U.S. patent number 3,586,964 [Application Number 04/820,409] was granted by the patent office on 1971-06-22 for inductive transducers.
This patent grant is currently assigned to Associated Engineering Limited. Invention is credited to Heinrich Strauch.
United States Patent |
3,586,964 |
Strauch |
June 22, 1971 |
INDUCTIVE TRANSDUCERS
Abstract
The present invention relates to inductive transducers and
discloses constructions of such transducers having two
concentrically arranged coil and core assemblies wherein the cores
are formed of ferromagnetic material and have their ends
terminating at the working face of the transducer. In operation,
the coils of the transducer may be connected in the opposed arms of
a bridge circuit, thereby producing a substantially linear output
from the bridge circuit.
Inventors: |
Strauch; Heinrich (Leamington
Spa, EN) |
Assignee: |
Associated Engineering Limited
(Leamington Spa, EN)
|
Family
ID: |
10170252 |
Appl.
No.: |
04/820,409 |
Filed: |
April 30, 1969 |
Foreign Application Priority Data
|
|
|
|
|
May 8, 1968 [GB] |
|
|
21873/68 |
|
Current U.S.
Class: |
324/234;
324/207.12; 336/83; 336/96; 324/207.19; 336/84R; 336/234 |
Current CPC
Class: |
H01F
37/00 (20130101) |
Current International
Class: |
H01F
37/00 (20060101); G01r 033/00 (); H01f 015/02 ();
H01f 027/24 () |
Field of
Search: |
;336/84,83,96,233,234,96,212 ;323/34 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Kozma; Thomas J.
Claims
I claim:
1. An inductive transducer comprising a first ferromagnetic core, a
first coil extending around said core, part of said first core
being turned back round the outside of said first coil, both ends
of said first core terminating in a plane forming the working face
of the transducer, a second ferromagnetic core disposed around the
periphery of said first coil, a second coil extending around said
second core, part of said second core being turned back round the
outside of said second coil, both ends of said second core
terminating at said working face, the spacing between the two ends
of one of said cores being substantially greater than the spacing
between the two ends of the wires of the other said core.
2. A transducer as claimed in claim 1, wherein said first and
second cores each include a plurality of ferromagnetic wires having
part of their length turned back round the outside of the first and
second coils respectively.
3. A transducer as claimed in claim 1, including a screen of
ferromagnetic material arranged between the two coil and core
assemblies.
4. A transducer as claimed in claim 1, wherein the first coil is
wound between two portions of insulating material surrounding the
first core, and the second coil is wound between two portions of
insulating material surrounding the second core.
5. A transducer as claimed in claim 2, wherein the first and second
cores are formed from iron wires and the first and second coils are
wound from copper wires.
6. A transducer as claimed in claim 1, wherein the second coil has
approximately the same axial extent as the first coil and also has
approximately the same number of turns as the first coil and
wherein, in order to have the same total resistance as the first
coil, said second coil is wound of a wire having a lower specific
resistance than the wire forming the first coil.
7. A transducer as claimed in claim 1, having a substantially
circular cross section to the axial direction of said cores and
wherein the radial spacing between the two ends of the first core
is substantially greater than the radial spacing between the two
ends of the second core.
8. A transducer as claimed in claim 7, wherein the first and second
cores each include a plurality of ferromagnetic wires, and the
folded back portion of the wires of said first and second cores are
substantially equally spaced in a circumferential direction around
the transducer.
9. A transducer as claimed in claim 1, wherein the coil and core
assemblies are encapsulated in an insulating material, such an an
epoxy resin.
10. A circuit arrangement including an inductive transducer as
claimed in claim 1, wherein said first and second coils are
connected to form the opposed arms of a bridge circuit, so that the
effect of a change of inductance occurring in one of said coils is
at least partially compensated by the effect of a change of
inductance in the same sense occurring in the other of said coils,
thereby producing a substantially linear output from the
bridge.
11. An arrangement as claimed in claim 10, wherein said first and
second coils are connected respectively in first and second arms of
an AC bridge circuit, means for deriving an output from across one
end of said first and second arms, means connecting the other end
of said first and second arms together and to earth potential, and
an AC input connected across the common point of the third and
fourth arms of the bridge circuit and earth.
12. An arrangement as claimed in claim 11, in which the AC input is
a high frequency carrier wave and the output of the bridge is fed
through a synchronous detector and a filter to a recorder or an
oscilloscope.
Description
This invention relates to inductive transducers.
According to this invention an inductive transducer comprises a
first ferromagnetic core, a first coil extending around said first
core, part of said first core being turned back round the outside
of said first coil, both ends of said first core terminating in a
plane forming the working face of the transducer; a second
ferromagnetic core disposed around the periphery of said first
coil, a second coil extending around said second core, part of said
second core being turned back round the outside of said second
coil, both ends of said second core terminating at said working
face; the spacing between the two ends of one of said cores being
substantially greater than the spacing between the two ends of the
other said core.
The first and second cores may each include a plurality of
ferromagnetic wires having part of their length turned back round
the outside of the first and second coils respectively.
A screen of ferromagnetic material is preferably arranged between
the two coil and core assemblies.
Preferably said first and second coils are connected in opposed
arms of a bridge circuit, so that the effect of a change of
inductance occuring in one said coil is at least partially
compensated by the effect of a change of inductance in the same
sense occurring in the other said coil, thereby producing a
substantially linear output from the bridge.
In the preferred arrangement, said first and second coils are
connected in first and second arms respectively of an AC bridge,
the output of the bridge being connected across one end of said
first and said second arms, the other end of said first and second
arms being connected together and to earth, and the AC input being
connected across the common point of the third and fourth arms and
earth.
One embodiment of the invention will now be described by way of
example with reference to the accompanying drawings, in which:
FIG. 1 is a diagrammatic longitudinal section of an inductive
transducer in accordance with the invention,
FIG. 2 is an elevation of the transducer, in the direction of arrow
II IN FIG. 1,
FIG. 3 is a circuit diagram showing the measuring circuit of the
transducer,
FIG. 4 is a diagram illustrating an aspect of the operation of the
transducer, the view being similar to that of FIG. 1 and
FIG. 5 is a graph showing inductance plotted against armature
displacement.
Referring to FIGS. 1 and 2, the transducer includes a core 1
consisting of 16 iron wires, which is surrounded by an inner coil 2
wound between two insulating rings 3, 4. The coil 2 is of copper
wire and has its ends led out to terminals a, b of the transducer.
The wires of the core 1 are folded back, at the end furthest from
the working face 13 of the transducer, around the outside of the
coil 2, and are substantially equally spaced around the periphery
(as seen at 1A). The assembly is then encapsulated in an epoxy
resin 5, which in turn is surrounded by a screen 6 of ferromagnetic
material which is connected to terminal e. This terminal is
connected to earth potential.
The screen 6 is surrounded by an outer core 7 consisting of 16 iron
wires equally spaced round the circumference, and the outer core is
in turn surrounded by an outer coil 8. The latter has approximately
the same axial extent as the inner coil 2, and in order to have the
same number of turns and the same total resistance as the inner
coil, as is preferred, may be wound of a copper wire having a lower
specific resistance. The outer coil 8 is wound between two
insulating rings 9, 10, of which the ring 10 nearer the working
face of the transducer is of substantially triangular or trapezium
section, and the ends of the coil are led out to terminals c, d.
The wires of the outer core 7 are folded back, at the end furthest
from the working face, around the outside of the outer coil 8 (as
seen at 7A), and are surrounded adjacent the working face by an
insulating collar 11. The assembly is then encapsulated in epoxy
resin 12, which fills the interstices between parts 7--11.
The radial spacing between the two ends of the wires of the inner
core 1 is preferably 15 to 20 times the radial spacing between the
ends of the wires of the outer core 7.
As seen in FIG. 3, the inner and outer coils 2, 8, are connected to
form two arms of a bridge circuit; and since one end (b, c) of each
coil is earthed in this arrangement, these ends may be connected to
the earthing connection e to the screen 6, so that the transducer
need only have three terminals a, d, e, one of which is the earth
terminal. The other two arms of the bridge circuit each comprise an
inductance and a resistor, and the components of one of these arms
may be adjustable for setting-UP purposes.
In operation, a carrier wave of a frequency of, for example, 100
kHz. is supplied across the bridge from a HF oscillator 20, and the
output of the bridge is taken, through a transformer 21, by way of
an AC amplifier 22 to a synchronous detector 23 and a filter 24,
from which the output, after further amplification in a DC
amplifier 25, is fed to a recorder of an oscilloscope 26.
Referring to FIG. 4, it can be seen that the magnetic field lines
31 from the outer iron core 7 do not extend so far across the air
gap as the magnetic field lines 30 from the inner iron core 1. This
is due to the difference in the radial spacing between the ends of
the wires of the iron core 7 and those of the iron core 1.
Therefore any material 32, forming an armature, coming towards the
active face of the transducer will first penetrate the magnetic
field lines 30 and, at a smaller distance from the active face, it
will penetrate the field lines 31.
FIG. 5 shows the corresponding inductance L of coil 2 and coil 8,
plotted against the distance D, of the armature 32 from the active
face of the transducer. Line 33 represents the inductance of coil 2
and line 34 that of coil 8.
If the transducers are connected as in the bridge circuit of FIG. 3
then the inductance of coil 8 (line 34) will be subtracted from the
inductance of coil 2 (line 33), resulting in line 35; this
represents a linear, or substantially linear, output from the
bridge. Moreover, the gap D may be decreased to zero, i.e. with the
armature 32 in contact with the ends of the wires of the cores 1,
7, and, as will be seen from FIG. 5, this gives the maximum output
and therefore enables the transducer to have the maximum
sensitivity.
The transducer in accordance with the invention may be applied to
the measurement of gap or displacement, for example, where the
transducer casing is secured in a housing and the armature is a
part movable relative to the housing; or to the measurement of
vibration (i.e. fluctuating displacement). The transducer may be
applied to the measurement of pressure by provision of an armature
in the form of a diaphragm which deflects with pressure and
cooperates with the working face; and also to the measurement of
temperature, where the armature is in a fixed position relative to
the working face but has a magnetic permeability which varies with
temperature, and thus varies the inductance of the transducer.
It will be appreciated that, in the case for example of a
displacement transducer, the inner and outer elements will tend to
be subjected to the same temperature, and thus any changes in
inductance or resistance are compensated by arranging the
transducers in two arms of a bridge circuit as shown in FIG. 3.
Moreover, any changes in the magnetic permeability of the armature
as a result of changing temperature will be compensated, since the
change in one arm of the bridge will be offset by that in the
other.
The working face 13 need not necessarily be a flat plane; for
example, where the transducer is mounted in a bearing and the
armature is formed by a rotating shaft carried in the bearing, the
working face may lie in a curved plan corresponding to the
curvature of the bearing surface.
Furthermore, instead of making the cores from ferromagnetic wires,
solid cores of a ferromagnetic material may be used.
* * * * *