In-Depth Exploration of Complex Control Systems (Part 2)

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Complex control systems include: cascade control systems, proportional control systems, ratio control systems, split-range control systems, automatic selective control systems, feedforward control systems, and three-pulse control systems, among others. In the previous and middle sections, we introduced the cascade control system, the proportional control system, the ratio control system, the range control system, and the automatic selective control system respectively. This article introduces feedforward control systems and three-shot control systems. 6) Feedforward control system: The difference between a feedforward control system and a feedback control system is that, when disturbances occur, a feedback control system adjusts based on the deviation between the measured value and the set value ； A feedforward control system adjusts according to the magnitude of the disturbance. A feedback control system must be such that the regulator adjusts the control parameters only when there is a deviation in the parameter being measured, thereby overcoming the impact of disturbances on that parameter. However, in some process flows, there is often a lag; once a disturbance occurs, it takes some time for the parameter being regulated to change, and during this period the regulator does not make any adjustments, falling behind the effect of the disturbance. When we use a feedforward control system, once a disturbance occurs and before the parameter being controlled has a chance to change, the transmitter already detects the magnitude of the disturbance, and the regulator then makes adjustments. This timely and appropriate adjustment prevents the parameter under measurement from deviating due to the interference. In practical process control applications, pure feedforward control systems are rarely used to adjust the controlled parameters; instead, schemes such as feedback control combined with feedforward control, or ratio control combined with feedforward control, are generally employed depending on the specific process being controlled. The three-input control system discussed below is an example of feedforward control combined with a cascade control loop. 7) Three impulse control system: The most typical application of the three impulse control system is in the boiler feedwater system, where the liquid level in the boiler drum changes due to factors such as variations in steam demand. When the steam load increases suddenly and significantly, the steam pressure inside the drum drops instantly, which intensifies the boiling of the water and causes the number of bubbles to increase rapidly. These bubbles appear not only on the surface of the water but also below it. Since the volume of these bubbles is many times larger than that of the water, this leads to an increase in the liquid level inside the drum. Since this increased liquid level does not reflect the actual amount of liquid stored in the drum, it is referred to as a \"false liquid level\". If a three-impulse control system is not introduced, the level controller will send incorrect signals, causing the water inlet valve to close further; as a result, the actual liquid level drops even more, the water in the drum boils more intensely, and the amount of bubbles increases rapidly, posing a risk of explosion in the boiler. The three impulse control system is a solution designed for controlling the liquid level in the boiler drum. It takes into account the parameters of feedwater flow rate and steam output flow rate; the liquid level of the drum serves as the controlled variable. Through an operator, this value is used as the setpoint for the feedwater flow rate control loop, thereby achieving balance among the drum liquid level, feedwater flow rate, and steam flow rate. The steam flow rate is used as a feedforward signal for the feedwater flow rate secondary control loop; in fact, this is a feedforward + cascade control loop. What was mentioned above is just an example of a three-impulse control system. In practical applications, depending on different process requirements, there are various three-impulse control schemes; there are also single-impulse control and double-impulse control methods. An appropriate control scheme should be devised based on the needs.