IEEE Robotics & Automation Magazine - December 2018 - 49

τM,q
dc
Motor

τL, qL

τC
JM

KL
Terminal Device

KC
(a)

Resonance Freq. (rad/s)

2) The couplings can be manufactured as dust-proof, fluidproof, and rust-proof and engineered to handle extreme
operating conditions.
Even if such a coupling renders the device heavier, this is
less relevant in an underwater environment. Considering,
thus, the tradeoff between the coupling/device weight and its
functionality, the design choice we opted for was a disk-type
MC (Magnetic Technologies, http://www.magnetictech.com)
capable of transmitting a torque of 1 Nm with a gap between
its two plates of about 6 mm (2A and 2B).
The actuator (11) is a 12-Vdc gear Maxon DCX 22 motor
with an 83:1 gear ratio. For motor position control, we used
two magnetic encoders (8). The encoder magnets are placed
on two gears, with a different number of teeth, connected to a
third gear mounted on the motor shaft. The combined readings of the two encoders enable the reconstruction of the absolute position of the motor shaft over a range of several turns,
thanks to the different gear ratios between the motor and the
two sensors. Preliminary experiments proved that neither
flooding in mineral oil nor proximity with the coupling magnetic field interferes with the sensors.
The encoders and actuator are connected to the SoftHand printed circuit board, which communicates with the
outer environment by means of the cables passing though
the penetrator. The employed bus carries power/ground and
two lines that implement an RS485 communication (see
[15] for details).
Thanks to the modular design of the manipulation system
and the immaterial coupling provided by the MC, various terminal devices (robotic hands, grippers, and/or tools) can be
readily connected to the outer motor shaft using a custommade tool change system (described in the following section).
The developed core system, along with the two terminal
devices currently designed, is shown in Figure 3. The maximum lift, pinch grasp, and power grasp forces exerted by the
SoftHand are 400, 20, and 76 N, respectively. The lift force
value refers to the force that the robotic hand/gripper exerts
to hang a corresponding weight, as in Figure 4(c). The gripper's maximum lift force results in about 150 N.

Tool Change System
The tool change system consists, essentially, of a snap mechanism between the motor group and each tool group. Two
springs (16) are fastened to the motor group coupling device
(15) to realize the snap mechanism, depicted in Figure 5; placing them on the motor group makes a unique motor-coupling
mechanism suitable for a
set of tools, each equipped
with its MC half. The couLaboratory testing assessed
pling of different tools is
rendered fast and simple,
the pressure-tolerant
thanks to both the MC
magnetic attractive force
nature of the two terminal
and the spring load, while
the uncoupling is possible
devices, which were able
by means of a custommade tool housing. Once
to withstand a pressure
the motor group is positioned and secured inside
of 50 bar without visible
the housing, the spring
work (i.e., the snap mechdamage or degradation in
anism between the tool
and the motor group) is
performance.
deactivated. In such a configuration, the tool group
(3) is fastened to the tool housing, both longitudinally and axially. In particular, the longitudinal fastening is accomplished by
a set of magnets that connect to a ferritic stainless-steel ring (4)
fastened to the MC half residing in the tool group (2A); the
same magnets block the tool MC half, allowing the tool angle
of a previously uncoupled tool to be maintained when recoupling with it. The axial fastening makes use of special guides
obtained in the tool housing that act as a prismatic joint for the
tool group. Thus, the motor group can be uncoupled by the
tool group (which remains secured to the tool housing) and
retrieved by the operator/robot simply by applying an axial
force on it. The coupling/uncoupling procedure is schematically depicted in Figure 5, along with the tool change mechanism configuration in each phase.

800
700
600

ωR (q )
max ωR = 737 rad/s
min ωR = 656 rad/s

500
400

4
6
8
10
Motor Angle q (rad)
(b)

F
(c)

Figure 4. (a) A two-inertia representation of the system and analyzed oscillator subsystem. The values J M and J L are the combined
motor/first coupling half and second coupling half/load inertias, respectively; x M, x C, and x L are the motor, coupling, and load
torques, respectively; q and q L are the motor and load angles, respectively; and K C and K L are the coupling and load torsional spring
stiffness coefficients, respectively. (b) The resonance frequency ~ R (q) (in red) of the oscillator of (a) with respect to q. (c) A depiction
of the SoftHand terminal device exerting a lifting force equal to F (lift force value definition).

december 2018

IEEE ROBOTICS & AUTOMATION MAGAZINE

http://www.magnetictech.com

IEEE Robotics & Automation Magazine - December 2018

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