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BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to the field of power window systems, and more particularly to devices and methods for maintaining a constant rate of speed at which an electric or other motor raises and lowers an automobile window. 
     2. Description of the Related Art 
     In the field of luxury automobiles, a customer&#39;s perception of quality is largely based on an automobile&#39;s level of refinement. One aspect of refinement centers on whether the operating speed of an automobile&#39;s power windows is smooth and consistent when the windows are raised and lowered. A problem common to all automobile power window systems is that the load on a window motor changes depending on whether the motor is pushing the window up or pulling it down. The force of gravity causes the load on the motor to be greater during upward travel than it is during downward travel. If this load inequality is not compensated for, the window will descend at a faster rate than it is able to ascend. This change in speed is much more noticeable with large, heavy pieces of glass such as those used in luxury vehicles. 
     A DC brush motor, which is commonly used to raise and lower automobile windows, converts electrical energy to mechanical energy by creating a magnetic field that pushes or pulls against permanent magnets on the motor case. The force on a current carrying conductor in a magnetic field is the product of the magnetic field strength and the current in the conductor. This relationship is described by the equation F=iL×B, where F is force, i is the magnitude of the current in the conductor, L is the direction current is traveling in the conductor, and B is magnetic field strength. The standard solution for overcoming the problem of unequal rates of power window ascent and descent has been to use pulse width modulation (PWM) to control the amount of current in a window motor&#39;s armature windings. By using PWM to vary the amount of voltage applied to a power window motor, the current flowing through the conductor may be varied and the speed-torque curve of the motor can be shifted up or down. Thus, by increasing the voltage applied to the motor during the upward travel of the window, and decreasing the voltage during the downward travel, the window&#39;s rates of ascent and descent can be matched. 
     It is desirable to have an alternative, mechanical solution to the problem of window motor load variation that does not require the additional electrical components necessary for employing PWM. 
     BRIEF SUMMARY OF THE INVENTION 
     It is the purpose of this invention to disclose devices and methods for varying the amount of torque applied to raising and lowering a power window in an automobile for equalizing that window&#39;s rates of ascent and descent without the use of additional electrical components. The invention is suitable for any window that has a vertical component of movement, thereby being affected by gravity in one direction but not in the opposite direction. 
     In accordance with the present invention, there is provided a power window assembly having a motor drivingly connected to a worm gear. The motor contains an armature that is rotatably driven by a magnetic field created by permanent magnets on the motor case. The worm of the worm gear is preferably a rigid, axial extension of the armature that communicates the rotational motion of the armature to the gear, which in turn drives the window up and down through conventional mechanisms. 
     In one embodiment of the invention, the armature is axially shifted in and out of the magnetic field in order to adjust the torque output of the motor. By shifting the armature partially out of alignment with the magnetic field, fewer of the armature&#39;s windings are exposed to the magnetic field, and the amount of torque generated by the motor drops accordingly. Axial displacement of the armature and the worm, which is preferably rigidly attached to the armature, is achieved by application of axial thrust forces to the armature and the worm that are generated by the gearing relationship between the worm and the gear. 
     When the window is being lowered, gravity assists the motor in pulling the window down, thereby obviating the need for maximum torque output. Therefore, the armature is shifted partially out of alignment with the magnetic field during downward travel. Conversely, when the window is being raised, and the load on the motor is increased relative to when the window is being lowered, due to the force of gravity resisting upward movement of the window, the armature preferably shifts into full alignment with the magnetic field to increase torque production. By varying the degree to which the armature is displaced when the window is lowered, a motor can be tailored to a particular window so that the window is raised and lowered at similar speeds. 
     In another embodiment of the invention, the torque provided for raising and lowering the window is varied by using a gear having an asymmetrical tooth profile. By forming the gear teeth with one side of each tooth having a greater pitch than the opposite side of the tooth, the efficiency of the gear varies depending on whether it is rotating clockwise or counterclockwise. 
     In order to compensate for the force of gravity, the gear is preferably oriented within the power window assembly to transmit torque more efficiently when rotating to raise the window and less efficiently when rotating to lower the window. By varying the tooth profile, a gear can be tailored to a particular window in order that the window ascends and descends at similar speeds. 
     In yet another embodiment of the invention, the torque provided for raising and lowering the window is varied by the incorporation of a worm having an asymmetrical thread profile. By forming the threads of the worm with one side of each thread having a greater pitch than the opposite side of the thread, the efficiency of the worm varies depending on whether it is rotating clockwise or counterclockwise. Similar to the asymmetrical tooth embodiment described above, the thread profile can be tailored to a particular window in order that the window ascends and descends at similar speeds. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1   a  is a schematic side view illustrating an embodiment of the present invention when operating to raise a window. 
         FIG. 1   b  is a schematic side view illustrating the embodiment of the present invention shown in  FIG. 1   a  when operating to lower a window. 
         FIG. 2   a  is a schematic side view illustrating an alternative embodiment of the present invention when operating to raise a window. 
         FIG. 2   b  is a schematic side view illustrating the alternative embodiment of the present invention shown in  FIG. 2   a  when operating to lower a window. 
         FIG. 3   a  is an exploded side view illustrating the engagement between the gear and the worm of the alternative embodiment of the present invention shown in  FIG. 2   a  when operating to raise a window. 
         FIG. 3   b  is an exploded side view illustrating the engagement between the gear and the worm of the alternative embodiment of the present invention shown in  FIG. 2   b  when operating to lower a window. 
         FIG. 4   a  is an exploded side view illustrating the engagement between the gear and the worm of an alternative embodiment of the present invention when operating to raise a window. 
         FIG. 4   b  is an exploded side view illustrating the engagement between the gear and the worm of an alternative embodiment of the present invention when operating to lower a window. 
     
    
    
     In describing the preferred embodiment of the invention which is illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific term so selected and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. For example, the word connected or terms similar thereto are often used. They are not limited to direct connection, but include connection through other elements where such connection is recognized as being equivalent by those skilled in the art. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiment of the invention shown in  FIGS. 1   a  and  1   b  is incorporated into an automobile electric window motor assembly, which is preferably a dc brush motor containing an armature  2  that rotates within a magnetic field created by permanent magnets  4  on the motor case. The armature  2  is rigidly connected to the worm  6  of a worm gear  8 . Because of the gearing relationship between the armature  2 , the worm  6  and the cooperating gear  10 , this type of assembly generates an axial thrust force (along the axis of the arrow of  FIG. 1   a ) on the worm  6  and the armature  2 , causing them to push against the motor case when rotating in one direction, and to push against the gear housing when rotating in the opposite direction, as shown in  FIG. 1   b.    
     The critical feature of the embodiment of the invention shown in  FIGS. 1   a  and  1   b  is that the armature  2  and the worm  6  are free to travel a short distance back and forth along a line defined by their shared axis. This can result from a gap being formed between the components of the motor, or a spring structure permitting such axial movement. By allowing the armature  2  and the worm  6  to shift several millimeters (in one example) along that line, the armature windings  12  can be displaced relative to the permanent magnets  4 , thereby altering the alignment between the armature windings  12  and the magnetic field of the permanent magnets  4 . This change in alignment results in a change in torque, as described below. 
     When a car&#39;s power window is being lowered, the amount of torque produced by the motor must be less than when it is being raised in order for the window&#39;s rates of ascent and descent to match. Referring to  FIG. 1   b , a reduction in torque is achieved by allowing the axial thrust force discussed above to move the armature  2 , and, thereby, shift the armature windings  12  partially outside of the magnetic field when the window is being lowered as shown by the arrow in  FIG. 1   b . The amount of shift is exaggerated in  FIG. 1   b  for illustrative purposes. This reduction in alignment between the windings  12  and the field increases the reluctance in the motor&#39;s magnetic circuit, resulting in decreased torque production and less force pulling the window down than the motor would generate with the windings  12  fully aligned with the magnetic field (as in  FIG. 1   a ). By decreasing the force pulling the window down to the desired degree, the rate of window descent can be matched to the rate of ascent. 
     Referring back to  FIG. 1   a , when the motor is driven in the “up” direction, the armature  2  is thrust in the opposite axial direction, thereby shifting the armature windings  12  into full alignment with the magnetic field. Greater exposure of the windings  12  to the field results in an increase in the motor&#39;s torque output. The result is that the window is more forcefully pushed upwardly in order to overcome the downward force of gravity. By varying the degree to which the armature  2  is displaced relative to the permanent magnets  4 , a motor can be tailored to suit a specific load profile. Generally, for the same motor, the heavier a particular window is, the greater the degree of armature  2  displacement necessary to match the rates of descent and ascent for that window. 
     In an alternative embodiment of the invention, the amount of torque applied to raising and lowering a window is varied by modifying the gearing arrangement in the power window assembly. Referring to  FIGS. 2   a - 3   b , the profile of each tooth  20  of the gear  22  is asymmetrical in the manner of a conventional asymmetrical gear. That is, the pitch of one side  24  of each tooth  20  is greater with respect to the gear wheel  26  than the pitch of the opposite side  28  of each tooth  20 . The horizontal and vertical components of the force imparted to the gear  22  by the worm  30  thus vary depending upon the direction in which the worm  30  is driving the gear  22 . This is best illustrated in the exploded views of the threads  32  of the worm  30  and the teeth  20  of the gear  22  shown in  FIGS. 3   a  and  3   b , wherein the lengths of the vectors V and H represent the relative magnitudes of the vertical and horizontal forces imparted by the threads  32  to the teeth  20 . Horizontal and vertical refer to the orientation in the figures as shown. 
     In  FIGS. 2   a  and  3   a , the gear  22  and the worm  30  are rotating in the clockwise direction, when viewed from the front and the right end, respectively, and the worm  30  is engaging the sides  28  of teeth  20  that have the lesser pitch. The horizontal component of the force imparted to the gear  22  is thus greater than the vertical component. In  FIGS. 2   b  and  3   b , the worm  30  and the gear  22  are rotating in the counterclockwise direction, when viewed from the front and the right end, respectively, and the worm  30  is engaging the sides  24  of teeth  20  that have the greater pitch. The horizontal component of the force imparted to the gear is thus lesser than the vertical component. 
     The result of this disparity in pitch is that the efficiency of the gear  22  is greater when the worm  30  turns clockwise (as shown in  FIGS. 2   a  and  3   a ) as compared to when it turns counterclockwise (as shown in  FIGS. 2   b  and  3   b ). This approach does not change the fundamental torque production of the armature (as in the embodiment of the invention described above), but changes the efficiency of torque transferred through the gearing. 
     Although a dc brush motor has been described and shown as the driving means for the gear  22 , those skilled in the art will appreciate that all other suitable driving means, such as various other rotary motors, can alternatively be used to drive the asymmetrical gear  22  while still achieving the same directionally-dependent torque efficiency described. 
     Referring to the exploded views of the gear  40  and the worm  42  shown in  FIGS. 4   a  and  4   b , it is contemplated that the teeth  44  of the gear  40  may alternatively be symmetrical (i.e., having the same pitch on both sides), and that the threads  46  of the worm  42  may instead be asymmetrical, thus achieving the same directionally-dependant efficiency relationship described above. In another alternative, an embodiment of the worm gear is contemplated wherein both the gear teeth and the threads of the worm are asymmetrical (not pictured). 
     Referring back to  FIGS. 2   a  and  2   b , the gear  22  is oriented deliberately within the power window assembly in order that the worm  30  engages the sides  28  of the gear teeth  20  having a lesser pitch when the worm  30  rotates to raise the window, and engages the sides  24  of the teeth  20  having a greater pitch when the worm  30  rotates to lower the window. Therefore, the gear  22  will receive torque from its cooperating worm  30  more efficiently when the window is being raised and less efficiently when it is being lowered. The additional torque efficiency provided during upward travel of the window compensates for the additional load placed on the motor due to gravity. By tailoring the gear tooth profile to a specific window, the gear  22  is able to transfer the desired torque to raise and lower the window at the same rate despite the different motor load. Generally, the heavier the window, the greater the disparity in pitch that will be necessary. 
     It is contemplated that the embodiments of the invention described above may be combined to form alternative, hybrid embodiments of the invention that incorporate features that have heretofore been discussed only in separate embodiments. For example, an embodiment is contemplated that incorporates variable armature alignment as well as an asymmetrical gear tooth profile. Alternatively, the window motor assembly can incorporate variable armature alignment as well as an asymmetrical thread profile on the worm. 
     This detailed description in connection with the drawings is intended principally as a description of the presently preferred embodiments of the invention, and is not intended to represent the only form in which the present invention may be constructed or utilized. The description sets forth the designs, functions, means, and methods of implementing the invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and features may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention and that various modifications may be adopted without departing from the invention or scope of the following claims.

Summary:
A device and method for matching the rates of speed at which an electric motor that is drivingly connected to a worm gear raises and lowers a window in an automobile power window assembly. By axially displacing the motor&#39;s armature, and/or by varying the thread and tooth profiles of the worm and the gear, the amount of torque produced by the motor and transmitted through the worm gear can be altered. In order to compensate for the effect of gravity on the motor load and on the window&#39;s speed of ascent and descent, more torque is provided when the window is being raised and less torque is provided when the window is being lowered.