Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

184 Chapter 5 Tracked Vehicle Suspensions and Drivetrains
very capable layout results. The main chassis is geared to the shared axle
so it is always at the half-angle between the front and rear tracks, which
allows it to be raised up yet still be level when folding both tracks down.
This trick raises the entire chassis, but it also offloads the weight of the
robot from the track guide blades, increasing rolling efficiency when
high traction is not needed. This reconfigurable layout combines the high
mobility of tracks with good smooth-road rolling efficiency.
There are two basic layouts for four-tracked vehicles. They are both
train-like in that there are two two-tracked modules connected by some
sort of joint. The two modules must be able to move in several directions
relative to each other. They can pitch up and down, yaw left and right,
and, ideally, roll (twist).
The simplest connection that allows all three degrees of freedom is the
ball joint. If the joint is passive, steering is accomplished in the same
way as a two-tracked vehicle, except that now both modules must turn at
just the right time to keep skidding between the modules to a minimum.
This turns out to be tricky. The ball joint also limits the range of steering
angle simply because the socket must wrap around the ball enough to
adequately capture it. A universal joint has a greater range of motion, and
is easier to use if the joint is to be powered.
The articulated joint, an active universal joint, overcomes the steering
problem by allowing the tracks to rotate at whatever speed limits skidding.
This steering method makes this layout very agile. The Hagglund
Bv206, which uses this layout, is considered nearly unstoppable in
almost any terrain from soft snow to steep hills. It is even amphibious,
propelled through the water by the tracks. Because it cannot be skid
steered, it can’t turn in place. Nevertheless, it is a very capable layout.
Steering the Hagglund Bv206 is done with a standard steering wheel,
which turns the articulated joint and forces the two modules to bend. The
tracks are driven through limited slip differentials, allowing the inner and
outer tracks in each module to travel at different speeds just like in an
Ackerman steered wheeled vehicle.
Six-Tracked Drivetrains
There is at least one track layout (Figure 5-18) incorporated on an existing
telerobotic vehicle that uses six tracks. It is an extension of the
Urbie design, but was actually invented before Urbie’s layout. The twotracked
layout is augmented by flipper tracks on both the front and
back, independently tilted, but whose tracks are driven by the main
track motors. This layout allows the vehicle to stand up like the one