Page 58 - Introduction to Autonomous Mobile Robots

Locomotion

Figure 2.26 43
The microrover Nanokhod, developed by von Hoerner & Sulger GmbH and the Max Planck Institute,
Mainz, for the European Space Agency (ESA), will probably go to Mars [138, 154].

2.3.2.4 Walking wheels
Walking robots might offer the best maneuverability in rough terrain. However, they are
inefficient on flat ground and need sophisticated control. Hybrid solutions, combining the
adaptability of legs with the efficiency of wheels, offer an interesting compromise. Solu-
tions that passively adapt to the terrain are of particular interest for field and space robotics.
The Sojourner robot of NASA/JPL (see figure 1.2) represents such a hybrid solution, able
to overcome objects up to the size of the wheels. A more recent mobile robot design for
similar applications has recently been produced by EPFL (figure 2.27). This robot, called
Shrimp, has six motorized wheels and is capable of climbing objects up to two times its
wheel diameter [97, 133]. This enables it to climb regular stairs though the robot is even
smaller than the Sojourner. Using a rhombus configuration, the Shrimp has a steering wheel
in the front and the rear, and two wheels arranged on a bogie on each side. The front wheel
has a spring suspension to guarantee optimal ground contact of all wheels at any time. The
steering of the rover is realized by synchronizing the steering of the front and rear wheels
and the speed difference of the bogie wheels. This allows for high-precision maneuvers and
turning on the spot with minimum slip/skid of the four center wheels. The use of parallel
articulations for the front wheel and the bogies creates a virtual center of rotation at the
level of the wheel axis. This ensures maximum stability and climbing abilities even for very
low friction coefficients between the wheel and the ground.