An actuator in its broadest definition is a device that produces linear or rotary motion from a source of power under the action of a source of control.
       Actuators take fluid, electric or some other source of power and convert it through a motor, piston or other device to perform work. Basic actuators are used to move valves to either fully opened or fully closed positions. Actuators for control or position regulating valves are given a positioning signal to move to any intermediate position with a high degree of accuracy. Although the most common and important use of an actuator is to open and close valves, current actuator designs go far beyond the basic open and close function. The valve actuator can be packaged together with position sensing equipment, torque sensing, motor protection, logic control, digital communication capacity and even PID control all in a compact environmentally protected enclosure.

       Figure 1: Basic electric actuator component

       As automation is adopted in more facilities, physical work is being replaced by machines and their automatic controls. The need for valve actuators to provide the interface between the control intelligence and the physical movement of a valve has grown. There is an important need for the increased working safety and the environmental protection that valve actuators can provide. Some areas are hazardous or hostile to human beings. In these circumstances an automated actuation device can reduce the risk to the individuals. Certain critical valves need to be opened or closed rapidly in the event of emergency circumstances. The valve actuator can prevent serious environmental catastrophes as well as minimize damage to facilities in such circumstances. With some processes requiring high pressures and large line sizes, the amount of power required to open or close a valve can be significant. In these circumstances the enhanced mechanical advantage and application of high output motors can facilitate easy operation of large valves.