JALURO is an open source two-wheeled lunar exploration robot. The robot is the entry of Team FREDNET in the Google Lunar X-Prize competition. JALURO, which stands for Just Another LUnar RObot, carries its payload underslung between two wheels, as shown in the picture below.
Charlie Masi, of Control Engineering, has this to say about the design:
Being underslung, it is in stable static equilibrium. Having independently controlled motors for each wheel, it is fairly simple to drive forward, backward, and to make turns in any direction with any radius from zero to infinity. It really is a neat concept!
The problem is that the underslung chassis acts like a pendulum or, more precisely, a rocking chair. Notice that the rocking amplitude seems almost random, depending mainly on the shape of any disturbing impulses. The problem is most obvious when the vehicle comes to a stop. The chassis can be seen to rock forward and back for some time after the forward motion ceases. This will be worse on the Moon, where there is no damping from air resistance to slow the motion down...
The good news is that this problem is apparently surmountable, with an equation of motion that is "fairly straightforward." Take a look at this test video of JALURO:
FREDNET is the only open-source entry in the X-Prize competition; they have a pretty exciting idea - that space is open for everyone! The following video contains a description of their program and vision.
(FREDNET mission statement video)
I'm sure there are lunar exploration robots elsewhere in science fiction, but somehow I was reminded of Arthur C. Clarke's spider tripod robot from his 1972 novel Rendezvous With Rama. It is also a "neat concept", and it has some obvious control challenges. (If you think a three-legged robot is impossible, take a look at the STriDER Tripedal Dynamic Experimental Robot .)