Both the system and human have a step in the physical world and one outside the physical world.
The objective of this pipeline is to transmit information to users through their sense of touch.
This is indeed a simplified pipeline, each step being studied by several research communities.
-While each step of the pipeline is essentially studied by one or two scientific fields, the role of Human-Computer Interaction is to connect them in a meaningful way.
+While each step of the pipeline is essentially studied by one or two scientific fields, the role of Human-Computer Interaction is to connect them in a meaningful and useful way for people.
\begin{figure}[htb]
\centering
This is less used because forces are harder to measures, especially if precision is required.
The advantage of such systems is their much higher stiffness than inductance systems.
+EMS \cite{tamaki10}
+
There are many ways to compute tactile signals, especially due to the diversity of actuation mechanisms and associated effects.
Vibrotactile feedback is certainly the most common type of haptic feedback, because of its low price and simplicity.
\defword{Eccentric Rotating Mass} (ERM) actuators are widely used.
The mechanical effect results from the centrifugal force of the rotating mass.
There is a delay before the motor spins fast enough, and inertia when the signal stops.
Therefore there is no easy way to control both the frequency and amplitude of the produced mechanical effect with these actuators.
-\defword{Voice coil actuators} actuators work similarly to speakers.
+\defword{Voice coil actuators} actuators work similarly to speakers, and they can actually be controlled with sound systems.
+The only difference is the frequency range, which is 1-1000Hz, compared to 200-20000Hz for sound.
They produce vibration with a coil and a permanent magnet~\cite{mortimer07,yao10}.
The precision of these actuators enables fine control of both frequency and amplitude.
Such precision is either used for encoding abstract messages called Tactons~\cite{brewster04,hoggan07}, or reproduce tactile effects such as button clicks~\cite{nashel03,lylykangas11}.
The squeeze film effect uses a high-frequency signal (tens of thousand Hertz) that we cannot perceive directly.{}
This vibration creates an air cushion between the finger and the surface so that this surface feels smoother~\cite{biet07}.
-\paragraph{Sense of touch}{}
+Finally, there are non-contact tactile technologies that transmit tactile feedback through air.
+The first technique uses an array of ultrasound actuators.
+The interferences of ultrasound waves create a stress field that triggers the mechanoreceptors of the skin~\cite{hoshi10,carter13}
+The second technique uses air vortexes~\cite{guptas13,sodhi13}.
+The displacement of a large membrane inside a box moves the air inside, which can escape trough a small circular hole.
+The vortex is created with the pressure, and moves forward on several meters before dissipating.
+
+\paragraph{Sense of touch}
\section{Research questions}
+
%Leverage the sense of touch to improve interaction.
Many places where information can get lost: command resolution, non-linear mechanical effect, bad contact between device and user, effect out of perceptual range, haptic illusions.
-\subsection{Towards a haptic semiology}
+\subsection{Towards a haptic semiology}{}
Lack of Bertin-like haptic semiology \cite{bertin83} \texttt{=>} diversity of sensations, stimulation mechanisms, haptic variables
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