Vehicle control device and vehicle control method

Abstract

In one aspect of a vehicle control device and a vehicle control method according to the present invention, when an abnormality occurs in the turning function of a steer-by-wire system, braking moment control for causing a vehicle to generate a yaw moment by using frictional breaking force applied to the wheels of the vehicle is performed in response to a steering operation input to the steering wheel. In the braking moment control, braking-moment braking force to be applied to a first wheel that is a turning wheel and a second wheel that is not a turning wheel is distributed in accordance with the state of the vehicle. As a result, braking moment control can function stably even when there is a change in the vehicle state.

Description

Vehicle control device and vehicle control method

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

[0002] The vehicle steering control device disclosed in Patent Document 1 includes an actuator that assists the steering of the steered wheels based on the steering torque, and in the event of a malfunction of the actuator, a braking force difference based on the steering torque is applied to the braking devices of the left and right wheels.

[0003] Japanese Patent Application Laid-Open No. 2000-190863

[0004] Incidentally, when an abnormality occurs in the steering function of the steer-by-wire system, a yaw moment is generated by creating a difference in friction braking force between the left and right wheels by the friction braking device, and control is executed to turn the vehicle. In this case, if the distribution of friction braking force for turning to each wheel is constant during turning, there is a possibility that the vehicle's traveling distance (traveling time) may be restricted or its turning performance may be reduced, depending on the vehicle condition.

[0005] The present invention has been made in consideration of the current situation, and its purpose is to provide a vehicle control device and a vehicle control method that can perform braking moment control, which generates a yaw moment in a vehicle by applying frictional braking force to the wheels, in a stable manner even when the vehicle state changes.

[0006] In one aspect of the vehicle control device according to the present invention, when an abnormality occurs in the steering function of the steer-by-wire system, braking moment control is executed in which a yaw moment is generated in the vehicle by friction braking force applied to the vehicle wheels in accordance with the steering operation input to the steering wheel, and in the braking moment control, braking forces for the braking moment applied to the first wheel that is the steered wheel and the second wheel other than the steered wheel are distributed in accordance with the state of the vehicle. Also, in one aspect of the vehicle control method according to the present invention, when the steering function of the steer-by-wire system fails and the steering wheel is turned to the right, if the temperature of a friction braking device that applies friction braking force to the right rear wheel of the vehicle is 100 degrees Celsius, friction braking force is applied to the right rear wheel, and if the temperature of the friction braking device that applies friction braking force to the right rear wheel is 300 degrees Celsius, friction braking force is applied to the right front wheel of the vehicle.

[0007] According to the present invention, braking moment control can be made to function stably even if the vehicle state changes.

[0008] 1 is an overall configuration diagram showing a vehicle control system. FIG. 2 is a block diagram showing a control system for braking / driving and steering in the vehicle control system. FIG. 3 is a flowchart showing a process of braking moment control. FIG. 4 is a flowchart showing a process of braking moment control. FIG. 5 is a diagram illustrating an example of braking / driving force distribution when a frictional braking force is applied to the rear wheels when a required acceleration is zero. FIG. 6 is a diagram illustrating an example of braking / driving force distribution when a frictional braking force is applied to the front wheels when a required acceleration is zero. FIG. 7 is a diagram illustrating an example of braking / driving force distribution when a frictional braking force is applied to the rear wheels when an acceleration is required. FIG. 8 is a diagram illustrating an example of braking / driving force distribution when a frictional braking force is applied to the rear wheels when a deceleration is required. FIG. 9 is a diagram illustrating an example of a braking / driving force distribution that generates a moment for steering the front wheels. FIG. 10 is a diagram illustrating a state in which the turning direction required by the driver differs from the direction of the front wheels. FIG. 11 is a diagram illustrating an example of braking / driving force distribution when the front wheels are being steered when a straight ahead travel is required. 1 is a diagram illustrating an example of the distribution of braking and driving forces when returning the front wheels to near a neutral position. FIG. 2 is a flowchart illustrating a process of braking moment control including slip correction control. FIG. 3 is a flowchart illustrating a process of braking moment control including slip correction control. FIG. 4 is a flowchart illustrating a process of slip correction control. FIG. 5 is a diagram illustrating an example of the distribution of braking and driving forces by slip correction control.

[0009] Hereinafter, an embodiment of a vehicle control device and a vehicle control method according to the present invention will be described with reference to the drawings. Fig. 1 is an overall configuration diagram showing a vehicle control system 100 mounted on a vehicle 10. Fig. 2 is a block diagram showing a control system for braking / driving and steering of the vehicle control system 100.

[0010] The vehicle 10 is a four-wheeled automobile equipped with a pair of left and right front wheels 11, 12 and a pair of left and right rear wheels 13, 14. The front wheels 11, 12 are first wheels that correspond to steered wheels among the wheels 11, 12, 13, 14 of the vehicle 10. The rear wheels 13, 14 are second wheels that are not steered wheels.

[0011] The vehicle 10 also includes, as a braking / driving force actuator 70, a first electric motor 71 that applies braking / driving force to the front wheels 11, 12 and a second electric motor 72 that applies braking / driving force to the rear wheels 13, 14. The braking / driving force control unit 31 acquires information on the operation amount of an accelerator pedal 73 from an accelerator pedal sensor 74 and outputs a control signal to the electric motors 71, 72 that corresponds to a target driving force based on the operation amount of the accelerator pedal 73, thereby applying driving forces corresponding to the operation amount of the accelerator pedal 73 to the front wheels 11, 12 and the rear wheels 13, 14. The braking / driving force control unit 31 also operates the electric motors 71, 72 as generators, thereby controlling regenerative braking that uses rotational resistance during power generation as braking force, and generates regenerative braking forces on the front wheels 11, 12 and the rear wheels 13, 14.

[0012] Vehicle 10 also includes, as a steering device, a steer-by-wire system 40 in which steering wheel 51 is mechanically separated from steered front wheels 11, 12. Steer-by-wire system 40 has a reaction force actuator 41 that applies an operation reaction torque to steering wheel 51, a wheel actuator 42 that applies a steering force to front wheels 11, 12, a steering angle sensor 43 that detects the steering angle δ of front wheels 11, 12 (in other words, the front wheel tire angle), and a steering control unit 32 that controls reaction force actuator 41 and wheel actuator 42.

[0013] Steering wheel 51 is also equipped with an operation angle sensor 52 that detects an operation angle θ, which is the rotation angle of steering wheel 51. Note that steering angle sensor 43 detects the rotation angle of a steering motor that constitutes wheel actuator 42 and the rack position in a rack and pinion mechanism that converts the rotational force of the steering motor into linear movement, thereby detecting the steering angle δ of front wheels 11, 12.

[0014] Steering control unit 32 acquires information about the steering angle θ of steering wheel 51 detected by steering angle sensor 52 (in other words, information about the steering operation), and calculates a target steering angle and a target reaction torque from the steering angle θ, etc. Then, steering control unit 32 controls wheel actuator 42 so that the steering angle δ detected by steering angle sensor 43 approaches the target steering angle, and also controls reaction force actuator 41 so that the target reaction torque is applied to steering wheel 51.

[0015] The vehicle 10 also includes a friction braking system that applies friction braking force to each of the wheels 11, 12, 13, and 14. The friction braking system includes a brake control unit 33, a brake pedal sensor 62 that detects the amount of operation of a brake pedal 61, brake actuators 15, 16, 17, and 18 that are provided on each of the wheels 11, 12, 13, and 14, and brake thrust sensors 21, 22, 23, and 24 that detect the thrust generated by the brake actuators 15, 16, 17, and 18, respectively.

[0016] The brake control unit 33 controls the friction braking force applied by the brake actuators 15, 16, 17, and 18 to the respective wheels 11, 12, 13, and 14 based on the output of the brake pedal sensor 62 and the outputs of the brake thrust sensors 21, 22, 23, and 24. The brake actuators 15, 16, 17, and 18 are friction braking devices that apply friction braking force to the respective wheels 11, 12, 13, and 14.

[0017] The braking / driving force control unit 31, steering control unit 32, and brake control unit 33 each include a microcomputer 31A, 32A, and 33A, respectively. Each of the microcomputers 31A, 32A, and 33A includes a microprocessor unit (MPU), read-only memory (ROM), random access memory (RAM), and other components (not shown), and various functions are realized by causing the MPU to run programs stored in the ROM, which serves as a storage unit.

[0018] The braking / driving force control unit 31, the steering control unit 32, and the brake control unit 33 constitute a vehicle control device 30 that controls the vehicle 10. The microcomputers 31A, 32A, and 33A function as a control section 30A of the vehicle control device 30.

[0019] The vehicle 10 also includes, as sensors for detecting the behavior of the vehicle 10, a yaw rate sensor 81 that detects the yaw rate γ of the vehicle 10 and a lateral acceleration sensor 82 that detects the lateral acceleration Gy of the vehicle 10. The vehicle 10 also includes a wheel speed sensor 83 that detects the wheel speeds V1, V2, V3, and V4 of the wheels 11, 12, 13, and 14. The control unit 30A (microcomputers 31A, 32A, and 33A) acquires output signals from the yaw rate sensor 81, the lateral acceleration sensor 82, and the wheel speed sensor 83.

[0020] In vehicle control system 100, steering control unit 32 (microcomputer 32A) diagnoses whether there is an abnormality in the steering function of steer-by-wire system 40. An abnormality in the steering function is a state in which the steering angle δ of front wheels 11, 12, which are steered wheels, cannot be controlled to a target steering angle corresponding to the operation angle θ of steering wheel 51 (in other words, the steering operation input to steer-by-wire system 40). An abnormality in the steering function occurs due to factors such as a malfunction of wheel actuator 42, an abnormality in the drive circuit of wheel actuator 42, or malfunctions of various sensors.

[0021] When steering control unit 32 detects an abnormality in the steering function of steer-by-wire system 40 (in other words, when it acquires an abnormality signal in the steering function), it outputs a braking force command that generates a difference in friction braking force between the left and right wheels, thereby generating a yaw moment in vehicle 10 that corresponds to the operating angle θ of steering wheel 51, and turns vehicle 10 in the operating direction of steering wheel 51. In other words, when an abnormality occurs in the steering function of steer-by-wire system 40, steering control unit 32 changes the direction of vehicle 10 by control that generates a yaw moment using the difference in friction braking force between the left and right wheels (hereinafter referred to as braking moment control), instead of changing the direction of vehicle 10 by controlling the steering angle δ of front wheels 11, 12.

[0022] 3 and 4 are flowcharts showing one example of the procedure (flow of a vehicle control method) for braking moment control executed by the steering control unit 32. Note that instead of a system in which the steering control unit 32 executes braking moment control, a system can be adopted in which a control unit such as a vehicle integrated control unit that is higher in level than the steering control unit 32 monitors the steering function of the steer-by-wire system 40 and executes braking moment control when an abnormality occurs in the steering function.

[0023] In step S201, the steering control unit 32 detects an abnormality in the steering function of the steer-by-wire system 40. Then, in the next step S202, the steering control unit 32 determines whether an abnormality has occurred in the steer-by-wire system 40 that makes it impossible to steer the front wheels 11, 12.

[0024] If the steer-by-wire system 40 is normal and the front wheels 11, 12 can be steered, the steering control unit 32 proceeds to step S203. In step S203, the steering control unit 32 calculates the target steering angle δtg of the front wheels 11, 12 based on the operation angle θ of the steering wheel 51.

[0025] Then, in step S204, steering control unit 32 controls wheel actuator 42 to execute normal steering control to bring steering angle δ of front wheels 11, 12 closer to target steering angle δtg. In other words, when steer-by-wire system 40 is normal, steering control unit 32 realizes turning movement of vehicle 10 using steer-by-wire system 40.

[0026] On the other hand, if an abnormality has occurred in the steering function of the steer-by-wire system 40 and the front wheels 11, 12 cannot be steered by the steer-by-wire system 40, the steering control unit 32 proceeds to step S205 and subsequent steps, and realizes the turning movement of the vehicle 10 by braking moment control instead of drive control of the steer-by-wire system 40. In step S205, the steering control unit 32 determines whether the temperature of a brake actuator (friction braking device) provided on the wheel to be braked, which applies friction braking force by braking moment control, is equal to or lower than a first threshold temperature TH1.

[0027] Here, the wheels to be braked, to which frictional braking force is applied in order to realize the turning function by braking moment control, are the turning inner wheel of the rear wheels 13, 14 and the turning inner wheel of the front wheels 11, 12. The steering control unit 32 determines the turning direction based on the operating angle θ of the steering wheel 51.

[0028] As will be described later, when the scrub radius of the front wheels 11, 12 is set to a negative value, a force for steering the front wheels 11, 12 in the turning direction can be generated by either making the driving force applied to the inner wheel of the turning front wheels 11, 12 greater than the driving force applied to the outer wheel of the turning front wheels, or by applying a frictional braking force to the outer wheel of the turning front wheels 11, 12. In this case, the wheel to be braked, to which a frictional braking force is applied in order to realize the turning function by braking moment control, is the outer wheel of the turning front wheels 11, 12.

[0029] The temperature of the brake actuators 15, 16, 17, and 18 (friction braking devices) is, for example, the temperature of the brake rotor. As will be described later, the steering control unit 32 estimates the temperature of the brake actuator by estimating the temperature increase due to braking moment control from the initial temperature of the brake actuator, for example, from the brake pressure and vehicle speed, and in step S205, compares the estimated temperature with a first threshold temperature TH1.

[0030] The initial temperature can be a set temperature stored in advance in memory, and the steering control unit 32 compares the initial temperature with the first threshold temperature TH1 in step S205 at the start of braking moment control. If the vehicle 10 is equipped with temperature sensors that detect the temperatures of the brake actuators 15, 16, 17, and 18, the steering control unit 32 detects the temperatures of the brake actuators 15, 16, 17, and 18 from the outputs of the temperature sensors, and can use the detected temperatures in the temperature determinations in steps S205 and S207.

[0031] On the other hand, the first threshold temperature TH1 is a temperature that exceeds the allowable operating temperature of the brake actuators 15, 16, 17, 18 (friction braking devices), and a state in which the temperature of the brake actuator exceeds the first threshold corresponds to a temperature range (in other words, an operational prohibited temperature range) in which deformation of the brake rotor and a decrease in the frictional braking force relative to the brake pressure occur. The first threshold temperature TH1 is set to, for example, about 350 degrees Celsius.

[0032] If the temperature of any one of the brake actuators provided for the wheels to be braked, which apply frictional braking force through braking moment control, exceeds the first threshold temperature TH1, the steering control unit 32 determines that braking moment control is not possible, and proceeds from step S205 to step S206. In step S206, the steering control unit 32 calculates, as a control target, the frictional braking force to be generated by the brake actuators 15, 16, 17, and 18 in order to stop the vehicle 10, and performs a setting to stop application of driving force to all wheels without receiving a drive request from the driver. In this way, when the temperature of the brake actuators provided for the wheels to be braked excessively rises and braking moment control, in other words, turning motion through application of frictional braking force, is no longer possible, the steering control unit 32 performs a process to prompt the vehicle 10 to stop (automatic stop process) to ensure the safety of the vehicle 10.

[0033] On the other hand, if the temperatures of the brake actuators provided for the wheels to be braked, which apply frictional braking force through braking moment control, are all equal to or lower than the first threshold temperature TH1, the steering control unit 32 determines that braking moment control is possible, and proceeds from step S205 to step S207. In step S207, the steering control unit 32 determines whether the temperature of the brake actuator provided for the turning inner wheel of the rear wheels 13, 14 is equal to or higher than the second threshold temperature TH2. The second threshold temperature TH2 is a temperature lower than the first threshold temperature TH1 (TH2<TH1) and is a threshold for distinguishing whether the temperature condition is such that continued use of the brake actuator is permitted. For example, the second threshold temperature TH2 is set to approximately 200 degrees Celsius.

[0034] If the temperature of the brake actuator provided on the turning inner wheel of the rear wheels 13, 14 is lower than the second threshold temperature TH2, the steering control unit 32 proceeds to step S208. Then, in step S208, the steering control unit 32 performs distribution setting such that the target wheel to which friction braking force (braking force for braking moment) is applied in braking moment control is the turning inner wheel of the rear wheels 13, 14, in other words, setting such that a turning moment is generated by the left and right braking force difference at the rear wheels 13, 14.

[0035] On the other hand, if the temperature of the brake actuator provided for the turning inner wheel of the rear wheels 13, 14 is equal to or higher than the second threshold temperature TH2, the steering control unit 32 proceeds to step S209. Then, in step S209, the steering control unit 32 performs allocation setting such that the target wheel to which friction braking force is applied in braking moment control is the turning inner wheel (or the turning outer wheel) of the front wheels 11, 12, in other words, setting such that a turning moment is generated by the left and right braking force difference at the steered front wheels 11, 12.

[0036] That is, in braking moment control, the steering control unit 32 sets the rear wheels 13, 14 as the target wheels for generating a left-right braking force difference as a standard setting, and generates a left-right braking force difference at the rear wheels 13, 14 until the temperature of the brake actuators for the rear wheels 13, 14 reaches or exceeds the second threshold temperature TH2. Then, when the application of frictional braking force to the rear wheels 13, 14 continues and the temperature of the brake actuators used to apply the frictional braking force reaches or exceeds the second threshold temperature TH2, the steering control unit 32 switches the target wheels for generating a left-right braking force difference from the rear wheels 13, 14 to the front wheels 11, 12.

[0037] In this way, in braking moment control, the steering control unit 32 allocates the braking force for the braking moment applied to the front wheels 11, 12 (first wheels) and the rear wheels 13, 14 (second wheels) in accordance with the temperatures of the brake actuators 17, 18 (friction braking devices) of the rear wheels 13, 14, which is the state of the vehicle 10. With this configuration, it is possible to continue braking moment control for a longer period of time (longer driving distance) while suppressing excessive temperature rise in the brake actuators.

[0038] Furthermore, when a left-right braking force difference is generated between the front wheels 11, 12, the accuracy of yaw moment control may be reduced due to unstable steering caused by the alignment characteristics (scrub radius) of the front wheels 11, 12. In contrast, the rear wheels 13, 14, which are not steerable wheels, do not turn even if a left-right braking force difference is generated, so it is possible to obtain a yaw moment corresponding to the left-right braking force difference with high accuracy.

[0039] For this reason, in braking moment control, the steering control unit 32 first performs control that generates a left-right braking force difference at the rear wheels 13, 14, which is expected to provide high control accuracy. Then, when the temperature of the brake actuators for the rear wheels 13, 14 rises, the control shifts to control that generates a left-right braking force difference at the front wheels 11, 12, although this reduces the control accuracy of the turning moment. Therefore, it is possible to increase the duration (distance) of braking moment control while increasing the accuracy of braking moment control as much as possible.

[0040] When the steering control unit 32 sets the target wheel for generating a left-right braking force difference to either the rear wheels 13, 14 or the front wheels 11, 12 in step S208 or step S209, the process proceeds to step S210 and onward to execute braking force control. In step S210, the steering control unit 32 calculates a target yaw rate γ* from the operation angle θ of the steering wheel 51 and the vehicle speed V, and calculates a target acceleration α* from the operation amount of the accelerator pedal 73 and the operation amount of the brake pedal 61.

[0041] Next, in step S211, the steering control unit 32 calculates the target braking / driving forces of each of the wheels 11, 12, 13, and 14 based on the target yaw rate γ*, the target acceleration α*, and the distribution setting of friction braking force in braking moment control. Then, in step S212, the steering control unit 32 outputs a braking force control command to the brake control unit 33 based on the target braking / driving forces of each of the wheels 11, 12, 13, and 14, and outputs a driving force control command to the braking / driving force control unit 31.

[0042] In step S213, the steering control unit 32 estimates the temperatures of all of the brake actuators 15, 16, 17, and 18 based on the brake pressure and vehicle speed. The steering control unit 32 compares the estimated temperatures obtained in step S213 with a threshold temperature in steps S205 and S207. The brake control unit 33 performs a process of estimating the temperatures of the brake actuators 15, 16, 17, and 18, and the steering control unit 32 acquires information about the temperatures estimated by the brake control unit 33 and can use this information for braking moment control.

[0043] As mentioned above, the second threshold temperature TH2 used to determine whether to switch the wheels to which the braking moment braking force is distributed is set to about 200 degrees Celsius. Therefore, when the steering function of the steer-by-wire system 40 has failed and the steering wheel 51 is turned to the right, for example, if the temperature of the brake actuator 18 that applies frictional braking force to the right rear wheel 14 of the rear wheels 13, 14 is 100 degrees Celsius, frictional braking force will be applied to the right rear wheel 14, and if the temperature of the brake actuator 18 is 300 degrees Celsius, frictional braking force will be applied to the right front wheel 12 of the front wheels 11, 12.

[0044] The distribution of braking / driving forces in braking moment control will be described in more detail below. Figures 5 and 6 illustrate an example of switching of braking / driving force distribution based on the temperature of the brake actuator in a state where the driver has operated the steering wheel 51 to the right from the neutral position, requesting a right turn of the vehicle 10, and the driver's acceleration / deceleration request (in other words, the requested acceleration / deceleration required of the vehicle 10) is zero, i.e., in a constant speed traveling state. The driver's acceleration / deceleration request is determined from the amount of operation of the accelerator pedal 73 and the amount of operation of the brake pedal 61.

[0045] 5 shows the initial distribution of braking / driving forces when the steering function of the steer-by-wire system 40 fails and the steering control unit 32 starts braking moment control. As braking moment control, the steering control unit 32 distributes braking force for braking moment (friction braking force) to the right rear wheel 14, which is the turning inner wheel of the rear wheels 13, 14, thereby generating a difference in braking / driving force between the left rear wheel 13 and the right rear wheel 14 and generating a turning moment that turns the vehicle 10 to the right. Note that the distribution of braking force for braking moment shown in FIG. 5 may be such that braking force for braking moment is applied to one of the left and right wheels and not to the other, or may be such that friction braking force is applied to both the left and right wheels while generating a difference between the friction braking force applied to one of the left and right wheels and the friction braking force applied to the other.

[0046] The steering control unit 32 also outputs a regenerative braking command to generate a regenerative braking force to the front wheels 11, 12. Furthermore, the steering control unit 32 executes drive control to output a drive command to the braking / driving force control unit 31 to generate a drive force to the rear wheels 13, 14, thereby realizing the running of the vehicle 10.

[0047] Here, when the driving force is a positive physical quantity and the braking force is a negative physical quantity, the steering control unit 32 controls the friction braking force applied to the right rear wheel 14 by braking moment control, the regenerative braking force applied to the front wheels 11, 12 by regenerative braking control, and the driving force applied to the rear wheels 13, 14 by drive control so that the sum of these forces becomes zero. As described above, the steering control unit 32 adjusts the distribution of braking and driving forces to a state where the acceleration / deceleration request is zero by making the sum of the friction braking force, regenerative braking force, and driving force zero for the entire vehicle. Note that Figure 5 shows a state in which there is a difference in the driving forces applied to the left and right rear wheels 13, 14. This indicates that the driving forces of the left and right rear wheels 13, 14 differ due to the function of a differential gear that transmits the output of the second electric motor 72 to the left rear wheel 13 and the right rear wheel 14.

[0048] As described above, in braking moment control, if regenerative braking force is generated on the front wheels 11, 12 to which no braking moment braking force is allocated, energy efficiency is improved through regeneration and temperature rise in the brake actuators 15, 16 of the front wheels 11, 12 can be suppressed. Furthermore, in the state in which braking moment braking force is allocated to the right rear wheel 14 in Fig. 5, the same value of regenerative braking force is applied to the front wheels 11, 12, which are steered wheels, and no difference in braking / driving force occurs between the front wheels 11, 12. Therefore, steering of the front wheels 11, 12 due to a difference in braking / driving force between the front wheels 11, 12 does not occur, and the turning moment for turning the vehicle 10 can be controlled with high precision by adjusting the frictional braking force applied to the right rear wheel 14.

[0049] 5 continues, the temperature of the brake actuator 18, which applies braking force for braking moment to the right rear wheel 14, which is the turning inside wheel, will rise due to frictional heat. Therefore, when the temperature of the brake actuator 18 reaches or exceeds the second threshold temperature TH2, the steering control unit 32 determines that the brake actuator 18 cannot be continued to be used, and switches the braking / driving force distribution to the distribution shown in FIG.

[0050] The braking / driving force distribution shown in Figure 6 is a distribution in which the braking / driving force distribution shown in Figure 5 is swapped between the front wheels 11, 12 and the rear wheels 13, 14. In other words, as braking moment control, the steering control unit 32 distributes braking force for braking moment to the right front wheel 12, which is the turning inner wheel of the front wheels 11, 12, thereby creating a braking / driving force difference between the left front wheel 11 and the right front wheel 12 and generating a turning moment that turns the vehicle 10 to the right.

[0051] The steering control unit 32 also applies a regenerative braking force to the rear wheels 13, 14, and also applies a driving force to the front wheels 11, 12. Here, when the driving force is a positive physical quantity and the braking force is a negative physical quantity, the steering control unit 32 controls so that the sum of the friction braking force applied to the right front wheel 12 by braking moment control, the regenerative braking force applied to the rear wheels 13, 14 by regenerative braking control, and the driving force applied to the front wheels 11, 12 by drive control becomes zero.

[0052] When the driver operates the steering wheel 51 to the left from the neutral position to request a left turn of the vehicle 10, the steering control unit 32 first distributes braking force for braking moment to the left rear wheel 13 to generate a turning moment that turns the vehicle 10 to the left. Then, when the temperature of the brake actuator 17 that applies friction braking force to the left rear wheel 13 becomes equal to or higher than the second threshold temperature TH2, the steering control unit 32 switches to a state in which a turning moment that turns the vehicle 10 to the left is generated by distributing braking force for braking moment to the left front wheel 11.

[0053] In this way, the steering control unit 32 switches the wheel to which the braking force for the braking moment is distributed, in accordance with the temperature rise of the brake actuator, from the inner turning wheel of the rear wheels 13, 14 to the inner turning wheel of the front wheels 11, 12. This braking moment control makes it possible to continuously generate the turning moment in the turning direction requested by the driver while preventing the temperature of the brake actuator from rising excessively.

[0054] 7 illustrates an example of braking / driving force distribution when the driver requests the vehicle 10 to turn right and accelerate, and the temperature of the brake actuator 18 for the right rear wheel 14, which is on the inside of the turn, is below the second threshold temperature TH2. Here, the steering control unit 32 distributes the braking force for the braking moment to the right rear wheel 14, which is the inside wheel of the turn, of the rear wheels 13, 14.

[0055] The steering control unit 32 also applies driving force to the rear wheels 13, 14 to realize driving, and applies driving force to the front wheels 11, 12 in response to the driver's request for acceleration. Note that when the temperature of the brake actuator 18, which applies friction braking force to the right rear wheel 14, reaches or exceeds the second threshold temperature TH2 in the braking / driving force distribution state shown in Figure 7, the steering control unit 32 switches the braking / driving force distribution between the front wheels 11, 12 and the rear wheels 13, 14.

[0056] 8 illustrates an example of braking / driving force distribution when the driver requests the vehicle 10 to turn right and decelerate, and the temperature of the brake actuator 18 for the right rear wheel 14, which is the inside wheel of the turn, is below the second threshold temperature TH2. Here, the steering control unit 32 distributes braking force for braking moment to the right rear wheel 14, which is the inside wheel of the turning rear wheels 13, 14. The steering control unit 32 also generates driving force to the rear wheels 13, 14 to realize traveling, and generates regenerative braking force to the front wheels 11, 12 in response to the deceleration request. Note that, in the braking / driving force distribution state shown in FIG. 8 , if the temperature of the brake actuator 18, which applies frictional braking force to the right rear wheel 14, becomes equal to or higher than the second threshold temperature TH2, the steering control unit 32 switches the braking / driving force distribution between the front wheels 11, 12 and the rear wheels 13, 14.

[0057] As described above, when the steering control unit 32 distributes the braking force for the braking moment to the rear wheels 13, 14, it satisfies the acceleration / deceleration request of the driver by controlling the braking / driving force applied to the front wheels 11, 12. Furthermore, when the steering control unit 32 distributes the braking force for the braking moment to the front wheels 11, 12, it satisfies the acceleration / deceleration request of the driver by controlling the braking / driving force applied to the rear wheels 13, 14. Therefore, when an abnormality occurs in the steering function of the steer-by-wire system 40, in addition to the function of changing the traveling direction of the vehicle 10, for example, toward the shoulder, it is also possible to realize acceleration / deceleration according to the driver's request and to limp home, which allows the vehicle 10 to reach the stopping position intended by the driver.

[0058] In addition, when the temperature of the brake actuator of the inner turning wheel of the rear wheels 13, 14 becomes high, the steering control unit 32 distributes braking force for the braking moment to the front wheels 11, 12 to generate a difference in braking force between the left and right wheels, thereby utilizing the alignment characteristics (scrub radius) of the front wheels 11, 12 to steer the front wheels 11, 12 in the turning direction required by the driver.

[0059] 9 shows a state in which, with the steering wheel 51 turned to the right, the temperature of the brake actuator 18 for the right rear wheel 14, which is the turning inner wheel of the rear wheels 13, 14, has reached or exceeded the second threshold temperature TH2, and the braking force for the braking moment is distributed to the front wheels 11, 12. Note that the scrub radii of the front wheels 11, 12 are assumed to be negative. At this time, the steering control unit 32 distributes the braking force for the braking moment to the left front wheel 11, which is the turning outer wheel of the front wheels 11, 12.

[0060] In a vehicle 10 in which the scrub radii of the front wheels 11, 12 are set to be negative, application of a braking force to the front wheels 11, 12 generates a moment that steers the front wheels 11, 12 toward the left-right central axis of the vehicle body. Here, in the case where the steering mechanism of the front wheels 11, 12 connects the front wheels 11, 12 with a tie rod and changes the steering angle of the front wheels 11, 12 as a unit, when the same braking force is applied to both front wheels 11, 12, the moments generated by the front wheels 11, 12 cancel each other out, and the steering angle of the front wheels 11, 12 does not change.

[0061] In contrast, if there is a difference in the braking forces applied to the front wheels 11, 12, the moments generated by the front wheels 11, 12 will differ, thereby changing the steering angles of the front wheels 11, 12. For example, if a larger braking force is applied to the left front wheel 11 of the front wheels 11, 12, the front wheels 11, 12 will be steered to the right, and if a larger braking force is applied to the right front wheel 12 of the front wheels 11, 12, the front wheels 11, 12 will be steered to the left.

[0062] 9 , if braking force is applied to the left front wheel 11, which is the outside turning wheel, it is possible to actively steer the front wheels 11, 12 toward the right, which is the turning direction, and the vehicle 10 can be turned to the right by the tire lateral force generated by the steering of the front wheels 11, 12. In this way, the switching of the distribution of the braking force for the braking moment in accordance with the temperature of the brake actuator is not limited to switching from the inside turning wheel of the rear wheels 13, 14 to the inside turning wheel of the front wheels 11, 12, but can also be switching from the inside turning wheel of the rear wheels 13, 14 to the outside turning wheel of the front wheels 11, 12 when the scrub radius is negative.

[0063] Furthermore, when the steering function of the steer-by-wire system 40 fails, the steering control unit 32 can return the steering angle of the front wheels 11, 12 to near the neutral position by utilizing the fact that the front wheels 11, 12 can be steered in a desired direction by generating a braking force difference between the front wheels 11, 12.

[0064] 10 shows a case where the front wheels 11, 12 are steered to the right, opposite the right-turning direction requested by the driver, when the steering function of the steer-by-wire system 40 fails. In this state, even if the steering control unit 32 applies frictional braking force to the turning inner wheel of the rear wheels 13, 14 to turn the vehicle 10 right, the tire lateral force generated by the front wheels 11, 12 acts in a direction that turns the vehicle 10 left, reducing the effectiveness of the braking moment control.

[0065] 11 shows a case where the front wheels 11, 12 are steered to the right despite the driver requesting straight ahead when the steering function of the steer-by-wire system 40 fails. In this case, because the front wheels 11, 12 are steered to the right, in order to make the vehicle 10 go straight, it becomes necessary to apply a braking force to the left rear wheel 13, and to generate a turning moment by the braking force difference between the rear wheels 13, 14 that cancels out the turning moment generated by the steering angle of the front wheels 11, 12.

[0066] In other words, when the steering function of steer-by-wire system 40 fails and front wheels 11, 12 are steered, more specifically, when the steering angle of front wheels 11, 12 exceeds a predetermined angle, it is necessary to continuously apply frictional braking force to one of rear wheels 13, 14 in order to keep vehicle 10 moving straight. If frictional braking force is continuously applied, the temperature of the brake actuator will rise and uneven tire wear may occur.

[0067] Here, if the steering function of the steer-by-wire system 40 fails, the steering control unit 32 generates a braking force difference between the front wheels 11, 12 to return the steering angle of the front wheels 11, 12 to near the neutral position, a turning function can be stably obtained by braking moment control, and the need to continue applying frictional braking force when driving the vehicle 10 straight is reduced.

[0068] The flowcharts in Figures 12 and 13 are flowcharts showing the procedure for braking moment control, including the process of returning the front wheels 11, 12 to near the neutral position, performed by the steering control unit 32. Note that steps S301 to S304 in the flowchart in Figure 12 are the same as steps S201 to S204 in the flowchart in Figure 3, and steps S307 to S313 in the flowchart in Figure 13 are the same as steps S207 to S213 in the flowchart in Figure 4. For this reason, the description of the processing content in steps S301 to S304 and steps S307 to S313 will be omitted, and the processing in steps S305 to S306 will be described in detail below.

[0069] If the steering control unit 32 determines in step S302 that the steer-by-wire system 40 is unable to steer the front wheels 11, 12, the process proceeds to step S305. In step S305, the steering control unit 32 determines whether the absolute value of the steering angle δ [deg] of the front wheels 11, 12 is equal to or less than a predetermined angle δth.

[0070] The steering angle δ indicates the neutral position as zero, and whether it is a steering angle to the right from the neutral position or a steering angle to the left from the neutral position is determined by the sign of plus or minus. Therefore, in step S305, the steering control unit 32 determines whether the front wheels 11, 12 are near the neutral position.

[0071] The steering angle δ that steering control unit 32 determines in step S305 is either a value detected by steering angle sensor 43 or an estimated value of steering angle δ. Here, steering control unit 32 can, for example, generate a difference in braking / driving force between rear wheels 13, 14 so as to generate a target yaw rate or a target lateral acceleration, and estimate the steering angle δ of front wheels 11, 12 based on the difference between the target yaw rate or target lateral acceleration and the actually generated yaw rate or lateral acceleration.

[0072] If steering control unit 32 determines in step S305 that the absolute value of steering angle δ exceeds predetermined angle δth and that front wheels 11, 12 have been steered to the right or left by more than the predetermined amount, the process proceeds to step S306. In step S306, steering control unit 32 performs processing to make the absolute value of steering angle δ of front wheels 11, 12 fall below predetermined angle δth, in other words, processing to return steering angle δ of front wheels 11, 12 to near the neutral position, without using steer-by-wire system 40. In other words, predetermined angle δth in step S305 is a threshold value for distinguishing, when braking moment control is performed, whether the current steering angle δ is an angle that can be left as is or an angle that requires processing to return it to a smaller angle.

[0073] On the other hand, if steering control unit 32 determines in step S305 that the absolute value of steering angle δ is equal to or less than predetermined angle δth, in other words, that steering angle δ is near the neutral position, it bypasses step S306 and proceeds to step S307. In other words, if the absolute value of steering angle δ is equal to or less than predetermined angle δth, steering control unit 32 determines that it is not necessary to perform processing to return steering angle δ of front wheels 11, 12 to near the neutral position.

[0074] In step S306, the steering control unit 32 generates a braking force difference between the front wheels 11, 12 so that the absolute value of the steering angle δ of the front wheels 11, 12 is equal to or less than a predetermined angle δth. For example, if the scrub radii of the front wheels 11, 12 are negative and the front wheels 11, 12 are steered to the right from the neutral position, the steering control unit 32 applies a frictional braking force to the right front wheel 12 by the brake actuator 16 in step S306, thereby steering the front wheels 11, 12 back toward the neutral position.

[0075] Furthermore, if the scrub radius of the front wheels 11, 12 is negative and the front wheels 11, 12 are steered to the left from the neutral position, the steering control unit 32 applies frictional braking force to the left front wheel 11 by the brake actuator 15 in step S306, thereby steering the front wheels 11, 12 back toward the neutral position. After returning the front wheels 11, 12 to the neutral position as described above, the steering control unit 32 proceeds to step S307 and subsequent steps, where braking moment control is performed to switch the wheels that generate a braking force difference between the left and right wheels from the rear wheels 13, 14 to the front wheels 11, 12 in accordance with the temperature rise of the brake actuator.

[0076] When the steering control unit 32 performs the process of steering the front wheels 11, 12 back toward the neutral position in step S306, it applies a braking force for braking moment, which is a friction braking force, to the turning inner wheel of the rear wheels 13, 14, thereby generating a turning moment. This prevents the turning moment from being generated during the process of steering the front wheels 11, 12 back toward the neutral position, and thus prevents the turning operation from being delayed.

[0077] 14 is a state diagram for explaining the control operation of the process of returning the front wheels 11, 12 to near the neutral position. Note that Fig. 14 illustrates a case where the scrub radius of the front wheels 11, 12 is negative, the driver requests a right turn, and the front wheels 11, 12 are being steered to the right when the steering function of the steer-by-wire system 40 fails.

[0078] At this time, the steering control unit 32 applies frictional braking force to the right front wheel 12 of the front wheels 11, 12, thereby generating a moment that steers the right front wheel 12 to the left, and steers the left front wheel 11 to the left in response to the steering of the right front wheel 12 to the left, thereby returning the front wheels 11, 12 to near the neutral position. In addition, in parallel with the returning of the front wheels 11, 12, the steering control unit 32 distributes braking force for braking moment to the right rear wheel 14, which is the turning inner wheel of the rear wheels 13, 14, thereby generating the rightward turning moment requested by the driver.

[0079] When the steering control unit 32 has completed the process of returning the front wheels 11, 12 to the neutral position, it transitions to the normal braking moment control, for example, as shown in Fig. 5. Here, because the front wheels 11, 12 have been returned to the neutral position before transitioning to the control state shown in Fig. 5, the control accuracy of the turning moment generated by applying a frictional braking force to the right rear wheel 14 is ensured, and turning performance is improved.

[0080] Furthermore, when the driver requests that the vehicle 10 travel straight, the steering control unit 32 actively returns the front wheels 11, 12 to the neutral position, thereby allowing the vehicle 10 to travel straight without allocating braking force for the braking moment. In other words, in order to travel straight with the front wheels 11, 12 steered, it is necessary to generate a turning moment that offsets the turning moment caused by the steering by using a difference in braking / driving force between the left and right wheels. In contrast, if the front wheels 11, 12 are returned to the neutral position, there is no need to generate a turning moment by using a difference in braking / driving force between the left and right wheels, which makes it possible to prevent temperature increases in the brake actuator and uneven tire wear.

[0081] However, if a wheel generating a turning moment slips during braking moment control, the desired turning moment cannot be generated, and turning performance deteriorates. One mode of braking moment control that can prevent such deterioration of turning performance due to wheel slip will be described below.

[0082] 15 to 17 are flowcharts showing one example of a procedure for braking moment control that can prevent a deterioration in cornering performance due to slippage. As will be described in detail later, the braking moment control shown in the flowcharts of Fig. 15 to 17 is a control that prevents a deterioration in cornering performance due to slippage by distributing the braking force for braking moment applied to the front wheels 11, 12 (first wheels) and the rear wheels 13, 14 (second wheels) in accordance with the slip state of the vehicle 10.

[0083] Note that steps S401 to S411 in Figures 15 and 16 are the same processes as steps S201 to S211 in Figures 3 and 4, and steps S413 to S414 in Figure 16 are the same processes as steps S212 to S213 in Figure 4. For this reason, detailed explanations of the steps other than step S412 in the flowcharts of Figures 15 and 16 will be omitted, and the processing in step S412 will be explained in detail.

[0084] In step S412, the steering control unit 32 performs slip correction control, which is control for correcting the distribution of braking / driving force when the wheel generating the turning moment slips. The flowchart in Figure 17 shows the details of the processing content in step S412.

[0085] In step S412A, the steering control unit 32 determines whether slip is occurring in one or more wheels (one of the left and right wheels or both left and right wheels) among the front wheels 11, 12 or rear wheels 13, 14 that generate the turning moment. Here, the steering control unit 32 determines that slip has occurred when the tire slip ratio calculated from the output of the wheel speed sensor 83 exceeds a threshold value.

[0086] If slip is not occurring on either the left or right wheels generating the turning moment, the steering control unit 32 proceeds to step S413 without performing slip correction control. On the other hand, if slip is occurring on at least one of the wheels generating the turning moment, the steering control unit 32 proceeds to step S412B. In step S412B, the steering control unit 32 determines whether the wheel opposite to the wheel generating the slip is slipping, or whether slip is occurring on both the left and right wheels.

[0087] If the wheel opposite to the slipping wheel is not slipping, the steering control unit 32 proceeds to step S412C. In step S412C, the steering control unit 32 reduces the braking / driving force applied to the slipping wheel while increasing the braking / driving force applied to the wheel opposite to the slipping wheel, so as to ensure the required turning moment while suppressing slip of the slipping wheel.

[0088] For example, when a turning moment is being generated by applying frictional braking force to one of the rear wheels 13, 14, and the outer wheel of the rear wheels 13, 14 slips, the steering control unit 32 reduces the driving force of the rear wheels 13, 14 and increases the frictional braking force applied to the inner wheel that is not slipping. This suppresses slippage of the outer wheel, maintains the difference in braking / driving force between the rear wheels 13, 14, and ensures the required turning moment. At this time, the steering control unit 32 adjusts the braking / driving force applied to the front wheels 11, 12 to achieve the required acceleration / deceleration.

[0089] On the other hand, if both the left and right wheels are slipping, the steering control unit 32 proceeds to step S412D. In step S412D, the steering control unit 32 determines whether the temperature of the brake actuator for the inside turning wheel to which frictional braking force is applied in order to generate a turning moment is equal to or lower than the first threshold temperature TH1.

[0090] If the temperature of the brake actuator on the inside wheel of the turn exceeds the first threshold temperature TH1, the steering control unit 32 proceeds to step S412F, and similarly to step S206, calculates, as a control target, the friction braking force to be generated by the brake actuators 15, 16, 17, and 18 in order to stop the vehicle 10. On the other hand, if the temperature of the brake actuator on the inside wheel of the turn is equal to or lower than the first threshold temperature TH1, the steering control unit 32 proceeds to step S412E.

[0091] In step S412E, the steering control unit 32 corrects the target braking / driving forces (front / rear distribution amounts) of each of the wheels 11, 12, 13, and 14 calculated in step S411 so as to generate the necessary turning moment while suppressing slip. For example, if slip occurs in the rear wheels 13, 14 while a braking force for braking moment, which is a friction braking force, is being applied to the inside of the turning rear wheels 13, 14, the steering control unit 32 reduces the braking / driving force applied to the rear wheels 13, 14 in step S412E to suppress slip of the rear wheels 13, 14. Furthermore, in step S412E, the steering control unit 32 compensates for the shortage of turning moment caused by the weakening of the braking / driving force applied to the rear wheels 13, 14 by applying a braking force for braking moment to the inside of the turning front wheels 11, 12.

[0092] 18 is a diagram showing an outline of the braking / driving force correction (distribution correction) in step S412E, taking as an example a state in which the driver requests a right turn. Because a right turn is requested, the steering control unit 32 executes braking moment control to apply a braking moment braking force, which is a friction braking force, to the right rear wheel 14, which is the turning inner wheel of the rear wheels 13, 14, and also executes drive control to apply a drive force to the rear wheels 13, 14.

[0093] If the left rear wheel 13 and the right rear wheel 14 slip in this state, the steering control unit 32 reduces the braking / driving force applied to the rear wheels 13, 14 through braking moment control and drive control in order to suppress this slip. When the steering control unit 32 reduces the braking / driving force applied to the rear wheels 13, 14, the difference in braking / driving force between the rear wheels 13, 14 becomes smaller, which reduces the turning moment generated at the rear wheels 13, 14 and makes it impossible to satisfy the turning moment required by the driver.

[0094] Therefore, the steering control unit 32 generates the turning moment required by the driver by applying a braking force for braking moment, which is a friction braking force, to the right front wheel 12, which is the turning inner wheel of the front wheels 11, 12, to make up for the lack of turning moment. In this way, slip correction control can prevent a decrease in turning performance due to the occurrence of slip, and allows braking moment control to function stably.

[0095] 18, when one of the wheels generating the turning moment slips, the steering control unit 32 can switch the distribution to apply friction braking force to the turning inner wheel of the front wheels 11, 12 and the turning inner wheel of the rear wheels 13, 14. Furthermore, the steering control unit 32 can execute control to distribute the braking force for the braking moment applied to the front wheels 11, 12 and the rear wheels 13, 14 in accordance with the slip state of the wheels, without executing control to distribute the braking force for the braking moment applied to the front wheels 11, 12 and the rear wheels 13, 14 in accordance with the temperature of the brake actuator.

[0096] The technical ideas described in the above embodiments can be used in appropriate combinations as long as no contradictions arise. Furthermore, although the contents of the present invention have been specifically described with reference to preferred embodiments, it is obvious that a person skilled in the art can adopt various modified embodiments based on the basic technical ideas and teachings of the present invention.

[0097] For example, when transitioning from a state in which braking force for braking moment is distributed to the rear wheels 13, 14 to a state in which braking force for braking moment is distributed to the front wheels 11, 12, which are steered wheels, the vehicle control device 30 (steering control unit 32) can transiently distribute braking force for braking moment to both the rear wheels 13, 14 and the front wheels 11, 12. Furthermore, the vehicle control device 30 (steering control unit 32) can distribute braking force for braking moment to both the rear wheels 13, 14 and the front wheels 11, 12 at a ratio corresponding to the ratio between the temperature of the brake actuator for the turning inner wheel of the rear wheels 13, 14 and the temperature of the brake actuator for the turning inner wheel (or the previous outer wheel) of the front wheels 11, 12.

[0098] Furthermore, vehicle 10 may be, for example, a work vehicle such as a forklift or construction equipment equipped with a steer-by-wire system that steers rear wheels 13, 14. In the case of vehicle 10 equipped with a steer-by-wire system that steers rear wheels 13, 14, vehicle control device 30 distributes a braking force for braking moment to the inner turning wheel of front wheels 11, 12 when the temperature of a friction braking device that applies a friction braking force to the inner turning wheel of front wheels 11, 12 is lower than first threshold temperature TH1, and distributes a braking force for braking moment to the inner turning wheel of rear wheels 13, 14, which are steered wheels, when the temperature of the friction braking device that applies a friction braking force to the inner turning wheel of front wheels 11, 12 is higher than first threshold temperature TH1. In other words, the steered wheels steered by the steer-by-wire system may be either the front wheels 11, 12 or the rear wheels 13, 14.

[0099] The vehicle 10 may have at least four wheels, i.e., a pair of left and right wheels (first wheels) that are steered and a pair of left and right wheels (second wheels) that are not steered, and may have six wheels. The vehicle may also have an internal combustion engine as a drive source instead of the electric motors 71, 72, or may have one of the front wheels or rear wheels that is provided with a driving force by an internal combustion engine and the other wheel that is provided with a braking / driving force by an electric motor.

[0100] Furthermore, in the above embodiment, the vehicle control device 30 (steering control unit 32) distributes the braking force for the braking moment applied to the first wheel, which is the steered wheel, and the second wheel other than the steered wheel in accordance with the temperature of the friction braking device and the slip state of the wheels, which are the vehicle conditions, but the vehicle conditions are not limited to the temperature of the friction braking device and the slip state of the wheels. For example, the vehicle control device 30 (steering control unit 32) can distribute the braking force for the braking moment applied to the first wheel, which is the steered wheel, and the second wheel other than the steered wheel in accordance with physical quantities that affect the turning moment generated by the difference in braking / driving force between the left and right wheels, such as wheel load, tire air pressure, degree of tire deterioration (amount of wear), and friction coefficient of the road surface.

[0101] 10...vehicle, 11, 12...front wheels (steered wheels, first wheels), 13, 14...rear wheels (second wheels), 15, 16, 17, 18...brake actuator (friction braking device), 30...vehicle control device, 30A...control section, 32...steering control unit, 40...steer-by-wire system, 51...steering wheel, 71...first electric motor, 72...second electric motor

Claims

1. A vehicle control device provided on a vehicle having a steer-by-wire system for steering the first wheel, which is mechanically separated from the steering wheel, and a friction braking device for applying friction braking force to the wheels, where the steerable wheels among the wheels of the vehicle are designated as first wheels and the other wheels among the wheels are designated as second wheels, wherein a control unit provided in the vehicle control device, when receiving an abnormality signal of the steering function in the steer-by-wire system, executes braking moment control that generates a yaw moment in the vehicle by the friction braking force applied to the wheels in accordance with the steering operation input to the steering wheel, and in the braking moment control, distributes the braking force for the braking moment applied to the first wheel and the second wheel in accordance with the state of the vehicle.

2. A vehicle control device as described in claim 1, wherein the control unit distributes the braking force for the braking moment to either the inner turning wheel of the first wheel or the inner turning wheel of the second wheel based on the temperature of the friction braking device that applies a friction braking force to the inner turning wheel of the second wheel, among the conditions of the vehicle.

3. A vehicle control device as described in claim 2, wherein the vehicle has a first electric motor that applies a braking / driving force to the first wheel and a second electric motor that applies a braking / driving force to the second wheel, and the control unit executes regenerative braking control to generate a regenerative braking force on the second wheel when distributing the braking force for the braking moment to the first wheel, and executes regenerative braking control to generate a regenerative braking force on the first wheel when distributing the braking force for the braking moment to the second wheel.

4. A vehicle control device as described in claim 3, wherein the control unit, when a required acceleration / deceleration required of the vehicle is zero, executes drive control to generate a driving force at the first wheel when distributing the braking force for the braking moment to the first wheel, and executes drive control to generate a driving force at the second wheel when distributing the braking force for the braking moment to the second wheel, and controls so that when the driving force is a positive physical quantity and the braking force is a negative physical quantity, the total value of the frictional braking force due to the braking moment control, the regenerative braking force due to the regenerative braking control, and the driving force due to the drive control becomes zero.

5. A vehicle control device as described in claim 2, wherein the control unit distributes the braking force for the braking moment to the inner turning wheel of the second wheel when the temperature of the friction braking device that applies a friction braking force to the inner turning wheel of the second wheel is lower than a second threshold temperature.

6. A vehicle control device as described in claim 5, wherein the control unit applies a predetermined braking force to the wheel when the temperature of the friction braking device that applies a friction braking force to the turning inner wheel of the second wheel exceeds a first threshold temperature that is higher than the second threshold temperature.

7. A vehicle control device according to claim 2, wherein the first wheels are front wheels of the vehicle, and the second wheels are rear wheels of the vehicle.

8. A vehicle control device as described in claim 1, wherein the control unit distributes the braking force for the braking moment to the inner turning wheel of the first wheel and the inner turning wheel of the second wheel based on a slip state of the second wheel among the conditions of the vehicle.

9. A vehicle control device as described in claim 1, wherein the scrub radius of the first wheel is set to be negative, and the control unit distributes the braking force for the braking moment to the outer turning wheel of the first wheel when distributing the braking force to the first wheel in accordance with the state of the vehicle.

10. A vehicle control device as described in claim 1, wherein when the control unit acquires an abnormality signal of the steering function in the steer-by-wire system, if the steering angle of the first wheel exceeds a predetermined angle, the control unit applies a frictional braking force to the first wheel by the friction braking device so that the steering angle of the first wheel falls below the predetermined angle.

11. A vehicle control method executed by a control unit mounted on a vehicle having a steer-by-wire system that steers front wheels, which are steerable wheels of the vehicle and are mechanically separated from the steering wheel, and a friction braking device that applies friction braking force to the wheels of the vehicle, wherein, when the steering function of the steer-by-wire system has failed and the steering wheel is turned to the right, if the temperature of the friction braking device that applies friction braking force to the right rear wheel of the rear wheels of the vehicle is 100 degrees Celsius, a friction braking force is applied to the right rear wheel, and if the temperature of the friction braking device that applies friction braking force to the right rear wheel is 300 degrees Celsius, a friction braking force is applied to the right front wheel of the front wheels.