Photoacoustic gas sensor and gas detection method

Abstract

To provide a photoacoustic gas sensor and a gas detection method capable of suppressing a decrease in detection precision.SOLUTION: A photoacoustic gas sensor (1) includes: a light source (11) for emitting light following a drive signal; a detection section (41) for detecting acoustic waves from a gas molecule of a detected gas absorbing emitted light and outputting a detection signal; a gas cell (40) that has a hole (43) through which gas passes and introduces the gas from the hole to a gas detection space; and a control section (50) for outputting a drive signal so as to change a drive frequency of a light source and at the same time searching a resonance frequency on the basis of an acquired detection signal. The control section outputs a drive signal such that a drive frequency of a light source becomes the searched resonance frequency.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a photoacoustic gas sensor and a gas detection method.

Background Art

[0002] In recent years, development of a gas sensor (gas measuring device) has been underway that includes a light source that emits infrared light and is configured such that the infrared light passes through a gas containing a gas to be detected, and that detects the concentration of the gas to be detected by utilizing the absorption characteristics of the infrared light by the gas to be detected. The gas to be detected is, for example, alcohol or carbon dioxide. For example, Patent Document 1 discloses a photoacoustic gas sensor that measures the gas concentration by picking up the vibration of gas molecules that have absorbed light as sound using a high-performance microphone or the like.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] Here, since a photoacoustic gas sensor picks up sound (acoustic waves that are rough waves) using a microphone or the like, the structure of the photoacoustic gas sensor affects the detection accuracy. For example, Patent Document 2 discloses that a photoacoustic sensor having a housing structure has a resonance frequency of 5 Hz to 25 kHz. In a photoacoustic gas sensor having a housing structure, it is possible to improve the detection accuracy of the gas to be detected by setting the driving frequency of the light source to the resonance frequency. However, the resonance frequency may vary due to changes in the use environment or forces applied during the implementation of the photoacoustic gas sensor, and the detection accuracy may decrease due to the variation in the resonance frequency.

[0005] An object of the present disclosure made in view of such circumstances is to provide a photoacoustic gas sensor and a gas detection method capable of suppressing a decrease in detection accuracy.

Means for Solving the Problems

[0006] (1) A photoacoustic gas sensor according to an embodiment of the present disclosure includes a light source that emits light according to a drive signal, a detection unit that detects an acoustic wave from gas molecules of a gas to be detected that has absorbed the emitted light and outputs a detection signal, a gas cell that has a hole through which gas passes and introduces the gas from the hole into a gas detection space, a control unit that searches for a resonance frequency based on the acquired detection signal while outputting the drive signal so as to change the drive frequency of the light source, and the control unit outputs the drive signal so that the drive frequency of the light source becomes the resonance frequency searched.

[0007] (2) As an embodiment of the present disclosure, in (1), the control unit searches for a resonance frequency at the time of initial startup.

[0008] (3) As an embodiment of the present disclosure, in (1) or (2), the control unit searches for a resonance frequency at a predetermined time.

[0009] (4) As an embodiment of the present disclosure, in any one of (1) to (3), the control unit uses the assumed magnitude of variation of the detection signal as a threshold value, and searches for a resonance frequency when there is a variation magnitude of the detection signal equal to or greater than the threshold value.

[0010] (5) As an embodiment of the present disclosure, in any one of (1) to (4), the control unit corrects the detection signal based on at least one piece of information among the temperature, humidity, and pressure of the gas.

[0011] (6) As one embodiment of the present disclosure, in any of (1) to (5), The control unit calculates a resonance frequency based on at least one piece of information among the temperature, humidity, and pressure of the gas, and searches for the resonance frequency using the calculated resonance frequency.

[0012] (7) As one embodiment of the present disclosure, in any of (1) to (6), The light source is a quantum infrared light emitting element.

[0013] (8) As one embodiment of the present disclosure, in any of (1) to (7), The light source is provided with a built-in sensor on the same element substrate and can measure the light emission amount.

[0014] (9) The gas detection method according to one embodiment of the present disclosure is A gas detection method executed by a photoacoustic gas sensor including a light source that emits light according to a drive signal, a detection unit that detects an acoustic wave from gas molecules of a gas to be detected that has absorbed the emitted light and outputs a detection signal, a gas cell that has a hole through which gas passes and introduces the gas into a gas detection space from the hole, and a control unit, wherein the control unit searches for a resonance frequency based on the acquired detection signal while outputting the drive signal so as to change the drive frequency of the light source, and outputs the drive signal so that the drive frequency of the light source becomes the searched resonance frequency.

Advantages of the Invention

[0015] According to the present disclosure, it is possible to provide a photoacoustic gas sensor and a gas detection method capable of suppressing a decrease in detection accuracy.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

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Figure 7

Figure 8

DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, a photoacoustic gas sensor 1 (see FIG. 1) and a gas detection method according to an embodiment of the present disclosure will be described with reference to the drawings. In each figure, the same or corresponding parts are denoted by the same reference numerals. In the description of the present embodiment, the description of the same or corresponding parts will be omitted or simplified as appropriate.

[0018] (Photoacoustic Gas Sensor) FIG. 1 is a diagram showing the configuration of the photoacoustic gas sensor 1 according to the present embodiment. The photoacoustic gas sensor 1 measures the presence or concentration of a gas to be detected in a gas by the photoacoustic method. In the present embodiment, the photoacoustic gas sensor 1 will be described as a device that measures the concentration of the gas to be detected. The photoacoustic gas sensor 1 includes a light source 11, a detection unit 41, a gas cell 40, and a control unit 50. Further, the photoacoustic gas sensor 1 may include a substrate 30. FIG. 1 is a cross-sectional view showing a cross-section of the photoacoustic gas sensor 1 including these components. Details of the components of the photoacoustic gas sensor 1 will be described later.

[0019] Here, the gas cell 40 has holes 43 through which gas (e.g., ambient air) passes, and the gas is introduced from the holes 43 into the gas detection space 42. The photoacoustic gas sensor 1 measures the concentration of the gas to be detected in the introduced gas. The gas to be detected is a specific gas to be detected, and can be, for example, carbon dioxide, water vapor, carbon monoxide, nitric oxide, ammonia, sulfur dioxide, alcohol, formaldehyde, methane, propane, chlorofluorocarbon, alternative chlorofluorocarbon, R32, R1234y, etc.

[0020] The photoacoustic gas sensor 1 measures the concentration of the gas to be detected in the gas introduced into the gas detection space 42 and outputs an electrical signal indicating the measurement result. The photoacoustic method measures the gas to be detected by picking up the vibration of the gas molecules that have absorbed light as sound (acoustic wave) with a high-performance microphone or the like.

[0021] The photoacoustic gas sensor 1 is configured to include a gas cell 40 on the main surface 30a of the substrate 30. Here, the main surface 30a is one of the surfaces of the substrate 30 having the largest area. As shown in FIG. 1, orthogonal coordinates are set so that the xy plane is parallel to the main surface 30a of the substrate 30. The z-axis direction is the direction perpendicular to the main surface 30a of the substrate 30. The z-axis direction may be referred to as the height direction or the up-down direction. The positive z-axis direction corresponds to the upward direction. In the present embodiment, the light source 11 and the detection unit 41 are arranged inside the gas detection space 42 above the main surface 30a.

[0022] (Substrate) The substrate 30 has a function of holding the gas cell 40, the light source 11, and the detection unit 41. In the present embodiment, the substrate 30 also holds the control unit 50. In the present embodiment, the substrate 30 is a PCB (printed circuit board), on which the light source 11, the detection unit 41, and the control unit 50 are mounted, and electrical connections between them are also made. The material of the substrate 30 is, for example, paper, glass cloth, ceramics, polyimide, liquid crystal polymer, etc.

[0023] (Gas cell) The gas cell 40 has a gas detection space 42 for detecting the gas to be detected. The gas cell 40 is provided on the main surface 30a of the substrate 30. That is, the gas detection space 42 is an internal space surrounded by the outer wall of the gas cell 40 and the substrate 30. The material of the outer wall of the gas cell 40 can be, for example, metal, glass, resin, composite materials thereof, etc. Examples of the resin include phenolic resin, epoxy resin, polyimide resin, bismaleimide triazine resin, fluororesin, polyphenylene oxide resin, LCP (liquid crystal polymer), PP (polypropylene), PEEK (polyether ether ketone), PA (polyamide), PPE (polyphenylene ether), PC (polycarbonate), or PPS (polyphenylene sulfide), PMMA (polymethyl methacrylate resin), PAR (polyarylate resin), etc., and rigid resins obtained by mixing two or more of these. Further, the gas cell 40 is preferably made of a material with a low light absorption coefficient and a high reflectance in order to efficiently confine the light emitted from the light source 11 within the gas cell 40. Specifically, a resin housing coated with an alloy containing aluminum, gold, silver, a dielectric, or a laminate thereof is preferred. When the inner surface of the gas cell 40 is formed by vapor deposition or plating on the resin housing, it is possible to improve high productivity and weight reduction as compared with the case of being formed of a metal material. Also, the gas cell 40 may be formed by cutting, but from the viewpoint of productivity, it is preferably formed by injection molding or press working. Further, the gas cell 40 and the substrate 30 may be mechanically joined by an adhesive, screw, claw, fitting, grommet, welding, solder, etc.

[0024] (Light source) The light source 11 emits light including wavelengths absorbed by the gas to be detected. The light source 11 is also referred to as a light emitting part. The light source 11 may be provided on the main surface 30a of the substrate 30 inside the gas detection space 42. In this embodiment, the light is infrared light. That is, the light source 11 is composed of an infrared light emitting element. The light emitting element emits light according to a drive signal (specific examples are voltage or current pulses). It is preferable that the light emitting element can be driven at high speed. For example, it is preferable that the drive signal is a pulse of 15 kHz or more. The light emitting element may be, for example, an LED (Light Emitting Diode), a laser (Light Amplification by Stimulated Emission of Radiation), a MEMS heater, a lamp, etc. In this embodiment, the light source 11 is an LED (infrared LED) that emits infrared light. Also, the light source 11 is preferably a quantum type infrared light emitting element that emits infrared light using electrons or holes in a semiconductor so that it can be driven at a high frequency.

[0025] Here, the photoacoustic gas sensor 1 may further include a drive part that outputs a drive signal to drive the light emitting element. In this embodiment, the drive part is included in the control part 50. That is, a part of the control part 50 functions as a drive part.

[0026] Also, the light source 11 may be provided with auxiliary members having auxiliary optical functions such as wavelength selection, light collection, scattering, and wavelength conversion. Specifically, the auxiliary members are a wavelength selection filter, a lens, a phosphor, a diffraction grating, etc.

[0027] Also, when the light source 11 is a quantum type light emitting element such as an LED, an optical sensor (built-in sensor) capable of measuring the light emission amount of the light source 11 may be provided on the same element substrate. Since the light source 11 deteriorates according to use, by monitoring the light emission amount, correction of the deterioration becomes possible, and the life of the device can be extended.

[0028] Here, the wavelength of the infrared ray may be 2 μm to 12 μm. The region of 2 μm to 12 μm has many absorption bands specific to various gases and is a wavelength band particularly suitable for use in the photoacoustic gas sensor 1. For example, there are absorption bands of methane at a wavelength of 3.3 μm, carbon dioxide at a wavelength of 4.3 μm, and alcohol (ethanol) at a wavelength of 9.5 μm.

[0029] (Detection unit) The detection unit 41 detects a change corresponding to the abundance of the gas to be detected in measuring the concentration of the gas to be detected present in the gas in the gas detection space 42. The detection unit 41 may be provided on the main surface 30a of the substrate 30 inside the gas detection space 42. The light emitted from the light source 11 passes through the gas detection space 42 and is absorbed according to the abundance of the gas to be detected. The detection unit 41 detects a signal based on the light emitted from the light source 11 and outputs a detection signal. The detection unit 41 is a microphone in this embodiment, but is not limited thereto and may be a barometric pressure sensor or a pressure sensor, etc. Also, in this embodiment, the signal based on light is an acoustic wave (vibration sound) from the gas molecules of the gas to be detected that has absorbed the light. That is, the detection unit 41 detects the acoustic wave and outputs a detection signal. The detection unit 41 preferably has a configuration covered by a metal housing in order to suppress the influence of electromagnetic noise generated when the light source 11 is driven.

[0030] (Gas detection space) The gas detection space 42 has a function of separating the space by an outer wall and accommodating a gas such as air in its internal space. The gas accommodated in the gas detection space 42 is replaced through the hole 43.

[0031] (Hole) The hole 43 is provided in a part of the outer wall (side wall and ceiling) partitioning the gas detection space 42 of the gas cell 40. Gas passes through the hole 43 and the gas in the gas detection space 42 is replaced. There may be a plurality of holes 43. Here, the hole 43 may be provided with a dust-proof filter (dust-proof filter). The dust-proof filter may be, for example, a non-woven fabric, a Teflon (registered trademark) sheet, etc.

[0032] (Control unit) The control unit 50 controls the entire photoacoustic gas sensor 1. Further, the control unit 50 may perform arithmetic processing to obtain the concentration of the gas to be detected. That is, the control unit 50 may calculate the concentration of the gas to be detected based on the detection signal from the detection unit 41. The control unit 50 may be configured to include one or more processors. The processor may be, for example, a general-purpose processor or a dedicated processor specialized for specific processing, but is not limited thereto and can be any processor. The control unit 50 may be provided outside the gas detection space 42 (see FIG. 1) or may be provided inside the gas detection space 42 (see FIG. 5). Further, the control unit 50 may be provided on the main surface 30a of the substrate 30, but may be embedded in the substrate 30, for example, or may be provided on the back surface opposite to the main surface 30a.

[0033] (Photoacoustic Gas Sensor Design) In the photoacoustic gas sensor 1 according to the present embodiment, the gas detection space 42 is partitioned by an outer wall and is provided as an internal space of a structure (gas cell 40) that is close to, for example, a rectangular parallelepiped. Therefore, by utilizing the resonance mode of the gas cell 40, that is, the fact that sound waves of a specific frequency (natural frequency) are greatly amplified, the sound picked up by the microphone (detection unit 41) can be increased. If the sound picked up by the microphone increases, the influence of noise can be reduced, so that the measurement accuracy of the photoacoustic gas sensor 1 can be improved. By setting the driving frequency of the light source 11 to a resonance frequency substantially equal to the natural frequency of the resonance mode of the gas cell 40, the output of the detection unit 41, which is a microphone, can be increased.

[0034] Here, regarding the light emission state of the light source 11, if the light-emitting state is defined as ON and the non-light-emitting state is defined as OFF, the driving frequency of the light source 11 is determined by the interval of ON. In the present embodiment, the light source 11, which is an infrared light-emitting element, emits light so as to repeat ON and OFF according to a driving signal. For example, if the driving signal is a pulse that takes a high level and a low level, the light source 11 turns ON when the driving signal is at the high level and turns OFF when the driving signal is at the low level. Therefore, the driving frequency of the light source 11 can be determined by the driving signal. Here, the duty ratio (the time that is ON with respect to the period) of the driving signal is not limited, but may be set, for example, to be 10% or more and 50% or less.

[0035] FIG. 2 is a diagram illustrating changes in the output of the microphone (detection unit 41) when the driving frequency of the light source 11 is changed. The gas cell 40 used in the experiment of FIG. 2 has a size of 1.06×0.96×0.85 cm 3 and has a hole 43 with a diameter of 1 mm. A driving current of 350 mA is applied to the light source 11, which is an infrared LED, and the duty ratio is 30%. When the driving frequency of the light source 11 was changed by 1 kHz in the range of 16 to 23 kHz, the output of the microphone (detection unit 41) became larger at 21 kHz compared to others. In the example of FIG. 2, the resonance frequency is considered to be 21 kHz.

[0036] (Calibration) Here, the natural frequency (f n : n = 1, 2, 3...) of the resonance mode of the gas cell 40 can be calculated by the following formula (1). To simplify the explanation, assume that the shape of the gas cell 40 is a cube, and L x , L y , L z represent the lengths of one side of the gas cell 40 in the x-axis direction, y-axis direction, and z-axis direction, respectively, and L x = L y = L z = L holds. If L is the volume of the gas detection space 42 as V, then it is the cube root of V.

[0037]

Equation

[0038] Here, λ n is the wavelength of the natural frequency. c is the speed of sound. k is the specific heat ratio of the gas. R is the gas constant. T is the temperature of the gas. M is the molar mass of the gas. FIG. 3 shows the resonance modes (n = 1, 2, 3) in the gas cell 40. Here, as the resonance frequency, a driving frequency approximately equal to the natural frequency in the case of n = 1 may be used, or a driving frequency approximately equal to the natural frequency in cases other than n = 1 may be used. For example, the value of n may be selected so as to avoid the influence of noise present in the environment where the photoacoustic gas sensor 1 is used.

[0039] As shown in the above formula (1), the natural frequency of the resonance mode of the gas cell 40 can vary depending on, for example, at least one of a change in the temperature of the gas (change in T), expansion or contraction of the gas cell 40 due to the influence of stress, etc. (change in L), and a change in the humidity or pressure of the gas (change in M). For example, the photoacoustic gas sensor 1 is calibrated at the time of product shipment so that the driving frequency of the light source 11 is adjusted to be the resonance frequency (hereinafter referred to as the "initial resonance frequency") based on the natural frequency of the gas cell 40 at the time of product shipment. However, as described above, the natural frequency of the gas cell 40 changes due to changes in the usage environment or forces applied during the installation of the photoacoustic gas sensor 1. Therefore, in order to suppress a decrease in detection accuracy, it is necessary to calibrate the resonance frequency even after product shipment. The photoacoustic gas sensor 1 according to the present embodiment includes a function for the control unit 50 to search for the resonance frequency.

[0040] The control unit 50 executes calibration of the resonance frequency during a predetermined period when the concentration of the gas to be detected is not being measured. The control unit 50 searches for the resonance frequency based on the detection signal (microphone output in the present embodiment) from the acquired detection unit 41 while outputting a driving signal so as to change the driving frequency of the light source 11 as the calibration of the resonance frequency.

[0041] FIG. 4 is a diagram for explaining a method of searching for a resonance frequency. The control unit 50 detects a change in the microphone output while changing the drive frequency of the light source 11. As an example, the control unit 50 compares the microphone outputs at each drive frequency while increasing the drive frequency of the light source 11 in 100 Hz steps. The control unit 50 may use the drive frequency of the light source 11 at which the microphone output becomes the maximum value as the current correct resonance frequency (searched resonance frequency). Here, in the search for the resonance frequency, instead of the maximum value, the control unit 50 may use, for example, a maximum value (not necessarily the maximum value but a value indicating a peak) for the determination. The control unit 50 outputs a drive signal so that the drive frequency of the light source 11 becomes the searched resonance frequency. That is, the resonance frequency is updated from the initial resonance frequency or the previously searched resonance frequency by calibration.

[0042] Here, as described above, the natural frequency of the gas cell 40 changes due to a change in the usage environment or a force applied when the photoacoustic gas sensor 1 is mounted. Further, the calibration of the resonance frequency needs to be executed when the concentration of the gas to be detected is not being measured. Therefore, the predetermined period for executing the calibration of the resonance frequency may be the following timing.

[0043] The control unit 50 may search for the resonance frequency at the initial startup of the photoacoustic gas sensor 1. The initial startup of the photoacoustic gas sensor 1 is the timing when the photoacoustic gas sensor 1 is mounted on the main board after being shipped, for example, when it is incorporated into an electronic device or the like. For example, the control unit 50 may detect the initial startup of the photoacoustic gas sensor 1 by a change in the value of a register used as a flag. For example, when the control unit 50 detects that a register set to "0" at the time of product shipment of the photoacoustic gas sensor 1 is rewritten to "1" when mounted on the main board, the control unit 50 may execute the calibration of the resonance frequency.

[0044] The control unit 50 may search for the resonance frequency at a predetermined time. The predetermined time may be, for example, a set time, a time determined by a set time interval, etc. The set time may be, for example, a time when the concentration measurement of the gas to be detected is not performed (e.g., 2 o'clock at night). Also, for example, the elapse of a certain time interval (e.g., one week) from the previous calibration of the resonance frequency may be included in the execution conditions for searching for the resonance frequency.

[0045] The control unit 50 may search for the resonance frequency when the magnitude of the assumed fluctuation of the detection signal is used as a threshold and the detection signal has a fluctuation magnitude equal to or greater than the threshold. For example, when the fluctuation of the microphone output during normal operation is at most about 1 mV, 3 mV is used as the threshold, and the resonance frequency may be searched when the detection signal has a fluctuation of 3 mV or more. In this case, since there may be an operational abnormality of the photoacoustic gas sensor 1, etc., the calibration of the resonance frequency may be executed as a process close to initialization (reset). Also, for example, when the fluctuation of the microphone output is about 1 mV per day even during normal operation, if there is no fluctuation (e.g., less than 0.1 mV) over a long period (e.g., three days), the calibration of the resonance frequency may be executed.

[0046] Also, the control unit 50 can also execute the calibration of the resonance frequency by combining these timings (at initial startup, at a predetermined time, and at a timing based on the fluctuation of the detection signal). By executing the calibration of the resonance frequency at an appropriate timing, a detection accuracy of a certain level or higher can be maintained.

[0047] The control unit 50 may execute the initialization and in-operation processes shown in FIGS. 6 and 7, for example. The initialization process is the process performed at the initial startup of the photoacoustic gas sensor 1. The in-operation process is the process performed in a state where the gas concentration can be measured after the initialization of the photoacoustic gas sensor 1. As shown in FIG. 6, when the power of the photoacoustic gas sensor 1 is turned on, if it is the first startup after the shipment of the photoacoustic gas sensor 1 (Yes in step S1), the control unit 50 searches for the resonance frequency as described above (step S2) and proceeds to the in-operation process. Also, if it is not the first startup after the shipment of the photoacoustic gas sensor 1 (No in step S1), the control unit 50 proceeds to the in-operation process.

[0048] As shown in FIG. 7, when the power of the photoacoustic gas sensor 1 is turned on and the initialization is completed, the control unit 50 waits until the next measurement (step S11). When the next measurement is started, if the usage time of the photoacoustic gas sensor 1 has elapsed for a certain time (Yes in step S12), the control unit 50 searches for the resonance frequency as described above (step S14) and performs the measurement (step S15). Also, if the usage time of the photoacoustic gas sensor 1 has not elapsed for a certain time (No in step S12), the control unit 50 performs the measurement (step S15).

[0049] Here, the control unit 50 may execute the initialization and in-operation processes shown in FIGS. 6 and 8, for example. In the in-operation process shown in FIG. 8, step S13 is executed instead of step S12. However, the processes of steps S11, S14, and S15 in FIG. 8 are the same as those in FIG. 7. As shown in FIG. 8, when the power of the photoacoustic gas sensor 1 is turned on and the initialization is completed, the control unit 50 waits until the next measurement (step S11). When the next measurement is started, if the previous measurement result is outside the expected value range of the signal fluctuation amount (Yes in step S13), the control unit 50 searches for the resonance frequency as described above (step S14) and performs the measurement (step S15). Also, if the previous measurement result is within the expected value range of the signal fluctuation amount (No in step S13), the control unit 50 performs the measurement (step S15).

[0050] As described above, the gas detection method executed by the photoacoustic gas sensor 1 according to the present embodiment includes the control unit 50 searching for the resonance frequency based on the acquired detection signal while outputting a drive signal so as to change the drive frequency of the light source 11. Further, the gas detection method may further include outputting a drive signal so that the drive frequency of the light source 11 becomes the searched resonance frequency.

[0051] Here, it is preferable that the temperature, humidity, and pressure of the gas are known during a predetermined period in which calibration of the resonance frequency is performed. For example, in the environment where the photoacoustic gas sensor 1 is used, it is preferable that information on the temperature, humidity, and pressure of the gas can be obtained from a temperature sensor, a humidity sensor, a pressure sensor, etc. At this time, the control unit 50 may perform calibration after correcting the detection signal (microphone output in the present embodiment) based on at least one piece of information among the temperature, humidity, and pressure of the gas. By performing the correction, the accuracy of the resonance frequency to be searched can be improved. Here, a known method may be used for correcting the detection signal based on at least one piece of information among the temperature, humidity, and pressure.

[0052] Further, the control unit 50 may calculate the resonance frequency based on at least one piece of information among the temperature, humidity, and pressure of the gas, and search for the resonance frequency using the calculated resonance frequency. For example, if calibration of the resonance frequency is performed while changing the range of 16 to 23 kHz in 100 Hz steps, the execution time may be significantly lengthened. For example, the control unit 50 may calculate the resonance frequency using the information on the temperature, humidity, and pressure of the gas by Equation (1) or the like, and perform calibration by changing the drive frequency of the light source 11 within a narrow range including the calculated resonance frequency. The execution time of the calibration can be shortened, and the resonance frequency can be searched efficiently.

[0053] As described above, the photoacoustic gas sensor 1 and the gas detection method according to the present embodiment can suppress a decrease in detection accuracy by performing calibration of the resonance frequency even after the product is shipped, according to the above configuration and steps.

[0054] Although the embodiments of the present disclosure have been described based on the drawings and examples, it should be noted that those skilled in the art can easily make various modifications or corrections based on the present disclosure. Therefore, it should be noted that these modifications or corrections are included in the scope of the present disclosure.

Description of Reference Numerals

[0055] 1 Photoacoustic gas sensor 11 Light source 30 Substrate 30a Main surface 40 Gas cell 41 Detection unit 42 Gas detection space 43 Hole 50 Control unit

Claims

1. A light source that emits light in accordance with a drive signal; a detection unit that detects an acoustic wave from gas molecules of the detection target gas that have absorbed the emitted light, and outputs a detection signal; a gas cell having a hole through which a gas passes and introducing the gas from the hole into a gas detection space; a control unit that searches for a resonance frequency based on the acquired detection signal while outputting the drive signal so as to change the drive frequency of the light source; The control unit outputs the drive signal so that the drive frequency of the light source becomes the searched resonant frequency.

2. The photoacoustic gas sensor according to claim 1 , wherein the control unit searches for a resonant frequency at an initial startup.

3. The photoacoustic gas sensor according to claim 1 , wherein the control unit searches for a resonant frequency for a predetermined period of time.

4. 3. The photoacoustic gas sensor according to claim 1, wherein the control unit searches for a resonant frequency when the magnitude of the fluctuation in the detection signal is equal to or greater than a threshold value that is an expected magnitude of the fluctuation in the detection signal.

5. 3. The photoacoustic gas sensor according to claim 1, wherein the control unit corrects the detection signal based on at least one of information on a temperature, humidity, and pressure of the gas.

6. 3. The photoacoustic gas sensor according to claim 1, wherein the control unit calculates a resonant frequency based on at least one of information on a temperature, humidity, and pressure of the gas, and searches for a resonant frequency using the calculated resonant frequency.

7. 3. The photoacoustic gas sensor according to claim 1, wherein the light source is a quantum type infrared light emitting element.

8. 3. The photoacoustic gas sensor according to claim 1, wherein the light source includes a built-in sensor on the same element substrate, and the light emission amount can be measured.

9. A gas detection method performed by a photoacoustic gas sensor including: a light source that emits light in response to a drive signal; a detection unit that detects acoustic waves from gas molecules of a detection target gas that have absorbed the emitted light and outputs a detection signal; a gas cell that has a hole through which gas passes and introduces the gas from the hole into a gas detection space; and a control unit, The control unit: searching for a resonance frequency based on the acquired detection signal while outputting the drive signal so as to change the drive frequency of the light source; outputting the drive signal so that the drive frequency of the light source becomes the searched resonant frequency.

Images