Method for controlling a motor vehicle with four-wheel steering, which includes taking into account constraints on the steering of the rear wheels

Abstract

The invention relates to a method for controlling a motor vehicle (10) which includes at least one steering actuator for the wheels of at least one rear axle and, for each of the front wheels and the wheels of the at least one rear axle, a differential braking actuator, the method comprising: a / calculating actuator commands on the basis of a control request representative of a desired yaw moment of the vehicle by means of a command allocation method; b / distributing the commands to the actuators, the commands being calculated by taking into account constraints on the minimum and maximum lateral forces that can be applied to the rear wheels of the vehicle in order to modify their steering angle.

Description

Description Title of the invention: Method for controlling a four-wheel steering motor vehicle, taking into account constraints on the steering of the rear wheels. Technical field

[0001] The present invention relates to the field of automotive vehicle equipment control.

[0002] It relates more specifically to a vehicle motion control method, or "vehicle motion control" (VMC) in English, the vehicle having four steering wheels and implementing a plurality of actuators acting in particular on the steering angle and on the differential braking of the wheels.

[0003] The invention also relates to a control system configured to implement the control method and to a vehicle comprising such a control system. Previous technique

[0004] It is known to implement four-wheel steering (4RD or 4WS) in motor vehicles. The steering of the two rear wheels is typically implemented by a single actuator but can also be implemented by two separate actuators, each acting on one of the two rear wheels.

[0005] A motor vehicle with four-wheel steering can be steered more easily and has better stability and maneuverability compared to a two-wheel steering vehicle where only the front wheels can be turned.

[0006] The thesis [1] describes a vehicle motion control system, the vehicle having four steering wheels and a differential braking actuator for each of the four wheels.

[0007] The described control system can notably be implemented to optimize the controls allowing the distribution of a desired yaw moment on the differential braking actuators of the four wheels and on the actuator controlling the steering of the rear wheels.

[0008] Figure 1 shows the block diagram of such a control system for controlling the yaw rate of a motor vehicle. As illustrated, this system operates in a closed loop.

[0009] First, a reference model 1 is used to determine the desired yaw rate of vehicle V re f, that is, the time derivative of the yaw angle The desired yaw rate depends, in particular, on the driver's actions, for example on the steering wheel or the vehicle's pedals, and / or the actions of a control unit of a partially or fully autonomous vehicle. Without limitation, the reference model may be defined by a bicycle model known as such, by a closed-loop controller, or by any means allowing the definition of a yaw setpoint representative of the desired behavior.

[0010] A high-level software controller 2 determines a yaw moment M z corresponding to the desired yaw rate. Then, a command allocation unit 4 implements a command allocation process, also known as control allocation, to determine which commands to optimize taking into account the limitations of the chassis 3.

[0011] The controls to be optimized by the allocation process include the braking force (or longitudinal force) at the left front wheel F x fb the braking force at the right front wheel F x f r , the braking force at the right rear wheel F xrr , the braking force at the left rear wheel F xri and the lateral force to the rear wheels F yr related to the steering of the rear wheels.

[0012] These commands are transmitted to low-level software controllers 5, 6, 7, 8, 9 corresponding elements that translate these force commands respectively into braking accelerations r b>fl , r bjfr , r b4 , r b>rr for each of the four front left, front right, rear left, and rear right wheels respectively, and in the steering angle of the rear wheels r .

[0013] The vehicle's actuators and vehicle 10 then implement the commands determined by the control system. Sensors on the vehicle measure its movement, including the yaw rate V, which is compared to the desired yaw rate of the reference model.

[0014] The high-level controller 2 then determines a new yaw moment M z to be reproduced according to the desired yaw rate and the measured yaw rate and the command allocation process is reproduced.

[0015] Thus, the closed-loop system as described implements an optimal allocation of commands on the differential braking actuators, each acting on one of the four wheels of the vehicle, and on the rear wheel steering actuator, said to operate four-wheel steering or 4RD.

[0016] The commands sent to the various actuators can be calculated using a command allocation method known as such, for example implementing a constrained optimization algorithm known as such. An example of a suitable command allocation algorithm is an active set algorithm, as described in the thesis [2]. This algorithm optimizes the commands based on a matrix linking the actuator commands to the setpoints given by the closed-loop controller.

[0017] This matrix, called the control effectiveness matrix, is determined by the equations of vehicle physics. However, the constraints imposed on the algorithm can be chosen so that the commands it calculates meet predetermined criteria for performance, efficiency, or safety.

[0018] According to thesis [1], the constraints considered include actuator-related constraints (i.e., the maximum action amplitude and maximum ramp of each actuator, the ramp corresponding to the time derivative of the amplitude) as well as physical constraints related to tire grip on the road and the maximum forces that can be applied to the tires while maintaining grip. These tire-related constraints define the friction ellipse.

[0019] According to thesis [1], the friction ellipse is used to define the maximum braking force Fx^max that can be imposed on each wheel {i, j} where i identifies the front f or the rear r and j identifies the left side 1 or the right side r of the vehicle.

[0020] CN application 105867168 A describes a command allocation model for an electric vehicle but does not disclose 4RD actuator command.

[0021] The control system described in the thesis [1] only takes into account the physical constraints of the actuators (maximum amplitude and ramp) and the adhesion of the tires.

[0022] In practice, however, the 4RD actuator must comply with safety constraints that require limiting the amplitude and ramp settings according to the vehicle's speed. Furthermore, the 4RD actuator has a direct impact on the driver's and passengers' experience while driving.

[0023] Existing solutions can therefore still be improved to better take into account the feelings of the driver and passengers as well as safety constraints.

[0024] There is therefore a need to improve existing motor vehicle actuator control processes, particularly to improve user safety and / or comfort.

[0025] The aim of the invention is to meet at least part of this need. Summary of the invention

[0026] To this end, the invention relates in one of its aspects to a method for controlling a motor vehicle comprising at least one steering actuator for the wheels of at least one rear axle of the vehicle and, for each of the front wheels and the wheels of at least one axle, a differential braking actuator, the method comprising: a) Calculate actuator commands based on a command request representative of a desired yaw moment of the vehicle using a command allocation method; b) Distribute the commands to the actuators, step a) calculating the actuator commands implementing the command allocation method such that: the maximum lateral force constraint that can be applied to the wheels of at least one rear axle F yrfn has Xis equal to the minimum of the maximum stress F of the lateral force applied to the wheels of at least one rear axle corresponding to a maximum steering angle of 15 rfn has X | and the maximum stress F yr / nax^llipse of the lateral force applied to the wheels of F at least one rear axle defined by the friction ellipse, the minimum lateral force stress that can be applied to the wheels of F at least one rear axle of vehicle F yr / t ùn is equal to the maximum of the minimum stress F yr could nflC that of the lateral force applied to the wheels of F at least one rear axle corresponding to a minimum steering angle and the minimum stress F yr / nin piii pse of the lateral force applied to the wheels of at least one rear axle defined by the friction ellipse, the steering control of the wheels of at least one rear axle being symmetrical, and

[0027] F yrjnaxpccei = 2C r | ô rpU ix | where C r is the drift stiffness of the tires of the wheels of F at least one rear axle and 5 r ,my X is defined according to a desired perceived wheelbase Ldes of the vehicle.

[0028] In one embodiment, the maximum steering angle ô r / nax is such that

[0032] where Rsteer is the ratio between the steering wheel angle and steer and the angle of the front wheels f DA is the angular dynamics that characterizes the relationship between the steering wheel angle and the lateral acceleration seen during the turn, ô stœr>max is the maximum steering wheel angle, oh f>max is the maximum desired angle of the front wheels, a y maT is the maximum lateral acceleration, v is the speed of the vehicle, L is the physical wheelbase of the vehicle.

[0033] The vehicle may include a single rear axle, particularly when the vehicle is a car.

[0034] In one embodiment, step a / of calculating the actuator commands implements the command allocation process with the following constraints:

[0035] Fyrjnax min (F 'yrynaxyuxeh fyrynax,ellipse)

[0036] ) F yrynax is the maximum lateral force that can be applied to the rear wheels of the vehicle and F yrfnin is the minimum lateral force constraint that can be applied to the rear wheels of the vehicle.

[0042] where C r is the lateral stiffness of the rear wheel tires, p is the coefficient of friction, F ZrJ is the vertical force on the wheel {r, j} where r identifies the rear F of the vehicle and j identifies the left or right side of the vehicle, Fx rJis the longitudinal force on the wheel {r, j}, and

[0046] where Rsteer is the ratio between the steering wheel angle and steer and the angle of the front wheels f DA is the angular dynamics that characterizes the relationship between the steering wheel angle and the lateral acceleration of the vehicle during its turn, ô stœr>max is the maximum steering wheel angle, oh fma , is the maximum desired angle of the front wheels, a y m " is the maximum lateral acceleration, v is the vehicle speed, L is the vehicle's physical wheelbase and L to is a desired perceived wheelbase of the vehicle.

[0047] Preferably, the method according to the invention is implemented by a control system as described with reference to [Fig. 1]. This system preferably comprises a computer configured to implement the method according to the invention.

[0048] Advantageously, using these constraints in the control optimization process improves vehicle stability and driving feel. These constraints allow for consideration not only of safety aspects related to control allocation but also those related to driving experience.

[0049] Thus, the constraints on the rear wheel steering actuator take into account the maximum steering wheel angle. s w depending on a predetermined maximum lateral acceleration a y!max and the current speed of the vehicle v.

[0050] The maximum lateral acceleration has y>m "is determined in such a way as to propose a maximum stress envelope for the vehicle. This envelope is preferably adjusted according to the capabilities of the chassis, particularly with regard to road contact, including the tires. Preferably, a y!max can be greater than or equal to 2 m / s2 and / or less than or equal to 14 m / s 2 The value of a yimax This can be adjusted during vehicle testing depending on the chassis. Advantageously, the steering angle can be limited based on the perceived lateral grip of the vehicle's rear axle.

[0051] The maximum steering wheel angle for maximum lateral acceleration is determined by

[0053] The maximum desired steering angle of the front wheels is determined by the equation

[0054]

[0055] The maximum steering angle of the wheels is determined. r>max depending on the vehicle's current speed v, and the maximum steering wheel angle ô s w maTand a desired perceived wheelbase L^. The desired perceived wheelbase allows adjustment of the vehicle's agility and turning radius and is preferably defined according to the desired speed and handling characteristics. Choosing L^ allows, for example, achieving sporty, neutral, or comfortable handling.

[0056] L des is preferably greater than or equal to 0.5 x L and / or less than or equal to 1.5 x L. For example, L des can be between 0.6 x L and 1 x L for speeds between 0 and 180 km / h and a desired sporty behavior. For neutral behavior, L des can generally be close to L.

[0057] Next, the desired rear wheel steering angle is determined. r>des allowing for behavior similar to that of a two-wheel steering vehicle with the wheelbase

[0058] 5 r ,d es = arctan

[0059] The maximum rear wheel steering angle rsmax is therefore defined by

[0061] Note that ô r>max can be positive or negative.

[0062] Preferably, oh rmaT is a limited value on a ramp. In other words, we preferentially limit the rate of change of this value. This helps to avoid variations abrupt changes that can lead to undesirable behaviors. For example, the variation of ô r>max can be limited to 10 7s, preferably to 5 7s or 2 7s.

[0063] The force applied to the rear wheels corresponds to ô r>mi ", that is to say the maximum stress of this force, is defined by

[0065] where C r is the drift stiffness of the rear wheel tires, expressed as a unit of force per unit angle.

[0066] Opposite steering of the rear wheels relative to the front wheels is permitted at all speeds so that the rear wheels can contribute to achieving the desired yaw moment even at high speeds. The minimum constraint for the force applied to the rear wheels is determined by

[0067] F yrjnin / iccel ~ ~ F yrpiaxpccel

[0068] Finally, by taking into account the constraints defined by the friction ellipse, we obtain the constraints for the rear wheel steering actuator:

[0069] F'yrynax — mill Fyrynax / icceb Fyrynax, ellipse)

[0070] Fyrynin ~ mOX (F ' yryninpcceh F ' yr,min,ellipse)

[0071] Preferably, step a / of actuator control optimization is carried out with the additional constraints Fxfjtnax lïiax^ F xynBxpKtipnneup ” F ' x^jmax^llipse^ or Fx^^nax is the maximum longitudinal force constraint that can be applied to wheel {i, j} where i identifies the front or rear of the vehicle and j identifies the left or right side of the vehicle; F x / na^ctionneur is the maximum longitudinal force that can be exerted by a differential braking actuator on a wheel, and on wheel {i, j} and Fy tj East the lateral force on the wheel {i, j}.

[0072] Preferably, the vehicle also includes one or more power steering and / or electric steering actuators and / or one or more wheel drive actuators. The controls for these actuators are also optimized during optimization step a.

[0073] Advantageously, the vehicle features a second rear wheel steering actuator.

[0074] The invention also relates to a motor vehicle control system comprising a command allocation unit configured to allocate commands to the vehicle actuators from a command request representative of a desired yaw moment of the vehicle, the command allocation unit being configured to implement the method as described above.

[0075] The invention also relates to a motor vehicle comprising a control system as described above. Brief description of the drawings

[0076] [Fig.1] The [Fig.1] is a block diagram of a prior art vehicle motion control system.

[0077] [Fig.2] Fig.2 is a graph representing the result of a simulation comparing the evolution of the yaw rate obtained by a control method according to the prior art to that obtained by a method according to the invention.

[0078] [Fig.3] The [Fig.3] is a graph representing the result of a simulation comparing the evolution of the yaw moment obtained by a control method according to the prior art to that obtained by a method according to the invention.

[0079] [Fig.4] The [Fig.4] is a graph representing the result of a simulation comparing the evolution of the lateral acceleration obtained by a control method according to the prior art to that obtained by a method according to the invention.

[0080] [Fig.5] The [Fig.5] is a graph representing the result of a simulation comparing the evolution of the vehicle's speed obtained by a control method according to the prior art to that obtained by a method according to the invention.

[0081] [Fig.6] Fig.6 is a graph representing the result of a simulation comparing the evolution of the braking torque on the front left wheel obtained by a control method according to the prior art to that obtained by a method according to the invention.

[0082] [Fig.7] The [Fig.7] is a graph representing the result of a simulation comparing the evolution of the braking torque on the right front wheel obtained by a control method according to the prior art to that obtained by a method according to the invention.

[0083] [Fig.8] Fig.8 is a graph representing the result of a simulation comparing the evolution of the braking torque on the left rear wheel obtained by a control method according to the prior art to that obtained by a method according to the invention.

[0084] [Fig.9] Fig.9 is a graph representing the result of a simulation comparing the evolution of the braking torque on the right rear wheel obtained by a control method according to the prior art to that obtained by a method according to the invention.

[0085] [Fig.10] The [Fig.10] is a graph representing the result of a simulation comparing the evolution of the rear wheel steering angle obtained by a control method according to the prior art to that obtained by a method according to the invention.

[0086] [Fig. 11] Figure 11 is a graph representing the result of a simulation comparing the evolution of the vehicle's trajectory obtained by a prior art control method with that obtained by a method according to the invention. Detailed description

[0087] Throughout this application, the terms "vertical," "lower," "upper," "bottom," "top," "top," "below," "under," "above," "on," and "vertical" refer to a motor vehicle in its rolling configuration, with its wheels in contact with the ground. The terms "front" and "rear" refer to the vehicle's orientation. The lateral direction of a wheel is parallel to the wheel's axis of rotation when the vehicle is in motion. The longitudinal direction of a wheel is perpendicular to both the vertical and the lateral direction.

[0088] [Fig.1] has been described in the preamble and will therefore not be commented on below.

[0089] Figures 2 to 11 illustrate the results of a simulation comparing a prior art vehicle control method with the method according to the invention. In this simulation, the method according to the invention implements the constraints Fxijmax, F yrjnax, F y r jni„ during the allocation and optimization stage of the commands for the differential braking actuators and the steering actuator for the rear wheels. The optimization of the commands is performed by an active ensemble algorithm as described in thesis [2],

[0090] The simulation uses a representative vehicle model that allows the use of a closed-loop control method. This is the MAD A model (advanced modeling of automotive dynamics) developed by Renault.

[0091] The simulation studies the response of the vehicle on a circle of 50 m radius with constant longitudinal acceleration and high adhesion, i.e. with a coefficient of friction p equal to 1.

[0092] Figure 2 shows the time evolution of the yaw rate setpoint pr1, pr2 and the effective yaw rate pmi, pm2, obtained respectively by a prior art method and by the method according to the invention. Figure 3 shows the time evolution of the yaw moment ml, m2 obtained respectively by a prior art method and by the method according to the invention. Figure 4 shows the time evolution of the lateral acceleration al, a2 obtained respectively by a prior art method and by the method according to the invention. Figure 5 shows the time evolution of the vehicle speed vl, v2 obtained respectively by a prior art method and by the method according to the invention.

[0093] These figures show that, when implementing the prior art method, the vehicle loses control of its trajectory and stalls after approximately 170 seconds. This is evidenced by a drop in lateral acceleration and a sudden increase in the yaw rate command. In comparison, these same figures show that the method according to the invention, which uses constraints on the rear wheel steering actuator, allows the vehicle to maintain control without stalling.

[0094] Figures 6 to 9 show the time evolution of the differential braking torque of each of the front left wheel bfll, bfl2, front right wheel bfrl, bfr2, rear left wheel brll, brr2, rear right wheel brrl, brr2 respectively obtained by a process according to the prior art and by the process according to the invention.

[0095] Figure 10 shows the time evolution of the rear wheel steering angle drl and dr2 obtained respectively by a prior art method and by the method according to the invention. It is clear from this figure that the rear wheel steering actuator operates differently from approximately 120 s onwards due to the constraints imposed by the method according to the invention, which improves vehicle handling. This is also evident from Figure 11, which shows the evolution of the vehicle trajectory tl and t2 obtained respectively by a prior art method and by the method according to the invention.

[0096] Between t = 120 s and t = 180 s, the impact of constraints on the rear wheel steering actuator on the allocation of the yaw moment setpoint is observed. Indeed, the rear wheel steering setpoint, represented by the evolution of the rear wheel steering angle, is limited, particularly with regard to the maximum lateral acceleration setpoint. Differential braking on all four wheels then takes over to ensure that the yaw moment setpoint is maintained.

[0097] After t = 120 s, the steering control of the rear wheels forces a return to a steering angle close to 0°. This implies a reduction in rear axle drift and therefore helps to prevent oversteer.

[0098] Differential braking, which aims to complement rear wheel steering control, is allocated based on the braking potential present on the inner wheels, so the forces are better distributed over the four wheels and the vehicle remains more stable on its trajectory.

[0099] Thus, this simulation illustrates the ability of the process according to the invention to take into account demands related to comfort and / or safety issues while allowing the reproduction of a yaw moment instruction.

[0100] Other variations and improvements may be envisaged without departing from the scope of the invention. In particular, the method according to the invention may take into account other actuators in addition to the differential braking actuators and the rear wheel steering actuator, notably one or more actuators power steering and / or electric steering and / or one or more wheel drive actuators. Although described with reference to the use of a single rear wheel steering actuator, the invention also applies to the case of a vehicle having two rear wheel steering actuators acting separately on each of the two rear wheels. List of documents cited

[0101] [1] “Optimal Coordination of Chassis Systems for Vehicle Motion Control. "Automatic Control Engineering," Kissai, M. (2019), doctoral thesis, Université Paris Saclay

[0102] [2] “Backstepping and control allocation with applications to flight control”, Harkegârd, O. (2003), doctoral thesis, Linköping University

Claims

Demands

1. A method for controlling a motor vehicle (10) comprising at least one steering actuator for the wheels of at least one rear axle of the vehicle and, for each of the front wheels and the wheels of at least one rear axle, a differential braking actuator, the method comprising: a) calculating actuator commands based on a command demand representative of a desired yaw moment of the vehicle by means of a command allocation method; b) distributing the commands to the actuators, step a) of calculating the actuator commands implementing the command allocation method such that: the maximum lateral force constraint that can be applied to the wheels of at least one rear axle Fyr / nax is equal to the minimum of the maximum constraint F yrfnaxpccei of the lateral force applied to the wheels of at least one rear axle corresponding to a maximum steering angle | ô rptax| and the maximum stress F yrpMX>e iii pse of the lateral force applied to the wheels of at least one rear axle defined by the friction ellipse, the minimum lateral force stress that can be applied to the wheels of at least one rear axle of vehicle F yr ^ n is equal to the maximum of the minimum stress F yrpmpccd of the lateral force applied to the wheels of at least one rear axle corresponding to a minimum steering angle and the minimum stress F yrynmplipse of the lateral force applied to the wheels of at least one rear axle defined by the friction ellipse, the steering control of the wheels of at least one rear axle being symmetrical, and F yrjnaxficcel = 2C r 18 rfnax | where C r is the drift stiffness of the tires on the wheels of at least one rear axle, and Ôrjnax is defined based on a desired perceived wheelbase of the vehicle.

2. A method according to claim 1, wherein the maximum steering angle d^max is such that ô„ = arctan . ( (There yfn ax) \ ^ TOX = arcsm( -^r™" J where Rsteer is the ratio between the steering wheel angle ô 8tœr and the angle of the front wheels f DA is the angular dynamics that characterizes the relationship between the steering wheel angle and the lateral acceleration seen during the turn, ô stœr!max is the maximum steering angle of the steering wheel, ôf imæt is the maximum desired angle of the front wheels, a y;max is the maximum lateral acceleration, v is the speed of the vehicle, L is the physical wheelbase of the vehicle.

3. A method according to any one of the preceding claims, step a / of optimizing the actuator controls being carried out with the additional constraints maximum longitudinal force constraint that can be applied to wheel {i, j} where i identifies the front or rear of the vehicle and j identifies the left or right side of the vehicle; Fxxx is the maximum longitudinal force that can be exerted by a differential braking actuator on a wheel, and / \ 2 p2 where Fz.j is the vertical force on the wheel {i, j} and F y tj is the lateral force on the wheel { i, j}.

4. A method according to any one of the preceding claims, the vehicle further comprising one or more power steering and / or electric steering actuators and / or one or more wheel drive actuators.

5. Method according to any one of the preceding claims, the vehicle comprising a second rear wheel steering actuator.

6. Method according to any one of the preceding claims, the desired perceived wheelbase being greater than or equal to 0.5 and 1.5 times the physical wheelbase of the vehicle.

7. By method according to any one of the preceding claims, the maximum lateral acceleration has y;max being greater than or equal to 2 m / s 2 and / or less than or equal to 14 m / s 2 .

8. A motor vehicle control system (10) comprising a command allocation unit (4) configured to allocate commands to the vehicle's actuators from a command request representative of a desired yaw moment of the vehicle, the order allocation unit being configured to implement the method according to one of the preceding claims.

9. Motor vehicle (10) comprising a control system according to the preceding claim.

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