[randantor]

624 used an ultrasonic sensor with great results this year (super cheap HC-SR04s from Amazon). The hard plastic of the totes and cans seemed to work very well with the sensor, and distances were accurate to a tolerance of about +- 0.5 inches.

We used the range information so that the robot wouldn't try to pick up an object unless the sensor read a distance less than a minimum threshold. Because of our lifter geometry, a tote needed to be much closer to the back of the bot than a can to be picked up, and having a distance measurement made it easy to have different thresholds for the 2 objects.

We tried an ultrasonic sensor last year, but the fabric covering of the ball seemed to confuse the sensor (absorbing echoes or something), and it was difficult to get a reliable measurement. It definitely seems to depend on the game piece, but if it's any sort of hard surface, ultrasonic sensors are worth considering.