Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy
Reexamination Certificate
2002-02-28
2004-08-17
Patel, Harshad (Department: 2855)
Measuring and testing
Volume or rate of flow
By measuring vibrations or acoustic energy
C367S152000
Reexamination Certificate
active
06776051
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ultrasonic transducer for transmitting and receiving ultrasonic waves, and a method for manufacturing the ultrasonic transducer, and an ultrasonic flowmeter using the ultrasonic transducer.
2. Description of Prior Art
In recent years, an ultrasonic flowmeter which measures the time of flight of an ultrasonic wave across the propagation path, and determines the passing speed of a fluid, thereby to measure the flow rate has come into use for a gas meter or the like.
FIG. 1
is a diagram showing the measurement principle of an ultrasonic flowmeter. As shown in
FIG. 1
, a fluid flows in the direction indicated by an arrow at a velocity V in a tube. A pair of ultrasonic transducers
101
and
102
are oppositely mounted on a tube wall
103
. The ultrasonic transducers
101
and
102
are respectively configured with piezoelectric vibrators such as piezoelectric ceramics as electrical energy/mechanical energy conversion elements. Herein, the ultrasonic transducer
101
is used as an ultrasonic transmitter, and the ultrasonic transducer
102
is used as an ultrasonic receiver.
The operation is as follows. Upon application of an alternating voltage with a frequency in the vicinity of the resonance frequency of the ultrasonic transducer
101
to the piezoelectric vibrator, the ultrasonic transducer
101
emits an ultrasonic wave into an external fluid along a propagation path denoted by L
1
in the diagram. Then, the ultrasonic transducer
102
receives the propagated ultrasonic wave, and converts it into a voltage. Subsequently, the ultrasonic transducer
102
is used as an ultrasonic transmitter, and the ultrasonic transducer
101
is used as an ultrasonic receiver. Upon application of an alternating voltage with a frequency in the vicinity of the resonance frequency of the ultrasonic transducer
102
to the piezoelectric vibrator, the ultrasonic transducer
102
emits an ultrasonic wave into the external fluid along a propagation path denoted by L
2
in the diagram. Then, the ultrasonic transducer
101
receives the propagated ultrasonic wave, and converts it into a voltage.
Further, with such an ultrasonic transducer, if an alternating voltage is successively applied thereto, ultrasonic waves are successively emitted from the ultrasonic transducer. Accordingly, it becomes difficult to determine the time of flight. For this reason, in general, a burst voltage signal using a pulse signal as a carrier wave is used as a driving voltage. Hereinafter, the measurement principle will be described in more details. Upon application of a burst voltage signal for driving to the ultrasonic transducer
101
, an ultrasonic pulse wave is emitted from the ultrasonic transducer
101
. The ultrasonic pulse wave propagates through the propagation path L
1
with a length L, and reaches the ultrasonic transducer
102
after (time of flight) t hours. With the ultrasonic transducer
102
, the propagated ultrasonic pulse wave can be converted into an electrical pulse wave at a high S/N ratio. By using the electrical pulse wave as a trigger signal, the ultrasonic transducer
101
is driven again to emit an ultrasonic pulse wave. This device is referred to as a sing-around device. The time required for an ultrasonic pulse to be emitted from the ultrasonic transducer
101
, and propagate through the propagation path to reach the ultrasonic transducer
102
is referred to as a sing-around period. The inverse thereof is referred to as a sing-around frequency.
In
FIG. 1
, a reference character V denotes the flow velocity of the fluid flowing in the pipe, C denotes the velocity of an ultrasonic wave in the fluid, and &thgr; denotes the angle between the direction of flow of the fluid and the direction of propagation of an ultrasonic pulse. When the ultrasonic transducer
101
is used as a transmitter, and the ultrasonic transducer
102
is used as a receiver, the following equation (1) holds:
f
1
=1/
t
1
=(
C+V
cos &thgr;)/
L
(1)
where t
1
denotes the sing-around period which is the time for an ultrasonic pulse emitted from the ultrasonic transducer
101
to reach the ultrasonic transducer
102
, and f
1
denotes the sing-around frequency.
In contrast, when the ultrasonic transducer
102
is used as a transmitter, and the ultrasonic transducer
101
is used as a receiver, the following equation (2) holds:
f
2
=1/
t
2
=(
C−V
cos &thgr;)/
L
(2)
where t
2
denotes the sing-around period therefor, and f
2
denotes the sing-around frequency.
Accordingly, the frequency difference &Dgr;f between both the sing-around frequencies is expressed as the following equation (3), so that the flow velocity V of the fluid can be determined from the length L of the propagation path for the ultrasonic wave, and the frequency difference &Dgr;f:
&Dgr;
f=f
1
−
f
2
=
2
V
cos &thgr;/
L
(3)
Namely, it is possible to determine the flow velocity V of the fluid from the length L of the propagation path for the ultrasonic wave, and the frequency difference &Dgr;f. Therefore, it is possible to determine the flow rate from the flow velocity V.
Such an ultrasonic flowmeter is required to have a high degree of precision. In order to improve the precision, the acoustic impedance of a matching layer becomes important which is formed on the transmitting and receiving surface of ultrasonic waves in the piezoelectric vibrator configuring the ultrasonic transducer for transmitting ultrasonic waves to a gas, or receiving the ultrasonic waves propagated through the gas. The acoustic impedance of the piezoelectric vibrator for generating the ultrasonic vibrations is about 30×10
6
. The acoustic impedance of air is about 400. The ideal value of the acoustic impedance of the acoustic matching layer is about 0.11×10
6
. Further, the acoustic impedance is defined as the following equation (4):
Acoustic impedance=(density)×(sound velocity)
Therefore, a low density material, such as a material obtained by solidifying a glass balloon or a plastic balloon with a resin material, is used for the acoustic matching layer for controlling the acoustic impedance at a low level. Alternatively, there has been adopted a method in which a hollow glass ball is thermally compressed, a molten material is foamed, or the like. The method is disclosed in Japanese Patent Publication No. 2559144, or the like.
For the acoustic matching layer used in a conventional ultrasonic transducer used for an ultrasonic flowmeter, there has been adopted a method in which a hollow glass ball is thermally compressed, a molten material is foamed, or the like, as described above. For this reason, there occur the following problems. The medium tends to be heterogeneous due to fracture of the glass ball under pressure, separation under insufficient pressure, foaming of the peeled molten material, or the like. Accordingly, variations occur in characteristics, which then generates variations in device precision. Further, there also occur the following problems. For example, since the acoustic matching layer is exposed to a gas, the surface is collapsed by the moisture, or the layer is easily deteriorated by a chemically active substance, resulting in inferior durability.
SUMMARY OF THE INVENTION
The present invention has been completed for solving such problems. It is an object of the present invention to provide a high sensitivity ultrasonic transducer, which is so configured as to reduce the variations in characteristics, thereby to enable the stabilization of the precision, as well as to enable the improvement of the durability, and the like, a method for manufacturing the ultrasonic transducer, and an ultrasonic flowmeter.
An ultrasonic transducer of the present invention is so configured as to include a piezoelectric element and an acoustic matching layer, wherein the acoustic matching layer is made of a dry gel of an inorganic oxide or an organic polymer, and the solid skeletal part
Hashida Takashi
Hashimoto Masahiko
Suzuki Masa-aki
Browdy and Neimark , P.L.L.C.
Patel Harshad
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