The Cornell Ranger is a walking robot with 4 legs and no knees. It's energy efficient but not particularly so compared to other walking robots. Ranger also shares one trait with the Energizer bunny that makes it different from all other walking robots; it keeps going and going. Ranger has previously walked 1km without human intervention. The builders discovered that another walking robot was reported to have walked 2.5km on a treadmill and a six-legged robot is supposed to have walked 2km. So Cornell Ranger set out to beat them and has now set a new record by walking over 9km - 45 laps non-stop around Cornell's Barton Hall running track. The Cornell Ranger was developed by Andy Ruina, Cornell professor of theoretical and applied mechanics and his team of students. For all the technical details on Ranger's construction see the technical report, Design and Construction of the Cornell Ranger, a world record distance walking robot (PDF format).