IEEE Robotics & Automation Magazine - June 2021 - 42

mechanisms of pneu-nets actuators, consisting of network-like
chambers inside a silicone body over a strainlimiting
layer.
PP-A achieves splaying of the four distal fingers, while
PP-B performs the abduction of the thumb and overall
bending of the palm. Instead of bending in the manner
of pneu-nets actuators, PP-A splays the distal fingers in
the palmar plane with a small hump after being pressurized.
The main body has four air chambers for actuation
[Figure 4(a)]. The four large grooves in front are used to
affix the four fingers. The proximal cavity is designed to
arrange the pneumatic tubes and sensor wires inside the
hands. For PP-B, the bending motion of the palm is
achieved by enlarging the horizontal width of the pneunets
actuator. The widths of the air chambers along the
palm were designed to be as wide as possible to generate
enough force to bend the palm along with the distal fingers.
The tubing tunnel connecting with the bottom
cavity in PP-A is used to run the tubes and lines through
the palm.
The soft palm uses the same key geometric parameters
Main Body
Silicone Layer A
Strain-Limiting Layer
Main Body
(a) Structure of PP-A
as the soft fingers, such as wall thickness t and the small
gap between segments a. The stiffness of the palm for
enabling a reliable support for the other fingers is considered
and simulated in the SOFA framework. In addition,
a humanoid esthetic design was implemented as a last
step, after ensuring that the soft actuators worked as
intended and would not affect the performance of the
pneumatic actuators.
Simulation
Figure 4(b) and (d) represents the simulation results of the
two parts of the soft palm. The three functions of the palm
can be clearly detected through the comparison of each
actuator before and after pressurization.
Fabrication
(b) Simulation of PP-A
Silicone Layer B
Strain-Limiting Layer B
Silicone Layer A
Thumb Base
Strain-Limiting Layer A
Main Body
(c) Structure of PP-B
(d) Simulation of PP-B
Figure 4. The (a) simulation and (b) structure of the soft palm.
(b) and (d) present a comparison of the palm actuators under
inflation and deflation states. The red chambers in (b) and (d)
represent the pressurized actuators in SOFA.
42 * IEEE ROBOTICS & AUTOMATION MAGAZINE * JUNE 2021
Strain-Limiting Layer Sealing
The strain-limiting layer is made of silkscreen fabric and
used to ensure that the actuators bend in the desired direction
during pneumatic pressurization [3]. The layer was
placed in the bottom mold, as in Step 2 of making the finger
in Figure 5, and glued together with the bottom sealing portions
of all three components. Subsequently, the fabrication
of the main body of the actuators was finished, and the
chambers inside the silicone body were all sealed. Note that
Step 3 of making PP-B does not require molds. The silkscreen
fabric was added manually in the reserved gap [Figure
5(h1) and (h2)]. Then, after pouring silicone into the
gap and curing, the silkscreen fabric was combined with the
main body.
Actuator Body Molding
The soft body was fabricated using Smooth-On silicone
rubber (Dragon Skin 10 Medium) and Sil-Poxy to obtain
a Shore-A hardness of 10. The A and B parts of the Dragon
Skin 10 were mixed equally by weight in a plastic cup
and then put into a vacuum chamber. Using a vacuum
pump, the air trapped in mixed silicone materials expands
to bubble and finally collapses under about 0.9 bar vacuum
pressure. After evacuation, the silicone material was
poured into the molds, as described in Steps 1 and 2 of
making the finger, PP-A, and PP-B in Figure 5. The molds
were generally left in an upright position to cure for 5 h at
room temperature.
IEEE Robotics & Automation Magazine - June 2021

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