GB2464279A

GB2464279A - Detection of a buried electric wire

Info

Publication number: GB2464279A
Application number: GB0818357A
Authority: GB
Inventors: Chris Ward
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2008-10-07
Filing date: 2008-10-07
Publication date: 2010-04-14
Anticipated expiration: 2028-10-07
Also published as: GB0818357D0; GB2464279B

Abstract

Apparatus for sensing a buried electric wire comprises an alternating magnetic field generator for generating magnetic flux lines around the wire over a length of the wire, and a pair of electrically conductive probes disposed on opposite sides of the generator along a line of symmetry which passes through the centre of its magnetic field. The probes are shielded from the generated magnetic field and configured to couple capacitively to respective spaced portions of the buried wire to detect their respective electric field strengths. An electric circuit with inputs connected to the probes is configured to generate a difference signal representative of the difference in electric field strength between the probes, whose magnitude is greater in the presence of the electric wire.

Description

DETECTION OF A BURIED ELECTRIC WIRE

The present invention relates to apparatus and a method for detecting an electric wire or other elongate conductor that may be buried for example beneath the surface of a road or path. The result of the detection may be used to record the location of the conductor, or to provide a visual or audible alarm, for example. The detector may be moved along a path in order to scan for the presence of a buried conductor.

Conventional metal detectors need as a target an area of metal that is at least a substantial fraction of the search coil area of the detector, to provide a sufficient probability of detection. More sophisticated detectors suitable for detecting wires beneath a surface rely on signals of opportunity, such as broadcast transmissions, or dedicated illuminators, to create electric currents in the wires, which are then sensed locally by a receiving magnetic loop. For the current to flow in the wire being detected, there is a need for capacitive loading further along the wire in order to sink the current, and this current, if near the wire end, is unlikely to be large; so sensitivity is compromised.

The purpose of the present invention is to provide a detector for a buried wire, with improved sensitivity, and without the need for electric current to be flowing in the wire being detected, nor any capacitive loading adjacent the wire.

The present invention provides apparatus for sensing a buried electric wire, comprising an alternating magnetic field generator for generating magnetic flux lines around the wire over a substantial length of the wire, and a pair of electrical field probes disposed on opposite sides of the alternating magnetic field generator along a line of symmetry which passes through the centre of its magnetic field, configured to couple capacitively to respective spaced portions of the buried wire to detect their respective electric field strengths, and an electric circuit with inputs connected to the probes and configured to generate a difference signal representative of the difference in electric field strength between the probes, whose magnitude is greater in the presence of the electric wire.

The invention also provides a land vehicle on which such apparatus is mounted with the said line of symmetry lying in a plane substantially parallel to the undersurface of the vehicle and thus to the ground in use.

The invention is capable of detecting the electric fields from the wire, without relying on any current flowing in the wire, nor on any capacitive loading. The invention works by exciting the wire with a magnetic dipole that does not directly couple to the electrical field probes, so that sensitivity is not compromised by large transmitted signals swamping returns from the target wire. Magnetic loops in existing devices have the defect that they emit an electric field which is coupled to the probes.

In order that the invention may be better understood, preferred embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings, in which: Figure 1 is a diagram of detecting apparatus embodying the invention; Figure 2 shows an alternative configuration of the apparatus of Figure 1; Figure 3 shows a configuration similar to that of Figure 2, but with digital processing circuitry; Figure 4 is a graph showing return signals on real and imaginary axes when a detector moves along a path over buried wires; and Figure 5 is a graph showing the magnitude of a return signal as a function of time.

A first embodiment of the invention is shown in Figure 1 of the drawings. An alternating magnetic field generator consists of a ferrite rod around which is wound an electric wire coil connected to a sinusoidal signal generator. The ferrite rod is electrically screened by a split cylindrical copper tube, i.e a copper tube which has a narrow gap to form an open circuit. The coil is mounted symmetrically about the centre of the ferrite rod, and the resulting magnetic field extends with a line of symmetry along the axis of the rod. The frequency could for example be 1 MIHz to 100 MHz, typically MIHz, generating an alternating field at the same frequency. The generator creates a magnetic dipole whose orientation switches at this frequency.

The tubular screen extends over the full length of the rod, forcing the magnetic field to remain within the tube up to the ferrite rod ends, so that the divergence of the field starts from two well-separated magnetic poles. The length of the rod may for example be 15cm, but may be varied to suit the detection application. The shorter the rod, the greater the intensity of the magnetic field, and hence the strength of the return signal. However, the longer the rod, the greater the radius of the magnetic lines of flux, allowing deeper penetration into the ground, or other medium, below the detection apparatus.

The electric screen prevents the emission of the electric field from the coil and from its electrical feed transition, which may for example be co-axial cable. It is important in this way to isolate the magnetic field generator electrically from the probes, to be described below. A high degree of screening integrity is required to achieve the full sensitivity of this system.

A pair of electric field probes are mounted symmetrically about the centre point of the ferrite rod, as shown in Figure 1. Each probe consists of a length of electrical conductor, running along a line perpendicular to the axis of the ferrite rod and through the centre of the ferrite rod. In this example, each probe comprises a thin strip of copper, approximately 5cm long, with its main surface lying in the plane parallel to the ferrite rod. The inner end of each probe is connected to a thin wire shielded by copper tube: the wire is electrically insulated from the tube by means of spaced insulative beads threaded around the wire. The probes are spaced equally from the axis of the ferrite rod.

The detection apparatus is mounted on a frame (not shown) which may form part of a land vehicle, with wheels or rails or some other guidance system so that it may travel along the ground in a specific direction. For greatest sensitivity, the frame and the vehicle should be such as to allow the alternating magnetic field generator arid the pair of probes to travel parallel to the ground surface and spaced closely from it, with the minimum possible disturbance from this path. The preferred configuration has the alternating magnetic field generator parallel to the path of motion of the apparatus, and the plane containing the pair of probes and their co-axial feed cables should be generally parallel to the ground surface. In one example, the ferrite rod is co-planar, or substantially co-planar, with the electric field probes; in a preferred example, however, the alternating magnetic field generator is in a plane below that of the electric field probes, i.e. rather closer to the ground.

Detection apparatus embodying the invention has been found to provide reliable detection of electric wires buried around 15mm below the surface of a mud path or road, which may be inhomogeneous and may comprise plant and tree roots, stones and moisture, causing clutter leading to false alarms: the reduction of false alarms is described below.

Each electric field probe is coupled capacitively through the vertical air space to a portion of the buried wire, in the event that a buried wire extends fully across the path of the detection apparatus. The wire may be bent, but provided there are portions adjacent both probes, the system will function satisfactorily. Accordingly, the separation of the probes may be determined according to the specific application, but it may typically be 20cm to 60cm.

The configuration of the electric field probes is designed specifically to minimise coupling between the magnetic field and the electric field circuits.

Whenever the apparatus is close to an electric wire, the wire will be encircled by magnetic flux lines, and variation in the magnetic dipole causes a corresponding variation, at the same frequency, in the potential difference between the ends of the wire. The differences in electric potential, coupled through the air capacitively to the electric field probes, are measured by circuitry shown in Figure 1 and described below.

The difference is indicative of the presence or absence of a wire or other elongate electrical conductor extending through the flux lines.

The electric wire connected to each probe is connected at the other end to an input to a high impedance amplifier, and the outputs of the two high impedance amplifiers are fed to a difference amplifier. Synchronous demodulation of the difference signal is used in order to reduce the detection bandwidth so as to be compatible with the search rate, determined by the motion of the detection apparatus relative to the ground, lowering the noise floor and improving sensitivity.

In the example shown in Figure 1, the output of the difference amplifier is split into two phase channels, with the I (In-phase) and Q (Quadrature) phase signals sent through a respective demodulator connected to a respective local oscillator at frequency w. The local oscillator signals are represented in Figure 1 by sine w and cos Wt. The I and Q mixer outputs are AC-coupled to the remainder of the processing circuit, so as to null out any fixed amplitude magnetic or electric field leakage signals and slowly changing imbalances caused by ground spacing variations and other causes. An alternative to the AC-coupling shown in Figure 1, best implemented digitally, is to generate a cancellation signal and to add this downstream of the mixers: the cancellation signal performs the same function of AC-coupling, but allows the conditions to be varied dynamically to suit the environment, for example by allowing a "hold" interval for slow scanning, or tailoring the high-pass characteristics to the local ground noise.

The signals on the I and Q channels are coupled to respective low-pass filters whose outputs then pass through logarithmic detectors and are then summed in an adder. The output of the adder represents the scalar magnitude of the return signal, but this includes clutter i.e spurious signals, and noise. Accordingly, this output is processed by passing it through a differential amplifier which thresholds it against a voltage which is time-varying in accordance with the magnitude of the input signal.

This amplifier operates as a noise riding threshold detector, as shown in Figure 1. The effect of this is to subtract the lower amplitude signals in a way which provides a constant false alarm rate, CFAR, a concept used, for example, in radar signal processing. Other CFAR algorithms may be employed in appropriate circuitry. The output signal of the CFAR circuitry may be used then to provide an audible alarm indicative of the presence of a buried wire, in real time; or a visual alarm; and it may also be used to record the location on the path of the vehicle at which this event occurred.

It will be appreciated that the location of the various circuitry components may vary according to the purpose of the apparatus, and it may be possible to separate the components, for example having some or all of the signal processing circuitry located remotely from the vehicle.

An alternative embodiment of the invention is shown in Figure 2, which differs from that of Figure 1 in that the high impedance amplifiers are located next to the corresponding probes, thereby reducing electrical interference between the connecting cables and the alternating magnetic field generator.

An embodiment of the invention utilising digital processing circuitry is illustrated in Figure 3, and shows JIQ mixers fed with voltage controlled oscillators for varying the phase shift applied to the signals. The demodulated signals at intermediate frequency, IFq and IF, are fed to respective analogue-to-digital converters, ADC, whose outputs are processed in a digital signal processor, DSP, to provide the output indicative of the presence of a buried wire. The digital signal processing circuitry includes variable threshold processing working with CFAR algorithms to improve the elimination of clutter, according to well-known principles.

With reference to Figure 4 of the drawings, a typical plot of the real and imaginary components, accumulated over time, of the intennediate frequency signal on a logarithmic scale, shows that there is a significant radius or amplitude over a relatively narrow range of phase angles. These correspond to real buried wires and some clutter.

With further reference to Figure 4, the largest intensity of traces, with a substantially smaller amplitude, is caused principally by variations in the height of the detector apparatus above ground, and bumps on the ground, and this is at an angle of around 30 degrees to 45 degrees to the range of phase angles associated with the target.

Accordingly, in order to improve signal processing and sensitivity, the signal processing algorithms may be used to rotate the phase angle so as to select only the signals associated with targets, i.e. to select a relatively narrow range of phase angles. Once this range has been selected, the scalar amplitude signal is processed using an appropriate CFAR algorithm to derive the output signal.

In the analogue implementation shown in Figures 1 and 2, the stronger of the two signal channels, either I or Q, may be selected for processing, with the other one being totally eliminated, for example by inserting switches in the respective channels immediately before the adder. The selection of the channel must then be made with the knowledge of which channel contains the majority of the signals of interest from targets.

Figure 5 illustrates very schematically a plot of scalar amplitude, or output voltage, against time, and shows four main peaks corresponding to the vehicle passing over buried wires. Clutter is represented by peaks which rise above the base line noise but which are not caused by wires.

In order to improve sensitivity, the apparatus may comprise two, three or more pairs of electric field probes arranged along the vehicle, along its path parallel to the axis of the ferrite rod or magnetic dipole. The signals from these pairs of probes will then contain peaks at the same relative positions, if those peaks correspond to actual buried wires; a lack of coincidence between the respective signals would suggest clutter or noise. Accordingly, correlation of the respective signals may be used to improve sensitivity, and this correlation may be done in the digital signal processor, DSP, of Figure 3, for example.

By analogy with methods of metal detection using hand-held scanners, the reversal of a scan and, for example, the employment of a zig zag path may typically be used to improve reliability, and subtle variations in audible or visual outputs may be used to increase the reliability of the analysis of the output. Accordingly, the detection apparatus path over the ground may be reversed, so that it traverses a wire in opposite directions. The signal returns from these path segments may then be correlated, again using the DSP, to improve sensitivity: of course the signals would have to be reversed in time before this correlation process.

Each of the pairs of electric field probes would preferably have its own adjacent alternating magnetic field generator. The outputs could be distinguished from one another by driving the alternating magnetic field generators with different frequencies, the outputs from the probes being split for analysis in the different frequency bands.

This may also be used to detect different aspects of the ground being scanned, for example to determine whether the ground is tightly compacted or is loose, or to determine whether there is significant moisture.

The alternating magnetic field generator could take the form of a rotating electromagnet, which would generate side bands offset from the centre frequency by the rotation frequency, to assist in discriminating against clutter. Alternatively, the effect of rotating the magnetic dipole could be caused electronically with the use of plural dipoles, for example crossed electromagnets.

It will be appreciated that sinusoidal drive of the alternating magnetic field generator could be replaced by other variations such as Walsh functions.

It will be appreciated that the speed of the vehicle relative to the ground will cause a Doppler effect in the return signal. The signal from the probes will be shifted in frequency from the centre frequency of the magnetic field dipole, dependent upon the relative velocity, and the direction of the shift will depend upon whether the vehicle is approaching or leaving the wire. The sharpness of the peaks of the return signals will also be a function of the ferrite rod length, or the length of the magnetic dipole, so these configurations may be optimised to suit the demands of specific applications.

Whilst specific types of electric field probes have been described, alternative sensors may be used. A variety of electric field sensors, for measuring the effect of the field e.g. on electrical charges, may be employed. Further, various means may be used to provide electrical isolation between the components of the apparatus, i.e. between the circuitry, the interconnecting wires or cables, the probes and the magnetic field generators. The specific layout and configuration illustrated in these examples is not intended to be limiting.

Further, whilst a major use of the present invention is in the detection of buried wires, it will be appreciated that the apparatus may be used for metal detection and for analysing the composition of a solid material such as the ground. Whilst it is envisaged that the detection apparatus will be moved relative to the target, the target could move relative to a stationary detection apparatus.

Claims

CLAIMS: 1. Apparatus for sensing a buried electric wire, comprising an alternating magnetic field generator for generating magnetic flux lines around the wire over a substantial length of the wire, and a pair of electrical field probes disposed on opposite sides of the alternating magnetic field generator along a line of symmetry which passes through the centre of its magnetic field, configured to couple capacitively to respective spaced portions of the buried wire to detect their respective electric field strengths, and an electric circuit with inputs connected to the probes and configured to generate a difference signal representative of the difference in electric field strength between the probes, whose magnitude is greater in the presence of the electric wire.
2. Apparatus according to Claim 1, in which the alternating magnetic field generator comprises an electromagnet having its dipole axis perpendicular to the line between the probes, the electromagnet being centred on that line.
3. Apparatus according to Claim 2, in which the alternating magnetic field generator comprises means for rotating the electromagnet relative to the remainder of the apparatus.
4. Apparatus according to Claim 2, comprising electrical shielding around the electromagnet with openings at each end pole of the electromagnet.
5. Apparatus according to Claim 4, in which the electrical shielding comprises a split covering of electrically conductive material making an open circuit around the dipole axis of the electromagnet.
6. Apparatus according to Claim 3, 4 or 5, in which the electromagnet comprises a coil driven by a signal generator configured to generate an alternating current.
7. Apparatus according to Claim 6, in which the alternating current is substantially sinusoidal.
8. Apparatus according to any preceding claim, in which the electric circuit comprises, for each probe, a high impedance amplifier whose input is connected to the probe and whose output is connected to a difference amplifier for generating the difference signal.
9. Apparatus according to any preceding claim, comprising plural said pairs of electrical field probes, separated along a path perpendicular to the said line of symmetry of each probe, for generating a series of said difference signals for the same buried wire.
10. Apparatus according to any preceding claim, comprising a signal processing circuit whose input is coupled to receive the difference signal, the signal processing circuit being configured to process the difference signal received over a period of time and to generate an output indicative of the presence or absence of the buried wire.
11. Apparatus according to Claim 10, in which the signal processing circuit is configured to identify the phase range in the difference signal for which there is greatest amplitude, and to threshold the signal in that phase range in order to reduce the influence of clutter or noise in the output.
12. Apparatus according to Claim 10 or Claim 11, in which the signal processing circuit comprises I and Q demodulation and filter channels for orthogonal phases of the difference signal, and a circuit for processing outputs from those channels to provide the said output.
13. Apparatus according to Claim 12, in which the I and Q channels comprise switches for selecting one of the channels for providing the said output.
14. Apparatus according to any one Claims 10 to 13, comprising an adaptive threshold circuit for comparing the signal with a time-varying threshold dependent upon the background clutter or noise to reduce the influence of clutter in the said output.
15. Apparatus according to Claim 14, configured to operate a constant false alarm rate, CFAR, algorithm.
16. Apparatus according to any of Claims 10 to 15, comprising a correlation processor for correlating two or more signals representative of the difference signal obtained over time, from the said probe or probes, as the apparatus moves relative to the buried wire or wires.
17. Apparatus according to any preceding claim, comprising a frame on which the probes and the alternating magnetic field generator are mounted.
18. Apparatus according to Claim 17, in which the alternating magnetic field generator is substantially co-planar with the probes.
19. Apparatus according to Claim 17, in which the alternating magnetic field generator is in a parallel plane spaced from the plane of the probes.
20. Apparatus according to Claim 18 or Claim 19, in which the alternating magneticfield generator is equidistant from the probes.
21. Apparatus according to any preceding claim, comprising electrical shields between the probes and the remainder of the apparatus but not between them and a region adjacent the apparatus which is to be scanned in use for the presence of a buried wire.
22. A land vehicle on which apparatus according to any preceding claim is mounted with the said line of symmetry lying in a plane substantially parallel to the undersurface of the vehicle and thus to the ground in use.
23. A land vehicle according to Claim 22, having wheels or rails or other means for guiding the vehicle along a path which is perpendicular to the said line of symmetry.
24. A method of scanning a surface for detecting buried wires, comprising moving apparatus according to any of Claims 1 to 21 or a vehicle according to Claim 22 or Claim 23 on a path over the surface and providing an output indicative of the presence of a buried cable.
25. A method according to Claim 24, comprising recording the location of the region where the output is indicative of a buried wire.
26. A method according to Claim 24 or Claim 25, as appendant to Claim 16, wherein the path includes a reversal over part of the same path, and the method comprises correlating the two or more signals obtained during the passage over the path in opposite directions.
27. A method according to any of Claims 24 to 26, as appendent to Claims 9 and 16, comprising correlating the signals from each pair of probes.
28. Apparatus for detecting the presence of a buried wire, substantially as described herein with reference to the accompanying drawings.
29. A method of detecting a buried wire, substantially as described herein with reference to the accompanying drawings.