Flame sensor

Details Flame sensor Flame sensor

2001-09-06

2004-01-13

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

Infrared responsive

C250S339140

Reexamination Certificate

active

06677590

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to a flame sensor. More particularly, this invention relates to a flame sensor capable of detecting a flame in places where solar rays and artificial rays of light are present without being affected by such rays of light.
2. Description of the Related Art
To detect a flame, there is a convenient method that detects resonance radiation generated by a high-temperature carbonic acid gas contained in the flame, as is well known in the art. A line spectrum of resonance radiation of the carbonic acid gas includes many wavelengths. To discriminate the line spectrum from ordinary artificial illumination and solar rays, it is appropriate to utilize a spectral line within the range of the infrared region or the ultraviolet region for detecting the flame.
Optical components belonging to both these regions do not much exist in artificial rays of light such as illumination, so disturbance by external light when sensing a flame is less in these regions.
To detect a flame in the presence of solar rays, a conventional method detects the line spectrum due to resonance radiation of the carbonic acid gas generated by the flame. To discriminate a continuous spectrum, such as solar rays and artificial light, from the line spectrum of the flame, this method compares and computes a plurality of outputs obtained from a monochromatic filter having a narrow-band that permits the passage of only the line spectrum of the flame and from monochromatic filters of a plurality of narrow-bands, which permit the passage of rays of light having one or a plurality of wavelengths, and the method discriminates whether light is the line spectrum of the flame or the continuous spectrum of the solar rays.
Another method utilizes flicker of light generated by the flame and detects the occurrence of the flame.
Among conventional methods that utilize resonance radiation of the carbonic acid gas, the method using the filter requires at least three monochromatic filters to achieve a flame sensor providing a small number of erroneous detections and capable of reliably sensing a flame. In addition, a computation circuit for sensing is complicated, and the flame sensor is unavoidably expensive.
Flame sensors using two or less filters involve the problem that the number of erroneous detections is great. Though economical, flame sensors utilizing the flicker of the flame also involve the problem that the number of erroneous detections is great. Therefore, the applicant of the present application has already proposed a flame sensor capable of reliably detecting a flame with equivalent certainty to the conventional flame sensors using three filters, and a flame sensor using three filters but using a simple computation circuit.
Solar rays, artificial rays or radiation from a stove emit not only visible rays, but also radiation in the infrared regions. However, this radiation is a continuous spectrum. In contrast, the spectrum of resonance radiation of the carbonic acid gas generated by the flame is a line spectrum in which energy concentrates in extremely narrow regions. Therefore, the technology described above utilizes the difference between the continuous spectrum and line spectrum for detecting the flame.
This technology, shown in
FIG. 11
, uses a broadband filter for permitting the passage of light of a band (W
10
) broader than a spectral line(W
20
) of resonance radiation of the carbonic acid gas generated by the flame and a narrow-band filter for permitting the passage of only the spectral line of resonance radiation of the carbonic acid gas, and has the band center of the broadband filter in alignment with that of the narrow-band filter. Intensity (optical energy) of light from the flame passing through these two filters is divided by the bandwidth of each filter to determine mean intensities.
When the intensity of the spectrum of light passing through the filters is a straight line-like continuous spectrum, energy of the rays of light passing through the two filters is proportional to the transmission bandwidth. Therefore, the mean intensities obtained by dividing this energy by the bandwidth are equal for the two filters.
However, when the rays of light passing through the filters are the line spectrum of resonance radiation of the carbonic acid gas, both of these two filters allow this line spectrum to pass therethrough and transmission energy is substantially equal. However, optical energy of the light passing through the broadband filter is divided by a greater bandwidth to calculate the mean intensity, whereas optical energy of the light passing through the narrow-band filter is divided by a smaller bandwidth. Consequently, a difference develops between these two mean intensities.
Therefore, the flame can be detected by judging whether or not a difference between the two mean intensities exceeds a threshold value.
In the technology described above, however, the band center of the broadband filter and the band center of the narrowband filter are in alignment with each other. Therefore, when the straight line-like continuous spectrum passes through the filters, the difference of the mean intensities is 0. To discriminate the straight line-like continuous spectrum from other spectra, the threshold value must be set to a small value near 0. However, it is difficult, from the aspect of production, to have the band center of the broadband filter in alignment with the band center of the narrowband filter. If the band centers of these two filters are not coincident, the difference of the mean intensities will not become 0 even when the straight line-like spectrum passes, resulting in inviting the occurrence of erroneous detections.
The explanation given above holds also true of the case where a first filter for allowing the passage of only light of the spectral line of the resonance radiation of the carbonic acid gas generated by the flame and a second filter for allowing the passage of light of a broader band than the spectral line are employed, the second filter being disposed in such a way that its band center is coincident with that of the spectral line, and the quantities of energy passing through these two filters is subtracted to detect a flame.
SUMMARY OF THE INVENTION
To solve the problems described above, the present invention aims to provide a flame sensor that can be easily produced and can accurately detect a flame.
A first aspect for accomplishing the objects described above provides a flame sensor that comprises a narrowband filter which passes only light of a band corresponding to a line spectrum of carbonic acid gas resonance radiation generated by a flame; a broadband filter which passes light of a band broader than the band corresponding to the line spectrum, and which has a band center different from a band center of the band corresponding to the line spectrum; a first light reception device which converts light passing through the narrowband filter to an electric signal; and a second light reception device which converts light passing through the broadband filter to an electric signal.
When the spectrum of the light passing through the filter is the continuous spectrum, energy of the rays of light passing through the two filters, the broadband filter and the narrow-band filter, is substantially proportional to the transmission bandwidth. Therefore, a difference between the mean intensities obtained by dividing this energy by each bandwidth is less than a predetermined value. The source of the difference between the mean intensities include the shape of the intensity distribution of the spectrum of rays of light passing through the filters and the distance between the band centers of the two filters.
In contrast, when only rays of light of a flame are present, the spectrum passing through the broadband filter and the narrow-band filter is mainly only the spectral line because the spectrum of the flame is the line spectrum, and energies passing through the broadband filter and the narrow-band filter are substantially

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