Braking control device

Abstract

A braking control device 80 comprises a control unit 92 that executes parking braking control in which, if a parking braking force is applied to a second wheel 13 after a vehicle 10 stops, a first normal braking force is applied to a first wheel 12 during a prescribed period after the vehicle 10 stops, and a friction material 44 is separated from a rotating body 43 corresponding to the second wheel 13 during the prescribed period.

Description

Brake control device

[0001] The present invention relates to a brake control device.

[0002] Patent Document 1 describes a brake system that applies a braking force to a wheel by pressing a friction material against a rotating body that rotates integrally with the wheel. The brake system is configured to press the friction material against the rotating body, and includes a normal braking device that applies a normal braking force to the wheel when there is a normal braking request based on the operation of the brake pedal, and a parking braking device that applies a parking braking force to the wheel when there is a parking braking request based on the operation of the parking brake switch.

[0003] During the running of the vehicle, the temperature of the rotating body and the friction material tends to increase because a normal braking force is repeatedly applied to the wheel. Also, the higher the temperature of the rotating body and the friction material, the more likely the amount of thermal expansion of the rotating body and the friction material is to increase. And when the vehicle is parked in a state where the temperature of the rotating body and the friction material is high, the amount of temperature drop of the rotating body and the friction material after applying the parking braking force to the wheel becomes large. In this case, the amount of decrease in the parking braking force becomes large in terms of the contraction of the rotating body and the friction material accompanying the temperature drop.

[0004] Therefore, when there is a parking braking request, the brake system waits in a state where only the normal braking force is applied to the wheel. That is, the brake system waits for the temperature of the rotating body and the friction material to decrease while maintaining the stop of the vehicle by applying the normal braking force. After the temperature of the rotating body and the friction material has decreased, the brake system applies the parking braking force to the wheel and releases the normal braking force that has been applied to the wheel. In this way, the brake system suppresses the decrease in the parking braking force accompanying the temperature drop of the rotating body and the friction material.

[0005] Japanese Unexamined Patent Application Publication No. 2022-114822

[0006] The above-described brake system applies the parking braking force to the wheel after the temperature of the friction material and the rotating body has decreased. For this reason, if the temperature of the rotating body and the friction material corresponding to the wheel to which the parking braking force is applied can be quickly decreased, it becomes possible to shorten the period during which the normal braking force is applied to the wheel.

[0007] A braking control device that solves the above problems comprises a first wheel which is one of the wheels of the vehicle, a first rotating body which rotates integrally with the first wheel, a first friction material which applies braking force to the first wheel by being pressed against the first rotating body, a second wheel which is a different wheel from the first wheel, a second rotating body which rotates integrally with the second wheel, a second friction material which applies braking force to the second wheel by being pressed against the second rotating body, and a parking braking device which applies parking braking force to the second wheel, and the parking A braking control device applicable to a vehicle equipped with a conventional braking device that has a mechanism different from a vehicle braking device, which applies a first normal braking force to the first wheel by pressing the first friction material against the first rotating body, and applies a second normal braking force to the second wheel by pressing the second friction material against the second rotating body, wherein, when applying the parking braking force to the second wheel after the vehicle has stopped, the control unit performs parking braking control which causes the first normal braking force to be applied to the first wheel during a predetermined period after the vehicle has stopped, and separates the second friction material from the second rotating body during the predetermined period.

[0008] The braking control device, through parking braking control, separates the second friction material from the second rotating body, thereby rapidly lowering the temperature of the second rotating body and the second friction material corresponding to the second wheel to which parking braking force is applied.

[0009] Figure 1 is a schematic diagram of a vehicle equipped with a braking control device. Figure 2 is a schematic diagram of the braking system installed in the vehicle in Figure 1. Figure 3 is a flowchart showing the processing flow executed by the braking control device to apply parking braking force to the vehicle in Figure 1. Figures 4(a) to 4(e) are timing charts showing the changes in the vehicle's state values ​​when the vehicle in Figure 1 is parked. Figure 5 is a flowchart showing the processing flow executed by a modified braking control device to apply parking braking force to the vehicle.

[0010] The following describes one embodiment of a vehicle equipped with a braking control device. <Configuration of this embodiment> As shown in Figure 1, the vehicle 10 is equipped with a braking operating member 11, a plurality of wheels 12, 13, a plurality of braking mechanisms 41, 42, a normal braking device 50, a parking braking device 60, a plurality of sensors 71 to 73 and a switch 74, and a braking control device 80.

[0011] The braking operation member 11 is a member operated by the driver when adjusting the deceleration of the vehicle 10. An example of the braking operation member 11 is the brake pedal. The plurality of wheels 12, 13 include a plurality of first wheels 12 and a plurality of second wheels 13. The first wheels 12 are different from the second wheels 13. An example of the plurality of first wheels 12 is two front wheels. An example of the plurality of second wheels 13 is two rear wheels. In Figure 1, only one of the plurality of first wheels 12 and only one of the plurality of second wheels 13 are shown.

[0012] <Braking Mechanism> The multiple braking mechanisms 41 and 42 consist of multiple first braking mechanisms 41, each individually provided for multiple first wheels 12, and multiple second braking mechanisms 42, each individually provided for multiple second wheels 13. In other words, the number of first braking mechanisms 41 is the same as the number of first wheels 12, and the number of second braking mechanisms 42 is the same as the number of second wheels 13.

[0013] Each of the multiple braking mechanisms 41 and 42 includes a rotating body 43, a friction material 44, a wheel cylinder 45, and a piston 46. The rotating body 43 rotates integrally with the corresponding wheels 12 and 13. Therefore, by pressing the friction material 44 against the rotating body 43, a braking force is applied to the corresponding wheels 12 and 13. When the wheel pressure, which is the hydraulic pressure in the wheel cylinder, increases, the piston 46 moves in the forward direction. When the piston 46 moves in the forward direction, the force with which the piston 46 presses the friction material 44 against the rotating body 43 increases. On the other hand, when the wheel pressure decreases, the piston 46 moves in the backward direction. When the piston 46 moves in the backward direction, the force with which the piston 46 presses the friction material 44 against the rotating body 43 decreases. Therefore, the braking mechanisms 41 and 42 can apply a greater braking force to the corresponding wheels 12 and 13 as the wheel pressure increases.

[0014] Hereafter, the braking force applied to the corresponding wheels 12 and 13 by the wheel pressure will be referred to as the "normal braking force." More specifically, the normal braking force applied to wheel 12 by the wheel pressure will be referred to as the "first normal braking force," and the normal braking force applied to wheel 13 by the wheel pressure will be referred to as the "second normal braking force." Furthermore, the rotating body 43 and friction material 44 of the first braking mechanism 41 correspond to the "first rotating body and first friction material," and the rotating body 43 and friction material 44 of the second braking mechanism 42 correspond to the "second rotating body and second friction material."

[0015] <Normal Braking System> The normal braking system 50 is configured to adjust the normal braking force by controlling the wheel pressure of a plurality of wheel cylinders 45. The normal braking system 50 is equipped with the same number of first pressure regulating actuators 51 as there are first wheels 12 and the same number of second pressure regulating actuators 52 as there are second wheels 13. The first pressure regulating actuators 51 and the second pressure regulating actuators 52 have a supply source for supplying brake fluid. Examples of supply sources include electric cylinders and electric pumps. The normal braking system 50 has a different mechanism from the parking brake system 60.

[0016] The first pressure regulating actuator 51 is connected to the wheel cylinder 45 of the first braking mechanism 41 via a fluid supply passage 53. Similarly, the second pressure regulating actuator 52 is connected to the wheel cylinder 45 of the second braking mechanism 42 via a fluid supply passage 54. In this way, the first pressure regulating actuator 51 adjusts the first normal braking force applied to the first wheel 12 by adjusting the wheel pressure of the wheel cylinder 45 of the first braking mechanism 41. Similarly, the second pressure regulating actuator 52 adjusts the second normal braking force applied to the second wheel 13 by adjusting the wheel pressure of the wheel cylinder 45 of the second braking mechanism 42.

[0017] <Parking Brake System> As shown in Figures 1 and 2, the parking brake system 60 has the same number of parking actuators 61 as there are second wheels 13. Multiple parking actuators 61 are integrated with the second braking mechanism 42 of the corresponding second wheel 13. Each parking actuator 61 includes an electric motor 62 and a linear motion conversion mechanism 63 that converts the rotational motion of the rotating shaft of the electric motor 62 into linear motion and transmits it to the piston 46 of the wheel cylinder 45.

[0018] When the electric motor 62 rotates in the first rotational direction, the piston 46 moves in the forward direction. When the piston 46 moves in the forward direction, the force with which the piston 46 presses the friction material 44 against the rotating body 43 increases. On the other hand, when the electric motor 62 rotates in the opposite direction to the first rotational direction, the piston 46 moves in the backward direction. When the piston 46 moves in the backward direction, the force with which the piston 46 presses the friction material 44 against the rotating body 43 decreases. In this way, the parking actuator 61 adjusts the braking force applied to the second wheel 13 by the operation of the electric motor 62. Hereafter, the braking force applied to the second wheel 13 by the parking actuator 61 will be referred to as "parking braking force".

[0019] The linear motion conversion mechanism 63 of the parking actuator 61 has a self-locking function. In other words, if the power of the electric motor 62 is not transmitted to the linear motion conversion mechanism 63, the components constituting the linear motion conversion mechanism 63 will not operate. Therefore, even if the power supply to the electric motor 62 is stopped while the parking braking force has increased due to the operation of the electric motor 62, the parking braking force is maintained by the self-locking function of the linear motion conversion mechanism 63. In this respect, the parking actuator 61 is a mechanism in the second braking mechanism 42 that maintains the position of the friction material 44 while it is pressed against the rotating body 43.

[0020] <Sensors and Switches> As shown in Figure 1, the multiple sensors output signals to the braking control device 80 according to the detection results. The multiple sensors 71 to 73 include a brake sensor 71, multiple wheel speed sensors 72, and an acceleration sensor 73. The brake sensor 71 detects information related to the driver's operation of the braking operating member 11. An example of the brake sensor 71 is a stroke sensor that detects the amount of operation of the driver's braking operating member 11. The amount of operation based on the detection signal of the brake sensor 71 is called the braking operation amount. The multiple wheel speed sensors 72 detect the rotational speed of the corresponding wheels 12 and 13. For example, the wheel speed sensor 72 outputs a pulse signal as a detection signal according to the rotational speed of the corresponding wheels 12 and 13. The rotational speed of the wheels 12 and 13 based on the detection signal of the wheel speed sensor 72 is called the wheel speed. The acceleration sensor 73 outputs a detection signal according to the acceleration of the vehicle 10 in the longitudinal direction. The acceleration based on the detection signal of the acceleration sensor 73 is called the longitudinal acceleration. The parking brake switch 74 outputs a signal to the braking control device 80 according to the operation status. The parking brake switch 74 is operated by the driver to switch the parking brake on and off.

[0021] As shown in Figure 1, the braking control device 80 adjusts the braking force of the vehicle 10 by controlling the normal braking device 50 and the parking braking device 60. The braking control device 80 includes a processing circuit 81 having a CPU 82 and a ROM 83. The ROM 83 stores a control program executed by the CPU 82 and various flags. When the CPU 82 executes the control program stored in the ROM 83, the processing circuit 81 functions as an acquisition unit 91 and a control unit 92.

[0022] <Acquisition Unit> The acquisition unit 91 acquires the braking operation amount, vehicle speed, vehicle acceleration, longitudinal acceleration, road surface gradient, EPB request flag, and EPB release request flag. The vehicle speed is calculated based on the wheel speeds of the multiple wheels 12 and 13. The vehicle acceleration is the time derivative of the vehicle speed. The road surface gradient is the gradient of the road surface on which the vehicle 10 is located. The road surface gradient is, for example, the absolute value of the difference between the vehicle acceleration and the longitudinal acceleration. The EPB request flag is a flag indicating whether or not there is a request from the driver for parking brake application. The EPB request flag is turned on, for example, when the parking brake switch 74 is operated while parking brake force is not applied to the second wheel 13. The EPB release request flag is a flag indicating whether or not there is a request from the driver for release of parking brake application. The EPB release request flag is turned on, for example, when the parking brake switch 74 is operated while parking brake force is applied to the second wheel 13. The EPB request flag and the EPB release request flag are stored, for example, in ROM 83.

[0023] <Control Unit> The control unit 92 controls the normal braking device 50 to adjust the first normal braking force applied to the first wheel 12 and the second normal braking force applied to the second wheel 13. For example, when the driver operates the braking operation member 11, the control unit 92 calculates the required braking force according to the amount of braking operation. Also, when another control device requests deceleration of the vehicle 10, the control unit 92 calculates the required braking force according to the request from the other control device.

[0024] Next, the control unit 92 calculates the first target wheel pressure, which is the target value for the wheel pressure of the first braking mechanism 41, and the second target wheel pressure, which is the target value for the wheel pressure of the second braking mechanism 42, based on the requested braking force. Then, the control unit 92 controls the first pressure regulating actuator 51 so that the wheel pressure of the first braking mechanism 41 becomes the first target wheel pressure. The control unit 92 also controls the second pressure regulating actuator 52 so that the wheel pressure of the second braking mechanism 42 becomes the second target wheel pressure.

[0025] The control unit 92 adjusts the parking braking force applied to the second wheel 13 by controlling the parking brake device 60. Specifically, when the EPB request flag is turned on, the control unit 92 causes the parking brake device 60 to apply parking braking force to the second wheel 13. On the other hand, when the EPB release request flag is turned on, the control unit 92 causes the parking brake device 60 to release the parking braking force that was applied to the second wheel 13. In other words, the control unit 92 sets the parking braking force applied to the second wheel 13 to "0".

[0026] When the driver is operating the vehicle 10, the driver operates the parking brake switch 74 after the vehicle 10 has arrived at its destination. That is, the EPB request flag is turned on when a parking brake request is generated. When the EPB request flag is turned on, it is possible that the rotating body 43 and friction material 44 in the first braking mechanism 41 and the second braking mechanism 42 have become hot due to the vehicle 10's previous driving. Therefore, if parking braking force is applied to the second wheel 13 while the rotating body 43 and friction material 44 in the second braking mechanism 42 are at a high temperature, the following problem may occur. That is, as the rotating body 43 and friction material 44 in the second braking mechanism 42 cool down, they contract due to heat, which can cause a decrease in the parking braking force applied to the second wheel 13 due to "thermal loosening." In this case, it is necessary to perform reclam control by moving the piston 46 forward again using the parking brake device 60 to increase the reduced parking braking force.

[0027] Therefore, when the EPB request flag is turned on, the control unit 92 applies parking braking force to the second wheel 13 when the temperature of the rotating body 43 and friction material 44 in the second braking mechanism 42 has sufficiently decreased. This will be explained in detail below.

[0028] When the EPB request flag is turned on, the control unit 92 controls the normal braking device 50 to start parking braking control, including the first braking process and the second braking process. In the first braking process, the control unit 92 applies a first normal braking force to the first wheel 12, but does not apply a second normal braking force to the second wheel 13. Also, in the first braking process, the control unit 92 does not apply parking braking force to the second wheel 13. In other words, when the first braking process is executed, in the first braking mechanism 41, the friction material 44 is pressed against the rotating body 43, whereas in the second braking mechanism 42, the friction material 44 is not pressed against the rotating body 43. In other words, in the second braking mechanism 42, the rotating body 43 and the friction material 44 are separated.

[0029] When normal braking is performed by the braking mechanisms 41 and 42, the rotating body 43 and the friction material 44 generate heat. Generally, when normal braking is performed, the rotating body 43 becomes hotter than the friction material 44. Therefore, by separating the rotating body 43 and the friction material 44, it is possible to suppress the heat absorbed by the friction material 44 from the hot rotating body 43. The decrease in parking braking force due to thermal relaxation is largely influenced by the thermal contraction of the friction material 44 as its temperature decreases. Therefore, by performing the first braking treatment, the temperature of the friction material 44 is more likely to decrease, and by suppressing thermal relaxation, the implementation of re-clamp control can be suppressed.

[0030] However, when the driver operates the parking brake switch 74, the driver is generally operating the braking operation member 11. In this regard, when the EPB request flag is turned on, the control unit 92 applies a first normal braking force to the first wheel 12 and a second normal braking force to the second wheel 13. Therefore, when the control unit 92 executes the first braking process, it maintains the state in which the first normal braking force is applied to the first wheel 12, while reducing the second normal braking force applied to the second wheel 13 to "0". In other words, after the first braking process has started, the vehicle 10 is kept stopped by the first normal braking force. Furthermore, after the first braking process has started, the control unit 92 does not reduce the first normal braking force even if the amount of braking operation decreases.

[0031] When vehicle 10 is stopped on a slope, the braking force required to maintain the vehicle 10's stop (hereinafter referred to as "parking maintenance braking force BPh") is greater than when vehicle 10 is stopped on a level road. Therefore, at the start of the first braking process, if the first normal braking force is less than the parking maintenance braking force BPh, it is preferable for the control unit 92 to increase the first normal braking force according to the road surface gradient so that the vehicle can maintain its stopped state even if the second normal braking force is reduced. More specifically, in the first braking process, it is preferable for the control unit 92 to increase the first normal braking force as the road surface gradient on which vehicle 10 stops increases.

[0032] As the control unit 92 continues the first braking process, the temperature of the rotating body 43 and the friction material 44 in the second braking mechanism 42 gradually decreases. When it is estimated that the temperature of the rotating body 43 and the friction material 44 in the second braking mechanism 42 has decreased sufficiently, the control unit 92 executes a second braking process that applies parking braking force to the second wheel 13. As an example, the start condition for the second braking process is met when the elapsed time since the second normal braking force became "0" due to the execution of the first braking process becomes a predetermined set time Tth. In other words, the start condition for the second braking process is met when the elapsed time since the friction material 44 separated from the rotating body 43 in the second braking mechanism 42 becomes a set time Tth. The set time Tth is preferably set in advance according to the specifications of the second braking mechanism 42. The set time Tth may be a fixed value, or it may be a variable value according to a parameter that affects the temperature of the second braking mechanism 42, such as the ambient temperature. Furthermore, the set time Tth may be a variable value corresponding to the estimated temperature of at least one of the rotating body 43 and the friction material 44.

[0033] In the second braking process, the control unit 92 causes the parking brake device 60 to apply parking braking force to the second wheel 13. That is, the control unit 92 drives the piston 46 in the forward direction using the linear motion conversion mechanism 63 by controlling the electric motor 62 of the parking brake device 60. When the parking braking force applied to the second wheel 13 exceeds a predetermined determination braking force as the piston 46 moves forward, the control unit 92 terminates the second braking process. As an example, the control unit 92 can determine whether the parking braking force applied to the second wheel 13 exceeds the determination braking force based on the current value of the electric motor 62 of the parking brake device 60. The determination braking force is set to a value greater than the parking maintenance braking force BPh. The determination braking force may be a fixed value or a value that changes according to the parking maintenance braking force BPh.

[0034] The control unit 92 terminates the first braking process after completing the second braking process. In other words, after a parking braking force greater than the parking maintenance braking force BPh is applied to the second wheel 13, the control unit 92 reduces the first normal braking force applied to the first wheel 12 to "0".

[0035] <Processing flow performed by the braking control device> The processing flow performed by the braking control device 80 will be explained with reference to the flowchart shown in Figure 3. This process is performed repeatedly in a predetermined control cycle when the vehicle 10 is stopped and no parking brake force is applied to the second wheel 13.

[0036] As shown in Figure 3, the braking control device 80 determines whether the EPB request flag is on or off (S11). If the EPB request flag is off (S11: NO), there is no parking brake request, so the braking control device 80 terminates this process. On the other hand, if the EPB request flag is on (S11: YES), there is a parking brake request, so the braking control device 80 determines whether the first normal braking force applied by the first braking mechanism 41 to the first wheel 12 is equal to or greater than the parking maintenance braking force BPh (S12).

[0037] If the first normal braking force is equal to or greater than the parking maintenance braking force BPh (S12: YES), that is, if the vehicle 10 can be kept stopped even if the second normal braking force becomes "0", the braking control device 80 eliminates the second normal braking force while maintaining the first normal braking force (S13). Once the second normal braking force is eliminated, the braking control device 80 determines whether the elapsed time since the second normal braking force was eliminated is equal to or greater than the set time Tth (S14). If the elapsed time since the second normal braking force was eliminated is less than the set time Tth (S14: NO), the braking control device 80 proceeds to step S14. In other words, the braking control device 80 waits for the rotating body 43 of the second braking mechanism 42 and the friction material 44 to cool down.

[0038] On the other hand, if the elapsed time since the second normal braking force was released is equal to or greater than the set time Tth (S14: YES), the braking control device 80 causes the parking braking device 60 to apply a parking braking force to the second wheel 13 (S15). Specifically, the friction material 44 is pressed against the rotating body 43 in the second braking mechanism 42 until the parking braking force is equal to or greater than the set braking force. Subsequently, the braking control device 80 releases the first normal braking force that was applied to the first wheel 12 (S16). After that, the braking control device 80 terminates this process. Therefore, from step S16 onward, the vehicle 10 is kept stopped by the parking braking force applied to the second wheel 13.

[0039] On the other hand, in step S12, if the first normal braking force is less than the parking maintenance braking force BPh (S12: NO), that is, if the second normal braking force becomes "0", the vehicle 10 will not be able to maintain its stop, the braking control device 80 increases the first normal braking force while eliminating the second normal braking force (S17). In this case, the braking control device 80 increases the first normal braking force so that the sum of the first normal braking force and the second normal braking force does not fall below the parking maintenance braking force BPh. Once the second normal braking force is eliminated, the braking control device 80 proceeds to step S14.

[0040] In the flowchart described above, steps S13, S16, and S17 correspond to the first braking process, and step S15 corresponds to the second braking process. <Operation and Effects of this Embodiment> The operation and effects of this embodiment will be described with reference to Figure 4. Figure 4 shows the changes in vehicle speed, normal braking force, parking braking force, and EPB request flag when the driver parks the vehicle 10.

[0041] As shown in Figures 4(a) to 4(e), prior to timing t11, the driver operates the braking operating member 11, thereby applying a first normal braking force to the first wheel 12 and a second normal braking force to the second wheel 13 via the normal braking device 50. In other words, the rotating body 43 and friction material 44 generate heat in the first braking mechanism 41, and the rotating body 43 and friction material 44 also generate heat in the second braking mechanism 42. Furthermore, as the first and second normal braking forces are applied to the first wheel 12 and the second wheel 13, the vehicle speed gradually decreases.

[0042] At timing t11, the vehicle speed becomes "0". In other words, vehicle 10 comes to a stop. In the example shown in Figure 4, the amount of braking applied by the driver is maintained constant even after vehicle 10 has come to a stop. For this reason, the first normal braking force and the second normal braking force are also maintained constant before and after timing t11.

[0043] At timing t12, a parking brake request is generated when the driver operates the parking brake switch 74, so the EPB request flag is turned on. In the example shown in Figure 4, the first normal braking force at timing t12 is less than the parking maintenance braking force BPh. Therefore, from timing t12 onwards, the first normal braking force increases while the second normal braking force decreases. At this time, even though the second normal braking force decreases, the first normal braking force increases so that the sum of the normal braking forces applied to the multiple wheels 12 and 13 does not become less than the parking maintenance braking force BPh.

[0044] When the timing t13 is reached, the increase in the first normal braking force ends, and at the same time, the decrease in the second normal braking force ends. At the timing t13, the first normal braking force becomes the parking maintenance braking force BPh. Also, at the timing t13, when the second normal braking force becomes "0", in the second braking mechanism 42, the friction material 44 separates from the rotating body 43. Therefore, after the timing t13, the cooling rate of the rotating body 43 and the friction material 44 in the second braking mechanism 42 becomes faster compared to the case where the rotating body 43 and the friction material 44 were in contact.

[0045] When the timing t14 is reached, at which the elapsed time from the timing t13 becomes the set time Tth, assuming that the rotating body 43 and the friction material 44 in the second braking mechanism 42 are sufficiently cooled, the application of the parking braking force to the second wheel 13 is started. That is, by starting the energization of the electric motor 62 of the parking brake device 60, the parking actuator 61 starts driving the piston 46 in the forward direction. Also, in the present embodiment, the application of the parking braking force to the second wheel 13 is started at a timing later than the timing t12 when the driver turns on the parking brake switch 74. On the other hand, at the timing t14, after the application of the parking braking force to the second wheel 13 is started, the first normal braking force applied to the first wheel 12 is maintained constant.

[0046] In this embodiment, the period from the timing t12 when the EPB request flag is turned on to the timing t14 when parking braking force is applied to the second wheel 13 is a cooling period for waiting for cooling of the rotating body 43 and the friction material 44 in the second braking mechanism 42. The cooling period includes the period from the timing t13 when braking force is not applied to the second wheel 13 to the timing t14. In other words, the cooling period includes the period during which the friction material 44 is separated from the rotating body 43 in the second braking mechanism 42. When the rotating body 43, which is at a higher temperature than the friction material 44, is separated from the friction material 44, the braking control device 80 can rapidly reduce the temperatures of the rotating body 43 and the friction material 44 in the second braking mechanism 42. That is, the braking control device 80 can shorten the period during which the operation of the first pressure regulating actuator 51 continues after the EPB request flag is turned on, or shorten the period from when the EPB request flag is turned on to when the application of parking braking force to the second wheel 13 is started.

[0047] At the timing t15, the parking braking force increases to the determined braking force. Therefore, at the timing t15, the energization of the electric motor 62 of the parking brake device 60 is stopped, and the piston 46 stops. As described above, the linear motion conversion mechanism 63 of the parking brake device 60 has a self-locking function. For this reason, even when the energization of the electric motor 62 of the parking brake device 60 is stopped, the parking braking force is maintained constant.

[0048] When the timing t16 is reached after the parking braking force has become the determined braking force, the first normal braking force applied to the first wheel 12 starts to decrease. At the timing t17, the first normal braking force becomes "0". That is, after the timing t17, while the first normal braking force and the second normal braking force are not applied to the first wheel 12 and the second wheel 13, respectively, the parking braking force is applied to the second wheel 13.

[0049] In the example shown in Figure 4, the timing t11 at which the vehicle 10 stops marks the start of the "predetermined period". The period from timing t11 to timing t17 within the "predetermined period" is the period during which the first normal force is applied to the first wheel 12. Furthermore, within the "predetermined period", the period from timing t13 onward is the period during which the second normal braking force is not applied to the second wheel 13.

[0050] <Examples of Modifications> This embodiment can be implemented with the following modifications. This embodiment and the following examples of modifications can be combined with each other to the extent that they do not contradict each other technically.

[0051] Immediately after the driver stops vehicle 10, the suspension connecting the wheels 12 and 13 to the vehicle body may be extended or contracted compared to the steady state due to the magnitude of the normal braking force when vehicle 10 stops. If the normal braking force applied to the wheels 12 and 13 corresponding to the suspension is abruptly reduced, the extension and contraction of the suspension will be abruptly released, which may affect the posture of vehicle 10. In other words, if the second normal braking force is abruptly reduced by executing parking brake control, the posture of vehicle 10 may be affected.

[0052] Therefore, when the control unit 92 of the braking control device 80 applies a parking brake to the second wheel 13 while applying a first normal braking force to the first wheel 12 and a second normal braking force to the second wheel 13, it limits the reduction gradient when reducing the second normal braking force applied to the second wheel 13 based on a physical quantity related to the attitude of the vehicle 10. For example, the physical quantity related to the attitude of the vehicle 10 is the pitch angle of the vehicle 10. In this case, it is preferable for the control unit 92 to reduce the reduction gradient of the second normal braking force as the pitch angle of the vehicle 10 increases. Alternatively, the physical quantity related to the attitude of the vehicle 10 may be a physical quantity related to the attitude of the vehicle 10 in the pitch direction, such as the sum of the normal braking forces applied to the multiple first wheels 12 and the multiple second wheels 13, the pitch rate of the vehicle 10, and the deceleration of the vehicle 10 just before stopping.

[0053] - When vehicle 10 is stopped on a flat road, the parking maintenance braking force BPh is smaller than when vehicle 10 is stopped on a slope. In other words, when vehicle 10 is stopped on a flat road, it may be possible to maintain the parking position of vehicle 10 even if the parking braking force decreases due to thermal relaxation. Therefore, the control unit 92 of the braking control device 80 may prohibit parking braking control when vehicle 10 is stopped on a flat road.

[0054] Furthermore, when the temperature of the rotating body 43 and friction material 44 in the second braking mechanism 42 is low, the decrease in parking braking force due to thermal relaxation is smaller than when the temperature is high. In other words, when the temperature of the rotating body 43 and friction material 44 in the second braking mechanism 42 is low, the parking of the vehicle 10 can be maintained even if the parking braking force decreases due to thermal relaxation. Therefore, the control unit 92 of the braking control device 80 may prohibit parking braking control when the temperature of the rotating body 43 and friction material 44 in the second braking mechanism 42 is low.

[0055] The following describes the differences between the braking control device 80 and the above embodiment in the second modified example, referring to the flowchart shown in Figure 5. As shown in Figure 5, the braking control device 80 determines whether the EPB request flag is turned on or off (S11). If the EPB request flag is off (S11: NO), the braking control device 80 terminates this process. On the other hand, if the EPB request flag is on (S11: YES), the braking control device 80 determines whether the magnitude of the road surface gradient is greater than or equal to a predetermined gradient determination value (S31). The gradient determination value is a determination value used to determine whether the road surface on which the vehicle 10 is located is a flat road or a slope. If the magnitude of the road surface gradient is less than the gradient determination value (S31: NO), that is, if the vehicle 10 is stopped on a flat road, the braking control device 80 causes the parking brake device 60 to apply parking braking force to the second wheel 13 (S32). In this case, parking braking force is applied to the second wheel 13 immediately after the EPB request flag is turned on. After that, the braking control device 80 terminates this process.

[0056] On the other hand, if the magnitude of the road surface gradient is greater than or equal to the gradient determination value (S31: YES), that is, if the vehicle 10 is stopped on an incline, the braking control device 80 determines whether the temperature of the rotating body 43 and friction material 44 in the second braking mechanism 42 is greater than or equal to a predetermined temperature determination value (S33). The temperature of the rotating body 43 and friction material 44 in the second braking mechanism 42 may be at least one of the temperatures of the rotating body 43 and the friction material 44 in the second braking mechanism 42. The temperature of the rotating body 43 and the temperature of the friction material 44 may be the actual temperature measured by a sensor or the like, or they may be an estimated temperature based on the ambient temperature and the history of the application of the second normal braking force. The temperature determination value is a determination value for determining whether the temperature of the rotating body 43 and the temperature of the friction material 44 are high or low.

[0057] If the temperature of the rotating body 43 and friction material 44 in the second braking mechanism 42 is above the temperature determination value (S33: YES), that is, if the decrease in parking braking force due to thermal relaxation is large, the braking control device 80 proceeds to step S12 in Figure 3. In this case, the braking control device 80 performs parking braking control to address the decrease in parking braking force due to thermal relaxation. On the other hand, if the temperature of the rotating body 43 and friction material 44 in the second braking mechanism 42 is below the temperature determination value (S33: NO), that is, if the decrease in parking braking force due to thermal relaxation is small, the braking control device 80 proceeds to step S32.

[0058] Based on the above, the braking control device 80 according to the second modified example can perform parking braking control only when necessary. In other words, the braking control device 80 according to the second modified example can suppress the execution of unnecessary parking braking control.

[0059] Furthermore, if the process in step S32 is performed, the braking control device 80 does not perform parking braking control. For this reason, the first normal braking force and the second normal braking force change according to the required braking force. For example, the first normal braking force and the second normal braking force decrease in accordance with the decrease in the braking operation amount.

[0060] - The second pressure regulating actuator 52 of the normal braking device 50 does not necessarily have to apply a second braking force to the second wheel 13 in accordance with the wheel pressure. For example, the second pressure regulating actuator 52 may be an electric actuator comprising an electric motor and a linear motion conversion mechanism that converts the rotational motion of the electric motor into the linear motion of a piston 46. In this case, the electric actuator applies a second normal braking force to the second wheel 13 by pressing the friction material 44 against the rotating body 43 with the piston 46 through the operation of the electric motor. The same applies to the first pressure regulating actuator 51.

[0061] - The parking actuator 61 of the parking braking device 60 does not necessarily have to apply parking braking force to the second wheel 13 by driving a piston 46. For example, if the vehicle 10 is equipped with an electric actuator as described above, the parking actuator 61 only needs to include a ratchet gear that rotates integrally with the output shaft of the electric motor, and a claw member that moves back and forth relative to the ratchet gear. In the modified example of the parking actuator 61, when the claw member engages with the ratchet gear, the ratchet gear becomes unable to rotate. Therefore, the parking actuator 61 can maintain the state in which braking force is applied to the second wheel 13 by engaging the claw member with the ratchet gear when the electric actuator is applying a second normal braking force to the second wheel 13. In this case, while the electric motor of the electric actuator is operating, the braking force applied to the second wheel 13 is the second normal braking force. In contrast, when the electric motor of the electric actuator stops operating, the braking force applied to the second wheel 13 becomes a parking braking force of the same magnitude as the second normal braking force.

[0062] - In parking braking control, the braking control device 80 may temporarily provide a period during which it does not apply the second normal braking force to the second wheel 13 during the time between when the EPB request flag is turned on and when parking braking force is applied to the second wheel 13. For example, a period during which it does not apply the second normal braking force to the second wheel 13 during the time between when the EPB request flag is turned on and when parking braking force is applied to the second wheel 13 may be provided intermittently.

[0063] - When vehicle 10 enters the parking lot from a public road, when vehicle 10 moves toward a parking space, or when the shift lever is switched alternately between D range and R range in a short period of time, it can be said that there is a high probability that vehicle 10 will be parked afterward. In that case, the braking control device 80 may determine whether there is a high probability that vehicle 10 will be parked in the immediate future based on information about the surroundings of vehicle 10 captured by the camera, the range information of the shift lever, and the current position information of vehicle 10. If there is a high probability of parking, the braking control device 80 may supplement the required braking force with only the first normal braking force. This modified example can suppress the heat generation between the rotating body 43 and the friction material 44 in the second braking mechanism 42 before vehicle 10 comes to a stop.

[0064] - If parking braking force is applied to the second wheel 13 immediately after the driver operates the parking brake switch 74, the driving noise of the parking brake device 60 is unlikely to cause discomfort to the driver. On the other hand, if parking braking force is applied to the second wheel 13 some time after the driver operates the parking brake switch 74, the driving noise of the parking brake device 60 may cause discomfort to the driver. Therefore, in the second braking process, the driving noise of the parking brake device 60 may be reduced by slowing down the rate of change of the parking braking force. In addition, in the first braking process, the driving noise of the normal braking device 50 may be reduced by slowing down the rate of change of the first normal braking force and the second normal braking force.

[0065] The vehicle 10 may be equipped with a temperature sensor that measures the temperature of the rotating body 43 of the second braking mechanism 42 and the friction material 44. In this case, the braking control device 80 may determine the timing for starting to apply parking braking force to the second wheel 13 based on the detection result of the temperature sensor during parking braking control. In other words, the braking control device 80 may set the timing for starting to apply parking braking force to the second wheel 13 to the timing when the temperature of the rotating body 43 of the second braking mechanism 42 and the friction material 44 falls below a predetermined temperature.

[0066] - In the first braking process, the braking control device 80 may set the first normal braking force to a predetermined braking force regardless of the road surface gradient. In this case, it is preferable that the predetermined braking force is set assuming the maximum road surface gradient at which the vehicle 10 can stop.

[0067] - The EPB request flag may be a flag that turns on when it is detected that the shift lever of the vehicle 10 has been operated from another range to the P range. - The braking control device 80 is not limited to a processing circuit 81 that includes a CPU 82 and a ROM 83 and executes software processing. For example, the braking control device 80 may include a dedicated hardware circuit that executes at least a part of the various processes performed in the above embodiment. An example of a dedicated hardware circuit is an ASIC. ASIC is an abbreviation for "Application Specific Integrated Circuit". In other words, the braking control device 80 may have any of the following configurations (a) to (c).

[0068] (a) A processing circuit comprising a processing unit that executes all of the above processes according to a program, and a program storage device such as a ROM that stores the program. (b) A processing circuit comprising a processing unit and a program storage device that execute a part of the above processes according to a program, and a dedicated hardware circuit that executes the remaining processes.

[0069] (c) A processing circuit equipped with dedicated hardware circuits to perform all of the above processing. Here, there may be multiple software execution devices equipped with processing units and program storage devices, and multiple dedicated hardware circuits.

Claims

1. A braking control device applicable to a vehicle comprising: a first wheel which is one of the wheels of the vehicle; a first rotating body which rotates integrally with the first wheel; a first friction material which applies braking force to the first wheel by being pressed against the first rotating body; a second wheel which is a different wheel from the first wheel; a second rotating body which rotates integrally with the second wheel; a second friction material which applies braking force to the second wheel by being pressed against the second rotating body; a parking brake device which maintains the position of the second friction material while the second friction material is pressed against the second rotating body and applies parking braking force to the second wheel; and a normal brake device which is a mechanism different from the parking brake device which applies a first normal braking force to the first wheel by pressing the first friction material against the first rotating body and applies a second normal braking force to the second wheel by pressing the second friction material against the second rotating body. A braking control device comprising a control unit that, when applying the parking braking force to the second wheel after the vehicle has stopped, applies a first normal braking force to the first wheel during a predetermined period after the vehicle has stopped, and performs parking braking control that separates the second friction material from the second rotating body during the predetermined period.

2. The braking control device according to claim 1, wherein when the control unit performs the parking braking control in a situation in which a first normal braking force is applied to the first wheel and a second normal braking force is applied to the second wheel, the control unit limits the reduction gradient when reducing the second normal braking force applied to the second wheel based on a physical quantity relating to the attitude of the vehicle.

3. The braking control device according to claim 1 or 2, wherein the control unit prohibits the parking braking control when the vehicle is stopped on a flat road.

4. The braking control device according to claim 1 or 2, wherein the control unit prohibits the parking braking control when the temperature of the second rotating body and the second friction material is below a temperature determination value.

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