arXiv:1911.02985v2 [cs.HC] 9 Dec 2019

Sonovortex: Aerial Haptic Layer Rendering by Aerodynamic
Vortex and Focused Ultrasound
Satoshi Hashizume∗

Amy Koike
Takayuki Hoshi
University of Tsukuba

Abstract

Yoichi Ochiai

In this paper, a method of rendering aerial haptics
that uses an aerodynamic vortex and focused ultrasound is presented. Significant research has been conducted on haptic applications based on multiple phenomena such as magnetic and electric fields, focusedultrasound, and laser plasma. By combining multiple physical quantities; the resolution, distance, andmagnitude of force are enhanced. To combine multiple tactile technologies, basic experiments on resolution and discrimination threshold are required. Separate user studies were conducted using aerodynamic
and ultrasonic haptics. Moreover, the perception of
their superposition, in addition to their resolution,
was tested. Although these fields cause no direct interference, the system enables the simultaneous perception of the tactile feedback of both stimuli. The
results of this study are expected to contribute to expanding the expression of aerial haptic displays basedon several principles.

ical contact or wearable devices. Second, it has ahigh programmability. In particular, it can be set
and rearranged at an arbitrary position in a threedimensional (3D) space, as it does not require physical actuators.
In this study, new aerial interactions were evaluated. The aim of the study was the development of
a new aerial haptics system to express a wide rangeof feedback. The proposed system (Figure 1) combines aerodynamic and acoustic fields. The aerodynamic vortex [1] from the aerodynamic field and thefocused ultrasound [2] from the acoustic field wereused to develop the device.
The tactile sensations of single and multiple fields
were then compared. Through a user study, it was
found that the aerodynamic vortex and focused ultrasound do not influence each other. By combining
different types of forces, the proposed system can display various textures. Based on the reports in theliterature, this is an early study that combines multifield physical quantities to render haptic textures.

1

2

Related Work

2.1

Aerial Haptics Feedback

Introduction

Aerial haptic feedback is a popular topic in research
fields such as real-world-oriented interaction, augmented reality (AR), and virtual reality (VR). Several methods have therefore been proposed to realize
aerial haptic feedback which include phenomena suchas magnetic forces, ultrasound, and air vortices.
An aerial haptic display has several advantages.
First, it projects a force over a distance without phys-

Several methods have been proposed for aerial haptic feedback without physical contact or wearable
devices. The technologies employed without wearable devices are based on aerodynamic vortices, focused ultrasound, laser induced plasma, and magnetic forces. These technologies have been applied
to touch panels [3] and VR [4] systems .
Ultrasonic technology, which uses ultrasound, can∗ hashizume@digitalnature.slis.tsukuba.ac.jp1

et al. [14] and Berkelman et al. [15] rendered a 3Dmodel in mid-air using an electromagnet array. In
direct tactile presentation, powerful tactile feedback
can be achieved without touching the screen.
Light is employed to provide sensation on thehands when the user is experiencing thermal radiation [16]. Nanosecond lasers applied to the skininduce a tactile sensation [17] . To date, radiofrequency and superconducting forces have not been
applied to aerial haptic feedback.
Figure 1: Sonovortex device.

2.2
provide tactile sensations in mid-air without the needfor a user actuator [5] [2]. The position of the focal
point can be changed using a phased array transducer
as it represents tactile behavior. The rendering ofvolumetric haptic shapes can also be achieve using
focused ultrasound [6]. MidAir [3] reflects a virtual
image in the air and provides tactile feedback using
an ultrasonic speaker based on the virtual image andfinger location. HaptoClone [7] enables real-time interaction with floating volumetric images using haptic feedback. This method is insufficient, given thatthe focused ultrasound force is very weak and only
limited focal points can be generated.
In [8], an air jet was used to produce contactless
haptic feedback with a low accuracy. In [4], virtual
objects were represented by air jets from an array of
nozzles. Vortex rings [1] have also been used as a
non-contact haptic feedback system, and air vortices
have been used to produce impact in midair [9] [10].
These previous approaches mainly used vortex rings
to add multi-modal sensation to a conventional display. Ueoka et al.[11] evaluated the manner in whichpeople perceive haptic stimuli generated by air vortex
rings and how the stimuli affect their emotional states
when stressed. However, both these methods have a
low fidelity due to the non-focusing stimulating area.
The tactile presentation of a magnetic field involves
both direct and indirect presentation. For direct tactile feedback, a magnet can be placed on the finger [12] . The authors in [13], presented magnetic
based sensing in addition to haptic feedback. Zhang

Cross-Field Haptics

This study combines multiple haptics technologies,
thus overcoming their individual drawbacks and improving the interaction width.
The wUbi-Pen [18] is a pencil-type tactile interface
that consist of a vibrator, linear vibrator, speaker,
and pin array. It provides functions such as feedback drag, drop, and movement. Minamizawa etal. [19] developed a device based on tactile presentation, which that combines one-point kinesthetic feedback and multipoint tactile feedback. The accuracyof the feedback was then improved by combining haptics technologies. Impacto [20] was designed to render the haptic sensation of hitting and being hit in
the VR environment. They combined tactile stimulation with electrical muscle stimulation. Cross-Field
Aerial Haptics [21] involves the drawing of a tactileinterface in the air by combining ultrasonic waves
and laser plasmas. Hashizume et al. [22] developed
a touch type haptic device that combines magnetic
and electrostatic fields. Their report also includes a
description of their implementation method.
Cross-field haptics is not a widely studied field.
Aerodynamic vortex and focused ultrasound are simultaneously utilized. Using both fields helps eliminate their drawbacks and it is intended to provide awider tactile presentation. The aerodynamic vortex
provides tactile sensations over large distances andforces. The focused ultrasound delivers distinguishable high-resolution tactile sensations.
2Air Cannon

According to [23][24], LS can then be expressed as
LS =

D

4VS
πD2

where VS is the slug volume.
Thus, for stable vortices, the following is true:
4VS
5 4.5
πD3

Focused Ultrasound(1)

An air cannon based on AIREAL, was developed inthis study [9]. Five 2-inch 15W Whisper subwoofers
were used as actuators. Sodhi et al. [9] determined
the total volume of air displaced by all five speakers
and the aperture diameter using the previous equations:
VS = 33, 670 mm3 , D = 2.1 cm
Figure 2: Air cannon and focused ultrasound visualized using dry ice and smoke.

3

Implementation

3.1

Aerodynamic Haptics

3.2

Ultrasound Haptics

Ultrasonic haptics are based on acoustic radiationpressure, which exerts a force on the surface of theskin (Figure 2, lower). Ultrasonic haptics can be applied to the skin for a long time-period; however, they
are relatively weak (10-20 mN). The sensation is similar to that of a laminar air flow within a narrow area.
The time delay ∆tij for the (i, j)-th transducer isgiven by:l00 − lij
∆tij =(2)c

An air vortex (Figure 3, right) is a ring of air thattypically has a toroidal shape and can travel at highspeeds over large distances. Vortex rings (Figure 2,
upper) can be formed by pushing air using a piston
through a circular aperture, or hole. The quality of
the formed vortex is dependents on the volume of air
pushed, the velocity of the piston, and the diameterof the aperture.
Gharib et al. [1] defined the stroke ratio RStroke
as a ratio of the length of the theoretical cylindricalslug of air pushed out of the nozzle LS to aperture ofdiameter D:
LS
RStroke =Dwhere l00 and lij are the distances from the focal
point to the (0, 0)-th reference and (i, j)-th transducers, respectively; and c is the speed of sound in the
air. The focal point can be moved by recalculationand the setting of the time delays for the next coordinates.
Haptic images are generated by an acoustic phasedarray system(Figure 3, middle). Haptic image Hi is
the summation of the time series of the focal points:

The stroke ratio characterizes the stability of the vorXHi =fp (x, y, z) × p × t(3)
tex as it exits the aperture, and it is used to define
the formation number. A typical value for the formation number is within the range of 3.6-4.5 for several where fp is the ultrasonic focal points, p is the acousvarious vortex systems.
tic pressure, and t is the time duration.
3

Whisper subwoofer

Air cannon

Phased array

40kHz transducers

NozzleDisplayed area

Phased array

Air cannon

Figure 3: Aerodynamic and ultrasound system.

3.3

Latency

4.1

Experiment of generated force

The amount of time required to produce a tactile
presentation using an aerodynamic vortex is different from that required using the focused ultrasound.
The focused ultrasound advances in the air at the
speed of sound. Therefore, the focused ultrasoundcan achieve a near simultaneous tactile presentation
with the generation of signals. The speed and delayof the aerodynamic vortex are greater than those ofultrasound. According to [10], the vortex speed isgiven by

The magnitude of the force generated by an ultrasonic wave and air cannon were considered. The precision electronic balance was set vertically and placed
15 cm from the ultrasonic device and air cannon (Figure 4). The mass displayed on the precision electronicbalance was then converted to force. The air cannon
was output at 30 Hz, and the power supply voltage
was varied from 5 to 17.5 V in increments of 2.5 V.
In addition the change in the magnitude of the generated force was examined. The ultrasonic wave was
output at a modulation frequency of 50 Hz, and theLSvs =output intensity was changed from 0 to 600 in incretcone
ments of 100. Moreover, the change in the magnitude
vsvvortex =of the generated force was investigated. To convert2
the output intensity to the output force, a conversion
where tc one is the time required for the displaced air
could be carried out using sin2 (πp/1248) [2]. The
to move through the aperture. The average speed
ultrasonic focal length was set as 15 cm.
of the aerodynamic vortex device used in this study
Figure 5 present the results, in which both the ulwas 7.2m/s [9]. A delay of 30mms was applied to the
trasonic wave and air cannon increase the force genfocused ultrasound to ensure that the aerodynamic
erated in proportion to the output intensity.
vortex and focused ultrasound reach the user simultaneously.

4.2

4

Experimentthresholdof

double-point

EXPERIMENTS AND REThe user study was conducted to investigate spatial
SULTS

resolution. The double-point threshold[25] for acousIn this section, a discussion on the user experiments tic radiation pressure induced by focused ultrasoundfor the evaluation proposed haptic system is pre- and air vortex pressure was evaluated. Five people
participated in the user study (20.2 years old on avsented.
4was set as 5.73 mN. The participants were stimulatedwith ultrasound for 5 s. The output force of the air
Air cannon
cannon was set as 7.67 mN, and the stimulation wasapplied five times. To provide a constant air vortex,
15cm
15cm
the air cannon was implemented at 15 Hz.
Results : The results are shown in Figure 6. In the(a) Setup of air cannon in(b) Setup of ultrasonic array in
caseof only ultrasonic waves (Figure 6 (a) and (b)),
experiment of generated force.
experiment of generated force.
the double-point threshold was approximately 6 mm
regardless of the modulation frequency. The doubleFigure 4: Setup of experiment on generated force
point threshold for an air cannon only (Figure 6 (c))
was 11 mm. The double-point threshold of an air canerage, with one female and four males). The par- non while an ultrasonic wave was constantly provided
ticipants were isolated from visual information using (Figure 6 (d) and (e)) was not much different fromblindfolds. Participants then placed their hands on that of an air cannon only. The double-point thresha table positioned 15 cm away from the haptic de- old of the ultrasonic wave while an air vortex was
vice(Figure 7 right). The platform could be moved constantly provided (Figure 6 (f) and (g)) was apwith an accuracy of 0.1 mm, as participants were told proximately 3 mm larger that of the ultrasonic only.
not to move their hands. The output force was set In the case in which the air cannon was affected, thewith reference to Figure 5.
variation of the double-point threshold by the particThe method of limits was used to measure the ipant was large.
double-point threshold. First, the standard stimulus
was applied to the palm of the hand of the partic- 4.3 Experiment of perceptual threshipant. The standard stimulus is a 10 s ultrasoundoldwave and five air vortex cycles. After administering
the standard stimulus, the platform was moved and The user study was conducted to evaluate the percepthe comparative stimulus was administered. After tual threshold for acoustic radiation pressure induced
administering the comparative stimulus, each partic- by the focused ultrasound and air the pressure voripant gave one of the following answers: ”(1) the two tex. Seven people participated in the user study (19.6
points are divided,” (2) ” the two points not are di- years old on average with two females and five males).
vided,” or (3) ”I do not know.” The distance between The participants were isolated from visual informathe standard and the comparative stimuli was ap- tion using blindfolds, and auditory information was
proximated until the participant answered ”the two eliminated using headphones with white noise (Figpoints are not divided” or ”I do not know” (descend- ure 7, left). Participants placed their hands on a table
ing series). The distance between the standard and positioned 15 cm away from the haptic device(Figurecomparative stimuli was then increased until the par- 7 right). The output force was set with reference toticipant answered with ”the two points are divided” Figure 5.
(ascending series). The test for the descending andFocused ultrasound : The focused ultrasoundascending series were carried out twice.
haptic stimulation was applied to the right palm of
The experiments were conducted when only ultra- each participant. Moreover, vibrotactile stimulation
sonic waves were applied, when only an air cannon modulated by 200-Hz and 50-Hz rectangular waves
was used, when an air cannon was used with a con- was applied. The output force was set as one of six
stant ultrasonic wave, and when an ultrasonic wave values (min:0.70 mN, max:10.9 mN) near the threshwas applied with a constant air vortex. The ultra- olds. The ultrasound output time was 200ms. Eachsonic waves were generated at modulation frequencies force condition was applied once (i.e., one trial) andof 50 Hz and 200 Hz. The focal length of the ultra- the number of trials was 10 per participant. The orsonic device was set as 15 cm, and the output force der of trials was randomized. In each trial, the parElectronicbalance

Ultrasonicarray

Electronicbalance5Force [m

12

16
14

4

12

2

8

0

6
4

Force [mN]

Force [mN]

10

6

0

10
8

20

40

60

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100

120

Ultrasonic output power [%]

6
4

2
0

2

0

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60

80

0

100

0

5

10

15

20

Air cannon output power [V]Ultrasonic output power [%](a) Experiment on force of ultrasonic(b) Experiment on force of air cannon

Figure 5: Results of experiment on generated force
Blindfold
Head Phone
Phased array
Air cannon

Displayed area

Displayed area

Figure 7: Overview of experimental setup

Figure 6:
Results of double-point threshold:(a)ultrasonic waves(50Hz), (b)ultrasonic waves(200Hz), (c)an air cannon, (d) an air cannon while
ultrasonic wave (50Hz) was constantly provided, (e)an air cannon while ultrasonic wave (200Hz) wasconstantly procided, (f) ultrasonic wave(50Hz) while
an air vortex was constantly provided. (g) ultrasonicwave(200Hz) while an air vortex was constantly
provided.

participant. The order of trials was randomized. In
each trial, the participants were asked whether they
perceived the stimuli on their forefingers.
Cross-field : Three types of user studies were conducted on the cross-field. First, focused ultrasoundand an aerodynamic vortex tactile stimulus were applied simultaneously. The output force of the air cannon was set as 7.67 mN. The vibrotactile stimulationof ultrasound was set as 200 Hz and 50 Hz. Second,
the air vortex perceptual threshold in space with aconstant ultrasound was evaluated. The output forceof the ultrasound was maintained at 9.7 mN. The output force of the air vortex was set to one of six values (min:0.66 mN, max:13.7 mN). Third, the focusedultrasound perceptual threshold in space with a constant aerodynamic vortex was evaluated. The output
force of the air vortex was set as 7.67 mN and the air
cannon was actuated to 20Hz. The output force of ultrasound was set as one of six values (min:0.70 mN,

ticipants were asked whether they perceived stimuli
on their forefingers.
Aerodynamic vortex : The aerodynamic vortex
haptic stimulation was applied to the right palm ofeach participant. The output force was set to oneof six values (min:0.66 mN, max:13.7 mN) near thethresholds. Each force condition was applied once
(i.e., one trial) and the number of trials was 10 per6

perceive in the air cannon presentation state. Thisis because the magnitude of the force that the air
cannon can apply is larger than the ultrasonic tactile stimuli. It is necessary to appropriately adjust
the magnitude of the applied force. However, when
the tactile stimulus was simultaneously applied, neither tactile sense could be perceived. When using
Sonovortex, it is effective to present the air cannon
tactile sense in the ultrasonic presentation state, or
to simultaneously present the air cannon and the ultrasonic.

Table 1: Results of simultaneous tactile presentation
with a focused ultrasound and aerodynamic vortex
The ultrasound
The perceptual
vibrotactile stimulation threshold of haptic
[Hz]
[%]
50
95.2
200
100
max:10.9 mN) and the vibrotactile stimulation was
modulated to 200 Hz and 50 Hz.
Results : The results are presented in Figure 8
and Table 1. The perception rate on the verticalaxis indicates the ease of tactile sensation. When theperception threshold was 100%, the examinee could
perceive the tactile sensation.
In the case wherein only the ultrasound was applied, the perception threshold was nearly 100% when
the output exceeded 4 mN. In the case wherein only
the air cannon was used, the perception threshold increased as the voltage increased. In addition, when
the output force exceeded 11 mN, the participants
could perceive feel the tactile sensation. When the air
cannon was operated with an ultrasonic wave, sensing when the output force reached 2 mN was difficult.
However, when the output force was higher than 2
mN, the results were the same as that wherein only
the air cannon was used. When ultrasonic waves wereapplied with an air cannon, the perception threshold
was generally low compared with the case wherein
only ultrasonic waves was applied. In particular,when the modulation frequency was 50 Hz, the perception threshold was less than 20 %. When presented simultaneously, participants could recognize aperception threshold of 95 % or greater.

5

In this study, a method was developed that combines multiple tactile technologies. This method generates a tactile sensation using an ultrasonic device
and air cannon. In addition, the ranges of possible
resolutions and thresholds were discussed. Cross-fieldhelps eliminate the drawbacks of each field and provide a wider tactile presentation. An aerodynamic
vortex provides tactile sensations over large distances
and with considerable force levels. The focused ultrasound delivers distinguishable high-resolution tactile
sensations.
However, there are still several drawbacks. Bothan air cannon and ultrasonic device generate environmental noise. In particular, given that the air
cannon produces a loud sound, using it in a quiet
space such as a hospital or company office is difficult.
However, this does not present a problem in noisy
spaces such as shops and towns. Moreover, to use
Sonovortex, an air cannon and phased array must be
deferred. For wearing, Sonovortex is heavy and large.
However, Sonovortex is expected to be incorporated
into the environment and used for digital signage andamusement.

Discussion and Conclusion

From the experiments conducted on double-point
thresholds, the double-point threshold was found toincrease the when air cannon and ultrasonic wave
were combined. In addition, from the experimentson perceptual thresholds, the air cannon was easy
to perceive in the ultrasonic presentation state; however, the ultrasonic tactile stimulus was difficult to

Acknowledgement
We would like to thank University of Tsukuba forsupporting this work. We are also thankful to all themembers of the Digital Nature Group at Universityof Tsukuba for their discussions and feedback.
7

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100

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perception rate [%](b) Experiment on perceptual threshold of air vortex

100

Perception rate [%](a) Experiment on perceptual threshold of ultrasonic

60

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Air cannon output force [mN](c) Experiment on perceptual thresholdof air vortex in ultrasonic space(d) Experiment on perceptual thresholdof ultrasonic in air vortex space

Figure 8: Results of perceptual threshold.

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