IEEE Robotics & Automation Magazine - December 2018 - 48

1) the original SoftHand anthropomorphic hand form
2) a gripper-like form with middle phalanges longer than
the hand.
The two devices are intended for medium/small and large
object grasping. The soft gripper terminal device is
designed with an industrial diving scenario in mind. In
this environment, it could be used to stabilize an ROV/
AUV by firmly catching hold of a structure in a docking
procedure. Then, a second robot arm could employ the
SoftHand to perform more fine maintenance operations.
Finally, more complex underwater manipulation tasks
could require an operator/robot to use both developed
tools (e.g., if a robot employed in an archeological operation must remove large debris to access smaller artifacts
with finesse). So we designed a custom tool change system
that enables fast and simple switching between different
terminal devices.
Design and Implementation
The design concept, shown in Figure 3(a), is a modular system composed of a waterproof servo-actuator core unit, a set
of soft terminal devices readily interchangeable through a tool
change system, and an optional waterproof handle device for
use by a human operator (e.g., as an ADS end effector). One
of the main design issues in submarine operations is electronic waterproofing. Two common techniques are
1) building a water- and pressure-tight enclosure to place
electronics
2) encasing all electronics in some resin, a methodology
known as potting.
We chose the first approach for the core module because it
allows easier system modification and intervention. Thus, we
designed a watertight enclosure that constitutes the central

T. Device 1:
Soft Hand

T. Device 2:
Soft Gripper
Tool Coupling
System

body of the underwater manipulation system. A section of the
complete assembly is presented in Figure 3(b): its maximum
diameter is 95 mm, its maximum length 170 mm, and its
mass approximately 2 kg. The enclosure components consist
of a cast-acrylic tube (7), an O-ring-sealed flange (13), and
openings for cable penetrators (14) to allow communication/
electrical power supply from the outer environment. To further protect the enclosure from hydrostatic pressure, the hull
was filled with mineral oil, thus reducing the pressure excursion between the inner and outer environments.
A second design issue relates to the necessary transmission of the motor torque from inside the watertight
enclosure to the outer underwater surrounding, where
the wet manipulation system interacts with objects to be
grasped. Obtaining a waterproof shaft sealing between
those two environments is not trivial: the rotating motion
of the shaft can cause momentary pathways for fluids to
leak through; with enough pressure/depth, water will
eventually penetrate past the seal and into the motor
housing. A more reliable solution for this issue is to make
the dynamic coupling between the actuated rotor (motor)
and the operative rotor immaterial. This is achieved using
a magnetically coupled drive consisting of two different
sets of magnets: one on the motor side and one on the
tool side. Magnetic couplings (MCs) are currently prized
for their effectiveness when submerged in water for two
main reasons [13], [14]:
1) The technology is intrinsically a torque limiter that can
help save the system's electromechanics. The coupling will
slip in the case of a severe rupture of the load torque. Moreover, this failure mode can be easily detected through voltage and current monitoring of the motor (see also the
section "Fault Detection").

3
2A
1B

Control 1:
Handle

Motor Coupling
System

10 11

2B
15

Core
Waterproof Unit

Control 2:
ROV/AUV Arm
(a)

12
13

1A
(b)

Figure 3. (a) The developed system scheme. The interchangeability of the terminal devices allowed by the MC drive and tool change
system is exploited. Tool components are highlighted in blue and control components are in red. (b) The core system and terminal
devices sections. The components are numbered as follows: 1A: SoftHand terminal device; 1B: soft gripper terminal device; 2A-2B:
inner and outer MC disks; 3: tool coupling device; 4: ferritic stainless steel ring; 5: tool pulley; 6: superior plate; 7: cast-acrylic tube; 8:
magnetic encoder group; 9: printed circuit board; 10: central support; 11: motor; 12: inferior flange; 13: inferior end cap; 14: cable
penetrator hole; 15: motor-coupling device; and 16: motor-coupling device springs. T.: terminal.

IEEE ROBOTICS & AUTOMATION MAGAZINE

december 2018

IEEE Robotics & Automation Magazine - December 2018

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