A valve positioner
is a valve actuator accessory that keeps a valve at an intermediate position
in response to a control signal.
Valves that use pneumatic actuators would normally come with pneumatic positioners prior to the invention of the modern electronic DCS or Digital Control System. The control signals were typically 3-15 psig where 3 psig would indicate fully closed and 15 psig fully open. However, this could be reversed if the set point calculation was easier using the reverse convention. A 6-30 psig range has also been used.
Since the invention of of the electronic DCS, a scale of 4 to 20 mA has been standardized. Again, most positioners allow you to define that 4 mA is either fully open or fully closed. In our digital age, recent developments such as HART protocol and low power CPU's allow the development of the valve positioner as a computer much like a car's engine computer. The HART protocol is a method of overlaying a digital signal on top of the 4-20 mA signal for remote configuration and allowing the operator to retrieve historical trending data from the device. In the most modern plants, the 4-20 mA signal has been abandoned and the valve positioner has become an TCP/IP device on the plant network.