CRAB rover

Suspension mechanism:

The suspension structure of the CRAB is based on two parallel bogies linked with pivot joints on the middle wheel and on top. It is a passive structure, i.e. no motors are used to modify its shape, and it adapts naturally to the terrain.
Such a structure has great advantages compared to an active structure:

• Simple control
• Lower failure risk due to lower system complexity
  
• Less energy consumption as no motors have to adapt the structure to the terrain shape

Hardware schematic:

Software schematic:

Here is a brief description of the software running on the CRAB. The divers parts are described below:

E* Path Planning:

The E* algorithm is a path planner which is based on a weighted region approach. Thus, the underlying technique is expressed within the continuous domain, and corresponds to a wavefront propagating from the goal, toward the rover. The environment in which the rover is evolving is represented as a grid, constituted of node, or cells.

More information about the E* algorithm can be found there.

Trajectory Controller:

Rover Controller:

Online Rover-Terrain Characterization:

The idea is to allow the rover to learn online (while moving) to link the rover-terrain characterics (RTC), called local data, with remote data corresponding to the terrain appearance (e.g. acquired by a camera). Such knowledge would allow the rover to estimate its RTC in a similar situation. 

The algorithm has to be able to decide whether a terrain is different from those already met, in its appearance, its characteristics or both. This corresponds to unsupervised learning.

The CRAB measures both local and remote data and computes their corresponding features, F_l and F_r respectively.

Samples associating F_l and F_r^t can be determined, where F_r^t corresponds to a delayed F_r, matching F_l.

Based on these samples, the probability distribution function (pdf) is learned, using ProBT.

P(F_l|F_r^t)

The same pdf is then used to asses the RTC (corresponding to F_l) of areas of which F_r only is known. These correspond to terrain ahead of the rover.

Movies:

• D-MAVT presentation (8.3Mb)
• CRAB rover overview (33Mb)
  

Pictures:

CRAB I

The CRAB I is the first version that was built in 2005. The length of the parallel bogies is equal, affecting unequally the mass repartition on the wheels.

CRAB II

The design of the CRAB I was upgraded in 2006 for this one, which has a better suspension mechanism, providing an equal mass repartition on the wheels.

In 2007, several elements of the CRAB II were modified. Among these, the most important changes lie in the addition of tactile wheels and the change of the steering mechanism.

CRAB evolution

Related publications:

Technical information:

Poster

Contacts:

Ambroise Krebs

Thomas Thüer

Cédric Pradalier