FR3142422A1 - Method of assisting the piloting of a motor vehicle to realign it parallel to an ideal trajectory, associated device and vehicle. - Google Patents

Abstract

The invention relates to a method of assisting the piloting of a motor vehicle to realign it parallel to an ideal trajectory in its lane, comprising the following steps: - acquisition of the initial values of a heading angle (θ0) and of a speed (V0) of the motor vehicle, - determination of a maximum value of a steering wheel angle generating a predetermined maximum lateral acceleration taking into account the initial value of the speed of the motor vehicle, - determination of a set point of steering wheel angle as a function of time comprising at least: a first setpoint portion modifying the steering wheel angle to reduce the heading angle of the vehicle up to a target value determined as a function of the initial value of the heading angle and of the maximum value of the steering wheel angle, and a second steering wheel angle setpoint portion modifying the heading angle of the vehicle until it is canceled. Figure for abstract: Fig.2

Description

Method for assisting in the piloting of a motor vehicle to realign it parallel to an ideal trajectory, associated device and vehicle. Technical field of the invention

The present invention relates generally to driving aids for motor vehicles.

It applies more particularly to cars and other motorized vehicles circulating on roads, but also applies to other areas such as robotics.

The invention relates to a method for assisting in the steering of a motor vehicle to realign it parallel to an ideal trajectory in its lane.

It also relates to a device for assisting in driving a motor vehicle adapted to implement said method and a vehicle comprising such a device for assisting in driving.

State of the art

In order to make motor vehicles safer, driver assistance systems and even partially to fully automated driving systems are currently being developed.

These systems include in particular an autonomous system called “Traffic Jam Pilot” (TJP) developed by the applicant, the aim of which is to assist drivers, in particular in the following situations: expressway, motorway, low-speed traffic jam.

This system has a level 2 autonomy according to the standard of the Society of Automotive Engineers (SAE International) and includes two vehicle control systems: longitudinal control and lateral control.

Longitudinal Control (ACC) ensures sufficient safety distance between the vehicle and a target vehicle by taking into account the speed of both vehicles.

Lateral control (also called Lane Centering Assist or LCA) helps guide the vehicle according to the trajectory of the road and the environment by modifying the vehicle's steering wheel angle.

To implement such a system, the motor vehicle is equipped with a series of sensors to acquire data characterizing the state of the motor vehicle and its environment, and a computer programmed to analyze this data in order to generate commands to slow down the vehicle if necessary and steer it into its lane.

However, these controls are set so as not to apply too abrupt a movement to the vehicle, for the comfort of the driver and passengers.

These checks are therefore not optimized to take into account an emergency situation such as driver discomfort, for example.

Presentation of the invention

In order to overcome the aforementioned drawbacks of the state of the art, the present invention proposes a solution to enable the vehicle to be realigned simply and quickly parallel to an ideal trajectory in its lane.

More particularly, the invention proposes a method for assisting in the steering of a motor vehicle to realign it parallel to an ideal trajectory in its lane, comprising the following steps:
- acquisition of the initial values of a heading angle and a speed of the motor vehicle,
- determination of a maximum value of a steering wheel angle generating a predetermined maximum lateral acceleration taking into account the initial value of the speed of the motor vehicle,
- determination of a steering wheel angle setpoint as a function of time comprising at least:
a first steering wheel angle instruction part modifying the vehicle heading angle to reduce the vehicle heading angle to a target value determined based on the initial heading angle value and the maximum steering wheel angle value, and a second steering wheel angle instruction part modifying the vehicle heading angle to cancel it.

Thus, thanks to the invention, it is possible to determine the evolution of a steering wheel angle setpoint over time as a function of the initial conditions of the vehicle and its environment at the time of activation of the method making it possible to quickly and simply realign the vehicle with a predetermined ideal trajectory, i.e. bring it back to a trajectory parallel to the predetermined ideal trajectory. In other words, the evolution over time of the steering wheel angle setpoint determined according to the invention makes it possible to quickly cancel the heading angle between a longitudinal axis of the vehicle and the ideal trajectory.

This process can be implemented in particular on any vehicle equipped with sensors enabling the acquisition of data relating to the vehicle's environment at all times, such as radar and camera already used in the TJP system.

Particularly advantageously, this method can be implemented as part of a vehicle safety procedure when a driver “non-activity” situation is detected by other vehicle sensors.

Preferably, the instruction calculated according to the method of the invention is added to the steering wheel angle instruction conventionally determined by the regulator ensuring lateral control LCA. Alternatively, this instruction can be taken into account alone to modify the steering wheel angle.

In any case, according to the method of the invention, the determined steering wheel angle setpoint is taken into account to modify the steering wheel angle of the vehicle, for example by controlling a power steering actuator so as to apply the setpoint to the steering wheel.

Preferably, by means of the method according to the invention, the determined steering wheel angle setpoint makes it possible to increase the yaw rate up to the target heading angle value and then to reduce the yaw rate until the heading angle is cancelled. Preferably, the heading angle is cancelled with a zero yaw rate.

The first part of the steering wheel angle command turns the steering wheel in one direction, while the second part of the steering wheel angle command turns the steering wheel in the other direction.

Other advantageous and non-limiting characteristics of the method according to the invention, taken individually or in all technically possible combinations, are as follows:
- to determine the maximum value of the steering wheel angle: a maximum value of a yaw rate of the vehicle is determined as a function of the maximum lateral acceleration of the vehicle and the initial value of the vehicle speed, and the maximum value of the steering wheel angle is determined as a function of the maximum value of the yaw rate, the initial value of the vehicle speed and characteristic quantities of the vehicle;
- a steering wheel angle setting is imposed which varies over time according to a predetermined mathematical function;
- said first and second steering wheel angle setpoint parts each vary according to an affine function of time;
- the affine functions of time according to which the first and second parts of the setpoint vary are two affine functions whose slopes have equal absolute values and opposite signs;
- the absolute value of the slope of each of the first and second affine functions is equal to a predetermined maximum steering wheel rotation speed;
- the determined target value of the heading angle is equal to a fraction of the initial heading angle or to a heading angle value reached when the steering angle reaches the maximum steering angle value;
- when the steering angle reaches said maximum steering angle value during the first setpoint part, a third intermediate setpoint part is determined according to which the steering angle is maintained equal to the maximum steering angle value for a duration determined as a function of the initial value of the heading angle and the maximum value of the yaw rate.

The invention also relates to a device for assisting in steering a motor vehicle to realign it parallel to an ideal trajectory in its lane, comprising a computer programmed to implement the method described above.

The invention also relates to a motor vehicle comprising a steering assistance device as described above and a power steering actuator, in which said computer of the assistance device is further programmed to control the power steering actuator so as to vary the steering wheel angle while taking into account said instruction.

Detailed description of the invention

The description which follows with reference to the attached drawings, given as non-limiting examples, will make it clear what the invention consists of and how it can be implemented.

On the attached drawings:

is a representation of the “bicycle” model applied to the motor vehicle moving in a traffic lane;

is a graph illustrating the evolution over time of the steering wheel angle setpoint determined according to the method of the invention when the target steering wheel angle value does not reach the maximum steering wheel angle value, and the evolution over time of a theoretical heading angle which results from the application of this setpoint to the steering wheel;

is a graph illustrating the evolution over time of the steering wheel angle setpoint determined according to the method of the invention when the target value of the steering wheel angle is equal to the maximum steering wheel angle value, and the evolution over time of a theoretical heading angle which results from the application of this setpoint to the steering wheel;

is a graph illustrating the evolution over time (in seconds) of the steering wheel angle setpoint determined according to the method of the invention when the target value of the steering wheel angle does not reach the maximum steering wheel angle value, the evolution over time of the corresponding steering wheel rotation speed setpoint as well as the evolution over time of a resulting simulated heading angle;

is a graph illustrating the evolution over time (in seconds) of the steering wheel angle setpoint determined according to the method of the invention when the target value of the steering wheel angle is equal to the maximum steering wheel angle value, and the evolution over time of the corresponding steering wheel rotation speed setpoint as well as the evolution over time of a resulting simulated heading angle;

is a graph illustrating the evolution over time (in seconds) of the total steering wheel angle setpoint (top curve) corresponding to the steering wheel angle setpoint determined according to the method of the invention and added to the steering wheel angle setpoint determined according to the conventional method implemented by the LCA system (solid line), the evolution over time of the corresponding total steering wheel rotation speed setpoint (solid line, middle curve) and the evolution over time of the lateral distance (gap) between the vehicle and the ideal trajectory when this total steering wheel angle setpoint is applied (bottom curve, solid line); these evolutions are compared to the evolutions over time of the steering wheel angle and steering wheel rotation speed setpoints determined by the LCA system alone (top and middle dotted curve) and of the evolution over time of the gap between the vehicle and the ideal trajectory resulting from the application of this steering wheel angle setpoint determined by the LCA system alone (dotted line of the bottom curve).

The invention relates to a method for assisting in the steering of a motor vehicle to realign it parallel to an ideal trajectory in its lane and a device for implementing it.

Device

On the , a “bicycle” type model of a motor vehicle 10 on a traffic lane VC is shown. In this model, the vehicle 10 is modeled by a frame and two wheels (as for a bicycle): the front steered wheel 11 and the rear non-steered wheel 12. The traffic lane VC is here delimited by the edge of the road 21 on one side and a broken line 22 on the other.

This motor vehicle 10 extends along a longitudinal axis X perpendicular to the common axes of the front 11 and rear 12 wheels. It comprises a conventional steering system for acting on the orientation of the steered wheels so as to be able to turn the vehicle. This conventional steering system notably comprises a steering wheel connected to connecting rods in order to pivot the steered wheels. In the example considered, it also comprises at least one power steering actuator for acting on the orientation of the steered wheels depending on the orientation of the steering wheel and/or depending on a request received from a computer. This power steering actuator can, for this, act on the steering column of the vehicle (which is fixed to the steering wheel) or on a rack (which connects the steering column to the steered wheels). Of course, the actuator could be arranged differently.

The computer is then designed to control the power steering actuator. For this purpose, it comprises at least one processor, at least one memory and various input and output interfaces.

Thanks to its input interfaces, the calculator is suitable for receiving input signals from different sensors.

Among these sensors, for example, a front camera and/or a radar is provided, making it possible to locate the position of the vehicle 10 in relation to its traffic lane VC.

The sensors may also include an angle sensor to measure the steering angle of the front steered wheels.

The vehicle sensors 10 also preferably include sensors for monitoring the driver's activity and/or level of attention. Such sensors may, for example, include a camera facing the driver's head, or pressure sensors located in the steering wheel.

Thanks to its output interfaces, the calculator is adapted to transmit a steering wheel angle instruction to the power steering actuator so as to control the latter so that it modifies the steering wheel angle according to the angle instruction.

By means of its memory, the computer stores data used in the context of the method described below. In particular, it stores a computer application, consisting of computer programs comprising instructions whose execution by the processor allows the computer to implement the method according to the invention described below.

The computer programmed to implement the method according to the invention thus forms a device for assisting in driving the vehicle according to the invention.

The computer also stores data and computer programs already known to enable the implementation of other driving assistance and driver activity detection processes.

These other processes relate in particular to keeping the vehicle in the centre of its lane. The computer programmed to implement a LCA type steering assistance process forms an LCA system. How to activate this LCA system and detect a centre line of the lane will not be described here.

This LCA system is programmed for example to implement a control law of the power steering actuator comprising a first setpoint element calculated in a closed loop and a second setpoint element determined by an open loop. The open loop has the function of taking into account the curvature of the road and of compensating for the effect of the bend on the states and the control. The closed loop has the function of keeping the vehicle in the center of its traffic lane while the latter is considered straight, i.e. rectilinear. The terms resulting from these two loops are added by means of an adder.

The method for detecting driver activity will also not be described in detail here. However, this method may notably be based on the detection of movements or actions of the driver or the monitoring of the attention (monitoring of eye movement, detection of closed eyes, of the position of the head for example) of the driver and allows, generally after a predetermined confirmation period, to detect an emergency situation linked to a lack of activity of the driver, for example when the driver feels unwell.

The driving assistance method according to the invention fits into this context. Indeed, the method according to the invention is preferably implemented when non-activity of the driver is detected.

The activation of the assistance device according to the invention, and therefore the implementation of the method according to the invention, is preferably independent of a choice by the driver, the assistance device implementing this method preferably being active by default.

Thus, after the detection of a confirmed non-activity of the driver, the assistance method according to the invention is implemented in order to realign the motor vehicle parallel to an ideal trajectory, also called in the following “reference trajectory” Id in the traffic lane.

This reference trajectory Id ( ) preferably coincides with the center line of the traffic lane VC, i.e. it is preferably the equidistant trajectory from the edge of the road 21 and the broken line 22 in the schematic example shown in the .

Realigning the vehicle means reorienting the vehicle so that its longitudinal axis X is parallel to the tangent T to the reference trajectory Id at the center of gravity CG of the vehicle, as detailed below. The implementation of the method according to the invention corresponds to a realignment maneuver.

Once the vehicle 10 has been realigned using the method according to the invention, the angle setpoint is for example calculated by the conventional LCA system which brings the vehicle 10 back towards the centre of the lane, i.e. makes the longitudinal axis X of the vehicle superimpose itself with the tangent T to the centre line of the traffic lane. This makes its trajectory coincide with the centre line of the traffic lane VC.

The assistance method according to the invention is used for the purpose of stopping the vehicle and regulating its position in the lane when a situation is detected in which the driver no longer exercises control of the vehicle, for example due to feeling unwell. In this context, the regulator ensuring lateral control, herein referred to as the LCA system, is used to regulate the position of the vehicle in the lane. The angle setpoint determined by the assistance method according to the invention is added here to the steering wheel angle setpoint conventionally determined by the LCA system.

Thus, the calculator of the assistance device according to the invention programmed to implement the method according to the invention is preferably also programmed to implement the LCA type steering assistance method. When the method for realigning the vehicle is implemented, the LCA system is activated and remains active until the end of the realignment maneuver.

Variables

Before describing in more detail the method according to the invention, we can introduce the different variables which will be used, some of which are illustrated in the .

The center of gravity of the vehicle will be noted “CG”.

The wheelbase of the vehicle 10, that is to say the distance between the axes of the two wheel sets, will be noted “L” and will be expressed in meters ( ).

We can consider an orthogonal reference frame (CG, X, Y, Z) attached to the vehicle. Its origin is the same as the center of gravity CG. The X axis corresponds to the longitudinal axis X of the vehicle. The Y axis corresponds to the lateral axis turned towards the left of the vehicle. When the vehicle is traveling on a horizontal road, the Z axis corresponds to the vertical axis. More generally, this Z axis is the axis normal to the road.

The steering wheel angle noted “δv” is the angle of rotation of the steering wheel around the steering column, the value of the steering wheel angle δv being zero when the wheels are aligned along the longitudinal axis X.

The steering angle that the front steered wheels 11 make with the longitudinal axis X of the motor vehicle 10 will be noted “δr” and will be expressed in rad.

Note that the steering angle δv and the steering angle δr are directly linked, with a gear ratio K and also dynamics that can be modeled as first or second order.

The longitudinal speed of the vehicle, along the X axis, will be noted V and will be expressed in m/s.

The heading angle between the X axis and the tangent T to the reference trajectory Id at the center of gravity CG will be noted “θ” and will be expressed in rad. In practice, the angles between the vehicle and the track lines, including the heading angle, are deduced from the data recorded by one or more cameras on the vehicle.

The yaw rate of vehicle 10, that is to say its rotation speed around the Z axis, will be noted “dθ/dt”.

The curvature of the reference trajectory Id of the traffic lane VC is denoted ρ (in m ^-1 ). This is the inverse of its radius of curvature at the level of the center of gravity CG of the motor vehicle 10.

In the example developed below, the reference trajectory Id is the center line of the traffic lane defined between the edge of road 21 and the middle of road 22, marked by a broken line on the .

Process

The method for assisting in driving the motor vehicle 10 according to the invention is designed to allow this vehicle 10 to reorient itself parallel to a reference trajectory Id of its traffic lane, in autonomous mode (without intervention by the driver). It is for example implemented when inactivity of the driver, for example due to driver discomfort, is detected.

In order to realign the vehicle 10 parallel to the reference trajectory Id, the steering assistance device generates the steering wheel angle instruction which allows the vehicle's heading angle to be returned to zero.

Where applicable, when the LCA system is active, this open-loop steering angle setpoint is added to the classic LCA system angle setpoint which includes the first closed-loop angle setpoint element and the second open-loop angle setpoint element related to road curvature, as mentioned previously.

Preferably, the steering wheel angle setpoint determined according to the steering assistance method of the invention is determined so as to return the vehicle's heading angle to zero as quickly as possible, while respecting the constraints of maximum steering wheel rotation speed and maximum lateral acceleration of the vehicle.

The maximum steering wheel rotation speed and the maximum lateral acceleration of the vehicle due to the realignment maneuver are two predetermined quantities that are taken into account as constraints. In the event of a turn, the lateral acceleration has two components that are added together and correspond to the contributions of the realignment maneuver controlled by the assistance device according to the invention, on the one hand, and of the tracking of the turn by the LCA system, on the other hand.

The maximum steering wheel rotation speed is determined in such a way as to limit the risk of injury to the driver when the steering wheel turns autonomously (without any action by the driver) under the control of the power steering actuator. It also takes into account the physical constraints of the steering that limit this speed. This maximum steering wheel rotation speed is, for example, equal to 100 degrees per second.

The lateral acceleration of the vehicle corresponds to the component of the acceleration of the vehicle in the direction of the Y axis of the reference frame attached to the vehicle.

The maximum lateral acceleration of the vehicle is the maximum value of the lateral acceleration of the vehicle guaranteeing driving comfort for the driver and/or allowing the driver to maintain control of the vehicle without risking a loss of grip. This maximum value depends on characteristic quantities of the vehicle (weight, position of the center of gravity, etc.) and also on the limits of feeling authorized for the driver for his comfort.

The maximum lateral acceleration is predetermined in any manner known to those skilled in the art. It is for example less than or equal to 3 m/s ² , preferably less than or equal to 2 m/s ² , for example between 0.6 and 0.8 m/s ² . When implementing the realignment method, the maximum lateral acceleration may be less than or equal to 10 m/s ² .

To calculate the open-loop angle setpoint, the method according to the invention is based on the vehicle dynamics equations in an established turn. The simulations carried out by the Applicant show that the behavior of the vehicle is consistent with the trajectory by making this assumption, even when the vehicle is not in an established turn regime.

The initial moment considered is that at which the assistance device according to the invention is activated, for example following the detection of discomfort in the driver of the vehicle.

At this initial moment, the calculator of the assistance device according to the invention is programmed to implement the following steps:
- acquisition of initial values of a heading angle θ ₀ and an initial speed V ₀ of the motor vehicle,
- determination of a maximum value of a steering wheel angle δvmax generating a maximum lateral acceleration Aymax predetermined by taking into account the initial value V ₀ of the speed V of the motor vehicle,
- determination of a steering wheel angle setpoint as a function of time comprising at least: a first setpoint part δv1(t) modifying the steering wheel angle to reduce the heading angle θ of the vehicle to a target value θtarget determined as a function of the initial value θ ₀ of the heading angle θ and the maximum value δvmax of the steering wheel angle and a second steering wheel angle setpoint part modifying the heading angle θ of the vehicle until it is zero.

At the initial instant, the heading angle θ of the vehicle is equal to an initial heading angle θ ₀ and its speed V is equal to an initial speed V ₀ and assumed to be constant. The initial speed of the vehicle and the initial heading angle of the vehicle are estimated or measured by any means known to those skilled in the art. The initial heading angle is deduced from the data recorded by the camera(s) and the initial speed is determined using vehicle sensors arranged on its wheels.

As mentioned earlier, the maximum lateral acceleration Aymax of the vehicle due to the vehicle realignment maneuver is a predetermined and known quantity.

The initial yaw rate is assumed to be zero. To model the vehicle's behavior, it is therefore assumed that the wheels are initially aligned along the vehicle's longitudinal axis X.

To determine the maximum value δvmax of the steering wheel angle δv, the calculator is programmed to:
- determine a maximum value dθmax of the yaw rate dθ/dt of the vehicle 10 as a function of the maximum lateral acceleration Aymax of the vehicle and the initial value V ₀ of the vehicle speed,
- determine the maximum value of the steering wheel angle δvmax as a function of the maximum value of the yaw rate dθmax, the initial value V ₀ of the vehicle speed and characteristic quantities of the vehicle.

At the initial speed V ₀ , the trajectory of the vehicle 10 must not exceed a certain curvature so that the lateral acceleration Ay of the vehicle does not exceed the maximum lateral acceleration Aymax. In a constant radius turn, the lateral acceleration Ay is related to the yaw rate dθ/dt by the following relationship:

Ay = V ₀ .dθ/dt.

Which gives for the maximum value dθmax of the yaw speed: dθmax = Aymax/V _{0 .}

The maximum value of the steering wheel angle δvmax is that which generates the maximum lateral acceleration Aymax in established cornering mode.

In established cornering mode, the steering angle δr of the front steered wheels is linked to the curvature ρ of the trajectory and to the lateral acceleration Ay by the following relation:

δr = L.ρ+K.Ay.

In this relationship, L is the length of the vehicle wheelbase and K is the value of the vehicle's understeer gradient. These are predetermined and known characteristic quantities of the vehicle.

The lateral acceleration Ay is equal to the square of the speed multiplied by the curvature of the trajectory, or Ay = V ₀ ² .ρ, and we have: V ₀ .dθ/dt = V ₀ ² .ρ.

The steering angle δr can be expressed as a function of the yaw rate and the vehicle speed: δr = dθ/dt.(L/V₀+ K.V₀).

Since the steering angle δv is equal to the steering angle δr multiplied by the gear ratio K_redof the power steering, we have: δv =K_red.dθ/dt.(L/V₀+ K.V₀).

In steady state, the maximum value δvmax of the steering wheel angle which generates the maximum lateral acceleration Aymax is therefore δvmax =K_red.dθmax.(L/V₀+ K.V₀).

The yaw rate can then be expressed as a function of the steering angle by: dθ/dt= δv/[K_red.( L/V₀+ K.V₀)].

According to the method of the invention, it is required that the steering wheel angle setpoint determined by the calculator varies over time according to a predetermined mathematical function.

This function is continuous. The first part of the setpoint varies the steering wheel angle in one direction (increase or decrease), while the second part of the setpoint varies the steering wheel angle in the opposite direction (decrease or increase).

This function is for example a continuous function. It can be in particular an affine function or more generally, a polynomial function.

More precisely, it is an affine or piecewise polynomial function.

In practice, in the example described here, said first and second parts of the steering wheel angle setpoint each vary according to an affine function of time.

The first part of the steering angle command to be determined modifies the steering angle to reduce the vehicle's heading angle θ to a target value θtarget.

This first part of the instruction is linear because we assumed the wheels were aligned at the initial moment. The initial value of the steering wheel angle is therefore zero.

The target value of the heading angle is determined as a function of the initial value θ ₀ of the heading angle θ and the maximum value δvmax of the steering wheel angle. It can also take into account the time function imposed for the variation of the steering wheel angle setpoint.

For example, the target heading angle value θtarget is equal to a fraction of the initial heading angle or to a heading angle value reached when the steering wheel angle reaches the maximum steering wheel angle value.

More precisely, the target θ value is adjusted by a compromise between efficiency and driver feel and must take into account the maximum steering wheel angle value so that the vehicle's lateral acceleration remains less than or equal to the maximum lateral acceleration. In the context of the proposed example, maximum efficiency is preferred, i.e. the fastest possible realignment is sought without any feel constraints.

A target value θtarget can then be the initial heading angle divided by 2, i.e. θtarget = θ ₀ /2, which theoretically allows, with the first part of the instruction varying linearly, to carry out the realignment as quickly as possible.

However, this target value can only be achieved if it is strictly less than the heading angle reached at the time when the steering angle value is equal to the maximum steering angle value.

Thus, the determined target value of the heading angle is equal to the fraction of the initial heading angle if it is strictly less than the heading angle reached at the time when the steering wheel angle value is equal to the maximum steering wheel angle value. Otherwise the target value is equal to the heading angle value reached when the steering wheel angle reaches the maximum steering wheel angle value.

So, in the example described here, δv1(t) = at and θtarget = θ ₀ /2.

By integrating the previous equation giving the yaw rate as a function of the steering angle dθ/dt= δv1(t)/[K_red.( L/V₀+ K.V₀)] = dθ/dt= a.t/[K_red.( L/V₀+ K.V₀)], we have:

θ(t) = ½ a.t²/ [K_red.( L/V₀+ K.V₀)] + θ₀.

The computer is programmed to determine the instant t _half for which the heading angle is equal to its target value, here θ(t) = θ ₀ /2.

We obtain the following equation:

When θ ₀ is positive, the variable a is negative and vice versa.

The steering angle at the moment when the heading angle is equal to its target value θ ₀ /2 is: δv1(t _half )=at _half.

The calculator is also programmed to impose a slope equal to the maximum steering wheel rotation speed VRmax in order to ensure rapid realignment of the vehicle, i.e. a=VRmax. The maximum steering wheel rotation speed VRmax is positive or negative depending on the direction of rotation of the steering wheel. Here, the sign of the steering wheel rotation speed is positive towards the left and negative towards the right. The variable a is: a= -Sign(θ ₀ ).VRmax.

The first part of the steering wheel angle instruction is then determined by the calculator as being δv1(t)=VRmax.t between the moment of activation of the assistance device t ₀ =0 and the calculated moment t _half : δv1(t)=VRmax.t if 0<t<t _half.

If the steering angle at the time when the heading angle is equal to its target value δv1(t _half )=VRmax.t _half is less than the maximum steering angle value, the first part of the angle setpoint is applied until time t _half .

The variation of the steering angle is thus carried out as quickly as possible.

The second part of the angle command returns the steering wheel angle to 0.

Preferably, the affine functions of time according to which the first and second setpoint parts vary are two affine functions whose slopes have equal absolute values and opposite signs.

Furthermore, the evolution over time of the steering wheel angle setpoint over time is preferably symmetrical. Graphically, the curve of the evolution over time of the steering wheel angle setpoint is preferably symmetrical with respect to a straight line parallel to the ordinate axis passing through the instant t _half .

In other words, the steering wheel angle setpoint varies from 0 to the angle setpoint value corresponding to the heading angle target value, and then from this value corresponding to the heading angle target value back to 0 symmetrically.

Thus, the absolute value of the slope of the second linear function used for the second part of the steering wheel angle setpoint is equal to the predetermined maximum steering wheel rotation speed. The steering wheel angle is thus brought back to 0 as quickly as possible.

In practice, the absolute value of the slope thus remains equal to the maximum rotation speed of the steering wheel: the variation of the steering wheel angle is ensured as quickly as possible.

We thus have: δv2(t)=-VRmax.t+b if t _half <t<2 _. t _half.

The parameter b is determined so that the steering wheel angle setpoint remains continuous. Thus δv2(t _half )=-VRmax.t _half +b = δv1(t _half )= -VRmax.t _half , and b=2VRmax.t _half .

Finally, the steering wheel angle instruction is zero for t greater than or equal to 2.t _half.

This case is schematically represented in the , which shows the steering wheel angle setpoint δv(t) as a function of time. This is equal to the union of the first and second setpoint parts δv1(t), δv2(t). The first setpoint part C1 corresponds to the evolution δv1(t)=VRmax.t up to t _half and the second setpoint part C2 corresponds to the evolution δv2(t)=-VRmax.t+2VRmax.t _half up to 2t _half (bottom curve). Correspondingly, the heading angle variation θ(t) is shown: the first part of the steering wheel angle setpoint brings the heading angle to the target value θ ₀ /2 (curve C3), while the second part of the setpoint brings the heading angle to zero (curve C4).

Using the steering wheel angle setpoint determined by the method according to the invention, the initially zero yaw rate is increased to the target heading angle value and then decreased until the heading angle is zero at zero yaw rate.

The steering wheel angle, which had an initial value of zero, is also returned to 0 at the end of the maneuver.

If, during the implementation of the first part of the instruction, the steering wheel angle reaches the maximum steering wheel angle value, the computer is programmed to saturate the steering wheel angle, i.e. to keep it equal to its maximum value during a third intermediate part of the instruction.

In this case, the first part of the angle setpoint is applied until time t ₁ at which the steering wheel angle becomes equal to its maximum value. In other words, if:

the calculator calculates the time t ₁ from which

Let with δv1(t ₁ ) = at ₁ = dθmax.Kred.(L/V ₀ +KV ₀ ), t ₁ =1/a.dθmax.Kred/V ₀ .(L+KV ₀ ² ).

The first part of the steering wheel angle instruction is then: δv1(t) = VRmax.t for 0<t<t ₁ , that is to say between the moment of activation of the assistance device t ₀ =0 and the calculated moment t ₁ .

The computer is programmed to maintain the steering wheel angle equal to its maximum value for a determined duration depending on the initial value of the heading angle θ ₀ and the maximum value of the yaw rate dθmax. In other words, the computer is programmed to determine a third part of the constant setpoint equal to the maximum value of the steering wheel angle between time t ₁ and another calculated time t ₂ . After this other calculated time t ₂ , the computer is programmed to determine the second part of the setpoint.

The heading angle at time _t1 is:

θ(t₁) = θ₁ = ½.dθmax².1/a.K_red.(L/V₀+K.V₀)+θ₀.

The evolution of the heading angle during the third intermediate part of constant steering wheel angle setpoint equal to the maximum value of the angle is obtained by integrating the equation mentioned above:

dθ/dt = δv3(t)/[K _red .(L/V ₀ +KV ₀ )].

If we impose a constant steering angle equal to its maximum value δvmax the integral of the function is equal to the constant times the variation in time:

θ(t) = δvmax/[K _red .(L/V ₀ +KV ₀ )].(tt ₁ )+θ ₁ .

The term δvmax/[K _red .(L/V ₀ +KV ₀ )] corresponds to the maximum yaw rate dθmax for a given maximum steering angle.

As mentioned above, the affine functions of time according to which the first and second parts of the setpoint vary are two affine functions whose slopes have equal absolute values and opposite signs. This allows the steering wheel angle to be varied as quickly as possible. The vehicle is thus reoriented as quickly as possible.

The evolution of the steering wheel angle setpoint over time then has a symmetrical shape.

Considering that the absolute values of the slopes are equal, the heading angle reduction when implementing the second part of the command is equal to the difference between the initial heading angle θ ₀ and the heading angle θ ₁ reached at the end of the first part of the command.

Let: θ(t₂) = θ₂= θ₀– θ_{1 .}

We obtain: θ₂= dθmax.(t₂-t₁)+θ₁ = θ₀–θ₁, let t₂= (θ₀–2θ₁)/dθmax+t₁.

Thus, the third part of the steering wheel angle setpoint is here δv3(t) = δvmax = K _red .dθmax.(L/V ₀ +KV ₀ ) = Aymax.K _red .(L/V ₀ ² +K) for t ₁ ≤ t<t ₂ , that is to say between the instant t ₁ at which the steering wheel angle becomes equal to its maximum value and the instant t ₂ calculated.

During the third setpoint part, the steering wheel angle is maintained equal to the maximum value of the steering wheel angle for the duration t ₂ -t ₁ which depends on the initial value of the heading angle θ ₀ and the maximum value of the yaw rate dθmax. It also depends on the value of the heading angle θ ₁ when the steering wheel angle reaches its maximum value. This duration is determined so that the return of the steering wheel angle to zero after the first and third setpoint parts, i.e. when implementing the second setpoint part, takes place over a duration identical to that of the implementation of the first setpoint part.

The heading angle is thus brought back to zero at time _t3 = _t2 + _t1 . The second part of the steering wheel angle setpoint, symmetrical to the first, is thus δv2(t)=-VRmax.t+c between the calculated times _t2 and _t3 with c = _VRmax.t3 . The setpoint is zero beyond time _t3 .

This case is schematically represented in the , which shows the steering wheel angle setpoint δv(t) as a function of time. The curve representing the steering wheel angle setpoint δv(t) as a function of time is the combination of three parts of curves C1’, C2’, C5’ representing respectively the first, second and third parts of setpoint δv1(t), δv2(t), δv3(t). The first part of curve C1’ corresponds to the evolution of the first part of setpoint δv1(t)=VRmax.t up to t₁ and the second part of curve C2’ corresponds to the evolution of the second part of setpoint δv2(t)=-VRmax.t+c between t₂and t₃(bottom curve). The third part of curve C5’ corresponds to maintaining the steering angle at its maximum value between times t₁and t₂by the third part of the instruction. Correspondingly, the variation in heading angle θ(t) is represented by a curve formed by the union of three portions of curve C3’, C6’ and C4’: the first part of the steering wheel angle instruction brings the heading angle to the target value θ1 (variation in heading angle represented by the portion of curve C3’) with a heading speed that increases to its maximum value, while the third part of the instruction corresponds to a linear decrease in slope equal to the maximum yaw rate (variation in heading angle represented by the portion of curve C6’) and the second part of the instruction brings the heading angle to zero (variation in heading angle represented by the portion of curve C4’) by reducing the yaw rate. The heading angle is cancelled at zero yaw rate. The steering wheel angle, which had an initial value of zero, is also returned to 0 at the end of the maneuver.

Figures 4, 5 and 6 show simulation results of the behavior of a vehicle subjected to the steering wheel angle setpoint determined according to the driving assistance method of the invention described above.

There shows the result of a simulation in which the closed loop of the LCA system is deactivated. These results show that the steering wheel angle setpoint generated according to the method of the invention (represented by curve C11) is sufficient in itself to realign the vehicle, i.e. to cancel its heading angle (represented by curve C10). The implementation of the method according to the invention is triggered at t ₀ = 12 seconds. The initial heading angle is θ ₀ = 2 degrees, the initial speed assumed to be constant is equal to 90 km/h. The value of the maximum lateral acceleration for this simulation is equal to 10m/s ² . At t = 13 seconds, the heading angle is almost zero, which means that the vehicle is realigned parallel to the reference trajectory. Curve C12 shows the evolution of the steering wheel rotation speed setpoint. The maximum steering wheel rotation speed here is 100 degrees/s. In practice, this C12 curve is the derivative of the steering wheel angle setpoint which allows the maximum imposed steering wheel rotation speed to be checked.

There shows the result of another simulation in a similar situation. The closed loop of the LCA system is disabled. The value of the maximum lateral acceleration is reduced compared to the example in so that saturation of the steering wheel angle is necessary. The implementation of the method according to the invention is triggered at t ₀ = 13.5 seconds. The initial heading angle is θ ₀ = 2 degrees, the initial speed assumed to be constant is equal to 80 km/h. The maximum lateral acceleration is equal to 2 m/s². At t = 14.5 seconds, the heading angle is very close to 0. The steering wheel angle setpoint calculated (curve C14) solely by the method according to the invention therefore makes it possible to bring the heading angle (curve C13) close to zero. Curve C15 shows the evolution of the steering wheel rotation speed setpoint. The maximum steering wheel rotation speed here is 100 degrees/s.

There shows the results of comparative simulations with and without the method according to the invention. The results of the simulation when the method according to the invention is implemented with the active conventional LCA system are shown in solid lines. The results of the simulation when the method according to the invention is not implemented are shown in dotted lines. The simulation then corresponds in the latter case to the results obtained solely using the conventional LCA system.

In practice, the steering wheel angle instructions generated by the method according to the invention and by the LCA system pass through a slope limiter and saturation. The curves shown show the results of simulations of the steering wheel angle and rotation speed instruction after this passage through the slope limiter and saturation. Curve C161 corresponds to the total steering wheel angle instruction generated by the LCA system alone, after saturation and speed limitation. Curve C162 corresponds to the total steering wheel angle instruction generated by the method according to the invention added to the steering wheel angle instruction generated by the LCA system, after saturation and speed limitation. It can be seen by comparing the curves of deviations from the ideal trajectory obtained with (solid line) and without (dotted line) the implementation of the method according to the invention that the use of the method according to the invention makes it possible to return the heading angle more quickly to zero with smaller deviations from the ideal trajectory.

The invention further relates to the motor vehicle 10 comprising the steering assistance device and the power steering actuator described above, in which said computer of the assistance device is programmed to control the power steering actuator so as to vary the steering wheel angle while taking into account said instruction.

The method and the assistance device according to the invention have the advantage of allowing a very rapid realignment of the vehicle parallel to the ideal trajectory considered, for example the center line of the traffic lane. This realignment respects the maximum values of steering wheel angle and steering wheel angle rotation speed predetermined for the vehicle while ensuring an optimized speed of execution. After the realignment carried out by the method and the device according to the invention, other methods and systems for conventional vehicle steering assistance take over, in particular to bring the vehicle back to the center of the lane, that is to say with a longitudinal axis tangent to the center line of the traffic lane, and possibly to stop the vehicle in an emergency situation.

Claims (10)

Method for assisting in the steering of a motor vehicle (10) to realign it parallel to an ideal trajectory (Id) in its traffic lane (VC), comprising the following steps:
- acquisition of the initial values of a heading angle (θ ₀ ) and a speed (V ₀ ) of the automobile vehicle (10),
- determination of a maximum value (δvmax) of a steering wheel angle generating a predetermined maximum lateral acceleration (Aymax) taking into account the initial value (V ₀ ) of the speed of the motor vehicle (10),
- determination of a steering wheel angle setpoint (δv(t)) as a function of time comprising at least:
a first setpoint part (δv1(t)) modifying the steering wheel angle to reduce the heading angle (θ) of the vehicle to a target value (θtarget) determined as a function of the initial value (θ ₀ ) of the heading angle and the maximum value (δvmax) of the steering wheel angle, and
a second part of the steering wheel angle instruction (δv2(t)) modifying the heading angle (θ) of the vehicle until it is cancelled,
- control of a power steering actuator so as to vary the steering wheel angle taking into account said determined steering wheel angle setpoint (δv(t)). Method according to claim 1, according to which, to determine the maximum value (δvmax) of the steering wheel angle:
- a maximum value (dθmax) of a vehicle yaw rate is determined as a function of the maximum lateral acceleration (Aymax) of the vehicle (10) and the initial value of the vehicle speed (10),
- the maximum value (δvmax) of the steering angle is determined as a function of the maximum value (dθmax) of the yaw rate, the initial value (V ₀ ) of the vehicle speed and characteristic quantities of the vehicle. Piloting method according to one of claims 1 and 2, according to which a steering wheel angle setting varying over time according to a predetermined mathematical function is imposed. A method according to claim 3, wherein said first and second steering wheel angle setpoint parts each vary according to an affine function of time. Method according to claim 4, according to which the affine functions of time according to which the first and second setpoint parts vary are two affine functions whose slopes have equal absolute values and opposite signs. The method of claim 5, wherein the absolute value of the slope of each of the first and second affine functions is equal to a predetermined maximum steering wheel rotation speed (VRmax). Method according to one of claims 1 to 6, according to which the determined target value (θtarget) of the heading angle is equal to a fraction of the initial heading angle or to a heading angle value reached when the steering wheel angle reaches the maximum steering wheel angle value. Method according to one of claims 1 to 7, according to which when the steering angle reaches said maximum steering angle value (δvmax) during the first setpoint part, a third intermediate setpoint part (δv3(t)) is determined according to which the steering angle is maintained equal to the maximum value of the steering angle for a duration determined as a function of the initial value (θ ₀ ) of the heading angle and the maximum value (dθmax) of the yaw rate. Device for assisting in the steering of a motor vehicle (10) to realign it with respect to an ideal trajectory in its traffic lane (30), comprising a computer programmed to implement the method of one of claims 1 to 8. Motor vehicle (10) comprising a steering assistance device according to claim 9 and a power steering actuator, in which said computer of the assistance device is programmed to control this power steering actuator.