Human-machine interface aspects Sequences can be the most difficult of the control schemes from an operator's standpoint. [...] the next step will activate. * Start/Stop/Pause/Resume Mode Switching-Define the mechanism that will give the operator the control needed. * Animation-Define the colors and flash behavior for each state of the sequence.
channel talk | Tips and Strategies for Systems Integrators Sequential control can be the most difﬁcult By Michael Whitt F 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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