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5.3.1 Sequencing
Situations often occur where it is necessary to activate a number of cylinders in some sequence. Thus
event 2 might have to start when event 1 is completed, event 3 when event 2 has been completed. For
example, we might have: only when cylinder A is fully extended (event 1) can cylinder B start
extending (event 2), and cylinder A can only start retracting (event 3) when cylinder B has fully
extended (event 2). In discussions of sequential control, it is common practice to give each cylinder
a reference letter A, B, C, D, etc., and to indicate the state of each cylinder by using a + sign if it is
extended or a - sign if retracted. Thus, a sequence of operations might be shown as A+, B+, A-, B-.
This indicates that the sequence of events is cylinder A extends, followed by cylinder B being
extended, followed by cylinder A retracting, followed by cylinder B retracting. Figure 5.15 illustrates
this with a displacement step diagram. Figure 5.16 shows a circuit that could be used to generate this
displacement-event diagram for two cylinders A and B.
FIGURE 5.15 Displacement-event diagram.
FIGURE 5.16 Two-actuator sequential operation.
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