The underside of the “turtle” shows the gearing mechanism for transferring the rotations of the independent axles to the sensor wheel. Rather than using an optical encoder – which is basically just too expensive – the sensor wheels pass through the slots of two IR emitter-detector pairs. These TTL logic devices pull up digital pins on the Arduino and this signal is passed to a Processing sketch. The wheels do generate consistent data on the whole, which is what we were aiming for.