JPH075092B2 - 4-wheel steering system for vehicles - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a four-wheel steering system for a vehicle.

(Prior Art) As shown in Japanese Patent Laid-Open No. 60-199771, some vehicles have steered not only the front wheels but also the rear wheels, so-called four-wheel steering. Then, in order to assist the external input necessary for steering the rear wheels, Japanese Patent Laid-Open No. 58-2
As shown in Japanese Patent No. 2757, there has been proposed a structure in which a hydraulic power steering mechanism is additionally provided to the rear wheel steering mechanism.

In this four-wheel steering, since the ratio of the rear wheel steering angle to the front wheel steering angle, that is, the steering ratio is changed according to the driving state of the vehicle, the rear wheel steering is electrically controlled, that is, the above power External input to the steering mechanism is generally performed by an electromagnetic actuator.

(Problems to be Solved by the Invention) By the way, as described above, when the steering of the rear wheels is electrically controlled, the control system may fail, for example, the actuator itself for steering the rear wheels or the drive of the actuator. It is conceivable that the rear wheel may not be controlled normally due to a failure of the transistor for the vehicle. Then, how to ensure safety in the event of such a failure is one of the challenges in implementing four-wheel steering.

Therefore, on the premise that the purpose of the present invention is to assist the rear wheel steering by the hydraulic power steering mechanism, the rear wheel control is not normally performed by providing the backup function to the rear wheel steering control. It is an object of the present invention to provide a four-wheel steering system for a vehicle that allows the vehicle to continue traveling as safely as possible.

(Means and Actions for Solving Problems) In order to achieve the above object, the present invention has the following configuration. That is, as shown in a block diagram in FIG. 1, in a four-wheel steering system for a vehicle in which not only front wheels but also rear wheels are steered, a rear wheel steering mechanism is attached to assist steering of the rear wheels. A hydraulic power steering mechanism, and releasing the hydraulic pressure in the power steering mechanism,
A hydraulic releasing means for disabling steering of the rear wheels by the power steering mechanism, and a neutral position linked to the rear wheel steering mechanism for returning the rear wheels to a neutral position after the hydraulic releasing means operates. Return means, steering detection means for detecting the steering state of the rear wheels, failure detection means for detecting that the control system for controlling the rear wheels has failed, and output from both detection means, Backup control means for activating the hydraulic pressure releasing means so that the hydraulic pressure releasing amount is larger when the control system is out of order and is larger when the steering amount is small, and the rear wheel is in the neutral position. It has a different structure.

When setting the hydraulic release amount according to the turning amount, for example, when the turning amount is large, the hydraulic releasing amount is set to zero when the turning amount is large, and when the turning amount is small, the hydraulic releasing amount is set to the maximum amount. As described above, the setting can be performed in two steps. In this case, returning to the neutral position when the turning amount is large is effectively prohibited, but this makes it possible to drastically change the running state of the vehicle irrespective of the driver's will. This is not only preferable for prevention, but also preferable for preventing the booster valve of the power steering mechanism from being damaged along with the return to the neutral position.

(Effects of the Invention) According to the present invention, when the control system for steering the rear wheels fails, the rear wheels can be forcibly returned to the neutral position and safe traveling can be continued.

When the steering amount is small, the hydraulic pressure release amount is larger than when the steering amount is large, so that the running state of the vehicle is highly likely to be significantly changed. The tendency to return to the neutral position becomes weaker than that, which is preferable in terms of safety and reduction of the driver's burden.

(Example) Hereinafter, an example of the present invention will be described with reference to the accompanying drawings.

In FIG. 2, 1R is the right front wheel, 1L is the left front wheel, 2R is the right rear wheel, 2L is the left front wheel, the left and right front wheels 1R, 1L are linked by the front wheel steering mechanism A, and the left and right rear wheels 2R , 2L are linked by a rear wheel steering mechanism B.

In the embodiment, the front wheel steering mechanism A includes a pair of left and right knuckle arms 3R and 3L and tie rods 4R and 4L, and a relay rod 5 that connects the pair of left and right tie rods 4R and 4L.
It consists of and. A steering mechanism C is linked to the front wheel steering mechanism A, and the steering mechanism C is of a rack and pinion type in the embodiment. That is, while the rack 6 is formed on the relay rod 5, the pinion 7 that meshes with the rack 6 is connected to the shaft 8
Is connected to the handle 9 via. As a result, when the handle 9 is turned to the right, the relay rod 5 is displaced to the left in FIG. 2 and the knuckle arms 3R, 3L are moved.
Around the rotation center 3R ', 3L'
The steering wheel is steered in the clockwise direction in the figure by an amount corresponding to the operation displacement amount, that is, the steering wheel steering angle. Similarly, when the steering wheel 9 is turned to the left, the left and right front wheels 1
R and 1L will be steered to the left.

Similarly to the front wheel steering mechanism A, the rear wheel steering mechanism B also has a pair of left and right knuckle arms 10R and 10L and a tie rod 11 respectively.
R and 11L, and a relay rod 12 that connects the tie rods 11R and 11L to each other. In the embodiment, the rear wheel steering mechanism B includes a hydraulic power steering mechanism D. The power steering mechanism D will be described with reference to FIG. 11 as well. A cylinder device 13 is attached to the relay rod 12, and the cylinder 13a is fixed to the vehicle body. A piston 13d defined by 13c is integrated with the relay rod 12.
The two chambers 13b and 13c in the cylinder 13a are connected to the pipe 14 or 1
It is connected to the control valve 16 via 5. Further, the control valve 16 is connected to pipes 18 and 19 extending from the reservoir tank 17, and one pipe 18 serving as an oil supply pipe is connected to an oil pump 20 driven by an engine (not shown). There is. The control valve 16 has a spool 16a, and its control rod 21 is a sliding type so-called booster valve type.
The input portion 21a integrated with the spool 16a of 21 is also used as a moving member of a steering ratio changing device E described later, and the output portion 21b of the control rod 21 is integrated with the relay rod 12 of the rear wheel steering mechanism B. Has been converted.

In the power steering mechanism D as described above, as is known, when the control rod 21 is displaced leftward in FIG. 2, the relay rod 12 is displaced leftward in FIG. The arms 10R, 10L rotate clockwise around the centers of rotation 10R ', 10L', and the rear wheels 2R, 2L are steered to the right. Then, during this steering, oil is supplied into the chamber 13b of the cylinder device 13 in accordance with the amount of displacement of the control rod 21 to assist in driving the relay rod 12 (power boosting action). Similarly,
When the control rod 21 is displaced to the right in FIG. 2, the rear wheels 2R, 2L are acted upon by the boosting action of the cylinder device 13 (the oil is supplied to the chamber 13b) according to the displacement amount.
Will be steered to the left.

The front wheel steering mechanism A also has a power steering mechanism F, like the rear wheel steering mechanism B. The power steering mechanism F includes a cylinder device 65 attached to the relay rod 5 of the front wheel steering mechanism A, and the cylinder 65a thereof is fixed to the vehicle body, while the inside of the cylinder 65a is fixed to two.
The piston 65d defined in the chambers 65b and 65c is the relay rod 5
Is integrated into. Two chambers 65b, 65 in this cylinder 65a
c is a steering mechanism C via a pipe 66 or 67.
It is connected to a rotary type control valve 68 provided on the shaft 8. The control valve 68 is connected to a pipe 70 extending from the flow dividing valve 69 connected on the discharge side of the oil pump 20 and a pipe 71 branched from the pipe 19. Such a power steering mechanism F is
Boost the operating force of the handle 9 (boost by supplying oil to the chamber 65b or 65c of the cylinder device 65)
Then, the operation of the power steering mechanism F itself is basically the same as that of the power steering mechanism D, and therefore detailed description thereof will be omitted.

The steering mechanism C and the rear wheel steering mechanism B are linked via a front wheel steering mechanism A and a steering ratio changing device E.
From this steering ratio changing device E, an input rod 22 extends forward, and a pinion 23 attached to the front end portion thereof meshes with a rack 24 formed on the relay rod 5 of the front wheel steering mechanism A. The output rod of the steering ratio changing device E is also used by the input portion 21a of the control rod 21 of the control valve 16 as described above.

An example of the steering ratio changing device E will be described with reference to FIG. 3, but in the embodiment, it has substantially the same configuration as that shown in the above-mentioned JP-A-60-199771. That is, the input portion 21a of the control rod 21 is slidably held in the vehicle width direction with respect to the vehicle body, and its movement axis is shown as l ₁ . The turning ratio changing device E also has a swing arm 31, and the swing arm 31 has its base end pivotally attached to the holder 32 by a pin 33 so as to be swingable. There is. The holder 32 has its rotation shaft 32a is held to the vehicle body rotatably about a perpendicular line l ₂ perpendicular to the moving axis l ₁ of the input unit 21a. And the pin 33
Is located at the intersection of these lines l ₁ and l ₂ and extends in a direction orthogonal to the orthogonal line l ₂ . Therefore, although the swing arm 31 can swing about the pin 33,
By rotating the holder 32, the tilt angle formed by the pin 33 and the moving axis l ₁ , that is, the tilt angle of the rocking orbital surface around the pin 33 with respect to the plane (reference plane) orthogonal to the moving axis l ₁ Is variable.

The tip portion of the swing arm 31 and the input portion 21a are connected by a connecting rod 34. That is, the connecting member 34 is
It is connected to the tip end of the swing arm 31 via a ball joint 35, and the input unit 21a via a ball joint 36.
Are linked to.

By the connecting rod 34 as described above, the distance between the ball joints 35 and 36 at each end of the swing arm 31 is always kept constant. Therefore, if the ball joint 35 is displaced in the left-right direction in FIG. 3, the input portion 21a is displaced in the left-right direction in FIG. 3 according to this displacement.

The swing of the swing arm 31 about the pin 33 is performed according to the operation displacement of the steering mechanism C, that is, the steering angle of the steering wheel. Therefore, in the embodiment, the connecting rod 34 is moved from the bevel gear to the connecting rod 34. The rotating plate 37 is connected. The rotating plate 37 is rotatably held by the vehicle body so that its rotating shaft 37a is located on the moving axis l _1, and the connecting rod 34 is connected to the eccentric portion of the rotating plate 37 by the ball joint. It penetrates slidably through 38. A bevel gear 39 connected to the input rod 22 is meshed with the rotating plate 37 formed of a bevel gear.

With such a rotating plate 37, the swing arm 31 is swung about the pin 33 by an amount corresponding to the steering angle of the steering wheel, but the axis of the pin 33 and the moving axis l ₁ are inclined. As a result, the ball joint 35 is displaced in the left-right direction in FIG. 3, that is, in the direction of the moving axis l _{1 in accordance} with the swing around the pin 33, and this displacement is transmitted through the connecting rod 34 to the input portion. 21a
And the input portion 21a is displaced.
The displacement of the ball joint 35 in the left-right direction in FIG. 3 is such that even if the swing angle of the swing arm 31 about the pin 33 is the same, the inclination angle of the pin 33, that is, the rotation angle of the holder 32. Is changed, the steering ratio is changed.

In order to change the inclination angle, a sector gear 40 as a worm wheel is attached to the rotation shaft 32a of the holder 32, and a worm gear meshing with the sector gear 40 is attached.
41 is rotatably driven by a stepping motor 44 as a tilt angle changing means via a pair of bevel gears 42 and 43. The turning angle, that is, the tilt angle of the holder 32 is detected by a turning ratio detecting sensor 45 including a potentiometer provided on the turning shaft 32a.

Here, the swing angle about the pin 33 of the swing arm 31 and the tilt angle of the swing arm 31 (the tilt angle of the pin 33) described above.
Will be described for the influence on the displacement of the ball joint 35 (input portion 21a) in the direction of the moving axis l ₁ . Now, the swing angle of the swing arm 31 about the pin 33 is θ, the reference plane orthogonal to the moving axis l ₁ is δ, and the swing angle of the swing track surface of the swing arm 31 is the reference plane δ. α, ball joint 35 pin
When the eccentric distance from 33 is r, the displacement X of the ball joint 3 in the direction of the moving axis l ₁ becomes X = rtan α · sin θ, which is a function with α and θ as parameters. Therefore, if the inclination angle α is fixed to a certain value, X will be θ
Function, that is, according to the steering angle of the steering wheel, and if the value of the inclination angle α is changed, the value of X changes even if the steering angle of the steering wheel is the same. Then, the change of the inclination angle α changes the steering ratio.

As an example of adjusting the tilt angle and changing the steering ratio as described above, there is a case as shown in FIG. In FIG. 4, the turning ratio is changed according to the vehicle speed, and when the front wheel turning angle in FIG. 4 is set to a certain value, the turning angle of the rear wheel turning angle with respect to the front wheel turning angle is changed. FIG. 5 shows how the steering ratio changes according to the vehicle speed.

Here, a pair of return springs 13e and 13f are attached to the rear wheel power steering mechanism D in order to forcibly bring the rear wheels 2R and 2L into a neutral position, that is, a straight traveling state. The springs 13e and 13f bias the rear wheel relay rod 12 from the left and right opposite directions with equal forces. Further, both oil chambers 13 of the power steering mechanism D are
b and 13c are connected via a communication passage 46, and an electromagnetic on-off valve 47 is connected to the communication passage 46. As a result, when the on-off valve 47 is closed, the oil chamber 13b or 1
Supplying hydraulic pressure to 3c causes rear wheels 2R and 2L to spring 13.
When the steering is turned against e or 13f and the on-off valve 47 is opened to make both oil chambers 13b and 13c at the same pressure, the rear wheels 2R and 2C are forced to the neutral position by the action of the springs 13e and 13f. To be done.
Of course, the urging force of the springs 13e and 13f is set to such a magnitude that it can take a neutral position against the external force received from the rear wheels 2R or 2L during turning.

Further, the sector gear 40 driven by the stepping motor 44 has both swing stroke ends thereof restricted by a pair of stoppers 48, 49 (see FIG. 3). The rotation range of the stepping motor 44 required over the entire swing range of the sector gear 40 (stroke end on the same phase side → stroke end on the opposite phase side) is set to “580” in the stepping number.

In FIG. 2, reference numeral 51 denotes a control unit composed of, for example, a microcomputer. The control unit 51 receives not only the output from the steering ratio sensor 45 but also the output from the vehicle speed sensor 53. ing. Further, from the control unit 51, the stepping motor 44 and the opening / closing valve are
In addition to being output to 47, an alarm device 54 including a lamp, a buzzer, etc. to warn that the control of the rear wheel steering angle has become impossible.
It is designed to be output to.

Next, the control content of the control unit 51 will be described with reference to the flowcharts shown in FIGS. 6 to 10. In the present embodiment, the stepping motor 44 is “step out” (stepping number and corresponding step number). In consideration of the possibility that the actual positional relationship described above may occur, the reference position adjustment, that is, "motor position initialization" is performed at any time. Then, in this "motor position initialization", the sector gear 40 is brought into contact with one of the stoppers 48 or 49 (in the embodiment, the stopper 49 which is on the opposite phase side when each member operates in the direction of the arrow in FIG. 3). The origin position of the stepping number "0" is set at this time, and the stepping number driven from this origin position is used as the motor position "MP" at that time.
The "motor position initialization" is performed at the start of control (immediately after starting the engine) and each time the vehicle speed becomes zero. Further, with the flowchart shown in this embodiment, "flag 1", "flag 2",
Although four types of flags, "flag E" and "flag 2WS", are used, the meaning of each flag is as follows.

Flag 1 This is for distinguishing whether or not "motor position initialization" is being performed, and means "0" or initialization completion, and "1" means initialization.

Flag 2 is set to "1" when "motor position initialization" is executed once, and is used to perform "motor position initialization" only once every time the vehicle speed changes from a non-zero state to zero. is there.

Flag E This is for distinguishing whether the control system of the rear wheel turning angle is normal or abnormal, where "1" indicates abnormal and "0" indicates normal.

Flag 2WS This is for distinguishing whether or not the rear wheels 2R and 2L are in the small turning ratio range. When "1" is in the small turning ratio range, "0" is not in the small turning ratio range. Indicates. It should be noted that whether or not this is a small turning ratio region is known in the embodiment by any one or a combination of two or more of the output of the turning ratio sensor 45, the stepping number of the stepping motor 44, and the vehicle speed. be able to.

Based on the above, according to the flowcharts shown in FIG. 6 to FIG.
The interrupt processing (FIGS. 8 to 10) for the main routine as shown in FIG. 6 will be described.

Interrupt processing 1 (FIG. 8) The interrupt routine shown in FIG. 8 is for driving the stepping motor 44, and at a predetermined time set by the timer (for example, the stepping motor 44 is set at 100 steps per second). When it is desired to drive, an interrupt is made to the main routine of FIG. 6 every 10 msec. In the figure, “CP” is a target rear wheel steering angle, that is, a target stepping number, required to have a steering ratio characteristic determined by a map using a vehicle speed as a parameter as shown in FIG. Also, as described above, “MP” indicates the swing position of the sector gear 40 (the turning position of the rear wheels 2R, 2L) from the origin position when the antiphase stopper 49 is the origin position, as the stepping number. It is a thing.

Based on the above, first in step S41,
It is determined whether or not the target stepping number CP and the current position MP match, and when CP = MP, the rear wheels 2R and 2L have the steering angles according to the predetermined steering ratio characteristics. In step S42, the energizing current to the stepping motor 44 is reduced (current down), and thereafter, in step S43, the timer is set within the above-mentioned predetermined time in preparation for the next interrupt.

When it is determined in step S41 that CP = MP is not satisfied, CP> MP is satisfied in step S45 after the supply current to the stepping motor 44 is increased (current town is released) in preparation for driving the stepping motor 44. It is determined whether or not. When it is determined that CP> MP is not established, the current position of the stepping motor 44 is located on the in-phase side of the target stepping number CP.
In step S46, the stepping motor 44 is driven toward the opposite phase side by one step. Then, following the operation of "1 stepping", the current position MP is updated by 1 stepping in step S47, and then step
Move to S43. On the other hand, if it is determined in step S45 that CP> MP, the stepping motor 44 is driven toward the same phase side by one step in step S48, and then the current position MP is updated in step S49, and the process proceeds to step S43. Transition.

Interrupt (Fig. 9) This interrupt processing is performed every time this pulse is generated (at the time of rising or falling of the pulse) because the vehicle speed sensor 53 is supposed to generate a pulse with the rotation of the meter cable of the speedometer. Is interrupted with respect to the main routine of FIG. The vehicle speed sensor 53 is, for example, a 20-pulse sensor (a sensor in which the number of pulses generated when the meter cable makes one rotation is 20), while the meter cable rotates 637 by rotating 1 km. Therefore, the number of pulses generated when traveling 1 km is "1
2740 pulses ”. The pulse generated from the vehicle speed sensor 53 is sequentially counted and integrated in step S51 and stored as P _CNT .

Interrupt 3 (FIG. 10) In this interrupt processing, the vehicle speed sensor 53 and the vehicle speed sensor 53 set as described above are used so that the integrated count pulse number described in FIG. 9 can be used as it is as the vehicle speed (km / h). Due to the relationship with the meter cable, an interrupt is made to the main routine shown in FIG. 6 every 282,575 msec. That is, in step S52, the P _CNT is directly set as the vehicle speed value (km / h), and then in step S53, the integrated count value P _{CNT in} step S51 of FIG. 9 is cleared.

It should be noted that FIGS. 9 and 10 are merely examples of vehicle speed detection, and the vehicle speed can be detected by appropriate means known in the related art.

Main Routine (Fig. 6) First, in step S1, the entire system is initialized, and in step S2, MP = 0, CP = -580, and flag 1 = "1" are set. That is, setting CP = −580 forces the sector gear 40 to cause the reverse phase side stopper 49 to perform the processing of step S45 to step S46 as is apparent from the description of FIG. 8 described above. It is for returning until it comes into contact, that is, for performing "motor position initialization", and the value of "580" must be set by returning only 580 steppings regardless of the position of the sector gear 40 at present. This is because it can be returned to the original position by being brought into contact with the antiphase stopper 49.

After that, after performing a system check to be described later in step S3, it is determined in step S4 whether or not the flag E is 0, that is, whether or not there is an abnormality in the control system of the rear wheel turning angle.

When it is determined in step S4 that the flag E is "0", that is, the control system is normal, the process proceeds to step S5, and it is determined whether the flag 1 is "1". In step S5, since the flag 1 is initially set to "1" in step S2, the process proceeds to step S6. In this step S6, it is determined whether or not CP = MP, but if CP = MP is not satisfied, a loop is returned from step S3 to step S6 again, and during this loop, the eighth Illustrated stepper motor 44
Drive (MP approaches −580), then CP
= MP. Then, when CP = MP, "motor position initialization" is completed, so MP = 0, CP = 0,
Flag 1 = 0 and flag 2 = 1.

When it is determined in step S5 that the flag 1 is not "1", it is determined in step S8 whether or not the current vehicle speed is zero. In this determination, when it is determined that the vehicle speed is not zero, that is, the vehicle is traveling, CP is set as a value according to the vehicle speed based on the map shown in FIG. 4 (FIG. 5) in step S9. . After that, in step S10, both flags 1 and 2 are set to "0", and the process returns to step S3.

If it is determined in step S8 that the current vehicle speed is zero, flag 2 or "0" is determined in step S11.
If the flag 2 is not "0", that is, if it is "1", the stepping motor 44 is not driven after "motor position initialization". It is useless to do
The process directly returns to step S3. If the flag 2 is determined to be "0" in step S11, "motor position initialization" is performed, and thus the process proceeds to step S12 (step S12).
Same as set in 2.)

On the other hand, if it is determined in step S4 that the flag E is not "0", that is, "1", it means that there is an abnormality in the control system for steering the rear wheels.
After shifting to S13, it is determined whether or not it is currently in the small turning ratio region. Then, when it is in the small turning ratio region, the step
By opening the on-off valve 47 in S14, the two oil chambers 13b and 13c of the power steering mechanism D are communicated with each other, thereby forcibly returning the rear wheels 2R, 2L to the neutral position, thereby reducing the steering ratio to 0. To do. Then, in step S15, the alarm 54 is activated to notify the driver that an abnormality has occurred in the control system. In addition, subsequently, in step S16, the supply current to the stepping motor 44 is decreased (current down), and then in step S17, all the interrupt processing of FIGS. 8 to 10 described above are stopped,
The control for steering the rear wheels is suspended.

If it is determined in step S13 that the flag 2WS is not "1", that is, if it is not in the small turning ratio region, the processes in step S15 and thereafter are performed without passing through step S14. In this way, when the turning ratio is large, the rear wheels 2R, 2L
However, in this case, it is unavoidable to protect the booster valve 16 of the power steering mechanism D in this case. To elaborate on this point,
If the turning ratio is large, the spool 16 of the booster valve 16
The a is retained in a state in which it is displaced to the right or left in FIG. 11 much more than when the rear wheels 2R and 2L are in the neutral position. In this state, if the rear wheels 2R, 2L are forcibly set to the neutral position, the relay rod 12, that is, the output portion 21b as a cylinder accommodating the spool 16a, is also largely displaced toward the neutral position, and as a result, the spool 16a. And output section 2
1b and a strong contact from the axial direction, this spool 16a
And will be damaged. On the other hand, when the neutral position is set in the small turning ratio region, the spool 16a and the output portion 21b are prevented from coming into contact with each other in the axial direction, and the damage thereof does not occur at all.

System check (FIG. 7) In this embodiment, it is checked whether or not the stepping motor 44 is normally driven. Therefore, by observing the level (“high” or “low”) of the contacting connection terminals when the excitation phase (each connection terminal) of the stepping motor 44 is “excited all” and when “all demagnetized”, Normal or normal is determined.

First, in step S21, the current supplied to the stepping motor 44 is increased, and both the flag 2WS and the flag E are set to "0". After this, steps S22 to S25
By the process of, all excitation phases are turned on (excitation), wait for a certain time to elapse, and monitor the excitation state. Then, if all the levels are "low" in step S25, it is determined to be normal.

When it is determined in step S25 that all are at the "low" level, in steps S26 to S28, after waiting for a certain period of time after turning off all the excitation phases of the stepping motor 44, the excitation state is Monitor. Then, if all the monitoring results in step S29 are "high" level, it is regarded as normal. After this, assuming that there is no abnormality in the drive control system of the stepping motor 44,
In step S31, the excitation state of the stepping motor 44 and its supply current are reset for the drive control in FIG.

If it is determined in step S25 that the levels are not all "low", for example, the stepping motor 44
This is an abnormal time such as an open failure of the driving transistor. At this time, the process proceeds to step S32 and flag E
Is set to "1". Further, when it is determined in step S29 that all are not at the “high” level, for example, when the transistor has a short circuit failure, when the coupler in the power supply path to the stepping motor 44 is disconnected, further, There is a possibility that there is a wire break or a short circuit, and this is an abnormal condition.
Move to 2 and set flag E to "1".

After step S32, it is determined whether or not the rear wheels 2R, 2L are currently in the small turning ratio region (this determination method has already been described), and in the small turning ratio region, step S34 In, the flag 2WS is set to "1".

Although the embodiment has been described above, the present invention is not limited to this, and includes the following cases, for example.

The actuator for changing the turning ratio is not limited to the stepping motor 44, but an appropriate one such as a DC motor may be adopted.

When the control unit 51 is configured by a computer, it may be digital type or analog type.

As the failure of the control system for steering the rear wheels, various cases such as the failure of the input portion necessary for controlling the steering ratio, for example, the vehicle speed sensor 53, are conceivable. Similarly to the case shown by, the turning ratio may be set to 0 only when the turning ratio is in the small turning ratio region.

To forcibly return the rear wheels to the neutral position, an electromagnetic actuator such as a motor or a hydraulic actuator operated by an electromagnetic valve may be separately provided. In this case, the opening / closing valve 47 is opened. The actuator may be operated in synchronization with the above.

The hydraulic pressure of the power steering mechanism D may be released by releasing the restraint of the rear wheel steering mechanism by the power steering mechanism D, for example, by releasing both chambers 13b and 13c to the reservoir tank 17, respectively. An appropriate method can be adopted.

(Effects of the Invention) As is apparent from the above, the present invention forcibly returns the rear wheels to the neutral position and safely travels even if the control system for steering the rear wheels fails. It is possible to continue.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of the present invention. FIG. 2 is an overall plan view showing an embodiment of the present invention. FIG. 3 is a skeleton diagram showing the rear wheel steering mechanism portion. FIG. 4 and FIG. 5 are graphs showing an example of steering ratio characteristics. 6 to 10 are flowcharts showing a control example according to the present invention. FIG. 11 is a sectional view of the power steering mechanism portion. A: Front wheel steering mechanism B: Rear wheel steering mechanism C: Steering mechanism D: Power steering mechanism E: Steering ratio changing device 1R, 1L: Front wheel 2R, 2L: Rear wheel 9: Handle 13: Cylinder device 13b, 13c: Oil chambers 13e, 13f: Return spring 44: Stepping motor 45: Steering ratio sensor 46: Communication passage 47: Open / close valve 51: Control unit

Claims (1)

[Claims] 1. A four-wheel steering system for a vehicle in which not only front wheels but also rear wheels are steered, and a hydraulic power steering mechanism attached to a rear wheel steering mechanism for assisting steering of the rear wheels. Release the hydraulic pressure in the power steering mechanism,
A hydraulic releasing means for disabling steering of the rear wheels by the power steering mechanism, and a neutral position linked to the rear wheel steering mechanism for returning the rear wheels to a neutral position when the hydraulic releasing means operates. Return means, steering detection means for detecting the steering state of the rear wheels, failure detection means for detecting that the control system for controlling the rear wheels has failed, and output from both detection means, Backup control means for activating the hydraulic pressure releasing means so that the hydraulic pressure releasing amount is larger when the control system is out of order and is larger when the steering amount is small, and the rear wheel is in the neutral position. A four-wheel steering system for a vehicle.