Vehicle control device and vehicle control method

Abstract

A braking control device (70) comprises a control unit (125) configured to perform vehicle-stopping braking control which coordinates a power unit (20) and a friction braking unit (40) during braking of a vehicle (10) so that, until the vehicle (10) stops, a regenerative braking torque is decreased to 0 (zero) while a frictional braking torque is increased. The control unit (125) is further configured to, in the vehicle-stopping braking control, cause the rate of increase in an axle torque at the time point of sign reversal of the axle torque to be smaller than the rate of change in the axle torque at a reference time point prior to the time point of sign reversal.

Description

Vehicle control device and vehicle control method

[0001] The present disclosure relates to a vehicle control device and a vehicle control method.

[0002] Patent Literature 1 discloses a control device that performs braking control to suppress a sudden change in the pitch attitude of a vehicle when the vehicle stops. The control device controls a motor generator so that regenerative braking torque decreases toward zero when the vehicle speed drops below a predetermined speed. Then, when the vehicle speed reaches zero, the control device increases friction braking torque by operating a hydraulic actuator.

[0003] JP 2016-28913 A

[0004] When the vehicle is about to come to a stop, a small driving torque such as creep torque is applied to the axle. Regenerative braking torque is applied to the axle. When at least one of driving torque and regenerative braking torque is applied to the axle, twisting occurs in the axle. Torque that causes twisting in the axle is called "axle torque." This axle torque is a torque that depends on the direction of the driving torque acting on the axle, the direction of the regenerative braking torque acting on the axle, the magnitude of the driving torque, and the magnitude of the regenerative braking torque.

[0005] When an axle torque acts on an axle in the forward direction, which is the rotation direction corresponding to the direction of travel of the vehicle, the axle torque has a positive value. On the other hand, when an axle torque acts on an axle in the reverse direction, which is the opposite of the forward rotation direction, the axle torque has a negative value.

[0006] While a driving torque such as creep torque acts on the axle in the forward direction, a regenerative braking torque acts on the axle in the reverse direction. Therefore, if the magnitude of the regenerative braking torque is greater than the magnitude of the driving torque, the axle torque will be negative. On the other hand, if the magnitude of the regenerative braking torque is smaller than the driving torque, the axle torque will be positive.

[0007] When the above-described braking control is being executed, the magnitude of the regenerative braking torque is relatively large at the start of the braking control, so the axle torque is a negative value. However, as the magnitude of the regenerative braking torque gradually decreases, the axle torque approaches 0 (zero). Then, when the magnitude of the regenerative braking torque becomes smaller than the magnitude of the driving torque, the sign of the axle torque reverses. The reversal of the sign of the axle torque means that the direction of the axle torque acting on the axle has reversed. When the direction of the axle torque acting on the axle reverses, the direction of torsion of the axle changes. When the direction of torsion of the axle reverses in this way, there is a risk that vehicle body vibrations will occur in the vehicle due to the reversal of the direction of torsion of the axle.

[0008] According to one aspect of the present disclosure, there is provided a vehicle control device applicable to a vehicle. The vehicle includes wheels, axles rotating integrally with the wheels, a power unit configured to apply regenerative braking torque and driving torque to the axles, and a friction braking unit configured to apply friction braking torque to the wheels. The vehicle control device includes a control unit configured to execute a vehicle-stop braking control during braking of the vehicle, by coordinating the power unit and the friction braking unit to reduce the regenerative braking torque to 0 (zero) while increasing the friction braking torque until the vehicle stops. The sum of the regenerative braking torque and the driving torque is an axle torque. The control unit is further configured, during the vehicle-stop braking control, to make the rate of change of the axle torque at a time point at which the axle torque reverses from positive to negative smaller than the rate of change of the axle torque at a reference time point prior to the time point at which the axle torque reverses from positive to negative.

[0009] According to another aspect of the present disclosure, there is provided a vehicle control method applicable to a vehicle. The vehicle includes wheels, axles rotating integrally with the wheels, a power unit configured to apply regenerative braking torque and driving torque to the axles, and a friction braking unit configured to apply friction braking torque to the wheels. The sum of the regenerative braking torque and the driving torque is axle torque. The vehicle control method includes, during braking of the vehicle, coordinating the power unit and the friction braking unit to perform stationary braking control to reduce the regenerative braking torque to 0 (zero) while increasing the friction braking torque until the vehicle stops, and, in the stationary braking control, making the rate of change of the axle torque at a time point at which the axle torque reverses from positive to negative smaller than the rate of change of the axle torque at a reference time point prior to the time point at which the axle torque reverses from positive to negative.

[0010] FIG. 1 is a schematic diagram of a vehicle equipped with a brake control device of a first embodiment. FIG. 2 is a timing chart when stopping the vehicle of FIG. 1. FIG. 3 is a flowchart explaining the processing flow of stationary braking control executed by the brake control device of FIG. 1. FIG. 4 is a flowchart explaining the processing flow executed by the brake control device of FIG. 1 during stationary braking control. FIG. 5 is a flowchart explaining the processing flow executed by the brake control device of a second embodiment during stationary braking control. FIG. 6 is a timing chart when stopping a vehicle equipped with the brake control device of the second embodiment. FIG. 7 is a flowchart explaining the processing flow executed by the brake control device of a third embodiment during stationary braking control. FIG. 8 is a timing chart when stopping a vehicle equipped with the brake control device of the third embodiment.

[0011] (First embodiment) A first embodiment of a vehicle control device and a vehicle control method will be described below. <Overall configuration of vehicle> Fig. 1 shows a vehicle 10 equipped with a brake control device 70. In the first embodiment, the brake control device 70 corresponds to the "vehicle control device." The vehicle 10 is equipped with a drive operating member 11, a brake operating member 12, a plurality of wheels 13, a power unit 20, a drive control device 30, a friction braking unit 40, and a plurality of sensors 61 to 63.

[0012] <Operation Members> The drive operation member 11 is a member that the driver operates when adjusting the acceleration of the vehicle 10. An example of the drive operation member 11 is an accelerator pedal. The brake operation member 12 is a member that the driver operates when adjusting the deceleration of the vehicle 10. An example of the brake operation member 12 is a brake pedal.

[0013] <Power Unit> The power unit 20 is configured to be able to apply a driving torque Td and a regenerative braking torque TbE to the axle 14 that rotates integrally with the wheels 13. The power unit 20 has at least the motor generator of the engine and the motor generator as a power source for the vehicle 10.

[0014] The rotation direction of the axle 14 corresponding to the direction of travel of the vehicle 10 is referred to as the "forward direction." The opposite direction to the forward direction is referred to as the "reverse direction." Torque acting on the axle 14 in the forward direction is indicated as a positive value, and torque acting on the axle 14 in the reverse direction is indicated as a negative value.

[0015] When the motor generator functions as a generator, a regenerative braking torque TbE is applied to the axle 14. When the regenerative braking torque TbE is applied to the axle 14, the rotation of the axle 14 is decelerated. In other words, when the vehicle 10 is not stopped, the regenerative braking torque TbE can be said to be a torque that acts on the axle 14 in the reverse direction.

[0016] <Axle Torsion> The wheel 13 is attached to the tip of the axle 14. Therefore, the integrated body including the wheel 13 and the axle 14 can be considered a "torsion pendulum." The part of the axle 14 to which torque is transmitted from the power unit 20 is defined as the transmission part, and the amount of rotation of the transmission part relative to the wheel 13 is defined as the rotation angle. When no torsion occurs in the axle 14, the rotation angle is 0 (zero) degrees. When at least one of the driving torque Td and the regenerative braking torque TbE is applied to the axle 14, torsion may occur in the axle 14. When torsion occurs in the axle 14 in this way, the magnitude of the rotation angle increases.

[0017] In the first embodiment, the torque that generates a torsion in the axle 14 and is applied to the transmission part of the axle 14 is referred to as "axle torque TqS." The axle torque TqS is the sum of the driving torque Td and the regenerative braking torque TbE applied to the axle 14. Therefore, when the driving torque Td is a positive value, the difference between the magnitude of the driving torque Td and the magnitude of the regenerative braking torque TbE becomes the axle torque TqS. In this case, when the magnitude of the regenerative braking torque TbE is smaller than the driving torque Td, the axle torque TqS becomes a positive value. When the magnitude of the regenerative braking torque TbE is larger than the driving torque Td, the axle torque TqS becomes a negative value. Furthermore, when the driving torque Td is a negative value, the axle torque TqS becomes a negative value. Specifically, the greater the sum of the magnitude of the driving torque Td and the magnitude of the regenerative braking torque TbE, the greater the magnitude of the axle torque TqS. When the axle torque TqS is a positive value, it can be said that the axle torque TqS is acting in the forward rotation direction on the axle 14. When the axle torque TqS is a negative value, it can be said that the axle torque TqS is acting in the reverse rotation direction on the axle 14.

[0018] As described above, the drive torque Td and the axle torque TqS can be either positive or negative. Therefore, the magnitude relationship between the drive torque Td and the axle torque TqS will be described below based on magnitudes including positive and negative signs.

[0019] The regenerative braking torque TbE is a value equal to or less than 0 (zero) and is not a positive value. Therefore, for convenience, the magnitude relationship of the regenerative braking torque TbE will be described below based on a magnitude that does not include positive or negative values. In other words, although the regenerative braking torque TbE is a negative value, an increase in the magnitude of the regenerative braking torque TbE will be expressed as an increase in the regenerative braking torque TbE, and a decrease in the magnitude of the regenerative braking torque TbE will be expressed as a decrease in the regenerative braking torque TbE. The same applies to other braking torques.

[0020] While the vehicle 10 is traveling, if the magnitude of the driving torque Td and the regenerative braking torque TbE changes, or if the driving torque Td changes sign, the axle torque TqS may reverse in sign. The reversal of the sign of the axle torque TqS means that the direction of torsion of the axle 14 reverses. When the direction of torsion of the axle 14 reverses, vehicle body vibration occurs due to the reversal of the direction of torsion of the axle 14. Furthermore, such vehicle body vibration increases the greater the rate at which the axle torque TqS increases when the sign of the axle torque TqS reverses. Here, the rate at which the axle torque TqS increases corresponds to the "rate of change of the axle torque."

[0021] <Drive control device> The drive control device 30 controls the power unit 20. The drive control device 30 is equipped with a processing circuit 31. An example of the processing circuit 31 is an electronic control device. In this case, the drive control device 30 has a CPU 32, a first memory 33, and a second memory 34. The first memory 33 stores a control program executed by the CPU 32. The second memory 34 stores the results of calculations by the CPU 32. The CPU 32 executes the control program in the first memory 33, allowing the processing circuit 31 to control the power unit 20.

[0022] The drive control device 30 is configured to be able to send and receive various information and commands to and from the braking control device 70 via the in-vehicle network. Therefore, when braking the vehicle, the drive control device 30 can operate the power unit 20 based on the command received from the braking control device 70.

[0023] The processing circuit 31 functions as a plurality of functional units by the CPU 32 executing the control program in the first memory 33. The plurality of functional units include a drive control unit 101 and a regeneration control unit 103.

[0024] When the drive operating member 11 is operated, the drive control unit 101 derives the required drive force such that the required drive force increases as the drive operation amount indicating the operation amount of the drive operating member 11 increases. The drive control unit 101 also derives the required drive force even when the drive operating member 11 is not operated. For example, when the drive operating member 11 is not operated and the vehicle speed VS is equal to or greater than the first threshold speed Vth1, the drive control unit 101 derives a negative required drive force. In this case, the required drive force corresponds to engine brake torque if the drive force output source is an engine, and corresponds to torque simulating engine brake torque if the drive force output source is a motor-generator. On the other hand, when the drive operating member 11 is not operated and the vehicle speed VS is less than the second threshold speed Vth2, the drive control unit 101 derives a positive required drive force. In this case, the required drive force corresponds to creep torque if the drive force output source is an engine, and corresponds to torque simulating creep torque if the drive force output source is a motor-generator. The creep torque is the drive torque Td for creeping the vehicle 10. Furthermore, when the drive operating member 11 is not being operated and the vehicle speed VS is less than the first threshold speed Vth1 and greater than or equal to the second threshold speed Vth2, the drive control unit 101 increases the required drive force as the vehicle speed VS decreases. Furthermore, the drive control unit 101 converts the required drive force into drive torque Td to derive required drive torque TdRq, which is the required value of the drive torque Td.

[0025] The drive control unit 101 adjusts the drive torque Td applied to the axle 14 by operating the power unit 20. The drive control unit 101 derives a drive torque command value Tdtr, which is a command value for the drive torque Td, based on the required drive torque TdRq. The drive control unit 101 then controls the power unit 20 based on the drive torque command value Tdtr. This allows the drive control unit 101 to adjust the drive torque Td applied to the axle 14.

[0026] The regenerative control unit 103 adjusts the regenerative braking torque TbE applied to the axle 14 by operating the power unit 20. The regenerative control unit 103 derives a regenerative braking torque command value TbEtr, which is a command value for the regenerative braking torque TbE, based on the required braking torque TbRq transmitted from the brake control device 70. Then, the drive control unit 101 controls the power unit 20 based on the regenerative braking torque command value TbEtr. This allows the regenerative control unit 103 to adjust the regenerative braking torque TbE applied to the axle 14.

[0027] Note that the drive control unit 101 may derive the drive torque command value Tdtr, and the regenerative control unit 103 may derive the regenerative braking torque command value TbEtr. In this case, the power unit 20 is controlled based on both the drive torque command value Tdtr and the regenerative braking torque command value TbEtr. That is, the power unit 20 applies to the axle 14 a torque that is the sum of the drive torque command value Tdtr and the regenerative braking torque command value TbEtr. This sum corresponds to the axle torque TqS. Therefore, it can be said that the power unit 20 can apply to the axle 14 an axle torque TqS that corresponds to the drive torque command value Tdtr and the regenerative braking torque command value TbEtr.

[0028] <Friction Braking Unit> The friction braking unit 40 applies a friction braking torque TbF to the vehicle 10. As a result, a friction braking force is generated at the wheels 13. The friction braking unit 40 includes a plurality of friction brakes 41 provided respectively for the plurality of wheels 13, and a braking actuator 50.

[0029] Each of the plurality of friction brakes 41 applies a friction braking torque TbF to the corresponding wheel 13. The friction brake 41 has a wheel cylinder 42, a rotating body 43, and a friction portion 44. The rotating body 43 rotates integrally with the wheel 13. Therefore, by pressing the friction portion 44 against the rotating body 43, the friction braking torque TbF is applied to the wheel 13. The force pressing the friction portion 44 against the rotating body 43 increases as the wheel pressure, which is the hydraulic pressure in the wheel cylinder 42, increases. Therefore, the friction brake 41 can apply a larger friction braking torque TbF to the wheel 13 as the wheel pressure increases.

[0030] The brake actuator 50 is configured to adjust the friction braking torque TbF applied to the multiple wheels 13 by controlling the wheel pressures of the multiple wheel cylinders 42. For example, the brake actuator 50 has a pressure source that can supply brake fluid to the multiple wheel cylinders 42. The pressure source is, for example, an electric pump and an electric cylinder. The brake fluid discharged from the brake actuator 50 is supplied to the wheel cylinders 42 via a supply flow path 51. In the following description, the sum of the friction braking torque TbF and the regenerative braking torque TbE is referred to as the total braking torque TbT.

[0031] <Sensors> The multiple sensors 61 to 63 output signals according to the detection results to the drive control device 30 or the braking control device 70. The multiple sensors 61 to 63 include, for example, an accelerator sensor 61, a brake sensor 62, and a wheel sensor 63.

[0032] The accelerator sensor 61 detects information related to the driver's operation of the drive operating member 11. An example of the accelerator sensor 61 is a stroke sensor that detects the amount of drive operation, which is the amount of operation of the drive operating member 11 by the driver. Note that the vehicle 10 may also be equipped with a sensor that detects the operating force of the drive operating member 11 by the driver.

[0033] The brake sensor 62 detects information related to the driver's operation of the brake operating member 12. An example of the brake sensor 62 is a stroke sensor that detects the braking operation amount, which is the amount of operation of the brake operating member 12 by the driver. Note that the vehicle 10 may also be equipped with a sensor that detects the operating force of the brake operating member 12 by the driver.

[0034] The vehicle 10 is equipped with wheel sensors 63 in the same number as the wheels 13. Each wheel sensor 63 detects the rotation speed of the corresponding wheel 13. The rotation speed of the wheel 13 based on the detection signal of the wheel sensor 63 is referred to as the "wheel speed VW."

[0035] <Brake Control Device> The brake control device 70 controls the brake actuator 50 of the friction braking unit 40. The brake control device 70 is configured to be able to send and receive various information and commands to and from the drive control device 30. Therefore, when braking the vehicle, the brake control device 70 can adjust the deceleration of the vehicle 10 by operating the brake actuator 50 and cooperating with the drive control device 30.

[0036] The braking control device 70 includes a processing circuit 71. An example of the processing circuit 71 is an electronic control device. In this case, the processing circuit 71 includes a CPU 72, a first memory 73, and a second memory 74. The first memory 73 stores a control program executed by the CPU 72. The second memory 74 stores the calculation results of the CPU 72. When the CPU 72 executes the control program in the first memory 73, the processing circuit 71 can operate the friction braking unit 40 to adjust the friction braking torque TbF. Furthermore, the processing circuit 71 can adjust the axle torque TqS acting on the axle 14 by transmitting instructions regarding the regenerative braking torque TbE and instructions regarding the drive torque Td to the drive control device 30.

[0037] The regenerative cooperative control executed by the processing circuit 71 will be described. The processing circuit 71 derives a required braking force, which is a required value of braking force for the vehicle 10. The braking force is the sum of the friction braking force and the regenerative braking force generated by the vehicle 10. When the brake operating member 12 is operated, the processing circuit 71 derives the required braking force such that the required braking force increases as the braking operation amount increases. When deceleration of the vehicle 10 is requested by another control device, the processing circuit 71 derives a value corresponding to the required deceleration of the vehicle 10 as the required braking force. For example, the processing circuit 71 converts the required braking force into braking torque to derive a required braking torque TbRq, which is a required value of the braking torque. The processing circuit 71 then transmits the required braking torque TbRq to the drive control device 30.

[0038] The processing circuit 71 of the brake control device 70 derives a target frictional braking torque TbFtr, which is a target for the frictional braking torque TbF applied to the vehicle 10, based on the regenerative braking torque TbE that the power unit 20 can generate and the required braking torque TbRq. If the regenerative braking torque TbE that the power unit 20 can generate is equal to the required braking torque TbRq, the processing circuit 71 derives 0 (zero) as the target frictional braking torque TbFtr. On the other hand, if the regenerative braking torque TbE that the power unit 20 can generate is less than the required braking torque TbRq, the processing circuit 71 derives the difference between the required braking torque TbRq and the regenerative braking torque TbE as the target frictional braking torque TbFtr. Then, the processing circuit 71 operates the brake actuator 50 based on the target frictional braking torque TbFtr.

[0039] <Functional Units> The processing circuit 71 functions as multiple functional units as a result of the CPU 72 executing the control program in the first memory 73. The multiple functional units are functional units for stopping the vehicle 10 by generating a braking force on the vehicle 10. The multiple functional units include a derivation unit 121, a stop-related value acquisition unit 123, and a control unit 125.

[0040] <Derivation Unit> The derivation unit 121 executes a derivation process at each predetermined control cycle when the vehicle 10 is decelerating due to the generation of a braking force. The derivation process is a process of acquiring various state quantities. For example, in the derivation process, the derivation unit 121 derives a required braking torque TbRq, a wheel speed VW, a vehicle body speed VS, and a vehicle body acceleration DVS. The vehicle body speed VS is the traveling speed of the vehicle 10 that can be derived using at least one of the wheel speeds VW of the multiple wheels 13.

[0041] In the derivation process, the derivation unit 121 derives a stop-maintaining braking force. The stop-maintaining braking force is a braking force required to maintain a stop on the road surface on which the vehicle 10 is traveling. The derivation unit 121 derives the stop-maintaining braking force based on information regarding the gradient of the road surface and the drive torque Td applied from the power unit 20 to the axles 14 while the vehicle is stopped. For example, when the road surface is a slope, the derivation unit 121 derives a braking force that is greater than when the road surface is not a slope. When it is possible to predict that the drive torque Td applied from the power unit 20 to the axles 14 while the vehicle is stopped, the derivation unit 121 derives a braking force that is greater than when it is possible to predict that the drive torque Td is not large as the stop-maintaining braking force. The derivation unit 121 then converts the stop-maintaining braking force into torque to derive the stop-maintaining braking torque Tbh.

[0042] <Stop-related value acquisition unit> The stop-related value acquisition unit 123 acquires a stop-related value at every predetermined control period when the vehicle 10 is decelerating due to the generation of braking force. The stop-related value is a value that decreases as the vehicle 10 approaches a predetermined stop position.

[0043] For example, the stopping-related value acquisition unit 123 estimates a stopping position, which is a position where the vehicle 10 will stop, based on the vehicle speed VS and the vehicle acceleration DVS. In this case, the stopping-related value acquisition unit 123 may estimate, as the stopping position, the position of the vehicle 10 when the vehicle speed VS becomes 0 (zero) at the current vehicle acceleration DVS. This stopping position corresponds to the "predetermined stopping position." Then, the stopping-related value acquisition unit 123 acquires, as the stopping-related value, a stopping distance DS, which is the distance from the current position of the vehicle 10 to the stopping position.

[0044] <Control Unit> When a braking force is generated on the vehicle 10 , the control unit 125 activates the friction braking unit 40 and controls the deceleration of the vehicle 10 by coordinating the power unit 20 and the friction braking unit 40 .

[0045] As shown in (A) to (F) of FIG. 2, the control unit 125 executes vehicle-stop braking control when the stopping distance DS is equal to or less than a predetermined threshold. The vehicle-stop braking control is a control that reduces the regenerative braking torque TbE to 0 (zero) while increasing the friction braking torque TbF until the vehicle 10 stops, thereby stopping the vehicle 10 in a state where the total braking torque TbT is smaller than the required braking torque TbRq. The start condition for the vehicle-stop braking control includes the execution of regenerative cooperative control. Note that in (A) of FIG. 2, the required braking torque TbRq is shown as a positive value for convenience, in consideration of the fact that the required braking torque Tbrq is a required value of braking torque.

[0046] The braking control during a stop includes a switching process, a reduction process, a holding process, and a degeneration process. The switching process corresponds to a "friction braking increase process," and the reduction process corresponds to a "regenerative braking decrease process."

[0047] When the stopping distance DS becomes equal to or less than the first threshold value DSth1, the control unit 125 starts a replacement process for the stationary braking control. In the example shown in FIG. 2 , the stopping distance DS becomes equal to or less than the first threshold value DSth1 at timing t13. The first threshold value DSth1 corresponds to the "predetermined threshold value." The replacement process is a process for replacing the regenerative braking torque TbE with the frictional braking torque TbF. In the replacement process, the control unit 125 operates the brake actuator 50 to increase the frictional braking torque TbF to a set braking torque Tbset that is equal to or greater than the stationary holding braking torque Tbh. In the example shown in FIG. 2 , the set braking torque Tbset is equal to the stationary holding braking torque Tbh. Therefore, the control unit 125 increases the target frictional braking torque TbFtr to the stationary holding braking torque Tbh and operates the brake actuator 50 based on the target frictional braking torque TbFtr. At the same time, the control unit 125 transmits to the regenerative control unit 103 an instruction to decrease the regenerative braking torque TbE by the amount of increase in the friction braking torque TbF resulting from the execution of the switching process.

[0048] When the regenerative control unit 103 receives the above instruction, the regenerative control unit 103 decreases the regenerative braking torque command value TbEtr by the increase amount of the friction braking torque TbF. Then, the regenerative control unit 103 operates the power unit 20 based on the regenerative braking torque command value TbEtr. This decreases the regenerative braking torque TbE by the increase amount of the friction braking torque TbF. As a result, changes in the total braking torque TbT due to the execution of the switching process are suppressed. When the friction braking torque TbF is maintained, the control unit 125 ends the switching process.

[0049] After the switching process is performed, the control unit 125 starts the reduction process when the stopping distance DS becomes equal to or less than the second threshold value DSth2. In the example shown in FIG. 2 , the stopping distance DS becomes equal to or less than the second threshold value DSth2 at timing t15. The second threshold value DSth2 is smaller than the first threshold value DSth1. In the reduction process, the control unit 125 transmits an instruction to the regenerative control unit 103 to reduce the regenerative braking torque TbE to 0 (zero). At this time, the control unit 125 may transmit an instruction to the regenerative control unit 103 to reduce the regenerative braking torque TbE at a standard reduction speed Vstd. The standard reduction speed Vstd is set so that the regenerative braking torque TbE becomes 0 (zero) before the vehicle speed VS becomes 0 (zero).

[0050] When the regenerative control unit 103 receives this instruction, the regenerative control unit 103 decreases the regenerative braking torque instruction value TbEtr toward 0 (zero). Then, the regenerative control unit 103 operates the power unit 20 based on the regenerative braking torque instruction value TbEtr.

[0051] The control unit 125 maintains the friction braking torque TbF during the reduction process. When the control unit 125 determines that the regenerative braking torque command value TbEtr has become 0 (zero), the control unit 125 terminates the reduction process. In the example shown in FIG. 2 , the regenerative braking torque command value TbEtr becomes 0 (zero) at time t19.

[0052] After completing the reduction process, the control unit 125 starts the retention process. In the retention process, the control unit 125 retains the target friction braking torque TbFtr. At this time, the control unit 125 also retains the state in which the regenerative braking torque TbE is 0 (zero).

[0053] If the control unit 125 determines that the vehicle 10 has stopped while the holding process is being executed, the control unit 125 transitions from the holding process to the degeneration process. For example, the control unit 125 determines that the vehicle 10 has stopped when the stopping distance DS is equal to or less than the third threshold value DSth3. In this case, the third threshold value DSth3 is smaller than the second threshold value DSth2. For example, the third threshold value DSth3 is 0 (zero). In the example shown in FIG. 2 , the control unit 125 determines that the vehicle 10 has stopped at timing t20.

[0054] In the degeneration process, the control unit 125 increases the friction braking torque TbF. For example, the control unit 125 increases the friction braking torque TbF to the required braking torque TbRq. At this time, the control unit 125 increases the target friction braking torque TbFtr to the required braking torque TbRq. Then, the control unit 125 operates the brake actuator 50 based on the target friction braking torque TbFtr. When the friction braking torque TbF becomes equal to or greater than the required braking torque TbRq, the control unit 125 ends the degeneration process. Then, the control unit 125 ends the stationary braking control.

[0055] As described above, the power unit 20 of the vehicle 10 outputs a positive or negative drive torque Td in accordance with instructions from the drive control unit 101, even when the driver does not operate the drive operating member 11. Specifically, as shown in FIG. 2E, when the vehicle body speed VS is equal to or greater than the first threshold speed Vth1, the power unit 20 applies a negative drive torque Td to the axle 14. Subsequently, when a braking force is applied to the vehicle 10 and the vehicle body speed VS becomes less than the first threshold speed Vth1, the power unit 20 gradually increases the drive torque Td applied to the axle 14 in accordance with the decrease in the vehicle body speed VS. After the vehicle body speed VS becomes less than the first threshold speed Vth1 and before the vehicle body speed VS becomes equal to the second threshold speed Vth2, the drive torque Td applied to the axle 14 becomes greater than 0 (zero). When the vehicle speed VS becomes less than the second determination speed Vth2, the power unit 20 applies a constant positive driving torque Td to the axle 14.

[0056] When the magnitude of the regenerative braking torque TbE is greater than the magnitude of the driving torque Td, the axle torque TqS becomes a negative value, as shown in (F) of FIG. 2. However, if the regenerative braking torque TbE is reduced by the execution of the reduction process of the stationary braking control, the axle torque TqS approaches 0 (zero). Then, when a positive driving torque Td is applied to the axle 14, and the magnitude of the driving torque Td becomes greater than the magnitude of the regenerative braking torque TbE, the axle torque TqS becomes a positive value. In other words, the sign of the axle torque TqS is reversed. As described above, during the execution of the reduction process of the stationary braking control, vehicle vibration occurs due to the reversal of the sign of the axle torque TqS. For this reason, when the sign of the axle torque TqS is reversed during the execution of the reduction process of the stationary braking control, it is preferable to reduce the rate of increase of the axle torque TqS at the time when the axle torque TqS is reversed.

[0057] Therefore, in the vehicle-stop braking control, the control unit 125 reduces the rate of increase of the axle torque TqS at the time when the axle torque TqS reverses between positive and negative signs compared to the rate of increase at a reference time point before the time when the axle torque TqS reverses between positive and negative signs. More specifically, in the vehicle-stop braking control, when the control unit 125 is reducing the total braking torque TbT so that the total braking torque TbT is smaller than the required braking torque TbRq, i.e., when the control unit 125 is executing the reduction process, the control unit 125 reduces the rate of increase of the axle torque TqS at the time when the axle torque TqS reverses between positive and negative signs.

[0058] In the vehicle-stop braking control, the control unit 125 executes processes, including a first adjustment process, a second adjustment process, and a third adjustment process, to reduce the rate of increase of the axle torque TqS at the point when the axle torque TqS reverses between positive and negative.

[0059] The control unit 125 starts the first adjustment process when a first determination condition is met. The first determination condition is met when, as the reduction process is performed, the axle torque TqS approaches the point at which its positive and negative values ​​reverse, i.e., when the axle torque TqS approaches 0 (zero). In this case, the control unit 125 calculates an expected arrival time from the current time until the point at which the axle torque TqS reverses its positive and negative values, based on the change in the axle torque TqS over time. Next, the control unit 125 may determine that the first determination condition is met when the expected arrival time is shorter than a predetermined first determination time. Alternatively, the control unit 125 may determine that the first determination condition is met when the axle torque TqS becomes equal to or greater than a predetermined first determination torque. In this case, the first determination torque is a negative value.

[0060] In the first adjustment process, the control unit 125 sets the rate of decrease of the regenerative braking torque command value TbEtr to a first decrease rate V1. That is, the control unit 125 instructs the regenerative control unit 103 to decrease the regenerative braking torque command value TbEtr at the first decrease rate V1. The first decrease rate V1 is greater than the standard decrease rate Vstd described above. Therefore, when the first adjustment process is performed, the rate of decrease of the regenerative braking torque command value TbEtr is greater than when the first adjustment process is not performed.

[0061] If a second determination condition is met during execution of the first adjustment process, the control unit 125 terminates the first adjustment process and starts the second adjustment process. The second determination condition is met after the first determination condition is met and when the axle torque TqS approaches the point at which the positive and negative signs of the axle torque TqS are reversed, compared to when the first determination condition was met. For example, the control unit 125 may determine that the second determination condition is met when the predicted arrival time is less than a second determination time that is shorter than the first determination time. Alternatively, the control unit 125 may determine that the second determination condition is met when the axle torque TqS is equal to or greater than a predetermined second determination torque. Here, the second determination torque is a negative value greater than the first determination torque.

[0062] In the second adjustment process, the control unit 125 sets the rate of decrease of the regenerative braking torque command value TbEtr to a second decrease rate V2. That is, the control unit 125 instructs the regenerative control unit 103 to decrease the regenerative braking torque command value TbEtr at the second decrease rate V2. The second decrease rate V2 is smaller than the standard decrease rate Vstd described above. Therefore, while the second adjustment process is being performed, the rate of decrease of the regenerative braking torque command value TbEtr is smaller than when the second adjustment process is not being performed. That is, by reversing the sign of the axle torque TqS during the second adjustment process, the rate of increase of the axle torque TqS at the time when the sign of the axle torque TqS is reversed is reduced.

[0063] If a third determination condition is met during execution of the second adjustment process, the control unit 125 terminates the second adjustment process and starts the third adjustment process. The third determination condition is met when the axle torque TqS becomes a positive value, i.e., after the axle torque TqS reverses its sign. Specifically, the control unit 125 may determine that the third determination condition is met when the elapsed time from the point in time when the axle torque TqS reverses its sign becomes equal to or greater than a predetermined third determination time. Alternatively, the control unit 125 may determine that the third determination condition is met when the axle torque TqS becomes equal to or greater than a predetermined third determination torque. Here, the third determination torque is a positive value.

[0064] In the third adjustment process, the control unit 125 sets the rate of decrease of the magnitude of the regenerative braking torque command value TbEtr to a third decrease rate V3. That is, the control unit 125 instructs the drive control unit 101 to decrease the regenerative braking torque command value TbEtr at the third decrease rate V3. The third decrease rate V3 is greater than the standard decrease rate Vstd described above. Therefore, during the third adjustment process, the rate of decrease of the regenerative braking torque command value TbEtr is greater than during the second adjustment process. In the first embodiment, the third decrease rate V3 is equal to the first decrease rate V1; however, in other embodiments, the third decrease rate V3 may be different from the first decrease rate V1.

[0065] The control unit 125 ends the third adjustment process when the regenerative braking torque command value TbEtr becomes 0 (zero). That is, the control unit 125 ends the third adjustment process and the reduction process simultaneously. When the control unit 125 executes the first adjustment process, the second adjustment process, and the third adjustment process, if only the reduction rate of the regenerative braking torque command value TbEtr is changed, the reduction rate of the total braking torque TbT during the reduction process may fluctuate. In this case, the total braking torque TbT may increase or decrease just before the vehicle 10 is stopped, which may cause the driver to feel uncomfortable. Therefore, when executing the first adjustment process, the second adjustment process, and the third adjustment process, the control unit 125 changes the target friction braking torque TbFtr so that the reduction rate of the total braking torque TbT is maintained constant. Specifically, in the first and third adjustment processes, when the rate of decrease of the regenerative braking torque command value TbEtr is increased, the target frictional braking torque TbFtr is increased to compensate for the insufficient braking torque by increasing the frictional braking torque TbF. In the second adjustment process, when the rate of decrease of the regenerative braking torque command value TbEtr is decreased, the target frictional braking torque TbFtr is decreased to compensate for the excessive braking torque by reducing the frictional braking torque TbF. In this way, the control unit 125 changes the target frictional braking torque TbFtr in accordance with the change in the rate of decrease of the regenerative braking torque command value TbEtr.

[0066] As described above, the axle torque TqS changes sign during the second adjustment process. The rate of increase of the axle torque TqS during the second adjustment process is smaller than the rate of increase of the axle torque TqS during the first adjustment process, which occurs before the second adjustment process. In this respect, the control unit 125 reduces the rate of increase of the axle torque TqS at the point in time when the axle torque TqS changes sign to a value smaller than the rate of increase at a reference point in time before the change. The reference point in this case is a point in time during the first adjustment process. Similarly, the rate of increase of the axle torque TqS during the second adjustment process is smaller than the rate of increase of the axle torque TqS during the third adjustment process, which occurs after the second adjustment process. In this respect, the control unit 125 reduces the rate of increase of the axle torque TqS at the point in time when the axle torque TqS changes sign to a value smaller than the rate of increase at a reference point in time after the change. The reference point in this case is a point in time during the third adjustment process.

[0067] <Smooth Stop Processing> The smooth stop processing will be described with reference to Fig. 3. The smooth stop processing is a series of processes that the processing circuit 71 executes when braking the vehicle while stationary. The processing circuit 71 repeatedly executes the smooth stop processing at each predetermined control cycle.

[0068] 3, the processing circuit 71 functions as the stopping-related value acquisition unit 123 to acquire the stopping distance DS (S11). Subsequently, the processing circuit 71 determines whether or not a braking request is present (S13). If the processing circuit 71 determines that a braking request is present (YES in S13), the processing circuit 71 proceeds to step S17. On the other hand, if the processing circuit 71 determines that a braking request is not present (NO in S13), the processing circuit 71 proceeds to step S15.

[0069] In step S15, the processing circuit 71 sets an end flag FLG2 (described later) to OFF. Then, the processing circuit 71 temporarily ends the smooth stop processing. In step S17, the processing circuit 71 determines whether the end flag FLG2 is set to OFF. If the end flag FLG2 is set to OFF (S17: YES), the processing circuit 71 proceeds to step S19. On the other hand, if the end flag FLG2 is set to ON (S17: NO), the processing circuit 71 temporarily ends the smooth stop processing.

[0070] In step S19, the processing circuit 71 determines whether the stopping distance DS is equal to or less than the first threshold value DSth1. If the stopping distance DS is greater than the first threshold value DSth1 (S19: NO), the processing circuit 71 temporarily terminates the smooth stop processing. On the other hand, if the stopping distance DS is equal to or less than the first threshold value DSth1 (S19: YES), the processing circuit 71 functions as the control unit 125 to execute stopping braking control. That is, when the stopping distance DS becomes equal to or less than the first threshold value DSth1, the processing circuit 71 starts stopping braking control.

[0071] Specifically, the processing circuit 71 determines whether the switching from the regenerative braking torque TbE to the friction braking torque TbF due to the execution of the switching process has been completed (S21). The processing circuit 71 determines that the switching has been completed if both of the following two conditions (A1) and (A2) are satisfied. On the other hand, the processing circuit 71 determines that the switching has not been completed if at least one of the two conditions (A1) and (A2) is not satisfied.

[0072] (A1) The friction braking torque TbF becomes equal to the set braking torque Tbset as a result of the replacement process. (A2) The regenerative braking torque command value TbEtr decreases by the amount of the increase in the friction braking torque TbF resulting from the replacement process.

[0073] If the processing circuit 71 determines that the switching has not been completed (S21: NO), the processing circuit 71 executes the switching process (S23) and temporarily ends the smooth stop process.

[0074] On the other hand, if the processing circuit 71 determines in step S21 that the switching has been completed (S21: YES), the processing circuit 71 determines whether the stopping distance DS is equal to or less than the second threshold value DSth2 (S31). If the stopping distance DS is greater than the second threshold value DSth2 (S31: NO), the processing circuit 71 temporarily terminates the smooth stop processing. In this case, the processing circuit 71 maintains the regenerative braking torque TbE and the friction braking torque TbF. On the other hand, if the stopping distance DS is equal to or less than the second threshold value DSth2 (S31: YES), the processing circuit 71 determines whether the stopping distance DS is equal to or less than the third threshold value DSth3 (S33). That is, the processing circuit 71 determines whether the vehicle 10 has stopped. If the stopping distance DS is greater than the third threshold value DSth3 (S33: NO), the processing circuit 71 proceeds to step S35. On the other hand, if the stopping distance DS is equal to or less than the third threshold value DSth3 (S33: YES), the processing circuit 71 proceeds to step S41.

[0075] In step S35, the processing circuit 71 determines whether the regenerative braking torque TbE is 0 (zero). If the processing circuit 71 determines that the regenerative braking torque TbE is not 0 (zero) (S35: NO), the processing circuit 71 executes a reduction process (S37). Then, the processing circuit 71 temporarily ends the smooth stop process.

[0076] On the other hand, if the processing circuit 71 determines in step S35 that the regenerative braking torque TbE is 0 (zero) (S35: YES), the processing circuit 71 executes the holding process (S39) and temporarily ends the smooth stop process.

[0077] In step S41, the processing circuit 71 determines whether the execution of the degeneration process has been completed. For example, if the friction braking torque TbF is equal to or greater than the required braking torque TbRq, the execution of the degeneration process is deemed to be completed. On the other hand, if the friction braking torque TbF is less than the required braking torque TbRq, the execution of the degeneration process is deemed to be incomplete. If the processing circuit 71 determines that the execution of the degeneration process has not been completed (S41: NO), the processing circuit 71 executes the degeneration process (S43). Then, the processing circuit 71 temporarily terminates the smooth stop process.

[0078] On the other hand, if the processing circuit 71 determines in step S41 that the execution of the degeneration processing has been completed (S41: YES), the processing circuit 71 sets the end flag FLG2 to ON (S45). Then, the processing circuit 71 ends the stationary braking control. That is, the end flag FLG2 is a flag that is set to ON when the execution of the degeneration processing of the stationary braking control has been completed. Thereafter, the processing circuit 71 temporarily ends the smooth stop processing.

[0079] <Vehicle Body Vibration Reduction Processing> The vehicle body vibration reduction processing will be described with reference to Fig. 4. The processing circuit 71 repeatedly executes the vehicle body vibration reduction processing at each predetermined control cycle.

[0080] As shown in Fig. 4, the processing circuit 71 determines whether or not the reduction process of the stationary braking control is being executed (S51). If the reduction process is not being executed (S51: NO), the processing circuit 71 terminates this process. On the other hand, if the reduction process is being executed (S51: YES), the processing circuit 71 determines whether or not a first determination condition is met (S52). If the first determination condition is not met (S52: NO), that is, if the axle torque TqS is not approaching the time when it reverses its positive and negative directions, the processing circuit 71 terminates this process.

[0081] On the other hand, if the first determination condition is met (S52: YES), i.e., if the axle torque TqS is approaching the time when it will reverse its positive and negative polarities, the processing circuit 71 starts the first adjustment process (S53). That is, the rate at which the regenerative braking torque command value TbEtr is reduced in the reduction process becomes the first reduction rate V1. Next, the control unit 125 determines whether the second determination condition is met (S54). If the second determination condition is not met (S54: NO), the processing circuit 71 proceeds to step S54. That is, the processing circuit 71 continues the first adjustment process.

[0082] On the other hand, if the second determination condition is met (S54: YES), the processing circuit 71 ends the first adjustment process and starts the second adjustment process (S55). That is, the rate of decrease of the regenerative braking torque command value TbEtr in the decrease process becomes the second decrease rate V2. Next, the control unit 125 determines whether the third determination condition is met (S56). If the third determination condition is not met (S56: NO), the processing circuit 71 proceeds to step S56. That is, the processing circuit 71 continues the second adjustment process.

[0083] On the other hand, if the third determination condition is met (S56: YES), that is, if the axle torque TqS has been reversed in sign, the processing circuit 71 ends the second adjustment process and starts the third adjustment process (S57). That is, the rate at which the regenerative braking torque command value TbEtr is reduced in the reduction process becomes the third reduction rate V3. When the termination condition for the reduction process is met, the control unit 125 terminates the reduction process and the third adjustment process. Thereafter, the control unit 125 terminates this process.

[0084] <Operations and Effects of First Embodiment> The operations and effects of the braking control device 70 when stopping the vehicle 10 will be described with reference to FIGS.

[0085] Before timing t11, the vehicle 10 is traveling with neither the drive operating member 11 nor the brake operating member 12 being operated. Therefore, the required braking torque TbRq is 0 (zero) and the drive torque Td is a negative value. As a result, before timing t11, the axle torque TqS is a negative value.

[0086] At time t11, when a braking request is generated, for example, when the driver begins to operate the brake operating member 12, the required braking torque TbRq begins to increase. Then, the regenerative braking torque TbE increases in accordance with the increase in the required braking torque TbRq. In the example shown in FIG. 2 , the increase in the required braking torque TbRq is equal to the increase in the regenerative braking torque TbE, so the friction braking torque TbF does not increase from 0 (zero). After time t11, the regenerative braking torque TbE increases, and the axle torque TqS decreases. Although not shown in the figure, after time t11, the vehicle speed VS gradually decreases at the point where the braking torque acts on the axle 14.

[0087] At time t12, when the vehicle speed VS becomes less than the first speed determination value, the drive torque Td begins to increase in response to the decrease in the vehicle speed VS. Furthermore, as the drive torque Td increases, the axle torque TqS begins to increase toward 0 (zero).

[0088] At timing t13, when the stopping distance DS becomes equal to or less than the first threshold value DSth1, a switching process for the stationary braking control is initiated. In the switching process, the frictional braking torque TbF is increased to the set braking torque Tbset (= stationary holding braking torque Tbh). At this time, the regenerative braking torque TbE is reduced by the increase in the frictional braking torque TbF. Therefore, the total braking torque TbT, which is the sum of the regenerative braking torque TbE and the frictional braking torque TbF, remains unchanged. Meanwhile, the axle torque TqS approaches 0 (zero) in response to the reduction in the regenerative braking torque TbE.

[0089] At timing t14, when the friction braking torque TbF reaches the set braking torque Tbset (=vehicle stop maintaining braking torque Tbh), the switching process ends. Therefore, during the period from timing t14 to timing t15, the regenerative braking torque TbE and the friction braking torque TbF are maintained constant.

[0090] At timing t15, when the stopping distance DS becomes equal to or less than the second threshold value DSth2, a reduction process of the stationary braking control is initiated. In the reduction process, the regenerative braking torque TbE is gradually reduced to 0 (zero). In the example shown in FIG. 2 , the start condition of the first adjustment process is also satisfied at the timing when the start condition of the reduction process is satisfied. Therefore, at timing t15, the regenerative braking torque TbE begins to decrease at a relatively large first reduction rate V1. Furthermore, the friction braking torque TbF begins to increase so that the reduction rate of the total braking torque TbT does not fluctuate due to the increase in the reduction rate of the regenerative braking torque TbE. Then, after timing t15, the axle torque TqS increases rapidly toward 0 (zero) due to the rapid reduction in the regenerative braking torque TbE.

[0091] At timing t16, when the start condition for the second adjustment process is satisfied, the first adjustment process ends and the second adjustment process begins. Therefore, at timing t16, the regenerative braking torque TbE begins to decrease at a relatively small second decrease rate V2. Furthermore, the friction braking torque TbF begins to decrease so that the decrease rate of the total braking torque TbT does not fluctuate due to the slow decrease rate of the regenerative braking torque TbE. Then, after timing t16, the regenerative braking torque TbE gradually decreases, causing the axle torque TqS to gradually increase toward 0 (zero). Specifically, as shown by the solid line in FIG. 2C , the decrease rate of the regenerative braking torque TbE during the second adjustment process is slower than the decrease rate of the regenerative braking torque TbE when the second adjustment process is not executed, as shown by the dashed line in FIG. 2C . As a result, as shown by the solid line in (F) of Figure 2, the rate of decrease of the axle torque TqS during execution of the second adjustment process is smaller than the rate of decrease of the axle torque TqS when the same process is not executed, as shown by the dashed line in (F) of Figure 2.

[0092] At timing t17 while the second adjustment process is being performed, the axle torque TqS becomes 0 (zero). In other words, the axle torque TqS reverses sign while the rate of increase in the axle torque TqS is slowing down. Here, the rate of increase in the axle torque TqS at the time when the axle torque TqS reverses sign is smaller than that at a reference time point before the time when the axle torque TqS reverses sign. The reference time point is, for example, any time point between timing t15 and timing t16. In this way, vehicle vibration caused by the reversal of sign of the axle torque TqS is suppressed.

[0093] At timing t18, when the start condition for the third adjustment process is met, the second adjustment process ends and the third adjustment process starts. Therefore, at timing t18, the regenerative braking torque TbE is reduced at a relatively large third reduction rate V3. Furthermore, to prevent the reduction rate of the total braking torque TbT from fluctuating due to the increase in the reduction rate of the regenerative braking torque TbE, the friction braking torque TbF is increased in accordance with the reduction rate of the regenerative braking torque TbE. Then, the abrupt reduction in the regenerative braking torque TbE causes abrupt increases in the axle torque TqS.

[0094] At time t19, when the regenerative braking torque TbE becomes 0 (zero), the reduction process ends and the holding process begins. In the holding process, the friction braking torque TbF is held at the set braking torque Tbset (= vehicle-stop holding braking torque Tbh), and the regenerative braking torque TbE is held at 0 (zero). Furthermore, after time t19, both the drive torque Td and the regenerative braking torque TbE no longer change, and the axle torque TqS also no longer changes.

[0095] When it is determined at timing t20 that the vehicle 10 has stopped, the holding process ends and the degeneration process begins. In the degeneration process, the friction braking torque TbF is increased to the required braking torque TbRq. Thereafter, at a timing after timing t20, when the friction braking torque TbF increases to the required braking torque TbRq, the degeneration process ends. In other words, the vehicle-stop braking control ends.

[0096] The first embodiment can achieve the following effects. (1) It can be inferred that the greater the magnitude of the axle torque TqS, the greater the degree of torsion of the axle 14. Furthermore, the slower the rate of increase of the axle torque TqS, the more gradually the degree of torsion of the axle 14 changes. Therefore, by gradually changing the degree of torsion of the axle 14 when the axle torque TqS switches from a negative value to a positive value, vehicle body vibration caused by the reversal of the torsion direction of the axle 14 is less likely to increase. Therefore, the brake control device 70 reduces the rate of increase of the axle torque TqS at the point when the axle torque TqS switches from positive to negative during vehicle stop braking control. The brake control device 70 can suppress vehicle body vibration caused by the reversal of the torsion direction of the axle 14 while suppressing the occurrence of vehicle body swaying during vehicle stop by executing vehicle stop braking control. As a result, the brake control device 70 can improve the comfort of the occupants of the vehicle 10 when the vehicle 10 is stopped by generating a braking force.

[0097] (2) During the vehicle-stop braking control, the brake control device 70 reduces the regenerative braking torque TbE to 0 (zero) while the vehicle-stop-maintaining braking torque Tbh is generated in the vehicle 10. As a result, the brake control device 70 can stop the vehicle 10 using the friction braking torque TbF even when the regenerative braking torque TbE becomes 0 (zero). In other words, the brake control device 70 can prevent a period during which no braking torque is applied from occurring during the execution of the vehicle-stop braking control.

[0098] Second Embodiment A second embodiment of the brake control device 70 and vehicle control method will be described. The second embodiment differs from the first embodiment in the method for reducing the increasing speed of the axle torque TqS at the time of positive / negative reversal. Therefore, the following description will mainly focus on the differences from the first embodiment. Furthermore, in the following description, the same reference numerals will be used to designate components corresponding to those in the first embodiment, and redundant description will be omitted.

[0099] In the second embodiment, the control unit 125 sets the set braking torque Tbset to a value obtained by adding a predetermined offset value Tboff to the vehicle-stop maintaining braking torque Tbh in the switching process of the vehicle-stop braking control. The offset value Tboff may be, for example, several percent to several tens of percent of the vehicle-stop maintaining braking torque Tbh.

[0100] During vehicle-stop braking control, the control unit 125 executes a process, including a fourth adjustment process, for reducing the rate of increase of the axle torque TqS at the point at which the axle torque TqS reverses its positive and negative polarities. The control unit 125 starts the fourth adjustment process when a fourth determination condition is met. The fourth determination condition is met when the axle torque TqS approaches the point at which the axle torque TqS reverses its positive and negative polarities while the reduction process is being executed. The control unit 125 may determine that the fourth determination condition is met when the predicted arrival time from the current time to the point at which the axle torque TqS reverses its positive and negative polarities is less than a predetermined fourth determination time. Alternatively, the control unit 125 may determine that the fourth determination condition is met when the axle torque TqS is equal to or greater than a predetermined fourth determination torque. In this case, the fourth determination torque is a negative value.

[0101] In the fourth adjustment process, the control unit 125 sets the rate of decrease of the regenerative braking torque command value TbEtr to a rate slower than the standard decrease rate Vstd. Therefore, while the fourth adjustment process is being performed, the regenerative braking torque TbE decreases more slowly than when the fourth adjustment process is not being performed. In other words, the control unit 125 reverses the sign of the axle torque TqS while the fourth adjustment process is being performed, thereby slowing down the rate of increase of the axle torque TqS at the time when the sign of the axle torque TqS reverses.

[0102] Furthermore, when the control unit 125 executes the fourth adjustment process, if only the rate of decrease of the regenerative braking torque command value TbEtr is changed, the rate of decrease of the total braking torque TbT during the execution of the reduction process may fluctuate. Therefore, when executing the fourth adjustment process, the control unit 125 changes the target frictional braking torque TbFtr so as not to fluctuate the rate of decrease of the total braking force. Specifically, when the rate of decrease of the regenerative braking torque command value TbEtr is to be reduced in the fourth adjustment process, the target frictional braking torque TbFtr is reduced. Furthermore, the control unit 125 terminates the fourth adjustment process when the target frictional braking torque TbFtr decreases to the vehicle-stop holding braking torque Tbh. After the control unit 125 terminates the fourth adjustment process, the rate of decrease of the regenerative braking torque TbEtr increases at the point where the rate of decrease of the regenerative braking torque command value TbEtr returns to the standard decrease rate Vstd.

[0103] <Vehicle body vibration reduction processing> The vehicle body vibration reduction processing will be described with reference to Fig. 5. The processing circuit 71 repeatedly executes the vehicle body vibration reduction processing at each predetermined control cycle. The processing circuit 71 functions as the control unit 125 to execute a plurality of processes constituting the vehicle body vibration reduction processing.

[0104] As shown in FIG. 5 , the processing circuit 71 determines whether a fourth determination condition is met (S61). If the fourth determination condition is not met (S61: NO), i.e., if the axle torque TqS is not approaching its point of time when its positive and negative values ​​reverse, the processing circuit 71 temporarily terminates this process. On the other hand, if the fourth determination condition is met (S61: YES), i.e., if the axle torque TqS is approaching its point of time when its positive and negative values ​​reverse, the processing circuit 71 starts a fourth adjustment process (S62). That is, as the rate of decrease of the regenerative braking torque command value TbEtr decreases, the target frictional braking torque TbFtr begins to decrease. Next, the processing circuit 71 determines whether the target frictional braking torque TbFtr is equal to or less than the vehicle-stop-holding braking torque Tbh (S63). If the target frictional braking torque TbFtr is greater than the vehicle-stop-holding braking torque Tbh (S63: NO), the processing circuit 71 proceeds to step S63. That is, the control unit 125 continues the fourth adjustment process. On the other hand, if the target friction braking torque TbFtr is equal to or less than the vehicle-stop maintaining braking torque Tbh (S63: YES), the processing circuit 71 ends the fourth adjustment process. Then, the processing circuit 71 ends this process.

[0105] <Functions and Effects of Second Embodiment> With reference to (A) to (F) of Figure 6, the functions and effects of the braking control device 70 when stopping the vehicle 10 will be described. Note that in (A) of Figure 6, as in Figure 2, the required braking torque TbRq is shown as a positive value for convenience.

[0106] Timing t21 is the timing at which a braking request is generated, similar to timing t11, and timing t22 is the timing at which the drive torque Td begins to increase, similar to timing t12.

[0107] At timing t23, when the stopping distance DS becomes equal to or less than the first threshold value DSth1, a switching process for the stationary braking control is initiated. During the switching process, the frictional braking torque TbF begins to increase toward the set braking torque Tbset. Specifically, the frictional braking torque TbF begins to increase toward a value obtained by adding the offset value Tboff to the stationary braking torque Tbh. After timing t23, the regenerative braking torque TbE is reduced by the increase in the frictional braking torque TbF. Therefore, the total braking torque TbT, which is the sum of the regenerative braking torque TbE and the frictional braking torque TbF, remains unchanged. Then, the axle torque TqS increases toward 0 (zero) in response to the decrease in the regenerative braking torque TbE and the increase in the drive torque Td.

[0108] At time t24, when the friction braking torque TbF reaches the set braking torque Tbset, the switching process ends. Therefore, from time t24 to time t25, the regenerative braking torque TbE and the friction braking torque TbF are held constant. Meanwhile, during this period, the drive torque Td is increasing, so the axle torque TqS increases toward 0 (zero).

[0109] At timing t25, when the stopping distance DS becomes equal to or less than the second threshold value DSth2, the reduction process of the stationary braking control is initiated. In the reduction process, the regenerative braking torque TbE is reduced toward 0 (zero). The reduction rate of the regenerative braking torque TbE is the standard reduction rate Vstd. Then, in response to the reduction in the regenerative braking torque TbE and the increase in the drive torque Td, the axle torque TqS increases toward 0 (zero).

[0110] At timing t26, the fourth adjustment process is initiated. As a result, the rate of decrease of the regenerative braking torque TbE becomes smaller than the standard decrease rate Vstd. Furthermore, the frictional braking torque TbF is reduced in accordance with the amount of decrease in the regenerative braking torque TbE so that the rate of decrease of the total braking torque TbT does not change. As the rate of decrease of the regenerative braking torque TbE decreases, the rate of increase of the axle torque TqS decreases. Specifically, as shown by the solid line in FIG. 6C, the rate of decrease of the regenerative braking torque TbE during execution of the fourth adjustment process is smaller than the rate of decrease of the regenerative braking torque TbE when the fourth adjustment process is not executed, as shown by the dashed line in FIG. 6C. As a result, as shown by the solid line in FIG. 6F, the rate of decrease of the axle torque TqS during execution of the fourth adjustment process is smaller than the rate of decrease of the axle torque TqS when the fourth adjustment process is not executed, as shown by the dashed line in FIG. 6F.

[0111] At time t27, the axle torque TqS becomes 0 (zero). In other words, the sign of the axle torque TqS is reversed while the rate of increase of the axle torque TqS is slowing down. Here, the rate of increase of the axle torque TqS at the time when the axle torque TqS reverses its sign is smaller than that at a reference time point before the time when the axle torque TqS reverses its sign. The reference time point is, for example, any time point between time t25 and time t26. In this way, vehicle vibration caused by the reversal of the sign of the axle torque TqS is suppressed.

[0112] At timing t28, when the friction braking torque TbF becomes the vehicle-stop maintaining braking torque Tbh, the fourth adjustment process ends. As a result, the rate of decrease of the regenerative braking torque TbE returns to the standard decrease rate Vstd, and the friction braking torque TbF is maintained at the vehicle-stop maintaining braking torque Tbh. As the rate of decrease of the regenerative braking torque TbE increases, the rate of increase of the axle torque TqS also increases.

[0113] At timing t29, the decrease process ends and the retention process starts, similar to timing t19. At timing t30, the retention process ends and the degeneration process starts, similar to timing t20.

[0114] The second embodiment can achieve the same effects as the effects (1) and (2) of the first embodiment. (Third Embodiment) A third embodiment of the brake control device 70 and vehicle control method will be described. The third embodiment differs from the first embodiment in the method of reducing the increasing speed at the time when the axle torque TqS reverses from positive to negative. Therefore, the following description will mainly focus on the differences from the first embodiment. Furthermore, in the following description, the same reference numerals will be used to designate components corresponding to those in the first embodiment, and redundant description will be omitted.

[0115] The control unit 125 according to the third embodiment performs processing, including a fifth adjustment processing and a sixth adjustment processing, to reduce the rate of increase of the axle torque TqS at the point at which the axle torque TqS reverses positive and negative during braking control at a stop.

[0116] The control unit 125 starts the fifth adjustment process when a fifth determination condition is met. The fifth determination condition is met when the axle torque TqS approaches the point at which it reverses its positive and negative values. The control unit 125 may determine that the fifth determination condition is met when the predicted arrival time from the current time to the point at which the axle torque TqS reverses its positive and negative values ​​is less than a predetermined fifth determination time. Alternatively, the control unit 125 may determine that the fifth determination condition is met when the axle torque TqS becomes equal to or greater than a predetermined fifth determination torque. In this case, the fifth determination torque is a negative value.

[0117] In the fifth adjustment process, the control unit 125 instructs the drive control unit 101 to reduce the drive torque Td. Therefore, while the fifth adjustment process is being performed, the rate of increase of the axle torque TqS decreases at the point where the drive torque Td decreases. In other words, by reducing the drive torque Td while the fifth adjustment process is being performed, the control unit 125 reduces the rate of increase of the axle torque TqS at the point where the axle torque TqS reverses between positive and negative.

[0118] If a sixth determination condition is met during the fifth adjustment process, the control unit 125 terminates the fifth adjustment process and starts the sixth adjustment process. The sixth determination condition is met when the axle torque TqS becomes a positive value, i.e., after the axle torque TqS reverses its sign. Specifically, the control unit 125 may determine that the sixth determination condition is met when the elapsed time from the point in time when the axle torque TqS reverses its sign is equal to or greater than the sixth determination time. Alternatively, the control unit 125 may determine that the sixth determination condition is met when the axle torque TqS becomes equal to or greater than the sixth determination torque. Here, the sixth determination torque is a positive value.

[0119] In the sixth adjustment process, the control unit 125 instructs the drive control unit 101 to increase the drive torque Td. Therefore, while the sixth adjustment process is being performed, the rate at which the axle torque TqS increases increases as the drive torque Td increases. In the sixth adjustment process, it is preferable that the drive torque Td be increased at a constant rate until it reaches the drive torque Td corresponding to the current vehicle speed VS. In other words, the control unit 125 ends the sixth adjustment process when the drive torque Td reaches the drive torque Td corresponding to the current vehicle speed VS.

[0120] <Vehicle body vibration reduction processing> The vehicle body vibration reduction processing will be described with reference to Fig. 7. The processing circuit 71 repeatedly executes the vehicle body vibration reduction processing at each predetermined control cycle. The processing circuit 71 functions as the control unit 125 to execute a plurality of processes constituting the vehicle body vibration reduction processing.

[0121] As shown in FIG. 7 , the processing circuit 71 determines whether the fifth determination condition is met (S71). If the fifth determination condition is not met (S71: NO), i.e., if the axle torque TqS is not approaching the time when its sign changes, the processing circuit 71 terminates this process. On the other hand, if the fifth determination condition is met (S71: YES), i.e., if the axle torque TqS is approaching the time when its sign changes, the processing circuit 71 starts the fifth adjustment process (S72). That is, the processing circuit 71 instructs the drive control device 30 to reduce the drive torque Td. Next, the processing circuit 71 determines whether the sixth determination condition is met (S73). If the sixth determination condition is not met (S73: NO), i.e., if the axle torque TqS has not yet reversed, the processing circuit 71 proceeds to step S73. That is, the processing circuit 71 continues the fifth adjustment process.

[0122] In step S73, if the sixth determination condition is met (S73: YES), that is, if the axle torque TqS is inverted in sign, the processing circuit 71 ends the fifth adjustment process and starts the sixth adjustment process (S74). That is, the processing circuit 71 instructs the drive control device 30 to increase the drive torque Td. When the drive torque Td is increased so that the drive torque Td output by the power unit 20 becomes the drive torque Td that should be output in accordance with the current vehicle speed VS, the processing circuit 71 ends the sixth adjustment process. Thereafter, the processing circuit 71 ends this process.

[0123] <Functions and Effects of Third Embodiment> The functions and effects of the braking control device 70 when stopping the vehicle 10 will be described with reference to (A) to (F) of Figure 8. Note that, in (A) of Figure 8, as in Figure 2, the required braking torque TbRq is shown as a positive value for convenience.

[0124] At timing t31, a braking request is generated, similar to timing t11. At timing t32, the driving torque Td begins to increase, similar to timing t12. At timing t33, the switching process begins, similar to timing t13, and at timing t34, the switching process ends, similar to timing t14. At timing t35, the reduction process begins, similar to timing t15.

[0125] When the fifth determination condition is met at timing t36, the fifth adjustment process is initiated. That is, from timing t36 onward, the drive torque Td begins to decrease at a constant rate. As a result, the rate at which the axle torque TqS increases decreases because the drive torque Td is decreasing even though the regenerative braking torque TbE is decreasing. Specifically, as shown by the solid line in FIG. 8E, the rate at which the drive torque Td decreases during the fifth adjustment process is smaller than the rate at which the drive torque Td decreases (=0 (zero)) when the fifth adjustment process is not executed, as shown by the dashed line in FIG. 8E. As a result, as shown by the solid line in FIG. 8F, the rate at which the axle torque TqS decreases during the fifth adjustment process is smaller than the rate at which the axle torque TqS decreases when the fifth adjustment process is not executed, as shown by the dashed line in FIG. 8F.

[0126] At timing t37, the axle torque TqS becomes 0 (zero). In other words, the sign of the axle torque TqS is reversed while the rate of increase of the axle torque TqS is slowing down. Here, the rate of increase of the axle torque TqS at the time when the axle torque TqS reverses its sign is smaller than that at a reference time point before the time when the axle torque TqS reverses its sign. The reference time point is, for example, any time point between timings t35 and t36. In this way, vehicle vibration caused by the reversal of the sign of the axle torque TqS is suppressed.

[0127] At timing t38, when the sixth determination condition is met, the fifth adjustment process ends and the sixth adjustment process begins. That is, the drive torque Td begins to increase at a constant rate. At timing t39, when the drive torque Td reaches the drive torque Td that the power unit 20 should output in accordance with the vehicle speed VS, the sixth adjustment process ends. Also at timing t39, the regenerative braking torque TbE becomes 0 (zero). Therefore, the decrease process ends and the retention process begins. Thereafter, at timing t40, similar to timing t20, the retention process ends and the degeneration process begins.

[0128] The third embodiment can achieve the same effects as the effects (1) and (2) of the first embodiment. <Modifications> The above-described embodiments can be modified as follows. The above-described embodiments and the following modifications can be combined with each other to the extent that no technical contradiction occurs.

[0129] As an example of a combination of multiple embodiments, the first embodiment and the second embodiment may be combined. That is, in the switching process according to the first embodiment, the set braking torque Tbset may be set to a value obtained by adding a predetermined offset value Tboff to the vehicle stop-maintaining braking torque Tbh. In this way, in the second adjustment process according to the first embodiment, the drive torque Td is reduced to the vehicle stop-maintaining braking torque Tbh, thereby further slowing down the rate at which the regenerative braking torque TbE is reduced.

[0130] The brake control device 70 may acquire the vehicle speed VS as the stopping-related value instead of the stopping distance DS. The brake control device 70 does not need to make the total braking torque TbT smaller than the required braking torque TbRq during braking control at a stop. In other words, the brake control device 70 may stop the vehicle 10 in a state where the total braking torque TbT is equal to the required braking torque TbRq during braking control at a stop.

[0131] The brake control device 70 does not need to execute the switching process in the stationary braking control according to the first and third embodiments. In Figures 2, 6, and 8, the timing at which the increase in the drive torque Td ends does not necessarily coincide with the timing at which the decrease process begins. The increase in the drive torque Td may end later than the start of the decrease process, or it may end earlier than the start of the decrease process.

[0132] 8, the start timing of the fifth adjustment process for reducing the driving force does not necessarily coincide with the end timing of the increase in the driving torque Td. The start timing of the fifth adjustment process may be later than the end timing of the increase in the driving torque Td, or may be earlier than the end timing of the increase in the driving torque Td.

[0133] The vehicle 10 may include an in-wheel motor as the motor generator. In this case, the "axle" corresponds to an output shaft of the in-wheel motor or a power transmission shaft from the in-wheel motor to the wheels 13.

[0134] For example, in the propeller shaft of a rear-wheel drive vehicle or a four-wheel drive vehicle, vibrations may occur due to the reversal of positive and negative torque acting thereon, similar to the axle 14. In this regard, the "axle" in the above embodiment includes a shaft member, such as a propeller shaft, provided in the power transmission path from the power unit 20 to the wheels 13.

[0135] The phrase "at least one" as used herein means "one or more" of the desired options. As an example, the phrase "at least one" as used herein means "only one option" or "both of two options" if the number of options is two. As another example, the phrase "at least one" as used herein means "only one option" or "any combination of two or more options" if the number of options is three or more.

[0136] The drive control device 30 and the braking control device 70 are not limited to processing circuits equipped with a CPU and ROM and executing software processing. For example, the drive control device 30 and the braking control device 70 may be equipped with dedicated hardware circuits that execute at least some of the various processes executed in the above-described embodiment. An example of a dedicated hardware circuit is an ASIC. ASIC is an abbreviation for "Application Specific Integrated Circuit." In other words, the drive control device 30 and the braking control device 70 may have any of the following configurations (a) to (c):

[0137] (a) A processing circuit comprising a processing device 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 device and program storage device that executes part of the above processes according to a program, and a dedicated hardware circuit that executes the remaining processes.

[0138] (c) A processing circuit comprising dedicated hardware circuits for executing all of the above processes, wherein the software execution device comprising a processing device and a program storage device, and the dedicated hardware circuits may be plural.

Claims

1. A vehicle control device applied to a vehicle, the vehicle comprising wheels, axles rotating integrally with the wheels, a power unit configured to apply regenerative braking torque and driving torque to the axles, and a friction braking unit configured to apply friction braking torque to the wheels, the vehicle control device comprising: a control unit configured to execute stationary braking control, when braking the vehicle, by coordinating the power unit and the friction braking unit to reduce the regenerative braking torque to 0 (zero) while increasing the friction braking torque until the vehicle stops, the sum of the regenerative braking torque and the driving torque is an axle torque, and the control unit is further configured, in the stationary braking control, to make the rate of change of the axle torque at a point in time when the axle torque reverses from positive to negative smaller than the rate of change of the axle torque at a reference point prior to the point in time of reversal.

2. A vehicle control device as described in claim 1, further comprising a stopping-related value acquisition unit configured to acquire a stopping-related value whose value decreases as the vehicle approaches a predetermined stopping position, wherein the sum of the regenerative braking torque and the friction braking torque is a total braking torque, and the stopping-time braking control is control that stops the vehicle with the total braking torque made smaller than a required braking torque for the vehicle when the stopping-related value is equal to or smaller than a predetermined threshold value, and wherein the control unit is further configured to make the rate of change of the axle torque at the point of time when the axle torque reverses positive and negative values ​​less than the rate of change of the axle torque at the reference point during the period when the total braking torque is being reduced so that the total braking torque becomes smaller than the required braking torque during the stopping-time braking control.

3. A vehicle control device as described in claim 2, wherein a braking torque capable of keeping the vehicle stopped on the road surface on which the vehicle is traveling is a stop-holding braking torque, and the stop-time braking control includes: a friction braking increase process that increases the friction braking torque to a set braking torque that is equal to or greater than the stop-holding braking torque; and a regenerative braking decrease process that is a process subsequent to the friction braking increase process and reduces the regenerative braking torque to 0 (zero) so as to reduce the total braking torque to the set braking torque, and wherein the control unit is further configured to make the rate of change of the axle torque at the point of time when the axle torque reverses positive and negative directions smaller than the rate of change of the axle torque at the reference point during execution of the regenerative braking decrease process.

4. The vehicle control device according to claim 3, wherein the control unit is further configured to reduce the rate of reduction of the regenerative braking torque during execution of the regenerative braking reduction process, thereby making the rate of change of the axle torque at the point of time when the axle torque reverses positive and negative directions smaller than the rate of change of the axle torque at the reference point.

5. A vehicle control device as described in claim 3, wherein the set braking torque is set to a value obtained by adding a predetermined offset value to the stop-holding braking torque, and the control unit is further configured to reduce the frictional braking torque to the stop-holding braking torque during execution of the regenerative braking reduction process, and to make the rate of reduction of the regenerative braking torque during the period in which the frictional braking torque is reducing smaller than the rate of reduction of the regenerative braking torque before the start of that period, thereby making the rate of change of the axle torque at the time of positive / negative reversal of the axle torque smaller than the rate of change of the axle torque at the reference time point.

6. The vehicle control device according to claim 3, wherein the control unit adjusts the drive torque by controlling the power unit, and the control unit is further configured to reduce the rate of change of the axle torque at the time point at which the axle torque reverses positive and negative by reducing the drive torque during the period in which the regenerative braking torque is being reduced in the regenerative braking reduction process, thereby making the rate of change of the axle torque smaller than the rate of change of the axle torque at the reference time point.

7. A vehicle control method applied to a vehicle, the vehicle comprising wheels, axles rotating integrally with the wheels, a power unit configured to apply regenerative braking torque and driving torque to the axles, and a friction braking unit configured to apply friction braking torque to the wheels, wherein the sum of the regenerative braking torque and the driving torque is axle torque, the vehicle control method comprising: when braking the vehicle, by coordinating the power unit and the friction braking unit, performing stationary braking control that reduces the regenerative braking torque to 0 (zero) while increasing the friction braking torque until the vehicle stops; and during the stationary braking control, making the rate of change of the axle torque at the point of time when the axle torque reverses from positive to negative smaller than the rate of change of the axle torque at a reference point prior to the point of reversal.

Images