Tape for measuring length - is wound around drum whose rotation is optically monitored to determine exact length unwound

摘要

The distance measuring equipment includes a flexible tape (4) which is wound spirally on to a cylindrical drum (5). The tape does not wrap over itself, but successive turns are spaced down the drum. The drum is mounted in a frame (12), and on the end of the drum axle is a flat round disc (11). The disc is optically encoded, and a light source and detector are provided inside a shield which encloses the disc to detect movement of the disc. The rotation of the disc is electronically counted in order to determine its angular position, and hence the length of tape unrolled.