Method of controlling a steer-by-wire steering system of a vehicle, corresponding steering system, vehicle and computer program product

Abstract

A method of controlling a steer-by-wire steering system (1 ) of a vehicle is described. The angular position (swa) of the steering wheel (2), the speed (vx) of the vehicle, and the current yaw rate (Ψm) of the vehicle are sensed. A first position (δf) of the steering rack (7) of the steering system (1) is calculated (20) based on the position of the steering wheel and the speed of the vehicle. A first yaw rate (Ψ0L,pass) of the vehicle is determined (231 ) based on the position of the steering wheel, the speed of the vehicle, and a curve indicative of the understeer gradient in stationary conditions. A second yaw rate (Ψ0L,ref) of the vehicle is determined (232) based on the position of the steering wheel, the speed of the vehicle, and a curve indicative of the desired understeer gradient. The first yaw rate (ΨOL,pass) is subtracted from the second yaw rate (ΨOL,ref) to determine a first difference factor (ΔΨ). Based on the first difference factor (ΔΨ) and the speed of the vehicle, a first corrective value of displacement (Δδf,OL) required to the steering rack (7) is determined (233) to neutralize the first difference factor (ΔΨ). A third yaw rate (ΨCL,ref,dyn) of the vehicle is determined (234) based on the position of the steering wheel, the speed of the vehicle, and parameters indicative of the physical characteristics of the vehicle. The current yaw rate (ΔΨ) is subtracted from the third yaw rate (ΨCL,ref,dyn) to determine a second difference factor (ΨCL,ref)- The second difference factor (ΨCL,ref) is transmitted to a PID controller (235) to determine a second corrective value of displacement (Δδf,CL) required to the steering rack (7). The actual position of the steering rack is controlled by an electrical actuator (6) based on the sum of the first position (δf) of the steering rack, the first corrective value of displacement (Δδf,OL) and the second corrective value of displacement (Δδf,CL)

Description

[0001] DESCRIPTION of the industrial invention entitled:

[0002] "Method of controlling a steer-by-wire steering system of a vehicle, corresponding steering system, vehicle and computer program product" of: Stellantis Europe S.p.A., of Italian nationality, Corso Giovanni Agnelli 200, 10135 Torino

[0003] Designated inventor: Giampaolo MASCHIETTI

[0004] Filed on:

[0005] ★★★★

[0006] TEXT OF THE DESCRIPTION

[0007] Technical field

[0008] The description relates to steer-by-wire (SbW) steering systems that can be implemented on vehicles, and in particular to a method of controlling such steering systems.

[0009] Prior art

[0010] In the steering systems traditionally implemented on vehicles, the mechanical connection between the steering wheel and the steering rack (which in turn is connected to the steering arms or tie rods to control the steering angle of the wheels) is continuous and made via the steering column and universal joints.

[0011] In order to reduce the size, weight, and mechanical complexity of the steering system, some modern vehicles are instead equipped with steer-by-wire or SbW steering systems, in which there is no continuous mechanical connection between the steering wheel and the steering rack: the steering column is replaced by an electronic control mechanism, which includes a sensor (mounted on the shaft of the steering wheel) to sense the rotation angle of the steering wheel, an electrical actuator to control the movement of the steering rack, and an electronic control unit that drives the electrical actuator based on the data sensed by the sensor. Thanks to the use of an electronic controller, steer-by-wire steering systems also allow the implementation of additional functions compared to those of traditional mechanical steering systems. For example, document US 2016 / 01 14832 A1 describes a steering control device that increases the steering reaction force of a steering reaction force actuator in a direction in which a lateral position of a host vehicle moves away from a travel path partition line when a turning amount of a turning actuator increases in the direction in which the lateral position of the host vehicle moves away from the travel path partition line. A steering reaction force control amount is computed for a reaction force suppression processing to reduce the steering reaction force control amount when the steering reaction force control amount has been at a threshold value or higher for a length of time. The reaction force suppression processing is carried out to control the steering reaction force actuator in lieu of the steering reaction force control amount. Meanwhile, the turning amount is maintained at the value occurring when the reaction force suppression processing began.

[0012] As another example, document US 2021 / 0016826 A1 describes a steer- by-wire steering system that includes a controller operable to operate a roadwheel actuator such that a position command to the roadwheel actuator based on a handwheel orientation is a magnitude corresponding to a handwheel orientation offset value in an opposite direction to reduce a difference between the handwheel orientation offset value and a predetermined handwheel zero value.

[0013] Document US 2023 / 0017618 A1 instead describes a steer-by-wire steering system for a vehicle, which includes a steering input device, a feedback actuator, and a steering assembly that can be connected to at least one steerable vehicle wheel. The steering assembly has a steering actuator device, a control device, and a feedback actuator monitoring device. The steer-by-wire steering system further has a synchronization monitoring device that is designed and configured, in at least one operating state of the steer-by-wire steering system, to determine a synchronization offset between a current position of the steering input device and a current position of the steering actuator device and / or of the at least one steerable vehicle wheel. The control device is further designed and configured, in at least one operating state of the steer-by-wire steering system, to further control the steering actuator device in accordance with the synchronization offset determined by the synchronization monitoring device.

[0014] As a further example, document CN 1 17302347 A describes a distributed steer-by-wire control method based on a virtual motor. The distributed steer-by- wire control method includes the steps of obtaining an expected yaw velocity and an expected side slip angle through a two-degree-of-freedom vehicle model; solving the reference rotation angles of the steering motors of the four wheels and the reference rotation angles of the virtual motors; solving the reference rotation speeds of the steering motors and the virtual motors of the four wheels; according to the reference rotation speeds and the actual rotation speeds of the steering motors and the virtual motors of the four wheels, generating the rotation speed compensation; the reference rotation speed is subtracted from the rotation speed compensation and the actual rotation speed at the same time, the obtained difference value and the rotation speed controller output current to the steering motor, the steering motor is controlled to deflect, and the steering action is completed.

[0015] Additional functionalities of steer-by-wire steering systems, which allow to increase the driving pleasure of the vehicle by improving its sportiness, are therefore desirable.

[0016] Object and summary

[0017] One or more embodiments aim to provide new driving functionalities for a vehicle, exploiting the steering control possibilities offered by steer-by-wire steering systems.

[0018] According to the solutions described herein, this object is achieved by a method having the features referred to in the following claims. Such a method can be implemented by a computer and / or an electronic processing unit, for example using the processing capacity of an on-board electronic control unit (ECU) of a vehicle.

[0019] The solutions described herein may also relate to a corresponding steering system for a vehicle, a corresponding vehicle, and a corresponding computer program product, loadable into the memory of an electronic processing unit of a vehicle. The claims form an integral part of the teachings provided herein in relation to the embodiments of the solutions described.

[0020] Brief description of the accompanying figures

[0021] One or more embodiments will now be described, by way of non-limiting example, with reference to the accompanying drawings, in which:

[0022] - Figure 1 is a diagram illustrating a steer-by-wire steering system; and

[0023] - Figure 2 is a block diagram illustrating the operation of a steer-by-wire steering system, according to one or more embodiments.

[0024] Detailed description

[0025] In the following description, various specific details are illustrated to provide a thorough understanding of various examples of embodiments according to the description. The embodiments can be obtained without one or more of the specific details, or with other methods, components, materials, etc. In other cases, known structures, materials, or operations are not illustrated or described in detail so that the various aspects of the embodiments are not obscured.

[0026] A reference to "an embodiment" or "a solution" within the context of this description indicates that a particular configuration, structure, or feature described in relation to the embodiment is included in at least one embodiment or solution. Phrases such as "in an embodiment" or "in a solution" that may be present at various points in the present description do not necessarily refer to the same embodiment or solution.

[0027] Furthermore, particular configurations, structures, or features may be combined in any suitable manner in one or more embodiments or solutions.

[0028] The references used herein are provided merely for convenience and therefore do not define the scope of protection or the extent of the embodiments.

[0029] As mentioned, in the various figures, the same references are used to indicate corresponding parts or elements, without repeating the related description for each figure for brevity.

[0030] As anticipated, Figure 1 schematically illustrates a steer-by-wire steering system 1 for a vehicle. The steering system includes a steering wheel 2 having a shaft (rotation axis) 3, on which a sensor (not visible in Figure 1 ) is mounted that senses the rotation angle of the steering wheel 2 and the torque applied by the driver turning the steering wheel 2. Furthermore, on the shaft 3 is mounted a feedback actuator 4, which can be operated to rotate the steering wheel 2 independently of the driver's action on it, and serves for example to transmit the road reactions to the steering wheel 2 and thus to provide the driver with feedback on the steering and driving behavior of the vehicle. The driver's steering request is transmitted to a control unit 5 via signal lines, using as input data the rotation angle of the shaft 2 measured by the rotation angle sensor. The control unit 5 controls, based on the signal produced by the rotation angle sensor and possibly other input variables (such as the vehicle speed, the yaw angle, and the like), an electrical steering actuator 6, which acts on the steering rack 7 (or steering box) to control the position of the steering arms or tie rods 8, and consequently the steering angle of the vehicle's steerable wheels (e.g., the front axle wheels).

[0031] Figure 2 is a block diagram illustrating the operation of the steering system

[0032] 1 during the vehicle's travel, according to the present invention. The steering system comprises substantially an algorithm 20 (e.g., implemented in the control unit 5, or in another electronic unit of the vehicle) that calculates the steering ratio T of the steering system, i.e., the relationship between the position 5f of the steering rack 7 and the angle swa (= steering wheel angle) of the steering wheel

[0033] 2 (or angular position of the steering wheel). Under normal driving conditions, algorithm 20 receives as input the vehicle speed vx, the angle swa of the steering wheel 2 detected by the appropriate sensor described earlier, and optionally a parameter DM indicative of the currently selected driving mode. Algorithm 20 determines the position 5f of the steering rack 7 (e.g., expressed as a percentage between 0% and 100%, or as a decimal value between 0 and 1 , where the lower end 0% or 0 indicates one of the end positions of the steering rack 7 and the upper end 100% or 1 indicates the other end position of the steering rack 7) based on the steering wheel angle swa (expressed in degrees, °) and the speed vx. In particular, under normal driving conditions, there may be a linear dependence (e.g., direct proportionality) between the steering wheel angle swa and the position of the steering rack 5f, and with the same rotation of the steering wheel, the displacement of the steering rack can be smaller as the vehicle speed vxis higher, i.e., the steering ratio T may decrease as the speed vxincreases (this to prevent that, at high speeds, a small correction of the angle swa made by the driver on the steering wheel can generate an excessive variation of the vehicle's trajectory). As illustrated in Figure 2, the position 5f of the steering rack is transmitted to the electrical steering actuator 6, which controls the position of the steering rack 7. The actuator 6 thus acts on the vehicle's mechanics M to control its steerable wheels and therefore the trajectory. Furthermore, the position <5 / of the steering rack can also be transmitted to a control logic 21 (e.g., implemented in the control unit 5 or in another control unit) that determines, based on it, a feedback torque T that has to be applied to the shaft 3 of the steering wheel by the feedback actuator 4, in order to provide the driver with the correct driving feeling. Once this feedback torque T is determined, it is transmitted to the feedback actuator 4. The actuator 4 thus acts on the vehicle's mechanics M to control the steering wheel. The feedback torque T and the steering wheel angle swa can be reported, in feedback, to algorithm 20.

[0034] One object of the present invention is to allow the driver to modify, upon request (i.e., selectively and only under certain conditions), the control logic of the steer-by-wire steering system to achieve a dynamic vehicle behavior that maintains a condition of controlled sliding or controlled drifting, in order to increase driving pleasure in certain sporty driving conditions, for example during the use of the vehicle on a track. To this end, an additional logic represented by the set of operations 23 is placed before the previously described logic 22, which can be selectively activated by the driver (for example, by acting on an appropriate driving mode selector) and / or can be activated only upon the occurrence of other conditions (e.g., only when the vehicle is in a controlled area such as a track or a test field).

[0035] Substantially, logic 23 (which can be implemented in the control unit 5 or in another control unit of the vehicle) receives as input the parameters vx, swa and DM already discussed earlier (e.g., by reading their values from a vehicle communication network such as the CAN network, optionally receiving such values redundantly via two separate CAN vehicle networks), and also receives as feedback from logic 22 the value of the steering ratio r, the position 5f of the steering rack and the feedback torque T.

[0036] With a set of operations 231 , logic 23 determines in open loop (e.g., using mathematical equations or calibration maps, such as look-up tables) a value ipoL.pass of the vehicle's yaw rate, based on passive measurements (steady-state, stationary conditions) carried out on a reference vehicle (not equipped with logic 23) starting from the input data (i.e., vehicle speed vx, steering wheel angle swa and wheel angle, the latter calculated based on the steering wheel angle swa) and based on a passive understeer gradient curve Kus(or understeer factor - a curve that substantially relates the steering wheel angle swa with the lateral acceleration ay of the vehicle). In particular, the set of operations 231 carried out in open loop starting from passive measurements allows to determine the value ipoL, passusing a bicycle model representative of the vehicle that is correlated with measurements performed on a "conventional" reference vehicle. Operations 231 allow to compare what happens in real-time on the vehicle according to the invention with what would happen if the same "passive" or conventional vehicle (i.e., not equipped with logic 23) were driven to the acceptable limit.

[0037] With a set of operations 232, logic 23 determines in open loop, using the same methodology of operations 231 , another value ip0L,ref of the vehicle's yaw rate, based on the same input parameters but based on a desired or target understeer gradient curve Kus. The target value Kuscan be an adjustable parameter (for example, equivalent to having a preset yaw angle (psi) or side-slip angle (beta)) or a value defined starting from the value of Kusmeasured on the reference vehicle driven by an experienced driver or tester, to which a safety coefficient can be applied to take into account factors related to the functional state of the vehicle.

[0038] In a subsequent subtraction node, the yaw rate ip0L,ref calculated according to the desired curve Kusis compared with the yaw rate ip0L,pass calculated according to the passive curve Kusto determine the difference between them, Alp — lpoL,ref ~ ^PoL,pass- With a set of operations 233, logic 23 determines in open loop (e.g., using an inverse static model of the vehicle dynamics), based on the difference ip and the vehicle speed vx, a value 80Lof displacement required to the steering rack 7 to neutralize the difference ip.

[0039] Therefore, substantially, a first part of logic 23 operates in open loop and compares the behavior acquired in "quasi -static" maneuvers performed on the reference vehicle model with the desired values of lateral acceleration, speed and yaw acceleration vs. steering wheel angle (defined by look-up tables, for example determined by specific tuning of the vehicle on which the new functionalities subject of the present invention are implemented). In particular, the open loop contribution of logic 23 is calculated via the inversion of the static bicycle model (logic 233) that receives as input a yaw rate error and provides as output a delta wheel angle that is necessary to obtain that delta yaw rate as input. In turn, the yaw rate error is calculated as the difference between a static yaw rate reference and a passive static yaw rate, i.e., based on the behavior of a "passive" vehicle, that is equipped with a traditional steering system, in "quasi-static" maneuvers. The passive static yaw rate data represents the passive vehicle and is obtained through tests (e.g., snail tests at different speeds). The static yaw rate reference data represents the desired behavior at SS, and the design parameter is the understeer coefficient (Kus), so a more agile or stable vehicle can be defined in different scenarios / driving modes.

[0040] With a set of operations 234, logic 23 determines in closed loop (using mathematical equations that describe the desired dynamics of the vehicle) a value iCL,ref,dyn of the vehicle's yaw rate, based on parameters indicative of the physical characteristics of the vehicle previously stored (e.g., moments of inertia, position of the center of gravity, front and rear track, wheelbase, etc.) starting from the input data (i.e., vehicle speed vx, steering wheel angle swa, wheel angle calculated based on the steering wheel angle swa, and lateral acceleration). For example, block 234 can solve the equations:

[0041] Jz^P=Fflf ~ Frlrmay= Ff + Fr where Jzis the polar moment of inertia around the z-axis (vertical axis) of the vehicle, p is the yaw acceleration of the vehicle, Ffis the sum (resultant) of the forces acting on the front axle of the vehicle, and Fris the sum (resultant) of the forces acting on the rear axle of the vehicle.

[0042] In a subsequent subtraction node, the yaw rate ipCL,ref,dyn calculated at step 234 is compared with the measured yaw rate ipmof the vehicle to determine the difference between them, ipCL,ref = ^PcL.ref.dyn ~ Proportional-lntegral-Derivative (PID) controller 235 receives as input the error parameter ipCLirefand produces, based on it, a further value A<5CLof displacement required to the steering rack 7.

[0043] Therefore, substantially, a second part of logic 23 operates in closed loop and is responsible for following the desired dynamic behavior. This not only allows covering dynamic scenarios such as double lane change, step steer, but also covering inaccuracies given by the model and disturbances. It is calculated through a PID controller (logic 235) that, starting from a yaw rate error, calculates a delta wheel angle that allows following the desired dynamic behavior. The yaw rate error is calculated as the difference between a dynamic yaw rate reference and the measured yaw rate. The dynamic yaw rate reference is calculated starting from the static yaw rate reference, adding the dynamic part (resulting from the solution of the differential equations of block 234) and offers the possibility, through calibration parameters, to decide how fast the yaw rate response to steering can be, or how much it can be damped.

[0044] In a subsequent addition node, the displacement required to the steering rack 8 '0Lcalculated in open loop is added to the displacement required to the steering rack A8CLcalculated in closed loop to produce a steering rack actuation command that requires an overall displacement A8f= A8f i0L+ A8f CLand which is transferred to the algorithm 20 discussed earlier. Therefore, substantially, the total request for "delta angle" (open loop + closed loop) is transformed into a steering rack displacement request, which is then sent directly to the steer-by- wire system for execution.

[0045] The displacement A8fis added to the displacement of the steering rack 7 that algorithm 20 calculates according to the "conventional" logic described earlier. In this way, the actual position of the steering rack is altered compared to what would be in "normal" driving conditions, in order to produce a certain sideslip angle (i.e., the angle difference between the direction in which the wheels are pointing and the direction in which the wheels are actually moving) and achieve a condition of drifting of the vehicle. The value of the side-slip angle can be predefined (e.g., depending on the selected driving mode) and / or can be chosen by the vehicle driver, using a specific section of the vehicle's interface (HMI), possibly integrated into the infotainment system already present on board the vehicle. In this way, it is possible to increase the sportiness of the vehicle perceived by the driver, deliberately bringing the vehicle into a controlled drifting condition, and thus without danger.

[0046] While the fundamental principles remain the same, the details of implementation and the embodiments may vary, even significantly, with respect to what has been described and illustrated here, by way of non-limiting example, without thereby departing from the scope of protection.

[0047] The scope of protection is determined by the attached claims.

Claims

CLAIMS1. A method of controlling a steer-by-wire steering system (1 ) of a vehicle, the method comprising:(i) sensing the angular position (swa) of a steering wheel (2) of the steering system (1 ), the travelling speed ( vx) of the vehicle, and the actual yaw rate of the vehicle;(ii) computing (20) a first position (51) of a steering rack (7) of the steering system (1 ) based on the angular position (swa) of the steering wheel (2) and on the travelling speed ( vx) of the vehicle;(iii) determining (231 ) a first value of yaw rate (ipoL.pass) of the vehicle based on the angular position (swa) of the steering wheel (2), on the travelling speed (vx) of the vehicle, and on a curve indicative of the understeer coefficient of the vehicle in stationary conditions;(iv) determining (232) a second value of yaw rate (ip0L,ref) of the vehicle based on the angular position (swa) of the steering wheel (2), on the travelling speed (vx) of the vehicle, and on a curve indicative of the desired understeer coefficient of the vehicle;(v) subtracting the first value of yaw rate (ipoL.pass) from the second value of yaw rate (ip0L,ref)t0determine a first corrective parameter (hip) ;(vi) determining (233), based on said first corrective parameter (&ijj) an on said travelling speed ( vx) of the vehicle, a first corrective value of displacement (^<V,OL) demanded to said steering rack (7) to neutralize said first corrective parameter (&ijj) ;(vii) determining (234) a third value of yaw rate (ipCL,ref,dyn) of the vehicle based on the angular position (swa) of the steering wheel (2), on the travelling speed (vx) of the vehicle, and on one or more stored parameters indicative of the physical characteristics of the vehicle;(viii) subtracting the actual yaw rate (ipm) of the vehicle from the third value of yaw rate (ipCL,ref,dyn)t0determine a second corrective parameter (ipCL,ref)>’(ix) transmitting said second corrective parameter (pCL,ref)t0 aproportional-integral-derivative controller (235) to determine a second corrective value of displacement ( 8fiCL) demanded to said steering rack (7); and(x) controlling the actual position of said steering rack (7) via an electrical actuator (6) of the steering system (1 ) based on the sum of said first position (51) of the steering rack (7), said first corrective value of displacement (A / ;0L) and said second corrective value of displacement2. The method of claim 1 , comprising:- determining (21 ), based on said first position (51) of the steering rack (7), a feedback torque (7) that has to be applied to a shaft (3) of the steering wheel (2) to provide a driving feedback to the driver of the vehicle; and- applying said feedback torque (7) to the shaft (3) of the steering wheel (2) via a further electrical actuator (4) of the steering system (1).

3. The method of claim 1 or claim 2, wherein the relation between the angular position (swa) of the steering wheel (2) and the first position (51) of the steering rack (7) is a linear dependence, and / or wherein the ratio between an angular displacement of the steering wheel (2) and a resulting linear displacement of the steering rack (7) increases as the travelling speed (vx) of the vehicle increases.

4. A steer-by-wire steering system (1) for a vehicle, the steering system (1 ) comprising:- a steering wheel (2) having a rotation shaft (3);- a sensor mounted on said rotation shaft (3) of the steering wheel and configured to sense the angular position (swa) of the steering wheel (2);- a steering rack (7) mechanically coupled to respective steering tie rods (8) to control the steering angle of the wheels of said vehicle;- a first electrical actuator (6) coupled to said steering rack (7) and configured to control the movement of the steering rack (7); and- an electronic control unit (5) configured to receive as input a signal indicative of said angular position (swa) sensed by said sensor, a signal indicative of the travelling speed (vx) of the vehicle, and a signal indicative of the actual yaw rate of the vehicle, said electronic control unit (5) being configured to control said first electrical actuator (6) according to the method of any of the previous claims.

5. A vehicle comprising a steer-by-wire steering system (1 ) according to claim 4.

6. A computer program product loadable in a memory of an electronic control unit of a vehicle, and comprising software instructions that, when the computer program product is executed by the electronic control unit, result in execution of the method according to any of claims 1 to 3 by the electronic control unit.

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