GB2262606A

GB2262606A - Metal detector with nulling coil.

Info

Publication number: GB2262606A
Application number: GB9126675A
Authority: GB
Inventors: Andrez Zbigniew Regini
Original assignee: RADIODETECTION Ltd; Radiodetection Ltd
Current assignee: RADIODETECTION Ltd; Radiodetection Ltd
Priority date: 1991-12-16
Filing date: 1991-12-16
Publication date: 1993-06-23
Anticipated expiration: 2011-12-16
Also published as: GB2262606B; GB9126675D0

Abstract

A metal detector has an AC driven transmission coil 1, a receiving coil 3 which detects the field generated by the transmission coil and a nulling coil 2, adjacent the transmission coil and generating a signal which is equal in magnitude and opposite in direction to the signal produced by the receiving coil in the absence of a metal object. The coils are preferably coaxial and coplanar with the nulling coil 28 receiving coil 3 connected in series opposition. The detector circuit 21 for the difference signal which indicates presence of metal may comprise a variable gain amplifier and a switched feedback path which varies the gain of the amplifier. The detector is insensitive to small variations in relative position between the transmission and receiving coils. <IMAGE>

Description

METAL DETECTOR The present invention relates to a detector for detecting a metal object, which detector makes use of the effect of that object on a magnetic field.

In a standard metal detector, there is a transmission coil through which is passed a current which generates a magnetic field, and a receiving coil which detects that field. The transmission and receiving coils are positioned so that, in plan, they overlap. The part of the receiving coil which lies within the transmission coil is then subject to a field in one direction, and the part of the receiving coil lying outside the transmission coil is subject to a field in the opposite direction. By suitable position of the transmission in the receiving coils, it is possible to arrange for the net field detected by the receiving coil to be substantially zero. Thus, in the absence of a metal object, the receiving coil gives null output.

When the detector approaches a metal object the magnetic field changes due to the presence of that object, and hence a different field is detected by the receiving coil. Therefore, the receiving coil is no longer in the null state, and an output is generated which can be detected. The magnitude of the change corresponds to the size and proximity of the metal object.

The disadvantage with the known arrangement is that any departure of the receiving coil from the null state generates an output; and that null state is dependent on the relative physical positions of the coils. Therefore, the detector is very sensitive to small changes in the relative position of the transmission and receiving coils. -An impact with the metal detector may jar the coils out of position, and the high sensitivity of the arrangement is such that even the normal movement of the detector may trigger a spurious output. Therefore, existing metal detectors have frequently to be re-calibrated to ensure that the receiving coil generates a null output in the absence of a metal object.

In order to overcome this problem, it was appreciated that it was necessary to arrange for the receiving coil to have a null state which was not critically sensitive on the relative location of the transmission and receiving coils. In a first aspect of the present invention, it is proposed that the receiving coil encloses, in plan, the transmission coil.

The signal produced by such a coil will not be null. Instead, it will correspond to the difference between the field within the transmission coil (which is in one direction) and the field between the transmission coil and the receiving coil (which is in the other direction). This field is equal in magnitude and opposite in direction to the signal that would be produced by the component of the field which is outside the receiving coil.

Then, according to the first aspect of the present invention, means are provided for generating a signal which is equal in magnitude and opposite in direction to the signal (hereinafter a "reverse" signal) produced by the receiving coil in the absence of a metal object. Hence, a null signal can be generated.

Such an arrangement is relatively insensitive to the relative positions of the transmission and receiving coils. While it is preferable that the transmission and receiving coils are co-axial (concentric), any departure from such an arrangement does not affect significantly the area which (in plan) lies between the transmission and receiving coils, and therefore does not significantly change the field experienced by the receiving coil corresponding to that area. Similarly, the field within the transmission coil remains constant, Hence, with a constant reverse signal, the neutral position can be maintained irrespective of small changes in the position of the transmission receiving coils.

Preferably, the means for generating the reverse signal includes a coil (hereinafter the "null" coil) located immediately adjacent the transmission coil.

Such a coil then receives a field substantially equal to the field within the transmission coil. By suitable scaling of the output of that coil e.g. by reducing the number of turns of the null coil relative to the receiving coil, the output of the null coil can be selected so that it is equal in size and opposite in magnitude to the signal generated by the transmission coil in the absence of a magnetic field.

Since the null coil is immediately adjacent the transmission coil, the field it experiences is relatively un-affected by a metal object.

It has further been appreciated that the use of such a null coil immediately adjacent the transmission coil and a generation of the reverse signal therefrom, permits geometrical arrangements to be achieved in which the receiving coil does not enclose (in plan) the transmission coil Thus, a metal detector involving three coils, being a transmission coil, a receiving coil and a null coil represents a second, independent, aspect of the present invention. With this aspect, for example, the receiving coil may be remote from the transmission and null coils. It will then experience a field from the transmission coil and generate a corresponding signal, and a null output can then be achieved by suitably balancing of that signal by output from the null coil.In such a system, the arrangement is relatively insensitive to small changes in the relative position of the coils, since the change in field experienced by the receiving coil will be small. It is even possible to use such a three coil arrangement in which the transmission and receiving coils overlap.

In the present invention, it may be necessary to adjust the sensitivity of the detector, for example, when it detects a large metal object. In a further development of the present invention, a suitable sensitivity circuit for a metal detector is provided, which sensitivity circuit includes a switched feedback loop. With the switch open, the output of the receiving coils is used directly to control an indicator for detecting a metal object. However, when the switch is closed, the output to the indicator is fed back to an amplifier whose bias is reduced in dependence on the size of that signal so that the detection threshold is reduced if the output of the detector coils is large. Hence, the single switch in the feed-back path controls the detection threshold while maintaining sensitivity.Thus the metal detector can be used or detecting relative changes in the location of a metal object rather than merely the presence or absence of such an object.

This feature of the present invention is applicable also to standard metal detectors and thus represents a third, independent, aspect of the present invention.

Embodiments of the present invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic view of the coil arrangement in an embodiment of a metal detector according to the present invention; Fig. 2 shows the magnetic field generated in such an arrangement; Fig. 3 shows the variation in magnetic fields in the embodiment of Fig. 1; Fig. 4 is a circuit diagram illustrating the interconnection of the coils in the embodiment of Fig.

1; and Fig. 5 shows a detection circuit, being part of the circuit of Fig. 4.

In an embodiment of the present invention, a first coil 1 is wound on a former, which is preferably circular in shape, and is connected, as will be described later, to means providing an alternating current thereto, so that it generates a magnetic field. The coil will therefore be referred to as the transmitting coil, or, more briefly as the Tx coil.

A second coil 2 is wound immediately over the transmitting coil 1 and therefore has a very close coupling to the coil 1. The coil 2 will be referred to as a nulling coil or N1 coil. A third coil 3 is then wound on a further former, again preferably circular, which has a diameter larger than that of the transmitting coil and is generally concentric and coplaner, with the transmitting coil 1 and the nulling coil 2. The coil 3 will be referred to as the receiving coil or Rx coil.

When an alternating current is fed to the transmitting coil 1, the magnetic field generated has a pattern shown by the magnetic flux lines 4 in Fig.

2. The flux passes through the centre of the transmitting coil and then passes around the outside of the transmitting coil. All the flux that passes through the centre of the coil will also pass around the outside. However, although the flux distribution within the transmitting coil may be assumed to be substantially constant, the flux density outside the coil and in the plane of the coil decreases at a rate proportional to the inverse square of the distance from the transmitting coil. The flux density is thus shown by the line 5 in Fig. 37 where it is assumed that there is a positive flux within the transmitting coil 1, and a negative flux outside it.

Since the receiving coil 3 encloses, in plan7 the transmitting coil 7, the field induced into it will be proportional to the net amount of flux that lies within the receiving coil 3. Thus, the net flux is represented by the flux in the area 6 in Fig. 3, less the fluxes in areas 7 and 8. This resulting flux is not zero, but is equal in magnitude and opposite polarity to the flux outside the transmitting coil 3, i.e. in the regions 9 and 10 in Fig. 3. There is thus a residual signal.

For the detection of a metal object7 it is desirable that the output is in a null state when no metal is present. Therefore, according to the present invention it is necessary to generate a signal which is equal in magnitude and explicit in polarity to that generated by the receiving coil 3 in the absence of a metal object. This is the purpose of the nulling coil 2. As was mentioned above, the nulling coil 2 is very closely coupled to the transmitting coil 1, and the field it detects, is at least substantially, the field corresponding to region 6 in Fig. 3. Therefore, by suitably adjusting (e.g. by selecting the number of turns of) the nulling coil 2, it is possible to ensure that the signal it generates from the flux corresponding to region 6 is equal in magnitude to that corresponding to regions 9 and 10.

When a metal object is brought into the vicinity of the detector, it will affect the shape of the field generated by the transmitting coil 1. The field that is within the periphery of the transmitting coil (i.e.

the flux in region 6), will not be affected significantly, as compared to the field outside the coil, i.e. the flux in regions 7 to 10. Hence, the metal object will cause little or no change in the output of the nulling coil 2, but will change significantly the output of the receiving coil 3.

Hence, a significant change can be generated from circuitry comparing the signals of the nulling coil 2 and the transmitting coil 3.

This circuitry is shown in Fig. 4. The transmitting coil 1 is connected between earth and the output of an oscillator 20, and the nulling coil 2 and receiving coil 3 are connected in series between earth and a detector circuit 21. Thus, the signal detected by the detector circuit 21 is represented by the net signal from the nulling coil 2 and the receiving coil 3. By suitably arranging the windings of the nulling coil 2 and the receiving coil 3, it is straightforward to arrange for the signal to the detector circuit 21 to be zero in the absence of a metal object.

The output of the detector circuit 21 is fed to an indicator 22, such as a loudspeaker, so that a suitable signal is given to the operator of the metal detector when the signals from the nulling coil 2 and a receiving coil 3 are not balanced, i.e. when a metal object is present. The oscillator 20 and the detector circuit 21 are also powered by a common power supply (PSU) 23.

This embodiment of the present invention is very insensitive to small movements of the receiving coil 3 relative to the transmitting coil. Suppose, for example, the transmitting coil 3 moves in the plane of the coils. The total amount of flux passing through the receiving coil 3 then undergoes little or no change. This can be seen by consideration of Fig. 3 where a shift of the receiving coil 3 e.g. to the right increases the area of region 8 but decreases the area of region 7. Assuming that the transmitting coil 1 remains within the receiving coil 3, the positive flux corresponding to region 6 will remain the same.

Thus, the net change of both the positive and negative flux is small, and so there is little or no change in the output of the receiving coil 3.

Hence, it is not essential that the transmitting coil 1 and the receiving coil 3 are concentric, and from consideration of Fig. 2 it can also be seen that it is not necessary that they lie in the same plane.

A concentric arrangement in the same plane is most insensitive to small changes in movement, but other geometrical arrangements are also relatively insensitive.

It can also be seen that it is possible for the receiving coils 3 to lie wholly outside the transmitting coil 1. In that case, by consideration of Fig. 3, the receiving coil 3 will be subject only to negative flux, but this can be counter balanced by an appropriate signal from the nulling coil 2. When small changes in the relative positions of the coil occur, this does not significantly affect the flux through the receiving coil because of the inverse decay of the flux density.

As shown in Fig. 5, the input to the detector circuit 21 from the series connection of the nulling coil 2 and the receiving coil 3 is fed via an amplifier 30 to an analysis circuit 31 (being a synchronsus rectifier) which determines the magnitude of that signal and controls the indicator 22 accordingly. The sensitivity of such a system is then determined by the gain of the amplifier 30.

There are situations, such as when the metal object is large, where it is desirable to change the detection threshold of the detection circuit. To achieve this, a feed-back path 32 leads from the output of the analysis circuit 31 via a reference setting circuit 33 and a switch 34 to the amplifier 30. The reference setting circuit 33 generates an output which is dependent on the signal to the indicator 22 and, when switch 34 is closed, can be arranged to alter the reference level (d.c. offset level) sf the amplifier 30 in dependence on that output. Hence, the output of the analysis circuit may be arranged to generate no output even though the input thereto may be non-zero. This process will reduce the detection threshold of the detector while still maintaining a constant sensitivity. This is useful, for example, if the metal detector is to be arranged to detect relative movement of a metallic object. To appreciate this, firstly assume that switch 34 is open and the metal detector is brought proximate a metal object. The detector circuit 21 will generate an output to the indicator 22. Then, if switch 34 is closed, that output is fed back via the reference setting circuit 33 to alter the reference position of the amplifier until, for example the signal to the indicator 22 is zero. The switch 34 may then be opened. Provided the input to the amplifier 30 remains constant, i.e. the relative position of the metal detector and the metal object remain unchanged, then there will be no signal to the indicator 22.If, however, there is relative movement of the metal object and metal detector, then the inputs to the amplifier 30 will change (either increasing or decreasing in dependence on the direction of relative movement), and a signal will then be passed to the indicator 22. Thus, rather than detecting the existence of the metal object, the metal detector detects relative movement thereof.

The advantage of the detector circuit arrangement of Fig. 5 is that the operator needs merely to close the switch 34 to adjust the sensitivity and/or reference position of the amplifier 30. The reference setting circuit 33 may be set so that it adjusts the sensitivity of the detector circuit 21 to a maximum in the absence of any input to the amplifier.

As was mentioned earlier, although the detector circuit of Fig. 5 has been designed with particular applicability to the detector of Figs. 1 to 4, it is also applicable to other metal detectors.

Claims

1. A metal detector comprising: a transmitting coil connected to a power source for generating a magnetic field; a detection coil arranged so as to surround (in plan) the transmitting coil such that a detection signal is included into the detection coil by the magnetic field; and means for determining the difference between the detection signal and a reference signal; whereby a metal object interacting with the magnetic field is detectable by variation of that difference.

2. A metal detector according to claim 1 wherein the transmitting and detection coils are co-axial.

3. A metal detector according to claim 1 or claim 2 wherein the transmitting and detector coils are coplanar.

4. A metal detector according -to any one of the preceding claims7 further including a further coil adjacent the transmitting coil, the further coil being arranged such that the reference signal is induced in the further coil due to its interaction with the magnetic field.

5. A metal detector comprising: a transmitting coil connected to a power source for generating a magnetic field; a detection coil arranged such that a detection signal is induced into the detection coil by the magnetic field; a further coil adjacent the transmitting coil arranged such that a reference signal is induced into the further coil by the magnetic field; and means for determining the difference between the detection signal and a reference signal; whereby a metal object interacting with the magnetic field is detectable by variation of that difference.

6. A metal detector according to claim 4 or claim 5 wherein the further coil is co-axial and co-planar with the transmitting coil.

7. A metal detector according to any one of claims 4 to 6, wherein the detection coil and the further coil are electrically connected.

8. A metal detector according to any one of the preceding claims wherein said reference signal and said detection signal are arranged to be equal in magnitude and opposite in direction in the absence of the metal object interacting with the magnetic field.

9. A metal detector according to any one of the preceding claims, wherein said means for determining said difference includes a detector circuit including amplifier means for generating a detection output in dependence on the difference, and a switched feedback path for selectively supplying the detection output to the amplifier means, the amplifier means being arranged to vary the biasing thereof in dependence on the detection output.

10. A metal detector comprising: a transmitting coil connected to a power source for generating a magnetic field; a detection coil arranged such that a detection signal is induced into the detection coil by the magnetic field; and a detector circuit including amplifier means for generating a detection output in dependence on the detection signal, and a switched feedback path for selectively supplying the detection output to the amplifier means, the amplifier means being arranged to vary the basing thereof in dependence on the detection output.

11. A metal detector according to claim 10 wherein the transmitting and detection coils are co-axial.

12. A metal detector according to claim 10 wherein the transmitting and detection coils are co-planar.

13. A metal detector substantially as herein described with reference to and as illustrated in the accompanying drawings.