GB2230853A

GB2230853A - Photoelectric smoke sensor

Info

Publication number: GB2230853A
Application number: GB9004603A
Authority: GB
Inventors: Takashi Suzuki; Katsuhiro Akimoto
Original assignee: Nittan Co Ltd
Current assignee: Nittan Co Ltd
Priority date: 1989-03-23
Filing date: 1990-03-01
Publication date: 1990-10-31
Anticipated expiration: 2010-03-01
Also published as: GB2230853B; JPH02123691U; GB9004603D0

Abstract

A photoelectric smoke sensor using light scattering or attenuating effects to detect smoke caused by a fire comprises a source (1) radiating light into a smoke detection space (3), a detector (4) receiving light attenuated or scattered by smoke in the smoke detection space, and detection circuitry (5, 6) for detecting a change in output of the light detection element, the detection circuitry including a device (6) for compensating the detector output for any change in an ambient temperature. The compensating device (6) may comprise a thermistor (Th) and amplification system having a temperature characteristic which balances that of the source (1) and detector (2). <IMAGE>

Description

PHOTOELECTRIC SMOKE SENSOR BACKGROUND OF THE INVENTION The present invention relates to a photoelectric smoke sensor for detecting smoke by use of a scattering phenomenon or an attenuating phenomenon of light in fire alarm equipment.

In fire alarm equipment, conventionally, photoelectric smoke sensors have been used as means for detecting smoke caused by a fire. Those photoelectric smoke sensors are classified into two groups according to the smoke detecting, one being of the scattered-light type, the other being of the attenuated-light type. That is, when smoke intruded into a specific space (hereinafter simply referred to as a smoke detection space) irradiated with light emitted from a light source, the scattered light is detected by a light detector in the sensor according to the scattered-light type sensor, while the attenuated light is detected by a light detector in the sensor according to the attenuated-light type.In those photoelectric smoke sensors, however, even if there is no smoke or the like in the smoke detection space, the outputs obtained from the light detectors may be changes by various primary factors. Those primary factors are classified into primary factors causing long-term changes and primary factors causing short-term changes. The primary factors causing long-term changes include reduction in quantity of light of a luminous element constituting a light source due to time aging thereof, reduction in sensitivity of a light detection element constituting a light detector due to time aging thereof, deposition of dust on a light source and/or a light detector, and so on. The primary factors causing short-term changes include entry of small insects into a smoke detection space, admixture of electrical noises, and so on.

Japanese Patent Unexamined Publication No. Sho.

53-134483 discloses such a system as shown in Fig. 5 as a system for compensating the changes in output due to those primary factors causing changes.

That is, luminous flux from a light source 50 is halved by a partition 51 so as to form two light paths 52 and 53, and two light detectors 54 and 55 so the luminous flux is incident into the light detectors 54 and 55 through the two light paths 52 and 53 respectively. One light path 52 is used as a compensating light path into which the outside air can flow from the circumference of the partition 51, while the other light path 53 is used as a detecting light path into which the outside air can flow easily, so that an alarm signal is obtained on the basis of a difference in degree of signals between the light paths 52 and 53. Thus, the reduction in light due to deposition of dust on the light source and the light detector or due to time aging of the light source can be compensated so that a stable sensor can be obtained.

Although the compensating system as described above is an effective means, the configuration there of is complicated.

Recently, however, a so-called analog system has become employed in fire alarm equipment. In the analog system, continuous change signals from each of sensors are sent to a receiver so that receiver can detect occurrence of a fire on the basis of a temporal change in each sensor, a relative change between the sensors, or the like. A long-term change in output of each sensor in such an analog system can be relatively easily compensated on the basis of comparison between present and past output values on the receiver side even if such a conventional compensating system as described above is not used.

Then, a problem is the short-term change in output of each sensor in such an analog system. The primary factors causing such a short-term output change include a temperature characteristic of a luminous element and a light detection element in addition to those primary factors described above.

That is a luminous element has such a photo-output versus ambient- temperature characteristic, for example, as shown in Fig. 3, so that the photo-output thereof changes in accordance with a change in the ambient temperature. Further, a light detection element has such a light-detection-sensitivity versus ambient-temperature characteristic, for example, as shown in Fig. 4, so that the light detection sensitivity changes in accordance with a change of in the ambient temperature. Those changes in both the elements appear as a geometrical change therebetween. In a fire sensor which is exposed in an atmosphere where temperature changes rapidly in case of occurrence of a fire and which is required to produce an accurate output in such an atmosphere, it is an importance problem how to compensate the changes in characteristics of both the elements due to a change in the ambient temperature.

SUMMARY OF THE INVENTION In order to solve the above problem, in the photoelectric smoke sensor according to the present invention, a detection means for detecting a change in output of a light detection element is provided with a temperature compensating means for compensating a change in output of the light detection element due to a change in an ambient temperature.

As the temperature compensating means, an amplifier with its amplification degree controlled by a thermistor having a resistance value which changes in accordance with a change in the ambient temperature, and as the amplifier, an operational amplifier is used.

In the photoelectric smoke sensor according to the present invention, temperature compensation is performed at an output end of the light detection element where the respective characteristic changes of the luminous element and the light detection element due to a change in ambient temperature appear geometrically, thereby compensating the change in output of the sensor due to the change in ambient temperature.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of an embodiment of the photoelectric smoke sensor according to the present invention; Fig. 2 is a characteristic curve diagram showing the relation between the ambient temperature and the photo-output at various parts of the sensor of Fig. 1; Fig. 3 is a characteristic curve diagram showing the relation between the ambient temperature and the photo-output of the luminous element; Fig. 4 is a characteristic curve diagram showing the relation between the ambient temperature and the light detection sensitivity of the light detection element; and Fig. 5 is a schematic constituent view of a photoelectric smoke sensor having conventional output compensating means DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 is a block diagram showing an embodiment of the photoelectric smoke sensor according to the present invention.

A luminous element 1 is intermittently driven by an output of an oscillating circuit 2 so as to emit light toward a smoke detection space 3. A light detection element 4 is provided at a position where the light emitted from the luminous element 1 is directly incident into the light detection element 4 through the smoke detection space 3 or at a position where the light emitted from the luminous element 1 and scattered by the smoke in the smoke detection space 3 is incident into the light detection element 4. The smoke sensor may be of the reduced-light type or of the scattered-light type depending on the position at which the light detection element 4 is provided. After amplified by an amplifier circuit 5, the output of the light detection element 4 is applied to a temperature compensating circuit 6 which acts as a temperature compensating means.

In this embodiment, the temperature compensating circuit 6 is constituted by an operational amplifier 7, four resistors R1, R2, R3, and R4, and a thermistor Th. The two resistors R1 and R2 are connected in series to each other between the output side of the amplifier circuit 5 and an inverted-input terminal of the operational amplifier 7. The thermistor Th is connected in parallel to the resistor R1 of the two resistors R1 and R2. The resistor R3 is connected as a feedback resistor between an output terminal of the operational amplifier 7 and the inverted-input terminal of the same. Further, the resistor R4 is connected to the operational amplifier 7 at its not-inverted input terminal so as to correct an off-set voltage.

Being apparent from the characteristic curves of Figs.

3 and 4, the respective rates of change of the characteristics of the luminous element 1 and the light detection element 4 used in this embodiment to the change in an ambient temperature are -1 %/0C and +0.4 %/0C respectively. Accordingly, as shown by a curve A in Fig. 2, the composite rate of change of the temperature characteristic of both the elements is about -0.5 r/"C.

Assuming that the resistance values of the resistors R1, R2, and R3 are 68 K#, 110 RQ, and 150 KQ respectively, the B-constant of the thermistor Th is 4400, and the resistance value of the thermistor Th at 25 C is 100 Kn, the amplification factor G of the temperature compensating circuit 6 per se is obtained by the following equation: R3 G = R1 // Th + R2 Corresponding to the change of the ambient temperature, the amplification factor changes in a manner as shown by a curve B in Fig. 2.

Accordingly, the composite characteristic of -the respective characteristics of the luminous element 1, the light detection element 4, and the temperature compensating circuit 6 corresponding to the change of the ambient temperature is substantially fixed to 100 % in a range of from -20 OC to +80 OC as shown by a curve C in Fig. 2.

After once held in a sample holding circuit 8, the output of the temperature compensating circuit 6 is converted into a digital signal in an analog-to-digital converting circuit 9, and then sent to a receiver 11 through a transmission circuit 10.

In this embodiment, the amplifier circuit 5 and the temperature compensating circuit 6 constitute the detection means for detecting the change in the output of the light detection element 4, and the configuration from the following sample holding circuit 8 to the receiver 11 may be changed in various manners by changing the signal transmission system, or the like.

In the photoelectric smoke sensor according to the present invention, as described above, since a detection means for detecting a change in an output of a light detection element is provided with a temperature compensating means for compensating a change of the detection output due to a change in the ambient temperature, a change of the composite characteristic of the luminous element and the light detection element due to the change of the ambient temperature can be compensated surely over temperatures in a wide range. This-. is very useful particularly in a fire sensor of an analog system in which continuously changing signals must be sent to a receiver.

Claims

1. A photoelectric smoke sensor comprising: a luminous element arranged to radiate light into a smoke detection space; a light detection element arranged to receive light attenuated or scattered by smoke in said smoke detection space; and a detection means for detecting a change in output of said light detection element, which is provided with a temperature compensating means for compensating a change in detection output due to a change in an ambient temperature.

2. A photoelectric smoke sensor as claimed in claim 1, wherein said temperature compensating means is an amplifier with its amplification degree controlled by a thermistor having a resistance value which changes in accordance with a change in an ambient temperature.

3. A photoelectric smoke sensor as claimed in claim 2, wherein said amplifier is an operational amplifier.

4. A smoke sensor substantially as described with reference to and as illustrated in Figure 1 and/or Figure 5 of the accompanying drawings.

5. A smoke sensor substantially as described with characteristics in accordance with any one or more of Figures 2 to 4 of the accompanying drawings.