Page 693 - Mechanical Engineers' Handbook (Volume 2)

684 General-Purpose Control Devices

• Divergence, that is, initiation of two or more independently executing sequences based
on a transitioncondition, as shown in Fig. 4d
The mechanisms for representing convergence, that is, resumption of a main sequence
after step selection or parallel sequence initiation, are also shown in Fig. 4c and Fig. 4d,
respectively.
Figure 4b illustrates the representation of the initial step of each sequence. The operation
of control sequences can be visualized by placing a token in each initial step upon the
initiation of system operation. A step is then said to be active while it possesses a token and
reset when it does not possess a token. The actions associated with the step are performed
while the step is active and are not performed when it is reset. The resetting of one or more
steps and the activation of one or more successor steps can then be envisioned as the proc-
esses of token passing, consumption, and generation, as shown in Fig. 5 and Fig. 6. It should
be noted that the selection and convergence of alternate paths within a sequence, as shown
in Fig. 5, simply involve the passing of a single token. In contrast, divergence to multiple
sequences involves the consumption of a single token and the generation of multiple tokens,
as shown in Fig. 6a, with the converse operation for termination of multiple sequences shown
in Fig. 6b.
When an action associated with a step is a Boolean variable, its association with the
step may be expressed in an action block, as shown in Fig. 7. The qualifiers which can be
used to specify the duration of the action are listed in Table 1. More complex actions can
be specified via one of the programming languages described in Section 2.3; in this case,
the action executes continuously while the associated ‘‘action control’’ shown in Table 1 has
the Boolean value ‘‘1.’’
An example of the application of SFCs to the control and monitoring of a single motion,
for example, in a robot control system, is given in Fig. 8. Here, the system waits until a
motion command is received via the Boolean variable CMD IN. It then initiates the appro-
priate motion by asserting the Boolean variable CMD. If a feedback signal DONE is not

Figure 5 Sequence selection and convergence.