Petruzella F.D. Programmable Logic Controllers

Process Control, Network Systems, and SCADA Chapter 14 313

ef ciency. In general, unlike distributive control systems,

a SCADA system usually refers to a system that coordi-

nates but does not control processes in real time .

In a typical SCADA system, independent PLCs per-

form I/O control functions on  eld devices while being

supervised by a SCADA/HMI software package running

connectors or wiring practices such that disconnection of

a single device is possible without disrupting the continu-

ity of the whole segment.

PROFIBUS-DP

PROFIBUS-DP (where DP stands for Decentralized

Periphery) is an open, international  eldb

us communi-

cation standard that supports both analog and discrete

signals. It is functionally comparable to DeviceNet.

The physical media are de ned via the RS-485 or  ber

optic transmission technologies. PROFIBUS-DP com-

municates at speeds up to 12 Mbps over distances up to

1200 meters. Figure14-46 illustrates a Siemens S7-200

Micro PLC system connection to a PROFIBUS-DP

network.

14.7 Supervisory Control

andData Acquisition (SCADA)

In some applications, in addition to its normal control

functions, the PLC is responsible for collecting data,

performing the necessary processing, and structuring the

data for generating reports. As an example, you could

have a PLC count parts and automatically send the data to

a spreadsheet on your desktop computer.

Data collection is simpli ed by using a SCADA (su-

pervisory control and data acquisition) system, shown in

Figure14-47 . Exchanging data from the plant  oor to a

supervisory computer allows data logging, data display,

trending, downloading of recipes, setting of selected pa-

rameters, and availability of general production data. The

additional supervisory control output capabilities allow

you to tweak your processes accurately for maximum

Figure 14-46 Micro PLC system connection to a PROFIBUS-DP network.

Source: Courtesy Siemens.

PROFIBUS-DP

module

PROFIBUS-DP

Figure 14-47 Supervisory control and data acquisition

(SCADA).

Alarms

PLC relay

controls

Analog

outputs

PID

process

control

Printer

Tables and

graphs

Hardcopy

Spreadsheet

Temperature

thermocouples

and resistance

temperature detectors

Pressure

Position

Force

Strain

Speed

Signal sources

Interface

system

Output loads

Computer

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314 Chapter 14 Process Control, Network Systems, and SCADA

Figure 14-48 Typical SCADA system.

Host computer

SCADA/HMI

software

Data

transfer

I/O

control

Field

devices

I/O

control

Field

devices

I/O

control

Field

devices

PLC

on a host computer, as illustrated in Figure 14-48 . Pro-

cess control operators monitor PLC operation on the

host computer and send control commands to the PLCs

if required. The great advantage of a SCADA system is

that data are stored automatically in a form that can be

retrieved for later analysis without error or additional

work. Measurements are made under processor control

and then displayed onscreen and stored to a hardcopy. Ac-

curate measurements are easy to obtain, and there are no

mechanical limitations to measurement speed.

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Process Control, Network Systems, and SCADA Chapter 14 315

1. Compare continuous and batch processes.

2. Compare centralized and distributive control

systems.

3. State the basic function of each of the following as

part of a process control system:

a. Sensors

b. Human-machine interface

c. Signal conditioning

d. Actuators

e. Controller

4. State the purpose of each of the following types of

screens associated with HMIs:

a. Trend values

b. Operational summary

c. Alarm summary

5. What is the main characteristic of a closed-loop

control system?

6. State the function of each of the following parts of

a closed-loop control system:

a. Set-point

b. Process variable

c. Error ampli er

d. Controller

e. Output actuator

7. Explain how on/off control works.

8. How does the proportional controller eliminate the

cycling associated with on/off control?

9. Explain how a motor-driven control valve action

can provide analog control.

10. How does time proportioning provide analog

control?

11. What process error or deviation is produced by a

proportional controller?

12. What term of a PID control is designed to eliminate

offset?

13. What does the derivative action of a controller

respond to?

14. List the three gain adjustments used in tuning the

response of a PID control loop.

15. Compare manual, autotune, and intelligent tuning

of a PID controller.

16. How many input and output values are normally

referenced in a PLC PID instruction?

17. What information is contained in the process

variable and control variable elements of a PID

instruction?

18. State the function of each of the following elements

of a PLC motion control system:

a. Programmable controller

b. Motion module

c. Servo drive

d. Servo motor

19. What does each axis of a robot arm function as?

20. List four types of communication tasks provided by

local area networks.

21. Name three common types of transmission media.

22. What are the three general levels of functionality of

industrial networks?

23. D e 

ne the term node

as it applies to a network.

24. Explain the physical layout of devices on a network

for each of the following network topologies:

a. Star

b. Bus

25. Compare device and process bus networks.

26. D e  ne the term protocol as it applies to a network.

27. What is the function of a network gateway?

28. D e  ne the term access method as it applies to a

network.

29. Summarize the token passing network access method.

30. Summarize the collision detection network access

method.

31. Summarize the polling network access method.

32. Compare parallel and serial data transmission.

33. Compare half-duplex and full-duplex data

transmission.

34. Explain how networking schemes minimize the

amount of wiring required.

35. What type of access control is used with DH+?

36. Compare the transmitting distances of RS-232

andRS-422/485 serial types.

37. What is DeviceNet used for?

38. List three pieces of information obtained from

DeviceNet devices by the network scanner.

39. What is ControlNet used for?

CHAPTER 14 REVIEW QUESTIONS

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316 Chapter 14 Process Control, Network Systems, and SCADA

44. What is Fieldbus used for?

45. Summarize the two main functions of a SCADA

system.

46. In what way does distributive control differ from

the supervisory control of a SCADA system?

40. Explain how redundant media works.

41. D e  ne the term bandwidth as it applies to a

network.

42. What is Ethernet/IP used for?

43. What type of protocol does Modbus use?

1. Distributive control systems have to be network

based. Why?

2. Assume an alarm is sounded in a control system

with an electronic HMI interface. How would you

proceed to identify and solve the problem?

3. How would an on/off controller respond if the dead-

band were too narrow?

4. In a home heating system with on/off control, what

will be the effect of widening the deadband?

5. a. Calculate the proportional band of a temperature

controller with a 5% bandwidth and a set-point of

500°F.

CHAPTER 14 PROBLEMS

b. Calculate the upper and lower limits beyond

which the controller functions as an on/off

unit.

6. Explain the advantage of using a 4- to 20-mA

current loop as an input signal compared to a 0- to

5-V input signal.

7. What does the term deterministic mean, and why is

it important in industrial communications?

8. How might a SCADA system be applied to deter-

mine the production rate of a bottled product over a

two-week period?

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317

system, CompactLogix system, FlexLogix sys-

tem, SoftLogix 5800 controller, and DriveLogix

system. Software is the essential difference

between PACs and PLCs. Basically, the ladder

logic conﬁ guration does not change but the

addressing of the instructions changes. Applica-

tion of the software that pertains to the Logix

control platform of controllers will be covered in

the various sections of this chapter. Knowledge

of basic ladder logic instructions and functions

(bit, timer, counter, etc.) covered in previous

chapters of the text is assumed and is thus not

repeated in this chapter.

Programmable logic controllers continue to

evolve as new technologies are added to their

capabilities. The PLC started out as a replace-

ment for banks of relays used to turn outputs

on and off as well as for timing and counting

functions. Gradually, various math and logic

manipulation functions were added. In order to

serve today’s expanding industrial control system

needs, leading automation companies have cre-

ated a new class of industrial controllers called

programmable automation controllers or PACs

( Figure 15-1 ). They look like PLCs in their physi-

cal appearance but incorporate advanced control

of communication, data logging, and signal pro-

cessing, motion, process control, and machine

vision in a single programming environment.

The Allen-Bradley programmable automa-

tion controller family includes the ControlLogix

ControlLogix Controllers

Figure 15-1 Programmable automation controllers (PACs).

Source: Image Used with Permission of Rockwell Automation, Inc.

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318

RSLogix 5000 programming software is used to set

up or con gure the memory organization of an Allen-

Bradley ControlLogix controller. RSLinx communication

software is used to set up a communications link between

RSLogix 5000 programming software and the Control-

Logix hardware as illustrated in Figure 15-3 . To estab-

lish communications with a controller, a driver must be

created in RSLinx software. This driver functions as the

software interface to a hardware device. The RSWho is the

network browse interface that provides a single window

to view all con gured network drivers.

Figure 15-4 shows an example of the ControlLogix’s

controllers properties and modules properties dialog

boxes used as part of the con guration process. The

parameters shown are typical of what general informa-

tion is required. After  rst con guring the controller,

Memory Layout

ControlLogix processors provide a  exible memory struc-

ture. There are no  xed areas of memory allocated for

speci c types of data or for I/O. The internal memory orga-

nization of a ControlLogix controller is con gured by the

user when creating a project with RSLogix 5000 software

( Figure 15-2 ). This feature allows the program data to be

constructed to meet the needs of your applications rather

than requiring your application to  t a particular memory

structure. A ControlLogix (CLX) system can consist of

anything from a stand-alone controller and I/O modules in

a single chassis, to a highly distributed system consisting

of multiple chassis and networks working together.

Conﬁ guration

Con guration of a modular CLX system involves es-

tablishing a communications link between the control-

ler and the process. The programming software needs to

know what CLX hardware is being used in order to be

able to send or receive data. Con guration information

includes information about the type of processor and I/O

modules used.

Part Objectives

After completing this part, you will be able to:

• Outline project organization

• De ne tasks, programs, and routines

• Identify data  le types

• Organize and apply the various data  le types

Part 1 Memory and

Project Organization

Figure 15-2 RSLogix 5000 screen.

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Memory and Project Organization Part 1 319

the I/O modules are con gured using RSLogix 5000

software. Modules will not work unless they have been

properly con gured. The software contains all the hard-

ware information needed to con gure any ControlLogix

module.

Project

RSLogix software stores a controller’s programming and

con guration information in a  le called a project. The

block diagram of the processor’s project  le is shown in

Figure 15-5 . A project  le contains all information relat-

ing to the project. The main components of the project  le

are tasks, programs, and routines. A controller can hold

and execute only one project at a time.

Figure 15-5 ControlLogix processor program ﬁ le.

Source: Image Used with Permission of Rockwell Automation, Inc.

Project

Controller tags

(global data)

Other routines

Program tags

(local data)

I/O data

System-shared data

Task

Main routine

Program

Figure 15-3 RSLinx and RSLogix software.

RSLogix

Ladder logic program

RSLinx

File View Communications

RSLinx

RSLogix

RSWho

Configure Drivers...

5000

ControlLogix

Controller

Figure 15-4 Controllers properties and modules properties dialog boxes.

Controllers propertiesGeneral

Vendor:

Type:

Revision:

Allen-Bradley

1756-L55ControlLogix5555Controller

10.24

Name:

Description:

Chassis Type:

Slot: 1

Controller 1

Prime Controller

1756-A7 7-Slot Chassis

Modules propertiesGeneral

Type:

Vendor:

Parent:

1756-IB16 16 Point 10V-31.2V DC Input

Allen-Bradley

Local

Name:

Description:

Comm Form:

Revision:

Electronic Keying Exact Match1

Digital_Input_16pt

Optional

Input Data

Slot: 0

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320 Part 1 Memory and Project Organization

The RSLogix 5000 controller organizer ( Figure 15-6 )

displays the project organization in a tree format showing

tasks, programs, routines, data types, trends, I/O con gu-

ration and tags. Each folder groups common functions

together. This structure simpli es the navigation and the

overall view of the whole project.

In front of each folder, there is an icon containing a

1 sign or a 2 sign. The 1 sign indicates that the folder

is closed. Click it to expand the tree display and display

the  les in the folder. The 2 sign indicates that the folder

is already open and its contents are visible. Clicking on

the right mouse button brings up many different, context-

sensitive popup menus. Often, you  nd that this is a short-

cut to access the property window or menu options from

the menu bar.

Tasks

Tasks are the  rst level of scheduling within a project. A

task is a collection of scheduled programs. When a task

is executed, the associated programs are executed in the

order listed. This list of programs is known as the program

schedule. Tasks provide scheduling based on speci c con-

ditions and do not contain any executable code. Only one

task may be executing at any given time. The number of

tasks a controller can support depends on the speci c con-

troller. The main types of tasks ( Figure 15-7 ) include:

• Continuous tasks execute nonstop but are always

interrupted by a periodic task. Continuous tasks

have the lowest priority. A ControlLogix continuous

task is similar to the File 2 in the SLC 500 platform.

Here the continuous task is named Main Task.

• Periodic tasks function as timed interrupts. They

interrupt the continuous task and execute for a  xed

length of time at speci c time intervals.

• Event tasks also function as interrupts. Rather than

being an interrupt on a timed basis, an event task

is triggered by an event that happened or failed to

happen.

Programs

Programs are the second level of scheduling within a

project. The function of the folders under Main Task is

to determine and specify the order in which the programs

Figure 15-6 Controller organizer tree.

Controller RSLogix

Tasks

Controller Tags

Program Tags

MainRoutine

Continuous

Program_01

Controller Fault Handler

Power-Up Handler

Program Tags

MainRoutine

Periodic

Unscheduled Programs

Data Types

Motion Groups

Trends

User-Defined

Program_02

Program Tags

Main

HMI

Strings

Predefined

Module-Defined

I/O Configuration

Figure 15-7 Continuous and periodic tasks.

Program 1

Program 2

Program 32

Continuous

task

Continuous task scan

Task auto-restart

Periodic task scan

5 ms

10 ms

15 ms

...

Program 1

Program 2

Program 32

Periodic

task

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execute. There is no executable code within a program.

Routines within programs will execute in the order listed

below their associated task in the controller organizer as

shown in Figure 15-8 . In this example, according to the

listed order, the Main Program is scheduled to execute

 rst, Program_A second, and Progam_B third. Programs

that are not assigned to a task are unscheduled. Unsched-

uled programs are downloaded to the controller but do

not execute. These programs remain unscheduled until

needed. Depending on the RSLogix 5000 software ver-

sion as many as 100 programs could be scheduled within

each task.

Routines

Routines are the third level of scheduling within a proj-

ect and provide the executable code for the project. Each

routine contains a set of logic elements for a speci c

programming language. When a routine is created it is

speci ed as ladder logic, sequential function chart, func-

tion block diagram or structured text ( Figure 15-9 ). Any

one routine must be completely in the same language.

The number of routines per project is limited only by the

amount of controller memory. Libraries of standard rou-

tines can be created that can be reused on multiple ma-

chines or applications. A routine can be assigned as one

of the following types:

• A main routine is one con gured to execute  rst

when the program runs. Each program will have one

main routine typically followed by several or many

subroutines.

• A subroutine is one that is called by another routine.

Subroutines are used for large or complex program-

ming tasks or tasks that require more than one pro-

gramming language.

• A fault routine is one that executes if the controller

 nds a program fault. Each program can have one

fault routine, if desired.

Petruzella F.D. Programmable Logic Controllers

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