| 摘要 |
In a measuring device such as a coilable tape rule, the movable elongated measuring element or blade is provided with a series of optical markings to indicate passage of distance. Photoelectric sensing means in the form of a pair of sensors carried by the stationary element such as the tape rule housing detects shifts in light level or transitions provided by the markings when the blade is illuminated and moved to traverse a distance being measured. A phase relationship of the sensors and the markings is utilized by logic circuitry to provide signals indicative of both amount and direction of movement. The logic signals are counted to indicate net distance traversed by the blade which indication is converted to units of measure and visually displayed. The blade can include conventional graduations and related indicia along with the optical markings so as to be both human readable and machine readable. Duplicate pairs of sensors are provided and signals associated with both pairs are compared for error detection, and the error detection circuitry has an externally applied reset controlled according to signals associated with the sensor pairs. |
