JP7581813B2 - Laser Gas Analyzer - Google Patents

Description

The present invention relates to a laser gas analyzer that analyzes the presence and concentration of various target gases in a space.

Gaseous gas molecules each have a unique optical absorption wavelength and an absorption line spectrum that indicates the absorption intensity. Laser light is light with a narrow spectral line width at a specific wavelength. In a laser gas analyzer, a laser element emits laser light of an optical absorption wavelength that is absorbed by the target gas, which is a gaseous gas molecule, and the target gas absorbs the laser light. The presence or absence of the target gas is detected based on the amount of laser light absorbed at that optical absorption wavelength. In addition, laser gas analysis can also detect the concentration of the target gas because the amount of laser light absorbed at the optical absorption wavelength is proportional to the concentration of the target gas.

It is necessary to select and analyze only a specific target gas from among the many gases present in the measurement space. Therefore, among the optical absorption wavelengths of the target gas and other gases in the measurement space, optical absorption wavelengths that are absorbed only by the target gas but not by other gases are selected.

If the gas pressure is low, the absorption line spectrum at the optical absorption wavelength of the gas being measured will be an ideal absorption line spectrum with a narrow spectral linewidth. However, in reality, the gas pressure is high, and the absorption line spectrum will be pressure broadened.

This pressure broadening is caused by collisions between gas molecules, and the absorption line spectrum where pressure broadening occurs has a wider spectral line width and a lower absorption intensity. In other words, when the gas pressure changes, the amount of light of the detected laser light also fluctuates due to changes in the amount of absorption of the gas being measured, which can lead to errors in the gas concentration.

A conventional technology for a laser gas analyzer that performs gas analysis taking such pressure spread into account is disclosed, for example, in Patent Document 1 (JP Patent Publication No. 2012-233900). As shown in Figure 1 of Patent Document 1, this conventional technology comprises a tunable laser light source 10, typically a semiconductor laser, that can sweep across the spectral line width of the absorption line spectrum of the target gas, a photodetector 12 that detects the intensity of the laser beam passing through the gas, and a control device 16 that includes a lock-in amplifier 24 and a microprocessor 26.

The laser gas analyzer of Patent Document 1 performs detection using wavelength modulation spectroscopy. The wavelength is swept by the drive current and a tunable laser light source 10 emits laser light modulated at a specific frequency. The laser light is detected by the photodetector 12, and the lock-in amplifier 24 performs lock-in detection of the signal at a multiple of the modulation frequency, and the gas concentration is calculated from the amplitude of this lock-in detection waveform. Lock-in detection improves the signal-to-noise ratio, making it suitable for measuring trace gases.

When the composition of multiple gases present in the measurement space is fixed, the amplitude of the lock-in detection wavelength obtained by the absorption of the measurement gas is a function of the wavelength modulation amplitude, and there is a maximum value. Therefore, when calibrating the standard gas, the signal-to-noise ratio can be maximized by adjusting the wavelength modulation amplitude so that the amplitude of the lock-in detection waveform is maximized. Then, the gas concentration can be calculated based on the correspondence (e.g., proportionality) between the gas concentration of the measurement gas and the amplitude of the lock-in detection waveform.

However, the actual gas composition in the measurement space may differ from that analyzed, for example in high-temperature combustion exhaust gas. Furthermore, if the gas concentration (or partial pressure) also fluctuates and the pressure spread changes, the spectral linewidth will fluctuate, and these effects will appear in the amplitude of the lock-in detection waveform, resulting in errors in the gas concentration measurement. In such cases, if the effects of the fluctuations in the spectral linewidth are not corrected, the gas concentration measurement will be uncertain.

Figure 7 is a quote from Patent Document 1 and shows the above phenomenon. The 2f signal on the vertical axis is the amplitude of the lock-in detection waveform, and the horizontal axis is the wavelength modulation amplitude. If the pressure differs, the shape of the function changes due to the influence of pressure spread, so if the gas concentration is simply calculated from the 2f signal, the value will contain an error.

In order to solve the above problem, Patent Document 1 reduces the pressure dependency of the gas concentration by setting the wavelength modulation amplitude at a point called the "point of action for pressure" shown in Figure 7, which is a point at which the fluctuation of the 2f signal is minimized with respect to pressure fluctuations.

In addition, Non-Patent Document 1 employs a method based on the area of the spectrum due to direct absorption without modulating the laser light (hereinafter referred to as the direct detection method) that is less susceptible to the effects of this pressure spread.

Patent document 2 proposes a method that combines the direct detection method with the method using wavelength modulation described in patent document 1 (hereinafter referred to as the wavelength modulation method).

JP 2012-233900 A JP 2011-117869 A

Tamura, Kazuto (2010), "Laser Gas Analyzer TDLS200 and its Application to Industrial Processes", Yokogawa Techniques Vol. 53 No. 2, pp. 113-116

At the "point of action against pressure" in Patent Document 1, the 2f signal is not necessarily invariant to pressure fluctuations, so there is a risk of residual gas concentration measurement error. In addition, because it is not an operating point where the 2f signal has a maximum value, there is a risk that the signal-to-noise ratio will be degraded compared to the maximum value.

In addition, the direct detection method described in Non-Patent Document 1 generally has a lower detection sensitivity than the wavelength modulation method, making it difficult to accurately measure low concentration gases.

In addition, in Patent Document 2, two analog circuits are provided for detecting using the direct detection method and the wavelength modulation method, and the two are switched between, which poses the problem of an increase in the circuitry.

The present invention has been made to solve the above problems, and its purpose is to provide a laser gas analyzer that minimizes the pressure dependence of gas concentration to improve measurement accuracy, while also improving the signal-to-noise ratio and measurement stability.

In order to achieve the above object, the laser gas analyzer of the present invention comprises a light-emitting unit having a laser element that emits laser light in a wavelength band including the optical absorption wavelengths of the absorption line spectrum of the gas to be measured, and a modulated light generating unit that can switch between a state in which the wavelength is repeatedly swept and modulated in the wavelength band including the optical absorption wavelengths of the absorption line spectrum of the gas to be measured and an unmodulated state, a light-receiving unit that receives the laser light that has passed through the space to be measured, and a light-receiving unit having a first measurement means that, in the modulated state, performs gas analysis on the detection signal output from the light-receiving element based on the amplitude of a frequency that is an integer multiple of the modulation frequency, and in the unmodulated state, performs gas analysis on the detection signal in the absence of the gas to be measured based on an area enclosed by a reference line and a detection signal in the presence of the gas to be measured, and performs correction processing on the gas concentration value measured by the first measurement means at a predetermined timing using a gas concentration value measured by the second measurement means.

The present invention provides a laser gas analyzer that can measure the concentration of a specific gas contained in a gas to be measured with high accuracy and high stability.

FIG. 1 is an overall configuration diagram of a laser gas analyzer according to an embodiment of the present invention. FIG. 1 is a block diagram of a signal processing system for a laser gas analyzer according to the present invention. A conceptual diagram of a laser sweep drive current and a detection signal for implementing the present invention. Detection signal by laser modulation method (2f detection method) Detection signal using a method that does not modulate the laser (direct detection method) Signal processing flow for implementing the present invention FIG. 1 shows the relationship between the 2f signal and the modulation amplitude and their pressure dependence in a conventional gas concentration measurement device.

The following describes an embodiment of the present invention.

The following describes a laser gas analyzer according to a first embodiment of the present invention with reference to the drawings. Figure 1 shows the overall configuration of the laser gas analyzer according to this embodiment.

The laser gas analyzer of this embodiment measures the concentration of a specific gas contained in the gas flowing inside the walls 50a and 50b. It can also detect the absence of gas if the gas concentration is 0 or below a predetermined value, and can also detect the presence or absence of gas.

The laser gas analyzer comprises a light emitting unit 10, a light receiving unit 20, and a communication line 40. The communication line 40 communicates between the light emitting unit 10 and the light receiving unit 20 by electrical signals. Also, a communication unit such as wireless or optical communication may be used instead of the communication line. Any of these communication units using a communication line, wireless, or optical communication may be used.

In such a laser gas analyzer, the light emitting unit 10 emits detection light 30. The detection light 30 is then projected into the measurement target space between the walls 50a and 50b.

At this time, a portion of the detection light 30 is absorbed by the specific gas. The remaining light that is not absorbed, i.e., the transmitted light, enters the light receiving section 20, and the amount of light is detected. The concentration of the specific gas is calculated from the detected amount of light.

Next, we'll explain each part in detail.

The light-emitting unit 10 includes a modulated light generating unit 11, a laser element 12, a collimating lens 13, a light-emitting unit window plate 14, a light-emitting unit container 15, and an optical axis adjustment flange 52a.

The light receiving section 20 includes a light receiving signal processing section 21, a light receiving element 22, a condenser lens 23, a light receiving section window plate 24, a light receiving section container 25, and an optical axis adjustment flange 52b.
First, the structure will be explained.

As shown in FIG. 1, holes are drilled in walls 50a, 50b of a pipe or the like in which a specific gas is present. Flanges 51a, 51b are fixed to the holes by welding or the like. Optical axis adjustment flanges 52a, 52b are attached to these flanges 51a, 51b so as to be mechanically movable. The positions of the light-emitting unit 10 and the light-receiving unit 20 can be adjusted by the optical axis adjustment flanges 52a, 52b.

Therefore, the optical axis adjustment flange 52a adjusts the emission angle of the detection light 30, and the optical axis adjustment flange 52b adjusts the incidence angle of the detection light 30. The optical axis adjustment flanges 52a and 52b allow the detection light 30 emitted from the light emitting unit 10 to be received by the light receiving unit 20 with the maximum light intensity.

The light-emitting unit container 15 and the light-receiving unit container 25 each house a light-emitting element, optical components, and electrical and electronic circuits, and isolate them from the outside air to protect them from wind, rain, dust, dirt, etc.

The light-emitting section window plate 14 and the light-receiving section window plate 24 are provided to close holes drilled in parts of the light-emitting section container 15 and the light-receiving section container 25. The light-emitting section window plate 14 and the light-receiving section window plate 24 are located in the optical path of the detection light 30, and while allowing the detection light 30 to pass through, they prevent gas, including specific gases, from entering the interior of the light-emitting section 10 and the light-receiving section 20. This prevents the light-emitting element, optical components, and electrical and electronic circuits from coming into direct contact with gas, protecting the inside. The mechanical structure is as follows.

Next, we will explain the optical functions of the light-emitting unit 10 and the light-receiving unit 20. The central wavelength of a specific absorption line spectrum absorbed by the gas to be measured is λ1.

Laser element 12 emits light at wavelength λ1 and its surrounding wavelengths.

The collimating lens 13 is made of a material that has high transmittance at wavelength λ1 and its surrounding wavelengths. The collimating lens 13 converts the detection light 30 into approximately parallel light, which can be transmitted to the light receiving unit 20 while minimizing loss due to diffusion.

For the light receiving element 22, a light receiving element having sensitivity at wavelength λ1 and wavelengths around it can be selected.

The focusing lens 23 is made of a material with high transmittance at wavelength λ1 and its surrounding wavelengths. The focusing lens 23 focuses the detection light 30 onto the light receiving element 22, making it possible to obtain a high signal strength.

Next, we will explain the signal processing functions of the light-emitting unit 10 and the light-receiving unit 20. First, we will explain the light-emitting unit 10.

The modulated light generating unit 11 is a signal processing and current driving circuit. It is necessary to irradiate laser light according to the absorption characteristics of the gas to be measured. In addition, the laser light needs to be wavelength-modulated modulated light. Therefore, the modulated light generating unit 11 supplies a driving current signal for emitting such laser light to the laser element 12.

The laser element 12 is, for example, a DFB laser diode (Distributed Feedback Laser Diode), a VCSEL (Vertical Cavity Surface Emitting Laser), or a DBR laser diode (Distributed Bragg Reflector Laser Diode).

The laser element 12 can variably control the emission wavelength by the drive current and temperature. Therefore, the laser element 12 is temperature controlled so that the emission central wavelength becomes the central wavelength of the absorption line spectrum of the gas to be measured. In addition, the drive current is controlled so that the wavelengths around the central wavelength of the absorption line spectrum of the gas to be measured are swept over time. Furthermore, in order to enable highly sensitive measurements by wavelength modulation spectroscopy, the drive current is controlled so that it can be switched at a predetermined timing between a case where an appropriate sine wave is superimposed and modulated (hereinafter referred to as the "2f detection method") and a case where a sine wave is not superimposed and modulated on the drive current utilizing the direct absorption phenomenon by the gas (direct detection method).

The light emitting point of the laser element 12 is located near the focal point of the collimating lens 13. The light emitted from the laser element 12 is diffused and enters the collimating lens 13, where it is converted into detection light 30, which is approximately parallel light. Note that, although this embodiment will be described using the collimating lens 13 as the parallel light conversion unit of the present invention, this is not intended to be limited to a collimating lens. For example, a parabolic mirror can be used as the parallel light conversion unit instead of a collimating lens.

The detection light 30, which is a substantially parallel light, passes through the light-emitting section window plate 14 and propagates inside the walls 50a and 50b, that is, into the space where gas, including the gas to be measured, is present.

Next, the light receiving unit 20 will be described. The light receiving unit 20 receives the detection light 30 that has passed through the light receiving unit window plate 24, and analyzes the light absorbed due to the absorption characteristics of the gas to be measured. The detection light 30 is focused by the focusing lens 23 and enters the light receiving element 22. Note that, although the focusing lens 23 is used in this embodiment, a parabolic mirror, a doublet lens, a diffractive lens, or the like may also be used instead of the focusing lens 23.
The light receiving signal from the light receiving element 22 is sent as an electric signal to the light receiving signal processing unit 21. The light receiving signal processing unit 21 processes the electric signal to calculate the gas concentration value.

The light receiving signal processing unit 21 is capable of gas detection by the 2f detection method and the direct detection method. Therefore, when the laser is driven by the 2f detection method, the light receiving signal processing unit 21 is provided with a process for locking in and detecting the harmonics of the modulation frequency of the wavelength-modulated laser light and calculating the amplitude of the detected waveform, enabling highly sensitive gas detection. Furthermore, when the laser is driven by the direct detection method, the light receiving signal processing unit 21 does not have a lock-in detection process, but instead has a process for calculating the area surrounded by the detection waveform and the reference line, enabling gas detection that is not affected by pressure spread, which will be described later. In addition, the light receiving signal processing unit 21 is provided with a process for calculating a correction value using the gas concentration calculated by this direct detection method and correcting the gas concentration value calculated by the 2f detection method with the correction value, enabling highly responsive, highly sensitive, and highly accurate gas detection.

Next, we will explain the signal processing of the present invention.

Figure 2 is a signal processing configuration diagram of the laser gas analyzer of this embodiment. The laser gas analyzer 1 includes a laser element 12, a laser element temperature control circuit 112, a wavelength sweep/modulation current control circuit 113, a light receiving element 22, an IV conversion circuit 122, a high-pass filter 123, and an amplifier circuit 124. It also includes a digital processing unit 140 that performs various signal processing after analog-to-digital conversion processing by the ADC 125. This digital processing unit includes a detection switching unit 126 that switches between the 2f detection method and the direct detection method, a lock-in detection unit 131 for the 2f detection method, a waveform acquisition unit 132, and an amplitude calculation unit 133, and includes a low-pass filter (LPF) 127 for the direct detection method, a thinning processing unit 128 that reduces the number of sampling data, a waveform acquisition unit 129, and an area calculation unit 130. Furthermore, a gas concentration calculation/correction calculation unit 134 is provided that calculates the gas concentration, calculates the correction value, and corrects the gas concentration based on the amplitude calculation value output from the amplitude calculation unit 133 and the area calculation value output from the area calculation unit 130. Also, a control unit 135 is provided that controls the wavelength sweep/modulation current control circuit 113, the detection switching unit 126, and the gas concentration calculation and correction calculation processing 134 in order to switch between the 2f detection method and the direct detection method at a predetermined timing.

The modulation signal generating unit 11 includes a laser element temperature control circuit 112 and a wavelength sweep and modulation current control circuit 113. The received light signal processing unit 21 includes an IV conversion circuit 122, a high-pass filter 123, an amplifier circuit 124, an ADC 125, a digital processing unit 140, and a control unit 135.

The laser element temperature control circuit 112 controls and stabilizes the temperature of the laser element 12 so that the central wavelength of the emission of the laser element 12 becomes the central wavelength of a specific absorption line spectrum of a specific measurement target gas. This is to prevent the output and wavelength of the laser element 12 from fluctuating due to changes in the ambient temperature, since the output and wavelength of the laser element 12 fluctuate depending on the temperature.

The wavelength sweep/modulation current control circuit 113 controls the drive current of the laser element 12 so that the wavelength emitted by the laser element 12 is swept over time among wavelengths around the central wavelength of the absorption line spectrum of a specific gas to be measured, and is modulated by a specified signal. These laser lights are controlled by the control circuit 113 to be repeatedly turned on and off.

At a given laser lighting timing, the wavelength of the laser light L can be varied by increasing or decreasing the drive current, so for example, by changing the magnitude of the drive current of the laser element 12, the wavelength of the laser light can be swept so as to cross the absorption lines of the gas components being measured. Furthermore, when the absorption by the gas being measured is detected with high sensitivity using the 2f detection method, a sine wave is superimposed on the drive current and modulated, and wavelength sweep and modulation are performed. Also, when the effect of pressure spread is reduced and absorption by the gas is detected using the direct detection method, only the wavelength sweep is performed without superimposing a sine wave on the drive current.

The drive method for wavelength sweeping and modulation is selected based on the command from the control unit 135. In this case, the modulation frequency by the sine wave is set to be higher than the frequency of the wavelength sweeping. As described later, for example, the former can be 50 kHz and the latter can be about 200 Hz.

The laser light L, which passes through the gas and is partially absorbed by the gas components to be measured, enters the light receiving element 22. The light receiving element 22 is an element that is sensitive to the wavelength of the laser light L, and can be selected as appropriate for the light receiving element 22, such as a photodiode, depending on the wavelength and signal strength of the laser light L.

At this time, the light receiving element 22 may also receive light emitted from a space where gas is present. Furthermore, if the light receiving element 22 is a photodiode, a dark current is generated. The repetition period of the sweep is selected to be short so that the fluctuations in the light receiving signal caused by these are sufficiently longer than the period of the repeated sweep of the laser light L. For example, the sweep can be set to several hundred Hz or more while the fluctuations in the dark current are several Hz to several tens of Hz.

The IV conversion circuit 122 is a circuit that converts the current signal from the light receiving element 22 into a voltage signal. For example, if the light receiving element 22 is a photodiode, a transimpedance amplifier can be selected that amplifies the current from the photodiode while converting it into a voltage. Here, under conditions where the laser light L is least attenuated, i.e., under conditions where there is no dust or the like on the optical path, amplification can be performed by appropriately providing an amplifier circuit (not shown) between the IV conversion circuit 122 and the high-pass filter 123 to the extent that the signal is not saturated.

The high-pass filter 123 removes the DC component contained in the signal from the IV conversion circuit 122. The signal from the IV conversion circuit 122 generally contains a DC component. The DC component is caused by, for example, light emitted from a space in which gas exists. If the light receiving element 121 is a photodiode, it is also caused by a dark current generated in the photodiode. Even if these DC components fluctuate, their time constant is sufficiently longer (i.e., they are low frequency) than the period in which the laser light L is repeatedly swept, so they are removed by the high-pass filter 123.

The cutoff frequency of the high-pass filter 123 is selected so that the repetition frequency (the inverse of the repetition period) of the laser light L and the frequency of the wavelength sweep and modulation signal of the laser light L are within the pass band of the high-pass filter 123. As a result, the on/off of the laser light L and the wavelength sweep and modulation signal of the laser light L pass through almost unchanged.

The reason why the high-pass filter 123 is provided immediately after the IV conversion circuit 122 is that if the signal is amplified to a DC signal by the amplifier circuit 124 described below, when the measurement conditions are poor, for example, when the light emitted from the space where the gas is present is strong, the dark current is large, and the transmittance of the laser light L is low, the absorption signal due to the gas that is effective for measurement becomes relatively small, and the detection sensitivity decreases. To prevent this situation, the high-pass filter 123 is provided immediately after the IV conversion circuit 122.

Such a signal wave from the high-pass filter 123 mainly includes a signal for turning on and off the laser light L and a wavelength sweep and modulation signal for the laser light L. Of these, the wavelength sweep and modulation signal wave when the laser light L is turned on fluctuates because the laser light L is scattered and attenuated due to, for example, fluctuations in the amount of dust coexisting in a space where gas is present. This scattering and attenuation is not wavelength-dependent within the wavelength sweep and modulation range of the laser light L, and passes through the high-pass filter 123.

The amplifier circuit 124 amplifies the signal from the high-pass filter 123 at an appropriate amplification rate without saturating the signal.
The ADC 125 is an element that converts the analog signal from the amplifier circuit 124 into a digital signal (AD conversion). An element with an appropriate sampling speed is selected for the ADC 125 so that the modulation components in the 2f detection method can be sufficiently detected. For example, if the laser modulation components are 50 kHz, the 2f detection method detects frequency components of 100 kHz, which is twice as high. Therefore, an AD conversion element with a sampling speed of, for example, 1 MHz or higher is selected so that these frequency components can be sufficiently detected.
The digital processing unit 140 processes the digital signal, and detects, calculates, and corrects the gas concentration by the 2f detection method or the direct detection method. Each detection method is selected by the detection switching unit 126 in conjunction with the laser drive at a predetermined timing described later.

The lock-in detection unit 131 that performs the 2f detection method is a processing unit that detects the signal of the modulation frequency component contained in the light receiving signal from the light receiving element 22. The frequency used for lock-in detection is twice the modulation frequency in the wavelength sweep/modulation current control circuit 113 as a reference. When the laser light L is swept to include the wavelength of the absorption line of the gas component to be measured, the output of the lock-in detection circuit 131 becomes a waveform with maximum and minimum values based on the absorption line of the gas component to be measured, as shown in Figure 4, where the horizontal axis is time and the vertical axis is signal intensity. Since the difference D between the maximum and minimum values is correlated with the gas concentration, the difference D can be detected to measure the gas concentration by performing calibration using standard gases set in advance for each concentration.

By using this type of lock-in detection, noise in frequency ranges other than the frequency used for lock-in detection can be significantly reduced, making it possible to detect only minute signals due to absorption by the gas being measured.

In the direct detection method, the processing unit utilizes the direct absorption phenomenon by gas for the light receiving signal from the light receiving element 22. In the direct detection method, the light receiving signal has a waveform with an extreme value at a position corresponding to the wavelength of the absorption line of the gas component to be measured, as shown in Figure 5. When the signal in a zero concentration state is used as the reference line, the area S enclosed by the absorption waveform correlates with the gas concentration. Therefore, as with the 2f detection method, by performing calibration using standard gases set in advance for each concentration, this area S can be detected to measure the gas concentration.

Comparing these two detection methods, the 2f detection method can detect minute absorption signals from the measured gas with high sensitivity, allowing accurate measurement of low concentration gases, but when the pressure of the measurement target gas atmosphere changes, the width W between extreme values and the difference D in amplitude shown in Figure 4 change due to the influence of the pressure spread described above. For example, when the pressure increases, the absorption spectrum line width broadens and the absorption intensity at a specific wavelength decreases, causing the width W between extreme values to increase and the difference D to decrease, resulting in measurement errors.
In the direct detection method, the absorption signal width also broadens due to pressure broadening, but the area basically changes linearly with respect to pressure changes, making it easy to correct, and stable measurements are possible even under pressure fluctuations. In addition, in the 2f detection method, in addition to pressure broadening, the spectrum width is also broadened by coexisting gas components, which affects the amplitude of the absorption waveform, but the area in the direct detection method is basically unaffected, making stable measurements possible.

On the other hand, the direct detection method has low absorption sensitivity to gas concentration, about 1/100 of that of the 2f detection method, meaning that it is difficult to obtain a sufficient absorption signal against noise and to accurately measure low concentration gases.

In addition, since the direct detection method does not superimpose a modulated signal, it does not require sampling as fast as the 2f detection method. For example, when the sweep period is about 5 ms, a sampling period of about 100 kHz and about 500 data points per sweep are sufficient. Therefore, it is possible to improve the measurement accuracy in the direct detection method, which has low absorption sensitivity, by applying digital filtering by a low-pass filter 127 to the output from the ADC 125, which has a high sampling period selected for the 2f detection method, and reducing the number of data points by thinning out the data, thereby applying an oversampling method that reduces the effect of quantization noise due to AD conversion, thereby reducing noise and improving resolution.
Furthermore, when the processing of this direct detection method is applied, a process for reducing noise and improving measurement accuracy may be performed by appropriately performing an averaging process using repeated data (not shown) after any of the processes of waveform acquisition 129, area calculation 130, and gas concentration calculation/correction calculation 134. In this case, it will take longer to obtain measurement data with the direct detection method than with the 2f detection method.

Next, the signal processing flow according to the present invention will be explained using Figure 6.

In signal processing, there are two modes to switch between: measurement mode and correction value calculation mode. When the measurement mode, which is the normal gas concentration measurement, is selected, a sine wave is superimposed on the laser drive current, modulated, and the gas concentration is detected using the 2f detection method. On the other hand, when the correction value calculation mode, which calculates the correction value, is selected, the modulation signal is not superimposed on the laser drive current, and the gas concentration is detected using the direct detection method. The measurement mode and correction value calculation mode are switched at a predetermined timing that is appropriately set, for example as shown in Figure 3. Generally, since pressure and other factors do not fluctuate frequently, it is sufficient to set and execute the correction value calculation at an appropriate interval according to the usage environment, for example, for a gas concentration measurement at a 1-second interval, the correction value calculation is performed at a 1-minute interval.

In the correction value calculation mode, the area enclosed by the reference line and the absorption signal is calculated using the direct detection method described above, and the gas concentration is calculated from the relationship between the standard gas and the absorption area obtained in advance. The gas concentration calculation value obtained using the direct detection method is divided by the gas concentration calculation value calculated using the 2f detection method to obtain the correction value α.

α=Gas concentration calculated by direct detection method/Gas concentration calculated by 2f detection method (Equation 1)
In this case, the gas concentration calculation by the 2f detection method is shown in the processing flow in FIG. 6 as a process of calculating a correction value using the gas concentration calculation value in the immediately preceding measurement mode. However, after the gas concentration is calculated by the direct detection method, no correction value calculation may be performed, and instead, a process may be provided in which a correction value is calculated using the gas concentration calculation value calculated in the correction value calculation mode immediately before gas concentration correction is performed in the immediately following measurement mode.

In the measurement mode, the 2f detection method is used to calculate the difference D between the maximum and minimum values after lock-in detection, and the gas concentration is calculated based on the relationship with the difference D in the previously acquired standard gas. After that, the calculated gas concentration value is multiplied by the correction value using the following formula to obtain the corrected calculated gas concentration value.

Corrected gas concentration calculation value = α × uncorrected gas concentration calculation value using the 2f detection method (Equation 2)
By carrying out the above processing, the correction value obtained from the direct detection method is used to correct the gas concentration calculation results of the 2f detection method, which is highly sensitive and can obtain measurement data in a relatively short time. This reduces the effects of pressure spread, making it possible to obtain gas concentrations with improved measurement stability and quick response.

In addition, in this embodiment, the circuitry up to the amplification process is standardized, and the subsequent direct detection method and lock-in detection using the 2f detection method are performed using digital circuits, reducing the number of parts, thereby enabling miniaturization and cost reduction. Furthermore, in the direct detection method, an oversampling technique is used to reduce noise and achieve high resolution, improving the detection accuracy using the direct detection method and thereby improving the accuracy of the correction value, thereby improving the accuracy of gas concentration measurement.

This makes it possible to provide a small, low-cost laser gas analyzer that has good responsiveness and high measurement accuracy.

The laser gas analyzer of the present invention is ideal for measuring combustion exhaust gas from boilers, waste incineration, etc., and for combustion control. It is also useful as an analyzer for steel gas analysis [blast furnaces, converters, heat treatment furnaces, sintering (pellet equipment), coke ovens], fruit and vegetable storage and aging, biochemistry (microorganisms) [fermentation], air pollution [incinerators, flue gas desulfurization and denitrification], exhaust gas from internal combustion engines of automobiles and ships (removal testers), disaster prevention [explosive gas detection, toxic gas detection, new building material combustion gas analysis], plant growth, chemical analysis [oil refining plants, petrochemical plants, gas generating plants], environmental [landing concentration, concentration inside tunnels, parking lots, building management], various physical and chemical experiments, etc.

10: Light emitting unit 11: Modulated light generating unit 12: Laser element 13: Collimating lens 14: Light emitting unit window plate 15: Light emitting unit container 20: Light receiving unit 21: Light receiving signal processing unit 22: Light receiving element 23: Condenser lens 24: Light receiving unit window plate 25: Light receiving unit container 30: Detection light 40: Communication line 50a, 50b: Wall 51a, 51b: Flange 52a, 52b: Optical axis adjustment flange 111: Laser element 112: Laser element temperature control circuit 113: Wavelength sweep/modulation current control circuit 121: Light receiving element 122: IV conversion circuit 123: High pass filter 124: Amplification circuit 125: AD converter (ADC)
126: Detection switching unit 127: Low pass filter (LPF)
128: Thinning processing unit 129: Waveform acquisition unit 130: Area calculation unit 131: Lock-in detection unit 132: Waveform acquisition unit 133: Amplitude calculation unit 134: Gas concentration calculation, correction calculation unit 135: Control unit

Claims (6)

A laser gas analyzer that measures the gas concentration of a measurement target gas present in a measurement target space,
a laser element that emits laser light having a wavelength band including an optical absorption wavelength of the absorption line spectrum of the measurement target gas;
a modulated light generating unit capable of switching between a state in which the wavelength is swept and modulated in a predetermined first period in a wavelength band including the optical absorption wavelength of the absorption line spectrum of the measurement target gas, and a state in which the wavelength is swept and unmodulated in a second period longer than the first period;
a light receiving element that receives the laser light that has passed through the measurement target space;
a first measuring unit that measures a gas concentration based on an amplitude of a frequency that is an integer multiple of a modulation frequency for a detection signal output from the light receiving element in the modulated state;
a second measurement unit which measures a gas concentration based on an area enclosed by a detection signal in the presence of the target gas and a detection signal in the presence of the target gas, the detection signal being used as a reference signal for the detection signal output from the light receiving element in the unmodulated state;
a gas concentration calculation/correction calculation unit that calculates a correction value using the gas concentration value measured by the second measurement unit, performs a correction process on the gas concentration value measured by the first measurement unit using the correction value, and analyzes the concentration or presence or absence of the measurement target gas;
A laser gas analyzer having a The gas concentration calculation and correction calculation unit includes:
Calculating a correction value by dividing the gas concentration value measured by the second measurement unit by the gas concentration value measured by the first measurement unit, and calculating a concentration value by multiplying the concentration value measured by the first measurement unit by the calculated correction value.
2. The laser gas analyzer according to claim 1. A light receiving signal processing unit that processes a detection signal output from the light receiving element,
The light receiving signal processing unit includes:
an AD conversion unit for converting the detection signal from an analog value to a digital value;
a detection switching unit that switches an output destination of the digital value between the first measurement unit and the second measurement unit;
3. The laser gas analyzer according to claim 1 or 2. The light receiving signal processing unit includes:
a first detection data processing unit that performs digital filtering on the digital value and then performs data thinning processing to output a reduced digital value;
4. The laser gas analyzer according to claim 3. a second detection data processing unit that averages the digital values acquired by sweeping the laser light a predetermined number of times;
4. The laser gas analyzer according to claim 3 . a second detection data processing unit that averages the reduced digital values ;
5. The laser gas analyzer according to claim 4 .