Control valves are valves used to control conditions such as flow, pressure, temperature and liquid level by fully or partially opening or closing in response to signals received from controllers that compare a "setpoint" to a "process variable" whose value is provided by sensors that monitor changes in such conditions
The opening or closing of control valves is done by means of electrical, pneumatic or hydraulic systems. Positioners are used to control the opening or closing of the actuator based on Electric, or Pneumatic Signals. These control signals, traditionally based on 3-15psi (0.2-1.0bar), more common now are 4-20mA signals for industry, 0-10V for HVAC systems, & the introduction of "Smart" systems, HART, Fieldbus Foundation and Profibus being the more common protocols.
Contact - The Valve Pipeline
Quarter-turn valves were invented because you only need to turn them a "quarter-turn" or 90* to open or close them (as opposed to gate/globe valves which are multi-turn and take many turns to do the same operation)