Strafe: Hexapod Robot

As part of a research project into bio mimetics, this robotic was rebuilt with the hopes of being able to replicate the omni directional movement of a  spider. The original platform was built by Thomas Michon, Bonnie Charles and Leif Jentoft at the FW Olin College of Engineering. As part of this project, the electronic controls were replaced with a Matlab driven servo controller, the transmission was redesigned, and the platform was redesigned. Most of the experimentation revolved around different algorithms regarding angle of the legs to control movement.

This project was completed with collaboration from Leif Jentoft during spring of 2007 using resources from the Bio-Mimetic Robotics Lab at Olin College supervised by Dr Gill Pratt.

background

I walked into this project after the initial design was done. At that point a PIC was running the electronics and the platform was extremely heavy. Mechanically, the system also faced challenges due to the way it was designed. It used cables to extend and retract the legs and was driven by a crank shaft and some cams that lost much of the power to friction. So even though the goal was to explore  the ways in which we could make the platform into an omni-directional walker, the first task that we would have to tackle would be a mechanical redesign. Looking at the way that legs worked we used the mechanical redesign to lead us to possible movement algorithm solutions.

redesigning the platform

Initially the big work was making the platform movable without too large of a motor. The existing transmission had too much power loss and the chassis was too heavy for effective movement. By limiting our design to structural components and thinner plastics we were able to significantly lighten the platform.

Cutting drivetrain losses was a little harder. The original device used a crankshaft of sorts to move steel cables through places in the holes in the chassis. Even though graphite was used to try to lessen the friction, the system had a hard time moving. To cut the friction, we redesigned the system to work using teflon coated mountain bike brake cables that rested inside of vinyl casings. Coupled with the weight decrease, we saw now had a platform that writhed mightily when activated.

the legs

Strafe, the robot, still could not move in a controlled fashion. Based on papers on the mechanics of legs we were able to determine that the platform was missing some critical characteristics that allow legs to work in nature. Most legs are essentially a combination of springs and pendulums. While their pendulum nature makes them efficient we felt like we could achieve a measure of success regarding just movement if we introduced a little spring into the legs; a determination we based on the theory the springy-ness of a leg in conjunction with proper positioning was the critical element that allowed legged systems to conserve the momentum needed for motion in a specific direction.

To add springy-ness to the legs, we cut the vinyl casings up and added 3 inch legs made of concentric spring loaded brass tubes. This system allowed for a degree of compression even when the leg was fully extended.

control

After the mechanical redesign, we wanted to get right to the experimentation with movement algorithms. To do this, we ripped out the old electronics control system and replaced it with a servo board that we could easily control. Since we did much of our modeling in Matlab, we chose to use a servo controller that had libraries built for it that interfaced directly with Matlab. Through several iterations of control code, we were able to record video of the platform moving in specific directions.

conclusions

At the end of the day, this project was a success. Both Leif and I planned to pick up the project again at a later date; but due to scheduling and just moving on/getting busy we were never able to go back to this exciting project.