Note: Descriptions are shown in the official language in which they were submitted.
CA 02793195 2012-09-14
MOTION DETECTOR WITH HYBRID LENS
Technical Field
This patent application relates to passive infrared motion detection devices
using
Fresnel lens arrays.
Background
Motion detectors are used in a variety of applications, but most commonly in
security systems and in lighting control system. Passive infrared motion
detectors
are one type of motion detector that uses optics, namely lenses and/or
reflectors,
to collect infrared light emitted by people and to direct that light onto a
pyroelectric
sensor that converts heat to an electric signal. That signal is processed to
detect
motion.
Many passive infrared motion detectors use Fresnel lenses printed or molded in
thin plastic sheets to focus light from an area onto the sensor. The amount of
light
reaching the sensor depends on the optical properties of the Fresnel lens, and
is
in direct relation to the size of the lens area. The lens material is not 100%
transparent and its thickness affects the signal level transmission. When the
lens
area is perpendicular to the direction of the area being covered, the losses
due to
lens thickness are the least because the light passes through minimal lens
material. Additional improvement on collecting the received optical signal can
be
further obtained when the sensor is perpendicular to the direction of the
light
coming from a Fresnel lens.
Since the need to efficiently collect light is more important for areas
farther from
the detector is greatest, the common Fresnel lenses and geometry of the sensor
and lenses are designed such that efficiency of collection is optimized for
the far
areas or zones, at the expense of efficiency for the closer areas. Most
typically, a
sheet having a large number of Fresnel lenses is curved to be essentially
cylindrical. The images collected by the lenses are focused onto the focal
plane
which is virtually located on a vertical axis located at the cylinder center,
Thus, to
obtain maximum efficiency from the far looking zones, the sensor is positioned
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CA 02793195 2012-09-14
normally on the vertical axis focal plane, and positioned at the same height
as the
row of Fresnel lenses that collect light from farther areas.
While the lenses collecting light from closer areas are located lower than the
sensor, the closer the area, the lower the row height is, thus having lower IR
light
transmission efficiency due to increased attenuation caused by longer light
path
through the lens material. This lower efficiency is compensated by the fact
that a
stronger signal is obtained from a closer object, and then also if needed, by
increased lens collecting area.
It is known to arrange Fresnel lenses on a support structure that is not
planar or
cylindrical. In some cases, the support structure is in an array, as in US
Patent
5,187,360, or a plurality of curved sheets, as in US Patent 5,221,919. In some
cases, the lens assembly is spherical, as in US Patent 7,635,846. When the
lenses are arranged on a spherical sheet and the detector is mounted at a
position
on a wall, the lenses of the detector are perpendicular to the direction of
the area
being covered.
The advantage of a spherical lens arrangement is that it allows more than only
the
far zones' collected energy to penetrate the lens material perpendicularly,
thus
obtaining an overall increased efficiency. However, it can be used only for
small
size lenslets, as each lenslet area is curved and only a small portion of each
lenslet maintains a "flat and perpendicular" characteristic. Therefore the far
looking
lens area size is limited and practically, reduces the efficiency of large
area
lenslets, such as the row of the far looking beams, where larger area is
needed,
but there is insufficient perpendicular added area for the needed received
light
energy. For the same reason, practically the spherical "above the rim" is
practically
of no use, and such designs typically use a "half spherical lens shape design.
Summary
It has been discovered that a combination of a cylindrical lens assembly
geometry
and a spherical lens geometry can overcome problems found in the prior art
Fresnel lens assembly designs.
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One advantage of combining cylindrical and spherical lens assemblies is that
the
lens efficiency can be increased, and with this higher efficiency, a smaller
lens
assembly can be used to provide a given amount of light collection on a
sensor.
The size of a lens assembly often large part of a detector size, and it is
advantageous to be able to reduce the size of a detector without compromising
detection effectiveness.
Another advantage of combining cylindrical and spherical lens assemblies is
that
both far field sensitivity and near field sensitivity can be maintained
without
compromising one for the other.
Another advantage of combining cylindrical and spherical lens assemblies is
that
lens losses are minimized substantially for all of the Fresnel lenses of the
lens
assembly.
Another advantage of combining cylindrical and spherical lens assemblies is
that
the lens assembly structure can benefit from stronger resistance to damage
from
handling or pressure on the lens assembly than a conventional cylindrical lens
assembly.
Another advantage of combining cylindrical and spherical lens assemblies is
that
creep zone coverage can be provided without compromising sensitivity or
effectiveness for the far field.
In some embodiments, there is provided a lens assembly for a passive infrared
motion detector having one or more rows of far field Fresnel lenses arranged
on a
substantially cylindrical sheet, the far field lenses being operative to
collect light
onto a sensor location, and a plurality of rows of mid/near field Fresnel
lenses
arranged on a substantially spherical sheet, the mid/near field lenses being
operative to collect light onto the sensor location.
In some embodiments, the cylindrical sheet and the spherical sheet are two
portions of a single molded sheet. In other embodiments, the cylindrical sheet
and
the spherical sheet are separate sheets, and the assembly has a middle support
member for holding the cylindrical sheet and the spherical sheet with respect
to
one another.
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In some embodiments, the cylindrical sheet comprises at least two rows of far
field
Fresnel lenses. In some embodiments, the spherical sheet comprises three rows
of Fresnel lenses.
In some embodiments, the far field lenses and the mid/near field lenses are
configured to collect light from a beam direction that is substantially
perpendicular
to the lenses.
In some embodiments, the Fresnel lenses each have an aperture sized to collect
substantially a same amount of light from a same light emitting object from
respective beam directions for an intended mounting position.
In some embodiments, there is provided an infrared motion detector comprising
an
infrared sensor and a lens assembly as defined above that is mounted in a
predetermined position with respect to the sensor.
In some embodiments, the detector includes a reflector mounted above the
sensor
for reflecting light from at least one of the mid/near field Fresnel lenses
onto the
sensor to provide one or more creep beams. The creep beams may be distinct
from and closer than beams of the near field lenses that reach the sensor
without
substantive reflection by said reflector.
In some embodiments, the sensor is located at a vertical position
corresponding to
the rows of far field Fresnel lenses. The rows of far field lenses may be two
in
number, and the sensor may be located between the rows.
In some embodiments, the detector has a housing, and the lens arrangement
comprises a single molded body having tabs connected to the housing for
supporting the body on the detector.
In some embodiments, the rows of far field lenses are one or two in number,
the
cylindrical sheet of the lens assembly is resistant to deformation resulting
from
external pressure due to handling of the detector.
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Brief Description of the Drawings
The invention will be better understood by way of the following detailed
description
of embodiments of the invention with reference to the appended drawings, in
which:
Figure 1A illustrates schematically a horizontal cross-section of a pair of
prior art
Fresnel lenses on a common cylindrically bent sheet focussing light onto a
sensor;
Figure 1 B illustrates schematically a vertical cross-section of a prior art
Fresnel
lens sheet, also bent to be cylindrical, having four rows of lenses focussing
light
onto a sensor;
Figure 2A is a front view of a passive infrared motion detector having a lens
assembly showing a single body having an array of Fresnel lenses, the body
having a lower spherical configuration and an upper cylindrical configuration.
Figure 2B is an illustration of a cylindrical geometry with an axial plane.
Figure 2C is a side view of the motion detector of Figure 2A showing far beams
reaching the cylindrical portion of the lens assembly.
Figure 2D is an illustration of a spherical geometry with an axial plane.
Figure 2E is a side view of the motion detector of Figure 2A showing mid/near
field
beams reaching the spherical portion of the lens assembly.
Figure 2F is a side cross-sectional view of a passive infrared motion detector
having a lens assembly showing a single body having an array of Fresnel
lenses,
the body having a lower spherical configuration and an upper cylindrical
configuration.
Figure 2G is a side cross-sectional view of a passive infrared motion detector
having a lens assembly with two lens bodies, one spherical and one
cylindrical.
Figure 3A is a side cross-sectional view of the embodiment of Figure 1 in
which a
reflector has been added to provide an image of the sensor at a position that
collects infrared light through lower Fresnel lenses from a "creep" zone below
the
detector.
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Figure 3B is a partial front elevation view of the embodiment of Figure 3a
showing
the shape of the reflector.
Figure 4A is a front view of a lens according to one embodiment having a
plurality
of rows of lenslets, two on a cylindrical portion and four on a spherical
portion of a
single body;
Figure 4B is a horizontal cross-section of the lens body of Figure 4A;
Figure 4C illustrates a "beam" arrangement for a wall-mounted passive infrared
detector showing close or near-field "beams" and far-field "beams".
Detailed Description
As illustrated in Figure 1A, a conventional lens body 14 is a cylindrical body
14
having Fresnel lenses or lenslets 16. The beam directions are essentially
perpendicular to the lens body 14 and the sensor 18 is arranged essentially at
the
center of the cylinder. Although the light passes through roughly the same
thickness of lens material for side beams as for center beams, the light
reaching
the sensor 18 from the side beams is received at an angle and may be less
efficiently detected.
As shown in Figure 1 B, a conventional lens body 14 is often configured to
direct
light from far beams almost normally onto the sensor, while for closer beams,
the
light is focussed onto the sensor 18 using lower lenses 16 that are below the
sensor 18, and thus the light reaches the sensor at an oblique angle. The
lower
lenses 16 are however less efficient than the upper lenses 16 because of the
angle they make with respect to their beams and the angle they make with
respect
to the sensor 18, along with the greater lens thickness seen by more oblique
rays
passing through the lens material that can cause greater absorption of light.
In some conventional detectors, the lens body 14 may be tilted with respect to
a
vertical direction to face downward.
This reduction in efficiency is not seen conventionally as a problem since the
intensity of light reaching the lens body 14 from near zones is much greater
than
for far zones. However, the size of the lens body 14 has to be increased to
provide
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the near beams and the ability to efficiently collect light firom the near
field is
compromised.
In the embodiment of Figure 2A, there is shown a wall-mounted infrared motion
detector having a lens assembly showing an upper cylindrical portion 1 and a
lower spherical portion 2. As illustrated in Figure 2B, the cylindrical
geometry has
a vertical axis plane, and the detector shown in Figure 2C shows upper lens
assembly portion 1 with a cylindrical geometry for focussing far field beams.
As
illustrated in Figure 2D, the spherical geometry has a vertical axis plane,
and the
detector shown in Figure 2E has a lower lens assembly portion 2 with a
spherical
geometry for focussing mid/near field beams.
In the embodiment of Figure 2F, the detector unit 10 has a housing 12, lens
assembly 14 that provides. a plurality of Fresnel lenses '16 molded or printed
thereon, a sensor 18 and a printed circuit board 20 having signal processing
and
communications circuitry.
The lenses 16 have a larger aperture in the cylindrical portion for collecting
the
greatest amount of light from the far field since the infrared light being
collected
from the far field is less intense. The apertures of the lenses 16 can be
arranged
to provide approximately the same signal strength at the detector 18 in
response
to the same object moving through the field of view. Thus the apertures of
lenses
16 collecting light from closer range can be smaller. Alternatively, one may
prefer
having the lenses only partially compensate for the distance of the objects
being
detected, wherein signals from far objects are less than signals from near
objects.
As illustrated, the cross-section of the lenses 16 are larger for the upper
lenses
associated with the far field than for the lower lenses 16 associated with the
near
field.
The design of the lens assembly 14 is different from a conventional spherical
lens
assembly design in which a top row of lenses provides far field beams. In
Figures
2E and 2F, not only is the spherical portion 2 made to stop short of a full
quarter
sphere, but the sensor position is well above the rim of the spherical
portion, and
the lenses 16 are designed accordingly to direct their light a higher than in
the
case of a conventional spherical lens assembly.
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The general shape of the lens body or assembly 14 is to be roughly
perpendicular
to the direction of the "beam", namely the direction from which light is
focussed by
a Fresnel lens 16 onto the detector. In the embodiment of Figure 2A, the lens
body
14 has a cylindrical upper portion and a spherical lower portion, and the body
14 is
a single piece of plastic material. It will be appreciated that these shapes
are
convenient approximations, and that other shapes can be used to efficiently
collect
light involving transmission through the lenses 16 and absorption by the
detector
18, where the angles the latter make with respect to the light affect
efficiency.
Molded or printed Fresnel lenses are known in the art, as are the materials
used to
make them, and need not be described in detail herein to understand how to
make
the embodiments described. Generally, thinner lenses absorb less infrared
light,
so lenses are made as thin as possible, limited by the required structural
strength.
Forming the lens assembly 14 as a single part, as in Figure 2A, has two
advantages, namely there is a single part to be mounted, and the strength
imparted by the spherical geometry reinforces the cylindrical portion of the
lens
assembly.
It will also be appreciated that the spherical lens of Figure 2A brings the
near field
lenses closer to sensor 18, and reduces the vertical height of the lens
assembly, in
comparison to the prior art lens assembly of Figure 1 B.
In the embodiment of Figure 2B, the motion detector has a first lens body 14a
that
is cylindrical, and a second lens body 14b that is spherical. A middle support
member 15 is provided to support the bottom of body 14a, as well as the top of
body 14b.
Dividing the lens body into two geometrical sections can have the advantage
that
body 14a can be made as a flat strip that is held in housing '12 in a curved
manner
to take the roughly cylindrical shape. The more complex spherically shaped
body
14b can thus be made smaller. The body 14a can also be made taller or shorter
depending on the particular needs to collect more or less light from the far
field
without need to change the design of the roughly spherical mid or near field
body
14b.
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The embodiment of Figure 3A is similar to the embodiment of Figure 2A and has
a
reflector 19 added to redirect light collected from near field lenses onto the
detector 18. The reflector 19 is placed above the light path of the top beam
and
preferably is tilted down by about 20 degrees from horizontal to reflect light
towards the sensor 18. In the front elevation view of the detector 18 and
reflector
19, shown in Figure 313, it can be seen that the reflector can have side wings
to
redirect light that is shifted to a side of the detector onto the detector 18.
It will be
appreciated that while the near field lenses 16 have their focal plane set to
collect
and direct light onto the detector 18, the reflector 19 provides an "image"
(or
multiple images) of the detector 18 that allows the near field lenses to
direct light
onto the detector from different viewing angles of the same lenslet, as well
as
detecting objects from a very close range, called the "creep Zone", more
efficiently.
This is achieved by the fact the light is directed to an image of the detector
through
the reflector that is farther than the actual detector, so the wings of the
reflector 19
can help refocus light onto the detector 18.
The reflector 19 can also be arranged to improve detection by reflecting onto
the
detector 18 light that was incident on the detector at a low angle and thus
was
reflected instead of absorbed. This practically increases the down looking
beams
collecting area and therefore increases their width and signal) strength.
It will be appreciated that reflector 19 can be shaped to improve light
collection
from the zone to be by the primary optics onto the sensor 18.
While the embodiment of Figures 3A and 3B show that Fresnel lenses whose
beams are reflected by the reflector 19 are on a spherical lens assembly, the
reflector can also be used to provide an enhancement of coverage with a non-
spherical lens assembly, such as a cylindrical lens assembly, individual flat
Fresnel lenses, or other suitable infrared optics.
Figure 4A shows a front elevation view of a single body lens array 14 having
rows
of Fresnel lenses 16a through 16d. The first two rows, 16a and 16b, are
arranged
on the cylindrical portion to focus light collected from far field beams, 17a
and 17b
respectively (see Figure 4C), onto sensor 18. The cylindrical shape is shown
in
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Figure 4B with the sensor 18 approximately at the axis of the cylinder. The
lens
body 14 has tabs with hooks to snap onto the detector housing 12.
When the lens body 14 is mounted on the housing 10, the top cylindrical rim
can
be made sufficiently reinforced to withstand local external pressure. In a
conventional lens body as shown in Figure 1A, the middle of the cylindrical
lens is
vulnerable to external pressure, namely a knock in the middle that arises
during
handling of the detector during installation can dent the lens. In the
embodiments
described above, the cylindrical lens portion is not very high, and thus is
not
vulnerable to denting as the prior are cylindrical lens of Figure 1A and 1B
would
be. Furthermore, the spherical lower part of the lens of Figures 2A and 3A
provides high resistance to external pressure.
Figure 4C is a schematic illustration of the "beams" 17a through 17d provided
by
Fresnel lenses 16a through 16d. The term "beam" is used in the art to refer to
the
volume in space from which the Fresnel lens collects light and directs it onto
the
detector, even if the term normally connotes transmission. The number and beam
direction of Fresnel lenses in an infrared motion detector to detect motion in
an
area is well known in the art, and need not be described in detail herein.
As shown, the lenses 16a and 16b collect light from the far field, about 13.7m
to
15m. The detector 10 is often mounted about 2.6m above floor level. It will be
appreciated by those skilled in the art that the beam distances and
arrangement is
a matter of design choice for the detector, and that a variety of beam
arrangements may be suitable. It will be noted that some rows of beams are
arranged to be close to each other to form a zig-zag of upper and lower beams.
In some embodiments, the arrangement of beams 17 can be used to help
discriminate between people and pets, as for example is described in commonly-
assigned US patent 6,215,399. When beams are arranged in an alternating height
level, it is possible to discriminate between pets and people by the different
signal
patterns produced by pets and people. This arrangement of lenses 16 can be
provided on the cylindrical portion of the lens assembly 14 and on the
spherical
portion for at least the mid field beams. The lenses 16a to 1 6f of Figure 4A
provide
the corresponding beams 17a to 17f shown in Figure 4C.
CA 02793195 2012-09-14
The intensity of infrared light reaching the detector unit 10 beyond 10m away
is
quite weak, so rows of lenses 16a and 16b are the largest to collect more
light.
The lenses 16a and 16b are also substantially perpendicular to their beam
directions, and this means that the rays pass through the least thickness of
lens
material. These lenses are arranged on the cylindrical portion of the lens
arrangement 14. The uppermost lens 16c of the spherical portion of the body 14
has an aperture almost as large as the lenses 16a and 16b, and covers an area
approximately within 7.6m to 9m from the unit 10. The next row of lenses 16d
in
the arrangement illustrated in Figures 4A and 4C have a smaller aperture and
cover an area within about 6m to 7.6m from the unit 10. The next rows of
lenses
16e cover the near field, namely within about 3m to 4m. The last rows of
lenses
16f cover the nearest field, between 1.5m and 2m. The near field lenses 16c to
16f
can be arranged into a variety of rows from 2 to 5 or more rows.
Since an area of 1m immediately below the unit 10 would allow an intruder to
escape detection by "creeping" against the wall to which the unit is mounted,
the
creep beam is provided to cover this area. The creep beam performance can be
further improved by using a reflector as in the embodiment of Figures 3A and
3B,
or separate lenses can be used. It will be understood that when a lens is used
without a reflector, a compromise is struck between the beam-lens angle and
the
post-lens beam-sensor angle, bearing in mind that the lens-related losses are
higher for the larger beam-lens angles and the beam-sensor angle also reduces
sensitivity.
As illustrated in Figure 5A, a test of the effect of the reflector 19 was
performed by
moving at a height of about 90cm a person's head in the area immediately below
the detector 10, and recording peak signal voltage at the center of squares as
illustrated every 50 cm. It will be appreciated that other lenses 16 of the
lens
assembly 14 provide beams outside of this small area near the detector 10.
As shown in Figure 5B, without the reflector 19, the closest central beam as
designed for the lens assembly was 150 cm wide and 50 cm deep, starting at 50
cm from the wall position of the detector 10, and includes 3 squares in a line
(hatched in Figure 5B). These parameters depend on the choice of lens assembly
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design. The area between the wall and 50cm from the wall provides little
chance
of detecting a person crawling.
However, the insertion of reflector 19 (in this case a metallic front surface
mirror)
as illustrated in Figures 3A and 3B, had the effect of changing the shape and
sensitivity of the zones, as shown in Figure 5C. Sensitivity with the
reflector
became greater in the area immediately below the detector than in any other
area,
and the beams have a V-shape formation for an effective creep zone coverage.
The peak signal strength measured without a reflector in the middle of the
50cm
by 50cm squares as shown in Figure 5B was:
17mV 20mV 17mV 19mV 18mV
15mV 122mV 105mV 100mV 17mV
22mV 21 mV 32mV 20mV 17mV
32mV 40mV 41 mV 47mV 50mV
The detection threshold was set to about 85mV, and the areas hatched are part
of
the detection zone. When the reflector 19 was used with the same lens assembly
and the same detector position, the peak signal strength was measured in the
same way again, as shown in Figure 5C as follows:
25mV 42mV 128mV 77mV 18mV
26mV 98mV 72mV 110mV 17mV
30mV 55mV 72mV 55mV 58rV
56mV 30mV 70mV 27mV 70m,V
The beam now effectively covers the creep zone with good sensitivity for a
person
crawling below the detector against its wall.
It will be appreciated that .the amount of light collected and directed onto
the
sensor from a given light emitting object can be tailored to be roughly the
same for
all beams by adjusting the size of the lenses 16 as a function of the range of
the
beam.
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While the Fresnel lenses are described as being in rows, it will be understood
that
any suitable arrangement of the lenses on the body 14 can be used, and that a
row need not necessarily have all lenses 16 of a row at the same horizontal
position. On a spherical body, the lenses near the pole at the bottom are much
smaller and can be arranged in any suitable pattern, without necessarily have
a
row arrangement.
While the lens arrangement 14 is described in the above embodiments as one or
more curved bodies, it will be appreciated that a facetted construction having
a
polyhedron or geodesic framework with planar or curved sheets of one or more
Fresnel lenses 16 to provide the required geometry can be substituted for a
continuous body construction.
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