Absolute Shaft Encoders from J Darbyshire and Son Limited

Absolute shaft encoders, sometimes named as shaft-angle encoders, are not only used to detect angular positions, but also suitable for linear movements that can be converted into rotary movements by a toothed belt, drive pinion, or wire winch.

The special feature of absolute shaft encoders is that they assign
a unique, digitally encoded signal to each individual measured
increment.

The method of transducing prevents the incorrect readings, even by a power cut, or a momentary malfunction. After the encoder is still connected, or the power is reconstituted, the position is able to provide readings. It is not necessary to move with a position of reference, because it is for encoders center type by incremental.

Catalogue of such Absolute Shaft encoders can be from here.