| Notes |
Scientific Reports | (2019) 9:8988 | https://doi.org/10.1038/s41598-019-45422-6 1
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Pneumatic actuator and fexible
piezoelectric sensor for soft virtual
reality glove system
Kahye Song1, Sung Hee Kim1, Sungho Jin2, Sohyun Kim3, Sunho Lee4, Jun-Sik Kim1,
Jung-Min Park1 & YoungsuCha1
The desire to directly touch and experience virtual objects led to the development of a tactile feedback
device. In this paper, a novel soft pneumatic actuator for providing tactile feedback is proposed and
demonstrated. The suggested pneumatic actuator does not use an external air compressor but it
is operated by internal air pressure generated by an electrostatic force. By using the actuator, we
designed a glove to interact with virtual reality. The fnger motions are detected by attached fexible
piezoelectric sensors and transmitted to a virtual space through Bluetooth for interconnecting with a
virtual hand. When the virtual fnger touches the virtual object, the actuators are activated and give the
tactile feedback to the real fngertip. The glove is made of silicone rubber material and integrated with
the sensors and actuators such that users can wear them conveniently with light weight. This device was
tested in a virtual chess board program, wherein the user picked up virtual chess pieces successfully.
In order to directly experience and feel the virtual reality (VR), various technologies connecting VR and the real
world have been developed1–3
. Head-mounted displays for the surrounding view and gloves for hand motion
recognition and tactile feedback are typical examples4–9
. In particular, the human-computer interface gloves are
essential devices for the users to experience the VR by conveying the user’s movements to the VR and transmitting the tactile feedback to the user10 (Fig. 1). Using this glove, the user can grab or place objects in VR and can
feel the textures of virtual objects11–13. In addition, the gloves can be used as text input device14. For performance
improvement, the interface gloves have been reported to be using various methods and materials including inertial measurement sensors, potentiometer-based sensing technique, or piezoresistive sensor15–19. Furthermore,
for more practical and comfortable usage, delicate interactive gloves based on novel materials and structures are
being developed.
Te core elements of the gloves are largely divided into sensors and actuators. Te sensors detect the movements of the users and send the motion information to the VR20. Among the various sensing materials, piezoelectric materials can be a good candidate for human-computer interaction21–24. Te piezoelectric materials are either
embedded in the gloves as a sensor for hand motion recognition or in an energy harvester using the motion25–27.
Flexible piezoelectric sensors have a few tens of micro-scale thickness, making them easy to mount on wearable
devices28–30. In this study, we select polyvinylidene fuoride (PVDF) as a piezoelectric material for the sensor.
PVDF materials have also found the applications in actuators, and energy harvesters31–34. When the PVDF sensor
is bent, we can measure voltage output from the sensor, analyze the value, and estimate the bending shape35,36.
In our previous study, we tested and validated the PVDF sensing ability to detect the changes in the fnger joints:
the sensor outputs were compared with the real angles obtained from the camera recording images, and they
matched well36.
Various actuators have been developed and installed for tactile feedback37–39. Actuators to provide mechanical
stimuli are more commonly used because they can accurately reproduce the actual texture40. Tey are found in
cell phones and pagers and can provide information about the contact force, texture, and roughness of an object.
However, the main limitation of vibration tactile stimulation and lateral strain stimulation is that the actuator
cannot provide information about the actual surface shape of the object41. In addition, mechanical actuators that
1
Center for Intelligent & Interactive Robotics, Korea Institute of Science and Technology, Seoul, Republic of Korea. 2
Department of Biomedical Engineering at Korea University, Seoul, Republic of Korea. 3
Department of Mechanical
System and Design Engineering at Seoul National University of Science and Technology, Seoul, Republic of Korea. 4
Department of Electrical Engineering at Korea University, Seoul, Republic of Korea. Correspondence and requests
for materials should be addressed to Y.C. (email: givemong@kist.re.kr)
Received: 11 March 2019
Accepted: 6 June 2019
Published: xx xx xxxx
OPEN
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require large systems are problematic in terms of weight and portability42. Sof actuators that provide smooth
and fexible tactile feedback can be an alternative to address those problems43–50. Te sof actuators have various
functional advantages, including their light weight and fexibility48,51. Because the sof actuators are usually made
of fexible materials such as polymers, they have a high strain density and are easy to fabricate as per the desired
shape52,53. In addition, fexible actuators with relatively simple mechanisms perform multiple degrees-of-freedom
motions that can be handled by complicated control systems and large-scale components of hard machines54–57.
Owing to its advantages, sof actuators have already been utilized in various felds, including medical and wearable applications44,53. Tus, herein, we developed and utilized a sof pneumatic actuator (SPA) for tactile feedback. Pneumatic actuators have advantage of light weight, simple system, high speed, and miniaturization58–62.
However, they need air pressure provided by an external compressor. Because of the existence of the compressor,
the entire system using the pneumatic actuators can be bulky. Notably, our actuator uses the internal air pressure
generated by an electrostatic force, without an external air compressor. To obtain the internal air pressure, the
fexibility of the actuator is very important.
For fexibility, we fabricated the actuator with silicone rubbers. Silicone rubber has an average modulus of
elasticity of several hundred kPa, a Poisson’s ratio close to 0.50, and a shear modulus of several tens kPa63,64. In
case of Ecofex, a commercial silicone rubber, its elastic modulus is 125 kPa63. Additionally, because silicone is
harmless to the human body, sof or porous silicone is used for rehabilitation, wearable application, as a surgical
material, and in daily life65–67. Terefore, Ecofex can be used as an actuator that touches the human body directly.
To sum up, in this paper, human-computer interface glove system with sensors and actuators is fabricated as
one-body. Without additional equipment, this glove senses and transmits hand movements and provides haptic
feedback. Te mounted actuator is fexible and provides very fast reaction rates. Also, we show the performance
test of the glove used in VR.
Pneumatic Soft Actuator
We developed a new type of pneumatic sof actuator activated by an electrostatic force. Te actuator has a small
size to give a fngertip tactile feedback. Te size and weight are as follows: diameter: 15mm, height: 5mm, weight:
0.57 g.
Operation mechanism. Te actuator can be divided into a ring part, where the electrostatic attractive force
works, and the center part, which is the contact part. Te silicone thickness of the ring part more than that of
the center part (ring: 500 μm, center: 200 μm). When diferent polarity voltages are applied to the ring part and
bottom electrode (Fig. 2a), the ring part moves downward by electrostatic attraction (Fig. 2b-1). As the actuator is
sealed, the air in the ring part moves to the center, and the central silicone expands and rises upward (Fig. 2b-2).
Te fngertip of a user can sense this swollen silicone, that is, the tactile feedback.
Figure 1. A glove for user interaction with VR. Te glove transmits the hand motion of the user to the VR and
transmits the stimulus to the user. Te fgures were created by the authors.
Figure 2. Actuator’s operating mechanism. Te fgures were created by the authors. (a) Diferent electrodes are
connected to the inner electrode of the actuator and the bottom electrode of the copper tape flm to induce an
electrostatic attractive force. (b) Te on-of state of the actuator and its operation mechanism.
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Actuator motion tracing. Te motion of the actuator was detected in real time with a high-speed 2D laser
scanner (LJ-V7001, Keyence, Japan). Data were collected through 1000 line scans per second.
In order to give a variety of tactile feedback while holding the virtual objects, a wide range of movements
should be possible. Te on/of switching of the actuator at the moment of catching or releasing a virtual object
must be fast and accurate. Moreover, it should remain “on” for holding the virtual object. In the same context, we
changed the on/of frequency of the actuator from 0.2Hz to 1Hz. Te input waveform is a square wave, and the
peak-to-peak value of the input voltage is 6 kV.
Figure 3 displays the displacement at the center of the actuator along with the frequency. Te displacements
are well maintained during the each on/of cycle, although it is observed damping phenomenon by elasticity during the switching time. Te peak-to-peak displacements at 0.2Hz (Fig. 3a), 0.5 Hz (Fig. 3b), and 1 Hz (Fig. 3c) are
about 0.10mm, 0.12mm, and 0.13mm, respectively, that is, the displacement increases as the frequency increases.
We note that the actuator has the ability to provide enough tactile feedback during the time of holding a virtual
object with the reaction speed under a few hundreds of milliseconds.
Moreover, the displacement can be varied by the input voltage amplitude (see Fig. 4). Te square waves with
diferent voltage levels at 1Hz were applied. When the input voltage is 2.4 kV, the displacement is about 0.11mm
(Fig. 4a). However, even though the input power is a square wave, the amplitude change seems like a triangle
wave. Tis may be because the input voltage is insufcient to follow the square wave input. When the power
source was 3 kV, it presented a square wave such as the input voltage (Fig. 4b). Te amplitude is about 0.11mm.
Finally, when the square input is 6 kV, the amplitudes increase to about 0.13 mm and the on state is well maintained (Fig. 3c). We conclude that the actuator is clearly controlled and the amplitude increases as the voltage
diference of the input source increase.
VR Glove With Pneumatic Actuator and Sensor
We designed an integrated glove system to work in VR, including the proposed pneumatic actuator (Fig. 5).
Specifcally, this glove system is divided into two parts, i) hardware part with a fexible glove including piezoelectric sensors, actuators, and interface board, and ii) sofware part with an interaction system between the real
world and VR. In the hardware part, the sensors in the fexible glove collect the joint data, and the interface board
Figure 3. Change in the actuator center displacement according to the input frequency. (a) 0.2Hz. (b) 0.5Hz.
(c) 1Hz.
Figure 4. Displacement change of the actuator center according to the input voltage change. (a) 0–2.4 kV. (b)
0–3 kV
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transfers the data to the computer system. When the virtual hands touch a virtual object in the interaction system,
the “on” signal is sent to the interface board. In the board, the high voltage converter is turned on, and the driving
voltage is sent to the actuator. Te program in the computer system converts the raw sensor data into fnger joint
angles for generating hand motions in the VR environment. Te total weight of VR gloves including the actuator,
sensor, board, and battery, is about 156.2 g.
Total system with VR. In this experiment, we used a virtual hand in a VR chessboard to capture a chess
horse (Fig. 6a–d)68. Te fnger movements were detected by the sensors and the data were transferred to the program, and the virtual hand of the screen moved based on the data (Fig. 6e–j). Specifcally, when the index fnger
was bent, the sensors gave the changed voltage signals as the output (Figs 6e (S1), 6g (S2) and 6i (S3)). Te sensor
signals were sent from the glove to the computer, and then processed through time integration and gain correction to obtain the angle sensed from the hand movements36.
In particular, the detailed calculation processes for the angle acquirement were as follows.
θ = ⋅ ∑ − × Δ =
=
( ) t A ( ( V t ) ( O t )) t n, 1, 2, ) (1) n
m
n
m m m
1
where θ( ) tn is a processed angle in tn, t = ∑ = Δt n m
n 1 m is the system time, V(tn) is the voltage value of sensor in tn,
and A is the gain. Δtm is the time interval of the m-th step, and O(tn) is the ofset voltage of the system in tn. We
integrated the value of the diference between the sensor output voltage and ofset voltage by the time to obtain
the real angles from the sensor outputs. Herein, the integral is to the numerical sum of a rectangular area with the
diference value and time interval. Using it, we obtained the ofset voltage of the raw sensor output.
=
∑
< <
∑ ≥
≤
= − − −
= − O t −
V t
s s
V t s
O t s
( )
( )
, 20 120
( )
100 , 120
( ), 20 (2)
n
m n s n m
n
n
m n
n m n
s n
19 20
119 20
n
where sn is the count value for the non-moving state and O(ts) is the last ofset value at the non-moving state. Te
ofset of the sensor was determined with an average of most recent 100 non-moving data. Each 20 data from the
start time and end time of non-moving state were excluded for improving the accuracy of the ofset value and
rapid reaction. To discriminate the moving state, we calculated the non-moving count value and used it as the
criterion of the ofset calculation, that is, sn. Te decision of the moving state was based on the quantity of voltage
change. If the voltage change was over a threshold value, Vth, we considered the sensor as moving. When the
count value was under 20, it was considered as a gap between the moving states, and we used the ofset at the last
non-moving state. Te count value for the non-moving state in tn is stated as follows,
=
− ≥
+ − ≤
−
− −
s V t V t V
s V t V t V
0, ( ) ( )
min (120, ( 1)), ( ) ( ) (3) n
n n th
n n n th
1
1 1
Figure 5. Te actual appearance of the integrated glove.
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Finally, the gain for estimating the real angle value was multiplied in the calculated value. Te gain is obtained
by the preliminary calibration operation, and all sensors have diferent gains36.
In this experiment, as shown in Fig. 6, the gain values of S1, S2, and S3 are 657.4 deg/V·s, 1151.0 deg/V·s,
and 1843.2 deg/V·s, respectively, and the threshold voltage is 0.05 V. To implement the tactile feedback in the
Figure 6. Actual operation of integrated glove for sensor and actuator. (a–d) Detects the movement of the
fnger and moves the same in the VR and grabs a virtual object. (e) Te voltage change and (f) the processed
angle by sensing from S1. (g) Te voltage change and (h) the processed angle by sensing from S2. (i) Te voltage
change and (j) the processed angle by sensing from S3. (k) Te amplitude variation of the actuator. Te actuator
was turned on at the moment of holding the object, and turned of when the object is released.
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interaction system between the VR and real hand, we transferred the active signal to the control voltage of AGH60P in the interface board when the virtual hand touches the chess horse.
To show whether the actuator was working at the time of holding the virtual object, an additional actuator
that receives the same control signal as that of the actuator of the index fnger was setup, and the movement was
measured with a line laser sensor. As a result, the virtual hand followed well according to the movement of the
hand, and the actuators also operated normally when the hand reached the virtual object (Fig. 6k). Depending on
the contact between the hand and the object, the actuator maintained the on/of state and gave the tactile feedback
to the user. Supplementary Video 1 shows this experiment.
Conclusion
In this study, we developed a new SPA and applied it as a glove system that interacts with the VR. Te pneumatic
actuator has an advantage that it can be operated without an external air compressor. We performed a series of
tests using the actuator showing that it can be adjusted periodically. Also it can be attached to the gloves to generate efective tactile feedback. In particular, when the user holds a virtual object, the actuator is well maintained
in the on state, and when the virtual object is released, the actuator is switched to the of state. Te actuator is
actuated by electrostatic attraction. When the air space is reduced by the electrostatic attraction, the central part
expands and is designed to give haptic feedback. Especially, the actuator showed a larger movement as the period
became faster and the applied voltage became larger. In addition, the designed silicone monolithic glove was able
to detect movement of fngers with the PVDF sensors and transmit data via Bluetooth. A voltage output by a piezoelectric sensor deformation provides fnger motion information. In order to distinguish the moving state from
the received information, the threshold value was specifed, and the gain value was obtained through the initial
calibration. We expect that our developed glove will be used in several ways by linking with various VR sofware.
Methods
Actuator fabrication method. Fabrication molds for the SPA were designed using Solidworks sofware
(Dassault Systems Solidworks Corp., USA) (Fig. S1a). Ten, the design was realized by the main part (VisiJet
M3 Crystal, 3D Systems Inc., USA) and supporter (VisiJet S300, 3D Systems Inc., USA) materials in a 3D printer
(ProJet HD3500, 3D systems Inc., USA) (Fig. S1b). Afer printing, the mold was heated in a convection oven
(DCF-31-N, Dae Heung Science, Korea) for melting the supporter material. Finally, the melted supporter was
completely removed from the mold in an oil bath in an ultrasonic cleaner (Sae Han Ultrasonic Co., Korea). Afer
washing and drying, a release agent (Ease release 200, Smooth-On, Inc., USA) was sprayed on the mold surface to
prevent the silicone from sticking to the mold.
After manufacturing the mold manufacture, the silicone was fabricated as the exterior of the actuator
(Fig. S1c). First, Ecofex 0030 part A (Smooth-On, Inc., USA), Ecofex 0030 part B (Smooth-On, Inc., USA), and
platinum silicone cure accelerator (Plat-cat, Smooth-On, Inc., USA) were mixed in a ratio of 1:1:0.04 (Fig. S1d).
Te well-mixed mixture was poured into the mold and cured at room temperature for 2 h. Te specimen with
a ring shape was carefully separated from the mold using tweezers afer fully curing. Ten, we inserted a hemispherical mold into the center of the silicone ring and poured the silicone mixture once more (Fig. S1e). It created
a diferent thickness of the ring and center of the actuator. Te hardened silicone was removed from the mold
(Fig. S1f). Te coiled wire and a carbon conductive adhesive tape (Nisshin EM Co., Ltd., Japan) with a hole in the
center were attached to the side of the ring silicone body of the actuator (Fig. S2a). Ten the polyethylene terephthalate (PET) flm (Saehan, Korea) was attached to the bottom of the silicone body and sealed well. Finally, we
attached a carbon conductive tape to the bottom of the PET flm as an electrode. A photograph of the completed
actuator is shown in Fig. S2b.
A high-voltage converter (AGH 60P-5, XP Power, Singapore) providing an output of 6 kV was utilized for
operating the actuator. Te high-voltage converter was connected to a power supply (MK3003P, MK power,
Korea), and its control pin was connected to a waveform generator (33500Bseries, Keysight technologies, USA),
which could output square waves. A thick-flm resistor (50 MΩ, Ohmite, USA) was connected between the output
pins of this converter for discharging. Te (+) and (−) ports of the high-voltage converter were connected to the
copper tape at the bottom and the wire in the actuator (Fig. 2a).
Fabrication method for soft virtual reality glove system. A piezoelectric flm (28 µm PVDF Silver
Ink, Measurement Specialties, Inc., USA) was cut to sizes of (length) 20mm×(width) 5mm for S1–S6/S9 - S11,
(length) 30 mm × (width) 5 mm for S8, and (length) 40 mm × (width) 5 mm for S7. Te capacitances of the
20 mm, 30 mm, and 40 mm length sizes of the piezoelectric sensors were measured using a graphical sampling
multimeter (DMM7510, Keithley Instruments Ltd., USA), and the values were 0.36 nF, 0.54 nF, and 0.70 nF,
respectively. Two 2mm×5mm copper tapes (1181, 3M, USA) were attached to the top and bottom of the sensor
and soldered to 0.7mm diameter electric wires.
A total of 11 sensors were attached to the glove, to detect the movements of the thumb, index fnger, and middle fnger, with a silicone adhesive (Sil-Poxy, Smooth-On, Inc., USA) (Fig. S3a): sensors attached for collecting
distal interphalangeal joint angle, proximal interphalangeal joint angle, metacarpophalangeal joint angle of the
thumb, index fnger, and middle fnger, and abduction/adduction angle between the fngers.
We also added three pneumatic actuators (A1–A3) to provide tactile feedback on the tips of the thumb, index
fnger, and middle fnger (Fig. S3).
In particular, the silicone-based glove was fabricated by using 3D-printed molds (Figs S4a and S4b). Te
silicone was poured into the printed molds (Fig. S4c). Afer curing, the glove contour fabrication was completed
(Fig. S4d). Moreover, the holders for the actuator attachment to the glove were also made using silicone (Fig. S4e).
Te rings were 3D-printed for the glove to wear on the fngers (Fig. S4f). Te holders and rings were attached to
the glove using the silicone adhesive.
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To measure the sensor output, control the actuators, and communicate with the computer, we utilized the
interface board as shown in Fig. S5a. Te board size was 80mm ×55 mm, including ATMEGA328P-AU as the
main microcontroller and F1E22 as a Bluetooth module. For the actuator control, three output nodes transmitted on/of values through the signal isolator into the control voltage pin of the high-voltage amplifer EMCO
AGH-60P (Fig. S5b). For the sensor measurement, the microcontroller collected the voltages from the 11 sensors
through the internal 10-bit analog–digital converter. We utilized an analog multiplexer because of the limitation
of the analog pins. In particular, three sensors were directly connected to the analog pins in the microcontroller, and other sensor outputs were measured through the multiplexer (Fig. S5c). Each sensor was connected
with a 10 MΩ load resistor, and one electrode was connected with+2.5V, produced from the voltage regulator
SPX1587AU-2.5. During data processing of the sensor output, a 30-Hz low-pass flter was used to attenuate the
60Hz noise from the power sources.
Sof virtual reality glove system was worn by one of the author and we agreed to publish the identifcation
information, images and videos included in this study.
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Acknowledgements
Tis work was supported in part by the Global Frontier R&D Program on ‘Human-centered Interaction for
Coexistence’ through the National Research Foundation of Korea grant funded by the Korean Government(MSIP)
(2011-0031425). We deeply thanks to Yoonjeong Cho for supporting us with the schematics.
Author Contributions
K.S., S.H.K. and Y.C. proposed the study. K.S. and S.K. fabricated actuator. S.J. and S.L. made sensor. S.H.K., J.S.K.
and J.M.P. designed and realized VR environment. K.S., S.J., S.K. and S.L. made integrated sof virtual glove and
performed the experiment. K.S. and S.H.K., analyzed the data and wrote paper. K.S. and Y.C. participated in
completing the manuscript.
Additional Information
Supplementary information accompanies this paper at https://doi.org/10.1038/s41598-019-45422-6.
Competing Interests: Te authors declare no competing interests.
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