Evaluation and Optimization of Rover Locomotion Performance

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Evaluation and Optimization of Rover Locomotion
Performance
ICRA07, Rome Workshop on Space Robotics

• Thomas Thueer Roland Siegwart

Machines that know what they do
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Outline

• Locomotion Concepts
• Metrics
• Aspects Locomotion Performance
• Example Rover Comparison - Simulation Hardware
• Improving Locomotion Performance
• Conclusion and Outlook

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Locomotion Concepts

• How to design wheeled rovers for rough terrain?

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Characteristics of Locomotion Mechanisms

• Trafficability capacities to drive over a loose
  terrain
• Main parameters
• Wheel-Ground Contact
• Distribution of Mass
• Maneuverability mainly the steering capacities
• Locomotion mechanism (steering of wheels)
• Type of contact with ground
• Terrainability capacities to cross obstacles and
  maintain stability
• Locomotion mechanism
• Mass distribution
• Type of contact, number and distribution of
  contact point

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Classification of Locomotion M. Yim, 1995

• Basic motion concept
• Roll-Legged Rolling type motion
• Wheel, tracks
• Swing-Legged Walking type motion
• Legs
• Temporal characteristic of contact
• Continuous-Footed Continuous ground contact
• Rolling, snake-like motion
• Discrete-Footed Discrete ground contact
• Walking like contact, jumping
• Type of contact
• Little-Footed Point contact with ground
• Idealized point contact of wheel or leg
• Big-Footed Surface contact with ground
• Track-type contact, real wheel (tire), big foot
  of a walker

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Most popular locomotion concepts

• SDB (Swing-legged, Discrete- and Big-footed)
• Most today's humanoid robots
• Adapted for flat ground
• Stability very critical in rough terrain
• SDL (Swing-legged, Discrete- and Little-footed)
• Walking robots with 4 or 6 legs
• Reasonably good stability with 6 and more legs
• System and control very complex
• RCL (Roll-legged, Continuous- and Little-footed)
• Wheeled rover with rigid tiers
• Good stability if of wheels and suspension is
  adapted
• Good maneuverability
• RCB (Roll-legged, Continuous- and Big-footed)
• Tracked rovers
• Good stability and tracking
• Bad maneuverability

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Comparison of locomotion concepts
Compe-tence Concept Trafficability Maneuver-ability Terrainability System Complexity Control Complexity
SBDe.g humanoid with big foots ok good bad high very high
SDL e.g. 6 leg robot very good good good very high high
RCL EPFL or RCL E wheel rover ok good ok-good low low
RCB Nanokhod good bad ok low low
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Wheeled Rovers (RCL)Concepts for Object Climbing

• Purely frictionbased

Change of center of gravity(CoG)
Adapted suspension mechanism with passive or
active joints
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Catalog of Existing Solutions I
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Catalog of Existing Solutions II
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Metrics

• Necessary for proper comparison of different
  systems
• Know what conclusion you want to derive
• Requirements
• Precise definition
• Measurable
• Objectivity / independent from specific
  parameters
• Ideally available in simulation and reality
• Apply to normalized systems
• Absolute / relative comparison
• Level of accuracy (requirements, level of
  knowledge of final design)

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Metrics Overview

• Metrics for different aspects of performance
• Terramechanics
• Obstacle negotiation capabilities
• Metrics for sub-systems
• Evaluation independently from rover
• Same performance of sub-system on different
  rovers
• E.g. Rover Chassis Evaluation Tools (RCET)
  activity for wheel characterization

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MetricsTerramechanical Geometrical Aspects

• Analysis of wheel ground interaction based on
  Bekker
• Drawbar pullEqual for all rovers if normalized,
  independent from suspension
• Slope gradeabilityDepends on suspension that
  defines normal force distribution on slope
• Static stability
• See slope gradeability
• Geometrical analysis not sufficient!

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MetricsObstacle Negotiation (Terrainability)

• Minimum friction requirement
• Minimizing risk of slippage/getting stuck in
  unknown terrain
• Optimization equal friction coefficients
• Minimum torque requirement
• Minimizing weight and power consumption
• Slip
• Bad for odometry, loss of energy

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Example Rover Comparison - Simulation Hardware

• Comparison of different rovers
• CRAB (sim. HW)
• RCL-E (sim. HW)
• MER rocker bogie type rover (sim.)

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Example Rover Comparison Simulation Setup

• Performance Optimization Tool (2DS RCET)
• Static, 2D analysis
• Fast calculation allows for parametrical studies
  optimization of structures
• Over actuated systems optimization of wheel
  torques
• Results reflect full potential of structure(not
  influenced by parameter tuning, control
  algorithm)
• Simulations
• Benchmark step obstacle (tough task for wheeled
  rovers)
• Rovers normalized (mass, wheels, track, CoG, load
  dist.)
• Models with respect to breadboard
  dimensions/weight

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Example Rover Comparison Simulation Results
Required friction coefficient -
Required torque Nm
CRAB RCL-E MERFWD MERBWD
Required friction coefficient - 0.64 0.95 0.57 1.0
Max. T Nm 6.0 7.3 6.7 8.9

• Equally good performance of CRAB and MER
• Different forward and backward performance of
  asymmetric systems as potential drawback

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Rover Comparison Experimental Setup

• Rovers
• Modular design same wheels and electronics
• GenoM software framework
• Motors Maxon RE-max 22 Watt EPOS controllers
• Equal footprint (0.65 m), similar weight (32-35
  kg)
• Test runs
• Control velocity, velocity with wheel
  synchronization
• Two types of obstacle coating (rough,
  carpet-like)
• Step (wheel diameter high)
• At least 3 runs log of currents, encoder values

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Example Rover Comparison Experimental Results
(1)

• CRAB
• Success rate SR 100
• Slippage Slip 0.3 m
• RCL-E
• Success rate SR 0 Wheels blocked because of
  insufficient torque
• Modification of controller settings Maximum
  current increased (2.5 A ? 3.5 A 8.6 Nm ? 12 Nm)
• Success rate SR 47
• Slippage Slip 0.41 m

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Example Rover Comparison Video of Testing
Hardware tests with CRAB and RCL-E
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Example Rover Comparison Experimental Results
(2)
saturation

• Rover CRAB
• Successful test run
• Peaks indicate obstacle climbing of wheels
• Current graph
• Saturation at 2.5A
• Negative currents occur
• Distance graph (encoders)
• Normal inclination ? wheel moving or slipping
• Reduced inclination ? wheel blocked

negative currents
wheels blocked
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Example Rover Comparison Experimental Results
(3)

• Rover RCL-E
• Failed test rover blocked (current limit at 2.5
  A)
• Rear wheel saturated
• Front and middle wheel slip
• Successful test(current limit at 3.5 A)
• Current back wheel gt 2.5 A
• Front and middle wheel currents similar as above
  
• Problems in climbing phase can be detected
  (oscillation of signal)

wheels slipping
wheel slipping - lack of grip
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Example Rover Comparison Simulation vs.
Experiments

• Qualitative Analysis
• Strong correlation predictions measurements
• Significantly higher torque (SR 0 , 2.5 A) and
  friction coefficient (SR 47 , 3.5 A) of RCL-E
  than CRAB (SR 100 , 2.5 A)
• Same ranking simulation/hardware for all metrics
• Quantitative Analysis
• Discrepancy of numerical values (40 )
• Static, ideal model
• Validation of simulations through hardware tests
• (Ref Thueer, Krebs, Lamon Siegwart, JFR
  Special Issue on Space Robotics, 3/2007)

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Challenging Environment on Mars

• Spirit and Opportunity Robots on Mars since
  24.1.2004

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Motion Control Tactile Wheels

• Improvement of locomotion performance through
  motion control
• Control types
• Torque control
• Kinematics based velocity control
• Need for tactile wheel
• Wheel ground contact angle required
• First prototype on Octopus
• Development of new metallic wheel

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Flexible Wheels

• Better tractive performance
• Lower total motion resistance

Courtesy of DLR Köln
Total sinkage mm Wheel deflection mm Max. soil slope Required wheel output torque Nm Combined output power (6 wheels) W Required input power W
Rigid wheel D35 cm, b15 cm, grouser height3.4 cm, i10 45.8 - 13.9 13.87 10.6 25.2
Flexible wheel D35 cm, b15 cm, grouser height0.1 cm, pressure on rigid ground5 kPa, i10 12.9 12.8 13.9 6.17 4.7 11.2
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Navigation Motion Estimation and Control in
Rough Terrain
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Conclusion

• Locomotion mechanisms and their characteristics
• Metrics for different aspects of performance
• Example of evaluation and comparison of systems
• Focus on obstacle negotiation aspect of
  locomotion performance
• Static 2D analysis in simulation
• Verfication and validation with hardware
• How to improve performance
• Motion control
• Tactile wheel as sensor for wheel ground contact
  angle

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Outlook

• Continuous Flight on Mars

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Thanks for your attention!

• Acknowledgement
• This work was partially supported through the ESA
  ExoMars Program and conducted in collaboration
  with Oerlikon Space, DLR and vHS
• Questions ?