Light reference system for railroad track surveying

Claim 1

In a reference system comprising a light beam projector, a beam receiver spaced therefrom, a road condition sensor including a single edged shadow board therebetween, the improvement wherein the receiver includes at least two photo-electric cells physically closely spaced apart in the direction of shadow board motion; and means for electrically connecting the outputs from the cells whereby to produce a command signal to stop shadow board movement when the shadow board is positioned at a datum position in the transmitted beam so as to distribute the transmitted light on the cells with a predetermined difference and in which a deadband producing transducer is provided physically closely spaced apart from the said two cells in the direction of the shadow board motion and inwardly of the shadow board edge when it is in datum position; means electrically connecting the output from the deadband producing transducer to the output from the cell close to the shadow board edge, when it is in datum position, to provide a command signal to commence shadow board movement into the beam when said deadband producing transducer is exposed by a predetermined amount to the transmitted beam, which means electrically connecting the output from the deadband producing transducer to the output from the cell close to the shadow board edge, include amplifying means for that cell and for said transducer and a summing and weighing network electrically connected to receive the output signals from each of the amplifying means and adapted to produce an operating signal when the output signals from the amplifying means are in a predetermined ratio to one another; and a trigger circuit activated by said operating signal and providing the command signal to commence shadow Board movement into the beam.