TECHNICAL FIELD 
     The disclosed invention relates generally to automotive head restraints. More particularly, the disclosed invention relates to automotive head restraints, particularly to rear seat head restraints, that are movable between an upright position when the presence of an occupant is sensed and a lowered or hidden position when no occupant presence is sensed. 
     BACKGROUND OF THE INVENTION 
     Seat head restraints have been used for many years in automotive vehicles. These head restraints generally extend vertically from the seat back. They are provided for neck and head protection during rear impacts and thus may prevent neck hyperextension. 
     At first head restraints were only required for the front seats of vehicles. However, as seat technology advanced and additional concerns arose over the need for head restraints for all passengers of the vehicle, today rear seat head restraints have become mandatory. As is frequently the case with new mandated technology, the rear head restraints on newer vehicles appear to be after-thoughts and frequently do not appear to have been a planned part of the seat design. 
     Current rear seat head restraints add additional height to the rear seats even when the head restraint is adjusted to the lowest height position. While providing a valuable addition to vehicle seating, this additional height partially blocks the view from the rearview mirror, especially for 5 th % female drivers. This situation exists even when there are no passengers on the rear seats. Upright head restraints on foldable rear seats may also interfere with the proper movement of the seat back from its upright position to its lowered position. 
     Accordingly, what is desired is a system that provides for movement of a head restraint between its upright position when a passenger is present and its lowered position when no passenger is present. A further improvement would be such a system in which movement of the head restraint between its upright position and its lowered position is undertaken automatically based on the presence or absence of a seat occupant. 
     As in so many areas of vehicle technology, particularly with respect to vehicle safety systems, there is always room for improvement. 
     SUMMARY OF THE INVENTION 
     The disclosed invention overcomes several of the problems of the prior art by providing a head restraint for any seat in the vehicle but primarily for rear seats that can be selectively moved between an upright and operating position and a lowered and substantially hidden position. Movement of the head restraint provides for improved driver rear visibility through the rearview mirror when there are no rear seat passengers. If a passenger is detected, the corresponding head restraint will be raised to a proper position for comfort and safety. 
     The seat assembly of the disclosed invention includes a seat having a seat base and seat back and a head restraint selectively and rotatably movable between a stowed and substantially hidden position and an upright position. The seat assembly further includes a head restraint motion assembly connecting the seat back and the head restraint and a sensor operatively associated with either or both of the seat base and the seat back for sensing if an occupant is present. Any suitable occupant presence detection sensors, such as an electro-resistive sensor, a weight sensor, and so forth can be used for occupant presence detection. 
     The head restraint includes at least one head restraint post connecting the head restraint to the head restraint motion assembly. The head restraint motion assembly itself includes a drive motor attached to a motor shaft that is attached to the head restraint post(s). 
     The duration of motor rotation can be controlled by on-off contact switches or by some other means to achieve the stowed or upright positions of the head restraint. The head restraint motion assembly includes a locking arrangement for locking the head restraint in its upright position. To stow the head restraint, the motor first disengages the locking arrangement and then continues the rotation to bring the head restraint to its stowed position. 
     The above advantages and other advantages and features will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this invention, reference should now be made to the embodiments illustrated in greater detail in the accompanying drawings and described below by way of examples of the invention wherein: 
         FIG. 1  illustrates the disclosed invention in which a seat frame and its associated stowed head restraint are shown in relation to the remainder of the seat back shown in broken lines; 
         FIG. 2  illustrates a view similar to that of  FIG. 1  but showing the head restraint in its upright position; 
         FIG. 3  illustrates a close up view illustrating the head restraint and the seat back upper beam fitted to the seat back frame; 
         FIG. 4  illustrates a view of the seat back upper beam fitted to the seat back frame; 
         FIG. 5  illustrates a perspective view of the head restraint motion assembly in relation to the head restraint; 
         FIG. 6  illustrates a detailed view of a head restraint post bracket; 
         FIG. 7  illustrates a view of the head restraint post bracket of  FIG. 6  taken from the opposite side; 
         FIG. 8  illustrates a detailed view of the post, motor, motor shaft and seat beam brackets of the head restraint motion assembly; 
         FIG. 9  is the same as  FIG. 8  but does not include the seat beam brackets; 
         FIG. 10  illustrates a close up view of the arrangement of  FIG. 8  in which the locking assembly is being engaged; 
         FIG. 11  illustrates a view similar to that of  FIG. 10  but shows additional detail of the locking arrangement; 
         FIG. 12  illustrates a detailed view of the arrangement of  FIG. 11 , particularly the pawl arrangement; 
         FIG. 13  illustrates a view of the head restraint in its lowered position relative to the seat back upper beam; 
         FIG. 14  illustrates a view similar to that of  FIG. 13  but shows the motor, motor shaft and the seat beam brackets in relation to the lowered head restraint; 
         FIG. 15  illustrates a close up view of the motor shaft in relation to a head restraint post bracket and a head restraint post in its stowed position; and 
         FIG. 16  is a flow chart depicting functionality of the disclosed invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following figures, the same reference numerals will be used to refer to the same components. In the following description, various operating parameters and components are described for different constructed embodiments. These specific parameters and components are included as examples and are not meant to be limiting. 
       FIGS. 1 and 2  illustrate a side view of a seat assembly for a vehicle, generally illustrated as  10 . The seat assembly  10  includes a seat back  12  (illustrated in broken lines) and a head restraint assembly  14 . The seat back  12  may be part of a seat structure for either a front seat, a rear seat, or a third row seat in the event that the vehicle has such a seat. 
     The head restraint assembly  14  includes a seat back frame  16  and an associated head restraint  18 . It is to be understood that the size and configuration of the seat back  12  and the components of the head restraint assembly  14  shown in the various views are only suggested and are used for illustrative purposes only. Accordingly, neither the illustrated size nor configuration of the seat back  12  and the head restraint assembly  14  are intended as being limiting. 
     As shown in  FIG. 1 , the head restraint  18  is in its retracted and lowered position. The head restraint  18  may be hidden from occupant view by either folding into the back side of the seat back  12  or resting in a pocket formed in an interior surface structure adjacent the seat back  12 . Either way, an objective of the disclosed invention is to eliminate the head restraint  18  from the driver&#39;s line-of-sight when no occupant is present in the seat. 
     As shown in  FIG. 2 , the head restraint  18  is in its upright position as would be the case if the sensor associated with the seat (preferably but not absolutely in the seat base [not shown]) senses the presence of a seat occupant. In this upright position, the head restraint  18  would provide comfort and the necessary support for the occupant&#39;s head in an impact event. 
       FIGS. 3 and 4  illustrate the back side of the seat back frame  16  of the head restraint assembly  14 . The head restraint  18  is supported typically by at least one head restraint post. Preferably, but not essentially, the head restraint  18  is supported by a pair of spaced apart head restraint posts  20  and  20 ′. As is known in the art (and as is illustrated in, for example,  FIG. 5 ), the head restraint posts  20  and  20 ′ represent the ends of a bent post, the curved part of which is embedded in the head restraint  18  upon molding, also as is known in the art. 
     The head restraint posts  20  and  20 ′ are pivotably attached to a seat back upper beam  22  of the head restraint assembly  14 . A pair of spaced apart slots  24  and  24 ′ are formed in the seat back upper beam  22  to movably accommodate the head restraint posts  20  and  20 ′. Formed on the outer sides of the spaced apart slots  24  and  24 ′ are seat beam brackets  26  and  26 ′ of which one bracket, seat beam bracket  26 , is shown in  FIG. 4 . 
       FIGS. 5 through 15  generally illustrate the operation of the head restraint assembly  14  and include such aspects of the disclosed invention as rotation control features and locking features. Fitted to the approximate ends of the posts  20  and  20 ′ are head restraint post brackets  28  and  28 ′ respectively. The brackets  28  and  28 ′ interconnect a motor drive shaft  29  which is driven by a motor  30 . It is to be understood that other drive arrangements may be adopted without deviating from the spirit and scope of the invention. 
     The head restraint post bracket  28  includes a catcher  32 , a head restraint post catcher  34 , and a catcher  36 . The head restraint post bracket  28 ′ includes a catcher  32 ′, a head restraint post catcher  34 ′, and a catcher  36 ′. A pair of spaced apart motor shaft cams  37  and  37 ′ are provided on the motor drive shaft  29  as are a pair of spaced apart motor shaft catchers  38  and  38 ′. 
     The locking arrangement of the disclosed invention incorporates a pawl system which includes pawls  40  and  40 ′ which pivot on pawl pivot pins  42  and  42 ′ respectively. The pawl  40  is provided with a pawl pin  44  and the pawl  40 ′ is provided with a pawl pin  44 ′. Slots, such as slot  39 , are formed in the seat beam brackets  26  and  26 ′ to allow movement of the pawl pins  44  and  44 ′. The pawls  40  and  40 ′ are urged to their engagement positions by resilient members such as pawl springs  46  and  46 ′. 
     In operation, the motor  30  is activated by a sensor such as an electro-resistive seat sensor or a similar device (not shown) used to detect passenger presence. The sensor is typically installed between the seat cover and the seat foam. When the passenger sits in the seat, the sensor is subjected to bending which increases its voltage due to its resistance change. Other sensors may be adapted for this purpose. 
     If the sensor detects the presence of an occupant, the stowed head restraint  18  is rotated to its upright position. Once in this position, the head restraint  18  is locked. 
     To achieve this position, the motor  30  causes the motor shaft  29  to rotate as shown in  FIGS. 8 and 9 . (The straight arrows in  FIGS. 8 and 9  are directed to the front of the vehicle.) A reverse angle view is shown in  FIG. 10  in which the vehicle front is also illustrated by a straight arrow. 
     Particularly, and describing the head restraint post bracket  28  and its associated components (with the understanding that the operation of the head restraint post bracket  28 ′ and its component is identical), the motor shaft catcher  38  engages the head restraint post bracket catcher  34  to rotate to its upright position. At this time the catcher  32  on the post bracket  28  rotates under the pawl  40  and achieves the locked position by engaging the contact surface of the pawl  40 . The pawl spring  46  maintains the pawl  40  in its engaged position thus holding the head restraint  18  in its upright and locked position. 
     In the event that the occupant sensor detects that there is no occupant present, the head restraint  18  is moved from its upright position (shown in  FIG. 2 ) to its stowed position (shown in  FIG. 1 ). To achieve stowage of the head restraint  18 , the motor shaft  29  is rotated by the motor  30  as shown in  FIGS. 11 and 12 . As rotation begins, the motor shaft cam  37  engages and lifts the pawl pin  44  such that the catcher  32  can rotate under the pawl  40 . 
     As the motor shaft  29  continues to rotate, the motor shaft catcher  38  engages the catcher  34  of the head restraint post bracket  28 . Thus engaged, the head restraint  18  is allowed to rotate with the motor shaft  29  to its stowed position as shown in  FIGS. 13 and 14 . The stowed arrangement of the head restraint post bracket  28 ′, the motor shaft  29 , the head restraint post  20 ′ and the associated locking and unlocking elements are illustrated in  FIG. 15 . 
     A flow chart illustrating one embodiment of the functionality of the disclosed invention is illustrated in  FIG. 16 . The disclosed functionality is suggested and is not intended as being limiting. 
     Initially, and at the start  100 , the vehicle power is turned on  120  and, subsequently, the seat presence detection threshold is defined  130 . This value may be pre-determined and quantified as a part of the occupant-sensing program. The seat sensor signal is then obtained  140  and is then corrected for signal noise  150 , bias and/or offset. If it is found that the value used in defining the seat presence detection threshold is not exceeded  160 , then the seat sensor signal is again obtained. 
     However, if it is found that the value used in defining the seat presence detection threshold is exceeded, then a pre-determined waiting period is undertaken  170 . After expiration of the pre-determined waiting period, if it is determined that the threshold value  180  is still exceeded, the head restraint is moved to its raised position  200  and is locked  220 . If, on the other hand, after the pre-determined waiting period expires it is found that the threshold value  180  is not exceeded, the head restraint is unlocked  190  and is moved to its stowed position  210 . Following movement of the head restraint to its raised position or to its stowed position (as the case may be), the headrest power is turned off  230 . If the vehicle power is turned off  240  the program is at an end  250  and awaits the next cycle. However, if the vehicle power is not turned off, the seat sensor signal  140  is obtained and the sequence of determining the position of the headrest is again followed as above. 
     The foregoing discussion discloses and describes exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the true spirit and fair scope of the invention as defined by the following claims.
A seat assembly including a head restraint for a vehicle seat that can be raised and lowered depending on the presence or absence of a seat occupant is disclosed. The seat assembly includes a seat having a seat base and seat back and a head restraint selectively movable between a stowed position and an upright position. The seat assembly further includes a head restraint motion assembly connecting the seat back and the head restraint and a sensor operatively associated with either or both of the seat base and the seat back for sensing an occupant. The head restraint includes at least one head restraint post connecting the head restraint to the head restraint motion assembly. The head restraint motion assembly includes a drive motor attached to a motor shaft attached to the head restraint post(s). A locking arrangement is provided to maintain the head restraint in its upright position.