Measuring and testing – Volume or rate of flow – By measuring electrical or magnetic properties
Reexamination Certificate
2003-09-25
2004-10-12
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Volume or rate of flow
By measuring electrical or magnetic properties
Reexamination Certificate
active
06802223
ABSTRACT:
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
F
from an exciting circuit
8
to exciting coils
3
A,
3
B 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
6
A,
6
B 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
4
A,
4
B 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
5
A,
5
B and the measurement tube
1
and the respective face electrodes
4
A,
4
B referred to above, and between the face electrodes
4
A,
4
B and guard electrodes
5
A,
5
B arranged so as to cover these face electrodes
4
A,
4
B and, in addition a difference amplifier (or differential amplifier)
6
C amplifies the difference e
AB
of the respective signals from the amplifiers
6
A,
6
B, 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
4
A,
4
B 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
4
A,
4
B. This anti-noise measure involves maintaining the guard electrodes
5
A,
5
B at the same potential as the face electrodes
4
A,
4
B and removing noise induced in the same phase between the face electrodes
4
A and
4
B by performing amplification by the difference amplifier
6
C after impedance conversion using the amplifiers
6
A,
6
B.
Also, in the magnetic flux circuit between the guard electrodes
5
A,
5
B and the exciting coils
3
A,
3
B, damping foil
7
A,
7
B, 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
4
A,
4
B and the amplifier
6
A,
6
B 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
4
A,
4
B 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.
2
D and FIG.
2
E. When a square wave exciting current i
F
as shown in
FIG. 2A
flows in the exciting coils
3
A,
3
B, the rising portion of the exciting magnetic flux &PHgr; 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
E
generated in the exciting magnetic flux path, as shown in FIG.
2
B.
Due to these changes of the exciting magnetic flux &PHgr;, 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
d
in FIG.
2
D.
It is therefore necessary for the construction within the detector
10
to be set up and arranged such that the eddy current i
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
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
3
A,
3
B 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
F
as shown in
FIG. 3
still occurs even though the exciting current i
F
is controlled with high frequency.
For this reason, thin conductive sheets called damping foils
7
A,
7
B are provided between the exciting coils
3
A,
3
B and the guard electrodes
5
A,
5
B 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
Futoo Makoto
Higuchi Takashi
Iijima Takuya
Kimura Tatsuya
Nakatani Hiroshi
Lefkowitz Edward
Miller T
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