Actuator

An actuator is something that converts energy into motion. It also can be used to apply a force. An actuator typically is a mechanical device that takes energy — usually energy that is created by air, electricity or liquid — and converts it into some kind of motion. That motion can be in virtually any form, such as blocking, clamping or ejecting. Actuators typically are used in manufacturing or industrial applications and might be used in devices such as motors, pumps, switches and valves.
Perhaps the most common type of actuator is powered b
y air and is called a pneumatic cylinder or air cylinder. 

An actuator also can be powered by electricity or hydraulics. Much like there are air cylinders, there also are electric cylinders and hydraulic cylinders in which the cylinder converts electricity or hydraulics into motion. Hydraulic cylinders, which use liquids, are often found in certain types of vehicles.
. An actuator is the mechanism by which a control system acts upon an environment. The control system can be simple (a fixed mechanical or electronic system), software-based (e.g. a printer driver, robot control system), or a human or other agent.

Types of Motion

Actuators can create a linear motion, rotary motion or oscillatory motion. That is, they can create motion in one direction, in a circular motion or in opposite directions at regular intervals. Hydraulic and air cylinders can be classified as single-acting cylinders, meaning that the energy source causes movement in one direction and a spring is used for the other direction.

                                                      HYDRAULIC ACTUATORS

Muscles as Actuators

Although actuators typically are discussed in terms of mechanical implements, muscles are sometimes given as an example of actuators. Energy is converted by the muscle into motion. For example, the calories that are in food that a person consumes represent energy that can be used by his or her muscles — which act as actuators — to create motion, such as running, kicking a ball or dancing.