Miniature impact actuator for haptic interaction with mobile devices

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

    ISSN:1598-6446 eISSN:2005-4092

    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 devices 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.


    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, Merkels disk, Ruffini ending, and Pacinian

    corpuscle) in human glabrous skin [1]. Merkels 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 Merkels

    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 motors 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: 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 (

    * Corresponding author.

  • Sang-Youn Kim and Tae-Heon Yang


    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.



    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


  • Miniature Impact Actuator for Haptic Interaction with Mobile Devices


    (Aturns) 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 Aturns, respectively (Fig. 2). The

    chosen magneto-motive force (40 Aturns) was applied

    in FEM simulation to obtain the output force of the

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


    N V PNI


    [A turns], (1)

    2 2


    V V PVI


    = =

    [W], (2)

    2( .),


    Wire DiaP

    = (3)

    2 ( ),tot m in out

    r N N d d = = + (4)

    8 234.5 . .1.724 10 ,

    234.5 . .

    Max Temp

    Min Temp


    + (5)


    NI: Magneto-motive force VI: Power consumption

    N: Number of turns V: Input voltage

    P: Pure conductor area : 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 Aturns 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.



    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)


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


  • Sang-Youn Kim and Tae-Heon Yang


    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...


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