mobility functional architecture and key design drivers

ASTRIUM CONFIDENTIAL Th

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bH]. ExoMars Rover Vehicle

Mobility Functional Architecture and Key Design Drivers ASTRA 2013 Nuno Silva // 17 May 2013


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Summary

1. Key Requirements 2. Localisation Function 3. Control Function 4. Perception, Navigation & Path Planning Function 5. Traverse Monitoring Function 6. Functional Architecture 7. Mobility Equipment 8. Mobility Operations 9. Mobility Modes 10.Rover Safety Whilst Driving 11.Conclusion

2 ExoMars Rover Vehicle Mobility Functional Architecture and Key Design Drivers

17 May 2013

Image credit: ESA


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1. Key Requirements

Martian Environment □ Terrain: slopes & rocks distributions, soil

□ Visual Environment

□ Sun light through atmosphere on Mars

□ As perceived by the Rover Cameras

□ Temperature: cold (on Mars) and hot (DHMR)

□ Limited solar energy: low power equipment

Autonomy □ 2 driving sols without ground in the loop

Performance □ 7m & 5º placement accuracy (MLG) after each 70m/sol

□ Drill placed within 0.15m &15º (7m away)

17 May 2013 ExoMars Rover Vehicle Mobility Functional Architecture and Key Design Drivers

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MER

Simulator

Image credit: NASA/JPL-Caltech


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2. Functional Analysis - Localisation

Need 1. Know where target is in order to reach it

□ Target in MLG need for Mars cardinal directions

2. Know Rover pitch and roll for terrain model

3. Know Rover position and attitude as it moves

□ Enabling target reached checking and corrections

How □ Absolute Localisation

□ Initialises Rover attitude in MLG using Sun and Mars gravity directions

□ Relative Localisation

□ Visual localisation accurately propagates initial state

□ Wheel Odometry and angular rates between visual frames


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Relative (VisLoc)

Absolute



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3. Functional Analysis - Control

Need □ Reach target

□ Follow & maximise nbr of safe paths

□ Hence, minimise corridor around path even with large terrain disturbances

How □ Use a highly manoeuvrable locomotion

actuator: 6 x 6 x 6 + 6W

□ Independent drive, steer or wheel walking

□ Simultaneous driving & steer, and driving &wheel walking (TBC)

□ Perform corrective manoeuvres in closed-loop

□ Dynamically change centre of rotation of Generic Ackermann & Point Turn

□ Translate Rover commands into individual & synchronised motor commands


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Open-Loop Closed-Loop


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4. FA - Perception, Navigation & Path Planning

Need □ Know the terrain around the Rover

□ Know how the terrain around the Rover compares with its driving capability

□ Have a safe, efficient & drivable path towards the target

□ Fast execution

How □ Use stereo-vision allowing to estimate a 3D

model of the surrounding terrain Note: loose soil is not detected this way

□ For each terrain location, assess if it is safe for the Rover to be there. If safe, characterise how difficult it is to drive over. Add necessary margins for knowledge & driving errors.

□ Use & tailor optimisation algorithms such that Rover drivability is considered in the path

17 May 2013 ExoMars Rover Vehicle Mobility

Functional Architecture and Key Design Drivers

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HiRISE DEM

MER



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5. Functional Analysis - Traverse Monitoring

Need □ Ensure Rover driving safety that Navigation chain

cannot avoid before hand (eg. slippage)

□ Ensure Rover driving safety when 1 failure occurs

□ Have all driving safety data available for monitoring

How □ Add functions estimating missing parameters that

have to be monitored

□ Add pre-processing of existing data making it compatible with FDIR PUS Service 12

□ Maximise use, within reason, of independent data

□ Limit monitors to safety cases, not performance checks


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MER: Spirit


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6. Functional Architecture


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MER: Spirit

CC

LocCam

CCNavCam

Sun Sensor AbsLoc

IMU (acc)

IMU(gyro)

RelLoc

VisLoc

WheelOdo

Estimator

Navigation

PreparationPerceptionTerrain Evaluation

Path Planning

Mobility Manager

Trajectory Control

Locomotion Manoeuver Control

Manoeuvre Motor Commands

GenAck, GenPT, Stop, ...

Position, Heading, Path Sequence

M

BEMA(x18 motors)

Actuator Drive

Electronics

Command

goto_target(x,y)

Referenceattitude

Traverse Monitoring

Coarse Tilt

Slippage

Safe Position

Position &Attitude

NavMap, Loc information

NavMap

NavMap, Pose & Target

Path

BCCAN Bus

ADE Manager

Mobility Equip.

Interface

Mobility Equip.

Interface

Mobility Equip. Interface

Corrected Manoeuvre Motor Commands

Locomotion

Pan & Tilt

Pan&Tilt

M


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7. Mobility Equipment

Localisation / Knowledge □ Sun Sensor: Sun direction in Rover frame

□ Accelerometer: Mars gravity vector in Rover frame

□ Locomotion (BEMA via ADE): wheel angles & speeds

□ Gyroscope: Rover angular rates

□ Localisation Cameras: for visual tracking of features as the Rover moves

□ Navigation Cameras: for perceiving the terrain surrounding the Rover

□ Pan & Tilt (via ADE): NavCam Pan & Tilt angles for 3D model of terrain

Actuation □ BEMA (via ADE): for driving the Rover

□ Bogies, wheels & associated drive, steer & walk actuators

□ Pan & Tilt (via ADE): for pointing NavCams


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MER: Spirit

Image credit: Neptec Design Group

Redundant Not Redundant

Partially Redundant

Sun Sensor X Gyroscope X Accelerometer X LocCam X NavCam X ADE X BEMA X


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8. Mobility Operations

Strategy □ Allow for gradual commanding from full autonomy to

lowest commanding level

□ Allow for switch-off of autonomy functions that might be inadequate for critical operations (eg. Egress)

□ Allow for “fast traverses” when in easy terrain

□ Keep determinism of operations with validated design

How □ Having a modular and hierarchical design

□ Allowing for different pre-determined and qualified levels of commanding

□ Structuring these into Mobility modes

□ Keeping it simple!


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MER: Spirit

MOB: Path Planning

MOB: Trajectory Control

MOB: Localisation

MOB: Locomotion

FollowPath

Generate Path

Determine Rover Position/

Attitude

CAN Bus

Locomotion Manoeuvre

LC_PATH_TRAV or LC_PATH_DRILL(Safe path)

LC_LOCOMOTION(Actuator/Manoeuvre)

LC_BUS(Actuator/Manoeuvre)

ADE

Data Pool

MOBSensors

MOB: Navigation

Terrain Evaluation

LC_NOM(Target)

LC_CHECK_PATH(Path)

LC_LLO

Command/Assess Goal


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9. Mobility Modes

Nominal Sequence □ Absolute Localisation (ABS_LOC) initialises the

Rover position and attitude

□ When Rover ready, Navigation (NAV) determines the drivability of the terrain surrounding the Rover

□ Afterwards, a path is planned (PP*) towards the target covering the next 2-3m

□ When complete, the Rover drives following that path (FPATH*) correcting for eventual deviations

□ It loops back to NAV until target reached

Special cases □ No Trajectory Control (LLO): eg. for Egress

□ Direct drive, monitoring only, safe mode


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MER: Spirit Mobility Modes

(showing Sub-Modes)

OFF

ABS_LOC

NAV

(LC_NOM)

LLO

PP_TRAV PP_DRILL(TBC)

FPATH_DRILL

LC_CHECK_PATH

LC_LLOLC_PATH_TRAV LC_PATH_DRILL

_EVAL

FPATH_TRAV

_STANDBY

_MOVE

_PER

_PREP

DDRIVE

MONO

LC_LOCOMOTION

Automated/Nominal

Reduced Autonomy

Note: - All modes have a nominal transition to ABS_LOC & MONO. - All nominal modes have a contingency transition to MSAFE.- These are not shown on the diagram.

MSAFE


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10.1 Rover Safety Whilst Driving

Rover safety is wider than only the elements related to driving □ Eg. Survival at night (following power issues)

□ In this presentation we only consider mobility related safety

Rover dynamic safety implications are different from spacecraft's (eg. Sun) □ Stop and wait is safe!

Not all driving problems are imminent safety threatening cases □ But may still require Ground intervention for resolution

Need to meet a success probability to reach target □ Maximise on-board resolution of non-safety threatening issues


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MER: Spirit Feared Event Cause

Loss of stability whilst driving

Steep slope Combined rock & slope Cliff

Collision Mars surface: terrain Deployment & Egress: lander

Stuck

Loose soil Large rocks Combined rock, slope, soil Overhang “Sit” on top of rock

Unsafe geometry (wheel in the air) Combined rock, slope, soil


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10.2 Rover Safety Whilst Driving

Feared event categories □ Safety critical?

□ Preventable by Navigation?

□ Can it safely be handled on-board?

□ Eg. dead-ends, edge of control corridor

□ If after using all safe options, no path is found, the Rover cannot keep driving and ground is required

Who feeds FDIR? □ Mobility Equipment Interface: equipment failures

□ Localisation: dangerous Rover tilt (fine), consistency, visual tracking…

□ Traverse Monitoring: slippage, rover tilt (coarse), position…

List of Monitors (preliminary): refer to paper


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MER: Spirit


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11. Conclusion

The ExoMars Rover Mobility will allow for the most autonomous Rover ever on the surface of Mars Such level of autonomy contributes

to science objectives of explaining the origin of life and the Universe The Mobility Sub-System has

reached TRL 6 in August 2011 and is now a pillar of the 2018 mission Next phases will be focused on

extensive testing on simulators and field testing on Mars yard


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LPM MDM


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Questions


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mobility functional architecture and key design drivers

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