Programmable Logic Controllers (eBook)

William Bolton (Autor)

eBook Download: PDF | EPUB

2015 | 6. Auflage
424 Seiten
Elsevier Science (Verlag)
978-0-08-100353-4 (ISBN)

Infos

This textbook, now in its sixth edition, continues to be straightforward and easy-to-read, presenting the principles of PLCs while not tying itself to one manufacturer or another. Extensive examples and chapter ending problems utilize several popular PLCs, highlighting understanding of fundamentals that can be used regardless of manufacturer. This book will help you to understand the main design characteristics, internal architecture, and operating principles of PLCs, as well as Identify safety issues and methods for fault diagnosis, testing, and debugging. New to This edition: - A new chapter 1 with a comparison of relay-controlled systems, microprocessor-controlled systems, and the programmable logic controller, a discussion of PLC hardware and architecture, examples from various PLC manufacturers, and coverage of security, the IEC programming standard, programming devices and manufacturer's software - More detail of programming using Sequential Function Charts - Extended coverage of the sequencer - More Information on fault finding, including testing inputs and outputs with an illustration of how it is done with the PLC manufacturer's software - New case studies - A methodical introduction, with many illustrations, describing how to program PLCs, no matter the manufacturer, and how to use internal relays, timers, counters, shift registers, sequencers, and data-handling facilities - Consideration of the standards given by IEC 1131-3 and the programming methods of ladder, functional block diagram, instruction list, structured text, and sequential function chart - Many worked examples, multiple-choice questions, and problems are included, with answers to all multiple-choice questions and problems given at the end of the book

Former Lecturer at Buckingham Chilterns University College, High Wycombe, UK, and now retired, William Bolton has worked in industry and academia as a senior lecturer in a college of technology, a member of the Nuffield Advanced Physics team, an adviser to a British government aid project in Brazil on technical education, as a UNESCO consultant in Argentina and Thailand, and as Head of Research and Development at the Business and Technician Education Council. He has written many engineering textbooks, including Mechatronics, 4th ed., Engineering Science, 5th ed., Higher Engineering Science, 2nd ed., Mechanical Science, 3rd ed., and Instrumentation and Control Systems.

Inhalt

Front Cover 1
Programmable Logic Controllers 4
Copyright 5
Contents 6
Preface 10
Prerequisite Knowledge Assumed 11
Changes from the Fifth Edition 11
Aims 11
Structure of the Book 12
Acknowledgments 13
Chapter 1: Programmable Logic Controllers 14
1.1 Controllers 14
1.1.1 Relay-Controlled Systems 16
1.1.2 Microprocessor-Controlled Systems 17
1.1.3 The Programmable Logic Controller 18
1.2 Hardware 20
1.3 PLC Architecture 22
1.3.1 Input/Output Unit 22
1.3.2 Sourcing and Sinking 24
1.4 PLC Systems 25
1.4.1 Security 28
1.5 Programs 29
1.5.1 The IEC Standard 30
1.5.2 Programming PLCs 32
Summary 33
Problems 34
Lookup Tasks 35
Chapter 2: Input/Output Devices 36
2.1 Input Devices 36
2.1.1 Mechanical Switches 39
2.1.2 Proximity Switches 42
2.1.3 Photoelectric Sensors and Switches 43
2.1.4 Encoders 44
2.1.5 Temperature Sensors 46
2.1.6 Position/Displacement Sensors 50
2.1.7 Strain Gauges 51
2.1.8 Pressure Sensors 53
2.1.9 Liquid-Level Detectors 54
2.1.10 Fluid Flow Measurement 54
2.1.11 Ultrasonic Proximity Sensors 55
2.1.12 Smart Sensors 55
2.1.13 Sensors Ranges 56
2.2 Output Devices 56
2.2.1 Relay 56
2.2.2 Directional Control Valves 57
2.2.3 Motors 59
2.2.4 Stepper Motors 62
2.3 Examples of Applications 66
2.3.1 A Conveyor Belt 66
2.3.2 A Lift 66
2.3.3 A Robot Control System 67
2.3.4 Liquid-Level Monitoring 68
2.3.5 Packages on Conveyor Belt Systems 68
Summary 69
Problems 70
Lookup Tasks 74
Chapter 3: Digital Systems 76
3.1 The Binary System 77
3.2 Octal and Hexadecimal 77
3.2.1 Octal System 78
3.2.2 Hexadecimal System 79
3.3 Binary Coded Decimals 79
3.4 Numbers in the Binary, Octal, Hex, and BCD Systems 80
3.5 Binary Arithmetic 81
3.5.1 Signed Numbers 82
3.5.2 One's and Two's Complements 82
3.5.3 Floating Point Numbers 83
3.6 PLC Data 84
3.7 Combinational Logic Systems 85
3.8 Sequential Logic Systems 86
3.8.1 Latches 86
3.8.2 Flip-Flops 88
Summary 88
Problems 90
Lookup Tasks 91
Chapter 4: I/O Processing 92
4.1 Input/Output Units 92
4.1.1 Input Units 92
4.1.2 Output Units 95
4.2 Signal Conditioning 98
4.2.1 Changing Voltage Levels 99
4.2.2 Op-Amp Comparator 101
4.2.3 Output Protection 102
4.3 Remote Connections 102
4.3.1 Serial and Parallel Communications 103
4.3.2 Serial Standards 104
4.3.3 Parallel Standards 107
4.3.4 Protocols 109
4.3.5 ASCII Codes 111
4.4 Networks 112
4.4.1 Distributed Systems 113
4.4.2 Network Standards 114
4.5 Examples of Commercial Systems 116
4.5.1 MAP 116
4.5.2 Ethernet 117
4.5.3 ControlNet 118
4.5.4 DeviceNet 118
4.5.5 Allen-Bradley Data Highway 119
4.5.6 PROFIBUS 119
4.5.7 Factory-Floor Network 119
4.6 Processing Inputs 119
4.7 I/O Addresses 121
Summary 122
Problems 123
Lookup Tasks 126
Chapter 5: Ladder and Functional Block Programming 128
5.1 Ladder Diagrams 128
5.1.1 PLC Ladder Programming 130
5.2 Logic Functions 133
5.2.1 AND 133
5.2.2 OR 134
5.2.3 NOT 136
5.2.4 NAND 137
5.2.5 NOR 138
5.2.6 Exclusive OR (XOR) 139
5.3 Latching 140
5.4 Multiple Outputs 140
5.5 Entering Programs 142
5.5.1 Ladder Symbols 143
5.6 Function Blocks 143
5.6.1 Logic Gates 143
5.6.2 Boolean Algebra 148
5.7 Program Examples 151
5.7.1 Location of Stop Switches 153
Summary 154
Problems 155
Lookup Tasks 163
Chapter 6: IL, SFC, and ST Programming Methods 164
6.1 Instruction Lists 164
6.1.1 Ladder Programs and Instruction Lists 166
6.1.2 Branch Codes 168
6.1.3 More Than One Rung 171
6.1.4 Programming Examples 172
6.2 Sequential Function Charts 173
6.2.1 Branching and Convergence 176
6.2.2 Actions 178
6.2.3 Programming a PLC 180
6.3 Structured Text 180
6.3.1 Conditional Statements 182
6.3.2 Iteration Statements 184
6.3.3 Structured Text Programs 185
6.3.4 Comparison with Ladder Programs 187
Summary 187
Problems 188
Chapter 7: Internal Relays 200
7.1 Internal Relays 200
7.2 Ladder Programs 201
7.2.1 Programs with Multiple Input Conditions 201
7.2.2 Latching Programs 203
7.2.3 Response Time 204
7.3 Battery-Backed Relays 205
7.4 One-Shot Operation 206
7.5 Set and Reset 207
7.5.1 Program Examples 211
7.6 Master Control Relay 212
7.6.1 Examples of Programs 216
Summary 217
Problems 219
Chapter 8: Jump and Call 228
8.1 Jump 228
8.1.1 Jumps Within Jumps 229
8.2 Subroutines 230
8.2.1 Function Boxes 231
Summary 234
Problems 234
Lookup Tasks 237
Chapter 9: Timers 238
9.1 Types of Timers 238
9.2 On-Delay Timers 239
9.2.1 Sequencing 241
9.2.2 Cascaded Timers 241
9.2.3 On/Off Cycle Timer 243
9.3 Off-Delay Timers 244
9.4 Pulse Timers 245
9.5 Retentive Timers 247
9.6 Programming Examples 248
Summary 249
Problems 251
Lookup Tasks 257
Chapter 10: Counters 258
10.1 Forms of Counter 258
10.2 Programming 258
10.2.1 Counter Application 262
10.3 Up- and Down-Counting 264
10.4 Timers with Counters 265
10.5 Sequencer 267
Summary 270
Problems 271
Lookup Tasks 279
Chapter 11: Shift Registers 280
11.1 Shift Registers 280
11.2 Ladder Programs 281
11.2.1 A Sequencing Application 283
11.2.2 Keeping Track of Items 283
Summary 285
Problems 286
Lookup Tasks 290
Chapter 12: Data Handling 292
12.1 Registers and Bits 292
12.2 Data Handling 293
12.2.1 Data Movement 293
12.2.2 Data Comparison 295
12.2.3 Data Selection 296
12.3 Arithmetic Functions 297
12.3.1 Arithmetic Operations 297
12.4 Closed Loop Control 298
12.4.1 Modes of Control 299
12.4.2 Control with a PLC 301
Summary 302
Problems 302
Lookup Tasks 306
Chapter 13: Designing Systems 308
13.1 Program Development 308
13.1.1 Flowcharts and Pseudocode 308
13.2 Safe Systems 311
13.2.1 PLC Systems and Safety 313
13.2.2 Emergency Stop Relays 315
13.2.3 Safety Functions 316
13.2.4 Safety PLCs 317
13.3 Commissioning 317
13.3.1 Testing Inputs and Outputs 318
13.3.2 Testing Software 319
13.3.3 Simulation 320
13.4 Fault Finding 321
13.4.1 Fault Detection Techniques 321
13.4.2 Program Storage 326
13.5 System Documentation 326
13.5.1 Example of an Industrial Program 327
Summary 349
Problems 349
Lookup Tasks 352
Chapter 14: Programs 354
14.1 Temperature Control 354
14.2 Valve Sequencing 360
14.2.1 Cyclic Movement 360
14.2.2 Sequencing 361
14.2.3 Sequencing Using a Sequential Function Chart 365
14.2.4 Car Park Barrier Operation Using Valves 365
14.2.5 Controlled Reset of Cylinders 369
14.3 Conveyor Belt Control 370
14.3.1 Bottle Packing 371
14.4 Control of a Process 377
14.5 A Selection Example: A Drinks Machine 380
14.6 A Data Comparison Example: A Fan Heater 380
Problems 383
Lookup Tasks 387
Appendix: Symbols 388
Ladder Programs 388
Function Blocks 389
Commonly Encountered Blocks 389
Logic Gates 390
Sequential Function Charts 391
Instruction List (IEC 61131-3 Symbols) 392
Structured Text 392
Operators 392
Conditional and Iteration Statements 393
Answers 394
Chapter 1 394
Chapter 2 394
Chapter 3 395
Chapter 4 396
Chapter 5 397
Chapter 6 398
Chapter 7 400
Chapter 8 401
Chapter 9 401
Chapter 10 402
Chapter 11 403
Chapter 12 404
Chapter 13 405
Chapter 14 408
Index 414

Textauszug

Chapter 1

Programmable Logic Controllers

Abstract

This chapter is an introduction to control systems, starting with a discussion of relay-controlled systems before discussing the programmable logic controller (PLC) and its general function, hardware forms, and internal architecture. PLCs are widely used for a range of automation tasks in such areas as industrial processes in manufacturing. The IEC standard 61131 is outlined. This overview is followed by more detailed discussion in the following chapters.

Keywords

Relay-controlled systems. Programmable logic controller

IEC 61131

This chapter is an introduction to the programmable logic controller (PLC) and its general function, hardware forms, and internal architecture. PLCs are widely used for a range of automation tasks in areas such as industrial processes in manufacturing. This overview is followed by more detailed discussion in the following chapters. For a summary of the history, development, features, and comparison with other control systems, see the Wikipedia entry for Programmable logic controller.

1.1 Controllers

What type of task might a control system handle? It might be required to control a sequence of events, maintain some variable constant, or follow some prescribed change. For example, the control system for an automatic drilling machine (Figure 1.1a) might be required to start lowering the drill when the workpiece is in position, start drilling when the drill reaches the workpiece, stop drilling when the drill has produced the required depth of hole, retract the drill, and then switch off and wait for the next workpiece to be put in position before repeating the operation. Another control system (Figure 1.1b) might be used to control the number of items moving along a conveyor belt and direct them into a packing case. The inputs to such control systems might come from switches being closed or opened; for example, the presence of the workpiece might be indicated by it moving against a switch and closing it, or other sensors such as those used for temperature or flow rates. The controller might be required to run a motor to move an object to some position or to turn a valve, or perhaps a heater, on or off.

Figure 1.1 An example of a control task and some input sensors: (a) an automatic drilling machine; (b) a packing system.

What form might a controller have? For the automatic drilling machine, we could wire up electrical circuits in which the closing or opening of switches would result in motors being switched on or valves being actuated. Thus, as a result, we might have a relay (Figure 1.2) closing or opening contacts which, in turn, switches on the current to a motor and causes the drill to rotate (Figure 1.3). Another switch might be used to activate a relay and switch on the current to a pneumatic or hydraulic valve, which results in pressure being switched to drive a piston in a cylinder and so results in the workpiece being pushed into the required position. Such electrical circuits would have to be specific to the automatic drilling machine. For controlling the number of items packed into a packing case, we could likewise wire up electrical circuits involving sensors and motors. However, the controller circuits we devised for these two situations would be different. In the “traditional” form of control system, the rules governing the control system and when actions are initiated are determined by the wiring. When the rules used for the control actions are changed, the wiring has to be changed.

Figure 1.2 A basic relay.

Figure 1.3 A control circuit.

1.1.1 Relay-Controlled Systems

Relay-controlled systems are hard-wired systems. Figure 1.2 shows the basic elements of a simple relay. When a current is switched on to flow through the relay solenoid, normally-closed (NC) contacts open and normally-open (NO) contacts close. These contacts can be used to give control in a system. As an illustration consider a relay being used to operate a pneumatic or hydraulic valve, this then results in pressure being applied to drive a piston to move a workpiece. We can represent the situation by a control drawing. Figure 1.4 shows the standard symbols used for relays and Figure 1.5 shows the control drawing with the vertical lines representing the power rails and the horizontal lines to systems connected between them. The sequence of events is read from the top horizontal line downwards. Thus, in the top line of Figure 1.5(a), when the Off–On switch is closed, the relay is activated. This closes the contacts on the second line and so the solenoid valve is switched on. A more usual control drawing is shown in Figure 1.5(b) which has the relay switched on by a momentary NO push-button switch. This closes two sets of contacts. Contacts 1 latch the push button switch so that when the push stops there is still connection of power to the relay. Contacts 2 switch on the solenoid valve. The relay, and hence power to the solenoid valve, is switched off when the normally closed push-button switch is pressed. The control drawings are obviously only part of the control system as there will need to be further lines for when the solenoid valve has moved the workpiece the required distance so that it stops its action.

Figure 1.4 Relay symbols.

Figure 1.5 Relay-controlled system control drawings.

Figure 1.6 shows another example of a relay control system. When the start push button is closed, the relay coil is switched on and latches the push button switch so that the relay remains on until the stop push button is pressed. The relay closes the NO contacts and opens the NC contacts. As a result, the green light is switched on and the red light switches off. When the stop push button is pressed, the current to the relay coil is switched off. This results in the NO contacts opening and the NC contacts closing and so the green light going off and the red light comes on. The next stage in the relay circuit might be a motor that is switched on by NO contacts, so the green light indicates when the motor is running and the red light when it is off.

Figure 1.6 Relay circuit to control red and green lights.

1.1.2 Microprocessor-Controlled Systems

Instead of hardwiring each control circuit for each control situation, we can use the same basic system for all situations if we use a microprocessor-based system and write a program to instruct the microprocessor how to react to each input signal from, say, switches and give the required outputs to, say, motors and valves. Thus we might have a program of the form:

If switch A closes Output to motor circuit If switch B closes Output to valve circuit

By changing the instructions in the program, we can use the same microprocessor system to control a wide variety of situations.

As an illustration, the modern domestic washing machine uses a microprocessor system. Inputs to it arise from the dials used to select the required wash cycle, a switch to determine that the machine door is closed, a temperature sensor to determine the temperature of the water, and a switch to detect the level of the water. On the basis of these inputs the microprocessor is programmed to give outputs that switch on the drum motor and control its speed, open or close cold and hot water valves, switch on the drain pump, control the water heater, and control the door lock so that the machine cannot be opened until the washing cycle is completed.

1.1.3 The Programmable Logic Controller

A programmable logic controller (PLC) is a special form of microprocessor-based controller that uses programmable memory to store instructions and to implement functions such as logic, sequencing, timing, counting, and arithmetic in order to control machines and processes (Figure 1.7). It is designed to be operated by engineers with perhaps a limited knowledge of computers and computing languages. They are not designed so that only computer programmers can set up or change the programs. Thus, the designers of the PLC have preprogrammed it so that the control program can be entered using a simple, rather intuitive form of language (see Chapter 4). The term logic is used because programming is primarily concerned with implementing logic and switching operations; for example, if A or B occurs, switch on C; if A and B occurs, switch on D. Input devices (that is, sensors such as switches) and output devices (motors, valves, etc.) in the system being controlled are connected to the PLC. The operator then enters a sequence of instructions, a program, into the memory of the PLC. The controller then monitors the inputs and outputs according to this program and carries out the control rules for which it has been programmed.

Figure 1.7 A programmable logic controller.

PLCs have the great advantage that the same basic controller can be used with a wide range of control systems. To modify a control system and the rules that are to be used, all that is necessary is for an operator to key in a different set of instructions. There is no need to rewire. The result is a flexible, cost-effective system that can be used with control systems, which vary...

Produkt-Details

• Erscheint lt. Verlag: 6.3.2015
• Sprache: englisch
• Themenwelt: Mathematik / Informatik ► Informatik ► Theorie / Studium
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• ISBN-10: 0-08-100353-6 / 0081003536
• ISBN-13: 978-0-08-100353-4 / 9780081003534