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discuss two different types of grippers
that are used in many mobile robot
projects. The parallel jaw gripper of
the PR2 robot shown in Figure 5 is
quite popular with many robot
designs as its jaw faces are always
parallel to each other. The other
popular type is the pincer type where
the two jaws swivel open and closed
about one or two axial points, with
one axis being like a pair of pliers, and
two axes being like the gripper in
Figure 6.
There are many types of grippers
used in industrial and special purpose
robots, including internal expansion
grippers, coffee grounds contained in
a rubber balloon, and tool interfaces
for numerous applications.
The robot shown in Figure 5 is
the Willow Garage PR2: an extremely
well designed and constructed robot
costing about $400,000. The figure is
from the IEEE Spectrum Robotics’
website and their June 2016 article
entitled, PR2 Learns a Trick to Grasp
Ungraspable Objects. PR2 has
encountered a large box (shown in
green in his ‘thoughts’) and is looking
at his small parallel jaw gripper, trying
to figure out how to grasp the box.
The box has a small hole in the middle
of one side. Fortunately, he was
supplied with a small dowel stored on
his shoulder that he can reach with his
gripper and insert it into
the hole to lift the box.
PR2 is shown in Figure
7 spending 15 minutes to
fold a towel — a task that a
human could do in a few
seconds. However, there
are few other robots that
could do folding.
The task of picking up
a box may seem simple for
you and me since we can look at the
box and deduce the angle of our view
and determine the approximate angle
that we must insert the dowel into
the hole; plus, determine if the dowel
is small enough or the hole is large
and deep enough. You and I would
place the end of the dowel in the top
of the hole and then wiggle it around
until it was parallel with the hole and
would slide it in.
This is the same method we use
when placing a drill bit back into a
drill index for storage, although there
are two levels of holes to deal with in
that scenario. It takes a few seconds
of maneuvering a drill bit to place it in
a drill index like the one shown in
Figure 8. The drill bit slides down
through the first hole and then we
wiggle it around until it fits through
the second hole. Easy enough for us
humans, but PR2 must use a complex
vision system set of algorithms and
have the correct dowel stored nearby.
The Human Hand: A
Multi-Purpose Gripper
If you ever get a chance, visit a
modern factory that has extensive
robot installations for many different
manufacturing processes. Some robots
may have paint spray end effectors;
others, spot welding heads, gas cutting
torch welders, vacuum grabbers,
internal expansion grippers, and so
many more types. Rather than trying to
develop the one-type-fits-all end
effector, each robot is fitted with a
specialized “hand” for a particular task.
Humans cannot remove our flesh
and bone hands that we use for
holding a golf club and then replace
them with a hand that can hold a
needle and thread to sew a tear in
our golf shirt. We don’t need to; our
hands are multi-purpose and adept at
Figure 8. Stowing drill bits in a
drill bit index takes a bit of
wiggling.
Figure 7. PR2 folding a towel, using its two parallel jaw
grippers and sophisticated vision system.
Figure 5. Willow Garage's PR2 mulling over a way to lift a box with his
small gripper.
Figure 6. ServoCity horizontal gripper.
