grass, and other surfaces that a wheeled robot can only
dream about. The large surface area of treads greatly
increases friction. A tracked vehicle can have trouble
making turns, and the treads can
pop off if they are made of flexible
rubber. Suitable tread material can
be hard to find.
A common approach is to
either use the Tamiya rubber Track
and Wheel set (Tamiya #70100), or
else hack a toy tank like the one in
Figure 7. You can keep the tracks
and all its sprockets and wheels on
the original toy, or transfer them to
body pieces you’ve specially made
for your robot like that in Figure 8.
Another problem is that rubber
treads can stretch over time. A
track tensioner mechanism is
recommended. A simple tensioning
technique is to make one of the
free-wheeling idlers adjustable. The
axle for the idler is inserted in a slot
rather than a hole. Fastener
hardware keeps the idler locked in
place over the slot. As the tracks
stretch, you can move the idler to
keep proper tension.
A better way to prevent loose
and sloppy tracks is to use material
that’s not stretchy. Figure 9 shows
the drive mechanics of a homebrew
tracked robot that uses plastic tread links. Each link is
separately molded out of rigid plastic; a stainless steel rod
connects the links. You can add or remove links to adjust
the length of the tread. These particular links are available
from online stores. Look for other sources for hard plastic
(and in some cases, metal) tank treads.
Why roll when you can walk? That’s the idea behind
robots with legs. Walking bots may have two, four, six, and
even eight legs — the six leg (hexapod) variety is perhaps
the most common. Four or more legs provide what’s
known as static balance, where at least three appendages
touch the ground at any time. With this arrangement, the
legs prevent the robot from toppling over.
Joints of each leg are defined as degrees of freedom
(DOF): the more DOF, the more agile the platform but the
more difficult it is to build. Two or three joints is typical for
four-, six-, and eight-legged robots. In most designs, each
joint requires its own motor, so for each DOF the cost,
weight, and complexity of the robot quickly escalates.
A 3DOF hexapod, for example, requires 18 motors —
three motors found in each of its six legs. Most walking
robots use servo motors for model radio control airplanes.
At an average cost of $12 per motor, that’s $216 for just
46 SERVO 05.2014
Figure 10. Multiple RC servo motors
act as leg joints in a walking robot. This
design uses 12 motors total; two servos
for each of its six legs.
Figure 11. The way a legged robot walks is its gait. Typically,
on a robot with six legs (hexapod), three legs touch the ground
at any one time in the form of a tripod, providing static balance.