WO2000072282A1

WO2000072282A1 - Self adjusting smoke detector

Info

Publication number: WO2000072282A1
Application number: PCT/IL1999/000269
Authority: WO
Inventors: Nitsan Nisim; David Kartoun; Sergey PASHIN
Original assignee: Rokonet Electronics Ltd.
Priority date: 1999-05-19
Filing date: 1999-05-19
Publication date: 2000-11-30
Also published as: AU3952899A

Abstract

A light-scattering smoke detector that has a test mode and a detection mode is provided, the detector comprising: a smoke chamber that receives smoke from the environment; an emitter that radiates light pulses that illuminate the smoke chamber; a photosensor that provides an output signal responsive to the intensity of light thereon; an optical channel that directs light from the emitter to the photosensor without its passing through the smoke chamber; a shutter that closes the channel in the test mode; and circuitry that receives output signals from the photosensor; and in the detection mode, in which the channel is open, the circuitry processes output signals from the photosensor to detect smoke and in the test mode the circuitry processes output signals from the photosensor to determine whether the detector is operating properly.

Description

SELF ADJUSTING SMOKE DETECTOR FIELD OF THE INVENTION

The invention relates to smoke detectors and in particular to smoke detectors that monitor their own performance and have a capability to calibrate and check themselves. BACKGROUND OF THE INVENTION

Many smoke detectors commonly used to raise a fire alarm when they detect smoke use light to detect smoke. These are hereinafter referred to as "photo-electronic smoke detectors", Typically , a photo-electronic smoke detector comprises a light emitter, such as a LED or an IRED, a photosensor such as a photodiode, and a detection chamber, hereinafter referred to as a "smoke chamber". Usually the emitter is controlled to cyclically illuminate the volume of the smoke chamber with a pulse of light. Smoke in the smoke chamber causes a change in an amount of light reaching the photosensor from the emitter. In response to the change, the photosensor generates a signal. If the change is greater than a certain magnitude the signal triggers an alarm. In some smoke detectors, hereinafter referred to as "light absorption smoke detectors", the emitter is aimed so that light from the light pulses is directly incident on the photosensor. If smoke enters the smoke chamber it absorbs some of the light and decreases the amount of light reaching the photosensor. In response to the decrease, the photosensor generates a signal. When the decrease exceeds a certain magnitude, the magnitude of the signal exceeds an alarm threshold and the smoke detector generates an alarm.

In other smoke detectors, hereinafter referred to as "light-scattering smoke detectors", light from the emitter that enters the smoke chamber is not aimed toward the photosensor. Inside surfaces of the smoke chamber are generally made of or coated with strongly light absorbent material and are shaped so as to minimize reflection of light from the emitter to the photosensor from the inside surfaces. In the absence of smoke in the smoke chamber, little or no light from the emitter is incident on the photosensor. When smoke is present in the smoke chamber, the smoke scatters light from the emitter. Some of the scattered light is incident on the photosensor and stimulates the photosensor to generate an output signal. If the scattered light reaching the photosensor exceeds a predetermined intensity, the signal strength exceeds an alarm threshold and the smoke detector generates an alarm.

A number of factors affect the reliability of photo-electronic smoke detectors. For example, changes in the ambient environment of a photo-electronic smoke detector may affect the sensitivity of the smoke detector's photosensor or the intensity with which its emitter illuminates the detector's smoke chamber. In addition, with time, the intensity of light from the emitter and/or the sensitivity of the photosensor generally decrease as a result of component aging and/or dust accumulation on their optical surfaces.

In a light-scattering smoke detector, dust and dirt accumulation on internal surfaces of the detector smoke chamber scatter some of the light from the emitter that enters the smoke chamber to the photosensor. This scattered light generates a background level of illumination incident on the photosensor that introduces a bias in the photosensor output signal. The bias effectively lowers the threshold detection. Furthermore, the bias changes with time as dirt and dust continue to accumulate on the smoke chamber's internal surfaces. Light scattered by dirt and dust in a scattering photo-electronic smoke detector that is incident on the detector photosensor is hereinafter referred to as "background light".

In order to assure reliable operation most photo-electronic smoke detectors incorporate some means for periodically testing whether the smoke detector is operating properly and/or for monitoring and/or adjusting performance of detector components. Some photo-electronic smoke detectors are designed so that they can be manually tested for proper operation. U.S. patent 4,099,178 to Ranney et al, the disclosure of which is incorporated herein by reference, describes a light-scattering smoke detector that comprises a direct line of sight path from the emitter to the photosensor of the detector. In normal operation the direct light path is closed. To test if the smoke detector is operating properly, the direct light path is manually opened so that light from the emitter is incident on the photosensor. The incident light simulates light that reaches the photosensor by being scattered from smoke in the detector smoke chamber. If the detector is operating properly this light generates an alarm.

U.S. patent 4,306,230 to Forss, the disclosure of which is incorporated herein by reference, describes a light-scattering smoke detector that automatically generates an alarm indicating that the detector is not functioning properly if the intensity of light radiated by the detector's emitter falls below a predetermined threshold. The described detector has a direct line of sight path from the emitter to the detector's photosensor that is always open. As a result, some light from the emitter is always incident on the photosensor when the emitter illuminates the smoke chamber. The detector generates an alarm when the intensity of light through the direct channel plus light scattered by smoke in the smoke chamber exceeds a predetermined threshold. If the intensity of light from the emitter to the photosensor through the direct channel falls below a predetermined threshold, the detector generates a "malfunction" alarm that indicates either emitter or photosensor failure. An absorption type smoke detector is described in U.S. patent 4,420,746, the disclosure of which is incorporated herein by reference. The detector stores a reference output to which the output of a photosensor is compared in order to determine if smoke in the detector reaches a density that warrants raising an alarm. As noted in the patent "cyclically and at relatively long intervals" the reference output is updated to compensate for changes in detector components and ambient environment.

SUMMARY OF THE INVENTION

An aspect of some preferred embodiments of the present invention relates to providing a light-scattering smoke detector that has improved self-monitoring and self-calibration capability. The smoke detector comprises a smoke chamber, a photosensor and an emitter that illuminates the smoke chamber, preferably with pulses of light radiated at regular intervals.

Preferably, the smoke detector comprises a processor and a memory.

According to an aspect of some preferred embodiments of the present invention, the smoke chamber is provided with an optical channel through which a small fraction of the light radiated by the emitter is transmitted to the photosensor without passing through the smoke chamber. The optical channel is operable to be open or closed. Unlike prior art light-scattering smoke detectors, in preferred embodiments of the present invention, the optical channel is used in both an open state and a closed state to monitor and calibrate detector functioning. During normal operation, when the detector is operating to detect smoke, the optical chamber is preferably open. During test operation, the optical chamber is preferably closed.

At any given moment, the photosensor provides an output signal that is substantially proportional to the intensity of light incident on it. Output signals provided by the photosensor when the emitter illuminates the smoke chamber with a pulse of light during normal operation and in the absence of smoke in the smoke chamber are hereinafter referred to as "reference signals". Output signals provided by the photosensor in the absence of smoke during test operation when the emitter illuminates the smoke chamber with a pulse of light are hereinafter referred to as "test signals".

According to an aspect of some preferred embodiments of the present invention, reference signals are used to determine if emitter intensity is low or dirt accumulation in the smoke chamber is excessive. Reference signals are proportional to the sum of the intensities of background light and light incident on the photosensor from the emitter that is transmitted through the optical channel. Preferably, each reference signal generated by the photosensor is tested by the processor to determine if the reference signal lies between appropriate lower and upper bounds that are stored in the detector memory. A reference signal below the lower bound indicates low emitter intensity. A reference signal greater than the upper bound indicates excessive dirt accumulation in the smoke chamber. If the reference signal does not lie between the lower and upper bounds the detector generates an appropriate "malfunction" alarm to alert a user that a problem requiring user intervention exists.

According to another aspect of some preferred embodiments of the present invention, reference signals are used to update a reference value stored in the detector memory. The reference value is used as a baseline to determine if a photosensor output signal indicates the presence of smoke. Preferably, each time that the emitter illuminates the smoke chamber, the processor processes the photosensor output signal according to an algorithm that uses the reference value to determine if the output signal indicates the presence of smoke. If the output signal indicates smoke, the detector generates a smoke alarm. If the signal does not indicate the presence of smoke, the output signal is considered, by definition, to be a reference signal (i.e. an output signal during normal operation in the absence of smoke in the smoke chamber).

In the absence of smoke it is expected that an output signal generated by the photosensor when the emitter illuminates the smoke chamber will be equal to the reference value. However, in accordance with some preferred embodiments of the present invention, the algorithm does not require equality of the reference value and the photosensor output signal to determine absence of smoke. If a "no smoke" finding is returned by the algorithm and the magnitude of the photosensor output signal is not equal to the reference value, the difference, if below some preferably predetermined level, is considered to indicate that the reference value requires adjustment. Accordingly, the stored "old" reference value is replaced by a new reference value that is preferably determined responsive to the difference between the magnitude of the photosensor output signal and the old reference value.

According to an aspect of some preferred embodiments of the present invention, the detector is periodically switched from normal operation to test operation by closing the optical channel so as to receive a test signal from the photosensor. The test signal is proportional to the intensity of background light alone. The processor preferably compares the test signal to a value stored in memory to determine if the background light exceeds a desirable upper limit. If it does the detector generates a malfunction alarm.

According to another aspect of some preferred embodiments of the present invention, the difference between the magnitude of the test signal and the stored reference level is determined. This difference is proportional to the intensity of light that reaches the photosensor through the optical channel and is therefore a measure of emitter intensity that is unbiased by background light. The difference is preferably compared by the processor to a value stored in memory to determine if emitter intensity is unacceptably low and if it is, the detector generates a malfunction alarm.

According to another aspect of some preferred embodiments of the present invention, light reaching the photosensor through the optical channel is light from the emitter that is reflected by a window between the emitter and the smoke chamber.

In some preferred embodiments of the present invention, the emitter, and preferably also the photosensor, is sealed from the smoke chamber by a window. Light from the emitter enters the smoke chamber through the window and background light and light scattered by smoke in the smoke chamber exit the smoke chamber to the photosensor through the window. The window transmits most of the light from the emitter that is incident on the window to the smoke chamber and reflects a small fraction of this incident light into the optical channel so that it reaches the photosensor. In accordance with some preferred embodiments of the present invention, the amount of light that reaches the photosensor through the optical channel is determined by the geometry of the optical channel and the amount of light scattered by the window. In other preferred embodiments of the present invention, the optical chamber comprises a light attenuator and the amount of light reaching the photosensor also depends upon the factor by which the attenuator attenuates light.

It should be noted that the window serves two functions. First, it provides for easy cleaning of the detector since the emitter and photosensor are protected by the window from dust and dirt and the window is relatively easily cleaned. Second, since the window reflects only a small portion of the incident emitter light, it prevents saturation of the photosensor circuit from light through the channel.

There is therefore provided in accordance with a preferred embodiment of the present invention a light-scattering smoke detector having a test mode and a detection mode comprising: a smoke chamber that receives smoke from the environment; an emitter that radiates light pulses that illuminate the smoke chamber; a photosensor that provides an output signal responsive to the intensity of light thereon; an optical channel that directs light from the emitter to the photosensor without its passing through the smoke chamber; a shutter that closes the channel in the test mode; and circuitry that receives output signals from the photosensor; and in the detection mode, in which the channel is open, the circuitry processes output signals from the photosensor to detect smoke and in the test mode the circuitry processes output signals from the photosensor to determine whether the detector is operating properly.

Preferably, the photosensor is sealed from the smoke chamber and light scattered by smoke in the smoke chamber from the smoke chamber passes through a window to reach the photosensor.

Additionally or alternatively, the emitter is preferably sealed from the smoke chamber and light from an emitter light pulse that illuminates the smoke chamber is incident on a window through which it passes to enter the smoke chamber. Preferably, some of the light from an emitter light pulse that is incident on the window is reflected by the window into the optical channel.

In some preferred embodiments of the present invention the circuitry comprises a memory. Preferably, a first reference value is stored in the memory, which first reference value is substantially equal to an output signal generated by the photosensor when an emitter light pulse illuminates the smoke chamber in the absence of smoke in the smoke chamber. In the detection mode, the circuitry preferably uses the first reference value to determine a value for a smoke index for an output signal that it receives from the photosensor and uses the value of the smoke index to determine if smoke is present in the smoke chamber.

In some preferred embodiments of the present invention the smoke index is equal to the quotient determined by dividing the difference between the magnitude of the output signal minus the first reference value by the first reference signal.

In some preferred embodiments of the present invention a second reference value is stored in the memory, which second reference value is equal to the magnitude of a photosensor output signal received by the circuitry while the shutter is closed. Preferably, the smoke index is equal to the ratio between a first number that is equal to the output signal minus the first reference signal and a second number that is equal to the first reference value minus the second reference value.

Additionally or alternatively, if the value of the smoke index for a light pulse is greater than a predetermined first threshold, the circuitry determines that smoke is present in the smoke chamber and generates a smoke alarm. Preferably, if the circuitry determines that smoke is absent, the circuitry determines whether the magnitude of the photosensor output signal responsive to the light pulse lies between a predetermined lower and upper bound and if it does not, generates a malfunction signal indicating that the detector is not operating properly. If the photosensor output signal lies between the lower and upper bounds and the ratio between the output signal and the previous output signal is less than a predetermined upper limit, preferably, the circuitry determines a new first reference value. Preferably, the upper limit is less than or equal to 1.50. More preferably, the upper limit is less than or equal to 1.30. Most preferably, the upper limit is substantially equal to 1.25.

In some preferred embodiments of the present invention, the circuitry determines the sum of the old first reference value plus a fraction less than one of the difference of the photosensor output signal minus the old first reference value and if the sum is less than a predetermined upper limit the new first reference value is determined to be equal to the sum. Preferably, if the sum is greater than the upper limit the new first reference value is determined to be equal to the upper limit.

Additionally or alternatively, the time period between consecutive light pulses emitted by the emitter divided by the fraction is preferably greater than 15 minutes. Preferably, the time period between consecutive light pulses emitted by the emitter divided by the fraction is preferably greater than 30 minutes. More preferably, the time period between consecutive light pulses emitted by the emitter divided by the fraction is substantially equal to 45 minutes.

In some smoke detectors, in accordance with preferred embodiments of the present invention, in the test mode, the circuitry processes output signals from the photosensor to determine whether the difference of the reference signal minus an output signal is less than a predetermined lower bound and if it is, generates a signal indicating that the intensity of light registered for light received from the emitter is low.

In some smoke detectors, in accordance with preferred embodiments of the present invention, in the test mode, the circuitry processes output signals from the photosensor to determine if they are greater than a predetermined upper limit and if an output signal is greater than the upper limit generates a signal indicating the chamber is dirty.

In some preferred embodiments of the present invention, the shutter comprises a baffle that is moved into and out of the optical channel to close and open the channel respectively. In some preferred embodiments of the present invention, the shutter is manually operated. In some preferred embodiments of the present invention, the shutter is automatically operated. In some preferred embodiments of the present invention, an actuator or motor moves the baffle. Preferably, the motor is a piezoelectric motor.

In some preferred embodiments of the present invention, the shutter comprises a liquid crystal that is electronically controlled to transmit or not transmit light. In some preferred embodiments of the present invention, the channel comprises an attenuator that controls the amount of light transmitted through the optical channel to the photosensor.

There is further provided, in accordance with a preferred embodiment of the present invention a method for detecting smoke comprising: a) illuminating a smoke chamber with light pulses radiated by an emitter; b) generating output signals responsive to light scattered by smoke in the chamber and incident on a photosensor; c) directing some light from each light pulse so that it is incident on the photosensor without the light passing into the smoke chamber; d) determining a first reference value from an output signal generated by the photosensor in the absence of smoke in the smoke chamber; e) for each light pulse, normalizing a difference between an output signal of the photosensor and the first reference value to a function of the first reference value; and f) determining that smoke is present in the smoke chamber if the normalized difference is greater than a predetermined threshold.

Preferably, the function of the first reference value used to normalize the difference is equal to the first reference value.

Alternatively, the method comprises; in a test mode: blocking light not passing through the smoke chamber from reaching the photosensor; and determining a second reference value in the absence of smoke in the smoke chamber from an output signal of the photosensor while in the test mode; wherein the function of the first reference value used to normalize the difference is equal to the first reference value minus the second reference value.

In some preferred embodiments of the present invention the method comprises; in a test mode: blocking light not passing through the smoke chamber from reaching the photosensor; determining whether an output signal of the photosensor, in the absence of smoke in the smoke chamber while in the test mode, lies between predetermined lower and upper bounds. Preferably, the method comprises providing an indication that the smoke detector is malfunctioning if the output signal does not lie between the predetermined lower and upper bounds.

Preferably, if the output signal is less than the predetermined lower bound the indication indicates that the intensity of light registered by the photosensor is not sufficient for proper smoke detection.

Additionally or alternatively, if the output signal is greater than the upper bound the indication preferably indicates that the intensity of background light from the smoke chamber is too large to enable proper smoke detection. BRIEF DESCRIPTION OF FIGURES

The invention will be more clearly understood by reference to the following description of preferred embodiments thereof read in conjunction with the figures attached hereto. In the figures, identical structures, elements or parts which appear in more than one figure are labeled with the same numeral in all the figures in which they appear. The figures are listed below and: Figs. 1A - 1C schematically show a smoke detector in respectively, normal operation without smoke in the detector smoke chamber, normal operation with smoke in the smoke chamber and in test operation, in accordance with a preferred embodiment of the present invention; Fig. 2 shows a flow chart of an algorithm that is used during normal operation by the detector shown in Fig. 1 to check detector operation and to determine when to generate a smoke alarm, in accordance with a preferred embodiment of the present invention; and

Fig. 3 shows a flow chart of an algorithm that is used during test operation by the detector shown in Fig. 1 to check detector operation and determine when to generate a malfunction alarm, in accordance with a preferred embodiment of the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Figs. 1A - 1C schematically show a cross sectional view of a smoke detector 20 in respectively, normal operation without smoke in the detector smoke chamber, normal operation with smoke in the smoke chamber and in test operation, in accordance with a preferred embodiment of the present invention. Parts and components of smoke detector 20 are not necessarily to scale and relative dimensions of the parts and components in Figs. 1A - 1C are chosen for ease and clarity of presentation.

Referring to Fig. 1A, detector 20 comprises a smoke chamber 22, an emitter 24, such as an LED or IRED, for illuminating smoke chamber 22 and a photosensor 26, such as a photodiode, for receiving light scattered by smoke in smoke chamber 22. Preferably, smoke detector 20 comprises a processor (not shown) and a memory (not shown).

Emitter 24 and photosensor 26 are preferably mounted in relatively deep recesses 28 and 30 respectively, in an optical block 32. Recesses 28 and 30 are preferably sealed with windows 34 and 36 respectively. Optical block 32 is mounted adjacent to smoke chamber 22. Smoke chamber 22 has portals 38, one of which is shown in Fig. 1, that enable smoke to enter, but substantially prevent light from entering, smoke chamber 22. Portals that enable smoke to drift into a smoke chamber and are appropriately "baffled" to prevent entry of light into the smoke chamber are well known in the art. A channel 40, hereinafter referred to as "optical channel" 40, extends from recess 30 to recess 28 along a line of sight from photosensor 26 to window 34. Optical channel 40 is controllable to be open or closed, preferably by a shutter 42. In normal operation, during which the detector is operating to detect smoke in smoke chamber 22, optical channel 40 is preferably open. In test operation, during which the intensity of background light is measured, optical channel 40 is preferably closed.

Shutter 42 preferably comprises a shutter channel 44 that intersects optical channel 40 and a baffle 46. Baffle 46 is moveable back and forth along shutter channel 44 so as to be inserted into or extracted from optical channel 40 and thereby to respectively close or open optical channel 40. In some preferred embodiments of the present invention, baffle 46 is moved along shutter channel 44 manually. In other preferred embodiments of the present invention, baffle 46 is moved along shutter channel 44 by an actuator or motor, such as a small mechanical motor or a piezoelectric motor. In other preferred embodiments of the present invention, the shutter is an electronic shutter such as a liquid crystal (LC) device that can be controlled to transmit or not transmit light. In Fig. 1A, smoke detector 20 is shown in normal operation; baffle 46 is outside of optical channel 40 and optical channel 40 is open.

Emitter 24 periodically emits, preferably at regular intervals, a light pulse to illuminate smoke chamber 22. Light from the light pulse is incident on window 34, which reflects a small fraction of the incident light into optical channel 40 and transmits most of the incident light into smoke chamber 22.

Some of the light that enters smoke chamber 22 from emitter 24 is reflected off inside surfaces of smoke chamber 22 in the direction of photosensor 26 so that it passes through window 36 and is incident on photosensor 26. This light, which is background light, is represented by dashed line 50. Some of the light, hereinafter referred to as "channel light", from emitter 24 that is reflected by window 34 into optical channel 40 is reflected along directions that intersect photosensor 26 and is incident on photosensor 26 if optical channel 40 is open. Channel light is represented by dashed line 52.

Preferably, optical channel 40 is fitted with a light attenuator 54 to control the amount of channel light 52 that reaches photosensor 26 when optical channel 40 is open. In some preferred embodiments of the present invention, the functions performed by shutter 42 and attenuator 54 are performed by a single device, such as an appropriate LC. In some preferred embodiments of the present invention, attenuator 54 is not used and the amount of channel light 52 that reaches photosensor 26 is determined by the geometry of optical channel 40 and the fraction of light incident on window 34 that window 34 reflects.

Both channel light 52 and background light 50 are proportional to the intensity of light radiated by emitter 24. Let the intensity of emitter light be represented by "I" and an output signal generated by photosensor 26 in response to light incident thereon be represented by "OS". Then, for output signals generated during normal operation, when there is no smoke in smoke chamber 22, as shown in Fig. 1A, the output signal OS can be written OS = αl + βl. In the equation, α and β are proportionality coefficients, αl represents the part of signal OS due to channel light 52 and βl represents the part of signal OS due to background light 50. Fig. IB schematically shows smoke detector 20 with smoke chamber 22 filled with smoke, which smoke is represented by stippled shading 56. Some of the light that enters smoke chamber 22 from emitter 24 is scattered by smoke 56 and is incident on photosensor 26. Light from emitter 24 that is scattered by smoke 56 to photosensor 26 is represented in Fig. IB by dashed line 58. Smoke scattered light 58 is substantially proportional to I and the density of smoke in the smoke chamber. Therefore, during normal operation, when there is smoke in smoke chamber 22, as shown in Fig. IB, OS can be written OS = αl + βl +γpl, where γpl is the contribution to OS from smoke scattered light 58, p is the density of smoke in smoke chamber 22 and γ is a coefficient of proportionality. Output signals OS generated by photosensor 26 during normal operation are processed by the processor to determine if the signals indicate the presence of smoke in smoke chamber 22. If a signal OS indicates smoke, detector 20 generates a smoke alarm. If smoke is not indicated, the signal OS is used, in accordance with a preferred embodiment of the present invention, as a reference signal to check the output intensity of emitter 26 and the intensity of background light 50. If the check indicates that output intensity of emitter 24 is unacceptably low or that background light intensity is unacceptably high, detector 20 generates a malfunction alarm. If no malfunction alarm is generated, the reference signal is also used to calibrate a reference value, "RV", which is stored in memory and used by the processor in processing output signals OS to determine if smoke is present in smoke chamber 22. Fig. 2 shows a flow diagram of an algorithm 60, in accordance with a preferred embodiment of the present invention, that governs how the processor in detector 20 processes output signals OS from photosensor 26 during normal operation of smoke detector 20. In step 62 of the flow diagram, emitter 26 illuminates smoke chamber 22 with a pulse of light. In step 64, the processor registers an output signal OS generated in response to light from the light pulse that reaches photosensor 26. OS is either equal to

(αl + βl), if there is no smoke in smoke chamber 22, or equal to (αl + βl -fγpl), if there is smoke in smoke chamber 22.

In step 66 a value "SI", hereinafter referred to as a "smoke index", is determined for OS using the reference value RV stored in memory. The reference value RV is an expected magnitude for a signal OS that is generated by photosensor 26 in the absence of smoke, i.e. RV is substantially equal to (αl + βl) at the time that the smoke index is determined. An initial "factory" reference value RV is stored in the detector memory when the detector is calibrated upon completion of its manufacture. With time, I, α and β change due to component aging and dirt accumulation, or as a result of changes in the ambient environment of detector 20 that affect components in detector 20. Therefore, in accordance with a preferred embodiment of the present invention, the value of RV is regularly adjusted to compensate for these changes. As a result, at any particular time, the value of RV is a relatively accurate predictor of the magnitude of a signal OS generated in the absence of smoke. Preferably, RV is adjusted during normal operation of detector 20 every time emitter 24 illuminates smoke chamber 20 according to steps of algorithm 60 discussed below.

Preferably, SI = (OS - RV)/RV. Substituting into this last expression the formulae for OS and RV given above, SI can be written, SI = [γ/(α + β)]p. From this last expression it is seen that, as long as RV is appropriately updated, smoke index SI is substantially proportional to the density p of smoke in the smoke chamber and is relatively independent of emitter intensity and the affects of dirt accumulation in the smoke chamber. SI therefore is a relatively robust and stable indicator of smoke in chamber 22. Following the determination of SI, in step 68 SI is compared to an alarm threshold,

"AT", which is stored in the memory of detector 20. Preferably, the value of AT is chosen so that when SI is greater than AT it indicates that density of smoke in the smoke chamber is greater than an appropriate industry standard smoke density for which it is determined that a fire alarm should be raised. A common standard is the American UL268 standard that requires that a threshold for smoke density at which a smoke detector raises an alarm have a value that lies between 0.5%/ft obscuration and 4%/ft obscuration.

If SI is greater than AT, the processor advances to step 70 and generates a smoke alarm. If SI is less than AT, algorithm 60 assumes that OS is due only to background light 50 and channel light 52 and can be used as a reference signal. The processor then advances to step 72 in algorithm 60 and preferably checks to see if OS lies between a lower limit "EL" and an upper limit "DL" that are stored in memory, i.e. the processor checks if DL > OS > EL. EL is determined so that if OS violates the equation because OS is less than EL, it indicates that the intensity of light emitted by emitter 26 and/or the sensitivity of photosensor 26 are unacceptably low. DL is chosen so that if OS violates the equation because OS is greater than DL, it indicates that background light 50 is unacceptably high. OS exceeds DL generally as a result of accumulation of an excessive amount of dirt on inside surfaces of smoke chamber 22 that increases the reflectance of the inside surfaces. If OS does not satisfy the equation DL > OS > EL, then the processor proceeds to step 70 and detector 20 generates a malfunction alarm.

If no malfunction alarm is generated, the algorithm assumes that the emission intensity of emitter 24, the intensity of background light 50 and the sensitivity of photosensor 26 are all within acceptable limits. The processor then proceeds to step 76, which represents an algorithm for adjusting RV. Preferably, in step 76 the processor first determines if the ratio of OS to RV is greater than a predetermined upper limit. If the ratio is greater than the upper limit, the value of RV is not adjusted and algorithm 60 returns to step 62 and the cycle is repeated for a next pulse of light radiated by emitter 24. Preferably, the upper limit for the ratio is less than 1.5. More preferably the upper limit is less than 1.30. Most preferably, the upper limit is substantially equal to 1.25.

If the ratio is less than the upper limit, the processor determines a sum equal to RV plus a fraction that is less than one of any difference that exists between OS and RV. If "S" represents the determined sum, then S = RV + (OS - RV)/K, where K is a number greater than 1. If S is less than a predetermined upper limit then the value for RV stored in memory is set equal to S. If S is greater than or equal to the upper limit, then RV is respectively set equal to the upper limit or left unchanged. Preferably, the upper limit for S is less than 1.75 x the initial factory value for RV. More preferably, the upper limit is less than 1.6 x the factory value. Most preferably, the upper limit is 1.5 x the factory value. Algorithm 60 then returns to step 62.

By adjusting RV as described above and using an appropriate value for K, rapid increases in OS between light pulses of emitter 24 that might be due to smoke, build up from pulse to pulse of emitter 24 until they trigger an alarm. On the other hand, slower changes in OS due to drift in the characteristics of components of detector 20 do not build up and trigger an alarm. Smoke density from a fire generally increases OS from its value with no smoke present to a value above threshold in a time period from seconds to minutes. Changes in OS of magnitude that might trigger an alarm and that are due to drift in component characteristics takes place in time periods on the order of hours. The time period that it takes for RV to be adjusted for a change in OS is on the order of KΔT where ΔT is the time between light pulses emitted by emitter 24. Therefore K and ΔT are preferably chosen so that KΔT is greater than 15 minutes. More preferably, KΔT is greater than one half-hour. Most preferably KΔT is substantially equal to 45 minutes. For example, in a preferred embodiment of the present invention, in which ΔT is on the order of 5 seconds, most preferably K would be about 500. In accordance with a preferred embodiment of the present invention, detector 20 is periodically switched from normal operation to test operation, during which the intensity of background light is measured and the intensity of light from emitter 24 registered by photosensor 26 is determined. The intensities are tested to determine if they lie within acceptable limits. In test operation, optical channel 40 is closed, channel light 52 is blocked from reaching photosensor 26 and output signals provided by photosensor 26 are proportional only to background light 50. Output signals generated by photosensor 26 during test operation of detector 20 are checked to determine if background light exceeds acceptable limits. Output intensity of emitter 24 is checked by subtracting the value of an output signal from the stored reference value to determine an intensity for channel light. The determined intensity for the channel light is checked to see if it indicates that output intensity of emitter 24 is unacceptably low.

In some preferred embodiments, for example, in embodiments in which shutter 42 is manually operated, detector 20 is switched to test operation and checked to assure reliable operation at time intervals that are warranted by ambient conditions in which detector 20 operates. In preferred embodiments of the present invention in which shutter 42 is automatically controlled, test operation may easily be performed automatically at regular and frequent intervals determined, for example, by the rate at which emitter 20 radiates light pulses. For example, in some preferred embodiments of the present invention in which shutter 42 is automatically operated, detector 20 is programmed to switch to test operation following every "n-th" light pulse emitted by emitter 24.

Tests performed during test operation of detector 20 are different from a test of background light and output intensity during normal operation performed in step 72 of algorithm 60 described in the discussion of Fig. 2. The test performed in step 72 tests a reference signal, which is the sum of background light and channel light, to determine if background light and output intensity of emitter 26 are within acceptable bounds. The test cannot distinguish if a change in a tested reference signal is the result of a change in background light, emitter intensity or a decrease in photosensor sensitivity. As a result, in some instances, the test may not recognize a component malfunction.

For example, generally, with time, emission intensity of emitter 26 decreases while the intensity of background light increases. Changes with time of emitter intensity are thus generally offset by changes with time of background light. Therefore, in a reference signal provided by photosensor 26, the output intensity of emitter 24 can decrease below an acceptable level and be masked by an increase in background light so that the test in step 72 does not detect the drop in output intensity of emitter 24. On the other hand, the tests performed during test operation of detector 20, since they test background light and channel light separately are not subject to this sort of error.

Fig. 3 shows a flow diagram of an algorithm 80, in accordance with a preferred embodiment of the present invention that governs how detector 20 processes output signals OS during test operation of smoke detector 20.

In step 82 of algorithm 80 detector 20 is switched from normal operation to test operation and shutter baffle 46 is inserted into optical channel 40. In step 84 emitter 24 illuminates smoke chamber 22 with a pulse of light. In step 86 the processor registers an output signal OS generated by photosensor 26 in response to background light 50 that is caused by the light pulse. The magnitude of OS is equal to βl. OS is then tested in step 88 to check if it is less than a background light test value "BTV". If OS is greater than BTV the algorithm moves to step 90 and detector 20 generates a signal indicating that smoke chamber 22 is dirty. If OS is less than BTV the chamber is considered to be in proper operating condition and algorithm 80 proceeds to step 92.

In step 92 OS is subtracted from the reference value RV that is in memory. The difference (RV - OS) is equal to αl, the magnitude of a signal generated by photosensor 26 in response to channel light 52 alone. (RV is equal to the updated value of (αl + βl) and therefore (RV - OS) = αl.) In step 94, (RV - OS) is checked to determine if it is greater than an emitter test value "ETV". If it is not, program 80 proceeds to step 96 and generates a signal to indicate that either the emission intensity of emitter 24 and/or the sensitivity of photosensor 26 are unacceptably low. If (RV - OS) is greater than ETV program 80 proceeds to step 98 in which program 80 switches detector 20 back to normal operation. In some preferred measurements of the present invention, the smoke index is defined differently than the way the smoke index SI is defined in algorithm 60. In some preferred embodiments of the present invention, the smoke index is defined so that it incorporates measurements of background light performed during test operation of detector 20. Let SI* represent a smoke index defined to incorporate measurements of background light. Preferably, SI* = (OSN - RV)/(RV - OSχ) where OS^ and OSχ represent output signals generated by photosensor 26 during normal and test operation respectively. SI* is a more stable and robust indication of smoke than smoke index SI, if OSj is measured frequently.

To understand this, let the coefficients α, β and γ in the expressions for output signals generated by photosensor 26 be modified to explicitly show their dependence on the sensitivity of photosensor 26. Then, if "σ" represents the sensitivity of photosensor 26, α and γ can be written, α =α'σ, β = β'σ and γ = γ'σ and OSN ^an^ OS- can be written

OS_N = (α'σl + β'σl +γ'σpl) and OS_T = β'σl.

In these expressions α' is constant, since preferably both emitter 24 and photosensor 26 are sealed from smoke chamber 22 and the outside environment of detector 20 by windows 34 and 36 respectively, γ' is substantially constant and changes slowly only to the extent that dirt accumulation on window 36 decreases the transmittance of window 36 for light from emitter 24. β' is not constant and changes with time as dirt and dust accumulate in smoke chamber 22. Substituting the expressions for OS^ and OSχ into the expression for SI* gives SI* = [γ'/α']p. The expression for SI* is accurate if at the time that (RV - OSj) is determined, the value of RV and OS- are current, i.e. RV is accurately equal to αl + βl = α'σl + β'σl and OSj is accurately equal to β'σl at the time (RV - OSχ) is determined. Assuming that when detector

20 is in normal operation, RV is measured every time emitter 24 illuminates smoke chamber 22 with a pulse of light, as provided for by algorithm 60, RV will be accurate. OSχ will also be substantially accurate if OSχ is measured and stored in memory frequently enough so that accumulation of dirt and dust in chamber 22 is relatively small between measurements of OSj.

SI can similarly be expressed in terms of α', β' and γ', by substituting for α, β and γ in the formula for SI given above the expressions for α, β and γ' in terms of photosensor sensitivity σ. Performing the substitution gives, SI = [γ'/(α' + β')]p. From the expression for SI* it is seen that the coefficient of the smoke density p in SI*,

[γ'/α'], is independent of β' and is therefore substantially constant. On the other hand, from this expression for SI it is seen that the coefficient of p in SI, [γ'/(α' + β')], is dependent on β'. The coefficient of smoke density p in SI is therefore more labile than the coefficient of p in SI*. SI* is therefore, generally, a more stable and robust indicator of the presence of smoke than is SI. In the description and claims of the present application, each of the verbs, "comprise"

"include" and "has", and conjugates thereof, are used to indicate that the object or objects of the verb include but are not necessarily a complete listing of all components, elements or parts of the subject or subjects of the verb.

The present invention has been described using non-limiting detailed descriptions of preferred embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. Variations of embodiments described will occur to persons of the art. The scope of the invention is limited only by the following claims.

Claims

1. A light-scattering smoke detector having a test mode and a detection mode comprising: a smoke chamber that receives smoke from the environment; an emitter that radiates light pulses that illuminate the smoke chamber; a photosensor that provides an output signal responsive to the intensity of light thereon; an optical channel that directs light from the emitter to the photosensor without its passing through the smoke chamber; a shutter that closes the channel in the test mode; and circuitry that receives output signals from the photosensor; and in the detection mode, in which the channel is open, the circuitry processes output signals from the photosensor to detect smoke and in the test mode the circuitry processes output signals from the photosensor to determine whether the detector is operating properly.

2. A smoke detector according to claim 1 wherein the photosensor is sealed from the smoke chamber and light scattered by smoke in the smoke chamber from the smoke chamber passes through a window to reach the photosensor.

3. A smoke detector according to claim 1 or claim 2 wherein the emitter is sealed from the smoke chamber and light from an emitter light pulse that illuminates the smoke chamber is incident on a window through which it passes to enter the smoke chamber.

4. A smoke detector according to claim 3 wherein some of the light from an emitter light pulse that is incident on the window is reflected by the window into the optical channel.

5. A smoke detector according to any of the preceding claims wherein the circuitry comprises a memory.

6. A smoke detector according to claim 5 wherein a first reference value is stored in the memory, which first reference value is substantially equal to an output signal generated by the photosensor when an emitter light pulse illuminates the smoke chamber in the absence of smoke in the smoke chamber.

7. A smoke detector according to claim 6 wherein, in the detection mode, the circuitry uses the first reference value to determine a value for a smoke index for an output signal that it receives from the photosensor and uses the value of the smoke index to determine if smoke is present in the smoke chamber.

8. A smoke detector according to claim 7 wherein the smoke index is equal to the quotient determined by dividing the difference between the magnitude of the output signal minus the first reference value by the first reference signal.

9. A smoke detector according to claim 7 wherein a second reference value is stored in the memory, which second reference value is equal to the magnitude of a photosensor output signal received by the circuitry while the shutter is closed.

10. A smoke detector according to claim 9 wherein the smoke index is equal to the ratio between a first number that is equal to the output signal minus the first reference signal and a second number that is equal to the first reference value minus the second reference value.

11. A smoke detector according to any of claims 8 - 10 wherein, if the value of the smoke index for a light pulse is greater than a predetermined first threshold, the circuitry determines that smoke is present in the smoke chamber and generates a smoke alarm.

12. A smoke detector according to claim 11 wherein if the circuitry determines that smoke is absent, the circuitry determines whether the magnitude of the photosensor output signal responsive to the light pulse lies between a predetermined lower and upper bound and if it does not, generates a malfunction signal indicating that the detector is not operating properly.

13. A smoke detector according to 12 wherein if the photosensor output signal lies between the lower and upper bounds and the ratio between the output signal and the previous output signal is less than a predetermined upper limit, the circuitry determines a new first reference value.

14. A smoke detector according to claim 13 wherein the upper limit is less than or equal to 1.5.

15. A smoke detector according to claim 13 wherein the upper limit is less than or equal to 1.30.

16. A smoke detector according to claim 13 wherein the upper limit is substantially equal to 1.25.

17. A smoke detector according to any of claims 13 - 16 wherein the circuitry determines the sum of the old first reference value plus a fraction less than one of the difference of the photosensor output signal minus the old first reference value and if the sum is less than a predetermined upper limit the new first reference value is determined to be equal to the sum.

18. A smoke detector according to claim 17 wherein if the sum is greater than the upper limit the new first reference value is determined to be equal to the upper limit.

19. A smoke detector according to claim 18 wherein the time period between consecutive light pulses emitted by the emitter divided by the fraction is greater than 15 minutes.

20. A smoke detector according to claim 19 wherein the time period between consecutive light pulses emitted by the emitter divided by the fraction is greater than 30 minutes.

21. A smoke detector according to claim 20 wherein the time period between consecutive light pulses emitted by the emitter divided by the fraction is substantially equal to 45 minutes.

22. A smoke detector according to any of claims 5 - 21 wherein, in the test mode, the circuitry processes output signals from the photosensor to determine whether the difference of the reference signal minus an output signal is less than a predetermined lower bound and if it is, generates a signal indicating that the intensity of light registered for light received from the emitter is low.

23. A smoke detector according to any of the preceding claims wherein, in the test mode, the circuitry processes output signals from the photosensor to determine if they are greater than a predetermined upper limit and if an output signal is greater than the upper limit generates a signal indicating the chamber is dirty.

24. A smoke detector according to any of the preceding claims wherein the shutter comprises a baffle that is moved into and out of the optical channel to close and open the channel respectively.

25. A smoke detector according claim 24 wherein the shutter is manually operated.

26. A smoke detector according to claim 24 wherein the shutter is automatically operated.

27. A smoke detector according to claim 24 wherein an actuator or motor moves the baffle.

28. A smoke detector according to claim 27 wherein the motor is a piezoelectric motor.

29. A smoke detector according to any of claims 1 - 23 wherein the shutter comprises a liquid crystal that is electronically controlled to transmit or not transmit light.

30. A smoke detector according to any of the preceding claims wherein the channel comprises an attenuator that controls the amount of light transmitted through the optical channel to the photosensor.

31. A method for detecting smoke comprising: a) illuminating a smoke chamber with light pulses radiated by an emitter; b) generating output signals responsive to light scattered by smoke in the chamber and incident on a photosensor; c) directing some light from each light pulse so that it is incident on the photosensor without the light passing into the smoke chamber; d) determining a first reference value from an output signal generated by the photosensor in the absence of smoke in the smoke chamber; e) for each light pulse, normalizing a difference between an output signal of the photosensor and the first reference value to a function of the first reference value; and f) determining that smoke is present in the smoke chamber if the normalized difference is greater than a predetermined threshold.

32. A method according to claim 31 wherein the function of the first reference value used to normalize the difference is equal to the first reference value.

33. A method according to claim 31 comprising; in a test mode: blocking light not passing through the smoke chamber from reaching the photosensor; and determining a second reference value in the absence of smoke in the smoke chamber from an output signal of the photosensor while in the test mode;; wherein the function of the first reference value used to normalize the difference is equal to the first reference value minus the second reference value.

34. A method according to any of claims 31 - 33 comprising; in a test mode: blocking light not passing through the smoke chamber from reaching the photosensor; determining whether an output signal of the photosensor, in the absence of smoke in the smoke chamber while in the test mode, lies between predetermined lower and upper bounds;

35. A method according to claim 34 comprising providing an indication that the smoke detector is malfunctioning if the output signal does not lie between the predetermined lower and upper bounds.

36. A method according to claim 35 wherein, if the output signal is less than the predetermined lower bound the indication indicates that the intensity of light registered by the photosensor is not sufficient for proper smoke detection.

37. A method according to claim 35 or claim 36 wherein, if the output signal is greater than the upper bound the indication indicates that the intensity of background light from the smoke chamber is too large to enable proper smoke detection.