Electricity: measuring and testing – Conductor identification or location – Inaccessible
Reexamination Certificate
2002-09-12
2004-11-23
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Conductor identification or location
Inaccessible
C324S329000
Reexamination Certificate
active
06822429
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an inductive sensor arrangement for detecting a ferromagnetic or non-ferrous electrically conducting object buried in a surrounding medium. The inductive sensor arrangement comprises at least one field coil for generating a penetrating alternating magnetic flux in the medium and at least one sense coil for sensing magnetic flux field disturbances caused by the ferromagnetic or non-ferrous electrically conducting object. The invention further relates to a method, and its application in a hand-held machine tool, for detecting a ferromagnetic or non-ferrous electrically conducting object hidden in a surrounding medium by use of the inventive inductive sensor arrangement.
Generally, metal detectors measure the change of a chosen parameter as an operator sweeps a sensing head across a surface of a medium wherein a disturbing piece of a metal could be hidden. The parameter may be a capacitance, an inductance or any other physical parameter that would provide a distinction of one material from another.
Specifically, detectors may be used to find reinforcing metal bars (hereinafter “rebars”), usually consisting of a ferrous material embedded in a media such as concrete, brick, plaster and the like. There are detectors on the market that can fulfill this requirement but the accurate ones must be swept across the surface of the medium, e.g. concrete. By such a “sweeping movement”, it is possible to determine the position and the (length) direction of the hidden object, e.g. the rebar, from the received response signals. The metal coverage area may either be determined automatically using a rather complicated system or manually. In a manual determination, it is usual practice to manually mark the coverage and the direction of the rebar on the surface of the medium. Such manual scanning and determination requires not only time but also a specific skill and knowledge of the user or operator.
U.S. Pat. No. 5,729,143 describes a metal detector including a receive coil and a transmit coil arranged in parallel overlapping winding planes and connected in an inductive bride. Such an arrangement is a typical example of a metal detector that requires the specific skill and knowledge of the operator for interpreting the signal response.
Another type of apparatus for detecting metallic objects in a non-metallic material is described in U.S. Pat. No. 2,546,771. This detecting apparatus comprises a pair of series connected secondary coils positioned with their radii in substantially parallel planes adjacent to such material to be introduced into a detecting gap between two primary windings, respectively, diametrically encircling only one of each of the secondary coils. The primary coils are energized by an alternating current with such polarities that their magnetic fluxes are additive through such material. The secondary coils are connected in series bucking relation to produce a zero resultant output voltage across the secondary coils when the magnetic field of the primary coil is undistorted in the absence of any metallic object eventually hidden in the material probe. This known detecting apparatus, however, due to its physical dimensions and arrangement of gapped primary coils, is not suitable for detecting ferromagnetic or non-ferrous electrically conducting objects in a surrounding medium.
EP 0 366 221 AZ discloses a buried metal detection device comprising yoke-connected coils that are rotated to produce an external alternating electromagnetic field to be aligned in different directions, and monitoring variations in the energy supply drawn by the coil as an indication for the presence or non-presence of a metallic object in a surrounding medium.
EP 1 092 988 A2 discloses an inductive sensor arrangement for detecting metal objects hidden in a surrounding medium comprising a field coil pair for generating an alternating magnetic flux by a sequential excitation with an AC-current, and a pair of sense coils, respectively mounted inside each associated field coil in an orientation to the axes of each of the field coils such that essentially no voltage is induced in the sense coils in an environment free of a metallic object. If a metallic object comes into the vicinity of the inductive sensor, four characteristic voltage value sets are produced in the sense coil pair, wherein the voltage sets are processed by an algorithm for defining a position and a distinction criterion with respect to the hidden metallic object. This sensor arrangement has the advantage of a single point measurement resulting in an accurate position discrimination for a hidden metallic object like a rebar in concrete. Such an inductive sensor, however, has the limitation that reliable detection of a ferrous object like a rebar is only possible within angles of ±60° or less, usually only ±45°. To make the detector functional over a full 360° range such that arbitrary rebar angles may be detected, the EP-document also proposes that a second pair of field/sense coils is placed at 90° positions with respect to the first pair of field/sense coils to cover the orthogonal ±45° range. Such a four field/sense coil arrangement, however, if intended as an add-on rebar detection tool for a hand-held appliance such as a drill hammer, becomes dimensionally bulky and relatively expensive.
DE 196 48 833 A1 describes a device for detecting and identifying hidden objects like plastic mines in a ground. This device comprises two side-by-side arranged sensor coils that are operated at different excitation frequencies. Depending on various physical properties of the hidden object such as electrical conductivity, permeability etc. the impedance of a receiver coil arranged in an overlapping configuration of the two sensor coils is modified differently depending on the respective material properties. Again, the scanning of a specific ground area and the interpretation of the receive signals requires experience and skill.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an inductive sensor arrangement and a method for detecting ferromagnetic or non-ferrous electrically conducting objects like rebars hidden in a medium which, on the one hand, is capable of full 360° range detection and a depth determination and, on the other hand, allows a compact and small sensor design.
An inductive sensor arrangement, in accordance with the invention, comprises three field coils arranged in essentially the same geometric plane and adjacently positioned at defined angular distances from each other in relation to and around a central axis orthogonal to the plane; and three sense coils, one sense coil being respectively mounted inside each field coil in an orientation to the axis of the respectively associated field coil such that essentially no voltage is induced in the sense coil from the respectively associated field coil in an environment free of a ferromagnetic or non-ferrous electrically conducting object or material. Preferably, the three field/sense coil pairs are positioned at equal angular distances approximately 120° from each other, and the axes of the sense coils are respectively arranged orthogonally with respect to the axis of their associated field coil.
A method for detecting a ferromagnetic or non-ferrous electrically conducting object hidden in a surrounding medium by the inductive sensor arrangement, in accordance with the invention, comprises the steps of excitation of the three field coils with defined current ramps sequentially applied to each of said field coils to produce a changing magnetic flux penetrating the medium originating from three different physical positions corresponding to the field coils arrangement, collecting nine distinct output voltages from the three sense coils, i.e.
while the first field coil is excited by the defined current ramp, a first output voltage is acquired from the associated first sense coil and second and third output voltages from the remaining sense coils, respectively,
while the second field coil is excited
Binder Albert
Golder Roger
Hilti Aktiengesellschaft
Patidar Jay
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