AU658059B2

AU658059B2 - Improved electromagnetic position transducer

Info

Publication number: AU658059B2
Application number: AU37143/93A
Authority: AU
Inventors: John Edward Hayter
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-04-22
Filing date: 1993-04-22
Publication date: 1995-03-30
Anticipated expiration: 2010-12-20
Also published as: AU3714393A

Description

1 658059

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT Name of Applicant: Actual Inventor: Address for Service: JOHN EDWARD HAYTER JOHN EDWARD HAYTER CULLEN CO., Patent Trade Mark Attorneys, 240 Queen Street, Brisbane, Qld. 4000, Australia.

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Invention Title: IMPROVED ELECTROMAGNETIC POSITION

TRANSDUCER

The following statement is a full description of this invention, including the best method of performing it known to me: .000 0 THIS INVENTION relates to an electromagnetic position or displacement transducer.

The transducer of this invention is an improvement, or modification of, the transducer described in my Australian patent no. 625407, and the disclosure of patent no. 625407 is incorporated herein by reference.

A position or displacement transducer should enable small changes in the position of an object, such as a float, to be detected with high sensitivity, and converted with linearity into an electrical output, which may then be easily processed or displayed.

The position or displacement transducer of this invention comprises a first wire coil of elongate form juxtaposed with at least a portion of an elongate permeable member, the first wire coil being energised, in use, to create a magnetic field therearound, a second wire coil wound around at least part of the elongate member and connected to measuring means, and a conductive loop member located around the juxtaposed coil and elongate member and slidable axially therealong, the coupling of the magnetic field to the second wire coil via the elongate permeable member being dependent on the axial position of the loop member, whereby in use, the axial position of the loop member relative to the 25 elongate member is indicated by the measuring means.

The first wire coil is suitably air-cored. For structural stability, the air-cored coil may be wound on a rigid tube of suitable non-magnetically permeable material, such as paper or PVC plastics material.

Electromagnetic position transducers are notoriously non-linear, and many transducers have to be individually calibrated. In the transducer of this invention, the density of the coil turns and/or the shape of the coil can be varied at the design stage to compensate for any non-iinearities.

The conductive loop member is typically a ring Sof aluminium or other suitable conductive material. If f: the transducer is used to measure liquid level, the loop 1 rN, (2 ,r

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member is typically housed in a buoyant casing so that the loop member floats on the liquid surface and rises and falls with the level of liquid.

The loop member is preferably a continuous loop, formed of one or more turns of a conductive 0 o material (or shorted turn).

The permeable member is typically a ferromagnetic rod, metal strip or lamination, wire or the like.

Tests have shown that when the coil is electromagnetically coupled to an elongate permeable member juxtaposed with it, maximum coupling occurs approximately midway along the length of the elongate member. In other words, the degree of coupling rises from a minimum at one end to a maximum around the middle of the elongate member, then drops to a minimum at the other end. To obtain improved linearity, the transducer of the preferred embodiment of this invention utilises only half of the elongate member. That is, the coil is juxtaposed with a half length of the elongate member.

The other half of the elongate member may be coiled or folded to reduce space.

The transducer of this invention is able to be constructed in small or miniaturised form, and provides 20 linear measurement of the position, and hence displacement, of the loop member. The transducer is particularly, but not solely, suitable for use in measuring small displacements. A preferred use of the transducer is as a liquid level gauge, such as a fuel tank gauge.

In order that the invention may be more fully understood and put into practice, a preferred embodiment thereof will now be described with reference to the accompanying drawings in which: Fig. 1 is a perspective view of a fuel gauge utilising the transducer of this invention; and Fig. 2 is a perspective view of the electromagnetic components of the fuel gauge of Fig. 1.

As shown in Fig. i, the fuel gauge 10 of the preferred embodiment comprises a sealed casing having a stem !I and a cap portion 12. The casing is typically constructed of thermo moulded plastics material such as PVC plastics. Two pairs of wires 13, 19 extend from the 4 cap portion 12. A first pair of wires 13 is connected to a voltage source, preferably AC. The other pair of wires 19 is connected to an ammeter or other suitable measuring device.

A buoyant ring 15 is slidably located on stem 11. When the stem 11 is placed in a liquid reservoir, such as a fuel tank, the ring 15 floats on the surface of the liquid and rises or falls with variation in liquid level.

The electromagnetic components of the fuel gauge 10 are shown in Fig. 2. The wires 13 are connected to an air-cored coil 14 wound on a rigid paper tube for support.

st.. The density (and shape) of the turns of coil 14 15 can be varied at the design stage to suit particular applications or to achieve a particular effect, e.g. to improve linearity.

The coil 14 is juxtaposed with an elongate permeable member 16, which typically is a metal strip or 20 lamination wrapped in an insulating tape. The coil 14 and metal strip 16 may be held side by side by adhesive tape 17.

The wires 19 are connected to opposite ends of a coil wound around metal strip 16.

The coil 14 and the bottom half of metal strip 16 are located within the casing of stem 11. The top half of the metal strip 16 can be wound or folded within the cap portion 12 to reduce the overall length of the transducer A conductive loop 18 is located within the ring member 15. The loop 18 is typically made from an aluminium ring which is both eectrically conductive and lightweight. The loop may alternatively be formed by one or more shorted turns of a conductive coil.

In use, an alternating vo.Lage is applied to coil 14 via wires 13 to create a time varying magnetic field. This field is coupled to the strip or lamination 16 by the conductive loop 18, the degree of coupling being dependent on the position of loop 18 relative to the coil 14 and strip 16. The electromagnetic coupling between coil 14 and strip 16 induces a current in the coil wound around strip 16, and this current is measured by the ammeter (not shown) connected to wires 19. In this manner, the ammeter provides an indication of the position of ring 18 (and hence float 18) along stem 11, and any changes in that position.

The transducer of the preferred embodiment is particularly suitable as a fuel gauge, as it can be made in a compact form at low cost. Furthermore, the transducer has improved linearity as only half of the metal strip 16 is used, and the coil 14 can be designed to ,Iompensate for other nonlinearities.

15 The foregoing describes only one embodiment of the invention, and modifications which are obvious to those skilled in the art may be made thereto without departing from the scope of the invention as defined in the following claims.

6.e1.: 0 0* 0

Claims

1. An electromagnetic transducer comprising a first wire coil of elongate form juxtaposed with at least a portion of an elongate permeable member, the first wire coil being energised, in use, to create a magnetic field therearound, a second wire coil wound around at least part of the elongate member and connected to measuring means, and a conductive loop member located around the juxtaposed coil and elongate member and slidable axially therealong, the coupling of the magnetic field to the second wire coil via the elongate permeable member being dependent on the axial position of the loop member, whereby in use, the axial position of the loop member relative to the elongate member is indicated by the measuring means.

2. A transducer as claimed in claim 1, wherein the first wire coil is juxtaposed with approximately half the length of the elongate member.

3. A transducer as claimed in claim 1 or 2, 20 wherein the elongate member comprises at least one metal strip.

4. A transducer as claimed in any preceding claim, wherein the first wire coil is air-cored.

A transducer as claimed in claim 4, wherein the first wire coil is wound on a rigid non-permeable tube.

6. A transducer as claimed in any preceding claim, "wherein the transducer comprises a sealed casing having a stem and cap portions, the first wire coil and said 0 portion of the elongate member being located in the stem 30 portion, and the remainder of the elongate member being curled or folded within the cap portion, the loop member being slidably located around the stem portion.

7. A transducer as claimed in any preceding claim, wherein the loop member comprises a metal ring or coil located within a buoyant tubular casing.

8. A transducer as claimed in any preceding claim, wherein the density of turns of the first wire coil Svaries along the length thereof.

9. A transducer as claimed in any preceding claim, wherein the measuring means comprises ammeter means f or measuring current induced in the second wire coil. A transducer substantially as hereinbefore described with reference to Figs. 1 and 2. DATED this fifteenth day of September 1994 JOHN EDWARD HAYTER By his patent attorneys CULLEN CO. IN k ABSTRACT An electromagnetic position transducer (10) comprises an elongate coil (14) juxtaposed with an elongate permeable member A conductive ring (18) is located around the juxtaposed coil and elongate member, and is slidable therealong. A voltage applied to the coil (14) creates a time varying electromagnetic field which is coupled to the elongate member, the degree of coupling being dependent on the position of the conductive ring. The field around the elongate member induces a current in a coil wound around the elongate member, and this current provides an indication of the position of the conductive ring. S o o