mobility challenges and possible solutions for low …
TRANSCRIPT
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MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
Slide 1
MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION
F. Herrmann (1), S. Kuß (1), B. Schäfer (1)
1 German Aerospace Center (DLR), Institute of Robotics and Mechatronics Oberpfaffenhofen, D-82234 Wessling, Germany
ASTRA 2011, April 12 - 14, Nordwijk
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Slide 2MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
MotivationExploration of small bodies is challenging
MicrogravityEnvironmental conditionsDeep space missions
Testing of microgravity mobility systems is impossible on earthSimulation (not valid without any tests)Alternative tests (mock-up)Microgravity tests
Hardware developmentTest-rigs BreadboardFlight model
Electronics and controller development for Deep space mission requirementsHigh miniaturizationSimulation support
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Slide 3MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
Small bodies environment
MicrogravityGravitational force depends on
- mass distribution/density- distance of body centre- position on target body
Undefined soil conditionsGround shapeMaterialBehaviour while interacting
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Slide 4MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
Mobility system requirements
Provide measurements on different locationsMaximise science possibilities
Robust conceptSimple but effectiveControllable & adaptiveIndependent from soil characteristics
Deep space qualifiedSurvival of cruise phase
• radiation• temperatures
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Slide 5MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
Finding a solutionMulti body system (MBS) simulation model
Small body (or representative) environmentMobile system
Gravitation model of the target bodySimple (mostly sufficient)Sophisticated (if needed)
Contact modelsPolygonal contact model PCMSoil contact model SCM (DLR developed)
Parameter variations Test out suitable model parametersSensitivity analysis to environment parameters
10³ - 10510³ - 10510³ - 10510³ - 2*105[kN/mn+2]Scaling coefficient k*
110.8 – 1.51.1 – 1.8[-]Deformation coefficient n
0.00.00.01.0[kPa]Cohesion
20-3030-3931-3330-32[deg]Internal friction angle
1800180014001300-2300[kg/m³]Bulk density
8-120.7 – 1.5--[mm]Grain size dist
pebblyCoarseIntermediateFine[-]Soil class
MRS-DMRS-CMRS-BMRS-AUnitParameter
10³ - 10510³ - 10510³ - 10510³ - 2*105[kN/mn+2]Scaling coefficient k*
110.8 – 1.51.1 – 1.8[-]Deformation coefficient n
0.00.00.01.0[kPa]Cohesion
20-3030-3931-3330-32[deg]Internal friction angle
1800180014001300-2300[kg/m³]Bulk density
8-120.7 – 1.5--[mm]Grain size dist
pebblyCoarseIntermediateFine[-]Soil class
MRS-DMRS-CMRS-BMRS-AUnitParameter
Parameter Unit Value Young’s modulus [N/m²] 4.72e5 Poisson ratio [-] 0.4 Layer depth [m] 0.02 Areal damping [Ns/m³] 1.0e8 Damping depth [m] 0.02 Friction coefficient μ [-] 0.45
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Slide 6MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
Simulation: Wheeled rover in microgravity (1)
Example model6-wheeled roverExoMars (breadboard) kinematicsMass of 102 kg reproduce ground loads of a 300 kg rover on MarsRover behaviour covered by hardware test experience
Scenario 1Earth gravityAscending slope of 11 degCrossing an obstacle
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Slide 7MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
10 % of g
Simulation: Wheeled rover in microgravity (2)
2.5 % of g
Test: Reducing gravity step by stepScenario 2: 10 % of earth gravityScenario 3: 2.5 % of earth gravity
Not consideredPossible change of soil behaviour due to microgravityMicrogravity-specific modification possibilities
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Slide 8MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
Simulation: Wheeled rover in microgravity (3)
Scenario 4: 1.0 % of earth gravityStill 1000 x higher gravity than usually on small bodies!
ResultsGreat impact of microgravity on traction performanceConventional kinematics do not work in this environmentLess wheel loads mean less applicable torqueDisturbances can lead to uncontrollable dynamics, e.g. wheel lift-offVery slow reaction due to microgravity
1.0 % of g
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Slide 9MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
Hopping mechanisms
Previous missionsPhobos hopper (43 kg)
spring-driven brackets10 hops20 meters each
MINERVA I & II (0.6 kg)Flywheel drivenLifetime: 36 hrs
Both were lost before operating on the target’s surface
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Slide 10MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
Trade off: Definition of a hopper concept (1)Requirements: MASCOT (DLR-RY)
10 kg lander packageTarget body 1999 JU3
surface gravity: 1.7e-5 gExample: Only two concepts
Arm conceptExcenter driven concept
Other tested conceptsSpring driven conceptsFlywheel
Important parametersRobustness of motionEstimated power consumptionMechanical issues
bearing & mounting designcomplexity
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Slide 11MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
Trade off: Definition of a hopper concept (2)
Example scenarioGravity: 1.7 * 10-5 gDifferent soil characteristics left/rightPCMv0 = 0.5 * vesc = 0.16 m/s
Lever arm concept
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Slide 12MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
Trade off: Definition of a hopper concept (3)Example scenario
Gravity: 1.7 * 10-5 gDifferent soil characteristics left/rightPCMv0 = 0.5 * vesc = 0.16 m/s
Excenter driven concept
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Slide 13MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
Trade off: Definition of a hopper concept (4)
Reasons for simulation-supported trade-offConcept decision in early phase (A)Not yet all information available
target propertiesfinal system parameters (mass..)
Many open questionsIt is easy with parameter variation to compare concepts
Results of the trade-offExcenter tappet concept is the most promising for given mission requirements
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Slide 14MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
Parameter Variation: Deviation of mass moment inertia (1)
When concept is fixedGet information about system behaviourImprove dynamicsSupport design processComponent selection
Parameter variation exampleHopping scenarioVariation of the inertia tensor (4x)Observe impact on dynamic behaviour
Desired resultsSpecification of acceptable inertia deviation
Other possible variationsPosition of CoMDrive control strategies
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Slide 15MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
dev01
x y z
x 0,0784 0 0
y 0 0,1152 0
z 0 0 0,1505
dev02
x Y z
x 0,0784 0,015 0
y 0,015 0,1152 0
z 0 0 0,1505
Parameter Variation: Deviation of mass moment inertia (2)Note: Slow motions due to microgravity
Realtime duration of this action: 400 s / 6:40 min
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Slide 16MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
dev04
x y z
x 0,0784 0 0,015
y 0 0,1152 0
z 0,015 0 0,1505
dev03
x y z
x 0,0784 0 0
y 0 0,1152 0,015
z 0 0,015 0,1505
Parameter Variation: Deviation of mass moment inertia (3)
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Slide 17MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
Component development (1)
Goal of the ParVar: identify required drive speed for small hopParameter variation
4 x K_L (proportional gain for position control): 5…2045 x T (time constant for drive action): 0.1 … 1sec180 variations
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Slide 18MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
ResultsHeight (z-position)Required motor torque
Component development (2)
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Slide 19MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
Component development (3)
Best resultK_L = 5T = 0.445 s
MotorLess than 5 mNm without margins and securityRuns less than 0.5 s Maximum drive speed: 820 rad/s or 7830 rpmRelocation distance: 0.79 mEstimated motor current: 0.55 A
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Slide 20MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
Component development (4)
Results are used for calculatingInput & output speed of the gearRequired current
This leads to suitable componentsMotorGearController / power electronics
Resulting actionSmall hopDuration: 130 s (low gravity!)
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Slide 21MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
DLR RM activities overview
MBS modelSimulation
??
Mock-upTests under earth gravity
BreadboardMicrogravity tests
Flight modelAsteroid
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Slide 22MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
DLR-RM test facility: Mock-up (1)Testing on earth
Impossible without modificationsMock-up: Highly scaled test model
Off-the-shelve componentsLess massMore powerIncreased excenter massesDifferent mass distributionGravity compensation: pendulumSimulation verification
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Slide 23MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
First test resultsPendulum: 2 m
ComparisonTestSimulation
DLR-RM test facility: Mock-up (2)
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Slide 24MOBILITY CHALLENGES AND POSSIBLE SOLUTIONS FOR LOW-GRAVITY PLANETARY BODY EXPLORATION> Florian Herrmann > 14.04.2011
OutlookMore mock-up tests
Improved test modespendulum length: up to 10 m
Control strategiesstart & stop positiondrive speed
Different ground conditions Breadboard microgravity tests
Drop towerParabolic flight
Simulation supportMock-up testsMicrogravity testsFlight model
1200
686
MASCOT is under the lead of DLR-RY (Bremen) and proposed for the Hayabusa-2 mission of JAXA