Abstract:
Electromechanical transducer element for converting mechanical stress into electrical signals, said transducer comprising: at least one transducer elements ( 119,120 ), said element having first and second surfaces; at least one signal electrode layer ( 209 ) arranged between two transducer elements, said signal electrode layer being a metal layer arranged in direct contact with first surfaces of the two transducer film elements. Bosses may be arranged adjacent to and/or partly onto at least one electrode layer. The invention relates also to a manufacturing method where adjacent to and/or partly onto a signal electrode and/or a ground electrode a thicker layer of isolating material is deposited, or for composition of the film bosses are arranged in the signal and/or ground electrode.

Description:
This application is a Continuation of PCT/FI01/01125 filed Dec. 19, 2001. 

   FIELD OF INVENTION 
   The present invention relates to an electromechanical transducer element for converting mechanical stress into electrical signals and, in particular, to an flexible unitary electret film transducer element, and to a method for its fabrication. These kind of transducers can be used for example as pressure, force, acceleration and vibration transducers. 
   PRIOR ART 
   As for an electromechanical transducer elements, piezoelectric crystals, piezoelectric sheet (e.g. polyvinylidene fluoride PVDF) and voided electret sheet belong to the prior art. In the commonest film type transducer structures, the metal electrodes are screenprinted silverpaste either on the transducer element or on a plastic film laminated together with transducer element, and connecting cable part is implemented using screened coaxial cable, which is connected to the electrode layers by means of crimped connectors. A drawback with this type of structures is the overall mechanicall strength, and difficulties to implement an integrated structure with necessary preamplifier part. 
   The electret field, or the permanent electric charge, is achieved by injecting charges into dielectric material. 
   A dielectric voided electret film and manufacturing process for same, applicable for use as electromechanical material for a transducer, is described in U.S. Pat. No. 4,654,546, said dielectric film comprising permanently charged, biaxially oriented, foamed, usually homogenous film layer containing flat lens-like, shredded or cavitated gas bubbles which can also be called voids or cells. The term “dielectric cellular electret film” is used here to refer to generally voided type electromechanical films having a permanent electric charge injected into material. Voided electret films are highly elastic and compress in thickness under pressure. In dielectric cellular electret film, flat lens-like gas bubbles effectively limit the mobility of electret charges in the dielectric material, because the gases have an electric resistance five decades better than the best solid insulating materials have. At the same time, compared to hard structure of piezoelectric materials, they act as an elastic soft layer during the conversion of vibrations into electric signals allowing pressure variations to cause microscopic changes in its thickness. The change in thickness causes change in capacitance and produces an electrical output voltage in proportion to the force. 
   WO-publication 96/06718 presents a procedure for pressure inflation of a pre-foamed plastic film, that makes it possible to manufacture strongly foamed film products, involving a high foaming degree and allowing the thickness of the product to be increased without increasing the amount of plastic material. The term “dielectric swelled cellular electret film” is used herein to refer to a foamed film-like plastic product as described in that WO-publication and having a permanent electric charge injected into material. 
   WO 97/39602 presents a stringed musical instrument transducer for converting string vibrations into electric signals, which transducer is composed of electromechanical 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, the signal electrode is arranged on the insulate sheet. As it is printed, it becomes typically 20 microns above the level of the insulate sheet. Therefore, when the transducer is under continuous pressure, which is the case in many applications, the transducer element compresses more from the signal electrode area than from the area beside the signal electrode. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is to eliminate the drawbacks of prior art and achieve an improved transducer of a completely new type, in which a dielectric swelled cellular electret film is used to transform the mechanical stress into electric signals, and wherein no dielectric firm plastic layer to carry the conductive electrodes will be needed in the transducer structure. Thus the transducer becomes mechanically stronger, thinner and the electrical properties become excellent because the firm plastic layers are not absorbing and dampening the vibration or other mechanical energy. Further, because of saved thickness exclusive firm plastic films, the amount of transducer elements can be increased, without adding too much thickness, and thus the output voltage and therefore the signal-to-noise ratio are further improved. Further, due possible increase in thickness of elastic soft dielectric cellular layers the structure becomes softer. Even further, the electrodes become more durable than screen-printed electrodes and can be easily connected by soldering to the preamplifier or connecting wire instead of using crimped connectors where there are plastic layers in between. Thus the electrical properties of connections become excellent and also more durable. Further, it is possible to simultaneously arrange the screening for the connection and the transducers of the invention are very cost effective to manufacture. 
   A further object of the invention is to produce a transducer as simple as possible, having no separate transducer part and no separate conductor for connecting it to a signal processing device, but which has a unitary, flexible and laminated structure and in which the connections for connecting it to a preamplifier can be disposed sequentially or side by side and which in itself is able to produce a balanced signal (differential transducer). 
   The invention relates also to a manufacturing method where adjacent to and/or partly onto a signal electrode and/or a ground electrode is deposited a thicker layer of isolating material, or for compression of the film bosses are arranged in the signal and/or ground electrode whereby the elastic sensor film when the sensor is continuously pressed can during a strong pressure be compressed entirely or at the highest points only. With this construction the sensor generates many times higher voltage under a high pressure than a conventional sensor. 
   In one embodiment of the invention, an durable and cost effective accelometer type transducer, wherein both the transducer and its preamplifier are arranged on same side of printed circuitry board, is achieved. 
   The invention is in detail defined in the attached claims. 
   With the invented manufacturing method, it is possible to produce ultra thin and flexible transducers of desired length, design and width, in which the electrodes in the transducer part are continuous extending from the transducer part to the preamplifier and which are unitary, flexible and thin laminate in construction. Fabrication is faster and more economic than with conventional methods. 
   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 an exploded perspective view illustrating the different components that comprise the transducer of the invention without extra dielectric layers carrying the electrodes at the transducer area, with connectors in preamplifier end arranged side by side, 
       FIG. 1   b  presents an exploded perspective view illustrating the different components that comprise the transducer of the invention without extra dielectric layers carrying the electrodes at the transducer area, with sequentially arranged connecting areas. in preamplifier end, 
       FIG. 2  presents the signal electrodes of the transducer of the embodiment in  FIG. 1   a,    
       FIG. 3  presents one side ground electrodes of the transducer of the embodiment in  FIG. 1   a,    
       FIG. 4  presents an exploded perspective view illustrating the possible screening of the connector end, 
       FIG. 5  presents a microscope picture of dielectric cellular electret bubble film. 
       FIG. 6  presents an exploded perspective view illustrating the different components that comprise another type of a transducer according to the invention, and 
       FIG. 7  presents an exploded perspective view illustrating a net of different components from which the components, that comprise a second type of a transducer according to the invention, are achieved. 
   

   DETAILED DESCRIPTION 
   The transducers of invention in  FIGS. 1   a  and  1   b  consists of a connector part  114  including connectors connecting the transducer to a preamplifier, a connection part  115  corresponding to a connection cable in a conventional transducer and a transducer part  116  for converting the vibrations into electric signals. As may be noted the transducers in  FIGS. 1   a  and  1   b  have no separate transducer part and no separate conductor for connecting it to a signal processing device, but are of a unitary, flexible and laminated structure extending from the end of transducer part  116  unitary as a connection part  115  up to the connector part  114  and in which the connections for connecting it to a preamplifier can be disposed in sequentially or side by side. 
   Referring now to  FIG. 1   a , signal electrode  209  is a thin metal film, for example tinbronze-alloy or tinned copper with thickness of preferably 0.035 up to 0.1 mm. It is to be noted that many thin metal films and thickness are suitable for the application. 
   On both sides of the signal electrode  209  there are swelled dielectric cellular electret films  119 ,  120  having flat gas bubbles  301  (FIG.  5 ), and on the outer sides of the cellular electret films  119 ,  120 , ground electrodes  211 ,  212 . Signal electrode  209  has a form where the electrode is broad in the transducer part and narrow in the connection part. In the connector part the signal electrode has an area corresponding the connection area of the connector  124 . Ground electrodes  211 ,  212  each comprises of thin metal film. Both the ground electrodes  211 ,  212  are connected together with a connector  124  in the connector part  114 . 
   Cellular electret films  119 ,  120  in the transducer area may each comprise of several film layers. Each film  119 ,  120  is charged. Preferably positive charges are injected onto the underside of sheet  119  and onto the top side of sheet  120 . Negative charges may be injected onto the top side of sheet  119  and onto the underside of sheet  120  but it is not essential. The films  127 ,  128  in the connection part are preferably uncharged operating thus as isolating film layers between the electrodes. It is also possible to extend the cellular electret films  119 ,  120  all the way to the connector part  114  but preferably use only partially charged film so that there is no charges in the connection part  115 , to avoid the connection part become active. The ground electrodes  211 ,  212  can also be sputtered, evaporated or chemically metallized to the outer sides of the bubble films  119 ,  120 . It is also possible to arrange the signal electrode  209  directly on the face of bubble film  119  or  120  by for example chemical metallizing process or screenprinting. In this embodiment, to increase the output voltage, it is also possible to use two, or even more, signal electrodes  209  by using three or more transducer elements  119 - 120  and in between each said element having one signal electrode  209  and at the outermost faces of the outermost transducer elements having the ground electrodes  211 - 212 . Further, by using two signal electrodes, two ground electrodes and three transducer elements, and having the two signal electrodes in connection part arranged side-by-side, an differential transducer can be obtained. 
   The outermost film layers  221 ,  223  are not essential for the transducers operation but in some embodiments of the invention, they can act as extra elastic layer in between the vibrating members or as insulation layer for example against printed circuitry board. 
     FIG. 4  shows how the ground electrode  211  may have an extension  224  on the side to form shielding against electrical interference in the connector end  114 . Because the connector area in the signal electrode is open for electromagnetic interference, it must be shielded. Typically this is taken care by metal housing of the preamplifier circuitry, but by this way, an very small preamplifier circuitry can be integrated into the connector end. The components of the circuitry, preferably one field-effect (FET) transistor and one resistor, are connected to the transducers electrodes  209 ,  211 ,  212  and the screening extension  224  is folded around the connector end  114  by using double sided tape  226 , which also forms the necessary insulating in between the components and extension  224 . Leads are connected to the circuitry for taking the signals to the amplifier. By having the preamplifier circuitry as close as possible to the transducer unit, the capacitance of the connection part is lowest possible and the signal-to-noise ratio becomes significantly better. 
   The transducers in  FIGS. 1   a  and  1   b  and  4  are fabricated as follows: 
   Referring to  FIG. 2  signal electrodes  209  and ground electrodes  211 ,  212  are made of a thin metal film  231 ,  232 ,  233 . Firstly the thin metal film  231 ,  232 ,  233  is screenprinted both sides with an insulating material in the areas to form the electrodes. Secondly the metal films  231 ,  232 ,  233  are taken into chemical corrode process where all metal except the areas coated with insulating material, is corroded away. Thirdly, the metal film is taken into next chemical process, where the insulating material is removed for example with alkaline solution. After this, a metal film  231 ,  232 ,  233 , where the wanted electrodes are connected to each others and frame surrounding them with very narrow keepers  234 , is remained. In the corners of each metal film  231 ,  232 ,  233  there is a hole  235  to ease the assembly. It is to be noted that there is other ways too to make similar metal film  231 ,  232 ,  233  containing electrodes for several transducers and with keepers connected to each others and frame. One way is to laser cut the same pattern from a metal film, other way is die-cutting the metal film with suitable tool having the same pattern. Water cutting can also be used. 
   Cellular electret film elements  119 ,  120  size large enough, consisting typically a laminate of 1-3 dielectric cellular electret films, preferably swelled, and metal films  231 ,  232 ,  233  are glued together so that first against metal film  232  with ground electrodes, transducer element  119  and insulating layer  127  are glued, and next, on the other side of the transducer element  119  and insulating layer  127 , the metal film  231  with signal electrodes is glued, and next, to the other side of metal film  231 , second transducer element  120  and second insulating layer  128  are glued, and next, on the other sides of the transducer element  120  and insulating substrate  128 , metal film  233  with second ground layers is glued. In this way a laminate is obtained from which the transducers can be cut away by for example by die-cutting, laser cuffing or water cutting. Further the connectors  124  can be connected by pressing them to connector end  114 . 
   It is also possible first to lay a sheet of glue on both sides on a sheet of dielectric cellular film. Such thin, typically 50 microns thick, glue sheets are manufactured for example by 3M. Obtained sheet of dielectric cellular film  101  ( FIG. 7 ) with glue covered by silicon papers is further diecut to form a suitable pattern with metal electrodes  100 ,  103 ,  104  including holes  236 . This sheet is further first glued, for example against the net of ground electrodes  103  by removing the silicone paper from the areas to come against ground electrode and holes  236  in align. All extra material is removed after glueing. 
   Next, the silicone paper on the other side of the electret film is removed and the net  100  forming the signal electrodes is installed against metal net  100  with holes  236  in align. Further another sheet of electret film  102 , with glue sheets on both sides covered with silicone paper, and diecut to suitable form, is glued against the the net of signal electrodes  100  by first removing the silicone paper from the areas to come against the signal electrodes. The last is to attach the second net  104  of ground electrodes against it. Further it is possible for attaching the sensor to PCB board, have a suitable glue sheet first diecut to form a suitable pattern and attached it to this of transducers, as can be seen in FIG.  6 . By this way, a sheet containing several transducers side by side is obtained, from which separate transducers can easily be removed because the net is hold together by only tiny keepers  105 , which are easily cut when pressed. Transducers are very well screened, cheap to manufacture, reliable to connect to preamplifier, and good in perform. 
   The sensors have very good output because two electret film sheets  107 ,  108  are connected in parallel to signal electrode, as referred in FIG.  6 . Further the pattern can be made such that signal electrode  106  has edges  113   a  arranged wholly inside the sensor structure in order to get a disturbance-free signal output, and the ground electrodes  109 ,  110  have small extensions  113   b , and other ground electrode  110  has opening  113   c  for an extension  113   d  at the signal electrode  106 . The extensions  113   b ,  113   d  can be bent 90 degrees and soldered directly to holes in a PCB board  112  after the sensor is first glued against it. Because the outer ground electrode  110  is arranged with a pattern where there is no hole for extension  113   d  at the signal electrode  106 , it becomes completely shielded against electromagnetic interference from that side. The PCB can have the necessary components for preamplifier and there may be an isolating layer  111  between the PCB  112  and ground electrode  110 . PCB can be same size as sensor and have the components on the other side or it can be larger and components can be on the same side as the transducer. If the transducer is on the same side as components, it can have a suitable mass added over it to work as accelometer type transducer. If the components are on the opposite side to the transducer, the PCB itself can form the mass for the transducer. It is also possible to exclude the ground electrode  110  and have the ground electrode arranged on the printed circuitry board  112  against which the transducer is applied. For a person skilled in art it is also possible to arrange a differential type transducer with this manufacturing technique, by arranging the screening with other ways for example by metal housing over the printed circuitry board. 
   In the manufacturing method where adjacent to and/or partly onto a signal electrode and/or a ground electrode is deposited a thicker layer of isolating material, or for compression of the film bosses are arranged in the signal and/or ground electrode whereby the elastic sensor film when the sensor film is less compressed on the points against the bare signal electrode than against the isolation layer which partly can overlap the signal electrode, whereby a sensor elements achieved with a corresponding signal electrode film, and wherein the sensor film against the isolation layer in hard pressure compresses less than in other parts. The isolation can be printed also on corresponding points in the ground electrode, or in the ground electrode only. When printed onto electrode, it can be same material as the electrode, ie. for example silver paste. We have noticed in many experiments, that the output voltage remains much higher with this kind arrangement when the transducer is in continuous high pressure or should withstand continuous high pressure impacts. Thus an improved signal gain is achieved, even two or more times better signal gain can be achieved, and the dynamics and linearity are also improved. 
   This can be achieved with an etching method described above whereby all the electrode surfaces can be made. Etching can also be realised so that when a certain figure is printed with isolating material on the surface of the metal, and the object is held in the process only a certain defined time, only a part of the metal is corroded (and forms a hollow). 
   There is multiple applications for this type transducers, for example keyboard switches, even force sensitive, electronic stethoscopes, and musical instruments pickups just to name a few; switches, accelometers and vibration sensors in general. 
   This procedure allows a considerably larger number of thin, flexible transducers of desired length, width and shape and having a continuous structure without joints than by conventional methods to be fabricated by the same amount of work while the manufacturing costs remain low. Further, referred to the FIGS  1   a  and  1   b , the transducers can be manufactured very thin without any extra flexible firm insulating substrates to carry the electrodes. Because there is thickness saved due no extra firm insulating substrates, there can be more of active layers, easily 4 layers, which further improves the output voltage and thus also the signal-to-noise ratio. 
   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.