Detector

Details Detector Detector

1999-03-19

2001-07-03

Evans, F. L. (Department: 2877)

Radiant energy

Invisible radiant energy responsive electric signalling

Infrared responsive

C340S578000

Reexamination Certificate

active

06255651

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a detector, and more particularly, but not exclusively, to a detector which is suitable for use as a fire detector.
DESCRIPTION OF THE PRIOR ART
One type of fire detector is a flame detector and is described in the Applicant's granted European Patent EP-B-0 064 811. The fire detector described in the aforementioned Patent was extremely successful. However, there was a risk that a false alarm might be given. The reason for this is described briefly below with reference to FIG.
1
.
The fire detector described in the aforementioned granted Patent comprised two sensors. Each sensor provided a signal, one indicative of energy at or around 4.3 &mgr;m the other at an energy of around 3.8 &mgr;m. Energy detected at 4.3 &mgr;m indicated that CO
2
was present. Energy detected at 3.8 &mgr;m was used as a reference signal. A signal, indicative of the presence of a fire occurred if a first signal exceeded both a variable threshold and a fixed threshold. In this way the detector ensured that an alarm was triggered as a result of a valid signal (arising from a fire) rather than from one arising due to background noise.
When the detected value of a valid signal (which exceeded the threshold), was compared with a reference value, a quotient was obtained. The quotient always exceeded a non-zero value.
This quotient was used to remove or reduce background noise. Other methods of reducing background noise were also possible, for example direct comparison of the detected and threshold values or comparison of their respective differences with a reference value.
Occasionally however false alarms occurred. Usually false alarms were due to a signal processor error. The processor was arranged to vary the threshold of the reference sensor, i.e., the sensor which detected radiation energy at 3.8 &mgr;m. This ensured that the threshold for triggering an alarm was increased with increasing similarity and decreased with decreasing similarity in the event of an increase/decrease in background noise. Thus, the greater the similarity between the variations with time of the output signals of each sensor, the greater the quotient and the higher the threshold to which an alarm trigger value was automatically shifted. This was found to assist the detector in discriminating between blackbody radiation which varied at a flame flicker frequency, (typically around 1 to 20 Hz).
FIG. 1
shows diagramatically a graph of blackbody radiation energy against wavelength. A radiation peak, centered around 4.3 &mgr;m occurs as a result of Carbon Dioxide (CO
2
) emission. In the detector described above, energy from the CO
2
peak was detected and when the detected value exceeded a predetermined threshold, an alarm was triggered. The threshold was variable and could be set prior to a fire detector being installed. Thus the detector could be configured to detect a small fire at a distance of, for example 5 m, or a larger fire at a distance of, for example 25 m.
However, the aforementioned detector was occasionally prone to false alarms. These occurred not as a result of threshold detection problems but rather as a corollary of the logic circuitry and software which determined so called blackbody rejection characteristics. Referring again briefly to
FIG. 1
, the dotted line A, below the main blackbody radiation curve B, depicts radiation from a “cold” blackbody. Because curve A represents a “cold” blackbody, whose peak energy emission is less than that of a flame, the peak of curve A is at a longer wavelength than that of curve B. Curve B is derived from a relatively hot blackbody such as a process heater, gas turbine or a boiler at which the detector was usually pointed. Typically radiation depicted by curve A may be from a relatively cool object such as a human body or part of a body which is exposed to the detector. When this occurred, the gradient of curve A, at or around 4.3 &mgr;m, was positive. It can be seen from curve B that its gradient was always negative at, or around, 4.3 &mgr;m. It has been found that this has been the reason for the problem which occasionally caused a false alarm. The detector effectively sensed activity at or around 4.3 &mgr;m and from knowledge of what was occurring at, or around, the 3.8 &mgr;m waveband, a processor calculated a threshold value. This threshold value was effectively used as part of a checking function which involved a cross-correlation algorithm.
Because the expected value of the intersect of the curve of radiation detected at 3.8 &mgr;m and blackbody radiation curve B (illustrated by point P on
FIG. 1
) was always higher than the value detected at the 4.3 &mgr;m (illustrated by point Q on
FIG. 1
) the cross-correlation function effectively “assumed” the function had a constant negative gradient. Interpolation between points P and Q was therefore always performed by the function according to a linear function (y=mx+c), where m=2 (Ep-Eq) and E
p
is the detected energy at point P (at 3.8 &mgr;m) and Eq is the energy detected at point Q (at 4.3 &mgr;m). Spurious signals detected from a random “cold” blackbody source, such as a hand waving or a person moving in front of the detector at a critical distance sometimes gave rise to false alarms if the motion was detected within a “flame flicker frequency” (typically 1 to 20 Hz). In these instances it was falsely predicted that such radiation exceeded the alarm threshold and an alarm was triggered.
It is an object of the present invention to provide a solution to the aforementioned problem.
SUMMARY OF THE INVENTION
According to the present invention there is provided a detector comprising: a first sensor arranged to provide signals indicative of incident radiation at two different wavebands, and at least a second sensor arranged to detect radiation at a third waveband; means for processing signals derived from the first sensor, so as to obtain an expected value of radiation incident on the second sensor; means for comparing the expected value with the actual value incident on the second sensor and means to trigger an alarm in the event of a preset threshold being exceeded by the detected value.
Thus the problem with prior art detectors is overcome by providing an independent detector for comparing actual radiation with expected radiation.
Preferably separate channels are provided with the first sensor, a first channel being able to detect radiation at a first wavelength, typically at or around 3.8 &mgr;m and a second wavelength, typically at or around 4.8 &mgr;m. By arranging the first sensor to detect at these wavelengths a prediction of energy centered around 4.3 &mgr;m, is able to be obtained. Thus a relatively broad band of energy sensed by the first sensor ensures that blackbody rejection characteristics across all wavelengths are very good. The channel detected by the sensor is hereinafter referred to as a guard channel.
A second sensor, detects radiation at 4.3 &mgr;m, and provides what is hereinafter referred to as a flame channel signal.
Means may be provided to detect the amount of energy between 3.8 &mgr;m and 4.8 &mgr;m and approximate this to a linear function. However, any suitable measurement of the energy at these two wavelengths provides sufficient information to interpolate the amount of blackbody radiation at an intermediate wavelength of around 4.3 &mgr;m. Detected energy from the emission from carbon dioxide may be superimposed onto energy received from any blackbody, in the detector field of view. Thus proper segregation between “non-flame” signals and flame signals is achieved.
A guard channel may provide a signal for cross correlation with the flame channel. The cross correlation signal provides an accurate prediction of the non-flame energy present in the flame detection waveband. The prediction of the amount of non flame energy is independent of the temperature of the radiation source and allows the detector to provide effective blackbody rejection over a wide range of source temperatures.
Most preferably the guard channel includes an

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