Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

202 Chapter 7 Walkers
when crossing streams or difficult terrain. They stand on three legs
while the forth leg is moved around until it finds, by feel, a suitable
place to set down.
These examples demonstrate that four-legged walkers can have
geometries that are either dynamically or statically stable or both.
Animals have highly developed sensors, a highly evolved brain, and fantastically
high power-density muscles, that allow this variety of motion
control. Practical walking robots, because of the limitations of sensors,
processors, and fast acting powerful actuators, usually end up being statically
stable with two to eight legs.
The design of dynamically-stable, walking mobile robots requires an
extensive knowledge of fairly complicated sensors, balance, high-level
math, fast-acting actuators, kinematics, and dynamics. This is all beyond
the scope of this book. The rest of this chapter will focus on the second
major category, statically-stable walkers.
LEG ACTUATORS
First, let’s look and leg geometries and actuation methods. There are
three major techniques for moving legs on a mobile robot.
• Linear actuators (hydraulic, pneumatic, or electric)
• Direct-drive rotary
• Cable driven
Hydraulics is not covered in this book, but linear motion can be done
effectively using two other methods, pneumatic and electric. Pneumatic
cylinders come in practically any imaginable size and have been used in
many walking robot research projects. They have higher power density
than electric linear actuators, but the problem with pneumatics is that the
compressed air tank takes up a large volume.
Linear actuators have the advantage that they can be used directly as
the leg itself. The body of the actuator is mounted to the robot’s chassis
or another actuator, and the end of the extending segment has a foot
attached to it. This concept has been used to make robots that use
Cartesian and cylindrical coordinate walkers. These layouts are not covered
in this book, but the reader is urged to investigate them since they
can simplify the development of the control code of the robot.