Sequential Function Charts
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Sequential Function Charts can be used to describe control sequences precisely.  They are also used for defining Recipe procedures, along with the similar Procedure Function Charts


Below is an example of a detailed  sequence to add material to a vessel

With the SFC Drawing capabilites of ControlDraw you can easily produce these charts


Originally, Grafcet was developed in France in the late 70s and it became the basis for the definition of the international standard IEC 848 (“Preparation of function charts for control systems”).


SFC's comprise a number of steps and transitions connected to each other by directed links.


Steps:

Every step can execute one or more actions.

A step can be either active or inactive. When it becomes active, it's actions are executed (but see action types)


Transitions:

There are transitions between every step.

When a Step is active and the following transition become true the successor step becomes active and the preceding step becomes inactive.

Each transition has associated events which are called transition conditions.

The transition conditions must combine to make a simple Boolean result, True or False.


Step Actions:

Every action is associated with a step. The step can have none or several actions associated. If

there is no associated action to the step, it will be considered as a WAIT function.

Each step/action association shall have an associated action qualifier. The action qualifier can

have values according to IEC 61131-3


Program Flow

A step becomes active when it's preceding transition activates (that is the transition is true, while the step before that transition is active.)

A step becomes inactive when it's subsequent transition becomes true

In Parallel branches, all the initial steps become active when the preceding transition activates

A parallel branch end when all the steps before the branch end are be active, and the subsequent transition becomes true



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