The invention relates to an electromagnetic flow meter of the type
having a cylindrically shaped tube section with two magnetic poles
each having a core section carrying a winding attached to the outside
and on opposite sides of the tube section. Smaller dimensions and
lower losses in the magnetic circuit are achieved by providing a
casing or yoke unit formed of sheet metal members and having U-shaped
core sections. The core sections have central webs adapted to the
curvature of the pole shoes and rectangular coils are disposed respectively
between the associated pole shoes and sheet metal yoke members.
What is claimed is:
1. An electromagnetic flow meter, comprising, a cylindrically shaped
tube section, two magnetic poles each having a core section carrying
a winding attached to the outside of and on diametrically opposite
sides of said tube section, each of said poles having a pole shoe
curved to fit the curvature of said tube section, two outer yoke
members formed of sheet metal and having casing portions thereof
in surrounding relation to said tube section, each said yoke member
being formed with a U-shaped section to form said core section having
legs which join the corresponding one of said casing portions and
which includes a central web section in abutting engagement with
the corresponding one of said pole shoes, and each said winding
having substantially rectangular coils disposed between the corresponding
one of said pole shoes and the corresponding ones of said outer
yoke member casing portions.
2. A flow meter according to claim 1 characterized in that each
said pole shoe extends over an angle of about 90.degree..
3. A flow meter according to claim 1 characterized in that said
sheet metal has a thickness of about 1 mm.
4. A flow meter according to claim 1 characterized in that said
core section legs have a height of 6 to 12 mm.
5. A flow meter according to claim 1 characterized in that said
outer yoke members are two substantially like sheet metal members
which have overlapping ends which are connected.
6. A flow meter according to claim 5 characterized in that said
overlapping ends of said sheet metal members are planar and parallel.
7. A flow meter according to claim 5 characterized in that said
sheet metal members have lugs which circumferentially overlap said
8. A flow meter according to claim 1 characterized in that said
central web section has a substantially square surface.
The invention relates to an electromagnetic flow meter in which
the magnetic system consists of two magnetic poles which can be
placed on a measuring tube from the outside and each have a core
section carrying a winding and, secured thereto, a pole shoe adapted
to the curvature of the measuring tube, and an outer yoke of such
a shape that the axial screws for clamping the measuring tube extend
outside the yoke.
In a known electromagnetic flow meter of this kind (DE-OS 32 25
22 6 FIG. 28a), the measuring tube which is provided with an internal
insulating layer consists of non-magnetic metal and has a thin wall
thickness. By using the pole shoes, one obtains a favourable field
distribution in the measuring tube. The predominant part of the
housing is disposed within the axial screw circle which is standardised
according to the diameter of the tubing. To accommodate the windings,
the housing is enlarged radially outwardly at opposite sides and
projects outwardly between each two axial screws. This leads to
comparatively large dimensions. In addition, there are high losses
in the magnetic circuit. The invention is based on the problem of
providing an electromagnetic flow meter of the aforementioned kind
with smaller dimensions and lower losses in the magnetic circuit.
This problem is solved according to the invention in that the yoke
consists of at least one sheet metal member, that the core sections
are U-shaped sheet metal members of which the central web is adapted
to the curvature of the pole shoe which is likewise of sheet metal
and the outer ends of the limbs are in one piece with the adjoining
yoke section, and that the windings are substantially rectangular
curved flat coils of which the sides are circumferentially offset
and are disposed between the associated pole shoe and a yoke section
extending within the axial screw circle.
By using sheet metal members for the yoke and the core sections,
the amount of iron is reduced to a bare minimum. Consequently, the
eddy current losses are extraordinarily low. Further, the path in
the iron is comparatively short because its radial extent in the
zone of the windings is restricted to the low height of the core
sections. Since the circumferential extent of the core section is
not limited by the axial screws, there is a comparatively large
contact face between the central web of the U-shaped bent sheet
metal member and the associated pole shoe. All this permits the
number of windings to be considerably reduced. In the form of flat
coils, the windings can therefore be readily arranged so that the
entire housing of the flow meter extends within the axial screw
circle. The thin wall thickness of the sheet metal member contributes
to this. One can even use ceramic measuring tubes of such a wall
thickness that they will withstand the forces occurring during operation,
particularly the axial clamping forces. In addition, the amount
of self-induction is lower, which leads to shorter decay periods
in the voltage peaks of the measuring signal occurring in the windings
after the usual switching over of the direction of the direct current,
thereby increasing the part of the measuring period during which
pseudo-stationary conditions exist. Further, the device is cheap
because it is made from sheet metal. The construction of the device
is suitable for a large variety of different measuring tube diameters,
it being possible to use flat coils having the same electrical parameters
and therefore the same evaluating circuit.
Preferably, the core section extends through an angle of about
40 to 50 degrees. This leads to the desired large contact surface
between the pole shoe and the core section so that the remaining
air gap will now have a strong influence on the magnetic circuit.
The pole shoe should itself extend through an angle of about 90
degrees. This circumferential extent substantially corresponds with
that of the winding so that the latter is securely held between
the pole shoe and the yoke. In addition, a pole shoe of these dimensions
results in a field within the measuring tube that is favourable
for the measurement.
The sheet metal is preferably about 1 mm thick. Having regard to
the width of the sheet metal, this thickness is sufficient for the
magnetic flux but the eddy current losses are extremely low.
Further, the limbs of the U-shaped sheet metal member should have
a height of only 6 to 12 mm. The radial extent by which the yoke
projects beyond the measuring tube is correspondingly low and the
magnetic path is correspondingly short.
Preferably, the yoke consists of two substantially identical sheet
metal members which carry the core section centrally and are interconnected
at least at one of their overlapping ends. At the point of overlap,
there is a large surface so that the unavoidable air gap will not
strongly influence the magnetic circuit.
In particular, the ends of the two sheet metal members are planar
In another embodiment, the sheet metal member is wider than the
U-shaped sheet metal member. The marginal zones will then serve
for magnetic screening.
In particular, the sheet metal member may carry lugs at both sides
of the U-shaped bent sheet metal member, the lugs externally overlapping
the sides of the flat coil that are axially offset. The screening
therefore extends substantially over the entire range of the winding.
These lugs can be very readily manufactured during stamping and
bending of the sheet metal member.
The central web of the U-shaped bent sheet metal member preferably
has a substantially square surface. This leads to the largest possible
contact area with the pole shoe for the shortest possible circumferential
It is recommended that the flat coils have about one thousand windings
and an ohmic resistance of about 50 ohm. One can standardise the
coils to these values for different sizes of measuring tube so that
a unitary evaluating circuit would be suitable for different conduit
In another embodiment, the sheet metal of the pole shoe may carry
screening of electrically conductive and magnetically non-conductive
material which overlaps the inside of the flat coil. The winding
is therefore completely surrounded by earthed components. In particular,
the screening may be a copper sheet.
Fastening of the sheet metal members to each other is preferably
by way of rivets. This gives a very cheap assembly.
A preferred example of the invention will now be described in more
detail with reference to the drawing, in which:
FIG. 1 is a cross-section through a flow meter according to the
FIG. 2 is an end elevation of one of the two sheet metal members,
FIG. 3 is a plan view of the upper sheet metal member and
FIG. 4 shows the winding current I against the measuring voltage
The electromagnetic flow meter shown in FIG. 1 serves to determine
the quantity of flow in the internal cross-section 1 of a measuring
tube 2 which is here of ceramic material. For this reason, a magnetic
field is produced in the measuring tube 2 with the aid of two pole
shoes 3 and 4. At two diametrically opposite measuring electrodes
5 and 6 disposed in the symmetrical plane, one can then derive a
measuring voltage U.sub.m which is proportional to the amount of
flow. The ends of the measuring tube 2 lie between the flanges of
two tubes. These flanges are clamped together with the aid of axial
screws 7 disposed on an axial screw circle 8. The diameter of this
circle and the number of screws arranged thereon are standardised
depending on the nominal diameter.
The housing 10 of the flow meter is disposed entirely within the
space left by the screws 7. It consists of two sheet metal members
11 and 12 which are interconnected along their parallel ends 13
to 16 by means of rivets 17 18 to result in an overlap of large
area. At the middle of each sheet metal member there is a sheet
metal core section 19 having a central web extending over about
40.degree. to 50.degree. and two limbs 21 and 22 with a height of
6 to 12 mm. The latter are made in one piece with the adjoining
bent yoke sections 23 or 24 of the corresponding sheet metal member
11. The central web 20 has the same curvature as the associated
pole shoe 3 which, in turn, extends through about 90.degree. and
is adapted to the outer circumference of the measuring tube 2. The
interconnection between the pole shoe 3 and central web 20 is by
way of rivets 25. Contact is over a large substantially square area.
Between the respective yoke sections 23 and 24 and the projecting
parts of the pole shoe 3 there is a winding in the form of a flat
coil 26 or 27. These flat coils each have about 1000 windings and
an ohmic resistance of about 50 ohm. Their axially offset sides
are overlapped by lugs 28 to 31. The sheet metal member 11 is therefore
axially wider than the U-shaped bent sheet metal member 19. The
sheet metal member 11 is first stamped and subsequently bent to
the desired shape.
Screening 32 in the form of a copper sheet is held to the outside
of each pole shoe 3. The fastening can be by way of spot welding
as shown. A corresponding copper sheet 34 is associated with the
other coil 27. The flats coils are therefore entirely surrounded
by earthed components. The connections of the electrodes 5 and 6
can be led outwardly through lateral apertures 35 and 36 in the
housing 10. A further stamped opening produces an earthing terminal
In operation, the windings are fed with a current I which, as is
shown in FIG. 4 has its direction reversed at predetermined time
intervals of, for example, 120 ms. After each reversal, the measuring
voltage U.sub.m starts with a peak which generally decays so that
the actual measurement can take place only a certain time after
switching over, that is to say, within the period a. The preceeding
decay period b is comparatively short for the flow meter here in
question so that, with a given switching period, a comparatively
long measuring period a is available. This is achieved because the
self-induction of the flat coils 26 and 27 is low by reason of the
small number of windings. The small number of windings is possible
because the magnetic return circuit consists of the bare minimum
of iron and therefore only small eddy current losses arise because
the radially extending parts of this return circuit are short and
because all surfaces where an air gap would occur are formed by
large areas of overlap. Since the housing 10 lies completely within
the axial screw circuit 8 no regard need be had to the gap between
adjacent axial screws. One obtains a small and handy device which
can be economically produced.