Anna Freitas
On July 4, 1997, the Mars Pathfinder, encapsulated in a 17’ by 17’ tetrahedral airbag, bounced along the surface of Mars. Cocooned within the cushioning was a mobile robot the size of a microwave oven—Sojourner—the first rover to successfully operate on Mars. For 83 Mars days the Sojourner gathered critical information about the Martian environment, including the mineral composition of rocks and the topology near its landing site, which provided evidence that Mars once had water.
Designing a Mars rover posed a stubbornly intricate challenge as researchers needed to make a robot to traverse a surface no one had seen up-close. Sojourner, like the subsequent Mars rovers, was designed with a six-wheel, rocker-bogie suspension. While the rocker-bogie suspension contributed to compact construction as well as equal weight distribution, this suspension posed its own set of dangers. If the rover hits an obstacle or goes over a large incline, rotation around the pivot that connects the rocker to the bogie risks tipping the rover. Additionally, the traditional rocker-bogie mechanism has issues with singularity: when the suspension reaches a geometric position where the bogie is stuck, it renders the robot immobile. Last, as traditional rocker-bogie suspension robots are prone to tipping, they are also slow. In light of these limitations, A. Cihan et al. designed a mobile robot to combine the rocker-bogie suspension with Chebyshev’s lambda mechanism.
Referred to as the rocker double lambda bogie mechanism, the new suspension consists of two, symmetric lambda mechanisms to make up the bogie. The key feature of this novel suspension is that the lambda mechanism converts rotational motion into linear motion. Likewise, the force generated when a mobile robot encounters a steep incline or an obstacle will have an output of linear motion rather than that of rotational motion, meaning that no rotation of the bogie will occur. As a result, when the rocker double lambda bogie mobile robot should not tip over.
To determine if these modifications increased stability, the scientists tested the new mechanism in a self-made robot. Like the Mars rovers, an effective robot needs to be able to navigate rough terrain and not tip over when moving on an incline of greater than 30 degrees. The researchers found that the robot was stable while climbing up these steep inclines. Additionally, the new suspension addressed the issue of the suspension getting stuck, with no observed singularity. Taken together, the findings of this study suggest that the new mechanism may be a significant improvement on the widely implemented, traditional rocker-bogie.
On average, rocker-bogie robots are only able to travel about one meter per second to maintain stability. Future experiments are needed to assess whether the rocker double lambda bogie robots can achieve higher speeds than their predecessors without tipping. A significantly higher speed would be beneficial in that these robots could collect more samples, or samples from locations much further than the landing site, in a given period.
Overall, the researchers validated that the rocker double lambda bogie mechanism can be implemented in a robot. Their preliminary findings suggest that a suspension integrating the new mechanism may be a good alternative to the traditional rocker-bogie. After further research to validate its improvements in stability, singularity, and speed, the rocker double lambda bogie mechanism may be implemented more widely in mobile robots, including future Mars rovers. This new rover would be less prone to immobilization on uneven terrains and able to collect environmental data more efficiently.