VARIABLE TOE ANGLE CONTROL SYSTEM FOR A VEHICLE

Abstract

A variable toe angle control system for a vehicle that can be incorporated with a fail-safe mechanism When a fault of the system is detected, at least one of toe-angle actuators is actuated to make toe angles of two wheels agree with each other. When one of the wheels has become fixed in position without regard to a control signal supplied to the corresponding actuator, the actuator for the other wheel is actuated so as to make the toe angles of the two wheels equal to each other. When at least one toe-angle sensor is found faulty, the actuators are both actuated until the actuators reach positions corresponding to stoppers. When information for determining target values of the toe angles of the right and left wheels is found faulty, the actuators are both actuated until the actuators reach positions corresponding to prescribed reference toe positions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

Now the present invention is described in the following with reference to the appended drawings, in which:

FIG. 1 is a schematic view of a vehicle to which the present invention is applied;

FIG. 2 is a flowchart of the control process of the system of the present invention;

FIG. 3 is a diagram showing the mode of operation in the standard abnormal mode;

FIG. 4 is a diagram showing the mode of operation in the sensor system abnormal mode; and

FIG. 5 is a diagram showing the mode of operation in the drive system abnormal mode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an outline of a vehicle to which the present invention is applied. The vehicle V comprises a front wheel steering device 3 for directly steering a right and left front wheel 2L and 2R according to a steering angle of a steering wheel 1, a right and left actuator 5L and 5R for individually changing the toe angles of a right and left rear wheel 4L and 4R by varying the lengths of parts, such as laterals links, of rear wheel suspension units supporting the right and left rear wheels 4L and 4R, respectively, a right and left toe angle sensor 6L and 6R for individually detecting the toe angles of the right and left rear wheels 4L and 4R, an accelerator sensor 7 for detecting a fore-and-aft acceleration acting upon the vehicle body to use it as a reference signal for determining a target control value in the variable toe angle control, a steering angle sensor 8 for detecting the steering angle of the steering wheel 1, and a control unit 9 for controlling the displacements of the actuators 5L and 5R according to the outputs of the various sensors. The fore-and-aft acceleration can also be computed from the signals of wheel speed sensors provided on the corresponding wheels 2L, 2R, 4L and 4R and pedal stroke sensors provided on an accelerator pedal and brake pedal.

Each actuator 5L, 5R may consist of a rotary motion/linear motion converter combining an electric motor fitted with a reduction gear and a thread mechanism, a cylinder device that linearly actuates a piston rod by using hydraulic pressure or any other known linear actuator. Each toe angle sensor 6L, 6R may consist of a potentiometer or any other known displacement sensor, but preferably consists of an electromagnetic sensor or other non-contact sensor for an improved durability.

According to such a variable toe angle control system, the toe-in and toe-out of the rear wheels 4L and 4R can be freely controlled under a prescribed condition by simultaneously actuating the right and left actuators 5L and 5R in a symmetric manner. If one of the right and left actuators 5L and 5R is extended while the other is retracted, the two rear wheels 4L and 4R may be steered either in the right or left direction.

The target value for each rear wheel 4L, 4R can be obtained from a map of optimum toe angle with respect to such running conditions as the front wheel steering angle and fore-and-aft acceleration. While feeding back the output (actual toe position) of each toe angle sensor 6L, 6R, an optimum torque that should be produced from the corresponding actuator 5L, 5R is computed from the deviation of the actual toe angle from a target toe angle, and an electric motor is duty controlled so as to produce the optimum torque through a current feedback. By thus forming a multiple feedback loops, a highly responsive and stable control is enabled.

FIG. 2 shows a control flow of the system of the present invention. First of all, it is determined if the overall system is operating normally (step 1). If the overall system is operating normally (Yes), a normal variable toe angle control is executed as described above (step 2). If an abnormal operation of the overall system is detected (No), it is determined if this abnormal operation is such that the basic data, such as front wheel steering angle and acceleration, for setting a control target value for the rear wheels is not available or the intelligent part of the variable toe angle control system itself is faulty (step 3). If the fault is in the intelligent part of the system, and not in drive system or the sensing system of the variable toe angle control system (Yes), a standard abnormal mode is executed (step 4).

In this case, because the proper toe angle cannot be computed although the actuators 5L and 5R as well as the driver circuits thereof are in good order, the variable toe angle control is terminated, and the two actuators are actuated toward a prescribed position such as a zero toe angle position (FIG. 3). The vehicle is then able to operate in a same manner as a conventional vehicle, however, without any toe angle control.

An abnormal condition other than that of the intelligent part of the variable toe angle control system can be determined as a failure to achieve a prescribed relationship between the speed difference ΔV between the two rear wheels 4L and 4R, the average speed V of the right and left wheels and the steering angle δ for more than a prescribed time period. More specifically, the speed difference ΔV between the two rear wheels 4L and 4R is substantially inversely proportional to the yaw rate γ, and a relationship γ=V/L·δ (where L is the wheel base) holds between the yaw rate γ, average speed V of the right and left wheels and steering angle δ under a steady state condition. If this relationship fails to hold in a pronounced way, it can be concluded that the wheel speed sensor is faulty.

If the variable toe angle control system is found to be faulty in step 3 (No), it is determined whether the fault is in the drive system of either one of the actuators 5L and 5R or either one of the toe angle sensors 6L and 6R (step 5). If the drive systems of the actuators 5L and 5R are in good order but at least one of the toe angle sensors 6L and 6R is found to be faulty (Yes), a sensor system abnormal mode is executed (step 6). In this case, the actuators 5L and 5R can operate in a normal fashion, but the actuator corresponding to the faulty toe angle sensor is unable to determine its own position. Therefore, the two actuators are both actuated to safe positions (with some toe-in) defined by mechanical stoppers by using an open loop so that the vehicle is allowed to travel in a stable manner (FIG. 4).

Any abnormal condition in the toe angle sensors 6L and 6R can be detected when an abrupt change in the output is detected or when the output deviates from a normal range for more than a prescribed time period. Also, a redundant sensor having two outputs can be used. In this case, a fault is detected if a prescribed relationship fails to be hold between the two outputs.

If a fault is detected in any of the drive systems such as one of the actuators (No), a drive system abnormal mode is executed (step 7). In this case, because the toe angle sensors 6L and 6R are normal and the actuators 5L and 5R are able to determine their own positions, the normal actuator is actuated to a position which is symmetric to the position of the failed actuator (FIG. 5). Thereby, the two wheels balance with each other, and the vehicle is able to travel in a stable manner.

An abnormal condition of any of the drive systems can be determined from an abnormal relationship between the duty ratio (or voltage) of the electric power applied to the electric motor and the feedback current (or the motor torque) or from an abnormal supply voltage to the electric motor.

The abnormal toe angle is changed to a target position at a speed appropriate to the nature of the component that has failed or the running condition of the vehicle as quickly as possible without causing any discomfort to the vehicle operator.

Thus, the present invention allows a fail-safe system for a variable toe angle control system at a relatively low cost.

In the present invention, without regard to whether a fault is detected in any of the drive systems or in any of the sensor systems, when changing the toe angle of the normal side is changed in relation to the toe angle of the faulty side which is known, the toe angles of the right and left wheels may be controlled to asymmetric values such as to change the oversteer/understeer tendency of the vehicle, or as to enable the vehicle to travel along a straight path when the vehicle is traveling on a split-μ road surface.

Although the present invention has been described in terms of a preferred embodiment thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims. For instance, the foregoing embodiment was directed to an application to rear wheels of a vehicle, but may also be applied to front wheels with slight modification to the embodiment which is obvious to a person skilled in the art.

The contents of the original Japanese patent application on which the Paris Convention priority claim is made for the present application are incorporated in this application by reference.

Claims

1. A variable toe angle control system for a vehicle, comprising: a pair of actuators for individually varying toe angles of a right and left wheel;a pair of sensors for individually detecting the toe angles of the right and left wheels; anda control unit for determining a target value for each of the right and left wheels according to operating conditions of the vehicle and supplying a control signal to each of the actuators according to an output of the corresponding sensor and the corresponding target value;wherein the control unit is configured to detect a fault of the system and actuate at least one of the actuators to make the toe angles of the two wheels agree with each other when a fault is detected in a part of the system.

2. The variable toe angle control system according to claim 1, wherein when one of the wheels becomes fixed in position without regard to the control signal supplied to the corresponding actuator, the control unit actuates the actuator for the other wheel so as to make the toe angles of the two wheels equal to each other.

3. The variable toe angle control system according to claim 1, further comprising a stopper for each of the right and left wheels, wherein when at least one of the sensors is found to be faulty, the two actuators are actuated until the actuators reach positions corresponding to the stoppers.

4. The variable toe angle control system according to claim 1, wherein when information for determining target values of the toe angles of the right and left wheels is found to be faulty, the two actuators are actuated until the actuators reach positions corresponding to prescribed reference toe positions.

5. A method for controlling toe angles of a vehicle in a variable toe angle control system, comprising: preparing a pair of actuators for individually varying toe angles of a right and left wheel;preparing a pair of sensors for individually detecting the toe angles of the right and left wheels;determining a target value for each of the right and left wheels according to operating conditions of the vehicle;supplying a control signal to each of the actuators according to an output of the corresponding sensor and the corresponding target value;detecting a fault of the system; andactuating at least one of the actuators to make the toe angles of the two wheels agree with each other when a fault is detected in a part of the system.

6. The method according to claim 5, further comprising, when one of the wheels has become fixed in position without regard to the control signal supplied to the corresponding actuator, actuating the actuator for the other wheel so as to make the toe angles of the two wheels equal to each other.

7. The method according to claim 5, further comprising preparing a stopper for each of the right and left wheels, and, when at least one of the sensors is found to be faulty, actuating both the actuators until the actuators reach positions corresponding to the stoppers.

8. The method according to claim 5, further comprising, when information for determining target values of the toe angles of the right and left wheels is found to be faulty, actuating both the actuators until the actuators reach positions corresponding to prescribed reference toe positions.