Actuators provide, where necessary, the output to support the IoT functionality, by converting an electrical power input into a type of motion. They can be split into several categories, depending on the type of motion generated and the transmission medium used for converting the electrical input. The aim of this section is to provide a high-level taxonomy of actuators, together with indication of the likely applications to encounter. Given the nature of the interaction with an IoT system and the fact that IoT inherited some of the components from industrial processes that it allowed replacing, the range of output actions is significantly wider than the sensed inputs and hence not suitable for an exhaustive review.

There are two main types of output movement: linear or rotary. The linear actuators will produce movement along a direction or may act as a push-pull mechanism within a system. The rotary ones will produce a rotational movement, typically through the use of a motor.

Based on the type of system that they require interaction with, actuators can be split into two five categories: electrical, magnetic, mechanical, hydraulic, or pneumatic. Each of these comes with a rather separate range of applications, given the physical characteristics of the employed mechanism. Given the stimulus in the IoT domain is invariably electrical, only the first two categories (electrical and magnetic) are relevant in this context.

Electrical actuators are perhaps the most versatile ones, as they allow interacting with a variety of other systems, typically through an electromagnetic switch/latch or motor. Due to the relatively high level of control, such applications will provide significant accuracy in terms of movement; the typical example for accurate movement is a step motor, which allows rotational movement for a specific angle, allowing a precise positioning of a device in a rotational plan. A rotational electric actuator can also be used as an (equally precise) linear actuator through the use of a lead screw within a shaft or drive nut.

The magnetic actuators include either distant magnetic interaction or magnetically controlled active materials [45]. The distant interaction requires either a moving coil, a moving magnet, or a moving iron. A mobile coil will start moving in a static magnetic field when electricity is applied to it; similarly, a mobile magnet placed between two magnet poles can be switched between poles through an electrical coil stimulation. The moving iron actuators are based on the tendency of a soft magnet placed into a coil to move so that the resulting magnetic energy is minimal. The magnetically controlled active materials include magnetorestrictive materials, which deform when affected by a magnetic field, and MRF (magnetorheological fluid), which solidify when subjected to a magnetic field.

Beyond their taxonomy, it is worth pointing out some of the typical usage of actuators as part of the IoT applications. There are three areas that have seen a significant upsurge due to the advent of IoT actuators: switches and latches, robotic controls, and accurate positioning.

Level Sensors

Level sensors are devices used for taking level measurements for liquid substances. Such substances may include water, petroleum, fuel, etc. Level sensors can be integrated for IoT applications, such as environmental monitoring, for sensing changes in sea level, providing early warnings for floods, in the manufacturing sector for monitoring the production process with tank liquid level readings and even in the smart city application area, in smart home appliances such as fridges. Level sensor capabilities may include monitoring the level of nonliquid substances, which however behave in a liquid-like manner. Such substances may include, for instance, powders.

Level sensors convert analog data, w'hich is the level of the substance measured, in a digital format. These sensors can be classified into two main categories:

  • • Point level sensor: These level sensors are used to provide information about whether the substance has reached a certain threshold sensing point or not.
  • • Continuous level sensor: Contrary to point level sensors, continuous sensors constantly provide information regarding the amount of substance in the container.

It is possible to classify level sensors even further, based on the means they take the measurements. For instance, if the sensor must be in contact with the liquid in question in order to take the level measurements, then this sensor is classified as an invasive sensor. On the other hand, sensors that employ ultrasonic or microwave technology in order to perform their tasks, are called noncontact sensors.

Gas Sensors

Gas sensors are used in order to sense various qualities in the air, or detect the concentration of different gases, such as carbon monoxide, carbon dioxide, oxygen, etc. Their integration in IoT can be observed in many application areas. For instance, such sensors are be applied in smart cities and smart homes for carbon monoxide and other toxic gasses detection. Additionally, such technology can benefit the environment, by constantly monitoring air pollution levels and therefore aid in preventative measures to be rendered. Oxygen level recordings can be proven to be beneficial for patients in the healthcare and assisted living application areas. Additionally, in the

Public Safety application area, oxygen monitoring sensors can be employed for vital checks in rescue crew.

Gas sensors may be divided into potable and fixed-type sensors. Portable sensors are usually battery-powered wearables, which can provide feedback to the user if the gas amount to be detected exceeds some levels. On the other hand, fixed-type sensors are integrated within the environment.

Water Quality Sensor

Water quality sensors can be used to measure the concentration of different elements in the water, such as pH. temperature, chlorine concentration, etc. Such sensors can be widely used for multiple IoT application domains; however, integration would be highly beneficial in the environmental area. With such technology, constant water pollution monitoring would be possible; therefore, it would be easy to detect timely water contamination. Proper measures can be taken in order to avoid further damage. Additionally, water quality sensors can be used for quality control purposes in the agriculture application area.

Most commonly used water quality sensors include chlorine sensors, which can monitor chlorine percentage in water, pH sensors, which can indicate the acidity of water, etc.

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