Abstract:
A parking brake assembly for an automotive drum brake is provided. The parking brake assembly replaces a conventional mechanical strut with an electronic strut actuator subassembly that provides for parking brake engagement and disengagement, sensing of brake lining wear and adjustment of brake shoe-to-drum clearance.

Description:
TECHNICAL FIELD 
     This invention relates to a leading/trailing type drum brake assembly in which the parking brake lever, adjuster strut and associated components are replaced with a single electronic subassembly that controls the parking brake, senses brake lining wear and adjusts the brake shoe-to-drum clearance to account for lining wear. The parking brake assembly includes a strut actuator subassembly that comprises a strut tube, an electric motor, a position encoder, a power screw and a connector fork. 
     BACKGROUND OF THE INVENTION 
     Vehicle brakes are used to slow the vehicle and also to maintain the vehicle in a parked position. Drum brakes are a common type of vehicle brake. Drum brake assemblies utilize opposing arcuate brake shoes that are movable away from one another into engagement with an inner surface of a drum to slow the vehicle. Typically, a hydraulic wheel cylinder forces the brake shoes into engagement with the drum. The same brake shoes may also be used to maintain the vehicle in a parked position. For example, drum brake assemblies have employed a separate mechanical linkage, or parking brake lever, that is actuated by a cable to maintain the brake shoes in engagement with the drum. Such mechanical parking brakes have presented various problems that have been partially addressed by the use of electric parking brake mechanisms. 
     Some electric parking brake mechanisms have eliminated the parking brake lever and completely redesigned the drum brake assembly to incorporate an electric parking brake mechanism. These mechanisms have required high torque motors and a gearbox to generate sufficient force to apply the brakes, adding significant cost to the drum brake assembly. These high torque motors tend to be relatively large and cannot simply replace components presently included in brake drum assemblies, but instead require substantial redesign of the brake assembly package. Furthermore, high torque motors utilize a significant amount of power that is undesirable in modern vehicles. 
     Presently available electric brake systems have the further disadvantage that they rely on conventional brake adjustment mechanisms. Such conventional adjustment systems are mechanical and only operate when the wheels are moving in a reverse direction. In addition, neither known electric brake systems nor conventional brake systems provide real-time sensing of brake lining wear and determination of a worn brake lining condition. 
     Therefore, an electric parking brake assembly is needed that requires relatively little power to apply, that may be easily retrofitted into a conventional drum brake assembly, and that provides real-time adjustment of the brake shoe-to-drum clearance and sensing of brake lining wear. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an electrically and hydraulically (hereinafter referred to as “electro-hydraulic”) actuated electronic parking brake assembly for a leading/trailing type automotive drum brake. The parking brake assembly includes a strut actuator subassembly that replaces conventional components including the parking brake lever, adjuster strut and associated parts, used in a standard drum brake. The strut actuator subassembly is easily incorporated into a conventional brake assembly and comprises a strut tube, an electric motor having a driveshaft, a position encoder, a power screw and a connector fork. Due to the design of the strut actuator subassembly, a low power (and low torque) motor can be used. 
     The brake assembly of the present invention includes a backing plate having primary and secondary shoe hold-down pin and spring assemblies with primary and secondary brake shoes. Each of the brake shoes has a portion in spaced relation from one another. A drum is disposed about the brake shoes. A wheel cylinder is supported by the backing plate and is arranged between the portions of the brake shoes for forcing the portions away from one another to engage the brake shoes with the drum in a brake position. A strut actuator subassembly, located adjacent to the wheel cylinder, interconnects the brake shoes proximate to the portions. The strut actuator subassembly comprises a strut tube, a power screw, a connector fork, an electric motor with a driveshaft and a position encoder. The strut tube has one open end having female threads along a portion of the interior and an opposing end that tapers to a two-pronged fork. The power screw has male threads along a portion of its length and an unthreaded rod portion at one end. The connector fork has an annular female receiver portion at one end suitable for receiving the rod portion of the power screw and a two-pronged fork at the other end. The electric motor and position encoder are located within the strut tube, and the electric motor is axially connected with the power screw via the driveshaft. The strut tube and the power screw connect at their respective female and male threaded ends. The power screw, at its rod end, connects to the female end of the connector fork. The forked end of the strut tube and the connector fork connect with slots in portions of each web of opposing brake shoes thereby securing the strut actuator subassembly between the opposing brake shoes. The strut tube, electric motor with driveshaft, position encoder, power screw and connector fork are in axial relationship with one another and when connected define the transverse structure of the strut actuator subassembly. 
     In operation of an electro-hydraulic apply parking brake of the present invention, the strut actuator subassembly serves as a parking brake latch. Hydraulic pressure is applied by the electro-hydraulic actuator to the wheel cylinder to advance the brake shoes against the drum. As the brake is applied hydraulically, the electric motor of the strut actuator subassembly is energized. The strut actuator subassembly turns the power screw to advance the connector fork and strut tube out against the hydraulically extended brake shoes. Due to the design of the power screw, it can only be driven in positive direction. Accordingly, when the hydraulic pressure to the wheel cylinder is released along with the power to the electric motor, the brake shoes become locked to against the drum. The parking brake is released by the application of a hydraulic pressure sufficient to remove the axial load on the power screw, the connector fork and the strut tube. The electric motor then re-energizes, reverses and retracts the power screw and strut tube away from the shoes allowing them to return to their predetermined disengaged position when the hydraulic pressure is removed. The connector fork returns along with the return of the brake shoes by action of the return springs. 
     The present invention also advantageously provides regular and precise brake adjustment and sensing of braking lining wear. During operation, the electric motor is periodically energized, such as during the ignition start cycle, causing the powers crew to turn which advances the brake shoes out until they touch the drum. The motor then reverses for a predetermined number of counts of the position encoder to obtain the desired shoe-to-drum clearance. This provides regular and precise brake adjustment. The position encoder also concurrently determines the distance that the shoe has traveled and compares it with the original brake lining position to provide a reasonable estimate of a worn out lining condition. Upon such indication of wear, the position encoder sends out a signal to notify the operator of the worn lining condition. 
     Accordingly, the above invention provides an electric parking brake mechanism that provides regular and precise brake adjustment and lining wear indication. The mechanism requires relatively little power to operate and may be easily incorporated, such as by retrofit, into a conventional drum brake assembly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages of the present invention can be understood by reference to the following detailed description when considered in connection with the accompanying drawing wherein: 
     FIG. 1 is a front elevation view of a drum brake assembly of the present invention. 
     FIG. 2 is an exposed planar view of the strut actuator subassembly, located at section  2 — 2  of FIG. 1, showing details of the components. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A drum brake assembly  10  for a vehicle is shown in FIG.  1 . The assembly  10  includes a backing plate  12  having primary  14  and secondary  16  shoe hold down pin-and-spring assemblies with primary  18  and secondary  20  brake shoes supported respectively thereon. A drum  21  is disposed about the brake shoes  18 ,  20 . Each of the brake shoes  18 ,  20  include brake shoe webs  22  that support friction linings  24  adjacent drum  21 . Each of webs  22  has a portion  30  in spaced relation from the other that extends to a second portion  32 . Preferably, an anchor block  36  is arranged between brake shoes  18 ,  20  to support the second portions  32  in spaced relationship to one another. 
     A wheel cylinder  40  is supported by backing plate  12  and is arranged between portions  30  of brake shoes  18 ,  20 . Wheel cylinder  40  includes pistons on either end, as is well known, that move in opposite directions to force the portions  30  away from one another to engage brake shoes  18 ,  20  with drum  21  when in a brake position. Hydraulic fluid is forced into a chamber between the pistons by a hydraulic brake actuator. Portions  30  have ears  42  that are received in indentations in the pistons (not shown). As portions  30  are forced away from one another, brake shoes  18 ,  20  pivot outward about pin-and-spring assemblies  14 ,  16 . 
     A strut actuator subassembly  48 , located at section  2 — 2 , is connected to slots  52 ,  53  in webs  22  interconnecting brake shoes  18 ,  20  at ends  50 ,  51  adjacent wheel cylinder  40  and proximate the portions  30 . Opposing ends  50 ,  51  of the strut actuator subassembly  48  engage webs  22  to hold the portions  30  in spaced relation. Ends  50 ,  51  also serve to secure strut actuator subassembly  48  between brake shoes  18 ,  20  within brake assembly  10 . Upper and lower return springs  46 ,  47  interconnect the portions  30  and second portions  32  for retracting brakes shoes  18 ,  20  from drum  21  against strut actuator subassembly  48  and anchor block  36 . In this manner, anchor block  36  and strut actuator subassembly  48  together determine the spacing of brake shoes  18 ,  20  from drum  21 . As linings  24  wear, brake shoes  18 ,  20  must be moved closer to drum  21  to maintain desirable brake application characteristics, such as consistent brake pedal travel and brake response time. To this end, strut actuator subassembly  48  serves as a lining wear sensor and a brake shoe-to-drum clearance adjuster to accommodate for wear in brake shoes  18 ,  20 . 
     Referring to FIG. 2, strut actuator subassembly  48  comprises a strut tube  54  that is open at one end having female threads and tapers to a fork at opposite end  50 . Strut actuator assembly  48  further comprises an electric motor  56  with a driveshaft  57 , a position encoder  58 , a power screw  60  having male threads along a portion of its length and a unthreaded rod portion at one end, and a connector fork  62  having a female end suitable to receive the rod portion of the power screw and a fork at end  51 . As shown in FIG. 1, electric motor  56  and position encoder  58  are located within strut tube  54 . The electric motor  56  is axially connected via driveshaft  57  to power screw  60 . Power screw  60  connects via its rod portion to connector fork  62  that connects at end  51  with slot  53 , and strut tube  54  connects with its fork at end  50  with slot  52 . In operation, the electric motor  56  turns driveshaft  57  to rotate power screw  60 . Power screw  60  advances connector fork  62  and strut tube  54  in opposing directions. When electric motor  56  is operating in the positive direction, power screw  60  (with connector fork  62 ) and strut tube  54  move outwardly to engage brake shoes  18 ,  20  with drum  21 . This outward movement provides for parking brake engagement, sensing of brake lining wear and determination of the shoe-to-drum distance. When the motor is operating in the reverse direction, power screw  60  and strut tube  54  move inwardly allowing the brake shoes  18 ,  20  to retract from drum  21  and return to their disengaged position. This inward movement provides for parking brake disengagement and adjustment of the brakes to a pre-selected brake shoe-to-drum clearance. 
     The electric parking brake of the present invention is actuated in response to a parking brake signal received from an electro-hydraulic parking brake mechanism (not shown). Hydraulic pressure is applied by an electro-hydraulic actuator (also not shown) to the wheel cylinder  40  that advances brake shoes  18 ,  20  against the drum  21 . The electro-hydraulic actuator concurrently sends a signal to the strut actuator subassembly  48  to energize electric motor  56 . Electric motor  56  turns driveshaft  57  in the positive direction moving power screw  60  (with connector fork  62 ) and strut tube  54  outwardly to engage the brake shoes  18 ,  20  with the drum  21 . Because the power screw  60  cannot be driven backwards, when the hydraulic pressure to wheel cylinder  40  is released with the power to the electric motor  56 , the brake shoes  18 ,  20  become locked against the drum  21 . The parking brake is released when the electro-hydraulic actuator is re-initiated by application of a hydraulic pressure that is sufficient to remove the axial load on the power screw  60 , connector fork  62  and the strut tube  54 . The electric motor  56  is concurrently re-energized, reverses the direction of driveshaft  57  and retracts power screw  60  and strut tube  54 , allowing brake shoes  18 ,  20  to return to a disengaged position. 
     Brake lining wear sensing and adjustment are also controlled by the strut actuator subassembly  48 . Upon brake installation, position encoder  58  records the original position of the brake shoes according to the pre-selected brake-to-drum clearance. Then, during operation, the electric motor  56  is periodically energized, such as during the ignition start cycle, causing the power screw  60  (with connector fork  62 ) and the strut tube  54  to move outwardly and advance the brake shoes  18 ,  20  out until they touch the drum  21 . The electric motor  56  then reverses for a predetermined number of counts according to position encoder  58  to obtain the desired shoe-to-drum clearance. The position encoder  58  concurrently determines the distance that brake shoes  18 ,  20  have traveled and compares it with the original set-point position. The position encoder  58  outputs a signal to notify the operator of a worn brake lining condition when a predefined point is reached. 
     In the present invention, electric motor  56  is preferably a low power, low torque motor since it need only generate enough force to move the weight of the brake shoes  18 ,  20  against the resistance of return spring  46  to touch drum  21 . 
     The invention has been described in an illustrative manner, and it is to be understood that the terminology that has been used is intended to be in the nature of words of description rather than of limitation. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.