Scarab Design Carnegie Mellon 13-14 December 2007.

Scarab DesignCarnegie Mellon13-14 December 2007

CMU | 13 December 2007 2

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


Design approach

Strong, slow & reliable

Serial work machine

Face disparate needs of drilling & driving

Arrive at capabilities that complement each other


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


Mounting drill

• Fixed to chassis vs. articulated• Strength & stiffness of load path

through chassis & suspension back to the ground

• Dual as instrument mast


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


Suspension

1.3 m1.4 m

CG h = 0.6 m

Stability pyramid


Wheel actuation

• Local amplifier• Brushless motor• 5:1 planetary• 80:1 harmonic drive• 400:1 total reduction• Rim pull ~ vehicle weight


Agile suspension

• Passive matching of terrain• Large stroke for terrain

approaching wheel diameter in size

• Steady platform for sensing


Agile suspension

Twist course video


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


Lowering drill

• Major benefit for drill system

• Sensors inspect site prior to kneeling

• Scarab poses with belly just above ground


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


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


Scalability

• Body is readily modifiable to suit payloads

• Configuration is scalable in both directions


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


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)


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


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


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


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

∅∅°

=∅

∅


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


Nominal ride height


JPL Sample-Return Rover

SRR1– 4-wheel skid, rotary actuated

shoulder, differential body pose

SRR2K– 4-wheel steering added

Scarab Design Carnegie Mellon 13-14 December 2007.

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