GB2225114A

GB2225114A - Determining physical parameters or properties of metallic samples using inductive techniques

Info

Publication number: GB2225114A
Application number: GB8924865A
Authority: GB
Inventors: Leonard Marmaduke Steele; David John Anson
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-11-04
Filing date: 1989-11-03
Publication date: 1990-05-23
Also published as: GB8924865D0; GB8825847D0

Abstract

In a method and apparatus for determining a physical parameter or property of a metallic sample, such as the carat value of a sample of gold (3), employing an inductive technique and wherein the sample (3) is compared against a reference sample (4) associated apparatus is zeroed, for instance, using a bridge circuit, by inducing a field in at least the metallic sample (3) in accordance with the skin effect. Inductor coils (7), (8) each connected in a respective arm of the bridge circuit are arranged to induce currents in the samples (3), (4). With a high frequency signal energising the coils so that flux does not penetrate the samples to any great extent the air gap between coil (8) and reference sample (4) is mechanically adjusted by raising or lowering supporting spikes (10) to bring the bridge into balance. The coils are then energised with a low frequency signal so that flux extends into the samples and the bridge balanced by a potentiometer in one of the bridge arms. The adjustment of the potentiometer provides an indication of the carat value. <IMAGE>

Description

INDUCTIVE METAL DETERMINATION DESCRIPTION This invention relates to apparatus for and a method of determining a physical parameter or property, such as, the composition, of a metallic sample, for instance, an alloy sample, employing an inductive technique and is especially, but not exclusively, related to such apparatus and method for determining the carat value of a gold sample by inductive comparison with a reference, such as, a reference sample.

An object of the present invention is to determine very accurately the composition of an alloy, such as, a gold sample, against a reference, employing an inductive technique which to-date has provided unreliable results.

Another object of the invention 1S to provide apparatus for and a method of supplying a fast read- out of the composition of an alloy, such as, the carat value of a sample of gold, for use in the assaying field.

Accordingly, one aspect of the invention resides in a method of determining a physical parameter or property of a metallic sample employing an inductive technique, wherein the sample is compared against a reference and wherein associated apparatus is zeroed by inducing a field in the metallic sample in accordance with the skin effect.

Another aspect of the invention provides apperatus for determining a physical parameter or property of a metallic sample employing an inductive technique, which apparatus comprises means arranged to induce a skin effect field in the sample, to enable the apparatus to be zeroed for subsequently inductively comparing the metallic sample against a reference.

Thus, for zeroing the apparatus, the skin effect field induced in the metallic sample is preferably a high frequency magnetic field which penetrates to a minimal depth at the surface thereof, in accordance with the associated skin effect characteristic.

In an embodiment of invention, the skin effect field is induced also in a reference sample during zeroing of the apparatus. Subsequently, the metallic sample is compared against the reference using a low frequency induction technique, whereby the field induced in the sample(s) is caused to penetrate further thereinto than did the high frequency induced, skin effect field.

Preferably, the skin effect field is induced in the sample(s) by means of an induction coil with respect to which the sample(s) may be supported stably, for instance, using a three point contact arrangement for precise location thereof.

In a preferred arrangement, the magnetic flux generated by the coil(s) intersects as much of the sample(s) as possible, with the coil forming an arm of a bridge circuit, in which case, the air gap between the sample(s) and coil(s) can be adjusted to bring the bridge circuit into balance, thereby zeroing the circuit or, subsequently, providing an indication as to the parameter or property of the sample to be determined.

Such balancing may be achieved by adjustment means, preferably in another arm of the bridge circuit, to provide such indication of the parameter o- property of the sample being determined.

As mentioned above, in an embodiment of inventive apparatus, as well as in the inventive method, a reference sample is employed, against which the metallic sample is compared. However, any other suitable form of reference may be used, for instance, a voltage or other electromagnetic reference, whether it be in analogue or digital form.

Thus, only the metallic sample whose physical parameter or property to be determined is used, with the reference being provided in a less cumbersome form, say, a voltage in a comparator circuit, which enables the apparatus to be incorporated in a portable housing capable of being brought into contact with or in the close vicinity of a metallic sample, for instant determination of, say, the carat value of a sample of gold.

In order that the invention may be more fully understood, a preferred embodiment of apparatus for determining the carat value of a gold sample, as well as an associated inventive method, will now be described by way of example and with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic, side elevational view of apparatus in accordance with the invention; Fig. 2 is a diagrammatic plan view of the apparatus shown in Fig. 1; and Fig. 3 is a diagram of an electronic circuit used in association with the apparatus shown in Figs.

1 and 2.

Referring firstly to Figs. 1 and 2 of the drawings, apparatus for determining the carat value of a gold sample, such as, a bullion bar 3, comprises a housing 1 with an electrically-insulating top 2, preferably made of a plastics material, for example, Nylon (Registered Trade Mark). Associated with the electrically-insulating top 2 are two sensing areas each comprising an aperture 5,6 in the top 2, an inductive element 7,8 in the form of a coil with only half its ferrite core facing upwardly, and a sample supporting arrangement represented by three steel spikes 9,10. Associated with each sensing area is the gold bullion bar sample 3, which is supported upon the spikes 9 and whose carat value is to be determined, and a gold bullion bar reference sample 4 which is supported upon the spikes 10 and whose carat value is known.

The spikes 9,10 provide a stable support for their respective samples 3,4.

The two inductive coils 7,8 are arranged such that their magnetic fluxes cut as much of the respective samples 3,4 as possible and such that they form respective arms 37,38 of an a.c. bridge circuit, as shown in Fig.

3, the other two arms of the circuit being formed by a resistor 31 (R1) and a resistor 32 (R2) in parallel with a potentiometer 33 (Rx).

When an electrically-conductive material, such as the gold bullion bar unknown and reference samples 3,4 are placed in their respective sensing areas, they cut the magnetic fields generated by their associated coils 7,8, such that eddy currents flow in them, thereby causing a phase shift in the currents flowing through the coils. Minute changes in phase shift can be detected by a precision instrumentation amplifier 34 associated with the a.c. bridge circuit of Fig. 3, and amplified by the amplifier 35.

If the electrical conductivities of the two samples 3,4 are the same, then theoretically the balance of the bridge circuit is maintained. If, however, the conductivities are different but the two samples 3,4 have perfectly machined surfaces so that the respective air gaps between them and their associated coils can be made identical, then the carat value of the unknown sample 3 can be determined.

Unfortunately, the casting of gold bullion bars produces surface undulations, so that the surface texture from one bar to another is not constant, thereby providing spurious determinations of carat values therefor.

In order to overcome this problem , and in accordance with the invention, the air gap between the reference sample 4 and its associated coil 8 can be adjusted by raising or lowering the supporting spikes 10 in dependence upon rotation of an air gap adjustment control 11. Such adjustment rotates a worm drive 12 via a spindle 13 and a corresponding wheel 14 and thread 15 upon which are mounted the spikes 10 on a threaded table 16. The worm drive 12, wheel 14, thread 15 and table 16 are arranged to be of the anti-backlash type, so that there is no play in the raising and lowering arrangement for the spikes 10.

During such adjustment, a high frequency (1) oscillator 39 energises the coils 7,8, the frequency being of the order of 1MHz. Such high frequency induces a magnetic field in the samples 3,4 in accordance with the skin effect, with the magnetic flux not penetrating the samples to any appreciable extent, thereby enabling the effective air gap between the samples 3,4 and their respective coils into balance in the bridge circuit.

This constitutes an effective zeroing of the bridge circuit after which another, lower frequency (fO ) oscillator 40 is switched on and the bridge circuit is brought into balance by adjusting the potentiometer 33 via the knob 17, such adjustment being read-off to provide an indication of the carat value of the unknown bullion bar sample 3.

Thus, using the skin effect technique to zero the bridge circuit balance in a high frequency mode, enables the apparatus to be used to determine carat values very accurately for different unknown gold samples.

It is to be appreciated that this inventive method and any associated apparatus can be used to determine other physical parameters, properties and characteristics of metallic materials, by employing the skin effect technique of zeroing or calibration.

The output from the amplifier 35 is fed to an L.E.D. bar driver circuit 41 which drives an "N" segment L.E.D.bar:-grah 42 to provide a visual indication of the carat value of the bar sample 3. However, it is to be appreciated further that other types of "readout" may be employed.

Claims

1. A method of determining a physical parameter or property of a metallic sample employing an inductive technique, wherein the sample is compared against a reference and wherein associated apparatus is zeroed by inducing a field in the metallic sample in accordance with the skin effect.

2. A method according to claim 1, wherein the skin effect field induced in the metallic sample is a high frequency magnetic field which penetrates to a minimal depth at the surface thereof, in accordance with the associated skin effect characteristic.

3. A method according to claim 1 or 2, wherein a skin effect field is induced also in a reference sample during zeroing of the associated apparatus.

4. A method according to claim 1, 2 or 3, wherein subsequent to the zeroing of the associated apparatus, the metallic sample is compared against the reference or reference sample, as the case may be, using a low frequency induction technique, whereby the field induced in the or each sample is caused to penetrate further thereinto than did the high frequency induced, skin effect field.

5. A method according to any preceding claim, wherein the skin effect field is induced in the or each sample, as the case may be, by means of an induction coil.

6. A method according to claim 5, wherein the or each sample is supported stably with respec.

to its respective induction coil.

7. A method according to claim 6, wherein the or each sample, as the case may be, is supported stably above its respective induction coil.

8. A method according to claim 6 or 7, wherein the or each induction coil is located precisely with respect to its associated sample by means of a three point contact arrangement.

9. A method according to claim 8, wherein said arrangement comprises three spikes, preferably of steel.

10. A method according to any of claims 5 to 9, wherein the magnetic flux generated by the or each coil intersects as much of the respective sample as possible, with the or each coil forming an arm of a bridge circuit.

11. A method according to claim 10, wherein the air gap between the or each sample and its respective induction coil is adjusted to bring the bridge circuit into balance.

12. A method according to claim 11, wherein such air gap adjustment is effected by means of an anti-backlash worm drive.

13. A method according to claim 10, 11 or 12, wherein said skin effect inducing means induces a high frequency magnetic skin effect field in the or each sample, as the case may be, and the air gap between the or each sample and respective induction coil is adjusted accordingly to bring the bridge circuit inzc balance, thereby zeroing the apparatus.

14. A method according to claim 13, wherein a low frequency magnetic field is induced in the or each sample, as the case may be, after zeroing of the apparatus, and the bridge circuit is again brought into balance, to provide an indication of the physical parameter or property of the metallic sample being determined.

15. A method according to claim 14, wherein the bridge circuit is brought into balance again by adjustment means which provides said indication.

16. A method according to claim 15, wherein said adjustment means is in another arm of the bridge circuit.

17. A method according to claim 14, 15 or 16 wherein a visual indication of the physical parameter or property of the metallic sample being determined is provided.

18. A method according to claim 17, wherein the visual indication is provided in the form of an 'N' segment L.E.D. bar graph driven by an L.E.D. bar driver circuit to which an output is fed from an amplifier associated with the bridge circuit.

19. A method according to any preceding claim when employed to determine the carat value of a gold sample.

20. Apparatus for determining a physical parameter or property of a metallic sample employing an inductive technique, which apparatus comprises means arranged to induce a skin effect field in the sampler to enable the apparatus to be zeroed for subsequently inductively comparing the metallic sample against a reference.

21. Apparatus according to claim 20, wherein a bridge circuit is arranged to be brought into balance to zero the apparatus.

22. Apparatus according to claim 20 or 21, wherein said skin effect inducing means is arranged to induce a high frequency magnetic field to a minimal depth at the surface of the metallic sample, in accordance with the associated skin effect characteristic.

23. Apparatus according to claim 20, 21 or 22 including means arranged to induce a skin effect field in a reference sample when the apparatus is being zeroed or the bridge is being balanced, as the case may be.

24. Apparatus according to any of claims 20 to 23 including means arranged to compare the metallic sample against the reference or reference sample, as the case may be, using an induction technique subsequent to zeroing of the apparatus, whereby the so-induced low frequency field penetrates further into the or each sample than did the high frequency skin effect field.

25. Apparatus according to any of claims 20 to 24, wherein said skin effect field inducing means comprises a magnetic induction coil.

26. Apparatus according to claim 25 including means arranged to support the or each sample stably with respect to its respective coil.

27. Apparatus according to claim 26, wherein the or each sample is supported above its respective coil.

28. Apparatus according to claim 26 or 27, wherein the or each induction coil is located precisely with respect to its associated sample by means of a three point contact arrangement.

29. Apparatus according to any of claims 25 to 28, wherein the or each magnetic induction coil is arranged such that its magnetic flux intersects as much of the respective sample as possible and such that it forms an arm of a bridge circuit.

30. Apparatus according to claim 29, wherein at least one of the other arms of the bridge circuit includes adjustment means for bringing the bridge circuit into balance.

31. Apparatus according to any of claims 25 to 30, wherein the air gap between the or each sample and its associated induction coil is adjustable.

32. Apparatus accrding to claim 31 including anti-backlash worm drive means for effecting such adjustment.

33. Apparatus according to any of claims 20 to 32, wherein said skin effect field inducing means is arranged to induce a high frequency magnetic skin effect field in the or each sample and the air gap between the sample and its respective induction coil is adjustable accordingly, to zero the apparatus or to bring the bridge circuit into balance, as the case may be.

34. Apparatus according to any of claims 20 to 33, wherein said skin effect field inducing means is also arranged to induce a low frequency magnetic field in the or each sample, after zeroing of the apparatus or balancing of the bridge circuit, whereby the apparatus can once again be zeroed or the the bridge circuit can be balanced again, as the case may be, to provide an indication of the physical parameter or property of the metallic sample being determined.

35. Apparatus according to claim 34 including adjustment means arranged to zero the apparatus again or to balance the bridge again, as the case may be, and to provide such indication.

36. Apparatus according to claim 35, wherein said adjustment means includes a potentiometer in another arm of the bridge circuit.

37. Apparatus according to claim 34, 35 or 36 including means arranged to provide a visual indication of the parameter or property of the sample being determined.

38. Apparatus according to claim 37, wherein said visual indication means comprises an 'N' segment L.E.D. bar graph driven by an L.E.D. bar driver circuit to which an output can be fed from an amplifier associated with the bridge circuit.

39. Apparatus according to any of claims 20 to 38 arranged to determine the carat value of a gold sample.

40. Apparatus for determining the carat value of a sample of gold, substantially as hereinbefore described with reference to the acompanying drawings.

41. A method of determining the carat value of a sample of gold, substantially as hereinbefore described.