If intelligent systems are compared to humans, then sensors are the sensory organs of humans. Different types of sensors perceive the surrounding environment and transmit data to the system for calculation, enabling real-time analysis, judgment, and response to the situation. With the continuous deepening of digitalization and intelligence, the application of various sensors has greatly expanded, playing a huge role in creating a better life for humanity.

A smartphone has hundreds of sensors: CMOS image sensors for cameras, ambient light sensors for checking environmental brightness, geomagnetic sensors for navigation, gyroscopes... It is precisely based on these sensors that various application software in the phone can work smoothly, and the phone can become a portable smart device that integrates work, life, and entertainment, bringing about huge changes in people's lifestyles. The visible and infrared photoelectric sensors on Fengyun satellites can obtain atmospheric information day and night, accurately predict weather, and even have sensors working on the moon and Mars, helping humans explore the mysteries of the universe.

Beyond the senses

Sensors are the "eyes" of information systems. It is like human organs such as eyes, ears, and skin, perceiving the surrounding environment and helping us understand the colorful world. The difference is that sensors are more sensitive and powerful than human senses. The diversity of information contained in the objective world far exceeds the capacity of our senses. The human eye cannot observe infrared and ultraviolet radiation, the ear cannot hear infrasound waves and ultrasound, and cannot perceive the magnetic field that constantly affects the invisible. These information beyond the sensory range can be sensed by sensors.

With the development of productivity, humans increasingly need to perceive the world in all aspects. In 1821, scientists developed the world's first sensor - a temperature sensor - based on the principle that materials generate voltage due to temperature differences. Initially, people directly utilized physical effects such as light, heat, electricity, force, and magnetism to prepare various sensors, which were large in size, low in sensitivity, and inconvenient to use. In the 1970s, integrated sensors that integrated sensitive components with signal circuits emerged, such as thermocouple sensors, Hall sensors, photosensitive sensors, etc. This type of sensor is composed of solid components such as semiconductors, dielectrics, and magnetic materials, and outputs analog signals. Since the end of the last century, digital sensors have developed rapidly, achieving digital signal output through analog/digital conversion modules. Digital sensors integrate intelligent processing units, which can automatically collect and process data, and adjust working parameters according to the environment. The photosensitive components in digital cameras are their representative products.

Overall, the working principle of sensors is that the electrical properties of certain substances vary with environmental factors. For example, platinum has different electrical resistivity at different temperatures, silicon's resistance decreases under visible light irradiation, and quartz generates charges on its surface under pressure. By utilizing the correspondence between resistance and temperature, temperature sensors can be made, and insulation structures can be added to sensitive components. Based on the relationship between temperature changes of sensitive components and infrared radiation energy, infrared sensors can be made. On this basis, non-contact temperature sensors can also be manufactured based on the relationship between target temperature and infrared radiation energy. The forehead temperature gun commonly used to measure body temperature utilizes this principle. With the help of rich physical and chemical effects, people have developed a variety of powerful sensors, such as an "electronic nose" with a sensitivity 1000 times higher than a dog's nose, which can "smell" gas molecules, and an infrared camera that can observe objects in the dark.

Founding Intelligence

Digitization is the quantification of the attributes of things and the use of numbers to express them as abstract results. With the help of modern information technology, people can store, process, and disseminate various digital information. Sensors can convert various information contained in things into electrical signals, and use digital to analog conversion circuits to express electrical signals digitally, which is an effective tool for digitization. When you take out your phone to take photos or videos, the photosensitive sensor will convert the received light intensity signal into an electrical signal, and then express and store it in numbers according to certain rules, ultimately forming the image on the phone screen.

Digitalization is based on obtaining information from sensors. The amount of information that digital systems need to process is very large, which cannot be obtained solely by manual labor or traditional equipment. With sensors, it can be obtained in real-time, efficiently, accurately, and quickly. Therefore, there are urban big data, weather big data, medical big data, agricultural big data, etc. By utilizing various sensors, people can hold remote meetings, learn online courses, scan QR codes for payment, and even live stream sales, thus developing a digital economy format. The various technologies involved in the digital economy, such as cloud computing, the Internet of Things, artificial intelligence, and 5G communication, are closely related to sensors.
Without sensors, there would be no digitization and intelligence. Sensors are the first level of intelligent systems, and their level determines the level of intelligent systems and their instruments and equipment. Sensor technology has become the forefront of research in the field of high-end information devices internationally, playing an irreplaceable role in artificial intelligence, smart cities, 5G communication, aerospace, life and health, and other fields. For example, a car will be equipped with over 100 sensors for pressure, temperature, position, sound, light, electricity, etc., which will be processed by the onboard computer to help the driver make judgments. The intelligent analysis of data reduces the difficulty of driving cars, making them safer and easier to drive. Moreover, autonomous vehicles obtain real-time road information through sensors, and once obstacles are detected, they are promptly avoided through intelligent analysis. High rise buildings, bridges, tunnels, and other structures in cities also require real-time monitoring of safety conditions through sensors such as video, temperature, pressure, and smoke. When the data is collected, the intelligent system will analyze it in a timely manner and extract a small amount of key information for users to make decisions. Even in the future, human senses can become more powerful through sensors and build intelligent systems.

Explore new scenarios

Currently, various types of sensors are evolving towards digitalization, intelligence, miniaturization, green, low-carbon, wearable, and other directions, further improving performance and reducing costs, presenting a vigorous development trend. Among them, the research and development of intelligent sensors, flexible sensors, and new principle sensors have representative significance and are expected to shape new ways of work and life.
Developing intelligent sensors is an important trend. With the help of intelligent sensing technology, people design and manufacture various sensing units and microsystems with the functions of acquiring, storing, and analyzing information, achieving low-cost and high-precision information collection. Intelligent sensors are widely used in robotics, autonomous driving, intelligent manufacturing, quantitative motion monitoring, and can also be used to develop non-invasive or minimally invasive health monitoring devices. The popular dynamic blood glucose meter in recent years is a good example. Diabetes patients will put flexible sensors into their bodies painlessly, and the sensors will measure blood glucose every 5 minutes and transmit it to mobile phone applications. Patients can observe changes in their blood sugar curve and adjust their blood sugar levels in a timely manner through methods such as diet and exercise. Some patients even bid farewell to medication and insulin therapy as a result. In addition, people are also developing biodegradable electronic devices to better assist smart sensors in low-carbon and environmentally friendly living.

Developing flexible sensors is another trend. Many application scenarios require sensors to be prepared on flexible matrix materials, with characteristics such as transparency, flexibility, stretchability, free bending or even folding, portability, and wearability. At present, commonly used sensing materials for preparing flexible sensors include carbon based materials (carbon black, carbon nanotubes, graphene, etc.), metal nanomaterials (metal nanowires, metal nanoparticles, etc.), polymer and protein fibers, etc. For example, a cross-linked supramolecular polymer thin film electrode material with stretchability, tear resistance, and self-healing ability can be used to manufacture next-generation wearable and implantable flexible electronic devices. Integrating multifunctional flexible sensors with flexible printed circuits can create "smart belts" that can be worn on different parts of the body, allowing for real-time monitoring and analysis of physiological information, helping people, especially those with sensory degradation, to understand their own health status.

New principle sensors are also constantly emerging. In the field of basic research, new laws have been discovered one after another, and people are using these scientific new understandings to prepare sensors. At the same time, technological progress has also put forward new requirements for basic research. In daily life, people hope to improve the pixel, sensitivity, speed and other performance parameters of cameras; In high-speed experiments, a stripe camera capable of recording femtosecond scale information is required; In quantum communication, photodetectors with sensitivity up to a single photon are required; In aerospace technology, it is necessary to achieve detection of high-speed moving objects and cold targets, and so on. This requires scientists to further explore the physical world, discover new phenomena and laws, and improve sensor performance.

With the rapid development of technology and the continuous application of new materials and processes, sensors with stronger performance, more diverse types, and higher levels of intelligence will create more new work and life scenarios, helping people "feel" a better life.

(The author is an academician of the CAS Member and a researcher of the Chinese Academy of Sciences Shanghai Institute of Technical Physics)