autonomous systems lab 1 evaluation and optimization of rover locomotion performance machines that...
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Evaluation and Optimization of Rover Locomotion Performance
Machines that know what they do
Thomas Thueer & Roland Siegwart
ICRA’07, RomeWorkshop on Space Robotics
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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 terrainMain parameters:
• Wheel-Ground Contact• Distribution of Mass
Maneuverability: mainly the steering capacitiesLocomotion mechanism (steering of wheels)Type of contact with ground
Terrainability: capacities to cross obstacles and maintain stability
Locomotion mechanismMass distributionType of contact, number and distribution of contact point
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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 definitionMeasurableObjectivity / 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
TerramechanicsObstacle negotiation capabilities
Metrics for sub-systemsEvaluation independently from roverSame performance of sub-system on different roversE.g. Rover Chassis Evaluation Tools (RCET) activity
for wheel characterization
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bMetricsTerramechanical & 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 stabilitySee slope gradeabilityGeometrical analysis not sufficient!
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bMetricsObstacle Negotiation (Terrainability)
Minimum friction requirementMinimizing risk of slippage/getting stuck
in unknown terrainOptimization: equal friction coefficients
Minimum torque requirementMinimizing weight and power
consumption
SlipBad for odometry, loss of energy
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Example: Rover Comparison - Simulation & Hardware
Comparison of different roversCRAB (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 analysisFast calculation allows for parametrical studies:
optimization of structuresOver actuated systems: optimization of wheel torquesResults reflect full potential of structure
(not influenced by parameter tuning, control algorithm)
SimulationsBenchmark: 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
CRAB RCL-E MER
FWD
MER
BWD
Required
friction
coefficient [-]
0.64
0.95 0.57 1.0
Max. T [Nm] 6.0 7.3 6.7 8.9
Req
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Req
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[Nm
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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
RoversModular design: same wheels
and electronicsGenoM software framework Motors: Maxon RE-max 22 Watt; EPOS controllersEqual footprint (0.65 m), similar weight (32-35 kg)
Test runsControl: velocity, velocity with wheel synchronizationTwo 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)
CRABSuccess rate: SR = 100 %Slippage: Slip = 0.3 m
RCL-ESuccess rate: SR = 0 %
Wheels blocked because of insufficient torqueModification 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)
Rover: CRAB Successful test run Peaks indicate obstacle
climbing of wheels Current graph
Saturation at 2.5ANegative currents occur
Distance graph (encoders)Normal inclination
wheel moving or slipping
Reduced inclination wheel blocked
saturation
wheels blocked
negative currents
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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 saturatedFront and middle wheel slip
Successful test(current limit at 3.5 A)
Current back wheel > 2.5 AFront 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 AnalysisStrong 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 AnalysisDiscrepancy 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 typesTorque controlKinematics based velocity
control Need for tactile wheel
Wheel ground contact angle required
First prototype on OctopusDevelopment of new “metallic“ wheel
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Flexible Wheels Better tractive performance Lower total motion resistance
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 D=35 cm, b=15 cm, grouser height=3.4 cm, i=10 %
45.8 - 13.9
13.87 10.6 25.2
Flexible wheel D=35 cm, b=15 cm, grouser height=0.1 cm, pressure on rigid ground=5 kPa, i=10 %
12.9 12.8 13.9
6.17 4.7 11.2
Courtesy of DLR Köln
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bNavigation – 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 simulationVerfication and validation with hardware
How to improve performanceMotion controlTactile wheel as sensor for wheel ground contact angle
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Outlook Continuous Flight on Mars
3.2 m
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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 vH&S
Questions ?