Anticipatory vibrotactile cueing facilitates grip force adjustment
Shogo Okamoto

Michael Wiertlewski

Vincent Hayward

Department of Mechanical Sci. and Eng.
Nagoya University, Nagoya, Japan

Department of Mechanical Eng.
Northwestern University, Evanston, USA

UPMC Univ Paris 06, Institut desSystèmes Intelligents et de Robotique,
Paris, France

A BSTRACT

encoder

Human grip forces are automatically adjusted upon occurrence ofan external disturbance experienced by an object that is held by
a thumb and index finger. We investigated some of the cues that
may be used by the brain to perform rapid grip restabilization. To
this end we ask subjects to grip and hold an instrumented and actuated parallelepiped-shaped handle between the index finger and thethumb. Under computer control, the handle could be jerked fromthe still grip and could independently provided vibration of 250 or
100 Hz to the gripping fingers. We found that the latency of the
motor corrective action was 139 ms on average, but when a vibrotactile stimulation was applied 50 ms before the application of thepulling force, the latency was reduced on average to 117 ms. The
average latency of the conscious response to the vibrotactile stimuli
was 230 ms, suggesting that vibrotactile stimulation was capable ofinfluencing the reflex action.
1B) Differential capstan drive

grip handle
A) Overall viewnormal force sensor

I NTRODUCTION

Grip force adjustment is automatically performed during manipulation. This adjustment is largely unconscious and is particular usefulin the case of rapid, unexpected variations in the loading and unloading of the grip [7]. The result is an automatic stabilization of
the grip preventing slip, yet avoiding overly large grip forces to be
employed. To achieve this function, it is likely that several populations of mechanoreceptors provide information to multiple spinal
and supraspinal sensorimotor loops operating at different levels of
the nervous system. Among these populations, mechanoreceptors
associated with cutaneous inputs play an important role since digital
anesthesia results in substantial attenuation of the gripping sensorimotor corrective behavior [5, 2, 1], where an increase of the grip
force could be interpreted as a strategic response of the nervous
system to weakened inputs [3, 14]. Also, vibrotactile stimulation
to the finger pads affect the grip force when the gripped object experiences perturbation [12]. It can be concluded from these studies
that cutaneous inputs are able to provide early-warning signals, inadvance of the catastrophic failure of a grip. Grip force adjustment
is an intriguing form of behavior that one would desire to replicate
for robotic [16, 20, 10, 17], and rehabilitation purposes [8, 9].
The shortest latency between the onset of a load and grip force
adjustment was found to be approximately 70 ms [2, 7] but according to the loading conditions, this value can vary up to 170 ms [6].
We focus here on grip force adjustment from the aspect of interface design for a more stable human grasp. Prior studies havelooked at the effect of added stimulation during the change of gripof load [12, 15]. From these studies it can be concluded that additional stimulation can modify the subsequent gripping behavior.
Here, our hypothesis is that stimulation of the fingertips canshorten the latency of the onset of grip force adjustment. To test thishypothesis we organized experimental conditions where the fingertips could be cued ahead of the slip-inducing, sudden load increase.
In an attempt to segregate the mechanical cues that could be de-IEEE World Haptics Conference 201314-18 April, Daejeon, Korea
978-1-4799-0088-6/13/$31.00 ©2013 IEEE

handle accelerometerleaf suspension
tangential force sensor
nail accelerometer
grip surface
recoil actuators
normal force sensor
C) Side view of grip handle

D) Perspective view

Figure 1: Apparatus including an instrumented grip handle and apulling mechanism. A: Overall view. B: Differential capstan drive withtwo motors. C: Side view of the grip handle. D: Perspective view ofthe grip handle.

terminant in provoking a grip adjustment, we also measured thefingertip bulk deformation during behavior.
Finally, this is one of the paired articles on the study of vibrotactile stimuli and their effects on grip force adjustment. Anotherarticle mainly discusses vibrotactile cues induced by slippage [19].
2

M ATERIAL AND M ETHODS

2.1 Apparatus
Figure 1 shows the experimental setup. The main component wasa grip handle to be held between the thumb and the index fingers
and driven by two DC motors (2642W012CR, Faulhaber) in tandem using a differential capstan mechanism. Such mechanisms canachieve arbitrarily large bi-directional traction for an arbitrarily low
pre-tension in the cable, thus insuring smooth operation and minimal friction losses [4]. The present system provides 20 N of continuous pulling force for less than 0.1 N of friction. Position was525

2.2 Participants, Task, and Stimuli
Five volunteer participants, which excluded the authors, from theresearch staff of the institution joined the experiments. During
the experiments, they wore headphones playing pink noise to maskout the faint sounds emitted by the vibration actuators and by thepulling motors.
The participants sat on a chair and held the grip handle between
their thumb and index fingers. They were instructed to hold the grip
handle in a relaxed manner with a natural grip sufficient to maintainit under gravity loading. They were asked to “hold the grip handle
such that you do not lose it, and respond to the vibrations as quickly
as possible.”
In some test conditions, after a randomized a time period (2–5 s),
an impulsive command could be sent to the motors and the grip handle was rapidly accelerated upward. The result of this acceleration
was a sudden increase of the load applied to the fingers, see Fig. 2
for a typical example. The interaction force peaked on average at
5 N, 130 ms after the onset of the command sent to the motors andthe force rate reached 38.5 N/s on average, reflecting the dynamics
of the handle-grip mechanics. In certain test conditions, vibrotactile stimulation was applied to the participants’ thumb and index
fingers. The magnitude of vibration was approximately ±10 m/s2 .
2.3 Testing Conditions
We prepared for four conditions that differed according to the loading and vibrotactile stimuli. The participants experienced these four
conditions in practice phases that lasted a few minutes. During themain task, each condition was presented ten times. In total, for individual participant, forty trials were performed in a randomizedorder. After each trial, the participants released the grip handle andrelaxed. Upon visual cueing, the participant gripped the grip handle
and conducted the next trial.
Loading only (Condition 1). The load was suddenly increased
as described previously.

526

vibration [m/s ] load traction [N]5.0A) Traction
tractionincrease

010B) Vibrotactile cue

0C) Grip force

-10
6.0

load traction [N]displacement [mm] grip rate [N/s]measured by a potentiometer (Baumer, BTIV 24S 16.24K, resolution: 0.0146 mm). The motors were commanded and the data wascollected through a multifunction DAQ card at a sampling rate of
10 kHz (PCI-6221, National Instruments).
The grip handle had three uni-axial force sensors organized asfollows. The normal grip force component was measured by twoforce sensors mounted in the load paths between the two plates
at each end of the handle (X-axis: 9313AA1, Kistler). The plateinteracting with the index included a leaf suspension mechanismsthat separated out the tangential component of the interaction force
which was measured by a high resolution, high bandwidth force
sensor (Z-axis: 9217A, Kistler). The fingers were in contact withsmooth, sanded finish plastic surfaces.
Two accelerometers were used to measure the deformations offinger pad during loading. The acceleration of the grip handle wasmeasured by a miniature solid state accelerometer (MMA7260QT,
Freescale Semiconductor, nominal bandwidth: 350 Hz) and theacceleration of the nail of the index finger measured by a second miniature accelerometer (ADXL300, Analog Devices, nominal
bandwidth: 1600 Hz). The differential acceleration gave a measureof the deformation of the finger, bulk strain rate by a first step ofintegration and bulk deformation by a second step of integration.
Because of the short time window and in the absence of slip, a precise estimate of the finger can be obtained this way. Moreover, full
slip can be detected when the rate and amplitude of the deformation
exceed thresholds.
Two recoil-type actuators (Haptuator Mark 2, Berrezag Amir)
could vibrate the grip handle. They were mounted orthogonally toloads paths of the force sensors to minimize interference with theforce measurements.

grip rate

grip
increase

060D) Grip force rate
20 N/s040E) Displacementof handle
maximum
0
-50

0

100

200
time [ms]

300

400

Figure 2: Time course of the loading stimulus, vibrotactile stimulus,
motor response and net grip handle displacement. A: Traction loading force component reaching a maximum, typically at 130 ms after
the torque command to the motors. B: Vibrotactile stimulus. C and D:
grip force response and rate. E: Net displacement of the grip handle,
reaching a maximum at a different time from the load due to complex biomechanical dynamics coupled to the inertial dynamics of the
motor/handle subsystem.

Vibrotactile stimulation only at 250 Hz (Condition 2). Vibrotactile stimulation at 250 Hz was produced as described previously, but not followed by sudden loading. The aim of this condition was to obtain and estimate the latencies of conscious response
to vibrotactile stimulation.
Load and vibrotactile stimuli at 250 Hz. (Condition 3).
Vibrotactile stimulation at 250 Hz was presented 50 ms before theonset of loading to the participants. The aim of this condition was toobserve the effects of vibrotactile stimuli on grip force adjustment.
Load and vibrotactile stimuli at 100 Hz. (Condition 4).
This condition was the same as Condition 3 with the difference thatthe vibration frequency was 100 Hz. The aim was to investigate
whether there was some frequency specificity visible in the motor
response given the likely contribution of the Pacinian receptor population.
Follow-up trials (Condition 5). After forty trials in Conditions
1–4, we repeated tests in Condition 2 for ten trials in order to evaluate the conscious responses to vibrotactile stimuli. In these followup trials, the participants were informed that only vibrotactile cues
would be presented to them, and had to respond to them as quickly
as possible.

Condition

Table 1: Averages and standard deviations of three parameters
among all participants. ‘ld’ and ‘vib’ indicate load and vibrotactile
cue, respectively.

1 (ld only)
2 (vib only)
3 (ld + vib)
4 (ld + vib)
5 (vib only)

Latency[ms]
139 ± 30229 ± 97116 ± 23117 ± 24
172 ± 55

Grip increase [N]
8.76 ± 4.21–
9.91 ± 4.55
10.5 ± 4.88–

Disp. grip
handle [mm]32.4 ± 17.0–
32.0 ± 15.0
30.5 ± 13.8–

2.4 Data Analysis
Several data can be extracted from these experiments. Figure 2
shows typical records of the time course of events, including traction loading, grip force response and the net displacement of thegrip handle. If the command is applied at time, t = 0, approximately
100 ms later, the onset of grip force adjustment was typically observed. Using these data, three types of parameters were evaluatedfor each trial. The largest and smallest values of these parameters
among those of each condition of individual participants were automatically pruned, and then the rest were used for statistics.
The latency between traction force and the grip force adjustment
was defined at the time difference of their respective onsets in Conditions 1, 3, and 4. For conditions 2 and 5, this value was defined
as time difference between the end of the vibrotactile cue and theonset of grip force adjustment. In Conditions 3 and 4, the end of thevibrotactile cue and the beginning of the rise in traction force coincided, as a result of the very high stiffness of the mechanical transmission between the motors and the handle. We are then allowed
to compare the responses among conditions using the same temporal baseline. The derivative of grip force signals was computedby finite difference after FFT filtering with a cut-off frequency of
100 Hz. An example can be seen in Fig. 2. The threshold as 20 N/s
was used to mark the beginning of grip force change. This threshold is approximately two to three times the noise levels. For sometrials, the grip force change rate did not reach this threshold level.
These trials were excluded from the reports.
The increase in grip force was the difference in grip force between its maximum and the average of the last 100 ms before theonset of the traction force, or the end of the vibrotactile stimulation. The grip force values were filtered in the way same as justdescribed. This value is known to depend on conditions of perturbative loads, and has been used as an important parameter to characterize grip force adjustment. Also, the magnitude of grip force isdirectly related to stable grasping.
The maximum deflection was defined as the difference in theposition of the grip handle between the highest value after the force
stimulus and the average of 100 ms before the onset of traction.
Unfiltered position signals from the encoder were used to calculate
this value. It is also an important parameter to evaluate the successof grip force adjustment. Smaller values indicate a more successful
recovery from disturbance.
3

R ESULTS

3.1 Latency of grip force adjustment
As a whole, the latencies between the pulling force and grip
force adjustment, vibration stimulation only (Condition 2) clearlyrecorded the largest values among four conditions. Condition 2
was, then, followed by the loading only condition (Condition 1) thatwas potentially significantly larger than the condition with combined loading and vibration at 250 Hz and at 100 Hz (Conditions
3 and 4). The latencies in Conditions 3 and 4 were nearly indistinguishable.

Figure 3: Latency between pulling force and grip force adjustment.
Averages and standard diviations among trials. Top) Entire trials.
Bottom) By participants P1–P5. Dots correspond to conditions 1–
4 from left to right for individuals. **: p < 0.01 by Tukey test. +++:
p < 0.001 by t-test.

Figure 3 (top) and Table 1 show the average and standard deviation values among all trials by condition. Figure 3 (bottom) shows
the latencies for each trial by participants. The latencies of the vibration only condition (Condition 2) were clearly larger than thoseof the other conditions with an average value of 229 ms hence of
279 ms from the onset of the cue. The latencies of load only condition 1 (Condition 1) recorded the second highest values. The otherconditions followed.
The Tukey test showed significant differences between Condition 2 and the others (conditions 1 and 2: q(3, 145) = 7.22,
p < 0.01, conditions 2 and 3: q(3, 145) = 9.34, p < 0.01, conditions 2 and 4: q(3, 145) = 9.23, p < 0.01). We also applied pairwised t-tests to the pairs of conditions, which additionally indicatedsignificant differences between conditions 1 and 3 (t(73) = 3.89,
p < 0.001, two-tailed) and conditions 1 and 4 (t(73) = 3.59, p <0.001, two-tailed). As shown in Fig. 3 (bottom), these trends wereobserved for all individuals. For P3, the latencies of condition 2
seem smaller than those of condition 1. However, note these areconservative estimates and that we should add 50 ms to the valuesof condition 2 when comparing the latencies of conditions 1 and 2.
3.2 Increase in grip force
In term of the increases in grip forces, no statistically significant differences were found among conditions. However, as a weak trend,
the presence of vibration seemed to provoke a larger increase ofgrip force of about 1 to 2 N (Condition 3 and 4). Fig. 4 (top) andTable 1 show the average and standard deviation values of all trials

527

16.0
condition 3

grip force [N]condition 4
12.0
condition 1
8.0

condition 2
4.0

force rate [N/s]080.0condition 1

condition 3

condition 4
40.0
condition 2

displacement [mm]020.0condition 4
condition 3

10.0
condition 1
condition 200Figure 4: Increase in grip force. Averages and standard diviations
among trials. No statistically significant differences were exhibitedamong conditions.

by conditions. Fig. 4 (bottom) shows the results by individual participants. The vibration only condition (Condition 2) was excluded.
On average, the increases in grip forces were the smallest in theload only condition (Condition 1), followed by the load combinedwith vibrations conditions (Conditions 3 and 4).
As shown in Fig. 4 (bottom), for three participants among five
(P1–P3), these trends held. For P5, the increases in grip force werehighest under condition 3, and those of conditions 1 and 4 were approximately equal. For P4, condition 4 exhibited the highest values,and conditions 1 and 3 recorded similar values on average.
3.3 Handle Displacement
Table 1 shows the averages and standard deviations of the displacements of grip handle. These values did not significantly vary among
the conditions.
3.4 Detailed analysis of the results for one participant
Fig. 5 shows the average time course of the grip force, grip force
rate, and the displacement of the grip handle for participant P5.
These profiles are the averages of ten trials for each condition. Data
from each trial referenced to zero time and displacement. As shownin the top panel figure, for this participant, the increase in grip force
was the largest in the condition where the loading and a vibration
at 250 Hz were both present (Condition 3). This increase value wasfollowed by the values obtained in the condition of loading only
(Condition 1) and loading and vibrations at 100 Hz (Condition 4).
The latencies (top and middle panels), obtained with loading plus
vibration, were the shortest but comparable (conditions 3 and 4),
followed by the loading only condition and lastly with vibrations
only (Condition 2). As shown in the bottom of Fig. 5, while there
was no unified tendency among participants, for this participant the

528

400
time [ms]800

Figure 5: Average results of one participant. Top) Grip force. Middle)
Derivative of grip force. Bottom) Position of the grip handle. Condition 1 is load stimulus only. Condition 2 is vibrotactile stimulus only.
Condition 3 is load + vibrotactile stimuli (250 Hz). Condition 4 is load
+ vibrotactile stimuli (100 Hz).

displacements under Condition 4 were the largest. As to the displacement of grip handle, while there was no unified tendencies
observed between the participants, for this participant, the presenceof vibrations made a noticeable difference.
3.5 Finger Pad Deformation
Some interesting data was obtained from the measurement of differential acceleration between the nail and the contact surface, expressing the mechanical loading applied to the finger. Figure 6(top) shows the accelerations of finger deformations for three participants. Each plot is the average of ten trials under the loading
only condition (Condition 1) after low-pass filtering at 200-Hz.
The middle panel shows the bulk deformation after two steps ofintegration of the differential acceleration signal. There is clearlysome amount of drift in the estimates due to imperfect calibrationof the accelerometers, yet the profile reveal quite different finger
mechanics from one subject to another. The particular examples
were selected to show the great differences in bulk deformation of
three fingers. The relation to the tangential load profiles shown inthe bottom panel shows complex dynamics that could be the resultof combined nonlinear biomechanics and neural response.
Focusing on the initial portions of the responses, the finger pad
deformation started approximately 15 ms later than the onset of the
traction force. These times of onset were very similar among all theparticipants. Furthermore, the initial slopes of acceleration values
were also comparable among them. They reach their first maximums around 30 ms later the onset of the pulling force. Then, theydiffered significantly among the participants. Similarly, the profiles

acceleration [m/s ]

1.0
P4

P2

P3

0

-0.1
4.0
deformation [mm]

P2

2.0
P4
0

P3

tangential loading [N]

1.5

P4
0
P2

-1.5

P3
0

100
time [ms]200

Figure 6: Finger pads’ deformations and their tangential forces. Top)
Acceleration of the deformations. Middle) Displacements. Bottom)
Tangential forces.

of tangential forces were very similar for the first 30 ms for all theparticipants and differed later.
It can be surmised from these observations that the initial dynamics of the interaction are dominated by the tissue mechanics
characterized by the typical nonlinear stiffening of the finger tissues
undergoing large deformation [13, 18] and that this initial phase of
30 ms is followed by mechanical events precipitated by the neural
response that can vary greatly among individuals. For example, one
participant recorded a maximum pad deformation of approximately
4 mm on average, but for another participant this deformation was
smaller than 1 mm. The mechanics of the fingertip depend, not only
in tangential loading, but also on normal loading. Another source ofdifferences were the grip forces before and after application of the
disturbances ([base-line → increase of grip force] = [3.3 → 2.8],[3.8→15.7], [5.3→8.3] N for P2, P3, and P4, respectively) which
could affect the biomechanical as well as the neural response. P2’s
grip-force increases were the lowest but the finger deformation was
the largest among the participants. P3’s grip-force increases weregreatest but the finger skin deformations were lowest. It is however
risky to draw any conclusion from this apparent correlation.
4 D ISCUSSION AND C ONCLUSION
Under our experimental conditions, the response latencies to a sudden traction loading pulling force were 139 ± 30 ms on average(Condition 1). Our definition of the latency is conservative and la-tencies and the neural response to disturbance is likely to be much
shorter since the mechanics are expected to lag the neural activation of the muscles. Give or take the differences in the definitionsof what latency is, our results are in line with those of the literature [2, 7, 6]. These value are also known to vary in a wide range (70to 170 ms) with the nature of the cueing given by particular typesof disturbances owing to the differential engagements of spinal andsupraspinal sensorimotor control loops.
Briefly, 50 ms vibrotactile cueing could influence the onset ofgrip force adjustment of a sudden unexpected change of load by
23 ms on average from a baseline value of 139 ms. This can becompared to the 229 ± 97 ms response to the same vibrotactile cue
presented in the absence of loading disturbance, which can be interpreted as a conscious reaction time to a vibrotactile cue or to the
172 ± 55 ms of reaction time when the cue was anticipated. It isthus permitted to conclude that the participant fast behavior was
not the result of a conscious reaction and that the vibrotactile cues
did modify their reflex reaction.
Mrotek et al. [11] pointed out the contributions of preparatory
activities to grip force adjustment. They compared the latenciesof grip force adjustment obtained during holding to those duringlifting-object phases. They reported that the latencies in latter phasewere smaller. During the lifting phase, tactile activities of finger
pads are comparatively higher owing to the presence of inertial
forces, raising the mechanical loading and thus the level of sensory
inputs. They also found that the lifting phase led to larger increases
in the grip forces. Here, we stimulated the tactile receptors using
vibrotactile cue before the onset of the perturbative loading. The
latency of grip force adjustment decreased and the grip forces has atendency to increase. These effects could be related.
Finally, we did not find any significant differences between thetwo frequencies: 250 and 100 Hz. However, this does not lead to aconclusion that there is no specification of mechanoreceptor classes
because the frequency tuning method does not always insure theactivation of specific receptor class.
We also could estimate the finger pad deformation in the courseof a sudden disturbance load. Interestingly, the initial phase of mechanical response did not vary significantly among the few individuals that we tested, that is during the first few tens of milliseconds
after the onset of the disturbance. On the other hand, the mechanical responses vary visibly among individuals, suggesting different
sensorimotor strategic neural responses to similar inputs.

ACKNOWLEDGEMENTS
The authors wish to thank Dr. Alexander Terekhov for discussing
the experimental procedures and results, Sheng-Chao Wong, andRafał Pijewski for mechanical and electrical design of the equipment. The authors also thank Mark Farrington for input on theexperimental platform. This work has been supported by the Technical Committee on Haptics, Student Exchange Program. MW and
VH acknowledge the support of European Research Council, Advanced Grant PATCH, agreement No. 247300.
R EFERENCES[1] A.-S. Augurelle, A. M. Smith, T. Lejeune, and J.-L. Thonnard. Importance of cutaneous feedback in maintaining a secure grip during manipulation of hand-held objects. Journal of Neurophysiology,
89(2):665–671, 2003.
[2] K. J. Cole and J. H. Abbs. Grip force adjustments evoked by load
force perturbations of a grasped object. Journal of Neurophysiology,
60(4):1513–1522, 1988.
[3] J. R. Flanagan and A. M. Wing. The role of internal models in motion
planning and control: Evidence from grip force adjustments during
movements of hand-held loads. Journal of Neuroscience, 17(4):1519–
1528, 1997.

529

[4] V. Hayward, P. Gregorio, O. Astley, S. Greenish, M. Doyon,
L. Lessard, J. McDougall, I. Sinclair, S. Boelen, X. Chen, J.-P. Demers, J. Poulin, I, Benguigui, N. Almey, B. Makuc, and X. Zhang.
Freedom-7: A high fidelity seven axis haptic device with application
to surgical training. In A. Casals and A. T. de Almeida, editors, Experimental Robotics V, volume 232 of Lecture Notes in Control andInformation Science, pages 445–456. Springer Verlag, 1998.
[5] R. S. Johansson, C. Häger, and L. Bäckström. Somatosensory control
of precision grip during unpredictable pulling loads. III. Impairments
during digital anaesthesia. Experimental Brain Research, 89:204–213,
1992.
[6] R. S. Johansson, C. Häger, and R. Riso. Somatosensory control ofprecision grip during unpredictable pulling loads. II. Changes in load
force rate. Experimental Brain Research, 89:192–203, 1992.
[7] R. S. Johansson and G. Westling. Roles of glabrous skin receptors andsensorimotor memory in automatic control of precision grip when lifting rougher or more slippery objects. Experimental Brain Research,
56:550–564, 1984.
[8] P. J. Kyberd and P. H. Chappell. Characterization of an optical andacoustic touch and slip sensor for autonomous manipulation. Measurement Science and Technology, 3(10):969, 1992.
[9] P. J. Kyberd, M. Evans, and S. Winkel. An intelligent anthropomorphic hand, with automatic grasp. Robotica, 16(5):531–536, 1998.
[10] T. Maeno, S. Hiromitsu, and T. Kawai. Control of grasping force
by detecting sticwslip distribution at the curved surface of an elastic
finger. Proceedings of IEEE International conference on robotics andautomation, pages 3895–3900, 2000.
[11] L. A. Mrotek, B. A. Hart, P. K. Schot, and L. Fennigkoh. Grip responses to object load perturbations are stimulus and phase sensitive.
Experimental Brain Research, 155:413–420, 2004.
[12] M. Nakamoto, M. Konyo, T. Maeno, and S. Tadokoro. Reflective
grasp force control of humans induced by distributed vibration stimuli
on finger skin with ICPF actuators. Proceedings of IEEE International
conference on robotics and automation, pages 3899–3904, 2006.
[13] N. Nakazawa, R. Ikeura, and H. Inooka. Characteristics of humanfingertips in the shearing direction. Biological Cybernetics, 82:207–
214, 2000.
[14] D. A. Nowak, J. Hermsdörfer, G. S, J. Philipp, L. M., and N. Mai.
The effects of digital anaesthesia on predictive grip force adjustments
during vertical movements of a grasped object. European Journal of
Neuroscience, 14:756–762, 2001.
[15] T. Sakurai, M. Konyo, S. Okamoto, and S. Tadokoro. Research ofconditions of stimulus for inducing grasping force control reflex. Proceedings of the 2000 IEEE/SICE International Symposium on System
Integration, pages 408–413, 2010.
[16] M. R. Tremblay and M. R. Cutkosky. Estimation of friction using
incipient slip sensing during a manipulation task. Proceedings ofthe 1993 IEEE International Confernce on Robotics and Automation,
1:429–434, 1993.
[17] J. Ueda, A. Ikeda, and T. Ogasawara. Grip-force control of an elastic
object by vision-based slip-margin feedback during the incipient slip.
IEEE Transactions on Robotics, 21(6):1139–1147, 2005.
[18] Q. Wang and V. Hayward. In vivo biomechanics of the fingerpad skinunder local tangential traction. Journal of Biomechanics, 40(4):851–
860, 2007.
[19] M. Wiertlewski, S. Endo, A. Wing, and V. Hayward. Slip-induced
vibration influences the grip reflex: A pilot study. Proceedings of the2013 IEEE World Haptics Conference, 2013.
[20] Y. Yamada, H. Morita, and Y. Umetani. Vibrotactile sensor generating
impulsive signals for distinguishing only slipping states. Proceedingsof the 1999 IEEE/RSJ International Conference on Intelligent Robots
and Systems, pages 844–850, 1999.

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