Method for controlling a four-wheel-steered motor vehicle, comprising controlling a rear-wheel steering actuator via a feedforward signal

Abstract

The invention relates to a method for controlling a motor vehicle (10) comprising at least one actuator for steering the rear wheels and, for each of the four wheels of the vehicle, a differential braking actuator, wherein the method comprises: a) optimising the actuator commands according to a control request representative of a desired yaw moment for the vehicle by means of a command allocation method; b) adding a feedforward control signal δFFD to the command for the rear wheel steering actuator obtained in step a); c) distributing the commands to the actuators.

Description

Description Title of the invention: Method for controlling a four-wheel steering motor vehicle, comprising controlling a rear wheel steering actuator by a direct-acting signal Technical field

[0001] The present invention relates to the field of controlling motor vehicle equipment.

[0002] More specifically, it relates to a method for controlling vehicle movement, or "vehicle motion control" (VMC) in English, the vehicle comprising four steered 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 such a control method and a vehicle comprising such a control system. Prior art

[0004] It is known to implement motor vehicles with four-wheel steering (4RD or 4WS for "four-wheel steering" in English). 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 than a two-wheel steering vehicle where only the front wheels can be steered.

[0006] Thesis [1] describes a vehicle motion control system, the vehicle having four steered wheels as well as a differential braking actuator for each of the four wheels.

[0007] The described control system can in particular be implemented to optimize the controls making it possible to distribute 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] [Fig.l] 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 the vehicle that is to say the time derivative of the desired yaw angle Wref. The desired yaw rate depends in particular on the actions of the driver, for example on the steering wheel or the pedals of the vehicle, and / or on the actions of a control unit of a partially or fully autonomous vehicle. Without this being limiting, the reference model 1 can be defined by a bicycle model known as such, by a closed-loop regulator or by any means making it possible to define 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 method to determine the commands to be optimized taking into account the limitations of the chassis 3.

[0011] The commands to be optimized by the allocation process include the braking force (or longitudinal force) at the left front wheel braking force at the right front wheel the braking force at the right rear wheel F xrr , the braking force at the left rear wheel F xrl and the lateral force at the rear wheels F yr linked to the steering of the rear wheels.

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

[0013] The vehicle actuators and the vehicle 10 then implement the commands determined by the control system. Vehicle sensors are used to measure the vehicle's motion, including the vehicle's yaw rate F, 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 based on the desired yaw rate and the measured yaw rate and the command allocation process is repeated.

[0015] Thus, the closed-loop system as described establishes 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, known as the four-wheel steering actuator or 4RD actuator.

[0016] The commands sent to the different actuators can be calculated by 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 instructions given by the closed-loop regulator.

[0017] This matrix, called the control effectiveness matrix, is determined by the vehicle's physics equations. However, the constraints imposed on the algorithm can be chosen so that the instructions calculated by it meet predetermined performance, safety, and service criteria.

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

[0019] According to the 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] However, the inventors have found during simulations and during real tests that the closed-loop system described with reference to [Fig.l] does not allow satisfactory control of the vehicle's turning radius, particularly at low speeds. In particular, it has been found that the difference between the desired yaw rate for the vehicle dynamics and the actual yaw rate is not sufficient to generate a sufficiently high rear wheel steering angle.

[0021] Patent EP 2085293 B1 describes a device for controlling the steering angle of the rear wheels of a motor vehicle.

[0022] There is a need to improve existing vehicle control methods and vehicle control systems, particularly to improve vehicle maneuverability.

[0023] The aim of the invention is to meet at least part of this need. Summary of the invention

[0024] To this end, the invention relates in one of its aspects to a method for controlling a motor vehicle comprising at least one rear wheel steering actuator and, for each of the four wheels of the vehicle, a differential braking actuator, the method comprising:

[0025] a / optimizing the actuator controls based on a control request representative of a desired yaw moment of the vehicle by means of a control allocation method;

[0026] b / add a direct action control signal ô FFD at the command of the rear wheel steering actuator obtained in step a / ;

[0027] c / distribute the commands to the actuators, is the steering angle of the front wheels, L is the physical wheelbase of the vehicle, L des is a desired felt wheelbase of the vehicle, the f and l rare the distances between the vehicle's center of gravity and the front and rear axles, respectively.

[0029] As is customary in the state of the art, the wheelbase refers to the distance between the front and rear axles of the vehicle.

[0030] Thus, the use of a feedforward control signal which is added to the steering actuator command at the output of the command allocation process makes it possible to greatly improve the maneuverability of the vehicle thanks to an additional action on the steering of the rear wheels.

[0031] Preferably the desired felt wheelbase L des is defined to have a value close to the actual wheelbase value L and to reproduce a desired behavior. For example, it can be between 75% and 125% of the physical wheelbase L.

[0032] According to an advantageous embodiment, the vehicle further comprises one or more power steering and / or electric steering actuators and / or one or more wheel drive actuators.

[0033] The invention also relates to a control system for a motor vehicle comprising a command allocation unit configured to allocate commands to the actuators of the vehicle from a command request representative of a desired yaw moment of the vehicle, the command allocation unit being configured to implement the method described previously.

[0034] The invention also relates to a control system for a motor vehicle configured to implement the method described above, the control system comprising a command allocation unit configured to allocate commands to the actuators of the vehicle from a command request representative of a desired yaw moment of the vehicle, the control system further comprising a calculation unit configured to add the direct action control signal ô FFD to the wheel steering actuator control rear of the vehicle.

[0035] The invention finally relates to a motor vehicle comprising a control system as described above. Brief description of the drawings

[0036] [Fig.l] [Fig.1] is a block diagram of a prior art vehicle motion control system.

[0037] [Fig.2] [Fig.2] is a block diagram of a vehicle motion control system according to the invention.

[0038] [Fig.3] [Fig.3] is a graph representing the result of a simulation comparing the evolution of the steering angle of the rear wheels of a motor vehicle obtained by a control method according to the prior art with that obtained by a method according to the invention.

[0039] [Fig.4] [Fig.4] is a graph representing the movement of the vehicle's steering wheel during the simulation implemented in [Fig.3], Detailed description

[0040] [Fig.l] was described in the preamble and will therefore not be commented on below.

[0041] [Fig.2] illustrates a block diagram of a control system implementing the control method according to the invention.

[0042] [Fig.2] differs from [Fig.l] in that a direct acting control signal (or "feedforward" in English) ô FFD is determined by a calculation unit 11 and added to the control signal of the vehicle's steering actuator.

[0043] The value of the control signal ô FFD is chosen so as to reproduce a desired wheelbase of the vehicle L des . The L wheelbase des represents an equivalent physical wheelbase of an ideal two-wheel steering vehicle whose behavior we wish to reproduce.

[0044] Thus, we calculate the turning radius R 2RD:des of this ideal two-wheel steering vehicle for a given front wheel steering angle. Then, we calculate the rear wheel steering angle of the vehicle such that the vehicle's turning radius R 4RD is equal to the turning radius R 2RD.des of the ideal two-wheel steering vehicle.

[0045] From the vehicle dynamics equations as disclosed for example in the book "Vehicle dynamics modeling of complex systems", J.-P. Brossard, Presses Polytechniques Romandes, ISBN 2889150143, 2013, we obtain the expression of the turning radius R4 RD of the four-wheel steering vehicle:

[0046] RI RD =-^- —i [ îf( sinô r ) 2 (cosôy) " + / ;'( sinô, ) 2 |'c<>sf>, ) 2 + 2 / r / ^sin(S / )cos(ôj)sin( ô r )cos(ô f ) + L 2 (cos(6y)cos(ô r ) ) 2 ] ( sin ( l5 HAS)) L

[0047] where oh r is the steering angle of the rear wheels, ô f is the steering angle of the front wheels, L is the physical wheelbase of the vehicle, l f and l r are the distances between the vehicle's center of gravity and the front and rear axles, respectively.

[0048] The steering angle of the rear wheels is of limited amplitude. For example, depending on the characteristics of the actuator considered, the maximum steering angle of the rear wheels can be less than or equal to 5 0 or 3.5°. In particular, it can be equal to 5°, or 0.0873 radians. This angle being small compared to 1, it is possible to consider that

[0049] if n<5, ~ <5,-

[0050] cosô r "1

[0051] sin(ôy-5 r ) ~ sin(ôy) -<5 r cos(<5 / )

[0052] (sin(<5f -<5 r ) ) 2 "(sin5y) 2 + Ô^(cosÔy) 2 -2Ô r cosÔysinÔ^

[0053] Therefore, we obtain

[0055] The turning radius R 2RD of the ideal two-wheel steering vehicle whose wheelbase L is to be reproduced des can be expressed from the same equations by considering that ô ris equal to 0 because the rear wheels do not turn. We obtain

[0056]

[0057] To determine the steering angle ô r rear wheels allowing to reproduce the desired position L des , we solve

[0058] p2 ^4RD

[0059] We obtain the following second degree equation:

[0060] AÔ 2 r + Bd r + C = 0

[0063] Solving this equation gives two solutions:

[0065] We retain the solution ô r2 . Furthermore, we want the rear wheels to turn in the opposite direction to that of the front wheels. We finally obtain:

[0066] Q FFD - _ s ig n ( <5y ) x abs(5 r7 )

[0067] This value is dynamic and constantly recalculated, preferably at the frequency updating the allocation of orders. For example, FFD can be recalculated every 5 ms, 10 ms or 20 ms.

[0068] Figures 3 and 4 illustrate the results of a simulation comparing a state-of-the-art vehicle control method with the method according to the invention.

[0069] The simulation uses a representative vehicle model that allows the use of a closed-loop control allocation process. This is the MADA (Advanced Modeling of Automotive Dynamics) model developed by Renault.

[0070] The simulation studies the vehicle's response to a sequence of steering wheel rotations. [Fig.4] represents the evolution of the steering wheel's steering angle during the simulation. [Fig.3] represents the response of the rear wheels.

[0071] In [Fig.3], the drl curve shows the evolution of the steering angle of the rear wheels with a method according to the prior art. It appears that the steering of the rear wheels is very limited and therefore does not allow satisfactory maneuverability of the vehicle to be obtained.

[0072] In comparison, curve dr2 shows the evolution of the steering angle of the rear wheels with a method according to the invention. It can be seen that the rear wheel steering actuator responds strongly to the steering wheel when the latter is turned significantly. The steering of the rear wheels quickly reaches its maximum amplitude, thus significantly improving the vehicle's handling.

[0073] Thus, this simulation illustrates the ability of the method according to the invention to improve the maneuverability of the vehicle thanks to an additional action on the steering of the rear wheels.

[0074] Other variants and improvements may be provided 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, in particular one or more power steering and / or electric steering actuators and / or one or more wheel drive actuators. List of cited documents

[0075] [1] “Optimal Coordination of Chassis Systems for Vehicle Motion Control. Automatic Control Engineering », Kissai, M. (2019), doctoral thesis, Université Paris Saclay

[0076] [2] “Backstepping and control allocation with applications to flight control”, Harkegârd, O. (2003), doctoral thesis, Linkôpings universitet

Claims

Claims

1. Method for controlling a motor vehicle (10) comprising at least one rear wheel steering actuator and, for each of the four wheels of the vehicle, a differential braking actuator, the method comprising: a / optimizing the actuator controls as a function of a control request representative of a desired yaw moment of the vehicle by means of a control allocation method; b / adding a direct action control signal ÔFFD to the control of the rear wheel steering actuator obtained in step a / ; cl distributing the controls to the actuators, front wheel steering, L is the physical wheelbase of the vehicle, L des is a desired felt wheelbase of the vehicle, the f and l r are the distances between the vehicle's center of gravity and the front and rear axles, respectively.

2. Method according to the preceding claim, the vehicle further comprising one or more power steering and / or electric steering actuators and / or one or more wheel drive actuators.

3. A control system for a motor vehicle (10) configured to implement the method according to one of the preceding claims, the control system comprising a command allocation unit (4) configured to allocate commands to the actuators of the vehicle from a command request representative of a desired yaw moment of the vehicle, the control system further comprising a calculation unit (11) configured to add the direct action control signal Ô F FD at the command of the vehicle's rear wheel steering actuator.

4. Motor vehicle comprising a control system according to the preceding claim.

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