Abstrict An ultrasonic flow meter for measuring a flow rate in a flow path
is provided having ultrasonic sensors each of which includes a semicircular
or substantially circular disk shaped transducer. Such transducers
are detachably mountable on an envelope of the flow path. The thickness
of the ultrasonic sensors in the direction of the length of the
flow path is reduced to improve a detection resolution. A plurality
of the ultrasonic sensors are mounted on the flow path while keeping
a predetermined distance therebetween in a length direction of the
flow path, so that the flow rate can be measured from the difference
of propagation times of ultrasonic waves between the ultrasonic
sensors.
Claims What is claimed is:
1. An ultrasonic flow meter comprising: at least two ultrasonic
sensors each for emitting and detecting ultrasonic waves; wherein
the ultrasonic sensors are disposed at a predetermined interval
in a length direction of a flow path so as to measure a flow rate
from a difference in propagation times of the ultrasonic waves between
the ultrasonic sensors; wherein the ultrasonic sensors each comprise
a semicircular disk shaped transducer having a notch which comes
into tight contact with an envelope of the flow path so as to be
detachably mounted on the envelope of the flow path; wherein the
ultrasonic waves which are emitted from the ultrasonic sensors are
propagated in all directions as spherical waves, including directly
along the flow path in upstream and downstream directions so as
to be directly detected by each other; and wherein a ratio (t/L)
of a thickness t of the ultrasonic sensors in the length direction
of the flow oath to a length L between the ultrasonic sensors does
not exceed a reference value of a detection resolution of the ultrasonic
sensors.
2. The ultrasonic flow meter according to claim 1 wherein the
notch is semicircular so as to be detachably mounted on a semicircular
portion of the envelope of the flow path.
3. The ultrasonic flow meter according to claim l, wherein the
ultrasonic flow meter comprises at least three ultrasonic sensors
which are disposed at predetermined intervals along the length direction
of the flow path.
4. The ultrasonic flow meter according to claim 1 wherein the
ultrasonic sensors are fixed to the envelope of the flow path with
an adhesive agent so as to come into tight contact with a semicircular
portion of the envelope of the flow path.
5. The ultrasonic flow meter according to claim 1 wherein the
ultrasonic sensors are fixed to the envelope of the flow path by
a mounting device having a resilient member such that the notches
of the transducers come into tight contact with a semicircular portion
of the envelope of the flow path.
6. An ultrasonic flow meter comprising: at least two ultrasonic
sensors each for emitting and detecting ultrasonic waves; wherein
the ultrasonic sensors are disposed at a predetermined interval
in a length direction of a flow path so as to measure a flow rate
from a difference in propagation times of the ultrasonic waves between
the ultrasonic sensors; wherein the ultrasonic sensors each comprise
a substantially circular disk shaped transducer having a notch,
and the ultrasonic sensors are detachably mounted on an envelope
of the flow path; wherein the ultrasonic waves which are emitted
from the ultrasonic sensors are propagated in all directions as
spherical waves, including directly along the flow path in upstream
and downstream directions so as to be directly detected by each
other; and wherein a ratio (t/L) of a thickness t of the ultrasonic
sensors in the length direction of the flow path to a length L between
the ultrasonic sensors does not exceed a reference value of a detection
resolution of the ultrasonic sensors.
7. The ultrasonic flow meter according to claim 6 wherein the
ultrasonic sensors are detachably mounted on the envelope of the
flow path such that a center of the flow path is substantially aligned
with a center of the substantially circular disk shaped transducer.
8. The ultrasonic flow meter according to claim 6 wherein the
ultrasonic flow meter comprises at least three ultrasonic sensors
which are disposed at predetermined intervals along the length direction
of the flow path.
9. The ultrasonic flow meter according to claim 6 wherein the
ultrasonic sensors are fixed to the envelope of the flow path with
an adhesive agent such that the notch of the transducer comes into
tight contact with a semicircular portion of the envelope of the
flow path.
10. The ultrasonic flow meter according to claim 6 wherein the
ultrasonic sensors are fixed to the envelope of the flow path by
a mounting device having a resilient member such that the notches
of the transducers come into tight contact with a semicircular portion
of the envelope of the flow path.
11. An ultrasonic sensor for measuring a flow rate in a flow path,
the ultrasonic sensor comprising a disk shaped transducer having
a notch which comes into tight contact with an envelope of the flow
path so as to be detachably mounted to the envelope of the flow
path, wherein a ratio (t/L) of a thickness t of the ultrasonic sensor
in a length direction of the flow oath to a length L between the
ultrasonic sensor and an additional ultrasonic sensor does not exceed
a reference value of a detection resolution of the ultrasonic sensor.
12. The ultrasonic sensor according to claim 11 wherein the notch
is semicircular so as to be detachably mounted on a semicircular
portion of the envelope of the flow path.
13. An ultrasonic sensor for measuring a flow rate in a flow path,
the ultrasonic sensor comprising a substantially circular disk shaped
transducer having a notch, wherein ultrasonic sensors are detachably
mounted on an envelope of the flow path, wherein a ratio (t/L) of
a thickness t of the ultrasonic sensor in a length direction of
the flow path to a length L between the ultrasonic sensor and an
additional ultrasonic sensor does not exceed a reference value of
a detection resolution of the ultrasonic sensor.
14. The ultrasonic sensor according to claim 13 wherein the ultrasonic
sensor is detachably mounted on the envelope of the flow path such
that a center of the flow path is substantially aligned with a center
of the substantially circular disk shaped transducer.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ultrasonic flow meter and an
ultrasonic sensor for measuring a flow rate of liquid in a flow
path by an ultrasonic wave and, more specifically, to an ultrasonic
flow meter having ultrasonic sensors which can be easily mounted
to the flow path and which are capable of accurately measuring the
flow rate of the liquid even when the inner diameter of the flow
path is small.
2. Description of the Prior Art
Hitherto, an ultrasonic flow meter for measuring a flow rate of
liquid in a flow path has been used in a semiconductor manufacturing
apparatus or in equipment in a plant. The ultrasonic flow meter
is an instrument for measuring a flow rate by measuring a difference
of propagation times of an ultrasonic wave in liquid in the flow
path between upstream and downstream directions. Various proposals
regarding the ultrasonic flow meter using the difference of propagation
times of the ultrasonic wave have been provided as described below.
JP-A-10-122923 discloses an ultrasonic flow meter having two annular
ultrasonic wave transducers provided on the outer peripheral surface
of a measuring tube so as to be tightly fitted thereon at a predetermined
interval, in which, when the measuring tube is filled with a fluid,
an ultrasonic wave generated by a transducer is transmitted through
a tube wall to a fluid, then is propagated toward the center of
the transducer (center of the straight pipe), and then is redirected
at a right angle and transmitted forward and backward along the
length of the tube.
JP-A-61-132823 discloses an ultrasonic flow meter having a pair
of ring-shaped transducers, each having a radius at a hollow portion
thereof which is equivalent to an inner diameter of a flow path,
which are mounted via acoustic insulating material at an adequate
interval so that the center axis of the ring-shaped transducers
and the center axis of the flow tube are aligned.
U.S. Pat. No. 5594181 discloses an ultrasonic flow meter in which
two ring-shaped piezoelectric transducers are fitted at a predetermined
interval into a solid portion of a pipe for measuring a flow rate
of a liquid flowing between the transducers.
The ultrasonic flow meters described above are constructed to measure
propagation times of ultrasonic waves by means of two ring-shaped
ultrasonic transducers provided on the measuring tube while switching
each ultrasonic transducer alternately between an ultrasonic transmission
mode and an ultrasonic reception mode, and then calculating the
flow rate in the tube. Such ultrasonic flow meters using ring-shaped
ultrasonic transducers are useful for measuring a flow rate in a
narrow tube which accommodates a minute amount of flow.
U.S. Pat. No. 3987674 discloses an ultrasonic flow meter in which
a storage case including an ultrasonic transducer integrated therein
is fixed along the length of the flow path by means of a mounting
device or the like, and in which another storage case is disposed
and fixed thereon at a predetermined interval so that the ultrasonic
transducers face each other (clamp-on system).
JP-A-2001-74527 discloses a multipass system ultrasonic flow meter
in which a plurality of pairs of transducers are provided, wherein
each pair propagates an ultrasonic wave in the direction of flow
of fluid and in the opposite direction therefrom, so that the number
of traverse lines increases and thus variations in measurement caused
by variations in distribution of flow velocity are reduced.
JP-A-2002-221440 discloses an ultrasonic flow meter including measuring
units provided on a measuring tubular body at an interval, wherein
the measuring units each include an arcuate transducer fixed on
part of the peripheral portion along the circumference of the tubular
body with adhesive agent.
In recent years, in a process of manufacturing semiconductors,
there is a trend to use a small amount of high-priced chemicals
as a result of miniaturization of the process, to reduce manufacturing
cost, or for environmental reasons. Therefore, the demands for more
accurate management of the flow rate of chemicals in a flow path
of a small aperture are increasing.
However, the ultrasonic flow meters and the ultrasonic sensors
in the prior art have the following problems.
The ultrasonic flow meter employing ring-shaped sensors may be
designed to secure a long distance between the sensors, and thus
a small flow rate can be measured with a high degree of accuracy.
However, when mounting the ring-shaped sensors on the flow path,
the piping of the flow path has to be disassembled, and hence ring-shaped
sensors cannot be employed in an apparatus in which the piping of
the flow path cannot be disassembled (Problem 1).
In the case of a clamp-on system, as shown in the layout of ultrasonic
sensors 20 in FIG. 10A, since the distance (L) between the ultrasonic
sensors 20 has to be reduced when the aperture of a flow path 5
is reduced as in FIG. 10B, the difference in propagation times of
ultrasonic waves generated by the flow of liquid becomes small,
making it difficult to achieve measurement with a high degree of
accuracy (Problem 2).
In the case of the ultrasonic flow meter employing the multipass
system, though it is adapted to reduce errors or variations in measurement
resulting from variations in distribution of flow velocity by increasing
the number of traverse lines, it is difficult to realize multiple
traverse lines by providing a plurality of ultrasonic sensors when
the aperture of the flow path is small (Problem 3).
As shown in FIG. 11B, in the case of the ultrasonic flow meter
on which arcuate ultrasonic transducers 21 are fixed with an adhesive
agent, the length L between the ultrasonic sensors 21 may be increased,
and the clamp-on system is also possible. However, since the dimension
t.sub.o of the transducer along the flow path 5 in the direction
of the length of the flow path (the direction of liquid flow) is
large, detection resolution in the direction along the flow path
5 (t.sub.o/L) is reduced, and thus sufficient measuring accuracy
cannot be obtained (Problem 4). This is because the detection resolution
of the ultrasonic sensors is reduced due to the large ratio of the
dimensions of the two-way transmission sensors to the distance between
the ultrasonic sensors.
Still, further, the ultrasonic sensor employing a transducer which
is large in a dimension in the direction along the length of the
flow path has a problem in that the sensitivity of the transducer
to transmission of the ultrasonic wave being propagated in the direction
along the flow path is low (Problem 5). This is because the area
of a sound source increases when the dimension of the transducer
in the direction of the length of the flow path is large, and thus
directivity of the ultrasonic waves transmitted by the ultrasonic
sensor increases correspondingly and, as a consequence, efficiency
of propagation of the ultrasonic waves in liquid is lowered, whereby
sensitivity to transmission is lowered.
SUMMARY OF THE INVENTION
In view of the above-described Problems 1 to 5 it is an object
of the present invention to provide an ultrasonic flow meter having
ultrasonic sensors which can be easily mounted to the flow path
and which are capable of measuring the flow rate with a high degree
of accuracy even when the inner diameter of the flow path is small.
According to the present invention, an ultrasonic flow meter is
provided which includes at least two ultrasonic sensors each for
emitting and detecting ultrasonic waves. The ultrasonic sensors
are disposed at a predetermined interval in a length direction of
a flow path so as to measure a flow rate from a difference in propagation
times of the ultrasonic waves between the ultrasonic sensors. Each
of the ultrasonic sensors comprises a semicircular disk shaped transducer
that has a notch which comes into tight contact with an envelope
of the flow path so as to be detachably mounted on the envelope
of the flow path. The ultrasonic waves which are emitted from the
ultrasonic sensors are propagated in all directions as spherical
waves along the flow path in upstream and downstream directions
so as to be directly detected by each other.
Alternatively, the ultrasonic sensors of the flow meter according
to the present invention may each comprise a substantially circular
disk shaped transducer having a notch. The substantially circular
disk shaped transducer detachably mounted to an envelope of the
flow path so as to substantially align a center of the flow path
substantially with a center of the substantially circular disk.
According to the present invention, since the ultrasonic sensors
are formed with a notch, the ultrasonic sensors can be fixed to
the flow path with an adhesive agent or mounted to the flow path
by means of a mounting device through a one-touch operation, and
thus the scope of application of the flow rate measurement may be
enlarged.
Since the ultrasonic sensors of the present invention may be mounted
on the flow path at an increased distance as in the case of the
ring-shaped ultrasonic sensors in the prior art, measurement of
the flow rate may be performed with a high degree of accuracy even
when the aperture of the flow path is small.
Since measurement by combining measured values from pairs of sensors
may be performed by disposing more than two sensors on the flow
path, measurement of a high degree of accuracy is achieved also
for transient flow including laminar flow, turbulent flow, or for
pulsating flow.
Since detection resolution of the ultrasonic sensors may be increased
by reducing the thickness of the ultrasonic sensors, reduction of
errors resulting from the detection resolution is achieved.
In addition, by reducing the thickness of, and increasing the outer
diameter of, the semicircular or substantially circular disk shaped
transducers of the ultrasonic sensors, reduction of a resonance
frequency of the ultrasonic sensors is achieved. Accordingly, attenuation
of ultrasonic vibrations may be controlled, and the effect of noise
or the like may be reduced, whereby measurement of flow rate with
high degree of accuracy is achieved.
BRIEF DESCRIPTION OF THE DRAWINGS
This patent or application contains at least one drawing executed
in color. Copies of this patent or patent application publication
with color drawings(s) will be provided by the Office upon request
and payment of the necessary fee.
FIG. 1A is a front view showing a configuration of an ultrasonic
sensor including a semicircular disk formed with a notch;
FIG. 1B is a bottom view of the ultrasonic sensor;
FIG. 2A is a front view showing a configuration of an ultrasonic
sensor including a substantially circular disk formed with a notch;
FIG. 2B is a bottom view of the ultrasonic sensor shown in FIG.
2A;
FIG. 3A is a cross-sectional view showing a state in which the
ultrasonic sensor including a semicircular disk formed with a notch
is fixed to a flow path with an adhesive agent;
FIG. 3B is a cross-sectional view showing a state in which the
ultrasonic sensor including a substantially circular disk formed
with a notch is fixed to the flow path with an adhesive agent;
FIG. 4A is a front view showing a state in which the ultrasonic
sensor is fixed to the flow path by means of a mounting device;
FIG. 4B is a side view of the state shown in FIG. 4A;
FIG. 5 is a perspective view showing a state in which the ultrasonic
sensor is fixed to the flowpath with an adhesive agent;
FIG. 6A shows a result of a three-dimensional calculation of sound
pressure distribution showing propagation of the ultrasonic wave
based on Cellular Automaton (CA) on a cross section A in FIG. 5;
FIG. 6B shows a result of a three-dimensional calculation of sound
pressure distribution showing propagation of the ultrasonic wave
based on Cellular Automaton (CA) on a cross section B in FIG. 5;
FIG. 7 is a block diagram of a control unit of the ultrasonic flow
meter;
FIG. 8 is a flowchart relating measurement of delay time required
when the control unit calculates the flow rate;
FIG. 9 is a flowchart relating measurement of the flow rate in
the case in which three ultrasonic sensors are mounted to the outer
periphery of the flow path;
FIG. 10A shows a length L between the ultrasonic sensors in the
arrangement of the clamp-on ultrasonic sensors of the ultrasonic
flow meter according to the prior art;
FIG. 10B shows the length L between the ultrasonic sensors in the
arrangement in which the aperture of the flow path is reduced according
to the prior art;
FIG. 11A shows the relationship between the thickness of and the
mounting distance between the ultrasonic sensors of the ultrasonic
flow meter according to the present invention; and
FIG. 11B shows the relationship between the thickness of and the
mounting distance between the ultrasonic sensors of the ultrasonic
flow meter according to the prior art.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, an embodiment of an ultrasonic flow
meter having ultrasonic sensors according to the present invention
will be described. The ultrasonic flow meter according to the present
invention is constructed in such a manner that the ultrasonic sensors
can be easily mounted to the flow path and the flow rate can be
measured with a high degree of accuracy even when the inner diameter
of the flow path is small.
As shown in FIG. 1A, an ultrasonic sensor 1 is a sensor including
an ultrasonic transducer, which is formed to be thin in the direction
of length of the flow path 5 and which includes a semicircular
disk with an outer radius of Rb having a notch 2 defining a semicircular
space with an inner radius of Ra. As shown in FIG. 1B, the semicircular
disk is formed to have thickness t. The notch 2 of the ultrasonic
sensor 1 is formed for mounting the ultrasonic sensor 1 to the flow
path 5 (shown in FIG. 3 and FIG. 4), and is configured so that the
inner surface of the ultrasonic sensor 1 and the envelope of the
flow path 5 come into contact with each other. The thickness t of
the ultrasonic sensor 1 in the direction of the length of the flow
path 5 (the direction of liquid flow) is designed so that the ratio
of the thickness t with respect to the length L between the ultrasonic
sensors (t/L) does not exceed a reference value s corresponding
to a detection resolution of the ultrasonic sensor. In other words,
the ratio (t/L) is determined to be (t/L).ltoreq.s. The reference
value s is the detection resolution required for the ultrasonic
sensor for obtaining the accuracy required for measuring a flow
rate.
The notch 2 shown in FIG. 1 and FIG. 2 is formed for detachably
mounting the ultrasonic sensor 1 to the envelope of the flow path
5.
The ultrasonic sensor 1 shown in FIG. 1 includes a semicircular
disk having a semicircular notch, while the ultrasonic sensor 1
shown in FIG. 2A includes a substantially circular disk with an
outer radius of Rd formed with a notch 2 having a semicircular portion
with a radius of Rc and a portion cut out so as to have an inner
surface extending substantially perpendicularly from a diameter
of the semicircular portion to an outer periphery of the substantially
circular disk, and having a width corresponding to the diameter
2Rc of the semicircle so that the center of the flow path 5 (FIG.
3B) substantially coincides with the center of the substantially
circular disk. As shown in FIG. 2B, the substantially circular disk
having the notch 2 is formed to have a thickness of t.
The ultrasonic sensor 1 shown in FIG. 1 and FIG. 2 is formed of
a piezoelectric material such as a PZT (lead zirconate titanate)
ceramic material. The ultrasonic sensor 1 is provided with an electrode
(not shown) for applying a voltage to a transducer or for picking
up the voltage generated at the transducer. A lead wire is connected
to the electrode of the transducer so that application of the voltage
and detection of the voltage may be performed from the outside.
The ultrasonic sensor is adapted to be driven by a voltage of 100
KHz to 1 MHz in frequency.
The cutout range of the notch 2 of the ultrasonic sensor 1 is from
the semicircular shape shown in FIG. 1 to the shape formed by cutting
out part of the substantially circular disk as shown in FIG. 2.
Variations in size of the notch 2 within this range do not affect
the measuring accuracy.
FIG. 3A is a cross-sectional view showing a state in which the
ultrasonic sensor 1 including the semicircular disk formed with
the notch 2 is fixed to the flow path 5 with an adhesive agent 4.
FIG. 3B shows a state in which the ultrasonic sensor 1 including
the substantially circular disk formed with the notch 2 is fixed
to the flow path 5 with an adhesive agent 4. As shown in FIGS. 3A
and 3B, the ultrasonic sensor 1 is mounted so that substantially
half of the outer periphery of the flow path 5 comes into tight
contact with the inner periphery of the ultrasonic sensor 1 and
the ultrasonic sensor 1 is fixed to the outer periphery of the flow
path 5 via the adhesive agent 4. Ultrasonic vibrations are transmitted
from the inner peripheral portion of the ultrasonic sensor 1 that
is in contact with the outer periphery of the flow path 5 via the
adhesive agent 4.
While an example in which the ultrasonic sensor 1 is fixed to the
flow path 5 using the adhesive agent 4 is shown in the fixing method
shown in FIGS. 3A and 3B, a method for fixing the ultrasonic sensor
1 to the flow path 5 by means of a mounting device will be described
below.
FIGS. 4A and 4B show a state in which the ultrasonic sensor 1 including
the substantially circular disk formed with the notch 2 is fixed
to the flow path 5 by means of the mounting device 3. As shown in
FIGS. 4A and 4B, the mounting device 3 includes an upper limit stopper
3b, a lower limit stopper 3c, and a spring 3a. The upper limit stopper
3b is shaped like a final bracket, and is attached with the spring
3a as a resilient member on the upper portion. The upper portion
of the lower limit stopper 3c comes into contact with the other
end of the spring 3a. The upper limit stopper 3b is formed with
a notch in the lower portion so as to prevent the ultrasonic sensor
1 from coming into contact therewith. The lower limit stopper 3c
is shaped like a final bracket, and is formed with an arcuate notch
in the upper portion thereof so as to come into contact with part
of the outer periphery of the ultrasonic sensor 1. The midsection
of the lower limit stopper 3c penetrates through the lower portion
of the upper limit stopper 3b, so that the lower limit stopper 3c
is capable of moving in a vertical direction. The upper limit stopper
3b and the lower limit stopper 3c are connected with each other
via the spring 3a. The lower limit stopper 3c and the upper limit
stopper 3b are formed of plastic or the like so as to absorb vibrations
from the ultrasonic sensor 1 and to prevent such vibrations from
being transmitted to other portions.
When mounting the ultrasonic sensor 1 to the flow path 5 by means
of the mounting device 3 the semicircular portion of the notch
2 of the ultrasonic sensor 1 comes into contact with a predetermined
position of the envelope of the flow path 5 and then the mounting
device 3 is slid into place while pushing up the lower limit stopper
3c so as to clamp the ultrasonic sensor 1 so that the notch on
the upper portion of the lower limit stopper 3c comes into contact
with part of the outer periphery of the ultrasonic sensor 1. Consequently,
the lower portion of the upper stopper 3b comes into contact with
the flow path 5 and the ultrasonic sensor 1 is forced down by a
spring pressure applied from above so that the envelope of the flow
path 5 and the ultrasonic sensor 1 are kept constantly in tight
contact with each other. By using the mounting device 3 the ultrasonic
sensor 1 can be mounted to the flow path 5 through a one-touch operation
without using the adhesive agent 4.
Although an example in which the ultrasonic sensor 1 including
the substantially circular disk formed with the notch 2 is fixed
to the flow path 5 by means of the mounting device 3 is shown in
FIGS. 4A and 4B, the ultrasonic sensor 1 including the semicircular
disk formed with the notch 2 may also be fixed to the flow path
5 by means of the mounting device 3.
More than two ultrasonic sensors 1 are preferably mounted to the
outer periphery of the flow path 5 with the adhesive agent 4 or
by means of the mounting device 3.
Wave motion of the ultrasonic waves emitted by the ultrasonic sensor
1 will be described. Generally, the sound source of an ultrasonic
wave may be considered to be a group of minute point sound sources.
The respective point sound sources are vibrated and the waves formed
thereby overlap with each other to form a wave surface. The ultrasonic
sensor 1 of the ultrasound flow meter according to the present invention
may be considered in the same manner. In other words, a contact
point between the flow path 5 and the notch 2 of the ultrasonic
sensor 1 is the sound source, and when the sound source vibrates
in the radial direction of the ultrasonic sensor 1 the wave generated
by the vibration thereof is propagated in fluid in the flow path
5. Since the oscillating wave from the ultrasonic sensor 1 is propagated
as a spherical wave, the ultrasonic wave is propagated in all directions
in the flow path 5 including a wave propagated in the radial direction
of the ultrasonic sensor 1 and a wave propagated to the upstream
and the downstream directions in the flow path 5. The ultrasonic
wave has such a nature that even when a plurality of ultrasonic
waves collide, they do not affect each other, and their direction,
amplitudes, and wavelength are not changed (Independency of Wave).
Therefore, the ultrasonic wave emitted by the ultrasonic sensor
1 is propagated directly in the upstream and the downstream directions
in the flow path 5 and is directly detected as a signal by another
one of the ultrasonic sensors 1. By contrast, ultrasonic waves which
are propagated toward the center of the flow path 5 and which are
then redirected at a right angle, and then propagated in the upstream
and the downstream directions in parallel with the flow path 5 are
not detected. Therefore, even when the ultrasonic sensor 1 has a
notch, the sound wave can be propagated in the upstream and the
downstream direction. FIG. 6A and 6B show a result of a three-dimensional
calculation of sound pressure distribution showing propagation of
an ultrasonic wave based on Cellular Automaton (CA). FIG. 5 is a
perspective view showing a state in which the ultrasonic sensors
are fixed to the flow path with adhesive agent; FIG. 6A shows a
result of calculation on a cross section A in FIG. 5 and FIG. 6B
shows a result of calculation on a cross section B in FIG. 5 in
which the contact portion between the flow path 5 and the ultrasonic
sensor 1 is considered to be a sound source surface, and is divided
into minute portions for calculation. In FIGS. 6A and 6B, measurement
is conducted by using a tube of having an outer diameter of 3 mm
and an inner diameter of 2 mm as the flow path, fluid flowing in
the flow path at a rate of 1500 m/s, and an ultrasonic wave having
a frequency of 100 kHz. It is clear from these drawings that the
ultrasonic wave emitted by the ultrasonic sensor 1 is propagated
as a spherical wave directly in the upstream and the downstream
directions of the flow path 5.
Subsequently, the ultrasonic flow meter for measuring the flow
rate by driving the ultrasonic sensor 1 will be described.
FIG. 7 is a block diagram of a control unit 7 of the ultrasonic
flow meter, the control unit 7 controlling a plurality of ultrasonic
sensors 1. As shown in FIG. 7 the control unit 7 includes a measuring
system switching circuit 7a for selecting ultrasonic sensors for
transmitting the ultrasonic wave and for receiving the ultrasonic
wave from the ultrasonic sensors 1a, 1b, and 1c, a pulse generating
circuit 7b for generating a pulsed drive signal in the transducer
of the selected ultrasonic sensor, a receiving circuit 7c for receiving
the signal from the selected ultrasonic sensor, an AD converter
7d for converting signal outputted from the receiving circuit 7c
into digital data, a computer 7f for controlling transmission and
reception of the ultrasonic sensor and calculating the flow rate
from received digital data, and a display unit 7g for displaying
the measured flow rate and the like.
Subsequently, referring to flow charts shown in FIG. 8 and FIG.
9 measurement of delay time required when the control unit 7 shown
in FIG. 7 calculates the flow rate will be described.
As shown in FIG. 8 the computer 7f outputs a sensor selection
signal for selecting the ultrasonic sensors 1 for transmitting and
for receiving the ultrasonic wave to the measuring system switching
circuit 7a (Step S1). Then, the computer 7f outputs a pulse-activating
signal for allowing the selected ultrasonic sensor 1 to transmit
the ultrasonic wave to the pulse generating circuit 7b (Step S2).
After having outputted the pulse-activating signal to the pulse
generating circuit 7b, the computer 7f activates a timer built in
the computer 7f to perform clocking (Step S3). The pulsed drive
signal outputted from the pulse generating circuit 7b is applied
to the ultrasonic sensor 1 for transmission from the measuring system
switching circuit 7a. The timer is adapted to notify the CPU of
the computer 7f that a preset time period has elapsed immediately
after the elapse thereof.
The signal from the ultrasonic sensor 1 for reception is entered
into the receiving circuit 7c via the measuring system switching
circuit 7a. The receiving circuit 7c performs amplification and
noise processing of the signal and outputs it to the AD converter
7d. The AD converter 7d converts the analogue signal received from
the receiving circuit 7c into digital data and outputs it to an
input/output circuit of the computer 7f.
The computer 7f reads digital data from the AD converter 7d, which
is connected to the input/output circuit, and stores received digital
data to a memory (Step S4). The computer 7f reads digital data at
predetermined time intervals from the AD converter 7d and stores
in the memory continuously while incrementing the address of the
memory. The computer 7f continues this operation until receiving
a time-up signal from the timer (Step S5).
The computer 7f reads data from the address in which digital data
received from the AD converter 7d is stored in sequence after receiving
the time-up signal from the timer, compares read data with a reference
value which is set in advance, determines the address of the memory
having data exceeding the reference value, and calculates a delay
time. Since digital data from the AD converter 7d is written into
the address of the memory at predetermined time intervals, the delay
time is calculated by calculating an offset value of the address
from the first address where data was written for the first time,
and multiplying the time intervals of writing (Step S6).
Subsequently, referring to a flowchart shown in FIG. 9 measurement
of the flow rate in the case where three ultrasonic sensors 1a,
1b, and 1c shown in FIG. 7 are mounted to the outer periphery of
the flow path 5 will be described.
The computer 7f outputs a signal to the measuring system switching
circuit 7a so as to select the ultrasonic sensor la for transmission
and the ultrasonic sensor 1b for reception (Step S10). Then, the
delay time calculating process shown in the flowchart in FIG. 8
is performed to calculate the delay time (Step S1). The calculated
delay time is stored as tab in the memory (Step S12).
Subsequently, the computer 7f outputs a signal to the measuring
system switching circuit 7a so as to select the ultrasonic sensor
1b for transmission and the ultrasonic sensor la for reception (Step
S13). Subsequently, the delay time calculating process shown in
the flowchart in FIG. 8 is performed to calculate the delay time
(Step S14). The calculated delay time is stored as tba in the memory
(Step S15). The flow velocity is calculated from the delay time
tab and the delay time tba, and then the flow rate Lab is obtained
from the flow velocity.
In the same procedure as described above, the respective flow rate
Lac and Lbc are obtained from the combinations of the ultrasonic
sensors 1a and 1c, and the ultrasonic sensors 1b and 1c, and then
the average value of the Lab, Lac and Lbc is calculated to obtain
the flow rate L. Consequently, the obtained flow rate is higher
in the degree of accuracy in comparison with the flow rate obtained
from a pair of ultrasonic sensors 1. By measuring the flow rate
with the plurality of combinations of ultrasonic sensors, errors
of measurement resulting from the effect of the pulsated flow may
be reduced.
As described thus far, according to the ultrasonic flow meter of
the present invention, since the ultrasonic sensor 1 can be fixed
to the flow path 5 with the adhesive agent 4 or may be mounted through
the one-touch operation of the mounting device without disassembling
piping of the flow path 5 by the provision of notch 2 on the ultrasonic
sensor 1 the scope of application for measuring the flow rate may
be enlarged.
In addition, since the ultrasonic sensors can be mounted to the
flow path 5 at an increased distance as in the case of the ring-shaped
ultrasonic sensor in the prior art, measurement of the flow rate
may be performed with a high degree of accuracy.
Furthermore, since measurement by combining measured values from
the pairs of sensors may be performed by disposing more than two
sensors on the flow path, measurement with a high degree of accuracy
is achieved also for transient flow including laminar flow, turbulent
flow or pulsating flow.
As shown in FIG. 11A, by decreasing the thickness t and increasing
the outer diameter D of the ultrasonic sensor 1 a resonance frequency
may be maintained at a lower value while increasing the detection
resolution. By maintaining the resonance frequency at a lower value,
attenuation of ultrasonic vibrations may be controlled, and thus
a high degree of accuracy is achieved.
For example, assuming that the reference value s of the detection
resolution of the ultrasonic sensor 1 is 1/100 by determining the
ratio of the thickness t of the ultrasonic sensor shown in FIG.
11A with respect to the length L between the sensors (t/L) to be
1/100 or below, errors resulting from the detection resolution may
be maintained below 1/100.
By contrast, FIG. 11B shows the thickness t.sub.o of the ultrasonic
sensor 21 and the length L between the sensors of the prior art.
Since the thickness t.sub.o of the ultrasonic sensor 21 in the prior
art in the direction of the length of the flow path 5 (the direction
of liquid flow) is large, detection resolution of the ultrasonic
sensor 21 is lowered.
In the ultrasonic flow meter using the ring-shaped sensor in the
prior art, ultrasonic waves are converged from the circumference
of the ring sensor to the center point of the flow path and are
then redirected in a perpendicular direction and propagated in the
upstream and the downstream directions of the flow path.
According to the ultrasonic flow meter of the present invention,
since the thickness of the ultrasonic sensor in the direction of
the flow path may be reduced, detection resolution may be improved,
and the directivity of the ultrasonic wave from the ultrasonic sensor
increases. Thus, the ultrasonic waves are directly propagated in
the direction along the flow path. Consequently, sensitivity to
transmission of the ultrasonic waves is improved and the effect
of the noise or the like may be reduced, whereby measurement with
a high degree of accuracy is achieved.
Using the ultrasonic flow meter arranged as described above, measurement
of the flow rate with a flow path having an inner diameter of 0.5
mm and measurement of a minimum flow velocity of 0.1 m/s for calculating
the flow rate has been conducted, and effectiveness of the ultrasonic
flow meter according to the present invention has been verified. |