The invention relates to an inductive sensor head for detecting a ferrous, ferric and/or nonferrous electrically conducting objects buried in a surrounding medium.
Metal detectors for detecting ferrous or non-ferrous objects in media like walls of concrete, brick, plaster or the like or in the ground based upon the disturbance or modulation of the inductive coupling between two coils are known in the art. For example, U.S. Pat. No. 5,729,143 describes a microprocessor controlled metal detector which uses a transmitter coil providing a periodically varying magnetic field in combination with a receiver coil connected thereto in an inductive bridge. The detector comprises means for automatically balancing the two overlappingly arranged coils and electronically compensating any initial coil misalignments or unwanted signals, in particular, during an initial calibration step. In a known metal detector, one of the coils, the field coil, generates an alternating magnetic field while the other coil, the sense coil, measures changes caused by a ferrous or non-ferrous material coming into the magnetic flux field while moving the detector over the medium containing the hidden disturbing object.
A problem with the known metal detectors is, on the one hand, the relatively large size, which is unavoidable due to the side-by-side arrangement of the field coil and the sensor coil and, on the other hand, the fact that the detector must be swept over a certain search area in a kind of scanning process.
It is an object of the invention to provide an inductive sensor head which is small in size and may be used as a hand-held tool or may be integrated into an electric hand-held tool, preferably, a drill hammer.
It is a further object of the invention to provide an inductive sensor head which provides sufficient clear information about a hidden ferrous or non-ferrous electrically conducting object without the necessity of sweeping the sensor head over a certain working area of the medium in which said object may be buried.
The invention provides an inductive sensor head for detecting of ferrous or non-ferrous electrically conducting objects hidden. In particular, such a sensor head comprises at least one larger diameter field coil with a small axial length-to-diameter-ratio and at least one twin pair of co-axially arranged sense coils both having a small diameter compared to the diameter of the field coil. Preferably, the inductance of the sense coil is significantly higher than the inductance of the field coil. The higher the inductance the more sensitive the sense coil is to magnetic changes and the less gain is needed in the amplifiers that follow such elements. The common axis of the twin pair of sense coils extends perpendicular to the axis and in a diameter direction to the field coil, and the axis is positioned in a plane of the winding plane of the field coil or in a plane essentially parallel to the winding plane of that field coil. Further, the two sense coils are positioned in an equal distance from the center of the field coil such that they are penetrated by the same magnetic flux direction of the flux field emanating from the field coil when excitated by an electric current.
For achieving better positional information, in particular for resolving depth information in relation to a hidden object, e.g., a reenforcing bar (xe2x80x9crebarxe2x80x9d in the following) from a single position measurement cycle, a significant improvement of the invention is achieved if a twin pair of coaxially positioned field coils is provided. The mutual axial distance of the two field coils can be rather close and may preferably be less than their internal diameter. As a rule, the distance between the field coils is arranged such that the difference in magnetic field strength on a rebar is sufficiently large that it can be accurately measured. In addition, two twin pairs of sense coils with orthogonally arranged axes are positioned in a center plane parallel and approximately at a halfway distance between the winding planes of the two field coils.
As will be described in the following further details, the invention also provides an advantageous driving circuit for the combination of a twin pair of field coils and a double twin-set of sense coils, wherein additional correction coils are provided in series connection with each of the two field coils in order to minimize magnetic offsets due to the fact that the sense coils cannot be or are difficult to be exactly positioned in the magnetic null position of both field coils.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by it use, references should be had to the drawings and description matter in which there are illustrated and described preferred embodiments of the invention.