Cool RobotCool RobotMechanical Design of a Solar-Mechanical Design of a Solar-

Powered Antarctic RobotPowered Antarctic Robot

Alex PriceAlex PriceAdvisor: Dr. Laura RayAdvisor: Dr. Laura Ray

Thayer School of Engineering at Dartmouth CollegeThayer School of Engineering at Dartmouth College

2

Project GoalsProject GoalsTraverse the Antarctic south polar plateau

autonomously on renewable energyRelatively cheap (about $20,000)Travel 500 kilometers in 2 weeksEasy to handle, transport, and maintain

– As lightweight as possible (also for energy reasons)– Small enough to fit inside the Twin Otter aircraft. – Easily assembled and tested after delivery– Scientific instruments easily added and integrated

3

Antarctic PlateauAntarctic Plateau Large central flat plateau

– High altitude (2800 meters)– Cold (-20° to -40° C in summer)– Dry and sunny, but windy– Firm, clean snow– Flat, but with wind-sculpted

“sastrugi” snow drifts Possible Robot Missions

– Automated distributed sensing Magnetometers Ionosphere studies

– Ground-penetrating Radar– Traverse team support– Ecological Studies

Sastrugi

4

Specifications and SolutionsSpecifications and SolutionsSpecifications:

– Average Speed of 0.4 m/s, top speed at least twice that– Maximum dimensions to fit in Otter:

1.5 m long 1.2 m wide 1.2 m tall

– Less than 75 kg empty ; 15 kg payload capacity. – Maximum ground pressure of 3 psi

Design to achieve those goals:– Specialized lightweight construction– Optimized dimensions– Careful component selection (tires, bearings, etc.)– Custom wheels, hubs, and drive train components

5

Overall Robot DesignOverall Robot Design

Solar panels attached over chassisand wheels by support arms

Tube on top of chassis box may be required to support center of top panel

Insulation is likely not required.

6

Solar Power in the AntarcticSolar Power in the Antarctic In summer, sun never sets, but is

always at a low angle Sun is brighter in high, dry climate

– As bright as 1200 W/m2 on a clear day– Few cloudy days in the central plateau

Significant reflected light from snowfield– Proportional to sun azimuth– Snow albedo of as high as 0.95

Diffuse component of insolation as large as 100 W/m2 from atmospheric scattering

Sunny day insolation fairly constant, but scattering and cloud cover varies with the time of year.

Sun Azimuth vs Day of Yearat 85° South Longitude

0

5

10

15

20

25

30

0 60 120 180 240 300 360

Day of Year (1 = Jan. 1st)

Azimuth (deg. above horizon)

Max Azimuth Min Azimuth

Variation in azimuth between max and min decreases to zero at 90°, at the pole.

7

Solar Power in the AntarcticSolar Power in the Antarctic

Available Power in Average Summer Sun: 1000 W/m2 of solar power available on an average sunny day

Sun azimuth angle 20° from horizon (average for November-February)

Robot facing front towards sun (worst case) ; Snow albedo 90%

Panel capacities are based on nominal 1-sun (1000 W/m2) input: 100% = 200 W/m2 energy output (20% efficient cell in direct sun)

Front128%

Top (direct sun only)

34%

Back (in shadow)

11%

Sides34%(reflected light only)

8

Scaling CapabilityScaling Capability

Design can be scaled well to a variety of sizes for different mission goals.

Performance of Different Robot Size Options (in average sun)

0%

20%

40%

60%

80%

100%

120%

140%

LargeMedium

9x6 9x5 8x5SmallRobot Configuration

Empty Mass (% of 75 kg goal)

Power Available (% of full power)

Maximum Speed (% of 1 m/s)

Notes on configurations: Large = 11x9x8 cells (9x9 on top)Med. = 10x9x6 cells (9x8 on top)9x6, 9x5, and 8x5 sizes are "cubic" with that size panel on each of the 5 sides. Small = 7x7x4 cells (7x4 on top), also cubic, with only 2 motors.

9

Tire SelectionTire Selection

Ideal tire would be lightweight and would have good traction, low ground pressure, and low rolling resistance; but no such tires are available within budget.

ATV tires

Russian Snow Bug tire

Custom cut tire

Apollo 17 rovermesh wheel Roleez ballon tire Mars Rover solid wheel

10

Tire SelectionTire Selection

Best tire of available selection was Carlisle’s 16x6-8 knobby ATV tire– About 6.5 pounds, very stiff, good tread pattern

11

Wheel DesignWheel Design

Commercially available wheel options are not suitable. – Aluminum racing wheels are all too large– Available 8”x5.5” wheels are too heavy (> 2.3 kg)– Require the use of heavy bolts and hubs

Thus, a custom wheel had to be designed to meet the requirements of the design

ITP aluminum Carlisle steel standard 1st design iteration

12

Wheel DesignWheel Design

Factor of Safety of 3 against static failure in worst-case loading

Factor of Safety of at least 2 against fatigue failure in worst-case driving conditions

Only 0.9 kg, and uses smaller bolts & hub Tubeless if 2 halves are sealed

13

Hub DesignHub Design

Standard 4-inch bolt circle Welds to drive shaft, bolts to wheel tabs Factor of safety of at least 2.5 against fatigue

failure in worst-case loading

14

Assembled WheelsAssembled Wheels

Wheel + hub + nuts and bolts = 1.1 kg– Far better than the commercially available 3+ kg

Total assembly (with tire and covers) = 4 kg Total weight savings on robot = 8 to 9 kg

15

Drive TrainDrive Train

Very efficient motor and gearboxCustom hollow aluminum shaft and supports

Option 1: Cantilevered support tube with press-fit bearing, minimizes loads on gearbox.

Option 2: Bearing pair to carry load, motor mounted loosely so bearings will support the bending loads.

16

Integration and AssemblyIntegration and Assembly

Heaviest components mounted in the center Motors, controllers, power electronics, and scientific

instruments mounted symmetrically on chassis

17

Future Plans and GoalsFuture Plans and Goals Complete Design and Test Components

– Wheels and Hubs NC machined– Drive Train design completion– Assemble and test drive train– Assemble and test solar panels

July - Chassis operational on batteries

August - Solar power systems tested and operational

September - Robot operational on solar power

Next year - Testing in Greenland and in Antarctica!

18

ConclusionsConclusions Design has been

optimized within the strict parameters

Robot should easily meet the mission goals

Future versions could be lighter and faster.

Autonomous navigation at the south pole is a daunting task, but we are well on our way to achieving that goal.

Building a robot is a lot of work, but has been and will continue to be a great experience.

19

AcknowledgementsAcknowledgements Laura Ray Alex Streeter ENGS 190/290 group Guido Gravenkötter Gunnar Hamann Mike Ibey Kevin Baron Pete Fontaine Leonard Parker Paula Berg Cathy Follensbee

Jim Lever Dan Denton CRREL Marc Lessard Gus Moore ‘99 Michael at Wilson Tire Don Kishi at Carlisle Tire National Science Foundation

Everyone at Thayer School who has made this possible

Full reference and bibliography information is included in the report.