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PID is also optimized – the valve position control system. Sometimes, when selecting an actuator, it is necessary to take into account both speed and efficiency, which often leads to the use of two different types of actuators. A valve position control system is a control strategy that takes into account speed, effectiveness, and cost-effectiveness. A valve position control system requires two control loops to operate together, whereas split-range control needs only one control loop. Strictly speaking, a valve position control system is a solution that is more similar to cascade control and override control. Figure 56-1 Valve position control system: In the control system shown in Figure 56-1, control valve B is used to ensure the speed and effectiveness of the control system, while control valve A is used to ensure economy and rationality. In short, control valve B is an effective means of controlling the variable being regulated; however, to improve efficiency, reduce costs, or ensure the effectiveness of the control mechanism, it is necessary to maintain valve B at an appropriate opening degree. This is the main purpose of the valve position control system. The valve position control system is also used to eliminate the separation range between the large and small valves connected in parallel. The valve position control system provides an adjustable range, but it also offers the sensitivity of small valves; when throttling only large valves, step control cannot meet high requirements. The valve position control system also eliminates the discontinuity at the split range points. The valve position control system must work in conjunction with the process PID; this is essentially an optimization strategy based on PID. The PID parameters of the two control loops must be considered together due to coupling. Coupling effects can be minimized by making the valve position control system much slower than the process PID. The slowdown of the valve position control system is consistent with the concept that optimization is carried out gradually in order to minimize disruption to the control loop; thus, the changes that lead to optimization (such as daily and seasonal temperature variations and raw material compositions) are typically slow. Figure 56-2: The small valve’s rapid control of the large valve ensures the operating range of the small valve. If the characteristics of the two valve positions are identical, split-range control can be considered; Figure 56-3 shows the valve position control for the low-pressure main pipe pressure in a steam pressure reduction system