Note: Descriptions are shown in the official language in which they were submitted.
32
Field of the Invention
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This invention relates to testing systems for photo- -
electric smoke detectors.
Background of the Invention
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Photoelectric smoke detectors are commonly used to detect
the presence of smoke particles in the air by sensing the
scattering of light energy by smoke particles in a sample ~ -
chamber of the detector. The sample chamber is commonly made
with walls which have a dull black finish to minimize light
reflection. A source of light is positioned to shine a beam
of light into the chamber. Should there be smoke present in
the chamber, the beam of light energy is scattered by the smoke
. particles and a portion of such scattered light is sensed by
photoelectric sensor, such as, a photodiode or the like, to
cause an alarm upon sensing an amount of scattered light energy
which results from a predetermined smoke concentration. ~ -
One of the concerns in using a photoelectric smoke
detector is that the photoelectric sensor and the source of ~ ~;
light, such as a light emittlng diode and associated electronic ~ ~ -
circuitry, are prone to failure or change ln characteristics ; ;
over extended periods. It is, therefore, important to provide
on the detector a type of test unit that, when actuated by the
consumer, indicates the device is still functioning as intended.
Several approaches have been taken ln the past to provide
test units for photoelectric smoke detectors, such as that
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disclosed in United States patent 3,868,184. This patent
discloses a particle simulating device for testing the operative-
ness of the photoelectric sensing cell and its associated alarm
circuit. The particle simulating device comprises a thin wire
mounted on a bell crank which is manually operated from outside
of the detector to swing the wire through the detection zone
at the intersection of lamp and photocell axis, so as to scatter
light from lamp to the cell in the same way an amount of
particles in the zone would accomplish. This arrangement lacks
precision in simulating a predetermined smoke concentration,
since it is very difficult to mount in the detector a suffi-
ciently thin wire to simulate a light scattering which would be
caused by a concentration of smoke in the range of 1.5 to 3%
obscuration.
In an attempt to improve the preclsion of the mechanical
device for simulating predetermined concentrations of smoke, a
wire may be positioned to swing into the sensitive area of the
chamber, which is not at the intersection of the lamp and photo-
cell axis. This permits the use of a larger diameter wire to
scatter light which has strayed from the light beam. The
difficulty with this arrangement is that a very exact location
is needed in positioning the wire in the sensitive zone to
provide a consistent scattering of an amount of light. This
entails the use of a complex mechanical linkage which will always
ensure the exact positioning for the wire in the sensitive zone
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and must be such that the swinging movement of the wire is
always the same and is not directly dependent upon the extent
to which a test button is pressed by the consumer.
Another approach for testing photoelectric smoke detectors
is disclosed in United States patent 4,099,178. In that
instance r the test means includes a light source and light '
responsive device and has a separate test light path other than
through the smoke chamber with light modifying means in the
test path to simulate a preselected smoke density. The test
means includes a normally closed gate for the test light path
the gate being manually opened to transmit light along the
test path to the light responsive device to cause actuation
of the alarm in the absence of smoke to indicate detector
operability. This arrangement entails the incorporation into
the device of a separate test light path other than the normal
path and the use of a mechanical linkage or the like to
provide externally of the device means for opening and closing
of the gate which permits the light from the light source to
travel along the test light path. The selection of the light
modifying means in the test path is critical, because with `~
variations in its characteristics, there is a result in varia-
tion in the sensitivity at which the detèctor is tested. Thus,
to ensure a testing of the device in a desired sensitivity range,
it is important to have rigid standards which must be met by
the light modifying means, such as opaque tape, to maintain
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a test in desired sensitivity range.
With the above form of testing devices for smoke detectors,
there is little, if any, flexibility in adjusting for changes
in the characteristics of the detector components. This may
arise, for example, in changing sources of supply for the photo-
electric sensor, the light source or other electronic components.
With mechanical devices for prodiving light scattering adjustment
of the amount of light to be scattered is very difficult, if not
impossible, with most units. Therefore, once the particular
size and location of the mechanical test unit has been deter-
mined, it is important to continue with the same source of
supply of the components to ensure that the manufactured units
are tested at the same sensitivity.
Summary of the Invention
The test arrangement, according to this invention, over- ;
comes the above problems in a simple manner, yet provides a
reasonably accurate testing of the detector sensitivty and
operativeness over the life of the unit.
The test arrangement may be incorporated in a photoelectric
smoke detector of the type having a smoke chamber, a light
source and a photoelectric sensor responsive to light scattered
by smoke in the chamber to cause an alarm at a predetermined
concentration of smoke. The test means, when actuated, is
adapted to increase light intensity from the light source to a
level which simulates in an essentially smoke-free chamber an
,
amount of light scattered and reflected by chamber walls
onto the sensor equivalent to that which would be scattered
by at least the said predetermined concentration of smoke. The
level of increased light intensity causes a device to indicate
S to the person testing the unit that the detector is operable
and is responding at a desired sensitivity.
Thus, the method according to this invention for testing
the sensitivity of a photoelectric smoke detector, comprises
increasing the light intensity of the smoke detector light
source to a level which simulates in an essentially smoke-free
chamber a sensed amount of light energy scattered and reflected
onto the sensor by chamber walls, which is equivalent to that
which would be scattered by at least the predetermined concen-
tration of smoke.
:
Description of the Drawings
Preferred embodiments of the invention are shown in
the drawings wherein:
Figure 1 is a schematlo representation of a preferred
type of electronic circuitry for a battery powered photo-
electric smoke detector having test means;
Figure 2 is a preferred embodiment for the sample
chamber of the photoelectric detector; and
Figure 3 shcws the sample chamber of Figure 2 opened
about its hinged axis to show the interior configuration thereof.
Detailed Description of the Preferred Embodiments
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Photoelectric smoke detectors may be used in commercial,
industrial or residential establishments for detecting smoke.
The detector may be battery powered or powered by a constant
voltage supply which may be AC converted into DC voltage. It
may be desirable in some instances to combine the use of a
photoelectric smoke detector with a dual ionization chamber
type of smoke detector to ensure that most forms of smoke
particles, caused by various types of fires, are sensed.
It is understood that the term light to describe the
energy emitted by the light source includes all radlant energy
which is capable of being scattered by smoke particles in a
sample chamber. Such radiant energy may be in the range of
infrared, visible and some of the ultraviolet region. The use
of the infrared or ultraviolet radiation, as emitted by the
light source, has the distinct advantage, because the
photoelectric sensor may be tuned to be sensitive to only
energy in the infrared or ultraviolet range to reduce the
effect external visible light has on the detector.
Figure l of the drawings shows a representative
electronic circuit for use in association with a photoelectric `~
smoke detector having test means according to this invention.
A timing circuitry l10 periodically activates the light source
14 and, upon sensing scattered light by a light sensor 16
caused by the presence of smoke particles, the electronic
circuit generally designated lO sounds an alarm such as an
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audible piezoelectric alarm 18. In this embodiment, the
circuit 10 is powered by a nine volt battery 20.
As shown in Figure 2, a preferred arrangement for a smoke
detector chamber is shown where the light source is housed in
chamber barrel portion 22 with leads 24 extending outwardly
therefrom and which are part of the circuit of Figure 1.
Correspondingly, the photoelectric sensor is housed in chamber
barrel portion 26 and has leads 28 which form part of the
circuitry. The chamber is provided with a plurality of
openings 30 which permit smoke in the air to enter the chamber.
Various forms of light blocking devices are found about the
openings, such as baffle 32 which, if the chamber is looked at
from a horizontal attitude, overlaps outer wall 34 to prevent
light from entering the chamber directly through the opening
30. Other forms of baffling arrangements are located about the
other openings to minimize, if not essentially eliminate, the
amount of outside light which may enter the chamber.
Preferably the chamber may be formed from injection
molded plastic and made in two parts hinged about area 36. The
two parts may be closed together and clipped shut by clip 38.
Further details of the interior of the sample chamber are
shown in Figure 3 where the chamber parts lay open. The beam
of light 40 from source 14, focused by lens 41, is directed
along the path shown in dotted line. Should there be smoke in
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the chamber, a portion of light energy scattered by smoke
particles impinges on sensor 16 along the path 42 shown in dot.
sy computer analysis, it has been found that, to provide an
acceptable degree of light scattering by smoke particles, the
minimum chamber size should have an optical depth of approximately
2 to 3 centimeters. An optimum positioning of the sensor 16,
relative to the light source 14 for sensing the amounts of
light energy scattered by smoke particles, is 30 away-from
the light beam, as shown in the drawing.
The chamber walls, such as wall 34, have on its inside
surface a plurality of vanes or flutes 44 which serve to minimize
the reflectivity of the surface and cause some scattering of
impinging light. The reflectivity is further reduced by
providing a dull black finish on the inside of the chamber walls. -
Various numbers and positioning of baffles, such as 46, are
located about the interior of the chamber to provide blocking
for light entering thropgh the openings 30 to the chamber.
Thus, when there is little, if any, smoke particles or other
particles in the sample~chamber, generally designated 48, light
emitted from the light source 14 will follow path 40 ~nd partially
be absorbed by wall 50 and remaining energy is reflected and
scattered from that portion of the wall onto other walls in
the chamber where most of the remaining light energy is absorbed.
A very small amount of light energy is, howe~er, scattered onto
the sensor 16 which is insufficient to cause an alarm. The
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shaping of the barrels 22 and 26 for the light source and
sensor include a plurality of vanes 52, which are shaped to
absorb anystray light from emitted beam 40 and to absorb any
stray light travelling down barrel 26 along path 42 towards the
sensor 16. In this embodiment, it is accomplished by sloping
the surfaces of the vanes to essentially trap and reflect or
scatter backwards some of the light energy, and due to the dull
black finish, absorb most of the remaining stray light energy.
There are several types of electronic circuits which
may be used in operating photoelectric smoke detectors, thus
the one shown in Figure 1 is illustrated to exemplify a preferred
type of circuitry. Depending upon the conditions for smoke
detection, it may desirable to work in a range which is
~ outside the visible region to reduce the effect that ambient
visible light may have on the detector. Thus, the light source -
14 may be a light emitting diode which has the characteristics,
when powered, to emit radiant energy in the infrared range,
which may be in the area of 900 nanometers. Thus, the photo-
electric sensor 16 may be a photodiode which is tuned to sense
principally radiant energy in the corresponding range of 900
nanometers. It is, of course, appreciated that the system may
be operated in the visible or ultraviolet range and that other
than light emitting diodes may be used with the photoelectric
sensor tuned accordingly. It is essential, however, that the ~ ;
chosen wavelength for the radiant energy be such that it will
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be effectively scattered by smoke particles which may enter
the chamber 48 of the detector.
Aside from the standard arrangement for the circui~ry
10 for the smoke detector, the circuitry includes a test device
which, when actuated, determines the operability of the
circuit and if it is reacting at a desired sensitivity.
Electronically, various techniques may be employed
to cause an increase in light intensity upon actuation of the
test means; one of which is exemplified by the following
preferred embodiment.
The tester arrangement is generally designated 54 and
forms part of the resistor network generaly designated 56.
The tester works on the principle of increasing the level of
light or radiant energy intensity emitted by the light source
14 to provide an amount of energy scattered and reflected
by the interior surfaces of the chamber 48, which impinges
upon light sensor 16 to simulate the same amount of energy
which would be scattered by a predetermined concentration of
smoke particles. The increased level of light intensity may,
therefore, be determined in simulating a desired smoke con-
centration to thus either actuate the alarm 18 or some other
device to indicate detector operability and that it is sensitive
at a desired level.
To further assist in the understanding of the test
means operation, reference is made to the details of the detector
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circuitry. The circuit is powered by a single nine volt
battery which, in normal operation, has an expected life in
excess of one year. The circuit 10 is designed so that it will
continue to operate accurately even as the battery discharges
with use over time and cannot maintain its full specified
voltage under load conditions. Connected in parallel with the
battery is a suitably large capacitor 58 which reduces
fluctuation in the voltage delivered to the circuit as rapidly
v~rying current demands are imposed by the circuit. A
capacitor 60 controls the internal timing of the timing
circuitry 110. The timing circuitry 110 periodically pulls pin
62 from a floating high voltage state (hereinafter referred to
as "off") to a near ground state (hereinafter referred to as
non") for a short time duration according to this embodiment of
approximately 200 microseconds.
The reference voltage device 112 produces a very accurate
reference voltage level at pin 64, which is independent of the
supply voltage and which forms part of establishing a reference
il~ adjusting the sensitivity of the photodiode 16.
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A voltage level detection circuit 114 triggers the
piezoelectric alarm when the voltage difference across pins 66,~
~; 68 drops below a certain critical level.
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When pin 62 in "on", current flows from the positive
terminal of the battery through resistor 70 and the zener diode
72 to pin 52 which is near ground. A well defined voltage
independent of the power supply is established on the base of
transistor 74 and, therefore, a well defined voltage is
established at the emitter of transistor 74. Resistor 76 of
the resistor network is in parallel with a thermistor 78. ~s
is appreciated, the thermistor 78 compensates for changes in
the characteristics of the photosensitive diode 16 due to
fluctuations in temperature. Resistors 76, 78 allow a well
defined and thus temperature compensated current to flow
through the emitter of transistor 74. A current independent of
the power supply voltage passes through the collector of
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transistor 74 and light emitting diode 15 to illuminate same at
a well defined level of radiant energy which may, as explained,
be in the range of 900 nanometers.
Transistor 80, as associated with the photodetection side ~
of the circuitry, is inactive when pin 62 i5 near ground and in ~ -
its "on" state, because the base of the transistor will also be
near ground. The voltage dividing network generally designated~
;~ 82 which consists of parallel connection of capacitor 84 with
;~ ~ series connection of the resistor 86, potentiometer 88 and
resistor 90, is;connected from the well established reference
voltage at pin 64 on the reference voltage device 112 to
ground. An adjustable yet stable voltage is produced at the
wiper contact of potentiometer 88.
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This voltage is fed to pin 66 of the voltage level
detection circuit 114 through a resistor 92. Pin 66 is
connected through a circuit decoupling capacitor 94 to the
anode of photodiode 16. The cathode of the photodiode 16 is
connected to pin 68 of the voltage level detection circuit 114
and through resistor 98 to ground. The voltage dividing
network 86 is thus provided with the voltage established at the
wiper contact of potentiometer 88. An amount of radiant energy -
scattered by smoke in the chamber impinges upon the photodiode
16 and causes the resistance of the photodiode to decrease to a
point such that the voltage across pins 66, 68 of the voltage
detection circuit 114 will drop below the critical level and
the alarm 18 will be triggered.
In factory calibration of a production unit, the detector
is placed in a controlled and clean environment. Potentiometer
88 is adjusted to the point where the alarm 18 begins sounding
when light emitting diode 14 is illuminated. By experimentj it
has been determined for the particular parameters of this
;~ circuit that the wiper contact voltage of the potentiometer 88
can be increased a determined amount; namely 2 millivolts, to
give the desired detector sensitivity. The device, when
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subsequently used in an environment, will sound an alarm for a
~ ~ desired smoke sensitivity of concentration equal to
;~ approximately 1.5~ obscuration.
When pin 62 on the timing circuitry 110 is "off", which
is the large percentage of the time, transistor 74 is inactive
and LED I4 is not illuminated. A small current flows from the
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positive terminal of the battery 20 through the series
connection of resistor 100, the parallel connection of resistor
76 and thermistor 78 to the base of the transistor 80. Since
the collector of transistor 80 is at ground, transistor 80
enters a mode in which the effective resistance across its
collector and emitter is very low. The emitter of transistor
80 is pulled to approximately ground. This ensures that, when
pin 62 on the timing circuitry 110 is "off", any extraneous
transient current from the cathode of the photodiode 16 will be
shunted to ground. This arrangement, therefore, avoids
erroneous triggering of the alarm because the voltage at pin 68 -
of the voltage level detection circuit cannot rise.
The testing device 54, according to the shown embodiment,
has a manually operated switch 102 which is normally open.
This switch may be closed to test for the operation and
sensitivity of the detector circuit. When the switch 102 is
closed, resistor 104 is connected in parallel with resistor 76
and thermistor 78 to provide additional current through the LED ;
14 when pin 62 on the timing circuitry 110 is "on". Resistor
104 has a value which has been estahlished by experiment for
the particular circuit parameters to increase the current ~
flowing through the light emitting diode 14 to a value which ;~;
causes the LED to increase the level of light intensity. The
level of increased radiant energy is such as to provide an
amount of energy scattered and reflected off the walls
impinging upon the photodiode 16, assuming the chamber to be
essentially clear of smo~e, to simulate an amount of light
which would be scattered by a
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smoke obscuration of 3~. Pushing test button 102, assuming the
circuitry to be operative, causes the alarm 18 to sound so that
the person testing the device is assured that the circuit is
operational and sensitive to at least 3% smoke obscuration and
it would respond in normal operation to an environmental
obscuration of approximately 3~ or more. At the time of
testing, should there by a concentration o~ smoke in the
chamber which is less than that needed normally to set off an
alarm, the amount of scattered and reflected light impinging on
the sensor, when the test button is pushed, will be greater
than the desirea sensitivity test level of 3%. Other
background noise may appear in the form of the previously
explained very small amount of light directed onto the sensor
by chamber walls. This is not considered a problem, because
such background noise is usually at a relatively low value. It
is appreciated that a lesser percentage obscuration may be
tested for in terms of detector sensitivity by simply chosing
another value for resistor 104 to produce a lower level of
increased light intensity which would simulate, for example,
1.5% smoke obscuration.
This manner of testing the photoelectric smoke detector
a~oids the need for any complicated mechanical linkage for
swinging into the sensitive region of the chamber a light
scattering device. The complete testing operation is
accomplished electronically where the integrity of both the
electronic circuitry and the light source and light sensor are
tested. Any desired sensitivity of the device greater than
that for which the device is set for, can also be established
by way of making appropriate alterations to the resistor
network 56.
Aside from the calibration adjustments provided by
voltage divider network 82 for the photosensitive diode 16,
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a calibration adjustment may also be provided for the light
emitting diode 14. This may be accomplished by substituting
for fixed resistor 104 a potentiometer. After calibration of
the photosensitive diode 16 in the manner previously discussed,
it may also be determined by experimentation that, to
compensate for a change in the characteristics of the light
emitting diode 14 or change in chamber characteristics, the
potentiometer in place of fixed resistor 104 may be varied to
provide the needed voltage pulse height through photoelectric
diode 16 which would be the same as that caused by a smoke
obscuration of desired concentration. Thus, by providing an
electronic form of testing the detector, there is the added
advantage of providing a unit which can compensate for
variations in the characteristics of the photoelectric diode,
light emitting diode and other characteristics of the ~ -
circuitry. As a result, it is not necessary to maintain rigid
controls on the characteristics of components and thus permit
obtaining components for the detector from several different
sources of supply. This appreciably facilitates manufacture of
the unit and, by reducing the rigid standards on component
characteristics, also reduces the cost of manufacture of the
unit.
In the illustrated embodiment, switch 102 is incorporated
on the device itself and is simply a mechanical switch. How- -
ever, a remote means of testing the circult integrity can be
accomplished by a relay mechanism or other suitable means. It
is also possible to automatically test the circuit sensitivity
by adding a second timing device to periodically switch in
resistor 104. The sounding of the alarm would be suppressed
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and would only be sounded if the circuit did not detect the
simulated increased obscuration by increasing the level of
light intensity from light emitting diode 1~.
In operation of the unit, should the device detect a
smoke obscuration of 1.5~ or more, the alarm will scund. The
system then checks for smoke obscuration every five seconds
after the first detection, where the alarm is resounded after
the first five second interval, should the smoke remain in the
chamber. This recheck on the presence of smoke is continued
until the smoke leaves the chamber.
The electronic circuit 10 may also include battery
voltage sensing device which will actuate an alarm or some
other indication means to let the consumer know when the
battery has achieved its low battery setting, which in this
instance, is considered to be 7.5 volts. At that time, the
battery should be replaced to maintain proper operation of the
circuitry.
It is also appreciated that, in actuating the test device
54 for determining detector operability, a device other than
the alarm 18 may be actuated to indicate circuit integrity.
Such a device may be a light or some other system of
indication. In particular when the unit is tested from remote
central control panel, an appropriate light may be employed to
indicate circuitry operativeness.
Although various embodiments of the invention have been
described herein in detail, it will be understood by those
skilled in the art that variations may be made thereto without
departing from the spirit of the invention or the scope of the
appended claims.
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