SnoMote

Although weather data from glacial regions is considered important and valuable, this data is difficult to obtain. Currently, human expeditions must be sent to collect this data, which is costly, time consuming, and dangerous. Yet, this approach yields data about a very limited area, and covers only a short duration in time. As part of the solution, a set of fixed weather stations have been installed, known as the Greenland Climate Network. While these weather stations provide a continuous data feed, only 18 such stations exist covering an area of over 650,000 sq. mi. Since this data is so sparse, scientists would like a team of mobile weather stations at their disposal to collect dense weather measurements at the time and place of their choosing.


However, arctic regions present a large assortment of terrain challenges. Large quantities of fresh surface snow can be present during certain times of the year. This fresh snow is soft, creating a potential sinking hazard for wheeled vehicles. Over time the winds harden the snow surface making it more amenable to locomotion. However, these same winds also sculpt the snow into dune-like structures that can be as large as one meter, again impeding motion. Tracked vehicles have been developed to overcome the specific challenges of arctic travel. The most famous of these devices is the snowmobile, but other variations exist ranging in size from small single person vehicles to bus-sized multi-passenger coaches. These platforms have been successful in military, commercial, and science applications since their development in the 1940s.

SnoMote Mk1

For these reasons a snowmobile chassis was selected as the base for the ``SnoMote'' prototype mobile sensor. A set of three prototype robotic rovers were constructed in our lab in anticipation of field testing. The rovers are based on an RC snowmobile chassis and have been retro-fitted with a Connex 400XM processor from Gumstix. This motherboard contains a 400MHz ARM processor, wireless 802.11g ethernet, and bluetooth capabilities. Additionally, a Robostix board was added, which includes an Atmel ATMega 128 RISC microcontroller, providing both SPI and I2C serial ports, general purpose IO pins, PWM outputs, and an ADC unit. The original steering mechanism was replaced by a servo motor to provide proportional steering control, while an H-bridge amplifier provides modulated voltage to the DC drive motor for variable speed control. Both motors are controlled by the Gumstix processor using the PWM outputs.

For navigation, a GPS unit connects to the embedded processor via the bluetooth interface, while a magnetic compass provides heading information via the I2C serial bus. Sensor data and internal state information are exchanged between scientists and other rovers over the bidirectional wifi link. Additionally, a 0.3 Megapixel wireless camera on-board each SnoMote provides real-time images.

To simulate the science objectives of the mobile sensor network, a weather-oriented sensor suite was added to each rover. Ultimately this science package will include an anemometer and a solar radiation sensor, among others. However, the size and expense of these types of sensors were not a good fit with the small footprint of the prototype platform. Instead, a set of solid-state sensors were selected that could measure meaningful weather related data and still fit within the confines of the rover's chassis. The final instument suite includes sensors to measure temperature, barometric pressure, and relative humidity.

SnoMote Mk2

During the field tests of the Mk1 rovers, it was discovered that the snowmobile-inspired platform suffered from stability issues. Standard snowmobiles operate with a motor driving a single track system located in the central rear body of the chassis which is guided by two runners located near the nose of the chassis on either side. Typically this design lends itself for maneuverability when combined with a rider who shifts their weight to vary applied forces and restore balance. Without the physical presence of a rider, the standard design lacks the ability to systematically redistribute weight and is vulnerable to toppling.

A dual tread drive train system was implemented in response to these problems. The new system improved performance over the original design in two major ways. First, it nearly doubled the surface area in contact with the snow. By increasing the contact area, issues of sinking and traction loss in soft snow are reduced. Secondly, by modifying the rear sector of the chassis, the surface contact footprint was converted from a nearly triangular pattern to a more rectangular shape. This greatly improves the stability characteristics of the platform, reducing the likelihood of roll-over.

The original platform also suffered from steering deficiencies. The stock steering linkages of the snowmobile lacked the necessary rigidity to effectively maneuver through the depths of snow present as the test site. At the same time, the stock suspension system was too stiff, directly translating surface changes into body roll, instead of compensating ride height. To correct these issues, the stock single-arm plastic linkage was replaced with an aluminum double-wishbone design. The new parallel linkage system extended the ski separation by 30%. The newly installed suspension additionally included a spring-over-oil damper system, which was easily adjustable in the field. This allowed the stiffness to be tuned to the quality of the terrain.

Acknowledgments

The funding for this project has been provided by the Earth Science Technology Office (ESTO) at the National Aeronautics and Space Administration (NASA), under the Advanced Information System Technologies (AIST) program.