CRANFIELD UNIVERSITY Charalambos Andrew Georgiou Thermal

156 Pages·2013·6.67 MB·English

by  Computer Centre

CRANFIELD UNIVERSITY

Charalambos Andrew Georgiou

Thermal Performance of a Multi-Axis Smoothing Cell

School of Applied Sciences
Precision Engineering

Masters by Research
Academic Year: 2010 - 2011

Supervisors: Paul Comley, Paul Shore
May 2011

CRANFIELD UNIVERSITY

School of Applied Science
Precision Engineering

MSc by Research

Academic Year 2010 - 2011

Charalambos Andrew Georgiou

Thermal Performance of a Multi-Axis Smoothing Cell

Supervisor: Paul Comley, Paul Shore

May 2011

© Cranfield University 2011. All rights reserved. No part of this
publication may be reproduced without the written permission of the
copyright owner.

ABSTRACT
Multi Axis Robots have traditionally been used in industry for pick and place, de-
burring, and welding operations. Increasing technological advances have
broadened their application and today robots are increasingly being used for
higher precision applications in the medical and nuclear sectors. In order to use
robots in such roles it is important to understand their performance. Thermal
effects in machine tools are acknowledged to account for up to 70% of all errors
(Bryan J. , 1990) and therefore need to be considered.
This research investigates thermal influences on the accuracy and repeatability
of a six degree of freedom robotic arm, which forms an integral part of a
smoothing cell. The cell forms part of a process chain currently being developed
for the processing of high accuracy freeform surfaces, intended for use on the
next generation of ground based telescopes. The robot studied was a FANUC
710i/50 with a lapping spindle the end effector.
The robot geometric motions were characterised and the structure was
thermally mapped at the latter velocity. The thermal mapping identified the key
areas of the robot structure requiring more detailed analysis. Further
investigation looked into thermal variations in conjunction with geometric
measurements in order to characterise the robot thermal performance. Results
showed thermal variations of up to 13ºC over a period of six hours, these
produced errors of up to 100µm over the 1300mm working stroke slow. Thermal
modelling carried out predicted geometric variation of 70µm to 122µm for
thermal variations up to 13ºC over a period of six hours. The modelling was
50% to 75% efficient in predicting thermal error magnitudes in the X axis. With
the geometric and modelling data a recommendation for offline compensation
would enable significant improvement in the robots positioning capability to be
achieved.
Keywords: Robot, Thermal Performance, Thermal Model, Geometric
Performance, Optics
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ACKNOWLEDGEMENTS
I would like to thank God and all those who have kept me in their prayers, my
family and friends who have assisted me in writing my thesis and made many
sacrifices for me. No trophy or victory in this world comes without sacrifice. I
wish to thank all those during this time who offered support; specifically my
parents Andreas and Rita Georgiou, who have unconditionally shown love for
me. My sisters and cousin Maria Georgiou and Petros Sofocleous who gave me
the opportunity to have the English and grammar checked during their working
hours by their account manager Christina Georgiou. She also deserves thanks
for her dedication throughout weekends which has been relentless. Natalie
Georgiou for the reminders to keep me from distractions, my cousin Paul
Pangalos who made many suggestions to me, sacrificing much time in making
comments and suggestions for writing up my thesis and also for ideas for my
VIVA presentation and offering his experience in working with other students.
Theodore Veniamin who also offered advice in writing up together with his
experience from a non-technical point of view who was always willing to lend a
hand if I needed it. Wynand Swart from South Africa, after I sent a request for
help over e-mail not having met him before who helped me with the DH
Parameters for Fanuc Robot 710iC/50 for the kinematic model of my robot, and
his secretary Han Li. Gratitude also goes to my supervisors Paul Comley who
also spend many hours going over my work and making comments, I could not
ask for a better supervisor, and Paul Shore for overseeing my work. All the
technicians and staff who I would join for tea breaks and who enjoyed my
‘unique’ humour, biscuits and cakes. Andy, Xavier, John, Kevin, Adam, Alan,
Roger, Yan, Ian, Renaud, and others. PhD and MSc by Research students
Marco Castelli, Graham McMeecking, Tom Morris, Yinka Abedayo, Eva,
Dominica, the Italian summer students, Andreas Bacci, Marco Targa, Andrea,
Andrea and Simone and Yakubo, Saadia Hakim.
This whole experience has taught me what research is, it is not a perfect world
where everything is set out for you, there are obstacles which are hard to
overcome but not impossible, for everything I am grateful.
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TABLE OF CONTENTS

ABSTRACT ......................................................................................................... i
ACKNOWLEDGEMENTS................................................................................... iii
LIST OF FIGURES ............................................................................................. 7
LIST OF TABLES ............................................................................................... 9
LIST OF EQUATIONS ...................................................................................... 10
List of symbols/acronyms ................................................................................. 10
List of acronyms ............................................................................................ 10
List of Symbols ............................................................................................. 11
1 Introduction ............................................................................................... 14
1.1 Background ......................................................................................... 14
1.2 Machine performance .......................................................................... 16
1.3 Research scope .................................................................................. 17
1.4 Work plan ............................................................................................ 17
2 Literature review: History and published work ........................................... 18
2.1 Performance factors ............................................................................ 18
2.1.1 Machine design ............................................................................. 18
2.1.2 Environment and thermal effects .................................................. 21
2.1.3 Robot design................................................................................. 23
2.2 Geometric performance measurement ................................................ 28
2.2.1 Direct methods.............................................................................. 29
2.2.2 Indirect methods ........................................................................... 35
2.3 Thermal measurement ........................................................................ 37
2.3.1 Contact devices ............................................................................ 37
2.3.2 Non-contact .................................................................................. 40
2.4 Compensation strategies for machine tools and robots....................... 42
2.4.1 Temperature control ..................................................................... 43
2.4.2 Offline compensation techniques .................................................. 45
2.4.3 Online compensation techniques .................................................. 46
3 Experimental methodology ........................................................................ 49
3.1 Geometric assessment ........................................................................ 49
3.1.1 Smoothing axes accuracy and repeatability ................................. 49
3.1.2 ISO standard................................................................................. 53
3.2 Thermal mapping ................................................................................ 54
3.2.1 Thermal mapping procedure ......................................................... 54
3.2.2 Implementation of thermal profiles to modelling ........................... 55
3.3 Thermal assessment ........................................................................... 58
3.3.1 Initial temperature measurement system ...................................... 59
3.3.2 Implementing the temperature measurement system ................... 60
3.3.3 Thermal effects ............................................................................. 66
4 Thermal modelling ..................................................................................... 69
4.1 Modelling strategies ............................................................................ 69
4.1.1 Initial estimation ............................................................................ 69
4.1.2 Linear distortion modelling: non-uniform thermal distribution ........ 70
4.2 Geometric model ................................................................................. 76
4.2.1 Inverse kinematics ........................................................................ 76
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4.2.2 Forward kinematics ....................................................................... 77
4.3 Geometric and thermal modelling ....................................................... 83
4.3.1 Applying thermal load to geometric model .................................... 83
4.4 Thermal results ................................................................................... 88
4.4.1 Results for thermal modelling of robot arm ................................... 88
5 Robot performance results ........................................................................ 94
5.1 Geometric evaluation of the robot ....................................................... 94
5.1.1 X Axis geometric accuracy ........................................................... 94
5.1.2 Y Axis geometric accuracy ........................................................... 96
5.1.3 Z Axis geometric accuracy ............................................................ 98
5.1.4 Circular motion of robot end effector ........................................... 100
5.1.5 ISO Standard geometric assessment ......................................... 102
5.2 A Thermal mapping system ............................................................... 104
5.2.1 Thermal profiling mapping: Thermal gradients for modelling ...... 108
5.3 Thermal performance ........................................................................ 119
5.3.1 Geometrical measurement ......................................................... 119
5.3.2 Thermal measurement ................................................................ 128
6 Discussion ............................................................................................... 135
6.1 Geometric, thermal effects, accuracy of model ................................. 135
6.1.1 X Axis ......................................................................................... 136
6.1.2 Y Axis ......................................................................................... 137
6.1.3 Z axis .......................................................................................... 139
6.1.4 Discussion of circular motion repeatability .................................. 140
6.1.5 Discussion of ISO motion repeatability ....................................... 141
6.2 Thermal performance ........................................................................ 142
6.2.1 Thermal measurement system ................................................... 142
6.2.2 Thermal output of robot .............................................................. 143
6.3 Recommendation for further work ..................................................... 144
7 Conclusions ............................................................................................. 145
8 References .............................................................................................. 147

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