Braking control device

A braking control device, when stopping a vehicle in a state where a braking force is being applied to the vehicle, executes a reducing control of reducing the braking force corresponding to a braking request before the vehicle stops and executes an increasing control of increasing the braking force corresponding to the braking request before the reducing control in order to suppress vehicle pitching behavior generated when the braking force is applied to the vehicle. The control device sets braking force increase amount in the increasing control based on a difference distance that is a difference between a first distance correlated with a vehicle traveling distance from a reduction start timing when the reducing control is executed until the stopping of the vehicle and a second distance correlated with a vehicle traveling distance from the reduction start timing when the reducing control is not executed until the stopping of the vehicle.

TECHNICAL FIELD

The present disclosure relates to a braking control device.

BACKGROUND ART

Conventionally, there has been a problem that, when a vehicle stops, a pitching behavior occurs in a vehicle body due to swing-back of a nose dive, and comfort of an occupant is impaired. Therefore, for example, JP 2005-348497 A discloses a control device that suppresses the occurrence of nose dive by smoothing deceleration immediately before the vehicle stops. In addition, this literature describes increasing the deceleration to Gmax before the control for smoothing the deceleration to prevent a delay in stopping the vehicle.

CITATIONS LIST

Patent Literature

Patent Literature 1: JP 2005-348497 A

SUMMARY

Technical Problems

However, in the control device described above, Gmax can be estimated as the maximum deceleration from the name, and a large deceleration feeling may be given to the driver before stopping. In addition, in the control device described above, since the deceleration is once increased and then reduced, the change gradient of the braking force tends to become large. For this reason, there is a concern about generation of an oil striking sound due to a rapid change in hydraulic pressure (high flow speed of brake fluid), and the like. As described above, in the device for preventing delay in stopping the vehicle, there is room for improvement in the control device from the viewpoint of the comfort of the occupant at the time the vehicle stops.

An object of the present disclosure is to provide a braking control device capable of preventing delay in stopping a vehicle and improving comfort of an occupant in the braking of the vehicle.

Solutions to Problems

A braking control device of the present disclosure includes a braking unit that applies a braking force to a vehicle in response to a braking request; and a control device that, when stopping the vehicle in a state where the braking force is being applied to the vehicle, executes a reducing control of reducing the braking force corresponding to the braking request before the vehicle stops and executes an increasing control of increasing the braking force corresponding to the braking request before the reducing control in order to suppress a pitching behavior of the vehicle generated when the braking force is applied to the vehicle; where the control device sets an increase amount of the braking force in the increasing control based on a difference distance that is a difference between a first distance correlated with a traveling distance of the vehicle from a reduction start timing when the reducing control is executed until the stopping of the vehicle and a second distance correlated with a traveling distance of the vehicle from the reduction start timing when the reducing control is not executed until the stopping of the vehicle.

Advantageous Effects

According to the present disclosure, the difference between the first distance and the second distance, that is, the traveling distance that increases by the reducing control is calculated, and the increase amount of the braking force in the increasing control is set based on the calculation result. As a result, the increasing control can be executed with the minimum increase amount of the braking force necessary for preventing the increase in the traveling distance necessary for stopping. According to the present disclosure, the increase gradient and the maximum value of the braking force can be set based on the increase amount so as to minimize the sense of deceleration that can be caused by the increasing control as much as possible. As described above, according to the present disclosure, a delay in the stopping of the vehicle can be prevented and the comfort of the occupant can be improved.

DESCRIPTION OF EMBODIMENT

Hereinafter, an embodiment of the present disclosure will be described based on the drawings. Each drawing used for description is a conceptual diagram. As shown inFIG.1, a braking control device1of the present embodiment includes a brake pedal11, a booster12, a master cylinder unit13, a reservoir14, a brake switch15, a stroke sensor16, an actuator (corresponds to “braking unit”)5, and a brake ECU (corresponds to “control device”)6.

The brake pedal11is an operation member that allows the driver to operate the brake. The brake switch15is a sensor that detects whether or not the brake pedal11is operated. The stroke sensor16is a sensor that detects the pedal stroke (hereinafter referred to as “stroke”) of the brake pedal11. The brake switch15and the stroke sensor16output the detection signal to the brake ECU6.

The booster12is a device that assists brake operation, and is, for example, a hydro-booster equipped with an accumulator, an electromagnetic valve, and the like. In this case, the brake pedal11is provided with a stroke simulator (not illustrated) that generates a reaction force with respect to the brake operation. The booster12uses an accumulator to generate a servo pressure corresponding to a stroke at the rear side of the master piston133described later. The master piston133moves forward by being pressed by the servo pressure. This configuration has a by-wire configuration in which the brake pedal11and the master cylinder unit13cooperatively move by control. The booster12is preferentially operated, for example, when a large braking force is required.

The master cylinder unit13is a device that generates a master pressure corresponding to the operation of the brake pedal11. Specifically, the master cylinder130is a cylinder member and includes a first master chamber131and a second master chamber132in which a master pressure is generated. The master cylinder unit13is configured so that the same hydraulic pressure is formed in the first master chamber131and the second master chamber132.

The first master chamber131is formed between the first master piston133and the second master piston134. The second master chamber132is formed between the second master piston134and the bottom portion of the master cylinder130. A first spring135is interposed between the first master piston133and the second master piston134. A second spring136is interposed between the second master piston134and the bottom portion of the master cylinder130. The reservoir14stores the brake fluid and resupplies the brake fluid to the master cylinder130(master chambers131,132). Communication between the reservoir14and the master chambers131and132is blocked when the master pistons133and134move forward by a predetermined amount.

The actuator5is a device that adjusts the hydraulic pressure (hereinafter referred to as “wheel pressure”) of each wheel cylinder181to184based on the master pressure supplied from the master cylinder130. The actuator5is disposed between the master cylinder130and the wheel cylinder181to184. The actuator5adjusts the wheel pressure in accordance with an instruction from the brake ECU6. For example, a disc brake device or a drum brake device (not illustrated) provided on each of the wheels Wf and Wr is driven according to the wheel pressure, and a braking force is generated at each of the wheels Wf and Wr.

In response to an instruction from the brake ECU6, the actuator5executes pressure increasing control for setting the wheel pressure to the same level as the master pressure, pressurization control for setting the wheel pressure to be higher than the master pressure, pressure reducing control for reducing the wheel pressure, or holding control for holding the wheel pressure. The actuator5executes, for example, anti-skid control (or also referred to as ABS control), sideslip prevention control (ESC control), automatic pressurization control, or the like based on the instruction from the brake ECU6. The automatic pressurization control is pressurization control performed according to a set target deceleration regardless of the presence or absence of a brake operation by the driver, for example, in automatic driving, adaptive cruise control, or the like.

Specifically, the actuator5includes a hydraulic circuit5A and a motor90. The hydraulic circuit5A includes a first piping system (corresponds to a “second supply unit”)50aand a second piping system (corresponds to a “first supply unit”)50b. The first piping system50ais connected to the wheel cylinders181and182of the rear wheel Wr. The second piping system50bis connected to the wheel cylinders183and184of the front wheel Wf. Each of the wheels Wf, Wr is provided with a wheel speed sensor73.

The first piping system50a includes a flow path A, a first differential pressure valve51, holding valves52and53, a pressure reducing flow path B, pressure reducing valves54and55, a pressure adjusting reservoir56, a reflux flow path C, a pump57, an auxiliary flow path D, and a pressure sensor71. In the description, the term “flow path” can be replaced with a term such as, for example, a fluid path, a hydraulic pressure path, an oil path, a pipeline, a passage, or a piping. The flow path A is a flow path connecting the master cylinder130and the wheel cylinders181and182.

The first differential pressure valve51is a normally open type linear solenoid valve provided in the flow path A. Based on the magnitude of the applied control current and the drive of the pump57, the first differential pressure valve51makes the hydraulic pressure in the flow path on the wheel cylinders181and182side higher than the hydraulic pressure in the flow path on the master cylinder130side. That is, the first differential pressure valve51is an electromagnetic valve capable of adjusting the differential pressure between the master pressure and the wheel pressure.

The first differential pressure valve51is closed to increase the wheel pressure until the differential pressure reaches the target differential pressure, and is opened by the force of the differential pressure to reduce the wheel pressure when the differential pressure becomes higher than the target differential pressure. The first differential pressure valve51is provided with a check valve51ain parallel.

The holding valves52and53are normally open type electromagnetic valves which are disposed in the flow path A and whose opening and closing are controlled by the brake ECU6. The flow path A is branched into two flow paths A1and A2at a branch point X on the wheel cylinders181,182side of the first differential pressure valve51so as to correspond to the wheel cylinders181and182. The holding valve52is provided in the flow path A1, and the holding valve53is provided in the flow path A2.

The pressure reducing flow path B connects a portion between the holding valve52and the wheel cylinder181in the flow path A1and the pressure adjusting reservoir56, and connects a portion between the holding valve52and the wheel cylinder182in the flow path A2and the pressure adjusting reservoir56.

The pressure reducing valves54and55are normally open type electromagnetic valves which are disposed in the pressure reducing flow path B and whose opening and closing are controlled by the brake ECU6. The pressure reducing valve54corresponds to the wheel cylinder181, and the pressure reducing valve55corresponds to the wheel cylinder182. The pressure adjusting reservoir56is a so-called low pressure reservoir including a cylinder, a piston, and a biasing member. The reflux flow path C is a flow path that connects the pressure reducing flow path B and the pressure adjusting reservoir56, and the branch point X.

The pump57is driven according to the rotation of the motor90, and suctions the brake fluid from a suction port and ejects the brake fluid from an ejection port. The pump57is provided in the reflux flow path C. The suction port is connected to a portion on the pressure adjusting reservoir56and the pressure reducing flow path B side in the reflux flow path C. The ejection port is connected to a portion on the branch point X side in the reflux flow path C. That is, the pump57suctions the brake fluid from the second master chamber132via the pressure adjusting reservoir56and ejects the brake fluid to the branch point X by the rotation of the motor90.

The auxiliary flow path D is a flow path connecting a pressure adjusting hole56aof the pressure adjusting reservoir56and a portion on the master cylinder130side than the first differential pressure value51in the flow path A. The pressure adjusting reservoir56is configured so that a valve hole56bcloses with increase in the inflow amount of the brake fluid to the pressure adjusting hole56adue to increase in stroke. A reservoir chamber56cis formed on the flow paths B and C side of the valve hole56b. The pressure sensor71is a sensor that detects the master pressure. The pressure sensor71transmits the detection result to the brake ECU6. Since the second piping system50bhas a configuration similar to the first piping system50a, the description thereof will be omitted. The actuator5is a device including a motor90and a pump57driven by the motor90and configured to be able to adjust a wheel pressure which is a braking hydraulic pressure.

The second piping system50bhas a configuration similar to the first piping system50a, and includes a flow path Ab that corresponds to the flow path A and connects the master cylinder130and the wheel cylinders183and184, a second differential pressure valve91that corresponds to the first differential pressure valve51, holding valves92and93that correspond to the holding valves52and53, a pressure reducing flow path Bb that corresponds to the pressure reducing flow path B, pressure reducing valves94and95that correspond to the pressure reducing valves54and55, a pressure adjusting reservoir96that corresponds to the pressure adjusting reservoir56, a reflux flow path Cb that corresponds to the reflux flow path C, a pump97that corresponds to the pump57, and an auxiliary flow path db that corresponds to the auxiliary flow path D. Two pumps57and97are driven by one motor90. The pumps57and97are controlled by the control of the motor90. As the explanation of the first piping system50acan be referred to for the detailed configuration of the second piping system50b, the description thereof will be omitted. In addition, the vehicle is provided with an acceleration sensor72that detects acceleration in the front-rear direction.

Here, each control state by the brake ECU6will be briefly described using the control on the wheel cylinder181as an example. In a state where there is no control on the actuator5, the first differential pressure valve51and the holding valve52are in the open state, the pressure reducing valve54is in the closed state, and the master pressure is supplied to the wheel cylinder181. In the pressure reducing control, the holding valve52is in the closed state and the pressure reducing valve54is in the open state. In the holding control, the holding valve52and the pressure reducing valve54are in the closed state. Furthermore, the holding control can also be executed by closing the pressure reducing valve54and throttling the first differential pressure valve51without closing the holding valve52. In the pressurization control, the first differential pressure valve51is in the differential pressure state (closed state until reaching the target differential pressure), the holding valve52is in the open state, the pressure reducing valve54is in the closed state, and the pump57is driven. As described above, the actuator5is a device that applies braking force to the vehicle in response to the braking request (instruction from the brake ECU6).

The brake ECU6is an electronic control unit including a CPU, a memory, and the like. Specifically, the brake ECU6is configured to execute various types of control by one or a plurality of processors. Various sensors such as the brake switch15, the stroke sensor16, the pressure sensor71, and the wheel speed sensor73are connected to the brake ECU6by a communication line (not shown). The brake ECU6determines whether or not the booster12and the actuator5need to be activated based on the detection results of these various sensors.

When determining that the actuator5needs to be activated, the brake ECU6calculates a target wheel pressure, which is a target value of the wheel pressure, for each wheel cylinder181to184, and controls the actuator5. The target wheel pressure corresponds to the target deceleration and the target braking force. The brake ECU6can calculate each wheel pressure based on the detection value of the pressure sensor71and the control states of the first differential pressure valve51and the second differential pressure valve91.

(Control at the Time of Stopping)

When stopping the vehicle while the braking force is being applied to the vehicle, the brake ECU6executes reducing control for reducing the braking force corresponding to the braking request before the vehicle stops and executes increasing control for increasing the braking force corresponding to the braking request before the reducing control in order to suppress the pitching behavior of the vehicle generated when the braking force is applied to the vehicle. The reducing control is a control for suppressing the nose dive by reducing the braking force (deceleration) of the vehicle more than the target braking force (braking force corresponding to the braking request) immediately before the stopping of the vehicle. The increasing control can also be said to be a raising control of the target braking force.

In the present embodiment, the braking force of the vehicle is the sum of the braking force of the front wheel Wf (hereinafter referred to as “front wheel braking force”) and the braking force of the rear wheel Wr (hereinafter referred to as “rear wheel braking force”). The front wheel braking force is calculated based on the wheel pressures of the wheel cylinders183and184of the front wheel Wf. The rear wheel braking force is calculated based on the wheel pressures of the wheel cylinders181and182of the rear wheel Wr.

The brake ECU6sets the increase amount of the braking force in the increasing control based on a difference distance which is a difference between a first distance correlated with the traveling distance of the vehicle from the reduction start timing when the reducing control is executed until the stopping of the vehicle and a second distance correlated with the traveling distance of the vehicle from the reduction start timing when the reducing control is not executed until the stopping of the vehicle.

More specifically, the brake ECU6includes a first calculation unit61, a second calculation unit62, a difference calculation unit63, and an increase setting unit64. The first calculation unit61sets the reduction start timing for starting the reducing control based on the braking force and the vehicle speed, and calculates the first distance correlated with a traveling distance of the vehicle from the reduction start timing when the reducing control is executed until the stopping of the vehicle. The first distance may be, for example, a traveling distance from a current time point until stop (hereinafter, also referred to as “stop required distance”) with the current time point (time point of calculation of the first distance) as a reference. The stop required distance correlates with a traveling distance of the vehicle from the reduction start timing until the stopping of the vehicle.

The second calculation unit62calculates the second distance correlated with a traveling distance of the vehicle from the reduction start timing when the reducing control is not executed until the stopping of the vehicle based on the braking force and the vehicle speed. The second distance may be, for example, a stop required distance with the current time point (time point of calculation of the first distance) as a reference, similar to the first distance. In this case, the second distance is a stop required distance when the reducing control is not executed, and can also be said to be a reference stop distance. The first distance and the second distance are calculated by the same calculation method.

The difference calculation unit63calculates a difference between the first distance and the second distance. That is, the difference calculation unit63calculates the stop required distance which increases by executing the reducing control. For example, the brake ECU6constantly (at predetermined intervals) calculates the first distance, the second distance, and the difference from the current braking force and the vehicle speed.

The increase setting unit64sets the increase amount of the braking force in the increasing control based on a difference distance, which is a difference between the first distance and the second distance. The increase amount of the braking force can be calculated based on, for example, the maximum value of the braking force, the duration of the increasing control, and the increase gradient of the braking force. That is, the increase amount of the braking force can be said to be a difference between the time integrated value of the braking force when the increasing control is executed and the time integrated value of the braking force when the increasing control is not executed. The increase setting unit64sets the increase gradient, the duration, and the maximum value of the braking force in the increasing control so as to realize the calculated increase amount. The increase setting unit64sets these set values to values that take into consideration the comfort of the occupant.

The increase setting unit64of the present embodiment calculates a minimum increase amount of the braking force (hereinafter referred to as “necessary increase amount”) necessary for preventing an increase in the stop required distance. The increase setting unit64sets the calculated necessary increase amount to the braking force to be increased by the increasing control. The increase setting unit64calculates the necessary increase amount every time the difference distance is calculated.

The increase setting unit64sets the maximum value of the braking force in the increasing control based on, for example, the necessary increase amount, a predetermined increase gradient, and a predetermined control duration. The predetermined increase gradient is selected, for example, from a predetermined numerical range set in advance within a range that does not give the driver a sense of discomfort. Furthermore, the predetermined increase gradient is set to, for example, a gradient at which no oil striking sound is generated in consideration of the reduction gradient in the reducing control. The predetermined control duration is set, for example, such that the time from the start of the increasing control to the completion of the reducing control is less than the reaction time of the driver to be described later. The brake ECU6may set at least one of the duration of the reducing control and the duration of the increasing control based on the increase amount of the braking force in the increasing control.

Common Effects of the Present Embodiment

According to the present embodiment, the difference between the first distance and the second distance, that is, the stop required distance that increases by the reducing control is calculated, and the increase amount of the braking force in the increasing control is set based on the calculation result. As a result, the increasing control can be executed with the minimum increase amount of the braking force necessary for preventing the increase in the stop required distance. According to the present embodiment, the increase gradient and the maximum value of the braking force can be set based on the increase amount so as to minimize the sense of deceleration that can be caused by the increasing control as much as possible. As described above, according to the present embodiment, it is possible to prevent a delay in the stopping of the vehicle and improve the comfort of the occupant.

(Example of Reducing Control and Increasing Control)

An example of the reducing control and the increasing control will be described with reference toFIG.2. When the wheel pressure is increased and the braking force is generated at time t1, the brake ECU6constantly calculates the first distance, the second distance, and the difference distance in a state where the braking force is being generated. The first distance and the second distance are calculated on the assumption that the braking force at the time of calculation is maintained constant until the vehicle stops. The brake ECU6calculates how many seconds it will take the vehicle to come to a stop, that is, the stop timing (time t6) based on the second distance. The brake ECU6inversely calculates each timing (control start vehicle speed etc.) from the stop timing (vehicle speed=0).

The brake ECU6calculates a reduction start timing (time t5), which is a timing to start the reducing control, based on the reduction gradient of the braking force in the reducing control set in advance (hereinafter referred to as a “predetermined reduction gradient”) and the minimum value of the braking force. The predetermined reduction gradient is a gradient at which the oil striking sound of the brake fluid is less likely to occur, and is a gradient within an allowable range set with emphasis on comfort. In consideration of variations in device operation, each value is set such that the reduction gradient in the last predetermined time (predetermined time before completion of the reducing control) of the set duration of the reducing control becomes 0 (that is, the braking force becomes constant).

In addition, the brake ECU6sets the minimum value of the braking force in the reducing control based on the gradient and the creep torque of the road surface on which the vehicle is traveling. The creep torque is a torque generated without operating the accelerator pedal at the time the vehicle stops due to the configuration of the engine and the torque transmission mechanism. The minimum value of the braking force in the reducing control is set to a value at which the vehicle can maintain the stop even by the influence of the road surface gradient and the creep torque.

The brake ECU6calculates a vehicle speed (hereinafter referred to as “reduction start vehicle speed”) at the reduction start timing from the current braking force and the vehicle speed. The brake ECU6starts the reducing control when the current vehicle speed reaches the reduction start vehicle speed. In this example, the reduction gradient is set in advance, but the reduction start vehicle speed may be set in advance.

The brake ECU6calculates an increase start timing (time t4) which is a timing to start the increasing control based on the calculated stop timing (time t6) and a preset specified time. The specified time is a time for executing the increasing control and the reducing control. In other words, the brake ECU6executes the increasing control and the reducing control at a specified time (time t4to t6). In other words, the brake ECU6starts the increasing control before a specified time from the stop timing (time t6).

The specified time in the present embodiment is set to a time within the reaction time of the driver. The reaction time of the driver (also referred to as “recognition/reaction time”) is a time (e.g., one second) from a time point at which the driver recognizes the deceleration of the vehicle until the driver performs some operation by the recognition. The reaction time of the driver can be acquired (calculated/estimated) in advance based on experiments and statistical data.

The brake ECU6calculates a vehicle speed (hereinafter referred to as “increase start vehicle speed”) at the increase start timing from the current braking force and the vehicle speed. The brake ECU6starts the increasing control when the current vehicle speed reaches the increase start vehicle speed. The brake ECU6calculates the maximum duration (continuable time) of the increasing control from the calculated increase start timing (time t4) and the reduction start timing (time t5).

The brake ECU6calculates a necessary increase amount based on the difference distance each time the difference distance is calculated. The brake ECU6calculates a minimum increase gradient (hereinafter referred to as a “minimum increase gradient”) of the braking force required in the increasing control based on the necessary increase amount and the maximum duration. When the calculated minimum increase gradient is less than a specified gradient set in advance, the brake ECU6sets the change gradient of the braking force in the increasing control to a predetermined increase gradient less than the specified gradient (minimum increase gradient predetermined increase gradient<specified gradient).

The brake ECU6calculates the maximum value (and the maximum value duration) of the braking force in the increasing control based on the predetermined increase gradient, the necessary increase amount, and the maximum duration. The brake ECU6executes the increasing control with the set predetermined increase gradient and maximum value during the maximum duration (times t4to t5).

In the present example, the reducing control reduces the braking force from the maximum value of the braking force in the increasing control. Thus, the reduction gradient of the braking force in the reducing control is slightly larger than the predetermined reduction gradient used in the calculation. The predetermined reduction gradient is set to be within an allowable range (small) even if the reduction gradient becomes large by the increasing control.

On the other hand, when the minimum increase gradient is greater than or equal to the specified gradient or the necessary increase amount is greater than or equal to the specified value, the brake ECU6reduces the duration of the reducing control within a preset allowable range (e.g., a range that does not impair the comfort of the occupant) and calculates the minimum increase gradient again, for example, so that the maximum duration of the increasing control becomes large. When the minimum increase gradient does not become less than the specified gradient with the change within the allowable range, the brake ECU6determines not to execute the increasing control and the reducing control.

The brake ECU6may calculate the maximum value of the braking force in the increasing control based on the predetermined increase gradient, the necessary increase amount, and the maximum duration without calculating the minimum increase gradient, and determine whether or not the maximum value is less than the specified value. In this case, the brake ECU6executes the increasing control with the maximum value when the maximum value is less than the specified value, and changes the increase gradient of the increasing control or the duration of the reducing control within the allowable range when the maximum value is greater than or equal to the specified value and calculates the maximum value again.

In the present example, the brake ECU6increases only the braking force of the rear wheel Wr in the increasing control. That is, in the increasing control, the brake ECU6controls the first piping system50aof the actuator5according to the target braking force (target wheel pressure) of the rear wheel Wr to increase the wheel pressure of the rear wheel Wr. The brake ECU6merely needs to increase at least one of the braking force of the front wheel Wf and the braking force of the rear wheel Wr in the increasing control.

The duration of the reducing control and the increasing control in the present embodiment is set based on the natural frequency of the pitching behavior (pitch natural frequency of the vehicle) that occurs at the time of vehicle stop. For example, the variation time of the braking force in the reducing control and the variation time of the braking force in the increasing control are set so as not to be less than or equal to ¼ (or less than or equal to ½) of the cycle of the pitching behavior. For example, when the braking force varies at less than or equal to a half cycle of the pitching behavior, the behavior by the control and the pitching behavior at the time of stopping tend to resonate. Based on this finding, the minimum value of the variation time (reduction time) of the braking force in the reducing control and the minimum value of the variation time (increase time) of the braking force in the increasing control are set such that resonance between the behavior by the control and the pitching behavior is suppressed. Note that the cycle of the pitching behavior may vary depending on the vehicle, but is a value less than the reaction time of the driver.

(Control from Start of Braking to Increasing Control)

As illustrated inFIG.2, when the braking is started at time t1, the front-rear distribution of the wheel pressure is set such that the wheel pressure of the rear wheel Wr becomes larger than the wheel pressure of the front wheel Wf. The distribution rate of the wheel pressure of the rear wheel Wr with respect to the sum of the wheel pressure of the front wheel Wf and the wheel pressure of the rear wheel Wr is a predetermined value larger than 50%. After time t2, when the braking force reaches the target braking force and becomes stable, each wheel pressure is gradually changed so that the wheel pressure of the front wheel Wf and the wheel pressure of the rear wheel Wr match while maintaining the target braking force. That is, the brake ECU6brings the wheel pressure of the front wheel Wf and the wheel pressure of the rear wheel Wr close to each other so that the wheel pressures of the front and rear wheels are at the same level. As a result, the wheel pressure (front wheel braking force) of the front wheel Wf gradually increases, and the wheel pressure (rear wheel braking force) of the rear wheel Wr gradually reduces. At time t3, the wheel pressures of the front and rear wheels become the same value (the same level), and the distribution rate of the front wheel braking force in the present embodiment becomes a predetermined value (predetermined value>50%).

Here, an overall control flow in braking will be described with reference toFIG.3. As a premise, the brake ECU6calculates the first distance, the second distance, the difference distance, the reduction start timing (reduction start vehicle speed), the necessary increase amount, the increase start timing (increase start vehicle speed), and the like at predetermined intervals (constantly) based on the current braking force or the target braking force as described above.

The brake ECU6determines whether or not braking is currently being performed by operating the brake pedal11or automatic brake control based on the wheel pressure (S101). When braking is currently being performed (S101: Yes), the brake ECU6determines whether or not the vehicle is stopped based on detection results of various sensors (e.g., the acceleration sensor72and the wheel speed sensor73) (S102). When the vehicle is stopped (S101: No), the brake ECU6generates braking force by predetermined braking force distribution.

When the vehicle is not stopped (S102: No), the brake ECU6determines whether or not the vehicle speed is lower than or equal to the reduction start vehicle speed (S103). The vehicle speed can be calculated from the detection result of, for example, the wheel speed sensor73. When the vehicle speed is lower than or equal to the reduction start vehicle speed (S103: Yes), the brake ECU6executes the reducing control (S104). When the vehicle speed is not lower than or equal to the reduction start vehicle speed (S103: No), the brake ECU6determines whether or not the vehicle speed is lower than or equal to the increase start vehicle speed (S105). When the vehicle speed is lower than or equal to the increase start vehicle speed (S105: Yes), the brake ECU6executes the increasing control (S106). In the increasing control, only the rear wheel braking force is increased.

When the vehicle speed is not lower than or equal to the increase start vehicle speed (S105: No), the brake ECU6determines whether or not the deceleration is in a stable state and the necessary increase amount is larger than or equal to the specified value (S107). For example, when the gradient of the target deceleration (differential value of deceleration: jerk) is less than or equal to a threshold value, the brake ECU6determines that the deceleration is stable. When the gradient of the target deceleration is less than or equal to the threshold value and the necessary increase amount is larger than or equal to the specified value (S107: Yes), the brake ECU6sets the target value of the front wheel distribution rate of the braking force to a predetermined value (the target wheel pressures of the front and rear wheels are the same value) and gradually increases the distribution rate of the front wheel braking force (S108).

When the gradient of the target deceleration is larger than the threshold value or when the necessary increase amount is less than the specified value (S107: No), the brake ECU6sets the front-rear distribution of the braking force so as to suppress the pitching behavior due to the increase in the braking force (S109). Specifically, the brake ECU6makes the distribution rate of the wheel pressure of the rear wheel Wr to be larger than 50%. When the braking force is stable and execution of the increasing control is necessary, the brake ECU6matches the wheel pressures of the front and rear wheels for preparation of the increasing control and the reducing control (ensure comfort). Thus, the brake ECU6executes change in the front-rear distribution of the braking force, increasing control, and reducing control according to the situation.

Other Effects of the Present Embodiment

The brake ECU6sets at least one of the duration of the reducing control and the duration of the increasing control based on the pitch natural frequency of the vehicle. According to this configuration, the behavior of the vehicle by the reducing control and the increasing control can be suppressed from resonating with respect to the pitching behavior of when the vehicle stops. That is, an increase in pitching behavior can be suppressed.

In addition, the brake ECU6sets the minimum value of the braking force in the reducing control based on at least one of the gradient of the road surface on which the vehicle is traveling and the creep torque of the vehicle. According to this configuration, sufficient braking force can be exerted even at the time of completion of the reducing control.

In addition, the actuator5includes a second piping system50bthat supplies the brake fluid to the wheel cylinders183and184of the front wheel Wf to generate the front wheel braking force by the wheel pressure of the front wheel Wf, and a first piping system50athat supplies the brake fluid to the wheel cylinders181and182of the rear wheel Wr to generate the rear wheel braking force by the wheel pressure of the rear wheel Wr. Then, the brake ECU6controls the actuator5so that the wheel pressure of the rear wheel Wr becomes larger than the wheel pressure of the front wheel Wf with the start of braking, and thereafter, the wheel pressure of the front wheel Wf and the wheel pressure of the rear wheel Wr are brought close to each other until the increasing control is executed.

According to this configuration, at the start of braking, the distribution rate of the wheel pressure of the rear wheel Wr is made larger than 50%, whereby the nose dive is suppressed and the pitching behavior is suppressed. At the execution of the increasing control, the wheel pressure of the front wheel Wf and the wheel pressure of the rear wheel Wr are close to each other (at the same level), and the balance of the flow speed of the brake fluid by the increasing control and the reducing control is easily maintained. That is, the flow speed of the brake fluid in one of the front and rear wheels is suppressed from becoming extremely large. As a result, the generation of the oil striking sound is suppressed. Furthermore, in applying the same braking force to the vehicle, increasing the wheel pressure of the two wheels makes it easier to avoid a minute oil pressure control region than increasing the wheel pressure of the four wheels, so that the pressure adjusting accuracy can be improved.

Furthermore, the brake ECU6increases only the rear wheel braking force as the increasing control. According to this configuration, the nose dive (pitching behavior) is less likely to occur even by the increase in the braking force, and the reduction in the comfort of the occupant is suppressed.

The brake ECU6also sets the duration of the increasing control and the duration of the reducing control such that the sum of the duration of the increasing control and the duration of the reducing control becomes less than the reaction time of the driver. According to this configuration, the increasing control and the reducing control are completed before the driver recognizes and reacts to the increase/reduction of the braking force. Therefore, it is possible to suppress the change of the pedal operation of the driver in response to the control and to stop the vehicle in accordance with the prediction of stopping at the time of the operation of the driver, that is, in accordance with the calculation result of the brake ECU6.

Here, the reaction time may be a fixed value set in advance, or may be set based on a map or the like set in advance. For example, the brake ECU6may set the reaction time based on the state of the driver. The brake ECU6estimates the wakefulness (such as drowsiness) and the age of the driver based on a detection result from a device that detects or predicts the state of the driver, for example, a driver monitor that images the driver or a device that detects a vital sign (such as pulse) of the driver. The brake ECU6sets the reaction time to be shorter the more the driver is awakened. The brake ECU6also sets the reaction time to be longer the higher the age. This enables control corresponding to the state of the driver. The age of the driver may be set in advance. It can also be said that the sum of the durations of the increasing control and the reducing control is set to a predetermined time set based on the reaction time of the driver.

The present disclosure is not limited to the embodiment described above. For example, the braking force to be controlled is not limited to the hydraulic braking force, and may be a regenerative braking force or a braking force generated by an electric parking brake. Furthermore, the increase amount of the braking force in the increasing control can be set even with the actuator5having one piping system (one channel). The braking unit (actuator5) may include an electric cylinder. In addition, the generation of the braking force is not limited only to the operation of the brake pedal11, and may be performed by automatic brake control. The present disclosure can also be applied to a vehicle with one pedal or an automatic drive vehicle.

In addition, the booster12may not be provided. In this case, for example, the brake pedal11is connected to the stroke simulator in the normal state, and is connected to the master cylinder in the event of an electric failure. Then, only at the time of electric failure, the master cylinder supplies the brake fluid to the wheel cylinder. If the operation feeling of the brake pedal11is ignored, the booster12may be a vacuum booster that assists the brake operation force by utilizing the intake negative pressure of the engine.