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This application claims benefit to U.S. Provisional Application No. 60/236,978, dated Sep. 29, 2000. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a liftgate assembly having force assist struts. More specifically, the invention relates to a liftgate assembly having adjustable force assist struts. 
     DESCRIPTION OF THE RELATED ART 
     Liftgates for motor vehicles require counterbalancing. The counterbalance allows the operator thereof to lift the liftgate with a minimal of effort. Further, the counterbalance prevents the liftgate from falling after the liftgate has been opened. This avoids injury to the operator as the liftgate will not fall on him or her. 
     Struts are usually used as the counterbalance for the liftgate. The struts are pneumatic cylinders typically filled with a gas material. A rod extends out from the pneumatic cylinder whereas the pressure created by the gas within the pneumatic cylinder provides a force assist for two purposes. 
     The first purpose is to aid the user in lifting the liftgate to its open position. The liftgate, including a large windowpane, is heavy and many users of the liftgate would be challenged to fully open the liftgate. The struts utilize the gas pressure to force the liftgate upwardly to assist the user in raising the liftgate. 
     The second purpose for using struts is for maintaining the liftgate in an open position without requiring a latch or support member that needs to be released when closing the liftgate. The struts allow the users to access the cargo area easily without much effort. 
     A liftgate is normally over-counterbalanced to auto open beyond a neutral force zone at the closure position. When a liftgate latch releases, the user urges the liftgate through the neutral zone until the counterbalance acts to swing the liftgate fully open. And in closing the liftgate, the user must first pull down and then change hand position to push in overcoming the counterbalance bias. 
     There are disadvantages to using the struts for providing force assist for the liftgate. In many instances, the liftgate is raised by the struts to a position that is unreachable to those users who are not able to reach up to the fully open liftgate. These users must either tie tethers to the liftgate or find objects to step up to reach the fully open liftgate. 
     Another disadvantage to the strut lift assist is that there is little regulation as to the fully open position. The finish of the liftgate may be damaged when the liftgate is opened in a low clearance area, e.g., under an open garage door. If opening the liftgate in a low clearance area is done routinely, adjustment to the fully open position may be desirable. 
     Yet another disadvantage associated with the current arrangement of using struts to assist in forcing the liftgate to an open position is that it is temperature dependent. Because the gas pressure in the strut obeys the characteristics of an ideal gas, the strut&#39;s force is significantly dependent on ambient temperature. As the ambient temperature rises, so too does the temperature of the gas within the pneumatic cylinder of the strut. This increases the force that the strut is able to generate resulting in a liftgate that rises quickly and is more difficult to close. Likewise, as ambient temperature decreases, so too does the force that the strut is able to produce. This reduction of force may result in little or no force assisting requiring the user to provide a force equal to the weight of the liftgate and windowpane to open the liftgate. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, there is provided a liftgate force control assembly that adjusts the force required to move a liftgate that is pivotally secured to a motor vehicle. The liftgate force control assembly includes a track that is fixedly secured to the motor vehicle. A follower is movably secured to the track. A strut defines a moving end and a secured end. The secured end is pivotally secured to the liftgate and the moving end is pivotally secured to the follower. The strut defines a moment with respect to the hinge that secures the liftgate to the motor vehicle. A motor is connected to the follower. The motor moves the follower along the track changing the moment of the strut such that the force required to move the liftgate changes as the moment changes. 
     According to another aspect of the invention, there is provided a vehicle having a liftgate mounted thereon by hinges. The liftgate is pivotally movable to open and close an opening in the vehicle. A pair of struts is operably connected between the liftgate and the vehicle to effect a lifting force on the liftgate. Each of the struts is pivotally mounted at one end to one of the liftgate and the vehicle and slidably mounted for reciprocating movement at an opposite end to the other of the liftgate and the vehicle. The reciprocating movement changes a magnitude of the lifting force being transferred to the liftgate. A drive motor operably engages the slidable end of each of the struts and operable to effect the reciprocating movement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
     FIG. 1 is a perspective view of a first embodiment of the invention shown in a motor vehicle, partially cut away; 
     FIG. 2 is a side view, partially cut away of the first embodiment of the invention secured to a liftgate and a motor vehicle, partially cut away; 
     FIG. 3 is a schematic side view of a liftgate force assist strut in fully open and fully closed positions; 
     FIG. 4 is a perspective view of a second embodiment of the invention shown in a motor vehicle, partially cut away; 
     FIG. 5 is a side view, partially cut away of the second embodiment of the invention secured to a liftgate and a motor vehicle, partially cut away; 
     FIG. 6 is an electrical schematic of an electronic control for one embodiment of the invention; and 
     FIG. 7 is a logic chart of one embodiment of an inventive method for operating the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a motor vehicle is shown at  10 . The vehicle  10  includes a liftgate  12 . The liftgate  12  is secured to motor vehicle  10  by two hinges  14 . The liftgate  12  is in a closed position in FIG. 1. A bottom portion  16  of the liftgate  12  is latched to the motor vehicle  10  adjacent a bumper  18  using a latch  19  known in the art. 
     A power assist assembly is generally indicated at  20 . The power assist assembly  20  includes two struts  22 ,  24 . Each of the struts  22 ,  24  has a movable end  26 ,  28  and a secured end  30 ,  32 , respectively, wherein the movable end  26 ,  28  is defined as the end that is connected to the movable liftgate  12  and the secured end  30 ,  32  is defined as the end connected to the motor vehicle  10 . Each strut  22 ,  24  includes a gas cylinder  34 ,  36  and a rod  38 ,  40  that telescopes within the gas cylinder  34 ,  36 . When the rod  38 ,  40  is retracted into the gas cylinder  34 ,  36 , the liftgate  12  is in the closed position. When the rod  38 ,  40  fully extends out of the gas cylinder  34 ,  36 , the liftgate  12  is in the open position, as a shown in phantom in FIG.  2 . 
     With specific reference to the second strut  24 , the movable end  28  and the secured end  32  are secured to the liftgate  12  and the motor vehicle  10  in standard fashion. More specifically, the movable end  28  is secured to the liftgate  12  without providing for lost motion therebetween. Likewise, the secured end  32  of the second strut  24  is secured to a D-pillar  39  of the motor vehicle  10  without providing for lost motion therebetween. The second strut  24  aids the operator in lifting the liftgate  12  by exerting of force thereon in an upward direction. 
     With reference to the first strut  22 , the movable end  26  is secured to a power assist assembly  20 . The secured end  30  is fixedly secured to the D-pillar  39  of the motor vehicle  10  in a standard or conventional fashion. More specifically, the secured end  30  is secured to the D-pillar  39  without providing for lost motion therebetween. 
     Referring to FIG. 3, the power assist assembly  20  is shown in two positions with respect to one of the two hinges  14 . The power assist assembly  20  includes a track  42 . The track  42  is fixedly secured to an inside surface  44  of the liftgate  12 . It should be appreciated by those skilled in the art that the track  42  may be secured to the motor vehicle  10  effectively reversing the orientation of the power assist assembly  20  without adding an inventive element to the invention. 
     The track  42  provides a guide for a follower  46 . The movable end  26  of the first strut  22  is connected to the follower or bracket  46 . A pin  48  extends through the follower  46  and allows the movable end  26  to pivot with respect thereto. 
     A motor or actuator  50  is secured to the track  42  at one end thereof. The motor  50  is bidirectional and rotates a drive screw  52 . The drive screw  52  is connected to the follower  46  through a drive nut (not shown). Therefore, when the motor  50  rotates the drive screw  52 , the drive nut moves the follower  46  along the track  42  adjusting the moment arm  47  of the power assist assembly  20 . The follower  46  moves back and forth along the track  42  depending on the direction of rotation of the drive screw  52 . 
     Returning to FIG. 2, an electronic control unit  54  is shown in phantom. The electronic control unit  54  receives three inputs and provides an output. Two of the three inputs received by the electronic control unit  54  are the output of a sensor (not shown) identifying the position of the follower  46  and the output of a sensor (not shown) identifying ambient temperature. The output  56  of the electronic control unit  54  is sent to the motor  50 . The output provides information to the motor  50  regarding the direction in which the motor  50  is to rotate and for how long. Depending on the temperature, the follower  46  will be moved along the track  42  to increase or decrease the moment arm  47  of the power assist assembly  20 . The change in the moment arm  47  is required as a function of temperature because the gases found in the two struts  22 ,  24  are affected by temperature. More specifically, the assistance provided by the two struts  22 ,  24  decreases as the ambient temperature decreases. Likewise, the assistance provided by the two struts  22 ,  24  increases as the ambient temperature increases. 
     The electronic control unit  54  receives a third input  58 . The third input  58  identifies the position of the liftgate  12 . The electronic control unit  54  measures the amount of time required for the liftgate  12  to move between positions. Depending on the temperature and the position of the follower  46  in the track  42 , the electronic control unit  54  measures the wear upon the struts  22 ,  24 . Given identical temperatures and follower position, if the liftgate  12  moves between two arbitrary positions quicker than what it had in the past, electronic control unit  54  could identify gases leaving the struts  22 ,  24  reducing the effective power to assist thereby. The electronic control unit  54  then moves the follower  46  adjusting the moment arm  47  of the power assist assembly  20  to compensate for the leaking gases that might reduce the efficiency of the struts  22 ,  24 . 
     Referring to FIGS. 4 and 5, wherein like primed numerals represent similar elements as those indicated in the first embodiment, a second embodiment  20 ′ is shown. The second embodiment of the power assist assembly  20 ′ differs from the first embodiment only in its orientation. More specifically, the power assist assembly  20 ′ is secured to a D-pillar  39  of the motor vehicle  10 ′ and not the liftgate  12 ′. In this embodiment, the movable end  26 ′ is secured to the D-pillar  39 ′ and the secured end  30 ′ is secured to the liftgate  12 ′. This embodiment provides for more movement of the position of the movable end  26 ′. Greater movement translates into more control over more situations. 
     Referring to FIG. 6, an electrical schematic of the invention  20  is generally indicated at  60 . The circuit  60  supplies power to the motor  50  to drive it in either direction, clockwise or counterclockwise. The direction of rotation for the motor is based on the positions of two switches  62 ,  64 . Both switches are single pole double throw switches  62 ,  64 . Each of the switches  62 ,  64  are connected to one end of the motor  50  with a resistor  66 ,  68  and a capacitor  70 ,  72  connected in parallel therebetween, respectively. One end of each of the switches  62 ,  64  is also connected to power, a resistor  74 ,  76  and a capacitor  78 ,  80 , which are, in turn, connected to a capacitor  82 ,  84 , respectively. Each of these elements is all connected to a single terminal  86 ,  88 . 
     Two transistors  90 ,  92  have their collector terminals connected to the terminals  86 ,  88 . The transistors  90 ,  92  receive a signal from two comparators  94 ,  96 . The comparators  94 ,  96  produce an output that drives the transistors  90 ,  92  to switch the switches  62 ,  64  to allow the motor  50  to drive in one direction or another. 
     Each of the comparators  94 ,  96  have a feedback resistor  98 ,  100 . The feedback resistors  98 ,  100  are connected between the output of the comparators  94 ,  96  and the non-inverted input of the comparators  94 ,  96 . The feedback resistors  98 ,  100  cause the motor  50  to slightly overshoot the target destination. This will avoid the nuisance of the constant adjustment of the liftgate force assist assembly  10 . 
     A potentiometer  102  is operated by the drive screw  52 . The potentiometer  102  adjusts the input to the non-inverting input of the first comparator  94  and the inverting input of the second comparator  96 . This provides an indication as to where on the drive screw  52  the follower  46  is. 
     A thermistor  104  is used as a portion of a voltage divider, generally shown at  106 , having a second resistor  108 . The thermistor  104  is the temperature sensor that senses the ambient temperature of air at the location of the liftgate force control assembly  12 . The voltage divider  106  is connected to the inverted input of the first comparator  94  and the non-inverted input of the second comparator  96 . The voltage divider  106  also includes a diode  110 . The diode  110  creates a null window between the first and second comparator reference points to provide a stable state for both comparators  94 ,  96  when they are in the off state. 
     Referring to FIG. 7, a method for operation is generally indicated at  112 . The method  112  starts with receiving a door ajar signal at  114 . The door ajar signal is typically initiated when the latch  19  is activated. The method  112  then identifies whether it is in touch mode at  116 . Touch mode is when the user of the motor vehicle  10  determines the level of force assist is desired. More specifically, the user may determine that little force assist is necessary. This not only reduces the moment of the strut  22  but also may determine how high the liftgate  12  will rise automatically. This will aid those that cannot reach the highest open position the liftgate  12  is capable of reaching. It will also aid those that frequently open their liftgate  12  in a closed environment, e.g., in a garage under an open garage door. 
     If the method is operating in the touch mode, it identifies in what position an indicator switch (not shown) is at  118 . It is then determined whether adjustment is required at  120 . If so, the motor  50  rotates the drive screw  52  to move the follower  46  at  122 . 
     Once the touch mode has been completed, it is determined whether the method should operate in a temperature mode at  124 . If not, the method is terminated. 
     If the method is to operate in temperature mode, the temperature is measured at  126 . Once measured, it is determined whether adjustment to the force is required at  128 . More specifically, it is determined whether the pressure within the strut  24  has changed due to a change in temperature. If so, the position of the strut  24  is modified to return the strut  24  to providing the same force assist as it would have when the strut  24  operated in the temperature that it last recorded when the door ajar signal was received last. 
     If adjustment is to be made, it is done so at  130 . To ensure continual adjustment due to fluctuations in temperature change does not occur, the feedback resistors  98 ,  100  allow the method  112  to overshoot the target temperature setting. Therefore, adjustment will not occur again until the temperature has changed to a degree that is represented by the last temperature reading plus an additional amount. The amount may be determined by a manufacturing setting or by a user of the motor vehicle  10 . 
     Once the adjustment has occurred, the method is returned at  132  for the next time the door ajar signal is received. As may be seen with FIG. 7, the method  112  can be separated into two halves, the touch mode, starting at decision diamond  116  and the temperature mode, starting at decision diamond  124 . These two halves operate independently of each other and, therefore, may be separated into separate embodiments for independent use. 
     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 liftgate force control assembly ( 20 ) adjusts the force required to move a liftgate ( 12 ) that is pivotally secured to a motor vehicle ( 10 ) using a hinge ( 14 ). The liftgate force control assembly includes a track ( 42 ) that is fixedly secured to the motor vehicle. A follower ( 46 ) is movably secured to the track. A strut ( 34, 36 ) has a movable end ( 26, 28 ) and a secured end ( 30, 32 ). The secured end ( 30, 32 ) is pivotally secured to the liftgate ( 12 ) and the movable end ( 26, 28 ) is pivotally secured to the follower ( 46 ). The strut ( 34, 36 ) defines a moment with respect to the hinge that secures the liftgate to the motor vehicle. A motor ( 50 ) is connected to the follower ( 46 ) to move the follower ( 46 ) along the track ( 42 ) changing the moment of the strut such that the force required to move the liftgate ( 12 ) changes as the moment changes.