Patent Publication Number: US-2021179041-A1

Title: Method for detecting a leak in a drive-by-wire brake system

Description:
BACKGROUND 
     The present disclosure relates to a method for detecting a leak in a drive-by-wire brake system for a vehicle. 
     Two-axle vehicles are fitted with hydraulic systems that have two brake circuits, thus ensuring that, if one of the two brake circuits fails, the vehicle can still be braked by the other brake circuit. In general, the wheel brakes are associated with the brake circuits in a diagonally split arrangement, in which a brake circuit acts on one front wheel and the respective diagonally opposite rear wheel, or in a front/rear split. The two brake circuits are connected to a master brake cylinder A brake pedal is coupled to the master brake cylinder and is actuated by a driver to build up a corresponding brake pressure in the two brake circuits. 
     SUMMARY 
     In one exemplary embodiment, a method of detecting a leak in a brake-by-wire hydraulic brake system includes determining if the brake system is in an ABS cycle and determining a pressure medium volume delivered for a measured brake pressure when the brake system is in the ABS cycle. The pressure medium volume is compared with a model value for the brake system at the measured brake pressure. Wheel slip of at least one wheel is identified when a difference between the pressure medium volume and the model value exceeds a specified threshold. At least one brake is isolated corresponding to the at least one wheel without wheel slip. 
     In a further embodiment of any of the above, the least one isolated brake includes closing an inlet valve to the at least one brake. 
     In a further embodiment of any of the above, the at least one brake includes a first brake and a second brake each corresponding to a first wheel and a second wheel of the at least one wheel without wheel slip. The first brake and the second brake are isolated from the brake system. 
     In a further embodiment of any of the above, the brake system includes a first brake circuit and a second brake circuit. The first brake is located in the first brake circuit and the second brake is located in the second brake circuit. 
     In a further embodiment of any of the above, the brake system includes a first brake circuit that has a pair of first brakes. A second brake circuit that has a pair of second brakes. 
     In a further embodiment of any of the above, the first brake circuit includes one of a pair of first inlet valves upstream of the each of the pair of first brakes. 
     In a further embodiment of any of the above, each of the pair of first brakes is located fluidly between one of the pair of first inlet valves and one of a pair of first outlet valves. 
     In a further embodiment of any of the above, the second brake circuit includes one of a pair of second inlet valves upstream of each of the pair of second brakes. 
     In a further embodiment of any of the above, each of the pair of second brakes is located fluidly between one of the pair of second inlet valves and one of a pair of second outlet valves. 
     In a further embodiment of any of the above, the brake system includes an electrically controllable pressure source that is in fluid communication with the first brake circuit and the second brake circuit. 
     In a further embodiment of any of the above, identifying wheel slip of the at least one wheel includes identifying the wheel slip with a wheel speed sensor. 
     In a further embodiment of any of the above, wheel slip indicates the at least one wheel has entered at least one of a locked or a sliding condition. 
     In another exemplary embodiment, a brake-by-wire hydraulic brake system for a vehicle includes a first brake circuit with a pair of first brakes, a second brake circuit with a pair of second brakes, and an electrically controllable pressure source is in fluid communication with the first brake circuit and the second brake circuit. A controller is configured to determine if the brake system is in an ABS cycle and determining a pressure medium volume delivered for a measured brake pressure when the brake system is in the ABS cycle. The pressure medium volume is compared with a model value for the brake system at the measured brake pressure. Wheel slip of at least one wheel is identified when a difference between the pressure medium volume and the model value exceeds a specified threshold. At least one brake of the pair of first brakes or the pair of second brakes that corresponds to the at least one wheel without wheel slip is isolated. 
     In a further embodiment of any of the above, the isolation of at least one of the pair of first brakes or the pair of second brakes includes closing an inlet valve to the one of the pair of first brakes or the pair of second brakes. 
     In a further embodiment of any of the above, the first brake circuit includes one of a pair of first inlet valves upstream of the each of the pair of first brakes. 
     In a further embodiment of any of the above, each of the pair of first brakes is located fluidly between one of the pair of first inlet valves and one of a pair of first outlet valves. 
     In a further embodiment of any of the above, the second brake circuit includes one of a pair of second inlet valves upstream of each of the pair of second brakes. 
     In a further embodiment of any of the above, each of the pair of second brakes is located fluidly between one of the pair of second inlet valves and one of a pair of second outlet valves. 
     In a further embodiment of any of the above, identifying wheel slip of the at least one wheel includes identifying the wheel slip with a wheel speed sensor. 
     In a further embodiment of any of the above, wheel slip indicates the at least one wheel has entered at least one of a locked or a sliding condition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features and advantages of the present disclosure will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows. 
         FIG. 1  schematically illustrates a brake-by-wire brake system. 
         FIG. 2  illustrates a method of detecting a leak in the brake-by-wire brake system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an example “brake-by-wire” hydraulic brake system  10  on a vehicle having a first brake circuit  12 - 1  and a second brake circuit  12 - 2 . A brake control unit  14  generates control signals for the valves of the first and second brake circuits  12 - 1  and  12 - 2  as output signals A on the basis of sensor signals E as input signals. 
     The brake system  10  includes a master cylinder  16  that can be actuated by a brake pedal  18  and a pressure medium reservoir  28  that is connected to the master cylinder  16 . An electrically controllable pressure source  30  includes an electrohydraulic actuator with an electric motor  32  as a drive motor and provides pressurized fluid to the first brake circuit  12 - 1  and the second brake circuit  12 - 2  through a first circuit block valve  24 - 1  and a second circuit block valve  24 - 2 , respectively. 
     The first brake circuit  12 - 1  is in fluid communication with a front left-hand wheel brake  22 - 1 FL and a rear right-hand wheel brake  22 - 1 RR for a front left-hand wheel  20 - 1 FL and a rear right-hand wheel  20 - 1 RR, respectively, through respective hydraulic lines. The first brake circuit  12 - 1  also includes an inlet valve  34 - 1 FL and an outlet valve  36 - 1 FL that forms a pressure modulation device for the front left-hand wheel brake  22 - 1 FL. Additionally, the first brake circuit  12 - 1  also includes an inlet valve  34 - 1 RR and an outlet valve  36 - 1 RR that forms a pressure modulation device for the rear right-hand wheel brake  20 - 1 RR. 
     The second brake circuit  12 - 2  is in fluid communication with a front right-hand wheel brake  22 - 2 FR and rear left-hand wheel brake  22 - 2 RL for a front right-hand wheel  20 - 2 FR and a rear left-hand wheel  20 - 2 RL, respectively, through respective hydraulic lines. The second brake circuit  12 - 2  also includes an inlet valve  34 - 2 FR and an outlet valve  36 - 2 FR that forms a pressure modulation device for the front right-hand wheel brake  22 - 2 FR. Additionally, the second circuit also includes an inlet valve  34 - 2 RL and an outlet valve  36 - 2 RL that forms a pressure modulation device for the rear left-hand wheel brake  22 - 2 RL. 
     To detect a rotational behavior of the wheels  20 - 1 FL,  20 - 1 RR,  20 - 2 FR, and  20 - 2 RL, there are respective speed sensors S 2 , which feed their sensor signals to the control unit  14  for evaluation to enable a corresponding slip control operation to be carried out at the wheels  20 - 1 FL,  20 - 1 RR,  20 - 2 FR, and  20 - 2 RL. 
     In a “brake-by-wire” operating mode, the inlet valves  34 - 1 FL and  34 - 1 RR are connected to a first circuit block valve  24 - 2  and inlet valves  34 - 2 FR and  34 - 2 RL are connected to a second circuit block valve  24 - 2 . Each of the first and second circuit block valves  24 - 1  and  24 - 2  are in fluid communication with the electrically controllable pressure source  30  for generating a system pressure. To measure the system pressure generated by the electrically controllable pressure source  30 , a pressure sensor S is arranged on the high-pressure side thereof. Furthermore, each of the outlet valves  36 - 1 FL,  36 - 1 RR,  36 - 2 FR, and  36 - 2 RL are connected to the pressure medium reservoir  28  and are in a normally closed position as opposed to the inlet valves  34 - 1 FL,  34 - 1 RR,  34 - 2 FR, and  34 - 2 RL, which are in a normally open position. 
     In the illustrated example, the master cylinder  16  is a dual-circuit tandem master cylinder and is connected to the pressure medium reservoir  28 . To form a redundant braking approach for the “brake-by-wire” brake system  10 , the master cylinder  16  can be connected to the wheel brakes  22 - 1 FL and  22 - 1 RR of the first brake circuit  12 - 1  via a first block valve  26 - 1  and to the wheel brakes  22 - 2 FR and  22 - 2 RL of the second brake circuit  12 - 2  via a second block valve  26 - 1 . The brake pressure generated in this case is measured with a pressure sensor S 4 . With the first and second block valves  26 - 1  and  26 - 2 , the hydraulic connection between the master cylinder  16  and the first and second brake circuit  12 - 1  and  12 - 2  is divided in the “brake-by-wire” operating mode. 
     During the brake-by-wire operating mode, a displacement sensor S 5  measures a pedal actuation of the brake pedal  18  brought about by the driver to determine a braking demand of the driver. A displacement simulator  40  is coupled hydraulically to the master brake cylinder  5  and receives the braking demand measure by the sensor S 5  and simulates a haptic feedback corresponding to the brake pressure generated, i.e. a corresponding pedal feel, to the brake pedal  18 . 
     In the illustrated example, the electrically controllable pressure source  30  is a single-circuit electrohydraulic actuator with a piston  38  actuated by the electric motor  32  via a rotation/translation mechanism. The piston  38  delimits a pressure space, which is connected to the pressure medium reservoir  28  in order to draw in the pressure medium. The position of the piston  38  is determined from the rotor position of the electric motor  32 , which is determined by a rotor position sensor S 3 , thus allowing the pressure medium volume delivered to be determined from the position of the piston  38 . 
       FIG. 2  illustrates a method  100  of detecting a leak in the brake system  10 . A leak during the “brake-by-wire” operating mode is detected by using the pressure sensor S to measure the brake pressure generated during a braking operation, e.g. an ABS control operation (Item  102 ). With the first and second circuit block valves  24 - 1  and  24 - 2  open, a pressure medium volume delivered for this brake pressure can be determined with a sensor S. Depending on the brake pressure, a model value for the pressure medium volume at the delivered brake pressure is determined by the control unit  14  with the model value indicating a theoretically correct value of the pressure medium volume required to build up the pressure value detected. The pressure medium volume determined by the rotor position sensor S 3  is compared with this model value. If the difference between the pressure medium volume determined and the model value (Item  104 ) thereof exceeds a specified threshold (Item  106 ), the presence of a leak in the brake system  10  is assumed, and if it does not exceed the specified threshold (Item  108 ), the brake system  10  does not have leak. 
     When a leak has been detected during the ABS control operation, the control unit  14  can then identify the specific wheel  20 - 1 FL,  20 - 1 RR,  20 - 2 FR,  20 - 2 RL associated with the leak. To identify the specific wheel  20 - 1 FL,  20 - 1 RR,  20 - 2 FR,  20 - 2 RL, the control unit  14  will look to the respective speed sensors S 2  for each wheel to determine if there is wheel slip. When the speed sensors S 2  have identified wheel slip, the corresponding wheel  20 - 1 FL,  20 - 1 RR,  20 - 2 FR,  20 - 2 RL has entered a locked and/or a sliding condition. When one of the wheels  20 - 1 FL,  20 - 1 RR,  20 - 2 FR,  20 - 2 RL has experienced wheel slip, it indicates that the corresponding brake  22 - 1 FL,  22 - 1 RR,  22 - 2 FR,  22 - 2 RL is able to provide sufficient pressure such that a leak associated with that wheel  20 - 1 FL,  20 - 1 RR,  20 - 2 FR,  20 - 2 RL is unlikely. 
     In the illustrated example, the control unit  14  will check for wheel slip of the front left-handed wheel  20 - 1 FL with the respective wheel speed sensor S 2  (Item  110 ). If the information conveyed to the control unit  14  from the respective wheel speed sensor S 2  indicates that there is not wheel slip at the front left-handed wheel  20 - 1 FL, the control unit  14  will signal the inlet valve  34 - 1 FL to close (Item  112 ). The control unit  14  will then return to item  104  and continue to compare the difference between the pressure medium volume determined and the model value to identify the presence of a leak as discussed above. 
     If the control unit determined that there was wheel slip at the front left-handed wheel  20 - 1 FL, the control unit  14  will check for wheel slip of the front right-handed wheel  20 - 2 FR with the respective wheel speed sensor S 2  (Item  114 ). If the information conveyed to the control unit  14  from the respective wheel speed sensor S 2  indicates that there is not wheel slip at the front right-handed wheel  20 - 2 FR, the control unit  14  will signal the inlet valve  34 - 2 FR to close (Item  116 ). The control unit  14  will then return to item  104  and continue to compare the difference between the pressure medium volume determined and the model value to identify the presence of a leak as discussed above. 
     If the control unit determined that there was wheel slip at the front right-handed wheel  20 - 2 FR, the control unit  14  will check for wheel slip of the rear left-handed wheel  20 - 2 RL with the respective wheel speed sensor S 2  (Item  118 ). If the information conveyed to the control unit  14  from the respective wheel speed sensor S 2  indicates that there is not wheel slip at the rear left-handed wheel  20 - 2 RL, the control unit  14  will signal the inlet valve  34 - 2 RL to close (Item  120 ). The control unit  14  will then return to item  104  and continue to compare the difference between the pressure medium volume determined and the model value to identify the presence of a leak as discussed above. 
     If the control unit determined that there was wheel slip at the rear left-handed wheel  20 - 2 RL, the control unit  14  will check for wheel slip of the rear right-handed wheel  20 - 1 RR with the respective wheel speed sensor S 2  (Item  122 ). If the information conveyed to the control unit  14  from the respective wheel speed sensor S 2  indicates that there is not wheel slip at the rear right-handed wheel  20 - 1 RR, the control unit  14  will signal the inlet valve  34 - 1 RR to close (Item  120 ). The control unit  14  will then return to item  104  and continue to compare the difference between the pressure medium volume determined and the model value to identify the presence of a leak as discussed above. 
     Although the illustrated example method  100  provides a specific order of wheels for checking wheel slip, the control unit  14  can check the wheels in a different order, such as by the wheels in each of the first and second brake circuit  12 - 1  or  12 - 2 . One feature of the above method is to maintain the greatest amount of braking power when a leak is determined and to prevent further loss of fluid from the brake system  10 . 
     For example, if a leak is identified at a single wheel  20 - 1  or  20 - 2 , only that single wheel would be isolated from the remaining brake system  10  such that there would be one remaining wheel from one of the first and second brake circuit  12 - 1 ,  12 - 2  functioning and both wheels  20 - 1  or  20 - 2  from the other of the first and second brake circuits  12 - 1 ,  12 - 2 . Additionally, if a leak was identified at a single wheel  20 - 1 ,  20 - 2  in the first and second brake circuits  12 - 1 ,  12 - 2 , respectively, then the other of the wheel  20 - 1 ,  20 - 2  in the first and second brake circuits  12 - 1 ,  12 - 2  would still be able to provide braking for the vehicle. 
     Although the different non-limiting examples are illustrated as having specific components, the examples of this disclosure are not limited to those particular combinations. It is possible to use some of the components or features from any of the non-limiting examples in combination with features or components from any of the other non-limiting examples. 
     It should be understood that like reference numerals identify corresponding or similar elements throughout the several drawings. It should also be understood that although a particular component arrangement is disclosed and illustrated in these exemplary embodiments, other arrangements could also benefit from the teachings of this disclosure. 
     The foregoing description shall be interpreted as illustrative and not in any limiting sense. A worker of ordinary skill in the art would understand that certain modifications could come within the scope of this disclosure. For these reasons, the following claim should be studied to determine the true scope and content of this disclosure.