The sensor is a kind of detection device that can feel the information measured, and can transform the detected information into the information output of the electrical signal or other required forms according to a certain rule, so as to meet the requirements of the transmission, processing, storage, display, recording and control of information. It is the first step to achieve automatic detection and automatic control. The national standard GB7665-87 is defined under the sensor as "a device or device that can be measured and converted into a available signal according to a certain rule, usually composed of sensitive elements and conversion elements."

      Different views can be used to classify sensors: their conversion principles (the basic physical or chemical effects of sensor work); their uses; their type of output signals and the materials and processes that make them.
According to the working principle of sensors, sensor classification can be divided into two categories: physical sensors and chemical sensors.

      The principle of sensor working principle is the application of physical effects, such as piezoelectric effect, magnetostrictive phenomenon, ionization, polarization, thermoelectric, photoelectric, magnetoelectric and other effects. The tiny changes in the measured semaphore will be converted into electrical signals.

      Chemical sensors include sensors that are causally related to the phenomena of chemical adsorption and electrochemical reaction, and the small changes in the measured signal will also be converted into electrical signals.

      Some sensors can not be divided into physical categories or chemical ones. Most sensors operate on the basis of physical principles. There are many problems in chemical sensor technology, such as the reliability problem, the possibility of scale production, the price problem and so on. This kind of problem has been solved, and the application of chemical sensors will be greatly increased.