Abstract:
A piezoelectric switch includes a piezoelectric element which is electrically connected to a printed circuit board by a pair of wires. A unitary plastic carrier is designed to retain both the piezoelectric element and the printed circuit board fixedly mounted thereon. In turn, the plastic carrier is sized and shaped to be co-axially disposed within the interior cavity of a generally cylindrically-shaped protective housing, wherein the plastic carrier is provided with at least one flexible finger which is configured to snap-fit into engagement within an annular groove formed in the inner surface of the housing so as to retain the carrier in place within the interior cavity. With the carrier fixed in place within the housing, an adhesive spacer disposes the piezoelectric element in contact with the inner surface of a touch plate provided in the housing. Accordingly, in use, the application of a tactile input onto the outer surface of the touch plate in turn causes the approximate center of the piezoelectric element to slightly deform. In response thereto, the piezoelectric element generates an output signal which passes to the printed circuit board. Based on the output signal received, the printed circuit board regulates the state of the piezoelectric switch.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to electrical switches and more particularly to piezoelectric switches.  
         [0002]     Piezoelectric switches, also commonly referred to simply as piezo switches, are well known and widely used in the electrical switch industry.  
         [0003]     A piezoelectric switch traditionally includes a piezoelectric element comprising a disc-shaped piezoelectric plate and a disc-shaped conductive plate that are affixed together by an adhesive. The piezoelectric plate is commonly constructed out of piezo crystal and serves as the positive terminal for the switch. The conductive plate is typically constructed out of a conductive metal, such as brass, and serves as the negative terminal for the switch. Each of the above-described plates of the piezoelectric element is electrically connected to a printed circuit board which in turn supports the output prongs (i.e., conductive terminals) for the switch.  
         [0004]     The piezoelectric element and printed circuit board of a piezoelectric switch are typically mounted within a hollowed out, non-conductive protective housing. Preferably, the housing is provided with an enlarged, button-shaped touch plate which serves as the contact surface for manually regulating the state of the switch. Typically, the piezoelectric plate of the piezoelectric element is affixed to the inner surface of the touch plate using double-sided adhesive tape which is often of a suitable thickness so as to serve as an insulating spacer. Accordingly, it is to be understood that the application of tactile pressure onto the outer surface of the touch plate is transferred to the piezoelectric element which, in turn, causes the piezoelectric element to very slightly deform. In response to its slight deformation, the piezoelectric element generates an electrical signal that is sent to the printed circuit board. If the electrical signal received by the printed circuit board meets a minimum threshold, the printed circuit board in turn switches the state of connection between the pair of output prongs for the switch (i.e., between an open state and a closed state).  
         [0005]     An example of a piezoelectric switch is disclosed in U.S. Pat. No. 6,064,141 (hereinafter the &#39;141 patent) which issued on May 16, 2000 in the name of Richard D. Wiciel, said patent being incorporated herein by reference. In the &#39;141 patent there is disclosed a piezoelectric switch that comprises a housing having an inner surface and an outer surface. A piezoelectric element is mounted on the inner surface of the housing. The piezoelectric element has a top layer made of piezo crystal and a bottom layer made of a conductive material such as brass. A printed circuit board is disposed within the housing for controlling the state of the switch, the printed circuit board being electrically connected to the piezoelectric element by an electrical conductor. In one embodiment, the electrical connector is in the form of a flex circuit which has a positive terminal, a negative terminal and a printed circuit board overlay portion that is affixed to the printed circuit board. The flex circuit sandwiches the piezoelectric element therewithin so that the positive terminal contacts the top layer and the negative terminal contacts the bottom layer. In another embodiment, the piezoelectric switch additionally comprises a pushbutton assembly movably mounted on the housing for providing the operator of the switch with a noticeable indication of a change in the state of the switch.  
         [0006]     Other known patents of interest include U.S. Pat. No. 4,430,595 which issued on Feb. 7, 1984 in the names of H. Nakasone et al., U.S. Pat. No. 4,761,582 which issued on Aug. 2, 1988 in the name of J. M. McKee, U.S. Pat. No. 5,332,944 which issued on Jul. 26, 1994 in the name of D. J. Cline, U.S. Pat. No. 5,231,326 which issued on Jul. 27, 1993 in the name of J. C. Echols, U.S. Pat. No. 5,442,150 which issued on Aug. 15, 1995 in the name of R. G. Ipcinski, and U.S. Pat. No. 5,636,729 which issued on Jun. 10, 1997 in the name of R. Wiciel, all of said patents being incorporated herein by reference.  
         [0007]     Piezoelectric switches of the type as described above offer a number of notable advantages over traditional electrical switches which include, among other things, multiple moving mechanical parts (e.g., pivotable contact arms).  
         [0008]     As a first advantage, piezoelectric switches of the type described above include no moving mechanical parts. Because it has been found that moving mechanical parts often falter from use over time, piezoelectric switches are consequently more reliable than traditional, mechanically-based, electrical switches, which is highly desirable.  
         [0009]     As a second advantage, piezoelectric switches of the type as described above can be hermetically sealed to protect against harmful environmental conditions (e.g., thermal influences and/or moisture). Specifically, because piezoelectric switches include no moving mechanical parts, the interior cavity of the housing for a piezoelectric switch can be filled with a potting compound, such as a silicon, to protect the electrical components of the switch from corrosive or otherwise damaging elements, which is highly desirable.  
         [0010]     Although well known and widely used in commerce, piezoelectric switches of the type as described above have been found to suffer from a notable drawback. Specifically, it has been found that the use of double-sided tape to affix the piezoelectric element to the inner surface of the touch plate can compromise the success rate of the switch in use. In particular, it has been found that operation of a piezoelectric switch is optimized when the application of an input force onto the contact plate in turn deforms the approximate center of the piezo crystal. However, because the entire surface of the piezo crystal is evenly affixed to the inner surface of the contact plate by an adhesive (which may, in turn, weaken over time), the application of an input force onto the housing is just as likely to cause the outer periphery of the piezo crystal to deform as it is to cause the approximate center of the piezo crystal to deform, which is highly undesirable.  
       SUMMARY OF THE INVENTION  
       [0011]     It is an object of this invention to provide a new and improved piezoelectric switch.  
         [0012]     It is another object of this invention to provide a new and improved piezoelectric switch that comprises a piezoelectric element mounted within a hollowed-out protective housing.  
         [0013]     It is still another object of this invention to provide a piezoelectric switch of the type as described above in which the piezoelectric element is mounted within the protective housing in such a manner so as to optimize the success rate of the switch.  
         [0014]     It is yet still another object of this invention to provide a piezoelectric switch of the type as described above which includes a minimum number of parts, which is inexpensive to manufacture and which is easy to use.  
         [0015]     Therefore, according to one feature of the present invention, there is provided a piezoelectric switch comprising (a) a housing shaped to define an interior cavity, the housing including a touch plate which comprises an inner surface and an outer surface, the outer surface of the touch plate being accessible for tactile input, (b) a piezoelectric element, the piezoelectric element comprising a piezoelectric plate and a conductive plate, the piezoelectric element being characterized by generating an electrical signal upon its deformation, (c) a printed circuit board assembly, the printed circuit board assembly comprising a printed circuit board that is electrically connected to the piezoelectric element, and (d) a carrier for holding the piezoelectric element, the carrier being separate from the housing, the carrier being sized and shaped to fit within the interior cavity of the housing, (e) wherein the carrier is designed to engage the housing to retain the carrier fixed in place within the interior cavity such that the piezoelectric element is coupled to the inner surface of the touch plate.  
         [0016]     Various other features and advantages will appear from the description to follow. In the description, reference is made to the accompanying drawings which form a part thereof, and in which is shown by way of illustration, an embodiment for practicing the invention. The embodiment will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing form the scope of the invention. The following detailed description is therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The accompanying drawings, which are hereby incorporated into and constitute a part of this specification, illustrate an embodiment of the invention and, together with the description, serve to explain the principles of the invention. In the drawings wherein like reference numerals represent like parts:  
         [0018]      FIG. 1  is a bottom perspective view of a piezoelectric switch assembly constructed according to the teachings of the present invention;  
         [0019]      FIG. 2  is an exploded perspective view of the piezoelectric switch shown in  FIG. 1 , the piezoelectric switch being shown with the potting material removed;  
         [0020]      FIG. 2A  is a fragmentary, top perspective view of the piezoelectric element shown in  FIG. 2 , the piezoelectric element being shown with a pair of wires connected thereto;  
         [0021]      FIG. 3  is a top plan view of the carrier shown in  FIG. 2 ;  
         [0022]      FIG. 4  is a bottom plan view of the carrier shown in  FIG. 2 ;  
         [0023]      FIG. 5  is a partially assembled, bottom perspective view, broken away in part, of the piezoelectric switch shown in  FIG. 2 ;  
         [0024]      FIG. 6  is a bottom perspective view, broken away in part, of the piezoelectric switch shown in  FIG. 1 , the piezoelectric switch being shown with the potting material removed;  
         [0025]      FIG. 7  is a bottom plan view of the piezoelectric switch shown in  FIG. 6 ; and  
         [0026]      FIG. 8  is a section view of the piezoelectric switch shown in  FIG. 7  taken along lines  7 - 7 , the piezoelectric switch being shown with the wires removed for greater ease in viewing.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0027]     Referring now to  FIG. 1 , there is shown a piezoelectric switch that is constructed according to the teachings of the present invention, the piezoelectric switch being identified generally by reference numeral  11 .  
         [0028]     Referring now to  FIG. 2 , piezoelectric switch  11  comprises a housing  13 , a piezoelectric element  15 , a carrier  17  and a printed circuit board assembly  19 .  
         [0029]     Housing  13  is a unitary member that is preferably constructed out of a rigid and durable material, such as plastic or aluminum. Housing  13  comprises a cylindrical wall  21  which includes an inner surface  23 , an outer surface  25 , a first end  27  and a second end  29  which together define a partially-enclosed interior cavity  31  which is sized and shaped to receive the remaining components of piezoelectric switch  11 , as will be described in detail below.  
         [0030]     An enlarged disc-shaped touch plate, or button,  33  is integrally formed onto first end  27  of cylindrical wall  21 , touch plate  33  comprising an inner surface  35  and an outer surface  37 . Preferably, the outer diameter of touch plate  33  is slightly greater than the outer diameter of cylindrical wall  21  in order to enclose first end  27  of cylindrical wall  21 . As will be described further below, the central portion of outer surface  37  serves as the contact surface for manually activating switch  11 .  
         [0031]     Outer surface  25  of cylindrical wall  21  is preferably threaded along the majority of its length. As a result, housing  13  (and, in turn, the entire switch  11 ) can be screwed into a corresponding threaded hole formed in the device in which switch  11  is to be installed.  
         [0032]     An annular groove  39  is formed in inner surface  23  of housing  13  at a location proximate second end  29 . As will be described further below, a portion of carrier  17  is designed to snap-fit into engagement within groove  39  in order to retain carrier  17  securely within interior cavity  31 .  
         [0033]     As seen most clearly in  FIG. 2A , piezoelectric element  15  comprises a disc-shaped piezoelectric plate  41  which is affixed onto a disc-shaped conductive plate  43  by an adhesive (not shown), the outer diameter of conductive plate  43  being slightly greater than the outer diameter of piezoelectric plate  41 . Piezoelectric plate  41  is preferably constructed out of piezo crystal and serves as the positive terminal for switch  11 . Conductive plate  43  is preferably constructed out of a conductive metal, such as brass, and serves as the negative terminal for switch  11 .  
         [0034]     A disc-shaped spacer  45  constructed out a deformable and non-conductive material (e.g., foam) is mounted on the free surface of piezoelectric plate  41  by an adhesive layer  47  (as seen most clearly in  FIG. 8 ). Furthermore, the free surface of spacer  45  is coated with an additional adhesive layer  49  which enables spacer  45  to be affixed against inner surface  35  of touch plate  33 . Accordingly, spacer  45  serves two principal functions: (1) as a means for securing piezoelectric element  15  against inner surface  35  of touch plate  33  so as to enable a suitable input force applied onto outer surface  37  of touch plate  33  to, in turn, stimulate piezoelectric plate  41  and (2) as a means for spacing piezoelectric element  15  slightly away from touch plate  33  so as to minimize the possibility of piezoelectric plate  41  being unintentionally stimulated by thermal influences present in the environment immediately surrounding housing  13 .  
         [0035]     Carrier  17  is a unitary member that is constructed out of a suitably rigid plastic material, such as LEXAN 241. As will be described further below, the particular design and function of carrier  17  within switch  11  serves as the principal novel feature of the present invention.  
         [0036]     Referring now to  FIGS. 2-4 , carrier  17  comprises a central, disc-shaped support plate  51  that includes a top surface  53  and a bottom surface  55 . A small circular hole  56  is formed in support plate  51  and is sized and shaped to allow for a pair of conductive wires to pass therethrough, as will be described further below.  
         [0037]     As seen most clearly in  FIGS. 2 and 3 , a pair of opposing, arcuately shaped support walls  57  is integrally formed onto top surface  53  in a spaced apart relationship along the outer periphery of support plate  51 . In addition, a pair of flexible fingers  59  is integrally formed onto top surface  53  along the outer periphery of support plate  51 , with one flexible finger  59  being disposed equidistantly between each adjacent pair of free ends for support walls  57 , as seen most clearly in  FIG. 6 .  
         [0038]     Together, support walls  57  and flexible fingers  59  are designed to support piezoelectric element  15  along the majority of its outer periphery, which is a principal object of the present invention. Specifically, as seen most clearly in  FIG. 3 , each support wall  57  is provided with an L-shaped notch  61  at its free end which is sized and shaped to support a portion of the outer periphery of conductive plate  43 . In addition, each flexible finger  59  is provided with a narrow slot  62  proximate its free end which is sized and shaped to receive a portion of conductive plate  43 . Furthermore, each flexible finger  59  is provided with an outwardly extending, ratchet-shaped tooth, or pawl,  63  at its free end which serves help retain conductive plate  43  of piezoelectric element  13  in place within slot  62 , as will be described further in detail below.  
         [0039]     As seen most clearly in  FIGS. 2 and 4 , a plurality of flexible fingers  65  is integrally formed onto bottom surface  55  in a spaced apart relationship along the outer periphery of support plate  51 , each finger  65  extending generally orthogonally away from bottom surface  55 . The free end of each finger  65  is shaped to include an outwardly extending, ratchet-shaped tooth, or pawl,  67 . As will be described further in detail below, pawl  67  of each flexible finger  65  is designed to engage within annular groove  39  so as retain carrier  17  fixed in place within interior cavity  31  of housing  13 .  
         [0040]     Carrier  17  is represented herein as comprising eight flexible fingers  65  which are equidistantly spaced along the outer periphery of the bottom surface  55  of support plate  51 . However, it is to be understood that the number and/or configuration of flexible fingers  65  could be modified without departing from the spirit of the present invention.  
         [0041]     Furthermore, a plurality of flexible fingers  69  is integrally formed onto bottom surface  55  in a spaced apart relationship, each finger  69  extending generally orthogonally away from bottom surface  55 . Flexible fingers  69  are spaced slightly in from the outer periphery of support plate  51 . In this manner, fingers  69  define a circular ring which is concentrically within the circular ring defined by fingers  65 , as seen most clearly in  FIG. 6 .  
         [0042]     Each flexible finger  69  is provided with a narrow slot  71  proximate its free end which is sized and shaped to receive a portion of the printed circuit board assembly  19 . Furthermore, each flexible finger  69  is provided with an outwardly extending, ratchet-shaped tooth, or pawl,  71  at its free end which serves help retain printed circuit board assembly  19  in place within slot  71 .  
         [0043]     Carrier  17  is represented herein as comprising four flexible fingers  69  which are equidistantly spaced apart from one another. However, it is to be understood that the number and/or configuration of flexible fingers  65  could be modified without departing from the spirit of the present invention.  
         [0044]     Referring back to  FIG. 2 , printed circuit board assembly  19  comprises a thin, generally square-shaped printed circuit board  75 , a pair of metal prongs  77  conductively coupled to printed circuit board  75 , electronics  79  soldered onto printed circuit board  75  and a pair of conductive wires  81  which electrically connect piezoelectric element  15  to printed circuit board  19 . As will be described further below, the input of contact onto piezoelectric element  15  creates an output current which passes from piezoelectric element  15  to printed circuit board  75  via wires  81 . In turn, electronics  79  analyzes said output current and, if it meets the minimum threshold, changes the conductive state of switch  11  by either opening or closing the electrical path which passes between conductive prongs  77 .  
       Assembly of Switch  11   
       [0045]     Piezoelectric switch  11  is designed for assembly in the following manner. Specifically, spacer  45  and conductive plate  43  are affixed to piezoelectric plate  41  by adhesives to form the unitary piezoelectric element  15  shown in  FIG. 2A . Piezoelectric element  15  is then electrically connected to printed circuit board  75  by conductive wires  81 . Specifically, a first wire  81 - 1  electrically connects piezoelectric plate  41  with printed circuit board  75  and a second wire  81 - 2  electrically connects conductive plate  43  with printed circuit board  75 , each of wires  81  being fed through hole  56  in support plate  51  of carrier  17 , as shown in  FIG. 2 .  
         [0046]     With piezoelectric element  15  electrically connected to printed circuit board  75 , both piezoelectric element  15  and printed circuit board  75  are mounted onto carrier  17  in snap-fit engagement therewith.  
         [0047]     Specifically, piezoelectric element  15  is mounted onto carrier  17  in the following manner. With spacer  45  directed away from carrier  17 , as shown in  FIG. 2 , piezoelectric element  15  is urged downward towards carrier  17  until the bottom surface of conductive plate  43  contacts the ratchet-shaped tooth  63  provided on each flexible finger  59 . Upon the application of a suitable downward force, conductive plate  43  causes each of fingers  59  to flex slightly outward to the extent necessary such that the majority of the outer periphery of the bottom surface of conductive plate  43  is disposed directly on top of notches  61  formed in arcuate support walls  57 . With conductive plate  43  disposed directly on top of notches  61  in support walls  57 , flexible fingers  59  resiliently snap back inward such that a portion of the outer periphery of conductive plate  43  aligns within the slot  62  formed in each finger  59 , the ratchet-shaped tooth  63  on each finger  59  engaging the top surface of conductive plate  43  so as to retain piezoelectric element  15  securely in place on carrier  17 , as shown in  FIG. 5 .  
         [0048]     Similarly, printed circuit board  75  is mounted onto carrier in the following manner. With conductive prongs  77  directed away from carrier  17 , as shown in  FIG. 2 , printed circuit board  75  is urged upwards towards carrier until the top surface of printed circuit board  75  contacts the ratchet-shaped tooth  73  provided on each flexible finger  69 . Upon the application of a suitable upward force, printed circuit board  75  causes each of fingers  69  to flex slightly outward to the extent necessary so that the outer periphery of printed circuit board  75  aligns within each slot  71 . At this time, flexible fingers  69  resiliently snap back inward with the ratchet-shaped tooth  73  on each finger  69  engaging the bottom surface of printed circuit board  75  so as to retain printed circuit board  75  securely in place on carrier  17 , as shown in  FIG. 5 .  
         [0049]     With piezoelectric element  15  and printed circuit board  75  mounted on carrier  17  in the manner as described above and as represented in  FIG. 5 , carrier  17  is slidably disposed in place within housing  13 . Specifically, with conductive prongs  77  directed away from housing  13 , carrier  17  is disposed in axial alignment within interior cavity  31  of housing  13 . Aligned in this manner, carrier  17  is then urged directly upward.  
         [0050]     It should be noted that, as carrier  17  is advanced into cavity  31 , fingers  65  on carrier  17  eventually contact second end  29  of housing  13 . However, upon the application of a suitable upward force on carrier  17 , second end  29  of housing inwardly biases fingers  65  to the extent necessary so that carrier  17  can project into interior cavity  31 .  
         [0051]     Further advancement of carrier  17  into cavity  31  ultimately causes adhesive layer  49  on the free end of spacer  45  to contact the inner surface  23  of housing  13 , thereby securing piezoelectric element  15  in place within housing  13 , as shown in  FIG. 8 . At the same time, the ratchet-shaped tooth  67  on each finger  65  aligns within annular groove  39  formed in housing  13 . As a result, each finger  65  resiliently snaps outward such that each tooth  67  projects into groove  39 , thereby securing carrier  17  in place within cavity  31 , as shown in  FIG. 6 .  
         [0052]     With carrier  17  (as well as piezoelectric element  15  and printed circuit board assembly  19 ) secured in place within interior cavity  31  of housing  13 , a supply of potting material  83  such as silicon is preferably deposited into cavity  31  in order to enclose the open second end  29  of housing  13 , as shown in  FIG. 1 . In this manner, potting material  83  serves to seal off open second end  29  of housing  13 , thereby protecting the electrical components of switch  11  (i.e., piezoelectric element  15  and printed circuit board assembly  19 ) from any potentially harmful environmental conditions, such as moisture.  
         [0053]     As such, it is readily apparent carrier  17  serves two principal functions: (1) to retain piezoelectric element  15  fixed in place within housing  13  and (2) to retain printed circuit board  75  fixed in place within housing  13 . Because carrier  17  is designed as a separate piece from housing  13  which can be easily snap-fit into place within interior cavity  31 , it is to be understood that carrier  17  renders piezoelectric switch  11  considerably less labor-intensive to assemble than most traditional piezoelectric switches, which is highly desirable.  
         [0054]     It should also be noted that the particular design of carrier  17  provides a high-level of support to piezoelectric element  15  along the majority of its outer periphery. As a result, even if adhesive layer  47  were to weaken over time, carrier  17  ensures that the application of a suitable force on outer surface  37  of touch plate  33  directs the deformation of piezoelectric plate  41  towards its center rather than along its outer periphery, thereby optimizing the success rate of switch  11 , which is highly desirable.  
       Operation of Switch  11   
       [0055]     With piezoelectric switch  11  assembled in the manner noted above, conductive prongs  77  of printed circuit board assembly  19  are electrically connected to the electrical circuit to be regulated by switch  11 . As noted briefly above, the threading provided on outer surface  25  of housing  13  enables switch  11  to be screwed in place within a corresponding threaded bore (e.g., provided in a panel for the device to be regulated by switch  11 ).  
         [0056]     With switch  11  electrically connected to a particular electrical circuit, it is to be understood that the application of a suitable contact force on outer surface  37  of button-shaped touch plate  33  in turn travels through spacer  45  and deforms the approximate center of piezoelectric plate  41 . This deformation causes piezoelectric plate  41  to generate an output signal which is passed to printed circuit board  75  via wire  81 - 1 .  
         [0057]     Electronics  79  are responsible for analyzing the output signal to determine whether it meets the minimum threshold. If the output signal is sufficient, electronics  79  opens or closes switch  11  (depending on its natural state). As a result, switch  11  serves to regulate the control of current between prongs  77  and, in turn, the electric circuit to which switch  11  is connected.  
         [0058]     The embodiments shown in the present invention are intended to be merely exemplary and those skilled in the art shall be able to make numerous variations and modifications to them without departing from the spirit of the present invention. All such variations and modifications are intended to be within the scope of the present invention as defined in the appended claims.