Method for controlling a four-wheel-steered motor vehicle, taking into account the recentring of the steering wheel

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 control method comprises: a) optimising the commands for the actuators according to a control request representative of a desired yaw moment for the vehicle by means of a command allocation method; b) detecting a change in the steering angle of the steering wheel of the motor vehicle in order to determine whether or not the steering wheel is recentred; c) adding a feedforward control signal δ r,FFD to the command for the rear wheel steering actuator obtained in step a); d) distributing the commands to the actuators, wherein the feedforward control signal δ r,FFD takes into account the recentring of the steering wheel.

Description

Description Title of the invention: Method for controlling a four-wheel steering motor vehicle taking into account the recentering of the steering wheel 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 a plurality of actuators, at least one actuator acting on the steering angle of the rear wheels.

[0003] The invention also relates to a control system configured to implement such a method as well as a motor 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 steering wheels 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 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 shown, this system operates in a closed loop.

[0009] First, a reference model 1 is used to determine the desired yaw rate of the vehicle i.e. the time derivative of the desired yaw angle Wref. The desired yaw rate depends in particular on the driver's actions, for example on the vehicle's steering wheel or pedals, 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 method include the braking force (or longitudinal force) at the left front wheel ^x / z, the braking force at the right front wheel F x / r _ 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 F b fl , F b fr , F b rb F 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 measure the vehicle's motion, including the vehicle's yaw rate, 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, called 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 according to 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] Patent EP 2085293 B1 describes a device for controlling the steering angle of the rear wheels of a motor vehicle.

[0019] Application JP2023044346A discloses a control system acting on the steering of the rear wheels to improve vehicle handling.

[0020] The inventors have found in simulations and in actual testing that a closed-loop system as described with reference to [Fig. 1] is likely to generate a rear wheel steering command that does not properly take into account the recentering of the vehicle's steering wheel.

[0021] There is a need to further improve the control methods of rear wheel steering actuators, particularly to improve driver feedback.

[0022] The aim of the invention is to meet at least part of this need. Summary of the invention

[0023] To do this, 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 control 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 / detecting a change in the steering angle of the steering wheel of the motor vehicle to determine whether the steering wheel is recentered or not; c / adding a direct action control signal ô r ^ FD to the command of T rear wheel steering actuator obtained in step a / ; d / distribute the commands to the actuators, where, if the steering wheel is recentered, the direct-acting control signal ô rFFD is defined by the equation is the maximum steering angle of the rear wheels, oh s 'steering wheel angle of the vehicle at time t and SsvvmoxO) est L' an maximum steering wheel turning angle reached since the start of steering wheel re-centering; and if the steering wheel is not recentered, the direct-acting control signal ô rFFD is defined by the equation &r,FFD &r,cin

[0024] 5 rcïra being a predetermined command of the rear wheel steering actuator.

[0025] Thus, the control method makes it possible to adjust a direct-acting control signal which is added to the command of the rear wheel steering actuator as determined by the command allocation method. This adjustment makes it possible to take into account the recentering of the steering wheel in particular in order to improve the feel of the driver of the vehicle.

[0026] According to an advantageous characteristic, the predetermined command <5 r, C i" of the rear wheel steering actuator is defined by the equation 5r,ci n = - sign(ôf) x abs(ô r2 )

[0028] where oh f is the steering angle of the front wheels, L is the physical wheelbase of the vehicle, L dcs 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.

[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] This value of 5 rcin which is added to the control of the steering actuator at the output of the control allocation process advantageously 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 a preferred embodiment, determining the recentering of the steering wheel comprises initializing a time counter when a decrease in the absolute value of the steering wheel steering angle is detected and, if the absolute value of the steering wheel steering angle does not increase before the time counter reaches a predetermined value, detecting the recentering of the steering wheel. This advantageously allows- usefully to filter out minimal variations in the steering wheel angle.

[0033] The invention also relates to a control system for a motor vehicle configured to implement the method according to one of the preceding claims, 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 r>FFD to the control of the vehicle's rear wheel steering actuator.

[0034] The invention finally relates to a motor vehicle comprising a control system according to the preceding claim. Brief description of the drawings

[0035] [Fig.l] [Fig.l] is a block diagram of a prior art vehicle motion control system.

[0036] [Fig.2] [Fig.2] is a block diagram of a vehicle motion control system according to the invention.

[0037] [Fig.3] [Fig.3] is a graph representing the evolution of the steering wheel angle and the evolution of the steering angle of the rear wheels as obtained by a control method according to the prior art.

[0038] [Fig.4] [Fig.4] is a graph representing the evolution of the steering wheel angle and the evolution of the steering angle of the rear wheels as obtained by a method according to the invention. Detailed description

[0039] Throughout the present application, the terms "vertical", "lower", "upper", "lower", "higher", "below", "under", "above", "on", "vertical" are to be understood with reference to a motor vehicle as it is in the rolling configuration, the wheels being in contact with the ground. The terms "front" and "rear" are to be understood with reference to the orientation of the vehicle.

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

[0041] [Fig.2] differs from [Fig.l] in that a direct acting control signal (or "feedforward" in English) is determined by a calculation unit 11 and added to the control signal of the vehicle's rear wheel steering actuator as determined by the command allocation unit.

[0042] An example of a control system as illustrated in [Fig. 2] and a control method implemented by this system are described and claimed in the patent application entitled “Method for controlling a four-wheel steering motor vehicle, comprising the control of a rear wheel steering actuator by a direct action signal” in the name of the Applicant and filed today.

[0043] According to the control method described in this application, the value of the direct-acting control signal is given by the equation 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 r are the distances between the vehicle's center of gravity and the front and rear axles, respectively.

[0046] Thanks to this control signal ô r>cin, the vehicle's maneuverability is significantly improved due to the additional action on the rear wheel steering.

[0047] [Fig.3] illustrates an example of the evolution of the control signal ô r , cin and the steering angle ô r of the rear wheel steering actuator obtained in response to the steering wheel turning angle ô sw by implementing the control method described above in the general case of a large steering wheel angle control.

[0048] The vehicle's response to a steering wheel turning sequence is studied.

[0049] The upper part of [Fig.3] represents the evolution of the steering wheel steering angle ô sw as a function of time t. We see that the steering wheel is initially in a zero steering position, the steering wheel is then turned until it reaches a maximum steering angle and then recentered towards the zero steering position.

[0050] In response, as seen in the lower part of [Fig.3], the process produces a command ô r cin represented by the drl curve which quickly exceeds the maximum angle ô r.sat that the steering actuator can reach, worth 5° in this example. Thus, the steering angle ô r of the rear wheels represented by the curve dr2 quickly reaches the saturation value.

[0051] We note that the value of the command ô r.cin remains greater than the maximum angle ô r >sat for a significant time after the start of the steering wheel re-centering towards the zero steering position. The steering wheel re-centering begins at time ti but the re-centering of the rear wheels only begins at time t2.

[0052] In other words, the steering of the rear wheels does not respond immediately to the re-centering of the steering wheel. This delay in taking into account is perceived negatively in terms of benefit.

[0053] [Fig.4] illustrates the result of a study which differs from that of [Fig.3] in that the method according to the invention is implemented.

[0054] The upper part of [Fig.4] represents the evolution of the steering wheel turning angle ô s w , which is identical to that shown in [Fig.3], The steering wheel turning angle increases until time tl and then decreases until reaching zero turning.

[0055] As visible in the lower part of [Fig.4], the steering instruction ô r , cin represented by the curve drl is identical to that represented in [Fig.3]. The curve dr3 represents the evolution of the steering angle setpoint of the rear wheels ô r obtained by the method according to the invention, by adding the direct action control signal ô r.FFD to the rear wheel steering actuator control.

[0056] We note that the instruction ô rdecreases as soon as the steering wheel begins to return to the zero steering position. Therefore, the rear wheel steering responds very quickly to the steering wheel re-centering, well before the time t2 at which the rear wheels start to re-center in the example in [Fig.3]. Advantageously, this greatly improves the feel when driving the vehicle.

[0057] Preferably, the detection of the recentering of the steering wheel in the method according to the invention implements a time or time step counter making it possible to filter out possible rapid variations in the movement of the steering wheel. When driving, the steering wheel is likely to be rotated rapidly in one direction or the other without there being any real intention to recenter the steering wheel.

[0058] The vehicle may in particular include a steering wheel movement sensor to detect changes in steering wheel rotation.

[0059] Thus, when the steering wheel turning angle begins to decrease in absolute value, that is, when |ô S W / ( t ) | dt is a minimum time step, a first time counter is activated. Once this counter exceeds a predetermined value, for example between 0.01 s and 1 s, it is confirmed that the steering wheel has been recentered.

[0060] When the steering wheel angle is no longer decreasing in absolute value, a second time counter is activated. Once this counter exceeds a predetermined value, for example between 0.01 s and 1 s, it is confirmed that the steering wheel recentering is complete.

[0061] Other methods can be implemented to detect steering wheel re-centering. For example, the steering wheel turning speed can be measured and the re-centering can be inferred from the change in this quantity.

[0062] The maximum steering wheel steering angle since the start of a re-centering phase can be obtained by updating this value as long as the steering wheel steering angle steering wheel increases in absolute value. In other words, we assign the value Uni que |<5g|y( z) | > &SW.niax (f'dt\

[0063] If the steering wheel angle does not increase in absolute value, the last calculated value of ô is kept SW nmx .

[0064] Other variants and improvements may be provided without departing from the scope of the invention. Although described in comparison with the vehicle control method described in the application "Method for controlling a four-wheel steering motor vehicle, comprising the control of a rear wheel steering actuator by a direct-acting signal" previously mentioned, the invention applies to any control method controlling a rear wheel steering actuator. The invention also applies to the case of a vehicle comprising two rear wheel steering actuators acting separately on each of the two rear wheels. List of cited documents

[0065] [1] “Optimal Coordination of Chassis Systems for Vehicle Motion Control. Automatic Control Engineering », Kissai, M. (2019), doctoral thesis, Université Paris Saclay

[0066] [2] « Backstepping and control allocation with applications to flight control », Hârkegârd, O. (2003), thèse de doctorat, 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 control 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 / detecting a change in the steering angle of the steering wheel of the motor vehicle to determine whether the steering wheel is recentered or not; c / adding a direct action control signal ô r FFD to the command of F rear wheel steering actuator obtained in step al; d / distribute the commands to the actuators, where, if the steering wheel is recentered, the direct action control signal & r ,FFD is defined by the equation is the steering angle maximum of the rear wheels, is the steering angle of the vehicle's steering wheel at time t and <5$ W niœr(0 est the maximum steering wheel turning angle reached since the start of steering wheel re-centering; and if the steering wheel is not re-centered, the direct-acting control signal ^r.FFi) is defined by the equation ur,FFD '^r.cin <5 r ,cin being a predetermined command of F rear wheel steering actuator.

2. A method according to claim 1, the predetermined command ô r , cin of F rear wheel steering actuator being defined by equation 5 r ,cin = -sign(ôf) x abs(ô r2 ) where oh f 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.

3. A method according to one of the preceding claims, wherein determining steering wheel re-centering comprises initializing a time counter when a decrease in the absolute value of the steering wheel steering angle is detected and, if the absolute value of the steering wheel steering angle does not increase before the time counter reaches a predetermined value, detecting steering wheel re-centering.

4. 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 r>FFD to the control of the vehicle's rear wheel steering actuator.

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

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