In the past decade, there have been significant changes in computing, software, and computing technology worldwide. Among the most popular are handheld devices such as personal computers, laptops, smartphones, and smartwatches.
It is unimaginable how our lives would be without the help of computing power. The curious part is that we are still only scratching the surface of the immense computational potential hidden within these seemingly intelligent machines.
With the advent of the Internet of Things, computing technology has reached a new level, redefining “intelligence” (how smart cities are building a better post-pandemic world), and the exciting moments are just beginning.
This article aims to answer the question: “How do sensors ‘sense’?” and focuses on the physical processes behind how sensors work.
As the name suggests, the Internet of Things is an umbrella that encompasses all types of devices. They either exist embedded in systems or as standalone entities. Regardless, the key is that they communicate with each other over the Internet. Each such device has an embedded transmitter and receiver that facilitates the communication process via the Internet.
However, every IoT system is different and may not be suitable for all applications. They are similar to humans; everyone excels at something—one cannot expect an actor to fly a plane or a pilot to act in a movie. Similarly, one cannot expect a single IoT system (and device) to do everything. Therefore, engineers design different systems to perform different tasks, providing the best results.
In modern business, the customer is king, and this is true across all industries. Thus, system designers always design, produce, and deliver IoT systems to provide a good user experience. Vera Kozyr’s book, “How to Develop IoT Hardware Products,” reiterates the time and effort all stakeholders invest in creating end-to-end, plug-and-play systems from a hardware product perspective.
Before exploring the internals of IoT devices, it is crucial to distinguish between devices and systems.
A device is like an individual member, while a system is like a team comprising individuals. Thus, devices are part of systems and vice versa.
Any system comprises multiple individual components (and subcomponents) that work together towards a common goal. Moreover, being part of a system (team) ensures higher productivity and better results. The primary components of an IoT system include:
Sensors that sense physical quantities
A central microcontroller in the field that controls all actions performed by the sensors and other components;
Cloud for data analysis and processing, analyzing and processing the data received;
Transmitters and receivers that establish communication between different sensors, sensor and microcontroller, and the central cloud server via the Internet;
A user interface that communicates with users and executes tasks as instructed by them.
A great example of an IoT system is a smartphone, which typically includes:
A global positioning system (GPS) module for determining location;
A temperature sensor to sense environmental temperature;
A microphone that can sense the user’s voice;
Proximity sensors to sense the distance between the user and the phone, locking the phone during calls.
Different applications on smartphones utilize different sensors. For example, Google Maps has a user interface (an application) that interacts with the GPS module to collect location coordinates. It processes data through an Internet connection to help users navigate to their destination.
A battery management system (BMS) is another example of an IoT system that uses multiple sensors. The BMS is an electronic system that protects and manages battery operations. In short, it is the personal caretaker of the battery.
Sensors act as gateways between the computer world and the real world. Thus, sensors need to convert anything they perceive in the real world into something that computers can understand.
And the common link between these two worlds is electricity.
Thus, we arrive at the technical definition of sensors: sensors in an IoT system sense the required physical quantities and convert them into electrical signals, transmitted directly or via a field microcontroller to a cloud-based central server.
IoT sensors are the sensors used in IoT systems.
Micro-Electro-Mechanical Systems (MEMS) are a type of micro-system technology (MST) composed of semiconductor materials (like silicon) that are on the micron scale.
Most sensors that detect mechanical energy use MEMS technology in some way. An accelerometer is a very typical example. This is primarily due to the rapid growth and enormous reliance on computing.
Since the manufacturing materials of MEMS technology are semiconductors, their main advantage is that they can be embedded in integrated circuits (ICs). Integrated circuits include other computing components that work on the data received from the sensors (also made of semiconductor materials).
In fact, the small size and chip integration greatly reduce costs. You can buy a MEMS-based accelerometer for less than £250 ($3.34). Moreover, MEMS-based sensors have the advantage of high sensitivity and the ability to detect minute changes, which was unimaginable in the past.
Depending on the application, a system can include one or more sensors that sense different physical quantities, thus having unique sensing mechanisms. The two most popular sensing mechanisms in MEMS technology that convert physical changes into electrical signals are:
1. Resistance-based sensing
2. Capacitive sensing
Both types of sensing mechanisms employ a simple principle—any change in physical quantities is captured by the change in resistance or capacitance of the material used in the sensor. Thus, larger changes in physical quantities indicate larger changes in resistance or capacitance of the material, and vice versa.
The main difference between the two types lies in how these two mechanisms work. Resistance-based sensing systems use resistance, while capacitive sensing systems use capacitance.
For over a century, we have been using resistive resistors to measure, analyze, control, and observe various physical quantities. As mentioned, when a physical quantity (like pressure) changes, the amount of change in resistance determines the amount of change in that quantity.
The change in resistance is governed by physical principles such as the photoconductive effect, thermal resistance effect of semiconductors, and piezoresistive effect.
Sensing through changes in physical geometry—The resistance of a material depends on the material’s geometric structure, length, and cross-sectional area. Any change in length and/or cross-sectional area will directly affect the resistance of the material.
Piezoresistive effect—Piezoresistive materials are special materials whose resistance changes when the material undergoes mechanical deformations such as pushing, pulling, or squeezing. Therefore, IoT sensors that measure pressure, vibration, and acceleration typically use piezoresistive materials.
While MEMS-based IoT sensors are very effective for mechanical and physical quantities, the operation of resistive sensors detecting non-mechanical quantities (such as light and temperature) is different. Thus, the sensing mechanisms change.
Light Sensing—To detect light, a special photosensitive material is needed. Plants detect light through special molecules called photoreceptors. Similarly, any light-sensing sensor uses a photosensitive material whose resistance decreases with increasing light intensity. A light-sensitive resistor, commonly known as LDR, is a very popular IoT sensor for detecting light.
Temperature Sensing—Similar to light sensing, temperature sensing also requires materials that can accommodate changes in environmental temperature. Most temperature sensors consist of thermistors, which are resistors that decrease in resistance with increasing temperature. For example, one parameter that prevents modern lithium-ion batteries from overcharging is the detection of battery temperature with the help of thermistors.
Chemical Sensors—These sensors are used to detect specific chemicals. The sensor contains a sensing layer made of a material whose resistance changes whenever it reacts with a chemical substance. For example, many IoT systems use MQ series (MQ9, MQ2, MQ7, etc.) gas sensors that can detect the presence of various gases such as carbon monoxide, liquefied petroleum gas, and methane.
Figure 1—Resistance-based Sensors
It can be said that the second most popular scientific equation, Ohm’s Law (V=IR), establishes a direct relationship between current, voltage, and resistance. The beauty of this law lies in the fact that any minute change in resistance can be instantaneously converted into an electrical signal (voltage or current).
Figure 2—Conversion of Physical Changes in Resistance Sensors to Electrical Signals
Thus, every resistance-based IoT sensor (including MEMS technology) directly or indirectly utilizes Ohm’s Law.
Capacitive sensing mechanisms capture changes in physical quantities by altering the capacitance of materials, just like resistance, depending on the physical geometry of the material.
However, almost all capacitive sensing systems primarily rely on changes in physical geometry—area, distance, and the capacitance capability of the material (described by the amount of charge it can store).
Touch sensors are one of the most common capacitive sensors in IoT systems. Smartphones use touch screens made up of many touch sensors. Essentially, it is a pressure sensor that can detect pressure/force from body contact.
When the screen is physically touched, the applied pressure alters the area or distance of the screen, triggering a change in the capacitance value beneath the screen.
This change in capacitance acts like an electronic switch, driving the electrical signal to the next level. Figure 3 illustrates how a touch sensor works.
Figure 3–2D and 3D Operation of Capacitive Touch Sensors
Similar to resistance-based sensing systems that use Ohm’s Law, capacitive systems have their unique relationship that maps changes in capacitance to voltage and current.
In resistance sensing, certain physical quantities such as light and temperature require a special type of material. There are pros and cons; on one hand, the change in resistance is unique to the measurement. However, on the other hand, this uniqueness requires entirely different measurement/sensing procedures.
In contrast, most capacitive sensing systems maintain a uniform sensing process, as this change is primarily due to changes in physical geometry. Additionally, compared to resistance sensors, they are relatively newer and currently limited to sensing mechanical systems using MEMS technology.
Moreover, the Internet of Things is just a part of sensor design. The system must effectively process the received data and provide application-centered results based on user needs.
Currently, IoT sensors have penetrated the manufacturing industry, automating most manual operations and forming a new branch called the Industrial Internet of Things (IIoT).
Unlike personal computers and smartphones, IoT technology has yet to bring about significant changes in our lives. Before that, the entire IoT ecosystem needs to continue developing.
References
[1] W. Y. Du, S. W. Yelich, “Resistive and Capacitive Based Sensing Technologies”, Sensors and Transducers Journal, April, 2008
[2] P&S Technologies, “P&S OPC271 Opto-Potentiometer”, TNT Audio, June, 2009
[3] Wikimedia Common Contributors, “Photoresistor 2.jpg”, Wikimedia Commons, The Free Media Repository, November, 2018
[4] “NTC Thermistor.jpg,” Wikimedia Commons, The Free Media Repository, September 2019
[5] Wikimedia Common Contributors, “R against T for a thermistor.png,” Wikimedia Commons, The Free Media Repository, July 2020
[6] Wikimedia Common Contributors, “PeizoAccelThoery.gif,” Wikimedia Commons, The Free Media Repository, July 2008
[7] Indiamart, “Standard MQ 9 Combustible Gas Sensor”
[8] D. Fischer, “Capacitive Touch Sensors,” Fujitsu Microelectronics Europe GmbH, Jan 2021
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Recruitment for IOTE2021 IoT Exhibition Officially Launched
Suzhou Station:
April 7-9, 2021 Suzhou International Expo Center
Shenzhen Station:
August 2021
Exhibition Contact:
Mr. Chen Jianghan 18676385933 (WeChat same number)
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