MECHANICAL DESIGN AND FIELD EVALUATION OF A ROBOTIC APPLE HARVESTER By ...

133 Pages·2016·5.07 MB·English

by  Joe Davidson

MECHANICAL DESIGN AND FIELD EVALUATION OF A ROBOTIC APPLE
HARVESTER
By
JOSEPH RYAN DAVIDSON
A dissertation submitted in partial fulfillment of
the requirements for the degree of
DOCTOR OF PHILOSOPHY
WASHINGTON STATE UNIVERSITY
School of Mechanical and Materials Engineering
MAY 2016
© Copyright by JOSEPH RYAN DAVIDSON, 2016
All Rights Reserved
© Copyright by JOSEPH RYAN DAVIDSON, 2016
All Rights Reserved
To the Faculty of Washington State University:
The members of the Committee appointed to examine the dissertation of JOSEPH RYAN
DAVIDSON find it satisfactory and recommend that it be accepted.
Changki Mo, Ph.D., Chair
Joseph Iannelli, Ph.D.
Aboutaleb ‘Amir’ Ameli, Ph.D.
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ACKNOWLEDGEMENT

I would like to thank Dr. Iannelli, Dr. Ameli, and the graduate committee for their
guidance, timely feedback, and enthusiastic interest in my work. My committee chair and
advisor, Dr. Mo, was an extremely supportive mentor and friend. I would also like to
acknowledge and thank the United States Department of Agriculture (USDA) who funded this
research through the National Robotics Initiative (NRI). In order to pick apples, a robotic apple
harvester has to know where the fruit are located. Abhisesh Silwal, with whom I spent many
hours slogging through muddy apple orchards, developed the machine vision system that
detected and localized the fruit. It was a pleasure collaborating with him, and I have tried to
diligently ensure that his contributions to the robotic system are highlighted and cited where
appropriate. My friend Cameron Hohimer provided substantial manufacturing advice and loaned
me his 3D printer to fabricate many of the parts used in the design. We also collaborated on the
design of the apple catching system. Last, but certainly not least, I would like to thank my wife
Sara, whose gentle nudging convinced me to pursue the Ph.D. program in the first place. Without
her unwavering support, none of this would have been possible.
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MECHANICAL DESIGN AND FIELD EVALUATION OF A ROBOTIC APPLE
HARVESTER
Abstract
by Joseph Ryan Davidson, Ph.D.
Washington State University
May 2016
Chair: Changki Mo
Every apple destined for the fresh market is picked by the human hand. Despite
substantial research to develop robotic apple harvesters, there are no robotic systems
commercially available. The highly unstructured orchard environment has been a major
challenge to the development of commercially viable robotic harvesting systems. The absence of
mechanical harvesters is a significant concern due to rising production costs and increasing
uncertainty about the future availability of manual labor. This dissertation presents the
mechanical design of a robotic apple harvester. The overall approach implemented was to
advance performance, as measured by speed and harvesting efficiencies, by simplifying the
harvesting task. A custom, seven degree-of-freedom manipulator was designed, fabricated, and
then integrated with a picking end-effector. The end-effector, which is the only system
component that makes contact with the fruit, is an underactuated, passively compliant design that
grasps the fruit with a spherical power grasp. The end-effector prototype was extensively
analyzed in the lab prior to field testing and shown to be robust to perception error.
Prior to field testing, a global camera was integrated with the mechanical system in order
to execute open loop, go-to picking with no intermediate visual servoing. The system was then
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evaluated in a commercial apple orchard in Prosser, Washington. Robotic manipulation adopted
‘undersensed’ picking methods developed through dynamic analysis of the hand picking process.
Because modern, planar orchard systems were selected for field studies, substantial
computational resources were not dedicated to motion planning. Detailed performance criteria
were used to report results, and each significant task in the harvesting process was individually
timed to help focus future efforts at reducing cycle time. The system successfully picked 127 of
the 150 fruit attempted for an overall success rate of 84%. The average picking time was 6.0 sec
per fruit. These fruit detachment efficiencies and execution times represent an approximately 3
sec improvement in performance levels reported for robotic apple harvesters. However,
substantial challenges to commercial implementation still remain. An overview of future work
needed to address some of these challenges is included in the summary.
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TABLE OF CONTENTS
Page
ACKNOWLEDGEMENT ............................................................................................................. iii
ABSTRACT ................................................................................................................................... iv
LIST OF FIGURES ....................................................................................................................... ix
LIST OF TABLES ........................................................................................................................ xii
1. INTRODUCTION ...................................................................................................................... 1
1.1 Motivation ..............................................................................................................................1
1.2 State-of-the Art ......................................................................................................................2
1.3 System Requirements and Design Criteria ............................................................................4
1.4 Design Approach ....................................................................................................................6
2.MANIPULATOR DESIGN ........................................................................................................ 8
2.1 Mechanical Design .................................................................................................................8
2.2 Manipulator Kinematics .......................................................................................................10
3.END-EFFECTOR DESIGN, ANALYSIS, AND FABRICATION ......................................... 14
3.1 Analysis of Link Coupling ...................................................................................................15
3.2 End-Effector Design ............................................................................................................19
3.3 End-Effector Fabrication ......................................................................................................21
3.4 Normal Force Simulation .....................................................................................................24
4. LABORATORY TESTING OF END-EFFECTOR AND INTEGRATED SYSTEM ............ 27
4.1 Normal Contact Forces ........................................................................................................27
4.2 End-Effector Robustness to Position Error ..........................................................................30
4.3 Laboratory Apple Picking ....................................................................................................33
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Page
4.3.1 Experimental Set-Up .....................................................................................................34
4.3.2 Apple Prioritization .......................................................................................................34
4.3.3 Path Planning .................................................................................................................35
4.3.4 Results ...........................................................................................................................35
4.3.4 Discussion .....................................................................................................................37
5.UNDERSENSED GRASPING ANALYSIS ............................................................................ 39
5.1 Undersensed Robotic Apple Harvesting ..............................................................................42
5.2 Experimental Method ...........................................................................................................44
5.2.1 Orchard Architecture and Cultivar Selection ................................................................44
5.2.2 Grasp Repeatability Analysis ........................................................................................47
5.2.3 Picking Patterns and Inertial Measurement Unit (IMU) Installation ............................50
5.2.4 Data Acquisition ............................................................................................................52
5.3 Results and Discussion .........................................................................................................53
6.FIELD EVALUATION OF THE INTEGRATED SYSTEM .................................................. 66
6.1 End-Effector Design Modifications .....................................................................................66
6.2 System Integration ...............................................................................................................67
6.2.1 Path Planning .................................................................................................................68
6.2.2 Harvesting Sequence .....................................................................................................73
6.2.3 Sequence Timing ...........................................................................................................73
6.3 EXPERIMENTAL SETUP ..................................................................................................75
6.4 RESULTS AND DISCUSSION ..........................................................................................76
7.SUMMARY .............................................................................................................................. 81
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Page
REFERENCES ............................................................................................................................. 85
APPENDICES
Appendix A – Manipulator Parts Listing ...................................................................................93
Appendix B – Manipulator (Arm Only) Drawings ....................................................................95
Appendix C – Matlab Kinematics Programs ...........................................................................102
Appendix D – Force Simulation Code .....................................................................................115

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LIST OF FIGURES

Figure 1. Comparison of reported robotic picking times with manual picking times for apples,
strawberries, oranges, and watermelons. ........................................................................................ 4

Figure 2. CAD model of the robotic manipulator (end-effector not shown). .............................. 10

Figure 3. Kinematic configuration and DH parameters of the custom manipulator. ................... 11

Figure 4. Apples on the tree show significant variability in shape, size, stem length, and
growing orientation. ...................................................................................................................... 16

Figure 5. (a) Drawing of the underactuated finger. (b) Representative model of the finger. ...... 17

Figure 6. Grasping simulation for stiffness ratio of .25. .............................................................. 19

Figure 7. Grasping simulation for stiffness ratio of 4. ................................................................. 19

Figure 8. Computer Aided Design (CAD) model of the prototype end-effector
(finger pads not shown). ............................................................................................................... 20

Figure 9. (a) Molds for the finger pads are included in the 3D printed parts. (b) Assembled
fingers with urethane flexures after the molds are removed. ........................................................ 21

Figure 10. Experimental setup used to determine flexural stiffness. ........................................... 23

Figure 11. Two finger grasp configuration determined by Matlab solver. .................................. 25

Figure 12. The normalized proximal force that develops at static-equilibrium for a
single finger. The x-y coordinate represents the center of a circle with radius of 40 mm. ........... 26

Figure 13. Experimental set-up for proximal normal force measurements. ................................ 28

Figure 14. Proximal and distal normal forces for each finger compared at five
different actuator loads. ................................................................................................................ 29

Figure 15. The proximal link normal forces that developed during a power grasp of
a sphere with a diameter of 80 mm. .............................................................................................. 30

Figure 16. Left: A configuration where the fingers successfully grasp the fruit and the stem
gripper pinches the stem. Right: The fingers grasp the fruit but the stem gripper pinches the
branch. ........................................................................................................................................... 31

Figure 17. Results of experiment to determine the end-effector’s robustness to position error. . 32

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