Scarab DesignCarnegie Mellon13-14 December 2007
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Unique combination of drilling & driving on the moon
Central issues Solutions
Drilling loads
Weigh enough
Mount drill on center
Lower drill to ground
Lunar terrain
Agile suspension
Adjustable suspension
Low cg
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Design approach
Strong, slow & reliable
Serial work machine
Face disparate needs of drilling & driving
Arrive at capabilities that complement each other
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Weighing enough
Robot weight on lunar surface must support drilling operation
• Up to 250 N downforce & 50 Nm torque required for drilling• Reserve 150 N passing through wheels for stability, torque & margin
against uplift and spin
Total weight on lunar surface > 400 N
400 N / 1.622 m/s2 250 kg vehicle mass
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Mounting drill
• Fixed to chassis vs. articulated• Strength & stiffness of load path
through chassis & suspension back to the ground
• Dual as instrument mast
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Mobility design
• 4 wheels, directly driven• Skid steered
– Simplicity & Lunokhod precedence
• Passive kinematic suspension• 1 mechanical release• Differential
– Maintain rectangular stability pyramid base
• Linkage differential– Suspension provided attach points– Frees drill workspace– Stiffness
• Pose adjustment– Actuate height of each side– Outboard of differencing effect
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Suspension
1.3 m1.4 m
CG h = 0.6 m
Stability pyramid
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Wheel actuation
• Local amplifier• Brushless motor• 5:1 planetary• 80:1 harmonic drive• 400:1 total reduction• Rim pull ~ vehicle weight
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Agile suspension
• Passive matching of terrain• Large stroke for terrain
approaching wheel diameter in size
• Steady platform for sensing
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Agile suspension
Twist course video
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Underbody shape
Maintains 30 cm belly clearance with a wheel on 30 cm positive obstacles
Keeps drill tip closest to ground when kneeling
30 cm 30 cm
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Lowering drill
• Major benefit for drill system
• Sensors inspect site prior to kneeling
• Scarab poses with belly just above ground
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Pose adjustment mechanism
• Raises & lowers by actuating wing angle (independent L & R)• Center link bisects wing angle: enables lift-and-level body averaging• Retains advantages of passive rocker bogie• Many ways to implement
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Mobility benefits
• Climbing slopes otherwise unable to• Leaning into cross-slopes for stability• Autonomous body roll leveling• Raising to avoid or recover from high centering• Changing wheelbase in
reaction to periodic terrain
• Inch-worming out of dug-in condition
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Scalability
• Body is readily modifiable to suit payloads
• Configuration is scalable in both directions
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Specifications
Mass: 280 kg Weight: 460 N 2750 N
Power (driving): 200 W (peak) Power (posing): 380 W (peak) Power (idle): 78 W
Speed: 5.0 cm/s (6.0 cm/s max)
Height (with drill tower): 2.2 m high stance, 1.6 m low stanceWidth (wheelbase): 1.4 mLength (wheelbase): 0.8 - 1.3 m Aspect (track/wheelbase): 1:1 low stance, 1:2 nominal, 1:7 highWheel diameter: 60 cm
Additional Material
CMU | 13 December 2007 18
Specifications
CG height: 0.64m nominal, 0.60m low, 0.72m high
Static pitchover: 42° nominal stance, 29° high, 45° low Static rollover: 53° nominal stance, 48° high, 55° low
Maximum / minimum straddle: 57 cm, Belly contact
Approach / departure angle: 105° nominal stanceBreakover angle: 115° nominal stance
Rim pull (single wheel): 2500 NDrawbar pull: 1560 N (medium-coarse grain sand)
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Design solution
• Drill implementation– Central location on vehicle to maximize weight for downforce– Direct mounting to chassis– Fixed drill structure
• Reduced actuation• Functions as navigation mast• Simplifies kinematics & mass properties
• Adjustable kinematic suspension– Body roll averaging over terrain– Bring drill to surface to operate– High stiffness platform to react drilling forces
• Skid steering– Reduced actuation– Increased stiffness
• Thermal approach– Utilize heat from radioisotope power supply – Shunt excess heat to radiator surface
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Vehicle requirements
• Drill dominated design– Bring drill to surface to operate– High stiffness platform to react forces
• Mobility over rough terrain– 30 cm obstacles– Steep soil slopes
• Environments– Fine, abrasive dust– Vacuum, 40 K ground, 3 K sky
• Power– Radioisotopic power supply
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NORCAT coring system
• 1 meter drilling, sampling & processing system– Lab R&D maturity
• Specs– ø30 cm borehole– ø1.5 cm continuous core– ~50 kg– 0.5 m x 0.5 m x 1.5 m volume
• Operations:– Drill to depth– Capture core, transfer– Meter core into pieces– Crush into fines– Transfer to oven
• Issues:– Loads, torques, vibrations– 1500 – 3000 cc cuttings pile
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Drill cuttings
cm 10 x cm to up cm 7.5 x cm from Range
cone 35 repose of Angle
cuttings cm 3000 to cm 1400
400% to 200% from factors Expansion
borehole cm 700 depth m borehole, cm
core continuous mm 15 takes drill RESOLVE
drilling faced full assume :Worstcase
33
3
3525
13
∅∅°
=∅
∅
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Potential attributes
• Internal actuation: shafts through shoulder & shaft-drive to hubs
• Actuated suspension to surmount extreme obstacle or extricate from twist
• Space-relevant wheels & tread: design, fab, mount• Hosting more of RESOLVE subsystems• Upscale chassis and body-averaging beam• Thermal isolation of cold drill and warm body
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Nominal ride height
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Nominal ride height
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Nominal ride height
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JPL Sample-Return Rover
SRR1– 4-wheel skid, rotary actuated
shoulder, differential body pose
SRR2K– 4-wheel steering added
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