Precision steer wheel control system with external solenoid

An apparatus for holding at a center position at least one steerable member mounted on a frame means for movement to either side of the center position, said apparatus comprising linkage means of variable length extending between said steerable member and said frame means wherein the length of said linkage means defines the center position, accumulator means for holding a supply of fluid, and means for pressurizing the fluid held in said accumulator means, said linkage means comprising centering means for providing a resistance force resisting steering forces tending to move said steerable member to either side of the center position, and trim means for transmitting said steering forces to said centering means;

• wherein said centering means comprises:
• centering cylinder means and centering piston means cooperating with said centering cylinder means to form first and second centering chambers, said centering piston means being arranged for movement in a first direction for compressing fluid within said first centering chamber and for movement in a second direction for compressing fluid within said second centering chamber,
• centering rod means arranged for movement with said centering piston means to either side of a neutral position corresponding to said center position,
• and centering fluid means for providing fluid communication between said accumulator means and said centering chambers so that a pressurized centering fluid in each of said first and second centering chambers provides said resistance force by biasing said centering piston means toward said neutral position upon movement of said steerable member toward either side of said center position;
• wherein said trim means comprises:
• a trim piston and trim rod means arranged for movement with said trim piston and providing a portion of said linkage means,
• trim cylinder means providing a portion of said linkage means and cooperating with said trim piston to form first and second trim chambers one on each side of said trim piston,
• and trim fluid means for providing a flow of trim fluid between said centering fluid means and each of said trim chambers and comprising trim valve means operable between a closed position for preventing said trim fluid flow so that said trim piston is held in a locked position in said trim cylinder, and an open position for allowing said trim fluid flow so that said trim piston is free to move away from said locked position in response to said steering forces, said trim piston movement causing trim fluid flow to one of said trim chambers and trim fluid flow from the other of said trim chambers to change the length of said linkage means for trimming movement of said steerable member to another center position;
• and wherein said trim fluid means further comprises:
• a first supply passage in said intermediate head member for communicating with at least one of said centering chambers to supply trim fluid to said first trim chamber,
• a second supply passage in said intermediate head member for communicating with at least one of said centering chambers to supply trim fluid to said second trim chamber,
• a first return passage in said intermediate head member for communicating with one of said trim chambers to return trim fluid to said centering fluid means,
• a second return passage in said intermediate head member for communicating with the other of trim chambers to return trim fluid to said centering fluid means,
• a first supply valve for preventing reverse fluid flow in said first supply passage,
• a second supply valve for preventing reverse fluid flow in said second supply passage,
• a first return valve for preventing reverse fluid flow in said first return passage,
• a second return valve for preventing reverse fluid flow in said second return passage;
• and actuating means activatable from a remote location for operating said trim valve means between said closed and open positions, said trim cylinder means and said centering cylinder means being interconnected by an intermediate head, and said trim valve means and said actuating means being carried by said intermediate head.