Abstract:
Disclosed are apparatus and methodology for minimizing and compensating for cracking in piezoelectric devices so as to maintain long term functionality of the devices. Compensation for cracking is achieved by applying solid conductive electrodes over the entire surface of the piezoelectric device and extending the electrodes beyond the perimeter of the piezoelectric device. In this way electrical connections are maintained even in the presence of cracking. Cracking of the piezoelectric device is limited by minimizing the local bending moment of the piezoelectric device by way of applying insulative support materials that may vary in thickness.

Description:
FIELD OF THE INVENTION 
       [0001]    The present subject matter relates to signal generators. In particular, the present subject matter relates to piezoelectric generators having solid copper electrodes. 
       BACKGROUND OF THE INVENTION 
       [0002]    The incorporation of electronic devices with tire structures has been shown to yield many practical advantages. Tire electronics may provide their own power source whose operation depends on tire related phenomena and may also include sensors and other components for obtaining information regarding various physical parameters of a tire, such as temperature, pressure, number of tire revolutions, tire rotational speed, etc. Such information may be useful in tire monitoring and warning systems, and may even be employed with feedback systems to monitor proper tire pressure levels. 
         [0003]    United States Published Patent Application 2003/0209063 (Adamson et al.) is directed to a system and method for generating electric power from a rotating tire&#39;s mechanical energy using piezoelectric fiber composites. 
         [0004]    United States Published Patent Application 2003/0056351 (Wilkie et al.) is directed to a piezoelectric micro-fiber composite actuator and a method for making the same. 
         [0005]    U.S. Pat. No. 6,093,997 (Zimnicki et al.) is directed to a piezoelectric resonator embedded within an electrically insulating substrate assembly, such as a multilayer printed circuit board. 
         [0006]    U.S. Pat. No. 5,747,916 (Sugimoto et al.) is directed to a piezoelectric transformer unit which transforms an input voltage into an output voltage and which includes a piezoelectric transformer element driven by high electric power. 
         [0007]    U.S. Pat. No. 5,305,507 (Dvorsky et al.) is directed to a method for encapsulating a ceramic device for embedding in composite structures. 
         [0008]    European Patent EP1724849 A2 (Guenther et al.) is directed to an electrically conductive supporting body with a piezoelectric material applied to the body. 
         [0009]    While various implementations of piezoelectric generators have been developed, and while various contact configurations have been implemented, no design has emerged that generally encompasses all of the desired characteristics as hereafter presented in accordance with the subject technology. 
       SUMMARY OF THE INVENTION 
       [0010]    In view of the recognized features encountered in the prior art and addressed by the present subject matter, an improved apparatus and methodology has been provided to extend the operational lifetime of piezoelectric devices even in the face of structural damage from normal operation through the provision of solid electrodes. 
         [0011]    In an exemplary embodiment, the present subject matter relates to a piezoelectric device comprising a layer of piezoelectric material having a first surface, a second surface, and a perimeter, a first layer of conductive material completely covering the first surface and extending beyond the perimeter of the piezoelectric material, and a second layer of conductive material completely covering the second surface and extending beyond the perimeter of the piezoelectric material. The first and second conductive layers extend beyond the perimeter of the piezoelectric material a sufficient distance so as to avoid the influence of any cracks created in the first or second conductive layers caused by cracks in the piezoelectric material. A first insulating layer supports the piezoelectric material and the first and second layers of conductive material. 
         [0012]    In further embodiments solder pads may be coupled to the first and second layers of conductive material, a second insulating layer may be provided that may have the same or a different thickness than the first insulating layer. In preferred embodiments the insulating material may be FR4, the conductive material may be copper, and the piezoelectric material may be lead zirconium titanate (PZT). 
         [0013]    In still further embodiments the device may include and elastomeric layer including a mesa portion on which the first insulating layer may be secured by way of an adhesive. In a particular configuration the adhesive may be Chemloc® and the elastomeric material may be rubber. 
         [0014]    The present subject matter also relates to methodologies for preserving piezoelectric device functionality in the presence of stress induced cracks. These methodologies comprising providing a layer of piezoelectric material having a first surface, a second surface, and a perimeter and completely covering the first and second surfaces with first and second layers of conductive material. Both the first and second layers of conductive material are extended beyond the perimeter of the piezoelectric material a sufficient distance so as to avoid the influence of any cracks created in the first or second conductive layers caused by cracks in the piezoelectric material. 
         [0015]    By employing these methodologies electrically conductive pathways may remain established by way of the conductive material extended beyond the perimeter of the piezoelectric material in the presence of cracks induced in the first and second conductive layers by cracks formed in the piezoelectric material. 
         [0016]    Additional objects and advantages of the present subject matter are set forth in, or will be apparent to, those of ordinary skill in the art from the detailed description herein. Also, it should be further appreciated that modifications and variations to the specifically illustrated, referred and discussed features and elements hereof may be practiced in various embodiments and uses of the invention without departing from the spirit and scope of the subject matter. Variations may include, but are not limited to, substitution of equivalent means, features, or steps for those illustrated, referenced, or discussed, and the functional, operational, or positional reversal of various parts, features, steps, or the like. 
         [0017]    Still further, it is to be understood that different embodiments, as well as different presently preferred embodiments, of the present subject matter may include various combinations or configurations of presently disclosed features, steps, or elements, or their equivalents (including combinations of features, parts, or steps or configurations thereof not expressly shown in the figures or stated in the detailed description of such figures). 
         [0018]    Additional embodiments of the present subject matter, not necessarily expressed in the summarized section, may include and incorporate various combinations of aspects of features, components, or steps referenced in the summarized objects above, and/or other features, components, or steps as otherwise discussed in this application. Those of ordinary skill in the art will better appreciate the features and aspects of such embodiments, and others, upon review of the remainder of the specification. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which: 
           [0020]      FIG. 1  diagrammatically illustrates an exemplary piezoelectric generator constructed in accordance with present technology; 
           [0021]      FIG. 2  diagrammatically illustrates a previously known piezoelectric generator construction; 
           [0022]      FIGS. 3   a - 3   c  diagrammatically illustrate various sandwich type construction methodologies that may be employed in the construction of a generator in accordance with present technology; 
           [0023]      FIG. 4   a  illustrates a tire patch including a generator constructed in accordance with present technology mounted thereon; 
           [0024]      FIG. 4   b  illustrates a cross-section of a portion at line  4 - 4  of the tire patch illustrated in  FIG. 4   a ; and 
           [0025]      FIG. 5  illustrates a dual piezoelectric sensor configuration constructed in accordance with the present technology. 
       
    
    
       [0026]    Repeat use of reference characters throughout the present specification and appended drawings is intended to represent same or analogous features or elements of the invention. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    As discussed in the Summary of the Invention section, the present subject matter is particularly concerned with an electrode configuration for a piezoelectric generator. 
         [0028]    Further, it should be appreciated that the term “generator” is meant to convey that flexure of the piezoelectric device of the present technology will produce an output voltage across output terminals provided on the device. Further still, as the device of the present technology may be employed as a sensor as well as a generator either separately or concurrently, the terms generator and sensor may be used hereinafter interchangeably. 
         [0029]    Selected combinations of aspects of the disclosed technology correspond to a plurality of different embodiments of the present invention. It should be noted that each of the exemplary embodiments presented and discussed herein should not insinuate limitations of the present subject matter. Features or steps illustrated or described as part of one embodiment may be used in combination with aspects of another embodiment to yield yet further embodiments. Additionally, certain features may be interchanged with similar devices or features not expressly mentioned which perform the same or similar function. 
         [0030]    Referring now to the drawings, and, first, briefly, to  FIG. 2 , there is illustrated a previously known piezoelectric generator  200  for which improvements in accordance with the present technology have been provided. As may be seen from  FIG. 2 , previously known piezoelectric generator  200  corresponds to a piezoelectric element  210  mounted on a support  220 . An etched patterns of conductive electrodes are provided on both the illustrated upper surface  212  of piezoelectric element  210  as electrode pattern  230  as well as an under side that is hidden in the  FIG. 2  view. These electrode patterns are connected respectively to terminals  232 ,  234  for the upper and under sides of the piezoelectric element  210 . 
         [0031]    Piezoelectric generators of the type illustrated in  FIG. 2  have been used in several environments including tires but it has been found that in such environments, cracking, as for example as illustrated by cracks  242 ,  244  often develop in the piezoelectric element  210  due to stress imparted to the piezoelectric element  210  from, such as, flexing of the tire in which the piezoelectric generator  200  is mounted. 
         [0032]    Those of ordinary skill in the art will appreciate that such cracks  242 ,  244  forming in the piezoelectric element  210  ultimately cause cracks and discontinuities in the conductive electrode patterns, thereby diminishing or even eliminating collection of charge at terminals  232 ,  234 . For example at location  246  where a crack crosses a single connection line from electrode pattern  230  to terminal  232 , no charge is able to reach terminal  232  thus rendering piezoelectric generator  200  completely inoperative. 
         [0033]    With reference now to  FIG. 1 , it may be noticed that piezoelectric generator  100 , constructed in accordance with present technology, has provided a piezoelectric device  110  mounted on a support  120  that is completely covered by an electrode  130 . It should be appreciated that piezoelectric device  110  is actually hidden by electrode  130  in  FIG. 1 . 
         [0034]    Further, electrode  130  actually extends beyond the edges of piezoelectric device  110  as illustrated by conductive extension portion  136  that is formed as an integral part of electrode  130 . Conductive extension portion  136  of electrode  130  extends beyond the perimeter of piezoelectric device  110  a sufficient distance so as to avoid the influence of any cracks created in the conductive layer  130  caused by cracks in the piezoelectric device  110  below electrode  130 . Electrode  130  and its extension portion  136  are coupled by additional conductive trace  142  to solder pad  132 . It will be appreciated that a similar electrode and extension portion are provide on the under side of piezoelectric device  110  and are connected by way of conductive trace  144  to solder pad  134 . In an exemplary configuration, conductive electrode  130  including extension portion  136 , trace  142 ,  144  and solder pads  132 ,  134  may be implemented in copper. Of course, other suitable conductive materials may also be employed. 
         [0035]    Providing such a solid conductive electrode does not prevent cracking of the piezoelectric material but rather allows for continued ability to harvest energy from even severely cracked piezoelectric devices at least so long as the perimeter conductive trace  136  remains intact and the pieces of the cracked piezoelectric device  110  remain adhered to the support  120 . 
         [0036]    With reference now to  FIGS. 3   a - 3   c  attention will be directed to various constructional aspects of a piezoelectric device constructed in accordance with present technology. As previously mentioned, the present subject matter relates to a sandwich type construction. As illustrated in  FIG. 3   a , an exemplary embodiment of such construction is shown to correspond to a piezoelectric layer  310  sandwiched between conductive layers  320 ,  322  all supported by an insulating support layer  330 . In an exemplary configuration piezoelectric layer  310  may correspond to a lead zirconium titanate (PZT) layer, conductive layers  320 ,  322  may correspond to copper layers and insulating support layer  330  may correspond to a fire retardant insulating material commonly called FR4. 
         [0037]    In a further exemplary embodiment as illustrated in  FIG. 3   b , another insulating support layer  332 , which may also correspond to FR4, may be added to complete the sandwich structure. A particular advantage of the inclusion of dual insulating support layers resides in the capability to minimize the peak local bending moment of the piezoelectric layer  310  and thereby minimize cracking by optimizing the thickness of the support layer or layers. 
         [0038]    It should be evident to those of ordinary skill in the art that such optimization requires judicious selection of the thickness of the insulating layer or layers. For example, at an extremely large thickness, all bending of the piezoelectric layer will be prevented thereby eliminating generation of signal producing voltages. Obviously, as the thickness becomes less and less, additional bending becomes possible thus eventually eliminating the benefits obtained from reductions in the local bending moment. An additional benefit of providing a double layer of insulating support material results from the encapsulation of the piezoelectric layer by such double insulating layers thereby providing additional physical protection for the piezoelectric layer. 
         [0039]      FIG. 3   c  illustrates yet another embodiment of the sandwich structure. In this instance, insulating layer  332 ′ is made thicker than insulating layer  332 . Such a change in thickness from one insulating layer to another offsets the piezoelectric layer from the central neutral plane of the sandwich structure and provides an opportunity to further adjust energy generation. Insulating layer  332 ′ may be provided by way of a thicker layer of insulating material or by providing two or more separate layers of material. It should be appreciated that the two or more layers may themselves be of different thickness as well as of the same thickness or combinations thereof. 
         [0040]    With reference now to  FIGS. 4   a  and  4   b , there is illustrated and exemplary tire patch  400  incorporating a sandwich structured piezoelectric device constructed in accordance with the present technology.  FIG. 4   b  illustrates an enlarged cross-section of a portion of tire patch  400  taken at lines  4 - 4  of  FIG. 4   a . As illustrated in  FIG. 4   a , a tire patch  400  corresponding to an elastomeric support  410  and an integrally formed central mesa  412  may be used to support the sandwich structure piezoelectric device  420 . Piezoelectric device  420  may be secured to the upper surface of mesa  412  by way of an adhesive layer. In an exemplary embodiment, elastomeric support  410  and integral mesa  412  correspond to rubber and adhesive  430  may correspond to Chemloc® adhesive available from LORD Corporation. In an exemplary construction, Chemloc® may be applied to a surface of the sandwich structure, the structure placed in a mold with rubber material and then cured. 
         [0041]    As illustrated in  FIGS. 4   a  and  4   b , sandwich structure piezoelectric device  420  may correspond to a device covering substantially the entire upper surface of mesa  412  and may, in fact, actually overhang the upper surface of mesa  412  slightly as illustrated by overhang  414  in  FIG. 4   b.    
         [0042]    With reference now to  FIG. 5 , there is illustrated another embodiment of a sandwich structure piezoelectric device  500  constructed in accordance with present technology. As may be seen, sandwich structure piezoelectric device  500  corresponds to a pair of sandwich structure piezoelectric devices  510 ,  510 ′ mounted on a common insulating support layer  520 . As with previous embodiments, insulating layer  520  may correspond to a layer of FR4. Furthermore, an additional insulating layer, also possibly FR4, may be provided as described with respect to  FIG. 3   b . Still further, such additional layer of insulating material may be varied in thickness or provided as plural layers to achieve thickness variations as discussed with reference to  FIG. 3   c.    
         [0043]    With further reference to  FIG. 5 , it will be noticed that sandwich structure piezoelectric devices  510 ,  510 ′ are electrically coupled in parallel by way of conductive traces  532 ,  534  through connection terminal  530  and  542 ,  544  through connection terminal  540 . It should be appreciated based on previous descriptions herein above that sandwich structure piezoelectric devices  510 ,  510 ′ each correspond to sandwich structures that include a first conductive layer that, in the instance of the sandwich structure piezoelectric devices  510 ,  510 ′ illustrated correspond to the layers illustrated as connected together by way of traces  542 ,  544  while the second conductive layers correspond to those connected together by way of traces  532 ,  534 . 
         [0044]    It should further be appreciated that while the illustrated pair of sandwich structure piezoelectric devices  510 ,  510 ′ are shown connected in parallel, other connection schemes are possible including serial connection as well as individual connection by way of additional, separate terminal pairs. Further still, it should be appreciated that the illustrated pair of commonly mounted device  510 ,  510 ′ may be jointly mounted to create a tire patch as illustrated in  FIG. 4   a . Furthermore, those of ordinary skill in the art will appreciate, given the present disclosure, that multiple pairs of commonly mounted devices may be provided and electrically coupled in any desired fashion from parallel to serial to individual and combinations thereof all within the scope of the present subject matter. In a yet further configuration, a plurality of piezoelectric devices may be commonly mounted and connected in parallel by use of single layers of conductive material covering both sides of the plurality of devices. 
         [0045]    While the present subject matter has been described in detail with respect to specific embodiments thereof, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing may readily produce alterations to, variations of, and equivalents to such embodiments. Accordingly, the scope of the present disclosure is by way of example rather than by way of limitation, and the subject disclosure does not preclude inclusion of such modifications, variations and/or additions to the present subject matter as would be readily apparent to one of ordinary skill in the art.