Nuts & Volts - September 2004

Personal Robotics

the hinges. There is just a lot of repetition involved. The other difficulty I came into was peeling the protective plastic off the Lexan. It took me nearly an hour to accomplish this. Before peeling, I should have taken some time to clean up the laser-singed plastic cuts.

The three-jointed legs add another dimension to hexapod walking. With a two-axis leg, you get some scrubbing on the ground as the hip swings the leg fore and aft. You can mince about, taking little steps, but it is somewhat unappealing. Here, with the third axis and a lot of floating-point mathematics and

inverse kinematics, you can start to calculate your path based on the geometry of the robot. You can set the ride height of the robot and trace the tip of the foot through a straight line or an arc to get much smoother walking. You can even crab-walk sideways.

The symmetrical design of this kit has always appealed to me in a hexapod, but it also opens the door for some interesting challenges.

Take, for example, the standard 3 x 6 hex; it has two banks of legs: left and right, each leg on a side moving similarly, but out of phase with its leading and trailing neighbors. In effect, each leg mimicks the legs around it. The legs on opposite sides are mirror images, yet again, out of phase.

In short, there is a lot of symmetry. According to the instructions, that is how this kit is meant to be built. I however, decided to torture myself by building the kit without mirrored symmetry. I, instead, opted for a single axis of symmetry, with all of the legs built identically and simply moving as rotations of each other

The EH3-R shows its tripod gait.

around the vertical axis.

What this means is that, by building and thus commanding each leg identically with no head or tail, the software is identical for each leg. There are no exceptions or reversing servos; each leg is a carbon copy of the others.

The commands that drive the legs will simply be coded as vectors — angle and magnitude — for the whole robot, and each leg will follow this path according to its position relative to the vector of commanded motion, not relative to the chassis. This means that there really is no “forward motion,” just crabbing. This can be further extended to arcing motions, where the whole robot follows an arc through space.

I chose to outfit this beast with a ServoPod from New Micros, Inc., both because the 18 servos do not even phase the 80 MHZ DSP onboard, but also because the language onboard — IsoMax — was specifically crafted to run highly optimized floating point calculations.

My first shot at the software — written over the course of two evenings — employs a lot of interesting elements. The first is a 36 step, pre-canned gait. This gait pattern is non-scalar and can be offset from the body along the axis of the hip,