Patent Publication Number: US-7589439-B2

Title: Electromechanical transducer element, method for forming an electromechanical transducer element and transducer formed by said method

Description:
This application is a continuation of prior PCT application No. FI 03/00035, filed Jan. 17, 2003. 

   FIELD OF INVENTION 
   The present invention relates to an electromechanical transducer element for converting force and pressure changes and vibrations into electrical signals and to a method for its fabrication. Present invention is especially useable as musical instrument transducer for converting vibrations into electrical signals and, in particular, to an flexible unitary under-saddle transducer element, 
   PRIOR ART 
   WO 97/39602 presents a stringed musical instrument transducer for converting string vibrations into electric signals, which transducer is composed of elastic, voided electret-film sheets and is capable of converting string vibrations into electric signals. The electrodes required by the electromechanical sheet are disposed on the surface of one or more thin and flexible dielectric materials, said electrodes forming electrically conductive surfaces of the transducer for connecting the transducer to a signal processing device, and which transducer is constructed of a unitary, thin and flexible layered sheet structure. 
   In the transducer described in WO 97/39602, signal and ground electrodes are arranged on the insulate sheet. As electrodes are printed with silver-paste, they are typically about 20 μm thick layers on the insulate sheets, which can be for example 100 μm thick polyester. Lack of the prior art transducers where voided electret-film is used as electromechanical film, is that when the transducer is under continuous high pressure, which is the case in many applications like under-saddle transducer in acoustic guitar, the electromechanical film compresses constantly and its output gets lower and lower upon time. This happens because under high pressure the gas inside voids diffuses and therefore the elasticity of the film drops which further causes the distance of the layers with opposite charges inside film getting smaller. 
   For example from U.S. Pat. No. 4,885,783 it is known to use electrical insulating material in order to increase the gas breakdown voltage and to lessen the deleterious effects of accidentally exceeding the voltage. U.S. Pat. No. 4,885,783 pertains to electrical-to-mechanical transducers. More particularly, the application pertains to an electro-static transducer in which an elastomeric dielectric material is disposed between a pair of opposed conductive plates across which an electrical potential difference is maintained. A plurality of strips, beads or nodules of elastomeric dielectric material are disposed between plates and in contact therewith, thereby separating plates by a distance “d” such that, for a given gas maintained between plates at a pressure “P”, the product Pd is significantly less than the value required to achieve the Paschen minimum breakdown voltage of the gas. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is to eliminate the drawbacks of prior art and achieve an improved transducer, in which a dielectric swelled cellular (voided) electret film is used to transform the mechanical stress into electric signals. In the present invention, adjacent to the signal electrode and/or possibly partly onto it, is deposited a layer of isolating material, for example by screen-printing a lacquer layer, and partly onto the ground electrode is deposited another layer of silver-paste, which also can be dielectric lacquer. This way arranging bosses or stripes against the elastic voided electromechanical film, the film, when the transducer is continuously under high pressure, like is the case with under the saddle transducers due the tension of the strings, compresses most only at the sides of the sensor. In the middle, over the actual signal electrode area, is left a area (space) where the voided film cannot compress entirely due the fact the thicker sides prevent from it to happen. With this construction the transducer generates much higher voltage output, typically about 6 dB more, which is essential for good signal-to-noise ratio and studio quality sound production, than with a conventional prior art transducer. Also, the output level remains better constant upon time. 
   It is also possible to otherwise generate bosses to the signal electrode and/or ground electrode to achieve the similar effect of the invention, for example by etching in case if pure metal electrodes are used. 
   The invention is in detail defined in the attached claims. 
   The structure of the invention thus allows the application of an effective and economic production technique with significantly improved electrical properties. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, the invention is described in more detail by the aid of examples by referring to the attached drawings, in which 
       FIG. 1   a  presents a cross-sectional view of the transducer, in this case a musical instrument transducer, according to the invention, 
       FIG. 1   b  presents a cross-sectional view of the transducer according to the invention, which have been under high pressure 
       FIG. 2   a  presents a screen-print film for printing the signal and ground electrode layers of the transducer in  FIGS. 1   a  and  1   b.    
       FIG. 2   b  presents a screen-print film for printing the ground electrode layers of the transducer in  FIG. 1   a  and  1   b.    
       FIG. 2   c  presents, according the present invention, a screen-print film for printing the dielectric layers adjacent to signal electrode and additional silver-paste layers onto ground electrode layer 
       FIG. 2   d  presents, according the present invention, another screen-print film for printing the dielectric layers adjacent to signal electrode and additional silver-paste layers onto ground electrode layer 
       FIG. 3  presents a microscope picture of swelled dielectric cellular electret bubble film. 
   

   DETAILED DESCRIPTION 
   The transducers of invention in  FIG. 1   a  consists of a two plastic films,  101  and  102 , for example polyester, with thickness typically 100 μm. On the upper side of the film  101  is printed a ground electrode layer  103 , screen-printed according to  FIG. 2   b , with thickness about 20 μm. Under the film  101  has first been printed at same time the signal electrode layer  104  and ground-loop electrode  105 , accordingly to  FIG. 2   a , both typically having thickness of 20 microns. Following this printing is printed dielectric layer  106  accordingly to  FIG. 2   c , also having thickness of about 20 μm. 100 μm thick polyester film  102  has on upper side 20 μm ground electrode layer  107 , printed with  FIG. 2   b . Partly onto the ground electrode layer is printed another about 20 μm layer  108  with silver-paste, by using film as in  FIG. 2   c .  FIG. 2   d shows another kind arrangement, where there comes additional, thin, for example about 0.3 mm wide, crossing lines  111  over both signal and ground electrodes. This kind arrangement is needed if the transducer has greater width in both x- and y-directions. 
   The films  109 ,  110  are active electromechanical films, being composed of permanently charged dielectric electret films  74  containing flat lens-like gas bubbles  75  or blisters (so called electret bubble film,  FIG. 3 ). In typical electromechanical transducer application films  109 ,  110  have originally been about 50 μm elastic electric films with about 35% gas of the thickness, which further have been swelled to about 70 microns thickness (about 55% gas of the thickness) and charged. The cross-sectional view in  FIG. 1   b  clearly shows how in the structure of the present invention, when the transducer is under high pressure, over the area of the signal electrode, there is a space for the voided transducer film not to compress entirely. Typically two layers of elastic electret films are used for higher output. If the total thickness of the two layers  108 , 109  is 140 μm, they can compress in the side areas  106 ,  107  down to about 65 μm. In the area of the signal electrode they can compress only down to about 105 μm. This will remain constant, significantly higher output level upon time under high pressure. 
   As is known in prior art transducers, number of electromechanical layers and their order can vary a lot. Signal and ground electrodes, as well as the additional isolation and/or conductive layers, can also be printed directly into elastic charged electret films which further can be laminated together. Another embodiment of the invention is for example to take two sheets of elastic electret film and having signal electrode printed on one side of them and ground electrode on opposite sides. By further printing the additional layers onto signal electrodes, to comprise the bosses or stripes, and laminating the two sheets of electret films together by having the signal electrodes against each other, extremely thin transducer can be achieved where no additional polyester or else layers are needed. Yet the structure will have same innovative benefit. 
   It is obvious to the person skilled in the art that different embodiments of the invention are not restricted to the examples described above, but that they can be varied within the scope of the claims presented below. The number of films and layers on top of each other can be chosen in accordance with the need in each case and the transducer can also have a shape other than rectangular in top view.