Cool RobotMechanical Design of a Solar-Powered Antarctic Robot Alex Price Advisor: Dr. Laura Ray Thayer School of Engineering at Dartmouth College
Project Goals • Traverse the Antarctic south polar plateau autonomously on renewable energy • Relatively cheap (about $20,000) • Travel 500 kilometers in 2 weeks • Easy 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
Antarctic 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
Specifications and Solutions • Specifications: • 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
Overall 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.
Solar 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. Variation in azimuth between max and min decreases to zero at 90°, at the pole.
Solar Power in the Antarctic Top (direct sun only) 34% 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% Back (in shadow) 11% Front 128% Sides 34% (reflected light only) Panel capacities are based on nominal 1-sun (1000 W/m2) input: 100% = 200 W/m2 energy output (20% efficient cell in direct sun)
Scaling Capability • Design can be scaled well to a variety of sizes for different mission goals.
Tire 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 Custom cut tire Russian Snow Bug tire Apollo 17 rover mesh wheel Roleez ballon tire Mars Rover solid wheel
Tire Selection • Best tire of available selection was Carlisle’s 16x6-8 knobby ATV tire • About 6.5 pounds, very stiff, good tread pattern
Wheel 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
Wheel 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
Hub 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
Assembled 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
Drive Train • Very efficient motor and gearbox • Custom hollow aluminum shaft and supports Option 2: Bearing pair to carry load, motor mounted loosely so bearings will support the bending loads. Option 1: Cantilevered support tube with press-fit bearing, minimizes loads on gearbox.
Integration and Assembly • Heaviest components mounted in the center • Motors, controllers, power electronics, and scientific instruments mounted symmetrically on chassis
Future 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!
Conclusions • 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.
Acknowledgements • 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 • Laura Ray • Alex Streeter • ENGS 190/290 group • Guido Gravenkötter • Gunnar Hamann • Mike Ibey • Kevin Baron • Pete Fontaine • Leonard Parker • Paula Berg • Cathy Follensbee • Full reference and bibliography information is included in the report.