Springer Series on Touch and Haptic Systems

Massimiliano Di Luca Editor

Multisensory
Softness
Perceived Compliance from Multiple
Sources of Information

Chapter 3

Vibrotactile Sensation and Softness
Perception
Yon Visell and Shogo Okamoto

3.1 Introduction
This chapter describes how mechanical vibrations can affect the perception of several
material and surface properties, with an emphasis on the perception of compliance.
Vibrations are fluctuations of force or displacement. They are generated by interactions with objects and as such they are produced during numerous human activities.
They accompany, for example, frictional sliding of surfaces, tapping, rolling movements, displacement and compression of granular and aggregate materials, fracturing
and breakage processes.
Prior research has demonstrated that the haptic channel is sensitive to vibrations,known as the vibrotactile sense, and can be used to discriminate touched surfaces ofobjects to extract properties such as roughness or surface regularity, and to identify
events, such as contact onset and contact slip. Despite its importance, the vibrotactile sense has received little attention to date as a potential cue for compliance
perception, especially when compared to other haptic perceptual channels or cues,
such as cutaneous contact area, proprioception, and kinesthesia. One reason can betraced to the contact mechanical origin of vibromechanical signals which consistof high-frequency fluctuations in force or displacement. In many of the mechanicalinteractions listed above, vibration energy is produced through impacts between thesurfaces of objects at a macroscopic scale or through interaction between surface
microgeometry (asperities) as it happens during sliding friction (Akay 2002). In
such cases, materials with high stiffness yield wide frequency bandwidth transientor sustained signal elements. In contrast, for compliant objects the effective stiffness

Y. Visell (B)Drexel University, Philadelphia, PA, USAe-mail: yonvisell@gmail.com
S. Okamoto
Nagoya University, Nagoya, Japan© Springer-Verlag London 2014
M. Di Luca (ed.), Multisensory Softness, Springer Series on Touch and Haptic Systems,
DOI 10.1007/978-1-4471-6533-0_3

31

32

Y. Visell and S. Okamotoof the impacting structures is low and the material may also be more damped. These
properties yields only low energy at high frequencies during interaction.
Nevertheless, several studies have suggested that vibromechanical signals caninfluence perception during object palpation for a wide range of object compliances(Giordano et al. 2012; Kobayashi et al. 2008; Ben Porquis et al. 2011; Takahiro
et al. 2010; Okamoto 2010; Ikeda et al. 2013; Rust et al. 1994; Okamura et al. 2001;
Kuchenbecker et al. 2006; Kildal 2010, 2012; Visell et al. 2011). It is well established that high-frequency mechanical vibrations generated during manually tapping,
scraping with a probe, or scanning with a finger can influence the perception of properties such as hardness (as reviewed below) and roughness (Klatzky and Lederman
1999; Hollins and Risner 2000; Bensmaia and Hollins 2003, 2005; Klatzky et al.
2003; Okamura et al. 1998). For example, amplifying vibrations generated during
manual surface scanning, or imposing sinusoidal vibrations, increases perceived surface roughness (Hollins et al. 2000). Thus, it is reasonable to ask whether there exist
high-frequency cues that are capable of influencing compliance judgements.

3.1.1 Vibrotactile Sensory Information
Vibromechanical stimulation of the skin affects both cutaneous receptors and receptors embedded in deep tissues, including muscles and tendons (Ribot-Ciscar et al.
1989; Freeman and Johnson 1982; Johnson 2001; Vedel and Roll 1982). The former stimulation include fast-adapting (FA) mechanoreceptors sensitive to phasic
signals, in the form of Meissner and Pacinian corpuscles. These mechanoreceptive
channels respond to transient or high-frequency mechanical stimuli. Also present
are slower adapting (SA) mechanoreceptors that respond primarily to tonic signals
produced by sustained or slowly-varying mechanical stimuli. In previous studies, thevibrotactile sense has been particularly associated with FA receptors. Mechanoreceptive afferents, which communicate the neural result of mechanical stimuli to thecentral nervous system, have been associated with receptive fields near to receptors that they terminate on. The size of the respective receptive fields for FA or SA
mechanoreceptive afferents can range from a few square millimeters to several squarecentimeters, depending on the receptor type, innervation density, and biomechanicalfactors. Among physiologically identified FA receptors, Meissner corpuscles havesmall receptive fields, while Pacinian corpuscles lie deeper in the skin and possesslarger receptive fields. The skin is sensitive to vibrotactile stimuli over a broad rangeof frequencies, up to nearly 1,000 Hz. Meissner corpuscles respond preferentiallyto vibrotactile signals in the range from 10 to 100 Hz, while Pacinian corpuscles
respond to higher frequencies, but neither type exhibits narrow frequency-selective
tuning like that present in the auditory system. In glabrous skin, which is found on
the volar surface of the hand and feet, FA afferents comprise about 70 % of the
cutaneous population. Vibrotactile sensitivity, measured in terms of the absolute ordifference threshold for detection, varies as a function of body location and stimulus
properties including contact conditions and frequency. Surveys of tactile sensitivity

3 Vibrotactile Sensation and Softness Perception

33

at different body locations and for different stimulus parameters, including frequency
and amplitude of stimulation, can be found in the following reviews (Morioka et al.
2008; Morioka and Griffin 2002; Verrillo 1966).

3.2 Contact Mechanics and Softness Cues
Haptic compliance perception involves discerning the deformability of objects
touched with the hand or foot, or even objects felt using a tool. As discussed in
Chap. 1, compliance C = 1/k can be quantified in terms of mechanical stiffness k,which in turn depends on the Young’s modulus and geometry of the material. In thesimplest case, the deformation x of a material can be described via a linear, quasistatic relation between force F = −kx and displacement x, or between continuum
mechanical quantities of stress σ = −Eε and strain ε.
The problem of softness perception consists of using haptic sensations to perceptually recover the compliance or material elasticity of an object. Thus, the notionof softness involves the extraction of object properties from stresses and strains, orforces and displacements, that are felt during exploration. Most prior research has
investigated compliance perception via manual touch (Harper and Stevens 1964;
Scott-Blair and Coppen 1940; Freyberger and Färber 2006; Tan et al. 1995; Srinivasan and LaMotte 1995; LaMotte 2000; Friedman et al. 2008; Bergmann Tiest
and Kappers 2009). However, the haptic perceptual system is able to discriminate
objects of different compliance in a multitude of ways, including touching with atool (LaMotte 2000) or with the hand or foot (Giordano et al. 2012; Kobayashi et al.
2008).
The perceptual system is capable of judging softness in different ways dependingon the information that is available. The contributions from different sources depend
upon the actions, tasks, or exploratory procedures being performed, the propertiesof the objects (their material composition, geometry, and microgeometry), and thecontact mechanical setting involved. Their integration thus must account for such
sources of variability and should proceed accordingly (see Chap. 5 for a model). The
same can be said about what kind of vibrations are available during different typesof interactions and thus how vibrotactual information could be used for softness
perception.
It is useful to consider four basic interaction patterns, which are represented
in Fig. 3.1. They consist of direct skin contact with a compressed elastic object,indirect skin contact with such an object, transient contact with a touched object, andfrictional sliding. Through these, it is possible to gain some insight into potential rolesof vibrotaction in softness perception. In the next four sections we will analyse thevibrotactile information available in each of these interactions and what experimental
results are available about the perception of softness.

34

Y. Visell and S. Okamoto
DirectContact

IndirectContact

TransientContact

Frictional
Sliding

Fig. 3.1 Four prototypical scenarios where vibrotactile information is available and can potentially
specify object softness: a Compressing a soft object with direct finger contact; b Compressing suchan object via a rigid intermediary; c Producing a short, ransient contact through tapping; d Sliding
the finger across the object surface

3.2.1 Direct Skin Contact
The first interaction type we consider involves direct contact between the skin andan elastic, compressed object, as shown in Figure 3.1a. This setting can be modelled using the Hertzian theory of non-adhesive linear elastic contact between twocompressible bodies (Johnson 1995). One body consists of the skin and pulp of thefinger, and the other constitutes the touched object. As the total normal force appliedby the finger increases, the area of contact between the bodies grows. Simultaneously, the skin and underlying tissues deform, as can be quantified by an increase instrain energy density near the contact region. This gives rise to at least two potential
perceptual cues (Bicchi et al. 2000; Scilingo et al. 2010) (see Chap. 11):
1. The rate of increase of contact area between finger and object surface with thenormal force between finger and object surface
2. The rate of increase in strain energy in the volume of the finger near the contact
region with normal force
The rate of increase in normal force can often be assumed to be slow, and the accompanying dynamics to be damped, due to the highly viscoelastic nature of the materials
involved. Often the mechanics can be modelled as quasi-static. In such cases, transient mechanical signals can be presumed to be insignificant. Commensurate withthis assumption, it could be hypothesized that, at a physiological level, the neural
input from slowly-adapting (SA) afferents from the finger provide most sensory
information, while inputs from fast-adapting (FA) afferent channels are less important. One could argue that vibromechanical cues contribute to softness perception bysimulating transient strain patterns over the skin like those produced when touchingan object, but there is little evidence to suggest that such cues contribute significantly to softness perception during direct skin contact, perhaps because contact
area itself is so highly weighted, when available: As noted in Chaps. 1, 2, and 5,
physiological and psychophysical studies have shown that haptic perceptual sensitivity to softness is highest when there is direct skin contact with a deformable surface.

3 Vibrotactile Sensation and Softness Perception

35

An alternative possibility, discussed further in Sect. 3.3, is that vibration feedback
could bias estimates of applied force during object compression.

3.2.2 Indirect Skin Contact
When touch is mediated by a rigid link, such as a handheld stylus or rigid mechanism, the cutaneous perceptual cues (1,2) noted above do not provide information
about compliance, since contact area and skin strain reflect the force between thefinger and a rigid surface but do not independently evidence the displacement of
the surface. Adding a rigid link to the interaction with an object having deformablesurfaces makes the interaction equivalent to the one obtained with an object withrigid surfaces. In such cases where cutaneous information is not directly informative
about compliance, discrimination performance is significantly reduced (Srinivasan
and LaMotte 1995). With rigid surfaces compliance estimation requires the combination of force and displacement information (see Chaps. 1 and 5). To estimate
object compliance during indirect touch, cutaneous force cues could be combinedwith displacement information obtained from visual and proprioceptive sense data
(see Chaps. 1, 2, and 5 for more information).
Several studies have demonstrated that individuals are able to estimate compliance under such settings (for example, estimating the compliance of a spring-loaded
mechanism) (Srinivasan and LaMotte 1995; Tan et al. 1995; Bergmann Tiest andKappers 2009; Jones and Hunter 1990). The results generally demonstrate reduced
sensitivity when compared to the case of direct skin contact with a deformable object
is available.
What does this suggest about possible roles of vibrotactile sensation in complianceestimation via indirect touch? It could be hypothesized that in such a setting, vibration
could affect compliance estimates in one of two ways: by either biasing estimateddisplacements !x or estimated forces !F (Fig. 3.2). As noted in Chap. 5, both typesof bias are possible.
As vibrations are normally produced during object deformation, amplifying thesevibrations could lead to a change in perceived compliance. This is partly supported
in the literature (Visell et al. 2011; Kildal 2012, 2010).
Fig. 3.2 Vibration stimuli
may bias complianceestimates by altering perceived
force F or displacement x

36

Y. Visell and S. Okamoto

3.2.3 Transient Contact
So far we have discussed potential increases in perceived compliance due to vibration.
In principle, however, vibrotactile cues could have the opposite effect namely they
could decrease perceived compliance. Tapping on a hard surface elicits characteristic
vibrations, in the form of transient mechanical signals, with broad frequency content,
due to the rapid changes in contact forces. During such an interaction, a stiffer object
may yield a more perceptually prominent vibrotactile signal. A model of the forces
involved can be given by the Hertz theory of viscoelastic contact. A simplified versionof the Hertz model that is suitable for the analysis of transient contact forces during
impact with a viscoelastic object is due to Hunt and Crossley (1975), and can be
written as
f impact = K (z) − D(z)ż,(3.1)

where z represents the depth of penetration beyond the undeformed surface of theobject, K models the growth rate of contact surfaces, and D captures the dissipation.
The tapping force excites vibrations in the object that can often be described by asource-filter model, consisting of an impulse response h(t) equal to the response to an
impulsive force, so that the net displacement is given by y(t) = z(t) ∗ h(t), where ∗denotes convolution in time. For a stiff object this response combines contributions ofbroadly distributed frequency content arising from the contact force and contributionsof high-frequency resonant modal frequencies. Either source may lie within the rangeof frequencies humans are sensible to, hence they could provide a potential perceptualcue to contact and to object compliance. Indeed, the notion that tapping on a surface isa suitable action for exploring surface hardness is familiar from everyday experience,and has further been explored experimentally (Lederman and Klatzky 1987).
The transient forces that are generated during tapping can yield vibromechanicalsignals that can be readily reproduced via a haptic interface using sufficiently wide
bandwidth motors or actuators. Among the earliest work exploring the use of contactgenerated vibration cues to communicate information about touched objects arerobotic teleoperation studies in which a human operator of a master robot uses a slave
robot to manipulate objects in a remote environment. The operator is provided withvibrotactile feedback that reproduces accelerations measured near the end effectorof the slave device (Massimino and Sheridan 1993; Kontarinis and Howe 1995). The
goal of such an arrangement, which can be described as a form of sensory substitution(Visell 2009), is to reproduce transient accelerations experienced at the end effector,
simulating a setting in which the master and slave device were coupled via a rigid
link capable of transmitting high-frequency vibrations, in the frequency range ofseveral hundred Hz. The research investigated the extent to which feedback of thistype could improve operator performance on basic tasks, such as peg insertion, but
the results were mixed.
Later, researchers sought to enhance force or contact information in computer
simulated virtual environments with vibrations that were designed to mimic the3 Vibrotactile Sensation and Softness Perception

37

physical response of real objects, either by means of a physical model (for example,
the Hunt-Crossley impact model mentioned above) or based on the measured
response of tapped materials (Kontarinis and Howe 1995; Okamura et al. 2001;
Kuchenbecker et al. 2006). Studies have shown that by superimposing transient
vibrations on contact forces, perceived surface hardness can be increased and material identity can be modified or enhanced (Kuchenbecker et al. 2006). Similar results
have been observed with transient audio feedback, in the form of tapping sounds (seeChap. 4 for a comprehensive review). In general these methods of rendering, which
employ a signal delivered through the auditory, visual, or tactile sense modalities
which are triggered by the movement of the participant, have collectively been given
the name of “event-based haptics”.

3.2.4 Frictional Sliding
Friction involves tangential forces produced during the sliding of objects. Texture
refers to small-scale modifications of mechanical interaction responses during sliding or during indentation. The forces involved comprise both slowly-varying nominal
or constitutive responses and fluctuating components generated by surface or material imperfections. The latter signal components relate relative displacement of theobjects concerned to high-frequency frictional force components (Ibrahim 1994;
Akay 2002), whose frequency bandwidth can overlap that of the vibrotactile sense.
In principle, perceptual information about interaction parameters, such as applied
force or displacement, and material properties, such as surface hardness, are available through such signals. In everyday terms, even for objects with very soft surfaces,such as textiles like velvet or silk, texture-like force components can provide information about material properties. Additionally, as demonstrated in the well-known
parchment skin illusion (Jousmäki and Hari 1998), amplifying the sound of frictional
rubbing can create a perceptual experience that the sliding surface is dryer, rougher,or harder than is nominally the case.
In contrast to softness sensations elicited by pressing on a surface, softness cues
produced by stroking with the finger are more difficult to interpret, since it is morechallenging to analyze the physical interactions between the finger (or a probe) andthe surface, and thus to relate softness perception to surface parameters. Nonetheless, it is plausible that individuals may use information acquired by stroking or
scanning with a finger in order to estimate object compliance. First, because thesliding dynamics may directly depend on bulk material properties such as elasticity.
Second, because surface properties may elicit prior expectations for object softness.
Textile softness is often perceived via rubbing with the fingers, as has been extensively studied in areas of the literature on applied perception and ergonomics (PenseLheritier et al. 2006; Chen et al. 2009). In order to obtain objective measures of fabricsoftness, several researchers have investigated the relation between reported textile
softness and vibromechanical cues. Rust et al. (1994) were able to predict textile
softness ratings using vibromechanical measurements obtained from a novel engineering instrument. Lang and Andrews (2011) observed a connection between the

38

Y. Visell and S. Okamoto

object rigidity and sliding-produced vibrations in a probe . These studies were motivated by the idea that stroking a harder material can lead to a larger microscopic
movements of a rigid probe as it comes in contact with the microscale defects of thesurface upon which it slides. The magnitude and frequency of the vibrations causedby these contacts depend on the surface hardness—i.e., the microscale defects ofthe surface of a compliant materials would deform rather than making the probe.
When the probe is mechanically coupled with a hard surface, their collective rigidity
relatively increases, which leads to a larger resonance frequency and vibratory accelerations. When the probe is coupled with a soft surface, their comprehensive rigidity
decreases, which leads to a smaller resonance frequency and significant damping
ratio. Hence, the mean acceleration values depending on contact forces approximately reflect the compliance of surfaces. What is still unclear is whether humans
can judge softness based on the cues generated by stroking alone as it is the caseinstead of perceived softness by tapping.

3.3 Effects of Low-Frequency Vibration on Softness
Prior literature has indicated that low-frequency vibrotactile stimuli, in the frequency range from 3 to 5 Hz, can evoke the perceptual sense of material softness(Ben Porquis et al. 2011; Takahiro et al. 2010). The softness experience that is evokedby such stimuli grows slowly, with a percept of vaguely defined onset. This slow but
still noticeable softness sensation could be of practical interest, because it holds thepotential to provide any handheld or grounded devices having vibrotactile channels with added value, namely the distinctive ability to produce artificial softness
sensations.

3.3.1 Prospective Mechanism
Skin-mediated softness percepts are produced by the spatial distribution of pressure
on the skin. Intensive pressure on a small area results in an experience of the contact
with hard object, whereas widely extensive pressure is perceived as that with soft
material, as shown in Fig. 3.3. According to the Hertzian contact theory (Johnson
1995), the radius a of a circular contact area made by two spherical bodies, here
representing the finger pad and a soft object, is
a=

!

3w R
4 E

"1/3

,

(3.2)

where w, R, and E are the contact force, composite radius and Young’s modulus of
the two bodies. The composite variables are3 Vibrotactile Sensation and Softness Perception

1
1
1
+
=
R
Rf
Rs

39

(3.3)

and
2

1 − νf1 − ν2s
1
+
,
=
E
Ef
Es

(3.4)

where Ri , E i , and νi are the radius, Young’s modulus, and Poisson ratio of bodyi. The suffixes f and s describe the finger and surface to be touched by the finger,
respectively. The values of R f , E f , and ν f are assumed to be known. This can bejustified by the assumption that a human observer should be roughly familiar with
their own finger pad’s size and softness. In case the soft object is a flat surface,Rs = ∞ and R ≈ R1 . In this case, the two softness parameters νs and E s may be
estimated from a and w. Note that the value range of Poisson ratio is narrow andtypically near 0.3. The effect of ν2s can therefore be viewed as insignificant comparedto E s . The cutaneous and kinesthetic receptors of the human finger are capable ofestimating both contact area and applied force. Humans can make use of slowly
adapting type I (SA I) mechanoreceptive units and pressure-sensitive nocireceptors
distributed beneath skin to estimate the pressure distribution and applied force causedby contact with soft surfaces. Receptors in the muscles and tendons of the finger andwrist are also sensitive to forces applied to finger pad.
At least three different engineering interfaces have been designed around theaforementioned principles, albeit by means of very different devices (see Chap. 14).
Bicchi et al. (2000) and Scilingo et al. (2010) fabricated a finger pad contactor consisting of several concentric actuated cylinders with different radii. This device madeit possible to control the pressure and contact area between a finger pad for testing
the hypothesis that contact area plays a key role in softness percepts. In contrast,
Fujita and Ohmori (2001) and Kimura et al. (2010) used balloon and sheet-based
tactile displays, respectively, to elucidate and demonstrate the effects of contact area
on softness percepts. The results of these studies affirmed the primary contributionof contact area to softness perception, although the explanatory hypotheses that each
proposed have not been completely unanimous. Nonetheless, changes in the contact
area can be said to effectively influence softness perception.
It is evident that the pressure distribution in the contact area has a strong connection with softness perception whereas its specific role leaves room for discussion.
As described above, SA I units and some nociceptors are sensitive to pressure orsustained indentation. There is a distinct possibility that the activation of SA I units
by vibrotactile stimuli induces softness percepts. For the low-frequency band or static mechanical stimuli, SA I units have the lowest thresholds, i.e., they are moresensitive than other units.
As described in the following section, larger low-frequency vibratory amplitudes
are associated with softer percepts. The changes in the indentation can result inchanges in deformed skin area, as shown in Fig. 3.3. With large skin indentation,
large populations of SA I units are expected to be activated. Additionally, the size of

40

Y. Visell and S. Okamoto

Contact with soft object withbroad but small pressure

Large indentation andlarge activated area

Contact with hard object
with intense pressure

Vibrotactile
contactor

Small indentation andsmall activated area

Fig. 3.3 Top Pressure distribution caused by contacts with objects. Down Finger pad deformation
via vibrotactile indentation

population of activated SA I units is more predictive than the impulse rate of single
unit about the area of the contact (Suzuki et al. 1999).

3.3.2 Low-Frequency Softness Rendering
Fig. 3.4 shows a schematic view of an experiment that evaluated effects of lowfrequency on softness perception (Okamoto 2010; Takahiro et al. 2010). The
experiment was based on the method of constant stimuli. In it, participants compared a low-frequency (5 Hz) vibrotactile stimulus and a physically soft sample andjudged which stimulus was felt softer. Vibrations were generated using a voice coil

Balances
Weight Tactile display Weight Softness
sampleFig. 3.4 Comparison between low-frequency vibrotactile stimuli and softness specimen

3 Vibrotactile Sensation and Softness Perception

41

Stiffness [kN/m]

20
15
10
5
0

0

0.5

1.0

1.5

2.0

Peak-to-peak displacement of vibrotactile stimuli [mm]Fig. 3.5 Low-frequency vibrotactile stimuli versus softness of specimen

motor with a contactor consisting of a plastic plate. The vibrotactile amplitudes were0–1.6 mm under a sustained load of 0.5 N. The physically soft specimens were cylindrical silicone rubber samples whose spring coefficient ranged 4.7–22.3 N/mm. Twobalances were used to ensure equal loads were applied to the two stimuli. Fig. 3.5
shows the compliance of the silicone perceived to be matching the one of vibrating
device. The perceived hardness decreased as a function of the vibration amplitude. In
this experiment, the true contact area between the vibrotactile contactor and finger padvaried with stimulus amplitude in such a way larger amplitudes led to larger contact
areas. In a subsequent investigation, the contact area was held constant, and the same
effects of the amplitudes on the softness percepts were observed (Ben Porquis et al.
2011). Furthermore, there is evidence that low-pass-filtered white-noise vibrations
can have a similar impact on softness percepts (Ikeda et al. 2013).

3.4 Volumetric Softness
During direct and indirect object contact (Fig. 3.1), vibration can accompany thecompression when the displacement of the volume releases energy. Such vibration
can be a potential cue to softness, but vibration can be released during a variety
of other inelastic processes. It is thus not surprising that vibromechanical energy
alone, when is not correlated with action, is unlikely to affect perceived softness. For
example, when touching a washing machine, one is seldom left with the impressionof owning a “soft” appliance.
A haptic interface that provides similar vibration feedback to what is felt during such situations might be expected to yield increased sensations of softness. A
schematic illustration is shown in Fig. 3.6. Several published demonstrations andperceptual studies have provided evidence that vibrotactile feedback can modulate volumetric softness (Visell et al. 2011; Kildal 2010, 2012). Other perceptual

42

Y. Visell and S. OkamotoFig. 3.6 The compression of a surface accompanied by suitable vibration feedback can yield
increased sensations of volumetric softness, due to a straightforward sensorimotor contingency
between the generation of internal vibromechanical energy during object compression

effects, including force-to-visual displacement gain modulation [or “pseudo-haptics”(Lecuyer et al. 2000)] are also known to affect volumetric softness perception.
In one study of effects of vibration feedback on volumetric compliance perception,
Kildal demonstrated that a rigid box pressed with the finger or a stylus could feelas though it compressed in height (Kildal 2010, 2012). This sensation was evokedby vibration feedback that was coupled to applied force. The stimuli consisted ofvibrations supplied by a resonant vibrotactile actuator driven by a voltage (amplitude)
signal. The stimulus design was motivated by a mechanical model consisting of aspring loaded mechanism moving over a corrigated surface. A vibration transient
was supplied whenever the normal force on the device changed by a quantity !F.
Averaging over vibration transients yields a stimulus s(t) with RMS amplitude givenbydF
dx= S0 k ,(3.5)
srms (t) = S0
dtdt
where S0 is a constant amplitude factor, F is normal force, t is time. The parameters
k and x are the virtual stiffness and displacement of the simulated mechanical model
responsible for producing the feedback.
Visell et al. (2011) investigated whether action-synchronized vibromechanicalstimuli felt when pressing on a surface could yield influence the perceived compliance of a walking surface. The vibration feedback they presented to participants
depended on the force that was applied to the walking surface. These signals werecomparable to those experienced when stepping on a natural material that produces
acoustic energy when compressed (e.g., snow, gravel, leaves, soil), or displacing afoot operated mechanism that generates friction noise (e.g., a pedal or slider). Vibromechanical results of stepping onto a walking surface have been found to be related
for the identification of the type of material that natural and man-made walking surface are composed of (Giordano et al. 2012). The authors’ investigation was based ona pair of experiments. The first sought to ascertain the dependence of perceived compliance on on the vibration stimulus waveform, and on the relation between applied
force and feedback amplitude. The stimuli consisted of vibration feedback that variedin two respects: the driving waveform (sinusoidal, white noise, or poisson noise) or3 Vibrotactile Sensation and Softness Perception

43

0.6
0.4

X

0.2

TP1
WC1
TD1
TC1
WP1
TD0
SD1
WD1
SP1
SC1
TP0
SD0
TC0
WC0
SC0
SP0
WD0
WP0

Mean compliance rating

Experiment 1 Stimuli
0.8

0.0

Fig. 3.7 In one experment, Visell et al. (2011) found that nine different types of vibration feedback
were all able to elicit a significantly increased percept of compliance when compared with the “novibration” case. Image from Visell et al. (2011)

and the temporal dependence on force (proportional, time-derivative, or constant).
Results indicate that vibrotactile feedback could elicit an increase in perceived compliance and that the effect grew in proportion to the feedback amplitude (Fig. 3.7).
In a second experiment, the authors manipulated the compliance of the walking surface that users stepped on, using a novel haptic interface (Fig. 3.8). The goal was todetermine whether compliance and vibration are perceptually integrated in an organized way, and to calibrate the perceptual effect in physical units, yielding a quantityof change in physical compliance that could be achieved with a given vibrotactile
feedback amplitude. Results point out that vibrotactile feedback provides a perceptual cue for compliance (Fig. 3.9). The vibration amplitudes required to produce anappreciable bias in perceived compliance were very low, near to the absolute threshold at which subjects could detect the vibrotactile stimuli when stepping. Thus, the

A
a
b
d

e

c

d

g

e

h

e

f
B

m
a

a'

k

m

c
170 cmFig. 3.8 Effects of vibration on volumetric softness. Image from Visell et al. (2011)

44

Y. Visell and S. OkamotoFig. 3.9 Effects of vibration on volumetric softness

amplitudes used were considerably smaller than the ones experienced during normal
walking on natural granular materials, such as gravel, as humans are well aware of
the vibrations present there. It is not entirely clear why the stimuli were so effectivein this experiment, which did not involve training and did not require awareness thatvibration feedback was being provided. The compliance estimation task adopted inthis study resembles prior experiments in which subjects used their hands to estimate
the haptic compliance of spring mechanisms or other objects with non-deformablesurfaces (LaMotte 2000; Tiest and Kappers 2009; Jones and Hunter 1990). Basedon those results, and on considerations of contact mechanics, it was expected that
subjects in the experiments described here required both force and displacement
information in order to judge compliance. In this light, it appears that added vibration feedback results in a modification of force and/or displacement informationthat increases compliance estimates. Prior research has shown that localized vibration stimulation of the foot sole can increase perceived force at the same location(Kavounoudias et al. 1999, 1998), which could be thought to influence compliancejudgments. However, an increase of force estimates would tend to reduce complianceestimates, whereas the aforementioned results reviewed show an opposite tendency.
Thus, a more likely explanation, which is also consistent with the results of Kildal(2010, 2012), seems to be that vibration elicited an increased sensation of displacement, as if an object or material was compressed or displaced. If so, an observer
could be presumed to infer an increase in compliance that grows linearly with theincreased sensation of displacement. The results of (Visell et al. 2011) were consistent with relative increases in estimated displacement of 25.0 and 33.5 % in low- andhigh-amplitude vibration conditions (amplitude 0.43 and 0.86 m/s2 ) as tested in the
study, suggesting a monotonically increasing relation between vibration amplitudeand compliance perception. The model proposed in Chap. 5 suggest that the increase
in sensed displacement may be related to a biased detection of the unperturbed position of the object. The first contact with a vibrating object is obtained earlier than
the resting state position. This should increase the amount of displacement sensed
afterwards. But on this basis one could ask: Why indeed is the washing machine notperceived to be “soft”?

3 Vibrotactile Sensation and Softness Perception

45

3.5 Conclusions
A wide variety of mechanical signals arise during haptic interaction with structured surfaces and objects. Among these, there are several sources of high-frequency
mechanical vibration that might influence haptic softness perception. They range
from transient vibrations induced during tapping on a surface, to subtle fluctuating forces during frictional sliding, to vibrations resulting from inelastic processes
accompanying the compression of solid objects. The heterogeneous nature of thecontact mechanical interactions involved precludes, to some extent, a unified explanation of all such effects. However, several studies reviewed above have provided
evidence for such effects. Together, they indicate that vibrotactile cues can influence
the perception of object compliance. This is notable because in standard accounts ofhaptic perception, such cues are not normally considered to be relevant to softness
perception. Rather, compliance is often described as being primarily mediated via
proprioceptors in the muscles and joints, and via cutaneous receptors for force orcontact deformation.
In most cases, vibrotactile cues have a comparatively weaker influence on perceived softness than can be achieved by manipulating the material properties of theobjects themselves or by manipulating force-displacement relations reproduced bya haptic interface. Nonetheless, the notion that a hard surface can be rendered moreor less compliant through mechanical vibrations alone is powerful and compelling.

References
Akay A (2002) Acoustics of friction. J Acoust Soc Am 111:1525
Ben Porquis L., Konyo M, Tadokoro S (2011) Representation of softness sensation using vibrotactile
stimuli under amplitude control. In: Robotics and automation (ICRA), 2011 IEEE international
conference on, IEEE, pp 1380–1385
Bensmaia S, Hollins M (2003) The vibrations of texture. Somatosens Mot Res 20(1):33–43
Bensmaia S, Hollins M (2005) Pacinian representations of fine surface texture. Percept psychophys
67(5):842
Bergmann Tiest WM, Kappers AML (2009) Cues for haptic perception of compliance. IEEE Trans
Haptics 2(4):189–199
Bicchi A, Schilingo EP, De Rossi D (2000) Haptic discrimination of softness in teleoperation: therole of the contact area spread rate. IEEE Trans Rob Autom 16(5):496–504
Chen X, Shao F, Barnes C, Childs T, Henson B (2009) Exploring relationships between touch
perception and surface physical properties. Int J Des 3(2):67–76
Freeman AW, Johnson KO (1982) Cutaneous mechanoreceptors in macaque monkey: temporaldischarge patterns evoked by vibration, and a receptor model. J physiol 323(1):21–41
Freyberger F, Färber B (2006) Compliance discrimination of deformable objects by squeezing with
one and two fingers. In: Proceedings of euroHaptics 2006, pp 271–276
Friedman R, Hester K, Green B, LaMotte R (2008) Magnitude estimation of softness. Exp Brain
Res 191(2):133–142
Fujita K, Ohmori H (2001) A new softness display interface by dynamic fingertip contact area
control. In: 5th world multiconference on systemics cybernetics and informatics, pp 78–82

46

Y. Visell and S. Okamoto

Giordano BL, Visell Y, Yao H-Y, Hayward V, Cooperstock JR, McAdams S (2012) Identificationof walked-upon materials in auditory, kinesthetic, haptic, and audio-haptic conditions. J Acoust
Soc Am 131:4002
Harper R, Stevens SS (1964) Subjective hardness of compliant materials. Q J Exp Psychol 16:
204–215
Hollins M, Fox A, Bishop C (2000) Imposed vibration influences perceived tactile smoothness.
Perception 29(12):1455–1466
Hollins M, Risner S (2000) Evidence for the duplex theory of tactile texture perception. Percept
Psychophys 62(4):695–705
Hunt K, Crossley F (1975) Coefficient of restitution interpreted as damping in vibroimpact. J Appl
Mech 42:440
Ibrahim R (1994) Friction-induced vibration, chatter, squeal, and chaos-part i: Mechanics of contact
and friction. Appl Mech Rev 47(7):209–226
Ikeda A, Suzuki T, Takamatsu J, Ogasawara T (2013) Producing method of softness sensation by
device vibration. In: 2013 IEEE international conference on systems, man, and cybernetics, pp3384–3389
Johnson KL (1995) Contact mechanics. Cambridge University, CambridgeJohnson KO (2001) The roles and functions of cutaneous mechanoreceptors. Curr Opin Neurobiol
11(4):455–461
Jones LA, Hunter IW (1990) A perceptual analysis of stiffness. Exp Brain Res 79:150–156
Jousmäki V, Hari R (1998) Parchment-skin illusion: sound-biased touch. Curr Biol 8(6):R190–R191
Kavounoudias A, Roll R, Roll J-P (1998) The plantar sole is a’dynamometric map’for humanbalance control. Neuroreport 9(14):3247–3252
Kavounoudias A, Roll R, Roll J-P (1999) Specific whole-body shifts induced by frequencymodulated vibrations of human plantar soles. Neurosci Lett 266(3):181–184
Kildal J (2010) 3D-press: haptic illusion of compliance when pressing on a rigid surface. In: International conference on multimodal interfaces and the workshop on machine learning for multimodal
interaction, ACM, New York, pp 21
Kildal J (2012) Kooboh: variable tangible properties in a handheld haptic-illusion box. In: haptics:
perception, devices, mobility, and communication, Springer, Berlin, pp 191–194
Kimura F, Yamamoto A, Higuchi T (2010) Development of a 2-Dof softness feeling display for tactile tele-presentation of deformable surfaces. In: 2010 IEEE international conference on robotics
and automation, pp 1822–1827
Klatzky R, Lederman S (1999) Tactile roughness perception with a rigid link interposed between
skin and surface. Percept Psychophys 61(4):591–607
Klatzky R, Lederman S, Hamilton C, Grindley M, Swendsen R (2003) Feeling textures througha probe: effects of probe and surface geometry and exploratory factors. Percept Psychophys
65(4):613
Kobayashi Y, Osaka R, Hara T, Fujimoto H (2008) How accurately people can discriminate thedifferences of floor materials with various elasticities. IEEE Trans Neural Syst Rehabil Eng
16(1):99–105
Kontarinis DA, Howe RD (1995) Tactile display of vibratory information in teleoperation and virtualenvironments. Presence Teleoper Virtual Environ 4(4):387–402
Kuchenbecker KJ, Fiene J, Niemeyer G (2006) Improving contact realism through event-based
haptic feedback. IEEE Trans Visual Comput Graphics 12(2):219–230
LaMotte R (2000) Softness discrimination with a tool. J Neurophysiol 83(4):1777
Lang J, Andrews S (2011) Measurement-based modeling of contact forces and textures for haptic
rendering. IEEE Trans Visual Comput Graphics 17(3):385–391
Lecuyer A, Coquillart S, Kheddar A, Richard P, Coiffet P (2000) Pseudo-haptic feedback: canisometric input devices simulate force feedback? In: Virtual reality, 2000. Proceedings. IEEE,
pp 83–90
Lederman SJ, Klatzky RL (1987) Hand movements: a window into haptic object recognition. Cogn
Psychol 19(3):342–368

3 Vibrotactile Sensation and Softness Perception

47

Massimino MJ, Sheridan TB (1993) Sensory substitution for force feedback in teleoperation. Presence Teleoper Virtual Environ 2(4):344–352
Morioka M, Griffin MJ (2002) Dependence of vibrotactile thresholds on the psychophysical measurement method. Int Arch Occupational Environ Health 75(1–2):78–84
Morioka M, Whitehouse DJ, Griffin MJ (2008) Vibrotactile thresholds at the fingertip, volar forearm,large toe, and heel. Somatosens Mot Res 25(2):101–112
Okamoto S (2010) Tactile transmission system and perceptual effects of delayed tactile feedback.
PhD thesis, Tohoku University
Okamura A, Dennerlein J, Howe R (1998) Vibration feedback models for virtual environments. In:
1998 IEEE international conference on robotics and automation, 1998. Proceedings, vol 1. pp
674–679
Okamura AM, Cutkosky MR, Dennerlein JT (2001) Reality-based models for vibration feedbackin virtual environments. IEEE/ASME Trans Mechatron 6(3):245–252
Pense-Lheritier A-M, Guilabert C, Bueno M, Sahnoun M, Renner M (2006) Sensory evaluation of
the touch of a great number of fabrics. Food Qual Prefer 17(6):482–488
Ribot-Ciscar E, Vedel J, Roll J (1989) Vibration sensitivity of slowly and rapidly adapting cutaneous
mechanoreceptors in the human foot and leg. Neurosci Lett 104(1):130–135
Rust J, Keadle T, Allen D, Shalev I, Barker R (1994) Tissue softness evaluation by mechanicalstylus scanning. Text Res J 64(3):163–168
Scilingo EP, Bianchi M, Grioli G, Bicchi A (2010) Rendering softness: integration of kinesthetic
and cutaneous information in a haptic device. IEEE Trans Haptics 3(2):109–118
Scott-Blair G, Coppen F (1940) The subjective judgement of the elastic and plastic properties ofsoft bodies. Br J Psychol 31:61–79
Srinivasan M, LaMotte R (1995) Tactual discrimination of softness. J Neurophysiol 73(1):88–101
Suzuki T, Mabuchi K, Nishimura H, Saito T, Kakuta N, Kunimoto M, Shimojo M, Ishikawa M(1999) The electrical control of pressure sensations: the relationship between stimulation signals
and subjective intensities and areas. In: first joint BMES/EMBS conference, pp 457–457
Takahiro Y, Okamoto S, Konyo M, Hidaka Y, Maeno T, Tadokoro S (2010) Real-time remotetransmission of multiple tactile properties through master-slave robot system. In: 2010 IEEE
international conference on robotics and automation, IEEE, pp 1753–1760
Tan H, Durlach N, Beauregard G, Srinivasan M (1995) Manual discrimination of compliance using
active pinch grasp: The roles of force and work cues. Percept Psychophys 57(4):495–510
Tiest WMB, Kappers AM (2009) Cues for haptic perception of compliance. IEEE Trans Haptics
2(4):189–199
Vedel J, Roll J (1982) Response to pressure and vibration of slowly adapting cutaneous mechanoreceptors in the human foot. Neurosci Lett 34(3):289–294
Verrillo RT (1966) Vibrotactile thresholds for hairy skin. J Exp Psychol 72(1):47
Visell Y (2009) Tactile sensory substitution: Models for enaction in hci. Interact Comput 21(1–2):38–53
Visell Y, Giordano BL, Millet G, Cooperstock JR (2011) Vibration influences haptic perception of
surface compliance during walking. PLoS One 6(3):e17697