JP2023132453A - Laser type gas analyzer - Google Patents

Abstract

To provide a laser type gas analyzer capable of highly accurately measuring the concentration of a specific gas included in the gas to be measured.SOLUTION: One embodiment of the present invention is a laser type gas analyzer 1 comprising: a modulating light generation unit 11 that supplies a drive current to a laser element 12; a light receiving element 22 that receives a laser light 30; a filter unit 125 that extracts, from detection signals outputted from the light receiving element 22, an integral multiple of the modulation frequency of the laser light 30 that is wavelength modulated; a waveform analysis unit 135 that calculates amplitude information on unnecessary regions that do not include absorption of a gas to be measured, using some of the detection signals before subtraction that are outputted from an AD converter 128; a subtraction circuit 126 that subtracts the amplitude information on the unnecessary regions from the detection signals outputted from the filter unit 125; and a measurement unit 130 that performs gas analysis on the detection signals after subtraction that is outputted from the AD converter 128 by way of the subtraction circuit 126.SELECTED DRAWING: Figure 2

Description

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

In a laser gas analyzer, a laser element emits laser light at a light absorption wavelength that is absorbed by a target gas, which is a gaseous gas molecule, and causes the target gas to absorb the laser light. The presence or absence of the gas to be measured is detected based on the amount of light absorbed. In addition, the laser gas analyzer can also detect the concentration because the amount of laser light absorbed at the optical absorption wavelength is proportional to the concentration of the gas to be measured. Note that it is necessary to select and analyze only a specific gas to be measured from among the many gases present in the space to be measured. Therefore, among the optical absorption wavelengths of the measurement target gas and other gases in the measurement target space, a light absorption wavelength that absorbs only the measurement target gas but not other gases is selected.

Among these, wavelength modulation spectroscopy is generally used to measure trace gases, and for example, Patent Document 1 is available. In wavelength modulation spectroscopy, a wavelength tunable laser light source emits a laser beam whose wavelength is swept by a drive current and modulated at a specific frequency.The laser beam is detected by a photodetector, and the detection signal is locked-in. Lock-in is detected at an integral multiple of the modulation frequency. Then, the gas concentration can be calculated based on a correspondence relationship such as a proportional relationship between the gas concentration of the gas to be measured and the amplitude information of the lock-in detection waveform.

Japanese Patent Application Publication No. 2012-177612

By the way, the detection signal having an integral multiple of the modulation frequency corresponding to the gas concentration of the gas to be measured is ideally a signal derived only from gas absorption.
However, the actual detection signal includes an unnecessary signal having the same frequency as the gas absorption signal and is unrelated to the gas absorption signal due to nonlinearity of the laser diode and distortion of the signal processing circuit. As a result, the gas absorption signal among the detection signals becomes relatively small, resulting in a problem of insufficient resolution of the gas absorption signal and an accompanying decrease in measurement accuracy.

Furthermore, in recent years, there has been an increasing need to be able to analyze minute amounts of gases to be measured on the order of ppm, which has increasingly led to insufficient resolution of gas absorption signals.

Therefore, the present invention has been made to solve the above problems, and its purpose is to provide a laser gas analyzer that reduces the influence of unnecessary components and improves the resolution of detection signals and measurement accuracy. There is a particular thing.

One aspect of the present invention is a laser-type gas analyzer that performs gas analysis of a gas to be measured existing in a space to be measured, which emits laser light in a wavelength band including a light absorption wavelength of an absorption line spectrum of the gas to be measured. Modulated light generation that supplies the laser element with a drive current generated so that the wavelength is repeatedly swept and modulated in a wavelength band that includes the light absorption wavelength of the absorption line spectrum of the measurement target gas and the laser element that emits the light. a light-receiving element that receives the laser light that has passed through the measurement target; and a filter that extracts a frequency that is an integral multiple of the modulation frequency of the wavelength-modulated laser light from the detection signal output from the light-receiving element. an AD converter that performs analog-to-digital conversion of the detection signal output from the filter unit, and a part of the detection signal before subtraction output from the AD converter that does not include absorption of the gas to be measured. a waveform analysis section that calculates amplitude information of the unnecessary region; a subtraction circuit that subtracts the amplitude information of the unnecessary region from the detection signal output from the filter section; and a waveform analysis section that calculates the amplitude information of the unnecessary region. and a measuring section that performs gas analysis based on a lock-in detection signal obtained by lock-in detecting the subtracted detection signal at an integral multiple of the modulation frequency.

One aspect of the present invention includes a DA converter that performs digital-to-analog conversion of the amplitude information of the unnecessary area between the waveform analysis section and the subtraction circuit, and the subtraction circuit converts the amplitude information that has been digital-to-analog converted. It is characterized by subtraction processing.

One aspect of the present invention is a laser-type gas analyzer that performs gas analysis of a gas to be measured existing in a space to be measured, which emits laser light in a wavelength band including a light absorption wavelength of an absorption line spectrum of the gas to be measured. Modulated light generation that supplies the laser element with a drive current generated so that the wavelength is repeatedly swept and modulated in a wavelength band that includes the light absorption wavelength of the absorption line spectrum of the measurement target gas and the laser element that emits the light. a light-receiving element that receives the laser light that has passed through the measurement target; and a filter that extracts a frequency that is an integral multiple of the modulation frequency of the wavelength-modulated laser light from the detection signal output from the light-receiving element. an AD converter that performs analog-to-digital conversion of the detection signal output from the filter unit, and a part of the detection signal before subtraction output from the AD converter that does not include absorption of the gas to be measured. a waveform analysis section that calculates amplitude information of an unnecessary region; a subtraction circuit that subtracts the amplitude information calculated by the waveform analysis section from the drive current generated by the modulated light generation section; and from the AD converter. The present invention is characterized by comprising a measuring section that performs gas analysis based on a lock-in detection signal obtained by lock-in detecting the output subtracted detection signal at an integral multiple of the modulation frequency.

One aspect of the present invention includes a DA converter between the subtraction circuit and the laser element that converts the drive current subtracted by the subtraction circuit into digital-to-analog, and the digital-to-analog converted drive current is It is characterized in that it is supplied to the laser element.

According to the present invention, it is possible to provide a laser gas analyzer that can analyze a gas to be measured with high precision.

FIG. 1 is an overall configuration diagram of a laser gas analyzer according to the present embodiment. FIG. 2 is a signal processing block diagram of the laser gas analyzer according to the first embodiment. FIG. 3(a) is a conceptual diagram of the waveform of the drive current, and FIG. 3(b) is a conceptual diagram of the waveform of the lock-in detection signal. FIG. 3 is an enlarged view of a lock-in detection signal. FIG. 5(a) is a waveform diagram of a detection signal before analog-to-digital conversion in a conventional laser gas analyzer, and FIG. 5(a) is a detection signal when there is no gas absorption, and FIG. It is a conceptual diagram in which a part of a) is enlarged, and FIG. 5(c) is a detection signal (without subtraction processing) when there is gas absorption. This is a lock-in detection signal obtained by performing lock-in detection on the detection signal of FIG. 5(c). FIG. 7(a) is a detection signal before analog-to-digital conversion in the laser gas analyzer according to the present embodiment, and FIG. 7(b) is a detection signal after subtraction when there is no gas absorption. This is the detection signal after subtraction when there is gas absorption. This is a lock-in detection signal obtained by performing lock-in detection on the subtracted detection signal in FIG. 7(b). FIG. 3 is a diagram showing a signal processing flow of the laser gas analyzer according to the first embodiment. FIG. 3 is a signal processing block diagram of a laser gas analyzer according to a second embodiment. FIG. 7 is a diagram showing a signal processing flow of a laser gas analyzer according to a second embodiment.

Hereinafter, the laser gas analyzer according to the present embodiment will be described in detail with reference to the accompanying drawings. Note that the present invention is not limited to the embodiments described below, and can be implemented with appropriate modifications within the scope without changing the gist thereof.

<Overview of laser gas analyzer>
FIG. 1 is an overall configuration diagram of a laser gas analyzer according to an embodiment of the present invention. As shown in FIG. 1, the laser gas analyzer 1 includes a light emitting section 10, a light receiving section 20, and a communication line 40.

The laser gas analyzer 1 analyzes a gas to be measured existing in a space to be measured. In the laser gas analyzer 1, a laser beam 30 emitted from a light emitting unit 10 is irradiated onto a gas to be measured flowing inside walls 50a and 50b (a space to be measured) forming a flow path for the gas to be measured. . The laser beam 30 that has passed through the gas to be measured enters the light receiving section 20, and the concentration of a specific gas can be determined from the detected amount of light. Furthermore, if the gas concentration is 0 or below a predetermined value, it is possible to detect the absence of gas, and therefore the presence or absence of gas can also be detected.

The light emitting unit 10 and the light receiving unit 20 are removably attached to walls 50a and 50b that constitute a flow path for the gas to be measured. The walls 50a and 50b are walls such as a flue through which a specific gas flows, and each has a hole. The flanges 51a and 51b are fixed to those holes by welding or the like. Optical axis adjusting flanges 52a and 52b provided on the light emitting section 10 and the light receiving section 20 are mechanically detachably attached to these flanges 51a and 51b. The light emitting section 10 and the light receiving section 20 are arranged at opposite positions with the walls 50a and 50b in between, but the positions can be adjusted by the optical axis adjustment flanges 52a and 52b.

The optical axis adjustment flange 52a can adjust the emission angle of the laser beam 30, and the optical axis adjustment flange 52b can adjust the incidence angle of the laser beam 30. The optical axis adjustment flanges 52a and 52b allow the laser beam 30 emitted from the light emitting section 10 to be received by the light receiving section 20 with the maximum amount of light.

[Light emitting section 10]
The light emitting section 10 will be explained. As shown in FIG. 1, the light emitting section 10 includes a modulated light generating section 11, a laser element 12, a collimating lens 13, a light emitting section window plate 14, a light emitting section container 15, and an optical axis adjustment flange 52a. Configured with the necessary features. As shown in FIG. 1, the modulated light generating section 11, the laser element 12, and the collimating lens 13 are arranged inside the light emitting section container 15. The light emitting unit container 15 isolates each built-in component from the outside air and protects it from wind, rain, dust, dirt, and the like.

The modulated light generation unit 11 generates a drive current whose wavelength is repeatedly swept and modulated in a wavelength band including the light absorption wavelength of the absorption line spectrum of the gas to be measured. The modulated light generation unit 11 then supplies the laser element 12 with a drive current for emitting modulated laser light. Thereby, for gas concentration analysis, modulated light whose wavelength is modulated according to the absorption characteristics of the gas to be measured can be irradiated.

The laser element 12 emits light at the center wavelength (hereinafter referred to as "λ1") of a specific absorption line spectrum absorbed by the gas to be measured, and wavelengths around the center wavelength. The laser element 12 variably controls the emission wavelength by driving current and temperature control.

The temperature of the laser element 12 is controlled so that the emission center wavelength becomes the center wavelength λ1 of the absorption line spectrum of the gas to be measured. Further, the laser light 30 emitted from the laser element 12 is controlled by the drive current supplied from the modulated light generation section 11 so as to temporally sweep the wavelength around the center wavelength of the absorption line spectrum of the gas to be measured. Furthermore, it is modulated by superimposing an appropriate sine wave so that it can be measured with high sensitivity by wavelength modulation spectroscopy (WMS). Wavelength modulation spectroscopy is also called 2f detection method.

The laser element 12 used is not particularly limited, but may be, 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 collimating lens 13 is made of a material that has high transmittance at the center wavelength λ1 of the absorption line spectrum of the gas to be measured and wavelengths around it. The collimating lens 13 converts the laser beam 30 into substantially parallel light, which can be transmitted to the light receiving section 20 while suppressing loss due to diffusion.

A light emitting point of the laser element 12 is arranged near the focal point of the collimating lens 13. The emitted light from the laser element 12 enters the collimating lens 13 while being diffused, and is converted into laser light 30 which is substantially parallel light. In this embodiment, the collimating lens 13 is used as the parallel light converting section, but it is not intended to be limited to the collimating lens. For example, a parabolic mirror can be used instead of the collimating lens 13 as the parallel light converter.

The laser light 30, which is substantially parallel light, passes through the light emitting unit window plate 14 and propagates inside the walls 50a and 50b, that is, into the space where the gas containing the gas to be measured exists. The light emitting unit window plate 14 is provided so as to make a hole in a part of the light emitting unit container 15 and cover the hole. The light emitting unit window plate 14 is located in the optical path of the laser light 30 and allows the laser light 30 to pass therethrough while preventing gas containing a specific gas to be measured from entering the light emitting unit 10 . This prevents each component placed inside the light-emitting container 15 from coming into direct contact with the gas, thereby protecting each component inside the light-emitting container 15.

[Light receiving section 20]
The light receiving section 20 will be explained. 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, and a light receiving section container 25. The light-receiving unit container 25 houses the light-receiving element 22, optical components, and electrical and electronic circuits therein, and isolates them from the outside air to protect them from wind, rain, dust, dirt, and the like.

The light receiving section 20 receives the laser beam 30 that has passed through the light receiving section window plate 24, and analyzes the absorbed light based on the light absorption characteristics of the gas to be measured. The light receiving section window plate 24 is provided so as to make a hole in a part of the light receiving section container 25 and close the hole. The light receiving unit window plate 24 is located in the optical path of the laser beam 30 and prevents gas containing a specific measurement target gas from entering the light receiving unit 20 while allowing the laser beam 30 to pass therethrough. This prevents each component placed within the light receiving section 20 from coming into direct contact with the gas, thereby protecting the interior. The laser beam 30 is condensed by a condensing lens 23 and enters the light receiving element 22 . In this embodiment, the condenser lens 23 is used, but instead of the condenser lens 23, a parabolic mirror, a doublet lens, a diffraction lens, or the like may be used.

The light receiving element 22 receives the laser beam 30 that has passed through the gas to be measured. It is possible to select a light-receiving element that is sensitive at the center wavelength λ of the absorption line spectrum of the gas to be measured and its surrounding wavelengths. The light reception signal from the light receiving element 22 is sent to the light reception signal processing section 21 as an electrical signal.

The condenser lens 23 is made of a material that has high transmittance at the center wavelength λ1 of the absorption line spectrum of the gas to be measured and wavelengths around it. Since the laser beam 30 is focused on the light receiving element 22 by the condensing lens 23, high signal strength can be obtained.

The light-receiving signal processing unit 21 processes the electric signal received by the light-receiving element 22 and calculates the gas concentration. Highly sensitive gas detection is possible by lock-in detecting harmonics of the modulation frequency of the wavelength-modulated laser beam 30 and calculating amplitude information of the detected waveform.

As shown in FIG. 1, the communication line 40 is connected to the light emitting section 10 and the light receiving section 20. Communication occurs between the light emitting section 10 and the light receiving section 20 using electrical signals. Further, instead of the communication line, a communication unit such as wireless or optical communication may be used.

<Description of blocks constituting the laser gas analyzer of the first embodiment>
The laser gas analyzer according to the first embodiment will be described below with reference to FIG. 2.

FIG. 2 is a signal processing block diagram of the laser gas analyzer 1 according to the first embodiment. In the block diagram of FIG. 2, the modulated light generation section 11 and the received light signal processing section 21 of the light emitting section 10 and the light receiving section 20 of the laser gas analyzer 1 shown in FIG. 1 will be explained in detail. 2, it is assumed that the laser gas analyzer 1 has the usual configurations even if not shown in FIG.

As shown in FIG. 2, the light emitting section 10 includes a modulated light generating section 11, a laser element 12, and a laser element temperature control circuit 112. The modulated light generation section 11 includes a wavelength sweep/modulation current setting section 113 and a DA converter 114.

The wavelength sweep/modulation current setting unit 113 modulates the wavelength of the laser beam 30 emitted by the laser element 12 with a predetermined signal so that the wavelength of the laser beam 30 emitted by the laser element 12 is swept near the absorption line with the center wavelength λ1 of the absorption line spectrum of the gas to be measured. The driving current of the laser element is controlled so that the Further, in the wavelength sweep/modulation current setting section 113 , a driving method for the wavelength sweep/modulation current setting section 113 is selected based on a command from the control section 160 of the light receiving section 20 . For example, the drive current is repeatedly turned on and off, thereby controlling the laser light 30 to be repeatedly turned on and off. At this time, the wavelength is repeatedly swept at a predetermined lighting timing. At this time, it is preferable that the modulation frequency by the sine wave is set higher than the wavelength sweep frequency.

DA converter 114 converts the digital signal into an analog signal. Therefore, the DA converter 114 performs DA conversion on the drive current sent from the wavelength sweep/modulation current setting section 113 and sends it to the laser element 12 .

The laser element temperature control circuit 112 controls and stabilizes the output and wavelength of the laser element 12 to be constant. Since the output and wavelength of the laser element 12 vary depending on the temperature, the temperature is controlled to be constant by the laser element temperature control circuit 112 so that the output and wavelength do not fluctuate due to changes in ambient temperature. Further, the laser element temperature control circuit 112 is controlled based on commands from the control section 160 of the light receiving section.

The laser element 12 emits a laser beam 30 whose wavelength is swept and modulated by the swept drive current so as to cross the entire absorption line at the center wavelength λ1 of the absorption line spectrum of the gas to be measured. The laser light 30 is modulated by superimposing a sine wave on a drive current.

The laser beam 30 shown in FIG. 2 that is incident on the light receiving element 22 is partially absorbed by the gas to be measured, but is composed of the remaining light not absorbed by the gas to be measured, that is, transmitted light.

The light receiving element 22 is an element that is sensitive to the wavelength of the laser beam 30, and can be appropriately selected depending on the wavelength and signal strength of the laser beam 30, such as a photodiode, for example. At this time, the light receiving element 22 may also receive light emitted from a space where gas exists. Further, when 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 fluctuations in the received light signal due to these factors are sufficiently longer than the period of repetition of the sweep of the laser beam 30.

As shown in FIG. 2, the received light signal processing section 21 includes an IV conversion circuit 122, a high-pass filter 123, a first amplifier circuit 124, a band-pass filter 125, a subtraction circuit 126, a second amplifier circuit 127, and an AD converter 128. , a measurement section 130, a waveform analysis section 135, a DA converter 150, and a control section 160. The measurement unit 130 includes a lock-in detection unit 131, a peak/bottom calculation unit 133 for calculating a signal according to the gas concentration from the lock-in detection signal, and a gas concentration calculation correction unit 134.

The IV conversion circuit 122 is a circuit that converts a 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 that amplifies the current from the photodiode while converting it into voltage can be selected. Here, under the condition that the laser beam 30 is least attenuated, that is, under the condition that there is no dust or the like on the optical path, amplification may be appropriately performed using an amplification circuit (not shown) to the extent that the signal is not saturated.

The high-pass filter 123 removes the DC component contained in the detection signal from the IV conversion circuit 122. The detection signal from the IV conversion circuit 122 generally contains a DC component. The direct current component is caused, for example, by light emitted from a space where gas exists. For example, if the light-receiving element 22 is a photodiode, it is also caused by dark current generated in the photodiode. Even if these DC components fluctuate, their time constants are sufficiently longer than the cycle of repeating the sweep of the laser beam 30. That is, since it is a low frequency, the DC component is removed by the high-pass filter 123, resulting in a waveform that straddles the reference voltage 0V.

The cutoff frequency of the high-pass filter 123 is set so that the repetition frequency (reciprocal of the repetition period) of turning on and off the laser beam 30 and the frequency of the wavelength sweep/modulation signal of the laser beam 30 are within the pass band of the high-pass filter 123. choose. As a result, the turning on/off of the laser beam 30 and the wavelength sweep/modulation signal of the laser beam 30 pass through with almost no change.

The reason why the high-pass filter 123 is provided immediately after the IV conversion circuit 122 is that if the first amplifier circuit 124 described below amplifies the DC signal, if the measurement conditions are poor, for example, the light emitted from the space where gas exists will be strong. This is because when the dark current is large and the transmittance of the laser beam 30 is low, the absorption signal by the gas that is effective for measurement becomes relatively small, and the detection sensitivity decreases. In order to prevent such a situation, a high-pass filter 123 is provided immediately after the IV conversion circuit 122 to remove the DC component in advance.

The signal wave from the high-pass filter 123 mainly includes a signal for turning on/off the laser beam 30 and a wavelength sweep/modulation signal for the laser beam 30. Among these, the wavelength swept/modulated signal wave when the laser beam 30 is turned on varies because the laser beam 30 is scattered and attenuated due to, for example, a variation in the amount of dust coexisting in a space where gas exists. This signal fluctuation due to scattering and attenuation of the wavelength sweep/modulation signal wave has no wavelength dependence in the wavelength sweep/modulation range of the laser beam 30 and passes through the high-pass filter 123.

The first amplification circuit 124 amplifies the detection signal that has passed through the high-pass filter 123 at an appropriate amplification factor without saturating it.

The bandpass filter 125 constitutes a filter section that extracts a frequency that is an integral multiple of the modulation frequency of the wavelength-modulated laser beam 30 from the detection signal output from the light receiving element 22. The bandpass filter 125 extracts, for example, a frequency signal twice the modulation frequency (hereinafter referred to as "2f signal").

In the wavelength modulation method, the gas concentration is detected based on a signal that is an integral multiple of the modulation signal as a gas absorption signal. As mentioned above, for example, a 2f signal is used, but this signal is much smaller than the wavelength sweep/modulation signal of the laser beam 30. Thereby, the 2f signal can be extracted from the bandpass filter 125 and amplified by an appropriate amplification factor without saturating the signal by the amplifier circuit, and after being converted into a digital signal, it can be used to accurately detect the gas concentration.

The second amplification circuit 127 amplifies the detection signal output by the bandpass filter 125 at an appropriate amplification factor without saturating the detection signal. As will be described later, in this embodiment, the subtraction circuit 126 removes unnecessary components from the detection signal output from the bandpass filter 125. However, before the unnecessary components are removed, the signal strength of the unnecessary components is large. Processing is performed to set the amplification factor low to avoid overrange. After removing unnecessary components, processing is performed to set the amplification factor high in order to make the gas absorption signal as large as possible and send the signal to the AD converter 128. The amplification process is controlled by the control section 160.

The AD converter 128 performs AD conversion on the detection signal output from the bandpass filter 125. The AD converter 128 converts the analog signal sent from the second amplifier circuit 127 into a digital signal. As shown in FIG. 2, the AD converter 128 is branched into a lock-in detector 131 and the AD converter 128 is branched into a waveform analyzer 135. It is sent to the analysis section 135. The AD converter 128 appropriately selects an element with a sampling rate so that the modulation component can be sufficiently detected. For example, if the modulation component of the laser is 50 kHz, the 2f detection method detects a frequency component of 100 kHz, which is twice that frequency. Therefore, for example, an AD conversion element having a sampling rate of 1 MHz or more is selected so that these frequency components can be sufficiently detected.

The waveform analysis unit 135 uses a portion of the detection signal output from the AD converter 128 to calculate amplitude information (unnecessary components) of an unnecessary region that does not include absorption of the measurement target gas. A signal whose phase and frequency are synchronized with the 2f signal according to the calculated amplitude information is sent to the DA converter 150.

The DA converter 150 is provided between the waveform analysis section 135 and the subtraction circuit 126, and performs DA conversion on the signal output from the waveform analysis section 135. As a result, the amplitude information of the unnecessary region calculated by the waveform analysis section 135 is converted into digital/analog and sent to the subtraction circuit 126.

The subtraction circuit 126 subtracts the amplitude information of the unnecessary region calculated by the waveform analysis section 135 from the detection signal output from the bandpass filter 125. In this way, based on the calculation results of the waveform analyzer 135, signals corresponding to amplitude information in unnecessary regions that do not include absorption of the gas to be measured are removed, and the signal strength of unnecessary components is reduced to a level below the gas absorption signal. Can be made smaller.

The lock-in detection section 131 performs phase detection (lock-in detection) at twice the frequency of the modulation frequency in the wavelength sweep/modulation current setting section 113 included in the signal from the second amplifier circuit 127 as a reference.

The peak/bottom calculating section 133 calculates the peak/bottom for calculating a signal according to the gas concentration from the lock-in detection signal. The gas concentration calculation correction section 134 performs gas concentration detection, calculation, and correction processing from the signals processed by the lock-in detection section 131 and the peak/bottom calculation section 133. The processing by the gas concentration calculation correction section 134 is controlled by the control section 160.

The control unit 160 controls the wavelength sweep/modulation current setting unit 113, the laser element temperature control circuit 112, the second amplifier circuit 127, the waveform analysis unit 135, the lock-in detection unit 131, and the gas based on the information on the gas to be measured. Controls the concentration calculation correction section 134.

<About calculation processing for gas analysis>
FIG. 3(a) is a conceptual diagram of the waveform of the drive current a1, and FIG. 3(b) is a conceptual diagram of the waveform of the lock-in detection signal b1. The drive current a1 shown in FIG. 3(a) is generated by the modulated light generation unit 11 so that the wavelength is repeatedly swept and modulated in a wavelength band including the center wavelength λ1 of the absorption line spectrum of the gas to be measured. be done. FIG. 3(b) corresponds to the waveform after detection by the lock-in detection unit 131 in FIG.

FIG. 4 is an enlarged view of the lock-in detection signal b1 shown in FIG. 3(b). As shown in FIGS. 3(b) and 4, it is desirable that the lock-in detection signal b1 has a waveform having an extreme value based on the absorption line of the gas component to be measured.

The difference D between the bottom and peak signal intensities of the lock-in detection signal b1 shown in FIG. 4 has a correlation with the gas concentration. Therefore, by performing calibration using standard gases set in advance for each concentration, it is possible to detect the difference D and measure the gas concentration.

<Problems with conventional technology>
By the way, in gas analysis of a gas to be measured, ideally a detection signal (2f signal) having an integral multiple of the modulation frequency corresponding to the gas concentration of the gas to be measured is a signal derived only from absorption of the gas to be measured. . However, in reality, due to nonlinearity of the laser diode and distortion of the signal processing circuit, an unnecessary 2f signal (unnecessary component) having the same frequency as the gas absorption signal is included, which is unrelated to the gas absorption signal. If this unnecessary component is larger than the gas absorption signal, the amplification factor of the amplifier circuit will be determined to such an extent that the unnecessary component will not be saturated, and the gas absorption signal will become relatively small.

FIG. 5 is a waveform diagram of a detection signal before analog-to-digital conversion in a conventional laser gas analyzer, FIG. 5(a) is a detection signal when there is no gas absorption, and FIG. 5(b) is a It is a conceptual diagram which expanded a part of Fig.5 (a). Unlike the configuration of this embodiment shown in FIG. 2, the conventional laser gas analyzer does not include the subtraction circuit 126 and the waveform analysis section 135, and the detection signal from the bandpass filter 125 shown in FIG. , and is directly sent to the lock-in detection section 131 through the AD converter 128.

If there is no gas absorption in the laser beam 30, ideally the detection signal (2f signal) from the bandpass filter 125 is not generated, so the signal strength is 0. However, in reality, as shown in FIGS. 5(a) and 5(b), there are unnecessary 2f signals that are unrelated to the gas absorption signal.

FIG. 5C is a waveform diagram of a conventional detection signal when the laser beam 30 has gas absorption. The region (I) in FIG. 5C is a signal of the region where the laser beam 30 absorbed the gas to be measured. On the other hand, the region (II) in FIG. 5(c) is a signal in an unnecessary region that does not include gas absorption. If the gas absorption signal is smaller than the signal in this unnecessary region, the waveform diagram of the detection signal will be as shown in FIG. , is set to such an extent that signals in unnecessary areas are not saturated. Therefore, the gas absorption signal cannot be sufficiently amplified.

FIG. 6 shows a conventional lock-in detection signal after lock-in detection. As shown in FIG. 6, in the lock-in detection signal, the difference D in signal strength becomes relatively small in the region (III) including gas absorption. Therefore, there was a problem that sufficient resolution for the gas absorption signal could not be obtained, resulting in a decrease in measurement accuracy.

Therefore, in this embodiment, as shown in FIG. 2, a waveform analysis section 135 and a subtraction circuit 126 are incorporated in the rear part of the bandpass filter 125, and the waveform analysis section 135 collects amplitude information of an unnecessary region where there is no gas absorption. The amplitude information of this unnecessary area is subtracted from the detection signal output from the bandpass filter 125 in the subtraction circuit 126. As a result, the amplitude information in the unnecessary region was controlled to be reduced to a level below the gas absorption signal. This process allows highly accurate gas analysis of the gas to be measured.

<About subtraction processing in this embodiment>
In this embodiment, as shown in FIG. 2, the detection signal output from the bandpass filter 125 passes through the subtraction circuit 126 without being subjected to subtraction processing, and after being analog-to-digital converted by the AD converter 128. , are sent to the waveform analysis section 135.

At this time, the waveform diagram of the detection signal from the bandpass filter 125 before subtraction is similar to, for example, FIG. 5(c) when there is gas absorption. As described above, in FIG. 5C, the region (I) is a signal in a region including gas absorption, and the region (II) is a signal in an unnecessary region not including gas absorption. The waveform analysis unit 135 calculates the amplitude information of the unnecessary region (II) in FIG. The subtraction circuit 126 shown in FIG. 2 subtracts the signal from the DA converter 150 from the signal from the bandpass filter 125 to remove the amplitude information in the unnecessary region (II) in FIG. 5(c). I can do it.

FIG. 7 is a waveform diagram of a detection signal before analog-to-digital conversion in the laser gas analyzer according to the present embodiment, and FIG. 7(a) is a detection signal after subtraction when there is no gas absorption. FIG. 7(b) shows the detection signal after subtraction when there is gas absorption.

As shown in FIG. 7(a), since unnecessary components are removed from the signal of the bandpass filter 125, the signal strength before AD conversion becomes sufficiently small and becomes almost 0 in the case where there is no gas absorption. .

On the other hand, Fig. 7(b) shows the signal intensity when there is gas absorption, but since unnecessary components were subtracted, the gas absorption signal was amplified sufficiently to the extent that it did not become saturated, which is clear compared to Fig. 5(c). Thus, a detection signal having an extreme value can be obtained.

FIG. 8 shows a lock-in detection signal obtained by performing lock-in detection on the subtracted detection signal of FIG. 7(b). The lock-in detection signal obtained in this embodiment can obtain a large difference D in signal strength compared to the conventional example shown in FIG. By obtaining such a large difference D, it is possible to obtain high resolution for the gas absorption signal, and it is possible to improve the accuracy of gas analysis.

<About unnecessary ingredients>
The amplitude information (unnecessary component) of the unnecessary region not including gas absorption shown in (II) of FIG. 5(c) will be explained.

In this embodiment, before gas analysis is performed, for example, during factory adjustment before shipping, a gas absorption waveform is generated approximately at the center of the laser sweep section using the gas to be measured (standard gas). The laser drive current in the wavelength sweep/modulation current setting section 113 and the set value of the laser element temperature control circuit 112 are adjusted so as to do so. As a result, the region (time domain) in which the gas absorption waveform exists and the region in which the gas absorption waveform does not exist within the sweep section (within a predetermined sweep time) are stored in the control unit 160. For example, in order to perform wavelength sweep/modulation current setting and temperature control based on the output results from the lock-in detection section 131 to the gas concentration calculation correction section 134, information from the lock-in detection section 131 and the gas concentration calculation correction section 134 is used. Information on unnecessary components can be collected in the control unit 160 and transmitted from the control unit 160 to the waveform analysis unit 135.

<Description of flowchart using the laser gas analyzer of the first embodiment>
FIG. 9 is a flowchart using the laser gas analyzer of the first embodiment. As shown in step S01 of FIG. 9, the modulated light generation section 11 generates a drive current. The modulated light generation unit 11 supplies the laser element 12 with a drive current generated such that the wavelength is repeatedly swept and modulated in a wavelength band including the center wavelength λ1 of the absorption line spectrum of the gas to be measured. In the wavelength sweep/modulation current setting section 113 configuring the modulated light generation section 11 , a driving method for the wavelength sweep/modulation current setting section 113 and the laser element temperature control circuit 112 is selected based on a command from the control section 160 . An example of the drive current is shown in FIG. 3(a).

The DA converter 114 shown in FIG. 2 converts the digital signal sent from the wavelength sweep/modulation current setting section 113 into an analog signal and sends it to the laser element 12.

Next, as shown in steps S02 and S03 in FIG. 9, the laser element 12 emits laser light 30 in a wavelength band that includes the center wavelength λ1 of the absorption line spectrum of the gas to be measured, and The light passes through the space and is received by the light receiving element 22. The signal received by the light receiving element 22 is processed by the IV conversion circuit 122, the high pass filter 123, and the first amplifier circuit 124, and is transmitted to the band pass filter 125.

As shown in step S04 of FIG. 9, the bandpass filter 125 extracts a frequency signal (2f signal) that is an integral multiple of the modulation frequency, for example, twice the modulation frequency.

As shown in step S05 in FIG. 9, the AD converter 128 shown in FIG. 2 performs AD conversion on the 2f signal output from the bandpass filter 125.

Next, as shown in step S06 in FIG. 9, the detection signal AD-converted in step S05 branches depending on whether it is before or after subtraction. That is, as shown in FIG. 2, a subtraction circuit 126 is provided before the AD converter 128, but when the subtraction circuit 126 is performing a predetermined subtraction process (after subtraction [YES]), the step The process advances to step S09, and if the predetermined subtraction process is not performed in the subtraction circuit 126 (before subtraction [NO]), the process advances to step S07.

In step S07, the waveform analysis unit 135 calculates amplitude information of an unnecessary region that does not include absorption of the gas to be measured. That is, in step S07, the waveform analysis unit 135 uses a portion of the detection signal output from the bandpass filter 125 to calculate amplitude information of an unnecessary region that does not include absorption of the gas to be measured. As already explained, in this embodiment, the unnecessary area shown in (II) of FIG. 5C can be known in advance, and therefore the amplitude information of the unnecessary area can be calculated.

The amplitude information signal of the unnecessary region calculated by the waveform analyzer 135 is subjected to DA conversion by the DA converter 150 and sent to the subtraction circuit 126 .

In step S08 in FIG. 9, the subtraction circuit 126 subtracts the amplitude information calculated by the waveform analysis unit 135 from the 2f signal output from the bandpass filter 125 in step S04. As a result, in the detection signal output from the band-pass filter 125, the signal strength of the amplitude information in the unnecessary region that does not include gas absorption can be reduced to a level below the gas absorption signal. For example, the detection signal shown in FIG. 7(b) can be obtained.

After the subtraction processing, the process returns to step S05 again to AD convert the detection signal after the subtraction, and then in step S06, since the subtraction processing has been completed, the process proceeds to step S09. In step S09, the measurement unit 130 shown in FIG. 2 calculates the concentration based on the detection signal output from the bandpass filter 125. Note that the setting of the amplification factor from the control unit 160 to the second amplifier circuit 127 can be changed in various ways depending on whether or not subtraction processing is being performed.

<Block diagram configuring the laser gas analyzer of the second embodiment>
FIG. 10 shows a signal processing block diagram of a laser gas analyzer according to a second embodiment of the present invention. In the following description, only the parts that are different from the first embodiment will be described.

As shown in FIG. 10, a subtraction circuit 115 is provided between the wavelength sweep/modulation current setting section 113 and the laser element 12. The subtraction circuit 115 subtracts the amplitude information calculated by the waveform analysis section 135 from the drive current signal output from the wavelength sweep/modulation current setting section 113.

A DA converter 114 is provided between the subtraction circuit 115 and the laser element 12. The signal subjected to subtraction processing by the subtraction circuit 115 is digital-to-analog converted by the DA converter 114. The digital-to-analog converted signal is sent to the laser element 12.

In the second embodiment, a signal corresponding to the data obtained by the waveform analysis section 135 is transmitted to the light emitting section 10, and the signal is subtracted from the signal of the wavelength sweep/modulation current setting section 113 by the subtraction circuit 115. , unnecessary components are removed by the light emitting section 10. Other operations and obtained detection signals are the same as in the first embodiment.

In the laser gas analyzer 1 of the first embodiment, since the subtraction process is performed on the light receiving unit 20 side, the influence of noise on the transmission path is small, and gas concentration can be detected with high accuracy. On the other hand, in the laser gas analyzer 2 of the second embodiment, unlike the first embodiment, the DA converter 150 in the light receiving section 20 is not required, and the subtraction process is also installed in the light emitting section 10 in advance. Since it can be performed by digital signal processing using a microcomputer, FPGA, etc., the number of parts can be reduced, which is advantageous in terms of circuit scale.

FIG. 11 is a flowchart of signal processing of the laser gas analyzer according to the second embodiment of the present invention. In the following description, only the parts that are different from the first embodiment will be described.

In the second embodiment, after step S07, the subtraction circuit 115 extracts the amplitude information (unnecessary component) is subtracted (step S10). As a result, amplitude information (unnecessary components) in a region that does not include gas absorption is removed from the drive current signal output from the wavelength sweep/modulation current setting section 113. Further, the signal subjected to subtraction processing by the subtraction circuit 115 in the DA converter 114 is subjected to DA conversion. After that, the process advances to step S02.

The digital-to-analog converted signal is sent to the laser element 12, and then sent to step S02. Thereafter, the process proceeds from step S02 to step S06, and then proceeds to step S09, where the concentration of a specific gas contained in the gas to be measured is analyzed. Note that the setting of the amplification factor from the control unit 160 to the second amplifier circuit 127 can be changed in various ways depending on whether or not subtraction processing is being performed.

The laser gas analyzer of the present invention is most suitable for measuring combustion exhaust gas from boilers, garbage incineration, etc., and for combustion control. Other areas include gas analysis for steel [blast furnaces, converters, heat treatment furnaces, sintering (pellet equipment), coke ovens], fruit and vegetable storage and ripening, biochemistry (microorganisms) [fermentation], air pollution [incinerators, flue gas desulfurization, Denitration], Exhaust gas (removal tester) from internal combustion engines of automobiles, ships, etc., Disaster prevention [Explosive gas detection, toxic gas detection, combustion gas analysis for new construction materials], Plant cultivation, Chemical analysis [Oil refinery plants, petrochemicals] It is also useful as an analyzer for environmental purposes (land concentration, tunnel concentration, parking lot, building management), and various physical and chemical experiments.

1 Laser gas analyzer 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 Condensing lens 24 Light receiving unit window plate 25 Light receiving unit container 30 Laser beam 40 Communication lines 50a, 50b Walls 51a, 51b Flanges 52a, 52b Optical axis adjustment flange 112 Laser element temperature control circuit 113 Wavelength sweep/modulation current setting unit 114 DA converter 115 Subtraction circuit 122 IV conversion circuit 123 High-pass filter 124 Amplification circuit 125 Band-pass filter 126 Subtraction circuit 127 Amplification circuit 128 AD converter 131 Lock-in detection section 132 Detection frequency setting section 133 Peak/bottom calculation section 134 Gas concentration calculation section 135 Waveform analysis section 150 DA converter 160 Control Department

Claims (4)

A laser gas analyzer that performs gas analysis of a gas to be measured existing in a space to be measured,
a laser element that emits laser light in a wavelength band that includes the light absorption wavelength of the absorption line spectrum of the gas to be measured;
a modulated light generation unit that supplies the laser element with a drive current generated such that the wavelength is repeatedly swept and modulated in a wavelength band including the light absorption wavelength of the absorption line spectrum of the measurement target gas;
a light receiving element that receives the laser beam that has passed through the measurement target;
a filter section that extracts a frequency that is an integral multiple of a modulation frequency of the wavelength-modulated laser light from the detection signal output from the light receiving element;
an AD converter that converts the detection signal output from the filter section from analog to digital;
a waveform analysis unit that calculates amplitude information of an unnecessary region that does not include absorption of the measurement target gas using a part of the detection signal output from the AD converter before subtraction;
a subtraction circuit that subtracts amplitude information of the unnecessary region from the detection signal output from the filter section;
a measurement unit that performs gas analysis based on a lock-in detection signal obtained by lock-in detecting the subtracted detection signal output from the AD converter via the subtraction circuit at an integral multiple of the modulation frequency; A laser gas analyzer characterized by having: between the waveform analysis section and the subtraction circuit;
It has a DA converter that converts the amplitude information of the unnecessary area into digital/analog,
2. The laser gas analyzer according to claim 1, wherein the subtraction circuit performs subtraction processing on amplitude information converted from digital to analog. A laser gas analyzer that performs gas analysis of a gas to be measured existing in a space to be measured,
a laser element that emits laser light in a wavelength band that includes the light absorption wavelength of the absorption line spectrum of the gas to be measured;
a modulated light generation unit that supplies the laser element with a drive current generated such that the wavelength is repeatedly swept and modulated in a wavelength band including the optical absorption wavelength of the absorption line spectrum of the measurement target gas;
a light receiving element that receives the laser beam that has passed through the measurement target;
a filter section that extracts a frequency that is an integral multiple of a modulation frequency of the wavelength-modulated laser light from the detection signal output from the light receiving element;
an AD converter that converts the detection signal output from the filter section from analog to digital;
a waveform analysis unit that calculates amplitude information of an unnecessary region that does not include absorption of the measurement target gas using a part of the detection signal output from the AD converter before subtraction;
a subtraction circuit that subtracts amplitude information calculated by the waveform analysis unit from the drive current generated by the modulated light generation unit;
A measurement unit that performs gas analysis based on a lock-in detection signal obtained by lock-in detecting the subtracted detection signal output from the AD converter at an integral multiple of the modulation frequency. Laser type gas analyzer. between the subtraction circuit and the laser element,
a DA converter that converts the drive current subtracted by the subtraction circuit into digital-to-analog;
A digital-to-analog converted drive current is supplied to the laser element.
The laser gas analyzer according to claim 3, characterized in that:

Images