Miniature impact actuator for haptic interaction with mobile devices

International Journal of Control, Automation, and Systems (2014) 12(6):1283-1288 DOI 10.1007/s12555-013-0499-5

ISSN:1598-6446 eISSN:2005-4092http://www.springer.com/12555

Miniature Impact Actuator for Haptic Interaction with Mobile Devices

Sang-Youn Kim and Tae-Heon Yang*

Abstract: The crucial procedure in haptic interaction with mobile devices is to convey an appropriate

haptic signal to a user according to the device’s condition. This haptic effect is achieved by creating

vibrotactile signals with a large frequency bandwidth. However, it is quite challenging to generate vi-

brotactile signal with large frequency bandwidth in mobile devices because a vibration motor is fabri-

cated to be embedded into the mobile devices. This paper presents a tiny vibrotactile actuator that can

selectively stimulate human mechanoreceptors by creating a haptic signal with a large frequency

bandwidth. To maximize the haptic effect in limited size, we simulate the magneto-motive force

created by a solenoid by changing a wire diameter of the solenoid. In order to evaluate the haptic per-

formance of the proposed actuator, we construct an experimental setup for measuring the force and the

displacement of the proposed actuator. Using the experimental setup, the output force is measured by

varying input current. The experiment clearly shows that the proposed actuator creates enough output

force to stimulate human skin across a large frequency bandwidth and to convey a variety of vibrotac-

tile sensations to users. The proposed actuator not only offers more reliable input than plain onscreen

keypads in smaller spaces but also allows users to manipulate keypads more effectively.

Keywords: Haptics, mobile device, vibration, vibrotactile actuator.

1. INTRODUCTION

Recently, haptic feedback in mobile devices has been

regarded as one of the dominant factors to increase the

level of immersion because the visual display unit of a

mobile device is not large enough to provide realistic and

exciting sensations to users. A user can communicate

and/or interact efficiently with a mobile device by adding

haptic information to the auditory and visual information.

Haptic feeling consists of kinesthetic and tactile

sensations. Kinesthetic information refers to sensory data

obtained from receptors of joints, muscles, ligaments,

and etc. On the other hand, tactile sense refers to sensory

data acquired through receptors of skin. If both infor-

mations are conveyed to users at the same time, a user

can intuitively and immersively interacts with mobile

devices. However, most haptic actuators for creating

kinesthetic feedback are too bulky to be inserted into

mobile devices. Therefore, for haptic interaction with

mobile devices, researchers have focused on producing

tiny vibrotactile actuators that stimulate the skin.

There are four major mechanoreceptors (Meissner

corpuscle, Merkel’s disk, Ruffini ending, and Pacinian

corpuscle) in human glabrous skin [1]. Merkel’s disk

responds to quasi-static deformations of the skin, such as

force or displacement, in the frequency range of 0.3-3 Hz.

It plays an important role in detecting spatial structures

in static contact, such as an edge or a surface. The

Ruffini ending produces a buzzing sensation in the

frequency range of 40-500 Hz. The Meissner corpuscle,

which has a frequency range of 3-40 Hz, detects dynamic

deformations of the skin such as the sensation of flutter.

The Pacinian corpuscle, which has a frequency response

in the range of 40-500 Hz, is the most sensitive to

vibration amplitude and is particularly known to serve as

the detector of acceleration or vibration.

Judging from above facts, three mechanoreceptors,

except the Ruffini ending, can be selectively stimulated

if a haptic actuator has a large frequency bandwidth. For

example, 1 Hz vibrotactile signal can stimulate Merkel’s

disk and 20 Hz signal can stimulate the Meissner cor-

puscles. If a vibrotactile actuator creates 80 Hz vibration

signal, Pacinian corpuscle are responded. Therefore, it is

necessary to consider an actuator that can generate

vibrotactile information over a large frequency band-

width in order to provide a variety of sensations to users.

Many mobile devices have employed eccentric motors

with operating frequencies of 100-250 Hz. Since the

vibrational force created by an eccentric motor is

proportional to the square of the motor’s rotational speed,

© ICROS, KIEE and Springer 2014

__________

Manuscript received November 9, 2013; accepted May 9,2014. Recommended by Associate Editor Shinsuk Park under thedirection of Editor Hyouk Ryeol Choi. This research was supported by the National Research Founda-tion of Korea (NRF) funded by the Ministry of Education, Scienceand Technology (grant number: 2013M3C1A3059588). This workwas also supported by the “Development of pulse analysis systemfor personalized medicine by converging hemodynamics andpulse diagnostics” (K14310) funded by the Medical EngineeringR&D Group of Korea Institute of Oriental Medicine. Thisresearch has been done for research year (2013) in Koreatech. Sang-Youn Kim is with the Interaction Lab., Advanced Tech-nology Research Center, KoreaTech, Chungjello 1600, Byeongcheon-myeon, Cheonan, Chungnam 330-708, Korea (e-mail: [email protected]). Tae-Heon Yang is with the Center for Mass and RelatedQuantities, Korea Research Institute of Standards and Science,267 Gajeong-ro, Yuseong-gu, Daejeon 305-340, Korea (e-mail:[email protected]).

* Corresponding author.

Sang-Youn Kim and Tae-Heon Yang

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we can hardly control the magnitude of the vibrational

force without changing the vibrational frequency, and we

can hardly produce a satisfactory low-frequency

vibrotactile signal under 100 Hz. Another critical

problem is that its response time is too slow to create

haptic feedback in real-time. Hence, the vibrotactile

effect obtained from an eccentric motor is limited to

creating an alert signal.

To improve response time of the haptic actuator,

Samsung Electro-Mechanics has developed a linear

resonance actuator (LRA) consisting of an elastic spring,

a permanent magnet with a flux path, and a solenoid coil

[2]. When an alternating electric current is applied to the

solenoid coil, the permanent magnet with the flux path

attached beneath the spring is linearly actuated and this

linear actuation causes vibration at a particular frequency.

Due to this structure, LRA can have fast vibration

response characteristic. However, the strategy of

vibration near the resonant frequency brought a new

issue for creating a variety of haptic sensations. The

limited frequency bandwidth of the LRA precludes it

from the creation of a variety of haptic sensations.

Therefore, it is necessary to consider a vibration actuator

which generates appropriate vibrations over a large

frequency bandwidth.

Piezo ceramic actuators have been developed for

producing vibrations with a wide frequency range from a

small device [3-6]. I. Poupyrev et al. developed a piezo-

actuator and embedded it to mobile devices for haptic

feedback [3]. M. Wagner et al. developed a helically

wound piezo actuator, helimorph, to increase the stroke

of the piezo actuator [4]. Since these actuators are

actuated over a broad bandwidth, they can convey

various tactile sensations (such as the clicking of a button,

and the surfing of menus) to users. Cruz and D. Grant

developed a piezoelectric actuator and a design tool for

delivering a variety of haptic effect to users [5]. J.

Lylykangas et al. designed a tactile stimulation system,

which is a platform for creating vibrotactile sensation by

piezo actuators, in order to investigate preferred

perception of tactile feedback from non-physical buttons

[6]. Research in Motion released a one button haptic

feedback system with four separate piezoelectric

actuators in order to improve the manipulability of a

mobile phone [7].

Even though these actuators based on piezo-ceramic

materials can selectively stimulate mechanoreceptors, it

is vulnerable to shocks. Another drawback to piezo

actuators is that their vibrational force is not strong

enough to stimulate mechanoreceptors except at their

resonant frequencies. Therefore, two or more piezo

actuators are operated simultaneously to create a strong

vibrotactile sensation.

For overcoming the weakness to shocks, impact type

motors have been proposed for haptic interaction with

mobile devices. The rapid responsiveness of the

vibration generated from the impact type motors makes

the motor create numerous vibration patterns according

to surfing and scrolling menus [8,9]. Furthermore,

impact type vibrotactile actuator can produce realistic

button sensation on the touch panel of mobile devices

[8,9]. ALPS Electric co. developed an impact actuator

(we call it Force Reactor™) [10]. Although, s-type

(small type) of the Force Reactor™ is small enough to be

incorporated into mobile devices, its vibration strength is

not strong enough to oscillate the whole body of mobile

devices. Engineering Acoustics Inc. developed a tiny

vibrator, a C-2 Tactor, to create a strong and localized

sensation on the body [11]. Tactile Labs Inc. developed a

vibrotactile actuator with a bandwidth of 50-500 Hz

capable of producing up to 3G of acceleration [12]. Even

though those vibrotactile actuators produce strong

vibration over wide range of frequency, it is not easy to

create vibrotactile sensation at frequency range below

50 Hz. Another minor problem is that the size of the

actuators is large to be embedded into mobile devices.

Therefore, this paper presents a miniature vibrotactile

actuator that creates a strong haptic effect sufficient to

stimulate human skin over a wide working frequency

range including low frequency.

2. DESIGN AND SIMUATION OF A NEW IMPACT

ACTAUTOR

Fig. 1 shows the components of the newly proposed

impact haptic actuator. The proposed impact actuator

consists of a steel housing, two solenoid coils fixed in a

steel housing, and two permanent magnets passing in and

out of the solenoid coils. The proposed actuator also

includes a link bar that connects the two permanent

magnets. A steel flux path is installed for each permanent

magnet in order to concentrate the magnetic field

strength in the gap between the steel flux path and the

steel housing. There are steel ball bearings in the

proposed actuator to decrease friction between the steel

housing and the steel flux path. Furthermore, we attached

silicon to both sides of the solenoid coils to minimize the

noise from collisions between the steel flux path and the

solenoid coil.

Fig. 2(a) shows the design of solenoid coil. The size of

the solenoid coil was determined by considering the size

of the housing and the permanent magnets. The inner

width and length of the solenoid coil was determined to

be 2 mm × 2 mm, and the outer width and length was

chosen as 2.5 mm × 2.5 mm. The height of the coil was

chosen as 3.4 mm. In order to maximize the force of an

actuator with limited size, the magneto-motive force

Fig. 1. Componenet of a new actautor and its assembled

figure.


1285

(A·turns) generated from the coil was simulated by

changing the wire diameter of the coil as shown in Fig.

2(b). The magneto-motive force (NI) and the power

consumption (VI) were calculated using (1) and (2). The

power consumption of a solenoid coil to be incorporated

into mobile devices was determined to be 0.3 W. As

shown in Fig. 2(b), as the wire diameter of the solenoid

increases, the magneto-motive force increases. On the

contrary, the magneto-motive force decreases as the wire

diameter increases while keeping the power consumption

fixed at 0.3 W, 0.6 W and 0.9 W. Thus, the maximum

magnetomotive force by the solenoid coil should be

intersection points of the graph. Since the power

consumption is fixed at 0.3 W, the wire diameter of the

solenoid coil and the number of turns were determined as

0.05 mm and 40 A·turns, respectively (Fig. 2). The

chosen magneto-motive force (40 A·turns) was applied

in FEM simulation to obtain the output force of the

actuator. Fig. 2(c) shows the fabricated solenoid coil.

tot

N V PNI

ρ

× ×

=

×�[A turns],⋅ (1)

2 2

tot

V V PVI

R ρ

×

= =

× �[W], (2)

2( .),

4

Wire DiaP

π ×

= (3)

2 ( ),tot m in out

r N N d dπ π= = +� (4)

8 234.5 . .1.724 10 ,

234.5 . .

Max Temp

Min Tempρ

−

⎡ ⎤+= × × ⎢ ⎥

+⎣ ⎦ (5)

where

NI: Magneto-motive force VI: Power consumption

N: Number of turns V: Input voltage

P: Pure conductor area ρ: Resistivity

:tot� Total length of coil

Fig. 3. Working principle with FEM simulation of a new

impact actuator.

Fig. 4. Result for FEM Simulation.

Fig. 3 shows the working principle of the proposed

actuator and its simulation result when the magneto-

motive force of 40 A·turns was provided to the two

solenoid coils. The magnetic field from the upper

permanent magnet goes by the upper steel flux path and

then passes through the steel housing. After that, the

magnetic field returns to its original position. Both

permanent magnets can be moved up and down

according to the direction of the applied current. In order

to create a strong impact at the downside of the proposed

actuator, the upper and lower permanent magnets are

both pulled down by the upper and lower solenoid coils,

respectively. For generating a strong impact at the upside,

the two permanent magnets are pushed up by the

respective solenoid coils. In this manner, repulsive and

attractive forces are created by the Lorentz force between

the permanent magnets and the solenoid coils. The two

permanent magnets and the solenoid coils produce linear

Lorentz force according to the direction of the stroke. Fig.

4 shows the haptic result for FEM simulation. The output

force was around 40 mN when the input current of 0.1 A

is provided to the actuator.

3. DEVELOPMENT OF THE IMPACT

ACTUATOR

Fig. 5 shows the constructed actuator prototype and its

detailed components. As mentioned above, we attached

the steel flux paths to the ends of the permanent magnets.

The steel flux paths and the steel housing were made of

1008 carbon steel (0.008% or less carbon and 99.8-

99.9% iron), which is a good ferromagnetic material, to

conduct magnetic flux from the permanent magnet with

very low resistance. The moving part consists of two

(a) (b)

(c)

Fig. 2. Parametric design of a solenoid coil and its

picture.


1286

permanent magnets, two steel flux paths, and a link bar.

The link bar connects the two permanent magnets, and it

linearly guides the moving part. This moving part is

vertically actuated to create strong impacts at both ends

of the housing. Each solenoid coil is made out of SS41

steel and it is fabricated by a precision machining (wire

cutting) process. The solenoid coil was wound using a

coil winding machine. The solenoid diameter is 2.5 mm

and its height is 3.4 mm. The two permanent magnets

were installed at the ends of the link bar with their north

poles facing each other. The two steel flux paths were

mounted at the outside ends of the corresponding

permanent magnets. The moving part travels linearly

inside the two solenoid coils due to the Lorentz force.

The two solenoid coils and the moving part were located

inside the steel housing that has two covers (the upper

and lower covers).

The wires from the two solenoid coils were connected

with a flexible PCB. A copper wire with a diameter of

0.05 mm is wound around the core of the solenoid. The

solenoid coil was designed to have 40 A·turns as we

mentioned in Section 2. When current is applied to the

solenoids, the permanent magnet moves from the initial

position to the other end and collides with a silicon

bumper attached to the end of the solenoid coils. This

collision generates strong and sharp impact vibration.

The size of the developed impact vibration actuator is

3 mm × 3 mm × 15.7 mm. Since the volume of the

proposed actuator (141.3 mm3) is smaller than that of

commercial linear resonance actuators (360 mm3), the

proposed actuator can be easily embedded in mobile

devices.

4. PERFORMANCE OF THE IMPACT

ACTUATOR

Fig. 7 shows an experimental setup to evaluate the

performance of the proposed actuator. The experimental

setup includes a microstage, equipped with a single load

cell (CAS BCL-1L). A digital indicator (CAS CI-510A)

was used to display the output. A function generator

(Agilent 33220A) was connected to the impact actuator

to provide current input. The impact actuator was placed

on the micro stage that moves in the vertical direction

(Z-axis). In order to delicately measure the impact force

using the load cell, we attached a contactor to the end of

the moving part as shown in Fig. 7. The forces generated

by the impact actuator were measured by varying the

input frequency and the position of the micro stage. The

indicator displayed the force generated by the impact

actuator. We measured forces in six trials and took their

average.

The average output force was around 70 mN. The

output force is large enough to generate strong impact

vibration for mobile devices [13]. The most important

factor in the vibrotactile actuator is to create a full stroke

in a wide frequency range, because maximum impact

vibration is generated with a full stroke. Therefore, we

needed to investigate whether the actuator’s stroke

decreases as the frequency varies. Fig. 8 shows a

schematic diagram of the measurement system for

sensing the displacement of the proposed actuator as a

function of the operating frequency.

Fig. 7. Experimental setup for measuring the force of the

impact actuator.

Fig. 5. Fabricated components and assembling process.

Fig. 6. Developed impact actautor.


1287

Fig. 8. Experimental setup for measuring the displace-

ment of the impact actuator.

Fig. 9. Frequency response of the actuator.

Fig. 9 shows the frequency response when voltage

input is provided to the proposed actuator. We produced

square waves with a function generator and supplied the

square waves to the proposed actuator. The contactor’s

displacement was measured by a laser Doppler

vibrometer. The measured signal was displayed on an

oscilloscope. As the input current increases from 60mA

to 120mA, the operating frequency range expands. The

proposed vibrotactile actuator produced its full stroke in

the frequency range of 0 to about 100 Hz. Therefore, we

can conclude that the proposed actuator selectively

stimulates mechanoreceptors with large enough feedback

vibration to convey haptic sensation to a user. Further-

more, we did not find any resonance effect because there

is no spring in the proposed vibrotactile actuator.

5. CONCLUSION

Although there are a lot of commercial haptic

actuators which are inserted into mobile devices for

creating tactile sensation, it is not easy to selectively

stimulate human’s mechanoreceptors. This paper

proposed a miniature impact actuator that provides

enough working frequency and output force to stimulate

all mechanoreceptors. The proposed impact actuator

could be embedded in mobile devices, and it could

selectively stimulate mechanoreceptors in the human

somatosensory system by adjusting frequency. The

proposed actuator was composed of the moving part

(which consists of two permanent magnets, two steel flux

paths, and a link bar), two solenoids, a steel housing, and

two covers. In the moving part, the two permanent

magnets was installed at the ends of the link bar with

their north poles facing each other and the magnets

passed in and out of the solenoid coils. According to the

current input, the moving part ran from the initial

position to the other end and collided with a silicon

bumper attached to the end of the solenoid coils in order

to generate vibration. A parametric design for a solenoid

was conducted for maximizing the impact force of the

proposed actuators. In order to demonstrate the

feasibility of the proposed actuator, we constructed an

experimental setup and verified that the proposed

actuator created sufficient output force to generate strong

impact vibration, and furthermore, it generated a wide

range of frequencies to express a variety of haptic

sensations. Our work underscores the importance of the

proposed haptic actuator to enable users to experience

immersion while interacting with mobile devices.

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[2] S. D. Kweon, I. O. Park, Y. H. Son, J. Choi, and H.

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[3] I. Poupyrev, S. Maruyama, S., and J. Rekimoto,

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pp. 24-32, 2009. [10] ALPS, Alps Electric Press Release Force Reactor

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[13] T.-H. Yang, D. Pyo, S.-Y. Kim, Y.-J. Cho, Y. D. Bae, Y. M. Lee, J. S. Lee, E. H. Lee, and D.-S. Kwon, “A new subminiature impact actuator for mobile devices,” Proc. of IEEE World Haptic Con-ference, Istanbul, Turkey, pp. 95-100, 21-24 June, 2011.

Sang-Youn Kim received his B.S. degree from Korea University, Seoul, Korea and an M.S.E (1996) and a Ph.D. (2004) in Mechanical Engineering at Korea Advanced Institute of Science and Technology (KAIST). He was a researcher in Virtual Reality Research Center in 2002. From 2004 to 2005, he was a researcher at Human Welfare

Robot System Research Center. In 2005, he was a research staff member at Samsung Advanced Institute of Technology (SAIT). He is currently an assistant professor of internet-media engineering at Korea University of Technology and Education. His current research interests include Human-Computer Interaction, 3D object modeling, Virtual Reality, and Haptics (haptic rendering, tactile display).

Tae-Heon Yang received his B.S. degree in Mechanical Engineering from Yonsei University, Seoul, Korea, and his M.S. and Ph.D. degrees in Mechanical Engi-neering from Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2006, 2008, and 2012, respectively. He is currently a senior research scientist at Korea Research In-

stitute of Standards and Science (KRISS), Daejeon, Korea. His research interests include Haptics (tactile actuator, tactile sen-sor and haptic interface), polymer devices (electroactive poly-mer and pressure resistive material), and national measurement standards of pressure and vacuum.

Documents

Miniature impact actuator for haptic interaction with mobile devices