Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

contact point improves mobility greatly. Three wheels can be laid out in
several ways. Five varieties are pictured in the following figures. The
most common and easiest to implement, but with, perhaps, the least
mobility of the five three-wheeled types, is represented by a child’s tricycle.
On the kid-powered version, the front wheel provides both propulsion
and steers. Robots destined to be used indoors, in a test lab or other
controllable space, can use this simple layout with ease, but it has
extremely poor mobility. Just watch any child struggling to ride their tricycle
on anything but a flat smooth road or sidewalk. Powering only one
of the three wheels is the major cause of this problem. Nevertheless,
there have been many successful indoor test platforms that use this layout
precisely because of its simplicity.
In order to improve the mobility and stability of motorcycles, the three
wheeled All Terrain Cycle (ATC) was developed. This vehicle demonstrates
the next step up in the mobility of three wheeled vehicles. The
rear two wheels are powered through a differential, and the front steers.
This design is still simple, but although ATCs seemed to have high
mobility, they did not do well in forest environments filled with rocks
and logs, etc. The ATC was eventually outlawed because of its major
flaw, very poor stability. Putting the single wheel in front lead to reduced
resistance to tipping over the front wheel. This is also the most common
form of accident with a child’s tricycle.
Increasing the stability of a tricycle can be easily accomplished by
reversing the layout, putting the two wheels in front. This layout works
fine for relatively low speeds, but the geometry is difficult to control
when turning at higher speeds as the forces on the rear steering wheel
tend to make the vehicle turn more sharply until eventually it is out of
control. This can be minimized by careful placement of the vehicle’s
center of gravity, moving it forward just the right amount without going
so far that a hard stop flips the vehicle end over end. A clever version of
this tail dragger-like layout gets around the problem of flipping over by
virtue of its ability to flip itself back upright simply by accelerating rapidly.
The vehicle flips over because there is no lever arm to resist the
torque in the wheels. Theoretically, this could be done with a tricycle
also. At low speeds, this layout has similar mobility to a tail dragger and,
in fact, they are very similar vehicles.
Steering with the front wheels on a reversed tricycle removes the
steering problem, but adds the complexity of steering and driving both
wheels. This layout does allow placing more weight on the passive
rear wheel, significantly reducing the flipping over tendencies, and
mobility is moderately good. The layout is still dragging around a passive
wheel, however, and mobility is further enhanced if this wheel is
powered.
Chapter 4 Wheeled Vehicle Suspensions and Drivetrains 137