Servo - January 2008

techniques or by dragging a cursor over the 3D terrain using the mouse.

A basic route can be created by dragging a 3D rover model to each successive waypoint in the terrain model. The operator’s experience is necessary to select reasonable routes and to plan drives with the tight operational communication windows. The operators must select the appropriate traverse and localization methodologies.

Once a sequence of desired locations relative to the terrain is decided, motion commands can be generated for path planning using a grid-based estimation of surface traversability applied to local terrain. Motion commands are then stored and packed to be sent to Mars.

• Rover perception (Mars). Reliable perception is mandatory when the robot is navigating, because dead reckoning information is clearly unreliable, especially in rocky terrain. Cameras are used because they provide a lot of information if compared to other sensors.

In particular, Visual Odometry is used when navigating. This algorithm computes the rover displacement between two successive camera images. Specifically, an image is taken before a rover’s movement. After the movement, another image is taken mostly of the same terrain area. Features present in both images are then associated, and the corresponding displacement is computed. As a result, an estimate of the current rover motion is computed.

agreement in the robotics community about the morphology and the design principles of rovers for space exploration. These are necessarily application-driven. Rovers must adhere to the concept of “Robotic Field Geologist,” i.e., a semi-autonomous rover is considered as a replacement for a team on Earth, able “to do in one day what a field geologist can do in about 45 seconds.”

In other words, the goal is to create a cyber astrobiologist, deploying scientific instruments (i.e., the scientific payload) over wide areas. The mechanical design and the cognitive architecture of planetary rovers are arranged to guarantee the traversal of uneven and rocky terrain according to specific requirements.

From the mechanical point of view, typical key requirements are: (1) the rover must traverse over obstacles of 25 cm in maximum dimension; ( 2) the rover must traverse over slopes of a nominal tilt of 16 degrees; ( 3) moreover, it should cope with hard, high traction terrains and soft deformable soils; ( 4) and with a temperature between -100 and 20 Celsius, in order to carry the scientific payload; and ( 5) each vehicle is approximately 1.4 m long and 1.2 m wide.

Furthermore, lithium-ion batteries are used for saving mass and volume.

From a mechanical perspective, the current state-of-the-art locomotion techniques assume that rovers are six-wheel drive, four-wheel steered vehicles with a specifically designed suspension system.

In particular, the rover suspension system is a mechanical assembly called rocker-bogie that connects the six wheels to the body of the rover itself (see Figure 4). In order to increase robustness to possible hardware faults, all six wheels are independently driven by DC motors. Moreover, in order to increase the mobility capabilities, front and rear wheels are independently steered, allowing the rover to turn in place, as well as to execute more complex turns.

The two front and rear wheels are steered by identical DC motors. A differential connects the two rocker-bogie systems to the main rover body.

The rocker-bogie design is characterized by a number of interesting properties: (1) safe traversal of obstacles whose dimensions are in the same order of magnitude of the diameter of the wheels (about 30 cm); ( 2) withstanding a tilt of 45 degrees in any direction without overturning; ( 3) absorbing the most of the impact load during terrain traversal, which is particularly important for the scientific payload carried around during mission execution; and ( 4) passively keeping all

• Rover locomotion (Mars). Given the rough nature of the Martian surface, cutting-edge state estimation techniques must be used to precisely allow a rover to reach a given target position. In general, reaching a target is a three-step process: terrain traverse, homing, and fine positioning. Once a given area is reached with a sufficient precision, scientific operations can be arranged, and the results sent back to Earth.