JP3638261B2 - Smoke density measuring device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a smoke concentration measuring apparatus using a light transmission method.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, a light transmission type smoke concentration measuring device (smoke meter) is known for measuring the concentration of particulate matter discharged from a combustion engine or airborne particulate matter. A conventional light transmission smoke density measuring device is arranged with the light source and light receiving part facing each other, the light path from the light source to the light receiving part intersects with the smoke passage, and the smoke density in the light path is measured from the attenuation rate of the transmitted light To do. In such a light transmission type measuring device, the smoke concentration is measured from the attenuation rate of light that has passed through the sample smoke only once, so the intersection (measurement region) between the light path and the sample smoke path. There was a fault that the sensitivity to low concentration was low. If an attempt is made to enlarge the measurement area in order to eliminate this drawback, the apparatus will be enlarged. In addition, since the crossing portion is small, there is a disadvantage that the temporal variation of the temperature and density of the smoke is large and the output value of the measuring device is not stable.
[0003]
In addition, in the Japanese Patent Application Laid-Open No. 8-43305, the present inventor performs vapor deposition on the circumference of a cylindrical glass tube, and slightly displaces it from the central axis so as to be perpendicular to the central axis from the side of the cylindrical glass tube. A smoke density measuring device has been proposed in which a smoke density is measured by irradiating parallel laser light and reflecting the laser beam multiple times in the measurement region. The number of reflections at this cylindrical glass tube is a function of the distance of the closest point of laser light from the center of the cylindrical glass tube. According to this apparatus, the optical path length becomes long and the smoke density can be measured with high sensitivity.
[0004]
[Problems to be solved by the invention]
In such a conventional apparatus, if the ideal thin parallel light can be irradiated toward the glass tube, the light can be transmitted through the measurement region any number of times. However, since the incident light actually has a certain beam width, the distance from each part to the center of the cylindrical axis is different from the cylindrical axis side to the outer part of the glass tube of the beam. Accordingly, as the number of reflections increases, the reflection point on the mirror shifts due to the laser beam portion, and light with different numbers of reflections eventually overlaps and enters the light receiving unit. For this reason, light with a short optical path length with a small number of reflections is also received at the same time, causing a problem that sensitivity is lowered. Therefore, the practical number of reflections was about seven.
[0005]
The present invention has been made in view of such a conventional problem, and provides a smoke density measuring device capable of measuring the density of smoke with high sensitivity and low density by increasing the number of reflections. The purpose is to do.
[0006]
[Means for Solving the Problems]
The invention of claim 1 of the present application includes a cylindrical glass tube in which a reflection film is formed at a position symmetrical with respect to the central axis of the outer peripheral surface and smoke to be measured is introduced, and the central axis of the glass tube A light source unit that has an incident optical axis perpendicular to the glass tube and that does not pass through a point on the central axis of the glass tube, and enters the laser beam from the side of the glass tube toward the reflective film, and is incident from the light source unit And a light receiving unit that receives reflected light that has been multiple-reflected from the reflective film of the glass tube, and the light source unit focuses light from the light source in a plane perpendicular to the central axis of the glass tube, A cylindrical lens having a focal point on the optical axis at a position having a predetermined deviation from a central point of the diameter of the glass tube parallel to the incident optical axis in a plane including incident light and total reflected light; It is a feature.
[0007]
The invention according to claim 2 of the present application is the smoke concentration measuring apparatus according to claim 1, wherein the light source unit is predetermined by a diameter of the glass tube parallel to the incident optical axis in a plane including the incident light and total reflected light. are those having a deviation incident laser beam, it is characterized in that it has a deviation setting means for setting the laser beam deviation of the light source unit.
[0008]
According to the present invention having such a feature, the reflection film is formed symmetrically with respect to the central axis of the cylindrical glass tube, and smoke to be measured is introduced into the glass tube. Light is incident from the side of the glass tube from the light source. The incident light does not pass through the center of the glass tube only in a plane perpendicular to the cylindrical axis by the cylindrical lens, but is converged at a point closest to the center and incident on the reflection film. Then, multiple reflections are made by the reflective film a number of times corresponding to the deviation from the center point, and light is attenuated according to the concentration of smoke passing through the glass tube. The light receiving unit receives the reflected light and calculates the density of smoke passing through the glass tube based on the received light level.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a transverse sectional view of a smoke concentration measuring apparatus according to an embodiment of the present invention, and FIG. 2 is a longitudinal sectional view thereof. As shown in these drawings, the smoke concentration measuring apparatus has a duct 1 to which smoke as a sample, for example, exhaust gas from a diesel engine or the like is added, and its tip is opened as a nozzle 2. A transparent glass tube 3 covering the outer periphery of the nozzle 2 is provided, and a suction duct 4 for sucking smoke passing through the nozzle 2 is further provided. Here, the glass tube 3 is assumed to have a transparent and accurate cylindrical shape, and a thin film is formed as a reflective film in a part thereof. As shown in FIG. 2, the thin film mirrors 5A and 5B are formed in a range of ¼ of the entire circumference symmetrically with respect to the center of the glass tube. These thin film mirrors 5 are formed in a band shape in a range of a certain width D from the outer periphery of the glass tube 3. The suction duct 4 may not be directly discharged to the atmosphere.
[0010]
As shown in FIGS. 1 and 2, a light source unit 10 is provided on the side of the glass tube 3. The light source unit 10 includes, for example, a laser diode 11 and a driving unit 12 that drives the laser diode 11. A cylindrical lens 14 is provided in the vicinity of the collimating lens 13 and the glass tube 3 for converting the laser light into parallel laser light. The cylindrical lens 14 is focused only in a plane perpendicular to the cylindrical axis in FIG. 2 (parallel to the paper surface), and is focused at the shortest point from the central axis of the glass tube 3. If it carries out like this, as it mentions later by the thin film mirror 5 of the glass tube 3, it will reflect repeatedly and will be radiate | emitted from a glass tube. The beam emitted from the glass tube 3 is transmitted to the light receiving unit by the lens 15.
[0011]
The light receiving unit includes a focusing lens 15 that focuses the emitted light, and a photoelectric converter, for example, a photodiode 16, that receives the focused laser light and converts it into an electrical signal. The output of the photodiode 16 is supplied to a latch unit 18 and a divider 19 through an amplifier 17. The latch unit 18 is a sample and hold circuit that holds the amplified output, and the output is given to the divider 19. The divider 19 divides these inputs and gives the output to the concentration calculator 20. The concentration calculation unit 20 calculates the concentration from the table based on this split calculation power. The deviation setting unit 21 moves the light source unit 10 in the vertical direction in FIG. 2 and sets a deviation d _X described later to an arbitrary value.
[0012]
Next, the operation of the present embodiment will be described. First, the laser diode 11 is driven by the drive unit 12 to emit laser light. The collimating lens 13 and the cylindrical lens 14 irradiate the measurement region in the glass tube 3 with a laser beam having a sufficiently thin diameter with respect to the cylindrical curvature. At this time, it is assumed that no smoke is guided to the duct 1. In this way, even if the laser beam passes through the measurement region, it is not attenuated by smoke or dust. Here, the laser beam is irradiated toward the thin film mirror 5B so as to be focused with a predetermined deviation d _x at a point closest to the center point slightly below the center point of the glass tube 3 indicated by a one-dot chain line. In this way, the focal point is formed at the same distance from the center point, and the light enters the substantially symmetrical position of the glass tube 3. Reflected again from this point, the same point data is again focused from the central point and reflected. In this way, the reflection position is gradually changed and the same operation is repeated. Further, since the curvature of the glass tube does not affect the direction perpendicular to the paper surface, the light beam is repeatedly reflected as shown in FIG.
[0013]
Here, the deviation between the diameter 3a of the glass tube 3 and the incident laser beam is d _x . Here, assuming that the radius of the outer diameter of the glass tube 3 is r ₁ and the radius of the nozzle 2 is r ₂ , the one-time measurement optical path length d _M and the reflection angle θ of the light beam are expressed by the following equations.
d _M = 2 × √ (r ₂ ² −d _X ² )
θ = sin ⁻¹ (d _X / r ₁ )
And it can be arbitrarily selected number of reflections by varying the deviation d _X. The number of passes N when light passes through the measurement region is the number of reflections + 1, and is expressed by the following equation.
N = INT {(π / 2−θ) / 4θ} × 2 + 1
The total optical path length L of light is expressed by the following equation.
L = N · d _M
[0014]
In this way, even if reflection is repeated, the light beam inside the glass tube does not gradually spread, and the number of reflections can be practically 20 or more. For example, if the outer diameter r ₁ of the glass tube is 79 mm and the deviation d _X is 1.6 mm, as shown in FIG. 3, it is reflected 19 times by the thin film mirrors 5A and 5B of the mirror and is above the lower thin film mirror 5B. Reflected. This reflected light has passed through the measurement area 20 times. This light is received by the photodiode 16, and the level of the reflected light is amplified and held in the latch unit 17.
[0015]
Here, the received light level is expressed by the following equation.
I = I ₀ exp (Lβ)
L is the total optical path length and β is a function proportional to the particle concentration. I ₀ is the incident light intensity.
[0016]
Next, smoke as a sample is introduced into the duct 1 and sucked from the suction duct 4 side, and passed through the measurement region. In this state, the measurement region is irradiated with laser light in the same manner. By doing so, the laser beam passes through the measurement region 20 times, and the laser beam attenuates according to the smoke concentration. Therefore, the smoke concentration can be calculated by measuring the degree of attenuation. If smoke such as dust passes through this measurement region, the laser light will be attenuated according to the concentration of dust. That is, the divider 19 calculates a ratio with the intensity of the laser beam in a state where the smoke held in the latch unit 18 is not allowed to pass through. By performing the division, the influence of the laser beam intensity I ₀ is removed, and the smoke concentration can be calculated. Since the optical path length L is constant, β can be calculated from the split calculation force. The particle concentration may be stored as a table in advance in the concentration calculation unit with respect to the divided value, and the value may be output, or the concentration may be directly calculated from this function.
[0017]
Here, in the present embodiment, the incident light is laser light having a deviation dX (1.6 mm in this case) from the horizontal diameter 3a of the glass tube 3, but the incident light is changed by changing the value of the deviation dX. The angle θ changes, and the number of passes N changes due to the change in the incident angle θ. The total optical path length L also changes. If the deviation dX of the incident light is appropriately changed, the total optical path length L can be significantly changed. Therefore, by setting the deviation dX corresponding to the smoke concentration by the deviation setting unit 20, the total optical path length can be selected as appropriate, thereby making it possible to accurately measure the density. Particularly in the case of a low concentration, the concentration can be measured with high accuracy by increasing the total optical path length L.
[0018]
In the present embodiment, as shown in FIG. 2, 1/4 thin film mirrors are formed in two places where the center of the glass tube 3 is point-symmetric, but the range is limited to 1/4. Instead, it is only necessary to form a thin film mirror with the center of the glass tube being point-symmetric. Therefore, a thin film mirror may be formed in a larger range or a smaller range. In addition, the thin film mirror may be intermittently formed only in a portion where light is actually incident and reflected.
[0019]
【The invention's effect】
As described above in detail, according to the present invention, the measurement optical path length can be increased in the measurement region through which smoke passes through multiple reflections by using the cylindrical lens. Therefore, the sensitivity is increased and measurement can be performed with high accuracy even when the smoke density is low. Further, by using an accurately shaped glass tube, a reflective film can be formed accurately. Therefore, the number of reflections can be significantly increased as compared with the conventional smoke density measuring apparatus, and measurement can be performed with high sensitivity. Further, by appropriately changing the number of reflections, an effect is obtained that the smoke density from a low density to a high density can be measured with high accuracy by one measuring device.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a smoke concentration measuring apparatus according to an embodiment of the present invention.
FIG. 2 is a longitudinal sectional view of an optical axis portion of a smoke concentration measuring device according to an embodiment of the present invention.
FIG. 3 is a diagram showing a state of multiple reflection on a glass tube.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Duct 2 Nozzle 3 Glass tube 4 Suction duct 5, 5A, 5B Thin film mirror 10 Light source part 11 Laser diode 12 Drive part 13 Collimate lens 14 Cylindrical lens 15 Focusing lens 16 Photo diode 18 Latch part 19 Divider 20 Density calculation part 21 Deviation setting section

Claims (2)

A cylindrical glass tube in which a reflection film is formed at a position symmetrical with respect to the central axis of the outer peripheral surface, and smoke to be measured is introduced;
A light source unit that has an incident optical axis perpendicular to the central axis of the glass tube, and that emits laser light from a side of the glass tube toward a reflection film that does not pass through a point on the central axis of the glass tube; ,
A light receiving unit that receives the reflected light incident from the light source unit and multiple-reflected on the reflective film of the glass tube,
The light source unit focuses light from the light source in a plane perpendicular to the central axis of the glass tube, and a focusing point on the optical axis is parallel to the incident optical axis in a plane including incident light and total reflected light. A smoke concentration measuring apparatus comprising a cylindrical lens having a predetermined deviation from a center point of the diameter of the glass tube.   The light source unit is configured to enter laser light with a predetermined deviation from the diameter of the glass tube parallel to the incident optical axis within a plane including the incident light and total reflected light. 2. The smoke concentration measuring apparatus according to claim 1, further comprising deviation setting means for setting a deviation of the laser beam.

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