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CROSS REFERENCES TO RELATED APPLICATIONS 
   None. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is related to door structures, but in particular, pertains to a self-contained structure allowing for ingress or egress of a domestic animal such as a dog, cat or other suitably sized animal through a pet door panel insert which adjustingly and without special tools installs primarily in sliding glass patio door structures, but can be installed adjacent to a hinged door having adjacent side panels or in a window. 
   2. Description of the Prior Art 
   Prior art pet entry structures include pet door panel inserts which adaptingly install in existing human entry door structures, such as a sliding glass patio door. Many such structures were an after market add-on device and, as such, the construction thereof was not of the same sturdiness and robustness as the structures designed for installation along with initial door installation. To be readily accommodated in a door frame, some inserts incorporated a spring loaded panel top which allowed a pet door panel insert to be installed on a temporary basis without having to install a pet door panel insert permanently. A drawback of this type of installation is that the pet door panel insert may be loose in the frame and may not provide a tight and sturdy installation. Other devices of more permanent design required accurate cutting and drilling in order to size the framework of the pet door panel insert to a door frame. Such custom sizing did not guarantee reuse of the pet door panel insert for installation at another door frame site as prior cutting and drilling may have rendered the structure unusable in a door frame of different dimension. What is needed is a pet door panel insert which can be easily and sturdily installed in an existing door frame without the use of drills, saws or other special tools, and which can be installed or reinstalled in door frames without damaging the pet door panel insert or the door frame. 
   SUMMARY OF THE INVENTION 
   The general purpose of the present invention is to provide a pet door panel insert. The pet door panel insert can be installed preferably in sliding glass patio door arrangements having a stationary door panel and an operator door panel, but can also be substituted for a panel of a door or a panel in a panel array and can even be installed in windows. The pet door panel insert includes a lower entry panel of any suitable composition and structure as known in the art, such as one or more flexible plastic panels, rigid or flexible swinging panels, spring loaded doors and the like, located at the lower region of a framework; an expandable panel assembly at the upper region of the framework; and one or more mid-panels, preferably of safety glass, but alternatively of clear plastic, wood, screen or other suitable material, located between vertically and horizontally aligned channels of the framework and between the lower entry panel and the expandable panel assembly. The pet door panel insert can be installed between the stationary door panel and adjacent door frame members at the stationary door panel end or against the door frame adjacent to the door frame members at the operator door panel end. A bottom framework channel of the pet door panel insert rests on the sill or existing track of the door frame, and one of the vertically aligned framework side channels aligns to either of the door jambs. The vertically expandable panel assembly is slidably engaged and positionably and expandingly adjustable in the upper portions of vertically aligned side channels of the framework and is incorporated to secure and to fix the top of the pet door panel insert to an existing channel or track at the top of the door frame. 
   According to one or more embodiments of the present invention, there is provided a framework having opposed vertically aligned left side and right side channels, a plurality of fixed horizontally aligned cross channels extending between the vertically aligned left and right side channels, a bottom engagement channel, a pet entry framework and pet door located at the lower region of the framework, and an expandable panel assembly slidably engaged and positionably adjustable between the upper vertical portions of the vertically aligned left and right side channels of the framework. The expandable panel assembly includes opposed vertically aligned left and right sleeve channels slidably engaged within the upper regions of the opposed vertically aligned left and right side channels, a deep extended wall cross channel engaging a filler panel where the filler panel extends between and slidingly engages opposed vertically aligned left and right sleeve channels, an apertured plate secured to the top of the filler panel, a horizontally aligned top engagement channel secured to the tops of the opposing vertically aligned left and right sleeve channels, opposed elongated machine screws threadingly engaged with stationary nuts captive within lower regions of the top engagement channel and extending downwardly from the top engagement channel to engage opposed adjustable nuts and further extending through the apertured plate to engage elongated body holes in the filler panel, and opposed front and rear cover slide plates extending between and adjustably fixed between opposed angled interior edges of the opposed vertically aligned left and right side channels. Various other horizontally aligned cross channels extend between the opposed vertically aligned left and right side channels and collectively provide for mutual full or partial support for a window panel, the expandable panel assembly, and other components of or associated with the invention. Sealing strips, gaskets and the like are incorporated throughout and within. 
   One significant aspect and feature of the pet door panel insert is an expandable panel assembly which is vertically positionable to accommodate door frame height. 
   One significant aspect and feature of the pet door panel insert is an expandable panel assembly having stationary structure and positionable structure where such structure is accessible from a standing position. 
   Still another significant aspect and feature of the pet door panel insert is the provision of elongated machine screws and adjustment nuts to position the positionable structure with respect to the stationary structure, thereby providing a pet door insert which can be adjustably installed within an existing framework. 
   Yet another significant aspect and feature of the pet door panel insert is its capability of being installed adjacent to an operator door panel or adjacent to a stationary door panel. 
   A further significant aspect and feature of the pet door panel insert is its capability to be installed without the use of complicated tools. 
   Another significant aspect and feature of the pet door panel insert is that it is reusable in another installation, as no cutting of framework members is required. 
   A still further significant aspect and feature of the pet door panel insert is vertically positionable front and rear cover slide plates frictionally engaged between the upper regions of a framework for covering elements of the positionable structure. 
   Having thus briefly described an embodiment of the present invention and set forth some significant aspects and features thereof, it is the principal object of the present invention to provide a pet door panel insert. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof and wherein: 
       FIG. 1  is a foreshortened isometric front view of the pet door panel insert, the present invention; 
       FIG. 2  is an exploded isometric front view of the expandable panel assembly shown with the upper regions of the left side channel and the right side channel, the front cover slide plate, the rear cover slide plate, and other closely associated components; 
       FIG. 3  is an assembled isometric front view of the elements of  FIG. 2 , excluding the front and rear cover slide plates; 
       FIG. 4  is a front view of the expandable panel assembly including the upper regions of the left side channel and the right side channel; 
       FIG. 5 , a top view, illustrates the alignment of several components of the expandable panel assembly with the right side channel; 
       FIG. 6  is a cross section view along line  6 — 6  of  FIG. 1  where the front cover slide plate and the rear cover slide plate have been included; and, 
       FIG. 7  is a view of the elements of  FIG. 4  where the front cover slide plate has been slidably repositioned upwardly. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  is a foreshortened isometric front view of the pet door panel insert  10 , the present invention. Readily observed major components, structures and other features of the invention include a framework  12 , an expandable panel assembly  14  at the upper region of the framework  12 , front and rear cover slide plates  16  and  18 , respectively, shown distanced from the general structure of the pet door panel insert  10 , a pet entry framework  20  located at the lower region of the framework  12  having a flexible or otherwise configured pet door  22  allowing passage of a pet through the lower structure of the pet door panel insert  10 , and one or more window panels  24 , preferably of safety glass, clear plastic or other suitable material, fixed between the members of the framework. The framework  12  includes a plurality of extruded channels having extruded full length screw acceptance bores. The plurality of channels includes opposed vertically aligned left and right side channels  26  and  28 , respectively, and a plurality of horizontally aligned cross channels extending between the inner edges of the left and right side channels  26  and  28  and suitably fixed such as by the use of screws therebetween. The plurality of horizontally aligned cross channels include a top cross channel  30 , a lower region cross channel  32  located between the window panel  24  and the pet entry framework  20 , and a bottom cross channel  34 . A horizontally aligned bottom engagement channel  36 , which aligns to the structure of a door sill, suitably attaches, such as by screws, to the underside of the bottom cross channel  34 . Weatherstrips  38  and  40  frictionally engage the horizontally oriented structure of the bottom engagement channel  36  and are incorporated to provide a seal with the structure of a door sill. 
     FIG. 2  is an exploded isometric front view of the expandable panel assembly  14  shown with the upper regions of the left side channel  26  and the right side channel  28 , the front cover slide plate  16 , the rear cover slide plate  18 , and other closely associated components.  FIG. 3  is an assembled isometric view of the elements of  FIG. 2  excluding the front and rear cover slide plates. With reference to  FIGS. 2 and 3 , the expandable panel assembly  14  and associated structure at the upper region of the pet door panel insert  10  is further described. In addition to the top cross channel  30  extending between the inner edges of the left side channel  26  and the right side channel  28 , a horizontally aligned full length top cross channel  42  extends between the innermost regions of the left side channel  26  and the right side channel  28  and is suitably secured therein and thereto by screws  44  and  46  extending respectively through the left side channel  26  and the right side channel  28  and thence into a screw acceptance bore in the full length top cross channel  42 . Screws  48  and  50  secure the top cross channel  30  to the left side channel  26  and the right side channel  28  in a similar manner. The screws  44 ,  46 ,  48  and  50  serve not only to secure the upper structure but also are utilized indirectly, as described later in detail, to provide for securing of the front cover slide plate  16  and the rear cover slide plate  18  over and about major portions of the expandable panel assembly  14 . As shown in  FIG. 3 , the expandable panel assembly  14  aligns between the upper regions or ends of the left side channel  26  and the right side channel  28  and is expandable vertically therefrom to extend and mate with the upper structure of a door frame. The expandable panel assembly  14  includes stationary structure  51 , best shown in  FIG. 2 , and positionable structure  53 , best shown in  FIG. 3 . The stationary structure  51  includes an upwardly opening extended wall cross channel  52  and a filler panel  54  of suitable thickness and material, such as, but not limited to, plastic, wood and the like, having a greater vertical length than the extended wall cross channel  52 . The filler panel  54  is accommodated by the extended wall cross channel  52  and also has a greater horizontal length than the extended wall cross channel  52  such that the ends of the filler panel  54  extend beyond the end edges of the extended wall cross channel  52  for sliding accommodation by positionable structure including the left sleeve channel  56  and the right sleeve channel  58 . The filler panel  54  includes vertically oriented elongated body holes  60  and  62  extending vertically therethrough. An apertured plate  64  secures to the upper surface of the filler panel  54  by screws  66  and  68 . Hardware items including a left adjustment nut  84 , a right adjustment nut  86 , a left lock washer  88 , and a right lock washer  90  are also part of the stationary structure  51 . 
   The positionable structure  53 , best shown in  FIG. 3 , includes the left sleeve channel  56 , the right sleeve channel  58 , a top engagement channel  70 , weatherstrips  72  and  74 , elongated machine screws  76  and  78 , and in  FIG. 2  other hardware items including a left nut  80 , a right nut  82 , and screws  92  and  94 . The elongated machine screws  76  and  78  provide for vertical adjustable expansion of the expandable panel assembly  14  by variably positioning the positionable structure  53  with respect to the stationary structure  51 . The elongated machine screws  76  and  78  extend through the top engagement channel  70  and are firmly fixed therein by left and right nuts  80  and  82  which are captured within the slotted under region of the top engagement channel  70 , such as shown in  FIG. 4 , and extend further through left and right adjustment nuts  84  and  86  and lock washers  88  and  90 , respectively, and through the apertured plate  64  to be accommodated by the body holes  60  and  62  of the filler panel  54 . The expandable panel assembly  14  is supported along the horizontal by intimate contact of the extended wall cross channel  52  with the full length top cross channel  42  and along the vertical by slidable engagement of the left sleeve channel  56  and the right sleeve channel  58 , respectively, with the upper regions of the left side channel  26  and the right side channel  28 , as also shown in  FIG. 4 . 
     FIG. 4  is a front view of the expandable panel assembly  14  including the upper regions of the left side channel  26  and the right side channel  28 . Shown in particular is the alignment of the expandable panel assembly  14  between the upper regions of the opposed left side channel  26  and the right side channel  28 . More specifically, the left sleeve channel  56  is shown in captured alignment with the left side channel  26  and the right sleeve channel  58  is shown in captured alignment with the right side channel  28 . The front cover slide plate  16  is shown in dashed lines in alignment between the angled interior edges  96  and  97  ( FIGS. 3 and 5 ) and against the extended wall cross channel  52 . Weatherstripping  93  is captured in external grooves in the opposed left side channel  26  and right side channel  28 . 
     FIG. 5 , a top view, illustrates the alignment of several components of the expandable panel assembly  14  with the right side channel  28 . Opposed angled interior edges  96  and  97  of the right side channel  28  assist alignment of the right sleeve channel  58  as well as provide structure for capture of the front cover slide plate  16  and the rear cover slide plate  18 , as later described in detail. Front cover slide plate  16  and rear cover slide plate  18  also include beveled edges  17  and  19  that are preferably complementary to and face the opposed angled interior edges  96  and  97 , respectively. When right side channel  28  is tightened inwardly, the beveled edge  17  is engaged and held by angled interior edge  96  and beveled edge  19  is engaged and held by angled interior edge  97 . When right side channel  28  is loosened outwardly, beveled edge  17  can slide vertically with respect to angled interior edge  96  and beveled edge  19  can slide vertically with respect to angled interior edge  97 . If the outward loosening is not excessive, front cover slide plate  16  and rear cover slide plate  18  may remain captured but vertically slidable and then be subsequently reversibly secured at a desired elevation by tightening inwardly with the right side channel  28 . The provision of bevels  17  and  19  on the cover slide plates  16  and  18  in combination with the angled inward edges  96  and  97  extends the range of non-excessive loosening wherein the plates remain captured but slideable, relative to that which would be observed if the cover slide plates had only square or non-beveled edges interacting with angled inward edges  96  and  97 . Such an arrangement allows for convenient access to internal adjustment parts beneath the front and rear cover slide plates and allows the installation of the pet door panel insert with fewer personnel since the slide plates can be conveniently secured at a desired elevation. 
     FIG. 6  is a cross section view along line  6 — 6  of  FIG. 1  where the front cover slide plate  16  and the rear cover slide plate  18  have been included. Illustrated in particular is the fixation of the nut  82  within the lower region of the top engagement channel  70 , as well as the relationship of other structures of the instant invention. This cross section view depicts the top engagement channel  70  and weatherstrips  72  and  74  engaging a door frame top member  98 . 
     FIG. 7  is a view of the elements of  FIG. 4  where the front cover slide plate  16  has been slidably repositioned to cover the space between the front lower edge of the top engagement channel  70 , the inner front edges of the left sleeve channel  56  and the right sleeve channel  58 , and the front edge of the apertured plate  64 , as well as the hardware therebetween. The rear cover slide plate  18  positions in the same manner. 
   Mode of Operation 
   The pet door panel insert  10  can be installed in a door structure such as a sliding glass patio door structure between the stationary door panel and adjacent door frame members at the stationary door panel end or against the door frame adjacent to the door frame members at the operator door panel end. The lower framework channel of the pet door panel insert  10  rests on the sill of the door frame and one of the vertically aligned framework side channels aligns to either of the door jambs. The vertically expandable panel assembly  14  is slidably engaged and positionably and expandingly adjustable in the upper vertical portions of vertically aligned side channels of the framework and is incorporated to secure and to fix the top of the pet door panel insert  10  to an existing door frame top member, channel or track at the top of the door frame. 
   The pet door panel insert  10  is set in the chosen location after first ensuring that the expandable panel assembly  14  is at the lowest vertical position or that it is not extended to an extent that would disallow fitting of the pet door panel insert  10  into the confines of the door frame. The bottom engagement channel  36  with weatherstrips  38  and  40  is then aligned to the door sill. The left adjustment nut  84  and the right adjustment nut  86  are then rotated manually or with the assistance of a small wrench, preferably in simultaneous clockwise rotation. Such rotation provides jackscrew-like operation where the left adjustment nut  84  and the right adjustment nut  86 , which bear in direct intimate contact with the apertured plate  64  which in turn bears directly on the upper surface of the filler panel  54 , reactingly urges the elongated machine screws  76  and  78  upwardly. Such upward movement of the elongated machine screws  76  and  78  also causes the remaining portion of the positionable structure  53  to be expanded upwardly until the weatherstrips  72  and  74  of top engagement panel  70  suitably engage the door frame member  98 . Upward positioning or expansion of the expandable panel assembly  14  is guided by the upper regions of the left side channel  26  and the right side channel  28  in sliding engagement over and about the exterior portions of the left sleeve channel  56  and the right sleeve channel  58 , respectively, and by the sliding engagement of the interior portions of the left sleeve channel  56  and the right sleeve channel  58  over and about the ends of the filler panel  54  which extend beyond the extended wall cross channel  52 . The front cover slide plate  16  and rear cover slide plate  18  are firmly engaged between the angled interior edges  96  and  97  of the left side channel  26  and the right side channel  28 , as well as intimately engaged against opposed sides of the extended wall cross channel  52 . One or more screws of either screw pair consisting of screws  44  and  48  or screw pair consisting of screws  46  and  50  may require a slight rotation to be backed off a few turns to be loosened to allow a slight outward repositioning of the appropriate left or right side channel  26  or  28  with respect to one of the outer ends of the top cross channel  30  and the full length top cross channel  42 . Such repositioning relaxes the frictional engagement of the angled interior edges  96  and  97  of the left side channel  26  and the right side channel  28  against the vertically aligned ends of the front cover slide plate  16  and the rear cover slide plate  18  so that the front cover slide plate  16  and the rear cover slide plate  18  can be slidingly repositioned along the angled interior edges  96  and  97  of the left side channel  26  and the opposed sides of the extended wall cross channel  52 , as shown in  FIG. 7 , thereby covering the space between the lower edges of the top engagement channel  70 , the inner edges of the left sleeve channel  56  and the right sleeve channel  58 , and the edges of the apertured plate  64 , as well as covering the hardware therebetween. Subsequent to positioning of the front cover slide plate  16  and the rear cover slide plate  18 , the appropriate screws are retightened to draw the left or right side channels  26  or  28  inwardly to positionally tightly secure the front cover slide plate  16  and the rear cover slide plate  18  in place between the angled interior edges  96  and  97  of the left side channel  26  and the right side channel  28 . Positioning of the front and rear slide plates  16  and  18  completes the sealed barrier between the outside environment and the inside environment and along with the underlying structure, including the extended wall cross channel  52 , the full length top cross channel  42  and the top cross channel  30 , and provides an aesthetically pleasing overall appearance from both sides of the pet door panel insert  10 . 
   One may appreciate that the expandable panel assembly of the present invention could also be used in an alternative configuration wherein expansion occurs at the bottom of a pet door panel insert and serves to raise the entire pet door panel insert assembly situated thereabove. In such an alternative configuration, the capacity to raise and hold the cover slide panels in a convenient raised position facilitating access to the adjustment nuts is particularly useful. Additionally, in a third alternative, an expandable panel assembly may be provided at both the top and bottom of a pet door panel insert. Such an alternative configuration would allow the height of the pet passage and associated door to be altered. For example, as a young puppy grows and matures, the desired position of the pet passage and associated door may be raised once or in several steps by reducing the expansion at the top and increasing the expansion at the bottom. Additionally, one may appreciate that internal spaces, such as the spaces between the cover slide plates, may be insulated to further increase the energy efficiency of the pet door panel insert. Such insulation may be a block of foam insulating material sized to fit within the space between the sliding cover plates or foam-in-place type insulation injected into the space subsequent to installation. The foam-in-place insulation may be included as a component of kit including the pet door panel insert, instructions, and a can of foam-in-place insulation. 
   Various modifications can be made to the present invention without departing from the apparent scope thereof. 
   PARTS LIST 
   
     
       
             
             
           
         
             
                 
             
           
           
             
               10 
               pet door panel insert 
             
             
               12 
               framework 
             
             
               14 
               expandable panel assembly 
             
             
               16 
               front cover slide plate 
             
             
               17 
               front bevel edge 
             
             
               18 
               rear cover slide plate 
             
             
               19 
               rear bevel edge 
             
             
               20 
               pet entry framework 
             
             
               22 
               pet door 
             
             
               24 
               window panel 
             
             
               26 
               left side channel 
             
             
               28 
               right side channel 
             
             
               30 
               top cross channel 
             
             
               32 
               lower region cross channel 
             
             
               34 
               bottom cross channel 
             
             
               36 
               bottom engagement channel 
             
             
               38 
               weatherstrip 
             
             
               40 
               weatherstrip 
             
             
               42 
               full length top cross channel 
             
             
               44 
               screw 
             
             
               46 
               screw 
             
             
               48 
               screw 
             
             
               50 
               screw 
             
             
               51 
               stationary structure 
             
             
               52 
               extended wall cross channel 
             
             
               53 
               positionable structure 
             
             
               54 
               filler panel 
             
             
               56 
               left sleeve channel 
             
             
               58 
               right sleeve channel 
             
             
               60 
               body hole 
             
             
               62 
               body hole 
             
             
               64 
               apertured plate 
             
             
               66 
               screw 
             
             
               68 
               screw 
             
             
               70 
               top engagement channel 
             
             
               72 
               weatherstrip 
             
             
               74 
               weatherstrip 
             
             
               76 
               elongated machine screw 
             
             
               78 
               elongated machine screw 
             
             
               80 
               left nut 
             
             
               82 
               right nut 
             
             
               84 
               left adjustment nut 
             
             
               86 
               right adjustment nut 
             
             
               88 
               left lock washer 
             
             
               90 
               right lock washer 
             
             
               92 
               screw 
             
             
               93 
               weatherstripping 
             
             
               94 
               screw 
             
             
               96 
               angled interior edge 
             
             
               97 
               angled interior edge 
             
             
               98 
               door frame top member

Summary:
A pet door panel insert which has a vertically expandable panel assembly in the upper region thereof installs in a sliding glass patio door or other entity arrangement simply without complicated tools. A positionable structure in the expandable panel assembly is controllingly spaced from a stationary structure of the expandable panel assembly and is actuated upwardly to engage the upper structure of a door frame to sturdily affix the pet door panel insert within the door frame. Opposed vertically adjustable front and rear cover slide plates are frictionally engaged by the upper region of a framework to seal the expandable panel assembly against the outside environment.