DK144255B - DEVICE FOR FIREFIGHT DETECTION - Google Patents

Description

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Dl The Patent and Trademark Office

(21) Application No. 4j597 / 73 (51) lnt.CI.3 6 08 B 17/00 (22) Filing date 10 Aug. 1973 (24) Race day 10 Aug. 1973 (41) Aim. available 12. bold). 1974 (44) Posted 25 Jan 1982 (86) International application # - (86) International filing day - (85) Continuation day - (62) Master application no -

(30) Priority Aug 11 1972, 37633/72, GB 14 Nov. 1972, 52587/72, GB

(71) Applicant CHUEE FIRE SECURITY LIMITED, Sunbury-on-Tha.mes, GB.

(72) Inventor William Thomas Peberdy, GB: Eric MacDonald, GB.

(74) Associate Engineer Hofman-Bang & Bout ard.

(54) Fire detection device.

The invention relates to an apparatus for detecting fires of the kind specified in the preamble of claim 1.

Fire detection with optical detection has usually been based on a measurement of smoke presence. In a first type of alarm, the light from a source falls directly on a light detector, where the light intensity of the detector is reduced by the presence of smoke, and in a second type of alarm a light beam is prevented from extending the direct from the light source to the detector, but in the presence of smoke CM / the light will be refracted and / or reflected by the smoke particles and -3 "thus being able to hit the light detector. The latter type is usually 1 - usually preferred.

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Detectors in which the photosensitive device is directly illuminated by means of a light beam may have the light source and the photosensitive member enclosed in a detector cell, or may be designed such that the photosensitive member is also exposed to ambient light. An example of the former embodiment is known from the specification of Swedish Patent No. 322,713, which is adapted to detect aerosols or suspended particles in the air in the detector cell.

The suspended particles reflect light from a pulsating radiation source to a photosensitive organ which will not receive light when the air is particulate free.

If the photosensitive means is also exposed to ambient light, the disadvantage is that a significant portion of the detector signal will be evoked by ambient light and have such amplitude that it is difficult to detect changes in light intensity from the light source. It has been proposed to use a laser to produce a concentrated light beam which is refracted in the presence of hot air or other heated gases as a result of fire, which in certain cases deflects away from the photosensitive body. Even if a fire causes a well-defined change in the light intensity, the impact from the ambient light will still be present and the method also has the disadvantage that the laser beam sight line must be set very accurately and that the laser's tension must be very stable, which is difficult to achieve in buildings where a normal displacement of the walls can cause the laser beam to deviate from the line of sight towards the light detector. Although such movements can be measured and used to correct the position of the laser by means of a servo system, this solution is expensive.

The object of the invention is to provide a detector which is more reliable than the known detectors mentioned above. This object is achieved by the design of claim 1, wherein the apparatus is based on the heat effect of the fire causing refraction of the emitted radiation pulses, the direction of which is not nearly as critical as with the known apparatus.

In order to measure the presence of hot gas, the apparatus of the invention does not utilize the change in the position of the incident light with respect to the surface of the detector, since the wide beam of light will always cover the entire surface of the detector, but is instead based on variations in light intensity within the cross-section of the light beam. , whereby the surface of the detector receives a time-varying radiation.

By using a pulsating radiation source in conjunction with a frequency selective circuit in the receiver, most noise signals, i.e. signals due to radiation from the surroundings are filtered away from the receiver's output signal. Generally, the modulation of the light beam caused by the fire generated by the fire of the gases through which the light beam passes will have a frequency between 1 Hz and 150 Hz, especially in the range of 2 Hz to 25 Hz.

A semiconductor light source, e.g. of the gallium arsenide type, for the transmission of infrared light emitted as pulses at a frequency of 1,000 Hz, which pulses each last approx. 2 microseconds. Thereby, signals generated by ambient light can be filtered off together with pulse signals from illumination at a frequency lower than 1,000 Hz. Furthermore, a signal at 1,000 Hz is easier to amplify than a direct current signal so that very weak signals can be measured, allowing a wide beam of light to be used. By emitting the light as pulses, the advantage is that the gallium arsenide light source can be applied to a large power, since the light source is only connected for very short time intervals.

Since the apparatus according to the invention has a light beam which completely overlaps the radius of the radiation-sensitive organ, minor variations in the radiation direction due, for example, to the displacements of the wall, cause another part of the cross-section of the beam to hit the light-sensitive surface. The total amount of radiation received by the receiver is essentially independent of such movements.

As a detector, a silicon phototransistor may advantageously be used. In one embodiment of the invention, the phototransistor is biased to a constant value by irradiating it with a secondary light source having a substantially constant light intensity. Alternatively, an electrical bias could be applied to the base of the phototransistor. As will be explained later, the bias for ^ 144255 is intended to displace the phototransistor's working point along the characteristic curve to a point where a further increase of the collector current due to changes in the ambient light intensity will not substantially affect the gain of the transistor. .

The invention will be explained in greater detail by the following description of an embodiment, with reference to the drawing, in which fig. 1 shows a block diagram of the apparatus according to the invention, while FIG. 2 shows a circuit diagram of the receiver's high frequency amplifier stage.

In FIG. 1, the radiation source 10 is a gallium arsenide diode which emits light in the infrared portion of the light spectrum. A bi-convex lens is provided in front of the diode 10. Len divergent light beam has a width of approx. 30 cm at a distance of 30 m.

The pulse is supplied with pulsating signals from a freely vibrating multivibrator 12 which oscillates at a frequency of approx. 1,000 Hz and operating a monostable pulsating circuit 14 »whose output pulse width equals approx. 2 microseconds. Light pulses are coupled directly to a low impedance switching circuit 16 which controls the energy supply to the photodiode 10.

Ted the receiver, behind a bi-convex lens 18, is placed a phototransistor 20. The presence of daylight, artificial light, heating and other external infrared radiation produces a direct current signal at the phototransistor 20's output, where the signal level changes depending on the environment. The gain of a transistor varies as a function of the collector current up to a certain value over which the gain is substantially constant. Therefore, below this value, the circuit for amplifying the AC signals will vary depending on the environment. In the embodiment shown, changes in ambient infrared radiation are reduced to a negligible fraction of the total illumination of the transistor by applying transistor 20 to a constant intensity base illumination using a photo diode 22, thereby obtaining a sufficient bias of the transistor to bring it into an area of constant gain, and thus a strong alternating current signal can be amplified despite variations in ambient radiation.

The phototransistor output signal is fed to a high frequency amplifier stage (see also Fig. 2) with transistors TR1 and TR2 and a transistor TR3 in an emitter follower circuit. The resistor R7 and capacitors C5 and C4 provide a counterconnection between transistors TR2 and TR1, with capacitor C4 decoupling the transistor TR1's emitter resistance at high frequencies. Capacitor C7 decouples the transistor TR2 emitter. Resistors R8 and R5 constitute another feedback path which is decoupled by capacitor C6. The time constants of the circuits leading to the transistors TR1 and TR2 are chosen such that the circuit is sensitive to frequencies in the range of 100 to 330 kHz, ie. at frequencies much higher than the pulse frequency of photodiode 10 at 1,000 Hz. The receiver's frequency sensitivity is adapted to the rise time of the pulses from the diode 10 and the rise time of the signals from the phototransistor 20. This achieves a better separation from interfering electric light than can be obtained with a circuit sensitive to the frequency 1000 Hz.

The other amplifier stages are designed in the usual way. The output of the high frequency amplifier 24 is fed to a waveforming circuit 26 which extends the signals over time and passes them to an integrating circuit 28 of the heat sensitive channel and to an integrating circuit 38 common to the smoke sensitive channel and a fault channel. In the heat-sensitive channel, the signal from the integrating circuit is passed to a sound frequency amplifier 30 which is sensitive to frequencies at which the light beam is modulated as it passes through a gas heated by fire. The signals are then passed to a rectifier circuit 32 and further through a time delay circuit 34, the output of which is arranged to trigger a thyristor and to activate a fire-relay relay in the output circuit 36. The time constant of the circuit 34 is selected so that the thyristor does not receive signals due to thermal disturbances in the air or a temporary interruption of the light beam.

In the smoke-sensitive channel, the signal from the integrating circuit 38 is passed to one comparator 40's input terminal, and the second input terminal U42S5 6 is blinded to a dormant signal level store 44 with a storage capacitor receiving a signal from the integrating circuit 38. Between the integrating circuit 38. circuit 38 and storage circuit 44 are inserted a sub-circuit 42 which halves the signal level. When smoke is caused by changes in the output of the integrating circuit 38 at 50% of the level of the storage circuit, this will immediately affect the input signal of the comparator 40 and the effect on the other terminal will be delayed so that the comparator generates a signal. which is passed through a time delay circuit 46 with a time constant of approx. 3 seconds to an output circuit 48, where the signal triggers a thyristor and activates the aforementioned fire warning relay.

The fault channel generates a fault-warning signal due to false signals, which are generated, for example, by some obstruction of the light beam or by a fault in the light source.

In the fault channel, a difference amplifier 50 compares the output of circuit 38 with a signal of a fault reference level circuit 52. from this circuit causes the differential amplifier 50 to produce an output error signal which is passed through a time delay circuit 54 to an output circuit 56 with a fault relay. In order to disregard signals corresponding to a brief interruption of the light beam, the time delay circuit has a time constant of approx. 1 second.

After 3 seconds, blocking the light beam would result in a signal through the smoke-sensitive channel. To prevent this, the error state will hold the detection of a smoke signal until the error condition is reset. The apparatus is designed such that if the smoke alarm signal is detected before the error signal, the smoke alarm signal will be retained.