There are many ways to classify sensors, but two commonly used methods are based on the measured physical quantity and the working principle of the sensor. Based on the measured physical quantity, sensors can be divided into: temperature sensors, humidity sensors, pressure sensors, displacement sensors, flow sensors, liquid level sensors, force sensors, acceleration sensors, torque sensors, etc. Based on the working principle, sensors can be categorized as electrical sensors, magnetic sensors, photoelectric sensors, potentiometric sensors, charge sensors, semiconductor sensors, resonant sensors, and electrochemical sensors. Additionally, sensors can be classified into direct conversion sensors and indirect conversion sensors based on their signal detection and conversion processes. Below, we will mainly explore the working principles and applications of sensors classified by their working principles.

1. Electrical Sensors
   Electrical sensors are widely used in non-electrical measurement technology. Common types include resistive sensors, capacitive sensors, inductive sensors, magneto-electric sensors, and eddy current sensors.

• Resistive sensors use the principle of a rheostat to convert the measured non-electrical quantity into a resistance signal. They generally include potentiometer-type, contact rheostat-type, resistance strain gauge-type, and piezo-resistive sensors. Resistive sensors are mainly used to measure parameters such as displacement, pressure, force, strain, torque, airflow velocity, liquid level, and liquid flow rate.
• Capacitive sensors are made based on the principle of changing the geometrical dimensions of the capacitance or altering the properties and content of the dielectric to cause a change in capacitance. They are primarily used to measure parameters such as pressure, displacement, liquid level, thickness, and moisture content.
• Inductive sensors are made using the principle of changing the inductance or mutual inductance by altering the geometric dimensions of the magnetic circuit or the position of the magnet, or utilizing the piezomagnetic effect. They are mainly used to measure parameters such as displacement, pressure, force, vibration, and acceleration.
• Magneto-electric sensors convert the measured non-electrical quantity into an electrical quantity using the principle of electromagnetic induction. They are mainly used to measure parameters such as flow rate, rotational speed, and displacement.
• Eddy current sensors are made based on the principle of metal moving in a magnetic field to cut magnetic lines of force, forming an eddy current within the metal. They are primarily used to measure parameters such as displacement and thickness.

2. Magnetic Sensors
   Magnetic sensors are made using some physical effects of ferromagnetic materials and are mainly used to measure parameters such as displacement and torque.

3. Photoelectric Sensors
   Photoelectric sensors occupy an important position in non-electrical measurement and automatic control technology. They are made using the photoelectric effect and optical principles of photoelectric devices and are mainly used to measure parameters such as light intensity, luminous flux, displacement, and concentration.

4. Potentiometric Sensors
   Potentiometric sensors are made using principles such as the thermoelectric effect, photoelectric effect, and Hall effect. They are mainly used to measure parameters such as temperature, magnetic flux, current, speed, light intensity, and thermal radiation.

5. Charge Sensors
   Charge sensors are made using the principle of the piezoelectric effect and are mainly used to measure force and acceleration.

6. Semiconductor Sensors
   Semiconductor sensors are made using principles such as the piezo-resistive effect, internal photoelectric effect, magneto-electric effect, and changes in substances produced by semiconductor contact with gases. They are mainly used to measure parameters such as temperature, humidity, pressure, acceleration, magnetic fields, and harmful gases.

7. Resonant Sensors
   Resonant sensors are made using the principle of changing the resonant frequency by altering the inherent electrical or mechanical parameters. They are mainly used to measure pressure.

8. Electrochemical Sensors
   Electrochemical sensors are made based on ion conductivity. Depending on the formation of their electrical characteristics, electrochemical sensors can be divided into potentiometric sensors, conductivity sensors, coulombic sensors, polarographic sensors, and electrolytic sensors. Electrochemical sensors are mainly used to measure parameters such as the composition of gases, liquids, or solids dissolved in liquids, the pH of liquids, conductivity, and redox potential.