Instrumentation in Process Control

372 Pages·1972·10.55 MB·English

by  E. J. Wightman (Auth.)

Instrumentation
in Process Control
E. J. WIGHTMAN, C.Eng., M.I.Mech.E.,
A.F.R.Ae, S., M.I.E.E.
(Formerly Engineering Director, Industrial Instrument
Division, Smiths Industries Ltd.)
LONDON
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First published in 1972
© E. J. Wightman 1972
ISBN 0 408 70293 1
Printed in Hungary
Preface
The object of this book is to highlight fundamental characteristics
of transducers used for making measurements of physical variables
such as pressure, temperature, flow, density, speed and displace­
ment. Methods are outlined by which it is possible to relate trans­
ducers and their characteristics to digital data processing techniques
which utilise these measurements in control applications.
This book deals specifically with the problems associated in the
gathering of data from physical processes and may be divided
into three parts covering chapters dealing with transducers, signal
processing and computing. The author investigates the various
methods of transmitting or indicating the data and compares the
various ways whereby it may be processed. These include storage
and its application to modify or control the operating conditions
of the physical process from which it is derived.
It is a widely recognised rule of thumb guide to the cost of a
computer installation that only 30% is attributable to the comput­
er; the remaining 70% is made up from transducer, signal condi­
tioning and transmission, and control costs. Designers of the ele­
ments from which such systems are constructed are faced with
detailed problems of the analysis of performance of each element,
and project engineers responsible for the whole system design and
installation have to take responsibility for achieving targets for
overall technical performance and cost. This book may be regarded
as a bridging operation between plant engineer, instrument designer
electronic engineer and computer engineer.
In covering such a broad field of interest it has been presumed
that the reader will have some knowledge of one or two specialist
subjects, but wishes to expand his repertoire beyond his normal
domain, and to be briefed in such fundamentals as will enable
better communication with colleagues working in related fields.
The subjects from which material in the text have been chosen
represent typical problem areas in the fields of physical measure­
ment and of electronic signal processing. The depth of treatment
has been varied according to state of the art, but has generally
been confined to those factors that contribute to limitations in
techniques of measurement and data processing. SI units, or dual
Si/Imperial units of measurement have been adopted in the great
majority of examples chosen to illustrate these limiting factors.
Much of the material from which this book has been compiled
is derived from commercial sources, because the primary object
is to highlight the practical problems of achieving system per­
formance of high accuracy, based on physical parameter measure­
ments that by their very nature suffer from many limitations.
There are often limits to what can be measured at all, and therefore
controlled in practice. Certain commercially available transducers
may approach laboratory standards of accuracy, but the accuracies
obtainable even from the best of these can usually be more than
matched by subsequent processing instrumentation.
A glossary of words and phrases comprising much of the termi­
nology used in data acquisition system engineering is given in the
Appendix.
The author wishes to express his appreciation to Peter Rush
(Editor, Instrument Practice) for his initial encouragement when
making a start to this book. The author is particularly indebted
to Mr. R. E. Fischbacher, Deputy Director of Sira Institute for
making constructive criticism throughout the drafting stages and
for assisting in checking the manuscript.
E. J. W.
Acknowledgements
The author is indebted to the following Companies who have
contributed information relating to their instruments and systems.
Avery Hardoll Ltd Guest International Ltd
Bell and Honell Ltd, Electron­ Harrison Reproduction Equip­
ics and Instruments Division ment Ltd
Computer Controls Ltd Kent Instruments Ltd
Dresser Industries Inc. K.D.G. Instruments Ltd
Electro-Mechanisms Ltd. Meterflow Ltd
(Licensees Shaevitz Engineer­ Moore, Reed & Co. Ltd
ing, U.S.A.) Muirhead & Co. Ltd
Elliott Bros. (London) Ltd, Penny Giles Ltd
Aircraft/Instrument Division S.E. Laboratories Ltd
Ellison Instrument Division, Schaewitz Engineering
Dietrich Standard Corp. Singer/General Precision Inc.
Evershed & Vignoles Ltd Smiths Industries Ltd, Industrial
Fairchild Controls Inc. Instrument Division
Ferranti Ltd Sperry Gyroscope Division
General Precision Inc. of Sperry Rand Ltd
George Kent Ltd (Stroud) Stow Laboratories Inc.
The author is indebted to the following publishers:—United Trade
Press Ltd., for permission to include references from articles first
published in Instrument Practice—"Digital Process Control Trans­
ducers" (1967) in Chapter 5, "Principles Applications and Trends
in Electronic Timer Counter Design" (1967) in Chapter 9, "Auto­
matic Monitoring and Data Acquisition via the Public Switched
Telephone Networks" (1968) in Chapter 12; Morgan Grampian
Ltd., for permission to base Chapter 11 on an article Hybrid Com­
puting Techniques Applied to Data Acquisition by the author,
which first appeared in "Instrument Review", February, 1968 and
for permission to publish extracts from "Digital Instruments Vol­
ume 1" to illustrate digital control systems; to E.D. Publications
Ltd., for permission to include A Dictionary of Data Logging
first published in Automation September 1967, in the Glossary
and to Newnes-Butterworth for permission to include extracts from
'Questions and Answers on Computers' by Clement Brown.
CHAPTER 1
Introduction
1.1 Objectives
The basic acquisition and handling of data for the control of plant
and processes is becoming a highly specialised field for the instru­
ment engineer. The increasing complexity of systems, whether they
be industrial, marine, aeronautical or biological, provide stimula­
tion and challenge.
A great deal of process instrumentation still involves pneumatic
techniques, but the demands of signal transmission over long dis­
tances and of computer compatibility inevitably mean that electro­
nics technology is strongly ascendant. In order to make an instru­
mentation system of any complexity work to utmost advantage a
basic understanding of all the physical disciplines, together with
considerable initiative and versatility, are required of both designer
and user.
The aim of this book is to introduce the would-be designer and
user to a few of the golden rules relating to instrument systems
and to enable them to avoid some of the pitfalls which can be
found in ill-conceived systems.
Although measurement and data acquisition systems have to be
tailored to suit specific market applications (e.g. aeronautical, ma­
rine or industrial) the basic techniques are similar; only packaging,
environment, duty cycle and manner of storing data differ to any
great extent. Examples of specialised forms of instrument system
in these fields are described at the end of the book where their
characteristics and performance specifications are compared and
related.
2 INSTRUMENTATION IN PROCESS CONTROL
The relationship between analogue and digital systems and their
associated interfacing, transmission and conversion problems are
of great importance to the systems engineer. Some requirements
will be dictated by the system; others by the means of data process­
ing, telemetry demands, duty cycle, or servicing requirements.
The successful operation of instrument systems in the presence
of hostile environment of electrical and physical noise is perhaps
the greatest challenge to an instrument engineer. Throughout this
book every opportunity will be taken to indicate possible sources
and effects of noise at each stage of the process and, where pos­
sible, to show what effective measures may be taken to deal with
them.
1.2 Primary Measurements
In any form of data acquisition system the quality of the output
can be no better than the quality of the input; intermediate pro­
cessing can contribute very significantly to system errors but, as
will be demonstrated in later chapters, a properly designed system
will ensure that translation losses are kept to a tenth of the error
attributable to the data sources themselves. To control any physical
variable it is necessary first to be able to measure it with appro­
priate accuracy. Chapters 2 to 8 are devoted to transducers and
measurement techniques for obtaining basic physical data. More
important, they treat the limitations inherent in transducer design
which set a limit to performance, and attempt to convey a better
understanding of what is meant by the use of the word 'perform­
ance'.
This fundamental need to obtain good basic measurements can­
not be over-emphasised and the importance of selecting the best
transducer for the job is only parallelled by the necessity to install
the transducer to comply with the manufacturer's design criteria.
Such points as avoidance of mechanical strain due to incorrect
mounting, ensuring that power supply and environmental tempera­
ture limits are not exceeded, and (as will be discussed later) arrang­
ing that signal leads are properly screened and routed, may appear
to be obvious at first sight. They are nevertheless all too frequently
overlooked somewhere along the fragmented path between the
designer's plant layout and the plumber's wrench.
Although a very wide range of variables may be measured on
process and production plant, these can be reduced to a small
INTRODUCTION 3
number for the great majority of installations. Among the most
common are:
Temperature
Pressure
Density
Flow
Displacement
These measurements are the subject of individual chapters.
The majority of transducers produce an analogue output. That
is, the output signal from the transducer (whether it be pressure,
current or voltage) is proportional to the measured variable (or
some other known function of it). A few produce a digital output,
either a digit (pulse or step) for every successive increment of the
input, or a coded discrete signal, representative of the numerical
value of the input. Transducers which produce a frequency output
are strictly analogue in nature, but are often called quasi-digital
because of the ease with which frequency can be converted to
digital form by counting over a determinate period.
For each of the physical variables there is a choice of transducers.
The means of measurement typically suited to the measurement of
these parameters are described with some of their limitations.
Notwithstanding some degree of standardisation of transducer
outputs, there exists a choice of signal levels from millivolts dc from
thermocouples, 0-5 mA, 0-10 mA, 4-20 mA from process industry
transducers, demodulated single-phase ac and 400 Hz synchro out­
puts—all requiring individual matching and means of adjusting
zero offset and span. Additionally certain specialised transducers
provide an ac output whose frequency follows some linear or spe­
cial relationship such as a square root function, and digital trans­
ducers (usually displacement types) supply binary coded signals in
serial or parallel form, depending on the mode of operation. Pneu­
matic signals may be in the form 3-15 lbf/in2.
This assortment of transducer signals represents a cross-section
of the range available from which a system may be required to
operate. Only in rare situations is a system engineer fortunate
enough to have multichannel information to process at a standard
signal level; in general a system will comprise a mixture of low
level (millivolt) dc, high level (several volts) ac or dc, and pulse
trains from transducers such as flowmeters or tachogenerators.
This text does not set out to compass the range of available trans-
4 INSTRUMENTATION IN PROCESS CONTROL
ducers; indeed it would not be practicable within such limited
scope. However, the examples given should serve to illustrate that
where a choice of techniques is available in making a particular
measurement, certain forms of signal (e.g. pulse train from turbine
flowmeter or shaft velocity sensor) offer distinct advantages in
many respects.
Nevertheless, electrical transducers producing outputs varying
from low level dc to fm are widely used, and pneumatic trans­
ducers and controllers are a popular choice especially in the petro­
chemical industries. Transducers for data acquisition tend to be
associated with electrical signal output to minimise transmission
loss. The cost of pneumatic-to-electrical converters in a pneumatic
system would be considerable if widespread data logging is re­
quired. When the best available transducer for each of the variables
is selected, it will become evident that the chances are that a mixed
bag of dc low level and high level voltages, ac demodulated signals
and variable frequency pulse trains may result. Consequently, a
generalised data acquisition system based on standard inputs is
seldom feasible.
Since one of the prime objectives of any data acquisition systems
is the repeatable measurement of data within accuracy limits to
an order better than the limits acceptable for control, considerable
emphasis is directed towards those techniques of transducer con­
struction that directly contribute to transducer performance. De­
scription of these relevant features should provide pointers that
will assist in understanding the limitations of transducers in gen­
eral, and highlight those areas of detail that may enable an objective
appraisal of any new form of transducer to be carried out, par­
ticularly in terms of the potential accuracy obtainable. Since the
performance of any system, whether for statistical data recording
or control, is ultimately dependent on the integrity of the basic
data fed into the input, insufficient attention to detail in this area
can all too easily invalidate the output results.
1.3 Signal Conversion Techniques
Prior to 1950, most data acquisition and control systems were
basically analogue, comprising proportional measuring, indicating,
computing, and positioning instruments. A classic example of a
simple form of 'analogue' data logger is the multi-point strip chart
recorder.