Creepy
Hybrid
Part 1
By Alan Marconett
How It All Began
Ever wish you could build a walking robot like a hexapod but without
spending all that money for the 18 or more servos and all those leg parts?
Many hobbyists build wheeled robots, but legged robots hold a great
fascination for us! Maybe it’s because we walk ourselves.
This project began as an effort to develop a minimalist robotic platform for
legs and an introduction to leg software, but it took an interesting turn.
You may be wondering about the title. Well, it’s a
hybrid because it has both legs and wheels! And, watching
this little bot move around with only two legs pulling it
happens to be a little bit creepy. I decided to call him
“Strider,” after the pony Frodo rode from Minas Tirith back
to the Shire in Lord of the Rings. By combining a pair of
wheels with a pair of legs, we can build a “legged” that is
much more interesting and instructive.
This “Creepy Hybrid” project will provide a platform
with which to experiment with a pair of legs as cheaply as
possible. It also offers 3DOF (three degrees of freedom)
legs that one could later expand into a full quadruped or
hexapod robot. It should also be possible to experiment
with 2DOF legs, as well (with some sacrifice in mobility).
The Design
Strider is designed around the available leg kits from
Lynxmotion, although it should be possible to substitute any
leg design the builder might want to try. The Lynxmotion
leg kits are built up from SES (Servo Erector Set) parts — a
really clever way to combine “Erector Set” type parts with
R/C servos for the construction of robots! You could make
your own legs similar to the SES parts, but why! The SES
parts are very nicely made, and you’d spend considerable
time making your own.
Each leg is driven by three rather standard R/C hobby
servos. By keeping the battery weight well aft on the
chassis, inexpensive R/C servos can be used. If you’re going
to expand this project into a quadruped or hexapod later,
be sure to pick servos that you can grow with. I had Hitec
HS475 servos left over from my hexapod upgrade that fit
the bill nicely.
The SES parts are drilled to match the servo horn
available for most R/C type hobby servos. I especially like
the metal servo horns with their drilled and taped holes,
and they are available for the Hitec servos. Insure your
servos fit the legs you intend to use!
SERVO 09.2009 51
