This application is based upon U.S. Provisional Patent Application Ser. No. 60/535,083 filed Jan. 8, 2004, and is a Divisional Application of Ser. No. 11/021,206 filed Dec. 23, 2004.
BACKGROUND OF THE INVENTION
 This invention deals generally with changing tables for infants, and more specifically with a height adjustment apparatus for a changing table installed on top of a structure such as a child's play yard.
 Children's playpens, now more often called play yards, are a commonly available accessory for use with infants and small children. It is not at all unusual for parents to take a folding play yard along on a trip or visit in order to provide a space for the child to play and sleep. There is also another accessory for children that is just as vital, but not taken out of the child's home as frequently. That is a changing table or bassinet.
 However, there are now available some changing tables that simply attach to a play yard and therefore are not at all difficult to take out of the home along with the play yard. Most of these tables take advantage of the height of the sides of the play yard and are attached to the top of the play yard sides.
 U.S. Pat. No. 5,339,470 to Shamie discloses one such combination playpen and changing table that re-hangs the bottom of the playpen from short straps attached to the horizontal top rails of the playpen in order to provide a somewhat higher surface for changing the child. U.S. Pat. No. 5,553,336 to Mariol uses a different approach by providing a complete assembly that resembles a second layer of the play yard that fits into wells on the top four corners of the play yard. Another type of changing table available is supported off one corner of a play yard, and it is rotated beyond the walls of the play yard when not in use.
 The problem with all such arrangements is that they furnish only one height for the changing table, and that one height, which is keyed to the existing height of the play yard walls, is usually too low for an adult to use with comfort, because adults of average height must bend down to the level of the changing table. Furthermore, any changing table with a single pre-designed height can not possibly suit all adults.
 It would be very beneficial to have an uncomplicated changing table that attaches to the top of a play yard and is easily adjustable to accommodate adults of different heights.
SUMMARY OF THE INVENTION
 The present invention provides a simple structure for attaching a height adjustable changing table to the top of a play yard or other structure. To accept the support structure of the changing table, the play yard or other supporting structure requires only that a socket with an open top be attached to an upper edge of the supporting structure. It is probably most convenient and most stable if two sockets are mounted near two upper corners of a play yard. A vertical support for the changing table then fits into each socket, and both the changing table and the entire height adjustment assembly are attached to the changing table without any further modification to or addition of internal structures for either the play yard support or the changing table support.
 The changing table support is attached to the changing table structure by conventional means such as rivets or screws, and the height adjustment assembly fits within the changing table structure, but the height adjustment assembly is external to both the support on the changing table and the socket on the play yard.
 The main part of the height adjustment assembly of the preferred embodiment is a height adjustment fixture that contains three sections. One section is a coupling, which in the preferred embodiment is a ring that is horizontally oriented as it fits around the changing table vertical support just below the location where the changing table support protrudes out of the changing table. An extension oriented parallel to the axis of the coupling ring is attached to and slightly offset from the coupling ring and contains the other two sections of the fixture. The second section is a cylindrical tube, the axis of which is parallel to the axis of the coupling ring and the axis of the changing table support. This tube fits within the changing table, and into a cylinder integrated into the changing table structure. The tube slides within the cylinder to maintain alignment between the changing table and the third section which is a rack assembly.
 The rack assembly is a linear strip attached to the outside of the cylindrical tube, with the linear strip containing a series of slots and separating lands so that it resembles the rack of a gear and rack assembly. In one embodiment of the invention, a gear mounted on the changing table engages the lands, and turning the gear raises or lowers the changing table.
 The preferred embodiment of the invention uses a spring loaded lever that engages the slots, but permits the height of the changing table to be manually lifted or lowered when the lever is pushed to move its latching end out of the slots.
 The particular benefits of the height adjustment assembly of the invention are simple structure, inexpensive manufacturing, ease of use, and attachment to any supporting structure with nothing more than the addition of one or two tubular sockets at the top of the supporting structure. In particular, there is no requirement to put holes in any of the tubular structure of the supporting structure or install complex locking mechanisms within the supports of either the changing table or the supporting structure to which it is attached.
BRIEF DESCRIPTION OF THE DRAWINGS
 FIG. 1 is a partial sectional side view of the height adjustment assembly of the preferred embodiment of the invention installed within a changing table that is shown atop a play yard.
 FIG. 2 is a perspective view of the height adjustment fixture of the preferred embodiment of the invention of FIG. 1.
 FIG. 3 is a side view of an alternate embodiment of the height adjustment assembly of the invention.
 FIG. 4 is a perspective view of another alternate embodiment of a height adjustment assembly of the invention.
 FIG. 5 is a perspective view of the coupling and latch assembly of the alternate embodiment of FIG. 4.
DETAILED DESCRIPTION OF THE INVENTION
 FIG. 1 is a partial sectional side view of height adjustment assembly 10 of the preferred embodiment of the invention installed within changing table 12 shown atop play yard 14, and FIG. 2 is a perspective view of height adjustment fixture 11 of the preferred embodiment of the invention. The view of height adjustment fixture 11 in FIG. 2 is from the opposite side of the view shown in FIG. 1 in order to better show the structure of height adjustment fixture 11.
 Height adjustment fixture 11 of FIGS. 1 and 2 includes a series of at least two slots 16 alternating with lands 18 which, when installed within changing table 12, are aligned in a vertical orientation. Slots 16 and lands 18 are formed on support structure 20, and tube 22 is attached to support structure 20 on surface 24 opposite lands 18. Height adjustment fixture 11 also includes coupling ring 26 which is attached to support structure 20 at the end of support structure 20 remote from opening 21 of tube 22. Coupling ring 26 is attached to support structure 20 at surface 28 which is the surface most remote from lands 18, so that axis 27 of coupling ring 26 is parallel to axis 23 of tube 22 and to axis 31 of changing table support 30, and coupling ring 26 has no interfering structures on either side of it along its axis 27.
 As shown in FIG. 1, the clearance on both sides of coupling ring 26 permits height adjustment fixture 11 to be installed within changing table 12 and perform its function although it need not be permanently attached to changing table 12. For changing table 12 to accommodate height adjustment fixture 11, changing table 12 itself needs three structures. One is changing table support 30 which can be a basic tube structure that is used to attach most accessories to supporting structures such as play yard 14. Changing table support 30 is attached to changing table 12 by any conventional means such as hole 32 and fasteners 34.
 The attachment of changing table 12 to play yard 14 is also conventional in that holding fixture 36 with socket 38 is attached to play yard 14 at an upper location, such as adjacent to a corner where horizontal rail 40 is attached to vertical support 42. Socket 38 in holding fixture 36 can then accept any simple rod or tube, such as changing table support 30, as long as the tube is of smaller diameter than socket 38. However, such simple holders for accessory structures provide no height adjustment since the lower part of the accessory structure would merely sit upon holding fixture 36.
 The simple structure of height adjustment fixture 11 furnishes a vital part of the height adjustment with no modification of either support 30 or holding fixture 36. In the preferred embodiment shown in FIG. 1, height adjustment fixture 11 is shown installed in changing table 12. This is accomplished by slipping coupling ring 26 over changing table support 30 so that the bottom surface 29 of coupling ring 26 rests upon holding fixture 36. Tube 22 of height adjustment fixture 11 then fits into a socket concentric with tube 22 within changing table 12 and aids in aligning height adjustment fixture 11 with changing table 12. Tube 22 also holds spring 44. Spring 44 is a compression spring, which is shown with dashed lines in FIG. 1 and inserted within tube 22 in FIG. 2, has its upper end against an inner surface of changing table 12, and serves to overcome any friction between tube 22 and its socket. Spring 44 thus assures that height adjustment fixture 11 continues to rest on holding fixture 36 of play yard 14 as changing table 12 is raised.
 In regard to stability and alignment, it is worthwhile to note that the most typical installation of height adjustment assembly 10 of the present invention is in pairs with one on each of two supports 30 located at opposite ends of changing table 12. The two supports are installed into holding fixtures 36 located near adjacent corners of play yard 40. Such an arrangement provides an appropriate length for the changing table and does not require any locking apparatus to prevent supports 30 from rotating within fixture 36.
 Height adjustment assembly 10 requires one more device attached to changing table 12. That is lever 46 which holds changing table 12 in one of the height positions provided by slots 16. Lever 46 rotates around pivot point 48 when lever handle 50 is pushed toward lands 18, and as lever 46 rotates, latch 52 moves out of the slot 16 in which it is lodged. This permits changing table 12 to be manually raised or lowered. When lever handle 50 is released, spring 54 forces lever handle away from slots 16, thus moving latch 52 into an available slot 16. Since height adjustment assemblies are typically installed near the ends of changing table 12, and when pushing lever handle 50 the user's hands are in contact with the underside of changing table 12, it is not at all difficult to operate two lever handles simultaneously and also raise or lower changing table 12.
 FIG. 3 is a side view of alternate embodiment 60 of the height adjustment assembly of the invention. Support structure 20, tube 22, and coupling ring 26 of height adjustment assembly 60 are identical to and serve the same function as the same parts of height adjustment fixture 11 in FIGS. 1 and 2 so they are identified by the same numbers. The difference in height adjustment assembly 60 is in the recess and land structure. Alternating slots 62 and lands 64 of height adjustment assembly 60 are constructed to operate as rack 61 in conjunction with gear 66 so the spacing of lands 64 must match the spacing of teeth 68 on gear 66. The height adjustment is then accomplished by rotating gear 66 in the appropriate direction.
 Gear 66 is mounted upon and attached to the changing table (not shown in FIG. 3) in a manner similar to lever 46 of FIG. 1, so that as gear 66 literally climbs up rack 61, the changing table is raised. This eliminates the requirement to manually lift the changing table to adjust the height.
 Gear 66 can be rotated by any conventional means. One rotating device is shown in FIG. 3. Knob 70 is mounted on surface 72 of the changing table (not shown) and drives gear 66 by conventional linking means 74 either directly or through a reduction drive. One requirement for any height adjustment assembly is that it have some sort of latching device to prevent the changing table from moving down with increased weight. The latching device is provided for height adjustment assembly 60 at knob 70. Knob 70 includes latch 76 that is pushed outward by spring 78 and moves into slots 80 to prevent both knob 70 and gear 66 from turning unless latch 76 is pulled into knob 70 by moving tab 79 against spring 78. Latch 76 is only one of many conventional devices to permit locking knob 70. Another such device is a tab on a spring loaded shaft that mates with slots around the shaft unless moved axially to disengage the tab from the slots for turning the shaft.
 FIG. 4 is a perspective view of another alternate embodiment of a height adjustment assembly 84 of the invention with only partial structure 86 of the changing table shown. Similar to the changing table 12 of FIG. 1, changing table structure 86 has changing table support 88 attached so that support 88 can be installed into a holding fixture (not shown) on the supporting structure upon which it will be supported. The essential difference between height adjustment assembly 84 and height adjustment assembly 10 of FIG. 1 is that latches 90 are integrated with coupling 92 into coupling and latch assembly 94. Coupling and latch assembly 94 thus minimizes the number of parts and simplifies the design of changing table structure 86. All that is required within changing table structure 86 are socket 96 (shown with invisible lines) within which coupling and latch assembly 94 fits and holes 98 to mate with latches 90. As with height adjustment assembly 10 of FIG. 1, height adjustment assembly 84 permits changing table support 88 to slide within coupling 92 when latches 90 are depressed, and changing table structure 86 and the entire changing table are raised as coupling and latch assembly 94 slides within socket 96. Releasing latches 90 then permits them to mate with other holes 98 and hold the changing table at a different height.
 FIG. 5 is a perspective view of coupling and latch assembly 94 of height adjustment assembly 84 of FIG. 4. Latches 90 are mounted upon spring 100 so that simply pressing latches 90 in toward the center of support structure 102 flexes spring 100 and releases the latches from holes 98 in changing table structure 86 (FIG. 4). This simple mechanism on support structure 102 eliminates the need for an external latching mechanism on the changing table. It should be understood that the rectangular configuration of support 102 and the use of pairs of latches 90 and twin holes 98 (FIG. 4) are not vital to the function of height adjustment assembly 84, and many other configurations of these structures can be used.
 The invention thus provides a convenient and easily manufactured height adjustment for a changing table used in conjunction with a play yard or any other supporting structure, and it requires no complex modification of the supporting structure or the changing table supports because the height adjustment assembly is external to those supports.
 It is to be understood that the form of this invention as shown is merely a preferred embodiment. Various changes may be made in the function and arrangement of parts; equivalent means may be substituted for those illustrated and described; and certain features may be used independently from others without departing from the spirit and scope of the invention as defined in the following claims. For example, springs other than leaf spring 54 as shown can be used, or height adjustment fixture 11 can use holes or any other type of recesses within support structure 20 instead of slots 16, and the latch can be a spring loaded retractable pin instead of pivoting latch 52. Furthermore, neither the changing table support nor the coupling of the height adjustment assembly need to have cylindrical cross sections, nor the coupling even be a ring, as long as they mate, can be assembled together, and the support can move within the coupling.