This paper describes a new tactile device which produces stress
fields in 3D space. Combined with midair and/or 3D stereoscopic
displays, this device provides highfidelity tactile feedback for
interaction with visual objects. The principle is based on a
nonlinear phenomenon of ultrasound; acoustic radiation pressure.
The fabricated prototype device consists of 324 airborne
ultrasound transducers, and the phase and intensity of each
transducer are controlled individually. The total output force
within the focal region is 1.6 gf. The spatial resolution is 20 mm.
The prototype can produce sufficient vibrations up to 1 kHz. An
interaction system including the prototype is also introduced
which tracks user’s hand and provides suitable touch feeling.
! Tactile display, Airborne ultrasound, Acoustic
radiation pressure
I.3.7 [Computer Graphics]: ThreeDimensional
Graphics and Realism.Virtual Reality
" ##
There has been a wide interest in midair and/or 3D displays and
such displays have been seen in many SF movies [1]. People in
the movies see and manually interact with touch screens or virtual
objects floating in front of them. Recently, novel technologies are
developed to render images hovering in air without special glasses.
FogScreen [2] and Heliodisplay [3] use a thin layer of fog as a
projection screen. Holo [4] provides floating images from an LCD
by utilizing a concave mirror. SeeReal Technologies is working
on a realtime, computergenerated, and 3D holography [5]
through the use of an eyetracking technology for reduction of
calculation amount. Furthermore, by applying the camerabased
and markerless hand tracking techniques demonstrated in
Holovizio [6] or GrImage [7], we will be able to handle the
projected images with our hands. Then, tactile feedback will be
the next demand. If tactile feedback is provided additionally, the
usability of the interaction systems will be highly improved.
There are three types of conventional strategies for tactile
feedback in free space. The first is attaching tactile devices on
user’s fingers and/or palms. Employed devices are, for example,
vibrotactile stimulators (CyberTouch [8]), motordriven belts
(GhostGlove [9]), and pinarray units (SaLT [10]). In this strategy,
the skin and the device are always in contact and that leads to
undesired touch feelings. The second is controlling the position of
tactile devices so that they contact with the skin only when tactile
feedback is required. In the masterslave system shown in [11],
the encountertype force feedback is realized by the exoskeleton
× !
master hand. The detailed tactile feedback for each finger is
provided by the electrotactile display attached on the finger part of
the master hand. The drawback of this strategy is that it requires
bulky robot arms. The last is providing tactile feedback from a
distance without any direct contact. For example, airjets are
utilized in [12] to realize noncontact force feedback. Although
airjet is effective for rough “force” feedback, its spatial and
temporal properties are quite limited and it cannot provide
detailed “tactile” feedback.
We have proposed a method for producing tactile sensation
with airborne ultrasound [13]. The method renders desired
pressure pattern in free space by using wave field synthesis with
high spatial and temporal resolution. Users can feel the pressure
with their bare hands. In [13], the prototype consisting of 91
ultrasound transducers was introduced and the feasibility of the
proposed method was discussed. It can move a focal point only
along Z axis. In this paper, we show an upgraded version of the
prototype consisting of 324 ultrasound transducers (Figure 1),
which can move a focal point threedimensionally. The structure
and performance of the device are described. An interaction
system is also shown which tracks user’s hand and provides tactile
feedback according to the hand’s position.
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Our method is based on a nonlinear phenomenon of ultrasound;
acoustic radiation pressure. The acoustic radiation pressure P [Pa]
is described as
2
2
ρc
p
α
c
I
α αE P = = =
(1)
where E [J/m
3
] is the energy density of the ultrasound, I [W/m
2
] is
the sound intensity, c [m/s] is the sound speed, p [Pa] is the RMS
sound pressure of the ultrasound, and ρ [kg/m
3
] is the density of
the medium. α [] is a constant ranging from 1 to 2 depending on
the reflection coefficient R [] at object surface; α ≡1+ R
2
. In case
the object surface perfectly reflects the incident ultrasound, α = 2,
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Third Joint Eurohaptics Conference and Symposium on Haptic Interfaces
for Virtual Environment and Teleoperator Systems
Salt Lake City, UT, USA, March 18-20, 2009
978-1-4244-3858-7/09/$25.00 ©2009 IEEE 256