Abstract:
A method of accommodating wear in an electrically operated brake assembly includes setting a home position when a brake shoe engages a brake drum. A stepper motor drives the brake shoes into engagement with the drum. Upon engagement of the brake shoes with the drum, the stepper motor slips. Slipping of a stepper motor indicates that a home position has been reached. Upon release of the brake, the brake shoe is moved away from the drum a fixed distance. The fixed distance is an optimal distance between the friction member and the rotating member such that regardless of wear, the distance relationship between the brake shoe and drum is maintained.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to an electrically operated brake assembly and a method of accommodating brake lining wear. 
   A typical brake system includes a hydraulic actuator in hydraulic communication with a hydraulic fluid reservoir. The hydraulic actuator forces brake pads or brake shoes toward a disk or rotor. The brake pads or brake shoes include a friction material that engages the rotating member creating a frictional contact that in turn results in a desired amount of braking torque. 
   Contact between the friction material lining the brake pads or shoes creates frictional contact with the drum or rotor that in turn creates the desired amount of braking torque to slow and stop the motor vehicle. Repeated braking operations progressively wear away friction material resulting in a progressively thinner brake lining. As the friction material wears, the distance between the brake lining and the rotor or drum increases. The increase in distance increases the amount of mechanical travel required to engage the brake. Various mechanical devices are known in the art to adjust and accommodate for the wearing of the brake lining. These devices adjust the brake pad or shoe in order to maintain a desired distance from the brake rotor or drum. 
   In some applications an electromechanical actuator provides at least a portion of the braking torque required to stop the motor vehicle. The electromechanical actuator is typically an electric motor that drives the brake pads or shoes into engagement with the rotor or drum. A sensor or sensors are used to determine the position of the brake lining relative to the rotating member and then accommodate and adjust for that distance to continually place the brake lining at an optimal distance relative to the rotating member. 
   As appreciated, the addition of such sensors and other devices increases the complexity and cost of any brake assembly. Further, sensors require calibration in order to provide for accurate position information. Additionally, brake assemblies operate in harsh environments of wide ranging temperature variation and exposure to various contaminants. 
   Accordingly, it is desirable to develop a brake assembly that automatically accommodates brake lining wear without the use of separate sensors. 
   SUMMARY OF THE INVENTION 
   This invention is an electrically actuated brake assembly that accommodates lining wear by setting a home position of the friction member when engaged with the rotating member and withdrawing the friction member a specified distance relative to the home position. 
   Slipping of the electric motor indicates engagement with the friction member and a home position of the friction member is set in response to indication of contact with the rotating member. This home position is reset with each actuation of the brake assembly. During operation the brake lining or friction member is released and moved away from the rotating member a fixed distance relative to the home position. As appreciated, by fixing the distance in which the brake lining or friction member moves away from the brake rotor or drum, the distance between the brake lining and the brake rotor or drum is maintained at a desired distance providing a consistent amount of braking torque. 
   Accordingly, the brake assembly of this invention provides a method and apparatus for automatically accommodating for brake lining wear without the use of additional sensors. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows: 
       FIG. 1  is a perspective view of a electrically actuated brake drum assembly; 
       FIG. 2  is a cross-sectional view of an electrically operated disc brake assembly; 
       FIG. 3A  is a cross-sectional view of a drum brake assembly in a disengaged position; 
       FIG. 3B  is a schematic view of a brake drum assembly in an engaged or home position; 
       FIG. 4A  is a schematic illustration of a disc brake assembly in a disengaged position; 
       FIG. 4B  is a schematic illustration of a disc brake assembly in a disengaged position or home position; 
       FIG. 5  is a graphical representation of a braking cycle; and 
       FIG. 6  is a graphical representation of the current drive pulses through a single braking cycle. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a drum brake assembly  10  includes a drum  12  rotating about a spindle  24  that frictionally engages brake shoes  14 . The brake shoes  14  are actuated by rotation of a cam  18 . The cam  18  is supported along a shaft  20  that is driven by a stepper motor  22 . The stepper motor  22  rotates the shaft  20  to rotate the cam  18 , which in turn will drive the brake shoes  14  toward the inner diameter of the drum  12 . Each of the brake shoes  14  includes a thickness of friction material  15 . The friction material  15  engages the inner diameter of the brake drum  12  to create a frictional contact required to create the desired amount of braking torque to slow and stop a motor vehicle. As the friction material  15  of the brake shoes  14  wears, the distance that the cam  18  must rotate increases. 
   Referring to  FIGS. 3A and 3B , a home position is determined by slippage of the stepper motor  22  as the frictional material  15  engages the drum  12 . The stepper motor  22  rotates a fixed amount according to a specified amount of current drive pulses. In this way, the precise rotation of the stepper motor  22  can be governed and controlled by measuring and providing a set number of drive current pulses. Although a stepper motor  22  is illustrated and preferred, it is within the contemplation of this invention that any type of electromechanical device capable of precisely determining a specific rotation or movement relative to a desired amount of current pulses may be used with this invention. 
   Referring to  FIG. 2 , a disc brake assembly is shown including a housing  40  that supports brake pads  34  including friction material  35 . At least one of the brake pads  34  is actuatable by a stepper motor  36 . The stepper motor  36  in this embodiment is a linear stepper motor  36  that moves the actuator  38  along a linear path to drive at least one of the brake pads  34  into contact with the rotor  32 . In this embodiment of the invention, the rotor  32  is surrounded on each side by the brake pads  34 . Each of the brake pads  34  includes an amount of the friction material  35 . The friction material  35  contacts the rotor  32  to create the frictional force creating the required braking torque. 
   Referring to  FIGS. 4A and 4B , the brake pads are movable between a disengaged position as shown in FIG.  4 A and an engaged position as shown in FIG.  4 B. The engaged position as shown in  FIG. 4B  causes the stepper motor  36  to slip as it attempts to overcome the forces created by contact with the rotor  32 . As appreciated, the electric motor  36  is limited in force and may only drive the brake pads  34  against the rotor  32  with a certain magnitude of force. When it is detected that the stepper motor  36  is no longer moving in response to a certain amount of drive current pulses, a home position is detected. Upon release of the brake, the stepper motor  36  reverses and moves the brake pads  34  a fixed distance relative to the home position. The fixed distance is selected to provide sufficient clearance between the brake pads  34  and the rotor  32  when not engaged, and to limit the distance the brake pad  34  must travel to engage the rotor  32 . 
   Referring to  FIG. 1 , the operation of the brake assembly  10  includes the steps of driving the brake shoes  14  into engagement with the drum  12  with the stepper motor  22 . Movement of the stepper motor  22  is accomplished by communicating a series of current drive pulses. As appreciated, stepper motors  22  typically operate by moving a specific rotational or linear distance relative to each current drive pulse. Once the stepper motor  22  has driven the brake shoe  14  against the drum  12 , movement of the rotating shaft  20  along with the stepper motor  22  stops relative to incoming current drive pulses. This slip indicates that the home position has been reached. The home position is the position where the brake shoes  14  are engaged completely with drum  12  and exert a desired amount of force to create the desired amount of braking torque. This amount of slip relative to the amount of current drive pulses indicates when the home position has been reached. 
   Upon release of the brake, the brake shoes  14  release to allow rotation of the drum  12 . The brake shoes are moved away from the drum  12  by reversing rotation of the stepper motor  22 . The stepper motor  22  reverses to move the brake shoes  14  away from the drum  12  a fixed distance. The fixed distance is determined as an optimal distance from the drum  12  to provide clearance and a desired amount of braking torque. 
   As appreciated, because the brake shoe  14  is moved away from the drum  12  a fixed distance after being released from the home position, the brake shoe  14  will always be at an optimal distance relative to the drum  12  regardless of the thickness of the friction material  15 . As the friction material  15  on the brake shoe  14  wears, the home position is adjusted toward the drum  12 . The home position is continuously adjusted as the friction material  15  wears resulting in the brake shoe  14  always being in a specified relationship relative to the drum  12 . The relationship between the brake shoe  14  and the drum  12  is continuously adjusted such that the distance between the brake shoe  14  and the drum  12  remains constant in the disengaged position regardless of the amount of friction material  15 . 
   Referring to  FIG. 2 , during operation of the disk brake assembly  30 , the stepper motor  36  advances the brake pads  34  into engagement with the rotor  32 . Movement of the brake pads  34  are detected relative to the number of current drive pulses communicated to the stepper motor  36 . Note that in this embodiment the stepper motor  36  drives the brake pads  34  linearly through actuator  38 . Once the brake pads  34  have engaged the rotor  32 , a home position is detected in response to motor slip. Disengagement of the brake pads  34  is accomplished by moving the brake pads  34  a fixed distance away from the set home position. Movement a fixed distance provides a consistent distance for travel for the brake pads  34  toward engagement regardless of the amount of friction material. 
   Referring to  FIG. 5 , a graphical illustration of the displacement of the brake shoe  14 , relative to the drum  12  is shown. The displacement  50  is shown relative to the braking force  54 . Note that the displacement of the shoe  14  indicated at  50  concurs with the amount of force  54  exerted. At point  60  where the brake is released, the distance or displacement is fixed. The fixed distance results in a new start position after each brake actuation. The new start position provides the optimal distance between the shoes  14  and the drum  12 . 
   Referring to  FIG. 6 , graphical illustration of the current drive pulses during movement or actuation of the friction member towards the rotating member is shown. Note that the forward movement indicated at  62  and above the X axis shows the amount of current drive pulses  66  as the shoe  14  approaches and engages the drum  12 . The current drive pulses are shown enlarged once the shoe  14  is in contact with the drum  12  as indicated at  67 . Current drive pulses indicated at  60  illustrate release of the brake and reversing of the stepper motor a fixed amount to provide the fixed displacement and the new start position as indicated at  56  in FIG.  5 . 
   The foregoing description is exemplary and not just a material specification. The invention has been described in an illustrative manner, and should be understood that the terminology used is intended to be in the nature of words of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed, however, one of ordinary skill in the art would recognize that certain modifications are within the scope of this invention. It is understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.