Driving Support Devices

Abstract

To suppress a driver's shaking although slip-down has occurred when an own vehicle has been stopped on a slope of a low μ road surface.SOLUTION: A DSS_ECU 45 provided in a driving support device comprises a slip-down determination part, a brake torque estimation part, a slip rate setting part, and a drive torque setting part. The slip-down determination part detects stepping-down on a brake pedal, and determines slip-down of an own vehicle M from a relationship between wheel speed of each drive wheel Aw (FI, Fr, RI, Rr) and vehicle speed. The brake torque estimation part estimates a brake torque acting on each drive wheel Aw at the time of slip-down. The slip rate setting part sets a slip rate λ of each drive wheel Aw at the time of slip-down on the basis of a preset slip distribution. The drive torque setting part sets a drive torque for driving each drive wheel Aw against the estimated brake torque, on the basis of the slip rate λ of each drive wheel Aw.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a driving assistance device that generates a driving torque for recovering grip force in each driving wheel against a braking torque when a vehicle slides downhill while stopped on a slope, even if the driver is depressing the brake pedal. [Background technology]

[0002] Generally, when a vehicle is traveling downhill or uphill on an extremely low μ road surface (a low friction coefficient road surface such as snowy road or icy road), the driver gently depresses the brake pedal to gradually decelerate and stop the vehicle, while being careful not to lock the wheels.

[0003] In this case, if the weight component (mg·sinθ) along the slope when the vehicle (mass m) is stopped on a slope (road surface gradient θ) is greater than the static friction coefficient μ of the slope, the tires will remain locked and the vehicle will slide (slide downward).

[0004] When a vehicle starts to slide, the driver becomes shaken and finds it difficult to recover from tire lock by pumping the brakes or gently operating the accelerator (to increase driving force) in a timely manner, and often ends up continuing to press the brake pedal.

[0005] In addition, when the vehicle slides down a slope, it is not limited to a linear slippage, but depending on the road surface shape (when there is a transverse gradient, etc.) and the difference in the static friction coefficient μ of the road surface that the four wheels are in contact with (split μ road surface), a natural yaw rate may occur in the vehicle, causing the vehicle's direction to gradually change. In that case, even if the driver operates the steering wheel to correct the direction of the vehicle, no lateral force is generated if the tire on the steering wheel side is locked and the grip force is not restored, making the driver more shaken and increasing his sense of anxiety.

[0006] As a countermeasure to this, for example, Patent Document 1 (JP 2017-94862 A) discloses a technology in which, when a control unit detects that the vehicle has stopped on a slope due to the driver's depression of the brake pedal, the brake device is actuated to execute a vehicle stop maintenance control that maintains the braking force by the brake fluid pressure. In this case, when the control unit detects a yaw angle acting on the vehicle of a predetermined value or more, the braking force of either the left or right wheel is reduced depending on whether the vehicle is stopped in a downhill or uphill state and the direction in which the yaw angle of the vehicle is generated relative to the slope, thereby reducing the generation of the yaw angle by providing a difference between the left and right braking forces. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2017-94862 A Summary of the Invention [Problem to be solved by the invention]

[0008] However, the technology disclosed in Patent Document 1 cannot be applied to a vehicle that is not equipped with a braking device that performs stop-hold control.

[0009] In addition, the technology disclosed in Patent Document 1 merely reduces the brake fluid pressure held by the braking device and applies a difference in braking force between the left and right sides to suppress the generation of a yaw angle. Therefore, it is difficult to guide the vehicle in a direction that matches the driver's intention while it is sliding down a slope, and there is a limit to suppressing the driver's agitation and reducing anxiety.

[0010] The present invention aims to provide a driving assistance device that can guide the vehicle in a direction in line with the driver's intention even if the vehicle rolls over when stopped on a slope with a low μ road surface, thereby suppressing the driver's agitation caused by the roll over and reducing the driver's sense of anxiety. [Means for solving the problem]

[0011] The present invention provides a driving support device including a driving source unit that applies driving force to each of front and rear drive wheels that can be driven independently, a brake detection unit that detects depression of a brake pedal, a vehicle body speed detection unit that detects the vehicle body speed of the host vehicle, a wheel speed detection unit that detects the wheel speed of each of the drive wheels, and a driving force control unit that controls the driving force of each of the drive wheels. The driving force control unit detects depression of the brake pedal by the brake detection unit, and determines whether the host vehicle is skidding or not from the relationship between the wheel speed of each of the drive wheels detected by the wheel speed detection unit and the vehicle body speed detected by the vehicle body speed detection unit. the vehicle is provided with a skid determination unit that determines whether or not a skid has occurred, a braking torque estimation unit that estimates a braking torque acting on each of the drive wheels when the skid determination unit determines that a skid has occurred, a slip ratio setting unit that sets a slip ratio of each of the drive wheels based on a preset slip distribution when the skid determination unit determines that a skid has occurred in the host vehicle, and a drive torque setting unit that sets a drive torque for driving each of the drive wheels against the braking torque estimated by the braking torque estimation unit, based on the slip ratio of each of the drive wheels set by the slip ratio setting unit. Effect of the Invention

[0012] According to the present invention, when the depression of the brake pedal is detected and it is determined that the vehicle is skidding based on the relationship between the wheel speed of each drive wheel and the vehicle body speed, the braking torque acting on each drive wheel is estimated, the slip ratio of each drive wheel is set based on a preset slip distribution, and the drive torque for driving each drive wheel against the braking torque is set based on the slip ratio of each drive wheel, so that even if the vehicle skids when the vehicle is stopped on a slope with a low μ road surface, the grip force of each drive wheel is restored and the vehicle can be guided in a direction according to the driver's intention. As a result, the driver's upset caused by skidding can be suppressed and the sense of anxiety can be reduced. [Brief description of the drawings]

[0013] [Figure 1]Schematic diagram of a driving support device [Diagram 2] Flowchart showing a routine for suppressing slippage control [Diagram 3] Flowchart showing forward slippage suppression control subroutine [Figure 4] Flowchart showing rearward slippage suppression control subroutine [Diagram 5] Flowchart showing the drive torque setting / output subroutine [Figure 6] A side view showing a state in which the vehicle slides forward down a slope. [Figure 7] A side view showing a state in which the vehicle slides backward down a slope. [Figure 8A] A diagram showing the slip distribution of each wheel when the driver inputs steering input during forward sliding [Figure 8B] A diagram showing the steering direction and slip distribution for each wheel when a natural yaw rate occurs during forward sliding. [Figure 9A] A diagram showing the slip distribution set for each wheel when the driver inputs steering input during backward rolling [Figure 9B] A diagram showing the steering direction and slip distribution for each wheel when a natural yaw rate occurs during a backward slide [Figure 10A] FIG. 10 is an explanatory diagram showing the vehicle behavior according to the slip distribution of each wheel set when steering to the right during forward sliding. [Figure 10B] FIG. 10 is an explanatory diagram showing the vehicle behavior according to the slip distribution of each wheel set when steering left during forward sliding. [Figure 10C] A diagram showing the vehicle behavior due to the slip distribution of each wheel and steering control when a clockwise yaw rate occurs during forward sliding. [Figure 10D] A diagram showing the vehicle behavior due to the slip distribution of each wheel and steering control when a counterclockwise yaw rate occurs during forward sliding. [Figure 11A] FIG. 11 is an explanatory diagram showing the vehicle behavior according to the slip distribution of each wheel set when steering to the right during a backward slide. [Figure 11B] FIG. 10 is an explanatory diagram showing the vehicle behavior according to the slip distribution of each wheel set when steering left during a backward slide. [Figure 11C] A diagram showing the steering direction set when a clockwise yaw rate occurs during a backward slide, slip distribution of each wheel, and vehicle behavior due to steering control. [Figure 11D] A diagram showing the steering direction set when a left-handed yaw rate occurs during a backward slide, slip distribution of each wheel, and vehicle behavior due to steering control. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] An embodiment of the present invention will be described below with reference to the drawings. A drive system, a steering system, and a control system are mounted on a vehicle (host vehicle) M shown in Fig. 1. The drive system is a four-wheel drive vehicle composed of a front drive section 1F and a rear drive section 1R. In the front drive section 1F, the output from a drive source 11, typically an electric motor or an engine, is transmitted from an output shaft 12a of a transmission 12 to left and right front drive wheels Fl, Fr via a front differential mechanism (front diff) 13 and left and right axle shafts 14.

[0015] The front differential 13 has a ring gear 13a that meshes with a drive pinion gear 12b provided on the output shaft 12a, and both ends of a ring gear shaft 13b that supports the ring gear 13a are connected to left and right axle shafts 14 via left and right multi-plate clutches (wet multi-plate clutches) 15l, 15r. The left and right multi-plate clutches 15l, 15r have outer plates 15a and inner plates 15b arranged alternately, with the outer plates 15a connected to the ring gear shaft 13b and the inner plates 15b connected to the left and right axle shafts 14.

[0016] The outer plate 15a and the inner plate 15b of both multi-plate clutches 15l, 15r are always in a half-clutch state, and the differential torque generated between the left and right front drive wheels Fl, Fr when traveling on a curved road is absorbed by drag torque. The fastening force of both multi-plate clutches 15l, 15r is variably controlled by a transfer control unit (T / C_ECU) 41, which will be described later, and the torque distribution to the left and right front drive wheels Fl, Fr is appropriately set when traveling on a low μ road such as a snowy road, or when there is a sudden torque change such as when starting or accelerating suddenly.

[0017] The rear drive unit 1R has left and right drive motors 12l and 12r, and the drive force of each of the drive motors 12l and 12r is transmitted to the left and right rear drive wheels Rl and Rr via a reduction mechanism (not shown). The drive source 11 and the left and right drive motors 12l and 12r correspond to the drive source unit of the present invention.

[0018] Meanwhile, the steering system has a steering mechanism 21, such as a rack-and-pinion mechanism, connected to the left and right front drive wheels Fl, Fr via tie rods 22. A steering shaft 24, to the tip of which a handlebar 23 is fixed, is connected to this steering mechanism 21. When the driver operates the handlebar 23, the left and right front drive wheels Fl, Fr are steered via the steering mechanism 21. An electric power steering (EPS) motor 25, serving as a steering drive unit, is connected to a portion of this steering shaft 24 adjacent to the steering mechanism 21 via a transmission mechanism (not shown).

[0019] The drive source 11 and the transmission 12 are controlled based on an output control signal and a gear shift control signal output from a power control unit (PW_ECU) 42. Furthermore, the assist torque (EPS torque) applied to the steering shaft 24 by the EPS motor 25 is controlled by an EPS control unit (EPS_ECU) 43, which is a steering control section. Furthermore, the outputs of the left and right drive motors 12l, 12r on the rear wheel side are controlled by a motor control unit (motor_ECU) 44.

[0020] Each of these control units 41-44 is connected to a DSS control unit (DSS_ECU) 45 as a driving force control unit that performs driving support (DSS: Driving Support System) via an in-vehicle network using, for example, CAN (Controller Area Network) communication, so as to be able to communicate bidirectionally. Each of the ECUs 41-45 constituting this control system is composed of a microcontroller having a RAM, a ROM, a rewritable non-volatile memory (flash memory or EEPROM), and peripheral devices. The ROM stores programs and fixed data required for executing each process in the CPU. The RAM is provided as a work area for the CPU, and temporarily stores various data in the CPU. The CPU is also called an MPU (Microprocessor) or a processor. Instead of the CPU, a GPU (Graphics Processing Unit) or a GSP (Graph Streaming Processor) may be used. Alternatively, the CPU, the GPU, and the GSP may be selectively combined and used.

[0021] The operation of each of the ECUs 41 to 44 is basically controlled according to a control signal from the DSS_ECU 45. Sensors required for controlling the operation executed by each of the ECUs 41 to 44 are connected to the input side of the DSS_ECU 45. The sensors include a brake switch 26 as a brake detection unit that detects the depression of the brake pedal by the driver, a wheel speed sensor 27 as a wheel speed detection unit that detects the wheel speed Vw of each of the drive wheels Fl, Fr, Rl, and Rr, a longitudinal acceleration sensor 28 that detects the longitudinal acceleration (G) when the vehicle body is moving and the road surface gradient direction whether the traveling direction of the host vehicle M is uphill or downhill based on this acceleration, a vehicle body speed detection unit 29 that detects the moving speed of the vehicle body, a steering angle sensor 30 as a steering angle detection unit that detects the steering angle generated by the driver's steering operation, a yaw rate sensor 31 as a yaw rate detection unit that detects the yaw rate acting on the vehicle body, and a brake fluid pressure sensor 32 that detects the brake pressure from the fluid pressure in the master cylinder when the driver depresses the brake pedal. In the following description, for convenience, the drive wheels Fl, Fr, Rl, and Rr will be collectively referred to as drive wheels Aw when referring to them comprehensively.

[0022] Furthermore, the vehicle speed Vv detected by the vehicle speed detection unit 29 is estimated from the amount of movement per unit time based on the position information from the GNSS (Global Navigation Satellite System) satellites, for example, if the vehicle is equipped with a car navigation system and position information from the GNSS satellites can be acquired. Alternatively, the vehicle speed Vv is estimated based on the change in acceleration detected by the longitudinal acceleration sensor 28. Alternatively, if the vehicle M is equipped with a forward recognition unit such as an on-board camera, the vehicle speed Vv may be estimated from the amount of movement per unit time of a specific point of interest based on information of the point of interest detected by the forward recognition unit.

[0023] Incidentally, when attempting to stop the host vehicle M on an extremely low μ road surface (a road surface with a low friction coefficient such as a snowy road or an icy road), the driver slowly depresses the brake pedal to gently decelerate the host vehicle M and carefully attempts to stop the vehicle so as not to lock the tires and cause slippage. However, if this extremely low μ road surface is a slope, and the weight component (mg·sinθ) of the host vehicle M along the slope exceeds the static friction coefficient μ of the slope due to the gradient of the road surface, the tires will lock and the vehicle will slide down. Even when a slide down occurs, many drivers will attempt to stop the host vehicle M by depressing the brake pedal.

[0024] However, since the tires are locked and slipping, it is difficult to stop the vehicle from sliding down quickly. In addition, in an unsteerable state during the slide, if the road surface has a transverse gradient or is a split μ road surface, a naturally occurring yaw rate (hereinafter referred to as a "natural yaw rate") may occur in the vehicle M, causing the vehicle's orientation to gradually change. If the DSS_ECU 45 detects a slide when the driver stops the vehicle M on an extremely low μ road surface, the DSS_ECU 45 performs driving assistance to recover the gripping force of the tires to quickly suppress the slide and stabilize the attitude of the vehicle M.

[0025] Specifically, the DSS_ECU 45 executes the skid suppression control according to a skid suppression control routine shown in Fig. 2. In this routine, first, in step S1, it is determined whether the brake switch 26 is ON to check whether the driver is depressing the brake pedal. If the brake switch 26 is OFF, the routine is terminated. If the brake switch 26 is ON, it is determined that the driver is depressing the brake pedal, and the process proceeds to step S2.

[0026] In step S2, it is determined whether the vehicle M is skidding or not based on the road surface gradient direction detected by the longitudinal acceleration sensor 28, the drive wheel speeds of the four wheels detected by the wheel speed sensor 27, and the vehicle body speed Vv detected by the vehicle body speed detection unit 29. If the vehicle body speed Vv and the wheel speed Vw are both 0 [Km / h] or are substantially the same speed, it is determined that skidding has not occurred, and the routine is terminated. On the other hand, if the vehicle is traveling on a slope where the road surface gradient direction is detected by the longitudinal acceleration sensor 28, and the wheel speed Vw is 0 [Km / h] and the vehicle body speed Vv is greater than 0 [Km / h] (Vv>0), it is determined that skidding due to slipping has occurred, and the routine proceeds to step S3. Therefore, even if a slight slip is detected just before stopping on an extremely low μ road surface, if the vehicle is not traveling on a slope, it is not determined that skidding has occurred.

[0027] Proceeding to step S3, the direction in which the vehicle M is skidding is checked based on the longitudinal acceleration direction (road surface gradient direction) detected by the longitudinal acceleration sensor 28. That is, if the longitudinal acceleration sensor 28 detects longitudinal acceleration while traveling downhill, it is determined that the skidding is occurring in the forward direction (forward) of the vehicle M. On the other hand, if the longitudinal acceleration sensor 28 detects longitudinal acceleration while traveling uphill, it is determined that the skidding is occurring in the backward direction (rear) of the vehicle M. The processing in step S3 corresponds to the skidding direction determination unit of the present invention.

[0028] If it is determined that the host vehicle M is sliding forward (see FIG. 6), the process proceeds to step S4, where forward slide suppression control is executed. If it is determined that the host vehicle M is sliding backward (see FIG. 7), the process branches to step S5, where backward slide suppression control is executed. The processes in steps S2 and S3 correspond to the slide judgment unit of the present invention.

[0029] The forward slide suppression control executed in step S4 described above is processed according to a forward slide suppression control subroutine shown in Fig. 3. Meanwhile, the rearward slide suppression control executed in step S5 is processed according to a rearward slide suppression control subroutine shown in Fig. 4.

[0030] First, the process of the forward slide suppression control subroutine shown in Fig. 3 will be described. In this subroutine, the steering angle detected by the steering angle sensor 30 is read in step S11, and the process proceeds to step S12 to check whether or not there is a steering input due to the driver's operation of the steering wheel 23 based on the steering angle. If the steering angle is detected, the process proceeds to step S13, and if the steering angle is not detected, i.e., if it is determined that the driver is not operating the steering wheel 23, the process branches to step S14.

[0031] The driver may steer while the vehicle is sliding down when he / she is trying to correct the turned attitude of the vehicle M or when he / she is intentionally trying to guide the vehicle M in a certain direction (for example, toward a snow surface where no ruts are formed in order to recover grip). On the other hand, the driver may not operate the steering wheel 23 when the vehicle M is not turning (yaw rate is generated) and is sliding down in a straight line or is shaking and unable to operate the steering wheel.

[0032] Therefore, when the steering of the driver is detected, driving assistance according to the driver's intention is executed in steps S13, S15, and S19. On the other hand, when the driver is not steering, driving assistance for stabilizing the attitude of the host vehicle M is executed in steps S14, S16, and S19.

[0033] When it is determined that the driver has input a steering wheel and the process proceeds to step S13, the required yaw rate is calculated or set by map search based on the steering angle detected by the steering angle sensor 30 and the vehicle speed, and the process proceeds to step S15. Since the vehicle speed during a downward slide is slow, the vehicle speed may be set to a fixed value. The process in step S13 and step S24, which will be described later, corresponds to the required yaw rate setting unit of the present invention.

[0034] In step S15, the slip ratio λ of each drive wheel Aw is set based on the distribution (slip distribution) of the slip ratios of all drive wheels Aw (=Fl, Fr, Rl, Rr) corresponding to the required yaw rate using the following equation (1), and the process proceeds to step S19. λ=[(Vv-Vw) / Vv]·100[%] …(1) That is, when all the drive wheels Aw are locked, even if the driver operates the steering wheel 23, the grip force of the tires is not recovered and no lateral force is generated, so it is difficult to turn the vehicle M in the desired direction. Therefore, the slip ratio λ of each drive wheel Aw is adjusted to provide a difference in the grip force of each drive wheel Aw, thereby turning the vehicle M in the steering direction of the driver.

[0035] Fig. 8A shows a schematic diagram of the slip distribution of all drive wheels Aw corresponding to the steering direction of the driver when the vehicle skids forward. Fig. 10A shows the vehicle behavior based on the slip distribution set when the driver steers to the right, and Fig. 10B shows the vehicle behavior based on the slip distribution set when the driver steers to the left.

[0036] When the driver steers the steering wheel 23 to the right during forward sliding, the driver is trying to turn the vehicle M to the right, so the slip ratio λ of the right rear drive wheel Rr is kept "large" (for example, λ≒100[%]) to maintain a nearly locked state, the slip ratio λ of the left rear drive wheel Rl and the right front drive wheel Fr is set to "medium" (for example, λ=70-50[%]), and the slip ratio λ of the left front drive wheel Fl is set to "small" (for example, λ=30[%] or less) to recover the grip force. As a result, as shown in FIG. 10A, the left front drive wheel Fl, which is the outer wheel side with the smallest slip ratio λ, is slowly driven, and the drive wheels Rl and Fr with the "medium" slip ratio λ are driven to follow it, making it possible to turn the vehicle M to the right.

[0037] Similarly, when the driver steers the steering wheel 23 to the left, the driver is trying to turn the vehicle M to the left, so contrary to right steering, the slip ratio λ of the left rear drive wheel Rl is set to "large" to maintain a nearly locked state, the slip ratio λ of the right rear drive wheel Rr and the left front drive wheel Fl is set to "medium," and the slip ratio λ of the right front drive wheel Fr is set to "small" to recover grip. As a result, the right front drive wheel Fr, which is the outer wheel with a "small" slip ratio λ, is slowly driven, and the drive wheels Rr and Fl with a "medium" slip ratio λ are driven to follow it, making it possible to turn the vehicle M to the left.

[0038] On the other hand, when the process branches from step S12 to step S14, the natural yaw rate detected by the yaw rate sensor 31 is read, and in step S16, it is checked whether the natural yaw rate is within a specified range. This specified range is a range in which the host vehicle M is considered to be sliding in a straight line direction, and is a fixed value previously determined and set through experiments, etc. If the direction of the natural yaw rate differs between right and left turns, the absolute value of this natural yaw rate is compared with the specified range.

[0039] If the natural yaw rate is determined to be within the specified range, the process proceeds to step S6 in Fig. 2. If the natural yaw rate is detected to be greater than the specified range, the process proceeds to step S17. The processes in steps S15 and S17 described above and steps S26 and S28 described below correspond to the slip ratio setting unit of the present invention.

[0040] In step S17, a target steering angle that offsets the natural yaw rate is set, and a slip ratio λ of each drive wheel Aw is set based on the slip distribution of each drive wheel Aw, and the process proceeds to step S18. The target steering angle is a steering angle that generates a yaw rate (reverse yaw rate) in the opposite direction to the natural yaw rate acting on the vehicle M, and is set based on the reverse yaw rate and the vehicle speed. Note that the vehicle speed during skidding is low, so the target steering angle may be a fixed value.

[0041] That is, when all the drive wheels Aw are locked, the DSS_ECU 45 sets the target steering angles of the left and right front drive wheels Fl, Fr, and the EPS_ECU 43 drives the EPS motor 25 to steer, but the tire grip force is not restored and no lateral force is generated. However, if the slip ratio λ on the outer wheel side is set to "small" and the grip force is restored, steering becomes possible. Therefore, the slip ratio λ of each drive wheel Aw is adjusted to provide a difference in the grip force of each drive wheel Aw, and the host vehicle M is turned in a direction that generates a reverse yaw rate, and the posture of the host vehicle M is returned to a straight line.

[0042] Fig. 8B shows a schematic diagram of the counter steering direction that generates a reverse yaw rate and the slip distribution of all the drive wheels Aw when a natural yaw rate occurs in the host vehicle M during forward sliding. Fig. 10C shows the vehicle behavior based on the slip distribution set when a clockwise natural yaw rate occurs, and Fig. 10D shows the vehicle behavior based on the slip distribution set when a counterclockwise natural yaw rate occurs.

[0043] When the vehicle M slides forward and experiences a natural right-handed yaw rate as shown in Fig. 10C, in order to generate a reverse yaw rate, the target steering angle is set to left steering (counter-steer direction) and the slip ratio λ of the right front drive wheel Fr, which is the outer wheel that requires the most driving force, is set to "small" to recover grip. To follow this, the slip ratios λ of the left front drive wheel Fl and the right rear drive wheel Rr are set to "medium". Furthermore, the slip ratio λ of the left rear drive wheel Rl is set to "large" to maintain an almost tire-locked state.

[0044] On the other hand, when the vehicle M slides forward and experiences a natural yaw rate in the left direction as shown in Fig. 10D, in order to generate a reverse yaw rate, the target steering angle is set to steering to the left (counter-steer direction) and the slip ratio λ of the right front drive wheel Fr, which is the outer wheel that requires the most driving force, is set to "small" to recover grip. To follow this, the slip ratios λ of the left front drive wheel Fl and the right rear drive wheel Rr are set to "medium". Furthermore, the slip ratio λ of the left rear drive wheel Rl is set to "large" to maintain an almost tire-locked state.

[0045] Next, the process proceeds to step S18, where data corresponding to the target steering angle is sent to the EPS_ECU 43, and the process proceeds to step S19. The EPS_ECU 43 calculates the rotation angle of the EPS motor 25 corresponding to the target steering angle, and drives the EPS motor 25 with the drive signal. The EPS motor 25 then operates the steering mechanism 21, and steers the left and right front drive wheels Fl, Fr by a set angle via the tie rod 22.

[0046] On the other hand, when the process proceeds from step S15 or step S18 to step S19, a process is executed to set and output the drive torque for each drive wheel Aw, and the process proceeds to step S6 in Fig. 2. The drive torque setting / output process for each drive wheel Aw in step S19 is executed according to a drive torque setting / main force process subroutine shown in Fig. 5. The process in this subroutine will be described later.

[0047] 2 to step S5, and the rearward slide suppression control subroutine shown in FIG. 4 is executed. First, in step S21, the DSS_ECU 45 executes a temporary reverse mode to temporarily switch the driving mode to the reverse mode, and transmits a mode switching signal to the PW_ECU 42 and the motor_ECU 44.

[0048] Then, the PW_ECU 42 and the motor_ECU 44 switch the mode to reverse and rotate and drive each drive wheel Aw in the backward direction according to the required drive torque set in a drive torque setting / output subroutine shown in Fig. 5, which will be described later. This temporary reverse mode is released when the driver releases the brake pedal, that is, when the brake switch 26 is turned off. The process in step S21 corresponds to the temporary reverse mode execution unit of the present invention.

[0049] Next, in step S22, the steering angle detected by the steering angle sensor 30 is read, and in step S23, it is checked based on this steering angle whether or not there is a steering input due to the driver's operation of the steering wheel 23. If a steering angle is detected, the process proceeds to step S24, and if it is determined that the driver is not operating the steering wheel 23 and a steering angle has not been detected, the process branches to step S25.

[0050] 7, the driver may steer when the vehicle M is sliding backwards mainly when trying to right the turned attitude of the vehicle M, or when trying to move the front drive wheels Fl, Fr or the rear drive wheels Rl, Rr of the vehicle M out of the ruts and move the vehicle M to a place where the tires are likely to grip, etc. On the other hand, when the driver is not operating the steering wheel 23, it may be that the vehicle M is sliding down in a straight line, or is shaking and unable to operate the steering wheel, etc.

[0051] Therefore, when the steering of the driver is detected, driving assistance according to the driver's intention is executed in steps S24, S26, and S30. On the other hand, when the driver is not steering, driving assistance for stabilizing the attitude of the host vehicle M is executed in steps S25, S28, S29, and S30.

[0052] If it is determined that there is a steering input from the driver and the process proceeds to step S24, the required yaw rate is calculated or set by map search based on the steering angle detected by the steering angle sensor 30 and the vehicle speed, and the process proceeds to step S26. Note that the vehicle speed during a downward slide is slow, so it may be set to a fixed value.

[0053] In step S26, the slip ratio λ [%] of each drive wheel Aw is set based on the slip distribution of each drive wheel Aw corresponding to the required yaw rate, and the process proceeds to step S30. When all drive wheels Aw are locked, no lateral force is generated in the tires, and it is difficult to turn the host vehicle M in the desired direction, so the slip ratio λ of each drive wheel Aw is adjusted to turn the host vehicle M in the driver's steering direction.

[0054] Fig. 9A shows a schematic diagram of the slip distribution of each drive wheel Aw corresponding to the steering direction of the driver when the vehicle rolls backward. Fig. 11A shows the vehicle behavior based on the slip distribution set when the driver steers to the right, and Fig. 11B shows the vehicle behavior based on the slip distribution set when the driver steers to the left.

[0055] If the driver steers the steering wheel 23 to the right while the vehicle is sliding backward, the driver is trying to turn the vehicle M to the left, so the slip ratio λ of the left front drive wheel Fl and the left rear drive wheel Rl, which bulge outward, is set to "small" to recover grip. Also, the slip ratio λ of the inner right front drive wheel Fr and the right rear drive wheel Rr is set to "medium" to make them follow the left front drive wheel Fl and the left rear drive wheel Rl. In this case, it is thought that the follow-up ability would improve if the slip ratio λ of the inner right front drive wheel Fr was set to "large", but since the right front drive wheel Fr is a steered wheel, it is necessary to recover a certain degree of grip by setting the slip ratio λ to "medium".

[0056] As a result, as shown in FIG. 11A, the left front drive wheel Fl and the left rear drive wheel Rl, which have a "small" slip ratio λ, are driven slowly, and the right front drive wheel Fr and the right rear drive wheel Rr, which have a "medium" slip ratio λ, are driven to follow them, thereby slowly turning the host vehicle M to the left.

[0057] Similarly, when the driver steers the steering wheel 23 to the left, the driver is trying to turn the vehicle M to the right, so contrary to right steering, the slip ratio λ of the right front drive wheel Fr and right rear drive wheel Rr on the outer wheels is set to "small" to recover grip force. Also, the slip ratio λ of the left front drive wheel Fl and left rear drive wheel Rl on the inner wheels is set to "medium" to follow the right front drive wheel Fr and right rear drive wheel Rr. In this case, too, since the left front drive wheel Fl on the inner wheel side is a steered wheel, the slip ratio λ is set to "medium" rather than "large" to recover a certain degree of grip force.

[0058] As a result, as shown in FIG. 11B, the right front drive wheel Fr and the right rear drive wheel Rr, which have a "small" slip ratio λ, are driven slowly, and the left front drive wheel Fl and the left rear drive wheel Rl, which have a "medium" slip ratio λ, are driven to follow them, causing the host vehicle M to turn slowly to the right.

[0059] On the other hand, when the flow branches from step S23 to step S25, the natural yaw rate detected by the yaw rate sensor 31 is read, and in step S27, it is checked whether the natural yaw rate is within a specified range. If it is determined that the natural yaw rate is within the specified range, the flow proceeds to step S6 in Fig. 2. If a natural yaw rate larger than the specified range is detected, the flow proceeds to step S28.

[0060] In step S28, a target steering angle that offsets the natural yaw rate and a slip ratio λ of each drive wheel Aw based on the slip distribution of each drive wheel Aw are set, and the process proceeds to step S29. The target steering angle is a steering angle that generates a yaw rate (reverse yaw rate) in the opposite direction to the natural yaw rate acting on the vehicle M, and is set based on the reverse yaw rate and the vehicle speed. Note that the vehicle speed during skidding is low, so it may be set to a fixed value.

[0061] That is, when all the drive wheels Aw are locked, the DSS_ECU 45 sets the target steering angles of the left and right front drive wheels Fl, Fr, and the EPS_ECU 43 drives the EPS motor 25 to steer, but the tire grip force is not restored and no lateral force is generated. However, if the slip ratio λ on the outer wheel side is set to "small" and the grip force is restored, steering becomes possible. Therefore, the slip ratio λ of each drive wheel Aw is adjusted to provide a difference in the grip force of each drive wheel Aw, and the steering wheel side of the host vehicle M is turned in a direction in which a reverse yaw rate is generated, and the posture of the host vehicle M is returned to a straight line.

[0062] Fig. 9B shows a schematic diagram of the steering direction that generates a reverse yaw rate and the slip distribution of all the drive wheels Aw when a natural yaw rate occurs in the host vehicle M during backward sliding. Fig. 11C shows the vehicle behavior based on the slip distribution set when a clockwise natural yaw rate occurs, and Fig. 11D shows the vehicle behavior based on the slip distribution set when a counterclockwise natural yaw rate occurs.

[0063] In the case where a natural yaw rate in the right direction occurs in the host vehicle M while sliding backward, in order to generate a reverse yaw rate, the target steering angle is set to right steering, and the slip ratio λ of the left front drive wheel Fl and the left rear drive wheel RI, which are the outer wheels that require the most driving force, is set to "small" to recover the grip force. In order to follow this, the slip ratio λ of the right front drive wheel Fr and the right rear drive wheel Rr is set to "medium".

[0064] Next, the process proceeds to step S29, where data corresponding to the target steering angle is transmitted to the EPS_ECU 43, and the process proceeds to step S30. The EPS_ECU 43 calculates a rotation angle of the EPS motor 25 corresponding to the target steering angle, and drives the EPS motor 25 with the drive signal, and the EPS motor 25 steers the left and right front drive wheels Fl, Fr by a set angle via the steering mechanism 21 and the tie rod 22. On the other hand, when the process proceeds from step S26 or step S29 to step S30, a process of setting and outputting drive torque for each drive wheel Aw is executed, and the process proceeds to step S6 in FIG. 2.

[0065] The drive torque setting / output of each drive wheel Aw, which is executed in step S19 in Fig. 3 or step S30 in Fig. 4, is processed according to a drive torque setting / main power processing subroutine shown in Fig. 5. First, in step S31, the DSS_ECU 45 reads the brake fluid pressure detected by the brake fluid pressure sensor 32, and then, in step S32, estimates the braking torque applied to each drive wheel Aw based on the brake fluid pressure. The processing in steps S31 and S32 corresponds to the braking torque estimation unit of the present invention.

[0066] Next, the process proceeds to step S33, where the required wheel speed Vw for each drive wheel Aw is calculated based on the slip ratio λ set for each drive wheel Aw using the above-mentioned formula (1), the process proceeds to step S34, where the required driving force torque for each drive wheel Aw is set based on the braking torque and the required wheel speed Vw for each drive wheel Aw, and the process proceeds to step S35. That is, in this step S34, the DSS_ECU 45 sets the required driving torque for rotating the tires of each drive wheel Aw against the braking torque applied to each drive wheel Aw. The processes in these steps S33 and S34 correspond to the driving torque setting unit of the present invention.

[0067] Then, when the process proceeds to step S35, the DSS_ECU 45 outputs the required driving torque set for each driving wheel Aw to each of the control units 41 to 44, and the process proceeds to step S6 in FIG.

[0068] Then, the PW_ECU 42 sets the output of the drive source 11 and the gear ratio of the transmission 12 based on the required drive torque for the left and right front drive wheels Fl, Fr. The T / C_ECU 41 adjusts the fastening force of the left and right multi-plate clutches 15l, 15r provided in the front differential 13 to set the wheel speeds of the left and right front drive wheels Fl, Fr. The motor_ECU 44 sets the drive torques of the left and right drive motors 12l, 12r based on the required drive torque to set the wheel speeds of the left and right rear drive wheels Rl, Rr. Then, by these controls, the tires of each drive wheel Aw are rotated slowly at a wheel speed corresponding to the slip ratio λ to recover the desired grip force. As a result, it becomes possible to guide the vehicle M in a direction according to the driver's intention, and the driver's upset due to sliding downhill is suppressed, thereby reducing anxiety.

[0069] Then, after executing the forward slide suppression control shown in step S4 of FIG. 2 or the rearward slide suppression control shown in step S5, the process proceeds to step S6, where the DSS_ECU 45 reads the vehicle speed Vv detected by the vehicle speed detection unit 29 and checks whether the slide of the host vehicle M has stopped. If it is determined that the slide has not stopped, the routine is terminated. On the other hand, if it is determined that the slide of the host vehicle M has stopped, the routine is ended. Note that if the driver releases the brake pedal (brake switch is OFF), the routine is terminated in step S1, and the slide suppression control is released.

[0070] In this manner, according to this embodiment, even if the vehicle M slides down when the vehicle M is stopped on a slope with a low μ road surface and the driver continues to depress the brake pedal and the tires remain locked, the DSS_ECU 45 outputs a drive torque corresponding to a predetermined slip ratio λ to each drive wheel Aw against the braking torque that continues the tire lock, so that the grip of the tires is restored and the vehicle can be guided in a direction according to the driver's intention. As a result, the driver's shaking when the vehicle M slides down is suppressed, and the sense of anxiety can be reduced.

[0071] In addition, when a natural yaw rate is detected when the vehicle M is sliding downhill, the DSS_ECU 45 generates a reverse yaw rate that offsets the natural yaw rate by adjusting the slip rate λ of each drive wheel Aw and the steering angle of the left and right front drive wheels Fl and Fr, which are the steered wheels, thereby correcting the posture of the vehicle M while sliding downhill. Therefore, even if the driver becomes shaken and is unable to steer the vehicle, the vehicle M can maintain a stable posture, giving the driver a sense of security.

[0072] The present invention is not limited to the above-described embodiment, and the left and right front drive wheels Fl, Fr may be driven independently by left and right drive motors. In this case, the drive source 11, the transmission 12, the front differential 13, and the T / C_ECU 41 are not required, and the left and right drive motors are controlled by the PW_ECU 42. [Explanation of symbols]

[0073] 1F: Front drive unit, 1R: Rear drive unit, 11...Drive source, 12…Gearbox, 12a...output shaft, 12b…Tribe pinion gear, 12l, 12r: Left and right drive motors, 13...Front differential mechanism, 13a…Ring gear, 13b...Ring gear shaft, 14...Axle shaft, 15a…Outer plate, 15b…inner plate, 15l, 15r...Multi-plate clutch, 21...Steering mechanism, 22…Tie rod, 23…Handle, 24...Steering shaft, 25…EPS motor, 26…Brake switch, 27...Wheel speed sensor, 28...Front and rear acceleration sensor, 29... Vehicle speed detection unit, 30...Steering angle sensor, 31...Yaw rate sensor, 32...Brake fluid pressure sensor, 41...Transfer control unit (T / C_ECU), 42...Power control unit (PW_ECU), 43...EPS control unit (EPS_ECU), 44...Motor control unit (Motor_ECU), 45…DSS control unit (DSS_ECU), Aw...Drive wheels, Fl…Front left drive wheel, Fr…Front right drive wheel, M: Vehicle, Rl…left rear drive wheel, Rr…Right rear drive wheel, Vv… Vehicle body speed, Vw… Wheel speed (required wheel speed), m… Own vehicle (mass, λ… Slip ratio, θ… Road surface gradient, μ… Static friction coefficient

Claims

1. A drive source unit that applies drive force to each of front and rear drive wheels that can be driven independently; A brake detection unit that detects depression of a brake pedal; a vehicle speed detection unit for detecting a vehicle speed of the host vehicle; a wheel speed detection unit for detecting a wheel speed of each of the drive wheels; a driving force control unit for controlling the driving force of each of the driving wheels; A driving assistance device comprising: The driving force control unit is a skid determination unit that detects depression of a brake pedal by the brake detection unit and determines whether or not the host vehicle is skidding based on a relationship between the wheel speeds of the drive wheels detected by the wheel speed detection unit and the vehicle body speed detected by the vehicle body speed detection unit; a braking torque estimation unit that estimates a braking torque acting on each of the drive wheels when the skid determination unit determines that a skid has occurred; a slip ratio setting unit that sets a slip ratio of each of the drive wheels based on a preset slip distribution when the skid determination unit determines that a skid has occurred in the host vehicle; a drive torque setting unit that sets a drive torque for driving each of the drive wheels against the braking torque estimated by the braking torque estimation unit, based on the slip ratio of each of the drive wheels set by the slip ratio setting unit; A driving assistance device comprising:

2. The vehicle further includes a steering angle detection unit that detects a steering angle generated by a steering wheel operation by a driver, The driving force control unit is a required yaw rate setting unit that sets a yaw rate required by the driver based on the steering angle detected by the steering angle detection unit; Further comprising: The slip ratio setting unit sets a slip ratio of each of the drive wheels corresponding to the required yaw rate set by the required yaw rate setting unit.

2. The driving support device according to claim 1.

3. a yaw rate detection unit that detects a yaw rate generated in the host vehicle; A steering motor that drives the steering and When the steering angle is not detected by the steering angle detection unit and a yaw rate is detected by the yaw rate detection unit, the slip ratio setting unit drives the steering motor at a target steering angle that offsets the yaw rate.

2. The driving support device according to claim 1.

4. The slip ratio setting unit sets the slip ratio of the steering wheel on the outer wheel side among the driving wheels when the host vehicle is steered to a minimum.

4. The driving support device according to claim 2 or 3.

5. The driving force control unit is a skid direction determination unit that determines a skid direction of the host vehicle when the skid determination unit determines that the host vehicle is skidding; a temporary reverse mode execution unit that executes a temporary reverse mode when the slide direction determination unit determines that the host vehicle is slide backward, and rotates and drives each of the drive wheels in a backward direction; The driving support device according to any one of claims 1 to 4, further comprising:

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