SPHERICAL ELECTRIC MOTOR DESIGN A Dissertation by Atilla Akil Master of Science, Wichita ...

68 Pages·2013·2.02 MB·English

SPHERICAL ELECTRIC MOTOR DESIGN

A Dissertation by
Atilla Akil
Master of Science, Wichita State University, 2005
Bachelor of Science, Hacettepe University, 1993

Submitted to the Department of Electrical Engineering and Computer Science
and the faculty of the Graduate School of
Wichita State University
in partial fulfillment of
the requirements for the degree of
Doctor of Philosophy

May 2013
© Copyright 2013 by Atilla Akil
All Rights Reserved
SPHERICAL ELECTRIC MOTOR DESIGN

The following faculty members have examined the final copy of this dissertation for form and
content, and recommended that it be accepted in partial fulfillment of the requirements for the
degree of Doctor of Philosophy with a major in Electrical Engineering.

__________________________________________
John M. Watkins, Committee Chair

__________________________________________
Mahmoud E. Sawan, Committee Member

__________________________________________
Ward T. Jewell, Committee Member

__________________________________________
Gamal Weheba, Committee Member

__________________________________________
Rena Hixon, Committee Member

Accepted for the College of Engineering

__________________________________
Vish Prasad, Dean

Accepted for the Graduate School

___________________________________
Abu S. M. Masud, Dean

iii
DEDICATION

To my father who passed away suddenly during this research

iv
I hear and I forget. I see and I remember. I do and I understand.

—Confucius (Chinese philosopher & reformer (551 BC– 479 BC))
v
ACKNOWLEDGMENTS
During this research and the writing of this dissertation, I have always felt great support
and motivation by many people. I would like to extend my sincere appreciation to my advisor,
Professor Emeritus M. E. Sawan. Without his comprehensive advice, it would not have been
possible to complete this research and to be confidently ready for the next challenges ahead of
me in my life. I would also like to extend my sincere appreciation to my committee chair,
Professor J. M. Watkins. His important suggestions and warm support have always been very
helpful and motivating.
I extend my sincere appreciation to Professors W. Jewell and R. Hixon for their warm
support and motivation. I also extend my sincere gratitude to Professor Gamal Weheba, who
helped me to build the main frame of this prototype in his laboratory. He has always given me
invaluable advice.
My sincere appreciation goes to my previous employer, Dr. Sophokles Anthimides, and
Trees For Life, Inc., where I did my internship during my master’s studies.
Especially, I thank my family. Their unlimited support and love have been the main
encouragement in my studies.
Thank you all very much.
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ABSTRACT

To date, electric motors have been rotary and linear with one degree of freedom (DOF)
motion. Advancements in technology have brought new, complex machines requiring multiple
degrees of freedom motion; however, this is currently being provided by connecting single-DOF
motors together, such as in a robotic ankle joint, where three servo motors are combined and 3-
DOF motion are obtained. However, the end-product is heavy, complex, and inefficient in many
ways. Several researchers have been studying spherical motors in order to obtain a 3-DOF
motion motor. The first attempt was in 1959 by Williams et al. [1]. Then other researchers began
investigating 3-DOF motion using a single actuator. Unfortunately, a spherical electric motor
(SEM) has not been commercialized yet because of some difficulties in realizing 3-DOF with a
single motor. Position resolution accuracy is very difficult to achieve in an SEM, and the bearing
system is another challenge. Transfer and magnetic bearings have been introduced as the
solution; however, under high torque, transfer bearings create high friction, and magnetic
bearings cannot handle the load. The actuation method remains the biggest challenge in
developing an SEM. In this research, the goal was to develop an SEM that could be applied to
industry relatively soon. First, a new actuation method was introduced by concentrating on
position accuracy. Similar methods have been used in liquid crystal display (LCD) and magnetic
suspension systems, whereby an array of coils on the stator surface are controlled to create a
moving stator pole coupled with a rotor pole that moves together, thus realizing 3-DOF. Second,
an electromagnetic torque model was derived for an SEM. Finally, a dynamic model was
developed, and a dynamic decoupling control system was designed. Results show that position
accuracy was achieved reasonably because the torque model is promising for calculating the
required currents based on torque values calculated by the computed torque model.
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TABLE OF CONTENTS
Chapter Page
1. INTRODUCTION ...............................................................................................................1

1.1 Motivation ................................................................................................................2
1.1.1 Spherical Electrical Motor ...........................................................................2
1.1.2 Applications .................................................................................................2
1.2 Objective ..................................................................................................................2
1.3 Contribution .............................................................................................................4
1.4 Structure of the Dissertation ....................................................................................4

2. TECHNICAL BACKGROUND ..........................................................................................5

2.1 Literature Survey .....................................................................................................5
2.1.1 SEM .............................................................................................................5
2.1.2 Control Systems for SEM ............................................................................7
2.2 Electric Motor Principles .......................................................................................10
2.2.1 Vectors and Fields......................................................................................10
2.2.2 Electromagnetism ......................................................................................10
2.2.3 Motion Production .....................................................................................11
2.2.4 Torque Production .....................................................................................12
2.2.5 Speed Production .......................................................................................12
2.3 Electric Motor Types .............................................................................................12
2.4 Conclusion .............................................................................................................12

3. WORKING PRINCIPLES AND STURCUTURE OF SEM.............................................13

3.1 Introduction ............................................................................................................13
3.2 Methodology ..........................................................................................................14
3.3 Structure of SEM ...................................................................................................16
3.4 Conclusion .............................................................................................................18

4. MAGNETIC FIELD AND TORQUE MODELING OF SEM .........................................19

4.1 Introduction ............................................................................................................19
4.2 Methodology ..........................................................................................................19
4.3 Magnetic Field and Exerted Force .........................................................................22
4.4 Conclusion .............................................................................................................29

viii
TABLE OF CONTENTS (continued)
Chapter Page
5. DYNAMIC MODELING OF SEM ...................................................................................30
5.1 Introduction ............................................................................................................30
5.2 Methodology ..........................................................................................................30
5.3 Dynamic Model .....................................................................................................32
5.4 Conclusion .............................................................................................................32

6. CLOSED-LOOP CONTROLLER FOR SEM ...................................................................33

6.1 Introduction ............................................................................................................33
6.2 Methodology ..........................................................................................................33
6.3 Design ....................................................................................................................34
6.4 Simulation ..............................................................................................................35
6.5 Conclusion .............................................................................................................42

7. CONCLUSION AND FUTURE WORK ..........................................................................43

REFERENCES ..............................................................................................................................45

APPENDIX ....................................................................................................................................51

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LIST OF FIGURES

Figure Page

1. General Principle of Electric Motors .................................................................................11
2. Bottom Part of Stator in Detail ..........................................................................................13
3. Low-Speed 2-DOF Joint SEM ...........................................................................................13
4. Low-Speed 3-DOF Joint SEM ...........................................................................................14
5. Interaction of Pole and Magnetic Field ..............................................................................14
6. Low-Speed 3-DOF Joint SEM with Rotor and Stator Poles..............................................15
7. High-Speed 3-DOF Wheel and Propeller SEM .................................................................17
8. Single Electromagnetic Coil and Rotor Pole .....................................................................19
9. Magnetization Curve (assumed to be straight) ..................................................................21
10. Bottom Part of Spherical Stator and Its South Pole Motion ..............................................24
11. Energized Stator Coils, Rotor Pole, and Neighbor Coil with Separation Angle ..............26
12. Expected Relationship f(δ) between Separation Angle and Torque ..................................27
13. Angular Trajectories with Zero Initial Conditions.............................................................36
14. Angular Velocities with Zero Initial Conditions ...............................................................36
15. Trajectory Tracking with Zero Initial Conditions ..............................................................37
16. Trajectory Tracking Error with Zero Initial Conditions ....................................................38
17. Input Torques with Zero Initial Conditions .......................................................................38
18. Angular Trajectories with Non-Zero Initial Conditions ....................................................40
19. Angular Velocities with Non-Zero Initial Conditions .......................................................40
20. Trajectory Tracking with Non-Zero Initial Conditions .....................................................41
21. Trajectory Tracking Error with Non-Zero Initial Conditions ............................................41
22. Input Torques with Non-Zero Initial Conditions ...............................................................42
x