A PAS system based on active noise reduction of multiple photoacoustic cells

Abstract

The present invention relates to the field of photoacoustic spectrum detection, and provides a PAS system based on active noise reduction of multiple photoacoustic cells, wherein the system comprises: a noise light source DFB laser, a measurement light source DFB laser, a beam combiner, a collimator, a first window, a first photoacoustic cell, a dichroic mirror, a second photoacoustic cell, and a second window; the measurement light source DFB laser emits a measurement light source, and the noise light source DFB laser emits a noise light source, and the two light sources are coupled by an optical fiber beam combiner and then emitted, and after being collimated by a collimator lens, they are emitted into the first photoacoustic cell through the first window, and after passing through the dichroic mirror, the light of the measurement light source is reflected back along the original path to enhance the photoacoustic effect, and the light of the noise light source is emitted into the second photoacoustic cell through the dichroic mirror and finally emitted through the second window. The present invention improves the signal-to-noise ratio of the system, thereby significantly improving the detection lower limit of the entire system, and has low cost and a wide range of applications.

Description

Technical Field

[0001] The invention relates to the field of photoacoustic spectroscopy detection, and in particular to a PAS system based on active noise reduction of multiple photoacoustic cells. Background Art

[0002] During long-term operation, oil-filled power transformers may experience latent faults such as partial discharge, spark discharge and transformer oil overheating, which may lead to the decomposition of insulating materials such as transformer oil or paper, thereby producing characteristic gases such as CH4, C2H6, C2H4, H2, CO, CO2, C2H2, and H2O.

[0003] By detecting and analyzing the type and content of characteristic gases dissolved in transformer oil, the type, degree and development trend of latent faults can be obtained.

[0004] At present, the detection technologies for characteristic gases dissolved in transformer oil mainly include gas chromatography, electrochemical sensor method and photoacoustic spectroscopy (PAS) method.

[0005] Among them, photoacoustic spectroscopy detection technology has the advantages of high sensitivity, good selectivity and large detection range.

[0006] According to the principle of photoacoustic spectroscopy detection technology, the PAS system modulates the incident laser to generate infrared light that changes periodically corresponding to the infrared absorption peak of the target gas to be measured. The target gas molecules absorb the energy of the modulated light source to achieve the transition of the molecules from the ground state to the excited state. After that, the molecules generate heat energy during the de-excitation process back to the ground state, and generate sound waves in the photoacoustic cavity of a certain volume. The generated sound waves will be detected by a highly sensitive microphone and converted into electrical signals and transmitted to the host computer. The electrical signals measured by the host computer can be inverted by the algorithm to obtain the concentration of each component gas.

[0007] In photoacoustic spectroscopy detection technology, the photoacoustic cavity, as the core module, is the place where the photoacoustic signal is generated and needs to avoid interference from external noise as much as possible.

[0008] At present, the shielding of photoacoustic cavity against external noise mainly relies on passive noise reduction measures, such as adding shielding layer and applying sound-absorbing material on the outer surface. However, the above measures have limited application scenarios and high cost due to various factors such as increased volume. Summary of the invention

[0009] The purpose of the present invention is to provide a PAS system based on active noise reduction of multiple photoacoustic cells, so as to solve the technical problems that the passive noise reduction adopted in the prior art leads to limited application scenarios and high costs.

[0010] The present invention provides a PAS system based on active noise reduction of multiple photoacoustic cells, which specifically includes:

[0011] Noise light source DFB laser, measurement light source DFB laser, beam combiner, collimator, first window, first photoacoustic cell, dichroic mirror, second photoacoustic cell, second window;

[0012] The measuring light source DFB laser emits a measuring light source, and the noise light source DFB laser emits a noise light source. The two light sources are coupled by a fiber optic combiner and then emitted. After being collimated by a collimating lens, they are emitted into the first photoacoustic cell through the first window. After passing through the dichroic mirror, the light of the measuring light source is reflected back along the original path to enhance the photoacoustic effect. The light of the noise light source passes through the dichroic mirror, enters the second photoacoustic cell, and finally exits through the second window.

[0013] Furthermore, it also includes: a connecting air tube, wherein the connecting air tube connects the first photoacoustic cell and the second photoacoustic cell.

[0014] Furthermore, the first window, the second window and the dichroic mirror have the same material and size.

[0015] Furthermore, the surfaces of the first window and the second window are coated with an anti-reflection film.

[0016] Furthermore, the surface of the dichroic mirror is coated with an optical film layer which reflects the measurement light source and transmits the noise light source.

[0017] Furthermore, the first photoacoustic cell and the second photoacoustic cell are made of the same material, are connected in the same direction, and a dichroic mirror is sealed at the connection.

[0018] Furthermore, the first photoacoustic cell and the second photoacoustic cell are photoacoustic cells of differential structure, each of which has a pair of acoustic resonant cavities of the same size and symmetrical positions.

[0019] The noise light source DFB laser and the measuring light source DFB laser perform signal synchronization processing, and the acoustic detectors in the first photoacoustic cell and the second photoacoustic cell perform signal synchronization processing.

[0020] The beneficial effects of the present invention are as follows: by introducing a noise light source for active noise reduction, utilizing the photoacoustic effect of the noise light source, deducting external noise interference and the noise generated by the boundary effect and relaxation effect inside the gas pool itself through signal synchronization processing, and then combining the orthogonal phase-locked algorithm to reduce the noise to the limit, thereby improving the system signal-to-noise ratio, thereby significantly improving the detection limit of the entire system, and the cost is low and the application range is wide. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The accompanying drawings, which constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

[0022] Figure 1 It is a schematic diagram of the system structure of the present invention;

[0023] in:

[0024] 1. Noise light source DFB laser; 2. Measurement light source DFB laser; 3. Beam combiner; 4. Collimating lens; 5. First window; 6. First photoacoustic cell; 7. Dichroic mirror; 8. Second photoacoustic cell; 9. Second window; 10. Connecting air pipe;

[0025] Figure 2 It is a schematic diagram of the effect of the anti-reflection film;

[0026] Figure 3 It is a schematic diagram of the effect of the optical film layer;

[0027] Figure 4 It is the waveform of conventional phase-locked amplification;

[0028] Figure 5 It is a fitting curve derived from the AD value after active noise reduction combined with phase-locked amplification of the present invention;

[0029] Figure 6 It is a comparison curve diagram of conventional phase locking and orthogonal phase locking of the present invention. DETAILED DESCRIPTION

[0030] Various exemplary embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings. It should be noted that unless otherwise specifically stated, the relative arrangement of components and steps, numerical expressions and numerical values ​​set forth in these embodiments do not limit the scope of the present invention.

[0031] At the same time, it should be understood that for the convenience of description, the sizes of the various parts shown in the drawings are not drawn according to the actual proportional relationship.

[0032] The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way intended to limit the invention, its application, or uses.

[0033] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0034] Technologies, methods, and apparatus known to ordinary technicians in the relevant field may not be discussed in detail, but where appropriate, such technologies, methods, and apparatus should be considered part of the authorization specification.

[0035] In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limiting. Therefore, other examples of the exemplary embodiments may have different values.

[0036] It should be noted that like reference numerals and letters refer to similar items in the following figures, and therefore, once an item is defined in one figure, it need not be further discussed in subsequent figures.

[0037] Please refer to Figure 1 , Figure 1 It is a schematic diagram of the system structure of the present invention.

[0038] The present invention provides a PAS system based on multi-photoacoustic cell active noise reduction, comprising:

[0039] Noise light source DFB laser 1, measurement light source DFB laser 2, beam combiner 3, collimating lens 4, first window 5, first photoacoustic cell 6, dichroic mirror 7, second photoacoustic cell 8, second window 9;

[0040] The measuring light source DFB laser 2 emits a measuring light source, and the noise light source DFB laser 1 emits a noise light source. The two light sources are coupled by the optical fiber combiner 3 and then emitted. After being collimated by the collimating lens 4, they are emitted into the first photoacoustic cell 6 through the first window 5. After passing through the dichroic mirror 7, the light of the measuring light source is reflected back along the original path to enhance the photoacoustic effect. The light of the noise light source passes through the dichroic mirror 7, is emitted into the second photoacoustic cell 8, and finally emitted through the second window 9.

[0041] It should be noted that the photoacoustic effect refers to the phenomenon that mechanical waves are generated when a substance is irradiated with light of periodic intensity modulation. When a certain medium is irradiated with light, the absorption of light by the medium will change its internal temperature, thereby causing changes in the structure and volume of certain areas within the medium; when a pulsed light source or a modulated light source is used, the rise and fall of the medium's temperature will cause the volume of the medium to expand and contract, thereby radiating mechanical waves outward. This phenomenon is called the photoacoustic effect.

[0042] In the present invention, when the optical fiber of the measuring light source is reflected back along the original path, the effect is further enhanced, which helps to improve the resolution and penetration of spectral imaging.

[0043] The system further includes: a connecting air pipe 10 , which is connected to the first photoacoustic cell 6 and the second photoacoustic cell 8 .

[0044] It should be noted that the connecting air pipe 10 is used to connect the air path to ensure that when the gas to be measured enters the photoacoustic cell, the gas components in the two photoacoustic cells are consistent. In addition, it also ensures that the noise generated by the noise light source DFB laser 1 in the two photoacoustic cells is consistent.

[0045] The first window 5 , the second window 9 , and the dichroic mirror 7 have the same material and size.

[0046] It should be noted that the same material and size of the above three structures can ensure that the window noise generated after the light passes through is consistent.

[0047] As an embodiment, the first window 5 and the second window 9 in the present invention are made of BK7, with a circular surface shape of one inch diameter and a thickness of 3 mm. The dichroic mirror 7 is made of fused quartz and has the same size as the first window 5 and the second window 9 .

[0048] The surfaces of the first window 5 and the second window 9 are coated with anti-reflection films.

[0049] As an embodiment, after the first window 5 and the second window 9 are coated with an antireflection film, the transmittance in the 1500nm-1700nm band is greater than 99.5%, thereby reducing the window noise generated by the window absorption. The specific effect is as follows: Figure 2 shown.

[0050] The surface of the dichroic mirror 7 is coated with an optical film layer that reflects the measurement light source and transmits the noise light source. The dichroic mirror divides the light beam into transmitted light and reflected light according to the wavelength range. The transmission band and the reflection band of the short-wave dichroic mirror are separated by the cut-off wavelength. For this dichroic mirror, the band less than the cut-off wavelength has a high transmittance, and the band greater than the cut-off wavelength has a high reflectivity.

[0051] It should be noted that the film layer can also adjust the transmission and reflection wavelength range of the optical film layer of the dichroic mirror 7 according to the absorption wavelength of the gas to be measured. Figure 3 shown.

[0052] The first photoacoustic cell 6 and the second photoacoustic cell 8 are made of the same material, are connected in the same direction, and a dichroic mirror 7 is sealed at the connection.

[0053] It should be noted that the first photoacoustic cell 6 and the second photoacoustic cell 8 are photoacoustic cells of differential structure, and have a pair of acoustic resonant cavities with the same size and symmetrical positions.

[0054] It should be noted that the noise light source DFB laser 1 and the measurement light source DFB laser 2 perform signal synchronization processing, and the acoustic detectors in the first photoacoustic cell 6 and the second photoacoustic cell 8 perform signal synchronization processing.

[0055] The purpose of synchronization processing is to ensure that the noise generated by the noise light sources in the two photoacoustic cells is consistent.

[0056] When the system of the present invention performs photoacoustic measurement, by deducting the noise generated by the noise light source DFB laser 1 in the second photoacoustic cell 8, the noise generated by external noise, photoacoustic cell boundary effect, and molecular relaxation effect in the first photoacoustic cell 6 can be simultaneously eliminated.

[0057] On the other hand, the present invention reduces the noise to the limit through an orthogonal phase-locked algorithm. The orthogonal phase-locked algorithm is an algorithm used for signal processing, and is often used to measure and extract the amplitude and phase information of a specific frequency component in an input signal.

[0058] Finally, the test results of the present invention are as follows:

[0059] During the measurement process, the gas in the photoacoustic cavity absorbs light energy and generates sound, which is detected by the microphone and converted into a voltage signal. Since this signal is very weak and contains strong noise, it needs to be amplified and initially filtered before further processing. The voltage signal is first amplified and then converted into a digital signal, and then phase-locked integration operation is performed through DSP.

[0060] Please refer to Figure 4-6 , Figure 4 It is the waveform of conventional phase-locked amplification; Figure 5 It is a fitting curve derived from the AD value after active noise reduction combined with phase-locked amplification of the present invention; Figure 6 It is a comparison curve diagram of conventional phase locking and orthogonal phase locking of the present invention.

[0061] It is easy to know that if the noise in the signal is completely filtered out, Figure 4 , both fitting lines in 5 should be straight lines, but Figure 4 ,5 The fitted curves all have different degrees of fluctuation, indicating that the processed signals still contain noise.

[0062] In order to compare the noise reduction effects of the two signal processing methods, the data fitting curves after processing by the two methods are compared. Figure 6 It can be seen from the figure that at the beginning of the detection, the photoacoustic signal starts from nothing and the signal is unstable, and the AD value fluctuates greatly. After the measurement system stabilizes, the signal data tends to be stable. Figure 6 The black line in the middle represents the data after noise reduction by a single digital phase-locked amplifier, and the red line represents the data after active noise reduction. It can be seen that compared with the traditional method, the data after active noise reduction has a smaller volatility. This is because the active noise reduction technology filters out some noise that the phase-locked amplifier cannot filter out, and can better track the change trend of the data before noise reduction. The concentration information in the photoacoustic cavity is inverted and calculated by the computer through the collection of AD values. Therefore, the fluctuation of the concentration value measured by the method proposed in the present invention is significantly smaller than that of the traditional method. That is, the method proposed in this article can reduce the noise in the photoacoustic signal and effectively improve the measurement accuracy of the photoacoustic spectroscopy sensor for online monitoring of low-concentration gases.

[0063] The beneficial effects of the present invention are as follows: by introducing a noise light source for active noise reduction, utilizing the photoacoustic effect of the noise light source, deducting external noise interference and the noise generated by the boundary effect and relaxation effect inside the gas pool itself through signal synchronization processing, and then combining the orthogonal phase-locked algorithm to reduce the noise to the limit, thereby improving the system signal-to-noise ratio, thereby significantly improving the detection lower limit of the entire system.

[0064] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

[0065] It should also be noted that the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, commodity or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, commodity or device. In the absence of more restrictions, the elements defined by the sentence "comprises a ..." do not exclude the existence of other identical elements in the process, method, commodity or device including the elements.

Claims

1. A PAS system based on multi-photoacoustic cell active noise reduction, characterized in that: include: A noise light source DFB laser (1), a measurement light source DFB laser (2), a beam combiner (3), a collimating lens (4), a first window (5), a first photoacoustic cell (6), a dichroic mirror (7), a second photoacoustic cell (8), and a second window (9); The measuring light source DFB laser (2) emits a measuring light source, and the noise light source DFB laser (1) emits a noise light source. The two light sources are coupled by an optical fiber combiner (3) and then emitted. After being collimated by a collimating lens (4), the light is emitted through a first window (5) and then emitted into a first photoacoustic cell (6). After passing through a dichroic mirror (7), the light of the measuring light source is reflected back along the original path to enhance the photoacoustic effect. The light of the noise light source is emitted through the dichroic mirror (7), then emitted into a second photoacoustic cell (8) and finally emitted through a second window (9); The surface of the dichroic mirror (7) is coated with an optical film layer that reflects the measurement light source and transmits the noise light source; The dichroic mirror (7) separates the light beam into transmitted light and reflected light according to the wavelength range; the dichroic mirror (7) separates the short-wave transmission band and the reflection band by a cut-off wavelength; The first photoacoustic cell (6) and the second photoacoustic cell (8) are made of the same material, are connected in the same direction, and a dichroic mirror (7) is sealed at the connection point; The first photoacoustic cell (6) and the second photoacoustic cell (8) are photoacoustic cells of differential structure, each having a pair of acoustic resonance cavities of the same size and symmetrical positions.

2. A PAS system based on multi-photoacoustic cell active noise reduction as claimed in claim 1, characterized in that: Also includes: A connecting air tube (10) is connected to the first photoacoustic cell (6) and the second photoacoustic cell (8).

3. A PAS system based on multi-photoacoustic cell active noise reduction as claimed in claim 1, characterized in that: The first window (5), the second window (9) and the dichroic mirror (7) have the same material and size.

4. A PAS system based on multi-photoacoustic cell active noise reduction as claimed in claim 1, characterized in that: The surfaces of the first window (5) and the second window (9) are coated with an anti-reflection film.

5. A PAS system based on multi-photoacoustic cell active noise reduction as claimed in claim 1, characterized in that: The noise light source DFB laser (1) and the measurement light source DFB laser (2) perform signal synchronization processing, and the acoustic detectors in the first photoacoustic cell (6) and the second photoacoustic cell (8) perform signal synchronization processing.

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