The automatic control system can perform certain tasks without the participation of people according to the design of the person. The key is the introduction of feedback. The feedback actually adds the output or state of the system to the input of the system and the input of the system to act on the system. The output state of the system is actually a variety of physical quantities, they have voltage, and some are flow, speed and so on. These quantities are often different from the nature of the input of the system and the range of values is different. Therefore, it cannot be directly combined with the input and needs to be measured and converted. The sensor acts just like this. It is like controlling the eyes and skin of the system, sensing various changes in the control system, and cooperating with other parts of the system to complete the control tasks.
In order to obtain information from the outside world, human beings must resort to the sensory organs. But human sensory organs are not omnipotent. To get more information, to further study natural phenomena and to create labor tools, human senses are not enough. As a tool to replace human senses, the history of sensors is more ancient than the emergence of modern science. The balance has been used as a tool for weight measurement in ancient Egypt and has been used until now. Temperature measurements using liquid expansion characteristics have appeared in the sixteenth century. Sensors based on the basic principles of electricity are based on the development of modern electromagnetics, but with the increasing reliability of active components such as vacuum tubes and semiconductors, this type of sensor has developed rapidly. When it comes to sensors, most of them refer to devices that have an electrical signal output.