Rocker-bogie
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The Rocker-Bogie system is the suspension arrangement used in the Mars rovers (mechanical robot) for both the Mars Pathfinder and Mars Exploration Rover missions. It is currently NASA's favored design.[1] The term "rocker" comes from the design of the differential which keeps the rover body balanced, enabling it to "rock" up or down depending on the various positions of the multiple wheels. The term "bogie", on the other hand, comes from the old railroad systems and refers to the train undercarriage with six wheels that can swivel to curve along a track.[2]
[edit] Design
The Rocker-Bogie design has no axles or springs, and allows the rover to climb over obstacles, such as rocks, that are up to twice the wheel's diameter in size while keeping all six wheels on the ground. The suspension system also enables the rover's body to withstand a tilt of 45 degrees in any direction without tipping over.[2] The system is designed to be used at slow speeds of around 10 cm/s, so as to minimize dynamic shocks and consequential damage to the vehicle when surmounting sizable obstacles.
The rover climbs over an obstacle by having its wheels lift each piece of the suspension over the obstacle one portion at a time. The design uses a two-wheeled rocker arm on a passive pivot attached to a main bogie that is connected differentially to the main bogie on the other side. The body of the rover is attached to the differential such that it is suspended at an angle that is the average of the two sides. The ride is further smoothed by the rocker which only passes on a portion of a wheel's displacement to the main bogie.
The rocker bogie on the MER (Mars Exploration Rovers) uses four separate bogies. On each side, a main bogie holds the front wheel on a long arm, and a shorter arm connects to a secondary bogie that holds the middle and rear wheels.
JPL states that this rocker bogie system reduces the motion of the main MER vehicle body by half compared to other suspension systems.[1]
Each of the rover's six wheels has an independent motor. The two front and two rear wheels have individual steering motors which allow the vehicle to turn in place. Each wheel also has cleats, providing grip for climbing in soft sand and scrambling over rocks. The maximum speed of the robots operated in this way is limited to eliminate as many dynamic effects as possible so that the motors can be geared down, thus enabling each wheel to individually lift a large portion of the entire vehicle's mass.
In order to go over an obstacle, the front wheels are forced against the obstacle by the rear wheels. The rotation of the front wheel then lifts the front of the vehicle up and over the obstacle. The middle wheel is then pressed against the obstacle by the rear wheels and pulled against the obstacle by the front until it is lifted up and over. Finally, the rear wheel is pulled over the obstacle by the front two wheels. During each wheel's traversal of the obstacle, forward progress of the vehicle is slowed or completely halted. This is not an issue for the operational speeds at which these vehicles have been operated to date.
One of the future applications of rovers will be to assist astronauts during surface operations. To be a useful assistant, the rover will need to be able to move much faster than human walking speed or at least equivalent. Other missions which have been proposed, such as the Sun-Synchronous Lunar Rover, require even greater speeds (4–10 km/h).
[edit] References
- ^ David P. Miller, Tze-Liang Lee: High-speed traversal of rough terrain using a rocker-bogie mobility system.
- ^ a b Brick Vista: mechanical, thermal, and mobility
- Donald B. Bickler: Articulated suspension system. US Patent 4,840,394, June 20, 1989
