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MULTI-LEVEL SOIL SENSING SYSTEMS TO
IDENTIFY SAFE TRAFFICABILITY AREAS FOR EXTRA-PLANETARY ROVERS
Francisco Comin
May, 16th 2013
2
Context and Motivation
MISSIONS
•MSL
•MER
•PATHFINDER
OBJECTIVES
•Climate and geology
•Human exploration
•Life on Mars?
Da
y N
igh
t
FAST
SAFE
Source: NASA
Source: NASA
PRIMARY ROVER
• EXOMARS test bed
• Mission-critical
• Wheeled and heavy
SCOUT ROVER
• DFKI Prototype
• Terrain characterisation
• Mobile and lightweight
Mission Concept
4
Soil Sensor Systems
• Soil sensor sub-systems and data fusion
PRIMARY ROVER
Remote
Sensing
DATA FUSION
STAGE 1
Wheeled-Bevameter
Or
PathBeater
DATA FUSION STAGE 2
NAVIGATION SYSTEM
SCOUT ROVER
Wheel-Leg
Soil
Interaction
Ground
Penetrating
Radar
Dynamic
Plate
Dynamic
Cone
Penetrometer
Outputs
GO MAYBE NO-GO
% TRAFF.
5
• Soil sensors deployment and detection properties
Soil Sensor Systems
6
Replicate pressure footprint
► Safety margin for low sinkage
► Maximum sinkage of 75mm
Wheel-Leg-Soil Interactions (UoS)
• Load Testing Foot (LTF)
7
Wheel-Leg-Soil Interactions (UoS)
• Sensor Components and Function
Output Shaft Encoder
► Absolute angular position
► Angular speed
Current Transducer
► Motor current consumption
Inertial Measurement Unit
► 3-axis linear accelerations
► 3-axis rotation rates
Belly Camera
► Wheel-leg terrain interface images
8
Wheel-Leg-Soil Interactions (UoS)
• Single wheel-leg lab testing and raw data acquisiton
9
Colour segmentation (blue) and blob detection
Masking of the area of interest
Sinkage measurement from wheel-leg hub to blob edge
Wheel-Leg-Soil Interactions (UoS)
• Vision-based sinkage detection
(1)
(2)
(3)
(4)
Shadows
Noise
10
Layers of materials with different properties
Discriminate soil type and layer thicknesses
Detect subsurface voids
Ground Penetrating Radar (UoS)
• GPR Simulations
Source: Cobham PLC Source: Cobham PLC
11
Hybrid Dynamic Plate / Cone (UoS)
IMU
• Hybrid Actuation Mechanism
Low mass of scout
requires dynamic impact
to generate desired force
Common drive motor to
save payload space and
mass
Dual-mode to adjust
impact force through
spring pre-load
Coupling mechanism
► Coupled for plate
► Un-coupled for cone
12
Selection of appropriate impact forces
Tests with three plate configurations
► Solid
► With Hole
► With Cone
Hybrid Dynamic Plate / Cone (UoS)
• Manual Drop Mass Tests
SSC-2
ES-3
13
Clearer differentiation in initial blows
Asymptotic trend for higher blow number
No significant differences between plate configurations
Hybrid Dynamic Plate / Cone (UoS)
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6
Dep
th p
er
Blo
w (
mm
)
Blows
• Plate Tests Results
Tests on ES-3
Solid Plate
Plate with Hole
Plate with Cone
Marker shape (Plate Configuration)
Loose Preparation
Medium Preparation
Dense Preparation
Marker colour (Density Preparation)
14
First two blows discriminate different soil preparations
Asymptotic trend for further blows
Steepness of curve also reveals soil characteristics
Hybrid Dynamic Plate / Cone (UoS)
0
10
20
30
40
50
60
70
0 2 4 6 8 10
Sin
kag
e (
mm
)
Number of Blows
ES-3 - Sinkage per blow (mm)
0
10
20
30
40
50
60
70
0 2 4 6 8 10
Sin
kag
e (
mm
)
Number of blows
SSC-2 - Sinkage per blow (mm)
• Cone Tests Results
15
Passive wheel with downward force
Four-bar mechanism allows vertical range of movement
No-Brake operation
► Encoder measures wheel rotation Ground speed/Slip estimates
► Slip, sinkage and vertical force Load bearing Bekker parameters
Full-Brake operation
► Triggered periodically in time or traversed distance
► Slip and horizontal force Shear strength Bekker parameters
Wheeled Bevameter (LSG)
• Operating Principle
No-Brake
Operation
Full-Brake
Operation
Periodically Normally
16
Edge mounted to lie on wheel path
Hinge actuator to change between stowed and deployed
Release/lock actuator for the four-bar mechanism
Wheeled Bevameter (LSG)
Stowed Position Deployed Position
• Deployment
17
Slope compliance
Obstacle impact damping
Automated stall detection
Wheeled Bevameter (LSG)
• Mechanical mock-up testing
18
Two long arms with a pyramidal cone tip at their ends
Stow for safety in case of steep slopes or obstacles
Load Bearing Test
► Whipping motion of arm, impact of cone on the soil
► Impact forces and penetration measured by an IMU
Shear Strength Test
► Short driving with cone in the soil
► Resistance force measure in arm deflection by strain gauges
PathBeater (LSG)
• Operating Principle
Safety
Check
Load Bearing
Test
Unsafe Shear Strength
Test
Safe
19
One arm mounted on each edge
Arms lie over wheel paths in deployed position
Spring-loaded whipping actuation
PathBeater (LSG)
• Deployment
Stowed Position
Deployed Position
Actuated Position
20
PathBeater (LSG)
• Slope Considerations
Steep slope or obstacle detection to avoid damage
21
PathBeater (LSG)
• Free fall and penetration tests
Free fall tests with different masses and cone angles
Automated penetration tests with sensor data acquisition
22
Conclusions
Collaborative mission concept for safer and faster navigation
High-mobility scout dedicated to terrain characterisation
Development of sensors with overlapping areas of detection:
► Combine their terrain characterisation properties
► Redundancy in detection for increased robustness
Multi-level sequential deployment with increasing reliability:
► Reduce power consumption
► Minimise stopping time of the Scout rover
Sensors usable jointly/separately in other mission concepts
Initial testing of the different sensor systems in lab conditions
23
Thanks for your attention!
Scout Rover Sensors
William A. Lewinger (UoS) e-mail: [email protected]
Francisco Comin (UoS) e-mail: [email protected]
Marcus Matthews (UoS) e-mail: [email protected]
Sinkage Detection
Said Al-Milli (UoS) e-mail: [email protected]
Conrad Spiteri (UoS) e-mail: [email protected]
Yang Gao (UoS) e-mail: [email protected]
Wheeled Bevameter
Lutz Richter (LSG) e-mail: [email protected]
PathBeater
Stephen Ransom (LSG) e-mail: [email protected]
Technical Manager
Chakravarthini Saaj (UoS) e-mail: [email protected]
