Hi,
I've seen the IR-approach, there's a really cool working system here: www.balbot.com The thing is I'm not sure of the idea since the surface has to be flat - or atleast that's what I think. You can't run up or down a slope for example since the difference between the front and back sensor will be different. The same drawbacks would apply to the ultrasonic transducer approach. Besides, I already got the gyro and accelerometer.... ;-)

I seem to recall someone mentioning the NV article a while back. I may even have seen it and if I have I think they used the same kalman filter code I'm refering to but I may very well be wrong on that point.

I've been working on this on and off for the last 7-8 months, so I'm not gonna give up and put in that third wheel just yet but thanks anyway sayzer ;-)

Thanks guys! Please keep the ideas comming!

/Henrik Olsson.