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In an earlier article on “IoT – Convergence of Domains” I talked about the influence that the technology of Internet-of-things has on Industry 4.0. IoT solutions have wrought a revolution and enabled automating processes and monitoring controls that were thought to be almost impossible.

At the heart of all these IoT solutions, the key enabler has been the advancement and evolution of Sensor technology. It's no exaggeration to say that sensors are the key enablers of IoT solutions and hence, they play a significant role in the Industrial Revolution 4.0.

Evolution of Sensors

Though Sensors are of different kinds, owing their functions to both electrical and mechanical means, it’s the former that has had a significant evolution. Electrical and electronic sensors have evolved rapidly with advancements in the semiconductor industry. It all began with the discovery of transistors and the process of doping... Dopants modify the property of the base material and enable the controlled flow of electrons.

Before we move on, what is doping? In common parlance, it refers to using certain performance-enhancing substances by athletes to improve their performances by unfair means over their competitors. In the semiconductor world, this term refers to the addition of minute quantities of foreign substances to alter the electrical behavior of the base material. Formally, doping is defined in semiconductor production as –

Doping is the intentional introduction of impurities into an intrinsic semiconductor for the purpose of modulating its electrical, optical, and structural properties. The doped material is referred to as an extrinsic semiconductor.

A lot of research in material science has helped in the advancement of semiconductor technology. The impure material and the concentration used in doping have different effects on the intrinsic semiconductor or the base material. These advancements in material science have led to the development and evolution of sensors. In short, IoT owes a great deal to advancements in the field of material science.

Broadly, sensors can be classified into two categories – namely, electrical (or electronic) sensors and mechanical sensors. Most sensors that impact IoT solutions are electronic in nature, barring a couple of important mechanical sensors.

Electronic Sensors

Electronic sensors are semiconductor-based devices that convert alterations in the physical environment into electrical pulse (or a potential difference). Ordinarily, these electrical signals are small in magnitude and difficult to process by themselves. They are amplified through an amplifier circuit consisting of an Operation-Amplifier (Op-Amp).  Thus, an amplified signal is then suitable for further processing and analyses.

The nature of the dopant (that is, the doping impurity) and the concentration in the base (or called substratum) determines the characteristics of the device. In all the different types of devices, the alterations are either in the current-carrying capacity of the device or in the resistance of the device.

Mechanical Sensors

Mechanical sensors work on the principle of detecting the tilt or the angular momentum – or some form of change in the mechanical property of the object under study. The detected tilt or angular motion can either be read off directly on a scale or converted to electrical signals for further processing.

Mechanical sensors are used extensively in mechatronics. They can be used to measure

• The tilt of a body (used extensively in aeronautics and aerospace applications)
• Measure the angular velocity
• Vibrations and acceleration sensing
• Study the fluid properties and flow of liquids

Functional classification of Sensors

Based on their usage, Sensors are classified in different ways. Besides the functional classes that were mentioned above in Mechanical Sensors, other functionalities could include:

• Gas Sensor:  Sensors used to detect certain specific gases or the composition of the gas in the environment under study
• Pressure Sensor: Sensor used to detect variations (or changes) in pressure or force
• Optical or Infra-Red Sensors: Sensors used to detect the changes in the light or Infra-red light respectively
• Proximity sensor: Sensor used to detect the proximity of objects to the sensing device
• Bio-Sensors: Sensors used to detect the biomarkers, parameters, bio-organisms, and particles
• Acoustic Sensors: Sensor used to detect sound
• Temperature Sensors: Sensor that measures variations in temperature
• Flow Sensors: Sensors that measure the fluid properties and its flow
• Humidity Sensors: Sensors that detect and measure the relative humidity in the environment
• Touch Sensors: Sensors that detect touch

And, the list can go on, limited only by the functions that they have been put to.  This categorization is based on their functionality or their usage.  These Sensors can be achieved by different technologies.

All these sensors and the mechanical sensors can also be broadly classified into:

• Contact Sensors – those sensors that have to be in touch or contact with the object or medium under measure.
• Contact-less Sensors – where the sensors do not physically touch the object being measured

The combination of the functional class with the contact class gives rise to a vast number of classes of sensors. 

We have not talked about the technology yet. Every sensing function can be achieved through different technologies or a combination of them.  We will explore some of the important technologies behind the sensors.

Sensor Technologies

The sensor transforms the detected physical change into an electrical signal. This can be achieved through different technologies. Based on the technology used in the sensor, they could be classified in different ways. Some of the important technology categories of the sensors are:

• Resistive sensors:  In this category of sensors, the resistance of the material medium undergoes a change. This change could be positive (indicating that the resistance increases when the physical change is increasing) or, it could have an inverse relationship. This is a very common technology that spans several functional domains of sensors. An example of resistive sensors includes the Temperature sensor.
• Capacitive Sensor: In this category, the capacitor of the sensor undergoes a change. Capacitance, like Resistance and Inductance, is an impedance device used in electrical circuits. The capacitance of a device is governed by the distance between the plates in the capacitor, as well as the dielectric of the insulating material that fills the space between the plates. The physical phenomenon that we want to measure alters one of these properties thereby causing the capacitance to change.
• Inductive sensors: These are very much like capacitive sensors. An inductor consists of a coil of a current-carrying conductor wound over a dielectric insulating material. When the current through the coil changes, due to inductance, certain currents called eddy currents are induced in the insulating material. By measuring and studying the eddy currents we can understand more about the phenomenon causing the change.
• Piezo-electric sensors: These are sensors that work on the principle of the Piezo-electric effect which is exhibited by some materials. When force is applied to such materials in a direction perpendicular to the flow of current through the material, it gives rise to an electric field and hence a current in a direction that is mutually perpendicular to the applied force and to the flow of current.
• Gyroscopic Sensors: These are sensors that are based on the principle of Gyroscopes. They are mechanical sensors. Such sensors are used for sensing angular tilt and angular velocity. They are extensively used in the aerospace industry.
• Chemical Sensors: These are sensors that can sense the chemical information of the medium and convert them into appropriate electrical signals.

This list is quite exhaustive. A lot more technologies go into the design of sensors. Each technology area of sensors is a detailed study by itself. Based on the combination of Technology, contact class, and functionality, there are innumerable different kinds of sensors.

In Conclusion

Sensors are one of the key elements of IoT solutions, which are the cause behind the Industrial Revolution 4.0. In this article, we have just taken a peep into the different classes of Sensors and the different categories that exist. The combination of the different categories results in a very large number of sensing devices to choose from, making it a large domain by itself.

About the Author

Dr. Anand Lakshmanan is a Senior-Member of IEEE, a Technologist, and an organization builder. He is currently pursing advisory and consulting roles for EdTech companies, and member of curriculum committee and Senate in Institutes of National repute.