WO2018188429A1 - 基于拍频效应的石英增强光声光谱气体检测装置及方法 - Google Patents

基于拍频效应的石英增强光声光谱气体检测装置及方法 Download PDF

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WO2018188429A1
WO2018188429A1 PCT/CN2018/078020 CN2018078020W WO2018188429A1 WO 2018188429 A1 WO2018188429 A1 WO 2018188429A1 CN 2018078020 W CN2018078020 W CN 2018078020W WO 2018188429 A1 WO2018188429 A1 WO 2018188429A1
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signal
frequency
gas
tuning fork
beat
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PCT/CN2018/078020
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English (en)
French (fr)
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董磊
武红鹏
弗兰克·蒂特尔
肖连团
贾锁堂
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山西大学
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Priority to US16/080,711 priority Critical patent/US11073469B2/en
Publication of WO2018188429A1 publication Critical patent/WO2018188429A1/zh

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    • 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
    • G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
    • 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
    • G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
    • G01N2021/1704—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids in gases

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  • the invention relates to a gas sensing technology, in particular to a quartz enhanced photoacoustic spectroscopy gas detecting device and method based on a beat frequency effect.
  • Photoacoustic spectroscopy is a well-developed gas detection technology that is widely used due to its zero background, non-selectivity to the wavelength of the source, and positive correlation between detection sensitivity and source power.
  • Quartz enhanced photoacoustic spectroscopy is a new photoacoustic spectroscopy technique (patent specification USZOOS/0117155AI) invented by Kosterev et al., Rice University, USA in 2002.
  • This technology uses an inexpensive tuning fork quartz crystal oscillator (referred to as: quartz tuning fork) to replace the expensive broadband microphone in traditional photoacoustic spectroscopy technology, as a photoacoustic converter to complete the detection of weak photoacoustic signals.
  • the excitation light source is modulated at the resonant frequency of the quartz tuning fork, the acoustic wave generated based on the photoacoustic effect converts the energy into the mechanical energy of the periodic vibration of the quartz tuning fork vibrating arm by resonating with the quartz tuning fork.
  • This part of the mechanical energy is converted into an electrical signal due to the piezoelectric effect of the quartz material and transmitted to the electrical signal detecting device via the electrode at the bottom end of the quartz tuning fork. Since the piezoelectric signal is positively correlated with the gas concentration, when measuring a certain gas, it is only necessary to previously perform concentration calibration on the measuring device with the known concentration of the gas, and the calibrated device can measure the gas.
  • the quartz tuning fork currently widely used in quartz enhanced photoacoustic spectroscopy sensors has a high quality factor Q (quality factor Q > 10000 for bare tuning forks under normal pressure) and a nominal frequency (referred to as the frequency specified in the crystal component specification) f 0
  • Q quality factor
  • f 0 nominal frequency
  • This high quality factor makes the quartz tuning fork have a very narrow frequency response interval (at normal pressure, the frequency response interval is about 2 Hz). Therefore, for the quartz enhanced photoacoustic spectral trace gas detection device, the modulation frequency of the excitation light source must be exactly the same as the resonance frequency of the quartz tuning fork.
  • the quartz tuning fork in the quartz-enhanced photoacoustic spectroscopy sensing device must be repeatedly and accurately determined, otherwise the quartz crystal oscillator will not be able to efficiently respond to the photoacoustic signal proportional to the gas concentration.
  • the repeated frequency measurement process not only increases the complexity and difficulty of the device, but also enables the quartz enhanced photoacoustic spectroscopy sensing device to perform continuous online monitoring of trace gases.
  • the response time of the above-mentioned quartz tuning fork is about 100 ms.
  • the response period of the conventional quartz enhanced photoacoustic spectral trace gas monitoring device is at least 500 ms, and such a long response time cannot be very good.
  • the object of the present invention is to solve the technical problem that the existing gas detecting device and method have long detection period, low detection sensitivity, and need to frequently calibrate the electrical parameters of the quartz tuning fork in the gas detecting process, and provide a quartz based on the beat frequency effect.
  • a quartz enhanced photoacoustic spectroscopy gas detecting device based on the beat frequency effect comprising a photoacoustic signal detecting module, a gas chamber, a light source module and a data collecting module;
  • the photoacoustic signal detecting module is a tuning fork quartz crystal oscillator; an entrance window and an exit window are respectively arranged on two side walls of the air chamber, and a gas inlet is arranged on a side of the lower end of the air chamber near the incident window, and the upper end of the air chamber is close to the exit a gas outlet is disposed on one side of the window, and the photoacoustic signal detecting module is vertically disposed in the inner cavity of the air chamber through the support, and the incident window and the exit window of the air chamber are located on the same optical path;
  • the light source module includes a laser light source, a first function generator, a second function generator, and a beam focusing device; a signal output end of the first function generator is connected to a current scan input port of the laser light source; and a signal of the second function generator The output end is connected to the current modulation input port of the laser light source, and a beam focusing device is disposed on the outgoing light path of the laser light source, and the laser beam passes through the beam focusing device and enters the micro-acoustic resonant cavity in the air inlet chamber through the incident window;
  • the data acquisition module includes a transimpedance preamplifier, a lock-in amplifier, a data acquisition card, and a computer for processing and recording related data and controlling the normal operation of the device; a signal input terminal and a tuning fork of the transimpedance preamplifier One electrode of the quartz crystal oscillator is connected, and the other signal input terminal is connected to the other electrode of the tuning fork quartz crystal oscillator and grounded; the signal input end of the lock-in amplifier is connected to the signal output end of the transimpedance preamplifier, and the synchronous signal input of the lock-in amplifier The end is connected with the synchronous output signal end of the second function generator; the signal input end of the data acquisition card is connected to the signal output end of the lock-in amplifier, and the RS232 interface of the data acquisition card is connected to the signal acquisition port of the computer.
  • the photoacoustic signal detecting module further includes a micro acoustic resonator matched with a tuning fork quartz crystal oscillator, and the micro acoustic resonator is located on the same optical path as the incident window and the exit window of the gas chamber;
  • the invention also provides a gas detection method using a quartz-enhanced photoacoustic spectroscopy gas detecting device based on a beat frequency effect, comprising the following steps:
  • the laser light source emits a laser beam driven by the first function generator and the second function generator to control the amplitude of the scanning voltage of the first function generator, so that the output center wavelength of the laser light source reaches a desired value, and the laser light source
  • the output wavelength scanning rate is 18 cm -1 s -1 ⁇ 200 cm -1 s -1
  • the modulation signal frequency of the second function generator is controlled to modulate the wavelength of the laser light source
  • the modulation signal frequency of the second function generator f and the resonant frequency f i-1 measured before the tuning fork quartz crystal sets the frequency difference of 10-210 Hz;
  • the modulated laser beam is focused and shaped by the beam focusing device, and the focused beam enters the photoacoustic signal detecting module in the air chamber through the incident window, and emerges from the exit window, and the laser and the air chamber are to be treated.
  • the photoacoustic signal detection module converts the photoacoustic signal into the transmission frequency of the tuning fork type quartz crystal electrode resonance frequency of the piezoelectric oscillator via the signal f i photoacoustic signal detection module tuning fork type quartz crystal Give a transimpedance preamplifier;
  • the transimpedance preamplifier amplifies the received piezoelectric signal and transmits it to the lock-in amplifier.
  • the lock-in amplifier demodulates the received piezoelectric signal with frequency f i by the demodulated signal of frequency f.
  • the post-modulation forming frequency ⁇ f i (subscript i indicates the ith measurement) is transmitted to the data acquisition card, and the detection bandwidth of the lock-in amplifier is 1 kHz to 100 kHz;
  • the data acquisition card transmits the captured beat frequency signal to the computer equipped with LabView software for processing, and then obtains the corresponding beat frequency signal map, and automatically finds the peak value of the beat frequency signal through the LabView software.
  • the signal value S of a peak point is taken on the frequency signal map, and the gas concentration value C to be measured is calculated by the following formula.
  • C is the gas concentration value to be tested
  • S N is the signal value of the device under high purity N 2 conditions
  • S 1 is the signal value of the corresponding peak point of the device under the standard gas condition of concentration C 0 ;
  • C 0 is the concentration value of the standard gas.
  • the lock-in amplifier demodulates the received piezoelectric signal into mixing and filtering, and the piezoelectric signal is mixed to form a beat signal having a frequency of ⁇ f i and then filtered and transmitted to the data acquisition card.
  • the method further includes the step 6): the LabView software automatically searches for the peak values of the beat signal, and uses the first peak value of the beat signal, the time interval between the peaks of two adjacent waves, and the respective peak points.
  • Cycle steps 1-6 to achieve continuous measurement of gas concentration.
  • the device of the present invention can complete the measurement of the basic electrical parameters (resonance frequency, quality factor) of the quartz tuning fork while measuring the concentration of the gas to be tested.
  • the repeated calibration of the quartz tuning fork electrical parameters during the gas detection process is avoided, so that the quartz tuning fork based gas sensor can be used for long-term continuous online monitoring of trace gases.
  • the invention can accurately obtain the electrical parameters of the quartz crystal oscillator and the gas to be tested by detecting the beat signal generated by the mixing process of the piezoelectric signal outputted by the quartz tuning fork and the demodulated signal of the lock-in amplifier.
  • the concentration is three orders of magnitude faster than the prior art quartz enhanced photoacoustic trace gas detection device, which solves the technical problem of long detection period in gas detection, low detection sensitivity and frequent calibration of quartz crystal electrical parameters.
  • the device of the invention greatly simplifies the traditional quartz enhanced photoacoustic spectrum trace gas detecting device, reduces the device cost and simplifies the use flow.
  • the invention has strong technical universality and environmental adaptability, and can be fully used for online gas monitoring in the fields of environmental monitoring, food safety monitoring and industrial production control.
  • FIG. 1 is a schematic structural view of a photoacoustic signal detecting module according to the present invention.
  • FIG. 2 is a schematic structural view of a photoacoustic signal detecting module installed in a gas chamber according to the present invention
  • Figure 3 is a schematic structural view of the device of the present invention.
  • Figure 5 is a piezoelectric signal diagram generated after the quartz tuning fork is pushed by the sound pulse
  • Figure 6 is a diagram of the beat frequency signal output by the lock-in amplifier
  • FIG. 7 is a diagram of a beat frequency signal after automatic processing by a computer
  • Figure 8 shows the signal values of the peak positions of the beat signal at different water vapor concentrations.
  • a quartz enhanced photoacoustic spectroscopy gas detecting device based on beat frequency effect in the embodiment includes a photoacoustic signal detecting module, a gas chamber 2 , a light source module and a data acquisition module. ;
  • the photoacoustic signal detecting module includes a tuning fork quartz crystal oscillator 1 and a micro acoustic resonator 3 matched with the tuning fork quartz crystal oscillator 1; the micro acoustic resonator 3 is horizontally disposed on both sides of the tuning fork quartz crystal oscillator 1 and
  • the utility model is composed of a stainless steel capillary perpendicular to the vibrating arm surface of the tuning fork quartz crystal oscillator. The central axes of the two stainless steel capillaries coincide and pass through the arm gap of the tuning fork quartz crystal oscillator 1; the two side walls of the gas chamber 2 are respectively provided with incident windows.
  • a gas inlet 22a is disposed on a side of the lower end of the gas chamber 2 adjacent to the incident window 21a
  • a gas outlet 22b is disposed on a side of the upper end of the gas chamber 2 near the exit window 21b
  • the photoacoustic signal detecting module is vertically supported by the support Straight disposed in the inner cavity of the gas chamber 2, the incident window 21a of the gas chamber, the micro acoustic resonator 3 and the exit window 21b are located on the same optical path; between the micro acoustic resonator 3 and the tuning fork quartz crystal oscillator 1
  • the coupling resonance can increase the detection sensitivity of the photoacoustic signal detection module by about 30 times.
  • the light source module includes a laser light source 5, a first function generator 6, a second function generator 7, and a beam focusing device 8; the signal output end of the first function generator 6 is connected to the current scanning input port of the laser light source 5, The signal output end of the two function generator 7 is connected to the current modulation input port of the laser light source 5, and the light beam focusing device 8 is provided on the outgoing light path of the laser light source 5.
  • the laser beam passes through the beam focusing device 8 and enters the inlet chamber through the incident window 21a. 2 in the micro-acoustic resonator 3;
  • the data acquisition module includes a transimpedance preamplifier 4, a lock-in amplifier 9, a data acquisition card 10, and a computer 11 for processing and recording related data and controlling the normal operation of the device; a signal of the transimpedance preamplifier 4 The input end is connected to one electrode of the tuning fork quartz crystal oscillator 1, and the other signal input end is connected to the other electrode of the tuning fork quartz crystal oscillator and grounded; the signal input end of the lock-in amplifier 9 is connected to the signal output end of the transimpedance preamplifier 4.
  • the sync signal input end of the lock-in amplifier 9 is connected to the synchronous output signal end of the second function generator 7 to ensure that the demodulated signal frequency of the lock-in amplifier 9 is the same as the modulation signal frequency f of the second function generator 7;
  • the signal input end of the capture card 10 is connected to the signal output end of the lock-in amplifier 9, and the RS232 interface of the data acquisition card 10 is connected to the signal acquisition port of the computer 11.
  • the laser light source 5 emits a laser beam under the driving of the first function generator 6 and the second function generator 7, and adjusts the signal amplitude of the first function generator 6, so that the output center wavelength of the laser light source 5 reaches a desired value.
  • the output wavelength scanning rate of the laser light source 5 is 18-200 cm -1 s -1 to ensure that the photoacoustic signal generated by the gas to be measured is a pulse sound signal; controlling the frequency of the modulation signal of the second function generator 7 to the laser light source
  • the wavelength of 5 is modulated, and the frequency f of the modulation signal of the second function generator 7 and the nominal frequency f 0 of the quartz tuning fork need to have a frequency difference of 10-210 Hz to ensure the generation of the beat signal during the subsequent signal demodulation process.
  • the modulation signal frequency f is determined according to the nominal frequency f 0 of the quartz tuning fork, f 0 is 32.7 kHz, the modulation signal frequency f is 32550 Hz, and the modulation signal is a sine wave; It is positively correlated with the wavelength of the laser output, so by changing the amplitude of the scanning signal voltage of the first function generator 6, the range of current variation of the laser light source 5 can be changed, thereby changing the output light wave of the laser light source 5. .
  • the modulated laser beam is focused and shaped by the beam focusing device 8 into a parallel beam having a spot diameter smaller than that of the quartz tuning fork arm, and the focused parallel beam enters the photoacoustic signal detecting module in the chamber 2 through the incident window 21a.
  • the micro-acoustic resonator 3 is permeable from the exit window 21b, and the laser acts on the air in the plenum 2 to generate a photoacoustic signal, and the photoacoustic signal detecting module converts the photoacoustic signal into a tuning fork quartz crystal oscillator 1
  • the transimpedance preamplifier 4 amplifies the received piezoelectric signal and transmits it to the lock-in amplifier 9.
  • the lock-in amplifier 9 solves the received piezoelectric signal of frequency f 1 with a demodulated signal of frequency f.
  • the frequency of the demodulated signal of the lock-in amplifier 9 is the same as the frequency f of the modulation signal of the second function generator 7.
  • the frequency signal is filtered and transmitted to the data acquisition card 10.
  • the detection bandwidth of the lock-in amplifier 9 is 1 kHz to 100 kHz to ensure that the detection bandwidth of the lock-in amplifier 9 is 200 times wider than the electronic bandwidth of the photoacoustic signal detection module. ;
  • the data acquisition card 10 transmits the acquired beat frequency signal to the computer 11 equipped with the LabView software for arithmetic processing, and obtains the corresponding beat frequency signal map, and automatically finds the peak values of the beat frequency signal through the LabView software. Taking the signal value S of a peak point on the obtained beat signal spectrum, and taking the following formula to calculate the concentration value C of the water vapor in the air,
  • C is the concentration of water vapor in the air
  • S N is the signal value of the device under high purity N 2 conditions
  • S 1 is the signal value of the device under the standard gas condition of concentration C 0
  • C 0 is the concentration value of the standard gas.
  • the standard gas having a concentration of C 0 in the present embodiment is water vapor having a concentration of 1%.
  • the quartz-enhanced photoacoustic spectroscopy gas detecting device and method based on the beat frequency effect is the same as the first embodiment, wherein the method further comprises the step 6):
  • the LabView software After the LabView software automatically finds the peak values of the beat signal, the first wave peak of the beat signal, the time interval between two adjacent wave peaks, and the beat frequency signal obtained by e-exponential fitting of each peak point are used.
  • the quality factor, ⁇ is the pi.
  • Cycle steps 1-6 to achieve continuous measurement of gas concentration.
  • FIG. 4 it is a scan signal map outputted by the first function generator 6.
  • the scanning voltage corresponding to the wavelength of the water vapor absorption line is located in the middle of the ramp voltage of the first half of the scanning signal. Since the damped vibration formed by the quartz tuning fork is pushed by the sound pulse for a period of time, in order to avoid the vibration vibration caused by the subsequent sound pulse to interfere with the previously formed damped vibration, the present embodiment adds a constant voltage after the end of the ramp voltage scanning to make the quartz tuning fork There is plenty of time to complete the damped vibration.
  • the period of the scanning signal in the present invention is less than 1 s to ensure that the wavelength scanning rate of the laser is greater than 18 cm -1 /s, thereby shortening the duration of the acoustic wave acting on the tuning fork arm enough to excite the quartz tuning fork.
  • the time scale of the transient vibration mode is less than 1 s to ensure that the wavelength scanning rate of the laser is greater than 18 cm -1 /s, thereby shortening the duration of the acoustic wave acting on the tuning fork arm enough to excite the quartz tuning fork.
  • FIG. 5 it is a piezoelectric signal diagram generated after the quartz tuning fork is pushed by the sound pulse. After the sound pulse disappears, the quartz tuning fork will continue to vibrate at its own resonance frequency f 1 , and the energy accumulated in the photoacoustic detection system will be consumed by the self-loss and external loss during the vibration process, generating a voltage having a frequency of the tuning fork resonance frequency f 1 . electric signal.
  • FIG. 6 it is a beat signal diagram outputted by the lock-in amplifier 9.
  • the piezoelectric signal with frequency f 1 and the demodulated signal with frequency f are multiplied within the lock-in amplifier to produce a beat signal with a frequency of ⁇ f.
  • the processing result is automatically calculated by the Labview software.
  • the signal values of the respective peaks of the beat signal are at different water vapor concentrations.
  • A is the signal value of the first peak of the beat frequency signal obtained by the system at different water vapor concentrations
  • B is the signal value of the second peak of the beat frequency signal obtained by the system at different water vapor concentrations
  • the beam focusing device 8 may be an optical fiber that couples the lens, or may be a general focusing lens.
  • the coupled beam may be a parallel beam or a converging beam.
  • the selection of the beam focusing device 8 is such that the laser beam from the laser source can be concentrated to a diameter (or a spot size at a focus position) of less than 0.3 mm.
  • the first signal peak (peak A) of the beat signal that is automatically processed by the computer may be affected by the forced vibration of the tuning fork caused by the sound pulse, so the second (peak B) and later peak signals are used to fit the beat.
  • the ringing period of the frequency signal may be affected by the forced vibration of the tuning fork caused by the sound pulse, so the second (peak B) and later peak signals are used to fit the beat.
  • the modulated beam interacts with the gas to be tested to generate sound waves and pushes the quartz tuning fork to vibrate. Since the period of the scanning signal provided by the first function generator is less than 1 s, the wavelength sweep rate of the laser light source is greater than 18 cm -1 /s, so that the sound wave generated by the gas after absorbing the light energy based on the collision retreat is a sound pulse. The sound pulse will push the tuning fork vibration arm to vibrate in a short time.
  • the sound energy stored in the photoacoustic detection module will cause the quartz tuning fork to continue to vibrate at its own resonance frequency f i , but due to the existence of the quartz tuning fork vibration The self-loss and environmental loss, the vibration amplitude of the quartz tuning fork will be attenuated in the form of e-index. Therefore, in each scanning period of the driving current, the vibrating arm of the quartz tuning fork reaches the amplitude of the vibration in a short time, and then the energy stored in the photoacoustic detecting module is consumed in the damped oscillation mode.
  • the piezoelectric signal generated by the quartz tuning fork is also a damped oscillating signal having a frequency equal to the tuning fork resonance frequency f i , and the amplitude of the piezoelectric signal corresponds to the concentration of the gas to be measured.

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Abstract

一种基于拍频效应的石英增强光声光谱气体检测装置及方法，该装置包括光声信号探测模块、气室（2）、光源模块和数据采集模块，通过探测石英音叉（1）输出的压电信号与锁相放大器（9）的解调信号经混频过程产生的拍频信号，可在毫秒量级的时间内精确获得石英晶振的电学参数以及待测气体的浓度，可用于环境监测、食品安全监测以及工业生产控制等领域的气体在线监测。

Description

基于拍频效应的石英增强光声光谱气体检测装置及方法 技术领域
本发明涉及气体传感技术，具体为一种基于拍频效应的石英增强光声光谱气体检测装置及方法。
背景技术
气态物质成分的精确测量在国防、航天、医疗、环境监测等各个领域都有着重要意义。光声光谱技术是一种发展较为成熟的气体检测技术，该技术由于具有零背景、对光源波长无选择性、探测灵敏度与光源功率成正相关等特性而被广泛应用。
石英增强光声光谱技术是于2002年被美国莱斯大学Kosterev等人发明的一种新型光声光谱技术(专利说明书为USZOOS/0117155AI)。该技术采用价格低廉的音叉式石英晶振(简称：石英音叉)代替传统光声光谱技术中价格昂贵的宽带麦克风，作为光声转换器以完成微弱光声信号的检测。具体来讲，波长对应于目标气体吸收线的激励光的能量在被待测气体吸收后，有一部分会由于气体分子的碰撞退激发以声波的形式被释放出来，并且声波的频率与激励光的调制频率相同。因此当激励光源以石英音叉的共振频率被调制时，基于光声效应产生的声波会通过与石英音叉共振将能量转化为石英音叉振臂周期性振动的机械能。这部分机械能会由于石英材料的压电效应被转换为电信号并经由石英音叉底端的电极传输至电信号探测装置被探测。由于压电信号与气体浓度成正相关，因此对某种气体进行测量时，仅需事先通过已知浓度的该种气体对 测量装置进行浓度标定，标定后的装置就能对该种气体进行测量。
目前被广泛用于石英增强光声光谱传感器的石英音叉是具有高品质因素Q(常压下裸音叉的品质因数Q>10000)且标称频率(指晶体元件规范中所指定的频率)f 0约为32.7kHz的商用石英音叉且石英音叉的振臂间隙约为0.3mm。如此高的品质因数使得该石英音叉具有极窄的频率响应区间(常压下，频率响应区间约为2Hz)。因此对石英增强光声光谱痕量气体检测装置而言，激励光源的调制频率必须与石英音叉的共振频率精确一致。然而，由于加工工艺、使用环境(包括气体成分、气体压强、环境温度等)、材料特性等因素的影响，石英音叉的实际共振频率很难与其标定的谐振频率长期保持一致。因此，石英增强光声光谱传感装置中石英音叉的共振频率必须被反复的、精确的测定，否则石英晶振将无法高效的对正比于气体浓度的光声信号产生响应。重复的频率测定过程不仅增加了装置的复杂程度及使用难度，也使石英增强光声光谱传感装置无法完成痕量气体的连续在线监测。另外，上述石英音叉的响应时间约为100ms，考虑数据采集及处理等必要相关测量过程，传统石英增强光声光谱痕量气体监测装置的响应周期至少为500ms，如此长的响应时间不能很好的满足当今社会各个领域对痕量气体实时高效检测的需求。
发明内容
本发明的目的是解决现有的气体检测装置及方法在气体检测过程中存在的探测周期长、探测灵敏度不高且需要频繁校准石英音叉电学参数的技术问题，提供一种基于拍频效应的石英增强光声光谱气体检测装置及方法。
本发明为解决上述技术问题所采用的技术方案是：一种基于拍频效应的石英增强光声光谱气体检测装置，包括光声信号探测模块、气室、光源模块和 数据采集模块；
所述光声信号探测模块为音叉式石英晶振；所述气室的两侧壁上分别设有入射窗口和出射窗口，气室下端靠近入射窗口的一侧设有气体入口，气室上端靠近出射窗口一侧设有气体出口，所述光声信号探测模块通过支座竖直设置在气室的内腔中，所述气室的入射窗口和出射窗口位于同一光路上；
所述光源模块包括激光光源、第一函数发生器、第二函数发生器和光束聚焦装置；第一函数发生器的信号输出端和激光光源的电流扫描输入端口相连；第二函数发生器的信号输出端与激光光源的电流调制输入端口相连，激光光源的出射光路上设有光束聚焦装置，激光光束通过光束聚焦装置后通过入射窗口入射进气室内的微型声音谐振腔中；
所述数据采集模块包括跨阻抗前置放大器、锁相放大器、数据采集卡以及用于处理和记录相关数据并控制装置正常运转的计算机；所述跨阻抗前置放大器的一个信号输入端与音叉式石英晶振的一个电极连接，另一个信号输入端与音叉式石英晶振的另一个电极连接后接地；锁相放大器的信号输入端连接跨阻抗前置放大器的信号输出端，锁相放大器的同步信号输入端与第二函数发生器的同步输出信号端连接；数据采集卡的信号输入端连接锁相放大器的信号输出端，数据采集卡的RS232接口连接计算机的信号采集端口。
进一步地，所述光声信号探测模块还包括与音叉式石英晶振相匹配的微型声音谐振腔，所述微型声音谐振腔与气室的入射窗口和出射窗口位于同一光路上；
本发明还提供了一种使用基于拍频效应的石英增强光声光谱气体检测装置的气体检测方法，包括如下步骤：
1)将待测气体连续不断的从气体入口充入气室中，并从气体出口流出；
2)激光光源在第一函数发生器和第二函数发生器的驱动下发出激光光束，控制第一函数发生器的扫描电压幅值，使激光光源的输出中心波长达到所需值，且激光光源的输出波长扫描速率为18cm -1s -1～200cm -1s -1；控制第二函数发生器的调制信号频率，对激光光源的波长进行调制；所述第二函数发生器的调制信号频率f与音叉式石英晶振前一次测到的共振频率f i-1(第一次测量使用标称频率f 0，下标i表示第i次测量)设定10-210Hz的频率差；
3)调制后的激光光束在光束聚焦装置的作用下被聚焦整形，聚焦后的光束通过入射窗口进入气室内的光声信号探测模块中，并从出射窗口透出，激光与气室中的待测气体作用并产生光声信号，光声信号探测模块将该光声信号转换成频率为音叉式石英晶振共振频率f i的压电信号后经光声信号探测模块的音叉式石英晶振的电极传输给跨阻抗前置放大器；
4)跨阻抗前置放大器将接收到的压电信号放大后传输至锁相放大器，锁相放大器以频率为f的解调信号对接收到的频率为f i的压电信号进行解调，解调后形成频率为Δf i(下标i表示第i次测量)拍频信号传输至数据采集卡中，所述锁相放大器的探测带宽为1kHz～100kHz；
5)数据采集卡将采集到的拍频信号传输至装有LabView软件的计算机中进行运算处理后得到相应的拍频信号图谱，并通过LabView软件自动寻找拍频信号的各个波峰值，从所得拍频信号图谱上任取一个峰值点的信号值S，带入以下公式计算待测气体浓度值C，
Figure PCTCN2018078020-appb-000001
式中，C为待测气体浓度值；S N为本装置在高纯N 2条件下的信号值；S 1 为本装置在浓度为C 0的标准气体条件下相应峰值点的信号值；S为选取峰值点的信号值；C 0为标准气体的浓度值。
进一步地，所述锁相放大器对接收到的压电信号的解调为混频和滤波，压电信号经混频形成频率为Δf i的拍频信号再经滤波后传输至数据采集卡中。
进一步地，还包括步骤6)：所述LabView软件自动寻找拍频信号的各个波峰值后，利用拍频信号的第一个波峰值、两相邻波峰值间的时间间隔以及对各个峰值点进行e指数拟合获得的拍频信号衰荡时间τ，根据公式Δf i＝|f-f i|计算石英音叉的本次测量实际共振频率f i；根据公式Q＝π·f i·τ计算石英音叉的品质因数Q，式中，Δf i为第i次测量的拍频信号的频率，f为第二函数发生器输出的调制信号的频率，f i为石英音叉第i次测量的实际共振频率，τ为拍频信号的衰荡时间，Q为石英音叉的品质因数，π为圆周率；
循环步骤1-6，实现气体浓度的连续测量。
本发明的有益效果是：
1.与现有技术中的石英增强光声光谱痕量气体检测装置相比，本发明所述装置可在测量待测气体浓度的同时完成石英音叉基本电学参量(共振频率、品质因数)的测量，避免了气体检测过程中对石英音叉电学参数的反复校准，从而使基于石英音叉的气体传感器可以被用于痕量气体的长时间连续在线监测。
2.本发明通过探测石英音叉输出的压电信号与锁相放大器的解调信号经混频过程产生的拍频信号，可在毫秒量级的时间内精确获得石英晶振的电学参数以及待测气体的浓度，比现有技术中的石英增强光声光谱痕量气体检测装置快三个数量级，解决了目前气体探测中探测周期长、探测灵敏度不高且需要频繁校准石英晶振电学参数的技术问题。
3.本发明所述装置大大简化了传统石英增强光声光谱痕量气体检测装置，降低了装置成本，简化了使用流程。
4.本发明具有极强的技术普适性以及环境适应能力，完全可用于环境监测、食品安全监测以及工业生产控制等领域的气体在线监测。
附图说明
图1为本发明中光声信号探测模块的结构示意图；
图2为本发明中光声信号探测模块安装在气室内的结构示意图；
图3为本发明装置的结构示意图；
图4为第一函数发生器输出的扫描信号图；
图5为石英音叉受到声音脉冲推动后产生的压电信号图；
图6为锁相放大器输出的拍频信号图；
图7为计算机自动处理后的拍频信号图；
图8为拍频信号各峰值位置在不同水蒸气浓度下的信号值。
具体实施方式
下面结合附图和实施例对本发明进行进一步说明。
实施例1
如图1、图2和图3所示，本实施例中的一种基于拍频效应的石英增强光声光谱气体检测装置，包括光声信号探测模块、气室2、光源模块和数据采集模块；
所述光声信号探测模块包括音叉式石英晶振1和与音叉式石英晶振1相匹配的微型声音谐振腔3；所述微型声音谐振腔3由一对水平设置在音叉式石英晶振1两侧且垂直于音叉式石英晶振1振臂面的不锈钢毛细管组成，两个不锈 钢毛细管的中心轴线重合且均穿过音叉式石英晶振1的振臂间隙；所述气室2的两侧壁上分别设有入射窗口21a和出射窗口21b，气室2下端靠近入射窗口21a的一侧设有气体入口22a，气室2上端靠近出射窗口21b一侧设有气体出口22b，所述光声信号探测模块通过支座竖直设置在气室2的内腔中，所述气室的入射窗口21a、微型声音谐振腔3和出射窗口21b位于同一光路上；所述微型声音谐振腔3与音叉式石英晶振1之间的耦合共振可将光声信号探测模块的探测灵敏度提高30倍左右。
所述光源模块包括激光光源5、第一函数发生器6、第二函数发生器7和光束聚焦装置8；第一函数发生器6的信号输出端和激光光源5的电流扫描输入端口相连，第二函数发生器7的信号输出端与激光光源5的电流调制输入端口相连，激光光源5的出射光路上设有光束聚焦装置8，激光光束通过光束聚焦装置8后通过入射窗口21a入射进气室2内的微型声音谐振腔3中；
所述数据采集模块包括跨阻抗前置放大器4、锁相放大器9、数据采集卡10以及用于处理和记录相关数据并控制装置正常运转的计算机11；所述跨阻抗前置放大器4的一个信号输入端与音叉式石英晶振1的一个电极连接，另一个信号输入端与音叉式石英晶振的另一个电极连接后接地；锁相放大器9的信号输入端连接跨阻抗前置放大器4的信号输出端，锁相放大器9的同步信号输入端与第二函数发生器7的同步输出信号端连接，以保证锁相放大器9的解调信号频率与第二函数发生器7的调制信号频率f相同；数据采集卡10的信号输入端连接锁相放大器9的信号输出端，数据采集卡10的RS232接口连接计算机11的信号采集端口。
使用上述实施例中基于拍频效应的石英增强光声光谱气体检测装置的气体 检测方法，包括如下步骤：
1)将空气连续不断的从气体入口22a充入气室2中，并从气体出口22b流出；
2)激光光源5在第一函数发生器6和第二函数发生器7的驱动下发出激光光束，调节第一函数发生器6的信号幅值，使激光光源5的输出中心波长达到所需值，且激光光源5的输出波长扫描速率为18-200cm -1s -1，以保证待测气体产生的光声信号为脉冲声音信号；控制第二函数发生器7的调制信号频率，对激光光源5的波长进行调制，第二函数发生器7的调制信号频率f与石英音叉的标称频率f 0需存在10-210Hz的频率差，以保证后续信号解调过程中拍频信号的产生，此步骤中，由于是第一次测量，根据石英音叉的标称频率f 0来确定调制信号频率f，f 0为32.7kHz，使调制信号频率f为32550Hz，且调制信号为正弦波；由于激光器电流与其激光输出波长正相关，所以通过改变第一函数发生器6的扫描信号电压幅值，可以改变激光光源5电流变化范围，从而改变了激光光源5的输出光波长。
3)调制后的激光光束在光束聚焦装置8的作用下被聚焦整形为光斑直径小于石英音叉振臂间隙的平行光束，聚焦后的平行光束通过入射窗口21a进入气室2内的光声信号探测模块的微型声音谐振腔3中，并从出射窗口21b透出，激光与气室2中的空气作用并产生光声信号，光声信号探测模块将该光声信号转换成频率为音叉式石英晶振1共振频率f 1(因为第一次测量，f i＝f 1)的压电信号后经光声信号探测模块的音叉式石英晶振1的电极传输给跨阻抗前置放大器4；
4)跨阻抗前置放大器4将接收到的压电信号放大后传输至锁相放大器9， 锁相放大器9以频率为f的解调信号对接收到的频率为f 1的压电信号进行解调，锁相放大器9的解调信号频率与第二函数发生器7的调制信号频率f相同，解调过程分为混频和滤波，压电信号经混频形成频率Δf 1＝208.33Hz的拍频信号再经滤波后传输至数据采集卡10中，所述锁相放大器9的探测带宽为1kHz～100kHz，以保证锁相放大器9的探测带宽比光声信号探测模块的电子带宽宽200倍以上；
5)数据采集卡10将采集到的拍频信号传输至装有LabView软件的计算机11中进行运算处理后得到相应的拍频信号图谱，并通过LabView软件自动寻找拍频信号的各个波峰值，从所得拍频信号图谱上任取一个峰值点的信号值S，带入以下公式计算空气中水蒸气的浓度值C，
Figure PCTCN2018078020-appb-000002
式中，C为空气中水蒸气的浓度值；S N为本装置在高纯N 2条件下的信号值；S 1为本装置在浓度为C 0的标准气体条件下的信号值；S为选取峰值点的信号值；C 0为标准气体的浓度值。
本实施中浓度为C 0的标准气体是浓度为1％的水蒸气。
实施例2
本实施例中的一种基于拍频效应的石英增强光声光谱气体检测装置及方法同实施例1，其中，所述方法还包括步骤6)：
所述LabView软件自动寻找拍频信号的各个波峰值后，利用拍频信号的第一个波峰值、两相邻波峰值间的时间间隔以及对各个峰值点进行e指数拟合获得的拍频信号衰荡时间τ＝0.01764s，根据公式Δf 1＝|f-f 1|计算石英音叉的共振频率f 1；根据公式Q＝π·f 1·τ计算石英音叉的品质因数Q，式中，Δf 1为拍频信 号的频率，f为第二函数发生器7输出的调制电流的频率，f 1为第一次测量石英音叉的实际共振频率，τ为拍频信号的衰荡时间，Q为石英音叉的品质因数，π为圆周率。
循环步骤1-6，实现气体浓度的连续测量。
如图4所示，为第一函数发生器6输出的扫描信号图。水蒸气吸收线波长对应的扫描电压位于扫描信号前半部分斜坡电压的中间位置。由于石英音叉被声音脉冲推动后形成的阻尼振动会持续一段时间，为了避免后续声音脉冲引起的振动干扰之前形成的阻尼振动，本实施例在斜坡电压扫描结束之后添加了一段恒定电压，使石英音叉有充足的时间完成阻尼振动。与传统石英增强光声光谱不同，本发明中扫描信号的周期要小于1s，以确保激光器的波长扫描速率大于18cm -1/s，从而将作用于音叉振臂的声波持续时间缩短至足以激发石英音叉瞬态振动模式的时间尺度。
如图5所示，为石英音叉受到声音脉冲推动后产生的压电信号图。在声音脉冲消失后石英音叉会以自身共振频率f 1继续振动，并在振动过程通过自损耗和外部损耗将积累在光声探测系统中的能量消耗完，产生频率为音叉共振频率f 1的压电信号。
如图6所示，为锁相放大器9输出的拍频信号图。由锁相放大器的工作原理可知，频率为f 1的压电信号与频率为f的解调信号在锁相放大器内部进行相乘，产了频率为Δf的拍频信号。
如图7所示，为拍频信号经NI公司生产的DAQ数据采集卡10传输至计算机11后，通过Labview软件自动计算处理的结果。图中，拍频信号的频率为Δf＝1/0.0048Hz＝208.33Hz，拍频信号的衰荡时间τ＝0.01764s。结合第二函 数发生器7输出的调制信号的频率f＝32550Hz，计算石英音叉的共振频率f 1＝f+Δf＝32758.33Hz；另外通过公式Q＝π·f 1·τ求得石英音叉的品质因数为1814.5(与通过传统电激励扫描方法获得的结果f 0’＝327550Hz,Q＝1816一致)；
如图8所示，为拍频信号各峰值处在不同水蒸气浓度下的信号值。其中A为系统处在不同水蒸气浓度下获得的拍频率信号的第一个峰值的信号值；B为系统处在不同水蒸气浓度下获得的拍频率信号的第二个峰值的信号值；C为系统处在不同水蒸气浓度下获得的拍频率信号的第三个峰值的信号值。由图8可知，拍频信号的各峰值对于不同浓度的水汽均表现出了良好的线性度，在实际测量中可以通过探测拍频信号的任意一个峰值来测定待测气体的浓度。
本发明中，所述光束聚焦装置8可以是耦合透镜的光纤，也可以是普通的聚焦透镜。耦合后的光束可以是平行光束也可以是汇聚光束，光束聚焦装置8的选择标准是可以将激光光源发出的激光光束汇聚至直径(或焦点位置处光斑尺寸)小于0.3mm。
通过计算机自动处理后的拍频信号的第一个信号峰值(峰值A)可能会受到声音脉冲引起的音叉受迫振动的影响，因此使用第二个(峰值B)及以后的峰值信号拟合拍频信号的衰荡周期。
本发明的原理如下：
激光光束无碰撞的穿过光声信号探测模块后，被调制的光束与待测气体相互作用产生声波并推动石英音叉振动。由于第一函数发生器提供的扫描信号的周期小于1s，因此激光光源的波长扫描率大于18cm -1/s，从而使气体吸收光能之后基于碰撞退激发产生的声波为声音脉冲。声音脉冲会在短时间内推动音叉振臂振动，在声音脉冲消失后，储藏在光声探测模块内的声波能量会使石英音 叉以自身共振频率f i继续振动，但由于石英音叉振动过程中存在的自损耗以及环境损耗，石英音叉的振动幅度会以e指数形式衰减。因此在驱动电流的每个扫描周期内，石英音叉的振臂会在短时间内达到振动的幅值，而后在阻尼振荡模式下将储存在光声探测模块内的能量消耗完。在此过程中，石英音叉产生的压电信号也会是一个频率等于音叉共振频率f i的阻尼振荡信号，且压电信号的幅值对应于待测气体的浓度。

Claims (5)

  1. 一种基于拍频效应的石英增强光声光谱气体检测装置，其特征在于：包括光声信号探测模块、气室(2)、光源模块和数据采集模块；
    所述光声信号探测模块为音叉式石英晶振(1)；所述气室(2)的两侧壁上分别设有入射窗口(21a)和出射窗口(21b)，气室(2)下端靠近入射窗口(21a)的一侧设有气体入口(22a)，气室(2)上端靠近出射窗口(21b)一侧设有气体出口(22b)，所述光声信号探测模块通过支座竖直设置在气室(2)的内腔中，所述气室的入射窗口(21a)和出射窗口(21b)位于同一光路上；
    所述光源模块包括激光光源(5)、第一函数发生器(6)、第二函数发生器(7)和光束聚焦装置(8)；第一函数发生器(6)的信号输出端和激光光源(5)的电流扫描输入端口相连；第二函数发生器(7)的信号输出端与激光光源(5)的电流调制输入端口相连，激光光源(5)的出射光路上设有光束聚焦装置(8)，激光光束通过光束聚焦装置(8)后通过入射窗口(21a)入射进气室(2)内的微型声音谐振腔(3)中；
    所述数据采集模块包括跨阻抗前置放大器(4)、锁相放大器(9)、数据采集卡(10)以及用于处理和记录相关数据并控制装置正常运转的计算机(11)；所述跨阻抗前置放大器(4)的一个信号输入端与音叉式石英晶振(1)的一个电极连接，另一个信号输入端与音叉式石英晶振的另一个电极连接后接地；锁相放大器(9)的信号输入端连接跨阻抗前置放大器(4)的信号输出端，锁相放大器(9)的同步信号输入端与第二函数发生器(7)的同步输出信号端连接；数据采集卡(10)的信号输入端连接锁相放大器(9)的信号输出端，数据采集卡(10)的RS232接口连接计算机(11)的信号采集端口。
  2. 根据权利要求1所述的一种基于拍频效应的石英增强光声光谱气体检测装置，其特征在于：所述光声信号探测模块还包括与音叉式石英晶振(1)相匹配的微型声音谐振腔(3)，所述微型声音谐振腔(3)与气室的入射窗口(21a)和出射窗口(21b)位于同一光路上。
  3. 一种使用权利要求1或2所述的基于拍频效应的石英增强光声光谱气体检测装置的气体检测方法，其特征在于，包括如下步骤：
    1)将待测气体连续不断的从气体入口(22a)充入气室(2)中，并从气体出口(22b)流出；
    2)激光光源(5)在第一函数发生器(6)和第二函数发生器(7)的驱动下发出激光光束，调节第一函数发生器(6)的扫描电压幅值，使激光光源(5)的输出中心波长达到所需值，且激光光源(5)的输出波长扫描速率为18cm -1s -1～200cm -1s -1；控制第二函数发生器(7)的调制信号频率，对激光光源(5)的波长进行调制，所述第二函数发生器的调制信号频率f与音叉式石英晶振前一次测到的共振频率f i-1(第一次测量使用标称频率f 0，下标i表示第i次测量)设定10-210Hz的频率差；
    3)调制后的激光光束在光束聚焦装置(8)的作用下被聚焦整形，聚焦后的光束通过入射窗口(21a)进入气室(2)内的光声信号探测模块中，并从出射窗口(21b)透出，激光与气室(2)中的待测气体作用并产生光声信号，光声信号探测模块将该光声信号转换成频率为音叉式石英晶振(1)共振频率f i的压电信号后经光声信号探测模块的音叉式石英晶振(1)的电极传输给跨阻抗前置放大器(4)；
    4)跨阻抗前置放大器(4)将接收到的压电信号放大后传输至锁相放大器(9)，锁相放大器(9)以频率为f的解调信号对接收到的频率为f i的压电信号进行解调，解调后形成频率Δf i(下标i表示第i次测量)拍频信号传输至数据 采集卡(10)中，所述锁相放大器(9)的探测带宽为1kHz～100kHz；
    5)数据采集卡(10)将采集到的拍频信号传输至装有LabView软件的计算机(11)中进行运算处理后得到相应的拍频信号图谱，并通过LabView软件自动寻找拍频信号的各个波峰值，从所得拍频信号图谱上任取一个峰值点的信号值S，带入以下公式计算待测气体浓度值C，
    Figure PCTCN2018078020-appb-100001
    式中，C为待测气体浓度值；S N为本装置在高纯N 2条件下的信号值；S 1为本装置在浓度为C 0的标准气体条件下相应峰值点的信号值；S为选取峰值点的信号值；C 0为标准气体的浓度值；
  4. 根据权利要求3所述的方法，其特征在于：所述锁相放大器(9)对接收到的压电信号的解调为混频和滤波，压电信号经混频形成频率为Δf i的拍频信号再经滤波后传输至数据采集卡(10)中。
  5. 根据权利要求3所述的方法，其特征在于，还包括步骤6)：
    所述LabView软件自动寻找拍频信号的各个波峰值后，利用拍频信号的第一个波峰值、两相邻波峰值间的时间间隔以及对各个峰值点进行e指数拟合获得的拍频信号衰荡时间τ，根据公式Δf i＝|f-f i|计算石英音叉的本次测量的实际共振频率f i；根据公式Q＝π·f i·τ计算石英音叉的品质因数Q，式中，Δf i为第i次测量的拍频信号的频率，f为第二函数发生器输出的调制信号的频率，f i为石英音叉第i次测量的实际共振频率，τ为拍频信号的衰荡时间，Q为石英音叉的品质因数，π为圆周率；
    循环步骤1-6，实现气体浓度的连续测量。
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CN114002184A (zh) * 2021-11-01 2022-02-01 安徽大学 多谐振增强型光声光谱多组分气体同时检测装置及方法
CN114002184B (zh) * 2021-11-01 2023-06-23 安徽大学 多谐振增强型光声光谱多组分气体同时检测装置及方法

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