METHOD FOR CONTROLLING A STEER-BY-WIRE STEERING SYSTEM AND STEER-BY-WIRE STEERING SYSTEM FOR A MOTOR VEHICLE

Abstract

A method can be used to control a steer-by-wire steering system for a motor vehicle. According to the method, a steering shaft sensor arranged on a steering shaft detects a steering angle input by a driver via a steering control element and a control unit based on a function of the detected steering angle and specifies a wheel steering angle for an electronically controlled steering actuator acting on at least one steered wheel. The control unit then calculates the wheel steering angle taking into account a specifiable correction angle. A steer-by-wire steering system can also be used to execute the method.

Claims

1.-10. (canceled).

11. A method for controlling a steer-by-wire steering system for a motor vehicle, comprising: detecting with a steering shaft sensor disposed on a steering shaft a steering angle input by a driver via a steering control element; and specifying with a control unit, as a function of the steering angle that is detected, a wheel steering angle for an electronically controlled steering actuator acting on a steered wheel, wherein the control unit takes into account a specifiable correction angle when calculating the wheel steering angle.

12. The method of claim 11 comprising detecting with the steering shaft sensor a steering torque introduced into the steering shaft, wherein the specifiable correction angle is calculated as a function of the steering torque.

13. The method of claim 12 comprising multiplying the steering torque by a selectable stiffness parameter to calculate the specifiable correction angle.

14. The method of claim 11 comprising calculating the specifiable correction angle as a function of a load that engages on the electronically controlled steering actuator.

15. The method of claim 14 comprising multiplying the load by a selectable stiffness parameter to calculate the specifiable correction angle.

16. The method of claim 15 comprising selecting the selectable stiffness parameter such that the specifiable correction angle corresponds to a spring stiffness of the steering shaft in a range of 0.5 to 4 Nm per angular degree.

17. The method of claim 11 comprising calculating the specifiable correction angle as a function of a driving speed of the motor vehicle.

18. The method of claim 11 comprising calculating the specifiable correction angle as a function of the steering angle that is detected.

19. The method of claim 11 comprising restricting the specifiable correction angle to a selectable angle interval.

20. A steer-by-wire steering system for a motor vehicle, comprising: a steering shaft sensor disposed on a steering shaft; an electronically controllable steering actuator acting on a steered wheel; and a control unit that is configured to specify a wheel steering angle for the electronically controllable steering actuator as a function of a steering angle detected by the steering shaft sensor, wherein the steer-by-wire steering system is configured to execute the method of claim 11.

Description

[0024] The invention is described in greater detail below on the basis of the exemplary embodiments represented in the attached illustrations.

[0025] FIG. 1 schematically shows an exemplary embodiment of the steer-by-wire steering system according to the invention,

[0026] FIG. 2 schematically shows the structure of the steering system and the associated control unit according to the exemplary embodiment according to FIG. 1 in a block diagram,

[0027] FIG. 3 schematically shows an example of the structure of the calculation unit of the control unit according to the exemplary embodiment according to FIGS. 1 and 2,

[0028] FIG. 4 schematically shows an example of the structure of the correction angle determination unit of the calculation unit according to FIG. 3,

[0029] FIG. 5 shows by way of example a characteristic curve of the correction angle determination unit according to FIG. 4.

[0030] The structure of a steer-by-wire steering system 1 for a motor vehicle according to a first exemplaty embodiment of the invention is represented schematically in figure (FIG. 1. Steer-by-wire steering system 1 has a steering control element 4 connected via a steering shaft 2 to a feedback actuator 9 in the form of a steering wheel. Steering system 1 further comprises an electronically controllable steering actuator 7 acting on two steered wheels 6 with a control device 8. Steering actuator 7 is connected via a steering gear 10 to a toothed rack 12. Steering gear 10 can comprise, for example, a pinion 11 which is in engagement with toothed rack 12. The translations of toothed rack 12 brought about by steering actuator 7 are transmitted via tie rods 13 to steered wheels 6 in order to adjust a wheel steering angle Ψ. Steer-by-wire steering system 1 finally comprises a control unit 5 which obtains steering angle φ of steering shaft 2 detected by a steering shaft sensor 3 as an input variable and specifies to steering actuator 7 an associated wheel steering angle Ψ as an output variable. Steering shaft sensor 3 can additionally be configured to detect a steering torque LM introduced into steering shaft 2. Steering shaft sensor 3 can either be formed as a separate sensor or integrated into feedback actuator 9. Control unit 5 furthermore serves to detect feedback effects from the carriageway which engage on steering actuator 7 as loads F, for example, in the form of a toothed rack force, and actuate feedback actuator 9 as a function of these loads F so that the driver feels the feedback effects from the carriageway in a familiar manner at steering control element 4.

[0031] FIG. 2 schematically shows the structure of steer-by-wire steering system 1 according to FIG. 1 as a block diagram. The steering position input by the driver at steering control element 4 is transmitted via steering shaft 2 to steering shaft sensor 3 which supplies measured steering angle φ and measured steering torque LM to a calculation unit 51 in control unit 5. Steering shaft sensor 3 comprises a steering angle sensor and a torque sensor which can be formed individually or as a measuring unit. Calculation unit 51 has as further input variables load F lying against toothed rack 12, vehicle speed v, as well as further state variables Z which characterize the vehicle state. From these input variables, calculation unit 51 calculates wheel steering angle Ψ and outputs this to a position controller 52 of control unit 5. The calculation of wheel steering angle Ψ is preferably performed taking into account a correction angle χ, the determination of which is preferably performed on the basis of steering torque LM and/or a load F and/or a driving speed v of the motor vehicle or other parameters, such as carriageway conditions, etc.. These variables can be measured, estimated or otherwise determined or identified or communicated.

[0032] Position controller 52 is provided to determine, from wheel steering angle 105 (nominal value) and an actual actuating angle ξ, a torque request signal T for steering actuator 7 which is suitable for adjusting wheel steering angle Ψ to steered wheels 6. Actual actuating angle ξ can be determined, for example, via a rotor position sensor at steering actuator 7. Alternatively, a separate sensor can be provided to detect actual actuating angle ξ, for example, in the form of a toothed rack position. Torque request signal T is output to a control device 8 of steering actuator 7 which converts this into associated motor currents I in order to actuate steering actuator 7.

[0033] Position controller 52 is furthermore provided to output a feedback signal for feedback actuator 9 on the basis at least of actuating angle ξ and wheel steering angle Ψ. The feedback signal can preferably be specified as a function of—in particular proportional to—the present position deviation between wheel steering angle Ψ and actuating angle ξ. The feedback signal can furthermore be dependent on load F.

[0034] FIG. 3 shows by way of example the structure of calculation unit 51 as a block diagram. Steering angle φ measured by steering shaft sensor 3 is therein supplied to a subtracter 55 and a steering torque determination unit 53. Steering torque determination unit 53 receives as further input variables steering torque LM measured by steering shaft sensor 3, load F, driving speed v, as well as further state variables Z which characterize the vehicle state. On the basis of these input variables, steering torque determination unit 53 calculates a virtual steering shaft torque M which is output to a correction angle determination unit 54. Correction angle determination unit 54 calculates, on the basis of virtual steering shaft torque M, a correction angle χ which is supplied to subtracter 55.

[0035] In the simplest case, virtual steering shaft torque M can be equal to measured steering torque LM. In other embodiments, measured steering torque LM can be offset by means of the remaining input variables of steering torque determination unit 53 to arrive at a virtual steering shaft torque M which is adapted to the driving situation. In further embodiments, virtual steering shaft torque M is calculated without taking into account measured steering torque LM.

[0036] Subtracter 55 calculates wheel steering angle Ψ from steering angle φ and correction angle χ. For example, the difference between steering angle φ and correction angle χ is assigned an assigned wheel steering angle Ψ in accordance with an assignment function. Alternatively, however, a preliminary wheel steering angle can also initially be assigned to steering angle φ, which preliminary wheel steering angle is then corrected by deducting correction angle χ from wheel steering angle Ψ.

[0037] FIG. 4 shows by way of example the structure of calculation unit 51 with a detailed representation of correction angle determination unit 54 as a block diagram. FIG. 4 shows a preferred embodiment, in the case of which a virtual steering shaft torque M is determined on the basis of load F and/or vehicle speed v, which virtual steering shaft torque M is converted by means of a stiffness parameter k into a preliminary correction angle α. Stiffness parameter k can be selected as a function of steering torque LM, steering angle φ, steering angle speed, vehicle speed, for example, by a function in which the stated variables serve as an input value. Depending on the embodiment, stiffness parameter k can also be represented in a characteristic field, or be acted upon as a function of the states with amplification factors. To this end, virtual steering shaft torque M is multiplied in a multiplier 56 by stiffness parameter k in correction angle determination unit 54. Virtual steering shaft torque M is preferably proportional to load F so that in this case, in order to calculate correction angle χ, load F or steering torque LM is multiplied by a selectable stiffness parameter k. The stiffness parameter is preferably selected so that the resultant virtual steering stiffness corresponds to a spring stiffness of steering shaft 2 in the range from 0.5 to 4 Nm per angular degree. This corresponds to the normal spring stiffness of a real torsion rod.

[0038] In the case of a real torsion rod arrangement, angle α would be the torsion angle. In the case of a real torsion rod arrangement, the torsion angle is restricted by an overload protection device. It is therefore preferably provided to restrict preliminary correction angle α subsequently in a restricter 57 to a selectable angle interval. Insofar as preliminary correction angle a lies within this interval, correction angle χ is equal to preliminary correction angle α. Insofar as preliminary correction angle α lies outside the angle interval, correction angle χ is fixed to a minimum correction angle χmin or a maximum correction angle χmax. The correction angle restricted in such a manner is output as correction angle χto subtracter 55.

[0039] The statements in relation to FIG. 3 otherwise correspondingly apply.

[0040] FIG. 5 schematically shows the characteristic curve of correction angle determination unit 54 according to FIG. 4. In a central torque range, a correction angle χ is assigned to virtual steering shaft torque M according to a linear relationship. Stiffness parameter k forms in this case the proportionality constant. In the case of virtual torques M outside the angle interval of restricter 57, correction angle χ is restricted to minimum or the maximum admissible correction angle χmin, χmax. As shown in FIG. 5, the angle interval is preferably selected to be symmetrical about zero.

[0041] When calculating correction angle χ, in contrast to the exemplary embodiment represented in FIGS. 1 to 5, many variants are conceivable. For example, restricter 57 can be dispensed with. Moreover, in one simplified embodiment, correction angle 102 can be specified as a constant, or calculated exclusively as a variable which is proportional to load F or steering torque LM. Finally, it is conceivable that virtual steering shaft torque M is calculated as a function of load F and the remaining state variables Z, driving speed v and/or steering angle φ are included in the calculation of a stiffness parameter k adapted to the driving situation. Minimum and/or maximum admissible correction angle χmin, χmax can also be specified as a function of the remaining state variables Z, driving speed v and/or steering angle φ and/or steering torque LM. Finally, partial stiffnesses which are added together to yield a total connection angle can also be calculated on the basis of individual state variables Z, v, φ.

LIST OF REFERENCE NUMBERS

[0042] 1 Steer-by-wire steering system

[0043] 2 Steering shaft

[0044] 3 Steering shaft sensor

[0045] 4 Steering control element

[0046] 5 Control unit

[0047] 6 Steered wheels

[0048] 7 Steering actuator

[0049] 8 Control device

[0050] 9 Feedback actuator

[0051] 10 Steering gear

[0052] 11 Pinion

[0053] 12 Toothed rack

[0054] 13 Tie rod

[0055] 51 Calculation unit

[0056] 52 Position controller

[0057] 53 Steering torque determination unit

[0058] 54 Correction angle determination unit

[0059] 55 Subtracter

[0060] 56 Multiplier

[0061] 57 Restricter

[0062] φ Steering angle

[0063] χ Correction angle

[0064] Ψ Wheel steering angle

[0065] α Preliminary correction angle

[0066] χmin Minimum correction angle

[0067] χmax Maximum correction angle

[0068] ξ Actual actuating angle

[0069] F Load

[0070] M Virtual steering shaft torque

[0071] v Driving speed

[0072] k Stiffness parameter

[0073] T Torque request signal

[0074] I Motor currents

[0075] LM Steering torque

[0076] Z State variable