NASA VALKYRIE: DESIGN OF A FOREARM AND

WRIST FOR MANIPULATOR INTEROPERABILITY

AND IMPROVED GLOVEBOX PERFORMANCE

MEIE 4701/4702

Technical Design Report

Tuesday, December 4, 2018

Department of Mechanical and Industrial Engineering

College of Engineering, Northeastern University

Boston, MA 02115

NASA Valkyrie: Design of a Forearm and Wrist for

Manipulator Interoperability and Improved

Glovebox Performance

Final Report

Design Advisor: Rifat Sipahi

Design Team

Matthew Bonanni, Max Choate,

David Coven, Bryant Grey-Stewart,

Ryan Loehr

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NASA VALKYRIE: DESIGN OF A FOREARM AND WRIST FOR

MANIPULATOR INTEROPERABILITY AND IMPROVED GLOVEBOX

PERFORMANCE

Design Team

Matthew Bonanni, Max Choate, David Coven,

Bryant Grey-Stewart, Ryan Loehr

Design Advisor

Rifat Sipahi

Abstract

The manipulation of hazardous materials within a glovebox is a promising application of humanoid

robotics. NASA R5 “Valkyrie” is an excellent candidate for this task; however, the constraints of a glovebox

limit the volume in which Valkyrie’s existing hardware can effectively operate. This capstone project

presents an evaluation of the capabilities of Valkyrie’s current forearms and presents the design,

development, and implementation of a new forearm for Valkyrie, specifically optimized for a glovebox

environment. This forearm also enables integration of other end effectors, particularly the Yale OpenHand,

by adopting its mounting configuration. By exploring a feasible kinematic chain beyond what is permitted

in a humanoid design, this new forearm improves Valkyrie’s glovebox range of motion by a factor of more

than 10 while satisfying existing payload and weight requirements.

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Table of Contents

1 Acknowledgments ................................................................................................................................. 1

2 Introduction ........................................................................................................................................... 2

2.1 Problem Statement ........................................................................................................................ 2

2.2 Project Motivation ........................................................................................................................ 2

3 Background Research ........................................................................................................................... 4

3.1 Valkyrie Forearm Background...................................................................................................... 4

3.2 Robotic Wrist Mechanisms ........................................................................................................... 4

3.3 Glovebox Constraints .................................................................................................................... 7

4 Specifications and Constraints .............................................................................................................. 9

4.1 Existing Forearm Capabilities and Requirements ......................................................................... 9

4.2 Requirements of the Redesigned Forearm .................................................................................... 9

5 Initial Design Concepts ....................................................................................................................... 10

5.1 Actuator Layout Concepts .......................................................................................................... 10

5.2 Comparison of Initial Designs .................................................................................................... 11

5.2.1 Pairwise Comparison .......................................................................................................... 11

5.2.2 Conclusions and RPR Selection .......................................................................................... 12

5.3 Initial Actuator Selection and Constraints .................................................................................. 13

5.4 Scope Adjustments and Solutions ............................................................................................... 14

6 Mathematical Modeling and Optimization ......................................................................................... 15

6.1 Denavit-Hartenberg Parameters .................................................................................................. 15

6.2 Gamut Computation and Optimization ....................................................................................... 17

6.3 Conclusions on Optimal Link Lengths ....................................................................................... 20

7 Gamut Demonstrator ........................................................................................................................... 22

7.1 Actuator Selection ....................................................................................................................... 22

7.2 Gamut Demonstrator Mechanical Design ................................................................................... 22

7.3 Gamut Demonstrator Design Analysis ....................................................................................... 30

7.4 Electrical Components and Controls ........................................................................................... 34

7.5 Fabrication of Components ......................................................................................................... 36

7.6 Assembly of Gamut Demonstrator ............................................................................................. 36

7.7 Test Bench Configuration ........................................................................................................... 37

7.8 Testing......................................................................................................................................... 38

8 Full-Strength Design ........................................................................................................................... 39

8.1 Actuator Selection for Full-Strength Forearm ............................................................................ 39

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8.2 Full-Strength Mechanical Design ............................................................................................... 41

8.3 Full-Strength Design Analysis .................................................................................................... 44

8.4 Full-Strength Electronics and Controls ....................................................................................... 46

9 Conclusions and Future Work ............................................................................................................. 47

9.1 Conclusions ................................................................................................................................. 47

9.2 Future Work ................................................................................................................................ 47

10 Intellectual Property ............................................................................................................................ 49

10.1 Description of Problem ............................................................................................................... 49

10.2 Proof of Concept ......................................................................................................................... 49

10.3 Progress to Date .......................................................................................................................... 49

10.4 Individual Contributions ............................................................................................................. 51

10.5 Future Work ................................................................................................................................ 52

11 References ........................................................................................................................................... 53

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List of Figures

Figure 1: Definitions of roll, pitch, and yaw rotation on Valkyrie forearm .................................................. 2

Figure 2: Yale OpenHand Model O .............................................................................................................. 3

Figure 3: Close-up of Valkyrie’s current humanoid end effector ................................................................. 4

Figure 4: Definition of roll, pitch, and yaw on model forearm ..................................................................... 5

Figure 5: Roll-Pitch-Roll robotic wrist mechanism [4] ................................................................................ 5

Figure 6: Roll-Pitch-Yaw robotic wrist [6] ................................................................................................... 6

Figure 7: Universal joint, simplified model of Valkyrie wrist mechanism [8] ............................................. 6

Figure 8: Parallel manipulator robotic wrist with 3-DOF [9] ....................................................................... 7

Figure 9: Representation of glovebox manipulation task. Top left, Valkyrie robot interacting with objects

in prototype glovebox environment. Top right, actual gloveboxes in use in nuclear facilities. Bottom,

dynamic simulation of Valkyrie interacting with glovebox model, with support polygon outline [11] ....... 7

Figure 10: Four initial sketches of 3-DOF arms for Valkyrie redesign, including pure linear and pure

rotary RPY, combination linear/rotary RPY, and rotary RPR .................................................................... 10

Figure 11: RPR wrist design in comparison to existing Valkyrie wrist ...................................................... 13

Figure 12: Traditional definitions of Denavit-Hartenberg parameters [14] ................................................ 15

Figure 13: Model representation of RPY robotic arm ................................................................................ 15

Figure 14: Model representation of RPR robotic arm ................................................................................. 16

Figure 15: Visualization of arms inside glovebox ...................................................................................... 16

Figure 16: Additional DOF afforded by Valkyrie body.............................................................................. 17

Figure 17: Sample point cloud of RPR arm ................................................................................................ 17

Figure 18: Convex hull of point cloud ........................................................................................................ 17

Figure 19: Reduced complexity model of RPR / RPY / current arm .......................................................... 18

Figure 20: Torques induced by arm component masses ............................................................................. 19

Figure 21: Gamut volume weight function for optimization ...................................................................... 19

Figure 22: Optimization surface of RPR model .......................................................................................... 20

Figure 23: Visualization of gamut envelope of Valkyrie current forearms ................................................ 21

Figure 24: Visualization of gamut envelope of RPR forearms ................................................................... 21

Figure 25: SolidWorks CAD of final gamut demonstrator design ............................................................. 23

Figure 26: Isometric view of elbow interface adapter ................................................................................ 23

Figure 27: Cross-sectional view of Roll 1 bearing structure ....................................................................... 24

Figure 28: Exploded view of Roll 1 bearing structure ................................................................................ 25

Figure 29: Pitch structure subassembly ...................................................................................................... 26

Figure 30: U-frame labeled with fastener holes .......................................................................................... 26

Figure 31: Two exploded halves of Pitch housing ...................................................................................... 27

Figure 32: Mounting plate CAD ................................................................................................................. 27

Figure 33: Roll 2 actuator and bearing structure ......................................................................................... 28

Figure 34: Wrist beam component .............................................................................................................. 28

Figure 35: Sliding connection between wrist beam and Yale Hand adapter plate ...................................... 29

Figure 36: Length parameters labeled on gamut demonstrator ................................................................... 29

Figure 37: FOS of Roll 1 bearing structure under maximum loading condition ........................................ 30

Figure 38: Cross-section FOS of Roll 1 elbow adapter under maximum loading condition ...................... 31

Figure 39: FOS of U-frame under maximum loading condition ................................................................. 31

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Figure 40: FOS of Pitch frame under maximum loading condition ............................................................ 32

Figure 41: FOS of Roll 2 bearing structure under maximum loading condition ........................................ 32

Figure 42: Cross-section FOS of Roll 2 bearing structure under maximum loading condition .................. 33

Figure 43: FOS of Link 3 under maximum loading condition .................................................................... 33

Figure 44: DYNAMIXEL MX-64AT motor [17] ....................................................................................... 35

Figure 45: OpenCM 9.04a microcontroller [18] ......................................................................................... 35

Figure 46: OpenCM 485 expansion board [19] .......................................................................................... 35

Figure 47: Test bench frame ....................................................................................................................... 37

Figure 48: Elbow connection plate (backside) for test bench ..................................................................... 37

Figure 49: Test clamps for securing elbow plates ....................................................................................... 37

Figure 50: Final arm and demonstration frame assembly ........................................................................... 38

Figure 51: Harmonic Drive FHA-11C drawings [13] ................................................................................. 40

Figure 52: SolidWorks CAD of full-strength design .................................................................................. 41

Figure 53: SolidWorks CAD of full-strength elbow adapter ...................................................................... 41

Figure 54: SolidWorks CAD of full-strength U-frame ............................................................................... 42

Figure 55: SolidWorks CAD of full-strength Link 2 bracket ..................................................................... 43

Figure 56: SolidWorks CAD of full-strength Roll 2 adapter ...................................................................... 43

Figure 57: SolidWorks CAD of full-strength Link 3 beam and Roll 2 interface ........................................ 44

Figure 58: Stresses in full-scale U-frame under maximum loading condition ........................................... 45

Figure 59: Stresses in full-scale Link 2 L-bracket under maximum loading condition .............................. 45

Figure 60: Stresses in full-scale Link 3 assembly under maximum loading condition ............................... 46

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List of Tables

Table 1 : NASA Valkyrie specifications ....................................................................................................... 4

Table 2: Pairwise comparison for desired objectives of forearm designs ................................................... 11

Table 3: Weighted design matrix for forearm mechanism concepts ........................................................... 12

Table 4: Denavit-Hartenberg parameters of two robotic arm types ............................................................ 16

Table 5: DH parameters of reduced complexity models ............................................................................. 18

Table 6: Final link lengths of new arm design ............................................................................................ 20

Table 7: Gamut volume measurement comparison ..................................................................................... 21

Table 8: Specifications for DYNAMIXEL motors [16] [17] ...................................................................... 34

Table 9: Selected specifications for Harmonic Drive FHA-11C-100-US200-E [13] ................................. 40

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List of Abbreviations

CAD Computer Aided Design

COTS Commercial Off-The-Shelf

DOF Degree(s) Of Freedom

FEA Finite Element Analysis

FOS Factor Of Safety

NASA National Aeronautics and Space Administration

RIVeR Robotics and Intelligent Vehicles Research

RPR Roll-Pitch-Roll

RPY Roll-Pitch-Yaw

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1 Acknowledgments

This research was carried out with support from Northeastern University’s RIVeR (Robotics and Intelligent

Vehicles Research) Laboratory, under Professor Taskin Padir. Professor Rifat Sipahi has provided valuable

feedback on this research and helped guide its development. The team would also like to thank Professor

Samuel Felton for his design feedback, and Murphy Wonsick for her valuable input regarding the current

arm and the needs of the Valkyrie team.

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2 Introduction

2.1 Problem Statement

The goal of the project is to design a forearm and wrist mechanism for NASA Valkyrie which can interface

with the Yale OpenHand, improve glovebox performance over the existing design, and simplify the

integration of future end effectors.

2.2 Project Motivation

NASA R5 “Valkyrie” is a humanoid robot designed by Johnson Space Center for the 2013 DARPA

Robotics Challenge. Valkyrie was awarded on loan to the Northeastern University Robotics and Intelligent

Vehicles Research (RIVeR) Laboratory for research starting in 2016. The RIVeR Lab is interested in testing

Valkyrie’s performance in a constrained environment to determine the viability of using an independent

robot to manipulate items in a glovebox. Currently, Valkyrie’s arms and hands are designed to be humanoid.

As shown in Figure 1, the forearm offers 3 degrees of freedom in the form of roll, pitch, and yaw, and is

tightly integrated with the humanoid hand as a single assembly

Figure 1: Definitions of roll, pitch, and yaw rotation on Valkyrie forearm

While the design accurately models a human forearm and hand, the constraints of a glovebox environment

present challenges to the operation of the forearm. The current forearm does not permit integration of other

end effectors, limiting what testing can be done with the platform. This restriction is particularly challenging

given RIVeR’s desire to test the Yale OpenHand Model O, shown in Figure 2, an open source robotic hand

designed jointly by iRobot, Harvard, and Yale. This manipulator may be a suitable candidate for use in a

glovebox environment.

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Figure 2: Yale OpenHand Model O

Additionally, the glovebox environment greatly restricts the motions that Valkyrie can achieve. To combat

these constraints and grant the RIVeR team the ability to test with other end effectors, a new, glovebox-

optimized forearm was proposed. The redesigned forearm is focused on enhancing the abilities of Valkyrie

when performing tasks in the constraints of a glovebox. Discussions with the RIVeR lab noted that a

humanoid design is not essential for this application, so the redesign breaks from humanoid convention to

allow an optimized range of motion within a glovebox. The dovetail mating design used by the Yale Hand

serves as a standard interface for connecting to an end effector. The operator could then equip Valkyrie

with any desired end effector so long as it consisted of the dovetail mate. Thus, the redesigned forearm

meets RIVeR’s requirements of a Yale Hand-compatible forearm with improved glovebox mobility, while

providing RIVeR with a powerful platform for testing future end effectors in a glovebox or other

constrained scenarios.

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3 Background Research

3.1 Valkyrie Forearm Background

NASA Valkyrie is a 44 degree of freedom (DOF) robot, standing 6’2” (1.88 m) tall and weighing 300 lbs

(136 kg). All movement is powered by fully-electric actuation throughout the body. [1] The current forearm

and hand assembly, shown in Figure 2, measures 135 mm in diameter just below the elbow connection

(while contained in its casing). From this location to the existing wrist joint, the total length is approximately

230 mm. It offers ±17° of motion in pitch and yaw, and ±180° in roll. In total, the current forearm and end

effector weigh 3.76 kg. A collection of these specifications is given in Table 1.

Table 1 : NASA Valkyrie specifications

Height (m) 1.88

Weight (kg) 136

Degrees of Freedom 44

Maximum Forearm Diameter (mm) 135

Forearm Length (mm) 230

Range of Motion (Pitch/Yaw) ±17°

Range of Motion (Roll) ±180°

Weight (kg) 3.76

Figure 3: Close-up of Valkyrie’s current humanoid end effector

3.2 Robotic Wrist Mechanisms

Several 3-DOF solutions for robotic wrists exist throughout the robotics and medical industry. Each solution

has advantages and disadvantages that must be considered in relation to their benefit to the Valkyrie forearm

redesign. For this purpose, roll, pitch, and yaw are defined as depicted in Figure 4:

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Figure 4: Definition of roll, pitch, and yaw on model forearm

One of the most commonly used solutions is the roll-pitch-roll (RPR) joint, also known as a “spherical

wrist”, shown in Figure 5, where rotary actuators drive separate pitch and roll axes of motion. [2] This

configuration provides a high range of motion in a robust design, desirable for many industrial applications.

However, RPR mechanisms suffer from a control singularity under inverse kinematics, which occurs when

the two roll axes are aligned. When this event occurs, the velocity of the roll actuators approaches infinity

for a finite velocity movement of the end effector. Additionally, since the two roll joints may counteract

each other, an infinite number of joint configurations are possible to achieve the same endpoint position.

[3] This is a software control issue that the arm must mitigate by limiting joint speeds and accounting for

singularity points during movements.

Figure 5: Roll-Pitch-Roll robotic wrist mechanism [4]

Another common wrist mechanism is the rotary-actuated roll-pitch-yaw (RPY) wrist depicted in Figure 6.

In an RPY wrist, the roll, pitch, and yaw axes are controlled by independent actuators, all of which are

offset from one another. This configuration provides a range of motion and robust configuration similar to

the RPR wrist. Additionally, RPY wrists do not suffer from the kinematic singularity problem with the

wrist fully extended, though they may suffer from singularities as pitch or yaw angle approaches 90 degrees.

[5]

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Figure 6: Roll-Pitch-Yaw robotic wrist [6]

Additional mechanisms do exist to achieve 3-DOF capabilities beyond the RPR and RPY systems

previously mentioned. Valkyrie currently uses a form of roll-pitch-yaw wrist utilizing a two-axis gimbal,

due to its close approximation of the behavior of a human wrist. Its roll is powered by a rotary actuator near

the elbow, like a human arm, while pitch and yaw are provided at a universal joint by a series of linear

actuators. This gimbal mechanism is commonly used in high-precision micromanipulators, where accuracy

of movement is more important than a wide range of motion. [7] Figure 7 depicts a universal joint similar

in layout to the actuated joint in Valkyrie’s wrist.

Figure 7: Universal joint, simplified model of Valkyrie wrist mechanism [8]

A fourth wrist mechanism that was investigated was the parallel manipulator depicted in Figure 8. For a 3-

DOF wrist, the manipulator consists of three linear actuators arranged in a helical pattern, with a central

linkage with ball joints on either end connecting the forearm to the end effector. The helical pattern allows

for control of the wrist in roll, pitch, and yaw from the same point. Such a mechanism is extremely stiff and

mechanically simple. [9] However, these mechanisms also experience nonlinear behavior, and have a

limited range of motion when compared to RPR or rotary RPY wrists.

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Figure 8: Parallel manipulator robotic wrist with 3-DOF [9]

3.3 Glovebox Constraints

The challenges and requirements of working within a glovebox environment are a complex topic that has

been extensively researched in recent years. For human operators, the most common issues involve their

ability to move their arms in the access portals, where their movement is limited. Additionally, operators

must wear thick, shielded gloves over their existing personal protective equipment. This restriction greatly

reduces both range and precision of movements inside the glovebox, as well as significantly limiting tactile

feedback, and thus operator capability [10].

Figure 9: Representation of glovebox manipulation task. Top left, Valkyrie robot interacting with objects in

prototype glovebox environment. Top right, actual gloveboxes in use in nuclear facilities. Bottom, dynamic

simulation of Valkyrie interacting with glovebox model, with support polygon outline [11]

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Operating Valkyrie in a glovebox environment presents a safer alternative to a human operator. This is

particularly applicable in processes such as decommissioning a contaminated glovebox, or otherwise

handing hazardous materials without needing a robotic system in each glovebox. Tasks performed during

decommissioning primarily consist of moving debris and objects from the glovebox workspace to an exit

port. These objects come in a variety of sizes and weigh up to 2 kg. Valkyrie is being used as a technology

demonstrator for future humanoid robots that could replace human operators working in contaminated

glovebox environments, by using the same ports and human-oriented tools. [11]

In other installations researched, 6-DOF robotic arms completely replace human operators within

gloveboxes. [12] The arms must reside entirely internal to the structure, operating from an overhead gantry

system to maximize range of motion and load bearing capability. However, this limitation makes them

unsuitable for certain tasks, such as decommissioning contaminated gloveboxes, as well as restricting a

single arm per glovebox whereas a humanoid robot can move from one glovebox to another.

A standard glovebox has 22 cm holes for the user to insert their hands into the 130 cm by 80 cm workspace.

The current forearm and wrist designs limit the movement of Valkyrie’s hand to the ±17º of rotation the

current wrist allows. Thus, the new forearm is to be designed such that it maximizes the accessible region

of the glovebox. Valkyrie must also manipulate objects on the floor of the glovebox without having to

remove its arms from the gloves of the glovebox environment.

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4 Specifications and Constraints

4.1 Existing Forearm Capabilities and Requirements

The existing forearm features 3-DOF, with its pitch and yaw being provided by a universal joint mechanism

and roll by a single motor mounted at the base of the arm. The universal joint can move within a ±17º range,

and the base motor a 360º range. The assembly weighs 3.76 kg total, which includes the cosmetic casings

attached to the arm. The Yale OpenHand weighs 716.3g and contributes to the mass budget for the separate

forearm. The maximum diameter of the existing forearm is 135 mm, measured at its base. RIVeR preferred

that any revision or redesign of this forearm remain within the same physical envelope of the original. The

overall length of the forearm was free to be adjusted as needed to allow a revision to reach additional

positions.

4.2 Requirements of the Redesigned Forearm

The redesigned forearm must primarily serve as a method to test the Yale Hand with the Valkyrie robot,

and thus must feature the Yale Hand as one possible end effector. The revision must not compromise the

range of motion of the forearm, with the intent being an improvement. Payload capacity should be such that

the arm can handle arbitrary objects within a glovebox environment, which, as discussed in background

research, sets a target of 2 kg or greater. Valid designs also must be able to handle the stall torque of

associated motors, with mitigations in place to avoid failures. The design should not exceed the 135 mm

diameter at any point to avoid interference with other parts of Valkyrie.

The forearm’s control system will not directly integrate into the current Valkyrie control hardware to protect

electronic systems onboard. Thus, the redesigned forearm features its own control solution and power

delivery system. Associated electronics are to be mounted on Valkyrie’s back and power and data

connections will run along Valkyrie’s arm. The connection to Valkyrie at the elbow will be a solely

mechanical one, rather than one incorporating data and power. The control software should, at a minimum,

allow for manipulation of the forearm to demonstrate improved movement and functional integration with

the Yale Hand.

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5 Initial Design Concepts

5.1 Actuator Layout Concepts

The development of forearm designs is focused on improving the forearm’s “gamut,” the range of possible

positions to which the forearm can be set. This expansion must be achieved while preserving the three

existing degrees of freedom. Since Valkyrie’s forearms are not required to retain a humanoid form, the

gimbal joint was removed in favor of an alternative design. The existing gimbal joint has limited pitch and

yaw rotation due to its linear actuation. Separating the pitch and yaw rotation to two distinct joint locations

enables a greater range of motion, specifically when Valkyrie’s elbows are constrained, as is the case when

working in a glovebox.

Once the concept of a non-humanoid forearm for Valkyrie was established, various designs were drafted

which preserve the three degrees of freedom. The four designs included a rotary roll-pitch-roll (RPR)

system, a purely rotary-actuated roll-pitch-yaw (RPY) design, a purely linear-actuated parallel manipulator

RPY design, and a hybrid RPY gimbal system (mimicking Valkyrie’s current design). These four designs

are illustrated in Figure 10.

Figure 10: Four initial sketches of 3-DOF arms for Valkyrie redesign, including pure linear and pure rotary

RPY, combination linear/rotary RPY, and rotary RPR

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The RPR concept offers a wide range of motion and high strength. The presence of two roll joints offers a

potentially shorter length, ultimately driving down the weight. The rotary actuator also provides a constant

torque across the entire range of motion. This design, however, presents a control challenge from a wrist

singularity where the two roll axes align, as discussed in Section 3.2.

The combined RPY (gimbal) design reflects the current design which exists on the Valkyrie forearm. Aside

from a familiar design and corresponding simplicity in control, this design is also very compact. However,

the wrist mechanism offers a limited range of motion. Due to the linear design, the effective strength of the

actuators would also decrease at the limits of its range of motion.

The purely rotary RPY design is very similar to the RPR design, with the capability for a wide range of

motion and providing a constant torque throughout that range of motion. The use of independent actuators

for pitch, yaw, and roll axes simplifies the overall design. The downside to this layout is the limited ability

to control the orientation of the robotic hand at any given endpoint, due to a finite number of possible

configurations.

The purely linear RPY design was briefly considered after initial research demonstrated its compact and

mechanically uncomplicated design. The concept, however, severely limits the potential range of motion

in pitch, yaw, and roll. The use of linear actuators additionally decreases the effective actuator strength at

the edges of the arm’s range of motion.

5.2 Comparison of Initial Designs

5.2.1 Pairwise Comparison

A pairwise comparison was created to narrow down each of the four contending designs into one potential

solution, displayed in Table 2 below. The objectives which were cross-evaluated for the comparison chart

were weight, strength, range of motion, simplicity, control, and cost.

Table 2: Pairwise comparison for desired objectives of forearm designs

Objectives Weight Strength Range of Motion Simplicity Control Cost Total Normalized

Weight X 1 1 1 1 1 5 0.333

Strength 0 X 0 1 0 1 2 0.133

Range of Motion 0 1 X 1 1 1 4 0.267

Simplicity 0 0 0 X 0 1 1 0.067

Control 0 1 0 1 X 1 3 0.200

Cost 0 0 0 0 0 X 0 0.000

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The outcomes of Table 2 display a high priority in limiting the weight of the full-strength forearm, as well

as increasing the range of motion. These points align with the design constraints and objectives given at the

start of the project. Primarily, the new mass of the forearm should not exceed the existing arm’s mass. This

constraint was cross-checked, as the addition of multiple rotary actuators into the forearm component will

quickly increase the weight of some of the designs. Increased range of motion is also a primary reason for

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the forearm redesign. Control of the forearm was treated as a necessary component of the new design for

the arm to be quickly integrated as a solution. The strength of the arm will also need to be verified during

the design process, specifically for the full-strength arm.

The full-strength design specification is for Valkyrie to lift a maximum of 2 kg mass with its arm, and the

design must be strong enough to carry the load and withstand its own weight. A minor consideration for

the forearm was the simplicity of the design. This variable differs from the control of the design, as control

relates to the ease of object manipulation by the arm, while simplicity implies the ease of fabrication. The

importance of design simplicity was minimized since each of the designs represent very similar levels of

complexity and will be attainable regardless of the final solution. While cost was also factored into this

pairwise comparison, it ranked the lowest due to available funds for the project as well as the need to design

to all other specifications first.

Each of the four previously stated designs was ranked according to the listed design objectives and given a

1-4 rating based on the team’s ranking of each design. A rating of “4” corresponded to a design which fit

the design criterion best, while a rating of “1” was given for a design which would poorly address the

objective. These rankings were multiplied by the normalized value of each objective’s total value from

Table 2, and used to produce the weighted design matrix of Table 3 below.

Table 3: Weighted design matrix for forearm mechanism concepts

Design Weight Strength Range of Motion Simplicity Control Cost Total

Weighting 0.333 0.133 0.267 0.067 0.200 0.000 -----

Linear-Rotary Actuated RPY 1.000 0.266 0.534 0.067 0.600 0.000 2.467

Pure Linear Actuated RPY 1.000 0.266 0.267 0.268 0.600 0.000 2.401

Pure Rotary Actuated RPY 0.333 0.532 1.068 0.134 0.800 0.000 2.867

Rotary Actuated RPR 0.667 0.532 1.068 0.201 0.400 0.000 2.867

Based on the results of Table 3, the best designs were the pure rotary-actuated RPY design and the rotary-

actuated RPR design.

5.2.2 Conclusions and RPR Selection

Since pure rotary RPY and rotary actuated RPR received equal weighted scores during the selection

process, additional factors were taken into consideration to inform selection of the final design. Under the

glovebox use conditions, only the roll-pitch-roll design would run the risk of encountering singularity

issues. However, after consulting with Professor Padir, it was determined singularity issues could be

eliminated by the control software.

The RPR design offers a key benefit in that the end effector may be rolled into an infinite number of

orientations within ±180° at a given endpoint, while the RPY has a finite number of orientations, often only

one, for that same point. This allows the hand to achieve a more favorable gripping orientation, increasing

its effectiveness.

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With this information, rotary-actuated RPR was chosen as the design for the forearm. A sketched

comparison of the current Valkyrie forearm and an early concept of the RPR configuration in the context

of Valkyrie, shown in Figure 11, was drawn to inform the design moving forward.

Figure 11: RPR wrist design in comparison to existing Valkyrie wrist

5.3 Initial Actuator Selection and Constraints

Actuators were chosen based on their ability to output the necessary torque while retaining a compact

form and low weight. Harmonic Drive advertises motors featuring “limited backlash, high torque output,

a compact size, as well as positional accuracy and reliability,” [13] granted by its strain wave mechanism.

These motors provide the torque density necessary to stay within the target forearm mass budget of 3 kg,

while still providing over 10 Nm of continuous operation torque. Additionally, these motors are already in

use throughout the entirety of the Valkyrie system.

The FHA-11C-100-US200-E was selected as the specific Harmonic Drive Actuator for use on the new

forearm. These motors provide 11 Nm of torque, weigh 0.62 kg, and feature advanced electronics

allowing various control modes. The motors also offer options for hollow shafts for ease of electrical

passthrough. [13] Three of these actuators were required for the design and would cost $2006 each.

During the actuator selection phase, costs and lead times became limiting factors of the design. The

budget would not allow for the purchase of three expensive Harmonic Drive actuators in addition to all

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the other costs associated with manufacturing the forearm. In addition, an unprecedented demand for

Harmonic Drive actuators had increased lead times on motors to roughly six months optimistically.

Receiving the motors so late in the project would not allow sufficient time for assembly and development

of controls. Any alternatives to the Harmonic Drive actuators either demanded similarly prohibitive costs

or failed to meet the mass and torque density requirements.

5.4 Scope Adjustments and Solutions

An additional request from RIVeR was to prove that the design demonstrated an increased gamut over the

existing Valkyrie forearm before the motors were purchased. From this point onward, the original design

split into two distinct segments: one serving as the proof of concept, the “gamut demonstrator,” and the

other showcasing and defining the path toward the initially envisioned design, the “full-strength design.”

As the gamut demonstrator’s sole objective was to demonstrate the increased gamut over the existing

forearm, several constraints could be relaxed. Primarily, the payload weight requirements were relaxed

such that less than a kilogram of weight is satisfactory. The gamut demonstrator would only need to move

the Yale Hand and light loads, which allows for the use of lower torque motors. These low torque motors

would allow the demonstration of the movement, while significantly reducing the overall cost compared

to an arm using Harmonic Drives. The lower loads additionally allow for other materials to be used in the

construction of the forearm, such as 3D printed materials, further reducing costs.

In parallel with this, a design was made using the Harmonic Drives as the selected actuators, designed to

handle the full target loads set in the original specifications. This design serves to demonstrate a version

of the redesigned forearm capable of lifting payload weighting 2 kg. Splitting the design allows for the

gamut demonstrator to prove the design in a low-cost solution, while simultaneously readying costlier

version for manufacturing and assembly if RIVeR desires its capabilities.

15

6 Mathematical Modeling and Optimization

6.1 Denavit-Hartenberg Parameters

To establish that the new forearm improves gamut over the current design, it was necessary to develop a

method of quantitatively analyzing them. First, the joint mechanics of each arm were described using

Denavit-Hartenberg (DH) parameters. In a kinematic chain, or series of linked joints, these parameters

describe the coordinate transformation from one joint to the next, as well as each joint’s axes of freedom.

The four parameters, a, α, d, and θ, are traditionally defined as shown in Figure 12.

Figure 12: Traditional definitions of Denavit-Hartenberg parameters [14]

Using this convention, the RPY and RPR configurations under consideration can be represented as in Figure

13 and Figure 14, respectively. The current arm design with a gimbaled wrist can be represented by the

RPY model with L2 = 0. Table 4 presents the resulting DH parameters.

Figure 13: Model representation of RPY robotic arm

16

Figure 14: Model representation of RPR robotic arm

Table 4: Denavit-Hartenberg parameters of two robotic arm types

RPY RPR / Current

a α d θ a α d θ

0 -π/2 L1 θ1 0 -π/2 L1 θ1

L2 -π/2 0 θ2 0 π/2 0 θ2

L3 π/2 0 θ3 0 0 L2 θ3

With these parameters determined, the arms were then modeled in MATLAB as rigid body trees. Using

forward kinematics, the 4x4 transformation matrix from the arm’s base to its end effector was computed

for a given set of joint angle values. This enables computation of the position of all links in the arms, as

depicted in Figure 15.

Figure 15: Visualization of arms inside glovebox

17

In analyzing the motion of these arms, it is also necessary to model the degrees of freedom afforded by the

rest of Valkyrie’s body. While full-body modeling software has been developed for Valkyrie and would

potentially be of use, the modeling of only forearms represents a significantly reduced scope, lending value

in its relative computational simplicity. [11] Instead of modeling all the rest of Valkyrie’s complex joints

in a physically accurate manner, it is possible to represent these as simple 6-DOF motions of the base of

the forearm, as depicted in Figure 16.

Figure 16: Additional DOF afforded by Valkyrie body

6.2 Gamut Computation and Optimization

Given these additional motions, the number of degrees of freedom grows to 9 for each arm design. It was

then necessary to formally define “gamut” and develop a method for evaluating it. By iterating through all

possible position values of all joints in an arm, and at each configuration checking whether any collisions

are occurring with the glovebox collars or walls, the software could generate a point cloud of resulting end

effector positions. A sample of this data is presented in Figure 17. The software could then determine the

convex hull of these points, depicted in Figure 18, and calculate its volume.

Figure 17: Sample point cloud of RPR arm

Figure 18: Convex hull of point cloud

This volume effectively serves as a quantitative measure of the arm’s gamut. The large number of DOF of

these models, however, results in very high computational complexity, increasing the time necessary to

compute a given end effector transformation since each requires 9 matrix multiplication operations. This

18

method increases the number of possible joint configurations as well. It is therefore desirable to reduce this

complexity to improve the efficiency of the model. In this scenario of glovebox operation, it can be shown

that the gamut envelopes are axisymmetric about the collar axis. Therefore, the models can be reduced to a

single plane, as depicted in Figure 19. Base motion up and down has been ignored for simplicity. These

results can be swept about the axis to compute the full cloud. This simplification reduces the quantity of

the models’ freedoms to 3, and results in the same set of DH parameters for all arm models, as shown in

Table 5.

Figure 19: Reduced complexity model of RPR / RPY / current arm

Table 5: DH parameters of reduced complexity models

a α d θ Joint Type

0 π/2 0 π Fixed

0 π/2 x1 -π/2 Prismatic

0 π/2 0 θ1 Revolute

0 -π/2 0 π/2 Fixed

0 -π/2 L1 -π/2 Fixed

0 π/2 0 θ2 Revolute

0 0 L2 0 Fixed

0 0 L3 0 Fixed

Now that the software could compute the gamut for a given arm configuration, the next step was to

determine the optimal arm design. An RPR layout had already been selected as described in Section 5.2, so

the remaining decisions were the lengths of the 3 links. It is important to note, however, that L2 and L3 do

not individually change the gamut envelope; rather, their sum does. Because of this relationship, L2 was

held at its minimum length to keep actuator 3 as close to the pitch joint as possible, minimizing exerted

moment due to gravity. Therefore, the 2 parameters for optimization are L1 and L3. As depicted in Figure

20, these lengths were constrained by the induced torque on actuators 1 and 2, which could not exceed the

operating torque, or approximately 20% of stall torque. For all of these gamut calculations, it was assumed

no payload was present.

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Figure 20: Torques induced by arm component masses

The length of link 1, however, does not impact the torque experienced by the actuators; rather, an increase

in this length would result solely in higher structural loads. The resulting stresses are difficult to model

analytically given the complex geometry of the design, so this constraint was ignored during the

optimization and will be discussed shortly.

The objective for this optimization was to maximize gamut. The gamut computation was slightly modified,

however: since objects inside the glovebox are stacked from the floor upwards, volume coverage at the

floor of the glovebox tends to be more valuable than coverage at the ceiling. Therefore, the software applied

a weight according to a decreasing cosine function, as depicted in Figure 21. This function enabled the

computation of a weighted gamut volume for use in optimization.

Figure 21: Gamut volume weight function for optimization

20

The software then optimized the model using a brute force technique by iterating through all possible

combinations of link lengths, checking at each point whether the combination was valid given the

constraints, and, if so, computing the gamut. This technique yielded a field of gamut values, from which

the highest result could be picked. To improve the computation and keep complexity low, the optimization

was run at a low resolution of 10. Then the parameter field was trimmed down around the optimal point,

and the optimization rerun. This process was repeated for a total of 3 iterations, yielding the data depicted

in Figure 22.

Figure 22: Optimization surface of RPR model

6.3 Conclusions on Optimal Link Lengths

It is evident from this result that the gamut of the arm improves as the length of either link is increased.

While a 1.5m arm is impractically long, this data demonstrates that the arm should be designed as long as

the motor torques and structural strength will allow. The link lengths were therefore selected based on an

iterative approach using finite element analysis, aiming for a safety factor of 1.3 as described in Section

7.2. Table 6 presents these values.

Table 6: Final link lengths of new arm design

Link Length (cm)

Link 1 22.5

Link 2 8.0

Link 3 17.5

21

Finally, this gamut modeling technique could be applied to the current and new arms. This analysis resulted

in the data presented in Table 7, and the gamut envelopes visualized in Figure 23 and Figure 24. It is evident

from this data that the team’s new design improves the arm’s glovebox gamut by approximately a factor of

10.3.

Table 7: Gamut volume measurement comparison

Model Gamut Volume (cm3)

Current Arm 1.12 ∙ 104

RPR 11.5 ∙ 104

Figure 23: Visualization of gamut envelope of

Valkyrie current forearms

Figure 24: Visualization of gamut envelope of RPR

forearms

22

7 Gamut Demonstrator

7.1 Actuator Selection

The gamut demonstrator will not need to bear the full payload of 2 kg, so the actuators have much lower

torque requirements than the fully capable design – the gamut demonstrator needs only to move itself. This

requirement relaxation, however, comes with an increased emphasis on motor availability and low cost.

ROBOTIS offers a line of hobbyist motors that suit the needs of the gamut demonstrator. To save money

and weight, the second roll actuator could be a weaker actuator than the one chosen for the first roll and

pitch actuators, as the required torque and applied bending moments would be much lower. The

DYNAMIXEL MX-28AT and the DYNAMIXEL MX-64AT were selected as they will supply the required

torque. When using a 12 V power supply, the MX-28AT provides a stall torque of 2.5 Nm and the MX-

64AT provides 6 Nm. These motors can move the unladen forearm based on the mass simulations of the

CAD modeling. ROBOTIS recommends that for stable motion, both motors should be run to a maximum

of 20% of their stall torque.

7.2 Gamut Demonstrator Mechanical Design

Based on the above design matrix, both the RPR and RPY configurations displayed similar promise as

candidates for a forearm layout. However, as described in Section 5.2.2, the RPR configuration was

ultimately selected for its better ability to control hand orientation at a given endpoint. This configuration

will allow Valkyrie to use the Yale Hand to manipulate items on the floor of a glovebox, regardless of the

object’s profile. Since strength is not a primary constraint on the gamut demonstrator, lighter and less

powerful motors were used as a cost savings measure.

After the selection of MX-64AT and MX-28AT servomotors, the mechanical design of the forearm was

developed. From a high-level standpoint, the forearm needs a mechanical interface for connecting to

Valkyrie, three actuators, and structural segments to link the actuators together. Additionally, the roll

actuators experience a large bending moment due to the mass of the arm and its payload. Since the bending

load capacity is not specified by the manufacturer and testing this limit would have been destructive, costly,

and impractical, a judgement was made to design bearing structures which bear the moment, shear, and

axial loads, transmitting only torque to the actuators. The final structure of the gamut demonstrator arm

now consists of an adapter between Valkyrie’s standard elbow interface, a bearing support structure for the

first roll actuator, a frame connecting the roll and pitch joints, a second frame connecting the pitch and

second roll joints as well as a second bearing support structure. Attached to that structure is a third link

connecting the second roll joint to the Yale Hand at a standard interface. This entire layout is shown in

Figure 25 below.

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Figure 25: SolidWorks CAD of final gamut demonstrator design

The arm interfaces with Valkyrie at the existing elbow mount. A cutout was added to the standard adapter

plate to fit the MX-64AT motor, while two-hole patterns allow the attachment of the motor and bearing

structure. The elbow plate connects to the existing elbow plate on Valkyrie’s upper arm via a two-flange

clamp. Due to unavoidable complexities of creating the 5-axis interface, the adapter plate was outsourced

for machining out of 6061 aluminum. The side of the model which interfaces with the first motor is shown

in Figure 26 below.

Figure 26: Isometric view of elbow interface adapter

Immediately following the elbow connection, the first actuator is mounted to provide roll for the entire

forearm. This motor is located as close as possible to the elbow connection to minimize its exerted moment

on the elbow. When the arm is working within a glovebox environment, this actuator can also operate fully

while located within the glovebox’s collars.

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Attached to the elbow adapter is the first roll actuator and surrounding bearing structure. The bearing

structure acts as a housing for the roll actuator as well as for a pair of bearings and the roll output shaft. It

consists of a machined outer housing acting as the load path between the shaft and the elbow adapter, the

roll output shaft, and thrust and crossed roller bearings held in place by a faceplate, as shown in Figure 27.

Figure 27: Cross-sectional view of Roll 1 bearing structure

The thrust bearing is used in conjunction with the crossed roller bearing, both with a 30 mm inner diameter

and sourced from McMaster. The thrust bearing bears compressive axial and moment loads, as transmitted

by the flange on the shaft. The crossed-roller bearing bears tensile axial, shear, and moment loads through

the body and flange of the shaft. The bearings are slightly preloaded onto the output shaft through the

faceplate to prevent radial or axial movement. The preload primarily compresses a Delrin gasket between

the top of the bearing housing and the bottom edge of the faceplate. This allows the assembly to

automatically adjust for manufacturing tolerances in the sensitive bearing stackup. The shaft, bearing

housing, and faceplate are all machined out of Al 6061. All components of the Roll 1 bearing structure are

shown in an exploded view in Figure 28.

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Figure 28: Exploded view of Roll 1 bearing structure

Attached to the end of the first roll output shaft is the pitch joint structure. This section of the forearm

consists of a U-frame, pitch actuator, and the pitch frame, as shown in Figure 29. The U-frame profile,

shown in Figure 30, was chosen to optimize the strength-to-weight ratio of the segment while also allowing

ease of interface with the roll output and housing the pitch motor. The flat profile of the frame provides

multiple surfaces for mounting points for both motors and an external plastic casing. Its length was

determined by the optimization described in Section 6.

The end of the U-frame opposite the Roll 1 connection holds the pitch actuator, with an extended cutout to

allow for clearance of both sides of the motor’s rotation. A commercial off-the-shelf (COTS) bracket from

DYNAMIXEL serves as the interface between the pitch actuator and the remainder of the forearm. To keep

the pitch bracket from interfering with the U-frame during its full range of motion, the profile of the U-

frame was minimized as much as possible. To accommodate these thinner walls while carrying the full

weight of the arm and actuator torque, the U-frame was milled out of Al 7075. This decision was made

following extensive FEA which demonstrated the necessity of a stronger aluminum alloy. Once this

material was chosen, the side walls of the U-Frame were locally shortened at the actuator to allow for easier

installation of the mounting screws into the flat base, and to enable the use of a smaller end mill to achieve

the necessary base fillet.

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Figure 29: Pitch structure subassembly

Figure 30: U-frame labeled with fastener holes

As shown in Figure 30, the U-frame also includes tapped holes while function as mounting points for an

external housing. This 3D printed housing, shown in Figure 31, serves multiple purposes. It routes the

cables running from Roll 1 to the pitch motor and prevents wires from being caught in the rotating shaft. It

Connection to

housing underside

Connection to

housing topside

Connection to

Pitch motor

27

also acts as a safety feature, preventing the operator’s fingers or other objects from accidentally getting

caught in the closing gap between the U-Frame and the pitch bracket. Finally, it improves the cosmetic

appearance of the arm.

Figure 31: Two exploded halves of Pitch housing

Directly mounted to the pitch frame is a mounting plate, designed to connect Roll 2 and its bearing structure

to the pitch frame. As discussed in Section 6.3, the gamut is driven not solely by the link 2 length, but by

the sum length of link 2 and link 3. Therefore, the link 2 length was minimized to reduce the moment

exerted by its mass on the upstream structure. Thus, the mounting plate was reduced to the minimum

structurally acceptable thickness while supporting connection points for all necessary screw holes, as shown

in Figure 32.

Figure 32: Mounting plate CAD

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Secured to other side of the mounting plate is the Roll 2 actuator and its surrounding bearing structure,

similar to that employed in Roll 1. This design uses a smaller shaft and bearings compared to Roll 1, and

its shaft directly interfaces with the final length segment of the arm. The cylindrical face of this shaft

contains cross-drilled threaded holes to interface with the wrist beam, which slides over the outer diameter

of the shaft. The Roll 2 bearing, shaft, and faceplate shown in Figure 33 will all be machined from 6061

aluminum stock.

Figure 33: Roll 2 actuator and bearing structure

The wrist beam, which will extend from Roll 2, is a long segment of 3D printed ABS material. This

component was 3D printed to reduce its mass due to the minimal stresses it will experience. The length of

this segment was determined to optimize gamut, as described in Section 6. The final length of this 3D

printed component, shown in Figure 34, is 72mm. Four clearance holes in the wrist beam align with the

four tapped holes of the shaft from Figure 33.

Figure 34: Wrist beam component

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At the end of the 3D printed wrist beam is an interface for connecting the arm to the Yale Hand. The wrist

beam contains the male side of a dovetail interface, a sliding mechanism based on the hand’s published

open source design. This design was validated via SolidWorks FEA for manipulating payloads up to 3 kg,

therefore qualifying it for both the gamut demonstrator and full-strength design. The dovetail was widened

by 50% and thickened for rigidity. The female side of this interface was 3D printed to replace the existing

adapter plate on the bottom of the Yale Hand assembly. These plates can be exchanged via screws

connecting to the base of the Yale Hand. The presence of this dovetail design will serve as a standard

interface for any future versions of the Yale Hand or any other desired end effectors. The sliding mechanism

of this interface can be seen in Figure 35.

Figure 35: Sliding connection between wrist beam and Yale Hand adapter plate

The total mass of the final gamut demonstrator is 2.34 kg. This value includes the mass of the machined

and 3D printed segments, sourced hardware, fasteners, motors, the protective housing, and the 715 g mass

of the Yale Hand itself. The total mass of the current arm on Valkyrie, which was set as the maximum mass

of the gamut demonstrator, is 3.76 kg. The length of the arm from the elbow axis to the pitch axis is 160

mm; the length of the arm from the pitch joint to the palm of the Yale Hand is 250 mm. These values,

shown in Figure 36, are derived from the optimal segment lengths from Section 6.3. The Link 1 length

developed for the gamut demonstrator is less than the value presented in Table 6 to account for the distance

between Valkyrie’s elbow pitch actuator and the elbow plate. The final link dimensions result in a total

length of the gamut demonstrator forearm from the elbow plate to the palm of the Yale Hand of 410 mm

when fully extended.

Figure 36: Length parameters labeled on gamut demonstrator

30

7.3 Gamut Demonstrator Design Analysis

Following the CAD modeling, the strength-critical components of the assembly were identified as the U-

frame segment and the roll bearing structures. Consideration was given to thicken walls around holes and

add fillets to reduce stress concentrations during modeling, but ultimately the strength of all of the forearm’s

structural components was validated by a series of SolidWorks Finite Element Analyses. These analyses

simulate the worst-case scenario in a glovebox environment, which occurs if the arm is to collide with a

wall while actuating both Roll 1 and Pitch joints. In this scenario, the motors of the arm would be operating

at stall torque without moving the forearm, producing a bending moment in addition to the weight of the

arm and Yale Hand. The goal was for all components to have a factor of safety greater than 1.3 under this

maximum loading condition. Furthermore, software-set torque limits below the actual motor stall torques

ensure that this condition never occurs.

Due to the slow velocity of the forearm’s motions, dynamic loads were deemed insignificant in comparison

to the static loads. Additionally, fatigue failure was deemed low-risk due to the low cycle counts and lack

of high-speed rotating components, and vibrational failure was deemed low-risk due to the clean vibrational

environment in which the arm is operating.

The Roll 1 bearing structure, was designed to maximize strength and stiffness, while maintaining an axially

compact form factor. Thin walls were avoided wherever possible, and multiple fasteners were used to secure

the bearing structure together and to the elbow adapter to better distribute loads across a greater area of the

structure. As a result, the entire structure is made from Al 6061 due to its low cost, ease of manufacturing,

and high availability, except for the Delrin gasket. The primary points of concern through the design and

analysis process were the stress concentrations at the fasteners and the mounting holes in the elbow adapter

plate. As shown in Figure 38, there were no significant stress concentrations within the shaft interface with

the motor. It must be noted that the elbow adapter has been converted from a standard component already

successfully in use on Valkyrie and has only been slightly modified from that state. Therefore, it was

decided that it was acceptable to use the adapter plate as designed.

Figure 37: FOS of Roll 1 bearing structure under maximum loading condition

31

Figure 38: Cross-section FOS of Roll 1 elbow adapter under maximum loading condition

The U-frame’s design includes multiple holes for mounting to both the DYNAMIXEL MX-64AT pitch and

Roll 1 actuators. The hole patterns required for mounting these motors necessitate holes to be placed in the

U-frame near one another as well as to the model’s edges. Initial FEA on the component displayed stress

concentrations along the thin walls which could potentially cause the material to fail under the worst-case

loading. Altering the layout of the holes is not a possibility due to the required interface with the motors.

To resolve this issue, the material of the U-frame was changed from Al 6061 to 7075, which provides a

much higher yield strength at only 5% higher weight, with material cost being a significant downside. FEA

was then performed on the U-frame with this new material, and the minimum factor of safety was calculated

at 2.7, as shown in Figure 39.

Figure 39: FOS of U-frame under maximum loading condition

32

The frame connecting the pitch actuator to the Roll 2 bearing was the sole COTS structural component in

the forearm. Made of Al 5052, the frame is sold by ROBOTIS for the MX-64 series of motors. Under the

worst-case loading conditions, this component has a factor of safety of 1.32. The primary areas of concern

were the inside corners of the cable routing cutouts on the flanges, causing a greater stress concentration

than the fastener holes on the frame.

Figure 40: FOS of Pitch frame under maximum loading condition

The Roll 2 bearing structure was driven by similar space constraints to fit the bearings. Like the Roll 1

structure, the Roll 2 bearing structure utilizes Al 6061 in its construction, with a Delrin gasket. Additionally,

fasteners were arranged as to better distribute loads around the structure. Again, primary areas of concern

were the fasteners themselves and their mounting holes, but FEA shown in Figure 41 and Figure 42 shows

that the structure is capable of handling the maximum loading condition. The fasteners and bearings are

also operating well below their designed maximum loads and are not at risk of failing.

Figure 41: FOS of Roll 2 bearing structure under maximum loading condition

33

Figure 42: Cross-section FOS of Roll 2 bearing structure under maximum loading condition

The longest component of the arm assembly is the wrist beam connecting the second roll actuator to the

Yale Hand adapter plate. This component will experience the least bending moment from the weight of the

arm compared to any other segment. Therefore, this beam was 3D printed from ABS to reduce weight and

enable the dovetail connection to be printed directly onto one end. This component was also verified via

SolidWorks FEA to support the claim that the ABS material will be sufficient to support the load of the

Yale Hand and motor stall torque. This factor of safety analysis, shown by Figure 43, verifies that the wrist

beam will be able to withstand any loads seen by the gamut demonstrator arm.

Figure 43: FOS of Link 3 under maximum loading condition

A point to note regarding the FEA results of Figure 43 is the relative inaccuracy of FEA loading of 3D

printed components. The ABS material itself is anisotropic and will have varying strength in different

34

directions, especially across different print orientations. The layer height and infill density of the print will

also affect the yield strength for the beam and is not accounted for by the factor of safety plot above. Some

3D printer software provides data for approximate multiplication factors on the strength properties based

on varying print settings. For the wrist beam analysis, a 70%-part infill was assumed, which will reduce the

part’s yield strength by approximately 35%. A 0.15 mm layer thickness was also assumed, which would

reduce the part’s strength by an additional 9%. This results in the printed part having approximately 59%

of the strength of a solid component of the same material, which was accounted for in the simulation run.

[15] With these factors included in the analysis of the wrist beam, the factor of safety for the component

remains at approximately 2.5. Fortunately, changes to the design can be easily implemented and the

component reprinted if any failures occur.

7.4 Electrical Components and Controls

The gamut demonstrator features two DYNAMIXEL MX-64AT motors (as shown in Figure 44) which

drive the first roll joint and the pitch joint, while an MX-28AT drives the motion at the wrist joint. The

specifications for these motors is noted in Table 8: Specifications for DYNAMIXEL motors. These motors

feature enough torque to drive the demonstrator, while including features such as integrated networking

which help streamline control and cable management. The networking allows for the motors to be daisy-

chained together, and for commands and power to be sent to each motor though another motor. Both models

can be controlled by setting a positional angle or a speed, and offer feedback including torque, position,

error correction, and other parameters.

Table 8: Specifications for DYNAMIXEL motors [16] [17]

MX-64AT MX-28AT

Width (mm) 40.2 35.6

Length (mm) 61.1 50.6

Height (mm) 41.0 35.5

Weight (g) 135.0 77.0

Operational Voltage (V) 12.0 12.0

Stall Torque (Nm) 6.0 2.5

Stall Current (A) 4.1 1.4

Gear Ratio 200:1 193:1

35

Figure 44: DYNAMIXEL MX-64AT motor [17]

Control of the motors is managed by the OpenCM 9.04 servo controller, shown in Figure 45. This is an

Arduino-based controller that includes interfaces for the DYNAMIXEL motors, as well as libraries supplied

by Robotis. The controller can operate connected to an external computer or independently with a preset

routine. The controller manages signaling and control of motors in the mounted configuration and is

configured to prevent the motors from exerting torques that could damage the arm. Power distribution is

managed by the OpenCM 485 expansion board shown in Figure 46, which is an attachment for the OpenCM

9.04. The OpenCM 485 connects to a 12V power source, and supplies power to the motors and controller.

Basic test routines involve setting the OpenCM 9.04 to limit the velocity and acceleration of the motors

upon initialization, then move the motors to a variety of predetermined positions, thus demonstrating the

movement capabilities of the forearm. Additionally, the OpenCM supports command via Serial over USB,

allowing any RIVeR computer to send movement commands to the forearm, thus providing a capable

platform to test in arbitrary movement routines.

Figure 45: OpenCM 9.04a microcontroller [18]

Figure 46: OpenCM 485 expansion board [19]

36

7.5 Fabrication of Components

All components for the gamut demonstrator were modeled in SolidWorks. To verify that the arm assembly

in CAD could properly be assembled after manufacturing, the design went through several design reviews.

The team met with Professor Rifat Sipahi and later Professor Samuel Felton to review proper functionality

of the forearm and discuss any potential oversights, particularly regarding the bearing structures. Following

this meeting, the team met with Ben Macalister, a Northeastern machinist, to ensure that all components

were machinable. Both the elbow adapter plate and the interface plate required 5-axis machining

capabilities, an unavoidable result of interfacing with the existing hardware on Valkyrie’s elbow. These

components were therefore outsourced to Protolabs to ease workload at Northeastern’s machine shop.

Drawings were then generated for each of the machined components which would be manufactured on-site.

Critical tolerances were explicitly stated to ensure that all components would properly interface with each

other and with off-the-shelf components. Once these machined components were received, they were

inspected to verify that all features were properly formed and determine if any significant rework was

necessary.

In parallel to the manufacturing of aluminum components, components to be 3D printed were sent to

Rebecca Knepple. These included the wrist beam, the Yale Hand adapter plate, the two halves of the

protective pitch housing, and an exterior cover for the Yale Hand. The team also designed clamps to connect

mate the halves of the elbow adapter together, and a ring to simulate the port opening of a glovebox. These

components, not part of the final forearm, were also 3D printed.

Components sourced from online vendors were also modified to properly interface in the assembly.

Threaded rod was cut to length for the Yale Hand connection, as was 2 mm steel rod stock to be used as

interface pins with the roll shaft. The gasket for each of the bearing structures was laser cut out of Delrin.

7.6 Assembly of Gamut Demonstrator

Assembly of the complete forearm began with minor rework on several manufactured components. All

post-manufacturing work was done to correct minor errors in manufacturing, such as tapped holes in place

of clearance holes or straightening edges to a more precise cut. Additionally, holes in the wrist beam were

drilled out to enlarge the under-printed diameter.

Components of the arm were first assembled as subsystems, starting with the motor faces and bearing

structures. The thrust and roller bearings were inserted on opposite ends of the shaft while secured in place

by the bearing structure. Each shaft was connected to the output face of the roll motors, via screws for Roll1

and via the 2 mm pins for Roll 2. The Delrin gasket and faceplate were installed, applying a preload onto

the shafts to secure them in place while rotating freely. The bearings were then screwed to their respective

adapter plates and connected to each end of the U-frame. After filing an additional edge of the U-frame, the

pitch motor was installed and connected to the Roll 2 bearing structure via the COTS pitch bracket and the

mounting plate. The wrist beam was inserted over the second roll shaft and secured via screws, and the

Yale Hand was placed onto the wrist beam through the sliding dovetail interface and held in place by the

threaded rod.

DYNAMIXEL cables were used to connect each of the three motors in series. A longer 350 mm cable was

required for connecting the pitch actuator to the first roll actuator to ensure rotation of ±180° at the first roll

37

joint. The protective pitch housing was then placed over the U-frame and pitch actuator in two halves and

secured to the U-frame via fasteners. The cables were inserted through the case to prevent interference with

rotating components.

7.7 Test Bench Configuration

A test bench setup was designed and built to interface with the forearm at the elbow plate and secure it

during any possible motions. The frame was designed to ensure that the forearm would not tip the structure

over, regardless of the location of the arm’s center of gravity or which motors were being run. Rails of

80/20 were cut to length and assembled with corner brackets to create the layout shown in Figure 47 below.

The metal component attached to the top of the frame is the elbow plate, simulating the plate present on

Valkyrie’s elbow. This design, shown in Figure 48, was modified to interface directly with the frame.

Figure 47: Test bench frame

Figure 48: Elbow connection plate (backside) for

test bench

The adapter plate on the forearm was secured to the elbow connection plate of Figure 48 via the 3D printed

clamps. Heat-set inserts were added to the clamps, shown in Figure 49, which were also painted black to

match the color scheme of the rest of the frame and not appear part of the arm itself. The forearm can be

removed from the test bench setup by removing the two screws holding the clamps together. A photograph

of the final assembly is presented in Figure 50.

Figure 49: Test clamps for securing elbow plates

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Figure 50: Final arm and demonstration frame assembly

7.8 Testing

Testing the forearm was essential to demonstrating that the entire gamut could be reached with the new

assembly layout. Each of the three actuators were first powered through their intended range of motion:

±180° for each of the roll motors and ±90° for the pitch motor. Test code was developed in C++ to rotate

each of the three motors to any given angle. The speed and acceleration of the motors’ movements were

tuned to values which provided smooth motion at a stable angular velocity. The code was later modified

to allow for all three motors to reach the user-defined endpoint at the same time to provide smoother

motion.

After basic movement of the motors was implemented, more advanced loops were written to showcase

the use cases the forearm can perform. These controls were uploaded onto the OpenCM expansion board

and stored locally on the board. After cycling through the forearm’s complete range of motion, the arm

entered three distinct control cycles. One set of controls resembled Valkyrie’s ability to reach upper-back

corners. The second loop rotated the Yale Hand in a circular motion (both continuous and with step

increments) to simulate the hand rotating a wheel. The third motion loop moved the hand to the floor on

both sides of the elbow to demonstrate grasping objects resting on the glovebox floor. These motions

were combined into a two-minute looping demonstration sequence which can be activated anytime via the

switch on the OpenCM board.

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8 Full-Strength Design

8.1 Actuator Selection for Full-Strength Forearm

Motor selection was the driving factor for the selection of other standard components, particularly in the

case of the full-strength design. Research of these components occurred in parallel with the mechanical

design of the full-strength forearm to ensure that the selected actuators could support the arm’s structure.

Because of the large moment arm acting on the pitch and Roll 1 actuators, a high-torque, low-speed actuator

is essential for this application. Previously discussed in Section 5, a Harmonic Drive actuator is the primary

choice for all three axes of motion in the full-strength design.

Out of the available actuator models, the FHA-11C-100-US200-E, pictured below in Figure 51, best fits

this application. The specifications sheet in Table 9 details the capabilities of this actuator. [13] The FHA-

11C can apply 11 Nm of torque and is therefore capable of statically holding a 2 kg mass in the Yale Hand

with the arm fully extended. Additionally, the actuator can withstand the bending moment applied at the

first roll joint due to the carried load and the mass of the arm. Furthermore, the FHA-11C features a compact

package, hole patterns for ease of mounting, integrated encoder, and an excellent torque density. Three

units cost a total of $6021.

The control system required for the Harmonic Drive actuator does introduce some complexity. Each servo

requires its own controller, in addition to the fact that each of the harmonic drive controllers runs off a 120V

power line, which is less convenient when physically mounted onto Valkyrie. Additionally, the control

relies on a custom solution using both Harmonic Drive’s own Windows-based software and EtherCAT for

communication between the motors, increasing the complexity of controls.

40

Figure 51: Harmonic Drive FHA-11C drawings [13]

Table 9: Selected specifications for Harmonic Drive FHA-11C-100-US200-E [13]

Gear Ratio 100:1

Maximum Torque 11 Nm

Maximum Speed 60 rpm

Torque Constant 2.6 Nm/Arms

Positioning Accuracy 90 arc-sec

Maximum Moment Load 40 Nm

Moment of Inertia 0.067 kgm2

Encoder Specification 2000 pulse/revolution 14-wire encoder

Encoder Resolution 800,000 p/rev

Mass 0.62 kg

External Dimensions 60x60x56 mm

Unit Cost $2,006

41

8.2 Full-Strength Mechanical Design

Following the same layout as the gamut demonstrator, the full-strength forearm design utilizes a roll-pitch-

roll wrist configuration with the same optimized link lengths. The Harmonic Drive actuators are both

heavier and output significantly more torque than the DYNAMIXEL motors but are significantly more

tolerant of bending moment and shear loads than the motors they will replace. Thus, the full-strength design

does not require moment-bearing structures on the two roll joints, greatly simplifying the mechanical design

of the forearm. Because of the increased mass of the Harmonic Drive actuators, structural weight reduction

is of increased importance with the full-strength design. The forearm structure consists of an adapter

between Valkyrie’s standard elbow interface, a frame connecting the roll and pitch joints, a second frame

connecting the pitch and second roll joints, and a third link connecting the second roll joint to the Yale

Hand at the same dovetail interface used in the gamut demonstrator. The layout of the full-strength forearm

is shown in Figure 52 below.

Figure 52: SolidWorks CAD of full-strength design

The forearm contains an interface for Valkyrie’s current elbow mount. Hole features have been added for

cable pass-throughs and mounting the Roll 1 actuator, but the remainder of the elbow interface remains

unchanged from the original component on Valkyrie, as shown in Figure 53. The adapter is made of Al

6061 and will be a 5-axis machined component.

Figure 53: SolidWorks CAD of full-strength elbow adapter

Attached to the output face of the Roll 1 actuator is the Link 1 U-frame, shown in Figure 54. This link

connects the Roll 1 and Pitch joints. The U-frame profile, retained from the gamut demonstrator, was chosen

42

to optimize the strength-to-weight ratio of the link while also retaining ease of interface and machinability.

The frame is easily modifiable to fit mounting points for protective housings and cables. To reduce mass,

the sides of the frame taper down to the bottom face and there are additional mass-saving cutouts. To

maintain the light weight of the structure and allow it to withstand the increased torque of the full-strength

actuators, the U-frame is designed to be milled out of Al 6061.

Figure 54: SolidWorks CAD of full-strength U-frame

Following the pitch joint is the Link 2 bracket, connecting the Pitch and Roll 2 actuators. Shown in Figure

55, the structure is a reinforced L-bracket to allow for the axes of rotation of the two roll joints to align.

Curved ribs are added to the sides, allowing for reinforcement of the bracket, maximizing clearance around

the pitch actuator, and minimizing stress concentrations at the edges between the ribs and the bracket.

Strength requirements necessitate that the L-bracket be machined out of Al 7075. M4 clearance holes are

drilled on both faces of the bracket for fasteners and cabling pass-throughs.

43

Figure 55: SolidWorks CAD of full-strength Link 2 bracket

Attached to the output face of the Roll 2 actuator is a small adapter between the Roll 2 output and the

machined Link 3 component, shown in Figure 56. The adapter consists of a flange with 6 M4 clearance

holes for fasteners that secure it to the output face of Roll 2 and a cross-drilled shaft with 4 M4 tapped holes

for interfacing with Link 3. There also is an additional 8 mm hole down the center of the shaft for a cable

pass-through. The design calls for the adapter to be machined out of Al 6061.

Figure 56: SolidWorks CAD of full-strength Roll 2 adapter

Secured to the Roll 2 adapter is the wrist beam. The beam consists of a square cross-section extruded to the

standard dovetail adapter. The cross-section features M4 clearance holes to interface with the cross-drilled

44

Roll 2 adapter shaft, as well as an 8 mm-diameter cutout for cabling, as shown in Figure 57. For increased

strength with the higher torque Harmonic Drive actuators, the wrist beam will be machined out of Al 6061.

However, the beam has the same interface dimensions as the 3D-printed gamut demonstrator wrist beam,

and the two parts can be used interchangeably.

Figure 57: SolidWorks CAD of full-strength Link 3 beam and Roll 2 interface

8.3 Full-Strength Design Analysis

As with the gamut demonstrator, the full-strength components are sized according to the length parameters

set by the mathematical modeling and have been designed for ease of manufacturing and assembly.

However, weight reduction is of increased importance with the full-strength design and was also discussed

throughout the design process. Due to the Harmonic Drive actuators’ increased strength against bending

moment loads, there is no need for bearing structures in the full-strength design. The remaining structural

components were designed to withstand the maximum torque output of the selected FHA-11C actuators of

11 Nm in the same worst-case loading condition as the gamut demonstrator: both Roll 1 and Pitch actuators

applying maximum torque combined with the weight of the arm. Critical components were identified as the

U-frame and the Link 2 L-bracket.

The U-frame utilizes large fillets wherever possible to reduce stress concentrations, particularly around the

weight reduction cutouts. Additionally, holes were spaced as far from edges as possible to further reduce

stress concentrations there. As a result of FEA under the worst-case loading scenario, it was determined

that the U-frame be composed of Al 6061. Analysis verified that the softer material would be sufficient for

the loading experienced, primarily due to the further location of holes from the edges of the material. Stress

concentrations were observed around the weight reduction cutouts on the sides, and around the motor

attachment holes to the Roll 1 actuator. Despite these concentrations, the lowest factor of safety observed

was 1.85 around the motor attachment holes.

45

Figure 58: Stresses in full-scale U-frame under maximum loading condition

The L-bracket connecting the Pitch to Roll 2 actuators was also a component of significant focus during

analysis due to the need to reduce mass while maintaining strength to withstand the increased motor torques.

Large fillets and hole distances from edges were used to reduce stress concentrations, and large ribs were

utilized in the design to improve the structural rigidity of the bracket. Additionally, as supported by the

FEA in Figure 59, the bracket will be machined out of Al 7075, allowing for an increased factor of safety

for minimal weight penalty.

Figure 59: Stresses in full-scale Link 2 L-bracket under maximum loading condition

46

The assembly for Link 3, consisting of the Roll 2 adapter and the wrist beam, was also analyzed under the

worst-case maximum loading condition. While it was observed that the wrist beam was mostly rigid under

loading, primary points of concern were the fastener clearance holes in the output shaft. As anticipated,

stress concentrations were focused around the clearance holes in the flange of the Roll 2 shaft and in the

cross-drilled holes in the Roll 2 adapter and the wrist beam, as shown in Figure 60. However, all stresses

were well below acceptable levels, and both components will be machined out of Al 6061.

Figure 60: Stresses in full-scale Link 3 assembly under maximum loading condition

8.4 Full-Strength Electronics and Controls

The FHA-11C would be controlled by the HA-800 servo driver supplied by Harmonic Drive, with one

servo driver being assigned to each individual motor. The HA-800 servo driver can be used to implement

multi motor configurations linked via EtherCAT and controlled by a computer. Systems such as the Intel

NUC are viable options for this computer to simplify contain arm’s controls as a self-contained unit. The

controllers can function in speed, positional, and torque control modes, providing multiple options for

control and several safeguards against torque related damage. A 120 V AC power source is needed for each

individual servo driver and will be accounted for if the system integrated into Valkyrie. Until that point,

120 V power from a wall source will be sufficient. The software stack for these controllers is a custom

solution from Harmonic Drive that runs on Windows 7 and 10. Networking with the controlling computer

will allow the RIVeR team to control the new forearm and end effectors easily alongside the rest of the

Valkyrie robot.

47

9 Conclusions and Future Work

9.1 Conclusions

The existing NASA Valkyrie hardware is a powerful humanoid platform that enables various forms of

robotic interaction testing by RIVeR. Despite this versatility, certain design choices for the standard

Valkyrie forearms limit the platform’s ability to manipulate objects in constrained environments such as a

glovebox. The redesigned forearm resolves this limitation by expanding the gamut of the Valkyrie forearms

and allowing for the use of arbitrary end effectors including the Yale OpenHand, while maintaining the

payload capacity of the existing hardware. This design provides RIVeR with the capabilities necessary for

testing within glovebox constraints and enables future investigations with different end effector hardware.

By dividing the development of the redesigned forearm into two steps, the gamut demonstrator and the full-

strength design, it was possible to design and demonstrate an improved forearm while remaining within the

time and budget constraints of a capstone design project. The gamut demonstrator functions as a low-cost

representation of the achievable gamut with an RPR joint configuration with the capability of bearing light

payloads. Its DYNAMIXEL actuators enable a complete, controlled solution that strongly validates the

potential of the full-strength design variant. The full-strength design shows the capabilities of the RPR

configuration when high torque motors are available and exists as a potential option for a forearm capable

of the same gamut while bearing heavier payloads.

Both designs benefited greatly from the decision to optimize the link length before attempting to

manufacture or design the hardware. Modeling for optimal link lengths offered a compromise between

extending link lengths to maximize gamut and increasing torque requirements which would mandate more

stringent actuator requirements. The optimization saved costs on motor selection and helped ground the

mechanical design to a certain link length target.

The gamut demonstrator was successfully assembled and tested, with several demo routines proving the

design is a functional solution to the problem of glovebox manipulation. Both the full-strength design and

gamut demonstrator have undergone FEA validation to ensure the forearms will not fail in operation. The

results are two hardware designs that significantly improve the possibilities for testing the Valkyrie platform

in constrained environments and in turn expand RIVeR’s use cases for the system.

9.2 Future Work

At the end of capstone on December 4th, all design and analysis resources regarding the full-strength design

will be shared with RIVeR. The SolidWorks CAD must be finalized before fabrication can occur. While

the design currently meets the weight restriction, further mass reduction on each of the link segments will

optimize the configuration. This weight of the design also does not incorporate fasteners and other COTS

components which must be added to the assembly. If desired, a slip ring system can be implemented around

the motors to allow for continuous rotation at the roll joints. Additionally, a protective external housing

similar in function to the housing developed for the gamut demonstrator should be designed. This housing

will serve the purpose of protecting the internal components of the forearm while also adding an

aesthetically similar design to the rest of Valkyrie.

48

The current controls of the arm only allow rotation of the motors to set positions. The software contains no

advanced controls for movement of the end effectors aside from manual entry of the rotational angles.

Updated controls will provide smoother motions for the forearm and functions for performing complex

tasks.

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10 Intellectual Property

10.1 Description of Problem

The existing NASA Valkyrie robot, containing a humanoid forearm, wrist, and hand, is a standard solution

for generic manipulation. This system possesses a constrained range of motion for performing handling

tasks in a glovebox environment due to the nature of its wrist design. Additionally, the forearm design is

solely compatible with the single humanoid hand initially designed with the robot, as the finger tendon

actuators are contained within the forearm structure. It cannot be modified for specialized tools due to the

fixed connection between the forearm and hand.

10.2 Proof of Concept

To better perform tasks in a glovebox environment, the augmented Valkyrie forearm must possess an

increased gamut and manipulation capabilities. The gamut of the current forearm design is analyzed with

software capable of simulating the forearm and hand in a 3D workspace. This software maps the volumetric

envelope which the newly developed forearm and hand can reach (gamut). Optimal lengths for each of the

segments within the forearm are defined as those which achieve the greatest gamut while not exceeding a

3.76 kg total mass and the torque limits of the actuators.

To manipulate objects, the new forearm integrates with the Yale OpenHand Model O. To accommodate

future end effectors, the connection between the forearm and end effector below the wrist has been designed

as a standard interface modified from the Yale Hand’s open source design. Specialized end effectors allow

Valkyrie to perform a much wider variety of tasks without needing to use human tools. This standard

interface will enable the integration of any future tools for Valkyrie, as well as future iterations of the Yale

Hand.

To definitively prove the increased gamut of the redesign without spending thousands of dollars on

Harmonic Drives, the project was split into the design of two forearms. The first design acts as a gamut

demonstrator, ignoring any payload requirements and focusing entirely on showing the improved gamut.

This considerably reduces the torque requirements of the motors, allowing the design to function with much

cheaper actuators. In addition to this design, the team details how to scale the arm up to its full capabilities

of motion and load manipulation. Since the upgrade uses a very similar design with different actuators, the

implementation of the fully capable forearm would be relatively simple.

10.3 Progress to Date

Team progress began with extensive background research regarding various concepts for three degree of

freedom robotic wrists. This investigation required a complete review of existing patents, research papers,

and technology currently implemented in industry. Four potential design concepts were generated: roll-

pitch-roll, roll-pitch-yaw with a gimbal mechanism, a pure linearly actuated roll-pitch-yaw, and a purely

rotary actuated roll-pitch-yaw. After consideration of characteristics prioritizing gamut, weight, and

strength requirements, the team narrowed these choices to either the roll-pitch-roll or purely rotary roll-

pitch-yaw design.

50

To clarify restrictions and intended specifications for the revised design, all team members have traveled

to MassRobotics to view Valkyrie, as well as met with Professor Taskin Padir and a Murphy Wonsick,

researcher from the RIVeR Laboratory. The revised design was not to exceed the mass and nominal

diameter of the existing forearm.

The team began the design process with preliminary hand sketches for each of the four previously

mentioned arm layout options. After further consideration of the hand’s specific use cases, the roll-pitch-

roll configuration provided the distinct advantage of allowing the end effector to have an infinite number

of positions at a given point in space, in addition to being an industry standard mechanism. Thus, the roll-

pitch-roll configuration was selected. A preliminary design was created in SolidWorks, containing the

existing adapter from Valkyrie’s elbow to the arm, two link segments between the actuators, and a dovetail

interface on the end of the forearm capable of mounting to a modified plate for the Yale Hand.

A MATLAB-based mathematical model was developed to compute the gamut for multiple arm

configurations. The simulation computed the increased volumetric envelope which a new roll-pitch-roll

design can attain over the existing design in a glovebox environment. The model was also used to optimize

the link lengths for achieving maximum gamut. Based on the constraints of the motor torque, the

optimization revealed that as the links increase in length, the gamut increases indefinitely. The maximum

gamut, however, was achieved at a prohibitively long arm length, so stress analysis informed the final

values for links’ lengths.

After computing the maximum torque and bending moment requirements, the team selected commercial

off the shelf components for both forearm designs. The foremost of these components were the actuators.

For the gamut demonstrator, the MX-28AT and MX-64AT actuators were selected from the DYNAMIXEL

line offered by ROBOTIS. These motors allow for manipulation of the gamut demonstrator while

minimizing cost and mass. The more powerful MX-64AT was selected for the first roll and pitch actuator,

and the MX-28AT was selected for the second roll actuator, due to the lesser torque requirement at that

extreme end of the arm. Three series of motors were considered for the fully capable forearm design:

Harmonic Drive, DYNAMIXEL PRO, and Maxon. These investigations confirmed that Harmonic Drive’s

FHA-11C-100-US200-E would be best suited for the fully capable design as it provided the necessary

torque and bending moment in a light, compact package.

The selection of actuators informed the mechanical design of both the gamut demonstrator and the fully

capable design. A U-frame was chosen for the structure of the first link between the first roll actuator and

the pitch actuator. This link also serves to house the pitch actuator. Housing structures were also designed

for the two roll actuators. These housings include a crossed roller bearing and thrust bearing, which bear

the radial, axial, and moment loads of the arm and allow the actuators to bear only torque.

Optimized link lengths and joint positions were confirmed, and a final FEA was performed on all machined

parts for the gamut demonstrator. Strength knockdown factors were used in FEA of 3D printed parts. A test

bench consisting of an 80/20 frame and a glovebox simulator were designed and constructed. Once the

designs of all machined components had been validated, the parts were manufactured and assembled.

Development of the control software for the gamut demonstrator occurred in parallel to the manufacturing

and assembly process. Upon completion of the gamut demonstrator, the control software was tested,

validating the functionality and gamut of the forearm design.

51

Concurrently, the design for the full-strength arm was completed. SolidWorks FEA was utilized to check

stresses at concentration points, validate material selection, and ensure that the full-strength design could

withstand expected operating loads.

10.4 Individual Contributions

David primarily worked on research into wrist actuation mechanisms and aspects of operating in a glovebox

environment, providing metrics for the design selection process and informing the initial design concepts.

Additionally, he worked to define operating constraints of the forearm design through a static loading

analysis. He also investigated alternatives to the Harmonic Drive actuators, and the impact the alternatives

would have on the overall design. Furthermore, he developed the initial design for the moment bearing

structure and the links of the gamut demonstrator, working with Matt and Ryan to finalize the assembly

design for manufacturing. He worked with Ryan to finalize the full-strength design as well. He also

performed all finite element analysis on the gamut demonstrator and full-strength designs, providing

materials and design feedback.

Max has created early-concept hand sketches of the four setups of motors for achieving 3-DOF at Valkyrie’s

wrist. Max also contributed to the alternative motor selection, ruling out any models with prohibitive costs

or torque outputs too low to meet the team’s needs. He suggested the team investigate crossed roller

bearings for their ability to handle bending moment in addition to axial and radial loads.

Ryan developed initial CAD for the design of the arm segments, the design which later became the full-

strength design after the project was separated into two categories. He created the adapter interface for

connecting to the Yale Hand and worked with David to develop the length segments in SolidWorks. Once

a bearing structure was deemed necessary, he developed the design of the Roll 1 and Roll 2 structures in

conjunction with David and Matt. Additionally, he worked with David to update and finalize the design of

the full-strength forearm.

Bryant focused on research regarding possible tooling for the robot arm, as well as work on the control

schemes. He was involved in selection and development of electronics on the gamut demonstrator and

ensuring the gamut demonstrator was operational. He will continue to work on manipulation routines and

more advanced control schemes with the demonstrator, as well as the eventual controls and electrical

considerations with the full-scale motor selection and design. He also contributed to debugging of FEA

models for the roller bearings.

Matt researched the capabilities of the existing forearm and Yale Hand and developed from this information

the requirements for the new design. Based on his research on modeling techniques for kinematic chains,

he then developed the mathematical modeling software that enabled quantitative gamut comparison of

design concepts and design optimization. This software was leveraged to simulate and visualize the

operational gamut and ultimately define the mechanical design. He also developed the mechanical design:

he conceptualized the roll actuator bearing structures and worked with Ryan and David to iterate and refine

their design, implementing the final thrust and crossed-roller bearing layout. He also performed necessary

tolerance stack and preload analyses. He refined the design of components throughout the forearm and

designed the protective housing.

52

10.5 Future Work

Further development of the full-strength design is necessary prior to fabrication and implementation, if

desired by RIVeR. The full-strength CAD requires the addition of COTS hardware, fasteners, and a

protective cover before the design can be considered complete. While FEA was performed on all individual

components in the full-strength design, additional analysis will be required once these changes are

implemented. Three Harmonic Drive actuators must be acquired to power the arm and metal components

must be machined. If desired, advanced controls software may be developed for improved manipulation of

the end effectors.

53

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