GB2278199A

GB2278199A - Apparatus for detecting and estimating length of hidden elongated conductive objects

Info

Publication number: GB2278199A
Application number: GB9309532A
Authority: GB
Inventors: Christopher Keith Richardson
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 1993-05-08
Filing date: 1993-05-08
Publication date: 1994-11-23
Anticipated expiration: 2013-05-08
Also published as: GB9309532D0; GB2278199B

Abstract

The apparatus comprises a coil 1 having two axially displaced turns 2, 3 which crossover to define a figure of eight. The turns are mutually symmetrical and substantially identical in shape so that in an homogeneous fluctuating electro-magnetic field, there is no output from the coil, but an output is generated when the turns are exposed to mutually differing fluctuating electro-magnetic fields. The coil is connected via a filter arrangement to a detector comprising a wideband amplifier and a demodulator diode, and to an indicator. The apparatus is intended to operate with an electromagnetic radiation field such as that produced by radio signals. The filter arrangement may comprise three filters, covering the range 10KHz to 10MHz which can be separately switched into the circuit. Since the frequency signal produced by an object will depend on the length of the object, an estimate of that length can be made from the response at different frequencies. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO CONDUCTIVE OBJECT DETECTORS This invention relates to apparatus suitable for detecting concealed elongate conductive objects and more especially, although not exclusively, it relates to apparatus for detecting buried conductive material such as metal pipes, cables or wires.

According to the present invention, apparatus suitable for detecting concealed elongate conductive objects comprises a coil having two axially displaced turns which crossover to define a figure of eight, the turns being mutually symmetrical and substantially identical in shape and configuration whereby consequent upon exposure of the coil as a whole to an homogeneous fluctuating electro-magnetic field, signals induced in one of the turns cancel with signals induced in the other of the turns and detector means coupled to the coil for the detection of a signal when the said turns are exposed to mutually differing fluctuating electro-magnetic fields.

As will be appreciated by those skilled in the art, radio frequency background signals are generally present in the ether across substantially the whole of the radio frequency spectrum and produce what might be regarded as a homogeneous fluctuating electro-magnetic field which will accordingly produce no detectable signal in the coil of the apparatus according to the present invention. If however a conductive object is placed in the vicinity of one turn of the coil, the homogeneity of the field is destroyed and a detectable signal is produced in consequence.

The apparatus relies upon this effect to detect the presence of a field associated with a conductive object buried in the ground or buried in a wall for example, which field produces mutually differing fluctuating electro-magnetic fields in the turns of the coil when it is brought into close proximity with the conductive object.

It is essential that when the coil is exposed to a substantially homogenous field as aforesaid, signals in the coil cancel thereby to produce zero output from the detector means. In order to facilitate the provision of a zero output signal in the presence of a homogeneous fluctuating electro-magnetic field, balancing means may be providing effective for compensating for small differences between the turns of the coil.

The balancing means may comprise a positionally adjustable conductor which forms a part of one of the turns at a point of overlap between the turns.

The detector means may comprise a wideband amplifier arranged to feed a demodulator and indicator means.

The demodulator may comprise a diode.

The indicator means may be a visual indicator.

The visual indicator may be a moving coil meter or the like.

Since it is apparent that concealed elongate conductive objects will be responsive to different frequencies in accordance with their length, the detector means may be arranged to include a filter arrangement via which the wideband amplifier is fed.

The filter arrangement may be adjustable continuously or in steps.

The filter arrangement may be adjustable in the range 10 KHz to 10 MHz.

One embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which; Figure 1 is a generally schematic plan view of a detector coil, and Figure 2 is a somewhat simplified circuit diagram of a detector circuit.

Referring now to Figure 1, a detector coil 1 comprises two turns 2 and 3 which are arranged to be insulated from one another and overlap at a point 4. In this example, the coils 2 and 3 are made of copper foil which might be conveniently supported on a printed circuit substrate. As hereinbefore explained, it is essential that radio frequency background signals induced in the two turns 2 and 3 of the coil 1 are similar so that they cancel whereby a zero signal is present in a coupling coil 4 which is arranged to feed a detector (as shown in Figure 2) via line 5.In order to ensure that the signals induced in the turns 2 and 3 of the coil 1 are equal and opposite, an adjustment arrangement may be provided (not shown) at the point 4 of overlap, which may simply comprise a small length of wire connected in series with the coils which may be twisted or deformed so as to produce zero output in the coupling coil 4.

Referring now to Figure 2, the line 5 is arranged to feed an amplifier 6, either directly, or via filters 7, 8 or 9 in accordance with the position of a switch 10. The amplifier 6 is connected to feed a detector arrangement as shown within a broken line 11 which comprises a diode 12 and resistive and capacitive filter components 13 and 14 respectively. The detector arrangement within the broken line 11 is arranged to feed an indicator which simply comprises a moving coil meter 15.

When the coil 1, as shown in Figure 1, is exposed to a fluctuating homogeneous electro-magnetic field, as would be produced by the radio frequency background radiation, a signal induced in the turn 2 will cancel with a signal induced in the turn 3 so that no pickup is detected in the coupling coil 4. If however the turn 3 is brought into close proximity with a metal object of any significant size and in particular an elongate conductive object, then the field produced in the elongate conductive object will produce a difference between the signals picked up in the turns 2 and 3 and a consequential signal which is picked up by the coupling coil 4 and fed via the line 5 to the detector circuit as shown in Figure 2.Assuming that the switch is in the position as shown in Figure 2, whereby the line 5 is coupled directly to the amplifier 7, then in the presence of a conductive elongate metal object, a signal will be detected by the indicator 14 indicative of the presence of the object. In the present example the filters 7, 8 and 9 are arranged to cover a frequency range of 10 KHz to 10 Mllz in three steps whereby different sensitivity is available in dependence upon the length of the concealed metal object. Thus it will be apparent that long objects will produce larger signals in the lower frequency ranges and conversely relatively short objects will be more responsive to higher frequencies. Thus by use of the switch 9 it is possible to determine approximately the length of the object detected.

Various modifications may be made to the arrangement described herein without departing from the scope of the invention, and for example any suitable form of amplitude detector may be used and the material from which the coil is made may be varied in accordance with the application in view. Moreover it will be appreciated that any suitable form of overlap adjustment may be provided in the coil whereby a balancing operation could be performed.

Claims

1. Apparatus suitable for detecting concealed elongate conductive objects comprising a coil having two axially displaced turns which crossover to define a figure of eight, the turns being mutually symmetrical and substantially identical in shape and configuration whereby consequent upon exposure of the coil as a whole to an homogeneous fluctuating electro-magnetic field, signals induced in one of the turns cancel with signals induced in the other of the turns and detector means coupled to the coil for the detection of a signal when the said turns are exposed to mutually differing fluctuating electro-magnetic fields.

2. Apparatus as claimed in Claim 1 comprising balancing means effective for compensating for small differences between the turns of the coil.

3. Apparatus as claimed in Claim 2, wherein the balancing means comprises a positionally adjustable conductor which forms a part of the turns at a point of overlap between the turns.

4. Apparatus as claimed in Claim 3, wherein the detector means comprises a wideband amplifier arranged to feed a demodulator and indicator means.

5. Apparatus as claimed in Claim 4, wherein the demodulator comprises a diode.

6. Apparatus as claimed in Claim 4 and Claim 5, wherein the indicator means is a visual indicator.

7. Apparatus as claimed in Claim 6, wherein the visual indicator is a moving coil meter or the like.

8. Apparatus as claimed in any preceding claim, wherein the detector means is arranged to include a filter arrangement

9. Apparatus as claimed in Claim 8, wherein the filter arrangement is adjustable continuously or in steps.

10. Apparatus as claimed in claim 9, wherein the filter arrangement is adjustable in the range 10 KHz to 10 MHz.

11. Apparatus as claimed in Claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.