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CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 11/680,285, filed Feb. 28, 2007, which is a continuation-in-part of International Application No. PCT/CA2006/000254, with an international filing date of Feb. 20, 2006. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to power liftgates for motor vehicles, and more particularly, to a power liftgate drive assembly having an absolute position encoder. 
         [0004]    2. Description of Related Art 
         [0005]    Motor vehicle liftgates or closure panels act to close and seal a rear cargo area of a van, minivan, or sport utility type of motor vehicle. Typically, these closure panels are mounted in a frame located at the rear of the vehicle, usually on a horizontally extending axis provided by a hinge. The liftgate is thus positioned to rotate between a closed position adjacent to the frame and an open position, in which the cargo area of the vehicle is accessible. The liftgate is often very heavy, and because of its mounting, it must be moved against gravity in order to reach the open position. Because of the liftgate&#39;s weight, it would be a great burden if a user was required to lift the liftgate into the open position and then manually hold it in place in order to access the vehicle&#39;s cargo area. 
         [0006]    In order to make it easier to open liftgates, most modern motor vehicles use gas or spring-loaded cylindrical struts to assist the user in opening and holding open liftgates. The struts typically provide enough force to take over the opening of the liftgate after the liftgate has been manually opened to a partially opened position at which the spring force and moment arm provided by the struts are sufficiently to overcome the weight of the liftgate, and to then hold the liftgate in an open position. 
         [0007]    Automated power systems to open and close vehicle liftgates are well known in the art. These systems typically use a power actuator to apply a force directly to the liftgate to enable opening and closing thereof. Such automated powered systems act as a direct replacement for the user-supplied force. 
         [0008]    With automated power systems to open and close vehicle liftgates it is desirable to provide a position sensor to monitor the position of the liftgate. Most position sensors, however, are limited by the fact that if power is temporarily lost or disconnected, and the liftgate is manually moved, the position sensor cannot detect the position of the liftgate until the position sensor is recalibrated or reset. Therefore, it is desirable to provide a power liftgate drive assembly having a position sensor capable of monitoring the position of the liftgate even after power is temporarily lost or disconnected and the liftgate is manually moved to another position. 
       SUMMARY OF THE INVENTION 
       [0009]    According to one aspect of the invention, a powered closure drive mechanism is provided for moving a closure between an open position and a closed position. The drive mechanism includes an elongated strut assembly extending between first and second ends. A rotatable lead screw is disposed within the strut assembly and a reversible motor turns the lead screw in first and second directions to move the first and second ends toward and away from each other to move the closure between the open and closed positions. A sensor assembly includes a worm fixed to the lead screw for rotation therewith and a gear meshingly engaged with the worm. The worm and gear are geared so that the gear rotates not more than one revolution in response to the closure moving between the open and closed positions. 
         [0010]    According to another aspect of the invention, a powered closure drive mechanism is provided for moving a rear liftgate on a motor vehicle between an open position pivoted away from the vehicle and a closed position adjacent the vehicle. The drive mechanism or electro-mechanical strut assembly includes an elongated strut housing extending between the liftgate and the vehicle. A rotatable lead screw is disposed longitudinally within the strut housing and a reversible motor turns the lead screw in a first direction and a second direction to move first and second ends of the strut housing toward and away from each other to move the liftgate between the open and closed positions. A sensor assembly is provided to determine a position of the liftgate between the open and closed positions. The sensor assembly includes a worm fixed to the lead screw for rotation therewith, a gear meshingly engaged with the worm, a two-pole magnet, and a sensor. The worm and gear are geared so that the gear rotates not more than one revolution in response to the liftgate moving between the open and closed positions. The magnet has a magnetic field and is mounted to the gear for rotation therewith. The sensor senses the magnetic field and generates an output signal to determine a rotational position of the magnet which corresponds to the position of the liftgate. 
         [0011]    According to yet another aspect of the invention, an absolute position encoder is provided for determining a position of a rear liftgate on a motor vehicle that is movable between an open position and a closed position by a strut assembly. The encoder includes a two-pole magnet and a sensor. The magnet has a magnetic field and is operatively coupled to the strut assembly to rotate not more than one revolution in response to the liftgate moving between the open position and the closed position. The sensor is adapted to be mounted to the strut assembly and senses the magnetic field of the magnet. The sensor outputs a signal in response to sensing the magnetic field to determine a rotational position of the magnet which corresponds to the position of the liftgate between the open and closed positions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Advantages of the 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, wherein: 
           [0013]      FIG. 1  is a perspective view of a motor vehicle with a rear liftgate in an open position; 
           [0014]      FIG. 2  is a side view of the motor vehicle with the rear liftgate in a closed position; 
           [0015]      FIG. 3  is a cross-sectional view of a liftgate strut assembly according to the invention; 
           [0016]      FIG. 4  is a fragmentary, enlarged cross-sectional view of the liftgate strut assembly; 
           [0017]      FIG. 5  is a partially cut-away perspective view of a sensor assembly; and 
           [0018]      FIG. 6  is a cross-sectional end view of the sensor assembly. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0019]    The present invention will be described below particularly with respect to its application in rear liftgates of motor vehicles. Those skilled in the art will, however, realize that the present invention may be applied to other types of vehicle closures and also to closures that are not mounted on vehicles. For example, the present invention may find application in trunk lids for motor vehicles, panels covers for light trucks, train doors, bus doors, and household closures such as windows and doors. In addition, it is contemplated that the present invention has utility for other automotive applications such as steering wheel position sensing, gas pedal position sensing, transmission gearbox encoder, headlight position control, and power seat position sensing. 
         [0020]    Referring now more particularly to the drawings, there is shown in  FIG. 1  a motor vehicle, generally shown at  10 , with a vehicle body or frame  12  which defines an opening  14  at a rear end thereof. A liftgate or door  16  (or more generally referred to as a “closure”) is adapted to fit within the opening  14 . The weight of the closure  16  biases it towards a closed position within the opening  14 , as shown in  FIG. 1 . 
         [0021]    A hinge assembly  18  is connected between an upper portion  20  of the vehicle body  12  and an upper portion  22  of the closure  16 , pivotally mounting the closure  16  to the vehicle body  12 . The hinge assembly  18  provides a generally horizontally extending axis  24  for pivotal movement of the closure  16  between the closed position, adjacent the vehicle body  12 , as shown in  FIG. 2 , and an open position, pivoted away from the vehicle body  12  such that the cargo area of the vehicle  10  is accessible, as shown in  FIG. 1 . 
         [0022]    A latch assembly  26  having cooperating parts mounted on the closure  16  and the vehicle body  12  is also shown in  FIG. 1 . The latch assembly  26  is provided for releasably locking the closure  16  in the closed position. The latch assembly  26  includes a latch  28  disposed within a lower portion  30  of the closure  16  and a complimentary latch striker  32  disposed within a lower portion  34  of the vehicle body  12 . 
         [0023]    A powered closure drive mechanism, generally shown at  36 , is provided for opening and closing the closure  16 . More particularly, the powered closure drive mechanism  36  is disclosed as a pair of electro-mechanical strut assemblies  38 . Each strut assembly  38  extends between a first end  40  and a second end  42 , the first  40  and second  42  ends being movable in opposite directions toward and away from each other. In the illustrated embodiment, one strut assembly  38  is mounted on each side of the vehicle  10 , extending between the closure  16  and the vehicle body  12 . It is appreciated by one of skill in the art that a single strut assembly  38  connected between the closure  16  and the vehicle body  12  will provide the necessary function of opening and closing the closure  16 . The first end  40  of the strut assembly  38  is operatively coupled to the vehicle body  12 , adjacent the upper portion  20  thereof. The second end  42  of the strut assembly  38  is pivotally coupled to an edge  44  of the closure  16 , between the upper  22  and lower  30  portions thereof. 
         [0024]    One strut assembly  38  is shown in detail in  FIG. 3 . The strut assembly  38  includes a housing  46  enclosing the various components of the strut assembly  38 . Internally, a motor  48  is disposed toward the second end  42  of the strut assembly  38 . The motor  48  is electrically connected to an electric energy source (not shown). It is contemplated that the motor  48  operates using electric energy that is standard in a motor vehicle protocol. The motor  48  is bi-directional allowing for rotation of a drive shaft  50  in two directions. The drive shaft  50  extends axially within the strut assembly  38  and is operatively coupled to a gearbox  52 . The gearbox  52  is disposed adjacent the motor  48 . 
         [0025]    The gearbox  52  includes an output shaft  54  that is driven by the drive shaft  50  of the motor  48  and extends coaxially therewith. The output shaft  54  of the gearbox  52  is operatively coupled to a lead screw  56  by a clutch assembly  58 , disposed adjacent the gearbox  52 . The clutch assembly  58  rotates the lead screw  56  in response to a rotational input from the output shaft  54  of the gearbox  52 . The clutch assembly  58  is an overload-type clutch in that it slips at a predetermined torque, but not below the predetermined torque. The clutch assembly  58  allows selective manual movement of the closure  16  between the open and closed positions. 
         [0026]    The lead screw  56  extends coaxially with the output shaft  54  of the gearbox  52  between a first end  60  disposed within the clutch assembly  58  and a second end  62  disposed at the first end  40  of the strut assembly  38 . A first portion  66  of the lead screw  56  adjacent the first end  60  is unthreaded while a remaining second portion  68  is threaded. A support nut  70  threadingly engages the threaded second portion  68  of the lead screw  56 . As the lead screw  56  rotates, the support nut  70  is driven linearly along the lead screw  56  in either a first direction or a second direction depending on the direction of rotation of the lead screw  56 . Linear travel of the support nut  70  along the lead screw  56  causes the first end  40  of the strut assembly  38  to move towards and away from the second end  42 , thereby causing the closure  16  to pivot between the open and closed positions. In one embodiment of the invention, the lead screw  56  is rotated approximately ten (10) revolutions to drive the support nut  70  between a first location, which corresponds to the closure  16  being in the closed position, and a second location, which corresponds to the closure  16  being in the open position. 
         [0027]    It is necessary to monitor exactly where the closure  16  is within its range of travel between the open and closed positions. To accomplish this, the strut assembly  38  also includes a sensor assembly  72  disposed between the clutch assembly  58  and the support nut  70 . The lead screw  56  extends through the sensor assembly  72 . The sensor assembly  72  includes a sensor housing  74  that defines an internal compartment  76 . A worm  78  and gear  80  are disposed within the internal compartment  76  and oriented generally orthogonal to each other. The unthreaded first portion  66  of the lead screw  56  extends axially through the worm  78  and the worm  78  is keyed or fixed to the lead screw  56  such that it rotates therewith. The gear  80  is mounted in meshing engagement with the worm  78  such that rotation of the worm  78  causes the gear  80  to rotate. The gear ratio between the worm  78  and the gear  80  is approximately 10:1 such that the gear  80  rotates not more than one (1) revolution for every ten (10) revolutions of the worm  78 , which corresponds to full travel of the closure  16  between the open and closed positions or alternatively between the closed and open positions. 
         [0028]    A diametrically charged or two-pole magnet  82  is generally disc-shaped and is fixedly secured to a distal end of the gear  80  and rotates therewith. Therefore, the magnet  82  rotates not more than one (1) revolution for full travel of the closure  16 . The magnet  82  has a north pole and a south pole which create a magnetic field. A board  84  with a chip  86  mounted thereon is fixedly secured to the sensor housing  74  adjacent the magnet  82 . The chip  86  includes at least one sensor mounted therein for sensing the magnetic field of the magnet  82  in order to resolve its rotational position. The chip  86  then outputs the rotational position of the magnet  82  to a controller  88  located within the vehicle. The controller  88  is electrically connected to the chip  86  and to the motor  48 . The chip  86  may output the rotational position of the magnet  82  in any number of suitable ways. For example, the chip  86  may output a linear analog signal that is proportional to position wherein approximately zero volts corresponds to the closed position of the closure  16  and approximately five volts corresponds to the open position of the closure  16 . One benefit of this powered closure drive mechanism  36  is that the chip  86  can always determine the absolute rotational position of the magnet  82  based on its magnetic field, even after a power disconnect during which the closure  16  is manually moved to a new position. The chip  86  is any suitable chip for sensing the magnetic field of the magnet  82  and outputting the rotational position of the magnet  82 , for example, the AS5040-10 bit Programmable Rotary Encoder manufactured by Austria Micro Systems AC. 
         [0029]    To initially calibrate the chip  86 , the magnet  82  needs to be set to a predetermined position relative to the lead screw  56 , so that approximately zero volts will correspond to the closed position of the closure  16  and approximately five volts will correspond to the open position of the closure  16 . Alternatively, the system can be assembled without paying attention to the alignment of the magnet  82  relative to the lead screw  56 . In this situation, with the closure  16  in the closed position the zero position is programmed into the chip  86 . 
         [0030]    It is appreciated that the lead screw threads can be selected such that any number of revolutions of the lead screw  56  is required to drive the support nut  70  between the first and second locations without varying from the scope of the invention. However, in order to accommodate a different number of revolutions of the lead screw  56 , the worm  78  and gear  80  must be selected such that the magnet  82  rotates not more than one (1) revolution for full travel of the closure  16 . 
         [0031]    The invention has been described in an illustrative manner. It is to be understood that the terminology, which has been used, is intended to be in the nature of words of description rather than of limitation. Many modifications and variations of the invention are possible in light of the above teachings. Therefore, within the scope of the appended claims, the invention may be practiced other than as specifically described.

Summary:
A powered closure drive mechanism is provided for moving a closure between open and closed positions. An elongated housing extends between first and second ends that are movable in opposite directions toward and away from each other. A rotatable lead screw is disposed longitudinally within the elongated housing. A reversible motor rotates the lead screw in a first direction and a second direction to urge the first and second ends of the housing toward and away from each other. A sensor assembly includes a worm fixed to the lead screw for rotation therewith and a rotatable gear meshingly engaged with the worm. The worm and gear are geared such that the gear rotates less than one revolution in response to the closure moving between the open and closed positions.