in the partial circuit, and the main reason is that the output impedance of the pre-stage current-voltage conversion circuit is large, the input impedance of the post-stage amplification circuit is small, and if the post-stage signal is directly connected, partial loss exists in the output resistance of the pre-stage. According to the obvious characteristics of high input impedance and low output impedance of the voltage follower, a voltage follower circuit is added to serve as a buffer stage and an isolation stage. In order to better extract a weak target signal from a large amount of mixed noise signals, the receiving and processing module adopts a phase-locked amplifying circuit with an AD630 chip as a core, a 2F connection SELA is used as a reference input end, a channel RinA is selected as a signal input end, Vout is a signal output end, the circuit can be matched with resistance values of R1 and R2 to adjust the amplification factor, and finally, signal acquisition can be carried out through a low-pass filter. The basic principle of the circuit is that an internal correlator is used for sequentially multiplying and integrating two paths of signals of a signal channel (signal _ in) and a reference channel (2F) to obtain a required effective signal (signal _ out). The signal to be measured of the circuit is a sine wave signal, the reference signal is a square wave signal, and the Fourier series of the signal to be measured can be obtained due to the periodicity of the square wave signal, as follows:
where y (t) is the amplitude of the reference signal, wrIs the reference signal frequency. Harmonic components on a required frequency band can be accurately obtained by adjusting the frequency and the phase of a reference signal, and meanwhile, under the condition that a required target signal is extracted from interference which is stronger than a measured optical signal by 100dB, a multiplier is not selected for the circuit to build a multiplication circuit, and an integrated phase-locked amplifier is directly selected for building a phase-locked amplification circuit. The reason is that the integrated lock-in amplifier can achieve only 100 muV channel lossThe voltage is regulated, extra low noise brought by the circuit is guaranteed, the modulator is used inside to amplify signals in an alternating current mode, and adverse effects of noise can be avoided. The phase-sensitive detector is utilized to realize the demodulation of the modulation signal, and simultaneously, the frequency and the phase are detected, the probability that the noise has the same frequency and phase with the signal is very low, and thus, the noise of the output signal is ensured to be very small.
Further, the interface unit comprises a 4-20mA output interface and an RS485/RS232 communication interface. The interface unit realizes external information transmission, the module design provides multiple communication modes of 4-20mA, RS485 and RS232 for the outside, and the integral signal link diagram of the interface unit is shown in figure 5.
As shown in FIG. 5, the master MCU is electrically connected to a digital-to-analog converter DAC1, the digital-to-analog converter DAC1 is communicatively connected to the temperature control module/sensor module via a communication channel (TEC _ CTL); the temperature control module/sensor module is in communication connection with the MCU through a signal channel (AD _ NTC); the temperature control module/sensor module is connected with the phase-locked amplifier; the phase-locked amplifier is connected with one analog-to-digital converter ADC1 through an AC serial port, the temperature control module/sensor module is connected with the other analog-to-digital converter ADC1 through a DC serial port, and the two analog-to-digital converters ADC1 are both connected with the MCU; the MCU is also connected with a CAN bus and has a plurality of communication modes of SYNC, 4-20mA, RS485 and RS 232. In fig. 1, the signal output by the DDS through the 2F channel is converted into a square wave through a comparator and transmitted to a lock-in amplifier.
Further, the sampling unit is a detachable unit. The sampling unit uses the design of can dismantling, but quick dismouting realizes the air sampling.
Further, the signal processing unit further comprises a lock-in amplifier.
Based on the embodiment 1, the invention also provides a detection method of the formaldehyde detection system based on the tunable laser spectrometry, which comprises the following steps:
s1, modulating the laser driving current by superposing a high-frequency sine wave and a low-frequency triangular wave;
s2, the laser emitted by the laser reaches the photoelectric detector through the gas to be detected for photoelectric conversion, and the harmonic signal of the gas absorption signal is extracted through the phase-locked amplifier;
and S3, obtaining a relative value of the gas concentration according to the relation between the gas concentration and the harmonic signal, calibrating the relative value, and finally obtaining the absolute value of the formaldehyde concentration.
The relationship between the gas concentration and the harmonic signal can be obtained by knowing the determined absorption line and setting parameters of temperature, pressure, optical path and laser frequency modulation amplitude.
The measurement principle of the detection method of the formaldehyde detection system based on the tunable laser spectrometry is as follows:
the current and the temperature are utilized to accurately modulate the output wavelength of the laser, so that the absorption spectrum line of the acetylene gas to be detected can be scanned, and qualitative and quantitative analysis of the gas components can be realized. The principle follows the Beer-Lambert law. Assuming incident light intensity as I0The gas concentration is X, the gas pressure is P, the distance of the laser passing through the gas is L, and the transmission light intensity is Iv. Under the condition of weak absorption, the light intensity attenuation satisfies the following conditions:
Iv＝I0T(v)＝I0exp[-S(T)g(v-v0)PXL] (1)
wherein S (T) is the absorption line intensity of the measured gas and is only related to the gas temperature; g (v-v)0) Is a linear function representing the shape of the absorption line, depending on factors such as gas temperature, pressure and composition. When the low concentration content is detected, the absorption intensity of a gas spectral line is weak, and the following formula is generally satisfied:
S(T)g(v-v0)PXL≤0.05 (2)
when the condition of the formula (2) is satisfied, the measured gas concentration result shows a better linear relationship, and the formula (1) can be approximately equal to the formula (3):
Iv≈I0[1-S(T)g(v-v0)PXL] (3)
the S (T) parameter of the conventional gas can be inquired in a molecular spectrum database HITRAN, and the S (T) at a certain temperature can be calculated by the formula (4):
wherein Q (T) is an intramolecular partition function, E is the molecular transition base layer energy, h is the Planckian constant, k Boltzmann' S constant, c is the speed of light, S (T)0) Is the reference temperature T0The line intensity of (b).
According to the formula, under the condition of determining information such as temperature, pressure, optical path and the like, the concentration information of the gas to be detected can be inverted. When the signal is weakly absorbed, the direct absorption measurement is easily interfered by various noises, and the light intensity attenuation caused by scattering, window pollution and the like cannot be distinguished, so that the measurement precision is influenced.
In order to improve the detection lower limit capability, the invention adopts a wavelength modulation method. Because the semiconductor laser has good tunable characteristic, high-frequency current with required frequency is added in the driving of the semiconductor laser, and the high-frequency modulation of laser can be conveniently realized. High-frequency sine waves are superposed with low-frequency triangular waves to modulate the driving current of the laser, the laser reaches a detector through gas to be detected to perform photoelectric conversion, and a second harmonic signal of a gas absorption signal is extracted through a phase-locked amplifier, so that 1/f noise of the laser in a low-frequency area and thermal noise of the detector can be effectively reduced, and the detection sensitivity is improved.
The invention relates to a detection method of a formaldehyde detection system based on tunable laser spectrometry, which comprises the following steps:
for step S1, the modulated laser frequency may be expressed as:
in the formula (I), the compound is shown in the specification,
is the triangular wave frequency, a is the modulation amplitude, f is the sine wave modulation frequency.
For step S2, the wavelength modulation spectroscopy uses a lock-in amplifier to detect the second harmonic component of the laser transmittance signal after the laser beam passes through the gas to be measured, and under the condition of weak absorption, the second harmonic signal of the gas to be measured is output as follows:
in the formula, θ is 2 π ft. For step S3, as can be seen from formula (6), when the determined absorption line is known and the parameters of temperature, pressure, optical path length and laser frequency modulation amplitude are given, the relationship between the gas concentration and the second harmonic can be obtained, as shown in formula (7).
For step S3, the result of the measurement by the wavelength modulation technique is a relative value of the concentration variation, and it is necessary to obtain an absolute value through calibration and finally obtain a concentration value of the measured gas:
in the formula, V2fIs a second harmonic component signal, I0The light intensity is a direct current component, and K is a calibration coefficient.
The method does not need complex pretreatment, and can detect the formaldehyde concentration of the ambient air in real time; the invention can reach the measuring optical path of more than 10m, even more than 40m, and can effectively reduce the 1/f noise of the laser in the low-frequency area and the thermal noise of the detector, thereby improving the detection sensitivity.
The foregoing has outlined rather broadly the preferred embodiments and principles of the present invention and it will be appreciated that those skilled in the art may devise variations of the present invention that are within the spirit and scope of the appended claims.
Claims (9)
1. Formaldehyde detection system based on tunable laser spectroscopy, characterized by, include:
the sampling unit is used for realizing air sampling;
the laser is used for emitting laser to the sampled air;
the multi-reflection absorption cell is used for reflecting laser rays incident to the sampling air for multiple times;
the photoelectric detector is used for receiving the optical signal, converting the optical signal into a current signal and carrying out current-voltage conversion on the current signal;
and the signal processing unit is used for converting the voltage signal after the conversion of the convection voltage into the concentration of the formaldehyde.
2. The tunable laser spectroscopy-based formaldehyde detection system according to claim 1, wherein the multiple anti-absorption cell comprises a cell body, a small mirror, a large mirror and a retro-reflector mounted in the cell body; the small reflector and the large reflector are both concave reflectors with the same curvature radius; the large reflector and the retro-reflector are positioned on the same side of the pool body, and the small reflector is positioned on the opposite side of the large reflector and the retro-reflector.
3. The tunable laser spectroscopy-based formaldehyde detection system according to claim 2, wherein a planar mirror is further disposed in the cell body of the multiple-reflection absorption cell; the plane reflector and the large reflector are positioned on the same side.
4. The tunable laser spectroscopy-based formaldehyde detection system according to claim 2, wherein the small mirrors are in plurality and are distributed side by side.
5. The tunable laser spectroscopy-based formaldehyde detection system according to claim 1, further comprising a laser driving module comprising a laser constant current driving circuit comprising an operational amplifier U1A, an operational amplifier U1B, a transistor Q1, a resistor R1, a resistor R2, a resistor R3, a resistor R4, a resistor R5, a resistor R6, a resistor R7, a resistor R8, a resistor R9, a capacitor C1, and a capacitor C2; the non-inverting input end of the operational amplifier U1A is grounded, and the inverting input end of the operational amplifier U1A is electrically connected with the resistor R4, the resistor R5, the resistor R1 and the capacitor C1 respectively; the output end of the operational amplifier U1A is electrically connected with a resistor R6; the non-inverting input end of the operational amplifier U1B is electrically connected with a resistor R6 and a resistor R2 respectively; the inverting input end of the operational amplifier U1B is electrically connected with the resistor R8, the resistor R9 and the capacitor C2 respectively; the output end of the operational amplifier U1B is electrically connected with a resistor R7; one end of the resistor R3 is electrically connected with the resistor R2, and the other end of the resistor R3 is electrically connected with the capacitor C2, the resistor R9 and the emitter of the triode Q1 respectively; the base electrode of the triode Q1 is electrically connected with the resistor R7; the collector of the transistor Q1 is connected to ground.
6. The tunable laser spectroscopy-based formaldehyde detection system according to claim 1, further comprising a laser temperature control module and a detector temperature control module; the laser temperature control module is used for controlling the temperature of the laser; the detector temperature control module is used for controlling the temperature of the photoelectric detector.
7. The formaldehyde detection system based on tunable laser spectroscopy according to claim 1, wherein the laser is further provided with a collimator for converging the laser beam emitted by the laser.
8. The tunable laser spectroscopy-based formaldehyde detection system according to any one of claims 1 to 7, wherein the signal processing unit further comprises a lock-in amplifier.
9. The method for detecting the formaldehyde detection system based on the tunable laser spectroscopy as claimed in claim 8, comprising the steps of:
s1, modulating the laser driving current by superposing a high-frequency sine wave and a low-frequency triangular wave;
s2, the laser emitted by the laser reaches the photoelectric detector through the gas to be detected for photoelectric conversion, and the harmonic signal of the gas absorption signal is extracted through the phase-locked amplifier;
and S3, obtaining a relative value of the gas concentration according to the relation between the gas concentration and the harmonic signal, calibrating the relative value, and finally obtaining the absolute value of the formaldehyde concentration.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CN202110770775.7A CN113640249A (en) | 2021-07-08 | 2021-07-08 | Formaldehyde detection system based on tunable laser spectrometry and detection method thereof |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CN202110770775.7A CN113640249A (en) | 2021-07-08 | 2021-07-08 | Formaldehyde detection system based on tunable laser spectrometry and detection method thereof |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CN113640249A true CN113640249A (en) | 2021-11-12 |
Family
ID=78416900
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CN202110770775.7A Pending CN113640249A (en) | 2021-07-08 | 2021-07-08 | Formaldehyde detection system based on tunable laser spectrometry and detection method thereof |
Country Status (1)
| Country | Link |
|---|---|
| CN (1) | CN113640249A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN114166796A (en) * | 2021-11-19 | 2022-03-11 | 杭州春来科技有限公司 | Non-methane total hydrocarbon measuring system based on laser spectroscopy and hydrogen flame ionization method |
Citations (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2003332677A (en) * | 2002-05-13 | 2003-11-21 | Sony Corp | Semiconductor laser drive circuit |
| CN104020114A (en) * | 2014-05-23 | 2014-09-03 | 中国人民解放军海军医学研究所 | Method for analyzing trace concentration of ammonia gas |
| CN208385834U (en) * | 2018-07-16 | 2019-01-15 | 深圳创维新世界科技有限公司 | Laser diode constant current driving system with modulation |
| CN210577014U (en) * | 2019-11-22 | 2020-05-19 | 深圳市皓龙激光设备有限公司 | Laser drive control circuit |
| CN211697465U (en) * | 2019-09-30 | 2020-10-16 | 杭州因诺维新科技有限公司 | Optical absorption cell and photoelectric gas analyzer |
-
2021
- 2021-07-08 CN CN202110770775.7A patent/CN113640249A/en active Pending
Patent Citations (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2003332677A (en) * | 2002-05-13 | 2003-11-21 | Sony Corp | Semiconductor laser drive circuit |
| CN104020114A (en) * | 2014-05-23 | 2014-09-03 | 中国人民解放军海军医学研究所 | Method for analyzing trace concentration of ammonia gas |
| CN208385834U (en) * | 2018-07-16 | 2019-01-15 | 深圳创维新世界科技有限公司 | Laser diode constant current driving system with modulation |
| CN211697465U (en) * | 2019-09-30 | 2020-10-16 | 杭州因诺维新科技有限公司 | Optical absorption cell and photoelectric gas analyzer |
| CN210577014U (en) * | 2019-11-22 | 2020-05-19 | 深圳市皓龙激光设备有限公司 | Laser drive control circuit |
Non-Patent Citations (4)
| Title |
|---|
| B. P. WERT ET AL.: "Design and performance of a tunable diode laser absorption spectrometer for airborne formaldehyde measurements", 《JOURNAL OF GEOPHYSICAL RESEARCH: ATMOSPHERES》, vol. 108, no. 12, 8 June 2003 (2003-06-08), pages 1 - 15 * |
| 刘立富 等: "基于TDLAS技术在垃圾焚烧中HCl的在线监测应用", 《激光与光电子学进展》, vol. 52, no. 11, 30 November 2015 (2015-11-30), pages 110101 * |
| 刘立富 等: "柴油发动机排放NH3快速在线监测系统的研究与应用", 《分析仪器》, no. 03, 28 May 2021 (2021-05-28), pages 14 - 19 * |
| 孔令华 等: "《光谱和光谱影像技术在各学科领域中的应用》", 31 March 2020, 吉林大学出版社, pages: 175 * |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN114166796A (en) * | 2021-11-19 | 2022-03-11 | 杭州春来科技有限公司 | Non-methane total hydrocarbon measuring system based on laser spectroscopy and hydrogen flame ionization method |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| CN109991189B (en) | Fixed point wavelength modulation gas concentration measuring device based on wave number drift correction and measuring method thereof | |
| US7957001B2 (en) | Wavelength-modulation spectroscopy method and apparatus | |
| US5307146A (en) | Dual-wavelength photometer and fiber optic sensor probe | |
| CN108981953B (en) | Laser absorption spectrum temperature measurement method and system based on interferometric modulation principle | |
| CN111829982A (en) | Multi-component gas concentration detection and temperature error correction method | |
| CN104730021A (en) | Method for calibrating non-dispersive infrared gas sensor | |
| CN109085133B (en) | Off-axis integral cavity atmosphere CH based on real-time reflectivity correction4Concentration measuring device and measuring method thereof | |
| CN103528989B (en) | Near infrared moisture measuring instrument | |
| CN111982813B (en) | All-fiber open gas monitoring system and detection method | |
| CN108801977B (en) | Device and method for detecting carbon dioxide gas of non-calibrated trace carbon 12 and carbon 13 | |
| CN114993987B (en) | Temperature and gas concentration measuring method and system based on absorption spectrum amplitude modulation | |
| Zhang et al. | A compact laser absorption spectroscopy tomographic system with short spectral scanning time and adjustable frame rate | |
| CN109813639B (en) | Infrared light modulation technology-based synchronous measurement device and measurement method for concentration of particulate matters and gas | |
| CN110987870A (en) | System and method for monitoring gas concentration in real time based on wavelength modulation spectrum technology | |
| CN114384045A (en) | System and method for detecting trace gas concentration and path length in real time | |
| CN113640249A (en) | Formaldehyde detection system based on tunable laser spectrometry and detection method thereof | |
| CN210626326U (en) | Multi-gas concentration detection device and alarm device | |
| CN106017533A (en) | Rapid tuning real-time calibration fiber grating demodulation device and work method | |
| CN201269854Y (en) | Optical sensor for automobile exhaust | |
| CN114544547A (en) | Method and system for on-line recursive demodulation of wavelength modulation spectrum signals | |
| CN112729544B (en) | Laser wavelength locking system and method for scanning absorption spectrum | |
| JPH0219718Y2 (en) | ||
| CN114778475A (en) | Method and system for detecting wide-range gas by variable optical path NDIR | |
| CN215415025U (en) | Infrared gas detection device of multi-light-source single sensor | |
| CN218512308U (en) | Gas concentration detection device based on multi-harmonic information fusion laser absorption spectroscopy technology |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PB01 | Publication | ||
| PB01 | Publication | ||
| SE01 | Entry into force of request for substantive examination | ||
| SE01 | Entry into force of request for substantive examination |