As some of you know, I'm working on a "big" robot using a wheelchair base. At least 3 people mentioned the risk that the robot could run into objects/people and damage them. Airman mentioned the use of a remote kill switch. I happen to have a little remote control rig that I can use for this. However, I'd also like a more autonomous safety, such as proximity sensors.
What I'm not sure about is what to use for this purpose. I bought a couple of atmega8-16PU chips, so I figure I could use IR rangefinders... However, those seem to only give you a distance reading within some range... Such as 10" to 40". In this case, if my robot is initialized when it's already close to an object/person, the sensors would not tell me anything (or am I wrong there?). They might also be ineffective if my 300 pound robot is moving fast, and does not have time to completely stop within a 30" range.
What would be some smart and cheap options to look into?