Patent Publication Number: US-2009236932-A1

Title: Electrostatic acting device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     The priority application number JP2008-076741, Electrostatic Acting Device, Mar. 24, 2008, Naoteru Matsubara, upon which this patent application is based is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electrostatic acting device, and more particularly, it relates to an electrostatic acting device comprising a first substrate formed with a first electrode and a second substrate formed with a second electrode. 
     2. Description of the Background Art 
     An electrostatic acting device comprising a first substrate formed with a first electrode and a second substrate formed with a second electrode is known in general. 
     An electret power generator (electrostatic acting device) comprising a movable portion (first substrate) provided with an electrode (first electrode) having conductivity and a fixed portion (second substrate) provided with an electrode (second electrode) made of an electret holding charges is disclosed in general. In general, the electrodes provided on the movable portion and the fixed portion respectively are so arranged at a constant interval (gap between the electrodes) therebetween as to be opposed to each other, and the movable portion is so supported as to be held between spring members. Thus, the electret power generator is so formed as to generate power by causing electrostatic induction between the opposed electrodes when the movable portion vibrates in a direction parallel to the fixed portion. 
     SUMMARY OF THE INVENTION 
     An electrostatic acting device according to a first aspect of the present invention comprises a first substrate formed with a first electrode, a second substrate provided to be opposed to the first substrate at a prescribed interval, formed to be movable relatively to the first substrate and formed with a second electrode, and a gap control portion controlling an interelectrode gap between the first electrode and the second electrode, wherein a distance between at least partial regions of the first electrode of the first substrate and the second electrode of the second substrate is smaller than an interelectrode distance controlled by the gap control portion. 
     A power generator according to a second aspect of the present invention comprises a first substrate formed with a first electrode, a second substrate provided to be opposed to the first substrate at a prescribed interval, formed to be movable relatively to the first substrate and formed with a second electrode, and a gap control portion controlling an interelectrode gap between the first electrode and the second electrode, to be capable of generating power by electrostatic induction due to relative movement of the first substrate and the second substrate, wherein a distance between at least partial regions of the first electrode of the first substrate and the second electrode of the second substrate is smaller than an interelectrode distance controlled by the gap control portion. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view showing an overall structure of a power generator according to a first embodiment of the present invention; 
         FIG. 2  is a sectional view taken along the line  200 - 200  in  FIG. 1 ; 
         FIG. 3  is a plan view showing an overall structure of a power generator according to a second embodiment of the present invention; 
         FIG. 4  is a sectional view taken along the line  200   a - 200   a  in  FIG. 3 ; 
         FIG. 5  is a plan view showing an overall structure of a power generator according to a third embodiment of the present invention; 
         FIG. 6  is a sectional view taken along the line  200   b - 200   b  in  FIG. 5 ; and 
         FIGS. 7 and 8  are diagrams for illustrating a modification of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be hereinafter described with reference to the drawings. 
     First Embodiment 
     A structure of a power generator  100  according to a first embodiment of the present invention will be now described with reference to  FIGS. 1 and 2 . The power generator  100  is an example of the “electrostatic acting device” in the present invention. 
     The power generator  100  according to the first embodiment of the present invention comprises a housing  1 , a movable portion  2 , a fixed portion  3 , two gap control portions  4  and spring members  5  (see  FIG. 1 ) made of a coil spring, as shown in  FIG. 2 . 
     The housing  1  includes a platelike support member  11 , a support  12  and a lid portion  13 , as shown in  FIGS. 1 and 2 . The fixed portion  3  is placed on the support member  11 . The support  12  is so formed as to enclose the support member  11  in plan view and to extend a direction (direction Z) perpendicular to an extensional direction (directions X and Y) of the support member  11 . The lid portion  13  is arranged on an upper portion of the support  12  to close an opening portion of the support  12 . 
     The movable portion  2  includes a movable substrate  21  made of Si (silicon) and electrets  22  formed on an arrow Z 1  direction side of the movable substrate  21 , as shown in  FIG. 2 . Each of the electrets  22  is formed by injecting charges by corona discharge after forming a multilayer film of an SiO 2  film having a thickness of about 1 μm formed on the movable substrate  21  and an organic SOG film having a thickness of about 0.3 μm, formed on this SiO 2  film. The quantity of discharged charges at this time is about 2×10 14  cm −2 . The movable substrate  21  is an example of the “first substrate” in the present invention, and the electrets  22  are examples of the “first electrode” in the present invention. 
     According to the first embodiment, each electret  22  formed by the multilayer film of the SiO 2  film and the organic SOG film has a function as a film applying compressive stress to the movable substrate  21 . More specifically, a thermal treatment process is performed before injecting charges by corona discharge in formation of the electrets  22 . At this time, a thermal expansion coefficient of each SiO 2  film (in a state before charge injection) is different from that of the movable substrate  21  made of Si, and hence the SiO 2  film is so deformed as to apply such stress (compressive stress) that compressing the movable substrate  21 . In this case, while the SiO 2  film is so deformed as to expand in the direction X, the movable substrate  21  is so deformed as to contract in the direction X following expansion of the SiO 2  film. Then the SiO 2  film is made as an electret by charge injection in a state of applying compressive stress to the movable substrate  21 . Accordingly, each electret  22  (SiO 2  film made as an electret) is so formed as to be brought into the state of applying compressive stress to the movable substrate  21 . Thus, according to the first embodiment, the movable substrate  21  and the electrets  22  are so formed as to have substantial convex shapes on the arrow Z 1  direction side (on a fixed substrate  31  side) and to be entirely warped in the direction Z. The movable substrate  21  and the electrets  22  are so formed as to be warped in a direction (direction X) intersecting with a relative movement direction (direction Y) of the movable substrate  21  to the fixed substrate  31  in plan view. Each electret  22  has both functions as a film applying compressive stress to the movable substrate  21  and an electrode. 
     The fixed portion  3  includes the fixed substrate  31  made of glass, provided on the support member  11 , and the collectors  32  made of Al, formed on a surface of the fixed substrate  31  on an arrow Z 2  direction side. The fixed substrate  31  is an example of the “second substrate” in the present invention. The collectors  32  are examples of the “second electrode” in the present invention. The fixed substrate  31  is so provided as to be opposed to the movable substrate  21 . The movable substrate  21  is so formed as to be relatively movable in the direction Y (see  FIG. 2 ) with respect to the fixed substrate  31 . 
     According to the first embodiment, the two gap control portions  4  are made of Si, SiO 2  or the like, and has a function of controlling an interelectrode distance (gap) in the direction Z between the electrets  22  of the movable substrate  21  and the collectors  32  of the fixed substrate  31 , as shown in  FIG. 2 . More specifically, the two gap control portions  4  are so arranged in the vicinities of ends  21   a  and  21   b  of the movable substrate  21  on the direction X side as to extend in the direction Z respectively and so formed as to control the interelectrode distance (gap) in the direction Z between the electrets  22  and the collectors  32  by controlling lengths of the gap control portions  4 . 
     According to the first embodiment, a length L 1  between the vicinity of the central portion of each electret  22  of the movable substrate  21  and the vicinity of the central portion of the corresponding collector  32  of the fixed substrate  31  is rendered smaller than a length L 2  between substrates controlled by the gap control portions  4 . According to the first embodiment, the length L 1  between the vicinity of the central portion of each electret  22  and the vicinity of the central portion of the corresponding collector  32  is rendered smaller than a distance L 3  between the vicinity of each of the peripheral portions of each electret  22  and the vicinity of each of the peripheral portions of the corresponding collector  32 . A region where the distance between the vicinity of the central portion of each electret  22  of the movable substrate  21  and the vicinity of the central portion of the corresponding collector  32  of the fixed substrate  31  is the smallest is a region in a direction along the relative movement direction (direction Y) of the movable substrate  21  to the fixed substrate  31  in plan view. 
     As shown in  FIG. 1 , the four spring members  5  are provided for holding the movable substrate  21  on the support  12  of the housing  1 . The spring members  5  expand/contract, so that the movable substrate  21  can vibrate in the direction Y with respect to the fixed substrate  31 . 
     A power generating operation of the power generator  100  according to the first embodiment of the present invention will be now described with reference to  FIGS. 1 and 2 . 
     As shown in  FIGS. 1 and 2 , electrostatic induction occurs between the electrets  22  and the collectors  32  opposed to each other in a state where the movable substrate  21  stands still in the housing  1 , so that charges are stored in the collectors  32 . Then, the power generator  100  vibrates in the direction Y, so that the electrets  22  move parallel to the collectors  32 . Thus, the quantity of charges induced in the collectors  32  due to electrostatic induction is changed. Then, a current is generated in a load (not shown) connected to the collectors  32 . 
     According to the first embodiment, as hereinabove described, the length (L 1 ) between the vicinity of the central portion of each electret  22  of the movable substrate  21  and the vicinity of the central portion of the corresponding collector  32  of the fixed substrate  31  is rendered smaller than the length (L 2 ) controlled by the gap control portions  4 . Thus, the interelectrode gap (distance) between the vicinity of the central portion of each electret  22  and the vicinity of the central portion of the corresponding collector  32  is reduced with compared with a case of always holding the distance between the electrodes at a constant interval, and hence the quantity of power generation can be further increased. 
     According to the first embodiment, as hereinabove described, the vicinity of the central portion of the movable substrate  21  is warped, whereby the length (L 1 ) between the vicinity of the central portion of each electret  22  of the movable substrate  21  and the vicinity of the central portion of the corresponding collector  32  of the fixed substrate  31  is rendered smaller than the length (L 2 ) controlled by the gap control portions  4 . Thus, the interelectrode gap (distance) between the vicinity of the central portion of the movable substrate  21  and the vicinity of the central portion of the fixed substrate  31  can be reduced. 
     According to the first embodiment, as hereinabove described, the movable substrate  21  is so formed as to be warped in the direction (direction X) intersecting with the relative movement direction (direction Y) of the movable substrate  21  to the fixed substrate  31  in plan view. Thus, the length (L 1 ) between the vicinity of the central portion of each electret  22  of the movable substrate  21  and the vicinity of the central portion of the corresponding collector  32  of the fixed substrate  31  can be rendered smaller than the length (L 2 ) controlled by the gap control portions  4 . 
     According to the first embodiment, as hereinabove described, the length (L 1 ) between the vicinity of the central portion of each electret  22  of the movable substrate  21  and the vicinity of the central portion of the corresponding collector  32  of the fixed substrate  31  is rendered smaller than the distance (L 3 ) between the vicinity of each of the peripheral portions of each electret  22  of the movable substrate  21  and the vicinity of each of the peripheral portions of the corresponding collector  32  of the fixed substrate  31 . Thus, the interelectrode gap between the vicinities of the central portions of each electret  22  and the corresponding collector  32  is rendered smaller than the interelectrode gap between the vicinities of the peripheral portions of the electrets  22  and the corresponding collector  32 . Consequently, the quantity of power generation in the vicinities of the central portions of the electrets  22  and the collectors  32  can be rendered larger than the quantity of power generation in the vicinities of the peripheral portions of the electrets  22  and the collectors  32 . Accordingly, the overall quantity of power generation can be increased. 
     According to the first embodiment, as hereinabove described, the region where the distance between the vicinity of the central portion of each electret  22  of the movable substrate  21  and the vicinity of the central portion of the corresponding collector  32  of the fixed substrate  31  is the smallest is the region in the direction along the relative movement direction (direction Y) of the movable substrate  21  to the fixed substrate  31  in plan view. Thus, the interelectrode gap in the direction Y between the vicinities of the central portions of each electret  22  and the corresponding collector  32  is rendered smaller than the interelectrode gap in the direction Y between the vicinities of the peripheral portions of the electret  22  and the corresponding collector  32 . Consequently, the quantity of power generation generated in the vicinities of the central portions of the electrets  22  and the collectors  32  can be rendered larger than the quantity of power generation generated in the vicinities of the peripheral portions of the electrets  22  and the collectors  32 . 
     According to the first embodiment, as hereinabove described, the movable substrate  21  is formed to entirely have a substantial convex shape toward the collectors  32 . Thus, the interelectrode gap (distance) between the vicinities of the central portions of each electret  22  and the corresponding collector  32  can be reliably reduced as compared with a case of linearly forming the movable substrate  21 , and hence the quantity of power generation can be easily increased. 
     According to the first embodiment, as hereinabove described, the SiO 2  film applying compressive stress to the movable substrate  21  is formed on the surface of the movable substrate  21  on the fixed substrate  31  side (arrow Z 1  direction side). Thus, the movable substrate  21  can be easily deflected to be convex toward the collectors  32 . Thus, the interelectrode gap (distance) between the vicinities of the central portions of each electret  22  and the corresponding collectors  32  can be easily reduced as compared with the case of linearly forming the movable substrate  21 . 
     According to the first embodiment, as hereinabove described, the movable substrate  21  is so formed as to be warped to the fixed substrate  31  side by the SiO 2  films applying the compressive stress. Thus, the movable substrate  21  and the electrets  22  can be easily warped through the thermal treatment process, and hence the interelectrode gap (L 1 ) between the vicinities of the central portions of each electret  22  and the corresponding collector  32  can be easily reduced as compared with a case where the movable substrate  21  is planarized. 
     Second Embodiment 
     Referring to  FIGS. 3 and 4 , a gap control portion  4   a  is formed to be arranged in a support  12  in a power generator  100   a  according to a second embodiment, dissimilarly to the first embodiment where the gap control portions  4  are arranged between the movable substrate  21  and the fixed substrate  31 . The power generator  100   a  is an example of the “electrostatic acting device” in the present invention. 
     In the power generator  100   a  according to the second embodiment, the gap control portion  4   a  is so arranged as to be held between a first support  12   a  provided on a support member  11  side and a second support  12   b  provided on a lid portion  13  side, as shown in  FIG. 4 . The movable portion  2  is supported on a second support  12   b  side by spring members  5   a . Thus, interelectrode gaps between electrets  22  of the movable substrate  21  and collectors  32  of the fixed substrate  31  are controlled by controlling a length of a gap control portion  4   a.    
     According to the second embodiment, each electret  22  is so formed as to be brought into a state of applying such stress (compressive stress) that compressing the movable substrate  21  in a direction Y dissimilarly to the first embodiment. Thus, the movable portion  2  is so formed as to have a substantial convex shape on an arrow Z 1  direction side (fixed substrate  31  side) and to be entirely warped in a direction Z. The movable substrate  21  and the electrets  22  are so formed as to be warped in a relative movement direction (direction Y) of the movable substrate  21  to the fixed substrate  31  in plan view. 
     The remaining structure and power generating operation of the power generator  100   a  according to the second embodiment are similar to those of the first embodiment. 
     According to the second embodiment, as hereinabove described, an interelectrode gap (distance) between the vicinities of central portions of each electret  22  and the corresponding collector  32  can be easily rendered smaller than an interelectrode gap between the vicinities of peripheral portions of each electret  22  and the corresponding collector  32  similarly to the first embodiment, even when the electrets  22  apply such stress (compressive stress) that compressing the movable substrate  21  in the direction Y to the movable substrate  21 , and hence the quantity of power generation can be increased. 
     According to the second embodiment, as hereinabove described, the interelectrode gaps between the electrets  22  and the collectors  32  can be easily controlled also when the gap control portion  4   a  is held between the first support  12   a  and the second support  12   b.    
     The remaining effects of the second embodiment are similar to those of the first embodiment. 
     Third Embodiment 
     Referring to  FIGS. 5 and 6 , a gap control portion  4   b  is formed to be arranged between regions in the vicinities of central portions of a movable substrate  21  and a fixed substrate  31  in a power generator  100   b  according to a third embodiment, dissimilarly to the first embodiment where the gap control portions  4  are arranged between the regions in the vicinities of the peripheral portions of the movable substrate  21  and the fixed substrate  31 . The power generator  100   b  is an example of the “electrostatic acting device” in the present invention. 
     In the power generator  100   b  according to the third embodiment, a single gap control portion  4   b  is so provided as to extend in a direction Y as shown in  FIGS. 5 and 6 . This gap control portion  4   b  is arranged between the regions in the vicinities of the central portions of the movable substrate  21  and the fixed substrate  31 , as shown in  FIG. 6 . According to the third embodiment, electrets  22  are formed on a surface of the movable substrate  21  on an arrow Z 1  direction side. Each electret  22  is made of an SiO 2  film and an organic SOG film which are materials having thermal expansion coefficients smaller than that of the movable substrate  21 . 
     The electrets  22 , which are formed on the surface of the movable substrate  21  on the arrow Z 1  direction side, are so formed as to be brought into a state of applying stress expanding the movable substrate  21  in a direction Y. More specifically, a case of expansion and a case of contraction depend on a method of forming a film of an electret or a condition of a thermal treatment process after forming the film of the electret. According to the third embodiment, the SiO 2  films are formed to be contracted, so that the SiO 2  films apply such stress (such stress that expanding the movable substrate  21 ) pulling the movable substrate  21 , dissimilarly to the first embodiment where the SiO 2  films are expanded. Thus, the movable portion  2  is so formed as to have a substantial convex shape on an arrow Z 2  direction side (side opposite to the fixed substrate  31 ) and to be entirely warped in a direction Z. The movable substrate  21  and the electrets  22  are so formed as to be warped in a direction (direction X) intersecting with a relative movement direction (direction Y) of the movable substrate  21  to the fixed substrate  31  in plan view. 
     Thus, a length L 4  between the vicinity of the peripheral portions of each electret  22  of the movable substrate  21  and the vicinity of the peripheral portions of the corresponding collector  32  of the fixed substrate  31  is rendered smaller than a length L 5  between the vicinity of the central portion of the movable substrate  21  and the vicinity of the electret  22  of the central portion of the collector  32  of the fixed substrate  31  according to the third embodiment. A region where the distance between the vicinity of each of the peripheral portion of each electret  22  of the movable substrate  21  and the vicinity of each of the peripheral portion of the corresponding collector  32  of the fixed substrate  31  is the smallest is a region in a direction along the relative movement direction (direction Y) of the movable substrate  21  to the fixed substrate  31  in plan view. 
     The remaining structure of the third embodiment is similar to that of the first embodiment. 
     According to the third embodiment, as hereinabove described, the length L 4  between the vicinity of the peripheral portions of each electret  22  of the movable substrate  21  and the vicinity of the peripheral portions of the corresponding collector  32  of the fixed substrate  31  is rendered smaller than the length L 5  between the vicinity of the central portion of the electret  22  of the movable substrate  21  and the vicinity of the central portion of the collector  32  of the fixed substrate  31 . Thus, an interelectrode gap between the vicinity of the peripheral portions of each electret  22  and the vicinity of the peripheral portions the corresponding collector  32  is reduced as compared with a case of holding a distance of the electrodes at a constant interval, and hence the quantity of power generation can be increased. 
     According to the third embodiment, as hereinabove described, the gap control portion  4   b  is arranged in the vicinities of the central portions of the electrets  22  of the movable substrate  21  and the collectors  32  of the fixed substrate  31 , whereby the distances between the vicinities of the central portions of the electrets  22  of the movable substrate  21  and the vicinities of the central portions of the collectors  32  of the fixed substrate  31  are held constant. Thus, the interelectrode gap between the vicinities of the central portions of each electret  22  of the movable substrate  21  and the corresponding collector  32  of the fixed substrate  31  can be controlled. 
     According to the third embodiment, as hereinabove described, the region where the distance (L 4 ) between the vicinity of the peripheral portion of each electret  22  of the movable substrate  21  and the vicinity of the peripheral portion of the corresponding collector  32  of the fixed substrate  31  is the smallest is a region in the direction along the relative movement direction (direction Y) of the movable substrate  21  to the fixed substrate  31  in plan view. Thus, the interelectrode gap in the direction Y between the vicinities of the peripheral portions of each electret  22  and the corresponding collector  32  is rendered smaller than the interelectrode gap in the direction Y between the vicinities of the central portions of the electret  22  and the corresponding collector  32 . Consequently, the quantity of power generation generated in the vicinities of the peripheral portions of the electrets  22  and the collectors  32  can be rendered larger than the quantity of power generation generated in the vicinities of the central portions of the electrets  22  and the collectors  32 . 
     According to the third embodiment, as hereinabove described, the movable portion  21  is so formed as to have a substantial convex shape on the side opposite to the fixed substrate  31 . Thus, the interelectrode gaps (distances) between the peripheral portions of the convex movable substrate  21  and the fixed substrate  31  can be reliably reduced. 
     According to the third embodiment, as hereinabove described, the movable substrate  21  is so formed as to be warped to a side opposite to the fixed substrate  31  by the SiO 2  film applying compressive stress. Thus, the movable substrate  21  and the electrets  22  can be easily warped through the thermal treatment process, and hence the interelectrode gap (L 4 ) between the vicinities of the peripheral portions of each electret  22  and the corresponding collector  32  can be reduced as compared with a case where the movable substrate  21  is planarized. 
     The remaining effects of the third embodiment are similar to those of the first embodiment. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims. 
     For example, while the film made of SiO 2  is employed as the film applying stress to the movable substrate in each of the aforementioned first to third embodiments, the present invention is not restricted to this but a film made of a material other than SiO 2  may be applicable so far as the film applies stress to the movable substrate. 
     While the movable substrate is made of Si in each of the aforementioned first to third embodiments, the present invention is not restricted to this but a movable substrate made of a material other than Si may be formed. 
     While the fixed substrate is made of glass in each of the aforementioned first to third embodiments, the present invention is not restricted to this but the fixed substrate may be formed by a material other than glass. 
     While the movable substrate is curved in each of the aforementioned first to third embodiments, the present invention is not restricted to this but a structure in which the fixed substrate is curved may be applicable. 
     While the gap control portion(s) is(are) made of Si or SiO 2  in each of the aforementioned first to third embodiments, the present invention is not restricted to this but the gap control portion may be formed by a hard organic resin material or a hard metal material, for example, so far as the material is hard enough not to change the thickness of the material. 
     While the quantity of discharged charges in injecting charges is 2×10 14  cm −2  as a condition where the film (SiO 2  and organic SOG) on the movable substrate is made as an electret in each of the aforementioned first to third embodiments, the present invention is not restricted to this but charges injection may be performed under a condition of the different quantity of discharged charges. Results of evaluation of samples (A to D) prepared under conditions of the different quantities of discharged charges will be hereinafter described. More specifically, a sample A made of an electret formed under a condition of the quantity of discharged charges of 2×10 14  cm 2 , a sample B made of an electret formed under a condition of the quantity of discharged charges of 7×10 14  cm −2 , a sample C made of an electret formed under a condition of the quantity of discharged charges of 7×10 15  cm −2  and a sample D made of an electret formed under a condition of the quantity of discharged charges of 2×10 16  cm −2  are formed.  FIG. 7  shows a diagram showing results of changes of surface potentials with time in storing the aforementioned samples A to D in the atmosphere. It is understood from the results of  FIG. 7  that, in the samples A and B, the quantities of discharged charges of which are low, the surface potentials thereof are slightly reduced at an initial stage, but are stabilized at high surface potentials after tens of hours. In the sample D, the quantity of discharged charges of which is high, on the other hand, the surface potential thereof was reduced due to aged deterioration in the atmosphere and most of accumulated charges disappeared.  FIG. 8  is a current-voltage characteristics diagram of the electrets of the samples A to D and a sample (ReF.) where charges are not injected. From the results of  FIG. 8 , it has been proved that a leakage current is increased as the quantity of discharged charges of the sample is larger (in other words, aged deterioration is larger). The reason of a leakage current of an insulating film generally includes “tunneling”, “introduction of impurity level” and “introduction of defect level”. In this case, it can be presumed from the points of a large actual film thickness and charge injection, that is, energy is physically given, that the reason is “leakage current due to introduction of defect level”. Accordingly, it is important to control discharge of charge injection while suppressing defects in order to form an electret comprising high stable characteristics. As an example of it, it can be said that minimizing the quantity of discharged charges is effective.