A valve actuator is the mechanism for opening and closing a valve. Manually operated valves require someone in attendance to adjust them using a direct or geared mechanism attached to the valve stem.
A butterfly valve is a quarter-turn valve used to regulate flow. A metal disc in the body of the valve is positioned perpendicular to the flow in the closed position, and rotated one quarter of a turn to be parallel to the flow in the fully opened position. Intermediate rotations allow regulation of liquid flow.
An actuator is a component of a machine that is responsible for moving and controlling a mechanism or system, for example by opening a valve. In simple terms, it is a "mover". ... When it receives a control signal, an actuator responds by converting the signal's energy into mechanical motion.