Fluid powered actuator system

A fluid powered piston actuator system comprising a pressure source (PS), first and second piston actuators (10, 11) first and second valve devices (15, 16) for controlling fluid flow in respective first (21) and second (28) flow paths between said pressure source (PS) and said first and second piston actuators (10, 11) said valve devices (15, 16) including operably interconnected control elements (20, 27) for increasing or decreasing flow through said first flow path (21) and for simultaneously decreasing and increasing flow through said second flow path (28), means (35, 36) responsive to a difference between intermediate pressures in said first and second flow paths (21, 28) for urging said control elements (20, 27) in directions to reduce said difference, third (40) and fourth (42) piston actuators operable in response to a servo pressure signal for moving said control elements (20, 27) in respective opposite directions to a pair of shut off positions at which fluid flow is shut off in both of said first and second flow paths (21, 28), and a control valve (45) in fluid communication with said third (40) and fourth (42) piston actuators, said control valve (45) comprising a spool 50 movable in response to a difference in pressures in said first and second flow paths (21, 28), said spool (50) having first and second lands (58, 59) and said control valve (45) including means actuable by movement of the spool (50) for selectively applying said servo pressure signal to both sides of a selected one of said lands (58, 59) and to one of said third and fourth actuators (40, 42) and for simultaneously applying a lower pressure to both sides of the other of said lands (58, 59) and to the other of said third and fourth actuators (40, 42) to move said elements (20, 27) to one of said shut off positions.