Sensor Basics: Classification, Working Principle and Application

A sensor is a sophisticated detection device that converts measured physical phenomena into electrical signals or other information outputs following specific parameters. These devices are fundamental to modern data acquisition, processing, storage, display, and control systems (Wang et al., 2019). The evolution of sensor technology has enabled objects to simulate human senses, including touch, taste, and smell, bringing us closer to autonomous systems.

Miniaturization, digitalization, intelligence, multi-function, systematization, and networking are all properties of sensors. It's the first step toward achieving autonomous detection and control.

Sensors enable objects to have senses such as touch, taste, and smell, and they gradually become alive as a result of their evolution. Thermal elements, photosensitive elements, gas-sensitive elements, force-sensitive elements, magnetic-sensitive elements, humidity-sensitive elements, sound-sensitive elements, radiation-sensitive elements, color-sensitive elements, and taste-sensitive elements are the ten categories according to their basic sensing functions.

Sensors are frequently categorized based on their functioning principle, input data, and application scope. They may be loosely split into three types based on their diverse functioning principles: physical type, chemical type, and biological type.

Ⅰ. Physical sensors

1. Physical Sensor Definition

Sensors built with the physical qualities of specific transforming components and the special physical properties of certain functional materials are known as physical sensors.

Resistive sensors, which useresistance value changes caused by metals and semiconductor materials under the action of the measured; inductance and differential transformer sensors, which use the magnetoresistance to change with the measured; and piezoelectric crystal sensors, which use the measured force to change the resistance value. Piezoelectric sensors, for example, are based on the piezoelectric effect. It's worth noting that, in recent years, a number of sensors have been developed exploiting semiconductor materials' specific capabilities, such as pressure-sensitive, photo-sensitive, and magnetic-sensitive sensors based on the piezoresistive effect, photoelectric effect, and Hall effect.

2. Physical Sensor Types

Physical sensors can be divided into physical sensors and structural sensors.

The so-called physical sensor is a sensor that uses the inherent features and effects of specific functional materials to directly convert the measured value into electricity. A pressure sensor constructed of piezoelectric crystals, for example, measures pressure using the positive piezoelectric effect of the piezoelectric material itself; another example is a photoresistor, which measures conductivity by changing the semiconductor material's response to light intensity. The photoconductive effect is altered to create the sensor.

Structural sensors employ physical rules to turn measured information into electricity and are based on structure (such as shape, size, and so on). An air-gap inductive sensor, for example, must have an iron core of a certain size, a coil with a specific number of turns, and an armature with a specific gap with the iron core. It can only be guaranteed to be safe if it meets certain structural and dimensional standards. The magnetoresistance changes as the gap between the armature and the iron core changes, creating the corresponding electrical signal; another example is an electric sensor, which is made up of a magnetic shoe with a coil that travels in its gap. This framework It is ensured that when the coil travels, it can cut the magnetic line of force, inducing an electromotive force in the coil that corresponds to the observed speed v.

Although the structural sensor uses some physical laws, it must be guaranteed by a precisely designed structure; whereas the physical sensor primarily relies on the effect of the material itself to sense information, although there are some requirements on the structure, structural sensors regard structure as a necessary and sufficient condition for sensing information, unlike the physical sensor.

It is worth noting that, as a result of the rapid advancement of material science in recent years, the use of physical property sensors  has grown in scope and speed, which is attributed to their low cost and suitability for mass manufacturing.

Ⅱ. Chemical Sensors

1. Chemical Sensor Definition

Chemical sensors convert the composition and concentration of inorganic and organic chemical compounds into electrical signals using the electrochemical reaction concept.

2. Chemical Sensor Working Principle

The ion-selective electrode is the most popular, and it is used to detect the pH value or the activity of specific ions in the solution, such as K, Na, and Ca. Although the electrodes' measuring objects differ, their measurement principles are similar. They primarily rely on the electrochemical reaction between the electrode interface (solid phase) and the measured solution (liquid phase), or the electrode's selective response to ions in the solution. There is the possibility of making a difference. Because the work potential difference of the component in the reaction process or the current value affected by it is linearly connected to the logarithm of the measured ion activity, the measured ion activity is measured. The ion-selective sensitive membrane, which is separated into solid and liquid membranes, is the main component. Solid films include glass, single crystal films, and polycrystalline films, while liquid films include positive and negatively charged carrier films, as well as neutral carrier films.

3. Chemical Sensor Uses

The ion-selective electrode developed by combining membrane technology and field effect transistor has seen the most rapid progress in recent years. It is formed on the field effect transistor's gate. This ion-selective electrode can test not only inorganic solutions, but also glucose, ions in gases, and certain serum components. Chemical sensors are widely employed in the chemical industry for chemical analysis, online detection, and environmental monitoring.

Ⅲ. Biological Sensors

1. Biological Sensor Definition

Biosensors are another type of sensor that has grown in popularity in recent years. It's a sensor that detects and measures biochemical compounds using biologically active chemicals. Functional conversion recognition capacity refers to a biologically active agent's selective affinity for a certain chemical. This single recognition capacity is utilized to determine whether or not a substance exists and at what concentration it exists, and then to transform electrical impulses using electrochemical processes.

2. Biological Sensor Working Principle

The functional recognition substance, which is employed for particular recognition of the tested drug, is one of the two primary components of the biosensor. Enzymes, antigens, antibodies, microbes, and cells are examples of functional recognition substances. These identifiers are cured on a particular organic film using a unique process, resulting in a functional film that recognizes specific low- to high-molecular molecules. The electrical and optical signal conversion device is the second. The purpose of this device is to convert the chemical reaction caused by the identification of the measured object on the functional film into an easy-to-transmit electrical or optical signal. Electrodes such as oxygen electrodes and hydrogen peroxide electrodes are the most widely utilized electrodes. Recently, biosensors have been developed in which the functional layer is mounted on the field-effect transistor rather than the gate-drain, allowing for a very tiny overall volume.

3. Biological Sensor Uses

If an optical approach is employed to identify the recognition response on the functional screen, the measured substance, such as a fluorescent biosensor, is measured by a change in light intensity. The sensitivity and linearity of the sensor are directly related to the conversion device. The physical sensor's most notable feature is its ability to identify the measured substance at the molecular level, which means it has a wide range of applications not only in chemical industry monitoring but also in medical diagnosis.

Sort by input

Users would appreciate the convenience of classification based on sensor input information (or measured parameters), which can convey the sensor's function. Sensors can be classified into displacement, speed, acceleration, force, pressure, flow rate, temperature, light intensity, humidity, viscosity, and concentration using this sensory classification approach. This classification approach is familiar to both manufacturers and users.

The above-mentioned information is still fairly broad. The temperature sensor, for example, contains thermocouple temperature sensors, thermistor temperature sensors, PN junction diode temperature sensors, and pyroelectric temperature sensors, all of which are manufactured of different materials and methods. Wait. The most popular approach of naming and categorizing the working principle and the measured parameter is this method.

Sort by application

There are further classifications based on the scope of application and application objects, such as vibration measurement sensors, optical sensors, and liquid level sensors, in addition to the two approaches mentioned above. Sensors are frequently classified according to the measured organ, such as heart sound sensors, ECG sensors, pulse sensors, and so on, especially in medical measurement.

The world has entered the information age as a result of the new technological revolution. The first problem to tackle in the process of using information is obtaining accurate and trustworthy data, and sensors are the most common manner and means of obtaining data in the natural and production areas.

Various sensors are employed in modern industrial production, particularly in automated production, to monitor and manage various factors in the manufacturing process so that the equipment can operate in a normal or optimal state and the products may attain the highest quality. As a result, without a large number of high-quality sensors, modern manufacturing will fall apart.

Sensors have a larger role in basic science research. In the macroscopic view, we must observe the vast universe thousands of light-years away, in the microscopic view, we must observe the particle world as small as fm, and in the vertical view, we must observe the evolution of celestial bodies over hundreds of thousands of years and an instantaneous response as short as s.

Various extreme technological researches, such as ultra-high temperature, ultra-low temperature, ultra-high pressure, ultra-high vacuum, ultra-strong magnetic field, ultra-weak magnetic field, and so on, have also played an important role in deepening material understanding, developing new energy sources, and developing new materials. Without adequate sensors, it is obviously impossible to get a vast amount of information that cannot be gathered directly by human senses. The difficulty of getting object information is the initial barrier to many basic scientific studies, and the advent of new mechanisms and high-sensitivity detecting sensors frequently leads to breakthroughs in this field. Sensor development is frequently a forerunner in the development of several fringe disciplines.

Sensors have already made inroads into disciplines as diverse as industrial production, space research, ocean exploration, environmental protection, resource survey, medical diagnosis, biotechnology, and even the preservation of cultural artifacts.

It is no exaggeration to say that practically every modern undertaking, from the vastness of space to the vastness of the ocean to numerous complicated engineering systems, is inextricably linked to a variety of sensors.

It is clear that sensor technology plays a critical role in the development of the economy and the promotion of social progress. The development of this subject is extremely important to all governments throughout the world. Sensor technology is expected to make a significant leap in the near future, reaching a new level commensurate with its relevance.

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