Alan Anderson ) writes:
On Nov 5, 7:38=A0pm, Dr J R Stockton
wrote:
It should have six legs, so that while walking it stands stably on three
feet while moving the other three in the direction of travel.

Six legs simplifies the task of balancing greatly, to the point where
you don't need any special feedback or fancy motion to remain stable
on relatively level ground. However, given that you still need that
feedback and fine control to deal with unpredictable terrain and
obstacles, four legs are sufficient. Goats do just fine without six
legs, for example.

Tiny robots can have much longer and skinnier legs by comparison, and
can use the leverage to take advantage of the inherent stability while
not paying much penalty in mass or motive power requirements. Huge
robots benefit from spreading the weight over more legs at the same
time, though the legs themselves end up being a much larger fraction
of the entire object. But at a reasonably large range of masses, once
you require a certain amount of freedom in leg motion, more than four
merely adds unnecessary complexity.

Don't have the url handy, but a few years back there were videos on the web
of DARPA's Big Dog robot that moved across rough terrain, righted itself
after being pushed off-balance and could jump over obstacles (all on four
legs).

--John Park