a hydraulic actuator operates either an intake or an exhaust valve for an engine cylinder . a driver piston is adapted to be operably connected to open and close the engine cylinder valve . an electrically driven operator produces movement of a valve spool which controls flow of fluid to and from the driver piston . a feedback mechanism is coupled to the valve spool and responds to movement of the driver piston by moving the valve spool into a position at which fluid flows neither to nor from the driver piston . the feedback mechanism ensures that the stroke of the hydraulic actuator is proportional to the magnitude of the electric current applied to the operator regardless of variation of the fluid pressure applied to the driver piston .