JPH0811541B2 - 4-wheel steering system for vehicles - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention turns a front wheel and a rear wheel in response to an operation of a steering wheel, and a rear wheel rolling in which a steering ratio of a rear wheel to a front wheel corresponds to a vehicle speed. The present invention relates to an improvement of a four-wheel steering system for a vehicle that is configured to change according to the rudder characteristics.

(Prior Art) A four-wheel steering system for a vehicle enhances the turning performance of the vehicle by controlling the steering ratios of the front and rear wheels to the opposite phase to set the steering characteristic to an oversteer tendency during a normal low-speed turn, thereby improving the turning performance of the vehicle and turning the high-speed turn. By controlling the steering ratio to the same phase and setting the steering characteristics in the direction that strengthens the understeer, the vehicle stability is enhanced based on the predetermined steering ratio characteristics set in advance according to the vehicle speed. It is configured to change the steering angle of the rear wheels with respect to the front wheels.

In a vehicle equipped with the four-wheel steering system, when the vehicle decelerates during turning, the steering ratio is reduced, that is, the rear wheels are in the opposite phase to the front wheels, resulting in oversteering. A tuck-in phenomenon that suddenly faces inward and a scooping phenomenon that the car body spins may occur. In order to prevent the occurrence of this tuck-in phenomenon or the like, conventionally, for example, as shown in Japanese Patent Laid-Open No. 60-85066, the steering ratio of the rear wheels to the front wheels is changed according to the vehicle speed only when the vehicle is in a substantially straight traveling state. When the vehicle is in a turning state, the steering angle of the rear wheels is kept constant, or when the vehicle speed changes abruptly as shown in JP-A-60-85067. Is configured to change the steering angle of the rear wheels with a predetermined delay by a delay circuit or the like.

However, in the former configuration, when decelerating in the high-speed turning state where the front wheel and the rear wheel are in the same phase, the rear wheel is kept in the same phase state even if the vehicle shifts to the low speed state, so that the turning property is However, if the front wheels and the rear wheels are accelerated in a low-speed traveling state in which the front wheels and the rear wheels are in opposite phases, the rear wheels are held in the opposite-phase state even after shifting to the high-speed state. There is a problem that the running stability of the vehicle cannot be improved, and the original characteristics of the four-wheel steering system cannot be exhibited in these states.

Also, in the latter configuration, when the vehicle speed is changed rapidly,
Since the steering angle of the rear wheels changes slowly, a steering ratio corresponding to changes in vehicle speed cannot be obtained, and the effect of increasing running stability in the high speed region during acceleration becomes insufficient, and in the low speed region during deceleration. There is a problem that the effect of improving the turning ability cannot be obtained.

(Object of the Invention) The present invention has been made based on the above technical background, and at the time of low speed, the turning ratio of the front and rear wheels can be controlled to the opposite phase to improve the turning performance of the vehicle, and at the same time, at high speed. Sometimes the steering ratio is controlled to the same phase to improve the running stability of the vehicle, while maintaining the function of the four-wheel steering system. An object of the present invention is to obtain a four-wheel steering system for a vehicle that can prevent the occurrence of a phenomenon.

(Structure of the Invention) The present invention is a four-wheel steering system for a vehicle having a front wheel steering mechanism that steers the front wheels and a rear wheel steering mechanism that steers the rear wheels, and vehicle speed detection means for detecting a vehicle speed, When the vehicle speed is low, the steering ratio of the front and rear wheels is set to a large value in the opposite phase direction according to the decrease of the vehicle speed, and when the vehicle speed is high, the steering ratio of the front and rear wheels is in phase with the increase of the vehicle speed. A rear wheel steering angle control unit that controls the rear wheel steering mechanism based on the rear wheel steering characteristic set to a large value of the direction, and the rear wheel steering characteristic includes a direction in which the vehicle speed decreases in comparison with a direction in which the vehicle speed increases. And a hysteresis setting section that sets a hysteresis having a predetermined displacement width so that the characteristics are in the same phase direction as the front wheels, and the displacement width of the hysteresis is set to 0 at a predetermined vehicle speed or less. It was done.

According to the above configuration, during deceleration of the vehicle, the rear wheel steering characteristics are displaced in the same phase direction in accordance with the output signal from the hysteresis setting unit, and in the low vehicle speed range below the predetermined vehicle speed,
By setting the displacement width to 0, it is possible to prevent the rear wheels from abruptly changing from the same phase as the front wheels to the opposite phase except for this low speed region.

(Embodiment) FIGS. 1 and 2 show a schematic configuration of a four-wheel steering system for a vehicle, in which front wheels 1L, 1R and rear wheels 2L, 2R are supported by a front wheel steering mechanism 3 and a rear wheel steering mechanism 12, respectively. ing.

The front wheel steering mechanism 3 includes a pair of left and right knuckle arms 4
L, 4R and tie rods 5L, 5R, and these left and right tie rods 5
It consists of a relay rod 6 that connects L and 5R. A steering wheel 10 is connected to the front wheel steering mechanism 3 via a rack and pinion type steering mechanism 7. That is, the steering mechanism 7 includes a rack 8 formed on the relay rod 6, a steering shaft 11 having a threaring wheel 10 connected to an upper end thereof and a pinion 9 engaging with the rack 8 attached to a lower end thereof. The left and right front wheels 1L, 1R are steered in accordance with the operation of the wheel 10.

On the other hand, the rear wheel steering mechanism 12 is similar to the front wheel steering mechanism 3 in that the left and right knuckle arms 13L and 13R and the tie rod 1 are used.
The relay rod 15 connects the 4L and 14R to each other, and further includes a hydraulic power steering mechanism 16. The power steering mechanism 16 includes a power cylinder 17 fixed to the vehicle body and having the relay rod 15 as a piston rod. Inside the power cylinder 17, two hydraulic chambers are provided by a piston 17a integrally attached to the relay rod 15. It is divided into 17b and 17c, and these hydraulic chambers 17b and 17c are connected to the control valve 20 via pipes 18 and 19, respectively.

The control valve 20 has a reserve tank.
Two pipes of an oil supply pipe 22 and an oil discharge pipe 23 reaching 21 are connected, and a hydraulic pump 24 driven by an engine (not shown) is arranged in the oil supply pipe 22. The control valve 20 is of a known spool valve type, and has a cylindrical valve casing 20a integrally attached to the relay rod 15 via a connecting member 25, and is fitted in the valve casing 20a. A hydraulic pump 24 is provided in one hydraulic chamber 17b (17c) of the power cylinder 17 according to the movement of the spool valve.
The pressure oil is supplied from to assist the driving force for the relay rod 15. The above power cylinder 17
Inside, place the relay rod 15 in the neutral position (rear wheels 2L, 2R
The return springs 17d, 17d for biasing the steering angle θR to 0) are mounted.

The relay rod 6 of the front wheel steering mechanism 3 has a rack 26 at a position different from the rack 8 constituting the steering mechanism 7.
A pinion 27 attached to the front end of a rotary shaft 28 extending in the vehicle front-rear direction is meshed with the rack 26, and the rear wheels of the rotary shaft 28 are connected to the rear wheel mechanism 12 via a steering ratio control mechanism 29. Are linked to.

The turning ratio control mechanism 29 has a control rod 30 held slidably in the vehicle width direction with respect to the vehicle body, and one end of the control rod 30 has the control valve 20.
Is connected to the spool valve. Further, the turning ratio control mechanism 29 includes a swing arm 33 whose base end is swingably supported by a U-shaped holder 31 via a support pin 32, and the holder 31 is a casing fixed to the vehicle body. It is rotatably supported (not shown) via a support shaft 35 having a rotation axis perpendicular to the movement axis of the control rod 30. The support pin 32 of the swing arm 33 is located at the intersection of the two axes and extends in the direction orthogonal to the rotation axis. By rotating the holder 31 about the support shaft 35, the tip of the support pin 32 is supported. A tilt angle formed by the pin 32 and the movement axis of the control rod 30, that is, a plane in which a swing locus surface of the swing arm 33 about the support pin 32 is orthogonal to the movement axis (hereinafter referred to as a reference plane).
It is designed to change the angle of inclination with respect to.

Further, one end of a connecting rod 37 is connected to the tip end of the swing arm 33 via a ball joint 36, and the other end of the connecting rod 37 is connected to the other end of the control rod 30 via a ball joint 38. The control rod 30 is displaced in the vehicle width direction according to the displacement of the tip end of the swing arm 33 in the vehicle width direction.

The connecting rod 37 is slidably supported by a rotation imparting arm 40 via a ball joint 41 at a portion near the ball joint 36. The rotation imparting arm 40 is integrally provided with a large-diameter bevel gear 43 which is rotatably supported on the moving axis via a support shaft 42. As shown in FIG. A bevel gear 44 attached to the rear end of the rotation shaft 42 is meshed with the rotation shaft 42 to transmit the rotation of the steering wheel 10 to the rotation imparting arm 40.

Therefore, the rotation imparting arm 40 and the connecting rod 37 are moved by an amount corresponding to the turning angle of the steering wheel 10.
When the swing arm 33 is swung around the support pin 32 with the rotation of the
When the axis of 32 is aligned with the axis of movement of the control rod 30, the ball joint 36 at the tip of the swing arm 33
Oscillates on the above reference plane, and the control rod
Although 30 is held stationary, if the axis line of the pin 32 is tilted with respect to the movement axis line and the swing locus surface of the swing arm 33 deviates from the reference plane, the swing arm centering around the pin 32 is used. The ball joint 36 is displaced in the vehicle width direction with the swing of 33, and this displacement is transmitted to the control rod 30 via the connecting rod 37.
Is configured to move along the axis of movement to actuate the spool valve of control valve 20.

That is, even if the swing angle of the swing arm 33 about the axis of the pin 32 is the same, the displacement of the control rod 30 in the left-right direction is accompanied by a change in the tilt angle of the pin 32, that is, the rotation angle of the holder 31. Change.

Then, in order to change the inclination angle of the support pin 32 with respect to the moving axis, that is, the inclination angle of the holder 31 with respect to the reference plane, the support shaft 35 of the holder 31 has a sector gear 45 as a worm wheel, and the sector shaft 45 has a rotary shaft 46. The worm gear 47 is meshed. Further, a bevel gear 48 is attached to the rotary shaft 46, and a bevel gear 49 mounted on the output shaft 50a of the stepping motor 50 is meshed with the bevel gear 48, and the stepping motor 50 is operated to operate the sector gear 45. Is rotated to change the inclination angle of the holder 31 with respect to the reference plane to control the steering angles θR of the rear wheels 2L and 2R, and the sector gear 45
From the neutral position where the center line is perpendicular to the center line of the rotary shaft 46 of the worm gear 47, when turning in the clockwise direction when viewed from above the vehicle body, the steering ratio is changed from the rear wheels 2L, 2R to the front wheels 1L, 1R. It is configured to control in the same phase so as to face the same direction.

Further, in the casing supporting the holder 31, the stopper members 51 on the opposite phase side and the same phase side, which are pins on the left and right sides of the sector gear 45 serving as the rotating member, for restricting the rotating range of the sector gear 45, 52 is attached, and when the sector gear 45 rotates to the opposite phase side, when the rotation angle from the neutral position becomes, for example, −17.5 °, the sector gear 45 comes into contact with the opposite phase side stopper member 51 and further. Is restricted, and when the sector gear 45 rotates toward the same phase, when the rotation angle from the neutral position becomes, for example, 20 °, the sector gear 45 comes into contact with the stopper member 52 on the same phase side to move. It is configured to be regulated.

The control position of the stepping motor 50 when the sector gear 45 contacts the stopper member 51 on the opposite phase side is set to the initial position. In addition,
Reference numeral 39 is a rod stopper that regulates the maximum movement range of the relay rod 15 in the rear wheel steering mechanism 12.

As shown in FIG. 3, the stepping motor 50 is so constructed that its operation is controlled by the output from the control unit 100 incorporated in the microcomputer.
That is, in the control unit 100, a vehicle speed detection unit 101b that detects the traveling vehicle speed V of the vehicle based on the detection signal PCN of the vehicle speed sensor 101a, and a vehicle speed detection unit that includes the vehicle speed sensor 101a and the vehicle speed detection unit 101b. Based on the actual traveling vehicle speed V, it is determined whether or not the vehicle is in the decelerating state. When the vehicle is in the decelerating state, the steering ratio of the rear wheels to the front wheels is the same as the basic rear wheel steering characteristic as described later. A hysteresis setting unit 102 for obtaining a calculated vehicle speed V'to obtain a hysteresis displaced in the phase direction, and a calculated vehicle speed V'at the time of deceleration when the vehicle speed is in a low speed region below a predetermined value (for example, 10 km / h). The displacement width calculation unit 103 for setting the displacement width α of the hysteresis to be 0, and the calculation speed V ′.
A steering ratio setting unit 104 that sets the steering ratio of the rear wheels 2L, 2R with respect to the front wheels 1L, 1R, and a control signal is output to the stepping motor 50 according to the output signal from the steering ratio setting unit 104. And a motor control unit 105 as a rear wheel steering angle control unit.

The control unit 100 is supplied from an on-vehicle battery when the ignition key switch (not shown) is turned on, and operates as a system power source. To explain, the control unit 100 includes a CPU 106 as a control unit and a ROM 107 that stores predetermined control data, and the CPU 106 uses the output voltage from the constant voltage circuit 108 that keeps the battery voltage (12V) at a constant voltage of 5V. Works, C
CPU 106 runaway detection unit 109 that detects PU 106 runaway, output voltage is
Voltage drop detection unit 11 that detects when voltage drops below 4.5V 11
It is reset by receiving each output from the power-on reset unit 111 which outputs a reset signal when 0 and the ON operation of the ignition key switch is started.

Further, the output signal of the vehicle speed sensor 101a is input to the integration filter 113 via the interface 112, and after the integration filter 113 removes chattering, the waveform shaping circuit 1
The signal waveform is shaped in 14 and supplied to the CPU 106.

Further, the control unit 100 has a stepping motor driver 115 which receives the output of the CPU 106 and drives the stepping motor 50, and receives the current down command signal from the CPU 106 and receives the stepping motor driver.
Each phase has a current down section 116 for limiting the output power from the battery power source to 50 to 100 mA for each phase while the motor 50 is not controlled (when the rotation of the motor output shaft 50a is stopped).

Next, a signal processing procedure performed by the CPU 106 of the control unit 100 will be described. FIG. 5 shows a main routine of a signal processing program, and this function serves as the steering ratio setting unit 104. After the start by turning ON the ignition key switch, the system is first initialized in step S1, and in the next step S2, the current step number MP of the stepping motor 50 is set to 580 and its target step number CP is set to 0, respectively. At the same time, the flag indicating execution of the motor position initialization control mode is set to F1 = 1. The target step number CP is CP = 0 at the control initial position of the stepping motor 50, that is, the position where the sector gear 45 is in contact with the antiphase stopper member 51 and the steering ratio is the maximum antiphase steering ratio. And the number of steps of the pulse signal input to the motor 50 when controlling the motor 50 to its target control position from there, and the current step number MP is the above control of the current control position of the motor 50. It shows the number of steps from the initial position.

The flag F1 is set to F1 = 1 in the motor position initialization control mode in which the motor 50 is controlled to initialize its control position. However, the flag F1 controls the steering ratio according to the vehicle speed. In the control mode, it is reset to 1 = 0.

After that, the process proceeds to step S3, and it is determined whether or not the flag F1 is F1 = 1. When this determination is F1 = 1, that is, when the position initialization control mode of the motor 50 is performed, the process proceeds to step S4 and the target number of steps for the motor 50.
It is determined whether CP is equal to the current number of steps MP, and when this determination is CP ≠ MP, the process directly returns to step S3.
When the determination is CP = MP and the initialization of the control position of the motor 50 is completed, the process proceeds to step S5, the target step number CP and the current step number MP of the motor 50 are set to CP = MP = 0, and the flag F1 is set. Is reset to F1 = 0 and a flag F2 for identifying that the control position initialization of the motor 50 has been executed once is set to F2 = 1, and then the process returns to step S3.

On the other hand, when it is determined in step S3 that F1 is not 1 and the motor 50 is controlled to change the steering ratio,
In step S6, it is determined whether or not the traveling vehicle speed V detected by the vehicle speed detection unit 101b is 0 (stopped state). If this determination is YES, the flag F2 is further set to F2 = 0 in step S7. Determine if there is. When the determination F2 ≠ 0 in step S7, the process directly returns to step S3, but it is determined that F2 = 0 and the traveling vehicle speed V
When it is confirmed that the control position initialization of the motor 50 is not executed when the vehicle stops at 0, the flag F1 is set to F1 = 1 in step S8, and the motor 50 is set in the next step S9.
After setting the current step number MP and the target step number CP to MP = 580 and CP = 0 corresponding to the control initial position, the process returns to step S3.

If it is determined in step S6 that the traveling vehicle speed V is not 0 and the vehicle is in a traveling state, the process proceeds to step S10, and the calculated vehicle speed V obtained based on the traveling vehicle speed V in a second interrupt routine described later. ′,
The motor 50 is collated with a control data table showing a basic rear wheel steering characteristic corresponding to a vehicle speed stored in advance in the ROM 107.
After reading the value f (V ') of the target step CP, the both flags F1 and F2 are reset to F1 = F2 = 0 in the next step S11, and then the process returns to the step S3.

The basic rear wheel steering characteristic control data table stored in the ROM 107 is shown in the solid line in FIG. 6, and the steering ratios of the front and rear wheels 1L, 2L (1R, 2R) depend on the vehicle speed. When the vehicle speed changes and the vehicle speed is low, the rear wheels 2L, 2R are in the opposite direction to the front wheels 1L, 1R in order to improve the turning performance of the vehicle, that is, the steering ratio of the front and rear wheels is a negative value. The steering ratio of the front and rear wheels is set to a large value in the opposite phase direction as the vehicle speed decreases.

Further, when the vehicle speed reaches, for example, about 67 km / hour, the steering ratio becomes zero, and the rear wheels 2L, 2R are irrespective of the steering of the front wheels 1L, 1R.
The steering angle θR is maintained at θR = 0 and the vehicle enters the normal two-wheel steering state. When the vehicle speed is higher, the rear wheels 2L and 2R are in the same direction as the front wheels 1L and 1R, that is, the front and rear wheels are rolling in order to improve the grip of the rear wheels 2L and 2R during cornering and improve the running stability. The steering ratio is set to a positive value, and the steering ratios of the front and rear wheels are set to a large value in the same phase direction as the vehicle speed increases.

Further, FIG. 7 shows a first interrupt routine which is interrupted to the main routine when the time set in the timer built in the CPU 106 has elapsed, and this routine serves as the motor control unit 105. The function is fulfilled. In the first interrupt routine, it is first determined in step S12 whether the target step number CP of the motor 50 is equal to the current step number MP.
When this determination is CP = MP, that is, when the output of the pulse signal to the motor 50 is not necessary and the motor 50 is held at its control position, the process proceeds to step S13 and the current down command signal is output to the current down unit 116. , The voltage applied to the motor 50 is reduced to suppress the amount of heat generated, and then the timer for generating the next interrupt process is set in step S14, and then the process returns to the step after the interrupt in the main routine.

When the determination in step S12 is CP ≠ MP, the process proceeds to step S15 to cancel the output of the current down command signal to the current down unit 116, and then proceeds to step S16, where the target step number CP of the motor 50 is CP. And the current step number MP are compared. This judgment
If CP> MP, the process proceeds to step S17 and the motor 50
Switches its excitation phase so that it moves only one step in the same phase direction of the turning ratio, then updates the current step number MP to MP ← MP + 1 in step S18, and then proceeds to step S14.

On the other hand, when the determination in step S16 is CP <MP, the process proceeds to step S19, where the excitation phase is switched so that the motor 50 moves only one step in the opposite phase direction of the steering ratio, and in step S20, the current step number. After updating MP to MP ← MP-1, the process proceeds to step S14.

Further, FIG. 8 shows a second interrupt routine which is interrupted at a constant cycle, and the functions of the hysteresis setting unit 102 and the displacement width calculating unit 103 are fulfilled by this routine. That is, in this second interrupt routine, first in step S21, the vehicle speed detection unit 101b determines the current traveling vehicle speed V based on the detection signal CPN of the vehicle speed sensor 101a. Next, in step S22, it is determined whether the traveling vehicle speed V and the previous calculated vehicle speed V'obtained and stored in the previous interrupt processing are large or small, and the current traveling vehicle speed V is higher than the previous calculated vehicle speed V '. If it is determined to be greater than or equal to and it is confirmed that the vehicle is in the acceleration state or the constant speed state, the value of the traveling vehicle speed V is output as it is as the calculated vehicle speed V ', and the calculated vehicle speed V'is obtained in step S23. Is updated to V '← V, and then the main routine is returned to. In this case, the hysteresis displacement width is 0 (step S23).

As a result, when the vehicle is accelerating or traveling at a constant speed, the stepping motor 50 is controlled so that the steering ratio becomes a steering ratio according to the basic rear wheel steering characteristics shown by the solid line in FIG. The steering angle θR of the wheels 2L, 2R is set to a predetermined value. That is, in the low-speed turning state, the rear wheels 2L, 2R are steered so as to be in the opposite phase to the front wheels 1L, 1R, and good turning performance is obtained.On the contrary, in the high-speed turning state, the rear wheels 2L, 2R are the same. The steering stability is improved by being steered in the phase.

If it is determined in step S22 that the current traveling vehicle speed V is lower than the previous calculated vehicle speed V'and it is confirmed that the vehicle is in the decelerated state, step S24
And the reference vehicle speed in the low speed range set in advance
Determine the magnitude of Vx. Then, when it is confirmed that the traveling vehicle speed V is in the traveling area equal to or higher than the reference vehicle speed Vx, for example, in the medium-high speed area of 10 km / h, the hysteresis displacement width α stored in the ROM 107 in step S25. After reading, the magnitude of the traveling vehicle speed V and the value V'-α obtained by subtracting the displacement width α from the previous calculated vehicle speed V'are determined in step S26.

As a result of this determination, when it is confirmed that the traveling vehicle speed V is higher than the value V'-α or both are equal, the calculated vehicle speed V'is not updated and the steering ratio is maintained at the previous state. To do. When it is determined that the traveling vehicle speed V has become smaller than the above value V′−α, the deceleration further proceeds,
In step S26, the calculated vehicle speed V'is updated to V '← V + α, and the rear wheels 2L, 2R are adjusted in accordance with the turning ratio corresponding to the vehicle speed higher than the current traveling vehicle speed V by the displacement width α of the hysteresis. The steering angle θR of is set.

As a result, when the vehicle decelerates in the medium-high speed region, as shown by an arrow a in FIG. 6, the section corresponding to the displacement width α of the hysteresis, the rear wheels 2L, 2R with respect to the front wheels 1L, 1R.
After the turning ratio of is maintained constant, as indicated by arrow b,
According to one of the rear wheel steering characteristics at the time of deceleration indicated by the broken line obtained by moving the above basic steering characteristics in parallel to the low speed side, the rear wheel 2
The rudder angle θR of L and 2R is set. That is, at the same vehicle speed, the rear wheel steering characteristics during deceleration are displaced in the same phase direction as compared with the basic rear wheel steering characteristics.

When the deceleration further progresses from the above state and it is confirmed in step S24 that the traveling vehicle speed V is in the low speed region equal to or lower than the reference vehicle speed Vx, the process proceeds to step S23 and the hysteresis displacement width α is set to 0. And As a result, as shown by arrow C in FIG. 6, the rear wheel steering characteristics during deceleration match the basic rear wheel steering characteristics shown by the solid line in the low speed region, and the front and rear wheels have the maximum reverse phase. So that
The rear wheels will be steered rapidly.

As described above, when the vehicle speed is in the medium-high speed range during deceleration of the vehicle, the front wheels 1 are moved according to the rear wheel steering characteristics during deceleration displaced in the same phase direction as compared with the basic rear wheel steering characteristics.
Since the steering ratio of L, 1R and rear wheels 2L, 2R is set,
It is prevented that L and 2R are steered steeply in the opposite phase direction. Therefore, the vehicle is held understeer for a predetermined time, and the running stability is improved.

In addition, when the vehicle speed decreases to a low speed region where there is no risk of the vehicle tuck-in phenomenon, the steered ratios of the front and rear wheels are set to a large value in the opposite phase direction as the vehicle speed decreases. Since the steering ratio rapidly changes in the opposite phase direction in accordance with the basic rear wheel steering characteristics described above, good turning performance can be obtained during low speed turning.

Further, when the vehicle shifts from the deceleration state to the acceleration state, as shown by an arrow d in FIG. 6, after the turning ratio is maintained constant, the basic rear wheel turning characteristic shown by the solid line is obtained. Accordingly, the steered ratio changes in the same phase direction as the vehicle speed increases.

In the above-described embodiment, the vehicle speed decreases to the reference speed Vx, and at the same time, the hysteresis displacement width α is set to 0 so that the rear wheel steering characteristic during deceleration matches the basic rear wheel steering characteristic. As shown in FIG. 9, the displacement width α is gradually reduced from the time when the vehicle speed reaches the first reference speed V2, and the displacement width α is set to 0 when the vehicle speed is decelerated at the second reference speed V2. You may comprise.

In addition, in the above embodiment, in order to obtain accurate steering characteristics and to control the steering angle of the rear wheels with good responsiveness, the output shaft rotates according to the number of pulses of the input pulse signal. Although the four-wheel steering system of the open-loop control system using the stepping motor has been described, the configuration of the present invention can be applied to the four-wheel steering system of the closed-loop system using a DC motor or the like.

In the above embodiment, the steering ratio of the front and rear wheels of the vehicle is variably controlled according to the vehicle speed, but the rear wheels are directly driven by the stepping motor or the like according to the vehicle speed and the steering angle of the front wheels. You may comprise.

As described above, according to the present invention, when the vehicle speed is low, the steering ratio of the front and rear wheels is set to a large value in the opposite phase direction according to the decrease of the vehicle speed, and when the vehicle speed is high, the vehicle speed is increased. In a four-wheel steering system for a vehicle configured to change the steering angle of the rear wheels with respect to the front wheels based on the rear wheel steering characteristics in which the steering ratios of the front and rear wheels are set to large values in the same phase direction in accordance with the increase of In the rear wheel steering characteristic, a hysteresis having a predetermined displacement width is set so that the characteristic in the decreasing direction of the vehicle speed in the same direction as the front wheel in the same direction as the increasing direction of the vehicle speed is set to a predetermined vehicle speed. In the following, the hysteresis displacement range is set to 0
Therefore, when decelerating from a high-speed turning state, the deceleration is performed after the steering ratio is maintained constant for a predetermined time except for a low-speed region where there is no risk of a vehicle tack-in phenomenon. The steering ratio will change according to the rear wheel steering characteristics. Therefore, the rear wheels are prevented from being steered steeply in the opposite phase direction to be in the oversteer state, the tack-in phenomenon of the vehicle at the time of deceleration is likely to occur, and the sticking-in phenomenon does not occur, and the running stability is further improved.

Moreover, in the low speed region and during acceleration, the steering angle of the rear wheels is set in accordance with the basic steering characteristics of the rear wheels.Therefore, as the vehicle speed decreases during low speed turning, the rear wheels are reversed from the front wheels. Demonstrate the original characteristics of a four-wheel steering vehicle that can be steered largely in the phase direction to improve the turning ability of the vehicle and to enhance the running stability of the vehicle during acceleration or constant speed in a high-speed turning state. It is possible.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an embodiment of a vehicle four-wheel steering system according to the present invention, FIG. 2 is a schematic perspective view of the steering system, and FIG. 3 is a block diagram showing the function of a control unit.
FIG. 4 is a block diagram showing a specific configuration of the control unit, FIG. 5 is a flowchart showing a main routine processed by the CPU in the control unit, and FIG.
FIG. 7 is a characteristic diagram of vehicle speed and turning ratio, FIGS. 7 and 8 are flow charts showing first and second interrupt routines processed by the CPU, and FIG. 9 is another embodiment of the present invention. It is the rear wheel steering characteristic figure which concerns. 1L, 1R …… front wheel, 2L, 2R …… rear wheel, 3 …… front wheel steering mechanism,
12 ... Rear wheel steering mechanism, 101a ... Vehicle speed sensor, 101b ... Vehicle speed detection unit, 102 ... Hysteresis setting unit, 103 ... Displacement width calculation unit, 104 ... Turning ratio setting unit, 105 ... Motor control Section (rear wheel steering angle control section).

Claims (1)

[Claims] 1. A four-wheel steering system for a vehicle having a front-wheel steering mechanism for steering front wheels and a rear-wheel steering mechanism for steering rear wheels, wherein vehicle speed detection means for detecting vehicle speed and low vehicle speed are provided. The steering ratio of the front and rear wheels is set to a large value in the opposite phase direction as the vehicle speed decreases, and the steering ratio of the front and rear wheels becomes a large value in the same phase direction as the vehicle speed increases when the vehicle speed is high. The rear-wheel steering angle control unit that controls the rear-wheel steering mechanism based on the rear-wheel steering characteristics set in the above, and the rear-wheel steering characteristics include a characteristic in which the vehicle speed decreases in the direction in which the vehicle speed decreases in comparison with the direction in which the vehicle speed increases. And a hysteresis setting section for setting a hysteresis having a predetermined displacement width so as to be in the same phase direction as the above, and the displacement width of the hysteresis is set to 0 at a predetermined vehicle speed or less. A four-wheel steering system for a vehicle.