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i never really gave it a thought about the hand not having rotation of its
own accord, but rather gets it from the elbow. that solves a huge problem
i'm having with my mech. you see, i'm having difficulty with my mech
holding his weapon because it's too heavy. the hand is based loosely on
gla-gla's design and thus the wrist is attached only by a 1x1 plate clamp
(?) that rests on only one stud. if i can redesign the hand so that the
wrist still bends but is attached more firmly to the forearm and design the
forearm so that it rotates closer to the elbow, then that should work.
thanks for the revelation, henrik!
In lugnet.build.mecha, Henrik Johansson writes:
> In lugnet.build.mecha, Bram Lambrecht writes:
> > However, a device is made up of several objects. Each component, if
> > treated separately, has six degrees of freedom. Thus each component of,
> > for example, the human arm (reduced to an upper arm, lower arm, and
> > hand) has 6 DOF, for a total of 18. However, the parts are attached to
> > each other in a certain way which restricts their movement. Each
> > constraint reduces the DOF by 1. When you add them all up, you get 11
> > constraints, reducing the DOF of the arm to 7. Instead of counting the
> > constraints, it's easier to count the DOF at each joint.
> > o Shoulder: spherical joint, 3 DOF (rotation about x0,y0,z0)
> > o Elbow: hinge joint, 1 DOF (rotation about z1)
> > o Wrist: spherical joint, 3 DOF (rotation about x2,y2,z2)
> > ...for a total of 7 degrees of freedom.
> > I guess you would count each joint as a point of articulation (POA)?
> > In robotics, we generally treat a spherical joint as three different
> > joints whose axes all happen to intersect at the same point. Then there
> > is only one DOF per joint, making it easier to count :)
>
> Very interesting thread. A bit OT (and picky ;) mabye, but about the human arm:
>
> If you examine your arm closley you'll find that the rotation of the hand
> actualy comes from the elbow witch rotates the under arm. The wrist only actes
> as a dual hinge to tilt the hand sideway or up and down, there is no ratation.
>
> So I think it's more correct to put it this way,
> o Shoulder: spherical joint, 3 DOF (rotation about x0,y0,z0)
> o Elbow: dual hinge joint, 2 DOF (rotation about y1,z1)
> o Wrist: dual hinge joint, 2 DOF (rotation about y2,z2)
>
> Regards Henrik
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Message is in Reply To:
 | | Re: articulation points?
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| (...) Very interesting thread. A bit OT (and picky ;) mabye, but about the human arm: If you examine your arm closley you'll find that the rotation of the hand actualy comes from the elbow witch rotates the under arm. The wrist only actes as a dual (...) (22 years ago, 20-Nov-02, to lugnet.build.mecha, lugnet.technic, lugnet.robotics)
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