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plc programming for industrial automation
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plc programming for industrial automation
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PLC Programming for
Industrial Automation
Kevin Collins
Contents
Introduction
PLC Basics
Function of a PLC
Inputs and Outputs
PLC Architecture and Wiring Diagrams
Network Protocols
Questions
Ladder Programming
Conditional Logic
Ladder Diagrams
Normally closed contacts
Outputs and latches
Internal relays
Timers
The Pulse Generator
Counters
Questions and exercises
Sequential Programming
Introduction
A Simple Automation Sequence
Evolution of the Sequential Function Chart
Programming using the Sequential Function Chart
Entering the SFC program into the PLC
Modifying an SFC Program
Selective Branching
Parallel Branching
Appendix: Using the TriLogi software
Introduction
I have been teaching PLC programming for fifteen years and the question that
I hear most often from students is “Can you recommend a book on this?” In response
I have trotted out the titles of various standard text books but I have never come
across a book that really develops the skill of PLC programming instead of telling the
reader what PLCs are all about. I have finally decided to fill this gap in the market
myself.
“What sort of PLCs do you use?” is another popular question. It implies that
familiarity with one make and model of PLC will leave the programmer struggling
when asked to use a different type. I deliberately teach a generic style of
programming that allows the learner to switch between types of PLC as easily as
between different makes of electronic calculator. Every skill needs practice however
and my thanks are due to TriLogi for permission to use their excellent PLC simulator
software throughout this book. The students can load the software onto a computer
and practice the examples and exercises provided.
The third problem that authors have failed to address is the variety of
programming languages available. Ladder logic is by far the most popular
programming language in use because of its resemblance to hard-wire control
diagrams. On its own, however it is unsuitable for complex programs. As the
automation task grows so the ladder program expands organically, until only the
original programmer can find his way through the tangle of inputs and outputs, relays
and function blocks.
This problem has been solved by the use of Sequential Function Chart
(GRAFCET) methods but the obvious popularity of ladder logic persists. The solution
is to plan the program using a sequential function chart and then to enter it into the
PLC using ladder logic. In this way program is highly structured, standardised and
easy to debug and modify, while the familiarity of ladder logic is preserved.
The first two chapters of the book are used for programming basics. The
remainder concentrates on the control of automation sequences commonly found in
industry.
The examples used in the book have all been thoroughly tested and their
suitability for use in the classroom and in industry established.
Chapter 1
PLC Basics
1.1 Function of a PLC
A PLC is a microprocessor-based controller with multiple inputs and outputs.
It uses a programmable memory to store instructions and carry out functions to
control machines and processes.
The PLC performs the logic functions of relays, timers, counters and
sequencers. It has the following advantages:
Low cost
Reliability
Reprogramability
Fig 1.1 A programmable logic controller
1.2 Inputs and Outputs
The PLC inputs give it information about the machine or process that it is
controlling. These are typically switches and sensors. The switches are connected to
an input module that provides the interface between the switches or sensors and the
PLC.
Input module circuits have opto-isolators to protect the internal PLC
circuitry from damage.
Fig 1.2 An Opto-Isolator
PLC
Inputs
Outputs
Program
LED
Photo
transistor
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