Abstract:
The invention concerns a brush system for a commutated DC motor. As the brush wears, different cross-sectional shapes, at different positions, come into contact with the commutator. The center of contact for each cross-sectional shape can be different, thereby changing brush angle as wear occurs. The change in brush angle can be desirable, in order to offset other effects which occur as a result of wear. For example, motor speed can change as a result of brush wear. Changing brush angle can offset the change in speed.

Description:
RELATED APPLICATION 
     This application is a continuation of Ser. No. 08/964,780 filed Nov. 5, 1997, which is a continuation-in-part of Ser. No. 08/598,379 filed Feb. 8, 1996, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention concerns brushes for commutated DC motors in which the contact between brush and commutator can be programmed to change position as the brush wears. 
     2. Description of Related Art 
     FIG. 1 illustrates a generalized commutator  2  in a simple DC machine. Commutator contact  3  (shown hatched) and contact  6  (not hatched) connect to leads  9  and  12  of a coil  15 , respectively. Brushes  18  (of positive polarity, as indicated) and  21  (of negative polarity) deliver current to the contacts, and cause current  24  to flow in the coil  15 . 
     When the commutator  2  is in the position shown in FIG. 1A, the current  24  flows in the direction shown, with respect to reference dot  27 , which is considered fixed to the coil  15 . When the commutator  2  rotates to the position shown in FIG. 1B, the current  24  reverses in direction, with respect to the dot  27  (although, of course, the current still flows from the positive brush  18  to the negative brush  21 ). 
     Therefore, rotation of the commutator  2  causes current within the coil  15  to reverse direction. This reversal causes reversal of the magnetic field lines  30  with respect to the coil  15 , as indicated, which are generated by the current  24 . Even though the magnetic field lines  30  point leftward in both FIGS. 1A and 1B, it should be remembered that, in FIG. 1B, the coil  15  is inverted, with respect to FIG.  1 A. Consequently, the magnetic field lines  30  have become reversed, in FIG. 1B, with respect to the coil  15 . 
     The magnetic field lines interact with a stationary magnetic field  30 S, produced by a stator (not shown). Since the two magnetic fields  30  and  30 S want to align with each other, they urge the coil to rotate, in order to allow the alignment. However, since the field lines  30  associated with the coil  15  continually reverse in direction, the coil  15  continually rotates in pursuit of this alignment. (If a non-changing, DC current flowed in the coil  15 , the coil would stop rotating once the fields became aligned.) 
     FIG. 2 is an enlarged view of brush  18  of FIG.  1 . As the brush  18  wears, and material is removed by the wear, a spring (not shown) causes the brush  18  to advance in the direction of arrow  19 . The brush  18  advances along a reference line  33 , drawn exactly at the 12 o&#39;clock position. During this advancement, the center  36  of the region of contact remains fixed on reference line  33 , as indicated in FIGS. 2B and 2C. 
     In a DC motor generally, changing the position of point  36  changes the speed, or torque, or both, produced by the motor. It can be desirable to change the position of point  36  during the lifetime of the motor, for various purposes. 
     One purpose is to compensate for changes in speed which are caused by wear. For example, when the brush configuration changes from that of FIG. 2A to FIG.  2 B, the area of contact becomes larger, and the average time during which brush  18  shorts two or more adjacent commutator segments, and therefore two or more armature coils, increases. This change in contact area can change motor speed. It may be desirable to move point  36 , in an attempt to counteract the change in motor speed. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an improved brush for a motor. 
     A further object of the invention is to provide a brush for a motor in which the center of contact changes in a predictable and desirable manner, as the brush wears. 
     A further object of the invention is to provide a method of designing a brush for a motor. 
     In one form of the invention, a motor brush is configured such that, as wear occurs, the contact region between the brush and commutator moves circumferentially along the commutator. 
     In another form of the invention, this invention comprises an electric motor having a commutator, an improvement consisting of a brush in contact with the commutator and means for changing contact angle of the brush, in response to brush wear. 
     In still another form of the invention, this invention comprises. 
     An electric motor consisting of a commutator and brush means for contacting the commutator at a region which moves circumferentially along the commutator, as the brush wears. 
     In yet another form of the invention, the invention comprises a brush system for an electric motor, consisting of a support for holding a brush in contact with a commutator and means for changing circumferential position of the contact, as the brush shortens due to wear. 
     These and other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
     FIGS. 1A and 1B illustrate a simple DC machine; 
     FIGS. 2A,  2 B, and  2 C illustrate wear-induced movement of brush  18  in FIG. 1A; 
     FIG. 3 illustrates one form of the invention; 
     FIGS. 4A and 4B illustrate circumferential movement of the center of contact  50 , as brush  40  wears; 
     FIGS. 5A-5C illustrate another form of the invention; 
     FIGS. 6A and 6B illustrate yet another form of the invention; 
     FIGS. 7A,  7 B, and  7 C illustrate still another form of the invention, showing initial clockwise motion of center point  92 , followed by counter-clockwise motion. 
     FIG. 8A illustrates a brush of arbitrary, generalized configuration; 
     FIG. 8B is a perspective view of the brush shown in FIG. 8A; 
     FIG. 9 illustrates shapes and relative locations of three cross-sections of the brush of FIG. 8; 
     FIGS. 10A,  10 B, and  10 C illustrate a sequence of steps in designing a brush; 
     FIG. 11 illustrates an automotive vehicle; 
     FIGS. 12A and 12B are exemplary plots illustrating how contact point angle should be programmed based on elapsed motor running time. 
     FIGS. 13A,  13 B and  13 C illustrate a sequence of events occurring in the wear of a brush; 
     FIGS. 14A,  14 B and  14 C illustrate a sequence of events occurring in the wear of a brush; 
     FIG. 15 illustrates a particular type of brush; and 
     FIG. 16 illustrates one form of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3 illustrates one form of the invention, in which a guide bar, or rail,  42  is affixed to brush  40 . The guide bar  42  slides in a slot  43  contained in a support  44  which is fixed in position, as indicated by ground symbol  46 . 
     As the brush  40  wears, a spring  47  causes it to be biased or advanced towards the commutator  2 , as indicated by FIGS. 4A and 4B. FIG. 4A shows brush position early in the wear cycle. Contact point  50  is located on a reference line  53 , which is drawn at the 12 o&#39;clock position. FIG. 4B shows brush position later in the wear cycle. The center point  50  of the contact region has advanced clockwise, as indicated by the angle A between lines  53  and  56 . The point  50  no longer lies on the 12 o&#39;clock line  53 , but has moved circumferentially, by angle A. 
     In motor terminology, the angle of the central point of contact  50  is commonly called a “brush angle,” or “contact angle.” FIG. 4B shows such an angle A, with reference to the 12 o&#39;clock position. 
     As an alternate to the embodiment of FIG. 3, the slot  43  can be fabricated in the brush  40 , while the guide bar  42  can be held by the support  44 . 
     FIGS. 5A-5C illustrate another embodiment of the invention, wherein brush  40 A is constrained within a guide  41 . As wear occurs, the central point  50  will first move clockwise, as in FIG. 5B, as indicated by line  56 . As wear proceeds, the central point then moves counter-clockwise, as indicated by line  56 A in FIG.  5 C. 
     FIGS. 6A-6B illustrate another embodiment, wherein a brush  60  in FIG. 6A is carried by a guide  63 . Initially, the center of contact  66  lies on reference line  53 . 
     As the brush  60  wears, a cylindrical groove  67  in FIG. 6B becomes worn into the contact surface  68 . The contact surface  68  in FIG. 6B becomes conformal to the cylindrical surface of the commutator  2  at the region of wear. As the groove  67  is formed, the center of contact  66  becomes displaced away from the reference line  53 , as indicated by line  71 . 
     One reason for the displacement is that the brush  60  is constrained, by guide  63 , to move along axial line  73  in FIG.  6 A. This axial line  73  does not coincide with the reference line  53 , contrary to the case of FIG. 2A, wherein a corresponding axial line  75  does coincide with reference line  53 . (In FIG. 2A, the axial line  75  is shown slightly removed from the reference line  33 , in order to make both lines visible.) Also, the axial line  73  in FIG. 6A coincides with a chord of the commutator, which runs between points A and B. 
     Because the brush  60  moves along axis  73 , which coincides with a chord, the brush has a radial component of motion, as well as a circumferential component. 
     From another point of view, because of the constraint on movement of the brush in FIG. 6B, as the cylindrical groove  67  becomes created, its endpoints  80  and  83  do not maintain symmetry about reference line  53 . Consequently, the center of contact  66 , which lies mid-way between these points, will eventually move away from the reference line  53 . 
     FIG. 7 illustrates another embodiment of the invention, in which two types of motion of point  92  occur, namely, clockwise and counter-clockwise. Initially, the brush  90  is positioned as shown in FIG.  7 A. Contact point  92  lies on reference line  53 . A time later, after some wear occurs, the brush advances to the position shown in FIG.  7 B. Now, central point  92  has advanced to the right of the reference line  33 , and lies on line  94 . Yet later, after additional wear occurs, the central point  92  has retreated to the position shown in FIG. 7C, and lies on line  96 . Central point  92  has first moved clockwise, and then counterclockwise. 
     FIGS. 5A-5C and  7 A- 7 C illustrate a general feature of one form of the invention, namely, that the brush can be designed with different cross-sections, at different axial locations. As wear occurs, the different cross-sections become successively exposed, and change the location of center point  92  in FIGS. 7A-7C. The concept of using different cross-sections will be elaborated, with reference to FIGS. 8A-8B and  9 . 
     FIG. 8A is a cross sectional view of a brush  100  having the preselected configuration shown, and FIG. 8B illustrates the brush  100  in perspective view. 
     In FIG. 8A, an axis  103  is defined within the brush  100 . The brush is constrained to move along this axis, as by confining it within a guide such as guide  63  as in FIG.  6 A. Different stations  110 - 117  in FIG. 8A are defined along the axis  103 . For the cross-section at each station, the shape, size, and location with respect to the axis  103 , is designed to provide the proper region of contact with the commutator. 
     For example, FIG. 9 illustrates three cross-sections, shown on the right of the figure, as they would appear if viewed along arrow  120 , as indicated by eye  123 . Also, axis  103 , on the left, is indicated on the right as axial planes  103 A, each associated with one of the three cross-sections. 
     The cross-section at station  111 , shown at the right, is rectangular, and is displaced from the axial plane  103 A. The cross-section at station  112  is again rectangular, but larger in size, and intersects the axial plane  103 A. The cross-section at station  116  is divided into two parts, because hole  125  intersects this station. The axial plane  103 A intersects the hole, as indicated. 
     Therefore, in general, the invention contemplates a brush having different cross-sections at different axial stations. For example, notice the serpentine or “zig-zag” shape in FIGS. 7A-7C. As the brush wears, different cross-sections become exposed in sequence. The different cross-sections produce different central contact points, such as point  92  in FIGS. 7A-7C. 
     One method for design of the brush includes the following steps. First, the shape needed for each cross-section is determined, as well as the location with respect to axis  103  in FIG.  9 . As an example of the shapes, FIG. 10A illustrates three hypothetical cross-sections C 1 , C 2 , and C 3 . FIG. 10B illustrates an example of location-with-respect-to-axis- 103 : cross-section C 2  is displaced from axis  103  by distance D 2 , as indicated on the right. Other cross-sections have the positions shown, with respect to the axis  103 . 
     The shape and location of each cross-section determine the center of contact generated by that cross-section. Point  92  in FIG. 7A illustrates one such center. Of course, the center of contact, in strict terms, is also determined by the cylindrical arc which is cut into the cross-section by the commutator. One example of such an arc lies in arc  67  in FIG.  6 A. 
     Determination of the precise center of contact involves a straightforward geometric computation, based on the relevant parameters of the cross-section and the cylindrical arc. For example, for the cross-section at station  112  in FIG. 8A, one conceptually moves the cross section into contact with the commutator  2 , and conceptually cuts an arc into the cross-section. The center of the arc represents the center of contact. 
     As the next step in the design process, the rate of wear of the brush is determined, which, in effect, determines distances D 1  and D 2  in FIG.  10 A. That is, the geometric distances D 1  and D 2  are, in effect, also equivalent to durations of time, because, as time progresses, the brush wears down. 
     Finally, the outer edges of the cross-sections are connected, as indicated by the dashed lines D in FIG. 10C, producing the overall shape of the brush. 
     A brush for a motor has been described, which contacts a commutator, and in which radial motion, toward the center of the commutator, is induced by abrasion against the commutator. The wear removes material from the end of the brush. 
     As the material is removed, different cross-sections become exposed. The cross-sections can be different in shape, size, position, or any combination of these three characteristics. The different cross-sections will cause the center, or centroid, of electrical contact between the brush and the commutator to change. 
     FIG. 11 illustrates an automobile  200 . Such automobiles contain motor-driven devices, such as windshield wipers, electrically driven windows, electrically adjustable seats, electrically collapsible roofs in convertibles, and the like. 
     These devices typically contain (1) an electric, brush-type, DC motor, (2) a drive train driven by the motor, such as the wiper linkage  205  in FIG. 11, or a gear train (not shown), and (3) the driven device, such as a windshield wiper, or seat. During the lifetime of the drive train, wear occurs in various components, causing lash or “play” of the drive train to increase. 
     This increase in play causes several problems. One is vibration. For example, if a bushing, within which a shaft rotates, becomes worn and enlarged, then the shaft is no longer securely captured by the bushing, and can vibrate. In general, an increase in vibration in any machine is not desirable. 
     Another problem is an undesired increase in component velocities. For example, assume that a lever within a linkage pivots about its center. If wear causes the pivot point to move away from the center, the none end of the lever may rotate faster, and the other end rotates slower. These changes in velocity are not desired. Further, the change in position of the pivot point will probably introduce eccentricity into the system, further increasing vibration and/or cause components (e.g., wiper arms) that are attached to the system to make undesirable contact with other vehicle components (e.g., the metal encasement “A-Pillar”) of the windshield. 
     To combat these problems, the Inventor proposes that the motor driving the train be programmed to decrease in speed as the drive train increases in age. FIG. 12A illustrated one type of programming. During the initial period of the motor&#39;s life, until the motor reaches 100 hours of age in this example, the speed is held constant at, 3,000 rpm in this example. In the middle period of the motor&#39;s life, between 100 and 200 hours of age in this example, the speed is progressively decreased to about 2,000 rpm. Then, in the final parts of the motor&#39;s life, after 200 hours in this example, motor speed is held constant at about 2,200 rpm. 
     The top part of FIG. 12 illustrates one approach to attaining these programmed changes in speed. The brush contact angle, discussed above, is held at zero degrees until 100 hours of life is reached. At that time, the contact angle progressively advances toward ten degrees, as lifetime progresses toward 200 hours. Then, at 200 hours, the contact angle is held constant. 
     FIG. 13 illustrates one apparatus for attaining this advancement of contact angle. In FIG. 13A, the face  215  of brush  220  is positioned so that the contact point  225  progressively moves toward the 12 o&#39;clock position  230 , as wear occurs, as indicated in the sequence of FIGS. 13A,  13 B and  13 C. 
     FIG. 14 illustrates another apparatus. Brush  250  contains an extension  255 . The contact point  260  progressively moves toward the 12 o&#39;clock position  230 , as wear occurs, as indicated in the sequence of FIGS. 14A,  14 B and  14 C. The extension  255  may be mechanically weak. FIG. 15 illustrates a non-conductive backing  270 , which is fastened to the brush  250 . As the wear of FIG. 14 occurs, the non-conductive backing  270  also wears, and makes contact with the rotor R, but has no effect on the position of the contact point  260 . 
     It should be appreciated that the contact point  260  shown in FIG. 14A lies outside the area CP defined by boundaries dictated by the guides  63   a  and  63   b.  As the brush  250  wears, the contact point  260  moves counter-clockwise (as shown in FIGS. 14A-14C) such that the point  260  moves within the area CP as best illustrated in FIG.  14 C. Conversely, it should be appreciated that a brush could be provided such that the contact point moves from within the area defined by the boundaries of the guides to outside the area. 
     FIG. 15 illustrates another approach to modulating motor speed, based on total elapsed running time of the motor. A vehicle  300  contains an electric motor  305  which drives a drive train  310 . A control  315  performs the functions indicated. 
     The control logs the total hours of the motor. Timers, known in the art, are available to log the total time of the motor. It is emphasized that the total running time of the motor will, in general, be different than the total running time of the vehicle itself. In principle, the brushed of FIGS. 13 and 14 provide an index as to the total running time: brush wear in correlated with total running time. 
     As control  315  further indicates, when motor speed lies between X and Y, speed is maintained such that speed=(K) (HOURS)=b. This equation is of the familiar form y=mx=b, wherein x and y are cartesian coordinates, m is the slope, and b is the y-intercept. FIG. 12A indicates that b equals about 3,750 rpm. The slope will equal (S 1 −S 2 )/(100−200), or about (3,000−2,200)/(−100), which equals negative 8. 
     Thus, for the example of FIG. 12A, when total elapsed time is between 100 and 200 hours, the control  315  in FIG. 16 maintains speed according to this equation: 
     
       
         speed−(−8)(hours)=3,750. 
       
     
     As a specific example, when total elapsed time is 150 hours, the equation just given indicates that speed will be held at (−8) (150)=3750, or 2,550 rpm. 
     In control  315 , when total time exceeds Y, which is 200 hours in FIG. 12A, speed is held at S 2 , which is 2,200 rpm in FIG.  12 A. As FIG. 16 indicates, the triplet of motor  305 , drive train  310 , and control  315  can occur multiple times within the vehicle. For example, one triplet can run windshield wipers, another can run an adjustable seat, and so on. The overall control  315  in some, or all, of these can comprise the brush system described above. 
     Numerous substitutions and modifications can be undertaken without departing from the true spirit and scope of the invention. What is desired to be secured by Letters Patent is the invention as defined in the following claims.