Blade control system for bulldozer

A blade control system for a bulldozer, comprising:

• a bulldozer having a tractor body portion defining a longitudinal axis, a blade of said bulldozer for performing ground leveling work or grading work, means for controlling a vertical position of the blade during the work, and means responsive to said controlling means for moving said blade in a vertical direction;
• light projecting means for forming over a predetermined ground area a horizontal optical reference plane or an oblique optical reference plane inclined at an arbitrary angle, said light projecting means being located at a position remote from said bulldozer and in light communication therewith;
• a light receiving means mounted on said tractor body portion of said bulldozer for detecting said optical reference plane formed by said light projecting means to issue a level signal to said controlling means, whereby said controlling means controls said vertical position of said blade in response thereto, said light receiving means comprising at least a pair of photo receivers arranged along said longitudinal axis of said tractor body portion of said bulldozer and spaced apart from each other and forming a tractor body reference plane, said controlling means being responsive in part to a distance between the tractor body reference plane and a tractor bearing ground surface to calculate vertical distance to be moved by said blade, said controlling means being responsive in part to a first output signal issued from one of said photo receivers at a first location on said reference plane, and a second output signal issued from the other of said photo receivers at a second location on said reference plane, said second location being longitudinally spaced from said first location, said controller.