2.6 Ultra Sonic
If you use the ultrasonic sensor you may use a library downloaded from the Net.
But I haven´t found anyone that was supported by an interrupt. So often you will have to wait a long time, until you will get a new value. Sometimes about 40ms. If your robot drive with a velocity of 50cm per second (our is able to drive 1m/s) then it will last about 2cm before you get a new value.
Oh sh.. where is the line?