Alt-BEAM Archive

Message #08964



To: beam@corp.sgi.com
From: RAn2555212@aol.com
Date: Sun, 9 Jan 2000 21:13:33 EST
Subject: [alt-beam] request


i am an amateur and i would love to be on the BEAM mailing list.



8965 Sun, 09 Jan 2000 18:55:30 -0800 [alt-beam] Re: Pressure Sensors (getting back on topic) beam@sgiblab.sgi.com Bruce Robinson So, steering this thread back on topic (be patient, it's near the end)
...

Senior wrote:
>
> Hehehe allright... The work they've done is great. No other robots
> really match up. It just seems to me that they spent so much money and
> work and only came up with static balance.

I imagine there is a fair amount of dynamic balance involved. A quote
from their web site (keep in mind that Honda engineers studied walking
motion for a year before they even started conceptual design on these
robots) ...

"We have revealed that no significant effect is caused to walking
even without toes. More important support is ensured by base
sections of the toes, i.e., balls of feet, and joint areas" ++

So they decided not to have toes. That immediately limits the type of
walking motion they can develop.

More to the point ...

"A human eases the impact of walking with a combination between
structures and functions of movement. ...
... Although the robot must feature similar shock-absorption
mechanisms, the structural measure was not welcome because it
might deteriorate the robot's stability. Impact absorption was thus
ensured through precise control of each component." ++

Basically, they realized the importance of the spring-like shock
absorption of the human foot, but found that it complicated walking
tremendously.

A flat, unsprung foot approaches the ground, and sensors adjust the
angle so it is perfectly flat when weight is applied to it. From this
point on, you have a firmly fixed reference point. As long as the centre
of gravity is directly above some portion of the foot, the robot is
stable. Angles measured at the ankle can be used to help determine
foot-body relationships.

A slightly springy foot approaches the ground, adjusts so it is
perfectly flat, and then weight starts to shift onto it. The instant
weight is applied, the foot shifts relative to the ground. As the centre
of gravity moves above the foot surface, the foot will tilt slightly as
the springs adjust. This will cause the robot to wobble around slightly
as the foot takes the load ... it will also make joint angle
measurements less reliable for determining just where the body is in
relation to the feet. This means you have to keep compensating your
various sensor systems to allow for changing weight distribution over
the foot.

So Honda engineers opted to avoid the extra complications and control
the limb motion to minimize shock. This inherently limits the types of
motion the robot can achieve.

Here's where pressure sensors come into the picture. Many of the
suggestions (including mine) involve some kind of compressive foam --
which is a spring. Unless this foam is very thin, you will get
significant motion changes as the weight distribution shifts. There will
be a magnifying effect as well: divide the height of your robot by the
narrowest dimension of a foot ... this is how much any deflection of the
pressure sensors will be magnified by.

Credits: All references, and dimensions are from the Honda Humanoid
Robot web site:

http://www.honda.co.jp/english/technology/robot/index.html

++ The quotations are found on this page:

http://www.honda.co.jp/english/technology/robot/tec1.html

By the way, a biped walker is not that difficult to construct. It has
been done with two servos and mechanical linkages. Needless to say, it
doesn't look very humanoid, and it sure can't handle uneven terrain.

Regards,
Bruce



8966 Mon, 10 Jan 2000 03:40:15 GMT [alt-beam] Re: Alf & Dragonfly beam@sgiblab.sgi.com "Mike Kulesza" >Can someone fill me in on some more technical details of the coils you
>guys have used in these projects.
>
>What kind are they?
>Where did you get them? (Ben..Does Dick Smtih sell them?)
>Are you using magnets with the coils or something else?

I took mine from the insides 24V DC motors...I didnt need them (voltage too
high), and they were part of a fan structure anyway.

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