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You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION  
       [0001]     Embodiments of the invention relate to spa covers and more particularly to spa covers which are moveable vertically between a position directly atop the spa and a position elevated above the spa to act as a roof structure during spa use.  
       BACKGROUND OF THE INVENTION  
       [0002]     It is known to cover hot tubs or spas to prevent contamination due to environmental debris, such as leaves, pollens and the like, to prevent excessive evaporation when the spa is not in use and to act as a safety measure to prevent animals, small children and the like from falling into the water when unsupervised.  
         [0003]     Most conventional spa covers are fabric covered foam structures which rest atop the spa when the spa is not in use and which are removed, either by sliding off the tub or by folding at a middle and lifting to a position away from the top of the tub, such by a support frame wherein the cover is suspended vertically in the folded position adjacent a side of the tub. The fabric covers may be susceptible to chemical vapors, such as chlorine, are prone to tearing at seams exposing the foam layers to the elements and to the steam and vapors from the tub and are generally susceptible to normal wear and tear necessitating multiple replacements during the life of the average spa.  
         [0004]     It is known to provide fixed structures built around the hot tub to provide an element of protection for use during inclement weather or to prevent excessive exposure to the sun. Most often the structure is independent of the cover and remains in a fixed position around the tub, the roof portion being fixed above the spa to permit use of the spa. While these structures may meet the needs of the user by providing a rigid or semi rigid roof structure, they add additional expense by requiring a spa cover to be used as well.  
         [0005]     Conventional spa covers are not designed to handle the weight of a person or persons resting on the cover. As the spa covers are typically flat however, individuals may be encourage to walk or otherwise provide undue weight on the cover, such as when shoveling snow from a deck in which the spa is recessed, which results in damage not only to the cover but to the spa itself.  
         [0006]     Others have attempted to provide domed rigid or semi-rigid cover structures which, like the conventional cover, reside atop the spa when in use and which are pivoted or slid laterally away from the spa when the spa is to be used.  
         [0007]     It is known to provide a vertically actuable cover to a swimming pool or a spa. U.S. Pat. No. 3,566,420 to Peterson et al teaches hydraulic actuators used to raise and lower a cover from a swimming pool and U.S. Pat. No. 6,71 8,566 to Wilson teaches a plurality of telescoping and threaded sections which are used to raise and lower a cover over a spa.  
         [0008]     There remains interest in the industry to find reliable, relatively simple and inexpensive lift systems for raising and lowering roof structures over spas, which can act to replace a conventional spa cover when in a lowered, spa engaging position and which act as a roof when in the raised position.  
       SUMMARY OF THE INVENTION  
       [0009]     A cover assembly for a structure, such as a spa, is vertically actuated between a lowered position atop the spa to an elevated position above the spa where the cover acts as a roof over the spa. The actuation of the cover is accomplished using a unique rack and pinion system within telescoping tubular members which form actuation members upon which the cover is supported.  
         [0010]     In a preferred embodiment, the rack and pinion system comprises a rack and a worm gear which is rotatably driven by an electric motor. Limit switches act to stop the motor when the cover has reached the elevated or lowered position. The cover is supported on a plurality of actuation members, typically one at each corner of the cover, and each of the electric motors is connected through a single circuit so as to co-ordinate the actuation members to support and vertically actuate the cover. A cable is connected between the rack and a third and upper telescoping member of each actuation member for assisting in raising and lowering cover. The cable is guided by a pulley which is connected at a top end of the rack.  
         [0011]     Preferably, stops are formed at top and bottom ends of the telescoping tubular members to limit the upward travel of the telescoping members within each other to prevent the telescoping members from becoming disconnected during elevation of the cover and, in the case of the cable-assisted embodiment, to lift the intermediate member with the upper tubular member.  
         [0012]     In a broad aspect therefore, a cover assembly comprises a cover supported on at least one actuation member, the cover being actuable between a lowered position atop the structure to an elevated position above the structure so as to act as a roof thereover, wherein the at least one actuation member further comprises:  
         [0013]     a plurality of telescoping tubular members;  
         [0014]     a rack and pinion assembly positioned within the telescoping tubular members and connected thereto; and  
         [0015]     means for rotating the pinion in engagement with the rack for raising and lowering the plurality of telescoping tubular members connected thereto for actuating the cover between the lowered and the elevated positions. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a perspective view of a spa cover according to one embodiment, shown in a raised position;  
         [0017]      FIG. 2  is a perspective view according to  FIG. 1  shown in a lowered position;  
         [0018]      FIG. 3   a  is a longitudinal sectional side view of an actuation mechanism for raising and lowering the spa cover, shown in the lowered position;  
         [0019]      FIG. 3   b  is a longitudinal sectional side view according to  FIG. 3   a , shown in the raised position;  
         [0020]      FIG. 4   a  is a longitudinal sectional front view according to  FIG. 3   a , in the lowered position;  
         [0021]      FIG. 4   b  is a longitudinal sectional front view according to  FIG. 3   a , in the raised position;  
         [0022]      FIG. 5  is a detailed view of the actuation mechanism, more particularly a worm gear and rack;  
         [0023]      FIG. 6  is a perspective view of the worm gear and rack of  FIG. 5  shown in the lowered position and illustrating an alternate embodiment for connection of the upper tubular member  
         [0024]      FIG. 7  is a schematic sectional view of an embodiment of an actuation mechanism having two telescoping tubular members, the upper tubular member being connected to the rack;  
         [0025]      FIG. 8  is a schematic section view of an embodiment of the invention according to  FIGS. 3   a - 4   b  having three telescoping tubular members, the upper tubular member being connected to the rack through a cable and pulley;  
         [0026]      FIG. 9  is a schematic of a circuit for independently operating of a plurality of gear motors for rotationally powering worm gears; and  
         [0027]      FIG. 10  is a schematic of a circuit for operating the plurality of gear motors according to  FIG. 9 , in series. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]     Having reference to  FIGS. 1 and 2 , a cover apparatus  1  for a structure  2 , such hot tub or spa, comprises a cover  3  supported on actuation members  4 , typically positioned at each corner of the spa  2 . The actuation members  4  elevate the cover  3  from a lowered position, wherein the cover  3  rests atop or adjacent a top edge  5  of the spa  2 , to a raised position, wherein the cover  3  is supported over the spa  2  to act as a roof.  
         [0029]     As shown in  FIG. 3   a - 6  and in a preferred embodiment, the actuation members  4  comprise a rack and pinion system  10  for raising and lowering the cover  3 . Each actuation member  4  further comprises two or more telescoping tubular members  11 . The tubular members  11  may be, but are not limited to being circular, rectangular or square in cross-section. Preferably, each actuation member  4  comprises three telescoping tubular members  11 , a lower tubular member  12  secured to a base  13 , an intermediate tubular member  14  and an upper tubular member  15  upon which the cover  3  is supported. The rack and pinion system  10  is housed within the telescoping tubular members  11  and connected thereto for actuation of at least the intermediate tubular member  14 . The upper tubular member  15  is connected thereto through a cable  16  which is anchored, at a first end  17 , to the base  13  or to an upper end  18  ( FIG. 6 ) of a rack  19  of the rack and pinion system  10  for movement therewith and, at a second end  20 , to a lower end  21  of the upper tubular member  15 . The cable  16  is guided at an upper end  22  of the rack  19  by a pulley  23 .  
         [0030]     Preferably, a stop  24   a  is formed at an upper end  25  of the lower tubular member  12  for engaging a stop  24   b  at a lower end  26  of the intermediate tubular member  14  for retaining the intermediate telescoping tubular member  14  in the lower telescoping tubular member  12  and to prevent the intermediate tubular member  14  from being pulled out of the lower tubular member  12 , when the cover apparatus  1  is actuated to the raised position. Similarly, a stop  24   c  is formed at an upper end  28  of the intermediate member  14  for engaging a stop  24   d  at the lower end  21  of the upper tubular member  15  for retaining the upper tubular member  15  within the intermediate tubular member  14  during actuation to the elevated position.  
         [0031]     Best seen in  FIGS. 6-8  and most preferably, the rack and pinion system  10  comprises a worm gear  30  which is rotatably connected to a motor  31 , such as an electric gear motor, by a rod  32 . The worm gear  30  is rotated to raise and lower the rack  19 . For an actuation member  4  having two telescoping tubular members  11  ( FIG. 7 ), rack  19  directly raises upper telescoping member  15 . In the case of an actuation member  4  having three telescoping tubular members  11  ( FIG. 8 ), the rack  19  raises and lowers the upper telescoping member  15  via the cable  16 , the upper tubular member  15  raising and lowering the intermediate tubular member  14  through engagement of the stops  24   c , 24   d . In the preferred embodiment ( FIG. 6 ) the cable  16  is connected at the second end  20  to a ring  33  which is used for mounting the cable  16  inside the lower end  21  of the upper tubular member  15 .  
         [0032]     Preferably, each actuation member  4  is powered by an electric gear motor  31 . Most preferably, all of the gear motors  31   a ,  31   b ,  31   c ,  31   d  are connected through a single circuit so that when the circuit is activated, all of the actuation members  4  are caused to move at the same time. Limit switches  40  are positioned on the rack and pinion system  10  to stop the gear motor  31  when the cover  3  has reached the lowered or the raised position. As shown in  FIGS. 9 and 10 , the gear motor limit switches  40  may be independent ( FIG. 9 ) or in series ( FIG. 10 ).  
         [0033]     In a preferred embodiment, the cover  3  is a substantially rigid, domed pyramidal-shaped cover manufactured of a foam core and having a fabric covering such as is known in the industry to provide protection and insulation as is also known with conventional spa covers. Further, the domed shape is particularly advantageous for spas which are enclosed in a deck structure to prevent persons or animals from walking or lying on the cover  3  and to minimize the amount of snow buildup on the cover  3  in snow-prone climates.

Summary:
A vertically actuable cover for a spa comprises telescoping members which utilize a rack and pinion system to raise and lower the cover from the spa. In the lowered position the cover protects the spa and in the elevated position the cover acts as a roof over the spa.