Nuts & Volts - September 2004

Personal Robotics

rotated in space, and scaled in both height and length. The phase of the gait can be rotated to perform different gaits, as well. This gait generator gets fed into the inverse kinematics engine, which takes the desired ride height of the robot, the desired position of the tip, and calculates the joint angles required to produce that tip position.

Overall, this kit gets an A, as well. I would like to see the pieces cleaned up somehow and see some provisions for anchoring the wires. A simple Allen wrench and pliers were all I needed to build it, but be prepared to spend some time programming, if only because there are 18 servos to deal with.

The EH3-R in its parked position.

Shelob

Not content to leave well enough alone, Phil Davis — a consummate hobbyist by night and computer scientist by day — has realized that even three joints per leg leave a lot to be desired. To counter this, Phil has created Open Source Shelob (www. IsoBots.com).

I have been in this hobby for many moons, but I have never heard of a 4 x 6 before. The addition of the fourth joint allows for better joint positioning and trajectory tracking. It is like a 3 x 6 on adjustable stilts. In a 3 x 6, you have to accept that, as the tips of your feet translate through a line on the ground, you will generate a small amount of scrubbing unless you account for the geometry of the tip and, if you do, you generate waggle or bumping in the body. You are effectively constrained, not being able to adjust the angle of the last segment to the ground. The fourth joint allows this, but Phil wasn’t even happy with that. He has also designed in an articulated, segmented body to better accommodate uneven terrain.

Shelob is constructed from 1/8” plywood from www.Filener.com This perfectly valid building material is easily cut, finished, and painted and is relatively inexpensive. People

SEPTEMBER 2004

even build houses and boats out of the stuff and rumor has it that they used to build airplanes out of it, too.

What is especially important about Shelob is that her construction is open source in nature. By placing all the costs associated with the intellectual property into the public, everyone can benefit from the efforts of all who contribute. This should drive the quality up and the price down.

Phil’s processors of choice are several IsoPods — one per segment — on a CANbus network. This will hardly task each IsoPod, but it will leave a lot of possibilities open for sensing by utilizing the extra timer channels and A/D converters. This can also remove some of the burdens of inverse kinematics from the main processor, which is meant to perform sensor synthesis and decision making.

Presently, the code is based on inverse kinematics, as well, but I know that the eventual goal is based

on sensing, not calculating, though some forward kinematics will be useful for tracking the joints themselves through space. The code also includes velocity and acceleration profiling to keep things smooth and precise and — I guarantee — lots and lots of trig.