US Patent 7839296 Scale and readhead apparatus and method

A scale reading apparatus includes a readhead that is moveable relative to a scale. The readhead includes an incremental channel from which a series of reference mark pulses are produced as the readhead is moved relative to the scale. A reference mark channel is also provided that produce a reference mark gating pulse when the readhead passes a predetermined (absolute) position mark on the scale. An alignment detection unit monitors during use, phase alignment of the reference mark gating pulse and reference mark pulses. The alignment detection unit is arranged to monitor the phase of the leading and trailing edges of said reference mark gating pulse relative to the phase of the reference mark pulses. The apparatus may be used in position encoders and the like.