Abstract:
An electric caliper having a housing mounted adjacent a rotatable disc. First and second force applying elements support a friction element between the housing and the rotatable disc. An electric actuator is mounted in the housing and is operatively connected to the first and second force applying elements. The electric actuator is operable to move the first and second force applying elements and the friction element toward, and away from, the disc and cause the friction element to frictionally contact the disc. A method is also provided for applying first and second forces against a friction element in response to an operation of an electric motor. The first and second forces are applied with respective magnitudes so as to maintain a substantially constant wear between the edges of the first friction element.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates generally to vehicle brakes and more particularly, to an apparatus and method for an electric caliper brake.  
         BACKGROUND OF THE INVENTION  
         [0002]    Various types of brake systems are commonly used in vehicles, including hydraulic, anti-lock, also referred to as ABS, and electric, also referred to as “brake by wire.” For example, in a hydraulic brake system, the hydraulic fluid transfers energy from a brake pedal to a brake pad for slowing down or stopping rotation of a wheel of the vehicle. Electronics control the hydraulic fluid in the hydraulic brake system. In the electric brake system, the hydraulic fluid is eliminated. Instead, an electric caliper controls the application and release of the brake pad.  
           [0003]    Known electric calipers often use an electric motor to drive a ballscrew that in turn advances and retracts a nut connected to a friction brake pad. One example of a known electric caliper is illustrated in FIGS. 5 and 6. To operate an electric caliper  16  within a brake  18 , a force is applied to a brake pedal (not shown) to initiate operation of an electric motor  20  within a bore  21  of a housing  22 . The motor  20  is connected to a gear drive  24  comprising, for example, a pinion gear  26 , planetary gears  28  and ring gear  30 . The rotation of the motor  20  and pinion gear  26  moves the axes of the planetary gears  28  about a circular path within the ring gear  30  with respect to a centerline  31  of the bore  21 . The axes of the planetary gears  28  are mechanically connected to a screw portion  32  of a ballscrew  34 ; and thus, motion of the axes of the planetary gears in the circular path provides a rotation of the screw portion  32 . Rotation of the screw  32  causes a ballscrew nut  36  and attached piston  40  to move linearly with respect to the centerline of the housing bore  21 , for example, out of the bore  21  away from the housing  22 , that is, from right to left as viewed in FIG. 5. The displacement of the piston  40  engages an inner brake pad  42  with one side of a disc or rotor  44  mechanically connected to a wheel (not shown). The resulting pressure that builds up from forcing the inner brake pad  42  against the disc  44  creates a reactionary force  46 . The reactionary force  46  is transmitted back into the housing  22  through a rod (not shown) and into a thrust bearing (not shown) in a known manner. The housing  22  is displaced in the direction away from the disc  44 , that is, from left to right as viewed in FIG. 5; and a housing bridge  47  pulls the outer brake pad  48  toward an opposite side of the disc  44 , until both the inner brake pad  42  and outer brake pad  48  are exerting pressure on the disc  44  to slow down or stop the rotation of the wheel. An electric caliper as described above is more fully described in U.S. Pat. No. 6,139,460 entitled “Electric Caliper”, which is hereby incorporated in its entirety by reference herein.  
           [0004]    Referring to FIG. 7, assume the disc  44  is normally rotating in a clockwise direction as viewed from the pad  42  as indicated by the arrow  49 , as the brake pads  42 ,  48  are applied against the disc  44 , the rotation of the disc  44  results in slightly greater forces being applied at leading edges  37  of pads  42 ,  48  than the forces being applied at respective trailing edges  38 . Consequently, over time, the brake pads  42 ,  48  will experience slightly greater wear at their respective leading edges than at their respective trailing edges. The result is a tapered wear pattern across respective faces of the brake pads  42 ,  48  as indicated by the dashed lines  39 . Such an uneven wear pattern reduces the useful life of the brake pads  42 ,  48 .  
           [0005]    Therefore, there is a need for an electric caliper braking system that provides a more uniform application of forces across the face of the brake pad.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention provides an electric caliper that provides more uniform pattern of brake pad wear than current systems and extends the useful life of brake pads. The electric caliper of the present invention allows fine tuning of the brake pad leading edge and trailing edge taper wear performance. Further, the electric caliper of the present invention provides significant design flexibility, and such electric calipers can be provided over a greater range of vehicle applications.  
           [0007]    According to the principles of the present invention and in accordance with the described embodiments, the invention provides an electric caliper for a brake system. The electric caliper has a housing mounted adjacent a rotatable disc and first and second force applying elements supporting a friction element between the housing and the rotatable disc. An electric actuator is mounted in the housing and is operatively connected to the first and second force applying elements. The electric actuator is operable to move the first and second force applying elements and the friction element toward, and away from, the disc and cause the friction element to frictionally contact the disc. By applying forces at two different locations on the disc, the two forces are able to compensate for the nonuniform forces on the friction element that arise from the direction of rotation of the disc, thereby providing a more uniform wear pattern on the friction element.  
           [0008]    In one aspect of this invention, the housing has first and second bores; and the electric actuator includes an electric motor mounted in the housing. The electric actuator further includes first and second rotary-to-linear motion converters disposed in the first and second bores, respectively. The rotary-to-linear motion converters are connected between the electric motor and respective first force applying elements to linearly move the respective force applying elements in response to a rotation of the electric motor.  
           [0009]    In another embodiment of the invention, a method is provided for operating an electric caliper for a brake system having a rotatable disc. The caliper has a pair of opposed friction elements supported by a caliper housing, wherein each of the friction elements is disposed on, and movable with respect to, a different side of the disc. The method applies first and second forces against a first friction element in response to an operation of an electric motor.  
           [0010]    In an aspect of this invention, the first force is applied adjacent one edge of the first friction element; and the second force is applied adjacent another edge of the first friction element. The first and second forces are applied with respective magnitudes so as to maintain a substantially constant wear between the edges of the first friction element.  
           [0011]    These and other objects and advantages of the present invention will become more readily apparent during the following detailed description taken in conjunction with the drawings herein. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a partially cross-sectioned top view of one embodiment of an electric caliper in accordance with the principles of the present invention.  
         [0013]    [0013]FIG. 2 is a partially cross-sectioned end view of the electric caliper of FIG. 1 in which other parts of the brake are not shown.  
         [0014]    [0014]FIG. 3 is a partially cross-sectioned top view of another embodiment of an electric caliper in accordance with the principles of the present invention.  
         [0015]    [0015]FIG. 4 is a partially cross-sectioned end view of the electric caliper of FIG. 3 in which other parts of the brake are not shown.  
         [0016]    [0016]FIG. 5 is a partially cross-sectioned side view of a known electric caliper.  
         [0017]    [0017]FIG. 6 is a partially cross-sectioned end view of the electric caliper of FIG. 5.  
         [0018]    [0018]FIG. 7 is a schematic top view of the disc and brake pads of the electric caliper of FIG. 5 illustrating a tapered wear pattern on the brake pads. 
     
    
     DETAILED DESCRIPTION  
       [0019]    Referring to FIGS. 1 and 2, a brake  50  includes an electric caliper  51  that has a caliper housing  52  with first and second bores  54 ,  56 , respectively, therein. Rotary-to-linear motion converters, for example, first and second ballscrews  62 ,  76 , are disposed in the respective bores  54 ,  56 . An electric motor  58  is located in the first bore  54  and has an output shaft mechanically connected to a gear box  64  that is used to provide a mechanical advantage to the motor  58 . A nut portion  66  of the ballscrew  62  supports a first force applying element, for example, piston  68 , that contacts the inner pad  42 . Thus, operation of the motor  58  causes the ball nut  66  to move linearly with respect to a centerline of the housing bore  54 , for example, out of the bore  54 , that is, from right to left as viewed in FIG. 1, to force the inner pad  42  against the rotating disc  44 . In a manner as previously described, motion of the inner pad  42  against one side of the disc  44  causes the outer pad  48  to move toward the opposite side of the disc  44 , that is, from left to right as viewed in FIG. 1. Thus, that action of the ball nut  66  results in braking forces being applied by the inner and outer pads  42 ,  48  against the rotating disc  44 .  
         [0020]    The output shaft of the motor  58  is also mechanically connected to a first gear  70  that meshes with a second gear  72 . The gear  72  is mechanically connected to a screw portion  74  of a second ballscrew  76 . The nut portion  78  of the second ballscrew  76  supports a second force applying element, for example, a piston  80 , that also contacts the inner brake pad  42 . Thus, rotation of the motor  58  rotates the gears  70 ,  72  and the screw  74 . Rotation of the screw  74  causes the nut  78  to move linearly with respect to a centerline of the housing bore  56 , for example, out of the second bore  56  toward the inner pad  42 , that is, from right to left as viewed in FIG. 1.  
         [0021]    Thus, operation of the motor  58  results in both ball nuts  66 ,  78  moving in unison out of the bore  56  and jointly forcing their respective pistons  68 ,  80  against the inner brake pad  42 . Assume the disc  44  is rotating in a clockwise direction as viewed from the brake pad  42  and indicated by the arrow  82 . Piston  80  is applying a braking force near leading edges  37  of the respective brake pads  42 ,  48 ; and the piston  68  is applying brake forces near trailing edges  38  of the respective brake pads  42 ,  48 .  
         [0022]    With the known braking system of FIGS. 5 and 6, the braking force is applied substantially at the centerline of the pads  42 ,  48 . However, with the embodiment of FIG. 1, by utilizing a dual bore caliper  50  providing two force applying pistons  68 ,  80 , the point at which the net force is applied can be shifted from the centerline of the brake pads  42 ,  48  toward their respective trailing edges  38 . Such a proportioning or shifting of the net force on the pads  42 ,  48  is accomplished by adjusting the gear ratio of the gears  70 ,  72  and the pitch of the ballscrews  62 ,  76 . For example, as operation of the motor  58  causes the piston  68  to move through a first displacement, the gear ratio of the gear  70 ,  72  can be chosen such that the piston  80  moves through a slightly smaller or slightly greater displacement. Most often, the gear ratio  70 ,  72  is chosen such that the piston  68  applies a greater force near the trailing edges  38  of the pads  42 ,  48 . That greater force is used to compensate for the greater force that is provided at the leading edges  37  as a result of the direction of rotation of the disc  44 , in this example, the clockwise rotation.  
         [0023]    [0023]FIGS. 3 and 4 illustrate an alternative embodiment of an electric caliper that provides a more even wear of the brake pads  42 ,  48  than the known system of FIG. 1. An electric caliper  90  of a brake  92  has a housing  94  with first and second bores  96 ,  98  that house rotary-to-linear motion converters, for example, first and second ballscrews  100 ,  102 . An electric motor  104  is supported by the housing  94  and has an output mechanically connected to a gear train  106  comprised of a drive gear  108 , a first gear  110  and a second gear  112 . The first and second gears  110 ,  112  are connected to respective first and second gear boxes  114 ,  116 . The first and second gear boxes  114 ,  116  are mechanically connected to respective first and second screw portions  118 ,  120  of the respective first and second ballscrews  100 ,  102 . The first and second ballscrews  100 ,  102  have respective ball nuts  122 ,  124  that support respective force applying elements, for example, pistons  126 ,  128 , respectively, that contact the inner brake pad  42 .  
         [0024]    Operation of the motor  104  rotates the gears  108 - 112  and screws  118 ,  120  via gear boxes  114 ,  116 . Rotation of the screws  118 ,  120  causes respective ball nuts  122 ,  124  and pistons  126 ,  128  to move linearly with respect to a centerline of the housing bore  96 , for example, out of the bore  96 , that is, from right to left as viewed in FIG. 3. Moving the pistons  126 ,  128  out of the housing bore  96  forces the inner pad  42  against one side of the rotating disc  44 . That motion of the inner pad against the disc causes the outer pad  48  to move toward an opposite side of the disc  44  in a manner as previously described, thereby causing the brake pads  42 ,  48  to apply braking forces against the rotating disc  44 .  
         [0025]    In this embodiment, the point at which the net force is applied to the brake pads  42 ,  48  can be shifted from a centerline of the brake pads  42 ,  48  to a location closer to the trailing edges  38  of the respective pads  42 ,  48 . The distribution of the braking force across the pads  42 ,  48  is controlled by adjusting the gear ratio of the gear mechanisms  106 ,  114 ,  116  and the pitch of the ballscrews  100 ,  102 . The gear ratios are chosen such that a greater force is applied by the piston  126  with respect to the forces applied by the piston  128 . That greater force compensates for the inherently greater force inherently occurring at the leading edges  37  of the respective pads  42 ,  48  in response to a clockwise rotation of the disc  44  as indicated by the arrow  134 .  
         [0026]    In use, with the embodiments of FIGS.  1 - 4 , with each use of the brakes, the braking forces are distributed over the brake pads  42 ,  48  in a controlled manner, such that the wear pattern of the brake pads  42 ,  48  can be controlled and made more uniform. The multi-bore electric calipers described herein allow fine tuning of the brake pad leading edge and trailing edge taper wear performance. With a more uniform, that is, a more linear and less tapered, wear pattern on the brake pads  42 ,  48 , the useful life of the brake pads  42 ,  48  is substantially lengthened, thereby providing a higher quality and less costly braking system. Further, the multi-bore electric caliper described herein provides significant design flexibility. For example, the ability of the multi-bore electric caliper to control the force distribution across the brake pad allows the use of commercially available motors and ballscrews. In addition, the calipers can be designed and applied over a wide range of vehicle applications.  
         [0027]    While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. For example, in the embodiment of FIGS. 1 and 2, the motor  58  is mechanically connected to the screw portion  60  of the ballscrew  62  via a gear box  64 . As will be appreciated, in an alternative embodiment, the motor  58  can be directly coupled to the screw portion  62  without using the intervening gear box  64 . Similarly, in the embodiment of FIGS. 3 and 4, the motor  104  is mechanically connected to the screw portions  118 ,  120  via respective gear boxes  114 ,  116 . However, as will be appreciated in alternative embodiments, the gear boxes  114 ,  116  can be eliminated and the screw portions connected more directly to the motor  104  via the gear train  106 .  
         [0028]    In all of the described embodiments, a motor is mechanically connected to a rotatable, but non-translatable, screw portion of a respective ballscrew; and a respective nut that supports a force applying piston translates along the screw in response to the motor rotating the screw. As will be appreciated, in alternative embodiments, the motor can be mechanically connected to a rotatable, but non-translatable, nut of a ballscrew; and a respective screw portion is used to support a piston. In that embodiment, the screw portion translates with respect to the nut in response to the motor rotating the nut.  
         [0029]    As will be also be appreciated, although the rotary-to-linear motion converters are described as ballscrews, other varieties of screw and nut combinations may be used. Further, other rotary-to-linear motion converters may be used in place of the ballscrews. In addition, the combinations of electric motors and respective rotary-to-linear motion converters used to operate pistons  68 ,  80 ,  126 ,  128  can be replaced by other electric power actuators, for example, piezoelectric actuators, etc. Also, in the examples described herein, the electric calipers have two bores; however, as will be appreciated, in some applications, it may be necessary to employ more than two bores with respective rotary-to-linear motion converters in order to achieve a desired brake pad wear performance and profile.  
         [0030]    As will further be appreciated, in other alternative embodiments, the gear boxes  64 ,  114 ,  116  and gear train  106  can be replaced by any other power transmitting mechanism that is effective to transfer rotational power from a motor and rotate a screw portion of a respective ballscrew.  
         [0031]    Therefore, the invention in its broadest aspects is not limited to the specific details shown and described. Consequently, departures may be made from the details described herein without departing from the spirit and scope of the claims, which follow.