Chirp modulation photothermoelastic spectrum resonant frequency and concentration synchronous measurement method and system

Abstract

The invention discloses a chirp modulation photoinduced thermoelastic spectrum resonant frequency and concentration synchronous measurement method and system, and belongs to the field of photoinduced thermoelastic spectrums.The method comprises the steps that S1, a chirp modulation signal is generated to drive a laser, and a modulated laser beam is generated; s2, the modulated laser beam penetrates through gas and then enters the surface of a quartz tuning fork to excite demodulation of a photoinduced thermoelastic signal to obtain a first harmonic signal; s3, fitting the first harmonic signal by adopting a Lorentz linear function; s4, calculating the central resonant frequency position of the quartz tuning fork according to a fitting result; s5, judging whether the central resonant frequency position of the quartz tuning fork is located at the center of a first harmonic signal or not; if not, adjusting the initial frequency of the chirp modulation signal, and repeating the steps S1 to S5 until the central resonant frequency position of the quartz tuning fork is located at the center of the first harmonic signal; and S6, calculating a response coefficient according to a fitting result, and performing inversion to obtain gas concentration information. According to the invention, high-precision measurement of gas concentration and real-time correction of resonant frequency can be realized.

Description

Technical Field

[0001] The present invention belongs to the field of photothermoelastic spectroscopy and relates to a method for synchronously measuring resonance frequency and concentration, and in particular to a method and system for synchronously measuring resonance frequency and concentration of chirp-modulated photothermoelastic spectroscopy. Background Art

[0002] The core sensor element of photoinduced thermoelastic spectroscopy (LITES) is a quartz tuning fork (QTF). The frequency response curve of a QTF can be described by a Lorentzian linear function, and its characteristic parameters are highly sensitive to the external environment. The thermal expansion coefficient of the QTF substrate material causes the QTF resonant frequency to decrease linearly with increasing temperature, and the quality factor exhibits complex fluctuations with temperature. According to the currently used wavelength modulation technology, due to the narrow bandwidth of the QTF frequency response, small frequency drifts have a significant impact on the spectral response intensity based on linear function estimation, seriously affecting the long-term stability of the LITES system. Currently, the electrical excitation method is mostly used to measure the resonant frequency of a quartz tuning fork. By generating sinusoidal signals of different frequencies to excite the quartz tuning fork, the output signal amplitude of the quartz tuning fork at different frequencies is measured and processed to obtain a precise resonant frequency. This method has high measurement accuracy but a long measurement time of several minutes. In addition, the currently used wavelength modulation technology cannot achieve synchronous measurement of the resonant frequency and gas concentration, making it difficult to meet the demand for real-time compensation of the resonant frequency in concentration measurement applications. Summary of the Invention

[0003] The present invention provides a chirp-modulated photothermoelastic spectrum resonance frequency and concentration synchronous measurement method and system to overcome the defects of the prior art.

[0004] To achieve the above object, the present invention adopts the following technical solutions:

[0005] In a first aspect, the present invention provides a method for synchronously measuring the resonant frequency and concentration of chirped modulated photothermoelastic spectroscopy, comprising the following steps:

[0006] S1, generating a chirped modulation signal to drive the laser and produce a modulated laser beam;

[0007] S2, the modulated laser beam passes through the gas and is incident on the surface of the quartz tuning fork to stimulate a photothermoelastic signal, the photothermoelastic signal is collected and demodulated to obtain a first harmonic (1f) signal;

[0008] S3, fitting the first harmonic signal using a Lorentz linear function;

[0009] S4. Calculate the central resonance frequency (i.e., resonant frequency) position of the quartz tuning fork based on the fitting results;

[0010] S5. Determine whether the central resonance frequency position of the quartz tuning fork is located at the center of the first harmonic signal;

[0011] If not, adjust the initial frequency of the chirp modulation signal and repeat S1 to S5 until the central resonance frequency of the quartz tuning fork is located at the center of the first harmonic signal;

[0012] S6. Calculate the response coefficient based on the fitting results and invert to obtain the gas concentration information.

[0013] Furthermore, in said S1, said chirp modulation signal S chirp Expressed as:

[0014] S chirp =acos[(2π(bt+f0))t+φ]+c;

[0015] Where a is the modulation amplitude of the chirp modulation signal, b is the frequency change rate of the chirp modulation signal, t is time, f0 is the initial frequency of the chirp modulation signal, φ is the initial phase of the chirp modulation signal, and c is the offset of the chirp modulation signal.

[0016] Furthermore, in S1, the chirped modulation signal generates a modulated laser beam by modulating the injection current of the laser.

[0017] Furthermore, in S2, the chirped modulation signal is used as a reference signal to demodulate the photothermoelastic signal to obtain a first harmonic signal that conforms to the Lorentzian line distribution.

[0018] Furthermore, in S3, the Lorentz linear function y used for fitting is expressed as:

[0019]

[0020] Where y0 is the bias, k is the response coefficient, the k value is related to the gas absorption coefficient, A is the frequency response curve area of ​​the quartz tuning fork, w is the frequency response bandwidth of the quartz tuning fork, b is the frequency change rate of the chirp modulation signal, t is the time, f0 is the initial frequency of the chirp modulation signal, f c is the central resonant frequency of the quartz tuning fork.

[0021] Furthermore, in S4, the central resonance frequency of the quartz tuning fork is the frequency corresponding to the maximum value of the fitting curve.

[0022] Furthermore, in S5, if the central resonant frequency position of the quartz tuning fork deviates from the center of the first harmonic signal, the method for adjusting the initial frequency of the chirp modulation signal is as follows: if the first harmonic signal is in a frequency under-regulation state, the initial frequency of the chirp modulation signal is gradually increased, and S1 to S5 are repeated until the central resonant frequency position of the quartz tuning fork is located at the center of the first harmonic signal; if the first harmonic signal is in a frequency over-regulation state, the initial frequency of the chirp modulation signal is gradually reduced, and S1 to S5 are repeated until the central resonant frequency position of the quartz tuning fork is located at the center of the first harmonic signal.

[0023] Furthermore, in S5, after the frequency adjustment of the primary harmonic signal is completed, the central resonance frequency of the quartz tuning fork, the frequency change rate of the chirp modulation signal, and the initial frequency of the chirp modulation signal satisfy the following relationship:

[0024]

[0025] Where, f c is the central resonance frequency of the quartz tuning fork, f0 is the initial frequency of the chirp modulation signal, b is the frequency change rate of the chirp modulation signal, and t is time.

[0026] Furthermore, in S6, the response coefficient is inversely proportional to the gas concentration, and the gas concentration information is obtained by inverting the relationship between the calibration gas concentration and the response coefficient.

[0027] In a second aspect, the present invention provides a chirp-modulated photothermoelastic spectrum resonant frequency and concentration synchronous measurement system for implementing the above-mentioned method; the system includes a microcontroller, a waveform generator, a laser diode, a gas measuring cell, a quartz tuning fork, a phase-locked amplifier and an analog-to-digital converter; the microcontroller controls the waveform generator to generate a chirp-modulated signal; the chirp-modulated signal drives the injection current of the laser diode to generate a modulated laser beam; the modulated laser beam passes through the gas measuring cell and converges on the surface of the quartz tuning fork to stimulate a photothermoelastic signal; the photothermoelastic signal is demodulated by the phase-locked amplifier to obtain a first harmonic signal; and the first harmonic signal is input to the analog-to-digital converter to realize data acquisition.

[0028] The beneficial effects of the present invention are as follows: the present invention provides a method and system for synchronously measuring the resonant frequency and concentration of chirped-modulated photothermoelastic spectroscopy, which can achieve high-precision measurement of gas concentration and real-time correction of the resonant frequency, helping to improve the long-term stability of photothermoelastic spectroscopy in measurement applications. Specifically, the present invention uses a chirped-modulated signal to modulate the laser injection current. When the chirped-modulated laser beam is directly incident on the surface of a quartz tuning fork, the chirped-modulated signal with the same parameters is used as a reference signal to demodulate the photothermoelastic signal output by the quartz tuning fork to obtain a first harmonic signal. When the chirped-modulated signal frequency sweep range covers the characteristic frequency range of the quartz tuning fork, the amplitude of the first harmonic signal exhibits a Lorentzian linear function distribution and a maximum value. A Lorentzian linear fit is performed on the signal, and the frequency corresponding to the maximum value of the curve is the central resonant frequency of the quartz tuning fork. If the frequency corresponding to the maximum value deviates from the center of the first harmonic signal, the initial frequency of the chirped-modulated signal is adjusted to align the central resonant frequency of the quartz tuning fork with the center of the first harmonic signal. The above analysis and adjustment method enables real-time measurement and compensation of the central resonant frequency of a quartz tuning fork. Furthermore, gas absorption in the measurement optical path causes the overall attenuation of the first harmonic signal, allowing inversion to reveal gas concentration information. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a block diagram of the chirp-modulated photothermoelastic spectrum resonant frequency and concentration synchronous measurement system of the present invention;

[0030] Figure 2 This is a flow chart of the chirp-modulated photothermoelastic spectrum resonant frequency and concentration synchronous measurement method of the present invention;

[0031] Figure 3 Schematic diagram of the first harmonic signal in the chirp-modulated photothermoelastic spectrum resonant frequency and concentration synchronous measurement method of the present invention;

[0032] Figure 4 It is the first harmonic signal waveform of different response coefficients in the chirp-modulated photothermoelastic spectrum resonant frequency and concentration synchronous measurement method of the present invention;

[0033] The markings in the accompanying drawings are: 1. microcontroller; 2. waveform generator; 3. laser diode; 4. gas measuring cell; 5. quartz tuning fork; 6. lock-in amplifier; 7. analog-to-digital converter. DETAILED DESCRIPTION

[0034] The specific embodiments of the present invention are described below with reference to the accompanying drawings.

[0035] The present invention provides a chirp-modulated photothermoelastic spectrum resonance frequency and concentration synchronous measurement method and system.

[0036] like Figure 1As shown, the system includes a microcontroller 1 , a waveform generator 2 , a laser diode 3 , a gas measuring cell 4 , a quartz tuning fork 5 , a lock-in amplifier 6 and an analog-to-digital converter 7 .

[0037] The microcontroller 1 controls the waveform generator 2 to generate a chirp modulation signal. The chirp modulation signal S chirp It is expressed in the following formula:

[0038] S chirp =acos[(2π(bt+f0))t+φ]+c;

[0039] Where a is the modulation amplitude of the chirp modulation signal, b is the frequency change rate of the chirp modulation signal, t is time, f0 is the initial frequency of the chirp modulation signal, φ is the initial phase of the chirp modulation signal, and c is the offset of the chirp modulation signal.

[0040] The generated chirp modulation signal is used to drive the injection current of the laser diode 3 to generate a modulated laser beam. The modulated laser beam passes through the gas measurement cell 4 and converges on the surface of the quartz tuning fork 5, thereby stimulating a photothermoelastic signal. The output signal of the quartz tuning fork is conditioned and demodulated by the phase-locked amplifier 6 to obtain a first harmonic signal, which is finally input into the analog-to-digital converter 7 to realize data acquisition of the harmonic signal.

[0041] like Figure 2 As shown, the method is as follows: first, the photothermoelastic signal is collected, and the chirped modulation signal is used as the reference signal for demodulation to obtain the 1f signal. At this time, the obtained 1f signal conforms to the Lorentzian linear distribution. The Lorentzian linear function is used to fit the collected 1f signal. The fitting function is expressed as follows:

[0042]

[0043] Where y0 is the bias, k is the response coefficient, the k value is related to the gas absorption coefficient, A is the frequency response curve area of ​​the quartz tuning fork, w is the frequency response bandwidth of the quartz tuning fork, and f is the frequency response bandwidth of the quartz tuning fork. c is the central resonant frequency of the quartz tuning fork.

[0044] According to the fitting results of the Lorentz linear function, the central resonance frequency f of the quartz tuning fork can be obtained c and the response coefficient k. Figure 3 The frequency under-regulation, regulation completion and over-regulation of 1f signal are shown. If the 1f signal is in the frequency under-regulation state, gradually increase the initial frequency f0 of the chirp modulation signal and repeat the above steps until the resonant frequency f c Located at the center of the 1f signal; if the 1f signal is in a frequency over-modulation state, gradually reduce the initial frequency f0 of the chirp modulation signal and repeat the above steps until the resonant frequency f cLocated at the center of the 1f signal. When the 1f signal frequency is adjusted, the central resonance frequency of the quartz tuning fork is f c , the frequency change rate b of the chirp modulation signal and the initial frequency f0 of the chirp modulation signal conform to the following relationship:

[0045]

[0046] At this time, the number of valid data points in the Lorentz linear fitting process is the largest, and the fitting result is the most accurate. Calculate the k value of the 1f signal based on the Lorentz linear function fitting result. Figure 4 The 1f signal waveforms with different k values ​​are shown. The k value of the 1f signal is inversely proportional to the gas concentration. Based on the concentration calibration curve, the current gas concentration can be calculated.

[0047] In the present invention, unless otherwise specified, the scientific and technical terms used herein have the meanings commonly understood by those skilled in the art. In addition, the equipment and operating procedures used herein are those widely used in the relevant fields and conventional procedures.

[0048] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A chirp-modulated photothermoelastic spectroscopy resonance frequency and concentration synchronous measurement method, characterized by: The following steps are involved: S1, generating a chirped modulation signal to drive the laser and produce a modulated laser beam; S2, the modulated laser beam passes through the gas and is incident on the surface of the quartz tuning fork to stimulate a photothermoelastic signal, the photothermoelastic signal is collected and demodulated to obtain a first harmonic signal; S3, fitting the first harmonic signal using a Lorentz linear function; S4. Calculate the central resonance frequency position of the quartz tuning fork according to the fitting results; S5. Determine whether the central resonance frequency position of the quartz tuning fork is located at the center of the first harmonic signal; If not, adjust the initial frequency of the chirp modulation signal and repeat S1 to S5 until the central resonance frequency of the quartz tuning fork is located at the center of the first harmonic signal; S6. Calculate the response coefficient based on the fitting results and invert to obtain the gas concentration information.

2. The chirp-modulated photothermoelastic spectroscopy resonance frequency and concentration simultaneous measurement method according to claim 1, characterized in that: In S1, the chirped modulation signal S chirp Expressed as: S chirp =acos[(2π(bt+f0))t+φ]+c; Where a is the modulation amplitude of the chirp modulation signal, b is the frequency change rate of the chirp modulation signal, t is time, f0 is the initial frequency of the chirp modulation signal, φ is the initial phase of the chirp modulation signal, and c is the offset of the chirp modulation signal.

3. The chirp-modulated photothermoelastic spectroscopy resonance frequency and concentration simultaneous measurement method according to claim 1, characterized in that: In S1, the chirped modulation signal generates a modulated laser beam by modulating the injection current of the laser.

4. The chirp-modulated photothermoelastic spectroscopy resonance frequency and concentration simultaneous measurement method according to claim 1, characterized in that: In S2, the chirped modulation signal is used as a reference signal to demodulate the photothermoelastic signal to obtain a first harmonic signal that conforms to the Lorentzian line distribution.

5. The chirp-modulated photothermoelastic spectroscopy resonance frequency and concentration synchronous measurement method according to claim 1, characterized in that: In S3, the Lorentz linear function y used for fitting is expressed as: Where y0 is the bias, k is the response coefficient, A is the frequency response curve area of ​​the quartz tuning fork, w is the frequency response bandwidth of the quartz tuning fork, b is the frequency change rate of the chirp modulation signal, t is the time, f0 is the initial frequency of the chirp modulation signal, and f is the frequency response area of ​​the quartz tuning fork. c is the central resonant frequency of the quartz tuning fork.

6. The chirp-modulated photothermoelastic spectroscopy resonance frequency and concentration simultaneous measurement method according to claim 1, characterized in that: In S4, the central resonance frequency of the quartz tuning fork is the frequency corresponding to the maximum value of the fitting curve.

7. The chirp-modulated photothermoelastic spectroscopy resonance frequency and concentration simultaneous measurement method according to claim 1, characterized in that: In S5, if the central resonance frequency position of the quartz tuning fork deviates from the center of the first harmonic signal, the method for adjusting the initial frequency of the chirp modulation signal is: If the first harmonic signal is in a frequency under-regulated state, gradually increase the initial frequency of the chirp modulation signal and repeat S1 to S5 until the central resonance frequency of the quartz tuning fork is located at the center of the first harmonic signal. If the first harmonic signal is in a frequency over-modulation state, the initial frequency of the chirp modulation signal is gradually reduced, and steps S1 to S5 are repeated until the central resonance frequency of the quartz tuning fork is located at the center of the first harmonic signal.

8. The chirp-modulated photothermoelastic spectroscopy resonance frequency and concentration simultaneous measurement method according to claim 7, characterized in that: In S5, after the frequency adjustment of the first harmonic signal is completed, the central resonance frequency of the quartz tuning fork, the frequency change rate of the chirp modulation signal, and the initial frequency of the chirp modulation signal satisfy the following relationship: Where, f c is the central resonance frequency of the quartz tuning fork, f0 is the initial frequency of the chirp modulation signal, b is the frequency change rate of the chirp modulation signal, and t is time.

9. The chirp-modulated photothermoelastic spectroscopy resonance frequency and concentration simultaneous measurement method according to claim 1, characterized in that: In the above-mentioned S6, the response coefficient is inversely proportional to the gas concentration. By measuring the relationship between the calibration gas concentration and the response coefficient, the gas concentration information is obtained by inversion.

10. A chirp-modulated photothermoelastic spectroscopy resonant frequency and concentration synchronous measurement system, characterized by: Used to implement the method according to any one of claims 1 to 9; The system includes a microcontroller, waveform generator, laser diode, gas measurement cell, quartz tuning fork, lock-in amplifier, and analog-to-digital converter; A microcontroller controls a waveform generator to generate a chirp modulation signal; the chirp modulation signal drives the injection current of the laser diode to produce a modulated laser beam; the modulated laser beam passes through a gas measurement cell and converges on the surface of a quartz tuning fork to stimulate a photothermoelastic signal; the photothermoelastic signal is demodulated by a lock-in amplifier to obtain a first harmonic signal; and the first harmonic signal is input to an analog-to-digital converter for data acquisition.

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