A Linearvibrotactile Actuator for Mobile Devices

Sang-Youn Kim

, Bonggoo Kim

and Tae-Heon Yang

Interaction Lab., Advanced Technology Research Center, Korea University of Technology and Education (KoreaTech),

(330-708) 1600 Chungjeolno, Byeongchunmyun, Chungnam, Cheonan, Korea

R&D Center, Yeonhab Precision Co. Ltd, (330-844)219-27 Haksusosa-gil, Mokcheon-eup, Chungnam, Cheonan, Korea

Korea Research Institute of Standards and Science, Daejeon, Korea

Keywords: Vibrotactile Actuator, Haptics, Mobile Device, Vibration.

Abstract: Although, the current vibrotactile actuators are widely used for haptic interaction with mobile devices, they

have still problems to be solved before accepting in many mobile devices. The most critical problem is that

the conventional vibrotactile actuators creates vibrotactile signal with limited frequency bandwidth. The

vibrotactile actuator with large frequency bandwidth allows a user to delicately and immersively manipulate

mobile devices. This paper presents a new vibrotactile actuator which creates vibrotactile signals with a

large frequency bandwidth. In our actuators, vibrotactile signal is generated by interaction between

solenoids and a permanent magnet. Experiments are conducted to investigate whether the proposed actuator

generates enough output force to stimulate human skin across a large frequency bandwidth. The result of the

experiments demonstrates that the proposed actuator is suitable for the haptic interaction with mobile

devices.

1 INTRODUCTION

Recently, the mobile industry is experiencing rapid

growth. As computer graphics, multi-media, and 3D

sound technologies are incorporated into the mobile

device, the devices are expected to be smarter and

smarter. According to Microsoft, 4 billion mobile

phones are in use all around the world (Microsoft

Tag, 2012). A market share of smart phones, which

is 25% in 2012, will balloon to 54% (IHS iSuppli,

2012). Due to the smart phones, the function of a

mobile phone has shifted from a traditional

telephone to an entertainment device with which a

user enjoys internet, movies, games, and etc.

Since visual information is most important factor

in interacting with mobile devices, mechanical

keypads and buttons in mobile devices are being

replaced by touch screens to maximize the display

area. A touch screen without a mechanical keypad

has led to a native user interface (UI) which reduces

the learning curve of a user to adapt usage of an

application. Enlarged screens and native UI allow a

user to intuitive and immersive interaction with

mobile devices. However, it is not easy to increase

the level of the immersion to the level where users

are truly “immersive”. To increase the level of the

immersion, many researchers and developers

focused on creating haptic feedback. The reason is

that interaction based touch is first way, and it

allows a user to non-verbally and cognitively

interact with devices. Therefore, haptic information

coupled with visual and/or audio information

enables a user to inattentively interact with devices.

Haptic feeling consists of tactile sensation

(sensory information acquired by pressure receptors

in the skin) and kinesthetic sensation (sensory data

obtained by receptors in joints, muscles, and

ligaments). Many haptic actuators that directly

provide kinesthetic force or pressure are too bulky to

be inserted them into mobile phones. Therefore, for

creating haptic feeling in mobile devices, many

researchers focused on tactile actuators because the

tactile actuators can easily be constructed in small

size. Among others, vibrotactile actuators have been

most widely studied to reproduce haptic sensations

on mobile screens by generating short vibration

feedbacks, and they have been successfully

commercialized in many mobile devices.

There are four major mechanoreceptors

(Meissner corpuscle, Merkel’s disk, Ruffini ending,

and Pacinian corpuscle) in the human glabrous skin

and their operating frequency are different from each

other except Ruffini ending (Johansson and Vallbo,

125

Kim S., Kim B. and Yang T..

A Linearvibrotactile Actuator for Mobile Devices.

DOI: 10.5220/0004525301250128

In Proceedings of the 10th International Conference on Signal Processing and Multimedia Applications and 10th International Conference on Wireless

Information Networks and Systems (SIGMAP-2013), pages 125-128

ISBN: 978-989-8565-74-7

 2013 SCITEPRESS (Science and Technology Publications, Lda.)

1979); (Johnson et al., 2000.). An eccentric motor,

which is the first commercialized vibrotactile

actuator, can create operating frequency in the range

of 80 to 250Hz. However it is not easy to generate a

variety of vibrotactile senation because the eccentric

motor creates concentrated force which is in

proportional to the square of the number of the

motor’s revolution. Another problem is that the

eccentric motor’s response time is too late to be used

for conveying vibrotactile sensation to a user in real

time. Therefore, a linear resonance actuator (LRA)

(Kweon et al., 2008) is developed to improve the

response time of the conventional vibrotactile

actuators. The LRA creates vibrotactile sensation

using resonant effect and its response time is fast

enough to be used for vibrotactile actuator. However

the strategy of vibration near the resonant frequency

limits frequency bandwidth of haptic actuators.

Piezo ceramic actuators have been developed for

producing vibrations with a wide frequency range

from a small device (Poupyrev et al., 2002);

(Wagner et al., 2005); (Cruz and Grant, 2011);

(Lylykangas et al., 2011). Even though a piezo

actuator can have possibility to selectively stimulate

mechanoreceptors, its vibrational force is not strong

enough to stimulate mechanoreceptors except at

their resonant frequencies.

Therefore, in this paper, we propose a linear type

vibrotactile actuator which not only creates

generates vibrations over a large frequency

bandwidth but also generate strong haptic effect

sufficient to stimulate human skin.

2 CONCEPTUAL DESIGN AND

IMPLEMENTATION

In this section, we describe a proposed vibrotactile

linear actuator consisting of a steel housing, a steel

flux path, steel ball bearings, two solenoid coils

fixed in a steel housing, two permanent magnets

passing in and out of the solenoid coils, and a link

bar that connects the two permanent magnets. The

steel flux path concentrates the magnetic field

strength in the gap between the steel flux path and

the steel housing. The steel ball bearings decrease

friction between the steel housing and the steel flux

path. Silicon was attached to both sides of the

solenoid coils to minimize the noise from collisions.

Fig. 1 shows the component of the proposed actuator

and its conceptual design.

Figure 1: Components of the proposed actuator and its

conceptual design.

(c)

(a) (b)

(

)

Figure 2: Parametric design of solenoid coil and its picture

In order to maximize the force of an actuator

with limited size, we conducted a parametric design

for a solenoid. Fig. 2(a) shows the parametric design

of the 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. For this given size of the

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solenoid coil, the magneto-motive force (A • turns)

generated from the coil was simulated by changing

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

The wire diameter of the solenoid coil and the

number of turns needed to produce the desired

magneto-motive force (A • turns) were determined

to be 0.05 mm and 40 A•turns respectively (Fig. 2).

The chosen magneto-motive force (40 A • turns) was

applied in the FEM simulation to obtain the output

force of the actuator. Fig. 2(c) shows the fabricated

solenoid coil.

(i)

(ii)

(iii)

(iv)

(v)

(vi)

(i) : Upper Steel Flux Path, (ii) : Upper Permanent Magnet,

(iii) : Upper Solenoid Coil, (iv) : Lower Solenoid Coil,

(v) : Lower Permanent Magnet, (vi) : Lower Steel Flux Path

Figure 3: Working principle with FEM simulation of a

new impact actuator.

Figure 4: Result for FEM simulation.

Fig. 3 shows the working principle of the

proposed actuator and the 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 a linear Lorentz force according to the

direction of the stroke.

Figure 5: The developed actuator.

The vibrational impact force was 76mN when a

current input of 0.2 A is supplied to the actuator.

When there is no current input to the proposed

actuator, output force becomes 1.5mN which is

lower than absolute threshold (Lederman, 1997;

Katz, 1989).

Fig. 5 shows the constructed actuator prototype.

The solenoid diameter is 2.5 mm and its height is 3.4

mm. The two permanent magnets are installed at the

ends of the link bar with their north poles facing

each other. The two steel flux paths are 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 are located inside

the steel housing that has two covers (the upper and

lower covers). 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.

ALinearvibrotactileActuatorforMobileDevices

127

Since the volume of the proposed actuator

(141.3mm

) is smaller than that of commercial linear

resonance actuators (360 mm

), the proposed

actuator can be easily embedded in mobile devices.

3 CONCLUSIONS

In this paper, we presented a tiny vibrotactile

actuator, which is easily embedded into mobile

devices, consisting of the moving part, two solenoids,

a steel housing, and two covers. Since the proposed

actuator provides enough working frequency and

output force to stimulate human skin, it can

selectively stimulate human’s mechanoreceptors.

According to the current input, the moving part runs

from the initial position to the other end and collides

with a silicon bumper attached to the end of the

solenoid coils in order to generate vibration. Our

work underscores the importance of the proposed

haptic actuator to enable users to experience

immersion while interacting with mobile devices.

ACKNOWLEDGEMENTS

This research was supported by the Basic Science

Research Program through the National Research

Foundation of Korea (NRF) funded by the Ministry

of Education, Science and Technology (grant

number : 2011- 0009757).This research was also

supported by the Dual Use Program Cooperation

Center (Development of tactile display device for

Virtual reality-based flight simulator, 12-DU-EE-03)

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