erwan/phd-thesis
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

Complete related work

Browse Source
This commit is contained in:
Erwan Normand
2024-09-23 03:09:45 +02:00
parent ee2b739ddb
commit 0495afd60c
12 changed files with 213 additions and 180 deletions
Download Patch File Download Diff File Expand all files Collapse all files

133
1-introduction/related-work/2-wearable-haptics.tex
View File

@@ -20,7 +20,7 @@ An increasing wearability resulting in the loss of the system's kinesthetic feed
\item Exoskeletons are body-grounded kinesthetic devices.
\item Wearable haptic devices are grounded on the point of application of the tactile stimulus.
]
\subfigsheight{38mm}
\subfigsheight{34mm}
\subfig{pacchierotti2017wearable_1}
\subfig{pacchierotti2017wearable_2}
\subfig{pacchierotti2017wearable_3}
@@ -147,14 +147,19 @@ An \ERM is a \DC motor that rotates an off-center mass when a voltage or current
\footnotetext{\url{https://www.precisionmicrodrives.com/}}
A \LRA consists of a coil that creates a magnetic field from an \AC to oscillate a magnet attached to a spring, as an audio loudspeaker (\figref{precisionmicrodrives_lra}). They are more complex to control and a bit larger than \ERMs. Each \LRA is designed to vibrate with maximum amplitude at a given resonant frequency, but won't vibrate efficiently at other frequencies, \ie their bandwidth is narrow, as shown on \figref{azadi2014vibrotactile}.
A \VCA is a \LRA but capable of generating vibration at two \DoF, with an independent control of the frequency and amplitude of the vibration on a wide bandwidth. They are larger in size than \ERMs and \LRAs, but can generate more complex renderings.
A \LRA consists of a coil that creates a magnetic field from an \AC to oscillate a magnet attached to a spring, as an audio loudspeaker (\figref{precisionmicrodrives_lra}).
They are more complex to control and a bit larger than \ERMs.
Each \LRA is designed to vibrate with maximum amplitude at a given resonant frequency, but won't vibrate efficiently at other frequencies, \ie their bandwidth is narrow, as shown on \figref{azadi2014vibrotactile}.
A \VCA is a \LRA but capable of generating vibration at two \DoF, with an independent control of the frequency and amplitude of the vibration on a wide bandwidth.
They are larger in size than \ERMs and \LRAs, but can generate more complex renderings.
Piezoelectric actuators deform a solid material when a voltage is applied. They are very small and thin, and allow two \DoFs of amplitude and frequency control. However, they require high voltages to operate thus limiting their use in wearable devices.
Piezoelectric actuators deform a solid material when a voltage is applied.
They are very small and thin and provide two \DoFs of amplitude and frequency control.
However, they require high voltages to operate, limiting their use in wearable devices.
\begin{subfigs}{lra}{Diagram and performance of \LRAs. }[
\item Diagram. From Precision Microdrives~\footnotemarkrepeat.
\item Force generated by two \LRAs as a function of sine wave input with different frequencies: both their maximum force and resonant frequency are different \cite{azadi2014vibrotactile}.
\item Force generated by two \LRAs as a function of sinusoidal wave input with different frequencies: both their maximum force and resonant frequency are different \cite{azadi2014vibrotactile}.
]
\subfigsheight{50mm}
\subfig{precisionmicrodrives_lra}
@@ -165,21 +170,21 @@ Piezoelectric actuators deform a solid material when a voltage is applied. They
\subsection{Modifying Perceived Haptic Roughness and Hardness}
\label{tactile_rendering}
Tactile rendering of haptic properties consists in modelling and reproducing virtual tactile sensations comparable to those perceived when interacting with real objects \cite{klatzky2013haptic}.
By adding such tactile rendering as feedback to the touch actions of the hand on a real object \cite{bhatia2024augmenting}, the perception of the object's haptic property can be modified.
Rendering a haptic property consists in modeling and reproducing virtual sensations comparable to those perceived when interacting with real objects \cite{klatzky2013haptic}.
By adding such rendering as feedback timely synchronized with the touch actions of the hand on a real object \cite{bhatia2024augmenting}, the perception of the object's haptic property can be modified.
The integration of the real and virtual sensations into a single property perception is discussed in more details in \secref{sensations_perception}.
%, both the real and virtual haptic sensations are integrated into a single property perception, as presented in \secref{sensations_perception}, \ie the perceived haptic property is modulated by the added virtual feedback.
In particular, the visual rendering of a touched object can also influence the perception of its haptic properties, \eg by modifying its visual texture in \AR or \VR, as discussed in the \secref{visuo_haptic}.
\textcite{bhatia2024augmenting} categorize the haptic augmentations into three types: direct touch, touch-through, and tool mediated.
In \emph{direct touch}, the haptic device does not cover the interior of the hand to not impair the user to interact with the \RE, and is typically achieved with wearable haptics.
In touch-through and tool-mediated, or \emph{indirect feel-through} \cite{jeon2015haptic}, the haptic device is interposed between the hand and the \RE.
\textcite{bhatia2024augmenting} categorize the haptic augmentations into three types: direct touch, touch-through, and tool-mediated.
In \emph{direct touch}, the haptic device does not cover the inside of the hand so as not to impair the user's interaction with the \RE, and is typically achieved with wearable haptics.
In touch-through and tool-mediated, or \emph{indirect feel-through} \cite{jeon2015haptic}, the haptic device is placed between the hand and the \RE.
%We are interested in direct touch augmentations with wearable haptics (\secref{wearable_haptic_devices}), as their integration with \AR is particularly promising for free hand interaction with visuo-haptic augmentations.
Many haptic augmentations were first developed with touch-through devices, and some (but not all) were later transposed to direct touch augmentation with wearable haptic devices.
%We also focus on tactile augmentations stimulating the mechanoreceptors of the skin (\secref{haptic_sense}), thus excluding temperature perception, as they are the most common existing haptic interfaces.
As we chose in \secref{object_properties} to focus on the haptic perception of the roughness and hardness of objects, we overview bellow the methods to modify the perception of these properties.
Of course, wearable haptics can also be used in direct touch context to modify the perceived friction \cite{konyo2008alternative,salazar2020altering}, weight \cite{minamizawa2007gravity}, or local deformation \cite{salazar2020altering} of real objects, but they are rare \cite{bhatia2024augmenting} and will not be detailed here.
Since we have chosen to focus in \secref{object_properties} on the haptic perception of roughness and hardness of objects, we review below the methods to modify the perception of these properties.
Of course, wearable haptics can also be used in a direct touch context to modify the perceived friction \cite{konyo2008alternative,salazar2020altering}, weight \cite{minamizawa2007gravity}, or local deformation \cite{salazar2020altering} of real objects, but they are rare \cite{bhatia2024augmenting} and will not be detailed here.
% \cite{klatzky2003feeling} : rendering roughness, friction, deformation, temperatures
% \cite{girard2016haptip} : renderings with a tangential motion actuator
@@ -187,51 +192,60 @@ Of course, wearable haptics can also be used in direct touch context to modify t
\subsubsection{Roughness}
\label{texture_rendering}
To modify the perception of haptic roughness (or texture, see \secref{roughness}) of a real object, vibrations are typically provided to the skin by the wearable haptic device when running the finger over the surface.
This is because running the finger or a tool on a textured surface generates pressures and vibrations (\secref{roughness}) at frequencies that are too high for rendering capabilities of most haptic devices \cite{campion2005fundamental,culbertson2018haptics}.
Two main approaches are used to render virtual textures: \emph{simulation models} and \emph{data-driven models} \cite{klatzky2013haptic,culbertson2018haptics}.
To modify the perception of the haptic roughness (or texture, see \secref{roughness}) of a real object, vibrations are typically applied to the skin by the haptic device as the user moves over the surface.
%This is because running the finger or a tool on a textured surface generates pressures and vibrations (\secref{roughness}) at frequencies that are too high for rendering capabilities of most haptic devices \cite{campion2005fundamental,culbertson2018haptics}.
There are two main approaches to modify virtual textures perception: \emph{simulation models} and \emph{data-driven models} \cite{klatzky2013haptic,culbertson2018haptics}.
\paragraph{Simulation Models}
Simulations of virtual textures are based on the physics of the interaction between the finger and the surface, and are used to generate the vibrations that the user feels when running the finger over the surface.
%Simulations of virtual textures are based on the physics of the interaction between the finger and the surface, and are used to generate the vibrations that the user feels when running the finger over the surface.
Early renderings of virtual textures consisted of modelling the surface with a periodic function
The simplest texture simulation model is a 1D sinusoidal grating $v(t)$ with spatial period $\lambda$ and amplitude $A$ that is scanned by the user at velocity $\dot{x}(t)$:
\begin{equation}{grating_rendering}
v(t) = A \sin(\frac{2 \pi \dot{x}(t)}{\lambda})
\end{equation}
That is, this model generates a periodic signal whose frequency is proportional to the user's velocity, implementing the speed-frequency ratio observed with real patterned textures (\eqref{grating_vibrations}).
It gives the user the illusion of a texture with a \emph{fixed spatial period} that approximate the real manufactured grating textures (\secref{roughness}).
The user's position could have been used instead of the velocity, but it requires measuring the position and generating the signal at frequencies too high (\qty{10}{\kHz}) for most sensors and haptic actuators \cite{campion2005fundamental}.
As more traditional force feedback systems are unable to accurately render such micro-details on a simulated surface, vibrotactile devices attached to the end effector instead generate vibrations to simulate interaction with the virtual texture \cite{campion2005fundamental,culbertson2018haptics}.
High-fidelity force feedback devices can reproduce patterned textures with great precision and provide similar perceptions to real textures, but they are expensive, have a limited workspace, and impose to hold a probe to explore the texture \cite{unger2011roughness}.
With a voice-coil actuator attached to the middle phalanx of the finger, \textcite{asano2015vibrotactile} used this model to increase the perceived roughness (\figref{asano2015vibrotactile_2})
Participants moved their finger over real grating textures (\qtyrange{0.15}{.29}{\mm} groove and ridge width) with a virtual sine grating (\qty{1}{\mm} spatial period) superimposed, rendered after \eqref{grating_rendering}.
The perceived roughness increased proportionally to the virtual texture amplitude, but a high amplitude decreased it instead.
\textcite{ujitoko2019modulating} instead used a square wave signal and a hand-held stylus with an embedded voice-coil.
The perceived roughness of real surfaces can be then modified when touched by a tool with a vibrotactile actuator attached \cite{culbertson2014modeling,ujitoko2019modulating} or directly with the finger wearing the vibrotactile actuator \cite{asano2015vibrotactile}, creating a haptic texture augmentation.
Of course, the fingertip skin is not deformed by the virtual texture and only vibrations are felt, but it has been shown that the vibrations produced on the fingertip skin running over a real surface are texture specific and similar between individuals \cite{delhaye2012textureinduced,manfredi2014natural}.
A common method vibrotactile rendering of texture is to use a sinusoidal signal whose frequency is modulated by the finger position or velocity \cite{asano2015vibrotactile,friesen2024perceived,strohmeier2017generating,ujitoko2019modulating}.
%\textcite{friesen2024perceived} proposed
The model in \eqref{grating_rendering} can be extended to 2D textures by adding a second sinusoidal grating with an orthogonal orientation as \textcite{girard2016haptip}.
More complex models have also been developed to be physically accurate and reproduce with high fidelity the roughness perception of real patterned surfaces \cite{unger2011roughness}, but they require high-fidelity force feedback devices that are expensive and have a limited workspace.
\paragraph{Data-driven Models}
Because simulations of virtual textures can be very complex to design and to render in real-time, direct capture of real textures have been used instead to model the produced vibrations \cite{culbertson2018haptics}.
Because simulations of realistic virtual textures can be very complex to design and to render in real-time, direct capture and models of real textures have been developed \cite{culbertson2018haptics}.
\textcite{okamura1998vibration} first dragged a stylus over sandpapers and patterned surfaces to measure the vibrations produced by the interaction.
They found that the contact vibrations with patterns could be modelled as exponential decaying sine waves (\eqref{contact_transient}) that depend on the normal force and scanning velocity of the stylus on the surface.
This technique was employed by \textcite{ando2007fingernailmounted} to augment a smooth sheet of paper with a virtual patterned texture: With a \LRA mounted on the nail, they rendered the virtual contacts of the finger with \qty{20}{\ms} vibration impulses at \qty{130}{\Hz} (\figref{ando2007fingernailmounted}).
Participants matched the virtual textures to real ones, with \qty{0.25}{\mm} height and \qtyrange{1}{10}{\mm} width, but systematically overestimated the virtual width to be \qty{4}{\mm} longer.
\textcite{okamura1998vibration} were the first to measure the vibrations produced by the interaction of a stylus dragged over sandpaper and patterned surfaces.
They found that the contact vibrations with patterns can be modeled as exponentially decaying sinusoids (\eqref{contact_transient}) that depend on the normal force and the scanning velocity of the stylus on the surface.
This technique was used by \textcite{ando2007fingernailmounted} to augment a smooth sheet of paper with a virtual patterned texture: With a \LRA mounted on the nail, they rendered the virtual finger contacts with \qty{20}{\ms} vibration impulses at \qty{130}{\Hz} (\figref{ando2007fingernailmounted}).
Participants matched the virtual textures to the real ones, with \qty{0.25}{\mm} height and \qtyrange{1}{10}{\mm} width, but systematically overestimated the virtual width to be \qty{4}{\mm} longer.
More models have been developed to capture "natural" (such as sandpapers) textures \cite{guruswamy2011iir} with many force and speed measures while staying compact and capable of real-time rendering \cite{romano2012creating,culbertson2014modeling}.
Such models are capable from the user's measurements of velocity and force as inputs to interpolate and generate a virtual texture to render as vibrations (\secref{vibrotactile_actuators}).
This led the release of the Penn Haptic Texture Toolkit (HaTT) database, a public set of stylus records and models of 100 haptic textures \cite{culbertson2014one}.
A similar database but captured directly from the fingertip was released very recently \cite{balasubramanian2024sens3}.
One limitation of these data-driven models is that they can render only isotropic textures: their capture does not depend on the position of the measure, and the rendering is the same whatever the direction of the movement.
Alternative models have been proposed to both render isotropic and patterned textures \cite{chan2021hasti}.
Other models have been then developed to capture everyday textures (such as sandpaper) \cite{guruswamy2011iir} with many force and velocity measures \cite{romano2012creating,culbertson2014modeling}.
Such data-based models are capable of interpolating from the user's measures of velocity and force as inputs to generate a virtual texture in real time (\secref{vibrotactile_actuators}).
This led to the release of the Penn Haptic Texture Toolkit (HaTT) database, a public set of stylus recordings and models of 100 haptic textures \cite{culbertson2014one}.
A similar database, but captured from a direct touch context with the fingertip, has recently been released \cite{balasubramanian2024sens3}.
A limitation of these data-driven models is that they can only render \emph{isotropic} textures: their record does not depend on the position of the measure, and the rendering is the same regardless of the direction of the movement.
Alternative models have been proposed to both render both isotropic and patterned textures \cite{chan2021hasti}.
When comparing real textures felt through a stylus with their virtual models rendered with a voice-coil actuator attached to the stylus (\figref{culbertson2012refined}), the virtual textures were found to accurately recreated roughness perception, but hardness and friction were not rendered properly \cite{culbertson2014modeling}.
\textcite{culbertson2015should} further showed that the perceived realism of the virtual textures, and similarity to the real textures, depended mostly on the user's speed but not on the user's force as inputs to the model, \ie respond to velocity is sufficient to render isotropic virtual textures.
When comparing real textures felt through a stylus with their virtual models rendered with a voice-coil actuator attached to the stylus (\figref{culbertson2012refined}), the virtual textures were found to accurately reproduce the perception of roughness, but hardness and friction were not rendered properly \cite{culbertson2014modeling}.
\textcite{culbertson2015should} further showed that the perceived realism of the virtual textures, and similarity to the real textures, depended mostly on the user's speed but not on the user's force as inputs to the model, \ie responding to speed is sufficient to render isotropic virtual textures.
\begin{subfigs}{textures_rendering_data}{Augmentating haptic texture perception with voice-coil actuators. }[
\item Increasing and decreasing the perceived roughness of a real patterned texture in direct touch \cite{asano2015vibrotactile}.
\item Comparing real patterned texture with virtual texture augmentation in direct touch \cite{friesen2024perceived}.
%\item Comparing real patterned texture with virtual texture augmentation in direct touch \cite{friesen2024perceived}.
\item Rendering virtual contacts in direct touch with the virtual texture \cite{ando2007fingernailmounted}.
\item Rendering an isotropic virtual texture over a real surface while sliding a hand-held stylus on it \cite{culbertson2012refined}.
\item Rendering an isotropic virtual texture over a real surface while sliding a hand-held stylus across it \cite{culbertson2012refined}.
]
\subfigsheight{35mm}
\subfig{asano2015vibrotactile}
\subfig{friesen2024perceived}
\subfigsheight{38mm}
\subfig{asano2015vibrotactile_2}
%\subfig{friesen2024perceived}
\subfig{ando2007fingernailmounted}
\subfig{culbertson2012refined}
\end{subfigs}
@@ -245,16 +259,16 @@ The perceived hardness (\secref{hardness}) of a real surface can be modified by
\paragraph{Modulating Forces}
When tapping or pressing a real object, the perceived stiffness $\tilde{k}$ of its surface can be modulated with force feedback \cite{jeon2015haptic}.
This was first proposed by \textcite{jeon2008modulating} who augmented a real surface tapped in 1 \DoF with a grounded force-feedback device held in hand (\figref{jeon2009haptic_1}).
This was first proposed by \textcite{jeon2008modulating} who augmented a real surface tapped in 1 \DoF with a grounded force-feedback device held in the hand (\figref{jeon2009haptic_1}).
When the haptic end-effector contacts the object at time $t$, the object's surface deforms by displacement $x_r(t)$ and opposes a real reaction force $f_r(t)$.
The virtual force of the device $\tilde{f_r}(t)$ is then controlled to:
\begin{equation}{stiffness_augmentation}
\tilde{f_r}(t) = f_r(t) - \tilde{k} x_r(t)
\end{equation}
A force sensor embedded in the device measures the reaction force $f_r(t)$.
The displacement $x_r(t)$ is estimated with the reaction force and tapping velocity using a pre-defined model of various materials, as described by \textcite{jeon2011extensions}.
As shown in \figref{jeon2009haptic_2}, the force $\tilde{f_r}(t)$ perceived by the user being modulated, but not the displacement $x_r(t)$, the perceived stiffness is $\tilde{k}(t)$.
This stiffness augmentation technique was then extended to enable tapping and pressing with 3 \DoFs \cite{jeon2010stiffness}, to render friction and weight augmentations \cite{jeon2011extensions}, and to grasping and squeezing the real object with two contact points \cite{jeon2012extending}.
The displacement $x_r(t)$ is estimated with the reaction force and the tapping velocity using a predefined model of different materials as described in \textcite{jeon2011extensions}.
As shown in \figref{jeon2009haptic_2}, the force $\tilde{f_r}(t)$ perceived by the user is modulated, but not the displacement $x_r(t)$, hence the perceived stiffness is $\tilde{k}(t)$.
This stiffness augmentation technique was then extended to allow tapping and pressing with 3 \DoFs \cite{jeon2010stiffness}, to render friction and weight augmentations \cite{jeon2011extensions}, and to grasp and squeez the real object with two contact points \cite{jeon2012extending}.
\begin{subfigs}{stiffness_rendering_grounded}{Augmenting the perceived stiffness of a real surface with a hand-held force-feedback device. }[%
\item Diagram of a user tapping the surface \cite{jeon2009haptic}.
@@ -264,11 +278,10 @@ This stiffness augmentation technique was then extended to enable tapping and pr
\subfig[0.42]{jeon2009haptic_2}
\end{subfigs}
\textcite{detinguy2018enhancing} transposed this stiffness augmentation technique with the hRing device (\secref{belt_actuators}): While pressing a real piston with the fingertip by displacement $x_r(t)$, the belt compressed the finger by a virtual force $\tilde{k}\,x_r(t)$ where $\tilde{k}$ is the added stiffness (\eqref{stiffness_augmentation}), increasing the perceived stiffness of the piston (\figref{detinguy2018enhancing}).
%This enables to \emph{increase} the perceived stiffness of the real piston up to \percent{+14}.
More importantly, the augmentation proved to be robust to the placement of the device, as the increased stiffness was perceived the same on the fingertip, the middle phalanx and the proximal.
Conversely, the technique allowed to \emph{decrease} the perceived stiffness by compressing the phalanx prior the contact and diminish the belt pressure as the user pressed the piston \cite{salazar2020altering}.
\textcite{tao2021altering} proposed instead to restrict the deformation of the fingerpad by pulling a hollow frame around it to decrease perceived stiffness (\figref{tao2021altering}): it augments the finger contact area thus the perceived Young modulus of the object (\secref{hardness}).
\textcite{detinguy2018enhancing} transposed this stiffness augmentation technique with the hRing device (\secref{belt_actuators}): While pressing a real piston with the fingertip by displacement $x_r(t)$, the belt compressed the finger with a virtual force $\tilde{k}\,x_r(t)$ where $\tilde{k}$ is the added stiffness (\eqref{stiffness_augmentation}), increasing the perceived stiffness of the piston (\figref{detinguy2018enhancing}).
More importantly, the augmentation proved to be robust to the placement of the device, as the increased stiffness was perceived the same on the fingertip, middle phalanx, and proximal.
Conversely, the technique allowed to \emph{decrease} the perceived stiffness by compressing the phalanx before the contact and reducing the pressure when the user pressed the piston \cite{salazar2020altering}.
\textcite{tao2021altering} proposed instead to restrict the deformation of the fingerpad by pulling a hollow frame around it to decrease perceived stiffness (\figref{tao2021altering}): it augments the finger contact area and thus the perceived Young's modulus of the object (\secref{hardness}).
\begin{subfigs}{stiffness_rendering_wearable}{Modifying the perceived stiffness with wearable pressure devices. }[%
\item Modify the perceived stiffness of a piston by pressing the finger during or prior the contact \cite{detinguy2018enhancing,salazar2020altering}.
@@ -282,19 +295,19 @@ Conversely, the technique allowed to \emph{decrease} the perceived stiffness by
\paragraph{Vibrations Augmentations}
\textcite{okamura2001realitybased} measured impact vibrations $Q(t)$ when tapping on real objects and found they can be modelled as exponential decaying sine wave:
\textcite{okamura2001realitybased} measured impact vibrations $v(t)$ when tapping on real objects and found they can be modeled as exponential decaying sinusoid:
\begin{equation}{contact_transient}
Q(t) = A \, |v_{in}| \, e^{- \tau t} sin(2 \pi f t)
v(t) = A \, |v_{in}| \, e^{- \tau t} sin(2 \pi f t)
\end{equation}
With $A$ the amplitude slope, $\tau$ the sine decay rate and $f$ the sine frequency, which are measured material properties, and $v_{in}$ the impact velocity.
With $A$ the amplitude slope, $\tau$ the decay rate and $f$ the frequency, which are measured material properties, and $v_{in}$ the impact velocity.
It has been shown that these material properties perceptually express the stiffness (\secref{hardness}) of real \cite{higashi2019hardness} and virtual surface \cite{choi2021perceived}.
Therefore, when contacting or tapping a real object through an indirect feel-through interface that provide such vibrations (\figref{choi2021augmenting_control}) using a voice-coil (\secref{vibrotactile_actuators}), the perceived stiffness can be increased or decreased \cite{kuchenbecker2006improving,hachisu2012augmentation,choi2021augmenting}, \eg sponge feeling stiffer or wood feeling softer (\figref{choi2021augmenting_results}).
A challenge with this technique is to provide the vibration feedback at the right time, to be felt simultaneous with the real contact \cite{park2023perceptual}.
Therefore, when contacting or tapping a real object through an indirect feel-through interface that provides such vibrations (\figref{choi2021augmenting_control}) using a voice-coil (\secref{vibrotactile_actuators}), the perceived stiffness can be increased or decreased \cite{kuchenbecker2006improving,hachisu2012augmentation,choi2021augmenting}, \eg sponge feels stiffer or wood feels softer (\figref{choi2021augmenting_results}).
A challenge with this technique is to provide the vibration feedback at the right time to be felt simultaneously with the real contact \cite{park2023perceptual}.
\begin{subfigs}{contact_vibrations}{Augmenting perceived stiffness using vibrations when touching a real surface \cite{choi2021augmenting}. }[%
%\item Experimental setup with a voice-coil actuator attached to a touch-through interface.
\item Voltage inputs (top) to the voice-coil for soft, medium and hard vibrations, with the corresponding displacement (middle) and force (bottom) outputs of the actuator.
\item Perceived intensity of stiffness of real sponge ("Sp") and wood ("Wd") surfaces without added vibrations ("N") and modified by soft ("S"), medium ("M") and hard ("H") vibrations.
\item Voltage inputs (top) to the voice-coil for soft, medium, and hard vibrations, with the corresponding displacement (middle) and force (bottom) outputs of the actuator.
\item Perceived stiffness intensity of real sponge ("Sp") and wood ("Wd") surfaces without added vibrations ("N") and modified by soft ("S"), medium ("M") and hard ("H") vibrations.
]
%\subfig[.15]{choi2021augmenting_demo}
\subfigsheight{50mm}
@@ -302,7 +315,7 @@ A challenge with this technique is to provide the vibration feedback at the righ
\subfig{choi2021augmenting_results}
\end{subfigs}
Vibrations on contact have been employed with wearable haptics but, to the best of our knowledge, only to render \VOs \cite{pezent2019tasbi,teng2021touch,sabnis2023haptic}.
Vibrations on contact have been employed with wearable haptics, but to the best of our knowledge only to render \VOs \cite{pezent2019tasbi,teng2021touch,sabnis2023haptic}.
We describe them in the \secref{vhar_haptics}.
%A promising alternative approach
@@ -341,5 +354,11 @@ We describe them in the \secref{vhar_haptics}.
\subsection{Conclusion}
\label{wearable_haptics_conclusion}
Haptic systems aim to provide virtual interactions and sensations similar to those with real objects.
The complexity of the haptic sense has led to the design of numerous haptic devices and renderings.
While many haptic devices can be worn on the hand, only a few can be considered wearable as they are compact and portable, but they are limited to cutaneous feedback.
If the haptic rendering is timely associated with the user's touch actions on a real object, the perceived haptic properties of the object can be modified.
Several rendering methods have been developed to modify the perceived roughness and hardness, but not all of them have been already transposed to wearable haptics.
%, unlike most previous actuators that are designed specifically for fingertips and would require mechanical adaptation to be placed on other parts of the hand.
% thanks to the vibration propagation and the sensory capabilities distributed throughout the skin, they can be placed without adaption and on any part of the hand