Note: Descriptions are shown in the official language in which they were submitted.
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PHOTOELECTRIC SMOKE DETECTOR
FIELD OF THE INVENTION
The present invention relates to a light
scattering type photoelectric smoke detector, more
particularly, a light scattering photoelectric smoke
detector having a surface mounted photodiode detector
oriented to a photoemitting diode to provide for good
sensitivity and radio frequency interference immunity.
BACKGROUND OF THE INVENTION
Smoke detectors based on light scattering by
smoke particles have been known for a number of years.
Such detectors at present utilize solid state components
including photoemitting diodes and photodiode detectors,
the two devices being incorporated in a fixed mounted
arrangement within a smoke sensing chamber. The smoke
sensing chamber is generally designed to exclude most
ambient light influences while providing openings to permit
entry of the smoke particles into the sensing chamber.
There have been many such designs developed and patented
which rely upon the walls of the smoke chamber having
labyrinth designs to allow the passage of the smoke
particles while excluding ambient light from the interior
of the chamber. Examples of such designs are shown in
United States Patent 3,914,616, 4,168,438, 4,315,158,
4,672,217 and 4,851,819. In many of the chambers of the
above noted patents, the photoemitting diodes and
photodiode detectors are mounted in either the sides of the
chamber or on the bottom of the chamber, most typically
with a 60 degree scattering angle along a horizontal plane
between the photoemitting diodes and photodiode detectors.
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The design of many of the prior art smoke detection
chambers results in a generally horizontal flow of the
smoke particles through the chamber. Ideally, the
photoemitting diode and photodiode detector should be
mounted in such a way that the intersection of the
transmitted light from the photoemitting diode and the view
of the photodiode detector falls within the horizontal path
of the smoke particles. However, depending upon the
laminar flow rate of the smoke particles through the
chamber, the horizontal flow may be shifted from the
intersection thereby affecting the sensitivity of the smoke
detector. Also, the use of the 60 scattering angle
increases the distance between the photodiode detector and
photoemitting diode thereby affecting the sensitivity and
increasing the potential for dust particle interference.
In addition, many of the prior art mounting arrangements
generally require the use of leads between the photodiode
detector and/or the photoemitting diode and the printed
circuit board carrying the other electrical and electronic
components utilized in the detection and alarm circuitry.
The use of such leads increases the potential
susceptibility of the alarm units to radio frequency
interference (RFI) with exposure to the increased number of
radio frequency (RF) transmitters throughout the RF
spectrum presently in use. As a result, there has been an
increasing need to improve the designs to increase the
immunity to radio frequency interference (RFI ) to prevent
unwanted false alarms. One means of achieving increased
RFI immunity has been through the use of leadless, surface
mounted components to improve RFI immunity over the older
leaded designs. One example of such design is shown in
Nagaoka, U.S. Patent 5,138,302, in which the photodetecting
element is mounted directly on the printed circuit board
and provided with a prism element to focus the scattered
light on to the photodetecting element.
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Smoke is generally classified as black or grey.
Grey smoke particles are generally much easier to detect as
they tend to scatter the light from the photoemitting diode
very well. Hence, most designs of photoelectric smoke
detectors are reasonably effective at detecting grey smoke.
Black smoke particles, on the other hand, do not generally
scatter the light as well and many designs of photoelectric
smoke detectors have difficulty properly detecting the
presence and level of black smoke. This is particularly
the case with those detectors utilizing a 60 scattering
angle as, at this angle, the grey smoke to black smoke
sensitivity is only 4:1. In these detectors which are
usually set to detect grey smoke at about 3% per foot
obscuration, the level of black smoke required to indicate
an alarm state would be 12% per foot obscuration or higher.
Thus, there still exists a need to provide very sensitive
smoke detection of both black and grey smoke particles with
minimal radio frequency interference.
20 `
SUMMARY OF THE INVENTION
The present invention provides for a
photoelectric smoke detector comprising a case having
mounted therein a circuit board and a smoke detection
chamber. The smoke detection chamber has side walls with
openings provided therein for ingress and egress of smoke
particles, a top and a bottom. A photoemitting diode is
mounted in a mounting arrangement in the side wall of the
smoke detection chamber so that a light beam from the
photoemitting diode is transmitted across the smoke
detection chamber without directly impinging upon either
the top or the bottom surfaces. The bottom of the smoke
detection chamber has an opening therein with a shielding
arrangement thereabout open to the chamber, the opening and
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shielding arrangement being located between the centerline
of the smoke detection chamber and the mounting arrangement
in the side wall of the chamber. The circuit board is
mounted to overlie the bottom surface and includes a
photodiode detector mounted directly thereon and positioned
generally in the opening so that it views into the smoke
detection chamber through the opening and through the
shielding arrangement, the shielding arrangement shielding
the photodiode detector from incident light which may be
present in the smoke detector chamber while providing for a
diverging field of view of the photodiode detector. The
diverging field of view of the photodiode detector
intersects the light beam of the photoemitting diode to
define a detection volume contained within the smoke
detection chamber located close to the photoemitting diode
and the photodiode detector and spaced from the surfaces of
the smoke detection chamber to minimize the effect on the
photodiode detector of light from the photoemitting diode
which is reflected exterior to the detection volume.
In an aspect of the invention there is provided
a smoke detection chamber for use in a photoelectric smoke
detector. The smoke detection chamber comprises a side
wall band encircling a detection volume to the interior
thereof, a top and a bottom spaced from the side wall band
to provide for open region for ingress and egress of smoke
particles to and from the detection volume. The smoke
detection chamber further includes a mounting arrangement
for a photoemitting diode located in the side wall band of
30 the ~moke detection chamber so that a light beam from such
photoemitting diode may be transmitted across the smoke
detection chamber without directly impinging upon either
the top or the bottom. The bottom of the smoke detection
chamber has an opening therein with a shielding arrangement
thereabout open to the chamber for mounting of a photodiode
detector so that it may view the detection volume through
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the opening and through the shielding arrangement. The
opening and shielding arrangement are located between the
centerline of the smoke detection chamber and the mounting
arrangement in the side wall of the chamber. The shielding
arrangement is capable of shielding a photodiode detector
from incident light which may be present in the smoke
detector chamber while providing for a diverging field of
view of such photodiode detector. The intersection of the
centerlines of the mounting arrangement and the shielding
arrangement define the detection volume contained within
the smoke detection chamber located close to the mounting
arrangement and the shielding arrangement and spaced from
the surfaces of the smoke detection chamber to minimize the
impingement of light reflected exterior to the detection
volume on a photodiode detector contained within the
shielding arrangement.
In yet another aspect of the invention there is
provided a smoke detection chamber for use in a
photoelectric smoke detector. The smoke detection chamber
comprises side walls with openings provided therein for
ingress and egress of smoke particles, a top and a bottom.
A mounting arrangement for a photoemitting diode is located
in the side wall of the smoke detection chamber so that a
light beam from a photoemitting diode may be transmitted
across the smoke detection chamber without directly
impinging upon either the top or the bottom surfaces. The
bottom of the smoke detection chamber has an opening
therein with a shielding arrangement thereabout open to the
chamber for mounting of a photodiode detector so that it
may view into the smoke detection chamber through the
opening and through the shielding arrangement. The opening
and shielding arrangement are located between the
centerline of the smoke detection chamber and the mounting
arrangement in the side wall of the chamber. The shielding
arrangement is capable of shielding a photodiode detector
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from incident light which may be present in the smoke
detector chamber while providing for a diverging field of
view of such photodiode detector. The intersection of the
centerlines of the mounting arrangement and the shielding
arrangement define a detection volume contained within the
smoke detection chamber located close to the mounting
arrangement and the shielding arrangement and spaced from
the surfaces of the smoke detection chamber to minimize the
impingement of light reflected exterior to said detection
volume on a photodiode detector contained within the
shielding arrangement.
BRIEF DESCRIPTION OF THE DRAWINGS
The above as well as other advantages and
features of the present invention wlll be described in
greater detail according to a preferred embodiment of the
present invention in which:
Figure 1 is a side view in cross section of a
preferred embodiment of the smoke detector according to the
present inventioni
Figure 2 is a illustration of the intersection
of the fields of view of the photodiode detector and
photoemitting diode;
Figure 3 is a exploded perspective view of the
smoke chamber and printed circuit board of the smoke alarm
of Figure 1; and
Figure 4 is a side view in cross section of a
second embodiment of a smoke detection chamber according to
the present invention.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A first preferred embodiment of the smoke
detector of the present invention is illustrated in Figure
1 generally indicated by the numeral 10. The smoke
detector 10 is a ceiling or wall mountable unit having a
case 12 of extruded polymeric material. The case has a
base 14 to which is releasably attached a cover 16 having
openings 18 to permit entry of smoke into the interior of
the case 12. Openings 18 are provided with a fine mesh
screen 20 to prevent entry of insects and other large
particulate matter into the interior of the case 12.
Mounted within the case 12 in the base 14 is a
printed circuit board 22. The printed circuit board has
electronic components 24 which typically make up the smoke
alarm circuitry mounted to one or both sides of the printed
circuit board. Preferably electronic components 24 are
surface mounted components. The details of the alarm
circuitry are not shown as the design of such circuitry is
within the knowledge of ordinary workers in the art of
smoke alarm design. Mounted on the printed circuit board
22 is a photodiode detector 26 and a smoke detection
chamber 30. The photodiode detector 26 is mounted such
that the it views into the smoke detection chamber 30 as
will be described further below.
The first preferred embodiment of the smoke
detection chamber 30 illustrated in figures 1 and 3, is in
the form of a cylinder preferably having an internal
diameter between about 2 to 3 inches, however other shapes
are possible. As shown in figures 1 and 3, the smoke
detection chamber has side walls 32, a top 34 and a bottom
36. As will be appreciated, when the smoke detection
chamber 30 is mounted in the case 12 and the case 12 in
turn mounted on the ceiling, the bottom 36 of the chamber
30 will in fact form the upper surface of the chamber,
while the top 34 of the chamber 30 will form the lower
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surface. However for ease of understanding, these parts
are named in relation to their attachment to the circuit
board 22.
The smoke detection chamber 30 may be molded in
more than one piece depending upon the complexity of the
structures contained therein. For example, the side 32 and
bottom 36 may be molded in one piece and the top 34 may be
molded as a separate piece which may be releasably attached
` to the other section of the smoke chamber 30.
To reduce the possibility of reflected light
from decreasing the sensitivity of the smoke detector 10,
the smoke detection chamber 30 is constructed of a dark
colored plastic, usually a black plastic preferably having
a matte or flat finish. A particularly preferred material
for the smoke detection chamber 30 is black ABS plastic.
The sidewall 32 of smoke detection chamber 30 is
provided with openings 40 for ingress and egress of smoke
particles into the smoke detection chamber 30. As
illustrated in figure 3, openings 40 are preferably
provided at the periphery of the sidewall 32 adjacent both
the top 34 and bottom 36. The openings 40 are clear in
that they do not have any obstructions for the free flow of
smoke particles such as labyrinths or the like and are
provided with a fine mess screen 41 to reduce the entry of
insects and large particulate matter into the smoke chamber
30 while allowing passage of smoke particles into the
chamber.
Smoke detection chamber 30 in the sidewall 32 is
also provided with a mounting arrangement 42 for a
30 photoemitting diode 44. Mounting arrangement 42 has
extending walls 46 to provide-a tunnel within which is
mounted the photoemitting diode 44. Extending walls 46
preferably extend beyond the end of the photoemitting diode
44 to bound or limit the outer rays of the beam of light 48
emitted by the photoemitting diode 44 to provide for a
relatively narrow beam of light broadcasting across the
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g
smoke chamber 30. The mounting arrangement preferably
restricts the light beam of the photoemitting diode 44 so
that the beam only impinges upon the opposite sidewall 33
and not on either the top 34 or bottom 36. These extending
walls 46 also provide for shielding of the light beam 48 of
the photoemitting diode 44 to reduce the possibility of
stray light from the photoemitting diode 44 shining
directly onto the photodiode detector 26.
To further reduce the possibility of reflection
of light within the chamber 30, the smoke detection chamber
30 is preferably provided with reticulated structures 38 in
the sidewall 33 directly opposite the photoemitting diode
44 as well as on the top 34. These reticulated areas 38 of
the smoke detection chamber 30 reduce the possibility of
reflection of light from the wall 33 and the top 34 and in
addition provide an area where any dust which may enter the
smoke detection chamber 30 may collect without causing
scattering of light to reflect and impinge upon the
photodiode detector 26. These dust hiding areas are located
such that they are not both in the path of the light beam
from the photoemitting diode and also within the view of a
photodiode detector as described herein below. While these
areas may in one or the other of these locations, they are
not located so as to satisfy both conditions.
The smoke detection chamber 30 is preferably
mounted directly on the printed circuit board 22 by means
of mounting clips S0 inserted through openings provided in
the printed circuit board 22.
The bottom 36 of the smoke detection chamber is
30 provided with an opening 52 having a ~hielding arrangement
54 to overlay the photodiode detector 26 mounted on the
printed circuit board 22. The shielding arrangement 54 is
of a shape to reduce the possibility of incident light
falling onto the photodiode detector 26. The shielding
arrangement 54 may be of any suitable shape, for example a
right circular cylinder having openings in the top and
.
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right circular cylinder having openings in the top and
bottom or a rectangular or square structure with an open
top and bottom. In the embodiment illustrated in the
figures, the shielding arrangement 54 is a square shaped
structure having an open top and bottom. The wall 56 of
the shielding arrangement closest to the position of the
photoemitting diode 44 is of a height to prevent any stray
incident light from the photoemitting diode 44 from falling
directly on the photodiode detector 26. This wall 56 may
be provided with a reticulated groove 59 for holding any
dust particles which may come in contact with the shielding
arrangement 54 and hiding such dust particles from the
field of view of the photodiode detector 26. The opposite
wall 58 has a height or shape to prevent any incident light
which may enter the chamber 30 through the openings 40 for
ingress and egress of smoke particles from falling on the
photodiode detector 26. Wall 58 may also be provided with
an inclined portion 60 to provide for increased shielding
of the photodiode detector 26 from any incident light which
may enter the smoke chamber 30.
The photodiode detector 26 contained within the
shielding arrangement 54 has a field of view 62 which
intersects the light beam 48 of the photoemitting diode 44
generally perpendicularly in a vertical plane to provide
for the detection volume to be located within the smoke
detection chamber 30 close to both the photoemitting diode
44 and photodiode detector 26 yet spaced from the surfaces
of the smoke detection chamber 30 to minimize the effect on
the photodiode detector 26 of light from the photoemitting
diode 44 which is reflected exterior to the detection
volume. The arrangement of the photoemitting diode 44 and
photodiode detector 26 with a scattering angle at a
generally right angle in the vertical plane permits the
detection volume 64 to be located close~to the
photoemitting diode 44 where the intensity of the light
beam from the photoemitting diode 44 is higher and close to
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the photodiode detector 26 where its sensitivity is higher
thereby increasing the overall sensitivity of the smoke
alarm 10. In addition the use of the scattering angle at a
generally right angle results in a black smoke to grey
smoke sensitivity ratio of about 3.5:1, increasing the
sensitivity to the presence of smoke particles of many
colors, particularly black smoke particles.
To achieve the above, the shielding arrangement
54 is preferably located such that its centerline is
between the centerline of the smoke detection chamber 30
and the mounting arrangement 42. Preferably the shielding
arrangement is located such that its centerline is located
approximately 20% to 40% of the dimension of the smoke
chamber 30 away from the mounting arrangement, more
preferably approximately 25% to 35%, most preferably
approximately 30%. This location is possible because of
the use of the scattering angle at a generally right angle
and places the photodiode detector 26 close to the
photoemitting diode 44 to view the tightly bunched rays and
far away from the opposite end wall 33 to reduce the
possibility of reflected light falling on the photodiode
detector 26. The combination of the location of the
shielding arrangement 54 with the mounting of the
photodiode detector 26 on the circuit board 22 and the
location of photoemitting diode 44 in the sidewall 32
provides for a very broad field of view 62 of the
photodiode detector 26 and hence increased sensitivity
without having to resort to the provision of additional
optical elements such as lenses for focusing of the light.
This expanded field of view ~2 of the photodiode detector
26 provides for a large area of intersection 64 with the
light beam 48 of the photoemitting diode 44 to define the
detection volume.
In operation, when smoke particles enter the
smoke detector 10 through the openings 18 provided in the
cover 16 and then through the openings 38 and 40 provided
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in the wall 32 of the smoke detection chamber 30, the smoke
particles fall within the light beam 48 of the
photoemitting diode 44. Smoke particles which are present
in the detection volume defined by the area of intersection
64 of the light beam 48 from the photoemitting diode 44 and
the field of view 62 of the photodiode detector 26, cause
the light from the photoemitting diode 44 to be scattered
such that it is directed through the shielding arrangement
54 and on to the photodiode detector 26. When the amount
of light detected by the photodiode detector 26 passes a
predetermined threshold the smoke alarm circuitry is
activated and the detector indicates the alarm condition in
the usual manner.
A second embodiment of a smoke detection chamber
of the present invention is illustrated in figure 4
generally indicated by the numeral 70. The smoke detection
chamber 70 is generally rectangular in shape having
dimensions of about 1 inch by 1 inch by 2 inches, with the
photoemitting diode 72 mounted in the sidewall 74 such that
the light beam 76 from the photoemitting diode 72 is
broadcast along the long dimension of the rectangular
shape. Similar to the first embodiment, the bottom 78 of
the smoke detection chamber 70 is provided with an opening
80 for mounting of a photodiode detector 82 and a shielding
arrangement 84 surrounding the opening 80. Similarly, the
photoemitting diode 72 is mounted in the sidewall 74 within
a mounting arrangement 86 to bound or limit the outer rays
of the light beam 76 to provide a relatively narrow beam of
light broadcasting across the chamber 70. The sidewall 74
30 of the chamber 70 opposite the photoemitting diode 72 is
made up of a series of vanes or louvres 88 which are open
to the outside of the chamber 70. These vanes or louvres
88 both permit easy passage of smoke particles into the
interior of the chamber 70 as well as acting to cause the
light beam 76 from the photoemitting diode 72 to be
reflected to the outside of the chamber 70 as it strikes
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the sidewall surface rather than back into the chamber 70
as may be the case if the sidewall 74 were solid in this
region. Similar to the first embodiment, the chamber 70 is
provided with screening 90 around all openings as well as
having a top 92 which is provided with a reticulated
surface 94 for controlling reflection of light and for
holding of any dust particles which may enter the chamber.
Other details with respect to the relationship between the
location of the various elements, the detection volume 96
and operation are similar to those of the first embodiment.
A prototype smoke detector of the present
invention as illustrated in the Figures, has been
constructed having a smoke detection chamber 30 as shown in
figure 3, in which the photoemitting diode 44 and
photodiode detector 26 are mounted in the large circular
cylinder having an interior diameter in the long direction,
i.e. along the beam of light of the photoemitting diode, of
2.45 inches. The photoemitting diode 44 broadcasts across
the chamber 30 and the photodiode detector 26 views axially
with a 90 scattering angle from its mounting position
directly on the printed circuit board 22 with the
centerline of the photodiode detector and shielding
arrangement being located 0.706 inches away from the
sidewall 32 in which the mounting arrangement 42 is
located. The prototype utilized a Siemens BPW34FA silicon
photodiode, a polysulfone-bodied detector with visible
light rejection characteristics. The photoemitting diode
utilized was a Siemens SFH484 light emitting diode which
operates at a wavelength of 880 nm in the infrared range.
The alarm and control circuitry employed a Motorola
MC145010 IC chip along with required circuitry for
operation. The prototype alarm exhibited a high
sensitivity and high RFI immunity to false alarms through
the UV spectrum to 1 GHz. The large smoke detection
chamber with reticulated walls and top achieved low
background reflection with good dust hiding capability.
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The prototype design of the preferred embodiment of the
present invention had a normalized figure of merit (NFM)
which is a measure of the smoke detection sensitivity to
background reflection ratio greater than unity. This
translates to smoke alarm signals being three times greater
than the background reflection for alarm point settings of
three percent per foot obscuration. This high NFM affords
exceptional immunity to false alarms from dust
accumulation.
Excellent smoke access to the smoke detection
chamber is afforded by the smoke detection chamber having
screened openings around its periphery at both the top and
the bottom of the cylindrical shape together with baffles
to disrupt laminar smoke flow associated with low air
velocity and dead air typical of smouldering fire
conditions. The smoke detection case is similarly vented
around the circular periphery both at the top and the
bottom and also utilizes disruptive vanes to turbulate
laminar smoke flow. The design of the present invention
has smoke detection sensitivities of both low and high air
velocity within ten percent of each other thus indicating
the detector's excellent smoke entry design and the
positioning of the detection volume defined by the
intersection of the transmitted light from the
photoemitting diode and the view of the photodiode
detector.
The provision of the surface mounted photodiode
detector allows the detector to be mounted directly to the
printed circuit board along with the other surface mounted
devices in a single step, thus reducing manufacturing
costs. The surface mounted photodiode detector is able to
view downwards through the opening 52 of the smoke
detection chamber 30 directly at the smoke reaction volume.
This smoke reaction volume is located in the lower portion
of the smoke sensing chamber immediately accessible to the
smoke flow.
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The smoke detector of the present invention
improves alarm response consistency and reduces
manufacturing steps and costs. The design of the smoke
detector as described herein provides for a very uniform
S detection sensitivity for various smoke types and colors
under varying conditions with high RFI immunity. To
further increase the RFI immunity of the smoke detector,
the photodiode detector may be mounted on the side of the
circuit board opposite the smoke chamber along with the
other SMT components. In this situation an aperture may be
provided in the circuit board between the photodiode
detector and the opening in the bottom of the smoke
detection chamber such that the photodiode detector views
into the smoke chamber through the aperture in the circuit
board and the opening in the bottom of the smoke detection
chamber.
Although various preferred embodiments of the
present invention have been described herein in detail, it
will be appreciated 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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