Automatic grade rod and method of operation

Abstract

A grade rod and method of operation for use in a surveying system employing a laser beam which is rotated in a plane. A flexible tape formed with a graduated scale is mounted for movement in a loop within the housing. A slider on the tape carries a pair of closely spaced-apart light detectors as well as electronic circuitry which discriminates between laser beam pulses and sunlight. Output signals from the circuitry on the slider are fed into a control circuit in the housing through a flexible cable which is carried in a traveling loop. Scale indicators are mounted for movement with the slider along a fixed scale which is provided on the housing, and a manually operated cursor is mounted for movement along the housing. The control circuit includes means for driving the tape and slider in a search mode until a light pulse from the laser hits a detector, and then in a track mode for centering the detectors on the plane of the laser beam. A circuit is provided for returning the slider to its home position should the beam be interrupted for a period of time, and also for returning the slider to its home position should no laser beam be detected. A stutter-start circuit is employed for operation in a track mode for a short time period after activation before switching to the search mode.