Mau 04 Space Systems Design

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HST Robotic Servicing Mission

Final Report

Jay Loftus

Sarah RazzaqiSandra Mau

Wing Chan

December 3, 2004


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AcknowledgementsFor their continued support, advice, and education Team X-Site wish to thank the following

people at MDRobotics and University of Toronto:

Paul Fulford MDRobotics Course Coordinator

Tim Reedman MDRobotics Systems Coordinator

Ross Gillett MDRobotics Electrical CoordinatorTim Fielding MDRobotics Mechanical Coordinator

Perry Newhook MDRobotics Software Coordinator

Professor Chris Damaren University of Toronto Course CoordinatorLuke Stras Teaching Assistant (University of Toronto)


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III

Abstract

The Hubble Space Telescope has been in service since 1990. Currently, the Hubble is

approaching a point where significant maintenance is required. Its ability to perform scientific

research has been hindered by aging power sources and it may loose the ability to stay in orbit by

2009. NASAs proposed servicing mission involves the use of a launch vehicle called the Hubble

Robotic Vehicle (HRV). It will contain a De-orbit Module, Ejection Module, and two robotic

manipulator systems: Grapple Arm (GA) and Dexterous Robot (DR). The purpose of this launch

vehicle and its robotic systems is to avoid Extra Vehicular Activity (EVA) performed by

astronauts and the use of a space shuttle. The focus of this report will be on the design of the GA.

Our final design solution involves an arm that is approximately 12m in length that can reach the

entire servicing workspace. It will have the capability to capture Hubble as well as the Dexterous

Robot without causing damage to either system. The arm will have six degrees of freedom and

have the capability to be tele-robotically controlled as well as being autonomous at certain

mission stages. It will incorporate 4 cameras and a Laser Range Finder to assist in guidance and

capture. In short, our system achieves all the requirements, and attempts to use the most efficient

method available.


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IV

Table of ContentsACKNOWLEDGEMENTS ........................................................................................................ II

ABSTRACT.................................................................................................................................III

TABLE OF CONTENTS ........................................................................................................... IVNOMENCLATURE AND ACRONYMS ............................................................................... VII

INTRODUCTION......................................................................................................................... 8

PROJECT CONCEPT AND DESIGN PARAMETERS .......................................................... 9

MISSION OBJECTIVES................................................................................................................... 9

MISSION REQUIREMENTS ............................................................................................................. 9GRAPPLE ARM REQUIREMENTS ................................................................................................. 11

SYSTEMS.................................................................................................................................... 12

INTERFACES ............................................................................................................................... 12

SYSTEM BLOCKDIAGRAM ......................................................................................................... 13FUNCTIONAL FLOW BLOCKDIAGRAM ....................................................................................... 14AUTONOMY ............................................................................................................................... 18

GROUND CONTROL ARCHITECTURE........................................................................................... 19

MECHANICAL .......................................................................................................................... 20

FUNCTIONAL REQUIREMENTS .................................................................................................... 20

PERFORMANCE REQUIREMENTS................................................................................................. 20PERFORMANCE REQUIREMENT ANALYSIS.................................................................................. 20

Speed ..................................................................................................................................... 20

Force/torque ......................................................................................................................... 22

Joint Angle Calculation ........................................................................................................ 22GA Worst Case Capture Scenario ........................................................................................ 23Precision ............................................................................................................................... 24

WIRING ...................................................................................................................................... 25

BOOM ........................................................................................................................................ 25

Type of Structure................................................................................................................... 26

Material................................................................................................................................. 27Number and Length of Booms............................................................................................... 27 Final Boom Configuration.................................................................................................... 27

JOINTS........................................................................................................................................ 28 Joint Selection....................................................................................................................... 28

Actuation............................................................................................................................... 29Material................................................................................................................................. 30 Joint Motor and Gearboxes .................................................................................................. 32

FINAL JOINT SELECTION ............................................................................................................ 34

ELECTRONIC COMPONENT CASINGS .......................................................................................... 35

THERMAL/ENVIRONMENTAL CONTROL ..................................................................................... 35 Passive Control ..................................................................................................................... 37Thermal Blankets / Multilayer Insulation (MLI) .................................................................. 37


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Radiators............................................................................................................................... 38

Active Control ....................................................................................................................... 39LUBRICANTS .............................................................................................................................. 39

JOINT SENSORS POSITION, VELOCITY, FORCE .......................................................................... 40

END EFFECTORS......................................................................................................................... 41

Type....................................................................................................................................... 41 Force/Torque Sensors ........................................................................................................... 42Material................................................................................................................................. 43

Final Design.......................................................................................................................... 44SNARE MOTOR AND GEARBOX SELECTION................................................................................ 45

VISION SYSTEMS........................................................................................................................ 46

TIE DOWNS SELECTION.............................................................................................................. 48Type/Release Mechanism...................................................................................................... 48Location ................................................................................................................................ 51

ELECTRICAL............................................................................................................................ 53

ELECTRICAL REQUIREMENTS..................................................................................................... 53

FUNCTIONAL REQUIREMENTS .................................................................................................... 53PERFORMANCE REQUIREMENTS................................................................................................. 53

ELECTRICAL ARCHITECTURE ..................................................................................................... 53

CABLING .................................................................................................................................... 65POWER....................................................................................................................................... 65

POWERREQUIREMENTS ............................................................................................................. 65

Electrical Components.......................................................................................................... 65MISSION STAGES........................................................................................................................ 67

SOFTWARE................................................................................................................................ 70

REQUIREMENTS.......................................................................................................................... 70

SYSTEMS CONTEXT DIAGRAM ................................................................................................... 72DATA DICTIONARY .................................................................................................................... 75

CONTROL SYSTEMS............................................................................................................... 76

ACTUATOR& SENSORSELECTION............................................................................................. 77

Required Components ........................................................................................................... 77CONTROL ARCHITECTURE.......................................................................................................... 78

SIMULATION WITH MATLAB.................................................................................................... 81

GA AND DR INTERFACING................................................................................................... 90

MECHANICAL INTERFACE .......................................................................................................... 90

End Effector and DR Grapple Fixture.................................................................................. 90Capture Envelope.................................................................................................................. 91Loads..................................................................................................................................... 91Thermal Interfacing .............................................................................................................. 91

ELECTRICAL INTERFACE ............................................................................................................ 93

Power .................................................................................................................................... 93Data....................................................................................................................................... 93

SOFTWARE INTERFACE .............................................................................................................. 94


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30% POWER REDUCTION PROPOSAL .............................................................................. 99

COMPLIANCE TABLE............................................................................................................ 99

CONCLUSIONS ......................................................................................................................... 99

APPENDIX A............................................................................................................................ 102

A1: FULL FUNCTIONAL FLOW BLOCKDIAGRAM (FORHRV + GA + DR)............................. 102A2: FUNCTIONAL FLOW AND HAZARD CONTROLS ................................................................. 116

A3: RS 422 SPECIFICATIONS .................................................................................................. 123

A4: MIL-STD-1553B SPECIFICATIONS.................................................................................. 124A5: DR STOPPING CALCULATIONS......................................................................................... 125

A6: REFERENCES .................................................................................................................... 125

APPENDIX B ............................................................................................................................ 126

B1: TORQUE CALCULATIONS................................................................................................... 126

B2: TORQUE & POWERREQUIRED FORSNARE MOTORS ........................................................ 127B3: SPEED REQUIREMENT CALCULATIONS ............................................................................. 128

B4: GA BOOM ANALYSIS........................................................................................................ 129B5: HST CAPTURE SCENARIO ANALYSIS ................................................................................ 130B6: HEAT TRANSFER............................................................................................................... 132

B7: REFERENCES ..................................................................................................................... 133

APPENDIX C............................................................................................................................ 134

C1: CABLING MASS CALCULATIONS ...................................................................................... 134

Data Lines:.......................................................................................................................... 134 Power Lines: ....................................................................................................................... 134Video Lines.......................................................................................................................... 135

C2: REFERENCES .................................................................................................................... 138

APPENDIX D............................................................................................................................ 139

D1: PRECISION CALCULATIONS .............................................................................................. 139D2: SOFTWARE DESCRIPTION ................................................................................................. 140

D3: REFERENCES .................................................................................................................... 152


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Nomenclature and AcronymsDR Dexterous RobotEE End Effector

EM Ejection Module

EMGF Electro-Mechanical Grapple FixtureGA Grapple Arm

GC Ground Control

GF Grapple FixtureEVA Extra Vehicular Activity

FFBD Functional Flow Block Diagram

HRV Hubble Repair Vehicle

HST Hubble Space TelescopeLEO Low Earth Orbit

WFC Wide Field Camera


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Introduction

The Hubble Space Telescope (HST) is one of the most remarkable scientific achievements in

human history. It has been in service since 1990 and the National Aeronautics and Space

Administration (NASA) wish to extend its service life. Currently, the Hubble is approaching a

point where significant maintenance is required. It is experiencing degradation in scientific

capabilities, loss of power, and has an aging battery that is steadily losing charge capacity.

However, the most important concern is the loss of stability control. NASA projects that the HST

will have fewer than two operating gyros by mid 2007, which will cause an uncontrolled decent

through the atmosphere and impact on Earth. Thus, the main objectives of this mission will be to

provide HST with the capability to de-orbit, increase its operational lifetime by at least five

years, and enhance its scientific capabilities by installing new instruments.

NASAs proposed servicing mission involves the use of a launch vehicle called the Hubble

Robotic Vehicle (HRV). It will contain a De-orbit Module, Ejection Module, and two robotic

manipulator systems: Grapple Arm (GA) and Dexterous Robot (DR). The purpose of this launch

vehicle and its robotic systems is to avoid Extra Vehicular Activity (EVA) performed by

astronauts and the use of a space shuttle. If a servicing mission cannot be launched by 2008, then

HST will be placed in safemode to allow for a launch date of 2009 at the latest. However, toreduce risk and minimize the interruption to HST scientific operations, the HRV should be

launched by mid 2008.

The focus of this report will be a potential design of the Grapple Arm manipulator system. Its

goals will include capture and docking with Hubble as well as perform servicing operations

through the capture and positioning of the DR. This report will provide a detailed description of

the design process including Interfacing and Robotic Autonomy Concepts, breakdowns of

subsystem components such as Software, Mechanical, and Electrical Systems.


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Project Concept and Design Parameters

MISSION OBJECTIVES

The success of the Hubble Robotic Servicing Mission depends on the successful completion ofthree main mission objectives; capture of HST, berthing of Deorbit Module to HST, servicing of

HST. These objectives require that the two robotic manipulators carry out the following tasks.

- GA captures either of HSTs two grapple fixtures

- GA berths DM to HST

- GA retrieves DR and aids it in servicing taskso DR replaces the P6A and P8A connectors in the HST Diode Box 2 to harness

power from the HST SA3 for battery augmentation

o DR installs the power and data conduito DR installs the WFC3 camerao DR establishes data connection for gyros

- GA stows DR for re-entry

- GA stows itself for re-entry

MISSION REQUIREMENTS

In order to achieve the mission objectives outlined in the previous section, the roboticmanipulators must be designed to meet the following requirements as laid out by NASA in the

Mission Concept Review presented in May of 2004. Only the mission requirements that are

relevant to the design of the GA are presented here.

Level I Requirements

The following requirements re-iterate the mission objectives identified earlier.

Provide the capability to safely and reliably de-orbit HST at the end of its useful

scientific life Provide the capability to robotically extend the scientific life of HST for a minimum of 5

(TBR) years

Provide robotic installation of the WFC3 and COS instruments

Provide single-fault tolerance for the de-orbit mission Ensure that Level I performance is not degraded by robotic servicing

Level II Requirements

The HRV encompasses both the GA and DR. The successful completion of the tasks related to

docking and disposal require that the GA to capture HST. The life extension tasks are performedby the DR using the GA as a servicing platform.

- The HRV shall be capable of pursuit, proximity operations, capture, and docking with theHST


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- The HRV shall be capable of achieving pursuit, proximity operations, capture, and

docking with the HST from an initial ELV-provided orbit of TBR km- The HRV shall be capable of performing a controlled de-orbit of HST in accordance with

controlling the odds of human casualty as specified in NPD 8710.3A

- The HRV shall be capable of performing a controlled de-orbit of HST for a minimum of

seven years (EOL) from launch- The HRV shall support HST life-extension with battery and RSU augmentations

- The HST life extension provided by the HRV shall be for a minimum of five (TBR) years

from the completion of servicing- The HRV shall provide the means to install WFC3 in place of the WF/PC2 radial

scientific instrument and install COS in place of the COSTAR axial scientific instrument

Level III Requirements

The following requirements relate to the pursuit, proximity, operations, capture, and dockingwith HST.

- The HRV shall be capable of rendezvous with HST at amaximum altitude of 560 km

- The HRV shall be capable of capturing and docking with HST with the HST in an un-powered drift having body rates up to +/- .22 deg/sec (TBR) magnitude in each of three

axes simultaneously

- The HRV shall provide controlled thruster plume impingement on the HST to avoid HSTattitude responses and mechanical damage to HST appendages, and to minimize

contamination and plume heating to any portion of the HSTo Solar Array Torque shall not exceed 4.5 in-lb about V2 Axiso HST torque in each of V1, V2 V3 shall not exceed 9.35 in-lb about the HST CG

- The HRV design shall make it possible to keep the HRV out of the sweep range of theHST solar arrays and the high gain antennas during capture and docking, as well as after

docking

- HRV shall be capable of a minimum of four capture/docking attempts- The HRV shall provide two independent means of capturing HST

- The HRV shall capture HST using one of the following hardpoints: either of the two

grapple fixtures, the FSS berthing pins on the aft bulkhead, either of the two trunnions

located on the +/-V3 sides of HST, or the HST keel fitting- The HRV shall be docked to the HST using the three HST berthing pins

- The axes of the HRV coordinate system shall be parallel to the respective axes of the

HST coordinate system- The HRV shall maintain at least a one-inch static clearance between the HST and the

HRV (except at designed structural load path)

- The docked HRV shall provide 2 (TBR) inches of clearance to the HST P105 and P106connectors for contingency umbilical mate by robotic means

- During capture and docking operations, loads into each HST berthing pin shall not

exceed those defined in table 1 Aft Bulkhead Allowables

- A contingency ground abort capability shall exist for all autonomous HRV operations- The HRV shall provide video transmission of rendezvous, capture, and docking with

HST. Resolution and frame rate shall support ground supervision of critical operations


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- The HRV shall redundantly acquire range rate (relative to the HST) data using different

types of sensors systems- The HRV command link shall utilize authentication and encryption

The following requirements relate to the life extension via gyro/battery augmentation and science

instrument replacement. The GA aids the DR during this phase of the mission.

- Tele-robotic methods shall be used for servicing

- The three RSUs shall be mounted on the OTA structure via the WFC3 instrument- The command and telemetry of the RSUs shall be through the HST 486 computer

- The DM batteries shall provide a minimum of 300 Ah of augmentation at 6 years for

HST utilization- SA3 shall provide the primary power source for HST Battery Augmentation. The DM

power feed to HST shall be through the aft bulkhead J101 umbilical (P105, P106 as

contingency)- The HRV shall provide environmental protection that meets the Science Instruments and

Dexterous Robot requirements for all mission phases as specified in ST-ICD- 02/ 03,Robot user guide (TBD)

- During life extension operation, loads into each FSS berthing pin must be limited asdefined in table 1 Aft Bulkhead Allowables

- All thermal systems must be verified in a 1 g ground test

GRAPPLE ARM REQUIREMENTS

Based on the above mission requirements, functional and performance requirements can be

derived for the GA system, more specifically for each subsystem of the GA. These requirementsare presented in the sections describing the individual subsystems. The subsystems were

designed to meet these derived requirements.


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Systems

INTERFACES

Defining system boundaries and constraints is a key step in designing an effective and efficientsystem. The following sections outline the HRV system interfaces.

Interface Characteristics

GA Avionics to EM - RS 422 serial data interface

- Parallel power at 28VDC nominal

- Command and data transfer

GA Avionics to GA

(internal)

- Parallel MIL-STD-1553B Data Interfaces

- Parallel power supplies at 28VDC nominal

GA to EM - Deck mount for arm base

- GA storage mounts and latches

GA to HST - Grapple Arm EE / HST GF interface

GA to DR - Grapple Arm EE / HST GF interface- Redundant power and data plugs

DR Avionics to EM

(via GA)

- Parallel RS 422 serial data interfaces

- Parallel power supplies at 28VDC nominal

EM to Ground Control - Satellite based communication system

Ground Control to GA - GA EE video feed to ground control

- Tele-robotic operation commands from ground control to GA

- Autonomy operation commands from ground control to GA

Ground Control to DR - GA EE video feed to ground control

- Tele-robotic operation commands from ground control to GA

- Autonomy operation commands from ground control to GA

DR to EM - Temporary stowage fixtures inside cargo bayDR to HST - DR EE on P6A and P8A connectors

- DR EE on power and data conduit

DR to WF/PC II - EE on A-Latch, Blind Mate Connector and Ground Strap

- EE to physical handhold on camera

DR to WFC3 - EE to physical handhold on camera

- EE on A-Latch, Blind Mate Connector and Ground Strap

- EE to Rate Sensing Unit (RSU) connectors

DM to EM - Docking latches

- Power and data interface

DM to HST - Docking latches

- Power interface


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SYSTEM BLOCK DIAGRAM

This section presents the system block diagram for Team X-Sites GA Design. The system block

diagram graphically depicts the interfaces and subsystems for the HRV.


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FUNCTIONAL FLOW BLOCK DIAGRAM

After determining what our mission objectives, mission requirements, and mission constraints were, we c

model for our mission, in the form of an FFBD. The following contains only the portions of the FFBD dFFBD is located in Appendix A. The hazards alongside the text version of the FFBD is also found in Ap

HRV (Grapple Arm and Dexterous FFBD)

1. Launch & Pursuit

2 Proximity Operations

Launch &

Pursuit

1

Proximity

Operations

2

Capture

3

Servicing

4

EM

& D

GA in tie-down

position on EM

surface

1.1

DR in stow-

away position

inside isolatedcargo bay

1.2

As HRV prepfor capture,

power up GA

and run systemcheck

2.1


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3 Capture

Release pins tying

GA down

3.1

GA deploys/unfolds

3.2

GA runs full system

check and functional

simulations

3.3

GA assumes ready

to capture pose

3.4

Ground Control

monitors HST

through video,

determines HST rollrates and GF capture

windows

3.6

In a GF capture

window Gnd Ctrl

adjusts GA position

/orientation enablingready to capture

conditions

3.7

Ground Control

tele-robotically

controls GA EE to

surround HST GF

3.8

EE captures HST

GF and brings HST

to near rest relative

to HRV

3.9

GA releases HSTGF and folds up to

dormant position

3.11


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3 ServicingGeneral operations of GA and DR that are performed repeatedly for each service task is outlined in detai

therafter

4a GA takes out DR to perform servicing operations

Servicing tasks to be performed

Conduit

Deploy/ Battery

Augmentation

4.1

Change-out for

WFC3

4.2

Change-out for

RSU

4.3

GA deploys/unfolds

4a.1

DR cargo bay dooropens via ground

control

4a.2

GA uses EE camerato locate the EMGF

on DR.

4a.3

Predictive displaysproject the current

positions of GA EE

and DR EMGF

4a.4

GA EE captures DR

EMGF

4a.6

GA connection

allows DR to power

up

4a.7

EM releases

pins/bolts/straps on

launch locks holding

DR down

4a.8

GA autonomously

maneuvers DR out

of cargo bay bylinear retraction, and

video verification

4a.9


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4.2 Stow DR

5 EM Jettison & Disposal

DR folds into stowconfiguration

4.4.1

GA moves directlyover DR cargo bay

4.4.2

GA teleroboticallylowers DR into the

DR cargo bay towithin 0.16

accuracy.

4.4.3

Stowing latches fixDR in stow position

4.4.4

GA EE detachespower/data plugs

from DR EMGF

4.4.6

GA EE releases DREMGF and retracts

2m linearly back

4.4.7

DR back in stow-away

position inside isolatedcargo bay

5.1

GA folds back to

original tie-down

position on EM

surface, reattaching

the physical binds

5.2

Ground control

sends signal for EM

to detach from DM

5.3

EM performs

evasive maneuvers

to navigate away

from HST

5.4


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AUTONOMY

The choice between autonomous and telerobotic operation for the various phases of the mission

is dictated primarily by the time sensitivity and required precision of a given task. Tasks thatcannot be performed properly with a time lag (on the order of seconds) cannot be performed

telerobotically.

For capturing the HST GF we have determined that autonomy is in fact not required. The entire

HST GF capture sequence may be done telerobotically by a ground operator.

The largest obstacle in the capture of the HST GF is the time delay between the Ground Control

and the HRV introduced in the satellite communication system. The time delay is estimated to be

from 5-7 seconds[1]. This time delay, although seemingly large, is within tolerances for ourcapture sequence.

Our capture envelope consists of a circle that is 34 cm in diameter (see Interface Connection forGA and DR: Capture Envelope). We have also determined that the maximum magnitude of

speed for the HST GF before capture will be 1.4 cm/s. If the telerobotic operator were to centre

the GA EE directly over the GF shaft on his monitor, at most the operator would be off by 9.8cm. The capture envelope of the GA EE can allows the operator up to 17 cm leeway. Thus in a

telerobotic situation the operator still has 7 cm of leeway for capture.

Fig. Telerobotic GF Capture Analysis

In addition, predictive displays could analyze mission data and use simulated models of the GA

and HST in order to project the current positions of the GA and HST and minimize the time

delay error[1].

The capture of the DR GF is a simpler case of the HST GF capture since the DR will not be

rotating relative to the GA during capture. Thus the DR GF will be captured in a similar manner

as the HST GF.

The DR will be extracted from the cargo bay in linear path to prevent snagging and damage to

the DR. The linear path will need to be slow and precise in order to minimize possible damage tothe DR. Since speed is not required in this task we chose to complete this task telerobotically

keeping the implementation of this task as simple as possible. Furthermore, the teleroboticimplementation of extracting DR simplifies any maneuvers required in problem solving should

the DR become stuck during extraction. The GA force/torque sensors and stop the extraction if


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unexpected loads occur. Thus, telerobotics is not only the simplest method, but also the safest

method to extract the DR from its stowage bay.

In contrast to GF capturing, the positioning of the dexterous robot during servicing is not a time-

driven aspect of the mission. In that regard telerobotics could be used. However, the precision

required in placing the DR (0.16 as specified by NASA) is beyond the capabilities of atelerobotic operator. Thus the positioning of the DR during servicing will have to be carried out

autonomously. There will still be interaction with ground control in the form of commands to

initiate or terminate a task. Ground control will also monitor the mission at every step. Any GApath will still be confirmed by the ground control before the GA carries it out. But tele-robotic

operation of the grapple arm from ground control will be limited to failure modes only.

Finally, calculations from our partner team have shown that the stowing of the dexterous robot is

less than or equal to the precision requirements for the grapple arm already specified by NASA.

Thus the lack time driven requirements or need for greater precision led to the decision to alsocarry out the stowing of DR telerobotically. This task will have to be monitored from ground

control to ensure that the latches are securely in place. In fact, all tasks will be monitored byground control with checks and command confirmations worked into the software of both the

grapple arm and dexterous robot.

GROUND CONTROLARCHITECTURE

From the functions previously defined we can determine the necessary components of a groundcontrol station. The mission is balanced with telerobotic and autonomous tasks, so the function

of ground control is to coordinate telerobotic operations and to give initiate and terminate

commands for autonomous tasks.

At this point four major ground control elements have been identified and are shown in the flow

diagram below.

Ground Control

Tele-robotics-tele-robotic operatoruses video feed andsensor information to

control grapple arm

Health Monitoring-monitor sensor datafrom thermal sensors,

booms stress sensors,end effector force and

torque sensors andvideo feed-alert Communicationsif sensor readings

exceed limit levels

Communications-responsible for sendingand receiving commandsfrom GA and DR includingconfirmation messages,

initiate and terminate taskmessages, emergencymessages etc-monitor communications

between GA and DR toensure link is active-monitor satellite positions

and availability to ensurethat communications

between ground control and

manipulators is always

active

Performance-monitor mission toensure that missionobjectives are beingmet to within

requirements-suggest changes tomission protocols toimprove mission

performance if

necessary


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MechanicalThis section will incorporate the reasons for selecting our mechanical components such as joints,

booms, vision systems, and end effector.

FUNCTIONAL REQUIREMENTS1. The GA must provide means to grapple either Hubble Space Telescope (HST) Grapple

Fixture (GF).

2. The GA must provide means to grapple the Dexterous Robot (DR).

3. The GA must provide power and data connections to the DR4. The GA must provide a vision system to aid grappling HST and completing servicing

tasks.

5. The GA must position and orient the DR for servicing.

6. The GA must do no harm to HST.

PERFORMANCE REQUIREMENTS1. The GA must have open loop accuracy of less than 1.5 and 2.

2. The GA must have position repeatability of 0.16.

3. The GA shall be capable of capturing and docking with HST in an un-powered drift

having body rates up to 0.22/s magnitude in each of three axes simultaneously.4. The GA shall withstand the forces associated with stopping a 1000 lb mass from the

maximum tip velocity of the DR in 2 or 2.5.

5. The GA shall be able to reach all points in the workspace (defined by the DR) duringservicing.

PERFORMANCE REQUIREMENT ANALYSISSpeed

In a worst case scenario HST will be spinning at 0.22/sec in 3-axis at the time of HRV/HSTrendezvous. Assuming that the HST GF are directly above the centre of mass of HST and that

the radius of HST at that point is 2.1m we can calculate the maximum speed of the GF.

The maximum speed of the GF in any direction and at any time would be:

VGF_MAX = sqrt( 3 * (0.22/sec * pi rad/180 * 2.1m/rad) = 0.013966255 m/s ~ 0.014 m/s

At the time of the HRV/HST rendezvous the HRV will be ~5-8 m away from the HST1

. So, themaximum distance the EE will have to travel to reach the GF will be 8 m. Before the sequencefor capturing the HST GF begins the GA can be moved within 4 m of the HRV, so that the GF

will only have to travel at most 10 m to capture the GF.

Since the HRV is spinning at 0.22/sec max, this will allow a capture window time of :

1MCR_6s00-6s11 pg.48


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180/0.22/sec = 818 sec or 13 min 38 sec

If we use a FOS of 1.5 for the capture sequence (so we can move the GA EE away from the HST

at any time) for the capture sequence we will have ~ 9 min 5 sec to move to the GF, attempt to

snare the GF and slow HST or back the GA EE away if the snare is unsuccessful.

Here is a basic timeline for the capture attempt sequence:

Locate HST GF, move within the capture envelope of the GA EE < 4 min 2 sec

Attempt to snare HST GF 50 sec

Slow Hubble 10 secRemove GA EE from HST if snare unsuccessful < 4 min 2 sec

Sub-Timeline: Locating GF and moving GA EE so HST GF is within its capture envelope:

Manually move GA EE within 2 m of HST GF (at 0.06 m/s) t ~ 1 min 40 sec Lock auto control of GA EE onto HST GF t ~ 10 sec

Move GA EE within 1 m of HST GF (at 0.06 m/s) t ~ 17 secMove GA EE within 0.5 m of HST GF (at 0.03 m/s) t ~ 17 sec

Move GA EE into capture envelope around HST GF (at 0.01 m/s) t ~ 50 sec

Total t ~ 3 min 31 sec

From the spinning of HST GF and the max speed of the GA EE relative to HST we can

determine that the maximum speed of the GA EE will have to be:

0.06m/s + 0.014m/s = 0.074m/s ~ 7.5cm/s

Now from the maximum speed of the GA EE we can determine the maximum angular velocities

required for the joints. To determine the maximum angular velocities required by the joints wecan look at the worst case where the GA is extended to its longest working length and moving

away from the base of the GA at max speed.

We have determined the angles required for worst-case situation for grappling the HST GF in the

Joint Angle Calculations section below. Where L1 = 5.7 m, L2 = 5.92 m, L3 = 0.82 m, theta1 =

1.078 rad, theta2 = 0.8231 rad, theta3 = 1.2405 rad.

Thus, from the Jacobian2

we can determine the maximum angular velocities required given that

V = [x_dot; y_dot; _dot] = [0; 0.075m/s; 0]

Vangle = J-1

*V = [1_dot; 2_dot; 3_dot] = [0.017; -0.0322; 0.0152]

2 Controls Workshop Handout, by Prof.Damaren


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Therefore, the max angular velocity occurs at the elbow joint and is 0.0322 rad/s, which is

equivalent to 0.3075 rpm.

Force/torque

The worst-case scenario for torque is during the capture of the HST. The detailed calculation is

outlined in Appendix B: Torque Calculations.

The joint torques can be found based on the end effector forces

and torques through this relation:

3

The Jacobians actual values are found in Appendix B: Speed

Requirement Calculations. The max EE torque was found to be158.4 Nm.

3

2

1

T

T

T

= JT

z

y

x

T

F

F

=

10.4907-0.656913.9066-4.1782-

19.5901-3.7448-

Nm4.1580

0

=

Nm

Nm

Nm

4.158

4.158

4.158

So in the worst case our torques will need to withstand 158.4 Nm of torque. Thus, our required

torque for each joint is 158.4 Nm.

Joint Angle CalculationIn order to conduct the following analysis, it was assumed that the GA operates only in the pitchplane. This approximation is sufficient in determining most relevant arm parameters, including

the arm configuration during capture.

3 ibid


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The worst case scenario for arm configuration during capture is when the HRV approaches HST

from behind with the major axis of HST in line with the major axis of HRV. (Please see below

for rough layout.) This case may cause the arm to fully extend, which is not desirable. In

addition, this case defines a required workspace radius of sqrt(10.6^2 + 0.082^2) = 10.65 m.

In fact, upon performing the calculations for the 5m boom length GA, it was found that the armwould be very near to fully extended during capture. Thus, the arm boom length was increased to

5.5m, which was sufficient for this worst-case capture scenario. With these new boom lengths

the joint angles were computed to be:

1 = 47.22 = 61.83 = 71.1 (Arises from Assumption 8 defined below).

These angles are later used to compute joint motor speeds and torques. The assumptions andequations used in this analysis are presented below.

GA Worst Case Capture Scenario

Assumptions:1) GA operates primarily as a pitch-plane robot during capture.

2) HRV and HST are of the same diameter.

3) GA shoulder joint is placed right at edge of EM.

4) A 2m maximum separation distance exists between HRV and HST for capture.5) Dimensions of DM, and DM/EM connection are as given in NASA MCR i.e. DM height

= 60 in, DM/EM connection height = 21 in.

6) HST Grapple Fixture is located 21.5 ft from bottom of HST.7) Each joint is 20 cm in diameter and 22 cm in height.

8) Wrist is parallel to x-axis (as defined below) =180 deg.

The following diagram defines the variables used in this analysis.

HST

DM

EM

HST Solar Array

HST Grapple

FixtureGA


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l1 = (half diameter of shoulder pitch joint) + (boom length) + (half diameter of elbow pitch joint)= 0.1 + 5.5 + 0.1

= 5.7 metres

l2 = (half diameter of elbow pitch joint) + (boom length) + (height of wrist yaw joint) + (halfdiameter of wrist pitch joint)

= 0.1 + 5.5 + 0.22 + 0.1

= 5.92 metres

l3 = (half diameter of wrist pitch joint) + (height of wrist roll joint) + (wrist boom length) + (end

effector length)= 0.1 + 0.22 + 0.2 + 0.3

= 0.82 metres

The following equations are used in the analysis. (Reference: AER402 Course Notes)

px = x l3cos = l1cos1 + l2cos(1 + 2)

py = y l3sin = l1sin1 + l2sin(1 + 2)c2 = (px

2+ py

2- l1

2+ l2

2)/ 2 l1l2

c1 = ((l1+ l2c2) px + l2s2 py)/( px

2+ py

2)

3 = - 1 - 2

With location of end effector tip (x, y) given as x = 0 and y = 2 metres + 21.5 ft + 60 inches + 21

inches = 10.6106 metres. (Based on Assumptions 3 to 6)

Precision

The GA precision requirement as outlined in the MCR (i.e. position repeatability of 0.16 inches),was determined to be the driving precision requirement. Using a simplified analysis, this tip

precision was converted to a joint precision, which in turn provides a specification for the types

of joint sensors required.


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The worst-case scenario for this analysis is with a fully extended arm (i.e. 3 = 0, 2 = 0, and 1 =

any angle). Assuming that the arm starts in a position such that 1 = 0 as well, to find thenecessary joint angle precision, move the arm tip position 0.16 inches and determine the new

joint angle 1. The analysis was simplified by assuming that the 0.16-inch separation occurs

along the arc formed by moving the tip from a point A to a point B. This assumption is

conservative since assuming an arc length of 0.16 inches results in a straight-line separationbetween point A and B of less than 0.16 inches. Given this, the joint angle is computed to be:

1 = 0.16 inches / 12.44 metres = 0.0187

A more rigorous analysis could be carried out using forward and inverse kinematics, but this

simplified analysis is sufficient in determining joint sensor specifications. Thus, it wasdetermined that a 16-bit resolver would be needed to measure joint angles to the required

precision.

WIRINGFor the wire selection we used the SAE-AS22759-11 document.

For our data purposes we will use a STD-1553b serial data bus. The data signals will be 0-5V,

resistance of the data bus will be ~70 ohms4. Since I = V/R, I ~ 70 mA in our data bus. This

current means that a wire larger or equal to AWG 30 is needed5. We chose M22759-11-28 for

our data bus.

For our power purposes we have a 28VDC power line running through our GA. The max power

required by our ARM will be ~200 W. Since P = VI, I = 200W/28VDC ~ 7 amps in our powerlines. This current means that a wire larger or equal to AWG 18 is needed. We chose M22759-

11-18 for our power lines.

For our video cable we chose NTSC coaxial cable.

BOOM

The requirements that were considered when selecting a boom included the strength needed to

capture Hubble and Dexterous Robot, the structural stability (i.e. wont deform due to thermal

expansion/contraction), and the weight.

4 Electrical_Systems_Notes_UofT2004 by Ross Gillett5 Electrical_Systems_Notes_UofT2004 by Ross Gillett


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Type of Structure

We considered four different types of structures for the GA Boom: a tube, a square prism, an I-beam, and a truss structure.

The tube, square prism, and I-beam were compared using moments of area. The truss could not

be compared using this method since it does not have a constant cross-sectional shape and wouldrequire Finite Element Analysis for an accurate model, which would increase the complexity ofour design.

Ixx

indicates the resistance to bending about the x-axis. Iyy

indicates the resistance to bending

about the y-axis. Ioindicates the resistance to twisting about the center line (x-y intersection).

Table 1 - Properties of Potential Boom Structures

Shape ofCross-Section

Ixx

Iyy

Io

Circle8.15875E-06 8.15875E-06 1.63175E-05Square 6.73333E-06 6.73333E-06 1.34667E-05

I 1.32933E-05 4.58333E-06 1.78767E-05

Using a cross-sectional area of 0.004m2

, it was determined that the I-beam has the largest Ixx

and

Io, the Tube has the largest I

yy, and the tube has the second largest I

o. If there was not such a large

discrepancy between the I-beams Ixx

and Iyy

, it would appear to be the best of the three.

However, we think the tube shape is the best of the three for the GA Boom even though the Ixx

,

and Iofor the tube are not the largest, they are close.

Next, we wanted to compare the tube and the truss structure. The truss structure was statically

indeterminate, making it very difficult to analyze analytically. Finite Element Methods could beused to analyze the truss structure, but were not at our disposal. In the end, we decided upon the

tube structure for the GA Boom because it was the best of the uniform cross-section shapes, and

simple. Whereas, the truss structure was complicated with many weld points and could present a

hazard if an HST or HRV component became entwined in the truss structure.


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Material

For our choice of GA Boom material we chose from 4 materials; Aluminum, Steel, Titanium,

and Carbon Composite.

Table 2 - Properties of Potential Boom Materials

Material Density

(kg/m3)

Youngs Modulus

E11 (GPa)

Youngs Modulus E22

transverse (GPa)

Thermal

Coef.

(x106

K-1

)

Aluminum 2700 7.31E+10 7.31E+10 23

Carbon Fibre 1600 2.03E+11 1.12E+10 0.6-4.3

Titanium 4850 1.02E+11 1.02E+11 8-10

Steel AISI

C1020 7850 2.03E+11

2.03E+11 10-18

From this data it is clear that carbon fibre has the lowest density, lowest thermal coefficient andone of the strongest longitudinal Youngs Moduli. The only setback for carbon fibre is that the

transverse Youngs Modulus is quite weak. However, since the carbon fibre has a low density

multiple layers at different angles may be used to nullify the problem of the low transverseYoungs Modulus. In the end, we chose carbon fibre as the GA boom material since it would

provide the lightest structure, smallest error due to bending and thermal expansion.

Number and Length of Booms

The requirements used in considering the length and number of booms on the GA included

determining that Hubbles Grapple Fixtures is approximately 22ft from its base and the GA must

reach over the DM from the edge of EM cargo bay where it is stored. Based on theserequirements as well as the assumptions made in the Worst Case Capture Scenarios from above,it was determined that the total length of the GA had to be at least 32ft (~10m). A single boom of

10m would not be able to reach the entire work area once the HRV is stationed, so a single boom

could not be used. Two booms would be able to reach the entire work area once the HRV isstationed, as long as the two booms are of equal length (~5m). A structure of three booms is

excessive since it adds complexity to the structure with additional joints and necessary position

computation. In addition, the added dexterity of the three booms is not necessary since the workareas are all reachable through generally direct paths. Thus, we chose to use two booms of 5m on

the GA, since two booms keeps the complexity at a minimum while providing the reach and

dexterity to perform all of its tasks.

Final Boom Configuration

We selected carbon fibre for our boom material. Carbon fibres low density, large Youngsmodulus, and low thermal coefficient make it the optimal choice for a boom to be used for

outerspace applications.


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Our final boom configuration consists of two hollow cylindrical booms, each 5.5m in length, and

a smaller boom before the EE that is 0.3 m in length. Each boom has an outer diameter of 20cmand a wall thickness of 5mm. (Appendix B: GA Boom Analysis)

JOINTS

Joint Selection

In large part, the choices available in selecting joints are limited by the earlier choice made

regarding boom selection, specifically the number of booms. By choosing a two-boom plus wrist

configuration the GA begins to resemble the Canada Arm. Given this, the motivation forchoosing joints is to mimic and improve the function of a human arm. The most obvious choices

are elbow/hinge like revolute joints, and wrist like spherical or ball joints, with prismatic and

cylindrical joints also as options.

Prismatic joints are relatively simple in that they have only one translational DOF. A

translational DOF is not an obvious choice for a robotic manipulator like the GA, but it couldpotentially offer an extra degree of freedom to provide greater reach. However, a reachadvantage is not necessary if the GA dimensions are chosen to ensure that all areas of the

workspace are within reach. Thus, prismatic joints are not considered any further.

Cylindrical joints provide two DOF, one translational and one rotational, making them slightly

more complex than a pure translational joint. However, as discussed in the previous paragraph, a

translation degree of freedom is unnecessary for the GA. The allure of a cylindrical joint is therotational DOF, but the added complexity of the translational DOF makes the cylindrical joint a

poor choice for the GA.

The two most common joints with only rotational degrees of freedom are those that mostresemble the joints of a human arm. A revolute joint provides a single rotational DOF while a

spherical joint provides three rotational DOF. Both are acceptable choices for the GA, but the

choice of one over the other or a combination of the two is dictated by the configuration of thejoints on the arm.

In particular, the design requirements stipulate a robotic manipulator with six DOF total. Thearrangement of these DOF must be chosen such that the GA can reach all points in the

workspace. There are several ways to arrange the necessary DOF, though some DOF must

necessarily be located at a given point on the GA.

For one, to achieve the requirement of grappling the HST and maneuvering the DR within theworkspace, the GA must have sufficient DOF at its wrist. Pitch and yaw DOF are most essential

for positioning the DR, while roll DOF is essential for aligning with the HST and DR GF.

The remaining DOF are used for the crude positioning of the GA in the workspace. To achieve

side-to-side motion, the most obvious choice is to put a joint with yaw DOF at the shoulder. Apitch and roll joint could be used at the elbow to somewhat mimic this side-to-side motion, or at

least to cover the same area as a yaw joint at the shoulder. However, this would put two joints at


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the elbow and only one at the shoulder. While there is nothing obviously wrong with this in

terms of DOF, it does mean that there is more weight at the elbow than necessary. This translatesinto the need for a stronger and possibly heavier structure for the booms. Thus, the fourth DOF is

achieved using a yaw joint at the shoulder.

This leaves only pitch maneuvering, in particular at the elbow. But to achieve full reach in thepitch plane a DOF in pitch is also required at the shoulder. The use of a pitch and roll joint at the

elbow could have eliminated the need for a pitch joint at the shoulder, but again it is more

desirable to concentrate weight at the shoulder rather than the elbow.This results in a final configuration of pitch and yaw joints at the shoulder, a pitch joint at the

elbow and a pitch, roll and yaw joint at the wrist. Based on this configuration the choice of the

type of joint to use at each arm joint can be made.

It is only at the wrist where three DOF are required, so using a spherical joint at this location is a

possibility. A spherical joint is essentially a ball and socket joint. The clear advantage of aspherical joint is that it has three DOF built in. However, this also is a draw back of the joint, in

that it is more difficult to control three DOF than one. Ease of control is essential in achievingthe required precision for the GA. Precision also depends on actuation method and control

system configuration, which will both be discussed in the next section, but the difficultiesassociated with control of a three DOF spherical joint make it a less desirable choice for the GA.

This leaves only revolute joints. A revolute joint has only one DOF, so to implement revolutejoints for the GA there will need to be two revolute joints at the shoulder, one at the elbow and

three at the wrist. Using three revolute joints rather than one spherical joint does add weight far

from the pivot point, but the driving requirement in selecting joints is precision not mass.

ActuationAlong with the type of joint, the method of joint actuation is essential in achieving the required

precision. Also, the actuator must be able to handle the torques applied during HST capture andloads during the DR servicing mission. There are a few common choices for joint motors,

namely stepper motors or dc servomotors (dc brush and brushless).

Of the three choices, DC brush motors are by far the simplest. They can be used in both open and

closed loop configurations and they do not require an electronic driver to operate. However, this

simplicity is not enough to outweigh the major disadvantage of a brush motor, which is having a

lifetime on the order of 50-200 hours in a vacuum. This is a very short lifetime and given thenature of this project, simplicity of design is not a major concern. Thus, DC brush motors are not

a good choice for GA joint actuation.

DC brushless motors are significantly more complex than their brush counterparts. They can also

be used in either open or closed loop configurations, but their greatest advantages arise when

operated in a closed loop, most often with position feedback. When combined with good sensors,these motors can provide extremely precise position control. It is possible to find encoders

(sensors) with on the order of 100,000 pulses per revolution. In fact, the precision of a brushless

DC motor is governed more by the quality of the sensors than the motor itself. DC brushless


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motors are also significantly more efficient than stepper motors (which will be discussed later) as

a result of the commutation process. They also can provide up to 2-4 times the power of asimilarly sized stepper motor. The primary disadvantage of DC brushless motors is that they are

considerably more complex than the alternatives, and thus are more costly.

Stepper motors are also a potentially good choice for GA joint actuation. Steppers are primarilyopen loop devices, but they can achieve fairly accurate positioning even in this configuration.

The limitation of stepper precision is governed by the step size, and the degree to which microstepping is possible. For a typical four-phase stepper, micro steps are in the range of 25000. The

incremental step sizes however result in accelerations and decelerations, which lead to

inefficiencies in the motor and thus high power consumption.

Though a stepper motor would be a fair choice for this application, they are simply not as precise

as DC brushless motors. The precision requirement is the driving factor in choosing both the typeof joint and the method of actuation. Thus, the best choice of motor for this application is a DC

brushless motor. They are the most precise of the three potential choices and are also veryefficient.

Material

The main restrictions affecting the type of material used for the joints are:

- Tight projected mission schedule (must launch in 2008)

- The joint must be able to repeatedly and consistently move in an accurate, predictable manner- The joint should last at least the life-time of the mission (say at least 5 years to get a safety

margin)

The limitations on materials that can be derived from above requirements are:- Since the mission is soon up-coming, should use tried-and-true technology rather than

experimental materials

- Structure: The joint must be strong/tough to sustain repetitive movement (should not wear outquickly)

- Structure: The joint should not rust, decay or build up deposits which could hinder its

movement

In terms of materials, there are the typical materials used for many previous space applications

including stainless steel, aluminum, titanium, and carbon composites. Other newer or moreexotic materials examined include carbon-carbon machineable composites, metal matrix

composites, beryllium and alloys, and invar.

The pros and cons for each material are summarized in Table 3. The material chosen for the joint

was titanium. Since the requirements specified that time is a restraint, it was decided that typical

materials should be used since they have been proven space worthy. Although aluminum andstainless steel are less expensive than titanium, they are more likely to corrode or deform, which

would affect joint motion. Carbon composite is more of a material for the booms being stiff and


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light. Since the joint is constantly moving, some flexibility would be beneficial. Thus, titanium

was chosen because it is strong, highly resistant to corrosion and light.

Table 3 - Benefits and drawbacks of some prospective materials:

Materials Pros Cons

Typical MaterialsStainless steel 300

series, 15-5 PH and

17-7, Custom 455

These austenitic stainless steels are

frequently immune to general corrosion 6

May experience pitting and crevice

corrosion and undergo stress corrosion

cracking in some environments

Aluminum 7075

and 6061

Favorable strength-to-weight ratios make

them the structural metal of choice foraerospace applications.

They also can have excellent atmospheric

corrosion capabilities.

Protective properties of the aluminum oxide

films that form on these alloys can breakdown locally and allow extensive

corrosion. It frequently occurs at bolt and

rivet holes or at cutouts where the small

grain boundaries perpendicular to the metal

surface are exposed.

Titauium Ti6Al4V Very strong, highly resistant to corrosion,

light7

its limited use means that small-scale

production operations result in a relatively

expensive metalCarbon composite Most often used to make stiff, light

members of regular cross, section.

(Examples include robotic booms andsupports for solar arrays and panels.)8

Much more expensive than metal alloys.

Newer Materials Introducing new materials into a

design can be a significant cost andschedule driver and introduces risk.

Carbon-carbonmachinable

composites

Current projects include structural skinsfor an all carbon thermal protection system

for space vehicles. Oxidation resistant

coatings available.9

Degradation that carbon-carbon compositesare susceptible to are not detectable at an

early stage by traditional non-destructive

testing techniques. Once significant damagehas occurred to the material, fracture

toughness may have decreased

dramatically.10

Metal matrix

composites

Near-zero coefficient of thermal expansion,

high-temperature capability, high thermal

conductivity, and high specific stiffness andstrength11

Relatively difficult to manufacture, inspect,

and scale-up. To do all that would be very

costly.

Beryllium and alloys High stiffness, light weight, dimensional

stability over a wide temperature range. 12

Poor ductility and fracture toughness.

Invar Very low thermal co-efficient of expansion. Costly

6 http://corrosion.ksc.nasa.gov/html/corr_control_matsel.htm

7 http://www.titan-japan.com/ohanashi/ohanashe.htm

8 Spacecraft Design Course Notes: Mechanical Elements of Spacecraft Design

9 http://www.southernresearch.org/pls/portal/url/page/PUB_ENGINEERING/PROCESS_MANU

10 http://www.southernresearch.org/pls/portal/url/page/PUB_ENGINEERING/PROCESS_MANU

11 http://www.tms.org/pubs/journals/JOM/0104/Rawal-0104.html

12 http://www.wordiq.com/definition/Beryllium


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Joint Motor and Gearboxes

The joint motor and gearboxes chosen for our GA was from Maxon Motors (Table 4). Our gearratio with this motor was determined to be ~ 13,900:1. This was found by dividing our highest

torque requirement by the Maximum Continuous Torque of this motor. We decided to use this

motor because the maximum power consumption was within an acceptable range, maximum of12Watts. Also, Maxon is a proven company for electric motors for space applications.

N = Torque Required/ (Torque Available*Maximum Efficiency)= 94.7 Nm / (9.86mNm*0.69)

= 13,919.51*94.7Nm was derived from Appendix B: Torque Calculations

In order to reduce the speed of this motor from 5000RPM to our performance required speed of

~0.3075RPM and to increase the torque from 9.86mNm to 94.7Nm, three gearboxes were chosen(Table 4). These gearboxes will be placed in series in order to take full advantage of their

performance.


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Torque Calculation

Stage 1

Motor Torque * Motor Efficiency * Gearbox 3.7:1 * Gearbox Efficiency

= 9.86E-03 Nm x 0.69 x 3.7 x 0.8

= 0.02014 Nm

This satisfies the maximum amount of torque the 3.7:1 gearbox can handle, which is 0.75Nm.

Stage 2

Torque @ Stage1 * Gearbox 100:1 * Gearbox Efficiency

= 0.02014 Nm x 100 x 0.7= 1.4098Nm

This satisfies the maximum amount of torque the 100:1 gearbox can handle, which is 50Nm.

Stage 3Torque @ Stage2 * Gearbox 93:1 * Gearbox Efficiency

= 1.4098 Nm x 93 x 0.7= 91.8Nm

This satisfies the maximum amount of torque the 100:1 gearbox can handle, which is 120Nm.

Speed Calculation

Stage 1

Motor Speed / (Gearbox 3.7:1 * Gearbox Efficiency)= 5000RPM / (3.7*0.8)

= 1689.19 RPM

Stage 2

Speed @ Stage 1 / (Gearbox 100:1 * Gearbox Efficiency)

= 1689.19 RPM / (100*0.7)

= 24.13 RPM

Stage 3

Speed @ Stage 2 / (Gearbox 93:1 * Gearbox Efficiency)= 24.13 RPM / (93*0.7)

= 0.37 RPM

Even though the torque and speed do not exactly match the requirements, the speed can be

lowered to increase the torque. According to the data and speed versus torque curve [1] provided

by Maxon, the highest speed the motor can be run at is 5000RPM with a torque of 9.86mNm.

This speed should be avoided during actual operations to ensure long-life cycle of the motor. Thecurve also shows that running at a lower RPM, approximately 4000RPM for example, the torque

value can be increased slightly to just under 10mNm.


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Table 4 Motor and Gearbox values

(Please See Appendix B: Reference# 1,2,3,4 for further details)

FINAL JOINT SELECTION

There will be six joints all together on the GA (Fig 1). A yaw and pitch joint at the shoulder, ayaw joint at the shoulder, and a yaw, pitch and roll joint at the wrist. From our trade study in

previous assignments, the best joint material to use for the joints will be Titanium.

Fig. 1 - GA Stowed Position

(Illustration Not to Scale)

GearboxRecommended Input

Speed (RPM)Max. Continuous

Torque (Nm)Max. Efficiency

(%)

MassInertia(gcm

2)

Part # 166155 -Ratio 3.7 to 1


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The specified dimensions of each joint, Diameter = 20cm and Height of 22 cm, will be able to

accommodate the combined dimensions of the motor and gearboxes provided by Maxon[Appendix B: Reference #1, 2, 3, 4], which is approximately 8.1cm Diameter and 21cm in length.

The joint boom is to provide structural reinforcement between the End Effector and wrist joint.

This additional joint will help relieve torsional and shearing stresses on the wrist joints by havinga slightly higher wall thickness.

ELECTRONIC COMPONENT CASINGS

For the eight circuit boards distributed throughout the GA, the components outlined from our

EFBD in the previous assignment will be enclosed in Aluminum Casings. Since there are nostructural stresses, the wall thickness can be made extremely small similar to a computer case.

With a volume of 15cm x 15cm x 30cm, a thickness of 2mm, and a density of 2700 kg/m3, themass of each casing is approximately 0.60kg by using the formula Mass = Density x Volume.

THERMAL/ENVIRONMENTALCONTROL

The purpose of having thermal controls is to maintain the electronics and mechanical parts

within its designed operational temperature range and to minimize energy loss. There must be asufficient amount of thermal control material to keep the interior components from melting or

internal malfunctions (i.e. the casing, motors, electronics.) Just as an idea for the allowable

temperature ranges, electronics should be operated within -200C to 650C. Table 5 describes thetrade-offs between each type of thermal control.

Table 5 - Types of thermal controls availableType Pros Cons

Passive Control Less dynamic, less controlledPhase change device Useful for electrical equipment that

experiences short power spikes.Unable to absorb any more heat aftermelting, allowing temperature to increase

thereafter.

Thermal blankets orother insulation

Lighter and less expensive thanconventional tiles

Gold blanket The outer layer of the blanket is a type of

second surface mirror material with highreflectivity and high emissivity.

On the sun side, the sun reduces static

charge buildup because of photoemission

of electrons from the Kapton.

Will radiate some heat to space but their

insulating properties increase with thenumber of layers.13

Black blanket Same as gold except for outer layer whichhas a higher absorptivity and lower

emissivity than the gold Kapton outer

13 http://www.qrg.northwestern.edu/projects/vss/docs/thermal/3-what-materials-are-used-for-thermal-control.html


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layer, to cut down on radiation of

spacecraft heat to space.This layer is also electrically conductive.

By grounding the outer layer to the

spacecraft frame the static charge buildup

which occurs on the anti-Sunside isdissipated. 14

Paints or coatings Thermal coatings are very efficient and

lightweight. i.e. Teflon coating has a highemissivity and radiates heat to space. 15

Coating will degrade over time.

Radiators Generally much simpler than active

radiators and easily mass produced

Radiators in space have to be much larger

than that on earth to perform the samecooling16

Active Control More complex, needs additional

moderationHeaters Electrical heaters are used for fine

temperature control, and usually only for

short periods of time.

Fluid loops

Air coolant loop Lower heat transfer rateWater coolant Lower heat transfer rate

Freon coolant Lowest operation temperature of the three.

17

Heat exchangers Highly conductive heat path and

extremely high heat transfer rates.

Lightweight, can be used for a wide rangeof temperature, and can have variable

conductance.

Slow inert gasses can be generated18

Louvers A controlled rate of heat dissipation. High temperatures can occur if they are

pointed at the sun.



16 http://www.permanent.com/i-heat-x.htm

17 http://www.tsgc.utexas.edu/archive/subsystems/thermal.pdf

18 http://www.tsgc.utexas.edu/archive/subsystems/thermal.pdf


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Passive Control

This includes multi-layer insulation or thermal blankets and radiators. The insulation shields thearm from the cold vacuum of space and radiators on the joints to allow heat to dissipate from

motors and electronics when in use.

Thermal Blankets / Multilayer Insulation (MLI)

This type of insulation is made of thin layers of metallic foils separated by insulating oxides or

fabric in vacuum. Heat transfer across this material is primarily by thermal radiation from one

layer to the next. The performance of MLI depends heavily on the details of its configuration.Seams, attaching points, and folds are all paths for heat leakage through the insulation. The

effective emittance of MLI typically ranges from 0.002 to 0.02 depending on the area insulated.

Like the current Canadarm, the MLI will consist of alternating layers of godized Kapton, Dacron

scrim cloth and a Beta cloth outer coating (fireproof). The outer layer of the MLI consists of a0.001 inch Kapton layer, which acts as a type of second surface mirror material with high

reflectivity and high emissivity. The multiple layers within the blanket are 0.00025 inch Kapton

thin silver coating on each side. These layers are separated by a Dacron netting, which preventscontact of the adjacent silver layers. The inside blanket layer makes contact with the spacecraft

body. The blanket layers exchange heat between adjacent layers by radiation.

Surface area of booms = surface area of blankets = (10.5m x pi * 0.2m) x 2 booms = 13.2 m2

Thickness of blanket = 2.5 mm = 0.0025 m

(http://www.tsgc.utexas.edu/archive/design/phobos/)Volume of blanket = 0.0025 x 13.2 = 0.033m3 = 33000 cm3

Density = 1.42 g/mL=1.42 g/ cm3

Mass of blankets = 46860g = 46.8 kg


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Radiators

There will be a radiator located at on the surface of each joint, with a 10cmx10cm surface area.

The radiators are made of aluminum, and have a silver Teflon surface coating. This material is

referred to as a second surface mirror. The material consists of a 0.010inch layer of transparent

Teflon coated with a thin layer of silver and a protective layer of Inconel. The material is applied

to the space side of the aluminum plate.

The grapple arms internal heat is conducted to the inside of the radiator plate via a conductive

gold foil connected to mechanisms that produce a high heat. The Teflon has a high emissivity

and radiates the heat to space. In addition, the sunlight may strike the first Teflon surface, passthrough to the second silver layer, and be reflected back to space. This will protect heat from

getting into the GA.

Radiator thickness = 0.5 cm = 0.005m

Radiator surface area = 100 cm2 = 0.01m2Volume of radiator = 5x10-5m3

Density of aluminum = 2700 kg/m3

Mass of radiator (each) = 0.135 kg

Total mass of radiators = 0.135 x 6 = 0.81 kg

Emissivity of 5 mil Silvered Teflon = 0.78

Absorptivity of5 mil Silvered Teflon = 0.05 to 0.09

(http://www-personal.engin.umich.edu/~adoolin/index_files/

thermal%20management%20problem%202b.doc)

Assume highest operating ambient temperature = 20 deg C = 293 K

Energy radiated to space

= A(T4s T4a)

= 0.01 m2 * 5.67x10-8 W/m2/K4 * 0.78 (3^4 273^4 )= -3.26 W

Energy absorbed from space when directly exposed to solar radiation

= GA

= 1358 Watts/m^2 * 0.09 * 0.01 m2

= 1.22 W(Refer to Appendix B: Heat Transfer for derivation of equations)


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Active Control

This type of thermal control would involve some sort of mechanical and electrical action. In the

GAs thermal control system, the only active components are sets of thermal control loopconsisting of a thermostat and flexible heaters. If the thermostat dips below a certain range, the

heaters will be activated to drive the temperature back to acceptable values. These heaters will

be placed on each circuit board. The distributed architecture for the active thermal control units(thermostat and heater) is described in the EFBD. There are 2 sets for these units (forredundancy) in each joint, 2 for the snare motor, and 1 each for the elbows 2 pan-and-tilt camera

and motors.

It was determined that active controls that required fluid flow (such as fluid loops, heat

exchangers) were impractical since there is nowhere for the fluid to flow to (unless through the

booms to other joints) and it would increase the weight of the arm significantly.

LUBRICANTSThe requirements stress that lubricants should have a long-life performance to meet the missiondemands. The main types of lubricants were examined, liquids, greases, and solids. Again,

although the new lubricants were briefly examined, it was deemed that tried-and-true methods

were a safer choice.

The major drawback in using a liquid lubricant is that the lubricant can be lost through

vaporization, creep or inadequate supply. Though countermeasures can be used they may beunfeasible due to the small size of the joint. For example, to ensure adequate lubricant supply,

positive feed systems can be developed to control the flow to certain areas, but that would

require a reservoir of lubricant and more complexity, thus liquid was not chosen.

Typically, films are used only when it is not possible to use liquid or grease. The major

drawback in using a solid lubricant is that since films have finite lives, they should not be used

for rolling-element bearing applications that would experience more than a million cycles ofsliding.

Thus solid films were not chosen, unless in combination with a grease lubricant.

Grease lubricant is already used in a variety of space applications, such as ball bearings, journalbearings and gears. The reason grease is so favored is that the grease can act as a reservoir for

supplying oil to contacting surfaces. It also acts as a barrier to prevent oil loss by creep or

centrifugal forces.Thus, grease lubricants were chosen for the joints.


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JOINT SENSORS POSITION, VELOCITY, FORCE

Internal state sensors are devices to measure the position, velocity and acceleration of robotic

joints by providing a feedback for the robots position and motion control system. Only sensorsfor revolute joints were considered since the grapple arms joints are revolute.

A set of sensors was chosen to measure different performance parameters in the joint as well asto add redundancy where possible. Limit switches were chosen to determine whether a joint had

reached its limit of travel (i.e. rotated 360 degrees). For odometry, the most common and well-

known position sensor is the optical encoder. An absolute optical encoder will be used, ratherthan an incremental one, due to its higher quality and resolution. For angular velocity and

position, the syncro and resolver was chosen over tachometers due to the accurate output and less

noise in the signal. For force/torque, a senor based on strain gauges will be used. The straingauge element will be chosen such that its resistance change is linear over the operating force

range.

Table 6 - Benefits and drawbacks of various internal sensors

Sensor type Pros Cons

PositionLimit switches Simplest type of sensor for binary

conditions (i.e. reach limit of travel)

Potentiometers Inexpensive Only measures accuracy to 0.5%

Limited life due to physical wiper contactwear

Optical encoders

(absolute andincremental)

Long life, the most common Sensitive to shock

Absolute OpticalEncoders

Absolute position is always known.Typically higher quality and resolution may

be increased with multiple rings

More expensive than incremental

Incremental OpticalEncoders

Simpler and more common than absoluteencoder

Must be calibrated when first poweredup.

Angular velocityTachometers Inexpensive Output is noisy and has limited life due

to commutator brush wear. 19

Synchros and resolvers Extremely accurate output for a reasonable

cost.

Can determine both rotational position andvelocity, thus can be used for both

odometry and velocity tracking. 20

Requires an analog to digital converter

Torque/ForceServo accelerometer Requires servo motor (active part)

Strain gauges Most common basic element for force

sensor

19 http://scholar.lib.vt.edu/theses/available/etd-7698-132910/unrestricted/Chapter2.pdf

20 http://me.queensu.ca/courses/MECH497/CD/htmlmods/robot/SENSORSS.htm


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END EFFECTORS

Type

There are two main types of end effectors that can be used. The first utilizes specially shapedgrippers to grab onto the target (Fig. 2) and the second utilizes wire ropes to snare the target (Fig.

3).

The end effectors with specially shaped grippers are used in tasks that are small and complex innature. They are designed to interface with specific grapple fixtures and perform specific tasks.

These types of effectors can be seen on the arms of the SPDM, where it must connect with many

different interfaces in its dexterous tasks and performing services such as attaching tools, activatefasteners, slide doors, turn bolts, etc Due to the nature of the tasks, this type of end effector is

much smaller in comparison to the wire rope end effectors. The gripping forces and

misalignment tolerances are also much smaller. This is necessary because a missed grasp or anexcessive amount of force by the grippers can damage the target interface as well as its own

components such as the Socket (Appendix B: Reference #5).

The wire rope end effectors are typically used when there is a large tolerance for misalignments

and when a large amount of force is required to hold the target in place. These types of end

effectors are can be seen on large maneuvering objects such as Canada Arm 2 where it uses the

effectors to attach itself to the Space Station and other large objects like a Shuttle. Since thesetypes of effectors can absorb large amounts of force, they are required to be quite large in size.

This large size along with its wire rope configuration limits the amount of fixtures it can

interface with. Henceforth, the tasks performed by these types of end effectors must remain

simple. It should only interface with fixtures such as power and data, and connections that attacha payload where it is directed appropriately (Appendix B: Reference #5). Therefore, with its high

strength and limited capabilities this type of end effector is most suitable for attaching theDexterous Robot or the Hubble Space Telescope onto the Grapple Arm, which satisfies the

functional requirement of being able to connect to either the DR or HST as well as the

requirement of having a large capture envelop in case of HRV tracking errors and HST rolling.The yellow-coloured rectangular shaped objects on the outside of the cylinder in Figure 3 are the

power and data connections to the DR, which satisfies the functional requirement of providing

power to the DR.


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Fig. 2 Gripper Type End Effector Fig. 3 Snare Type End Effector

Force/Torque Sensors

Force sensors are typically strain gauges attached to the surface of the object being measured(Fig. 4). A strain gauge essentially converts mechanical motion into an electronic signal. The

deformation or strain that occurs in an object being stressed causes a change in capacitance,

inductance, or resistance. This change is proportional to the strain being experienced by thesensor and therefore, the amount of force being applied can be measured. An electrical strain

gauge is however, subject to instability and inaccuracies due to temperature, material properties,

the adhesive that bonds the gage to the surface, and electromagnetic interference. However, ithas been tried-tested-and-true over the past, and therefore should be used to reduce additional

research and development complexities.

Fibre optic force sensors are another method of measuring force. It uses reflected light from a

fibre optic probe to measure the force. There are no electrical signals and/or attachments. Thisallows for much greater precision and accuracy when compared to an electrical strain gauge

(Appendix B: Reference #6). However, fibre optic cables are subject to deterioration due to

radiation. Therefore, this type of force sensor should not be used.


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Fig. 4 Strain Gauge

Material

The materials that should be considered for use on the end effector include Titanium, Beryllium,and Aluminum. Titanium has higher then average fracture toughness compared to Beryllium

and Aluminum, it is light in weight, has high boiling and melting points, but its stiffness is only

average and is a more expensive material. Beryllium has higher then average stiffness, l

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Mau 04 Space Systems Design