Control procedure for a steer-by-wire steering system of a motor vehicle

Abstract

A control procedure is provided for controlling a hand counter-torque (36) acting on a steering wheel (14) of a steer-by-wire steering system (10), in which an actual steering wheel angle (16) of the steering wheel (14) is measured, an actual pinion angle (32) of a rack actuator (26) provided for steering wheels via a rack (22) and / or a measured value linearly dependent on the actual pinion angle is determined, wherein the actual pinion angle (32) corresponds directly and unfiltered to a time-dependent current steering position of the wheels, an elastic torsion of a steering wheel (14) rotated by the actual steering wheel angle (16) to the virtual torsion bar running in a rack positioned corresponding to the actual pinion angle (32) is calculated and an elastic torsional moment of the virtual torsion bar acts as a hand counter-torque (36) on the steering wheel (14).By simulating the torsional behavior of the saved torsion bar for the applied counter-moment (36), a transmission of a reaction force of a wheel in a steer-by-wire steering system (10) that is as true to nature as possible is made possible.

Description

[0001] The invention relates to a control method for controlling a manual counter-torque acting on a steering wheel of a steer-by-wire steering system of a motor vehicle. Furthermore, the invention relates to a control device and a steering system that can be used to implement such a control system.

[0002] From DE 10 2014 105 088 A1, a steering system for a motor vehicle based on a steer-by-wire approach is known, in which a manual counter-torque for the actuation of a steering wheel is applied by means of a rack for steering the wheels, for the actuation of which a control takes into account various measured and estimated values ​​and suppresses disturbances in certain frequency ranges.

[0003] From US 2023 / 0 303 158 A1 it is known to automatically compensate for vibrations on wheels in a steer-by-wire steering system using a rack actuator acting on a rack for steering the wheels and to additionally use the signal used to actuate the rack actuator to control a manual counter-torque on the steering wheel.

[0004] However, especially in sports and racing vehicles, there is a need to be able to detect any reaction force of a wheel as directly as possible, even without direct mechanical coupling, in a haptic manner on the steering wheel in order to be able to perceive as much important useful information about the current driving situation as possible.

[0005] The object of the invention is to demonstrate measures that enable the most realistic transmission of a wheel's reaction force in a steer-by-wire steering system.

[0006] The object is achieved according to the invention by a control method having the features of claim 1, a control device having the features of claim 9 and a steering system having the features of claim 10. Preferred embodiments of the invention are specified in the subclaims and the following description, which can each individually or in combination represent an aspect of the invention, the scope of protection being determined by the claims.

[0007] One aspect of the invention relates to a control method for controlling a manual counter-torque acting on a steering wheel of a steer-by-wire steering system, in which an actual steering wheel angle of the steering wheel is measured, an actual pinion angle of a rack actuator provided for steering wheels via a rack and / or a measured value linearly dependent on the actual pinion angle is determined, wherein the actual pinion angle corresponds directly and unfiltered to a time-dependent current steering position of the wheels, an elastic torsion of a virtual steering wheel rotated with the actual steering wheel angle to the torsion bar running in a rack positioned corresponding to the actual pinion angle is calculated and an elastic torsional moment of the virtual torsion bar acts as a manual counter-torque on the steering wheel.

[0008] It was recognized that the rack used to steer the wheels is essentially rigidly coupled to the wheels, apart from negligible elastic deformation components during operation. The rack can be articulated to the respective wheel carrier of the corresponding wheel via tie rods, with essentially no damping elements being provided in the path between a wheel carrier and the rack. Furthermore, it is possible to map the position of the rack very precisely via the rotation angle of the pinion of the rack actuator by deliberately avoiding displacement and / or force-damping elements in the coupling of the rack actuator to the rack.This utilizes the knowledge that the actual pinion angle for the pinion's rotation angle, or the measured value linearly dependent on this actual pinion angle, is determined from existing measured variables of the rack actuator, which are also used to monitor and check the actuation of the rack actuator. If changing wheel reaction forces occur, for example because the affected wheel is traveling over uneven ground, this generates a changing position of the rack and possibly a time-dependent change in the steering angle, which can be represented with the help of the directly or indirectly measured actual pinion angle as a difference between the actual pinion angle and the actual steering wheel angle. This difference leads to a rotation of the imaginary torsion bar torque and can thus be used as useful information for controlling the manual countertorque for the steering wheel, whereby the difference can be impressed at an identical frequency.The difference between the actual pinion angle and the actual steering wheel angle can therefore be used unfiltered and without changing the frequency spectrum in the calculation of the manual counter torque to be provided.

[0009] This considers that if the steer-by-wire steering system eliminates the need for a mechanical coupling between the steering wheel and the rack, a torsion bar would be provided connecting the steering wheel to the rack. This torsion bar would elastically twist as a result of the reaction torques acting on the wheels and introduced into the rack, as well as the steering torque introduced via the steering wheel, and would provide a resistance torque through its elastic torsion. This resistance torque can be calculated based on the actual steering wheel angle of the steering wheel and the actual pinion angle of the rack actuator, and can be simulated in the steer-by-wire steering system as a manual countertorque provided by a steering wheel actuator.To do this, it is only necessary to store the typical material properties of a torsion bar actually provided without a steer-by-wire steering system in the steer-by-wire steering system for the virtual representation of the torsion bar in order to simulate the manual counter-torque of an actually provided torsion bar. The torsion bar stiffness can be specified, whereby the torsion bar stiffness can preferably be individually varied to enable a stiffer or more sensitive coupling.If the corresponding steering angle of the wheels cannot be precisely achieved when turning the steering wheel due to sufficiently high reaction torques, for example when the wheels hit an obstacle from the side, this can be quantified by measuring the actual pinion angle or a measured value that is linearly dependent on it, in particular by comparing an actual pinion angle corresponding to the actual steering wheel angle. A corresponding torsion of the virtual torsion bar can be calculated, from which the corresponding manual countertorque can be determined and provided by the steering wheel actuator. This allows a purely mechanical coupling to be very precisely simulated in the steer-by-wire steering system, whereby the reaction torques acting on the wheels can be directly and as precisely as possible mapped haptically on the steering wheel in their excitation frequency, in particular with identical frequencies.Especially in a sports car or racing car, this immediate, tactile feedback of the reaction torque acting on the wheels can enable particularly high-performance driving at the limits of what is possible. Simulating the torsional behavior of the omitted torsion bar for the applied manual counter-torque enables the most realistic transmission of a wheel's reaction force in a steer-by-wire steering system.

[0010] The actual pinion angle can be determined in particular by an existing sensor system of the rack actuator, for example on a pinion of the rack actuator, whereby the rack force actuator can be mechanically coupled directly or indirectly to the rack. Instead of the actual pinion angle, another measured value can also be used that is linearly dependent on the actual pinion angle of the rack actuator. This measured value can in principle be measured at any point within the force flow in which the force introduction point of the rack actuator onto the rack is located. For example, instead of the actual pinion angle, a rotor angle of a drive motor driving the rack actuator can be measured, which, taking into account the existing gear ratio, allows a conclusion to be drawn about the actual pinion angle.If the actual pinion angle is referred to below, this should mean that the actual pinion angle can also be replaced by the linearly dependent measured value, without this having to be repeated again and again for the sake of simplified representation.

[0011] The steering wheel can be a steering device, regardless of its geometric shape, that can be manually manipulated, in particular rotated, by a driver to steer the motor vehicle. As a rule, the steering wheel can rotate a steering shaft that is rotatably mounted in the motor vehicle. The steering angle of the steering shaft can easily be measured by a rotation angle sensor known as a steering angle sensor and used as a measure of the rotation of the steering wheel. It is also possible to have a steering wheel actuator act on the steering shaft to provide a manual counter-torque. The steering shaft can, in particular, be a remnant of a torsion bar that would otherwise be provided without a steer-by-wire steering system, which would provide a mechanical coupling of the steering wheel to the rack.

[0012] The manual counter-torque is specifically designed to simulate a direct mechanical coupling between the steering wheel and the wheels steered by the steering wheel, even though the coupling in the steer-by-wire steering system is not mechanical but electrical, for example, through electrical actuation of actuators. Since the mechanical coupling is interrupted, it is possible to deliberately influence feedback regarding the wheel reaction forces by appropriately influencing the manual counter-torque. For example, the wheel reaction forces can be reflected more or less strongly in the manual counter-torque via a setting made by the driver, allowing the driver to receive more or less sensitive feedback.For example, in a racing situation it may be desirable to be able to experience even small excitations in the wheel reaction forces in a haptic manner on the steering wheel, while in travel mode it may be desirable to experience excitations in the wheel reaction forces in a way that is as little disruptive as possible on the steering wheel.

[0013] The rack can be part of a rack and pinion steering system for a motor vehicle, in which the rack can steer wheel carriers of the steerable wheels coupled via tie rods by a displacement transverse to the direction of travel of the motor vehicle (Y-direction), in particular to enable cornering.

[0014] The rack actuator can, for example, be designed as an electrically controllable device that can exert a force to displace the rack via a pinion or a ball screw. Furthermore, the rack actuator can be designed to absorb wheel reaction forces acting on the rack and / or to accommodate a displacement of the rack resulting from wheel reaction forces. This means that locking the rack actuator in specific positions on the rack, which corresponds to a specific angular position of the acting pinion, is not provided.A force equilibrium controlled by the rack actuator for a specific steering angle of the steering wheel can be canceled by acting wheel reaction forces and lead to a displacement of the rack and a rotation of the pinion, so that a deviation from the controlled force equilibrium can be detected by the difference between the actual pinion angle and the actual steering wheel angle.

[0015] The steering wheel actuator can, for example, be designed as an electrically controllable device that can generate the required manual counter-torque as quickly as possible. For this purpose, the steering wheel actuator can act, in particular, on the steering shaft of the steering wheel.

[0016] The steering wheel angle and / or the pinion angle are measured as a rotation angle, which is measured starting from a zero position, which corresponds to the vehicle traveling straight ahead. Depending on the direction of rotation, the measured rotation angle can be measured as a positive or negative value, so that the steering direction (right or left in the Y direction) can be determined from the sign of the measured rotation angle alone.

[0017] The direct and unfiltered correlation between the actual pinion angle and the steering position of the wheels is achieved in particular by a coupling that is as rigid and stiff as possible between the pinion of the rack actuator and the rack, as well as between the rack and the associated tie rod and the associated wheel carrier. In the coupling section between the wheel carrier and the pinion of the rack actuator, only rigid sub-elements are provided, which, although they may be connected to each other in an articulated manner, exhibit at most only such a negligible elastic deformation component under load that these sub-elements can be considered almost ideally rigid during expected normal operation.

[0018] In particular, it is intended that excitations influencing the actual pinion angle act on the steering wheel at identical frequencies via the provided manual countertorque. The frequency-identical coupling between the actual pinion angle and the calculated manual countertorque means that no low-pass or high-pass filtering is performed on the frequency spectrum of the excitations, and the frequency spectrum is passed on essentially unfiltered and without any change in the frequency spectrum to the steering wheel actuator to provide the manual countertorque. A true-to-life representation of the reaction forces occurring at the vehicle's wheels on the steering wheel is thus at most impaired by the physical inertia of the steering wheel actuator acting on the steering wheel. The inertia of the steering wheel actuator generally only dampens high-frequency vibrations that can no longer be resolved haptically by the driver anyway.A frequency-identical coupling between the difference between the actual steering wheel angle and the actual pinion angle and the calculated manual counter-torque is therefore only based on frequency ranges that are haptically perceptible to humans. However, active damping of frequencies in the frequency spectrum of the difference between the actual steering wheel angle and the actual pinion angle, which could unnecessarily impair haptic perception and / or cause unnecessary computing time and delays, is avoided.

[0019] Preferably, a time-dependent curve of the elastic torsional moment of the virtual torsion bar and / or the manual counter-torque is calculated from the difference between the actual steering wheel angle and the actual pinion angle. In particular, the difference between the actual steering wheel angle and the actual pinion angle is used exclusively on a linear scale to calculate the counter-torque. Taking into account additional transmission ratios in the virtual torque path via the virtual torsion bar, the difference between the actual steering wheel angle and the actual pinion angle is a good measure of the torsion of the virtual torsion bar that was actually saved by the steer-by-wire steering system.When converting the difference into the manual countertorque to be applied, a suitable factor for the amplitude of the frequency spectrum of the difference can be provided, although frequencies within the frequency spectrum are not altered by superimposed frequencies. In particular, deliberate low-pass filtering and / or high-pass filtering is also omitted, although unintentional suppression of frequencies cannot always be avoided for purely physical reasons of the components intended for transmitting the measured frequency spectrum of the difference. The transmission of the difference between the actual steering wheel angle and the actual pinion angle, as a value relevant for controlling the manual countertorque, can be carried out very quickly and thus almost simultaneously by eliminating filter operations.The haptic impression on the steering wheel can therefore be approximately synchronous with the wheel reaction forces introduced, within the scope of the temporal resolution of sensory impressions by a person.

[0020] The difference can be scaled linearly using a conversion factor, whereby only the amplitude but not the frequency response is changed. This allows the wheel reaction forces to be represented as manual counter-torque at the steering wheel with greater or lesser amplified power, but with a nearly identical frequency spectrum. A true-to-life representation of the reaction forces occurring at the vehicle's wheels at the steering wheel is thus only compromised by the physical inertia of a steering wheel actuator acting on the steering wheel, which is intended to generate the calculated manual counter-torque.

[0021] For calculating the manual counter-torque, it is particularly preferred to scale the actual pinion angle with a steering ratio to be mapped between the steering wheel and the wheels. When converting the actual pinion angle, the resulting scaling can also be used to take the desired steering ratio into account. This allows the steering comfort and steering behavior of the vehicle to be adjusted accordingly. Preferably, the driver can individually specify a suitable steering ratio to provide a more sluggish or more sensitive steering response, depending on personal preference.

[0022] In particular, the actual pinion angle is used immediately, especially without a dead-time element in the controlled system, to generate the manual counter-torque. Unnecessary time delays caused by complex calculations can be avoided, so that the haptic impression on the steering wheel can be almost synchronous with the introduced wheel reaction forces, within the temporal resolution of human sensory impressions.

[0023] Preferably, the actual steering wheel angle is transmitted at a higher frequency than adjusting a linear scaling of the difference based on additional measured values. In principle, it is possible to further adjust the calculated manual counter-torque based on additional measured values ​​to further improve steering feel. In this situation, however, the difference between the actual steering wheel angle and the actual pinion angle is determined and transmitted more frequently for calculating the manual counter-torque than the other measured values. This also makes it possible to haptically represent very high-frequency disturbance effects in the wheel reaction forces.

[0024] Particularly preferably, a rack and pinion actuator is coupled to the rack with minimal, particularly zero, play, particularly via a ball screw drive. Bearing play and / or tooth flank play between the rack and pinion actuator and the rack can thus be avoided, allowing the wheel reaction forces to be mapped as precisely as possible into the angular position of the pinion. The ball screw drive, also known as a recirculating ball screw, is characterized by lower friction and less stick-slip behavior compared to a conventional screw drive, thus avoiding frictional damping in the coupling section between the wheel carrier and the pinion of the rack and pinion actuator.

[0025] In particular, a steering wheel actuator is provided to act on the steering wheel and generate the calculated manual counter-torque. With the help of the steering wheel actuator, a steer-by-wire steering system can be implemented by removing the mechanical coupling between the steering wheel and the wheels to be steered and replacing it with the individually controllable steering wheel actuator on the steering wheel side, while the rack actuator acting on the steering rack is provided on the wheel side.

[0026] A further aspect of the invention relates to a control device for regulating a manual counter-torque acting on a steering wheel of a steer-by-wire steering system, comprising an input port for reading in an actual steering wheel angle of the steering wheel and an actual pinion angle of a rack actuator, an output port for actuating a steering wheel actuator acting on the steering wheel to provide the manual counter-torque, and a computer unit for calculating the manual counter-torque from the actual steering wheel angle and the actual pinion angle, wherein the computer unit is configured to carry out the control method, which can be designed and developed as described above. By simulating the torsional behavior of the omitted torsion bar for the acting manual counter-torque, the most realistic transmission of a reaction force of a wheel in a steer-by-wire steering system is possible.

[0027] A further aspect of the invention relates to a steering system for a steer-by-wire steering system of a motor vehicle, comprising a manually rotatable steering wheel for steering the motor vehicle, a steering angle sensor for measuring an actual steering wheel angle, a rack for changing a steering angle of wheels of the motor vehicle, a rack actuator for displacing the rack, a pinion angle sensor for directly or indirectly measuring an actual pinion angle of a rack actuator acting directly or indirectly on the rack, a steering wheel actuator acting on the steering wheel for providing a manual counter-torque and a control device connected to the steering angle sensor, the pinion angle sensor and the steering wheel actuator, which control device can be designed and further developed as described above, for regulating the manual counter-torque acting on the steering wheel.By simulating the torsional behavior of the saved torsion bar for the acting manual counter torque, a transmission of a wheel's reaction force in a steer-by-wire steering system that is as true to nature as possible is possible.

[0028] The invention is explained below by way of example with reference to the accompanying drawings using a preferred embodiment, wherein the features presented below can represent an aspect of the invention both individually and in combination, the scope of protection being determined by the claims. It shows: Fig. 1: a schematic diagram of a steer-by-wire steering system

[0029] The Fig.1 using the example of an "apaEPS" (axis-parallel arrangement of an engine / power pack) can be used for a non-mechanical coupling of a steering system of a motor vehicle. To steer the motor vehicle, a steering wheel unit 12 with a manually rotatable steering wheel 14 is provided. With the aid of a steering angle sensor, an actual steering wheel angle 16 can be measured as the angle of rotation of a steering shaft 18 connected to the steering wheel 14 and fed to a control device 20. With the aid of the control device 20, a rack 22 can be displaced in response to the measured actual steering wheel angle 16, which can thereby pivot tie rods 24. A wheel carrier can be connected to the respective tie rod 24, and a wheel to be steered can be connected to the wheel carrier. To displace the rack 22, a rack actuator 26 is provided which can be controlled by the control device 20 and which is connected via a gear, e.g.as shown here by a ball screw drive 30, with which the rack 22 is coupled with little friction and play.

[0030] An actual pinion angle 32 can be measured using a pinion angle sensor or indirectly calculated using the sensor system of the rack actuator 26. Using the actual pinion angle 32, the position of the rack 22 and the current steering angle of the wheels can be measured. Within the control device 20, the measured actual pinion angle 32 can be compared with a measured actual steering wheel angle 16, and a difference 34 can be calculated. Based on this difference 34, a manual counter-torque 36 can be calculated, which is fed as a target variable to a steering controller 38. The steering controller 38 controls a steering wheel actuator 40, which introduces the manual counter-torque 36 into the steering wheel 14, in particular via the steering shaft 18, so that the driver manually holding the steering wheel 14 receives haptic feedback.From the actual pinion angle 32, with knowledge of the actual steering wheel angle 16, a torsion of a virtual torsion bar is also calculated using the difference 34, which would have coupled the steering wheel 14 to the rack 22 if a purely mechanical steering system had been provided instead of the steer-by-wire steering system 10. From the torsion of the torsion bar, which is actually only considered virtually in the steer-by-wire steering system 10, an associated torsion bar torque can be calculated, which can be used as the manual countertorque 36. In particular, wheel reaction forces introduced into the rack 22 via the wheels can be mapped haptically in a virtually unfiltered manner and with identical frequencies, and at most only linearly scaled, via the manual countertorque 36 acting on the steering wheel 14.

[0031] From the difference 34 and / or the torsion bar torque 42 of the virtually considered torsion bar, a steering force assistance 44 can be calculated in the control unit 20, for example with the aid of a boost curve 43, which is to be provided by the rack actuator 26 via a suitable motor torque 58. However, a stabilization force 46 requested by a driver assistance system 45 can also be taken into account, in particular within the framework of a steering stabilization function. Furthermore, other influencing variables, such as motor signals 48 of the rack actuator 26, a driving speed 50 of the motor vehicle, and other measured values ​​52 in a sub-module 54, in particular for steering feel and feedback functions, can be taken into account, which could require an adjustment of the operation of the rack actuator 26.In a safety module 56, it can be checked and ensured that, despite a multitude of influencing variables, the rack actuator 26 is only operated in a suitable operating range.

Claims

[1] Control method for controlling a manual counter-torque (36) acting on a steering wheel (14) of a steer-by-wire steering system (10), in which an actual steering wheel angle (16) of the steering wheel (14) is measured, an actual pinion angle (32) of a rack actuator (26) provided for steering wheels via a rack (22) and / or a measured value linearly dependent on the actual pinion angle is determined, wherein the actual pinion angle (32) corresponds directly and unfiltered to a time-dependent current steering position of the wheels, an elastic torsion of a virtual torsion bar extending from the steering wheel (14) rotated with the actual steering wheel angle (16) to the rack positioned corresponding to the actual pinion angle (32) is calculated and an elastic torsional moment of the virtual torsion bar acts as a hand counter-torque (36) on the steering wheel (14). [2] Control method according to claim 1, in which excitations influencing the actual pinion angle (32) act on the steering wheel (14) at identical frequencies via the provided manual counter-torque (36). [3] Control method according to claim 1 or 2, in which a time-dependent course of the elastic torsional moment of the virtual torsion bar and / or the manual counter-torque (36) is calculated from a time-dependent course of a difference (34) between the actual steering wheel angle (16) and the actual pinion angle (32). [4] Control method according to one of claims 1 to 3, in which, for the calculation of the manual counter-torque (36), the actual pinion angle (32) is scaled with a steering ratio to be mapped between the steering wheel (14) and the wheels. [5] Control method according to one of claims 1 to 4, in which the actual pinion angle (32) is used immediately, in particular without a dead time element in the controlled system, for generating the manual counter-torque (36). [6] Control method according to one of claims 1 to 5, in which a transmission of the actual steering wheel angle (16), in particular an associated duty cycle and / or an associated clock frequency, is higher frequency than an adaptation of a scaling of the actual pinion angle (32) on the basis of further measured values. [7] Control method according to one of claims 1 to 6, in which the rack actuator (26) is coupled to the rack (22) with little play, in particular via a ball screw drive (30). [8] Control method according to one of claims 1 to 7, in which a steering wheel actuator (40) acts on the steering wheel (14) and the steering wheel actuator (40) generates the calculated manual counter-torque (36). [9] Control device (20) for controlling a manual counter-torque (36) acting on a steering wheel (14) of a steer-by-wire steering system (10), with an input port for reading in an actual steering wheel angle (16) of the steering wheel (14) and an actual pinion angle (32) of a rack actuator (26), an output port for actuating a steering wheel actuator (40) acting on the steering wheel (14) to provide the manual counter-torque (36) and a computer unit for calculating the manual counter-torque (36) from the actual steering wheel angle (16) and the actual pinion angle (32), wherein the computer unit is adapted to carry out the control method according to one of claims 1 to 8. [10] Steering system (10) for a steer-by-wire steering system of a motor vehicle, with a manually rotatable steering wheel (14) for steering the motor vehicle, a steering angle sensor for measuring an actual steering wheel angle (16), a rack (22) for changing a steering angle of wheels of the motor vehicle, a rack actuator (26) for displacing the rack (22), a pinion angle sensor for directly or indirectly measuring an actual pinion angle (32) of a rack actuator (26) acting directly or indirectly on the rack (22), a steering wheel actuator (40) acting on the steering wheel (14) for providing a manual counter-torque (36) and a control device according to claim 9 connected to the steering angle sensor, the pinion angle sensor and the steering wheel actuator (40) for regulating the manual counter-torque (36) acting on the steering wheel (14).

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