Step_1 activated by 1st cycle bit, but it will turn off during the 2nd cycle onwards. After which it will be activated by step_2 and stop_PB (during the return loop). Internal scan cycle (the very first cycle). The initial step should be activated by the PLC Do not worry about outputs at the moment. Only 1 step should be active at any one time.ģ. R1 and R2 will either be 1 (active) or 0 (inactive).Ģ. Represent the steps (step_1 and step_2) by internal Grafcet Design Sequence How to do the sequence. Labelled Connector : indicating where control is going to (destination Connector). Simultaneous De-activation : indicated a double horizontal line followed by transition. Start of Sequence Selection : indicated by a single horizontal line followed by two or more transitions.Įnd of Sequence Selection : indicated by one or more transitions followed by a single Horizontal line. Simultaneous Activation : indicated by transition followed by a double horizontal line. Step : a steady state with which command or actions may be associated. Transition : between steps and with which logic conditions are associated.ĭirected Link : connecting steps to transitions, and transitions to steps. Initial Step : defining the initial situation of the automated system. Initial Step (double box) Initiated by First Cycle Bitĩ Labelled Connector : indicating where control has come from (source The working of an automatic equipment may be described graphically by a group of: STEPS to which are associated certain ACTIONS TRANSITIONS which are conditions to be fulfilled DIRECTED LINKS linking steps and transitionsīlock Diagram of a PLC-controlled automatic system :Ĭylinder A retracted cylinder B retractedĠ WAIT step ( - doing nothing, in this case ) Chapter 9 : GRAFCET and Ladder Diagram The GRAFCET is a tool for describing the specifications of the control unit of an automated system.
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