JP2536197B2 - Rear wheel steering system for front and rear wheel steering vehicles - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rear wheel steering system for a front and rear wheel steering vehicle, which steer-controls rear wheels according to a yaw rate acting on a vehicle body.

[Prior Art] Conventionally, an apparatus of this type is provided with a yaw rate detecting means for detecting a yaw rate as shown in, for example, Japanese Patent Laid-Open No. 63-207772, and controls the rear wheels in a direction to suppress the yaw rate. Steering amount determining means for determining the steering amount of the rear wheels based on an arithmetic expression having a control term for increasing the steering control amount of the rear wheels in the yaw rate suppressing direction as the detected yaw rate increases. Prepare,
By steering the rear wheels according to the determined steering amount, the yaw rate that occurs in the vehicle body when a side wind is received, the vehicle turns, etc. is suppressed to improve the running stability of the vehicle. There is.

[Problems to be Solved by the Invention] However, in the above conventional device, in order to improve the running stability of the vehicle, the steering amount of the rear wheels in the yaw rate suppressing direction must be set to a relatively large value. However, increasing this value causes the following.
That is, increasing the steering amount of the rear wheels in the yaw rate suppressing direction means that the vehicle body is oriented more tangentially to the turning direction, and when the vehicle is making a steady circular turn, Although the driver desires to visually check the corner exit, the vehicle body faces outward, so in some vehicles, the above confirmation may be difficult and it may be difficult to drive the vehicle. When the rear wheels are steered in the direction that suppresses the yaw rate of the vehicle,
Since the stability of the vehicle at the time of turning is improved, the initial turning ability of the vehicle at the time of sudden steering may be felt to be dull.

The present invention has been made to address the above problems,
The purpose is to solve the above-mentioned various problems by adding the steering speed of the front wheels to the steering of the rear wheels in the yaw rate suppressing direction, and increase the degree of freedom of vehicle characteristics. An object is to provide a rear wheel steering device for a front and rear wheel steering vehicle.

[Means for Solving the Problems] In order to achieve the above object, the structural features of the invention according to claim 1 are, as shown in FIG. 1, a yaw rate detecting means 1 for detecting a yaw rate, and a yaw rate. Control amount determining means 2 for determining a control amount for steering the rear wheel RW in a direction of suppressing the control amount, the control amount increasing on the basis of the detected yaw rate as the detected yaw rate increases, and the determined control amount. In a rear wheel steering system for a front and rear wheel steering vehicle, which comprises a steering control means 3 for steering and controlling a rear wheel RW in accordance with the above, a steering speed detecting means 4 for detecting a steering speed of a front wheel FW.
And the changing means 5 for changing the determined control amount according to the detected steering speed of the front wheels FW.

Further, in the invention according to claim 2, the changing means 5 has a storage means 5a in which a function value having the steering speed of the front wheels FW as a variable is stored in advance, and based on the detected steering speed of the front wheels FW. This is because the control amount is changed by the function value derived from the storage means 5a.

Further, in the invention according to claim 3, the change means 5 keeps the determined control amount to a small value when the detected steering speed of the front wheels FW is small, and steers the detected front wheels FW. The control amount is changed to a large value when the speed increases.

In the present invention according to claim 4, the changing means 5 is
When the detected steering speed of the front wheels FW is small, the determined control amount is kept at a large value, and when the detected steering speed of the front wheels FW is increased, the determined control amount is set to a small value. It consists of something to change.

Further, the invention according to claim 5 is provided with a vehicle speed detecting means 6 for detecting a vehicle speed in addition to the configuration of the invention according to claim 1, and the control amount determining means 2 controls the rear wheel in a direction to suppress the yaw rate. The control amount for steering the RW is determined based on the detected vehicle speed and the detected yaw rate, the control amount increasing as the detected vehicle speed increases and increasing as the detected yaw rate increases. .

[Operation and Effect of the Invention] In the invention according to claim 1 configured as described above, the control amount determining means 2 is a control amount for steering the rear wheel RW in a direction in which the yaw rate is suppressed, and the yaw rate is detected. Based on the yaw rate detected by the means 1, a control amount that increases as the detected yaw rate increases is determined, and the steering control means 3 performs steering control of the rear wheels RW according to the determined control amount. When the amount is determined, the changing unit 5 changes the determined control amount according to the steering speed of the front wheels FW detected by the steering speed detecting unit 4. In this case, the changing means 5 stores the function value having the steering speed of the front wheels FW as a variable in the storage means 5a in advance, and the detected steering of the front wheels FW is performed, for example. The determined control amount is changed by the function value derived from the storage means 5a based on the speed.

Therefore, according to the inventions according to claims 1 and 2, the control amount for suppressing the yaw rate and ensuring the traveling stability of the vehicle is changed according to the steering speed of the front wheels FW, so that the traveling stability is ensured. In addition, various turning characteristics can be obtained according to the steering wheel operation, and the vehicle can be adapted to various traveling environments.

Further, in the invention according to claim 3 configured as described above, the changing means 5 keeps the determined control amount to a small value when the steering speed of the front wheels FW is small,
When the steering speed is increased, the determined control amount is changed to a large value. Therefore, according to the invention of claim 3, in addition to the effects of the inventions of claims 1 and 2, when the steering speed of the front wheels FW is small, such as during steady circle turning, the yaw rate is suppressed. Since the control amount for controlling the vehicle body can be relatively small and the vehicle body can be relatively turned in the turning direction, the driver can visually confirm the corner exit and drive the vehicle easily.

Further, in the invention according to claim 4 configured as described above, the changing means 5 keeps the determined control amount at a large value when the steering speed of the front wheels FW is small, and maintains the front wheels FW.
When the steering speed is increased, the determined control amount is changed to a small value. Therefore, according to the invention of claim 4, in addition to the effects of the invention of claims 1 and 2, it is possible to reduce the control amount for suppressing the yaw rate when the front wheels FW are steered steeply. Because you can
The initial turning property associated with the turning of the vehicle is improved.

Further, in the invention according to claim 5 configured as described above, in addition to the operation of the invention according to claim 1, the control amount determining means 2 also considers the vehicle speed detected by the vehicle speed detecting means 6. Thus, the control amount that increases as the detected yaw rate increases and that increases as the detected vehicle speed increases is determined. Therefore, according to the fifth aspect of the present invention, when the vehicle is traveling at high speed, the control amount for suppressing the yaw rate is set to be large, so that the traveling stability of the vehicle at high speed traveling can be better ensured. It

[Embodiment] A first embodiment of the present invention will be described below with reference to the drawings. FIG. 2 schematically shows an entire front and rear wheel steering vehicle according to the present invention. This front and rear wheel steering vehicle has front left and right wheels FW1, FW2
The left and right rear wheels RW1, RW2 are composed of a front wheel steering mechanism A for steering the vehicle, a rear wheel steering mechanism B, and an electric control circuit C.
It is equipped with a rear wheel steering device that steers in conjunction with W2 steering.

The front wheel steering mechanism A has a steering handle 11, and this steering handle 11 is connected to a pinion gear 13 via a steering shaft 12. The pinion gear 13 meshes with the rack bar 14, converts the rotational movement of the steering handle 11 into the reciprocating movement of the rack bar 14 and transmits the reciprocating movement. The left and right tie rods 15a and 15b and the left and right knuckle arms 1 are provided at both ends of the rack bar 14.
The left and right front wheels FW1 and FW2 are steerably connected via 6a and 16b, and the bar 14 steers the left and right front wheels FW1 and FW2 according to the axial displacement caused by the turning of the steering wheel 11. Steering shaft 12
A control valve 17 made up of a four-way valve is installed in the middle of the engine, and the valve 17 powers hydraulic oil from a hydraulic pump 21 driven by an engine 18 in accordance with steering torque acting on the steering shaft 12. It supplies the oil to one oil chamber of the cylinder 22 and discharges the hydraulic oil in the other oil chamber of the cylinder 22 to the reservoir 23. The power cylinder 22
By driving the rack bar 11 in the axial direction according to the supply and discharge of the hydraulic oil, steering of the left and right front wheels FW1 and FW2 is assisted.

The rear wheel steering mechanism B has a relay rod 31 for axially displacing and steering the left and right rear wheels RW1 and RW2, like the rack bar 14. Similar to the case of the front wheel steering mechanism A, the left and right rear wheels RW1 and RW2 are steerably connected to both ends of the relay rod 31 via the left and right tie rods 32a and 32b and the left and right knuckle arms 33a and 33b. The relay rod 31 is axially displaceably supported by a housing 34 supported by the vehicle body, and a power cylinder 35 is formed in the housing 34. The power cylinder 35 drives the relay rod 31 in the axial direction according to the supply and discharge of hydraulic oil, and the cylinder 35 is divided into left and right oil chambers 35b and 35c by a piston 35a fixed to the relay rod 31. These left and right oil chambers 35b, 35c
The springs 36a, 36b are incorporated so as to penetrate the relay rod 31 in a preloaded state, and the springs 36a, 36b urge the relay rod 31 to the neutral position by their elastic force. are doing.

A spool valve 37, which constitutes a hydraulic copying mechanism together with the power cylinder 35, is incorporated in the housing 34. The spool valve 37 is a valve sleeve housed in a housing 34 so as to be fluid-tight and slidable in the axial direction.
37a and a valve spool 37b fixed to the housing 34. The hydraulic oil from the hydraulic pump 38 driven by the engine 18 is transferred to the left oil of the power cylinder 35 in accordance with the displacement of the valve sleeve 37a to the left in the figure. The hydraulic oil in the right oil chamber 35c of the cylinder 35 is discharged to the reservoir 23 while being supplied to the chamber 35b. When the valve sleeve 37a is displaced to the right in the figure, the spool valve 37 supplies the hydraulic oil from the hydraulic pump 38 to the right oil chamber 35c of the power cylinder 35, and the hydraulic oil in the left oil chamber 35b of the cylinder 35. Is discharged to the reservoir 23.

A through hole 37a1 is provided at the right end of the valve sleeve 37a, and a lever 41 penetrates the through hole 37a1. A spherical nodular ridge 41a is provided in the middle of the lever 41, and the lever 41 engages slidably and slidably with the inner peripheral surface of the through hole 37a1 at the outer peripheral surface of the nodal ridge 41a. ing. Further, the lower end of the lever 41 is fitted into an annular groove 35a1 provided on the outer periphery of the piston 35a so as to be rotatable and slidable in the vertical direction, and the upper end of the lever 41 is rotatable to a pin 42. It is connected.

Both ends of the pin 42 have support holes 34a provided in the housing 34,
It has entered into 34b so that it can move forward and backward and cannot rotate. Also,
Rack teeth 42a are formed on the outer periphery of the pin 42, and a worm 44 fixed to the rotation shaft of the step motor 43 meshes with the rack teeth 42a. In this case, step motor 43
When is rotated in the positive (or negative) direction, the pin 42 is displaced rightward (or leftward).

The electric control circuit C includes a front wheel steering angle sensor 51 and a vehicle speed sensor.
52, a yaw rate sensor 53, and a rear wheel steering angle sensor 54 are provided. The front wheel steering angle sensor 51 detects the rotation angle of the steering shaft 12 and outputs a detection signal indicating the detected rotation angle. The vehicle speed sensor 52 detects the rotation speed of the output shaft of the transmission (not shown) and outputs a detection signal indicating the detected rotation speed. The yaw rate sensor 53 detects the rotational angular velocity of the vehicle body about the vertical axis and outputs a detection signal indicating the detected rotational angular velocity. The rear wheel steering angle sensor 54 detects the rotation angle of the rotation shaft of the step motor 43 and outputs a detection signal indicating the detected rotation angle.

Each of these sensors 51 to 54 is connected to a microcomputer 55, which is composed of a ROM 55b, a CPU 55c, a RAM 55d and an I / O 55e (input / output interface) which are connected to a bus 55a. ROM55b stores the program corresponding to the flowchart of FIG.
The coefficients K ₁ , K ₂ and K ₃ are stored in the form of a table. Coefficient K
₁ is a control variable which is multiplied by the front wheel steering angle θf and changes according to the vehicle speed V for steering the left and right rear wheels RW1, RW2 in opposite phase to the left and right front wheels FW1, FW2 when the vehicle turns at a low speed. , As shown in FIG. 4A, it is negative in a region where the vehicle speed V is low and becomes zero in a region where the vehicle speed V is high. Coefficient K
₂ changes according to the vehicle speed V for steering the left and right rear wheels RW1, RW2 in-phase with the left and right front wheels FW1, FW2 in the direction of suppressing the yaw rate when the yaw rate ωy is applied and the vehicle turns at a high speed. As shown in FIG. 4B, it is a control variable that is zero in a region where the vehicle speed V is small, and gradually increases to a large positive value as the vehicle speed V increases. The coefficient K _{3 is} also multiplied by the yaw rate ωy, which controls the in-phase steering amount (yaw rate suppression steering amount) to a small value when the vehicle is in a steady circular turn. As shown in FIG. It has a small positive value in a region where the speed dθf / dt is small (corresponding to a steady circular turn), and gradually becomes a large positive value as the front wheel steering speed dθf / dt increases.

The CPU 55c repeatedly executes the program from closing to opening of an ignition switch (not shown), and the RAM 55d temporarily stores variable data necessary for executing the program. The I / O 55e sends and receives signals to and from an external circuit, and the I / O 55e is connected to the sensors 51 to 54 and a drive circuit 56. The drive circuit 56 is a step motor for the number of steps corresponding to the rotation control signal from the microcomputer 55.
While rotating 43, the motor 43 is then controlled so as to be maintained at the position after the rotation.

Next, the operation of the embodiment configured as described above will be described.

When the ignition switch is closed, the CPU 55c starts executing the program in step 100 of FIG.
After the execution of this program is started, at step 102, the respective detection signals are read from the respective sensors 51 to 54, and the front wheel steering angle θf, the vehicle speed V, the yaw rate ωy and the rear wheel steering angle θr are respectively calculated from the respective detection signals. RA
It is stored in M55d. In this case, only the current values of the vehicle speed V, the yaw rate ωy, and the rear wheel steering angle θr are stored, but the past wheel steering angle θf is also stored for the subsequent processing. The front wheel steering angle θf and the rear wheel steering angle θ
At r, the left and right front wheels FW1 and FW2 and the left and right rear wheels RW1 and RW2 each have a positive value when steered to the right, and have a negative value when steered to the left. Further, the yaw rate ωy has a positive value when the vehicle turns right by steering the right front wheels FW1 and FW2 to the right, and has a negative value when turning the vehicle left.

Next, at step 104, the front wheel steering speed dθf / dt is calculated by time differentiating the stored front wheel steering angles θf from the past to the present, and is stored in the RAM 55d. After the processing of step 104, the vehicle speed V and the front wheel steering speed dθf / dt stored in the RAM 55d in step 106
Based on the above, the table in the ROM 55b is referred to, and each coefficient K ₁ , K ₂ , K ₃ corresponding to the vehicle speed V and the front wheel steering speed dθf / dt.
The target rear wheel steering angle θr * is calculated in step 108 by executing the calculation of the following equation 1.

θr * = K ₁ · θf + K ₃ · K ₂ · ωy Equation 1 After calculating the target rear wheel steering angle θr *, step 110
At the same target rear wheel steering angle θr *, the current left and right rear wheels RW1,
By subtracting the rear wheel steering angle θr representing the steering angle of RW2, the steering amount θr * −θ of the rear wheels RW1 and RW2 to be steered.
r is calculated and output to the drive circuit 56 via the rotation control signal I / O 55e for the step motor 43 corresponding to the steering amount θr * −θr.

The drive circuit 56 supplies a drive pulse according to the rotation control signal to the step motor 43, and the motor 43 rotates the worm 44 by an amount corresponding to the drive pulse. In this case, if the rotation control signal corresponding to the rear wheel steering amount θr * −θr is positive, the step motor 43 rotates forward and the pin 42
Is displaced to the right, and the upper end of the lever 41 is displaced to the right with the lower end as a fulcrum. As a result, the valve soub 37a is displaced to the right, the hydraulic oil from the hydraulic pump 38 is supplied to the right oil chamber 35c of the power cylinder 35, and the hydraulic oil in the left oil chamber 35b of the cylinder 35 is stored in the reservoir 23.
Therefore, the relay rod 31 is displaced leftward and the left and right rear wheels RW1, RW2 are steered rightward. On the other hand, the leftward displacement of the relay rod 31 causes the lower end portion of the lever 41 to be displaced leftward with the upper end portion as a fulcrum, and the valve sleeve 37a is displaced leftward. Then, when the valve sleeve 37a returns to the reference position, the supply and discharge of the hydraulic oil is stopped, and the leftward displacement of the relay rod 31 is also stopped. The steering wheel is steered to the right by an amount corresponding to the wheel steering amount θr * −θr.

If the rotation control signal corresponding to the rear wheel steering amount θr * −θr is negative, the step motor 43 rotates negatively and the pin 42 is displaced leftward. The wheels RW1 and RW2 have the rear wheel steering amount θr * − from the past state.
The steering wheel is steered to the left by an amount corresponding to θr.

After the processing of step 110, the program is step 102.
After that, the processes of steps 102 to 110 described above are repeatedly executed, and the left and right rear wheels RW1 and RW2 are steered to the target rear wheel steering angle θr *.

As a result of steering control of the left and right rear wheels RW1, RW2 in this way, when the left and right front wheels FW1, FW2 are steered to the right while the vehicle is traveling at low speed, the front wheel steering angle θf is positive and the coefficient K ₁
Is negative and the coefficient K ₂ is zero, the target rear wheel steering angle θr * is set to negative, and the left and right rear wheels RW1, RW2 are steered in the left direction, that is, in the opposite phase to the left and right front wheels FW1, FW2. To be done. Further, when the left and right front wheels FW1 and FW2 are steered to the left during the same low speed traveling, the coefficients K ₁ and K ₂ are the same as in the above case, and only the front wheel steering angle θf changes to a negative value. Wheel steering angle θr
* Is set to a positive value, and the left and right rear wheels RW1, RW2 are steered in the right direction, that is, in this case also, the left and right front wheels FW1, FW2 are steered in reverse phase. As a result, the turning performance of the vehicle while traveling at low speed is improved.

On the other hand, when the left and right front wheels FW1 and FW2 are steered to the right while the vehicle is traveling at high speed, the yaw rate ωy is positive, the coefficient K ₁ is zero, and the coefficients K ₂ and K ₃ are both positive. Because there is
The target rear wheel steering angle θr * is set to a positive value, and the left and right rear wheels RW1, RW
2 is steered in the right direction, that is, in phase with the left and right front wheels FW1, FW2 (in the yaw rate suppression direction). When the left and right front wheels FW1 and FW2 are steered to the left while driving at the same high speed, the coefficient K ₁ ,
K ₂ and K ₃ are the same as in the above case, only the yaw rate ωy changes to negative, so the target rear wheel steering angle θr * is set to negative and the left and right rear wheels RW1, RW2 are in the left direction, that is, in this case. Is steered in phase with the front left and right wheels FW1 and FW2 (in the yaw rate suppression direction). As a result, the running stability of the vehicle during high-speed running is improved.

Further, in such in-phase control (yaw rate suppression control), the coefficient K ₃ changes according to the front wheel steering speed dθf / dt,
The target rear wheel steering angle θr * changes. That is, as shown in FIG. 4C, when the front wheel steering speed dθf / dt is small as in the case where the vehicle is making a steady circular turn, the coefficient K ₃ is a small positive value, so the target rear wheel steering angle is The absolute value | θr * | of θr * becomes a small value, and the in-phase steering amount (the yaw rate suppressing steering amount) of the left and right rear wheels RW1, RW2 becomes small. Thus, when the vehicle is in a steady circular turn, the vehicle body relatively faces the turning direction, and the driver can visually check the corner exit, which facilitates driving the vehicle.

Next, a second embodiment of the present invention will be described. The vehicle according to the second embodiment places importance on the initial turning property at the start of turning, and is structurally the same as the case of the first embodiment shown in FIG. However, in such a case, the ROM 55b stores a program corresponding to the flowchart of FIG. 5 and has a coefficient different from that of the first embodiment.
K ₁ and K ₂ are stored in the form of a table.

The coefficient K ₁ is represented by a two-dimensional map, as shown in FIG. 6A, and shows a tendency that the front wheel steering speed dθ / dt increases and accordingly decreases, and also increases as the vehicle speed V increases. . Further, the coefficient K ₂ has a tendency to increase as the vehicle speed V increases, as shown in FIG. 6B. In addition,
Both coefficients K ₁ and K ₂ are always positive values.

The operation of the second embodiment will be described with reference to the flowchart of FIG. 5, and in the second embodiment as well,
Similar to the case of the first embodiment, the front wheel steering angle θf, the vehicle speed V, the yaw rate ωy,
The rear wheel steering angle θr and the front wheel steering speed dθf / dt are calculated. Using these variables, the coefficients K ₁ and K ₂ are derived in step 106a, and the derived coefficients K ₁ and K ₂ and the calculated yaw rate ωy are used in step 108a to obtain the following equation 2. By executing the calculation, the target rear wheel steering angle θr * is calculated.

θr * = K ₁ · K ₂ · ωy Equation 2 Next, the left and right rear wheels RW1 and RW1 are processed by the same processing of step 110 and the operation of the rear wheel steering mechanism B as in the case of the first embodiment.
2 is controlled to the calculated target rear wheel steering angle θr *.

As a result, in the second embodiment, the in-phase steering amount (yaw rate suppressing steering amount) of the left and right rear wheels RW1, RW2 with respect to the left and right front wheels FW1, FW2 increases as the vehicle speed V and the yaw rate ωy increase, so that the high-speed running is performed. The running stability of the vehicle is improved. Further, in such a case, when the front wheel steering speed dθf / dt increases, the in-phase steering amount decreases, so that the initial turning property of the vehicle accompanying turning becomes favorable.

As described above, according to the first and second embodiments, the coefficient that changes according to the front wheel steering speed dθf / dt is
The left and right rear wheels RW1, RW2 using the yaw rate ωy are added by adding the in-phase steering amount (yaw rate suppression steering amount) of the left and right rear wheels RW1, RW2 related to the yaw rate ωy in the form of multiplication.
Since the steering control is corrected, the degree of freedom in steering control of the vehicle by the yaw rate control is increased, and various steering characteristics of the vehicle can be realized.

As described above, in the case where the front wheel steering speed dθf / dt is added to the control term of the yaw rate ωY in the form of multiplication, the way of adding the difference varies depending on the type of vehicle. Also, it differs depending on the type of the arithmetic expression such as Equation 2 and the manner in which the value of the coefficient used in the arithmetic expression changes.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram corresponding to the claims corresponding to the present invention described in the claims, and FIG. 2 is a rear wheel steering device according to first and second embodiments of the present invention. FIG. 3 is a flow chart corresponding to the program according to the first embodiment, and FIGS. 4A to 4C are respective coefficients K ₁ , K ₂ , K according to the first embodiment. _{3 is} a characteristic diagram, FIG. 5 is a flowchart of a program according to the second embodiment, and FIGS. 6A and 6B are characteristic diagrams of coefficients K ₁ and K _{2 according} to the second embodiment. Explanation of symbols A ... front wheel steering mechanism, B ... rear wheel steering mechanism, C ... electric control circuit, FW1, FW2 ... front wheels, RW1, RW2 ... rear wheels, 51 ...
Front wheel steering angle sensor, 52 …… Vehicle speed sensor, 53 …… Yaw rate sensor, 54 …… Rear wheel steering angle sensor, 55 …… Microcomputer.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shin Koike 1 Toyota Town, Toyota City, Aichi Prefecture, Toyota Motor Co., Ltd. (72) Inventor Mizuho Sugiyama 1 Toyota Town, Aichi Prefecture, Toyota Motor Co., Ltd. ( 72) Inventor Hitoshi Iwata 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Co., Ltd. (56) Reference JP-A-1-172071 (JP, A)

Claims (5)

(57) [Claims] 1. A yaw rate detecting means for detecting a yaw rate, and a control amount for steering a rear wheel in a direction for suppressing the yaw rate, the control amount increasing on the basis of the detected yaw rate as the detected yaw rate increases. In a rear wheel steering system for a front-rear wheel steering vehicle, comprising: a control amount determining means for controlling the rear wheels, and a steering control means for steering-controlling the rear wheels according to the determined control amount. A rear-wheel steering system for a front-rear wheel steering vehicle, comprising: means for changing the control amount determined according to the detected steering speed of the front wheels. 2. The changing means has a storage means for storing in advance a function value having the steering speed of the front wheels as a variable, and the function value derived from the storage means on the basis of the detected steering speed of the front wheels. The rear wheel steering system according to claim 1, wherein the control amount determined by the above is changed. 3. The changing means keeps the determined control amount to a small value when the detected steering speed of the front wheels is small, and makes the determination when the detected steering speed of the front wheels becomes large. The rear wheel steering system according to claim 1, wherein the controlled amount to be changed is changed to a large value. 4. The changing means keeps the determined control amount at a large value when the detected steering speed of the front wheels is small, and the determination means when the detected steering speed of the front wheels becomes large. The rear wheel steering system according to claim 1, wherein the control amount to be controlled is changed to a small value. 5. A vehicle speed detecting means for sending out a vehicle speed, a yaw rate detecting means for detecting a yaw rate, and a control amount for steering the rear wheels in a direction to suppress the yaw rate, which are based on the detected vehicle speed and the detected yaw rate. A control amount determining means for determining a control amount that increases with an increase in the detected vehicle speed and an increase with an increase in the detected yaw rate, and steering control means for steering-controlling the rear wheels in accordance with the determined control amount. In a rear wheel steering system for a front and rear wheel steering vehicle, a steering speed detecting means for detecting a steering speed of a front wheel, and a changing means for changing the control amount determined according to the detected steering speed of the front wheel are provided. A rear-wheel steering device provided with front and rear wheel steering vehicles.