WO2007073772A1

WO2007073772A1 - Method and system to prevent vehicle overturning, estimator and controller for the system

Info

Publication number: WO2007073772A1
Application number: PCT/EP2005/014226
Authority: WO
Inventors: François DECHAMP
Original assignee: Renault Trucks
Priority date: 2005-12-29
Filing date: 2005-12-29
Publication date: 2007-07-05

Abstract

A method to prevent vehicle overturning when driving in a curve for a vehicle having an electronically controllable wheel steering mechanism and an electronically controllable wheel braking mechanism, wherein the method comprises the step (60) of estimating the risk that the vehicle overturns, and only if the risk estimation passes a predetermined threshold, the method comprises the steps (86, 90) of: - controlling the steering mechanism to automatically lower the steering angle of the vehicle wheels so as to decrease the risk of overturning, and - in parallel, controlling the braking mechanism to achieve a differential braking that turns the vehicle without steering the wheels so as to compensate the effect of the automatic steering angle lowering.

Description

METHOD AND SYSTEM TO PREVENT VEHICLE OVERTURNING, ESTIMATOR AND CONTROLLER FOR THE SYSTEM

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method and a system to prevent vehicle overturning, and an estimator and a controller for the system.

BACKGROUND OF THE INVENTION

Methods to keep control of the vehicle yaw angle when sliping using differential braking are known (refer for example to application US 6,325,469 in the name of Carson et al.). This method lowers the risk of accident.

However, there is no method to lower the risk that a vehicle overturns, particularly when the vehicle is driving too fast while undertaking a curve.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a method to prevent vehicle overturning when driving in a curve for a vehicle having an electronically controllable wheel steering mechanism and an electronically controllable wheel braking mechanism.

With the foregoing and other objects in view there is provided in accordance with the invention a method to prevent vehicle overturning that comprises the steps of estimating the risk that the vehicle overturns, and only if the risk estimation passes a predetermined threshold, the method comprises the steps of:

- controlling the steering mechanism to automatically lower the steering angle of the vehicle wheels so as to decrease the risk of overturning, and

- in parallel, controlling the braking mechanism to achieve a differential braking that turns the vehicle without steering the wheels so as to compensate the effect of the automatic steering angle lowering.

The above method allows to lower the risk of overturning by taking precedence over manual control in case it is established that overturning is likely to occur while maintaining as much as possible the vehicle within the curve.

The embodiments of the above method may comprise one or several of the following features: - during the braking mechanism control step, the braking effort to exert on at least one wheel is determined according to the overturning risk estimation value,

- during the braking mechanism control step, the braking effort to exert on at least one wheel is determined according to the difference between a desired yaw angle and an actual vehicle yaw angle, and

- the estimated risk is a function of the difference between:

- the vehicle lateral acceleration angular momentum around a longitudinal rotation axis that passes through the contact points between wheels at the exterior of the curve, and

- the vehicle weight angular momentum around the same longitudinal rotation axis.

The above embodiments of the method present the following advantages:

- using the overturning risk estimation value to control the braking mechanism makes the overturning prevention even more efficient; and

- using the difference between a desired vehicle yaw angle and an actual vehicle yaw angle to control the braking mechanism makes the maintaining of the vehicle within the curve more efficient.

The invention also relates to a system to prevent vehicle overturning when driving in a curve for a vehicle having an electronically controllable wheel steering mechanism and an electronically controllable wheel braking mechanism, the system comprising:

- an estimator to estimate the risk that the vehicle overturns, and

- a controller which, if the risk estimation passes a predetermined threshold, is able to:

- control the steering mechanism to automatically lower the steering angle of the vehicle wheels so as to decrease the risk of overturning, and

- in parallel, to control the braking mechanism to achieve a differential braking that turns the vehicle without steering the wheels so as to compensate the effect of the automatic steering angle lowering. The embodiment of the above system may comprise one or several of the following features: - the controller is able to determine the braking effort to exert on at least one wheel according to the overturn risk estimation value, and

- the controller is able to determine the braking effort to exert on at least one wheel according to the difference between a desired vehicle yaw angle and an actual vehicle yaw angle.

The invention also relates to an estimator suitable to be used in the above system, wherein the estimator is able to estimate the risk that the vehicle overturns from measurement carried out on the vehicle, this measurement being representative of the difference between: - the vehicle lateral acceleration angular momentum around a longitudinal rotation axis that passes through the contact points between wheels at the exterior of the curve, and

- the vehicle weight angular momentum around the same longitudinal rotation axis. The invention also relates to a controller suitable to use in the above system.

These and other aspects of the invention will be apparent from the following description, drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of a vehicle having a system to prevent vehicle overturning when driving in a curve;

Figure 2 is a schematic top view of the chassis and the wheels of the vehicle of figure 1 ;

Figure 3 is a flowchart of a method to prevent vehicle overturning.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

Figure 1 shows a vehicle 2.

For simplicity, only the details necessary to understand the invention are shown on figure 1. In the following description well-known functions or constructions by a person of ordinary skill in the art are not described in details.

Vehicle 2 has a conductor steering wheels 4 associated with a steering wheel angle sensor 6. Sensor 6 outputs a steering wheel angle signal to an onboard calculator 8. Vehicle 2 has a brake pedal 10 actuated by the conductor, associated with a pedal travel sensor 12. Sensor 12 outputs a pedal travel signal to calculator 8.

Wheel suspension mechanisms are associated with each wheel of vehicle 2. For simplicity, only two wheels 14 and 16 are shown. Also for simplicity, only one suspension mechanism 18 associated with wheel 14 is shown in figure 1. Suspension mechanism 18 achieves a flexible connection between the chassis of vehicle 2 and the axle of wheel 14. For example, this is achieved using deformable cylinder filled up with a dampening liquid or gas.

In order to measure the effort that the road exerts on wheel 14, a sensor is associated with each suspension mechanism. For example, a sensor 20 that measures the dampening liquid or gas pressure within mechanism 18 is provided. Sensor 20 outputs a pressure signal to calculator 8.

Wheel 14 is a front steering wheel that is steered by an electronically controllable wheel steering mechanism 22. Left and right front wheels can be steered independently from each other. Mechanism 22 is controlled by calculator 8.

Each wheel of vehicle 2 is provided with an electronically controllable wheel braking mechanism. Figure 1 shows two braking mechanisms 24 and 26 to brake wheels 14 and 16, respectively.

Mechanisms 24 and 26 are controlled by calculator 8. Each braking mechanism can be controlled independently from the other one to exert a braking effort on a wheel that can be different from the braking effort exerted on the other wheels. Thus, these braking mechanisms are able under the control of calculator 8 to achieve a differential braking during which the braking efforts applied to each wheel may be different. Finally, vehicle 2 has a vehicle overturning risk estimator 30 and a controller

32.

Estimator 30 is able to estimate the risk that vehicle 2 overturns from measurement carried out on vehicle 2. The risk that vehicle 2 overturns is a function of the difference between: - the vehicle lateral acceleration angular momentum around a longitudinal rotation axis that passes through the contact point between the wheels at the exterior of the curve, and - the vehicle weight angular momentum around the same longitudinal rotation axis.

Controller 32 is able to control steering mechanism 22 and each braking mechanisms 24, 26 according to the risk estimation outputted by estimator 30. The functions of estimator 30 and controller 32 will be described in more details in view of figure 3.

For example, estimator 30 and controller 32 are implemented within calculator 8. To this end, for example, calculator 8 is built from a conventional programmable electronic calculator able to execute program instructions recorded in a memory 34. Memory 34 stores the instructions necessary for the execution of the method of figure 3 when those instructions are executed by electronic calculator 8. Memory 34 also stores the value of every parameter necessary to the execution of the method of figure 3.

Finally, figure 1 shows a point C which is the gravity center of vehicle 2. Figure 2 shows a top view of vehicle 2 while vehicle 2 is driving along a curved path. The curved path is illustrated by a curved line 40. Wheels 14 and 16 are at the interior of the curve.

Figure 2 shows also front wheel 42 and rear wheel 44 of vehicle 2 which are at the exterior of the curve. The longitudinal axis of vehicle 2 that passes through gravity center C is illustrated by a dashed line 46. The longitudinal axis is parallel to the instantaneous displacement direction of vehicle 2. The longitudinal rotation axis over which vehicle 2 may overturn is illustrated by a dashed line 48. Line 48 is parallel to line 46 and passes through the contact points between wheels 42 and 44 and the road surface. The steering directions of steering front wheels 14 and 42 are shown by dashed lines 50 and 52, respectively. Lines 50 and 52 are parallel to the road surface.

Mechanism 22 controls steering angles β-_\ and β₂. Angles β-_\ and β₂ are the angle between lines 52 and 46 and lines 50 and 46, respectively. The yaw angle a between line 46 and a fixed direction is also shown in figure

2. For example, the fixed direction is the north direction and is illustrated by a dashed line 56. In figure 2 the braking effort on wheels 14, 16, 42 and 44 are illustrated by arrows

respectively. Arrows represent

the direction and the amplitude of the braking effort exerted on the wheels. The braking efforts are controlled by the corresponding braking mechanisms like mechanisms 20 and 26.

The lateral acceleration of vehicle 2 is also represented by an arrow a_L~ which extends from the gravity center C. The lateral acceleration is perpendicular to line 46. The lateral acceleration corresponds to a centrifugal force.

The notation introduced in view of figure 2 are used in the following parts of the specification.

The combination of estimator 30, controller 32, the different sensor like sensor 20, steering mechanism 22 and the electronically braking mechanisms form a system to prevent vehicle overturning when driving in curve 40.

The operation of such a system will now be described with reference to figure 3 in the particular situation depicted in view of figure 2.

Initially, in step 60, estimator 30 estimates the risk E that vehicle 2 overturns around axis 48. More precisely, during step 60, in an operation 62, sensor 20 measures the suspension pressure and outputs the corresponding measurement to estimator 30. Subsequently, in operation 64, estimator 30 computes a pressure drop ΔP in suspension mechanism 18.

This pressure drop ΔP is then filtered, in an operation 66, to cancel any pressure drop which is shorter than a predetermined time interval Δt.

In operation 68, the filtered pressured drop rate is outputted to controller 32 as a vehicle overturning risk estimation E. Then, in step 70, controller 32 compares risk estimation value to a predetermined threshold Si. If the risk estimation value is inferior to threshold S-i, then controller 32 executes a standard steering wheel procedure 72. Otherwise, controller 32 executes an emergency steering procedure 74. During the standard steering wheel procedure 72, initially, in operation 80, the desired yaw angle a_r is computed from the steering wheel angle signal outputted by sensor 6. Then, according to the actual yaw angle a and the computed desired yaw angle α_r, in an operation 82, controller 32 controls steering mechanism 22 to obtain steering angles β1 and β ₂ that achieve the desired yaw angle σ_r.

During the execution of the emergency steering procedure 74, in a step 86, controller 32 controls mechanism 22 to automatically lower steering angles β₁ and β₂ by a predetermined step Δβ. For example, controller 32 decreases angles β₁ and β ₂ according to the following relations : (1 )

where: - β1 k-1 and β 2k-1 are the actual values of steering angles β ₁ and β₂ respectively,

- β 1k and β 2k are the values of steering angles β 1 and β ₂to achieve at the end of step 86.

For example , step Aβ is determined according to the following relation:

(2)

where: - Ki is a predetermined coefficient, and

- E is the current risk estimation value outputted by estimator 30.

In parallel to step 86, in step 90, controller 32 controls the braking mechanisms to modify the yaw angle a of the vehicle without steering the wheel. More precisely, during step 90, controller 32 achieves a differential braking. For example, in operation 92, controller 32 determines the braking effort to exert on each wheel. For example, in operation 92, controller 32 uses the following relation:

(3)

(4)

where: - F is the braking effort determined according to the pedal travel signal outputted by sensor 12,

- E is the present risk estimation value,

- k₂ is a predetermined coefficient, and - Ffi, Fri , Ffe and F_re are the amplitudes of braking efforts on wheels 14, 16,

42 and 44, respectively.

Then, in an operation 94, controller 32 controls each braking mechanism to achieve the braking efforts determined during operation 92.

On the one hand, decreasing the steering angle β decreases the risk to overturn. However, on the other hand, lowering steering angle β makes the vehicle to go straight ahead and the vehicle risks to go out of the curve. To lower the risk to have the vehicle driven out of the curve, in parallel, in step 90, controller 32 controls each braking mechanisms to modify the yaw angle of vehicle 2 without steering wheels in order to maintain as much as possible vehicle 2 along line 40. As can be understood from relations (3) and (4), the braking effort is higher on the wheels which are in the interior of the curve to achieve a yaw angle that maintains vehicle 2 along line 40 without steering the wheels. More precisely, step 90 compensates as much as possible the modification of the yaw angle introduced by step 86. Therefore, the risk to overturn is decreased while maintaining vehicle 2 within the curve.

At the end of step 72 or at the end of steps 86 and 90, the method returns to step 60. Thus, if during step 70, the risk estimation value is still superior to threshold Si, then steering angles β-_\ and β₂ will be further lowered in the next execution of the emergency steering procedure 74. Consequently, steering angles β1 and β₂ are repeatedly decreased as long as the risk estimation value is superior to threshold S1. In parallel, the braking effort on the curve interior wheels is repeatedly increased so as to compensate for the lowering of steering angles β1 and β₂.

Many additional embodiments are possible. For example, the overturning risk estimation may be established according to the signal delivered by a gravity center position sensor and a lateral accelerometer. More precisely, according to the gravity center position, it is possible to determine the vehicle weight angular momentum and the lateral acceleration momentum. In another embodiment, braking effort to exert on each wheel during step 90 are determined according to the difference between:

- a desired yaw angle which is established, for example, from the steering wheel angle signal, and - the actual yaw angle which can be measured by a yaw angle sensor.

This last embodiment can be combined with the embodiment described in view of figure 3 in order to determine each braking effort according to the desired yaw angle, the actual yaw angle and the overturning risk estimation value E.

The above method to prevent vehicles overturning also applies to vehicles in which steering angles β 1 and β₂ are always equal.

LIST OF REFERENCES

2 vehicle

4 steering wheel

6 steering wheel angle sensor

8 calculator

10 brake pedal 12 pedal travel sensor

14 wheel

16 wheel

18 suspension mechanism

20 pressure sensor 22 electronically controllable wheel steering mechanism

24 braking mechanism

26 braking mechanism

30 risk estimator

32 controller 34 memory

40 curved path

42 front wheel

44 rear wheel C gravity center a yaw angle β steering angle braking efforts

Claims

1. A method to prevent vehicle overturning when driving in a curve for a vehicle having an electronically controllable wheel steering mechanism and an electronically controllable wheel braking mechanism, wherein the method comprises the step (60) of estimating the risk that the vehicle overturns, and only if the risk estimation passes a predetermined threshold, the method comprises the steps (86, 90) of:

2. The method according to claim 1 , wherein during the braking mechanism control step (90), the braking effort to exert on at least one wheel is determined according to the overturning risk estimation value.

3. The method according to claim 1 or 2, wherein during the braking mechanism control step (90), the braking effort to exert on at least one wheel is determined according to the difference between a desired yaw angle and an actual vehicle yaw angle.

4. The method according to any one of the previous claims, wherein the estimated risk is a function of the difference between:

5. System to prevent vehicle overturning when driving in a curve for a vehicle having an electronically controllable wheel steering mechanism and an electronically controllable wheel braking mechanism, the system comprising:

- an estimator (30) to estimate the risk that the vehicle overturns, and - a controller (32) which, if the risk estimation passes a predetermined threshold, is able to:

- control the steering mechanism to automatically lower the steering angle of the vehicle wheels so as to decrease the risk of overturning, and - in parallel, to control the braking mechanism to achieve a differential braking that turns the vehicle without steering the wheels so as to compensate the effect of the automatic steering angle lowering.

6. The system according to claim 5, wherein the controller (32) is able to determine the braking effort to exert on at least one wheel according to the overturn risk estimation value.

7. The system according to claim 5 or 6, wherein the controller (32) is able to determine the braking effort to exert on at least one wheel according to the difference between a desired vehicle yaw angle and an actual vehicle yaw angle.

8. An estimator suitable to be used in a system according to any one of claims 5 to 7, wherein the estimator is able to estimate the risk that the vehicle overturns from measurement carried out on the vehicle, this measurement being representative of the difference between:

- the vehicle lateral acceleration angular momentum around a longitudinal rotation axis that passes through the contact points between wheels at the exterior of the curve, and - the vehicle weight angular momentum around the same longitudinal rotation axis.

9. A controller suitable to be used in a system according to any one of claims herein the controller is able to:

- in parallel, to control the braking mechanism to achieve a differential braking that turns the vehicle without steering the wheels so as to compensate the effect of the automatic steering angle lowering.