U.S. patent number RE33,848 [Application Number 07/069,141] was granted by the patent office on 1992-03-17 for automatic windshield wiper speed control with piezoelectric sensor.
This patent grant is currently assigned to Nissan Motor Company, Limited. Invention is credited to Yasuhiro Shiraishi.
United States Patent |
RE33,848 |
Shiraishi |
March 17, 1992 |
Automatic windshield wiper speed control with piezoelectric
sensor
Abstract
A wiper speed control system for an automotive wiper controls
the operational speed of a wiper in accordance with rain
conditions. The control system includes a rain sensor detecting
rain conditions to produce an analog signal having an amplitude
depending upon the detected rain conditions. The analog sensor
signal is converted into a digital pulse signal by a converter for
application to a digital circuit system. A control signal is
produced by digitally processing the pulse signal. The control
signal is applied to a wiper driver circuit to adjust the
operational speed or timing in accordance with the control
signal.
Inventors: |
Shiraishi; Yasuhiro (Zama,
JP) |
Assignee: |
Nissan Motor Company, Limited
(Yokohama, JP)
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Family
ID: |
27320163 |
Appl.
No.: |
07/069,141 |
Filed: |
July 2, 1987 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
527776 |
Aug 30, 1983 |
04527105 |
Jul 2, 1985 |
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Foreign Application Priority Data
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Sep 2, 1982 [JP] |
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57-151729 |
Sep 2, 1982 [JP] |
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57-151730 |
Sep 20, 1982 [JP] |
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57-162176 |
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Current U.S.
Class: |
318/444;
15/250.001; 318/483; 318/DIG.2 |
Current CPC
Class: |
B60S
1/0822 (20130101); B60S 1/0859 (20130101); Y10S
15/15 (20130101); Y10S 318/02 (20130101) |
Current International
Class: |
B60S
1/08 (20060101); B60S 001/08 () |
Field of
Search: |
;318/443,444,483,643,DIG.2 ;15/25C,250.02,250.13 ;73/17R,171
;200/61.04,81R ;307/1R,116,117,118 ;340/601,602 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2621116 |
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Nov 1977 |
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DE |
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3017982 |
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Nov 1981 |
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DE |
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4931 |
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Jan 1978 |
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JP |
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86422 |
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Jul 1978 |
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JP |
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55-063941 |
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May 1980 |
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JP |
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57-130747 |
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Aug 1982 |
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JP |
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Other References
Electronic Design, vol. 27, p. 19, Sep. 13, 1979..
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Primary Examiner: Ro; Bentsu
Attorney, Agent or Firm: Foley & Lardner
Claims
What is claimed is:
1. A control system for controlling a windshield wiper
comprising:
rain sensor means for sensing the rate of rain and generating
sensor signals indicative of the sensed rain rate;
means responsively connected to the rain sensor means for
generating digital pulses having a frequency corresponding to the
sensed rain rate in response to the sensor signals;
means responsively connected to the means for generating digital
pulses for counting the digital pulses and generating a wiper
control signal every time a predetermined number of the digital
pulses has been counted;
a signal generator for outputting a constant-frequency digital
signal to said counting means, said counting means being responsive
to both the number of the digital pulses and the number of pulses
in the digital signal in generating said wiper control signal;
and
means responsively connected to the counting means for driving the
windshield wiper through one cycle whenever the wiper control
signal is generated.
2. A control system for controlling a windshield wiper
comprising:
a driver motor associated with the windshield wiper for driving the
windshield wiper at a variable rate;
a rain sensor including a vibrator with a sensor surface exposed to
rain and vibrating in accordance with the rate of rain, said rain
sensor producing a sensor signal having an amplitude which varies
in accordance with the magnitude of vibration of said vibrator;
means for converting the sensor signal into a train of digital
pulses having a frequency corresponding to the sensed rain rate in
response to the sensor signal; and
a drive signal generator receiving the train of digital pulses and
including counting means for counting the number of pulses in said
train of digital pulses, generating a drive signal for driving the
driver motor for one cycle of wiper operation whenever a given
number of pulses are received.
3. The control system as set forth in claim 2, wherein said rain
sensor further includes a piezoelectric element detecting the
magnitude of vibration of said vibrator and producing said sensor
signal with an amplitude corresponding to the vibration magnitude
of said vibrator.
4. The control system as set forth in claim 3, wherein said
.[.converter means.]. .Iadd.converting means .Iaddend.comprises
means for shaping said sensor signal into .[.a train.]. .Iadd.said
train .Iaddend.of digital pulses.
5. The control system as set forth in claim 4, wherein said drive
signal generator comprises a counter which counts the pulses of
said pulse train and produces said driver signal whenever the
.[.counter value.]. .Iadd.counted value .Iaddend.reaches a
predetermined value.
6. The control system as set forth in claim 5, wherein said driver
motor is responsive to said driver signal to drive said windshield
wiper through one cycle so as to control the timing between
intermittent strokes of the wiper in dependence upon said driver
signal.
7. The control system as set forth in claim 6, wherein said sensor
surface has an area of more than 100 mm.sup.2.
8. The control system as set forth in claim 3, wherein said
vibrator of said rain sensor is provided the natural resonant
frequency in a range of from approximately 10 KHz to approximately
25 KHz.
9. The control system as set forth in claim 3, wherein said rain
sensor is mounted on .[.a cowl top panel of the vehicle.].
.Iadd.the cowl top of a vehicle .Iaddend.and said sensor surface is
inclined approximately parallel to a windshield.
10. The control system as set forth in claim 3, wherein said rain
sensor is mounted on .[.a cowl top panel of the vehicle.].
.Iadd.the cowl top of a vehicle .Iaddend.and said sensor surface is
inclined facing forward at an angle of about 5.degree. to
25.degree..
11. A control system for controlling a windshield wiper
comprising:
rain sensor means for sensing the rate of rain and generating
sensor signals indicative of the sensed rain rate;
means responsively connected to the rain sensor means for
generating digital pulses having a frequency corresponding to the
sensed rain rate in response to the sensor signals;
means responsively connected to the means for generating digital
pulses for counting the digital pulses and generating a wiper
control signal every time a predetermined number of the digital
pulses has been counted; and
means responsively connected to the counting means for driving the
windshield wiper through one cycle whenever the wiper control
signal is generated;
wherein said rain sensor means comprises a vibration sensor which
detects the rain conditions by detecting vibrations created by the
impact of a raindrop onto a sensor surface thereof, said vibration
sensor comprising a vibrator having said sensor surface and a
piezoelectric element for detecting magnitude of vibration of said
vibrator to produce said sensor signals.
12. The control system as set forth in claim 11, wherein said
sensor surface of said rain sensor means has an area greater than
100 mm.sup.2.
13. The control system as set forth in claim 11, wherein said
vibrator of said rain sensor .Iadd.means .Iaddend.has a natural
resonant frequency in a range of from approximately 10 KHz to
approximately 25 KHz.
14. The control system as set forth in claim 11, wherein said
sensor surface is inclined to be approximately parallel with the
windshield.
15. The control system as set forth in claim 11, wherein said
sensor surface is inclined upward front-to-rear with respect to
vehicle travel at an angle of 5.degree. to 25.degree. to the
horizontal.
16. A control system for controlling a windshield wiper
comprising:
rain sensor means comprising a vibration sensor which detects rain
conditions by detecting vibrations created by the impact of a
raindrop onto a sensor surface thereof, said vibration sensor
comprising a vibrator having said sensor surface inclined to be
approximately parallel with the windshield and a piezoelectric
element for detecting magnitude of vibration of said vibrator to
generate sensor signals indicative of the sensed rain rate
means responsively connected to the rain sensor means for
generating digital pulses having a frequency corresponding to the
sensed rain rate in response to the sensor signals;
means responsively connected to the means for generating digital
pulses for counting the digital pulses and generating a wiper
control signal every time a predetermined number of the digital
pulses has been counted;
a signal generator for outputting a constant-frequency digital
signal to said counting means, said counting means being responsive
to both the number of digital pulses and the number of pulses in
said digital signal in generating said wiper control signal;
and
means responsively connected to the counting means for driving the
windshield wiper through one cycle whenever the wiper control
signal is generated.
17. A control system for controlling a windshield wiper
comprising:
rain sensor means comprising a vibration sensor which detects rain
conditions by detecting vibrations created by the impact of a
raindrop onto a sensor surface thereof, said vibration sensor
comprising a vibrator having said sensor surface inclined upward
front-to-rear with respect to the direction of vehicle travel at an
angle of 5.degree. to 25.degree. to the horizontal and a
piezoelectric element for detecting magnitude of vibration of said
vibrator to generate sensor signals indicative of the sensed rain
rate
means responsively connected to the rain sensor means for
generating digital pulses having a frequency corresponding to the
sensed rain rate in response to the sensor signals;
means responsively connected to the means for generating digital
pulses for counting the digital pulses and generating a wiper
control signal every time a predetermined number of the digital
pulses has been counted;
a signal generator for outputting a constant-frequency digital
signal to said counting means, said counting means being responsive
to both the number of digital pulses and the number of pulses in
said digital signal in generating said wiper control signal;
and
means responsively connected to the counting means for driving the
windshield wiper through one cycle whenever the wiper control
signal is generated.
18. A control system for controlling a windshield wiper
comprising:
a rain sensor including a vibrator with a sensor surface exposed to
rain and vibrating in accordance with the rate of rain, and further
including a piezoelectric element detecting the magnitude of
vibration of said vibrator and producing a sensor signal having an
amplitude which varies in accordance with the magnitude of
vibration of said vibrator;
means for converting the sensor signal into a train of digital
pulses having a frequency corresponding to the sensed rain rate,
said converting means comprising means for shaping said sensor
signal into .[.a train.]. .Iadd.said train .Iaddend.of digital
pulses;
a driver signal generator receiving the train of digital pulses and
comprises a counter which counts the number of pulses in said pulse
train and produces a driver signal whenever the counted number of
pulses reaches a predetermined value;
a pulse generator adapted to output a constant-frequency pulse
signal to said counter, said counter counting the pulses from both
said constant-frequency pulse signal and said digital signal to
produce said driver signal whenever the sum of the pulses from
.[.said converter means.]. .Iadd.said converting means .Iaddend.and
the pulses from said pulse generator reaches the predetermined
value; and
a driver motor associated with said windshield wiper and responsive
to said driver signal for driving said windshield wiper through one
cycle so as to control the timing between intermittent strokes of
the wiper in dependence on the driver signal.
19. An intermittent timing control system for an automotive wiper
system comprising:
a windshield wiper;
a wiper motor for driving said wiper, said wiper motor being
controllable to drive said wiper at a variable sweep speed and
operative for driving said wiper at variable intervals in an
intermittent mode of operation;
a rain sensor having a sensor surface exposed to rain for detecting
the rate of rain, vibrating in response to an impact of raindrops
on said sensor surface at a magnitude and frequency depending upon
the rate of rain, and producing a sensor signal having an amplitude
and frequency corresponding to the magnitude and frequency of
vibration of said sensor surface;
a pulse generation for producing a train of pulses corresponding to
said sensor signals and having a frequency depending upon the rate
of rain; and
counting means receiving said train of pulses and producing a drive
signal for operating .[.said driver motor.]. .Iadd.said wiper motor
.Iaddend.for one cycle of wiper operation whenever a given number
of pulses are received.
20. An intermittent timing control system for an automotive wiper
system comprising:
a windshield wiper;
a wiper motor for driving said wiper, said wiper motor being
controllable to drive said wiper at a variable sweep speed and
operative for driving said wiper at variable intervals in an
intermittent mode of operation;
a rain sensor having a vibrator adapted to oscillate at an
amplitude and frequency depending upon the rate of rain, said
vibrator having one surface exposed outwardly and inclined to be
approximately parallel with the windshield so that raindrops
directly collide with said one surface for causing oscillation of
said vibrator, and said vibrator producing a sensor signal having
an amplitude and frequency corresponding to the magnitude of
vibration and frequency thereof;
means responsively connected to the rain sensor for generating
digital pulses at a frequency corresponding to the frequency of
said sensor signals;
means, responsively connected to said generating means, for
counting the number of digital pulses;
a signal generator for outputting a constant-frequency digital
signal to said counting means, and said counting means being
responsive to both the number of digital pulses and the number of
pulses in said digital signal in generating .[.said wiper control
signal.]. .Iadd.a wiper control signal.Iaddend.; and
means responsive to said sensor signal indicative of the magnitude
and frequency of vibration of said vibrator for deriving an
intermittent timing signal for wiper intermittent operation based
on the sensor signal value. .Iadd.
21. An intermittent timing control system for a wiper system of a
vehicle, comprising:
a windshield wiper;
a wiper motor for driving said wiper, said wiper motor being
controllable to drive said wiper at a variable sweep speed and
operative for driving said wiper at variable intervals in an
intermittent mode of operation;
a rain sensor having a vibrator adapted to oscillate at an
amplitude and frequency depending upon the rate of rain, said
vibrator having one surface exposed outwardly of said vehicle so
that raindrops directly collide with said one surface for causing
oscillation of said vibrator, said one surface of said sensor being
mounted on a point on the body of said vehicle subject to positive
pressure from an air stream travelling over said vehicle and said
vibrator producing a sensor signal having an amplitude and
frequency corresponding to the magnitude of vibration and frequency
of said one surface; and
means responsive to said sensor signal indicative of the magnitude
and frequency of vibration of said vibrator for deriving an
intermittent timing signal for wiper intermittent operation based
on the sensor signal value. .Iaddend. .Iadd.22. The control system
as set forth in claim 21 wherein said one surface of said rain
sensor has an area greater than 100 mm.sup.2. .Iaddend. .Iadd.23.
The control system as set forth in claim 21 wherein said one
surface of said rain sensor is inclined at an angle relative to the
horizontal. .Iaddend. .Iadd.24. The control system as set forth in
claim 23 wherein said angle is in the range of 5 degrees to 25
degrees. .Iaddend. .Iadd.25. The control system as set forth in
claim 24 wherein said angle is variable in dependence on raid drop
speed. .Iaddend. .Iadd.26. The control system as set forth in claim
21 wherein said one surface is mounted on a cowl top panel of said
vehicle. .Iaddend.
.Iadd. A control system for controlling a windshield wiper of a
vehicle, comprising:
a driver motor associated with the windshield wiper for driving the
windshield wiper;
a rain sensor for sensing rain and producing sensor signals
indicative of the sensed rain, said rain sensor including a sensor
surface having a variable angle of inclination with respect to the
horizontal which is mounted on an external surface of said vehicle
body at an orientation in front of a front windshield, which
orientation subjects said sensor surface to positive pressure from
an air stream travelling over said vehicle; and
means responsive to said sensor signal for deriving a driving
signal for
said driver motor. .Iaddend. .Iadd.28. A control system as set
forth in claim 27 wherein said sensor surface is mounted on a cowl
top panel of
said vehicle. .Iaddend. .Iadd.29. A control system for controlling
a windshield wiper of a vehicle comprising:
a variable inclination rain sensor for sensing the rate of rain and
generating sensor signals indicative of the sensed rain date, said
sensor being mounted on an external portion of the vehicle body at
an orientation in front of a front windshield, which orientation
subjects said sensor to positive pressure from an air stream
passing over said vehicle;
means responsively connected to the rain sensor for generating
digital pulses having a frequency corresponding to the sensed rain
rate in response to the sensor signals;
means responsively connected to the means for generating digital
pulses for counting the digital pulses and generating a wiper
control signal every time a predetermined number of the digital
pulses has been counted; and
means responsively connected to the counting means for driving the
windshield wiper through one cycle whenever the wiper control
signal is
generated. .Iaddend. .Iadd.30. A rain sensor for use in an
intermittent timing control system for a wiper system of a vehicle,
which wiper system includes a windshield wiper, a wiper motor for
driving said wiper, said wiper motor being controllable to drive
said wiper at a variable sweep speed and operative for driving said
wiper at variable intervals in an intermittent mode of operation,
and a wiper control circuit associated with said wiper motor for
operating the latter at a controlled timing, said wiper control
circuit being responsive to a rain condition indicative sensor
signal for deriving said timing, wherein said rain sensor
comprises:
a sensor housing mounted on an external surface of an vehicle body
adjacent a front windshield;
a vibrator supported on said sensor housing and adapted to
oscillate at an amplitude frequency depending upon the rate of
rain, said vibrator having one surface exposed outwardly to the
atmosphere so that raindrops directly collide with said one surface
for causing oscillation of said vibrator, said one surface of said
vibrator being inclined at a given angle determined in relation to
the inclination of the front windshield, and said vibrator
producing said rain condition indicative sensor signal having an
amplitude and frequency corresponding to the magnitude of
vibration and frequency of said one surface. .Iaddend. .Iadd.31. A
rain sensor as set forth in claim 30 wherein said inclination angle
is in the range of 5 degrees to 25 degrees. .Iaddend. .Iadd.32. A
rain sensor as set forth in claim 30 wherein said one surface is
mounted on a cowl top panel of said vehicle. .Iaddend. .Iadd.33. A
rain sensor for use in an intermittent timing control system for a
wiper system of a vehicle, which wiper system includes a windshield
wiper, a wiper motor for driving said wiper, said wiper motor being
controllable to drive said wiper at a variable sweep speed and
operative for driving said wiper at variable intervals in an
intermittent mode of operation, and a wiper control circuit
associated with said wiper motor for operating the latter at a
controlled timing, said wiper control circuit being responsive to a
rain condition indicative sensor signal for deriving said timing,
wherein said rain sensor comprises:
a sensor housing mounted on an external surface of an vehicle body
adjacent a front windshield;
a vibrator supported on said sensor housing and adapted to
oscillate at an amplitude and frequency depending upon the rate of
rain, said vibrator having one surface exposed outwardly to the
atmosphere so that raindrops directly collide with said one surface
for causing oscillation of said vibrator, said one surface of said
vibrator having an area greater than or equal to 100 mm2, and said
vibrator producing said rain condition indicative sensor signal
having an amplitude and frequency corresponding to the magnitude of
vibration and frequency of said one surface. .Iaddend.
.Iadd.34. A rain sensor as set forth in claim 33, wherein said
vibrator one surface being inclined at an angle in the range of 5
degrees to 25 degrees in relation to the inclination of the front
windshield. .Iaddend. .Iadd.35. A rain sensor as set forth in claim
33 wherein said one surface is mounted on a cowl top panel of said
vehicle. .Iaddend. .Iadd.36. A rain sensor for use in an
intermittent timing control system for a wiper system of a vehicle,
which wiper system includes a windshield wiper, a wiper motor for
driving said wiper, said wiper motor being controllable to drive
said wiper at a variable sweep speed and operative for driving said
wiper at variable intervals in an intermittent mode of operation,
and a wiper control circuit associated with said wiper motor for
operating the latter at a controlled timing, said wiper control
circuit being responsive to a rain condition indicative sensor
signal for deriving said timing, wherein said rain sensor
comprises:
a sensor housing mounted on an external surface of a vehicle body
adjacent a front windshield;
a vibrator supported on said sensor housing and adapted to
oscillate at an amplitude and frequency depending upon the rate of
rain, said vibrator having one surface exposed outwardly to the
atmosphere so that raindrops directly collide with said one surface
for causing oscillation of said vibrator, said one surface of said
vibrator being inclined at a given angle determined in relation to
the inclination of the front windshield, and said vibrator
producing said rain condition indicative sensor signal having an
amplitude and frequency corresponding to the magnitude of
vibration and frequency of said one surface. .Iaddend. .Iadd.37. A
rain sensor as set forth in claim 36 wherein said inclination angle
is in the range of 5 degrees to 25 degrees. .Iaddend. .Iadd.38. A
rain sensor as set forth in claim 36 wherein said one surface is
mounted on a cowl top panel of said vehicle. .Iaddend.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to a wiper control system
for an automotive windshield wiper, which automatically controls
the operating speed of the wiper in dependence upon rain
conditions. More particularly, the invention relates to a wiper
control system which can avoid the influence of external conditions
on detection of rain conditions.
Published Japanese Utility Model Application No. 57-130747,
published on Aug. 14, 1982 discloses an automatic control system
for an automotive wiper. The automatic control system includes a
vibration pick-up which detects vibration of the windshield created
by the impact of rain drops. The output signal of the vibration
pick-up is amplified, filtered and then compared with a threshold
to produce a driver signal when the output signal level exceeds the
threshold level.
In this wiper control system, since the comparator compares an
analog vibration pick-up output with an analog threshold, and since
the amplifier, filter and comparator all perform analog processing,
they are apt to be influenced by external conditions such as errors
in the operational characteristics of the respective circuit
elements, or temperature conditions. These influences may cause the
wiper operation to deviate from that required.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provided a
wiper speed control system which satisfactorily and successfully
avoids the influence of external conditions on detection of rain
conditions.
Another and more specific object of the present invention is to
provide a wiper speed control system, which performs digital
processing to detect rain conditions and to control operation of
the automotive wiper mechanism.
To accomplish the foregoing and other objects, a wiper speed
control system according to the present invention includes a
control circuit which can convert an analog rain sensor signal into
a pulse signal suitable for digital processing. In practice, the
pulses of the pulse signal are counted and compared with a
predetermined threshold value to produce a driver signal when the
counter value becomes equal to the predetermined threshold
value.
By controlling the wiper operation according to the digitally
processed rain condition data, the influence on detection of the
rain conditions due to external conditions can be satisfactorily
and successfully avoided.
According to one aspect of the invention, a wiper control system
for an automotive wiper comprises a rain sensor for producing a
sensor signal having a value indicative of rain conditions, means
for producing a digital rain-condition data signal according to the
sensor signal value, means for producing a wiper control signal
based on said rain-condition data signal, and a wiper driver motor
responsive to the wiper control signal to drive a wiper at a rate
variable in accordance with the wiper control signal.
In another aspect, a wiper speed control system for an automotive
wiper comprises a rain sensor including a vibrator with a sensor
surface exposed to rain and vibrating in accordance with rain
conditions, the rain sensor producing a sensor signal having an
amplitude corresponding to the magnitude of vibration of the
vibrator, a converter means for converting the sensor signal into a
digital signal representative of the detected rain conditions means
for processing the digital signal to produce a driver signal, and a
wiper driver motor responsive to the driver signal for driving a
windshield wiper at a rate variable in accordance with the driver
signal.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be understood more fully from the
detailed description given below and from the accompanying drawings
of the preferred embodiment of the invention, which, however,
should not be taken to limit the invention to the specific
embodiments but for explanation and understanding only.
In the drawings:
FIG. 1 is a schematic block diagram of the preferred embodiment of
a wiper speed control system according to the present
invention;
FIG. 2 is a sectional view of a rain sensor employed in the wiper
speed control system of FIG. 1;
FIG. 3 is a schematic block diagram of a modification of the wiper
speed control system of FIG. 1;
FIG. 4 is a diagram illustrating location of a rain sensor on a
vehicle with regard to dynamic air pressure;
FIG. 5 is a diagram of application of the rain sensor to the wiper
speed control system of FIG. 3; and
FIG. 6 illustrates the vibration frequency spectrum acting on the
rain sensor.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, particularly to FIG. 1, a wiper
speed control system of the preferred embodiment of the present
invention generally comprises a rain sensor 10 which detect rain
conditions and produces a sensor signal having a value indicative
of the detected rain conditions. The rain sensor 10 is connected to
a pulse signal generator 12 which is adapted to shape the sensor
signal and to output a signal in the form of a train of pulses. The
pulse train produced by the pulse signal generator 12 has a
frequency corresponding to the sensor signal value and so
indicating the detected rain conditions. The pulse train is fed to
a counter 14. The counter 14 counts the pulses of the pulse train
and produces a wiper control signal whenever the counter value
reaches a predetermined value. As will be apparent to one of
ordinary skill in the art, counter 14 must be either a ring counter
or a counter which resets itself after the predetermined value is
reached so that it can restart counting the digital pulses from
pulse signal generator 12. The wiper control is fed to a driver
circuit 16 including a driver motor 18 to drive a wiper mechanism
20.
As shown in FIG. 2, the rain sensor 10 comprises a vibration sensor
having a sensor surface 102 inclined essentially parallel to the
windshield 23 of a vehicle. The sensor surface 102 is formed on a
vibrator panel 104 secured in a sensor housing 106 in turn rigidly
secured to a cowl top panel 22 near the windshield 23. A
piezoelectric element 108 is secured to the inner surface of the
vibrator panel 104 for vibration therewith.
In this design, the rain sensor 10 detects vibrations produced by
impact of rain drops on the sensor surface. This is, when rain
falls, rain drops impinge on the sensor surface. Shock applied to
the sensor surface and the vibrator panel 104 varies depending on
the size of the colliding rain drop, its speed at impact and
frequency of occurence of its collision. Therefore, the magnitude
of vibration caused by impact of rain drops varies depending upon
the size of the colliding rain drop, the collision speed and
frequency of occurrence of its collision. The piezoelectric element
108 vibrates with the vibrator panel and produces a sensor signal
having a value corresponding to the magnitude of vibration.
In summary, the sensor signal produced by the rain sensor in the
manner set forth above is an analog signal, the output voltage of
which varies in dependence upon the vibration amplitude of the
vibrator panel and, in turn, in dependence upon the rain
conditions. The pulse signal generator 12 converts the analog
signal into a pulse train suitable for digital processing for the
purpose of wiper speed control. The counter 14 is a digital counter
for counting the pulses from the pulse signal generator 12 and
internally compares the counter value with a predetermined
threshold value to produce a counter signal when the counter value
becomes equal to the threshold value.
When the wipers have been manually selected to operate
intermittently, the counter signal can serve as a driver signal
activating the driver circuit to operate the wiper mechanism 20. In
this case, the timing of occurrences of the counter signal
corresponds to the interval between successive wiper operations,
the frequency of which varies in dependence upon rain conditions.
However, according to the shown embodiment it is also possible to
control the wiper operational speed depending upon the detected
rain conditions, even in low- or high-speed continuous operation
modes. In this case, wiper speed control may be performed by
adjusting the voltage to be applied to the driver motor 18 in
accordance with the frequency of occurrence of the counter
signal.
FIG. 3 shows a modification of the foregoing preferred embodiment
of wiper speed control system according to the present invention.
In this modification, a rain sensor 10 is associated with the pulse
signal generator 12. As in the foregoing preferred embodiment, the
rain sensor 10 produces an analog sensor signal having a value
indicative of the rain conditions. The pulse signal generator 12
converts the analog sensor signal into a pulse train having a
frequency corresponding to the sensor signal value. The pulse
signal generator 12 is connected to a counter 32. The counter 32 is
also connected to an oscillator 34 which is adapted to output a
pulse train with a constant frequency. The counter 32 counts the
pulses from both the pulse signal generator 12 and the oscillator
34 and produces a counter signal when the counter value, which
represents the total number of pulses from the pulse signal
generator and the oscillator 34, become equal to the predetermined
threshold.
As shown in FIGS. 4 and 5, the rain sensor 10 is mounted on the
cowl top panel 22 of the vehicle body at a point subject to
positive pressure from the air stream, as indicated by hatched area
in FIG. 4. If the rain sensor 10 is mounted on the vehicle body at
a point subject to negative dynamic air pressure, the air pressure
may adversely influence detection of rain conditions, since the
angle of incidence of the rain will be affected by the influence of
the air pressure.
The sensor surface 102 of the rain sensor 10 is inclined at a given
angle with respect to the horizontal plane such that the rear edge
is higher than the front edge. The angle of inclination of the
sensor surface with respect to horizontal plane is empirically
selected to fall within the range of from 5.degree. to 25.degree..
Normally, rain drop speed is approximately 1 m/sec. to 15 m/sec. In
this normal rain condition, the selected inclination angle of the
sensor surface, i.e. 5.degree. to 25.degree. is satisfactory for
detecting rain condition under usual vehicle speed i.e.,
approximately 10 to 55 miles per hour.
Preferably, the rain sensor would be provided with a sensor surface
which can be variably inclined in dependence upon the rain drop
speed. Dynamic air pressure reflecting vehicle speed may be used as
a detecting parameter to control the inclination angle of the
sensor surface.
As in the foregoing embodiment, the rain sensor 10 includes a
vibrator panel 104 with a sensor surface 102 exposed by a sensor
housing 106. The vibrator panel 104 engages the sensor housing 106
only at its circumferential edge so that it can vibrate in response
to impact of rain drops on the sensor surface. A piezoelectric
element 108 is secured to the vibrator panel 104 via an electrode
310 formed therebetween. Another electrode 312 is formed on the
opposite surface of the piezoelectric element 108. The electrodes
310 and 312 are electrically connected to an amplifier 316. The
amplifier 316 is connected to the wiper speed control circuit,
including the pulse signal generator 12, the counter 32 and the
oscillator 34, to supply the sensor signal thereto.
The natural or natural resonant frequency of the vibrator panel 104
is chosen to fall in the range of 10 KHz to 25 KHz. This specific
natural frequency of the vibrator panel is experimentarily selected
to avoid the influence of vibrations created by the vehicle horn or
other equipment or engine vibrations. FIG. 6 shows the vibration
frequency spectrum created by vehicle horn and received by the rain
sensor. As will be appreciated from FIG. 6, the maximum frequency
of vibration created by the vehicle horn is approximately 9 KHz.
Therefore, by selecting the natural frequency of the vibrator panel
104 in the given range, i.e., 10 KHz to 25 KHz, the influence of
the vehicle horn upon detection of the rain conditions can be
satisfactorily avoided.
The maximum natural frequency of the vibrator panel 104 is selected
in view of the amplitude of the sensor signal. To achieve a natural
resonant frequency of the vibrator panel 304 beyond 25 KHz, the
vibrator panel has to be adequately thick in relation to the area
of the sensor surface. In this case, the amplitude of vibration
produced by impact of rain drops onto the sensor surface will be
too low to be used in the wiper control system. That is, by setting
the natural frequency of the vibrator panel so high, the sensivity
to rain will be lowered, thereby degrading the response
characteristics of the wiper control system.
In this modification, since the counter signal is produced when the
total number of pulses from both the pulse signal generator and the
oscillator reaches the predetermined threshold, the wiper mechanism
can be driven at the lowest interval in intermittent operational
mode or at the lowest speed in continuous operational mode, as
defined by pulse period of the pulse train from the oscillator. As
a result, the wiper mechanism will continue to operate even when
the vehicle passes through a tunnel in which rain drop cannot be
detected. This will be advantageous for providing clear vision for
the driver.
It should be noted that the sensor surface subjected to rain should
be larger than 100 mm.sup.2 in area in accordance with experimental
results. In experimental tests, it was found that a rain drop
impinging on a plane surface usually expands to an area of
approximately 100 mm.sup.2. It is necessary to provide a sensor
surface sufficiently large to receive the full volume of rain water
upon impact in order to detect the rain conditions. Otherwise, a
plurality of rain sensors, each of which has a sensor surface area
of less than 100 mm.sup.2, can be used so that the total effective
area of the sensor surface exceeds 100 mm.sup.2.
As set forth above, according to the present invention, an analog
sensor signal is converted into a pulse train so that the influence
of errors in the circuit element characteristics, temperature
conditions and so forth can be avoided.
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