Proceedings of the 2019 IEEE/SICE
International Symposium on System Integration
Paris, France, January 14-16, 2019

Inflatable Robotic Arm with Overlaid Plastic Sheet Structure
Ryosuke Tatara1 , Kazuki Ebisu2 , Naoki Nomaguchi1 ,Akihiro Kawamura3 , Ryo Kurazume3 and Sadao Kawamura2

Abstract— An inflatable structure has been focused by several
researchers in the field of soft robotics, owing to its features:
flexible, lightweight and compactable. In particular, inflatablerobotic arms have been expected to perform daily tasks nearhumans due to safe contact. To date, several inflatable roboticarms have been proposed, however, the number of robotic arms
composed of only plastic materials is quite limited. In addition,
the control performance of the robots has not been discussed
sufficiently.
This paper proposes a novel inflatable robotic arm composedof poly-laminated sheets and low-density polyethylene (LDPE)
sheets. The combination of the two materials provides highpressure resistance. Several advantages are also given: hightorque, high control performance, and high load capacity.
Moreover, both pressure and visual feedback controls are
utilized to realize precise motion control. The end-effector of
the robot is composed of pleated structures and grasps flexibly
according to object shapes. The performance of the robot isfinally described by some experiments.

I. INTRODUCTION
Service robots have been expected to play a role in thesociety because of the lack of workforce in the welfare
and nursing care fields. Safety is one of the most importantfactors for service robots to work in these fields since robots
need to perform tasks near people and other various things.
However, most of the conventional robotic arms have been
mainly made of metal materials to realize high stiffness andprecise control. It leads to risks of contacts with humans
and things due to high rigidity and heavyweight. On theother hand, inflatable structures have been focused by someresearchers in the field of soft robotics due to flexibility, lightweight and compactability. In particular, inflatablerobotic arms have been expected to perform daily tasks nearhumans and things safely. These robots can be applied toa wide range of applications where safety, lightness andflexibility are required such as human support robots, drone,
harvesting robots and mounting on space structures requiring
compactness.
Several inflatable robotic systems have been developed
so far. Sanan et al.. have been proposed a lightweight
and flexible inflatable link for robotic arms. They make1 Ryosuke
Tatara and Naoki Nomaguchi are with Graduate
School of Information Science and Electrical Engineering,
Kyushu University, Fukuoka, JAPAN tatara, nomaguchi@irvs.ait.kyushu-u.ac.jp
2 KazukiEbisu and Sadao Kawamura are with Graduate
School of Science and Engineering, Ritsumeikan University,
Shiga,
JAPAN
rr0049fi@ed.ritsumei.ac.jp andkawamura@se.ritsumei.ac.jp
3 Akihiro Kawamura and Ryo Kurazume are with Faculty of Information
Science and Electrical Engineering, Kyushu University, Fukuoka, JAPAN

kawamura, kurazume@ait.kyushu-u.ac.jp

978-1-5386-3615-2/19/$31.00 ©2019 IEEE

Fig. 1.

Inflatable robotic arm

it possible to control the contact force at the tip of thelink [1]. Viosembert et al. have focused on the inflatablestructure which connects joints and links continuously [2].
The structure is quite light, therefore the constraint due
to the weight is reduced. Ronghuai et al. have developed
flexible inflatable robotic arms for telepresence robots [3].
The robot is capable of imitating human arms and realizing
remote interaction. As mentioned above, several inflatablerobotic arms have been proposed so far. However, mostof conventional inflatable robotic arms partially consist ofhighly-rigid and heavy materials [4]–[7]. The number ofrobotic arms composed of only plastic materials is quite
limited.
To date, we have focused on the inflatable structures anddeveloped lightweight and flexible inflatable robotic arms
using plastic materials for all components [8], [9]. The
inflatable structures are made only of polyethylene material.
The inflatable robot is driven by differential air pressures of
the inflatable pneumatic actuators arranged on opposite sides
of joints. The control method of this robot is visual feedback
control using RGB cameras. In the previous researches, theperformance of the robotic arm has been confirmed by someexperiments of motion control, but it has some problems
with the durability of the plastic material and the control
method. The problem is that tensile strength and durabilityof the material are not high. In addition, the desired pressure
is applied by feedforward control of pneumatic regulatorsin the control method, and pressure feedback control is not
implemented since no pressure sensors are mounted on the
robotic arm. It is therefore difficult to realize precise motion
control.
This paper proposes a new inflatable robotic arm composed of poly-laminated sheets and LDPE. Figure 1 shows
the proposed inflatable robotic arm. The combination of689

the two materials provides high durability and high-pressure
resistance. Then, several advantages are also given: hightorque, high control performance, and high load capacity.
Moreover, both visual and pressure feedback controls are
utilized to realize precise motion control using a motion
capture system and pressure sensors. The end-effector of thisrobot is composed of pleated structures which deform andgrasps flexibly according to object shape. The performanceof the robot is finally described by some experiments ofpractical daily applications.
II. INFLATABLE ROBOTIC ARM
The proposed inflatable robotic arm consists of inflatable
pneumatic bag actuators, inflatable links, a robotic gripper
with a pleated structure as shown in Figs. 6 and 7. All parts
consist of bags of poly-laminated sheets sandwiched between
two LDPE sheets. Although the LDPE sheet is a popularmaterial for an inflatable parts and has many advantages suchas lightweight, softness, and safety, it is easy to break underhigh-pressure conditions. Therefore we attached the polylaminated sheet with the LDPE sheet. The poly-laminatedsheet has good characteristics such as high tensile strengthand durability. Each link weights 50 g and the actuators
weight from 90 g to 120 g depending on the number of bags.
The total weight of the arm is only 340 g. In this paper, thedetail of the manufacturing process of the components isshown below.
As shown in the conventional researches [8], [9], thetensile strength and durability of the inflatable robot madeonly of PE are not high. We have already tested some materials such as LDPE and high-density polyethylene (HDPE).
However, these pressure resistance are about 60 kPa and100 kPa respectively and are not adequate to realize themanipulation of daily things. In addition, the durability is
not adequate for repetitive use. Therefore, a poly-laminatedmaterial is newly introduced in this research. Firstly, themaking process of inflatable bags is described. Next, thedetails of the inflatable actuator and link are shown below.
A. Inflatable bag unit
A poly-laminated sheet is made by knitting LDPE fibers
which are produced by slitting oriented LDPE film. The
oriented LDPE film is manufactured by stretching LDPE
film with commercial film stretching system which leads toalign molecular direction in a specified direction. The tensile
strength of poly-laminated sheet is quite high, but the seal
performance is quite low. Thereby, the combination of polylaminated sheet and LDPE sheet is appropriate and provides
high durability and high-pressure resistance. The combination of the two materials gives high tensile strength andhigh seal performance. A poly-laminated sheet sandwichedby LDPE sheets is welded at once using heat sealer NL303DH (Ishizaki electric mfg. Co., Ltd.) and a pneumatic
cylinder CDG1BN63-200Z (SMC Co., Ltd.). By using thesedevices shown in Fig. 2, both welding temperature andpressing force are able to be controlled. In order to get
the optimal temperature and pressing force to weld theseFig. 2.

Welding machine

Fig. 3.

Actuator structurematerials, a number of experiments are conducted. In theexperiments, a simple square airbag is welded. According to
the experimental results, the optimal temperature, pressing
force and welding time are determined to 125 ◦ C, 1500 N
and 60 s, respectively. Finally, the pressure resistance of theinflatable bag using two materials is 300 kPa. It is five times
higher than the conventional one [8], [9].
B. Inflatable links and actuators
Firstly, the manufacturing process of an inflatable actuatoris introduced. An inflatable actuator is made by cutting the
sheets to the size as shown in Fig. 3(a). Poly-laminated sheet
and LDPE sheets are alternately overlaid as shown in Fig.
3(b). Next, the overlaid sheets are folded along the dotted line
and from six to eight sheets are accumulated as shown in Fig.
3(c). In order to connect chambers, the centers of the sheets
are welded and flow path holes are drilled. Figure 4 shows
flow path parts. Then, a connector component is attached to690

Fig. 6.

Fig. 4.

Inflatable links and actuators

Flow path

Fig. 7.

Robotic gripper with pleated sheet structure

expansion ratio between the flat part and the folded part
enables the gripper to grasp objects. The robotic gripper hasa load capacity of approximately 700 g at least and deforms
flexibly according to the target shape.
III. CONTROL METHODS

Fig. 5.

Link structure

the large airbag to connect to an external air tube as shownin Fig. 4. After welding the surrounding sides, all airbags areconnected with a plastic cable as shown in Fig. 3(d). Finally,
two actuators are arranged antagonistically inside and outsideof each joint. The joint torque to actuate the robotic arm isgenerated by inflating and deflating the actuators.
The making process of the inflatable link is almost thesame as the inflatable actuator as shown in Fig. 5. In order toform a cylindrical link into a shape, all sheets are folded andwelded as shown in Fig. 5(c). The links maintain rigidity bypressurizing in a cylindrical shape. Figure 6 shows a photoof links and actuators.
C. Robotic gripper with pleats structure
Pleated sheet structure is used for the robotic gripper andthe pleated structure is a bag-like structure, where the backside of the gripper is folded as shown in Fig. 7. Different

This section shows the control method of the proposed
inflatable robotic arm. The robotic arm is controlled byboth pressure and visual feedback controls. In conventional
researches [8], [9], the inflatable robotic arm is controlled byonly visual feedback control using RGB cameras. However,the control performance is depending on the calibration
precision of the cameras. In addition, the pressure control
of actuators has been based on feedforward control by just
inputting desired pressure into pressure regulators. There
always exist pressure errors due to time-delays and distancefrom regulators. It also decreases the performance of therobotic arm. Therefore, the motion capture system and thepressure feedback control system are newly introduced inthis paper.
A. Visual feedback control
Figure 8 shows the configuration of the robot. This
inflatable robot does not have any encoders on joints. The
posture of the robotic arm is measured by means of the
motion capture system Vero (Vicon Motion Systems Ltd).
It is assumed that the gripper position and a target object aremeasured by the motion capture system. Each joint angle
is estimated from the gripper position by inverse kinematics
technique. For example, the position of the gripper is measured using optical markers attached to the objects as shownin Fig. 7.

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Fig. 8. DOF configuration of the inflatable robotic arm (In this research,θ0 is constant value 45◦ .)The desired torque of each joint is then calculated byusing a PID control method. The control method is given
as follows:∫{}dτv = J T (θ) KP v ev (t)+KIv ev (t)dt+KDv ev (t) (1)dtev (t) = rt − r,

(2)

where
τv = (τv1 , τv2 , τv3 , τv4 )T : Desired torque of visual feedback
control
J 3×4 (θ) : Jacobian matrix
rt = (xt , yt , zt )T : Target gripper positionr = (x4 , y4 , z4 )T : Current gripper position
KP v , KIv , KDv : Proportional, integral and differential gainof visual feedback control.
Moreover, torques for gravity compensation are required.
It is calculated as follows:τg1 = 0
τg2 = (m2 glg2 + m3 gl2 + m4 gl2 )S02
+ (m3 glg3 + m4 gl3 )S023 +(m4 glg4 )S0234
τg3 = (m3 glg3 + m4 gl3 )S023+ (m4 glg4 )S0234
τg4 = (m4 glg4 )S0234

(3)
(4)
(5)
(6)

where
τg = (τg1 , τg2 , τg3 , τg4 )T : Gravity compensation torquem2 , m3 , m4 : Total sum of the mass of each link and actuator
lg2 , lg3 , lg4 : Distance from joints to center of gravity of thelink.
As a result, desired torques are calculated asτ = τv + τg ,Fig. 9. The result of preliminary experiment for torque-pressure characteristics

torque-pressure characteristics. The relation between torques
and pressures change depending on joint angles. The basepressure is applied to both upper and lower actuators beforehand to maintain a certain rigidity. The differential pressure
is applied to the upper actuator and the lower actuator withdifferent signs. We define the relationship between desired
torque and target pressure as

B. Pressure feedback control
The desired torques τ are converted into pneumatic pressures using the torque-pressure conversion characteristics.
These characteristics are obtained by some preliminary experiments. Figure 9 shows the result of experiments for(8)

Pt = Pb + ∆P

(9)

where
∆P : Differential pressure
A(θ) : Transformation matrix obtained by Fig. 9
Pt : Target pressure
Pb : Base pressure.
After the conversion from torque to pressure, pressure
feedback control is utilized. The feedback control not only
makes precision of motion control higher than the conventional control method but also suppresses unstable behavior
like hunting. As mentioned above, the current pressure
values of actuators are measured by pressure sensors AP33A (KEYENCE Co., Ltd.).
After denoising, target pressure are updated by the following equations:∫d
P = Pt +KP p ep (t)+KIp ep (t)dt+KDp ep (t) (10)
dt

(7)

where
τ = (τ1 , τ2 , τ3 , τ4 )T : Desired torque.

1∆P = ± A(θ)τ
2

ep (t) = Pt − Ps

(11)

where
P : Compensated target pressure
Ps : Pressure acquired by a pressure sensor
KP p , KIp , KDp : Proportional, integral and differential gainof pressure feedback control.
Consequently, the whole control flow is shown in Fig. 10
by a block diagram.

692

Fig. 10.

Fig. 11.

Block diagram

Environment of inflatable link rigidity characteristic experimentFig. 13. The relationship between displacement of the link made of polylaminated sheets and LDPE, and force at the steady stateFig. 12. The relationship between displacement of the link made only of
LDPE and force at the steady stateFig. 14. Result of comparing the effect of pressure feedback control bystep response of sensor pressure.

IV. EXPERIMENTS
A. Rigidity evaluation
Experiments to confirm the improvement with changing
material are conducted. In this research, the rigidity characteristics of the inflatable link are focused on as one of
the evaluation indices. Figure 11 shows the experimental
environment. A digital force gauge ZTS-500N (Imada Co.,
Ltd.) is used to measure the rigidity of the link. The relationship between the displacement of the link and the force
measured by the force gauge at the steady state is shownin Figs. 12 and 13. From these figures, it is confirmed that
not only pressure resistance but also the rigidity of the link
made of the proposed material are improved compared withconventional one.
B. Pressure feedback control
Next, the experiments to confirm the performance of thepressure feedback control are conducted. Figure 14 shows
the transient responses of the pressure denoised by an IIR
low-pass filter. In this figure, the target value is represented

by a blue line. The orange line shows the transient responseof pressure in the case where the pressure feedback control
is utilized. The gray line shows the case without pressure
feedback control. From the figure, the pressure measured bya pressure sensor converges to the target value. Furthermore,
it is also confirmed hunting, an unstable behavior, is notobserved by applying the feedback control during the experiments.
C. Visual feedback control
Finally, the experiment to confirm the performance ofpressure and visual feedback control are conducted. In theexperiment, the gripper position of the robot is controlled by
the proposed method. Figure 15 shows the transient responseof the deviation of the position of the gripper. In addition,
Fig. 16 show the initial and final states of motion controlof the visual feedback control experiment. As a result, it isconfirmed that the position of the gripper converges to thedesired value precisely.

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[5] S. Sanan, “Soft Inflatable Robots for Safe Physical Human Interaction,” pp. 13-23, Pittsburgh, PA, 2013.
[6] C. Majidi, “Soft Robotics: A Perspective Current Trends and Prospects
for the Future,” Soft Robotics, vol. 1, No. 1, pp. 5-11, 2014.
[7] S. Sanan, P. Lynn and S. Griffith, “Pneumatic Torsional Actuators
for Inflatable Robots,” Journal of Mechanisms and Robotics, Vol. 6 /031003-1, 2014.
[8] H. Kim, Y. Tanaka, A. Kawamura, S. Kawamura and Y. Nishioka, “Improvement of position accuracy for inflatable robotic arm using visualfeedback control method,” Proc. of the IEEE International Conference
on Advanced Intelligent Mechatronics, pp. 767-772, Busan, 2015.
[9] H. Kim, A. Kawamura, Y. Nishioka and S. Kawamura, “Mechanicaldesign and control of inflatable robotic arms for high positioning
accuracy,” Advanced Robotics, Vol. 32, Issue 2, pp. 89-104, 2018.
Fig. 15.

Fig. 16.

Step response of visual feedback control

Experimental environment of visual feedback control

V. CONCLUSIONSThis paper proposed an inflatable robotic arm made of
LDPE and poly-laminated sheets. By using the combinationof these two materials the pressure resistance performance,load capacity and working area of the inflatable robotic arm
were improved. The manufacturing process of the robot werealso described. Moreover, the control performance of therobot was improved by both visual and pressure feedback
controllers. Finally, the control performance of the robot was
confirmed by some experiments.
In future works, we will realize more practical daily
applications using the proposed inflatable robot.
ACKNOWLEDGEMENTS
This research is supported by the Japan Society for the
Promotion of Science Grant-in-Aid for Basic Research (B)(JP18H03764).
We would like to appreciate Kawamura laboratory researchers at Ritsumeikan University who have helped a lot
in making the robotic arm.
R EFERENCES[1] S. Sanan, M. H. Ornstein and C. G. Atkeson, “Physical humaninteraction for an inflatable manipulator,” Proc. of the 2011 Annual
International Conference of the IEEE Engineering in Medicine andBiology Society, pp. 7401-7404, Boston, MA, 2011.
[2] S. Voisembert, A. Riwan, N. Mechbal and A. Barraco, “A novel
inflatable robot with constant and continuous volume,” Proc. of the2011 IEEE International Conference on Robotics and Automation, pp.
5843-5848, Shanghai, 2011.
[3] R. Qi, T. L. Lam and Y. Xu, “Mechanical design and implementationof a soft inflatable robot arm for safe human-robot interaction,” 2014
IEEE International Conference on Robotics and Automation (ICRA),
pp. 3490-3495, Hong Kong, 2014.
[4] Maruyama D, Kimura H, Koseki M and Inou N, “Driving force andstructural strength evaluation of a flexible mechanical system with ahydrostatic skeleton,” Journal of Zhejiang University SCIENCE A,
vol. 11, No. 4, pp. 255-262, 2010.

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