From the development and structural principle of control instruments, what are the main types? What are their characteristics?

There are three main types: basic control instrument, unit combination instrument and control device based on microprocessor.

The basic control instrument is generally assembled with detection device and display device, which integrates detection, control and display, with simple structure, low price and convenient use. However, because of its poor universality and difficult signal transmission, it is generally used in some simple control systems.

Unit combination instrument is to divide the instrument into several units according to their different functions, and the units are connected with each other by unified standard signals. The control unit can receive the measured value and the given value signal, and then send out the control action signal related to them through certain operation according to their deviation. The unit combination instrument is highly flexible and universal, and each unit can be properly combined, and it can also be combined with modern equipment such as computers to form various complex new control systems.

With the development of computer technology, a variety of microprocessor-based controllers have appeared, which makes the controller enter a new stage in structure, function and reliability. In recent 20 years, there have been many controllers based on distributed control system or field bus, which not only have control function, but also have network communication function to meet the needs of mass production in the information society.