The objective for Basketbots was to score baskets in each of the six, twenty-second periods. Four points were awarded for the first and last baskets in each period and just one point for intervening baskets, thus making timing an important issue. As one of the three baskets was selected randomly in each period, the basket emitted an IR signal to indicate its activation. Tape in the center along the arena's length provided guidance for positioning and shooting.
Our team, the Great White Hope (henceforth referred to as GWH), decided to shoot from the robot's initial position instead of driving up to the baskets. This required a triangulation algorithm for determining the robot's position and direction with respect to the baskets. First, GWH uses five IR photodetectors placed at its front to look for the beacon emitted by the active basket. It rotates its whole body until it sees the beacon with one of the sensors and then centers in on the signal by comparing the intensity seen by each sensor.
In order to determine the distance to shoot, two L-shaped arms swing out from the back of the platform with a tape and contact sensor mounted at their tips. As the tape sensors cross the tape, and as the contact sensors bump the wall, the microprocessor records the number of steps the arm has rotated. Because the robot's geometry and the angle of rotation for each step are known, GWH can calculate the position of its arms. With one arm crossing the tape and both arms contacting the wall, GWH has three data points to use for computing its position and direction. Once it knows its exact location it can calculate the distance to each basket. When a new period begins, the catapult platform can turn to face the new basket without hesitation because GWH knows exactly where it is.
GWH has an additional catapult platform that rotates on top of the main body platform. Once the main body is roughly centered in on the basket, GWH uses two fine IR photodetectors mounted to the catapult platform to exactly center the catapult with the beacon. The processor controls the motor which rotates the catapult platform based on differential sensing of the two photodetectors. GWH adjusts the motor until both sensors report near equal intensity.
Once all is in place, the catapult fires at the active basket. The distance varies roughly between 6.5 and 7.5 ft (which can be calculated once the arms are extended and the position known), and GWH can fire different distances using pulse width modulation. It sends a different duty cycle pulse to the motor for each basket, thus increasing or decreasing the distance the ball travels.
Not all these strategies were used in the final product. The arms were abandoned due to time constraints and mechanical problems, the catapult platform would occasionally slip and was slower than hoped, and the differential IR sensors simply weren't necessary (nor easy to use without the catapult platform).