What is Sequential Control System?

This is a control system that manages a process defined by a series of tasks to be performed i.e. a sequence of operations, one after the other. Each operation in the sequence is performed either for a certain period of time (in which case, it is time-driven), or until the task is finished e.g. as indicated by a limit switch (in which case it is event-driven).

A time-driven sequence is open loop because there is no feedback while an event-driven task is closed loop because feedback signal is required to specify when the task is finished.

A typical example of a sequentially controlled system is the automatic washing machine. The first event in the wash cycle is to fill the tub with water. This is an event-driven task because the water is admitted till it gets to the proper level as indicated by a float and limit switch (i.e. a closed loop). The next two tasks, wash and spin drain, are each done for a specified period of time and are time-driven events i.e. open loop.

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