Abstract:
A flexible actuator comprises a thin film and at least one first enclosure with at least one first bendable element coupled to the first enclosure. The thin film may comprise a conductive layer and a first electret layer over a first surface of the conductive layer. The thin film is configured to be bendable. The first enclosure have a first electrode layer as part of the first enclosure. The first enclosure is provided over the first electret layer with the first electrode layer being spaced apart from the first electret layer. The first electrode layer is coupled with a first terminal of an audio signal input. The thin film is configured to interact with the first enclosure in response to audio signals supplied by the audio signal input and to generate sound waves.

Description:
INCORPORATION BY REFERENCE 
     U.S. Provisional Patent Application No. 61/035,300, titled “Electret Materials, Electret Speakers, and Methods of Manufacturing the Same” is incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to actuators, and more particularly, to flexible electret actuators and methods of manufacturing the same. 
     2. Background of the Invention 
     In the recent years, there have been continued developments for electronic products. One design concept for those developments has been providing lightweight, thin, portable, and/or small devices. In this regard, flexible electronic technology has been increasingly used in various applications, such as LCDs, flex circuits and flexible solar cells. Applications for flexible electronics, such as flexible speakers, may benefit from their low profile, reduced weight, and/or low manufacturing cost. 
     A loudspeaker may produce sound by converting electrical signals from an audio amplifier into mechanical motions. Moving-coil speakers are widely used currently, which may produce sound from the forward and backward motions of a cone that is attached to a coil of wire suspended in or movably coupled with a magnetic field. A current flowing through the coil may induce a varying magnetic field around the coil. The interaction of the two magnetic fields causes relative movements of the coil, thereby moving the cone back and forth. This compresses and decompresses the air, and thus generating sound waves. Due to structural limitations, moving-coil speakers are less likely to be made flexible or in a low profile. 
     An electrostatic speaker may operate on the principle of Coulomb&#39;s law that two conductors with equal and opposite charge may generate a push-pull force between them. The push-pull electrostatic force may cause vibration of a diaphragm, thereby generating sound. An electrostatic speaker may include two porous electrodes and a diaphragm placed between the electrodes to form a series of capacitors. The electrodes and the diaphragm may be separated by dielectric materials. The low-profile and lightweight diaphragm makes the electrostatic speaker superior to other types of speakers, such as dynamic, moving-coil or piezoelectric speakers, with respect to its transition response, expansion capability in high frequency, smoothness of sound, acoustic fidelity and low distortion. 
     With the simple structure, electrostatic speakers may be manufactured in various sizes to accommodate increasing demands for small and thin electronic devices. However, some electrostatic speakers may require a DC-DC converters for providing high voltage to the speakers. Considering the size, cost and power consumption of DC-DC converters, some electret materials have been developed to reduce or avoid the need of DC-DC converters. 
       FIG. 1  illustrates an exemplary electret speaker, which may include porous electrodes  110   a  and  110   b  with a number of holes  112   a  and  112   b  on each electrode having a porosity of at least 30 percent. The electrodes  110   a  and  10   b  may be made of metals or plastic materials coated with a conductive film. The holes  112   a  and  112   b  may be provided for allowing sound waves to pass through them. The electret speaker may further include a diaphragm  120 , which may include a conductive layer  122  sandwiched between electret layers  124   a  and  124   b . The electret layers  124   a  and  124   b  may store positive or negative charges. The electrodes  110   a  and  110   b , and diaphragm  120  may be held in place by holding members  130   a  and  130   b . Elements  140   a ,  140   b ,  142   a  and  142   b  may be made of insulating materials and may be used for separating the diaphragm  120  from the electrode plates  110   a  and  110   b  to form cavities  150   a  and  150   b  for the diaphragm  120  to vibrate. 
     In operating of an electret speaker of  FIG. 1 , each signal source  160   a  and  160   b  may output equal and opposite alternating signals to the electrodes  110   a  and  110   b  via conductive lines  162   a  and  162   b . The signals may cause a time-varying electric field to develop between the electrodes  110   a  and  110   b  and the electret layers  124   a  and  124   b , thus resulting in a push-pull force. The push-pull force may cause the diaphragm  120  to vibrate, resulting in sound waves that may pass through holes  112   a  and  112   b.    
     BRIEF SUMMARY OF THE INVENTION 
     One example consistent with the invention provides a flexible actuator that may comprise a thin film and at least one first enclosure with at least one first bendable element coupled to the first enclosure. The thin film may comprise a conductive layer and a first electret layer over a first surface of the conductive layer. The thin film is configured to be bendable. The first enclosure has a first electrode layer as part of the first enclosure. The first enclosure is provided over the first electret layer with the first electrode layer being spaced apart from the first electret layer. The first electrode layer is coupled with a first terminal of an audio signal input. The thin film is configured to interact with the first enclosure in response to audio signals supplied by the audio signal input and to generate sound waves. 
     In another example consistent with the invention, a flexible actuator may comprise a thin film and at least one first enclosure with at least one first bendable element coupled to the first enclosure. The thin film may comprise a conductive layer. The thin film is configured to be bendable. The first enclosure has a first electrode layer and a first electret layer as part of the first enclosure. The first electrode layer is coupled with a first terminal of an audio signal input. The thin film is configured to interact with the first enclosure in response to audio signals supplied by the audio signal input and to generate sound waves. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended, exemplary drawings. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
       In the drawings: 
         FIG. 1  is a sectional view of an exemplary electret speaker in the prior art; 
         FIG. 2  is a sectional view of an exemplary flexible electret actuator in examples consistent with the present invention; 
         FIG. 3  is a detailed section view of portions of an exemplary flexible electret actuator in examples consistent with the present invention; 
         FIG. 4  is a detailed section view of portions of an exemplary flexible electret actuator in examples consistent with the present invention; 
         FIG. 5  is a sectional view of an exemplary flexible electret actuator in examples consistent with the present invention; 
         FIG. 6  is a sectional view of an exemplary flexible electret actuator in examples consistent with the present invention; 
         FIG. 7  is a sectional view of an exemplary flexible electret actuator in examples consistent with the present invention; 
         FIG. 8  is a top view of an exemplary application of an exemplary flexible electret actuator in examples consistent with the present invention; and 
         FIG. 9  is a side view of an exemplary application of an exemplary flexible electret actuator in examples consistent with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  illustrates an exemplary flexible electret actuator in examples consistent with the present invention. Referring to  FIG. 2 , the flexible electret actuator  200  may comprise first enclosures  210   a , a first bendable elements  211   a , second enclosures  210   b , second bendable elements  211   b  and an electret diaphragm  220 . The first enclosures  210   a  and the first bendable elements  211   a  may comprise a first flexible layer  214   a  and a first electrode  216   a . The second enclosures  210   b  and the second bendable elements  211   b  may comprise a second flexible layer  214   b  and a second electrode  216   b . The flexible layers  214   a  and  214   b  may be made of plastic materials with plasticity or blended fibers. In one example, the flexible layers  214   a  and  214   b  may be made of metal meshes or thin metal plates. The thickness of each flexible layer  214   a  and  214   b  may be in a range of about 20 micrometers to about 10,000 micrometers. The flexible layers  214   a  and  214   b  may be made by at least one of the processes, including but not limited to, injection molding, pressing, forging, plastic thermoforming, mechanical manufacturing and continuous roll-to-roll processes. The first and second electrodes  216   a  and  216   b  may be made from conductive materials such as gold, silver, aluminum, copper, chromium, platinum, indium tin oxide (ITO), silver paste, carbon paste or other conductive materials, or a combination of some of them. The thickness of each electrode  216   a  and  216   b  may be in a range of about 0.01 micrometers to about 100 micrometers. The first and second electrodes  216   a  and  216   b  may be coated on the first and second flexible layers  214   a  and  214   b  by, for example, spraying-coating, spin-coating, dip-coating, sputtering, evaporation, electroplating or a screen-printing process. When the flexible layers  214   a  and  214   b  may be made of metal meshes or thin metal plates to remove the need for the first and second electrodes  216   a  and  216   b  in some examples. 
       FIG. 3  shows details of the first enclosures  210   a  and the first bendable elements  211   a.  Note that the second enclosures  210   b  and second bendable element  211   b  may have corresponding configuration as described below. Each first enclosure  210   a  may have an upper portion with a width C, side portions with a width D and a number of acoustic holes  212   a  on the upper portion. The upper portion and the side portions of each first enclosure  210   a  may provide a cavity  205   a  (                      with a width E and a length F. Each first bendable element  211   a  with a width B may have a thickness of A. The first bendable element  211   a  maybe made of bendable materials while the upper portion and the side portions of the first enclosures  210   a  may be made of rigid materials. As such, when the flexible electret actuator  200  is bent, the length F of the cavity  250   a  defined by the upper portion and the side portions remains the same. In other words, the first enclosures are substantially rigid to limit spacing variation between each first enclosure and the thin film area covering by the first enclosures when the flexible actuator is bent.
       FIG. 4  shows the electret diaphragm  220  which may include a conductive layer  222 , a first electret layer  224   a  and a second electret layer  224   b . The conductive layer  222  may be made of gold, silver, aluminum, copper, chromium, platinum, indium tin oxide (ITO), silver paste, carbon paste or other conductive materials, or a combination of some of them. The conductive layer  222  may be coated on the electret layer  224   b  by, for example, spraying-coating, spin-coating, dip-coating, sputtering, evaporation, electroplating or a screen-printing process. In one example, the electret layers  224   a  and  224   b  may be made of at least one of the following materials: fluorinated ethylene propylene (FEP), poly tetrafluoroethylene (PTFE), cyclic olefin copolymer (COC), polychlorotrfluoroethylene (PCTFE), poly(ethylene-tetrafluoroethylene) (ETFE), Teflon AF, polyimide (PI), polyetherimide (PEI), polystyrene (PS), polycarbonate (PC), polymethylmethacrylate (PMMA), polyvinyl chloride (PVC), and tetrafluoroethylene-per-fluoromethoxyethylene copolymer (PFA). The electret layers  224   a  and  224   b  may store either positive charges or negative charges. The electret layers  224   a  and  224   b  may improve its charge storage stability by corona charge. The electret-metal-electret structure of the diaphragm  220  may be fabricated by a conventional process. In one example, the electret layer  224   a  may be formed on the conductive layer  222  and the electret layer  224   b  through vacuum thermal compression, ultrasonic pressing, mechanical compression or a roll-to-roll process to form an electret-metal-electret structure. 
     The electret diaphragm  220  may be placed between the first enclosures  210   a  and the second enclosures  210   b  by a process, such as a roll-to-roll pressing process or a large-area imprinting process. In that regard, the electret-metal-electret structure of the diaphragm  220  may be affixed to portions of the first bendable elements  211   a  and the second bendable elements  211   b . In one example, the diaphragm  220  may be affixed to the first and second enclosures  210   a  and  210   b  by, for example, a thermal pressing process, ultrasonic pressing process, vacuum thermal compression, a roll-to-roll process or mechanical compression. In another example, the diaphragm  220  may be affixed to the first and second enclosures  210   a  and  210   b  by an adhesive element  270  (as shown in  FIG. 2 ). In one example, the adhesive element  270  may be a double-sided adhesive tape, epoxy resin or instant adhesive glues. The first and second bendable elements  211   a  and  211   b  may hold and support the diaphragm  220  to provide its tension. Referring again to  FIG. 2 , the first enclosure  210   a , the second enclosure  210   b  and the diaphragm  220  together provide a first cavity  250   a  and a second cavity  250   b  to ensure the efficiency of the diaphragm  220  and its displacement. The assembly of the first and second enclosures  210   a  and  210   b  and the diaphragm  220  may form a single unit of a flexible electret actuator  200 . A number of the units arranged together may constitute a flexible electret actuator as shown in  FIGS. 8 and 9 . 
     In operation of a flexible electret actuator  200  of  FIG. 2 , each signal source  260   a  and  260   b  may output an equal and opposite alternating signal to the electrodes  216   a  and  216   b  via conductive lines  262   a  and  262   b . The signals may cause a time-varying electric field to develop between the electrodes  216   a  and  216   b  and the electret layers  224   a  and  224   b , thus resulting in a push-pull force. The push-pull force may cause the diaphragm  220  to vibrate. The resultant sound waves may pass through holes  212   a  and  212   b  and thus generating sound. 
     Another example consistent with the present invention provides a flexible electret actuator wherein the electret layer is included as part of the first enclosures and the first bendable element. In this example, a flexible electret actuator may include first enclosures  510   a , first bendable elements  511   a , second enclosures  510   b  and second bendable elements  511   b .  FIG. 5  shows details of the first enclosures  510   a  which may include an electrode  516   a , a flexible layer  514   a , an electret layer  524   a , and acoustic holes  512   a . Since the flexible layer  514   a , the electret layer  524   a , the electrode  516   a  and the acoustic holes  512   a  are same as those corresponding elements described in connection with  FIGS. 2-4 , description of these elements will not be repeated. In this example, the electret layer  524   a  may be provided under the flexible layer  514   a  by at least one of the processes, including spraying, ultrasonic pressing process, thermal pressing process or mechanical compression. When the electret layer  524   a  is made of plastic with plasticity, the flexible layer  514   a  may be omitted as shown in  FIG. 6 . In the examples of  FIGS. 5 and 6 , the electrostatic charges stored in electret layers  524   a  and  524   b  may be positive or negative. 
     Referring to  FIG. 6 , the diaphragm  520  may be made of at least one of the following materials: fluorinated ethylene propylene (FEP), cyclic olefin copolymer (COC), polyimide (PI), polyetherimide (PEI), polystyrene (PS), polycarbonate (PC), polymethylmethacrylate (PMMA), polyvinyl chloride (PVC), and poly(ethylene terephthalate (PET). The thickness of the diaphragm  520  may be in a range of about 0.5 micrometers to about 200 micrometers. The diaphragm  520  may be coated with a conductive film to form a conductive diaphragm  520  by, for example, a spraying-coating, spin-coating, dip-coating, sputtering, evaporation, electroplating or screen-printing process. In one example, the conductive layer may be gold, silver, aluminum, copper, chromium, platinum, indium tin oxide (ITO), silver paste, carbon paste or other conductive materials. 
     Referring again to  FIG. 6 , the conductive diaphragm  520  may be affixed to portions of the first bendable element  511  a and the second bendable element  511   b  in the same way as described in connection with  FIGS. 2-4  above. In addition, a flexible electret actuator  500  of  FIG. 6  operates the same as described in connection with  FIGS. 2-4 . 
       FIG. 7  illustrates another example in consistent with the present invention. The flexible electret actuator  700  is the same as the flexible electret actuator  500  of  FIG. 6  except that one of the electret layers  724   a  and  724   b  stores positive charge and the other stores negative charges. In this example, electrodes  716   a  and  716   b  are connected to ground via conductive lines  780   a  and  780   b . In operation of a flexible electret actuator of  FIG. 7 , the signal source  760  may output an alternating signal to the conductive diaphragm  720  via conductive line  762 . The signal may cause a time-varying electric field to develop between the conductive diaphragm  720  and the electret layers  724   a  and  724   b , thus resulting in a push-pull force. The push-pull force may cause the diaphragm  720  to vibrate. The resultant sound waves may pass through holes  712   a  and  712   b  and thus generating sound. 
     It will be appreciated by those skilled in the art that changes could be made to the examples described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular examples disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.