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1-introduction/related-work/2-wearable-haptics.tex
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\section{Augmenting Object Perception with Wearable Haptics}
\label{wearable_haptics}
One of the roles of haptic systems is to render virtual interactions and sensations that are \emph{similar and comparable} to those experienced by the haptic sense with real objects, particularly in a visual \VE~\cite{maclean2008it,culbertson2018haptics}.
Moreover, a haptic augmentation system should \enquote{modulating the feel of a real object by virtual [haptic] feedback}~\cite{jeon2009haptic}, \ie a touch interaction with a real object whose perception is modified by the addition of virtual haptic feedback.
The haptic system should be hand-held or worn, \eg on the hand, and \enquote{not permanently attached to or integrated in the object}~\cite{bhatia2024augmenting}.
One of the roles of haptic systems is to render virtual interactions and sensations that are \emph{similar and comparable} to those experienced by the haptic sense with real objects, particularly in a visual \VE \cite{maclean2008it,culbertson2018haptics}.
Moreover, a haptic augmentation system should \enquote{modulating the feel of a real object by virtual [haptic] feedback} \cite{jeon2009haptic}, \ie a touch interaction with a real object whose perception is modified by the addition of virtual haptic feedback.
The haptic system should be hand-held or worn, \eg on the hand, and \enquote{not permanently attached to or integrated in the object} \cite{bhatia2024augmenting}.
\subsection{Level of Wearability}
@@ -14,7 +14,7 @@ Different types of haptic devices can be worn on the hand, but only some of them
An increasing wearability resulting in the loss of the system's kinesthetic feedback capability.
\begin{subfigs}{pacchierotti2017wearable}{
Schematic wearability level of haptic devices for the hand~\cite{pacchierotti2017wearable}.
Schematic wearability level of haptic devices for the hand \cite{pacchierotti2017wearable}.
}[
\item World-grounded haptic devices are fixed on the environment to provide kinesthetic feedback to the user.
\item Exoskeletons are body-grounded kinesthetic devices.
@@ -28,7 +28,7 @@ An increasing wearability resulting in the loss of the system's kinesthetic feed
Haptic research comes from robotics and teleoperation, and historically led to the design of haptic systems that are \emph{world-grounded} to an external support in the environment, such as a table (\figref{pacchierotti2017wearable_1}).
These are robotic arms whose end-effector is either held in the hand or worn on a finger and which simulate interactions with a \VE by providing kinesthetic forces and torques feedback (\figref{pacchierotti2015cutaneous}).
They provide high fidelity haptic feedback but are heavy, bulky and limited to small workspaces~\cite{culbertson2018haptics}.
They provide high fidelity haptic feedback but are heavy, bulky and limited to small workspaces \cite{culbertson2018haptics}.
More portable designs have been developed by moving the grounded part to the user's body.
The entire robotic system is thus mounted on the user, forming an exoskeleton capable of providing kinesthetic feedback to the finger, \eg in \figref{achibet2017flexifingers}.
@@ -42,9 +42,9 @@ Moreover, as detailed in \secref{object_properties}, cutaneous sensations are ne
\begin{subfigs}{grounded_to_wearable}{
Haptic devices for the hand with different wearability levels.
}[
\item Teleoperation of a virtual cube grasped with the thumb and index fingers each attached to a grounded haptic device~\cite{pacchierotti2015cutaneous}.
\item A passive exoskeleton for fingers simulating stiffness of a trumpet's pistons~\cite{achibet2017flexifingers}.
\item Manipulation of a virtual cube with the thumb and index fingers each attached with the 3-RSR wearable haptic device~\cite{leonardis20173rsr}.
\item Teleoperation of a virtual cube grasped with the thumb and index fingers each attached to a grounded haptic device \cite{pacchierotti2015cutaneous}.
\item A passive exoskeleton for fingers simulating stiffness of a trumpet's pistons \cite{achibet2017flexifingers}.
\item Manipulation of a virtual cube with the thumb and index fingers each attached with the 3-RSR wearable haptic device \cite{leonardis20173rsr}.
]
\subfigsheight{38mm}
\subfig{pacchierotti2015cutaneous}
@@ -65,9 +65,9 @@ Several actuators are often combined in a haptic device to obtain richer haptic
\subsubsection{Moving Platforms}
\label{moving_platforms}
The moving platforms translate perpendicularly on the skin to create sensations of contact, pressure and edges~\cite{pacchierotti2017wearable}.
The moving platforms translate perpendicularly on the skin to create sensations of contact, pressure and edges \cite{pacchierotti2017wearable}.
Placed under the fingertips, they can come into contact with the skin with different forces, speeds and orientations.
The platform is moved by means of cables, \eg in \figref{gabardi2016new}, or articulated arms, \eg in \figref{perez2017optimizationbased}, activated by motors grounded to the nail~\cite{gabardi2016new,perez2017optimizationbased}.
The platform is moved by means of cables, \eg in \figref{gabardi2016new}, or articulated arms, \eg in \figref{perez2017optimizationbased}, activated by motors grounded to the nail \cite{gabardi2016new,perez2017optimizationbased}.
The motors lengthen and shorten the cables or orient the arms to move the platform over 3 \DoFs: two for orientation and one for normal force relative to the finger.
However, these platforms are specifically designed to provide haptic feedback to the fingertip in \VEs, preventing interaction with a \RE.
@@ -76,18 +76,18 @@ However, these platforms are specifically designed to provide haptic feedback to
A pin-array is a surface made up of small, rigid pins arranged very close together in a grid and that can be moved individually.
When placed in contact with the fingertip, it can create sensations of edge, pressure and texture.
The \figref{sarakoglou2012high} shows an example of a pin-array consisting of \numproduct{4 x 4} pins of \qty{1.5}{\mm} diameter and \qty{2}{\mm} height, spaced at \qty{2}{\mm}~\cite{sarakoglou2012high}.
Pneumatic systems use a fluid such as air or water to inflate membranes under the skin, creating sensations of contact and pressure~\cite{raza2024pneumatically}.
Multiple membranes are often used in a grid to simulate edges and textures, as in the \figref{ujitoko2020development}~\cite{ujitoko2020development}.
The \figref{sarakoglou2012high} shows an example of a pin-array consisting of \numproduct{4 x 4} pins of \qty{1.5}{\mm} diameter and \qty{2}{\mm} height, spaced at \qty{2}{\mm} \cite{sarakoglou2012high}.
Pneumatic systems use a fluid such as air or water to inflate membranes under the skin, creating sensations of contact and pressure \cite{raza2024pneumatically}.
Multiple membranes are often used in a grid to simulate edges and textures, as in the \figref{ujitoko2020development} \cite{ujitoko2020development}.
Although these two types of effector can be considered wearable, their actuation requires a high level of mechanical and electronic complexity that makes the system as a whole not portable.
\begin{subfigs}{normal_actuators}{
Normal indentation actuators for the fingertip.
}[
\item A moving platform actuated with cables~\cite{gabardi2016new}.
\item A moving platform actuated by articulated limbs~\cite{perez2017optimizationbased}.
\item Diagram of a pin-array of tactors~\cite{sarakoglou2012high}.
\item A pneumatic system composed of a \numproduct{12 x 10} array of air cylinders~\cite{ujitoko2020development}.
\item A moving platform actuated with cables \cite{gabardi2016new}.
\item A moving platform actuated by articulated limbs \cite{perez2017optimizationbased}.
\item Diagram of a pin-array of tactors \cite{sarakoglou2012high}.
\item A pneumatic system composed of a \numproduct{12 x 10} array of air cylinders \cite{ujitoko2020development}.
]
\subfigsheight{37mm}
\subfig{gabardi2016new}
@@ -100,23 +100,23 @@ Although these two types of effector can be considered wearable, their actuation
\label{tangential_actuators}
Similar in design to the mobile platforms, the tangential motion actuators activate a rigid pin or surface in contact with the fingertip under the finger to create shearing sensation on the skin.
An articulated and motorized arm structure moves the effector in multiple directions over 2 \DoFs parallel to the skin, \eg in \figref{leonardis2015wearable}~\cite{leonardis2015wearable}.
Some actuators are capable of both normal and tangential motion over 3 \DoFs on the skin and can also make or break contact with the finger, \eg in \figref{schorr2017fingertip}~\cite{schorr2017fingertip}.
An articulated and motorized arm structure moves the effector in multiple directions over 2 \DoFs parallel to the skin, \eg in \figref{leonardis2015wearable} \cite{leonardis2015wearable}.
Some actuators are capable of both normal and tangential motion over 3 \DoFs on the skin and can also make or break contact with the finger, \eg in \figref{schorr2017fingertip} \cite{schorr2017fingertip}.
\subsubsection{Compression Belts}
\label{belt_actuators}
A simpler alternative approach is to place a belt under the finger, and to actuate it over 2 \DoFs by two motors placed on top of the finger~\cite{minamizawa2007gravity}.
A simpler alternative approach is to place a belt under the finger, and to actuate it over 2 \DoFs by two motors placed on top of the finger \cite{minamizawa2007gravity}.
By turning in opposite directions, the motors shorten the belt and create a sensation of pressure.
Conversely, by turning simultaneously in the same direction, the belt pulls on the skin, creating a shearing sensation.
The simplicity of this approach allows the belt to be placed anywhere on the hand, leaving the fingertip free to interact with the \RE, \eg the hRing on the proximal phalanx in \figref{pacchierotti2016hring}~\cite{pacchierotti2016hring} or Tasbi on the wrist in \figref{pezent2022design}~\cite{pezent2022design}.
The simplicity of this approach allows the belt to be placed anywhere on the hand, leaving the fingertip free to interact with the \RE, \eg the hRing on the proximal phalanx in \figref{pacchierotti2016hring} \cite{pacchierotti2016hring} or Tasbi on the wrist in \figref{pezent2022design} \cite{pezent2022design}.
\begin{subfigs}{tangential_belts}{Tangential motion actuators and compression belts. }[
\item A skin strech actuator for the fingertip~\cite{leonardis2015wearable}.
\item A 3 \DoF actuator capable of normal and tangential motion on the fingertip~\cite{schorr2017fingertip}.
%\item A shearing belt actuator for the fingertip~\cite{minamizawa2007gravity}.
\item The hRing, a shearing belt actuator for the proximal phalanx of the finger~\cite{pacchierotti2016hring}.
\item Tasbi, a wristband capable of pressure and vibrotactile feedback~\cite{pezent2022design}.
\item A skin strech actuator for the fingertip \cite{leonardis2015wearable}.
\item A 3 \DoF actuator capable of normal and tangential motion on the fingertip \cite{schorr2017fingertip}.
%\item A shearing belt actuator for the fingertip \cite{minamizawa2007gravity}.
\item The hRing, a shearing belt actuator for the proximal phalanx of the finger \cite{pacchierotti2016hring}.
\item Tasbi, a wristband capable of pressure and vibrotactile feedback \cite{pezent2022design}.
]
\subfigsheight{33.5mm}
\subfig{leonardis2015wearable}
@@ -154,7 +154,7 @@ Piezoelectric actuators deform a solid material when a voltage is applied. They
\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 sine wave input with different frequencies: both their maximum force and resonant frequency are different \cite{azadi2014vibrotactile}.
]
\subfigsheight{50mm}
\subfig{precisionmicrodrives_lra}
@@ -165,21 +165,21 @@ 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.
The integration of the real and virtual haptic sensations into a single property perception is discussed in more details in \secref{sensations_perception}.
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.
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 greatly influence the perception of its haptic properties, \eg by modifying its visual texture in \AR or \VR, as discussed in the \secref{visuo_haptic}.
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.
Also called direct feel-through~\cite{jeon2015haptic}, 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.
In touch-through and tool-mediated, or \emph{indirect feel-through}, the haptic device is interposed between the hand and the \RE or worn on the hand, respectively.
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.
%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 grounded haptic devices and later transposed to wearable haptic devices.
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 present bellow the methods to modify these properties with wearable haptic devices.
Of course, wearable haptics can also be used to modify the perceived friction \cite{konyo2008alternative,salazar2020altering}, weight \cite{minamizawa2007gravity}, or local deformation \cite{salazar2020altering} of real objects, but they are less common \cite{bhatia2024augmenting} and will not be detailed here.
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.
% \cite{klatzky2003feeling} : rendering roughness, friction, deformation, temperatures
% \cite{girard2016haptip} : renderings with a tangential motion actuator
@@ -187,31 +187,54 @@ Of course, wearable haptics can also be used to modify the perceived friction \c
\subsubsection{Roughness}
\label{texture_rendering}
To modify the perception of haptic roughness (or texture, see \secref{roughness}) of a real object, vibrations are provided to the skin by the wearable haptic device when running the finger over the surface.
Two approaches are used to render virtual textures: \emph{physics-based models} and \emph{data-driven models}~\cite{culbertson2018haptics}.
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}.
\paragraph{Physics-based Models}
\paragraph{Simulation Models}
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}.
Notably, \textcite{okamura1998vibration} rendered grating textures with exponentially decaying sinudoids that simulated the strokes of the grooves and ridges of the surface, while \textcite{culbertson2014modeling} captured and modelled the roughness of real surfaces to render them using the speed and force of the user.
\cite{chan2021hasti,guruswamy2011iir}
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.
\textcite{ando2007fingernailmounted} were the first to propose this approach that they experimented with a voice-coil mounted on the index nail (\figref{ando2007fingernailmounted}).
The sensation of crossing edges of a virtual patterned texture (\secref{texture_rendering}) on a real sheet of paper were rendered with \qty{20}{\ms} vibration impulses at \qty{130}{\Hz}.
Participants were able to match the virtual patterns to their real counterparts of height \qty{0.25}{\mm} and width \qtyrange{1}{10}{\mm}, but systematically overestimated the virtual width to be \qty{4}{\mm} longer.
Early renderings of virtual textures consisted of modelling the surface with a periodic function
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}.
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}.
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}.
\paragraph{Data-driven Models}
Because physics-based models to render realistic 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 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}.
An effective approach to rendering virtual roughness is to generate vibrations to simulate interaction with the virtual texture~\cite{culbertson2018haptics}, relying on the user's real-time measurements of position, velocity and force to modulate the frequencies and amplitudes of the vibrations, with position and velocity being the most important parameters~\cite{culbertson2015should}.
For example, when comparing the same virtual texture pairwise, but with different parameters, \textcite{culbertson2015should} showed that the roughness vibrations generated should vary with user speed, but not necessarily with user force.
Virtual data-driven textures were perceived as similar to real textures, except for friction, which was not rendered properly.
\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.
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}.
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.
\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 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}.
]
\subfigsheight{35mm}
\subfig{asano2015vibrotactile}
\subfig{friesen2024perceived}
\subfig{ando2007fingernailmounted}
\subfig{culbertson2012refined}
\end{subfigs}
\subsubsection{Hardness}
@@ -221,7 +244,7 @@ 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}.
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}).
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:
@@ -231,7 +254,7 @@ The virtual force of the device $\tilde{f_r}(t)$ is then controlled to:
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}.
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}.
\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}.
@@ -244,7 +267,7 @@ This stiffness augmentation technique was then extended to enable tapping and pr
\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}.
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}).
\begin{subfigs}{stiffness_rendering_wearable}{Modifying the perceived stiffness with wearable pressure devices. }[%
@@ -264,9 +287,9 @@ Conversely, the technique allowed to \emph{decrease} the perceived stiffness by
Q(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.
It has been shown that these material properties perceptually express the stiffness (\secref{hardness}) of real~\cite{higashi2019hardness} and virtual surface~\cite{choi2021perceived}.
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}.
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}.
\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.
@@ -279,7 +302,7 @@ 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