Abstract: A high resolution servo actuator is responsive to an electronic control or manual control. An output shaft having fast response and infinite positioning for stroking a manual servo valve on a pump is rotated by hydraulic power. A set of hydraulic working cylinders and centering cylinders are connected through a rack and pinion drive to a shaft. Centering cylinders utilize stop tubes to prevent its pistons from traveling past the center position and applying a constant force to center the shaft. A directional valve controls the flow of hydraulic fluid to the working cylinders for driving the shaft throughout its range of travel. An electrohydraulic servo valve converts the electronic control signal into hydraulic flow to the actuator. Shaft position feedback is developed and connected to the electronic control.