o far this year, we've taken several steps towards
bringing our 60 lb combat robot, Troublemaker out
of retirement. We've upgraded the batteries,
devised a new weapon, and destroyed a few test
subjects (R.I.P. Duct Tape Avenger). Troublemaker is
just about ready to jump back into the ring — we just need
to find a ring for it to jump back into.
Tracking down a competition with a 60 pound weight
class has been tricky, so in the meantime we wanted to
hack some of our other kits. After contracting some viral
inspiration from the Internet, we thought to turn a few of
our run-of-the-mill wheeled robots into jolly jaunting walking
Walking, however, might be an imprecise term. What
we were really looking to do was to investigate the
effectiveness of a home-hacked version of a locomotion
design characteristic of the RHex robot. Could we bring
similar biological inspiration to our kits? Would walking be
as simple as putting one robotic leg in front of the other?
Walk this way to find out ...
The initial inspiration for this project arose when one of
Evan's colleagues, Peter, asked if he had heard about the
OutRunner "running" robot. The OutRunner robot has
gotten some press recently as a Kickstarter funded project
from Robotics Unlimited that makes the bold claim that it is
the world's first remotely controlled running robot.
The OutRunner runs on six "legs" that look a bit more
like two rimless wheels with three spokes each, rather than
the bottom half of an Asimo. We don't mean to diminish
the impressiveness of the bot — its movement is
mesmerizingly fluid and it can reach speeds up to 20 miles
per hour. Any bot that can do that is a winner in our book.
However, the type of locomotion it uses looks a little less
like running and a little more like something often termed
We've heard this term used to generally describe a type
of locomotion that uses rotary motion, but instead of
driving wheels, the motors drive "legs." The term has its
genesis in a multi-university project funded by DARPA that
began in 1998 — the Robotic Hexapod. The goal of the
RHex project was to implement lessons from animal
locomotion to advance the state of the art of robotics.
Instead of slavishly trying to mechanically reproduce every
articulation of a cockroach leg, the RHex team sought to
mimic the functional aspects of its creepy crawley
inspiration. The culminating result was a six-legged robot
with springy legs with a rotary motion reminiscent of
regular old wheels.
There is much more than meets the eye with the RHex
design. Technically speaking, the efficiencies obtained by
the design of the RHex robot can be modeled as a Spring
Loaded Inverted Pendulum, or SLIP. A SLIP is a conceptual
model where a mass sits atop a spring, and the RHex team
built on the fact that the animal kingdom is replete with
examples of an animal's center of mass being — in a
mechanical sense — situated on a SLIP.
The RHex team hypothesized that SLIP mechanics
present a number of efficiencies for locomotion because
some energy expended during locomotion can be stored in
spring potentials and released on the next step instead of
simply going to waste.
Basically, the jaunty bouncing gait of many animals
helps improve the efficiency of their movement. However,
to realize significant energy savings from the bounce in a
robot step, the step needs to be forceful and fast — more
forceful and faster than what is easily achievable with the
type of actuators that might be used to truly replicate an
animal's gait (pneumatic actuators might be able to speedily
apply a lot of force once, but doing that over and over is
tough when you have to compress the air for the step first).
So, the RHex team designed a robot with springy legs that
spun about with rotary motion.
Achieving the right balance of speed and torque is
much easier with a rotary motor, even if animals (with the
exception of Sonic, the hedgehog) don't actually have legs
that spin 360 degrees continuously. The RHex draws its
name from the fact that it is equipped with six such
spinning legs. In addition to being more insect-like, the six
legs of the hexapod improve the balance and stability of
by Bryce Woolley and Evan Woolley
Go to www.servomagazine.com/index.php/magazine/
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SERVO 08.2014 69
Twin brothers hack whatever’s put in front of them, then tell you about it.