Programmable_Controllers_Engineer_Guides

All industrial processes need some form of control system if they are to
run safely and economically. In recent years a specialist control computer,
called a programmable controller, has evolved and revolutionized control
engineering by combining computing power and immense flexibility at
a reasonable price.
This book is concerned with the application and use of programmable
controllers. It is not an instructional book in programming, and is certainly
not a comparative guide to the various makes of machine on the market.
To some extent, choosing a programmable controller is rather like
choosing a word processor. You ask people for their views, try a few
simple examples in a shop, and buy the cheapest that you think meets
your requirements. Only after several months do you really know the
system. From then on, all other word processors seem awkward.
Programmable controllers are similar. Unless there are good reasons
for a particular choice (ready experience in the engineering or maintenance
staff, equipment being supplied by an outside contractor and similar
considerations), there are good and bad points with all (the really bad
machines left the market years ago).
At the Sheerness Steel Company where I work, the plant control is
based on about sixty programmable controllers consisting of Allen
Bradley PLC 2s and 5s, GEC (now CEGELEC) GEM-80s, ASEA (now
ABB) Masters and Siemens SIMATIC S5s, with small machines primar-
ily from Mitsubishi. These controllers are somewhat like the trees at
Galleons Lap in Winnie the Pooh; there never seems to be the same
number on two successive days, even if you tie a piece of string around
each one!
As with most plants, the background to this distribution of controllers
is largely historical chance (the original Mitsubishi came on a small
turn-key plant from an outside contractor, for example), but the ready
access to these machines is the reason for their prominence in this book.
Even within this range of PLC families, the coverage in this book is
not complete. The PLCs have been chosen to cover the application points
I wish to make, not as a complete survey of a manufacturer’s range.
In ‘previous lives’ I have worked with PLCs from AEG, GE, Landys
and Gyr, Modicon, Telemecanique, Texas Instruments and many other
companies. To these manufacturers I offer my sincere apologies for not
giving them more coverage, but to do so would have made a tedious book
and masked the application points I have tried to make. I could happily
use any of these machines, and there is not a major difference in style or
philosophy between them (the manufacturers would no doubt disagree!).
The guideline is therefore choose a machine that suits you, and do not
change manufacturers for purely economic reasons. Knowledge, consistency
of spares and a good relationship with a manufacturer are very valuable.
A book like this requires much assistance, and I would like to thank
Peter Bark and Dave Wilson of ABB, Adrian Bishop, Bob Hunt, Julian
Fielding, John Hanscombe, Hugh Pickard, Jennie Holmes and Hennie
Jacobs of Allen Bradley, Peter Backenist, David Slingsby and Stuart
Webb of GEC/CEGELEC, Peter Houldsworth, Paul Judge, Allan
Norbury, Dickon Purvis, Paul Brett and Allan Roworth of Siemens,
and Craig Rousell who all assisted with information on their machines,
commented constructively on my thoughts and provided material and
photographs.
My fellow engineers at Sheerness Steel also deserve some praise for
tolerating my PLC systems (and who will no doubt compare my written
aims with our actual achievements!).
A book takes some time to write, and my family deserve considerable
thanks for their patience.

Andrew Parr
Minster on Sea
eaparr2002@yahoo.co.uk

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