Patent Application
20080054577
Now the present invention is described in the following with reference to the appended drawings, in which:
Now the present invention is described in the following in more detail in terms of a concrete embodiment with reference to the appended drawings.
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.
Now is considered the motion of the vehicle when an acceleration/deceleration acts upon the vehicle while the vehicle is turning. The load (Wfl, Wfr, Wrl and Wrr) on each of the wheels of the vehicle while making a turn can be given as follows:
Wfl (left front wheel)=Wf/2−Kφf/(Kφf+Kφr)·h·W·αy/df−h·W·αx/(2·L)
Wfr (right front wheel)=Wf/2+Kφf/(Kφf+Kφr)·h·W·αy/df−h·W·αx/(2·L)
Wrl (left rear wheel)=Wr/2−Kφr/(Kφf+Kφr)·h·W·αy/dr+h·W·αx/(2·L)
Wrr (right rear wheel)=Wr/2+Kφr/(Kφf+Kφr)·h·W·αy/dr+h·W·αx/(2·L)
where
Because the lateral force is proportional to the load and lateral acceleration, the front and rear lateral forces (CFf, CFr) can be given by the following equations:
CFf=(Wfl+Wfr)·αy=(Wf−h·W·αx/L)·αy
CFr=(Wrl+Wrr)·αy=(Wr+h·W·αx/L)·αy
The yaw moment (ML) at this time can be computed from the lengths (Lf, Lr, Lf+Lr=L) of the arms from the gravitational center and lateral forces by using the following equations.
Now, the effect of the rear wheel toe angle on the turning behavior is considered in the following. If the rear wheels are given with a certain toe angle (θ), the lateral forces on the right and left rear wheels can be given by the following equations.
CFrl=−Krl·(β+Lr·γ/V+θ)/2
CFrr=−Krr·(β+Lr·γ/V−θ)/2
where
By noting that the lateral forces change in dependence on the load, the static load on the rear wheels is given by:
Wrl0=Wrr0=Lf·W/(2·L)
and the lateral displacement of the load is given by:
Krl=Kr·(L−K·αy) and Krr=Kr·(L+K·αy)
wherein
Therefore, the total lateral force acting on the rear axle (CFr) can be given by
CFr=CFrl+CFrr=−Kr·(β+Lr·γ/V−K·αy·θ)
and the total yaw moment (M2) produced owing to the rear wheel toe angle is given by
M2=−K·Kr·αy·θ·Lr (2)
Therefore, the to angle (θ) that is required for canceling the instability yaw moment owing to the acceleration/deceleration represented by Equation (1) can be given by
θ=h·W·αx/(Lr·Kr·K) (3)
The control rule that can be found from Equation (3) is that the rear wheel toe angle (θ) that is required to cancel the instability yaw moment that is produced as the vehicle makes a turn is given independently from the steering angle and lateral acceleration or the turning state of the vehicle. The rear wheel toe angle obtained from Equation (3) does not produce any lateral force and there is no risk of self-steer because the right and left lateral forces balance out when the vehicle is traveling straight, but can produce an appropriate corrective yaw moment that is required to cancel any instability moment according to the lateral acceleration produced in the vehicle.
A target value of the toe angle may be proportional to the acceleration/deceleration of the vehicle, but a certain time delay is inevitable in the output of the acceleration/deceleration sensor owing to noise removing processes or the like as well as the operation of the actuator. By computing the toe angle target value according to the pedal displacement of the accelerator pedal in the foregoing step, an improvement in the control response can be achieved owing to a control action similar to a feedforward control. An even further improvement in the control action similar to a feedforward control can be achieved by taking into account the time differential value of the pedal displacement (pedal speed). When such a differential value is used, point a (pedal speed at which the toe angle is zero) and point b (pedal speed at which the toe-out is maximized) in
A target toe angle is then computed according to the actual acceleration that acts upon the vehicle (step 5). The toe angle is determined in relation with the acceleration/deceleration in such a manner that a slight toe-in is selected under a constant vehicle speed condition. The toe-in is progressively increased with an increased deceleration while a greater toe-out is selected with an increased acceleration. The target toe angle value based on the accelerator pedal displacement is re-evaluated at this time. More specifically, the vehicle acceleration may vary even when the pedal displacement is the same depending on the running resistance of the vehicle, and a large pedal displacement may not necessarily produce a correspondingly high acceleration. In such a case, the target toe angle value is modified in this step.
A bump toe angle property is changed (step 6) to take into account the fact that the toe angle changes in the case of a common independent suspension system depending on the vertical displacement of the wheel or the extension and retraction of the damper. This property is due to the specific link geometry and the compliance of the bush that connects each link to the vehicle body, and is so undefined that it is highly difficult to take into account in designing an optimum toe angle property. However, according to the present invention, an optimum bump toe angle property is determined in advance according to the extent of the nose lift at the time of acceleration, and it is actively realized. Therefore, a desired bump to angle property can be achieve without depending on the link geometry, and the freedom in the design of the wheel suspension system such as the selection of a link layout that minimizes the intrusion into the passenger compartment can be enhanced.
The final target toe angle is thereby obtained (step 7). On the other hand, as long as the vehicle speed is lower than the prescribed value (No), a default toe angle target value is given (step 8).
When the final target toe angle is determined, it is determined if a variable roll stiffness control is in progress (step 9). If a variable roll stiffness control is in progress (Yes), the corresponding modification of the target toe angle (by changing the coefficient K in Equation (3) depending on the roll stiffness ratio) is effected (step 10).
The variable roll stiffness control of step 10 essentially consists of minimizing the change in the attitude of the vehicle as it makes a turn and/or traveling over an irregular road surface, for instance, by changing the damping property of each individual shock absorber (see Japanese patent laid open publication No. 5-319057) or changing the effective torsional stiffness of the stabilizer that connects the right and left suspension arms to each other (see Japanese patent laid open publication 2000-71739). When the roll stiffness is changed, the steering property while making a turn changes, and the foregoing process compensates such changes in the steering property.
The deviation of the actual toe angle from the target toe angle and the optimum operating speed of the actuator corresponding to the current operating condition of the vehicle are determined by looking up a predetermined map, and the actuator is feedback controlled so as to achieve the target toe angle (step 11).
On the other hand, when it is determined in step 2 that the vehicle is decelerating (No), an optimum target toe angle is computed according to the pedal displacement (speed) of the brake pedal and the magnitude of the deceleration in a similar manner as in the computing step at the time of acceleration, and a target toe angle is determined after making a rebound compensation that accounts for the change in the attitude of the vehicle owing to the nose dive at the time of deceleration (steps 12 to 17).
A slow-in, fast-out is considered to be desirable as a way of driving a vehicle on a winding road. On the other hand, when a yaw moment acts upon the vehicle, the steering property of a common vehicle is such that applying a brake causes an oversteer tendency and accelerating the vehicle causes an understeer tendency. In other words, it is possible that braking or accelerating while making a turn could impair the stability of the vehicle.
In the toe angle control of the system of the present invention, the instability yaw moment given by Equation (1) is canceled as expressed by Equation (2), and the rear wheel toe angle changes with the fore-and-aft acceleration as given in Equation (3). In other words, as shown in
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-231543 | Aug 2006 | JP | national |
