Electromagnetic flowmeter

A pair of magnetic field generating units and a pair of measuring electrodes are mounted on a measuring pipe. The pair of magnetic field generating units generate a substantially functional distribution magnetic field in the measuring pipe. Each magnetic field generating unit has a plurality of coils. A turn ratio of the plurality of coils is substantially equal to a turn ratio of pieces obtained by dividing a sin distribution coil into N portions when the number of coils is given as N (N is an integer of 2 or more). The plurality of coils are constituted by, e.g., coils with pole pieces and saddle coils.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an electromagnetic flowmeter suitable for 
measurements of an eccentric flow fluid. 
2. Description of the Related Art 
It is difficult for an electromagnetic flowmeter to accurately measure a 
flow rate of an eccentric flow fluid (drifted fluid), as is well-known to 
those skilled in the art. The eccentric flow fluid is defined as a fluid 
having a disturbed flow. An eccentric flow is formed in an electromagnetic 
flowmeter when an upstream straight pipe connected to the electromagnetic 
flowmeter does not have a sufficient length, when a solid substance or 
rust is nonuniformly attached to the inner wall surface of the upstream 
pipe of the electromagnetic flowmeter or a solid substance is precipitated 
on the bottom of the upstream pipe, or when a fluid to be measured is a 
slurry containing a solid substance susceptible to precipitation or 
floating. 
In order to accurately measure a flow rate of an eccentric flow fluid, an 
electromagnetic flowmeter having a function of generating a functional 
distribution magnetic field is developed. The functional distribution 
magnetic field is defined as a magnetic field having a magnetic flux 
density distribution close to a reciprocal number of a value of a 
weighting coefficient W which is virtually given in a measuring pipe 1, as 
shown in FIG. 1. Referring to FIG. 1, an angle 2.PHI..sub.B is a spreading 
angle defined by lines which contact an area for weighting coefficient 
W=2.0 with respect to the center of the measuring pipe. An angle 
2.PHI..sub.R is an angle defined by lines which contact an area for 
weighting coefficient W=1.2. 
Conventional electromagnetic flowmeters each having a function of 
generating a functional distribution magnetic field are shown in FIGS. 2A 
and 2B and FIGS. 3A and 3B. 
Referring to FIGS. 2A and 2B, a pair of substantially T-shaped yokes 2 are 
fixed to an outer wall surface of a measuring pipe 1 with small gaps 
therebetween. Coils 3 are respectively wound around yoke portions 2a 
extending in the radial direction of the measuring pipe. An outer casing 4 
having end flanges 4a is mounted outside the measuring pipe 1. An 
electrode 6 extends through the wall surface of the measuring pipe 1. A 
signal from the electrode 6 is extracted outside the casing 4 through a 
signal line 8. The coils 3 are powered through a line 7. 
Referring to FIGS. 3A and 3B, a pair of saddle coils 3 comprising 
excitation windings are mounted in contact with the outer wall surface of 
a measuring pipe 1. Other arrangements of the electromagnetic flowmeter 
shown in FIGS. 3A and 3B ar the same as those in FIGS. 2A and 2B. The same 
reference numerals as in FIGS. 2A and 2B denote the same parts in FIGS. 3A 
and 3B, and a detailed description thereof will be omitted. 
In order to obtain a functional distribution magnetic field in each 
conventional electromagnetic flowmeter described above, an angle .PHI. in 
FIGS. 2A and 3A is set to be a predetermined value. More specifically, the 
angle .PHI. is about 40.degree. (it varies in accordance with the shape of 
the coils and configuration of magnetic flux generators) when a length L 
of the coil is set to be a half of the inner diameter of the measuring 
pipe, i.e., L=(1/2)D. When the length L is infinite, the angle .PHI. is 
set to be about 30.degree.. When the length L is smaller than a half of 
the inner diameter of the measuring pipe, i.e., L&lt;(1/2)D, the angle .PHI. 
is generally larger than 40.degree.. 
As described above, the angle .PHI. is relatively large. Gaps between 
opposing yokes 2 and opposing coils 3 are relatively large and magnetic 
flux directing opposite directions passes through the gaps. Then, 
nonsymmetrical magnetic field exists in the gaps. For this reason, 
fluctuations of a measured signal obtained by the electrodes 6 occurs. 
A relationship between the functional distribution magnetic field and the 
measured signal will be described on the basis of extensive studies made 
by the present inventor. 
In an electromagnetic flowmeter having a relatively small magnetic field 
length in the fluid flow direction, a relationship between the pair of 
electrodes 6 and an electromagnetic force generated therebetween is given 
by equation (1) below: 
##EQU1## 
where D is the inner diameter of the measuring pipe 1, R=r/a, .theta. is 
the angle used for polar coordinate transformation, Bx is the magnetic 
flux density at a point P in the x direction, By is the magnetic flux 
density at the point P in the y direction, Wx is a weighting function 
representing a magnitude of an electromagnetic force generated between the 
electrodes at the point P in the x direction, Wy is a weighting function 
representing a magnitude of an electromotive force generated between the 
electrodes at the point P in the y direction, and Vz is the flow speed in 
the measuring pipe 1 in a direction perpendicular to the drawing surface. 
The Wx and Wy are represented by equations (2) and (3), respectively: 
EQU Wx=(R.sup.2 sin 2.theta.)/(1-2R.sup.2 cos 2.theta.+R.sup.4)(2) 
EQU Wy=(1-R.sup.2 cos 2.theta.)/(1-2R.sup.2 cos 2.theta.+R.sup.4)(3) 
Since a distance 2h in FIG. 5 is large near the electrodes of the 
conventional electromagnetic flowmeter using the functional distribution 
magnetic field, magnetic fluxes having opposite magnetization directions 
with respect to the y-y line are generated parallel to the y-y line 
obtained by connecting the electrodes 6 are generated near the electrodes 
6, as illustrated in FIGS. 5, 6 and 7. Therefore, the magnetic flux 
densities Bx and By are complicatedly distributed near the electrodes. As 
can be understood from equation (1), the magnetic flux densities Bx and By 
are combined with the weighting functions Wx and Wy to obtain Bx.Wy and 
By.Wx which influence an electromotive force E. For this reason, when an 
eccentric flow passes near the electrodes, the measured signal is 
fluctuated. 
In an electromagnetic flowmeter having no function of generating a 
functional distribution magnetic field, when a fluid to be measured 
eccentrically flows, a general flow rate measurement error occurs in 
addition to the fluctuation of the measured signal. 
SUMMARY OF THE INVENTION 
The present invention has been made in consideration of the above 
situation, and has as its object to provide an electromagnetic flowmeter 
having better performance than that of a conventional electromagnetic 
flowmeter. 
It is another object of the present invention to provide an electromagnetic 
flow meter having a small fluctuation magnitude of a measured signal. 
It is still another object of the present invention to provide a compact 
electromagnetic flowmeter having a small fluctuation of a measured signal. 
It is still another object of the present invention to provide an 
electromagnetic flowmeter having a short rise time of a magnetic flux. 
In order to achieve the above objects of the present invention, there is 
provided an electromagnetic flowmeter comprising: 
a measuring pipe through which a fluid to be measured flows; 
at least a pair of magnetic field generating units for applying a magnetic 
field to the fluid to be measured; and 
at least a pair of electrodes for detecting an electromotive force induced 
in the fluid by the magnetic field, 
wherein the pair of magnetic field generating units generate a 
substantially functional distribution magnetic field in the measuring 
pipe, and each of the pair of magnetic field generating units comprises a 
plurality of coils. 
When the number of coils is given as N (N is an integer of 2 or more), a 
turn ratio of each coil piece is explained by a product of a turn number 
of windings of a sine distribution coil and domain integration of a sine 
function. The total of the turn numbers of the pieces is equal to the turn 
number of the sin distribution coil. 
The plurality of coils comprise, e.g., coils with pole pieces and saddle 
coils. 
The electromagnetic flowmeter having the above arrangement is substantially 
free from an influence of an eccentric flow and can provide a measured 
signal having a small fluctuation magnitude. 
Additional objects and advantages of the invention will be set forth in the 
description which follows, and in part will be obvious from the 
description, or may be learned by practice of the invention. The objects 
and advantages of the invention may be realized and obtained by means of 
the instrumentalities and combinations particularly pointed out in the 
appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In order to best understand the present invention, the principle based on 
the present invention will be described below. 
When eccentric flows are present in areas having weighting functions of 0.5 
and 2.0 under the same magnetic flux density, an influence of the 
eccentric flow flowing in the area having the weighting function of 2.0 on 
measurement values is about four times that of the eccentric flow flowing 
in the area having the weighting function of 0.5. The area having the 
weighting function of 2.0 is located near the electrodes. It is very 
effective to reduce the influences of the eccentric flows by causing a 
magnetic field in the measuring pipe to come close to a functional 
distribution magnetic field (i.e., the strength of the magnetic field is a 
reciprocal number to the weighting function) to weaken a magnetic field 
near the electrodes. 
As shown in FIG. 8, when a distance 2h between the excitation coils 3 is 
reduced, the opposing magnetic fluxes are canceled to each other, and the 
magnetic flux between the coils is almost zero. As shown in FIG. 8, almost 
all the magnetic fluxes are formed in a direction perpendicular to a line 
obtained by connecting the electrodes 6. As a result, By .apprxeq.0 is 
obtained, so that equation (1) is modified into equation (1a). 
##EQU2## 
When a functional magnetic field is to be obtained by an excitation coil 
and the like, relation Bx=(1/Wy) is equivalently established. Equation 
(1a) can be simplified to obtain equation (1b) below: 
##EQU3## 
It is ideal to obtain the electromotive force E on the basis of equation 
(1b). Even if, however, equation (1b) is not perfectly satisfied, the 
influences of eccentric flows on the measurement values can be reduced 
when the value Wy.Bx adjacent to the electrodes is smaller than the Wy 
and/or the value Wy.Bx far from the electrodes is greater than the Wy. 
An excitation portion of an electromagnetic flowmeter according to the 
present invention aims at obtaining a structure for generating a magnetic 
flux given by equation (1a). 
An electromagnetic flowmeter according to an embodiment of the present 
invention will be described with reference to the accompanying drawings. 
Ideally, as indicated by the alternate long and two short dashed line DD in 
FIG. 9A, when excitation coils (sine distribution coils) whose thickness 
is distributed in a sin curve are formed so that end portions of the coils 
are located near the electrodes 12, a functional distribution magnetic 
field can be obtained. At the same time, the distance 2h between the coils 
can be almost zero, thereby satisfying equation (1a). 
According to this technique, however, the maximum thickness of each 
excitation coil is increased. For this reason, the size of the outer 
casing is increased, as indicated by the alternate long and two short 
dashed line EE. Therefore this excitation coil cannot be used in an 
electromagnetic flowmeter, so called wafer type, which is inserted between 
the pipes which are then bolted shown in FIG. 10. 
In order to solve this problem, the first embodiment having a structure 
shown in FIGS. 9A and 9B is provided. 
The structure of the electromagnetic flowmeter shown in FIGS. 9A and 9B 
will be described below. 
A pair of opposite electrodes 12 are mounted on the outer wall surface of a 
measuring pipe 11 through which a fluid to be measured flows. The 
electrodes 12 are perpendicular to a fluid flow direction and a direction 
of a magnetic flux and are located on a line y-y passing across the axis 
of the measuring pipe 11. The electrodes 12 are in direct contact with the 
fluid. The electrodes 12 are insulated from the measuring pipe 11 when the 
measuring pipe 11 is mode of a conductive material with, for example, a 
lining made of an insulative material. 
A pair of excitation portions comprise a pair of saddle excitation coils 13 
and a pair of coils 14 with pole pieces. The coils 13 and 14 cooperate to 
generate a magnetic field substantially equivalent to that generated by 
the sine distribution coil indicated by the alternate long and two short 
dashed line DD. 
Each coil 14 with a pole piece comprises a yoke 14 made of a magnetic 
material, a T-shaped pole piece 14a made of a magnetic material, and a 
coil 14b wound around the pole piece 14a. The coils 14 are located to 
oppose each other on the outer wall surface of the measuring pipe 11 in a 
direction perpendicular to the line y-y obtained by connecting the 
electrodes 12. At the same time, the pole piece 14a is located on a line 
x-x passing across the axis of the measuring pipe. 
The saddle coils 13 are located to surround the coils 14 and oppose each 
other adjacent to the outer wall surface of the measuring pipe 11. The end 
portions of the saddle excitation coils 13 are located near the electrodes 
12. 
An outer casing 15 protects the measuring pipe 11, the electrodes 12, and 
the coils 13 and 14. The outer casing 15 is made of a magnetic material or 
the like. The inner surface of the outer casing 15 is in tight contact 
with the pole pieces 14a, and the outer casing 15 serves as a magnetic 
path for the coils 14. 
A total turn count of the coils 13 and 14 is determined on the basis of a 
magnitude of a target electromotive force E and a magnitude of an 
excitation current. If a total turn count of the coils is given as 2T, 
total turn counts of the upper coils 13 and 14, and the lower coils 13 and 
14 are respectively T. 
The turn count of each coil 13 or 14 is equal to that of a half of the sine 
distribution coil having a turn count T. More specifically, the total turn 
count of each coil 13 or 14 is determined by equations (4) to (6) for 
dividing the total turn count T into n (n is an integer) portions: 
##EQU4## 
More specifically, the diameter of the measuring pipe 11 is given as 75 mm, 
the total turn count 2T=1,440 (turns), .PHI.m1=0.degree., 
.PHI.n1=40.degree., and .PHI.n2=90.degree.. These angles are obtained by a 
computer simulation and experiments. By using equations (4) and (5), 
##EQU5## 
That is, the coil 13 has 168 turns, and the coil 14 has 552 turns. 
In the structure shown in FIGS. 9A and 9B, two coils 13 and 14 cooperate to 
have a function substantially equivalent to the sine distribution coil. At 
the same time, the distance 2h between the ends of the coils 13 is small. 
Therefore, the magnetic flux near the electrodes 12 in the y-y direction 
is reduced, and the fluctuation of the measured signal can be reduced. 
In an electromagnetic flowmeter which requires a small excitation current 
and uses coils each having a large turn count, the saddle coil 13 is 
preferably made thin, as shown in FIG. 11, and is laid out in the entire 
40.degree. range. 
It is difficult to manufacture the coils 13 in the arrangement shown in 
FIG. 11. In order to solve this problem, a coil 13 may be obtained as a 
mass and may be located near the electrode 12 so as to reduce the distance 
2h as shown in FIG. 12. In this case, the turn count T.sub.1 of the coil 
13 is preferably reduced slightly, e.g., by about 10% from the calculated 
turn count. The structure in FIG. 12 is excellent in working efficiency 
and the like. 
In an electromagnetic flowmeter which requires a large excitation current 
and uses coils each having a small turn count, a plate-like coil having 
one or a plurality of turns can be used as the coil 13. FIG. 13A shows a 
plate-like coil having one turn, and FIG. 14A shows a plate-like coil 
having two turns. FIG. 13B shows a plane structure of the plate-like coil 
13. FIGS. 13C and 14B show equivalent circuits of magnetic generating 
portions shown in FIGS. 13A and 14A, respectively. 
When the structures shown in FIGS. 13A to 14B are employed, a magnetic flux 
along the line y-y can be eliminated or extremely reduced, and the 
fluctuation of the measured signal can be minimized. 
The plate-like coil 13 may be formed using a superconductor material. When 
a winding portion 14b of the coil with a pole piece has a small turn 
count, it may be made of a thick plate-like coil or a superconductor coil. 
The second embodiment of the present invention will be described with 
reference to FIG. 15. 
The second embodiment exemplifies an electromagnetic flowmeter having 
plural pairs of electrodes. Referring to FIG. 15, plural pairs of 
electrodes 12a and 12b are mounted on a measuring pipe 11. Saddle 
excitation coils 13 and coils 14 with pole pieces which have a function 
substantially equivalent to the sin distribution excitation coil indicated 
by the alternate long and two short dashed line in FIG. 15 are mounted on 
the outer wall surface of the measuring pipe 11. The saddle excitation 
coils 13 are located near the electrodes 12a and 12b. The turns of the 
coils 13 and 14 are calculated on the basis of equations (4) and (5). 
In this embodiment, the combination of the coils 13 and 14 also provides a 
function substantially equivalent to the sine distribution coil. In 
addition, the saddle coils 13 are located near the electrodes. Therefore, 
the electromagnetic flowmeter is substantially free from an influence of 
an eccentric flow by the sine distribution coil and provides a measured 
signal having a small fluctuation magnitude because the edges are adjacent 
to the electrodes. 
The third embodiment of the present invention will be described with 
reference to FIGS. 16A and 16B. This embodiment exemplifies an 
electromagnetic flowmeter in which a sin distribution coil is divided into 
four parts. Each excitation portion comprises three saddle coils 13A, 13B, 
and 13C and a coil 14 with a pole piece. The turn counts of the excitation 
coils 13a, 13b, 13c, and 14 are determined on the basis of equations (4) 
to (6). 
It is possible to slightly shift the coil positions from positions obtained 
by the calculations. In this case, for example, the turn counts and/or 
exitation current are corrected in accordance with experiments and the 
like. 
The present invention is not limited to the particular embodiments 
described above. In the above embodiments, the outer casing 15 comprises a 
cylinder. However a rectangular parallelepiped casing may be used. 
In the above embodiments, each excitation portion comprises a combination 
of a coil with a pole piece and a saddle coil (or saddle coils). However, 
the excitation portion may be constituted by only saddle coils, and the 
structure of the coil of the excitation portion may be arbitrarily 
determined. 
In each embodiment described above, the outer casing 15 is made of a 
magnetic material, and its inner surface is in tight contact with the 
outer end faces of the pole piece 14a, and the outer casing 15 serves as a 
feedback magnetic path for the coil 14. However, the present invention is 
not limited to this. The outer casing 15 may be made of a nonmagnetic 
material, and a space between the outer casing 15 and the measuring pipe 
11 may serve as a feedback magnetic path. In addition, as shown in FIG. 
17, a core 15A may be formed inside the outer casing 15. 
The outer casing 15 is preferably brought into tight contact with the rear 
end face of the pole piece 14a. However, the present invention is not 
limited to this. As shown in FIG. 17, a gap may be formed between the 
outer casing 15 and the pole piece 14a due to a structural reason or the 
like. In this case, an excitation current must be increased by about 20% 
to obtain a predetermined magnetic flux density, as compared with the case 
wherein the outer casing is in tight contact with the pole piece. However, 
high mounting precision of the outer casing is not required, the structure 
of the electromagnetic flowmeter can be simplified, and assembly 
efficiency can be improved. 
When the outer casing 15 is made of a nonmagnetic material, the outer 
casing itself has a function of an air core. For this reason, a larger 
excitation current must flow in each coil than that flowing in the 
magnetic outer casing. When this arrangement is employed, however, the 
rise time of the magnetic flux can be shortened, so that an 
electromagnetic flowmeter for high-speed measurement an be obtained. When 
the rise time of the magnetic flux can be shortened, a flow rate can be 
measured independently of electrochemical noise generated by the 
electrodes 12. 
When a laminated core is arranged inside the outer casing 15, the laminated 
core may be brought into tight contact with the inner surface of the outer 
casing or spaced apart therefrom by a predetermined distance. In addition, 
a gap may be formed between the inner surface of the laminated core and 
the rear end face of the pole piece 14a, or these members may be brought 
into tight contact with each other. A magnetic powder may be formed into a 
diced core with a resin in place of the laminated core. When the laminated 
core or the dust core itself serves as an outer casing, the outer casing 
15 may be omitted. 
The pole piece 14a may be made of a laminated core or a dust core. 
The present invention is not limited to an electromagnetic flowmeter of the 
type having electrodes which contact a fluid to be measured the flow rate 
of the fluid. It can be applied to an electromagnetic flowmeter of the 
type shown in FIG. 18, whose electrodes exist in the measuring pipe and 
are connected to the fluid by statistic electrostatic capacitance in terms 
of an alternating current. 
If the yoke portion 14c of the coil 14, is made larger, the magnetic flux 
density around the axis of the measuring pipe 11 will be reduced, possibly 
making it difficult to measure the flow rate accurately. In the 
embodiment, since the yoke portion 14c is relatively small, magnetic 
fluxes extend around the axis of the measuring pipe 11. Further, a large 
saddle-shaped coil is used in combination with the coil 14, thus improving 
the characteristics of the flowmeter which would be insufficient if only 
the coil 14 were used. 
In the electromagnetic flowmeter of each embodiment described above, the 
excitation portions for generating a functional distribution magnetic 
field are made of a plurality of coils. More specifically, the function 
substantially equivalent to the sine distribution coil can be obtained by 
the plurality of coils, thereby reducing an influence of the eccentric 
flow on the measured signal. The coil ends are located near the electrodes 
12 to reduce the fluctuations of the measured signals. In addition, in the 
above embodiment, the excitation portion is made of the coil 14 with a 
pole piece and the saddle coil 13. As compared with an arrangement using 
the sin distribution coil, the maximum thickness of the coil can be 
reduced to provide a compact electromagnetic flowmeter and facilitate the 
manufacture of coils. 
Due to the combination of two coils 13 and 14, the leakage of magnetic 
fluxes can be reduced. More specifically, in only the coil 14 were used, 
leakage magnetic fluxes would be generated as is shown in FIG. 19A. On the 
other hand, if only the coil 13 were used, leakage magnetic fluxes would 
be generated as is illustrated in FIG. 19B. Since the coils 13 and 14 are 
used in combination, the leakage magnetic fluxes cancel out those area 
shown by dotted lines in FIG. 19C, whereby the total amount of leakage 
magnetic fluxes is reduced. As a result, the power loss is reduced, and 
the magnetic flux generator can be driven with a small current. When a 
small current is supplied to the magnetic flux generator, the magnetism 
increases quickly, thereby frequency of magnetic excitation can be 
increased. The higher the magnetic excitation frequency, the more easily 
can signals be separated from so-called l/f noise. Hence the S/N ratio can 
be increased, thus enhancing the characteristic of the electromagnetic 
flowmeter. 
Additional advantages and modifications will readily occur to those skilled 
in the art. Therefore, the invention in its broader aspects is not limited 
to the specific details, and representative devices, shown and described. 
Accordingly, various modifications may be made without departing from the 
spirit or scope of the general inventive concept as defined by the 
appended claims and their equivalents.