AU2018202517A1 - Lane departure prevention device and lane departure prevention system - Google Patents

Abstract

LANE DEPARTURE PREVENTION DEVICE AND LANE DEPARTURE PREVENTION SYSTEM A lane departure prevention device mounted on a vehicle (1) including a braking device configured to apply a braking force to wheels includes: a vehicle control unit (172) configured to control the braking device such that a yawing moment in a direction of avoiding a departure of the vehicle (1) from a traveling lane of the vehicle (1) is applied when the vehicle (1) is predicted to depart from the traveling lane in which the vehicle (1) is currently traveling; and a calculation unit configured (173) to calculate an index value related to a variation in yaw rates of the vehicle (1) that is traveling, wherein the vehicle control unit (172) is configured to control the braking device such that the braking force becomes smaller when the index value is equal to or greater than a predetermined threshold value than when the index value is smaller than the predetermined threshold value. 121L 142 141 121 FR 122FL 122FR ACTUATOR 113RR 122RL 113FL 113RL 172 i l121 RR 121RL- 161 172 --- | LDA VEHICLE SPEKER173 WHEEL SPEAKER DATA S-- SPEED 152 v ACQUISITION SENSOR BAD ROAD UNIT 162 DETERMINA- - .. YAWRATEL 1 5 3 155 TION UNIT - SENSORS CAMERA ---------------------------- > .- ACCELERATION ~154 ECU SENSOR

Description

LANE DEPARTURE PREVENTION DEVICE AND LANE DEPARTURE PREVENTION SYSTEM

BACKGROUND OF THE INVENTION

Field of the Invention [0001] The present invention relates to the technical field of a lane departure prevention device and a lane departure prevention system.

Description of Related Art [0002] As a lane departure prevention device, there is a known lane departure prevention device that, when there is a possibility that the vehicle will depart from the traveling lane, controls the braking force applied to the wheels so that a yawing moment, large enough to prevent the vehicle from departing from the traveling lane, is applied to the vehicle.

[0003] However, it is sometimes preferable to temporarily stop the control for preventing the lane departure described above (hereinafter referred to as "lane departure prevention control" as necessary), for example, when it is estimated that the passenger does not desire to perform the lane departure prevention control. To address this problem, Japanese Patent Application Publication No. 2011-168194 (JP 2011-168194 A) proposes a technology for stopping the lane departure prevention control when the steering torque is larger than the threshold value.

SUMMARY OF THE INVENTION

[0004] The lane departure prevention control is performed by applying a braking force to the vehicle. This means that the lane departure prevention control, if performed when traveling on a bad road, may sometimes result in an unstable vehicle behavior. Therefore, when the vehicle is traveling on a bad road, it is preferable that the lane departure prevention control be automatically stopped regardless of the driver's intention.

[0005] Whether the traveling lane of the vehicle is a bad road can be determined using, for example, the slip rate of the vehicle. However, on a bad road such as a dirt road, the slip rate cannot be detected accurately because the wheels are not always in contact with the ground. In this case, it becomes difficult to determine whether the road is a bad road.

[0006] If whether the road is a bad road cannot be determined accurately, the lane departure prevention control cannot be stopped in an appropriate timing in some cases. This may lead to a situation where the lane departure prevention control is performed on a bad road with the result that the vehicle behavior becomes unstable.

[0007] The present invention provides a lane departure prevention device and a lane departure prevention system that can accurately determine whether the road is a bad road and, based on the determination, appropriately performs the lane departure prevention control.

[0008] A first aspect of the invention provides a lane departure prevention device that is mounted on a vehicle including a braking device configured to apply a braking force to wheels of the vehicle. The lane departure prevention device includes: a vehicle control unit configured to control the braking device such that a yawing moment in a direction of avoiding a departure of the vehicle from a traveling lane of the vehicle is applied when the vehicle is predicted to depart from the traveling lane in which the vehicle is currently traveling; and a calculation unit configured to calculate an index value related to a variation in yaw rates of the vehicle that is traveling. The vehicle control unit is configured to control the braking device such that the braking force becomes smaller when the index value is equal to or greater than a predetermined threshold value than when the index value is smaller than the predetermined threshold value.

[0009] According to the lane departure prevention device of the present invention, the braking force applied to the vehicle by the braking device is made relatively smaller when the index value related to the variation in the yaw rates of the vehicle that is traveling is equal to or greater than the predetermined threshold value. Note that the variation in the yaw rates is a value that becomes greater as the condition of the road surface on which the vehicle is traveling is worse (that is, as the road is worse). Thus, even in a situation where the wheels do not always contact the ground, for example, on a dirt road, the road surface condition is correctly reflected. The "predetermined threshold value" is set in advance as a threshold value that can determine whether the road surface condition is so bad that the behavior of the vehicle becomes unstable by applying the braking force. Therefore, the lane departure prevention device of the present invention can accurately determine whether the vehicle is traveling on a bad road, based on the relationship between the index value related to the variation in the yaw rates and the predetermined threshold value.

[0010] As described above, the braking force is made smaller according to the index value related to the variation in the yaw rate. By doing so, a large braking force is not applied to the vehicle in the case in which there is a possibility that the vehicle is traveling on a bad road as compared to the case in which the vehicle is not traveling on a bad road. Therefore, the lane departure prevention device of the present invention prevents the behavior of the vehicle traveling on a bad road from becoming unstable due to the application of the braking force.

[0011] In the first aspect, the calculation unit may be configured to calculate a root mean square of change rates of the yaw rates of the vehicle as the index value.

[0012] According to this aspect, the index value can be suitably used to determine whether the vehicle is traveling on a bad road. Note that the root mean square of the change rates of the yaw rate is calculated as a value that is larger as the variation in the yaw rates becomes larger.

[0013] In the first aspect, the vehicle control unit may be configured to control the braking device not to apply the braking force when the index value is equal to or greater than the predetermined threshold value continuously for a predetermined period or longer.

[0014] According to this aspect, the application of the braking force to the vehicle is stopped when the state in which the vehicle is determined to be traveling on a bad road continues for a predetermined period or longer. The "predetermined period" is set as a threshold value for determining that there is an extremely high possibility that the vehicle is traveling on a bad road or as a threshold for determining that the road surface condition is so bad that a small braking force, if applied, will cause the behavior of the vehicle to become unstable. Therefore, according to this aspect, the lane departure prevention device makes it possible to suitably prevent the behavior of the vehicle traveling on a bad road from becoming unstable due to the application of a braking force.

[0015] A second aspect provides a lane departure prevention system. The lane departure prevention system includes a braking device configured to apply a braking force to wheels of a vehicle and at least one electronic control unit mounted on the vehicle. The at least one electronic control unit is configured to: predict a possibility that the vehicle will depart from a traveling lane in which the vehicle is currently traveling; control the braking device such that a yawing moment is applied to the vehicle, the yawing moment being in a direction of avoiding a departure of the vehicle from the traveling lane, when the vehicle is predicted to depart from the traveling lane; calculate an index value related to a variation in yaw rates of the vehicle that is traveling; and control the braking device such that the braking force becomes smaller when the index value is equal to or greater than a predetermined threshold value than when the index value is smaller than the predetermined threshold value.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: [0017] FIG. 1 is a block diagram showing a configuration of a vehicle according to a first embodiment; [0018] FIG. 2 is a flowchart showing a flow of the lane departure prevention operation according to the first embodiment; [0019] FIG. 3 is a flowchart showing a flow of the BLDA control reduction operation according to the first embodiment; [0020] FIG. 4 is a timing diagram showing the RMS value of yaw rates when a vehicle travels on an ordinary road; [0021] FIG. 5 is a timing diagram showing the RMS value of yaw rates when a vehicle travels on a bad road; [0022] FIG. 6 is a timing diagram showing a period during which the BLDA reduction is performed; [0023] FIG. 7 is a timing diagram showing a control amount of the normal BLDA control; [0024] FIG. 8 is a timing diagram showing a control amount when the BLDA control is reduced; [0025] FIG. 9 is a timing diagram showing a control amount when the BLDA control is prohibited; [0026] FIG. 10 is a timing diagram showing a control amount when the BLDA control is reduced from the middle; [0027] FIG. 11 is a flowchart showing a flow of the BLDA control reduction operation according to a second embodiment; and [0028] FIG. 12 is a timing diagram showing a period during which the BLDA control is reduced and prohibited.

DETAILED DESCRIPTION OF EMBODIMENTS

[0029] A lane departure prevention device according to this embodiment will be described in detail below with reference to the drawings. Note that a vehicle 1 in the description below includes a lane departure prevention device according to this embodiment.

First embodiment [0030] A lane departure prevention device according to a first embodiment will be described with reference to FIG. 1 to FIG. 10.

Vehicle configuration [0031] First, a configuration of the vehicle 1 according to this embodiment will be described with reference to FIG. 1. FIG. 1 is a block diagram showing a configuration of the vehicle according to the first embodiment.

[0032] As shown in FIG. 1, the vehicle 1 includes a brake pedal 111, a master cylinder 112, a brake pipe 113FL, a brake pipe 113RL, a brake pipe 113FR, a brake pipe 113RR, a left front wheel 121FL, a left rear wheel 121RL, a right front wheel 121FR, a right rear wheel 121RR, a wheel cylinder 122FL, a wheel cylinder 122RL, a wheel cylinder 122FR, a wheel cylinder 122RR, a brake actuator 131, a steering wheel 141, a vibration actuator 142, a vehicle speed sensor 151, a wheel speed sensor 152, a yaw rate sensor 153, an acceleration sensor 154, a camera 155, a display 161, a speaker 162, and an Electronic Control Unit (ECU) 17.

[0033] The brake pedal 111 is a pedal depressed by the driver to brake the vehicle 1. The master cylinder 112 adjusts the pressure of the brake fluid (or any fluid) in the master cylinder 112 to the pressure corresponding to the depression amount of the brake pedal 111. In the description below, the pressure of the brake fluid is sometimes simply referred to as "hydraulic pressure".

[0034] The hydraulic pressure in the master cylinder 112 is transmitted to the wheel cylinders 122FL, 122RL, 122FRand 122RR via the brake pipes 113FL, 113RL, 113FR and 113RR, respectively. Therefore, the braking forces corresponding to the hydraulic pressures, transmitted to the wheel cylinders 122FL, 122RL, 122FR and 122RR, are applied respectively to the left front wheel 121FL, the left rear wheel 121RL, the right front wheel 121FR and the right rear wheel 121RR.

[0035] The brake actuator 131, which operates under control of the ECU 17, can adjust the hydraulic pressure transmitted to each of the wheel cylinders 122FL, 122RL, 122FR, and 122RR irrespective of the depression amount of the brake pedal 111. Therefore, irrespective of the depression amount of the brake pedal 111, the brake actuator 131 can adjust the braking force applied to each of the left front wheel 121FL, left rear wheel 121RL, right front wheel 121FR and right rear wheel 121RR. The brake actuator 131 is one example of the "braking device".

[0036] The steering wheel 141 is a device operated by the driver to steer the vehicle 1 (that is, steer the steered wheels). In this embodiment, the steered wheels are the left front wheel 121FL and the right front wheel 121FR. The vibration actuator 142, which operates under control of the ECU 17, can vibrate the steering wheel 141.

[0037] The vehicle speed sensor 151 detects the vehicle speed Vv of the vehicle 1. The wheel speed sensor 152 detects the wheel speed Vw of each of the left front wheel 121FL, the left rear wheel 121RL, the right front wheel 121FR and the right rear wheel 121RR. The yaw rate sensor 153 detects the yaw rate γ of the vehicle 1. The acceleration sensor 154 detects the acceleration G (more specifically, longitudinal acceleration Gx and lateral acceleration Gy) of the vehicle 1.

The camera 155 is a capturing device for capturing the external situation ahead of the vehicle 1. The detection data indicating the detection results of the sensors, from the vehicle speed sensor 151 to the acceleration sensor 154, and image data indicating the image captured by the camera 155 are output to the ECU 17.

[0038] The display 161, which operates under control of the ECU 17, can display arbitrary information. The speaker 162, which operates under control of the ECU 17, can output arbitrary sound.

[0039] The ECU 17 is configured as a control unit that controls the overall operation of the vehicle 1. In particular, the ECU 17 in this embodiment performs the lane departure prevention operation to prevent the vehicle 1 from departing from the currently traveling lane. That is, the ECU 17 functions as a control device for implementing the so-called Lane Departure Alert (LDA) or Lane Departure Prevention (LDP). The specific processing of the lane departure prevention operation will be described in detail later.

[0040] To perform the lane departure prevention operation, the ECU 17 includes a data acquisition unit 171, an LDA control unit 172, and a bad road determination unit 173 that are logically implemented processing blocks included in the ECU 17. The data acquisition unit 171 acquires data, which indicates the state of the vehicle, from the various sensors, 151 to 154, and the camera 155. The LDA control unit 172 performs the lane departure prevention operation.

The bad road determination unit 173 performs determination processing that determines the execution mode of the lane departure prevention operation. The LDA control unit 172 is one example of the "vehicle control unit", and the bad road determination unit 173 is one example of the "calculation unit".

Lane departure prevention operation [0041] Next, the lane departure prevention operation, performed by the ECU 17, will be described in detail below with reference to FIG. 2. FIG. 2 is a flowchart showing a flow of the lane departure prevention operation according to the first embodiment.

[0042] In FIG. 2, the data acquisition unit 171 acquires the detection data, which indicates the detection result of each of the vehicle speed sensor 151, the wheel speed sensor 152, the yaw rate sensor 153, and the acceleration sensor 154, as well as the image data which indicates an image captured by the camera 155 (step S101).

[0043] The LDA control unit 172 analyzes the image data acquired in the processing in step S101 and, based on the analyzed result, identifies the lane end of the traveling lane, in which the vehicle 1 is currently traveling, in the image captured by the camera 155 (step SI02) (in this embodiment, one example of the lane end is a "white line"). An existing technology can be used for the white line identification method and, therefore, the detailed description of the method will be omitted.

[0044] The LDA control unit 172 determines whether the traveling lane, in which the vehicle 1 is currently traveling, is a straight lane or a curved lane based on the white line identified in the processing in step SI 02. If it is determined that the traveling lane is a curved lane, the LDA control unit 172 calculates the curvature radius of the traveling lane (step SI 03). The curvature radius of the traveling lane is substantially equivalent to the curvature radius of the white line. Therefore, the LDA control unit 172 calculates the curvature radius of the white line, identified in the processing in step SI02, for using the calculated curvature radius as the curvature radius of the traveling lane.

[0045] In addition, the LDA control unit 172 calculates the current lateral position, lateral speed, and departure angle of the vehicle 1 (step SI 04), based on the white line identified in the processing in step SI02. The "lateral position" in this case means the distance from the center of the traveling lane to the vehicle 1 (typically, the distance to the center of the vehicle 1) along the lane width direction orthogonal to the traveling-lane extending direction (lane extending direction). The "lateral speed" means the speed of the vehicle 1 along the lane width direction. The "departure angle" means the angle formed between the traveling lane and the longitudinal axis of the vehicle 1 (that is, the angle formed between the white line and the longitudinal axis of the vehicle 1).

[0046] In addition, the LDA control unit 172 sets an allowable departure distance (step SI05). The allowable departure distance indicates the maximum allowable value of the departure distance of the vehicle 1 from the traveling lane (that is, the departure distance of the vehicle 1 from the white line) when the vehicle 1 departs from the traveling lane.

[0047] For example, the allowable departure distance may be set as follows. That is, the LDA control unit 172 may set the allowable departure distance from the viewpoint of satisfying the law and regulation requirements (for example, the request made by NCAP: New Car Assessment Program). The allowable departure distance may be set by any other method.

[0048] After that, the LDA control unit 172 determines whether there is a possibility that the vehicle 1 will depart from the currently traveling lane (i.e., whether the vehicle 1 is going to depart from the traveling lane) (step SI06). For example, the LDA control unit 172 calculates the future position of the vehicle 1 (for example, the position after several hundred milliseconds to several seconds have elapsed) based on the current speed, lateral position, and lateral speed of the vehicle 1. Then, the LDA control unit 172 compares the future position of the vehicle 1 with the center of the traveling lane to calculate the departure amount of the vehicle 1. An example of the departure amount is the amount of deviation in the lane width direction from the center of the traveling lane to the future position of the vehicle 1. The LDA control unit 172 then determines whether the departure amount of the vehicle 1 is larger than the departure determination value. If it is determined that the departure amount of the vehicle 1 is larger than the departure determination value (for example, if the vehicle 1 will extend across, or travel on, the white line in the future position), the LDA control unit 172 determines that there is a possibility that vehicle 1 will depart from the traveling lane.

[0049] If it is determined in step SI06 that there is no possibility that the vehicle 1 will depart from the traveling lane (step SI06: No), the lane departure prevention operation shown in FIG. 2 is terminated. After that, the LDA control unit 172 starts the lane departure prevention operation, shown in FIG. 2, again after the first predetermined period (for example, several milliseconds to several tens of milliseconds) has elapsed. That is, the lane departure prevention operation shown in FIG. 2 is repeated at periodic intervals corresponding to the first predetermined period.

[0050] On the other hand, if it is determined in step SI 06 that there is a possibility that the vehicle 1 will depart from the traveling lane (step SI06: Yes), the LDA control unit 172 warns the driver of the vehicle 1 that there is a possibility that the vehicle 1 will depart from the traveling lane (step SI07). More specifically, the LDA control unit 172 causes the display 16 to display an image indicating that there is a possibility that the vehicle 1 will depart from the traveling lane and/or causes the vibration actuator 142 to vibrate the steering wheel 141 to inform the driver that there is a possibility that the vehicle 1 will depart from the traveling lane.

[0051] In parallel with the processing in step SI07 described above, the LDA control unit 172 performs the BLDA (Brake-LDA) control (steps S108 to SI 11). At this time, the LDA control unit 172 turns on the BLDA control flag. The BLDA control is a control operation that applies a yawing moment to the vehicle 1 in the departure-avoiding direction so that the departure distance of the vehicle 1 from the traveling lane falls within the allowable departure distance.

[0052] In the BLDA control according to this embodiment, braking force is applied to at least one of the left front wheel 121FL, the left rear wheel 121RL, the right front wheel 121FR, and the right rear wheel 121RR so that a braking force difference is generated between the left wheels and the right wheels. As a result, a yawing moment in the departure-avoiding direction is applied to the vehicle 1. The BLDA control will be described below in more detail.

[0053] The LDA control unit 172 calculates a target yaw rate so that the vehicle 1 traveling away from the center of the traveling lane can travel along the target trajectory (that is, the target traveling line) toward the center of the traveling lane (step SI 08).

[0054] Next, the LDA control unit 172 calculates a yawing moment, which will be applied to the vehicle 1 for generating the target yaw rate in the vehicle 1, as the target yawing moment (step SI09). For example, the LDA control unit 172 may calculate the target yawing moment by converting the target yaw rate to the target yawing moment based on a predetermined conversion function.

[0055] Next, the LDA control unit 172 calculates the braking force that can achieve the target yawing moment. At this time, the LDA control unit 172 individually calculates the braking forces to be applied respectively to the left front wheel 121FL, the left rear wheel 121RL, the right front wheel 121FR, and the right rear wheel 121RR.

[0056] In addition, the LDA control unit 172 calculates a pressure command value for designating a hydraulic pressure necessary for generating the braking force calculated in step SI09 (step SI 10). At this time, the LDA control unit 172 individually calculates the pressure command value for designating the hydraulic pressures in each of the wheel cylinders 122FL, 122RL, 122FR and 122RR.

[0057] Next, the LDA control unit 172 controls the brake actuator 131 based on the pressure command values (step SI 11). As a result, the braking force corresponding to the pressure command value is applied to at least one of the left front wheel 121FL, the left rear wheel 121RL, the right front wheel 121FR, and the right rear wheel 121RR. That is, the braking force difference between the left wheels and the right wheels applies a yawing moment to the vehicle 1 in the direction to avoid departure.

[0058] After that, the LDA control unit 172 starts the lane departure prevention operation, shown in FIG. 2, again after the first predetermined period has elapsed. At this time, since the BLDA control flag is on, the lane departure prevention operation is started while still applying the yawing moment, caused by the BLDA control, to the vehicle 1. If it is determined in the determination in step SI06, which is performed again, that there is a possibility that the vehicle 1 will depart from the traveling lane (step SI06: Yes), the processing from step SI07 and the following steps is performed and, therefore, a yawing moment, which is caused by the BLDA control, is continuously applied to the vehicle 1. On the other hand, if it is determined in the determination in step SI06, which is performed again, that there is no possibility that the vehicle 1 will depart from the traveling lane (step SI 06: No), the BLDA control flag is turned off and, at the same time, the application of the yawing moment to the vehicle 1, which is caused by the BLDA control, is terminated. BLDA control reduction operation [0059] Next, the BLDA control reduction operation, which reduces or prohibits the BLDA control according to the situation, will be described below with reference to FIG. 3. FIG. 3 is a flowchart showing a flow of the BLDA control reduction operation according to the first embodiment. Note that the BLDA control reduction operation is performed in parallel with the lane departure prevention operation shown in the flowchart in FIG. 2.

[0060] In FIG. 3, when the BLDA control reduction operation is started, the bad road determination unit 173 first calculates the slip rate of the vehicle 1 using the data acquired by the data acquisition unit 171 (step S201). The slip rate can be calculated, for example, from the vehicle speed Vv, detected by the vehicle speed sensor 151, and the wheel speed Vw detected by the wheel speed sensor 152.

[0061] Next, the bad road determination unit 173 determines whether or not the slip rate, calculated in step S201, is equal to or greater than the first threshold value (step S202). The "first threshold value" is a value pre-set as a threshold value for determining whether the road on which the vehicle 1 is traveling is a bad road. If it is determined that the slip rate is equal to or greater than the first threshold value (step S202: Yes), the LDA control unit 172 turns on the BLDA reduction flag (step S203) because it is determined that the vehicle 1 is traveling on a bad road.

[0062] The BLDA reduction flag is a flag for determining whether to reduce the control amount calculated by the BLDA control (in other words, the braking force to be applied by the BLDA control) in steps SI 08 to SI 11 in FIG. 2. If the BLDA reduction flag is ON, the control amount calculated by the BLDA control is made smaller than when the BLDA control is performed at a normal time (that is, when the BLDA flag is OFF).

[0063] On the other hand, if it is determined that the slip rate is smaller than the first threshold value (step S202: No), the bad road determination unit 173 calculates the RMS (Root Mean Square) value of the change rates of the yaw rate γ of the vehicle 1 using the data acquired by the data acquisition unit 171 (step S204). In the description below, the value calculated in this step is referred to as "yaw rate RMS" as necessary. The yaw rate RMS, one example of "index value", is calculated as a value related to the variation of the yaw rates γ of the vehicle 1. The yaw rate RMS can be calculated using the yaw rate γ detected by the yaw rate sensor 153.

Instead of the yaw rate RMS, another value related to the variation of yaw rates may also be calculated.

[0064] Next, the bad road determination unit 173 determines whether or not the calculated yaw rate RMS is equal to or greater than the second threshold value (step S205). The "second threshold value" is a value pre-set as a threshold value for determining whether the road on which the vehicle 1 is traveling is a bad road. The relationship between the yaw rate RMS and the road surface condition will be described below with reference to FIG. 4 and FIG. 5. FIG. 4 is a timing diagram showing the RMS value of yaw rates when the vehicle travels on an ordinary road. FIG. 5 is a timing diagram showing the RMS value of yaw rates when the vehicle travels on a bad road.

[0065] As shown in FIG. 4 and FIG. 5, the yaw rate RMS is stabilized at a relatively low value when the vehicle travels on an ordinary road (that is, a road other than a bad road). On the other hand, the yaw rate RMS becomes relatively high when the vehicle travels on a bad road (for example, a dirt road surface). Therefore, if an appropriate second threshold value is set in advance, it can be determined whether the vehicle 1 is traveling on a bad road depending on whether the yaw rate RMS is a value equal to or greater than the second threshold value. As already described, whether the road is a bad road can be determined also based on the slip rate of the vehicle 1 (see step S202). However, on a bad road such as a dirt road surface, the wheels 121FL, 121RL, 121FR, and 121RR of the vehicle 1 do not always contact the ground and, therefore, it is difficult to accurately determine whether the road is a bad road based on the slip rate. On the other hand, the yaw rate RMS is a parameter independent of the grounding state of the wheels 121FL, 121RL, 121FR, and 121RR of the vehicle 1. This means that, even when whether or not the road is a bad road cannot be determined based on the slip rate, the yaw rate RMS can be suitably used to determine whether the road is a bad road.

[0066] Returning to FIG. 3, if it is determined that the yaw rate RMS is equal to or greater than the second threshold value (step S205: Yes), it can be determined that the vehicle 1 is traveling on a bad road and, therefore, the LDA control unit 172 turns on the BLDA reduction flag (step S203). On the other hand, if it is determined that the yaw rate RMS is smaller than the second threshold value (step S205: No), it can be determined that the vehicle 1 is not traveling on a bad road and, therefore, the LDA control unit 172 turns off the BLDA reduction flag (step S206).

[0067] As described above, the BLDA control reduction operation determines whether the vehicle 1 is traveling on a bad road and, according to the determination result, reduces the BLDA control. More specifically, if it is determined in step SI 06 in FIG. 2 that there is a possibility of lane departure (step SI 06: Yes), the LDA control unit 172 references the ON/OFF state of the BLDA reduction flag (in parallel with the processing in SI 07). If the BLDA reduction flag is ON, the LDA control unit 172 calculates the hydraulic pressure in the processing in SI 10 in such a way that the control amount of the BLDA control becomes smaller than usual. As a result, the braking force applied to the vehicle 1 by the BLDA control becomes relatively small. On the other hand, if the BLDA reduction flag is OFF, the LDA control unit 172 calculates the hydraulic pressure in the processing in SI 10 in such a way that the normal braking force is applied to the vehicle 1 by the BLDA control. Thus, calculating the hydraulic pressure in this way prevents the behavior of the vehicle 1 from becoming unstable due to the application of a relatively large braking force to the vehicle 1 when the vehicle travels on a bad road.

Example of specific operation [0068] Next, a specific operation when the BLDA control is reduced by the above-described BLDA control reduction operation (in particular, the operation in which the yaw rate RMS is used to determine whether the vehicle is traveling on a bad road) will be described with reference to FIG. 6. FIG. 6 is a timing diagram showing a period during which BLDA reduction is performed.

[0069] If the yaw rate RMS is equal to or greater than the second threshold value as shown in FIG. 6, it is determined that the vehicle 1 is traveling on a bad road. From that point in time, the control amount of the BLDA control is reduced (in the description below, this control is referred to as "BLDA reduction" as necessary). Note that the BLDA reduction is performed not only during the period in which the yaw rate RMS is equal to or greater than the second threshold value and but is performed continuously until the reduction continuation time Tl elapses from the time when it is determined that the yaw rate RMS is equal to or greater than the second threshold value. Such an operation is implemented by repeatedly performing the flow, such as the one shown in FIG. 3, at each reduction continuation time Tl.

[0070] Next, a change in the control amount during the BLDA control reduction operation will be described in more detail with reference to FIG. 7 to FIG. 10. FIG. 7 is a timing diagram showing a control amount of the BLDA control. FIG. 8 is a timing diagram showing a control amount when the BLDA control is reduced. FIG. 9 is a timing diagram showing a control amount when the BLDA control is prohibited. FIG. 10 is a timing diagram showing a control amount when the BLDA control is reduced from the middle.

[0071] As shown in FIG. 7, when the normal BLDA control is performed, the braking force is applied to each of the wheels 121FL, 121RL, 121FR, and 121RR of the vehicle 1 so that the yawing moment in the direction of preventing the lane departure of the vehicle 1 is generated. More specifically, the control amount of the BLDA control is gradually increased from the time when it is determined that there is a possibility that the vehicle 1 will depart from the lane, and is gradually decreased from the time when it is determined that the vehicle 1 has avoided departure from the lane.

[0072] As shown in FIG. 8, when the BLDA control is reduced (that is, when it is determined that the vehicle is traveling on a bad road), the control amount of the BLDA control (see the solid line in the figure) is made smaller than when the normal BLDA control (see the broken line in the figure) is performed. Controlling the control amount in this way prevents a relatively large braking force from being applied to the vehicle 1 when the vehicle is traveling on a bad road. Therefore, controlling the control amount in this way prevents the behavior of the vehicle 1 traveling on a bad road from becoming unstable due to the application of braking force.

[0073] As shown in FIG. 9, when the BLDA reduction flag is ON, the control amount may be reduced to zero (in the description below, this control is referred to as "BLDA prohibition" as necessary). During the BLDA prohibition, the braking force is not applied to the vehicle 1 by the BLDA control. Therefore, controlling the control amount in this way reliably avoids the situation in which the behavior of the vehicle 1 traveling on a bad road becomes unstable due to the application of braking force.

[0074] As shown in FIG. 10, when the BLDA reduction flag is turned on while the BLDA control is performed, the control amount may be reduced from that point. Controlling the control amount in this way makes it possible to prevent the behavior of the vehicle 1 traveling on a bad road from becoming unstable due to the continued application of a relatively large braking force.

[0075] As described above, even when it is difficult to determine, using the slip rate, whether the vehicle is traveling on a bad road, the lane departure prevention device according to the first embodiment uses the yaw rate RMS to determine whether the vehicle is traveling on a bad road. This allows the BLDA control to be reduced or prohibited in an appropriate timing, suitably preventing the behavior of the vehicle 1 from becoming unstable due to the BLDA control.

Second Embodiment [0076] Next, a lane departure prevention device according to a second embodiment will be described with reference to FIG. 11 and FIG. 12. Note that the second embodiment differs from the above-described first embodiment only in a part of the operations and that the other operations and device configurations are substantially the same as those of the first embodiment. Therefore, the parts different from those in the first embodiment will be described below in detail and the duplicated description of the other parts will be omitted. BLDA control reduction operation [0077] The BLDA control reduction operation according to the second embodiment will be described with reference to FIG. 11. FIG. 11 is a flowchart showing the flow of the BLDA control reduction operation according to the second embodiment. In FIG. 11, the same reference numeral is assigned to the same processing as the processing shown in FIG. 3 (that is, the processing of the BLDA control reduction operation according to the first embodiment).

[0078] In FIG. 11, when the BLDA control reduction operation according to the second embodiment is performed, the slip rate and the yaw rate RMS are used, as in the first embodiment, to determine whether the vehicle 1 is traveling on a bad road (step S202, step S205). If it is determined that the vehicle is traveling on a bad road (step S202: Yes, or step S205: Yes), the BLDA reduction flag is turned on (step S203). However, in the second embodiment, if the BLDA reduction flag is ON, the BLDA reduction (see FIG. 8) is performed but the BLDA prohibition (see FIG. 9) is not performed.

[0079] When the BLDA reduction flag is turned on, the LDA control unit 172 determines whether a predetermined period has elapsed since the last time the BLDA reduction flag was turned on (step S301). That is, the LDA control unit 172 determines whether the BLDA reduction flag has been turned on continuously for a predetermined period or longer. The "predetermined period" is set as a threshold value for determining that there is an extremely high possibility that the vehicle 1 is traveling on a bad road or as a threshold for determining that the road surface condition is so bad that a small braking force, if applied, will cause the behavior of the vehicle 1 to become unstable.

[0080] If the predetermined period has elapsed since the BLDA reduction flag was turned on (step S301: Yes), the LDA control unit 172 turns on the BLDA prohibition flag (step S302). On the other hand, if the predetermined period has not elapsed since the BLDA reduction flag was turned on (step S301: No), the LDA control unit 172 turns off the BLDA prohibition flag (step S303).

[0081] The BLDA prohibition flag is a flag for determining whether to perform the BLDA prohibition. Therefore, if the BLDA prohibition flag is ON, the control amount of the BLDA control becomes zero, meaning that no braking force is applied to the vehicle 1. On the other hand, if the BLDA prohibition flag is OFF, the BLDA control or the BLDA reduction is performed according to the BLDA reduction flag. Note that the BLDA prohibition flag, together with the BLDA reduction flag, is turned off (step S304) also if it is determined that the yaw rate RMS is smaller than the second threshold value (step S205: No).

Example of specific operation [0082] Next, a specific operation performed when the BLDA control is reduced by the BLDA control reduction operation according to the second embodiment will be described below with reference to FIG. 12. FIG. 12 is a timing diagram showing a period during which the BLDA control is reduced and prohibited.

[0083] As shown in FIG. 12, when the yaw rate RMS becomes equal to or greater than the second threshold value, it is determined that the vehicle 1 is traveling on a bad road and, from this point in time, the BLDA control reduction is performed. After that, in the second embodiment, when a predetermined period (indicated by the continuation time threshold value T2 in the figure) elapses with the yaw rate RMS equal to or greater than the second threshold value (that is, with the BLDA reduction flag turned on), the BLDA prohibition flag is turned on and, after that, the BLDA control is prohibited. As described above, in the second embodiment, the BLDA control is prohibited, not from the beginning, but only when the period during which the BLDA reduction is performed continues for a predetermined period or longer. The continuation time threshold value T2 is set as a period longer than the execution period of the flow in FIG. 11.

[0084] As described above, the lane departure prevention device according to the second embodiment gradually reduces the BLDA control. More specifically, when the yaw rate RMS first becomes equal to or greater than the second threshold value, the control amount is only reduced and, when that state continues for a predetermined period or longer, the control amount is set to zero (in other words, the BLDA control itself is prohibited). Therefore, as compared with the case where the BLDA control is only reduced or the case where the BLDA control is prohibited from the beginning, the BLDA control can be appropriately performed according to the situation. Therefore, the lane departure prevention device according to the second embodiment appropriately prevents the behavior of the vehicle 1 traveling on a bad road from becoming unstable due to the application of braking force.

[0085] The present invention can be changed as necessary without departing from the spirit or concept of the invention that can be read from the claims and the whole specification, and a lane departure prevention device that includes such a change is included in the technical concept of the present invention.

Claims (4)

1. A lane departure prevention device that is mounted on a vehicle, the vehicle including a braking device configured to apply a braking force to wheels of the vehicle, the lane departure prevention device comprising: a vehicle control unit configured to control the braking device such that a yawing moment in a direction of avoiding a departure of the vehicle from a traveling lane of the vehicle is applied when the vehicle is predicted to depart from the traveling lane in which the vehicle is currently traveling; and a calculation unit configured to calculate an index value related to a variation in yaw rates of the vehicle that is traveling, wherein the vehicle control unit is configured to control the braking device such that the braking force becomes smaller when the index value is equal to or greater than a predetermined threshold value than when the index value is smaller than the predetermined threshold value.

2. The lane departure prevention device according to claim 1, wherein the calculation unit is configured to calculate a root mean square of change rates of the yaw rates of the vehicle as the index value.

3. The lane departure prevention device according to claim 1 or 2, wherein the vehicle control unit is configured to control the braking device not to apply the braking force when the index value is equal to or greater than the predetermined threshold value continuously for a predetermined period or longer.

4. A lane departure prevention system comprising a braking device configured to apply a braking force to wheels of a vehicle, and at least one electronic control unit mounted on the vehicle, the at least one electronic control unit being configured to: predict a possibility that the vehicle will depart from a traveling lane in which the vehicle is currently traveling; control the braking device such that a yawing moment is applied to the vehicle, the yawing moment being in a direction of avoiding a departure of the vehicle from the traveling lane, when the vehicle is predicted to depart from the traveling lane; calculate an index value related to a variation in yaw rates of the vehicle that is traveling; and control the braking device such that the braking force becomes smaller when the index value is equal to or greater than a predetermined threshold value than when the index value is smaller than the predetermined threshold value.