Arc welding control

622 Pages·2003·7.28 MB·English

by  Jiluan Pan

Arc welding control

Arc welding
control
Pan Jiluan
Cambridge England
Published by Woodhead Publishing Limited,Abington Hall,Abington
Cambridge CB1 6AH,England
www.woodhead-publishing.com
Published in North America by CRC Press LLC,2000 Corporate Blvd,NW
Boca Raton FL 33431,USA
First published 2003,Woodhead Publishing Limited and CRC Press LLC
© 2003,Woodhead Publishing Ltd
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Woodhead Publishing ISBN 1 85573 687 X
CRC Press ISBN 0-8493-1772-X
CRC Press order number:WP1772
Typeset by SNP Best-set Typesetter Ltd.,Hong Kong
Printed by TJ International,Cornwall,England
Contents
Preface ix
List of symbols xii
Part I Dynamic behaviour of arc welding
1 Theoretical analysis 3
1.1 Introduction 3
1.2 Welding with constant wire-feed rate 4
1.3 Welding with regulated wire-feed rate 12
1.4 Welding with both regulated wire-feed rate and
self-regulation 15
2 Dynamic characteristics of power sources 19
2.1 Status of research 19
2.2 Description of dynamic characteristics 25
2.3 Transfer functions of power sources 26
2.4 Measurement of the transfer function 41
2.5 Influence of the transfer function on technological
properties 48
2.6 Summary 50
3 Experimental study 52
3.1 Introduction 52
3.2 CO welding 53
2
3.3 DC MIG welding 68
3.4 Pulsed MIG welding 75
3.5 Summary 81
v
vi Contents
Part II Welding arc control
4 Control method QH-ARC 101 85
4.1 Status of research on welding arc control 85
4.2 Principle of control method QH-ARC 101 88
4.3 Realisation of double-step characteristic 92
4.4 Circuit for alternating between two double-step output
characteristics 100
4.5 Static accuracy and dynamic characteristics 103
4.6 Circuit diagram of the power source 115
4.7 Technological behaviour 117
5 Control method QH-ARC 102 and 103 126
5.1 Introduction 126
5.2 Control method QH-ARC 102 129
5.3 Control method QH-ARC 103 134
5.4 Mathematical analysis of arc operating-point movement 148
5.5 Closed-loop control of weld penetration 168
6 Arc control of CO welding and AC MIG welding 179
2
6.1 CO welding 179
2
6.2 AC MIG welding 194
Part III Arc sensors and seam tracking
7 Sensors for weld-seam tracking 235
7.1 Status of research and development 235
7.2 Status of the application of arc sensors in industry 240
7.3 Principles of seam tracking with arc sensors 243
7.4 Scanning arc-welding torches 248
8 Mathematical models of arc sensors 261
8.1 Introduction 261
8.2 Static model 261
8.3 Dynamic model 271
9 Processing of arc-sensor signals 296
9.1 Introduction 296
9.2 Signal processing of an ideal welding arc 296
9.3 Analysis of signal characteristics 313
Contents vii
9.4 Filtering of signals 327
9.5 Signal processing in a practical welding process 343
10 An automatic seam-tracking machine with
an arc sensor 360
10.1 Introduction 360
10.2 Conventional automatic seam-tracking machine 360
10.3 Crawling type of tractor 374
Part IV Real time measurement of welding
temperature-field
11 Principles of the colorimetric imaging method 411
11.1 Introduction 411
11.2 Fundamentals of the colorimetric imaging method 417
12 Study of the colorimetric imaging method 429
12.1 Introduction 429
12.2 Facilities for acquisition of the infrared image 429
12.3 Sensor for experimental use 432
12.4 The colorimetric imaging system 435
12.5 Calibration of the system using a mathematical method 439
12.6 Experimental results and verification 440
12.7 Calibration procedure 444
12.8 Effects of measurement conditions 445
13 Design of a colorimetric imaging apparatus 450
13.1 Introduction 450
13.2 Formation of the thermal-radiation image 450
13.3 Theoretical analysis of the colorimetric imaging method 453
13.4 Division of the temperature field 457
13.5 Wavelength of the filter 462
13.6 Design of the sensor 470
13.7 Effects of measuring conditions 477
13.8 Calibration of the system 484
14 Practical measurement and application 488
14.1 Experimental apparatus 488
14.2 Measurement of the welding temperature field 488
viii Contents
14.3 Mathematical model of the temperature field in welding
based on measurement 492
14.4 Theoretical results and measured data for different
welding parameters 503
14.5 Extraction of the thermal-cycle parameters 505
14.6 Three-dimensional welding temperature field 507
14.7 Closed-loop control of weld penetration 511
14.8 Summary 525
Part V Automatic path programming of robot
15 Vision systems and automatic path programming 531
15.1 Recognition of the groove path 531
15.2 A robot with automatic path programming 548
References 582
Index 601
Preface
Welding is a relatively new manufacturing process that has only existed for
about a hundred years.However,it is an extremely important technology
because,without it,many modern industries would never have developed.
One such is the nuclear power plant industry. Nuclear power plants are
large complex structures that place incredible demands on structural
integrity.Without welding technology,such structures could never be built.
Industrial products need to be both robust and structurally sound.
Before liberation in China,welding technology did not exist.It was only
during the first five-year plan of economic construction,in the 1950s,that
an urgent need for welding technology was first recognised. Welding
technology became of prime importance in many universities and major
research institutes. During these first stages of development and imple-
mentation of welding technology in China,the USSR supported over 156
new construction projects.However,the welding technology and materials
industries could not meet this increased demand and it was because of this
that the author focused his key research on welding metallurgy (for ex-
ample,the hot crack problem) and other technologies which were used for
the welding of industrial products at that time.
By the 1970s,the welding materials and technology industries were more
or less fully developed.The prime focus moved then to welding automa-
tion.Due to the complexity of this process,welding automation was incred-
ibly difficult to develop and implement.For example:in order to seal-weld
the root side of a pressure vessel made of low-alloy steel,the worker would
have to enter the vessel at a preheated temperature of 100–200°C. Even
when the worker wore a wet asbestos suit,the working time could be only
20 minutes.This situation seriously affected the quality of the product and
the health of the worker. For mass production such as the welding of
pipelines, where the conditions were significantly better, a mechanised
welding process could be used.But with a low level of automation,incom-
plete penetration often occurred due to the misalignment of the arc in the
weld-groove.This,in turn,caused many rejects and significant economic loss.
ix
x Preface
With the economic development in his country,the author has devoted
much of his time to research into welding automation.The train of thought
in this particular field can be divided into two specific types.The first is to
use modern science to analyse the welding process through quantita-
tive methods. The second is to develop and perfect welding automation
through the application of modern technology.These two fields of research
both complement and promote one another.The focus of this research is
to grasp and solve the key individual problems of automation and then to
try to synthesis results so they can be implemented. In this monograph
the theory and developments made by the author are systematically
described.
It is the author’s intention that this book will not only promote welding
automation but will also play a significant role in guiding further develop-
ment in this important area of technology.
The book is divided into five parts, with fifteen chapters. Part I, ‘The
dynamic process of arc welding’, quantitatively describes the dynamic
behaviour of arc welding,the power sources that are used and their effect
on welding technology through the basis of control theory.Part II,‘Welding
arc control’, describes new ways of controlling the welding arc through
using modern electronics.The methods of arc control that are introduced
are unique,original and demonstrate outstanding performance.The author
demonstrates significant systematic achievements in this field that have
never before been published.These methods provide important means for
developing new welding processes and power sources.
Arc sensors are a type of sensor that have significant potential for auto-
matic seam tracking in arc welding.In Part III,‘Arc sensors and seam track-
ing’,the author establishes the first mathematical model of the sensor on
the basis of control theory,giving a clear picture of the essence and phe-
nomena of the sensor.Subsequently,the author has also analysed the output
signals of the sensors and proposed a space transformation method that can
improve the reliability and accuracy of the weld-groove recognition.Finally,
a rotating-arc sensor and two typical seam-tracking welding machines have
been developed by the author using the sensor described. Part IV,‘Real-
time measurement of welding temperature fields and its application’,
describes a new method developed by the author for measuring weldment
temperature fields using a colorimetric-image method. Experimental
studies demonstrate that the method is simple and convenient,the data is
reliable, and the speed fast.The method can be used in practice for real-
time control. This development is original and has not been published
before.
Part V,‘Automatic path programming of welding robots’, describes the
idea of recognising weld grooves with a three-dimensional vision system
and automatic programming of the welding path.It discusses the method