Method of optimising the output of a sensor for indicating the relative location of a mettalic object

A method of optimising the output of a sensor for indicating the relative location of a metallic object, the sensor being of the type having a primary electromagnetic coil arranged to generate a time varying magnetic field;

• and a secondary electromagnetic coil arranged to detect the time varying magnetic field as affected, directly or indirectly, by the object and to output, on the basis of the detected time varying magnetic field, a signal indicative of the relative location of the object, the method including the steps of:
• supplying the primary coil with an alternating current to result in the generated time varying magnetic field;
• locating the object in a first position and recording the signal output by the secondary electromagnetic coil for a range of respective frequencies of the supplied alternating current;
• locating the object in a second position and recording the signal output by the secondary electromagnetic coil for the range of respective frequencies of the supplied alternating current;
• calculating, for each of the respective frequencies, a value for the span to offset ratio of the measured signals on the basis of the respective signals measured for the object in the first and second positions, the calculating step including, for each respective frequency
• calculating the difference between the amplitudes of the signals measured for the object in the first and second positions, and
• dividing the difference by the amplitude of the signal measured for the object in the second position;
• and
• determining the frequency of the supplied alternating current which provides the maximum span to offset ratio on the basis of the calculations.