EP0113046B1

EP0113046B1 - Heat detector

Info

Publication number: EP0113046B1
Application number: EP19830111869
Authority: EP
Inventors: Toshiaki Nohmi Bosai Kogyo Co. Ltd. Okazaki
Original assignee: Nohmi Bosai Kogyo Co Ltd
Current assignee: Nohmi Bosai Ltd
Priority date: 1982-12-03
Filing date: 1983-11-26
Publication date: 1987-09-16
Also published as: EP0113046A2; JPS5988793U; DE3373736D1; JPH0330952Y2; ES528187A0; ES8501552A1; EP0113046A3

Description

This invention relates to a heat detector comprising of an optical fiber, which is vulnerable to fusing at a low temperature, as heat sensing element according to the preamble of the patent claim.
Such heat detectors are used for instance in fire detecting systems. From the BE-A-881 994 is known a fire detector which comprises an optical fiber, stable to approximately 57°C, which melts above approximately 68,5°C. In general such heat detectors use optical fibers being vulnerable in a low temperature range of some 60--300°C as heat sensing wires and which detect fires by decrease in light reaching the photoelectric elements as caused when the optical fibers are damaged by heat or flame in case of fire, and the light from the light emitting elements leaks out through the damaged portions.
For their heat sensing wires these heat detectors use an optical fiber which is composed of only a plastic core fusing at a low temperature (some 60-300°C), an optical fiber with its core and clad layer made of plastic fusing at a low temperature, or an optical fiber with its core made of quartz glass which does not fuse at a low temperature and its clad layer made of plastic fusing at a low temperature. Of these optical fibers, the ones composed of the plastic core alone or the core and clad layer, both being made of plastic, fuse and open with heat from or flame of fire while in the case of the one composed of the quartz glass core and the plastic clad layer, only the clad layer fuses and the core does not open. If these optical fibers have an open or partial fusion caused by heat or flame in case of fire, all or part of the light from the light source leaks out through the damaged portion, and consequently the photoelectric element receives no light or only a part of the light from the light source.
In practice, however, there exist the sunlight, illumination light etc. around the optical fiber. If the optical fiber opens or partially fuses due to heat or flame in case of fire, the environmental light such as the sunlight, illumination light or light from flame enters the optical fiber from the damaged portion. Therefore, even if the optical fiber is damaged by fire, the photoelectric element receives the environmental lights and the light from flame entering through the damaged portion in lieu of the light from the light source, and accordingly there is scarcely . any change in light intensity reaching the photoelectric element before and after the optical fiber is damaged, and it is quite difficult to discriminate the fire.
It is an object of this invention to provide a heat detector which uses an optical fiber as the heat detecting element and is capable of readily discriminating a fire by detecting, without fail, a decrease of light caused by damage in the optical fiber due to fire. Another object of this invention is to provide a heat detector which shows an improved noise immunity over the prior art heat detectors using optical fibers.
The hereinbefore defined objects of this invention are achieved by the use of the heat detector which is defined in the preamble of the specification by the characterizing features of the patent claim.
The invention will now be more specifically described in the following by way of example only and with reference to the accompanying drawing, wherein 1 is an optical fiber with its core and clad layer made of plastics having different refraction rates and which fuses at a low temperature (some 60-300°C), 2 is a light emitting element such as a light emitting diode provided at one end of optical fiber 1, 3 is a light emitting current feed circuit to feed light emitting current to light emitting element 2, 4 is a modulation circuit to modulate the light emitting current from light emitting current feed circuit 3 by signal with specific frequency f from oscillator 5 and to feed the modulated light to light emitting element 2, 6 is a photoelectric element such as solar cell provided at the other end of optical fiber 1, 7 is an amplifier to amplify the output of photoelectric element 6, 8 is a bandpass filter through which only the output modulated by specific frequency f out of the amplified output from amplifier 7 can pass, and 9 is a fire discriminating circuit comprising a comparator which gives out a fire signal when the output through the bandpass filter has fallen below the predetermined level.
In normal condition, the heat detector of this composition operates as follows. The light emitting current from light emitting current feed circuit 3 is normally modulated by modulation circuit 4 and fed to light emitting element 2. With this modulated light emitting current, light emitting element 2 emits modulated light to optical fiber 1. On the other hand photoelectric element 6 receives the modulated light from light emitting element 2 through optical fiber 1. The output of photoelectric element 6 is amplified by amplifier 7, and then fed to fire discriminating circuit 9 through bandpass filter 8. Fire discriminating circuit 9 does not give out a fire signal as long as the modulated light output is above the predetermined level.
If a fire breaks out and optical fiber 1 fuses to open with heat from or flame of the fire, propagation of the modulated light from light emitting element 2 is interrupted at the opened portion, and external light such as sunlight, illumination light, and light of the flame enter optical fiber 1 from the opened portion. Consequently, a photoelectric element 6 no longer receives the modulated light from light emitting element 2 but the external light and light of the flame, thus generating output which does not contain modulated light portion with specific frequency f, and which is amplified by amplifier 7 and sent to bandpass filter 8. Nevertheless, since this output sent to bandpass filter 8 contains no modulated light portion with frequency f, no output develops at bandpass filter 8. As a result of this, the modulated light output in fire discriminating circuit 9 falls below the predetermined value, i.e. O, fire discriminating circuit 9 determines it as fire and sends a fire signal to fire control panel which is not shown in the drawing.
It is also feasible to make the light emitting current from light emitting current feed circuit 3 a pulsed output, to modulate the light emitting current pulse by the output of oscillator 5 in modulation circuit 4 and to feed the modulated light emitting current pulse to light emitting element 2. In this case, operation of amplifier 7 or fire discriminating circuit 9 is synchronized with light emitting current output of light emitting current feed circuit 3. It is also possible to use an optical fiber which is composed of only plastic core fusing at a low temperature or an optical fiber with its core made of quartz glass which does not fuse at a low temperature and its clad layer made of plastic fusing at a low temperature. According to the present idea, occurrence of a fire is discriminated by emitting modulated light to the optical fiber from the light emitting element and checking if there is attenuation in the modulated light intensity reaching the photoelectric element. Therefore, it offers a heat detector using an optical fiber as heat sensing wire which is capable of readily discriminating fire by detecting, without fail, such attenuation of light from the light source as caused by damage in the optical fiber in case of fire.

Brief description of drawing

The drawing is a schematic diagram of an embodiment of the heat detector according to this idea.

Claims

A heat detector comprising a light emitting element (2), a circuit (3, 4, 5) which feeds modulated light emitting current to the light emitting element (2), an optical fiber (1) through which the modulated light from the light emitting element is fed and which is vulnerable to breaking, fusing at a low temperature, etc., a photoelectric element (6), which receives light from the optical fiber (1), and a fire discriminating circuit (9) which detects output of the modulated light portion of the output of the photoelectric element (6) to discriminate whether a fire has broken out or not, characterized in that the circuit (4) which feeds the modulated light emitting current is formed in such a way that the modulated light emitting current is intermittently fed to the light emitting element (2), and the fire discriminating circuit (9) is formed in such a way that it detects the output synchronously with the modulated light emitting current output.