Patent ID: 7080700

Claim:
A tractor assembly for moving within a borehole, comprising: an elongate body; a first gripper assembly slidably coupled to said body for selectively gripping an inner surface of the borehole; a second gripper assembly slidably coupled to said body for selectively gripping an inner surface of the borehole; a first propulsion assembly for propelling said body through the borehole when said first gripper assembly is gripping the inner surface of the borehole; a second propulsion assembly for propelling said body through the borehole when said second gripper assembly is gripping the inner surface of the borehole; and a valve system comprising: a gripper control valve having a first position for directing pressurized fluid to said first gripper assembly, said gripper control valve having a second position for directing pressurized fluid to said second gripper assembly; and a propulsion control valve having a first position for directing pressurized fluid to said first propulsion assembly, said propulsion control valve having a second position for directing pressurized fluid to said second propulsion assembly; said propulsion control valve being piloted by fluid pressures in said first and second gripper assemblies, said propulsion control valve moves from said first position to said second position only after said gripper control valve moves from said first position to said second position.