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FIELD OF INVENTION 
     The present invention relates to a window covering that may be raised without the need to apply a force to either a control mechanism or the window covering itself as the window covering is opened. In particular, the present invention relates to a window covering having a control mechanism configured to exert an upward force on the shade element and bottom rail that is of sufficient magnitude to raise the shade element and bottom rail without additional force being applied by the user during raising. 
     BACKGROUND OF THE INVENTION 
     Window shades and coverings are found in many applications and used to regulate the amount of light entering a room, and to provide aesthetic appeal to a decor. Such window shades and coverings take many forms, including roller shades, Roman shades, Venetian blinds, and cellular shades. Conventional cellular or pleated shades utilize cord locks or a transmission mechanism to raise, lower and position the window covering in a desired position. With window coverings utilizing a cord lock, cords run up through the folded fabric, across the inside of a head rail and exit through a locking mechanism. Other cellular shades include a transmission mechanism and a continuous loop cord that is pulled by a user to raise and lower the window shade. Roman shades and Venetian blinds also tend to include raising cords that are secured to a lower bar or bottom rail. 
     There are some disadvantages to these designs. Cords present the potential hazard of a child getting caught in or strangled by the exposed control cord. Cords also tend to distract from the aesthetics of a window covering in that they extend along the face of the window covering and, when the window shade is opened, must either be wrapped on a hook or just left on the floor. With window coverings that utilize cord locks, the cords also experience substantial wear due to friction against surfaces as a result of raising and lowering of the window covering. 
     Other window coverings include common roller shades, which operate in the absence of a cord. These roller shades include a wound torsion-spring retraction mechanism in combination with a clutch or locking mechanism mounted with a roller onto which the shade is rolled and collected. In operation, a roller shade is pulled down by a user to a desired location, where it is locked in place by the clutch or locking mechanism. To unlock and release the shade so that it may be raised, the user typically pulls on a bottom rail of the shade, extending the shade sufficiently to disengage the internal clutch or locking mechanism within. When the clutch or locking mechanism is disengaged and the user releases the shade, the shade is retracted using the torsion-spring driven retraction mechanism. Known roller shades, however, are only operable with flat shade material which rolls up neatly into a confined location. 
     The mechanism utilized in such roller shades is not compatible with other window coverings, such as cellular shades, Venetian blinds, and Roman shades. As roller shades are raised, the amount of shade being lifted decreases such that a constant force torsional spring member is capable of applying the necessary winding or upward force throughout the opening range. By contrast, a similar lifting mechanism is typically unsuitable in cellular shades, Venetian blinds, and Roman shades. In these types of window coverings the material of the shade element is typically gathered by raising a bottom member, such as a bottom rail, and increasing amounts of weight are gathered on the bottom member as the window covering is raised. The reason for this is that the shade material or shade element increasingly stacks on the bottom rail as the bottom rail rises, which increases the load on the lifting mechanism. 
     In order to address this increasing weight, very strong torsional springs have been used to accommodate the maximum weight of the shade. One drawback to this approach, however, is that the rate at which the window covering is retracted may be too fast and uncontrolled. One attempt to address this problem is found in U.S. Pat. No. 6,666,252, issued to Welfonder. This patent teaches the use of a fluid brake to control the rate at which the raising cords are retracted throughout the raising process. Another approach that has been used is shown in U.S. Pat. No. 6,056,036, issued to Todd, which employs a mechanical friction member to continuously slow the rate of retraction. One problem with these approaches has been that the spring utilized exerts a force that makes it difficult for a user to overcome when attempting to lower the shade. Excessive pulling force by the user often results in damage to the window covering. 
     Alternatively, variable force springs have been used. Such variable force springs are substantially more complicated in use and manufacture. 
     Therefore, there is a need for a window covering raising mechanism for window coverings such as Venetian blinds, cellular shades and Roman shades that is self-raising and overcomes the foregoing problems. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a self-raising window covering and a control mechanism for the window covering. In particular, the window covering is a self-raising window covering that includes a head rail, a shade element, such as a cellular panel, blind slats, or Roman shade material, a bottom rail, at least one raising cord operatively connected at a first end to the bottom rail, and a control mechanism. The head rail may define an elongated channel wherein the control mechanism is disposed therein. In some embodiments, the control mechanism includes a drive axle and a drive unit operatively connected with the drive axle. The drive unit, which may be a constant force spring, is adapted to provide a substantially constant rotational force on the drive axle. 
     At least one cord winding assembly is also provided in co-axial relation with the drive axle. Typically, the number of cord winding assemblies will be the same as the number of raising cords. However, in some instances, one cord winding assembly may be adapted to operate with multiple cords. The cord winding assembly includes at least one winding drum operatively connected to a second end of the raising cord and having a tapered portion. The cord winding assembly also includes a rotatable positioning member for moving the cord winding assembly laterally along the drive axle upon rotation of the positioning member. In a preferred embodiment, the positioning member is a threaded tubular member connected to the winding drum. The cord winding assembly is adapted to translate the rotational force on the drive axle to a raising force on the raising cord, wherein the raising force is greater than a total downward force exerted by the shade element and bottom rail throughout the range of opening and closing. In a preferred embodiment, the cord winding assembly is rotationally secured with the drive axle by a hub member adapted to engage the cord winding assembly and the drive axle. The hub member may be in a sliding relationship with the tapered portion of the cord winding assembly. 
     A clutch member or locking member is also operatively connected with the axle and adapted to releasably lock the drive axle in a desired position. In a preferred embodiment, the clutch member comprises a reciprocator disposed coaxially relative to the drive axle and movable between a released position and a locked position, and a spring member connected to the reciprocator and operable to either tighten or relax the hold of the reciprocator on the drive axle. The reciprocator is configured to cause the spring member to tighten on the drive axle in the locked position for blocking a rotation of the drive axle against the rotational force applied by the drive unit, and cause the spring member to relax the drive axle in the released position to permit a rotation of the drive axle under the rotational force applied by the drive unit 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view, partly in cutaway, of a preferred embodiment of a window covering according to the present invention; 
         FIG. 2  is an exploded perspective view of the single spring coil drive unit of  FIG. 1 ; 
         FIG. 3  is a side elevational cross section view of the single spring coil drive unit of  FIG. 1 ; 
         FIG. 4  is a side elevational cross section view of an alternative single spring coil drive unit; 
         FIG. 5  is a side elevational cross section view of a double spring drive unit; 
         FIG. 6  is a side elevational cross section view of an alternative double spring drive unit; 
         FIG. 7  is an exploded perspective view of the cord winding assembly shown in  FIG. 1 ; 
         FIG. 8A  is a front elevational view of the window covering of  FIG. 1  in a closed position and with the head rail in cross section; 
         FIG. 8B  is a front elevational view of the window covering of  FIG. 1  in a partially open position and with the head rail in cross section; 
         FIG. 9A  is a perspective view of a preferred clutch member when the window covering is in a fully raised position; 
         FIG. 9B  is a cross sectional view of the clutch member of  FIG. 9A ; 
         FIG. 10A  is a perspective view of the clutch member of  FIG. 9A  as the user pulls down on the window covering; 
         FIG. 10B  is a cross sectional view of the clutch member of  FIG. 10A ; 
         FIG. 11A  is a perspective view of the clutch member of  FIG. 9A  as the user releases the window covering; 
         FIG. 11B  is a cross sectional view of the clutch member of  FIG. 11A ; 
         FIG. 12A  is a perspective view of the clutch member of  FIG. 9A  as the user pulls down on the window covering to release the clutch member; 
         FIG. 12B  is a cross sectional view of the clutch member of  FIG. 12A ; 
         FIG. 13A  is a perspective view of the clutch member of  FIG. 9A  as the window covering self-raises; 
         FIG. 13B  is a cross sectional view of the clutch member of  FIG. 13A ; 
         FIG. 14  is a perspective view of an alterative embodiment of a window covering according to the present invention with a deceleration member; 
         FIG. 15A  is a side elevational cross section view of the deceleration member of  FIG. 14  disengaged from one cord winding assembly; 
         FIG. 15B  is a side elevational cross section view of the deceleration member of  FIG. 14  engaging one cord winding assembly; and 
         FIG. 15C  is a side elevational cross section view of the deceleration member of  FIG. 14  when the window covering is fully raised. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention disclosed herein is susceptible to embodiment in many different forms. Shown in the drawings and described in detail hereinbelow are preferred embodiments of the present invention. The present disclosure, however, is only an exemplification of the principles and features of the invention, and does not limit the invention to the illustrated embodiments. 
     Referring to  FIG. 1 , an embodiment of a self-raising window covering  10  according to the present invention is shown. A head rail  12  defining a channel is provided. A pair of drive units, such as spring units  14  and  16  are coaxially mounted about a drive axle  18 . Also mounted on drive axle  18  are cord winding assemblies  20  and  22 . Each of cord winding assemblies  20  and  22  includes a frustoconical winding drum  24  and  26 , and a threaded tubular member  32  and  34 , respectively. Raising cords  28  and  30 , which are shown as wound on winding drums  24  and  26 , are secured at an end to the winding drums  24  and  26 . In this embodiment, a clutch  36  is also provided and co-axially mounted on the drive axle  18 . Each of these components is discussed in greater detail below. Window covering  10  further includes a shade element, such as cellular shade material  38  and a bottom member, such as bottom rail  40 . The term “cord” as used may encompass a cord, strip, ribbon, string or any similar flexible elongated elements that are suitable for supporting the suspended shade element, and can be wound or unwound to deploy or retract the shade element. A relatively short length of cord  42  can also be provided so that the user can pull down the window covering and, as will be discussed in further detail, release the clutch so that the window covering will retract itself. 
     Referring to  FIG. 2 , a preferred embodiment of the spring unit  14  is shown. The spring unit  14  comprises a spring casing  42 , a spring axle  44 , a constant force coil spring  46  and a cover  48 . The coil spring  46  and the spring axle  44  are secured within the casing  42 , which is closed by cover  48 . A first end  50  of the coil spring  46  is secured to the spring axle  44 , which is coaxially connected to the drive axle  18  ( FIG. 1 ). In this preferred embodiment, the coil spring is configured to provide sufficient rotational force to the drive axle  18  and winding drums  24  and  26  to raise the shade element and bottom rail. Other alternative embodiments of spring units are also possible, such as shown in  FIGS. 3-6 . 
     For example, a suitable spring unit  114  shown in  FIG. 3  may include a coiled spring member  146  having a first end secured with a first spring axle  142  that connects to the drive axle  18  shown in  FIG. 1 , and a second end secured with a second spring axle  144  that is offset from the first spring axle  142 . The coiled spring  146  in a relaxed position may be initially wound around the second spring axle  144 . As the shade element is pulled downward, the coiled spring  146  may stretch out from the second spring axle  144  and progressively wind around the first spring axle  142 . This configuration of the spring unit  114  may be suitable when the used coiled spring  146  has a greater length to allow a longer deployment range of the shade element. 
       FIG. 4  illustrates another suitable spring unit  214 , which is similar to the embodiment shown in  FIG. 3  except that the second end of the coiled spring does not connect to any second spring axle. Instead, the coiled spring  246  winds on itself at its second end, while the first end  252  of the coiled spring  246  connects to a single spring axle  218  connected to the drive axle  18  shown in  FIG. 1 . 
     Still other suitable embodiments of spring units are shown in  FIGS. 5 and 6 . In  FIG. 5 , spring unit  314  includes an assembly of two coiled springs  346  and  348  that may be used to provide a greater raising force for the shade element. The first coiled spring  346  has its first end connected to a first spring axle  344 , and the second coiled spring  348  has its first end connected to a second spring axle  345 . The second end of the first coiled spring  346  and the second end of the second coiled spring  348  respectively connect to a third spring axle  318  located between the first and second spring axles  344  and  345  and connected to the drive axle  18 . As the shade element is pulled downward, the coiled springs  346  and  348  may respectively stretch out from the first and second spring axle  344  and  345  to progressively wind around the third spring axle  318  to apply an increased raising force on the drive axle  18 . In  FIG. 6 , the shown embodiment is very similar to that shown in  FIG. 5  except that the two coiled springs  446  and  448  that wind on the axle  418  connected to the drive axle do not connect with second spring axles. Although each of the embodiments shown utilizes a spring as the driving mechanism for the drive unit, it should be understood that any suitable mechanism for imparting a rotational force on the drive axle may be utilized. 
     Referring again to  FIG. 1 , the rotational force exerted upon a drive axle  18  causes the cord winding assemblies  20  and  22  to rotate and translate for winding the cords  28  and  30 , which thereby raises the shade element  38  vertically toward the head rail  12 . Further details on a preferred embodiment of a cord winding assembly are provided with reference to  FIG. 7 . 
     Cord winding assembly  20  is mounted co-axially with the drive axle  18  that passes through a fixed housing comprised of a frame  64  and upper cover  65 . The cord winding assembly  20  includes a winding drum  24  and a rotational positioning member, such as threaded tubular member  32 , fixedly connected at an end of the winding drum  24 . The cord winding assembly  20  is preferably mounted on the drive axle  18  via a hub member, such as adapter  60  that is configured to transmit rotational movement between the drive axle  18  and the cord winding assembly  20  while allowing a relative translation movement therebetween. In some embodiments, the adapter  60  may be coaxially mounted inside a central hole of the winding drum  24 , and include a through hole for mounting the drive axle  18 . To transfer rotational movement while permitting smooth relative translation between the winding drum  24  and the adapter  60 , a peripheral surface of the adapter  60  may be provided with radial portions that contact with ribs protruding radially inward from the surface of the central hole of the winding drum  24 . Further, the threaded tubular member  32  engages with toothed rollers  66 , which are rotatably mounted to frame  64  and bracket  68  fixedly secured in head rail  12 . Rotational movements thereby can be transferred between the drive axle  18  and the cord winding assembly  20 , while smooth relative translations with reduced frictions are permitted therebetween. In addition, the engagement via the adapter  60  and the threaded tubular member  32  allows an improved support of the load of the suspended components, e.g. shade element  38  and bottom rail  40 . 
     The winding drum  24  is tapered and is preferably frustoconical in shape, and may include striations or grooves to improve gripping of the cord  28  wound on the surface of the winding drum  24 . An end of the raising cord (not shown) is secured towards the larger diameter end  62  of the winding drum  24 . As the cord winding assembly  20  rotates and translates in a direction to wind the raising cord  28 , the raising cord is wrapped around increasingly narrower portions of the winding drum  24 . 
     Referring to  FIGS. 8A and 8B , the raising operation of the window covering is shown. When the shade element  38  is fully deployed, as shown in  FIG. 8A , the raising cord  28  is fully extended from a wider portion of the winding drum  24 . As the bottom rail  40  rises under the resilient force of the spring units  14  and  16 , as shown in  FIG. 8   b , the threaded engagement between the threaded tubular member  32  and rollers  66  causes the rotating cord winding assembly  20  to move laterally within the head rail  12 , such that the raising cord winds along the winding drum  24  towards its narrower end. 
     Because the rising bottom rail  40  causes the shade element  38  to collapse and stack up thereon, the total weight being raised by the resilient force applied by spring units  14  and  16  thus increases. The load on the spring units is now described with reference to one of the spring units. The load on one spring unit  14  is derived with an adequate scale factor from a momentum M on the drive axle  18  that can be approximated by the product between the suspended weight W, including the weight of the bottom rail plus the amount of shade element  38  stacked thereon, and a winding radius R of the winding drum  24 . As the bottom rail  40  rises, W will increase, and R will decrease because the raising cord  28  winds on increasingly narrower portions of the tapered winding drum  24  that slide with reduced frictions owing to the adapter  60  and threaded tubular member  32  and adapter  60 . Accordingly, even though the suspended weight W increases, the load M on one spring unit  14  can be kept at a level that varies slightly and can be overcome by the constant force spring  46  ( FIG. 2 ) to fully raise the bottom rail  40  and shade element  38 . In order to lower the window covering, a user exerts an approximately constant pulling force regardless of the position in height of the window covering. With the cord winding assemblies  20  and  22 , spring units  14  and  16  of constant force thus can be suitably used to raise a suspended weight charge W that increases as it rises. 
     In some embodiments, such as the one depicted, the shade element itself may have an effect on the total downward force or suspended weight. For example, where the shade element is a cellular window covering, an inherent upward spring bias to the material may serve to decrease the total downward force. The total contribution of this spring bias varies depending on the degree to which the cellular window covering is extended. 
     As explained, as the window covering opens, the total weight suspended increases and the total raising force decreases. As such, the rate at which the window cover raises decreases as it nears a fully opened condition. Therefore, the shortcoming typically found in roller shade where the shade is retracted to quickly and violently avoided. 
     Referring again to  FIG. 6 , the clutch member  36  is provided in order to lock the shade element  38  and bottom rail  40  in a desired position. Clutch member  36  is mounted coaxially with the drive axle  18  and is configured to unlock the drive axle  18  as the user pulls down the bottom rail  40  to stretch the shade element  38 , and to lock the drive axle  18  when the user releases the bottom rail  40  at the desired height. When the user pulls down slightly on the bottom rail again, the clutch disengages and allows the bottom rail  40  to be raised by the spring units  14  and  16 . Referring to  FIGS. 9A and 9B , the clutch member  36  includes a casing  70  that has fixed protrusions  72  and  74 . A collar  76  rotating with the drive axle  18  is provided, which reciprocates axially along the drive axle  18 . A reciprocator  78  is co-axially mounted over collar  76  and is movable both rotatably and axially therewith. A spring  80  having a first end  82  and a second end  84  is provided between collar  76  and reciprocator  78 . 
       FIGS. 9A and 9B  show the clutch when the window covering  10  is in a fully raised position. Spring  80  is in a relaxed condition with second end  84  in an abutting relationship with protrusion  74 . As shown in  FIGS. 10A and 10B , when the user pulls on the bottom rail (not shown), a clockwise rotation (as shown) of the axle  18  and the collar  76  occurs and causes the second end  84  of the spring  80  to disengage from protrusion  74 . Spring  80  tightens on collar  76  such that rotation of the collar  76  is transmitted to reciprocator  78  via the contact between first end  82  of the spring  80  and reciprocator  78 , which brings reciprocator  78  into abutment with protrusion  72 . As the reciprocator  78  abuts against protrusion  72 , the spring  80  relaxes again and the drive axle  18  may continue to rotate as the user further pulls on the bottom rail. Referring to  FIGS. 11A and 11B , as the user releases the bottom rail at a desired height, spring  80  tightens on collar  76  and the drive axle  18 , urged by the spring units  14  and  16  ( FIG. 1 ), rotates reciprocator  78  in a counterclockwise direction until it reaches a locking position where protrusion  72  abuts against a stop  79  on the reciprocator  78 . In this locking position, the spring  80  tightens to stop rotation of the drive axle  18  against the raising force exerted by spring units  14  and  16 . Referring to  FIGS. 12A and 12B , as the user pulls down slightly on the bottom rail, the spring  80  tightens and a resulting clockwise rotation of the drive axle  18  and collar  76  causes the reciprocator  78  to disengage from the locking position to a release position. When the user releases the bottom rail as shown in  FIGS. 13A and 13B , the spring units  14  and  16  cause the drive axle  18  to rotate in a counterclockwise direction to bring second end  84  of the spring  80  into engagement with protrusion  74 , and thereby loosening spring  80 , which permits drive axle  18  to continue rotating and fully opening the window covering. 
     An alternative embodiment of the window covering according to the present invention is shown in  FIG. 14 . In most aspects, this embodiment is the same as the ones previously discussed. Window covering  510  includes a head rail  512  having a pair of spring units  514  and  516  mounted with a drive axle  518 . Cord winding assemblies  520  and  522  are also provided. Raising cords  528  and  530  pass through shade element  538  and are connected with bottom rail  540 . In addition, at least one deceleration member  550  is provided. Deceleration member  550  is engageable with one cord winding assembly  522  to slow down the rise of the bottom rail  540  as it approaches the head rail. 
     The preferred embodiment of the deceleration member  520  is shown in  FIGS. 15A-15C . In the position of  FIG. 15A , the cord winding assembly  522  is disengaged from the deceleration member  550 . As the cord winding assembly  522  winds the cord  526 , the cord winding assembly  522  also moves towards the deceleration member  550 . As the cord winding assembly  522  engages with a plate  552  of the deceleration member  550  as shown in  FIG. 15B , the rotation of the cord winding assembly  522  causes the plate  552  to rotate. The plate  552  is connected to an axle sleeve  554 , which is in contact with a decelerating member, such as viscous oil liquid, contained inside a housing  556 . The sleeve  554  is configured to achieve a resistant contact with the decelerating member to decelerate the rotation of the cord winding assembly. For example, protrusions or fins may be provided on the axle sleeve  554 . The rate at which the bottom rail is raised by the spring units  514  and  516  is slowed as the bottom rail reaches the head rail so that the bottom rail more smoothly stops at a fully opened position. 
     The foregoing descriptions are to be taken as illustrative, but not limiting. Still other variants within the spirit and scope of the present invention will readily present themselves to those skilled in the art.

Summary:
The present invention relates to a self-raising window covering and a control mechanism for the window covering. In particular, the window covering includes a drive unit, such as constant force spring, that is adapted to apply a substantially constant rotational force on the drive axle. A cord winding assembly is coaxially mounted on the drive axle, and includes at least one winding drum operatively connected to a second end of the raising cord and having a tapered portion, as well as a rotatable positioning member for moving the cord winding assembly laterally along the drive axle upon rotation of the positioning member. The cord winding assembly is adapted to translate the rotational force on the drive axle to a raising force on the raising cord, wherein the raising force is greater than a downward force exerted by the shade element and bottom rail throughout the range of opening and closing. A clutch member or locking member is also operatively connected with the axle and adapted to releasably lock the drive axle in a desired position.