Classifications

• • G—PHYSICS
  • G08—SIGNALLING
  • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
  • G08B17/00—Fire alarms; Alarms responsive to explosion
  • G08B17/12—Actuation by presence of radiation or particles, e.g. of infrared radiation or of ions

Definitions

• the present invention relates to a method and apparatus for preventing the occurrence of false responses in optical detection devices, which are sensitive to changes or fluctuations in optical radiation emitted from a source.
• optical detection devices are flame-detectors, smoke detectors and the like.
• the response signal of such optical detection devices can be applied to provide a fire alarm signal or, for example, to supervise operation of burners, furnaces and the like.
• Flame detectors in which radiation from the flames is sensed have been proposed, utilizing radiation derived from the flames in the visible light range, infra-red (I.R.) range, or ultraviolet (UV) range.
• I.R. infra-red
• UV ultraviolet
• Known flame detectors to provide outputs representative of presence of a flame, and operating purely within the above referred to light ranges, frequently are not reliable, since signals are derived not only from radiation due to flames, but also caused by extraneous radiation, such as daylight, artificial light sources, radiant heaters providing I.R.
• the different spectral composition of radiation from flames is used in order to distinguish between radiation from flames and disturbing or interfering radiation.
• Two photoelectric sensors with different spectral sensitivity are exposed to radiation from the flame; for example, one photoelectric sensor is sensitive to blue light, and one is sensitive to red light.
• the photo cells may be serially connected. At the junction point between the two photo cells, a d-c signal will occur which depends on the spectral composition or the colour of the light radiation to which the sensors are exposed.
• Such a flame detector while functioning properly under most conditions may, however, react to interfering radiation which by chance has the same, or similar spectral composition as radiation from a flame.
• the invention therefore provides a method for preventing the occurrence of false responses in optical detection devices, which are sensitive to changes or fluctuations in optical radiation emitted from a source comprising the steps of: a) receiving optical radiation emitted by a source; b) selecting a predetermined range of wavelengths; c) detecting changes in the received optical flux and deriving therefrom a signal having time series data (signal traces) at the detector output; characterized by the step of d) analyzing the detected signal for its chaotic (i.e.
• aperiodic behaviour by establishing the associated fractal dimension of the signal and using the existence of this fractal property of the said time series data of the optical radiation emitted by the said source to discriminate against those sources of fluctuating optical radiation which are periodic or intermittent; and e) only providing a response at the output of the optical detection device in case of chaotic behaviour of the source.
• the invention further provides an apparatus for preventing the occurrence of false responses in optical detection devices, which are sensitive to changes or fluctuations in optical radiation emitted from a source comprising means for receiving optical radiation emitted by a source; means for selecting a predetermined range of wavelengths; means for detecting changes in the received optical flux and deriving therefrom a signal having time series data (signal traces) at the detector output; characterized by means for analyzing the detected signal for its chaotic (i.e.
• aperiodic behaviour by establishing the associated fractal dimension of the signal and using the existence of this fractal property of the said time series data of the optical radiation emitted by the said source to discriminate against those sources of fluctuating optical radiation which are periodic or intermittent; and means for only providing a response at the output of the optical detection device in case of chaotic behaviour of the source.
• the emitted and received optical radiation is in the infra-red (I.R.) range.
• the invention is based upon the following steps:
• the value of the fractal dimension is useful in further discriminating between different sources of chaotic optical radiation.
• the invention is further based upon the fact that certain optical radiation e.g. flame flicker is chaotic, i.e aperiodic.
• the chaotic behaviour of the flame can be objectively quantified by applying the concept of fractal dimension to the time series data from the detector output. Rotating or vibrating sources will be periodic, i.e. non-chaotic and will not have a fractal dimension, neither will beam interrupts.
• the roughened perimeter could be enscribed by a polygon of N sides of equal length e.
• Fractal character thus exhibits two distinctive features: (1) the measured length of a curve (or the area of a surface) depends on the measurement scale according (2) to a power law dependence, e -D for curves (and e ⁇ -D for surfaces) .
• the key property of the detected signal is chaotic behaviour.
• Establishing the fractal dimension s a simple way of establishing whether the signal is chaotic.
• the actual value obtained for the fractal dimension while it may prove a useful quantity in itself, is not as significant as the existence of a fractal dimension which holds over a broad range of time intervals (analogous to the wide range of e values for the case of the perimeter previously discussed) . Therefore, in particular, the invention is based upon the idea of using the fractal property of the time series data of the optical radiation e.g. the I.R. emitted by a flame to discriminate against those sources of fluctuating I.R. which though not flame generated, do satisfy the frequency test of existing flame detectors.
• fig. 1 represents schematically the operational principles of known I.R. flame detectors
• fig. 2 represents flame flicker data which are used to apply the concept of fractal dimension according to the present invention
• fig. 3 represents a graph derived from the data of fig. 2 from which according to the invention the fractal dimension of flicker can be obtained.
• a narrowband optical bandpass filter 1 restricts the I.R. radiation from a source entering the detection device to a narrow range of wavelengths around 4.4 microns. These wavelengths are e.g. emitted by flames 3 or hot surfaces but are sufficiently strongly absorbed by the atmosphere for no contribution to be left in sunlight at the Earth's surface. Consequently, any such wavelengths entering the detector will have been produced locally - either in a flame or from a hot surface. The detector is effectively "solar blind".
• the transmitted I.R. radiation is then detected by an I.R. detector 2.
• This is very sensitive and inherently suited to detecting changes in I.R. flux.
• the detector output is passed through an electrical bandpass filter 4 that restricts the transmitted signal to components in the range of 0.5 to 15 Hz. These frequencies are characteristic of flickering flames.
• a hot object periodically enters or leaves the detector's field of view.
• rotating machinery might periodically obscure or reveal a hot surface to the detector, or (ii) when the detector's line-of-sight to a hot object is intermittently obscured, such as by a group of people walking past.
• the resulting signal trace is markedly different from that generated by a flickering flame.
• the chaotic behaviour of the flame can be quantified by applying the concept of fractal dimension to the time series data from the detector output.
• the trace length of a signal can be measured with progressively smaller step lengths (finer discrimination) by any means suitable for the purpose for recognizing chaotic behaviour and determining fractal dimension.
• Fig. 2 An instructive way to consider the operation is shown in Fig. 2.
• stepping along the trace using means for measuring the trace length of a signal with progressively finer discrimination, e.g. a pair of suitable frequency dividers set to a particular step length.
• the measured length of the trace is the number of steps times the step length; obviously for large step lengths much detailed structure is missed out.
• progressively smaller step lengths ever smaller features of the trace can be followed and the total length measured increases.
• a plot of log (total measured length) against log(step length) will yield a straight line graph whose slope gives the fractal dimension.
• Fig. 3 shows such a result for a flickering flame.
• the horizontal axis represents Log ⁇ o (step length) whereas the vertical axis represents Log ⁇ Q (total measured length) .
• control card could be used to handle the processing for several detector heads, interfacing them to the existing fire- detection system.
• the present invention is not restricted to flame detection or hot surface changes detection, but the fractal test algorithm of the invention could also be applied to detect phenomena such as smoke (where the signal fluctuations due to real smoke, are chaotic, whereas those from obscuration of the beam or beam interrupts, are not), gas or other dispersing constituents of a mixture for which the signals representing fluctuations in concentration need to be distinguished from more periodic or intermittent confusing signals.

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• Business, Economics & Management (AREA)
• Emergency Management (AREA)
• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Photometry And Measurement Of Optical Pulse Characteristics (AREA)
• Fire-Detection Mechanisms (AREA)
• Measurement Of The Respiration, Hearing Ability, Form, And Blood Characteristics Of Living Organisms (AREA)

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Cited By (2)

Citations (6)

• 1994
  • 1994-08-30 DE DE69407190T patent/DE69407190T2/de not_active Expired - Fee Related
  • 1994-08-30 AU AU75375/94A patent/AU7537594A/en not_active Abandoned
  • 1994-08-30 SG SG9606075A patent/SG97742A1/en unknown
  • 1994-08-30 CA CA 2170519 patent/CA2170519A1/en not_active Abandoned
  • 1994-08-30 EP EP94925486A patent/EP0715744B1/en not_active Expired - Lifetime
  • 1994-08-30 WO PCT/EP1994/002888 patent/WO1995006927A1/en active IP Right Grant
  • 1994-08-30 DK DK94925486T patent/DK0715744T3/da active
• 1996
  • 1996-02-27 NO NO960783A patent/NO960783L/no unknown

Patent Citations (6)

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