We've looked at a few ultrasonics to detect once a boulder has entered our mecanum roller intake, but most don't really start working until 8in range or so. Any other ideas to sense a boulder with in 3-4in? We can probably get ultrasonics to work but I feel like there might be some "proper" way to do this. IR doesn't seem to work at close proximity. Thanks!
EDIT: Looked at this ultrasonic -
http://www.maxbotix.com/Ultrasonic_Sensors/MB1220.htm