Gas detection device and its calibration method

What is AI technical title?
AI technical title is built by Patsnap AI team. It summarizes the technical point description of the patent document.
The gas detection device addresses phase differences in photoacoustic sensors by using a differential output mechanism and output adjustment to correct sensor signals, enhancing detection sensitivity and accuracy.

JP2025133392APending Publication Date: 2025-09-11HITACHI GLOBAL LIFE SOLUTIONS INC

View PDF 1 Cites 0 Cited by

Patent Information

Application Number: JP2024031310
Authority / Receiving Office: JP · JP
Patent Type: Applications
Current Assignee / Owner
Filing Date: 2024-03-01
Publication Date: 2025-09-11

AI Technical Summary

Technical Problem

Photoacoustic sensors face challenges in achieving accurate detection sensitivity due to phase differences between first and second photoacoustic cells resulting from dimensional variations, which are exacerbated near resonance frequencies.

Method used

A gas detection device with first and second photoacoustic sensors having the same configuration, utilizing a differential signal output mechanism and output adjustment to correct the second sensor's output signal based on a reference gas, ensuring equivalent background noise levels, thereby reducing phase differences and enhancing detection sensitivity.

Benefits of technology

The solution effectively minimizes phase differences between output signals, ensuring sufficient detection sensitivity by correcting the second sensor's output to match the first sensor's background noise, resulting in more accurate differential signals and improved detection performance.

✦ Generated by Eureka AI based on patent content.

Smart Images

Figure 2025133392000001_ABST

Patent Text Reader

Abstract

To provide a gas detection device capable of ensuring sufficient detection sensitivity of a photoacoustic sensor by reducing an influence of a phase difference between output signals of a first photoacoustic cell and a second photoacoustic cell, and to provide its calibration method.SOLUTION: A gas detection device comprises a first photoacoustic sensor for detecting a specific gas component of a measurement gas, a second photoacoustic sensor for detecting a surrounding background noise, difference signal output means for outputting a difference signal by comparing a first output signal of the first photoacoustic sensor and a second output signal of the second photoacoustic sensor, and gas measurement control means for measuring a specific gas component included in a measurement gas on the basis of the difference signal. Output adjustment means for performing correction so as to bring the second output signal closer to the first output signal is provided, and the difference signal output means gives a difference signal between the second output signal corrected by the output adjustment means and the first output signal of the first photoacoustic sensor to the gas measurement control means.SELECTED DRAWING: Figure 4

Need to check novelty before this filing date? Find Prior Art

Description

[Technical Field]

[0001] The present invention relates to a gas detection device that detects components of a gas that causes an odor, and more particularly to a gas detection device that detects an odor using a photoacoustic sensor, and a calibration method for the same. [Background technology]

[0002] Recently, attempts have been made to improve the efficiency of quality control by quantifying odors (type of gas, concentration, etc.) using gas detection devices. Quantifying odors requires a gas detection device that can detect odor components according to the detection purpose. Photoacoustic sensors, which utilize the photoacoustic effect, have been proposed as gas sensors for detecting odor components.

[0003] The photoacoustic effect is a phenomenon in which, when light of a specific wavelength is intermittently (pulsed) irradiated onto molecules of a specific component that make up a gas (the odor molecules to be detected), the molecules of the specific component that absorb the light undergo thermal expansion and contraction, generating acoustic waves.

[0004] The photoacoustic sensor works by detecting acoustic waves generated by the thermal expansion and contraction of molecules that absorb light using a microphone. Therefore, the microphone may detect environmental sounds (hereinafter referred to as background noise) generated in the surrounding environment. Therefore, it is necessary to suppress the influence of this background noise. For example, Japanese Patent Laid-Open Publication No. 2021-179332 (Patent Document 1) describes the following:

[0005] Patent Document 1 discloses a gas detection device that includes a first photoacoustic sensor (hereinafter sometimes referred to as a measurement cell) into which air containing odor molecules is introduced and which is equipped with a first microphone that detects acoustic waves, and a second photoacoustic sensor (hereinafter sometimes referred to as a reference cell) into which air that does not contain odor molecules (or contains only negligible amounts of odor molecules) is introduced and which is equipped with a second microphone that detects acoustic waves, and that is equipped with a sensing circuit that detects odor molecules based on the difference between the detection signal of the first microphone and the detection signal of the second microphone.

[0006] In this way, by taking the difference between the detection signal from the first photoacoustic cell and the detection signal from the second photoacoustic cell, an output signal is obtained that is free of background noise, thereby suppressing the influence of background noise contained in the detection signal from the first photoacoustic cell. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2021-179332 Summary of the Invention [Problem to be solved by the invention]

[0008] Incidentally, it is important that the first and second photoacoustic cells have the same structure, dimensions, etc., so that the background noise generated in the first and second photoacoustic cells can be considered equivalent. However, photoacoustic cells need to be small, and structural constraints can cause variations in dimensions, resulting in variations in the output signals of the first and second photoacoustic cells (this will be briefly explained in an embodiment described later).

[0009] In particular, it was found that dimensional variations cause a phase difference between the output signals. Therefore, even if the difference between output signals at the same resonance frequency is calculated, the phase difference makes it impossible to obtain an accurate difference, resulting in a problem of deterioration in detection sensitivity (minimum sensitivity).

[0010] An object of the present invention is to provide a gas detection device that can reduce the influence of the phase difference between the output signals of a first photoacoustic sensor and a second photoacoustic sensor and ensure sufficient detection sensitivity of the photoacoustic sensor, and a calibration method thereof. [Means for solving the problem]

[0011] The present invention provides a gas detection device comprising a first photoacoustic sensor for detecting a specific gas component in a measurement gas, a second photoacoustic sensor having the same configuration as the first photoacoustic sensor for detecting ambient background noise, differential signal output means for comparing a first output signal of the first photoacoustic sensor with a second output signal of the second photoacoustic sensor and outputting a differential signal, and gas measurement control means for detecting the specific gas component based on the differential signal, wherein an output adjustment means for correcting the second output signal is connected to the differential signal output means, and the gas measurement control means has a function of determining and storing a correction term for correcting the second output signal so that the second output signal approaches the first output signal, based on the first output signal of the first photoacoustic sensor when the first light source of the first photoacoustic sensor is caused to emit light in the presence of a reference gas not containing the specific gas component, and the second output signal of the second photoacoustic sensor in the presence of the reference gas, and wherein in a measurement mode for detecting the specific gas component, the output adjustment means corrects the second output signal using the correction term stored in the gas measurement control means. [Effects of the Invention]

[0012] According to the present invention, it is possible to reduce the influence of the phase difference between the output signals of the first photoacoustic cell for measurement and the second photoacoustic cell for reference, thereby ensuring sufficient detection sensitivity of the photoacoustic sensor. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a configuration diagram illustrating the configuration of a measurement photoacoustic cell and a reference photoacoustic cell, which are the premise of the present invention, and the principle of detecting the difference between their output signals. FIG. [Figure 2] 2 is an explanatory diagram illustrating a phase difference of an output signal in the configuration shown in FIG. 1. FIG. [Figure 3] FIG. 10 is an explanatory diagram illustrating a change in minimum sensitivity due to a phase difference. [Figure 4] 1 is a configuration diagram showing the configuration of a gas detection device according to an embodiment of the present invention. [Figure 5] FIG. 5 is a flowchart illustrating a method for calibrating the gas detection device in FIG. [Figure 6] FIG. 6 is an explanatory diagram illustrating the effect of the calibration in FIG. 5. [Figure 7] FIG. 6 is an explanatory diagram illustrating a convergence condition for the determination processing step in the flowchart in FIG. 5. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The embodiment is an example for explaining the present invention, and for the sake of clarity, some details have been omitted and simplified as appropriate. The present invention can also be implemented in various other forms.

[0015] First, the configurations of the first and second photoacoustic sensors will be described with reference to FIG.

[0016] The first photoacoustic sensor 10A and the second photoacoustic sensor 10B have substantially the same configuration and the same dimensional relationship. This allows the background noise generated in the first photoacoustic cell 10A and the second photoacoustic cell 10B to be considered equivalent. Since the first photoacoustic sensor 10A and the second photoacoustic sensor 10B have the same configuration, they will be collectively described as the same photoacoustic sensor. Here, the reference numerals are assigned "A" and "B" to indicate the first photoacoustic sensor 10A and the second photoacoustic sensor 10B, respectively.

[0017] The first photoacoustic sensor 10A and the second photoacoustic sensor 10B are used selectively depending on the type of gas to be detected.

[0018] For example, when detecting gas component A, the measurement gas is introduced into the first photoacoustic sensor 10A and the second photoacoustic sensor 10B, and the photoacoustic effect is generated in the first photoacoustic sensor 10A using the LED element 17A, which serves as a light source. On the other hand, when detecting gas component B, the measurement gas is introduced into the second photoacoustic sensor 10B and the first photoacoustic sensor 10A, and the photoacoustic effect is generated in the first photoacoustic sensor 10B using the LED element 17B. Therefore, measurement can be performed by the first photoacoustic sensor 10A for a certain period of time, and by the second photoacoustic sensor 10B for another certain period of time.

[0019] In Fig. 1, cell bodies 11A and 11B of photoacoustic cells 10A and 10B are disposed on one surface of flat substrates (e.g., resin substrates) 12A and 12B. Control circuits and the like are mounted on the substrates 12A and 12B. In this manner, the substrates 12A and 12B and the cell bodies 11A and 11B are integrated as an assembly as a whole. Therefore, this assembly can be incorporated into a box made of synthetic resin, metal, or the like, and assembled as a gas detection device.

[0020] The control circuits of the substrates 12A and 12B have the function of controlling the light emission frequency and light intensity of the light source (light emitting means) described later, and the function of measuring acoustic waves from a microphone described later. For this reason, by arranging the cell bodies 11A and 11B on the surfaces of the substrates 12A and 12B on which the control circuits are provided, connection wiring etc. is made easy.

[0021] The cell bodies 11A and 11B include housings 13A and 13B made of a metal or resin material, and detection chambers 14A and 14B having a predetermined volume are formed inside the housings 13A and 13B. The detection chambers 14A and 14B are hollow spaces and are formed with substantially the same shape and spatial volume. For example, the spatial shape of the detection chambers 14A and 14B can be formed in a cylindrical shape or a rectangular parallelepiped shape.

[0022] The detection chambers 14A, 14B are formed with first through-holes 15A, 15B that fluidly connect the detection chambers 14A, 14B formed therein with the outside, and the substrates 12A, 12B are formed with second through-holes 16A, 16B that fluidly connect the detection chambers 14A, 14B with the outside.

[0023] The first through-holes 15A, 15B and the second through-holes 16A, 16B may be one or more, as long as they have an area large enough to smoothly introduce the required amount of gas into the detection chambers 14A, 14B. The through-holes 15A, 15B and 16A, 16B have the same area.

[0024] LED elements 17A and 17B are disposed inside detection chambers 14A and 14B, and emit light in a pulsed manner at a predetermined frequency to thermally excite specific gas components in the measurement gas within detection chambers 14A and 14B. As described above, the light emitted by LED elements 17A and 17B is switched depending on the gas component to be measured. LED elements 17A and 17B are mounted on the front surfaces of substrates 12A and 12B, and are controlled and driven by a control circuit (not shown). Instead of LED elements, a semiconductor laser or the like can be used as the light source.

[0025] Straight resonant ducts 18A and 18B are formed inside housings 13A and 13B and are fluidly connected to detection chambers 14A and 14B. These resonant ducts 18A and 18B have circular cross sections perpendicular to the axes of the resonant ducts 18A and 18B, and have the function of resonating and amplifying minute acoustic waves generated in detection chambers 14A and 14B. This makes it easy to measure the acoustic waves using a microphone, which will be described later.

[0026] Microphones 19A and 19B for detecting acoustic waves are provided on the surfaces opposite to the substrates 12A and 12B where the housings 13A and 13B are in contact. Sound pickup sections 20A and 20B of the microphones 19A and 19B are formed on the substrates 12A and 12B, and these are fluidly connected to the resonance ducts 18A and 18B.

[0027] The output signals of the microphones 19A and 19B are input to a differential amplifier circuit 21. In this case, when the first photoacoustic sensor 10A is used for measurement and the second photoacoustic sensor 10B is used for reference, the operating signal of the differential amplifier circuit 21 becomes a detection output signal from which background noise has been removed. The detection output signal represents the specific gas component to be measured.

[0028] The resonant frequency of each photoacoustic sensor having the above-described configuration is expressed by the following equation (1). f=C / 2π·√S / VL……(1) Here, "C" is the speed of sound, "V" is the volume of the detection chamber, "S" is the cross-sectional area of the resonant duct, and "L" is the length of the resonant duct.

[0029] Photoacoustic sensors must be small, and structural constraints can lead to dimensional variations. Because the detection chambers 14A and 14B have large volumes, the effects of dimensional variations are minimal. However, because the resonant ducts 18A and 18B are narrow passages, manufacturing variations can easily cause dimensional variations in the resonant duct cross-sectional area (S) and length (L).

[0030] Therefore, if there are different dimensional variations in the resonant ducts 18A and 18B of the photoacoustic sensors 10A and 10B, these variations will manifest as variations in the phase of the resonant frequency, as can be seen from equation (1). Note that the phase variations in the output signals are also affected by the resonance of the substrates 12A and 12B.

[0031] Therefore, when gas measurement is actually performed using a photoacoustic sensor, the differential signal fluctuates due to the influence of the phase difference of the output signals described above, and it is expected that the detection sensitivity (minimum sensitivity) will deteriorate.

[0032] FIG. 2 shows the phase difference when a measurement mode for measuring gas components is executed using two photoacoustic sensors with dimensional variations. The horizontal axis represents the light emission frequency of the light source, and the vertical axis represents the phase difference. In this case, the first photoacoustic sensor 10A is used for measurement, and the second photoacoustic sensor 10B is used for reference. Here, the first photoacoustic sensor 10A performs measurement with the LED element 17A emitting light, and the second photoacoustic sensor 10B performs measurement with the LED element 17B not emitting light.

[0033] As shown in Figure 2, the LED element 17A of the first measurement photoacoustic sensor 10A is changed in emission frequency from low to high, and the phase difference between the output signals of the two photoacoustic cells corresponding to the frequency at that time is plotted. As shown in Figure 2, it can be seen that there is a phase variation in the output signals between the two photoacoustic sensors, which have dimensional variations. In particular, in this example, the phase difference variation is large near the resonant frequency.

[0034] Furthermore, Figure 3 shows the frequency characteristics of the minimum sensitivity caused by this phase difference. The minimum sensitivity is correlated with the variation in the phase difference, and degradation of the minimum sensitivity occurs especially near the resonance frequency where the phase difference is large.

[0035] As such, there is a demand for a gas detection device that can reduce the influence of the phase difference between the output signals of two photoacoustic sensors and ensure sufficient detection sensitivity of the photoacoustic sensors. In order to meet this demand, the present invention proposes a gas detection device as shown in the embodiments described below. Although embodiments of the present invention will be described below, the present invention is not limited to these embodiments and can include various modifications within the scope of the technical concept of the present invention.

[0036] 4 shows the configuration of a gas detection device according to an embodiment of the present invention. The following description will mainly focus on the configuration of this embodiment. Other components will be simply described by name and function.

[0037] 4, gas detection device 30 is made up of user interface section 31 and detection section 32. Detection section 32 has the function of detecting gas components and transmitting the detection results to user interface section 31. User interface section 31 also has the function of reporting the detection results (for example, displaying them on a display), displaying the measurement mode and calibration mode, reporting an alarm, and inputting various input commands such as for the measurement mode and calibration mode.

[0038] The user interface unit 31 includes an input unit 33 that inputs various input commands, and a sensor control unit 34 that controls the detection unit 32 based on the various input commands. Furthermore, the sensor control unit 34 has functions such as running the detection unit 32 in a measurement mode or a calibration mode based on the input commands, and transmitting the detected gas components to an output unit 35 based on the detection results sent from the detection unit 32.

[0039] Output unit 35 has the function of sending a status signal to status display unit 36, which indicates whether it is in measurement mode or calibration mode, the function of sending measurement results to measurement result display unit 37, which displays the measurement results of gas components, and the function of sending measurement conditions to measurement condition display unit 38, which displays the current measurement conditions. The operating status of gas detection device 30 can be monitored by checking the display contents of status display unit 36, measurement result display unit 37, and measurement condition display unit 38.

[0040] 1, the detection unit 32 includes a first photoacoustic sensor 10A and a second photoacoustic sensor 10B. In the following, as described above, the first photoacoustic sensor 10A is described as a measurement cell, and the second photoacoustic sensor 10B is described as a reference cell for background noise compensation. Of course, as mentioned above, when measuring different gas components, the photoacoustic sensors can be used in the reverse relationship.

[0041] The photoacoustic sensors 10A and 10B are provided with light sources 16A and 16B (LED elements in this example) and signal receiving units 19A and 19B (microphones in this example). Light source control units 40A and 40B are connected to the LED elements 16A and 16B. A light source control signal (an on / off signal of a predetermined frequency) from a gas measurement control unit 42 is input to the light source control units 40A and 40B, and the LED elements 16A and 16B blink and emit light in synchronization with this. On the other hand, in this embodiment, the second photoacoustic sensor 10B is a reference cell and is therefore not emitting light in the measurement mode.

[0042] The photoacoustic sensors 10A and 10B are connected to output adjusters 41A and 41B (corresponding to output adjusting means in the claims), which are a feature of this embodiment. The output adjusters 41A and 41B have a function of compensating for the influence of the phase difference between the photoacoustic sensors 10A and 10B, respectively. The output signals of the photoacoustic sensors 10A and 10B are input to the output adjusters 41A and 41B connected to them, respectively.

[0043] Here, when measuring a gas component using the first photoacoustic sensor 10A for measurement, the output adjuster 41A does not function, and the output signal of the photoacoustic sensor 10A is output as is. On the other hand, the output signal of the second photoacoustic sensor 10B for reference is adjusted by the output adjuster 41B. The output adjuster 41B corrects the output signal of the second photoacoustic sensor 10B to reduce the influence of the above-mentioned phase difference. This correction method will be described later.

[0044] Furthermore, when measuring gas components using the second measurement photoacoustic sensor 10B, the output adjuster 41B does not function, and the output signal of the photoacoustic sensor 10B is output as is. On the other hand, the output signal of the reference first photoacoustic sensor 10A is adjusted by the output adjuster 41A. The output adjuster 41A corrects the output signal of the first photoacoustic sensor 10A to reduce the influence of the phase difference described above.

[0045] In the measurement mode, the output signal (uncorrected) from the output adjustment unit 41A and the output signal (corrected) from the output adjustment unit 41B are input to a differential single conversion circuit (corresponding to a differential signal output means in the claims) 21 consisting of a differential amplifier circuit, which outputs a differential signal. This differential signal is obtained by subtracting the corrected output signal (background noise N) from the output signal (signal S + background noise N) due to the light emission of the first photoacoustic sensor 10A, which includes background noise, from the corrected output signal (signal S) from the second photoacoustic sensor 10B.

[0046] The detection output signal from differential single conversion circuit 21 is sent to gas measurement control unit 42 and further to sensor control unit 34. The function of sensor control unit 34 is as described above, and executes the display function for status display unit 36, measurement result display unit 37, and measurement condition display unit 38.

[0047] The gas measurement control unit (corresponding to the gas measurement control means in the claims) 42 has a function of detecting a specific gas component contained in the measurement gas based on the differential signal from the differential single conversion circuit 21. It also has a function of calculating a correction term (amplitude adjustment value) to be set in the output adjustment units 41A and 41B and setting the correction term in the output adjustment units 41A and 41B. Therefore, the output signal from the photoacoustic sensor is corrected by the correction term. This correction term is the final correction value after performing calibration, which will be described later.

[0048] Furthermore, gas measurement control unit 42 is connected to correction condition storage unit 43 so as to be able to send and receive data, and stores correction conditions (correction terms due to calibration, which will be described later) for output regulators 41A and 41B. Gas measurement control unit 42 is also connected to measurement condition storage unit 44 so as to be able to send and receive data, and stores the mode execution status, whether measurement mode or calibration mode is being executed, and the measurement conditions. Gas measurement control unit 42 is also connected to data storage unit 45 so as to be able to send and receive data, and stores the detection results regarding the detected gas components.

[0049] In the gas detection device configured as above, a calibration mode that performs correction to reduce the influence of the phase difference between the two photoacoustic sensors 10A and 10B, which is a feature of this embodiment, will now be described.

[0050] In this embodiment, in order to reduce the influence of the phase difference for each frequency between the first photoacoustic sensor 10A and the second photoacoustic sensor 10B, the output signal of the second photoacoustic sensor 10B is corrected so that the output signal when the LED element 16A of the first photoacoustic sensor 10A is made to emit light and the output signal when the LED element 16B of the second photoacoustic sensor 10B is not made to emit light are substantially the same output signal when the first photoacoustic sensor 10A and the second photoacoustic sensor 10B are placed under the same environmental conditions (a reference gas not containing the gas component to be measured / for example, the same air conditions).

[0051] In other words, the output signal generated when the LED element 16A of the first photoacoustic sensor 10A is illuminated in the presence of the reference gas can be considered to be the background noise of the first photoacoustic sensor 10A in measurement mode. Therefore, if the output signal of the second photoacoustic sensor 10B in the presence of the reference gas is corrected to be equivalent to the output signal of the first photoacoustic sensor 10A, the output signals from the two photoacoustic sensors 10A, 10B can be considered to be substantially the same background noise. Therefore, this corrected background noise of the second photoacoustic sensor 10B corresponds to the background noise of the first photoacoustic sensor 10A in measurement mode.

[0052] When measuring a specific gas component in the measurement mode, the background noises from the two photoacoustic sensors 10A and 10B have similar amplitudes, so that a more accurate differential signal can be obtained than in the past.

[0053] In this embodiment, the output signal of the second photoacoustic sensor 10B is corrected by adjusting the amplitude of the output signal. Specifically, the output signal of the second photoacoustic sensor 10B is multiplied by a correction term α (amplitude adjustment) so that the level of the output signal of the second photoacoustic sensor 10B approaches the level of the output signal of the first photoacoustic sensor 10A.

[0054] Next, a method for adjusting the amplitude of the second photoacoustic sensor 10B (calibration method) will be described with reference to the flowchart shown in Fig. 5. The flowchart shown in Fig. 5 shows the flow of the calibration mode, and is mainly a process executed by the computer constituting the gas measurement control unit 42.

[0055] Here, when the calibration mode is executed, a reference gas that does not contain the specific gas component to be measured, for example, a standard gas (nitrogen), is introduced into the detection chambers 14A and 14b (see FIG. 1) of the first photoacoustic sensor 10A and the second photoacoustic sensor 10B, and the same environmental conditions are maintained.

[0056] Furthermore, in order to match the measurement mode, in the calibration mode, the LED element 16A of the first photoacoustic sensor 10A emits light, and the LED element 16B of the second photoacoustic sensor 10B does not emit light.

[0057] Furthermore, there are several methods for determining whether the correction term α has converged to a predetermined appropriate value, but in the following embodiment, we will explain as an example a method in which it is determined that convergence has occurred when the difference between the two output signals becomes equal to or less than a predetermined threshold value.

[0058] <Step S10> In step S10, a photoacoustic sensor to be caused to emit light is selected. In this embodiment, the first photoacoustic sensor 10A is used as the measurement cell, and therefore the first photoacoustic sensor 10A is selected in step S10. In the following, the description will be given assuming that the first photoacoustic sensor 10A is selected. On the other hand, if the second photoacoustic sensor 10B is used as the measurement cell, the second photoacoustic sensor 10B is selected. This selection is input by the input unit 33.

[0059] Next, when the photoacoustic sensor 10A is selected, the process proceeds to step S11. <Step S11> In step S11, the light emission frequency of the first photoacoustic sensor 10A to be emitted is set to a predetermined value, a blinking signal is sent to the LED element 16A, and an output signal is output from the microphone 19A. Here, the output adjuster 41A connected to the first photoacoustic sensor 10A does not function as an adjuster and outputs the output signal as is. In other words, the correction term α is set to "1".

[0060] As described above, the output signal at this time is an output signal due to light emission containing background noise under the reference gas (signal S+background noise N). However, the output signal due to light emission from the first photoacoustic sensor 10A (signal S+background noise N) cannot actually be handled separately as signal S and background noise N, but becomes background noise N as a whole.

[0061] Furthermore, an output signal (background noise N) is output from the microphone 19B of the second photoacoustic sensor 10B, which is not activated, in synchronization with the first photoacoustic sensor 10A. This output signal is corrected by the output adjuster 41B, but since this is the initial state, the correction term α is set to "1".

[0062] Once the output signals of the first photoacoustic sensor 10A and the second photoacoustic sensor 10B have been output, the process proceeds to step S12. As is well known, the output signals are temporarily stored in RAM in order to perform the calculations described below.

[0063] <Step S12> In step S12, the difference (Δ) between the value of the output signal of the first photoacoustic sensor 10A obtained in step S11 and the value of the output signal of the second photoacoustic sensor 10B obtained in step S11 is obtained via the differential single conversion circuit 21.

[0064] Then, it is determined whether the difference (Δ) is smaller than a predetermined threshold (Δref). If it is determined that the difference (Δ) is smaller than the predetermined threshold (Δref) (YES determination), it is determined that the output signal of the second photoacoustic sensor 10B at this light emission frequency does not need to be corrected, and the process proceeds to step S15.

[0065] On the other hand, if it is determined that the difference (Δ) is greater than the predetermined threshold (Δref) (NO determination), it is determined that the output signal of the second photoacoustic sensor 10B at this emission frequency needs to be corrected, and the process proceeds to step S13. Note that step S12 can be omitted because a similar processing step is included in the subsequent step S14.

[0066] <Step S13> In step S13, since it was determined in step S12 that the output signal of the second photoacoustic sensor 10B needs to be corrected, amplitude adjustment is performed. That is, the output signal of the first photoacoustic sensor 10A and a correction term α that brings the output signal closer to the output signal of the first photoacoustic sensor 10A are newly set, and the output signal of the second photoacoustic sensor 10B synchronized with the first photoacoustic sensor 10A is multiplied by the correction term α to output an amplitude-adjusted output signal.

[0067] Here, the new correction term α can be set by using, for example, a gradient method. Once the output signal of the first photoacoustic sensor 10A and the output signal of the second photoacoustic sensor 10B corrected with the new correction term α are output, the process proceeds to step S14.

[0068] <Step S14> In step S14, the difference (Δ) between the value of the output signal of the first photoacoustic sensor 10A obtained in step S13 and the value of the output signal (amplitude adjusted) of the second photoacoustic sensor 10B is obtained via the differential single conversion circuit 21.

[0069] Then, it is determined whether the difference (Δ) is smaller than a predetermined threshold (Δref). If it is determined that the difference (Δ) is smaller than the predetermined threshold (Δref) (YES determination), it is determined that the output signal of the second photoacoustic sensor 10B at this light emission frequency does not need to be corrected, and the process proceeds to step S15.

[0070] On the other hand, if it is determined that the difference (Δ) is still greater than the predetermined threshold (Δref) (NO determination), it is determined that the output signal of the second photoacoustic sensor 10B at this time needs to be recorrected, and the process returns to step S13 again. Then, a new correction term α for recorrection is set by the gradient method described above, and the same operation is executed.

[0071] By repeatedly executing this correction loop, it is finally determined that the difference (Δ) is smaller than the predetermined threshold (Δref) (YES judgment). Then, if it is determined that the output signal of the second photoacoustic sensor 10B does not need to be corrected, the process proceeds to step S15.

[0072] Here, this correction loop is based on the idea that the output signals of the first photoacoustic sensor 10A and the second photoacoustic sensor 10B are considered to be out of sync, and that the output signals of both are brought closer together to compensate for this sync.

[0073] <Step S15> In step S15, the final correction term α is stored in association with the light emission frequency of the first photoacoustic sensor 10A as the final correction term α of the output adjuster 41B of the second photoacoustic sensor 10B that does not emit light. The correction condition storage unit 43 shown in FIG. 4 is allocated as the storage location. The final correction term α is used by the output adjuster 41B to correct the output signal of the second photoacoustic sensor 10B in the actual measurement mode. Once the final correction term α has been stored, the process proceeds to step S16.

[0074] <Step S16> In step S16, it is determined whether or not the final correction terms α for all the required light emission frequencies have been stored. In this determination, the processing steps of steps S11 to S15 described above are executed for a plurality of light emission frequencies, for example, as shown in Fig. 2. In other words, if it is determined in step S16 that all the final correction terms α have not been stored, the process returns to step S11 and executes the same processing steps.

[0075] By performing the processing steps described above, the output signal of the second photoacoustic sensor 10B is multiplied by the correction term α to bring it closer to the output signal of the first photoacoustic sensor 10A (in this case, the overall background noise N).

[0076] Therefore, the output signal (background noise) of the second photoacoustic sensor 10B reflecting the correction term α can be regarded as the output signal (background noise) of the first photoacoustic sensor 10A, and as a result, the influence of the phase difference between the output signals of the two photoacoustic sensors 10A and 10B can be reduced.

[0077] Furthermore, since the adjusted output signal of the second photoacoustic sensor 10B can be regarded as the output signal of the first photoacoustic sensor 10A (overall background noise N), a more accurate background noise can be obtained when removing the background noise from the output signal of the first photoacoustic sensor 10A in the measurement mode. Once step S16 is completed, the process proceeds to step S17.

[0078] <Step S17> In step S17, the normal gas component measurement mode is executed. The correction term α set in the output adjuster 41B of the second photoacoustic sensor 10B is stored in the correction condition storage unit 43 by the calibration mode described above.

[0079] Therefore, in the measurement mode, the correction term α corresponding to the light emission frequency of the LED element 16A of the first photoacoustic sensor 10A is read out and set in the output adjuster 41B of the second photoacoustic sensor 10B, thereby reducing the influence of the phase difference between the output signals of the two photoacoustic sensors 10A and 10B and producing a more accurate differential signal than conventionally possible, thereby improving detection sensitivity.

[0080] Figure 6 shows the change in detection sensitivity (minimum sensitivity) at several emission frequencies in the calibration mode described above. Focusing on the resonance frequency, it can be seen that compared to the initial minimum sensitivity, the minimum sensitivity is improved when the first correction term α is applied, and the minimum sensitivity is further improved when the nth correction term α is applied.

[0081] Although the flowchart described above is for the case where the first photoacoustic sensor 10A is the measurement cell and the second photoacoustic sensor 10B is the reference cell, the same applies to the case where the second photoacoustic sensor 10B is the measurement cell and the first photoacoustic sensor 10A is the reference cell. In this case, the correction term α is set in the output adjuster 41A corresponding to the first photoacoustic sensor 10A.

[0082] Thus, according to this embodiment, the output signal of the first photoacoustic sensor 10A under the reference gas can be regarded as background noise of the first photoacoustic sensor 10A in the measurement mode. Therefore, if the output signal of the second photoacoustic sensor 10B under the same reference gas is corrected to be equivalent to the output signal of the first photoacoustic sensor 10A, the output signals from the two photoacoustic sensors 10A and 10B can be regarded as background noises with amplitudes similar to each other.

[0083] Therefore, when measuring gas components in the measurement mode, the output signal (background noise) of the second photoacoustic sensor 10B is similar to the background noise contained in the output signal of the first photoacoustic sensor 10A in the measurement mode, so a more accurate differential signal can be obtained.

[0084] The timing for executing the calibration mode can be determined arbitrarily. For example, calibration can be performed when the gas detection device is manufactured. This provides the advantage of improved usability, as calibration is completed at the time of manufacture, eliminating the need for calibration at a factory or warehouse.

[0085] In addition, calibration can be performed when the gas detection device is installed in a factory or warehouse. This makes it possible to compensate for changes in background noise depending on the installation environment. Furthermore, calibration can be performed at a predetermined start time (for example, the start time of work every morning). This makes it possible to reflect the latest correction term α.

[0086] Furthermore, in this embodiment, the correction term α is calculated for each of a plurality of emission frequencies in step S16, but it is also possible to calculate the correction term α only for the emission frequency required for gas measurement. For example, the correction term α may be calculated by specifying the resonance frequency of the measurement cell. Furthermore, the correction term α may be calculated by specifying a predetermined frequency in a band of about ±20% centered on the resonance frequency. These settings can be made in steps S11 to S16.

[0087] Furthermore, the convergence conditions in steps S12 and S14 can be set arbitrarily. In this embodiment, the convergence condition is the difference threshold value (Δref) of the output signal, but the convergence condition can also be determined by the number of correction loops n. In other words, the number of times the correction loop is executed can be set, and once correction has been performed that number of times, a determination can be made that convergence has been achieved.

[0088] Furthermore, the convergence condition can be determined from the amount of change between the previous and current values of the differential signal. For example, the convergence condition shown below can be set. The convergence condition is set to 1 / N of the amount of change between the first and next differential signal (Df(2) - Df(1)), and the correction loop is set to complete when 1 / N of the amount of change in the differential signal due to the next correction (Df(n) - Df(n-1)) falls below 1 / N of the amount of change between the first and next differential signal (Df(2) - Df(1)). In other words, 1 / N of the amount of change in the differential signal between the first and next (Df(2) - Df(1)) becomes the convergence judgment threshold.

[0089] Furthermore, using the same concept, it is possible to set the convergence condition using the minimum sensitivity. In this case, the above-mentioned differential signal Df(n) is replaced with the minimum sensitivity Sm(n) and the above-mentioned calculation is performed.

[0090] As shown in the table in Figure 7, the correction term α is derived using the gradient method. Then, the minimum sensitivity and convergence judgment value are found for each correction term α, and this convergence judgment value is compared with the convergence judgment threshold defined as 1 / N of (Sm(2) - Sm(1)) to determine the convergence state.

[0091] For example, in the above formula, if "N=5" and the initial correction term α="1" results in a minimum sensitivity of "14.23", and then the gradient method uses correction term α="0.7" and the minimum sensitivity is "9.961", then "0.8538", calculated as (14.23-9.961) / 5, becomes the convergence judgment threshold for convergence judgment.

[0092] Therefore, in FIG. 7, when the correction term α=0.118, the convergence judgment value becomes 0.717, which is below the convergence judgment threshold value mentioned above, so it can be determined that convergence has occurred.

[0093] In this embodiment, the output signal of the second photoacoustic sensor 10B, which does not emit light, is corrected by the correction term α, in other words, the output signal is made to approximate the output signal (background noise) of the first photoacoustic sensor 10A to reduce the influence of the phase difference, but the influence of the phase difference can also be reduced by other methods. In short, it is sufficient to provide a function that makes the output signal of the second photoacoustic sensor 10B approximate the output signal of the first photoacoustic sensor 10A in the calibration mode.

[0094] As described above, the present invention is configured to correct the output signal of one of the photoacoustic sensors so as to reduce the influence of the phase difference between the output signals of the two photoacoustic sensors. This reduces the influence of the phase difference between the output signals of the measurement photoacoustic sensor and the reference photoacoustic sensor, ensuring sufficient detection sensitivity of the photoacoustic sensor.

[0095] The present invention is not limited to the above-described embodiments, but includes various modifications. The above-described embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those including all of the described configurations. Furthermore, it is possible to replace part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. It is also possible to add, delete, or replace other configurations with respect to the configuration of each embodiment. [Explanation of symbols]

[0096] 10A...first photoacoustic sensor, 10B...second photoacoustic sensor, 11A, 11B...cell body, 12A, 12B...substrate, 13A, 13B...housing, 14A, 14B...detection chamber, 17A, 17B...LED element, 18A, 18B...resonant duct, 19A, 19B...microphone, 21...differential amplifier, 40A, 40B...light source control unit, 41A, 41B...output adjuster, 42...measurement control unit.

Claims

1. a first photoacoustic sensor for detecting a specific gas component of the measurement gas; a second photoacoustic sensor having the same configuration as the first photoacoustic sensor for detecting ambient background noise; a differential signal output unit that compares a first output signal of the first photoacoustic sensor with a second output signal of the second photoacoustic sensor and outputs a differential signal; a gas detection device including a gas measurement control means for detecting the specific gas component based on the differential signal, an output adjusting means for correcting the second output signal is connected to the differential signal output means; the gas measurement control means has a function of determining and storing a correction term for correcting the second output signal from the first photoacoustic sensor when a light source of the first photoacoustic sensor is caused to emit light in a reference gas that does not contain the specific gas component and the second output signal from the second photoacoustic sensor in the reference gas so that the second output signal approaches the first output signal; In the measurement mode for detecting the specific gas component, the output adjustment means corrects the second output signal using the correction term stored in the gas measurement control means. A gas detection device characterized by:

2. 2. The gas detection device according to claim 1, In the measurement mode, the first light source of the first photoacoustic sensor emits light at a predetermined frequency, and the second light source of the second photoacoustic sensor does not emit light. A gas detection device characterized by:

3. 3. The gas detection device according to claim 2, the output adjustment means is provided between the second photoacoustic sensor and the differential signal output means to correct the second output signal; In the measurement mode, the differential signal is obtained by the differential signal output means from the corrected second output signal and the first output signal of the first photoacoustic sensor, and the obtained differential signal is provided to the gas measurement control means. A gas detection device characterized by:

4. 4. The gas detection device according to claim 3, The predetermined frequency is a plurality of frequencies, a resonance frequency, or a plurality of frequencies in a predetermined band centered on the resonance frequency. A gas detection device characterized by:

5. 1. A method for calibrating a gas detection device comprising: a first photoacoustic sensor for detecting a specific gas component in a measurement gas; a second photoacoustic sensor having the same configuration as the first photoacoustic sensor for detecting ambient background noise; differential signal output means for comparing a first output signal of the first photoacoustic sensor with a second output signal of the second photoacoustic sensor to output a differential signal; and gas measurement control means for detecting the specific gas component based on the differential signal, an output adjustment means for correcting the second output signal is provided between the second photoacoustic sensor and the differential signal output means; and The gas measurement control means a first step of obtaining the first output signal of the first photoacoustic sensor when a first light source of the first photoacoustic sensor is caused to emit light at a predetermined frequency in a reference gas that does not contain the specific gas component, and the second output signal of the second photoacoustic sensor when a second light source of the second photoacoustic sensor is not caused to emit light; a second step of comparing the first output signal with the second output signal, and determining and storing a correction term for correcting the second output signal so that the second output signal approaches the first output signal; a third step of setting the stored correction term in the output adjusting means to correct the second output signal; a fourth step of comparing the first output signal with the second output signal corrected in the third step, and when a difference between the corrected second output signal and the first output signal satisfies a predetermined convergence condition, storing the correction term at this time as a final correction term; A method for calibrating a gas detection device, comprising:

6. 6. A method for calibrating a gas detection device according to claim 5, comprising: The fourth step is If the difference is smaller than a predetermined difference threshold, the correction term at this time is stored as the final correction term, and if the difference is larger than the predetermined difference threshold, the correction term at this time is changed to a correction term for re-correction; The third step is to set the correction term for re-correction in the output adjusting means to correct the second output signal.