Override control

The characteristic of override control is that when the deviation between the parameter being regulated and the set value exceeds a certain threshold, another control scheme (system configuration or set value) is used for regulation; once the deviation returns to a certain level, control switches back to the normal scheme. Therefore, the use of a selector is necessary, but the selector here is not necessarily an independent device or module; it can also be an internal function of a device or module, such as a dedicated single-loop override regulator. I don’t see your system diagram, but based on what you’ve said, it’s likely that when the liquid level fluctuates within a certain range, only the main disturbance affecting the liquid level – namely the steam flow – is adjusted. Structurally, this constitutes a single-loop control system for steam; from a control perspective, it represents a feedforward control mechanism for the liquid level ； When the liquid level fluctuations exceed a certain range, the liquid level signal (via a regulator) is used as the setpoint for steam regulation; structurally, this constitutes uniform control of both the liquid level and the steam, while from a control perspective it represents a feedforward control system with feedback based on the primary parameter (the liquid level). If this switching is carried out automatically by a selector, such a system can be referred to as override control; the override mechanism determines the problem on its own, generally without the need for human intervention.