JPH01103581A - Control method of four-wheel steering device for automobile - Google Patents

Abstract

PURPOSE:To improve the advantages of high-speed running stability much more by providing a rear wheel slip detecting means in a control unit, and controlling a rear wheel steering angle in the direction of increasingly strengthening an under-steering tendency when a rear wheel slip occurs. CONSTITUTION:A rear wheel slip detecting unit detects the rotating speeds of each wheel based on wheel-speed signals inputted from each car-speed sensors 15L, 15R and 16L, 16R, and picks out the lowest rotating speed out of the rotating speeds of each wheel detected at a state where the brake is not applied. Then, the rotating speeds of rear two wheels are compared with the value obtained by multiplying the above lowest rotating speed by a constant, and when at least either of the rotating speeds of the rear two wheels is greater, it is judged that a rear wheel slip occurs. And at this time, the amount of steering of the rear wheels 13 in the direction of under-steering (in-phase steering gear change direction) is allowed to be increased much more than usual, thus an inward yaw-moment in a vehicle generated due to the rear wheel slip is eliminated, and a spin of the vehicle can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method of controlling a four-wheel steering system for an automobile.

Conventional Technology Various types of four-wheel steering devices for automobiles have been developed in the past, which steer the rear wheels in accordance with the steering of the front wheels. .

Conventionally, the four-wheel steering device described above generally determines the rear wheel steering angle (including the steering direction) from the front wheel steering angle detected by the steering wheel operation angle and the vehicle speed detected by the vehicle speed sensor. In the vehicle speed range, the rear wheels are steered in the opposite direction to the front wheels (reverse phase steering) to improve turning performance, while in the high vehicle speed range, the rear wheels are steered in the same direction as the front wheels (in-phase steering) and the vehicle speed is As increases, the ratio θR/θ of the rear wheel steering angle OR to the front wheel steering angle θF increases.
Normally, control is performed to gradually increase F in order to improve stability during medium and high speed driving.

The front wheel steering force is used instead of the front wheel steering angle, which is one of the parameters for determining the rear wheel steering angle, and the setting characteristic of the ratio θR/F of the rear wheel steering angle θR to the steering force F is determined from the front wheel steering force and the vehicle speed. A method of controlling the rear wheel steering angle based on this method has also been developed in recent years.

Problems to be Solved by the Invention As mentioned above, conventional four-wheel steering vehicles aim to increase the tendency of understeer and improve stability by steering the rear wheels in the same phase when steering at medium to high vehicle speeds. However, if the in-phase steering amount of the rear wheels is too large, the turning performance of the vehicle will deteriorate, so by changing θR/θF or θR/F according to the vehicle speed at that time, as described above, the turning performance can be reduced. The understeer tendency is controlled to be increased within a range that does not significantly worsen the understeer tendency.

However, in a four-wheel drive vehicle or a rear-wheel drive vehicle, if rear wheel slip occurs due to excessive rear wheel drive torque, the rear wheel lateral force decreases and an inward yaw moment is generated in the vehicle, causing the above initial setting θR/θF to change. The amount of in-phase steering of the rear wheels determined in this manner will not be sufficient, and the vehicle will be in a state where it is likely to spin.

The present invention aims to address the above-mentioned problems.

Means for Solving the Problems The present invention provides a rear wheel steering system that uses the front wheel steering angle or steering force as one of the parameters for determining the rear wheel steering angle as described above. detects the rotational speed of the front wheels, and when the rotational speed of at least one rear wheel is greater than the rotational speed of the other wheel, which is the slowest, it is determined that slipping has occurred in the rear wheels, and the rear wheel steering This is characterized by performing control to increase the angle in a direction that strengthens the understeer tendency.

As described above, when rear wheel slip occurs due to excessive rear wheel drive torque, it is reliably detected and the amount of rear wheel steering in the understeer direction is increased, thereby suppressing a decrease in rear wheel lateral force. This eliminates the inward yaw moment that occurs in the vehicle and prevents the vehicle from spinning.

Example Embodiments of the present invention will be described below with reference to the accompanying drawings.

In FIG. 1, 1 is a steering wheel, and 2 is a front wheel.A known front wheel steering system from the steering wheel 1 to the front wheels 2 includes a control valve 3 that operates according to the steering torque of the steering wheel 1, and a control valve 3 that operates according to the steering torque of the steering wheel 1. A known hydraulic power steering device is provided which includes a power cylinder 5 having left and right oil chambers into which hydraulic pressure discharged from a hydraulic pump 4 is introduced to either the left or right side when activated.

Reference numerals 6a and 6b are left and right oil pressure sensors that detect the oil pressure in the left and right oil chambers of the power cylinder 5, and the oil pressure sensors 6a,
The left and right oil pressure signals detected by the left and right oil pressure signals 6b are input to an operational amplifier 7, which calculates the difference between the left and right oil pressures, and inputs a signal corresponding to the oil pressure difference to the control unit 9.

In the above, since the hydraulic power steering device generates a hydraulic auxiliary force according to the steering torque of the steering wheel l, the above-mentioned hydraulic pressure difference corresponds to the steering torque of the steering wheel l, and therefore, the difference in hydraulic pressure corresponds to the steering torque of the steering wheel l. The signal input to 9 is a steering force signal corresponding to the front wheel steering force. 8 is a vehicle speed sensor that emits a vehicle speed signal corresponding to the vehicle speed.
The vehicle speed signal is also input to the control unit 9.

The control unit 9 determines a rear wheel steering angle target value from the above-inputted steering force signal and vehicle speed signal based on a preset vehicle speed-steering force-rear wheel steering angle characteristic, and based on that, controls the motor, etc. An output signal is issued to the rear wheel steering actuator lO, and the rear wheel 13 is steered via the rear wheel steering mechanism 12 consisting of an electromagnetic clutch 119, for example, a link mechanism, and the rear wheel is a rear wheel that detects the rear wheel steering angle. The vehicle is steered to the target narrow road steering angle by feedback control based on the rear wheel steering angle signal from the steering angle sensor 14.

15L, f5R and 16L, 16R are wheel speed sensors that respectively detect the rotational speed of the left and right front wheels and the left and right rear wheels, and wheel speed signals ωFL of these wheel speed sensors,
ωFR and ωRL+ωRR are control unit 9
is input.

In FIG. 1, 17 is a motor rotation speed sensor.
When a motor is used as the rear wheel steering actuator 10 as shown in the figure, the motor speed is controlled by the signal from the motor rotation speed sensor 17.

The control unit 9 is provided with a rear wheel slip detection section for detecting rear wheel slip using a method as shown in FIG.

That is, the rear wheel slip detection section includes front and rear wheel speed sensors 15L, 15R, and 16L.

When detecting the rotational speed of the wheels based on the input of the wheel speed signal of 16R, first check whether the brake is applied using a brake switch, etc. Among the rotational speeds, the lowest rotational speed ω0 is extracted.

Next, compare the rotational speeds ωRL and ωRR of the rear two wheels with the value ωO-, which is the sum of the above-mentioned lowest rotational speed ω0 and a constant K (for example, set to about 1.1), and find that ωRL≧
If ω0・K or ωRR≧ω0・K, it is assumed that the rear wheels are slipping, and the steering force-rear wheel steering angle setting characteristic at the vehicle speed at that time is determined based on the characteristics as shown by the solid line (a) in Fig. 3, for example. Assuming that the rear wheel steering angle target value is determined using
By determining the rear wheel steering angle target value based on the characteristic of b), the amount of steering of the rear wheels 13 in the understeer direction (in-phase steering direction) is greatly increased compared to normal conditions, thereby reducing the amount of vehicle steering caused by rear wheel slip. Eliminates inward yaw moment and prevents the vehicle from spinning.

If both the left and right rear wheel rotational speeds are not greater than ωO-, it is determined that there is no rear wheel slip, and for example, the solid line water (
By determining the rear wheel steering angle target value based on the set characteristic in a), a suitable rear wheel steering angle in the understeer direction can be provided at the current vehicle speed.

The above embodiment shows an example in which rear wheel slip is detected based on the rotational speed of four wheels, front, rear, left, and right, but rear wheel slip may also be detected based on the rotational speed of three wheels, two rear wheels and one front wheel. In this case as well, the value ωO- obtained by multiplying the lowest speed ω0 of the three rotational speeds by a constant K is compared with the rotational speed of the rear two wheels, and either one of the two rear wheels or If both rotational speeds are ω0·K or higher, it is determined that the rear wheels are slipping.

In the case of a vehicle equipped with a hydraulic power steering device as shown in the embodiment in Fig. 1, the easiest way to detect the steering force is from the oil pressure of the power cylinder 5 as shown in the figure, but other methods can also be used to detect the steering force from the rotation of the electric motor. If the vehicle is equipped with an electric power steering device to provide assistance, the steering force can be detected by the motor torque determined from the current value flowing through the electric motor, and if the vehicle is not equipped with a power steering device, the steering force can be detected by the torque applied to the steering shaft, for example. Alternatively, the steering force may be detected from the front wheel cornering force, etc. Furthermore, the front wheel steering angle may be detected instead of the steering force, and the rear wheel steering angle target value may be determined from the front wheel steering angle and the vehicle speed. The present invention is also applicable to a conventional rear wheel steering device that is configured to do so.

Further, although the embodiment shown in FIG. 1 shows an example in which the present invention is applied to a four-wheel drive vehicle, the present invention can also be applied to a rear-wheel drive vehicle in which only the rear two wheels are drive wheels, and the actuator 10 ．． The rear wheel steering mechanism 12 and the like are not limited to the illustrated embodiment, and any conventionally known mechanism can be employed.

Effects of the Invention As described above, according to the present invention, when rear wheel slip occurs due to excessive rear wheel drive torque, the reduction in rear wheel lateral force accompanying the rear wheel slip is reduced by adjusting the rear wheel steering amount in the understeer direction. By increasing this, it is possible to suppress the inward yaw moment of the vehicle and prevent spin.
This is extremely effective in that it can further improve the benefits of a four-wheel steering system that aims to improve high-speed running stability.

[Brief explanation of the drawing]

FIG. 1 is an explanatory plan view showing one embodiment of the present invention, FIG. 2 is a flowchart showing a rear wheel slip detection method, and FIG. 3 is a diagram showing an example of steering force-rear wheel steering angle characteristics. ■... Handle, 2... Front wheel, 5... Power cylinder, 6a, 6b... Oil pressure sensor, 7... Operational amplifier, 8... Vehicle speed sensor, 9... Control unit, 10 ...Rear wheel steering actuator, 13...
Rear wheel, 14... Rear wheel steering angle sensor, 15L. 15R, 16L, 16R...Wheel rotation speed sensor. that's all

Claims (1)

[Claims] In a vehicle equipped with a front and rear wheel steering system that uses the front wheel steering angle or steering force as one of the parameters for determining the rear wheel steering angle, the rotational speed of the left and right rear wheels and at least one front wheel is detected and the brakes are applied. When the rotational speed of at least one rear wheel is higher than the rotational speed of the other wheel with the lowest rotational speed by at least a certain amount, it is determined that slipping has occurred in the rear wheels, and the rear wheel is steered in the direction of understeer. 1. A method of controlling a four-wheel steering device for an automobile, the method comprising: controlling an amount of steering to be increased.