Datum sensing using optical grating

Apparatus for detecting a datum or reference position in a range of movement of a member, the apparatus comprising a light source, grating means movable with the member;

• the grating means including a first grating comprising first and second grating halves having parallel lines which are symmetrically arranged on respective sides of a central axis with the lines increasing in pitch in directions extending away from the central axis, a second grating comprising third and fourth grating halves respectively identical to the first and second grating halves but with the third and fourth grating halves being separated by a spacing, whereby light from the light source illuminates the first and second gratings to cause the generation of interference images of the first and third gratings and the generation of interference images of the second and fourth gratings, said interference images varying in light intensity with movement of the member, and detector means for detecting the intensity of the interference images, for generating individual signals representative of the intensity of the interference images and for generating a difference signal representative of the difference between the said individual signals, said difference signal passing through zero at the datum position of the member.