GB2327752A

GB2327752A - Calibrating smoke detector

Info

Publication number: GB2327752A
Application number: GB9715728A
Authority: GB
Inventors: Stephen Henry Ellwood
Original assignee: RAFIKI PROTECTION Ltd
Current assignee: RAFIKI PROTECTION Ltd
Priority date: 1997-07-26
Filing date: 1997-07-26
Publication date: 1999-02-03
Anticipated expiration: 2017-07-26
Also published as: GB2327752B; GB9715728D0

Abstract

An optical scatter smoke sensor comprises a base 10 and a removable head 11. An optical emitter 18 and detector 19 are disposed on the base 10 and project into a smoke chamber 15 in the head 11, when the head 11 is mounted to the base 10. The optical emitter 18 and detector 19 are normally obscured from each other and the alarm is triggered when the output of the detector 19 exceeds a threshold value, due to scatter in the chamber 15. When the head 11 is removed from the base 10, the optical emitter 18 and detector 19 are no longer obsured from each other and the output of the detector 19 is measured again, so that the sensitivity of the sensor can be adjusted in order to compensate for contamination and/or component ageing.

Description

optical scatter smoke sensor This invention relates to an optical scatter smoke sensor for activating an alarm in the event that smoke is detected.

Optical scatter smoke sensors are well known. One such sensor comprises an optical emitter and detector disposed inside a sensing chamber having a smoke inlet. The emitter and detector are arranged such that there is no line of sight between them, although some light from the emitter does reflect off the walls of the chamber and into the detector: this is known as the standing scatter level. When smoke enters the chamber through the inlet, it scatters radiation from the emitter and thereby substantially increases the amount of radiation detected by the detector. A sensing circuit senses this increase in radiation and triggers an alarm.

A disadvantage of this arrangement is that dust, grease and other matter builds up on the surfaces of the emitter and detector, thereby reducing the sensitivity of the sensor.

Another disadvantage is that the optical emitter and detector become aged with prolonged use, again reducing the sensitivity of the detector. A further disadvantage is that dust and grease etc. can build up on the walls of the chamber, thereby causing the standing scatter level to increase or decrease, depending on the type of contamination.

It is known to use an additional receiver within the chamber, which directly monitors the output of the emitter, so as to take account of contamination on the face of the emitter and/or detector as well as ageing of the other components in the sensor. However, a disadvantage of this arrangement is that it is complex and costly. Another solution to the problem is to use an additional emitter within the chamber, but again this increases the cost and complexity of the sensor.

None of the above-mentioned solutions solve the problem of contamination inside the chamber. However, it has been proposed to separately deal with this by including an averaging circuit which averages the standing scatter level signal over a period of hours and adjusts the alarm threshold level to compensate for any change in the average signal detected.

The general solution to the contamination problem is to make the parts which get the most contaminated easily removable and cheap to replace. U.K. Patent application number 2 296 763 discloses a smoke sensor in which the optical sensing chamber is removable from the rest of the sensor for cleaning. This technique allows the contamination on the walls of the chamber to be easily removed. Furthermore, the emitter and detector are isolated from the sensing chamber by windows, which are removed when the sensing chamber is removed. Accordingly, the sensor does not suffer from the problem of contamination on the walls of the chamber or the faces of the emitter and detector.

This solution to the problem of contamination is difficult to achieve in the low cost desired, owing to the number of extra mechanical components that are required to produce a fully sealed chamber. Furthermore, the effect of ageing of the emitter and detector is not eliminated.

We have now devised an optical scatter smoke sensor which alleviates each of the above-mentioned problems.

In accordance with this invention there is provided an optical scatter smoke sensor comprising a base, a head detachably mounted to the base, an optical smoke sensing chamber disposed in the head, an optical emitter disposed on the base and arranged to emit radiation into said chamber when the head is mounted to the base, an optical detector disposed on the base, means for measuring a first output signal of the detector when the head is fitted to the base, so as to determine the amount of radiation radiating through the chamber from the emitter, means for triggering an alarm when the first output signal exceeds a threshold value, means in the chamber for at least partially obscuring the detector from the emitter, means for measuring a second output signal of the detector when the head is removed from the base, so as to determine the amount of radiation radiating directly from the emitter and means for varying the sensitivity of the alarm, according to the value of said second output signal.

When the head is removed, the obscuring means is not present between the detector and emitter. In use, the sensor is initially powered with the head removed, so that the detector receives direct radiation from the emitter. The head is then fitted and the sensor is set to the desired smoke concentration that will trigger the alarm.

Under normal operating conditions, little or no radiation from the emitter reaches the detector, owing to the obscuring means inside the chamber. However, when smoke enters the chamber more of the emitted radiation reaches the detector by means of reflection and/or refraction caused by smoke particles. It will be appreciated that the sensitivity of the alarm falls as the value of the output of the detector when the head is removed decreases and vice-versa.

Each time the head is removed for cleaning, the amount of direct radiation received by the detector is measured again.

Thus, the sensitivity of the alarm is varied each time the head is removed, in order to compensate for contamination and/or component ageing.

In one embodiment, the sensitivity is varied by adjusting the threshold according to the magnitude of said second output signal.

The threshold value can be adjusted by multiplying the original threshold value by the ratio of the new second output signal to the old second output signal.

Alternatively, the threshold value can be adjusted by multiplying the previous threshold value by the ratio of the new second output signal to the previous second output signal.

Thus, any variation in the second output signal of the detector, caused by component ageing or by contamination build up on the component faces or chamber walls, is overcome by varying the threshold value by a corresponding percentage.

In an alternative embodiment, the sensitivity is varied by adjusting the amount of gain applied to the first output signal of the detector. Thus, the gain is increased when the value of the second signal decreases and vice-versa.

Preferably storage means are provided on the base for storing the value of the second output signal.

Preferably means are provided on the base for storing an original or a previous threshold value.

Preferably means are provided for causing the output of the detector to be measured when the head is removed. In one embodiment a switch on the base is actuated when the head is removed. This switch may also serve to provide a fault signal that the head has been removed. In an alternative embodiment the output of the detector is continuously monitored, means being provided for recognising that the head is removed by monitoring for a sudden change in output of the detector.

Preferably the measuring and triggering means comprises a comparator which compares the first output signal of the detector with the threshold value and triggers an alarm, in the event that the output exceeds the threshold value.

Typically the signal received by the detector under alarm conditions is very small and thus the detector is preferably connected to the comparator via an amplifier.

Preferably the measuring and triggering means comprises processing means.

The second output signal of the detector is much higher than the first output signal, since radiation is able to pass directly from the emitter to the detector. This high output may be outside the measurement range that is used to monitor the first output signal. Thus, preferably means are provided for attenuating the input to the processing means when the head is removed.

Also, in accordance with this invention there is provided a method of recalibrating an optical scatter smoke sensor having a base, a head detachably mounted to the base, an optical smoke sensing chamber disposed in the head, an optical emitter disposed on the base and arranged to emit radiation into said chamber when the head is mounted to the base, an optical detector disposed on the base, means for measuring the output signal of the detector, means for triggering an alarm when the first output signal exceeds a threshold value, and means in the chamber for at least partially obscuring the detector from the emitter, the method comprising removing the head, energising the emitter, measuring and storing the output value of the detector, fitting the head, adjusting the sensitivity of the alarm according to the value of the stored signal.

An embodiment of this invention will now be described by way of example only and with reference to the accompanying drawing, the single figure of which is an exploded perspective view of an optical smoke sensor in accordance with this invention.

Referring to the drawing, there is shown an optical scatter smoke sensor for fitting to a ceiling. The sensor is connected to an alarm control panel by wires (not shown).

The sensor comprises a cylindrical base 10 for attaching to the ceiling and a removable head 11. The base 10 and head 11 comprise moulded plastics housings.

A connection block (not shown), having terminals for connecting to the wires of the alarm circuit, is mounted in the base 10. A circular printed circuit board (PCB) 12, mounted inside the base 10, has terminals connected to terminals on the connection block. The PCB 12 is retained by a removable bottom cover 13 of the base 10.

The head 11 comprises a circular optical smoke chamber 14 having an upper wall 15 formed of black plastics material.

A removable black plastics disc 16 forms the lower wall of the chamber 14. An annular peripheral wall of the chamber 14 is formed by a tubular mesh screen 26, which allows smoke to enter the chamber 14, but prevents insects etc. from entering.

A light emitting diode (LED) 18 and photodiode 19 extend from the PCB 12 through apertures 25 in the bottom cover 13 of the base 10 and into the smoke chamber 14, where they are received in respective housings 20, 21. A plurality of light baffles 22 are arranged on the upper wall 15 of the chamber 14, in order to prevent light from entering the chamber 14 through the screen 26.

The LED 18 is directed directly at the photodiode 19.

When the head 11 is fitted to the base 10, a baffle wall 24 disposed on the upper wall 15 of the smoke chamber 14 blocks the path of direct light between the LED 18 and the photodiode 19. Lenses (not shown) are preferably mounted to the housings 20, 21 for increasing the optical gain of the sensor. A microswitch actuator 27 extends from the bottom wall 13 of the base 10, the head 11 being arranged to depress the actuator 27 when it is fitted to the base 10.

In use, the sensor is initially energised with its head 11 removed, such that light from the emitter 18 is directed directly onto the photodiode 19. The magnitude of the output signal S2 from the photodiode 19 is measured and stored in a measuring device disposed on the PCB 12.

The head 11 is then fitted to the base 10 and the sensor is calibrated to trigger an alarm when the concentration of smoke inside the chamber rises above a certain level, the level being adjustable.

Under normal conditions, there is no smoke inside the chamber 14 and only a very small amount of light from the LED 18 reaches the photodiode 19. However, when smoke enters the chamber 14 through the screen 26, a greater percentage of the light emitted by the LED 18 reaches the photodiode 19. Thus, it will be appreciated that the amount of light reaching the photodiode 19 from the LED 18 is directly proportional to the smoke concentration inside the chamber 14.

The alarm is therefore triggered when the output signal Sl of the photodiode 19 exceeds a threshold level STH, where STH = S2 X n, n being the smoke concentration level at which the alarm is to be triggered.

The output signal Sl of the photodiode 19 falls as the LED 18 and the photodiode 19 age. S, also falls as dirt and grease etc. build up on the lenses and on the surfaces of the LED 18 and photodiode 19. Thus, in these circumstances the output signal Sl will not reach the threshold level STH until the smoke concentration inside the chamber 14 is much higher: this has the disadvantage that the fire will not be detected at an early stage.

Dirt and grease etc., also build up on the walls of the smoke chamber and this can cause reflections inside the chamber, causing the output signal S1 of the detector 19 to rise above the threshold level STH, thereby triggering a false alarm.

In order to overcome some of these problems, the head can be regularly removed for cleaning, so that the dirt and grease etc. on the lenses and on the walls of the chamber can be removed. However, this does not solve the problem of component ageing and nor does it solve the problem of contamination on the surfaces of the LED 18 and photodiode 19.

The present invention overcomes these problems by taking a new measurement of the output of the photodiode 19, each time the head 11 is removed. This new output signal 52New will vary with contamination and ageing of the components. The PCB 12 preferably comprises a microprocessor and memory, which take and store a measurement of the value of the new direct output signal S2Nw, when the microswitch 27 signals that the head 11 has been removed.

When the head 11 is replaced, the microprocessor modifies the gain applied to the output signal SX of the detector 19 by a factor of S2NCW/S2. Thus, the gain is increased when S2New falls as a result of contamination and/or component ageing and vice-versa.

Preferably, the magnitude of the output signal Sl of the detector 19 is measured when the head is replaced, the threshold level being a fixed value above this value of S.

Thus, account is taken of any changes in the standing scatter level under no smoke conditions.

Claims

1) An optical scatter smoke sensor comprising a base, a head detachably mounted to the base, an optical smoke sensing chamber disposed in the head, an optical emitter disposed on the base and arranged to emit radiation into said chamber when the head is mounted to the base, an optical detector disposed on the base, means for measuring a first output signal of the detector when the head is fitted to the base, so as to determine the amount of radiation radiating through the chamber from the emitter, means for triggering an alarm when the first output signal exceeds a threshold value, means in the chamber for at least partially obscuring the detector from the emitter, means for measuring a second output signal of the detector when the head is removed from the base, so as to determine the amount of radiation radiating directly from the emitter and means for varying the sensitivity of the alarm, according to the value of said second output signal.

2) An optical scatter smoke sensor as claimed in claim 1, in which the sensitivity is varied by adjusting the threshold according to the magnitude of said second output signal.

3) An optical scatter smoke sensor as claimed in claim 2, in which the threshold value is adjusted by multiplying the original thres:iold value by the ratio of the new second output signal to the cld second output signal.

4) An optical scatter smoke sensor as claimed in claim 2, in which the threshold value is adjusted by multiplying the previous threshold value by the ratio of the new second output signal to the previous second output signal.

5) An optical scatter smoke sensor as claimed in claim 1, in which the sensitivity is varied by adjusting the amount of gain applied to the first output signal of the detector.

6) An optical scatter smoke sensor as claimed in any preceding clan, in which storage means are provided on the base for storing the value of the second output signal.

7) An optical scatter smoke sensor as claimed in any preceding claim, in which means are provided on the base for storing an original or a previous threshold value.

8) An optical scatter smoke sensor as claimed in any preceding claim, in which means are provided for causing the output of the detector to be measured when the head is removed.

9) An optical scatter smoke sensor as claimed in claim 8, in which a switch on the base is actuated when the head is removed.

10) An optical scatter smoke sensor as claimed in claim 8, in which the output of the detector is continuously monitored, means being provided for recognising that the head is removed by monitoring for a sudden change in output of the detector.

11) An optical scatter smoke sensor as claimed in any preceding claim, in which the measuring and triggering means comprises a comparator which compares the first output signal of the detector with the threshold value and triggers an alarm, in the event that the output exceeds the threshold value.

12) An optical scatter smoke sensor as claimed in claim 11, in which the detector is preferably connected to the comparator via an amplifier.

13) An optical scatter smoke sensor as claimed in any preceding claim, in which the measuring and triggering means comprises processing means.

14) An optical scatter smoke sensor as claimed in any preceding claim, in which means are provided for attenuating the input to the processing means when the head is removed.

15) An optical scatter smoke sensor substantially as herein described with reference to the accompanying drawing.

16) A method of recalibrating an optical scatter smoke sensor having a base, a head detachably mounted to the base, an optical smoke sensing chamber disposed in the head, an optical emitter disposed on the base and arranged to emit radiation into said chamber when the head is mounted to the base, an optical detector disposed on the base, means for measuring the output signal of the detector, means for triggering an alarm when the first output signal exceeds a threshold value, and means in the chamber for at least partially obscuring the detector from the emitter, the method comprising removing the head, energising the emitter, measuring and storing the output value of the detector, fitting the head, adjusting the sensitivity of the alarm according to the value of the stored signal.

16) A method as claimed in claim 15, in which the sensitivity is varied by adjusting the threshold according to the magnitude of said second output signal.

17) A method as claimed in claim 16, in which the threshold value is adjusted by multiplying the original threshold value by the ratio of the new second output signal to the old second output signal.

18) A method as claimed in claim 16, in which the threshold value is adjusted by multiplying the previous threshold value by the ratio of the new second output signal to the previous second output signal.

19) A method as claimed in claim 15, in which the sensitivity is varied by adjusting the amount of gain applied to the first output signal of the detector.

20) A method of recalibrating an optical scatter smoke sensor, the method being substantially as herein described with reference to the accompanying drawing.