Subject:
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Re: articulation points?
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Newsgroups:
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lugnet.robotics
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Date:
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Thu, 21 Nov 2002 05:21:52 GMT
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Original-From:
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Steve Baker <sjbaker1@airmail(AvoidSpam).net>
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Viewed:
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1654 times
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Gordon Bentley wrote:
> The main reason it would make sense to quote the total DOF is that is
> determines the volume the arm needs or is able to work in. For example
> imagine a snake-bot with a dozen 2 DOF joints. It can reach around objects
> to position it's effector. So it is more versatile than the basic 3 joint
> arm. As you say though, the value of this is dependant on the application.
The trouble with this is that it's not necessarily true.
Take 100 Lego turntables and connect them up into a vertical
column.
By this measure, it has 100 DOF - but as a mechanism, it has no more
flexibility than a single turntable (1-DOF). Now, take that same
pile of turntables and plug them together with a 1 stud offset and
you have another 100 DOF mechanism that is totally flexible like
a snake - and can position it's end in two translational and one
rotational degrees of freedom - a 3-DOF mechanism in my terms.
I define those two mechanisms as 1-DOF and 3-DOF respectively.
The other definition describes them both as 100-DOF.
My definition is a useful one - it tells you a lot about how
useful the mechanism is. The other definition is useless - it
tells you nothing helpful.
Furthermore, in your example of the snake versus a 3 joint arm,
the snake may not be as able to reach around things if all of it's
joints are close together with one long straight section at one
end - compared to a 3 joint arm with long sections between each
joint.
So counting up the DOF's of each internal joint doesn't MEASURE
or PREDICT anything very useful.
Hence, I think it's silly to mindlessly add up the degrees of
freedom of each of the internal joints and quote that as the
number of degrees of freedom of the total mechanism.
IMHO, you should treat the device as a black box and count the
total degrees of freedom of the entire device.
---------------------------- Steve Baker -------------------------
HomeEmail: <sjbaker1@airmail.net> WorkEmail: <sjbaker@link.com>
HomePage : http://web2.airmail.net/sjbaker1
Projects : http://plib.sf.net http://tuxaqfh.sf.net
http://tuxkart.sf.net http://prettypoly.sf.net
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Message is in Reply To:
 | | Re: articulation points?
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| The main reason it would make sense to quote the total DOF is that is determines the volume the arm needs or is able to work in. For example imagine a snake-bot with a dozen 2 DOF joints. It can reach around objects to position it's effector. So it (...) (22 years ago, 21-Nov-02, to lugnet.robotics, lugnet.build.mecha, lugnet.technic)
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