FR3097508A1 - A method of regulating the speed of a motor vehicle implementing an adaptive cruise control function - Google Patents

Abstract

The invention relates to a method of regulating the speed of a motor vehicle (1), the vehicle implementing an adaptive speed regulation function, hereinafter ACC function, the method comprising the steps of: determining a torque setpoint to be applied to the wheels of the vehicle, as a function of a setpoint acceleration; - determine the steering angle of the vehicle; - estimate the longitudinal component of the lateral forces applied to the vehicle during steering; - determining a compensation torque to be applied to the wheels to compensate for the longitudinal component of the lateral forces; - modulate the setpoint torque by adding the setpoint compensation torque. Figure for the abstract: Fig. 1

Description

Method for regulating the speed of a motor vehicle implementing an adaptive speed regulation function

The invention relates to the field of driving assistance systems fitted to motor vehicles. Driving assistance systems, often referred to by the acronym ADAS (for " Advanced driver-assistance systems "), aim to lighten the load on the driver, in particular by freeing him from certain tasks, by improving his attention and/or its perception of the environment, by detecting certain risks, by automatically performing actions in response to the detection of these risks, etc.

To perform the desired functions, the driving assistance systems operate using sensors fitted to the vehicle and making it possible to perceive the environment of the vehicle, and in particular the vehicles located nearby. This perception of the environment by the driving assistance system makes it possible to prevent the risk of accidents by guaranteeing compliance with predefined rules, such as for example that of requiring the vehicle to remain at a minimum distance from the vehicle which precedes it.

Among the various driving assistance functions, an adaptive longitudinal regulation function is known, commonly designated by the acronym ACC (for “ adaptive cruise control ”). This function aims to maintain the vehicle at a set speed defined by the driver, while regulating the speed of the vehicle to manage the following of a slower vehicle if necessary. Thus, if a vehicle traveling at a speed lower than the setpoint speed defined by the driver precedes the vehicle concerned, the driving assistance system will manage the braking of the latter. However, the driver remains responsible for driving the vehicle and must be able to intervene at any time if the situation so requires.

The operation of an adaptive cruise control system aims to control the acceleration of the vehicle, through the powertrain and the controlled braking system. For this purpose, the system must determine a setpoint acceleration to then determine the torque to be applied to the wheels. In order to determine the setpoint acceleration, the regulation system generally uses the sum of two controllers:

• an adaptive controller by feedforward action (commonly designated by the English name "Feedforward"), which is based on the a priori knowledge of the vehicle: the mass of the vehicle is known or estimated, the slope of the road is known thanks to an accelerometer longitudinal, and depending on these parameters, it is possible to determine what torque must be applied to the wheels to achieve a given vehicle acceleration;
• a feedback controller, (generally referred to as “feedback”), which loops back on the error between a target acceleration and a measured acceleration.

In certain situations, for example when the driver steers his vehicle sharply, an unwanted deceleration of the vehicle occurs. This deceleration is corrected by the feedback controller, but with some delay. In the case of steering, the deceleration is due to the lateral forces to which the vehicle is subjected. Indeed, these forces include a component parallel to the longitudinal axis of the vehicle which is not zero, and which is directed towards the rear of the vehicle, thus opposing its advancement.

The object of the present invention is to remedy the drawbacks of the state of the art, and more particularly those set out above, by proposing a speed regulation method for a motor vehicle implementing an adaptive speed regulation function which makes it possible, when the wheels of the vehicle are steered, to limit any unwanted deceleration as much as possible.

To this end, the invention relates to a method for regulating the speed of a motor vehicle, the vehicle implementing an adaptive speed regulation function, hereinafter ACC function, the method comprising the steps consisting of:

- determining a setpoint torque to be applied to the wheels of the vehicle, as a function of a setpoint acceleration;

- determine the steering angle of the vehicle;

- estimate the longitudinal component of the lateral forces applied to the vehicle during steering;

- determining a compensation torque to be applied to the wheels to compensate for the longitudinal component of the lateral forces;

- modulating the setpoint torque by adding the setpoint compensation torque.

Thus, based on the steering angle of the wheels, the method according to the invention makes it possible to estimate the torque which it is necessary to apply to the wheels in order to avoid any deceleration due to the lateral forces undergone by the vehicle following the steering of the wheels. Indeed, knowledge of the steering angle and certain parameters of the vehicle, such as its speed and its mass, makes it possible to easily estimate the lateral forces applied to the vehicle during steering, and therefore the torque necessary to compensate these efforts and thus avoid any unwanted deceleration. Knowing the torque required to compensate for the lateral forces allows it to be taken into account at the level of the Feedforward type controller, thus avoiding a momentary deceleration until it is compensated by the Feedback type controller.

In one embodiment, the compensation torque is given by the following formula:

where R _wheel is the radius of the driving wheels;

m is the mass of the vehicle;

L _rear is the distance between the axis of rotation of the rear wheels and the center of gravity G of the vehicle;

L _front is the distance between the axis of rotation of the front wheels and the center of gravity G of the vehicle;

V is the vehicle speed;

α is the front wheel steering angle;

K is the oversteer coefficient of the vehicle.

In one embodiment, the steering angle is determined by means of an angle sensor on the steering wheel.

In one embodiment, mass is determined using one or more mass sensors.

In one embodiment, the mass is estimated as a function of the variation of the driving force generated at the wheels of the vehicle and of the variation of the acceleration of the vehicle

The invention also relates to an assisted driving system comprising a computer, at least one sensor and actuators arranged to implement the method as defined above.

The invention also relates to a motor vehicle equipped with an assisted driving system as defined above and/or implementing a method as defined above.

The present invention will be better understood on reading the following detailed description, made with reference to the accompanying drawings, in which:

FIG. 1 represents a motor vehicle equipped with an assisted driving system allowing the implementation of a method in accordance with the invention.

FIG. 2 represents a model of the vehicle and the forces undergone during steering.

FIG. 1 represents a motor vehicle able to implement one or more driving assistance functions, and which is for this purpose equipped with an assisted driving system 2. In the remainder of this description this vehicle will be referred to as host vehicle 1. The assisted driving system 2 comprises in particular a computer 10, actuators 12, as well as a plurality of detectors, in the example a plurality of sensors 14a, 14b, 16a, 16b, 18 distributed on the front , the rear and the sides of the host vehicle 1. These sensors may include one or more of the following types of sensors: ultrasonic sensors, radars, lidars, day vision cameras, night vision cameras, etc. The assisted driving system 2 comprises in the example four cameras 18 (i.e. a front camera, a rear camera and two side cameras) and a plurality of sensors 14a, 14b, 16a, 16b of the ultrasonic and/or radar sensor type and /or lidar. The computer 10 receives the data provided by all the sensors 14a, 14b, 16a, 16b and by all the cameras 18.

Thanks to all of these sensors, the computer 10 is able to detect the presence of obstacles in the environment of the vehicle, and in particular the presence of vehicles in this environment, whether on the current track of the host vehicle 1 or on adjacent lanes.

The computer 10 is also connected to a plurality of actuators 12, which include devices able to act in particular on the accelerator and the braking system of the host vehicle, in order to control its speed. Advantageously, the actuators 12 also comprise devices capable of acting on the direction of the vehicle in order to control its trajectory.

The assisted driving system is configured to implement, in an assisted driving mode, one or more assistance functions, including in particular an adaptive cruise control function, hereinafter the ACC function. As mentioned above, such a function makes it possible to regulate the speed of the vehicle without intervention from the driver (the latter must however retain control of the vehicle, and in particular control of the direction, since this is a assistance function), respecting a safety distance instruction vis-à-vis the vehicle preceding the host vehicle, that is to say a minimum gap between these two vehicles. The ACC function is also capable of managing the slowing down of the vehicle, if the slowing down of the vehicle in front of it requires it. Preferably, the assisted driving system 2 comprises at least one camera and one radar type sensor to implement the ACC function described above.

The assisted driving system, to implement the ACC function, determines a setpoint acceleration, and, on the basis of this setpoint acceleration, determines the torque that it is necessary to apply to the wheels. The setpoint acceleration depends in particular on the difference between the speed of the vehicle and the setpoint speed entered by the driver when implementing the ACC function (this setpoint speed can be modulated by the automated driving system according to the gap with the preceding vehicle).

As mentioned above, a significant steering can lead to an undesired temporary decrease in the speed of the vehicle (for example because the vehicle is traveling on a roundabout), due to the lateral forces which include a non-zero component along the axis longitudinal of the vehicle. The deceleration induced by these lateral forces can reach values close to 0.05 g, or even higher. A description is given below, in relation to FIG. 2, of how the method in accordance with the invention makes it possible to take account of the steering of the vehicle in order to anticipate and counter these lateral forces, by modulating the setpoint acceleration determined by the system assisted driving 2.

FIG. 2 is a diagram of a vehicle represented in the form of a “bicycle” model, that is to say a simplified model with a front wheel and a rear wheel. In the diagram of FIG. 2, the vehicle is in a configuration in which the front wheel is turned through an angle α. In such a configuration, it can be seen that the lateral forces Fy _front , Fy _rear which are exerted respectively on the front wheel and on the rear wheel are parallel to the axis of rotation of the corresponding wheel. The rear lateral force Fy _rear is therefore perpendicular to the longitudinal axis of the vehicle (corresponding to the axis AB, axis passing through the centers of rotation A, B, of the front and rear wheels).

On the other hand, the front lateral force Fy _front is not perpendicular to the longitudinal axis AB of the vehicle. It follows that the front lateral force Fy _front has a non-zero longitudinal component. It is this longitudinal component, directed towards the rear of the vehicle, which opposes the advancement of the vehicle, and which is therefore likely to cause an undesired slowing down of the vehicle during steering if it is not taken into account.

The longitudinal component of the forward lateral force Fy _front is equal to:

In the formula above, α is the steering angle of the front wheel, and Fy _front is the value of the front lateral force, i.e. the lateral force that applies to the front wheel .

Given that :

In this equation:

α is the steering angle of the front wheel;

m is the mass of the vehicle;

γ _lat is the lateral acceleration of the vehicle;

I is the yaw moment of inertia of the vehicle;

φ is the yaw angle of the vehicle, so that the second derivative of this angle is equal to the yaw acceleration of the vehicle;

L _rear is the distance between the axis of rotation of the rear wheels and the center of gravity G of the vehicle (distance GB in FIG. 2);

L _front is the distance between the axis of rotation of the front wheels and the center of gravity G of the vehicle (distance GA in FIG. 2).

In the double equation above, we neglect the term cos α, which is close to 1 when the angle α is less than or equal to 15°, knowing that the order of magnitude of the steering angle of the wheels is about 10° on a roundabout.

Moreover, neglecting the yaw acceleration (case of an established turn), the yaw rate divided by the steering angle is written:

With :

K is the oversteering coefficient of the vehicle;

C _front is the pneumatic drift stiffness of the front axle, which is a constant for a given tire and load;

C _rear is the pneumatic stiffness of the rear axle, which is also a constant for a given tire and load.

In addition, in an established turn, the lateral acceleration γ _lat corresponds to the product of the yaw rate by the longitudinal velocity V.

Thus, neglecting the yaw acceleration, we obtain the following equation:

Where V is the vehicle speed (in ms ^{- 1} ), and K the oversteer coefficient. The oversteering coefficient K is a constant for a given vehicle, which is a function in particular of the tire cornering rigidities and of the geometry of the running gear, as shown above.

We then deduce the longitudinal component Fx of the front lateral force:

Knowledge of this longitudinal component Fx makes it possible to determine the _wheel torque C to be applied to the wheel in order to compensate for it:

Either :

Thus, the method in accordance with the invention makes it possible to estimate very simply the torque which it is necessary to apply to the wheels in order to compensate for the deceleration caused by a steering of the vehicle. Indeed, it suffices to know the mass of the vehicle, the steering angle, the speed of the vehicle, as well as the distance of the front and rear running gear from the center of gravity G of the vehicle, the radius of the tires, as well as the coefficient of oversteering.

The mass of the vehicle can be determined by means of sensors, or estimated using the process covered by patent application FR 3 054 660 entitled “Process for estimating the mass of a motor vehicle”. Such a method makes it possible to estimate the mass of a vehicle in motion, based on the variation of the engine force generated at the wheels of the vehicle and on the variation of the acceleration of the vehicle.

The steering angle can be determined by means of a steering wheel angle sensor, by knowing the gear ratio of the steering.

Claims (7)

Method for regulating the speed of a motor vehicle (1), the vehicle implementing an adaptive speed regulation function, hereinafter ACC function, the method comprising the steps consisting in:
- determining a setpoint torque to be applied to the wheels of the vehicle, as a function of a setpoint acceleration;
- determine the steering angle of the vehicle;
- estimate the longitudinal component of the lateral forces applied to the vehicle during steering;
- determining a compensation torque to be applied to the wheels to compensate for the longitudinal component of the lateral forces;
- modulating the setpoint torque by adding the setpoint compensation torque. Method according to the preceding claim, in which the compensation torque is given by the following formula:

where R _wheel is the radius of the drive wheels;
m is the mass of the vehicle;
L _rear is the distance between the axis of rotation of the rear wheels and the center of gravity G of the vehicle;
L _front is the distance between the axis of rotation of the front wheels and the center of gravity G of the vehicle;
V is the vehicle speed;
α is the front wheel steering angle;
K is the oversteer coefficient of the vehicle. Method according to one of the preceding claims, in which the steering angle is determined by means of an angle sensor on the steering wheel. Method according to one of Claims 2 and 3, in which the mass is determined by means of one or more mass sensors. Method according to one of Claims 2 and 3, in which the mass is estimated as a function of the variation of the driving force generated at the wheels of the vehicle and of the variation of the acceleration of the vehicle. Assisted driving system (2) comprising a computer (10), at least one sensor (14a, 14b, 16a, 16b, 18) and actuators (12) arranged to implement the method according to one of Claims 1 at 5. Motor vehicle (1) fitted with an assisted driving system (2) in accordance with the preceding claim.