Page 692 - Mechanical Engineers' Handbook (Volume 2)

1 Characteristics of General-Purpose Control Devices 683

• The IEC 61131-3 standard defines a set of sequential function chart (SFC) constructs,
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specifically intended to make the IEC 60848 concepts usable for the programming of
programmable logic controllers (PLCs).
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• The IEC 61512-1 standard defines SFC-like constructs for the description of the
sequential operations involved in the control of batch chemical processes.
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• The IEC 61499-1 standard defines an execution control chart (ECC) construct which
provides for the sequential execution of control algorithms in function blocks for
distributed automation systems, similar to the Harel state chart notation of the Unified
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Modeling Language (UML) .
As shown in Fig. 4a, an SFC is constructed from three basic types of elements:
• A step, representing the current state of the controller and controlled system within
the sequential control plan
• A set of associated actions at each step
• A transition condition that determines when the state of the controller and controlled
system is to evolve to another step or set of steps
An SFC consists of a set of independently operating sequences of control actions built up
out of these basic elements via two mechanisms:
• Selection of one of a number of alternate successors to a step based on mutually
exclusive transition conditions, as shown in Fig. 4c

Figure 4 Sequential function chart constructs.