JPH0796390B2 - 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 according to an operation of a steering wheel, and changes a steering ratio of the front and rear wheels to a steering ratio corresponding 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 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.

By the way, in a vehicle equipped with the above-mentioned four-wheel steering system, when decelerating at the time of turning, the steering wheel is steered in a direction in which the steering ratio decreases, that is, in a direction in which the rear wheels have a reverse phase with respect to the front wheels. 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 the front wheels and the rear wheels are decelerated in the high-speed turning state in which the phases are the same, the rear wheels are kept in the same phase state even if the vehicle shifts to the low-speed state,
If it is not possible to improve the turning performance, and conversely the vehicle accelerates in a low-speed running state in which the front wheels and the rear wheels are in the opposite phase, the rear wheels are held in the opposite phase state even after shifting to the high speed state. Therefore, 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, even when the vehicle is suddenly changed, the steering angle of the rear wheels changes gently, so that the steering ratio according to the vehicle speed cannot be obtained, and the effect of increasing the running stability during acceleration is obtained. However, there is a problem that the turning performance cannot be improved during deceleration.

(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. An object of the present invention is to obtain a four-wheel steering system for a vehicle that can effectively prevent the occurrence.

(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, A rear wheel steering angle control unit that controls the rear wheel steering mechanism based on the rear wheel steering characteristic that is set so that the rear wheel steering angle changes according to the change in vehicle speed, and the vehicle speed deceleration in the rear wheel steering characteristic. The hysteresis setting section that sets the 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 vehicle speed increasing direction, and the hysteresis displacement width in the anti-phase control region And a displacement width changing unit for making the value larger than the displacement width of the same phase control region.

According to the above configuration, during deceleration of the vehicle, the steering ratio characteristic is displaced in the same phase direction according to the output signal from the hysteresis setting unit, and the phase control regions of the front and rear wheels are in the opposite phase control region, or By changing the displacement width of the hysteresis in the displacement width changing part depending on whether it is in the phase control region, it is possible to prevent the rear wheel from suddenly changing from the same phase as the front wheel to the opposite phase, and to reverse the front and rear wheels. The frequency of being in phase will be reduced.

(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 4L, 4
R and tie rods 5L and 5R, and the left and right tie rods 5L and 5
It consists of a relay rod 6 connecting R. Also,
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 connected to the upper end thereof, and a pinion 9 engaging with the rack 8 attached to the lower end thereof.
11, and is configured to steer the left and right front wheels 1L, 1R according to the 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 knuckle arms 13L and 13R and the tie rods 14 are used.
The relay rod 15 connects the 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 a hydraulic pump 24 driven by an engine (not shown) is arranged in the oil supply pipe 22. It is set up. The control valve 20 is of a known spool valve type, and is a cylindrical valve casing 20a integrally attached to the relay rod 15 via a connecting member 25.
And a spool valve (not shown) fitted in the valve casing 20a, and supplies pressure oil from the hydraulic pump 24 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, 17d for urging the relay rod 15 to a neutral position (position where the steering angle θR of the rear wheels 2L, 2R is 0) are mounted.

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

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 is connected to a spool valve of the control valve 20. 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.
Reference numeral 31 is rotatably supported by a casing (not shown) fixed to the vehicle body 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.

Also, a ball joint is attached to the tip of the swing arm 33.
One end of the connecting rod 37 is connected via 36,
The other end of the connecting rod 37 is connected to the other end of the control rod 30 via a ball joint 38, and the control rod 30 is moved in the vehicle width direction according to the displacement of the tip of the swing arm 33 in the vehicle width direction. It is designed to be displaced.

The connecting rod 37 is the ball joint.
At a portion near 36, the rotation imparting arm 40 is slidably supported via a ball joint 41. 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 around the moving axis by an amount corresponding to the rotation angle of the steering wheel 10, and the swinging arm 33 swings around the support pin 32 accordingly. When moved, when the axis of the support pin 32 coincides 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, the swing arm around the axis of the pin 32
Even if the swing angles of 33 are the same, the displacement of the control rod 30 in the left-right direction changes with the change of the tilt 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. A bevel gear 48 is attached to the rotating shaft 46, and the bevel gear 48 has an output shaft of a stepping motor 50.
Bevel gear 49 mounted on 50a is meshed, and by operating stepping motor 50 to rotate sector gear 45, the inclination angle of holder 31 with respect to the reference plane is changed to change the steering angle of rear wheels 2L, 2R. When θR is controlled and the sector gear 45 is rotated in the clockwise direction when viewed from above the vehicle body from the neutral position where the center line is perpendicular to the center line of the rotation shaft 46 of the worm gear 47, the turning ratio is The wheels 2L and 2R are configured to be controlled in the same phase so as to face the same direction as the front wheels 1L and 1R.

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,
When the sector gear 45 rotates to the opposite phase side and the rotation angle from the neutral position becomes, for example, −17.5 °, the sector gear 45 contacts the opposite phase side stopper member 51 and further rotation is restricted, Further, when the sector gear 45 is rotated 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 restrict the movement. ing. 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. 39 is the rear wheel steering mechanism 12
Is a rod stopper that restricts the maximum movement range of the relay rod 1 in FIG.

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 ratios of the front and rear wheels are changed from the basic steering ratio characteristic to the in-phase direction as described later. The hysteresis setting unit 102 for obtaining the calculated vehicle speed V'to obtain the displaced hysteresis and the phase control region of the front-rear wheel turning ratio are determined, and the calculated vehicle speed V'at deceleration is provided accordingly. A displacement width setting unit 103 that sets the hysteresis displacement width α to a predetermined value, and a steering ratio setting unit 104 that sets the steering ratio of the rear wheels 2L, 2R to the front wheels 1L, 1R according to the calculated speed V ′. From this steering ratio setting unit 104 Output signal R
And a motor control unit 105 as a rear wheel steering angle control unit that outputs a control signal to the stepping motor 50 according to the above.

The control unit 100 operates as a system power source, which is 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. Then, the control unit 100 becomes 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.

The output signal of the vehicle speed sensor 101a is an interface signal.
After being input to the integration filter 113 via 112, chattering is removed by the integration filter 113, the waveform shaping circuit 114
The signal waveform is shaped by and is 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 50.
Output current from the battery power source to the motor 50 is not controlled (when the rotation of the motor output shaft 50a is stopped)
In addition, the current down section 116 that limits each phase to, for example, 100 mA
have.

Next, a signal processing procedure performed by the CPU 106 of the control unit 100 will be described. FIG. 5 shows the main routine of the signal processing program. By this routine, the function as the turning ratio setting unit 104 is fulfilled. After starting by turning on the ignition key switch, first initialize the system in step S ₁ .
In the next step S ₂ , the current step number MP of the stepping motor 50 is set to 580 and the target step number CP thereof is set to 0, and the flag F ₁ indicating execution of the motor position initialization control mode is set to F ₁ = 1. Set to. The target step number CP is the control initial position of the stepping motor 50, that is, the position where the sector gear 45 contacts the antiphase stopper member 51 and the steering ratio is the maximum antiphase steering ratio.
0, from which the number of steps of the pulse signal input to the motor 50 when controlling the motor 50 to its target control position is shown, and the current number of steps MP is the motor 50.
It shows the number of steps of the current control position from the control initial position. The flag F ₁ is set to F ₁ = 1 in the motor position initialization control mode in which the motor 50 is controlled to initialize its control position, but the steering ratio is controlled according to the vehicle speed. In the vehicle speed sensitive control mode, it is reset to F ₁ = 0.

Then, the procedure proceeds to step S _3, the flag F ₁ makes a determination of whether F ₁ = 1. When this determination is F ₁ = 1 (that is, when the position initialization control mode of the motor 50 is performed), the process proceeds to step S ₄ and the target step number CP for the motor 50 is set.
Is equal to the current step number MP, and when this determination is CP ≠ MP, the process directly returns to step S ₃ . Further, when the determination control position initializing the motor 50 in the CP = MP is completed, the process proceeds to step S _5, the target step number CP and the current step number MP of the motor 50 to CP = MP = 0,
Then, after resetting the flag F ₁ to F ₁ = 0 and setting the flag F ₂ for identifying that the control position initialization of the motor 50 has been completed once to F ₂ = 1, Back to S _3.

On the other hand, it is determined in step S ₃ that F ₁ = 1 is not satisfied,
When controlling the motor 50 for changing the steering ratio, it determines whether or not the travel speed V detected by the vehicle speed detection section 101b is in the 0 (stopped state) the process proceeds to step S _6, YES is the determination when the further the flag F ₂ in step S ₇ determines whether F ₂ = 0. Then, it returns to the step S _3, control position initialization of the motor 50 at the time of stop of the F ₂ = 0 and the determination has been traveling speed V is 0 when the determination in step S ₇ is F ₂ ≠ 0 If it is confirmed that the step F is not executed, the flag F ₁ is set to F ₁ = 1 in step S ₈ , and the current step number MP and the target step number CP of the motor 50 are set in the next step S _9. After setting the MP = 580, CP = 0 corresponding to the control the initial position and the process returns to step S _3.

Further, the step when the in step S _6, the travel speed V is the vehicle not 0 is determined to be running state
Proceeds to S _10, control data table showing the basic steering ratio characteristic that the second calculation speed V 'obtained based on the running speed V in interrupt routine, corresponding to vehicle speed stored in advance in ROM107 below Then, the value f (V ') of the target step number CP of the motor 50 is read out and the control signal R is output, and then both flags F ₁ and F ₂ are set to F ₁ = F in step S _11. After reset ₂ = 0 and the process returns to step S _3. The control data table of the basic turning ratio characteristics stored in the ROM 107, as shown by the solid line in FIG. 6, shows the turning ratio of the front and rear wheels 1L, 2L (1R, 2R) according to the vehicle speed. When the vehicle speed changes and the vehicle speed is low, the rear wheels 2L, 2R are steered in the opposite direction to the front wheels 1L, 1R, that is, in the opposite phase, in order to improve the turning performance of the vehicle. On the other hand, when the vehicle speed reaches about 67 km / hour, the steering ratio becomes zero,
The steering angle θR of the rear wheels 2L, 2R is θR regardless of the turning of the front wheels 1L, 1R
= 0 is maintained and the vehicle enters the normal two-wheel steering state. When the vehicle speed is higher, the rear wheels 2L, 2R are rotated in the same direction as the front wheels 1L, 1R, i.e., in the same phase, in order to improve the grip of the rear wheels 2L, 2R during cornering and improve running stability. The steering ratio is set so that the steering ratio is positive.

Further, FIG. 7 shows a first interrupt routine which is interrupt-processed with respect to the main routine when the time set in the timer incorporated in the CPU 106 has elapsed,
By this routine, the function as the motor control unit 105 is fulfilled. In the first interrupt routine, the target step number CP of the motor 50 at a first step S ₁₂ determines whether the current is equal to the 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 S ₁₃ and the current down command signal is output to the current down unit 116. it allows suppressing the heat value by lowering the voltage applied to the motor 50, and then returns to steps after interruption of the main routine after setting the timer for generating the next interrupt process in step S _14.

Further, when the decision in step S ₁₂ is CP ≠ MP, after releasing the output of the current down command signal for the current down unit 1169 proceeds to step S _15,
The process proceeds to step S _16, the target step number CP of the motor 50
And the current step number MP are compared. If this determination is CP> MP, the routine proceeds to step S ₁₇ and the motor 5
0 switching the excitation phase to move by one step in the same phase direction of the steering ratio, and then proceeds to step S ₁₄ after updating the current step number MP to MP ← MP + 1 in step S _18. On the other hand, when the decision in step S ₁₆ is CP <MP is switched the excitation phase as the motor 50 moves one step in the opposite phase direction of the steering ratio proceeds to step S _19, in step S ₂₀ currently it proceeds to step number MP to step S ₁₄ after updating the MP ← MP-1.

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 102 and the displacement width setting unit 103 is fulfilled by this routine. That is, the detection signal P of this second interrupt routine, the vehicle speed sensor 101a at step S ₂₁
Based on CN, the vehicle speed detector 101b obtains the current traveling vehicle speed V. Then, the running speed V at step S ₂₂
And the magnitude of the previous calculated vehicle speed V ′ obtained and stored in the previous interrupt processing is determined, and it is determined that the current traveling vehicle speed V is greater than or equal to the previous calculated vehicle speed V ′. there when it is in an acceleration state or constant speed state is confirmed, the value of the running speed V operation speed V in step S ₂₃ 'as it is output as, calculating the vehicle speed V'
Is updated to V '← V, and then the main routine is returned to.

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 steering ratio characteristic 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.

Further, in step S _22, if the current driving speed V is determined to be smaller than the previous calculation speed V ', the vehicle is in a deceleration state is confirmed, step S ₂₄
Then, it is determined whether or not the current front and rear wheels are in the same phase control region based on the previous control signal R output from the turning ratio setting unit 104. And if that is the front and rear wheels in the same phase control region is confirmed, the displacement width of the hysteresis of the phase control region which is stored in the ROM107 in step S ₂₅ alpha ₁
After reading, the value α ₁ is set as the displacement width α output as a control signal in step S ₂₆ .

In step S _24, the front and rear wheels when it has been confirmed that the reverse phase control region, in step S ₂₇ of the hysteresis of the reverse phase control region which is stored in the ROM107 as a larger value than the displacement range alpha ₁ After reading the displacement width α ₂ ,
In step S ₂₈ , this value α ₂ is set as the displacement width α output as a control signal. Thereafter, it is determined in step S _29, and the current running speed V, and the magnitude of the value V'-alpha minus displacement width alpha of hysteresis is outputted as a control signal from the previous calculation speed V '. 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. And the deceleration goes on,
When the traveling speed V is determined to become smaller than the value V'-α, 'the V' operation the vehicle speed V is updated to ← V + alpha in step S _29, the displacement width of the hysteresis than the current traveling speed V The steering angle θR of the rear wheels 2L, 2R is set in accordance with the steering ratio corresponding to the vehicle speed that is higher by α.

As a result, when the vehicle is decelerated when the front and rear wheels are in the in-phase control region, the hysteresis displacement range α _{1 in} the in-phase control region is as shown by the arrow a in FIG.
After the steering ratio of the rear wheels 2L, 2R with respect to the front wheels 1L, 1R is maintained constant in the section corresponding to, the basic steering ratio characteristics are reduced by the displacement width α ₁ as indicated by arrow b. The rear wheels 2L, 2 according to the steering ratio characteristics during deceleration indicated by the broken line
The steering angle θR of R is set. And the steering ratio of the front and rear wheels is 0
Then, at the time of entering the anti-phase control region, as shown by arrow C, the section corresponding to the difference between the displacement widths α ₂ and α ₁ , the rear wheel
After the rudder angles of 2L and 2R are maintained at 0, as shown by arrow D, the basic turning ratio characteristic is decelerated by parallel translation to the low speed side by the displacement width α ₂ of the antiphase control region. The steering angle θR of the rear wheels 2L, 2R is set according to the turning ratio characteristic.
That is, at the same vehicle speed, the steering ratio characteristic during deceleration is displaced in the same phase direction as compared with the basic steering ratio characteristic described above, and the displacement width is different depending on the phase control region of the front and rear wheels. The displacement width changing unit composed of the displacement setting unit 103 is changed so as to have a larger value when the front and rear wheels are in the opposite phase region than in the same phase region.

When the vehicle shifts from the decelerating state to the accelerating state, as shown by arrows e and f in FIG. 6, a constant turning ratio is maintained in a section corresponding to the hysteresis displacement width α ₁ or α _2. After this is maintained, the steering ratios of the front and rear wheels are controlled according to the basic steering ratio characteristics shown by the solid line.

Moreover, when the front and rear wheels are in the anti-phase control region, the steering ratio characteristic during deceleration is configured to be displaced in the same phase direction more than in the in-phase control region. It is possible to effectively prevent the occurrence of the tuck-in phenomenon of the vehicle during the deceleration and the occurrence of the pushing-in phenomenon. On the contrary, when the front and rear wheels are in the same phase control region, the displacement amount of the steering ratio characteristic is smaller than that in the opposite phase control region. It is possible to prevent the vehicle from being held in the understeer state for a long period of time, prevent the turning ability of the vehicle from being greatly impaired when decelerating from the high-speed turning state, and obtain good driving operability.

In the same phase control region, it is not always necessary to displace the turning ratio characteristic to have hysteresis,
The displacement width α ₁ may be set to 0, and the turning ratio characteristic may be displaced in the same phase direction only when the front and rear wheels are in the antiphase control region.

In addition, in the above embodiment, in order to obtain accurate steering characteristics and to control the steering angle of the rear shaft 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.

(Effects of the Invention) As described above, according to the present invention, the front and rear wheels are steered according to the operation of the steering wheel, and the steering ratio of the front and rear wheels is changed with the steering ratio characteristic corresponding to the vehicle speed. In the configured four-wheel steering system for a vehicle, the rear wheel steering characteristic is provided with a predetermined displacement width so that the characteristic is 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 the hysteresis setting unit for setting the hysteresis having and the displacement width changing unit for setting the displacement width of the hysteresis in the antiphase control region to a value larger than the displacement width of the same phase control region are provided, the front and rear wheels are decelerated during deceleration of the vehicle. The frequency of setting to the opposite phase is reduced, the running stability is improved, it is possible to effectively prevent the tuck-in phenomenon of the vehicle and the entry phenomenon, and it is possible to reduce the vehicle from the high-speed turning state. When the speed is increased, the front and rear wheels are prevented from being held in the understeer state for a long time, and the turning ability of the vehicle can be prevented from deteriorating.

Furthermore, since the steering angle of the rear wheels is set in accordance with the above basic turning ratio characteristics during acceleration, etc., the turning performance of the vehicle can be improved during low-speed turning, and the traveling stability of the vehicle during high-speed turning can be improved. The original characteristic of the four-wheel steering vehicle that enhances the property can be exhibited.

[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, 101b …… Vehicle speed sensor, 101b …… Vehicle speed detection unit, 102 …… Hysteresis setting unit, 103 …… Displacement width setting unit (displacement width changing unit), 104 …… Turning ratio setting unit , 105 ...
... Motor control unit (rear wheel steering angle control unit).

Claims (1)

[Claims] 1. A four-wheel steering system for a vehicle having a front-wheel steering mechanism for steering the front wheels and a rear-wheel steering mechanism for steering the rear wheels. The rear-wheel steering angle control unit that controls the rear-wheel steering mechanism based on the rear-wheel steering characteristics that are set so that the rear-wheel steering angle changes, and the rear-wheel steering characteristics include the vehicle speed in the deceleration direction of the vehicle speed. 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 compared to the increasing direction, and the hysteresis displacement width in the antiphase control region is the same phase control region. A four-wheel steering system for a vehicle, comprising: a displacement width changing unit that makes the displacement width larger than the displacement width.