The last few decades have experienced dramatic changes in the world of computers, software, and computing technology. As an engineer, it is fascinating to be a part of an era that boasts huge amounts of computing power. The most popular ones are personal computers, laptops, hand-held devices like smartphones and smart-watches.
It has become impossible to imagine and lead a life without the assistance of computing prowess. And the best part is, we are still skimming the surface of the vast computing potential lying dormant within such seemingly intelligent machines.
With the advent of Internet-of-Things (or IoT) that has taken the computing technology to the new level and redefined the word “smart” (How Smart Cities Can Help Build a Better Post-Pandemic World), it is fair to state that the excitement has only begun.
This article aims to answer the question, “how does a sensor sense?” and focuses on the physics of a sensor’s working.
What is Internet-of-Things (IoT)?
Engineers and scientists tend to nomenclate (picking a name for something) so that the newly coined term is self-explanatory. The term IoT is no different.
As the name suggests, IoT is an umbrella encompassing all types of devices. They are either embedded into a system or exist as an individual entity. Either way, the key is that they communicate (or talk) with each other via the internet. Every such device has an embedded transmitter and receiver that effectuates the communication process using the internet.
However, every IoT system is not the same and is not necessarily apt for all applications. As a matter of fact, they are akin to us humans. Every individual is great at something. You cannot expect an actor to fly an aircraft and a pilot to act in a film. Similarly, you cannot expect a single IoT system (and device) to do everything. Hence, engineers design different systems to perform different tasks to provide the best possible results.
In modern-day business, the customer is king and this is true across all industries. Hence, the system designers always design, produce, and ship IoT systems to provide a seamless user experience. IoT Hardware Product Development: How-To by Vera Kozyr, reiterates the time and efforts invested by all the stakeholders into creating an end-to-end, plug-and-play style system from the perspective of a hardware product.
Before exploring the innards of an IoT device, it is important to differentiate between a device and a system.
A device is like an individual member, while the system is like a team involving the individual. Thus, a device is a part of a system, while the vice-versa is not true.
Components of the IoT system
Any system comprises multiple individual components (and sub-components) that collectively work towards achieving a common goal. Moreover, being a part of a system (team) is ensures higher productivity and achieves better results. The major components of an IoT system are:
- The Sensors to sense physical quantities
- On-site central micro-controller that controls all the actions performed by sensors and other components
- Cloud, Data Analysis and Processing to analyze and process the received data
- Transmitter and Receiver to establish a communication between different sensors, sensors and micro-controller and the central cloud server via internet
- User Interface to communicate with and perform tasks instructed by the user
IoT Sensors: The Bridge to Real World
A good example of an IoT system is a smartphone that usually consists of:
- A Global Positioning System (GPS) module to determine the location
- A temperature sensor to sense the ambient temperature
- A microphone to sense the user’s voice and,
- A proximity sensor to sense the user’s distance from the phone and lock it during a call.
Different applications on smartphone use different sensors. For example, Google Maps has a user interface (an app) to interact with the GPS module and gather location co-ordinates. It processes the data via an internet connection to help the user route to his/her destination.
Battery Management System (BMS) is another example of an IoT system that uses multiple sensors. A BMS is an electronic system that protects and manages the operations of the battery. In short, it is the personal caretaker of the battery. I’ve explained the functioning of a smartphone BMS in my article – Battery Management System in Smartphones — in energyio.tech.
A sensor acts like a gateway between the computing world and the real world. Consequently, the sensor needs to convert whatever it senses in the real world into a special something that a computing machine understands.
Thankfully, the common link between the two worlds is electrical energy!
Hence, we arrive at the sensor’s technical definition – a sensor in an IoT system senses the desired physical quantity and converts it into an electrical signal transmitted to the central cloud-based server directly or via an on-site micro-controller.
An IoT sensor is, well, a sensor used in an IoT system.
Micro-Electromechanical Systems (MEMS) and The Sensing Mechanism of IoT Sensors
Micro-electromechanical Systems (or MEMS) is a microsystems technology (MST) consisting of minute components made up of semiconductor material like silicon with size lying in the micrometer range.
If not all, most sensors detecting mechanical energy use MEMS technology in one way or the other. An accelerometer is an extremely popular example. This is primarily due to the rapid growth and vast dependence on computers.
Since MEMS technology’s manufacturing material is a semiconductor, the primary advantage is that it can be embedded into an integrated circuit (IC). An IC includes other computing components (also made up of semiconductor material) that act on the data received from the sensors.
In fact, the small size and chip integration dramatically reduce the cost. You can literally buy a MEMS-based accelerometer for less than ₹250 ($3.34). Also, MEMS-based sensors boast high sensitivity and detect minute changes, which were unimaginable with predecessors.
Types of Sensing Mechanisms and Working Principle
Depending on the application, a system may comprise one or more sensors, sensing a different physical quantity, thereby having a unique sensing mechanism. The two of the most popular sensing mechanisms in MEMS technology that convert a physical change into an electrical signal are:
- Resistive based sensing
- Capacitive based sensing
The sensing mechanism in both the types uses a simple principle – any change in the physical quantity is captured by a change in electrical resistance or capacitance of the material used in the sensor. Thus, a larger change in the physical quantity shows a larger change in the resistance or capacitance of the material and vice-versa.
The major difference between the two types is the working of the two mechanisms. A resistive based sensing system uses, well, a resistor while a capacitive based sensing system uses a capacitor.
Don’t worry if you haven’t heard of a resistor and capacitor before this article. You can read the difference between them. Think of the two components as two people with their own unique set of traits.
Resistive Based Sensing Mechanism (Using MEMS Technology)
We have been using resistive resistors to measure, analyze, control and observe various physical quantities for over a century. As mentioned earlier, whenever a physical quantity (like pressure) changes, the amount of change in the electrical resistance determines how much the quantity has changed.
The change in the electrical resistance is governed by physics principles like Photoconductive Effect, Thermoresistive Effect of Semiconductors and Piezoresistive Effect .
- Sensing via Changes in Physical Geometry – The electrical resistance of a material depends on the material’s geometry, length, and cross-sectional area. Any change in the length or/and cross-sectional area will directly affect the resistance of the material.
- Piezoresistive Effect – A piezoresistive material is a special material whose electrical resistance changes when the material experiences a mechanical deformation like a push, pull or squeeze. Hence pressure, vibration, and acceleration measuring IoT sensors commonly use piezoresistive materials.
Other Resistive Based Sensing Mechanisms Used in IoT Sensors
Although MEMS-based IoT sensors are extremely effective for mechanical, physical quantities, resistive-sensors’ operation detecting non-mechanical quantities like light and temperature is not the same. Thus, the sensing mechanism changes.
- Light Sensing – To detect light, a special light-sensitive material is required. Plants detect light with the help of special molecules called photoreceptors. Similarly, any light-sensing sensor uses photoresistors – a material whose electrical resistance decreases as the light’s intensity increases. A light-dependent resistor or commonly known as LDR is a very popular IoT sensor used to detect light.
- Temperature Sensing – Similar to light sensing, temperature sensing also requires materials that are receptive to changes in the ambient temperature. Most temperature sensors consist of a thermistor – a material whose electrical resistance decreases with increasing temperature. For example, one of the parameters used to prevent over-charging of modern-day lithium-ion batteries is to detect the battery temperature with thermistors’ help.
- Chemical Sensors – These sensors are used to detect a particular chemical. The sensor contains a sensing layer made up of a material whose resistance changes whenever it reacts with the chemical. For example, many IoT systems use the MQ series (MQ9, MQ2, MQ7, etc.) gas sensor. It detects the presence of various types of gases like carbon monoxide, LPG and methane.
Conversion to Electrical Signals
Arguably, the second most popular scientific equation, Ohm’s Law (V = IR), establishes a direct relationship between electrical current, voltage and resistance. The beauty of this law is that any small change in the resistance can be converted to an electrical signal (voltage or current) in a jiffy.
Hence, every resistive based IoT sensor (including MEMS technology) uses Ohm’s Law directly or indirectly.
Capacitive Based Sensing Mechanism in IoT Sensors
A capacitive-based sensing mechanism captures the change in physical quantity by changing the material’s capacitance and, like resistance, depends on the material’s physical geometry.
However, almost all capacitive based sensing systems predominantly rely on changes in the physical geometry – area, distance, and the material’s capacitive ability described by the amount of charge it can store.
A touch sensor is one of the most common capacitive based sensors in an IoT system. A smartphone uses a touch screen consisting of numerous touch sensors. Essentially, it is a pressure sensor that detects the pressure/force from physical touch.
When the screen is stimulated by physical touch, the pressure exerted changes the area or/and distance, which triggers a change in the value of the capacitance underneath the screen.
This change in capacitance acts like an electrical switch that drives an electrical signal to the next stage. Fig 3 illustrates the working of a touch sensor.
Similar to the resistive based sensing systems that use Ohm’s Law, capacitive based systems have their own unique relation that maps a change in the electrical capacitance to voltage and current. Unfortunately, the mathematical equation is beyond the scope of this article.
Capacitive vs. Resistive Sensing
In resistive-sensing, some physical quantities like light and temperature, require a special type of material. This is a boon and a bane! On one side, the resistance variation is unique to the quantity being measured. But on the other side, this uniqueness requires an entirely different measuring/sensing procedure.
Instead, most capacitive based sensing systems maintain a uniform sensing procedure as the change is primarily due to variations in physical geometry. Moreover, they are relatively new compared to its resistive counterpart and are currently limited to sensing mechanical systems using MEMS technology.
I hope I was able to explain the working of some of the commonly used sensors in IoT systems. Moreover, sensor design fabrication is only one part of an IoT. The system has to effectively process the received data and provide application-centric results by catering to the user requirements.
As it stands now, IoT sensors have penetrated the manufacturing industry and automated most manual operations leading to an entirely new branch called The Industrial IoT (IIOT).
Unlike personal computers and smartphones, the IoT technology is yet to enforce a dramatic transformation in our lives. Until then, the entire IoT ecosystem needs to continue evolving.
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 Indiamart, “Standard MQ 9 Combustible Gas Sensor”
 D. Fischer, “Capacitive Touch Sensors,” Fujitsu Microelectronics Europe GmbH, Jan 2021