Abstract:
A method of locating a concealed conductor is performed by generating a magnetic field with a direction in which the field is a maximum. The magnetic field is rotated until that direction is directed towards the conductor. In this way, a signal is induced in the conductor which has a maximum value when that direction is directed towards the conductor. The induced signal is detected in the conductor, using a detector arranged to determine the direction of the conductor relative to the detector. The detector detects when the induced signal has the maximum value.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the location of a concealed conductor, such as an underground cable or pipe. 
     2. Summary of the Prior Art 
     It is well known to detect the path (ie, to locate) a buried cable by applying a recognizable signal to that cable, and detecting the magnetic fields generated by the signal on the cable at remote sites. Such techniques are very well known, and widely used, but all depend on access to the cable at some point to apply the signal thereto. This may not always be possible, particularly when the people carrying out the locating are not employees of the utility to whom the cable belongs. Of course, an electrical cable carrying an alternating mains current will itself generate a magnetic field, which can be detected, but it is impossible to distinguish the field thus generated by one cable from that generated by another. It is therefore not usually possible to use the mains current as a way of locating a specific cable. 
     If an alternating magnetic field is generated at a particular site, that will induce a current on the cable which can be detected at a remote site due to the magnetic field which the current generates in the cable. Therefore, it is possible to induce a current on a cable without having direct access thereto, and subsequently to detect that cable. However, there are often many cables or metal pipes within range of an induction source and signals will be generated on all those simultaneously, so that detection at a remote site cannot identify which cable or pipe is being detected. It is thus not possible to trace the path of a specific cable or metal pipe. For simplicity, the subsequent discussions will use the term “cable” to refer to the concealed conductor. However, the invention is not limited to the detection of cables, but also metal pipes or other concealed conductors. 
     SUMMARY OF THE INVENTION 
     The present invention arises from the realisation that a rotating induction field will induce peak signals on different cables at different times, in dependence on the rotation of the field. It is further possible to detect the origin of a magnetic field using an appropriate detector such as a pair of perpendicular coils. Thus, if the preferred direction of detection is rotated in synchronism with the rotation of the induction field, it is possible to identify one cable out of a group of cables. 
     Although it is possible to generate a rotating induction field by rotating a single coil, it is preferable to use at least one pair of crossed coils, and vary the currents thereto in quadrature. In a further development of this, two pairs of crossed coils are used, with the parallel coils of each pair being proximate and generating fields in anti-parallel. With such an arrangement, the fields from the parallel coils cancel each other in one direction, and reinforce each other in another. If the detector coils are then located in the direction in which the fields cancel, there will be no direct coupling from the transmitter (ie, the coils generating the induction field) to the receiver (the coils detecting the induced current). 
     The above discussion assumes that there is a single rotating induction field. Whilst this can distinguish cables at different positions around the point of rotation, it cannot distinguish cables which lie in the same direction from the point of rotation of the field, but at different distances. 
     However, if two induction field sources are used, the two fields can be rotated so that the point at which the maxima of the two fields coincide is known from the spacing of the two sources, and the directions of the maxima. The point of coincidence can be moved by rotating one or both or the fields. Thus, the point of coincidence maxima can be caused (by rotation of either or both of the fields) to map-out an area perpendicular to the axes of rotation (this assumes that the fields rotate about parallel axes). If a cable is located at a point of coincidence of the maxima, as the maxima map-out the area, corresponding signals will be induced on the cable. Thus, signals can be induced on different cables at different times, irrespective of the position of the cables relative to the sources of the induction fields. When the rotating fields move, they can be thought of as mapping out a grid, so that the position of a given cable within that grid can be identified. 
     In practice, if coils are used to define the induction fields, the minima of such fields are defined more sharply than the maxima. Therefore, once an initial identification of the location of a cable has been identified, by observing induction at the location of the maximum (or maxima) of the induction field(s), it is preferable subsequently to investigate minima, as this then enables the location of the previously detected cable to be identified more precisely. 
     In practice, the alternating current on a cable, even when a mains current, exhibits not only the principal frequency of the alternating current but also other frequencies. It has been found that the frequency spectrum varies from cable to cable even when all the cables are carrying mains current. The most usual variation is that a cable will exhibit frequency gaps and these frequency gaps are not always the same for all cables. 
     The spectrum of frequencies of the current on the cable generates a corresponding spectrum in the magnetic field generated by the current in the cable. Thus, by obtaining a spectrum of the vertical field from each cable, and investigating anomalies in the spectrum, such as gaps in the frequencies, it is possible to identify the cable. This arrangement may be used in combination with the cable location arrangement described previously, but is an independent aspect of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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 diagram showing the principles underlying the present invention; 
     FIG. 2 is a schematic view of the principal components of a conductor location device embodying the present invention; 
     FIG. 3 illustrates the principles underlying another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring first to FIG. 1, consider a pair of crossed (ie, perpendicular) coils  10 . If alternating currents are applied thereto, a magnetic field will be generated. By suitable adjustment of the currents in the coils  10 , that field can be arranged to have a direction of maximum intensity. Suppose now that coils  10  are rotated in the direction of arrow  11 . The direction of maximum intensity will thus also rotate. Thus, at some point in rotation in the coils, the direction of maximum intensity will be directed towards one underground cable  12  which is buried below the ground surface  13 . Coupling to other cables  14  will be less. However, if the coils  10  rotate, the maximum coupling will pass from the cable  12  successively to the other cables  14 . 
     The above description assumes that the coils  10  are rotating in the direction of arrow  11 . However, by varying the currents to the coils  10  in quadrature, the direction of maximum field intensity may be caused to rotate without physical movement of the coils  10 . The same effect of successive maximum coupling to the cables  12 ,  14  will occur. 
     Suppose now that the coils  10  are used to detect magnetic fields due to currents in the cables  12 ,  14  which have been induced by a rotating induction field as previously described. When the maximum of the induction field is aligned with cable  12 , the induced current in that cable  12  will be a maximum. If there is a fixed relationship between the coils which generate the induction field (transmitter coils) and the coils which detect the magnetic fields in the cables  12 ,  14  (receiver coils), then the receiver coils will detect the field from the cable  12  and, due to the fact that the coils  10  are crossed, will be able to identify the direction of that field. Thus, by synchronizing the direction of maximum induction field and detection direction, and by rotating both, it is possible to identify separately each of the cables  12 ,  14 . 
     Of course, since the induction field does not have a sharp maximum, currents will be induced into others of the cables  14 , even when the maximum is directed towards cable  12 . Thus, at the detector, some signals will be detected from those cables  14 . However, since those signals will be significantly less than when the maximum of the induced field is directed towards the corresponding cable  14 , it is possible to recognise when the induction corresponds to a maximum, and when it does not, so that the appropriate cable  12 ,  14  can still be identified unambiguously. 
     FIG. 2 shows an embodiment for achieving such an arrangement. In FIG. 2, the transmitter coils  20  comprise two pairs of parallel coils  21 ,  22  with the coils of the two pairs being perpendicular. The coils of each pair have currents applied thereto in opposite directions, as shown by the arrows on the coils  21  to  24 . The reason for this is that the magnetic fields generated by the coils of each pair will then reinforce each other in directions in the plane defined by the coils  21  to  24 , but will cancel each other out in directions perpendicular to that plane. Thus, at the detector coils  30 , the field from coil  21  will be cancelled by the field from coil  22 , and the field from coil  23  will be cancelled by the field from coil  24 . There will thus be no direct coupling between the transmitter coils  20  and the receiver coils  30 . 
     The device of FIG. 2 is then positioned over an area where cables are to be located. The fields due to the transmitter coils  21  to  24  are then rotated, either by physically rotating the coils  21  to  24  or by varying the currents therein in quadrature as previously described, and thus there will be induction of currents in cables below the device as has previously been described with reference to FIG.  1 . The magnetic fields generated by those induced currents can then be detected by the detector coils  30 . 
     One problem with the embodiments described with reference to FIGS. 1 and 2 is that it is not possible to distinguish between two cables which lie in the same direction from the transmitter coils. Whilst the magnitude of the current induced in those two coils will be different, due to their different distances from the transmitter coils, the maxima induced in those coils will occur at the same time. The arrangement shown in FIG. 3 seeks to overcome this problem, and proposes that there are two sets of transmitter coils  40 ,  41 , each of which generates a field which is rotatable as described with reference to FIG.  1 . Although FIG. 3 shows arrangements in which the sets of transmitter coils  40 ,  41  each comprise a crossed pair of coils, as in FIG. 1, the arrangement of transmitter coils  21  to  24  shown in FIG. 2 may also be used for each of the transmitters  40 ,  41 . 
     Each coil  40 , 41  has, at any time, a line extending therefrom corresponding to the line of the maximum field intensity, and, except in the case where those lines are parallel, they will intercept at some point. By rotating the fields to suitable angles relative to each vertical, the point of intersection can be changed. If the angle φ 1  and φ 2  of the maximum field intensities are known, and the separation of the coils  40 , 41  is known, the point of intersection can be determined. If the point of intersection coincides with a cable, a signal will be induced in the cable by both fields and this can be detected as has previously been described. 
     At any time, there will be only one point at which the maxima of each of the fields from the two sets of transmitter coils  40 ,  41  coincide. As the fields are rotated, that point will move below the surface  13  above which the sets of transmitter coils  40 ,  41  are located. The result is that the point of intersection of the maxima can be considered to scan out a grid  42  below the sets of transmitter coils  40 ,  41 . Thus, signals can be induced in cables  43  below the ground  13  at different times. A similar set of detector coils corresponding to the sets of transmitter coils  40 ,  41  can be used which similarly scans an area to detect specific cables. 
     In practice, it is preferable that the frequencies F 1  and F 2  of the transmitter coils  40 ,  41  are different. 
     It may then be possible to plot a transfer function against frequency for each position in the grid  42 , which may enable different types of cables to be identified. 
     In such an arrangement, the alignment of the transmitter and receiver coils must be maintained accurately. If they are not to be physically interconnected (and the optimum spacing of the transmitter and receiver coils may prevent this or at least make the device excessively unwieldy, some other positioning system is required, such as an optical system. 
     In the above discussion, cables are detected when they are aligned with the maximum (or maxima in the case of the embodiment of FIG. 3) of the induction field(s). However, the maxima are not sharp and thus the accuracy of location is not high. The minima of the fields are much sharper, but such minima cannot be used initially to locate the cables since they generate no induction. However, if the cables are first pre-located by observing maxima, and then the general location of each cable thus identified is investigated using the minima of the field, to detect the position of the field at which there is no coupling, a more accurate determination of the cable location can be obtained. 
     As has previously been mentioned, a cable carrying an alternating current generates a magnetic field with a spectrum of frequencies, and that spectrum may contain anomalies, such as gaps, which can be identified by e.g. the detector coils  30  and a suitable processor for processing the signals generated in the coils  30 . By investigating the gaps in the spectra from the different cables, it is further possible to distinguish different cables in terms of those gaps and so identify the path of the cable. Such identification is not wholly reliable, since it depends on the gaps in the field spectra, and it is not impossible for two cables to have the same gaps. However, in most cases, such investigation of spectral anomalies, such as gaps, is sufficiently reliable, particularly when the general area of the cable has already been located, so that the path of the cable can then be followed using a location based solely on the spectral gaps of a cable which has already been identified.