Page 36 - Introduction to Autonomous Mobile Robots

Locomotion

The six different events are 21
1. lift right leg;
2. lift left leg;

3. release right leg;
4. release left leg;
5. lift both legs together;

6. release both legs together.
Of course, this quickly grows quite large. For example, a robot with six legs has far more
gaits theoretically:

N = 11! = 39916800 (2.3)

Figures 2.7 and 2.8 depict several four-legged gaits and the static six-legged tripod gait.

2.2.2 Examples of legged robot locomotion
Although there are no high-volume industrial applications to date, legged locomotion is an
important area of long-term research. Several interesting designs are presented below,
beginning with the one-legged robot and finishing with six-legged robots. For a very good
overview of climbing and walking robots, see http://www.uwe.ac.uk/clawar/.

2.2.2.1 One leg
The minimum number of legs a legged robot can have is, of course, one. Minimizing the
number of legs is beneficial for several reasons. Body mass is particularly important to
walking machines, and the single leg minimizes cumulative leg mass. Leg coordination is
required when a robot has several legs, but with one leg no such coordination is needed.
Perhaps most importantly, the one-legged robot maximizes the basic advantage of legged
locomotion: legs have single points of contact with the ground in lieu of an entire track, as
with wheels. A single-legged robot requires only a sequence of single contacts, making it
amenable to the roughest terrain. Furthermore, a hopping robot can dynamically cross a gap
that is larger than its stride by taking a running start, whereas a multilegged walking robot
that cannot run is limited to crossing gaps that are as large as its reach.
The major challenge in creating a single-legged robot is balance. For a robot with one
leg, static walking is not only impossible but static stability when stationary is also impos-
sible. The robot must actively balance itself by either changing its center of gravity or by
imparting corrective forces. Thus, the successful single-legged robot must be dynamically
stable.