Programmable Logic Controllers (eBook)

William Bolton (Autor)

eBook Download: PDF | EPUB

2011 | 4. Auflage
304 Seiten
Elsevier Science (Verlag)
978-0-08-046295-0 (ISBN)

Infos

This is the introduction to PLCs for which baffled students, technicians and managers have been waiting. In this straightforward, easy-to-read guide, Bill Bolton has kept the jargon to a minimum, considered all the programming methods in the standard IEC 1131-3 - in particular ladder programming, and presented the subject in a way that is not device specific to ensure maximum applicability to courses in electronics and control systems.

Now in its fourth edition, this best-selling text has been expanded with increased coverage of industrial systems and PLCs and more consideration has been given to IEC 1131-3 and all the programming methods in the standard. The new edition brings the book fully up to date with the current developments in PLCs, describing new and important applications such as PLC use in communications (e.g. Ethernet - an extremely popular system), and safety - in particular proprietary emergency stop relays (now appearing in practically every PLC based system).

The coverage of commonly used PLCs has been increased, including the ever popular Allen Bradley PLCs, making this book an essential source of information both for professionals wishing to update their knowledge, as well as students who require a straight forward introduction to this area of control engineering.

Having read this book, readers will be able to:
* Identify the main design characteristics and internal architecture of PLCs
* Describe and identify the characteristics of commonly used input and output devices
* Explain the processing of inputs and outputs of PLCs
* Describe communication links involved with control systems
* Develop ladder programs for the logic functions AND, OR, NOT, NAND, NOT and XOR
* Develop functional block, instruction list, structured text and sequential function chart programs
* Develop programs using internal relays, timers, counters, shift registers, sequencers and data handling
* Identify safety issues with PLC systems
* Identify methods used for fault diagnosis, testing and debugging programs

Fully matched to the requirements of BTEC Higher Nationals, students are able to check their learning and understanding as they work through the text using the Problems section at the end of each chapter. Complete answers are provided in the back of the book.

* Thoroughly practical introduction to PLC use and application - not device specific, ensuring relevance to a wide range of courses
* New edition expanded with increased coverage of IEC 1131-3, industrial control scenarios and communications - an important aspect of PLC use
* Problems included at the end of each chapter, with a complete set of answers given at the back of the book

Inhalt

Cover 1
Programmable Logic Controllers 4
Contents 6
Preface 8
Changes from third edition 8
Aims 9
Structure of the book 9
1 Programmable logic controllers 11
1.1 Controllers 11
1.1.1 Microprocessor controlled system 12
1.1.2 The programmable logic controller 13
1.2 Hardware 14
1.3 Internal architecture 15
1.3.1 The CPU 16
1.3.2 The buses 16
1.3.3 Memory 17
1.3.4 Input/output unit 18
1.3.5 Sourcing and sinking 20
1.4 PLC systems 20
1.4.1 Programming PLCs 24
Problems 25
2 Input-output devices 27
2.1 Input devices 27
2.1.1 Mechanical switches 29
2.1.2 Proximity switches 30
2.1.3 Photoelectric sensors and switches 31
2.1.4 Encoders 32
2.1.5 Temperature sensors 34
2.1.6 Position/displacement sensors 36
2.1.7 Strain gauges 37
2.1.8 Pressure sensors 38
2.1.9 Liquid level detector 39
2.1.10 Fluid flow measurement 39
2.1.11 Smart sensors 40
2.2 Output devices 40
2.2.1 Relay 40
2.2.2 Directional control valves 41
2.2.3 Motors 44
2.2.4 Stepper motors 46
2.3 Examples of applications 49
2.3.1 A conveyor belt 49
2.3.2 A lift 49
2.3.3 A robot control system 49
2.3.4 Liquid level monitoring 50
Problems 51
3 Number systems 54
3.1 The binary system 54
3.2 Octal and hexadecimal 55
3.2.1 Octal system 55
3.3 Binary arithmetic 57
3.3.1 Signed numbers 58
3.3.2 One's and two's complements 59
3.3.3 Floating point numbers 60
3.4 PLC data 61
Problems 62
4 I/O processing 63
4.1 Input/output units 63
4.1.1 Input units 63
4.1.2 Output units 66
4.2 Signal conditioning 69
4.3 Remote connections 72
4.3.1 Serial and parallel communications 73
4.3.2 Serial standards 73
4.3.3 Parallel standards 76
4.3.4 Protocols 78
4.3.5 ASCII codes 79
4.4 Networks 79
4.4.1 Distributed systems 81
4.4.2 Network standards 82
4.4.3 Examples of commercial systems 83
4.5 Processing inputs 85
4.6 I/O addresses 86
Problems 87
5 Ladder and functional block programming 90
5.1 Ladder diagrams 90
5.1.1 PLC ladder programming 91
5.2 Logic functions 94
5.2.1 AND 94
5.2.2 OR 95
5.2.3 NOT 97
5.2.4 NAND 97
5.2.5 NOR 98
5.2.6 Exclusive OR (XOR) 99
5.3 Latching 100
5.4 Multiple outputs 101
5.5 Entering programs 103
5.5.1 Ladder symbols 103
5.6 Function blocks 104
5.6.1 Logic gates 105
5.6.2 Boolean algebra 107
5.7 Program examples 110
5.7.1 Location of stop switches 112
Problems 113
6 IL, SFC and ST programming methods 118
6.1 Instruction lists 118
6.1.1 Ladder programs and instruction lists 119
6.1.2 Branch codes 122
6.1.3 More than one rung 124
6.1.4 Programming examples 124
6.2 Sequential function charts 125
6.2.1 Branching and convergence 127
6.2.2 Actions 129
6.3 Structured text 130
6.3.1 Conditional statements 132
6.3.2 Iteration statements 132
6.3.3 Structured text programs 133
Problems 134
7 Internal relays 142
7.1 Internal relays 142
7.2 Ladder programs 143
7.2.1 Programs with multiple input conditions 143
7.2.2 Latching programs 144
7.3 Battery-backed relays 146
7.4 One-shot operation 147
7.5 Set and reset 148
7.5.1 Program examples 151
7.6 Master control relay 152
7.6.1 Examples of programs 155
Problems 156
8 Jump and call 164
8.1 Jump 164
8.1.1 Jumps within jumps 165
8.2 Subroutines 166
Problems 167
9 Timers 169
9.1 Types of timers 169
9.2 Programming timers 170
9.2.1 Sequencing 171
9.2.2 Cascaded timers 171
9.2.3 On-off cycle timer 172
9.3 Off-delay timers 173
9.4 Pulse timers 175
9.5 Programming examples 176
Problems 177
10 Counters 183
10.1 Forms of counter 183
10.2 Programming 184
10.2.1 Counter application 184
10.3 Up and down counting 188
10.4 Timers with counters 189
10.5 Sequencer 190
Problems 192
11 Shift registers 199
11.1 Shift registers 199
11.2 Ladder programs 200
11.2.1 A sequencing application 203
11.2.2 Keeping track of items 204
Problems 204
12 Data handling 207
12.1 Registers and bits 207
12.2 Data handling 208
12.2.1 Data movement 208
12.2.2 Data comparison 210
12.3 Arithmetic functions 212
12.3.1 Arithmetic operations 212
12.4 Closed loop control 213
12.4.1 Modes of control 214
12.4.2 PID control with a PLC 215
Problems 216
13 Designing systems 220
13.1 Program development 220
13.1.1 Flow charts and pseudocode 220
13.2 Safe systems 224
13.2.1 PLC systems and safety 225
13.2.2 Emergency stop relays 227
13.2.3 Safety PLCs 228
13.3 Commissioning 228
13.3.1 Testing inputs and outputs 228
13.3.2 Testing software 229
13.3.3 Simulation 230
13.4 Fault finding 230
13.4.1 Fault detection techniques 231
13.4.2 Program storage 236
13.5 System documentation 237
13.5.1 Example of an industrial program 237
Problems 258
14 Programs 260
14.1 Temperature control 260
14.2 Valve sequencing 264
14.2.1 Cyclic movement 264
14.2.2 Sequencing 266
14.2.3 Sequencing using a sequential function chart 269
14.2.4 Car park barrier operation using valves 270
14.3 Conveyor belt control 275
14.3.1 Bottle packing 275
14.4 Control of a process 279
Problems 281
Appendix: Symbols 286
Ladder programs 286
Function blocks 287
Logic gates 288
Sequential function charts 289
Instruction List (IEC 1131-3 symbols) 289
Structured text 290
Answers 291
Index 298
A 298
B 298
C 298
D 299
E 299
F 299
G 299
H 299
I 299
J 299
L 299
M 300
N 300
O 300
P 300
R 301
S 301
T 302
U 302
V 302
W 302
X 302

Textauszug

2 Input-output devices

This chapter is a brief consideration of typical input and output devices used with PLCs. The input devices considered include digital and analogue devices such as mechanical switches for position detection, proximity switches, photoelectric switches, encoders, temperature and pressure switches, potentiometers, linear variable differential transformers, strain gauges, thermistors, thermotransistors and thermocouples. Output devices considered include relays, contactors, solenoid valves and motors.

2.1 Input devices

The term sensor is used for an input device that provides a usable output in response to a specified physical input. For example, a thermocouple is a sensor which converts a temperature difference into an electrical output. The term transducer is generally used for a device that converts a signal from one form to a different physical form. Thus sensors are often transducers, but also other devices can be transducers, e.g. a motor which converts an electrical input into rotation.

Sensors which give digital/discrete, i.e. on-off, outputs can be easily connected to the input ports of PLCs. Sensors which give analogue signals have to be converted to digital signals before inputting them to PLC ports. The following are some of the more common terms used to define the performance of sensors.

1. Accuracy is the extent to which the value indicated by a measurement system or element might be wrong. For example, a temperature sensor might have an accuracy of ±0.1 °C. The error of a measurement is the difference between the result of the measurement and the true value of the quantity being measured errors can arise in a number of ways, e.g. the term non-linearity error is used for the error that occurs as a result of assuming a linear relationship between the input and output over the working range, i.e. a graph of output plotted against input is assumed to give a straight line. Few systems or elements, however, have a truly linear relationship and thus errors occur as a result of the assumption of linearity (Figure 2.1(a)). The term hysteresis error (Figure 2.1(b)) is used for the difference in outputs given from the same value of quantity being measured according to whether that value has been reached by a continuously increasing change or a continuously decreasing change. Thus, you might obtain a different value from a thermometer used to measure the same temperature of a liquid if it is reached by the liquid warming up to the measured temperature or it is reached by the liquid cooling down to the measured temperature.

Figure 2.1 Some sources of error: (a) non-linearity, (b) hysteresis

2. The range of variable of system is the limits between which the input can vary. For example, a resistance temperature sensor might be quoted as having a range of –200 to +800°C.

3. When the input value to a sensor changes, it will take some time to reach and settle down to the steady-state value (Figure 2.2). The response time is the time which elapses after the input to a system or element is abruptly increased from zero to a constant value up to the point at which the system or element gives an output corresponding to some specified percentage, e.g. 95%, of the value of the input. The rise time is the time taken for the output to rise to some specified percentage of the steady-state output. Often the rise time refers to the time taken for the output to rise from 10% of the steady-state value to 90 or 95% of the steady-state value. The settling time is the time taken for the output to settle to within some percentage, e.g. 2%, of the steady-state value.

Figure 2.2 Response of a sensor or measurement system to a sudden input. You can easily see such a response when the current in an electrical circuit is suddenly switched on and an ammeter reading observed.

4. The sensitivity indicates how much the output of an instrument system or system element changes when the quantity being measured changes by a given amount, i.e. the ratio ouput/input. For example, a thermocouple might have a sensitivity of 20 μV/°C and so give an output of 20 μV for each 1°C change in temperature.

5. The stability of a system is its ability to give the same output when used to measure a constant input over a period of time. The term drift is often used to describe the change in output that occurs over time. The drift may be expressed as a percentage of the full range output. The term zero drift is used for the changes that occur in output when there is zero input.

6. The term repeatability is used for the ability of a measurement system to give the same value for repeated measurements of the same value of a variable. Common cause of lack of repeatability are random fluctuations in the environment, e.g. changes in temperature and humidity. The error arising from repeatability is usually expressed as a percentage of the full range output. For example, a pressure sensor might be quoted as having a repeatability of ±0.1% of full range. Thus with a range of 20 kPa this would be an error of ±20 Pa.

7. The reliability of a measurement system, or element in such a system, is defined as being the probability that it will operate to an agreed level of performance, for a specified period, subject to specified environmental conditions. The agreed level of performance might be that the measurement system gives a particular accuracy.

The following are examples of some of the commonly used PLC input devices and their sensors.

2.1.1 Mechanical switches

A mechanical switch generates an on–off signal or signals as a result of some mechanical input causing the switch to open or close. Such a switch might be used to indicate the presence of a workpiece on a machining table, the workpiece pressing against the switch and so closing it. The absence of the workpiece is indicated by the switch being open and its presence by it being closed. Thus, with the arrangement shown in Figure 2.3(a), the input signals to a single input channel of the PLC are thus the logic levels:

Figure 2.3 Switch sensors

Workpiece not present 0

Workpiece present 1

The 1 level might correspond to a 24 V d.c. input, the 0 to a 0 V input.

With the arrangement shown in Figure 2.3(b), when the switch is open the supply voltage is applied to the PLC input, when the switch is closed the input voltage drops to a low value. The logic levels are thus:

Workpiece not present 1

Workpiece present 0

Switches are available with normally open (NO) or normally closed (NC) contacts or can be configured as either by choice of the relevant contacts. An NO switch has its contacts open in the absence of a mechanical input and the mechanical input is used to close the switch. An NC switch has its contacts closed in the absence of a mechanical input and the mechanical input is used to open the switch.

The term limit switch is used for a switch which is used to detect the presence or passage of a moving part. It can be actuated by a cam, roller or lever. Figure 2.4 shows some examples. The cam (Figure 2.4(c)) can be rotated at a constant rate and so switch the switch on and off for particular time intervals.

Figure 2.4 Limit switches actuated by: (a) lever, (b) roller, (c) cam

2.1.2 Proximity switches

Proximity switches are used to detect the presence of an item without making contact with it. There are a number of forms of such switches, some being only suitable for metallic objects.

The eddy current type of proximity switch has a coil which is energised by a constant alternating current and produces a constant alternating magnetic field. When a metallic object is close to it, eddy currents are induced in it (Figure 2.5(a)). The magnetic field due to these eddy currents induces an e.m.f. back in the coil with the result that the voltage amplitude needed to maintain the constant coil current changes. The voltage amplitude is thus a measure of the proximity of metallic objects. The voltage can be used to activate an electronic switch circuit, basically a transistor which has its output switched from low to high by the voltage change, and so give an on-off device. The range over which such objects can be detected is typically about 0.5 to 20 mm.

Figure 2.5 Proximity switches: (a) eddy current, (b) reed switch, (c) capacitive

Another type is the reed switch. This consists of two overlapping, but not touching, strips of a springy ferromagnetic material sealed in a glass or plastic envelope (Figure 2.5(b)). When a magnet or current-carrying coil is brought close to the switch, the strips become magnetised and attract each other. The contacts then close. The magnet closes the contacts when it is typically about 1 mm from the switch. Such a switch is widely used with burglar alarms to detect when a door is opened; the magnet being in the door and the reed switch in the frame of the door. When the door opens the switch opens.

A proximity switch that can be used with metallic and non-metallic objects is the capacitive proximity switch. The capacitance of...

Produkt-Details

• Erscheint lt. Verlag: 1.4.2011
• Sprache: englisch
• Themenwelt: Sachbuch/Ratgeber
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• ISBN-10: 0-08-046295-2 / 0080462952
• ISBN-13: 978-0-08-046295-0 / 9780080462950