An actuator is a device, which can affect a change in the environment by converting electrical energy into some form of useful energy. Some examples are heating or cooling elements speakers, lights, displays, and motors. The actuators can be classified into three categories. electrical, hydraulic, and pneumatic actuators depending on their operation. Hydraulic actuators simplify mechanical motion using fluid or hydraulic power. Pneumatic actuators use the pressure of compressed air and electrical use electrical energy. 

For example, a smart home system consists of many sensors and actuators. The actuators are used to lock/unlock the doors, switch on/off the lights or other electrical appliances, alert users of any threats through alarms or notifications, and control the temperature of a home (via a thermostat). A sophisticated example of an actuator used in loT is a digital finger, which is used to turn on/off the switches (or anything that requires small motion) and is controlled wirelessly.

The function of the actuator is opposite of the sensor. It behaves like a tool. By interpreting the electrical impulses sent from the control system and converting them into mechanical motion, it actually introduces changes to its physical surroundings by means of a variety of simple actions, including but not limited to opening and closing valves, changing other devices' position or angle, activating them or emitting sounds or light. In simple words, the actuator work like a 'mover'. This consists of a digital-to-analog converter, an output filter, and some sort of amplifier. Increasingly, digital techniques like digital signal processors (DSPs) and class-D amplifiers are being employed in actuators to make them more accurate, responsive, and energy-efficient. 

Sensors and actuators are omnipresent in modern industrial applications, and the emergence of the Internet of Things has opened up completely new possibilities for IoT sensor and actuator applications not only within the industrial sector but also in the field of commercial and domestic use. The Internet of Things has put sensors to work for powerful, cloud-based analytical software to develop intelligent solutions for machines, people, and the environment alike. 

For example, car breaks convert mechanical energy to heat energy. For systems, nearly all sensors take some physical parameters and turn them into electrical signals. Similarly, nearly all actuators in IoT systems take electrical signals and convert them into some sort of physical output. Physical parameters include electrical (voltage, current, power, resistance, capacitance, inductance, frequency, phase, etc.), mechanical (position, speed, acceleration, weight, compass heading, gravity, force, tension, pressure, flow, torque, magnetic field, etc.), acoustic (sound, vibration, seismic, etc.), image (light intensity, cameras, displays, infrared (IR), Light Detection and Ranging (LIDAR, etc.), chemical (potential hydrogen (pH)concentrations, composition, purity, etc.), medical (heart rate, respiration, blood pressure, temperature, electroencephalogram (EEG), etc.) and many more. 

Types of Actuators 

Actuators are small and rarely visible during operation, but the effects of their work can be felt in vehicles, industrial machines, or any other electronic equipment involving automation technologies. They can be separated into four main categories based on their construction pattern and the role they play in a specific IoT environment: 

Linear Actuators: These sensors are used to enable motion of objects or elements in a straight line. 
Motors: This type of actuator helps in the rotational movements of device components or whole objects. 
Relays: This type of actuator defines electromagnet-based actuators to operate power switches in lamps, heaters or even smart vehicles.
Solenoids: These are mostly used in home appliances like locking or triggering mochani they also help as controllers in IoT-based gas and water leak monitoring systems 

Types of Actuators - actuators in iot
Fig. Types of Actuators

Sensor to Accutator Flow - actuators and types in iot
Fig. Sensor to Accutator Flow