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DESIGN OF A HIGH TRACTION FLEXIBLE
WHEEL FOR THE NEXT GENERATION OF
MANNED LUNAR ROVERS
7th Americas regional conference of the ISTVS
Louis Corriveau
November 4th 2013
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PRESENTATION OVERVIEW
Context & Objectives
Requirements Definition
Mission constraints
Terramechanics approach
Dynamic model
Concept Generation
Methodology
Convergence
Prototype
Preliminary Results
Vertical stiffness and damping
Lateral Stiffness
Traction and wheel dynamics
Concluding Remarks
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Credit : Duncan Jones, 2009
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CONTEXT & OBJECTIVES
Sustainable human activity further than low Earth orbit
Manned lunar missions lasting more than a few months
Lunar vehicles used daily
Safe, reliable and durable
Fast, controllable and long range
Design of a flexible wheel
Maximize drawbar pull, control and comfort
Minimize energy requirements
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Credit : NASA
How to obtain a deformation of at least 10% of the wheel diameter,
while having enough damping to dissipate the energy of deformation?
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REQUIREMENTS DEFINITION
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MISSION CONSTRAINTS
Performance requirements
Speed of up to 20 km/h on a flat terrain
Mass of 250 kg per wheel (410 N on the moon)
Wheel mass up to 5 kg
The moon
Environment
Resistant to space radiation (up to 10 Ge/nucleon)
Resistant to temperature cycling (-233 C, +123 C)
Terrain
Drive up a 27 slope, which is the mean slope of a crater
Avoid high sinkage which can lead to mission abortion
Soil
Generate traction on soil with similar trafficability parameters as on the moon
Resistant to lunar dust infiltration and augmented wear
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TERRAMECHANICS APPROACH
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Optimal values
D = 1.25 m
b = 0.25 m
Pg = 2.5 kpa
Kv = 4100 N/m
NWVPM
New wheel
parameters
Wheel diameter, width
and ground pressure
Lunar trafficability
parameters Drawbar pull
Torque
Vertical stiffness
Sinkage
Wong, 2010
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K C
X
DYNAMIC MODEL
Mass-spring-damper model
Mass is predefined from the mission requirements
Spring stiffness is known from the terramechanics analysis
Damper coefficient is unknown
Load case
Wheel maximum deformation of 15 cm
Comfort
Minimize the acceleration of the mass
Control
Minimize the force trying to lift the vehicle off the ground
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Damping coefficient
Optimal value : 2025 Ns/m
Limit value : 500 Ns/m
m
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CONCEPT GENERATION
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Spring
Cantilever Beams
Cables
Strips in bending
CONCEPT CONVERGENCE
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Materials
Fully metallic
Composites
Damper
Active and passive
Inertial
Magnetic
Piezoelectric
Friction
1
2 3 4 Requirements Concept
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PROTOTYPE
3D model
Detailed design
Mass estimation
Titanium and aluminum construction
Manufacturing drawings
Finite element model of the spring part
Equivalent beam model based on experimental tests
Quickly determine a design point for the first prototype
First prototype
All stainless steel
Find major design flaws
Ascertain the hypothesis
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PRELIMINARY RESULTS
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MTS SHOCK DYNAMOMETER
Most critical aspects of the design
Vertical stiffness
Vertical damping
Wheel compression
From 50 to 100 mm
3 cycles at each frequency
[0.05 0.1 0.5 1 2 3 4 5 6]Hz
Force, displacement and velocity were recorded
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VERTICAL STIFFNESS
Almost constant over the spectrum analyzed
Gap between the compression and rebound curves
Energy dissipated, i.e. hysteresis
Stiffness 5 times higher than the requirements
FEM model is conservative
Easier to reach a lower stiffness than the opposite
Mass will decrease along with stiffness
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0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 10 20 30 40 50
Fo
rce [k
N]
Displacement [mm]
0.05 Hz 0.1 Hz 0.5 Hz 1 Hz 2 Hz 3 Hz 4 Hz 5 Hz 6 Hz
Average stiffness
Prototype : 20 000 N/m
Rubber tire : 60 000 N/m
Requirement : 4100 N/m
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VERTICAL DAMPING
Log variation over the spectrum analyzed
Equivalent damping found using the Gehman model
Used to characterize the behavior of rubber tires
Order of magnitude in line with a rubber tire
Damping coefficient is slightly less than the requirement
Need to identify the parameters affecting the damping coefficient
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1
10
100
1000
10000
0.01 0.1 1 10
Dam
pin
g C
oeff
icie
nt
[Ns/
m]
Fréquence [Hz]
Equivalent damping
Prototype : 339 Ns/m
Rubber tire : 270 Ns/m
Requirements : 500-2000 Ns/m
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MTS MULTIAXIAL LOAD FRAME
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Important aspect of the design
Lateral stiffness
Constant vertical load
[230 430 630 830 1030 1530 2030]N
Lateral deformation
From 0 to 50 mm
The forces and displacements were recorded
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LATERAL STIFFNESS
Why is lateral stiffness important?
Keeps the vehicle parallel to the ground while traversing a slope
Maintains control of the vehicle while turning and in emergency manoeuvers
Skid steered vehicles
Increases with vertical load
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8
10
12
14
16
0 500 1000 1500 2000 2500 Late
ral st
iffn
ess
[N/m
m]
Vertical load [N]
At a vertical load of 2000 N
Prototype : 15.5 N/mm
Rubber tire : 33 N/mm
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SINGLE-WHEEL TEST BENCH
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Verify the performance
Drawbar pull
Objectives
Confirm the dynamic behavior
Validate the terramechanics model (performance predictions)
Constant speeds
Wheel speed
Soil bin speed
Fixed slip ratio of 30%
Constant vertical load
508 N Drawbar Pull
Terramechanics : 324 N
Experiment : 319 N
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CONCLUDING REMARKS
First prototype is encouraging
Vertical stiffness is higher than the requirement
Vertical damping is comparable to an ATV rubber tire, but is lower than requirement
Lateral stiffness needs to be increased
Terramechanics correlation with the experimental results is accurately validated
Fractional factorial design
Experimental model of the design
Influence of chosen geometrical parameters
Optimization theory to determine which combination of parameters give:
Maximum drawbar pull
Necessary admissible torque
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QUESTIONS
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