JPH0655593B2 - 4-wheel steering system for vehicles - Google Patents

Description

The present invention relates to a four-wheel steering system for a vehicle.

(Prior Art) Some vehicles, as disclosed in Japanese Patent Laid-Open No. 60-199771, have both the front wheels and the rear wheels steered, which is so-called four-wheel steering.

In this four-wheel steering, since the steering characteristics such as 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 wheels are electrically steered. It is generally controlled.

(Problems to be Solved by the Invention) By the way, as described above, in the case of electrically controlling the steering of the rear wheels, as an actuator for adjusting the steering ratio, from the viewpoint of the ease of the control and the like. It is conceivable to use a stepping motor (pulse motor). This stepping motor has its stepping number (pulse number)
Therefore, the rotation angle is uniformly determined, and the desired rotation position, that is, the steering ratio can be easily and accurately controlled by the stepping number with the reference position as the origin. This is extremely advantageous in that the steering ratio open control is performed to speed up the responsiveness of the control, that is, the steering ratio can be changed quickly by following the change in the operating state of the vehicle. Even when the steering ratio is feedback-controlled, the actual steering ratio can be swiftly converged to the target steering ratio.

However, this stepping motor is not rotated by the rotation angle corresponding to the input stepping number when, for example, an unexpectedly large external force is applied, and as a result, the rotation position corresponding to the stepping number and the actual rotation position are increased. There is a "deviation", that is, "step-out", and this is one obstacle to obtaining the desired steering ratio.

Therefore, the object of the present invention is to presume that a stepping motor is used as an actuator for adjusting the steering characteristic, and a large "step-out" of the stepping motor.
It is an object of the present invention to provide a four-wheel steering system for a vehicle that can prevent the above.

(Means and Actions for Solving Problems) In order to achieve the above-mentioned object, in the present invention, the stepping motor has been made paying attention to the fact that the driving torque increases as the driving speed decreases. is there. That is, when a step-out is detected, this drive speed is reduced to secure a large drive torque and "step-out" is performed.
Are prevented from progressing. In addition to this, when the operation of the vehicle is once stopped and then newly restarted, in order to suppress the occurrence of step out of the stepping motor again as much as possible, based on the drive speed at the time of the previous operation, The initial drive speed of the stepping motor is set. Specifically, as shown 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 stepping motor for adjusting steering characteristics of rear wheels with respect to front wheels, A steering characteristic control unit that controls the stepping motor based on a predetermined steering characteristic, a step-out detecting unit that detects a step-out of the stepping motor, and a step-out of the stepping motor is detected, Drive speed adjusting means for reducing the drive speed of the stepping motor, non-volatile storage means for storing the corrected drive speed, and the stepping operation based on the drive speed stored in the storage means when the vehicle is restarted. An initial value setting means for setting an initial value of a motor drive speed is provided.

(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 is 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, 3L and tie rods 4R, 4L, respectively.
And a relay rod 5 connecting the pair of left and right tie rods 4R and 4L. 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 formed.
Is connected to the handle 9 via the shaft 8.
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 and 3L are centered around their rotation centers 3R 'and 3R'. The steering wheel 9 is steered clockwise by an amount corresponding to the operation displacement amount of the steering wheel 9, that is, the steering wheel steering angle. Similarly,
When the steering wheel 9 is turned to the left, the left and right front wheels 1R and 1L are steered to the left according to the operation displacement amount.

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, 10L and tie rods 11R, 11L, and a relay rod 12 connecting the tie rods 11R, 11L. In the embodiment, the rear wheel steering mechanism B is configured to include the hydraulic power steering mechanism D. Explaining the power steering mechanism D, 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 3c is integrated with the relay rod 12. Two chambers 13 in this cylinder 13a
b and 13c are connected to a control valve 16 via a pipe 14 or 15. Further, pipes 18 and 19 extending from the reservoir tank 17 are connected to the control valve 16, and an oil pump 20 driven by an engine (not shown) is connected to one pipe 18 which serves as an oil supply pipe. There is. The control valve 16 is a so-called booster valve type (spool type) in which the control rod 21 is a sliding type.
The input unit 21a of No. 1 is also used as a moving member of the steering ratio changing device E described later, and the output unit 21b of the control rod 21 is integrated with the relay rod 12 of the rear wheel steering mechanism B.

In such a power steering mechanism D, 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 and 10L rotate clockwise around the rotation centers 10R 'and 10L', and the rear wheels 2R and 2L are steered to the right.
Then, at the time of this steering, oil is supplied into the chamber 13b of the cylinder device 13 according to the displacement amount 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 wheel 2R is actuated by the boosting action of the cylinder device 13 (the oil is supplied to the chamber 13b) according to the displacement amount. 2L will be steered to the left. In addition, 13e and 13f in FIG. 2 are springs for urging the rear wheels 2R and 2L toward the neutral position.

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. The cylinder 65a is fixed to the vehicle body while the cylinder 65a is fixed to the vehicle body.
A piston 65d that defines the inside of a into two chambers 65b and 65c
Is integrated with the relay rod 5. The two chambers 65b and 65c in the cylinder 65a are provided with pipes 66 or 6
7 is connected to a rotary control valve 68 provided on the shaft 8 of the steering mechanism C. The control valve 68 is a pipe 7 extending from a diversion valve 69 connected on the discharge side of the oil pump 20.
0 and a pipe 71 branched from the pipe 19 are connected. Such a power steering mechanism F boosts the operating force of the handlebar 9 (power boosting 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 the steering ratio changing device E, an input rod 22 extends forward, and a pinion 23 attached to the front end of the input rod 22 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.

The turning ratio changing device E constitutes a turning characteristic adjusting means, and an example thereof will be described with reference to FIG. 3, but in the embodiment,
The structure is substantially the same as that shown in the above-mentioned JP-A-60-197977. That is, the input portion 21a of the control rod 21 is held slidably in the vehicle width direction with respect to the vehicle body, and its movement axis is l.
It is shown as ₁ . Further, the steering ratio changing device E is
It has a swing arm 31, and this swing arm 31
The base end portion is pivotally attached to the holder 32 by a pin 33 so as to be swingable. 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. The pin 33 is located at the intersection of the lines l ₁ and l ₂ and extends in the direction orthogonal to the orthogonal line l ₂ . Therefore, the swing arm 31 is connected to the pin 3
3, the holder 32 is swung, and the tilt angle formed by the pin 33 and the moving axis l ₁ , that is, the moving axis of the swinging orbital surface about the pin 33 is rotated by the holder 32. The tilt angle with respect to the plane (reference plane) orthogonal to l ₁ is variable.

The tip end 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 connected to the swing arm 31 via the ball joint 35.
Is also connected to the input portion 21a via a ball joint 36. With such a connecting rod 34, the distance between the ball joints 35 and 36 at each end of the swing arm 31 is always kept constant. Therefore, the ball joint 35
3 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 swinging of the swinging 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 on the movement axis l _1, and the connecting rod 34 is connected to the ball joint with respect to the eccentric portion of the rotating plate 37. 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. Then, the ball joint 35 is displaced in the left-right direction in FIG. 3, that is, the direction of the moving axis l _{1 in accordance} with the swing around the pin 33, and this displacement is caused by the connecting rod 34.
It is transmitted to the input section 21a via the, and the input section 21a is displaced. When the ball joint 35 is displaced in the left-right direction in FIG. 3, the tilt angle of the pin 33, that is, the rotation angle of the holder 32 is the same even if the swing angle of the swing arm 31 about the pin 33 is the same. If it changes, it will be changed (turning ratio change).

In order to change the tilt angle, the rotation shaft 32a of the holder 32
On the other hand, a sector gear 40 as a worm wheel is attached, and a worm gear 41 meshing with the sector gear 40 is rotationally driven by a stepping motor 44 as a tilt angle changing means via a pair of bevel gears 42 and 43. It is like this. And this holder 3
The rotation angle, that is, the inclination angle of 2 is detected by a steering ratio detection sensor 45, which is provided on the rotation shaft 32a and includes a potentiometer or the like.

Here, the swing angle of the swing arm 31 about the pin 33 and the tilt angle of the swing arm 31 (the tilt angle of the pin 33) are the directions of the moving axis l _{1 of the} ball joint 35 (input portion 21a). The effect on the displacement of will be described.
Now, the swing angle about the pin 33 of the swing arm 31 is θ, the reference plane orthogonal to the moving axis l ₁ is δ, and the swing arm 3 is
Assuming that the inclination angle of the rocking orbital surface of No. 1 with the reference plane δ is α and the eccentric distance from the pin 33 of the ball joint 35 is r, the displacement X of the ball joint 3 in the direction of the moving axis l ₁ is 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 becomes a function of θ, that is, the steering angle of the steering wheel. If the value of the inclination angle α is changed,
Even if the steering angle is the same, the value of X will change. 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.

The swinging stroke ends of the sector gear 40 driven by the stepping motor 44 are restricted by a pair of stoppers 48 and 49 (see FIG. 3). The rotation range of the stepping motor 44 required over the entire swing range of the sector gear 40 (from the same-phase side stroke end to the opposite-phase side stroke end) is "580" in the stepping number.

In FIG. 2, reference numeral 51 denotes a control unit composed of, for example, a microcomputer.
In addition to the output from the steering ratio sensor 45, a vehicle speed sensor 53
The output from is input. From the control unit 51, the stepping motor 44
And an alarm device 54 including a lamp and a buzzer.

Next, the control content of the control unit 51 will be described based on 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 stepping number "0" is set as the origin position at this time, and the stepping number driven from this origin position is set as the motor position "MP" at that time, and this "motor position initialization" is performed. Is performed at the start of control (immediately after starting the engine) and each time the vehicle speed becomes zero. Further, in the flowchart shown in this embodiment, three types of flags, "flag 1", "flag 2", and "flag 3" are used, but 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 3 “1” when step out of stepping motor 44 is detected
When the step-out is detected once, it is used to prohibit the step-out check until the "motor position initialization" is completed.

Based on the above, according to the flow charts shown in FIGS. 6 to 10, the explanation will be given for each figure, or for the sake of explanation, the interrupt processing (FIGS. 8 to 8) for the main routine as shown in FIG. It will be described from FIG.

Interrupt Processing 1 (FIG. 8) The interrupt routine shown in FIG. 8 is for driving the stepping motor 44, and is performed at a predetermined time set by the timer (for example, the stepping motor 44 is set to 10 times per second).
(If you want to drive at a rate of 0 steps, every 10 msec)
Then, the main routine of FIG. 6 is interrupted. Then, each step is driven by one stepping number. Therefore, the longer the interrupt time (T) is, the lower the driving speed of the stepping motor 44 is, and this interrupt time is 1 step as shown in FIG. Each time it is detected, the length is gradually increased step by step. Further, “CP” in the figure is a target rear wheel turning angle, that is, a target stepping number, required to have a turning ratio characteristic determined by a map using a vehicle speed as a parameter as shown in FIG. 4 (FIG. 5), for example. Further, as described above, “MP” is the stepping number of the swing position of the sector gear 40 (the steering position of the rear wheels 2R, 2L) from the origin position when the antiphase stopper 49 is the origin position. It is shown in.

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, 2L are turned by a predetermined amount. Since the turning angle is in accordance with the ratio characteristic, the energizing current to the stepping motor 44 is decreased (current down) in step S42, and thereafter, in step S43, the timer is set to a predetermined value described later in preparation for the next interrupt. Set to time T.

If it is determined in step S41 that CP = MP is not established, the current supplied to the stepping motor 44 is increased (current town is released) in preparation for driving the stepping motor 44, and then in step S45, C
It is determined whether or not P> MP. And CP> M
When it is determined that it is not P, the stepping motor 4
Since the current position of 4 is located on the same phase side as the target stepping number CP, the stepping motor 44 is driven toward the opposite phase side by one stepping in step S46. Then, following the operation of "1 stepping", the current position MP is updated by 1 stepping in step S47, and then the process proceeds to step S43. On the contrary, when CP> MP is determined in step S45, the stepping motor 44 is operated in step S48.
Is driven by one stepping toward the same phase, the current position MP is updated in step S49, and the process proceeds to step S43.

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 in accordance 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.
The number of pulses generated when the vehicle travels 1 km is "12740 pulses". The pulses generated from such a vehicle speed sensor 53 are sequentially counted and integrated in step S51 and stored as PCNT.

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). 282,575mse in relation to the meter cable
An interrupt is made to the main routine shown in FIG. 6 every c. That is, in step S52, the PCNT
Is set as the vehicle speed value (km / h) as it is, and then in step S53, the integrated count value PCNT 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, the final drive speed of the stepping motor 44 used in the previous operation is set. N is read. Of course, this table number N is stored in a non-volatile memory so that it is not cleared when the current is cut off. In the embodiment, the driving speed of the stepping motor 44 is 100 PP as the table number N sequentially increases from 0, 1, 2, 3, ...
S, 90PPS, 80PPS ... 10
Decreased by PPS, maximum speed corresponding to N = 0 10
It is 0PPS. Then, in step S3,
It is determined whether or not the table number N read in step S2 is 0, that is, whether or not the table number corresponds to the fastest driving speed. If N = 0,
In step S4, the table number corresponding to the driving speed one step higher than the table number read in step S1 is converted, and then the process proceeds to step S5. If N = 0, the process directly proceeds to step S5. That is, in the embodiment, as a general rule, the drive speed that is one step higher than the drive speed stored in the previous operation is set as the initial value in the current operation, as is clear from the explanation in the next step S5. It has become.

In step S5, MP = 0, CP = -58
0, flag 1 = “1”, and the stepping motor 44 drive interrupt time T is set as an initial value based on the drive speed in the previous operation, as described in steps S2 to S4. Further, setting CP = -580 forces the processing from step S45 to step S46 to be forced, and the sector gear 40 causes the reverse phase side stopper 49, as is apparent from the description of FIG. 8 described above. For returning to contact, that is, "motor position initialization"
To set the value of "580".
This is because, regardless of the current position of the sector gear 40, if it is returned only by 580 stepping, it can be brought into contact with the antiphase stopper 49 and returned to the original position.

Then, in step S6, it is determined whether or not the flag 1 is "1". In step S6, since the flag 1 is initially set to "1" in step S5, the process proceeds to step S7. In this step S7, it is determined whether or not CP = MP, but CP = M
If it is not P, a loop is returned from step S6 to step S7 again, and the stepping motor 44 shown in FIG. 8 is driven during this loop (MP approaches −580). CP = MP
Becomes Then, when this CP = MP, "motor position initialization" is completed, so in step S8, MP = 0, CP = 0, flag 1 = 0, flag 2 =
1 and flag 3 = 0.

When it is determined in step S6 that the flag 1 is not "1", it is determined in step S9 whether the current vehicle speed is zero. When it is determined in this determination that the vehicle speed is not zero, that is, the vehicle is traveling, the CP is determined in step S10 in FIG.
It is set as a value according to the vehicle speed based on the map shown in FIG. After that, after both the flag 1 and the flag 2 are set to "0" in step S11, a step-out check is performed in step S12 to be described later, and then the process returns to step S6.

When it is determined in step S9 that the current vehicle speed is zero, it is determined in step S13 whether or not the flag 2 is "0", and when the flag 2 is not "0", that is, "1". ",""Motor position initialization"
Since the stepping motor 44 is not driven later, it is unnecessary to perform this "motor position initialization" again, and the process directly returns to step S6. Step S1
If it is determined that the flag 2 is "0" in 3, the process proceeds to step S14 to perform "motor position initialization" (the same as the setting in step S2).

Step-out check (FIG. 7) First, in step S21, it is determined whether or not flag 3 = 0, that is, whether or not step-out has occurred after completion of "motor position initialization", and flag 3 = 0. If it is determined that step-out has not occurred at the time of step S21, the process proceeds to step S22. In this step S22, the actual turning ratio detected by the turning ratio sensor 45 is A / D.
It is read as the converted value θs. Then, step S23
, The stepping number corresponding to θs is MPX
Is set as
Whether -MPX | is smaller than a preset set value, that is, the actual turning ratio (MPX) and the control value (M
P) is within a permissible error range (for example, "5" in step setting number in the above-mentioned "setting value").

In step S24, when | MP-MPX | is larger than the set value, it means that "step out" has occurred, and in this case, the process proceeds to step S25. In this step S25, the table number N corresponding to the current drive speed is higher than the table number N (upper limit value of N) in which the slowest drive speed is set in the table in which the drive speed of the stepping motor 44 is set. It is determined whether the smaller one is smaller. When it is determined that the current table number N is smaller than the upper limit value, the process proceeds to step S26, and the table number N is a table number for which the driving speed is set to be one step lower than the current driving speed. Will be updated. Then, in step S27, after setting to the time T corresponding to the updated table number (reduction of drive speed), flag 3 is set to 1 in step S28. Then, in step S29, the table number (driving speed) updated in step S27.
Are stored in the non-volatile memory.

On the other hand, in step S24, when | MP-MPX | is smaller than the set value, it means that step-out has not occurred.
Return as it is. If it is determined in step S21 that the flag 3 is not 0, it is not necessary to perform the step-out check until the "motor position initialization" is completed after the step-out is performed once. To do.

Here, in this embodiment, in the step S25,
If it is determined that the table number N is not smaller than the upper limit value, it means that there is no table for setting the driving speed of the stepping motor 44 to a smaller value. In this case, the process proceeds to step S30, it is no longer possible to prevent the step-out, and the alarm device 54 is activated, and at step S31, the interruption time T is set so that the driving torque of the stepping motor 44 becomes extremely large. Is set to an abnormally low set value, that is, an extremely low speed, and in step S32, CP is set to the rear wheels 2R, 2L.
Is set to a value corresponding to a steering ratio of 0, which is the neutral position.
Then, the detected value θs of the turning ratio sensor 45 is read (step S33), and this θs is set to MP (step S).
34), in step S35, after waiting until the time when MP and CP substantially match (the time when the rear wheels 2R, 2L are returned to the substantially neutral position), wait for all interrupts in step S36 and transfer. Stop the control of the steering ratio.

Although the embodiment has been described above, when the control unit 51 is configured by a computer, it may be either a digital type or an analog type. Further, as the initial value of the drive speed of the stepping motor 44 set in step S5, the drive speed used in the previous operation may be used as it is.

(Effects of the Invention) As is clear from the above description, when the stepping motor for adjusting the steering characteristics is out of step, the driving speed of the stepping motor is reduced to make it less likely that step out will occur. Therefore, it is preferable for suppressing the step-out of the stepping motor to a minimum and controlling the steering according to a predetermined steering characteristic. Further, in the present invention, the drive speed (initial value) of the new operation this time is set based on the drive speed of the stepping motor used in the previous operation. It is preferable for preventing the deterioration.

Of course, in the present invention, the driving speed of the stepping motor is set as high as possible unless the stepping motor is out of step, so that the control response is ensured as fast as possible and the accuracy of the control is improved. Can be secured.

BRIEF DESCRIPTION OF THE 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. A: Front wheel steering mechanism B: Rear wheel steering mechanism C: Steering mechanism E: Steering ratio changing device 1R, 1L: Front wheels 2R, 2L: Rear wheels 9: Steering wheel 44: Stepping motor 45: Steering ratio sensor 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 stepping motor for adjusting the steering characteristics of the rear wheels with respect to the front wheels, and a predetermined steering characteristic. A steering characteristic control means for controlling the stepping motor based on the stepping-out characteristic, a step-out detection means for detecting a step-out of the stepping motor, and a stepping motor drive speed when the stepping-out of the stepping motor is detected. Drive speed adjusting means, a non-volatile storage means for storing the corrected drive speed, and an initial drive speed value of the stepping motor based on the drive speed stored in the storage means when the vehicle is restarted. A four-wheel steering system for a vehicle, comprising: