Position encoder using statistically biased pseudorandom sequence

A position sensor comprising a body at least partially surrounded by a housing, the body having a grating element attached thereto or integral therewith, the grating element comprising a surface, the surface comprising a coded distribution of regions of high and low reflectivity or transmissibility, the sensor also comprising at least one EMR source and at least one array of EMR sensitive detectors, the source irradiating the surface and the array receiving incident EMR reflected from or transmitted through the surface, the source and the array fixed with respect to the housing, a pattern thereby produced by incident EMR on the array resulting from the regions of high and low reflectivity or transmissibility on the surface of the grating element, the coded distribution having a sequence of bits, each bit comprising only one region of high reflectivity or transmissibility and only one region of low reflectivity or transmissibility, each bit representing a binary state 1 or a binary state 0 depending on whether it respectively corresponds to a transition from regions of high to low, or alternatively a transition from regions of low to high, reflectivity or transmissibility on the surface of the grating element, the bits being arranged at a substantially uniform code pitch, the incident EMR on the array therefore determining a substantially spatially periodic intensity pattern of incident EMR on the array, characterised in that the coded distribution comprises over 50% of dominant bits of either a binary state 1 or 0, and less than 50% of regressive bits, the dominant and regressive bits arranged consecutively as a pseudo-random binary code of length m, the pseudo-random binary code sampled as a series of n consecutive bits, wherein m and n are integers greater than one, and m is greater than n, and also determining the substantially spatially periodic intensity pattern incident on the array, the coded distribution arranged such that any said series of n consecutive bits also comprises over 50% dominant bits and less than 50% regressive bits, the pattern on the array processed by a processor to derive the absolute position of the coded distribution with respect to the housing, and hence provide a measure of the absolute position of the body with respect to the housing.