Sequential control can be the most difficult by ProQuest


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Sequential control can be the most difficult
By Michael Whitt

                                  or almost any project, there will be three                                                     consists of States and Transitions. Each box, in the
                                  types of control: Discrete devices, www.isa.                                                   SFC diagram, represents a miniature “step” pro-
                                  org/link/200804CT; Continuous, www.isa.                                                        gram that executes until the “transition” condi-
                           org/link/CT_1108; and Sequential.                                                                     tion shown beneath it becomes true. When that
                                               The control logic specification                                                    occurs, the program transits to the next Step, and
 Key sections for a control spec             should define each of the major                                                      executes those tasks.
 Section 1 – Process overview                sequential-control subsystems                                                          In this chart, the long, horizontal, double-line
 Section 2 – Operability & visualization     and give a general outline of how                                                   represents an “Or” function.
 Section 3 – Device control                  the system needs to behave.                                                            A Step can contain a single command, or it can
 Section 4 – Continuous control                A sequential control element is                                                   even contain another SFC.
 Section 5 – Sequence control                one in which a series of activities                                                    In either case, part of the transition logic should
 Section 6 – Alarms & reports                need to occur in a specific order.                                                   contain confirmation that the action(s) command-
 Section 7 – Data flow & historization          Every process control facility,                                                   ed in the Step logic has actually occurred, thus
 Section 8 – Fault tolerance & recovery      even if it is primarily a “continu-                                                 making the logic deterministic.
                                             ous” process, has sequential as-
                           pects. Startup and shutdown are two examples                                                          Sequence structure
                           that are hard to escape.                                                                              However documented, the “specifier” should insist
                              Sometimes, the sequences are operator-in-                                                          the programmer be able to answer the following
                           tensive, where the operator gets the system to a                                                      questions for each Sequence:
                           stable operating condition and then puts it into                                                      1. What are the Start Conditions for this Sequence?
                           automatic mode. However, other systems are not.                                                          a. Start the Sequence Timer
                           Batch applications, for example, are sequence                                                            b. Raise the Sequence Active Flag…
                           (recipe)-heavy.                                                                                       2. Are all sequence-related devices in Automatic? It
                              One of the best ways to describe a sequence is                                                         may be desirable to have a Zone-Auto/Manual
                           to use a Sequential Function Chart (SFC). An SFC                                                          feature to allow the Operator to put all the de-
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