A control valve receives the information from various sensors and transmitters in a control loop and processes that data to manage the control of the fluid parameter. The following figure shows a typical control loop for a control valve. There are many different types pf control valves, such as straight stroke control valve, single seat control valve, cage guided control valve, micro flow control valve, cryogenic control valve, bellow seal control valve, fluorine lined control valve, high pressure control valve, angle stroke control valve, ball control valve, etc. http://www.lozosevalve.com/straight-stroke-control-valve/cryogenic-control-valve/