Image Number 3 for United States Patent #6651686.
An actuator system for a system valve exposable to a source of system pressure. A piston within a sensing vessel is caused to move longitudinally from a first position to a second position in response to a designated system pressure. A first piston stem carried by the piston is mechanically coupled to an actuator control valve, which is interposed in a fluid path between an actuator and a source of actuator pressure. Movement of the piston from its first to second position causes the coupled actuator control valve to block the fluid path linking the actuator and its pressure source, thereby causing a predetermined change in pressure at the actuator. As a result, the actuator shifts from its unactuated to its actuated state, thereby causing the system valve to shift from a first system valve position to a second system valve position.