Abstract:
A piezoelectric sensing device is described for measuring material thickness of target such as pipes, tubes, and other conduits that carry fluids. The piezoelectric sensing device comprises a substrate such as a flexible circuit material, a piezoceramic element, and a solder layer disposed therebetween. These features are arranged in manner that provides a low-profile measurement device suitable for high-temperature applications such as those applications in which the temperature exceeds 120° C. Embodiments of the piezoelectric sensing device can be configured for use as stand-alone units separately located on the target or for use as a string of sensing elements coupled together by way of the flexible circuit material.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates to measuring material thickness using ultrasonic transducers and in one embodiment to a piezoelectric sensing device that comprises a flexible circuit material and a piezoelectric ceramic. 
         [0002]    Several industries (e.g., oil and gas, refinery, chemical, power generation) require the transport of fluid (e.g., liquids or gases) through pipes. Nondestructive testing systems can be placed on the outer surface of these pipes to monitor corrosion/erosion of the pipes, including corrosion/erosion on the interior of pipe walls. These systems are usually implemented as part of manual inspection over the course of time, wherein the pipe wall thickness and changes in the thickness are monitored over time. In some cases, the probe or other nondestructive testing device is permanently coupled to the outer surface of the pipe to continuously monitor corrosion/erosion at that location to determine pipe corrosion/erosion rates and to determine whether that pipe location is in need of preventative maintenance to prevent a pipe failure. 
         [0003]    One example of a nondestructive testing system used to monitor corrosion/erosion of a pipe is an ultrasonic testing system. When conducting ultrasonic testing of a pipe, an ultrasonic pulse is emitted from a probe coupled to the outer surface of the pipe and passed through the pipe wall. As the ultrasonic pulse passes into and through the pipe wall, various pulse reflections called echoes are reflected back to the probe as the pulse interacts with the outer surface of the pipe, internal structures within the pipe wall, and with the back wall of the pipe wall. The echo signals can be displayed on a screen with echo amplitudes appearing as vertical traces and time of flight or distance as horizontal traces. By tracking the time difference between the transmission of the ultrasonic pulse and the receipt of the echoes, various characteristics of the pipe can be determined, including pipe wall thickness. If the thickness of the pipe wall at the location of the ultrasonic testing system decreases over time (e.g., as would be shown be a reduction in the time of flight of the back wall echo), this can be an indication of corrosion/erosion. 
         [0004]    Various factors influence the configuration of devices and in particular the materials for use in these non-destructive testing systems. Operating conditions such as the operating temperature in some applications, for example, can exceed the temperature thresholds of materials such as copolymers of polyvinylidene fluoride (PVDF) (e.g., P(VDF-TrFE)). Processing conditions including temperatures related to certain processing steps during manufacture are also limiting. Performance factors such as accuracy and sensitivity to small defects and to small changes in material thickness are other factors that preclude the use of particular materials and combinations thereof. However, while improved performance can be achieved using certain configurations of materials, these configurations often result in physical characteristics (e.g., height profile) that limit the applicability of the resultant devices. 
         [0005]    It would therefore be advantageous to provide a device suited for ultrasonic testing and measurement of material thickness, with improved performance and physical features but that is also configured for high operating temperatures and high process temperatures. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    In one embodiment a piezoelectric sensing device comprises a substrate, a solder layer disposed on the substrate, and a piezoelectric element coupled to the substrate via the solder layer, the piezoelectric element comprising a ceramic. In one example of the piezoelectric sensing device, the substrate, the solder layer, and the piezoelectric element are arranged as a layered structure that has a profile height that does not exceed 3 mm. In one example o the piezoelectric sensing device, the substrate comprises a material that is compatible with operating temperatures in excess of 120° C. 
         [0007]    In another embodiment a measurement system for measuring material thickness of a target. The measurement system comprises a substrate comprising a flexible circuit material having an area with an electrode with a t-shaped geometry. The measurement system also comprises a solder layer disposed on the electrode and a piezoelectric element disposed on the solder layer. The piezoelectric element comprising a ceramic body having a first electrode, a second electrode, and a wrap tab that is coupled to each of the first electrode and the second electrode. The measurement system further comprises a connection for conducting inputs and outputs to and from the piezoelectric element. In one example of the measurement system, the flexible circuit material, the solder layer, and the piezoelectric element are arranged as a layered structure that has a profile height that does not exceed 3 mm. 
         [0008]    In yet another embodiment an apparatus for monitoring material thickness of a target. The apparatus comprises a transducer array secured to the target and instrumentation coupled to the transducer array. In one example of the apparatus, the transducer array comprises a piezoelectric sensing device. In one example of the apparatus, the piezoelectric sensing device comprises a layered structure that has a flexible circuit material, a solder layer, and a ceramic body coupled to the flexible circuit material via the solder layer. In one example of the apparatus, the layered structure has a profile height that does not exceed 3 mm. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    So that the manner in which the features of the invention can be understood, a detailed description the invention may be had by reference to certain embodiments, some of which are illustrated in the accompanying drawings. It is to be noted, however, that the drawings illustrate only certain embodiments of this invention and are therefore not to be considered limiting of its scope, for the scope of the invention encompasses other equally effective embodiments. The drawings are not necessarily to scale, emphasis generally being placed upon illustrating the features of certain embodiments of invention. Thus, for further understanding of the invention, reference can be made to the following detailed description, read in connection with the drawings in which: 
           [0010]      FIG. 1  is a schematic diagram of an exemplary embodiment of a measurement system. 
           [0011]      FIG. 2  is an exploded assembly view of an exemplary embodiment of a piezoelectric sensing device. 
           [0012]      FIG. 3  is a side, cross-section, assembled view of the piezoelectric sensing device of  FIG. 2 . 
           [0013]      FIG. 4  is a front view of another exemplary embodiment of a piezoelectric sensing device. 
           [0014]      FIG. 5  is a side, cross-section view of the piezoelectric sensing device of  FIG. 4 . 
           [0015]      FIG. 6  is a front view of yet another exemplary embodiment of a piezoelectric sensing device. 
           [0016]      FIG. 7  is a side, cross-section view of the piezoelectric sensing device of  FIG. 6 . 
           [0017]      FIG. 8  is a schematic diagram of an implementation of a piezoelectric sensing device such as the piezoelectric sensing devices of  FIGS. 2-5 . 
           [0018]      FIG. 9  is a schematic diagram of another implementation of a piezoelectric sensing device such as the piezoelectric sensing devices of  FIGS. 2 ,  3 ,  6 , and  7 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    Referring now to the figures, there is illustrated in  FIG. 1  an exemplary embodiment of a measurement system  10  with improved sensitivity and construction, the latter of which is beneficial for implementation of the measurement system  10  at operating temperatures greater than, e.g., 120° C., and in areas where access by other measurement systems is limited. The measurement system  10  can comprise a transducer array  12  and instrumentation  14 , which is operatively coupled to the transducer array  12  via a connection  16 . The transducer array  12  can comprise one or more sensing elements  18 , each of the sensing elements  18  having a piezoelectric element  20  coupled to a substrate  22 . 
         [0020]    Transducer array  12  can be disposed on a target, such as a pipe, a tube, and related conduits that can be subject to corrosion and erosion by way of the fluid that is transported therein. The disposition of the transducer array  12  permits ultrasonic signals generated by the piezoelectric element  20  to impinge on the material of the target. These ultrasonic signals are reflected such as by surfaces of the material, wherein the reflected signals are detected by the piezoelectric element  20 . 
         [0021]    In one embodiment, instrumentation  14  can include an ultrasonic test unit  24  that generates waveform pulses (generally, “inputs”), which are applied to the piezoelectric element  20  via the connection  16 . The waveform pulses cause a mechanical change (e.g., a dimensional change) in the piezoelectric element  20 . This change can cause an acoustic wave, which is transmitted through the material of the target. Conversely, the piezoelectric element  20  generates a voltage difference when acoustic waves reflected from the material under inspection contact the surface of the piezoelectric element  20 . This voltage difference is detected as receive signals (generally, “outputs”) by the ultrasonic test unit  24  or other signal processing electronics. 
         [0022]    The ultrasonic test unit  24  can include various control means, which are useful to determine the amplitude, timing, and transmit sequence of the waveform pulse generated by the piezoelectric element  20 . The waveform pulse is generally in the frequency range of about 5 MHz to about 20 MHz. By tracking the difference between the transmission of the waveform pulse and the receipt of the received signal and measuring the amplitude of the reflected wave, various characteristics of the material can be determined. In one example, the thickness of the material of the target, as well as any corresponding changes in the thickness, can be determined using time-of-flight analysis, the subject matter of which will be recognized by those artisans having skill in the transducer and related arts. 
         [0023]    In one embodiment, the sensing elements  18  are separately arranged and are constructed as individual sensing units. Communication between these individual units and the ultrasonic test unit  24  is facilitated by the connection  16 , and in one construction the connection  16  has a plurality of cables (not shown). These cables are coupled to each of the sensing elements  18 . Exemplary cables can include coaxial cables and optical fibers, as well as single and plural strands of copper and/or related materials that can conduct the inputs and outputs (e.g., the waveform pulses and the received signals) to and from the piezoelectric element  20  as contemplated herein. 
         [0024]    In another embodiment, the sensing elements  18  are arranged on a common substrate, generally demarcated in the present example with the numeral  26 . This arrangement is defined by one or more of the piezoelectric elements  20  being disposed on the common substrate  26 . The piezoelectric element  20  of the sensing elements  18  can be spaced apart from one another along for example a strip of material, and as discussed in one or more embodiments below, this material can comprise a flexible circuit material that can conform to the shape of the target. In one example, conductors are incorporated in the flexible circuit material, with each conductor terminating at the piezoelectric element  20  and at the end of the common substrate  26 . The connection  16  can include one or more connectors (not shown), which are coupled to the conductors, and which can be incorporated or otherwise affixed onto the free end. The connector can be coupled to a mating connector or other device such as a bundle of coaxial cables extending from the ultrasonic test unit  24 . This combination can communicate the inputs and outputs between the piezoelectric element  20  and the instrumentation  14 . 
         [0025]    The number of the sensing elements  18  in the transducer array  12  can vary, and in one construction the number can vary from one to twenty. In one particular example the number is fourteen. An alternative selection of the number can be based on any one or combination of the dimensions of the target under inspection, the preferred spacing of the sensing elements  18  on the target, and the type of defect being detected. When implemented in connection with the common substrate  26 , the spacing between the approximate centers of the piezoelectric element  20  can be from about 10 mm to about 100 mm. Moreover, in implementations where the sensing elements  18  are arranged as individualized units, each can be located on the target independently of other ones of the sensing elements  18  of the transducer array  12 . Thus the space between adjacent ones of the piezoelectric element  20  and the location of the piezoelectric element  20  relative to features (e.g., edges) of the target can be optimized for each of the sensing elements  18  as desired. 
         [0026]    Although the transducer array  12  is depicted as a linear array (e.g., wherein the sensing elements  18  form a single row with one or more columns) other configurations are also envisioned. In one embodiment, the transducer array  12  can include one or more rows and one or more columns of sensing elements  18 . In another embodiment, the sensing elements  18  are arranged in formations that are different that arrays of rows and columns. By way of example, one formation for transducer array  12  can comprise a first row of sensing elements  18  and a second row of sensing elements  18 , wherein the second row is positioned in perpendicular relation to the first row, thus forming a “t” shape. 
         [0027]    Focusing now on the construction of the sensing elements  18 , reference can be had to  FIGS. 2 and 3 . Here there is depicted an exemplary embodiment of a piezoelectric sensing device  100  which can be deployed as one or more of the sensing elements  18  of  FIG. 1 . In one embodiment, the piezoelectric sensing device  100  can comprise a substrate  102  and a piezoelectric element  104  with a ceramic body  106 . The ceramic body  106  can be configured with an electrode  108 , a ground  110 , and a wrap tab  112  constructed of gold plating or comparable conductive material that is deposited on the ceramic body  106 . The substrate  102  can comprise a flexible circuit material  114 , shown in this example with a first layer  116  and a second layer  118 , and with a receiving area  120  that is configured to receive the piezoelectric element  104 . The receiving area  120  can have electrodes  122  for connecting to, e.g., the electrode  108  and the ground  110 . The electrodes  122  can include a first or ground electrode  124  and a second or hot electrode  126 . The electrodes  122  can conform to an electrode geometry  128  that is defined by an isolation gap  130  between the electrodes  122  and/or a shape geometry  132  as applied to one or both of the electrodes  122 . In one example the shape geometry  132  comprises a t-shaped geometry  134  for the hot electrode  126 . 
         [0028]    In one embodiment, the piezoelectric sensing device  100  may also include a solder layer  136  that comprises one or more materials such as tin, lead, silver, bismuth, and indium. The solder layer  136  is deposited during assembly and is used to couple the piezoelectric element  104  to the receiving area  120  of the substrate  102 . When assembled, the combination of the substrate  102 , the piezoelectric element  104 , and the solder layer  136  are arranged as a layered structure  138  with a profile height P. Embodiments of the piezoelectric sensing device  100  can be configured so that the profile height P does not exceed about 7 mm, and in one example the profile height is from about 0.25 mm to about 1 mm. These values are smaller than conventional devices, which permits use of the piezoelectric sensing device  100  in places that are generally not accessible with measurement devices of conventional construction. 
         [0029]    Materials for use in the ceramic body  106  are selected for their properties including for example compatibility with processing conditions during assembly such as the reflow temperatures required to reflow the solder layer  136 . These reflow temperatures typically are in excess of 200° C. and in one exemplary process the reflow temperatures is about 220° C. Other properties to consider include, but are not limited to, dielectric constant of the material, wherein the materials that are selected for the ceramic body  106  should have a dielectric constant that renders good electrical impedance matching, while minimizing the overall dimensions of the piezoelectric element  104 . These dimensions include, for example, dimensions for the rectangular shape of  FIG. 2  of about 3 mm by about 5 mm, although the length and width can vary, respectively, from about 2 mm to about 8 mm. In other examples, the shape of the piezoelectric element  104  can comprise a square, a circle, and/or an ellipse. With reference to the profile height P discussed above, it is further contemplated that piezoelectric element  104  is formed with an overall thickness from about 0.1 mm to about 1 mm. 
         [0030]    In one embodiment, it may be desirable to use piezoelectric ceramics such as Navy Type II materials and related ceramics (e.g., lead zirconium titanate piezoelectric), although other materials having similar properties and composition are likewise contemplated and may be used. For purposes of constructing the ceramic body  106  (and the piezoelectric element  104  in general), in one example a brick of Navy Type II material can be diced into plates having a thickness on the order of 0.6 mm. These plates can be finished by way of finish grinding operations so that the thickness of the resulting plates is about 0.2 mm. Linear grinding, lapping, and back grinding are all acceptable finish grinding operations. The plates can thereafter be cut into strips with a width of about 9 mm and the electrodes can be formed, poled, and tested. Plating operations such as sputtering can be used to deposit the gold (Au) plating and the finished plates can be diced to form the individual piezoelectric elements (e.g., the piezoelectric element  104 ). In one example, a single brick of Navy Type II material can yield approximately 2880 of the piezoelectric element  104 . It will be appreciated that the electrodes  122  can be formed using certain deposition, etching, sputtering, and related processing techniques and processes recognized within the scope and spirit of the present disclosure. 
         [0031]    The layers (e.g., the first layer  116  and the second layer  118 ) of the flexible circuit material  114  can comprise materials such as a polyamide-based film, as well as other materials and films that comprise one or more of polyester (PET), polyimide (PI), polyethylene napthalate (PEN), and polyetherimide (PEI). The layers can be constructed together to form a laminate that is compatible with the processing conditions, operating temperatures, and physical characteristics (e.g., the profile height P) discussed herein. Conductors such as electrical conductors like metal foil may be included among the layers, or in other examples the conductors can be incorporated amongst the layers such as by using electroplating and related plating and deposition techniques. These conductors can extend to the electrodes  122  as well as to peripheral edges and areas of the substrate  102 . This configuration is useful to conduct the pulse and electrical signals to and from the piezoelectric element  104 , an example of which was discussed above in connection with the common substrate (e.g., the common substrate  26  of  FIG. 1 ). 
         [0032]    Referring next to  FIGS. 4-7 , there is provided exemplary embodiments of a piezoelectric sensing device  200  ( FIGS. 4 and 5 ) and  300  ( FIGS. 6 and 7 ). For purposes of the discussion that follows below, like numerals are used to identify like components as between  FIGS. 2-7 , except that the numerals are increased by 100 (e.g.,  100  is  200  in  FIGS. 4 and 5 , and  200  is  300  in  FIGS. 6 and 7 ). The piezoelectric sensing devices  200  and  300  are useful for implementation in one or more of the configurations of the transducer array  12  discussed in connection with  FIG. 1  above. 
         [0033]    The piezoelectric sensing device  200  that is depicted in  FIGS. 4 and 5 , for example, is suited for use in connection with the configuration of the transducer array  12  ( FIG. 1 ) wherein each of the sensing elements  18  is arranged as individual units. In one embodiment, the piezoelectric sensing device  200  can comprise a substrate  202  and a piezoelectric element  204 . The substrate  202  can comprise a flexible circuit material  214  with a receiving area  220  in which is positioned the piezoelectric element  204 . The receiving area  220  can have electrodes  222  including a ground electrode  224  and a hot electrode  226 . A solder layer  236  can be disposed on one or more of the electrodes  222  using screen printing techniques recognized in the art. 
         [0034]    The flexible circuit material  214  can comprise a frontside  240  and a backside  242  on which are located the electrodes  222 . The piezoelectric sensing device  200  can also comprise one or more cable connections  244  with cable connection pads  246  and strain reliefs  248 . The cable connection pads  246  can include a ground pad  250  and a hot pad  252 , each being coupled to, respectively, the ground electrode  224  and the hot electrode  226  by way of one or more vias  254 . The vias  254  extend through the flexible circuit material  214 , thereby coupling the cable connection pads  246  on the frontside  240  to the electrodes  222  on the backside  242 . In one example, a ground plane  256  is also incorporated into the flexible circuit material  214 . The ground plane  256  is coupled to the ground electrode  224  and the ground pad  250 . 
         [0035]    The piezoelectric sensing device  300 , as depicted in  FIGS. 6 and 7 , can be implemented when the transducer array  12  ( FIG. 1 ) utilizes a common substrate (e.g., the common substrate  26  ( FIG. 1 )). In one embodiment, the piezoelectric sensing device  300  can comprise a substrate  302  and a piezoelectric element  304 . The substrate  302  can comprise a flexible circuit material  314  with one or more receiving areas  320  configured for receiving the piezoelectric element  304  thereon. The receiving areas  320  can have electrodes  322  including a ground electrode  324  and a hot electrode  326 . A solder layer  336  is also included for securing the piezoelectric element  304  to the electrodes  322 . 
         [0036]    The piezoelectric sensing device  300  can comprise a common substrate  358  in which a plurality of conductors  360  are incorporated. The conductors  360  can include hot conductors  362  and ground conductors  364 , each being illustrated as extending from a free end  366  of the common substrate  358 . Disposed on the free end  366  is a connector  368  such as a multi-pin connector that is coupled to each of the conductors  360 . The connector  368  is likewise configured to couple to a mating connector (not shown) as might be associated with the instrumentation (e.g., instrumentation  14  ( FIG. 1 )) contemplated herein. 
         [0037]    Discussing now the implementation of piezoelectric sensing devices such as the piezoelectric sensing devices  100 ,  200 , and  300  discussed above, reference is now directed to  FIGS. 8 and 9 . The  FIGS. 8 and 9  illustrate, respectively exemplary embodiments of a piezoelectric sensing device  400  and  500 , these embodiments being configured for use in measurement systems such as the measurement systems described above and in more detail below. Like numerals are also used to identify like components as between the  FIGS. 2-9 . However, although some of the features and concepts of the piezoelectric sensing devices of the present disclosure may not be depicted or discussed in connection with  FIGS. 8 and 9 , it is contemplated that such features and concepts are applicable to the piezoelectric sensing devices  400  and  500  as well as embodiments and derivation thereof. 
         [0038]    There is depicted in  FIG. 8 , for example, a plurality of piezoelectric sensing devices  400 , each of which can comprise a substrate  402  and a piezoelectric element  404 . The substrate  402  can include a flexible circuit material  414  with a ground electrode  424 , a hot electrode  426 , and a solder layer  436  that is used to secure the piezoelectric element  404  to the substrate  402 . The flexible circuit material  414  includes a frontside  440  and a backside  442 . In one embodiment, the piezoelectric sensing devices  400  are implemented as part of a measurement system  470 , which can comprise a transducer array  472 , instrumentation  474 , and a connection  476  such as one or more cables  478  that are coupled to the piezoelectric element  404 . The measurement system  470  can also comprise a connection terminal  480  to aggregate the cables  478 , acting in one example as a central hub for communicating signals to and from the instrumentation  474  and the piezoelectric sensing devices  400  of the transducer array  472 . 
         [0039]    In one embodiment, the piezoelectric sensing devices  400  are secured to a surface  482  of a target  484  using a couplant  486  such as an adhesive that is disposed on the backside  442  of the substrate  402 . To further ensure proper functioning and coupling of the piezoelectric sensing devices  400  to the surface  482 , one or more outer structures  488  can be utilized such as a protective layer  490  and a fastening mechanism  492 . These outer structures  488  can be incorporated as part of the piezoelectric sensing devices  400  or in one embodiment the outer structures  488  comprise one or more pieces separate from the piezoelectric sensing devices  400 . Assembly of the pieces of the outer structures  488  can occur at the time of implementation and installation of piezoelectric sensing devices  400  and the measurement system  470  generally. 
         [0040]    The couplant  486  can be disposed on surfaces of the substrate  402 , as depicted in  FIG. 8 , as well as on the piezoelectric element  404 . Care should be taken during application to avoid degradation of the performance of the piezoelectric element  404 . In addition to performance characteristics, it may be desirable that materials for use as the couplant  486  are compatible with the material characteristics of the substrate  402  and the target  484 . In one example, adhesives such as acrylic adhesives can be applied at as a layer with a nominal initial thickness of about 1 mm. Other adhesives and related materials that may be likewise acceptable include, but are not limited to, cyanocrylates, epoxies, solvent-based adhesives, and cold-flow adhesives, as well as combinations and derivations thereof. 
         [0041]    The protective layer  490  is used to prevent damage to the underlying structure, e.g., the piezoelectric sensing devices  400 . Materials can likewise have electrically insulating properties thus providing protection from the outer environment as well as preventing arcing, shorting, and other electrical-induced failures that can occur. Exemplary materials for use as the protective layer  490  can include silicon, nylon, neoprene, polymeric materials, and combinations and derivations thereof. 
         [0042]    The fastening mechanism  492  can be in the form of the band-like structure illustrated in  FIG. 8 . When the target  484  is a pipe or other circumferential device, such structures can be affixed about the circumference. These structures can incorporate secondary fastening and tightening features that reduce the diameter of the band about the pipe, thereby applying a force onto the piezoelectric sensing devices  400 . For other configurations of the target  484 , such as for targets with flat or irregular constructions, the fastening mechanism  492  may be configured with devices that are designed for the specific configuration of the target  484 . These devices may include magnets and magnetized implements that can cause to be applied to force onto the piezoelectric sensing devices  400 . 
         [0043]    Referring now to  FIG. 9 , it is seen that the piezoelectric sensing device  500  can comprise a substrate  502  and a piezoelectric element  504 . The substrate  502  can comprise a ground electrode  524  and a hot electrode  526 , and a solder layer  536  is included as contemplated herein. The substrate  502  is arranged as a common substrate  558  with a free end  566  on which is disposed a connector  568 . The piezoelectric sensing device  500  is part of a measurement system  570 , which can comprise a transducer array  572 , instrumentation  574 , and a connection  576  coupled therebetween. To secure the piezoelectric sensing device  500 , a couplant  586  is used and further protection is afforded by a protective layer  590  and a fastening mechanism  592 . In one embodiment, the connection  576  can comprise a single cable  594  that is coupled to the connector  568  and to the instrumentation  574 . The single cable  594  can comprise, for example, a mating connector  596  that is configured to mate with the connector  568 . 
         [0044]    This written description uses examples to disclose embodiments of the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.