Cell structure for photoacoustic gas sensor and signal processing circuit

Abstract

The present invention relates to a unique photoacoustic sensor (PAS) cell structure for a photoacoustic gas sensor, and a signal processing circuit for processing an output signal of a gas sensor.

Description

Cell structure and signal processing circuit for photoacoustic gas sensors

[0001] The present invention relates to a unique photoacoustic sensor cell (PAS cell) structure for a photoacoustic gas sensor and a signal processing circuit for processing the output signal of the gas sensor.

[0002] Since the photoacoustic effect was discovered by Bell in the 1880s, various studies have been conducted to detect gases using acoustic signals generated through the interaction of light and gas. Initially, large equipment and high costs were required, but recently, thanks to the development of MEMS technology and small light sources, applications as low-cost, small sensors are also being made.

[0003] The prerequisite for the photoacoustic effect is the absorption of light (photons) by particles, and particles in an excited state after absorbing photons release thermal energy through a non-radiative relaxation process. This released thermal energy increases the surrounding temperature and pressure through localized heating.

[0004] Therefore, when the intensity of the incident light is periodically modulated, infrared rays absorbed by the gas release energy as heat through a non-optical relaxation process, and this heat causes thermal vibrations in surrounding gas molecules, resulting in pressure changes according to compression and expansion cycles, and pressure changes can be detected through a sound pressure detection means such as a microphone (MIC).

[0005] At this time, if a microphone (MIC) is placed within a resonant structure (i.e., an acoustic resonator) having a resonant frequency identical to the modulation frequency of the incident light, a photoacoustic signal amplified by acoustic resonance can be obtained.

[0006] The sound pressure signal in a photoacoustic gas sensor is explained by the following mathematical equation 1.

[0007]

[0008] Here ε is the adiabatic coefficient of the gas, L is the optical path, Q is the signal quality factor, Vc is the volume of the photoacoustic gas sensor cell, ω is the angular frequency, Ms is the sensitivity of the acoustic sensor, ε represents the infrared absorption coefficient of the gas in a specific wavelength range, N represents the gas density, Cm represents the concentration of the gas being measured, and P0 represents the energy of the irradiated light. Therefore, the output signal of the photoacoustic gas sensor is the gas absorption coefficient ( It exhibits a characteristic that increases proportionally as the optical path (L), signal quality (Q), acoustic sensor sensitivity (Ms), and irradiated light energy (P0) increase, and increases as the cell volume (Vc) and angular frequency (ω) decrease.

[0009] Physically, the gas absorption coefficient ( Since the ) cannot be controlled, from the perspective of system design, a method must be devised to secure high sensitivity (Ms) and optical energy (P0) of the acoustic sensor, increase the optical path (L) while reducing the volume (Vc) of the gas sensor cell, and lower the angular frequency (ω) in the resonance structure or signal processing process while improving the signal quality (Q). In addition, since the sound pressure change caused by gas concentrations below ppm (sub-ppm) is an ultrafine signal in the range of pV to nV, it must be possible to separate and detect ultrafine signals of a certain period that exist within external noise (e.g., power noise, vibration, thermal noise, etc.).

[0010] In addition, a signal processing circuit that uses the output from a non-dispersive infrared gas sensor or a photoacoustic gas sensor as an input signal, amplifies it, removes noise, and provides an output voltage cannot accurately measure gas concentration if the frequency and phase of the measured sound pressure are not precisely aligned; therefore, it is necessary to provide a reference signal (Vref.) through accurate frequency measurement along with phase alignment.

[0011] Additionally, a method to continuously measure arbitrary frequencies occurring within the resonant structure needs to be devised to provide a reference signal in the absence of gas or during the measurement preparation process.

[0012] Accordingly, the present invention aims to provide a PAS cell structure that maximizes sensing sensitivity by combining a resonator with a high-efficiency optical waveguide (i.e., eliminating noise from a long optical path and the surrounding environment, and amplifying the signal).

[0013] In addition, the present invention aims to provide a signal processing circuit capable of providing a reference signal by continuously measuring an arbitrary frequency during the measurement preparation process, while simultaneously amplifying and effectively extracting a minute signal provided by a gas sensor during measurement.

[0014] To achieve the above objective, a signal processing circuit according to one embodiment of the present invention is a signal processing circuit for amplifying an input signal from a gas sensor with a low-noise amplifier (LNA) and filtering the amplified signal to output a DC voltage proportional to the gas concentration, comprising: a frequency-voltage converter that generates a voltage signal proportional to the frequency of the input signal; an MCU that provides two reference signals having exactly the same frequency as the voltage signal input from the frequency-voltage converter and having a phase difference of 90 degrees from each other; two mixers that mix each reference signal and the input signal to generate output signals of the following cos component and sin component, respectively; and a low-pass filter connected to each mixer to generate output signals Vx and Vy, respectively.

[0015] In addition, the gas sensor output (Asig.), which is the amplitude of the input signal, and the change in phase difference (θ) are given by the following equation,

[0016] ,

[0017] It is obtained as.

[0018] Meanwhile, a photoacoustic gas sensor cell according to one embodiment of the present invention, in a photoacoustic gas sensor cell (PAS cell) for a gas sensor that provides an input signal to a signal processing circuit, comprises: an optical waveguide having a white cell structure; a resonator group composed of one or more resonators formed vertically on the upper surface of the optical waveguide; and a sound pressure detection means for measuring sound pressure generated within the resonators, wherein the resonator group comprises two resonators, one each attached to a front top region and a back top region in the optical waveguide having a white cell structure where the sound pressure is relatively high.

[0019] In addition, the resonator group may further include two resonators, one attached to the front bottom region and the other to the back bottom region, where the sound pressure is relatively high, in the optical waveguide of the white cell structure.

[0020] Additionally, it further includes a sound wave generating means, such as a microspeaker or a resonant element mounted within the PAS cell, wherein the sound wave generating means is driven at a final frequency generated by the MCU when the gas to be analyzed is present.

[0021] According to the present invention, a PAS cell structure is provided that maximizes sensing sensitivity by combining a resonator with an optical waveguide of a high-efficiency structure (i.e., eliminating noise from a long optical path and the surrounding environment, and amplifying the signal).

[0022] In addition, according to the present invention, a signal processing circuit is provided that has a function of continuously measuring an arbitrary frequency during the measurement preparation process to provide a reference signal, and simultaneously processes the output signal of a gas sensor during measurement, and accurately extracts a minute signal component having a specific frequency inherent in the noise to calculate the gas concentration.

[0023] FIG. 1 is a configuration diagram of a photoacoustic gas sensor according to one embodiment of the present invention.

[0024] FIG. 2 is a configuration diagram of a PAS cell according to one embodiment of the present invention.

[0025] Figure 3 is the result of a computer simulation of the sound pressure change of the PAS cell in Figure 2.

[0026] Figure 4 is a graph showing the change in sound pressure according to the energy of the light source in the PAS cell of Figure 2.

[0027] FIG. 5 is a configuration diagram of a PAS cell according to another embodiment of the present invention.

[0028] Figure 6 is a graph showing the magnitude of sound pressure in each region where the resonator is located in the PAS cell of Figure 5.

[0029] Figure 7 is a graph showing the phase change according to the frequency change of sound pressure in each region where the resonator is located in the PAS cell of Figure 5.

[0030] FIG. 8 is a configuration diagram of a PAS cell according to another embodiment of the present invention.

[0031] Figure 9 is a graph showing the magnitude of sound pressure in each region where the resonator is located in the PAS cell of Figure 8.

[0032] Figure 10 is a graph showing the phase change according to the frequency change of sound pressure in each region where the resonator is located in the PAS cell of Figure 8.

[0033] FIG. 11 is a block diagram of a signal processing circuit for a photoacoustic gas sensor according to one embodiment of the present invention.

[0034] Figure 12 is a tuneable phase shifter circuit used in the circuit of Figure 11.

[0035] FIG. 13 is a block diagram of a signal processing circuit for an improved photoacoustic gas sensor according to another embodiment of the present invention.

[0036]

[0037] The terms used herein are used merely to describe specific embodiments and are not intended to limit the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. Terms such as “comprising,” “having,” or “equipping” in this specification are intended to indicate the existence of the features, numbers, steps, actions, components, parts, or combinations thereof described herein, and should not be understood as precluding the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof.

[0038] Unless otherwise defined in this specification, all terms used herein, including technical or scientific terms, have the same meaning as generally understood by those skilled in the art to which the present invention pertains.

[0039] Terms such as those defined in commonly used dictionaries should be interpreted as having meanings consistent with their meanings in the context of the relevant technology, and should not be interpreted in an ideal or overly formal sense unless explicitly defined in this specification.

[0040] Hereinafter, the present invention will be described in detail through embodiments of the present invention with reference to the attached drawings.

[0041]

[0042] FIG. 1 is a configuration diagram of a photoacoustic gas sensor according to one embodiment of the present invention.

[0043] The photoacoustic gas sensor of FIG. 1 is prepared by preparing an optical waveguide with an efficient structure having a long optical path or an optical focusing structure, and combining a resonator to amplify the photoacoustic effect on one side thereof to maximize sensing sensitivity. A sound pressure detection means, such as a microphone (MIC), is provided at the end of the resonator to detect pressure changes. In the present invention, the optical waveguide structure equipped with a resonator is referred to as a photoacoustic sensor cell (PAS cell).

[0044] A light source is mounted on the PAS cell, and a circuit (not shown) for signal processing is combined to form a photoacoustic gas sensor. Unlike in Fig. 1, the gas inlet and gas outlet formed in the optical waveguide of the PAS cell can be designed at any location on the side of the PAS cell.

[0045] Meanwhile, a photoacoustic gas detector is mounted on one side of the optical waveguide of the PAS cell. The gas detector of Fig. 1 is manufactured with a TO-8 metal package structure, but it may have a different package structure. The detector of Fig. 1 can be manufactured by forming a broadband window that passes infrared rays with wavelengths of 2 to 15 μm by mounting a CaF2 filter inside the metal package, injecting the gas to be measured through the inlet and outlet, and sealing the package through processes such as welding and cutting.

[0046] When a gas detector absorbs infrared radiation, the enclosed target gas absorbs infrared radiation of a specific wavelength. During the transition from the excited state to the ground state, it releases heat, causing the surrounding target gas to vibrate. An internal microphone (MIC) measures this vibration and provides a signal externally. At this time, the microphone provides a signal of a specific frequency; this signal is amplified and noise is removed through a signal processing circuit, after which it is output as a DC voltage through a modulation process.

[0047]

[0048] FIG. 2 is a configuration diagram of a PAS cell according to one embodiment of the present invention.

[0049] From the perspective of system design, the PAS cell should be configured to secure high sensitivity (Ms) and light energy (P0) of the acoustic sensor, increase the optical path (L), and reduce the volume (Vc) of the gas sensor cell.

[0050] The PAS cell of FIG. 2 has a long optical path and a resonator for amplifying the photoacoustic effect is coupled to one side thereof to maximize sensing sensitivity. A photoacoustic gas detector of FIG. 1 may be mounted at the end of the optical waveguide of the PAS cell. Light emitted from an IR light source passes through an optical waveguide having a long optical path, such as a white-cell structure, and is absorbed by the target gas, thereby reducing the light intensity; accordingly, the output of the photoacoustic gas detector at the end of the optical waveguide is also reduced.

[0051] If a specific resonator—that is, a type of small-volume pipe—is formed vertically or horizontally on one side of an optical waveguide and a microphone (MIC) for sound pressure measurement is mounted at the end of the pipe, sound pressure having a magnitude proportional to the optical intensity (Po) can be detected within the resonator. The microphone (MIC) mounted on the resonator provides a signal measuring the minute sound pressure generated when the optical path is amplified.

[0052] Through this configuration, a resonator is formed perpendicularly to an optical waveguide with a long optical path, thereby securing a PAS cell with a significantly extended optical path. The optical waveguide and the resonator form a PAS cell with a T-shaped cross-section, and the resonator can be formed horizontally on one side of the optical waveguide (i.e., parallel to the normal direction of that side) or perpendicularly on the upper surface of the optical waveguide (i.e., in the normal direction of the upper surface as shown in Fig. 2).

[0053] According to this structure, if the target gas is not present in the PAS cell, the indirect detector—that is, the photoacoustic gas detector at the end of the optical waveguide—outputs a constant voltage; however, if the target gas is present in the PAS cell, sound pressure is generated inside the resonator, a specific sound pressure is measured by the direct detector (i.e., the microphone at the end of the pipe), and the indirect gas detector at the end of the optical waveguide exhibits a reduced output due to the decrease in light intensity.

[0054] In FIG. 2, the gas sensor consists of an optical waveguide with a white cell structure, a resonant tube formed vertically on its upper surface, a buffer volume, and a microphone (MIC) for measuring sound pressure inside the buffer volume. This structure corresponds to a direct photoacoustic gas sensor, and in addition to the white cell structure, a Herriott cell structure may be used to extend the optical path while keeping the volume of the optical waveguide small. The buffer or buffer volume is a space for maintaining the sinusoidal pressure distribution generated within the resonator in the form of a standing wave, and refers to a volume designed and applied according to boundary conditions.

[0055]

[0056] Figure 3 is the result of a computer simulation (COMSOL-Multiphysics) of the sound pressure change of the PAS cell in Figure 2.

[0057] Figure 3 shows the magnitude of the sound pressure generated across the entire PAS cell. It can be observed that relatively high sound pressure is generated in the edge region where the mirror is mainly located in the white cell structured optical waveguide (i.e., the front red region and the rear blue region), while relatively low sound pressure is generated in the center of the cell. The rear region of the optical waveguide (i.e., the blue region) shows a negative sound pressure value, which implies a phase inversion and can be considered as a region where negative (-) pressure is generated compared to the positive (+) pressure in the front region (i.e., the red region).

[0058]

[0059] Figure 4 is a graph showing the change in sound pressure according to the energy of the light source in the PAS cell of Figure 2.

[0060] Figure 4 shows the magnitude of sound pressure according to the energy of the light source, and the simulation results show that as the energy of the light source increases, the sound pressure increases linearly as presented in Equation 1.

[0061] Based on the results of FIGS. 3 and FIGS. 4, it is expected that an optimized PAS cell can be designed by considering the change in sound pressure generated according to the position of the resonator, and that the performance of the photoacoustic gas sensor can be improved by applying this. This will be explained in detail with reference to FIGS. 5 and FIGS. 8.

[0062]

[0063] FIG. 5 is a configuration diagram of a PAS cell according to another embodiment of the present invention, FIG. 6 is a graph showing the magnitude of sound pressure in each region where a resonator is located in the PAS cell of FIG. 5, and FIG. 7 is a graph showing the phase change according to the frequency change of sound pressure in each region where a resonator is located in the PAS cell of FIG. 5.

[0064] The PAS cell of Fig. 5 is a case in which two resonators (i.e., a front top resonator and a back top resonator) are attached, one each, to the region with the highest sound pressure (i.e., the location of the highest positive (+) pressure and negative (-) pressure) in an optical waveguide with a white cell structure.

[0065] Figure 6 shows the magnitude of sound pressure in each resonator region (Front top / Back top), and the sound pressure in the front top is relatively larger than the sound pressure in the back top. Figure 7 shows the phase of sound pressure occurring in the same region, and it can be confirmed that the phase difference between the two sound pressures at the resonant frequency is almost 180 degrees.

[0066] Therefore, by using the PAS cell structure presented in Fig. 5 to measure the sound pressure in each region, calculating the difference between the two sound pressures, and removing background noise, the relative sound pressure can be increased. In other words, the signal caused by ambient noise is primarily removed, and the generated sound pressure can be increased by nearly double.

[0067]

[0068] FIG. 8 is a configuration diagram of a PAS cell according to another embodiment of the present invention, FIG. 9 is a graph showing the magnitude of sound pressure in each region where a resonator is located in the PAS cell of FIG. 8, and FIG. 10 is a graph showing the phase change according to the frequency change of sound pressure in each region where a resonator is located in the PAS cell of FIG. 8.

[0069] The PAS cell of Fig. 8 shows a configuration in which the same resonance structure is installed at the bottom of the PAS cell structure presented in Fig. 5.

[0070] The PAS cell of Fig. 8 is a case in which a total of four resonators (i.e., front top and front bottom resonators and back top and back bottom resonators) are attached, one each at the top and bottom, in the region with the highest sound pressure (i.e., the location of the highest positive (+) pressure and negative (-) pressure) of the optical waveguide with a white cell structure.

[0071] Figure 9 shows the magnitude of sound pressure in each resonator region (Front top / bottom, Back top / bottom). It can be seen that the magnitude of sound pressure at the front top and front bottom is almost similar, and that the back top and back bottom also have almost the same sound pressure. In addition, it can be seen that overall, the sound pressure at the front is relatively larger than the sound pressure at the back.

[0072] However, as shown in Fig. 10, it was confirmed that the phase of the sound pressure in each region showed only a 180-degree phase change according to the position change between the front and back, regardless of the top and bottom.

[0073] Consequently, by calculating the difference in sound pressure at the front top / back top or front bottom / back bottom, respectively, and forming a structure that adds these values, it becomes possible to create an improved structure capable of increasing sensing sensitivity by more than four times.

[0074]

[0075] FIG. 11 is a block diagram of a signal processing circuit for a photoacoustic gas sensor according to one embodiment of the present invention.

[0076] FIG. 11 is a signal processing circuit of a photoacoustic gas sensor presented in the applicant's previously unpublished domestic application No. 10-2024-173545 (filed Nov. 28, 2024), for example, as shown in FIG. 5, which differentially amplifies signals from microphones (Mic1) and (Mic2) that directly measure sound pressure at two resonators (Front top, Back top) using a low-noise amplifier (LNA), and passes the amplified signals through a bandpass filter (BP) to obtain a sound pressure signal.

[0077] The sound pressure signal is provided to a phase sensitive detector (PSD1, PSD2), and after obtaining the second harmonic signal and DC component of the sound pressure signal frequency input through the phase sensitive detector (PSD), a DC voltage output (Vout) proportional to the measured gas concentration is obtained by passing through a low-pass filter (LP1, LP2, LP3).

[0078] In this case, since accurate gas concentration cannot be measured if the frequency and phase of the measured sound pressure signal are not accurately aligned, it is necessary to provide a reference signal (Vref.) through accurate frequency measurement along with phase alignment.

[0079] When the phase of the reference signal is θref. and the phase of the input sound pressure signal is θsig., if there is a difference in the phases of the two signals, the voltage (Vout) output through the phase detector (PSD) and the low-pass filter (LP) will have a phase difference θ (= θsig. - θref.) as shown in Equation 2 below, making it difficult to accurately measure the gas concentration.

[0080]

[0081] Therefore, to eliminate this, a number of variable phase shifters (TPS, QPS) must be included in the signal processing circuit of Fig. 11.

[0082]

[0083] Figure 12 illustrates a tuneable phase shifter circuit used in the circuit of Figure 11.

[0084] This circuit is a feedback voltage (V) corresponding to the DC voltage that has passed through the low-pass filter (LP1) in the circuit of FIG. 11. CTRL Using ), the change in capacitance and the corresponding change in phase are induced as shown in Equation 3 below to correct the phase difference.

[0085]

[0086] However, since the phase change caused by this circuit is small, multiple TPS (Tunnable phase shifters) are required in the signal processing circuit of Fig. 11, which may lead to increased complexity and cost for the entire circuit.

[0087]

[0088] FIG. 13 is a block diagram of a signal processing circuit for an improved photoacoustic gas sensor according to another embodiment of the present invention.

[0089] The improved circuit presented in FIG. 13 compensates for the shortcomings described above in relation to FIG. 11 and FIG. 12, and the signal processing process is explained as follows.

[0090] The sound pressure generated in the PAS cell is measured in the manner described above using the PAS cell of the structure presented in Fig. 5 or Fig. 8.

[0091] When the sound pressure signal from the PAS cell is differentially amplified by a low-noise amplifier (LNA), and the differentially amplified signal is passed through a band-pass filter (BPF) and then amplified through an amplifier (AMP), a specific frequency (f) in A signal having )) can be obtained.

[0092] A frequency-voltage converter (FV) outputs a DC voltage proportional to the corresponding frequency to a microcontroller unit (MCU), and after the MCU accurately analyzes the frequency of the input signal, if it provides two reference signals (e.g., signals with an amplitude of "1" such as a sine wave or square wave) that have exactly the same frequency as the input signal and have a phase difference of 90 degrees from each other to an analog mixer, then each output as shown in Equation 4 can be obtained.

[0093]

[0094] Here, Asig. is the amplitude of the sound pressure signal from the sensor.

[0095] In the above formula, if the phase difference between the sensor's sound pressure signal (θsig.) and the reference signal (θref.) is briefly denoted as θ (= θsig. - θref.), and since the magnitude of Vref. is 1 as previously defined, the average value of the voltage signal passing through the low-pass filters (LP1, LP2) is given by the following Equation 5.

[0096]

[0097] As can be seen from Equation 5, although there is a change in the phase angle θ due to the product of the two signals, the change in the amplitude and phase difference of the sensor output can be calculated using Equation 6 below.

[0098]

[0099] Therefore, even if there is a phase difference (θ) between voltage signals (Vx, Vy), the sensor output (Asig.), which is the amplitude of the input signal, can be accurately calculated. That is, the magnitude of the input signal can be accurately calculated by performing calculation and processing in the MCU using vector rotation (calculation of the inverse rotation matrix of the phase difference θ).

[0100] Meanwhile, the microspeaker illustrated in FIG. 13 is mounted inside the PAS cell and driven by the final frequency generated by the MCU when the gas to be analyzed is absent. This is to ensure continuous sound pressure measurement in the PAS cell even in an atmospheric state where the gas to be analyzed is not present, thereby ensuring active operation and operational stability of the PAS gas sensor system. The function of the microspeaker can be replaced by a resonant element such as a piezoelectric resonator or other types of sound wave generating means.

[0101] In cases where a microspeaker is not installed, the final frequency generated by the MCU when the target gas is present is stored in the MCU, and a method to ensure operational stability by operating the PAS gas sensor system with that frequency when the target gas is subsequently introduced and measurement resumes can also be considered.

[0102]

[0103] Furthermore, the present invention is not limited solely to the embodiments described above, and it is specified that those skilled in the art may add, delete, and modify various configurations within the scope of the technical concept of the present invention as described in the claims.

Claims

1. A signal processing circuit for amplifying an input signal from a gas sensor using a low-noise amplifier (LNA) and filtering the amplified signal to output a DC voltage proportional to the gas concentration, A frequency-voltage converter that generates a voltage signal proportional to the frequency of the input signal; An MCU that provides two reference signals having exactly the same frequency as the voltage signal input from the frequency-voltage converter and having a phase difference of 90 degrees from each other; Two mixers that mix each of the above reference signals and the above input signals to generate output signals of the following cos component and sin component, respectively; and Includes a low-pass filter connected to each of the above mixers to generate output signals Vx and Vy, respectively. Signal processing circuit.

2. In Claim 1, The gas sensor output (Asig.), which is the amplitude of the above input signal, and the change in phase difference (θ) are given by the following equations: obtained as, Signal processing circuit.

3. A photoacoustic gas sensor cell (PAS cell) for a gas sensor that provides an input signal to a signal processing circuit according to claim 1 or 2, Optical waveguide with white cell structure; A resonator group composed of one or more resonators formed in a direction perpendicular to the upper surface of the optical waveguide; and It includes a sound pressure detection means for measuring sound pressure generated within the resonator, The above resonator group comprises two resonators, one attached to the front top region and the other to the back top region, where the sound pressure is relatively high in the optical waveguide of the white cell structure. Photoacoustic gas sensor cell.

4. In Claim 3, The above resonator group further includes two resonators, one attached to each of the front bottom region and the back bottom region, where the sound pressure is relatively high in the optical waveguide of the white cell structure. Photoacoustic gas sensor cell.

5. In Claim 3, It further includes a sound wave generating means mounted within the above PAS cell, and The above sound wave generating means is driven at the final frequency generated by the MCU when the gas to be analyzed is absent, Photoacoustic gas sensor cell.

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