Proceedings of the 2011 IEEE
International Conference on Robotics and Biomimetics
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Spectrum-Based Vibrotactile Footstep-Display
for Crinkle of Fragile Structures
Shogo Okamoto, Shun Ishikawa, Hikaru Nagano, and Yoji Yamada
Abstract— This study developed a vibrotactile display method
for the crinkle/scrunch of fragile structures. For such structures, tactile stimuli are more easily perceived by humans thanthe very small reaction forces that occur during a crinkle. The
developed method synthesizes vibrotactile acceleration stimuli
for a crinkle based on the preliminarily measured acceleration
spectra. We developed a footstep display that presents thesynthesized stimuli to human soles in response to the humans’foot speed. The display presents the sensations acquired by stepping on fragile structures. Experimental participants identified
the types of materials of cylindrical fragile structures among
paper, aluminum foil, and polypropylene film, at probabilities
significantly higher than the chance level.

I. I NTRODUCTION
Force and tactile feedbacks associated with the fracture ofstructures or materials are inevitable for haptic simulations.
Force feedback is effective for simulating the manipulationof fragile materials or surgical and sculptural procedures.
For example, methods for displaying reaction forces whencutting soft tissues with scissors have been studied [1], [2]. In
addition, force feedback models for dental burring or cutting
have been studied [3], [4].
In the case of fragile structures, where the yield forces
are too small to be perceived, tactile stimuli have greater
perceptual importance than force stimuli. Based on thesetactile stimuli, humans can detect the fracture and identify
the types of broken structures or materials. However, todate, tactile feedback methods for fractures have rarely beenstudied. Most of the related studies have focused on thedisplay of forces.
The objective of this study is to develop a tactile display
method for the crinkle/scrunch of fragile structures. The
tactile stimuli associated with a fracture depend on the typeof material or the speed of the fracture motion. Hence, thisstudy aims at producing tactile stimuli in accordance with theactive movements, especially the velocities, of the humans
crinkling the structures. Moreover, the goal of this method
is the presentation of stimuli that would enable humans to
identify the types of materials used for the structures.
Although it is conventional to present tactile stimuli tohuman hands and fingers, we present stimuli to human footsoles. The virtual stimuli acquired when breaking/scrunchingfragile objects on a floor are developed. These stimuli are
expected to enhance the enjoyment or validity of virtual
walkthrough systems (e.g. [5], [6]) for such things as virtualThis work was partly supported by MEXT KAKENHI 23135514 and theHori Sciences and Arts Foundation.
The authors are with the Department of Mechanical Science and Engineering, Graduate School of Engineering, Nagoya University, Japan. E-mail:see http://www.mech.nagoya-u.ac.jp/asi/en/

978-1-4577-2137-3/11/$26.00 © 2011 IEEE

museum. The presentation of virtual reality stimuli to thesoles has seldom been attempted. For example, Visell et al.
tried to apply a sound synthesis model for walking andprovide vibrotactile stimuli to the soles to present the senseof virtual floor materials [7]. In addition, they investigated
the identifiable vibrotactile stimuli for feet while standing
still [8]. However, these researches did not consider thetactile stimuli associated with the fracture of structures.
For purposes other than virtual reality, many researchers
are interested in the presentation of stimuli to the soles.
Velázquez et al. arranged vibrators in the soles of shoes andstudied the human perception of spatial stimuli patterns [9].
They also made use of their device for walk navigation.
Watanabe et al. attempted to navigate walking cycles using
shoes with vibrotactile feedback [10]. Such feedback for thefeet were provided for gait training [11]. It was reported thatvibratory stimuli to the soles affect the center of gravity or
its awareness of a man standing upright [12], [13].
We employ vibrotactile stimuli to the soles. Vibrotactile
stimuli have been used to synthesize various sensations on
hands and fingers. The industrial use of vibrotactile stimulators is beneficial because of the abundance of actuators andthe possible simplicity of tactile displays.
II. O BSERVATION OF ACCELERATION S IGNALS DURINGF RACTURE OF F RAGILE S TRUCTURES
To develop a vibrotactile stimuli model, we measured
the acceleration stimuli generated by the crinkle of fragile
structures. We adopted cylinders made of paper, aluminum
foil, or polypropylene film (Fig. 1a) as the fragile structures. These structures crinkle without producing noticeable
reaction forces. The thicknesses of these materials were 80,
11, and 30 μm, respectively. Each cylinder had a diameterof 40 mm and a height of 20 mm. As shown in Fig. 1b,
a human that wore socks but no shoes stepped on andscrunched the structure on a 2-mm thick acrylic panel. Thispanel was part of the footstep display (see also Fig. 5). An
accelerometer (ADXL335, Analog Devices, bandwidth of
550 Hz) attached to the other side of the panel just beneaththe structure measured the acceleration when the structure
was scrunched. The bandwidth of the accelerometer covered
the frequency of our interest up to 500 Hz. The peak humansensitivity for the vibrotactile stimuli lies at approximately
100 Hz for acceleration stimuli [14]. The frequencies over500 Hz are hardly effective in this study because theseaccelerations were smaller than the perceptual thresholds,
which are larger than 0.3 m/s2 [14], for all the materials.
A potentiometer fastened to the participant’s ankle measured2459

Fig. 1. Observation of acceleration stimuli due to fracture of fragile objects.
a) Cylindrical fragile object. b) Side view of setup. c) Bird’s eye view ofsetup.

Fig. 2.
Accelerations along Z-axis caused by fracture of paper-madecylinder. a) Accelerations with foot velocity of 50 mm/s. b) Accelerations
with foot velocity of 200 mm/s.

the position of the foot along the Z-axis. The ankle joint
was bound to a metal plate by fabric bands such that the soleremained parallel to the panel (Fig. 1c). The foot position andacceleration caused by the crinkle were recorded at 5 kHz.
Fig. 2 shows the accelerations along the Z-axis that wereobserved when the paper cylinder was broken. Figs. 2aand 2b show those when the foot velocity was 50 and
200 mm/s, respectively. The velocity values stated above
are average velocities for the time that the foot was incontact with the cylinder and moving downward by 10 mm.
The foot contacted the cylinder at t = 0 in the figures.
Even when the same cylinder was broken, the profiles of
the accelerations including the magnitudes or decay times,
varied with the contact velocity. Fig. 3 shows the amplitude
spectra of the accelerations in Figs. 2a and 2b. These spectra
differ depending on the velocity. For example, when the footvelocity was 50 mm/s (Fig. 3a), spectrum peaks were foundaround 20, 70, and 280 Hz. On the other hand, when thevelocity was 200 mm/s (Fig. 3b), the peaks were around 10,
40, and 180 Hz. In terms of the magnitude across the grossfrequency bands, it was larger for the velocity of 200 mm/s
than that for the velocity of 50 mm/s. These data suggest thatvelocity-dependent tactile stimuli are required for displaying
the crinkle of fragile structures. It should be noted that theabove mentioned accelerations included vibrations in theexperimental setup such as the top panel in addition to thosein the cylinder.

Fig. 3. Spectra of accelerations caused by fracture of paper-made cylinder.

III. S PECTRUM - BASED VIBROTACTILE SYNTHESISMODEL FOR FRACTURE OF FRAGILE STRUCTURES

The amplitude or power spectrum determines the subjective quality of vibrotactile stimuli, and the phase information
for the components does not significantly affect the perception [15], [16]. Hence, we also focus on the amplitude
spectrum of the vibrotactile stimuli.
Let yi (i = 0, . . . , N − 1) denote the temporal sequenceof the acceleration caused by the crinkle of the structure.
We acquire the spectrum of the acceleration by computing
the discrete cosine transformation of yi . The spectrum isdetermined by
N −1

πk(i + 12 )2 
Ck =) (k = 0, . . . , N − 1) (1)yi cos (N i=0Nwhere Ck is the acceleration for a frequency of (F k)/N .

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F and N are the sampling frequency of the accelerationand the number of samples, respectively: F = 5000 Hz andN = 2500. This is a type-II discrete cosine transform (DCT)with a factor of 2/N , which makes the DCT indices theamplitudes of the frequency elements.
As described above, the accelerations sensed by the accelerometer included the vibrations in the experimental setup,mainly those of the top panel, in addition to those from thecrinkle of the structure. Hence, in order to separate theseaccelerations, the subtraction of the spectra is performed.
Ck,panel,obj is the spectrum of the accelerations that are
observed when the structure on the top panel is broken.
Ck,panel is the spectrum of the accelerations observed when
the top panel is stepped on without a structure. Thus, we
determine the spectrum of the accelerations that come fromthe fracture of the structure as 
Ck = |Ck,panel,obj | − |Ck,panel |Ck = 0 if |Ck,panel,obj | − |Ck,panel | < 0.

(2)

The spectrum of the acceleration depends on the footvelocity. In order to present the difference in the spectrum
caused by the foot velocity, the spectrum is synthesizedusing the pre-measured spectra. The amplitude spectrum forvelocity v is determined by
 |Ck (vi+1 )| − |Ck (vi )|Ck (v) =(v − vi ) + |Ck (vi )|vi+1 − vi·sign(Ck (vi+1 )) (3)(vi < v < vi+1 ), (k = 0, . . . , N − 1).
This equation is used to determine the spectrum for v
from the interpolation of the spectra for vi and vi+1 . The
spectra for vi were measured first. These vi values were
(v1 , v2 , v3 , v4 ) = (50, 100, 200, 300) mm/s. In the case ofv > v4 or v < v1 , Ck (v) is extrapolated. This synthesisof the spectrum loses its phase information. However, as
described above, the phase information is ineffective for theperception of vibrotactile stimuli.
Ck (v) is re-transformed into the temporal sequence ofaccelerations. These are determined by
yi =

N
−1

k=1

Ck (v) cos (πk(i + 12 ))N

(i = 0, . . . , N − 1). (4)These yi values are applied to human soles as the acceleration stimuli for the foot velocity v.
Examples of the above process are shown in Fig. 4. Fig. 4ashows the acceleration observed when the paper cylinder
was stepped on at 100 mm/s. Fig. 4b shows that observed
when the top panel without the cylinder was stepped on at
100 mm/s. Fig. 4c shows the spectra of Figs. 4a and 4b.
The subtraction of these two spectra following (2) gives the
spectrum of the acceleration that comes from the fractureof the cylinder. Fig. 4d shows the acceleration acquired bycomputing the inverse DCT of this subtracted spectrum. The
vibrotactile display mentioned later was used to present thisacceleration as the pseudo acceleration stimuli of the fractureof a paper cylinder that did not include the vibrations of theFig. 4. Example of synthesized acceleration data. a) Acceleration acquiredby fracture of paper on top panel at contact speed of 100 mm/s. b)
Acceleration acquired when stepping on top panel at contact speed of
100 mm/s. c) Acceleration spectra of Figs. 4a and 4b. d) Accelerationacquired by subtracting spectrum of b from a in frequency region.

top panel. The acceleration in the range of t < 0 in Fig. 4d
was not presented to the users. That of t ≥ 0 was presented
to them.
IV. E XPERIMENTAL E QUIPMENT
A footstep display was used to present the crinkle ofstructures by applying vibrotactile stimuli to human soles.
The display presented non-spatially distributed acceleration
stimuli to the entire sole area. We usually acquire spatiallydistributed stimuli on our soles when stepping on objects.
However, in this study, in order to test the validity of thespectrum-based vibrotactile synthesis method, we used thesimplified vibrotactile display. Because we assumed that thefragile structures did not produce noticeable reaction forces
during their crinkles, the device did not have a force display
function.

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Hardware: The device used voice coil speakers (23 W,
4 Ω) as vibrotactile stimulators (Fig. 5). Four voice coils
were used, and an acrylic top panel was placed on them.
Springs located beneath the voice coils pushed them tightly
against the top panel. The top panel was deformable a several
millimeters when the participants stepped on. Fabric tape
was used to fix each user’s ankle joint to a linear slider
with a potentiometer (Fig. 1c). The meter measured theposition and velocity of the feet. A computer using a realtime Linux received the output from the potentiometer andsent a voltage input to the voice coils at a refresh rate of
5 kHz. A MAX9704 amplifier (MAXIM Integrated Products,
10 W, class D) was inserted between the computer and voicecoils.
Usage: A user wearing socks but no shoes sat in a chairand stepped on the top panel. They stepped so that his or her
foot arch came down at the center of the panel. Then, according to the foot speed measured by the potentiometer, thevibrotactile acceleration stimuli determined by the method in
section III were produced. The display started producing thestimuli just when the foot reached the panel.
Frequency Response: In order to realize the acceleration stimuli on the sole, the voltage inputs to the voicecoils were determined based on the frequency response of
the experimental equipment. We measured the frequency
response of the equipment while a load of 4 kg was applied
to the top panel. This load was close to the foot force ofthe typical users. Fig. 6 shows the frequency response of
the equipment when the voltage inputs to the voice coils
were 1.0 Vrms. We also measured the responses for voltage
inputs of 0.1–1.5 Vrms. The acceleration outputs of theequipment changed almost linearly with the voltage inputs
when the frequency was fixed. We prepared a weighting
function, W (f, a), that determined the applied voltage tothe voice coils from frequency f and desired acceleration
output a based on the frequency response of the experimental
equipment. The function linearly interpolated the voltage
values from the recorded values. The acceleration spectrum
was weighted by W (f, a). Then, the inverse DCT was used
to transform it into a temporal sequence of voltage data that
was applied to the voice coils. The voltage data were given
by
ei =

N
−1

k=1

W(

πk(i + 12 )Fk 
, Ck (v)) cos (
)
N
N

(5)

(i = 0, . . . , N − 1).

This voltage input was used in the experiment of section V.
V. E XPERIMENTIn order to evaluate to what extent the developed tactile
display and algorithm presented differences in the materials
of structures, we conducted an identification experiment. The
participants first stepped on and broke the real cylindricalstructures made of paper, aluminum foil, and polypropylene
film. Then, they experienced virtual stimuli that mimicked

Fig. 5.

Foot step display based on voice-coil actuators

Fig. 6. Frequency response of experimental system with voltage inputs of
1.0 Vrms. Frequency vs. peak-to-peak acceleration output.

each of the materials through the footstep display and reported which material they felt corresponded to what theyexperienced (experiment 1). In experiment 1, we meant
to investigate how well the participants could identify thevirtual materials without any instruction or practice. After
experiment 1, the participants were given the correspondences between the real and virtual materials. Then, they
participated in the same identification tasks as before (experiment 2). In experiment 2, we meant to investigate how well
the participants could identify the materials when they knew
the stimuli for the real and corresponding virtual materials.
A. Tasks and ParticipantsParticipants: The participants were 5 voluntary malestudents from the authors’ laboratory, excluding the authors.
Hence, the following experiments are regarded as preliminary
experimental studies. None of the participants were experts
or had any experience with the study of vibrotactile displays.
In order to avoid the possibilities of falling and fatigue, theysat in a chair to participate in the experiments. Before theexperiments, they experienced crinkling the three types ofcylindrical structures on the top panel. They experienced
these fractures until they felt familiar with the stimuli of
real cylinders, which took approximately 3 min. In order toshut out the sounds produced by the crinkle of real cylinders
or that produced by the voice coil speakers, the participants
listened to pink noise through noise-cancelling headphones.
Experiment 1: In experiment 1, after experiencing thecrinkling of the real cylinders, the participants performed
an identification task with no instruction about the virtual stimuli. They stepped on the virtual stimuli that wererandomly presented to them (Fig. 7). Then, they orally

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TABLE I
AVERAGE ANSWER RATIOS IN E XPERIMENT 1 IN THE FORM OF
CONFUSION MATRIXParticipants’
answers

Paper
0.68∗
0.16
0.16

Paper
Alum.
PP.

Presented stimuli
Aluminum
0.14
0.56∗
0.3

PP.
0.04
0.36
0.6

TABLE IIAVERAGE ANSWER RATIOS IN E XPERIMENT 2 IN THE FORM OF
CONFUSION MATRIX

Fig. 7.
Experimental scene. Participant sitting on chair stepped onvibrotactile foot-step display. Left: foot is held up. Right: foot placed on
top panel.

reported whether the type of material that they felt waspaper, aluminum foil, or polypropylene film. They steppedon the top panel at arbitrary foot speeds and were allowed toexperience the virtual stimulus as many times as they wanted.
In addition, they were encouraged to experience the stimulus
at least a few times before making a determination. Each
type of virtual material was presented 10 times. In total, 30
stimuli/trials were presented to the participants.
Experiment 2: After experiment 1, the participants
experienced the virtual stimuli while the names of thecorresponding materials were given to them. This teachingprocess took approximately 3 min. After this instruction,
they conducted the same identification task as that in experiment 1.
B. Stimuli
The materials used for the real and virtual stimuli were
paper, aluminum foil, and polypropylene film. We measured
the acceleration produced by stepping on cylindrical structures made from these three types of materials in advance.
The measurement methods followed those in section II. We
measured the acceleration when the foot velocities were
(v1 , v2 , v3 , v4 ) = (50, 100, 200, 300) mm/s. The measured
acceleration was translated into the acceleration stimuli forthe footstep display using the methods in section III. The
foot velocity, v, that was used for synthesizing the stimuli
was measured in real-time. It was the average velocity of
the foot while it moved from 20 mm to 10 mm above the
top panel. The acceleration stimuli were synthesized basedon this average velocity. The synthesis of the acceleration
spectrum was performed using (1)–(3). The footstep display
started the stimuli just when the participant’s foot reached
the top panel.
C. Results
Experiment 1 (no instruction): Table I shows the average answer ratios in experiment 1 in the form of a confusion
matrix. The diagonal elements are the correct answer ratios.
Here, a correct answer refers to a match between the typeof material presented to the participant and the answer givenby them. In terms of the correct answer ratios, those forParticipants’
answers

Paper
Alum.
PP.

Paper
0.82∗∗
0.1
0.08

Presented stimuli
Aluminum
0.04
0.76∗∗
0.2

PP.
0
0.26
0.74∗

the paper were the highest, followed by the polypropylene
film and aluminum foil. The participants often confused thealuminum foil and polypropylene film. Fig. 8 shows theaverages and standard deviations of the correct answer ratios
for each material. The standard deviations were computed
across the 5 participants. We applied a t-test to determine
whether they were higher than the chance level of 0.33. We
found that the correct answer ratios were higher than the
chance level for the paper (t0 (4) = 2.79, P = 0.025, onetailed), and aluminum foil (t0 (4) = 2.60, P = 0.030, onetailed), but not for the polypropylene film (t0 (4) = 1.84,
P = 0.070, one-tailed).
Experiment 2 (after instruction): Table II shows theaverage answer ratios in experiment 2. The trend for theseratios is similar to that of experiment 1. The correct answer
ratios for the paper were the highest, followed by thealuminum foil and polypropylene film. The aluminum foil
and polypropylene film were often confused. Fig. 9 shows
the averages and standard deviations of the correct answer
ratios for each material. The correct answer ratios werehigher than the chance level for all types of materials: thepaper (t0 (4) = 5.31, P = 0.0030, one-tailed), aluminum foil(t0 (4) = 4.14, P = 0.0072, one-tailed), and polypropylene
film (t0 (4) = 2.77, P = 0.025, one-tailed). In experiment2, the participants could identify the types of materials at
significantly higher ratios than the chance level.
Comparison of Experiments 1 & 2: In order to determine the significant differences in the correct answer ratios
between experiments 1 and 2, we applied a one-way ANOVA
to the correct answer ratios with the group factor being thetype of experiment. As a result, no significant difference wasobserved between the two experiments (F0 (1, 28) = 3.08,
P = 0.090). Based on Tables I and II, it seems that becauseof the instruction, in experiment 2, the participants could
identify the types of materials better than they could inexperiment 1. However, the differences in the correct answer
ratios were not very significant.

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Fig. 8.
Average correct answer ratios and standard deviations among
participants in Experiment 1 (no instruction). * means significance levelof 0.05 for correct answer ratios to be higher than chance level.

little perceivable reaction force. Tactile stimuli that depended
on the velocity of the crinkle movement were realized bysynthesizing the acceleration spectra recorded beforehand.
We presented the sensation on human foot soles associatedwith the crinkle of fragile objects using this synthesizing
method. The vibrotactile display for the soles was built basedon voice coil speakers. We validated the synthesizing method
using a material-identification experiment. The experimental
participants could identify the types of materials (paper,
aluminum foil, and polypropylene film) at rates significantly
higher than the chance levels. The developed vibrotactile
method for displaying the crinkle of objects is expected
to provide virtual reality systems with immersive sensory
feedback.
R EFERENCES

Fig. 9.
Average correct answer ratios and standard deviations among
participants in Experiment 2 (after instruction). * and ** mean significancelevels of 0.05 and 0.01, respectively, for correct answer ratios to be higher
than chance level.

D. Short Discussions
In experiments 1 and 2, the correct answer ratios for thepaper were higher than those for the aluminum foil andpolypropylene film. According to the introspective reports
from the participants, they found it difficult to identify thealuminum foil and polypropylene film even when using thereal cylindrical structures. This difficulty in differentiating
the real stimuli might have been translated into the low
identification ratios of these two types of virtual materials
in the experiments.
In terms of experiment 1, even without any instruction,
we expected that the participants could identify all three
types of materials. However, they confused the aluminum foil
and polypropylene film. This simply suggests that we should
improve the validity of the synthesis algorithm for the stimuli
and footstep display. For example, the lack of dynamics,
including the proper responses toward the different footforces, may have to be considered for better virtual stimuli.
On the other hand, in experiment 2, after instruction, thecorrect answer ratios for the aluminum foil and polypropylene film improved. This implies that the footstep display
presented something different to the participants for eachtype of materials. They were then able to identify the virtualmaterials based on these differences. In addition, it should
be noted that under the current experimental design, there isno denying that the correct answer ratios of the participants
might have improved as a result of habituation to the trials.
VI. C ONCLUSIONS[1] A. M. Okamura, R. J. Webster III, J. T. Nolin, K. W. Johnson, andH. Jafry, “The haptic scissors: Cutting in virtual environments,” Proc.
the 2003 IEEE International Conference on Robotics and Automation,
pp. 828–833, 2003.
[2] H. Wakamatsu, X. Zhang, and S. Honma, “Teleoperational force
display system in manipulation of virtual object using scissors-type
cutting device,” Proc. of the 3rd Asia-Pacific Conference on Control& Measurement, pp. 312–316, 1998.
[3] D. Wang, Y. Zhang, Y. Wang, Y. Lee, P. Lu, and Y. Wang, “Cutting
on triangle mesh: Local model-based haptic display for dental preparation surgery simulation,” IEEE Transactions on Visualization andComputer Graphics, vol. 11, no. 6, pp. 671–683, 2005.
[4] M. Agus, A. Giachetti, E. Gobbetti, G. Zanetti, and A. Zorcolo,“Real-time haptic and visual simulation of bone dissection,” Presence,
vol. 12, no. 1, pp. 110–122, 2003.
[5] J. Chim, R. W. H. Lau, H. V. Leong, and A. Si, “Cyberwalk: A webbased distributed virtual walkthrough environment,” IEEE Transactions on Multimedia, vol. 5, no. 4, pp. 503–515, 2003.
[6] M. M. Rafferty, D. G. Aliaga, V. Popescu, and A. A. Lastra, “Images
for accelerating architectural walkthroughs,” IEEE Computer Graphics
and Applications, vol. 18, no. 6, pp. 38–45, 1998.
[7] Y. Visell, J. R. Cooperstock, B. L. Giordano, K. Franinovic, A. Law,
S. McAdams, K. Jathal, and F. Fontana, “A vibrotactile device fordisplay of virtual ground materials in walking,” Proc. EuroHaptics
2008, pp. 420–426, 2008.
[8] Y. Visell, A. Law, and J. R. Cooperstock, “Touch is everywhere: Floor
surfaces as ambient haptic interfaces,” IEEE Transactions on Haptics,
vol. 2, no. 3, pp. 148–159, 2009.
[9] R. Velázquez, O. Bazán, and M. Magaña, “A shoe-integrated tactile
display for directional navigation,” Proc. the 2009 IEEE/RSJ International Conference on Robots and Inteligent Systems, pp. 1235–1240,
2009.
[10] J. Watanabe, H. Ando, and T. Maeda, “Shoe-shaped interface forinducing a walking cycle,” Proc. of the 2005 International Conference
on Augmented Tele-Existence, pp. 30–34, 2005.
[11] K. L. Lurie, P. B. Shull, K. F. Nesbitt, and M. R. Cutkosky, “Informing
haptic feedback design for gait retraining,” Proc. the 2011 IEEE World
Haptics Conference, pp. 19–24, 2011.
[12] A. Kavounoudias, R. Roll, and J.-P. Roll, “Specific whole-body shifts
induced by frequency-modulated vibrations of human plantar soles,”Neuroscience, vol. 266, pp. 181–184, 1999.
[13] R. Roll, A. Kavounoudias, and J.-P. Roll, “Cutaneous afferents fromhuman plantar sole contribute to body posture awareness,” Neuroreport, vol. 13, no. 15, pp. 1957–1961, 2002.
[14] M. Morioka and M. J. Griffin, “Perception thresholds for verticalvibration at the hand, seat and foot,” Proc. of the European Acoustic
Association Forum Acusticum 2005, pp. 1577–1582, 2005.
[15] S. J. Bensmaı̈a and M. Hollins, “Complex tactile waveform discrimination,” Journal of the Acoustical Society of America, vol. 108, pp.
1236–1245, 2000.
[16] S. A. Cholewiak, K. Kim, H. Z. Tan, and B. D. Adelstein, “A
frequency-domain analysis of haptic gratings,” IEEE Transactions on
Haptics, vol. 3, no. 1, pp. 3–14, 2010.

This study developed a vibrotactile display method to capture the crinkle of fragile structures, which are broken with
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