An actuator is a machine, or rather a part of a machine used to convert externally available energy into motion based on the control signals.  Much like how hands and legs enable humans to move around and perform actions, actuators let machines perform various mechanical movements. The topic for discussion for this article is actuators. We will explain what is an actuator, how actuators work, and what are the different types of actuators used in industrial and domestic applications.
From the perspective of systems engineering, functions of any engineering product can be classified into three distinct categories; the collection of input, processing and producing an output.
For electromechanical systems, the input is detected and measured by a device called a sensor. The task of a sensor is to sample the signals available to it and convert them into a form understandable by the system. The system then processes the information and decides how to respond. But how exactly does a system respond?
The answer is, with the help of an Actuator. Typically, an actuator consists of:
The most apparent and basic classification of actuators is based on the type of motion that it produces.
The actuators that can provide a circular motion at their output can be classified under the category of rotary actuators. When it comes to rotational motion, it is hard to think of any other device than the motors, which we shall discuss in the next section of this article.
The actuators that can provide motion in a straight line at their output can be classified under the category of linear actuators. Hydraulic or Pneumatic actuators are the most common linear actuators used in the industry. We will also discuss these devices in detail.
The energy source can be another means of classification for the actuators.
Electromagnetic actuators make use of electricity and magnetism to perform actuation. These actuators are among the most commonly used actuators in the industries.
Servo drives can be powered by an AC or DC power supply and consist of a motor, feedback unit, control unit, and sometimes a gearbox. The working of a servo motor greatly differs from that of ordinary AC or DC motors. To operate a servo motor, a control signal is required in addition to the power.
Initially, when a voltage is applied to the terminals of a servo motor, it begins to rotate. The position of the shaft is continuously monitored by a rotary encoder, and the voltage-current levels are kept in check by the voltmeter-ammeter combination. The controller then computes the motor’s actual speed, compares it with the target speed, and adjusts the voltage and current levels to reduce the error between the target speed and actual speed.
Stepper motors are used for applications where the angular position of the shaft needs to be accurately controlled. The control scheme of the stepper motor is simple, accurate and doesn’t require any feedback. This is the reason why they are often more affordable.
The stator of the stepper motor contains multiple teeth, each acting as a pole for the rotor. When a particular pole or a set of poles are energised, the rotor reorients itself to allow maximum MMF to pass through it. When the next step of the poles is energised, the rotor shifts its position. This allows the rotor to complete a revolution in several distinct steps, and that’s how the motor gets its name.
A solenoid actuator consists of a conducting coil wound on a ferromagnetic core with a flat head on one side and a spring connected on the other. The whole apparatus is placed in a hollow cylindrical body. Whenever current flows through the wire, the coil acts as an electromagnet, attracting the ferromagnetic core in one direction and compressing the spring during the process.
Once the power supply is removed, the spring pushes the core back to the original position. The strength of the actuator depends upon the number of turns in the coil. The setup looks and acts a lot like a piston.
The actuators that make use of liquids or gasses are called fluid power actuators. On a very superficial level, we can think of a fluid power actuator as a moving disk inside a hollow cylinder filled with fluid forming a piston. The movement of the disk appears as the motion of the actuator. Advanced fluid actuators with dual-acting cylinders make use of fluid for both extension and retracement strokes.
These actuators make use of liquids as a driving force to produce mechanical work. Hydraulic Actuators are probably the most widely used linear actuators in real-life applications. These devices are used when stable, but high actuating thrust/forces are required in a small region.
The design and construction of pneumatic actuators are very similar to that of hydraulic actuators. The difference is that instead of using a liquid, energy from compressed gases or vacuum is used to facilitate the actuation process.
These actuators are used to interconvert rotary and linear motion in machines. Some examples of mechanical actuators are rack and pinion arrangements, crankshafts, gears, pulleys, and chains.
Thermal actuators make use of materials that expand or contract by the application of heat. These actuators can be used to sense temperatures and shut off a supply to the system they are a part of. Thermal actuators combine the functions of a temperature switch and an actuator in a single package.
Apart from the commonly used actuators, some actuators are still under research and find their application in limited fields.
Piezoelectric materials exhibit a contraction/expansion whenever a voltage is applied to them. By applying a controlled signal, this property of piezoelectric materials can be used to build actuators for small but highly precise and rapid positioning mechanisms. 
Shape Memory Alloys (SMAs) undergo a change in their molecular arrangement when they are heated or cooled. When a force is applied to alloys like Nitinol (Nickel-Titanium), they experience a deformation that can be reversed with heating.
Heating can be done directly by application of thermal energy or with the help of electric power. This property of SMAs can be used to build actuators 
It gets challenging to downsize conventional actuators like electric motors beyond a certain limit, making them unsuitable for miniature machines. This is where Supercoiled Polymer Actuators (SPAs) come in. Supercoiling is a property of DNA strands that makes it possible for them to relieve stress by twisting around themselves. SPAs are inspired by a similar design that lets them reversibly change their shape and size when stimulated. These structures respond quickly and can last for millions of cycles. 
Hydrogel actuators demonstrate a change in their shape with changes in the temperature, light, pH and concentration of certain substances. The fact that hydrogels can be effective only in aqueous medium limits their applications to some specific specialised fields.
Research shows that some hydrogel actuators can be optically and sonically camouflaged as their properties are similar to that of water.  
An actuator that can generate sufficient force has suitable load-speed characteristics, works in the operating range with high efficiency, and comes with a robust design is considered ideal for a given application.
Industrial automation and robotics are the two fields where it is just impossible to imagine getting anything done without actuators. These parts enable production machines to move from one place to another and grab objects. Actuators are also widely used in heavy construction equipment and agricultural machinery to enable several different sets of movements. Another beautiful application of actuators can be in solar panels. As the sun rises and sets during the day, the solar panels equipped with actuators keep changing their angle to harness maximum solar energy.
Coming to household applications, actuators can be found in almost every smart home appliance, from furniture to robotic vacuum cleaners that require any sort of manoeuvre. A lot of toys too contain some small actuators built-in them. The applications are endless.
Actuators are present in almost all the industrial and domestic appliances we use today. Yet innovations in the domain get little coverage in media compared to cutting edge technologies like Artificial Intelligence (AI), Blockchain, Internet of Things (IoT), 3D printing, etc.
Over the next couple of years, new actuators with better performance parameters will be conceptualised, and the ones that are under research will make their way to industries.
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