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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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