JP2535511B2 - 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 configured to change according to the steering ratio characteristic.

(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. Based on the predetermined rear wheel steering ratio characteristic preset according to the vehicle speed, so as to enhance the running stability of the vehicle by controlling the steering ratio to the same phase and setting the steering characteristic in the direction that strengthens the understeer. It is configured to change the steering angle of the rear wheels with respect to the front wheels.

By the way, in a vehicle equipped with the four-wheel steering system, when decelerating during turning, the steering ratio is reduced in a direction in which the rear wheels are in a phase opposite to that of the front wheels, resulting in oversteer and a vehicle body. 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. 61-85066, 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.

Further, in the latter configuration, when the vehicle is rapidly accelerated, the steering angle of the rear wheels changes gently, so that a predetermined turning ratio cannot be obtained even in a high-speed turning state, and the effect of increasing running stability can be obtained. Is insufficient, and the rear wheel steering ratio characteristic changes according to the amount of change in vehicle speed per unit time such as deceleration, so that the driver cannot accurately grasp this rear wheel steering ratio characteristic. There was a problem.

(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 in the same phase to improve the running stability of the vehicle, and the function of the four-wheel steering system is maintained. The rear wheel steering ratio characteristic does not change according to deceleration, etc., and the steering angle of the rear wheel changes according to the predetermined rear wheel steering ratio characteristic, and the driver changes this rear wheel steering ratio. It is an object of the present invention to obtain a four-wheel steering system for a vehicle that can accurately grasp the characteristics.

(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 rear wheels to the front wheels is increased according to 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 ratio characteristic set to a large value in the same phase direction, and the rear-wheel steering ratio characteristic, in which the vehicle speed increases in the deceleration direction of the vehicle speed. And a hysteresis setting unit that sets a hysteresis having a predetermined displacement width so that the characteristics are in the same phase direction as the front wheels in the same direction as the direction.

According to the above configuration, when the vehicle decelerates, the rear wheel steering ratio characteristic changes in the same phase direction according to the output signal from the hysteresis setting unit, so that the rear wheel rapidly changes from the same phase as the front wheel to the opposite phase. Will be prevented.

(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 has a rack 8 formed on the relay rod 6, a steering wheel 10 at the upper end, and the rack 8 at the lower end.
A steering shaft 11 having a pinion 9 that meshes with the steering shaft 11 is provided, and the left and right front wheels 1L and 1R are steered according to an operation of the steering 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 nuttle arms 13L and 13R and the tie rods 14 are arranged.
The relay rod 15 connects L 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. Two pipes, an oil supply pipe 22 and an oil discharge pipe 23, which reach the reserve tank 21, are connected to the control valve 20, and the oil supply pipe 22 has a hydraulic pump 24 driven by an engine (not shown). Is provided.

The control valve 20 is of a known spool valve type, and is connected to the relay rod 15 by a connecting member 25.
Cylindrical valve casing integrally attached via
20a and a spool valve (not shown) fitted in the valve casing 20a are provided, and the hydraulic pressure from the hydraulic pump 24 is supplied to one hydraulic chamber 17b (17c) of the power cylinder 17 according to the movement of the spool valve. The driving force for the relay rod 15 is assisted. In the power cylinder 17, return springs 17d and 17d for urging the relay rod 15 to a neutral position (positions where the steering angles θR of the rear wheels 2L and 2R are 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 end of the rotary shaft 28 is connected to the rear wheel steering mechanism via a steering ratio control mechanism 29. 12 connected.

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 the 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 rotate about the moving axis by an amount corresponding to the rotation angle of the steering wheel 10, and the swing arm 33 swings about the support pin 32 accordingly. When the axis of the support pin 32 is aligned with the movement axis of the control rod 30, the ball joint 36 at the tip of the swing arm 33 only swings on the reference plane, and the control rod 30 moves. Although held in a stationary state, when the axis of the pin 32 tilts with respect to the movement axis and the swing locus surface of the swing arm 33 deviates from the reference plane, the swing arm 33 centered around the pin 32 moves. The ball joint 36 is displaced in the vehicle width direction due to the swing, and this displacement is transmitted to the control rod 30 via the connecting rod 37, and the control rod 30 moves along the movement axis to And it is configured to actuate the spool valve of the trawl 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 control rod 30
The displacement in the left-right direction changes with the change of the inclination angle of the pin 32, that is, the rotation angle of the holder 31.

Then, in order to change the inclination angle of the support pin 32 with respect to the movement axis, that is, the inclination angle of the holder 31 with respect to the reference plane, the support shaft 35 of the holder 31 includes a sector gear 45 as a worm wheel, and the worm gear on the rotary shaft 46. 47 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 that supports the holder 31, the stopper members 51 on the opposite phase side and the same phase side are formed on the left and right sides of the sector gear 45 serving as the rotating member from the pins that restrict the rotation 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. The above rotation 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 contacts the stopper member 52 on the same phase and moves. Are 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. Reference numeral 39 is a rod stopper that restricts 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 102 that detects the traveling vehicle speed V of the vehicle based on the detection signal PCN of the vehicle speed sensor 101, and a vehicle speed detection unit that includes the vehicle speed sensor 101 and the vehicle speed detection unit 102 detects the vehicle speed. 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 ratios of the front and rear wheels are in phase from the basic rear wheel steering ratio characteristic as described later. Calculated vehicle speed V'to obtain hysteresis displaced in the direction
And the calculated vehicle 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 operates as a power supply system power source supplied from an in-vehicle battery when an ignition key switch (not shown) is turned on, and its internal configuration will be specifically described with reference to FIG. , The control unit 100 is C as a control unit
A PU 106 and a ROM 107 that stores predetermined control data are provided.
The CPU 106 operates by the output voltage from the constant voltage circuit 108 that keeps the battery voltage (12V) at a constant voltage of 5V,
CPU runaway detection unit 109 that detects runaway of 6, output voltage is 4.5V
It is reset by receiving each output from the voltage drop detection unit 110 that detects a decrease below and the power-on reset unit 111 that outputs a reset signal when the ON operation of the ignition key switch is started.

Further, the output signal of the vehicle speed sensor 101 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 that limits the output current from the battery power source for 50 to, for example, 100 mA during each non-control of the motor 50 (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 F1 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, and the current number of steps 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, F1 = 0 is reset.

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 102 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. Then, when the determination in step S7 is F2 ≠ 0, the process directly returns to step S3, but when it is determined that F2 = 0 and the traveling vehicle speed V is 0, the control position initialization of the motor 50 is executed. If it is confirmed that the flag F1 is not set, the flag F1 is set to F1 = 1 in step S8, and the motor is set in the next step S9.
After setting the current step number MP and the target step number CP of 50 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 is compared with the control data table that shows the basic rear wheel steering ratio characteristic corresponding to the vehicle speed stored in advance in the ROM 107.
After reading the value f (V ') of the target step number CP of 50, the flags F1 and F2 are both reset to F1 = F2 = 0 in the next step S11, and then the process returns to step S3.

The basic rear wheel steering ratio characteristic control data table stored in the ROM 107 is shown in FIG. 6 as a solid line, and the front and rear wheels 1L, 2L (1R, 2R) are steered according to the vehicle speed. When the ratio changes and the vehicle speed is low, the rear wheels 2L and 2R are steered in the opposite direction to the front wheels 1L and 1R, that is, in the opposite phase, in order to improve the turning performance of the vehicle. While the ratio becomes negative, when the vehicle speed reaches, for example, about 67 km / hour, the steering ratio becomes zero, and the steering angle θR of the rear wheels 2L, 2R becomes θR = 0 regardless of the steering of the front wheels 1L, 1R. The vehicle is kept in the normal two-wheel steering state.
If the vehicle speed is higher, the rear wheels 2L, 2 when cornering
The rear wheels 2L and 2R are steered in the same direction as the front wheels 1L and 1R, that is, in the same phase, in order to improve the grip force of the R and improve running stability, and the steering ratio is set to be positive. ing.

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 150. 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 unnecessary 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. Allows motor 5
The applied voltage to 0 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.

Further, when the determination in step S12 is CP ≠ MP, the process proceeds to step S15, and after the output of the current down command signal to the current down unit 116 is canceled,
Proceed to step S16, and target step number CP of the motor 50
And the current step number MP are compared. This judgment is CP> MP. In some cases, the process proceeds to step S17, the excitation phase is switched so that the motor 50 moves by one step in the same phase direction of the turning ratio, and then the current step number MP is updated to MP ← MP + 1 in step S18, and then step S14 described above.
Move on to. 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. MP to MP ← MP-1
After updating to, move to step S14.

Further, FIG. 8 shows a second interrupt routine in which interrupt processing is performed at a constant cycle, and the function as the hysteresis setting unit 103 is fulfilled by this routine. That is, in the second interrupt routine, first, in step S21, the vehicle speed detection unit 102 obtains the current traveling vehicle speed V based on the detection signal PCN of the vehicle speed sensor 101. Next, in step S22, the magnitude of the traveling vehicle speed V and the previous calculated vehicle speed V'obtained and stored in the previous interruption processing is determined, and the current traveling vehicle speed V is calculated as the previous calculated vehicle speed V '.
If it is determined that the vehicle is in an accelerating state or a constant speed state, it is determined that the value of the traveling vehicle speed V is the calculated vehicle speed V ′ in step S23.
Is output as it is, the calculated vehicle speed V'is updated to V '← V, and then the process returns to the main routine.

As a result, when the vehicle is accelerating or traveling at a constant speed, the tempering motor 50 is controlled so that the steering ratio becomes the basic rear wheel steering ratio characteristic shown by the solid line in FIG. Then, the steering angles θR of the rear wheels 2L, 2R are 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 with respect to 1L, 1R, and good turning performance is obtained.
On the contrary, in the high-speed turning state, the rear wheels 2L and 2R are steered so as to be in the same phase, and the traveling stability is improved.

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 low speed state, step S24
Then, the magnitude of the traveling vehicle speed V and a value V'-α obtained by subtracting a predetermined hysteresis width α from the previously calculated vehicle speed V'are determined. 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 set to the previous state. maintain. When the vehicle speed V is further reduced and it is determined in step S24 that the traveling vehicle speed V has become smaller than the value V'-α, the calculated vehicle speed V'is updated to V '← V + a in step S25, and the current vehicle speed V'is updated. The steering angles θR of the rear wheels 2L, 2R are set in accordance with the steering ratio corresponding to the vehicle speed higher than the traveling vehicle speed V by the amount corresponding to the hysteresis width α.

As a result, during deceleration, as shown by the arrow a in FIG. 6, the section corresponding to the hysteresis width α, the front wheels 1L, 1R
After the steering ratios of the rear wheels 2L and 2R for the rear wheels are maintained constant, the basic rear wheel steering ratio characteristics described above are translated to the low speed side as shown by the arrow b. The steering angle θR of the rear wheels 2L, 2R is set according to the steering ratio characteristic. That is, at the same vehicle speed, the rear wheel steering ratio characteristic during deceleration is displaced in the same phase direction as compared with the basic rear wheel steering ratio characteristic during acceleration or constant speed.

In this way, when the vehicle is decelerating, the front wheels 1L, 1R and the rear wheels 2L, 2R are rotated according to the rear wheel steering ratio characteristics during deceleration that are displaced in the same phase direction as compared with the basic rear wheel steering ratio characteristics described above. Since the steering ratio is set, the rear wheels 2L and 2R are prevented from being steered steeply in the opposite phase direction even when the vehicle speed decreases from the high-speed turning state. Therefore, the vehicle is held on the understeer for a predetermined time to improve the running stability, and the occurrence of the tack-in phenomenon and the scooping phenomenon of the vehicle is suppressed.

Further, when the vehicle speed further decreases, the steering ratio changes according to the rear wheel steering ratio characteristic during deceleration set in parallel with the basic rear wheel steering ratio characteristic described above. As a result, the driver can easily understand the change state of the turning ratio. That is, since the basic rear wheel steering ratio characteristic and the rear wheel steering ratio characteristic at the time of deceleration are parallel and the rate of change of the steering ratio becomes constant according to the deceleration, the driver can easily perform this. You can grasp.

When the vehicle shifts from the decelerating state to the accelerating state, as shown by an arrow C in FIG. 6, after a constant steering ratio in a section corresponding to the hysteresis width α is maintained, a basic line indicated by a solid line is shown. The steering ratios of the front and rear wheels are set according to the typical rear wheel steering ratio characteristics.

In the above embodiment, in order to obtain accurate steering characteristics and to control the steering angle of the rear shaft with good responsiveness, a stepping motor whose output shaft rotates according to the number of pulses of the input pulse signal is used. I explained about the open-loop control type four-wheel steering system that I used.
The configuration of the present invention can also be adopted in a closed-loop four-wheel steering device that uses a 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.

(Effects of the Invention) As described above, according to the present invention, the front and rear wheels are steered in accordance with the operation of the steering wheel, and the steering ratio of the front and rear wheels is changed by the rear wheel steering ratio characteristic corresponding to the vehicle speed. In the four-wheel steering system for a vehicle configured as described above, the rear wheel steering ratio characteristic has a predetermined displacement in the same phase direction as the front wheels in the decelerating direction of the vehicle speed as compared to the increasing direction of the vehicle speed. Since a hysteresis setting unit that sets a hysteresis with a width is provided, even when the vehicle decelerates from a high-speed turning state, the rear wheels are prevented from being steered steeply in the opposite phase direction and oversteered. Tuck-in phenomenon of the vehicle is easy and the phenomenon of biting does not occur, traveling stability is further improved, and the steering angle of the rear wheels is set according to the above-mentioned basic rear wheel steering ratio characteristic during acceleration. Therefore, the original characteristics of the four-wheel steering system can be exhibited such that the turning performance of the vehicle can be enhanced when the vehicle turns at a low speed, and the traveling stability of the vehicle can be enhanced when the vehicle turns at a high speed.

Moreover, the rear wheel steering ratio characteristic does not change in accordance with deceleration, etc., and it is possible to establish a certain correspondence between the rear wheel steering ratio characteristic during deceleration and the basic rear wheel steering ratio characteristic. Therefore, there is an effect that the driver can easily grasp this rear wheel turning ratio characteristic.

[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, and FIGS. 7 and 8 are flow charts showing first and second interrupt routines processed by the CPU. 1L, 1R …… front wheel, 2L, 2R …… rear wheel, 3 …… front wheel steering mechanism,
12 …… Rear wheel steering mechanism, 101 …… Vehicle speed sensor, 102 …… Vehicle speed detection unit, 103 …… Hysteresis setting unit, 104 …… Turning ratio setting unit, 105 …… Motor control unit (Rear wheel steering angle control unit ).

Claims (1)

(57) [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 when the vehicle speed is high, the steering ratio of the rear wheels to the front wheels is set in the same phase direction as the vehicle speed increases. A rear-wheel steering angle control unit that controls the rear-wheel steering mechanism based on the rear-wheel steering ratio steering characteristic set to a large value, and the rear-wheel steering ratio steering characteristic includes the rear-wheel steering ratio steering characteristic in the deceleration direction of the vehicle speed in comparison with the increasing direction of the vehicle speed. A four-wheel steering system for a vehicle, comprising: a hysteresis setting unit that sets a hysteresis having a predetermined displacement width such that the characteristics of the above are in the same phase direction as the front wheels.