scheme of production.
I have written this book to provide engineering students and professionals with a broad range of knowledge and skills that are essential in using PLCs to implement control schemes in various industries. The text is comprehensive in that it covers control logic design, PLC hardware and software, PLC instructions, and PLC programming.
The text is designed for flexible use so that readers can easily choose the desired depth of coverage. Chapters 1 through 5 focus on electrical control fundamentals, number systems, logic basics, Boolean algebra, and simplifying logic circuits. This group of chapters provides readers who are studying manufacturing, industrial engineering, and mechanical engineering with the necessary electrical control backgrounds to effectively learn PLCs. Chapters 6 and 7 introduce two methods of designing PLC control logics and circuits. The knowledge in these two chapters is very important for PLC programmers, yet is absent in many PLC texts. Chapters 8, 9, and 10 introduce PLC memory, file structure, discrete input and output modules, and analog input and output modules. Chapters 11 through 18 cover all important basic as well as advanced PLC programming instructions.
The text is generic in nature but also specific for Allen-Bradley SLC 500 PLCs. Readers shall be able to learn generic principles and apply them to a specific PLC model with little need for help from PLC manuals.
I would like to thank the many people who helped to make this text possible. First, I want to thank many former students who took my PLC courses at Eastern Michigan University. Some of their creative ideas and PLC programming projects are included in many chapters. Their contributions to this text are very appreciated.
I would like to thank Jim Dodd, Robert Weinstein, Janet Romano, Cathleen Prisco, and other editing and production staff at Industrial Press for their patience and professional skills in working on this text.
I am greatly indebted to my dear wife, Grace, for her support and encouragement.
Su-Chen Jonathon Lin
PROGRAMMABLE
LOGIC
CONTROLLERS
Introduction to Programmable Logic Controllers
PLCs are digital electronic apparatus with a programmable memory for storing instructions to implement logic, sequencing, timing, counting, and arithmetic functions. Since its inception in 1969, PLC technology has staged through six generations of development. Memory size and added functions have been increased from one generation to the next.
There are four major components in a PLC: processor unit, input modules, output modules, and programming device. A processor unit is the brain of the PLC that consists of three elements: central process unit, memory, and power supply. Input modules provide the physical connection between the processor unit and input field devices such as limit switches. Output modules provide the physical connection between the processor unit and output field devices such as motor starters. A programming device allows creating PLC programs and entering the programs to PLCs.
The basic operation principle of a PLC is program scanning. A PLC processor scans a program in a cyclic manner, starting from left to right at the top rung, then proceeding to the next rung. During scanning, the processor simultaneously updates the status of input and output instructions in both the program and the memory, as well as interacting with input and output modules.
The benefits of using PLCs are very obvious. They are flexible and programmable, reliable, and cost saving. PLCs provide ease of installation and implementation, as well as ease of maintenance and troubleshooting. There are five categories of PLCs according to their size and configuration, including micro PLCs, small PLCs, medium PLCs, large PLCs, and very large PLCs.
1.1 Definition of PLC
The term PLC is the abbreviation for Programmable Logic Controller. PLCs were initially called Programmable Controllers (PCs). The use of this term, PCs, caused some confusion when personal computers (PCs) became popular. To avoid this confusion, PCs are referred to as personal computers and PLCs programmable logic controllers. The National Electrical Manufacturing Association (NEMA) defines a programmable controller as follows:
“A programmable controller is a digital electronic apparatus with a programmable memory for storing instructions to implement specific functions, such as logic, sequencing, timing, counting, and arithmetic to control machines and processes.”
More specifically, a PLC can be considered as an industrial computer that is especially designed for use in industrial, rugged environments. It performs the following functions:
•Receives and interprets the signals from various input switches and sensors
•Implements the control logics designed in the form of programs
•Outputs the control signals to activate the power devices such as motor starters, solenoids, contactors, etc.
1.2 History of PLCs
The inception of PLCs resulted from the necessity and the emergence of computer technology. Electromechanical relays that provide logic controls for industrial systems had been successfully implemented for many generations. The main problems with those hard-wired, relay-based systems include:
•Lack of flexibility of reprogramming
•Limit to relay types of applications
•Susceptive to mechanical failure due to physical contact and wear
•Prone to wiring errors
•Difficulty of trouble shooting
•Limited to small-to-medium size of control systems
•Costly to implement when the size of control systems increase
GM’s