Hardware

Schematic

A schematic of all the AVR connections is available here in GIF format.

Processor

The Omnibot uses an ATmega162 AVR from Atmel as it's central processor. This chip runs at 16Mhz, has 4 timers, 6 PWM generators, a hardware multplier, and a whole lot of external interrupts. The processor, motor driver chips, and support circuitry was all built onto a peice of pad-per-hole perf-board from radioshack. A 7805 voltage regulator was used to provide 5V logic power from the 7.2V battery.

Motors

Three Futaba S3003 servos, with all of the electronics gutted, were used to drive the wheels. The DC motors were driven directly using two SN754410 Quad Half H-Bridge chips.

Encoders

Once the servos were gutted, we performed the painstaking task of building optical encoders into the casing to count the rotations of the first gear in the gear train. Our encoders were built very much like these ones, built by Giuseppe Marullo. We used two Hamamatsu P5587 photoreflector IC's mounted onto a small peice of perf board cut to fit into the space where the pot used to be inside the servo. We had to grind out all of the plastic in that space using a dremel, then cut the two holes for the photo-reflectors, then finally use a hot glue gun to glue the circuit boards into place. The encoders actually turned out pretty well, although getting them working right was a HUGE pain. They were very sensitive about their location (i.e. how close to the gear, what angle), and the slightest movement could break them. But we were lucky and when the glue dried they were in a good position and gave us some pretty nice looking square wave output.

Body

The body was cut out of a peice of Lucite acrylic, purchases at home depot. In order to get a nicely shaped hexagon, and to make sure the wheels were all aligned right we first designed the body on Solidworks and used a printout of the Solidworks design as a template for the cuts and wheel mounting holes.

Batteries

We used a custom NiMH battery pack from Digikey as our only power source. Digikey sells battery packs with pretty much any number of any size battery you want wrapped up in plastic with two solder-tabs sticking out. We used a 6 cell pack of 4/5 A's to give us 7.2V with 2 Amp Hrs.