U.S. patent application number 09/904545 was filed with the patent office on 2002-02-21 for vortex flow meter.
Invention is credited to Ehara, Takashi, Ezaki, Yasuhiko, Fukami, Yasumasa, Kasahara, Kazuyuki, Matsuda, Toshihiko, Matsuguma, Motohiko, Sakai, Toshisuke, Yamaguchi, Masashi.
Application Number | 20020020225 09/904545 |
Document ID | / |
Family ID | 27554816 |
Filed Date | 2002-02-21 |
United States Patent
Application |
20020020225 |
Kind Code |
A1 |
Sakai, Toshisuke ; et
al. |
February 21, 2002 |
Vortex flow meter
Abstract
A vortex flow meter includes a measuring tube in which a fluid
is carried, a vortex generator provided in the measuring tube for
developing a Karman vortex in the fluid, a magnetic field generator
generating a magnetic field across the measuring tube at the
downstream of the vortex generator, a pair of electromotive force
measuring electrodes provided at the downstream of the vortex
generator for measuring an electromotive force generated by the
Karman vortex passing across the magnetic field, a pair of
reference electrodes provided at the upstream and downstream of the
electromotive force measuring electrodes, respectively, for
measuring a potential at each location, and a detector circuit
electrically connected to the electromotive force measuring
electrodes and the reference electrodes for calculating the flow of
the fluid from the electromotive force and the potential measured
by the reference electrodes. The flow is calculated by subtracting
the potential difference of the reference potential measuring
electrodes from the electromotive force measured by the
electromotive force measuring electrodes. This allows the flow
meter to have an increased measuring range, meet a wide range of
flow conditions, have a simple construction, and have a measurement
accuracy.
Inventors: |
Sakai, Toshisuke; (Fukuoka,
JP) ; Matsuda, Toshihiko; (Fukuoka, JP) ;
Kasahara, Kazuyuki; (Fukuoka, JP) ; Fukami,
Yasumasa; (Fukuoka, JP) ; Ehara, Takashi;
(Fukuoka, JP) ; Matsuguma, Motohiko; (Saga,
JP) ; Ezaki, Yasuhiko; (Fukuoka, JP) ;
Yamaguchi, Masashi; (Fukuoka, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W.
SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
27554816 |
Appl. No.: |
09/904545 |
Filed: |
July 16, 2001 |
Current U.S.
Class: |
73/861.22 |
Current CPC
Class: |
G01F 1/3287 20220101;
G01F 1/325 20220101 |
Class at
Publication: |
73/861.22 |
International
Class: |
G01F 001/32 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 17, 2000 |
JP |
2000-215527 |
Aug 29, 2000 |
JP |
2000-258677 |
Sep 4, 2000 |
JP |
2000-266825 |
Sep 4, 2000 |
JP |
2000-266826 |
Sep 12, 2000 |
JP |
2000-276006 |
Sep 26, 2000 |
JP |
2000-292054 |
Claims
What is claimed is:
1. A vortex flow meter comprising: a measuring tube in which a
fluid is carried; a vortex generator provided in the measuring tube
for developing a Karman vortex in the fluid; a magnetic field
generator for generating a magnetic field applied at a downstream
of the vortex generator across the measuring tube; a pair of
electromotive force measuring electrodes provided at a downstream
of the vortex generator for measuring an electromotive force
generated when the Karman vortex passes across the magnetic field;
a pair of reference electrodes provided at an upstream and
downstream of the electromotive force measuring electrodes,
respectively, for measuring a potential at each location; and a
detector circuit electrically connected to the electromotive force
measuring electrodes and the reference electrodes for calculating a
flow of the fluid from the electromotive force and the potential
measured by the reference electrodes.
2. The vortex flow meter according to claim 1, wherein the detector
circuit comprises a differential amplifier for eliminating
common-mode disturbing noise components induced at the
electromotive force measuring electrodes and the signal between the
paired reference electrodes.
3. The vortex flow meter according to claim 2, wherein the detector
circuit further comprises: a first positive amplifier for
amplifying the electromotive force generated between the
electromotive force measuring electrodes and inputting the
amplified electromotive force to a first input of the differential
amplifier; and a second positive amplifier for amplifying the
electromotive force generated between the reference electrodes and
inputting the amplified electromotive force to a second input of
the differential amplifier.
4. The vortex flow meter according to claim 1, wherein the detector
circuit further comprises: a coupling capacitor for coupling the
electromotive force measuring electrodes to the differential
amplifier; and a voltage follower circuit for generating a
reference potential from the potential measured at the reference
electrodes.
5. The vortex flow meter according to claim 4, wherein the voltage
follower circuit generates the reference potential in which the
potential measured by the reference electrodes is superimposed with
a certain potential.
6. The vortex flow meter according to claim 4, wherein the detector
circuit further comprises a high impedance circuit for determining
the reference potential.
7. The vortex flow meter according to claim 1, wherein the detector
circuit further comprises an amplifier for amplifying an output of
the differential amplifier.
8. The vortex flow meter according to claim 1, wherein each of the
reference electrodes has a diameter not greater than 1/2 of a width
of the vortex generator.
9. The vortex flow meter according to claim 1, wherein the
reference electrodes have the same shapes each other.
10. The vortex flow meter according to claim 1, wherein each of the
electromotive force measuring electrodes has a diameter not greater
than 1/2 of a width of the vortex generator.
11. The vortex flow meter according to claim 1, wherein the
electromotive force measuring electrodes have the same
diameters.
12. The vortex flow meter according to claim 1, wherein each of the
electromotive force measuring electrodes has, in the measuring
tube, a length ranging 2 to 2.5 times as large as a width of the
vortex generator.
13. The vortex flow meter according to claim 1, wherein the
electromotive force measuring electrodes have the same length in
the measuring tube.
14. The vortex flow meter according to claim 1, wherein a distance
between the electromotive force measuring electrodes is 2 to 2.5
times as large as a width of the vortex generator.
15. The vortex flow meter according to claim 1, wherein a width
between the magnetic field generators in a radial direction of the
measuring tube is 1.5 to 2 times as large as a width of the vortex
generator.
16. The vortex flow meter according to claim 1, wherein the
measuring tube has an undulated portion over an inner wall surface
thereof.
17. The vortex flow meter according to claim 16, further comprising
a separate undulated member provided on an inner wall of the
measuring tube.
18. The vortex flow meter according to claim 17, wherein the
separate undulated member is electrically conductive and contacts
directly with a portion of one of the reference electrodes.
19. The vortex flow meter according to claim 17, wherein the
separate undulated member is a coil spring.
20. The vortex flow meter according to claim 19, wherein the coil
spring has a portion having a pitch not greater than a diameter of
the reference electrodes, the portion accepting one of the
reference electrodes.
21. The vortex flow meter according to claim 1, wherein a cross
section of the measuring tube has a track shape having linear
portions orthogonal to the magnetic field and arcuate portions
bridging over the linear prtions, the arcuate portions arranged
symmetrical about a direction orthogonal to the magnetic field, a
distance between the arcuate portions being greater than a width
between the linear portions.
22. The vortex flow meter according to claim 21, wherein each of
the arcuate portions has a semi-circular shape.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a flow meter for measuring
the flow of a fluid, such as air or liquid running in a measuring
tube, accurately throughout a wide range.
BACKGROUND OF THE INVENTION
[0002] Among flow meters for measuring the flow of a fluid which
runs in a measuring tube, a Karman-vortex flow meter is known. A
conventional Karman-vortex flow meter disclosed in Japanese Patent
Laid-open No.60-40914 develops a Karman vortex in the flow of a
fluid, and a generating frequency (referred to as a frequency
hereinafter) of the vortex is measured for calculating the rate of
the flow. The calculation is based on the fact that the
Karman-vortex generating frequency is proportional to the flow. For
measuring the Karman-vortex frequency, the meter disclosed in the
Japanese Patent Laid-open No.60-40914 employs an ultrasonic or
oscillation technique. It is known that when ultrasonic or
oscillating waves directed to a Karman vortex has the frequency or
phase change. The change in the ultrasonic or oscillating waves may
be measured with only a large, complex, expensive meter even if the
measuring tube is relatively small. Because the measuring accuracy
of such an expensive meter depends primarily on the generating
mechanism of the Karman vortex, the accuracy is easily declined by
a condition such as an ambient temperature or disturbing turbulence
which affects the generation of a Karman vortex.
[0003] Japanese Patent No.3113946 discloses that a Karman-vortex
frequency is measured with a magnetic field. The modified flow
meter will be explained. FIG. 7 is a cross sectional view of a
conventional flow meter. The meter includes a measuring tube 1 in
which an electrically conductive fluid flows and a vortex generator
2 provided in the measuring tube 1. The vortex generator 2
generates a Karman vortex 3. The meter also includes a pair of
electromotive force measuring electrodes 4a and 4b, a detector 5
electrically connected to the electromotive force measuring
electrodes 4a and 4b for measuring a voltage between the electrodes
4a and 4b to calculate a flow rate of the fluid 5 running in the
measuring tube 1, and a pair of magnetic field generators 7a and 7b
mounted around the measuring tube 1. The magnetic field generators
7a and 7b are two magnets mounted to both sides of the measuring
tube 1, respectively, so that the two, N and S, poles come opposite
to each other. More specifically, the magnetic field generators 7a
and 7b are arranged so that the orientation of the magnetic field
from the N pole to the S pole is vertical to the axis of the vortex
generator 2 and to the electromotive force measuring electrodes 4a
and 4b. At the downstream of the vortex generator 7 in the flow, a
pair of lines of Karman vortices are generated in which alternate
vortices of opposite rotation are developed at a frequency
proportional to the representative dimension of the vortex
generator 2. The electromotive force measuring electrode 4b is
located at the downstream of the vortex generator 2. The
electromotive force measuring electrode 4a opposite to the
electromotive force measuring electrode 4b is located at the
downstream of the vortex generator 2 and at the upstream of the
electromotive force measuring electrode 4b. FIG. 7 illustrates the
electromotive force measuring electrode 4a arranged unitarily with
the vortex generator 2 for simplicity.
[0004] The Karman vortex 3 generated by the vortex generator 2
changes the velocity of the flow thus causing a change in the
magnetic flux of the magnetic field developed between the magnetic
field generators 7a and 7b. The change in the magnetic flux then
generates an inductive electromotive force between the
electromotive force measuring electrodes 4a and 4b. The number of
voltage changes is proportional to the number of vortices and is
measured by the detector circuit 5 for calculating the flow.
[0005] However, as the electrodes in the conventional Karman-vortex
flow meter are directly provided at the downstream of the vortex
generator, they detect vortices in a area where the vortices do not
depart completely from the vortex generator and before the vortices
grow up to a measurable size. Therefore, the meter receives an
influence of fluctuations of the Karman vortices.
[0006] Also, as the electrodes are arranged at a point and thus has
a small sensing area, the accuracy of measurements may stay low.
Particularly, when the electric conductivity is low, a small flow
is hardly measured.
[0007] The meter is susceptible to disturbing noises and thus
requires a scheme for diminishing the affect of noises. Having to
include a sophisticated filter circuit, the meter has the overall
arrangement intricate, become expensive, thus creating a secondary
drawback.
[0008] Moreover, when the flow of the fluid is small where Reynolds
number is less than 3000 calculated from Re=UL/v (where U is the
average flow velocity in the cross section, L is the representative
length, and v is the kinetic viscosity coefficient), the velocity
distribution significantly varies by the resistance of the inner
wall of the measuring tube. This makes Karman vortices generate
unstably, thus declining the accuracy and repeatability of
measurements and making the flow be hardly calculated
accurately.
SUMMARY OF THE INVENTION
[0009] A vortex flow meter includes the follows:
[0010] A measuring tube in which a fluid is carried;
[0011] A vortex generator provided in the measuring tube for
developing a Karman vortex in the fluid;
[0012] A magnetic field generator for generating a magnetic field
to be applied at the downstream of the vortex generator across the
measuring tube;
[0013] A pair of electromotive force measuring electrodes provided
at the downstream of the vortex generator for measuring an
electromotive force which is generated when the Karman vortex
passing across the magnetic field;
[0014] A pair of reference electrodes provided at the upstream and
downstream of the electromotive force measuring electrodes for
measuring the potentials at the locations, respectively; and
[0015] A detector circuit electrically connected to the
electromotive force measuring electrodes and reference electrodes
for calculating the flow of the fluid from the electromotive force
and the potential measured by the reference electrodes.
[0016] The flow meter measures the flow while offsetting a change
of the flow caused by a change of the measuring environments and
conditions, hence increasing the measurement range, decreasing the
cost with no use of extra components for a noise reduction, and
improving an accuracy of a measurement.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a cross sectional view of a flow meter according
to Embodiment 1 of the present invention;
[0018] FIG. 2 is a block diagram of a detector circuit in the flow
meter of Embodiment 1;
[0019] FIG. 3 is a block diagram of a detector circuit in a flow
meter according to Embodiment 2 of the present invention;
[0020] FIG. 4 is a cross sectional view of a flow meter according
to Embodiment 3 of the present invention;
[0021] FIG. 5 is a cross sectional view of an arrangement with a
flow meter according to Embodiment 4 of the present invention;
[0022] FIG. 6A is a cross sectional view of a flow input section of
the flow meter of Embodiment 4;
[0023] FIG. 6B is a cross sectional view of the flow meter of
Embodiment 4; and
[0024] FIG. 7 is a cross sectional view of a conventional flow
meter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] (Embodiment 1)
[0026] A flow meter according to Embodiment 1 of the present
invention will be described referring to FIG. 1 and FIG. 2. Like
components are denoted by like numerals as those of the
conventional flow meter and will be explained in no more detail.
FIG. 1 is a cross sectional view of the flow meter of Embodiment 1,
and FIG. 2 is a block diagram of a detector circuit in the flow
meter of Embodiment 1.
[0027] As shown in FIG. 1, the flow meter includes a measuring tube
1, a vortex generator 2 provided in the measuring tube 1 for
developing Karman vortices, a pair of electromotive force measuring
electrodes 4a and 4b, a pair of reference electrodes 6a and 6b, a
detector circuit 5 for measuring an inducted electromotive force to
calculate a rate of the flow and offsetting the influence of
disturbing noises, and a pair of magnetic field generators 7a and
7b. A Karman vortex 3 is generated while alternate vortices of
opposite rotation are developed at a frequency being proportional
to the representative dimension of the vortex generator 2. The
small inner diameter of the measuring tube 1 has an inner wall
affect the Karman vortex 3 significantly. Too large diamter makes
the flow velocity slow down and may hardly produce a Karman vortex.
At an appropriate flow velocity, the Reynolds number preferably
ranges from 3,000 to 100,000. The vortex generator 2 of this
embodiment has a triangular column shape, and may has any possible
shape developing a Karman vortex. The vortex generator 2 of this
embodiment is mounted to the inner wall of the measuring tube 1 so
that one of the sides of the generator is vertical to the direction
of the flow. In the flow meter of this embodiment, the measuring
tube 1 has an inner diameter of 7 mm for measuring a range of the
flow rate from 1 L/min to 10 L/min. Under that condition, it is
most desirable that the triangular column body of the vortex
generator 2 is an isosceles triangle in the cross section having a
width of 2 mm and a height of 3 mm.
[0028] The electromotive force measuring electrodes 4a and 4b are
arranged in parallel with each other at the downstream of the
vortex generator 2 so that their axes may extend at a right angle
to the axis of the vortex generator 2 and the direction of the
flow. Each line of flow passing the electromotive force measuring
electrode 4a and passes the electromotive force measuring electrode
4b. The magnetic field generators 7a and 7b are permanent magnets
mounted on both sides of the measuring tube 1 to sandwich the
electromotive force measuring electrodes 4a and 4b with two, N and
S, poles facing opposite to each other. When the range of the flow
measurement is set from 1 L/min to 10 L/min, the density of
magnetic flux in the measuring tube 1 has to be increased by the
magnetic field generators 7a and 7b. Therefore, a rare-earth group
permanent magnet having a width 1.5 times greater than that of the
vortex generator 2 is used as the magnetic field generators 7a and
7b.
[0029] The reference electrodes 6a and 6b measure a potential
difference between the upstream and the downstream of the
electromotive force measuring electrodes 4a and 4b. The reference
electrodes 6a and 6b are also arranged in parallel with the
electromotive force measuring electrodes 4a and 4b. This allows a
flow line passing the reference electrodes 6a and 6b passes the
electromotive force measuring electrodes 4a and 4b. More
specifically, the four electrodes are aligned in a row along the
direction of the flow from upstream to downstream.
[0030] The Karman-vortex flow meter has to be carefully sized for
the representative dimension of each component and the Reynolds
number of a fluid for steadily developing a Karman vortex 3 and for
not generating a noise with a disturb in the measurement tube
1.
[0031] FIG. 1 shows a width D is which the representative dimension
of the vortex generator 2, diameters da and db of the electromotive
force measuring electrodes 4a and 4b respectively, a diameter dc of
each of the reference electrodes 6a and 6b, a length h of each of
the electromotive force measuring electrodes 4a and 4b in the
measuring tube 2, a distance L between the electromotive force
measuring electrodes 4a and 4b, and a width Dm along the tube
diameter of each of the magnetic field generators 7a and 7b. Those
dimensions are determined in precise balance so as to steadily
develop the Karman vortex 3 but not any disturbing noise, as
explained below.
[0032] For example, the diameter dc of each of the reference
electrodes 6a and 6b is not greater than 1/2 of the width D of the
vortex generator 2. This holds 1/2 or smaller of the Reynolds
number. Accordingly, as laminar flows appear about the reference
electrodes 6a and 6b, the Karman vortex 3 is rarely interrupted.
Also, the reference electrodes 6a and 6b develop few vertices. Such
a change in the flow velocity does not disturb the magnetic flux
between the electromotive force measuring electrodes 4a and 4b and
does not affect to measure the flow without declining the
accuracy.
[0033] Preferably, the diameters da and db of the respective
electromotive force measuring electrodes 4a and 4b may not be
greater than 1/2 the width D of the vortex generator 2 preferably.
This holds 1/2 or smaller of the Reynolds number and can create
laminar flows about the electrodes 4a and 4b. As a result, the
Karman vortex 3 is hardly disturbed or fractured. The flow
disturbed by the electromotive force measuring electrode 4a is
spread and runs to the downstream before reaching the electromotive
force measuring electrode 4b. In case that the diameters da and db
of the electromotive force measuring electrodes 4a and 4b are
equal, any noise developed at the electromotive force measuring
electrodes 4a and 4b can be minimized. More specifically, the
diameters da and db being different from each other may vary the
resistance to fluid and the friction between the two electrodes 4a
and 4b. Accordingly, the flow is generated irregularly, thus
causing noises. The electrodes 4a and 4b having the same diameters
minimize to generate noises. In this embodiment, the diameters da
and db equal to each other allows a substantially uniform noise to
be generated. The detector circuit 5 includes a differential
amplifier for removing, from the detection signal, a disturbing
noise induced on the electromotive force measuring electrodes 4a
and 4b and declines the noise in the signal processing process.
[0034] In case that the length h in the tube of the electromotive
force measuring electrodes 4a and 4b is 2 to 2.5 times as large as
the width D of the vortex generator 2, the inner diameter of the
measuring tube 1 is preferably 3 to 4 times as large as the width D
for taking an appropriate balance between the flow and the size.
Those dimensions permit the electromotive force measuring
electrodes 4a and 4b to sandwich two strings of the Karman vortex 3
at their distant ends with a minimum height and thus measure a
change of the electromotive force favorably at the center of the
string of the Karman vortex 3, thus measures the flow accurately.
The detector circuit 5 offsets eddies generated in the wake of the
distal ends of the electromotive force measuring electrodes 4a and
4b or disturbing noises developed by turbulence of the flow
excluding the Karman vortex 3. Thus, an overall noise is reduced,
and the flow measurement can be improved in the accuracy.
[0035] In case that the distance L between the electromotive force
measuring electrodes 4a and 4b is 2 to 2.5 times as large as the
width D of the vortex generator 2, the distance L can be smaller
than the distance between two strings of the Karman vortex 3.
Therefore, as there is one or no vortex passing between the
electromotive force measuring electrodes 4a and 4b, the level of
noise can be reduced. And thus, one Karman vortex 3 in the string
corresponds one of the detection signal (one pulse), the frequency
can be counted very easily, and the flow measurement can further be
improved in the accuracy.
[0036] In case that the width Dm along the tube diameter of the
magnetic field generators 7a and 7b is 1.5 to 2 times as large as
the width D of the vortex generator, the magnetic field developed
in the measuring tube 1 is targeted to the area in the measuring
tube 1 where the Karman vortex 3 is developed. Accordingly, a noise
generated from undesired eddies developed on the inner wall of the
measuring tube 1 can be reduced. In the flow meter of this
embodiment, the width Dm is 1.5 times as large as the width D.
[0037] In the flow meter sized as described above, when the Karman
vortex 3 runs across the magnetic field between the electromotive
force measuring electrodes 4a and 4b, the vortex changes the
magnetic field and generates pulses of the electromotive force at
the electromotive force measuring electrodes 4a and 4b.
Simultaneously, the reference electrodes 6a and 6b measures a
potential difference around this area, i.e., a reference potential
difference between the electromotive force measuring electrodes 4a
and 4b. To the potential difference, the measuring conditions
(direct current factors and disturbing noises excluding the Karman
vortex) are reflected. As a result, the detector circuit 5 offsets
the reference potential difference in the electromotive force and
thus removes undesired noise or signal components in the detection
signal.
[0038] The detector circuit and an operation of the circuit in the
flow meter of Embodiment 1 will be described in more detail. As
shown in FIG. 2, the detector circuit 5 includes operational
amplifiers 10a, 10b, 10c, 10d, and 10e, a comparator 11, a
band-pass filter 12, resistors, and coupling capacitors.
[0039] The operational amplifier 10a has a positive input terminal
connected to a coupling capacitor 14a and has a negative input
terminal to which an output is feed back via a resistor 13c, thus
forming a first positive amplifier. The electromotive force
measuring electrode 4a is connected to the positive input terminal
via the coupling capacitor 14a, and the electromotive force
measuring electrode 4b is connected to the negative input terminal
via a resistor 13b which determines an input potential. Similarly,
the operational amplifier 10b has a positive input terminal
connected to a coupling capacitor 14b and has a negative input
terminal to which an output is fed back via a resistor 13f, thus
forming a second positive amplifier. The reference electrode 6a is
connected to the positive input terminal via the coupling capacitor
14b, and the reference electrode 6b is connected to the negative
input terminal via a resistor 13e which determines an input
potential. The gain of each of the first and second positive
amplifiers is determined by the two resistors connected to the
negative input terminal. The electromotive force signals developed
on the electromotive force measuring electrodes 4a and 4b and the
reference electrodes 6a and 6b is amplified to a desired level by
the gain.
[0040] The operational amplifier 10c is a buffer amplifier. The
operational amplifier 10d operates as a differential amplifier for
positive-amplifying an output of the first positive amplifier and
for subtracting, from the amplified output, an output of the second
positive amplifier. That is, the differential amplifier removes a
common-mode signal caused by the disturbing noise from the
detection signal and amplifies the detection signal to a desired
level easily detected by the comparator 11. An output signal of the
operational amplifier 10d passes through the band-pass filter 12
and has the waveform shaped by the comparator 11 before released
out from the detector circuit 5. In brief, the electromotive force
signal induced by the Karman vortex 3 passing between the
electromotive force measuring electrodes 4a and 4b is subjected to
the amplification and the noise elimination of the first and second
positive amplifiers and differential amplifier. And then, the
electromotive force is shaped to a squire wave by the comparator 11
and then released as a string of pulses having a frequency
proportional to the rate of the flow. As described previously, one
pulse corresponds one Karman vortex 3 and is counted to measure the
flow accurately.
[0041] (Embodiment 2)
[0042] A flow meter according to Embodiment 2 of the present
invention will be described referring to FIG. 3. Like components
are denoted by like numerals as those of Embodiment 1 and will be
explained in no more detail. FIG. 3 is a block diagram of a
detector circuit according to Embodiment 2.
[0043] An operational amplifier 10a positive-amplifies a signal
received via a coupling capacitor from the electromotive force
measuring electrode 4a with a negative feedback. The amplified
signal and a signal received from the electromotive force measuring
electrode 4b are differentially-amplified by an operational
amplifier 10b. The amplifier 10b removes a common-mode signal
caused by disturbing noises from the amplified output and
amplifies, the detection signal to a desired level. The detection
signal then passes through a band-pass filter 12 and is amplified
by a positive amplifier includes mainly an operational amplifier
10c to a desired level easily detected by a comparator 11. In
brief, the electromotive force induced by the Karman vortex 3
passing between the electromotive force measuring electrodes 4a and
4b is amplified by the differential amplifier and the positive
amplifiers. The electromotive force has the waveform shaped by the
comparator 11 and is released as a string of pulses having a
frequency proportional to the rate of the flow.
[0044] An operational amplifier 10e operates as a voltage follower
circuit. This circuit has an extremely high input impedance and a
low output impedance. The reference electrodes 6a and 6b are
connected to the grounding side of voltage dividing resistors 13k
and 131 loaded with a source voltage Vcc.
[0045] Therefore, an electrode potential actually measured between
the reference electrodes 6a and 6b is superimposed on a desired
partial determined from the source voltage Vcc divided by resisters
and has the impedance converted into low. This allows the output of
the operational amplifier 10d (the reference potential applied to
the detector circuit 5) to remain constant even if the reference
electrode potential actually measured varies. As a result, an
influence of external potential fluctuations is eliminated. A noise
can be canceled by the differential amplifier including mainly the
operational amplifiers 10a and 10b even when the electromotive
force developed at the electromotive force measuring electrodes 4a
and 4b is small. Consequently, a change in the magnetic field
derived from the Karman vortex 3 can favorably be measured.
[0046] As described above, the detector circuit 5 of Embodiment 2
eliminates a fluctuation in the actual reference electrode
potential with the voltage follower circuit and the reference
electrodes 6a and 6b. And the circuit 5 removes the common-mode
disturbing noise from the electromotive force received from the
coupling capacitor with the differential amplifier circuit, thus
detecting the electromotive force induced by a change of the
magnetic field caused by the Karman vortex. Accordingly, the flow
meter according to this embodiment has a simple construction, is
less expensive, and measures the flow accurately. (Embodiment
3)
[0047] A flow meter according to Embodiment 3 of the present
invention will be described referring the cross sectional view of
FIG. 4. Also, like components are denoted by like numerals as those
of Embodiment 1 and will be explained in no more detail.
[0048] Undulation members 8a and 8b extending uniformly along the
inner wall of the measuring tube 1 are provided. The members are
particularly implemented in this embodiment by coil springs which
are less expensive.
[0049] As described previously, a Karman vortex 3 is steadily
developed and has the frequency be proportional to the rate of flow
when a fluid to be measured is carried in turbulent flows. This
condition may be expressed with the Reynolds number. In a proper
range of flow velocities, the Reynolds number ranges from 3,000 to
100,000. The Reynolds number may however be varied depending on the
shape of the inner wall of the measuring tube 1. Particularly, the
Reynolds number ranges from 2,320 to 3,000 at the transition
between a laminar flow and a turbulent flow. For measuring a flow
in the range of the Reynolds number, the inner wall of the
measuring tube 1 has to be roughed uniformly on the surface
upstream of the vortex generator 2 in order to develop turbulent
flows steadily.
[0050] For that reason, Embodiment 3 employs the two coil springs
8a and 8b are provided on the upstream and downstream of the vortex
generator 2 along the inner wall of the measuring tube 1,
respectively. The spring coils 8a and 8b, as each having a uniform
pitch, are most favorable materials for implementing uniform
undulation over the inner wall. Also, the spring coils 8a and 8b
are easily assembled and has the effect precisely predicted. The
coil springs 8a and 8b prevent the measuring tube 1 from being
delicately machined in the inner wall, hence contributing to the
easy fabrication of the flow meter with the measuring tube 1
undulated on the inner wall without costly processes.
[0051] The coil springs 8a and 8b are made of electrically
conductive material and directly connected with the reference
electrodes 6a and 6b, respectively. The reference electrodes 6a and
6b, which are directly connected with the coil springs 8a and 8b,
have the performance for measuring the reference potential
improved. Upon having the pitch between two adjacent windings
partially greater than the diameter of the reference electrodes 6a
and 6b, the coil springs 8a and 8b hold the reference electrode 6a
and 6b securely and tight with a yielding force, respectively. The
coil springs 8a and 8b, upon being of a close-coiled helical type,
have increased areas contacting with the fluid thus improving the
effectiveness. For inhibiting the reference electrodes 6a and 6b
from being deformed by the yielding force of the coil springs 8a
and 8b , the coil springs 8a and 8b has to be made from the same
material and to have the same diameter as the reference electrodes
6a and 6b.
[0052] An operation of the flow meter according to Embodiment 3
will be explained. A fluid to be measured running in the measuring
tube 1 is disturbed by the coil spring 8a to be a turbulent flow.
Accordingly, a Karman vortex 3 is developed with the vortex
generator 2 as proportional to the velocity of the flow. As the
Karman vortex 3 crosses the magnetic field between the
electromotive force measuring electrodes 4a and 4b, the vortex
creates regularly a string of pulses of electromotive force.
Simultaneously, a potential difference between the electromotive
force measuring electrodes 4a and 4b to which the other factors (a
direct current and disturbing noises other than the Karman vortex)
is reflected is measured with the reference electrodes 6a and 6b.
As a result, an undesired noise or signal component in the
detection signal can readily be removed by the detector circuit 5
offsetting the component in the reference potential signal.
[0053] The flow meter according to Embodiment 3 has the coil
springs 8a and 8b provided for implementing uniform undulation over
the inner wall of the measuring tube 1, allowing a Karman vortex 3
to be developed steadily. This can extend the measuring range and
improve the resistance to noises. Upon having the uniform
undulation implemented by separate members, the measuring tube 1
can be less expensive.
[0054] (Embodiment 4)
[0055] A flow meter according to Embodiment 4 of the present
invention will be described referring to FIGS. 5 and 6. Like
components are denoted by like numerals as those of Embodiment 1
and will be explained in no more detail. FIG. 5 is a cross
sectional view of an arrangement of the flow-meter of Embodiment 4.
FIG. 6A is a cross sectional view of a flow input section of the
flow meter and FIG. 6B is a cross sectional view of the flow meter
of Embodiment 4.
[0056] As shown in FIG. 5, the flow meter has a metal yoke 9 for
reducing a leakage of magnetic fluxes generated by magnetic field
generators 7a and 7b.
[0057] As shown in FIG. 6B, the cross section of a measuring tube
lb of Embodiment 4 consists of a track shape having an arcuate
portion. The shape has linear portions orthogonal to the magnetic
field generated by the magnetic field generators 7a and 7b and
arcuate portions bridging over the linear portions. This permits
the distance dg between the magnetic field generators 7a and 7b to
be minimized without disturbing a Karman vortex 3 generated by a
vortex generator 2. The arcuate portions are curved outward and
symmetrical about a direction perpendicular to the magnetic field.
The symmetry about the direction perpendicular to the magnetic
field is equivalent to the symmetry about a line extending between
and in parallel with the linear portions. The arcuate portions are
also symmetrical about the magnetic field in Embodiment 4. For
reducing the distance dg in the cross section of the measuring tube
1b, the distance X between the inner sides of arcuate portions
(referred to as an arcuate distance) is not smaller than the length
Y between the linear portions (referred to as a linear distance).
More particularly, the arcuate distance X is a long axis length of
the track shape while the linear distance Y is a short axis
length.
[0058] The arcuate portion may preferably have a semi-circular
shape of which the diameter is equal to the linear distance Y. This
allows the measuring tube 1b to be fabricated much easily and less
expensively. The measuring tube having this shape is smoothly
jointed with the linear edges in the direction of a tangent, thus
rarely producing secondary eddies which disturb the measurement and
can have an increased measuring range and improved measuring
accuracy.
[0059] The smaller the arcuate distance X, the higher the velocity
of the flow will increase to provide a higher Reynolds number,
hence making the measurement easy. If the arcuate distance X is too
small, the inner wall surface disturbs the flow and prevents a
Karman vortex 3 from being generated. Therefore, the arcuate
distance X and the width W of the vortex generator 2 preferably
have the ratio of W/X ranging 0.2 to 0.4. The measuring tube 1b of
Embodiment 4, for having a measuring range from 1 L/min to 10
L/min, has the linear distance of 3.8 mm, the diameter of the
arcuate portion of 3.8 mm, and the arcuate distance of 6.8 mm. The
vortex generator 2 in the measuring tube 1b may preferably be a
triangular column having an isosceles triangle in the cross
section, a width of 2 mm, and a height of 3 mm.
[0060] The magnetic field generator 7a and 7b are accompanied with
a metal yoke 9 and spaced by a small distance dg from each other
for reducing the leakage and thus increasing the magnetic flux. In
case that the measuring tube 1b of this embodiment has the linear
distance of 3.8 mm, the diameter of the arcuate portion of 3.8 mm,
and the arcuate distance of 6.8 mm for having a measuring range
from 1 L/min to 10 L/min, the magnetic field generators 7a and 7b
may be implemented by a ferrite magnet. More preferably, the
magnetic material is a rare-earth permanent magnet, such as
neodymium (Ne), samarium (Sm), or cerium (Ce), which has a higher
magnetic flux density and a favorable thermal property for further
improving the accuracy of measurement.
* * * * *