An actuator is a device that makes something move or operate. An actuator initiates movement of a machine part upon obtaining a part of the input energy as a feedback control signal. In other words, an actuator converts the energy into physical motion or mechanical movement. The main purpose of an actuator is to control the movements within the machines.聽

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On the basis of the movement of the mechanical part, actuators can be divided into two broad categories as listed below:

If an actuator tends to move an object along a straight line, then such an actuator is known as a linear actuator. A simple linear actuator typically consists of a nut, a cover, and a sliding tube. The sliding tube transfers motion to the object, whereas the combination of nut and cover holds the actuator in place.

Rotary actuators help the object connected to it or a part of the machine rotate about its axis or simply allows it to exhibit a circular motion. It makes use of a pneumatic cylinder that provides the necessary torque force required to turn or rotate the mechanical part of the machine periodically.聽

A rotary vane actuator consists of a vane attached to a central spindle. The compressed air tends to push the vane, which in turn causes the spindle to rotate. The air behind the vane gets released through a port. When the vane reaches its precalibrated angle of rotation, the airflow is reversed, causing the spindle to move back to its original position. The process is then repeated again and again after regular intervals. As compared to rack and pinion actuators, the rotary vane actuators generate less torque and, therefore, can rotate up to a limited distance. These types of rotary actuators are generally used for lighter loads.

Rack and pinion actuators consist of two cylinders and two pistons interconnected with the help of a rack. A pinion gear is present in contact with the rack gear. When there is a pressure difference between the ports of the actuator set up, the rack is able to move to and fro, causing the pinion gear to rotate accordingly. When the rack moves completely towards one side, the pinion gear rotates in the clockwise direction. When the rack moves back to its original position, the pinion gear tends to rotate in the counter-clockwise direction. Rack and pinion actuators offer greater torque as compared to the rotary vane actuators. They are best suited for the applications that require more speed and less wear such as automation, pharmaceutical industries, etc.

On the basis of the source of energy used to power an actuator, there are eight types of actuators:

Electric actuators make use of electrical energy to operate. The signal transmission rate of electric actuators is quite high. Also, they are easy to operate and offer high accuracy and sensitivity. Electric actuators are classified into two broad categories, namely, electromechanical actuators and electrohydraulic actuators.

Electromechanical actuators convert the input electrical energy into linear or rotary movement. In certain cases, an electromechanical actuator is also used to exhibit a combination of both linear and rotary motion in a mechanical component.聽

Electrohydraulic actuators are powered with the help of electrical energy. The output of electrohydraulic actuators is fed to a hydraulic accumulator. The accumulator then generates the force used to move the object.

Hydraulic actuators are powered by a variety of liquids or fluids. They make use of a fluid-filled cylinder with a piston suspended at the centre. A hydraulic actuator typically helps the objects to move along a straight line and exhibit a linear motion.聽

Pneumatic actuators use compressed air for their operation. Pneumatic actuators are highly reliable and accurate. The use of pressurized gases to induce mechanical movement is usually preferred because it improves precision and allows the object to exhibit fine motion. Even a slight change in pressure can generate a huge amount of force. Pneumatic actuators are used in applications where it is required to start or stop a machine quickly.

Thermal actuators convert thermal energy into kinetic energy. These types of actuators contain a temperature-sensitive element that expands and contracts as per the temperature variations in the surroundings. The expansion and contraction of the temperature-sensitive element cause the piston to move. Thermal actuators are reliable and best suited for hazardous locations. Thermal actuators are used in applications such as temperature control, freeze protection, etc.

Mechanical actuators are one of the most used actuators in everyday life. For instance, a pulley system, gears, rack and pinion system, etc., are prominent examples of mechanical actuators. The muscular force is applied to the input of the mechanical actuators, the actuator then provides a significant amount of leverage to the efforts applied, and finally, the desired movement of the object is achieved.聽

Supercoiled polymer actuators are majorly used in robotics and in the medical field to create prosthetic limbs. They make use of a coil that contracts and expands upon heating or cooling, thereby using pressure to induce motion.

Magnetic actuators work upon the principle of magnetic actuation based on the Lorentz force equation. The force required to cause the part of the machine to get displaced from its original position with the help of magnetic actuators is produced by placing a current-carrying conductor in a static magnetic field. The magnetic field produced around the current-carrying conductor interacts with the static magnetic field and generates a high magnitude force.

Piezoelectric actuators are the actuators that take electronic signals as input and are used to control the mechanical movement of a device. Piezoelectric actuators do not use guides and pistons to transfer energy, instead, they make use of the pressure variations caused due to compression and expansion of materials like ceramic to produce high-frequency strokes. These types of actuators have a high-frequency response and greater displacement resolution.

Sliding doors are one of the most prominent examples where one can observe the real-life application of an actuator. The actuators used in sliding doors obtain their input from the motion sensors. When the sensor detects a movement, the actuator gets activated and causes the panels of the sliding doors to move sideways, thereby controlling the movement of the door.

Electric car seats form a classic example where the application of actuators can be observed easily. The position of the car seat can be adjusted with the help of electrically powered motors. To move the seat back and forth, the switch placed on the side is turned on. The circuit then gets completed, and the electrical signal flows through the circuit wires to power the motor. The motor acts as the actuator, which helps to control the movement of the seat.

Actuators find their prime application in various industries. For instance, automobiles, chemicals, pharmaceuticals, food, and various other industries require different types of actuators to convert energy into motion. Some of the most commonly used actuators in such industries include electric motors, pneumatic control valves, pneumatic motors, stepper motors, etc.

The escalators were initially called revolving stairs. The motion is induced in the escalators with the help of an actuating mechanism. The actuating mechanism used to control the movement of an escalator obtains its input from the motion sensor. Once the motion gets detected by the sensor, a signal gets transferred to the actuating mechanism, which further activates the movement of the escalator belt.

The advanced braking systems used to stop moving heavy vehicles such as a bus, truck, etc., generally consists of pneumatic actuators. Pneumatic brakes are also known as air brakes. Using an actuator-based braking system in place of the conventional braking system is quite advantageous as it improves the optimal usage of the allocated space and promotes energy conservation.

A variety of gym equipment and exercise machines make use of actuators to control their operation. The common types of actuators used in such applications include the pneumatic actuator, electrical actuator, etc. For instance, the actuator present in the internal machinery of a treadmill is used to convert electrical energy into mechanical movement of the belt. Similarly, a robotic bicep exercise machine also uses electric actuators.

Windshield wipers usually make use of linear actuators for their operation. The battery of the vehicle is connected to the wiper switch, which is further connected to the wiper motor. The wiper motor is attached to the actuating mechanism and the wiper blade. It forms an electrical circuit that is required to control the operation of windshield wipers. When the wiper switch is turned on, the circuit gets completed. This causes the actuating mechanism to get activated and induce linear and periodic motion into wiper blades. Hence, one can easily picturise the working of a linear actuator by looking at a pair of wiper blades, sweeping dirt and water off the windshield.

The motor present inside an electric fan is yet another example of an electric actuator. This is because it converts the electrical signal into the mechanical motion of the fan blades. It can easily be observed that the blades of a fan tend to exhibit a rotary motion, which is why rotary actuators are used in the internal circuitry of the fan.

One of the major applications of supercoiled polymer actuators lies in the construction of artificial limbs and muscles. Such advanced prosthetic limbs are made up of artificial supercoiled polymer threads. The property of such polymers by virtue of which they expand or contract upon heating or cooling is utilized in this particular area of application. The use of supercoiled polymer actuators in prosthetic limbs is advantageous because it has a high power-to-weight ratio and a large dynamic range. It also enables the user to achieve fast controllable actions.

A number of consumer electronic items make use of thermal actuators. For instance, washing machines, dishwashers, etc., make use of thermal actuators to control and operate their dispenser latches. The thermal actuators, in this case, are used in combination with a thermistor that has a positive temperature coefficient. This combination of actuator and thermistor is known as a wax motor.

A pneumatic mailing system makes use of cast-iron tubes bolted together with the help of a bolted flange. The tubes are buried deep inside the ground. Rotary blowers and air compressors are used to provide the necessary air pressure to push the mails through the tubes. The pneumatic mailing system has been obsolete now, but it constitutes a classic example of pneumatic actuators used in real life.