Electricity: measuring and testing – Of geophysical surface or subsurface in situ – For small object detection or location
Reexamination Certificate
1998-12-14
2001-07-31
Snow, Walter E. (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
For small object detection or location
C324S247000
Reexamination Certificate
active
06268731
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a locator for locating an electrically conductive object. It is particularly suitable for measuring the position and orientation of buried underground objects such as cables and pipes.
2. Summary of the Prior Art
There is currently an increasing proliferation of underground objects such as cabling, piping, ducting etc. carrying utilities such as gas, electricity and telephone lines. Consequently it has become critical that persons involved in e.g. excavation work are aware of the location of said objects before commencing work so that unnecessary expense and inconvenience are not incurred through accidental damage.
Thus a typical target object for a locator is a cable comprising a conductive component such as metal sheathing or wiring. An electromagnetic field susceptible of detection by a locator can be produced in such a cable by e.g. the application of a signal to the cable sheathing or wiring via a suitable transmitter, or an alternating current carried by the cable.
Buried fibre-optic communication systems have especially high costs associated with their damage because of the difficulty of repairing broken fibre-optic cables and the potentially large numbers of customers who may be inconvenienced by the damage. Fibre-optic cables however usually have a protective metal sheath which can be used to make them locatable as described above.
WO-A-95-30913 disclosed a locator in the form of a ground penetration probe which had spaced antennae within the probe, each of which antennae detected electromagnetic signals from a buried underground object, such as a cable. The electromagnetic signals from the antennae were analyzed to determine the separation of the locator and the objection in the direction of the spacing of the antennae, and also in the perpendicular direction. This enabled a display to show visually the separation of the locator and the object.
In the arrangement disclosed in WO-A-95-30913, the antennae had identical aerial arrays, the aerial arrays being formed by detection coils. Although it was possible for each antenna to have an array with one horizontal and one vertical coil, WO-A-95-30913 also disclosed arrangements in which each antenna had three mutually perpendicular coils to detect magnetic fields, one coil in the direction of the spacing of the antennae, and the other two in two mutually perpendicular directions.
SUMMARY OF THE INVENTION
The present invention is based on the realization that sufficient information can be derived from antennae which are not all identical. At its most general, therefore, the present invention proposes a locator with at least two spaced apart antennae, with those antennae having aerial arrangements which are not identical.
In deriving the position of a concealed object relative to the locator, an important measurement is the distance between the locator and the concealed object measured in the direction of elongation of the locator. This direction (hereinafter the X direction) is particularly important when the locator is a ground penetration probe.
Although the antennae used in the present invention are not all identical, they must, in total, provide sufficient different measurements of the electromagnetic field from the concealed object to enable the location of that concealed object to be determined. Therefore, the arrangement of the electromagnetic field detectors (aerials) of the antennae must satisfy certain minimum conditions. Thus, the two or more antennae for measuring electromagnetic fields together have at least four electromagnetic field detectors (aerials), of which the first and second aerials measure field components in a direction (the Y direction) perpendicular to the direction of separation of the first and second antennae. These components are then used to determine the separation of the antennae and the concealed object in a direction (the X direction) parallel to the direction of separation of the first and second antennae. The third aerial in the X direction measures components and the fourth aerial measures components in a direction (the Z direction) mutually perpendicular to the X and Y directions. The aerials are distributed amongst the antennae so that at least one antenna measures a component direction not measured by at least one of the other antennae. Suitable processing means uses the differences between (and sometimes the absolute values of) the measurements made by the aerials to derive positional and orientation information of a target object with respect to the locator.
An advantage of arrangements according to the present invention is that a simpler locator having fewer aerials may be provided, but the locator is nonetheless able to supply an operator with similar direction, distance, and orientation information available to an operator of the locator of WO-A-95-30913. Thus information can be supplied which is similar to the information supplied by the embodiment of the locator of WO-A-95-30913 but with a locator having only four aerials.
Normally, the antennae will be located within a housing locator so that one antenna is near an end of the locator, and the other antennae are then spaced apart within that housing. The locator will then be used with the one antenna near an end thereof preferably being an end which is bought closest to the concealed object. That one antenna will therefore detect the strongest signals and thus it is preferable that one antenna is formed by the first and third aerials referred to earlier. This means that the measurement in the Y direction, and one of the measurements in the X direction are based on the strongest fields detected by the locator.
The second aerial is then in a second antenna spaced from the first in the X direction. It would also be possible for the fourth aerial to be included in the second antenna, or even in the first, but it is preferable that it is part of the third antenna, and that third antenna is located between the first and second antennae, with all three antennae sharing a common axis.
In a further development the second antenna comprises the second and a fifth aerial, the fifth aerial being able to measure field components in the X direction. The measurement by the locator of an additional X component may be used by the processing means to supplement the positional and orientation calculations described above. However, it is often the case, especially in urban situations, that the electromagnetic field produced by an object is distorted by fields produced by other conducting objects, such as parallel cables. When this occurs, X component amplitude and phase information measured by the third and fifth aerials may be used by an appropriately configured processing means to compensate for the distortion.
Common mode interference can also be a problem during electromagnetic detection. This occurs when electromagnetic signals are received not only from the object but also from sources such as a transmitter or adjacent metalwork. Therefore, in another development the third antenna comprises the fourth and a sixth aerial, the sixth aerial being able to measure field components in the Y direction. Having a third Y component measurement can enable the processing means (again appropriately configured) to detect and compensate for common mode interference.
A type of aerial suitable for the purposes of our proposal is a coil aerial. When an electromagnetic field passes through such a coil, the axis of the coil is the direction of the field component measured by the aerial. However the shapes and dimensions of suitable coils may be various.
Typically the antennae are arranged along a receiver which may be in the shape of a blade lying substantially in a plane containing the X and Y directions. Where the locator is a hand-held device, the proximal end of the receiver is normally attached to one end of the locator body, the body having a handle at the other end. The first antenna will then usually be located towards the distal end of the receiver.
When locating
Frost Nicholas James
Hopwood Michael Peter
Snow Walter E.
Woodbridge Richard C.
Woodbridge & Associates P.C.
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