Practical Programmable Logic Controllers for Automation and Process Control

Transcription

1 Practical Programmable Logic Controllers for Automation and Process Control

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5 Presents Practical Programmable Logic Controllers (PLCs) for Automation and Process Control Revision 4.1 Written by Dinesh Patil B.E. (Instrumentation & Control) 1 st Class, Dip Industrial Electronics Latest Revision by Rodney Jacobs, NH Dip, M Dip Tech, Pr Tech Eng, BA (Hons), D Tech Website: idc@idc-online.com

6 IDC Technologies Pty Ltd PO Box 1093, West Perth, Western Australia 6872 Offices in Australia, New Zealand, Singapore, United Kingdom, Ireland, Malaysia, Poland, United States of America, Canada, South Africa and India Copyright IDC Technologies All rights reserved. First published 2006 All rights to this publication, associated software and workshop are reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher. All enquiries should be made to the publisher at the address above. Disclaimer Whilst all reasonable care has been taken to ensure that the descriptions, opinions, programs, listings, software and diagrams are accurate and workable, IDC Technologies do not accept any legal responsibility or liability to any person, organization or other entity for any direct loss, consequential loss or damage, however caused, that may be suffered as a result of the use of this publication or the associated workshop and software. In case of any uncertainty, we recommend that you contact IDC Technologies for clarification or assistance. Trademarks All logos and trademarks belong to, and are copyrighted to, their companies respectively. Acknowledgements IDC Technologies expresses its sincere thanks to all those engineers and technicians on our training workshops who freely made available their expertise in preparing this manual.

7 Contents 1 Introduction to the PLC Introduction Basic Block Diagram of the PLC Size of the PLC System Components of the PLC Systems PLC and Process Interaction Number Systems and Codes 8 2 Processors, Power Supply and Programming Devices Introduction To The PLC Hardware Processors (CPU) PLC Power Supply Programming Device 17 3 Memory System and I/O Interaction Memory Systems Digital I/O Interaction Analog I/O Interaction 26 4 Digital Input/Output Systems Basics of Discrete I/O Systems Types of Discrete Field Devices Types of Discrete Input Modules Types of Discrete Output Modules 34 5 Analog Input/ Output Systems Basic of Analog I/O Systems Types of Analog Field Devices Types of Analog Input Modules Types of Analog Output Modules 47 6 Special Function I/O and Serial Communication Interfacing Introduction Fast Response Input Module Counter Module Positioning Module Stepper Motor Positioning Module 61 7 Good Installation Practices Introduction PLC Modules PLC Rack PLC Panel Internal Wiring PLC Panel Power Supply Cabling Between PLC And Field Devices 74

8 7.7 Cabling PLC And Control Room Computers PLC Earthing Specific PLC Installation Requirements Control Room Requirements 83 8 Fundamentals of PLC Programming Introduction PLC Programming Steps Programming Languages Basic Logic Instructions Timers Counter Program Flow Control Instructions Data Load and Transfer Instructions Arithmetic or Math Instructions Data Acquisition Introduction A Typical Data Acquisition System Aliasing and the Sampling Theorem Data Coding System Analog and Digital Control Introduction Analog Inputs Signal Filtering Analog Display Analog Control Application of PID Control Alternative Forms of Analog Control Fault Tolerance - Spreading the Risk How Reliable Is Our Equipment? Project Planning Key Questions Two Key Strategies The People! Primary Loop Control and Interlocking The Field Devices Communications DCS Structure PLC System Options The Costs Utilities Spare PLCs I/O Allocation Backing up Current Plant Data Power Supplies 152

9 12 Peripheral Equipment Future Directions with Smart Instruments Highway Addressable Remote Transducer (HART) ASCII Communications Devices Intelligent Communication Devices Data Communications Introduction RS RS Modbus Serial Ethernet and TCP/IP Introduction Mbps Ethernet Mbps Ethernet ( Fast Ethernet ) Gigabit Ethernet Industrial Ethernet IP (Internet Protocol) ARP and RARP TCP UDP Field buses Background Plant Automation Hierarchies HART DeviceNet Profibus Foundation Fieldbus Operator Interfaces Introduction Ergonomic Considerations High Security PLC Systems Introduction and Terminology Background to Safety Control Systems Safety Systems Concepts Resistance to Random Hardware Failures Architectures for Safety PLCs Objections to Standard PLCs Used for Safety Characteristics of Safety PLCs Hardware Characteristics of a Safety PLC Software Characteristics of a Safety PLC Design Safety PLCs Redundant Architectures for PLCs- High Availability with High Integrity Conclusion of Safety PLCs Application Software Safe Networking 277

10 17.15 Classifications and Certification Summary of High Security PLCs System Programming and Implementation Introduction Taking Process Inputs Creating I/O List Deciding Hardware Configuration of PLC system I/O Address Assignment Developing Program Structure Tips for Developing a PLC Program Program Verification and Simulation Creating Documentation Best Practice Documentation What Is a Manual? Types of Manuals Planning the Manual Drafting the Manual Reviewing the Manual HMI (Human Machine Interface) Introduction Design Consideration of HMI Hardware Interface Between PLC and HMI Software Interface Between PLC and HMI Electrical Design and Construction Introduction PLC Enclosure Panel Layout Electrical Wiring of Panel Gudielines for Panel Electrial Design Functional Specification of the System Introduction System Requirement Specification Function Specification of the System Hitech Mining Circulation of the Functional Specification Checklist of Factors for Selection of an I/O Module Selection of the System Testing of the System Fuzzy Logic Introduction Understanding Fuzzy Logic The Rules of Fuzzy Logic Fuzzy Logic Example Using Rules And Patches The Achilles Heel of Fuzzy Logic 387

11 24 Configuration of the System Initial Concepts Training on the System Input/Output Database Creation General Database Design Tips Installation and Commissioning Introduction Control Room MCC Requirements Installation of Equipment Loop Testing Manual Control Automatic Control System Handover Maintenance and Training Working Example of PLC Programs Introduction Control Philosophy Typical PLC Program for Drives Understanding Advanced Programming Techniques 413 Bibliography 419 Appendices Appendix A Glossary 421 Appendix B ASCII Table 437 Appendix C Number Systems 439 Appendix D Logic Fundamentals 449 Appendix E Practicals: Part Appendix F Basic Program 471 Appendix G Functional Logic Diagrams 479 Appendix H Practicals: Part 2 483

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13 1 Introduction to the PLC In this chapter, we will learn the following: Introduction to the PLC Basic block diagram of the PLC Size of the PLC system Components of the PLC system PLC and process interaction Number system and codes 1.1 Introduction to the PLC In the past, processes were controlled manually, which was a very tedious job. During the early years of control, man constructed hardware relay wiring to control the same process. However, relays could not meet all the needs of modern times, and a faster solution was required. Simply, when a change of control logic was required, the entire hardware wiring needed to be changed. This was time consuming as well as tiresome. After a lot of toil, man finally designed the PLC. He overcame all the constraints and attained flexibility to carry out the necessary modifications. PLC means Programmable Logic Controller. The word Programmable differentiates it from the conventional hard-wired relay logic. It can be easily programmed or changed as per the application s requirement. The PLC also surpassed the hazard of changing the wiring. The PLC as a unit consists of a processor to execute the control action on the field data provided by input and output modules. In a programming device, the PLC control logic is first developed and then transferred to the PLC.

14 2 Practical Programmable Logic Controllers (PLCs) for Automation and Process Control What can a PLC do? It can perform relay-switching tasks. It can conduct counting, calculation and comparison of analog process values. It offers flexibility to modify the control logic, whenever required, in the shortest time. It responds to the changes in process parameters within fraction of seconds. It improves the overall control system reliability. It is cost effective for controlling complex systems. Trouble-shooting becomes simpler and faster. An operator can easily interact with the process with the help of the HMI (Human-Machine Interface) computer. 1.2 Basic block diagram of the PLC Figure 1.1 shows the basic block diagram of a common PLC system. Figure 1.1 Block diagram of a PLC As shown in the above figure, you will find the heart of the PLC in the center, i.e., the Processor or CPU (Central Processing Unit). The CPU regulates PLC program, data storage, and data exchange with I//O modules. I/O Modules are the second important and basic components of any PLC system. Input and output modules are the media for data exchange between field devices and CPU. It tells CPU the exact status of field devices and also acts as a tool to control them.

15 Introduction to the PLC 3 The programming device is the third significant component. A programming device is a computer loaded with programming software, which allows a user to create, transfer and make changes in the PLC software. Memory is the fourth notable component. Memory provides the storage media for PLC program as well as different data. 1.3 Size of the PLC system PLCs are classified on basis of their sizes: A small system is one with less than 500 analog and digital I/Os. A medium system has I/Os ranging from 500 to 5,000. A system with over 5,000 I/Os is considered large. 1.4 Components of the PLC system Figure 1.2 illustrates a sample PLC system and its components. Figure 1.2 The main/chief components of a PLC

16 4 Practical Programmable Logic Controllers (PLCs) for Automation and Process Control This is the actual system. What does it look like? Does it seem difficult to understand? Let us compare Fig. 1.2 component by component with Fig. 1.3, which is its simplified version. Figure 1.3 Simplified component configuration of a PLC 1) CPU or processor The main processor (Central Processing Unit or CPU) is a microprocessor-based system that executes the control program after reading the status of field inputs and then sends commands to field outputs. It is easy to perform arithmetic functions, manipulate data and calculate Boolean logic. The PLC s memory contains the manufacturer s operating system and housekeeping functions. It also has program written by the user and data stored by the user related to the process or equipment being controlled. In Fig. 1.3, the processor module is plugged into the first slot of the central or local rack, which is the normal practice. The CPU card is connected with I/O modules through back-plane connections inside the rack or chassis. Hence, it is possible for the CPU to read the status of all input modules through the data bus. This is called local I/O chassis. If the I/Os are located at remote places, the CPU accesses them through a remote I/O chassis. In such a case, the remote chassis will have a remote I/O communication module. It collects the data from I/O modules and sends it to the CPU through an I/O rack communication link. This is particularly useful when the process is divided up, and located in remote parts.

17 Introduction to the PLC 5 The CPU card has ports for communicating with the programming device as well as the operator s station. 2) I/O section The I/O section consists of a rack and individual I/O modules, which are plugged into the rack and a DC power supply. A standard approach is to connect to the main processor rack with communication cables to a series of other I/O racks. I/O modules act as Real Data Interface between field and PLC CPU. The PLC knows the real status of field devices, and controls the field devices by means of to the relevant I/O cards. Various I/O modules are available. Each one of these will be discussed in more detail in the following sections. 3) Programming device A CPU card can be connected with a programming device through a communication link via a programming port on the CPU. Thus, it is possible to transfer programs from the programming device to the CPU, monitor the CPU s program online and make necessary changes in the CPU s program. More details of a programming device will be discussed in the next chapter. 4) Operating station An operating station is commonly used to provide an "Operating Window" to the process. It is usually a separate device (generally a PC), loaded with HMI (Human Machine Software). This operating station can change any process set point, observe all process parameters, process alarms, etc. 1.5 PLC and process interaction It is very interesting to see how the interaction between the PLC and the process takes place through I/O modules. This section will explain the role of different I/O modules as a part of PLC and process interaction. The field devices or field data are broadly classified as follows: Digital or discrete or on/off type field devices Analog or continuous type field devices For the sake of simplicity, the first examples of digital type field devices, PLC and process interaction can be studied Digital I/O and PLC Fig. 1.4 shows how different types of digital or discrete field devices are connected to the PLC through digital I/O modules.

18 6 Practical Programmable Logic Controllers (PLCs) for Automation and Process Control Figure 1.4 Digital I/O process interaction Discrete or digital inputs are the most common types of field inputs. Selector switches, pushbuttons, limit switches, temperature switches, level switches, flow switches, etc., are common examples. All types of switches that permit digital contact are included in this class. Depending on the field device contacts, they are normally connected to 110VAC, 230V AC and, 24VDC types of digital input cards installed in PLC rack. It is assumed to be 24VDC in Fig Solenoid valves, relays and auxiliary contactors are discrete or digital output devices commonly used in the field. Depending on the field devices 110VAC, 230VAC and 24VDC types of digital output cards are installed in the PLC rack. It is assumed to be 24VDC in Fig Depending on the state of field devices, i.e., Logic 1 or Logic 0 (True/False) the corresponding voltage signal will be received at a digital input module. The CPU collects the status of Logic 0 or Logic 1 status from I/P module for program execution. At the end of program execution, the CPU will release the On/Off command to the digital output module in form of Logic 0 or Logic 1.

19 Introduction to the PLC 7 The digital output module passes this status on to field devices in form of voltage so that they will be turned on/off depending on the state of the output Analog I/O and PLC Figure 1.5 shows how different types of analog or continuous types of field devices are connected with the PLC through analog I/O modules. Figure 1.5 Analog I/O process interaction Analog or continuous devices are generally used for getting feedback of the process parameter control. For example, temperature transducers (RTDs, thermocouples), level, pressure, flow, etc., transmitters. Basically, all types of transducer devices giving continuous signal are included in this class. Depending on field transducer devices, they are normally connected to 0-20 or 4-20 ma DC, 0-10VDC, RTDs, thermocouples, etc., type of analog input card, installed in the PLC rack. Analog output actuators, commonly used in the field, include continuous actuators, I/P converter for valves, reference for drives, etc.

20 8 Practical Programmable Logic Controllers (PLCs) for Automation and Process Control The corresponding signal received at the analog input module, depends on the value of field devices (i.e., either 4-20mA or some continuous value) and varies from a minimum to a maximum over the entire calibrated range.. This signal is converted by the input module, into a count using ADC (analog-to-digital converter). The CPU collects the same count from an analog I/P module for program execution. At the end of program execution, the CPU will once again release a count proportionate to the control value to the analog output module. The analog output module passes the same count (either as a 4-20mA or some continuous signal) using DAC (digital-to-analog converter) to the field actuating devices. The 4-20mA signal actuator will either open/close the valve depending on it s setup. 1.6 Number systems and codes The commonly used number systems can be listed as: Decimal number system Binary number system Octal number system Hexadecimal (Hex) number system One may also encounter BCD (binary coded decimal) in codes. The following definitions / terms are commonly encountered: Bit: A single binary digit that can have either value 0 or 1. Base: This denotes the total number of digits used by the number system. LSB (Least Significant Bit): This is the bit that represents the smallest value. MSB (Most Significant Bit): This is the bit that represents the largest value. Byte: A group of 8 bits. Nibble: A group of 4 bits. Word: A group of 16 bits Decimal number system The decimal number system is the most common number system (and one which we should all have learnt in school!). The decimal number system has a base of 10. Base 10 means it uses ten unique numbers (0 to 9) for the entire number system. A specific weight value is allocated to each digit from the right to the left. Therefore, the value of a decimal number depends on the digit as well as its location.

21 Introduction to the PLC 9 For example, If we take 2413 as a decimal number, its weight can be shown as, x 10 exp. 0 = 3x1 = 3 1 x 10 exp. 1 = 1x10 = 10 4 x 10 exp.2 = 4x100 = x 10 exp.3 = 2x1000 = 2000 = 2413 (Decimal) Binary number system The most important base for the microprocessor is base 2. Normally, it is also known as Binary, since binary means only two values. The binary system uses only two digits zero (0) and one (1) to represent all numbers. Two states exists for any digital device one as On state or Logic 1 and other as Off state or Logic 0. As the base was 10 in the decimal system, here, the base is 2 in the binary system. The weight of the digits is calculated in terms of base 2 like for example, x 2 exp. 0 = 1x1 = 1 1 x 2 exp.1 = 1x2 = 2 0 x 2 exp.2 = 0x4 = 0 0 x 2 exp.3 = 0x8 = 0 1 x 2 exp.4 = 1x16 = 16 = 19 (Decimal) Octal number system The base for this number system is 8. That is why it is called Octal. The binary system uses eight digits from zero (0) to seven (7) to represent all the numbers. Octal Digit Binary Equivalent

22 10 Practical Programmable Logic Controllers (PLCs) for Automation and Process Control Basically, it can be considered as a shorthand version of binary. Octal notation was used to provide an easy and more readable view of the binary data. But it is not frequently used, when compared to other number systems. For example, the octal number 24 is represented as, x 8 exp.0 = 4x1 = 4 2 x 8 exp.1 = 2x8 = 16 = 20 (Decimal) Hexadecimal (Hex) number system The base of this number system is 16. It provides a shorter notation of the numbers, compared octal system. It is very popular and was introduced by IBM. The hexadecimal number system uses 16 digits. These are: Numbers 0 to 9, and it uses letters A, B, C, D, E, and F to represent the decimal equivalents of 10, 11, 12, 13, 14 and 15. Hex Digit Binary Equivalent Hex Digit 8 9 A B C D E F Binary Equivalent As shown in above table, a hex digit is representing four binary digits (0000 to 1111). For example, the hex number 2F is represented as, F F x 16 exp.0 = Fx1 = 15 2 x 16 exp. 1 = 2x16 = 32 = 47 (Decimal) Conversion of numbers Table 1.1 gives an overview of all number systems we have seen so far. Note that all number systems start with digit 0.

23 Introduction to the PLC 11 Table 1.1 Number formats Decimal Binary Octal Hex A B C D E F BCD (Binary Coded Decimal) Binary coded decimal uses binary numbers in coded format to represent the decimal numbers (unlike the normal way; discussed in the earlier section). It is also called as 8421 BCD code. It basically employs four binary bits, with the weights 1, 2, 4 and 8 assigned to it. This is illustrated in the table given below: Decimal Digit BCD Equivalent Decimal Digit 8 9 BCD Equivalent For example, the decimal number 79 is represented in BCD format as, = 79 (Decimal)

24 12 Practical Programmable Logic Controllers (PLCs) for Automation and Process Control Each decimal number is replaced by its BCD equivalent, which is a four-digit binary number. The BCD number format is useful for the PLC while handling a large number of input and output data. Nowadays, PLC s possess a certain number format conversion programming instructions. One can easily convert numbers from one format to another with the help of these instructions. Should you wish to do some form of conversion, program the PLC with the necessary code and it will be done.