Abstract:
A micro-actuator has a pair of conductive metallic layers connectable to an electrical potential source so as to induce a force between the metallic layers upon application of an electrical field. A layer of dense elastomer material is sandwiched between the pair of conductive metallic layers such that there will be a change in the volume of the elastomer material in response to relative movement between the conductive metallic layers. The elastomer material has at least one void within the elastomer material, whereby the micro-actuator exhibits void-enhanced growth and reduction in size in response to the effected force induced by the applied electrical field. At least one of the metallic layers is a flexible electrode plate. The other metallic layer may be rigid and essentially non-deformable. The elastomer material has substantial plurality of voids.

Description:
FIELD OF THE INVENTION 
     This invention relates to micro-actuators usable to produce controlled movements. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 2,896,507 describes an imaging member which includes an elastically deformable layer sandwiched between a pair of electrode plates. In operation, an electrical field is established across the deformable layer, thus causing this layer to deform. The deformation produces relative movement between the electrode plates. U.S. Pat. No. 3,716,359 discloses improved thin flexible metallic layer electrode plates comprising a plurality of different metals such as, for example, gold, indium, aluminum, silver, magnesium, copper, cobalt, iron, chromium, nickel, gallium, cadmium, mercury, and lead. Various techniques for forming the metallic layers on the elastomer layer are described including, for example, by vacuum evaporation. U.S. Pat. No. 4,163,667 describes the use of a composition of titanium and silver for use as the flexible conductive metallic layer electrode plates in imaging members. 
     It has been found that using solid elastomer material for the elastically deformable layer does not produce a significant amount of relative movement between the pair of electrode plates in response to the application of a reasonable electrical field because there is very little change in the volume of the elastomer. According to a feature of the present invention, the provision of a significant amount of free space or voids in the elastically deformable layer increases the relative motion of the pair of electrode plates. 
     DISCLOSURE OF THE INVENTION 
     It is an object of the present invention to provide a deformable micro-actuator of an elastomer material that produces a significant amount of relative movement between the pair of electrode plates in response to the application of a reasonable electrical field. 
     It is another object of the present invention to provide a deformable micro-actuator of an elastomer material having a significant amount of free space or voids in the elastomer material such that a significant amount of relative movement between the pair of electrode plates is produced in response to the application of a reasonable electrical field. 
     According to a feature of the present invention, a micro-actuator has a pair of conductive metallic layers connectable to an electrical potential source so as to induce a force between the metallic layers upon application of an electrical field. A layer of dense elastomer material is sandwiched between the pair of conductive metallic layers such that there will be a change in the volume of the elastomer material in response to relative movement between the conductive metallic layers. The elastomer material has at least one void within the elastomer material, whereby the micro-actuator exhibits void-enhanced growth and reduction in size in response to the effected force induced by the applied electrical field. 
     In a preferred embodiment of the present invention at least one of the metallic layers is a flexible electrode plate. The other metallic layer may be rigid and essentially non-deformable. The elastomer material has substantial plurality of voids. 
     The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiments presented below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which: 
     FIG. 1 is a partially schematic, cross-sectional view of a micro-actuator according to the present invention; 
     FIG. 2 is a perspective view of the micro-actuator of FIG. 1; 
     FIG. 3 is a partially schematic, cross-sectional view of a micro-actuator according to a second embodiment of the present invention; 
     FIG. 4 is an enlarged view of a portion of the micro-actuators of FIGS.  1 - 3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, there is shown in partially schematic, cross-sectional view, a micro-actuator  10 . FIG. 2 is a perspective view of the micro-actuator of FIG. 1 with portions of the figure cut away for clarity. Micro-actuator  10  includes an optional support substrate  12 , a thin, flexible conductive layer first electrode plate  14 , and an optional layer of insulating material  16 . Insulating material  16 , in turn, carries a deformable elastomer layer  18 . 
     Overlying elastomer layer  18  (FIGS. 1 and 2) or  18 ′ (FIG. 1) is a rigid, essentially non-deformable conductive metallic second electrode plate  20 . First and second electrode plates  14  and  20 , respectively, are connected to an electrical potential source  22  by leads  24 . Potential source  22  may be A.C., D.C., or a combination thereof. The potential source may also include suitable switching apparatus, not shown. Conductive layer first electrode plate  14  may be formed as a rigid, essentially non-deformable conductive metallic plate, eliminating the need for support substrate  12 . Similarly, second electrode plate  20  may be formed as a thin, flexible conductive layer on a support substrate, not shown. 
     In another embodiment shown in FIG. 3, a micro-actuator  10 ′ incorporates insulating material in a deformable elastomer layer  18 ′. Thus the need for layer  16  of FIGS. 1 and 2 is obviated. 
     In operation, an electric field is established across deformable elastomer layer  18  (FIGS. 1 and 2) or  18 ′ (FIG. 3) in a direction normal the planes of first and second electrode plates  14  and  20  by applying a potential from source  22  to the electrode plates. The mechanical force of attraction between first and second electrode plates  14  and  20  due to the electric field causes deformable elastomer layer to compress. Of course, first and second electrode plates  14  and  20  will repulse and cause the elastomer layer to deform in expansion if like electrical poles are applied to electrode plates  14  and  20 . 
     Deformable elastomer layer  18  may comprise any suitable elastomer material, such as for example natural rubber or synthetic polymers with rubber-like characteristics (silicone rubber, styrenebutadiene, polybutadiene, neoprene, butyl, polyisoprene, nitrile, urethane, and ethylene rubbers). Elastomers having relatively high dielectric strength will allow the devices to be operated at higher voltage levels, which in many instances may be preferred. 
     Suitable selection of a particular elastomer material which exhibits an elastic modulus appropriate for a predetermined intended use is within ordinary skill given the description herein. For example, a relatively more stiff elastomer will typically recover more rapidly when an electric field is removed. On the other hand, an elastomer material having a relatively low elastic modulus is typically capable of greater deformations for a given value of electric field. 
     It has been found that using solid elastomer material for deformable elastomer layer  18  or  18 ′ does not produce a significant amount of relative movement between electrode plates  14  and  20  for a reasonable amount of force because there is very little change in the volume of the elastomer. According to a feature of the present invention, the addition of a significant amount of free space, referred to herein as voids  26  (FIG. 4) in deformable elastomer layer  18  increases the relative motion of first and second elastomer. Voids  26  are filled with air. 
     The voids give rise to effective values for the Young&#39;s modulus E eff  and permittivity ∈ eff  of deformable elastomer layer  18  or  18 ′. To first order, these effective values are obtained via a simple volumetric weighting of the respective values of the void and the portion of fully dense solid elastomer. Specifically, the effective values are given by 
     
       
           E   eff   =E (1−β)  (1) 
       
     
     and 
     
       
           ∈   eff =∈ 0 β+∈(1−β)  (2) 
       
     
     where E and ∈ are the Young&#39;s modulus and the permittivity of the solid elastomer, respectively, and        β   =       Δ                   V   void         V   tot                              
     where ΔV void  is the volume occupied by the voids and V tot  is the total volume occupied by deformable elastomer layer  18  or  18 ′ in its undeformed state. For practical applications, ΔV void  is limited to approximately 25% of the total volume. Therefore, 0&lt;β&lt;0.25. 
     It suffices to perform the analysis with optional layer  16  absent. When a voltage is applied between electrode plates  14  and  20 , a strain develops in the deformable elastomer layer which is given by                  Δ                 t       t   0       =       1     E   eff                           F   e                     (     Δ                 t     )       A               (   3   )                                
     where t 0  is the undeformed thickness of deformable layer  18  or  18 ′, Δt is the deformation, A is the area of electrode plate  14 , and F e  is the electrostatic force of attraction between electrode plates  14  and  20 . Furthermore,                  F   e          (     Δ                 t     )       =       -     1   2                ɛ   eff        A                   V   0   2         2          (     t   -          Δ                 t            )     2                   (   4   )                                
     where |Δt| is the absolute value of Δt and V 0  is the voltage applied between electrode plates  14  and  20 . An expression for the strain is obtained by substituting equations (1), (2) and (4) into (3),                        Δ                 t       t   0       =                  -     1   2                       1     E   eff                           ɛ   eff                     V   0   2           (     t   -          Δ                 t            )     2                     =                  -     1   2                       1     E                   (     1   -   β     )                             [         ɛ   0                   β     +     ɛ                   (     1   -   β     )         ]                     V   0   2           (     t   -          Δ                 t            )     2                     =                    -     1   2                       1   E                       ɛ                   V   0   2           (     t   -          Δ                 t            )     2         -       1   2                     1     E                   (     1   -   β     )                       (     t   -          Δ                 t            )     2                           (   5   )                                
     The strain is negative indicating a compressive deformation. The first term in equation (5) is the strain due to a fully dense solid elastomer (without voids). Thus, the additional deformation due to the voids is            Δ                   t   void       =       -       t   0     2                       1     E                   (     1   -   β     )                             ɛ   0                   β                   V   0   2           (     t   -          Δ                 t            )     2           ,                          
     which shows that the addition of voids increases the deformation and hence the relative motion of electrode plates  14  and  20 . 
     For efficient operation of this system, the thickness t 0  of deformable elastomer layer  18  or  18 ′ should be much smaller that either of the side dimensions of the area A of electrode plate  14 . Thus, for example, if electrode plate  14  has a square area of A=10,000 μm, the thickness to should be less than 10 μm. 
     It is instructive to estimate the voltage required to achieve a typical deformation. Consider a device in which the undeformed thickness of deformable elastomer layer  18  or  18 ′ is t 0 =10 μm. Using solid elastomer values of E=10 4  N/m 2  and ∈=3 ∈ 0  where ∈ 0 =8.85×10 −12  farad/m. It follows that a voltage of approximately V 0 =27 volts produces a deformation of Δt=0.1 μm. 
     As an example, the thickness of deformable elastomer layer  18  or  18 ′ may be in the range of from about 0.5 microns to about 200 microns, depending upon the dielectric properties of the elastomer. 
     Electrode plate  20  should have good lateral conductivity, excellent stability, and little internal stress; as well as being highly adherent to deformable elastomer layer  18  or  18 ′. Suitable materials for electrode plate  20  include gold, silver, chromium, nickel, aluminum, conducting polymer, etc. Electrode plate  20  may be formed such as by chemical reaction, precipitation from a solution, electrophoresis, electrolysis, electroless plating, vapor deposition and others. The thickness of electrode plate  20  may, for example, be in the range of from about 200 angstroms to about 5,000 angstroms depending upon any desired flexibility, and the requisite strength and conductivity. 
     It will be recognized that the number of voids, the size of the individual voids, and the total volume of the voids relative to the volume of elastically deformable elastomer layer  18  are variables selectable during the design of a particular system. 
     PARTS LIST 
       10  micro-actuator 
       12  support substrate 
       14  first electrode plate 
       16  insulating material 
       18  elastically deformable elastomer layer 
       20  second electrode plate 
       22  potential source 
       24  leads 
       26  voids