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Sequentially Controlled Systems
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A sequentially controlled system controls a process that is defined as a series of tasks to be performed—that is, a sequence of operations, one after the other. Each operation in the sequence is performed either for a certain amount of time, in which case it is time-driven, or until the task is finished (as indicated by, say, a limit switch), in which case it is event-driven. A time-driven sequence is open-loop because there is no feedback, whereas an event-driven task is closed-loop because a feedback signal is required to specify when the task is finished. The classic example of a sequentially controlled system is the automatic washing machine. The first event in the wash cycle is to fill the tub. This is an event-driven task because the water is admitted until it gets to the proper level as indicated by a float and limit switch (closed loop). The next two tasks, wash and spin-drain, are each done for a specified period " of time and are time-driven events (open loop). A timing diagram for a washing machine is shown in Figure 1.10. Another example of a sequentially controlled system is a traffic signal. The basic sequence may be time-driven: 45 seconds for green, 3 seconds for yellow, and 45 seconds for red. The presence or absence of traffic, as indicated by sensors in the roadbed, however, may alter the basic sequence, which is an event-driven control. Many automated industrial processes could be classified as sequentially controlled systems. An example is a process where parts are loaded into trays, inserted into a furnace for 10 minutes, then removed and cooled for 10 minutes, and loaded into boxes in groups of six. In the past, most sequentially controlled systems used switches, relays, and electromechanical timers to implement the control logic. These tasks are now performed
more and more by small computers known as programmable logic controllers (PLCs), which are less expensive, more reliable, and easily reprogrammed to meet changing needs—for example, to put eight items in a box instead of six.
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