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This article is divided into three parts: upper, middle, and lower. Complex control systems include: cascade control systems, proportional control systems, ratio control systems, range control systems, automatic selective control systems, feedforward control systems, and three-shot control systems, among others. In the previous article, we introduced cascade control systems and proportional control systems; this article focuses on ratio control systems, range control systems, and automatic selective control systems. 3) Ratio control system: In the production process, it is often necessary to mix two or more materials in precise proportions (ratios) for manufacturing purposes, and accurate control of the proportions of these materials is required. In practical applications, a ratio control system is typically used for this purpose. Next, we take the single-loop proportional control system as an example: the active variable F1 has no feedback control and is therefore variable; F2 is the passive variable, which changes along with F1. In a stable state, F2 can be kept at F2 = KF1 (where K is the proportional coefficient). As can be seen from the block diagram, F2 forms a closed-loop control circuit, which is why it is called a single-loop proportional control system. Figure: Ratio control system – The single-loop ratio control system is generally used in situations where the F1 parameter does not change much, such as the output flow rate of a metering pump. If the change in F1 affects the subsequent process, then a double closed-loop ratio control system should be used, or a cascade ratio control system based on the specific requirements of the process may be employed. 4) Split-range control system: The main purpose of setting up a split-range control system is to increase the adjustable range R, so as to meet the requirements of special control systems ； Improve control quality, save energy, and enhance the operating conditions of control valves ； It meets the requirements for low flow rates during startup and shutdown, as well as high flow rates during normal operation, ensuring good control quality in all scenarios ； Ensure stability and safety under normal operation as well as in emergency conditions. Taking the block diagram below as an example, the material is heated through hot water valve A and steam valve B, with the requirement that the temperature of the material at the outlet of the heat exchanger remain constant. To save steam, steam is added only when hot water valve A is fully open and the temperature has not yet reached the desired set value, at which point steam valve B is opened. Figure: Split-control system. As can be seen from the figure, even after valve A for hot water is fully opened, the temperature remains below the set value; the temperature controller then outputs a pressure of over 60 kPa. At this point, the steam valve opens to supply additional steam, thereby bringing the temperature up to the set value. In this example, the output of the regulator is divided into two sections to control two air-operated control valves; depending on the process requirements, it is also possible to design one air-operated control valve, one air-shut control valve, or two air-shut control valves. To meet the special requirements of industrial processes, split-range control can also divide the entire output signal of the controller into several signal segments; each segment controls a control valve, with each control valve operating only within a specific portion of the overall range of the controller’s output signal. 5) Automatic selective control system: In situations such as overload of equipment like compressors, pumps, and blowers, when the reactor reaches its limit of operation, or during emergencies related to the start-up or shutdown of large-scale production facilities, conventional control systems are no longer sufficient to meet the control requirements. Relying on operators to handle these issues would result in high stress and busy work for the operators, as well as an increased risk of errors. Therefore, an automatic selective control system, also known as a bypass control system, is used to ensure safety in production and to manage automatic start-up and shutdown processes. This control method involves adding the logical relationships arising from the constraints in the production process to the conventional control system. When the production process approaches those constraints, and before any unsafe conditions arise, it switches to a controller designed to handle such unsafe situations; this controller takes over from the conventional control system until the production process returns to a safe range, at which point the conventional control system resumes its normal operation. This type of automatic selection control is sometimes also referred to as an automatic protection control system. Taking the figure below as an example: The flow rates of ammonia and air send signals to a divider through flow transmitters; after a ratio calculation is performed, the resulting ratio signal is used as the measurement value for both the KC ratio regulator and the KY override regulator. The output signals from these regulators are then fed to an LS low selector, which in turn controls the valve. Figure: During normal operation of the automatic selective control system, the output of the KY override regulator is 100 kPa, and the LS low selector sends the signal to the control valve only through the KC ratio regulator. If there are fluctuations in the production process and the ratio of ammonia to air flow exceeds the allowable safety limit, the KY override regulator activates, reducing its output and closing the control valve until the ratio of ammonia to air flow returns to the normal range, after which the KC ratio regulator resumes operating automatically. (The control valves on ammonia pipelines are generally selected to be air-operated valves.) The diagram is not included here; if needed, it can be viewed in the original source: https://www.sohu.com/a/308376089_120129629?sec=wd