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Sequential control can be the most difficult

For almost any project, there will be three types of control: Discrete devices A discrete device (or discrete component) is an electronic component with just one circuit element, either passive (resistor, capacitor, inductor, diode) or active (transistor or vacuum tube), other than an integrated circuit. , www.isa. org/link/200804CT; Continuous, www.isa. org/link/CT_1 108; 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 shut·down  
A cessation of operations or activity, as at a factory.


the closing of a factory, shop, or other business


shut down
 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 Sequential function chart (SFC) is a graphical programming language used for PLCs. It is one of the five languages defined by IEC 61131-3 standard. The SFC standard is defined in IEC 848, "Preparation of function charts for control systems", and was based on GRAFCET.  (SFC SFC
sergeant first class
). 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 1. (probability) deterministic - Describes a system whose time evolution can be predicted exactly.

Contrast probabilistic.
2. (algorithm) deterministic - Describes an algorithm in which the correct next step depends only on the current state.

Sequence structure

However documented, the "specifier" should insist the programmer (1) A hardware device used to customize a programmable logic chip such as a PAL, GAL, EPROM, etc. See PROM programmer.

(2) A person who designs the logic for and writes the lines of codes of a computer program.
 be able to answer the following questions for each Sequence:

1. What are the Start Conditions for this Sequence?

a. Start the Sequence Timer timer,
n radiographic timing device that functions as an automatic exposure timer and a switch to control the current to the high-tension transformer and filament transformer. The face of the timer is calibrated in seconds and fractions of seconds.

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, orto 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?J

a. What are the Jump Conditions^,

b. What are the Jump Actioj;

5) Should this Step have a^ause" 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 The Standpoint is a newspaper published in the British Virgin Islands. It was originally published under the name Pennysaver, largely as a shopping-coupon promotional newspaper, but since emerged as one of the most influential sources of journalism in the . 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 (Human Machine Interface) The user interface in a manufacturing or process control system. It provides a graphics-based visualization of an industrial control and monitoring system.  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 A/M Away Message (AOL Instant Messaging)
A/M Automatic / Manual
A/M Aeromobile (Italian: airplane)
A/M Ampere Per Meter
A/M approach and moor (US DoD) 
) 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 re·work  
tr.v. re·worked, re·work·ing, re·works
1. To work over again; revise.

2. To subject to a repeated or new process.


This properly defined set will go far towards guaranteeing the systems integrator An individual or organization that builds systems from a variety of diverse components. With increasing complexity of technology, more customers want complete solutions to information problems, requiring hardware, software and networking expertise in a multivendor environment.  delivers a system that will behave as desired.

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LLC - Logical Link Control
. All Rights Reserved.
Copyright 2008 InTech
No portion of this article can be reproduced without the express written permission from the copyright holder.
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Author:Michael Whitt
Date:Dec 1, 2008
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