Communications – electrical: acoustic wave systems and devices – Signal transducers – Underwater type
Reexamination Certificate
2003-05-07
2004-09-07
Lobo, Ian J. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Signal transducers
Underwater type
C310S334000
Reexamination Certificate
active
06788620
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an acoustic matching member used for an acoustic matching layer of an ultrasonic sensor, an ultrasonic transducer for transmitting/receiving ultrasonic waves, a method for manufacturing them, and an ultrasonic flowmeter using them.
2. Related Background Art
In recent years, an ultrasonic flowmeter has been used as a gas meter and the like, where a time for ultrasonic waves to propagate through a propagation path and a velocity of fluid moving therein are measured so as to determine a flow rate of the fluid.
FIG. 13
shows the principles of measurement by the ultrasonic flowmeter. As shown in
FIG. 13
, within a measurement tube including a flow path, fluid flows at a velocity of V in the direction shown by the arrow in the drawing. In a tube wall
103
, a pair of ultrasonic transducers
101
and
102
is disposed so as to oppose each other. The ultrasonic transducers
101
and
102
are configured with a piezoelectric vibrator such as a piezoelectric ceramic functioning as an electric/mechanical energy transducer, and therefore exhibit resonant characteristics like a piezobuzzer and a piezoelectric oscillator. In this case, the ultrasonic transducer
101
is used as an ultrasonic transmitter and the ultrasonic transducer
102
is used as an ultrasonic receiver.
These ultrasonic transducers operate as follows: when an AC voltage at a frequency close to a resonant frequency of the ultrasonic transducer
101
is applied to the piezoelectric vibrator, the ultrasonic transducer
101
operates as an ultrasonic transmitter so as to emit ultrasonic waves to a propagation path in the fluid flowing in the tube, which is indicated by L
1
in the drawing, and the ultrasonic transducer
102
receives the ultrasonic waves that have propagated and converts them to voltage. Subsequently, the ultrasonic transducer
102
conversely is used as an ultrasonic transmitter and the ultrasonic transducer
101
is used as an ultrasonic receiver. That is, by applying an AC voltage at a frequency close to a resonant frequency of the ultrasonic transducer
102
to the piezoelectric vibrator, the ultrasonic transducer
102
emits ultrasonic waves to a propagation path in the fluid flowing in the tube, which is indicated by L
2
in the drawing, and the ultrasonic transducer
101
receives the ultrasonic waves that have propagated and converts them to voltage. In this way, each of the ultrasonic transducers
101
and
102
serves as the receiver and the transmitter, and therefore, in general, they are called an ultrasonic transmitter/receiver.
In such an ultrasonic flowmeter, the continuous application of an AC voltage results in the continuous emission of ultrasonic waves from the ultrasonic transducer, which makes it difficult to measure the propagation time. Therefore, normally, a burst voltage signal is used as a driving voltage, where a pulse signal is used as a carrier wave. A more detailed description of the measurement principles will be given below. By applying a burst voltage signal to drive the ultrasonic transducer
101
and allow the ultrasonic transducer
101
to emit an ultrasonic burst signal, this ultrasonic burst signal propagates through a propagation path L
1
with a length of L to arrive at the ultrasonic transducer
102
after the time t has elapsed. The ultrasonic transducer
102
can convert the ultrasonic burst signal that has propagated only into an electric burst signal at a high S/N ratio. This electric burst signal is amplified electrically and is applied again to the ultrasonic transducer
101
to allow an ultrasonic burst signal to be emitted. This device is called a sing around device. A time required for an ultrasonic pulse to be emitted from the ultrasonic transducer
101
and propagate through the propagation path to arrive at the ultrasonic transducer
102
is called a sing around period, and the reciprocal of the sing around period is called a sing around frequency.
In
FIG. 13
, V denotes a flow velocity of fluid that flows through the tube, C (not illustrated) denotes a velocity of an ultrasonic wave in the fluid and &thgr; denotes an angle between the flowing direction of the fluid and the propagation direction of the ultrasonic pulse. When the ultrasonic transducer
101
is used as an ultrasonic transmitter and the ultrasonic transducer
102
is used as an ultrasonic receiver, the following formula (1) will be satisfied, where t
1
denotes a sing around period that is a time for an ultrasonic pulse emitted from the ultrasonic transducer
101
to arrive at the ultrasonic transducer
102
, and f
1
denotes a sing around frequency:
f
1
=1
/t
1
=(
C+V
cos &thgr;)/
L
(1)
Conversely, when the ultrasonic transducer
102
is used as an ultrasonic transmitter and the ultrasonic transducer
101
is used as an ultrasonic receiver, the following formula (2) will be satisfied, where t
2
denotes a sing around-period and f
2
denotes a sing around frequency:
f
2
=1
/t
2
=(
C−V
cos &thgr;)/
L
(2)
Therefore, a frequency difference &Dgr;f between the both sing around frequencies will be the following formula (3), so that the flow velocity V of the fluid can be determined from the length L of the propagation path of ultrasonic waves and the frequency difference &Dgr;f:
&Dgr;
f=f
1
−f
2
=2
V
cos &thgr;/L (3)
That is to say, the flow velocity V of the fluid can be determined from the length L of the propagation path of ultrasonic waves and the frequency difference &Dgr;f, and a flow rate can be determined from the velocity V.
Such an ultrasonic flowmeter requires high accuracy. In order to improve the accuracy, an acoustic impedance of an acoustic matching layer becomes important, where the acoustic matching layer is formed on a surface for transmitting/receiving ultrasonic waves of the piezoelectric vibrator constituting the ultrasonic transducer for transmitting the ultrasonic waves to gas or receiving the ultrasonic waves that have propagated through gas.
FIG. 12
is a cross-sectional view showing a configuration of a conventional ultrasonic transducer
20
. Reference numeral
10
denotes an acoustic matching layer functioning as an acoustic matching device,
5
denotes a sensor case,
4
denotes electrodes, and
3
denotes a piezoelectric member functioning as a vibration device. The sensor case
5
and the acoustic matching layer
10
or the sensor case
5
and the piezoelectric member
3
are bonded with an epoxy adhesive and the like. Reference numeral
7
of
FIG. 12
denotes driving terminals, which are respectively connected to the electrodes
4
of the piezoelectric member
3
. Reference numeral
6
denotes an insulation seal for securing electrical insulation of the two driving terminals. Ultrasonic waves generated from vibrations of the piezoelectric member
3
oscillate at a specific frequency, and the oscillation is conveyed to the case via the epoxy adhesive, and further is conveyed to the acoustic matching layer
10
via the epoxy adhesive. The matched oscillation propagates as an acoustic wave through gas as a medium that is present in the space.
This acoustic matching layer
10
has a role of allowing the vibrations of the vibration device to propagate effectively through the gas. The acoustic impedance Z will be defined as the following formula (4) using a sound velocity C and a density &rgr; of the substance:
Z=&rgr;×C
(4)
The acoustic impedance is different significantly between the piezoelectric member as the vibration device and the gas as a medium to which ultrasonic waves are emitted (hereinafter called “emission medium”). For instance, the acoustic impedance of a piezo-ceramic such as PZT (lead zirconate titanate), which is a common piezoelectric member, is about 30×10
6
kg/m
2
/s. Whereas, for the gas as the emission medium, the acoustic impedance (Z
3
) of air, for example, is about 400 kg/m
2
/s. On a boundary surface between the substances wit
Hashida Takashi
Hashimoto Kazuhiko
Hashimoto Masahiko
Nagahara Hidetomo
Shiraishi Seigo
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