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In lugnet.build.mecha, Jennifer Clark writes:
> In lugnet.build.mecha, Eric Sophie writes:
> > What constitutes a Point of Articulation?
> > Example
> > My Mecha is kick arse! It has lot's of movement. It rocks! It has 12 POA
> > giving the Mecha 24 DOF! Whoooo hooooo!
>
> Pedant's corner:
>
> This reminds me of an argument I've had several times, and to be honest have
> yet to come to a definitive answer.
Oh? Hmmm...do tell.
> The maximum number of degrees of freedom
> an object can have in three dimensional space is six; translation in the X,
> Y, and Z planes and rotation about in the X, Y, and Z axes. Giving a device
> more points of articulation does not necessarily increase the degrees of
> freedom, although it can increase the envelope or ability of the device to,
> for example, work round corners.
Right! I see. In my own way I work around certain "movements" or joints by
making two separete joints cover the same envelope or Area or Control.
I mean in that, for example. The Arms of my Super Mech-Bot don't have an upper
Rotation built in to them. Yet I can acheive the same position of the arms by
making the upper arm Swing out and upper arm shoulder Splay. (dang, did I
say that right?) .)
> There can be other oddities, where an object can move in more than one plane
> and/or rotation and have less degrees of freedom than the movement initially
> implies; think of a ball bearing rolling down a spiral race. This has
> translation in all three planes but only one parameter is needed to specify
> any possible location in those three planes,
Whoa, that's heavy! and so very true. There is a beautiful set of forces
harmonizing right there in what you just pointed out. Enlightening.
> i.e. the distance of the ball
> from the start of the race. If the ball rolls in a predictable way then the
> same parameter can be used to derive the rotation of the ball at an
> arbitrary position as well.
Hmmm, so in a way what I'm saying about the Super Mech-Bot makes much more
literal sense to me now. I mean, when I was building the Armature, I knew
that as much as I tried, the upper arm rotation was always going to be a
problem.
I could get the same results by combining two other joints to bring the Arm
into the same position as the one could. That was the end results of that
study. Anyway that's how I am relating to it. :)
> Think of an excavator boom with a joint at one end attaching it to the
> chassis and a rotating bucket on the other. To me this has 2 DOF; the
> translation of the boom (which will follow an arc) and the rotation of the
> bucket. Ok, so translating the boom will also rotate the bucket, but this is
> not an independent parameter so it is still only contributes 1 DOF. If you
> split the boom and add another similar joint to give the standard excavator
> boom, the boom can now cover an area rather than an arc, so you now have 3
> DOF. If you now split the boom again and another joint, similar to many long
> reach demolition excavators, however, the boom still only covers an area so
> you still have only 3 DOF. No matter how many joints of this type you add,
> you still have only 3 DOF.
Interesting, so in there lays the ambigutity.
> Is there a case to be made here for a description based on internal state of
> a machine vs. external state? It is possible to conceive of a machine where
> many internal states can correspond to one external or viewed one, for
> example. Presumably the answers are out there rather than the mere
> conjecture I put forward here.
It's a good point. It can go a few ways within each meaning.
I suppose the whole thing is best when used verbally (of course which can be
broken down into its basic logics) I show people all the time about my
Robots. Ussually it will go like this:
Q: Where does it move?
A: Here in the Arm is a POA, it moves to a fro in these DOF.
Ok that was fun.
BTW, I had to take a Vitamin, jog a block, do push ups and sit ups, listen
to Brain and Memory enhancing tapes and videos, do my home work, seek
knowledge and wisdom, eat vegetables, watch public television, just to keep up!
Wow! is she smart or what!
Legomaster
www.mylegomaster.com
> Of course, if your mecha inhabits a higher dimension, and therefore
> genuinely uses the extra DOFs available, I take it all back :-)
>
> Jennifer
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Message has 1 Reply:
 | | Re: articulation points?
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| o.k. so let me get this right: poa doesn't necessarily dictate dof. poa refers to the axes a given object can rotate around, but dof refers to the limitations (or lack thereof) in space a given object can move around in. according to eric, i may use (...) (22 years ago, 20-Nov-02, to lugnet.build.mecha, lugnet.technic, lugnet.robotics)
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Message is in Reply To:
| | Re: articulation points?
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| (...) Pedant's corner: This reminds me of an argument I've had several times, and to be honest have yet to come to a definitive answer. The maximum number of degrees of freedom an object can have in three dimensional space is six; translation in the (...) (22 years ago, 20-Nov-02, to lugnet.build.mecha, lugnet.technic, lugnet.robotics)
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