Method of controlling a road vehicle with variable stiffness and steered rear wheels when driving along a curve

Abstract

A method of controlling a road vehicle (1) having variable stiffness and steered rear wheels (3) when driving along a curve. The method comprises the steps of determining an actual attitude angle (β) of the road vehicle (1), setting a desired attitude angle (β TGT ), determining an actual yaw rate (dψ / dt) of the road vehicle (1), setting a desired yaw rate (dψ / dt TGT ), and changing the steering angle (α) of the rear wheels (3) and the stiffness distribution of the connections of the four wheels (2, 3) to the vehicle frame (12) in a simultaneous and coordinated manner depending on the difference (ε TGT ) between the actual attitude angle (β) and the desired attitude angle (β β ) and the difference (ε TGT ) between the actual yaw rate (dψ / dt) and the desired yaw rate (dψ / dt ψ ).

Description

[0001] Cross-references to related applications

[0002] This patent application claims priority to Italian Patent Application No. 102020000002746, filed on February 12, 2020, the entire disclosure of which is incorporated herein by reference. Technical Field

[0003] The present invention relates to a method for controlling a road vehicle with variable roll stiffness and steering rear wheels when traveling along a curve. Background Technology

[0004] Although there aren't many available models primarily based on passive mechanical systems that only allow a "fixed" steering angle to be assigned to the rear wheels, four-wheel steering vehicles (also known as 4WS vehicles) have been produced by the automotive industry for many years. In these passive mechanical systems, the rear wheels are coupled to the front wheels to assign a fixed steering angle to the rear wheels when the front wheels are steering.

[0005] More modern powertrains equipped with (electric or hydraulic) active actuators controlled by an electronic control unit are able to control the steering of the rear wheels by imparting a steering angle that can vary within a predetermined range to the rear wheels.

[0006] When driving at high speeds (e.g., above 60 km / h) along a curve, rear-wheel steering increases vehicle stability, especially near the point of traction loss. To improve vehicle stability at high speeds along a curve, the rear wheels are steered in the same direction as the front wheels (i.e., in the same direction of steering as the front wheels, meaning the rear wheels also steer to the right when the front wheels steer to the right) to reduce yaw angle, thus making the vehicle easier to control. In other words, rear-wheel steering in the same direction counteracts the centrifugal force that tends to cause the rear of the vehicle to slip when driving along a curve, allowing the rear of the vehicle to conform to an ideal trajectory, thereby increasing stability and efficiency. In this case, the rear wheels are steered at a steering angle that is typically less than 1-2° and based on the steering angle of the front wheels.

[0007] In low-speed (e.g., up to 60 km / h) maneuvering situations, the steering of the rear wheels significantly reduces the turning radius. In particular, to improve vehicle handling at low speeds, the rear wheels are steered in the opposite direction with a maximum steering angle of 3-5° based on the steering angle of the front wheels (i.e., in the opposite direction to the steering of the front wheels, meaning that when the front wheels turn right, the rear wheels turn left, and vice versa).

[0008] Patent application EP3153382A1 describes a method for controlling a road vehicle with steering rear wheels while traveling along a curve; the control method includes the following steps: determining the actual attitude angle of the road vehicle; determining the desired attitude angle; and changing the steering angle of the rear wheels based on the difference between the actual attitude angle and the desired attitude angle.

[0009] The article "Combined effect of active suspension and rear wheel steering control system on vehicle lateral stability" (Liang Wu et al., "IBEC 2003" and SAE Conference Transactions 2002-01-20, Issue 48, Volume 1, 28 March 2017, XP055738637, US ISSN: 0148-7191, DOI: 10.4271 / 2017-01-0257) describes a method for controlling a road vehicle with variable stiffness and steered rear wheels when driving along a curve. SUMMARY

[0010] The object of the present invention is to provide a method for controlling a road vehicle with variable stiffness and steered rear wheels when driving along a curve, which maximizes the performance of the road vehicle when driving along a curve without destabilizing the vehicle, while being easy and economical to implement.

[0011] According to the invention, there is provided a method for controlling a road vehicle with variable roll stiffness and steered rear wheels when driving along a curve; the road vehicle comprising a chassis, four wheels, at least one first actuator designed to change the stiffness distribution of the connection of the four wheels to the chassis and at least one second actuator designed to change the steering angle of the rear wheels; the control method comprising the steps of:

[0012] determining the actual attitude angle of the road vehicle;

[0013] setting a desired attitude angle;

[0014] determining the actual yaw angular velocity of the road vehicle;

[0015] setting a desired yaw angular velocity; and

[0016] changing the steering angle of the rear wheels and the stiffness distribution of the connection of the four wheels to the chassis in a simultaneous and coordinated manner, as a function of the difference between the actual attitude angle and the desired attitude angle and the difference between the yaw angular velocity and the desired yaw angular velocity;

[0017] The control method is characterized in that it further comprises the step of prioritizing the steering action of the rear wheels when driving along a curve and lateral acceleration is less than 4 to 6 m / s 2 in order to maximize the dynamic response of the vehicle to the steering command of the front wheels requested by the driver, adjusting the stiffness distribution of the connection of the four wheels to the chassis by means of the first actuator accordingly.

[0018] The appended claims describe the preferred embodiments of the invention and form an integral part of the description. BRIEF DESCRIPTION OF DRAWINGS

[0019] The application will now be described with reference to the accompanying drawings, which show non-limiting embodiments of the application, in which:

[0020] Figure 1 is a schematic plan view of a road vehicle with rear wheels controlled according to the application;

[0021] Figure 2 is Figure 1 a schematic plan view of the road vehicle of

[0022] Figure 3 is Figure 1 a schematic plan view of the road vehicle of

[0023] Figure 4 is Figure 1 a schematic perspective view of the road vehicle of

[0024] Figure 5 is a block diagram showing the logic of the control method according to the application. DETAILED DESCRIPTION

[0025] In Figure 1 , reference 1 generally indicates a road vehicle with two front wheels 2 and two rear drive wheels 3 receiving torque from a powertrain 4.

[0026] The powertrain 4 comprises an internal combustion heat engine 5, provided in a longitudinal front position and provided with a crankshaft 6, and a servo-assisted driveline 7, having a configuration known as "transaxle", which transmits the torque generated by the internal combustion engine 5 to the rear drive wheels 3. The driveline 7 comprises a drive shaft 8, which is connected on one side to the crankshaft 6 and on the other side is mechanically connected to a transmission 9, provided with at least one clutch and arranged in a longitudinal rear position. The transmission 9 is connected in a chain-like manner to an electronically controlled self-locking differential 10, from which a pair of half shafts 11 each starts integrally with a respective rear drive wheel 3.

[0027] The wheels 2 or 3 are each mechanically connected to a chassis 12 of the road vehicle 1 by means of a suspension 13 (partially shown in Figure 1 ), provided with electronically controlled actuators 14, which allow the elastic stiffness and damping of the suspension elements to be varied (increased or decreased). By way of example, the actuators 14 can regulate the vertical installation stiffness and damping constant of the suspension 13 in parallel with the traditional elastic elements of the suspension 13 (helical springs and passive hydraulic shock absorbers). By way of example, this regulation can be carried out by means of different elements, such as electronically controlled anti-roll bars and magneto-rheological shock absorbers.

[0028] The electronically controlled actuators 14 allow to vary the stiffness of the connection of the four wheels 2 and 3 of the road vehicle 1 to the chassis 12 and, therefore, to vary the distribution of the vertical load acting on the wheels 2 and 3 among the four wheels 2 and 3 in dynamic conditions, i.e. in the presence of longitudinal or transverse accelerations. In other words, by varying the stiffness of the electronically controlled actuators 14, it is possible to vary the distribution of the vertical load on the four wheels 2 and 3 and, therefore, the punctual vertical load acting on each of the wheels 2 and 3.

[0029] According to Figure 2 , the rear driving wheels 3 are steered wheels, i.e. they are carried by corresponding suspensions 13 so that they can rotate together (i.e. with the same degree of rotation) to the right or to the left about a vertical axis, thus varying the corresponding steering angle a (shown in Figure 3 ); in particular, (electric or hydraulic type) actuators 15 are provided, which actively control the variation of the steering angle a of the rear driving wheels 3.

[0030] The steering of the rear driving wheels 3 increases the stability of the road vehicle 1 when driving along a curve at high speed (for example, above 60 km / h); in particular, in order to improve the stability of the road vehicle 1 when it drives along a curve at high speed, the rear driving wheels 3 are steered in phase (i.e. in the same direction as the steering of the front wheels, which means that when the front wheels 2 are steered to the right, the rear driving wheels 3 are also steered to the right). In other words, the in-phase steering of the rear driving wheels 3 counteracts the centrifugal forces that tend to make the rear of the vehicle slip when driving along a curve, thus making the rear of the vehicle adapt to the ideal trajectory, thus increasing stability and effectiveness. In this case, the rear driving wheels 3 are steered with a steering angle a that is usually less than 1-2° and is usually based on the steering angle of the front wheels 2 (i.e. the greater the steering angle of the front wheels 2, the greater the steering angle a of the rear driving wheels 3).

[0031] The steering of the rear driving wheels 3 significantly reduces the steering radius in the case of low-speed manoeuvres (for example, up to 60 km / h); in particular, in order to improve the low-speed manoeuvrability of the road vehicle 1, the rear driving wheels 3 are counter-steered (i.e. in the opposite direction with respect to the steering of the front wheels 2, which means that when the front wheels 2 are steered to the right, the rear wheels 3 are steered to the left, and vice versa) with a maximum angle of 3-5° based on the steering angle of the front wheels 2 (i.e. the greater the steering angle of the front wheels 2, the greater the steering angle a of the rear driving wheels 3).

[0032] The road vehicle 1 comprises an electronic control unit 16 ("ECU") which, as will be described in greater detail hereinafter, regulates the behaviour of the road vehicle 1 when it is travelling along a bend, inter alia by acting on the electronically controlled actuators 14 of the suspension 13 and by controlling the steering angle a of the rear driven wheels 3. The control unit 16 can be physically constituted by a single device or by different devices which are separate from one another and communicate with one another through a CAN network of the road vehicle 1.

[0033] According to a possible (but not limiting) embodiment, the control unit 16 periodically estimates (for example, with a frequency of at least a few tens of Hz and in a known manner) the grip of the wheels 2 and 3 on the road surface, determines the radius of curvature of the trajectory T of the road vehicle 1 (i.e. determines the degree of curvature of the trajectory T) and determines the speed of travel V of the road vehicle 1. According to the grip of the wheels 2 and 3 (and therefore according to the stability of the road vehicle 1), according to the radius of curvature of the trajectory T and according to the speed of travel V, the control unit 16 periodically determines the desired attitude angle b TGT ; in particular, in order to determine the desired attitude angle b TGT . Figure 3 When travelling along a bend, the control unit 16 determines the attitude angle b of the road vehicle 1 (i.e. the angle between the longitudinal axis x of the road vehicle 1 and the direction of the speed of travel V of the road vehicle 1 at the centre of gravity B) in a known manner. By way of example, the control unit 16 estimates the trajectory T followed by the road vehicle 1 using measurements provided in real time by a three-axis gyroscope and by a GPS tracking unit; in particular, the trajectory T is determined by twice integrating the accelerations measured by the three-axis gyroscope and the measurements provided by the GPS tracking unit are used to periodically eliminate the position errors which arise in the integration process. Furthermore, the control unit 16 estimates the speed of travel V of the road vehicle 1 at the centre of gravity B using measurements provided in real time by the three-axis gyroscope; in particular, the speed V of the road vehicle 1 at the centre of gravity B is determined by integrating the accelerations measured by the three-axis gyroscope once (ensuring that the speed of travel V of the road vehicle 1 at the centre of gravity B is actually tangent to the trajectory T followed by the road vehicle 1, otherwise, in the case of significant deviations, at least one further iteration is made on the calculated values in order to correct the parameters used).

[0034] When travelling along a bend, the control unit 16 determines the actual (real) attitude angle b of the road vehicle 1 in real time (for example, as will be described hereinafter). Furthermore, when travelling along a bend, the control unit 16 determines the desired (ideal) attitude angle b TGT which allows the performance (i.e. the speed of travel along a bend) to be maximised and at the same time places the vehicle 1 in a stable condition (i.e. in a condition of complete safety).

[0035] According to a possible (but not limiting) embodiment, the control unit 16 periodically estimates (for example, with a frequency of at least a few tens of Hz and in a known manner) the grip of the wheels 2 and 3 on the road surface, determines the radius of curvature of the trajectory T of the road vehicle 1 (i.e. determines the degree of curvature of the trajectory T) and determines the speed of travel V of the road vehicle 1. According to the grip of the wheels 2 and 3 (and therefore according to the stability of the road vehicle 1), according to the radius of curvature of the trajectory T and according to the speed of travel V, the control unit 16 periodically determines the desired attitude angle b TGT ; in particular, in order to determine the desired attitude angle b TGT ., the control unit 16 can use a predetermined map (usually created experimentally during the design and construction phase of the road vehicle 1 and stored in a memory) or a mathematical model of the road vehicle 1.

[0036] While travelling along a curve, the control unit 16 determines, in a known manner, the actual yaw angular speed dψ / dt of the road vehicle 1; the yaw angular speed dψ / dt is the rate of change of the yaw angle ψ (shown in Figure 4 ), i.e. the first time derivative of the yaw angle ψ. As is known, the yaw angle ψ represents the amplitude of the swing of the road vehicle 1 about the vertical axis Y passing through the centre of gravity B of the road vehicle 1.

[0037] While travelling along a curve, the control unit 16 determines, according to Figure 5 , the actual attitude angle β of the road vehicle 1, sets the desired attitude angle β TGT , determines the actual yaw angular speed dψ / dt of the road vehicle 1, sets the desired yaw angular speed dψ / dt TGT , and then varies the steering angle a of the rear wheel 3 and the stiffness distribution of the connections of the four wheels 2 and 3 to the chassis 12 in a simultaneous and coordinated manner according to the difference ε TGT between the actual attitude angle β and the desired attitude angle β β and the difference ε TGT between the actual yaw angular speed dψ / dt and the desired yaw angular speed dψ / dt ψ .

[0038] Figure 5 The above is illustrated schematically and it is disclosed that the control unit 16 comprises: a calculation module 17 which determines the actual attitude angle β of the road vehicle 1; a calculation module 18 which determines the actual yaw angular speed dψ / dt of the road vehicle 1; a calculation module 19 which sets the desired attitude angle β TGT and the desired yaw angular speed dψ / dt TGT ; a subtracter module 20 which calculates the difference ε TGT between the actual attitude angle β and the desired attitude angle β β ; a subtracter module 21 which calculates the difference ε TGT between the actual yaw angular speed dψ / dt and the desired yaw angular speed dψ / dt ψ ; and finally a calculation module 22 which controls the electronic control actuator 14 (thus varying the stiffness distribution of the connections of the four wheels 2 and 3 to the chassis 12) and the actuator 15 (thus varying the steering angle a of the rear wheel 3) in a simultaneous and coordinated manner.

[0039] According to the preferred embodiment, the calculation module 22 determines the desired steering angle a of the rear wheel 3 according to the two differences ε β and ε ψ together (i.e. in a simultaneous and coordinated manner).TGT the desired stiffness distribution D of the connection of the four wheels 2 and 3 with the frame 12 TGT (by means of which the electronic control actuator 14 is controlled).

[0040] According to the preferred embodiment, the calculation module 22 determines the desired steering angle a of the rear wheel 3 by combining open-loop control logic and feedback control logic together TGT and the desired stiffness distribution D of the connection of the four wheels 2 and 3 with the frame 12 TGT .

[0041] Moreover, according to the preferred embodiment, the desired steering angle a of the rear wheel 3 TGT and the desired stiffness distribution D of the connection of the four wheels 2 and 3 with the frame 12 TGT is determined by a single and general mathematical model (thus, this model coordinates together the action for the steering of the rear wheel 3 and the action for the stiffness distribution).

[0042] According to the preferred embodiment, the actual attitude angle b and the desired attitude angle b TGT are compared in the time domain; that is, the actual attitude angle b is compared directly with the desired attitude angle b TGT from moment to moment. On the other hand, the actual yaw rate d y / dt and the desired yaw rate d y / dt TGT are compared in the frequency domain. The time domain analysis applied to the attitude angle b represents its standard description with respect to the time variable, unlike the frequency domain analysis applied to the yaw rate d y / dt TGT which, on the other hand, represents its description in terms of frequency range (spectrum). In the frequency domain, the yaw rate d y / dt TGT is considered as a superposition of complex sinusoidal waves, each of which represents a given frequency (the corresponding phase is usually neglected).

[0043] According to the preferred embodiment, the calculation module 22 of the control unit 16 is programmed so that, in the medium-low lateral acceleration conditions (generally referred to simply as "linear" domain, in which the lateral acceleration is generally less than 4 to 6 m / s 2 , i.e. when the lateral acceleration is less than the threshold of 4 to 6 m / s 2 , for example equal to 5 m / s 2 ), the actuation requirements increase the dynamic response of the road vehicle 1 to follow the instructions of the driver, imposed by steering the front wheels 2, when driving along a curve. The coordination between the steering angle a of the rear wheel 3 and the desired stiffness distribution D of the connection of the four wheels 2 and 3 with the frame 12 TGT prioritizes the determination of the steering angle a of the rear wheel 3, thus favoring the rapid generation of lateral forces from the rear wheel 3, then, if necessary, the stiffness distribution D is associated.TGT In other words, when driving along a curve under medium to low lateral acceleration conditions, the main action is performed solely by changing the steering angle α of the rear wheel 3, while the stiffness distribution D of the connection between the four wheels 2 and 3 and the frame 12 is controlled. TGT It is adjusted only as a driven action relative to the change in the steering angle α of the rear wheel 3 (i.e., it is adjusted to adapt to the change in the steering angle α of the rear wheel 3).

[0044] According to a preferred embodiment, the calculation module 22 of the control unit 16 is programmed to maximize lateral stability under high lateral acceleration conditions (which typically means approaching the lateral stability limit, i.e., when the lateral acceleration is close to the lateral stability limit). For example, the lateral acceleration is 1-2 m / s² different from the maximum lateral acceleration representing the lateral stability limit. 2 It approaches the lateral stability limit; that is, the difference between the current lateral acceleration and the maximum lateral acceleration representing the lateral stability limit is less than 1 to 2 m / s². 2 The threshold within the range and, for example, equal to 1.5 m / s 2 At this point, the lateral acceleration approaches the lateral stability limit. From another perspective, the current lateral acceleration approaches the lateral stability limit when it reaches (exceeds) 85% of the maximum lateral acceleration representing the lateral stability limit. It should be noted that the maximum lateral acceleration is not a constant value applicable to all conditions, but is periodically updated (estimated) by the control unit 16; in particular, due to the interaction between the tire and the road surface, the maximum lateral acceleration varies substantially depending on the grip conditions.

[0045] The steering angle α of the rear wheel 3 and the desired stiffness distribution D of the connection between the four wheels 2 and 3 and the frame 12. TGT The coordination priority between the wheels maximizes the ability to generate lateral forces on the front half-shaft (consisting of the two front wheels 2) and the rear half-shaft (consisting of the two rear wheels 3). This generation capability varies depending on the cornering conditions, i.e., whether there are longitudinal forces for driving or braking (at the beginning or end of the corner) or only for cornering. In this case, the stiffness distribution D through the connection between the four wheels 2 and 3 and the frame 12 is... TGT The associated action plays a major role, which is then associated with the in-phase steering of the rear wheel 3 at the steering angle α. In other words, when traveling along a curve under high lateral acceleration conditions, the desired stiffness distribution D at the connection between the four wheels 2 and 3 and the frame 12 is uniquely altered by changing the desired stiffness distribution D. TGT The rear wheel 3 performs the primary action, while the steering angle α of the rear wheel 3 serves only as a secondary action relative to the desired stiffness distribution D. TGT The changes are adjusted (i.e., adjusted to fit the desired distribution D). TGT (Changes).

[0046] In this particular dynamic situation, in order to maximize the transverse force generating capacity of the front wheels 2 and of the rear wheels 3, it is convenient to reduce the height of the center of gravity of the road vehicle 1 by adjusting the stiffness of the connection of the four wheels 2 and 3 to the frame 12, by selectively acting on the electronically controlled (front and rear) actuators 14 on the inside or on the outside of the bend.

[0047] According to the preferred embodiment, the calculation module 22 of the control unit 16 is programmed so that, when driving along the bend and when the motion conditions are between the two above-mentioned cases (i.e. between the medium-low lateral acceleration conditions and the high lateral acceleration conditions), the adjustment can be made in a coordinated manner, which takes into account the dynamic requirement of reducing the response time of the rear axle by following the fast response of the forces generated by the rear wheels 3 to the steering command requested by the driver of the front wheels 2 and the requirement of adjusting the elastic stiffness, for example in order to reduce the roll movement of the road vehicle or to optimize the variation of the characteristic angles of the suspensions 13 in the presence of combined effects of steering and vertical vibrations. In other words, when the motion conditions are between the two above-mentioned cases (i.e. between the medium-low lateral acceleration conditions and the high lateral acceleration conditions), the main actions both include the variation of the desired stiffness distribution D TGT of the connection of the four wheels 2 and 3 to the frame 12 and the variation of the steering angle a of the rear wheels 3, i.e. the same level of adjustment is made on both the desired stiffness distribution D TGT of the connection of the four wheels 2 and 3 to the frame 12 and on the steering angle a of the rear wheels 3 (i.e. they are both "main" actions, one of which is not prioritized over the other).

[0048] In the embodiment illustrated in the attached figures, the control unit 16 acts on the electronically controlled actuators 14 to vary the dynamic response of the suspensions 13 connecting the wheels 2 and 3 to the frame 12; i.e. the electronically controlled actuators 14 allow to vary the dynamic response of the suspensions 13 connecting the wheels 2 and 3 to the frame 12 and, consequently, the distribution of the roll stiffness of the connection of the four wheels 2 and 3 to the frame 12.

[0049] As mentioned above, the electronically controlled actuators 14 can comprise four electronically controlled shock absorbers (one for each wheel 2 or 3) and / or two electronically controlled anti-roll bars (front and rear, respectively).

[0050] The embodiments described herein can be combined with each other without thereby going beyond the scope of protection of the present application.

[0051] The control method described above has different advantages.

[0052] First of all, the control method described above maximizes the performance of the road vehicle when driving along the bend (i.e. maximizes the bend coverage speed) without destabilizing the road vehicle 1 (i.e. approaching the stability limits of the vehicle 1 but maintaining an adequate safety margin).

[0053] Furthermore, the control method described above is particularly safe, since it always leaves the road vehicle 1 in a controlled state and is always able to act promptly and effectively when required.

[0054] The control method described above maximizes the dynamic response speed of the vehicle to the steering command requested by the driver to the front wheels 2.

[0055] Finally, in addition to maximizing the performance of the road vehicle as described above, the control method described above allows a uniform control of the road vehicle 1 over its entire range of use, i.e. from low to high lateral acceleration ranges, by implementing a combined regulation of the actuators, and meets the need to regulate the characteristic angles of the suspensions 13 and reduce the body roll movements.

[0056] List of reference signs

[0057] 1 vehicle

[0058] 2 front wheels

[0059] 3 rear wheels

[0060] 4 powertrain

[0061] 5 engine

[0062] 6 crankshaft

[0063] 7 driveline

[0064] 8 drive shaft

[0065] 9 mechanical transmission

[0066] 10 self-locking differential

[0067] 11 half shaft

[0068] 12 chassis

[0069] 13 suspension

[0070] 14 electronically controlled actuator

[0071] 15 actuator of the steered rear wheels

[0072] 16 control unit

[0073] 17 calculation module

[0074] 18 calculation module

[0075] 19 calculation module

[0076] 20 subtracter module

[0077] 21 subtracter module

[0078] 22 calculation module

[0079] B center of gravity

[0080] β attitude angle

[0081] ψ yaw angle

[0082] Z vertical axis

[0083] Fx longitudinal force

[0084] Fy lateral force

[0085] X longitudinal axis

[0086] Y lateral axis

[0087] Z vertical axis

[0088] α steering angle of rear wheel

[0089] D stiffness distribution

Claims

1. A control method of a road vehicle (1) having variable stiffness and steered rear wheels (3) when driving along a curve, said road vehicle (1) comprising: frame (12); two front wheels (2) and two rear wheels (3); at least one first actuator (14) designed to vary the stiffness distribution of the connection of the two front wheels (2) and of the two rear wheels (3) to the frame (12); and at least one second actuator (15) designed to vary the steering angle (a) of the rear wheels (3), the control method comprising the steps of: determining the actual yaw angle (b) of the road vehicle (1); Set the desired attitude angle (β TGT ) determining the actual yaw rate (d y / dt) of the road vehicle (1); setting a desired yaw angular velocity (dψ / dt TGT ); and Based on the actual attitude angle (β) and the desired attitude angle (β) TGT The difference (ε) between β ) and the actual yaw rate (dψ / dt) and the desired yaw rate (dψ / dt) TGT The difference (ε) between ψ The steering angle (α) of the rear wheel (3) and the stiffness distribution of the connection between the two front wheels (2) and the two rear wheels (3) and the frame (12) are changed simultaneously and in a coordinated manner. characterized in that said control method further comprises the step of prioritizing the steering action of said rear wheels (3) when driving along a curve and for lateral accelerations lower than 4 to 6 m / s 2 in order to maximize the dynamic response of said vehicle (1) to the steering command of said front wheels (2) requested by the driver, by adjusting accordingly the stiffness distribution of the connection of both said front wheels (2) and both said rear wheels (3) to said chassis (12) by means of said first actuator (14).

2. The control method according to claim 1, characterized by, When driving along a curve and the lateral acceleration is less than 4 to 6 m / s 2 , the primary action is performed exclusively by changing the steering angle (a) of the rear wheels (3), while the stiffness distribution (D TGT ) of the connection of the two front wheels (2) and the two rear wheels (3) to the frame (12) is only adjusted with respect to the change in the steering angle (a) of the rear wheels (3) as a secondary action.

3. The control method according to claim 1, characterized by, When driving along a curve and the lateral acceleration is less than 4 to 6 m / s 2 The stiffness distribution (D TGT ) of the connection of the two front wheels (2) and the two rear wheels (3) to the frame (12) is adjusted to the change of the steering angle (a) of the rear wheels (3) only.

4. The control method according to claim 1, characterized by, the control method further comprising the step of, when driving along a curve and the lateral acceleration is close to the stability limit, preferentially selecting, in a manner consistent with the curve coverage phase, the stiffness distribution of the connection of the two front wheels (2) and of the two rear wheels (3) to the frame (12), adjusting the steering angle (a) of the rear wheels (3) accordingly.

5. The control method according to claim 4, characterized by When driving along a curve and the lateral acceleration is close to the stability limit, the main action is performed exclusively by changing the stiffness distribution (D TGT ) of the connection of the two front wheels (2) and the two rear wheels (3) to the frame (12), while the steering angle (a) of the rear wheels (3) is only adjusted as a slave action with respect to the stiffness distribution (D TGT ) of the connection of the two front wheels (2) and the two rear wheels (3) to the frame (12).

6. The control method according to claim 4, characterized by When driving along a curve and the lateral acceleration is close to the stability limit, only the steering angle (a) of the rear wheels (3) is adjusted to adapt the stiffness distribution (D TGT ) of the connections of the two front wheels (2) and the two rear wheels (3) to the frame (12).

7. The control method according to claim 4, characterized by, The control method also comprises the step of lowering the center of gravity of the road vehicle (1) by varying the stiffness distribution (D TGT ) of the connection of the two front wheels (2) and of the two rear wheels (3) to the frame (12) when driving along a curve and when the lateral acceleration is close to the stability limit.

8. The control method according to claim 1, characterized by, The control method further comprises the step of simultaneously adjusting the steering angle (a) of the rear wheels (3) and the stiffness distribution (D 2 ) of the connection of the two front wheels (2) and the two rear wheels (3) with the frame (12) with the same hierarchy when driving along a curve and the lateral acceleration is greater than 4 to 6 m / s TGT ) and less than the stability limit.

9. The control method according to claim 1, characterized by, the difference between the current lateral acceleration and the maximum lateral acceleration representative of the lateral stability limit is less than a threshold value in the range of 1 to 2 m / s 2 the lateral acceleration is considered to be close to the lateral stability limit when the difference between the current lateral acceleration and the maximum lateral acceleration representative of the lateral stability limit is less than a threshold value in the range of 1 to 2 m / s 10. The control method according to claim 1, characterized by, The lateral acceleration is considered to be close to the lateral stability limit when the current lateral acceleration reaches 85% of the maximum lateral acceleration representative of the lateral stability limit.

11. The control method according to claim 1, characterized by, The control method further comprises the step of determining, as a function of the difference (ε β ) between the actual yaw rate (dψ / dt) and the desired yaw rate (dψ / dt TGT ), together with the difference (ε ψ ) between the actual attitude angle (β) and the desired attitude angle (β TGT ), a desired steering angle (α TGT ) of the rear wheels (3) and a desired stiffness distribution (D TGT ) of the connections of the two front wheels (2) and of the two rear wheels (3) to the frame (12).

12. The control method according to claim 11, characterized by, determining the desired steering angle (a TGT ) of the rear wheels (3) and the desired stiffness distribution (D TGT ) of the connection of the two front wheels (2) and the two rear wheels (3) to the frame (12) by combining open-loop control logic and feedback control logic.

13. The control method according to claim 11, characterized by, said desired steering angle (a TGT ) of said rear wheels (3) as well as said desired stiffness distribution (D TGT ) of said connection of both said front wheels (2) and both said rear wheels (3) to said frame (12) are determined by a single general mathematical model.

14. The control method according to claim 1, characterized by, comparing the actual attitude angle (β) and the desired attitude angle (β TGT ) in the time domain.

15. The control method according to claim 1, characterized by, comparing the actual yaw rate (dψ / dt) and the desired yaw rate (dψ / dt TGT ) in the frequency domain.

16. The control method according to claim 1, characterized by, The first actuator (14) varies the dynamic response of the suspensions (13) connecting the front wheels (2) and the rear wheels (3) to the frame (12) and comprises electronically controlled shock absorbers, or the first actuator (14) varies the dynamic response of the suspensions (13) connecting the front wheels (2) and the rear wheels (3) to the frame (12) and comprises electronically controlled anti-roll bars.

Images