【2026 Complex Control】Complex control and control scheme design are two things at different levels
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Control scheme design is a method for solving problems. Complex control is an extension of PID, and like PID, it is a tool that should be used flexibly in the design of control schemes. Therefore, the design of so-called complex control schemes actually involves two things at different levels, which should not be confused.
Control performance can be improved by selecting different PID configurations and PID parameters, as well as through feedforward control, cascade control, or even a combination of feedforward and cascade control. Achieving performance improvements through structural modifications is essentially the same as achieving them through PID parameter tuning.
Just as with PID parameter tuning, instrument/control engineers should know how to improve control performance through structural improvements. This aspect of the work generally does not need to be considered in the control scheme design, or it is taken into account only in the final stages of such design, based on actual conditions.
Cascade control is designed to overcome disturbances and non-linearity on the control side, but the cascade approach reflects the idea of solving problems in layers, and it is used more frequently in the design of control schemes. The problems solved and the application scenarios of cascade control and cascade mode are not significant. This has not been accurately understood before either.
Certain principles should be followed in the design of control schemes: 1. Understand the intended purpose and operational approach of the current control scheme, but at the same time avoid being overly constrained by that scheme and its approach to solving the problems at hand； 2. Break down complex problems or adapt them into single pairings, and resolve them using a single-loop PID in sequence ； 3. The MV must be consistent from start to finish; control rights cannot be switched during operation ； 4. By simulating the dynamic processing process of control schemes under various constraints, find the simplest and most optimal solution.
Staged control is a commonly used form of complex control. Essentially, split-range control remains a single-loop PID; nevertheless, it should be used with caution. When there is 1 CV and 2 MVs, the method of combining MVs can certainly be used to select step control. However, it should be noted that hierarchical control or valve position control with different set values can also be selected using the CV division method.
For example, when using vent valves and nitrogen make-up valves to control the pressure of storage tanks, staged control of the MV combination is often the preferred approach. In fact, it is often a better choice to have two pressure control circuits for the storage tanks with different set values, each controlling the stepwise regulation of the CV valves for venting and nitrogen filling. The two single circuits can be tuned independently, and the issue of the air release valve and the nitrogen make-up valve turning on simultaneously during rapid pressure changes is also easily resolved. Using proportional control for regulating the total flow rate via large and small valves is an erroneous design that fails to understand the intended purpose of such a system.
Override control is designed for safety and abnormal operating conditions; if it is used to address control issues on a frequent basis, it is often an improper use of override control. Overshoot control requires a switch in MV control rights, so it should be used with caution in the design of control schemes. However, in practical design, many factors that seem related to safety lead to the choice of override control. Although override control is rarely used, its abuse is quite common. Simplicity is not necessarily right, and complexity is definitely not right. Split-range control + override control are generally not simple enough.
For example: In a water treatment process, the water is sent to an FWKO tank and then flows to a gas/liquid separator to remove the gases entrained in the water. Then the water is sent downstream for further treatment. The flow rate is regulated by a flow controller. The operational requirement is to maintain two liquid levels by controlling two flow control valves. Level and flow measurement can be provided as required. For the FWKO tank, the liquid level is the control variable, while the outlet flow valve LCV-101 is the actuating variable. For the separator, due to the nonlinear dynamics of the pump, a cascade level (LC-102) to flow rate (FC-101) control loop is employed to improve level control.
The challenge is that the capacity of the separator is much smaller than that of the FWKO. Fluctuations in the FWKO outlet flow can easily affect the separator level controller LC-102. During the first few years of operation, shutdowns were frequently caused by the interlock due to high liquid levels in the separator.
To prevent high liquid level interlocks, the same high liquid level controller LC-103 on the separator is used in conjunction with the FWKO tank liquid level controller LC-101 to form an override protection system. However, the override control often takes effect, preventing LCV-101 from functioning consistently. So this solution can solve the problem, but it is not simple or optimal enough.