SCALE READING APPARATUS

PCT No. PCT/GB97/00594 Sec. 371 Date Oct. 20, 1997 Sec. 102(e) Date Oct. 20, 1997 PCT Filed Mar. 5, 1997 PCT Pub. No. WO97/34126 PCT Pub. Date Sep. 18, 1997An opto-electronic scale reading apparatus includes a scale 10 and a readhead 14 movable one relative to another. The readhead produces a pair of quadrature pulses which enable the generation of an incremental count corresponding to the displacement of the readhead 14 relative to a reference mark 80. The reference mark 80 is a permanent magnet mounted on the scale substrate inside a non-ferromagnetic casing 90. A pair of differential Hall sensors 62A,B within the readhead, and associated processing circuitry generate a step-change ZERO pulse when the readhead 14 and scale 10 are in a particular relative position. This relative position can be adjusted to enable corresponding adjustment of the relative phase of the quadrature pulses emitted from the readhead and the ZERO pulse, by means of a ferromagnetic adjusting screw 92. Movement in the position of the screw 92 relative to the permanent magnet 80 changes the position of highest magnetic flux density in the region of the magnet, thereby altering the position of which the ZERO pulse will be emitted.