Servo - September 2009

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!