Automatic depth control for endless chain type trencher

Claim 1

In a trencher of the type having an elongated digging frame having one end thereof pivotally mounted on a vehicle so that the other end is free for movement in a vertical plane parallel to the path of travel of the vehicle, said digging frame having two spaced sprockets traversed by an endless chain carrying a plurality of spaced digging scoops, means on the vehicle for driving the forward sprocket and power means for varying the pivotal angle of said digging frame relative to the horizontal, thereby determining the depth of the trench dug by said blades, the improvement comprising: 1. means for periodically sweeping a laser beam over the working area where the trench is to be dug, said beam defining a reference plane of known height relative to the desired depth of the trench; 2. an upstanding mast having its bottom end pivotally mounted directly on said digger frame adjacent said other free end thereof and above the rear sprocket for movement in a vertical plane; 3. means for maintaining said mast in a true vertical position irrespective of the angular position of said digging frame whereby the height of said mast varies as a trigonometric function of said pivotal angle and not directly proportional to the trench depth; 4. laser beam sensors carried by the top of said mast; 5. control circuit means responsive to said sensors for operating said power means to vary the angular position of said digging frame to maintain the trench depth at said desired level beneath said beam reference plane and