It seems that it is much easier to stay up with three or four legs than two. Just lock them into a slightly outward direction and you cannot fall. If you're on one or two though, you've really got to balance the rest of the system much more carefully.

Your comment got me thinking. From a technical point of view of traversing over unknown and uneven terrain, there really is no reason to go with bipedal, especially if it's easier to do control systems for tri or quadraped robots. Of the handful that can be bipedal (primates, bears, birds), it's not their preferred mode of movement. However, it's probably a psychological disadvantage for the robots to be 3 or 4 legged and work along side humans in a job that requires it to be treated as a human, like a waiter, house companion, etc. That alone is reason enough to continue bipedal robotic research.

There are only two real advantages to bipeds:

1) Height 2) Use of spare limbs for tools

Height doesn't seem to be much of an advantage for robots at this stage, and I think that robots could have 6 limbs with no problem. Therefore, biped robots are really only a novelty.

3) Use of human devices that are biped specific

Ladders, automobiles and heavy machinery, household equipment, etc. all rule out quadrupeds.

While it would be possible to custom-build all of this stuff to be usable by a quadruped, it means you then have to have two of everything if you plan to cohabitate with a robot assistant. From the perspective of assistance robots, it's not going to be possible to accomplish everything without bipedal bots.

I think wheeled bots (like Trevor's Monty) will also be a great option--probably a tenth the cost of a walking mechanism, an order of magnitude more speed and endurance, etc. But they'll still be unsuitable for many assistance robot tasks. Stairs, for example. And unlike the Daleks, leveling the building will not suffice as a solution to the problem of climbing stairs.

Well, not necessarily. Those devices aren't biped specific- they're specific to thing with grasping mechanisms.

Who's to say a quadruped robot can't have 4 feet + 1 or 2 grapsing mechanisms? Or have two front feet that are also capable of grasping?

While you can certainly have a 4 legged robot, 2 armed helping you around the house or a hospital, only using grasping mechanisms to climb the ladder to adjust the satellite dish on the roof, help you with the dishes, or get you out of bed, I think it's important to consider the reaction people have to a robot's form factor given its job.

You and I might think it's really cool to have 4 feet and two arms (like a centaur!), but if a robot reminds someone of a gigantic insect, and it's suppose to be a household companion/housekeeper, people might reject having it around the house. That's a good reason to have bipedal robots.

But on the other hand, if you get close, but not exactly with bipedal robots, people might get even more freaked out with the uncanny valley.

Good point.

Personally, I'd love to have a robotic centaur helping me around the house.

Think Centaur.

For comparison, hikers on rough terrain usually bring a hiking staff and aim to keep at least two points of contact at all times.

I can see two legs being more efficient than four, but wheels are usually vastly more efficient than either..

Like those ED-209 bloopers...