Classifications

• • G—PHYSICS
  • G08—SIGNALLING
  • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
  • G08B17/00—Fire alarms; Alarms responsive to explosion
  • G08B17/10—Actuation by presence of smoke or gases, e.g. automatic alarm devices for analysing flowing fluid materials by the use of optical means
  • G08B17/103—Actuation by presence of smoke or gases, e.g. automatic alarm devices for analysing flowing fluid materials by the use of optical means using a light emitting and receiving device
  • G08B17/107—Actuation by presence of smoke or gases, e.g. automatic alarm devices for analysing flowing fluid materials by the use of optical means using a light emitting and receiving device for detecting light-scattering due to smoke
• • G—PHYSICS
  • G08—SIGNALLING
  • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
  • G08B17/00—Fire alarms; Alarms responsive to explosion
  • G08B17/10—Actuation by presence of smoke or gases, e.g. automatic alarm devices for analysing flowing fluid materials by the use of optical means
  • G08B17/11—Actuation by presence of smoke or gases, e.g. automatic alarm devices for analysing flowing fluid materials by the use of optical means using an ionisation chamber for detecting smoke or gas
  • G08B17/113—Constructional details

Definitions

• the present invention relates to suspended particle detectors, in particular smoke detectors, and is concerned with that type of detector including a housing, a radiation source arranged to shine a beam of 5.
• radiation typically visible light or infrared, across the housing and a sensor responsive to radiation scattered by the particles within the housing and connected to an evaluation circuit arranged to provide an alarm when the density of suspended particles reaches 10. a predetermined value.
• amplifier or comparator will result in a variation in the particle density at which an alarm is indicated. This variation can be reduced by using higher quality components but this naturally substantially increases the cost of the detector.
• a suspended particle detector includes a housing containing a 5. radiation ⁇ fource arranged to shine radiation across the interior of the housing, a first radiation sensor arranged to receive radiation scattered from the wall of the housing and radiation scattered from particles suspended in the housing and to produce .a first signal 10. indicative of the total intensity of radiation incident on it and a second radiation detector arranged to produce a second signal indicative of the intensity of the radiation source, the detector also including signal combining means arranged to combine the two 15. signals in opposition to produce a composite signal, the detector being constructed and arranged so that the composite signal has a first polarity when the density of suspended particles in the housing is less than a predetermined threshold value and the opposite 20. . polarity when the said density is greater than the threshold value and evaluation means arranged to detect when the polarity of the composite signal reverses and to produce an alarm signal.
• the senor of the present invention operates 25. in a very different manner to the known construction referred to above since there is no attempt to make the composite output signal independent of background radiation intensity as previously but on the other hand the threshold particle density at which an 30. alarm is indicated is genuinely independent of the
• detector of the present invention need only detect a change in polarity of the composite signal rather than an absolute value of this signal which is inherently more simple and reliable. This latter feature means that the electrical components used in the detector
• The. second sensor may be positioned to detect the intensity of the background radiation in the manner
• the second sensor is arranged to be directly subject to the radiation from the radiation source which makes possible the use of a relatively insensitive and thus cheap sensor. 5. It is preferred that the two sensors rely on a similar sensing principle so that any change. of sensitivit resulting from ageing or temperature changes will be similar for the two sensors. Preferably these are both silicon junction photodiodes and the second sensor may 10. be a low cost, glass encapsulated silicon rectifier diode.
• the housing preferably includes a block of non- translucent material in which there is a first passage in which the radiation source is situated and a
• second passage communicating with the first passage in which the second sensor is situated. This is found to be a simple manner of ensuring that radiation from the source, e.g. visible light or infrared, impinges directly on the second sensor which is shielded from both
• the detector preferably includes adjustment means arranged to vary the magnitude of the second signal at a given density of suspended particles.
• the adjustment means may be electrical but are preferably mechanical
• the detector may be calibrated by introducing
• 10. includes an amplifier to the input of which the signal combining means is connected and to the output of which a logic.unit is connected, the amplifier being so arranged that if there is no input the output is of the said opposite polarity so that an .alarm signal
• Figure 1 is a diagrammatic cross sectional elevation of a detector chamber of a smoke detector in accordance with the present invention
• Figure 2 is a block diagram of the detector
• FIG. 3 is a graph showing the magnitude of the various signals at different points in the circuitry.
• the detector chamber shown in Figure 1 comprises a base 2 of non-translucent material connected to
• an infrared light emitting diode which is arranged to radiate a substantially collimated pulsed infrared beam through the passage 9 and then across the interior of the chamber. Also situated in the base is a first or smoke scattered radiation sensor 5, in this case
• a photodiode in front of which is an assembly 6 comprising a lens and an optical filter.
• the sensor 5 has a field of view which extends across the interior of the chamber and intersects the path of the pulsed beam from the light-emitting diode 3 over a volume 8.
• a further passage 10 in the base 2 within which is a second or reference sensor 4 comprising a further photodiode.
• a second or reference sensor 4 comprising a further photodiode.
• an adjustable radiation attenuator 7 comprising a
• the light emitting diode 3 is pulsed and 30.
• the reference sensor 4 receives radiation whose intensity
• OM?I is dependent only on the position of the screw 7 and the intensity of the diode 3 and at any particular setting of the screw 7 its output signal is therefore indicative only of the intensity of the. radiation from the diode 3.
• the sensor 5 receives two components of radiation, the first being background radiation, that is to say ' radiation scattered from the wall of the chamber, and the second being smoke-scattered radiation, that is to say radiation scattered by the smoke particles,
• the circuitry shown in Figure 2 comprises a pulse generator 21 connected to the light emitting diode 3
• the two sensors 4 and 5 are connected to a signal combining circuit 22 comprising a direct inverse parallel connection which is connected so that its output, i.e. the difference
• the amplifier is a discrete component operational amplifier operating from a zener diode regulated 5 volt supply (not shown) and its output is connected to a logic
• unit 24 arranged to detect when the polarity of the output of the amplifier changes.
• the quiescent output ofthe amplifier is set close to and slightly above the logic threshold of the unit 24.
• the gain of the amplifier is such that the amplitude of the output pulses is
• OMPI and amplified signal pulse excursions are limited by the available output of the amplifier.
• the logic unit consists of a CMOS counter which is clocked by an auxiliary output from the pulse 5.
• generator 21 and connected to "reset” each time the amplifier output presents a logic “low” level during the positive transition of the clock signal and to "count” each time the amplifier presents a logic “high” level during the positive transition of the. clock 10. signal.
• an output signal is produced which fires an output switch 25. This causes current to flow from the output connection 26 to the negative supply connection 27 thus allowing the actuation of an 15. alarm and in this case a light emitting diode indicator 28 and a similar repeat remote indicator 29 which is present also in this embodiment.
• the pulse generator 21 is a complementary astable oscillator operating from a current source of about 20. 80 microamperes derived from the output terminal 26.
• the amplifier integrates and amplifies the composite 25. " signal over the duration of the radiation pulse to produce an appropriate input for the logic unit when the positive clock transition occurs at the end of each radiation pulse.
• the graph of Figure 3 shows the magnitude of 30. the various signals against time, all the signals pulsing in synchronism with the radiation source with the same .duration, i.e. 150 microseconds.
• the x axis, indicated by 30, represents the logic threshold level of the logic unit whilst the line 31 which is
• diode when smoke is present in the housing i.e. as a result of both background and smoke-scattered radiation.
• 34 represents the output of the reference sensor 5 and this is exactly the same for every pulse since the intensity of radiation does not vary.
• 35 and 36 represent the composite signal when no smoke is present in the housing and smoke of greater than threshold density is present in the housing respectively.
• the reference sensor 4 always produces an output represented by curve 34 at each
• the housing increases the output of the sensor 4 increases to that represented by value 33 and the composite*signal rises towards the curve represented by the curve 36 which is both positive and above the logic threshold.
• OMPI is indicated, though it will be appreciated that three is an arbitrary number chosen substantially -to exclude the possibility of transient signals or variations in the logic threshold resulting in an alarm being 5. incorrectly indicated.

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• Chemical & Material Sciences (AREA)
• Analytical Chemistry (AREA)
• Business, Economics & Management (AREA)
• Emergency Management (AREA)
• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Fire-Detection Mechanisms (AREA)
• Investigating Or Analysing Materials By Optical Means (AREA)
• Sampling And Sample Adjustment (AREA)

Priority Applications (2)

Applications Claiming Priority (1)

Publications (1)

Family

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Family Applications (1)

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Cited By (1)

Families Citing this family (6)

Citations (3)

• 1982
  • 1982-07-14 GB GB08220406A patent/GB2123548B/en not_active Expired
• 1983
  • 1983-07-13 EP EP83304076A patent/EP0099729A1/en not_active Withdrawn
  • 1983-07-13 JP JP58502413A patent/JPS59501283A/ja active Pending
  • 1983-07-13 WO PCT/GB1983/000171 patent/WO1984000429A1/en active Application Filing
  • 1983-07-14 ES ES524120A patent/ES524120A0/es active Granted
  • 1983-07-14 IL IL69222A patent/IL69222A0/xx unknown
• 1984
  • 1984-02-29 DK DK1393/84A patent/DK139384D0/da not_active IP Right Cessation
  • 1984-03-13 NO NO840972A patent/NO840972L/no unknown
  • 1984-03-13 FI FI841014A patent/FI841014A/fi not_active Application Discontinuation

Patent Citations (3)

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