MediumANT
A six-legged, walking robot, created using servos and Pololu Wixels.
Cockroaches don’t need wheels!
While wheels are an efficient method of transportation, animals such as cockroaches and cheetahs can move as fast or faster than a similarly-sized wheeled vehicle, while also having more mobility over different types of terrain.
With this inspiration, I partnered with Dr. Shai Revzen of the UMich BIRDS Lab to create a six-legged foam core ant-like robot, MediumANT. While not as fast as a wheeled robot due to its simple construction, MediumANT is able to cross bumpy and rough terrain that a wheeled robot could not.
Design
The Legs
MediumANT’s legs are shaped so that, when first coming into contact with the ground, the majority of the bottom surface (the right part of the top white foam) is in contact with the ground, providing the most traction.
The legs are attached to the robot using a hinge, and to the motors using the clear plastic offsets, which have the bottom attached to the motor and an axle through the top going to the hole in the leg.
This hinge-offset combination allows the leg to pivot and grab the ground, then lift the robot of the ground while walking.
The Body
The body is made up of a bottom, top, three supporting ridges, and several walls.
The lengthwise walls come in two pairs are responsible for holding the rear and front servos, along with supporting the top.
An additional box that forms two lengthwise walls for the center servos and two width-wise walls for holding the battery pack is inserted in the middle.
All the walls also help support the top, which holds the electronics on four screw standoffs.
The Electronics
The robot is controlled by a pair of Pololu Wixel microcontrollers, a Pololu Micro Maestro servo controller, and six Parallax High-Speed Continuous Rotation Servos.
The Wixel (chip on right) reads the positions of the legs from a special motor signal line that first passes through a RC filter to clean the signal. It uses the signal to set the motor positions, and tells the second Wixel connected to the computer when it is ready.
The computer Wixel then provides the next set of motor positions and polls until the robot Wixel is ready again.
The robot Wixel sends servo commands to the Micro Maestro (bottom left) over a UART line to move the servos to the proper positions.
This cycle continues to allow the robot to walk like an ant.
The Code
All the code for this project was written in C11 and compiled by the Small Device C Compiler (SDCC) to Intel MCS-51 assembly. A set of one-byte commands was used to communicate between the two Wixels, where a command byte had its MSB set, and data bytes did not. This simplified programming, as errors could be easily caught, as there would be leftover data bytes or an unexpected command byte. The Wixels could take variable-length commands, simply by specifying a length parameter as the first data argument.
This command processing allowed a “division of labor” between the two Wixels. The robot Wixel simply had to worry about moving the servos, it didn’t have to plan out a movement pattern. And the computer Wixel could have its movement pattern easily updated, without having to worry about how the servos were moved.