Catchy Robot

I just got my NXT LEGO set, and I had to challenge the system and myself a bit, so I set out to create a version of the tribot that could catch balls. That was challenging in several ways:

• The robot had to detect that the ball was approaching (and at what speed). I solved that by tilting the ultrasonic sensor, and then sample the distance to the ball twice with a 100 ms interval after it came withing 50 cm. That way I could get a quite reliable estimate at the speed, and hence when the claws should close.
• The robot had to know wether it caught the ball, so that it would only return if it actually had something to return with. I solved that by installing a bar over the claws that gets pushed up by the ball when the 'bot catches it. It triggers a small lever to push the push button sensor when the wheels and axels turn.
  
• The original tribot actually twists quite a bit in the construction, so the "ball detector" didn't work reliably until I reinforced it, so that the claws ended up the same place each time it caught the ball.

I think in all it came out quite well. You can see a movie of the bot here:

http://www.youtube.com/watch?v=w2n24bk81iQ