channel talk | Tips and Strategies for Systems Integrators
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-