GB2215063A

GB2215063A - Detecting buried conductors

Info

Publication number: GB2215063A
Application number: GB8803787A
Authority: GB
Inventors: Yukinobu Miyamoto; Yasuhiro Wasa
Original assignee: NEC Corp; Tokyo Gas Co Ltd
Current assignee: NEC Corp; Tokyo Gas Co Ltd
Priority date: 1988-02-18
Filing date: 1988-02-18
Publication date: 1989-09-13
Also published as: GB8803787D0

Abstract

In an arrangement for detecting buried conductors by sensing the flow of electric current therethrough the magnetic field generated around the conductor (13) is measured at points (P1???P7) above the ground level (14). The line normal to the measured magnetic fields at each point is projected and the intersection of these lines serves to establish the position of the conductor (13). To obtain a higher degree of accuracy and a more rapid measurement method at least three sets of magnetic sensors are used for detecting the magnetic field with the results from the intersections being statistically averaged to improve accuracy. Each sensor comprises two sensing elements whose areas cross at right angles, the axis of one set of sensing elements of the sensors being in parallel and the areas of the other set of sensing elements being colinearly aligned. <IMAGE>

Description

Detecting Buried Conductors This invention relates to the detection of buried conductors in which an electric current flowing through the conductor is detected from above the ground using the magnetic field formed around the conductor.

Methods are known by which the magnetic field of electrical conductors can be detected, see for example JP 59-14249. In detecting buried cables magnetic detecting means are generally used as such devices are easy to handle and provide a comparatively high detection accuracy. Such devices may also be used in a wide range of applications.

In the accompanying drawings Figure 4a shows a prior art method of detecting the position of a buried conductor 13 from above the surface of the ground 14 and wherein a current flowing through the conductor 13 produces a cylindrical magnetic field around the conductor which forms the central axis. This magnetic field is measured at least at two points such as P1 and P2 and the direction which is normal to the magnetic field B1 and B2 defines at the intersection the position of the buried conductor 13. Such a system is disclosed in JP 59-163586.

In this known art if more tan two sensors are used in an arrangement then each sensor needs to independently detect the magnetic field in three axial directions. If only two measured points are used then considerable error may occur in establishing the position of the buried conductor especially if noise or other interference is present. In the known art the method of detecting the magnetic field at a number of points by moving a set of sensors involves a relatively long time period and efficiency is low. For such a reason detection of buried conductors cannot be made quickly if an emergency arises.

In this invention a detector for buried conductors is proposed which may accurately and rapidly detect the position of the buried cable.

According to this invention there is provided a method for detecting the position of conductors buried beneath the ground, or otherwise inaccessible, wherein the magnetic field generated around the conductor by a current flowing therethrough is determined using a magnetic sensor, whereby the direction of the conductor in relation to the magnetic sensor may be established, in which method three or more magnetic sensors are used in a spatial distribution, the direction of the conductor thus established by each sensor being processed to define a mean position of the conductor.

Embodiments of the invention are described by reference to the accompanying drawings showing examples.

In the drawings: Figure 1 shows schematically a construction Ol magnetic sensor according to the invention, Figure 2 shows a block diagram of the circuit of the detector, Figure 3 shows three arrangements of magnetic sensors in the detector, and wherein Figure 3a shows five sets of magnetic sensors arranged colinearly, Figure 3b shows two sets of magnetic sensors arranged in parallel lines and staggered, and Figure 3c shows two lines of magnetic sensors arranged in alignment, Figure 4a shows a method of establishing the position of a buried conductor by prior art methods, and Figure 4b shows a method of detecting a buried conductor using the detector means according to this invention.

Referring firstly to Figure 4b this explains the theory of the present invention. A buried conductor 13 has an electric current flowing therethrough which generates a magnetic field around the conductor. This magnetic field is measured at a number of points P1 through P7 above the ground level 14. At each measurement point a line which is normal to the direction of magnetic field at each point is projected and the intersection is defined as the position of the buried conductor 13. The measuring points are set up and arranged along a straight line with the axes of the magnetic sensors in parallel and additionally extending parallel to the ground surface 14. The sensors are all directed in the same direction.In the case of establishing a position using the magnetic field from two axes alone, the intersection point will tend to scatter because of inherent errors in the measurement of the magnetic field at respective points. By using measurements from a plurality of points the errors may be minimised and the accuracy improved through a statistical averaging method whereby a mean value may be calculated.

Additionally the reliability of the data produced can be assessed from the distribution of the scattering at the intersection points.

Figures la and 1b of the drawings show construction for the detector and as shown in Figure la seven sets of magnetic sensors 1...7 are arranged on a straight line and accommodated within a casing 1. Figure 1b shows a view of each magnetic sensor which is constructed to have two axes being a horizontal direction sensor element 3 to detect the magnetic field in the direction horizontal to the ground surface and a vertical sensor element 4 which detects the magnetic field in a vertical direction. In the present invention it is sufficient to provide the sensors with two axes.

Figure 2 shows a block diagram of the electrical circuit and the output 5 from each sensor (S1...S7) is fed to the input of a multiplexer 6. In the multiplexer 6 only one of the input signals is selected by means of a central processor unit 9. The particular selected sensor signal is passed through a rectifier 7 and an analogue/digital converter 8 and thereafter fed to the central processor unit 9. The magnetic field at the measured sensor point is computed by the central processor unit 9 which calculates the position, reliability and other parameters of the buried conductor on the basis of the diagram of Figure 4b. The result of this computation is shown on a display 10.

Referring to Figure 4a, the magnetic fields B1 and B2 formed by the current flowing through the buried conductor 13 are measured at two points P1 and P2. If the distribution of the magnetic field is cylindrical about the conductor 13 then respective directions which are normal to the magnetic field measured at points P must intersect through the centre of the buried cable 13.

In reality however, the magnetic field will be distorted by influence of noise or magnetic materials under the ground and do not therefore adopt a perfectly cylindrical distribution about the conductor 13. Thus normal directions measured with respect to the magnetic field do not necessarily intersect at a single point. Thus, as shown in Figure 4b, the intersection point of two sets of normal lines which have been selected may scatter considerably from the intersection of two other normal lines. The scattering of the intersection points will become smaller as the magnetic field distribution nears the buried conductor and therefore the extent of distortion of the magnetic field can be ascertained by establishing the scatter distribution at the intersection points. Thus the accuracy of an intersection point in relation to an ideal magnetic field distribution can be established. This can be effected by statistical treatment of the parameters to establish the value of "reliability".

Three methods may be used for statistically expressing the scattering factor of the intersection point and in the first method tile scattering may be expressed by a visual method by plotting the two intersection lines which are normal to the magnetic field onto a diagram such as that shown Figure 4b. In a second method, the mean value of the intersection points which have been established by the aforementioned method can be assessed and quantitively provide a numerical value based on a mean. The percentage of intersections which should exist within a given range previously established can then be ascertained. In a third method the value can be expressed quantitively using standard deviation related to the scattering of the intersection points using a mean value of intersection positions as the centre.

In any of the three methods described it is fundamental that the magnetic field direction is first measured at a number of points above the ground by using the detector according to the invention and then the reliability of the detected result in establishing the buried conductor is expressed using the distribution or scattering of a number of intersection points which can be obtained from lines normal to the direction of the magnetic field.

In addition, the method of calculating the reliability and asymmetry of the measured magnetic field distribution can be used in addition to the scattering of the intersection points.

As the magnetic field distribution formed by a current flowing through the buried conductor 13 has a cylindrical distribution about the conductor as a central axis, a linearly symmetrical distribution is obtained with the position of the buried conductor 13 at the centre.

When the actual magnetic field distribution is distorted by aforementioned causes it may be calculated and displayed by the deviation from the line symmetry (asymmetry). The asymmetry becomes greater if another buried conductor exists in close proximity. By using the asymmetry therefore it is possible to establish the presence of an adjacent buried conductor as well as obtaining reliability in the measured result.

If the number of measuring points is made greater than above mentioned then the accuracy of calculating the position of the buried conductor is enhanced and reliability increased in establishing the value.

The aforementioned embodiment relates to an arrangement of magnetic sensors which are positioned on a straight line as shown in Figure 3a. A number of such magnetic sensors are arranged with respective axes parallel and the one axis directed in the same direction.

A similar effect may be obtained if the magnetic sensors are arranged alternately on pairs of parallel lines as illustrated in Figure 3b or if the magnetic sensors are arranged on parallel lines in alignment as shown in Figure 3c. The arrangements shown in Figures 3b and 3c provide an advantage in that the magnetic sensors can be arranged in a higher density if the sensors have the same size.

In the invention more than three sets of magnetic sensors serving to detect magnetic fields in two directions at mutual right angles are arranged so that the axes perpendicular to the ground of each sensor are parallel and the axes horizontal to the ground are in alignment. The magnetic field formed by the current flowing through the buried conductor is measured at a plurality of points above the ground by the sensors and the line normal to the magnetic field direction is established to provide the respective intersection points. The intersection points specify the position of the buried conductor at their intersections and by this means the position of the buried conductor can be detected with high accuracy and in a short period of time using a simple construction of magnetic sensor.

Claims

1. A method for detecting the position of conductors buried beneath the ground, or otherwise inaccessible, wherein the magnetic field generated around the conductor by a current flowing therethrough is determined using a magnetic sensor, whereby the direction of the conductor in relation to the magnetic sensor may be established, in which method three or more magnetic sensors are used in a spatial distribution, the direction of the conductor thus established by each sensor being processed to define a mean position of the conductor.

2. Method in accordance with claim 1, wherein each sensor comprises two magnetic detectors mutually at right angles, each detector of each sensor being respectively all aligned colinearly or all aligned in parallelism.

3. Method in accordance with claim 1 or 2, wherein the sensors are distributed spatially in a linear array.

4. Method in accordance with any preceding claim, wherein the signals from each sensor are processed by sequentially scanning the sensor outputs, effecting an analogue to digital conversion and then computing the conductor position based on a mean value from each sensor output.

5. Apparatus for carrying out a method in accordance with any preceding claim, the apparatus comprising at least three magnetic sensors each comprising two sensing elements whose axes cross at right angles and arranged within a casing with the axes of one set of sensing elements of the sensors in parallel and the axes of the other set of sensing elements of the sensors colinearly aligned.

6. An apparatus in accordance with claim 5 wherein all the magnetic sensors are arranged on the same line within the casing.

7. An apparatus in accordance with claim 5 wherein the magnetic sensors are arranged on two parallel lines with the sensors in each line being mutually staggered.

8. Apparatus in accordance with claim 5 wherein the magnetic sensors are arranged along two parallel lines within the casing with the sensors in each line being in alignment.

9. Method for detecting the position of conductors buried beneath the ground substantially as herein described and exemplified with reference to the drawings.

10. Apparatus for detecting the position of conductors buried beneath the ground substantially as hereinbefore described, exemplified, and illustrated with reference to the drawings.