Patent Publication Number: US-6336367-B1

Title: Vibration transducer unit

Description:
This invention relates to a vibration transducer unit according to the preamble of claim  1 . 
     Vibration transducers can be used for example in saddle or bridge transducers for acoustic guitars, double bass and violin by means of which the vibrations of the strings are converted to electric signals. They are generally mounted under the saddle of the guitar or to the bridge of double bass and violin. They consist of a transducer part with electromechanical material such as elastic electret bubble film, piezoelectric crystals or polymers and electrically conductive electrode layers, as well as a connection cable, by means of which the signals are taken to a separate preamplifier. 
     In U.S. Pat. publication No. 5,319,153 a piezoelectric transducer for instruments is presented. Therein a strip-formed polyvinylidene fluoride (PVDF) film is used as an active electromechanical material. A connection cable is connected to the transducer part by soldering. 
     A drawback with vibration transducer units according to the prior art is their complicated structure and high price. When the preamplifier part and the transducer part are placed separately, a connection cable is needed between them. This causes extra costs and the connection cable often causes electrical interference especially if it has to be substantially long. If the electromechanical material used is film-like material, like PVDF rendered into piezoelectric, or elastic electret bubble film with permanent electric charge, the transducer must be very close the preamplifier. This is because this kind transducers have relatively low capacitance compared to standard cables capacitances. Additionally it is difficult to place a separate preamplifier in an electric guitar, double bass or violin. Furthermore, the transducer part and the preamplifier have to be protected by means of separate shields, in order to avoid mechanical damage and electrical interference. 
     The aim of the present invention is to remove the drawbacks of the prior art technique and to achieve a vibration transducer unit, which is small and compact, and by means of which additional interference can be avoided. In the solution according to the present invention the preamplifier and the transducer part are, according to the attended claims, integrated into one unit. A connection cable between the transducer part and the preamplifier is not needed. As a result a very compact unit is achieved including both mechanical and electrical protection of said unit. 
    
    
     In the following the invention is described in detail by the aid of examples, by referring to the attached set of drawings, wherein 
     FIG. 1 presents an explosion picture of a vibration transducer unit according to the invention, 
     FIG. 2 a  presents an explosion picture of one embodiment vibration transducer unit according to the invention, 
     FIG. 2 b  presents an vibration transducer unit according to the invention placed in an electric guitar, 
     FIG. 3 presents an explosion picture of one embodiment vibration transducer unit according to the invention, 
     FIG. 4 presents an explosion picture of one embodiment vibration transducer unit according to the invention, 
     FIG. 5 a  presents an explosion picture of one embodiment vibration transducer unit according to the invention, 
     FIG. 5 b  presents a vibration transducer unit according to the invention with a heat-shrink tubing threaded on it, 
     FIG. 6 a  presents an explosion picture of one embodiment vibration transducer unit according to the invention, 
     FIG. 6 b  presents two vibration transducer units according to the invention placed on a double bass, and 
     FIG. 7 presents a cross-section of an electret bubble film. 
    
    
     FIG. 1 presents a vibration transducer unit according to the present invention, with a strip-formed transducer part  11 , and a connector  12  connected to the end of it. A suitable method for manufacturing such transducer element is described in the WO 97/39602. Connector  12  attaches the transducer part  11  to the circuit board  13  consisting preamplifier control circuitry  14  as well as two output conductors  15  and a ground conductor  16 . The transducer  11 , the connector  12  and the circuit board  13  have been protected with a metallic protection shield, said shield composed of both a bottom part  17  and top part  19  protecting the whole unit. The bottom part is composed of a plate  17 , which is as wide as the transducer part  11  to which it is attached for example by gluing. It has side walls  18  substantially surrounding the connector  12 . The shield consists of a plate  191  being as wide as the transducer part  11 , significantly surrounding the space of the circuit board, said top part having a form of a bottomless rectangular box  10 , which protects and shields the circuit board and the components on it, and inside which the walls  18  fit. The top and the bottom parts of the shield can be attached to each others for example by soldering. In this embodiment the output conductors  15  operate both as signal output conductors and power input conductors for the preamplifier. The ground conductor  16  is connected to either the top or the bottom part of the protection shield. It is to be noted that if one or both faces of the transducer are electrically conductive material and the glue in between it and the plate  19  and/or  17  is also electrically conductive, the ground conductor  16  is unnecessary. The transducer part  11  of the invention can be composed of for example one or more dielectric films laminated together, the material of which can be for example polyester. Necessary signal and ground electrodes can be arranged to form suitable shape on the surface of the films for example by silk-screen printing or etching. Between the signal and ground electrodes there is an active electromechanical film, for example being composed of one or several permanently charged dielectric electret films  74  containing flat lens-like gas bubbles  75  or blisters (so called electret bubble film, FIG.  7 ). Such film suitable as electromechanical material for a vibration transducer is presented in U.S. Pat. publication No. 4,654,546. In WO publication 96/06718 a method is presented, by means of which this kind of film can be swelled in order to form a swelled electret film with flat or teared gas blisters. Solid electret film or piezoelectric plastic film, such as PVDF, can also be used as the electromechanical material. 
     Referring now to the FIG. 2 a , the protection shield can also be made in such a manner that the top part  21  has a strip  22 , which can be bent to form the bottom part of the protection shield for the preamplifier and control circuitry  23 . In this case the separate bottom plate  17  can be omitted. FIG. 2 b  shows a transducer unit  24  according to the invention, placed in connection with a bridge  25  and saddles  26  of an electric guitar. 
     One more transducer unit according to the invention is presented in FIG.  3 . In this embodiment the transducer part  31  is integrated onto the same uniform, very thin (the thickness being for example 0,1 mm) circuit board  33  together with the preamplifier and control circuitry  34 . In this embodiment the circuit board  33  is as long as the whole transducer unit. The connector  12  is not needed, and the structure becomes very thin, especially on the transducer part side. There is a metallized area  35  on the top side of the circuit board. This metallized area  35  operates as a signal contact for the transducer  31 . The upper side of the transducer  31  is placed against protection shield  36 , which is grounded. The protection shield  36  and the transducer  31  are glued together. The bottom side  37  of the circuit board  33  is metallized to provide the shield for the transducer. The bottom side  37  of the circuit board  33  is metallized at suitable areas to be enable the protection shield to be grounded directly to the circuit board  33  for example by soldering. Output conductors  38  are soldered to the circuit board  33 . 
     In FIG. 4 one embodiment vibration transducer unit according to the invention is presented. In this embodiment a protection plate  41  is made of a strip-like metal plate. Said plate  41  is bent up as shown at one end of the structure, providing space for the preamplifier and control circuitry portion  42 . The plate  41  connects to the electrical ground of the vibration transducer unit via suitable conductive, for example metallized, areas on the circuit board  43 . The bent portion  45  of the plate  41  also forms the top part of the protective shield for the preamplifier and control circuitry portion  42 . The bottom surface  44  of the circuit board  43  is metallized to provide the bottom part of the electrically protective shield for the preamplifier and control circuitry portion  42 . In this embodiment, electrically protective shield for the preamplifier and control circuitry part  42  is solely formed by the bent portion  45  of the plate  41  and the metallized surface  44  of the circuit board  43 , without any additional shield parts. A piece of heat shrink tubing or other suitable means can be placed over the end of the vibration transducer unit to provide better mechanical and moisture protection. In this embodiment the protective shield does not need to have separate parts for providing walls or bottom for the shield. Between the protective plate  41  and circuit board  43  there is the transducer element  46  and they all are glued together. 
     FIG. 5 a  shows another embodiment of the invention. There the protection shield  51  for the preamplifier circuitry  52  and the circuit board  53  are made of one piece of flexible, very thin circuit board material, such as Kapton®. The signal electrode  54  for the transducer element  55  is arranged on top side of the circuit board  53  and the transducer element  55  is glued on top of it. The bottom side of the circuit board  53  is metallized and it provides the electrical shield for .the signal electrode. On one end of the circuit board there are the components  56  of the circuitry and output conductors  57 . The circuit board  53  has an extension  51 , with metallized outer surface, which extension is wrapped around the circuitry  52  to provide the electrical shield. FIG. 5 b  shows how the extension  51  now completely wrapped around the circuitry  52 , which can be covered with suitable heat-shrink tubing  58 . 
     In FIG. 6 a  one embodiment vibration transducer unit according to the invention is presented. In this embodiment the metallic protection plate  61  provides mainly mechanical reinforcement for the transducer part  62 . However, it is recommended that the metal plate  61  connects to the electrical ground of the vibration transducer part  62  via suitable conductive outer layer, made from for example graphite. The plate  61  can also be made of non conductive dielectric material such as polyester. Transducer part  62  is preferably made according to WO 97/39602, containing a transducer element, electrically conductive layers and conductive contacts being attached by crimping or by other suitable means. Said conductive contacts are soldered onto a flexible circuit board  63 . The circuit board  63  is made wider to provide extension  64  at the preamplifier and control circuitry  65  end. In this embodiment, the circuit board  63  is made of very thin and flexible material such as Kapton®. The surface  66  of the circuit board  63  is metallized and it is wrapped around over the preamplifier and control circuitry portion  65  to provide protective shield for it. In this embodiment the electrically protective shield for the preamplifier and control circuitry portion  65  is solely formed by the circuit board  63 , which enables very easy and costeffective manufacturing and assembly processes. The flexible protective shield  64 ,  66  can also be made of a separate piece of flexible conductive material, which is connected to the electrical ground of the preamplifier and control circuitry part  65  by suitable means. In this case a separate insulator, made of nonconductive material, is first wrapped around or threaded over the preamplifier and control circuitry part, and the protective shield is then wrapped around or threaded over the insulator. Referring to FIG. 6 b , a piece of heat shrink tubing  73  or other suitable means can be placed over the end of the vibration transducer unit to provide better mechanical protection and neater outlook. In FIG. 6 b , a vibration transducer unit according to the invention has been placed on the bridge  67  of a double bass, in a slot formed by a curved body part  68  and an end of a wing part  69 , close to the body part  68 . The transducer is secured in the middle of the slot by wedge a  70  made from for example wood. Additionally, a second vibration transducer unit  71  according to the invention is placed under the bridge foot  72 , between the bridge and the top plate (sound board) of the instrument. A similar transducer unit or set of multiple transducer units can also be placed on some other stringed instruments, for example cello or violin, by similar means. 
     It is obvious for a skilled person in the art, that the embodiments of the invention are not solely restricted to the examples presented above, but they can be varied within the scope of the claims presented hereafter. Thus the vibration transducer unit according to the invention can also be used for example in other stringed instruments, as for example in pianos or grand pianos, in order to convert vibrations into electric signals. Embodiments of the transducer unit according to the invention can also be used in other than musical instrument applications, actually in all applications in which mechanical vibrations are converted to electrical signals. Embodiments of the transducer unit according to the invention can be used for converting vibrations to electrical signals between any two or more separate vibrating objects, or inside any slot within any vibrating object, or even for measuring vibrations from any vibrating surface if an object having suitable mass is attached to the opposite surface of the transducer unit. Additionally, embodiments of said transducer unit can be used for converting vibrations below or above the audible range in frequency (infra-sonic and ultra-sonic, respectively) to electrical signals.