Photoacoustic gas sensor and gas detection method

Abstract

To provide a photoacoustic gas sensor and a gas detection method having a wide measurement range.SOLUTION: A photoacoustic gas sensor (1) includes: a light source (11) for emitting light according to 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) having multiple operation modes. The multiple operation modes include: a first mode for performing concentration calculation of a detected gas from the amplitude of a detection signal; and a second mode for calculating a variation in a resonance frequency on the basis of an acquired detection signal while outputting a drive signal so as to change a drive frequency of the light source, and for performing concentration calculation of a detected gas on the basis of a variation in the 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 advanced, which 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 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

Summary of the Invention

Problems to be Solved by the Invention

[0004] Here, the measurement range of the gas sensor is determined. Usually, in order to widen the measurement range, changes such as optical design and circuit design are required. However, changes in optical design and circuit design greatly affect the detection accuracy of the gas to be detected. Therefore, a technique for widening the measurement range without design changes is demanded.

[0005] In view of such circumstances, an object of the present disclosure is to provide a photoacoustic gas sensor and a gas detection method having a wide measurement range.

Means for Solving the Problems

[0006] (1) The photoacoustic gas sensor according to an embodiment of the present disclosure is a light source that emits light according to a drive signal, A detection unit that detects an acoustic wave from gas molecules of the detected gas that has 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 having a plurality of operation modes; The plurality of operation modes include a first mode in which the concentration of the detected gas is calculated from the amplitude of the detection signal, and a second mode in which, while outputting the drive signal so as to change the drive frequency of the light source, a change in the resonance frequency is calculated based on the acquired detection signal, and the concentration of the detected gas is calculated based on the change in the resonance frequency.

[0007] (2) As one embodiment of the present disclosure, in (1), The control unit selects the first mode or the second mode based on the amplitude of the detection signal.

[0008] (3) As one embodiment of the present disclosure, in (1) or (2), The control unit selects the first mode or the second mode based on the concentration of the detected gas.

[0009] (4) As one embodiment of the present disclosure, in any one of (1) to (3), The control unit selects the first mode or the second mode based on the resonance frequency.

[0010] (5) As one embodiment of the present disclosure, in any one of (1) to (4), The control unit switches between the first mode and the second mode when it becomes equal to or higher than a first threshold value.

[0011] (6) As one embodiment of the present disclosure, in any one of (1) to (5), The control unit switches between the first mode and the second mode when it becomes less than a second threshold value.

[0012] (7) As one embodiment of the present disclosure, in any one of (1) to (6), The control unit switches between the first mode and the second mode at a predetermined time.

[0013] (8) As one embodiment of the present disclosure, in any one of (1) to (7), In the case of the first mode, the control unit can change the setting of the amplification factor of the detection signal.

[0014] (9) As one embodiment of the present disclosure, in any one of (1) to (8), In the case of the second mode, the control unit outputs the drive signal so that the amount of light of the light source is smaller than the amount of light in the case of the first mode.

[0015] (10) As one embodiment of the present disclosure, in any one of (1) to (9), The control unit corrects the detection signal based on at least one piece of information among the temperature, humidity, and pressure of the gas.

[0016] (11) As one embodiment of the present disclosure, in any one of (1) to (10), The light source is a quantum type infrared light emitting element.

[0017] (12) As one embodiment of the present disclosure, in any one of (1) to (11), The gas to be detected in the second mode includes the gas to be detected in the first mode, and the types of the gas to be detected in the second mode are more than those of the gas to be detected in the first mode.

[0018] (13) As one embodiment of the present disclosure, in any one of (1) to (12), The type of the gas to be detected in the first mode is one type, and the types of the gas to be detected in the second mode are two or more types.

[0019] (14) As one embodiment of the present disclosure, in any one of (1) to (13), The light source is provided with a built-in sensor on the same element substrate and can measure the light emission amount.

[0020] (15) A gas detection method according to an embodiment of the present disclosure is 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, and a control unit that has a plurality of operation modes. A gas detection method executed by a photoacoustic gas sensor, As the plurality of operation modes, performing concentration calculation of the gas to be detected from the amplitude of the detection signal; calculating a variation in 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 performing concentration calculation of the gas to be detected based on the variation in resonance frequency.

Effect of the Invention

[0021] According to the present disclosure, a photoacoustic gas sensor and a gas detection method with a wide measurement range can be provided.

Brief Description of the Drawings

[0022]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

[0023] Hereinafter, an optoacoustic 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 this embodiment, descriptions of the same or corresponding parts will be omitted or simplified as appropriate.

[0024] (Optoacoustic Gas Sensor) FIG. 1 is a diagram showing the configuration of an optoacoustic gas sensor 1 according to this embodiment. The optoacoustic gas sensor 1 measures the presence or concentration of a gas to be detected in a gas by the optoacoustic method. In this embodiment, the optoacoustic gas sensor 1 will be described as a device that measures the concentration of the gas to be detected. The optoacoustic gas sensor 1 includes a light source 11, a detection unit 41, a gas cell 40, and a control unit 50. Further, the optoacoustic gas sensor 1 may include a substrate 30. FIG. 1 is a cross-sectional view showing a cross-section of the optoacoustic gas sensor 1 including these components. Details of the components of the optoacoustic gas sensor 1 will be described later.

[0025] Here, the gas cell 40 has a hole 43 through which a gas (for example, ambient air) passes, and the gas is introduced from the hole 43 into the gas detection space 42. The optoacoustic 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.

[0026] The optoacoustic 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 optoacoustic method measures the gas to be detected by picking up the vibration of gas molecules that have absorbed light as sound (acoustic wave) with a high-performance microphone or the like.

[0027] The photoacoustic gas sensor 1 is configured to include a gas cell 40 on the main surface 30a of a 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 such that the xy plane is parallel to the main surface 30a of the substrate 30. The z-axis direction is a 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 vertical 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 disposed inside the gas detection space 42 above the main surface 30a.

[0028] (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, or the like.

[0029] (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. Resins include, for example, 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, compared with the case of being formed of a metal material, high productivity and weight reduction can be achieved. 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, screws, claws, fitting, grommets, welding, soldering, etc.

[0030] (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 example: a pulse of voltage or current). 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, or the like. In this embodiment, the light source 11 is an LED (infrared LED) that emits infrared light. Further, 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.

[0031] Here, the photoacoustic gas sensor 1 may further include a drive unit that outputs a drive signal to drive the light emitting element. In this embodiment, the drive unit is included in the control unit 50. That is, a part of the control unit 50 functions as a drive unit.

[0032] Further, the light source 11 may be provided with auxiliary members having auxiliary optical functions such as wavelength selection, condensing, scattering, and wavelength conversion. Specifically, the auxiliary members are a wavelength selection filter, a lens, a phosphor, a diffraction grating, or the like.

[0033] Further, 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 deteriorates according to use, by monitoring the light emission amount, it is possible to correct the deterioration and extend the life of the device.

[0034] 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.

[0035] (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. Further, 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 with a metal housing in order to suppress the influence of electromagnetic noise generated when the light source 11 is driven.

[0036] (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.

[0037] (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. The 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.

[0038] (Control unit) The control unit 50 controls the entire photoacoustic gas sensor 1. Further, the control unit 50 executes arithmetic processing for obtaining the concentration of the gas to be detected. That is, the control unit 50 calculates 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. Details of the calculation of the concentration of the gas to be detected by the control unit 50 will be described later.

[0039] (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, for example, close to 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 becomes larger, 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.

[0040] Here, regarding the light emission state of the light source 11, if the state of emitting light is defined as ON and the state of not emitting light 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 to be 10% or more and 50% or less as an example.

[0041] FIG. 2 is a diagram illustrating the change 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.

[0042] (Concentration calculation) 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, it is assumed that the shape of the gas cell 40 is a cube, and L x , L y , L z are the lengths of one side in the x-axis direction, y-axis direction, and z-axis direction of the gas cell 40, 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.

[0043]

Equation

[0044] 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 that is approximately equal to the natural frequency in the case of n = 1 may be used, or a driving frequency that is approximately equal to the natural frequency in cases other than n = 1 may be used. The driving frequency that is approximately equal to the natural frequency may include, for example, being 0.9 times or more and 1.1 times or less of the natural frequency. Also, the driving frequency that is approximately equal to the natural frequency may include, for example, a deviation of about -1 kHz to +1 kHz between the driving frequency and the natural frequency. 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.

[0045] For example, the photoacoustic gas sensor 1 may be calibrated at the time of product shipment so that the driving frequency of the light source 11 is 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. The control unit 50 may output a driving signal so that the driving frequency of the light source 11 becomes the initial resonance frequency, and may perform concentration calculation of the gas to be detected from the amplitude of the detection signal. That is, the control unit 50 acquires the vibration of the gas molecules of the gas to be detected that have absorbed light as a detection signal that is the microphone output, and calculates the concentration of the gas to be detected according to the amplitude (magnitude) thereof. Here, the control unit 50 has a plurality of operation modes. The plurality of operation modes includes a first mode and a second mode described later. When the operation mode of the control unit 50 that performs concentration calculation of the gas to be detected from the amplitude of the detection signal is the first mode, high-precision measurement using the resonance frequency is possible in the first mode. For example, when the concentration of the gas to be detected present in the gas is a low concentration (less than 30000 ppm as an example), measurement by the first mode may be executed.

[0046] Here, as shown in the above formula (1), the natural frequency of the resonance mode of the gas cell 40 varies depending on the change in the molar mass of the gas (change in M). The change in the molar mass of the gas corresponds to a change in the concentration of the gas to be detected or the like in the gas. Therefore, if it can be treated that there is almost no change in temperature, the shape of the gas cell 40, etc., it is possible to calculate the concentration of the gas to be detected from the variation in the resonance frequency (for example, the variation from the initial resonance frequency). Here, when the operation mode of the control unit 50 that calculates the concentration of the gas to be detected based on the variation in the resonance frequency is set as the second mode, measurements can be performed in a measurement range different from the first mode in the second mode. Therefore, by having the control unit 50 have the first mode and the second mode, the photoacoustic gas sensor 1 can expand the measurement range without changing the optical design, circuit design, etc. For example, when the concentration of the gas to be detected present in the gas is high (for example, 30,000 ppm or more), the measurement by the second mode may be executed.

[0047] Here, in the second mode, the control unit 50 executes a process of searching for the resonance frequency. The control unit 50 searches for the resonance frequency based on the detection signal (microphone output in this embodiment) from the acquired detection unit 41 while outputting a drive signal so as to change the drive frequency of the light source 11.

[0048] FIG. 4 is a diagram for explaining the search for the resonance frequency. The control unit 50 detects the 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 resonance frequency (the 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 extreme value (a value that indicates a peak but is not necessarily the maximum value) for the determination. The control unit 50 calculates the variation of the resonance frequency by obtaining the difference between the searched resonance frequency and the set resonance frequency. Here, the set resonance frequency may be, for example, the initial resonance frequency, the previously searched resonance frequency, or the resonance frequency under known conditions (for example, when the concentration of the gas to be detected is 0%). In this way, in the case of the second mode, the control unit 50 calculates the variation of the resonance frequency based on the acquired detection signal while outputting a drive signal to change the drive frequency of the light source 11.

[0049] Here, when the temperature of the gas (air) is 15 ° C, CO, which is the gas to be detected in the gas 2 shows a specific experimental example in which the concentration changes and the resonance frequency fluctuates. In the rectangular parallelepiped-shaped gas cell 40 for experiments, when the CO 2 concentration is 0%, the molar mass is about 28.96 g / mol, and the resonance frequency is measured to be 19.14 kHz. Also, when the CO 2 concentration is 10%, the molar mass is about 30.46 g / mol, and the resonance frequency is measured to be 18.66 kHz. In the case of the second mode, the control unit 50 measures the concentration of the gas to be detected in the gas by converting the variation of the resonance frequency into a change in the concentration of the gas to be detected. Explaining using this specific example, for example, 19.14 kHz, which is the resonance frequency when the CO 2 concentration is 0%, may be the above-mentioned "set resonance frequency". If the searched resonance frequency is 18.66 kHz, the control unit 50 determines that the change in the concentration of the gas to be detected, CO 2 is an increase of 10% (that is, the current CO 2It may be calculated as (the concentration is 10%). In calculating the detected gas based on the variation of the resonance frequency, the control unit 50 may use Equation (1), or store the data of the experimental examples as described above in the memory of the control unit 50 or the like, and calculate the detected gas based on the stored data.

[0050] Also, in the second mode, a change in the molar mass of the gas (a variation in the molecular weight in the gas) can be detected. Therefore, it is also possible to detect the influence of the mixing of other gases other than the detected gas. In the above example, when the change in the CO 2 concentration is at most an increase of 5%, from the calculation result of the control unit 50 (the CO 2 concentration increases by 10%), it is estimated that other gases other than CO 2 have been mixed in. Therefore, the second mode can be applied to detecting the presence or absence of gas leakage in factories or the like that use various types of gases.

[0051] The control unit 50 may select (use properly) the first mode or the second mode as follows. First, in the first mode, the concentration of the detected gas is calculated from the amplitude of the detection signal. However, when the concentration of the detected gas is high, the amplitude of the detection signal may reach the upper limit (saturate). Therefore, the control unit 50 may select the first mode or the second mode based on the amplitude of the detection signal. Specifically, the control unit 50 first performs measurement in the first mode, and may switch from the first mode to the second mode when the concentration of the detected gas calculated in the first mode becomes equal to or higher than a threshold value (for example, 30,000 ppm) (see Fig. 6). Also, the control unit 50 may switch from the second mode to the first mode when the concentration of the detected gas calculated in the second mode becomes less than the threshold value (for example, 30,000 ppm) (see Fig. 6). Here, as another example, the control unit 50 may switch from the first mode to the second mode when the amplitude of the detection signal is equal to or higher than a threshold value in the first mode (see Fig. 7). Also, the control unit 50 may switch from the second mode to the first mode when the resonance frequency becomes equal to or higher than a threshold value in the second mode (see Fig. 7). In this way, the control unit 50 may select the first mode or the second mode based on the amplitude of the detection signal, the concentration of the detected gas, or the resonance frequency. Also, the control unit 50 may switch between the first mode and the second mode when, for example, it becomes equal to or higher than a first threshold value, or when it becomes less than a second threshold value. The first threshold value and the second threshold value may be the same or different.

[0052] Here, in the case of the first mode, the control unit 50 may be able to change the setting of the amplification factor of the detection signal so that the amplitude of the detection signal does not immediately reach the upper limit (saturate). In other words, the control unit 50 may have a function of switching the amplification factor of the microphone output and may be able to switch the measurement range within the low concentration range of the detected gas.

[0053] Further, in the case of the second mode, the control unit 50 may output a drive signal such that the light quantity of the light source 11 becomes smaller than that in the first mode. For example, in the case of the second mode, the control unit 50 may output a drive signal that is a pulse with a smaller change width (amplitude) of voltage or current than in the first mode. Since the light quantity of the light source 11 is small, the amplitude of the detection signal in the second mode becomes smaller than that in the first mode. Therefore, the amplitude of the detection signal does not reach the upper limit (saturate). Here, in the second mode, it is sufficient to be able to search for the resonance frequency (relative comparison of the amplitude of the detection signal), and the magnitude of the light quantity of the light source 11 does not affect the detection accuracy. Also, by reducing the light quantity of the light source 11, it is possible to reduce the power consumption.

[0054] Here, the control unit 50 may switch between the first mode and the second mode at a predetermined time. The predetermined time may be, for example, when a certain state has elapsed for a certain time, or may be a preset time. The control unit 50 may switch from the first mode to the second mode when a certain time has elapsed in the first mode (see FIG. 8). Also, the control unit 50 may switch from the second mode to the first mode when a certain time has elapsed in the second mode (see FIG. 8). The set time may be a regular time such as every hour. As described above, the second mode can be applied to detection of the presence or absence of gas leakage in factories and the like that use various types of gases. Therefore, the measurement in the second mode may be periodically executed so that gas leakage detection in factories and the like is performed.

[0055] Here, when the photoacoustic gas sensor 1 is used in a facility where leaks of two types of gases are assumed and one type of gas (single gas) is detected in the first mode, the control unit 50 may perform the following mode transition. First, after the photoacoustic gas sensor 1 is activated, the presence of gas may be detected in the second mode. When the presence of some gas (miscellaneous gas) is detected in the second mode, the control unit 50 may switch from the second mode to the first mode (see Fig. 9). In the first mode, as described above, the concentration of the single gas is detected. And when leaks of two types of gases are assumed, the respective gas concentrations may be calculated from the concentration of the single gas detected in the first mode. By such a method, without the need to measure the concentrations of the two types of gases respectively, the concentration of the other gas that is not a single gas can be estimated, and the cost of the entire system including the photoacoustic gas sensor 1 can be reduced. Here, when a certain period of time has elapsed in the first mode, the control unit 50 may switch from the first mode to the second mode (see Fig. 9).

[0056] Here, it is preferable that the temperature, humidity, and pressure of the gas are known in the measurement of the gas to be detected. 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 correct the detection signal (the microphone output in this embodiment) based on at least one of the information on the temperature, humidity, and pressure of the gas. By performing the correction, the measurement accuracy can be further improved. Here, a known method may be used for correcting the detection signal based on at least one of the temperature, humidity, and pressure information.

[0057] The photoacoustic gas sensor 1 according to the present embodiment can execute a gas detection method by switching between the above-described plurality of operation modes. The gas detection method includes, as a plurality of operation modes, calculating the concentration of the gas to be detected from the amplitude of the detection signal, and calculating the variation of 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, and calculating the concentration of the gas to be detected based on the variation of the resonance frequency.

[0058] As described above, the photoacoustic gas sensor 1 and the gas detection method according to the present embodiment can expand the measurement range without changing the optical design, circuit design, etc. with the above configuration.

[0059] 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 deformations or modifications based on the present disclosure. Therefore, it should be noted that these deformations or modifications are included in the scope of the present disclosure.

Explanation of Reference Numerals

[0060] 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 according to a drive signal; A detection unit that detects an acoustic wave from gas molecules of a detected gas that has absorbed the emitted light and outputs a detection signal; A gas cell having a hole through which gas passes and introducing the gas from the hole into a gas detection space; A control unit having a plurality of operation modes, and The plurality of operation modes include a first mode in which the concentration of the detected gas is calculated from the amplitude of the detection signal, and a resonance frequency is calculated based on the acquired detection signal while outputting the drive signal so as to change the drive frequency of the light source. A photoacoustic gas sensor including a second mode in which the concentration of the detected gas is calculated based on the change in the resonance frequency.

2. The photoacoustic gas sensor according to claim 1, wherein the control unit selects the first mode or the second mode based on the amplitude of the detection signal.

3. The photoacoustic gas sensor according to claim 1, wherein the control unit selects the first mode or the second mode based on the concentration of the detected gas.

4. The photoacoustic gas sensor according to claim 1, wherein the control unit selects the first mode or the second mode based on the resonance frequency.

5. The photoacoustic gas sensor according to any one of claims 2 to 4, wherein the control unit switches between the first mode and the second mode when the value becomes equal to or greater than a first threshold value.

6. The photoacoustic gas sensor according to any one of claims 2 to 4, wherein the control unit switches between the first mode and the second mode when the value becomes less than a second threshold value.

7. The photoacoustic gas sensor according to claim 1, wherein the control unit switches between the first mode and the second mode at a predetermined time.

8. The photoacoustic gas sensor according to any one of claims 1 to 4, wherein in the case of the first mode, the control unit can change the setting of the amplification factor of the detection signal.

9. The photoacoustic gas sensor according to any one of claims 1 to 4, wherein in the case of the second mode, the control unit outputs the drive signal so that the amount of light of the light source is smaller than the amount of light in the case of the first mode.

10. The photoacoustic gas sensor according to any one of claims 1 to 4, wherein the control unit corrects the detection signal based on information on at least one of the temperature, humidity, and pressure of the gas.

11. The photoacoustic gas sensor according to any one of claims 1 to 4, wherein the light source is a quantum infrared light-emitting element.

12. The photoacoustic gas sensor according to any one of claims 1 to 4, wherein the gas to be detected in the second mode includes the gas to be detected in the first mode, and the types of the gas to be detected in the second mode are more than those of the gas to be detected in the first mode.

13. The photoacoustic gas sensor according to any one of claims 1 to 4, wherein the type of the gas to be detected in the first mode is one type, and the types of the gas to be detected in the second mode are two or more types.

14. The photoacoustic gas sensor according to any one of claims 1 to 4, wherein the light source is provided with a built-in sensor on the same element substrate and is capable of measuring the light emission amount.

15. A gas detection method performed 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 a gas passes and introduces the gas from the hole into a gas detection space, and a control unit that has a plurality of operation modes, wherein as the plurality of operation modes, performing concentration calculation of the gas to be detected from the amplitude of the detection signal; calculating a variation in 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 performing concentration calculation of the gas to be detected based on the variation in resonance frequency.

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