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A friend and colleague of mine who studies robotics says that bipedal robots present much greater challenges for stability and locomotion than those with more legs.

So why is there so much effort to develop bipedal robots? Are there any practical advantages?

Of course, I see the advantage of having arm-like free appendages, but it seems like 4 legs and 2 arms would generally be a better design.

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  • $\begingroup$ A lot of responses are saying that environments are designed for humans, and so humanoids are the best solution. It's a valid argument, but, IMO, not that convincing without more support. It could just point to a lack of imagination. $\endgroup$
    – capybaralet
    Commented Jan 4, 2015 at 15:50
  • $\begingroup$ I like the "science" argument: that studying the problem teaches us useful things. $\endgroup$
    – capybaralet
    Commented Jan 4, 2015 at 15:51
  • $\begingroup$ Please do not edit your question this way. It changes the context, and makes the answers less applicable. If you knew that to begin with, why ask the question? $\endgroup$
    – Spiked3
    Commented Jan 5, 2015 at 17:39
  • $\begingroup$ Well, my question is basically: "Why only two legs?" I didn't think people would be silly enough to think that I meant "Why arms?" $\endgroup$
    – capybaralet
    Commented Jan 8, 2015 at 15:06
  • $\begingroup$ Honest question: What alternative would I have? Ask another question (I assume it would be flagged as a duplicate)? Just use comments (then less people read it and I get more answers I consider irrelevant). $\endgroup$
    – capybaralet
    Commented Jan 8, 2015 at 15:08

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I would say:

  1. Compactness of the structure: imagine humanoid robots that are required to operate in household environment: two-legged robots might have less difficulties in navigating among tables and chairs standing very close each other.

  2. Energy consumption: of course the power needed increases proportionally with the number of legs.

  3. Human likeness: an aspect that we don't have to neglect if one day we want to have robots among us, both for social reasons and technological reasons. Regarding the latter, generally speaking, copying the human body entails a number of benefits for robots that are designed to act in human context: this applies equivalently to the robot upper-body (e.g. manipulation) and to its lower-body (e.g. locomotion).

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  • $\begingroup$ 1. I'm struggling to think of an environment that humans can navigate and dogs cannot. 2. Really? You could of course use smaller legs if you had more of them. $\endgroup$
    – capybaralet
    Commented Jan 3, 2015 at 21:03
  • $\begingroup$ Simply think of a dog-shaped robot that needs to navigate and at the same time grab objects from the top of the table: the robot will be definitely bigger. The task shapes the body. Besides, a smaller dog, to save energy, has not point in that. $\endgroup$
    – Ugo Pattacini
    Commented Jan 3, 2015 at 23:02
  • $\begingroup$ However, just as conclusive remark: stabilization and locomotion of bipedal robots is no longer a big challenge to tackle... $\endgroup$
    – Ugo Pattacini
    Commented Jan 3, 2015 at 23:07
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    $\begingroup$ @user2429920 The DARPA challenge specifically includes driving/operating a car/vehicle, a perfect example environment where 4 legs would be cumbersome. Not saying cant be done, but a good example where fitting in helps. $\endgroup$
    – Spiked3
    Commented Jan 9, 2015 at 23:38
  • $\begingroup$ @Spiked3 - OK, gtk, but that just pushes the question on to DARPA... $\endgroup$
    – capybaralet
    Commented Jan 11, 2015 at 0:44
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In addition to the reasons given by Ugo, studying humanoid robots in general helps in the medical field too.

  • Trying to understand what sensors/systems robots need helps us understand what sensors/systems we use
  • If we can make a fully functioning humanoid robot, then individual parts such as an arm or a leg can be converted to a better functioning prosthetic limb for people.

And one more reason I can think of

  • Why study anything hard? All the subproblems worked on by individual researchers can be easily applied to other types of robots.
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    $\begingroup$ Very appropriate comments: the return on investment in the medical field would be quite relevant. $\endgroup$
    – Ugo Pattacini
    Commented Jan 3, 2015 at 13:22
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Really simple; Bipedal robots are designed to operate in environments made for ..... bipedal people.

A swimming bipedal robot is probably not a good idea.

Since climbing stairs, stepping over curbs, sitting in vehicles, opening doors and reaching into cabinets are all environment issues due to a design for humans, you can save a lot of contingency planning by using a bipedal model. Yes, it requires some complex math that evolution gave the human bipeds, but the big picture is that in the end, it is much simpler than alternatives.

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What you are asking is "why build something that is hard to make when there are easy to make solutions?" Simple answer: "because you learn more when you do something that is hard."

Also if yu want a robot that can function in a world with humans then if is best if the robot is in the shape of a human. We have constructed out world based on our shape and size. Doors and stairs and walkway paths and the with of isles in the super market and a million other things are build for use by humans.

We could argue that today "everything" is wheel chair accessible so why not build a robot that looks like a powered wheel chair? OK sounds good until you try siting in a wheel chair for a full day and see how much better walking works.

But really you build the robot based on the task it has to do. Most don't move at all and are bolted to the concrete floor.

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I find it quite astonishing that I was actually pondering on this particular type of matter regarding bipedal robots, or more specifically humanoids, very recently.

Humanoids, given that their physical structure extensively resembles that of a human, are more suitable in operating in environments where humans would normally operate in. This alone has the potential to alleviate the tasks that would otherwise seem redundant and repetitive but also situations where it would be too dangerous for humans to work in.

Intrinsically, our environment is filled with objects that have been specifically designed for us to interact with. Whether it's the faucet in your sink, the knob at the door or the hand-drill, all these items have been designed for us to easily manipulate. Ideally if we want robots to operate in such an environment and utilize our tools, the robot would have to be built in a way that would allow it to accomplish such a feat in an efficient manner. Consequently the most viable robot that can satisfy these stipulations are humanoids.

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