Patent Publication Number: US-2022234562-A1

Title: Braking system of vehicle capable of regenerative braking and hydraulic braking and method of controlling the same

Description:
TECHNICAL FIELD 
     The present disclosure relates to a braking system of a vehicle capable of regenerative braking and hydraulic braking and a method of controlling the same. 
     BACKGROUND 
     The statements in this section merely provide background information on the present disclosure and do not necessarily constitute related art. 
     Regenerative braking is a way of braking that drives a motor as a generator using the driving inertia of a vehicle and uses a resistance produced by driving the motor as a braking force. 
     In the case of a hybrid electric vehicle (HEV), a regenerative braking unit and a hydraulic braking unit cooperate to brake the vehicle (hereinafter, “cooperative braking”), thereby providing stable braking force to the vehicle. 
     The vehicle further includes an electric booster unit in order to boost a driver&#39;s pedal effort. The electric booster unit uses rotational torque of an electric motor provided in the electric booster unit to boost a force applied to the inside of a master cylinder from an operating rod. Also, when generating a pedal force, the electric booster unit is configured to give the driver a required pedal feel. Specifically, the electric booster unit is configured to generate a proper amount of pedal force corresponding to a pedal stroke as a reaction disc is compressed by the electric booster unit. 
     Meanwhile, when a vehicle with a conventional cooperative braking function performs regenerative braking, electronic stability control (ESC) is used to reduce hydraulic pressure by an amount compensated by regenerative braking during regenerative braking. To this end, conventional vehicles require ESC with a specification that enables cooperative control of regenerative braking and hydraulic braking. Specifically, ESC requires a pressure reducing device such as an accumulator in order to reduce hydraulic pressure, which requires a higher specification for ESC. This may lead to a cost increase. 
     Moreover, when regenerative braking is disabled while a vehicle with a conventional cooperative braking function is in the midst of cooperative braking, the amount of hydraulic oil in the braking system is increased using ESC, in order to increase the braking force of hydraulic braking. A pump is activated to draw oil from the master cylinder in order to increase the amount of hydraulic oil in the braking system. As a result, the pressure generated inside the master cylinder is reduced, and the driver will have an unnatural feel that the pedal force is lowered. 
     SUMMARY 
     In view of the above, the present disclosure primarily aims to minimize the driver&#39;s unnatural feel on the brake when regenerative braking is disabled during cooperative braking. 
     The problems to be solved in the present disclosure are not limited to the foregoing problems, and other problems not mentioned herein would be clearly understood by one of ordinary skill in the art from the following description. 
     As explained above, according to this embodiment, the braking system allows a motor piston to move backward while the hydraulic pressure in the master cylinder is temporarily reduced, when regenerative braking is disabled during cooperative braking, thereby minimizing the driver&#39;s unnatural feel on the brake. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a braking system according to an embodiment of the present disclosure. 
         FIG. 2  is a graph for explaining how the operation works when the braking system brakes the vehicle in a cooperative braking mode. 
         FIG. 3A  and  FIG. 3B  are a cross-sectional view for explaining how the operation works when a regenerative braking interrupt signal is received in the cooperative braking mode of the braking system according to an embodiment of the present disclosure. 
         FIG. 4  is a flowchart of a method of controlling a braking system according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     A conceptual diagram of the braking system  1  depicted in  FIGS. 1 to 3  in the present disclosure is a simplified view of how the braking system  1  works for ease of comprehension, and it should be noted that there might be differences with a concrete shape of an actual braking system  1 . 
       FIG. 1  is a cross-sectional view of a braking system according to an embodiment of the present disclosure. 
     Referring to  FIG. 1 , a braking system  1  according to an embodiment of the present disclosure includes all or part of a pedal master unit  10 , an electric booster unit  20 , a housing  30 , a pedal feel generating unit  40 , and an electric control unit  50  (e.g., a controller). 
     The pedal master unit  10  is configured to transmit a pedaling force to a master cylinder  14  when a pedal  11  is applied by the driver. The pedal master unit  10  includes all or part of the pedal  11 , an operating rod  12 , a push rod  13 , the master cylinder  14 , and a return spring  15 . 
     The pedal  11  is a part the driver pushes to slow or stop the vehicle. When the driver presses the pedal  11  to apply a certain amount of pressure or higher to one end of the operating rod  12 , the other end of the operating rod  12  compresses the reaction disc  32 . In this case, a stroke of the pedal  11  is sensed by a pedal travel sensor (not shown) which is provided separately. The one end of the operating rod  12  may be disposed to abut a central part of the reaction disc  420 . 
     The operating rod  12  is a medium by which the driver&#39;s pedal effort is transmitted to the reaction disc  420 . The one end of the operating rod  12  is connected to the pedal  11 . The pedal effort F RD  transmitted to the reaction disc  420  is delivered to the master cylinder  14  by the operating rod  12 . In an initial state in which the pedal  11  starts to be applied, the other end of the operating rod  12  may be separated from the reaction disc  420 . As the pedal  11  is applied, the other end of the operating rod  12  moves forward toward the reaction disc  420 . 
     At least part of the push rod  13  is inserted into the master cylinder  14 . The push rod  13  reciprocates in a lengthwise direction of the master cylinder  14  within the master cylinder  14 , and when the push rod  13  moves forward, it may pressurize a brake fluid stored in the master cylinder  14 . 
     The master cylinder  14  is configured to hold a brake fluid within. The brake fluid within the master cylinder  14  is pressurized, and a hydraulic pressure used for braking is generated. The generated hydraulic pressure is transmitted to a plurality of wheel brakes (not shown). 
     The return spring  15  is disposed inside the master cylinder  14 , and contracts or expands by reciprocating motion of the push rod  13 . Preferably, the return spring  15  may be a coil spring. However, the present disclosure is not necessarily limited to this, and the return spring  15  may be a plate spring or be made of an elastic material such as rubber. Also, although not shown in this disclosure, the return spring  15  may be disposed inside a housing of the electric booster unit  20 . The return spring  15  may be disposed inside the master cylinder  14  or inside the electric booster unit  20  so as to be compressed by part of a force transmitted by either or both of the operating rod  12  and the electric booster unit  20 . 
     The electric booster unit  20  is configured to boost the driver&#39;s pedal effort. The electric booster unit  20  includes all or part of a motor  22 , a gear device  24 , a screw shaft  26 , and a motor piston  28 . 
     The motor  22  is configured to rotate forward or backward in response to a signal from the electric control unit  50 . 
     The gear device  24  is configured to transmit a rotational torque of the motor  22  to the screw shaft  26 . The gear device  24  includes all or part of a first gear  240 , a second gear  242 , and a third gear  244 . 
     The first gear  240  receives the rotational torque transmitted from the motor  22  and transmits it to the second gear  242 . The second gear  242  transmits the rotational torque transmitted from the first gear  240  to the third gear  244 . The third gear  244  transmits the rotational torque transmitted from the second gear  242  to the screw shaft  26 . The speed of rotation may be reduced or increased by a certain percentage while the rotational torque is transmitted to the first gear  240  to the third gear  244 , based on the ratio of the number of teeth among the first to third gears  240  to  244 . 
     The screw shaft  26  is configured to convert the rotational toque transmitted by the gear device  24  into linear motion. The screw shaft  26  includes all or part of a first shaft  260  and a second shaft  262 . 
     The first shaft  260  rotates while restrained by the third gear  244 . The second shaft  262  is configured to convert the rotating motion of the first shaft  260  into linear motion. Preferably, the first shaft  260  may be comprised of a pinion, and the second shaft  262  may be comprised of a rack. One end of the second shaft  262  is connected to the motor piston  28 . Due to this, as the motor  22  is driven, the second shaft  262  moves forward toward the reaction disc  420  or moves backward, i.e., in the opposite direction. 
     The motor piston  28  reciprocates in the lengthwise direction of the master cylinder  14  by a force transmitted by a combination of the gear shaft  24  and the screw shaft  26 . The motor piston  28  is configured in such a way that one side is compressed by the second shaft  262  and the other side compresses the reaction disc  420 . 
     The motor piston  28  is disposed in proximity to the first shaft  260  if the pedal  11  is not pressed, that is, there is no braking request signal. 
     The housing  30  is formed to surround at least part of the pedal master unit  10 , at least part of the electric booster unit  20 , and at least part of the pedal feel generating unit  40 . The housing  30  includes a spring mount  32 . 
     The spring mount  32  is fixed to the housing  30 , and at least part of the pedal spring unit  44  is attached to one side of the spring mount  32 . The spring mount  32  is formed to support the pedal spring unit  44  when the pedal spring unit  44  is compressed by the driver&#39;s pedal effort. 
     When the pedal  11  is pressed by the driver, the pedal feel generating unit  40  provides a pedal feel to the driver. The pedal feel generating unit  40  includes all or part of a disc unit  42  and the pedal spring unit  44 . 
     The disc unit  42  is configured to be compressed by one or more of the operating rod  12  and the motor piston  28 . A reaction force to the pedal feel generated by the disc unit  42  is transmitted to the push rod  13 . As the push rod  13  pressurizes the brake oil stored in the master cylinder  14  and at least part of the pressurized brake oil is delivered to a plurality of wheel brakes (not shown), a hydraulic braking force F hyd  may be generated. 
     The disc unit  42  includes a reaction disc  420  and a reaction disc container  422 . 
     The reaction disc  420  is configured to be compressed by the operating rod  12 . When one end of the operating rod  12  is compressed by the force the driver applies on the pedal  11 , the other end compresses the reaction disc  420 . 
     Moreover, the reaction disc  420  is configured to be compressed by the motor piston  28 . Meanwhile,  FIG. 1  of the present disclosure illustrates that the reaction disc  420  and the motor piston  28  are in contact with each other even in the initial state. However, if no braking request signal is generated by the electric control unit  50 , this may mean that the motor piston  28  is separated from the reaction disc  420 . 
     Meanwhile, as the pedal  11  is pressed, an end of the operating rod  12  moves forward toward the reaction dic  420  and abuts the reaction disc  420 . If the pedal  11  is pressed further, an outer circumference of the reaction disc  420  is compressed by the motor piston  28 , and the central part of the reaction disc  420  is compressed by the operating rod  12 . To this end, a longitudinal section of the motor piston  28  may be approximately annular, and the operating rod  12  may penetrate through an open central part of the motor piston  28 . In this case, the operating rod  12  and the reaction disc  420  are coaxially arranged. However, the present disclosure is not limited to this, any braking system with a device capable of compressing the reaction disc  420  by applying the pedal  11  and driving the motor  22  may be included in this disclosure. For example, the outer circumference of the reaction disc  420  may be compressed by the operating rod  12 , and the central part of the reaction disc  420  may be compressed by the motor piston  28 . Even in this case, the operating rod  12  and the reaction disc  420  may be coaxially arranged. 
     The reaction disc  420  is made of a compressible material. For example, at least part of the reaction disc  420  may be made of a rubber material. When the reaction disc  420  is compressed by one or more of the operating rod  12  and the motor piston  28 , a reaction force created by the compressing force is transmitted to the driver through the operating rod  12 , and constitutes part of the driver&#39;s pedal feel. Hereinafter, the pedal force generated as the reaction disc  420  is compressed by an external force is referred to as F RD . 
     The reaction disc container  422  is formed in such a way as to contain at least part of the reaction disc  420  in a space formed therewithin. When one side of the reaction disc container  422  is compressed by one or more of the operating rod  12  and the motor piston  28 , the other side of the reaction disc container  422  compresses the push rod  13 . 
     One side of the pedal spring unit  44  is connected to the operating rod  12 , and the other side is connected to the spring mount  32 . The pedal spring unit  44  generates a tensile force or compressing fore as the relative distance between the pedal  11  and the spring mount  32  increases or decreases. A reaction force created by the compression of the pedal spring unit  44  is transmitted to the driver through the operating rod  12 , and constitutes part of the driver&#39;s pedal feel. Hereinafter, the pedal force generated as the reaction disc  420  is compressed by an external force is referred to as F spring . 
     The pedal spring unit  44  includes a spring  440  and a damper  442 . In this disclosure, the spring  440  and the damper  442  are illustrated as being connected in series but not necessarily limited to this, and the spring  440  and the damper  442  may be connected in parallel. 
     A total pedal force F pedal  provided to the driver may be determined as the sum of the pedal force F RD  generated by the reaction force to the compressing force of the disc unit  42  and the pedal force F spring  generated by the reaction force to the compressing force of the pedal spring unit  44 . 
     The electric control unit  50  generates a braking request signal based on a pedaling signal transmitted from a pedal travel sensor (not shown). The braking request signal is an electrical signal that allows at least part of a plurality of wheel brakes (not shown) to generate a braking force. 
     The electric control unit  50  calculates the total braking force F total  required to brake the vehicle based on the pedaling signal. Also, the electric control unit  50  determines whether to use regenerative braking or not, and applies a regenerative braking force F reg  and controls the electric booster unit  20  differently, depending on whether regenerative braking is used or not. Here, the total braking force required may be the sum of hydraulic braking force and regenerative braking force. A plurality of braking modes may be set. For example, the electric control unit  50  may brake the vehicle by setting a hydraulic braking mode which uses hydraulic braking force alone, a regenerative braking mode which uses regenerative braking force alone, and a cooperative braking mode which uses both hydraulic braking force and regenerative braking force. 
       FIG. 2  is a graph for explaining how the operation works when the braking system brakes the vehicle in a cooperative braking mode. 
     Although the electric control unit  50  may brake the vehicle by setting the braking mode to the hydraulic braking mode and the regenerative braking mode as explained above,  FIG. 2  only illustrates the cooperative braking mode. 
     Upon receiving a braking signal, the electric control unit  50  calculates a corresponding total braking force required. After calculating the total braking force required, the electric control unit  50  calculates the hydraulic braking force and the regenerative braking force so as to meet the total braking force required, and accordingly brakes the vehicle. A regenerative braking interruption signal is turned ON after a certain period of braking time, and the electric control unit  50  performs control to decrease the regenerative braking force and increase the hydraulic braking force, from a first point in time X 1  where the regenerative braking interruption signal is received. 
     The hydraulic braking force increases while the regenerative braking force decreases between the first point in time X 1  and a second point in time X 2 . The amount of hydraulic braking force at the second point in time X 2  may be equal or similar to the amount of total braking force required. After the second point in time X 2 , braking is performed using the hydraulic braking force alone. 
       FIG. 2  illustrates a process in which the electric control unit  50  receives a regenerative braking interruption signal during braking of the vehicle and decreases the regenerative braking force to 0 and increases the hydraulic braking force up to the total braking force required. Even if the electric control unit  50  changes the setting from the cooperative braking mode to the hydraulic braking mode, the braking system  1  may still operate as in the graph of  FIG. 2 . Also, although  FIG. 2  illustrates that the regenerative braking force is greater than the hydraulic braking force in the cooperative braking mode, this is merely an example and they are not limited to what is illustrated in  FIG. 2 . 
     The hydraulic pressure in the master cylinder  14  drops suddenly due to a sudden rise in hydraulic braking force between the first point in time X 1  and the second point in time X 2 . In this case, the pedal force is also lowered, and the driver may have an unnatural feel on the brake. The present disclosure involves a braking system  1  for minimizing such an unnatural feel, and a concrete method for this will be described in detail in the description of  FIG. 3 . 
       FIG. 3  is a cross-sectional view for explaining how the operation works when a regenerative braking interrupt signal is received in the cooperative braking mode of the braking system according to an embodiment of the present disclosure. 
       FIG. 3 a    is a view illustrating how the pedal  10  is applied by the driver when braking. When the pedal  10  is applied by the driver, the operating rod  12  compresses the reaction disc  420 . The electric booster unit  20  operates as well to assist the driver&#39;s pedal effort and therefore compresses the reaction disc  420  together with the motor piston  28 . 
       FIG. 3 b    is a view illustrating how the braking system  1  operates when the hydraulic pressure in the master cylinder  14  drops suddenly between the first point in time X 1  and the second point in time X 2 . The braking system  1  has to raise the hydraulic braking force within a short time between the first point in time X 1  and the second point in time X 2 . That is, it is necessary to supply brake oil to a plurality of wheel brake mechanisms in order to raise the hydraulic braking force. The brake oil is supplied to a plurality of wheel brake mechanisms through the master cylinder  14  from a reservoir (not shown). At this point, the amount of brake oil in the master cylinder  14  is decreased, and the hydraulic pressure in the master cylinder  14  drops suddenly. Due to the sudden drop in hydraulic pressure in the master cylinder  14 , the driver&#39;s pedaling force is lowered, giving the driver an unnatural pedaling feel. To eliminate such an unnatural feel, the electric control unit  50  drives the motor  22  to move the motor piston  28  backward. 
     The electric control unit  50  calculates a first hydraulic pressure in the master cylinder  14  at the first point in time X 1  and calculates a second hydraulic pressure in the master cylinder  14 . Hydraulic pressures may be calculated by using a hydraulic measurement device (not shown) placed separately in the master cylinder  14 . The electric control unit  50  may store data on the hydraulic pressure in the master cylinder  14  relative to a displacement of the motor piston  28 . In this case, the electric control unit  50  may calculate the amount of hydraulic pressure in the master cylinder  14 , not by using the hydraulic measurement device (not shown) but by using the displacement of the motor piston  28 . 
     The electric control unit  50  may calculate the sudden drop in hydraulic pressure between the first point in time X 1  and the second point in time X 2  by using the difference between the first hydraulic pressure and the second hydraulic pressure. After calculating the sudden drop in hydraulic pressure, the displacement of the motor piston  28  relative to this sudden drop in hydraulic pressure may be calculated. Afterwards, the electric control unit drives the motor  22  to move the motor piston  28  backward by the calculated displacement. 
     As described above, the reaction disc  420  is made of an elastic material and therefore causes the operating rod  12  to compress the central part of the reaction disc  420  in such a manner as depicted in (b) of  FIG. 3 . As the motor piston  28  moves backward, the force compressing the reaction disc  420  decreases, which increases the force applied to the operating rod  12 . Accordingly, even if the hydraulic pressure in the master cylinder drops suddenly, the driver will not have an unnatural pedaling feel by moving the motor piston  28  backward. 
       FIG. 4  is a flowchart of a method of controlling a braking system according to an embodiment of the present disclosure.  FIG. 4  illustrates a flowchart of the cooperative braking mode, among the above-described hydraulic braking mode, regenerative braking mode, and cooperative braking mode. A method of braking when the electric control unit  50  sets the braking mode to the hydraulic braking mode or the regenerative braking mode will be omitted since it is apparent to those of ordinary skill in the art. 
     The electric control unit  50  determines whether a braking signal is received or not (S 10 ). The electric control unit  50  does not perform the control shown in  FIG. 4  until it receives a braking signal. The braking signal is generated when the driver applies the pedal  11  or when it is determined that the vehicle requires braking while autonomous driving. 
     If it is determined that a braking signal is received, the electric control unit  50  calculates a total braking force required (S 20 ). The total braking force required is calculated based on a stroke length sensed by a pedal travel sensor. After calculating the total braking force required, the electric control unit  50  determines which mode to use for braking, among the hydraulic braking mode, the regenerative braking mode, and the cooperative braking mode. The description of  FIG. 4  will be given on the assumption that the electric control unit  50  sets the braking mode to the cooperative braking mode to perform braking. 
     After calculating the total braking force required, the electric control unit  50  calculates required generative braking force and required hydraulic braking force (S 30 ). Here, a braking force corresponding to the total braking force required needs to be generated by adding the required generative braking force and the required hydraulic braking force. 
     The electric control unit  50  calculates a displacement of the motor piston  28  which is required to generate the calculated required hydraulic braking force (S 40 ). The electric control unit  50  may store the amount of hydraulic braking force generated relative to the displacement of the motor piston  28 , in the form of a look-up table (LUT) or the like. 
     After calculating the displacement of the motor piston  28 , the electric control unit  50  drives the electric booster unit  20  based on the displacement of the motor piston  28  (S 50 ). That is, the electric control unit  50  sends a driving signal to the motor  22  of the electric booster unit  20  and performs control so as to move the motor piston  28  forward or backward by the calculated displacement. As the motor piston  28  moves forward toward the master cylinder  14 , hydraulic pressure is supplied to a plurality of wheel brakes (not shown). 
     The electric control unit  50  determines whether the regenerative braking mode is turned OFF during braking in the cooperative braking mode (S 60 ). That is, upon receiving a regenerative braking interruption signal, the electric control unit  50  determines that the regenerative braking mode is turned OFF. 
     If the regenerative braking mode is turned OFF, the electric control unit  50  calculates a first hydraulic pressure applied into the master cylinder  14  at a first point in time X 1  where the regenerative braking interruption signal is received (S 70 ). The electric control unit  50  may calculate hydraulic pressure by using a hydraulic measurement device (not shown) provided to measure the hydraulic pressure in the master cylinder  14 , or may calculate hydraulic pressure by using data on the hydraulic pressure in the master cylinder  14  relative to the displacement of the motor piston  28  which is stored in a memory of the electric control unit  50 . 
     The electric control unit  50  calculates a second hydraulic pressure applied into the master cylinder  14  at a second point in time X 2  after a preset time interval from the first point in time X 1  (S 80 ). In this step, the electric control unit  50  calculates the hydraulic pressure in the master cylinder  14  by using a hydraulic measurement device (not shown). The second point in time X 2  may be a point in time where the hydraulic braking force becomes equal to the total braking force required, but is not limited thereto. 
     After calculating the first hydraulic pressure and the second hydraulic pressure, the electric control unit  50  calculates the difference in calculated hydraulic pressure between the first point in time and the second point in time (S 90 ). Next, the electric control unit  50  calculates the displacement of the motor piston  28  to be compensated for based on the pressure difference, by using the difference in calculated hydraulic pressure (S 100 ). The displacement to be compensated for based on the pressure difference may be measured in advance by an experiment and stored in a memory in the form of a LUT. 
     The electric control unit  50  drives the electric booster unit  20  based on the displacement of the motor piston  28  to be compensated for (S 110 ). The electric control unit  50  sends a control signal to the motor  22  to move the motor piston  28  backward in the opposite direction of the master cylinder  14 , so that the pressure applied to the operating rod  12  is kept constant over a period of time between the first point in time and the second point in time.