Abstrict The invention consists in a capacitative electromagnetic flow meter
in which excitation is performed at a frequency above the commercially
available frequency and having a characteristic correction filter
that corrects the gain frequency characteristic of the exciting
current such that the exciting flux waveform has a flat section.
In the detection unit, the value of the electrostatic capacitance
between the face electrodes 4A, 4B and guard electrodes 5A, 5B is
made smaller than the value of the electrostatic capacitance between
the detecting face electrodes 4A, 4B and the fluid to be measured.
The exciting coils are fixed to a cylindrical yoke, being electrostatically
screened by coil fixing plates. Fixing by an earth ring is performed
with this cylindrical yoke and the two ends of the measurement tube
being symmetrical with respect to the tube axis and electrode axes.
In addition, fixing is effected by filling the entire interior of
the detection unit with epoxy resin.
Claims What is claimed is:
1. A capacitative electromagnetic flow meter comprising: a measurement
tube made of insulating material through which flows a fluid to
be measured; an exciting coil wound on a magnetic pole arranged
facing a periphery of said measurement tube, that supplies flux
in a direction orthogonal to a tube axis direction of said measurement
tube; a pair of face electrodes arranged facing a periphery of an
outer wall of said measurement tube in directions respectively orthogonal
to a direction of said flux and said tube axis direction of said
measurement tube; guard electrodes arranged so as to cover said
face electrodes from said periphery of said outer wall, maintaining
a prescribed separation with said face electrodes; an exciting circuit
that supplies an exciting current of a frequency of at least a commercially
available frequency to said exciting coil; a pre-amplifier that
amplifies a detection signal detected through an electrostatic capacitances
between said fluid to be measured and said pair of face electrodes,
respectively, and between said face electrodes and said guard electrodes;
a cable whereby said face electrodes and guard electrodes are connected
with said pre-amplifier; a signal processing unit that outputs a
flow rate of said fluid to be measured from an output signal of
said pre-amplifier; a cylindrical yoke forming a magnetic return
circuit of an exciting magnetic field arranged coaxially with said
measurement tube and so as to cover a periphery of said exciting
coil; a coil fixing plate of non-magnetic material electrically
configured and fixed to said cylindrical yoke, covering said exciting
coil; and an earth ring provided at both ends of said measurement
tube, whereby a metal pipe that is coaxially arranged with said
cylindrical yoke and said cylindrical yoke are configured and fixed
symmetrically and electrically connected with respect to an axis
connecting centers of said pair of face electrodes and a tube axis
of said measurement tube, at a periphery of said cylindrical yoke,
wherein said exciting circuit comprises filter means that controls
a waveform of said exciting current such that an exciting flux waveform
has a flat section; and a value of an electrostatic capacitance
formed between said face electrodes and said guard electrodes is
smaller than a value of the electrostatic capacitance between said
fluid to be measured and said face electrodes.
2. The capacitative electromagnetic flow meter according to claim
1 wherein an epoxy resin is used to fill a space between said measurement
tube and said cylindrical yoke and a space between said measurement
tube and said face electrodes and guard electrodes, respectively,
and to fix said epoxy resin.
3. A capacitative electromagnetic flow meter comprising: a measurement
tube made of insulating material through which flows a fluid to
be measured; exciting means arranged facing a periphery of said
measurement tube, that supplies flux in a direction orthogonal to
a tube axis direction of said measurement tube; a pair of face electrodes
arranged facing a periphery of an outer wall of said measurement
tube in directions respectively orthogonal to a direction of said
flux and said tube axis direction of said measurement tube; guard
electrodes arranged so as to cover said face electrodes from said
periphery of said outer wall, maintaining a prescribed separation
with said face electrodes; an exciting circuit that supplies an
exciting current of a frequency of at least a commercially available
frequency to said exciting coil; a pre-amplifier that amplifies
a detection signal detected through an electrostatic capacitances
between said fluid to be measured and said pair of face electrodes,
respectively, and between said face electrodes and said respective
guard electrodes; a cable whereby said face electrodes and guard
electrodes are connected with said pre-amplifier; a signal processing
unit that outputs a flow rate of said fluid to be measured from
an output signal of said pre-amplifier; a cylindrical yoke forming
a magnetic return circuit of an exciting magnetic field arranged
coaxially with said measurement tube at a periphery of said exciting
coil; a coil fixing plate of non-magnetic material electrically
configured and fixed to said cylindrical yoke, fixing said exciting
coil; and an earth ring provided at both ends of said measurement
tube, whereby a metal pipe that is coaxially arranged with said
cylindrical yoke and said cylindrical yoke are configured and fixed
symmetrically and electrically connected with respect to an axis
connecting centers of said pair of face electrodes and a tube axis
of said measurement tube, at a periphery of said cylindrical yoke;
wherein said coil fixing plate and said cylindrical yoke have both
ends cut away in a direction of said tube axis, in a shape of a
line where a central axial plane of a flux that intersects orthogonally
a center of said tube axis of said measurement tube intersects a
periphery of said cylindrical yoke.
4. The capacitative electromagnetic flow meter according to claim
3 wherein said cutaway shape is adjusted by making a dimension
in a direction of an axis joining centers of said pair of face electrodes
a minimum and adjusting a dimension in a tube axis direction of
said measurement tube.
5. The capacitative electromagnetic flow meter according to claim
3 wherein said pre-amplifier is inserted in a head between a detection
unit and signal processing unit and an interior of said detection
unit that is surrounded by said measurement tube, and said metal
pipe and said earth ring and an entire interior of said detection
unit as far as said head is filled and fixed with an epoxy resin.
6. The capacitative electromagnetic flow meter according to claim
3 wherein a surface of said face electrodes and said guard electrodes
is coated with a soft rubber and a space between said measurement
tube and said cylindrical yoke and a space between said measurement
tube and said face electrodes and guard electrodes are filled and
fixed with an epoxy resin.
7. The capacitative electromagnetic flow meter according to claim
6 wherein, as said soft rubber, silicone resin is applied.
Description CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims benefit of priority to Japanese
application numbers JP 2002-278426 filed Sep. 25 2002 JP 2002-346918
filed Nov. 29 2002 and JP 2003-169705 filed Jun. 13 2003 the
entire content of which is incorporated by reference herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an electromagnetic flow
meter that measures the flow rate of a fluid to be measured that
is flowing through a measurement tube, and more particularly relates
to a capacitative electromagnetic flow meter.
[0004] 2. Description of the Related Art
[0005] There are two types of electromagnetic flow meter, namely,
the liquid-contacting electrode type electromagnetic flow meter,
in which an electrode is directly in contact with the fluid to be
measured and the electro motive force (hereinbelow called the e.m.f.)
generated in the fluid to be measured is directly detected, and
the non-liquid-contacting electrode type electromagnetic flow meter
(hereinbelow termed a capacitative electromagnetic flow meter),
in which the electrode is not directly in contact with the fluid
to be measured and the e.m.f. generated in the fluid be measured
is detected through the electrostatic capacitance between the fluid
to be measured and the electrodes.
[0006] Furthermore, an electromagnetic flow meter is required to
obtain a stable flow rate signal from which the effect of noise
has been removed. However, this noise has various causes, so a large
number of types of electromagnetic flow meter exist, depending on
the different means used to effect such removal (see for example
Laid-open Japanese Patent Publication No. H. 8-304132 (referred
to as Patent Reference 1)).
[0007] Various types of anti-noise measures are known that have
been subsequently developed to improve the capacitative electromagnetic
flow meter disclosed in this Patent Reference 1 (for example Laid-open
Japanese Patent Publication No. 2001-116598 (referred to as Patent
Reference 2)). The construction and action of these will be described
with reference to FIG. 1 to FIG. 3.
[0008] First of all, the construction thereof will be described
with reference to FIG. 1. As shown in this Figure, this capacitative
electromagnetic flow meter comprises a detection unit 10 and a signal
processing unit 11 that is used to find the flow rate from the detected
signal e detected by the detection unit 10.
[0009] The detection unit 10 applies magnetic flux by forming a
return magnetic circuit, not shown, with respect to the fluid 2
to be measured, by passing an exciting current i.sub.F from an exciting
circuit 8 to exciting coils 3A, 3B wound on magnetic poles 7 arranged
facing the outer wall of the measurement tube 1 made of an insulating
substance, through which the fluid 2 to be measured flows.
[0010] Amplifiers 6A, 6B are used to amplify the e.m.f. proportional
to the flow rate of the fluid 2 to be measured, mentioned above,
through the electrostatic capacitance between a pair of face electrodes
4A, 4B that are arranged facing the outer wall of the tube 1 where
measurement is conducted in a direction orthogonal to the direction
of this magnetic flux and guard electrodes 5A, 5B and the measurement
tube 1 and the respective face electrodes 4A, 4B referred to above,
and between the face electrodes 4A, 4B and guard electrodes 5A,
5B arranged so as to cover these face electrodes 4A, 4B and, in
addition a difference amplifier (or differential amplifier) 6C amplifies
the difference e.sub.AB of the respective signals from the amplifiers
6A, 6B, thereby performing detection of the detection signal e.
[0011] Next, flow rate measurement is conducted by passing this
detection signal e to a signal processing unit 11 which samples
positions other than the region of rise of the detection signal
e (termed differentiation noise).
[0012] In this system, the impedance between the face electrodes
4A, 4B and the fluid 2 to be measured is extremely high, so various
types of anti-noise measures are provided in the detection unit
10.
[0013] One of these anti-noise measures is in respect of noise
that is induced between the face electrodes 4A, 4B. This anti-noise
measure involves maintaining the guard electrodes 5A, 5B at the
same potential as the face electrodes 4A, 4B and removing noise
induced in the same phase between the face electrodes 4A and 4B
by performing amplification by the difference amplifier 6C after
impedance conversion using the amplifiers 6A, 6B.
[0014] Also, in the magnetic flux circuit between the guard electrodes
5A, 5B and the exciting coils 3A, 3B, damping foil 7A, 7B, to be
later described, may be arranged.
[0015] In addition, grounding of such a detection unit 10 is achieved
by connecting to ground G by connecting the earth E of a metal pipe
casing liquidly connected with the periphery, not shown of the measurement
tube 1 and a common potential earth C of the circuit.
[0016] Noise, called differentiation noise, as described above,
is superimposed on the detection signal e of a capacitative electromagnetic
flow meter constructed in this way.
[0017] This noise is induced in the detection loop formed between
the two face electrodes 4A, 4B and the amplifier 6A, 6B by induction
due to electromagnetic coupling with the exciting magnetic flux
and the difference of the potential fluctuations between the two
ground points G and the respective face electrodes 4A, 4B that occur
when the exciting magnetic flux fluctuates is superimposed on the
rising portion of the detection signal e as noise.
[0018] The details of this will be described using FIG. 2A, FIG.
2B, FIG. 2C, FIG. 2D and FIG. 2E. When a square wave exciting current
i.sub.F as shown in FIG. 2A flows in the exciting coils 3A, 3B,
the rising portion of the exciting magnetic flux .PHI. shows a waveform
whose characteristic is somewhat blunted by the response time constant
of the diamagnetic field action in the exciting magnetic circuit,
as shown in FIG. 2C, by the eddy current i.sub.E generated in the
exciting magnetic flux path, as shown in FIG. 2B.
[0019] Due to these changes of the exciting magnetic flux .PHI.,
noise in differential form i.e. differentiation noise is superimposed
on the rising portion of the detection signal e as described above,
as in the portion N.sub.d in FIG. 2D.
[0020] It is therefore necessary for the construction within the
detector 10 to be set up and arranged such that the eddy current
i.sub.E generated in the exciting magnetic circuit is kept to a
minimum.
[0021] Also, in order to detect the stable component of the flow
rate signal, as shown in FIG. 2E, the flow rate is found by sampling
with the timing of a sampling signal S.sub.P at which the value
of the differentiation noise has become small.
[0022] Apart from the differentiation noise described above, low-frequency
noise, called "fluid noise" is superimposed on the detection
signal e. The mechanism of generation of this fluid noise is inferred
to be that low-frequency potential fluctuations are produced in
the fluid 2 to be measured, due to irregular movements of the ions
that are transported by the fluid 2 to be measured. Such fluid noise
increases when the flow rate of the fluid 2 to be measured becomes
faster.
[0023] In order to separate this fluid noise and the e.m.f. that
is proportional to the flow rate, the frequency of the exciting
current is made higher than the frequency of the commercial supply
(or commercial frequency) and the exciting circuit is set such that
the flux waveform settles down in a short time.
[0024] However, since the inductance of the exciting coils 3A,
3B has a characteristic having a resonant point in the high frequency
region in the vicinity of 50 kHz, the phenomenon of oscillation
of the exciting current i.sub.F as shown in FIG. 3 still occurs
even though the exciting current i.sub.F is controlled with high
frequency.
[0025] For this reason, thin conductive sheets called damping foils
7A, 7B are provided between the exciting coils 3A, 3B and the guard
electrodes 5A, 5B in order to eliminate the resonant point of the
oscillation.
[0026] As described above, in a conventional capacitative electromagnetic
flow meter, the excitation frequency of the exciting current is
made higher than the commercially supplied frequency in order to
avoid the effect of fluid noise and damping foil is provided in
the flux path in order to suppress oscillation of the exciting current
produced by this raising of the exciting frequency.
[0027] However, since such damping foil exists, because it is arranged
in the flux path, generation of eddy currents cannot be avoided
and the problem arises of fluctuations of potential on the damping
foil being detected as noise, due to electrostatic coupling with
the exciting coils. There were therefore, in addition, the drawbacks
that the construction was complicated due to the need for anti-noise
measures such as the requirement to provide measures such as arranging
an insulating layer between the guard electrodes and the damping
foil.
[0028] Also, as described above, since the output impedance from
the face electrodes is extremely high, the input impedance of the
amplifier needs to have a high value of the order of a few G .OMEGA..
Slight changes in the insulating characteristics of this portion
produce errors of measurement, so the interior of the measurement
tube around the face electrodes and guard electrodes was filled
with epoxy resin, with the object of preventing any decrease in
insulation.
[0029] However, if the method was adopted of fixing these components
by packing with epoxy resin, stress was generated between the face
electrodes and guard electrodes when this heated resin contracted,
causing cracks, with the risk of decrease in insulation. Furthermore,
since the face electrodes and guard electrodes were of large size,
if mechanical vibration of the entire detection unit was produced
by the fluid flowing through the interior during measurement, differences
were produced in the output impedance of the two face electrodes,
resulting in the production of induction noise. Also, friction noise
was generated by the vibration of the signal cable.
SUMMARY OF THE INVENTION
[0030] Accordingly, one object of the present invention is to provide
a novel, stable capacitative electromagnetic flow meter which is
little affected by differentiation noise (electromagnetic induction
noise) or electrostatic induction noise or friction noise and which
is little affected by fluid noise and with excellent resistance
to vibration and humidity, by reducing to the utmost problems generated
by the diamagnetic effect in the flux path.
[0031] In order to achieve the above object, the present invention
is constituted as follows. Specifically, according to the present
invention,
[0032] a capacitative electromagnetic flow meter comprises:
[0033] a measurement tube made of insulating material through which
flows a fluid to be measured;
[0034] an exciting coil wound on a magnetic pole arranged facing
the periphery of the measurement tube, that supplies flux in a direction
orthogonal to the tube axis direction of the measurement tube;
[0035] a pair of face electrodes arranged facing the periphery
of the outer wall of the measurement tube in directions respectively
orthogonal to the direction of the flux and the tube axis direction
of the measurement tube;
[0036] guard electrodes arranged so as to cover the face electrodes
from the periphery thereof, maintaining a prescribed separation
with the face electrodes;
[0037] an exciting circuit that supplies exciting current of a
frequency of at least the commercially available frequency to the
exciting coil;
[0038] a pre-amplifier that amplifies the detection signal detected
through the electrostatic capacitances between the fluid to be measured
and the pair of face electrodes, respectively, and between these
face electrodes and the respective guard electrodes;
[0039] a cable whereby the face electrodes and guard electrodes
are connected with the pre-amplifier;
[0040] a signal processing unit that outputs the flow rate of the
fluid to be measured from an output signal of the pre-amplifier;
[0041] a cylindrical yoke forming a magnetic return circuit of
the exciting magnetic field arranged coaxially with the measurement
tube and so as to cover the periphery of the exciting coil;
[0042] a coil fixing plate of non-magnetic material electrically
connected and fixed to the cylindrical yoke, covering the exciting
coil; and
[0043] earth rings provided at both ends of the measurement tube,
whereby a metal pipe that is coaxially arranged with this cylindrical
yoke and the cylindrical yoke are arranged and fixed symmetrically
and electrically connected with respect to the axis connecting the
centers of the pair of face electrodes and the tube axis of the
measurement tube, at the periphery of the cylindrical yoke; wherein
[0044] the exciting circuit comprises filter means that controls
the waveform of the exciting current such that the exciting flux
waveform has a flat section; and the value of the electrostatic
capacitance formed between the face electrodes and the guard electrodes
is smaller than the value of the electrostatic capacitance between
the fluid to be measured and the face electrodes.
[0045] Consequently, since, according to the present invention,
the frequency of the exciting current is high and is controlled
within a prescribed settling time, damping foil is unnecessary and
eddy currents of the magnetic circuit are suppressed, thereby making
it possible to arrange for the exciting flux waveform to have a
flat section, so flow rate measurement can be achieved in a stable
fashion with high accuracy without being subject to the effects
of differentiation noise, electrostatic noise or fluid noise.
[0046] Also, since electrostatic induction noise from the exciting
coils is screened by the coil fixing plate and the electrostatic
capacitance between the face electrodes and the guard electrodes
is small and the amplification gain of the induction noise superimposed
on the detection signal is low, a capacitative electromagnetic flow
meter can be obtained that is resistant to induction noise.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] A more complete appreciation of the present invention and
many of the attendant advantages thereof will be readily obtained
as the same becomes better understood by reference to the following
detailed description when considered in connection with the accompanying
drawings, wherein:
[0048] FIG. 1 is a layout diagram showing prior art;
[0049] FIG. 2A, FIG. 2B, FIG. 2C, FIG. 2D and FIG. 2E are diagrams
showing the operation of the prior art;
[0050] FIG. 3 is a diagram showing the action of the prior art;
[0051] FIG. 4 is a layout diagram showing an embodiment of the
present invention;
[0052] FIG. 5 is a side view of a detection unit according to a
first embodiment of present invention;
[0053] FIG. 6 is a cross-sectional view of the detection unit according
to the first embodiment of the present invention;
[0054] FIG. 7 is a diagram of the principles of the present invention;
[0055] FIG. 8 is a diagram of principles of the present invention;
[0056] FIG. 9A and FIG. 9B are diagrams of the characteristic of
an exciting circuit according to the first embodiment of the present
invention;
[0057] FIG. 10A, FIG. 10B, FIG. 10C, FIG. 10D, FIG. 10E and FIG.
10F are diagrams of the action of an exciting circuit according
to the first embodiment of the present invention;
[0058] FIG. 11 is a diagram given in explanation of fluid noise;
[0059] FIG. 12 is a model diagram of a detection circuit between
the face electrodes and guard electrodes according to the first
embodiment of the present invention;
[0060] FIG. 13A and FIG. 13B are diagrams of the shape of the face
electrodes according to the first embodiment of the present invention;
[0061] FIG. 14 is a side view of a detection unit according to
a second embodiment of the present invention;
[0062] FIG. 15 is a cross-sectional view of the detection unit
according to the second embodiment of the present invention;
[0063] FIG. 16A and FIG. 16B are layout diagrams of face electrodes
and guard electrodes according to the present invention;
[0064] FIG. 17A and FIG. 17B are diagrams showing the construction
of a return magnetic circuit according to a third embodiment of
the present invention;
[0065] FIG. 18A and FIG. 18B are diagrams of a method of setting
up a return magnetic circuit according to the third embodiment of
the present invention;
[0066] FIG. 19A and FIG. 19B are diagrams of the beneficial action
of the third embodiment of the present invention;
[0067] FIG. 20 is a diagram of the setting of the exciting frequency
of the third embodiment of the present invention; and
[0068] FIG. 21A, FIG. 21B and FIG. 21C are constructional views
of the face electrodes and guard electrodes.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0069] (First Embodiment)
[0070] Referring now to the drawings, wherein like reference numerals
designate identical or corresponding parts throughout the several
views, and more particularly to FIG. 4 to FIG. 13 thereof, one embodiment
of the present invention will be described.
[0071] First of all, the layout of an embodiment of the present
invention will be described with reference to FIG. 4 to FIG. 6.
The capacitative electrostatic flow meter of the first embodiment
of the present invention comprises a detection unit 10 that detects
the flow rate of fluid, a signal processing unit 11 that finds a
flow rate signal from the detection signal of this detection unit
10 and an exciting circuit 8 that supplies exciting current to an
exciting coil in the detection unit 10.
[0072] FIG. 5 is a side cross-sectional view of the detection unit
10. FIG. 6 is a cross-sectional view of the detection unit 10. The
exciting circuit 8 and signal processing unit 11 are integrally
constituted and accommodated, together with a pre-amplifier 6 in
a box on top of the detection unit 10 as shown in FIG. 5 and FIG.
6.
[0073] A separated type of device would also be possible, in which
these are separately arranged.
[0074] In these Figures, 1 is a measurement tube made of insulator
such as ceramics. Exciting coils 3A and 3B, wound on a magnetic
pole 7 are oppositely arranged at the periphery of the measurement
tube 1 and are connected in series.
[0075] The exciting coils 3A and 3B are set and arranged such that
flux is supplied in the direction orthogonal to the tube axis direction
of the measurement tube 1 by means of a cylindrical yoke 71 arranged
coaxially with the measurement tube 1 and positioned at the periphery
of the exciting coils 3A, 3B.
[0076] 4A and 4B are face electrodes made of non-magnetic material
having high electrical conductivity, such as copper sheet and are
oppositely arranged at the periphery of the measurement tube 1
so that the axis joining their centers is arranged so as to intersect
orthogonally both the direction of the magnetic field of the exciting
flux and the axis of the tube through which the fluid 2 to be measured
flows. Also, the guard electrodes 5A, 5B are set fixed in position
so as to present an electrostatic capacity below a prescribed value,
as will be described, so as to completely cover the respective face
electrodes 4A and 4B and with the distance between the face electrodes
4A, 4B and the respective guard electrodes 5A, 5B fixed.
[0077] The coil fixing plates 9A, 9B are constructed of metal sheet
made of non-magnetic material having a high electrical conductivity,
such as copper sheet and completely cover the exciting coils 3A
and 3B; they are fixed, electrically connected with the cylindrical
yoke 71.
[0078] 1A1 and 1A2 are earth rings that form flanges provided at
both ends of the measurement tube, both ends of a metal pipe 1B
and the cylindrical yoke 71 mentioned above being fixed thereto.
[0079] In addition, earth terminals E provided on the earth rings
1A1 1A2 connected with a common earth point C of the circuit are
connected, selecting a ground point G at a stable potential.
[0080] 10A and 10B are signal cables, constituted as double shielded
cables constructed by covering a core wire, a shield provided at
the periphery of this core wire and a guard shield with insulator.
The core wires of these signal cables 10A, 10B are respectively
connected with face electrodes 4A, 4B and the shields thereof with
the guard electrodes 5A, 5B and the guard shields thereof with the
common earth C of the circuit. These signal cables 10A and 10B are
fixed maintaining a prescribed distance, by arranging an insulating
spacer, not shown, along the periphery of the cylindrical yoke 71
and are connected with the amplifiers 6A, 6B of the pre-amplifier
6 through an electrical conduction passage provided in the metal
pipe 1B.
[0081] In more detail, these signal cables are connected with the
pre-amplifier 6 by running along in the same plane as the direction
of the exciting magnetic field described above, being fixed at fixed
intervals with insulator interposed along the periphery of the cylindrical
yoke 71 described above and intersecting at right angles at the
top of the cylindrical yoke 71.
[0082] The core wire, shield and guard shield constituting the
signal cables 10A, 10B are all made of non-magnetic material of
high electrical conductivity, such as copper.
[0083] Also, the face electrodes 4A, 4B and guard electrodes 5A,
5B are respectively connected with the non-inverting input, inverting
input and output of respective amplifiers 6A, 6B and the difference
of the outputs of the respective amplifiers 6A, 6B is amplified
by a difference amplifier 6C to provide a detection signal e that
is connected with an analogue/digital conversion circuit (hereinbelow
termed ADC circuit) 11A of the signal processing unit 11.
[0084] The detection signal e is converted to a digital signal
in this ADC circuit 11A, the output of which is then sent to a flow
rate measurement processing circuit 11B, where it is processed to
convert it to a flow rate signal.
[0085] In the exciting circuit 8 a square wave signal of 300 Hz
is generated by means of a square wave generator 8A and this signal
is supplied to a characteristic correction filter 8B. After correction
processing of the gain frequency characteristic so as to have a
flat section in the exciting flux waveform, as will be later described,
in this characteristic correction filter 8B, the output of this
characteristic correction filter 8B is applied as an exciting current
i.sub.F to the exciting coils 3A, 3B mentioned above through a current
control amplifier 8C.
[0086] Next, further details of the setting and action of the various
units of the capacitative electromagnetic flow meter according to
the present invention set up in this way will be described.
[0087] First of all, FIG. 7 and FIG. 8 are views given in explanation
of the principles of measurement of a capacitative electrostatic
flow meter according to the present invention. FIG. 7 is a view
given in explanation of the principles of detection and is a model
diagram showing how an e.m.f. is generated that is proportional
to the flow rate between the face electrodes 4A and 4B, the central
axis connecting the face electrodes 4A and 4B of the side face of
the measurement tube 1 being designated as the x axis, the axial
direction of the exciting flux .PHI. at right angles to this being
designated as the y axis and the axial direction of the tube through
which the fluid moves being designated as the z axis.
[0088] In order to obtain a stable capacitative electromagnetic
flow meter of high accuracy, various anti-noise measures are necessary,
as described above. First of all, as such anti-noise measures, as
described in the prior art example, measures are required in respect
of differentiation noise produced by electromagnetic induction,
fluid noise generated in the fluid and induction noise generated
by electromagnetic coupling and electrostatic coupling in the detection
unit 10 induced into the detection circuit that detects the e.m.f.
[0089] In FIG. 7 in order to avoid the effects of differentiation
noise and fluid noise, an exciting flux .PHI. of high frequency
having an ideally flat section, to be described in detail later,
is applied and the various sections are designed such that accurate
separation and extraction of only the flow rate signal component
is achieved, by reducing to the utmost the eddy current generating
element component of the magnetic flux circuit that disturbs the
waveform of the exciting flux .PHI., intersecting orthogonally with
this exciting magnetic flux .PHI., and reducing to the utmost the
noise that is induced in the detection loop formed by the detection
electrodes constituted by the face electrodes 4A, 4B and the guard
electrodes 5A, 5B (hereinbelow referred to as the detection electrodes)
and the pre-amplifier 6.
[0090] Consequently, in the detection loop formed by the face electrodes
4A, 4B and the pre-amplifier 6 as shown in FIG. 8 the signal cables
10A, 10B are arranged so as to remove noise induced by electromagnetic
coupling, by reducing the area S shown hatched and making the flux
that intersects with this detection loop as small as possible, by
arranging for this to be formed in a plane that is parallel with
the exciting flux .PHI..
[0091] Also, with a detection unit 10 as shown in FIG. 5 and FIG.
6 noise that is induced in the same phase is removed by the difference
amplifier 6C by adopting a construction that is electrically symmetrical
with regard to both electrostatic aspects and electromagnetic aspects,
so that the face electrodes 4A and 4B are respectively at the same
potential with respect to ground G.
[0092] Furthermore, since, while the e.m.f. of the fluid is at
the millivolt level, the portion represented by the exciting coils
3A, 3B is a high potential level of a few tens of volts, the noise
resulting from the induction of the signal from the exciting circuit
into the face electrodes 4A and 4B and the guard electrodes 5A,
5B by electrostatic conduction is screened by covering the exciting
coils 3A, 3B without a gap by means of the coil fixing plates 9A,
9B constituted of non-magnetic material such as copper and connecting
with the ground G through the cylindrical yoke 71 and earth rings
1A1 1A2.
[0093] In this way, a symmetrical construction is achieved such
that the noise that is induced in the two face electrodes 4A, 4B
and the guard electrodes 5A, 5B is a minimum, of the same phase
and same level, and the earth rings 1A1 1A2 are connected to a
stable ground G.
[0094] Also, since the metal members in the magnetic flux circuit
of the exciting flux .PHI., apart from the magnet poles 7 and cylindrical
yoke 71 constituting the return magnetic circuit, are made of non-magnetic
material of high electrical conductivity such as copper in order
to suppress generation of eddy currents, the arrangement is such
that flux components produced by eddy currents are as far as possible
not generated.
[0095] In addition, the various elementary components and members
that constitute the detection unit are fixed in such a way that
their setting conditions are not changed by vibration or humidity.
[0096] The set-up of the various units is in accordance with the
above concepts.
[0097] The detailed setting and action of the exciting current
i.sub.F will now be described with reference to FIG. 9 to FIG. 11
with reference to the detailed setting of the various units based
on the above concepts.
[0098] At first, the frequency of this exciting current i.sub.F
is set to a high frequency that is beneficial for distinguishing
from the fluid noise, at which the amount of fluid noise is small.
The reasons for this are described with reference to FIG. 11.
[0099] FIG. 11 shows an example of the measurement results of the
fluid noise described above, the frequency being shown along the
horizontal axis and the noise power dBm being shown along the vertical
axis; the characteristics when the flow rate is 2.5 m/sec and when
it is 0.5 m/sec are shown. As shown in this Figure, typically the
fluid noise increases when the flow rate of the fluid 2 to be measured
increases.
[0100] However, irrespective of the flow rate of the fluid 2 to
be measured, the frequency tends to be attenuated from about 10
Hz, with the noise power tending to converge to a level of -70 dBm
in the vicinity of 200 Hz. The exciting frequency is therefore set
by the square wave generator 8A at an oscillation frequency of at
least 200 Hz more than the commercially supplied frequency, at which
the e.m.f. generated by the flow rate of the fluid 2 to be measured
has a high S/N that is unaffected by fluid noise; for example this
value may be set at 300 Hz.
[0101] Next, the detailed setting of the correction of the waveform
of the exciting flux .PHI. produced by the exciting current i.sub.F
will be described with reference to FIG. 9 and FIG. 10. The object
of this, as already described, is to achieve an earlier rise of
the exciting flux .PHI., thereby achieving an earlier rise of the
differentiation noise contained in the detection signal e and also
the settling time.
[0102] As shown in FIG. 9A, typically, the exciting current i.sub.F
has a gain frequency characteristic of a first-order delay circuit
in which the exciting current i.sub.F is attenuated from the vicinity
of the fundamental frequency (f.sub.0) of the exciting magnetic
field. The exciting flux .PHI. created by the exciting current i.sub.F
therefore also has a waveform tracking this.
[0103] Thereupon, as shown in FIG. 9B, this exciting current i.sub.F
is supplied through the current control amplifier 8C with a waveform
of the exciting current i.sub.F having a differentiation characteristic
in which the rising section of the exciting waveform .PHI. rises
steeply, through the characteristic correction filter 8B having
a high band pass characteristic, in which the gain frequency characteristic
of the exciting circuit including the harmonic components of the
fundamental frequency (f.sub.0) of the exciting magnetic field is
flat.
[0104] This characteristic correction filter 8B is a high band
filter circuit of a characteristic as shown in FIG. 9B, producing
an exciting current i.sub.F waveform having a gain frequency characteristic
that is flat up to for example the third harmonic (f.sub.3) frequency
band having a frequency of three times the fundamental frequency
(f.sub.0) of the exciting magnetic field.
[0105] Furthermore, if correction of higher order harmonics, for
example the fifth harmonic (f.sub.5) component is necessary, the
characteristic correction filter 8B is set to a gain frequency characteristic
that is flat up to the higher order frequency region.
[0106] FIG. 10 is a view given in explanation of the action and
effect of the exciting current and the exciting flux waveform when
this correction filter 8B is employed. FIG. 10A, FIG. 10B and FIG.
10C show the prior art case, when no characteristic correction filter
8A is employed; FIG. 10D, FIG. 10E and FIG. 10F show the case where
the characteristic correction filter 8B is employed.
[0107] In this Figure, FIG. 10A and FIG. 10B show the exciting
flux .PHI., FIG. 10B and FIG. 10E show the detection signal e and
FIG. 10C and FIG. 10F show the sampling signal S.sub.P of the detection
signal e. The continuous line in FIG. 10B and FIG. 10E shows the
case where the fluid 2 to be measured is stationary and the broken
line shows the waveform of the detection signal e when the fluid
2 to be measured is flowing.
[0108] As shown in these Figures, the rise of the exciting flux
.PHI. is blunted as shown in FIG. 10A but the rise is hastened as
shown in FIG. 10D by shaping of the exciting current waveform by
the characteristic correction filter 8B.
[0109] As a result, the differentiation noise contained in the
detection signal e shown in this FIG. 10B is improved such that
convergence takes place in a short-time as shown in this FIG. 10E.
Consequently, with the timing of the sampling signal S.sub.P, stable,
highly accurate flow rate measurement can be achieved, since only
the flow rate component that is unaffected by the differentiation
noise can be extracted.
[0110] In regard to the characteristic of this characteristic correction
filter 8B, an optimum response characteristic of the exciting circuit
can be selected by observing the output waveform of the exciting
flux .PHI. or the waveform of the detection signal e.
[0111] Next, the details of the setting of the electrostatic capacitance
between the face electrodes 4A, 4B and the guard electrodes 5A,
5B will be described, using FIG. 12 by means of a model diagram
of the detection circuit of one of the face electrodes 4A and amplifier
6A.
[0112] C1 is the electrostatic capacitance formed between the face
electrodes 4A and the fluid 2 to be measured i.e. by the material
of the measurement tube 1; one side of this is connected with the
electrostatic capacitance C2 between the face electrodes 4A and
the guard electrode 5A while the other side thereof is connected
with the ground potential G through the fluid resistance Rs of the
fluid 2 to be measured.
[0113] In addition, the connection point of the electrostatic capacitances
C1 and C2 is connected with the input of the amplifier 6A and the
output thereof is connected with the other one of the terminals
of the electrostatic capacitance C2.
[0114] If the input impedance of the amplifier 6A is sufficiently
high, the noise v.sub.N that is superimposed on the guard electrode
5A and the output v.sub.A of the amplifier 6A are:
v.sub.A=(1+j.omega.C1Rs).multidot.C2/C1.multidot.v.sub.N.
[0115] Consequently, if the electrostatic capacitance C2 between
the face electrode 4A and the guard electrode 5A is larger than
the electrostatic capacitance C1 between the face electrodes 4A
and the fluid 2 to be measured, the noise that is superimposed on
the guard electrode 5A is amplified.
[0116] Consequently, if for example the measurement tube 1 is made
of ceramics, plastics or the like, which has a smaller dielectric
constant than ceramics, is inserted between the face electrode 4A
and the guard electrode 5A and, in addition, the separation between
the face electrode 4A and the guard electrode 5A is set to be larger
than the thickness of the measurement tube 1.
[0117] If the dielectric constant of this ceramics is taken as
9 and the dielectric constant of the plastics is taken as 3 even
if the separation between the face electrode 4A and the guard electrode
5A is the same as that of the measurement tube 1 the noise that
is induced in the guard electrode 5A or the noise that is generated
in the amplifier 6A will be reduced to one third.
[0118] Apart from the electrical noise generating factors described
above, noise is generated by heat or vibration of the detection
unit 10 as a whole. An example of countermeasures in this respect
is again described with reference to the constructional diagrams
of FIG. 5 and FIG. 6 described above.
[0119] When there is an abrupt change in the temperature of the
fluid of the measurement tube 1 the adhesive or supporting members
for mounting of the face electrodes 4A, 4B and guard electrodes
5A, 5B undergoes elongation/compression force due to the difference
in thermal expansion coefficient, so these are formed of copper
sheet or the like having a high electrical conductivity and are
fixed, arranged in a construction capable of withstanding deformation
due to thermal distortion, with plastics or the like, by packing
with epoxy resin or the like, maintaining a prescribed separation,
as described above.
[0120] Also, these face electrodes 4A, 4B are arranged to suppress
as far as possible generation of flux components in the y axis direction
referred to above, by being made of a shape to prevent generation
of eddy currents, by inserting slits as shown in FIG. 13 and in
particular FIG. 13B.
[0121] Also, the signal cables 10A, 10B referred to above are fixed
by adhesive, maintaining a fixed separation, with an insulator such
as epoxy resin interposed, at the periphery of the cylindrical yoke
7. Non-magnetic material of high electrical conductivity, such as
copper, is employed for all of the core wires, shields and guard
shields so that no vibration is produced even if the exciting magnetic
field .PHI. fluctuates periodically, and so as to make it possible
to prevent friction noise, which might be generated by friction
with the insulators interposed respectively between these.
[0122] (Second Embodiment)
[0123] The aspect in which the second embodiment differs from the
first embodiment is that, in order to produce a construction of
even higher reliability than the construction of the detection unit
10 described in the first embodiment, the entire high impedance
section from the face electrodes 4A, 4B up to the difference amplifier
6 is fixed in a compact fashion by packing with insulating material
having strong insulating characteristics, in order to improve resistance
to vibration and insulation of the detection unit 10 as a whole.
[0124] A construction is therefore achieved in which generation
of electromagnetic induction noise induced by change of input impedance
of the two inputs of the difference amplifier 6 and friction noise
due to friction of the conductors of the signal cables 10A, 10B
and the insulator can be prevented.
[0125] Hereinbelow, items in the capacitative electromagnetic flow
meter according to the second embodiment of the present invention
that are the same as in the case of the first embodiment are given
the same reference numerals and further description thereof is omitted.
A second embodiment of the present invention is described below
with reference to FIG. 14 FIG. 15 FIG. 16 and FIG. 4. FIG. 14
is a side cross-sectional view of the detection unit 10 and FIG.
15 is a cross-sectional view of the detection unit. Also, FIG. 16
is a view given in explanation of the construction of the face electrode
4A and guard electrode 5A.
[0126] First of all, the names and functions of various items are
the same as in the case of the first embodiment, so the description
will be suitably abbreviated by describing only the differences
thereof with respect to the first embodiment. In the head section
21 where the detection unit 10 and signal processing unit 11 are
coupled, the pre-amplifier 6 that amplifies the signal from the
detection electrodes is installed separately from the ADC circuit
11A and the flow rate measurement circuit 11B of the signal processing
unit 11 of the downstream stage.
[0127] The signal processing unit 11 may be arranged within a box
at the top of the detection unit 10 as described in the first embodiment
or in a separate casing separated therefrom.
[0128] Next, the construction of the detection electrodes will
be described with reference to FIG. 16A and FIG. 16B. FIG. 16A is
a cross-sectional view of one of the face electrodes 4A and guard
electrode 5A; FIG. 16B is a side view seen from the outside direction
of the electrode axis x joining the face electrodes 4A and 4B. A
metal plate of high electrical conductivity, for example face electrode
4A made of copper sheet or the like, is sintered to the outside
wall of the measurement tube 1 made of ceramics or the like.
[0129] In addition, a guard electrode 5A of a shape extending further
than the electrode dimension in the z axis direction in which the
fluid 2 to be measured moves is arranged so as to cover this face
electrode 4A (the y axis direction indicates the direction of application
of the exciting flux).
[0130] This guard electrode 5A is molded of metal sheet of high
electrical conductivity, like the face electrode 4A, and constitutes
a cavity in the direction of the tube axis, of a downwardly diverging
shape seen from the direction of the xy cross section of FIG. 16A.
As shown in FIG. 16B, this guard electrode 5A covers the face electrode
4A and is arranged so as to ensure a prescribed gap with respect
to the face electrode 4A. As described in the first embodiment,
the electrostatic capacitance between the detection electrodes is
arranged to be smaller than the electrostatic capacitance between
the face electrode 4A and the fluid 2 to be measured.
[0131] The size of this guard electrode 5A does not need to be
sufficiently large to cover all of the periphery of the measurement
tube, which it divides into two together with the other guard electrode
5B but need only be sufficiently large to enable the prescribed
e.m.f. to be extracted. Usually, as shown in FIG. 16A and FIG. 16B,
the extension of this guard electrode 5A in the circumferential
direction of the measurement tube 1 is sufficient if it is about
60.degree. and the dimension of this guard electrode 5A in the tube
axis direction z through which the fluid 2 to be measured flows
is sufficient if it is about 60% of the length of the measurement
tube 1.
[0132] These guard electrodes 5A, 5B are fixed to the measurement
tube 1 by adhesive glass tape.
[0133] Also, as in the first embodiment, the shape of the face
electrode 4A may be a shape in which slits are inserted so as to
decrease generation of eddy currents.
[0134] One of the core wires of the signal cables 10A, 10B is connected
with the face electrodes 4A, 4B while the shield thereof is connected
with the guard electrodes 5A, 5B, respectively, the other thereof
being connected with the pre-amplifier 6. The guard shield is earthed
by being connected with the common potential C of this pre-amplifier
6 shown in FIG. 4.
[0135] Next, the method of packing the detection unit 10 and pre-amplifier
6 with epoxy resin will be described. In the cylindrical yoke 71
of the detection unit 10 in a condition assembled as in FIG. 14
and FIG. 15 apart from the holes 71a, 71b, through which the signal
cables 10A, 10B are passed, that are provided in the vicinity of
the tube wall of the measurement tube 1 where the tube axis of the
measurement tube 1 and the axis connecting the centers of the pair
of electrodes intersect, there are provided, separately or in common,
a hole 71c through which the exciting signal whereby the exciting
current is applied to the exciting coils 3A, 3B from the exciting
circuit 8 cable is passed and a hole 71d for introduction of the
epoxy resin that is sealed therein and for escape of air during
this process, in locations other than those of the holes 71a, 71b,
so as not to disturb the exciting flux.
[0136] Epoxy resin mixed with a prescribed curing agent is then
slowly poured in from a prescribed resin introduction pipe socket
from the head 21 of the signal processing unit 11 while shaking
and tilting the detection unit 10 to left and right and forwards
and backwards and while allowing the air in the interior thereof
to escape, so as to cover the pre-amplifier 6 of the head 21.
[0137] With this construction, the face electrodes 4A, 4B and the
guard electrodes 5A, 5B are fixed within the measurement tube 1
in a compact arrangement having a prescribed mechanical strength
and fixing can be achieved solely by means of the epoxy resin filling.
[0138] Also, depending on the application, by pouring in epoxy
resin as far as the signal processing unit 11 at the rear section
arranged at the top of the head 21 insulating of this portion also
can be ensured.
[0139] In this way, epoxy resin is poured into the entire space
within the detection unit 10 enclosed by the measurement tube 1
metal pipe 1B and earth rings 1Al and 1A2. In addition, fixing with
epoxy resin is achieved as far as the head 21 of the detection unit
10.
[0140] With this second embodiment of the present invention, electromagnetic
induction noise and friction noise produced by vibration of the
signal cables 10A, 10B that connect these detection electrodes and
the difference amplifier 6 can be prevented, since the high impedance
section is laid in a compact fashion over the minimum distance from
the detection electrodes to the difference amplifier 6 and is fixed
by epoxy resin. Furthermore, deterioration of insulation is reduced
by fixing the high impedance section with resin.
[0141] Also, since the detection electrodes 4A, 4B are sintered
to the measurement tube 1 and the guard electrodes 5A, 5B compact
and are made of a thickness capable of withstanding prescribed stress,
there is no risk of failure even on occurrence of thermal contraction
due to temperature change of the fluid 2 to be measured or on curing
of the epoxy resin. Furthermore, since the pre-amplifier 6 is arranged
at the head 21 of the detection unit 10 a heat-screening construction
can be achieved, making it possible to use the flow meter even when
the fluid 2 to be measured is at high temperature.
[0142] (Third Embodiment)
[0143] The difference of the third embodiment from the first embodiment
is that both ends in the tube axis direction intersecting with the
central axial plane of the flux of the coil fixing plates 9A, 9B
and cylindrical yoke 71 that form the magnetic return circuit are
cut away in prescribed shapes in order to suppress generation of
eddy currents, so that differentiation noise components contained
in the detection signal can be reduced by more rapid rise of the
exciting current and a high exciting frequency range of more than
200 Hz can be set, at which the fluid noise is less than a prescribed
range.
[0144] Also, in order to cut off electrostatic coupling produced
by the cutting away of the coil fixing plates 9A, 9B and the cylindrical
yoke 71 lugs are provided on the guard electrodes 5A, 5B to strengthen
the electrostatic shielding effect of the guard electrodes 5A, 5B
in the tube axis direction and a configuration is adopted having
only a small gap.
[0145] Furthermore, by applying a coating of silicone resin to
the surface of the face electrodes 4A, 4B and guard electrodes 5A,
5B before filling with epoxy resin and curing, stress produced by
heat absorption by the epoxy resin is absorbed by the coating material,
making it possible to prevent production of cracks between the face
electrodes 4A, 4B and guard electrodes 5A, 5B and the epoxy resin.
[0146] Hereinbelow, regarding the capacitative electrostatic flow
meter according to the third embodiment of the present invention,
items that are the same as items in the first embodiment shown in
FIG. 4 to FIG. 6 are indicated with the same reference symbols and
further description thereof is omitted.
[0147] FIG. 17 is a view showing how the cylindrical yoke 71 and
the end of one of the coil fixing plates 9A that form the magnetic
return circuit are cut away in order to reduce eddy currents and
FIG. 17A is a perspective view seen from above the side-face in
the direction of the central axis of the flux (hereinbelow called
the y axis).
[0148] In these Figures, one exciting coil 3A is fixed by means
of a screw or the like, not shown, to the cylindrical yoke 71 by
the coil fixing plate 9A shown in FIG. 17B. The coil fixing plate
9A that contacts the exciting coil 3A, and part of the end of the
cylindrical yoke 71 are cut away in a prescribed shape, whose details
will be described later, along the direction of the tube axis of
the measurement tube 1 (hereinbelow called the z axis). The other
coil fixing plate 9B and cylindrical yoke 71 on the other side are
given an identical cutaway shape in a symmetrical position.
[0149] Although the magnetic pole 7 is not shown in FIG. 18B, the
exciting means constituted by the exciting coils 3A, 3B and magnetic
pole 7 may provide a prescribed exciting flux intensity and may
alternatively be constituted solely by the exciting coils 3A, 3B.
[0150] Next, the method of defining the cutaway (or cut-out) shape
will be described with reference to FIG. 18A and FIG. 18B. FIG.
18A shows in qualitative fashion the relationship of the area of
the cutaway portions and the decreasing effect of differentiation
noise produced by eddy currents superimposed on the detection signal,
and the increase in magnetic field noise produced by electrostatic
coupling of the exciting power source of the exciting coils 3A,
3B with the face electrodes 4A, 4B and the guard electrodes 5A,
5B (hereinbelow termed electrical field noise).
[0151] As shown in these Figures, regarding the area of the cutaway
portions, there is a trade-off relationship (antinomy) between the
decrease of differentiation noise produced by eddy currents and
increase of electric field noise produced by electrostatic induction,
so the area must be set in a range such that both effects are less
than the prescribed allowed values.
[0152] For example, according to prescribed model tests, as shown
in these Figures, if this cutaway area is made to be a cutaway area
of 20% to 30% of the projection area of the coil fixing plates 9A,
9B that contact the exciting coil 3A and cylindrical yoke 71 in
the y axis direction, the noise can be kept below the prescribed
allowed noise level.
[0153] Various modifications of this cutaway shape are possible.
FIG. 18B is a view showing the cutting away of the coil fixing plate
9A and cylindrical yoke 71 projected onto the xz axis plane. As
shown in this Figure, since the eddy currents that form the diamagnetic
field in the y axis direction can be cut off by cutting away a prescribed
limiting dimension lx in the direction of the axis that joins the
pair of electrode axes (hereinbelow called the x axis), effective
adjustment of the optimum cutaway dimension of (lx.times.lz) can
be achieved by increasing the lz dimension in the z axis direction,
in a range such that electric field noise is not increased.
[0154] As a result of such adjustment of the eddy currents, an
early rise of the exciting flux .PHI. can be achieved, so an exciting
frequency can be set at which the frequency component of fluid noise
can easily be separated.
[0155] The benefits obtained by the third embodiment when set up
as follows will now be described.
[0156] As shown in FIG. 19A, if, for example, the exciting frequency
is set at 100 Hz (T=10 msec) and the detection signal e is sampled
with a sampling pulse Sp, as the signal component becomes larger,
fluctuations of the differentiation noise can be neglected but,
as shown by the broken line, if the exciting frequency is set at
200 Hz (T/2=5 msec), the ratio of the differentiation noise that
is sampled with respect to the detection signal e of FIG. 18B becomes
large, with the result that its effect on measurement accuracy becomes
large and cannot be neglected.
[0157] In this respect, if the eddy currents are reduced by means
of the cutaway portions described above as described in this embodiment,
the rise of the exciting flux .PHI. occurs earlier, so, even at
an exciting frequency of 200 Hz, the ratio of differentiation noise
of the detection signal e of FIG. 19B is decreased, as shown by
the continuous line.
[0158] As a result, even though, with regard to the exciting frequency,
lowering of the fluid noise and increase of differentiation noise
are in a trade-off relationship, it becomes possible to select a
frequency, specifically a frequency in the vicinity of 200 Hz and
over, in a range of exciting frequency at which both of these are
below the prescribed noise levels, as shown in FIG. 20.
[0159] Next, the shield structure of the guard electrode 5A (5B)
will be described with reference to FIG. 21A and FIG. 21B. FIG.
21A and FIG. 21B are respectively a perspective view given in explanation
of the construction of the region of the guard electrode 5A seen
from the x axis direction of one side face of the measurement tube
1 and a cross-sectional view of the guard electrode region seen
from the z axis direction of the measurement tube 1.
[0160] As shown in FIG. 21B, a lug 5Aa is provided at both ends
in the said axis direction of the measurement tube 1 of the guard
electrode 5A, so as to produce a shape in which the gap is a small
as possible while maintaining sufficient space for introduction
of the epoxy resin to be achieved.
[0161] In the third embodiment, the coil fixing plates 9A, 9B and
both ends of the cylindrical yoke 1 that constitute the magnetic
return circuit are cut away, so the risk of increased electrical
field noise due to electrostatic capacitative coupling of the exciting
coils 3A, 3B and the guard electrode 5A is increased but electrostatic
coupling can be reduced by shielding by providing lugs at both ends
of the guard electrodes 5A, 5B.
[0162] Also, as shown in FIG. 21C, on the surface of the face electrode
4A (4B) and guard electrode 5A (5B), silicone resin or soft rubber
such as chloroprene rubber is applied in the other portions prior
to the introduction of the epoxy resin, in the high impedance portion
between the face electrode 4A (4B) and guard electrode 5A (5B) and
the lead of the signal cable, such that the insulation is not lowered
by this soft rubber. Filling with epoxy resin of high insulating
properties is performed after this soft rubber has dried.
[0163] By coating the surfaces of the face electrodes 4A, 4B and
guard electrodes 5A, 5B in this way with silicone resin or the like,
stress produced on thermal contraction of the epoxy resin and due
to differences in the thermal expansion/contraction between the
epoxy resin and the face electrodes 4A, 4B and guard electrodes
5A, 5B can be absorbed by means of this applied silicone resin,
so cracking or loosening can be prevented.
[0164] As described above, with the present invention, by simplifying
the construction of the exciting magnetic circuit, the rise of the
exciting flux can be made to occur earlier, so an exciting frequency
can be employed at which the effect of fluid noise can be reduced.
Also, a symmetrical shape construction is adopted in respect of
the paired face electrodes and guard electrodes and detection circuit
in which difference amplification is performed and a shield construction
whereby a high ratio of removal of in-phase noise can be achieved
electrically is employed, such that the flow rate signal obtained
by difference amplification from the paired face electrodes and
guard electrodes can be detected with high S/N ratio.
[0165] Furthermore, according to the present invention, a structure
is produced which is of high strength capable of withstanding even
the stress produced by thermal expansion/contraction, by filling
the interior of this detection unit with insulator, thereby making
it possible to provide a capacitative electromagnetic flow meter
of highly reliability that is resistant to the effects of induction
noise or friction noise produced by vibration.
[0166] Also, according to the present invention, by making the
electrostatic capacitance of the face electrodes and guard electrodes
smaller than the electrostatic capacitance between the face electrodes
and the fluid to be measured, the amplification gain of the induction
noise and the amplifier noise is suppressed, so a stable capacitative
electromagnetic flow meter of high accuracy can be provided.
[0167] Also, according to the present invention, generation of
eddy currents can be prevented, so the risk of disturbing the magnetic
flux circuit is small. Also, thanks to the provision of prescribed
mechanical strength, a capacitative electromagnetic flow meter can
be achieved which is resistant to thermal deformation.
[0168] Also, according to present invention, the area of intersection
with the detection loop can be made small and the electromagnetic
induction noise can be reduced. Also, since the cable between the
detection electrodes and the pre-amplifier is non-magnetic and fixed,
a capacitative electromagnetic flow meter can be obtained in which
there is little generation of vibration noise of the cable produced
by vibrations of the fluid or noise due to electromagnetic induction.
[0169] Also, according to present invention, a capacitative electromagnetic
flow meter can be produced in which there is no noise generation
due to vibration of the fluid and no noise generation due to vibration
of the exciting coils.
[0170] Also, since, according to the present invention, the length
of the signal cable is made a minimum, the difference amplifier
is accommodated compactly in the head and the entire high-impedance
portion from the aforementioned face electrodes and guard electrodes
to the aforementioned pre-amplifier is fixed, generation of noise
due to vibration can be kept to a minimum and a capacitative electromagnetic
flow meter can be provided of excellent resistance to humidity.
[0171] Also, since, according to the present invention, the coil
fixing plates and cylindrical yoke are cut away in a prescribed
shape, suppressing generation of eddy currents and enabling a higher
exciting frequency to be employed due to earlier rise of the exciting
flux, a capacitative electromagnetic flow meter can be obtained
that is little affected by fluid noise.
[0172] Also, according to the present invention, since silicone
resin or soft rubber is applied between the face electrodes and
guard electrodes and epoxy resin, the stress generated when thermal
contraction of the epoxy resin occurs is absorbed by the soft rubber,
so a construction can be achieved in which no cracks are generated
between the face electrodes, guard electrodes and epoxy resin.
[0173] Obviously, numerous additional modifications and variations
of the present invention are possible in light of the above teachings.
It is therefore to be understood that within the scope of the appended
claims, the present invention may be practised otherwise than as
specially described herein. |