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Stuart O Anderson
12-07-2007, 05:47 AM
Hello,

By way of introduction, I am a Ph.D. student at the Carnegie Mellon Robotics
Institute, where I work with humanoid robots performing gait and balance
tasks.

It is generally understood that the bent knees exhibited by humanoid robots
like ASIMO or HRP-2 are an artifact of the algorithms used to generate a
stable gait. A straight knee is a singularity that reduces the rank of the
kinematic jacobian, meaning that the foot velocity cannot have a component
parallel to the line connecting the hip and ankle when the knee is
straight. These robots control their balance during gait by accelerating
their center of mass using the inverse jacobian to determine necessary joint
accelerations. When the jacobian is near a singularity the inverse can
become badly conditioned, resulting in large joint accelerations or no
solutions. To avoid this, the robots keep their knees bent during gait.
This approach to generating stable gaits is often referred to as the Zero
Moment Point, or ZMP, method, because stability is enforced by ensuring that
the center of pressure lies within the region of support at every instant
during gait. Recently, researches have proposed a variety of methods to
work around the need for straight knees while still using the ZMP criterion
for gait generation.

An alternative approach that uses straight knees during stance and seems
more closely tied to biomechanics is passive dynamic walking as demonstrated
by McGeer, Ruina, Kuo, and Wisse. This article in science news provided a
good overview of the field:
http://www.sciencenews.org/articles/20050806/bob8.asp
Some videos of a powered passive dynamic walker, that make a good contrast
to videos of asimo, from a different survey in Science:
http://www.sciencemag.org/cgi/content/full/307/5712/1082/DC1

Stuart Anderson
PhD Candidate
Robotics Institute
Carnegie Mellon University


On Dec 7, 2007 1:27 PM, Steven Abbott wrote:

> Hello,
>
> I don't contribute that often, finding it far more interesting to just
> "listen", but as its a friday afternoon!....
>
> I would think that by making asimo walk about like that it should do
> something rather interesting to his centre of gravity, perhaps making it
> slightly lower? The greater level of flexion in running would shift the
> centre of gravity still further, which would make sense as running would
> probably be even less stable.
>
> As to why, well, however human honda want to make asimo, his internal bits
> and pieces are going to inevitably have a very different weight
> distribution to that of a biological human. It may be that he is slightly
> top heavy, hence the need to shift the centre of gravity a bit when
> walking?
>
>
>
> --
> Steven Abbott
> Researcher
> Bioengineering Research Group
> Anglia Ruskin University
> 01245 493131 - ext 3317
>
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