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from the IEEE."2009 IEEE International Conference on Robotics and Automation
Kobe International Conference Center
Kobe, Japan, May 12-17, 2009

Transmission of Tactile Roughness through Master-slave Systems
Shogo Okamoto, Masashi Konyo, Takashi Maeno and Satoshi Tadokoro

Abstract— In this study, a tactile-roughness transmission system applicable to master-slave systems with a communication
time delay is developed. The master-side system constructs a local model of target objects placed in the slave-side environment.
Tactile feedbacks presented to an operator at the master side areproduced by combining the physical properties of target objects
in the local model and the kinetic information of the operator.
The time delay between the operator’s motion and the tactile
feedback is cancelled because the stimuli are synchronized withthe exploratory motions.
The proposed system is applied to the transmission of tactileroughness. The tactile stimuli presented to the operator are vibratory stimuli whose amplitude and frequency are controlled.
These stimuli are locally synthesized by combining the surface
wavelength of target objects and the operator’s hand velocity.
Using the developed tactile-roughness transmission system, an
experiment for transmitting the perceived roughness of grating
scales was conducted. As a result, the roughness perceived by
the operators was found to highly correlate with the roughnessof the scales in the slave-side environment with a coefficient of
0.83.

I. INTRODUCTION
The transmission of tactile sensations is a technologythat enables human operators to perceive textures being
touched by robots. This technology provides the operatorswith benefits such as the facilitation of texture recognition,
an improvement in the reality of perception, and providing
a stable grasp by preventing slippage.
For the majority of tactile transmission systems, tactile
stimuli are presented to the operator on the basis of the information sensed by tactile sensors. In related studies, various
tactile transmission systems have been developed. For instance, palpation systems for minimal invasive surgery [1][2],tactile transmission to aid robotic manipulation for masterslave systems [3–5], transmission of perceived textures [6],or presentation of tactile sensations for prosthetic arms [7]have been reported. However, none of the studies addressed
the problem of a communication time-delay between thetactile-display-side and tactile-sensor-side systems.
For tactile transmission systems, a communication timedelay between the master and the slave side systems impairs
tactile exploration. One of the problems induced by the timedelay is a temporal gap between the exploratory motionsof the operators and the corresponding tactile feedbacks
presented to them. The allowable time delay is approximately
40–60 ms. When the time delay is significant, the operators
S. Okamoto, M. Konyo and S. Tadokoro are with the Graduate School ofInformation Sciences, Tohoku University, Japan, okamoto, konyo,

tadokoro@rm.is.tohoku.ac.jp
T. Maeno is with the Graduate School of System Design and Management, Keio University, Japan

978-1-4244-2789-5/09/$25.00 ©2009 IEEE

Fig. 1.

Block Diagram of the Proposed Tactile Telepresence System

do not perceive the textures accurately, or the validity of thesystem is deteriorated [8].
The objective of this study is to develop a system for thetransmission of tactile roughness using a master-slave systemwith a communication time delay. The system cancels the
temporal gap between the exploratory motions and the tactile
stimuli. In order to cancel the gap, a tactile telepresence
system based on the physical parameters has been proposed[9]. However, the previous system transmitted only vibratory
frequencies as roughness stimuli and could not achieve
the transmission of perceived roughness. In this study, thetransmission of vibratory amplitudes as well as frequencies
for realizing the transmission of tactile roughness is achieved.
II. PROPOSED TACTILE TELEPRESENCE SYSTEM
A. Stimuli Generation in Synchronization with Exploratory
Motions at Master Side
Figure 1 illustrates a block diagram of the proposedsystem. The proposed tactile transmission system does notdirectly transfer the information sensed by a tactile sensor
at the slave side to the operator at the master side. This isbecause the transmitted information includes a time delay.
Instead, the master-side system constructs a local model ofthe target objects placed in the slave-side environment. The
tactile feedbacks are generated by combining the exploratory
motions of the operators and the physical properties in thelocal model. The temporal gap between the motions and thestimuli due to the communication time delay does not occur
because the stimuli are synchronized with the motions.
The slave-side system estimates the physical parametersof the target textures and transmits them to the master sidesystem. The slave-side system continues to estimate andtransmit the parameters in order to renew the model at themaster-side. A delay in the renewal of the model cannot be
avoided due to the communication time delay or the delay intactile sensing; however, this problem is relieved by sensing
the physical parameters in real time.

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Though, the proposed system cancels the delay between
the operator’s motions and the tactile feedbacks presented
to them, the transmission of the physical properties oftarget textures still includes a delay. Therefore, the proposed
method does not accurately transmit the tactile sensations of
uneven textures whose surface property frequently changes.
However, the method is effective for the texture that is evenor smoothly changes.

Fig. 2.

Block Diagram of the Master-side System

B. Application to Tactile Roughness
We apply the proposed system to the transmission oftactile roughness.
We determine the physical parameters and the types of
exploratory movements that affect roughness perception fromliterature on psychophysics. The perceived roughness of grating scales used in this study strongly depends on the spatial
wavelengths or the groove widths of the scales [11–13].
The vibratory frequencies of cutaneous deformations that are
determined by the hand speeds affect perceived roughness[13][14]. Therefore, we focus on the surface wavelength of
textures λ and the hand velocity v(t).
The developed system employs vibrotactile stimuli forpresenting tactile stimuli to the operators. The frequency
and amplitude of vibrotactile stimuli are known to affectthe subjective magnitude of stimuli [15][16]. By varying thefrequency and amplitude of vibratory stimuli, the system
controls the perceived roughness that is presented to theoperators. In other words, the magnitude and speed of
cutaneous deformations are controlled.
III. ROUGHNESS-TRANSMISSION SYSTEM
A. Master-side
1) Generation of Tactile Stimuli Via Vibration: A method
for generating tactile stimuli at the master-side system is
described. When a human finger explores a roughness specimen with a surface wavelength λ at a velocity v, a vibration
with a fundamental frequency f = v/λ is perceived by thehuman finger. As a substitute for this stimulus, sinusoidal
deformations with frequency f are applied to the finger via
a vibrator. The voltage applied to the vibrator isy = A sin (2πf t) + B,

(1)

where A and B are the amplitude and bias, respectively.
The bias was 75 V, which was half of the maximal voltage
applied to the vibrator.
2) Tactile Display System: The vibrator is a piezo-stacktype actuator (NEC/TOKIN ASB510C801P0). The maximal
deformation generated by the vibrator is approximately 55µm for A = 75 V. The deformation changed linearly withthe applied voltage. As to the frequency characteristics ofthe vibrator, the gain reached -3 dB at approximately 310
Hz. The characteristic was relatively flat within the rangeof frequencies used for the tactile stimuli. The output forceof vibrator is 800 N, i.e., large enough so that the vibration
does not attenuate due to the finger force of the operator.
A block diagram of the tactile display system [8] is shownin fig. 2. The vibrator is installed on a linear slider whose

Fig. 3.
Setup

Image of the Master-side

Fig. 4.
Contact between the
Finger and the Vibrator

position on a linear guide is measured using an opticalencoder. A control computer receives the position of theslider and generates the value of the electric voltage tobe applied to the vibrator. The control frequency of thecomputer was 5 kHz.
Figure 3 illustrates an image of the tactile display device.
The operators held their right middle finger on the vibrator
and moved their hand along the linear guide (X-axis). The
vibrator generated the deformation along the Z-axis. The
operators were instructed not to touch any parts of their hands
to the equipment except the right middle finger, as shown infig. 4.
The users agreed that the vibrations transmitted by theequipment resembled the ones they perceived when they
explored rough surfaces such as grating scales through astylus.
B. Slave-side
1) Tactile Sensor: The tactile sensor of the developed
tactile-roughness-transmission system needs to estimate thespatial wavelength of texture surfaces. The estimation
method is described in subsection IV-A. In addition, thesensor needs to have an elasticity similar to that of humanfingers because the developed system determines the amplitude of vibratory stimuli presented to the operators from themagnitude of the sensor’s deformation.
The developed system installed a human-finger-like tactile
sensor [17]. Its structure is illustrated in fig. 6. The sensor has
the strain gauges as transducers embedded inside a silicone
rubber body. Ridges that emulate the epidermal ridges ofhuman fingers are arranged around the outer layer of thesensor. The width of the ridge is 0.6 mm. The strain gauges
are placed beneath the ridges and designed to sense the1468

C. Connection of Master and Slave Systems
The developed tactile-roughness transmission system comprises the master-side system and the slave-side systemdescribed in III-A, III-B, respectively. They are connectedby Ethernet. The tactile sensor in the slave-side system isposition-controlled by the position of the operator’s hand at
the master side. The communication rate was set to 250 Hz.

Fig. 5.

IV. ESTIMATION OF SURFACE WAVELENGTHSAND VIBRATORY AMPLITUDE OF TACTILESENSOR

Schematic Diagram of the Tactile-Sensor-Side System

As mentioned in II, the frequency f and amplitude A ofthe vibratory stimulus are controlled for displaying tactileroughness to the operators.
f is determined by the surface wavelength λ and the handvelocity v using the relationship f = v/λ. The tactile sensor
is required to estimate λ. The method for the estimation of
λ using the tactile sensor is described in IV-A.
A is determined from the amplitude of the strain signals of
the tactile sensor. The amplitude of the strain signals is also
described as a function of λ and v, g A (λ, v). The functiongA (λ, v) is experimentally identified in IV-B.
A. Surface-Wavelength Estimation by FFT
Fig. 6. Tactile Sensor used in the
Tactile Transmission System

Fig. 7. Trapezoidal Grating Scale:
Cross-sectional View and Overall
View

vibratory information of the ridges when the sensor contacts
an object. The dimensions of the sensor should be as large
as those of average human finger. However, currently, thesensor is three times as large as a human middle finger, andits thickness is 10 mm.
The wavelength-estimation method used in this study hasa generality that is applicable to other tactile sensors. However, the sensor is capable of estimating friction coefficients
between the target objects and their Young’s moduli. The
application of the sensor can be extended to the transmissionof perceived friction and softness, which the authors plan todevelop.
2) Slave-side System: A schematic view of the slaveside system is presented in fig. 5. The sensor is attached
to a single axial arm (YAMAHA MR12T) through a Zaxial slider. The arm is activated along the X-axis and iscontrolled by a computer. The outputs of the strain gauges of
the tactile sensor are transmitted to the AD board of control
computer by means of a strain amplifier. The strain signals
were sampled at 1 kHz.
The roughness specimens scanned by the tactile sensor are
grating scales with alternate grooves and ridges, as shown infig. 7. The ratio of RW (ridge width) to GW (groove width)is 1 and the surface wavelength λ is defined as RW + GW .
The tactile sensor is thrust 1 mm deep into the specimens
with the static reaction force in the normal direction being
approximately 0.6 N. However, this figure is not importantin this study, and it was confirmed that the sensor worked
properly with the pressing depth of up to 2.5 mm (2.1 N).

1) FFT-based Wavelength Estimation Method: When thetactile sensor scans the target texture, the sensor vibrates
because of collisions. The spatial wavelength of the object
can be estimated from the frequency of this vibration on thebasis of the relationship λ = v(t)/f (t), where f (t) is thevibratory frequency of the sensor at t, v(t) is the velocity ofthe sensor.
From the time sequential data of the strain outputs, FFT is
computed. The power spectrum density acquired by meansof the FFT reveals the vibratory frequencies of the sensor.
FFT-based estimation is suitable for the proposed tactileroughness transmission system because FFT deals withmultiple frequency components in real time. However, thisstudy transmits the fundamental frequency only. In order toextract a single fundamental frequency, the frequency withthe maximal power was defined as f (t).
The window size of the FFT was experimentally determined to be 32 ms. The size was selected from among thefollowing values –16, 32, 64 and 128 ms– so that the estimation error was minimal when the sensor scanned the grating
scales with surface wavelengths ranging from 0.5–3.0 mm
at an average exploratory velocity of humans. The averagevelocity was assumed to be v(t) = 197.0 sin (2π1.07t) mm/s
from a past study where the operators conducted a tactile
exploration of virtual textures using the same tactile display
device as that used in this study [8].
2) Experimental Validation of Estimation Method: Figure
8 illustrates the surface wavelengths estimated by the FFTbased method when the sensor scanned the roughness specimens at constant speeds. Three different scanning speeds
were used. They were the mean and mean plus-minus twotimes the standard deviation of the average exploratory velocity mentioned above, and their values were 73.5, 125.4, and1469

Fig. 8. Estimated Surface Wavelengths of Roughness Samples by FFTbased Method When the Sensor Scanned Samples at Constant Speeds

Fig. 9. Estimated Surface Wavelengths of Roughness Samples When the
Sensor Scanned Samples at Sinusoidal Velocity

177.4 mm/s, respectively. The sensor scanned the specimens
over a length of 175 mm. The figure indicates the average ofthe estimated wavelengths per ms. Apparently, the estimation
errors were larger when the sensor scanned the specimens
with wavelengths ranging from 2–3 mm at the minimal speedof v = 73.5 mm/s. This was due to the frequency resolutionof the FFT. The lower the vibratory frequency, the further
significant is the effect of resolution on the estimation error.
Figure 9 presents the estimation results when the scanning velocity was sinusoidal. The velocity was v(t) =197.0 sin(2π1.07t). The estimation was conducted on thebasis of the data acquired for five cycles, i.e., 4.665 s. The
dots in the figure are average values per ms and standard
deviations. The estimation was approximately correct except
for the wavelengths ranging from 2–3 mm as well as that at
the constant speeds.
B. Determination of Amplitude of Sensory Signal as a Function of Wavelength and Velocity
The function g A (λ, v), which determines the amplitudeof sensory outputs of the tactile sensor, was experimentally
determined. The amplitude of the output signal was obtained
from the FFT of the signals. The maximal power among thepower spectrum was used as the amplitude of the signalsof the sensory outputs. In order to determine the function,
the maximal signal power was recorded by varying λ and v
in the experiment. As a result, it was found that the signal
power was significantly affected by λ, but rarely affected byv. Therefore, the signal power was found to a function of

λ. The experimental conditions, results, and the determinedfunction are presented in detail in this following sections.
1) Experimental Conditions: In the experiment, the sensor
scanned the specimens at constant speeds. Then, the FFT ofgauge signals generated during the scanning was computed
and the maximal power among the power spectral density
was recorded. The scanning speed was in the range of 50–
230 mm/s, and the wavelength was varied in the range of
0.8–2.0 mm.
2) Experimental Results: Figure 10 shows the plots ofthe computed signal powers and scanning speeds for theroughness specimens with different surface wavelengths.
The signal power was recorded every 1 ms and averaged.
Contrary to our predictions, the signal power was not significantly affected by the scanning speed. Within the investigatedrange, the powers did not exhibit a specific trend respective tothe scanning speeds, and were comparatively flat. Hence, theeffect of the sensor speed on the signal power was ignored.
However, from a dynamic point of view, it should be notedthat the power spectral densities were found to vary due tothe stick-slip effect.
Figure 11 shows the relationship between the signal power
and the surface wavelength of the roughness specimens.
The error bars indicate the standard deviations among thescanning speeds. The relationship between the signal power
and the wavelength of the specimens is nonlinear, and thepower exhibits a local maximum for the wavelength of 1.2–
1.6 mm. The width of the distal ridges of the tactile sensor
is 0.6 mm, which is almost the same as the groove width ofthese roughness specimens. The local maximum is observed
only if these two parameters match. Therefore, it depends
on the dimensions of the tactile sensor, and the sensor needs
to be downsized in the future for improving the reality oftactile sensations. The relationship between the power andthe wavelength is approximated (R 2 = 0.94) by using afourth-order function as follows:gA (λ) = 30770λ4 − 162663λ3 + 309743λ2−248938λ + 72298.

(2)This equation is used to determine the amplitude of thevibratory stimuli presented to the operator in V. Since inthis study, the master-slave system does not have a degreeof freedom in Z-axis, the pressing force of the sensor to
the roughness samples is constant. In case that the proposed
method is extended to the system with Z-axial movements,
the pressing force needs to be involved in the local modeland (2).
V. EXPERIMENT: TRANSMISSION OF TACTILEROUGHNESS
An experiment to transfer the tactile roughness of thegrating scales to the operator is conducted. Three types ofexperiments are conducted.
In experiment 1, the roughness-transmission experimentis conducted while the proposed measure is applied to themaster-slave system. For comparison, experiments 2 and 3
are conducted when the proposed measure is not applied,1470

where Pmax was 6000 in order to adjust the output voltage
level to the maximam output voltage of the equipment.
B. Simulated Temporal Gap
A method for simulating the time delay between the hand
movements and tactile feedbacks in experiments 2 and 3 isdescribed. The delayed stimuli are produced from bufferedhand movements. The voltage applied to the vibrator is givenby
2πv(t − D)t
)+B
(6)
λ
where D is the simulated time delay. In experiment 2, D is
120 ms, which is two times the detection threshold of thetime delay [8]. In experiment 3, which emulates jitter, D is
subjective to a normal distribution whose mean and standard
deviation are 120 ms and 40 ms, respectively. D is sampled
every 200 ms, i.e., 5 Hz in experiment 3.
y(t) = A(λ) sin (Fig. 10. Power-v Plot: Relationships between Signal Powers and Scanning
Speeds by Surface Wavelength of Roughness Specimens

C. Task & Procedure

Fig. 11. Power-λ Plot: Relationships between Signal Powers and Surface
Wavelength of Roughness Specimens

and time delays are simulated between the hand movementsof operators and the tactile feedbacks. In experiment 2, thedelay is a constant value. In experiment 3, the delay varies,
emulating jitter. The manner in which these simulated timedelays are generated is described in V-B.
In order to equalize the quality of tactile stimuli in all typesof experiments, the stimuli are produced by the same method,which is described in V-A. Then, after the experiments, theseexperimental results are compared with each other.
The participants were eight staff and students of Tohoku
University who were in their 20s and 30s. All eight participants participated in experiments 1, 2, and 3. First, they
performed experiment 1. Then, they performed experiments
2 or 3. Four participants performed experiment 2 followedby experiment 3 while the others performed the experimentsin the opposite order.
A. Vibratory Stimuli
For the vibratory stimuli, f and A of the vibration were
controlled. The relationship f = v/λ determined f . The
amplitude of sensory outputs of tactile sensor determined A
using (2). The applied voltage y(t) to the vibrator wasA(λ)=

f (λ, v) =y(t) =

B
Pmax
λ/v,

gA (λ),A(λ) sin (2πf (λ, v)t) + B,

(3)
(4)
(5)

The participants explored the roughness samples at theslave side through the developed master-slave system. At thesame time, they were allowed to touch the grating scales
by their bare fingers as comparison stimuli. The numberof comparison stimuli was 12 and their wavelengths were0.4–2.6 mm. The participants chose the comparison stimulus
which was felt closest to the tactile roughness they perceived
through the master-slave system. They could report the
middle of the one stimulus and the another stimulus as theanswer. They were allowed to touch the comparison stimuli
whenever they wanted using their right hand or left hand.
Some of the participants touched the vibrator with their right
middle finger and touched the comparison stimuli with theirleft hand. The other touched the vibrator and the comparison
stimuli alternatively with their right hand. At the slave side,
seven types of stimuli were prepared. Their wavelengths were0.8–2.0 mm.
The one trial was limited to 20 s and the order of stimuli
presentation was random. Each stimulus was presented threetimes and the answers from the second and third trials were
employed as the formal answers. Before the experiment, theparticipants practiced for the task as long as they wanted.
Individual practice was approximately 5 min. The numberof participants was four. They heard pink noise throughheadphones.
D. Experimental Result
Figure 12 shows the results of experiment. The verticalaxis is the average wavelength of the comparison samples
that the participants reported, and the horizontal axis is thatof the presented samples. The error bars are the standard
deviations among the participants. The answers reached the
local peak at λ=1.2–1.6 mm, and drew a s-shaped curve. The
s-shaped curve reflected the profile of (2) and fig. 11.
Figure 13 shows the average and standard deviations ofcoefficients of correlation between the roughness samples
presented at the slave-side and the samples reported by theparticipants. In experiment 1, in which that the proposed1471

and were synchronized with the exploratory motions of theoperators.
A tactile-roughness transmission experiment was conducted using the developed system. The result indicatedthat the proposed method could more effectively transmit
the perceived roughness to the operators than the system inwhich the delay was not compensated.
R EFERENCES

Fig. 12. Results of Roughness-Transfer Experiment: Presented Roughness
Stimuli and Stimuli Selected by Participants

Fig. 13. Average Coefficient of Correlationship Between the Presented
Roughness Samples and Reported Roughness

measure was applied, the participants’ answers had strong
correlation with the presented samples (r = 0.83). Experiment 2, a constant delay existed, exhibited the second highest
coefficient. Experiment 3, jitter was simulated, exhibited thelowest coefficient. Welch’s t-test showed significant difference between experiment 1 and 3 (t = 2.70, df = 10.13,
p = 0.027, one-tailed).
From above experimental results and analysis, the operators could most accurately experience the roughness of textures presented at the slave-side, when the delay between the
exploratory motions and tactile feedbacks was cancelled by
the proposed method in experiment 1. Also, according to theinterviews after the experiments, the participants recognized
the existence of the delay, and the tactile feedbacks wereunnatural in experiments 2 and 3. Thus, it was experimentallyconfirmed that the proposed method can transmit roughness
sensations of textures through teleoperation with a communication time delay.
VI. CONCLUSION
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