The Next Step SPACE ROBOTICS INITIATIVE Skyworker CDR 11/18/99-1 Skyworker - Mobile Manipulator Critical Design Review Field Robotics Center November.

The Next StepSPACE ROBOTICS INITIATIVE

Skyworker CDR 11/18/99-1

Skyworker - Mobile ManipulatorSkyworker - Mobile ManipulatorCritical Design ReviewCritical Design ReviewSkyworker - Mobile ManipulatorSkyworker - Mobile ManipulatorCritical Design ReviewCritical Design Review

Field Robotics Center

November 18, 1999

William “Red” WhittakerPeter StaritzChris Urmson



• Constellation of SSP satellites in GEO

• 1GW of energy to the ground

• Microwave transmission antenna 1 km in diameter

• Mass of 4800 MT (10X as massive as ISS)

• Assembled over 1 year, maintained for 30 years

Space Solar Power (SSP) FacilitiesSpace Solar Power (SSP) FacilitiesSpace Solar Power (SSP) FacilitiesSpace Solar Power (SSP) Facilities

4000 m



Assembly, Inspection, MaintenanceAssembly, Inspection, MaintenanceAssembly, Inspection, MaintenanceAssembly, Inspection, Maintenance

• Extremely large scale structures

• Poor accessibility

• Long life cycle

• Dangerous environment

• Necessitates a robotic workforce – Assembly, Inspection, Maintenance (AIM)

Radiator

Parabolic Reflector

Radiator

Parabolic Reflector



ObjectivesObjectivesObjectivesObjectives

• Demonstrate the viability of using robots for orbital construction

• Prove the validity of using structure walkers for orbital AIM

• Demonstrate SSP AIM relevant tasks using robotics

• Simulate prospective SSP AIM robots and tasks



Representative TasksRepresentative TasksRepresentative TasksRepresentative Tasks• Walk, turn, and transition across planes on a truss

structure

• Pick up and place a payload at arbitrary locations and orientations in space

• Carry a payload while walking, turning, and transitioning

• Conduct calibration and inspection tasks

• Connect power and communications cables

• Cooperatively carry massive or large payloads

• Perform tasks that require multiple robot collaboration



DemonstrationDemonstrationDemonstrationDemonstration

• Prototype Robot– Pick up and carry a model transmitting element the length of the

truss, turn while carrying, couple the element to the structure– Connect Power Management and Distribution (PMAD) to the

element– Perform a mock calibration

• Simulation– Large scale construction utilizing multiple robots– Coordinated installation of full scale transmitting elements– Demonstrate extended lifetime operations



Configuration - Key MetricsConfiguration - Key MetricsConfiguration - Key MetricsConfiguration - Key Metrics

• Continuous Gait

• Forces exerted / Forces experienced

• Workspace

• Control Complexity

• DOF

• Mass

• Cost

• Energy Consumption



SkyworkerSkyworkerSkyworkerSkyworker

• Continuous Gait– Reduced forces on

structure – Low energy

consumption– Constant contact with

structure– Requires 4 joint

synchrony

• 11 Degrees of freedom

• Extensive Workspace



Skyworker - SpecificationsSkyworker - SpecificationsSkyworker - SpecificationsSkyworker - Specifications

• Tetherless Mobile Manipulator

• Processor: Pentium166

• Walking Speed: 10cm/s

• Mass: <37kg

• Dimensions:

3m x 0.5m x 0.1m

• Degrees of Freedom: 11

• Material: Aluminum

• Power: 200W peak



Force AnalysisForce AnalysisForce AnalysisForce Analysis

• Mass Estimates

• Forces Due to imperfect GC system

• Maximum torque 16 N-m

• Maximum force 12 N

Mass Quantity Group MassJoints 2 kg 11 22 kgLinks 4.5 kg 3 13.5 kgGrippers 3.0 kg 3 9 kg

Total 44.5 kg

Original Mass Estimates



Joint DevelopmentJoint DevelopmentJoint DevelopmentJoint Development

• Determined maximum torque, speed, and travel needed for gait

• Modularity considerations

• Motor / Reduction Combination– 16 Nm torque +– 44 degrees/second +

• Skyworker actuators– 57.5 degrees/second at

32 Nm torque

Joint Required Max Torque Required Max Speed Max Theta1 16 Nm 32.1 deg/s +/- 180 deg2 Variable Variable +/- 90 deg3 Variable Variable +/- 90 deg4 Variable Variable +/- 90 deg5 10 Nm 43.2 deg/s +/- 180 deg6 Variable Variable +/- 90 deg7 1 Nm 32.7 deg/s +/- 180 deg8 9 Nm 35.1 deg/s +/- 180 deg9 Variable Variable +/- 90 deg

10 Variable Variable +/- 90 deg11 Variable 35.1 deg/s +/- 180 deg

– 62.5 degrees/second at 16 Nm torque



ReductionReductionReductionReduction

• Two Stage Transmission

• Stage 1 - Planetary Gearhead– Integral Unit– 4.8 to 1 reduction– 1.3 deg no-load backlash– 80% efficiency

• Stage 2 - Harmonic Drive– High reduction ratio with zero

backlash – Low mass - high torque ratio– Efficiencies ranging from 70% to

80%

Ratio 100 to 1Rated torque at 2000 rpm 7.8 NmRated average torque 11 NmLimited repeated peak 28 NmLimit momentary peak 54 NmMaximum Input Speed 5000 rpmAverage Input Speed 3500 rpmMass .09 kg

CSF 2A-GR-14 Harmonic Drive



Motor SelectionMotor SelectionMotor SelectionMotor Selection• Motors Selection Criteria

– Power Minimization– Mass – Available with integral encoder and planetary gearhead– Space relevance

• Maxon Motors– DC Graphite Brushed– Rated for 42 volts, operating at 30 volts– .340 kg– 4800 rpm & 24 Nm output torque requires 48 watts



Joint OverviewJoint OverviewJoint OverviewJoint Overview

3 Inline revolute (Size A)

2 Offset (Cantilever) revolute 3 Inline revolute (Size B)

3 Axial revolute



Axial Revolute JointAxial Revolute JointAxial Revolute JointAxial Revolute Joint

• Most Complicated

• Interface between F/T sensor and Gripper

• Mass 1.48 kg

• Shear key



Anatomy of a JointAnatomy of a JointAnatomy of a JointAnatomy of a Joint

Force Torque Interface Cap

Force Torque Interface

Bearing Secure

Bearings

Output Shaft

Harmonic Housing

Harmonic Drive

Potentiometer Belt

Feedback Drum

Bearing Baseplate

Gripper Interface

Force Torque Closing Plate

Potentiometer Pulley

Potentiometer

Motor/Planetary/Encoder Package



Structure OverviewStructure OverviewStructure OverviewStructure Overview

• High bending and torsional stiffness

• Weight minimization– Truss reduction

• Access via removable bottom plates– Also serve as internal

attachment points

• Each link is unique– Little opportunity for

modularity



GripperGripperGripperGripper



Gripper’s Force AnalysisGripper’s Force AnalysisGripper’s Force AnalysisGripper’s Force Analysis• Clamping force

needed to counteract effects of stride

• Maximum force required: 500N

x

y

z

Time (s)

Time (s)

For

ce (

N)

For

ce (

N) Singularity in stride



ConceptConceptConceptConcept

• Dynamic gait requires a robust and fast gripping mechanism

• Robustness• Simple Design - Single jaw actuated• Low Power - Limited motor torque

required• Error Correction - Designed with

mechanical allowance for imperfect approach

• Speed• Fast Approach - Direct approach

allowed by configuration• Fast Mechanism - High speed

advantage provided by linkage• Fast Motor - High RPM attained with low

torque requirement



MechanismMechanismMechanismMechanism

• Gripping Mechanism: “Vise Grip” Four bar linkage

– Speed Advantage: Moving jaw adequately slowed at final closing

– Force Advantage: Motor force multiplied at locking

– Power Advantage: Zero power required when locked



CoatingCoatingCoatingCoating

• Potential for wear of aluminum gripping face mandated protective coating

• Stainless Steel Coating• Reduced wear• Increased coefficient of

friction

• Thermally sprayed coating courtesy of the State University of New York at Stony Brook’s Center for Thermal Spray Research



Gravity CompensationGravity CompensationGravity CompensationGravity Compensation

• Skyworker requires gravity compensation to operate properly

• Marionette style cable support counteracts the force of gravity

• Combination Active/Passive system– Vertical axis is passive– Horizontal axis are active

• Modify a heritage gravity compensation system



Robot Interface ModificationsRobot Interface ModificationsRobot Interface ModificationsRobot Interface Modifications

• Four attachment points

• Sliding interface to allow transition between walking and manipulating postures

• Arc center located at CG



Feedback ControllerFeedback ControllerFeedback ControllerFeedback Controller

Optical Angle Sensor

Picture of shuttle withangle sensor board

(to be taken w/ digital camera)

Output voltages linear function of angles.



Feedback ControllerFeedback ControllerFeedback ControllerFeedback Controller

Control Issues

• Gantry pendulum is a fourth order system

• Model as a second order system– Second order model sufficiently accurate over a small range of

inputs.– Skyworker will only move over a small range of velocities.– Tune PID controller for good responses over these inputs.– Hack: Zero integral term with change of direction (faster

response)



Skyworker: Power ElectronicsSkyworker: Power ElectronicsSkyworker: Power ElectronicsSkyworker: Power Electronics

• Power Budget:– Motors: 140W peak power required– Motor controllers, communications, sensors, digital

electronics, CPU, and miscellaneous: +5, +10, -10 volt supplies, 60W maximum

– Worst case: 200W – Mass Constraint: 4kg (batteries+converters)

• Skyworker must be capable of performing operations for a minimum of 20 minutes prior to recharging



Battery TechnologiesBattery TechnologiesBattery TechnologiesBattery Technologies• Batteries we considered:

• New high-rate discharge NiMH batteries will be used, because they provide a high power/weight ratio along with other desirable properties

NiCd NiMH New NiMH Li-IonEnergy/kg Fair Good Good ExcellentEnergy/cm3 Fair Good Good ExcellentCharge Rate High Moderate High LowEase of Charging Easy Easy Easy DifficultMax. Discharge Rate(Power)

High Moderate High Low

Cost/W Low Low Low HighMass/W Low Moderate Low HighVolume/W Moderate Moderate Moderate High



Panasonic High Rate Discharge NiMHPanasonic High Rate Discharge NiMHPanasonic High Rate Discharge NiMHPanasonic High Rate Discharge NiMH

Cell size Sub C Mass per cell 55g Cell voltage 1.25V down to 1V during discharge Current 3000mA-hours

8 to 9 amps continuously for 20 minute demo 3.4 usable watt-hours per cell

Charge rate 1 hour quick charge with delta-V charger (Standard NiCd battery charger)

Max discharge rate 10 Amps (10 Watts/cell minimum) Other nice features No memory effect

500 charge/discharge cycles Comparison to NiCd Equal or better in every way to NiCd, with twice the energy

density of NiCd and (amazingly) no more expensive



Power SystemPower SystemPower SystemPower System

• Two separate battery packs– Motor pack

• 30 cells• 102 watt-hours

– Electronics pack • 20 cells• 68 watt-hours• Further optimization possible (to equalize run time)• Powers three switching power supplies that produce +5V, +10V

and –10V

• Safety System– E-stop switches located on robot and at control station– Power switching circuitry prevents simultaneous connection of

multiple power sources



Testing BatteriesTesting BatteriesTesting BatteriesTesting Batteries



Battery and Tethered OperationBattery and Tethered OperationBattery and Tethered OperationBattery and Tethered Operation

• Charging– Using external delta-V chargers to charge batteries without

removing them from Skyworker

• Battery Monitoring System– Battery voltage monitoring circuitry will let Skyworker know

that it’s batteries are nearly drained

• When Skyworker’s batteries are recharging, it can run off of a tether that supplies 36V and 24V to Skyworker’s motors and voltage regulators.



Electrical Wiring DiagramElectrical Wiring DiagramElectrical Wiring DiagramElectrical Wiring Diagram

A/D

µP

A/D

MC MC

MC

MC MC

MC

MC

MC MC

MC

MC

MC

DC/DC 5V

DC/DC 10V

DC/DC -10V

A/D

MC MC

SC SC SC

SC

30V BATT 24V BATT

FT FT

FT

Legend:

+24V Power -10V Power +10V Power +36V Power RS232 Bus Sensor Bus Data Bus A/D Data Bus 1 Data Bus 2 Data Bus 3



Joint Labeling SchemeJoint Labeling SchemeJoint Labeling SchemeJoint Labeling Scheme

1

2

3

4

5

6

7

8

9

10

11

The Next Step SPACE ROBOTICS INITIATIVE Skyworker CDR 11/18/99-1 Skyworker - Mobile Manipulator Critical Design Review Field Robotics Center November.

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