Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

210 Chapter 7 Walkers
Figure 7-10 Extra wide feet
provide two-legged stability
theoretically have very high mobility. Many research robots have been
built that use four or six legs and are impressively agile, if very slow.
Although it would seem impossible to build a two-legged staticallystable
robot, there is a trick that toys and some research robots use that
gives the robot the appearance of being dynamically stable when they are
actually statically stable. The trick is to have feet that are large enough to
hold the robot upright on one foot without requiring the foot to be in
exactly the right place. In effect, foot size reduces the required accuracy
of foot placement so that the foot can be placed anywhere it can reach
and the robot will not fall over.
The wide feet must also prevent tipping over sideways and are so wide
that they overlap each other and must be carefully shaped and controlled
so they don’t step on each other. Two-legged walking, with oversized
and overlapping feet, is simply picking up the back foot, bringing it forward,
and putting it down. The hip joints require a second DOF in addition
to swinging fore and aft, to allow rotation for turning. Each leg must
have at least three DOF, and usually requires four. The layout shown in
Figure 7-10 can only walk in a straight line because it lacks the hip rotation
joint. Notice that even with only two legs and no ability to turn, this
layout requires six actuators to control its six degrees of freedom.
This layout provides a good educational tool to learn about walking.
Although in the final implementation it may have eight DOF and its