166
Luciola: A Millimeter-Scale Light-Emitting Particle Moving in
Mid-Air Based On Acoustic Levitation and Wireless Powering
YUKI UNO, The University of TokyoHAO QIU, The University of TokyoTORU SAI, The University of TokyoSHUNTA IGUCHI, The University of Tokyo
YOTA MIZUTANI, The University of Tokyo
TAKAYUKI HOSHI, The University of TokyoYOSHIHIRO KAWAHARA, The University of TokyoYASUAKI KAKEHI, Keio University
MAKOTO TAKAMIYA, The University of Tokyo
In this paper, we present an approach to realize the levitation of a small object with an embedded electronic circuit. Luciola
is a light-emitting particle with a diameter of 3.5mm and a weight of 16.2mg moving in mid-air in a 10.4cm ´ 10.4cm ´ 5.4cm
space through acoustic levitation using two 40-kHz 17 ´ 17 ultrasonic transducer arrays placed face-to-face at a distance of
20cm and wirelessly powered by 12.3-MHz resonant inductive coupling. The novelty of this paper is the acoustically levitatedelectronic object by the combined application of ultrasonic levitation and wireless powering to the levitated electronic object.
A new shape of the levitated object and a new placement of the receiver coil to simultaneously realize acoustic levitation
and wireless powering are proposed, achieving a stable wireless powering to a rotating levitated object at the bottom of an
acoustic potential. To enable the levitation of a particle, a custom IC chip is essential in reducing the size and weight of theparticle. In the design of the custom IC chip, a new voltage detector circuit enabling an accurate voltage detection and acorrect output during the start-up is proposed to achieve an intermittent lighting of the LED to increase the maximum
distance between the transmitter and the receiver coil. Luciola is applied to a self-luminous pixel in a mid-air display anddrawings of characters in mid-air are demonstrated.
CCS Concepts: • Human centered computing → Human computer interaction (HCI)
General Terms: Design, Hardware
Additional Key Words and Phrases: Millimeter-scale; levitation; ultrasound; wireless powering; IC chip
ACM Reference format:
Yuki Uno, Hao Qiu, Toru Sai, Shunta Iguchi, Yota Mizutani, Takayuki Hoshi, Yoshihiro Kawahara, Yasuaki Kakehi, and
Makoto Takamiya. Luciola: A Millimeter-Scale Light-Emitting Particle Moving in Mid-Air Based On Acoustic Levitation andWireless Powering. Proc. ACM Interact. Mob. Wearable Ubiquitous Technol. 1, 4, Article 166 (December 2017), 17 pages.
DOI: https://doi.org/10.1145/3161182

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DOI: https://doi.org/10.1145/3161182
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1 INTRODUCTION
In this paper, we propose a smart millimeter-scale particle levitated in the mid-air, which has internal electronics. In theparticle scale, it has been hard to install active levitation mechanisms on the object due to their size, weight, and power
consumption. Thus, external fields such as electric, magnetic, air-flow have been adopted to generate levitation force andcontrol particles in mid-air. Among others, acoustic levitation is one of the most promising ways to keep the objects
suspended in the air. By using ultrasonic phased array speakers, the system enables two-dimensional or three-dimensional
position control of levitated particles. This approach has an advantage with quick and precise movement control of particles.
On the other hand, the particles have a strong constraint on their weight, size, and shapes to maintain its balance in midair. Thus, previous systems tended to use non-electric materials, which don’t have any further functions themselves, just for
position controls.
Our ultimate goal is making a swarm of smart levitated particles. Each tiny particle has abilities to change theirappearances by sensing the surroundings and talk to each other. As a first step toward this concept, we propose Luciola, alevitating-particle consisting of the LED, the receiver coil for wireless powering, and a custom IC chip for the rectifier, thevoltage regulator, and the intermittent lighting of the LED implemented in 180-nm CMOS process. The 3D position, as wellas the ON/OFF timing of the LED, can be computationally controlled, making it possible to control the lighting of the LED
depending on the position and the context.
This paper includes the following contributions:(1)
An acoustically levitated electronic object is realized for the first time.
(2)
A new shape of the levitated object and a new placement of the receiver coil are proposed to simultaneously realize
the acoustic levitation and the wireless powering.
(3)
In the custom IC chip, a new voltage detector circuit enabling an accurate voltage detection and a correct output
during the start-up is proposed to achieve the intermittent lighting of the LED to increase the maximum distance between
the transmitter and the receiver coil.
From the following sections, we will show the detail of our concept, design, implementation, technical evaluations andpotential application scenarios.

2 RELATED WORKS
The novelty of this paper is the acoustically levitated electronic object, combining an ultrasonic levitation and a wirelesspowering to the levitated object. Though each technology of the acoustic levitation and the wireless powering is not new,the simultaneous realization of the acoustic levitation and the wireless powering presents a new challenge. To realize it, a
new shape of the levitated object and a new placement of the receiver coil are proposed, which will be described in section
4.3. In this section, related past works of the levitated small objects and the wireless powering to small objects are reviewed.

2.1 Levitated Small Objects
To overcome the constraints of gravity and move the smart objects freely in the air, many attempts have been made byresearchers. In this section, we introduce past research works that tries to lift the object and suspend in the air. In this paper,
drones (multicopters) are not discussed, because self-powered millimeter-scale drones are not available. Conventional
approaches to lifting up objects and keep it suspended in the air use invisible external forces in the field. Typical examples
of such invisible fields include magnetic, electric, air-flow, and acoustic field. ZeroN [1] is one of the earliest attempt to allow
users to take physical components of tabletop tangible interfaces off the surface and place them in the air. An
electromagnetic suspension system is used to keep a spherical dipole magnet suspended. Thus the levitating object needs tobe made of magnetic material. A floating tangible user interface named floatio [2] takes advantage of the Coandă effect. A
polystyrene light ball of 3.5cm diameter is suspended by the airflow generated by a blower fan. The restriction of thisapproach is that the floating object must be a smooth sphere and lightweight. Fairly lights [3] is a system that draws aerial
images in mid-air using a laser-induced plasma. The size of the volumetric image is at millimeter-scale. People can touchand feel the energy of the plasma, but it is impossible to embed computational functionality in the voxel.
In this paper, an ultrasonic field is used to levitate a millimeter-scale particle, because the spatial resolution of theultrasonic levitation is high (e.g. 0.5mm [4]). Since 1970’s, the ultrasonic levitation system using a single transducer and areflector plate to make a standing wave has been developed for a containerless crystal growth in space experiments [5].
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After that, using one or two ultrasonic transducers, a manipulation of two droplets in air [6], the levitation of small living
animals in air [7], and a non-contact manipulation in air [8] are demonstrated. The past innovation was a two-dimensional
ultrasonic transducer array [9] to generate a focal point by a beamforming. The two-dimensional ultrasonic transducer arrayis originally developed for a haptic interface [9], and a commercial product is available [10]. The two-dimensional ultrasonictransducer array is applied to the three-dimensional non-contact levitation of small particles [11]. In Lapillus bug [12] atransducer array is used to suspend a physical particle in mid-air as a means to provide a new interaction of objects in thephysical 3D environment. In Pixie Dust [4], two sets of two transducer arrays facing each other can produce a 2D grid of
focal points and hold multiple particles at 4.2-mm intervals. Using Pixie Dust, a mid-air screen with projection isdemonstrated. Thanks to an intelligent phase control algorithm called a holographic method [13], the efficiency and thecontrollability of the ultrasonic levitation are greatly improved. Based on the holographic method, three-dimensional midair displays using levitated particles are demonstrated as Levipath [14], Floating Charts [15], and JOLED [16]. Problem of all
the past works in the ultrasonic levitation is that the levitated particle is non-electric and has no electronic functions. In thispaper, an electronic particle is introduced into the ultrasonic levitation to realize a self-luminous pixel in the threedimensional mid-air display.

2.2 Wireless Powering to Small Objects
Target applications of the past works regarding the wireless powering to small objects are mainly implanted medical
devices or RFIDs, and the wireless powering to levitated objects is not reported. In [17] for the implanted glucose monitoring,
the total system including the receiver coil for the wireless powering is integrated on a 1.4mm ´ 1.4mm IC chip and thepower is wirelessly supplied at 900MHz from a transmitter coil with the distance of 1cm. The received power is 6µW, whichis too low to light the LED requiring 3.5mW in Luciola. Similarly, in [18] for the RFIDs, the total system including thereceiver coil for the wireless powering is integrated on a 3.7mm ´ 1.2mm IC chip and the power is wirelessly supplied at
24GHz from a transmitter coil with the distance of 50cm. The received power is 1.5µW, which is too low to light the LED.
Thus, the conventional wireless powering does not fit the ultrasonic levitation in terms of the size, the distance, and thereceived power.

3 OVERVIEW OF LUCIOLA SYSTEM
Fig. 1 shows a photograph of Luciola on a fingertip. Fig. 2 shows a photograph of Luciola levitated by the ultrasonic arrayand emitting light by wireless powering. Fig. 3 shows an overview of the Luciola system. Two 40-kHz 17 ´ 17 ultrasonictransducer arrays are placed face-to-face at a distance of 20cm and the ultrasound generated from each transducer is focused
to a focal point by tuning the phase of each transducer [9]. The focal point is the location where the acoustic levitation force
is generated. Luciola floats in mid-air as a result of being trapped at the focal point. By moving the focal point in 3D space
using a computer, Luciola moves in mid-air in a 10.4cm ´ 10.4cm ´ 5.4cm space. The transmitter coil with a diameter of
31mm for wireless powering is placed near the focal point and transmits the power to the receiver coil in Luciola. The
transmitter for the wireless powering includes a switch to control the ON/OFF timing of the LED in Luciola. By
synchronously controlling the focal point of the ultrasound and the ON/OFF timing of the switch of the wireless powering
using the PC, Luciola displays 3D images in mid-air.

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166:4 • Y. Uno et al.
LuciolaLuciola

4.5mm

31mm
Transmitter coil for wireless powering

Fig. 1. Luciola particle on fingertip.

Fig. 2. Luciola levitated by ultrasonic array andemitting light by wireless powering.

17×17 ultrasonictransducer array(Upper)
10cm
z

PC

LuciolayxTransmitter coilfor wireless powering

12.3MHz10cm

17×17 ultrasonictransducer array(Lower)17cm

17cm

Fig. 3. Overview of Luciola system.
In this paper, two different setups for the Luciola system are used. In the first setup shown in Fig. 4, two 17 ´ 17 ultrasonictransducer arrays are placed face-to-face [4]. In the second setup shown in Fig. 5, a 17 ´ 17 ultrasonic transducer array anda reflector plate are placed face-to-face [6, 19]. The transmitter coil is placed below the reflector plate. In the first setup,
movement in the x, y, and z directions is possible. In contrast, in the second setup, movement in the x and y directions is
possible, and the levitation height is fixed. The first setup is used for 3D movement in mid-air, while the second setup is
used for drawing above a piece of paper on the reflector.

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10cm

10cm
17cm
20cm
17cm

(a)

(b)

Fig. 4. (a) Overall view and (b) enlarged view of two 17 ´ 17 ultrasonic transducer arrays placed face-to-face in
Luciola system.

20cm

Luciola17cm
17cm

Fig. 5. Overall view of 17 ´ 17 ultrasonic transducer array and reflector plate placed face-to-face in Luciolasystem.

4 IMPLEMENTATION
Fig. 6 shows a block diagram of the Luciola system. In this section, detailed implementations of the Luciola system are
explained.

4.1 Ultrasonic Transducer Arrays For Levitation and Movement in Mid-Air
The 40-kHz 17 ´ 17 ultrasonic transducer arrays shown in Fig. 4 and 5 are the same as those in [4]. In each ultrasonictransducer array, 285 ultrasonic transducers (Nippon Ceramic Co., Ltd., T4010B4) with a diameter of 1cm and the soundpressure above 117dB, and 72 drivers (STMicroelectronics, L293DD) with an output voltage of 24Vpeak-to-peak are used. The
measured power consumption of the ultrasonic transducer array is 134W.
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FPGA

17×17
ultrasonictransducer array

FPGA

17×17
ultrasonictransducer arrayPC

ArduinoPTX

PoweramplifierSignalgenerator

PRX

CTX 87pF

SWTX

12.3MHz

Node 2
Wireless
powering

In+

Node 1LTX
1.25μH

InLRX
CRX
22.8nH 6.8nF

Custom IC chip
Chargepump(Rectifier&
Voltageboost)

VDD
CSTR
4.7μF

Voltagedetector Out
LED

SWRX
R1
500Ω

PLED

Transmitter for
wireless powering

LED

Particle for Luciola (3.5-mm diameter)

Fig. 6. Block diagram of Luciola system.

4.2 Transmitter For Wireless Powering
Fig. 6 includes a circuit schematic of the transmitter used for wireless powering. A 12.3-MHz sine wave voltage from asignal generator (Agilent Technologies, DSO5054A) is amplified by a power amplifier (NF Corporation, HSA4101). The
output power (PTX) of the power amplifier is 3.2W. LTX and CTX are the LC resonator used for resonant inductive coupling.
The transmitter coil is made of a copper wire with a diameter of 1mm and has five turns with a diameter of 31mm. The
height of the transmitter coil is 5.5mm. The measured inductance of LTX is 1.25µH, and the quality factor of LTX at 12.3MHz
is 161. The quality factor of the whole transmitter including the power amplifier is 46. In wireless powering using a resonant
inductive coupling, a high quality factor is very important to increase the power transmission efficiency. A resonantcapacitor (CTX) of 87pF is selected to achieve the LC resonance at 12.3MHz. For CTX, a capacitor with a rated voltage of 3kV
is used, because a very high voltage is generated in the LC resonance.
To control the ON/OFF switching of the LED in Luciola, a switch (SWTX) is inserted between the power amplifier andthe LC resonator. A relay (Panasonic Electric Works, TQ2-L2-4.5V) with an ON resistance of less than 50mW is used for
SWTX. SWTX and the ultrasonic array are synchronously controlled by a PC through Arduino Uno.

4.3 Proposed Shape of Levitated Object and Placement of Receiver Coil to Simultaneously
Realize Acoustic Levitation and Wireless Powering
The novelty of this paper is the acoustically levitated electronic object. To realize an electronic object, a power supply isrequired. In this work, a battery-free power supply using wireless powering is used, because batteries are too heavy to induce
acoustic levitation. When the ultrasonic levitation and the wireless powering are combined, we encountered a new problem.
As shown in Fig. 7 (a), in ultrasonic levitation, the levitated object rotates [20] similarly to a spinning top at the bottom of
the acoustic potential [4] generated by the transducer array. In previously published ultrasonic levitations [4, 8, 11-16],
spherical objects are levitated. As shown in Fig. 7 (a), however, when the receiver coil for wireless powering is embedded ina spherical object, wireless powering is unstable, because the receiver coil randomly rotates above the transmitter coil andthe power transmission efficiency randomly fluctuates. To achieve a stable wireless powering, the receiver coil in theProceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies, Vol. 1, No. 4, Article 166. Publication date:
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levitated object should be parallel to the transmitter coil. To solve the problem, a new shape of the levitated object and a
new placement of the receiver coil to simultaneously realize acoustic levitation and stable wireless powering are proposed.
As shown in Fig. 7 (b), the proposed shape is a solid of revolution excluding a sphere. The solid of revolution is a solid figure
obtained by rotating a plane figure around a straight line (= the axis of revolution) that lies on the same plane. In the
proposed shape, the sphere is omitted, because the sphere has infinite axes of revolution and the axis of revolution is not
fixed at the bottom of the acoustic potential, as shown in Fig. 7 (a). As shown in Fig. 7 (b), the receiver coil is proposed to beplaced on a plane perpendicular to the axis of revolution. By using the proposed shape and placement, the axis of revolutionof the acoustically levitated object is fixed to the z-axis direction and the receiver coil rotates on the x-y plane. As a result,a stable wireless powering is achieved, because the receiver coil is always kept parallel to the transmitter coil. Is this paper,a hemisphere is selected as the shape of the levitated object. The other solids of revolution, however, including a circularcone and a cylinder shown in Fig. 7 (c) can be used.

4.4 Particle Used in Luciola
Fig. 6 includes a circuit schematic of the particle for Luciola. The particle includes a receiver coil (LRX), a resonantcapacitor (CRX), a storage capacitor (CSTR), a LED, and a custom IC chip. LRX and CRX are the LC resonator used for resonant
inductive coupling. LRX and CRX are connected in parallel to increase the voltage at node 2. The functions of the particle are(1) to receive the 12.3-MHz wireless power using the receiver coil (LRX), (2) to rectify the AC voltage into the DC voltage
using the charge pump circuits, (3) voltage boosting using the charge pump circuits, (4) storage of the charge in CSTR, (4)voltage regulation of VDD using the voltage detector and SWRX, and (5) the intermittent lighting of the LED (described in
section 4.5) using the voltage detector and SWRX. To achieve the functions, the custom IC chip includes charge pump circuits
to rectify the AC voltage into DC voltage and to boost the DC voltage, a voltage detector to regulate the DC voltage (VDD)
for the LED, a switch (SWRX) for the charge storage in CSTR, and a resistor (R1) to limit the current for the LED to avoid its
breakdown. In the Luciola, the rectifier circuit to convert AC voltage of the wireless powering to DC voltage is very
important, because the power required to drive the LED with the 12.3-MHz AC voltage is 54 times larger than that with the
DC voltage according to our preliminary experiment. An AC-drive LED consumes larger power than a DC-drive LED,
because the LED impedance decreases with increasing frequency due the parasitic capacitance of the LED and the voltage
applied to the LED is reduced.
It is difficult to design the particle for Luciola, because the requirements for the acoustic levitation and the wirelesspowering are opposite. The acoustic levitation requires a small and lightweight particle, while the wireless powering
requires a large receiver coil and heavy electronic circuits. To enable the levitation of the particle, the custom IC chip isessential in reducing the size and weight of the particle. The size of the particle is preferably less than 4.2mm (= half the
wavelength of the 40-kHz ultrasound). To investigate the trade-off between the acoustic levitation and wireless powering,
two types of the particles, 3.5- and 4.5-mm-diameter hemispherical particles, are fabricated.
Axis of revolution

Acoustic potential
Levitated object
Receiver coil
Both coils are NOT parallel.
SphereWireless powering

Unstable wireless powering

Transmitter coil(a) Conventional

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Axis of revolution

Acoustic potentialzLevitated object

yx

Receiver coil
Both coils are parallel.

Hemisphere
Wireless powering

Stable wireless powering

Transmitter coil(b) Proposed

Axis of revolution
Receiver coil

Circular cone

Cylinder(c) Other proposed options

Fig. 7. (a) Conventional spherical object in ultrasonic levitation. (b) Proposed shape of levitated object andplacement of receiver coil to simultaneously realize acoustic levitation and wireless powering. (c) Other
proposed options.

Fig. 8 shows a teardown photograph of the particle used in Luciola. The diameter of the flexible PCB is 3.5mm. The LED
is mounted on the front of the flexible PCB. On the back of the flexible PCB, the custom IC chip, CRX, and CSTR are mounted.
LRX of 22.8nH is implemented with a 2.5-turn spiral inductor on the flexible PCB with 50-µm copper thickness. The four
components (the LED, the custom IC chip, CRX, and CSTR) are carefully arranged so that the center of gravity of the circular
flexible PCB is on the axis of revolution to achieve a stable wireless powering to a rotating levitated object as discussed in
section 4.3 The quality factor of LRX at 12.3MHz is 16. CRX of 6.8nF is selected to achieve LC resonance at 12.3MHz.
To minimize the size and weight of the particle, minimum-size off-the-shelf components are used. Specifically, amultilayer ceramic capacitor (Murata Manufacturing, GRM033R71A682KA01D) with a size of 0.6mm ´ 0.3mm ´ 0.3mm isused for CRX of 6.8nF, a multilayer ceramic capacitor (Murata Manufacturing, GRM035R60J475ME15D) with a size of 0.6mm´ 0.3mm ´ 0.5mm is used for CSTR of 4.7µF, and an SMD chip LED (Kingbright, KPG-0603SEC-E-TT) with a size of 0.65mm´ 0.35mm ´ 0.2mm is used. The chip size of the custom IC chip is 1mm ´ 1mm and its thickness is reduced from the initialvalue of 0.70mm to 0.15mm to minimize the size and weight of the particle. A hemispherical case with a diameter 3.5mm
fabricated with a 3D printer (Stratasys, Objet260 Connex3) using MED610 photopolymer is attached to the flexible PCB. Fig.
9 shows the layout and micrograph of the custom IC chip. The chip is fabricated using a 180nm CMOS process and the chipsize is 1mm ´ 1mm. Instead of 1.8V core transistors, 3.3V I/O transistors are used in this design.

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Flexible PCB
Hemispherical case

Front

Back
Custom IC chip

LRX
CSTR

LED

3.5mm

CRX

3.5mm

3.5mm

Fig. 8. Teardown photograph of particle used in Luciola.
LayoutIn-

Charge pumpLED

Voltagedetector

SWRX
GND

LED

1.0mm

VDD

Micrograph

In+

1.0mm

Fig. 9. Layout and micrograph of custom IC chip.
Fig. 10 shows a breakdown of the measured weight of each component in the 3.5-mm-diameter hemispherical particle.
The hemispherical case, a potting, the flexible PCB, and solder account for 31%, 25%, 21%, and 12% of the total weight of
16.2mg, respectively. The potting is a gelatinous compound covering the custom IC and the bonding wires to provide
resistance to shock and vibration. Owing to the reduced-weight design, the electronic components (CSTR, Custom IC, CRX,
and LED) account for only 10% of the total weight.

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Custom IC CRX LED
CSTR

Solder

10%

2.0mg
(12%)

Hemispherical
case
5.1mg
(31%)

Flexible PCB
3.4mg
(21%)

Potting
4.0mg
(25%)

Total: 16.2mg

Fig. 10. Breakdown of measured weight of each component in 3.5-mm-diameter hemispherical particle.

4.5 Proposed Voltage Detector With Accurate Voltage Detection and Correct Output
During Start-Up For Intermittent Lighting of LED
4.5.1 Intermittent Lighting of LED. To light the LED in Luciola, a power of more than 3.6mW is required. To continuously
light the LED, a power of more than 3.6mW must be delivered to the LED in the wireless powering. In this case, however,
the maximum distance between the transmitter coil and the receiver coil is limited. In this paper, a charge storage circuit toenable intermittent lighting of the LED is used to increase the maximum distance. Initially, in Fig. 6, CSTR is disconnectedfrom LED using SWRX and is charged by the charge pump. When VDD is increased to a predefined voltage, SWRX is turned
on to connect CSTR and the LED, and the LED is turned on. Using the charge storage circuit, the LED intermittently lights
even if the wireless power is less than 3.6mW. For example, if the power delivered to CSTR is 0.36mW, the charge storage
circuit enables the intermittent lighting of the LED with a duty cycle of 10% (10% ON time and 90% OFF time), thereby
increasing the maximum distance between the transmitter and the receiver coil.
4.5.2 Proposed Voltage Detector with Accurate Voltage Detection and Correct Output during Start-up In the circuit designof the custom IC chip in the particle, a new voltage detector circuit with an accurate voltage detection and a correct output
during the start-up is proposed to achieve the intermittent lighting of the LED. The voltage detector in Luciola has two
requirements: (1) an accurate voltage detection and (2) a correct output during the start-up. In Luciola, VDD should be
regulated between 3.3V (= voltage rating of transistors) and 2.1V, and the voltage variations of 3.3V and 2.1V due to processvoltage-temperature (PVT) variations are not allowed, because an operation above 3.3V VDD will damage the transistors andan operation below 2.1V VDD will not light the LED. At the start-up, VDD increases from 0V, because Luciola is a wirelessly
powered battery-free system. During the start-up, Out in Fig. 6 should be always high to keep SWRX turned off. If Out
incorrectly goes to low during the start-up, SWRX is turned on and the charge stored on CSTR is accidentally discharged,
which results in the malfunction of the intermittent lighting of the LED. When a conventional low-power bandgap reference
circuit [21] to generate a constant reference voltage (= 1.23V) insensitive to PVT variations was used in the voltage detector,
we encountered a new problem and proposed a new voltage detector to solve this problem. As shown in Fig. 11 (a), theoutput voltage (VREF1) of the bandgap reference during the start-up is in the metastable state and is incorrect, resulting in
the malfunction of the intermittent lighting. The metastable state originates from feedback circuits included in the bandgap
reference. To achieve the correct output during the start-up, a 2-transistor voltage reference [22] is used. As shown in Fig.
11 (b), the output voltage (VREF2) of the 2-transistor voltage reference during the start-up is correct, resulting in the normal
operation of the intermittent lighting. The problem of the 2-transistor voltage reference, however, is that VREF2 is sensitive
to process variations. For example, when the threshold voltage of the transistors has ±0.05V variations, VREF2 will have ±0.1V
variations [22].
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To solve the problems, a new voltage detector circuit with an accurate voltage detection and a correct output during thestart-up is proposed. Figs. 12 and 13 show the circuit schematic and timing chart of the proposed voltage detector,
respectively. In the proposed voltage detector, the 2-transistor voltage reference [22] and bandgap reference [21] arecombined. During the initial wake-up, the 2-transistor voltage reference is used to obtain a correct output. After the wakeup, the bandgap reference is used to realize an accurate voltage detection. The mode transition from the 2-transistor voltage
reference to the bandgap reference is managed by logic circuits shown in Fig. 12. The details of the operation of the proposed
voltage detector are explained. During the start-up mode in Fig. 13, the 2-transistor voltage reference instead of the bandgap
reference is used for the correct output. The incorrect outputs (Detect_2.1V and Detect_3.3V) of the bandgap reference
during the start-up mode are ignored to prevent the incorrect turn-on of SWRX. When the increasing VDD crosses 1.6V,
Detect_1.6V of the 2-transistor voltage reference changes from low to high, which reset the logic circuits. After that, when
the increasing VDD crosses 3.6V, Detect_3.6V of the 2-transistor voltage reference changes from high to low and the LED isturned on, which means the end of the start-up mode and a normal operation mode using the bandgap reference starts.
When the decreasing VDD crosses 2.1V, Detect_2.1V of the bandgap reference changes from high to low and the LED isturned off. After that, when the increasing VDD crosses 3.3V, Detect_3.3V of the bandgap reference changes from high to
low and the LED is turned on. The turn-on time of the LED (tON) is constant, while the turn-oﬀ time of the LED (tOFF) depends
on the received wireless power. When the LED current is assumed to be 2mA, tON is calculated as 4.7µF ´ (3.3V – 2.1V) /2mA = 2.8ms, which is close to the cognitive limit of human eyes. By increasing CSTR, tON is increased at the cost of increasing
the physical size of the capacitor. In Fig. 13, 3.3V and 2.1V are constant regardless of PVT variations, while 3.6V and 1.6V
depend on the process variations. The inaccuracy of 3.6V and 1.6V is acceptable, because 3.6V and 1.6V are used only at theinitial start-up. In this way, the proposed voltage detector achieves both the accurate voltage detection and the correct output
during the start-up, achieving the intermittent lighting of the LED to increase the maximum distance between the transmitterand the receiver coil. The simulated power supply current of the proposed voltage detector is 546nA at VDD of 3.3V.

VDD
3.3V

No variations

VREF1

1.23V
Incorrect output
due to metastability0VTime(a) Bandgap reference
VDD

3.3V

Correct output

Variations

0.4V

VREF2 = 0.4V± 0.1V

0V

Time
(b) 2-transistor voltage reference

Fig. 11. (a) Output voltage (VREF1) of bandgap reference during start-up. (b) Output voltage (VREF2) of the 2transistor voltage reference during start-up.
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166:12 • Y. Uno et al.
VDD

VDD

R1
46
V
80 DD

Comparator+

Voltage divider

Bandgap reference

30
V
80 DD

VREF1
(1.23V)+

Detect_3.3V

-

20
V
80 DD

+

2-transistor
voltage reference
9
V
80 DD

R80

Pros: Accurate VREF1
after start-up
Cons: Incorrect VREF1
during start-up

Detect_2.1V-

Pros: Correct VREF2
during start-up
Cons: Inaccurate VREF2
after start-up

VREF2(0.4V)

Detect_1.6V

Logic circuits

R2

Out

-

+

Detect_3.6V-

Fig. 12. Circuit schematic of proposed voltage detector.

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Luciola: A Millimeter-Scale Light-Emitting Particle Moving in Mid-Air Based on Acoustic Levitation and… • 166:13

End of start-up
Start-up

Normal operation

3.6V
3.3V
2.1V
1.6V

Bandgap reference

2-transistor voltage reference

VDD

0V

Time

High
Detect_1.6V
Low
Time

High
Detect_3.6V
Low
Incorrect outputs

Time

High
Detect_2.1V
Low
Time
High

Detect_3.3V
Low
Time
3.6V
3.3V

Out

2.1V

0V

Time
Intermittent
lighting

ON
LED

ton

toff

OFF

Time

Fig. 13. Timing chart of proposed voltage detector.

5 EVALUATION
5.1 Range of Motion of Luciola
To investigate the 3D range of the levitation, the moving range of motion of Luciola is measured. In the setup with thetwo 40-kHz 17 ´ 17 ultrasonic transducer arrays placed face-to-face at a distance of 20cm shown in Fig. 4, the transmittercoil is removed and the 3.5-mm-diameter hemispherical particle is slowly moved from the origin in the x- and z- directions.
Because the two ultrasonic transducer arrays have symmetry about X-axis and Y-axis, only the motion in the positive XProceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies, Vol. 1, No. 4, Article 166. Publication date:
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166:14 • Y. Uno et al.
direction is measured. Similarly, because the two ultrasonic transducer arrays have symmetry about the Z-axis, only the
motion in the positive Z-direction is measured. Fig. 14 shows the measured range of motion of Luciola along the X- and Zdirections. For each condition, ten trials are performed. The average maximum range of motion from the origin along theX- and Z- directions is 52mm and 27mm, respectively.
z

27mm

y
52mm

3.5mm

0

-52mm

-52mm

x

52mm

-27mm

Fig. 14. Measured range of motion of Luciola along X- and Z-directions.

5.2 Range of Lighting of LED Using Wireless Powering
The range of the lighting of the LED using the wireless powering, and the increased range thanks to the intermittent
lighting are investigated. Similar to the symmetry discussion in section 5.1, the dependence of the duty cycle of the LED on
the position of Luciola in the XZ plane is measured. Fig. 15 shows the dependence of the measured duty cycle of the LED on
the position of Luciola in the XZ plane for the 3.5- and 4.5-mm-diameter hemispherical particles. The output power (PTX) of
the power amplifier is 3.2W. The position is varied in steps of 1mm. For both hemispherical particles, the maximum range
in the X-direction is around 15mm, which corresponds to the radius of the transmitter coil. The maximum z at x = 0 with aduty cycle of 0% in the 3.5-mm-diameter particle is 8.3mm, while for the 4.5-mm-diameter particle it is 16.4mm. Next, theincreased range owing to the intermittent lighting is investigated. For the 3.5-mm-diameter particle, the maximum z at x =0 with a duty cycle of 100% is 6.0mm, while that at x = 0 with a duty cycle of 0% is 8.3mm, indicating that the range oflighting of the LED is extended by 38% owing to the intermittent lighting. Similarly, for the 4.5-mm-diameter particle, themaximum z at x = 0 with a duty cycle of 100% is 14.0mm, while that at x = 0 with a duty cycle of 0% is 16.4mm, indicatingthat the range of lighting of the LED is extended by 17% owing to the intermittent lighting.

6 APPLICATIONS
Utilizing the advantages of Luciola of (1) levitating in mid-air and moving in three dimensional directions, (2) selfluminous pixel, and (3) ON/OFF control of the LED depending on the position of Luciola, drawings of characters in mid-air
and a levitating and moving micro book light in mid-air are shown. The speed of Luciola of between 0.5cm/s and 1.5cm/s isused.
Fig. 16 shows a setup photograph and a long-exposure photograph of a drawing of an answer of a sum in mid-air using
the setup shown in Fig. 5. The exposure time is 20s. The size of the drawn “8” is 20mm ´ 13mm. Luciola moves from theorigin to the starting point of the drawing. At the starting point, LED of Luciola is turned on. Then, Luciola draws “8”. After
finishing the drawing of “8”, LED of Luciola is turned off and Luciola returns to the origin. This demonstrates the possibilityof interaction with printed materials. The advantage of displaying graphics and characters above the book and the paper isthat an electronic display for the book and the paper is not required and conventional books and papers can be used, which
greatly increases the opportunities to present and overlay information above the conventional physical objects.

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Luciola: A Millimeter-Scale Light-Emitting Particle Moving in Mid-Air Based on Acoustic Levitation and… • 166:15

z

x

0
16.4
3.5mm

15

6.0 5
0

0

5

10 15
x (mm)

20

50Duty cycle of LED
100%
80%
60%
40%
20%
0%

10

10

z (mm)

+38%

4.5mm

15
14.0

z (mm)

8.3

+17%

0

5

10 15
x (mm)

20

Fig. 15. Dependence of measured duty cycle of LED on position of Luciola in XZ plane for 3.5- and 4.5-mmdiameter hemispherical particles.

Luciola

20mm
13mm

Fig. 16. Setup photograph and long-exposure photograph of drawing of answer of sum in mid-air.
Fig. 17 shows a long-exposure photograph of a drawing of characters of “LUCIOLA” one by one in mid-air using the
setup shown in Fig. 4. By synchronously controlling the position of the focal point of the ultrasound and the ON/OFF timingof the switch of the wireless powering using PC, any alphabetical letters can be drawn in mid-air. The exposure time is 15s
to 25s meaning that people can hardly recognize these characters unless consciously watching the movement of the Luciolaparticle at this moment. But the moving speed of the particle could be improved. In the demonstrations in Figs. 16 and 17,
the motion pattern is on an XY plane. However, three-dimensional movement is supported as shown in Figs. 14 and 15, andthus drawing of three-dimensional graphics is also possible.
Fig. 18 shows a levitating and moving micro book light in mid-air. In the setup of Fig. 5, a book is placed on the reflector
plate and the book works as the reflector plate. The particle for Luciola is levitated approximately 12mm above the bookwith the LED-side down, moves from left to right along the line of the book, and shines a light on the characters on thebook. The transmitter coil for the wireless powering is placed below the book. In the demonstration of Fig. 18, the X-Y
coordinate of the moving is set manually. In the future implementations, however, the X-Y coordinate of the moving could
be automatically set using an eye-gaze tracking system.

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166:16 • Y. Uno et al.

Fig. 17. Long-exposure photograph of drawing of characters of “LUCIOLA” in mid-air.

LuciolaFig. 18. Levitating and moving micro book light in mid-air.

7 LIMITATIONS AND FUTURE WORKSIn this paper, only one Luciola particle was in operation for demonstration on the stage. In the next step, however, it
would be nicer if multiple Luciola particles can be independently operated on the stage. A margin for the levitation, however,is small, and it is difficult to levitate more than two Luciolas independently. There are two ways to control multiple particles
simultaneously; (1) increasing the number of the ultrasonic transducer array and (2) reducing the size and weight of theparticle.
Increasing the number of the ultrasonic transducer array cause another problem, because it will negatively affect thehuman auditory system. The upper limit of the sound pressure level at 40kHz is 110dB [9], while the sound pressure levelof a single ultrasonic transducer used in this study is 117dB. Therefore, the sound pressure level of the 17 ´ 17 ultrasonictransducer array is much larger than 110dB, and ear protection is required.
In [12], two particles are independently levitated by switching two focal points at 500Hz using the setup in Fig. 5 and asame ultrasonic transducer array as this work. The particles used in [12], however, are 2.5-mm-diameter polystyrene
3particles with the weight of 0.5mg and the density of 0.06g/cm , which are smaller and lighter than the particles in this work
Proceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies, Vol. 1, No. 4, Article 166. Publication date:
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Luciola: A Millimeter-Scale Light-Emitting Particle Moving in Mid-Air Based on Acoustic Levitation and… • 166:17
3(3.5-mm-diameter, 16.2mg and 1.4g/cm ). Therefore, the next step of Luciola will be to downscale the particle smaller andlighter to 2-mm in diameter. A limiting factor for the miniaturization is the receiver coil because the receiver coil is thelargest in the particle as shown in Fig. 8 and the heaviest in the electric components as shown in Fig. 10. Therefore, designinga smaller and lighter receiver coil is important. Reducing the diameter of the receiver coil, however, will reduce the rangeof wireless powering as shown in Fig. 15. In conclusion, as discussed in section 4.4, the trade-off between the acoustic
levitation and wireless powering is a fundamental problem in Luciola.

ACKNOWLEDGMENTS
This work is partially supported by JST ERATO Grant Number JPMJER1501, Japan.

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Received May 2017; revised October 2017; accepted November 2017

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