December 2008

Sequential control can be the most difficult

By Michael Whitt

For almost any project, there will be three types of control: Discrete devices, www.isa.org/link/200804CT; Continuous, www.isa.org/link/CT_1108; and Sequential.

The control logic specification should define each of the major sequential-control subsystems and give a general outline of how the system needs to behave.

A sequential control element is one in which a series of activities need to occur in a specific order.

Every process control facility, even if it is primarily a "continuous" process, has sequential aspects. Startup and shutdown are two examples that are hard to escape.

Sometimes, the sequences are operator-intensive, where the operator gets the system to a stable operating condition and then puts it into automatic mode. However, other systems are not. Batch applications, for example, are sequence (recipe)-heavy.

One of the best ways to describe a sequence is to use a Sequential Function Chart (SFC). An SFC consists of States and Transitions. Each box, in the SFC diagram, represents a miniature "step" program that executes until the "transition" condition shown beneath it becomes true. When that occurs, the program transits to the next Step, and executes those tasks.

In this chart, the long, horizontal, double-line represents an "Or" function.

A Step can contain a single command, or it can even contain another SFC.

In either case, part of the transition logic should contain confirmation that the action(s) commanded in the Step logic has actually occurred, thus making the logic deterministic.

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Sequence structure

However documented, the "specifier" should insist the programmer be able to answer the following questions for each Sequence:

1. What are the Start Conditions for this Sequence?

 a.  Start the Sequence Timer

 b.  Raise the Sequence Active Flag…

2.  Are all sequence-related devices in Automatic? It may be desirable to have a Zone-Auto/Manual feature to allow the Operator to put all the devices in automatic, or to automatically put them in Automatic when the sequence is activates.

3. What are the Exit Conditions for this Sequence? 

4.  What are the Exit Actions for this Sequence?

 a. Stop the Sequence Timer

 b. Lower the Sequence Active Flag…

Sequence step structure

The programmer should be able to answer the following questions for each Sequence Step:

1.  What are the Start Conditions for this Step?

2.  What actions should take place in this Step?

 a. Start the Step Timer and Raise the Step Active flag

 b. Open Valve X, etc.

3.  How long should this Step be active before setting
 an alarm?

4.  Should this Step have a "Jump" function? If so,

 a. What are the Jump Conditions?

 b. What are the Jump Actions?

5. Should this Step have a "Pause" function? If so,

 a. What are the Pause Conditions?

 b. What are the Pause Actions?

 c. What are the Resume Conditions?

 d. What are the Resume Actions?

6. Should this Step have a Safe-State program? If so,

 a. What are the Safe-State Conditions?

 b. What are the Safe-State Actions?

 c. What are the Resume Conditions?

 d. What are the Resume Actions?

7.  What are the Exit Conditions for this Step? This is  the Transition Logic.

8.  What are the Exit Actions for this Step?

 a. Stop the Step Timer

 b. Lower the Step Active Flag…

Human-machine interface aspects

Sequences can be the most difficult of the control schemes from an operator's standpoint. Being able to know whether the sequence is operating correctly can be the difference between a good batch and a bad one.

At a minimum, the HMI needs to provide key information
to the operator as to the Active Step and Step Time. Alarm status should feed in as well, and it should tie into the HMI Alarm Manager.

It is important to pre-define the behavior of each sequence. Some of the considerations are:

  • Auto/Manual (A/M) Mode Switching-In some cases, it may be desirable to give the Operator the ability to single-step through a sequence (Manual Mode). In Manual Mode, even if the transition conditions are met, the transition logic will not be satisfied until the Operator presses the Next Step button. At that time, 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.

A properly defined set of Sequential Control Requirements will greatly reduce misunderstandings and related rework.

This properly defined set will go far towards guaranteeing the systems integrator delivers a system that will behave as desired.

ABOUT THE AUTHOR

Michael Whitt (mwhitt@mesainc.com) is an ISA Senior Member and the Manager of Integrated Systems at Mesa Associates, Inc.