JPS61240026A - Multiple visual field type flame detector - Google Patents

Abstract

PURPOSE:To prevent error signals from developing by reducing the lowering of the relative light transmittances of optical fibers even when the optical fibers are bent by a method wherein graded index type optical fiber is employed as the optical fiber subject to bending work and at the same time the only one peak of transmittance is made so as to correspond to the predetermined angle of incidence. CONSTITUTION:Light signals from three visual fields 15, 16 and 17 are led through a 3-core junction fiber cable 5 to a photoelectric conversion part 10. The photoelectric conversion part 10 has a structure laminating a detector of infrared light (1-2.5mum in wavelength) to a detector of visible-near infrared light (0.4-0.9mum in wavelength) so as to detect lights with in said two wavelength regions with each of three laminated type photoelectric conversion elements in order to obtain six electric signals in total. Only the variation with the range of about 50Hz-1kHz, which is characteristic to the light signals emitted from the primary combustion zone of flame, is selectively amplified with a frequency discriminating amplifier 11, changed-over at a certain period (for example 50 ms) by a multiplexer 12, converted by an A/D converter 13 and finally introduced in a flame judging part 14 in order to judge the presence or absence of flame by whether at least one of the six electric signals exceeds the pre-set threshold value or not.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a flame detector, and more particularly to a multi-view flame detector using a plurality of optical fibers.

[Conventional technology and its problems]

In various combustion devices, automatic control is increasingly being used to save labor and optimize control, but in this case, it is more difficult to accurately determine whether each burner should be ignited or extinguished than with manual control. Even more strongly desired. In the case of a small-capacity burner such as an ignition burner, a flame contact type flame detector is often used, which detects ions in the flame as an ion current by bringing an electrode rod into contact with the flame. However, there is a risk of burnout when using this type of equipment for a long period of time. It is used.

The light emitted by a flame has a wide range of wavelengths, from the ultraviolet to the infrared, and which wavelength range is effective in determining the presence or absence of a flame depends on the type of fuel, combustion conditions (air-fuel ratio, It depends largely on the presence or absence of exhaust gas recirculation, etc.). Generally, direct current light in the ultraviolet region is used for gas fuel, and flickering light (in the visible-infrared region) is used when burning oil or coal.
A method of detecting fluctuating light) is often used.

FIG. 3 shows a conventional flame detector using optical fibers.

The light from the flame within the field of view 24 in the figure is collected by a condensing lens 26 which is a light guide. A photoelectric conversion unit 23 installed in a place where heat radiation from the flame is small via the optical fiber 21 inside the protection tube 22
After converting it into an electric signal, the detection circuit 25 determines whether there is a flame or not.

In boilers for business use, the burner capacity is large and the number of installed burners is large, for example, 48 to 50, so there is a risk of flame interference between adjacent burners, which tends to cause fluctuations in the flame position of each burner. Furthermore, mixed combustion of exhaust gas has been strengthened as one of the nitrogen oxide (Not) reduction combustion methods, and the phenomenon of flame blow-off is becoming more likely to occur.
The positional fluctuations of the flame to be detected tend to increase.

In view of the above-mentioned problems, the inventors have separately proposed a multi-view flame detector that can follow even if the combustion conditions change and the flame position increases.

1 Figure 4 shows the structure of the light guiding section (light receiving section) of this multi-field flame detector. In the figure, the light receiving section has three different fields of view with fiber angles θf (θf=o, 15", 30"), and even if the flame position changes, at least one of each fiber is always in the flame position. In order to detect the primary combustion region (the core of the flame), one of the three optical fibers is selected and the detection light is transmitted to the determination section via the connector 28. In this structure, in order to reduce the size of the tip of the light guide part, θ
As a method of forming the angle f, heat bending is performed, and the metal mount portion 27 is placed and integrated.

However, by bending the optical fiber, the relative light transmittance decreases to less than half of the value before the bending process, and there is a risk that there will be multiple transmittance peaks, resulting in the generation of erroneous signals.

[Object of the present invention]

The present invention was constructed in view of the above-mentioned problems, and it is an object of the present invention to provide a flame detector in which the decrease in relative light transmittance is small even when an optical fiber is bent, and which does not generate false signals.

[Summary of the invention]

In summary, the present invention minimizes the decrease in relative light transmittance by using the optical fiber/Nl to be bent as a gray rad index type optical fiber, and also has one transmittance peak corresponding to a predetermined angle of incidence, thereby reducing erroneous signals. It is designed to prevent this from occurring.

〔Example〕

Examples of the present invention will be specifically described below.

In Figure 1, field of view 15 (fiber angle θf-0 degrees), field of view 16 (θf-15 degrees), field of view 17 (θf-30 degrees),
The optical signal from the light guide section (light receiving section) disposed inside the ceramic cover 6 is transmitted from the tip 1. Optical fiber relay connector 2. Optical fiber 3. Another fiber optic relay connector 4. The signal is introduced into the photoelectric conversion unit 10 via the relay fiber cable (3 cores) 5 and converted into an electrical signal. Of the electrical signals after conversion, only the high frequency fluctuations of about 50 Hz or more are selectively amplified by the frequency discrimination amplifier 11, guided to the multiplexer 12, where the signal is switched by a certain amount M (for example, 50 m), and the A/
After being converted into a digital signal by the D converter 13, the signal is introduced into a flame determination section 14 to determine the presence or absence of flame.

In the above configuration, the tip of the light guide, which has a large amount of heat radiation from the flame, is housed in the ceramic cover (made of alumina) 6 as described above, and the light from the flame is introduced through the slit at the tip. It has become.

Also, the light guide section 1. The relay connector 2.4 and the optical fiber 3 are connected to the protective outer tube 7. It is cooled by cooling air flowing inside the protective inner tube 8, and the cooling air is blown out from the tip of the protective outer tube 7.
This is to prevent dust from adhering to the light guide tip 1.

Optical signals from the three fields of view (15, 16, 17) are guided to the photoelectric conversion unit 10 by a three-core relay fiber cable 5, and the photoelectric conversion unit includes an infrared light (1 to 2.5 μm wavelength) detection element and , visible-near infrared light (0.4-0.9 μm wavelength)
It has a structure in which detection elements are stacked, and each of the three stacked photoelectric conversion elements detects light in two wavelength ranges.

As a result, a total of six electrical signals are obtained. Note that by adopting a stacked photoelectric conversion element, the structure can be simplified and the cost can be reduced.

Next, the frequency discrimination amplifier 11 selectively amplifies only the fluctuations of about 50Hz to IKHz that are specific to the optical signal from the second combustion region of the flame, and it amplifies only the fluctuations of about 50Hz or less due to backlight, adjacent/opposed flames, etc. This eliminates the effects of low-frequency fluctuation light and prevents the generation of erroneous signals. The flame determination unit 14 determines whether or not there is a flame, depending on whether at least one of the six electrical signals exceeds a preset threshold value at each switching cycle in the multiplexer 12.

FIG. 2 shows the structure of the tip of the light guide.

Three quartz-based graded indexes, that is, distributed refractive index (G
1) type optical fiber 18 is installed. All three optical fibers have all coatings such as jackets and sheath materials removed, and consist only of a core portion and a cladding portion around it, increasing heat resistance. In this case field of view 15
The optical fiber of 1 is not bent, but the optical fiber corresponding to the field of view 16°j 17 is thermally bent to achieve a predetermined fiber angle θr.

Figures 6 (A) and (B) show the step index (S
The difference in refractive index distribution between the r) type optical fiber and the above-mentioned Gl type optical fiber is shown. In the SL type optical fiber, the refractive index of the core portion 29 is uniform in the core radial direction, whereas
In the GI type optical fiber, the refractive index of the core portion 29 is distributed type, and the core portion 29 has a characteristic that the convergence to the center is higher than that of the SI type.

FIGS. 7(A) and 7(B) schematically show the optical transmission path in the core section 29 of each SR type and CI type optical fiber.
Since the core of the mold has a uniform refractive index, it is difficult for incident light from the a direction to be transmitted, but the mold is not easily affected by zero bending. Here, Fig. 5 shows the relative light transmittance of the SL type optical fiber, and in the case of θf-0@, 1.0 is achieved, whereas in the case of θf = 15° and 30'', Furthermore, by bending the fiber, multiple transmittance peaks will occur for each fiber, such as P and ~P#, which can cause false signals. This shows that performance deteriorates.

FIG. 8 shows the optical transmission characteristics when a Gl type optical fiber is thermally bent. As is clear from the drawing, even when the Gl-type fiber is bent, the relative transmittance maintains approximately 0.5 and has a transmittance peak only at a predetermined angle of incidence, which prevents the generation of false signals and improves the Performance can be improved.

〔effect〕

Since the present invention is as described above, it is possible to significantly improve the performance of a multi-view flame detector that is currently being proposed separately.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram showing an embodiment of the flame detector according to the present invention, Fig. 2 is a sectional view of the tip of the light guide section of the flame detector according to the present invention, and Fig. 3 is a conventional one using an optical fiber. Figure 4 is a cross-sectional view of the tip of the light guide of a conventional flame detector, Figure 5 is a diagram showing the light transmission characteristics of the tip of the light guide of a conventional flame detector, and Figure 6 is a diagram showing the light transmission characteristics of the tip of the light guide of a conventional flame detector. Figure (A) is SL type, same (B) is GI type.
Fig. 7 is a schematic diagram showing the refractive index distribution of each type optical fiber, (A) is a schematic diagram showing the optical transmission path of the 31 type optical fiber, and (B) is a schematic diagram showing the optical transmission path of the Gl type optical fiber. FIG. 3 is a diagram showing the light transmission characteristics at the tip of the light guide section of a flame detector. l...Light guiding part (light receiving part) tip, 3... Optical fiber,
10... Photoelectric conversion unit, 18... Gl type optical fiber. Figure 1 Figure 2 Figure 3 Figure 4 Figure 5

Claims (2)

[Claims] (1) At least some of the optical fibers located in the light receiving section are bent at different angles to make the incident angles of the optical fibers different, and at least some of the optical fibers are bent. A multi-view flame detector characterized in that the optical fiber is a distributed index optical fiber (GI type optical fiber). (2) A photoelectric conversion section in which a photoelectric conversion element in the infrared wavelength range and a photoelectric conversion element in the visible-near infrared wavelength range are stacked is connected to the light receiving part. The multi-field flame detector according to item (1).