Vehicle stop holding device

Abstract

To enable holding of vehicle stop while suppressing an increase in actuation opportunity of a brake that increases a brake force applied to a steering wheel.SOLUTION: A vehicle stop holding device 60 includes: a setting part 105 that, as a function part, when a vehicle is in a stop state on a road surface, sets a brake force lower limit value FbL on the basis of a road surface gradient θS, a wheel angle θW of a steering wheel, and a drive force Fd applied to the steering wheel; and a brake instruction part 107 that, when the brake force applied to the steering wheel falls below the brake force lower limit value FbL, performs first processing of actuating a break 40 such that the brake force becomes equal to or greater than the brake force lower limit value FbL, and, when the brake force is equal to or greater than the brake force lower limit value FbL, performs second processing of causing the brake 40 to hold the brake force.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a vehicle stop maintaining device that maintains a vehicle stopped on a slope. [Background technology]

[0002] Patent Document 1 discloses an example of a braking control device that performs hill-hold control, which maintains a vehicle stopped on a slope even after the driver releases the brakes when the braking force applied by the driver stops the vehicle on the slope. The braking control device sets a hill-hold braking force based on the gradient of the slope, the angle between the slope's inclination and the vehicle's longitudinal direction, and the steering angle of the vehicle's steering wheels. The braking control device then activates the vehicle's braking device based on the hill-hold braking force. The hill-hold braking force is necessary to keep the vehicle stopped on a slope and to keep the wheels from rotating. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-182034 Summary of the Invention [Problem to be solved by the invention]

[0004] While the hill-hold control is in operation, the steering angle of the steered wheels may change due to the driver's steering operation, etc. When the steering angle changes in this way, the magnitude of the hill-hold braking force also changes. Therefore, the brake control device activates the brakes in accordance with the change in the hill-hold braking force. If the brakes are activated to increase the braking force, operating noise due to the activation of the brakes will be generated while the vehicle is stopped. [Means for solving the problem]

[0005] A vehicle stop maintaining device for solving the above problem is applied to a vehicle that includes a steered wheel, which is a wheel with a variable steering angle, a drive device that operates to apply a drive force to the steered wheel, and a brake device that adjusts the braking force applied to the steered wheel, and maintains the vehicle stopped on a road surface. The vehicle stop maintaining device includes: a setting unit that sets a lower limit of the braking force that can stop the rotation of the steered wheel on the road surface based on the gradient of the road surface, the wheel angle that is the angle between the inclination direction of the road surface and the front-to-rear direction of the steered wheel, and the driving force applied to the steered wheel when the vehicle is stopped on the road surface; and a braking instruction unit that executes a first process to activate the brake device so that the braking force applied to the steered wheel becomes less than the lower limit, and executes a second process to cause the brake device to maintain the braking force when the braking force applied to the steered wheel is greater than or equal to the lower limit. [Effects of the Invention]

[0006] The vehicle stop maintaining device has the effect of being able to maintain the vehicle at a stop while suppressing an increase in the frequency of activation of the braking device, which increases the braking force applied to the steered wheels. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic diagram showing the configuration of a vehicle equipped with a vehicle stop maintaining device according to an embodiment. [Figure 2] FIG. 2 is a block diagram showing the functional configuration of the vehicle stop maintaining device of FIG. [Figure 3] FIG. 3 is a perspective view that schematically shows a state in which a vehicle is stopped on a slope. [Figure 4] FIG. 4 is a schematic diagram of the vehicle of FIG. 3 when viewed from above, and a schematic diagram showing the front wheels of the vehicle. [Figure 5] FIG. 5 is a schematic diagram showing the relationship of forces acting on the front wheels of a vehicle stopped on a slope. [Figure 6] FIG. 6 is a diagram showing a case where the front wheels are steered in a direction that increases the gravitational rotational force when the vehicle is stopped on an uphill road. [Figure 7] FIG. 7 is a diagram showing a case where the front wheels are steered in a direction to reduce the gravitational rotational force when the vehicle is stopped on an uphill road. [Figure 8] FIG. 8 is a diagram showing a case where the front wheels are steered in a direction that increases the gravitational rotational force when the vehicle is stopped on a downhill road. [Figure 9] FIG. 9 is a diagram showing a case where the front wheels are steered in a direction to reduce the gravitational rotational force when the vehicle is stopped on a downhill road. [Figure 10] FIG. 10 is a flowchart showing a series of processes executed by a processing circuit included in the vehicle stop maintaining device of FIG. DETAILED DESCRIPTION OF THE INVENTION

[0008] An embodiment of the vehicle stop maintaining device will be described below with reference to FIGS. <Overall vehicle configuration> FIG. 1 is a schematic diagram of a portion of a vehicle 10 equipped with a stop-holding device 60. The vehicle 10 has front wheels 11 and rear wheels 12 as wheels. In the vehicle 10, the front wheels 11 are wheels whose steering angle can be changed. When the steering angle of the front wheels 11 changes, the traveling direction of the vehicle 10 changes. In this respect, the front wheels 11 correspond to the "steering wheels" in the vehicle 10.

[0009] The vehicle 10 is equipped with an operation system including a steering member 15 and a brake operating member 16. The steering member 15 is a member that is operated by the driver of the vehicle 10 when changing the direction of travel of the vehicle 10. An example of the steering member 15 is a steering wheel. The brake operating member 16 is a member that is operated by the driver when adjusting the deceleration of the vehicle 10. An example of the brake operating member 16 is a brake pedal.

[0010] The vehicle 10 includes a steering device 20, a drive device 30, and a braking device 40. The steering device 20 operates to change the steering angle of the front wheels 11 in response to the operation of the steering member 15 by the driver. For example, the steering device 20 is configured to operate in response to the driving of an electric motor 21.

[0011] The drive device 30 operates to apply drive force to the front wheels 11, which are also steered wheels. The drive device 30 includes a power unit 31 having at least one of an engine and an electric motor, and a drive control unit 32 that controls the power unit 31. An example of the drive control unit 32 is an electronic control device. The drive control unit 32 is configured to be able to send and receive various information and commands to and from the vehicle stop maintaining device 60 via an in-vehicle network.

[0012] The braking device 40 adjusts the braking force applied to the front wheels 11 and the rear wheels 12. The braking device 40 includes a braking actuator 41 and a braking control unit 42 that controls the braking actuator 41. The braking actuator 41 is configured to be able to individually adjust the front wheel braking force FbF, which is the braking force applied to the front wheels 11, and the rear wheel braking force FbR, which is the braking force applied to the rear wheels 12. An example of the braking control unit 42 is an electronic control device. The braking control unit 42 is configured to be able to send and receive various information and commands to and from the vehicle stop maintaining device 60 via an in-vehicle network.

[0013] <In-vehicle sensors> The vehicle 10 has a plurality of sensors. Detection signals from the sensors are output to the vehicle stop maintaining device 60. The plurality of sensors include a steering sensor 51, a brake sensor 52, a longitudinal acceleration sensor 55, a lateral acceleration sensor 56, and a wheel speed sensor 57.

[0014] The steering sensor 51 detects information related to the operation of the steering member 15 by the driver. For example, the steering sensor 51 detects the direction and amount of operation of the steering member 15 by the driver. When the steering member 15 is operated, the front wheels 11 are steered in a direction corresponding to the direction in which the steering member 15 is operated. At this time, the front wheels 11 are steered by an amount corresponding to the amount of operation of the steering member 15. In this embodiment, the steering amount of the front wheels 11 is referred to as the "steering angle θf." The steering angle θf is the steering amount of the front wheels 11 based on the position when the steering member 15 is not steered. The steering angle θf can be derived based on a change in the detection signal of the steering sensor 51.

[0015] The brake sensor 52 detects information related to the driver's operation of the brake operating member 16. For example, the brake sensor 52 is a stroke sensor that detects the amount of operation of the brake operating member 16 by the driver. The vehicle 10 may also have, as the brake sensor, an operating force sensor that detects the operating force of the brake operating member 16 by the driver or a correlation value of the operating force.

[0016] The longitudinal acceleration sensor 55 detects the acceleration of the vehicle 10 in the longitudinal direction. The lateral acceleration sensor 56 detects the acceleration of the vehicle 10 in the lateral direction. The acceleration based on the detection signal of the longitudinal acceleration sensor 55 is referred to as the "longitudinal acceleration Gx." The acceleration based on the detection signal of the lateral acceleration sensor 56 is referred to as the "lateral acceleration Gy."

[0017] A wheel speed sensor 57 is provided for each wheel. Each wheel speed sensor 57 detects the rotation speed of the corresponding wheel. The rotation speed of the wheel based on the detection signal of the wheel speed sensor 57 is referred to as the "wheel speed VW."

[0018] <Stopping holding device> When the vehicle 10 is stopped on the road surface, the vehicle stop maintaining device 60 executes various processes for maintaining the stopped state in which the vehicle 10 is stopped on the road surface.

[0019] The vehicle stop maintaining device 60 includes a processing circuit 61. One example of the processing circuit 61 is an electronic control device. In this case, the processing circuit 61 includes a CPU 62, a first memory 63, and a second memory 64. The first memory 63 stores a control program executed by the CPU 62. The second memory 64 stores the results of calculations by the CPU 62.

[0020] 2, the CPU 62 executes the control program in the first memory 63, causing the processing circuit 61 to function as multiple functional units. The multiple functional units are functional units for maintaining a stopped state. The multiple functional units include a calculation unit 101, an acquisition unit 103, a setting unit 105, and a braking instruction unit 107.

[0021] <Arithmetic section> The calculation unit 101 calculates the road surface gradient θS, the vehicle angle θ, and the wheel angle θW when the vehicle 10 is stopped on the road surface.

[0022] FIG. 3 is a schematic diagram of a case where the road surface 201 on which the vehicle 10 is stopped is an uphill road. FIG. 4A is a schematic diagram of the vehicle 10 of FIG. 3 viewed from above. As shown in FIG. 3, the road surface gradient θS is the inclination angle of the road surface 201 with respect to the horizontal plane 200. The calculation unit 101 calculates the road surface gradient θS based on the longitudinal acceleration Gx and the lateral acceleration Gy. As shown in FIG. 4A, the resultant force of the longitudinal acceleration Gx and the lateral acceleration Gy is called the "total acceleration Gtl." In other words, of the total acceleration Gtl, the component in the longitudinal direction of the vehicle 10 corresponds to the longitudinal acceleration Gx. Of the total acceleration Gtl, the component in the lateral direction of the vehicle 10 corresponds to the lateral acceleration Gy. When the vehicle 10 is stopped, the calculation unit 101 calculates the road surface gradient θS so that it increases as the magnitude of the total acceleration Gtl increases. The total acceleration Gtl can be calculated based on the following relational expression (D1).

[0023]

number

[0024] The calculation unit 101 calculates the vehicle angle θ based on the longitudinal acceleration Gx and the lateral acceleration Gy. For example, when the longitudinal acceleration Gx is a negative value and the lateral acceleration Gy is 0 (zero), the calculation unit 101 derives 0 (zero) degrees as the vehicle angle θ. When the longitudinal acceleration Gx is a negative value, the calculation unit 101 derives the vehicle angle θ so that the vehicle angle θ increases as the lateral acceleration Gy increases. On the other hand, when the longitudinal acceleration Gx is a positive value and the lateral acceleration Gy is 0 (zero), the calculation unit 101 derives 180 degrees as the vehicle angle θ. When the longitudinal acceleration Gx is a positive value, the calculation unit 101 derives the vehicle angle θ so that the vehicle angle θ decreases as the lateral acceleration Gy increases. In this way, the calculation unit 101 can calculate the vehicle angle θ to be an angle corresponding to the angle between the inclination direction X of the road surface 201 and the longitudinal direction Y of the vehicle. That is, when the vehicle longitudinal direction Y tilts clockwise in the tilt direction X as shown in FIG. 4(A), the calculation unit 101 calculates the vehicle angle θ so that it becomes greater than 0 (zero).

[0025] FIG. 4(B) shows the front wheels 11 of the vehicle 10 stopped on an uphill road. Specifically, the front wheels 11 before being steered are shown by two-dot chain lines, while the front wheels 11 after being steered are shown by solid lines. As shown in the figure, the wheel angle θW is the angle between the inclination direction X of the road surface 201 and the wheel fore-and-aft direction Z. The wheel fore-and-aft direction Z is the front-and-aft direction of the front wheels 11. Specifically, the wheel fore-and-aft direction Z is the direction along the road surface 201 that is perpendicular to the axle of the front wheels 11. When the vehicle 10 is traveling straight, the wheel fore-and-aft direction Z coincides with the vehicle fore-and-aft direction Y.

[0026] The calculation unit 101 derives the wheel angle θW based on the vehicle angle θ and the steering angle θf of the front wheels 11. Here, when the front wheels 11 are steered in a direction that increases the degree of deflection of the vehicle 10 with respect to the road surface 201, that is, when the front wheels 11 are steered in the same direction as the vehicle angle θ increases from 0 (zero) degrees to 180 degrees, the steering angle θf will be a positive value. At this time, when the front wheels 11 are steered in a direction that decreases the degree of deflection of the vehicle 10 with respect to the road surface 201, that is, when the front wheels 11 are steered in the same direction as the vehicle angle θ decreases from 180 degrees to 0 (zero) degrees, the steering angle θf will be a negative value. Increasing the degree of deflection of the vehicle 10 with respect to the road surface 201 means increasing the angle between the inclination direction X of the road surface 201 and the vehicle longitudinal direction Y. On the other hand, decreasing the degree of deflection of the vehicle 10 with respect to the road surface 201 means decreasing the angle between the inclination direction X of the road surface 201 and the vehicle longitudinal direction Y. The angle formed by the inclination direction X of the road surface 201 and the vehicle longitudinal direction Y is minimum when the vehicle angle θ is 0 (zero) degrees, and maximum when the vehicle angle θ is 180 degrees. When the front wheels 11 are steered from the state shown by the two-dot chain line in FIG. 4B to the state shown by the solid line, the steering angle θf becomes a negative value. Conversely, when the front wheels 11 are steered from the state shown by the solid line in FIG. 4B to the state shown by the two-dot chain line, the steering angle θf becomes a positive value.

[0027] The calculation unit 101 derives the difference between the vehicle angle θ and the steering angle θf as the wheel angle θW. When the front wheels 11 are steered so as to transition from the state shown by the two-dot chain line to the state shown by the solid line as in the example of Fig. 4(B), the steering angle θf becomes a negative value, and therefore the wheel angle θW decreases as the front wheels 11 are steered. Conversely, when the front wheels 11 are steered so as to transition from the state shown by the solid line to the state shown by the two-dot chain line, unlike the example of Fig. 4(B), the steering angle θf becomes a positive value, and therefore the wheel angle θW increases as the front wheels 11 are steered.

[0028] <Acquisition part> Returning to FIG. 2, the acquisition unit 103 acquires the road surface gradient θS, the wheel angle θW, and the driving force Fd applied to the front wheels 11 by the drive unit 30.

[0029] <Settings section> The setting unit 105 sets the braking force lower limit value FbL based on the road surface gradient θS, the wheel angle θW, and the driving force Fd acquired by the acquisition unit 103 when the vehicle 10 is stopped on the road surface 201. The braking force lower limit value FbL is the lower limit value of the braking force that can stop the rotation of the front wheels 11 on the road surface 201. Here, "rotation of the front wheels 11" means that the axles of the front wheels 11 and the front wheels 11 rotate together.

[0030] The process for setting the braking force lower limit value FbL will be described with reference to FIGS. Even when the vehicle 10 is stopped, a driving force Fd may be applied from the drive unit 30 to the front wheels 11, which are also driving wheels. The driving force Fd applied to the front wheels 11 when the driver is not operating the accelerator pedal is called a "creep driving force." The driving force Fd can be expressed, for example, as the axle torque per driving wheel. A front wheel braking force FbF is also applied to the front wheels 11. Furthermore, when the road surface 201 is a slope, gravity acting on the front wheels 11 acts as a gravitational rotational force Fgw. The gravitational rotational force Fgw is the magnitude of the component of gravity acting on the front wheels 11 in the wheel fore-and-aft direction Z. The gravitational rotational force Fgw tends to rotate the front wheels 11 down the slope. The gravitational rotational force Fgw can be calculated based on the road surface gradient θS and the wheel angle θW. For example, the setting unit 105 can calculate the gravitational rotational force Fgw by referring to the following relational expression (D2): In the relational expression (D2), "g" is the gravity acting on the front wheel 11. "MW" is the amount of weight M of the vehicle 10 that is supported by one front wheel 11. The body of the vehicle 10 is supported by four wheels. Therefore, an example of the front wheel load MW is the value obtained by dividing the weight M by 4.

[0031]

number

[0032] The front wheel braking force FbF can be calculated by referring to the following relational expression (D3). In relational expression (D3), "Pb" is the force that presses the friction material against the disc that rotates integrally with the front wheel 11 in the friction brake for the front wheel 11. "Sb" is the area of ​​the piston of the friction brake. "μb" is the friction coefficient between the disc and the friction material. "Kb" is the conversion coefficient from pressing force to friction force.

[0033]

number

[0034] When the road surface 201 is an uphill road as shown in FIG. 5, or more precisely when the wheel angle θW is in the range of 0 (zero) degrees or more and less than 90 degrees, the front wheels 11 can be kept stopped from rotating on the road surface 201 if the condition shown in the following relational expression (D4) is satisfied.

[0035]

number

[0036]

number

[0037] <When the vehicle stops on an uphill road> When the road surface 201 is an uphill road, as shown in the above relational expression (D4), if the applied braking force FbFA is equal to or greater than the absolute value of the difference between the gravitational rotational force Fgw and the applied driving force FdA, the front wheels 11 can be kept stopped from rotating. Therefore, when the road surface 201 is an uphill road, the setting unit 105 sets the front wheel braking force FbF corresponding to the absolute value of the difference between the gravitational rotational force Fgw and the applied driving force FdA as the braking force lower limit value FbL. For example, the setting unit 105 can derive the braking force lower limit value FbL by referring to the following relational expression (D6). When the road surface 201 is an uphill road, the setting unit 105 can derive the braking force lower limit value FbL by substituting the absolute value of the difference between the gravitational rotational force Fgw and the applied driving force FdA for "FbA1" in relational expression (D6).

[0038]

number

[0039] Therefore, when the wheel angle θW is changed as shown in FIG. 6(A), the setting unit 105 changes the braking force lower limit value FbL so that it increases in accordance with the change in the wheel angle θW, as shown in FIG. 6(B).

[0040] FIG. 7 shows a case where the front wheels 11 are steered in a direction that increases the vehicle angle θ on an uphill road. In FIG. 7(A), the front wheels 11 before steering are shown by a two-dot chain line, while the front wheels 11 after steering are shown by a solid line. As shown in the figure, when the road surface 201 on which the vehicle 10 is stopped is an uphill road, and the front wheels 11 are steered in a direction that increases the vehicle angle θ, the steering angle θf becomes a positive value. Therefore, the magnitude of the wheel angle θW increases. As a result, the gravitational rotational force Fgw decreases, as shown in FIG. 7(B).

[0041] Therefore, when the wheel angle θW is changed as shown in FIG. 7(A), the setting unit 105 changes the braking force lower limit value FbL so that it becomes smaller in accordance with the change in the wheel angle θW, as shown in FIG. 7(B).

[0042] <When the vehicle stops on a downhill road> When the road surface 201 is a downhill road, if the applied braking force FbFA is equal to or greater than the sum of the gravitational rotational force Fgw and the applied drive force FdA, the front wheels 11 can be maintained in a stopped state. Therefore, when the road surface 201 is a downhill road, the setting unit 105 sets the front wheel braking force FbF corresponding to the sum of the gravitational rotational force Fgw and the applied drive force FdA as the braking force lower limit FbL. For example, the setting unit 105 can derive the braking force lower limit FbL by referring to the above relational expression (D6). When the road surface 201 is a downhill road, the setting unit 105 can derive the braking force lower limit FbL by substituting the sum of the gravitational rotational force Fgw and the applied drive force FdA for "FbA1" in the relational expression (D6).

[0043] FIG. 8 shows a case where the front wheels 11 are steered in a direction that increases the vehicle angle θ on a downhill road. In FIG. 8(A), the direction in which the front wheels 11 are turned downward in the direction of inclination is the direction that increases the vehicle angle θ. Also in FIG. 8(A), the front wheels 11 before steering are shown by two-dot chain lines, while the front wheels 11 after steering are shown by solid lines. As shown in the figure, when the road surface 201 on which the vehicle 10 is stopped is a downhill road, and the front wheels 11 are steered in a direction that increases the vehicle angle θ, the steering angle θf becomes a positive value. Therefore, the magnitude of the wheel angle θW increases. As a result, the gravitational rotational force Fgw increases, as shown in FIG. 8(B).

[0044] Therefore, when the wheel angle θW is changed as shown in FIG. 8(A), the setting unit 105 changes the braking force lower limit value FbL so that it increases in accordance with the change in the wheel angle θW, as shown in FIG. 8(B).

[0045] FIG. 9 shows a case where the front wheels 11 are steered in a direction that reduces the vehicle angle θ on a downhill road. In FIG. 9(A), the front wheels 11 before steering are shown by a two-dot chain line, while the front wheels 11 after steering are shown by a solid line. As shown in the figure, when the road surface 201 on which the vehicle 10 is stopped is a downhill road, and the front wheels 11 are steered in a direction that reduces the vehicle angle θ, the steering angle θf becomes a negative value. Therefore, the magnitude of the wheel angle θW decreases. As a result, the gravitational rotational force Fgw decreases, as shown in FIG. 9(B).

[0046] Therefore, when the wheel angle θW is changed as shown in FIG. 9(A), the setting unit 105 changes the braking force lower limit value FbL so that it becomes smaller in accordance with the change in the wheel angle θW, as shown in FIG. 9(B).

[0047] <Brake instruction section> Returning to FIG. 2, the braking instruction unit 107 executes a first process when the front wheel braking force FbF becomes less than the braking force lower limit value FbL. In the first process, the braking instruction unit 107 operates the braking device 40 so that the front wheel braking force FbF becomes equal to or greater than the braking force lower limit value FbL. For example, in the first process, the braking instruction unit 107 transmits an instruction to the braking control unit 42 of the braking device 40 to increase the front wheel braking force FbF to equal to or greater than the braking force lower limit value FbL.

[0048] The braking instruction unit 107 executes a second process when the front wheel braking force FbF is equal to or greater than the braking force lower limit value FbL. In the second process, the braking instruction unit 107 causes the braking device 40 to maintain the magnitude of the front wheel braking force FbF. For example, in the second process, the braking instruction unit 107 transmits an instruction to the braking control unit 42 to maintain the front wheel braking force FbF.

[0049] When the road surface 201 on which the vehicle 10 is stopped is a slope, the braking command unit 107 can also execute the second process without determining whether the front wheel braking force FbF is equal to or greater than the braking force lower limit FbL, depending on the steering direction of the front wheels 11. That is, when the road surface 201 is a slope, the braking command unit 107 executes the second process when the front wheels 11 are steered in a direction that reduces the magnitude of the component of gravity acting on the front wheels 11 in the wheel fore-and-aft direction Z, i.e., the gravitational rotational force Fgw, in a situation where the road surface 201 is a slope. For example, when the front wheels 11 are steered on an uphill road as shown in the example of FIG. 7, the braking command unit 107 executes the second process. For example, when the front wheels 11 are steered on a downhill road as shown in the example of FIG. 9, the braking command unit 107 executes the second process.

[0050] <Maintaining stop processing> 10, a vehicle stop maintaining process, which is a series of processes executed by the processing circuit 61 of the vehicle stop maintaining device 60 to maintain the vehicle stopped state, will be described. The processing circuit 61 repeatedly executes the vehicle stop maintaining process at every predetermined control period.

[0051] In step S11, the processing circuit 61 determines whether the vehicle 10 is stopped. For example, if the duration of the state in which the wheel speed VW of the front wheels 11 is 0 (zero) is equal to or longer than a predetermined time, the processing circuit 61 determines that the vehicle 10 is stopped (S11: YES). Then, the processing circuit 61 shifts the processing to step S13. On the other hand, if the wheel speed VW of the front wheels 11 is not 0 (zero) or if the duration is less than a predetermined time, the processing circuit 61 determines that the vehicle 10 is not stopped (S11: NO). Then, the processing circuit 61 temporarily ends the vehicle stop maintenance processing.

[0052] In step S13, the processing circuit 61 functions as the calculation unit 101 to calculate the road surface gradient θS, the vehicle angle θ, and the wheel angle θW. In the next step S15, the processing circuit 61 functions as the acquisition unit 103 to acquire the driving force Fd applied to the front wheels 11 from the drive unit 30 in addition to the road surface gradient θS and wheel angle θW calculated in step S13.

[0053] In the following step S17, the processing circuit 61 determines whether the front wheels 11 have been steered since the vehicle 10 stopped on the road surface 201. If the processing circuit 61 determines that the front wheels 11 have been steered (S17: YES), the processing circuit 61 proceeds to step S19. On the other hand, if the processing circuit 61 determines that the front wheels 11 have not been steered (S17: NO), the processing circuit 61 temporarily ends the vehicle stop maintenance processing.

[0054] In step S19, the processing circuit 61 functions as the braking instruction unit 107 to determine whether the front wheels 11 have been steered in a direction that reduces the gravitational rotational force Fgw. If the processing circuit 61 determines that the front wheels 11 have been steered in a direction that reduces the gravitational rotational force Fgw (S19: YES), the processing circuit 61 proceeds to step S29. On the other hand, if the processing circuit 61 determines that the front wheels 11 have been steered in a direction that increases the gravitational rotational force Fgw (S19: NO), the processing circuit 61 proceeds to step S21.

[0055] In step S21, the processing circuit 61 sets the braking force lower limit value FbL by functioning as the setting unit 105. At this time, the processing circuit 61 sets the braking force lower limit value FbL based on the road surface gradient θS, the wheel angle θW, and the driving force Fd applied to the front wheels 11.

[0056] In the following step S23, the processing circuit 61, functioning as the braking instruction unit 107, acquires the front wheel braking force FbF. Then, in step S25, the processing circuit 61, functioning as the braking instruction unit 107, determines whether the front wheel braking force FbF is less than the braking force lower limit value FbL. If the processing circuit 61 determines that the front wheel braking force FbF is less than the braking force lower limit value FbL (S25: YES), the processing circuit 61 proceeds to step S27. On the other hand, if the processing circuit 61 determines that the front wheel braking force FbF is equal to or greater than the braking force lower limit value FbL (S25: NO), the processing circuit 61 proceeds to step S29.

[0057] In step S27, the processing circuit 61 executes the first process by functioning as the braking instruction unit 107. For example, in the first process, the processing circuit 61 transmits the braking force lower limit value FbL to the braking control unit 42 as an instruction value for the front wheel braking force FbF. Thereafter, the processing circuit 61 proceeds to step S31.

[0058] In this case, when the brake control unit 42 receives the command value of the front wheel braking force FbF from the vehicle stop maintaining device 60, it operates the brake actuator 41 so that the front wheel braking force FbF becomes the command value. This allows the brake control unit 42 to increase the front wheel braking force FbF.

[0059] In step S29, the processing circuit 61 executes the second process by functioning as the braking instruction unit 107. In this case, the processing circuit 61 transmits an instruction to maintain the front wheel braking force FbF to the braking control unit 42. Thereafter, the processing circuit 61 shifts the process to step S31.

[0060] In step S31, the processing circuit 61 determines whether or not an end condition for maintaining the stopped state is satisfied. For example, when an instruction to start the vehicle 10 is input by the driver operating the accelerator pedal, the processing circuit 61 determines that the end condition is satisfied. When the processing circuit 61 determines that the end condition is satisfied (S31: YES), the processing circuit 61 shifts the processing to step S33. On the other hand, when the processing circuit 61 determines that the end condition is not satisfied (S31: NO), the processing circuit 61 shifts the processing to step S13. That is, the processing circuit 61 continues to execute the vehicle stop maintenance processing.

[0061] In step S33, the processing circuit 61 instructs the braking control unit 42 to cancel the state in which braking force is applied to the wheels 11, 12. Then, the processing circuit 61 temporarily ends the vehicle stop maintenance process.

[0062] When the braking control unit 42 receives the release instruction from the vehicle stop maintaining device 60, it operates the braking actuator 41 to set the front wheel braking force FbF and the rear wheel braking force FbR to 0 (zero). <Actions and Effects of This Embodiment> (1) The processing circuit 61 sets a braking force lower limit value FbL when the vehicle 10 is stopped on the road surface 201. Then, the processing circuit 61 executes a first process when the front wheel braking force FbF is less than the braking force lower limit value FbL, and executes a second process when the front wheel braking force FbF is equal to or greater than the braking force lower limit value FbL. In other words, the processing circuit 61 does not activate the braking device 40 when it is possible to restrict the rotation of the front wheels 11 in a direction that would cause the vehicle 10 to slide down a slope without increasing the front wheel braking force FbF.

[0063] For example, when the road surface 201 is an uphill road, the driving force Fd acts on the front wheels 11 in a direction that restricts the rotation of the front wheels 11 in a direction that would cause the vehicle 10 to slide downhill. The braking force lower limit FbL is set taking into account the driving force Fd applied to the front wheels 11. Therefore, the processing circuit 61 can reduce the number of times that the front wheel braking force FbF falls below the braking force lower limit FbL, and therefore can reduce the number of times that the braking device 40 is activated to increase the front wheel braking force FbF. Furthermore, even if the front wheel braking force FbF falls below the braking force lower limit FbL, the processing circuit 61 can reduce the amount of deviation between the front wheel braking force FbF and the braking force lower limit FbL. In other words, the processing circuit 61 can prevent the increase in the front wheel braking force FbF while the vehicle is stopped from becoming greater than necessary.

[0064] Therefore, the vehicle stop maintaining device 60 can maintain the vehicle 10 at a standstill while suppressing an increase in the chances of the braking device 40 operating to increase the front wheel braking force FbF. (2) In the vehicle stop maintaining device 60, the processing circuit 61 sets the braking force lower limit value FbL and selects the first process or the second process when the front wheels 11 are steered while the vehicle is stopped. In other words, even when the vehicle is stopped, if the front wheels 11 are not steered, the processing circuit 61 does not set the braking force lower limit value FbL or select the first process or the second process because the vehicle can be maintained stopped. Therefore, the vehicle stop maintaining device 60 can suppress an increase in the control load when the vehicle is stopped.

[0065] (3) When the vehicle 10 is stopped on a slope, if the front wheels 11 are steered in a direction that increases the wheel angle θW, the gravitational rotational force Fgw acting on the front wheels 11 decreases. When the gravitational rotational force Fgw decreases, the total force acting on the front wheels 11 in a direction that causes the vehicle 10 to slide down the slope decreases.

[0066] Therefore, in the vehicle stop maintaining device 60, when the front wheels 11 are steered in a direction that increases the wheel angle θW while the vehicle 10 is stopped on a slope, the processing circuit 61 executes the second process without setting the braking force lower limit value FbL. This reduces the number of opportunities for the vehicle stop maintaining device 60 to set the braking force lower limit value FbL even when the front wheels 11 are steered, thereby suppressing an increase in the control load.

[0067] <Example of change> The above embodiment can be modified as follows: The above embodiment and the following modifications can be combined with each other within the scope of technical compatibility.

[0068] The vehicle stop maintenance process may be a process that omits the determination in step S19 shown in Fig. 10. In this case, the processing circuit 61 sets the braking force lower limit value FbL even when the front wheels 11 are steered in a direction that reduces the gravitational rotational force Fgw.

[0069] In the first process, the braking instruction unit 107 may transmit a braking force greater than the braking force lower limit value FbL as the instruction value for the front wheel braking force FbF to the braking control unit 42. For example, the braking instruction unit 107 transmits the sum of the braking force lower limit value FbL and a predetermined offset value to the braking control unit 42 as the instruction value for the front wheel braking force FbF.

[0070] The processing circuit 61 may calculate the braking force lower limit FbL when the vehicle 10 stops, set the braking force lower limit FbL to a reference front wheel braking force FbFst, which is an estimate of the actual braking force, and execute the second processing within a range in which the braking force lower limit FbL after steering is equal to or less than the reference front wheel braking force FbFst. For example, the wheel angle θW, gravitational rotational force Fgw, and gravitational rotational force Fgw when the vehicle 10 stops may be set to the reference wheel angle θWst, the reference gravitational rotational force Fgwst, and the reference driving force Fdst, respectively. When the vehicle 10 stops, the wheels are stopped, so it is clear that the reference front wheel braking force FbFst applied to the front wheels 11 is equal to or greater than the braking force lower limit FbL calculated from the reference gravitational rotational force Fgwst and the reference driving force Fdst. Therefore, for example, the range of the wheel angle θW in which the braking force lower limit value FbL based on the wheel angle θW is equal to or less than the reference front wheel braking force FbFst is calculated, and when steering into the range of the wheel angle θW, the second process is selected to maintain the braking force, thereby keeping the vehicle 10 stationary. In this case, the processing circuit 61 executes the second process based on the actual braking force estimated from the driving force and gravity. Therefore, even if a difference occurs between the acquired braking force Fb and the actually generated braking force, the stop maintaining device 60 can prevent the vehicle 10 from moving during execution of the second process. Note that the reference wheel angle θWst, the reference gravitational rotational force Fgwst, and the reference driving force Fdst may be values ​​at the time when it is determined that the vehicle 10 is stationary. Alternatively, these may be values ​​at a time when a predetermined time has elapsed since it was determined that the vehicle 10 is stationary.

[0071] Furthermore, even if the processing circuit 61 selects the first processing based on a comparison between the acquired braking force Fb and the braking force lower limit value FbL in the above embodiment, if the processing circuit 61 selects the second processing based on a comparison between the reference front wheel braking force FbFst and the braking force lower limit value FbL as described above, the processing circuit 61 may be configured to execute the second processing.

[0072] Some vehicles have a function of steering the rear wheels 12 in response to steering of the steering member 15 by the driver. When a vehicle stop maintenance device is applied to such a vehicle, the vehicle stop maintenance device may set a lower limit value for the rear wheel braking force FbR applied to the rear wheels 12. In this case, if a driving force Fd is applied to the rear wheels 12 of the vehicle, the vehicle stop maintenance device may set the lower limit value for the rear wheel braking force FbR taking into account the driving force Fd applied to the rear wheels 12. If the rear wheel braking force FbR is less than the lower limit value, the vehicle stop maintenance device executes a first process to operate the brake device 40 so that the rear wheel braking force FbR is equal to or greater than the lower limit value. On the other hand, if the rear wheel braking force FbR is equal to or greater than the lower limit value, the vehicle stop maintenance device executes a second process to maintain the rear wheel braking force FbR.

[0073] The electronic control device that functions as the braking control unit 42 may also function as the vehicle stop maintaining device 60. The processing circuit 61 is not limited to a circuit having a CPU and ROM and executing software processing. In other words, the processing circuit 61 may have any one of the following configurations (a), (b), and (c):

[0074] (a) The processing circuit 61 includes one or more processors that execute various processes according to a computer program. The processor includes a CPU and memory such as RAM and ROM. The memory stores program code or instructions configured to cause the CPU to execute processes. Memory, i.e., computer-readable media, includes any available media that can be accessed by a general-purpose or special-purpose computer.

[0075] (b) The processing circuit 61 includes one or more dedicated hardware circuits that perform various processes. Examples of dedicated hardware circuits include application specific integrated circuits (ASICs) or FPGAs. ASIC is an abbreviation for "Application Specific Integrated Circuit." FPGA is an abbreviation for "Field Programmable Gate Array."

[0076] (c) The processing circuitry 61 includes one or more processors that execute some of the various processes in accordance with a computer program, and one or more dedicated hardware circuits that execute the remaining processes among the various processes.

[0077] <Other technical ideas> The technical ideas that can be understood from the above-described embodiment and modified examples will be described. [Note] When the vehicle is stopped on a slope, it is preferable that the setting unit sets the lower limit value so that the lower limit value decreases as the wheel angle approaches 90°.

[0078] The expression "at least one" used herein means "one or more" of the desired options. As an example, the expression "at least one" used herein means "only one option" or "both of two options" if the number of options is two. As another example, the expression "at least one" used herein means "only one option" or "any combination of two or more options" if the number of options is three or more. [Explanation of symbols]

[0079] 10...Vehicle 11...Front wheels (an example of a steering wheel) 12...Rear wheels (an example of a steering wheel) 30...Driver 40...braking device 41...Braking actuator 42...Braking control unit 60…Station holding device 61...Processing circuit 105...Settings section 107...Braking instruction section 201…road surface

Claims

1. A vehicle stop maintaining device that is applied to a vehicle that includes steering wheels that are wheels whose steering angle is changeable, a drive device that operates to apply a driving force to the steering wheels, and a braking device that adjusts the braking force applied to the steering wheels, and that maintains the vehicle stopped on a road surface, a setting unit that sets a lower limit of the braking force that can stop the rotation of the steered wheels on the road surface, based on the gradient of the road surface, the wheel angle that is the angle between the inclination direction of the road surface and the front-rear direction of the steered wheels, and the driving force applied to the steered wheels when the vehicle is stopped on the road surface; a braking instruction unit that, when the braking force applied to the steered wheels falls below the lower limit, executes a first process to operate the braking device so that the braking force becomes equal to or greater than the lower limit, and, when the braking force applied to the steered wheels is equal to or greater than the lower limit, executes a second process to cause the braking device to maintain the braking force. Parking holding device.

2. The braking instruction unit executes the second process when the steered wheels are steered in a direction that reduces a magnitude of a component of gravity acting on the steered wheels in a front-rear direction of the steered wheels while the vehicle is stopped on a slope. The vehicle stop maintaining device according to claim 1.

3. When the vehicle is stopped on an uphill road, the setting unit sets the lower limit value so that the lower limit value decreases as the magnitude of the difference between the magnitude of a component of gravity acting on the steered wheel in the front-rear direction of the steered wheel and the driving force applied to the steered wheel decreases. The vehicle stop maintaining device according to claim 1 or 2.

4. When the vehicle is stopped on a downhill road, the setting unit sets the lower limit value so that the lower limit value decreases as the sum of a magnitude of a component of gravity acting on the steered wheel in a front-to-rear direction of the steered wheel and a driving force applied to the steered wheel decreases. The vehicle stop maintaining device according to claim 1 or 2.

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