JPH01254841A - Signal processing method for gas sensor - Google Patents

Abstract

PURPOSE:To detect the concentration of gas to be measured with high accuracy by imposing modulation on the oscillation waveform of laser light by using a rectangular wave which oscillates between two wavelengths with which the wave height of the absorption spectrum of disturbing gas to be measured is equal in level. CONSTITUTION:The laser light emitted by a semiconductor laser element 1 is passed through a lens 2 and transmitted through the gas 4 to be measured such as sulfur dioxide, and the light is converged by a lens 6 on an infrared detector 7, which converts the light into an electric signal. The other laser light beam which is split by a half-mirror 3, on the other hand, is reflected by a mirror 8, transmitted through a reference cell 9 which contains gas whose concentration is already known, and passed through a lens 10, and the light is converted by a photodetector 11 into an electric signal. Here, the output of the photodetector 11 is analyzed by a microcomputer 46. This analyzed value is used to detect the wavelength with which the spectrum value of the disturbing gas is equal in level and the rectangular wavelength which oscillates between the wavelengths is used to modulate the oscillation waveform of the element 1. Thus, the concentration of the gas to be measured is detected with high accuracy.

Description

[Detailed Description of the Invention] [Summary] Regarding a gas sensor that detects the concentration of gas using a semiconductor laser element, when the absorption spectrum of the interfering gas species overlaps with the spectrum of the gas to be measured, the interfering gas species The purpose of this research is to promote a signal processing method for gas sensors that prevents the measured concentration value of the measured gas from fluctuating due to concentration fluctuations in the gas. In the method of emitting and transmitting the laser beam into a gas to be measured and detecting the infrared absorption spectrum of the gas with a photodetecting element to detect the concentration of the gas to be measured, the oscillation wavelength of the laser beam is , by modulating using a rectangular wave that oscillates between two wavelengths such that the absorption spectrum of the interfering gas close to the absorption spectrum of the gas to be measured has the same wave height.

[Industrial application field]

The present invention relates to a gas sensor using a semiconductor laser element,
A signal processing method for a gas sensor that can measure the concentration of a gas to be measured with high accuracy even when the absorption spectrum of a gas to be measured by the sensor overlaps with the absorption spectrum of an interfering gas contained in the gas to be measured. Regarding.

Gas sensors using infrared semiconductor laser elements have various advantages such as small size, high sensitivity, and ease of maintenance.
Used in pollution gas detection devices, etc.

[Conventional technology]

FIG. 3 is a block diagram of a conventional infrared laser type gas sensor.

As shown in FIG. 3, a conventional gas sensor includes a laser element 1 that outputs a variable wavelength laser beam, a lens 2 that converts the laser beam into parallel light, a half mirror 3 that splits the parallel light, and a half mirror 3 that splits the parallel light. A long optical path cell 5 allows one of the laser beams divided by 3 to pass through the gas to be measured 4, a lens 6 condenses the transmitted light from the specially manufactured optical path cell, and the light condensed by the lens 6 is detected. and a photodetector 7 that converts the signal into an electrical signal.

Further, the other laser beam split by the half mirror 3 is reflected using the mirror 8, and the reference cell 9 containing a gas of known concentration and the laser beam transmitted through the reference cell 9 are detected via the lens 10. After that, the photodetector 11 converts into an electric signal, the signal processing circuits 12 and 13 process the output signals of the photodetectors 7 and 11, respectively, and the output signals of each signal processing circuit 12.13 are calculated. Divider 1 that displays concentration
4 and a display 15.

To explain the operation of such a gas sensor using drawings, a laser beam emitted from a semiconductor laser element 1 is converted into a parallel beam by a lens 2, and the gas to be measured, for example, a polluting gas such as sulfur dioxide gas (SO□), is Pass through 4. The light transmitted through this gas is focused by a lens 6 onto an infrared detector 7 and converted into an electrical signal. As shown in FIG. 4, the semiconductor radar device 1 described above can continuously change the wavelength by changing the current applied to the device.

[Problem to be solved by the invention]

Conventionally, when detecting the concentration of a gas to be measured, such as SO□ gas, by such a method, as shown in FIG. In order to improve the measurement, the modulation of the laser oscillation wavelength is, for example, the wavelength (A) of the valley part of the absorption spectrum. The wavelength (B) is modulated using a rectangular wave 22 that oscillates between, for example, (8,8470 tt m ), and a rectangular output waveform 23 is obtained at the detection 37 .

Here, the vertical axis in Figure 5 shows the transmittance (%) of infrared light in the gas under test, and the horizontal axis shows the oscillation wavelength of the laser light (μm).
shows. Note that the spectrum 24 of the interfering gas is exaggerated and drawn in a large size for the sake of explanation.

By the way, the signal received by the detector 7 is in a state in which the spectrum 24 of the interfering gas simultaneously contained in the gas to be measured, that is, water vapor (11□0), overlaps with the rectangular shape in the spectrum 24 of this interfering gas. Only the portion of the offset voltage of the rectangular wave corresponding to the difference value (α) between the wavelength (A) corresponding to the trough of the wave 22 and the portion corresponding to the wavelength (B) corresponding to lIJ of the rectangular wave 22 is measured. 7. Detector of spectral measurements of the gas to be measured; There is a problem in that the output waveform 23 fluctuates and fluctuates as the concentration of the interfering gas changes.

Conventionally, as a method to prevent the influence of the absorption spectrum of the interfering gas, a method was proposed in which two Raded elements were prepared and the absorption spectra of the interfering gas and the gas to be measured were measured separately and then corrected (Japanese Patent Application No. 1983). 154488)) or a method of detecting the concentration of interfering gas using a hygrometer and correcting the spectrum value of the gas to be measured output by the detector using the detected value (see Japanese Patent Laid-Open No. 62-261032). ).

However, this method requires two laser elements,
Furthermore, it is necessary to provide a measurement circuit in a separate system in conjunction with this laser element, which complicates the device configuration of the gas sensor, increases the size of the device, and increases the cost of the device.

SUMMARY OF THE INVENTION An object of the present invention is to provide a concentration detection method for a gas sensor that eliminates the above-mentioned problems and allows the concentration of a gas to be measured to be detected easily and with high accuracy without requiring a complicated device configuration.

[Means to solve the problem]

As shown in FIG. 1, the concentration detection method for a gas sensor according to the present invention solves the above problems, as shown in FIG.
A rectangular wave 32 that oscillates between two wavelengths in which the wave height of the interference gas absorption spectrum 31 of the measured gas is at the same level.
It is constructed by modulating using .

[For production]

The method of the present invention modulates the oscillation wavelength of the laser beam using a rectangular wave that oscillates between two wavelengths where the absorption spectrum of the interfering gas simultaneously contained in the gas to be measured has the same level. This prevents interference due to the absorption spectrum of the interfering gas from entering the signal received by the device.

〔Example〕

An embodiment of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a detailed explanatory diagram of the present invention. Here, the spectrum 31 of the interfering gas is exaggerated and drawn in a large size for the sake of explanation.

As shown in the figure, the spectrum 21 of the gas to be measured (So□)
Spectrum 31 of the interfering gas (11□0) superimposed on
The measured gas (SO
Wavelength A (8) corresponding to the valley part of spectrum 21 of 2)
, 8451) The oscillation wavelength of the laser element is modulated by a rectangular wave 32 that oscillates between wavelength C (8,8458) μm corresponding to the position E of the disturbing gas spectrum 31. In this way, since the measurement value is not affected by the offset voltage α of the interfering gas spectrum unlike the conventional method described above, highly reliable and stable measurement can be performed.

The configuration of a gas sensor for carrying out such a method is shown in FIG.

As shown in the figure, the apparatus configuration for implementing the method of the present invention is such that the output of the photodetector 11 of the conventional apparatus shown in FIG.
The microcomputer 46 analyzes it, uses this analyzed value to detect the wavelengths A and C at which the spectrum values of the interfering gas are at the same level, and uses the rectangular wave 32 that oscillates between the wavelengths A and C to drive the laser element. Modulates the oscillation wavelength of 1.

As shown in Figure 2, the reason for adding a device such as the microcomputer 46 is that the oscillation wavelength of the laser element changes slightly over time, so it is possible to adjust the oscillation wavelength of the laser with high precision just before concentration measurement. This is because it is necessary to measure the

In this way, there is no need to separately provide a semiconductor laser element for detecting the spectral value of the interfering gas, and there is no need to provide a measurement circuit associated with this separately provided laser element. As shown in the detector output waveform 33 in Fig. 1, when the influence of interfering gas is eliminated,
The concentration of the gas to be measured can be detected with high accuracy.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, the concentration of the gas to be measured can be easily detected with high accuracy using a simple method without the influence of interfering gases. If used in a gas sensor, a highly accurate gas sensor can be obtained.

[Brief explanation of the drawing]

Fig. 1 is a detailed explanatory diagram of the present invention, Fig. 2 is a block diagram of a gas sensor for carrying out the method of the present invention, Fig. 3 is a block diagram of a gas sensor used in the conventional method, and Fig. 4 is a laser element. FIG. 5 is an explanatory diagram of the conventional method. In the figure, 1 is a laser element, 2 is a lens, 3 is a half mirror, 14 is a gas to be measured, 5 is a long optical path cell, 6 is a condensing lens, 7 is a photodetector, 9 is a reference cell, 10 is a lens , 11 is a photodetector, 12.13 is a signal processing circuit, 4 is a divider, 15 is a display, 21 is a measured gas absorption spectrum, 24.31 is an interfering gas absorption spectrum, 32 is a square wave, 33 indicates the detector output waveform, and 46 indicates the microcomputer. A C-+ (, Brn) 滲ｯ柊E7 がめ方 5 天梢 明 fig. Ka 2! R/lλ is under I\・ρ"Sribu's 7°'CJv7 Figure 3 → Mark 77 [+Denshi Ichijo-1chi1suvri*; Figure 4

Claims (1)

[Claims] By changing the bias current of the semiconductor laser element, an infrared laser beam with a changing wavelength is emitted, the laser beam is transmitted into the gas to be measured, and the infrared absorption spectrum of the gas is detected by a light detection element. In a method for detecting the concentration of a gas to be measured, the oscillation wavelength of the laser beam is set to two wavelengths at which the wave heights of the absorption spectrum (31) of a disturbing gas close to the absorption spectrum of the gas to be measured are at the same level. 1. A signal processing method for a gas sensor, characterized in that modulation is performed using a rectangular wave (32) that oscillates between.