JPS62116357A - Rear wheel steering controller for front and rear wheel steering car - Google Patents

Abstract

PURPOSE:To always secure the steering stability of a car by fixing the determined steering angle ratio when the steering angle of a front wheel is larger than a prescribed value and releasing the fixation of the determined steering angle ratio after the lapse of a prescribed time from the time when the steering angle of the front wheel becomes smaller than a prescribed value. CONSTITUTION:The steering angle ratio of the rear wheel steering angle for the front wheel steering angle is determined by a means 2 according to the car speed detected by a means 1. When a means 3 detects the steering angle of a front wheel and the detected value is larger than a prescribed value, a means 4 fixes the determined steering angle. When the means 4 fixes the determined steering angle ratio, a means 5 releases the fixation of the determined steering angle ratio after the lapse of a prescribed time from the time when the detected front wheel steering angle becomes smaller than a prescribed value. A means 6 outputs the control signal corresponding to the determined steering angle ratio, and in correspondence with the control signal, the rear wheel steering angle is set on the basis of the determined steering angle ratio by a rear wheel steering mechanism 30, and the rear wheels are steered.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a rear wheel steering control device for a vehicle with front and rear wheel steering, and in particular, controls the ratio of the rear wheel steering angle to the front wheel steering angle (steering angle ratio) according to the vehicle speed. The present invention relates to an improvement of a rear wheel steering control device for a vehicle with front and rear wheel steering.

[Conventional technology]

Conventionally, rear wheel steering control devices for front and rear wheel steered vehicles have been disclosed, for example, in Japanese Patent Laid-Open No. 59-81272 and Japanese Patent Laid-Open No. 59-8127.
As disclosed in Publication No. 3, when the vehicle is running at a lower speed than a predetermined vehicle speed value, the steering angle ratio is set so that the rear wheel steering angle is in the opposite phase to the front wheel steering angle. Set it to a value of
Furthermore, a system has been proposed in which the steering angle ratio is set to a value such that the rear wheel steering angle is in phase with the front wheel steering angle when the vehicle is traveling at a higher speed than a predetermined vehicle speed value. According to this rear wheel steering control device, the turning radius of the vehicle can be made small during low-speed driving, and the turning radius of the vehicle can be improved. Furthermore, when driving at medium to high speeds, the turning radius of the vehicle can be increased to allow the vehicle to change lanes quickly and easily.

[Means to solve the problem]

However, in the conventional device described above, since the steering angle ratio is controlled only by the vehicle speed, during deceleration, the steering angle ratio becomes smaller as the vehicle speed decreases, and the turning radius of the vehicle becomes smaller. Therefore, if the vehicle is decelerated during a turn, the turning radius tends to become smaller as the vehicle speed decreases, leading to the problem that the vehicle will spin. or,
When accelerating during a turn, the turning radius of the vehicle increases as the vehicle speed increases, resulting in a bulging trajectory that bulges outward compared to the turning trajectory of a normal two-wheel steering vehicle. Let's take a look at the problem. On the other hand, in Japanese Patent Application No. 60-36759, the present applicant detected the front wheel steering angle, and when the absolute value of this front wheel steering angle was small, the steering angle ratio was updated sequentially according to the vehicle speed. proposed a vehicle rear wheel steering control device that does not update the steering angle ratio when the absolute value is large. If this is done, it is possible to prevent the course direction of a vehicle during turning from being changed regardless of the driver's intention, without impairing the 11I performance of the conventional weighing device. In other words, assuming that a vehicle that has been traveling straight enters a curved road, turns along the same road, and then continues straight ahead again, the driver must judge the vehicle's course conditions, such as the width of the road and the angle of the road. At the same time, the vehicle is slowed down to enter a curved road, and then the front wheels are steered to turn the vehicle. Since the steering angle ratio at the start of the turn is determined to a value corresponding to the vehicle speed immediately before the turn, that is, the vehicle course condition, the performance of conventional devices of this type is not impaired. Additionally, the steering angle ratio during a turn is maintained at the above value regardless of vehicle speed, so if the front wheel steering angle is held constant, the rear wheel steering angle will not change and the turning radius of the vehicle will also change. do not. In this way, even if the turning vehicle is decelerated or accelerated, the turning radius of the vehicle does not change, so it is possible to avoid changing the course of the vehicle against the driver's will. However, even in the above device, the rear wheel steering control is started when the front wheel steering angle reaches a predetermined value, so if the front wheel steering angle becomes less than the predetermined value even just before the end of a turn, the rear wheels will turn according to the vehicle speed. There is a risk of being steered. Therefore, the vehicle may become unstable just before the end of the turn.
This has the problem that the steering stability deteriorates significantly.

[Purpose of the invention]

The present invention has been made in order to solve the above-mentioned conventional problems, and it prevents the vehicle from being caught in the vehicle and the bulge caused by acceleration and deceleration during a turn, and also prevents the unstable running state of the vehicle immediately before the end of the turn. The purpose of the present invention is to provide a rear wheel steering control system for a vehicle with front and rear wheel steering, which can eliminate the problem and improve steering stability.

[Means to solve the problem]

The present invention is a rear wheel steering control device for a front and rear wheel steered vehicle that controls the ratio of the rear wheel steering angle to the front wheel steering angle in accordance with the vehicle speed, the gist of which is shown in FIG. As shown, a steering angle ratio determining means 2 determines the steering angle ratio of the rear wheel steering angle to the front wheel steering angle according to the detected vehicle speed; A front wheel steering angle detection means 3 detects the steering angle, a steering angle ratio fixing means 4 fixes the determined steering angle ratio when the detected front wheel steering angle is larger than a predetermined value, and the steering angle ratio fixing means 4 determines the determined steering angle. steering angle ratio fixation release means 5 for unfixing the determined steering angle ratio after a predetermined period of time has elapsed since the detected front wheel steering angle became smaller than a predetermined value when the ratio was fixed; an output means 6 for outputting a control signal corresponding to the ratio; and a rear wheel steering mechanism 30 for steering the rear wheels by setting a rear wheel steering angle based on the determined steering angle ratio in response to the control signal. The above objective has been achieved. Further, in an embodiment of the present invention, the steering angle ratio fixing release means includes:
The fixed steering angle ratio is released after a predetermined period of time has passed since the detected front wheel steering angle becomes equal to or less than a predetermined value smaller than a predetermined value when the steering angle ratio is fixed by the steering angle ratio fixing means. It is composed of

[Effect]

In the present invention, when the detected front wheel steering angle is larger than the first predetermined value, the determined steering angle ratio is fixed. Therefore, without impairing the performance of the conventional scale device, it is possible to prevent the direction of travel of a vehicle during turning from being changed irrespective of the driver's intention. Also, when the determined steering angle ratio is fixed, after a predetermined time has elapsed since the detected front wheel steering angle became smaller than a predetermined value,
The fixed steering angle ratio is now released. Therefore,
The steering angle ratio can be changed after the vehicle has entered a stable running state. Thereby, the steering stability of the vehicle can be ensured. Further, the steering angle ratio fixing release means detects: 1) one wheel steering angle;
The steering angle ratio fixing means is configured to release the determined steering angle ratio from fixing after a predetermined period of time has passed if the steering angle ratio becomes less than a predetermined value smaller than the predetermined value when the steering angle ratio is fixed by the steering angle ratio fixing means. In this case, the steering angle ratio can be changed after the vehicle has more reliably shifted to a stable running state. This makes it possible to further ensure the steering stability of the vehicle.

【Example】

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of a rear wheel steering control device for a front and rear wheel steered vehicle to which the present invention is applied will be described in detail with reference to the drawings. FIG. 2 shows a front wheel steering mechanism 20, a rear wheel steering mechanism 30, and an electric control device 40 that controls the rear wheel steering mechanism 30 of a vehicle to which the present invention is applied. The front wheel steering 1 structure 20 includes a rack and binion mechanism 21 and a pair of left and right relay rods 22a and 22b connected to the rack portion of this mechanism 21. The rack and pinion mechanism 21 is connected to a steering handle 24 via a steering shaft 23 at its rack and pinion part,
The rotational movement of the steering handle 24 is controlled by the relay rods 22a, 2.
2b is converted into a reciprocating motion. The left and right relay rods 22a and 22b connect to the left and right front wheels 26a via left and right tie rods and left and right knuckle arms 25a and 25b (not shown).
, 26b, and the left and right front wheels 26a, 26b are steered by the left and right relay rods 22a122b. The rear wheel steering mechanism 30 includes a swing lever 32 for steering the rear wheels 31a and 31b in conjunction with the steering of the front wheels 26a and 26b, and a swing lever 32 for shifting the movable fulcrum 32a of the swing lever 32. It includes a linear actuator 33 and a relay rod 34 that interlocks the left and right rear wheels 31a and 31b. The swing lever 32 includes a fixed point 32b to which a front connecting rod 35 connected to the left knuckle arm 25a is pivoted, and a fixed point 32c to which a rear connecting rod 36 is pivoted, and the movable fulcrum 32a is connected to the fixed point 32b. and the fixed point 32c, the fixed point 32C is displaced in the opposite direction to the fixed point 32b. Furthermore, when the movable fulcrum 32a is on the opposite side of the fixed point 32b to the fixed point 32c, the fixed point 32c is displaced in the same direction as the fixed point 32b. Furthermore, when the movable fulcrum 32a is on the fixed point 32c, the fixed point 32C is not displaced. In the above manner, the steering angle ratio is set according to the position of the movable fulcrum 32a. The linear actuator 33 includes an actuator rod 33a connected to a movable fulcrum 32a, and by moving the actuator rod 33a in the lateral direction of the vehicle body, the movable fulcrum 32a is displaced in the same direction. The relay rod 34 is connected to the right rear wheel 31a, 31
b are connected to each other via left and right tie rods (not shown) and left and right knuckle arms 37a and 37h. A rear connecting rod 36 is connected to the left knuckle arm 37a, and the swinging of the swinging lever 32 causes the rear wheels 31a,
31b is steered. The electric control device 40 includes a vehicle speed sensor 41 that detects the vehicle speed V, a front wheel steering angle sensor 44 that detects the steering angle of the front wheels,
A position sensor 43 calculates a target steering angle ratio K based on the vehicle speed sensor 41 and front wheel steering angle sensor 44 and detects a set steering angle ratio KS set by the swing lever 32.
In cooperation with the linear actuator 3, the set steering angle ratio Ks of the swing lever 32 becomes equal to the target steering angle ratio K.
3. The vehicle speed sensor 41 is configured to pick up the rotation of the output shaft of the transmission and generate a pickup signal with a frequency proportional to the vehicle speed V. The pickup signal from the vehicle speed sensor 41 is converted into a rectangular wave signal by a waveform shaper 41a and supplied to the microcomputer 45. The front wheel steering angle sensor 44 is connected to the relay rods 22a, 22
b, that is, the steering angle of the front wheels, to generate an analog signal. The analog signal of this front wheel steering angle sensor 44 is sent to an analog-to-digital converter (hereinafter △/
The digital signal is converted into a digital signal by a D converter 44a and supplied to the microcomputer 45. The position sensor 1/43 is configured to detect the moving position of the linear actuator rod 33a, that is, the current position of the movable fulcrum 32a, and generate an analog signal representing the set steering angle ratio KS. The analog signal from the position sensor 43 is converted into a digital signal by an A/D converter 43a and supplied to the microcomputer 45 as a set steering angle ratio KS. The microcomputer 45 constitutes the steering angle ratio determining means 2, the steering angle ratio fixing means 4, the steering angle ratio fixing means 5, and the output means 6 shown in FIG. 1, and is shown in FIG. A read-only memory (hereinafter referred to as ROM) 45a that stores a program corresponding to the tough flow rate and parameters for calculating the target steering angle ratio K, etc., and a central processing unit Fl (CPU) that executes the program; 15
b, a programmable memory (RAM) 45c that temporarily stores variables necessary for program execution, an input/output interface (Ilo) 715d that inputs each signal of each of the sensors 41, 43, and 44; ROM45a, C
The bus 45e commonly connects the PU45b, RAM45c, and l1045d. Said l1045
d is a digital-to-analog converter (D/A) that converts the digital signal output from the microcomputer 45 into an analog signal and drives and controls the linear actuator 33.
converter) 33b is connected. The operation of the vehicle rear wheel steering control device configured as described above will be explained using the flowchart shown in FIG. When an ignition switch (not shown) is closed to start the vehicle, the CPU 45b executes step 100.
The execution of the program is started in step 102, and various variables and flags are set to initial values of zero. In this step 102, various variables and flags that are set to zero include the vehicle speed V, which is a variable representing the detected vehicle speed, the front wheel steering angle δ, which is a variable representing the steering angle of the front wheels, and the variable set by the swing lever 32. In the table, the target steering angle ratio, which is a variable representing the steering angle ratio to be adjusted, and the steering angle ratio currently set by the swing lever 32 are expressed as the set steering angle ratio Ks, which is a variable, and the timer time. There is a timer time T which is a variable, and a steering angle ratio fixing flag KHFLG whose value is 1 and indicates that the steering angle ratio is fixed, and whose value is 0 and which indicates that the steering angle ratio is released. Next, the program proceeds to step 104, in which the waveform shaper 41 receives the signal from the vehicle speed sensor 41.
A vehicle speed (■) is calculated based on the vehicle speed signal supplied via a, and this vehicle speed (■) is stored in the RAM 45c. Next, the process proceeds to step 106, in which a target steering angle ratio K for the detected vehicle speed (2) is calculated based on the parameters stored in the ROM 45a, for example according to the steering angle ratio characteristics shown in FIG. 4, and is stored in the RAM 45c. . As a result, as shown in Fig. 4, the target steering angle ratio has a value whose absolute value changes continuously from a large negative value to a large positive value as the vehicle speed V increases. Also, the vehicle speed is set to
is set to zero when is equal to the predetermined value Vo. Note that a negative steering angle ratio means that the rear wheels are steered in the opposite phase to the front wheels, and a positive steering angle ratio means that the rear wheels are steered in the same phase as the front wheels. A ratio of zero means that the rear wheels are not steered. After calculating the target steering angle ratio in step 106, the program proceeds to step 108.
, reads the front wheel steering angle δ [supplied from the front wheel steering angle sensor 44 via the A/D converter 44a, and stores it in the RAM 45G.
to be memorized. Next, the process proceeds to step 110, and in step 110, the CPU 45b determines whether the front wheel steering angle δf stored in step 108 is greater than the first predetermined value θ. If the determination in step 110 is negative, that is, if the front wheel steering angle δf is determined to be less than or equal to the first predetermined value θ, the process proceeds to step 112. In this step 112, the steering angle ratio fixed flag KHF
It is determined whether LG is 1 or not. If it is determined no in this step 112, that is, the steering angle ratio fixed flag KHF
If LG is determined to be zero, the process proceeds to step 114, where the CPU 45b reads the set steering angle ratio Ks supplied from the displacement sensor 43 via the A/D converter 438. Next, proceed to step 116;
At step 116, CP tJ 45 b determines whether the set steering angle ratio KS is equal to the target steering angle ratio K. If the determination in step 116 is negative, that is, if it is determined that the set steering angle ratio Ks is not at least equal to the target steering angle ratio 1, the program proceeds to step 118,
In this step 118, the steering angle ratio K is set to the target steering angle ratio.
The difference between s and s is shown in Table 4.The digital signal is output to the D/A converter 33b. The D/A converter 33b converts this digital signal into an analog signal and outputs a drive control signal having a magnitude corresponding to the difference to the linear actuator 33. The linear actuator 33 is an actuator rod 3
3a, the movable fulcrum 32a is displaced in the direction in which the swing lever 32 sets the target steering angle ratio K. The program then returns to step 114 where the CPU 45
b until the set steering angle ratio KS becomes equal to the target steering angle ratio K.
The cyclic processing of steps 114-118 continues. When the set steering angle ratio KS becomes equal to the target steering angle ratio K through the circulation processing of steps 114 to 118, the CPU 45b
is determined to be correct in step 116, and the program proceeds to step 120. In this step 120, CP
U45b outputs a digital signal representing the difference between the target steering angle ratio and the set steering angle ratio KS, that is, zero, to the D/A converter 33b. As a result, the D/A converter 33b stops outputting the drive control signal to the linear actuator 33, and the displacement of the movable fulcrum 32a by the linear actuator 33 is stopped. After processing this step 120, the program moves to step 1
Returning to step 04, the CPU 45b continues the circulation process of steps 104 to 120, and the steering angle ratio K is set based on the steering angle ratio characteristic as shown in FIG. When J3 is in such a state, the left and right front wheels 26a, 26 are rotated in response to leftward (or rightward) rotation of the steering handle 24.
When b is steered to the left (or right), this steering force is applied to the left knuckle arm 25a1, the front connecting rod 35, the swinging lever 32, the rear connecting rod 36, and the left knuckle arm 3.
The signal is transmitted to the 7a1 relay Otsudo 34 and the right knuckle arm 37b, and the left and right rear wheels 31a and 31b are moved to the swing lever 3.
The vehicle is steered according to the steering angle ratio set by No.2. At this time, if the vehicle speed V is smaller than the predetermined value Vo, the steering angle ratio is set to a negative value whose absolute value decreases as the vehicle speed In other words, the left and right front wheels 26a and 26b are steered in opposite phases, and the steering angle becomes smaller as the vehicle speed increases. Furthermore, if the vehicle speed ■ is equal to the predetermined value Va, the steering angle ratio is set to zero, so the left and right rear wheels 31a and 31b are not steered. Furthermore, if the vehicle speed V is greater than the predetermined value Vo, the steering angle is set to a positive value whose absolute value increases as the vehicle speed V increases, so that the left and right rear wheels 31a and 31b are steered toward the left (or right). ) That is, the left and right front wheels 26a and 26b are steered in the same phase, and the steering angle increases as the vehicle speed (2) increases. On the other hand, during the circulation processing of steps 104 to 120, if the steering wheel 24 is rotated leftward or rightward by more than the first predetermined value θ, the CPU 45b controls the front wheel steering angle δf in step 110. is larger than the first predetermined value θ, it is determined to be positive, and the program returns to step 122.
Proceed to. In this step 122, the steering angle ratio fixed flag KHFLG is set to 1, and then the process returns to step 104, and the CPU 45b performs steps 104 to 110.
Even if the vehicle speed {circle around (2)} changes and the target steering angle ratio K changes accordingly, the set steering angle ratio Ks does not change and is fixed because the processing from step 112 onwards is not performed. In the above state, when the steering wheel 24 is returned to within the first predetermined value θ, the CPU 45b makes a negative determination in step 110 because the front wheel steering angle δr is smaller than the first predetermined value θ, and advances the program to step 112. In this step 112, since the steering angle ratio fixed flag KHFLG was set to 1 in the previous step 112, it is determined to be positive.
Proceed to step 124. In this step 124, the front wheel steering angle δf is set to a second predetermined value θ which is smaller than the first predetermined value θ.
It is determined whether the value is greater than 0 or not. If it is determined to be positive in this step 124, that is, if it is determined that the front wheel steering angle δ[ is larger than the second predetermined value θ0, the program returns to step 104. Also, if the above-mentioned step 124 makes a negative determination,
That is, when it is determined that the front wheel steering angle δf is less than or equal to the second predetermined value 00, the 10-gram controller advances to step 126, and in step 126, a timer is started. Next, the program proceeds to step 128, in which it is determined whether the timer time T is smaller than the predetermined value To. If this step 128 is determined to be positive, that is, the timer time T is set to the predetermined value Wi T
If it is determined that the timer time T is smaller than the predetermined value To, the timer time T is counted until it becomes larger than the predetermined value To. If the determination at step 128 is negative, that is, if the timer time T is determined to be greater than or equal to the predetermined value To, the process proceeds to step 130, where the timer is reset. Next, the program proceeds to step 132, in which the steering angle ratio fixing flag KHFLG is set to zero. The program then returns to step 114 and
Processing from step 114 onwards is performed. As can be understood from the above operation description, according to this embodiment, when the front wheel steering angle δf is smaller than the first predetermined value θ, the steering angle ratio is determined based on the characteristics shown in FIG. V continuously changes from negative to positive as V increases, and the turning radius of the vehicle increases as vehicle speed V increases. Therefore, the slip angle of the vehicle, which increases as the vehicle speed V increases and increases as the turning radius of the vehicle decreases, does not exceed a certain value, and as a result, the steering stability of the vehicle is maintained well. The vehicle's small turning performance at low speeds is fully demonstrated. On the other hand, in a vehicle that is controlled according to the steering angle ratio characteristics, when deceleration is performed during a turn, the radius of rotation of the vehicle decreases as the vehicle speed decreases, resulting in the occurrence of a engulfing phenomenon of the vehicle.
Also, when accelerating, the turning radius of the vehicle increases as the vehicle speed increases, causing bulge, but the front wheel steering angle δf is the first
When the predetermined value θ is exceeded, the steering angle ratio is fixed so that rear wheel steering control based on the vehicle speed is not performed. As a result, the turning radius of the vehicle does not change, so that the occurrence of the above-mentioned entrainment phenomenon and bulge running can be prevented. Further, the steering angle ratio fixing is canceled when the front wheel steering angle δf is set to a second predetermined value θ0.
Since the steering angle ratio is within the specified range and is carried out after a predetermined period of time has elapsed, the steering angle ratio can be changed while the vehicle is running stably, and the steering stability of the vehicle can be maintained. In the above embodiment, the steering angle ratio fixing is released when the detected front wheel steering angle δf becomes smaller than the second predetermined value θ0, which is smaller than the first predetermined value θ when fixing the steering angle ratio.
Although the steering angle ratio fixing is performed after a predetermined period of time has elapsed to further ensure the steering stability of the vehicle, the present invention is not limited to this, and the steering angle ratio fixing release is performed based on the first predetermined value θ and the second predetermined value θ.
The predetermined value θ0 may be set equal to the predetermined value θ0, and the detection may be performed after a predetermined time has elapsed from when the detected front wheel steering angle δf becomes smaller than the first predetermined value θ for fixing the steering angle ratio. Further, in the embodiment described above, the rear wheel steering machine MA30 includes the swing lever 32 and the movable fulcrum 32a of the swing lever 32.
a linear actuator 33 that displaces the left and right rear wheels 3;
1a and 31b, the present invention is not limited to this, and the rear wheel steering mechanism is a swinging mechanism in which a movable fulcrum 32a is displaced by a linear actuator 33. Instead of the lever 32, there is a left and right relay 0 that interlocks the left and right rear wheels 31a and 31b.
Directly drive Tsudo 34! ＃′ru linear actuator (
(not shown) may be provided.

【Effect of the invention】

As explained above, according to the present invention, it is possible to prevent the vehicle from being caught in the vehicle and the bulge caused by acceleration and deceleration during a turn, and also to eliminate the unstable running state of the vehicle before the end of the turn i, thereby stabilizing the vehicle's handling. It has the excellent effect of improving physical properties.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the essential structure of a rear wheel steering control device for a front and rear wheel steered vehicle according to the present invention, and FIG. 2 is an implementation of the rear wheel steering control device for a front and rear wheel steered vehicle according to the present invention. Showing a front wheel steering mechanism, a rear wheel steering mechanism, and an electric control device in an example,
FIG. 3 is a plan view including a partial block diagram, and FIG. 3 is a flowchart showing the operation of the microcomputer in the same embodiment.
The figure is a diagram showing an example of the steering angle ratio with respect to the vehicle speed in the same embodiment. 20-...Front wheel steering tis, 21...Rack and binion mechanism, 22a, 22
b, 34... Relay rod, 24... Steering handle, 26a, 26b... Front wheel, 30... Rear wheel steering mechanism, 31 a, 3 l b... Rear wheel, 32... Rocking dynamic lever, 33... linear actuator, 40... electric control device, 41... vehicle speed sensor, 43... position sensor, 44... front wheel steering angle sensor, 45... microcomputer.

Claims (2)

[Claims] (1) A rear wheel steering control device for a front and rear wheel steered vehicle that controls the ratio of a rear wheel steering angle to a front wheel steering angle according to a vehicle speed, comprising: a vehicle speed detecting means for detecting a vehicle speed; , a steering angle ratio determining means for determining a steering angle ratio of a rear wheel steering angle to a front wheel steering angle, a front wheel steering angle detecting means for detecting a steering state of the front wheels, and when the detected front wheel steering angle is larger than a predetermined value, a steering angle ratio fixing means for fixing the determined steering angle ratio, and a predetermined period of time elapsed since the detected front wheel steering angle became smaller than a predetermined value when the determined steering angle ratio was fixed by the steering angle ratio fixing means. Later, a steering angle ratio lock release means for unfixing the determined steering angle ratio, an output means for outputting a control signal corresponding to the determined steering angle ratio, and a rear wheel turning angle determined in response to the control signal. A rear wheel steering control device for a front and rear wheel steered vehicle, comprising: a rear wheel steering mechanism that steers the rear wheels by setting based on a steering angle ratio; (2) The steering angle ratio fixing release means is configured to: after a predetermined period of time has elapsed since the detected front wheel steering angle becomes equal to or less than a predetermined value that is smaller than a predetermined value when the steering angle ratio is fixed by the steering angle ratio fixing means; A rear wheel steering control device for a front and rear wheel steered vehicle according to claim 1, which is configured to release the fixation of the determined steering angle ratio.