Abstract:
To provide a tactile vibration applying device that efficiently outputs vibrations using an electrostatic or piezoelectric actuator. The tactile vibration applying device includes the electrostatic or piezoelectric actuator formed in a flat shape, and expanding and contracting in a thickness direction, a first elastic body having an elastic modulus smaller than an elastic modulus of the actuator in the thickness direction and disposed to contact a surface of the actuator on a side of the first electrode, and a first cover covering a surface of the first elastic body opposite to a surface of the first elastic body contacting the actuator, pressing the actuator and the first elastic body in the thickness direction of the actuator, and holding the first elastic body in a state that the first elastic body is compressed more than the actuator.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a Continuation Application of International Application No. PCT/JP2016/079027, filed on Sep. 30, 2016, which is incorporated herein by reference. The present invention is based on Japanese Patent Application No. 2015-253590, filed on Dec. 25, 2015, claiming the domestic priority of the former, the entire contents of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a tactile vibration applying device. 
       BACKGROUND ART 
       [0003]    One of various information transmission items is an item using a system of applying vibrations to human beings. In recent years, it is required to apply various senses to human beings by applying vibrations to the human beings while changing, for example, the frequency and amplitude complicatedly. Users thus select vibration actuators according to purposes. When selecting the vibration actuator, the users typically consider the power consumption. The vibration actuator includes a tactile vibration applying actuator and a voice vibration applying actuator. The tactile vibration is a low-frequency vibration, whereas the voice vibration is a high-frequency vibration. 
         [0004]    An eccentric motor that rotates an eccentric mass member, a device that vibrates a vibrating member by a voice coil motor (also referred to as “linear resonant actuator”), an electrostatic actuator, and a piezoelectric actuator have been known as an actuator that generates a vibration. 
         [0005]    The eccentric motor is typically driven by direct current (DC) and thus the eccentric motor operates only in a single direction. Consequently, the eccentric motor only transmits changes in magnitude of vibrations and in timing of vibrations, and thus the eccentric motor is unsuitable for applying complicated vibrations. Additionally, the eccentric motor has relatively large power consumption. 
         [0006]    The voice coil motor is driven by a magnet, a coil, and a mass spring system, and thus the voice coil motor can apply complicated vibrations by receiving various input signals. The voice coil motor constitutes an LCR circuit and thus has an electrical resonance frequency. Consequently, the voice coil motor has small power consumption and large amplitude at the resonance frequency, whereas the voice coil motor has large power consumption and small amplitude when its frequency deviates from the resonance frequency. The voice coil motor is thus unsuitable for use in a wide frequency band and if the voice coil motor is used in such a wide frequency band, it requires a control algorithm for changing input signals. Meanwhile, the power consumption is large in a band that deviates from the resonance frequency. 
         [0007]    The electrostatic actuator and the piezoelectric actuator constitute an RC circuit and thus the electrostatic and piezoelectric actuators do not have an electrical resonance frequency unlike the voice coil motor. Consequently, the electrostatic actuator and the piezoelectric actuator have small power consumption in a wide frequency band. The electrostatic actuator is disclosed in Japanese Patent No. 5281322 and Japanese Translation of PCT International Application Publication No. JP2014-506691A. WO 2013/145411 discloses a speaker using the electrostatic actuator. 
       SUMMARY 
     Technical Problems 
       [0008]    If an electrostatic actuator or a piezoelectric actuator is used by itself, the amplitude of a vibration is small. These actuators thus have output vibrations insufficient for use as a tactile vibration applying actuator. 
         [0009]    An object of the present invention is to provide a tactile vibration applying device that efficiently outputs vibrations using an electrostatic or piezoelectric actuator. 
       Solutions to Problems 
       [0010]    A tactile vibration applying device according to the present invention includes an electrostatic or piezoelectric actuator formed in a flat shape, having a first electrode and a second electrode opposing to each other in a thickness direction, and expanding and contracting at least in the thickness direction; a first elastic body having an elastic modulus smaller than an elastic modulus of the electrostatic or piezoelectric actuator in the thickness direction and disposed in contact with a surface of the electrostatic or piezoelectric actuator on a side of the first electrode; and a first cover covering a surface of the first elastic body opposite to a surface of the first elastic body contacting the electrostatic or piezoelectric actuator, pressing the electrostatic or piezoelectric actuator and the first elastic body in the thickness direction of the electrostatic or piezoelectric actuator, and holding the first elastic body in a state that the first elastic body is compressed more than the electrostatic or piezoelectric actuator. 
         [0011]    The elastic modulus of the first elastic body is smaller than the elastic modulus of the actuator in the thickness direction. In a state where the first elastic body is pressed against the first cover, the first elastic body is compressed more than the actuator. The first cover keeps such a state as an initial state. In a state where the first cover presses the actuator and the first elastic body, the compression amount of the actuator is small. The expansion and contraction of the actuator is thus hardly affected even if the first cover presses the actuator. 
         [0012]    When a voltage is applied to the first electrode and the second electrode of the actuator, the actuator expands and contracts in the thickness direction. The displacement of the surface of the actuator on the side of the first electrode according to the expansion and contraction of the actuator is transmitted via the first elastic body to the first cover. Additionally, the elastic deformation force of the first elastic body is changed by the expansion and contraction of the actuator and such a change in the elastic deformation force of the first elastic body is transmitted to the first cover. The first elastic body is compressed in the initial state and thus vibrations can be efficiently applied to the first cover. That is, even if vibrations of the actuator alone are small, tactile vibrations can be applied to the first cover. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  is a cross-sectional view of a tactile vibration applying device according to a first embodiment. 
           [0014]      FIG. 2  is a cross-sectional view of internal components of the tactile vibration applying device shown in  FIG. 1  before the tactile vibration applying device is mounted on a cover. 
           [0015]      FIG. 3  shows electrical connection of an electrostatic actuator and a drive circuit constituting the tactile vibration applying device shown in  FIG. 1 , and deformation of the electrostatic actuator when a voltage is applied to the electrostatic actuator. 
           [0016]      FIG. 4  is a cross-sectional view of a tactile vibration applying device according to a second embodiment. 
           [0017]      FIG. 5  is a cross-sectional view of internal components of the tactile vibration applying device shown in  FIG. 4  before the tactile vibration applying device is mounted on a cover. 
           [0018]      FIG. 6  is a cross-sectional view of a tactile vibration applying device according to a third embodiment. 
           [0019]      FIG. 7A  is a perspective view of a substrate of an electrostatic actuator according to a fourth embodiment. 
           [0020]      FIG. 7B  is a perspective view of the electrostatic actuator according to the fourth embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     1. First Embodiment 
     (1-1) Structure of Tactile Vibration Applying Device  1   
       [0021]    The structure of a tactile vibration applying device  1  is described with reference to  FIG. 1 . The tactile vibration applying device  1  applies tactile vibrations to human beings. The tactile vibration, in contrast to a voice vibration, is detected by human beings through a tactile sense, and is a low-frequency vibration as compared to the voice vibration. 
         [0022]    As shown in  FIG. 1 , the tactile vibration applying device  1  includes an actuator  10 , a first elastic body  20 , a second elastic body  30 , a first cover  40 , a second cover  50 , and a peripheral cover  60 . 
         [0023]    The actuator  10  is an electrostatic actuator or a piezoelectric actuator. According to a first embodiment, the actuator  10  is an electrostatic actuator. The actuator  10  is flat. While the outline of the actuator  10  is, for example, rectangular, the outline may be formed in any shape. The actuator  10  expands and contracts at least in a thickness direction. When the actuator  10  is an electrostatic actuator, the actuator  10  expands and contracts also in a flat surface direction. That is, when the actuator  10  is an electrostatic actuator, the actuator  10  is molded with an elastomer. 
         [0024]    Specifically, the actuator  10  includes, as shown in  FIG. 1 , a first electrode  11 , a second electrode  12 , a dielectric layer  13 , a first insulating layer  14 , and a second insulating layer  15 . These electrodes and layers are flat. The elastic modulus (Young&#39;s modulus) of the entire actuator  10  in the thickness direction is denoted by E1 (10) . The elastic modulus of the entire actuator  10  in the flat surface direction is denoted by E2 (10) . The loss coefficient tan δ of the entire actuator  10  is denoted by tan δ (10) . 
         [0025]    The first electrode  11  and the second electrode  12  are disposed to oppose to each other with a distance therebetween in the thickness direction of the actuator  10 . The dielectric layer  13  is interposed between the first electrode  11  and the second electrode  12 . The first insulating layer  14  is disposed to contact a surface of the first electrode  11  opposite to a surface thereof opposing the dielectric layer  13 , and covers the first electrode  11 . The second insulating layer  15  is disposed to contact a surface of the second electrode  12  opposite to a surface thereof opposing the dielectric layer  13 , and covers the second electrode  12 . 
         [0026]    The first electrode  11  and the second electrode  12  are formed in the same shape, and molded by mixing a conductive filler with an elastomer. The first electrode  11  and the second electrode  12  are thus flexible and stretchable. Examples of the elastomer forming the first electrode  11  and the second electrode  12  include silicone rubber, ethylene-propylene copolymer rubber, natural rubber, styrene-butadiene copolymer rubber, acrylonitrile-butadiene copolymer rubber, acrylic rubber, epichlorohydrin rubber, chlorosulfonated polyethylene, chlorinated polyethylene, and urethane rubber. It is only required that the conductive filler mixed with the first electrode  11  and the second electrode  12  is conductive particles, and for example, particles of a carbon material or a metal can be used for the conductive filler. 
         [0027]    The dielectric layer  13 , the first insulating layer  14 , and the second insulating layer  15  are molded with an elastomer. The dielectric layer  13 , the first insulating layer  14 , and the second insulating layer  15  are thus flexible and stretchable. A material that functions as a dielectric body in the electrostatic actuator  10  is used for the dielectric layer  13 . The dielectric layer  13  is particularly formed to be the thickest among the components constituting the actuator  10 , and the dielectric layer  13  can expand and contract in the thickness direction and the flat surface direction. An insulating material is used for the first insulating layer  14  and the second insulating layer  15 . 
         [0028]    Examples of the elastomer forming the dielectric layer  13 , the first insulating layer  14 , and the second insulating layer  15  include silicone rubber, acrylonitrile-butadiene copolymer rubber, acrylic rubber, epichlorohydrin rubber, chlorosulfonated polyethylene, chlorinated polyethylene, and urethane rubber. 
         [0029]    The first elastic body  20  and the second elastic body  30  are made of the same material and formed in the same flat shape. The peripheral edge shape of the first elastic body  20  and the second elastic body  30  is the same as that of the actuator  10 . The first elastic body  20  is disposed to contact a surface of the actuator  10  on a side of the first electrode  11  (a top surface of the actuator  10  in  FIG. 1 ), that is, the entire surface of the first insulating layer  14 . The second elastic body  30  is disposed to contact a surface of the actuator  10  on a side of the second electrode  12  (a bottom surface of the actuator  10  in  FIG. 1 ), that is, the entire surface of the second insulating layer  15 . 
         [0030]    A material with a small elastic modulus E (20)  and a small loss coefficient tan δ (20)  is used for the first elastic body  20 , and a material with a small elastic modulus E (30)  and a small loss coefficient tan δ (30)  is used for the second elastic body  30 . That is to say, a material that is soft and has low attenuation characteristics is suitable for the first elastic body  20  and the second elastic body  30 . In particular, the elastic moduli E (20)  and E (30)  of the first elastic body  20  and the second elastic body  30  are smaller than the elastic modulus E1 (10)  of the actuator  10  in the thickness direction. 
         [0031]    The ratio of the elastic modulus E (20)  of the first elastic body  20  to the elastic modulus E1 (10)  of the actuator  10  in the thickness direction is particularly equal to or less than 15%. The ratio of the elastic modulus E (30)  of the second elastic body  30  to the elastic modulus E1 (10)  of the actuator  10  in the thickness direction is also equal to or less than 15%. These ratios are preferably equal to or less than 10%. 
         [0032]    Additionally, the first elastic body  20  and the second elastic body  30  respectively have the loss coefficients tan δ (20)  and tan δ (30)  that are equal to or less than the loss coefficient tan δ (10)  of the actuator  10  under predetermined conditions. The predetermined conditions include a usage environment in which the temperature is in the range of −10° C. to 50° C. and the vibration frequency is equal to or less than 300 Hz. 
         [0033]    For example, silicone rubber that meets the conditions described above is suitable for the first elastic body  20  and the second elastic body  30 . For example, urethane rubber is not suitable for the first elastic body  20  and the second elastic body  30  because the urethane rubber has relatively good attenuation characteristics. However, urethane rubber may be used for the first elastic body  20  and the second elastic body  30  depending on target characteristics. 
         [0034]    The first cover  40  is flat and covers a surface of the first elastic body  20  (a top surface of the first elastic body  20  in  FIG. 1 ). This surface of the first elastic body  20  is opposite to a surface of the first elastic body  20  contacting the actuator  10 . The second cover  50  is flat and covers a surface of the second elastic body  30  (a bottom surface of the second elastic body  30  in  FIG. 1 ). This surface of the second elastic body  30  is opposite to a surface of the second elastic body  30  contacting the actuator  10 . 
         [0035]    The peripheral cover  60  is formed in a cylindrical shape to cover the peripheral surfaces of the actuator  10 , the first elastic body  20 , and the second elastic body  30 . The peripheral cover  60  is provided at the outer peripheral edge of the first cover  40  and formed integrally with the first cover  40 , constituting an integral member. That is, the integral member of the first cover  40  and the peripheral cover  60  is formed in a capsule shape to cover the surface of the first elastic body  20  and the peripheral surfaces of the actuator  10 , the first elastic body  20 , and the second elastic body  30 . The peripheral cover  60  is fixed to the second cover  50 , which is separated from the integral member. The peripheral cover  60  is slightly spaced apart from the outer peripheral surfaces of the actuator  10 , the first elastic body  20 , and the second elastic body  30 . That is, the peripheral cover  60  allows for stretching of the actuator  10 , the first elastic body  20 , and the second elastic body  30  in the flat surface direction. 
         [0036]    The entire surfaces of the actuator  10 , the first elastic body  20 , and the second elastic body  30  are covered by the first cover  40 , the second cover  50 , and the peripheral cover  60 . The first cover  40  and the second cover  50  press the actuator  10 , the first elastic body  20 , and the second elastic body  30  in the thickness direction of the actuator  10 . The first cover  40  and the second cover  50  are fixed in this state through the peripheral cover  60 . 
         [0037]    The first cover  40  and the second cover  50  respectively have elastic moduli E (40)  and E (50)  larger than the elastic modulus E1 (10)  of the actuator  10  in the thickness direction, the elastic modulus E (20)  of the first elastic body  20 , and the elastic modulus E (30)  of the second elastic body  30 . Various materials that meet the conditions described above such as a resin, a metal, and an elastomer can be used for the first cover  40  and the second cover  50 . 
         [0038]    The relationship of the elastic moduli of the members constituting the tactile vibration applying device  1  is represented by the following formula (1). The first cover  40  and the second cover  50  hold the actuator  10 , the first elastic body  20 , and the second elastic body  30  in a compressed state. Here, the elastic modulus E (20)  of the first elastic body  20  and the elastic modulus E (30)  of the second elastic body  30  are smaller than the elastic modulus E1 (10)  of the actuator  10  in the thickness direction. The first elastic body  20  and the second elastic body  30  are thus compressed more than the actuator  10 . 
         [0000]      [Formula 1] 
         [0000]        E   (40)   ,E   (50)   &gt;E 1 (10)   &gt;E   (20)   =E   (30)   (1)
 
         [0039]    Additionally, the second cover  50  is electrically connected to the first electrode  11  and the second electrode  12 , and functions as a circuit board unit including a drive circuit  51  for controlling a voltage applied to the first electrode  11  and the second electrode  12 . Meanwhile, the first cover  40  is a tactile vibration applying part for a user. That is, the user receives tactile vibrations by contacting the first cover  40 . 
       (1-2) Description of Deformation of Internal Components 
       [0040]    Next, the internal components  10 ,  20 , and  30  of the tactile vibration applying device  1  before and after being held by the first cover  40  and the second cover  50  are described with reference to  FIGS. 1 and 2 . The internal components  10 ,  20 , and  30  are held by the first cover  40  and the second cover  50  as shown in  FIG. 1 . The thickness of the actuator  10  is denoted by W 10  and the width of the actuator  10  in the flat direction is denoted by L 10 . The thickness of the first elastic body  20  is denoted by W 20  and the thickness of the second elastic body  30  is denoted by W 30 . 
         [0041]    Meanwhile,  FIG. 2  shows the internal components  10 ,  20 , and  30  before being held by the first cover  40  and the second cover  50 . The thickness of the actuator  10  seems to be equal to W 10 , but in practice, the thickness is denoted by W 11  that is slightly larger than W 10 . On the other hand, the width of the actuator  10  in the flat direction seems to be equal to L 10 , but in practice, the width in the flat direction is denoted by L 11  that is slightly smaller than L 10 . The thickness of the first elastic body  20  is denoted by W 21  that is much larger than W 20 , and the thickness of the second elastic body  30  is denoted by W 31  that is much larger than W 30 . The internal components  10 ,  20 , and  30  before and after compression thus satisfy the following formulae (2) and (3). 
         [0000]    
       
         
           
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         [0042]    Formula (2) is a relational expression of compressibility, whereas formula (3) is a relational expression of compression amount. The compressibility of the first elastic body  20  and the second elastic body  30  is larger than that of the actuator  10  in the thickness direction. The compression amount of the first elastic body  20  and the second elastic body  30  is also larger than that of the actuator  10 . 
       (1-3) Operations of Actuator and Tactile Vibration Applying Device 
       [0043]    Next, operations of the actuator  10  and the tactile vibration applying device  1  are described with reference to  FIGS. 1 and 3 . As shown in  FIG. 3 , the first electrode  11  and the second electrode  12  of the actuator  10  are connected to the drive circuit  51 . The drive circuit  51  may apply an alternating voltage (a periodic voltage including positive and negative values) to the first and second electrodes, or may apply a periodic voltage including positive offset values to the second electrode  12  while the first electrode  11  is connected to the ground potential. In particular, the first electrode  11  disposed on a side that a human being approaches is connected to the ground potential and this further improves the safety. 
         [0044]    When the amount of charge accumulated in the first electrode  11  and the second electrode  12  increases, the dielectric layer  13  is compressed and deformed. That is, the thickness of the actuator  10  reduces and the width of the actuator  10  in the flat direction increases as shown in  FIG. 3 . On the other hand, when the amount of charge accumulated in the first electrode  11  and the second electrode reduces, the dielectric layer  13  returns to its original shape. That is, the thickness of the actuator  10  increases and the width of the actuator  10  in the flat direction reduces. The actuator  10  expands and contracts in the flat surface direction as well as in the thickness direction. 
         [0045]    When the actuator  10  expands and contracts, the tactile vibration applying device  1  operates as follows. A state where the first elastic body  20  and the second elastic body  30  are compressed in the thickness direction as shown in  FIG. 1  is referred to as an initial state of the tactile vibration applying device  1 . When the thickness of the actuator  10  is reduced by an increase in the amount of charge, the first elastic body  20  and the second elastic body  30  are deformed so as to have a smaller compression amount than in the initial state. On the other hand, when the thickness of the actuator  10  is increased by a reduction in the amount of charge, the first elastic body  20  and the second elastic body  30  operate to return to the initial state. That is, the first elastic body  20  and the second elastic body  30  are deformed so as to have a larger compression amount than in a case where the amount of charge increases. 
         [0046]    As the drive circuit  51  applies a periodic voltage to the first electrode  11  and the second electrode  12 , the actuator  10  interposed between the first elastic body  20  and the second elastic body  30  changes its shape as follows: flat shape→curved shape projecting upward in  FIG. 1 →flat shape→curved shape projecting downward in  FIG. 1 →flat shape. 
         [0047]    The displacement of a surface of the actuator  10  on a side of the first insulating layer  14  according to the deformation of the actuator  10  is transmitted via the first elastic body  20  to the first cover  40 . Additionally, the elastic deformation force of the first elastic body  20  is changed by the expansion and contraction of the actuator  10  and such a change in the elastic deformation force of the first elastic body  20  is transmitted to the first cover  40 . The first elastic body  20  and the second elastic body  30  are compressed in the initial state and thus vibrations can be efficiently applied to the first cover  40 . That is, even if vibrations of the actuator  10  alone are small, tactile vibrations can be applied to the first cover  40 . 
         [0048]    If the loss coefficient tan δ (20)  of the first elastic body  20  and the loss coefficient tan δ (30)  of the second elastic body  30  are very large, vibrations are absorbed by the first elastic body  20  and the second elastic body  30  even though the actuator  10  expands and contracts. In such a case, vibrations of the actuator  10  are hardly transmitted to the first cover  40  even though the actuator  10  expands and contracts. 
         [0049]    According to the present embodiment, however, materials with small loss coefficients tan δ (20)  and tan δ (30)  are used for the first elastic body  20  and the second elastic body  30 . In particular, the loss coefficient tan δ (10)  of the actuator  10 , the loss coefficient tan δ (20)  of the first elastic body  20 , and the loss coefficient tan δ (30)  of the second elastic body  30  satisfy the following formula (4). Vibrations caused by the expansion and contraction of the actuator  10  are transmitted to the first cover  40  without being absorbed by the first elastic body  20  and the second elastic body  30 . 
         [0000]      [Formula 4] 
         [0000]      tan δ (10) ≧tan δ (20) =tan δ (30)   (4)
 
         [0050]    As indicated by formula (1), the elastic modulus E (20)  of the first elastic body  20  and the elastic modulus E (30)  of the second elastic body  30  are smaller than the elastic modulus E1 (10)  of the actuator  10  in the thickness direction. The actuator  10  is thus hardly compressed in the initial state where no voltage is applied to the first electrode  11  and the second electrode  12 . Consequently, if the first cover  40  and the second cover  50  press the actuator  10 , this does not affect the expansion and contraction of the actuator  10 . Therefore, the actuator  10  can expand and contract reliably. 
       (1-4) Effects of First Embodiment 
       [0051]    As described above, the tactile vibration applying device  1  according to the first embodiment can reliably generate tactile vibrations by efficiently transmitting small vibrations of the actuator  10  to the first cover  40 . The first cover  40  and the second cover  50  press the actuator  10 , the first elastic body  20 , and the second elastic body  30  with the actuator  10  interposed between the first elastic body  20  and the second elastic body  30 . The actuator  10  thus expands and contracts in the initial state without any external influence. It is possible to efficiently obtain tactile vibrations in the first cover  40 . The second cover  50  is a circuit board unit including the drive circuit  51 . As the second cover  50  also functions as a circuit board unit, it is possible to achieve compactness and an efficient arrangement. 
         [0052]    The ratio of the elastic modulus E (20)  of the first elastic body  20  to the elastic modulus E1 (10)  of the actuator  10  in the thickness direction is equal to or less than 15%. The ratio of the elastic modulus E (30)  of the second elastic body  30  to the elastic modulus E1 (10)  of the actuator  10  in the thickness direction is also equal to or less than 15%. The first elastic body  20  and the second elastic body  30  can thus be compressed more than the actuator  10  in the initial state. 
         [0053]    Materials with small loss coefficients tan δ (20)  and tan δ (30)  are used for the first elastic body  20  and the second elastic body  30 . The first elastic body  20  and the second elastic body  30  can thus transmit vibrations caused by the expansion and contraction of the actuator  10  to the first cover  40  without absorbing the vibrations. This is reliably achieved when silicone rubber is used for the first elastic body  20  and the second elastic body  30 . The loss coefficient tan δ (20)  of the first elastic body  20  and the loss coefficient tan δ (30)  of the second elastic body  30  are equal to or less than the loss coefficient tan δ (10)  of the actuator  10  in the usage environment in which the temperature is in the range of −10° C. to 50° C. and the vibration frequency is equal to or less than 300 Hz. The first elastic body  20  and the second elastic body  30  can thus reliably transmit vibrations caused by the expansion and contraction of the actuator  10  to the first cover  40  without absorbing the vibrations. 
         [0054]    The first elastic body  20  and the second elastic body  30  are formed to be flat with an elastomer. The deformation of the actuator  10  according to the expansion and contraction of the actuator  10  can be reliably transmitted to the first cover  40 . Even if the actuator  10  is an electrostatic actuator or a piezoelectric actuator, vibrations in the first cover  40  have a low frequency. The structure described above enables the tactile vibration applying device  1  to easily generate tactile vibrations in a frequency band lower than that of voice vibrations. 
         [0055]    Additionally, as the actuator  10  is an electrostatic actuator formed of an elastomer, the tactile vibration applying device  1  generates low-frequency tactile vibrations more reliably. 
       2. Modification of First Embodiment 
       [0056]    An electrostatic actuator is used for the actuator  10  in the first embodiment. A piezoelectric actuator may be used for the actuator  10 . In this case, the dielectric layer  13  is replaced by a piezoelectric body. That is, the piezoelectric body is interposed between the first electrode  11  and the second electrode  12 . The actuator  10  operates in the same manner as in the first embodiment to generate tactile vibrations in the first cover  40 . 
         [0057]    The first cover  40  according to the first embodiment may be a touch panel member. In this case, the drive circuit  51  applies a periodic voltage to the actuator  10  according to an operation of the touch panel member by a user, for example. According to the operation of the touch panel member by the user, tactile vibrations are applied to the user contacting the touch panel member. 
       3. Second Embodiment 
       [0058]    A tactile vibration applying device  100  according to a second embodiment is described with reference to  FIGS. 4 and 5 . In the tactile vibration applying device  100  according to the second embodiment, the same structures as in the tactile vibration applying device  1  according to the first embodiment are denoted by the same reference numerals, and descriptions thereof are omitted. 
         [0059]    The tactile vibration applying device  100  includes an actuator  10 , a first elastic body  20 , a second elastic body  30 , a peripheral elastic body  170 , a first cover  140 , a second cover  150 , and a peripheral cover  160 . The tactile vibration applying device  100  according to the second embodiment is different from the tactile vibration applying device  1  according to the first embodiment in the shape of the first cover  140 , the second cover  150 , and the peripheral cover  160 . Further, the peripheral elastic body  170  is additionally provided. 
         [0060]    The peripheral elastic body  170  is made of the same material as the first elastic body  20  in a cylindrical shape. The peripheral elastic body  170  is disposed on an outer peripheral surface of the first elastic body  20  to be integral with the first elastic body  20 . An inner peripheral surface of the peripheral elastic body  170  conforms an outer peripheral surface of the actuator  10 . The peripheral elastic body  170  is disposed to contact the entire peripheral surface of the actuator  10 . 
         [0061]    A material with a small elastic modulus E (170)  and a small loss coefficient tan δ (170)  is used for the peripheral elastic body  170  as well as the first elastic body  20 . That is to say, a material that is soft and has low attenuation characteristics is suitable for the peripheral elastic body  170 . In particular, the peripheral elastic body  170  satisfies formula (5). That is, the peripheral elastic body  170  has the elastic modulus E (170)  smaller than an elastic modulus E2 (10)  of the actuator  10  in a flat surface direction. The ratio of the elastic modulus E (170)  of the peripheral elastic body  170  to the elastic modulus E2 (10)  of the actuator  10  in the flat surface direction is equal to or less than 15%. This ratio is preferably equal to or less than 10%. 
         [0000]      [Formula 5] 
         [0000]        E 2 (10)   &gt;E   (170)   (5)
 
         [0062]    Additionally, the peripheral elastic body  170  satisfies formula (6) under predetermined conditions. That is, the peripheral elastic body  170  has the loss coefficient tan δ (170)  that is equal to or less than the loss coefficient tan δ (10)  of the actuator  10  under predetermined conditions. Under the predetermined conditions, the temperature and the vibration frequency are the same as in the first embodiment. 
         [0000]      [Formula 6] 
         [0000]      tan δ (10) ≧tan δ (20) ≧tan δ (30) ≧tan δ (170)   (6)
 
         [0063]    For example, silicone rubber that meets the conditions described above is suitable for the peripheral elastic body  170  as well as the first elastic body  20 . For example, urethane rubber is not suitable for the peripheral elastic body  170  because the urethane rubber has relatively good attenuation characteristics. However, urethane rubber may be used for the peripheral elastic body  170  depending on target characteristics. 
         [0064]    The first cover  140  is flat and covers a surface of the first elastic body  20  (a top surface of the first elastic body  20  in  FIG. 4 ) and one end surface of the peripheral elastic body  170  (a top surface of the peripheral elastic body  170  in  FIG. 4 ). The second cover  150  is flat and covers a surface of the second elastic body  30  (a bottom surface of the second elastic body  30  in  FIG. 4 ) and the other end surface of the peripheral elastic body  170  (a bottom surface of the peripheral elastic body  170  in  FIG. 4 ). The first cover  140  and the second cover  150  respectively have different sizes from the first cover  40  and the second cover  50  according to the first embodiment, but have substantially the same functions. 
         [0065]    The peripheral cover  160  covers the entire outer peripheral surface of the peripheral elastic body  170 . An inner peripheral surface of the peripheral cover  160  presses inward the peripheral elastic body  170  in the flat surface direction of the actuator  10 . That is, the peripheral cover  160  tightly contacts the outer peripheral surface of the peripheral elastic body  170 . 
         [0066]    The peripheral cover  160  has an elastic modulus E (160)  larger than the elastic modulus E2 (10)  of the actuator  10  in the flat surface direction and the elastic modulus E (170)  of the peripheral elastic body  170 . Various materials that meet the conditions described above such as a resin, a metal, and an elastomer can be used for the peripheral cover  160 . According to the present embodiment, the peripheral cover  160  is made of the same material as the first cover  140  to be integral with the first cover  140 . 
         [0067]    The peripheral cover  160  holds the actuator  10  and the peripheral elastic body  170  in a compressed state in the flat surface direction. Here, the elastic modulus E (160)  of the peripheral elastic body  170  is smaller than the elastic modulus E2 (10)  of the actuator  10  in the flat surface direction. The peripheral elastic body  170  is thus compressed more than the actuator  10 . 
         [0068]    Next, the internal components  10  and  170  of the tactile vibration applying device  100  before and after being held by the peripheral cover  160  are described with reference to  FIGS. 4 and 5 . The internal components  10  and  170  are held by the peripheral cover  160  as shown in  FIG. 4 . The width of the actuator  10  in the flat surface direction is denoted by L 10 . The width of the peripheral elastic body  170  is denoted by L 170 . 
         [0069]    Meanwhile,  FIG. 5  shows the internal components  10  and  170  before being held by the peripheral cover  160 . The width of the actuator  10  in the flat surface direction is denoted by L 11  that is substantially equal to L 10 . The width of the peripheral elastic body  170  is denoted by L 171 . With this, the internal components  10  and  170  before and after compression thus satisfy the following formulae (7) and (8). 
         [0000]    
       
         
           
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               Formula 
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               7 
             
             ] 
           
         
       
       
         
           
             
               
                 
                   
                     
                       
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                           170 
                         
                       
                       
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                         171 
                       
                     
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                     [ 
                     
                       Formula 
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                       8 
                     
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                       171 
                     
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                       170 
                     
                   
                 
               
               
                 
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         [0070]    Formula (7) is a relational expression of compressibility, whereas formula (8) is a relational expression of compression amount. The compressibility of the peripheral elastic body  170  is larger than that of the actuator  10  in the flat surface direction. The compression amount of the peripheral elastic body  170  is also larger than that of the actuator  10  in the flat surface direction. 
         [0071]    In the tactile vibration applying device  100  according to the second embodiment, the flat surface of the actuator  10  is interposed between the first elastic body  20  and the second elastic body  30 . Additionally, the peripheral surface of the actuator  10  is sandwiched between portions of the peripheral elastic body  170 . The operation of the tactile vibration applying device  1  according to the first embodiment in the thickness direction is thus equivalent to the operation of the tactile vibration applying device  100  according to the second embodiment in the flat surface direction. That is, not only a tactile vibration transmitted through the first elastic body  20  and the second elastic body  30  but also a tactile vibration transmitted through the peripheral elastic body  170  is generated in the first cover  140  and the peripheral cover  160 . Consequently, tactile vibrations caused by the expansion and contraction of the actuator  10  are applied to a user contacting the first cover  140  and the peripheral cover  160  more efficiently. 
       4. Third Embodiment 
       [0072]    A tactile vibration applying device  200  according to a third embodiment is described with reference to  FIG. 6 . In the tactile vibration applying device  200  according to the third embodiment, the same structures as in the tactile vibration applying device  100  according to the second embodiment are denoted by the same reference numerals, and descriptions thereof are omitted. 
         [0073]    The tactile vibration applying device  200  includes an actuator  10 , a first elastic body  20 , a second elastic body  30 , a peripheral elastic body  170 , a first cover  240 , a second cover  150 , and a peripheral cover  160 . The tactile vibration applying device  200  according to the third embodiment is different from the tactile vibration applying device  100  according to the second embodiment only in the first cover  240 . The first cover  240  functions as a tactile vibration applying part for a user and includes a plurality of projections  241  on its surface. The projections  241  may be formed in various shapes including a cylindrical shape, a prismatic shape, a frustoconical shape, and a truncated pyramid shape. The area of a distal end surface of each projection  241  is much less than the area that the user contacts. 
         [0074]    As tactile vibrations in the first cover  240  are transmitted via the projections  241  to the user, the surface pressure applied to the user is increased. The user thus feels tactile vibrations more easily. For example, silicone rubber or the like is used for the projections  241  as well as the first elastic body  20 . This is because the projections  241  are soft to some degree to reduce stimuli to the user and thus tactile vibrations are appropriately applied to the user. Silicone rubber has a small loss coefficient tan δ as described above and thus when the first cover  240  vibrates, vibrations are hardly attenuated even if the projections  241  are interposed between the first cover  240  and the user. Tactile vibrations are thus reliably applied to the user. 
       5. Fourth Embodiment 
       [0075]    An actuator  310  according to a fourth embodiment is described with reference to  FIGS. 7A and 7B . In the actuator  10  according to each of the embodiments described above, the first electrode  11 , the second electrode  12 , the dielectric layer  13 , the first insulating layer  14 , and the second insulating layer  15  are flat and laminated to constitute the actuator  10 . A long and flat base  310   a  is prepared as shown in  FIG. 7A . The base  310   a  is constituted in the same manner as the actuator  10  according to the first embodiment and is different from the actuator  10  only in the shape. 
         [0076]    The long base  310   a  shown in  FIG. 7A  is wound to form an actuator  310  shown in  FIG. 7B . The actuator  310  shown in  FIG. 7B  includes three layers of the actuator  10  shown in  FIG. 1 , which are laminated together. The actuator  310  enables a multi-layer actuator structure to be easily achieved and large tactile vibrations to be generated. 
         [0077]    The actuator  310  is formed by winding the base  310   a  in the fourth embodiment. The multi-layer actuator structure may be formed by laminating a plurality of the actuators  10  shown in  FIG. 1 . 
       6. Other Modifications 
       [0078]    The actuator  10  contacts the first cover  40 ,  140  with the first elastic body  20  being interposed between the first cover  40 ,  140  and the actuator  10  in the embodiments described above. The actuator  10  contacts the second cover  50 ,  150  with the second elastic body  30  being interposed between the second cover  50 ,  150  and the actuator  10  in the embodiments described above. The second elastic body  30  may be eliminated and the actuator  10  may directly contact the second cover  50 ,  150 . That is, a surface of the actuator  10  on a side of the first electrode  11  contacts the first cover  40 ,  140  with the first elastic body  20  being interposed between the first cover  40 ,  140  and the actuator  10 , but a surface of the actuator  10  on a side of the second electrode  12  directly contacts the second cover  50 ,  150 . In this case, tactile vibrations are applied to the first cover  40 ,  140  by the first elastic body  20 . The operation of the second cover  50 ,  150  is restricted on the surface of the actuator  10  on the side of the second electrode  12  more than in a case where the second elastic body  30  is interposed between the second cover  50 ,  150  and the actuator  10 . The displacement amount of the actuator  10  is thus reduced easily. While the tactile vibration applying device may eliminate the second elastic body  30 , tactile vibrations transmitted to the first cover  40 ,  140  are reduced.