Patent Publication Number: US-2022234564-A1

Title: Compact electromechanical brake

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an at least partly electrically actuated automobile vehicle brake providing greater compactness. 
     In most current automobiles, the service brake is provided by disc brakes and/or drum brakes. 
     They perform the service braking function, which aims to slow down or even stop the automobile vehicle. The parking brake function aiming to bring a stationary vehicle to a standstill and the emergency brake function are usually achieved with the system providing the parking brake. 
     The parking brake can be applied by pulling a lever arranged in the passenger compartment which is connected to a mechanism located in a drum brake or disc brake via a brake cable. The mechanism tends to move the braking shoes of the drum brakes apart to apply them against the drums. In the case of a disc brake, the brake pads are clamped to the brake disc. 
     To improve driver comfort and safety in the application of the parking brake, the car industry has developed an electric parking brake. 
     For example, an actuator provided with an electric motor is arranged at least on each rear brake, and causes the brakes to be applied. 
     The driver presses for example a button disposed on the dashboard to activate the electric parking brake. 
     Electric parking brake systems include DC electric motors, the use of which is recommended by the German Automobile Industry Association in its “VDA Recommendation”. 
     In the case of a disc brake, it is generally the piston that is moved by the actuator and applies the brake pads against the brake disc. In the case of a drum brake, the actuator is for example arranged between the brake shoes and serves as a fixed point when applying service braking via a wheel cylinder. The actuator is, for example, of the screw-nut type and moves the shoes apart. 
     No sensors are implemented in the actuator to know the relative position of the mechanical elements of the brake. The measurement of current and/or voltage across the electric motor makes it possible to know the state of the applied brake. Indeed, an increase in current and/or voltage corresponds to an applied state of the parking brake. 
     Furthermore, it is currently desired that none of the components contacts a mechanical stop during the release stroke to avoid damaging the brake, for example, it is desired that the piston does not abut against the bottom of the caliper. 
     The relative position of the mechanical elements of the brake during the application of parking braking is currently obtained by means of an estimation by knowing the applied position given by the value of current and/or voltage across the motor and, for example, by counting the number of revolutions of the nut of the actuator. However, the estimation of the relative position of the mechanical elements of the brake at the end of the brake release phase is not very accurate. In order to take this inaccuracy into account and to prevent the brake components from abutting, an error margin is allowed around the estimated position. Furthermore, an additional safety margin is taken to account for dimensional inaccuracies inherent to the brake manufacture. As a result, the dimensioning of the brake incorporates these error and safety margins. The brake then has a certain overall size. Furthermore, due to the inaccuracy in the position of the components in the released state, the starting position for the next braking operation is not known. In addition, the stroke to be covered by the component(s) is lengthened which increases the brake application time, and also the time during which the actuator emits noise. 
     Document US2014/0345989 describes a drum brake including an electric actuator for the parking brake implementing a lever. Thereby, It has a large overall size. 
     DISCLOSURE OF THE INVENTION 
     It is therefore a purpose of the present invention to provide an at least partly electrically actuated automobile vehicle brake which is compact and provides greater control accuracy. 
     The aforementioned purpose is achieved by an automobile vehicle brake including at least one electric actuator for moving one or more elements of the brake so as to apply a braking force to an element rotatably integral with a wheel, said actuator including at least one mechanical stop, said actuator being controlled in the brake release phase so that at least one of the movable elements of the actuator bears against the stop. As the position of the stop is known to within the manufacturing clearance, the position of said at least one element is known with increased accuracy. It is then possible to dimension the brake as accurately as possible, i.e. without taking account of a margin due to the inaccuracy in the estimation of the retracted position. The stroke to be travelled in order to apply the brake is then reduced, and the brake application time is shorter. The time during which the actuator emits noise is actually also reduced, as well as the duration of discomfort to the driver and passengers of the vehicle. 
     In other words, an actuator is made in which the position of at least one movable element is voluntarily set to a rest state of the brake, so as to reduce uncertainties about its position, and thus have greater freedom in choosing dimensions of the actuator, for example so as to make it more compact. 
     In one example, the brake includes an electric actuator for applying the parking brake and a hydraulic actuator providing the service brake function. 
     In another example, the brake is entirely electrically actuated, with the actuator providing both parking and service braking. 
     In one example, the actuator is of the screw-nut type and the stop means include a tooth on the thread forming a stop for the nut. In another example, the actuator includes elastically deformable means capable of storing energy, the stop means forming a stop for at least part of the elastically deformable means. 
     One object of the present invention is then an electromechanical brake including at least one frictional member for contacting a friction part rotatably integral with the wheel and at least one actuator configured to apply said frictional member against said friction part, and an electric motor configured to activate said actuator, said actuator including at least one movable element upon activating the actuator, said movable element being movable between a braking position, in which the frictional member is in contact with the friction part, and a position beyond a release position, in which said frictional member is no longer in contact with the friction part. The actuator includes stop means arranged so that said movable element bears against said stop means, when it assumes a position beyond the release position, and the electric motor is controlled, in the brake release phase, to activate the actuator until the movable element bears against the stop means. 
     Preferably, the electromechanical brake includes a control unit including means for controlling the power supply to the electric motor and means for measuring the electric consumption of the electric motor, said control means being configured to stop power supply to the electric motor, when the electric consumption exceeds a given value. 
     Advantageously, the actuator is configured to apply at least parking braking and wherein the brake includes a second actuator for applying service braking. 
     For example, the brake is a drum brake including a drum, a backing plate, two shoes, the actuator being attached to the backing plate. 
     In one advantageous example, the actuator includes a screw-nut assembly, said nut forming said movable element, said screw including a head and a rod provided with a screw pitch, said head forming a piston bearing against one of the brake shoes, and the stop means include a tooth protruding from said thread arranged to contact the nut when it assumes a position beyond the release position. 
     For example, the actuator may include a body, elastically deformable means able to store energy upon applying activation of the actuator, at least part of said deformable means forming the movable element, and the body may include the stop means. 
     It is also an object of the present invention to provide a method for manufacturing an electromechanical brake according to the present invention, including: 
     determining the position of the at least one movable element beyond which the brake is released, 
     b) determining the position of the stop means relative to said release position so that the stop means are spaced from the release position by at least one manufacturing clearance associated with said method, 
     c) making the stop means in the actuator in the position determined in step b), 
     d) assembling the actuator and the frictional member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be better understood based on the following description and the appended drawings in which: 
         FIG. 1  is a schematic representation of the estimated movement of a movable element of an actuator of an electromechanical brake of the state of the art. 
         FIG. 2  is a schematic representation of the estimated movement of a movable element of an actuator of an electromechanical brake according to the invention. 
         FIG. 3  is a perspective view of an example of an electric parking brake drum brake according to the invention. 
         FIG. 4  is a partially cut-away perspective view of an example actuator for a drum brake according to the invention. 
         FIG. 5  is a schematic cross-section view of another example actuator for a drum brake according to the invention. 
     
    
    
     DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS 
     In  FIG. 3 , a “dual mode” drum brake  2  in an exemplary embodiment of the invention can be seen. 
     The drum brake  1  includes a drum (not represented) movably integral with the wheel (not represented), an X-axis revolving backing plate  2  fitted with a first and a second arc-of-circle shoes  3  and  4 , the shoes being radially movable so that they can be pressed against the cylindrical inner face of a drum not represented. 
     The shoes  3  and  4  each include a web  3   a ,  4   a  made of flat sheet in the form of a portion of a circular crown which carries a brake lining  3   b ,  4   b , and are mounted diametrically opposite each other with their ends bearing on both a hydraulic wheel cylinder  6  and a mechanical actuator  10  carried by the backing plate  2 . These shoes  3  and  4  are further biased towards each other by two return springs  8  and  9 , and each pressed against the backing plate  2  by a so-called side spring. 
     A wear adjustment connecting rod  7  extends along the wheel cylinder  6 , having a first end bearing on the web  3   a  of the first shoe  3  and a second end bearing on the web  4   a  of the second shoe  4  when the brake is at rest. 
     The wheel cylinder  6  is for being actuated when the drum brake  1  is used according to a first so-called “simplex” operating mode, which ensures progressive braking particularly adapted to brake the vehicle in service. It comprises a hydraulic chamber closed at its ends by two pistons which move away from each other when the hydraulic pressure increases and pushes the associated ends of the shoes  3  and  4 . 
     The mechanical actuator  10 , in turn, ensures parking and emergency braking by moving the associated ends of the shoes apart to ensure rapid and powerful locking of the vehicle wheels according to a so-called “duo-servo” operating mode, especially when the wheel cylinder  6  is inactive. This actuator is driven by an electric motor  22 . In the example represented, the wheel cylinder includes two opposite pistons, each of which actuates one of the shoes  3 ,  4  by spacing apart their two ends  3 . 1 ,  4 . 1  facing from each other, that is those located on the same side of the axis of rotation X, here referred to as the “movable ends” and located at the top of  FIG. 3 . At its opposite end  32 ,  4 . 2 , called the stop end, each shoe bears on the backing plate  2  by an anchoring element integral with the backing plate, thus forming a stop for this shoe. The anchoring element transmits at least part of the braking torque between the shoes and backing plate. The anchoring element of both shoes is formed by the actuator  10 . 
     The actuator  10  extends along an axis Y perpendicular to the axis X of the backing plate and parallel to the backing plate. It includes a casing  12  which is attached to the backing plate  2  on the side of the first face. The casing includes a housing formed by a through bore  13  extending along the axis Y and opening at both longitudinal ends of the casing  12 . A drive assembly is housed in the bore  13 . 
     The drive assembly is, for example, of the screw-nut type, such as that described in application WO2015/101486. It includes two bearing elements  16 ,  20  for forming a fixed point for the ends  3 . 2 ,  4 . 2  of the brake shoes  3 ,  4  respectively in service braking, and applying a force on the ends  3 . 2 ,  4 . 2  in parking braking by being moved towards outwardly of the casing along the axis Y. 
     The drum brake also includes a drive device  22  for the actuator, attached to the other face side of the backing plate opposite the face to the side of which the actuator  10  is mounted. 
     The drive device is an electric motor which may or may not be associated with a motor reducer. 
     A passageway (not illustrated) is provided through the backing plate  2  to allow drive transmission from the drive device to the drive assembly of the actuator  10 . 
     In  FIG. 4 , an example drum brake actuator  10  according to the invention can be seen. It includes a first threaded element  16  and a second element  18  having an internal thread receiving the first threaded element  16 , the first and second elements interacting with each other to form a screw-nut system. This screw-nut system produces the linear movement under the effect of a rotation of the second element  18  relative to the first element  16 . It transforms torque received by the second element  18  into two opposite axial forces on each of the elements  16 ,  18  respectively. 
     The first element  16  forms a piston bearing against the brake shoe  3  and a second piston  20  is interposed between the brake shoe  4  and the second element  18 . The second element  18  is to be rotatably driven by a toothed wheel  24  rotated by the drive device. In the example represented, the first element  18  is mounted in the toothed wheel  24  and is rotatably secured relative thereto by means of splines. 
     In this embodiment, the thread angle of this screw-nut system is chosen so that the force transmission obtained is irreversible, by choosing a thread angle which is less than the friction angle of the pair of materials used to make these two elements. 
     This choice of a screw-nut system, combined with the choice of such a thread angle, produces an irreversibility that provides the locking function in the parking braking position. That is, a force received by the pistons  16 ,  20  from the shoes  3 ,  4  is blocked by the non-slip between the threads of both elements of the screw-nut system. It is thus not transmitted to the motor system, making it unnecessary to block the motor or to keep it under load. 
     Further, as is visible in  FIG. 4 , the actuator  10  also includes an elastically deformable element along the axis of the actuation motion, so-called elastic element or also spring pack, to provide the function of force stabilisation in the bearing chain. In this exemplary embodiment, this elastic element is made by one of both pistons, in this case the second piston  20 . Depending on the forces transmitted in the direction of movement of the spreader, this elastic element has a determined spring constant to provide a stroke allowing to maintain or restore sufficient bearing of the shoes against the drum under different circumstances or changes of situation. 
     Alternatively, the spring pack is arranged between the second piston  20  and the second element  18 . 
     In particular, this elastic element is thus likely to store, by compression in the actuation assembly upon activating the actuator while the device is in the first braking position, a sufficient quantity of mechanical energy to maintain or bring the device into the second braking position if bearing of the wheel cylinder  6  is interrupted after activation of the actuator  10  without the need to activate the actuator again. 
     Such a situation occurs, for example, when the driver stops the vehicle and holds it stationary using the service brake, and then engages the parking brake before releasing the service brake control, for example when stopping to park on a slope. This elastic reserve compensates for the strokes required to move from one braking position to another, for example from simplex to duo-servo mode, while providing sufficient load to meet the needs for bringing the vehicle to a standstill. 
     The stroke of this elastic element also makes it possible, during a variation in the dimensions of elements  3 ,  4  of the drum brake, and without activation of the actuator  10 : 
     to maintain the bearing force of the shoes  3 ,  4  against the friction track in the case of dimensional variations in one direction, for example in the case of thermal shrinkage of the shoes or of the elements of the mechanical chain creating this bearing, such as the pistons or the mechanism which moves them apart, or for example in the case of thermal expansion of the drum; and 
     to limit the increase in forces in the mechanism in the case of dimensional variations in the other direction, which may be caused, for example, by thermal shrinkage of the drum when it cools down when stationary after having been heated as a service brake during a journey. 
     This elastic element thus makes it possible to limit and most often avoid any need for automatic reactivation of the system during parking, also called “re-clamping”, which can be energy consuming and subject to malfunctions that may have serious consequences. 
     In embodiments where motion of the actuator is achieved by an irreversible type mechanism, the elastic element is located downstream of the irreversible mechanism. 
     In the example in  FIG. 4 , the elastic element is here provided by the second piston  20  which comprises a piston head  202  having rearwardly a skirt within which a piston bottom  203  can slide. The piston head  202  and the piston bottom  203  bear against each other by means of a compressible elastic structure  201 , in this case an advantageously pre-stressed stack of conical steel washers, known as “Belleville” washers. The assembly is held together by crimping the end of the skirt around the rear of the piston bottom  203 . 
     According to the invention, the drum brake includes means  26  forming a stop for at least one of the movable elements of the actuator  10 , during the parking brake release phase. 
     The movable element has a release position Pr from which, in the brake release phase, it is certain that the parking brake has been released. The means  26  are arranged in the actuator in a position Pb beyond the release position Pr in the release phase. 
     The term “movable elements of the actuator” refers to the elements of the actuator that are moved upon applying the parking brake and releasing the same. 
     According to a first example, the stop means are arranged between the spring pack and the body of the actuator  10 . 
     When the actuator is activated to release the parking brake, the second element  18  slides along the axis Y of the actuator towards the first piston  16 , the second piston  20  moves in the direction S 2  under the action of the shoe  4  which is brought back inwardly by the return spring  8 . Further, the spring pack relaxes, causing the piston bottom  203  of the piston  20  to move in direction S 2 . The actuator is in a state where the brake is released, the bottom  203  of the second piston  20  is in the position Pr. The bottom  203  continues to move to bear against an axial stop and reach the rest position Pb. The position Pb of the stop  26  is such that the bottom  203  bears against it, when the spring pack is completely released. In the example represented, the stop  26  is a shoulder formed in the bore  13  of the actuator casing  12 . The end of the second element  18  for contacting the second piston  20  slides in a reduced diameter portion of the bore  13 . 
     The position Pb of the bottom  203  is then still the same at each release of the parking brake. This position is known to within the manufacturing clearance. 
     Consequently, at the beginning of each activation of the parking brake, the position of the second piston  20  is known to within the manufacturing clearance. Its position, upon activating the actuator  10 , can then be estimated relatively accurately by counting the number of revolutions of the second element  18  which is known from the number of revolutions of the motor and the reduction ratio between the motor and the second element  18 . The estimation uncertainty due to the uncertainty of the position of one of the actuator elements in the rest state is eliminated. The position Pb is downstream of the position Pr in the direction of movement of the movable element in the brake release phase. 
     In another exemplary embodiment represented in  FIG. 5 , the actuator  110  includes stop means formed by a tooth  126  on the thread of the first piston  16  against which the longitudinal end  18 . 1  of the second element  18 , oriented on the side of the first piston  16 , bears when the actuator is in the released position. The tooth  126  is schematically represented in  FIG. 5 . 
     The operation of the brake according to the invention will now be described from the example of  FIG. 5  and with the schematic representation of the movement D of the second element  18  in  FIG. 2 . 
     The positions in  FIG. 2  are the positions of the longitudinal end  18 . 1  of the second element  18  for contacting the tooth  126 . 
     It is considered that the parking brake is applied ( FIG. 5 ). The first element is in the position Ps. The pistons  16  and  20  are moved away from each other and apply the brake shoes against the drum. The longitudinal end  18 . 1  of the second element  18  is in the position Pf, when the parking brake is applied. 
     If the driver decides to deactivate the parking brake, he or she controls the activation of the actuator  10 , for example by pressing a button, the second element  18  is rotated via the electric motor and the toothed wheel, so that the pistons  16  and  20  slide towards each other. The second piston  16 , in particular its threaded rod, slides in the second element  18 , the piston head  16 . 1  moving closer to the longitudinal end  18 . 1  of the second element. The second element  18  turns until the pistons  16 ,  20  have travelled a stroke towards each other which is considered sufficient to ensure the parking brake release, the longitudinal end  18 . 1  of the second element  18  then has the position Pr. The second element  18  is then turned until the tooth  126  bears against the longitudinal end  18 . 1  of the second element, the longitudinal end  18 . 1  of the second element  18  is in the position Pb. A control unit UC then measures an increase in the current across the motor, and controls the power supply to the motor to be stopped. The actuator is stopped. 
     The position of the tooth  126  is advantageously chosen so that the contact between the tooth and the second element  18  takes place just after the release stroke has been travelled in order to obtain a very compact brake. 
     Thus, when designing the brake, in particular the actuator, the axial dimension of the actuator is chosen so that the second element  18  can travel the distance between Pr and Pb. As the position Pb is known to within the manufacturing clearance, the length of the actuator can be optimised and there is no need to provide an additional margin to take account of the uncertainty of the position of the second element in the released position. The distance between Pb and Pr is at least equal to the manufacturing clearance. In  FIG. 2 , this distance is slightly greater than the clearance j. 
     In  FIG. 1 , the movement of the second element  18  in an actuator of the state of the art can be seen. A rest position assumed by the second element is an estimated position Pe, which is estimated with an error margin e. Dimensioning of the actuator takes account of the error margin e and the manufacturing clearance j for the position Pb′ of the stop. Considering an actuator of the type shown in  FIG. 5 , the length of the actuator of the state of the art is at least 2e greater than that of the actuator according to the invention in  FIG. 5 . 
     By virtue of the invention, a reduction of 2 mm in the length of the actuator can be achieved. 
     Further, by optimising the stroke of the first element, i.e. by reducing it, the application of the parking brake during the next activation of the parking brake will be faster, as the first element has a shorter distance to travel. 
     According to another exemplary embodiment, a stop is provided on the body of the actuator against which the first element  18  bears in the stop position. 
     The present invention is also applicable to an electromechanical disc brake. 
     The present invention applies to floating caliper disc brakes and fixed caliper disc brakes. 
     In the examples described, the movable element(s) of the actuator move along an axis and possibly rotate about that axis. In another example, the actuator has at least one part that is only rotatably movable, for example of the cam type, the stop forming an angular stop. The stop means interrupt a rotational motion only. 
     In the example of  FIG. 4 , the stop means form an axial stop for the spring pack. In the example shown in  FIG. 5 , the stop means form a rotational stop and interrupt a translational movement. Further alternatively, the stop means form an axial stop for an element that is both rotatably and translationally movable. 
     An exemplary method for making the brake shown in  FIG. 5  will now be described. 
     Firstly, the minimum movement of the movable element(s) of the actuator is determined to make sure that the brakes are released. From this minimum movement, the position that the movable element(s) have to reach in the release phase, for the brake to be released, is determined. This position is the position Pr in  FIG. 2 . 
     The position of the stop Pb is then determined relative to the position Pr, taking account of the manufacturing clearance j. The minimum distance between Pr and Pb is at least equal to j. 
     The stop means are made on the body of the actuator or another part of the actuator, so that the movable part contacts the stop in the position Pb, i.e. Pr+j. The various parts of the actuator are adapted to this movement, especially the casing which has a greater compactness. 
     In the case of a movable element having a rotational movement only, the movement is an angular movement. 
     The present invention applies to all electromechanical brakes that are partly or entirely electrically actuated. It applies to brakes in which parking and/or service braking is achieved by electrical actuation. In the case of an electromechanical brake, in which service braking is provided by an electrical actuator, contact between at least one movable element of the actuator and the stop means takes place at each complete release of the service braking. 
     REFERENCES 
     
         
         
           
               1  drum brake 
               2  backing plate 
               3  first shoe 
               4  second shoe 
               3   a ,  4   a  webs 
               3   b ,  4   b  brake linings 
               6  hydraulic wheel cylinder 
               7  wear adjustment connecting rod 
               8 ,  9  return springs 
               10  mechanical actuator 
               12  actuator casing 
               13  actuator bore 
               16  first element/first piston 
               16 . 1  head of the second piston 
               18  second element 
               18 . 1  longitudinal end of second element 
               20  second piston 
               22  electric motor 
               24  toothed wheel 
               26 / 126  stop means 
               201  spring pack 
               202  head of the second piston 
               203  top of the second piston 
             X, Y axes 
             e error margin 
             j manufacturing clearance 
             Pb, Pb′ stop position 
             Pf braking position 
             Pr release position 
             UC Control unit