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This article presents the development of modular soft robotic wrist joint mechanisms for delicate and precise manipulation in the harsh deep-sea environment. The wrist consists of a rotary module and bending module, which can be combined with other actuators as part of a complete manipulator system. These mechanisms are part of a suite of soft robotic actuators being developed for deep-sea manipulation via submersibles and remotely operated vehicles, and are designed to be powered hydraulically with seawater. The wrist joint mechanisms can also be activated with pneumatic pressure for terrestrial-based applications, such as automated assembly and robotic locomotion. Here we report the development and characterization of a suite of rotary and bending modules by varying fiber number and silicone hardness. Performance of the complete soft robotic wrist is demonstrated in normal atmospheric conditions using both pneumatic and hydraulic pressures for actuation and under high ambient hydrostatic pressures equivalent to those found at least 2300 m deep in the ocean. This rugged modular wrist holds the potential to be utilized at full ocean depths (>10,000 m) and is a step forward in the development of jointed underwater soft robotic arms.
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ORIGINAL ARTICLE
A Modular Soft Robotic Wrist for Underwater Manipulation
Shunichi Kurumaya,
1
Brennan T. Phillips,
2–4
Kaitlyn P. Becker,
2
Michelle H. Rosen,
2
David F. Gruber,
5
Kevin C. Galloway,
6
Koichi Suzumori,
1
and Robert J. Wood
2,3
Abstract
This article presents the development of modular soft robotic wrist joint mechanisms for delicate and precise
manipulation in the harsh deep-sea environment. The wrist consists of a rotary module and bending module,
which can be combined with other actuators as part of a complete manipulator system. These mechanisms are
part of a suite of soft robotic actuators being developed for deep-sea manipulation via submersibles and
remotely operated vehicles, and are designed to be powered hydraulically with seawater. The wrist joint
mechanisms can also be activated with pneumatic pressure for terrestrial-based applications, such as automated
assembly and robotic locomotion. Here we report the development and characterization of a suite of rotary and
bending modules by varying fiber number and silicone hardness. Performance of the complete soft robotic wrist
is demonstrated in normal atmospheric conditions using both pneumatic and hydraulic pressures for actuation
and under high ambient hydrostatic pressures equivalent to those found at least 2300 m deep in the ocean. This
rugged modular wrist holds the potential to be utilized at full ocean depths (>10,000 m) and is a step forward in
the development of jointed underwater soft robotic arms.
Keywords: soft robotic arm, fiber-reinforced actuator, underwater manipulation
Introduction
The development of soft actuators is a highly active
research topic, with direct applications for human-friendly
robots,
1
human-like robot,
2–5
human-assist robots,
6–10
and
medical devices owing to their flexibility, compliance, and
straightforward fabrication. Soft actuators can achieve com-
plex motions with relatively simple control, offering an ad-
vantage over conventional mechatronic systems.
11–19
Soft
actuators are also practical due to their compatibility with
pneumatic and hydraulic pressure from various fluids and
gases, and have a demonstrated utility as a tough and robust
solution for use in harsh environments.
11
They also offer un-
ique advantages over traditional deep-sea actuators, such as
hydraulic cylinders and electrical linear actuators, which are
better suited for heavy-duty tasks. For these reasons, our re-
search group has been developing soft actuators for delicate
and precise manipulation in the deep sea. Initial progress was
made with the development of ‘‘boa’’-type fiber-reinforced
actuators and ‘‘bellows’’-type grippers, which were used to
successfully collect fragile biological specimens in the deep
sea.
20
This article focuses on the design, construction, and
characterization of wrist modules that are intended for use with
the existing soft grippers and other soft actuators currently in
development, as shown in Figure 1a, and is a step forward in
the development of complete underwater soft robotic arms.
Fiber-reinforced soft actuators can realize various motions
such as bending, twisting, expansion, extension, and contrac-
tion by varying the fiber arrangement.
21
When an elastic body
expands, its body extends orthogonal to the direction of wound
fibers, while motion is deflected depending on the fiber ar-
rangement. The influence of fiber angle on the deformation of
fiber-reinforced soft actuators has been investigated in previous
work,
22–24
however, other design parameters such as rubber
hardness, number of fibers, and rubber thickness require further
characterization. While various types of fiber-reinforced soft
1
Department of Mechanical Engineering, Tokyo Institute of Technology, Tokyo, Japan.
2
School of Engineering and Applied Science, Harvard University, Cambridge, Massachusetts.
3
Wyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge, Massachusetts.
4
Department of Ocean Engineering, University of Rhode Island, Narragansett, Rhode Island.
5
Department of Natural Sciences, Baruch College, City University of New York, New York, New York.
6
School of Engineering, Vanderbilt University, Nashville, Tennessee.
SOFT ROBOTICS
Volume 00, Number 00, 2018
ªMary Ann Liebert, Inc.
DOI: 10.1089/soro.2017.0097
1
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actuators have been developed, fabrication processes vary
widely and there are no existing standards for multiactuator
integration. Furthermore, contemporary manipulator systems
offer strength and multiple degrees of freedom, which are
difficult to achieve using existing soft actuators. To over-
come these challenges, we present a system based on fiber-
reinforced soft actuators that are both scalable and modular
(Fig. 1). Continuum robotic elements to marine applications
made of rigid and partially soft materials have been devel-
oped in related works.
25–27
Modularity has a demonstrated
utility in soft robotics to simplify control and increase system
redundancy.
28
Our bending and rotary modules can be com-
posed in series to form wrist, elbow, and shoulder joints.
In this article, we focus on two types of fiber-reinforced
soft actuators that are scaled and arranged as a wrist mech-
anism. We report the development and characterization of
these actuators by varying fiber number and silicone hardness
with both pneumatic and hydraulic pressure. Water was
chosen as a hydraulic fluid as it is a desirable medium for soft
actuators.
29
Demonstrations of the soft robotic wrist operat-
ing with both pneumatic and hydraulic pressure are presented
toward applications such as remote manipulation and robot-
assisted assembly. In addition, a functionality test under high
hydrostatic pressure was performed to assess the wrist’s
utility for deep-sea manipulation.
Design Overview
Several factors constrained the design of the soft wrist,
which we developed with the goal of integrating into a com-
plete manipulator system. One key aspect was modularity,
which offers several advantages: specific components can be
scaled independently, arranged in various sequences, and re-
paired and/or replaced with relative ease. Control of a full
manipulator system is also simplified with discrete motions for
each actuator. Using interchangeable gender-specific connec-
tors, these actuators can be connected in series to achieve a
desired working envelope. Internal passages for pressure lines
and sensor integration were included to streamline the as-
sembly. Both the connectors and the actuators themselves were
designed with compactness in mind, to aid in approximating
the behavior of a traditional multidegree-of-freedom joint.
FIG. 1. (a) The soft wrist
with bending (left) and rotary
(right)modulesattachedin
series. (b) Schematic over-
view of the rotary module
having a pressurized chamber,
asymmetric fibers, and inner
hole for wiring. This module
twists when the chamber is
pressurized. (c) Schematic
overview of the bending mod-
ule having two pressurized
chambers, circular fibers, and
an inner hole for tubing and
wiring. It bends in a direction
opposite to the pressurized
chamber. Color images avail-
able online at www.liebertpub
.com/soro
FIG. 2. Fabrication process of the rotary module. (a) Li-
quid silicone rubber (Smooth-Sil 950 by Smooth-On) is
poured in both halves of 3D-printed molds. (b) The two
mold halves are clamped together, and then, silicone rubber
is formed into a cylindrical shape. (c) Silicone cylinders and
three-dimensional (3D)-printed end connecters are com-
bined by silicone adhesive (Sil-Poxy by Smooth-On). Both
inner and outer cylinders are fabricated in the same manner.
(d) Both ends are tightened by fibers and reinforced by
nylon sheets. (e) Kevlar fibers are attached with a helical
arrangement. (f) The completed actuator is coated with
silicone adhesive. Color images available online at www
.liebertpub.com/soro
2 KURUMAYA ET AL.
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The rotary module is a fiber-reinforced actuator with one
degree of freedom consisting of an elastic cylinder sur-
rounded by a helical arrangement of fibers (Fig. 1c). This
design was optimized to achieve large angular motions with
minimal axial displacement. The module has an interior
passage for pressure and sensor lines, and a single sealed
chamber for pressurization. The operation principle is the
same as that of conventional fiber-reinforced actuators.
21,22
Expansion in the radial direction under applied internal
pressure leads to twisting by extension in the vertical direc-
tion of the fibers. When pressure is released from the cham-
ber, it returns to its original unactuated state.
The bending module is a fiber-reinforced actuator with a
single degree of freedom, consisting of an elastic cylinder
surrounded by a circular arrangement of fibers (Fig. 1b). This
module has an inner hole for wiring and two symmetrical
chambers for pressurization. When one chamber is pressur-
ized, the module bends in a direction opposite the pressurized
chamber, and when both chambers are pressurized the
module extends in the axial direction. Typical operation re-
quires one chamber to pressurize, while the opposite chamber
is relieved of pressure.
Soft Module Development
Rotary module
The rotary module consists of seven basic parts: inner and
outer cylinders, two end connectors, a pressure control tube,
fibers, and nylon sheets. The fabrication process of the rotary
module is described in detail in Figure 2. First, cylinder
molds consisting of upper, inner, and outer parts are three-
dimensional (3D) printed and liquid silicone rubber is poured
into these molds (Fig. 2a). Next, the filled molds are com-
bined and clamped together after removing any bubbles from
the rubber (Fig. 2b). Once the silicone rubber has cured, the
parts are removed. The two cured silicone cylinders and 3D-
printed end connectors are combined using silicone adhesive
(Fig. 2c). The supply tube is attached to one end connector.
The actuator is then tightened by fibers and reinforced by
nylon taffeta sheets (70 denier, heat sealable) outside and
inside, respectively. Nylon sheets serve not only to reinforce
against overpressurization but also as an anchor to affix fibers
(Fig. 2d). Kevlar fibers are then attached in a helical ar-
rangement to a pattern laser cut onto a nylon sheet (Fig. 2e).
Finally, the entire actuator is coated in silicone glue to pre-
vent slipping of the helical fibers (Fig. 2f).
Specifications of the rotary module are listed in Table 1.
Fiber angle is determined from previous work
20
that reported
an optimal twisting angle using fibers arranged at a 30angle.
The range of motion of a fiber-reinforced actuator under a
specific pressure usually depends on the aspect ratio of wall
thickness and diameter; the wall thickness of the rotary module
was chosen to be similar to a flexible microactuator described
in previous work.
22
The module twists in the direction of the
fibers’ offset angle, with some expansion and extension when
pressurized (Fig. 3). The fabricatedmodule twists77under an
air pressure of 172 kPa and can achieve 90rotation at 210 kPa.
These pressures are similar to those used in soft gripping ac-
tuators developed for deep-sea sampling.
20
Bending module
The bending module also consists of seven basic parts: two
half-cylinder chambers, two end connectors, a pressure control
tube, fibers, and nylon sheets. The fabrication process of the
bending module is described in detail in Figure 4 and is similar
to that of the rotary module. First, half-cylinder chamber molds
consisting of upper, inner, and outer parts are 3D printed and
liquid silicone rubber is poured into these molds (Fig. 4a).
Next, these molds are combined and clamped together after
removing bubbles (Fig. 4b). Once the silicone rubber has
cured, the parts can be removed. Two half-cylinder chambers
can be made in the same mold. A nylon sheet is put between
the chambers and glued together with silicone adhesive to
reinforce the inner sides of the chamber, and prevents exten-
sion in the center (Fig. 4c). 3D-printed end connectors are then
attached with silicone adhesive and the pressure control tube is
attached to an end connector (Fig. 4d). Both ends are tightened
by fibers and reinforced by nylon sheets outside and inside,
respectively (Fig. 4e). Kevlar fibers are then attached in a
circular arrangement and the actuator is coated with silicone to
prevent fibers from moving out of position (Fig. 4f).
Table 1. Specifications of the Rotary Module
Parameter Value
Diameter of the outer cylinder 46 mm
Diameter of the inner cylinder 26 mm
Length 120 mm
Length of silicone part 70 mm
Hardness of the silicone 50 shore A
Fiber angle 30
Number of outer fibers 12
FIG. 3. Top and side view of the rotary
actuator with 30fiber angle. It twists 77
and extends 11 mm in the axial direction
under an air pressure of 172 kPa. Color
images available online at www.liebertpub
.com/soro
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Specifications of the bending module are shown in Ta-
ble 2. Each design parameter was chosen to match that of the
rotary module. The fiber angle is maximized at 0fiber
angle, since bending motion occurs via symmetric extension
from a single chamber when pressurized (Fig. 5). Some
asymmetry in the bending angles of each side can occur due
to errors in fabrication.
Joint system
All modules have standardized gender-specific connectors
(Fig. 6a). The joint is mechanically connected by #6–32
screws and nuts. Space in the female joint is reserved for
tubing and wiring to run inside or outside the actuator
(Fig. 6b). Thanks to the standardized joint system, modules
are easily and quickly assembled and rearranged to achieve a
desired workspace. Once modules connect to each other, all
supply tubes and wires can run through the inner holes
(Fig. 6c). Inner holes in the current design are capable of
routing more than 20 3-mm-diameter tubes.
Characterization
Basic characteristics
To characterize the rotary and bending modules, four ex-
periments under pneumatic pressure were conducted. First, the
range of motion under varying pneumatic pressures was mea-
sured visually. Second, the torque of the rotary actuator and the
applied force of the bending actuator were measured by using a
materials characterization system (model: Instron 5544A, sin-
gle column) as shown in Figure 7. Third, the compliance of
each module was also measured using the same instrument.
Finally, the speed of actuation was observed. The torque Tand
rotation angle uof the rotary module are calculated using
Equations (1) and (2), respectively, where the pulley’s diameter
D,tensionF, and Instron displacement Ddare indicated in
Figure 7a:
T¼
D
2
F(1)
u¼
Dd
D(2)
For the bending module, bending force Fwas measured
and the bending angle hwas calculated using Equation (3),
where initial module length Land Instron displacement Dd
are indicated in Figure 9b:
h¼tan 1Dd
L(3)
The characterization results of the rotary module are shown
in Figure 8. The maximum rotary angle is measured to be 90,
the maximum torque is 0.43N$m, and the actuation time
reaching 0.25 N$m at 103 kPa is 5 s. This results in a primary
delay time constant of *3 (defined as the time required to
achieve 63.2% of maximum torque). The characterization re-
sults of the bending module are shown in Figure 9. The
maximum bend angle is 122, maximum force is 15.6 N, and
actuation time reaching 16 N at 138 kPa is 6 s, resulting in a
primary delay time constant of *2. Both modules display
hysteresis due to the material properties of silicone rubber.
Characteristics by varying design parameters
In addition to their geometrical design, fiber-reinforced ac-
tuators have various design parameters such as fiber angle,
Table 2. Specifications of the Bending Module
Parameter Value
Diameter of the outer cylinder 46 mm
Diameter of the inner cylinder 26 mm
Length 150 mm
Length of silicone part 100 mm
Hardness of the silicone 30 shore A
Fiber angle 0
Number of outer fibers 14
Fiber interval 6.7 mm
FIG. 4. Fabrication process of the bending module. (a)
Liquid silicone rubber (M4601 by Wacker Chemical) is
poured in both halves of 3D-printed molds. (b) The two
mold halves are clamped together, and then, silicone rubber
is formed into a half-cylindrical shape. (c) Nylon sheet is put
between two half cylinders and these are assembled by sil-
icone adhesive. (d) Fabricated silicone cylinders and 3D-
printed end connecters are combined with silicone adhesive.
(e) Both ends are tightened by fibers and reinforced by nylon
sheets outside and inside, respectively. (f) Kevlar fibers are
attached with a circular arrangement of fibers and the cyl-
inder is coated with silicone. Color images available online
at www.liebertpub.com/soro
4 KURUMAYA ET AL.
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number of fibers, and material elasticity that can be tuned to
achieve the desired workspace. Focusing on the number of
fibers and the hardness of silicone rubber, comparative tests
were conducted to determine their effect on the rotary and
bending actuators using both pneumatic and hydraulic pressure.
To determine the characteristics of the rotary module and
bending module, a compliance experiment was conducted
using the Instron materials characterization machine (as de-
scribed in section 4.1). Rotary and bending modules with
varying number of fibers and hardness of silicone rubber, as
shown in Figure 10, were fabricated for these experiments.
The specifications of the rotary and bending modules used in
the compliance experiments are listed in Tables 3 and 4. The
number of fibers can be also represented as a fiber interval, so
the bending modules are described in terms of fiber interval.
The relationship between torque and rotary angle or bending
force and angle (i.e., compliance) was measured with the
Instron machine moving from the actuated state at a partic-
ular pressure toward a length with no torque or bending force.
First, the compliance characteristics of the rotary module
with varying number of fibers are shown in Figure 11 and
with varying silicone rubber hardness in Figure 12. Com-
parison of the experimental values for torque and rotary angle
is presented in Table 5. From these results, the following
conclusions can be made.
(1) Rotary modules with hydraulic pressure show 3–11%
higher torque than those with pneumatic pressure.
(2) Rotary modules with pneumatic pressure exhibit
*75–100% increased rotary angle than those with
hydraulic pressure.
(3) The hardness of silicone rubber increases the maxi-
mum torque and rotary angle.
(4) The number of fibers increases the maximum torque
and rotary angle.
Rotary modules with hydraulic pressure show increased
torque and decreased rotary angle than those with pneumatic
pressurebecause air is more compressible than water. The hard-
ness of silicone dramatically influences compliance properties,
making it an important design parameter. Harder silicone (50
shore A) is preferable, but if the module is operated at lower
pressures, a softer silicone (30 shore A) is suitable. The number
of fibers also influences compliance properties and perfor-
mance increases in accordance with the fiber number. Fur-
thermore, rotary modules can unevenly expand when there are
too few fibers to constrain it. Although the number of fibers that
can be attached to the module is limited, a dense fiber ar-
rangement increases performance and durability.
The compliance characteristics of the bending module with
varying number of fibers and silicone hardness are shown in
FIG. 6. Images of the joint
system. (a) Male joint and
female joint connected to
each other by screws and
nuts. (b) Side holes for sensor
wiring. (c) Supply tubes can
pass through the inner hole.
Color images available online
at www.liebertpub.com/soro
FIG. 5. Side view of the bending actuator with 0fiber angle (a). It bends 115to the left (b) and 100to the right (c)
under an air pressure of 172 kPa. Color images available online at www.liebertpub.com/soro
SOFT ROBOTIC WRIST FOR UNDERWATER MANIPULATION 5
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Figures 13 and 14. A comparison of the experimental values
for torque and rotary angle is presented in Table 6. From these
results, the following conclusions can be made.
(1) Bending modules using hydraulic pressure exhibit
linear compliance properties.
(2) Increasing silicone rubber hardness decreases the
maximum bending force by 13–18% and reduces the
bending angle by *50%.
(3) Among the fiber intervals tested, the bending force
and bending angle are optimized with an interval of
6.7 mm.
As shown with the rotary module, bending modules with
hydraulic pressure show linear compliance properties due to
fluid compressibility. Similarly, the hardness of silicone dra-
matically influences compliance properties. Silicone rubber of
30 shore A hardness is preferable to that of 50 shore A due to
material resistance to deformation. The number of fibers also
influences compliance properties; however, bending force is
not increased in accordance with the fiber number. In this
experiment, we find that performance appears to be optimized
with a fiber interval of *6.7 mm, which likely corresponds to
a ratio related to the geometry of the actuator. If the fiber
interval is too small, the minimized surface area of rubber
limits expansion, and thus, extension area in the axial direction
is decreased. If fiber interval is too large, expansion in the
lateral direction is also increased and this limits the amount of
force applied to bending, and thus, the extension area in the
axial direction becomes small. A more exhaustive modeling
and experimental effort are required to define a truly optimal
relationship between fiber interval and actuator geometry.
FIG. 7. Experimental setup for actuator characterization.
(a) Rotary module with a counterweight of 2.5 kg to prevent
bending. Torque was calculated by measuring force Fusing
a 50-mm-diameter pulley, and rotary angle uwas visually
observed. (b) Bending module. Bending force is observed
as tension Fby Instron, and bending angle is calculated by
the actuator’s displacement. The circle indicates position of
the counter weight and arrowheads show the direction
displacement and force. Color images available online at
www.liebertpub.com/soro
FIG. 8. Characterization of the rotary module. (a) Rotary angle under applied air pressure, where the maximum rotary angle
is 90at an air pressure of 207 kPa. (b) Torque under applied each pressure, where the maximum torque is 0.43N$matanair
pressure of 138 kPa. (c) Compliance characteristics at an air pressure of 103 kPa. (d) The speed of actuation at an air pressure
of 138 kPa, where the actuation time reaching 0.25N$m is 5 s. Color images available online at www.liebertpub.com/soro
6 KURUMAYA ET AL.
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Soft Wrist Demonstration
A soft robotic wrist with two degrees of freedom is created
by combining rotary and bending modules, and additional
actuators can be attached to either end for various manipu-
lation tasks. The working range of motion at the tip of the
wrist was obtained using a Vicon motion capture system, and
encompasses a hemisphere of 140-mm radius bounded by the
FIG. 9. Characterization of the bending module. (a) Bending angle under applied each pressure, where the maximum
range of motion is 122at an air pressure of 172 kPa. (b) Bending force under applied each pressure, where the maximum
force is 15.6 N at an air pressure of 138 kPa. (c) Compliance characteristics at an air pressure of 138 kPa. (d) The speed of
actuation at an air pressure of 138 kPa, where the actuation time reaching 16 N is 6 s. Color images available online at www
.liebertpub.com/soro
FIG. 10. Rotary modules (Nos. 1–4) shown with various
number of fibers and hardness of silicone rubber. Specific
properties are listed in Table 3. Bending modules (Nos. 5–8)
shown with various number of fibers (fiber interval) and hard-
ness of silicone rubber. Specific properties are listed in Table 4.
Color images available online at www.liebertpub.com/soro
Table 3. Specifications of the Rotary Module
Used for Comparison
No.
Length of silicone
part (mm)
No. of
fibers
Hardness of silicone
rubber (shore A)
17024 50
27012 50
370 8 50
47012 30
Table 4. Specifications of the Bending Module
Used for Comparison
No.
Length of
silicone
part (mm)
No. of
fibers
Fiber
interval (mm)
Hardness of
silicone rubber
(shore A)
5 100 19 5 30
6 100 14 6.7 30
7 100 9 10 30
8 100 14 6.7 50
SOFT ROBOTIC WRIST FOR UNDERWATER MANIPULATION 7
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rotary actuator’s twisting range of *90(Fig. 15). This
workspace is similar in scale to the existing soft manipulators
developed recently.
30,31
We envision increasing the work-
space of the soft wrist by using additional modules.
A functionality test was conducted at a high-pressure test
facility (Woods Hole Oceanographic Institution) to demon-
strate the wrist’s performance under high hydrostatic pressures
(Fig. 16; see also Supplementary Video S1; Supplementary
Data are available online at www.liebertpub.com/soro). Ac-
tuation was successfully achieved using similar hydraulic
pressures presented in section 4, which were elevated above
the ambient pressure of 24 MPa (equivalent to 2300 m sea-
waterdepth)withnoapparentlimitationintheactuators
range of motion. Following the test, the actuators showed no
signs of damage and were completely functional at normal
atmospheric pressure.
FIG. 11. Compliance character-
istics of rotary modules by varying
the number of fibers under a main-
tained pressure of 103 kPa. Color
images available online at www
.liebertpub.com/soro
FIG. 12. Compliance character-
istics of rotary module by varying
the hardness of silicone rubber un-
der a maintained pressure of 63 kPa.
Color images available online at
www.liebertpub.com/soro
Table 5. Comparison of Experimental Values for Torque and Rotary Angle
at 103 kPa Pneumatic or Hydraulic Pressure
No. 1 No. 2 No. 3 No. 4
a
Maximum torque with pneumatic pressure 0.288 N$m 0.253 N$m 0.237 N$m 0.241 N$m
Maximum torque with hydraulic pressure 0.297 N$m 0.283 N$m 0.246 N$m 0.247 N$m
Maximum rotary angle with pneumatic pressure 21.820.418.527.9
Maximum rotary angle with hydraulic pressure 11.811.59.916.0
a
Maximum torque and rotary angle of No. 4 were pressurized at 63 kPa.
8 KURUMAYA ET AL.
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Conclusions and Future Work
We presented the design, fabrication, and characterization
of a new class of modular fiber-reinforced actuators. Two
different designs are demonstrated: a rotary actuator capable
of up to 90rotation with a maximum torque of 0.43 N$m,
and a bending actuator that can curve 120in opposing
directions and apply a force of 15.6 N. Actuation character-
istics of these modules using both pneumatic and hydraulic
pressure are presented, along with the impacts of varying
material selection and fiber arrangement. The actuators are
designed with a standardized joint mechanism, and their use
as a complete wrist assembly is demonstrated under normal
atmospheric pressure and high hydrostatic pressure.
Our rotary and bending modules are similar in scale to a
human wrist, making them ideal candidates for a human-scale
FIG. 13. Compliance characteris-
tics of bending modules by varying
the fiber interval under a maintained
pressure of 138 kPa. This measure-
ment was performed by releasing
tension. Color images available on-
line at www.liebertpub.com/soro
FIG. 14. Compliance characteris-
tics of bending modules by varying
the hardness ofsilicone rubber under
a maintained pressure of 138 kPa.
This measurement was performed
by releasing tension. Color images
available online at www.liebertpub.
com/soro
Table 6. Comparison of Experimental Values for Bending Force and Bending Angle
at 138 kPa Pneumatic or Hydraulic Pressure
No. 5 No. 6 No. 7 No. 8
Maximum bending force with pneumatic pressure 15.0 N 15.3 N 15.6 N 13.3 N
Maximum bending force with hydraulic pressure 15.1 N 15.4 N 14.6 N 12.7 N
Maximum bending angle with pneumatic pressure 36.746.344.724.6
Maximum bending angle with hydraulic pressure 16.322.220.811.5
SOFT ROBOTIC WRIST FOR UNDERWATER MANIPULATION 9
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manipulator system. This work is part of a series of steps
toward controllable soft robotic arms and grippers that are
designed for robust, remote manipulation. Using hydraulic
pressure, they are particularly suited for work in the marine
environment and have direct applications for sampling of
delicate biological specimens. We also envision other prac-
tical applications, such as robot-assisted assembly, logistics,
agriculture, and surgical manipulation.
In the near future, we anticipate field testing these actua-
tors as part of a complete manipulator system in the deep sea.
The design of the modules would benefit from further re-
finement, with a focus on increasing their ability to handle
higher pressures to achieve a higher range of motion and
force application. The actuators would also benefit from
having proprioceptive sensors toward a haptic control inter-
face and autonomous manipulation motions.
Acknowledgments
This study was funded by NSF Instrument Development
for Biological Research Award #s 1556164 to R. Wood and
K. Galloway and #1556123 to D. Gruber. Any opinions, find-
ings, and conclusions or recommendations expressed in this
material are those of the authors and do not necessarily reflect
the views of the National Science Foundation. The authors
express their sincere appreciation to James Weaver and Alex
Meckes for their assistance in 3D printing and mold design.
Author Disclosure Statement
No competing financial interests exist.
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Address correspondence to:
Shunichi Kurumaya
Department of Mechanical Engineering
Tokyo Institute of Technology
Tokyo 152-8552
Japan
E-mail: kurumaya.s.aa@m.titech.ac.jp
SOFT ROBOTIC WRIST FOR UNDERWATER MANIPULATION 11
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