Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE
COMPACT MOISTURE SENSOR
WITH COLLIMATOR LENSES AND PRISMATIC COUPLER
BACKGROUND OF THE INVENTION
Field of the Invention. The present invention
relates generally to an optical moisture sensor for
mounting upon the interior surface of a windshield, and
more particularly, to a compact optical moisture sensor
having a optical emitters, detectors, and optical
components mounted on a planar circuit board which is
positioned parallel to the interior surface. Collimator
lenses and a prismatic coupler are used to reflect and
refract light beams as the light beams travel from the
emitters to the outer surface of the windshield and back
to the detectors.
Summary of Related Art. Motor vehicles have long
been equipped with motor-driven windshield wipers for
clearing moisture from the external surface of the
windshield, at least within the driver's field of vision,
and generally over a larger area so as to enhance vision
through the windshield. In most vehicles today, the
windshield wiper system includes mufti-position or
variable speed switches which allow the driver to select a
wide, if not an infinitely variable, range of speeds to
suit conditions. Wiper controls are manually operated and
typically include a delay feature whereby the wipers
operate intermittently at selected time delay intervals.
Wiper control systems have recently been developed
which include a moisture sensor mounted on the windshield
to automatically activate the motor when moisture is
deposited upon the surface of the windshield or other
' vehicle window upon which a wiper may be employed, such as
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the rear window. By sensing rain or other moisture on the
glass surface, the wipers can be controlled accordingly.
Such wiper control systems free the driver from the
.
inconvenience of frequently adjusting the wiper speed as
the driving conditions change. Wiper control systems with
optical moisture sensors have been incorporated into the
production of several models of passenger cars. In order
to increase the commercial use and consumer acceptance of
the wiper control systems, there is a need for a more
i0 compact and less expensive optical moisture sensor.
Wiper control systems have employed a number of
different technologies to sense the moisture conditions
encountered by a vehicle, including conductive,
capacitive, piezoelectric, and optical sensors. Optical
sensors operate upon the principle that a light beam being
diffused or deflected from its normal path by the presence
of moisture on the exterior surface of the windshield.
The systems which employ optical sensors have the singular
advantage that the means of sensing (i.e. disturbances in
an optical path) is directly related to the phenomena
observed by the driver (i.e., disturbances in the optical
path that affords the driver vision). Thus, optical
systems generally have an advantage over other sensor
technologies in that they are closely related to the
problem corrected by the wipers.
McCumber et al. (U. S. Patent No. 4,620,141) disclose
an automatic control circuit for triggering a sweep of the
wiper blades in response to the presence of water droplets
on the exterior surface of a windshield. The rain sensor
devices for controlling the windshield wipers of a vehicle
as disclosed by McCumber et al. and Teder (U. S. Patent
Nos. 5,059,877 and 5,239,244) include a box-like housing
i
mounted upon the interior surface of the windshield. The
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presence of moisture on the surface of the windshield
affects the reflection of light at the air-glass
interface, and this change in reflected light is
electronically processed and utilized as the signal for
activating the windshield wipers.
The sensor housing in an optical moisture sensor
should securely engage the windshield and be optically
coupled to the windshield so as to effectively eliminate
the interface between the light emitters-detectors and
glass surface from an optical standpoint. U.S. Patent No.
5,262,640 to Purvis et al. describes an intermediate
adhesive interlayer for affixing the sensor housing to the
windshield. The sensor housing is affixed directly to the
surface of the windshield or other vehicle window by means
of an intermediate interlayer disposed between the sensor
housing and the interior surface of the windshield.
Tn optical moisture sensors, light from an emitter is
directed by a guide means into the windshield at an angle
of approximately forty-five degrees with respect to the
windshield. The light is then reflected by the outer
surface o.f the windshield at approximately a forty-five
degree angle and is directed by a guide means into a
detector. Water on the outside surface of the windshield
effects the overall transmittance of the optical path
between emitter and detector.
When the angle of entry of the light beam into the
windshield is greater than fifty degrees, a loss of signal
frequently occurs. When the angle of entry is less than
forty degrees, a loss of sensitivity occurs and the sensor
is not able to properly detect moisture on the windshield.
' Consequently, it is essential that the angle of entry of
the light beam from the emitter enter the windshield at
approximately forty-five degrees.
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The desired forty-five degree angle can be achieved
by mounting the optoelectronic devices (emitters and
detectors) at forty-five degree angles or by deflecting
the light as it travels between the devices and the glass
windshield. The sensors in which the emitters and
detectors are mounted at forty-five degree angles to the
windshield have required bulky, box-like enclosures.
Light may be deflected only by reflection, refraction or
diffraction. Reflecting mirrors are amenable to
deflections of sixty degrees or greater. A mirror
designed to implement a shallower deflection must be quite
large to accommodate a wide splay of rays. Diffractive
lenses are not very efficient and can be quite expensive.
A refractive service can efficiently deflect a beam
approximately twenty degrees or less. The preferred
forty-five degree angle for optical moisture sensors is
generally too small for a reflective system and too large
for a refractive system. Consequently, most of the
optical sensors have used optical devices deployed at a
suitable angle rather than devices for deflecting the
light at the desired angle.
The references cited above have optical devices
deployed at forty-five degrees, which requires a box like
enclosure. Additional examples of optical sensor mounting
configurations to achieve the forty-five degree angle
between the optical axis of the emitter and the glass
windshield are disclosed in Noack (U.S. Patent No.
4,355,271), Bendicks (U.S. Patent No. 5,323,637) and
Larson (U. S. Patent No. 4,859,867).
Stanton (U. S. Patent No. 5,414,257) discloses optical
sensor optoelectronic devices mounted on a circuit board
at an appropriate angle to change or deflect the optical
axis. Stanton teaches devices cast from flexible epoxy
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resin and the bending of the leads to the desired angle.
The problem with electronic devices with bent leads is
that most automated component insertion equipment cannot
insert components with bent leads. In addition to
5 increased costs to assemble the circuit boards, the bent
lead devices are less reliable from a performance
standpoint.-
The mounting of optoelectronic devices on circuit
boards is also disclosed in Schierbeek (U.S. Patent No.
4,956,592) and in Wiegleb et al. (DE3806881). The
optoelectronic devices are mounted on small circuit boards
which are aligned perpendicular to the windshield.
Reflective surfaces, each bending the light ninety degrees
in a rotational fashion, deflect the optical axis to the
required angle within the windshield. Although the
mounting configurations in these references do not require
lead forming, the use of such small circuit boards creates
other problems. The small circuit boards used to mount
the optoelectronic devices cannot accommodate the signal
processing circuitry, which must be located on a separate
circuit board. The use of multiple circuit boards and the
orientation of the circuit boards in the housing of the
sensor increases the size and cost of the sensor. The
required mounting angles for the optoelectronics in a
sensor could also be obtained by the use of flexible
circuit boards, but such material is more expensive and
less reliable than standard circuit boards.
Optoelectronic devices are customarily mounted and
aligned on a printed circuit board, which also
accommodates signal processing. Conventional
optoelectronic devices, including the new surface-mount
technology devices (SMT's), are generally designed so that
their optical axis is normal to the circuit board on which
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they are mounted. The use of a single circuit board
mounted co-planar with the surface of the windshield could
result in a low cost and compact sensor enclosure.
However, such design presents significant problems in
achieving the desired forty-five degree configuration
because the optical axis is perpendicular to the circuit
board.
One configuration which both reduces the cost and
reduces the size of the optical sensor is to use a single
detector to simultaneously.detect two or more emitters, as
disclosed in Noack. Such a configuration provides the
desired area of detection with a fewer number of
detectors. However, the light paths are widely splayed,
which requires a larger detector or additional optical
IS elements for concentrating the light.
Another area of concern in the manufacture of optical
moisture sensors is the mounting of the sensor to the
windshield. Vehicle manufactures desire a sensor which is
already installed at the windshield manufacturer, or a
sensor that is very easy to install on the vehicle
production line. The windshield manufacturer ships
windshield nested together such that there is very little
spacing for mounting a sensor.
Schofield (U.S. Patent No. 4,930,742) discloses the
use of a bracket, such as a rear view mirror bracket, for
mounting the optical moisture sensor. This approach
necessitates additional support structure or the addition
of silicone pieces to optically couple the moisture sensor
to the windshield. A bracket mounting systems results in
additional parts and increased costs.
Bendix (U. S. Patent No. 5,278,425) and Stanton teach
that a lens may be permanently affixed to the windshield
such that a sensor housing may be detachably mounted on
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the lens. The lens may impart focal power to the beam, as
in shown Bendix. Alternatively, the lens may couple the
beams to the windshield through planar surfaces normal to
the beam direction, as disclosed in Stanton. However,
both Bendix and Stanton require a lens that is
approximately as thick as the windshield. When stacking
the windshields for transportation from the glass
manufacturer to the vehicle assembly line, the additional
space necessitated for the lens adds additional handling
costs to the cost of the windshield.
Watanabe (U.S. Patent No. 4,701,613) discloses an
integral coupler lens having a series of V-grooves forming
a segmented prism with planar surfaces normal to the
direction of the beams. Segmented lenses have a greater
potential for parasitically admitting ambient light, which
reduces optical efficiency and degrades the signal from
the emitter. The resulting beam travels at a forty-five
degree angle with respect to the windshield, and thus is
not amenable to coplanar approaches.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is
provided a compact rain sensor for mounting on the inner
surface of a windshield. The rain sensor includes
collimator lenses and a detachable prismatic coupler to
facilitate the mounting of emitters and detectors on a
circuit board which is positioned parallel to the inner
surface of the windshield.
The thin and lightweight coupler of the present
invention is adhesively secured to the windshield. A
sensor housing is detachably secured about the outer edges
of the coupler. Within the sensor housing, surface-
mounted infrared emitters and detectors, as well as signal
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processing circuitry, are all mounted on a single printed
circuit board secured in the housing. h7hen the sensor
housing is mounted on the coupler, the printed circuit
board is parallel to the inner surface of the windshield.
Surface mounted emitters and detectors, which are very
cost and space efficient, are mounted on the circuit board
with an initial optical axis perpendicular to the circuit
board.
Compact collimator lenses performs three optical
I0 functions in directing the infrared light beams from the
emitters to the prismatic coupler. For each lens, the
first surface gathers the light beam and a second surface
reflects the light beam such that the optical axis, after
starting out perpendicular to the circuit board, is
reflected approximately sixty degrees from the initial
optical axis. A third surface of the lens forks a convex
lens to collimate the rays of the light beam.
After the light beam exits the collimator lens, the
light beam enters the prismatic coupler such that the
light beam is refracted approximately fifteen degrees in
the direction of the initial optical axis and is optically
coupled to the windshield. The resulting collimated light
beam is traveling at approximately a forty-five degree
angle with respect to the circuit board and with respect
to the initial optical axis perpendicular to the circuit
board. The inner surface of-the windshield, which is
parallel to the circuit board, receives the rays of the
light beam at the desired forty-five degree angle.
The light beam is reflected off of the outer surface
of the windshield and back through the windshield at
approximately a forty-five degree angle to the prismatic
coupler and collimator lens to a detector. Any water
present on the outer surface of the windshield effects the
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amount of light directed back to the detector. The light
beam is refracted approximately fifteen degrees by the
coupler and is reflected approximately sixty degrees by
the collimator lens such that the vertical axis is
perpendicular to the circuit board as the light beam is
detected by the detector.
In a windshield application, the sensor may be
provided with multiple emitter-detector optical systems to
provide an array of sensed areas. The emitters and
detectors are electrically connected to the windshield
wiper control circuitry to control operation of the wiper
system.
An object of the present invention is to reduce to
size of the sensor, especially the height of the sensor
housing extending from the inner surface of the
windshield. The combination of beam deflections by
reflection and refraction permits the use of surface
mounted emitters and detectors on a single circuit board
which is parallel to the inner surface of the windshield.
With the use of the surface mounted emitters and
detectors, the space between the circuit board and
prismatic coupler need only be tall enough to accommodate
the collimator lenses. The surface of the single circuit
board required in the present invention is mounted in
close proximity to the inner surface of the sensor
housing. The deflection of the light beams into and out
of the coupler permits a thin coupler to be used. By
mounting all of the components and control circuits on a
single circuit board and by mounting such circuit board
parallel to the inner surface of the windshield, a
significant reduction in the height of the sensor housing
can be achieved.
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Another object of the present invention is to provide
glass manufacturers and motor vehicle manufacturers with a
more efficient and cost effective means for mounting the
rain sensors on the windshield of a vehicle. In the
5 present invention, the coupler will generally be mounted
on the inner surface of the windshields by the glass
manufacturer prior to transporting the windshields to the
vehicle manufacturing plant. The vehicle manufacturer
conveniently mounts the sensor housing, which includes the
10 circuit board, onto the coupler as the vehicle is being
assembled. Because the coupler is small, thin, and
relatively inexpensive, the coupler can be mounted on all
of the windshields being transported from the glass
manufacturer to a specific assembly line at an automotive
15 plant without changing the conventional packaging
materials used by the glass manufacturer. As the
windshields are installed in a vehicle, the mounting of
the sensor can be completed by conveniently attaching the
sensor housing to the coupler.
20 A further object of the present invention is to
reduce the cost of manufacturing the sensor by mounting
all of the optoelectronic components and signal processing
circuitry on a single, planar circuit board. The surface
mounted technology and chip-on-board technology combined
with automated assembly techniques for production of the
circuit board provide an improved efficiency and cost
reductions in the manufacture of the sensors. The
configuration of the present invention eliminates the use
of multiple circuit board and lead formation on the
optical devices.
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An object of the present invention is to provide a
moisture sensor having high optical efficiency and
improved signal strength. The present invention utilizes
single surface lenses, which are more efficient than
segmented lenses.
In an alternative embodiment, the collimated light
beams from two infrared emitters are directed onto a
single detector. An object of the present invention is to
reduce the number of required optoelectronic components
IO without increasing the size or reducing the effectiveness
and efficiency of the moisture sensor.
BRIEF DESCRIPTION OF THE DRAWINGS
The above, as well as other advantages of the present
IS invention, will become readily apparent to those skilled
in the art from the following detailed description of a
preferred embodiment when considered in the light of the
accompanying drawings in which:
Fig. 1 is a fragmentary perspective view showing an
20 optical moisture sensor mounted upon the windshield of an
automobile;
Fig. 2 is an enlarged perspective view showing the
mounting of the prismatic coupler with an adhesive
interlayer on the inner surface of the windshield;
25 Fig. 3 is a transverse section of the optical
moisture sensor showing the sensor mounted on the
windshield, taken substantially along line 3-3 of Fig. 1;
Fig. 4 is a transverse section view of an alternative
embodiment of the moisture sensor having light beams from
30 two emitters directed into a single detector;
Fig. 5 is a perspective view of the prismatic coupler
for the alternative embodiment shown in Fig. 4; and
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Fig. 6 is a schematic, top plan view of the prismatic
coupler shown in Fig. 5 and including the positioning of
the optical other optical elements mounted on the circuit
board of the alternative embodiment to show the path of
the light beams between the emitters and the detectors in
the moisture sensor.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to Fig 1, there is shown generally a
moisture sensor 10 of the present invention and a portion
of an automobile, including a hood 12, side posts 14 and a
roof 16 defining an opening within which a windshield 18
is mounted. Windshield wiper blades 20, shown in their
at-rest position along the lower edges of the windshield,
are operable in a conventional manner to swing in arcs 22
and sweep accumulated moisture from the surface of the
windshield 18.
As shown in Figs. 2-3, the moisture sensor 10
includes a prismatic coupler 24, a circuit board 26 for
mounting the optoelectronic components and the signal
processing circuitry, and a sensor housing 28 for
enclosing the circuit board 26 and attachment to the
coupler 24.
The coupler 24 is secured to the inner surface 30 of
windshield 18 for the optical detection of moisture on the
outer surface 32 of the windshield. The moisture sensor
10 is typically mounted adjacent to the rear view mirror
on the inner surface 30 so as to minimize any view
obstruction for the passengers in the automobile. The
windshield I8 is generally relatively flat in the area
where the sensor 10 is to be mounted, so that the bottom
surface 34 of the coupler 24 may be planar. However,
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it is contemplated that the bottom surface 34 of the
coupler 24 may be correspondingly contoured to match a
curved windshield surface where appropriate. A double-
' sided adhesive interlayer 36 is used to secure the coupler
24 to the windshield 18. The interlayer 36 is made from
silicone or other similar flexible plastic material. The
coupler 24 may be secured to the windshield 18 by the
glass manufacturer prior to transporting the windshield 18
to the automotive assembly line.
The prismatic coupler 24 is made from polycarbonate
or other similar material for optically coupling the
moisture sensor 10 to the windshield 28. From a material
composition standpoint, the coupler 24 must be able to
withstand a wide range of temperature to which an
automobile may be subjected.
The prismatic coupler 24 shown in Fig. 2 includes
four pairs of prismatic regions formed on a base 42, each
pair having an emitter prism 38 and a detector prism 40.
The emitter prism 38 includes a surface 39 for receiving
light beams and the detector prism 40 has a corresponding
surface 41 where light beams exit the detector prism 40.
The surfaces 39,41 have a light convex curvature and are
formed at an angle of approximately twenty-one degrees
with respect to the base 42 of the coupler 24. Blocking
grooves 44 are formed on the base 42 between the prism
pairs 38,40. A plurality of mounting clips 46 are also
provided around the periphery of the base 42 for securing
the sensor housing 28 to the base 42 of the coupler 24.
The thickness of the prismatic coupler 24 is an
important consideration from a packing standpoint when
transporting the windshield from the glass manufacturer to
the automotive assembly line. Special racks and packaging
' material have been designed to pack the individual
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windshields as close as possible for shipping efficiency
while protecting the windshields during transport to
prevent scratching or other damage to the windshields.
The automotive windshields typically include a mounting
button on the windshield for mounting the rear view mirror
such that the shipping racks can accommodate such mounting
button. The prismatic coupler 24 of the present invention
is less than 5 mm, which is thinner than the typical
mirror mounting button. Consequently, the thin prismatic
coupler 24 permits the glass manufacturer to mount the
coupler 24 on the windshield production line without
having to change the packaging and material handling
processes used to deliver the windshields to the
automobile assembly line. The ability to mount the
coupler at the windshield production operations without
changing the packaging and material handling features is
an important consideration in gaining increased usage of
the moisture sensor and wiper control system by the
automotive companies.
The circuit board 26 is mounted on the base 48 of the
sensor housing 28, as shown in Fig. 3. The sensor housing
28 is made from a hard plastic or other rigid material and
includes four vertical walls 50 extending from the base
48. One of the objects of the present invention is to
minimize the size of the sensor housing, and specifically,
the height (h) of the walls 50 extending from the inner
surface 30 of the windshield 18_ The walls 50 of the
housing 28 include a flange 52 to facilitate retention of
the housing 28 by the clips 46 on the base 42 of the
coupler 24. The flange 52 of wall 50 adjacent the
detector prisms 40 is provided with a blocking edge 54 to
block out ambient light from the detectors on circuit
board 26.
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The present invention includes a single, planar
circuit board 26 with the optoelectronic components and
signal processing circuitry mounted on the circuit board
26. Conventional surface mounting techniques may be used
5 to mount the components on the circuit board 26. The
coupler 24 in Fig. 2 includes prismatic regions for four
emitter detector pairs. A system with four emitters
provides sufficient area for detection of moisture on the
windshield, which results in smooth wiper system
10 performance. However, the techniques of the invention may
be applied to other moisture sensing operation and may
include any number of emitter and detector pairs.
Referring now to Fig. 3, the cross sectional view of
the sensor shows a single emitter 56 and detector 58 pair.
15 Additional emitter 56 and detector 58 pairs may be mounted
on the circuit board 26 in a similar manner. The optical
path of the collimated light beam 60 as the rays of the -
light beam 60 travel from the emitter 56 to the outer
surface 32 of the windshield 18 and back to the detector
58 is also shown in Fig. 3.
The emitter 56 and detector 58 are surface mounted
devices, such as Siemens part numbers SFH-421 and BPW-
34FAS, respectively. The detector 58 may be a large
photodiode or a phototransistor. The emitter 56 and
detector 58 may also be implemented using silicon die
bonded directly to the circuit board 26 in a chip-on-board
approach. The emitter 56 radiates infrared energy such
that a light beam 60 is emitted primarily in a direction
that is perpendicular to the surface of the circuit board
26. The optical axis 62 of light beam 60 is normal to the
circuit board 26 upon leaving the emitter 56. The optical
axis 62 of light beam 60 travels through the nominal
center of the optical surfaces in the moisture sensor 10.
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The detector 58 is mounted so that the axis of highest
sensitivity is perpendicular to the circuit board 26. The
detector 58 also has an angle of acceptance such that
light beams striking the detector along the perpendicular
axis of highest sensitivity or within the angle of
acceptance about such axis will cause the detector 58 to
generate a control signal.
A collimator lens 64 is mounted adjacent the emitter
56 and centered along the optical axis 62 of light beam
60. A similar collimator lens 66 is mounted adjacent the
detector 58. Mounting posts (not shown) are used to
support and position the collimator lenses 64,66 on the
circuit board 26. The collimator lens 64 includes a
planar surface 68, which reduces the divergence of the
rays of the light beam 60. The mirror surface 70 of the
lens 64 acts as a folding mirror to reflect the light beam
60. The mirrorsurface 70 is positioned at an angle of
sixty degrees with respect to the surface of the circuit
board 26. When light beam 60 strikes the mirror surface
70, the light beam 60 is reflected by the process of total
internal reflection such that the optical axis is
reflected approximately sixty degrees from its initial
path perpendicular to the circuit board 26. The
collimator lens 64 also includes a convex lens surface 72
at the end of the lens 64. The convex lens surface 72
decreases the divergence of the rays of the light beam 60
to that of an almost collimated condition as the light
beam 60 exits the collimator lens 64.
The collimator lens 66 adjacent the detector 58 also
has a convex lens surface 74, a mirror surface 76, and a
planar surface 78 for reflecting the light beam 60 to the
detector 58. The optical axis 62 of the light beam 60 is
reflected such that the path of the light beam 60 is
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changed by approximately sixty degrees to achieve the
desired perpendicular angle of entry of the light beam 60
into the detector 58. Although the preferred angle at the
' detector is perpendicular, any light beams 60 within the
at~gle of acceptance will be detected by the detector 58.
' The angle of acceptance for surface mounted detectors is
generally in the thirty to sixty degree range.
The signal processing circuitry includes conventional
components 80A, 80H, 80C, 80D (Fig. 3) mounted on the
circuit board_ In additional, light barricades 82 may be
mounted on the circuit board to exclude ambient light from
the detector 58 and to prevent improper optical
commuri,xcat~.otn~ or crosstalk between the emitter 56 and
detector 58 in the housing 28_ The emitter 55 and
detector 58 are electrically connected to the signal
processing circuitry. the details of which do not form a
part of the present inwenti.on. Additional, details
concerning the operation of the signal processing
circuitry and the interface with the Controller and the
wiper control system may be obtained from U.S. Patent Nos.
4,620,141: 5,059,877; 5,239,249; and 5,262.640.
When the moisture sensor is in operation, the
controller (not shown) signals the eatitter 56 which causes
a light beam 60 to be emitted perpendicular to the Circuzt
board 26. As shown in Fig. 3, the bight beam is directed
through the surface of the collimator lens 64 which
reduces the divergence of the rays of the light beam 60.
The rays of the light beam 60 travel through the clear
material of the collimator lens 64 until striking the
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mirror surface 70, which reflects the light beam 60
approximately 60 degrees from the initial path of the
light beam 60. The resulting optical axis 62 is at
approximately a thirty degree angle with respect to the
circuit board 26. The convex lens surface 72 of the
collimator lens 64 collimates the light beam 60 for entry
into the coupler 24.
The light beam 60 enters the coupler 24 at the
surface 39 of detector prism 38. The surface 39 may
include a convex curvature to ensure to ensure that the
light beam 60 is fully collimated within the coupler 24.
The detector prism 38 formed on the base 42 of the coupler
24 causes the light beam to be refracted approximately
fifteen degrees. The light beam 60 is optically coupled
into the interlayer 36 and then into the windshield 18
such that the light beam enters at an angle of
approximately forty-five degrees.
The light beam 60 travels through the windshield 18,
continuing at an angle of approximately forty-five
degrees. At the outer surface 32 of the windshield 18,
the beam is totally reflected and passes back through the
windshield 18. If any moisture is present on the outer
surface 32 of the windshield 18, a portion of the light
beam 60 is not reflected and passes through the windshield
18. By detecting the light beam 60 reflected from the
outer surface 32, the detector 58 of the moisture sensor
10 generates a control signal which is indicative of the
amount of moisture on the outer surface 32 of the
windshield 18.
The light beam 60 which is reflected from the outer
surface 32 of the windshield 18 at approximately forty-
five degree angle passes through the interlayer 36, the
base 42 of the coupler 24, and the detector prism 40. The
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light beam 60 passes through the prism surface 41. The
detector prism 40 is formed at an angle similar to the
emitter prism 38 and results in the refraction of the of
the light beam 60 by about fifteen degrees. The convex
curvature of the prism surface 41 and of the convex lens
surface 74 makes the light beam 60 slightly convergent.
The light beam 60 passes through the collimator lens 66
until the light beam 60 is reflected from mirror surface
76 by total internal reflection. The light beam is
reflected approximately sixty degrees by the mirror
surface 76 such that the optical axis 62 of light beam 60
is once again normal to the circuit board 26.
The light beam passes through the detector collimator
lens 66 and exits the lens 66 at planar surface 78. The
planar surface 68 converges the light beam 60 to a point
on the detector 58. Although the detector generally has
the highest sensitivity when the light beams are
perpendicular to the circuit board 26, any light beams 60
within the acceptance angle of the detector 58 will be
detected. The detector 58 generates a control signal for
the signal processing circuitry on the circuit board 26.
The control signal is processed and transmitted to a
controller for controlling the operation of the windshield
wipers 20.
The preferred angle for the light beam 60 to enter
the windshield I8 is forty-five degrees. In general,
acceptable signals can be generated for an entry angle
between forty and fifty degrees. As noted above, an angle
above fifty degrees results in lost signals and an angle
below forty degrees results in lost sensitivity. To
obtain the forty-five degree angle when the light beam 60
initially starts out perpendicular to both the circuit
board 26 and the windshield 18, the light beam is
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reflected at approximately sixty degrees and is refracted
at approximately fifteen degrees. In general, the mirror
surfaces 70, 76 reflect the light beam 60 to change the
optical axis approximately sixty degrees, but acceptable
5 reflection may occur in the range between fifty and
seventy degrees. The prisms 38, 40 refract the light beam
fifteen degrees, but the acceptable range is ten to twenty
degrees. The important consideration is that the net
effect of the reflection and refraction is a light beam 60
10 entering the windshield 18 at approximately forty-five
degrees.
Ambient light often present a problem in moisture
sensors 10 which use an optical detection system. Ambient
light will generally enter the housing at an angle too
15 steep to be sensed by the detector 58. In addition, light
barricades 54,82 are formed in the housing and mounted on
the circuit board to further exclude ambient light and
prevent crosstalk between the emitter 56 and detector 58.
The blocking grooves 44 built into the coupler 24 serve to
20 trap crosstalk that may arise out of parasitic paths.
The convex lens surfaces 72, 74 on the lenses 64, 66
may be provided as aspheric surfaces. The use of aspheric
surfaces reduces optical aberration, which tends to
degrade the optical efficiency. The overall optical
configuration of the emitter half of the sensor 10 and the
detector half of the sensor 10 may be described as
infinite conjugate ratio systems. This optical
arrangement inherently provides low aberration, and thus
the aspheric surfaces do not deviate greatly from truly
spherical surfaces.
The optical axis 62 of light beam 60 strikes the
prismatic surfaces 39, 41 of the coupler 24 at an oblique
angle as opposed to deploying the coupling prisms with
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21
surfaces perpendicular to the optical axis. The oblique
deployment of the prisms 38, 40 permits the optical axis
62 to be diverted to a direction perpendicular to the
circuit board using a single reflector. This facilitates
the use of a single circuit board 26 and a more shallow
design of the coupler 24. The lenses 64, 66 perform
several functions to help reduce the number of parts
required in the sensor 10. A combination of the above
factors permits a small and compact housing 28 to be used
for the sensor 10.
Referring now to Figs. 4-6, an alternative embodiment
of the present invention is provided with a different
arrangement of the optical components. The main
difference between the sensor 10 of the first embodiment
and the sensor 84 of the alternative embodiment is the
elimination of half the detectors by directing two light
beams to the same detector.
In principle, a single detector may receive infrared
radiation from several emitters. In the present
invention, cost benefits can be obtained by combining two
emitters with a single detector without adversely
impacting the size or operating complexity of the moisture
sensor 84. Although the discussion is directed to a
sensor 84 with four emitters and two detectors, the sensor
84 may include any number of emitter-detector sets.
The moisture sensor 84 includes a detachable
prismatic coupler 86, a circuit board 88, and a sensor
housing 90. The sensor housing 90 has the same features
as the housing 28 described above. The changes to the
coupler 86 and to the arrangement of the optoelectronic
components on the circuit board 88 will be highlighted
below.
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22
In Fig. 5, the coupler 86 includes a base 92 with
four emitter prisms 94 and two detector prisms 96. The
width of the detector prisms 96 has been increased to
facilitate the receipt of two light beams 98 with optical
axis 100 at one detector. The orientation of the prisms
94, 96 on the base 92 has been changed from the first
embodiment to facilitate the transmittal of the light
beams 98 between the emitters 102 and the detectors 104.
The coupler 86 is mounted on the inner surface 108 of
windshield 106 to detect moisture on the outer surface 110
of the windshield.
The collimator lenses 112 adjacent the emitter 102
are similar to the collimator lens 64 in the first
embodiment. The collimator.lens 114 adjacent the detector
104 has two distinct lens segments which are integrally
formed to direct two light beams 98 into the single
detector 104. The emitters 102, detectors 104, and
collimator lenses 112, 114 are mounted on circuit board
88.
The collimator lens 112 is positioned above the
emitter 102 on the initial optical axis 100 extending
perpendicular from the emitter on the circuit board 88,
which is similar to the first embodiment. To achieve
proper alignment with the collimator lens 114, the
collimator lens must also be rotated. The collimator lens
112 is rotated approximately ten to twelve degrees about
the initial optical axis 100 extending perpendicular to
the emitter 102. When the light beam 98 passes through
the planar surface 116 and strikes the mirror surface 118,
the light beam 98 is reflected sixty degrees so that the
light beam 98 is at an angle of thirty degrees with
respect to the circuit board 88 and also the inner surface
108 of the windshield I06. In addition, because of the
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23
rotation of the lens 112 on a vertical axis, the light
beam 98 is also is also rotated in the plane of the
windshield 106. The rotation of the collimator lens 112
and the path of the light beam 98 are shown in Fig. 6, in
which only the optical system of the sensor 84 is
illustrated.
After traveling through the collimator lens 112, the
light beam 98 exits the lens 112 at the convex aspheric
lens surface 120. As noted above, the rays of the light
beam 98 are partially collimated. The light beam then
passes through the prism surface 122 of emitter prism 94
on the coupler 86. This surface is also rotated
approximately ten to twelve degrees in the plane of the
windshield so that the optical axis 100 of the light beam
98 is not further rotated. The prism surface 122 is
positioned such that the surface 122 is at an approximate
angle of twenty-one degrees with respect to the inner
surface 108 of the windshield 106.
The emitter prism 94 refracts the light beam 98 and
optical axis 100 approximately fifteen degrees such that
the light beam enters the windshield at the desired forty-
five degree angle. A slight astigmatic curvature is
included in the surface 122 of the prism 94 to fully
collimate the light beam 98. The light beam 98 is coupled
undeflected into the windshield 106 and is reflected by
the outer surface 110 of the windshield 106.
The light beam 98 passes through the detector prism
96 on the coupler 86. The prism surface 124 has a common
plane for two emitter optical paths. A slight convex
curvature may be added to the prism surface 124, but the
surface is reasonably flat. The surface 124 of the
detector prism 96 is formed at an angle of approximately
twenty-one degrees with respect to the inner surface 108
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of the windshield 106. The light beam 96 leaves the
windshield 106 at approximately forty-five degree angle
and is refracted approximately fifteen degrees by detector
prism 96 so that the light beam 98 is traveling at an
angle of approximately thirty degrees with respect to the
inner surface I08 of the windshield I06. Because of the
orientation of the detector prism 96, the light beam 98 is
rotated an additional five degrees in the plane of the
windshield. After passing through the detector prism 96,
the light beams 98 are at~an angle of approximately
sixteen degrees with respect to the longitudinal axis in
the plane of the windshield.
After the detector prism 96, the light beam 98 enters
the detector collimator lens 114 at the convex aspheric
IS surface 126, which converges the Light beam 98. The Light
beam strikes the mirror surface 128, which reflects the
light beam 98 approximately sixty degrees such that light
beam passes through planar surface 130 and is directed to
the detector 104 on circuit board 88.
Because of the rotation of the light beam 94 within
the plane of the windshield 106, the optical axis 100 of
the light beam 98 after leaving the collimator lens 114 is
not exactly perpendicular to the detector 104 on the
circuit board 88. The light beam 98 will typically be
approximately sixteen degrees from the axis extending from
the detector 104 perpendicular to the circuit board 88.
The detector 104 is mounted on the circuit board 88 such
that the axis of highest sensitivity is perpendicular to
the circuit board 88. Although the light beam 98 is
directed to the detector 104 at an angle approximately
sixteen degrees from vertical, the light beam 98 strikes
the detector 104 well within the acceptance angle of the
detector 104. '
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Fig. 6 shows the configuration of the optical
components and the path of the light beam 98 with optical
axis 100 when two emitters 102 are directed to a single
detector 104. The detector collimated lens 114 has the
5 appropriate convex surface 126, mirror surface 128, and
planar surface to accommodate light beams 98 form two
emitters 102. The convex surface 126 of the detector
collimated lens 114 is splayed apart to provide two
distinct surfaces 126A, I26B for converging the light
10 beams 98.
Fig. 6 also shows the slight rotation of the beams 98
within the plane of the windshield 106. The light beams
from the two emitters form a V-type arrangement. When two
sets are used in a single sensor 84, the third and fourth
15 emitters 102 and the second detector 104 are positioned as
shown. The housing 90 may be reduced in size because the
optical configuration permits to light beams to share
common active optical elements. The mirror surface 128
and planar surface 130 of the detector collimated lens 114
20 are common to two optical paths. This optical technique
of angular spectrum multiplexing saves space and reduces
the cost of the sensor 84. The mounting of the circuit
board 88 parallel to the inner surface 108 of the
windshield 106 provides significant space and cost
25 reduction in the manufacturing of the sensor 84.
In the sensor 84 (and also in sensor 10), the total
focal power of all of the optical surfaces which the light
beam 98 encounters before it is coupled to the windshield
106 must be sufficient to collimate the light beam 106.
The majority of the focal power is placed at the convex
lens surface 20 of the collimator lens 112, and a lessor
amount is placed at the prism surface 122 of the coupler
86. The focal power may be distributed differently by
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26
adjusting the curvature of the surfaces so long as the
changes to the prism surface 122 do not adversely effect
the necessary refraction of the light beam 98.
One option to adjust the focal power is to add focal
power earlier in the optical path by making the planar
surface 116 convex. It is also possible to add focal
power to the mirror surface 118 by making the mirror
surface 118 a parabolic section with the focal point of
the parabola located at the emitter 102. The prism
IO surface 122 may be implemented as a segmented surface, or
Fresnel lens, at the expense of optical efficiency. A
similar redistribution of focal power may be utilized on
the detector optics of the sensors 10, 84. However, in
general, it is preferable to place the focal power at the
end of the optical path from the emitter 102 to the
windshield so as to lengthen the focal length of the
optics. Increasing the focal length results in fewer rays
of the light beam 98 to missing the detector prism 96. In
addition, the longer focal lengths permit less critical
tolerance in many of the parts.
In addition to the front windshield of a motor
vehicle, the moisture sensor of the present invention can
also be used on other glass surfaces for the detection of
moisture.
