JPH08127356A - Rear wheel steering device for vehicle - Google Patents

Abstract

PURPOSE: To reduce a burden applied to an actuator for steering a rear wheel so as to attain extending a life of this actuator, by controlling a target steering amount to a contraction side by a target steering amount suppressing means, when a lateral force decreased condition, that maximum lateral force generatable in the rear wheel is decreased to a prescribed value or less, is detected by a lateral force decreased condition detecting means. CONSTITUTION: When control of ABS is executed in a rear wheel side (S240), a control device serves as a lateral force decreased condition detecting means for a rear wheel. Next, a hold value Khold is set to a variable K2 (S270), to calculate a target steering angle θr of the rear wheel (S280), and thereafter based on a difference from an actual rear wheel steering angle read from a rear wheel steering angle sensor 60, a voltage signal is output to a rear wheel steering actuator from an output interface (S290). These constituted a yaw rate feedback gain suppressing means, to control the target steering angle θr to a contraction side. Here by eliminating worsening a control effect and further in addition by reducing a burden to the rear wheel steering actuator, its life is extended.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vehicle rear wheel steering system for steering and controlling rear wheels.

[0002]

2. Description of the Related Art Conventionally, as a rear wheel steering device for a vehicle,
It is known to steer the rear wheels using the rotational angular velocity (yaw rate) of the vehicle in the vertical axis direction as a parameter. Such rear wheel steering control is called yaw rate proportional control, and the rear wheel steering amount is determined by multiplying the yaw rate by a coefficient (hereinafter referred to as yaw rate feedback gain) according to the vehicle speed.

By the way, there are cases where the vehicle is equipped with an ABS (anti-lock brake system). ABS
In general, the front-and-rear slip ratio of the wheels is high under the condition that the vehicle operates, and the running stability of the vehicle tends to decrease. The ABS operates to keep the front-and-rear slip ratio of the wheels within an appropriate range to suppress the deterioration of the running stability of the vehicle. It has been proposed to increase the yaw rate feedback gain in the rear wheel steering system during the ABS operation to enhance the rear wheel steering control effect to further compensate the running stability of the vehicle (Japanese Patent Laid-Open No. 4-176780). Gazette).

[0004]

However, in the above-mentioned conventional rear wheel steering device, when the ABS control is executed on the rear wheel side, the rear wheel steering control effect cannot be sufficiently obtained. When the ABS control is being executed on the rear wheels is when the slip ratio of the rear wheels becomes large, and at this time, only a small lateral force is generated on the rear wheels. Therefore, even if the yaw rate feedback gain is increased to increase the steering amount of the rear wheels, the generated lateral force is small, so that it is not possible to obtain a control effect commensurate with the steering amount of the rear wheels.

In the above-mentioned conventional rear wheel steering system, since the yaw rate feedback gain is increased to increase the steering amount of the rear wheels, the load on the actuator for steering the rear wheels becomes heavy. If the rear wheel steering control effect cannot be ensured as described above, there is no point in increasing the load on the actuator. As a result, the burden on the actuator is unnecessarily heavy and the life of the actuator is shortened.

The vehicle rear wheel steering system of the present invention has been made in view of these problems, and it is possible to prolong the life of the actuator by reducing the load on the rear wheel steering actuator. Has an aim.

[0007]

In order to achieve such an object, the following constitution is adopted as a means for solving the above problems.

That is, a rear wheel steering system according to claim 1 of the present invention is a rear wheel steering actuator for steering and controlling a rear wheel according to a control signal from the outside, and a rear wheel target based on a predetermined parameter. In a rear-wheel steering system for a vehicle, comprising: a control unit that calculates a steering amount and outputs a control signal based on the target steering amount to the rear-wheel steering actuator. Provided are lateral force reduction state detection means for detecting a lateral force reduction state that is equal to or less than a predetermined value, and target steering amount suppression means for controlling the target steering amount to a reduction side when the lateral force reduction state is detected. That is the summary.

In the vehicle rear wheel steering system having the above structure,
The lateral force reduction state detection means includes a slip ratio detection unit that detects a slip ratio of the rear wheel, and a determination unit that determines that the slip ratio of the rear wheel is equal to or more than a predetermined value as the lateral force reduction state. It may be configured to include (claim 2).

Further, in the vehicle rear wheel steering system having the above configuration, the predetermined value in the lateral force reduction state detecting means may be set to a magnitude determined by the centrifugal force applied to the rear wheels (claim 3). ).

Further, in the third aspect of the present invention, the lateral force reduction state detecting means includes a side slip detecting section for detecting a side slip angle and a side slip angular velocity of the vehicle, respectively, and the detected side slip angle and side slip of the vehicle. It is determined whether or not the angular velocities are equal to or more than predetermined values, respectively, and the affirmative determination is defined as the time when the maximum lateral force that can be generated on the rear wheel is equal to or less than the magnitude determined by the centrifugal force applied to the rear wheel. It may be configured to include a determination unit (the one according to claim 4).

Further, the target steering amount suppressing means may be provided with a target steering amount minute control unit for controlling the target steering amount to a predetermined minute amount close to a value of 0 (claim 5). .

[0013]

According to the vehicle rear wheel steering system of the present invention, when the lateral force reduction state detecting means detects a lateral force reduction state in which the maximum lateral force that can be generated on the rear wheels is equal to or less than a predetermined value, the target is set. The target steering amount is controlled to the reduction side by the steering amount suppressing means.
When this lateral force reduction state is detected, even if the steering amount of the rear wheels is increased, the control effect commensurate with the steering amount cannot be obtained. Therefore, in the lateral force reduction state in which the rear wheel steering control effect is small, the load on the rear wheel steering actuator is reduced by controlling the target steering amount to the reduction side and shifting the rear wheel steering amount to the reduction side. Let

According to another aspect of the present invention, there is provided a rear wheel steering system for a vehicle,
When the slip ratio of the rear wheels detected by the slip ratio detecting unit is equal to or more than a predetermined value, the determining unit determines that the lateral force is in a reduced state. For this reason, it becomes possible to detect the lateral force reduction state from a parameter such as a slip ratio that is relatively easy to detect.

According to another aspect of the present invention, there is provided a rear wheel steering system for a vehicle comprising:
The lateral force reduction state is defined when the maximum lateral force that can be generated on the rear wheel becomes less than or equal to the magnitude determined by the centrifugal force applied to the rear wheel. The maximum lateral force that can be generated by the rear wheel is equal to or less than the centrifugal force when the centrifugal force exceeds the maximum lateral force that can be generated by the rear wheel, and in this case, the rear wheel is in a skid state. In this case, even if the steering amount of the rear wheels is increased, the control effect is small. Therefore, by reducing the target steering amount, the load on the rear wheel steering actuator is reduced.

According to another aspect of the present invention, there is provided a rear wheel steering system for a vehicle comprising:
The sideslip angle and the sideslip angular velocity of the vehicle are detected by the sideslip detector, and whether or not the detected sideslip angle and sideslip angular velocity of the vehicle are each equal to or greater than a predetermined value,
The determination by the determination unit determines when the maximum lateral force that can be generated on the rear wheel becomes equal to or less than the magnitude determined by the centrifugal force applied to the rear wheel. Therefore, it becomes possible to detect the lateral force reduction state from the sideslip angle and the sideslip angular velocity of the vehicle.

According to a fifth aspect of the present invention, there is provided a rear wheel steering system for a vehicle,
The target steering amount is controlled by the target steering amount minute control unit to a predetermined minute amount close to 0. As a result, the smaller the steering angle of the rear wheels, the smaller the generated lateral force of the rear wheels. When the generated lateral force of the rear wheel becomes smaller, the longitudinal force of the rear wheel becomes larger due to the relationship between the lateral force of the wheel and the longitudinal force. Therefore, the longitudinal force of the rear wheel is secured, thereby improving the running performance of the vehicle. It becomes possible.

[0018]

Preferred embodiments of the present invention will be described below in order to further clarify the structure and operation of the present invention described above. FIG. 1 is a schematic configuration diagram of a vehicle rear wheel steering system as a first embodiment of the present invention, and FIG. 2 is a block diagram showing an electrical configuration of the rear wheel steering system.

As shown in FIG. 1, the vehicle rear wheel steering system according to this embodiment includes a front wheel steering mechanism 20 for connecting a right front wheel 10FR and a left front wheel 10FL of a vehicle (rear wheel drive) and a right rear wheel 10.
A rear wheel steering mechanism 30 that links the RR and the left rear wheel 10RL is provided.

The front wheel steering mechanism 20 includes a pair of left and right knuckle arms 21L and 21R and tie rods 22L and 22L.
R and a relay rod 23 that connects the tie rods 22L and 22R to each other. The relay rod 23 includes a rack and pinion type steering mechanism 25.
The pinion 26, which is a component thereof, is connected to a steering wheel 29 via a shaft 27.

When the steering wheel 29 is turned to the right or left by the steering mechanism 25, the relay rod 23 is displaced leftward or rightward in FIG.
The knuckle arms 21L, 21R rotate by an amount corresponding to the steering angle of the steering wheel 29, and the front wheels 10FL, 10FR
Is steered to the right or left.

Similarly to the front wheel steering mechanism 20, the rear wheel steering mechanism 30 also has a pair of left and right knuckle arms 31L and 31R.
And tie rods 32L and 32R, and tie rods 32L and
The relay rod 33 connects 32R to each other. A rear wheel steering actuator 35 is provided in the middle of the relay rod 33 of the rear wheel steering mechanism 30. The rear-wheel steering actuator 35 is an electronic control unit for four-wheel steering (hereinafter referred to as 4WS ECU).
The relay rod 33 is displaced leftward or rightward in FIG. 1 by supplying an electric signal from 40 to the internal electric motor 35a. When the relay rod 33 is displaced in the left-right direction, the knuckle arms 31L and 31R rotate, and the rear wheels 10RL and 10RR are steered to the right or left. The detailed configuration of the rear wheel steering actuator 35 will be described later.

This vehicle is composed of a steering sensor 51 for detecting a steering angle θf (= front wheel steering angle) of the steering wheel 29 and a vibrating gyro as sensors for detecting various parameters relating to the steering state of the rear wheels. A yaw rate sensor 53 that detects a yaw rate ω, a vehicle speed sensor 54 that detects a vehicle speed (vehicle speed) V, and the like are provided. Here, the steering sensor 51 and the yaw rate sensor 53 are positive in the clockwise direction from the neutral position. Further, the rear wheel steering actuator 35 includes
A rear wheel steering angle sensor 60 that indirectly detects the rear wheel steering angle by detecting the stroke amount of the relay rod 33 is provided. Even in the rear wheel steering angle sensor 60, the clockwise direction from the neutral position is positive. These sensors are EC for 4WS
It is connected to U40. The electric motor 35a of the rear wheel steering actuator 35 is connected to the 4WS ECU 40, and the rear wheel steering actuator 35 is controlled by the 4WS ECU 40.

As shown in FIG. 2, the 4WS ECU 40
Is configured as a logical operation circuit centered on a microcomputer, and more specifically, various arithmetic processes are executed by the CPU 40a and the CPU 40a that execute various arithmetic processes for controlling the rear wheel steering amount according to a preset control program. A ROM 40b in which control programs and control data necessary for the operation are stored in advance, a RAM 40c in which various data necessary for executing various arithmetic processes in the CPU 40a are temporarily read and written, and detection signals from the above sensors are input. Input interface 40d, CPU 40
The rear wheel steering actuator 35 according to the calculation result in a.
And an output interface 40e for outputting an electric signal to the electric motor 35a.

The 4WS ECU 40 is provided with a second input interface 40f, and an electronic control unit (hereinafter referred to as ABS ECU) 45 for an ABS (antilock brake system) mounted on the vehicle 45.
From the input interface 40f. The ABS is a known system that controls the braking force so that the wheels do not lock when the brakes are applied, and a detailed description thereof will be omitted here.

The ABS ECU 45 is configured as a logical operation circuit centered on a microcomputer, similar to the 4WS ECU described above, and includes a CPU 45a and a ROM 45.
b, a RAM 45c, an input interface 45d, an output interface 45e and the like. The input interface 45d serves as a sensor for detecting various parameters related to the ABS control, and detects a pedal switch 55 that outputs an on / off signal BSW according to whether or not a brake pedal is operated, and a wheel speed vFL of the left front wheel 10FL. Left front wheel speed sensor 56, right front wheel 10FR wheel speed vFR
A front right wheel speed sensor 57, a left rear wheel speed sensor 58 that detects the rear left wheel 10FL wheel speed vRL, and a right rear wheel speed sensor 59 that detects the right rear wheel speed 10RR. It is connected. These sensors 55-5
Each signal input from 9 BSW, vFL, vFR, vRL, vRR
Is output from the output interface 45e,
It is input to the 4WS ECU 40 as described above.

The detailed structure of the rear wheel steering actuator 35 will be described below. FIG. 3 is a vertical cross-sectional view of the rear wheel steering actuator 35.

As shown in FIG. 3, the rear wheel steering actuator 35 includes a cylindrical casing 61. Bearing members 63 and 65 are attached to both ends of the casing 61.
An actuator shaft 67 is arranged via the.

On the other hand, an electric motor 35a is mounted on the inner wall of the casing 61 via a resin supporting member 69. The electric motor 35 a includes a coil 71 as a stator that is directly fixed to the support member 69, a cylindrical iron rotor member 72 as a rotor, and a cylindrical permanent member mounted on the outer periphery of the rotor member 72. It is composed of a magnet 73. Bearings 75 and 76 are provided at both ends of the rotor member 72, and the rotor member 72 and the permanent magnet 73 are rotatably supported by the support member 69 by the bearings 75 and 76.

A power transmission member 79 is coaxially fixed to the rotor member 72 at one end of the rotor member 72, and a gear (sun gear) 81a of a planetary gear mechanism 81 is provided on the outer periphery of the power transmission member 79. It is arranged. Planetary gear mechanism 8
1 is configured by connecting two sets of planetary gears in series. The first set of planetary gears is composed of a sun gear 81a, four planetary gears 81b and 81c (two are not shown), and a ring gear 81d, and the second set of planetary gears is a sun gear 82a and four. Planetary gears 82b, 8
2c (two are not shown) and ring gear 81d (first
Common with the group).

A nut 85 is coupled to the shafts of the four planetary gears 82b and 82c of the second set of planetary gears. The nut 85 is rotatably supported inside the casing 61 by a bearing 87, but does not move in the axial direction. A trapezoidal screw 85a is formed on the nut 85, and the trapezoidal screw 85a meshes with the trapezoidal screw 67a formed on the actuator shaft 67. Therefore, when the nut 85 rotates, the screw thread of the trapezoidal screw 85a moves in the axial direction, and the trapezoidal screw 67a meshing with it moves, so that the actuator shaft 67 moves in the axial direction. That is, the nut 85 and the trapezoidal screw 67 a convert the rotational movement of the planetary gear mechanism 81 into the linear movement of the actuator shaft 67.

Therefore, when an electric signal is supplied from the 4WS ECU 40 to the electric motor 35a of the rear wheel steering actuator 35, the planetary gear mechanism 8 is driven by the electric motor 35a.
1 makes a rotational movement, and as a result, the actuator shaft 67 makes a linear movement in the axial direction as described above. Since the relay rod 33 of the rear wheel steering mechanism 30 is connected to the actuator shaft 67, the relay rod 33 is displaced leftward or rightward in FIG. 1, and as a result, the rear wheels 10RL and 10RR are moved rightward or leftward. Steered to. The steering angle at this time is 4
It changes according to the strength of the electric signal from the WS ECU 40.

The processing executed by the 4WS ECU 40 will be described in detail below with reference to a flowchart. The 4WS ECU 40 executes ABS control determination processing for determining whether or not ABS control is being executed, and rear wheel steering control processing for steering control of the rear wheels. First, the former A
The BS control determination process will be described with reference to the flowchart of FIG. This process is repeatedly executed as an interrupt process that is started every predetermined time when the vehicle starts running after the power is turned on. It is assumed that variables, flags, etc. are set to initial values by the initialization process immediately after the power is turned on.

As shown in FIG. 4, when the process is started,
First, the CPU 40a performs a process of inputting the signals BSW, vFL, vFR, vRL, vRR sent from the ABS ECU 45 via the input interface 40d and storing the signals in the RAM 40c (step S100).

Then, the CPU 40a proceeds to step S10.
Based on the signal from the ABS ECU 45 input at 0, it is determined whether or not the ABS mounted on this vehicle is in operation (step S11).
0). Specifically, from these signals, when the condition that one of the wheels is about to be locked is detected for the first time when the brake pedal is depressed and the wheel speed decreases, the ABS is detected.
Is determined to be in operation.

If it is determined in step S110 that the ABS is in operation, then the ABS operation is
Is it executed on the 0FL, 10FR side, or the rear wheel 1
A process is performed to determine whether the 0RL, 10RR side is being executed (steps S120, S130). The ABS equipped in this vehicle is provided with a front and rear two-system braking system in which the left and right front wheels 10FL and 10FR and the left and right front wheels 10RL and 10RR are respectively provided in the same pipe.
In 20 and S130, the brake hydraulic pressure of each system is detected, and the above determination is performed from them. Note that this determination may be performed by using a control signal sent to the solenoid valve provided in each system.

At step S120, it is determined that the ABS control is being executed on the front wheels 10FL, 10FR side, and at step S130, it is determined that the ABS control is being executed on the rear wheels 10RL, 10RR. CPU 40a is AB
A value 2 is set to the flag F indicating the operation status of S (step S140).

On the other hand, when it is determined in step S120 that the ABS control is not executed on the front wheels 10FL, 10FR side, the ABS control should be executed on the rear wheels 10RL, 10RR side (step S120). AB in S110
Since it is determined that S operation is in progress), the CPU 4
0a proceeds to step S140 and sets the value 2 to the flag F.

On the other hand, in step S120, it is determined that the ABS control is being executed on the front wheels 10FL, 10FR side,
In step S130, the ABS is controlled to the rear wheels 10RL, 10
If it is determined that it is not being executed on the RR side, the CPU 40a
Sets a value 1 to the flag F indicating the operating status of the ABS (step S150).

If it is determined in step S110 that the ABS control is not in operation, the CPU 40a
Sets the value 0 to the flag F (step S16).
0). After executing step S140, S150, or S160, the process returns to "return" to end this process.

That is, in this ABS control determination processing,
When the control of S is executed at least on the side of the rear wheels 10RL, 10RR, the value 2 is set in the flag F and the ABS is set.
When the control of is executed only on the front wheels 10FL, 10FR side, the flag F is set to the value 1, and the ABS control is not executed on the front wheels 10FL, 10FR side or the rear wheels 10RL, 10RR side. Sometimes flag F has a value of 0
Is set. Since the ABS control is started based on the front-rear slip ratio, the ABS control can be regarded as a state in which the front-rear slip ratio is equal to or higher than a predetermined value. Therefore, the ABS control determination process realizes the determination unit in claim 2 and the lateral force reduction state detection unit in claim 1. Note that steps S100 to S130
Constitutes so-called "rear wheel ABS start monitoring means".

Next, the rear wheel steering control process executed by the 4WS ECU 40 will be described with reference to the flowcharts of FIGS. 5 and 6. This process is 5 after the power is turned on.
It is composed of a routine that is repeatedly executed as an interrupt process that is activated every 0 [msec] and a routine that is repeatedly executed as an interrupt process that is activated every 4 [msec].

When the 50 ms routine shown in FIG. 5 is started, the CPU 40 a first inputs the output signal of the steering sensor 51 from the input interface 40 d and stores it as the front wheel steering angle θf in the RAM 40 c, and the yaw rate sensor 53. The detected yaw rate ω is input from the input interface 40d and stored in the RAM 40c,
Further, the vehicle speed V detected by the vehicle speed sensor 54 is input from the input interface 40d and stored in the RAM 40c (step S200).

Next, the CPU 40a performs a process of obtaining the variables K1 and K2 based on the vehicle speed V (step S21).
0). The vehicle speed V is stored in the ROM 40b of the 4WS ECU 40.
The map A showing the correlation between the variable K1 and the variable K1 and the map B showing the correlation between the vehicle speed V and the variable K2 are stored in advance. In step S210, the vehicle speed V input in step S200 is illuminated on the map A. The variable K1 is also calculated and the variable K2 is calculated by comparing the vehicle speed V with the map B.

An example of map A is shown in FIG. As shown in FIG. 7, the vehicle speed V is a predetermined value V1 (for example, 10 [km /
h]) or less, the variable K1 takes the value 0 and the vehicle speed V
Is a predetermined value V1 to a predetermined value V2 (for example, 30 [km / h])
Up to, the variable K1 rapidly increases, and when the vehicle speed V is higher than the predetermined value V2, the variable K1 gradually increases. Further, an example of the map B is shown in FIG. As shown in FIG. 8, the vehicle speed V is a predetermined value V1 (for example, 10 [km /
h]) or less, the variable K1 takes the value 0 and the vehicle speed V
Is larger than the predetermined value V1, the variable K2 gradually increases.

The variable K1 is the target steering angle θr of the rear wheels.
The variable K2 corresponds to the gain applied to the yaw rate ω, and the variable K2 corresponds to the gain applied to the steering angle θf of the front wheels when calculating

After the execution of step S210, the variable K2 is multiplied by 1.5 to calculate the upper limit value Kmax of the variable K2 (step S220). Next, a variable K corresponding to a predetermined relatively small vehicle speed Vt (for example, 20 [km / h])
The value of 2 is calculated from the map B shown in FIG. 8 and the value is set as the hold value Khold (step S230). The upper limit value Kmax and the hold value Kh
old is used in a 4 ms routine described later. After executing step S230, the process returns to "return" to end this routine once.

When the 4 ms routine shown in FIG. 6 is started, the CPU 40a first performs a process of determining whether the flag F obtained in the above 50 ms routine has a value 0, a value 1 or a value 2 ( Step S240). If it is determined in step S240 that the flag F is 0,
ABS control is for front wheels 10FL, 10FR side, rear wheels 10RL, 1
Since it has not been executed on any of the 0RR sides, the process proceeds to step S280, and the process of calculating the target steering angle θr of the rear wheels is performed. This process will be described in detail in step S200.
Using the front wheel steering angle θf and the yaw rate ω input in step S1 and the variables K1 and K2 calculated in step S210, the target steering angle θr of the rear wheels is calculated according to the following equation (1).

[0049]

[Equation 1]

When the target steering angle θr of the rear wheels is calculated in step S280, thereafter, based on the difference between the target steering angle θr and the actual rear wheel steering angle read from the rear wheel steering angle sensor 60,
A voltage signal is output from the output interface 40e to the electric motor 35a of the rear wheel steering actuator 35 so that the actual rear wheel steering angle becomes the target steering angle θr (step S2).
90). As a result, the rear wheel 10 is adjusted according to the target steering angle θr.
RL and 10RR are steered. Then "return" processing
And exit once.

On the other hand, in step S240, the flag F has the value 1
If it is determined that the ABS is controlled, the front wheels 10FL, 1 are controlled by the ABS.
Since it is executed only on the 0FR side, the processing is step S2.
Go to 50. In step S250, step S210
It is determined whether or not the variable K2 calculated in step S220 exceeds the upper limit value Kmax calculated in step S220. If it is determined that the variable K2 does not exceed the upper limit value Kmax in this step, a process of incrementing the variable K2 by a small amount ΔK is performed. Perform (step S260). After that, the above-mentioned step S280,
Proceed to S290. After that, by repeatedly executing this 4 ms routine, the variable K2 is gradually increased until it reaches the upper limit value Kmax.

On the other hand, in step S240, the flag F has the value 2
If it is determined that the ABS control is performed on at least the rear wheels 10RL and 10RR, the process proceeds to step S270, and the hold value Khold calculated in step S230 is set in the variable K2. To do. Then, it progresses to steps S280 and S290. Note that steps S240, S270 and S280 constitute a so-called "control gain suppressing means", and further,
A yaw rate feedback gain suppressing means for suppressing a yaw rate feedback gain, which is a control gain, is configured. This controls the target steering angle θr toward the reduction side, and realizes the target steering amount suppressing means of claim 1. .

The ABS control determination process and the rear wheel steering control process configured as described above are executed by the CP of the 4WS ECU 40.
By being repeatedly executed by U40a, the driving state of the vehicle changes as follows. First, when the ABS control is not executed, the rear wheel steering angle θr is optimally controlled by executing the yaw rate proportional control (step S280) using the yaw rate ω as a parameter.

From this state, when the front wheels 10FL, 10FR slip and ABS control is executed on the front wheels 10FL, 10FR side, the yaw rate feedback gain K2 is gradually increased and the ABS control is executed. The value is 1.5 times as large as the value of K2 when it is not present (step S2
50, S260), the rear wheel steering angle θr becomes large. Therefore, the running stability of the vehicle is compensated by this large steering control.

On the other hand, when the rear wheels 10RL, 10RR slip and ABS control is executed also on the rear wheels 10RL, 10RR side (only on the rear wheels 10RL, 10RR side), the yaw rate feedback gain K2 becomes a predetermined value. Hold value Kh
and the rear wheel steering angle θr is set to the front wheels 10FL, 10FL.
It is held on the reduction side compared to when the ABS control is executed on the FR side.

When the ABS control is being executed on the rear wheels 10RL, 10RR side, even if the steering amounts of the rear wheels 10RL, 10RR are increased as described in the section "Problems to be solved by the invention". As the control effect corresponding to the steering amount cannot be obtained, as described above, the rear wheels 10RL, 10
Even if the steering amount of RR is held to the reduction side, the control effect is not impaired, and the rear wheel steering actuator 35
It is possible to reduce the burden on the operator. As a result, the life of the actuator 35 can be extended.

Further, in this embodiment, the rear wheels 10RL, 10
Since whether or not the RR is in the slip state is determined based on whether or not the ABS control is being executed, the detection can be performed by using the existing device mounted on the vehicle. Therefore, the configuration of the device becomes easy.

The second embodiment of the present invention will be described below. The rear wheel steering system of the vehicle of the second embodiment is different from that of the first embodiment only in the contents of the 4ms routine of the rear wheel steering control processing executed by the CPU 40a of the 4WS ECU 40. The software configuration and hardware configuration of are the same. The 4 ms routine of the rear wheel steering control process will be described below with reference to the flowchart of FIG.

In FIG. 9, the same process numbers as those in the first embodiment are designated by the same step numbers. This embodiment is different from the first embodiment in that step S2
When it is determined that the flag F has a value of 1 in 40, the following processing is executed.

That is, when it is determined that the flag F has the value 2, it is determined whether or not the previously calculated variable K2 is below the lower limit value Kmin (step S300). The lower limit value Kmin is a predetermined value, which is a value close to the value 0. In step S300, the variable K2 is the lower limit value Kmin.
If it is determined that the value is not less than the value, the variable K2 is reduced by a small amount.
A process of decrementing by K is performed (step S31).
0). Then, as in the first embodiment, step S28.
0, the process proceeds to S290, and this routine is once terminated. After that, the variable K2 is gradually reduced to the lower limit value Kmin by repeatedly executing this 4 ms routine.

Incidentally, steps S240, S300, S3
10 and S280 constitute so-called "control gain suppressing means", and further constitute "yaw rate feedback gain suppressing means for suppressing the yaw rate feedback gain which is the control gain. This is the target steering angle θ.
The target steering amount suppressing means of claim 1 is realized by controlling r to the reduction side.

According to this configuration, the rear wheels 10RL, 10
RR slips, ABS control is rear wheels 10RL, 10RR
When executed on the side, the yaw rate feedback gain K
2 gradually decreases, and the rear wheel steering angle θr is controlled to the reduction side.

As described in detail above, according to the rear wheel steering system for a vehicle of the second embodiment, the ABS is controlled by the rear wheel 10R.
Unnecessary operation of the rear wheel rudder angle when being executed on the L, 10RR side can be suppressed. Therefore, similarly to the first embodiment, the load on the rear wheel steering actuator 35 can be reduced, and the life of the actuator 35 can be extended.

In particular, in this embodiment, the yaw rate feedback gain is reduced to a value close to 0 to set the rear wheel steering angle to a small value equal to or smaller than a predetermined angle, so that the generated lateral force of the rear wheels is suppressed. be able to. Figure 10
The force generated at the tire ground contact point is shown in Fig. 4, but as shown in this figure, the force F obtained by vector-combining the lateral force Fs of the wheel and the longitudinal force Ff does not exceed the friction circle R.
If the generated lateral force Fs is small, the longitudinal force Ff will be large. Therefore, after the slip of the rear wheels 10RL, 10RR is eliminated, the longitudinal force Ff of the rear wheels 10RL, 10RR can be secured, and the traveling performance of the vehicle can be improved.

The third embodiment of the present invention will be described below. The rear wheel steering system for a vehicle of the third embodiment differs from that of the first embodiment in the following points. For hardware, the ABS is equipped with a traction control system (TRC), and for software,
The vehicle state determination process of FIG. 11 is executed in place of the ABS control determination process of FIG. TRC is a well-known system that suppresses slip by controlling driving force. Other hardware configurations and software configurations are the same. The vehicle state determination process will be described below with reference to the flowchart of FIG.

As shown in FIG. 11, when the process is started, the CPU 40a of the 4WS ECU 40 first sends the wheel speed signal v sent from the TRC ECU (not shown).
FL, vFR, vRL, vRR are input via the input interface 40d and stored in the RAM 40c (step S1).
00). Next, the CPU 40a calculates the average wheel speed vF of the front wheels from vFL and vFR, and calculates the average wheel speed vR of the rear wheels from vRL and vRR (step S410).

Then, using the average wheel speed vF of the front wheels and the average wheel speed vR of the rear wheels, the slip ratio S is calculated according to the following equation (2) (step S42).
0).

[0068]

[Equation 2]

After the execution of step S420, the TRC installed in this vehicle is based on the calculated slip ratio S and the like.
Is executed (step S430). It should be noted that detailed conditions regarding this determination are not directly related to the present invention, and a description thereof will be omitted.

When it is determined in step S430 that the TRC is not in operation, the CPU 40a sets the flag F to the value 0 (step S440). On the other hand, step S43
If it is determined that the TRC is operating at 0, then
It is determined whether or not the slip ratio S calculated in step S420 is 6% or more (step S450).
Here, when it is determined that the slip ratio S is 6% or more, the process proceeds to step S440, and the value 0 is set in the flag F.

In step S450, the slip ratio S is 6
If it is determined that the slip ratio S is smaller than [%], then it is determined whether or not the slip ratio S is 4 [%] or more (step S460). If it is determined that the slip ratio S is 4% or more, that is, 4 to 6%, the flag F is set to the value 2 (step S480).

On the other hand, in step S460, the slip ratio S
Is smaller than 4 [%], the flag F has a value of 1
Is set (step S480). Step S44
After executing 0, S470, or S480, the process returns to "return" to end this process once.

That is, in this vehicle state determination processing, the TRC
When the slip ratio S is extremely large, 6% or more, even when the TRC is not operating,
When the value 0 is set in the flag F and the slip ratio S is 4 to 6 [%] during the TRC operation, the value 2 is set in the flag F and the slip ratio S is 4 [%] during the TRC operation. When it is smaller, the value 1 is set in the flag F.

In response to the value of the flag F set in this way, in this embodiment, the same rear wheel steering control processing (FIGS. 5 and 6) as in the first embodiment is executed. Therefore, when the slip ratio S is 4% or less, which is relatively small, the flag F
Is the value 1, the yaw rate feedback gain K2 is increased (step S260 in FIG. 6). When the slip ratio S is higher than 4 [%] and within 6 [%], the flag F has the value 2, so the yaw rate feedback gain K2 is suppressed to the hold value Khold. When the slip ratio S is 6% or more, the flag F has a value of 0, so the yaw rate feedback gain K2 is not corrected and remains at that value.

That is, when the slip ratio S becomes 4 [%] or more, the value becomes smaller than the value of the yaw rate feedback gain K2 increased when the slip ratio S is smaller than 4 [%] (the slip ratio S is 4 to 4). When the value is 6 [%], the hold value Khold is controlled, and when the slip ratio S is 6 [%] or more, the hold value Khold is controlled as it is.

When the slip ratio S is large, the rear wheels 10
Even if the steering amount of RL, 10RR is increased, the control effect commensurate with the steering amount cannot be obtained.
Even if the steering amounts of the rear wheels 10RL and 10RR are controlled to the reduction side, the control effect is not impaired, and the load on the rear wheel steering actuator 35 can be reduced. Therefore, similarly to the first embodiment, there is an effect that the life of the actuator 35 can be extended.

Further, in this embodiment, since the determination of the slip state of the rear wheels is made from the slip ratio calculated by TRC, the detection is performed by using the existing device mounted on the vehicle. You can Therefore, the configuration of the device becomes easy.

In the third embodiment, the determination unit according to claim 2 is realized by the processing of steps S420, S450 and S460, and the lateral force reduction state detecting means according to claim 1 is realized. .

The fourth embodiment of the present invention will be described below. The rear wheel steering system of the vehicle of the fourth embodiment differs from that of the first embodiment in the following points. The hardware is substantially the same as that of the first embodiment except that a lateral G sensor (not shown) for detecting the lateral acceleration of the vehicle is further provided. The software is different in that the vehicle state determination process of FIG. 12 is executed instead of the ABS control determination process of FIG. The other hardware configurations and software configurations are the same. Less than,
This vehicle state determination processing will be described with reference to the flowchart of FIG.

As shown in FIG. 12, when the processing is started, the CPU 40a of the 4WS ECU 40 first outputs the output signals from the steering sensor 51, the yaw rate sensor 53 and the vehicle speed sensor 54 to the input interface 40.
Input from d, and this is the front wheel steering angle θf, yaw rate ω,
The vehicle speed V is stored in the RAM 40c, the output signal of the lateral G sensor is input from the input interface 40d, and the output signal is stored in the RAM 40c as the lateral acceleration Gy (step S500).

Next, the CPU 40a uses the yaw rate ω, the vehicle speed V and the lateral acceleration Gy stored in the RAM 40c to calculate the vehicle side slip angular velocity Θ according to the following equation (3), and further uses the vehicle side slip angular velocity Θ: The vehicle side slip angle θ is calculated according to the following equation (4) (step S510).

[0082]

(Equation 3)

In step S510, the vehicle side slip angular velocity Θ
When the vehicle side slip angle θ is calculated, subsequently, a process of determining the slip state of the rear wheels 10RL, 10RR from these Θ, θ is performed (step S520). Rear wheel 10RL, 1
The 0RR slip state is a gripping state (hereinafter referred to as state I), a transition from grip to side slip (referred to as state II), and a side slip state (referred to as state III).
The two states can be distinguished, and these are correlated with the vehicle side slip angular velocity Θ and the vehicle side slip angle θ. 4WS
A map C indicating this correlation is stored in advance in the ROM 40b of the ECU 40 for the vehicle. In step S220, the vehicle lateral slip angular velocity Θ and the vehicle lateral slip angle θ calculated in step S510 are collated with the map C,
The slip condition of the rear wheel is determined.

An example of the map C is shown in FIG. FIG.
As shown in Fig. 4, the horizontal axis represents the vehicle side slip angle θ, and the vertical axis represents the vehicle side slip angular velocity Θ. The region defined by the two is divided into three regions, State I, State II, and State III. Has been. The line segment that divides each area has the relationship shown in the following expression (5).

[0085]

[Equation 4]

However, a, b and s are constants.

When it is determined in step S530 that the result of the determination is the state I, that is, the state where the rear wheels 10RL and 10RR are gripping, a value 0 is set in the flag F (step S560). On the other hand, when it is determined in step S540 that the determination result is state II, that is, the rear wheels 10RL and 10RR are in the transition state from grip to sideslip, a value 1 is set in the flag F (step S570). On the other hand, in step S550, the determination result is state III, that is,
When it is determined that the rear wheels 10RL and 10RR are in the side-sliding state, the flag F is set to the value 2 (step S57).
0). After that, the process returns to "return" to end this processing.

In response to the value of the flag F set in this way, in this embodiment, the same rear wheel steering control processing (FIGS. 5 and 6) as in the first embodiment is executed. Therefore, the rear wheel 10
When the RL and 10RR are in the gripped state, the flag F is 0, so the yaw rate proportional control is executed and the rear wheel steering angle θr is optimally controlled. On the other hand, when the rear wheels 10RL and 10RR are in the transition state from the grip to the sideslip, the flag F has the value 1, so the yaw rate feedback gain K2 is increased (step S260 in FIG. 6). When the rear wheels 10RL and 10RR are in the side-sliding state, the flag F has the value 2, so the yaw rate feedback gain K2 is suppressed to the hold value Khold. That is, when the rear wheels 10RL and 10RR are in the side-sliding state, the yaw rate feedback gain K2 increased when the rear wheels start to slide is controlled to a reduction side.

When the rear wheels 10RL, 10RR are in a side-sliding state, the maximum lateral force that can be generated on the rear wheels 10RL, 10RR is a predetermined value or less, that is, the rear wheels 10RL, 10RR.
Even if the steering amount of RR is increased, it is not possible to obtain the control effect commensurate with the steering amount.
Even if the steering amounts of RL and 10RR are controlled to the reduction side, the control effect is not impaired, and the load on the rear wheel steering actuator 35 can be reduced. Therefore, similarly to the first embodiment, there is an effect that the life of the actuator 35 can be extended.

In the fourth embodiment, the determination section in claim 4 is realized and the lateral force reduction state detecting means in claim 1 is realized by the processes of steps S510 to S550. Further, it is clear that the value used for the determination is a magnitude determined by the centrifugal force applied to the rear wheel, as described in claim 3.

The fifth embodiment of the present invention will be described below. The rear wheel steering system of the fifth embodiment differs from that of the fourth embodiment in the following points. In the fourth embodiment, 4WS
The CPU 40a of the ECU 40 executes the vehicle state determination process shown in FIG. 12 and then executes the same rear wheel steering control process as in the first embodiment. However, instead of this rear wheel steering control process, this execution is performed. In the example, the following rear wheel steering control process is executed. The hardware configuration is
Same as the embodiment.

The rear wheel steering control process will be described below with reference to FIG.
A description will be given along the flowchart of No. 4. The process of the rear wheel steering control is repeatedly executed as an interrupt process activated by the 4WS ECU 40 every predetermined time after the power is turned on.

When the rear wheel steering control routine shown in FIG. 14 is started, the CPU 40a first determines what the value of the flag F obtained in FIG. 12 is (step S60).
0). Here, if it is determined that the flag F is 0,
The processing of steps S610 to S640 called yaw rate proportional control is executed. Steps S610 to S
The process of 640 is the same as steps S200 and S2 described above.
10, S280, and S290 are the same, and detailed description thereof is omitted here.

On the other hand, if it is determined in step S600 that the flag F is 1, the process proceeds to step S640 and the target steering angle θr of the rear wheels calculated in step S630 is held.

On the other hand, if it is determined in step S600 that the flag F has the value 2, the following processing is executed. CP
The U 40a first performs a process of determining whether or not the absolute value of the target steering angle θr of the rear wheels calculated in step S630 is 0.3 [deg] or less (step S650). This processing is to determine whether or not the target steering angle θr of the rear wheels is near neutral.

If a negative determination is made in step S650, and it is determined that the target steering angle θr deviates from the vicinity of neutral and is greatly cut, then it is determined whether or not the target steering angle θr is larger than 0. Is performed (step S66)
0). Here, when it is determined that the target steering angle θr is larger than 0, the rear wheels are steered to the right, so the target steering angle θr is set to a small value in order to displace the rear wheel steering angle to the left. Decrement by the amount Δθr (step S670).
On the other hand, when it is determined in step S660 that the target steering angle θr is smaller than the value 0, the rear wheels are steered to the left, so the target steering angle is changed to the right to displace the rear wheel steering angle. θr is incremented by a small amount Δθr (step S680).

When the process of step S670 or S680 is executed, the process proceeds to step S640, and the rear wheels 10RL, 10RR are steered according to the calculated target steering angle θr.

On the other hand, if a positive determination is made in step S650 and it is determined that the target steering angle θr is near neutral, the CPU 40a determines that the rear wheel steering actuator 3
The process of stopping the energization of the electric signal to the electric motor 35a of No. 5 is performed (step S690). As a result, the control of the rear wheel steering is prohibited while the steering angle of the rear wheels is maintained near the neutral position. After that, the CPU 40a exits the process to "return" and ends the process.

The rear wheel steering control process thus constituted is C
By being repeatedly executed by the PU 40a, the driving state of the vehicle changes as follows. First, the rear wheels 10RL,
When 10RR is in a gripping state, the flag F has a value of 0, so that the normal yaw rate proportional control is executed and the rear wheel steering angle θr is optimally controlled. From this state, when the rear wheels 10RL and 10RR start to skid, the rear wheel steering angle θr determined by the yaw rate proportional control is obtained.
Is maintained. After that, when the rear wheels 10RL and 10RR are completely slipped, the rear wheel steering angle θr is gradually returned to the neutral position in steps S650 to S680, and then the electric motor 35a of the rear wheel steering actuator 35 is transferred to the rear wheel steering actuator 35 in step S690. The energization is stopped and the rear wheel steering control is prohibited.

Therefore, according to the vehicle rear wheel steering system of this embodiment, the rear wheels 10RL and 10RR are in a side slipping state,
When the maximum lateral force that can be generated on the rear wheels 10RL, 10RR is below a predetermined value, the rear wheel steering actuator 35
Since the actuator is stopped, the load on the actuator 35 can be reduced. Therefore, similarly to the first embodiment, there is an effect that the life of the actuator 35 can be extended. Further, in this embodiment, since the rear wheel steering angle θr is returned to the neutral position prior to the stop of the operation of the rear wheel steering actuator 35, the safety at the time of the stop can be enhanced. Further, since the rear wheel steering angle θr is gradually returned to the neutral position, it is possible to suppress a sudden change in the vehicle behavior at the time of the return, and to improve the riding comfort of the vehicle.

In the fifth embodiment, the processing of steps S600, 640 to S690 results in claim 5
The target steering amount minute control unit in is realized. this is,
The so-called "rear wheel steering prohibition means" has been realized.

In the first embodiment described above, the target steering amount suppressing means is realized by suppressing the yaw rate feedback gain as the control gain, but instead of this, the front wheel steering angle proportional to the front wheel steering angle is proportional. The target steering amount suppression means may be realized by suppressing the gain.

The embodiments of the present invention have been described above.
The present invention is not limited to these examples, and it goes without saying that the present invention can be implemented in various modes without departing from the scope of the present invention.

Further, it should be additionally noted that the embodiments of the present invention have various embodiments of the following technical matters in addition to the technical matters described in the claims.

(1) The vehicle rear wheel steering system according to claim 1, further comprising a wheel slip control means for controlling a slip state of the wheel to a predetermined state, and the lateral force reduction state detection means comprises: A rear wheel steering system for a vehicle, comprising: a determination unit that determines whether or not the control by the wheel slip control means is being performed on the rear wheel side and determines that the control is being performed by the lateral force reduction state.

(2) The vehicle rear wheel steering system according to claim 1, wherein the target steering amount control means controls the yaw rate feedback gain related to the yaw rate to a reduction side to reduce the target steering amount. A rear wheel steering system for a vehicle, comprising a gain reduction unit for controlling on the reduction side.

(3) A rear wheel steering actuator for steering and controlling the rear wheels according to a control signal from the outside, and a target steering amount for the rear wheels is calculated using the yaw rate as a parameter, and a control signal based on the target steering amount is calculated. To the rear wheel steering actuator, a wheel slip control means for controlling the slip state of the wheels to a predetermined state, and a target steering amount for increasing the target steering amount when the wheel slip control means is in operation. In a rear-wheel steering system for a vehicle, comprising: an increasing means; a lateral force reduction state detecting means for detecting a lateral force reduction state in which a maximum lateral force that can be generated on the rear wheels becomes a predetermined value or less; and the lateral force reduction state. And a target steering amount suppressing unit that controls the target steering amount increased by the target steering amount increasing unit to the reduction side when the rear steering device is detected.

[0108]

As described above, the rear wheel steering system for a vehicle according to claim 1 is for steering the rear wheels when the maximum lateral force that can be generated on the rear wheels is below a predetermined value. The load on the actuator can be reduced. Therefore, the life of the rear wheel steering actuator can be extended.

In the vehicle rear wheel steering system according to the second aspect, the lateral force reduction state can be detected from the relatively easy-to-detect parameter such as the slip ratio, and the device configuration can be simplified. .

According to another aspect of the present invention, there is provided a rear-wheel steering system for a vehicle, wherein when the centrifugal force exceeds the maximum lateral force that can be generated by the rear wheels and the rear wheels are in a side-slip state, the rear-wheel steering actuator is burdened. Can be reduced.

According to the fourth aspect of the present invention, the rear wheel steering system for a vehicle can easily detect the reduced lateral force state from the side slip angle and the side slip angular velocity of the vehicle.

In the vehicle rear wheel steering system according to the fifth aspect, the lateral force generated by the rear wheels can be reduced by reducing the steering amount of the rear wheels, and the longitudinal force of the rear wheels can be secured. As a result, the traveling performance of the vehicle can be improved.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a vehicle rear wheel steering system according to a first embodiment of the present invention.

FIG. 2 is a block diagram showing an electrical configuration of a rear wheel steering system.

FIG. 3 is a vertical cross-sectional view of a rear wheel steering actuator 35.

FIG. 4 is a flowchart showing ABS control determination processing executed by a 4WS ECU 40.

FIG. 5 is a flowchart showing a 50 ms routine of rear wheel steering control processing executed by the 4WS ECU 40.

FIG. 6 is a flowchart showing a 4 ms routine of a rear wheel steering control process executed by the 4WS ECU 40.

FIG. 7 is a graph showing a change in a variable K1 according to a vehicle speed V.

FIG. 8 is a graph showing a change in a variable K2 according to a vehicle speed V.

FIG. 9 is a flowchart showing a 4 ms routine of rear wheel steering control processing executed in the second embodiment of the present invention.

FIG. 10 is a lateral force Fs generated by a rear wheel at a tire ground contact point.
It is explanatory drawing which shows the correlation of the front-back force Ff.

FIG. 11 is a flowchart showing vehicle state determination processing executed in the third embodiment of the present invention.

FIG. 12 is a flowchart showing vehicle state determination processing executed in a fourth embodiment of the present invention.

FIG. 13 is a graph showing how the slipping state of the rear wheels changes depending on the vehicle side slip angular velocity Θ and the vehicle side slip angle θ.

FIG. 14 is a flowchart showing a rear wheel steering control process executed in a fifth embodiment of the present invention.

[Explanation of symbols]

 10FL ... Left front wheel 10FR ... Right front wheel 10RL ... Left rear wheel 10RR ... Right rear wheel 20 ... Front wheel steering mechanism 21L, 21R ... Knuckle arm 22L, 22R ... Tie rod 23 ... Relay rod 25 ... Steering mechanism 26 ... Pinion 27 ... Shaft 29 ... Steering wheel 30 ... Rear wheel steering mechanism 31L, 31R ... Knuckle arm 32L, 32R ... Tie rod 33 ... Relay rod 35 ... Rear wheel steering actuator 35a ... Electric motor 40a ... CPU 40b ... ROM 40c ... RAM 40d ... Input interface 40e ... Output Interface 40f ... Input interface 45 ... ABS ECU 45a ... CPU 45b ... ROM 45c ... RAM 45d ... Input interface 45e ... Output interface 51 ... Steering sensor 53 ... Yaw rate 54 ... Vehicle speed sensor 55 ... Pedal switch 56 ... Left front wheel speed sensor 57 ... Right front wheel speed sensor 58 ... Left rear wheel speed sensor 59 ... Right rear wheel speed sensor 60 ... Rear wheel steering angle sensor 61 ... Casing 63 , 65 ... Bearing member 67 ... Actuator shaft 67a ... Trapezoidal screw 69 ... Supporting member 71 ... Coil 72 ... Rotor member 73 ... Permanent magnet 75, 76 ... Bearing 79 ... Power transmission member 81 ... Planetary gear mechanism 85a ... Trapezoidal screw 87 ... Bearing V ... Vehicle speed ω ... Yaw rate θf ... Front wheel rudder angle θr ... Rear wheel rudder angle θ ... Vehicle side slip angle Θ ... Vehicle side slip angular velocity

Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location B62D 137: 00

Claims (5)

[Claims] 1. A rear wheel steering actuator for steering and controlling a rear wheel according to a control signal from the outside, a target steering amount of the rear wheel calculated from a predetermined parameter, and a control signal based on the target steering amount. A rear-wheel steering system for a vehicle, comprising: a control unit that outputs the rear-wheel steering actuator. A lateral force reduction state that detects a lateral force reduction state in which the maximum lateral force that can be generated on the rear wheels becomes a predetermined value or less. A rear wheel steering device for a vehicle, comprising: a detection unit; and a target steering amount suppression unit that controls the target steering amount to a reduction side when the lateral force reduction state is detected. 2. The vehicle rear wheel steering system according to claim 1, wherein the lateral force reduction state detecting means includes a slip ratio detecting section for detecting a slip ratio of the rear wheel, and a slip ratio of the rear wheel. A rear wheel steering system for a vehicle, comprising: a determination unit that determines that the lateral force is in a state where the lateral force is equal to or more than a predetermined value. 3. The rear wheel steering system for a vehicle according to claim 1, wherein the predetermined value in the lateral force reduction state detecting means has a magnitude determined by the centrifugal force applied to the rear wheels. 4. The vehicle rear wheel steering system according to claim 3, wherein the lateral force reduction state detecting means includes a side slip detecting section for detecting a side slip angle and a side slip angular velocity of the vehicle, respectively. It is determined whether or not the sideslip angle and the sideslip angular velocity of the vehicle are each equal to or greater than a predetermined value, and at the time of affirmative determination, the maximum lateral force that can be generated on the rear wheel is less than or equal to the centrifugal force applied to the rear wheel. A rear-wheel steering device for a vehicle, comprising: 5. The vehicle rear wheel steering system according to claim 1, wherein the target steering amount suppressing unit controls the target steering amount to a predetermined minute amount close to a value 0. Rear-wheel steering system for vehicles equipped with.