Note: Descriptions are shown in the official language in which they were submitted.
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COMPACT MOISTURE SENSOR WITH EFFICIENT,
HIGH OBLIQUITY OPTICS
This application is a division of Canadian Patent
Application No. 2,302,660, filed October 5, 1998.
BACKGROUND 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 optical emitters, detectors, and
optical components mounted on a planar circuit board which
is positioned parallel to the interior surface. A coupler
having collimator and focusing lenses is used to refract
light beams as the light beams travel from the emitters,
and are reflected from the outer surface of the windshield
back to the detectors.
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
multi-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
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 is
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 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).
Noak (U. S. Patent No. 9,355,271) discloses an optical
moisture sensor having optical components mounted in a
box-like housing attached to the interior surface of the
windshield. The moisture 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) also disclose a box-like housing mounted upon
the interior surface of the windshield for enclosing the
optics and electronics.
In optical moisture sensors, light from an emitter is
directed into the windshield at an angle of approximately
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forty-five degrees with respect to the windshield. The
light is then reflected by the outer surface of the
windshield at approximately a forty-five degree angle and
is directed into a detector. The presence of moisture on
the surface of the windshield affects the reflection of
light at the air-glass interface at the outer surface of
the windshield, and this change in reflected light is
electronically processed and utilized as the signal for
activating the windshield wipers.
McCumber et al. (U. S. Patent No. 4,620,191) 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.
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.
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. Stanton (U. S. Patent No. 5,414,257) discloses
an optical sensor having optoelectronic devices mounted on
a circuit board at an appropriate angle with respect to
the surface of the glass such that their optical axis'
extend at the appropriate forty-five degree angle or can
be deflected so as to do so. Stanton teaches devices cast
from flexible epoxy resin and the bending of the leads of
the devices at an angle to facilitate the angled mounting_
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The problem with bending the leads of the electronic
devices is that most automated component insertion
equipment cannot insert components with bent leads which
increases the cost of assembling the circuit boards. In
addition, the bent lead devices are less reliable from a
performance standpoint.
The mounting of optoelectronic devices on circuit
boards without bending the leads is disclosed in Zettler
(U. S. Patent No. 5,560,245). The emitters and detectors
are mounted on small satellite circuit boards which are
angled with respect to the main circuit board. The
satellite circuit boards are angled to aligned the
emitters and detectors at the appropriate forty-five
degree angle with the windshield. Although this mounting
configuration does 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.
Conventional optoelectronic devices, including the
new surface-mount technology devices (SMT's), are
generally designed so that the optical axis is
perpendicular to the circuit board on which they are
mounted. Teder (U.S. patent No. 5661,303) discloses the
use of a single circuit board mounted co-planar with the
surface of the windshield which results in a low cost and
compact sensor enclosure. However, this design requires
optical components having optical axis
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which are approximately parallel to the optical axis of
the optoelectronic devices. It is desirable to reduce the
size and cost of the optical components to further reduce
the size and cost of the moisture sensor.
Another way to reduce the size and cost of the
optical sensor includes reducing the number of
optoelectonic components. Noak discloses using a single
detector to simultaneously detect two or more emitters.
Muller (U. S. Patent No. 5015931) discloses that several
beams may be derived from a single nondirectional emitter.
Such configurations provides the desired area of detection
with a fewer number of detectors. McCumber et al. (U. S.
Pat. No. 9,620,141) teach that a balanced configuration
tends to reject the effect of ambient light. Emitters,
however, typically vary by about 2:1 in signal strength.
This has limited the ability of prior art optical moisture
sensor systems to achieve a good signal balance. The.
optical paths shown by Muller in '931 are of unequal
length. Thus, the paths would be of differing optical
efficiency and could not be used to make a well-balanced
system. Teder (U. S. patent No. 5661,303) uses four
emitters and two detectors to achieve four optical paths
of equal length, however it is desirable to reduce the
size and cost of the moisture sensor by using even fewer
components.
The optical moisture sensor should securely engage
the windshield and the optics contained therein should 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.
Purvis (U.S. Patent No. 5,262,690) describes an
intermediate adhesive interlayer for affixing the sensor
housing and the optics contained therein to the
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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.
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 windshields nested
together such that there is very little spacing for
mounting a sensor.
Schofield (U.S. Patent No. 9,930,743) 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 ('257)
teach that a lens may be permanently affixed to the
windshield such that a sensor housing may be detachably
mounted on the lens. The lens may impart focal power to
the beam, as shown in 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. It is desirable to
have a sensor which is attached to the windshield and is
thin enough not to interfere with nesting the windshields
during shipping.
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Modern solar-control windshields, such as windshields
sold under the trademark "EZ-KOOL" commercially available
from Libbey-Owens-Ford Co., absorb the infrared rays used
by many optical moisture sensors. Sensors without
coupling or light gathering optics are likely to be too
inefficient for use on these windshields. In German
Patent No. DE 3314770 to Kohler, et. al., lenses in a
coupler increase the sensed area and efficiency of a
moisture sensor. Watanabe (U. S. Patent No. 4,701,613)
discloses a series of V-grooves that couples rays into and
out of a windshield with an improved efficiency, however,
the devices are mounted at a forty-five degree angle with
respect to the glass surface because the grooves do not
gather diverging light rays and focus them onto the
detector. It is desirable to mount the optoelectronic
components on a single planar circuit board while
improving the efficiency of the optical moisture sensor
for use on modern solar-control windshields.
SUMMARY OF THE INVENTION
The present invention relates to a moisture sensor
for mounting on a first surface of a sheet of glass to
detect moisture in a sensing area on a second surface of
the sheet of glass. The moisture sensor includes a
coupler to be mounted on the first surface of the sheet of
glass for coupling light rays into and out of the glass
and a housing detachably secured to the coupler. A planar
circuit board is secured in the housing and has a device
surface which is disposed generally parallel to the first
surface of the sheet of glass. An emitter for emitting
light rays about an emission axis is mounted on the device
surface. The emission axis extends from the emitter
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approximately perpendicular to the device surface of the
circuit board. A collimator for collimating light rays
from the emitter into a collimated light beam has an
aperture with a physical center and an optical center
spaced apart from the physical center. An optical axis
extends through the optical center. The emitter and the
collimator are disposed such that the optical axis forms a
first oblique angle with respect to the emission axis.
A detector having a detection surface and a detection
axis extending from the detection surface is mounted on
the device surface of the planar circuit board such that
the detection axis is approximately perpendicular to the
device surface. The detector detects light striking the
detection surface and generates signals in response to the
detected light. The coupler also includes a focuser for
focusing the collimated light beam onto the detection
surface. The focuser has an aperture with a physical
center and an optical center spaced apart from the
physical center. An optical axis extends through the
optical center. The focuser and the detector are disposed
such that the optical axis forms a second oblique angle
with respect to the detection axis.
The sensor is provided with multiple emitter-detector
optical systems to provide an array of sensed areas. A
pair of emitters is used in conjunction with a pair of
detectors to achieve four separate optical paths of equal
length and four sensing areas on the glass surface. The
emitters and detectors form a balanced electrical system
which is electrically connected to the windshield wiper
control circuitry to control operation of the wiper
system.
An efficient and cost effective means for mounting
the moisture sensors on the windshield of a vehicle is
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provided. In the present invention, the coupler will
generally be mounted or. the inner surface of the
windshields by the glass manufacturer prior to transporting
the windshields to the vehicle manufacturing plant. The
S vehicle manufacturer conveniently mounts the sensor
housing, which includes the 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 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.
The cost of manufacturing the sensor is reduced 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.
A substantial portion of the light rays emanating
from each emitter are coupled into each of the two
detectors, providing a high optical efficiency.
Additionally, a pair of emitters and a pair of detectors
are used to form four optical paths of equal length to
provide a balanced optical system having four sensing
areas. The numbers of optoelectronic components is
reduced which decreases the cost of the sensor without
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reducing the effectiveness and efficiency of the moisture
sensor.
BRIEF DESCRIPTION OF THE DRAWINGS
5 The above, as well as other advantages of the present
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:
10 Fig. 1 is a fragmentary perspective view showing an
optical moisture sensor mounted upon the windshield of an
automobile;
Fig. 2 is an enlarged perspective view showing the
mounting of the moisture sensor of the present invention
on the inner surface of the windshield;
Fig. 3 is an enlarged perspective view showing the
mounting relationship between the housing and the coupler
of the present invention;
Fig. 4 is a transverse section view showing the
collimator mounted adjacent to the emitter in the present
invention;
Fig. S is a side view taken along line 5-5 showing
the collimating lens aperture;
Fig. 6 is a perspective view showing the collimator
extending from the coupler in the present invention;
Fig. 7 is a side view taken along line 7-7 showing
the collimating lens aperture;
Fig. 8 is a plan view of an alternative embodiment of
the present invention illustrating the four optical paths;
Fig. 9 is a schematic diagram illustrating the
optoelectronic components of the alternative embodiment of
the present invention; and
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Fig. 10 is a transverse section view of a second
alternative embodiment of the present invention
illustrating the collimating lens using a segmented lens.
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. The moisture sensor 10 is secured to the
windshield within the area swept by windshield wiper
blades 20.
As shown in Fig_ 2, the moisture sensor 10 includes a
coupler 24, a circuit board 26 for mounting electronic
components 27, and a sensor housing 28 attachable to the
coupler 24 for enclosing the circuit board 26.
The coupler 24 includes a mounting surface 29 which
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 (not shown) on
the inner surface 30 so as to minimize any view
obstruction for the passengers in the automobile, although
the sensor may be mounted elsewhere on the windshield.
The windshield 18 is generally relatively flat in the area
where the sensor 10 is to be mounted, so that the mounting
surface 29 of the coupler 24 may be planar. However, it
is contemrlated that the mounting surface 29 of the
coupler 24 maybe correspondingly contoured to match a
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curved windshield surface where appropriate. The sensor
may also be mounted on other windows including the rear
window.
A double-sided adhesive interlayer 34 is used to
5 secure the coupler mounting surface 29 to the windshield
18 or other window. The interlayer 34 is made from
silicone or other similar flexible, transparent plastic
material. The coupler 24 may be secured to the windshield
18 by the glass manufacturer prior to transporting the
10 windshield 18 to the automotive assembly line. A
rectangular sleeve 36 extends from the coupler 24 opposite
the mounting surface 29 and coupler tabs 53 extend
outwardly from the ends of the sleeve for securing the
coupler to the housing 28 as described below.
The coupler 24 also has a collimator 37 including a
collimating body 38 extending from the coupler and a
collimating lens 40 disposed adjacent to the collimating
body. The collimating lens 40 has an optical axis 41
which extends through the collimating body 38 at a forty-
five degree angle with respect to the inner surface of the
windshield 30. The coupler 24 further includes a focuser
42 having a focusing body 43 extending from the coupler
and a focusing lens 49 disposed adjacent to the focusing
body. The focusing lens 44 has an optical axis 45 which
extends through the focusing body 43 at a forty-five
degree angle with respect to the inner surface of the
windshield 30. The coupler 24, collimating body 38,
collimating lens 40, focusing body 43 and focusing lens 44
are preferably formed integrally from a single piece of
material. The collimating lens 40 is formed by shaping
the surface of the collimating body 38, and the focusing
lens 44 is formed by shaping the surface of the focusing
body 43, in a manner described below. Alternatively, a
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separate collimating lens 90 may be disposed adjacent to
the collimating body 38 and a separate focusing lens 94
may be disposed adjacent the focusing body 43.
The coupler is formed from a refractive material such
as polycarbonate, or polyester resin, although any suitable
material may be used which can withstand a wide range of
temperatures to which an automobile may be subjected. The
coupler 24 optically couples light rays into and out of
the windshield 18 so that the light rays are not deflected
as they pass from the collimating body 38 to the
windshield and from the windshield to the focusing body
93. In addition the coupler 29 provides a secure base for
mounting the collimating lens 40, focusing lens 49, and
housing 28 to the windshield 18.
The thickness of the coupler 29 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
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 (not shown) on the windshield for mounting the rear
view mirror such that the shipping racks can accommodate
such mounting button. The coupler 24 of the present
invention is less than 5 mm thick, which is thinner than
the typical mirror mounting button. Consequently, the
thin 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
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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
S automotive companies.
Referring now to Figs. 2 and 3, the sensor housing 28
is made from a hard plastic or other rigid material and is
opaque to block out unwanted light. For clarity, Fig. 3
shows the coupler 24 without showing the collimator or
focuser. The housing 28 includes a base 46 and four side
walls 48 extending from the base, preferably forming a
box-shaped enclosure. The housing 28 is sized to fit over
the sleeve 36 of the coupler 29 after the coupler has been
secured to the windshield 18. Grooves 50 are formed on
the interior of the housing walls 48 for receiving the
coupler tabs 53 and detachably retaining the housing 28 to
the coupler 24. The side walls 48 of the housing 23, as
well as the sleeve 36 of the coupler 29, are each made
slightly deformable to facilitate snapping the housing into
place over the coupler so that the coupler tabs 53 enter
the grooves S0. Optionally, notches may be cut in the
sleeve 36 of the coupler 29 to increase the deformation.
Once the housing 28 is snapped in place over the
coupler sleeve 36, any lateral forces applied to the
coupler 24 are transferred by housing walls 98 to sleeve
36. The housing walls 48 and coupler sleeve 36 have a
large surface area, and will have no tendency to
concentrate forces which leads to breakage. Further, the
non-circular shape of coupler sleeve 36 absorbs torsional
forces applied to housing 28. The present moisture sensor
will thus tend to remain firmly affixed to the windshield
in the event of a collision, or if it is handled clumsily
by a curious passenger. A notch Sl in a side wall 48 of
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the housing 28 facilitates its removal with a coin or
screwdriver. Preferably the coupler 24 is autoclaved onto
the windshield with the aid of a very high adhesion
silicone, although any suitable material may be used. The
5 shallow depth of the coupler attachment method permits
such an installation to be performed at the windshield
manufacturer, without impacting the pack density of the
windshield as described above. Vehicle manufacturers
dislike any process dealing with adhesives or other
10 chemicals, and prefer to have the moisture sensor coupler
come to them affixed to the windshield.
In addition to making the moisture sensor impact
resistant, the perimeter secured design is simple to
install. In contrast to moisture sensor attachment
15 methods featuring separate clips or other attachment
features, the present moisture sensor housing may be
snapped onto the coupler in a one handed operation. This
reduces the time it takes the vehicle manufacturer to
install the moisture sensor, reducing the cost of the
system.
A single, planar circuit board 26 is held in the
housing by tabs 52 which extend inwardly from the inner
surface of the housing walls. The circuit board 26
includes a device surface 54 on which the electronic
components 27 are mounted. The circuit board 26 is
mounted in the housing 28 so that the device surface 54 is
approximately parallel to the inner surface 30 of the
windshield 18 when the housing 28 is secured to the
coupler 24 and the coupler is secured to the windshield.
The electronic components 27 are mounted on the device
surface 54 of the circuit board 26 so that the top
surfaces of the electronic components are approximately
parallel to the device surface 54. Conventional surface
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mounting techniques may be used to mount the components on the
circuit board 26.
The electronic components 27 include an emitter 56, a
detector 58, and signal processing circuitry 59. Although a
single emitter 56 and detector 58 are shown, multiple emitters
and detectors may be used as described below. The emitter 56
is preferably an infrared light-emitting diode although any
suitable emitter may be used, and the detector 58 is
preferably a photodiode, although any suitable detector may be
used_ The emitter 56 and detector 58 are surface mounted
devices, such as Siemens part numbers SFH-421 and BPW-34FAS,
respectively. 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 signal processing circuitry includes conventional
components 59 mounted on the circuit board 26. In addition,
light barricades 61 may be mounted on the circuit board to
exclude ambient light from the detector 58 and to prevent
improper optical communication or crosstalk between the
emitter 56 and detector 58 in the housing 28. The emitter 56
and detector 58 are electrically connected to the signal
processing. 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; 5,239,244; and
5,568,027.
As shown in Fig. 4, the emitter 56 is typically composed
of a plastic housing or case 60, an infrared
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emitting die 62 mounted within a depression in the case,
and a clear epoxy filled region 64. The emitter 56
radiates rays of light 65, typically of a specific
wavelength such as infrared energy at 880 nM, although
other wavelengths may be used. The light rays 65 are
emitted as a divergent fan of rays which is symmetric
about an emission axis 66 extending from the emitter
primarily in a direction perpendicular to the device
surface 54 of the circuit board 26. The light rays 65
emanate from the emitter 56 over a splay of angles with
each ray traveling at an angle 6~ with respect to the
emission axis 66. The intensity of each of the rays 65
diverging from the emitter 56 is approximately the cosine
of 6e. Thus, the rays 65 from the emitter 56 are strongest
along emission axis 66. In the near field in which the
invention operates, the rays at an emitter angle AE of
greater than about fifty degrees are shadowed by the
emitter case 60, and thus are less intense.
As shown in Fig. 2, the detector 58 includes a
detection surface 67 extending approximately parallel to
the device surface 54. A detection axis 68 of highest
detection sensitivity extends from the detection surface 67
in a direction that is primarily perpendicular to the
detection surface 67 and the device surface 54 of the
circuit board 26. The detector 58 also has an angle of
acceptance (not shown) extending symmetrically about the
detection axis 68 such that light beams striking the
detector within the angle of acceptance will cause the
detector 58 to generate a control signal. The specific
emitter 56 and detector 58 to be used are chosen so that
the detector is sensitive to the wavelength of light
emitted by the emitter.
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When the housing 28 is secured to the coupler 29 as
shown in Fig. 2, the collimating body 38 and collimating
lens 40 extend towards the emitter 56 and the focusing
body 43 and focusing lens 49 extend towards the detector.
A portion of the light rays 65 emanating from the emitter
56 strike the collimating lens 90 and are collimated into
a beam 72 traveling through the collimating body 38 along
the collimating lens optical axis 41. The light rays
which strike the collimating lens 90 preferably range from
approximately 10 to approximately fifty degrees with
respect to the emission axis 66, although the lens may be
shaped to accept light rays from smaller or larger angles.
The collimating lens 90 is disposed relative to the
emitter 56 such that the optical axis 41 forms an oblique
angle 69 with respect to the emission axis 66. The
oblique angle 69 is preferably between 39 and 51 degrees
although it may be smaller or larger. The surface of the
collimating lens 40 must be shaped, as described below, to
form a collimated beam of sufficient intensity that the
detector 58 can produce a useable signal.
Similarly, the focusing lens 44 is disposed relative
to the detector 58 such that the optical axis 45 of the
focusing lens 44 forms an oblique angle 71 with respect to
the detection axis 68. The oblique angle 71 is preferably
between 39 and 51 degrees. The surface of the focusing
lens 44 is shaped, as described below, to focus the
collimated beam 72 onto the detection surface of the
detector. The collimated beam 72 is focused into a fan of
convergent rays having sufficient intensity at the
detection surface, 67 that the detector can produce a
useable signal. The fan of light rays converging onto the
detection surface preferably range from approximately 10 to
approximately fifty degrees with respect to the detection
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axis, although the fan of rays may form smaller or larger
angles with respect to the detection axis.
Light travels from the emitter 56 to the detector 58
along a optical path 73. The light rays from the emitter
which are collimated into the collimated beam 72 travel
along the optical path into the windshield 18 at a forty-
five degree angle with respect to the inner surface 30.
The collimated light beam 72 strikes the outer surface 32
at a sensing region 74 and is reflected along the optical
path 73 back through the windshield and into the focusing
body 43 at a forty-five degree angle with respect to the
inner surface 30. The optical axis 45 of the focusing
lens 99 is translated from the optical axis 41 of the
collimating lens 40 at the surface of the coupler 29 by a
distance T. No single light ray travels laterally along
this translation; rather it is an artifice indicating that
the optical center of the system shifts at the surface of
the coupler 24. Distance T is selected so that the
focusing lens 44 gathers the full width of beam 72. The
collimating surface 40 is a truncated surface of rotation,
symmetric about optical axis 41. Translation T of the
optical axis 41 to the optical axis 45 comes about because
of the nature of the asymetrics of the collimating and
focusing apertures. The outside surface of the glass acts
as a folding mirror. Because of the effects of this
folding mirror, rays close to the emission axis strike the
detector at an angle far from the detection axis. Thus, a
beam passing through the optical center of the collimator
would not pass through the optical center of the focuser
lens, which is shifted away from the detection axis.
Referring now to Figs. 5 and 6, the collimating lens
has a light receiving aperture 82 defined by the
perimeter 80. The perimeter 80 may be the physical edges
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of the collimating lens 40, or the perimeter 80 may define
the area of the lens surface which receives light emitted
directly from the emitter 56 and which collimates such
light as described above. Light rays that strike the lens
5 surface outside of the aperture 82 are not collimated and
are not effectively transmitted to the detector 58. The
light receiving aperture 82 has a width W, as shown,
measured in the direction of reference line 5-5. The
light receiving aperture 82 has a physical center 84
10 positioned in the center of the perimeter 80.
The optical center of a lens is defined as the point
at which an optical axis intersects the lens surface.
Also, by definition, a ray of light traveling along the
optical axis which enters the lens aperture through the
15 optical center goes straight, whereas all other rays
entering the lens aperture are deflected by the lens along
a path parallel to the optical axis. The collimating lens
40 has an off-center optical center 86 spaced apart from
the physical center 84 and therefore an off-center optical
20 axis 41. Preferably, the optical center 86 is displaced
from the physical center 84 by about 22$ of width W,
although any suitable displacement may be used. The
surface of the collimating body 38 of the coupler 24 may
optionally be covered with an opaque material to exclude
rays that do not strike the aperture 82, or such rays may
be allowed to pass through the coupler unobstructed.
Additionally, the surface of the collimating lens outside
the perimeter 80 defining the aperture 82 may optionally
be covered with an opaque material to exclude rays that do
not strike the aperture 82, or such rays may be allowed to
pass through the coupler unobstructed. Only those emitter
rays that pass through aperture 82 are useful for sensing
rain.
CA 02520651 1998-10-05
21
Referring now to Fig. 7, the focusing lens 44 has a
light transmitting aperture 90 defined by the perimeter
88. The perimeter 88 may be the physical edges of the
focusing lens 49, or the perimeter 88 may define the area
of the lens surface which transmits the collimated light
beam 72 in a focused beam to the detection surface 67 of
the detector 58. Light rays that exit the focusing lens
outside of the aperture 90 are not focused onto the
detector 58. Aperture 90 has a width W, as shown. The
focusing lens 49 has a physical center 92 positioned in
the center of the perimeter 88. The optical center 94 of
the lens 94 is off-center, that is, it is spaced apart
from the physical center 92 and, therefore, the focusing
lens optical axis 45 is also off-center. Preferably, the
optical center 94 is displaced from the physical center 92
by about 220 of width W, although any suitable
displacement may be used. The surface of the focusing
body 43 of the coupler 24 may optionally be covered with
an opaque material to exclude rays that do not strike the
aperture 90, or such rays may be allowed to pass through
the coupler unobstructed. Additionally, the surface of
the focusing lens 94 outside the perimeter 88 defining the
aperture 90 may optionally be covered with an opaque
material to exclude rays that do not strike the aperture
90, or such rays may be allowed to pass through the
coupler unobstructed.
When the moisture sensor is in operation, the
controller (not shown) signals the emitter 56 which causes
light rays 65 to be emitted symmetrically about the
emission axis. The light rays 65 which strike the
collimating lens aperture 82 are collimated into a beam 72
traveling along the optical path 73 which is parallel with
. the collimating lens optical axis 91. The light beam 72
CA 02520651 1998-10-05
22
is optically coupled into the interlayer 34 and then into
the windshield 18 along the optical path 73. The light
beam 72 travels through the windshield 18, continuing at
an angle of approximately forty-five degrees and is
reflected by the outer surface 32 of the windshield 18 at
the sensing region 74. The reflected beam passes back.
through the windshield 18 along the optical path 73 at a
forty-five degree angle with respect to the windshield
surface. The collimated light beam 72 travels through the
focusing body 43 and the focusing lens 44. The focusing
lens focuses the collimated beam 72 onto the surface of
the detector 58. If moisture 76 has accumulated on the
windshield in the sensing region 74, not all of the
collimated light beam 72 will be reflected back to the
focusing body 43 and the detector 58 will produce a signal
representative of the amount of light which is detected.
Although the detector generally has the highest sensitivity
when the light beams are perpendicular to the circuit
board 26, any light beams 72 within the acceptance angle
of the detector 58 will be detected. The signal
processing circuitry 59 receives the detector signal and
interprets the change in the signal as the presence of
moisture and controls the wipers accordingly.
For proper operation, the collimating lens 40 must be
positioned with respect to the emitter 56 so that a
sufficient amount of the light rays 65 which strike the
lens aperture 82 will be collimated. Referring again to
Fig. 4, the angle of a line intersecting the surface of
the collimating lens 40 with respect to the windshield is
shown at 6x. Values of ax vary over the surface of the
collimating lens. As mentioned above, it is preferable
that the rays of the collimated light beam 72 travel
within the windshield 18 at an angle 6~ of forty-five
CA 02520651 1998-10-05
23
degrees with respect to the windshield surface 30. In
order for the collimating lens to refract the emitter rays
to the required forty-five degree angle, it may be shown
by manipulating Snell's law that:
6X = arctan [(sin(8E) - n ~ sin(~))/(cos(~) - n ~ cos(Q; )))
Where n is the refractive index of the coupler 29.
The coupler 24 is preferably molded from polycarbonate,
having a refractive index of n=1.57 at 880 nM.
Alternatively, the coupler may be fabricated from glass,
acrylic, or some other clear material. From this
equation, it may be shown for example that for an emitter
angle AE of 10 degrees, a collimating lens surface angle of
76 degrees is required. At such a steep angle, about half
of the intensity of the beam from the emitter is reflected
off of the collimating lens surface, and thus does not
enter the windshield 18. The reflection increases
dramatically at even smaller emitter angles. Therefore,
this relationship between the emitter angle and the
collimating lens surface angle establishes a lower limit
on the distance between the collimator lens 40 and the
emission axis 66. Similarly, the same lower limit is
placed on the distance between the focusing lens 44 and
the detection axis 68. Light rays passing through the
focusing lens 44 closer than about 10 degrees to the
detection axis 68 will internally reflect off the inside
surface of the focusing lens 49 and reduce the intensity
of the focused beam that reaches the detection surface 67.
Other effects establish an upper limit on the
distance between the emission axis 66 and the collimator
lens 40. As 8e increases, obliquity reduces the strength
CA 02520651 1998-10-05
29
of the emitter beam which varies according to cos 0E as
described above. Also, at values of 8E of about fifty
degrees, the emitted light rays are shadowed by emitter
case 60. Thus, the range of angles which may be usefully
S coupled into the windshield 18 is confined to emitter
angles between about 10 and about fifty degrees. Again,
at smaller angles, too large a fraction of the beam is
reflected off of the collimator lens surface. At larger
angles, obliquity reduces the strength of the emitted
light, and the light is shadowed by the case of the
emitter. Within this range of emitter angles, effects of
reflection and obliquity roughly cancel. Thus, the
emitted light rays are reasonably uniform within the
prescribed range of emitter angles. This restriction of
the emitter beam carries a further advantage in that it
allows the design to fit within the S mm height
requirement of the coupler. A wider range of emitter rays
would require a taller coupler. Similarly, light rays
traveling in excess of about fifty degrees from the
detection axis 68 will be received poorly by the detector
58 due to high obliquity. As with the collimator 37,
restricting the angles of the rays received from the
focusing lens 44 by the detector 58 permits a shallow
coupler 29 design.
The surface of the collimating lens 40 is shaped to
allow the collimating lens to collimate a large portion of
the light rays traveling from the emitter when the
emission axis 66 forms an oblique angle with respect to
the optical axis 41. Preferably, the surface of the
collimating lens is a continuous, convex refractive
surface, although the surface may be segmented as
described below. The appropriate shape of the lens
surface can be determined using an optical design software
CA 02520651 1998-10-05
TM
system, such as the Zemax system by Focus Software in
Tucson, AZ. The resulting surface shape is best
represented by a polynomial asphere. The surface is given
by a sag function, which generates the distance z between
5 the surface and the radius from the optical axis. That
surface may be, for purposes of illustration:
z= ( c r= ) / ( 1 +J1- + c c r ) +a, rz +az r° +a~ r6 c~ ~ + . . .
Where:
Coefficient Value
r infinity
c 0
a, 0.22631484
aZ -0.018779505
a, 0.0010712278
a, 0
This method of describing an aspheric lens is
familiar to those skilled in the art of optical system
design. Alternatively, a spherical lens of radius 3.163
mm may be substituted, however, aberration is induced
which may reduce the intensity of the light transmitted by
the lens. The values given will allow slight divergency
of the collimated beam, easing the tolerance requirements
of the emitter placement.
Although only one optical path is required to make a
functioning moisture sensor, a single optical path may
provide a sensing area of inadequate surface area for
smooth operation of the wipers. Referring now to Fig. 8,
CA 02520651 1998-10-05
26
an alternative embodiment of the present invention is
provided with a different arrangement of optical components
providing multiple optical paths. The sensor 100 of the
alternative embodiment includes a first and second emitter
156a and 156b, and a first and second detector 158a and
158b, mounted to a circuit board device surface (not
shown) in a manner similar to that described above. The
first emitter 156a is located on the circuit board (not
shown) at a first corner 102a of a square 104 and the
second emitter 156b is located on the circuit board at a
second corner 102b opposite the first corner 102a. The
first and second emitters 156a and 156b include emission
axes (not shown) similar to the emission axis 69 of the
emitter 56 shown in Fig. 9. The first detector 158a is
located on the circuit board at a third corner 102c of the
square 104 and the second detector 158b is located on the
circuit board at a fourth corner 102d opposite the third
corner 102c. The first and second detectors 158a and 158b
include detection axes (not shown) similar to the
detection axis 68 of the detector 56 shown in Fig. 4.
The circuit board is mounted in a housing 28 shown in Fig.
3 in a manner similar to the circuit board 26 described
above.
The sensor 100 includes a coupler 106 having a
mounting surface (not shown) which is mounted to the
windshield in a manner similar to the coupler 29 described
above. The housing 28 is attached to the coupler 106 in a
similar manner as coupler 29 described above. The coupler
106 includes a first collimator 108a located adjacent the
first emitter 156a at the first corner 102a when the
housing 28 is attached to the coupler 106. The coupler
CA 02520651 1998-10-05
27
106 also includes a second collimator 108b located
adjacent the second emitter 156b at the second corner 102b
when the housing 28 is attached to the coupler 106. Each
collimator 108a and 108b includes two collimating bodies
109 and two collimating lenses 110. The two collimating
lenses 110 abut each other so that their optical axes 111
form an approximate ninety degree angle when viewed as
shown in Fig. 8. The collimating lenses 110 are
preferably formed integrally with the collimating bodies
109, although separate lenses may be disposed adjacent
each collimating body as described above.
Each of the collimating lenses 110 are similar to the
collimating lens 40 described above and to avoid
duplication shall not be described in such detail. Each
collimating lens 110 has a physical center, an optical
center, and an optical axis similar to the physical center
84, optical center 86, and optical axis 41 of the
collimating lens 40 as shown in Figs. 9, 5 and 6,
collimating lenses 110 of the first collimator 108a are
disposed adjacent the first emitter 156a such that each of
the optical axes forms an oblique angle with respect to
the emitter axis described above. The collimating lenses
110 of the second collimator 108b are disposed adjacent
the second emitter 156b such that each of the optical axes
forms an oblique angle with respect to the emitter axis
described above. The surface of the collimating lenses
110 are formed similarly to the collimating lens 40
described above such that the optical center is offset
from the physical center for the reasons described above.
CA 02520651 1998-10-05
28
The coupler 106 also includes a first focuser 114a
located adjacent the first detector 158a at the third
corner 102c when the housing 28 is attached to the coupler
106. The coupler 106 further includes a second focuser
114b located adjacent the second detector 158b at the
fourth corner 102d when the housing 28 is attached to the
coupler 106. Each focuser 114a and 114b includes two
focusing bodies 115 and two focusing lenses 116. The two
focusing lenses 116 abut each other so that their optical
axes 117 form an approximate ninety degree angle when
viewed as shown in Fig. 8. The focusing lenses 116 are
preferably formed integrally with the focusing bodies 115,
although separate lenses may be disposed adjacent each
focusing body as described above. A corner of each
collimating lens 110 and focusing lens 116 is removed to
allow the juxtaposition but the performance of the lenses
is not adversely affected.
Each of the focusing lenses 116 are similar to the
focusing lens 44 described above and to avoid duplication
shall not be described in such detail. Each focusing lens
116 has a physical center, an optical center, and an
optical axis similar to the physical center 92, optical
center 94, and optical axis 95 of the focusing lens 44 as
shown in Figs. 2 and 7. The focusing lenses 116 of the
first focuser 114a are disposed adjacent the first
detector 158a such that each of the optical axes forms an
oblique angle with respect to the emitter axis described
above. The focusing lenses 116 of the second focuser 114b
are disposed adjacent the second detector 158b such that
each of the optical axes forms an oblique angle with
respect to the emitter axis described above. The surface
CA 02520651 1998-10-05
29
of the focusing lenses 116 are formed similarly to the
focusing lens 44 described above such that the optical
center is offset from the physical center for the reasons
described above.
Four optical paths 173a, 173b, 173c, and 173d are
provided. The first optical path 173a extends from the
first emitter 156a through a collimator lens 110 and
collimator body 109 of the first collimator 108a, into the
windshield at a forty-five degree angle with respect to
the inner surface to a first sensing area 179a, back
through the windshield at a forty-five degree angle with
respect to the windshield inner surface, through a
focusing body 115 and focusing lens 116 of the first
focuser 114a to the first detector 158a. The second
optical path 173b extends from the second emitter 156b
through a collimator lens 110 and collimator body 109 of
the second collimator 108b, into the windshield at a
forty-five degree angle with respect to the inner surface
to a second sensing area 174b, back through the windshield
at a forty-five degree angle with respect to the
windshield inner surface, through the focusing body 115
and focusing lens 116 of the first focuser 114a to the
first detector 158a.
The third optical path 173c extends from the first
emitter 156a through a collimator lens 110 and collimator
body 109 of the first collimator 108a, into the windshield
at a forty-five degree angle with respect to the inner
surface to a second sensing area 174c, back through the
windshield at a forty-five degree angle with respect to
the windshield inner surface, through the focusing body
115 and focusing lens 116 of the second focuser 114b to
CA 02520651 1998-10-05
the second detector 158b. The fourth optical path 173b
extends from the second emitter 156b through the
collimator lens 110 and collimator body 109 of the second
collimator 108b, into the windshield at a forty-five
5 degree angle with respect to the inner surface to a fourth
sensing area 179d, back through the windshield at a forty-
five degree angle with respect to the windshield inner
surface, through the focusing body 115 and focusing lens
116 of the second focuser 114b to the second detector
10 158b.
In operation, the emitters 156a and 156b emit
diverging light rays into a hemisphere so that each of the
adjacent collimator lenses 110 receives an equal amount of
light. The two collimating bodies 109 and lenses 110 at
15 the first collimator 108a produce first and second
collimated light beams 172a and 172b, similar to the
collimated beam 72 described above. The first and second
collimated light beams 172a and 172b are splayed at right
angles to each other when viewed as shown in Fig. 8, and
20 each light beam travels along the first and third optical
path 173a and 173c respectively. The two collimating
bodies 109 and lenses 110 at the second collimator 108b
produce third and fourth collimated light beams 172c and
172d, similar to the collimated beam 72 described above.
25 The third and fourth collimated light beams 72c and 72d
are splayed at right angles to each other, and each light
beam travels along the second and fourth optical path 173b
and 173d respectively.
The first collimated light beam 172a is reflected by
3-0 the outside surface of the windshield at the first sensing
area 174a, back through the focusing body 115 and focusing
CA 02520651 1998-10-05
31
lenses 116 to the first detector 158a. If moisture is
present in the first sensing area on the outside surface
of the windshield, some of the collimated light beam will
not be reflected back into the focuser 114 and the first
detector 158a will emit a signal corresponding to the
change in the light detected. The signal will be
processed by signal processing circuitry (not shown)
similar to the signal processing circuitry 59 shown in
Fig. 2 and the wipers will be controlled accordingly.
Similarly, the second, third and fourth collimated light
beams will reflect off of the corresponding sensing areas,
and the first or second detector will detect any changes
in the light received. By using four sensing areas, the
moisture sensor 100 can provide improved wiper control and
enhanced visibility.
The arrangement of the optical components in the
alternate embodiment moisture sensor 100 provides a
balanced optical system because the four optical paths 102
are of equal length and equal optical efficiency. This
arrangement will compensate for differences in efficiency
between emitters 56, which may vary considerably. Both
detectors 58 will receive an equal amount of light from a
particular emitter, and the sum of the light received from
both emitters will be the same for each detector.
Referring to Fig. 9, a balanced electrical system 190
is shown for use in conjunction with the above described
balanced optical system to provide a balanced moisture
sensor system. A pulsed current source drives emitters
156a and 156b, which are preferably connected in series by
line 191. A light beam (represented by dashed lines 172a,
172b, 172c, 172d) traveling along an optical path couples
CA 02520651 1998-10-05
32
each emitter 156a, 156b to each detector 158a, 158b. Each
optical path has an equal length and a similar optical
efficiency. The detectors 158a, 158b function in current
mode, and are connected together into a common current
summing node 192. Signal processing and control circuitry
connected to node 192 detects the presence of rain. For a
perfectly balanced moisture sensor system, no current will
flow from node 192 to the signal processing and control
circuitry in the absence of rain. A balanced moisture
sensor system is desirable because it requires less
dynamic range from the signal processing circuitry and it
enhances the ability of the system to reject ambient
light.
riodern solar-control windshields, such as windshields
sold under the trademark "EZ-KOOL" commercially available
from Libby Owens Ford, Co., reduce the passage of infrared
light through the windshield. Optical moisture sensors
used on such windshields must have a high efficiency since
the windshield reduces the transmittance of the infrared
beam from the emitter to the detector. The moisture
sensor 100 described above provides an efficient sensor
capable of use with these solar-controlled windshields.
Moisture sensors as described above have been tested on
"EZ-KOOL" brand solar-control windshields using couplers
composed of polyester casting resin which produce 17
microamps per amp of emitter current which is sufficient
for typical signal processing circuitry. The moisture
sensor provided a combined sensing area of 57 square
millimeters using only two emitters and two detectors, and
production versions will probably have even greater sensing
areas.
CA 02520651 1998-10-05
33
Referring now to Fig. 10, an alternate embodiment of
a collimator lens is shown using a segmented lens, or
Fresnel lens 202 rather than the continuous, convex lens
90 discussed above. The Fresnel lens 202 may also be used
as the focusing lens in place of the continuous, convex
focusing lens 94 discussed above. Due to the similarity
between the collimating lens and the focusing lens as
discussed above, only a Fresnel lens collimator is
discussed. A similar Fresnel lens can be used for the
focuser which performs similarly to the continuous, convex
lens focuser 90 described above.
The Fresnel collimator lens 202 has the advantage
that the lens region, and thus the moisture sensor as a
whole, may be made still thinner. The resulting thinner
coupler 24 comes at the expense of some optical
efficiency, and a somewhat more complex mold needed to
form the coupler and lenses 202. Such a lens may be
constructed by projecting the surface of the collimator
lens of Figs. 4 and 6 onto the inside surface of the
coupler 24, permitted to extend to a depth D in a modulo
operation. This results in collimator lens 202 comprised
of a number of refracting segments 204. Note that, in
contrast to the common construction of a Fresnel lens, the
plane of projection of the light rays is not orthogonal to
the optical axis, but rather angled to provide reflection
at the outer surface of the glass as described above.
Alternatively, optical design programs such as the
aforementioned Zemax may be used to generate the required
surface directly, using suitable tilt commands to achieve
the desired plane of projection. As a further method of
CA 02520651 1998-10-05
39
generating the surface, the formula derived from Snell's
law above may be employed to generate the required angles.
The disadvantage of the segmented approach is that it
creates occlusion regions, such as shown at 206.
Occlusion regions 206 occur when light rays strike a non-
useful return segment 208. Such segments are needed to
keep the geometry of the lens within depth D. The
occlusion regions 206, however, are not capable of
directing light in the desired direction and degrade the
optical efficiency of the system. The multipath
configuration of the invention, as shown in Fig. 8, is not
modified. Similarly, the attachment method is unchanged.
The Fresnel approach may be fabricated with many segments,
as shown, or with as few as two. Also, while it is
preferred that the projection be onto the plane of the
inside wall of the coupler, the plane of projection may be
tilted somewhat toward the optical devices. Such an
implementation would require fewer occlusion regions.
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.
