Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

Chapter 9 Comparing Locomotion Methods 235
COMPLEXITY
A more nebulous comparison criteria that must be included in an evaluation
of any practical mechanical device is its inherent complexity. A
common method for judging complexity is to count the number of moving
parts or joints. Ball or roller bearings are usually counted as one part
of a joint although there may be 10s of balls or rollers moving inside the
bearing. A problem with this method is that some parts, though moving,
have very small forces on them or operate in a relatively hazard-free
environment and, so, last a very long time, sometimes even longer than
nonmoving parts in the same system.
A second method is to count the number of actuators since their number
relates to the number of moving parts and they are the usually the
source of greatest wear. The drawback of this method is that it ignores
passive moving parts like linkages that may well cause problems or wear
out before an actively driven part does. The first method is probably a
better choice because robots are likely to be moving around in completely
unpredictable environments and any moving part is equally susceptible
to damage by things in the environment.
Speed and Cost
There are two other comparison parameters that could be included in a
comparison of mobility methods. They are velocity of the moving vehicle
and cost of the locomotion system. Moving fast over rough and
unpredictable terrain places large and complex loads on a suspension
system. These loads are difficult to calculate precisely because the terrain
can be so unpredictable. Powerful computer simulation programs
can predict a suspension system’s performance with a moderate degree
of accuracy, but the suspension system still must always be tested in the
real world. Usually the simulation program’s predictions are proven
inaccurate to a significant degree. It is too difficult to accurately predict
and design for a specific level of performance at speeds not very far
above eight m/s to have any useful meaning. It is assumed that slowing is
an acceptable way to increase mobility, and that slowing can be done
with any suspension design. Mobility is not defined as getting over an
obstacle at a certain speed; it is simply getting over the obstacle at whatever
speed works.
Cost can be related to size, weight, and complexity. Fewer, smaller,
and lighter parts are usually cheaper. The design time to get to the simplest,
lightest design that meets the criteria may be longer, but the end
cost will usually be less. Since cost is closely related to size, weight, and