the motors. (To be fair, you’re
better off with higher quality
servos for your walking bot. It’s
not hard to spend $300 to
$500 on just the motors for a
well-equipped hexapod robot.)
Of all locomotion types, legs
require the greatest degree of
machining and assembly skill.
Flexing of legs can cause stress in
material; acrylic plastics can break
over time. Figure 10 shows a
2DOF hexapod constructed out of
expanded PVC plastic. The better
walking bots use aluminum
metal, polycarbonate plastic,
or ABS plastic.
The gait of a walking robot
refers to the pattern of its leg
movements. Gaits can and do
differ depending on the speed of
travel — a running gait is
completely different from a
walking gait, for instance.
Figure 11 shows the most common gait of a six-legged
robot. This gait is often referred to as an “alternating tripod
gait” because at least three legs are always touching the
ground at the same time, providing the static balance
The alternating tripod gait goes through a number of
sequences. For each side of the tripod, the legs are lifted,
lowered, and swept forwards or backwards in unison. This
provides for a reasonably fast walk while still maintaining
good static balance as the weight of the robot is shifted
from one side to the other.
Legs act either to lift or to power (see Figure 12).
Legs are lifted to orient them into a new position. During
lift, the leg does not provide propulsion, nor does it
contribute to the balance of the robot. Legs that power,
propel the robot in the opposite direction of the movement.
Because of the complexities of constructing legged
robots, most bot builders instead opt to purchase a kit
of premade parts.
So, now you have the basics of making your mobile
robot, well, mobile. For your next project, be sure to
carefully consider the pros and cons of each drive method.
Be sure to match the drive system to your budget,
construction skills, and robot play area. There’s nothing
worse than to see a helpless robot designed for rolling
over carpet get caught in the soft sand of your back yard.
Talk about cruel! SV
Figure 12. Like any walking creature, a legged robot moves by lifting
a leg segment up and down while sweeping the leg back and forth.
SERVO 05.2014 47
Gordon McComb is the author of the best-selling
Robot Builder’s Bonanza and Arduino Robot Bonanza,
both published by McGraw-Hill.