Vehicle braking apparatus

Abstract

To suppress braking force shortage during rainfall with a simple control logic.SOLUTION: A vehicle braking apparatus comprises a braking unit 10, an operation unit 2B, an on-off unit 20, and a control device 30. The braking unit 10 includes a friction braking unit 11 and a regenerative braking unit 12. The operation unit 2B is operated for braking, and the on-off unit 20 outputs an ON signal during rainfall and an OFF signal during non-rainfall. The control device 30 includes a determination unit 33 that determines establishment / non-establishment of a rainfall condition to be established when the ON signal is output by the on-off unit 20 and not to be established when the OFF signal is output, and a control unit 34 that controls the braking unit 10 during operation of the operation unit 2B in accordance with the establishment / non-establishment determination of the rainfall condition. The control unit 34 performs a first braking control in which the braking force of a non-driven wheel 1R is controlled to a predetermined first braking force when it is determined that the rainfall condition is not established, and performs a second braking control in which the braking force of the non-driven wheel 1R is controlled to second braking force greater than the first braking force when it is determined that the rainfall condition is established.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a vehicle braking device that brakes a vehicle. [Background technology]

[0002] Electric vehicles equipped with a motor for driving are braked not only by friction brakes but also by regenerative brakes. For example, in a two-wheel drive electric vehicle, the drive wheels are braked by hydraulic friction brakes as well as regenerative brakes, and the non-drive wheels are braked only by friction brakes. When braking with regenerative braking, the motor operates as a generator and charges the battery for driving the vehicle, improving the electric fuel efficiency of the electric vehicle. By increasing the proportion of braking force that is accounted for by regenerative braking, the electric fuel efficiency of the electric vehicle can be improved.

[0003] On the other hand, if the proportion of braking force exerted on the drive wheels by regenerative braking in the total braking force exerted on a two-wheel-drive electric vehicle is increased, there is a risk that the drive wheels will slip without the non-drive wheels slipping. Therefore, technologies have been proposed that increase the braking force of non-driven wheels when slippage of driven wheels is detected.For example, a control device has been proposed that increases the braking force of a hydraulic brake device when it determines that slippage has occurred during braking using regenerative torque (see Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2020-82928 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the control of increasing the braking force of non-driven wheels after slippage of the driven wheels is detected as in Patent Document 1, the braking force of the non-driven wheels is compensated for after the braking force of the driven wheels becomes insufficient. Therefore, there is a risk of insufficient braking force during the period from when slippage of the driven wheels is detected until the braking force of the non-driven wheels is increased. Furthermore, since it is essential to detect slippage of the driven wheels as a prerequisite for implementing control to increase the braking force of the non-driven wheels, the control logic is complicated.

[0006] Furthermore, when it is raining or snowing (hereinafter referred to as "rainfall"), the coefficient of friction of the road on which the vehicle is traveling is lower than when it is not raining, making it easier for the drive wheels to slip. Therefore, as mentioned above, it is difficult to resolve the lack of braking force during the period from when drive wheel slip is detected until the braking force of the non-drive wheels is increased.

[0007] The vehicle braking system of the present invention was invented in consideration of these problems, and one of its objectives is to suppress insufficient braking force during rainfall using simple control logic. However, in addition to this objective, another objective of the present invention is to achieve operational effects derived from the various components shown in the "Mode for Carrying Out the Invention" below, which are not obtainable with conventional technology. [Means for solving the problem]

[0008] The disclosed vehicle braking system can be realized as the following disclosed aspects (application examples), which solve at least part of the above-mentioned problems. Each of the aspects from aspect 2 onwards is an aspect that can be selected as an additional option, and each of the aspects can be omitted. None of the aspects from aspect 2 onwards discloses an aspect or configuration that is essential to the present invention.

[0009] Aspect 1. The disclosed vehicle braking device includes a braking unit, an operating unit, an on / off unit, and a control device. The braking unit includes a friction braking unit that brakes at least the non-driven wheels of the vehicle, including the driven and non-driven wheels, with friction brakes, and a regenerative braking unit that brakes only the driven wheels with regenerative brakes. The operating unit is operated by the driver of the vehicle to apply braking. The on / off unit outputs an on signal when it is raining and an off signal when it is not raining. The control device includes a determination unit that determines whether a rain condition is met when the on / off unit outputs the on signal and is not met when the off signal is output, and a control unit that performs braking control to control the braking unit in accordance with the determination of whether the rain condition is met by the determination unit when the operating unit is operated to apply braking. In the braking control, when the judgment unit determines that the rainfall condition is not met, the control unit performs a first braking control that controls the braking force of the non-driven wheel by the friction braking unit to a predetermined first braking force, and when the judgment unit determines that the rainfall condition is met, the control unit performs a second braking control that controls the braking force of the non-driven wheel by the friction braking unit to a second braking force greater than the first braking force.

[0010] Aspect 2. In the above aspect 1, it is preferable that the friction braking unit brakes not only the non-driven wheels but also the driven wheels with friction brakes, and that in the second braking control, the control unit controls the braking force of the non-driven wheels by the friction braking unit and the braking force of the driven wheels by the friction braking unit to be equal to each other.

[0011] Aspect 3. In the above aspect 1, it is preferable that the friction braking unit brakes not only the non-driven wheels but also the driven wheels with friction brakes, and that the control unit, in the second braking control, increases the braking force of the friction braking unit on the non-driven wheels more than the braking force of the friction braking unit on the driven wheels.

[0012] Aspect 4. In any one of Aspects 1 to 3 above, it is preferable that the control unit controls the total braking force, which is the sum of the braking force by the friction braking unit and the braking force by the regenerative braking unit, to a first total braking force in the first braking control, and controls the total braking force, which is the sum of the braking force by the friction braking unit and the braking force by the regenerative braking unit, to a second total braking force equal to the first total braking force in the second braking control.

[0013] Aspect 5. In any one of Aspects 1 to 3 above, it is preferable that the control unit controls the total braking force, which is the sum of the braking force by the friction braking unit and the braking force by the regenerative braking unit, to a first total braking force in the first braking control, and controls the total braking force, which is the sum of the braking force by the friction braking unit and the braking force by the regenerative braking unit, to a second total braking force greater than the first total braking force in the second braking control.

[0014] Aspect 6. In any one of Aspects 1 to 5 above, it is preferable that the control unit controls the braking force by the regenerative braking unit to a predetermined first regenerative braking force in the first braking control, and controls the braking force by the regenerative braking unit to a second regenerative braking force smaller than the first regenerative braking force in the second braking control. Aspect 7. In any one of Aspects 1 to 6 above, it is preferable that in the second braking control, the control unit allocates a braking force to the driving wheels, including the braking force by the regenerative braking unit, greater than the braking force of the non-driving wheels by the friction braking unit.

[0015] Aspect 8. In any one of Aspects 1 to 7 above, it is preferable that the vehicle braking system includes a vehicle speed sensor that detects the vehicle speed of the vehicle. In this case, it is preferable that the friction braking unit brakes not only the non-drive wheels but also the drive wheels with friction brakes, and that the control device has a setting unit that sets the distribution of the braking force of the drive wheels by the friction braking unit and the braking force by the regenerative braking unit in accordance with the vehicle speed detected by the vehicle speed sensor. It is also preferable that the setting unit sets the braking force by the regenerative braking unit to be greater than the braking force of the drive wheels by the friction braking unit when the vehicle speed detected by the vehicle speed sensor is within a predetermined speed range, and that the control unit implements the second braking control when the vehicle speed detected by the vehicle speed sensor is within the predetermined speed range. [Effects of the Invention]

[0016] According to the disclosed vehicle braking system, it is possible to suppress a lack of braking force during rainfall using a simple control logic. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a schematic diagram illustrating a vehicle braking device according to an embodiment; [Figure 2] 10(a) and 10(b) are maps used for control by a vehicle braking system according to an embodiment. [Figure 3] 4 is an example of a flowchart illustrating braking control performed by a vehicle braking device according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0018] An embodiment of a vehicle braking system will be described with reference to the drawings. The embodiment described below is merely an example, and is not intended to exclude various modifications or application of techniques not explicitly described in the embodiment. The configurations of the embodiment can be modified in various ways without departing from the spirit of the embodiment. Furthermore, they can be selected or combined as needed.

[0019] The vehicle braking system of this embodiment is a system for controlling the braking of a vehicle, and is applied to a vehicle whose wheels include drive wheels and non-drive wheels (also called "driven wheels"). For example, the vehicle braking system is not applied to a vehicle in which all wheels are drive wheels, such as a four-wheel drive vehicle, but is applied to a two-wheel drive vehicle, such as a front-wheel drive or rear-wheel drive vehicle. In a front-wheel drive vehicle, the front wheels are drive wheels and the rear wheels are non-drive wheels. In a rear-wheel drive vehicle, the front wheels are non-drive wheels and the rear wheels are drive wheels.

[0020] Furthermore, the vehicle to which the vehicle braking device is applied is an electric vehicle equipped with a motor that brakes the drive wheels with regenerative braking. Electric vehicles include electric vehicles (EVs) equipped with only a motor as a drive source, and hybrid vehicles (HEVs, Hybrid Electric Vehicles) equipped with a motor and an engine as a drive source. Hybrid vehicles include plug-in hybrid vehicles (PHEVs, Plug-in Hybrid Electric Vehicles). A plug-in hybrid vehicle is a hybrid vehicle that can externally charge the battery or receive external power from the battery. A plug-in hybrid vehicle is equipped with a charging port (inlet) for inserting a charging cable that supplies power from external charging equipment, and an outlet for external power supply.

[0021] I. ONE EMBODIMENT A vehicle braking device according to one embodiment will now be described with reference to FIGS. Fig. 1 is a schematic diagram showing a vehicle braking system according to an embodiment, Fig. 2 is a map used for control by the vehicle braking system according to an embodiment, and Fig. 3 is an example of a flowchart illustrating control performed by the vehicle braking system according to an embodiment. In the following embodiment, a front-wheel drive electric vehicle is exemplified as a vehicle to which the vehicle braking device is applied. In the description of the embodiment, the forward direction of the vehicle is defined as the front, and the backward direction is defined as the rear.

[0022] [1. Configuration] The configuration of the vehicle braking device of one embodiment is roughly divided into an input system configuration, a control system configuration, and an output system configuration. The input system configuration acquires information necessary for control and inputs that information to the control system configuration. In other words, the information output by the input system configuration is input to the control system configuration. The control system configuration generates a control signal based on the information input from the input system configuration and outputs that control signal to the output system configuration. The output system configuration operates according to the control signal output from the control system configuration. In other words, the control system configuration is the subject of control based on the information input from the input system configuration, and the output system configuration is the object of control by the control system configuration. The configuration of the output system, the configuration of the input system, and the configuration of the control system will be explained below in that order.

[0023] [1-1.Output system configuration] As shown in FIG. 1, the output system is provided with a braking unit 10 that brakes the wheels 1W of the vehicle. The wheels 1W are made up of front wheels 1F and rear wheels 1R. The wheels 1W illustrated here are the front wheels 1F which are driving wheels of the vehicle, and the rear wheels 1R which are non-driving wheels that do not drive the vehicle.

[0024] The braking unit 10 is provided with a friction braking unit 11 that applies braking by a friction brake and a regenerative braking unit 12 that applies braking by a regenerative brake. The friction braking unit 11 brakes the front wheels 1F and the rear wheels 1R (that is, all the wheels 1W). The friction braking section 11 is provided with a hydro unit 1H (also called a "hydraulic unit" and indicated as "H / U" in FIG. 1) that brakes each wheel 1W hydraulically.

[0025] The hydro unit 1H is a unit that hydraulically operates the disc brakes 1D of each wheel 1W. The disc brakes 1D are provided with a brake rotor 1L attached to each wheel 1W and brake pads 1P that are pressed against the brake rotor 1L. The hydro unit 1H hydraulically presses the brake pads 1P against the brake rotor 1L, and the frictional force between the brake rotor 1L and the brake pads 1P brakes each wheel 1W.

[0026] This hydro unit 1H applies a common hydraulic pressure to the disc brakes 1D of the front wheels 1F and rear wheels 1R, braking the front wheels 1F and rear wheels 1R with friction brakes. When a common hydraulic pressure is applied to the disc brakes 1D, the braking force of the front wheels 1F and the braking force of the rear wheels 1R are equal. The hydro unit 1H illustrated here has the function of increasing or decreasing the braking force of the wheels 1W by adjusting the level of hydraulic pressure applied to the disc brakes 1D.

[0027] The hydro unit 1H may be a unit that hydraulically operates the drum brakes of each wheel 1W. In this case, brake shoes are pressed against the brake linings of the drum brakes, and the wheels 1W are braked by the frictional force between the brake linings and the brake shoes (i.e., braked by friction brakes).

[0028] The friction braking unit 11 illustrated here is provided with an electric brake booster 1B that assists the braking force of the friction braking unit 11. The electric brake booster 1B is an auxiliary device that increases braking force using power from a motor (that is, electrically).

[0029] The electric brake booster 1B illustrated here assists in the braking force of only the rear wheel 1R among the wheels 1W braked by the friction braking unit 11. This electric brake booster 1B electrically increases only the hydraulic pressure applied to the disc brake 1D of the rear wheel 1R by the hydro unit 1H, thereby assisting in the braking force of the rear wheel 1R by the friction brake. However, the electric brake booster 1B is not essential and does not have to be provided in the vehicle.

[0030] The regenerative braking unit 12 brakes only the front wheels 1F. The regenerative braking section 12 is provided with a motor 1M that drives the vehicle, and an MCU 1C (Motor Control Unit) that controls the motor 1M. The motor 1M is a motor generator that operates as an electric motor when the vehicle is driven and as a generator when the vehicle is braked. The motor 1M is not connected to the rear wheels 1R, but only to the front wheels 1F.

[0031] The motor 1M, which operates as an electric motor, drives the front wheels 1F using power supplied from a drive battery (not shown). The motor 1M, which operates as a generator, brakes the front wheels 1F with regenerative braking and charges the drive battery (not shown). The MCU 1C is a control unit that controls the motor 1M. The MCU 1C can operate the motor 1M as an electric motor or a generator, and can also adjust the degree of regeneration of the motor 1M operating as a generator. By adjusting the degree of regeneration of the motor 1M, the strength of the regenerative braking that brakes the front wheels 1F can be adjusted.

[0032] [1-2. Input system configuration] The input system includes a brake pedal 2B (operation unit), a vehicle speed sensor 2S, and an on / off unit 20 that outputs an on signal or an off signal depending on the weather. The brake pedal 2B is a pedal operated by the driver of the vehicle to brake. This brake pedal 2B is depressed (braking operation) when braking the vehicle. The brake pedal 2B outputs an ON signal when depressed, and outputs an OFF signal when not depressed.

[0033] The input system includes an accelerator pedal 2A as well as a brake pedal 2B. The accelerator pedal 2A is a pedal that is depressed by the driver when driving the vehicle. It should be noted that instead of the brake pedal 2B and the accelerator pedal 2A, a single pedal that is operated when braking and driving the vehicle may be used. Also, the present invention is not limited to the brake pedal 2B, and any known operating device may be used as long as it is operated by the driver of the vehicle to brake. Alternatively, a brake switch (not shown) may be provided that outputs an ON signal when the brake pedal 2B is depressed and outputs an OFF signal when the brake pedal 2B is not depressed.

[0034] The vehicle speed sensor 2S is a sensor that detects the traveling speed Vs of the vehicle (hereinafter referred to as "vehicle speed"). This vehicle speed sensor 2S can be a device that calculates the vehicle speed Vs based on the rotation speed of each wheel 1W (i.e., wheel speed). Note that the vehicle speed sensor 2S is not limited to a device that calculates the vehicle speed Vs based on the rotation speed of each wheel 1W, and various known devices can be used.

[0035] The on / off unit 20 is provided with a wiper switch 21 for operating a wiper (not shown), and a rain sensor 22 for detecting the presence or absence of rain. The wiper switch 21 is a switch that is operated to an on or off state by the driver of the vehicle. When operated to the on state, the wiper switch 21 outputs an on signal to operate the wipers, and when operated to the off state, it outputs an off signal to stop the operation of the wipers.

[0036] The wiper switch 21 is turned on when the driver needs to operate the wipers, and can therefore be said to be a switch that outputs an on signal when rain, snow, hail, etc. (hereinafter referred to as "rain, etc.") adheres to the front windshield of the vehicle. When rain or the like adheres to the front windshield of the vehicle, it is raining or the like (hereinafter referred to as "raining"). Therefore, the wiper switch 21 can be said to be a switch that outputs an ON signal when it is raining and outputs an OFF signal when it is not raining (hereinafter referred to as "non-raining").

[0037] The wiper switch 21 has three on-states, which are set in order of decreasing wiper operation frequency: intermittent, low, and high. The on-state wiper switch 21 is operated to one of the intermittent, low, and high states. The wiper switch 21 operates the wipers intermittently when operated to the intermittent state, and operates the wipers continuously when operated to the low or high state. The number of times the wipers operate per unit time (operation frequency) is higher when the wiper switch 21 is in the high state than when it is in the low state.

[0038] The rain sensor 22 is a sensor that detects whether or not rain or the like is on the windshield of the vehicle. This rain sensor 22 outputs an ON signal if rain or the like is on the windshield of the vehicle, and outputs an OFF signal if not. Therefore, like the wiper switch 21, the rain sensor 22 can be said to be a switch that outputs an ON signal when it is raining and an OFF signal when it is not raining.

[0039] When an ON signal is output from the rain sensor 22, the wipers may be automatically operated. The rain sensor 22 may be a sensor that detects not only the presence or absence of rain or the like on the windshield of the vehicle, but also the amount of rain or the like on the windshield of the vehicle.

[0040] For example, if a rain sensor 22 is employed that can detect the amount of rain or the like on the vehicle's windshield in three levels: light, medium, and heavy, the wiper operation frequency may be set according to the detected amount of rain or the like. Specifically, when a light amount of rain or the like is detected by the rain sensor 22, the wipers may be operated intermittently. Alternatively, when a medium or heavy amount of rain or the like is detected by the rain sensor 22, the wipers may be operated continuously.

[0041] [1-3. Control system configuration] The control system includes a control device 30 . An ECU (Electronic Control Unit) can be used as the control device 30. An ECU is a computer that incorporates a processor (central processing unit), memory (main memory), storage device, interface device, timer function, etc., which are connected to each other via an internal bus.

[0042] The control device 30 may be a PHEV-ECU if the vehicle to which the vehicle braking device is applied is a plug-in hybrid vehicle, or an EV-ECU if the vehicle to which the vehicle braking device is applied is an electric vehicle. The PHEV-ECU and the EV-ECU are integrated control devices (integrated ECUs). The control device 30 may be an ECU that is lower in level than these integrated control devices.

[0043] The control device 30 performs control depending on whether various conditions are met while the vehicle is being braked (while the operating unit is being operated to brake). The control device 30 stores a map 31 in which a distribution of braking force by regenerative braking and braking force by friction braking (hereinafter simply referred to as "braking force distribution") is set in advance with respect to the braking force of the front wheels 1F.

[0044] This control device 30 has a setting unit 32 that sets braking force distribution, a determination unit 33 that determines whether various conditions are met, and a control unit 34 that performs braking control in accordance with the determination results by the determination unit 33. The braking control performed by the control unit 34 includes two types: first braking control and second braking control. The setting unit 32, the judgment unit 33, and the control unit 34 are elements that are conveniently classified according to their roles (functions) in the control device 30, and each may be written as an independent program, or may be written as a composite program that combines the functions of both.

[0045] As shown in Figures 2(a) and 2(b), a braking force distribution corresponding to the vehicle speed V is preset in the map 31. This map 31 sets the braking force distribution according to the characteristics of the motor 1M. Therefore, a map 31 such as that shown in Figures 2(a) and 2(b) is provided for each vehicle model (motor 1M). In FIG. 2(a), the vertical axis indicates vehicle speed V and the horizontal axis indicates elapsed time t, and in FIG. 2(b), the vertical axis indicates deceleration D and the horizontal axis indicates elapsed time t.

[0046] The map 31 shown in FIG. 2 will be described below along the elapsed time t. From the first time point t1 through the second time point t2, the third time point t3, and to the fourth time point t4, the lower the vehicle speed V, the higher the proportion of braking force due to regenerative braking (the lower the proportion of braking force due to friction braking). At the first time point t1, the braking force provided by the regenerative brake is allocated smaller than the braking force provided by the friction brake. For example, at the first time point t1, the braking force provided by the regenerative brake is set to 40%, and the braking force provided by the friction brake is set to 60%.

[0047] The second point in time t2 is the point in time when the braking force distribution between the regenerative brake and the friction brake is set to be equal, i.e., at the second point in time t2, the braking force distribution is set to 50% by the regenerative brake and 50% by the friction brake. If the region of vehicle speed V from the first point in time t1 to the second point in time t2 is defined as the "high speed region R1," in the high speed region R1, the distribution of braking force by the friction brake decreases as the vehicle speed V decreases, but the distribution of braking force by the friction brake is set to more than half.

[0048] At the third time point t3 and the fourth time point t4, the braking force by the regenerative brake is set to be greater than the braking force by the friction brake. For example, at the third time point t3, the braking force by the regenerative brake is set to 70% and the braking force by the friction brake is set to 30%. Also, at the fourth time point t4, the braking force by the regenerative brake is set to 90% and the braking force by the friction brake is set to 10%.

[0049] If the region of vehicle speed V from the second point in time t2 to the third point in time t3 is defined as the "medium-high speed region R2," and the region of vehicle speed V from the third point in time t3 to the fourth point in time t4 is defined as the "medium speed region R3," then in the medium-high speed region R2 and the medium speed region R3, the distribution of braking force by regenerative braking increases as the vehicle speed V decreases, and the distribution of braking force by regenerative braking is set to more than half.

[0050] From the fourth time point t4 to the fifth time point t5, the braking force distribution is set to be constant even if the vehicle speed V decreases. If the region of the vehicle speed V from the fourth time point t4 to the fifth time point t5 is defined as the "low-to-medium speed range R4," in the low-to-medium speed range R4, the braking force distribution is constant regardless of the vehicle speed V. From the fifth time point t5 through the sixth time point t6 to the seventh time point t7, the distribution of braking force by the friction brake increases as the vehicle speed V decreases. At the sixth time point t6, similar to the third time point t3, the distribution is set to 70% braking force by the regenerative brake and 30% braking force by the friction brake.

[0051] At the seventh point in time t7, the vehicle comes to a stop (vehicle speed V becomes zero), and only the braking force due to the friction brake is set. That is, at the seventh point in time t7, the braking force due to the friction brake is set to be distributed at 100%. If the region of vehicle speed V from the fifth point in time t5 to the sixth point in time t6 is defined as the "low speed region R5," and the region of vehicle speed V from the sixth point in time t6 to the seventh point in time t7 is defined as the "very low speed region R6," then in the low speed region R5 and the very low speed region R6, the braking force from the friction brake is set to increase as the vehicle speed V decreases.

[0052] The setting unit 32 sets the distribution of braking force by the friction braking unit 11 and the braking force by the regenerative braking unit 12 in accordance with the vehicle speed Vs detected by the vehicle speed sensor 2S. The setting unit 32 sets the braking force distribution based on the map 31. Specifically, the setting unit 32 reads out from the map 31 a braking force distribution corresponding to the vehicle speed V that is equal to the vehicle speed Vs detected by the vehicle speed sensor 2S. The distribution of braking force by the friction braking unit 11 and the braking force by the regenerative braking unit 12 is set so as to match the braking force distribution read out in this manner.

[0053] The determination unit 33 determines whether various conditions relating to braking control are satisfied. The various conditions for which the determination unit 33 determines whether they are satisfied include an operation condition and a rainfall condition. The operation condition is a condition for determining whether or not the vehicle is being braked (a state in which the operating unit is being operated to brake), and can also be said to be a condition for determining whether or not braking control should be performed.

[0054] This operation condition is satisfied when an ON signal is output by the brake pedal 2B, and is not satisfied when an OFF signal is output by the brake pedal 2B. Note that the determination of whether the operation condition is satisfied or not is not limited to the ON and OFF signals output by the brake pedal 2B, but may also use the ON and OFF signals output by the brake switch.

[0055] If it is determined that the operation conditions are met, it is then determined whether the rain conditions, which will be explained next, are met. The rain condition is a condition for determining whether or not the second braking control should be performed, and is met when an ON signal is output from at least one of the wiper switch 21 and the rain sensor 22. The rain condition is not met when an OFF signal is output from the wiper switch 21 and the rain sensor 22.

[0056] The conditions for which the exemplified determining unit 33 determines success or failure include the above-mentioned operation condition and rainfall condition, as well as a vehicle speed condition. The vehicle speed condition is a condition for determining whether the vehicle speed Vs is within a predetermined speed range Rp. The "predetermined speed range Rp" referred to here is a vehicle speed range that is empirically or experimentally set in advance as a vehicle speed range in which braking the front wheels 1F of a vehicle traveling on a low μ road with the braking force distribution set in the map 31 may cause the front wheels 1F to slip. The predetermined speed range Rp exemplified here includes the above-mentioned medium speed range R3, low-medium speed range R4, and low speed range R5. In these medium speed range R3, low-medium speed range R4, and low speed range R5, the braking force applied to the front wheels 1F by the regenerative braking unit 12 is set to be greater than the braking force applied to the front wheels 1F by the friction braking unit 11.

[0057] In other words, the predetermined speed range Rp is a vehicle speed range in which the ratio of braking force due to the friction brake is set in the map 31 to be smaller than a predetermined friction ratio. The "predetermined friction ratio" referred to here is set in advance empirically or experimentally as the lower limit (e.g., 30%) of the ratio at which the vehicle can be sufficiently braked by the braking force of the rear wheel 1R alone, even if the front wheel 1F slips, when the rear wheel 1R is braked with a braking force equal to the friction brake of the predetermined friction ratio of the braking force of the front wheel 1F.

[0058] The control unit 34 performs braking control when the determination unit 33 determines that the operation condition is met. Note that the control unit 34 does not perform braking control when the determination unit 33 determines that the operation condition is not met. This control unit 34 performs a first braking control when the judgment unit 33 determines that at least one of the vehicle speed condition and the rainfall condition is not met, and performs a second braking control when the judgment unit 33 determines that the vehicle speed condition and the rainfall condition are met.

[0059] The vehicle braking system of this embodiment performs the first braking control based on the knowledge that the friction coefficient of the road on which the vehicle is traveling is secured when the rainfall condition is not satisfied. If the friction coefficient of the road on which the vehicle is traveling is secured, the front wheels 1F are less likely to slip when braking the vehicle, so when it is determined that the rainfall condition is not satisfied, the first braking control, which will be described below, is performed.

[0060] In the first braking control, the braking force of the rear wheel 1R by the friction braking unit 11 is controlled to a predetermined first braking force. The "predetermined first braking force" here is set to the braking force by the friction brake out of the braking force distribution set by the setting unit 32. In this first braking control, the front wheel 1F is braked with the braking force distribution set by the setting unit 32.

[0061] As an example of the first braking control that is performed when the vehicle speed Vs detected by the vehicle speed sensor 2S is in the low-medium speed range R4, the front wheel 1F is braked with a braking force distribution of 90% by the regenerative brake and 10% by the friction brake, and the rear wheel 1R is braked with a braking force equal to the braking force by the friction brake that brakes the front wheel 1F.

[0062] As described above, increasing the allocation of regenerative braking to the front wheels 1F improves electricity efficiency (energy recovery rate). However, if more braking force is allocated to the front wheels 1F than to the rear wheels 1R, there is a risk that the front wheels 1F may not have enough margin to grip the road surface as much as the rear wheels 1R, especially when the coefficient of friction of the road surface is low.

[0063] Therefore, the vehicle braking system of this embodiment performs second braking control based on the knowledge that when the rainfall condition is met, the friction coefficient of the road on which the vehicle is traveling has decreased due to rain or the like (i.e., the road is low μ). If the friction coefficient of the road on which the vehicle is traveling has decreased due to rain or the like, the front wheels 1F are likely to slip when braking the vehicle, so when it is determined that the rainfall condition is met, the second braking control described below is performed.

[0064] In the second braking control, the braking force of the rear wheels 1R by the friction braking unit 11 is controlled to a second braking force that is greater than a predetermined first braking force. In other words, the second braking control is a control that brakes the rear wheels 1R with a braking force that is greater than the braking force by the friction brakes with the distribution set by the setting unit 32. In this way, the second braking control that brakes the rear wheels 1R with a second braking force that is greater than the first braking force can be said to be a control that increases the braking force of the rear wheels 1R to the second braking force in order to compensate for a lack of braking force of the front wheels 1F during braking of a vehicle traveling in conditions such as rain.

[0065] When the braking force allocated to the front wheels 1F is greater than the braking force allocated to the rear wheels 1R, the rear wheels 1R have a relatively greater margin of error than the front wheels 1F before they can grip the vehicle road, and the front wheels 1F tend to slip more easily than the rear wheels 1R. For this reason, the second braking control is premised on allocating a greater braking force to the front wheels 1F (the braking force of the front wheels 1F including the braking force due to the regenerative braking unit 12) than to the braking force of the rear wheels 1R (the braking force of the rear wheels 1R due to the friction braking unit 11).

[0066] The specific contents of the second braking control will be explained below by giving a first example, a second example, and a third example. In explaining these three examples, terms related to braking control will be defined as follows. In the first braking control, the braking force of the front wheels 1F by the friction braking unit 11 is called the "first front friction force," and the braking force of the rear wheels 1R by the friction braking unit 11 is called the "first rear friction force." Also, in the first braking control, the braking force of the front wheels 1F by the regenerative braking unit 12 is called the "first regenerative braking force."

[0067] The total braking force obtained by combining the first front friction force, the first rear friction force, and the first regenerative braking force is referred to as the "first total braking force." That is, the first total braking force is the total braking force obtained by combining the braking force by the friction braking unit 11 and the braking force by the regenerative braking unit 12 in the first braking control. Here, an example is shown in which the ratio of the first regenerative braking force to the first front friction force and the first rear friction force is 9:1:1.

[0068] In addition, in the second braking control, the braking force of the front wheels 1F by the friction braking unit 11 is called the "second front friction force," and the braking force of the rear wheels 1R by the friction braking unit 11 is called the "second rear friction force." In addition, in the second braking control, the braking force of the front wheels 1F by the regenerative braking unit 12 is called the "second regenerative braking force."

[0069] The total braking force obtained by combining the second front friction force, the second rear friction force, and the second regenerative braking force is referred to as the “second total braking force.” In other words, the second total braking force is the total braking force obtained by combining the braking force by the friction braking unit 11 and the braking force by the regenerative braking unit 12 in the second braking control. In a first example of the second braking control, a ratio of the second regenerative braking force to the second front friction force to the second rear friction force is 7:3:3. In second and third examples of the second braking control, a ratio of the second regenerative braking force to the second front friction force to the second rear friction force is 9:1:3.

[0070] The first example of the second braking control is a control in which the braking forces of all wheels 1W by the friction brakes are equal. That is, in the first example of the second braking control, the second front friction force and the second rear friction force are controlled to be equal to each other. In the first example of the second braking control, the electric brake booster 1B is not required, and the braking forces of all wheels 1W by the friction brakes are increased by increasing the hydraulic pressure using the hydro unit 1H of the friction braking unit 11.

[0071] The second example of the second braking control is control in which the braking force by the friction brake is different between the front wheels 1F and the rear wheels 1R. That is, the second example of the second braking control is control in which the second rear braking force is made higher than the second front friction force. In a second example of the second braking control, the braking force of the rear wheel 1R is increased by the electric brake booster 1B.

[0072] In the first and second examples of the second braking control described here, the first total braking force and the second total braking force are controlled to be equal to each other. In this case, the braking force of the front wheels 1F by the regenerative brake is suppressed by the amount that the braking force of the rear wheels 1R is increased by the friction brake (in other words, the friction brake and the regenerative brake are controlled in a coordinated manner). In other words, the second regenerative braking force is controlled to be smaller than the first regenerative braking force.

[0073] The third example of the second braking control is similar to the second example in that the braking force applied by the friction brakes is different between the front wheels 1F and the rear wheels 1R, but differs in that it is controlled to a second total braking force that is greater than the first total braking force. In the third example of the second braking control, control is performed in which, with respect to the braking force applied to each wheel 1W in the first braking control, the braking force applied to the front wheels 1F by the regenerative brakes is not suppressed, and only the braking force applied to the rear wheels 1R is increased by the electric brake booster 1B.

[0074] [2. Flowchart] The control procedure (flow) performed by the control device 30 will be described below with reference to the flowchart in Figure 3. The control described here is repeatedly performed at a predetermined cycle when the main power supply of the vehicle is turned on.

[0075] 3, first, it is determined whether the operation conditions are met (step S2). If it is determined that the operation conditions are not met, this control cycle is ended (return). If it is determined that the operation conditions are met, braking control is carried out (steps S4, S6, S8, S10). In the braking control, first, it is determined whether or not the vehicle speed condition is met (step S4).

[0076] If it is determined that the vehicle speed condition is met, it is determined whether the rain condition is met (step S6). If it is determined that the rain condition is met, the second braking control is performed (step S8). Then, this control cycle ends (return). On the other hand, if it is determined that the vehicle speed condition is not satisfied, the first braking control is performed (step S10). Also, if it is determined that the rainfall condition is not satisfied, the first braking control is performed (step S10). Then, this control cycle ends (return).

[0077] [3. Actions and Effects] The vehicle braking system of this embodiment is configured as described above, and therefore provides the following actions and effects. (1) In front-wheel drive electric vehicles, conventional technology that increases the braking force of the rear wheels after detecting front wheel slippage may result in insufficient braking force during the period from when front wheel slippage is detected until the braking force of the rear wheels is increased.

[0078] In contrast to this, according to the vehicle braking system of this embodiment, when it is determined that the rainfall condition is met, the second braking control is executed to increase the braking force of the rear wheels 1R by the friction braking unit 11 to the second braking force without checking whether or not the front wheels 1F are slipping. Therefore, in principle, the second braking control can avoid a lack of braking force during the period from when slippage of the front wheels 1F is detected until the braking force of the rear wheels 1R is increased. Therefore, a simple control logic can be used to suppress a lack of braking force during rain.

[0079] Incidentally, one possible technology for determining whether a vehicle road is a low μ road is a technology (hereinafter referred to as "Comparative Technology 1") that determines whether a vehicle road is a low μ road by analyzing an image of the vehicle road captured by an on-board camera.Comparative Technology 1 requires an on-board camera to capture an image of the vehicle road and also requires image analysis processing, which makes the configuration for determining whether the vehicle road is a low μ road complicated and leads to complex control logic.

[0080] In addition to Comparative Technology 1, another possible technology for determining whether the road on which a vehicle is traveling is a low μ road is a technology (hereinafter referred to as "Comparative Technology 2") that acquires information on whether the road on which a vehicle is traveling is a low μ road through vehicle-to-vehicle communication or road-to-vehicle communication.Comparative Technology 2 requires equipment that communicates with the outside of the vehicle and also requires processing of the information acquired through communication, which makes the configuration for determining whether the road on which a vehicle is traveling is a low μ road complicated and leads to complex control logic.

[0081] On the other hand, in the vehicle braking system of this embodiment, the establishment of the rainfall condition is determined based on the output signal of the on / off unit 20, such as the wiper switch 21 or rain sensor 22 already present in the vehicle, so the configuration for determining whether the vehicle is traveling on a low μ road is simple, which contributes to simplifying the control logic. In the vehicle braking system of this embodiment, when the rainfall condition is established, the second braking control is performed based on the knowledge that the friction coefficient of the vehicle is reduced by rain or the like (i.e., the road is a low μ road), so the control logic can be simplified.

[0082] (2) As in the first example of the second braking control, when the second front frictional force and the second rear frictional force are controlled to be equal to each other, even in a vehicle not equipped with an electric brake booster 1B, the second braking control can be implemented by increasing the hydraulic pressure of the hydro unit 1H that brakes all wheels 1W equally. This also simplifies the configuration of the vehicle braking system.

[0083] (3) When the second rear frictional force is increased without increasing the second front frictional force, as in the second and third examples of the second braking control, this contributes to further suppressing slip of the front wheels 1F. (4) When the first total braking force and the second total braking force are controlled to be equal to each other, as in the first and second examples of the second braking control, the total braking force does not fluctuate and remains constant when the control to be implemented is switched from one of the first braking control and the second braking control to the other. Therefore, there is no change in the braking feeling, which contributes to ensuring drivability.

[0084] (5) When the second total braking force is controlled to a second total braking force greater than the first total braking force, as in the third example of the second braking control, the vehicle can be reliably braked by the second braking control, which has a braking force higher than that of the first braking control. Furthermore, the second braking control can be performed by simply increasing the braking force of the rear wheels 1R using the electric brake booster 1B, without suppressing the braking force of the front wheels 1F by regenerative braking, relative to the braking force applied to each wheel 1W in the first braking control. This contributes to simplifying the control logic.

[0085] (6) When the second regenerative braking force is controlled to be smaller than the first regenerative braking force, as in the first and second examples of the second braking control, the braking force of the rear wheels 1R is relatively higher than the braking force of the front wheels 1F, and the insufficient braking force of the vehicle can be further suppressed. By suppressing the second regenerative braking force in this way, the friction braking by the friction braking unit 11 and the regenerative braking by the regenerative braking unit 12 can be coordinated to perform the second braking control.

[0086] (7) In addition, in the second braking control, the braking force of the front wheel 1F is distributed more than the braking force of the rear wheel 1R, which contributes to improving the electricity consumption by regenerative braking of the front wheel 1F. (8) In addition, since the map 31 is set so that the braking force of the regenerative braking unit 12 is greater than the braking force of the front wheels 1F by the friction braking unit 11 in the predetermined speed range Rp, the front wheels 1F are likely to slip. In contrast, since the second braking control is performed when the vehicle speed Vs detected by the vehicle speed sensor 2S is within the predetermined speed range Rp, slip of the front wheels 1F can be effectively suppressed.

[0087] [II. Other] The above-described embodiment is merely an example, and the vehicle braking system of this embodiment may include at least a braking unit, an on / off unit, an operating unit operated by the driver of the vehicle to apply the brakes, and a control device. The on / off unit may be configured with at least one of a wiper switch and a rain sensor, and the wiper switch or the rain sensor may be omitted.

[0088] Furthermore, the on / off unit may be any in-vehicle device that outputs an on signal when it is raining and an off signal when it is not raining. For example, a drive mode selector may be used as the on / off unit instead of or in addition to a wiper switch or rain sensor. The drive mode selector is a switch that allows the driver of the vehicle to select the drive mode of the vehicle. The drive mode selector has various settings for the drive mode, such as "wet mode" and "snow mode." The wet mode is the mode selected when driving on a wet road (a so-called "wet road"). The snow mode is the mode selected when driving on a snowy road.

[0089] The drive mode selector is switched to a wet mode or a snow mode when it is raining, and to another drive mode when it is not raining. This drive mode selector outputs an ON signal when switched to the wet mode or the snow mode, and outputs an OFF signal when switched to another drive mode. Therefore, the drive mode selector can function as an ON / OFF unit that outputs an ON signal when it is raining and an OFF signal when it is not raining.

[0090] The vehicle speed condition is an optional condition and may be omitted. Therefore, the second braking control may be performed when the operation condition and the rainfall condition are satisfied. The braking force distribution in the first braking control and the second braking control is not limited to the distribution set by the setting unit using a map as described above, but any distribution can be adopted without using a map or a setting unit, and various distributions can be set.

[0091] Alternatively, the friction braking unit may brake at least the rear wheels with the friction brakes, or may brake only the rear wheels with the friction brakes. The vehicle braking system of this embodiment is not limited to electric vehicles with drive wheels in the front and non-drive wheels in the rear, as described above, but can also be applied to electric vehicles with non-drive wheels in the front and drive wheels in the rear. In this case, the front wheels should be read as the rear wheels and the rear wheels as the front wheels.

[0092] In addition, the degree to which the braking force of the non-driven wheels is increased in the second braking control may be adjusted depending on the amount of rain, etc. To give a specific example, the difference between the second braking force and the first braking force (the degree to which the braking force of the rear wheels is increased in the second braking control) may be set (adjusted) to be greater when the rain sensor detects a large amount of rain, etc. or the wiper switch is in the high state than when the rain sensor detects a small amount of rain, etc. or the wiper switch is in the intermittent state.

[0093] In addition to the above-mentioned braking control, other controls may be implemented, such as an increase control that increases the braking force of the regenerative brakes by operating a paddle shifter or a shift knob attached to the steering wheel of the vehicle. When the "increase control" is implemented, it is preferable to implement a second braking control without suppressing the braking force of the drive wheels by the regenerative brakes (without controlling the second regenerative braking force to be smaller than the first regenerative braking force). [Explanation of symbols]

[0094] 10 Braking part 11 Friction brake part 12 Regenerative braking section 1F Front wheels (drive wheels) 1R rear wheel (non-drive wheel) 1W Wheel 20 On / Off Section 21 Wiper switch 22 Rain sensor 2B Brake pedal (operating part) 2S vehicle speed sensor 30 Control device 32 Setting section 33 Judgment section 34 Control Unit R3 Medium speed range (specified speed range) R4 Low to medium speed range (specified speed range) R5 Low speed range (specified speed range)

Claims

1. a braking unit including a friction braking unit that brakes at least the non-driven wheels of the vehicle, including the driven wheels and non-driven wheels, with a friction brake, and a regenerative braking unit that brakes only the driven wheels with a regenerative brake; an operating unit that is operated by a driver of the vehicle to apply a brake; an on / off unit that outputs an on signal when it is raining and an off signal when it is not raining; a control device including: a determination unit that determines whether a rainfall condition is met when the on / off unit outputs the on signal and whether the rainfall condition is not met when the off signal is output; and a control unit that performs braking control that controls the braking unit in accordance with the determination by the determination unit of whether the rainfall condition is met when the operating unit is subjected to a braking operation; In the braking control, when the determination unit determines that the rainfall condition is not established, the control unit performs first braking control to control the braking force of the friction braking unit on the non-driven wheels to a predetermined first braking force, and when the determination unit determines that the rainfall condition is established, performs second braking control to control the braking force of the friction braking unit on the non-driven wheels to a second braking force greater than the first braking force. A vehicle braking system comprising:

2. the friction braking unit brakes not only the non-driven wheels but also the driven wheels with friction brakes, In the second braking control, the control unit controls the braking force of the friction braking unit on the non-driven wheels and the braking force of the friction braking unit on the driven wheels to be equal to each other.

2. A vehicle braking system according to claim 1, wherein:

3. the friction braking unit brakes not only the non-driven wheels but also the driven wheels with friction brakes, In the second braking control, the control unit increases the braking force of the friction braking unit on the non-driven wheels to be greater than the braking force of the friction braking unit on the driven wheels.

2. A vehicle braking system according to claim 1, wherein:

4. The control unit controls the total braking force, which is a combination of the braking force by the friction braking unit and the braking force by the regenerative braking unit, to a first total braking force in the first braking control, and controls the total braking force, which is a combination of the braking force by the friction braking unit and the braking force by the regenerative braking unit, to a second total braking force equal to the first total braking force in the second braking control.

4. A vehicle braking system according to claim 1, wherein:

5. The control unit controls the total braking force, which is a combination of the braking force by the friction braking unit and the braking force by the regenerative braking unit, to a first total braking force in the first braking control, and controls the total braking force, which is a combination of the braking force by the friction braking unit and the braking force by the regenerative braking unit, to a second total braking force greater than the first total braking force in the second braking control.

4. A vehicle braking system according to claim 1, wherein:

6. The control unit controls the braking force by the regenerative braking unit to a predetermined first regenerative braking force in the first braking control, and controls the braking force by the regenerative braking unit to a second regenerative braking force smaller than the first regenerative braking force in the second braking control.

4. A vehicle braking system according to claim 1, wherein:

7. In the second braking control, the control unit allocates a braking force to the driving wheels including the braking force by the regenerative braking unit greater than a braking force to the non-driving wheels by the friction braking unit.

4. A vehicle braking system according to claim 1, wherein:

8. a vehicle speed sensor for detecting a vehicle speed of the vehicle; the friction braking unit brakes not only the non-driven wheels but also the driven wheels with friction brakes, the control device has a setting unit that sets a distribution of braking force of the driving wheels by the friction braking unit and braking force by the regenerative braking unit in accordance with the vehicle speed detected by the vehicle speed sensor, the setting unit sets the braking force of the regenerative braking unit to be greater than the braking force of the driving wheels of the friction braking unit when the vehicle speed detected by the vehicle speed sensor is within a predetermined speed range; The control unit performs the second braking control when the vehicle speed detected by the vehicle speed sensor is within the predetermined speed range.

4. A vehicle braking system according to claim 1, wherein:

Images