Apparatus for determining location of an element of magnetizable material in a construction structure

Apparatus for determining a location of a magnetizable element in a construction structure, said apparatus comprising:

• a scanning head displaceable in a scanning direction and including a magnet for producing a magnetic field in the structure and having a bottom pole face adjacent to the surface of the structure along which the scanning head is displaceable;
• at least one pair of magnetic field sensors secured in the region of the bottom pole face for measuring the magnetic field disturbed by the magnetizable element, the at least one pair of magnetic field sensors comprising a pair of field plates located on the bottom pole face, the field plates lying, in a spaced relationship relative to each other on a straight line extending parallel to the scanning direction during displacement of the scanning head;
• a differential circuit connecting the field plates together for generating a differential measurement value in each scanning position of said scanning head;
• means, associated with said differential circuit, for storing generated differential measurement values for all of the scanning positions of the scanning head during displacement of the scanning head along the entire displacement path thereof in the scanning direction;
• and
• means, associated with said storing means, for determining a location of the magnetizable element wherein the determining means comprises means for processing the stored differential measurement values for determining a maximum of the stored differential measurement values, which defines the location of the magnetizable element.