PREFACE
Designing software for control systems is difficult. Experienced controls engineers
have learned many techniques that allow them to solve problems. This book was written to
present methods for designing controls software using Programmable Logic Controllers (PLCs). It is my personal hope that by employing the knowledge in the book that you will be able to quickly write controls programs that work as expected (and avoid having to learn by costly mistakes.)
This book has been designed for students with some knowledge of technology,
including limited electricity, who wish to learn the discipline of practical control system
design on commonly used hardware. To this end the book will use the Allen Bradley ControlLogix processors to allow depth. Although the chapters will focus on specific hardware, the techniques are portable to other PLCs. Whenever possible the IEC 61131
programming standards will be used to help in the use of other PLCs.
In some cases the material will build upon the content found in a linear controls
course. But, a heavy emphasis is placed on discrete control systems. Figure 1.1 crudely
shows some of the basic categories of control system problems.
PROGRAMMABLE LOGIC CONTROLLERS
Topics:
• PLC History
• Ladder Logic and Relays
• PLC Programming
• PLC Operation
• An Example
Objectives:
• Know general PLC issues
• To be able to write simple ladder logic programs
• Understand the operation of a PLC
INTRODUCTION
Control engineering has evolved over time. In the past humans were the main
method for controlling a system. More recently electricity has been used for control and
early electrical control was based on relays. These relays allow power to be switched on
and off without a mechanical switch. It is common to use relays to make simple logical
control decisions. The development of low cost computer has brought the most recent revolution, the Programmable Logic Controller (PLC). The advent of the PLC began in the
1970s, and has become the most common choice for manufacturing controls.
PLCs have been gaining popularity on the factory floor and will probably remain
predominant for some time to come. Most of this is because of the advantages they offer.
• Cost effective for controlling complex systems.
• Flexible and can be reapplied to control other systems quickly and easily.
• Computational abilities allow more sophisticated control.
• Trouble shooting aids make programming easier and reduce downtime.
• Reliable components make these likely to operate for years before failure.
Ladder Logic
Ladder logic is the main programming method used for PLCs. As mentioned before, ladder logic has been developed to mimic relay logic. The decision to use the relay logic diagrams was a strategic one. By selecting ladder logic as the main programming method, the amount of retraining needed for engineers and tradespeople was greatly
reduced.
Modern control systems still include relays, but these are rarely used for logic. A
relay is a simple device that uses a magnetic field to control a switch, as pictured in Figure
2.1. When a voltage is applied to the input coil, the resulting current creates a magnetic
field. The magnetic field pulls a metal switch (or reed) towards it and the contacts touch,
closing the switch. The contact that closes when the coil is energized is called normally
open. The normally closed contacts touch when the input coil is not energized. Relays are
normally drawn in schematic form using a circle to represent the input coil. The output
contacts are shown with two parallel lines. Normally open contacts are shown as two
lines, and will be open (non-conducting) when the input is not energized. Normally closed
contacts are shown with two lines with a diagonal line through them. When the input coil is not energized the normally closed contacts will be closed (conducting).
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