Apparatus and method for measuring properties of a ferromagnetic material

A method for measuring stress in an object of ferromagnetic material, using an apparatus including a probe including an electromagnet core defining two spaced-apart poles for inducing a magnetic field in the object, so faces of the two spaced-apart poles define four corners of the probe, a drive coil wound around the electromagnet core, and at least four sensing coils configured to sense the magnetic flux density that links the electromagnet core and the object, wherein at least two of the at least four sensing coils are arranged in the vicinity of each of the two spaced-apart poles, and an alternating current supply configured to supply an alternating electric current to the drive coil to generate an alternating magnetic field in the electromagnet core and consequently in the object, wherein two of the at least four sensing coils are arranged on the face of each pole, each of the two sensing coils is between the face of the respective pole and a surface of the object, and each of the two sensing coils are attached at opposite ends of the face of the respective pole so that one sensing coil is near each of the four corners of the probe, the method comprising:

• placing the probe adjacent to the surface of the object;
• applying the alternating electric current to the drive coil so the magnetic field in the object oscillates about zero with an amplitude much less than saturation;
• detecting signals from the sensor coils;
• measuring signal components of the signals in phase and quadrature relative to the alternating electric current;
• and
• distinguishing changes in the signal components due to lift-off from changes due to stress, and hence deducing the stress in the object.

An apparatus for measuring material properties of an object of ferromagnetic material, the apparatus comprising:

• a probe comprising:
• an electromagnet core defining two spaced-apart poles for inducing a magnetic field in the object;
• a drive coil wound around the electromagnet core;
• and
• at least two sensing coils configured to sense the magnetic flux density that links the electromagnet core and the object, wherein at least one of the at least two sensing coils is arranged in the vicinity of each of the two spaced-apart poles;
• and
• an alternating current supply configured to supply an alternating electric current to the drive coil to generate an alternating magnetic field in the electromagnet core and consequently in the object,
• wherein the at least two sensing coils are wound around the electromagnet core adjacent to the two spaced-apart poles to form at least one matched pair, the at least one matched pair comprising a sensing coil in the vicinity of one pole of the two spaced-apart poles connected in series with a sensing coil in the vicinity of the other pole of the two spaced-apart poles to provide an output, the sensing coils of each of the at least one matched pair being wound or connected such that equal magnetic field changes directed towards the two spaced-apart poles would produce no net output.