GB1604973A

GB1604973A - Apparatus for locating metal and/or ferromagnetic particles

Info

Publication number: GB1604973A
Application number: GB2491878A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-05-31
Filing date: 1978-05-31
Publication date: 1981-12-16

Description

(54) APPARATUS FOR LOCATING METAL AND/OR FERROMAGNETIC PARTICLES (71) I, JOHN HUGH DAYLY WALTON, a British Subject, of 2 Broadfield Road, Folkestone, England, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to apparatus for locating metal and/or ferromagnetic particles.

According to the present invention there is provided apparatus for locating metal and/or ferromagnetic particles comprising a transformer having a primary, for energisation with alternating current, and a secondary arranged coaxially with said primary so as to be magnetically coupled thereto, and means for indicating the presence of voltage on the secondary, which voltage is a function of the presence of a metal and/or ferromagnetic particle in the vicinity of the transformer, wherein said secondary comprises sets of turns so arranged relative to one another and to the primary as to provide a detection zone extending frame one side of the transformer and encircled by a surface of null response to particles, which surface tapers towards the transformer.

The tapering surface of null response preferably has a half angle of taper, at a plane which cuts the primary and which is at right angles to the axis of the transformer, of less than 70n, preferably less than 60', and ideally less than 40 . Such surface may be a paraboloid.

Conventional apparatus for locating metal and/or ferromagnetic particles, for example a conventional metal detector, has a planar surface of null response, which usually lies in the plane of the primary of the transformer of the detector. An arrangement according to the first aspect is more suitable than the conventional arrangement for detecting particles in a region external to the transformer and can be usefully employed in medical applications-for example detecting metal particles in an eyeball.

Preferably the transformer has a ferromagnetic core having a rod member extending along its axis, the rod member being of high thermal conductivity, so as to provide means for dissipating temperature gradients present in the core. The member preferably has a thermal conductivity greater than that of aluminium, for example it may be copper. It will be seen that such a probe can be constructed so that drifts in the results are reduced.

For a better understanding of the present invention and to show how the same may be carrid into effect, reference will now be made, by way of example, to the accompanying drawings: Figure 1 is a diagrammatic illustration of an arrangement for detecting metal and/or ferromagnetic particles.

Figure 2 illustrates an alternative form of an arrangement for detecting metal and/or ferromagnetic particles.

Figure 3 is a circuit diagram pertaining to part of Figure 1 or 2, Figure 4 is a vector diagram pertaining to the circuit of Figure 3, and Figure 5 is a diagrammatic illustration of a conventional known form of metal detector.

Figure 1 shows a probe 15, with a probe casing 9 and a working face 14. The probe is of elongate form and may have a crosssectional dimension as small as one centimeter. A coaxial transformer arrangement, comprising a primary 1 and two secondary windings 2 and 3, is situated in close proximity to the working face 14 of the probe.

Secondary windings 2 and 3, which are unequal in diameter, are asymmetrically arranged with respect to primary 1. Primary 1 is energised by an AC generator 5 via connecting leads 6a. Secondary windings 2 and 3 are opposingly connected, in series, and via connecting leads 6b, to indicating means 4.

The number of turns on the two secondary windings is selected so that the voltage induced in them by the primary 1, in the absence of any metal or ferromagnetic particles, will be equal, (and opposite as they are serially opposed), and the indicating means will display zero. To this end the mutual inductances between windings I and 2 and between windings I and 3 can be made equal, i.e. the small winding 2 has more turns than winding 3 and so couples more strongly then winding 3, to that zone 10 which contains the axis 8 of the probe 9 and is at one side of the transformer. Hence, near to the axis in that zone, the magnetic effects of the two secondary windings do not cancel.

The relative coupling strengths are such that the magnetic effects of the two secondary windings do cancel on a surface of null response 7 which is a surface of revolution of a line about axis 8, and which intercepts axis 8 within the probe at apex I I. In this embodiment the surface is inclined at an angle of 30 to a plane which is perpendicular to he plane of the primary I, at the working surface 14. The relative proportions, diameters, number of turns and axial positions of all the windings 1, 2 and 3 influence the shape of this surface of null response.

The assembly is arranged so that the indicating means displays zero when no particles of metal or ferromagnetic material are in the vicinity of the probe.

If a particle of metal or ferromagnetic material is present in the vicinity of the probe, the magnetic field generated by the primary will induce in the particle currents which will in turn cause a magnetic field to be set up. This field will couple to the secondary windings and induce therein a voltage which is detectable by the indicating means. If the particle lies in the zone 10 bounded by the surface 7, its induced magnetic field will couple more strongly to coil 2 than to coil 3 and, as coils 2 and 3 are oppositely connected to the indicating means, the latter will show a resultant signal of a particular phase with reference to the phase of the voltage or current applied to the primary coil.If a particle is located outside the zone 10, but is in close enough proximity to the working face 14 of the probe 15 to be influenced by the field generated by the primary coil 1, then a field will be induced which will couple more strongly to coil 3 than to 2, and cause a resultant signal of opposite phase to be shown by the indicating means.

A particle situated on the surface of null response 7 will couple equally with coil 2 and coil 3, and so there will be no resultant signal.

To reduce the complications and disturbing effects of more massive metal objects situated outside the zone 10, the indicating means can be arranged to indicate only when a particle of metal or ferromagnetic material is present in the zone 10. It will be noted that a ferrous particle inside zone 10 will have a similar effect on the probe detector to a non-ferrous particle outside the zone 10, due to the nature of the phase changes in the secondary which each situation induces, i.e. due to the effects of the different particles on the coupling between windings.

To detect whether a particle is in the zone 10, a circuit that will detect the dissipation of energy in the zone is needed-such energy dissipation is indicative of either a ferrous or non-ferrous particle. Figure 3 shows such an arrangement. The primary I is supplied by generator 5 and the resultant difference current between that induced in winding 2 and that induced in winding 3 is fed to a ring modulator 26 also fed by the current supplied by generator 5. The difference current thus resolved in the direction of the current supplied to the primary is fed via switching arrangement 25 to relay 17. Figure 4 shows a vector diagram wherein arrows 21 and 22 represent the currents in windings 2 and 3 due to the presence of a particle in the vicinity of the transformer.Arrow 20 shows the resultant difference current resolved along the current axis I, i.e. the component of the difference current in the direction of the supply current. Relay 17 is activated only if this component 20 is less than a preset threshold 19, and lights lamp 18 indicating that there is not a particle present in zone 10.

Relay 17 is also operable to activate a second relay 27 so that when a particle is detected but is not in zone 10, display meter 28 is disabled. A particle outside the zone would result in vectors 23 and 24 representing the currents in windings 2 and 3, and a resultant difference current resolved along the current axis, in the opposite directon to 20. This would operate relay 17 and would disable display meter 28. If such a component of the difference current resolved along the current axis, in the opposite direction to 20. This would operate relay 17 and would disable display. Meter 28 can be arranged to display whether the particle when present in the zone 10 is ferrous or non-ferrous independence upon the relative phase of the difference current and the supply current fed through ring modulator 29 as illustrated in Figure 3.

The probe has a ferromagnetic core 12, for example a ferrite core, situated coaxially within the coils, to generate a greater magnetic flux and to increase the magnetic coupling between the coils and any metal particles which may be nearby. However, this arrangement is very sensitive to temperature changes which cause drift in the results.

A copper rod 13 or a rod of any material of high thermal conductivity (preferably greater than 236 W/M/K at 273 K) coaxial with the coils and the ferromagnetic core and arranged along the middle of the core, helps dissipate temperature gradients quickly and so avoids prolonged drift of the results.

Neither the core nor the rod are essential elements of the probe-an air core could be used.

Figure 5 shows a conventional form of metal detector wherein the surface of null response 7 is planar, and so the detector is not very sensitive to the presence of particles in zone 10 which is quite a distance from the surface 7. The circuit of Figure 3 may be used with this conventional detector.

Figure 2 shows an alternative arrangement of the apparatus shown in Figure 1 wherein winding 2 is not disposed inside the primary 1. This arrangement shows how a handle 16 may be used to support the transformer (1,2 and 3) and facilitate manipulation of the probe, for example moving it into corners. In another alternative arrangement (not shown) windings 1 and 3 may be interwound on the same former. This gives better coupling between the coils, and hence more sensitivity, but winding 2 must have more turns to ensure that the surface of null response is suitably shaped.

WHAT WE CLAIM IS: 1. Apparatus for locating metal and/or ferromagnetic particles, comprising: a transformer having a primary, for energisation with a alternating current, and a secondary arranged coaxially with said primary so as to be magnetically coupled thereto; and means for indicating the presence of voltage on the secondary, which voltage is a function of the presence of a metal and/or ferromagnetic particle in the vicinity of the transformer, wherein said secondary comprises sets of turns so arranged relative to one another and to the primary as to provide a detection zone extending from one side of the transformer and encircled; by a surface of null response to particles, which surface tapers towards the transformer.

2. Apparatus according to claim 1, wherein said surface of null response has a half angle of taper of less than 70t at a plane which cuts the primary and which is at right angles to the axis of the transformer.

3. Apparatus according to claim I or 2, wherein said secondary comprises electrically interconnected first and second windings which are arranged asymmetrically with respect to the centre plane of the primary.

4. Apparatus according to claim 3, wherin said first and second windings have different diameters.

5. Apparatus according to clam 3 or 4, wherein said first and second windings are disposed at different distances from the centre plane of the primary.

6. Apparatus according to claim 3, 4 or 5, wherein said first and second windings comprise different numbers of turns.

7. Apparatus according to any one of the preceding claims, wherein said surface of null response is a paraboloid.

8. Apparatus according to any one of the preceding claims wherein said indicating means includes locating means for indicating, when a metal and/or ferromagnetic particle is present in the vicinity of the transformer, whether it is in said zone or outside said zone.

9. Apparatus according to claim .8, wherein the locating means comprises means for comparing the phase of the voltage across the windings of the secondary with the phase of the current in the primary to determine whether or not a metal and/or ferromagnetic particle is present in said zone.

10. Apparatus according to clam 8 or 9, and comprising means for analysing the voltage on the secondary windings to distinguish between ferromagnetic and non-ferromagnetic particles, when in said zone.

11. Apparatus according to claim 10, wherein the analysing means has display means arranged to be disabled by the locating means when a particle is detected outside said zone.

12. Apparatus according to any one of the preceding claims wherein the transformer has a ferromagnetic core having a rod member extending along its axis, the rod member being of high thermal conductivity, so as to provide means for dissipating temperature gradients present in the core.

13. Apparatus according to claim 12 wherein said member is formed of a material whose thermal conductivity is greater than 236 W/M/K at 273K.

14. Apparatus according to claim 13, wherein a major constituent of said member is copper.

15. Apparatus according to claim 14, wherein said ferromagnetic core is formed of ferrite.

16. Apparatus substantially as hereinbefore described with reference to Figure 1, 2 or 3 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. not very sensitive to the presence of particles in zone 10 which is quite a distance from the surface 7. The circuit of Figure 3 may be used with this conventional detector. Figure 2 shows an alternative arrangement of the apparatus shown in Figure 1 wherein winding 2 is not disposed inside the primary 1. This arrangement shows how a handle 16 may be used to support the transformer (1,2 and 3) and facilitate manipulation of the probe, for example moving it into corners. In another alternative arrangement (not shown) windings 1 and 3 may be interwound on the same former. This gives better coupling between the coils, and hence more sensitivity, but winding 2 must have more turns to ensure that the surface of null response is suitably shaped. WHAT WE CLAIM IS:

1. Apparatus for locating metal and/or ferromagnetic particles, comprising: a transformer having a primary, for energisation with a alternating current, and a secondary arranged coaxially with said primary so as to be magnetically coupled thereto; and means for indicating the presence of voltage on the secondary, which voltage is a function of the presence of a metal and/or ferromagnetic particle in the vicinity of the transformer, wherein said secondary comprises sets of turns so arranged relative to one another and to the primary as to provide a detection zone extending from one side of the transformer and encircled; by a surface of null response to particles, which surface tapers towards the transformer.