JPH0796391B2 - 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 in opposite phases during normal low-speed turning to set the steering characteristics to oversteer, thereby turning the vehicle during high-speed turning. To improve the running stability of the vehicle by controlling the steering ratio to the same phase and setting the steering characteristic to understeer, the rear wheel with respect to the front wheel based on the predetermined steering ratio characteristic set in advance according to the vehicle speed. It is configured to change the steering angle.

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 in which the vehicle body suddenly faces inward and a scooping phenomenon in which the vehicle body spins may occur. Especially, this tendency is remarkable on a snowy road or an icy road where the friction coefficient between the road surface on which the vehicle travels and the wheels is small. 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 depending on the vehicle speed only when the vehicle is in a substantially straight traveling state. And the steering angle of the rear wheels is kept constant when the vehicle is in a turning state.
As shown in Japanese Patent Laid-Open No. 60-85067, when the vehicle speed changes rapidly, a delay circuit or the like is used to change the steering angle of the rear wheels with a predetermined delay.

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, 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. There is a problem that is insufficient.

Moreover, in both the former and latter cases, since no consideration is given to the magnitude of the frictional resistance generated between the wheel and the road surface, it is not possible to perform appropriate control according to the road surface condition.

(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, and the vehicle speed sharply decreases during turning regardless of the road surface condition. An object of the present invention is to obtain a four-wheel steering system for a vehicle that can easily prevent a tack-in phenomenon from occurring in the vehicle body even in such a case.

(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, Friction coefficient detection means for detecting the magnitude of the friction coefficient between the road surface and the wheels, and the rear wheel steering mechanism is controlled based on the rear wheel steering characteristics set so that the rear wheel steering angle changes in accordance with the change in vehicle speed. The rear wheel steering angle control unit and the rear wheel steering characteristic have a predetermined displacement width so that the characteristics are in the same phase direction as the front wheels in the deceleration direction of the vehicle speed as compared to the increasing direction of the vehicle speed. A hysteresis setting unit for setting a hysteresis and a displacement width changing unit for making the displacement width of the hysteresis smaller as the friction coefficient detected by the friction coefficient detecting means is larger are provided.

According to the above configuration, during deceleration of the vehicle, the steering ratio characteristics are displaced in the same phase direction in accordance with the output signal from the hysteresis setting unit, and this displacement width is the friction coefficient between the road surface and the wheels in the displacement width changing unit. In the road surface condition in which the friction coefficient is small and the tuck-in phenomenon of the vehicle is likely to occur, the rear wheels are prevented from abruptly changing from the same phase as the front wheels to the opposite phase.

(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 11 connected to the upper end thereof, and a pinion 9 engaging with the rack 8 attached to the lower end thereof.
And the left and right front wheels 1L, 1R are steered in accordance with 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 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. If 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 remains stationary. However, if the axis line of the pin 32 tilts 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 33 swings around the pin 32. Due to the movement, the ball joint 36 is displaced in the vehicle width direction, 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, and the control rod 30 moves. And it is configured to actuate the spool valve of the roll valve 20. That is, the swing arm 33 about the axis of the pin 32
Even if the rocking angle is the same, the displacement of the control rod 30 in the left-right direction changes as the tilt angle of the pin 32, that is, the rotation angle of the holder 31 changes.

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 15 in.

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 deceleration state. When the vehicle is in the deceleration state, the steering ratios of the front and rear wheels are in the same phase direction from the basic steering ratio characteristic as described later. The friction coefficient μ between the road surface and the wheel is determined based on the detection signal from the hysteresis setting unit 102 that obtains the calculated vehicle speed V ′ so that the displaced hysteresis is obtained, and the slip sensor 51a that measures the slip ratio of the wheel. A friction coefficient detecting section 51b for detecting, a displacement width calculating section 103 for calculating a displacement width α of hysteresis given to the calculated vehicle speed V ′ at the time of deceleration according to the friction coefficient μ, and the calculating speed V ′. Front wheels 1L, 1R according to
Steering ratio setting unit 104 for setting the steering ratio of the rear wheels 2L, 2R for
And a control signal R output from the turning ratio setting unit 104
And a motor control unit 105 as a rear wheel steering angle control unit that controls 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.

Further, the output signals of the vehicle speed sensor 101a and the slip sensor 51a are input to an integration filter 113 via an interface 112, chattering is removed by the integration filter 113, and then a signal waveform is shaped by a waveform shaping circuit 114 so that the CPU
Supplied to 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 a main routine of a signal processing program, and this function serves as the steering ratio setting unit 104. 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 is in contact with the anti-phase side stopper member 51 and the steering ratio is maximum anti-phase side steer CP =
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, when it is determined in step S ₃ that F ₁ = 1 is not satisfied and the motor 50 is controlled to change the steering ratio,
Proceed to Step S ₆ determines whether the traveling speed V detected by the vehicle speed detection section 101b is in the 0 (stopped state), when the determination is YES, further the flag F ₂ is in the step S ₇ F It is determined whether or not 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 the if it is not running is confirmed, the flag F ₁ is set to F ₁ = 1 at step S _8, the motor in the next step S ₉
Back current step number MP and the target step number CP 50 in step S ₃ after setting the MP = 580, CP = 0 corresponding to the control initial position.

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 After reading the value f (V ′) of the target step number CP of the motor 50, the both flags F ₁ and F ₂ are reset to F ₁ = F ₂ = 0 in the next step S ₁₁ . later returns to the step S _3. The control data table of the basic turning ratio characteristics stored in the ROM 107 is as shown in the solid line in FIG.
When the turning ratio of 1L, 2L (1R, 2R) changes and the vehicle speed is low, the rear wheels 2L, 2R move 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 becomes negative while the steering ratio becomes negative, while when the vehicle speed reaches, for example, about 67 km / hour, the steering ratio becomes zero, and the rear wheels 2L regardless of the steering of the front wheels 1L, 1R. The steering angle θR of 2R 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, 2R are connected to the front wheels 1L and 2R in order to improve the grip of the rear wheels 2L, 2R during cornering and improve running stability.
The steering ratio is set to be positive by steering in the same direction as L and 1R, that is, in the same phase.

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 Meirurechin 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 116 proceeds to step S _15, the process proceeds to step S _16, the motor 50 The magnitude relationship between the target step number CP and the current step number MP is determined. This judgment
If CP> MP, the process proceeds to step S ₁₇ and the motor 50
Switches its excitation phase so that it moves by one step in the same phase direction of the turning ratio, then updates the current number of steps MP to MP ← MP + 1 in step S ₁₈ , and then moves to step S ₁₄ . 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 which is interrupted at a constant cycle, and this routine fulfills the functions of the hysteresis setting section 102 and the displacement width calculating section 103. 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, current travel speed V in step S ₂₂ is determined to be smaller than the previous calculation speed V ', when the vehicle it was confirmed that the deceleration state, the detection of the slip sensor 51a at step S ₂₄ Friction coefficient detector 5 based on signal
In 1b, find the friction coefficient μ between the road surface and the wheels, and
In S ₂₅ , the hysteresis displacement range α according to this friction coefficient μ
Is calculated. That is, the displacement width α of this hysteresis is set as a linear function h (μ) of the friction coefficient μ as shown in FIG. 9, and the hysteresis corresponding to the friction coefficient μ is based on this linear function h (μ). Calculate the displacement width of.

As a result, as shown by the broken line in FIG. 6, the steering ratio characteristics during deceleration of the vehicle are compared with the basic steering ratio characteristics by the hysteresis displacement width α calculated according to the friction coefficient μ. As the friction coefficient μ decreases, the displacement range of the turning ratio characteristic corresponding to the friction coefficient μ increases.

Next, in step S _26, determines the current driving speed V, and the magnitude of the value V'-alpha minus displacement width alpha of the hysteresis 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. Then, the process proceeds further decelerated, when the travel speed V is determined to become smaller than the value V'-alpha, 'a V' operation the vehicle speed V is updated to ← V + alpha in step S _27, the current travel 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 higher by the amount corresponding to the hysteresis displacement width α.

As a result, when the vehicle decelerates, as shown by the arrow a in FIG. 6, the section corresponding to the displacement width α of the hysteresis obtained according to the friction coefficient μ, the steering of the rear wheels 2L, 2R with respect to the front wheels 1L, 1R After the ratio is maintained constant, as shown by an arrow b, the basic steering characteristic is rearranged according to one of the steering ratio characteristics at the time of deceleration indicated by the broken line which is translated to the low speed side. The steering angle θR of the wheels 2L, 2R is set. 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, and the displacement amount increases as the friction coefficient μ decreases.

In this way, when decelerating the vehicle, the steering ratios of the front wheels 1L, 1R and the rear wheels 2L, 2R are changed according to the steering ratio characteristics during deceleration displaced in the same phase direction as compared with the basic steering ratio characteristics described above. Since it is set, even when the vehicle speed decreases from the high speed turning state,
The rear wheels 2L, 2R are prevented from being steered steeply in the opposite phase direction. Therefore, the vehicle is kept in the understeer state for a predetermined time when decelerating, and the running stability is improved.

Further, in a road surface condition where the friction coefficient μ is small and slip is likely to occur, the change width α of the hysteresis is calculated by the displacement width changing unit so that the change width α of the hysteresis is larger than that in the normal road condition. The value is changed, and the wheels are kept in the understeer state for a long time, so that the running stability is further improved, and the occurrence of a vehicle tack-in phenomenon or the like can be effectively prevented. Moreover, since the turning ratio characteristic during deceleration is made parallel to the basic turning ratio characteristic, the rate of change of the turning ratio in each characteristic becomes constant according to the amount of deceleration, and the driver changes this. There is an advantage that it can be grasped easily.

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

It should be noted that, as the friction coefficient μ is smaller, for example, the displacement width α is set as a quadratic function g (μ) of the friction coefficient μ as the control data for calculating the displacement width α, as shown in FIG. Various functions can be used as long as α is large. Further, as the means for detecting the friction coefficient μ, a means for detecting unevenness of the road surface by a vehicle height sensor, a temperature sensor, a means for determining the freezing or raining condition of the road surface by a wiper operation detecting means or the like may be used. .

In the above embodiment, in order to obtain accurate steering characteristics and to make it possible to variably control the steering angle of the rear shaft with good response, the stepping in which the output shaft rotates according to the number of pulses of the input pulse signal Although the open-loop control type four-wheel steering device using a motor has been described, the configuration of the present invention can also be applied to a closed-loop type four-wheel steering device 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 a hysteresis setting unit that sets the hysteresis that has been set and a displacement width changing unit that sets the displacement width of the hysteresis to a smaller value as the friction coefficient between the road surface and the wheel is larger, when decelerating in the turning state of the vehicle The smaller the coefficient of friction, the more the steering ratio is kept constant for a long time, and then the steering ratio changes according to the steering ratio characteristics during deceleration. Wheels It is possible to prevent the steered vehicle 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 apt to occur, and the sticking-in phenomenon does not occur, so that the running stability is further improved. Moreover, since the steering angle of the rear wheels is set in accordance with the above basic turning ratio characteristic when increasing the speed, the turning performance of the vehicle can be enhanced during low-speed turning, and the vehicle travels during high-speed turning. The original characteristic of a four-wheel steering vehicle that enhances stability 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, FIGS. 7 and 8 are flow charts showing first and second interrupt routines processed by the CPU, and FIGS. 9 and 10 are displacement widths, respectively. It is a characteristic view which shows the relationship with a friction coefficient. 1L, 1R …… front wheel, 2L, 2R …… rear wheel, 3 …… front wheel steering mechanism,
12 ... Rear wheel steering mechanism, 101 ... Vehicle speed sensor, 101b ... Vehicle speed detection unit, 102 ... Hysteresis setting unit, 103 ... Displacement width calculation unit (displacement width changing unit), 104 ... Turning ratio setting unit , 105 ...
... Motor control unit (rear wheel steering angle control unit).

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location B62D 113: 00

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, a vehicle speed detecting means for detecting a vehicle speed, a road surface and wheels. Friction coefficient detecting means for detecting the magnitude of the friction coefficient of the vehicle, and the rear wheel steering mechanism for controlling the rear wheel steering mechanism based on the rear wheel steering characteristic set so that the rear wheel steering angle changes according to the change in vehicle speed. The angle control unit and the rear wheel steering characteristic are set with hysteresis having 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. The four wheels of a vehicle comprising a hysteresis setting unit and a displacement width changing unit that reduces the displacement width of the hysteresis in accordance with a larger friction coefficient detected by the friction coefficient detecting means. Steering device.