Programmable Logic Controllers

 
 
Newnes (Verlag)
  • 6. Auflage
  • |
  • erschienen am 29. April 2015
  • |
  • 424 Seiten
 
E-Book | ePUB mit Adobe DRM | Systemvoraussetzungen
E-Book | PDF mit Adobe DRM | Systemvoraussetzungen
978-0-08-100353-4 (ISBN)
 

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.
    • Englisch
    • Cambridge
    • 13,25 MB
    978-0-08-100353-4 (9780081003534)
    0081003536 (0081003536)
    weitere Ausgaben werden ermittelt
    1 - Front Cover [Seite 1]
    2 - Programmable Logic Controllers [Seite 4]
    3 - Copyright [Seite 5]
    4 - Contents [Seite 6]
    5 - Preface [Seite 10]
    5.1 - Prerequisite Knowledge Assumed [Seite 11]
    5.2 - Changes from the Fifth Edition [Seite 11]
    5.2.1 - Aims [Seite 11]
    5.3 - Structure of the Book [Seite 12]
    6 - Acknowledgments [Seite 13]
    7 - Chapter 1: Programmable Logic Controllers [Seite 14]
    7.1 - 1.1 Controllers [Seite 14]
    7.1.1 - 1.1.1 Relay-Controlled Systems [Seite 16]
    7.1.2 - 1.1.2 Microprocessor-Controlled Systems [Seite 17]
    7.1.3 - 1.1.3 The Programmable Logic Controller [Seite 18]
    7.2 - 1.2 Hardware [Seite 20]
    7.3 - 1.3 PLC Architecture [Seite 22]
    7.3.1 - 1.3.1 Input/Output Unit [Seite 22]
    7.3.2 - 1.3.2 Sourcing and Sinking [Seite 24]
    7.4 - 1.4 PLC Systems [Seite 25]
    7.4.1 - 1.4.1 Security [Seite 28]
    7.5 - 1.5 Programs [Seite 29]
    7.5.1 - 1.5.1 The IEC Standard [Seite 30]
    7.5.2 - 1.5.2 Programming PLCs [Seite 32]
    7.6 - Summary [Seite 33]
    7.7 - Problems [Seite 34]
    7.8 - Lookup Tasks [Seite 35]
    8 - Chapter 2: Input/Output Devices [Seite 36]
    8.1 - 2.1 Input Devices [Seite 36]
    8.1.1 - 2.1.1 Mechanical Switches [Seite 39]
    8.1.2 - 2.1.2 Proximity Switches [Seite 42]
    8.1.3 - 2.1.3 Photoelectric Sensors and Switches [Seite 43]
    8.1.4 - 2.1.4 Encoders [Seite 44]
    8.1.5 - 2.1.5 Temperature Sensors [Seite 46]
    8.1.6 - 2.1.6 Position/Displacement Sensors [Seite 50]
    8.1.7 - 2.1.7 Strain Gauges [Seite 51]
    8.1.8 - 2.1.8 Pressure Sensors [Seite 53]
    8.1.9 - 2.1.9 Liquid-Level Detectors [Seite 54]
    8.1.10 - 2.1.10 Fluid Flow Measurement [Seite 54]
    8.1.11 - 2.1.11 Ultrasonic Proximity Sensors [Seite 55]
    8.1.12 - 2.1.12 Smart Sensors [Seite 55]
    8.1.13 - 2.1.13 Sensors Ranges [Seite 56]
    8.2 - 2.2 Output Devices [Seite 56]
    8.2.1 - 2.2.1 Relay [Seite 56]
    8.2.2 - 2.2.2 Directional Control Valves [Seite 57]
    8.2.3 - 2.2.3 Motors [Seite 59]
    8.2.4 - 2.2.4 Stepper Motors [Seite 62]
    8.3 - 2.3 Examples of Applications [Seite 66]
    8.3.1 - 2.3.1 A Conveyor Belt [Seite 66]
    8.3.2 - 2.3.2 A Lift [Seite 66]
    8.3.3 - 2.3.3 A Robot Control System [Seite 67]
    8.3.4 - 2.3.4 Liquid-Level Monitoring [Seite 68]
    8.3.5 - 2.3.5 Packages on Conveyor Belt Systems [Seite 68]
    8.4 - Summary [Seite 69]
    8.5 - Problems [Seite 70]
    8.6 - Lookup Tasks [Seite 74]
    9 - Chapter 3: Digital Systems [Seite 76]
    9.1 - 3.1 The Binary System [Seite 77]
    9.2 - 3.2 Octal and Hexadecimal [Seite 77]
    9.2.1 - 3.2.1 Octal System [Seite 78]
    9.2.2 - 3.2.2 Hexadecimal System [Seite 79]
    9.3 - 3.3 Binary Coded Decimals [Seite 79]
    9.4 - 3.4 Numbers in the Binary, Octal, Hex, and BCD Systems [Seite 80]
    9.5 - 3.5 Binary Arithmetic [Seite 81]
    9.5.1 - 3.5.1 Signed Numbers [Seite 82]
    9.5.2 - 3.5.2 One's and Two's Complements [Seite 82]
    9.5.3 - 3.5.3 Floating Point Numbers [Seite 83]
    9.6 - 3.6 PLC Data [Seite 84]
    9.7 - 3.7 Combinational Logic Systems [Seite 85]
    9.8 - 3.8 Sequential Logic Systems [Seite 86]
    9.8.1 - 3.8.1 Latches [Seite 86]
    9.8.2 - 3.8.2 Flip-Flops [Seite 88]
    9.9 - Summary [Seite 88]
    9.10 - Problems [Seite 90]
    9.11 - Lookup Tasks [Seite 91]
    10 - Chapter 4: I/O Processing [Seite 92]
    10.1 - 4.1 Input/Output Units [Seite 92]
    10.1.1 - 4.1.1 Input Units [Seite 92]
    10.1.2 - 4.1.2 Output Units [Seite 95]
    10.2 - 4.2 Signal Conditioning [Seite 98]
    10.2.1 - 4.2.1 Changing Voltage Levels [Seite 99]
    10.2.2 - 4.2.2 Op-Amp Comparator [Seite 101]
    10.2.3 - 4.2.3 Output Protection [Seite 102]
    10.3 - 4.3 Remote Connections [Seite 102]
    10.3.1 - 4.3.1 Serial and Parallel Communications [Seite 103]
    10.3.2 - 4.3.2 Serial Standards [Seite 104]
    10.3.3 - 4.3.3 Parallel Standards [Seite 107]
    10.3.4 - 4.3.4 Protocols [Seite 109]
    10.3.5 - 4.3.5 ASCII Codes [Seite 111]
    10.4 - 4.4 Networks [Seite 112]
    10.4.1 - 4.4.1 Distributed Systems [Seite 113]
    10.4.2 - 4.4.2 Network Standards [Seite 114]
    10.5 - 4.5 Examples of Commercial Systems [Seite 116]
    10.5.1 - 4.5.1 MAP [Seite 116]
    10.5.2 - 4.5.2 Ethernet [Seite 117]
    10.5.3 - 4.5.3 ControlNet [Seite 118]
    10.5.4 - 4.5.4 DeviceNet [Seite 118]
    10.5.5 - 4.5.5 Allen-Bradley Data Highway [Seite 119]
    10.5.6 - 4.5.6 PROFIBUS [Seite 119]
    10.5.7 - 4.5.7 Factory-Floor Network [Seite 119]
    10.6 - 4.6 Processing Inputs [Seite 119]
    10.7 - 4.7 I/O Addresses [Seite 121]
    10.8 - Summary [Seite 122]
    10.9 - Problems [Seite 123]
    10.10 - Lookup Tasks [Seite 126]
    11 - Chapter 5: Ladder and Functional Block Programming [Seite 128]
    11.1 - 5.1 Ladder Diagrams [Seite 128]
    11.1.1 - 5.1.1 PLC Ladder Programming [Seite 130]
    11.2 - 5.2 Logic Functions [Seite 133]
    11.2.1 - 5.2.1 AND [Seite 133]
    11.2.2 - 5.2.2 OR [Seite 134]
    11.2.3 - 5.2.3 NOT [Seite 136]
    11.2.4 - 5.2.4 NAND [Seite 137]
    11.2.5 - 5.2.5 NOR [Seite 138]
    11.2.6 - 5.2.6 Exclusive OR (XOR) [Seite 139]
    11.3 - 5.3 Latching [Seite 140]
    11.4 - 5.4 Multiple Outputs [Seite 140]
    11.5 - 5.5 Entering Programs [Seite 142]
    11.5.1 - 5.5.1 Ladder Symbols [Seite 143]
    11.6 - 5.6 Function Blocks [Seite 143]
    11.6.1 - 5.6.1 Logic Gates [Seite 143]
    11.6.2 - 5.6.2 Boolean Algebra [Seite 148]
    11.7 - 5.7 Program Examples [Seite 151]
    11.7.1 - 5.7.1 Location of Stop Switches [Seite 153]
    11.8 - Summary [Seite 154]
    11.9 - Problems [Seite 155]
    11.10 - Lookup Tasks [Seite 163]
    12 - Chapter 6: IL, SFC, and ST Programming Methods [Seite 164]
    12.1 - 6.1 Instruction Lists [Seite 164]
    12.1.1 - 6.1.1 Ladder Programs and Instruction Lists [Seite 166]
    12.1.2 - 6.1.2 Branch Codes [Seite 168]
    12.1.3 - 6.1.3 More Than One Rung [Seite 171]
    12.1.4 - 6.1.4 Programming Examples [Seite 172]
    12.2 - 6.2 Sequential Function Charts [Seite 173]
    12.2.1 - 6.2.1 Branching and Convergence [Seite 176]
    12.2.2 - 6.2.2 Actions [Seite 178]
    12.2.3 - 6.2.3 Programming a PLC [Seite 180]
    12.3 - 6.3 Structured Text [Seite 180]
    12.3.1 - 6.3.1 Conditional Statements [Seite 182]
    12.3.2 - 6.3.2 Iteration Statements [Seite 184]
    12.3.3 - 6.3.3 Structured Text Programs [Seite 185]
    12.3.4 - 6.3.4 Comparison with Ladder Programs [Seite 187]
    12.4 - Summary [Seite 187]
    12.5 - Problems [Seite 188]
    13 - Chapter 7: Internal Relays [Seite 200]
    13.1 - 7.1 Internal Relays [Seite 200]
    13.2 - 7.2 Ladder Programs [Seite 201]
    13.2.1 - 7.2.1 Programs with Multiple Input Conditions [Seite 201]
    13.2.2 - 7.2.2 Latching Programs [Seite 203]
    13.2.3 - 7.2.3 Response Time [Seite 204]
    13.3 - 7.3 Battery-Backed Relays [Seite 205]
    13.4 - 7.4 One-Shot Operation [Seite 206]
    13.5 - 7.5 Set and Reset [Seite 207]
    13.5.1 - 7.5.1 Program Examples [Seite 211]
    13.6 - 7.6 Master Control Relay [Seite 212]
    13.6.1 - 7.6.1 Examples of Programs [Seite 216]
    13.7 - Summary [Seite 217]
    13.8 - Problems [Seite 219]
    14 - Chapter 8: Jump and Call [Seite 228]
    14.1 - 8.1 Jump [Seite 228]
    14.1.1 - 8.1.1 Jumps Within Jumps [Seite 229]
    14.2 - 8.2 Subroutines [Seite 230]
    14.2.1 - 8.2.1 Function Boxes [Seite 231]
    14.3 - Summary [Seite 234]
    14.4 - Problems [Seite 234]
    14.5 - Lookup Tasks [Seite 237]
    15 - Chapter 9: Timers [Seite 238]
    15.1 - 9.1 Types of Timers [Seite 238]
    15.2 - 9.2 On-Delay Timers [Seite 239]
    15.2.1 - 9.2.1 Sequencing [Seite 241]
    15.2.2 - 9.2.2 Cascaded Timers [Seite 241]
    15.2.3 - 9.2.3 On/Off Cycle Timer [Seite 243]
    15.3 - 9.3 Off-Delay Timers [Seite 244]
    15.4 - 9.4 Pulse Timers [Seite 245]
    15.5 - 9.5 Retentive Timers [Seite 247]
    15.6 - 9.6 Programming Examples [Seite 248]
    15.7 - Summary [Seite 249]
    15.8 - Problems [Seite 251]
    15.9 - Lookup Tasks [Seite 257]
    16 - Chapter 10: Counters [Seite 258]
    16.1 - 10.1 Forms of Counter [Seite 258]
    16.2 - 10.2 Programming [Seite 258]
    16.2.1 - 10.2.1 Counter Application [Seite 262]
    16.3 - 10.3 Up- and Down-Counting [Seite 264]
    16.4 - 10.4 Timers with Counters [Seite 265]
    16.5 - 10.5 Sequencer [Seite 267]
    16.6 - Summary [Seite 270]
    16.7 - Problems [Seite 271]
    16.8 - Lookup Tasks [Seite 279]
    17 - Chapter 11: Shift Registers [Seite 280]
    17.1 - 11.1 Shift Registers [Seite 280]
    17.2 - 11.2 Ladder Programs [Seite 281]
    17.2.1 - 11.2.1 A Sequencing Application [Seite 283]
    17.2.2 - 11.2.2 Keeping Track of Items [Seite 283]
    17.3 - Summary [Seite 285]
    17.4 - Problems [Seite 286]
    17.5 - Lookup Tasks [Seite 290]
    18 - Chapter 12: Data Handling [Seite 292]
    18.1 - 12.1 Registers and Bits [Seite 292]
    18.2 - 12.2 Data Handling [Seite 293]
    18.2.1 - 12.2.1 Data Movement [Seite 293]
    18.2.2 - 12.2.2 Data Comparison [Seite 295]
    18.2.3 - 12.2.3 Data Selection [Seite 296]
    18.3 - 12.3 Arithmetic Functions [Seite 297]
    18.3.1 - 12.3.1 Arithmetic Operations [Seite 297]
    18.4 - 12.4 Closed Loop Control [Seite 298]
    18.4.1 - 12.4.1 Modes of Control [Seite 299]
    18.4.2 - 12.4.2 Control with a PLC [Seite 301]
    18.5 - Summary [Seite 302]
    18.6 - Problems [Seite 302]
    18.7 - Lookup Tasks [Seite 306]
    19 - Chapter 13: Designing Systems [Seite 308]
    19.1 - 13.1 Program Development [Seite 308]
    19.1.1 - 13.1.1 Flowcharts and Pseudocode [Seite 308]
    19.2 - 13.2 Safe Systems [Seite 311]
    19.2.1 - 13.2.1 PLC Systems and Safety [Seite 313]
    19.2.2 - 13.2.2 Emergency Stop Relays [Seite 315]
    19.2.3 - 13.2.3 Safety Functions [Seite 316]
    19.2.4 - 13.2.4 Safety PLCs [Seite 317]
    19.3 - 13.3 Commissioning [Seite 317]
    19.3.1 - 13.3.1 Testing Inputs and Outputs [Seite 318]
    19.3.2 - 13.3.2 Testing Software [Seite 319]
    19.3.3 - 13.3.3 Simulation [Seite 320]
    19.4 - 13.4 Fault Finding [Seite 321]
    19.4.1 - 13.4.1 Fault Detection Techniques [Seite 321]
    19.4.2 - 13.4.2 Program Storage [Seite 326]
    19.5 - 13.5 System Documentation [Seite 326]
    19.5.1 - 13.5.1 Example of an Industrial Program [Seite 327]
    19.6 - Summary [Seite 349]
    19.7 - Problems [Seite 349]
    19.8 - Lookup Tasks [Seite 352]
    20 - Chapter 14: Programs [Seite 354]
    20.1 - 14.1 Temperature Control [Seite 354]
    20.2 - 14.2 Valve Sequencing [Seite 360]
    20.2.1 - 14.2.1 Cyclic Movement [Seite 360]
    20.2.2 - 14.2.2 Sequencing [Seite 361]
    20.2.3 - 14.2.3 Sequencing Using a Sequential Function Chart [Seite 365]
    20.2.4 - 14.2.4 Car Park Barrier Operation Using Valves [Seite 365]
    20.2.5 - 14.2.5 Controlled Reset of Cylinders [Seite 369]
    20.3 - 14.3 Conveyor Belt Control [Seite 370]
    20.3.1 - 14.3.1 Bottle Packing [Seite 371]
    20.4 - 14.4 Control of a Process [Seite 377]
    20.5 - 14.5 A Selection Example: A Drinks Machine [Seite 380]
    20.6 - 14.6 A Data Comparison Example: A Fan Heater [Seite 380]
    20.7 - Problems [Seite 383]
    20.8 - Lookup Tasks [Seite 387]
    21 - Appendix: Symbols [Seite 388]
    21.1 - Ladder Programs [Seite 388]
    21.2 - Function Blocks [Seite 389]
    21.2.1 - Commonly Encountered Blocks [Seite 389]
    21.3 - Logic Gates [Seite 390]
    21.4 - Sequential Function Charts [Seite 391]
    21.5 - Instruction List (IEC 61131-3 Symbols) [Seite 392]
    21.6 - Structured Text [Seite 392]
    21.6.1 - Operators [Seite 392]
    21.6.2 - Conditional and Iteration Statements [Seite 393]
    22 - Answers [Seite 394]
    22.1 - Chapter 1 [Seite 394]
    22.2 - Chapter 2 [Seite 394]
    22.3 - Chapter 3 [Seite 395]
    22.4 - Chapter 4 [Seite 396]
    22.5 - Chapter 5 [Seite 397]
    22.6 - Chapter 6 [Seite 398]
    22.7 - Chapter 7 [Seite 400]
    22.8 - Chapter 8 [Seite 401]
    22.9 - Chapter 9 [Seite 401]
    22.10 - Chapter 10 [Seite 402]
    22.11 - Chapter 11 [Seite 403]
    22.12 - Chapter 12 [Seite 404]
    22.13 - Chapter 13 [Seite 405]
    22.14 - Chapter 14 [Seite 408]
    23 - Index [Seite 414]
    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...

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    E-Book-Reader: Bookeen, Kobo, Pocketbook, Sony, Tolino u.v.a.m. (nicht Kindle)

    Das Dateiformat PDF zeigt auf jeder Hardware eine Buchseite stets identisch an. Daher ist eine PDF auch für ein komplexes Layout geeignet, wie es bei Lehr- und Fachbüchern verwendet wird (Bilder, Tabellen, Spalten, Fußnoten). Bei kleinen Displays von E-Readern oder Smartphones sind PDF leider eher nervig, weil zu viel Scrollen notwendig ist. Mit Adobe-DRM wird hier ein "harter" Kopierschutz verwendet. Wenn die notwendigen Voraussetzungen nicht vorliegen, können Sie das E-Book leider nicht öffnen. Daher müssen Sie bereits vor dem Download Ihre Lese-Hardware vorbereiten.

    Weitere Informationen finden Sie in unserer E-Book Hilfe.


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