From: burtp@rs6407.ecs.rpi.edu (Philip Nason Burt)
Subject: Re: Robot juggling survey
Organization: Rensselaer Polytechnic Institute, Troy, NY
Date: Thu, 21 Jan 1993 17:39:07 GMT

In article <C17LI8.pr.1@cs.cmu.edu> mason+@CS.CMU.EDU (Matt Mason) writes:
>Many of those roboticists do juggle.  But the theoretical computer scientists
>seem to be better jugglers.  I wonder why?  
>
I personaly feel that mechanical engineers are the best jugglers. :)
they have the best understanding of spacial relations (3d vector
stuff)

>guys, each doing two-ball fountain.  For each subject, the hand followed a
>roughly elliptical path.  But the locations of the release and catch points along
>the ellipse seemed to differ between the two subjects.  Sakaguchi et
>al designed their robot to use elliptical paths too.
>
>
>First, analysis and design of a catch would be a lot simpler if there were only
>one strike, rather than a sequence of bounces.  One way to do that would be to
>have a zero coefficient of restitution---a perfectly plastic impact.  Like when a
>beanbag hits a concrete floor.  I've noticed that when I drop a lacrosse ball
>into my hand, it doesn't bounce.  But if I repeat the experiment with my hand
>resting on a table, the ball bounces.  Apparently flexion of the elbow joint
>gives an effective restitution of zero.  But it only works with heavy enough

Your description of the elliptical hand path is a better explaination
of this phenomenon (I think).  When a ball is caught, your hand is
moving in the same direction and velocity as the ball - thus no
bounce.  Try juggling decent weight balls with your hands cupped
rather than grasping the balls.  It is sort of soothing to juggle like
this.

>balls.  Nerf balls of the same size don't work.  It makes me wonder if very light
>balls would make blindfold juggling impossible, or harder.
>
>To apply the same trick to clubs, you need to cancel the translational and
>rotational energy simultaneously.  In theory, this can be done by striking the
>club at the center of percussion.  I notice that if I catch a club in my usual
>fashion, but don't close the fingers, the club really does seem to strike without
>bouncing, hang motionless for an instant, then slip off.  This suggests that
>velocity-matching and closing of fingers play only a secondary role in catching. 
>Any reaction?  It also suggests that clubs could be tuned "scientifically", by
>matching mass and angular inertia to the dynamic characteristics of the human
>arm.  Maybe even customized to the individual juggler, since some of us are
>beefier than others.  Much as the "tuned track" was designed to match human
>running dynamics.

This is some pretty cool insight.  Maybe i will try making my own
weighted sticks to determine what is best for me.
>
>The details of release are also interesting.  Baseball pitchers let the ball roll
>off the end of the fingers to get speed and backspin, and they use knucklers to
>get slow rotation.  But in juggling things are different because the speeds are
>too low for aerodynamic effects.  (Aren't they?)  We care mostly about accuracy. 
>How do the details of release affect accuracy?  Does it help to let the ball roll
>out of the hand?  Do good jugglers all give similar rotation to the ball?

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*    ><>        phil burt        <><    *            burtp@rpi.edu           *
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