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Classification of selective control systems: Depending on the position of the signal selector within the system structure, selective control systems can be divided into two types: ① Systems in which the signal selector is located at the output of the regulator, and it is responsible for selecting the signal output by the regulator, as shown in Figure 1. The main feature of this selective control system is that two regulators share one control valve. Under normal production conditions, the output signals from both regulators are sent to the signal selector simultaneously; the control signal from the normal regulator is then selected and sent to the control valve, thereby enabling automatic control of the production process. At this time, the replacement regulator is in an open circuit state and has no control effect on the system. When production is abnormal, a replacement regulator is selected via a signal selector to control the system in place of the normal regulator. At this time, the normal regulator is in an open-circuit state and has no control effect on the system. When the production conditions of the system return to normal, through the automatic switching of the signal selector, it is still the original regulator that controls the normal progress of production. This type of selective control system is widely used in modern industrial production processes. http://yunrun.com.cn/upload/201901/22/201901221734191224.png Figure 1: Block diagram of a system in which the signal selector is located at the output of the regulator. ② A system in which the selector is placed before the regulator to select the signal output by the transmitter, as shown in Figure 2. This selective system is characterized by several transmitters sharing one regulator. There are usually two reasons for making such a choice: one is to select the highest or lowest measurement value ； The second is to select reliable measurement values. In a fixed-bed reactor, for example, in order to prevent the catalyst from being damaged due to excessive temperatures, several temperature monitoring points are installed at different locations within the catalyst bed. The temperature readings from these points are passed through a high-value signal selector, which selects the highest reading as the value to be used for automatic temperature control, thereby ensuring the safety of the catalyst layer in the reactor. http://yunrun.com.cn/upload/201901/22/201901221751381489.png Figure 2: Block diagram of the selection system in which the signal selector is located before the regulator. Original source: Principles, structure, and classification of selective control systems http://yunrun.com.cn/tech/2348.html