JP2003285763A - Rear wheel steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a sense of incongruity of a driver caused by parallel movement of a vehicle at the time of low speed, and solve a sense of incongruity of the driver caused by rapidly steering a rear wheel under a rear end extension restraining control. <P>SOLUTION: In this rear wheel steering device, a rear wheel is steered according to a steering angle of a front wheel. This device comprises a rear end extension restraining control means for restraining extension of the rear end of the vehicle; a same phase steering restraining means for restraining the steering angle so as not to steer the front wheel and the rear wheel into the same phase when the rear end extension restraining control means steers the rear wheel in a manner to restrain extension of the rear end; and/or a rear wheel steering speed restraining means for restraining the speed of the rear wheel steering in a specific range when the rear end extension restraining control means steers the rear wheel in a manner to restrain the extension of the rear end. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rear wheel steering system capable of steering not only front wheels but also rear wheels according to a motion state of a vehicle to improve traveling stability of the vehicle, and more particularly,
The present invention is applied to a rear wheel steering system that performs rear wheel squeeze suppression control that prevents the rear portion of a vehicle from protruding outside the traveling locus of the front portion of the vehicle when traveling at low speed.

[0002]

2. Description of the Related Art Conventionally, a rear wheel steering control capable of improving running stability and small turning performance of a vehicle by controlling a steering angle of a rear wheel corresponding to steering of a front wheel by a steering operation of a driver. The device is known. In such a rear-wheel steering system, how to set an optimal target rear-wheel steering angle according to the front-wheel steering angle and the situation of the vehicle becomes a problem. For example, “7.5 Steering system and steering stability of automobiles” (publisher: Sankaido, author: Kayaba Industry Co., Ltd., 1st printing date: September 10, 1996), page 7.5, “7.5 .
3 Side slip zeroization 4 WS control ”column (hereinafter“ Publication 1 ”
Say. ), Paying attention to the vehicle body slip angle (deviation angle between the vehicle traveling direction and the vehicle front-rear direction), so that this vehicle body slip angle is always zero, the vehicle body slip angle is proportional to the front wheel steering angle at a predetermined vehicle speed or less. An ideal vehicle characteristic with excellent maneuverability by controlling the rear wheel steering angle in the opposite phase in proportion to the front wheel steering angle while maintaining a small turning performance by controlling the rear wheel steering angle in the opposite phase Is obtained (hereinafter, this control is referred to as "steering angle proportional control". See FIG. 12).

In the steering angle proportional control described in Publication 1, in a low speed range where the vehicle starts from a stop, the rear wheels are steered in reverse phase with respect to the front wheels, thereby improving the small turning ability. It is what makes me. However, in this case, if the steering angle of the rear wheels is large, the rear end of the vehicle (particularly the shoulder at the rearmost end of the vehicle) will overhang outside the running trajectory of the front end of the vehicle, and the driver will be inadvertent. Turning the steering makes it easier for the vehicle to come into contact with obstacles on the side of the vehicle.

[0004] Therefore, in order to solve the problem of the overhang of the rear end of the vehicle and to secure a small turning property, the publication "105 of the Society of Automotive Engineers of Japan, Preprints 882, Showa 63-10" is published. A rear wheel steering angle is controlled so that the rear end of the vehicle follows the front end locus of the vehicle as described in "One Control Method for Suppressing Overhang of 4WS Vehicle" (hereinafter referred to as "Publication 2"). The method of doing is proposed. Specifically, the traveling locus of the vehicle front end is stored for each predetermined sampling distance, and the traveling direction of the vehicle front end at the sample point closest to the vehicle rear end at that time point is stored. The rear wheel steering angle value is controlled so that the traveling direction of the rear end of the vehicle and the traveling direction of the rear end of the vehicle match.

Further, the applicant of the present invention has filed Japanese Patent Application No. 2001-242.
In No. 842, of the two points where the line segments having the two points at both ends are parallel to the front-rear direction of the vehicle, the point on the vehicle front side is the front end point g,
A point on the rear side of the vehicle is set as a rear end point f, and an averaged trajectory obtained by smoothly averaging the traveling trajectories of the front end point g is calculated.
The invention calculates the target rear-wheel steering angle so that the target rear-wheel steering angle is limited so as not to go out of the inner region of the averaged trajectory.

[0006]

SUMMARY OF THE INVENTION In a rear wheel steering control device that performs the rear end overhang suppressing control for suppressing the rear end overhang as described above, the steering angle of the rear wheel is determined by the driver's steering operation. Does not necessarily correspond to. For example, immediately after starting the vehicle, if the steering angle of the rear wheels is zero and you try to make a large turn, first, the normal rear wheel steering angle control attempts to steer the rear wheels largely out of phase with the front wheels. To do. Here, in order to suppress the overhang of the rear end, the rear wheel may be in phase with the front wheel. However, when the front wheels and the rear wheels are in phase, the vehicle feels to move in parallel, which is contrary to the driver's intention to turn, and the driver feels a great deal of discomfort.

In order to deal with this, it is conceivable to prohibit the state where the front wheels and the rear wheels are in phase with each other in the rear end overhang suppressing control. In this case, since the front wheels and the rear wheels are not in phase, the driver's discomfort due to parallel movement can be prevented.
The protrusion at the rear end is the same as in a two-wheel steering vehicle, so there is no particular problem. However, when the vehicle runs for a while in this state, the rear end overhang suppression control attempts to control the rear wheels in a phase opposite to that of the front wheels, and the steering angle of the rear wheels is greatly cut. At this time, if the amount of steering angle of the rear wheels is large, the driver will have a large discomfort in the steered steering angle control of the rear wheels.

Therefore, the first object of the present invention is to eliminate the discomfort of the driver due to the movement of the vehicle in parallel when the vehicle is running at a low speed.

A second object of the present invention is to eliminate the uncomfortable feeling of the driver due to the abrupt steering of the rear wheels by the rear end extension control.

[0010]

In order to solve the above-mentioned first problem, in the present invention, as described in claim 1,
In a rear wheel steering device that steers the rear wheels according to the steering angle of the front wheels, a rear end overhang suppressing control unit that suppresses the rear end overhang of the vehicle is provided, and the rear end overhang suppressing control unit controls the rear end overhang. When the rear wheels are steered so as to suppress the steering, the in-phase steering angle limiting means is provided to limit the steering angle so that the front wheels and the rear wheels do not steer in phase. According to this, the front wheel and the rear wheel are not steered in phase when the rear-end overhang suppressing control unit steers the rear wheel so as to suppress the rear end overhang. Further, in the state where the rear end bulge suppression control means is not operating, the same phase of the front wheels and the rear wheels is allowed.

In order to solve the above-mentioned second problem, the present invention provides a rear wheel steering system for steering the rear wheels according to the steering angle of the front wheels. A rear-end overhang suppressing control unit for suppressing overhang is provided, and when the rear-end overhang suppressing control unit steers the rear wheels so as to suppress overhang of the rear end, the rear-wheel steering speed is specified. The rear wheel steering speed limiting means for limiting the range is provided. According to this, when the rear end overhang suppression control means steers the rear wheels to suppress the rear end overhang, the amount of change in the target rear wheel steering angle per unit time does not exceed a specific value. Since the value of the target rear wheel steering angle is changed, the rear wheel steering angle is slowly changed by the rear end overhang suppression control.

In order to solve the above first and second problems, the present invention provides a rear wheel steering system for steering the rear wheels according to the steering angle of the front wheels as described in claim 3.
When the rear end overhang suppression control means for suppressing the overhang of the rear end of the vehicle is provided and the rear end overhang suppression control means steers the rear wheels so as to suppress the overhang of the rear end, the front wheel and the rear wheel are controlled. In-phase steering angle limiting means for limiting the steering angle so that the wheels do not steer in-phase, and when the rear-end overhang suppressing control means steers the rear wheels so as to suppress rear-end overhang, The rear wheel steering speed limiting means for limiting the speed to a specific range is provided. According to this, the front wheel and the rear wheel are not steered in phase when the rear-end overhang suppressing control unit steers the rear wheel so as to suppress the rear end overhang. Further, in the state where the rear end bulge suppression control means is not operating, the same phase of the front wheels and the rear wheels is allowed. Further, when the rear end overhang suppression control means steers the rear wheels to suppress the rear end overhang, the target rear wheel steering angle is controlled so that the amount of change in the target rear wheel steering angle per unit time does not exceed a specific value. Since the value of the steering angle is changed, the rear wheel steering angle is slowly changed by the rear end bulge suppression control.

The above-mentioned specific range may be a predetermined value given in advance, or may be adjusted according to the running condition.

In order to solve the above first and / or second problems, in the present invention, as described in claim 4, in claim 2 or 3, a specific range of the rear wheel steering speed limiting means. Is set to a range in which the yaw rate fluctuation speed difference is within the allowable range around the target angular speed of the rear wheel steering angle. According to this, when the rear-end overhang suppressing control unit steers the rear wheels so as to suppress rear-end overhang, the value of the target rear-wheel steering angle is set so as not to exceed the allowable speed difference of the yaw rate fluctuation. Since it is changed, the rear-wheel steering angle is slowly changed by the rear-end extension suppression control.

In order to solve the above first and / or second problems, in the present invention, as described in claim 5, in claim 2 or 3, a specific range of the rear wheel steering speed limiting means. Is the maximum allowable yaw rate fluctuation speed difference / centered on the differential value of the rear wheel / front wheel steering angle ratio x steering angle.
The area with the range of yaw rate gain for the rear wheel was set as the specific range. According to this, when the rear-end overhang suppressing control unit steers the rear wheels so as to suppress rear-end overhang, the value of the target rear-wheel steering angle is set so as not to exceed the allowable speed difference of the yaw rate fluctuation. Since it is changed, the rear-wheel steering angle is slowly changed by the rear-end extension suppression control.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a conceptual diagram of a vehicle including a rear wheel steering control device according to the present invention. In the vehicle 1, each front wheel 3 is provided with a wheel speed sensor 9 that detects the rotational speed of the front wheel. The wheel speed sensor 9 emits a pulse each time the front wheel 3 rotates by a predetermined angle, and the traveling distance can be obtained by counting the number of the pulses. Therefore, the wheel speed sensor 9 has a function as a traveling distance sensor. There is. An actuator 15 for steering the rear wheels 5 is arranged behind the vehicle. The actuator 15 is provided with a rear wheel steering angle sensor 13 that detects the actual steering angle amount of the rear wheels 5. The steering wheel 7 that steers the front wheels 3 includes a front wheel steering angle sensor 1 that detects the actual steering angle amount of the front wheels 3.
1 is provided. In the vehicle, a shift position sensor 23 for detecting a shift position of a transmission of the vehicle, a yaw rate sensor 17 for detecting a yaw rate occurring in the vehicle 1, a controller 21 for controlling the actuator 15,
An ignition switch 19 for starting the vehicle engine is attached. The above front wheel steering angle sensor 1
The outputs of the rear wheel steering angle sensor 13, the wheel speed sensor 9, the shift position sensor 23, the yaw rate sensor 17, and the ignition switch 19 are input to the controller 21. The controller 21 calculates a target rear wheel steering angle according to the detection values of these various sensors, and gives a command to the actuator 15 so that the actual steering angle amount of the rear wheels becomes the target rear wheel steering angle.

Next, the outline of the control will be described with reference to FIG. 10, which is a model diagram of the vehicle. Here, each angle is expressed as an angle with the counterclockwise direction of the paper being positive. In FIG. 10, point g indicates the front end point of the vehicle, and point f indicates the rear end point of the vehicle.

Points a, b, c and d are sampling points on the traveling locus 30 of the front end point g, and the front end point g is the point d,
The vehicle travels in the order of points c, b, and a, and the point a is the latest sampling point and indicates the current position of the front end point g. The wheel speed sensor 9 emits a pulse each time the rotation angle of the front wheel 3 reaches a predetermined angle, and the latest sampling point is updated each time the wheel speed sensor 9 emits a pulse. Therefore, the sampling distance Dx, which is the distance between these sampling points, corresponds to the traveling distance of the vehicle when the front wheels 3 rotate by a predetermined angle.

The curve 32 is the actual traveling locus 3 of the front end point g.
It represents an averaged locus obtained by smoothly averaging 0 using arcs. The averaging locus 32 is updated as the sampling points are updated as described later. The point e is the averaged trajectory 32 from the latest sampling point a.
Predetermined distance toward the rear end point f direction = kDx (k is
It is a natural number obtained by rounding off the value of the total vehicle length Le / Dx, and k
Dx is a point that has returned by the entire vehicle length, that is, a distance substantially equal to the distance between the front end point g and the rear end point f), and is a point on the averaging locus 32 of the rear end point f. θfear represents the traveling direction angle of the front end point g on the averaged trajectory at the nearby point e. Note that, in FIG. 10, since the vehicle is turning to the left in the traveling direction, the area inside the averaging trajectory 32 of the front end point g is the left area in the traveling direction with respect to the averaging trajectory 32 (left side in FIG. 10). ).

Θet is the latest sampling point a and the rear end point f.
It is the deviation angle between the line segment connecting the and the latest sampling point a and the line segment connecting the rear end point neighboring point e on the averaging locus. The deviation angle θet is calculated as a positive value in the state shown in FIG. 10, and the positional relationship between the trailing end point f and the trailing end point neighboring point e on the averaged trajectory is reversed with respect to the latest sampling point a. If it is in the state of being operated, it is calculated as a negative value.

If it is desired to prevent the rear end point from projecting, the steering angle of the rear wheels may be adjusted so that the rear end point f passes through the inside area of the averaging locus 32. In the present embodiment, the rear end point f is the inner region of the averaging trajectory 32,
Further, the rear wheel steering angle is adjusted so as to approach the averaged trajectory 32, that is, to reduce the deviation angle θet.

FIG. 11 is a schematic diagram showing how the averaged trajectory 32 is updated each time the latest sampling point changes. The case where the front end point g of the vehicle travels a distance Dx from the previous sampling point a1 at time t1 and moves to a new sampling point a0 at time t0 is shown. At the previous sampling point, a point near the rear end point f is e1, the averaging locus is 32 (t1), its length is k · Dx, and its central angle is k · θato. When the sampling point is updated to the new sampling point a0, the averaging locus becomes 32 (t
From 1) to 32 (t0), the points near the rear end point are updated from e1 to e0.

At this time, in the present embodiment, the point e0 near the rear end point is placed on the previous averaging locus 32 (t1), and the averaging locus 32 (t0) is set to the sampling point a0 and the rear end point neighborhood. An arc having a length of k · Dx is set as the arc at the latest sampling point a0 with the point e0 as both ends. Length k · Dx
Corresponds to the central angle k · θat of the averaged trajectory 32 (t0). In this way, the points near the rear end point are always set to be inside the traveling locus of the front end point. By setting the averaging locus 32 as described above and adjusting the steering angle of the rear wheels so that the rear end point f passes through the inner region of the averaging locus 32, the rear end point can be prevented from overhanging.

Next, details of the control of the controller 21 will be described.
This will be described with reference to FIGS.

FIG. 2 is a main routine executed by the controller 21. When the ignition switch 19 is turned on, the main routine starts and the process proceeds to step 101.

At step 101, initialization processing of each variable is executed. As shown in FIG. 3, the specific contents of the initialization process in step 101 are such that both θfeao and θato are set to 0 rad, and the current time is set to t0. Further, a time obtained by subtracting the initial vehicle speed pulse input interval time Td0 which is a predetermined value from the current time t0 is set as t1. Further, the actual rear wheel steering angles δm detected by the rear wheel steering angle sensor 13 are substituted into variables δrlimits1, δrlimit, δrlimitst0, respectively. The variables δrlimits1 and δrlimitst0 are
Limit value of smoothed target rear wheel rudder angle (rad)
Δrlimits1 is a variable representing the previous value in the calculation cycle, and Δrlimitst0 is a value at time t0. δrlimit is a variable that represents the limit value of the rear wheel steering angle. In the initialization process, these δrlimits1, δrlimits, δrlimi
The current actual rear wheel steering angle δm is given to all tst0.

When the initialization process is executed, step 10
Go to 2. The processing from step 102 to step 110 is repeatedly looped thereafter. On the other hand, when an output pulse from the wheel speed sensor 9 is input while the loop processing is being executed, a vehicle speed sensor pulse input processing as shown in FIG. 4 is executed as interrupt processing at any time. In the vehicle speed sensor pulse input processing, the processing of updating the sampling points is performed.

In the vehicle speed sensor pulse input processing, the smoothed limit value δrlimits of the target rear wheel steering angle at the interruption time t0 is substituted into δrlimitst0, δrlimit is substituted into δrlimit1, and time t0 is calculated at time t1 and time t0 is calculated at time t0. Substitute the current time. The time of the latest sampling point is t0, and the time of the sampling point one time before is t1. δrlimits
Is a value calculated by a rear wheel limited steering angle smoothing calculation process, which will be described later. By substituting this value for Δrlimitst0, the smoothed limit value of the target rear wheel steering angle at the latest sampling point is held as Δrlimitst0. Further, Δrlimit is the actual rear wheel steering angle δm in the initial value, but thereafter is a calculated value of the limit value of the rear wheel steering angle calculated by the rear wheel limit steering angle calculation processing and the like which will be described later.
By substituting this value into δrlimit1, the rear wheel steering angle limit value δrlimit1 at the sampling point one time before is held. That is, at the latest sampling point, the interrupt time is t0, the smoothed limit value of the target rear wheel steering angle is δrlimitst0, and at the previous sampling point, the interrupt time is t1 and the rear wheel steering angle limit value is Δrlim
Let it1. In this way, the sampling point is updated every time this interrupt is executed.

In step 102, it is judged whether or not the control cycle Ts has passed, and if the control cycle Ts has not passed, the next process is suspended until the time passes. This control cycle T
For s, enter a value that is larger than the time required for the main routine to make a round. When the control cycle Ts has elapsed, the process proceeds to the next step 103. In step 103, the signals from various sensors such as the wheel speed sensor 9 and the front wheel steering angle sensor 11 are fetched to grasp the current conditions of the vehicle.

In step 104, a normal rear wheel steering angle calculation is performed based on the information from various sensors input in step 103. The normal rear wheel rudder angle calculation is an operation in which the controller 21 calculates the normal target rear wheel rudder angle δrt0 based on a control rule that is a basis for calculating the optimum target rear wheel rudder angle. In this embodiment, the steering angle proportional control is adopted as a basic control law. The steering angle proportional control is a method of setting a value proportional to the actual front wheel steering angle Δf as the target rear wheel steering angle. Further, here, the target rear wheel steering angle is changed according to the vehicle speed. Specifically, as shown in FIG. 5, the rear wheel / front wheel steering angle ratio ratio_map (v) corresponding to the vehicle speed v obtained from the output of the wheel speed sensor 9 is shown in FIG. Then, this is multiplied by the actual front wheel steering angle Δf to obtain the normal target rear wheel steering angle Δrt0. The rear wheel / front wheel steering angle ratio ratio_map (v) is set to a negative value below a predetermined vehicle speed and a positive value above a predetermined vehicle speed. Therefore, below the predetermined vehicle speed, the normal target rear wheel steering angle δrt0 is equal to the actual front wheel steering angle δf.
When the vehicle speed is equal to or higher than the predetermined vehicle speed, the normal target rear wheel steering angle Δrt0 is calculated in phase with the actual front wheel steering angle Δf.

In step 105, it is judged whether or not the vehicle speed sensor pulse input processing, which is the interrupt processing described above, has been performed. If the vehicle speed sensor pulse input processing is performed, the process proceeds to the rear wheel limit steering angle calculation in step 106, and if the vehicle speed sensor pulse input processing is not performed, step 10 is performed.
6 is not processed and the process proceeds to step 107. That is, the rear wheel limited steering angle calculation processing is performed only when the latest sampling point is updated.

In step 106, rear wheel limited steering angle calculation is performed. In this rear wheel limit steering angle calculation routine, the limit value Δrlimit of the target rear wheel steering angle is calculated. The details are shown in FIG. In the rear wheel limit steering angle calculation routine, the previous values θfeao and θ of the traveling direction angle θfea of the front end point on the averaged trajectory and the trajectory angle θat of the front end point on the averaged trajectory, respectively.
ato is saved in step 208 described later.

First, in step 201, the traveling direction angle θfe of the front end point is calculated from the absolute angle θb of the vehicle body, the actual front wheel steering angle δf, the rear wheel steering angle δr, the axle length Lf from the center of gravity to the front wheel, and the center of gravity to the rear wheel. Axle length Lr, wheel base L (= Lf + L
r), the length Lfe from the center of gravity to the front end, the yaw rate γ, and the vehicle speed v are used to calculate by the following equation.

(Equation 1) θfe = θb + Lr / L · δf + Lf / L · δr + Lfe · γ /
v Next, in step 202, the trajectory angle θat of the front end point on the averaging trajectory is set to the traveling direction angle θfe of the front end point, the previous value θfeao of the traveling direction angle of the front end point on the averaging trajectory, and the averaging trajectory. It is calculated by the following equation using the previous value θato of the trajectory angle of the front end point.

(Equation 2) θat = f ⁻¹ (sin (θfe−θfeao + (k−1) · θato) + (1-c
os ((k−1) ・ θato)) / θato) where f (θ) = (1−cos (k ・ θ)) / θ) With this equation, the central angle of the averaged trajectory as shown in FIG. The locus angle θat, which is the average value of k · θat, is obtained.

Next, at step 203, the traveling direction angle θfea of the front end point on the averaging locus at the updated latest sampling point a0 is calculated by averaging the previous value θfeao of the traveling direction angle of the front end point on the averaging locus. It is calculated by the following equation using the trajectory angle θat of the front end point on the trajectory and its previous value θato.

(Equation 3) θfea = θfeao + k · θat− (k−1) · θato The above equation 2 and equation 3 determine the trajectory angle θat of the front end point on the averaged trajectory and the traveling direction angle θfea. Since it is possible, the averaged trajectory at the latest sampling point can be specified.

Next, at step 204, the deviation angle θet, which is the angle formed by the rear end point f, the front end point g, and the point e near the rear end point on the averaging locus, is defined as the traveling direction angle of the front end point on the averaging locus. It is calculated by the following equation using θfea, the trajectory angle θat of the front end point on the averaged trajectory, and the absolute angle θb of the vehicle body.

(Equation 4) θet = θfea + k · θat / 2−θb Next, in step 205, the traveling direction angle θfear of the front end point on the averaging locus and near the rear end point e is given by
From the traveling direction angle θfea of the front end point on the averaging trajectory to the central angle k
・ Subtract θat and calculate as follows.

(Equation 5) θfear = θfea−k · θat Next, at step 206, the limit value θrelimit of the traveling direction angle θre of the rear end point f is set to the traveling direction angle θfear of the front end point.
Then, C1 · θat is subtracted from the value to calculate as follows. Here, C1 is a gain for limiting a predetermined steering angle, and is a value given in advance.

(Equation 6) θrelimit = θfear−C1 · θat The target rear wheel steering angle may be obtained so that the traveling direction angle θre of the rear end point f matches this limit value θrelimit.

Next, in step 207, using θrelimit calculated in step 206, the limit value δrlim of the target rear wheel steering angle, which is the final calculation target of this routine, is used.
Calculate it as follows: The calculation formula here is for calculating the steering angle value of the rear wheels for matching the traveling direction angle θre of the rear end point f with the limit value θrelimit.

(Equation 7) δrlimit = L / (Lf + Lre)  (θrelimit-θb)-(Lr
−Lre) / (Lf + Lre) · δf That is, the target rear wheel steering angle is the target rear wheel steering angle limit value δrlim
If the steering angle of the rear wheels is controlled to match it, the rear end point f
Passes through the inside area of the averaging locus, and the protrusion of the rear end point can be suppressed.

Finally, in step 208, the traveling direction angle θfea of the front end point on the averaging trajectory and the trajectory angle θat of the front end point on the averaging trajectory are respectively set to the previous values θfeao in preparation for calculation in the next sampling. And θato are substituted.

Here, the physical meaning of the target rear wheel steering angle limit value δrlimit calculated in step 207 will be described. Temporarily, in step 206, −C1 · θ
Assuming that there is no term of et and θrelimit = θfear, in step 207 the target rear wheel steering angle limit value δrlimit
Is calculated as a rear wheel steering angle value such that the traveling direction angle θre of the rear end point f matches the traveling direction angle θfear of the front end point on the averaging locus at the neighboring point e0. Thus -C1 · θ
Even if the limit value δrlimit is calculated by omitting the term of et, the rear end point f
Does not extend beyond the area inside the averaged trajectory 32 (t0). However, as shown in FIG. 4, when there is a deviation angle θet, even if the steering angle of the rear wheels is increased by relaxing the limit value of the rear wheel steering angle by an angle considering this deviation angle θet. It is possible to prevent the rear end point f from protruding from the inside area of the arc 32 (t0) which is the averaged trajectory. As described above, this calculation routine assumes only the case where the rear wheel steering angle is calculated in the opposite phase to the front wheel steering angle, so if the limit value of the rear wheel steering angle can be relaxed, relax it as much as possible. That is, it is desirable to increase the absolute value of the rear wheel steering angle limit value to improve the small turning performance. Therefore, in step 206, by adding the term −C1 · θet, the limit value θrelimit of the traveling direction angle of the rear end point is relaxed and obtained, and the relaxed limit value θrelimit of the traveling direction angle of the rear end point at the rear end point f is obtained. By calculating the target rear wheel steering angle limit value Δrlimit so that the rear end point traveling direction angles θre match, the target rear wheel steering angle limit value Δrlimit is relaxed as much as possible.

When C is increased, the trailing end point f is controlled so as to quickly ride on the averaging locus. In this case,
There is a possibility that the rear end point f may temporarily go outside the averaging locus due to control hunting or the like. Therefore, the value of C may be appropriately adjusted and set according to the characteristics of the vehicle.

When step 208 is executed as described above, the rear wheel limit steering angle calculation routine ends, and the process returns to the main routine shown in FIG. Next, in step 107, the rear wheel limited steering angle smoothing calculation routine is executed.

At step 107, rear wheel limited steering angle smoothing calculation is performed. Target rear wheel steering angle limit value δrlimit
Is calculated in the rear wheel limited steering angle calculation (step 207 in FIG. 6). This calculation is performed in steps 105 and 106 in FIG.
As shown in, it is executed only when the input of the vehicle speed sensor pulse is updated, that is, when the sampling point is updated. Therefore, the limit value Δrlimit of the target rear wheel steering angle becomes a numerical value for each sampling point and becomes discrete. If this value is directly adopted as the target rudder angle, the target rudder angle will be suddenly changed when the sampling point is passed.
Since the sampling interval becomes long during low-speed traveling, if the target steering angle is discrete, the steering of the rear wheels will also be discrete, and the feeling will be poor. Therefore, the smoothed limit value δrlimits of the target rear wheel steering angle, which is a continuous value complementing the discrete target rear wheel steering angle limit value δrlimit, is calculated,
This value is used as the target value of the rear wheel steering angle.

When the rear wheel limit steering angle calculation routine ends,
In step 107, the rear wheel limited steering angle smoothing calculation routine is executed. The details of this routine are shown in FIG.

In FIG. 8, as shown in step 401, the smoothed limit value δrl of the target rear wheel steering angle is set.
imits is the smoothed limit value δrlimits1 one control cycle before, with a specific gradient (2δrlimit−δrlimit1−
δrlimitst0) / (t0-t1) is multiplied by the control period Ts, and the increments calculated are added to obtain the following formula.

(Equation 8) δrlimits = δrlimits1 + Ts · (2δrlimit−δrlimit1
−δrlimitst0) / (t0−t1) This particular slope is expressed in fractions and the denominator is (t0−
t1), that is, the sampling interval between the time t0 of the latest sampling point and the time t1 of the previous sampling point, and the numerator is (2δrlimit−δrlimit1−δrlimitst0) =
(Δrlimit-δrlimit1) + (δrlimit-δrlimitst0)
And That is, the deviation between the target rear-wheel steering angle limit value δrlimit at the latest sampling point and the target rear-wheel steering angle limit value δrlimit1 at the previous sampling point is added to the target rear-wheel steering angle limit value δrlim at the latest sampling point.
It is a value obtained by adding the deviation between it and the smoothed limit value Δrlimitst0 of the target rear wheel steering angle at the latest sampling point. Therefore, at the next sampling, the smoothed limit value δrl of the target rear wheel steering angle
The imits are adjusted so as to approach the target rear wheel steering angle limit value δrlimit.

FIG. 13 is a diagram schematically showing the process of this interpolation calculation. The point i is the sampling point (t1
Target rear wheel steering angle limit value δrlimit1, j point is the target rear wheel steering angle limit value δrlimit at the latest sampling point (t0 time), and k point is the smoothed limit at the latest sampling point (t0 time) Indicates the value Δrlimitst0.
In order for the limit value δrlimits smoothed at the next sampling point to approach the target rear wheel rudder angle limit value, smoothing processing was performed when (t0-t1) further advanced from the latest sampling point (at t0). It suffices that the limit value Δrlimits reaches the point m at which the line segment ij is extended. Therefore, the line segment
Slope of km (2δrlimit−δrlimit1−δrlimitst0) / (t
The smoothed limit value Δrlimits of the current target rear wheel steering angle may be set so that (0−t1) becomes a gradient.

When step 401 is executed, step 402 is proceeded to next. In step 402, the current smoothed limit value δrlimits is substituted for the smoothed limit value δrlimits1 of one control period before as a preparation for executing step 401 in the next control period.

When step 402 is executed, the rear wheel limited steering angle smoothing calculation routine ends. And Figure 2
After returning to the main routine of step 108, the rear wheel steering angular velocity limit calculation of step 108 is executed.

At step 108, rear wheel steering angular velocity limit calculation is performed. Here, the amount of change in the smoothed limit value Δrlimits obtained in the above step is checked, and the limit value Δrlimits is corrected according to the amount of change. The value of the smoothed limit value δrlimits one control cycle before is δrlimi
It is saved as ts_old in step 608 described later.

For details of this processing, as shown in FIG. 8, first in step 601, the maximum allowable amount of the differential value of the limit value is obtained.
Drl_max and the minimum allowable amount of derivative of limit value Drl_min,
Rear wheel / front wheel steering angle ratio ratio_map (v) according to vehicle speed v, differential value dδsw / dt of steering angle δsw, maximum allowable yaw rate fluctuation speed difference Gtmax, and yaw rate gain Grd for rear wheels
It is calculated by the following equation using r (0).

(Equation 9) Drl_max = ratio_map (v) · dδsw / dt + Gtmax / Grdr (0) (Equation 10) Drl_min = ratio_map (v) · dδsw / dt−Gtmax / Grdr (0) Here, the steering angle δsw Rear wheel / differential value dδsw / dt
The target angular velocity of the rear-wheel steering angle is calculated by multiplying the front-wheel steering angle ratio ratio_map (v), and the range in which the target angular velocity of the rear-wheel steering angle is swung by Gtmax / Grdr (0) vertically, that is, the yaw rate fluctuation speed difference is within the allowable range. The range that can be entered is the allowable range.

Next, at step 602, the time derivative dδrlimits / dt of the smoothed limit value δrlimits.
Checks whether exceeds the maximum allowable value of the derivative of the limit value Drl_max. Also, in step 603, the time derivative dδrlimits / dt of the smoothed limit value δrlimits.
Check whether or not the minimum allowable amount of derivative of limit value Drl_min times. If the time derivative dδrlimits / dt of the smoothed limit value δrlimits exceeds the maximum allowable amount Drl_max of the derivative value of the limit value, in step 605,
The smoothed limit value δrlimits is multiplied by the maximum permissible amount Drl_max of the differential value of the limit value by the control cycle Ts, and the previous value δrlimits_old is added. Time derivative of smoothed limit value δrlimits dδrlimit
If s / dt has exceeded the minimum permissible differential value Drl_min of the limit value, in step 604, the smoothed limit value δrlimits is added to the minimum permissible differential value of the limit value Drl_m.
The control period Ts is multiplied by in, and the value obtained by adding the previous value δrlimits_old is substituted.

(Equation 11) δrlimits = δrlimits_old + Ts · Drl_max (Equation 12) δrlimits = δrlimits_old + Ts · Drl_min Next, at step 606, the sign of the steering angle δsw and the smoothed limit value δrlimit of the rear wheel steering angle.
Determine if the signs of s match. If they match, in step 607, 0 (zero) is substituted for the smoothed limit value δrlimits. That is, when the smoothed limit value δrlimits is calculated so that the front wheel steering and the rear wheel steering are in phase, by setting the smoothed limit value δrlimits to zero,
This prevents the rear wheels from being steered in phase.

Finally, in step 608, the current smoothed limit value δrlimits is substituted into δrlimits_old for the next calculation, the subroutine is terminated, and the process returns to the main routine of FIG.

In the above rear wheel steering angular velocity limit calculation, the rear wheel steering velocity in the case of performing the rear end overhang suppressing control is limited. Further, the steering angle of the rear wheels is limited so that the front wheels and the rear wheels are not steered in phase when the rear end overhang suppression control is performed.

Regarding the above-mentioned rear wheel steering speed limitation, FIG.
This will be described with reference to FIG. FIG. 14 shows steer angular velocity dδdw / dt and rear wheel steering angular velocity dδrlimits when steering the steering wheel.
It shows the characteristics of / dt. The center dotted line is dδrso / dt ＝
ratio_map (v) · dδsw / dt, which is the target angular velocity of the rear wheel steering angle obtained by multiplying the differential value dδsw / dt of the steering angle Δsw by the rear wheel / front wheel steering angle ratio ratio_map (v). The rear wheel / front wheel steering angle ratio ratio_map (v) has a negative value at a low vehicle speed as shown in FIG. 12, so the slope of the dotted line is negative.
The two parallel solid lines are the boundary lines dδrlimits / dt = ratio
_map (v) · dδsw / dt ± Gtmax / Grdr (0) is shown. When the steering angular velocity dδdw / dt and the rear wheel steering angular velocity dδrlimits / dt are within the parallel boundary line, the rear wheel steering velocity is limited. For example, if the steer angular velocity is zero with the driver holding the steering wheel, the rear wheel steering angular velocity is ±
It is limited to the range of dt-Gtmax / Grdr (0). Then, the limit range of the rear wheel steering angular velocity increases and decreases in inverse proportion to the vertical direction of the steering angular velocity.

When the rear wheel steering angular velocity limit calculation is completed, the target rear wheel steering angle limit calculation of step 109 is executed. Here, the target rear wheel steering angle δrt, which is the final calculation target of the controller 21, is calculated. The details are shown in FIG. In step 501, it is determined whether the actual front wheel steering angle δf is positive or zero. If the actual front wheel steering angle δf is positive or zero,
Proceeding to step 502, if the actual front wheel steering angle δf is negative,
Go to step 503. In step 502, the normal target rear-wheel steering angle δrt0 calculated in step 104 and the smoothed limit value δrlimits of the target rear-wheel steering angle calculated in step 107 are large in consideration of the sign. This value is used as the target rear wheel steering angle Δrt.
On the other hand, in step 503, the normal target rear wheel steering angle δrt0
And of the smoothed limit value δrlimits,
Taking into account the sign, the smaller value is used as the target rear wheel steering angle Δrt.

When the vehicle is traveling at a low vehicle speed, the rear wheel steering angle is calculated in a phase opposite to that of the front wheels as shown in FIG. Here, when the front wheel is cut in the positive direction, the rear wheel is cut in the negative direction, and Δrlimits and Δrt0 are negative or zero. Here, when the absolute value of δrt0 is smaller than the absolute value of δrlimits, δrlimits <δrt0 <0, and the larger normal target rear wheel steering angle δrt0 remains the target rear wheel steering angle δr.
used as t. Also, from the absolute value of δrlimits, δrt
When the absolute value of 0 is large, δrt0 <δrlimits <0, and the larger smoothed limit value δrlim
Its is used as the target rear wheel steering angle δrt. When the front wheel is cut in the negative direction, the rear wheel is cut in the positive direction, and Δrlimits and Δrt0 are positive values.
Here, when the absolute value of δrt0 is larger than the absolute value of δrlimits, 0 <δrt0 <δrlimits, and the smaller normal target rear wheel steering angle δrt0 is used as it is as the target rear wheel steering angle δrt. When the absolute value of δrt0 is smaller than the absolute value of δrlimits, 0 <δrlimits <δrt0,
The smaller smoothed limit value Δrlimits is used as the target rear wheel steering angle Δrt. When driving at high speed, the rear wheels are in phase with the front wheels. In this case, step 6
At 07, if δrlimits is in phase with the steer angle, δrlimi
Since ts = 0 and the absolute value of δrlimits is always smaller than δrt0, the normal target rear wheel steering angle δrt0 is directly used as the target rear wheel steering angle δrt.

Finally, at step 110, the actuator 1 is adjusted so that the steering angle of the rear wheels becomes equal to the target rear wheel steering angle δrt.
Control 5 In this control, the actual steering angle of the rear wheel 5 is detected by the rear wheel steering angle sensor 13, and this is the target rear wheel steering angle δ.
Servo control may be performed using PID control or the like so that it becomes equal to rt. As a result, the rear wheels 5 are controlled to an appropriate steering angle, and the rear end of the vehicle is prevented from overhanging at low vehicle speeds.

The above-described embodiment of the rear wheel steering system according to the present invention has the following effects. That is, the controller 21 limits the target rear wheel steering angle δrt so that the rear end point f of the vehicle does not extend beyond the inner region of the averaged locus 32 or the like obtained by smoothly averaging the running locus of the front end point g. Even if the rear wheels 5 are reverse-phase steered with respect to the front wheels 3 while the vehicle 1 is traveling in the low speed range, the rear end points f do not project to the outer area with respect to the averaging locus 32 of the front end points g. The target rear wheel steering angle δrt is limited, and the problem of the vehicle rear end point f overhanging can be solved.

Further, the target rear wheel steering angle δ is calculated based on the averaged locus 32 obtained by smoothly averaging the running locus 30 of the front end point g.
Since rt is limited, even when the traveling locus 30 of the front end point is disturbed, the influence of the disturbance is less likely to be reflected in the calculation result of the target rear wheel steering angle δrt. Therefore, according to the present invention, it is possible to further improve the turning performance of the vehicle.

Further, the target rear wheel steering angle limit value δrlimits
In calculating, the deviation angle θ, which is a relaxation amount related to the deviation between the rear end point near the rear end point e on the averaging trajectory and the rear end point f of the vehicle.
It is calculated in consideration of et. Therefore, when the deviation has occurred, it is possible to calculate in a direction in which the absolute value of the target rear wheel steering angle limit value Δrlimits can be set to a larger value. Therefore, when the rear wheels 5 are reverse-phase steered with respect to the front wheels 3, it is possible to reliably solve the problem of overhanging of the rear end point f of the vehicle 1 and improve the maneuverability.

In addition, when updating the arc 32 which is the averaging locus, it can be performed based on only the information of the arc 32 calculated last time when the latest sampling point is updated and the information of the latest sampling point. . Therefore, when calculating the arc 32 which is the averaged trajectory, the controller 21
Needs to store only the information on the latest sampling point a, and it is not necessary to store information on a plurality of past sampling points. Further, due to the arc 32, the traveling locus 30
, The geometrical characteristics peculiar to the circular arc can be used, and the averaged locus can be easily calculated as compared with a complicated method such as polynomial approximation. Therefore, the work space and memory capacity required by the controller 21 can be saved, and the operation speed can be improved.

When the target rear-wheel steering angle limit value δrlimit is interpolated to the smoothed limit value δrlimits which is a continuous value, at least the limit value δrlimit1 calculated at the past sampling point input time t1 is used. In consideration of the smoothed limit value δrlimits at the latest sampling point input time t0, the current smoothed limit value δrlimit from the latest sampling point input time t0 to the next sampling point input time
Since s is calculated, the current smoothed limit value δrlimits, which is the result of the interpolation calculation, includes the increase / decrease tendency of the past limit value and the deviation between the past limit value δrlimit and the smoothed limit value δrlimits. It is a reliable value that takes into consideration.

Further, the front end point g and the rear end point f are set at the frontmost end portion and the rearmost end portion of the vehicle 1, respectively.
Therefore, the rear end of the vehicle, which is most likely to come into contact with the obstacle on the side of the vehicle due to the overhang of the rear end of the vehicle, does not extend outside the averaging locus 32 related to the front end of the vehicle. It is possible to more reliably prevent contact with an obstacle on the side of the vehicle due to overhang of the end.

It has been found that, depending on the shape of the vehicle, there are appendages that extend forward and leftward from the front end point g, and appendages that extend rearward and leftward and rightward from the rear end point f. If it is, the front end point g and the rear end point f may be replaced with the positions of the end portions of the appendages for calculation.

As described above, the rear wheel steering system of this embodiment is a system for steering the rear wheels in accordance with the steering angle of the front wheels, and the rear wheel limited steering angle for calculating the limited steering angle of the rear wheels. A calculation routine 106, a rear wheel limit steering angle smoothing calculation routine 107 for smoothing the calculated value, a target rear wheel steering angle limit calculation routine 109 for limiting the normal target rear wheel steering angle of the rear wheel to the limit value, and an actual rear wheel A rear-end squeeze restraint control means including a rear-wheel steering actuator driving step 110 for steering is provided.

Further, the rear wheel steering speed dδrlimits / dt which is a differential value of the limit value of the rear end overhang suppression control means is set to a specific range r.
The rear wheel steering speed limiting means is composed of steps 601 to 604 for limiting to atio_map (v) · dδsw / dt ± Gtmax / Grdr (0). Therefore, the smoothed limit value δrlimits is further restricted so that the rate of change, which is the differential value, does not deviate from the specific range ratio_map (v) ・ dδsw / dt ± Gtmax / Grdr (0). The rear wheels do not change significantly with respect to the operation.

FIG. 15 shows a steering angle of 545 ° (left) when a 2WS vehicle (rear wheel steering angle is zero), a steering angle proportional control, a rear end extension control, and a control according to the embodiment of the present invention are adopted. ) (Front wheel left steering angle 38.93 °) and a vehicle simulation result assuming a U-turn at a constant vehicle speed of 5 km / h are shown. It is assumed that the actual rear wheel steering angle δm at the time of starting when the control according to the embodiment of the present invention is adopted is zero. When the driver steers the steering wheel when the engine is stopped, if the rear wheels are steered along the running trajectory before the stop,
This is because it may move in an unexpected direction.

As is clear from the results shown in FIG. 15, the target rear wheel steering angle δrt0 changes sharply in the portion c, whereas when the control according to the embodiment of the present invention is adopted, d
As shown in, the change in the target rear wheel steering angle Δrt0 is gradual.

Further, when the limit value of the rear end overhang suppression control means is in phase with the steering angle δs, the rear end overhang suppression control means is disabled by invalidating the limit value of the rear end overhang suppression control means. In the case where the rear wheels are steered so as to suppress the rear end overhang, the in-phase steering angle limiting means including steps 606 to 607 for limiting the steering angle so that the front wheels and the rear wheels are not steered in phase is provided. There is. As a result, the vehicle does not move in parallel due to the rear end overhang suppression control.

In the above-mentioned embodiment, the rear end overhang suppression control is proposed by another method, for example, a method of checking the overhang without averaging the traveling locus, or the second embodiment of 001-242842. The method of dividing the averaging locus into a plurality may be used instead.

[0080]

According to the inventions of claims 1 and 3,
When the vehicle is running at low speed, the driver's discomfort caused by the vehicle moving in parallel can be eliminated.

According to the second and third aspects of the present invention, it is possible to eliminate the driver's discomfort due to the abrupt steering of the rear wheels due to the rear end extension control.

According to the fourth aspect of the present invention, since the yaw rate fluctuation due to the steering of the rear wheels is within the allowable speed difference,
The discomfort of the driver can be eliminated.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of a four-wheel steering vehicle including a rear wheel steering system according to the present invention.

FIG. 2 is a flowchart of a main routine executed by a controller.

FIG. 3 is a flowchart showing an initialization process according to the embodiment of the present invention.

FIG. 4 is a flowchart showing interrupt processing according to the embodiment of the present invention.

FIG. 5 is a flowchart showing a normal rear wheel steering angle calculation process in the embodiment of the present invention.

FIG. 6 is a flowchart showing a rear wheel limited steering angle calculation process in the embodiment of the present invention.

FIG. 7 is a flowchart showing rear wheel limited steering angle smoothing calculation processing according to the embodiment of the present invention.

FIG. 8 is a flowchart showing a rear wheel steering angular velocity limit calculation process in the embodiment of the present invention.

FIG. 9 is a flowchart showing a target rear wheel steering angle limit calculation in the embodiment of the present invention.

FIG. 10 is a model diagram showing movement of the vehicle in the embodiment of the present invention.

FIG. 11 is a schematic diagram showing how the averaged trajectory is updated each time the latest sampling point changes.

FIG. 12 is a diagram showing a rear wheel / front wheel steering angle ratio map in steering angle proportional control.

FIG. 13 is a diagram schematically illustrating a process of interpolation calculation according to the embodiment of the present invention.

FIG. 14 is a steer angular velocity dδd according to the embodiment of the present invention.
7 is a graph showing characteristics of w / dt and rear wheel steering angular velocity dΔrlimits / dt.

FIG. 15 is a graph showing changes in vehicle speed v, target rear wheel steering angle δrt, and steering angle when a vehicle motion simulation is performed.

[Explanation of symbols]

1 vehicle 3 front wheel 5 rear wheel 7 steering 9 wheel speed sensor 11 front wheel steering angle sensor 13 rear wheel steering angle sensor 17 yaw rate sensor 19 ignition switch 21 controller 23 shift position sensor 30 actual traveling locus 30 traveling locus 32 curve 32 averaging locus a, a0, a1, b, c, d Sampling point C1 Gain for predetermined steering angle limit Drl_max Maximum allowable amount of differential value of limiting value Drl_min Minimum allowable amount of differential value of limiting value Dx Sampling distance dδdw / dt Steer Angular velocity dδrlimits / dt Rear wheel steering angular velocity e, e0, e1 Neighbor point of rear end point on averaging trajectory g Front end point Grdr (0) Yaw rate gain Gtmax for the rear wheel Maximum allowable yaw rate fluctuation speed difference k ・ θat, k ・ θato average Center angle L of the trajectory of change L Wheel base Le / Dx Total vehicle length Lf Axle length from center of gravity to front wheel Lfe Length from center of gravity to front end Lr Axle length ratio_map (v) Rear wheel / front wheel steering angle ratio t0 Time of latest sampling point t1 Time of previous sampling point Td0 Initial vehicle speed pulse input interval time Ts Control cycle v Vehicle speed γ Yaw rate δf Actual front wheel steering angle δm Actual Rear wheel rudder angle δr Rear wheel rudder angle δrlimit Target rear wheel rudder angle limit value at the latest sampling point δrlimit1 Target rear wheel rudder angle limit value at the previous sampling point δrlimits Smoothed limit value δrlimits1 1 control cycle before Smoothed limit value δrlimitst0 Smoothed limit value at the latest sampling point δrt, δrt0 Normal target rear wheel steering angle δsw Steering angles θat, θato Trajectory angle of front end point on averaged trajectory θb Absolute angle θet deviation of vehicle body Angle θfe Traveling direction angle of front end point θfea, θfeao Traveling direction angle of front end point on averaging trajectory θfear Traveling direction of front end point on averaging trajectory, which is near to rear end point e Angle θre Travel direction angle of rear end point θrelimit Limit value of travel direction angle of rear end point Step 106 Rear wheel limit steering angle calculation step 107 Rear wheel limit steering angle smoothing calculation routine Step 108 Rear wheel steering angle speed limit calculation step 109 Target rear wheel steering Angle limit calculation

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) B62D 137: 00 B62D 137: 00

Claims (5)

[Claims] 1. A rear wheel steering system for steering a rear wheel according to a steering angle of a front wheel, comprising rear end bulge suppression control means for suppressing bulging of a rear end of a vehicle, and rear end bulge suppression control means. Is equipped with in-phase steering angle limiting means for limiting the steering angle so that the front wheel and the rear wheel do not steer in phase when the rear wheel is being steered so as to suppress overhang of the rear end. Steering device. 2. A rear wheel steering system for steering a rear wheel according to a steering angle of a front wheel, comprising rear end bulge suppression control means for suppressing bulging of a rear end of a vehicle, and rear end bulge suppression control means. The rear-wheel steering device includes rear-wheel steering speed limiting means for limiting the rear-wheel steering speed within a specific range when the rear-wheel steering is performed to suppress rear-end overhang. . 3. A rear wheel steering system for steering rear wheels according to a steering angle of front wheels, comprising rear end overhang suppressing control means for suppressing overhang of a rear end of a vehicle, and rear end overhang suppressing control means. When steering the rear wheels so as to suppress the rear end overhang, the in-phase steering angle limiting means for limiting the steering angle so that the front wheels and the rear wheels do not steer in phase, and the rear end overhang suppressing control means A rear-wheel steering device comprising rear-wheel steering speed limiting means for limiting a rear-wheel steering speed to a specific range when the rear wheels are being steered so as to suppress rear-end overhang. 4. The specific range of the rear wheel steering speed limiting means according to claim 2 or 3, wherein the yaw rate fluctuation speed difference is within an allowable range around the target angular speed of the rear wheel steering angle. Rear wheel steering device characterized by. 5. The specific range of the rear wheel steering speed limiting means according to claim 2 or 3, wherein a maximum permissible yaw rate fluctuation speed difference / rear direction is centered on a rear wheel / front wheel steering angle ratio × a steering angle differential value. A rear wheel steering system characterized in that an area having a width of yaw rate gain for a wheel is set as a specific range.