JP2002202257A - Gas detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform accurate gas detection by reducing a drift of an electric signal when a speckle pattern changes by reducing the brightness difference of the speckle pattern. SOLUTION: This gas detector 1 is equipped with a gas container 3 for containing a detected gas, a light source 5 for emitting divergent light into the container 3, a light diffusion surface 11 formed on an inner wall surface of the container 3 to diffuse the divergent light, and a photoreceptor 9 for receiving light from the container 3 concurrent with the launch of the divergent light. A light diffusion plate 21 is disposed in the container 3 to directly receive the divergent light incoming from the light source 5. The diffusion plate 21 is rotated in a prescribed cycle by a motor 23. Since the divergent light from the light source 5 is diffused by the diffusion plate 21 and the diffusion plate 21 is rotated in the prescribed cycle by the motor 23, the brightness difference of the speckle pattern of a laser beam received by the photoreceptor 9 is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas detection apparatus utilizing laser absorption spectroscopy, and more particularly to introducing a gas to detect the presence or absence of a target gas and to measure the concentration of the gas, thereby performing an interaction with a laser beam. The present invention relates to a gas detector having a gas cell to be made.

[0002]

2. Description of the Related Art Laser absorption spectroscopy is a type of spectroscopic technique used for analysis and the like, utilizing the property that a substance has a large absorption at a specific light wavelength. By applying this technology in the field of gas analysis, it becomes possible to detect and measure the concentration of a target gas.

FIG. 5 shows an example of an optical system of a conventional gas detector.
Shown in This optical system includes a laser light source 51, a gas cell 52, and a light receiver 53. The gas cell 52 generally includes a container 54 for storing gas, a gas introduction hole 55 and a gas discharge hole 56, and a first window 58 and a second window 59 through which a laser beam 57 passes. The laser light 57 emitted from the laser light source 51 enters through the first window 58, is partially absorbed by the gas contained in the container 54, emits from the second window 59, and reaches the light receiver 53.
When the wavelength of the laser beam 57 is changed near the absorption wavelength of the gas to be detected, a gas absorption curve as shown in FIG. 6 is observed. Using laser light of a wavelength as close as possible to the wavelength showing large absorption on this curve, the degree of absorption of the laser light by gas,
That is, the gas concentration (including gas detection) can be known by measuring the amount of change in the amount of laser light received by the light receiver 53.

[0004] By the way, when the gas concentration is actually measured, errors due to various causes occur. Among them, errors due to drift caused by laser light interference have a large absolute value and it is difficult to take countermeasures. Specifically, multiple reflections occur when the laser light passes through the window of the gas cell or through the optical system inside the light receiver, which interferes with the original laser light and appears as a change in light intensity. When this intensity fluctuation is superimposed on the absorption curve, it is observed as if the concentration of the detected gas has changed.

In order to avoid optical multiple reflection as much as possible, conventionally, a device as shown in FIG. 7 has been devised. In this example, antireflection films 61 are formed on both sides of a wedge-shaped glass plate.
Is used for the window material of the gas cell 52. By doing so, the influence of interference due to multiple reflection can be reduced.

[0006]

However, even if multiple reflections are suppressed by the gas cell 52, a similar problem occurs at the light receiver 53. For example, when a photodiode (hereinafter, referred to as PD) is used for the light receiver 53, multiple reflection occurs between the window of the PD container and the surface of the PD element, and this causes a measurement error. The error caused by the interference is
Since it is possible to occur in various places in the optical system, it is difficult to improve all optical components so that interference does not occur.

Accordingly, the present inventors have already filed an application for a gas detection apparatus in which a measurement error due to multiple reflection of light used for measurement is suppressed (Japanese Patent Laid-Open No. 2000-206035).

In the gas detecting device 71 disclosed in the above-mentioned Japanese Patent Laid-Open Publication No. 2000-206035, a light diffuser for diffusing a coherent laser beam to break the coherence is provided in an optical path inside or outside the gas cell. As a result, optical noise (hereinafter referred to as interference noise) caused by interference is reduced. As an example of the gas detection device disclosed in the above publication, in a gas detection device 71 shown in FIG. 8, the shape of a container 72 for containing a gas to be detected having a gas introduction hole 72a and a gas discharge hole 72b is a sphere. A light diffuser 73 of a reflection type is formed on the entire inner wall surface of the container 72,
The light source 74 and the light receiver 75 are mounted on the outside of the wall surface of the light emitting device.

When the gas detector 71 shown in FIG. 8 is employed, the light diffuser 73 is formed on the entire inner wall surface of the container 72, so that the divergent light from the light source 74 is emitted from the light diffuser 73.
When the light is diffused, speckles are generated by the light diffusion of the light diffuser 73 having a rough surface with minute unevenness. The speckles are irregularly distributed dark spots and bright spots that occur when an optically rough surface is illuminated with highly coherent light such as laser light as described above. Speckle is formed by the complex interference of many disordered component waves that are re-emitted at fine parts of a rough surface.

Generally, when coherent light such as laser light is illuminated on an optically rough surface, the illuminated surface reflects the laser light to generate a speckle pattern. The difference between the light and dark of the speckle pattern is reduced due to a decrease in the spatial coherence of the laser light.

If the speckle pattern generated as described above does not change its state, accurate gas detection and gas concentration measurement can be performed without affecting the measurement error.

However, when the container 72 is deformed due to a temperature change due to the heat emitted from the light source 74, a temperature change of the gas sent into the container 72, a temperature change due to the heat of a circuit incorporated in the apparatus, etc., the heat is formed on the wall surface of the container 72. Since the surface of the light diffuser 73 is also deformed, the speckle pattern also moves slowly and changes. This change in the speckle pattern is a phenomenon that occurs when the device 71 is screwed and installed, even if the container 72 touches the device 71 and a slight distortion occurs. When the speckle pattern changes due to a change in temperature or external stress as described above, a drift occurs in an electric signal corresponding to the amount of light received by the light receiver 75, and this drift causes a measurement error, making it impossible to perform accurate gas detection. The problem arises.

In view of the above, the present invention has been made in view of the above-mentioned problems, and the spatial coherence of laser light is reduced by actively changing the conditions of light diffusion to thereby reduce the speckle pattern. It is an object of the present invention to provide a gas detection device that can reduce the difference in brightness and darkness, reduce drift of an electric signal when a speckle pattern changes, and perform accurate gas detection.

[0014]

According to a first aspect of the present invention, there is provided a gas detecting apparatus for receiving a gas to be detected, and a light incident on the gas container. Light source 5, a light diffusion surface 11 formed on at least a part of an inner wall surface of the gas container 3 so as to diffuse the light, and light from the gas container 3 accompanying the incidence of light from the light source 5. And a light detector 9 for detecting the presence or absence and concentration of the gas to be detected based on the amount of light received by the light receiver 9, provided in the gas container 3, A light diffusing unit 21 for directly receiving and diffusing light from the light source 5 and a moving unit 23 for moving the light diffusing unit 21 at a predetermined cycle are provided.

According to a second aspect of the present invention, there is provided the gas detecting device according to the first aspect, wherein the moving means 23 is provided.
However, the present invention is characterized in that a rotating mechanism for rotating the light diffusing means 21 is formed.

According to a third aspect of the present invention, there is provided the gas detecting device according to the first aspect, wherein the moving means 23 is provided.
However, a vibration mechanism for vibrating the light diffusion means 21 is provided.

According to a fourth aspect of the present invention, there is provided a gas detector according to any one of the first to third aspects.
The light diffusing means 21 reflects light from the light source 5.

According to a fifth aspect of the present invention, there is provided a gas detection apparatus according to any one of the first to third aspects,
The light diffusing means 21 transmits light from the light source 5.

According to a sixth aspect of the present invention, there is provided a gas detection apparatus according to any one of the first to fifth aspects.
A plurality of the light diffusing means 21 and the moving means 23 are provided separately from a portion which directly receives light from the light source 5.

[0020]

Embodiments of the present invention will be specifically described below with reference to the drawings. FIG. 1 is a configuration diagram showing a first embodiment of the gas detection device of the present invention. As shown in FIG. 1, the gas detection device 1 mainly includes a gas container 3 and a light source 5.
And a light receiver 9.

First, the gas container 3 is configured as a gas cell for storing a gas to be detected. The gas container 3 in the present embodiment is formed to have a spherical inner wall surface. The material of the gas container 3 is a metal such as aluminum or stainless steel, or glass according to the type of the gas to be detected. Specifically, for example, carbon dioxide (CO
₂ ), non-corrosive gases such as methane (CH ₄ ) and nitrous oxide (N ₂ O) are preferably made of metal, such as ammonia (NH ₃ ) and hydrogen chloride (HCl). Glass is preferred for highly corrosive gases.

The inner wall surface of the gas container 3 is formed as a light diffusion surface 11 composed of minute uneven surfaces. More specifically, a metal such as aluminum or stainless steel having a rigidity is cut out by, for example, a lathe or a milling process to form a gas container 3. A diffusion surface 11 is formed. In addition, the light-diffusing surface 11 may be a metal such as aluminum or stainless steel, a frosted surface made of a reflective material such as glass, an aluminum or gold vapor-deposited surface, or a reflectance. A powder coated with a large powder (polytetrafluoroethylene, barium sulfate, sulfur, etc.) can be used.

The gas container 3 has a gas inlet 13 for introducing a gas to be detected and a gas outlet 15 for discharging the gas to be detected.
Are provided. As a result, a certain amount of the gas to be detected is introduced into the gas container 3 through the gas introduction hole 13 and accommodated therein. When the detection and concentration measurement of the gas to be detected contained in the gas container 3 are completed, the gas container 3
Is discharged from the gas discharge hole 15.
Thereafter, a gas to be detected, which is a target of the next detection and concentration measurement, is introduced from the gas introduction hole 13 and gas replacement is performed.

The gas container 3 has a first through hole 17 and a second through hole 19. The divergent light from the light source 5 enters the gas container 3 from the first through hole 17. Further, diffused light from the inside of the gas container 3 is emitted from the second through hole 19 toward the light receiver 9. The airtightness between the first through hole 17 and the light source 5 and the airtightness between the second through hole 19 and the light receiver 9 are provided by using a sealing member such as an O-ring.

Next, the light source 5 is configured as a laser light source employing a semiconductor laser (LD) in the present embodiment. Although not shown, the light source 5 has a unit structure in which, for example, a semiconductor laser, an electronic cooling element, a reference gas cell, and a reference light receiver are housed in a closed container. The electronic cooling element precisely controls the temperature of the semiconductor laser disposed thereon. The reference gas cell and the reference light receiver are
Using the backward light from the semiconductor laser, the oscillation wavelength of the semiconductor laser is adjusted to the center wavelength of the absorption spectrum of the gas to be detected.

The light source 5 having the above configuration is arranged such that the element end face side from which the forward light is emitted from the semiconductor laser views the inside of the gas container 3 from the first through hole 17 of the gas container 3. Thus, the light source 5 emits divergent light without requiring components such as a collimator lens and an optical isolator.

Next, the light receiver 9 is formed of, for example, a photodiode, and is provided so as to view the inside of the gas container 3 from the second through hole 19 of the gas container 3. In addition, the second through hole 19 is connected to the gas container 3 through the first through hole 17.
Are disposed so as not to directly receive the divergent light from the light source 5 emitted into the inside. As a result, the light receiver 9 is
It is arranged without directly receiving the divergent light from.

When the divergent light is emitted from the light source 5, the light receiver 9 receives the diffused light diffused by the light diffusing surface 11 inside the gas container 3 containing the gas to be detected. And
An electric signal corresponding to the amount of the diffused light received by the gas to be detected is output. Note that the gas detection device 1 of the present embodiment
Then, the electric signal output from the light receiver 9 is sampled at every predetermined frequency (for example, every 10 kHz).

As shown in FIG. 1, a light diffusing plate 21 as a light diffusing means is disposed inside the gas container 3 described above. The light diffusion plate 21 includes a light transmission type and a light reflection type. The light transmission type is commercially available as a polytetrafluoroethylene plate or a acrylic resin plate polished with sandpaper or the like, or an optical glass polished with an abrasive, and processed into a glass form, or as a "light diffusion plate" or "dephaser" Optical components are employed. The light-reflective type has a reflective surface such as metal or glass finished in a frosted state, an aluminum or gold deposited on these surfaces, or a powder having a large reflectivity (polytetrafluoroethylene, Those coated with barium sulfate, sulfur, etc. are employed.

The light diffusing plate 21 is formed in a flat plate shape as shown in FIG. 1, and is arranged in an optical path of the divergent light emitted from the light source 5 and in a range where the divergent light is directly received. The range in which the divergent light is directly received is at least the level of the light intensity of 1 / e ² when the peak of the light intensity of the light beam from the light source 5 which is the divergent light is set to 1. Good.

The light diffusing plate 21 can be moved by moving means. As the moving means, a motor 23 forming a rotating mechanism as shown in FIG. 1 can be considered. Light diffusion plate 21
Rotates by the drive of the motor 23. Motor 23
Is different from the above-mentioned sampling frequency (10 kHz),
And it is not an integral multiple, for example, 0.9 kHz or 1.
It is set so as not to be synchronized with the sampling frequency such as 1 kHz.

The light diffusing plate 21 shown in FIG. 1 is arranged such that the flat surface is orthogonal to the optical axis S of the divergent light (laser light) emitted from the light source 5. Also, the motor 23
Are arranged such that their output shaft 23a is on the optical axis S. That is, the light diffusing plate 21 has a flat surface that receives the divergent light orthogonal to the optical axis S of the divergent light from the light source 5 and rotates around the optical axis S.

In the above configuration, when the light source 5 is driven in a state in which the gas to be detected is accommodated in the gas container 3 from the gas introduction hole 13, the divergent light from the light source 5 passes through the first through hole 17. Is irradiated inside the gas container 3. This divergent light
The light is diffused variously by the light diffusion surface 11 on the inner wall surface of the gas container 3. Further, the laser light diffused by the diffusion surface is reflected a plurality of times inside the gas container 3. Then, the laser light that has passed through various paths due to multiple reflections is received by the light receiver 9 through the second through hole 19.

The light source 5 receives the light from the light receiver 9 inside the gas container 3.
Is absorbed by the gas to be detected. As a result, in the photodetector 9, the level of the electric signal output varies due to the change in the amount of received light. That is, the presence / absence of gas is detected based on the level fluctuation of the electric signal, and the concentration of the gas is detected based on a high level difference.

Here, inside the gas container 3, a part of the divergent light emitted from the light source 5 reaches the light diffusion plate 21 and diffuses before reaching the light diffusion surface 11. The rotation of the light diffusion plate 21 causes the light diffusion plate 21 to rotate at a predetermined cycle.
Changes the diffusion condition of the laser light diffused by. The laser light diffused by the light diffusion plate 21 is reflected a plurality of times by the light diffusion surface 11 on the inner wall surface of the gas container 3 as described above, is variously diffused, and reaches the light receiver 9.

Therefore, in the gas detection device 1 having the above-described structure, the container is deformed due to a temperature change due to heat emitted from the light source 5, a temperature change of gas exposed to the outside air, a temperature change due to heat of a circuit incorporated in the device, and the like. The movable light diffusing plate 21 can be changed so that the resulting change in the speckle pattern can be ignored.
As a result, the speckle pattern positively and randomly changes and moves to reduce the difference in brightness, so that interference noise due to the speckle pattern is reduced, and
Can reduce the drift of the electrical signal output by
Accurate gas detection can be performed without measurement errors.

In the above-described embodiment, the light diffusing plate 21 has its flat surface orthogonal to the optical axis S of the diverging light from the light source 5 and the motor 23 rotates the light diffusing plate 21 about the optical axis S. However, the present invention is not limited thereto, and the above-described effects can be obtained even with the configuration described below.

Specifically, in FIG. 2, as a second embodiment, the flat surface of the light diffusing plate 21 is arranged orthogonal to the optical axis S of the divergent light from the light source 5, and the output shaft 23a of the motor 23 is Optical axis S
The light diffusing plate 21 is rotated and moved as a position out of the range.

In FIG. 3, as a third embodiment, the flat surface of the light diffusing plate 21 is inclined so as not to be orthogonal to the optical axis S of the divergent light from the light source 5 (or the output shaft 23a of the motor 23). The diffusion plate 21 is eccentrically rotated. In the case of the configuration of FIG. 3, the rotation center of the light diffusion plate 21 (the output shaft 23a of the motor 23) may be either on the optical axis S or at a position off the optical axis S.

FIG. 4 shows a fourth embodiment in which, in addition to the light diffusing plate 21 which directly receives the divergent light of the light source 5, another light diffusing plate 21 is rotated. In this case, the light diffusion plate 21 and another light diffusion plate 21 which directly receive the divergent light, and the form of the position of the rotation center by the motor 23 may be any of the forms shown in FIGS. As described above, if there is another light diffusion plate 21, the condition of light diffusion is further changed, interference noise due to the speckle pattern is reduced, and more accurate gas detection can be performed.

Further, in the embodiments shown in FIGS. 1 to 4, the motor 23 is described as the moving means, but a vibration mechanism for moving the light diffusion plate 21 by vibration may be used as the moving means. Good. In this case, as the vibration mechanism, for example, a vibrator used as a vibration calling device such as a mobile phone, a voice coil motor, a linear actuator such as a PZT element using a piezo effect,
It is conceivable to employ an ultrasonic generator or the like. Also,
The vibration frequency is the above sampling frequency (10 kHz)
Are different frequencies and are not integral multiples of
For example, it is set so as not to be synchronized with the sampling frequency, such as 0.9 kHz or 1.1 kHz. The direction of vibration includes a direction along the optical axis S, a direction crossing the optical axis S, or a composite direction thereof.

1 to 4, the shape of the light diffusing plate 21 is not limited to a flat plate, but may be concave or convex toward the divergent light of the light source 5.
Alternatively, a concave and convex surface may be formed by combining a concave surface and a curved surface.

Also, in the gas detection device 1 according to each of the above-described embodiments, the case where the inner shape of the gas container 3 forming the gas cell is a sphere is described, but the gas container 3 has at least one of the inner wall surfaces. The shape is not limited to a sphere as long as the light diffusion surface 11 is provided on the portion, and may be, for example, an ellipsoid, an ellipsoid, a cone, or the like.

In the gas detector 1 according to each of the above-described embodiments, a single light receiver 9 for receiving laser light emitted from the light source 5 and diffused inside the gas container 3 is illustrated as a single unit. As described above, a plurality of light receivers 9 may be provided at portions not directly receiving divergent light from the light source 5.

[0045]

As described above, in the gas detector according to the present invention, the light emitted from the light source is diffused by the light diffusing means and the condition of light diffusion is changed by the moving means. The difference between the light and dark of the speckle pattern on the surface is reduced, the electric signal according to the received light amount can be detected stably, and the drift of the electric signal when the speckle pattern changes reduces the gas detection accurately. It can be carried out.

As the light diffusing means, a reflection type or a transmission type is used.
In addition, the same effect as described above can be obtained by various configurations such as using a rotating mechanism or a vibration mechanism as the moving means.

A plurality of light diffusing means and moving means may be provided separately from the part which directly receives light from the light source. According to this, the condition of light diffusion is further changed and interference noise due to speckle pattern is generated. Is reduced, and more accurate gas detection can be performed.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a first embodiment of a gas detection device of the present invention.

FIG. 2 is a configuration diagram showing a second embodiment of the gas detection device of the present invention.

FIG. 3 is a configuration diagram showing a third embodiment of the gas detection device of the present invention.

FIG. 4 is a configuration diagram showing a fourth embodiment of the gas detection device of the present invention.

FIG. 5 is a diagram showing an example of an optical system of a conventional gas detection device.

FIG. 6 is a view showing a gas absorption curve.

FIG. 7 is a diagram showing a configuration of a conventional gas detection device.

FIG. 8 is a diagram showing another configuration of a conventional gas detection device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Gas detector, 3 ... Gas container, 5 ... Light source, 9 ... Light receiver, 11 ... Light diffusion surface, 21 ... Light diffusion means (light diffusion plate),
23. Moving means (motor).

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Takeshi Tsukamoto F-term (reference) 2G057 AA01 AB02 AB04 AB06 AC03 BA05 BB08 DA10 2G059 AA01 BB01 DD12 DD13 EE01 within Anritsu Corporation 5-10-27 Minamiazabu, Minato-ku, Tokyo EE12 GG01 GG02 HH01 JJ26 KK04 LL04 NN01 NN02 NN06

Claims (6)

[Claims] 1. A gas container (3) for containing a gas to be detected, a light source (5) for entering light into the gas container, and at least one of inner wall surfaces of the gas container so as to diffuse the light. A light diffusing surface (11) formed in a portion, and a light receiver (9) for receiving light from the gas container accompanying the incidence of light from the light source, based on the amount of light received by the light receiver. A gas detecting device for detecting presence or absence and concentration of the gas to be detected, provided inside the gas container, for directly receiving and diffusing light from the light source; Moving means (2) for moving
3) A gas detection device comprising: 2. The gas detection device according to claim 1, wherein the moving means (23) forms a rotating mechanism for rotating the light diffusing means (21). 3. The gas detecting device according to claim 1, wherein the moving means (23) forms a vibration mechanism for vibrating the light diffusing means (21). 4. The light diffusion means (21) for reflecting light from the light source (5).
The gas detection device according to claim 3. 5. The light diffusion means (21) for transmitting light from the light source (5).
The gas detection device according to claim 3. 6. The light diffusing means (21) and the moving means (23) are provided in a plurality in addition to a part which directly receives light from the light source (5). The gas detection device according to claim 5.