The sensor is a kind of detecting device, which can sense the measured information, and can transform the detected information into electrical signal or other necessary form of information output according to certain regularity. In order to meet the information transmission, processing, storage, display, recording and control requirements. It is the first step to realize automatic detection and automatic control. The national standard GB7665-87 defines the sensor as: "the device or device that can sense the specified measurement and convert it into usable signals according to certain rules, usually consisting of sensitive elements and conversion elements".

Sensors can be classified from different perspectives: their conversion principles (the basic physical or chemical effects on which the sensors work); their uses; the types of signals they produce and the materials and processes by which they are made.
   According to the working principle of sensors, the classification of sensors can be divided into two categories:

Physical sensors and chemical sensors. The classification of working principle of sensors is based on physical effects, such as piezoelectric effect and magnetostrictive phenomenon. Ionization, polarization, thermoelectric, photoelectric, magnetoelectric and other effects. Any small change in the measured semaphore will be converted into an electrical signal.

Chemical sensors, including those with chemical adsorption, electrochemical reaction and other phenomena as a causal relationship, the small changes in the measured semaphore will also be converted into electrical signals.

Some sensors can neither be classified as physical nor chemical. Most sensors operate on the basis of physical principles. There are many technical problems of chemical sensors, such as reliability, the possibility of large-scale production, price problems and so on. If these problems are solved, the application of chemical sensors will have a huge growth.

Sensor Classification

——Sensor Classification: according to its use, Sensors can be classified:

pressure-sensitive and force-sensitive sensors, position sensors, liquid level sensors, energy sensors, speed sensors, acceleration sensors, radiation sensors, thermal sensors.

——Sensor Classification: according to its principle, Sensors can be classified:

vibration sensors, humidity sensors, magnetic sensors, gas sensors, vacuum sensors, biosensors and so on.

—— Sensor classification: according to its output signal, the sensor can be divided into:

 (1) analog sensor: the measured non-electrical quantity will be converted into analog electrical signal.

 (2) Digital sensors: conversion of measured non-electrical quantities into digital output signals (both direct and indirect).

 (3) Pseudo-digital sensor: converts the measured semaphore into an output of a frequency signal or a short-period signal (including direct or indirect conversion).

 (4) switch sensor: when a measured signal reaches a specific threshold value, the sensor outputs a set low or high level signal accordingly. Sensor classification: according to material classification: under the action of external factors, all materials will make a corresponding, characteristic response.

——Sensor classification: according to material classification:

under the action of external factors, all materials will make corresponding, characteristic response. The materials that are most sensitive to external action, namely, those with functional properties, are used to make sensor sensitive elements. From the point of view of applied materials, sensors can be divided into the following categories:

 (1) according to the categories of materials used: metals, polymers, ceramics, mixtures;

 (2) according to the physical property of the material: conductor, insulator, semiconductor, magnetic material;

 (3) according to the crystal structure of the material: single crystal, polycrystalline, amorphous material.

——Sensor classification: according to its manufacturing process, sensors can be divided into:

integrated sensors, thin film sensors, thick film sensors, ceramic sensors