Position detector comprising a periodically varying magnetic member

A position detector 1 comprises first and second members eg scale 9 and housing 10 Fig. 1, a transducer 6,7, at least one analogue to digital converter (155,110, Fig.6) and a digital signal processor (20, Fig.6). The first member carries a train of elements of magnetic material eg balls or washers disposed side by side in a row and in contact with one another on a central axis, the train having a dimension varying perpendicular to the central axis. The second member carries a transducer comprising magnetic field transmitting 7' and sensing 6a-6d portions. The transmitting means produce a magnetic field trough the train. The sensing means comprises at least one pickup coil 6a1-6d3 to detect the varying magnetic field produced during relative movement of the first and second members as a result of variation in the distance between the pickup coil(s) and the train elements. The resulting varying amplitude signal is converted by analogue to digital converter(s) which provides a digital output to the digital signal processor. The processor samples the signal at pre-determined intervals to determine the relative positions of the members. The position detector includes automatic calibration (self diagnosis) for accurate resolution (1 žm or less).