(2021) 8:16
Yamada and Mitsuda Robomech J
https://doi.org/10.1186/s40648-021-00203-7
Open Access
RESEARCH ARTICLE
A vacuum-driven rubber-band gripper
Ko Yamada1
and Takashi Mitsuda2*
Abstract
Robotic grippers that gently handle objects of various shapes are required for various applications these days. Conventional finger-shaped grippers are multifunctional and can grip various objects; however, grasping an item without
slippage requires planning the positioning of the fingers at appropriate locations on the item. Hence, a ring-shaped
soft gripper that coils itself around objects like a rubber band is suggested in this paper. The proposed gripper
comprises a soft tube containing laminated sponges interleaved with plastic sheets. Evacuation of the air within the
sponges shrinks them and decreases the diameter of the ring, thereby allowing the gripper to firmly hold objects. The
gripper is therefore flexible enough to coil around objects of various shapes without gaps. Furthermore, the rigidity
of the compressed sponges inside the gripper prevents wobbling of the gripped objects. The air within the gripper
can be used to adjust the gripping force. The minimum diameter of the gripper after evacuating the air within the
sponges is approximately one-fourth of the original diameter. Thus, the proposed gripper is expected to be used in
various applications as it automatically conforms to the different shapes while simply gripping objects gently and
securely.
Keywords: Pneumatic actuator, Flexible structure, Soft robot, Gripper, Robot hand, Vacuum-driven gripper
Introduction
Robotic grippers that can handle objects automatically
without manual intervention or human assistance are
essential for mechanization. In recent years, the demand
for grippers that can gently handle items of various
shapes has increased. However, it is challenging for conventional grippers comprising hard parts and motors to
hold fragile objects, such as food items, without breaking or slippage. In this context, there is a growing interest in soft robots [1, 2], which are composed of flexible
materials. Many grippers have been designed and developed with flexible fingers that can be bent by manipulating the internal air pressure [3–6]. A few of these have
even been commercialized [7, 8]. Recently, flexible pneumatic grippers capable of adjusting their own stiffness
have been developed [9–13]. However, finger-shaped
grippers require fine control of the finger movements
*Correspondence: mitsuda@is.ritsumei.ac.jp
2
College of Information Science and Engineering, Ritsumeikan University,
Kusatsu-shi, Japan
Full list of author information is available at the end of the article
or appropriate positioning of the fingers to prevent the
object from slipping through the them.
In contrast, grippers without fingers have also been
proposed. A universal gripper [14] grasps an object by
holding it using a flexible bag containing particles and
stiffening the bag by manipulating its internal vacuum
pressure to cause jamming transition of the particles [15].
In the origami magic ball [16], an object is placed within
a conical gripper and grasped by shrinking the cone via
changes to the internal vacuum pressure; such grippers
do not require precise control for grasping objects. However, the former must be pressed against the object to be
held, which is not suitable for fragile items, while the latter can only grasp objects with circular cross-sectional
shapes. We therefore develop a gripper that can grasp
objects of various shapes without pressing them; the proposed ring-shaped gripper encircles the object like a rubber band by evacuating the air within. This gripper can
deform along the surface of an object without any gaps
even when the object’s cross section is non-circular. Gaps
between an object and the gripper’s surface decreases the
contact area; this affects the gripper’s stability when holding objects. In addition, a smaller contact area increases
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Yamada and Mitsuda Robomech J
(2021) 8:16
the contact pressure; such pressure can deform flexible
objects. Therefore, when objects (such as, food items or
fragile objects) are grasped by the gripper, they should be
grasped gently by applying low contact pressure.
The vacuum-driven ring-shaped gripper developed in
this study is novel. Although, ring-shaped grippers have
been developed by Bridgestone Corporation [17] and
Wang et al. [18], their grippers are operated using compressed air. It comprises a hard cylinder and rubber bag.
The flexible bag when inflated by compressed air wraps
around an item which is inside the cylinder. The cylinder
does not change its shape; therefore this could pose an
obstacle for observing the state of gripping of the item
inside. In contrast, the gripper developed in this study
changes its shape according to the item being gripped.
Therefore, the grasping state is observable, and the gripper is adequate for operations in narrow spaces.
In this paper, we first present the design of the gripper.
Then, the experimental results of a telescopic actuator
having the same internal structure as the gripper, which
was developed to optimize the gripper design, are presented. Next, we describe the mechanical properties of
the developed gripper, including its deformation characteristics, ability to encircle various objects, and gripping force. Finally, we present its attachment method to
a robotic hand and performance for handling various
objects.
Page 2 of 12
foam, density 25 kg/m3) and polypropylene sheets (thickness 0.15 mm) were alternately laminated and placed
within a bag. When the air inside the bag is evacuated,
the sponges contract and create dents between the polypropylene sheets, as shown in Fig. 1. This contraction of
the sponges shorten the actuator along the direction of
its long axis. The length of the actuator varies with the
internal vacuum pressure because the compressive force
of the pressure and restoring force of the sponges determine the amount of shortening. Based on this mechanism, we developed a ring-shaped gripper that shrinks
when the inside air is evacuated, as shown in Fig. 2. As
described in the following sections, the gripper is flexible
and can encircle objects of various shapes. The internal
vacuum pressure in the gripper can be used to adjust
Design
Overview
A sponge sealed within a flexible airtight bag shrinks
when the air in the bag is evacuated. We first developed a telescopic actuator, as shown in Fig. 1, using this
property. Rectangular sponges (INOAC Corp., urethane
Fig. 2 Schematic illustration of a rubber-band gripper. The rubber
band gripper shrinks and wraps around objects of various shapes
placed in the ring by evacuating the inside air through the air tube
Fig. 1 Schematic illustration of a developed telescopic actuator. The telescopic actuator is composed of rectangular sponges that are interleaved
with polypropylene sheets sealed in a flexible airtight bag. When the inside air of the bag is evacuated, the sponges are contracted, which shortens
the actuator
Yamada and Mitsuda Robomech J
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the tightening force. When the internal air pressure is
returned to atmospheric pressure, the resilience of the
sponges restores the gripper to its original shape and
releases the gripped object.
As alternatives to the developed actuator, bellows
can be used as pneumatically driven telescopic actuators. However, such bellows must be embedded with
hard rings to prevent collapsing when the interior air is
evacuated. This is not desirable for gripping fragile items
because the hard rings may come into contact with the
item, which could distort the flexibility of the gripper’s
cross-sectional shape. In the developed gripper, the stiffness of the contracted sponge moderately prevents the
gripper from being crushed under atmospheric pressure, and the stiffness of the interleaved polypropylene
sheets is small. Therefore, the gripper is flexible and can
fit to the surfaces of various objects. Contrarily, a gripper
made of bellows cannot stably grasp objects because its
flexibility is retained even after grasping objects. Increasing the internal vacuum pressure enables the gripper to
be rigid; however, the contact pressure on the gripped
object may also increase. Further, the developed gripper
has increased rigidity after grasping the objects owing to
the compressed sponges. Therefore, the proposed gripper
can grasp objects stably even though it is composed of
soft materials and very flexible in the resting state.
Page 3 of 12
Relationship between shrinkage ratio of telescopic
actuator and sponge shape
The amount of shrinkage of the gripper after evacuation
of air varies depending on the shape of the sponges. To
increase the range of object sizes that can be grasped by
the gripper, the maximum shrinkage ratio, defined as the
ratio of decrease in the inner diameter of the ring to its
initial inner diameter, should be large. To optimize the
internal structure of the rubber-band-like gripper, we
investigated the relationship between the shape of the
rectangular parallelepiped-like sponges and shrinkage
ratio of the telescopic actuator.
The width, height, and thickness of the sponge are
defined as shown in Fig. 1. We investigated the effects of
the heights and widths of the sponges on the shrinkage
ratio along the long axis of the actuator by fixing their
thickness to 30 mm. Sponge samples of 15 sizes with different heights (30, 40, and 50 mm) and widths (10, 20, 30,
40, and 50 mm) were prepared; then, three sponges of
each type were connected in series with interleaved polypropylene sheets and encased in a natural rubber sheath
of thickness 0.25 mm. The distance between the polypropylene sheets (i.e., width of the sponge) was measured
when the internal air pressure was decreased to − 75 kPa.
Figure 3 shows the relationship between the width of
the sponge in the initial state and the shrinkage ratio at
Fig. 3 Relationship between the width of a sponge at the initial state and shrinkage ratio when the inside air pressure of the actuator is − 75 kPa.
The shrinkage ratio was smaller for thinner sponges. The height of the sponges did not affect the shrinkage ratio when the width of the sponge was
small
Yamada and Mitsuda Robomech J
(2021) 8:16
− 75 kPa for each height; the smaller the width of the
sponge, the greater is the shrinkage ratio. When the
width of the sponge is small, the height has little effect on
the shrinkage ratio. Thus, a greater shrinkage ratio of the
actuator may be obtained when the width of the sponge
is less than 10 mm. However, it is difficult to build a gripper with very thin sponges laminated together; hence, in
the present study, sponges of width 10 mm are used to
build the gripper.
Relationship between length of the telescopic actuator
and internal air pressure
We examined the length of the telescopic actuator and
its relation to the internal air pressure. Ten sponges
of height 30 mm, thickness 30 mm, and width 10 mm
were connected in series with interleaved polypropylene
sheets and encased in a natural rubber sheath of thickness 0.25 mm. Wooden pieces of height 30 mm, thickness
30 mm, and width 12 mm were attached to both ends of
the connected sponges instead of acrylic plates to fix the
actuator to a measuring instrument. Figure 4 shows the
relationship between the length of the actuator, excluding
the outer membrane and wooden pieces (i.e., length of
the connected sponges), and the internal air pressure. The
actuator was compressed significantly from atmospheric
pressure to approximately − 10 kPa and then further contracted gradually. When the air inside the actuator was
evacuated, the ends of the sponges were bent, causing
Page 4 of 12
dents between the polypropylene sheets, as shown in
Fig. 1. This deformation and compression properties
of the sponges thus affect the shortening mechanism of
the actuator. This rapid shortening of the actuator at low
vacuum pressure therefore enables the gripper to grasp
objects gently, as demonstrated in the following sections.
Contraction force of the telescopic actuator
The rubber-band-like gripper encircles an object by
contraction forces generated by the decreasing internal
vacuum pressures. After wrapping around the object,
the contraction forces generate a wrapping force on the
object (i.e., the force perpendicular to the contact surface). To investigate this wrapping force of the gripper,
we examined the relationship between the contraction
force of the telescopic actuator and its length. The contraction force was measured with a universal testing
machine (UTM; IMADA Co., Ltd., MX2-500 N) and a
force gauge (IMADA Co., ZTA-200 N) according to the
following procedure.
First, one end of the actuator was fixed to the base
of the UTM, and the internal air pressure of the actuator was adjusted to the target value, thereby contracting the actuator. Next, the other end of the contracted
actuator was fixed to the force gauge after adjusting the
position of the force gauge to avoid pulling the actuator.
Then, the contraction force was measured after reducing the internal air pressure to − 75 kPa. Further, the
Fig. 4 Relationship between the length of the telescopic actuator and internal air pressure. The telescopic actuator contracted significantly from
atmospheric pressure to approximately − 10 kPa and then contracted gradually. The length of the telescopic actuator does not include that of the
membrane and the wooden pieces attached to both ends of the actuator
Yamada and Mitsuda Robomech J
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contraction forces were measured for various lengths of
the actuator by adjusting the internal air pressures to
0, − 3, − 5, − 10, − 20, − 30, − 40, and − 60 kPa. Three
measurements were obtained for each length of the
actuator, and the averages were calculated. Because the
contraction force is a result of the difference between
the internal air pressure and the pressure when affixed
to the force gauge, it is assumed that the contraction
force is greater when the length of the actuator is closer
to its natural length than when compressed.
Figure 5 shows the relationship between the length of
the telescopic actuator and contraction force. The contraction force was smaller for a shorter length of the
actuator, which is in agreement with the inference. The
nearly constant contraction force from 45 to 85 mm
length was attributed to the fact that the length of the
telescopic actuator changed significantly from atmospheric pressure to approximately − 10 kPa. When the
length of the actuator was in this range, the change
in the internal air pressure was small. Therefore, the
contraction force, which is be proportional to the difference between the initial air pressure and vacuum
pressure during measurement (i.e., − 75 kPa in this
experiment), did not change significantly. The increase
in the contraction force with length change from 85 to
100 mm can be attributed to the change in the crosssectional area owing to the indenting of the sponges, as
mentioned earlier.
Page 5 of 12
Figure 6 shows the relationship between the contraction force and difference between the internal air pressure when the actuator is fixed to the instrument and
pressure during measurement (− 75 kPa); the contraction
force is observed to be almost proportional to the pressure difference. The steep increase in the contraction
force for pressure differences exceeding 70 kPa (i.e., when
the length of the actuator was close to its natural length)
was owed to the change in the cross-sectional area of the
actuator.
The contraction force of the telescopic actuator thus
varies with length, unlike pneumatic cylinders. However,
the contraction force is almost constant except when the
length of the actuator is close to its longest and shortest values. Therefore, when the actuator is used as a ring
gripper, the wrapping force is approximately constant
regardless of the object size, except when the size is close
to the maximum or minimum inner diameter of the ring.
Material of the outer membrane for the rubber‑band
gripper
To increase the shrinkage ratio of the rubber-band gripper, it is preferable to have a flexible outer membrane
that does not interfere with the contraction of the
sponge. According to beam theory, the bending stiffness is determined by the product of Young’s modulus
and second moment of area. Therefore, to select a suitable outer membrane for the gripper ring, three grippers
Fig. 5 Relationship between the contraction force of the telescopic actuator when the inside air pressure is − 75 kPa to the length of the actuator.
The contraction force was almost constant when the length was in the range of 45–85 mm, which was due to the fact that the telescopic actuator
changed its length significantly from atmospheric pressure to approximately − 10 kPa. When the length of the actuator was in this range, the inside
air pressure was nearly constant, resulting in a constant contraction force. A steeper increase over 85 mm could be attributed to the change in the
cross-sectional area by dents on the surface of the actuator
Yamada and Mitsuda Robomech J
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Fig. 6 Relationship between the contraction force and difference between the inside air pressure when the actuator is fixed to the instrument
and that at measurement (− 75 kPa). The contraction force was almost proportional to the differential air pressure. The steep increase over − 70 kPa
could be attributed to the change in the cross-sectional area by dents on the surface of the actuator
Table 1 Thickness, Young’s modulus, and bending stiffness of
each material
Table 2 Relationship between outer membrane material and
shrinkage ratio
Material
Thickness [mm]
Young’s
modulus
[MPa]
Bending
stiffness
[Nm2]a
Material
Cotton-lined PVC film
0.3
185
1.3 × 10–5
Polyethylene tarpaulin
0.07
402
3.5 × 10−7
Natural rubber
0.25
3
1.2 × 10–7
a
Theoretical value for a rectangular cross-sectional beam of width 30 mm
with different outer membranes of varying thicknesses
and Young’s moduli were developed. The grippers each
enclosed 60 sponges of height 30 mm, thickness 30 mm,
and width 10 mm, which were glued together in a circle with interleaved polypropylene sheets of thickness
0.15 mm. The initial inner diameter of each gripper was
150 mm.
Table 1 lists the materials of the outer membrane
used in the grippers. The bending stiffness values in the
table were estimated according to beam theory. A cotton-lined poly(vinyl) chloride (PVC) film was chosen
as the outer membrane because the PVC film without
lining deformed plastically even under small forces. The
cotton lining therefore increases the Young’s modulus
and prevents plastic deformation. The polyethylene
tarpaulin used in the experiment was a plain weave of
polyethylene tape of width 2–4 mm, and the bending
Shrinkage
ratioa
Cotton-lined PVC film
0.7
Polyethylene tarpaulin
0.8
Natural rubber
0.8
a
Shrinkage ratio is the quotient of decrease in inner diameter divided by the
inner diameter before shrinkage
stiffness of the tarpaulin shown in Table 1 is equivalent
to that of a polyethylene film of the same thickness.
Therefore, the bending stiffness of the tarpaulin may be
smaller than this estimate.
Table 2 shows the shrinkage ratio when the interior
of each gripper was depressurized to − 75 kPa. The cotton-lined PVC film having the highest bending stiffness
showed the least shrinkage ratio. The shrinkage ratios
of the polyethylene tarpaulin and natural rubber were
almost equal. However, the gripper using the polyethylene tarpaulin required a longer time to return to its
original state when released from airtightness. In contrast, the gripper using the natural rubber membrane
returned to its original state quickly. The resilience of
the natural rubber membrane might thus shorten the
expansion time of the gripper. Accordingly, we adopted
Yamada and Mitsuda Robomech J
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natural rubber for the outer membrane of the proposed
gripper.
Experiments
In this section, the wrapping performance of the gripper
with natural rubber membrane is described along with a
method of mounting the device to a robot arm.
Relationship between internal air pressure and inner
diameter of the gripper
The relationship between the inner diameter of the gripper and its internal air pressure was investigated. The
Page 7 of 12
inner diameter of the gripper was measured as the internal pressure was varied from 0 to − 10 kPa in increments
of 1 kPa, from − 10 to − 70 kPa in increments of 10 kPa,
and at − 75 kPa. Figure 7 shows the deformations of the
gripper, and Fig. 8 shows the relationship between the
internal air pressure and shrinkage ratio. The shrinkage
ratio of the gripper rapidly increased with increase in the
vacuum pressure up to approximately − 10 kPa, as with
the telescopic actuator, and then gradually increased. At
vacuum pressures beyond − 30 kPa, the inner diameter
changed very slightly.
Fig. 7 Deformations of a rubber band gripper at various vacuum pressures (0, − 5, − 10, and − 30 kPa). The gripper significantly contracts even
at the low vacuum pressure (− 5 kPa). The gripper significantly shrinks at a low vacuum pressure (− 5 kPa). The inner diameter of the gripper was
nearly a minimum at − 30 kPa
Yamada and Mitsuda Robomech J
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Page 8 of 12
Fig. 8 Relationship between the internal air pressure and shrinkage ratio of the inner diameter of a rubber-band gripper. The gripper shrank rapidly
when the inside air pressure decreased. The shrinkage ratio changed gently when the inside vacuum pressure exceeded − 10 kPa, and it became
constant when the air pressure exceeded − 30 kPa
Performance of encircling an object
The wrapping performance of the gripper was assessed
using a bottle of diameter 100 mm, regular triangular
prisms of sides 76 mm and 90 mm, a plate of dimensions 115 mm × 18 mm, a cone with a base diameter of
100 mm and side of 70 mm, and a cruciform prism made
of three prisms. Figure 9 shows the gripper states when
the internal air pressure was decreased to − 75 kPa. The
gripper was able to encircle the bottle and plate without
any gaps. The gripper could also encircle the triangular
prisms, but small gaps were observed; larger gaps were
seen for the smaller triangular prism. The gripper could
wrap around the plate without any gaps; therefore, the
presence of gaps is not determined by the curvature but
by the angles of the corners and the peripheral length of
the object. When the peripheral length of an object is
small, it is difficult to encircle it when it has sharp corners because the amount of deformation of the gripper
is limited when it is shrunk before contacting the object.
If the peripheral length of the object is large, the gripper
can encircle it even when it has sharp corners, as shown
in Fig. 9.
For the cruciform prism, gaps were observed in the
concave areas. The developed gripper wraps around the
objects by reducing its circumference. Therefore, the
gripper cannot follow the concavity of the object because
the circumference needs to be extended to deform along
the surface of the object. For the case of the cone, the
gripper enclircled along the slope surface by twisting. To
check the ability of the developed gripper to wrap around
the slope, the gripper was contracted with its bottom
5 mm in contact with the top part of a 100 mm diameter pillar. Figure 10 shows this wrapping motion. The
upper part of the gripper continued to contract even after
the bottom of the gripper touched the pillar so that the
gripper twisted by 90°. The inner diameter of the gripper, when twisted by 90°, was equal to the minimum
inner diameter after contraction without encircling any
objects, as shown in Fig. 7. Therefore, when the gripper is wrapped around a cone, the minimum slope angle
θ that the gripper can twist by to contact the slope is
determined by the minimum inner diameter of the gripper dmin under vacuuming, diameter of the cone with the
bottom of the gripper touching dc, and height of the gripper h as follows:
d c − d min
θ = cos−1
(1)
2h
Contact pressure
The relationship between the contact and vacuum
pressure of the gripper was examined using a pillar of
Yamada and Mitsuda Robomech J
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Page 9 of 12
Fig. 9 Performance of gripping objects of various shapes: a a bottle; b a small triangular prism; c a large triangular prism; d a plate; e a cone; f a
cross-shaped prism. The gripper wrapped it around the bottle, plate, and cone without any gaps. The gripper wrapped it around triangular prisms.
However, larger gaps were observed for the smaller triangular prism. For a cone, the gripper twists and wraps around the slope. For a cross-shaped
prism, gaps appeared in concave areas
Fig. 10 Twisting motion of the gripper when its bottom contacted the top of a pillar. When the inside air of the gripper was evacuated, the upper
part of the gripper continued to contract even after the bottom part contacted the pillar so that the gripper twisted until 90°
100 mm diameter. The contact pressure was measured
by an air-pack type contact pressure sensor (AMI3037SB-SET, AMI TECHNO, Inc.) when the internal air
pressure changed from − 20 to − 80 kPa in increments
of 10 kPa. The contact pressure was measured three
times and averaged. As shown in Fig. 11, the contact
pressure increased with the internal vacuum pressure.
Because the wrapping motion of the gripper almost
ends at a vacuum pressure lower than − 30 kPa, as
shown in the previous section, the contact pressure can
be increased by increasing the vacuum pressure. Contact pressure can also be reduced by stopping air suction when the gripper is in contact with the object. The
wrapping force (i.e., the integral of contact pressure
on all contact surfaces) is proportional to the contact
pressure if the contact area is constant and the contact
pressure is uniform. However, this assumption may not
hold true if the cross-sectional shape of the object is
not a circle. The future scope of the study involves an
Yamada and Mitsuda Robomech J
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Page 10 of 12
Fig. 11 Relationship between the contact pressure and internal air pressure of the rubber band gripper when wrapped around a pillar. The contact
pressure increased with the internal vacuum pressure
investigation of the relationship between object shape
and contact pressure.
Installation on the robot arm
The structure of the apparatus for attaching the developed gripper to a robot arm is shown in Fig. 12. The
gripper and three links (195 mm in length) were fixed
with strings (cotton, 1 mm diameter). The links are also
fixed with strings to a metal ring of diameter 110 mm
that is attached to the robot arm. The gripper, links,
and metal ring are loosely connected to function like a
ball joint. In addition, the strings are flexible so that the
Fig. 12 Schematic illustration of the mounting device. The three links are connected with strings to the gripper and a metal ring for fixing to a
robot arm. The strings are flexible so that the links move freely without interfering with the deformation of the gripper
Yamada and Mitsuda Robomech J
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links move freely without interfering with the deformation of the gripper.
Figure 13 shows the method by which this apparatus is
used to lift a plastic bottle (950 g), mug (289 g), toy duck
(61 g), book (185 g), box (144 g), and square bread loaf
(146 g). For the mug, the gripper grasped the handle part.
The results show that the gripper fixed to a robot arm
can handle objects of various shapes. For the bread, the
evacuation of air was stopped at − 3 kPa to ensure that
the soft bread was not crushed when encircled without
gaps. This result shows that the gripper can grasp a soft
object whose cross section is not circular even though it
requires control of the internal air pressure according to
the contact condition.
Finally, the maximum weight that the gripper can
hold was examined when grasping polyvinyl chloride pillars of 38 mm and 60 mm diameter, a glass bottle of 100 mm diameter, and a steel quadrangular prism
Page 11 of 12
(12 mm × 100 mm). The maximum weight that the gripper could handle without slippage was 2.4, 3.7, 3.3, and
3.0 kg, respectively. The maximum weight depends on
the shape and material of the items; nonetheless, the
results achieved indicate that the developed gripper is
capable of holding heavy objects.
Conclusions
In this study, a flexible ring-shaped gripper that wraps
around an object like a rubber band was developed
by evacuating the air inside the gripper. Experimental results showed that the gripper could wrap around
objects of various shapes, such as bottles, boxes, cones,
and plates without any gaps, owing to the flexibility of
the sponges and the outer membrane. Encircling an
object without any gaps increases the contact area and
decreases the contact pressure. Therefore, the developed
Fig. 13 The gripper picks up various objects a a bottle (950 g); b a mug (289 g); c a duck toy (61 g); d a book (185 g); e a box (144 g); f square bread
loaf (146 g). The gripper was able to lift objects of various shapes by wrapping around them. For the bread, exhausting the inside air of the gripper
was stopped at – 3 kPa to avoid crashing after the gripper wrapped the square bread without gaps
Yamada and Mitsuda Robomech J
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gripper can stably grasp objects without considering the
point of contact, which is required for a finger-shaped
one. In this work, we have also demonstrated that a robot
arm with a gripper can lift a heavy object. Increasing the
internal air pressure of the gripper increased its wrapping
force; at the same time, the gripper was able to grasp soft
bread without crushing it by decreasing the internal air
pressure.
The above observations suggest the following possibilities for future work. First, the developed gripper cannot
achieve contact with the concavity of an object because
the gripper shrinks by shortening its circumferential
length. To fit the concavity, the length of the gripper
needs to be extended partially. A larger contact area with
the object is appropriate for grasping a fragile object.
Therefore, developing a structure that can deform for
indentation is a challenging direction of study.
Next, the developed mounting apparatus had flexible
strings connecting the links to the gripper. This method
allowed flexible deformation of the gripper but caused
swaying of the gripped object because the links were
always free to move. The rocking motion should be minimized for positioning the object precisely. In addition,
the connecting method limits the use of the gripper to
hang objects because gravity causes the gripper to position only objects that hang downwards. If the gripper can
follow the posture of the robot arm, it could be used for a
wider range of applications.
Finally, a theoretical analysis of the gripper’s deformation is an important consideration. It is necessary to
investigate how the sponge and outer membrane interact
with each other during contraction of the gripper. This
study shows that the developed gripper can wrap around
an object with a slope. The maximum inclinations of such
slopes were analyzed for a cone but not for other objects,
such as triangular pyramids. Analysis of wrapping objects
in three dimensions is therefore required.
Page 12 of 12
of Information Science and Engineering, Ritsumeikan University, Kusatsu-shi,
Japan.
Received: 25 December 2020 Accepted: 7 April 2021
Availability of data and materials
Not applicable.
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Declarations
Publisher’s Note
Authors’ contributions
The first author conducted the study under the supervision of the second
author. The first and second authors wrote the manuscript. Both authors read
and approved the final manuscript.
Funding
This work was supported by JSPS KAKENHI Grant Number JP18K04064.
Competing interests
The authors declare no competing financial interests.
Author details
1
Graduate School of Information Science and Engineering, Ritsumeikan
University, 1-1-1 Noji-Higashi, Kusatsu-shi, Shiga 525-8577, Japan. 2 College
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