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 BACKGROUND OF THE INVENTION
1. Field of the Invention
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.
2. Description of the Related Art
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.
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)).
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.
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.
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.
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.
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).
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.
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.
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.
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.
Noise, called differentiation noise, as described above, is superimposed
on the detection signal e of a capacitative electromagnetic flow
meter constructed in this way.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
In order to achieve the above object, the present invention is
constituted as follows. Specifically, according to the present invention,
a capacitative electromagnetic flow meter comprises: 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 the periphery of the measurement tube, that supplies flux
in a direction orthogonal to the tube axis direction of the measurement
tube; 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; guard electrodes arranged so as to cover
the face electrodes from the periphery thereof, maintaining a prescribed
separation with the face electrodes; an exciting circuit that supplies
exciting current of a frequency of at least the commercially available
frequency to the exciting coil; 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; a cable whereby the face electrodes and guard
electrodes are connected with the pre-amplifier; a signal processing
unit that outputs the flow rate of the fluid to be measured from
an output signal of the pre-amplifier; 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; a coil fixing plate of non-magnetic material
electrically connected and fixed to the cylindrical yoke, covering
the exciting coil; and 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 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.
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.
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
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:
FIG. 1 is a layout diagram showing prior art;
FIG. 2A, FIG. 2B, FIG. 2C, FIG. 2D and FIG. 2E are diagrams showing
the operation of the prior art;
FIG. 3 is a diagram showing the action of the prior art;
FIG. 4 is a layout diagram showing an embodiment of the present
invention;
FIG. 5 is a side view of a detection unit according to a first
embodiment of present invention;
FIG. 6 is a cross-sectional view of the detection unit according
to the first embodiment of the present invention;
FIG. 7 is a diagram of the principles of the present invention;
FIG. 8 is a diagram of principles of the present invention;
FIG. 9A and FIG. 9B are diagrams of the characteristic of an exciting
circuit according to the first embodiment of the present invention;
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;
FIG. 11 is a diagram given in explanation of fluid noise;
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;
FIG. 13A and FIG. 13B are diagrams of the shape of the face electrodes
according to the first embodiment of the present invention;
FIG. 14 is a side view of a detection unit according to a second
embodiment of the present invention;
FIG. 15 is a cross-sectional view of the detection unit according
to the second embodiment of the present invention;
FIG. 16A and FIG. 16B are layout diagrams of face electrodes and
guard electrodes according to the present invention;
FIG. 17A and FIG. 17B are diagrams showing the construction of
a return magnetic circuit according to a third embodiment of the
present invention;
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;
FIG. 19A and FIG. 19B are diagrams of the beneficial action of
the third embodiment of the present invention;
FIG. 20 is a diagram of the setting of the exciting frequency of
the third embodiment of the present invention; and
FIG. 21A, FIG. 21B and FIG. 21C are constructional views of the
face electrodes and guard electrodes.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
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.
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.
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.
A separated type of device would also be possible, in which these
are separately arranged.
In these FIG. 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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..
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.
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.
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.
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.
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.
The set-up of the various units is in accordance with the above
concepts.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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:
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.
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.
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.
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.
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.
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.
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.
Second Embodiment
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.
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.
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.
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.
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.
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.
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).
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.
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.
These guard electrodes 5A, 5B are fixed to the measurement tube
1 by adhesive glass tape.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Third Embodiment
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.
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.
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.
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.
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).
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.
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.
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).
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.
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.
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.
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.
The benefits obtained by the third embodiment when set up as follows
will now be described.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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. |