Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

Chapter 9 Comparing Locomotion Methods 231
robot. A rule of thumb to use when figuring out the robot’s width is to
make it about 62 percent of the length of the robot.
The components of the system each have their own volume, and moving
parts sweep out a sometimes larger volume. These pieces of the robot
are independent of the function of the robot, but take up volume.
Including the volume of the mobility system’s pieces is useful. As will be
seen later, weight is critical, so the total mass of the mobility system’s
components needs to be included. Since mass is directly related
(roughly, since materials have different densities) to the volume of a
given part, and volume is easier to calculate and visualize, volume
negates any need to include mass.
Efficiency
Another good rule of thumb when designing anything mechanical is that
less weight in the structure and moving parts is always better. This rule
applies to mobile vehicles. If there were no weight restriction and little
or no size restriction, then larger and therefore heavier wheels, tracks, or
legs would allow a vehicle to get over more obstacles. However, weight
is important for several reasons.
• The vehicle can be transported more easily.
• It takes less of its own power to move over difficult terrain and, especially,
up inclines.
• Maintenance that requires lifting the vehicle is easier to perform and
less dangerous.
• The vehicle is less dangerous to people in its operating area.
For all these reasons, smaller and lighter suspension and drive train
components are usually the better choice for high mobility vehicles.
There are three motions in which the robot moves: fore/aft, turn, and
up/down, and each requires a certain amount of power. The three axes of a
standard coordinate system are labeled X, Y, and Z, but for a mobile robot,
these are modified since most robot’s turn before moving sideways. The
robot’s motions are commonly defined as traverse, turn, and climb. A
robot can be doing any one, two, or all three at the same time, but the
power requirements of each is so different that they can easily be listed
independently by magnitude. Climbing uses the most power and turning in
place usually requires more power than moving forwards or backwards.
This does not apply to all mobility systems but is a good general rule.