Comparison of different soft grippers for lunch box packaging
Wang et al. Robot. Biomim.
Comparison of different soft grippers for lunch box packaging
Zhongkui Wang 0 2
Mingzhu Zhu 1
Sadao Kawamura 0 2
Shinichi Hirai 0 2
0 Department of Robotics, Ritsumeikan University , Noji‐Higashi 1‐1‐1, Kusatsu 525‐8577 , Japan
1 Ritsumeikan Global Innovation Research Organiza‐ tion, Ritsumeikan University , Noji‐Higashi 1‐1‐1, Kusatsu 525‐8577 , Japan
2 Department of Robotics, Ritsumeikan University , Noji‐Higashi 1‐1‐1, Kusatsu 525‐8577 , Japan
Automating the lunch box packaging is a challenging task due to the high deformability and large individual differences in shape and physical property of food materials. Soft robotic grippers showed potentials to perform such tasks. In this paper, we presented four pneumatic soft actuators made of different materials and different fabrication methods and compared their performances through a series of tests. We found that the actuators fabricated by 3D printing showed better linearity and less individual differences, but showed low durability compared to actuators fabricated by traditional casting process. Robotic grippers were assembled using the soft actuators, and grasping tests were performed on soft paper containers filled with food materials. Results suggested that grippers with softer actuators required lower air pressure to lift up the same weight and generated less deformation on the soft container. The actuator made of casting process with Dragon Skin 10 material lifted the most weight among different actuators.
Soft pneumatic actuator; Soft gripper; Fabrication; Grasping; Lunch box packaging
In Japan, people often eat box lunches for the
convenience and great varieties. Every day, several million box
lunches are produced and consumed in Japan.
Considering the hygiene and freshness, lunch boxes are usually
manufactured and distributed locally. So far, the
packaging of lunch boxes is still performed by human labors due
to the fragility, variety, high deformability, and the
individual differences in shape and physical property of food
]. To reduce labor cost, automation systems
for lunch box packaging are highly demanded in food
A typical lunch box (Fig. 1a) usually consists of rice and
dishes distributed in soft paper containers. The paper
containers (Fig. 1b) usually have a frustum shape and
highly deformable. Picking and placing such containers
filled with food materials is the main task for lunch box
packaging. The traditional rigid grippers and vacuum
packaging systems, which have been widely used in food
industry, have difficulties to perform such a task because
the rigid gripper may damage the food material and the
vacuum system needs a flat surface to allow suction. New
grasping mechanism providing gentle grasps is required
to cope with this task.
In recent years, pneumatic soft robotic grippers have
drawn great attention from researchers because of their
flexibility and adaptability. Pioneer works in
developing pneumatic soft gripper were conducted by Suzumori
et al. in the 1990s of the last century. They proposed a
four-fingered gripper made of fiber-reinforced rubber
with three cylindrical air chambers and experimentally
tested different grasping modes [
]. Similar ideas were
also proposed and applied in constructing flexible arm
], soft robotic glove for at-home rehabilitation
, and a manta swimming robot [
]. Another idea for
constructing pneumatic soft actuator is to use pleated
chamber morphology and was firstly proposed by Ilievski
et al. [
]. Marchese et al. [
] summarized the design and
fabrication of soft fluidic elastomer robots and divided
such robots into three types: ribbed, cylindrical, and
pleated, based on their chamber morphologies.
Comparing with the first two types, the pleated type is capable of
bending to higher curvatures and exerting higher
maximum forces because of its ability to accommodate the
largest energy input. Therefore, this idea was widely used
to actuate soft robots, such as the soft planar grasping
], the soft gripper for biological sampling
on deep reefs [
], and a soft gripper for object
According to [
], the main disadvantage of pleated
design is the complex fabrication process which involves
several casting processes. To simplify the fabrication
process, 3D printing technology has been adopted and
several gripper designs have been proposed. MacCurdy
et al. [
] presented a two-finger gripper using
printable hydraulic technology. Peele et al. [
] proposed a 3D
printable soft actuator using projection
stereolithography. Most recently, Yap et al. [
] presented a high-force
soft gripper fabricated using common 3D printer and
fused deposition modeling (FDM) technology. This
gripper is promising for handling heavy objects and it can lift
a weight up to 5 kg with a maximum payload-to-weight
ratio of 1805%. However, the authors concluded that this
gripper is not suitable for applications where low
pressure and delicate force are required due to the relatively
hard material property of NinjaFlex.
In our previous work, we have presented a 3D printed
soft gripper using Objet260Connex printer (Stratasys,
MN, USA) [
] and integrated a curvature sensor to
capture the bending behavior of the actuators [
also proposed a simplified line-segment model to
calculate the deformation behavior of the actuator [
previous actuator was printed as two separate parts and
glued together to form seamless chambers. In this study,
we presented two ways to print the actuator in one shot
to further simplify the fabrication process. For
comparisons, we also presented another two grippers fabricated
by traditional casting procedure.
Design of the soft actuator
The soft actuator design is based on the idea of the
pleated type morphology of the fluidic elastomer robot.
As shown in Fig. 2, the actuator has a similar size of an
Asian male’s finger and consists of twelve soft air
chambers. Among the chambers, eleven of them have a wall
thickness of 1.5 mm and one larger chamber at the end
has a wall thickness of 3 mm. The thicker wall of the
larger chamber makes the actuator end stiffer than the
rest of the actuator to mimic the function of human nail.
A 1.2-mm groove was designed to cross the bottom of
all chambers to allow air passing through. A hole with
a diameter of 4 mm was designed on the left-side wall
to allow the insertion of the air hose. Rippled structure
was designed on the bottom surface of the actuator to
increase the grasping stability and mimic the human
fingerprint. Performance effects of geometry variations of
the soft actuator design present an important and
interesting issue, but it is out of the scope of this paper, in
which we are mainly focusing on the variations of
material and fabrication process.
Design of connector and base
To connect multiple soft actuators and construct a
gripper, we designed a rigid connector and a gripper base.
The connector consists of two parts: the bottom half and
top half as shown in Fig. 3a, b. During assembling, the
soft actuator was firstly fitted into the bottom half and
the air hose was inserted through the hole on the
connector. Then, the top half of the connector was covered
on the top of the actuator and both halves were fixed
and screwed together. The cavity height formed by both
halves of the connector was designed 1 mm shorter than
the actuator height. Therefore, the actuator can be fixed
stably after screwing together the connector.
Considering the circular shape of the grasping target (the paper
container in Fig. 1b), we designed a gripper base (Fig. 3c)
with three female connectors distributed circularly. A
snap-lock mechanism consisting of a male (on the
bottom half of the connector) and a female (on the base)
interfaces was designed for assembling the connector to
the base without using screws. The assembly is shown
in Fig. 3d. Three actuators were chosen because we
believe that three is the minimum number of actuators to
achieve a stable grasping and the grasping stability will be
increased by using more actuators.
Two methods and four materials were used to fabricate the
soft actuators. Two methods are: (1) the traditional
casting process, (2) 3D printing using the Objet350 Connex3
printer (Stratasys, Minnesota, USA) and the Agilista printer
(Keyence, Japan). Four materials are: (1) the Dragon Skin
10 (Smooth-on Inc., PA, USA), (2) the Ecoflex
(Smoothon Inc., PA, USA), (3) the TangoPlus or TangoBlackPlus
(Stratasys, MN, USA), which mainly consists of propenoic
acid, ethyl ester, and trimethylbicyclo, and has a hardness
of Shore A26-A28 and an elongation at break of 170–220%,
and (4) the AR-G1L (Keyence, Japan), which mainly
consists of silicone and acrylate monomer, and has a hardness
of Shore A35 and an elongation at break of 160%.
We fabricated molds using 3D printer Zortrax M200
(Zortrax, Olsztyn, Poland). The Dragon skin and Ecoflex
materials were used to fabricate two types of soft
actuators. First, we poured the Dragon skin into the chamber
bottom mold (Fig. 4a) and then dipped the chamber top
mold (Fig. 4b) into the bottom mold and fixed the two
molds using screws. After curing, the model with open
chambers was carefully removed from the molds.
Second, the Dragon skin material was poured into the cover
mold (Fig. 4c) to make a cover. Third, the cured chambers
and cover were glued together by spreading a thin slice
of Dragon skin material on both gluing surfaces. After
curing, we completed the first type actuator as shown in
Fig. 4d. This actuator has open spaces between
neighboring chambers. Therefore, it is easy to bend and requires
lower pressure to actuate. To have a stronger actuator
and a closed appearance, we poured Ecoflex material into
the wrapping mold (Fig. 4e) and dipped the previously
completed actuator (Fig. 4d) into it. After curing and
removing from the mold, we completed the second-type
actuator as shown in Fig. 4f.
We fabricated the soft actuators (Fig. 5) using the
Objet350 Connex3 and Agilista printers. It takes around
one and half hours for Objet350 Connex3 and around
two hours for Agilista to print the soft actuators. The
connectors and base were printed by Objet350 Connex3.
After printing, the actuators fabricated by Agilista were
put inside water to dissolve the support materials. For the
actuator fabricated by Objet350 Connex3, we removed
the support material inside the groove at the bottom side
(Fig. 2b) to allow air passing through. Since the support
material is granular and not very sticky, we could easily
separate the support material from the chamber walls
Fig. 4 The casting process: a the chamber bottom mold, b the
chamber top mold, c the cover mold, d the soft actuator with sepa‑
rate chambers (actuator No. 1), e the wrapping mold, and f the soft
actuator with wrapped chambers (actuator No. 2)
by simply pressing the chambers from external surfaces.
Therefore, we did not remove all the support materials
from the chambers and the actuator could be inflated. In
fact, the actuator responded faster with support
material inside the chambers compared to the actuator with
empty chambers because less air was required to inflate
the chambers. After fixing the connector on the soft
actuator, we assembled the gripper. To compare the
performance with and without support material, we also
fabricated the same actuator by Objet350 Connex3 without
using support material as shown in Fig. 12a.
After fabricating the gripper base (Fig. 3c), we
assembled the grippers (Fig. 6) using the proposed four types
of soft actuators. The materials used in this study have
an increasing order of hardness as: Ecoflex, Dragon skin,
TangoPlus, and AR-G1L. Therefore, the gripper in Fig. 6a
has the smallest initial grasping opening under gravity.
The gripper with the wrapped actuators (Fig. 6b) was
found to have the largest initial grasping opening because
the air chambers were connected by Ecoflex material.
For experimental tests, we employed an air compressor
(JUN-AIR 3-4) and an electro-pneumatic regulator (SMC
ITV2030) to provide constant air pressures. We
experimentally tested the performance of different actuators
under different air pressures and different soft grippers
grasping the paper container filled with food materials.
Single actuator test
Figure 7 shows experimental snapshots of different
actuators under different air pressures. Under gravity, actuator
No. 1 has the largest bending because Dragon skin is the
softest comparing with TangoPlus and AR-G1L. Actuator
No. 2 bent the least under gravity because the opening
regions between neighboring chambers were wrapped by
Ecoflex material. During experiments, the input pressure
was started from 10 kPa and increased every 10 kPa until
the actuator bends over 90◦. The definition of bending
angle α is indicated in Fig. 7(b-3). Due to the softness, the
actuator No. 1 bent over 120◦ under a pressure of 20 kPa.
Instead of bending, actuator No. 2 expanded significantly
along the pressure increasing and did not reach 90◦ under
a pressure of 50 kPa, after which we stopped the pressure
increasing. Compared to actuator No. 3, actuator No. 4
generated less bending because the material AR-G1L is
stiffer than material TangoPlus.
Three actuators for each type were fabricated and
tested. Bending angles under different air pressures were
calculated using ImageJ (https://imagej.nih.gov/ij/). The
relationships between the input pressure and the
averaged bending angle are plotted in Fig. 8 with the standard
deviations indicated by the error bars. Since only three
test points were available for actuator No. 1, we did not
plot the approximation of the data. We found the
nonlinearity and the largest individual difference in
actuator No. 2. This can be explained by the two materials
combination and complex manual fabrication process.
The 3D printed actuators showed better linearity and
less individual differences compared to casting
fabrication. Apparently, actuator No. 3 fabricated by 3D printer
Objet350 Connex3 has the best performance (Fig. 8c)
in terms of the linearity and individual difference. The
influence of support material can also be seen in Fig. 8c.
Without using support material, bending angles became
a little less due to the lighter weight, but the linear
relationship against the input pressure was similar
compared to actuator with internal support material.
Weight grasping test
Weight grasping tests were performed using the
grippers and a paper container filled with different weights
of red beans. The grippers were mounted onto a
commercial Denso robot arm (Fig. 9a), and a pick-and-place
motion was programed. The motion was started from
position 1 (P1 in Fig. 9b). Firstly, the gripper moved
down (motion 1) to position 2 (P2) where the target was
placed. Once arrived P2, the gripper was pressurized and
attempted to grasp the target. After 5 s, the gripper was
lifted up (motion 2) and back to P1, where the gripper
was programmed to wait for 10 s. Finally, the gripper was
brought down (motion 3) to P2 again to release the
target. Ten seconds of grasping without dropping was
considered as a successful test. The weight test protocol is:
(1) the target weight was started from 20 g and increased
every 10 g, (2) the input air pressure was started from
a value where the gripper succeeded 10 times of the
pick-and-place tests and increased every 10 kPa, (3) we
increased the input pressure if the gripper failed to pick
up a weight more than three times, (4) the tests ended at
a target weight of 90 g because the container was full and
90 g is heavy enough for representing most of the side
dishes in a typical Japanese lunch box.
Results of weight grasping tests are given in Table 1.
Experimental snapshots of different grippers grasping
a 50 g target are shown in Fig. 9c through f. We found
that the gripper with softer actuators required lower
air pressures to lift up the same weight of target. For
example, gripper with actuator No. 1 could lift up a
70 g target with a pressure of 20 kPa, but it required 40
and 50 kPa for grippers with actuators No. 3, and No.
4, respectively, to lift up the same target. Actuator No.
2 performed the worst among the four actuators, and
most of the failed grasps were caused by the unbalanced
inflations of three actuators. We also found that the
gripper with softer actuators generated less deformation
on the paper container if we compared the container
deformation in Fig. 9c, e, f. Significant deformation
(Fig. 9d) was generated by the gripper with actuator No.
2 due to the individual differences of the actuators. In
addition, we found that the influence of support
material was not significant. Removing internal support
material could slightly improve the grasping
performance. We believe that this may be caused by the softer
and more compliant property of the actuator without
Food material grasping
Grasping tests on the side dishes shown in Fig. 1b were
conducted using actuator No. 1, and experimental
snapshots are shown in Fig. 10. The weights are 25.3, 24.2,
31.6, and 23.2 g for ohitashi, hijiki, fried chicken, and
salmon fish, respectively. Based on the experiments, it
was easy to grasp and lift ohitashi and hijiki, but relatively
hard to successfully lift up the salmon fish due to the
irregular shape of the salmon fish.
The durability tests were performed using the air
control system shown in Fig. 11a. Two SMC valves
(VQ1105M-M5) were used to control the air input and output.
A MOSFET was used to switch the valves on–off, and
a pressure sensor (MIS-2500) was used to monitor the
air pressure. The Arduino was programmed to realize a
0.2 Hz actuation frequency and count the actuation
circles. We tested one actuator for type No. 1 at a pressure
of 20 kPa, and two actuators for types No. 3 and No. 4 at
their working pressures: 30 and 40 kPa for No. 3, and 50
and 60 kPa for No. 4. Test results for actuators of types
No. 3 and No. 4 are shown in Fig. 11b. Actuator No. 1 was
tested for more than 6000 circles and it does not seem
to break or leak. Therefore, the results were not shown.
We found that actuator No. 4 outperformed actuator No.
3 at low working pressure but underperformed at high
working pressure. We believe that this is caused by the
stiffer property of the AR-G1L material. We also
investigated the influence of support material. We printed the
actuator as two separate parts: the open chambers and
a cover, as shown in Fig. 12a. By doing this, the internal
surface of the chambers can be very smooth without
support material. The chambers were then sealed by gluing
the cover on it using a rubber targeted glue (ThreeBond
1521B). We tested two materials using this design. One is
the TangoPlus material and the other is the
TangoBlackPlus (Fig. 12a), which has the same hardness as
TangoPlus but in black color. Test results are shown in Fig. 12b.
Actuators with separate design and TangoPlus material
reached more than 1000 circles at 30 kPa and around 500
circles at 40 kPa. Surprisingly, actuators made of
TangoBlackPlus material realized more than 12,000 circles (only
6000 circles are shown in Fig. 12b) at 30 kPa and more
than 4000 circles at 40 kPa.
Grasping and handling a highly deformable object, such as
a paper container filled with food material, is difficult due
to the deformability and the complex contact conditions.
Robotic gripper made of soft materials is able to adapt
these difficulties and provides a possibility for handling
such objects even without accurate control. In this study,
we fabricated four types of pneumatic soft actuators using
different materials and different fabrication processes.
By comparing the performances among different type
of actuators, we found that the 3D printed soft actuators
had better linearity in the pressure-bending relationship
and showed less individual differences thanks to the high
printing resolution of the 3D printer. Actuator fabricated
by two materials (actuator No. 2) showed nonlinear
behaviors and significant individual differences due to the
inhomogeneity and complex manual processes. Grasping tests
showed that actuator made of softer materials required
lower air pressure to grasp and lift the same weight of
target. Meanwhile, softer actuators generated less
deformation on the deformable target compared to harder
actuators. Individual differences in actuator No. 2 resulted in
uneven bending and further imposed unbalanced grasping
forces on the target. Weight tests showed that actuator No.
1 could lift up to 90 g with a pressure of 20 kPa. Actuators
No. 3 and No. 4 could lift up 70 and 80 g of targets with
pressures of 40 and 60 kPa, respectively. Durability tests
showed that 3D printed actuators, especially, the ones with
support materials remaining inside chambers, had lower
durability compared to the actuators fabricated by casting
process. Without using support materials could slightly
improve the grasping performance and significantly
improve the durability of the actuator No. 3. Different 3D
printable materials (TangoPlus or TangoBlackPlus) also
affected the durability of the actuators. Apparently, at this
moment, the traditional casted actuator (type No. 1) still
outperformed the 3D printed actuators in both grasping
performance and durability test despite the complex
fabrication process. Nevertheless, along the rapid development
of 3D printing technology, more printable soft materials
and better performances are expected within a measurable
period of time.
In this paper, only preliminary results were
presented regarding soft gripper handling highly
deformable objects. Many open questions are still remained
untouched. Quantitative analysis on shape adaptability of
soft gripper will be investigated together with optimized
design of soft actuator structure. Durability tests on
more 3D printed actuators and statistical results will be
presented in the future. Improving the durability
performance in the design point of view is another issue needs
to be investigated.
ZW proposed the actuator and gripper design, fabricated the actuator No. 3
and No. 4, drafted most of the manuscript. MZ fabricated the actuator No. 1
and No. 2 and drafted subsection of casting process. SK and SH supervised the
study. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
This work was supported by Kakenhi 15H02230 and Kakenhi 17K14633, Japan
Society for the Promotion of Science (JSPS), Japan.
Springer Nature remains neutral with regard to jurisdictional claims in pub‑
lished maps and institutional affiliations.
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