Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
1999-12-30
2001-10-02
Trieu, Van T. (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S552000, C324S326000
Reexamination Certificate
active
06297736
ABSTRACT:
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.
REFERENCES:
patent: 1718352 (1929-06-01), Guilford
patent: 1766378 (1930-06-01), Guilford
patent: 4639674 (1987-01-01), Rippingale
patent: 5093622 (1992-03-01), Balkman
patent: 5425367 (1995-06-01), Shapiro et al.
patent: 5438266 (1995-08-01), Tsang
patent: 5773971 (1998-06-01), Tavernetti
patent: 251429 (1926-05-01), None
patent: 2175096A (1986-05-01), None
E.B. Glennie, Inductive Detection of Underground Metallic Pipes Proc. IEE, vol. 122, No. 4, Apr. 1975, pp. 345-348.
J.C. Lim, Synthesis of a Single Null Response in an otherwise Omnidirectional Pattern using a Circular Array, Proc. IEE, vol. 122, No. 4, Apr. 1975, pp. 343-344.
Frost Nicholas James
Lewis Andrew Biggerstaff
Radiodetection Limited
Trieu Van T.
Woodbridge Richard C.
Woodbridge & Associates P.C.
LandOfFree
Locating concealed conductors does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Locating concealed conductors, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Locating concealed conductors will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2559927