Abstrict In a Karman vortex flow meter, the variation of pressure caused
near both sides of a Karman vortex forming unit inserted into the
stream of fluid is applied to an oscillating member which is oscillating
in a self-excited mode at a resonance frequency, so that the vortex
frequency; i.e., the volume flow rate is detected from the modulation
of the resonance frequency which is caused by variation of pressure
or density attributing to the vortexes formed.
Claims What is claimed is:
1. A Karman vortex flow meter for detecting a flow rate of fluid
from the frequency of vortex formation comprising:
a Karman vortex forming means inserted into a stream of a fluid
for forming vortices near both sides thereof;
an oscillating means, inserted into the fluid, oscillating at a
resonant frequency that varies with fluid density, for producing
an oscillating signal;
means for transmitting variations in pressure from the vortex forming
means to the oscillating means whereby changes in pressure caused
by vortex formation induce a change in density near the oscillating
means, thereby modulating the frequency of the oscillating signal;
and
means for detecting the frequency vortex formation including means
for demodulating the oscillating signal thereby producing a signal
whose frequency is indicative of the fluid flow rate.
2. A Karman vortex flow meter as claimed in claim 1 further including
means for detecting a density of the fluid from the average frequency
of the oscillating signal, and
means, coupled to the means for detecting a density and to the
means for detecting the frequency, for calculating a mass flow rate
of the fluid from the density of the fluid and the frequency of
the vortex.
3. A Karman vortex flow meter as claimed in claim 1 wherein said
oscillating means comprises a vibrating board and an acoustic vibration
system provided on at least one side of said vibrating board, said
acoustic vibration system including a cavity and a cylinder.
4. A Karman vortex flow meter as claimed in claim 2 wherein said
oscillating means comprises a vibrating board and an acoustic vibration
system provided on at least one side of said vibrating board, said
acoustic vibration system including a cavity and a cylinder.
5. A Karman vortex flow meter as claimed in either of claims 1
or 2 wherein said Karman vortex forming means is provided with
a pressure introducing opening through which the vibration in pressure
of the fluid is applied said mechanical vibrating means.
6. A Karman vortex flow meter as claimed in either of claims 3
or 4 wherein said Karman vortex forming means is provided with
a pressure introducing opening through which the vibration in pressure
of the fluid is applied to said mechanical vibrating means.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a Karman vortex flow meter which detects
the frequency of Karman vortexes formed downstream of a prismatic
member inserted in a stream of fluid, to measure the flow rate of
the fluid.
2. Prior Art
A method in which the attenuation of an ultrasonic wave emitted
in a fluid is measured with a signal transmitting sensor and a signal
receiving sensor has been employed to detect the vortex pressure
variation of a Karman vortex flow meter of this type.
However, the method is disadvantageous in the following points:
The method is not economical because it uses the two sensors which
are expensive. Furthermore, in the method, the extremely small sound
pressure variation of the ultrasonic wave is detected in an analog
mode, and therefore the detection is liable to be affected by noise.
In order to eliminate the difficulty, the circuit is unavoidably
intricate in arrangement.
SUMMARY OF THE INVENTION
Accordingly, an object of this invention is to provide a Karman
vortex flow meter simple in construction in which the above-described
difficulties have been eliminated.
The foregoing object and other objects of the invention have been
achieved by the provision of a Karman vortex flow meter for detecting
a flow rate from the frequency of vortexes formed by a Karman vortex
forming unit inserted into a stream of fluid; in which, according
to the invention, the variation in pressure of the fluid caused
near both sides of the Karman vortex forming unit is applied to
an oscillating member producing a modulated signal whose frequency
changes with the density of the fluid, to detect a Karman vortex
frequency by demodulating the modulated signal.
The nature, principle and utility of the invention will become
more apparent from the following detailed description when read
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIG. 1 is a sectional view showing a Karman vortex flow member
according to one embodiment of the present invention;
FIG. 2 is a sectional view taken along line A--A in FIG. 1 showing
a vortex forming unit;
FIG. 3 is a vertical sectional view of a detector shown in FIG.
1;
FIG. 4 is a block diagram showing an electrical circuit of the
detector in FIG. 3;
FIGS. 5A to 5C are an explanatory diagrams used for a description
of the operating principle of this invention, in which FIG. 5A is
an explanatory diagram of an oscillation system, and FIGS. 5B and
5C are equivalent circuit diagrams of the oscillation system shown
in FIG. 5A;
FIG. 6 is an explanatory diagram used for a description of the
output signal of the detector;
FIG. 7 is a sectional view showing a Karman vortex flow meter according
to another embodiment of the present invention;
FIG. 8 is a block diagram showing an alternative electrical circuit
of the detector in FIG. 3 according to a further embodiment of the
present invention;
FIG. 9 is an explanatory diagram used for a description of the
output signal of the detector.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A first embodiment of this invention will be described with reference
to FIGS. 1 through 5.
In FIG. 1 reference numeral 1 designates a pipe; 4 a vortex forming
means for producing a Karman vortex street; and 3 a vortex detection
means. The vortex forming means 4 as shown in FIG. 2 comprises
an upstream prismatic element 41 isosceles-triangular in section
and a downstream prismatic element 42 isosceles-trapezoidal in section.
The two prismatic elements 41 and 42 are inserted in the stream
of fluid in the pipe in such a manner that they are perpendicular
to the steam of fluid with a predetermined distance therebetween.
The downstream prismatic element 42 has slits 5a and 5b on both
sides of its one end portion in such a manner that the slits are
communicated with each other. The slits are used to apply the variation
in pressure of a vortex formed to the detector 3.
The vortex detection means is shown in FIGS. 3 and 4. Reference
numeral 8 designates a bottomed cylindrical vibrator which comprises:
a bottom 8a to the inner surface to which a piezo oscillator 9 is
fixedly bonded; and a flange 8b at the opening. The vibrator 8 is
made of a thin metal plate about 0.1 mm in thickness. The piezo
oscillator 9 is made up of a disc-shaped piezo substrate 9a 0.1
to 0.2 mm in thickness, a first electrode 9b formed on one surface
of the substrate 9a, and second and third electrodes 9c and 9d formed
on the other surface of the substrate 9a. The one surface of the
substrate 9a on which the first electrode 9b is formed is held in
contact with the inner surface of the bottom 8a of the vibrator
8 so that the first electrode 9b is electrically connected to the
vibrator 8. The vibrator 8 and the piezo oscillator 9 form a vibrating
board 10.
Further in FIG. 3 reference numeral 11 designates a bottomed cylindrical
container. The inner wall of the upper open end portion of the container
11 is female-threaded as indicated at 11a. One end of a cylinder
12 is fixedly secured to the outer surface of the bottom 11b of
the container 11 in such a manner that the cylinder 12 is coaxial
with the container 11. A circular through-hole 11c having a diameter
equal to the inside diameter of the cylinder 12 is formed in the
bottom 11b of the container 11 in such a manner that it is communicated
with the inside of the cylinder 12 and the pressure introducing
openings 5a and 5b. In FIG. 3 reference numeral 13 designates a
bottomed cylindrical housing whose outer wall is male-threaded as
indicated at 13a. The housing 13 is screwed into the container 11
so that the flange 8b of the vibrator 8 is held between the housing
13 and the container 11 whereby the vibrator 8 is fixedly held in
the space defined by the housing 13 and the container 11. Further
in FIG. 3 reference numeral 14 designates a first space formed
by the vibrating board 10 and the container 11; and 15 a second
space defined by the vibrating board 10 and the housing 13. Through-holes
16 and 16 are formed in the bottom 13b of the housing 13. A printed
circuit board 18 forming a detecting circuit 17 is fixedly bonded
to the inner surface of the bottom 13b of the housing 13. The first
and second spaces 14 and 15 are fluid-tightly separated from each
other by the vibrating board 10.
The detector 3 further comprises lead wires 19a which are extended
from the detecting circuit 17 through the through-hole 16 to an
external circuit outside of the second space 15 a lead wire 19b
connected between the vibrator 8 and the detecting circuit 17 and
lead wires 19c and 19d connected between the detecting circuit 17
and the electrodes 9c and 9d of the piezo oscillator 9.
The arrangement and operation of the detecting circuit 17 will
be described with reference to FIG. 4.
In FIG. 4 reference numeral 21 designates an amplifier whose output
voltage is applied through the electrode 9c to the piezo substrate
9a; and 22 a feedback circuit for detecting a voltage generated
by the piezo substrate 9a through the electrode 9d and positively
feeding back it to the amplifier 21. The vibrating board 10 is formed
as described above, and the piezo substrate 9a is vibrated (expanded
and contracted) radially when AC voltage is applied across the electrodes
9b and 9c. Therefore, when the piezo substrate 9a vibrates in this
manner, the bottom 8a of the vibrator 8 is vibrated in the axial
direction of the vibrator 8. As a result, an AC voltage corresponding
to the strain of the piezo substrate 9a is generated across the
electrodes 9d and 9b, and positively fed back to the amplifier 21
through the feedback circuit 22. As a result, the vibrating board
10 is maintained resonated at a natural frequency F; that is, it
is placed in self-oscillation state.
The detecting circuit 17 further comprises: an impedance conversion
circuit 23 which receives the output AC voltage 21a of the amplifier
21 having a frequency equal to the natural frequency F, to facilitate
a signal processing operation (described later) which is carried
out for the voltage; and a waveform shaping circuit 24 which subjects
the output signal of the conversion circuit 23 to waveform shaping,
to output a pulse train signal 24a having the frequency F. As is
apparent from the above description, the detecting circuit 17 is
made up of the amplifier 21 the feedback circuit 22 the impedance
conversion circuit 23 the waveform shaping circuit 24 and the
printed circuit board 18 bearing these circuits.
Further in FIG. 4 reference numeral 25 designates a means for
detecting the frequency of vortex formation by demodulating the
aforementioned signal 24a thereby outputting a signal 25a whose
frequency is indicative of the fluid flow rate.
When the detector 3 with the vortex forming unit 4 shown in FIG.
3 is arranged in the pipe 1 as shown in FIG. 1 the fluid 2 flows
into the space 14 through the pressure introducing openings 5a and
5b and the through-hole 11c, and into the space 15 through a through-hole
43 formed in the pipe 1 and the through-holes 16. When, under this
condition, the vibrating board 10 is self-oscillated in the above-described
manner, the vibrating board will vibrate at the resonance frequency
of a vibration system including the first space 14 the part of
the through-hole 11c, the inside of the cylinder 12 and the vibrating
board 10.
The resonance output is much higher than the Karman vortex generating
output As the fluid flows in the pipe 1 Karman vortex pressure
variation occurs near both sides of the vortex forming unit, and
the above-described resonance frequency is modulated in synchronization
with the variation. On the other hand, it has been found through
experiments that the Karman vortex pressure increased and decreased
from the average pressure in the pipe 1. Accordingly, the pulse
train signal 24a outputted by the waveform shaping circuit 24 is
such that the resonance frequency of the vibrating board 10 based
on the average density of the fluid in the pipe 1 is modulated with
the Karman vortex generating frequency based on the volume flow
rate of the fluid. Thus, the volume flow rate of the fluid flowing
in the pipe 1 can be detected from the frequency of the modulation
of the pulse train signal with the vortex. This will become more
apparent from FIG. 6 in which reference character 25a designates
the Karman vortex modulation frequency component extracted from
the pulse train signal 24a by the signal conversion circuit 25
thus representing the Karman vortex frequency (volume flow rate).
Referring to FIG. 4 reference numeral 27 designates a frequency
detecting section which comprises the vibrator 9 the detecting
circuit 17 and the signal conversion circuit 25 to detect the
modulation frequency Fk of the vibration system 40.
The vibration of the vibrating board 10 will be described with
reference to FIGS. 5A to 5C.
As shown in FIG. 3 the side wall 8c of the vibrator 8 is confronted
through an extremely small gap with the inner wall of the container
11 permitting the space 15 to be much larger in volume than the
space 14 so that, even when the vibrating board 10 vibrates as described
above, the pressure in the space 15 is substantially maintained
unchanged. The sum of the opening areas of the through-holes 16
is considerably large so that the natural frequency of the space
15 is much lower than that of the vibration system 40 consisting
of the vibrating board 10 the space 14 the through-hole 11c, and
the inside space of the cylinder 12. Therefore, for simplification
in description, the essential components in FIG. 3 may be as shown
in FIG. 5A. In FIG. 5A, reference character Mm designates the mass
of the vibrating board 10 S is the area of the bottom 8a of the
vibrator 8 Cm is the compliance corresponding to the spring constant
Km of the vibrating board 10 Cm=1/Km, Ma is the mass of the fluid
2 under test in the cylinder 12 and Ca is the acoustic capacity
in the space 14. The acoustic capacity Ca is represented by the
following equation (1):
where W is the volume of the space 14 X is the acoustic velocity
in the fluid 2 and .rho. is the density of the fluid 2.
In FIG. 5A, the vibration system is so designed that the height
h of the space 14 and the sectional area S.sub.1 of the inside space
of the cylinder 12 are both extremely small, and both the mass of
the fluid in the space 14 and the acoustic capacity in the inside
space of the cylinder 12 can be disregarded. Therefore, with the
acoustic vibration system converted into a mechanical vibration
system so that vibration system shown in FIG. 5A can be represented
by an electrical equivalent circuit shown in FIG. 5B. In FIG. 5B,
MaO and CaO are the mass and the acoustic compliance which are represented
by the following equation (2): ##EQU1## If, in FIG. 5B, the following
equation (3) is established in which .omega. is the angular frequency
of a vibration, then the circuit of FIG. 5B can be converted into
the circuit of FIG. 5C. If the resonance frequency of the circuit
shown in FIG. 5C is represented by F, then the following equation
(4) will be established: ##EQU2##
From equations (2), (3) and (4), the following expression (5) is
obtained: ##EQU3##
As is apparent from equation (4), when the essential components
shown in FIG. 1 are so designed that the equivalent circuit shown
in FIG. 5C is formed, the vibrating board resonates at the frequency
F, and the frequency F of the pulse train forming the output signal
24a of the waveform shaping circuit shown in FIG. 4 corresponds
to the mass Ma of the fluid 2 under test.
A second embodiment of the invention is as shown in FIG. 7. The
second embodiment is different from the first embodiment shown in
FIG. 1 in that a detecting section 44 and a vortex forming unit
45 are separate from each other. Thus, the detector 44 and the vortex
forming unit can be individually replaced when necessary; that is,
the maintenance of the Karman vortex flow meter can be achieved
at a cost lower than that of the first embodiment. The detector
44 can be obtained by removing the downstream prismatic element
42 from the detector 3 shown in FIG. 3.
A third embodiment of the invention will be described with reference
to FIG. 3 and 8.
This third embodiment is similar to the first, with the addition
that in FIG. 8 reference numeral 25 designates a signal conversion
circuit which receives the signal 24a and applies a signal 25b corresponding
to the average frequency F of the pulse train forming the signal
24a and a modulation frequency signal 25a due to a Karman vortex
street to an arithmetic section 26. Upon reception of the output
signals 25a and 25b, the arithmetic section 26 performs an arithmetic
operation according to an equation (described later), to output
a mass flow rate signal 26a.
Accordingly, the density of the fluid 2 can be detected from the
frequency F.
The arithmetic circuit 26 in FIG. 8 is so designed as to output
the mass flow rate (=density.times.volume flow rate) signal 26a
which is the product of the density obtained through calculation
according to equation (4) and the Karman vortex frequency. Therefore,
the measurement of a mass flow rate can be achieved with the simple
circuit having only one sensor.
Thus, with the detector, a fluid such as gas which is low in density
can be measured with high sensitivity and with high accuracy. Furthermore,
the effect of the variation in vibration of the vibrating board
10 with the vibration in temperature of the fluid 2 under test on
the measurement accuracy can be minimized, with the result that
the fluid 2 can be measured over a wide range of temperatures.
As is apparent from the above description, in the Karman vortex
flow meter of the invention, the variation of pressure caused near
both sides of the Karman vortex forming unit inserted in the stream
of fluid is applied to the acoustic vibration system including the
cavity and the cylinder, so that the volume flow rate of the fluid
is detected from the modulation of the resonance frequency of the
system which is attributable to variations in pressure caused by
Karman vortices. Therefore, the Karman vortex flow meter is simple
in construction and low in manufacturing cost.
Furthermore, with the Karman vortex flow meter of the invention,
the density of the fluid can be detected from the average resonance
frequency of the vibration system which is a function of the mass
of the fluid in the cylinder, with the result that the mass flow
rate (density.times.volume flow rate) can be detected by using the
density and the volume flow rate of the fluid.
Furthermore, in the Karman vortex flow meter of the invention,
the acoustic compliance of the cavity is made smaller than the compliance
of the vibrating board, so that the action of the fluid under test
which increases the mass of the vibration system including the vibrating
board during the vibration of the vibrating board is made more effective
by the action of the cavity and the cylinder. Therefore, the Karman
vortex flow meter of the invention can measure even gas with high
accuracy, and it has a wide range of temperatures for measurement.
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