Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

Chapter 4 Wheeled Vehicle Suspensions and Drivetrains 131
35 kilo-pascals) for the majority of vehicles of all types. Everything from
the largest military tank to the smallest motor cycle falls within that
range, though some specialized vehicles designed for travel on loose
powder snow have pressures of as low as five kilo-pascals. This narrow
range of pressures is due to the relatively small range of densities and
materials of which the ground is made. Vehicles with relatively low
ground pressure will perform better on softer materials like loose sand,
snow, and thick mud. Those with high pressures mostly perform better
on harder packed materials like packed snow, dirt, gravel, and common
road surfaces. The best example of this fact are vehicles designed to
travel on both hard roads and sand. The operator must stop and deflate
the tires, reducing ground pressure, as the vehicle is driven off a road and
onto a stretch of sand. Several military vehicles like the WWII amphibious
DUKS were designed so tire pressure could be adjusted from inside
the cab, without stopping. This is now also possible on some modified
Hummers to extend their mobility, and might be a practical trick for a
wheeled robot that will be working on both hard and soft surfaces.
This also points to the advantage of maintaining as even a ground
pressure as possible on all tires, even when some of them may be lifted
up onto a rock or fallen tree. Suspension systems that do this well will
theoretically work better on a wider range of ground materials.
Suspension systems that can change their ground pressure in response to
changes in ground materials, either by tire inflation pressure, variable
geometry tires, or a method of changing the number of tires in contact
with the ground, will also theoretically work well on a wider range of
ground materials.
This chapter focuses on suspension systems that are designed to work
on a wide range of ground materials, but it also covers many layouts that
are excellent for indoor or relatively benign outdoor environments. The
latter are shown because they are simple and easy to implement, allowing
a basic mobile platform to be quickly built to ease the process of getting
started building an autonomous robot. Vehicles intended for use in
any arbitrary outdoor environment tend to be more complicated, but
some, with acceptably high mobility, are surprisingly simple.
SHIFTING THE CENTER OF GRAVITY
A trick that can be applied to mobile robots that extends the robot’s
mobility, independent of the mobility system, is to move the center of
gravity (cg) of the robot, thereby changing which wheels, tracks, or legs
are carrying the most weight. A discussion of this concept and some lay-