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									  Interception versus tracking when catching free-falling objects

 Pascal Prévost, Joe McIntyre, Patrice Senot, Alain Berthoz and Jean-Jacques
                        LPPA, Collège de France – CNRS, Paris, France

The main problem regarding the catching of a free-falling object by hand is to move the hand
to the right place at right time. Furthermore, the projected force of impact between the hand
and the object must be taken into account. To solve this problem with robots, a tracking
strategy was chosen in which the tangential velocity of the robotic hand was matched to that
of the flying object prior to closure of the grasp (Hong and Slotine, 1995). This strategy is
advantageous, in that it 1) reduces the degrees-of-freedom, 2) increases the margin-of-error for
the timing of hand closure and 3) reduces the relative momentum at impact. The goal of this
study was to examine the strategies employed by human subjects when performing a similar
free-falling catching task.
Six subjects were asked to catch a bar dropped by the experimenter (2 heights) or by himself
(1 height), with each hand. Bar and hand kinematics were captured by means of an optical 3D-
system. We found that hand and bar velocities were not exactly matched in amplitude, but
nevertheless were in the same direction at the moment of grasping. This suggests humans also
adopt a tracking strategy, as opposed to a purely interception strategy, to catch a falling object
with the hand.

Hong, W. & Slotine J.-J.E. (1995). Experiments in hand-eye coordination using active vision.
Experimental Robotic IV, Sprinter-Verlag, Proceeding of the 4th International Symposium on
Experimental Robotics, ISER’95, Stanford, Califonia, June 30-July-2.

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