SEAT BELT RETRACTOR

A seat belt retractor including a spool and a measurement mechanism for determining a measure of the amount of seat belt webbing unwound from the spool. The measurement mechanism includes cycloidal gear transmission. A sensor is provided for detecting movement of a magnet mounted on a sensing gear and a controller may be provided for determining a measure indicating the amount of rotation of the spool and the amount of webbing withdrawal based on the movement of the magnet.