CHAPTER 3. Visual grasp algorithm
Figure 3.10: Steps of the process about reconstruction of the object's surface: on
the left a teddy bear, on the right a bottle.
Images in a number equal to n
img
= 13 have been taken for both the objects
by a common webcam mounted inside the palm of the hand in Figure 3.7. The
reconstruction surface is sampled with n
s
= 1500 points, while the reconstruction
parameters have been chosen as: m = 10
-3
kg, k = 0.3 · 10
-3
N/m, b = 0.09 ·
10
-3
Ns/m, and F
a
= 5 N. For the grasp planner, k
, k
c
m
, k
e
, k
a
have been chosen
all equal to 1, in order to have an equivalent weight for all the contributions,
while the threshold
f
has been tuned to a value of 0.002 N. The floating security
distance
f
has been set to 2 cm, which is deliberately a huge value for a better
visualization of the trajectories. The computational time for the whole process is
about 1.5 s on a Pentium 1.7 GHz: in particular, the planner stage is the slower
one, since the stage about the reconstruction of the object employs only 80 ms,
for n
s
= 1500 points, to accomplish its job.
In Figure 3.10 some intermediate steps of the reconstruction algorithm are
shown, while the finger trajectories and the final grasp configurations, respectively
for the teddy-bear and for the little bottle, are shown in Fig. 3.11. Both cases of
k
c
m
= 1 (left) and k
c
m
= 0 (right) are considered (the bold point represents the
position of the object's center of mass of the reconstructed object). In particular,
for the case k
c
m
= 1 it is evident that both the grasp planes of the final grasps
contain the center of mass of the objects, while for the case k
c
m
= 0 the plane of
the final grasp is far from the center of mass to achieve a more extended areas of
the grasp polygon.
More in detail, Figure 3.11 shows how the teddy-bear is grasped with three
fingers achieving a desirable stable planar equilateral grasp (120
o
apart) for both
cases of k
c
m
= 1 and k
c
m
= 0. The yellow lines represent the sequence of recon-
struction points selected by the planner during the evolution of the reconstruction
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