Sequence of events

The Sequential Function Chart ( AS , English: Sequential Function Chart , SFC ) is one of five in the 61131 EN -3 standardized programming languages and is used for programming a programmable logic controller in the form of a Petri net . In Siemens STEP 7 , the sequence language is known as S7-GRAPH.

functionality

A sequence control is a chain of control steps that are linked to one another by transition conditions ( transitions ).

Commands, also called actions, are integrated directly at the step symbols. When a control step with an attached action is reached, an output of the control is set and an actuator is activated (e.g. pump on, solenoid valve open) or an internal program jump is carried out.

The transitions that are located between the individual control steps are each linked with an input bit (step enabling condition ) that has been sent from a sensor to an input of the control (e.g. limit switch for level, limit switch, light barrier). A signal change of the input bit triggers the switching of the transition so that the subsequent control step is activated.

The first control step does not normally have its own action, as this is also an initialization step. This means that the program starts with the initialized step during the first operating cycle. Each control step is linked to the next control step by a transition.

  ╔════════╗
  ║ ║
  ║ START ║ Initial state
  ╚═══╤════╝
      │
     ─┼─Füllstand_low step enabling condition 
      │
  ┌───┴────┐ ┌───┬────────────┐
  │ fill ├──┤ S │pump schalten Switch on control step with command pump
  │ active │ └───┴────────────┘
  └───┬────┘
      │
     ─┼─Level_high step enabling condition
      │
  ┌───┴────┐ ┌───┬────────────┐
  │ fill ├──┤ R │pump │ Switch off control step with command pump
  │ done │ └───┴────────────┘
  └───┬────┘
      │
      ↓
    BEGIN

advantages

The sequence control is particularly well suited for error analysis, as the current control step and the transition conditions are visualized at any time.

distribution

Sequence control is particularly widespread in large systems. Despite standardization, not every PLC manufacturer offers the sequential function chart as programming software.