1. NAVAL
POSTGRADUATE
SCHOOL
MONTEREY, CALIFORNIA
THESIS
Approved for public release; distribution is unlimited
DESIGN, INTEGRATION, AND TEST OF A MODULAR
SPACECRAFT-BASED ROBOTIC MANIPULATOR LINK
by
Daniel A. Alvarez
December 2014
Thesis Advisor: Marcello Romano
Co-Advisor: Markus Wilde
3. i
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DESIGN, INTEGRATION, AND TEST OF A MODULAR SPACECRAFT-
BASED ROBOTIC MANIPULATOR LINK
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6. AUTHOR(S) Daniel A. Alvarez
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Naval Postgraduate School
Monterey, CA 93943–5000
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13. ABSTRACT (maximum 200 words)
This thesis reports the design, integration, and testing of a modular non-fixed base robotic manipulator link for
equipping mobile vehicles (e.g., spacecraft, terrestrial, or sea vehicles). In particular, the developed manipulator link
will be used as a test bed for spacecraft-based robotic operations at the NPS Spacecraft Robotics Laboratory. The
design of the link is new and unique in that it is completely modular, allowing for reconfiguration of the manipulator
or the replacement of links during operations. The wireless link carries all necessary components to command a
servomotor and receive torque, velocity, and positional feedback data. In addition, common structural interfaces mean
that the link can attach and detach from the robotic base and other links without any changes to the electrical or
mechanical architecture of the system. The design and integration process developed in this thesis enable construction
of additional links to result in a full multi-link robotic manipulator.
The inertial parameters of the integrated prototype link were also experimentally measured. These inertial
parameters are necessary and sufficient to model a spacecraft-manipulator system of n links via computer simulation.
This thesis provides the foundation of such a simulation by modeling a one-link spacecraft-manipulator system, which
was corroborated with experimental data. Construction of future links provides a flexible means by which more
complex simulations can be experimentally validated.
14. SUBJECT TERMS
Manipulator, Link, Modular, Spacecraft, Non-Fixed, Free-Floating
15. NUMBER OF
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193
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5. iii
Approved for public release; distribution is unlimited
DESIGN, INTEGRATION, AND TEST OF A MODULAR SPACECRAFT-BASED
ROBOTIC MANIPULATOR LINK
Daniel A. Alvarez
B.S., California Institute of Technology, 2010
Submitted in partial fulfillment of the
requirements for the degree of
MASTER OF SCIENCE IN ASTRONAUTICAL ENGINEERING
from the
NAVAL POSTGRADUATE SCHOOL
December 2014
Author: Daniel A. Alvarez
Approved by: Marcello Romano
Thesis Advisor
Markus Wilde
Co-Advisor
Garth Hobson
Chair, Department of Mechanical & Aerospace Engineering
7. v
ABSTRACT
This thesis reports the design, integration, and testing of a modular non-fixed base robotic
manipulator link for equipping mobile vehicles (e.g., spacecraft, terrestrial, or sea
vehicles). In particular, the developed manipulator link will be used as a test bed for
spacecraft-based robotic operations at the NPS Spacecraft Robotics Laboratory. The
design of the link is new and unique in that it is completely modular, allowing for
reconfiguration of the manipulator or the replacement of links during operations. The
wireless link carries all necessary components to command a servomotor and receive
torque, velocity, and positional feedback data. In addition, common structural interfaces
mean that the link can attach and detach from the robotic base and other links without any
changes to the electrical or mechanical architecture of the system. The design and
integration process developed in this thesis enable construction of additional links to
result in a full multi-link robotic manipulator.
The inertial parameters of the integrated prototype link were also experimentally
measured. These inertial parameters are necessary and sufficient to model a spacecraft-
manipulator system of n links via computer simulation. This thesis provides the
foundation of such a simulation by modeling a one-link spacecraft-manipulator system,
which was corroborated with experimental data. Construction of future links provides a
flexible means by which more complex simulations can be experimentally validated.
9. vii
TABLE OF CONTENTS
I. INTRODUCTION........................................................................................................1
A. RESEARCH MOTIVATION .........................................................................1
B. STATE OF THE ART.....................................................................................2
1. First Non-fixed Base Manipulator: EFFORTS-I (1987) ..................3
2. Non-fixed Base Manipulators in the 1990s........................................4
3. Non-fixed Base Manipulators in the 2000s........................................6
4. Naval Postgraduate School Spacecraft Robotics Laboratory..........8
C. RESEARCH OBJECTIVES.........................................................................10
II. BACKGROUND THEORY......................................................................................13
A. MASS AND CENTER OF MASS IDENTIFICATION ALGORITHM ..13
B. MOMENT OF INERTIA IDENTIFICATION ALGORITHM................15
C. KINEMATICS OF A ROBOTIC MANIPULATOR .................................20
1. Kinematic Fundamentals ..................................................................21
2. Kinematics of a Fixed-Base Robotic Manipulator..........................24
3. Kinematics of a Free-Floating Base Manipulator...........................29
III. DESIGN ......................................................................................................................33
A. REQUIREMENTS.........................................................................................33
B. INITIAL DESIGN .........................................................................................34
1. Initial Design Components................................................................34
a. Servomotor ..............................................................................34
b. Servomotor Driver...................................................................36
c. Torque Sensor .........................................................................37
d. Analog to Digital Converter....................................................39
e. Battery Power Supply..............................................................41
f. Wireless Transceiver...............................................................42
g. DC/DC Converters ..................................................................43
h. Auxiliary Components ............................................................45
i. Summary of Components Chosen for Initial Design ............46
2. Initial Design Electrical Architecture ..............................................47
3. Initial Design Mechanical Architecture...........................................49
C. FINAL DESIGN.............................................................................................52
1. Final Design Components..................................................................52
a. Wireless Transceiver...............................................................53
b. Analog to Digital Converter....................................................55
c. Auxiliary Components ............................................................57
d. Summary of Components Used in Final Design ...................60
2. Final Design Electrical Architecture................................................61
a. Custom Circuit Board.............................................................64
3. Final Design Mechanical Architecture.............................................70
IV. INTEGRATION.........................................................................................................71
A. INITIAL INTEGRATION............................................................................71
10. viii
B. FINAL INTEGRATION ...............................................................................75
C. SOFTWARE...................................................................................................80
1. Sending and Receiving Servomotor Data ........................................81
a. Servomotor Driver Command Modes.....................................81
b. Servomotor Excitation Commands.........................................82
c. Servomotor Feedback Commands..........................................84
d. Illustrative Example in Current Mode...................................85
2. Receiving Torque Sensor Data .........................................................89
3. Unit Conversion for Manipulator Link Data..................................89
V. EXPERIMENTAL CAMPAIGN .............................................................................93
A. MASS AND CENTER OF MASS IDENTIFICATION .............................93
B. MOMENT OF INERTIA IDENTIFICATION...........................................96
1. Experiment Set-Up.............................................................................96
2. Experiment Description.....................................................................98
3. Results and Analysis ........................................................................100
C. COMPUTER SIMULATION.....................................................................109
D. FREE-FLOATING EXPERIMENTS........................................................117
1. Experiment Set-Up...........................................................................117
2. Experiment Description...................................................................120
3. Results and Analysis ........................................................................121
4. Model Comparison with Experimental Results ............................125
VI. CONCLUSION ........................................................................................................131
A. SUMMARY OF WORK..............................................................................131
B. FUTURE WORK.........................................................................................133
C. RESEARCH SIGNIFICANCE...................................................................138
APPENDIX A. MOMENT OF INERTIA DATA............................................................141
APPENDIX B. ARDUINO CODE.....................................................................................151
APPENDIX C. SERVOMOTOR MANUFACTURER CORRESPONDENCE...........165
APPENDIX D. ANALYTIC EXPRESSION OF BASE INERTIA AND DYNAMIC
COUPLING MATRICES FOR FREE-FLOATING SPACECRAFT-LINK
SYSTEM...................................................................................................................167
LIST OF REFERENCES....................................................................................................169
INITIAL DISTRIBUTION LIST.......................................................................................173
11. ix
LIST OF FIGURES
Figure 1. EFFORTS-I Free-Floating Robotic Manipulator (1987), from [18] .................3
Figure 2. University of Osaka’s Free-Floating Robotic Manipulator (1993), from
[19].....................................................................................................................5
Figure 3. ARFR Free-Flying Robotic Manipulator (1992), from [20]..............................5
Figure 4. Stanford Aerospace Robotics Lab Free-Flying Robots (1993), from [21]........6
Figure 5. MIT Experimental Free-Flying Robot (2009), from [22]..................................7
Figure 6. Underside of MIT Robot, from [22] ..................................................................7
Figure 7. Tsinghua University Humanoid Free-Flying Space Robot (2011), from
[23].....................................................................................................................8
Figure 8. Granite Flat-Floor in Naval Postgraduate School SRL......................................9
Figure 9. SRL Four-Link Robotic Manipulator (2012).....................................................9
Figure 10. Spacecraft-Manipulator Link System on SRL Flat-Floor................................11
Figure 11. Force Diagram of Manipulator Link................................................................14
Figure 12. Force and Vector Diagram of Manipulator Link i, From [28].........................16
Figure 13. Representation of a Point P in Different Coordinate Planes, from [30] ..........22
Figure 14. Chain of Coordinate Transformations, from [30]............................................23
Figure 15. Characterization of Link i in a Robotic Manipulator, after [30]......................25
Figure 16. Characterization of a Non-fixed Base Robotic Manipulator, from [29]..........30
Figure 17. Harmonic Drive FHA-8C-30-12S17b-E Servomotor......................................36
Figure 18. Harmonic Drive DEP-090-09 Servomotor Driver...........................................37
Figure 19. Futek TFF400 Torque Sensor..........................................................................39
Figure 20. Advantech ADAM-4012 ADC ........................................................................41
Figure 21. Inspired Energy NH2054 Li-Ion Battery.........................................................42
Figure 22. Quatech ABDG-SE-IN5420 ............................................................................43
Figure 23. Traco Power 75-2415WI V DC/DC Converter ...............................................44
Figure 24. Cosel ZUS32415 DC/DC Converter................................................................44
Figure 25. RS 485 to RS 232 Connector...........................................................................45
Figure 26. TTL to RS 232 Connector................................................................................46
Figure 27. SKF Explorer 608-2z Ball Bearings ................................................................46
Figure 28. Initial Design Electrical Power Architecture...................................................48
Figure 29. Initial Design Communications Architecture ..................................................49
Figure 30. CAD Model of Link with Dimension..............................................................50
Figure 31. CAD Model of Joint with Dimensions ............................................................50
Figure 32. CAD Model of Link and Joint Together..........................................................51
Figure 33. Arduino Due and XBee (Left) Replaced Quatech Transceiver (Right)...........55
Figure 34. Front and Back of Custom Circuit Board ........................................................56
Figure 35. Custom Circuit (Left), Along with Arduino Due, Replace ADAM ADC,
Cosel DC/DC Converter, and RS 485 to TTL Connector (Right)..................57
Figure 36. Arduino RS 232 Shield....................................................................................58
Figure 37. Arduino with RS 232 Shield (Left) Replaces TTL to RS 232 Connector
and Electrical Junctions (Right).......................................................................58
Figure 38. Custom Male RS 232 Connector to RJ12........................................................59
12. x
Figure 39. Custom RJ12 to Male RS 232 Connector (Left) Replaces need for Null
Modem and Gender Changer (Right) ..............................................................60
Figure 40. Arduino to Motor Driver Connection in its Final Configuration.....................60
Figure 41. Final Design Electrical Power Architecture ....................................................63
Figure 42. Final Design Communications Architecture Diagram.....................................64
Figure 43. Custom Circuit Board Diagram .......................................................................65
Figure 44. Voltage Divider Circuit....................................................................................67
Figure 45. Unity Gain Amplifier Circuit...........................................................................68
Figure 46. Voltage Gain Amplifier Circuit .......................................................................69
Figure 47. Ball Bearings for Manipulator Joint ................................................................72
Figure 48. Motor with Torque Sensor at Manipulator Joint..............................................72
Figure 49. Harmonic Drive Motor Controller Integrated..................................................73
Figure 50. ADAM-4012 ADC Integrated .........................................................................73
Figure 51. Battery Adapter, Arduino and DC/DC Converter Integrated ..........................74
Figure 52. Initial Integration with Wire Connections .......................................................74
Figure 53. Initial Integration of Manipulator Link with Components Illustrated .............75
Figure 54. 24 V TEP DC/DC Converter Rewired for Final Integration ...........................76
Figure 55. 24 V TEP DC/DC Converter Re-Integrated ....................................................76
Figure 56. Preparing XBee Shield, RS 232 Shield and Custom Circuit Board for
Integration........................................................................................................77
Figure 57. Connection and Integration of Custom Circuit Board.....................................77
Figure 58. Re-Integration of Arduino Board with Xbee and RS 232 Shields...................78
Figure 59. Final Integration of Manipulator Link with Components Illustrated...............79
Figure 60. Initial versus Final Integration Comparison 1 .................................................79
Figure 61. Initial versus Final Integration Comparison 2 .................................................80
Figure 62. Servomotor Command Mode Guide, from [33] ..............................................81
Figure 63. Code to Set Up Servomotor for Current Mode Operation...............................86
Figure 64. Code Sending Current Commands to Servomotor ..........................................87
Figure 65. Code to Receive Positional Feedback from Servomotor.................................88
Figure 66. Code to Receive Torque Sensor Feedback ......................................................89
Figure 67. Simulink Subsystem Performing Unit Conversions ........................................91
Figure 68. Weighing the Manipulator Link.......................................................................94
Figure 69. Force Diagram of Manipulator Link................................................................95
Figure 70. Manipulator Link Integrated with FSS ............................................................97
Figure 71. Sample Profile: 1000 mA Experiment Trial 1.................................................99
Figure 72. Moment of Inertia Experiment in Progress....................................................100
Figure 73. Raw Data for 1000 mA Experiment, Trial 1..................................................101
Figure 74. Moment of Inertia Data for 1000 mA Experiment, Trial 1............................103
Figure 75. Moment of Inertia Histograms for 1000 mA Experiments............................104
Figure 76. Cumulative Moment of Inertia Histograms per Excitation Current Level ...105
Figure 77. One-Way ANOVA Analysis of Moment of Inertia Values...........................106
Figure 78. Moment of Inertia Mean with Standard Error at 95% Confidence Level.....106
Figure 79. Final Moment of Inertia Histogram for 800 mA and 1000 mA
Experiments ...................................................................................................108
Figure 80. One-Link Spacecraft Manipulator System Referenced to Spacecraft Frame 110
13. xi
Figure 81. One-Link Spacecraft Manipulator Referenced to Inertial Frame ..................114
Figure 82. Simulink Model of One-Link Spacecraft-Manipulator System.....................117
Figure 83. Vicon Camera (Left) and Tracking Marker (Right) ......................................118
Figure 84. Vicon Tracking Software Snapshot ...............................................................119
Figure 85. Manipulator Link and FSS in Free-Floating Configuration ..........................120
Figure 86. Commanded Motor Angular Velocity Profile for Free-Floating
Experiments ...................................................................................................121
Figure 87. Free Floating Experiment Trial 1 Results......................................................122
Figure 88. Free Floating Experiment Trial 2 Results......................................................122
Figure 89. Free Floating Experiment Trial 3 Results......................................................123
Figure 90. Behavior of FSS Attitude in Free-Floating Experiment Trial 2 ....................124
Figure 91. Input Joint Velocity ( 1q ) to Model ................................................................126
Figure 92. Model versus Measured Data for Single Excitation ......................................127
Figure 93. Model versus Measured Data with Linear Correction for Single Excitation128
Figure 94. Motor Operation Illustrating the Relationship Between Current and
Torque............................................................................................................134
Figure 95. Future Work to Improve Mechanical Design ................................................135
Figure 96. Future Work to Improve Battery Slot............................................................136
Figure 97. Future Spacecraft-Based Manipulator Configurations Using Modular
Links ..............................................................................................................138
15. xiii
LIST OF TABLES
Table 1. Denavit-Hartenberg Parameters and Definitions.............................................24
Table 2. Robotic Manipulator Parameters and Definitions............................................25
Table 3. Servomotors Analyzed for Initial Design ........................................................35
Table 4. Motor Drivers Analyzed for Initial Design......................................................36
Table 5. Torque Sensor Analyzed for Initial Design .....................................................38
Table 6. Analog to Digital Converters Analyzed for Initial Design..............................40
Table 7. Summary of Components Chosen for Initial Design.......................................47
Table 8. Manipulator Link Overall Design Parameters.................................................51
Table 9. Wireless Transceivers Analyzed for Final Design...........................................54
Table 10. Analog to Digital Converters Analyzed for Final Design ...............................56
Table 11. Summary of Components Used in Final Design..............................................61
Table 12. Summary of Components Used in Custom Circuit Board...............................61
Table 13. Manipulator Link Final Design Parameters.....................................................70
Table 14. Servomotor Excitation Commands..................................................................83
Table 15. Examples of Set Commands Sent to Servomotor Driver.................................84
Table 16. Servomotor Driver Feedback Data Commands ...............................................84
Table 17. Examples of Set and Get Commands Sent to Servomotor Driver...................85
Table 18. Moment of Inertia Identification Experiments Architecture ...........................98
Table 19. Moment of Inertia Parameters per Experiment..............................................105
Table 20. Final Moment of Inertia Parameters ..............................................................108
Table 21. One-Link Spacecraft-Manipulator System Parameters .................................110
Table 22. Denavit-Hartenberg Parameters.....................................................................111
Table 23. Visual Comparison of Spacecraft-Link Position and Attitude for Actual
Experiment, Corrected Experimental Data, and Model Prediction ...............129
17. xv
LIST OF ACRONYMS AND ABBREVIATIONS
AC Alternating Current
ADC Analog to Digital Converter
ARFR Astronaut Reference Flying Robot
CCD Charge-Coupled Device
COM Center of Mass
DC Direct Current
DCE Data Communications Equipment
DH Denavit-Hartenberg
DTE Data Terminal Equipment
EFFORTS Experimental Free-FlOating RoboT Satellite
EVA Extra-Vehicular Activities
FSS Floating Spacecraft Simulator
GJM Generalized Jacobian Method
HD Harmonic Drive
IDE Integrated Development Environment
ISS International Space Station
MOI Moment of Inertia
NPS Naval Postgraduate School
PCB Printed Circuit Board
RPM Revolutions Per Minute
SRL Spacecraft Robotics Laboratory
VR Virtual Reality
19. xvii
ACKNOWLEDGMENTS
I would like to thank the following people for their invaluable assistance in the
completion of this thesis. This project would never have come to fruition had it not been
for each and every one of you.
My advisor, Dr. Marcello Romano—Thank you for your expertise and patience as
I toiled with the struggles of hardware.
My co-advisor, Dr. Markus Wilde—Thank you for your expertise and
mentorship. I have since forgiven your abrupt move to the hurricane state, but only
because of your unwavering guidance, every step of the way, even from three hours in
the future. Traveling through hyperspace ain’t like dusting crops, but we’ll get there
eventually.
CDR Zach Staples and Dr. James Calusdian—Thank you for teaching me
everything I know about the black art of electronics. You allowed me to walk in with a
bag of parts, take over half of your lab, and leave with a thesis project. Words cannot
express my gratitude.
My wife-to-be Brittany—Thank you for allowing me to spend a year in Monterey
playing with robots as you planned our entire wedding. Thank you for supporting my
endeavors and always being at my side, no matter the circumstance. You are the love of
my life.
My parents—Thank you for being the greatest role models any son could ask for.
Thank you for teaching me about the value of education and making me the man I am
today.
21. 1
I. INTRODUCTION
A. RESEARCH MOTIVATION
Robotic manipulators have played a pivotal role in the industrialization of modern
society. Of all households in the United States today, 95% own a car, 97% own a
television, and 91% own a cellphone. In 1950, these percentages were 60%, 9%, and 0%,
respectively, [1], [2], [3]. This tremendous improvement, in not only availability but also
quality, is due to the development of mass production, which has been largely enhanced
by the use of robotic manipulators in the manufacturing process. As technology continues
to move forward, robotic manipulators remain at the cutting edge, enabling advancements
in robotic surgery [4], IED disposal [5], and high-precision machining [6].
The robotic manipulators used in the roles listed above share the commonality of
utilizing a ground-stabilized, fixed base. In this configuration, the motion of the
manipulator arm leaves the manipulator base essentially unperturbed. As commercial,
military, scientific, and educational uses of space systems is advancing, the need to utilize
the advantages of robotic manipulators in orbit is growing. The best examples of robotic
manipulators in space are the Shuttle/Space Station Remote Manipulator Systems,
commonly referred to as the “Canadarm” manipulators. The first Canadarm was equipped
on the space shuttle, and used to successfully deploy, maneuver, and capture payloads,
[7], [8]. The second Canadarm is operated on the International Space Station (ISS), where
it enables vital operations like berthing of resupply vehicles and positioning astronauts
and equipment for Extra-Vehicular Activities (EVA). In both instances, the heavy mass
of the space shuttle (~90 tons) or ISS (~416 tons) [9] compared to the Canadarm
manipulators (~2 tons) [10] make the dynamics of the spacecraft-manipulator system
closely mimic a fixed-base manipulator. With the retiring of the space shuttle and a desire
to place robotic manipulators on satellites as small as a CubeSat [11], future manipulators
in space will represent a significant percentage of the total spacecraft mass. In this case,
non-fixed base dynamics, where the motion of the robotic manipulator arm will perturb
the manipulator base, become paramount.
22. 2
B. STATE OF THE ART
Due to the tremendous impact that robotic manipulators have had on the
development of industry on Earth, it is natural to expect that successful
commercialization of space would require similar capability. Thus, non-fixed base
robotic manipulators have been studied as far back as the 1980s, and, particularly
recently, continue to be an active field of scientific research. As mentioned previously,
however, the demonstration of motion and control for non-fixed base robotics is difficult
because three-dimensional microgravity is difficult to achieve in a laboratory
environment. The simulation of the dynamics of space systems is limited to the following
approaches, that are either limited in the dimensionality, quality, or duration.
Experiments on parabolic flights, in which an aircraft generates reduced
gravity conditions at the peak of a parabolic trajectory. While this type of
experiment does imitate the free-falling conditions of a body in orbit, the
duration is limited to about 20 seconds and opportunities are limited,
typically making it impractical for laboratory experimentation [12].
Submerge robots in water and use buoyancy to counteract gravity. This is
commonly used for astronaut training, but the viscosity of the fluid alters
the dynamics of a robotic manipulator [13].
Test a fixed-based robotic manipulator on a moving platform, which is
excited according to mathematical simulations that attempt to mimic non-
fixed base conditions. With a good computational model, a variety of
manipulation experiments can be performed in a controlled and repeatable
environment. However, the quality of the simulated microgravity
environment is restricted by the fidelity of the computational model, and
there are additional constraints like servo-control bandwidth or data
processing time delays [14].
Suspend robotic manipulators with tethers, such that the free-floating
environment is replicated in two dimensions. This results in a controlled
and repeatable test bed to test non-fixed base dynamics in two dimensions.
The challenge, however, is to employ proper counter-balancing such that
the tension in the tether remains constant and does not affect the dynamics
[15].
Levitate robotic manipulators on a precision-flat surface with compressed-
air bearings so that the robots are operating in a two-dimensional non-
fixed base configuration [16].
23. 3
The latter experimental environment provides a robust test bed for non-fixed base
robotic manipulators in two dimensions due to its availability, flexibility, and relatively
low cost. This thesis focuses on a robotic manipulator link that is intended to be tested
with air bearings on a flat-floor, and this literature review will constrain its scope to just
that environment.
1. First Non-fixed Base Manipulator: EFFORTS-I (1987)
The earliest example of a self-contained robotic manipulator tested on air bearings
found in this literature review is the Experimental Free-FlOating RoboT Satellite-I
(EFFORTS-I) robot by Kazuya Yoshida at the Tokyo Institute of Technology. Having
developed the Generalized Jacobian Method (GJM) analytically around the same time
[17], Yoshida built EFFORTS-I as an experimental validation of his analysis [17].
EFFORTS-I featured a two-link design, each 0.25m in length. Connected to each link
was a servomotor at the joints that allowed for positional feedback. In addition,
EFFORTS-I had a wireless transceiver on the robotic base as well as an on-board gas
tank. Unfortunately, however, the compact size of EFFORTS-I did not allow for a large
air tank, and testing duration was limited to 1–2 min [18]. In spite of this, EFFORTS-I
was able to successfully perform capturing maneuvers with a non-moving satellite object,
as shown in Figure 1 [18].
Figure 1. EFFORTS-I Free-Floating Robotic Manipulator (1987), from [18]
24. 4
2. Non-fixed Base Manipulators in the 1990s
Research and development of non-fixed base robotic manipulators surged
substantially in the early 1990s, very likely fueled by the construction of the ISS. Japan
continued to lead the field with the University of Osaka’s single-arm free-floating
manipulator [19] and the Ministry of International Trade and Industry’s (MITI, at the
time, Figure 2) dual-arm Astronaut Reference Flying Robot ( [20] ARFR, Figure 3). The
University of Osaka free-floating manipulator was primarily intended to be an
improvement over the EFFORT robots developed by Yoshida [19]. Its single arm could
be configured with either two or three links, each about 0.3 m long and weighing about 1
kg [19]. The joints contained a servomotor and encoder utilizing proportional-derivative
(PD) feedback control. Furthermore, the end-effector was used to capture a free-floating
target through the use of a visual-sensing charge-coupled device (CCD) camera [19]. The
ARFR was considerably larger, weighing 150 kg and using a 180 Wh battery [20]. It
featured a new dual arm design, with on-board computer, sensor-processing unit, and a
torque sensor on its end-effector “gripper.” It also featured on-board thrusters, providing
coarse position (±4 cm) and attitude (±4°) control [20].
Stanford University also developed a series of non-fixed base robotic
manipulators in its Aerospace Robotics Laboratory (Figure 4) in the early 1990s. These
robotic vehicles featured dual-arm manipulators with pneumatic end-effector grippers,
proximity sensors, on-board computing, and thrusters and momentum wheels for
position/attitude control [21]. The robots performed a variety of tasks, from docking and
capture, to more complex maneuvers that involved several robots working in teams [21].
25. 5
Figure 2. University of Osaka’s Free-Floating Robotic Manipulator (1993), from [19]
Figure 3. ARFR Free-Flying Robotic Manipulator (1992), from [20]
26. 6
Figure 4. Stanford Aerospace Robotics Lab Free-Flying Robots (1993), from [21]
3. Non-fixed Base Manipulators in the 2000s
The study of non-fixed base robotics has proliferated throughout the world in the
2000s. Interest has peaked recently with the desire to place robotic manipulators on
satellites as small as a CubeSat. CubeSats and other small satellites (SmallSats) are being
used to perform imagery and communications missions, and may one day replace or at
least abate the need for EVA missions [11].
Two examples of recent non-fixed base manipulators are MIT’s experimental
free-flying robot (Figure 5) and Tsinghua University’s humanoid free-flying space robot
(Figure 7). The technology used in these free-flying robot demonstrators has improved
substantially over time. MIT’s robot features eight thrusters, wireless communications,
on-board computing, four on-board NiMH batteries, and dual arm manipulators with
servomotors and torque sensors at the base [22]. In addition, position and motion are
measured through the use of accelerometers, position sensors (using optical mouse
sensors, see Figure 6), as well as a SICK laser range system and overhead cameras in the
laboratory [22]. Tsinghua University’s humanoid free-flying space robot features
thrusters, wireless communications, on-board computing, rechargeable Li-ion batteries,
dual-arm manipulators, and a CCD camera at the “head” of the robot which enables
object tracking based on an optical virtual reality system [23].
27. 7
Figure 5. MIT Experimental Free-Flying Robot (2009), from [22]
Figure 6. Underside of MIT Robot, from [22]
28. 8
Figure 7. Tsinghua University Humanoid Free-Flying Space Robot (2011), from [23]
4. Naval Postgraduate School Spacecraft Robotics Laboratory
The Spacecraft Robotics Laboratory (SRL) at the Naval Postgraduate School
(NPS) has been researching spacecraft robotics for over a decade. In 2012, a large granite
flat-floor 16 m2
in area was installed (See Figure 8), and has since been used to test a
variety of dynamic models and GNC algorithms from free-flying robotic vehicles, called
Floating Spacecraft Simulators (FSS). These vehicles have been used to perform
numerous tasks, from optimal control to docking maneuvers to object avoidance [24],
[25], [26]. Recently, the SRL has researched placing a manipulator arm on its spacecraft
simulators. The first iteration of a free-floating robotic manipulator was the four-link
manipulator shown in Figure 9, which used four servomotors, wireless communications,
and on-board computing. Position and attitude data were measured through an overhead
Vicon metrology and tracking system [27]. This research represents a major step forward
on the second iteration development of a non-fixed spacecraft-based robotic manipulator
at the SRL.
29. 9
Figure 8. Granite Flat-Floor in Naval Postgraduate School SRL
Figure 9. SRL Four-Link Robotic Manipulator (2012)
30. 10
C. RESEARCH OBJECTIVES
Having investigated numerous non-fixed base robotic manipulators in the
literature review, the objective of this research was to design, integrate, and test a new
design. Rather than building an entire manipulator arm, this research focused on a single
link, featuring, for the first time to the best knowledge of the author, a completely
modular design. The link contains an on-board servomotor, encoder, torque sensor,
power supply, and microcontroller with wireless communications. Thus, the design can
eventually be scaled into a manipulator arm of essentially arbitrary length, each link
having motion and torque feedback. The flexibility of the design allows the
reconfiguration of the spacecraft-manipulator system from a single-link arm to a multi-
link arm, without the need for modifications of the structure, by using standardized
docking interfaces. A review of current literature shows that such a design has not been
implemented thus far, and will enable original research in the field of non-fixed
spacecraft-based robotic manipulators.
As mentioned, this project represents a major step forward on the second
generation of a spacecraft-based robotic manipulator at the SRL. The first generation,
shown in Figure 9, featured a four-link design mounted to a free-floating base with
positional feedback at each joint. Aside from featuring a modular design, the second
generation manipulator has a joint interface that is compatible with the latest FSS
vehicles. This allows for a second, third, or even fourth manipulator arm to be mounted to
the same spacecraft base since each individual link can be removed or replaced with
minimal effort. A depiction of the single link developed in this thesis attached to a FSS is
shown in Figure 10.
This thesis has used the terms “free flying” and “free floating” when describing
non-fixed base manipulators, and follows the convention used at the SRL. A “free flying”
spacecraft refers to a configuration in which the base, while not fixed, has active
translation and attitude control capability. In the case of the SRL FSS, air bearings allow
for the spacecraft simulators to levitate on the granite flat-floor, thus greatly reducing the
effect of friction, and compressed air micro-thrusters enable translation and attitude
control. If the thrusters are disabled and only the air bearings are used, the spacecraft
31. 11
simulators are in a “free-floating” configuration, whereby linear and angular momentum
are uncontrolled, and thus constant in a frictionless environment with no external actions.
Once the prototype manipulator link was built, an additional objective of this
thesis was to attach it to one of the FSS and test it. Aside from demonstrating a
functioning design, it was desired to derive the manipulator link’s inertial parameters
from the motion of the system. The inertial parameters of interest in this case are the
mass of the manipulator link, location of the center of mass, and moment of inertia about
the vertical axis of the center of mass (COM). These inertial parameters are sufficient
(and necessary) to simulate the dynamics and kinematics of the system. Due to the
modular design, the inertial parameters should stay essentially unchanged for all future
links, which allows for the system to be modeled for an arbitrary number of manipulator
links. A final objective of this thesis was to model a one-link spacecraft-manipulator
system and corroborate kinematic data with the actual prototype manipulator.
Figure 10. Spacecraft-Manipulator Link System on SRL Flat-Floor
33. 13
II. BACKGROUND THEORY
This chapter will cover the background theory of the experimental portion of this
thesis. The goal of the experiments are to determine the inertial parameters (mass, center
of mass, and moment of inertia) of the manipulator link, and then model the spacecraft-
link system to verify the accuracy of the inertial parameters. Deriving the mass and
location of the center of mass (COM) can be accomplished by performing a
force/moment balance with two scales. The identification of the moment of inertia
follows the inertial parameter identification procedure illustrated in “Handbook of
Robotics,” [28]. Finally, the theory behind the kinematics of a robotic manipulator
system is covered in sufficient detail to provide validation for a one-link manipulator
system.
A. MASS AND CENTER OF MASS IDENTIFICATION ALGORITHM
The mass of the manipulator link and the location of the COM can be derived by
performing a force and moment balance measurement about the COM. This process is
elucidated by referencing the force diagram shown in Figure 11. In practice, this is
accomplished by weighing the manipulator at two different locations using lab scales.
The variable x1 represents the distance to the center of mass from location 1, and x2 + x3
represents the distance from location two. Eventually, the parameter of interest for
modeling purposes is the distance from the link joint to the center of mass, which is x2.
The force mCOMg represents the effective gravitational force of the link at the center of
mass, and thus mCOM is the mass of the entire link.
34. 14
Figure 11. Force Diagram of Manipulator Link
The total mass of the manipulator link (mCOM) can be derived by performing a
force balance, as shown in Equations 2.1 and 2.2.
1 2 0COMF m g m g m g (2.1)
1 2COMm m m (2.2)
Thus, the mass of the total system is obtained by determining m1 and m2, which
can be measured using a pair of lab scales.
The variables x1 and x2 give the location of the COM, but are initially unknown.
The distance between the scales that weigh m1 and m2 (x1 + x2 + x3), however, can be
measured. Since mCOM was already determined above, we can derive x1 and x2 by
balancing the moments of the system about the COM. The procedure is detailed in
Equations 2.3 and 2.4.
2 3 2 1 1( ) 0COMM x x m g x m g (2.3)
Adding x1m1g + x1m2g to both sides of the equation we get:
2 3 2 1 2 1 2 1 1( )x x m g x m g x m g x m g . (2.4)
35. 15
Factoring like-terms yields:
2 3 21 1 1 2( )x m g xx m mx g . (2.5)
And since m1 + m2 = mCOM is known, we can solve for x1:
31
1
1 2
2 2x xx m
x
m m
. (2.6)
It is noted that this procedure derives the center of mass in only one dimension.
The z-coordinate of the COM is of no consequence since the link is intended to operate in
a two-dimensional environment where all rotations will be about the z-axis. The y-
coordinate of the COM is ignored in this analysis due to the manipulator link’s thin
geometry, whereby any y-axis offset in the COM will have negligible effect.
B. MOMENT OF INERTIA IDENTIFICATION ALGORITHM
The moment of inertia of the manipulator link is derived based on the Inertial
Parameter Identification section of “Handbook of Robotics” [28]. In the planar case, only
the moment of inertia about the vertical axis is of interest. Therefore, the moment of
inertia can be treated as a scalar quantity. The scalar moment of inertia is defined as the
ratio between torque and angular acceleration about a rotation axis, or I , where I
is the moment of inertia, is the torque exerted, and is the angular acceleration.
Because robotic manipulators are typically comprised of multiple links with multiple
joints, however, this section provides the inertial parameter derivation for a general n-link
manipulator in three dimensions. For the test configuration used in this thesis, where a
single link is attached to a spacecraft base and excited about a single joint in planar
motion, the moment of inertia reduces to the simple equation shown above. The entire
derivation is provided for future reference to re-calculate the inertial parameters of an n-
link manipulator arm.
A force and vector diagram for arbitrary link i in an n-link manipulator is shown
in Figure 12. The [3 × 1] vectors doi and ri represent the position of joint i and the COM
of link i referenced in inertial coordinate system 0, O0. The vector cn is then defined as
36. 16
the position of the COM referenced to joint i such that 0n i i c r d , and fi is a [6 × 1]
matrix representing the forces (Fi) and torques ( iτ ) exerted on joint i.
Figure 12. Force and Vector Diagram of Manipulator Link i, From [28]
Furthermore, the [6 × 1] spatial velocity of joint i, iυ , is introduced, collecting the
angular velocity vector iω and the linear velocity vector Vi.
i
i
i
ω
υ
V
(2.7)
The [6 × 6] spatial inertia matrix Ii collects all mass and inertia terms of link i,
referenced about the joint position.
T
i i n
i
i n i
m
m m
I c
I
c E
(2.8)
The matrix iI is the inertia tensor about joint i, E is the identity matrix, and
a is
the skew-symmetric matrix of vector a, defined as follows:
xx xy xz
i yx yy yz
zx zy zz
I I I
I I I
I I I
I (2.9)
37. 17
1 0 0
0 1 0
0 0 1
E (2.10)
0
0
0
z y
z x
y x
a a
a a
a a
a . (2.11)
To compute the actions (forces and torques) exerted on joint i, the time derivative
of the spatial momentum is taken, shown in Equation 2.12:
i
i i i i i i i i
i
d
dt
τ
f I υ I υ υ I υ
F
. (2.12)
Using the above definitions, and noting that i oi i i v d ω v , the following holds.
T T
i i i n oi i iiii i n
i i
ii n i i n i i oi i i
mm
m m m m
I ω c d ω vωI c
I υ
vc E c ω d ω v
(2.13)
T T
ii i i i n i i i i n i i
i i i
ii i n i i i n i i i i
m m
m m m m
ωω v I c ω I ω c ω v
υ I υ
v0 ω c E ω c ω ω v
(2.14)
Substituting Equations 2.13 and 2.14 back into Equation 2.12 and simplifying
yields:
i i i i i oi i n
i
i oi i i n i i i n
m
m m m
I ω ω I ω d c
f
d ω c ω ω c
. (2.15)
We then make the following assignment for in order to solve for the inertial
parameters.
38. 18
0 0 0
0 0 0 ( ) ( )
0 0 0
xx
xy
x y z
xz
i i x y z i i
yy
x y z
yz
zz
I
I
I
L l
I
I
I
I ω ω I (2.16)
Substituting Equation 2.16 into Equation 2.15 yields:
( ) ( )
( )
i
oi i i i
i i n
oi i i i
i
m
L L
m
l
0 d ω ω ω
f c
d ω ω ω 0
I
. (2.17)
The vector on the right then lists all the inertial parameters, which can be solved
by taking the pseudo-inverse of the matrix on the left, as shown in Equations 2.18 and
2.19.
( ) ( )
( )
i
oi i i i
i i n i i
oi i i i
i
m
L L
m
l
0 d ω ω ω
f c A
d ω ω ω 0
I
(2.18)
1T T
i i i i i
A A A f (2.19)
The procedure described above provides a general method to derive the inertial
parameters of an n-link manipulator in three dimensions. Significant simplifications can
be made for the scenario applicable to this thesis. In order to experimentally derive the
moment of inertia of the manipulator link, the one-link manipulator system will be put in
a configuration where the base is held fixed and the motor at the joint is excited by a
constant torque. The resulting acceleration will then be measured by taking the time
derivative of the angular velocity, which is fed back from the motor’s encoder.
Because there is only one link and the spacecraft base will be held fixed, the
distance (d01) between inertial coordinate frame O0 and the joint frame O1 will be
constant, and any time derivative of d0i will be zero. Furthermore, the one-link
manipulator system will operate in only two dimensions and utilize a motor that excites
39. 19
only with a torque about the z-axis at the joint. No other actions will be present at the
joint. This will cause the angular velocity at the joint, w1, to only have a z component and
the only moment of inertia parameter of interest is thus Izz. These simplifications are
reiterated in Equations 2.20–2.22.
1
0
0
τ
0
0
0
z
f (2.20)
01
0
0
0
d 0 (2.21)
1
0
0
z
ω (2.22)
Plugging in these simplifications back into Equation 2.18 yields Equation 2.23.
1
1
2
1
2
1
1 2
2
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
τ 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0
x
yz z
zz z
xxz z
xyz z
xzz z
yy
yz
zz
m
m c
m c
m c
I
I
I
I
I
I
f
(2.23)
And now the simplifications provide the following relation for the moment of
inertia of interest, Izz.
40. 20
z zz zI (2.24)
Rearranging to solve for the moment of inertia yields the expected result, which is
shown in Equation 2.25.
I (2.25)
The z subscript has been dropped for legibility. Any mention of torque, angular
velocity, and moment of inertia in the experimental campaign section is assumed to be
about a z-axis. The parallel axis theorem can now be used to determine the moment of
inertia about the center of mass, shown in Equation 2.26.
2
2COM COMI I m x (2.26)
The variable mCOM is the mass of the manipulator link and x2 is the distance from
the joint to the COM, as shown in Figure 11. Once the mass of the manipulator (mCOM),
center of mass location (x2), and moment of inertia about the center of mass (ICOM) are
known, a system with an arbitrary number of links can be modelled via computer
simulation.
C. KINEMATICS OF A ROBOTIC MANIPULATOR
As mentioned, an additional objective of this thesis is to model a free-floating
one-link manipulator to corroborate kinematic data once the integration of the
manipulator link was complete. In order to model a robotic manipulator system, it is
required to know the inertial parameters, for which the procedure has been elucidated in
the previous sections. Corroborating data from a computer simulation and actual
experimentation provides an effective means to test the fidelity of the computer model,
laboratory environment, and inertial parameter estimation.
This section explains the theory behind the kinematics of a robotic manipulator
that is required in order to model a free-floating system. Starting with an overview of
kinematic fundamentals for a fixed-base robotic manipulator, the concept of the Jacobian
is introduced, which relates the motion of each individual manipulator joint to the
movement of the end-effector through a matrix called the Jacobian. This is typically
41. 21
referred to as mapping the joint space to the operational space of a robotic manipulator.
Taking the inverse of the Jacobian allows for movement of each manipulator joint to be
controlled by desired movement of the end-effector.
When the base of the manipulator is not fixed, the complexity of the system
increases substantially since the movement of the base must be predicted, and referenced
to a separate inertial coordinate system. This thesis references the commonly-used
Generalized Jacobian Method (GJM), which was developed at the Tokyo Institute of
Technology by Profs. Kazuya Yoshida and Yoji Umetani in the 1980s [17], and
expounded upon by Prof. Wilde (Florida Institute of Technology, former NRC post-
doctoral associate at NPS) and Prof. Romano (NPS) [29]. The major advantage of the
Generalized Jacobian method is that it provides a closed-form solution to the dynamics of
a robotic manipulator system, without being based on heavy numerical computation. This
chapter does not delve into the dynamics and Lagrangian Mechanics upon with the GJM
is derived, but instead goes insofar as to show all components necessary to model the
kinematics of a free-floating base manipulator system.
1. Kinematic Fundamentals
The study of kinematics is often described as the geometry of motion. In robotics,
the kinematics of a manipulator system describe its movement based on the configuration
of joints and links in the manipulator. Fundamentally, this is achieved through a series of
coordinate transformations from the base of the manipulator to its end-effector. These
transformations consist of translations and rotations of Cartesian coordinate systems
collocated with each joint of the manipulator. Thus, the movement of each link of a
robotic manipulator can be expressed in a local frame, which in turn is transformed into
the position and orientation of a common inertial frame.
42. 22
Figure 13. Representation of a Point P in Different Coordinate Planes, from [30]
Figure 13 and Equations 2.27–2.28 display the most fundamental representation
of a vector in different coordinate systems. In this case, both p0
and p1
are the position
vectors of the same point P, but are referenced to different coordinate systems in space.
Equation 2.27 relates these vectors through the use of the offset vector and rotation
matrix .
0 0 0 1
1 1 p o R p (2.27)
By using the homogenous representation of a vector (e.g.
1
), Equation
2.27 can be represented using a single matrix,
0 0
0 1 1
1
1T
R o
A
0
, as is done in Equation
2.28.
0 0 0 1
0 0 11 1
1
1 1 1T
p R o p
p A p
0
(2.28)
Matrix 0
1A is called a homogenous transformation matrix and the operation
described in Equation 2.28 is called a homogenous transformation. The usefulness of
homogenous transformations is elucidated in Equations 2.29–2.30, where it can be seen
43. 23
that a simple inversion of the homogenous transformation matrix 0
1A allows the reverse
transformation.
11 0 0 1 0
1 0
p A p A p (2.29)
1 1 1 1 00 0 0
1 0 0 11 1 1
0
11
TT
R R oR R o
A
00
(2.30)
Extrapolating the homogenous transformation methodology across a series of
coordinate systems allows any point to ultimately be represented in the base frame.
Figure 14 and Equations 2.31 and 2.32 display how a series of n coordinate systems can
be transformed back to the base coordinate system. In particular, cumulative homogenous
transformation matrix 0
nT represents the product of each individual homogenous
transformation matrix and directly relates the final coordinate system n to the initial
coordinate system 0.
Figure 14. Chain of Coordinate Transformations, from [30]
0 0 1 1 1 0
1 2
n n
n n
p A A A p T p (2.31)
44. 24
0 0
0 0 1 1
1 2
1
n n n
n n
R o
T A A A
0
(2.32)
In practice, the kinematics of a robotic manipulator are determined by locating
coordinate systems at each manipulator joint. Despite Equation 2.32 providing a robust
method to transform from any coordinate frame to the base frame, there is still a certain
ambiguity in determining 1i
i
A , based on how the rotation matrix 1i
i
R is defined (e.g.,
roll, pitch, yaw vs ZYZ rotation). Thus, a standard convention has been adopted in
robotics called the Denavit-Hartenberg (DH) Convention. In the DH Convention, the
homogenous transformation matrix 1i
i
A (from link coordinate system i-1 to link
coordinate system i) is comprised of the four DH Parameters explained in Table 1.
Table 1. Denavit-Hartenberg Parameters and Definitions
DH Parameter Definition
di Translation from frame i-1 to frame i along axis zi-1
ϑi Rotation from xi-1 to xi about zi-1
αi Rotation from zi-1 to zi about xi
ai Translation along axis xi
Using the DH Parameters, the homogenous transformation matrix 1i
i
A is given
by Equation 2.33.
1
cos sin cos sin sin cos
sin cos cos cos sin sin
0 sin cos
0 0 0 1
i i i i i
i i i i i ii
i
i i i
a
a
d
A (2.33)
2. Kinematics of a Fixed-Base Robotic Manipulator
In order to ultimately relate the motion of a manipulator’s end-effector with its
base, the position vectors of the manipulator links (Equation 2.27) are differentiated once
with respect to time.
45. 25
0 0 0 1 0 1
1 1 1 p o R p R p (2.34)
The time derivative of the rotation matrix is given by 0 0 0
1 1 1 R ω R , yielding:
0 0 0 1 0 0 1
1 1 1 1 p o R p ω R p . (2.35)
In order to generalize for any particular joint in a robotic manipulator the
convention of Figure 15 is adapted for the composition of the position vector. All
variables are explicitly defined in Table 2 and Equations 2.36–2.37 re-derive Equation
2.35 using this convention.
Figure 15. Characterization of Link i in a Robotic Manipulator, after [30]
Table 2. Robotic Manipulator Parameters and Definitions
Robotic Manipulator Parameter Definition
0
ip
Vector defining the location of coordinate frame i,
referenced to base coordinate frame 0
1
1,
i
i i
r
Vector from coordinate frame i-1 to i, referenced to
coordinate frame i-1
46. 26
Robotic Manipulator Parameter Definition
xi Unit X vector of coordinate frame i
yi Unit Y vector of coordinate frame i
zi Unit Z vector of coordinate frame i
Joint i
Joint i of the manipulator, collocated with
coordinate frame i-1
Link i
Link i of the manipulator, which connects joint i to
joint i+1 along 1
1,
i
i ir
0
iR
Rotation matrix from coordinate frame i to base
coordinate frame 0
0
1,i iω
Angular velocity of coordinate frame i with respect
to coordinate frame i-1, referenced to base
coordinate frame 0
Motion of frame i from base frame:
1
0 0 0 1
1 1,i i
i
i i i
p p R r . (2.36)
This changes Equation 2.35 to:
1
0 0 0 1 0 0 1
1 1. 1 1 1,i i
i i
i i i i i i i
p p R r ω R r . (2.37)
With all positions and velocities referenced in the base frame, Equation 2.37
simplifies to:
1
0 0 0 0 0
1. 1 1,i i i i i i i p p r ω r . (2.38)
The “0” superscript, indicating the expression of a vector or matrix is referenced
to the base coordinate frame 0, can then be assumed and omitted for legibility:
1 1. 1 1,i i i i i i i p p r ω r . (2.39)
Having adopted this convention, Equations 2.40 and 2.41 describe the relative
linear and angular velocity of coordinate frame i when coordinate frame i-1 is rotating
47. 27
about the axis we have defined as zi-1. Equations 2.42–2.43 utilize Equation 2.39 to give
us general motion equations of coordinate frame i in the base coordinate frame. Note that
r is constant since the links cannot change in length.
1. 1, 1,i i i i i i r ω r (2.40)
1, 1i i i i ω z (2.41)
Motion of frame i, from base frame, when frame i-1 is rotating about zi-1:
1 11, 1, 1 1, 1, 1 1,( )i i ii i i i i i i i i i i i p p ω r ω r p ω ω r (2.42)
1
1 1 1, 1 1,
i
i i i i i i i i
ω ω R ω ω ω (2.43)
1 1,i i i i i p p ω r (2.44)
1 1i i i i ω ω z . (2.45)
Equations 2.44 and 2.45 allow for the motion of coordinate frame i to be fully
defined in reference to the base coordinate frame, and only dependent on the motion of
coordinate frame i-1. Thus, the motion of a manipulator’s end-effector can be fully
determined by the final coordinated frame, which in turn is characterized by the sum of
the motion of all previous coordinate frames relative to the base frame. This allows us to
relate the angular velocity of each individual joint, iq (in this derivation, iq is equal to the
time derivative of DH Parameter i ), with the motion of the end effector, E
E
E
p
v
ω
. As
mentioned, the matrix that relates these parameters is called the Jacobian, J. In robotics
terms, the this is referred to mapping the joint space of the manipulator into the
operational space, Ev = Jq .
The Jacobian component that relates linear velocity, JT, is derived in Equations
2.46–2.49. The Jacobian component that describes angular velocity, JR, is derived in
Equations 2.50–2.53. Collectively, these two components make up the full Jacobian
matrix, shown in Equation 2.54, which can map the joint space into the operation space
and vice-versa, shown in Equations 2.55 and 2.56.
48. 28
Linear velocity component of Jacobian (JT) relates linear velocity of end-effector,
Ep , with the angular velocity of each individual joint, iq :
1
1
i
n
E n T i
i
q
p p j (2.46)
1, -1 1 1( )iT i i i i ,E i i E iq j ω r z p p . (2.47)
Since i iq , this simplifies to:
1 1( )iT i E i j z p p (2.48)
1 2 1n nT T T T T
J j j j j . (2.49)
Angular velocity component of Jacobian (JR) relates angular velocity of end-
effector, Eω , with the angular velocity of each individual joint, iq :
1
i
n
E n R i
i
q
ω ω j (2.50)
1iR i i iq j z . (2.51)
Since i iq , this simplifies to:
1iR ij z (2.52)
1 2 1n nR R R R R
J j j j j . (2.53)
Full Jacobian Matrix:
T
R
J
J
J
. (2.54)
And thus we have derived the expression of how the Jacobian matrix, J, maps the
joint space to the operational space (Equation 2.55) and vice-versa (Equation 2.56):
49. 29
E
E
E
p
v Jq
ω
(2.55)
1
2
1
1
E
n
n
q
q
q
q
q J v
(2.56)
3. Kinematics of a Free-Floating Base Manipulator
The Jacobian matrix developed above enables the mapping of joint space motion
onto operational space motion for a fixed base condition. For a free-floating base
manipulator, however, the manipulator base coordinate system is not an inertial reference
frame. Therefore, the kinematics of a spacecraft-manipulator system must be referenced
to a separate inertial coordinate system. Figure 16 displays an example robotic
manipulator. The base spacecraft body is still represented by coordinate system 0. In
contrast to a fixed-base manipulator system, however, the base coordinate system is
moving with respect to inertial coordinate frame J. The position vector of the center of
mass of the spacecraft-manipulator system, CMtot, is given as J
Cr . A free-floating
manipulator system will rotate around the system center of mass in such a way that linear
(PJ
) and angular (LJ
) momentum are conserved, and J
Cr remains fixed.
50. 30
Figure 16. Characterization of a Non-fixed Base Robotic Manipulator, from [29]
The conservation of momentum is expressed in Equation 2.56, where 0
J
x is the
movement of the base coordinate frame 0 in the inertial frame J, 0H is the base inertia
matrix, and 0mH is the dynamic coupling matrix. The base inertia matrix 0H
characterizes the inertia properties of the base spacecraft. The dynamic coupling matrix
0mH expresses the contribution to the kinetic energy of the spacecraft-manipulator
system from the coupling between the base spacecraft and manipulator [29]. While the
full expressions for these matrices is provided, this thesis does not delve into the
Lagrangian Mechanics and dynamics from which they are derived. For a full analytical
derivation of the dynamics and kinematics of a free-floating manipulator system, the
reader is pointed to “A Thorough Symbolic Analytic Derivation of the Equations of
Motion for a Free-Floating Spacecraft with an On-Board Robotic Manipulator” (in
preparation for publication) by Wilde, Romano, and Grompone [29]. Instead, this section
focuses on the kinematics of the spacecraft-manipulator system, which can be fully
defined by deriving the motion of the base spacecraft (coordinate frame 0), 0
J
x , and then
applying the kinematic relations derived previously.
51. 31
Equations 2.57–2.63 fully characterize the conservation of linear (PJ
) and angular
(LJ
) and momentum for a free-floating spacecraft-manipulator system, as developed by
Yoshida and Umetani [17]. Since it is valid to assume that the robotic manipulator starts
at rest, the linear and angular momentum will always equal zero. Equation 2.64
rearranges Equation 2.57 in order to solve for the motion of the base spacecraft, 0
J
x .
0 0 0 0
J
J
mJ
P
H x H q
L
(2.57)
Base inertia matrix:
0
0
0
J
tot tot c
J
tot c
m m
m
H
E r
r HS
, (2.58)
where:
1 0 0
0 1 0
0 0 1
E (2.59)
0
0
0
z y
z x
y x
a a
a a
a a
a (2.60)
0 0 0
1
N
J J J J
i i i i
i
m
H I r r IS (2.61)
TJ J i J
i i i iI R I R . (2.62)
Dynamic coupling matrix:
1
0
1
0i i
N
i
i
N
J
i
m
i
i
m
m
I j jr
j
H
Ti
R i T
, (2.63)
52. 32
and thus the motion of the base spacecraft (frame 0) can be solved:
1
0 0 0
J
m
x H H q . (2.64)
The dynamic coupling of each joint back to the manipulator base in the free-
floating scenario make the computations of the robotic system considerably more
complex, and this section only provides a cursory overview. However, having fully
defined the base inertia and dynamic coupling matrices, we can solve for the motion of
the manipulator base, as shown in Equation 2.64, which in turn allows us to apply the
fixed base kinematics derived prior. Later in this thesis this theory is applied in
comparing the simulation of a one-link manipulator with actual experimental data.
53. 33
III. DESIGN
This chapter gives an overview of the manipulator link’s design. The design
process started in March 2013, as a research and development project of Prof. Markus
Wilde, co-advisor of this thesis. The author joined the project in April 2014, after the
system requirements had been defined and much of the initial design had been
established. The following sections describe the design requirements and the initial
design, leading up to the final design completed in October 2014, which incorporated
numerous design improvements and lessons learned.
A. REQUIREMENTS
The modular manipulator design builds on the existing manipulator designs
surveyed above. The manipulator link is designed to be attached to the existing SRL FSS.
Therefore, attitude and position control thrusters and on-board computing are available
on the base-vehicle. Furthermore, position and motion data is available through the
overhead laboratory Vicon system. The laboratory manipulator system is required to be
modular, i.e. it must be assembled or reconfigured under orbital operating conditions. The
design goal was to minimize the required number of interfaces between the base-vehicle
and the manipulator links, as well as in between adjacent manipulator links. Therefore,
the link must contain a servomotor, encoder, torque sensor, rechargeable Li-ion battery,
and wireless communication within the mechanical structure, and must be equipped with
standardized structural interfaces. The requirements for on-board power supply and
wireless communications are critical for meeting the design goal of modularity, enabling
the links to be controlled by the base-vehicle’s on-board computer without the need to
route data and power wires through the manipulator joints. The installation of a torque
sensor provides a robust test bed for future research in spacecraft docking, collisions and
avoidance maneuvers (e.g., it will be possible to determine whether commanded torque is
equal to measured torque at every joint), and compliant control. The overarching
requirements for the single manipulator link designed as part of this thesis are
summarized as follows.
54. 34
Easily interchangeable modular design
No wires routed through joints
Wireless data relay to base robotic vehicle
On-board power supply
On-board servomotor and encoder
On-board torque sensor
Highest accuracy components for reasonable cost and mass
B. INITIAL DESIGN
1. Initial Design Components
In order to realize the modular manipulator link design, selecting suitable components
was among the first and most important tasks. From the onset it was known that a
servomotor with absolute encoder, servomotor driver, torque sensor, wireless transceiver,
and battery would be needed in order to maintain the modular design. From there, an
analog to digital converter (ADC) was needed to digitize data from the torque sensor, to
the wireless transceiver. The last set of components was then chosen to ensure electrical
compatibility throughout. The following sections will go through the design trades made,
ending with a description of the components ultimately chosen for the initial design.
a. Servomotor
The capabilities of the servomotor and its encoder determine the performance,
motion envelope, accuracy, and precision of the manipulator. Therefore, motor selection
was one of the primary tasks during initial design. Arguably the most important
component of the manipulator link, a lot of emphasis was put in choosing a high-
precision motor. Out of a wide range of available servomotors, pre-selection resulted in
the three different motors listed in Table 3. The table is color-coded based on how well
the servomotors of interest met the aforementioned requirements.
55. 35
Table 3. Servomotors Analyzed for Initial Design
Name Faulhaber 2642-
024 CR
HD FHA-8C-30-
US200-E
HD FHA-8C-30-
12S17b-E
Image
Encoder Type Incremental Incremental Absolute
Max Torque 0.5 Nm 1.8 Nm 1.8 Nm
Voltage 24 V DC 24 V DC 24 V DC
Max Current 0.97 A 3 A 3 A
Max Velocity 400 RPM 200 RPM 200 RPM
Accuracy Unknown 0.04 deg 0.04 deg
Mass 0.135 kg 0.4 kg 0.5 kg
Cost $1000 (est) $2467 $2500
Chosen for Initial
Design?
No No Yes
Despite being heavier and more expensive, the Harmonic Drive FHA-8C-30-
12S17b-E servomotor was chosen because of the combination of high torque, high
accuracy due to the use of a zero-backlash Harmonic Drive gearbox, and an absolute
encoder.
Figure 17 shows a Harmonic Drive FHA-8C-30-12S17b-E motor as delivered.
The motor is capable of generating up to 1.8 Nm of torque and its internal absolute
encoder gives feedback of position and angular velocity with 131,071 counts per
revolution.
56. 36
Figure 17. Harmonic Drive FHA-8C-30-12S17b-E Servomotor
b. Servomotor Driver
The combination of operating voltage, current, and encoder type and signal
standard determine the servomotor driver to be used with a given servomotor. The
servomotor driver is responsible for the proper amplification of the motor control signals,
and for reading and interpreting the data from the encoder. In addition, the servomotor
drivers for high-performance servomotors also prevent damaging the servomotor by
implementing built-in protections like current constraints and a safety shut-down if a
command cannot be executed. The servomotor drivers analyzed are listed in Table 4.
Table 4. Motor Drivers Analyzed for Initial Design
Name AllMotion EZSV10 HD DCJ-055-09 HD DEP-090-09
Image
Encoder Type Incremental Incremental Absolute
Voltage 24 V DC 24 V DC 24 V DC
Current 2 A 9 A 10 A
Dimensions 24 x 35 x 15 mm 97 x 64 x 33 mm 196 x 99 x 31 mm
Comm Ports RS 232 / RS 485 CANopen / RS 232 EtherCAT / RS 232
Mass 0.05 kg (est) 0.14 kg 0.45 kg
Cost $199 Included with motor $700
Chosen for
Initial Design?
No No Yes
57. 37
The selection of the servomotor de facto pre-selected the servomotor driver. Thus,
the Harmonic Drive DEP-090-09 servomotor driver was selected because of its
compatibility with the Harmonic Drive FHA-8C-30-12S17b-E and support of an absolute
encoder. The main drawback of this motor driver, however, is its large size. With
dimensions of 196 x 99 x 31 mm, the motor driver is by far the largest component that
must fit inside the manipulator link. If a reduction of this volume is desired in the future,
the use of a custom servomotor driver circuit board is recommended. The protections that
are programmed into this off-the-shelf driver (e.g. prevention of a power surge greater
than 3 A), however, would need to be carefully included into the custom software and
hardware. A picture of the actual Harmonic Drive DEP-090-09 servomotor driver ordered
is shown in Figure 18.
Figure 18. Harmonic Drive DEP-090-09 Servomotor Driver
c. Torque Sensor
As discussed in the literature review, robotic manipulators have frequently used
torque sensors on the end-effector of the manipulator arm. Incorporating a torque sensor
at every joint is a novel feature of this design. If, for example, torque sensor feedback
indicates that the commanded torque does not equal measured torque, it can indicate that
an obstacle has been encountered in the task space. Thus, this design enables future
58. 38
research that was previously not possible. Two different torque sensors were analyzed
and are compared in Table 5.
Table 5. Torque Sensor Analyzed for Initial Design
Name Futek TFF350 Futek TFF400
Image
Torque Range ±11 Nm ±3 Nm
Max Excitation 18 V DC 18 V DC
Output 2 mV/V 2 mV/V
Resolution 0.01 Nm 0.003 Nm
Dimensions 50.8 x 37.6 mm 50.8 x 50.2 mm
Mass 0.082 kg 0.25 kg
Cost $640 $1040
Chosen for Initial
Design?
No Yes
While the Futek TFF350 torque sensor featured higher torque capacity, less mass,
and a lower cost, the Futek TFF400 torque sensor was chosen for its enhanced resolution.
Also, 3 Nm capacity was more than sufficient to cover the torque generated by any of the
motors investigated. Furthermore, as the output voltage scales linearly with the torque
measured and must be converted through an analog-digital converter, a smaller maximum
torque capacity translates into an additional increase in resolution.
A picture of the actual Futek TFF400 torque sensor ordered is shown in Figure
19. The torque sensor itself does not contain any electronics, and instead has a
Wheatstone bridge, which generates an analog signal. The strain gauges of the
Wheatstone bridge convert the signal according a linear relationship between the torque
(±3 Nm) and a voltage relation of 2 mV per excitation voltage. In the modular link
the torque sensor is supplied with an excitation voltage of 15 V (maximum allowable is
18 V). Therefore, the output signal will vary ±30 mV. It is important that the torque
59. 39
sensor is fed by a constant excitation voltage so that the torque-to-signal voltage ratio
remains unchanged.
Figure 19. Futek TFF400 Torque Sensor
d. Analog to Digital Converter
The analog voltage output of the torque sensor must be converted into a digital
signal for data transmission and further processing. In order to achieve a high resolution
of the torque signal, the analog-to-digital converter (ADC) must have a high bit-
resolution and its input signal range must also match the torque signal range (±30 mV) as
closely as possible. To keep the implementation effort of the initial link design at a
minimum, it was initially decided to use an off-the-shelf, commercial ADC instead of
designing a custom electronic circuit. Table 6 shows two available ADCs that best fit the
design requirements.
60. 40
Table 6. Analog to Digital Converters Analyzed for Initial Design
Name Taskit RS232-ADC24 Advantech ADAM-4012
Image
Resolution 24 bits 16 bits
Analog Channels 8 1
Voltage Range 0-2.5 V ± 150 mV
Comm Port RS 232 RS 485
Mass 0.03 kg 0.05 kg (est)
Dimensions 62 x 55 x 17 mm 112 x 70 x 47 mm
Voltage 12 V DC 24 V DC
Current 0.1A (est) 0.05 A
Cost $150 $180
Chosen for Initial Design? No Yes
Even though the Taskit RS232-ADC24 analog to digital converter featured
greater resolution, more channels, a convenient RS 232 exit port, and less volume, the
Advantech ADAM-4012 ADC was chosen for a number of reasons: (1) It can
accommodate both positive and negative voltages created by the torque sensor; (2) it can
be supplied from the same 24 V DC bus as the servomotor driver, thus eliminating an
additional voltage converter; (3) it was readily available from US-based vendors, while
the Taskit ADC would have had to be ordered in Europe, potentially leading to project
delays. While the use of the ADAM-4012 ADC did introduce a RS 485 to RS 232
converter into the design, this was deemed an acceptable design impact. A picture of the
actual Advantech ADAM-4012 ADC ordered is shown in Figure 20. It is noted, and
elaborated upon later, that the ADAM-4012 was replaced by a custom circuit in the final
design.
61. 41
Figure 20. Advantech ADAM-4012 ADC
e. Battery Power Supply
Unlike before where various components were compared and contrasted, the
following components were selected “single source”, because of their robustness and
reliability. The Inspired Energy series of rechargeable Li-ion batteries have been
successfully used in the SRL for years, and are currently equipped on all the SRL FSS
vehicles. The particular Inspired Energy NH2054 model selected is shown in Figure 21.
It has a capacity of 6.2 Ah at a nominal voltage of 14.4 V DC, which is sufficient for
hours of experimentation before needing to be recharged.
62. 42
Figure 21. Inspired Energy NH2054 Li-Ion Battery
f. Wireless Transceiver
The wireless transceiver selected for the initial design is the Quatech ABDG-SE-
IN5420. A picture of the actual model ordered is shown in Figure 22. The Quatech was
selected to serve as a wireless virtual serial port for the FSS on-board computer, in order
to interface bi-directionally with the servomotor driver and the torque sensor ADC via RS
232. Since the Quatech is designed as a wireless serial device server for industrial
applications, it is perfectly suited for this task. It is noted, however, that in the final
design we opted to instead use an Arduino microcontroller with wireless XBee shield, in
order to reduce the number of components and the associated mass and volume.
63. 43
Figure 22. Quatech ABDG-SE-IN5420
g. DC/DC Converters
The electronic components of the initial design operate at two different voltages.
The servomotor driver, wireless transceiver, and ADC all operate at 24 V, but the torque
sensor has a maximum excitation of 18 V. Therefore, two DC/DC converters are needed
to convert the 14.4 V DC coming from the battery. The converter selected for the 24 V
DC bus is the Traco Power 75-2415WI V DC/DC, shown in Figure 23. It features an
input voltage range of 9-36 V, which enables it to track the changes in battery voltage
(14.4 V to 12 V) as the charge gets depleted, and outputs a constant 24 V required to
power the motor, driver, and ADC. Due to availability of DC/DC converters, it was
decided to excite the torque sensor with constant 15 V. The 15 V are derived from the 24
V bus by a Cosel ZU32415 DC/DC Converter, shown in Figure 24.
64. 44
Figure 23. Traco Power 75-2415WI V DC/DC Converter
Figure 24. Cosel ZUS32415 DC/DC Converter
65. 45
h. Auxiliary Components
A number of additional electronic and mechanical components were required for
the initial link design. In order to ensure communication compatibility, transitional
connectors were used, shown in Figure 25 and Figure 26. Also, SKF Explorer model 608-
2z ball bearings were used for the link joint, shown in Figure 27.
Figure 25. RS 485 to RS 232 Connector
66. 46
Figure 26. TTL to RS 232 Connector
Figure 27. SKF Explorer 608-2z Ball Bearings
i. Summary of Components Chosen for Initial Design
A summary of all the components chosen for the initial design, excluding wires
and their connecting terminals (e.g. crimped pins) is provided in Table 7. How these
components were wired together is detailed in the following Electrical Architecture
section.
67. 47
Table 7. Summary of Components Chosen for Initial Design
Component Type Component Chosen
Servomotor HD FHA-8C-30-12S17b-E
Servomotor Driver HD DEP-090-09
Torque Sensor Futek TFF400
ADC Advantech ADAM-4012
Wireless Transceiver Quatech ABDG-SE-IN5420
Power Supply Inspired Energy NH2054 Li-Ion Battery
DC/DC Converter Traco Power 75-2415WI
DC/DC Converter Cosel ZU32415
Connector RS 485 to RS 232 Connector
Connector TTL to RS 232 Connector
Ball Bearing SKF Explorer 608-2z Ball Bearings
2. Initial Design Electrical Architecture
The electrical power architecture of the initial design is shown in Figure 28.
Ultimately, each link is powered by a 14.4 V battery power supply, which then shares a
common ground with every component. The Traco Power (TEP) DC/DC converter
increases the voltage to 24 V to power the servomotor, motor driver, wireless transceiver,
and ADC. It also supplies power to the 24 V to 15 V Cosel DC/DC converter, which, as
mentioned before, supplies the torque sensor with the needed steady voltage.
68. 48
Figure 28. Initial Design Electrical Power Architecture
The communications diagram of the initial design is shown in Figure 29. The
Quatech wireless transceiver transfers data to and from the FSS on-board computer, and
has two RS 232 serial ports to connect to the motor driver and RS 485 to RS 232
connector. The servomotor driver serves as a data relay with the transceiver, both
amplifying commands to the motor and sending back position/velocity data from the
motor’s encoder. The RS 485 to RS 232 connector was required because the ADC output
port was RS 485. Finally, the ADC was required to digitize the analog signal coming
69. 49
from the torque sensor. Because the torque sensor’s Wheatstone bridge has strain gauges
that are set to output 2 mV per V of excitation, the torque sensor’s analog signal will vary
between ±30 mV.
Figure 29. Initial Design Communications Architecture
3. Initial Design Mechanical Architecture
The mechanical architecture of the manipulator link was designed using Siemens
NX 7.5 CAD software. The models of the initial design components were provided by the
various manufacturers and imported as STEP files. The link and joint structures were
custom designed and 3D printed in-house. Mechanically, all components were assembled
using standard hardware with pre-printed holes, and the ball bearings at the joint were
press-fit into place. Figure 30 illustrates the components inside the manipulator link as
well the overall link dimension. Similarly, the components of the joint and overall joint
dimensions are shown in Figure 31.
70. 50
Figure 30. CAD Model of Link with Dimension
Figure 31. CAD Model of Joint with Dimensions
71. 51
A CAD model of the integrated manipulator link and joint assembly is shown in
Figure 32. Using NX 7.5 density and measurement tools, as well as manufacturer-
provided data on the various components, a list of the estimated overall mechanical
parameters (total length, height, and mass) is provided in Table 8.
Figure 32. CAD Model of Link and Joint Together
Table 8. Manipulator Link Overall Design Parameters
Parameter Estimated Value
Total Length 392 mm
Total Width 77 mm
Total Height 162 mm
Mass 2.7 kg
72. 52
C. FINAL DESIGN
1. Final Design Components
As alluded to in previous sections, a number of design improvements were
performed during the design and integration process that required replacing certain
components for the final design. The first and most important design change came in
deciding to introduce an Arduino Due microcontroller to replace the Quatech wireless
serial server, giving the link a certain degree of computational autonomy. Arduino is an
Italian-based family of single-board microcontrollers that come with a ready-made
integrated development environment (IDE), which allows for programming in the C++
language. The Arduino Due, in particular, is an improvement over Arduino’s original
Uno board by utilizing an enhanced 32-bit ARM core microcontroller and a greater
number of digital I/O pins, analog input pins, and serial RX/TX pins [31].
Arduino boards are also compatible with a number of both first and third party
“shields”, which make the addition of external hardware straightforward. One of these
shields is a called an “XBee Shield”, which connects the Arduino to a serial XBee radio
transceiver. Because the Arduino can be configured to provide both wireless
communications (in conjuction with an XBee Shield) and analog-to-digital conversion (in
conjunction with a custom circuit board, discussed below), it could replace two major
components (Quatech wireless transceiver and the Avantech ADC) from the initial
design, along with their corresponding connectors. In addition, the fact that the Arduino
can be programmed to read, transmit, and filter data as desired made it an appealing
option, and it was implemented in the final design.
Other smaller optimizations involving replacing certain connectors were also
incorporated into the final design. The Arduino was equipped with an “RS 232 Shield”
which, as the name implies, turns one of the Arduino’s serial ports into an RS 232 port.
This made the Arduino directly compatible with the servomotor driver. Despite this
improvement, this particular connection proved to be rather cumbersome, and one of the
lessons learned (detailed later) during the integration process was that both the Arduino
and motor driver are Data Communications Equipment (DCE) ports, which makes them
73. 53
incompatible unless both a gender changer and null modem connector are used. Thus, by
utilizing a custom-made RS 232 connector in which the TX and RX pins were manually
switched, the gender changer and null modem were replaced, further simplifying the
design.
This section lists the specific components of the final design chosen to replace
components from the initial design. In addition, the practice of illustrating lessons learned
is commenced. These lessons learned highlight some of the more challenging problems
that arose during the integration process. Several of the components in the final design
were chosen as a result of the lessons learned. The section is concluded with a
comprehensive list of all the components used in the final design (including those kept
from the initial design).
a. Wireless Transceiver
Replacing the Quatech wireless transceiver with an Arduino Due and XBee
Shield was a significant design change, and its advantages were discovered early enough
in the design process that the Arduino was added even in the initial integration. Aside
from the fact that the Arduino is more compact than the relatively large Quatech
transceiver, its custom programmability allowed for numerous improvements. One of the
improvements that made use of the Arduino’s programmability is organizing the data
such that the motor parameters (position, angular velocity, and current) were grouped
with the torque value from the torque sensor and sent as an organized data packet. This
allowed for easy interpretation of the data across the wireless communications. A further
improvement that could be implemented in the future is to manipulate the data such that
it is not transmitted in units that are specific to a particular servomotor (counts), and
instead send data in generic units (radians) that can be interpreted by a generic program.
An important thing to note when using XBee transceivers is that they do not come
with configuration software. However, a configuration program called XCTU is available
for free on the internet. The configuration of both the receive and transmit antennas must
be identical, and the baud rate of the XBees must match the baud rate of the receiving
computer port as well as the transmitting Arduino port.
74. 54
Lesson Learned:
XBee radio transceivers do not come with configuration software. The
XCTU program should be downloaded and used to configure the XBees.
In particular, the baud rate must match that of the receiving computer
port as well as the transmitting serial port of the Arduino.
A summary comparison of the Quatech wireless transceiver and the Arduino Due
with XBee shield is provided in Table 9. A picture of the actual components ordered is
provided in Figure 33.
Table 9. Wireless Transceivers Analyzed for Final Design
Name Quatech ABDG-
SE-IN5420
Arduino Due
with XBee Shield
Image
Input Voltage 5-36 V DC 6-16 V DC
Current 1.5 A 800 mA
Dimensions 120 x 120 x 29 mm 113 x 53 x 25 mm
Comm Ports RS 232 TTL / Analog
Mass 0.5 kg (est) 0.08 kg
Cost $399 $350
Custom
Programmable?
No Yes
Chosen for Final
Design?
No Yes
75. 55
Figure 33. Arduino Due and XBee (Left) Replaced Quatech Transceiver (Right)
b. Analog to Digital Converter
Another improvement of the Arduino Due over the original Uno board is that the
internal ADC was improved to 12 bits. Therefore, it can be used to replace the Advantech
ADC and its corresponding RS 485 to RS 232 connector. While the Advantech ADC
does have better resolution at 16 bits, the signal from the torque sensor (±30 mV) does
not map to the entire ADC’s range (±150 mV). If the torque sensor’s signal (±30 mV) is
amplified to use close to the full input voltage range of the Arduino Due (0-3.3 V), the
resulting torque resolution still reaches the maximum resolution of the torque sensor
(0.003 Nm).
The amplification of the torque sensor output voltage required a custom-designed
circuit built around a voltage amplifier and was not implemented in the initial integration.
However, the benefits of introducing a custom circuit board to the link design became
apparent during the integration of the initial link design. This custom circuit board was
intended to amplify the torque sensor’s signal to the Arduino Due’s input voltage range,
which allowed for the Advantech ADC and its bulky RS 485 to RS 232 connector to be
removed from the design. In addition, the 24 V to 15 V Cosel DC/DC converter was
included in the circuit board, which allowed for the removal of yet another component. A
comparison of the Advantech ADC and Arduino Due with the custom circuit board is
provided in Table 10. Figure 34 and Figure 35 show a picture of the custom circuit board
and a view of the components it allowed to be replaced respectively.
76. 56
Table 10. Analog to Digital Converters Analyzed for Final Design
Name Advantech
ADAM-4012
Arduino Due
with Custom Circuit Board
Image
Resolution 16 bits 12 bits
Channels 1 12
Input Voltage ± 150 mV ±30 mV
Operating Voltage 24 V DC 15 V
Current 50 mA 800 mA
Comm Ports RS 485 TTL / Analog
Mass 0.05 kg (est) 0.10 kg
Cost $180 $100
Chosen for Final
Design?
No
Yes
Figure 34. Front and Back of Custom Circuit Board
77. 57
Figure 35. Custom Circuit (Left), Along with Arduino Due,
Replace ADAM ADC, Cosel DC/DC Converter,
and RS 485 to TTL Connector (Right)
c. Auxiliary Components
As mentioned previously, the design change to implement an Arduino came early
enough that it was implemented in the initial integration. However, several other design
improvements that this enabled were not implemented until later in the final design
process. Among these was the introduction of an RS 232 Shield in addition to the XBee
Shield, which eliminated the need to use a TTL to RS 232 connector. The RS 232 Shield
is shown in Figure 36 and a comparison with the components it replaced is shown in
Figure 37.
78. 58
Figure 36. Arduino RS 232 Shield
Figure 37. Arduino with RS 232 Shield (Left) Replaces TTL to RS 232 Connector
and Electrical Junctions (Right)
While the addition of an RS 232 Shield allowed the Arduino to connect directly to
the servomotor driver, it was discovered that the Arduino and servomotor driver were
incompatible without the use of both a connector gender changer and null modem. In
serial communication, all devices utilizing RS 232 should conform to standard TIA/EIA-
574 [32], dictating that they are either classified as Data Communication Equipment
(DCE) or Data Terminal Equipment (DTE). The Arduino and servomotor driver were
incompatible because they both act as DCE. This problem was overcome by ordering a
79. 59
custom male RS 232 to RJ12 connector and manually switching the transmit TX and
receive (RX) wires. By making this switch, the RS 232 to RJ12 connector is no longer
TIA/EIA-574 compliant, but serves the purpose of simplifying the link design by
eliminating the need to include both a gender changer and a null modem. The ground
wire in the connector was left in the same location and the other wires were left unused as
they are not required for serial communication. The custom-made RS 232 to RJ 12
connector is shown in Figure 38 and a comparison with the components it replaced is
shown in Figure 39. A picture of the Arduino connection to the motor driver in its final
configuration is shown in Figure 40.
Lesson Learned:
In serial communication, all devices utilizing RS 232 are either Data
Communication Equipment (DCE) or Data Terminal Equipment (DTE).
Because the Arduino and Servomotor driver are both DCE, they are
incompatible without the use of a null modem or custom-made connector.
Figure 38. Custom Male RS 232 Connector to RJ12
80. 60
Figure 39. Custom RJ12 to Male RS 232 Connector (Left) Replaces need for
Null Modem and Gender Changer (Right)
Figure 40. Arduino to Motor Driver Connection in its Final Configuration
d. Summary of Components Used in Final Design
A summary of all the components used in the final design, excluding wires and
their connecting terminals (e.g., crimped pins) is provided in Table 11. How these
components were wired together is detailed in the following Electrical Architecture
section. Table 12 lists the components used in the custom circuit board, which is also
detailed in the Electrical Architecture section.
