A control valve is a valve with a power positioning actuator for moving the valve plug to any position relative to valve port or ports in response, and in proportion to, an external input received.
The opening or closing is commonly operated by electrical, hydraulic, or pneumatic actuators. Usually a modulating valve can be set to any position between fully open and fully closed; positioners are used to ensure the valve attains the desired degree of opening. Air-actuated valves are used for their simplicity as they only require a compressed air supply.
The pneumatic control signal is traditionally set on a pressure range of 3-15 psi or an electrical signal of 4-20 Ma for industry, or 0-10 V for HVAC systems. With development of cheap industrial microprocessors, the electrical signal offers a “SMART” communication signal with Hart or Fieldbus protocols or Profibus protocols.