Electro-pneumatic positioners are mechanical devices that control the position of a pneumatic actuator based on digital input signals provided by a programmable controller, such as a programmable logic controller. Generally, the controller is connected to an I/P (current to pressure) converter, which is in turn connected to one or more control valves. The I/P converter controls the control valves that are each in fluid communication with both a supply of pressurized fluid, such as pressurized air, and one or more volumes within the pneumatic actuator. Based on a command from the controller to the I/P converter, the I/P converter opens the control valve to allow the pressurized fluid from the supply into one of the volumes of the pneumatic actuator. This pressurized air acts on the surface of a piston defining part of the volume within the pneumatic actuator, thereby moving the piston and an actuator arm coupled to the piston into a desired position.
Spool valves and poppet valves are commonly used as control valves in electro-pneumatic applications. Generally, spool designs have a compact form factor due to the linear displacement of a cylindrical spool element within a housing. Poppet valve assemblies can be used in a wide range of operational environments, but can have a larger form factor than spool designs. Poppet valve assemblies generally include a combination of two normally open poppet valves and two normally closed poppet valves. A pair of valve plugs are coupled to a single axially-displaceable shaft to form the first set of normally closed and normally open poppet valves. A second shaft having a second pair of valve plugs forms the second set of normally closed and normally open poppet valves. When each shaft is in a first valve position, the valve plug for the normally closed poppet valve is sealingly engaged with a corresponding valve seat, and the valve plug for the normally open poppet valve is disengaged from a corresponding valve seat. When the shaft is axially displaced to a second shaft position, the valve plug for the normally closed poppet valve is moved to a position such that it is disengaged from the corresponding valve seat, and the valve plug for the normally open poppet valve is moved to a position such that it sealingly engages the corresponding valve seat. When the shaft is axially displaced to return to the first shaft position, the valve plug for the normally closed poppet valve is moved to a position such that it sealingly engages the corresponding valve seat, and the valve plug for the normally open poppet valve is moved to a position such that it is disengaged from the corresponding valve seat. The cycle repeats as the valve shaft is axially reciprocated from the first position to the second position, and back to the first position.