Sequential function charts

Sequencing, of fermentation, 11 40-41 Sequencing project results, assembly and analysis of, 12 510-512 Sequential function charts (SFC), 20 705-707... 

The basic constructs of sequential function charts are presented in Fig. 8-61. The basic construct of a sequential function chart is the step-transition-step. But also note the constructs for OR and AND. At the divergent OR, the logic proceeds on only one of the possible paths, specifically, the one whose transition is the first to attain the true condition. At the divergent AND, the logic proceeds on all paths simultaneously, and all must complete to proceed beyond the convergent AND. This enables sequential function charts to provide parallel logic. 

In the sequential function chart in Fig. 8-606 for the pumps, the logic is initiated with both pumps stopped and pump A as the lead pump. When LH actuates, the lead pump is started. A divergent OR is used to create two paths ... 

Start-up methods. In this study, methods of start-up of a continuous cooling crystallizer are considered. Although several start-up methods are possible two typical ones as shown in Figure 2 were chosen. This description is based on the SFC (Sequential Function Chart). The SFC is conq)Osed of step and transition . Step , which is denoted with a box, shows the state of apparatus. Transition with a bold line indicates a condition that makes a move of the step . The items on the side of step are actual operations. The feed solution was charged to the crystallizer from the feed line. The feed was stopped when the liquid level arrived at the set up value, i.e. the hold up was fulfilled, l en, the solution was cooled to the operating temperature. Seed crystals were added when the solution temperature became constant at the set up value. After that, the continuous feeding and discharge were started. In this study, the experimental duration was more than 10 residence times in order to ensure the attainment of steady state. This method of start-up is defined as SU Method 1. 

NOTE 1 Typical examples of LVL are given in lEC 61131-3. They include ladder diagram, function block diagram and sequential function chart. 

For precise modelling of batch operations a formalism that describes the system behaviour at this level of details is needed. Such formalism must allow analysing the interactions of the different process units and steps that allows developing the best co-ordination control system (Fig. 4). The Petri Net and Sequential Function Chart (SFC) have been used to adequately represent and develop the co-ordination system of batch processes. 

Sequential Function Chart (SFC) has been standardised (lEC 1131-3) as a representation format for sequential control logic in manufacturing and batch processes industries. 

A Sequential Function Chart is a graphical method of representing a sequential control system by using a sequence of steps and transitions (Fig. 5). Each step is a command or action that is either active or inactive. The flow of control passes from one step to the next through a conditional transition that can be true or false. 

Fig. 17. Sequential Function Chart for the recipes described in Table 3 (R1 and T1 only).

To create an information flow chart, a complete list of steps for a batch process must be documented and displayed. From this representation, it is straightforward to prepare a sequential function chart. Figure 22.4 shows the flow-chart symbols that indicate the points of decisions, processing operations, input-output structure, and the sequence they should follow, which will be illustrated later in Example 22.1. 

A sequential function chart (SFC) describes graphically the sequential behavior of a control program. More sophisticated than the information flow chart, it is derived from two earlier approaches, Petri Nets and Grafcet (David, 1995 Desrochers and Al-Jaar, 1995). SFCs consist of steps that alternate between action... 

Figure 22,5 A generic sequential function chart (active steps are 3 and 6). Ri through denote specified actions.

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