Note: Descriptions are shown in the official language in which they were submitted.
CA 02467233 2004-05-13
ULTRASONIC SENSOR
Descri.ption
The present invention relates to an ultrasonic sensor
which is used for a vehicle and the like.
An ultrasonic sensor is used as an obstacle detection
sensor for detecting obstacle by transmitting and receiving
an ultrasonic wave. This ultrasonic sensor has a
piezoelectric vibrator. The piezoelectric vibrator vibrates
to transmit an ultrasonic wave and receives a reflected wave
from an obstacle, so that an obstacle can :be detected.
However, the above ultrasonic sensor has a disadvantage
that a reverberation occurs after normal vibration of the
piezoelectric vibrator. Therefore, detection accuracy of the
sensor may be reduced and the ultrasonic sensor may not be
able to detect an obstacle at a short distance if the
reverberation continues long.
In order to adjust the reverberation period to be
within a suitable range, a capacitor to compensate for an
electric capacity of the piezoelectric vibrator can be used.
In JP-A-H8-237796, as shown in. FIG. 8, an ultrasonic
sensor has a capacitor J2 connected in parallel with a
piezoelectric vibrator Ji in order to reduce a temperature
drift of a sensor output. When a temperature of the
ultrasonic sensor changes, an electric capacity of the
capacitor J2 changes in the opposite direction of an electric
capacity change of the piezoelectric vibrator .71. That is,
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the electric capacit:y change of the piezoelectric vibrator ,71
can be cancelled.
However, characteristics of the piezoelectric vibrator
J1 and the capacitor J2 are uneven. Therefore, an increase
amount of electric capacity of the piezoelectric vibrator ,T1
and a decrease amount of electric capacity of the capacitor
J2 are not equal. Accordingly, a temperature compensation
cannot be performed correctly and a temperature drift occurs.
Further, a temperature range to compensate for electric
capacity may be limited according to characteristics of the
capacitor J2. Therefore, the technique disclosed in JP-A-H8-
237796 is not satisfactory to adjust a reverberation period.
In JP-A-H11-.103496, an ultrasonic sensor has a
capacitor alternately connected and disconnected in parallel
with a piezoelectric vibrator in order to improve a sound
pressure of an ultrasonic wave to be transmitted and a
sensitivity to a reflected wave. The capacitor is connected
when the piezoelectric vibrator transmits an ultrasonic wave.
To the contrary, the capacitor is disconnected when the
piezoelectric vibrator receives a reflected wave Thus, a
resonance frequency and an anti-resonance frequency are
changed to improve a sound pressure of an ultrasonic wave to
be transmitted and a sensitivity to a reflected wave.
However, a reverberation period cannot be suitably adjusted
only by connecting or disconnecting a capacitor. Therefore,
the technique disclosed in JP--A-H11-103496 is not
satisfactory to adjust a reverberation period.
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In view of the foregoing problems, it is an object of
the present invention to provide an ultrasonic sensor which
can compensate for an electric capacity of a piezoelectric
vibrator in order to adjust a reverberation period to be
within a suitable range.
In order to achieve the above object, an ultrasonic
sensor includes a piezoelectric vibrator, a reverberation
measuring means and a compensating means.
The piezoelectric vibrator vibrates to transmit an
ultrasonic wave, receives a reflected wave of the transmitted
ultrasonic wave and produces an output signal corresponding
to vibration of the piezoelectric vibrator. The
reverberation measuring means measures a reverberation period
of the piezoelectric vibrator from the output signal. The
compensating means compensates for an operation
characteristic of the piezoelectric vibrator in accordance
with the measured reverberation period.
Thus, the ultrasonic sensor can compensate for electric
capacity of the piezoelectric vibrator and the reverberation
period can be adjusted to be within a predetermined range.
Accordingly, the ultrasonic sensor can decrease a
reverberation period, which may cause reduction in detection
accuracy and impossibility of detection of an obstacle at a
short distance.
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Accordingly, in one aspect the invention provides
an ultrasonic sensor comprising: a piezoelectric vibrator
which vibrates to transmit an ultrasonic wave, receives a
reflected wave of the transmitted ultrasonic wave and
produces an output signal corresponding to vibration of the
piezoelectric vibrator; a reverberation measuring means for
measuring a reverberation period of the piezoelectric
vibrator from the output signal; and a compensating means
for compensating for an operation characteristic of the
piezoelectric vibrator in accordance with the measured
reverberation period, wherein the compensating means
compensates for the operation characteristic of the
piezoelectric vibrator so that the reverberation period is
adjusted to be the shortest when the reverberation period
measured by the reverberation measuring means is not within
a predetermined range.
In another aspect the invention provides an
ultrasonic sensor comprising: a piezoelectric vibrator which
vibrates to transmit an ultrasonic wave, receives a
reflected wave of the transmitted ultrasonic wave and
produces an output signal corresponding to vibration of the
piezoelectric vibrator; a reverberation measuring means for
measuring a reverberation period of the piezoelectric
vibrator from the output signal; and a compensating means
for compensating for an operation characteristic of the
piezoelectric vibrator in accordance with the measured
reverberation period, wherein the compensating means
normally compensates for the operation characteristic of the
piezoelectric vibrator so that the reverberation period is
adjusted to be the shortest.
In another aspect the invention provides an
ultrasonic sensor comprising: a piezoelectric vibrator which
vibrates to transmit an ultrasonic wave, receives a
3a
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reflected wave of the transmitted ultrasonic wave and
produces an output signal corresponding to vibration of the
piezoelectric vibrator; a reverberation measuring means for
measuring a reverberation period of the piezoelectric
vibrator from the output signal; and a compensating means
for compensating for an operation characteristic of the
piezoelectric vibrator in accordance with the measured
reverberation period, wherein: the compensating means
includes at least one of a capacitor and a coil; the
compensating means compensates for the operation
characteristic of the piezoelectric vibrator by changing a
combination of the capacitor and the coil which are
connected to the piezoelectric vibrator; the reverberation
measuring means measures a reverberation period of the
piezoelectric vibrator for all or some of combinations of
the capacitor and the coil which are connected to the
piezoelectric vibrator; and the compensating means selects a
combination from the combinations of the capacitor and the
coil connected to the piezoelectric vibrator, whose
reverberation periods are measured by the reverberation
measuring means.
In another aspect the invention provides an
ultrasonic sensor comprising: a piezoelectric vibrator which
vibrates to transmit an ultrasonic wave, receives a
reflected wave of the transmitted ultrasonic wave and
produces an output signal corresponding to vibration of the
piezoelectric vibrator; a reverberation measuring means for
measuring a reverberation period of the piezoelectric
vibrator from the output signal; and a compensating means
for compensating for an operation characteristic of the
piezoelectric vibrator in accordance with the measured
reverberation period, wherein: the compensating means
includes at least one of a capacitor and a coil; the
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compensating means compensates for the operation
characteristic of the piezoelectric vibrator by changing a
combination of the capacitor and the coil which are
connected to the piezoelectric vibrator; the ultrasonic
sensor is mounted in a vehicle in order to detect an
obstacle around the vehicle; and the compensating means
changes the combination of the capacitor and the coil
connected to the piezoelectric vibrator when a speed of the
vehicle is more than a predetermined value.
In another aspect the invention provides an
ultrasonic sensor comprising: a piezoelectric vibrator which
vibrates to transmit an ultrasonic wave, receives a
reflected wave of the transmitted ultrasonic wave and
produces an output signal corresponding to vibration of the
piezoelectric vibrator; a reverberation measuring means for
measuring a reverberation period of the piezoelectric
vibrator from the output signal; and a compensating means
for compensating for an operation characteristic of the
piezoelectric vibrator in accordance with the measured
reverberation period, wherein: the compensating means
includes at least one of a capacitor and a coil; and the
compensating means compensates for the operation
characteristic of the piezoelectric vibrator by changing a
combination of the capacitor and the coil which are
connected to the piezoelectric vibrator; the ultrasonic
sensor is mounted on a front side of a vehicle in order to
detect an obstacle in front of the vehicle; and the
compensating means changes the combination of the capacitor
and the coil connected to the piezoelectric vibrator when
the vehicle moves back.
In another aspect the invention provides an
ultrasonic sensor comprising: a piezoelectric vibrator which
vibrates to transmit an ultrasonic wave, receives a
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reflected wave of the transmitted ultrasonic wave and
produces an output signal corresponding to vibration of the
piezoelectric vibrator; a reverberation measuring means for
measuring a reverberation period of the piezoelectric
vibrator from the output signal; and a compensating means
for compensating for an operation characteristic of the
piezoelectric vibrator in accordance with the measured
reverberation period, wherein: the compensating means
includes at least one of a capacitor and a coil; and the
compensating means compensates for the operation
characteristic of the piezoelectric vibrator by changing a
combination of the capacitor and the coil which are
connected to the piezoelectric vibrator; the ultrasonic
sensor is mounted on a rear side of a vehicle in order to
detect an obstacle behind the vehicle; and the compensating
means changes the combination of the capacitor and the coil
connected to the piezoelectric vibrator when the vehicle
moves forward.
Additional objects and advantages of the present
invention will be more readily apparent from the following
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detailed description of preferred embodiments when taken
together with the ac;companying drawings, in. which:
FIG. 1 is a circuit diagram of an ultrasonic sensor
according to a first embodiment of the present invention;
FIG. 2 is a flow diagram of adjusting processing of a
reverberation period in the ultrasonic sensor according to
the first embodiment;
FIG. 3A is a waveform diagram of an output signal from
a piezoelectric vibrator when a reverberation period does not
exceed a predetermined threshold time Tth and FIG. 3B is a
waveform diagram of an output signal when a reverberation
period exceeds the predetermined threshold time Tth;
FIG. 4 is a circuit diagram of an ultrasonic sensor
according to a second embodiment of the present invention;
FIG. 5 is a circuit diagram of an ultrasonic sensor
.according to a third embodiment of the present invention;
FIG. 6 is a circuit diagram of an ultrasonic sensor
according to a fourth embodiment of the present invention;
FIG. 7 is a circuit diagram of an ultrasonic sensor
having a single switch for ON/OFF of capacitor; and
FIG. 8 is a circuit diagram of an ultrasonic sensor
according to a pricr art.
[First Embodiment]
In FIG. 1, an ultrasonic sensor is applied as an
obstacle detection sensor for both transmitting and receiving
an ultrasonic wave. For example, this obstacle detection
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sensor is mounted in a vehicle and detects an obstacle near
the corners of the vehicle. This obstacle detection sensor
transmits an ultrasonic wave from a piezoelectric vibrator 1
and receives a reflected wave from ar.L obstacle corresponding
to a detection object. Thus, this obstacle detection sensor
detects an existence of an obstacle.
The ultrasonic sensor includes the piezoelectric
vibrator 1, multiple capacitors 2a, 2b and 2c as capacitive
components, multiple switches 3a, 3b and 3c, a vibrator
driving circuit. 4, an output signal processing circuit 5 and
a switch driving circuit 6.
The piezoelectric vibrator 1 is attached to a housing
(not shown). The piezoelectric vibrator 1 vibrates to
transmit an ultrasonic wave and receives a reflected wave
from an obstacle. An output signal corresponding to
vibration of the piezoelectric vibrator 1 is outputted to the
output signal processing circuit 5.
The capacitors 2a-2c are used to compensate for a
change of electric capacity of the piezoelectric vibrator 1
based on a temperature change or a difference of resonance
characteristics based on product unevenness. Each of the
capacitors 2a-2c is connected i_n parallel with the
piezoelectric vibrator 1. Although three capacitors are used
in the first embodiment, the number of capacitors is not
limited particularly. An electric capacity of each of the
capacitors 2a-2c is determined according to electric capacity
characteristics of the piezoelectric vibrator 1. Here, it is
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not important whether electric capacities of the capacitors
2a-2c are the same or not.
The switches 3a-3c are connected in series with the
capacitors 2a-2c respectively and are driven to ON or OFF by
the switch driving circuit 6. Thus, the capacitors 2a-2c can
be connected to or disconnected j=rom the piezoelectric
vibrator 1.
The vibrator driving circuit 4 drives the piezoelectric
vibrator 1. Specifically, the vibrator driving circuit 4
applies a voltage at a predetermined level to the
piezoelectric vibrator 1 and causes the piezoelectric
vibrator 1 to vibrate and transmit an ultrasonic wave.
The piezoelectric vibrator 1 receives a reflected wave
from an obstacle and vibrates. An output signal
corresponding to vibration of the piezoelectric vibrator 1 is
outputted to the output signal processing circuit 5. The
output signal processing circuit 5 amplifies the output
signal because it is attenuated to be lower than the voltage
applied to the piezoelectric vibrator 1 by the vibrator
driving circuit 4. Thereafter, the output signal processing
circuit 5 performs various calculations to output s. sensor
output signal of the ultrasonic sensor. For example, the
sensor output signal is inputted to an alarming buzzer
driving circuit. The alarming buzzer driving circuit
determines an existence of an obstacle or a distance to an
obstacle and performs a processing such as sounding the
alarming buzzer.
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Furthermore, in the first embodiment, the output signal
processing circuit 5 measures a reverberation period of the
output signal from the piezoelectric vibrator 1. In order to
measure the reverberation period, for example, a timer is
provided in the output signal processing circuit 5. When the
measured reverberation period exceeds a predetermined
threshold time, the output signal processing circuit 5
determines a combination of ON/OFF condition of the switches
3a-3c in order to adjust the reverberation period to be
shorter than the predetermined threshold time. In order to
determine the combination of ON/OFF condition of the switches
3a-3c, for example, the output signal processing circuit 5
stores a switch control characteristic in advance. The
switch control characteristic is a relationship between the
measured reverberation period and the combination of ON/OFF
condition of the switches 3a-3c, which is to be selected at
the corresponding measured reverberation period. The output
signal processing circuit 5 determines the combination of
ON/OFF condition of the switches 3a-3c according to the
measured reverberation period based on the switch control
characteristic. The output signal processing circuit 5
outputs a switch condition signal, which represents the
determined combina-tion of ON/OFF coridition of the switches
3a-3c, to the switch driving circuit 6.
The switch driving circuit 6 drives the switches 3a-3c
to ON or OFF based on the switch condition signal from the
output signal processing circuit 5. Thus, each of the
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capacitors 2a-2c is connected to or disconnected from the
piezoelectric vibrator 1. Therefore, a compensation for
electric capacity of the piezoelectric vibrator 1 is
performed and a reverberation period can be adjusted to be
shorter than the predetermined threshold time.
Specifically, as shown in FIG. 2, the following
adjusting processing is performed by the output signal
processing circuit 5 in order to adjust a reverberation
period.
First, at step S110, a reverberation period T is
measured by the output signal processing circuit 5. Before
the reverberation period T is measured, as described above,
the piezoelectric vibrator 1 transmits an ultrasonic wave,
receives a reflected wave from an obstacle and vibrates. The
piezoelectric vibrator 1 outputs an output signal
corresponding to vibration of the piezoelectric vibrator 1 to
the output signal processing circuit 5. The output signal
processing circuit 5 amplifies the output signal. By using a
timer or the like, the output signal processing circuit 5
measures a time it takes for the amplified output signal to
be attenuated to be a predetermined magnitude. The measured
time is the reverberation period T.
Next, at step S120, it is decided whether the
reverberation period T is longer than a predetermined
threshold time Tth. Here, the predetermined threshold time
Tth depends on a structure of the ultrasonic sensor, a place
where the ultrasonic sensor is used and the like.
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Specifically, the predetermined threshold time Tth is set in
a range that a reverberation in an obstacle detection does
not affect a subsequent obstacle detection. For example,
when the ultrasonic sensor is used as a corner sonar of a
vehicle, the predetermined threshold time Tth is set to be in
a range from lms to 2ms, preferably, lower than 1.4ms.
When it is decided that the reverberation period T is
shorter than the predetermined threshold time Tth at step
S120, the adjusting processing is immediately finished. That
is, the output-signal processing circuit 5 does not output a
switch condition signal to the switch driving circuit 6 and
an ON/OFF condition of the switches 3a-3c is not changed.
When it is decided that the reverberation period T is
longer than the predetermined threshold time Tth at step S120,
the adjusting processing proceeds to step S130. That is, the
output signal processing circuit 5 outputs a switch condition
signal to the switch driving circuit 6 and an ON/OFF
condition of the switches 3a-3c is changed. As described
above, the switch condition signal is determined based on the
switch control characteristic by the output signal processing
circuit 5. The switch condition signal, that is, a
determination whether the number of capacitors connected to
the piezoelectric vibrator 1 is increased or decreased
depends on a resonance characteristic of the piezoPlectric
vibrator 1. Specifically, the number of capacitors is
determined so that a resonance characteristic of the circuit
including the piezoelectric vibrator 1 and the capacitors
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becomes a resonance characteristic for attenuating a sigrial
of the same frequency as that of the output signal from the
piezoelectric vibrator 1.
When the ON/OFF condition of the switches 3a-3c i_s
performed at step S130, a combined electric capacity of the
capacitors 2a-2c is changed and a reverberation period is
also changed.
Thereafter, the adjusting processing is finished once
and is started again from step S110. That is, the
reverberation period T is measured again. Then, it is
decided again whether the measured reverberation period T is
longer than the predetermined threshold time Tth at step S120.
When it is decided that the measured reverberation
period T is shorter than the predetermined threshold time Tth
at step S120, it is decided that adjustment of reverberation
period is successful. Therefore, the adjusting processing is
finished. To the contrary, when it is decided that the
measured reverberation period T is longer than the
predetermined threshold time Tth at step S120, the adjusting
processing proceeds to step S130 and the above processing is
repeated until the measured reverberation period T is
adjusted to be shorter than the predetermined threshold time
Tth.
In waveform diagrams in FIGS. 3A and 3B, the
predetermined threshold time Tth is set to be 1.4ms and the
predetermined magnitude Vth to decide a reverberation period
is set to be 1V. When a reverberation period is shorter than
CA 02467233 2004-05-13
the predetermined threshold time Tth (=1.4ms) as shown in FIG.
3A, the ON/OFF condition of the switches 3a-3c is not changed
and a connection condition of the capacitors 2a-2c is not
changed.
When a reverberation period is longer than the
predetermined threshold time Tth (=1.4ms) as shown in FIG. 3B,
the ON/OFF condition of the switches 3a-3c is changed and a
connection condition of the capaci1tors 2a-2c is changed.
Thus, a reverberation period can be reduced as shown in the
waveform diagram.
As described above, the switch driving circuit 6
changed the ON/OFF condition of the switches 3a-3c and a
combination of. the capacitors 2a-2c connected to the
piezoelectric vibrator 1 is changed. Thus, a compensation
for electric capacity of the piezoelectric vibrator ]. can be
performed and a reverberation period can be adjusted to be
shorter than a predetermined threshold time. Accordingly,
the ultrasonic sensor can restrict increase of a
reverberation period, which will cause reduction in detection
accuracy and impossibility of detection of an obstacle at a
short distance.
Further, a timing for changing the ON/OFF condition of
the switches 3a-3c by the switch driving circuit 6 is not
explained above. However, a timing that a function of the
ultrasonic sensor is not affected is preferable. That is, a
timing that the ultrasonic sensor is to be driven is avoided.
For example, when the ultrasonic sensor is used as a vehicle
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sonar, the ultrasonic sensor is to be driven when the vehicle
is put into a garage. Therefore, when a vehicle speed is
more than 20km/h, it is not decided to be a timing that the
ultrasonic sensor is to be driven and the above ON/OFF
condition is changed. Further, when the ultrasonic sensor is
mounted at a vehicle front side, it is preferable that the
ON/OFF condition of the switches 3a-3c is changed when the
vehicle goes back. To the contrary, when the ultrasonic
sensor is mounted at a vehicle rear side, it is preferable
that the ON/OFF condition of the switches 3a-3c is changed
when the vehicle goes forward.
Further, in the first embodiment, a reverberation
period is adjusted to be shorter than a predetermined
threshold time. To the contrary, it is possible that upper
and lower limits of reverberation period are set. In this
case, when a reverberation period is longer than the upper
limit or is shorter than the lower limit, it is decided that
adjustment is impossible and, for example, an alarming buzzer
is sounded. Thus, when a change of a reverberation, period
becomes larger, for example, in case where a wire in a sensor
circuit is broken or the ultrasonic sensor is frozen, an
abnormality of the ultrasonic sensor can be alarmed.
[Second Embodiment]
As shown in FIG. 4, an ultrasonic sensor according to
the second embodiment is different from that of the first
embodiment in the following point. That is, a temperature
sensor 7 is provided in order to estimate a temperature of
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the ultrasonic sensor. A temperature output signal f:rom the
temperature sensor 7 is inputted to the output signal
processing circuit 5.
A position where the temperature sensor 7 is mounted is
not limited particularly. However, it is required that a
temperature in the position where the temperature sensor 7 i_s
mounted correlates with a temperature in a position where the
ultrasonic sensor is mounted. That is, it is preferable that
the temperature sensor 7 is mounted near the ultrasonic
sensor.
In the second embodiment, the output signal processirig
circuit 5 stores a switch control characteristic for
temperature compensation in advance. The switch control
characteristic for temperature compensation is a relationship
between the estimated temperature of the ultrasonic sensor
based on the temperature detected by the temperature sensor 7
and the combination of ON/OFF condition of the switches 3a-3c,
which is to be selected at the corresponding estimated
temperature of the ultrasonic sensor. The output signal
processing circuit 5 determines the combination of ON/OFF
condition of the switches 3a-3c according to the estimated
temperature based on the switch control characteristic for
temperature compensation. The output signal processing
circuit 5 outputs a switch condition signal, which represents
the determined combination of ONOOFF condition of the
switches 3a-3c, to the switch driving circuit 6.
Specifically, the switch condition signal, that is, a
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determination whether the number of capacitors connected to
the piezoelectric vibrator 1 is increased or decreased
depends on a temperature characteristic of the piezoelectric
vibrator 1. That is, a resonance characteristic of the
piezoelectric vibrator 1 changes when a temperature of the
piezoelectric vibrator 1 changes. Accordingly, the resonance
characteristic is adjusted by changing the number of
capacitors connected to the piezoelectric vibrator 1.
Specifically, the number of capacitors is determined so that
a resonance characteristic of the circuit including the
piezoelectric vibrator 1 and the capacitors becomes a
resonance characteristic for attenuating a signal of the same
frequency as that of the output signal from the piezoelectric
vibrator 1.
Thus, even. when electric capacity of the piezoelectric
vibrator 1 changes according to a temperature change and
resonance characteristic is changed, a combination of the
connected capacitors can be changed. Accordingly, a
reverberation period can be adjusted even when a temperature
of the ultrasonic sensor changes.
[Third Embodiment]
As shown in FIG. 5, an ultrasonic sensor according to
the third embodiment has multiple coils 8a-8c instead of the
multiple capacitors 2a-2c in the above embodiments.
Similarly to the above embodiments, the coils 8a-8c can also
compensate for electric capacity of the piezoelectric
vibrator 1.
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[Fourth Embodiment]
As shown in FIG. 6, an ultrasonic sensor according to
the fourth embodiment has capacitors 9a-9c and coils 10a, lOb
instead of the multiple capacitors 2a-2c in the first and
second embodiments. The capacitor 9a and the coil l0a are
connected in series and both of them are connected in
parallel with the piezoelectric vibrator 1. Similarly, the
capacitor 9b and the coil 10b are also connected as described
above. The capacitor 9c is connected in parallel with the
piezoelectric vibrator 1.
Similarly to the above embodiments, the above
combination of the capacitors 9a-9c and the coils 10a, 10b
can also compensate for electric capacity of the
piezoelectric vibrator 1. Further, the above combination of
the capacitors and the coils is merely an example. The
combination of capacitors and coils can be changed according
to an electric capacity of each capacitor and an inductance
of each coil.
[Other Embodiments]
In the above embodiments, the ON/OFF condition of the
switches 3a-3c is changed in order to adjust a reverberation
period of the output signal from the piezoelectric vibrator 1
only when the reverberation period exceeds a predetermined
threshold time. However, it is possible that the ON/OFF
condition of the switches 3a-3c is normally changed :i.n order
to adjust a reverberation period to be the shortest. in this
case, the output s:Lgnal processing circuit 5 normally outputs
CA 02467233 2004-05-13
a switch condition signal according to the output signal from
the piezoelectric vibrator 1. Based on the above switch
condition signal, the switch driving circuit 6 changes ON/OFF
condition of the switches 3a-3c.
Further, in the above embodiments, the output signal
processing circuit 5 stores the switch control characteristic
in advance. The output signal processing circuit 5
determines a combination of ON/OFF condition of the switches
3a-3c based on the switch control characteristic. To the
contrary, it is possible that the ON/OFF condition is
actually changed for all of possible combinations of ON/OFF
condition of the switches 3a-3c in order to obtain the
shortest reverberation period. In this case, the output
signal processing circuit 5 measures a reverberation period
for all of possible combinations of ON/OFF condition of the
switches 3a-3c. The output signal processing circuit 5
selects a combination with the shortest reverberation period
from all of the possible combinations.
The ultrasonic sensor according to the second
embodiment is an ultrasonic sensor that the temperature
sensor 7 is added to the ultrasonic sensor accordinc; to the
first embodiment. However, the ultrasonic sensor according
to the second embodiment can be applied to that of the third
or fourth embodiment.
In the above embodiments, the capacitors 2a-2c, 9a-9c
and the coils 8a-8c, 10a, 10b are connected in parallel with
the piezoelectric vibrator 1. However, it is possible that
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these are connected in series with the piezoelectric vibrator
1. In this case, a switch is connected in parallel with each
of the capacitors and the coils. When the switch is turned
on, current does not flow in corresponding one of the
capacitors and the coils. That is, each of the capacitors
and the coils is bypassed.
In the above embodiments, the multiple switches 3a-3c
are used for changing the ON/OFF condition of the capacitors
or the coils. However, as shown in FIG. 7, it is possible
that a single switch is used for changing the ON/OFF
condition of the capacitors or the coils.
17
