U.S. patent application number 15/003516 was filed with the patent office on 2016-05-19 for ultrasonically clearing precipitation.
The applicant listed for this patent is EchoVista GmbH. Invention is credited to David TREVETT, Patrick TREVETT.
Application Number | 20160137167 15/003516 |
Document ID | / |
Family ID | 49119084 |
Filed Date | 2016-05-19 |
United States Patent
Application |
20160137167 |
Kind Code |
A1 |
TREVETT; David ; et
al. |
May 19, 2016 |
ULTRASONICALLY CLEARING PRECIPITATION
Abstract
A system for clearing precipitation from a window comprises one
or more transducers (1-8) fixed to the window. The transducers are
driven by a generator (13) to produce surface acoustic waves that
propagate through the window. The window may be a laminated window
such as a windscreen (10) for a vehicle. A sensing system (122) may
be used for detecting the presence of precipitation to actuate the
clearing system.
Inventors: |
TREVETT; David; (Poole,
GB) ; TREVETT; Patrick; (Poole, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
EchoVista GmbH |
Seligenstadt |
|
DE |
|
|
Family ID: |
49119084 |
Appl. No.: |
15/003516 |
Filed: |
January 21, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/EP2014/065559 |
Jul 18, 2014 |
|
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15003516 |
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Current U.S.
Class: |
15/3 |
Current CPC
Class: |
B60S 1/0818 20130101;
B08B 7/028 20130101; B60S 1/02 20130101 |
International
Class: |
B60S 1/02 20060101
B60S001/02; B08B 7/02 20060101 B08B007/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 22, 2013 |
GB |
1313061.2 |
Claims
1. A system for clearing precipitation from a window, the system
comprising: a window; one or more transducers; and a generator for
generating an ultrasonic drive signal to drive the one or more
transducers, wherein the one or more transducers are fixed to a
surface of the window and driven by the generator to produce waves
such that the waves propagate substantially only through a surface
region of the window.
2. A system according to claim 1, wherein the window is a laminated
window comprising a laminate layer sandwiched between a top and
bottom layer of glass, and wherein the generator is configured to
drive the one or more transducers to produce waves such that the
waves propagate substantially only through the top layer of
glass.
3. A system according to claim 2, wherein the generator is
configured to drive the one or more transducers to produce waves
such that the waves propagate substantially only through a surface
region of the top layer of glass.
4. A system according to claim 3, wherein the generator is
configured to drive the one or more transducers to produce waves
such that the waves penetrate into the top layer of glass to a
depth of less than 3 mm from the surface of the window.
5. A system according to claim 1, wherein each of the one or more
transducers are configured to operate in the frequency range of 400
kHz to 1.5 MHz.
6. A system according to claim 1, wherein the generator comprises a
pulse generator and wherein the waves comprise pulsed waves.
7. A system according to claim 1, wherein the generator is
configured to cause the one or more transducers to sweep through a
range of frequencies.
8. A system according to claim 1, wherein the generator is
configured to cause the transducer to produce waves which are
frequency modulated.
9. A system according to claim 1, wherein the generator is
configured to drive the one or more transducers to produce waves
such that mode conversion of the waves ultrasonically propels the
precipitation.
10. A system according to claim 1, wherein the generator is
configured to drive the one or more transducers to produce waves
such that mode conversion of the waves ultrasonically vaporises the
precipitation.
11. A system according to claim 1, further comprising an impedance
matching circuit configured to match an impedance of the generator
to an impedance of the one or more transducers.
12. A system according to claim 1, wherein the surface of the
window is coated with a hydrophobic coating.
13. A system according to claim 1, further comprising an impedance
matching circuit configured to match an output impedance of the
generator with an input impedance of the one or more
transducers.
14. A system according to claim 1, further comprising: a control
system having a sensor configured to sense ultrasonic waves emitted
by one or more of the said transducers for detecting the presence
of precipitation; and a controller responsive to the sensor for
controlling the operation of the generator.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation under 35 U.S.C. .sctn.120
of International Application No. PCT/EP2014/065559, filed Jul. 18,
2014, which claims priority to United Kingdom Application No. GB
1313061.2, filed Jul. 22, 2013. The entire contents of the
above-referenced patent applications are hereby incorporated by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to ultrasonically clearing
precipitation from a window. Embodiments of the invention relate to
clearing precipitation from a laminated windscreen of a
vehicle.
[0004] 2. Description of the Related Technology
[0005] Conventionally, a driver of a vehicle uses wipers to remove
precipitation from the one or more windows to maintain a clear view
through the window. However, the wipers are rubber or plastic and
assembled to a metal fixing with a motor and the lifetime of the
wipers depend on how long it takes for the parts to perish.
Commercially available products such as RainX.RTM. can be applied
to the surface of a window for easy cleaning of the window.
However, since the wipers contact the surface of the window they
also remove products applied to the window surface when they are in
use and further application of the product is then necessary.
SUMMARY
[0006] According to a first aspect of the invention, there is
provided a system for clearing precipitation from a window, the
system comprising a window, one or more transducers, and a
generator for generating an ultrasonic drive signal for the one or
more transducers, wherein the one or more transducers are fixed to
the surface of the window and driven by the generator to produce
surface acoustic waves, wherein the surface acoustic waves
propagate substantially only through a surface region of the
window.
[0007] There is provided a system according to the first aspect of
the invention, comprising a control system having a sensor arranged
to sense ultrasonic waves emitted by one or more of the said
transducers for detecting the presence of precipitation, and a
controller responsive to the sensor for controlling the operation
of the system or apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] Various features and advantages of the present disclosure
will be apparent from the detailed description which follows, taken
in conjunction with the accompanying drawings, which together
illustrate, by way of example only, features of the present
disclosure, and wherein:
[0009] FIG. 1A is a schematic illustration showing a vehicle having
transducers located in a peripheral region of a windscreen.
[0010] FIG. 1B is a schematic illustration showing a windscreen and
electronics for operating the transducers.
[0011] FIG. 2 is a schematic illustration showing a cross-section
through a vehicle windscreen with a transducer bonded to its
surface.
[0012] FIG. 3A is a schematic illustration showing a transducer
emitting surface acoustic waves into precipitation on the surface
of a windscreen.
[0013] FIG. 3B is a schematic illustration showing an angled
windscreen with a transducer emitting surface acoustic waves into
precipitation on the surface of the windscreen.
[0014] FIG. 3C is a schematic illustration showing droplet
propulsion and atomisation of precipitation using surface acoustic
waves.
[0015] FIG. 4 is a schematic illustration showing the contact angle
of precipitation for hydrophobic and hydrophilic coatings.
[0016] FIG. 5 is a schematic illustration showing surface acoustic
waves emitted from a transducer in a pulsed mode.
[0017] FIG. 6 is a schematic illustration showing a method for
matching impedance lines in the system.
[0018] FIG. 7A is a schematic diagram showing a design for an
inter-digital transducer for operation at a frequency of 500
kHz.
[0019] FIG. 7B is a schematic illustration showing the wavelengths
of different types of waves at a frequency of 500 kHz through glass
3 mm thick.
[0020] FIG. 7C is a schematic illustration of different wave
types.
[0021] FIG. 8 is a graph showing the calculated wave speeds as a
function of frequency for different types of waves traveling
through 3 mm thick automotive glass.
[0022] FIG. 9A is a schematic illustration showing a design for a
transducer for operation at a frequency of 1 MHz.
[0023] FIG. 9B is a schematic illustration showing a design for a
transducer for operation at a frequency of 500 kHz.
[0024] FIG. 10 is a schematic illustration of a visor having
transducers attached to a visor.
DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS
[0025] In the description, the term "acoustic wave" is used to
refer to a wave produced by a transducer that is being driven; it
does not refer to the frequency of a wave being in the audible
acoustic range for people.
[0026] Precipitation includes rain, sleet, snow, ice, drizzle,
mist, fog, hail or other types of precipitation. When precipitation
falls onto a window of a vehicle, for example the windscreen, it
impedes the view for a driver.
[0027] When precipitation falls onto a window it is attracted to
the surface of the window by surface tension. The precipitation,
for example liquid water, can form many droplets across the window
surface. The applicant's research has found that each of the many
droplets will be a different size, have a different diameter and
have a different shape which may be regular or irregular. An
example of a droplet size may be approximately 0.4 millilitres (ml)
having a diameter of approximately 1 centimetre (cm) for example
but could be much smaller. The front window or windscreen of a
vehicle such as a car is inclined, for example, at an angle of
34.quadrature.. The angle may be greater. Some vehicles have
windscreens inclined at a greater angle, for example 35.quadrature.
or more. A rear window may be inclined at a larger angle than the
front windscreen. Large liquid droplets will run down the
windscreen faster than small liquid droplets due to their larger
mass and greater influence under gravity. Other effects such as
surface tension of the droplet and air flow over the surface can
affect how a droplet moves across the surface. Surface tension may
affect small droplets more than larger droplets. As the droplet
size decreases, the internal pressure of the droplet increases. For
example, smaller droplets require a larger angle before running off
the windscreen compared to larger droplets. In an illustrative
embodiment, the merging of droplets using ultrasonic waves during
operation of the system may be useful since the larger droplets may
be more easily cleared from the windscreen compared to smaller
droplets due to their larger mass and influence under gravity and
airflow. In addition, the surface tension effects without air flow
are constant and may be independent of temperature.
[0028] Embodiments of the present invention use ultrasonic waves to
remove precipitation from the surface of a window. In general,
ultrasonic waves are acoustic waves with a frequency above 100
kiloHertz (kHz) and up to around 50 MegaHertz (MHz) or higher. The
ultrasonic waves used in embodiments of the invention have a
frequency in the range of about 400 kHz to 1.5 MHz. Transducers are
used to produce acoustic waves in a range of frequencies. The
transducers do not operate at a single frequency but instead
operate across a range of frequencies (i.e. bandwidth) either side
of a central frequency. The operating frequency of a transducer is
to be understood as relating to the main operating frequency or
central frequency of the transducer within the bandwidth of
frequencies.
[0029] In embodiments of the invention, transducers are bonded to a
window surface and driven to emit acoustic waves having ultrasonic
frequencies. The range of frequencies of acoustic waves emitted
from the transducer are dependent on the design of the
transducer.
[0030] FIG. 1A shows an embodiment of the invention wherein
transducers (1-8) are positioned along the peripheral area (9) of a
windscreen (10) of a vehicle. The transducers are bonded or glued
to the periphery of the windscreen. There are limitations on the
attachment locations of the transducers for droplet removal from a
windscreen. The transducers must be positioned so as not to
obstruct the view of the driver or other occupants of the vehicle.
The position of the transducer on the windscreen may influence the
efficiency of the transducer in terms of ability to clear
precipitation from the windscreen. Any suitable number of
transducers may be used for removing precipitation from the
windscreen. There may be a plurality of individually spaced
transducers along the peripheral area of one or more sides of the
window or windscreen. The transducer may also be arranged to form a
continuous strip either at the sides or top and bottom of the
windscreen, or both the sides and the top and bottom. An
inter-digital transducer (IDT) may be used.
[0031] FIG. 1B schematically shows the driving electronics for the
transducers. The transducers are connected to a driving electronics
system via wiring (123) where the driving system comprises a power
supply (11), a control unit (12), a frequency generator (13), a
power amplifier (14), and a pulse generator (15). The power supply
may be a 12V or 24V vehicle battery. The driving system may be
controlled by a rain sensor (122) and/or other manual controls
(121). The rain sensor can be a propriety item or can be formed
using the transducers already part of the system with the
appropriate additional circuitry.
[0032] The transducers are bonded to the windscreen and energised
or driven by the driving electronics. Suitable bonding agents are
commercially available and are used to fix each transducer to the
windscreen. The bonding agent is used to form a uniform bonding
layer between each transducer and the surface of the window. In an
illustrative embodiment of bonding the transducers to the
windscreen, the bonding agent is mixed in a vacuum to prevent air
bubbles forming within the bonding layer. If gas bubbles are
present in the bonding layer, ultrasonic frequencies will be highly
attenuated and it could impede the efficiency of the transducers.
An example of a suitable bonding agent is epoxy resin. In an
embodiment, the epoxy resin may be prepared or provided in a vacuum
bag ready for mixing prior to application to the windscreen and
transducers, wherein the vacuum bag comprises two compartments
separated by a barrier and wherein the barrier is broken in order
to mix the epoxy within the vacuum bag. In an embodiment, the
bonding layer is thin to minimise the refraction of sound through
the multi-layered system of the glass, bonding layer and
transducers. The bonding agent may have other special properties
such as acoustically matching the impedance of the bonding agent to
the impedance of the window surface to which they are being
attached, in order to efficiently couple or transmit acoustic waves
into the window by minimising unwanted reflections from the window
surface. Each transducer comprises a set of electrodes as the
active elements next to a piezoelectric layer, and a ground
electrode. In some embodiments the transducers are attached with
one electrode (for example, a ground electrode) facing away from
the window and one electrode (for example, a cut electrode) adhered
to the outer surface of the windscreen. In an embodiment, the
transducers are bonded to the surface such that the transducer
surface is parallel with the surface of the windscreen or other
surface to which they are being attached.
[0033] Each transducer is driven by the frequency generator 13 and
power amplifier 14 of FIG. 1B to emit a range of frequencies. The
range of frequencies or bandwidth of frequencies emitted may be
fixed and chosen by the designer. Alternatively the frequencies or
bandwidth may be chosen by an operator. The operator may be, for
example, a driver of the vehicle in which the transducer is
installed. For example, the driver may have the option to select
the range of frequencies emitted according to the amount of
precipitation to be removed from a window, such as for heavy rain
or light drizzle conditions. This can also be conducted
automatically using a rain sensor. This may take the form of a dial
or buttons for the driver to select within the vehicle according to
the conditions. Driving the transducer causes the transducer to
emit acoustic waves. The transducer design may determine the full
range of operating frequencies that the transducer is capable of
being driven to produce. The acoustic waves emitted depend on
factors such as transducer design, contact angle between the
transducer and surface to which the transducer is bonded, driving
power, among other factors. The frequency and dimensions of a
transducer may be chosen to affect the spread of the emitted
acoustic beam from the transducer, for example, the higher the
frequency selected the more focussed the emitted acoustic beam may
be. The wavelength of each type of acoustic wave emitted is a
function of the spacing between the electrodes of the
transducer.
[0034] The transducers can be driven in continuous or pulsed mode.
A pulsed generator can be used to drive the transducers in a pulsed
mode. In pulsed mode the acoustic waves will be emitted from the
transducer in pulses. The frequency generator may provide frequency
modulated signals to produce frequency modulated acoustic waves. In
an example, the frequency of the waves is driven through a range of
frequencies by frequency sweeping.
[0035] Each wave consists of nodes and antinodes--nodes are regions
of a wave having minimum amplitude and antinodes are regions of a
wave having maximum amplitude. Standing waves occur when there is a
stable superposition of waves in a system. For example, a
transmitted wave and reflected wave may combine to form a standing
wave due to cancellation or amplification of their frequency
components. In an example, a wave traveling along the surface of
the window may be reflected at the window edge due to an acoustic
impedance mismatch between the window material and the surrounding
medium. The reflected wave can interfere with the wave traveling in
the opposite direction such that the phases of the two waves cancel
each other out or combine to cause a standing wave to form. The
applicant's research has found that, in an example, droplets
sitting on a windscreen surface will feel the influence of acoustic
waves traveling through or along the windscreen. The droplets may
be observed to vibrate or move along the windscreen at different
speeds which may depend upon the positions of nodes or antinodes of
the waves traveling through or along the windscreen. When the
transducers bonded to the periphery of the windscreen are driven,
there may be a distribution of ultrasonic vibration in the
windscreen, for example, the presence of maxima and minima
corresponding to a spatial interference pattern. Droplets moving at
a greater speed compared to other droplets may be caused by regions
of the windscreens at which antinodes are located or areas close to
where the transducers are located. Vibrating droplets or slow
moving droplets may be located at or near a node on the windscreen
or further from where the transducers are located. Droplets located
close to the transducers may experience a direct sound field
wherein surface acoustic waves (SAWs) are emitted and encounter a
droplet before they have been reflected somewhere in the
windscreen. Droplets located at greater distances from where the
transducers are located may be mainly subjected to a reverberant
energy field wherein ultrasonic waves may encounter the droplet
from all directions or may be reflected at boundaries before
encountering a droplet. Using pulsed energy may reduce the level of
the reverberant field, such as for the embodiment of FIG. 5.
[0036] In other examples, droplets may vibrate and cause smaller
droplets to combine with other/separate droplets to form larger
droplets, which may then run off the surface of the windscreen
removing the precipitation.
[0037] FIG. 2 schematically shows a cross-section through an
example of the windscreen (10) of the vehicle of FIG. 1A. The
windscreen consists of a laminate layer (20) sandwiched between two
sheets of glass (21-22). The glass is suitable for automotive use.
Due to current safety legislation measures, it is required that a
vehicle windscreen be laminated, where a polyvinyl butyral (PVB)
laminate is compressed between two layers of annealed glass. A
nominal amount of adhesive may be sprayed onto the glass surface
and heat applied to compress the laminate layer between the layers
of glass. This reduces the problem associated with the destruction
of a single layer of tempered windcreen glass on impact, such as
during an accident, which were previously used. The laminated glass
also serves to contain the passenger airbag within the cabin area
of the vehicle in such circumstances. The laminate layer is a
reinforcing layer that may be 0.38 millimetres (mm) thick and the
sheets of glass either side of the laminate layer may each be 3 mm
thick such that the total thickness of the windscreen may be around
6.4 mm thick. There may or may not be an optional coating layer
(23) on the top surface of the windscreen. A transducer (1-8) is
bonded to the windscreen near the edge of the windscreen or in a
peripheral region of the windscreen. A bonding layer (24)
attaches/fixes the transducer to the surface of the windscreen. The
transducers may be affixed to the windscreen and concealed from
external view. The transducers may be hidden below a rubber or
plastic seal (25) which surrounds most conventional windscreens and
runs along the periphery of the windscreen. The transducers are
positioned so that the operation of the transducers is not affected
by the presence of the rubber seal. The windscreen has an outer
region (27) of the windscreen which is exposed to the weather, and
an inner region (26) which relates to the interior of the vehicle
to which the windscreen is fitted.
[0038] FIG. 3A is a schematic showing a transducer of FIG. 1A, 1B
or 2 bonded to the surface of a windscreen which has precipitation
on its surface. In this example a water droplet (30) is present on
the surface of the windscreen. The transducer is driven to produce
waves (31) that travel (39) along the surface of the windscreen as
shown, also referred to as SAWs. The frequencies of the SAWs
emitted are in the range 400 kHz to 1.5 MHz, and preferably the
main operating frequency is 1 MHz. Each transducer may have a main
operating frequency within a bandwidth of frequencies. For example,
a transducer designed to have a main operating frequency of 1 MHz
may also be driven to operate at 500 kHz within its bandwidth of
frequencies. However, a transducer having a main operating
frequency of 1 MHz that is driven at 500 kHz may not perform as
efficiently as a transducer having a main operating frequency of
500 kHz and driven at 500 kHz. The SAWs are emitted from the
transducer. The laminate layer within a vehicle windscreen is
highly attenuating to ultrasonic waves and causes a damping effect.
Therefore the ultrasonic waves may be confined to a region at the
surface such that they do not penetrate deeply into the glass or
the laminate layer. The wave frequency and mode of the transmitted
wave may be chosen so as not to interact substantially with the
plastic or laminate layer. Therefore, the SAWs are coupled to the
surface of an object, or windscreen, and may not penetrate into the
laminate layer within the windscreen. Suitable waves for this
application may include Lamb waves, Rayleigh waves or other
shear-type waves. Other types of waves are likely to be attenuated
and dissipate their energy into the laminate layer. In addition to
SAWs traveling at the surface of the windscreen, other waves (32)
may also be emitted into the body of the windscreen. The other
waves emitted may include longitudinal or shear-type waves. In some
embodiments, the launch angle of the waves into the top layer of
automotive glass may be chosen to produce the desired type of
ultrasonic wave traveling through the glass.
[0039] A calibration of the transducers may be performed to
optimise the operating efficiency of the system.
[0040] The SAWs will be coupled to the surface of the windscreen
and on reaching the edge of the windscreen will be partially
reflected (33) back into the windscreen along its surface as shown
in FIG. 3A. The reflections will be due to an acoustic impedance
mismatch between the glass material of the windscreen and the
medium of the outer region or peripheral region (34) of the
windscreen. For the frequencies of interest in the range 400 kHz to
1.5 MHz, the ultrasonic waves will be highly attenuated through
air.
[0041] The applicant's research has found that when the SAWs
encounter a water droplet, some of the wave energy will be
transferred to the droplet via mode conversion (35) and
longitudinal waves (37) may travel through the droplet. When
ultrasonic waves or SAWs are applied to water droplets, the
droplets may be atomised (also known as jetting or vaporisation).
There are three progressive stages that can be observed including
streaming (36), propulsion and atomisation.
[0042] The contact angle (38) of the water droplet to the surface
will affect the angle at which the SAWs will encounter the droplet.
This angle has been labelled as .quadrature.R and may relate to the
Rayleigh angle of Rayleigh waves traveling along the surface of the
windscreen. The SAWs may be Rayleigh waves or Lamb waves. If the
SAWs are Lamb waves, these may relate to anti-symmetric Lamb waves
or a flexural mode. As the SAWs enter the droplet, mode conversion
takes place and longitudinal waves are transmitted into the water
droplet. Due to the mode conversion and transfer of energy the SAW
amplitude decreases and may also be referred to as a "leaky" wave
(43). The longitudinal waves transmitted into the droplet causes
streaming to occur within the droplet whereby internal rotational
mixing and some cavitation takes place. The next stage is exhibited
by "propulsion" of the droplets, where they move rapidly at right
angles to the transducers or IDT electrodes.
[0043] FIG. 3B is a schematic showing the front windscreen on the
vehicle of FIG. 1A or 1B which is inclined at an angle, in this
embodiment the angle is .quadrature.=34.quadrature.. In some
embodiments of the invention the surface of the windscreen may be
curved. The applicant's research has found that when droplets are
propelled (40) to move along a surface, each water droplet has a
leading edge (41) at the front and a trailing edge (42) at the
back. The leading edge has a different shape to the trailing edge.
The contact angle of the droplet with the surface is different for
the leading edge and trailing edge. As discussed above, the contact
angles at the edges of the droplet depend upon the surface
treatment of the surface on which the droplets are sitting or
moving across. For water droplets moving on an inclined surface the
water droplet will have a different contact angle (38) with the
surface at the advancing edge and trailing edge when compared to
the contact angles of a water droplet on a flat surface. As shown a
SAW emitted from the transducer is coupled into the droplet at the
angle .quadrature.R. In this example the SAW encounters the droplet
at the trailing edge, however other SAWs may encounter the droplet
at the leading edge due to being emitted from a different location
around the periphery of the windscreen, or due to reflections. The
other SAWs encountering the droplet at the leading edge will be
coupled into the droplet at a different angle than
.quadrature.R.
[0044] In the example of FIG. 3C, a droplet is shown as being
vaporised or atomised (44). SAWs traveling along the windscreen are
able to transfer energy into the droplet causing the droplets to
vibrate or resonate (45) or move along the surface. With enough
energy the droplets may resonate, causing internal rotational
mixing and cavitation, and burst into many smaller droplets (46)
thereby being atomised. As such, the precipitation is cleared or
removed from the surface of the windscreen.
[0045] The internal pressure changes within the droplet cause the
droplet shape to change. The droplet shape changes by becoming
skewed (48) along the direction in which the SAWs travel as shown
in FIG. 3C. In this Figure, the droplet is shown on a level surface
to highlight the effect that the SAWs have on the shape of the
droplet. However, in other embodiments the droplet shape may also
be effected by the angle of inclination of the surface. When SAWs
are applied to the droplet, firstly, the droplets are seen to
"stream" wherein the internals of each droplet rotates, possibly
caused by cavitation. Secondly, the droplets undergo propulsion and
may be observed to vibrate or be propelled to move across the
surface of the windscreen. Applying SAW energy may allow the
droplet to overcome surface tension and move or become propelled
along the surface of the windscreen. The direction of propulsion of
the droplet may be in the same direction in which the SAWs travel.
The driving power of the transducers may be increased so as to
effectively propel the droplets across the surface of the
windscreen. During the "propulsion" process the droplets may
collide with one another and form larger droplets, which due to
their greater mass and reduced surface tension, may move more
rapidly across the windscreen. The mechanism of propulsion may be
due to streaming associated with the creation and collapse of
cavities inside the liquid or due to "bubble activity" (47) arising
from circulation of the liquid inside the droplet. Thirdly, the
droplets may be atomised wherein the droplet is split into much
smaller droplets. In this way precipitation may be cleared from a
windscreen using SAWs.
[0046] The applicant's research has found that since each droplet
will have a different size with varying diameters, each droplet
will have a different resonant frequency and may vibrate at a
preferred frequency within a range of frequencies. When a SAW with
the correct frequency to match that of the resonant frequency of
the droplet is encountered, the droplet will resonate in a high
energy state. To "hit" the resonant frequency of each droplet the
transducers may be driven to sweep through a range of frequencies.
The many SAWs traveling through the windscreen, including reflected
waves, may combine via superposition to locally increase their
amplitude at some frequencies. The driving frequency of the
transducers can be varied by sweeping through a range of
frequencies. In an embodiment, this enables the SAW frequency to
"hit" the resonant frequency of a droplet. In this way, the
differently sized droplets may be vaporised. As the droplet size
decreases, its internal pressure increases. It may therefore be
easier to vaporise smaller droplets than larger droplets.
[0047] As mentioned earlier, a droplet size may be around 0.4 ml
with a diameter of around 1 cm. For droplet resonance to occur an
integer number of half-wavelengths may need to fit within the
droplet diameter. For example, for a SAW having a wavelength of 1
cm and a droplet diameter of 1 cm, there will be two
half-wavelengths of the SAW that may fit within the droplet to
cause resonance of the droplet.
[0048] In other examples the precipitation may not be a water
droplet but may instead be hail, snow or a layer of ice or other
precipitation. For precipitation other than water droplets such as
for rain, the application of the ultrasonic waves may vary to
achieve a similar effect of clearing the precipitation from a
window. For example, ultrasonic waves may be employed to break down
a sheet of ice or frost formed over the windscreen during
winter.
[0049] As discussed, to further improve the process of clearing
precipitation, the outer surface of the windscreen can be treated
with an optional coating. FIG. 4 is a schematic showing the effect
that an optional coating layer (23) on the surface of the
windscreen may have on a droplet on its surface. This optional
coating will be located on the surface of the windscreen at the
outer region. For example, the optional coating (23) on the outer
surface may be a hydrophobic coating (49) which may be added by
spraying or wiping onto the surface. A hydrophobic coating causes
any water droplets on the layer to be repelled from its surface so
as to minimise the contact area of the water droplet with the
hydrophobic coating. In an illustrative embodiment a hydrophobic
coating layer on the windscreen is preferred to aid the removal or
clearing of precipitation from the windscreen surface. In an
example of a transducer emitting SAWs across a hydrophobic coating,
the contact angle .quadrature.R1 of SAWs with the droplet will be
large. Alternatively, the optional coating (23) may be a
hydrophilic coating (50). If a hydrophilic coating (50) were
applied to the surface of the windscreen, the contact area of the
water droplet will be much larger and the contact angle
.quadrature.R2 of SAWs with the droplet will be small. For removing
precipitation from the surface of the windscreen, a hydrophobic
coating layer is preferred over a hydrophilic coating layer. Since
the contact angle of the SAWs with the water droplet is larger for
a hydrophobic layer than a hydrophilic layer
(.quadrature.R1>.quadrature.R2), there may be more efficient
mode conversion of the SAWs into the droplet and the droplet may be
propelled across the surface of the windscreen more effectively. A
hydrophobic coating reduces surface tension by changing the contact
angle between the droplet and the windscreen surface. The air flow
over the surface of the windscreen may also assist in removing
precipitation from the windscreen. The contact angle between a
droplet and a surface will also depend upon the viscosity of the
droplet and the type of material of the surface. For example,
whether the droplet is water or oil, or the surface is automotive
glass or a plastic or polycarbonate material such as are used in
visors for motorcycle helmets.
[0050] FIG. 5 is a schematic showing the transducers of FIGS. 1A to
3B operating in pulsed mode. The transducers may be driven in
pulsed mode since pulsing the waves (51), for example at
half-second intervals, may stop the temperature of the transducers
rising too much because it reduces the build-up of heat in the
system. Thus allowing for a higher amplitude of input signal to
remove precipitation more rapidly.
[0051] Frequency modulation may be employed to more effectively
clear precipitation than amplitude modulation which may not be as
effective. The transmission efficiency may also be optimised by
acoustic impedance matching.
[0052] The power efficiency of the system may be optimised, since
about two thirds of the energy can be transferred or lost as heat.
Acoustic losses may include scattering or absorption within the
system, for example at glass impurities or defects. To prevent
heating effects, the circuitry and materials such as the
transducers on the window may be optimised for impedance
matching.
[0053] FIG. 6 schematically shows an impedance matching circuit
(60) which matches the electrical impedance of the power amplifier
(14) of the driving circuit to the impedance of the transducers
(1-8). This improves the efficiency of the power circuit or system
as a whole and reduces loss of energy from the system. There may be
unwanted reflections arising from a mismatched impedance line.
[0054] Other impedances may be matched or improved matching
obtained by minimising the acoustic impedance difference between
the transducer and the surface to which the transducer is bonded.
An anti-reflection coating can be used on the surface of the
transducer to enhance the coupling of ultrasonic waves from the
transducer into the surface to which it is bonded. The transducer
design may be optimised to minimise the acoustic impedance
mismatches between surfaces such as to maximise the coupling of
waves.
[0055] Two types of transducer design will now be discussed for
square and circular transducer designs. Many piezoelectric
transducers are commercially available in the circular form.
However, the circular design is not favoured in this application
because the circular transducer design radiates acoustic energy
equally in all radial directions. The square form is preferred
because it radiates acoustic energy in directions that are
perpendicular to its electrodes. Therefore the acoustic energy may
be more closely controlled during application to a windscreen for
removing precipitation. The cutting or shaping of a transducer may
change its resonant frequency. In an illustrative embodiment, the
electrode finger spacing of the IDT may be adjusted according to
the characteristics of the transducer and the windscreen.
[0056] FIG. 7A is a schematic diagram showing an embodiment for one
possible design for a transducer (1-8) of FIG. 1A or 1B,
appropriate for operation at 500 kHz. The transducer shown is an
inter-digital transducer (IDT). An IDT may be fabricated from
piezoelectric material or by modifying the outer electrode of a
standard piezoeletric transducer by cutting through the electrode
and leaving the piezoelectric material uncut as much as possible.
IDTs are designed to work by matching the spacing (71) of the
electrode fingers to the wavelength of the waves that may be needed
to be excited, depending on the application at hand. This may
correspond to the frequency at which the transducer resonates. In
this way it is possible to fabricate an IDT which can generate SAWs
at ultrasonic frequencies. The IDT can effectively be tuned to
match the physical constants of the outer glass layer of the
windscreen to optimise the efficiency of the system. The transducer
design shown may produce waves that penetrate to a depth of less
than 3 mm from the surface of the windscreen or windshield. This
will prevent the waves from suffering damping effects due to the
laminate layer which is located at a depth of 3 mm into the
windshield.
[0057] For the transducer in FIG. 7A, the dimensions of the
transducer are shown. The dimensions of the electrodes may be
specifically chosen to select an operating frequency for the
transducer. In this example, the diameter (72) of the transducer is
40 mm and the first (73) and second electrodes (74) are separated
by a gap (75) between the electrodes of 4 mm. The second electrode
(74) is shown to be 11 mm wide, giving an electrode finger spacing
(71) of 15 mm. The electrode shown operates at 500 kHz.
[0058] FIG. 7B is a schematic illustration showing the wavelengths
of different types of waves emitted from the transducer design of
the embodiment shown in FIG. 7A for 3 mm thick automotive glass.
For an electrode finger spacing of 15 mm, the wavelength of Lamb
waves (76) (or Rayleigh waves) emitted will be around 7.8 mm,
longitudinal waves (77) will be 10.8 mm and shear waves (78) will
be around 7.0 mm. The wavelengths of these three types of wave
emitted will be larger than the 3 mm thickness of the glass through
which they will be traveling. The phase velocity or speed of these
waves in automotive glass can be calculated using the well-known
acoustic equation (c=f.quadrature., for phase velocity,
c=frequency, f.times.wavelength, .quadrature.). In this example,
the Lamb waves travel at around 3900 metres per second (m/s), the
quasi-longitudinal waves at around 5400 m/s and the shear waves at
3500 m/s. The spacing of the electrode fingers is roughly twice the
wavelength of the Lamb waves. The electrode finger spacing is
around 3/2 wavelengths for the longitudinal waves such that the
longitudinal waves are not excited in this particular embodiment of
the invention.
[0059] FIG. 7C shows examples of the types of waves that may be
emitted from the different transducer designs. Other waves may also
be emitted, for example Rayleigh waves, longitudinal waves or shear
waves.
[0060] The speed (and hence wavelength) of waves in the automotive
glass of the windscreen may vary with frequency, material
properties (for example, Young's modulus, density, or Poisson
ratio), and thickness of the glass. These parameters may be known
within a certain tolerance or experimentally measured. For example,
a laser vibrometer may be used to accurately determine the spatial
field of vibration within the windscreen during the operation of
the system for the purposes of obtaining a more accurate measure of
the wave speeds within the glass for refining and improving the
efficiency of the transducer designs.
[0061] FIG. 8 shows a graph of calculated phase velocities (80) of
each type of wave (76-78) as a function of the wave frequency (81)
through 3 mm thick automotive glass. These calculations are based
on the assumption that the material through which the waves are
travelling is a thin plate having the following parameters:
thickness of glass=3 mm; Young's modulus of glass=70 GigaPascals
(GPa); density of glass=2500 kilograms per metre cubed (kg/m3); and
Poisson ratio of glass=0.23. The Lamb wave shown may be slightly
high and relate to an anti-symmetric Lamb wave or flexural mode.
The phase velocity of the longitudinal and shear waves in a chosen
direction through automotive glass are relatively constant (82) at
all frequencies shown. The phase velocity of the Lamb (or Rayleigh)
waves increases (83) with higher frequencies. As may be seen, the
shear wave speed is almost identical to the wave speed of the Lamb
waves (or Rayleigh waves) at 500 kHz. This may be beneficial for
effectively clearing precipitation from the surface of the
windscreen.
[0062] Some square transducer designs have also been used in
certain embodiments of this invention. The square transducers used
are 2 cm by 2 cm and are fabricated from standard piezoelectric,
such as lead zirconate titanate (PZT) material. To form the
electrodes of the transducer, grooves are mechanically or laser cut
into the piezoelectric material. All of the electrodes will be
operated simultaneously using the same electrical signal.
[0063] FIGS. 9A and FIG. 9B show two square transducer designs
usable for the transducers of FIG. 1A or 1B. Transducers designed
to operate at higher frequencies, for example at a frequency of
around 1 MHz, may be smaller and generate shorter wavelength SAWs
compared to transducers designed to operate at lower frequencies,
for example at a frequency of around 500 kHz, which may be larger
and generate longer wavelength SAWs. The preferred operating
frequency may be 1 MHz and may be for a transducer having a square
design (as opposed to a transducer having a circular design). In
some embodiments, the thickness of the transducers are a few
millimetres thick and the larger the area of the transducer the
thicker the piezoelectric layer. The electrode layer sits on top of
a piezoelectric material and the electrode layer is much thinner
than the piezoelectric layer. In some embodiments the electrode
layer is a thin film much less than 1 mm thick. The electrode layer
may be cut using a laser or cut mechanically, however cutting using
a laser may provide a better finish to the electrode fingers than
mechanical cutting which may leave burred edges on the
electrodes.
[0064] FIG. 9A shows a transducer design for a piezoelectric
material (90) thickness of d=3 mm, having electrodes (91) that are
1.35 mm wide which corresponds to an operating frequency of 1 MHz
(for a wavelength of 2.7 mm at a phase velocity of 2700 m/s), for
an area of 2 cm by 2 cm. In this example, the electrodes (91) are
adhered to the surface of the windscreen and the uncut side of the
piezoelectric material faces away from the windscreen. Each
electrode finger width corresponds to half of the acoustic
wavelength for each of the emitted waves. Other similar example
transducer designs may have electrodes that are 1.08 mm and 0.9 mm
wide for operating frequencies of 1 MHz (for a wavelength of 2.16
mm at a phase velocity of 2160 m/s) and 1.2 MHz (for a wavelength
of 1.8 mm at a phase velocity of 2160 m/s) respectively.
[0065] FIG. 9B shows an alternative transducer design for an IDT.
This is a 2.8 cm by 2.8 cm transducer design for a square
transducer which may be fabricated from a circular piezoelectric
material (90) (PZT material), as shown on the left of FIG. 9B. The
piezoelectric layer in this example is 4 mm thick. In this example,
the cut electrodes are adhered to the surface of the windscreen
with the other side, or uncut side of the piezoelectric layer,
facing away from the windscreen. A ground electrode may face away
from the windscreen without being adhered to the windscreen
surface. The gaps (93) between the electrodes (92) may be cut using
a high powered laser. The electrodes sit on top of the
piezoelectric material. In this example, the gaps between the
electrodes are 0.4 mm wide and resemble a square waveform pattern
(95) across the electrode layer. Cutting through the electrode
layer from top to bottom will form two separate parts of the
transducer, each designed for an operating frequency of 500 kHz
(for a wavelength of 4.32 mm at a phase velocity of 2160 m/s). In
this example it may only be necessary to cut through the thin
electrode layer without cutting into the piezoelectric layer,
provided that a gap is created in the electrode layer to produce
the two separate electrodes. The electrodes of the two parts are
able to slot into one another. Each electrode has an opposite
polarisation to the other. When the electrodes are combined by
slotting into one another the adjacent electrodes produce
alternating polarisations (96) from one electrode to the next.
Electrodes may be oppositely polarised to adjacent or other
electrodes so that one of the electrodes is energised with one
polarity of signal and the other electrode with the opposite
polarity. An example of the droplet motion (94) that may be
observed relative to the transducer electrodes is shown.
[0066] The example transducer designs described may be capable of
vaporising droplets of precipitation on a windscreen or other glass
surface.
[0067] FIG. 3A has been shown with a laminate layer, however in
other examples a laminate layer may not always be present, such as
for a motorcycle helmet visor. In this example, the visor may be
fabricated from a plastic or polycarbonate material. The chosen
operating frequencies of the transducers for clearing precipitation
from the visor in this example, may not be the same as the
frequencies used in the embodiments discussed for clearing
precipitation for the windscreen. For example, the frequencies of
the transducers in some embodiments for clearing precipitation from
a visor may be lower than those used for clearing precipitation
from a windscreen.
[0068] FIG. 10 shows an embodiment of the invention in which
transducers (1-8 or 31-33) are attached to a visor (30), for
example a visor of a motorcycle helmet or other helmet, for
clearing precipitation or other debris or material. In the example
of a visor, the transducer may be fixed directly to the surface of
the visor and operate in a similar manner as described above (for
embodiments relating to a laminated windscreen in a vehicle),
wherein SAWs are used to remove precipitation from the surface of
the visor. The transducers may be driven by a drive system (11-15).
In some embodiments the transducers are driven to operate at
frequencies within the range of 100 kHz to 1 MHz. The transducers
are connected to the drive system by electronic wiring (35) and a
plug and socket (36). The transducers may be bonded directly to the
visor or bonded to a removable clip (34) adapted to clip onto the
edge of the visor, thus allowing for a change or replacement of a
damaged visor. Similarly, the transducers are positioned in a
peripheral region of the visor so as not to obscure the rider's
vision. The visor shown does not contain a laminate layer and
therefore any suitable wave may be used for clearing precipitation
including SAWs. The waves may be coupled to the surface of the
visor or contour of the visor so as to be effectively transmitted
through the entire surface of the visor.
[0069] Other examples where a transducer and driving system may be
used include a detection system for detecting the presence of rain
drops or precipitation and therefore initiate the system for
clearing the precipitation. In an example detection system two or
more transducers may be employed. A first transmitting transducer
may emit ultrasonic waves to a second receiving transducer. The
receiving transducer may be able to monitor the energy of the
ultrasonic waves received from the transmitting transducer. If a
calibration is performed when no precipitation exists on the
surface being investigated, there will be a base level of acoustic
energy received at the receiving transducer. When precipitation is
present on the surface the precipitation will absorb acoustic
energy and the receiving transducer will observe a drop in the
acoustic energy received below the calibrated base level, therefore
indicating the presence of precipitation. At this point, the
ultrasonic system for clearing precipitation from the windscreen
surface may be switched on and the ultrasonic power can be
automatically changed in line with the severity of precipitation.
In this way, the transducers used for operating the system can also
be used for controlling it.
[0070] Some embodiments provide the advantage of improving the
power efficiency of the transducers or increasing the ultrasonic
wave energy in the windscreen. The system may become more effective
and efficient by carefully "tuning" the IDT. For example the IDT
impedance may be matched to the windscreen or glass, or the input
frequency may be pulsed. In other embodiments, several frequencies
may be used to overcome standing waves on the glass, for example by
sweeping through a range of frequencies, or using frequency
modulation. In other illustrative embodiments bending waves or Lamb
waves may be found to be more effective for inducing "streaming" or
propulsion of a droplet, whilst minimising the amount of shear
waves emitted which may reduce the effectiveness of the system.
[0071] Other embodiments provide the advantage of minimising the
heating of the transducer, for example by using pulsed waves, which
in turn allows for more power to be supplied to the system to allow
for a greater area of the windscreen to be cleared of
precipitation.
[0072] The use of IDTs and SAWs has the advantage of minimising any
damping effect that may be caused by the existence of a laminate
layer. This may minimise issues of providing enough power for
droplet removal from a windshield without causing the internal
laminate layer within the windshield to delaminate.
[0073] Other advantages of illustrative embodiments may be that the
hydrophobic coating is not be removed or wiped off since ultrasonic
transducers are used to clear precipitation from a surface that has
been treated and there are no visibly moving parts across the
surface of the windscreen.
[0074] Embodiments of the invention may be applied not only to
laminated automotive windscreens and visors but also to laminated
windows of buildings and to laminated windows used in any other
situation, for example ships and boats.
[0075] Embodiments of the invention may also be applied to
un-laminated windows.
[0076] The preceding description has been presented only to
illustrate and describe examples of the principles described. This
description is not intended to be exhaustive or to limit these
principles to any precise form disclosed. Many modifications and
variations are possible in light of the above teaching.
* * * * *