Introduction to Industrial Automation

Figure 7.3  Indicative form of an instruction in IL language.

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  Introduction to Industrial Automation

between the ST language and other traditional languages is the way the flow of a program is con-

trolled. An ST program can run from start to the end many times within a second. Conversely, 

a LAD or FBD program cannot reach the last instruction unless the execution of the previous 

instructions is complete. The ST language is designed to consociate with the other languages of 

the standard. For example, a LAD program can call a subroutine written in the ST language.

  5. Sequential Function Chart (SFC)—Basically a Graphical Language that Uses Program 

Segments Written in IL Language.

    The SFC language is based on the GRAFCET (graphical function chart), which is a 

software tool of French origin and is very similar to Petri networks. The GRAFCET lan-

guage (IEC 848 standard) was established in 1977 by a French working group to provide a 

graphical tool for describing and defining sequential industrial processes that are controlled 

by logical controllers. The basic SFC programming components (symbols) are:

 

1.  Steps or system operating states associated with actuator actions 

STEP

ACTION

STEP

ACTION

 

2.  Transitions related to logical conditions of system variables such as the condition “IF t = 

30° C” or the wait condition “WHEN TIMER IS ON” 

 

3.  Directional interconnections between steps and transitions.

    Figure 7.4 presents a general example of an SFC language program in order to understand 

its utilization.

–  In each branch only one step can be active.

–  When a transition is satisfied, which means that all the conditions for this transition 

are met, then the previous step is deactivated and the next one is forced to be executed.

Initial 

Step

Action 1

Step 1

Action 4

Step 3

Step 2

Action 3

Transition 1

Lad

Program

(    )

(    )

(    )

Lad

Program

(    )

(  )

(  )

(  )

(  )

(  )

Transition 3

Transition 2

Transition 5

Transition 4

Initial

step

Action 2

Action 1

Step 1

Action 4

Step 3

Step 2

Action 3

LAD

program

(    )

LAD

program

(  )

(  )