Brake force control unit for a vehicle

Abstract

Brake force control unit (10) for a vehicle, comprising a brake device (16) for applying a braking force (Fbt) to a vehicle (14) and a control unit (18) for controlling the brake device, wherein the control unit is configured to perform a brake force holding control for controlling the brake device in order to maintain a braking force applied to the vehicle when the vehicle is stopped on an incline in an uphill direction, and to perform a brake force reduction control for controlling the brake device so that the braking force is gradually reduced when it is determined that a driving request to drive the vehicle is generated during an execution of the brake force holding control, characterized in that the vehicle (14) has a driving assistance device (32) which is configured to perform driving assistance control which automatically brakes and drives the vehicle (14) without requiring any braking / driving action by the driver, wherein the control unit (18) is configured to control the braking device (16) in such a way that the braking force during an execution of the brake force reduction control is greater when a driving request is generated by the driving assistance control than when a driving request is generated by the driving operation of the driver.

Description

Background of the invention 1. Field of the invention

[0001] The present invention relates to a brake force control unit for a vehicle such as an automobile. 2. Description of the state of the art

[0002] A hill start assist control system is known as a brake force control system in a vehicle such as an automobile, as described, for example, in published Japanese patent application No. JP 2002-104 147 A. This system is activated when a vehicle stops and starts uphill on an incline. A hill start assist control system consists of a brake force maintenance control and a brake force reduction control.

[0003] Brake force hold control is executed when a vehicle decelerates and comes to a stop by braking while traveling uphill on an incline, and a preset brake force hold condition is met. The stopped state of the vehicle is maintained by holding the braking force applied to the vehicle. Brake force reduction control is executed when a drive request to start the vehicle is generated in a state where brake force hold control is currently being performed. The braking force is released after a predetermined time has elapsed since the drive request was generated, or the braking force is gradually reduced while a required driving force is increased.

[0004] In particular, in the brake force reduction control, the braking force is preferably reduced gradually in such a way as to prevent the vehicle from sliding / rolling backward down the incline due to a lack of braking force, and in addition to preventing the driver from feeling stuck / stuck due to a remaining excessive braking force and a starting deceleration of the vehicle.

[0005] Furthermore, a driving assistance control system is known for vehicles such as automobiles, which allows a vehicle to be braked and driven without requiring braking / driving input from a driver, such as inter-vehicle distance control (distance control between vehicles) in all vehicle speed ranges. Hill start assist control can be implemented not only in vehicles that are braked and driven by a driver, but also in vehicles where driving assistance control is used.

[0006] If braking force is controlled to decrease immediately to reduce the likelihood of the driver experiencing a feeling of being stuck when a drive request is generated by the driver's input, there is a high probability that the vehicle may roll backward due to insufficient braking force when a drive request is generated by the driver assistance system. Conversely, if braking force is controlled to decrease slowly to reduce the likelihood of the vehicle rolling backward when a drive request is generated by the driver assistance system, there is a high probability that the driver will experience a feeling of being stuck due to excessive braking force remaining when a drive request is generated by the driver's input.

[0007] Furthermore, in a vehicle such as an automobile, the actual braking force generated may be lower than the target braking force for the control system. This can be due to a reduction in the braking force control system's amplification or a deterioration of the friction elements in the braking device over time. In a situation where the braking force is lower than the target value when the braking force reduction control is activated, there is a high probability that the vehicle may slide or roll backward due to insufficient braking force.

[0008] In particular, even if the vehicle rolls backward while a driver is driving, the driver can apply the brakes to quickly counteract the rollback. However, if the vehicle rolls downward while the driving assistance control is in progress and the driver is not currently applying the brakes, he or she cannot immediately apply the brakes to counteract the rollback.

[0009] From DE 102 42 122 A1, a method for preventing a motor vehicle from rolling backward when starting on an incline is known, in which the wheel brake pressure is maintained at a high level until the drive torque generated by a vehicle engine reaches a predetermined minimum drive torque. According to the disclosure, the minimum drive torque is calculated based on the longitudinal forces on the wheels and the inertial force of the vehicle during a stopping maneuver. The current drive torque during a starting maneuver is calculated and compared with the calculated minimum drive torque. If the current drive torque is greater than or equal to the minimum drive torque, the wheel brakes are released.

[0010] DE 10 2005 036 217 A1 discloses a device for ensuring that a motor vehicle remains stationary, comprising a distance-related longitudinal dynamics control module contained in an electronic control unit. This module decelerates the motor vehicle to a standstill while maintaining a specific distance from a target object. In this device, at least until the vehicle comes to a standstill, the longitudinal dynamics control module specifies a target braking torque to a brake control module. At a defined initial point in time after the motor vehicle has been detected as stationary, the longitudinal dynamics control module transmits a transfer signal to the brake control module. Thereupon, the brake control module independently maintains a wheel braking torque, acting as a parking brake, regardless of the target braking torque specified by the longitudinal dynamics control module.At a defined second point in time after a start-up command, the longitudinal dynamics control module, starting with the actual braking torque present at that time, again specifies a target braking torque to reduce the wheel braking torque. Summary

[0011] Based on the prior art, the present invention aims to provide a brake force control unit that is improved to reduce the risk of a vehicle slipping / rolling backward when brake force reduction control is performed during the execution of the driving assistance control, while avoiding the possibility of a driver experiencing a feeling of being stuck / trampled when brake force reduction control is performed during a driving operation being carried out by the driver. This objective is achieved according to the invention with a brake force control unit having the features of claim 1.

[0012] According to the present invention, a brake force control unit for a vehicle is provided, comprising: a brake device for applying a braking force to a vehicle, and a control unit for controlling the brake device, and the control unit is configured to perform a brake force maintenance control for controlling the brake device in order to maintain a braking force applied to the vehicle when the vehicle is stopped on an incline in an uphill direction, and to perform a brake force reduction control for controlling the brake device so that the braking force is gradually reduced when it is determined that a driving request to drive the vehicle is generated during an execution of the brake force maintenance control.

[0013] The vehicle has a driving assistance device which is designed to perform driving assistance control, which automatically brakes and drives the vehicle without requiring any braking / driving input from the driver.

[0014] The control unit is designed to control the braking device in such a way that the braking force during an execution of the brake force reduction control is greater when a driving request is generated by the driving assistance control than when a driving request is generated by the driver's driving operation.

[0015] According to the configuration above, during an execution of the brake force reduction control, the braking force is controlled so that it is greater when a driving request is generated by the driving assistance control than when a driving request is generated by the driver's driving input.

[0016] Therefore, compared to a case where the braking force when a drive request is generated by the driving assistance control is the same as when a drive request is generated by the driver's input, it is possible to reduce the probability of the vehicle sliding / rolling if the brake force reduction control is executed during the execution of the driving assistance control. Furthermore, since the braking force is not increased when a drive request is generated by the driver's input, it is possible to avoid an increase in the probability that the driver might experience a feeling of being stuck if a brake force reduction control is executed during the driver's input.

[0017] Furthermore, the braking force generated by the driving assistance control unit when a driving request is generated by the driver's input is greater than the braking force generated by the driver's input. Therefore, even if the actual braking force generated is lower than the target braking force for the control unit, due to a reduction in the braking force gain or similar mechanism, it is possible to reduce the probability of the vehicle rolling downwards due to a lack of braking force when the braking force reduction control is executed during the driving assistance control operation.

[0018] According to one aspect of the present invention, the vehicle has a device for estimating a required driving force, which is configured to estimate a required driving force of the driving request in both cases, i.e., when the driving request is generated by the driving assistance control and when the driving request is generated by a driving action of the driver, and the control unit is configured to calculate a target braking force, which gradually decreases when the estimated required driving force increases, and to control the braking device in such a way that a braking force applied to the vehicle becomes the target braking force.

[0019] According to the above aspect, a target braking force is calculated to be gradually reduced in accordance with an increase in the required driving force, which is estimated by the device for estimating a required driving force, and the braking device is controlled in such a way that a braking force applied to the vehicle becomes the target braking force.

[0020] Therefore, the target braking force can be calculated to a suitable value according to the required driving force, in contrast to a case where the target braking force is not calculated and thus gradually decreases with an increase in the required driving force. Accordingly, it is possible to effectively reduce the probability of the vehicle rolling when the brake force reduction control is performed during the execution of the driving assistance control, while effectively avoiding the probability of the driver experiencing a feeling of being stuck when the brake force reduction control is performed while the driver is in the middle of driving.

[0021] According to another aspect of the present invention, the vehicle has a device for estimating a downward force / downward force / downward force, which is configured to estimate a downward force / downward force that causes the vehicle to descend along the slope due to gravity, and the control unit is configured to calculate a target braking force such that the target braking force at the time when a quantity of the required driving force becomes equal to the downward force / downward force is greater when the driving request is generated by the driving assistance control than when the driving request is generated by the driver's driving operation.

[0022] According to the above principle, the target braking force is calculated at the point in time when a magnitude of the required driving force equals the downward force / gravity force estimated by the downward force / gravity force estimator, such that it is greater when the driving request is generated by the driving assistance control than when the driving request is generated by the driver's driving input. Therefore, a braking force can be applied to the vehicle even when a magnitude of the required driving force equals the downward force / gravity force estimated by the downward force / gravity force estimator, thus effectively reducing the probability of the vehicle sliding downhill when the braking force reduction control is executed during the driving assistance control operation.

[0023] Furthermore, according to another aspect of the present invention, the control unit is configured to calculate the target braking force, when the driving request is generated by the driving assistance control, to a value which is greater than the target braking force when the driving request is generated by the driving action of the driver.

[0024] According to the above, when the driving request is generated by the driving assistance control, the target braking force is calculated to a value greater than the target braking force when the driving request is generated by the driver's input. Therefore, when the driving request is generated by the driving assistance control, the braking force applied to the vehicle during the braking force reduction process can be increased compared to when the driving request is generated by the driver's input. Thus, if the braking force reduction control is implemented during the execution of the driving assistance control, the probability of the vehicle skidding can be effectively reduced.

[0025] Furthermore, according to another aspect of the present invention, the control unit is configured to calculate a target braking force when the driving assistance control is being carried out, such that the difference between a target braking force when the driving request is generated by the driving assistance control and a target braking force when the driving request is generated by the driver's driving action gradually decreases.

[0026] According to the above aspect, the target braking force, when the driving assistance control is being executed, is calculated such that the difference between the target braking force when the driving request is generated by the driving assistance control and the target braking force when the driving request is generated by the driver's driving input gradually decreases. Therefore, the difference in the target braking forces gradually decreases as a point in time is approached at which the magnitude of the required driving force becomes equal to the downward force / gravity force, which is estimated by the downward force / gravity force estimator. This makes it possible to reduce the probability that the braking force will become excessively large when the driving assistance control is being executed in a situation approaching the aforementioned point in time.

[0027] Furthermore, according to another aspect of the present invention, the control unit is configured to calculate a target braking force such that a reduction rate of the target braking force when the driving request is generated by the driving assistance control is smaller than a reduction rate of the target braking force when the driving request is generated by the driving action of the driver.

[0028] According to the above principle, the target braking force is calculated such that the rate of reduction of the target braking force when the driving request is generated by the driving assistance control is lower than the rate of reduction of the target braking force when the driving request is generated by the driver's driving input. Therefore, the braking force applied to the vehicle during the process of reducing the braking force can be increased, and the braking force can be applied to the vehicle for a longer period when the driving request is generated by the driving assistance control, compared to when the driving request is generated by the driver's driving input. Thus, it is possible to effectively reduce the probability of the vehicle rolling away when the braking force reduction control is performed during the execution of the driving assistance control.

[0029] Furthermore, according to another aspect of the present invention, the control unit is configured to calculate a target braking force such that a target braking force, when the driving request is generated by the driving assistance control, is delayed with respect to a target braking force, when the driving request is generated by the driving operation of the driver.

[0030] According to the above aspect, a target braking force is calculated such that a target braking force, when the driving request is generated by the driving assistance control, is delayed with respect to a target braking force, when the driving request is generated by the driving action of the driver.

[0031] Therefore, if the driving request is generated by the driving assistance control, braking force can be applied to the vehicle for a longer period than if the driving request is generated by the driver's driving input. Accordingly, it is possible to effectively reduce the probability of the vehicle rolling away if the brake force reduction control is implemented while the driving assistance control is in operation.

[0032] Furthermore, according to another aspect of the present invention, the control unit is configured to calculate a target braking force which gradually decreases in a preset first pattern when the driving request is generated by the driver's driving action, to calculate a target braking force which gradually decreases in a preset second pattern in order to be greater than a target braking force calculated in the first pattern when the driving request is generated by the driving assistance control, and to control the braking device such that a braking force applied to the vehicle becomes the target braking force.

[0033] According to the above aspect, when the driving request is generated by the driver's driving action, a target braking force is calculated which gradually decreases in a preset first pattern, and when the driving request is generated by the driving assistance control, a target braking force is calculated which gradually decreases in a preset second pattern to be greater than the target braking force calculated in the first pattern.

[0034] Therefore, if the driving request is generated by the driving assistance control, the braking force applied to the vehicle can be increased compared to a case where the driving request is generated by the driver's input. Thus, it is possible to effectively reduce the probability of the vehicle sliding off the road if the brake force reduction control is implemented during the execution of the driving assistance control.

[0035] Other objectives, other features and associated advantages of the present invention are easily understood from the description of the embodiments of the present invention, which is described with reference to the following drawings. Brief description of the characters Fig. Figure 1 is a schematic configuration diagram showing a brake force control unit for a vehicle according to an embodiment of the present invention. Fig. Figure 2 is a flowchart showing a brake force reduction control routine of the hill start assist control in the first to fourth embodiments. Fig. Figure 3 is a flowchart showing a brake force reduction control routine of the hill start assist control in the fifth to seventh embodiments. Fig. Figure 4 is a time diagram showing a change in a target braking force Fbt with respect to a change in a required driving force Fdreq in the first embodiment. Fig. Figure 5 is a time diagram showing a change in a target braking force Fbt with respect to a change in a required driving force Fdreq in the second embodiment. Fig. Figure 6 is a time diagram showing a change in a target braking force Fbt with respect to a change in a required driving force Fdreq in the third embodiment. Fig. 7 is a time diagram showing a change in a target braking force Fbt with respect to a change in a required driving force Fdreq in the fourth embodiment. Fig. Figure 8 is a time diagram showing a change in a target braking force Fbt in the fifth embodiment. Fig. Figure 9 is a time diagram showing a change in a target braking force Fbt in the sixth embodiment. Fig. Figure 10 is a time diagram showing a change in a target braking force Fbt in the seventh embodiment. Detailed description

[0036] The present invention will now be described in detail with reference to the attached figures. [First embodiment]

[0037] As in Fig. As shown in Figure 1, the brake force control unit 10, according to embodiments of the present invention, comprises a brake device 16, which applies braking forces to the left and right front wheels 12FL and 12FR and the left and right rear wheels 12RL and 12RR in order to exert a braking force on a vehicle 14, and an electronic control unit of a brake control (brake control ECU) 18 for controlling the brake device 16. The left and right front wheels 12FL and 12FR are steered wheels, and although they are in Fig. 1 not shown, is steered by a steering device in response to a steering input from a driver. In the following description and Fig. 1. The “electronic control unit” is described as “ECU”.

[0038] The brake device 16 includes a hydraulic circuit 20 as a brake actuator, and wheel cylinders 24FR, 24FL, 24RR and 24RL, each provided at wheels 12FL to 12RL. The brake device 16 applies braking forces to the wheels by controlling pressures in the wheel cylinders 24FR to 24RL through the hydraulic circuit 20, and thus changes the braking force of the vehicle 14. Although it is in Fig. Not shown in Figure 1, the hydraulic circuit 20 contains an oil reservoir, an oil pump, various valve devices, and the like.

[0039] As in Fig. As shown in Figure 1, the brake device 16 has a master brake cylinder 28 for pumping brake fluid in response to the driver pressing down on a brake pedal 26. The master brake cylinder 28 is equipped with a pressure sensor 30 for detecting a master brake cylinder pressure Pm, and a signal indicating a master brake cylinder pressure Pm, detected by the pressure sensor 30, is input to the brake control ECU 18.

[0040] The brake control ECU 18 controls the brake pressure of each wheel based on a master cylinder pressure Pm, thereby controlling the braking force of each wheel according to the amount of depressurization of the brake pedal 26, i.e., the amount of braking applied by the driver. The brake pressure of each wheel cylinder is individually controlled by the hydraulic circuit 20, which in turn is controlled by the brake control ECU 18 as needed. Therefore, the brake system 16 can control the braking force of each wheel individually, regardless of the driver's braking input.

[0041] Furthermore, the brake control ECU 18 controls the braking force of each wheel as needed based on information transmitted by a driving assistance ECU (electronic control unit) 34 of a driving assistance device 32, as will be described in detail later. A signal indicating a longitudinal acceleration Gx of the vehicle 14, which is detected by a longitudinal acceleration sensor 36, is also input into the brake control ECU 18.

[0042] As in Fig. As shown in Figure 1, a signal indicating a vehicle speed V, detected by a vehicle speed sensor 38, and a signal indicating information ahead of the vehicle 14, detected by a radar sensor 40, are input into the driver assistance ECU 34. Furthermore, a signal indicating whether or not to implement inter-vehicle distance control as a driver assistance control is input into the electronic control unit 34 by an ACC switch 42, which is operated by the driver, and the electronic control unit 34 displays the inter-vehicle distance control status on a display 44. In particular, the detection of information ahead of the vehicle 14 can be performed by a camera, such as a CCD camera, or by a combination of a radar sensor and a camera.

[0043] Although it is not in Fig. As shown in Figure 1, the ACC switch 42 includes an intermediate vehicle distance control start button, an intermediate vehicle distance control end button, an intermediate vehicle distance setting button for setting a reference intermediate vehicle distance Lc, and a vehicle speed setting button for setting a reference vehicle speed Vc. The driver assistance ECU 34 performs the intermediate vehicle distance control in accordance with the settings of these buttons. That is, the electronic control unit 34 starts the intermediate vehicle distance control when the intermediate vehicle distance control start button is pressed and stops the intermediate vehicle distance control when the intermediate vehicle distance control end button is pressed.Although not shown in the figure, while the inter-vehicle distance control is being carried out, the radar sensor 40 emits radio waves in a millimeter wave band to the front of the vehicle 14 and captures reflected waves to obtain information in front of the vehicle 14.

[0044] If the difference ΔV (=V-Vc) between a vehicle speed V, detected by the vehicle speed sensor 38, and the reference vehicle speed Vc is a positive value, the electronic control unit 34 sends a braking request to the brake control ECU 18 as needed, so that the magnitude of the vehicle speed difference ΔV becomes less than or equal to a reference value Va (a positive constant). The electronic control unit 18 controls the braking device 16 such that the braking forces of the wheels 12FL to 12RL increase when the braking request is received.

[0045] On the other hand, if the vehicle speed difference ΔV is a negative value, the electronic control unit 34 issues an acceleration request to a vehicle control ECU (electronic control unit) 46 as needed, so that the vehicle speed difference ΔV becomes less than or equal to a reference value Vb (a positive constant). When the acceleration request is received, the electronic control unit 46 controls an output from a drive unit 48 such that the driving forces of the left and right rear wheels 12RL and 12RR, which are the drive wheels, increase (brake force control of a constant-speed vehicle control system).

[0046] It is noted that in the illustrated embodiment, the vehicle 14 is a rear-wheel drive vehicle; however, the vehicle to which the present invention is applied can be any of a front-wheel drive vehicle, a rear-wheel drive vehicle, or an all-wheel drive vehicle. Furthermore, the drive unit 48 can be any drive unit known from the prior art, such as a drive unit with an engine and an automatic transmission, a drive unit with an engine and a continuously variable transmission, a hybrid system, a fuel cell system, or an electric motor.

[0047] Furthermore, the driver assistance ECU 34 performs an inter-vehicle distance control to manage the distance between the vehicle itself and a vehicle ahead. For example, the electronic control unit 34 determines the presence or absence of a vehicle ahead based on information in front of the vehicle 14, which is detected by the radar sensor 40, and, if a vehicle ahead is present, estimates a distance L between the vehicle itself 14 and the vehicle ahead.If the difference ΔL (=L-Lc) between the distance L and the reference distance Lc is a positive value that exceeds a reference value La (a positive value), the electronic control unit 34 outputs a signal indicating a target acceleration Gat as an acceleration request to the vehicle control ECU 46, such that the distance difference ΔL is less than or equal to the reference value La and greater than or equal to a reference value Lb (a negative value). When a signal indicating the target acceleration Gat is input, the electronic control unit 46 calculates a target output torque of the drive unit 48 to achieve the target acceleration Gat and controls the drive unit such that the output torque of the drive unit becomes the target output torque.

[0048] On the other hand, if the distance difference ΔL is a negative value, which is less than the reference value Lb, the driving assistance ECU 34 sends a signal indicating the target deceleration Gbt to the brake control ECU 18 as a brake request, so that the distance difference ΔL becomes greater than or equal to the reference value Lb and less than or equal to the reference value La. When the signal indicating the target deceleration Gbt is received, the electronic control unit 18 calculates the target braking forces on wheels 12FL to 12RL based on the target deceleration Gbt and controls the brake device 16 so that the braking force at each wheel becomes the corresponding target braking force (brake force control of the inter-vehicle distance control).

[0049] The absolute values ​​of the reference values ​​La and Lb are variably adjusted according to the vehicle speed, so they decrease as the vehicle speed V decreases. Therefore, for example, if the vehicle ahead decelerates and comes to a stop, the reference value La gradually decreases, causing the vehicle to decelerate and come to a stop in such a way as to maintain a predetermined distance between the vehicle and the vehicle ahead. Conversely, if the vehicle ahead starts and accelerates in a situation where both the vehicle and the vehicle ahead are stopped, the vehicle also starts and accelerates, while maintaining a gradually increasing distance from the vehicle ahead.

[0050] In particular, when the vehicle 14 stops and starts while traveling uphill, the driving assistance ECU 34 works in conjunction with the brake control ECU 18 to execute the brake force hold control and brake force reduction control of the hill start assist control. That is, when the vehicle 14 decelerates and stops by braking while traveling uphill on an incline and a preset brake force hold condition is met, the brake control ECU 18 executes the brake force hold control, which maintains the braking force applied to the vehicle. Furthermore, when a drive request to start the vehicle 14 uphill on the incline is generated and a signal specifying a target acceleration Gat is output to the driving control ECU 46, a signal specifying the target acceleration Gat is also output to the electronic control unit 18.The electronic control unit 18 estimates a required driving force Fdreq, which corresponds to the target acceleration Gat, and executes the brake force reduction control, which gradually reduces the braking force applied to the vehicle 14, according to the required driving force Fdreq. The brake force reduction control is described in detail later.

[0051] If no braking request is entered, the brake control ECU 18 sets the control mode of the brake device 16 to the normal brake force control mode, so that the braking forces of the wheels 12FL to 12RL become the braking forces corresponding to the master cylinder pressure Pm. If no acceleration request is entered, the drive control ECU 46 controls the output of the drive unit 48 based on an accelerator pedal opening φ, which is detected by an accelerator pedal opening sensor 52 provided on an accelerator pedal 50, and the like. If necessary, reference is made, for example, to the published Japanese patent application JP 2003-34240A for the inter-vehicle distance control described above.

[0052] The brake control ECU 18, the driving assistance ECU 34, and the driving control ECU 46 can each contain a microcomputer with a processor, ROM, RAM, and an input / output interface, all interconnected by a bidirectional, standard bus. Programs such as brake control and inter-vehicle distance control are stored in the ROM of the respective microcomputer, and each control function is executed by the corresponding processor (CPU) according to the program. Furthermore, the electronic control units (ECUs) 18, 34, and 46 exchange necessary signals via CAN bus, which is not shown in the diagram.

[0053] As will be described in detail later, in the first embodiment, the brake control unit (ECU) 18 executes the brake force reduction control of the hill start assist control according to the flowchart shown in Fig. 2 is shown. In particular, the brake force holding control of the hill start assist control, which is executed when the vehicle 14 stops on an incline in the uphill direction and conditions known from the prior art are met, can be implemented in any manner known from the prior art. <bremskraftverringerungssteuerungsroutine>

[0054] Next, a brake force reduction control routine of the hill start assist control in the first embodiment is described with reference to the flowchart which is shown in Fig. The brake force reduction control is described in the flowchart shown in section 2. Fig. As shown in Figure 2, the brake force reduction control is initiated when the vehicle's braking force holding control has completed its operation and is executed repeatedly at predetermined time intervals. The following description describes the brake force reduction control according to the flowchart shown in Figure 2. Fig. 2 is simply referred to as "the control". The same applies to the brake force reduction control routine, which, according to the flowchart shown in Fig. 3 is shown, in the second embodiment described later.

[0055] First, in step 10, a determination is made as to whether a reduction of braking force is complete or not, for example by determining whether the target braking force Fbt, which is calculated in the previous step 70 or 90 (described later), is less than or equal to a control end reference value Fbt0 (a positive constant). If a positive determination is made, the braking force reduction control is terminated, and if a negative determination is made, the control proceeds to step 20.

[0056] In step 20, a determination is made as to whether the braking force is reduced by the reduction of the target braking force Fbt or not. If the determination is affirmative, the control system proceeds to step 50, and if the determination is negative, the control system proceeds to step 30.

[0057] In step 30, a determination is made as to whether or not a driving request is generated to propel vehicle 14. If an affirmative determination is made, the control proceeds to step 50, and if a negative determination is made, the control proceeds to step 40. Although it is not in Fig. As shown in Figure 2, when the control system proceeds to step 50, a downward force / gravitational force Fdown, which causes the vehicle to descend a slope due to gravity, is estimated based on the longitudinal acceleration Gx of the vehicle 14 and stored in RAM. Thus, step 30 and the longitudinal acceleration sensor 36 function as a downward force estimator, which estimates the downward force / gravitational force that causes the vehicle to descend a slope due to gravity. The downward force Fdown can be estimated based on the slope angle and the mass of the vehicle 14, and the slope angle can be detected or estimated based on information from a navigation device.

[0058] When the inter-vehicle distance control (IPDC) initiates a control action to start and drive the vehicle, it is determined that a drive request is generated by the IDC. Similarly, when the driver shifts gears from P or N to D, or when the accelerator pedal opening φ increases from 0, a drive request is generated by the driver. Furthermore, when the gear position is in D, or when the accelerator pedal opening φ is a positive value, a drive request is generated by the driver.

[0059] In step 40, the brake force holding control continues by maintaining the braking forces applied to wheels 12FL-12RL by the brake device 16. This prevents the vehicle 14 from rolling down the incline due to gravity.

[0060] In step 50, a determination is made as to whether the driving request is generated by the inter-vehicle distance control or not. If a negative determination is made, that is, if it is determined that the driving request is generated by the driver's driving input, the control proceeds to step 80; and if an affirmative determination is made, the control proceeds to step 60.

[0061] In step 60, a required driving force Fdreq, corresponding to a target acceleration Gat, is estimated, for example, as the product of the target acceleration Gat and a mass (a positive constant) of the vehicle 14. The required driving force Fdreq is positive if it is a driving force in the acceleration direction of the vehicle 14. Thus, step 60 and the vehicle control unit (ECU) 46 act as a device for estimating a required driving force to determine the driving force required by the vehicle request. The required driving force Fdreq can be estimated based on an accelerator pedal opening φ, a drive torque of the drive unit 48, and the like.

[0062] In step 70, a target braking force Fbt of vehicle 14 is calculated according to the following equation (1). In particular, the downhill force Fdown is positive if it is a force acting in the downhill direction, and the target braking force Fbt is positive if it is a force acting in the deceleration direction of vehicle 14. ΔFbt is an increase correction amount (a positive constant) of the braking force. Fbt=Fdown−Fdreq+ΔFbt

[0063] In step 80, a required driving force Fdreq is estimated based on an accelerator pedal opening φ, which is detected by the accelerator pedal opening sensor 52, such that it increases as the accelerator pedal opening φ increases. Thus, step 80 and the accelerator pedal opening sensor 52 act as an estimating device for the required driving force to determine the driving force required for the driving request. In this step, the required driving force Fdreq can also be estimated based on the amount of depressurization of the accelerator pedal 50, the drive torque of the drive unit 48, and the like.

[0064] In step 90, a target braking force Fbt of vehicle 14 is calculated according to the following equation (2). Fbt=Fdown−Fdreq

[0065] Once step 70 or 90 above is complete, the control process continues to step 100. In step 100, the target braking forces Fbtfl, Fbtfr, Fbtrl, and Fbtrr of the front wheels 12FL and 12FR and the rear wheels 12RL and 12RR are calculated, each based on the target braking force Fbt and a front / rear brake force distribution ratio. Furthermore, the braking forces are controlled by adjusting the brake device 16 so that the braking forces of the respective wheels become the corresponding target braking forces. Therefore, the braking force Fb of the vehicle 14 is gradually reduced as the required driving force Fdreq increases, until the reduction in braking force is complete and a positive determination is made at step 10. <Betrieb der ersten Ausführungsform>

[0066] As understood from the description above, when a drive request to propel the vehicle 14 is generated during brake force maintenance control, an affirmative determination is made at step 30, and the steps after step 50 are executed, thereby performing brake force reduction control. If a drive request is generated by the inter-vehicle distance control (hereinafter referred to as "in the case of inter-vehicle distance control"), an affirmative determination is made at step 50, so that a target braking force Fbt of the vehicle 14 is calculated in steps 60 and 70. On the other hand, if a drive request is generated by the driver's driving action (hereinafter referred to as "in the case of driver action"), a negative determination is made at step 50, so that a target braking force Fbt of the vehicle 14 is calculated in steps 80 and 90.In both cases, in step 100, the braking force Fb of the vehicle 14 is gradually reduced by controlling it to become the target braking force Fbt.

[0067] The target braking force Fbt of vehicle 14 is calculated in step 70 according to equation (1) and in step 90 according to equation (2). Therefore, when comparing the cases of inter-vehicle distance control and driver operation, in a case where the downhill force Fdown and the required driving force Fdreq are equal, the target braking force Fbt in the case of inter-vehicle distance control is greater by the increase correction amount ΔFbt than the target braking force Fbt in the case of driver operation.

[0068] For example, Fig. 4 A time diagram showing a change in a target braking force Fbt in relation to a change in a required driving force Fdreq in the first embodiment. Fig. 4. The solid line and the dashed line indicate the target braking forces Fbt in the case of inter-vehicle distance control and in the case of driver input, respectively, and the single-point catenary indicates a required driving force Fdreq. Furthermore, the two-point catenary indicates a change in the vehicle's driving force Fd in the case of inter-vehicle distance control, where the value of the downhill force Fdown is assumed to be zero. The same applies to the ones described later. Fig. 5 to 7.

[0069] As in Fig. As shown in Figure 4, the required driving force Fdreq changes in the same way in both cases of inter-vehicle distance control and driver input. A driving request is generated at time t0, and it is assumed that the target braking force Fbt in the case of inter-vehicle distance control becomes equal to the downhill force Fdown at time t2. Furthermore, it is assumed that the target braking force Fbt becomes 0 at time t3 in the case of driver input, and that the target braking force Fbt becomes 0 at time t5 in the case of inter-vehicle distance control. In particular, an actually generated braking force Fb does not exceed the downhill force Fdown, so it is Fdown, even though in the section from time t0 to time t2 the target braking force Fbt is shown to be greater than the downhill force Fdown in the case of inter-vehicle distance control.

[0070] In the case of driver input, the target braking force Fbt becomes zero at time t3 when the required driving force Fdreq equals the downhill force Fdown, and after time t3 the vehicle's driving force Fd becomes positive and gradually increases. Therefore, compared to the case where the target braking force Fbt becomes zero at any time other than time t3, it is possible to reduce the probability that the vehicle will roll down the slope due to gravity and that the driver will experience a feeling of being stuck due to excessive braking force.

[0071] On the other hand, in the case of inter-vehicle distance control, the target braking force Fbt at time t3 is not zero, but rather ΔFbt. Thus, the vehicle's driving force Fd is zero until time t4 between time t3 and time t5, and after time t4, the vehicle's driving force Fd becomes a positive value and gradually increases. Therefore, the risk of the vehicle sliding down the incline due to gravity can be effectively reduced. Since starting the vehicle is not triggered by the driver's input, the driver experiences no sensation of the vehicle being stuck, even if the start is delayed.

[0072] In the first embodiment and the fifth embodiment, which will be described later, the increase correction amount ΔFbt of the braking force is a positive constant. However, the distance by which the vehicle 14 slides down an incline when the braking force becomes zero early is greater when the incline angle is greater and thus the absolute value of the vehicle's longitudinal acceleration Gx is greater. Therefore, the increase correction amount ΔFbt can be variably adjusted according to an absolute value of the longitudinal acceleration Gx such that the increase correction amount ΔFbt increases when the absolute value of the vehicle's longitudinal acceleration Gx increases.According to this modification, regardless of the size of the incline, it is possible to effectively reduce the risk of the vehicle sliding down the slope, while reducing the likelihood of the vehicle starting excessively due to excessive braking force. [Second embodiment]

[0073] In the second embodiment, the steps of the process that differ from step 70 are described in Fig. 2 shown in the flowchart in the same way as in the first embodiment, wherein the brake force reduction control of the hill start assist control is carried out.

[0074] In step 70 of the second embodiment, for example, if it is assumed that a difference Fdreq-Fdreq(-1) between a current required driving force Fdreq and a required driving force Fdreq(-1) stored one cycle earlier is ΔFdreq, and K2 is a positive, constant coefficient less than 1, a target braking force Fbt of vehicle 14 is calculated according to the following equation (3). In particular, when calculating the target braking force Fbt, the difference ΔFdreq is set to zero immediately after a required driving force is generated by the inter-vehicle distance control, since there is no required driving force Fdreq(-1). Fbt=Fdown−Fdreq+K2ΔFdreq

[0075] Fig. 5 is a time diagram similar to Fig. 4, which shows a change in the target braking force Fbt in relation to a change in a required driving force Fdreq in the second embodiment. As can be seen from the comparison between the solid line and the dashed line in Fig. As understood in section 5, the decreasing change in the target braking force Fbt is smoother in the case of inter-vehicle distance control than in the case of driver input. The target braking force Fbt in the case of inter-vehicle distance control is a positive value at time t3 and becomes zero at time t6, which is delayed from time t3. Therefore, as in the first embodiment, the vehicle's driving force Fd is zero until time t4 between time t3 and time t6, and after time t4, the vehicle's driving force Fd becomes a positive value and increases gradually.

[0076] In the second embodiment described above, the coefficient K2 is a positive constant. However, the distance the vehicle 14 slides down an incline when the braking force becomes zero early is greater when the incline angle is larger, and thus the absolute value of the vehicle's longitudinal acceleration Gx is greater. Therefore, the coefficient K2 can be variably adjusted according to the absolute value of the vehicle's longitudinal acceleration Gx such that the coefficient K2 increases when the absolute value of the longitudinal acceleration Gx increases. According to this modification, it is possible to effectively reduce the risk of the vehicle 14 sliding down the incline, regardless of the magnitude of the incline angle, while simultaneously reducing the probability of the vehicle being excessively decelerated from a standstill due to an excessively large braking force. [Third embodiment]

[0077] In the third embodiment, the steps of the process that differ from steps 60 and 70 are described in Fig. 2 shown in the flowchart in the same way as in the first embodiment, wherein the brake force reduction control of the hill start assist control is carried out.

[0078] In step 60 of the third embodiment, as in step 60 of the first embodiment, a required driving force Fdreq, which corresponds to a target acceleration Gat, is estimated, but the required driving force Fdreq is stored in the RAM.

[0079] In step 70, based on a required driving force Fdreq(-n) (a required driving force stored n cycles prior) stored in RAM before a preset delay time Δt (a positive constant), a target braking force Fbt of vehicle 14 is calculated according to the following equation (4). Specifically, the required driving force Fdreq(-n) is set to zero in a section before the delay time Δt has elapsed after the required driving force was generated. When the calculation of the target braking force Fbt is complete, the information on the required driving force Fdreq(-n) used in the calculation is deleted from RAM. Fbt=Fdown−Fdreq(−n)

[0080] Fig. 6 is a time diagram similar to Fig. 4, which shows a change in a target braking force Fbt in relation to a change in a required driving force Fdreq in the third embodiment. In the case of inter-vehicle distance control, as shown by the solid line in Fig. As specified in section 6, before time t1, when the delay time Δt elapses from time t0, at which the driving request is generated, the required driving force Fdreq(-n) is set to zero. Therefore, before time t1, the target braking force Fbt is calculated as a function of the downhill force Fdown, so that the target braking force Fbt is a constant value of the downhill force Fdown, and the target braking force Fbt is reduced after time t1.

[0081] Accordingly, in the case of inter-vehicle distance control, the target braking force Fbt is a value that is delayed by the delay time Δt from the target braking force Fbt in the case of driver input. The target braking force Fbt in the case of inter-vehicle distance control is a positive value at time t3 and becomes zero at time t7, which is delayed from time t3 by the delay time Δt. Thus, as in the first and second embodiments, the vehicle's driving force Fd is zero until time t4 between times t3 and t7, and when time t4 has passed, the vehicle's driving force Fd becomes positive and gradually increases.

[0082] In the third embodiment and the seventh embodiment, which will be described later, the deceleration time Δt is a positive constant. However, the distance the vehicle 14 slides down an incline when the braking force becomes zero early is greater when the incline angle is larger and thus the absolute value of the vehicle's longitudinal acceleration Gx is greater. Therefore, the deceleration time Δt can be variably set according to an absolute value of the longitudinal acceleration Gx such that the deceleration time Δt increases when the absolute value of the vehicle's longitudinal acceleration Gx increases.According to this modification, regardless of the size of the incline, it is possible to effectively reduce the risk of the vehicle sliding down the slope, while reducing the likelihood of the vehicle starting excessively due to excessive braking force.

[0083] As can be understood from the descriptions above, according to the first and third embodiments, it is possible to gradually reduce the target braking force Fbt in accordance with an increase in a required driving force Fdreq. In particular, in the case of inter-vehicle distance control, it is possible to effectively reduce the probability of the vehicle 14 sliding down an incline while avoiding increasing the probability that a driver might experience a feeling of being stuck when the driver performs a driving operation. Since starting the vehicle is not caused by a driving operation by the driver, the driver does not experience a feeling of being stuck, even if the vehicle's start is delayed.

[0084] Furthermore, according to the first to third embodiments, the target braking force Fbt in the case of driver operation is not affected by the setting of the target braking force Fbt in the case of inter-vehicle distance control. That is, the target braking force Fbt becomes zero at time t3 when the required driving force Fdreq equals the downhill force Fdown. Therefore, in the case of driver operation, it is possible to reduce the probabilities of the vehicle 14 rolling down the slope due to gravity and of the driver experiencing a feeling of being stuck, without being affected by the setting of the target braking force Fbt in the case of inter-vehicle distance control.

[0085] Furthermore, according to the first and second embodiments, the target braking force Fbt after time t0 is greater in the case of inter-vehicle distance control than the target braking force Fbt in the case of driver input. According to the third embodiment, the target braking force Fbt after time t1 is greater in the case of inter-vehicle distance control than the target braking force Fbt in the case of driver input.

[0086] Therefore, in a situation where the braking force is lower than the target braking force Fbt due to a reduction in the gain of the brake force control / regulation or a variation in the coefficient of friction of the brake force generators of the brake device 16, it is possible to reduce the probability that the vehicle 14 will roll downhill along an incline due to a lack of braking force. [Fourth embodiment]

[0087] In the fourth embodiment, steps 70 onwards are different steps of the flowchart which is shown in Fig. 2 is shown, in the same way as in the first embodiment, wherein the brake force reduction control of the hill start assist control is carried out.

[0088] In step 70 of the fourth embodiment, for example, a target braking force Fbt of vehicle 14 is calculated according to the following equation (5), where ΔFd is the difference Fdown-Fdreq between the downhill force Fdown and the required driving force Fdreq, and K4 is a positive, constant coefficient less than 1. As can be understood from comparing equations (5) and (2), the target braking force Fbt in the case of inter-vehicle distance control is greater than the target braking force Fbt in the case of driver operation by an increase correction amount K4ΔFbtΔFd, and the increase correction amount K4ΔFbtΔFd gradually decreases. Here, ΔFbt is an increase correction amount (a positive constant) of the braking force, as in the first embodiment. Fbt=Fdown−Fdreq+K4ΔFbtΔFd

[0089] Fig. 7 is a time diagram similar to Fig. 4, which shows a change in the target braking force Fbt in relation to a change in a required driving force Fdreq in the fourth embodiment. As can be seen from the comparison between the solid line and the dashed line in Fig. As understood in section 7, the target braking force Fbt in the case of inter-vehicle distance control is greater than the target braking force Fbt in the case of driver input, and their difference gradually decreases and becomes zero at time t3. Thus, the vehicle's driving force Fd is zero from time t0 to time t3 and becomes a positive value, gradually increasing after time t3. As in the first embodiment, it is Fdown, since the braking force Fb that is actually generated does not exceed the downhill force Fdown itself, even in a section where the target braking force Fbt in the case of inter-vehicle distance control is greater than the downhill force Fdown.

[0090] Consider, for example, a situation where the control gain of the brake force control is reduced to 90, compared to the normal value of 100 in the case of inter-vehicle distance control. It is assumed that the downhill force Fdown is 100 N, and that the braking force Fbreq (not shown), required to hold the vehicle in the stopped state, is 100 N at time t0. However, the actual braking force Fb at time t0 is 90 N due to the reduction in control gain, and vehicle 14 rolls downhill due to the insufficient braking force.

[0091] According to the fourth embodiment, the target braking force Fbt in the case of inter-vehicle distance control is greater by the increase correction amount K4ΔFbtΔFd than the target braking force Fbt in the case of driver operation, and an actual braking force Fb at time t0 is greater than 90N, so that it is possible to reduce the probability that the vehicle 14 may slide down an incline due to insufficient braking force.

[0092] Furthermore, the braking force Fbreq, required to hold the vehicle in a stopped state, gradually decreases as the required driving force Fdreq increases. For example, if the braking force Fbreq decreases to 10 N, the reduction in the actual braking force Fb due to the decrease in control gain is 1 N. That is, the amount of the braking force increase correction needed to compensate for the reduction in the actual braking force Fb also gradually decreases as the required driving force Fdreq increases.

[0093] According to the fourth embodiment, the increase correction amount K4ΔFbtΔFd gradually decreases as the required driving force Fdreq increases. Therefore, in the case of inter-vehicle distance control, it is possible to reduce the probability that the vehicle's start may be excessively delayed if the increase correction amount of the braking force is excessively large, while simultaneously reducing the probability that the vehicle 14 will roll downhill along the incline due to a lack of braking force.

[0094] In particular, according to the fourth embodiment, the increase correction amount K4ΔFbtΔFd becomes zero at time t3, when the required driving force Fdreq equals the downhill force Fdown. Therefore, it is possible to reduce the probability that the vehicle's start might be excessively delayed compared to a case where the increase correction amount is a positive value at time t3.

[0095] In the fourth embodiment described above, the coefficient K4 is a positive constant. However, the distance by which the vehicle 14 descends along an incline due to the lack of braking force increases as the incline angle increases, and thus the absolute value of the vehicle's longitudinal acceleration Gx increases. Therefore, the coefficient K4 can be variably adjusted in accordance with the absolute value of the longitudinal acceleration Gx such that the coefficient K4 increases in the range less than 1 when the absolute value of the vehicle's longitudinal acceleration Gx increases. According to this modification, it is possible to effectively reduce the risk of the vehicle 14 descending the incline, regardless of the magnitude of the incline angle, while simultaneously reducing the probability of the vehicle being excessively decelerated upon starting due to excessive braking force. [Fifth embodiment]

[0096] In the fifth embodiment and the sixth and seventh embodiments, which are described later, the required driving force Fdreq is not estimated in either the case of inter-vehicle distance control or driver input, and the target braking force Fbt is controlled in both cases to decrease in a preset reduction pattern. Therefore, in these embodiments, the braking force reduction control of the hill start assist control is carried out according to the [reference to relevant section]. Fig. The flowchart shown in section 3 was carried out.

[0097] As can be seen from the comparison between Fig. 3 and Fig. As can be understood in section 2, in the fifth to seventh embodiments, steps 10 to 50 and step 100 are performed similarly to the first to fourth embodiments. However, if a positive determination is made in step 30, step 35 is executed. In step 35, a determination is made as to whether a predetermined holding time Δt0 (a positive constant), which is set in advance, has elapsed from the time at which the driving request is generated. If a negative determination is made, the control system proceeds to step 40, and if a positive determination is made, the control system proceeds to step 50. Thus, the braking force applied to the vehicle is maintained until the predetermined holding time Δt0 has elapsed.

[0098] Furthermore, in the fifth to seventh embodiments, steps 60 and 80 from the first to fourth embodiments are not performed. If an affirmative determination is made in step 50, that is, if it is determined that the driving request is generated by the inter-vehicle distance control, the control proceeds to step 75, and if a negative determination is made, the control proceeds to step 95.

[0099] In particular, in step 75 of the fifth embodiment, the target braking force Fbt of vehicle 14 is calculated according to the following equation (6). In equation (6) below and equation (7), which is described later, Fbtf is a target braking force Fbt calculated in the previous cycle, and ΔFbdec is a reduction amount (a positive constant) of the braking force for each cycle. When step 75 is executed for the first time, Fbtf is set to the braking force (the same as the downhill force Fdown) that has been maintained up to that point. Furthermore, in equation (6) below, ΔFbt is an increase correction amount (a positive constant) of the braking force, as in the first embodiment. Fbt=Fbtf−ΔFbdec+ΔFbt

[0100] On the other hand, in step 95 of the fifth embodiment, the target braking force Fbt of the vehicle 14 is calculated according to the following equation (7). In particular, the target braking force Fbt is also calculated according to equation (7) in steps 95 of the sixth and seventh embodiments, which are described later. Fbt=Fbtf−ΔFbdec

[0101] Thus, when the predetermined holding time Δt0 has elapsed, the target braking force Fbt of vehicle 14 is reduced in the first pattern by being calculated according to equation (7) in the case of driver input, and it is reduced in the second pattern by being calculated according to equation (6) in the case of inter-vehicle distance control. Therefore, compared to the cases where the downhill force Fdown is constant, the target braking force Fbt in the case of inter-vehicle distance control is greater than the target braking force Fbt in the case of driver input by the increase correction amount ΔFbt, and the target braking force Fbt in both cases decreases by ΔFbdec in each cycle.

[0102] Fig. Figure 8 is a time diagram showing changes in the target braking force Fbt in the fifth embodiment in the cases of driver input and inter-vehicle distance control. As in Fig. As shown in Figure 8, the reduction of the target braking force Fbt is started at time t0' when a predetermined holding time Δt0 has elapsed since time t0, at which the driving request is generated.

[0103] The target braking force Fbt in the case of inter-vehicle distance control, indicated by the solid line, is also a positive value at time t8, when the target braking force Fbt in the case of driver input, indicated by the dashed line, becomes zero, and it becomes zero at time t9, which is ΔFbt / ΔFbdec later than time t8. As in the first embodiment, the actually generated braking force Fb does not exceed the downhill force Fdown even in a section where the target braking force Fbt in the case of inter-vehicle distance control is greater than the downhill force Fdown, so that it is Fdown.

[0104] Therefore, in the case of inter-vehicle distance control, compared to the case of driver operation, the target braking force Fbt can be increased to increase the braking force Fb applied to the vehicle, and the braking force Fb can be applied to the vehicle for a longer period of time. [Sixth embodiment]

[0105] In step 75 of the sixth embodiment, the target braking force Fbt of the vehicle 14 is calculated according to the following equation (8), where K6 is a positive, constant coefficient less than 1. Fbt=Fbtf−K6ΔFbdec

[0106] Thus, when the predetermined holding time Δt0 has elapsed, the target braking force Fbt of vehicle 14 is reduced in the first pattern by being calculated according to equation (7) in the case of driver input, and it is reduced in the third pattern by being calculated according to equation (8) in the case of inter-vehicle distance control. The rate of reduction of the target braking force Fbt in the third pattern is smaller than the rate of reduction of the target braking force Fbt in the first pattern. Thus, compared to the cases in which the downhill force Fdown is equal, the target braking force Fbt in the case of inter-vehicle distance control is greater than the target braking force Fbt in the case of driver input, and the difference between them gradually increases over time.

[0107] Fig. 9 is a time diagram similar to Fig. 8, which shows changes in the target braking force Fbt in the cases of driver input and inter-vehicle distance control. As in Fig. As shown in Figure 9, in both cases—driver control and inter-vehicle distance control—the reduction of the target braking force Fbt begins at time t0', once the predetermined holding time Δt0 has elapsed. The target braking force Fbt in the case of inter-vehicle distance control, indicated by the solid line, decreases more slowly than the target braking force Fbt in the case of driver control, indicated by the dashed line, and is still a positive value at time t8 when the latter target braking force Fbt becomes zero, and becomes zero at time t10, which is later than time t8.

[0108] Therefore, after the predetermined holding time Δt0 has elapsed from the time at which the required driving force is generated, in the case of inter-vehicle distance control the target braking force Fbt can be made greater than in the case of driver operation in order to increase the braking force Fb applied to the vehicle, and the braking force Fb can be applied to the vehicle for a longer time.

[0109] In the sixth embodiment described above, the coefficient K6 is a positive constant. However, the coefficient K6 can be variably adjusted according to an absolute value of a longitudinal acceleration Gx such that the coefficient K6 decreases in the range less than 1 as the absolute value of the longitudinal acceleration Gx of the vehicle increases, in contrast to the coefficient K2 in the second embodiment described above. According to this modification, it is possible, regardless of the magnitude of the incline, to effectively reduce the risk of the vehicle 14 rolling down the incline, while simultaneously reducing the probability of the vehicle being excessively decelerated from a standstill due to excessive braking force. [Seventh embodiment]

[0110] In step 75 of the seventh embodiment, before a predetermined deceleration time Δt (a positive constant) from time t0' has elapsed, the target braking force Fbt of the vehicle 14 is adjusted to the braking force (the same as the downhill force Fdown) that has been maintained up to that point. The target braking force Fbt is calculated according to equation (7) above after time t0'', at which the predetermined deceleration time Δt from time t0' has elapsed.

[0111] Accordingly, while in the case of driver input the target braking force Fbt of vehicle 14 decreases from the time at which the predetermined holding time Δt0 has elapsed, in the case of inter-vehicle distance control the decrease begins with a delay time Δt after the predetermined holding time Δt0 has elapsed. Therefore, compared to the cases in which the downhill force Fdown is equal to the delay time Δt after the time at which the predetermined holding time Δt0 has elapsed, the target braking force Fbt in the case of inter-vehicle distance control is greater by ΔFbdecΔt than the target braking force Fbt in the case of driver input.

[0112] Fig. 10 is a time diagram similar to Fig. 8, which shows changes in the target braking force Fbt in the cases of driver input and inter-vehicle distance control. As in Fig. As shown in Figure 10, the target braking force Fbt in the case of the inter-vehicle distance control, which is indicated by the solid line, is still a positive value at time t8, at which time the target braking force Fbt in the case of the driver's driving action, which is indicated by the dashed line, becomes zero, and becomes zero at time t11, which is later than time t8 by the deceleration time Δt.

[0113] Therefore, in the case of inter-vehicle distance control, the holding time of the braking force can be increased by Δt compared to the case of driver input. Furthermore, after the deceleration time Δt has elapsed from the time at which the predetermined holding time Δt0 has expired, the target braking force Fbt can be increased to increase the braking force Fb applied to the vehicle, and the braking force Fb can be applied to the vehicle for a longer period of time.

[0114] As can be understood from the descriptions above, according to the fifth to seventh embodiments, in the case of inter-vehicle distance control, it is possible to effectively reduce the probability that the vehicle 14 will roll downhill along an incline, while simultaneously reducing the probability that the driver will experience a feeling of being stuck when performing a driving operation. Since starting the vehicle is not caused by the driver's driving operation, the driver will not experience a feeling of being stuck, even if the vehicle's start is delayed.

[0115] Furthermore, according to the fifth to seventh embodiments, the target braking force Fbt after time t0' is greater in the case of inter-vehicle distance control than the target braking force Fbt in the case of driver input. Therefore, in a situation where the braking force becomes lower than the target braking force Fbt due to a reduction in the gain of the brake force control / regulation or a variation in the coefficient of friction of the brake force generators of the brake device 16, it is possible to reduce the probability that the vehicle 14 will roll down an incline due to a lack of braking force.

[0116] Although the present invention has been described in detail with reference to specific embodiments, it will be obvious to those skilled in the art that the present invention is not limited to the embodiments described above, and various other embodiments are possible within the scope of the present invention.

[0117] For example, in the embodiments described above, the driving assistance control is an inter-vehicle distance control in every vehicle speed range. However, the driving assistance control can be any driving assistance control that automatically controls and drives the vehicle without requiring any control or driving input from the driver, such as in an automatic driving control system.

[0118] For example, in the first to fourth embodiments, a required driving force Fdreq is estimated, and a target braking force Fbt of the vehicle 14 is calculated based on the required driving force Fdreq, with the braking force Fb of the vehicle being gradually reduced according to the required driving force Fdreq. However, a target reduction gradient of the braking force Fb of the vehicle 14 can be calculated based on the required driving force Fdreq and controlled such that a reduction gradient of the braking force Fb becomes the target reduction gradient, whereby the braking force Fb of the vehicle can be gradually reduced.

[0119] In modifications of the first and third embodiments, the vehicle's braking force Fb is reduced by Fdown + ΔFbt at the target reduction gradient. In a modification of the second embodiment, a target reduction gradient is calculated based on the required driving force Fdreq, such that the target reduction gradient is smaller in the case of the drive assistance control than in the case of driver input.

[0120] Furthermore, in the fifth to seventh embodiments, in both cases of the driving assistance control and the driver's driving operation, the target braking force is reduced linearly by a constant reduction amount ΔFbdec in each cycle. However, the reduction amount ΔFbdec can be set variably according to an absolute value of a longitudinal acceleration Gx, so that it decreases when an absolute value of the vehicle's longitudinal acceleration Gx increases. According to this modification, compared to the case in which the reduction amount ΔFbdec is constant, regardless of the magnitude of an incline, it is possible to more effectively reduce the risk of the vehicle rolling downhill along the incline, while simultaneously reducing the probability of the vehicle being excessively decelerated due to excessive braking force during acceleration.

[0121] Furthermore, in the fifth to seventh embodiments, the target braking force Fbt can be gradually reduced in a non-linear manner, and the reduction gradient of the target braking force Fbt in the case of the driving assistance control can differ from the reduction gradient in the case of driver input. In these modifications as well, the target reduction gradient in the case of the driving assistance control is preferably smaller than the target reduction gradient in the case of driver input.

[0122] Furthermore, in the embodiments described above, if an affirmative determination is made in step 20, the control proceeds to step 50. However, if an affirmative determination is made in step 20, a determination can be made as to whether a driving request is generated as in step 30 or not, and if an affirmative determination is made, the control can proceed to step 50; however, if a negative determination is made, the control can terminate.

[0123] Furthermore, in the embodiments described above, the target braking forces Fbt are calculated according to equations (1) to (8) above. However, as long as the target braking force Fbt is calculated as in the Fig. 4 to 10, where the reduction is shown, the calculation of the target braking force Fbt is carried out according to equations which are different from the above equations (1) to (8).

[0124] Furthermore, the first to third embodiments can be implemented in any combination, and the fifth to seventh embodiments can be implemented in any combination.

[0125] Furthermore, although in the embodiments described above the braking device is a hydraulic braking device 16, the braking device can be an electromagnetic braking device as long as a braking force applied to the vehicle can be controlled by controlling a braking force applied to each wheel.< / bremskraftverringerungssteuerungsroutine>

Claims

[1] Brake force control unit (10) for a vehicle, comprising a brake device (16) for applying a braking force (Fbt) to a vehicle (14) and a control unit (18) for controlling the brake device, wherein the control unit is configured to perform a brake force holding control for controlling the brake device to maintain a braking force applied to the vehicle when the vehicle is stopped on an incline in the uphill direction, and to perform a brake force reduction control for controlling the brake device so that the braking force is gradually reduced when it is determined that a driving request to drive the vehicle is generated during an execution of the brake force holding control, characterized by , that the vehicle (14) has a driving assistance device (32) which is configured to perform driving assistance control which automatically brakes and drives the vehicle (14) without requiring any braking / driving action by the driver, wherein the control unit (18) is configured to control the braking device (16) in such a way that the braking force during an execution of the brake force reduction control is greater when a driving request is generated by the driving assistance control than when a driving request is generated by the driving operation of the driver. [2] Brake force control unit (10) for a vehicle according to claim 1, characterized by, that the vehicle (14) has a required driving force estimation device (46 and step 60) which is configured to estimate a required driving force (Fdreq) of the driving request in both cases, i.e., when the driving request is generated by the driving assistance control and when the driving request is generated by a driving action of the driver, and wherein the control unit (18) is configured to calculate a target braking force (Fbt) which gradually decreases in accordance with an increase in the estimated required driving force, and to control the braking device (16) such that a braking force (Fb) which is applied to the vehicle (14) becomes the target braking force (Fbt). [3] Brake force control unit (10) for a vehicle according to claim 1, characterized by, that the vehicle (14) has an estimating device for a downslope force (36 and step 30) which is configured to estimate a downslope force (Fdown) which causes the vehicle to descend along the slope due to gravity, and wherein the control unit (18) is configured to calculate a target braking force (Fbt) such that the target braking force at the time at which a quantity of the required driving force becomes equal to the downslope force is greater when the driving request is generated by the driving assistance control than when the driving request is generated by the driving operation of the driver. [4] Brake force control unit (10) for a vehicle according to claim 3, characterized by, that the control unit (18) is configured to calculate the target braking force (Fbt) when the driving request is generated by the driving assistance control to a value which is greater than the target braking force when the driving request is generated by the driving operation of the driver. [5] Brake force control unit (10) for a vehicle according to claim 4, characterized by , that the control unit (18) is configured to calculate a target braking force (Fbt) when the driving assistance control is being carried out, so that a difference between a target braking force when the driving request is generated by the driving assistance control and a target braking force when the driving request is generated by the driving action of the driver gradually decreases. [6] Brake force control unit (10) for a vehicle according to claim 3, characterized by, that the control unit (18) is configured to calculate a target braking force (Fbt) such that a reduction rate of the target braking force when the driving request is generated by the driving assistance control is less than a reduction rate of the target braking force when the driving request is generated by the driving operation of the driver. [7] Brake force control unit (10) for a vehicle according to claim 3, characterized by , that the control unit (18) is configured to calculate a target braking force (Fbt) such that a target braking force when the driving request is generated by the driving assistance control is delayed with respect to a target braking force when the driving request is generated by the driving operation of the driver. [8] Brake force control unit (10) for a vehicle according to claim 1, characterized by, that the control unit (18) is configured to calculate a target braking force (Fbt) which gradually decreases in a preset first pattern when the driving request is generated by the driver's driving action, to calculate a target braking force (Fbt) which gradually decreases in a preset second pattern to be greater than a target braking force calculated in the first pattern when the driving request is generated by the driving assistance control, and to control the braking device (16) such that a braking force (Fb) applied to the vehicle (14) becomes the target braking force (Fbt).

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