STEER-BY-WIRE STEERING SYSTEM WITH ADAPTIVE RACK-AND-PINION POSITION ADJUSTMENT

Abstract

A method and system for rack-and-pinion position adjustment for a steer-by-wire steering system for a motor vehicle. A module provides adjustment of the rack-and-pinion position, by determining a position error from a difference between desired and estimated value(s) of the rack-and-pinion position and the rack-and-pinion speed in a feedback structure, from which a control variable is determined for controlling the rack-and-pinion and a disturbance variable compensation for the control variable is carried out in a feedforward structure by means of a rack force estimation.

Claims

1.-10. (canceled)

11. A method for the adjustment of a rack-and-pinion position for a steer-by-wire steering system for a motor vehicle, comprising: providing a module configured to adjust a position of a rack-and-pinion, determining, with the module, a position error from a difference between a desired and an estimated value of the rack-and-pinion position and a rack-and-pinion speed in a feedback structure, determining, from the position error, a control variable to control the rack-and-pinion, and carrying out a disturbance variable compensation for the control variable via a feedforward structure via a rack-and-pinion force estimation.

12. The method of claim 11, further comprising carrying out a friction force compensation of the control variable in another feedforward structure via an estimation of a coefficient of static friction and a friction model.

13. The method of claim 12 wherein the friction force compensation and/or the disturbance variable compensation is carried out via a non-linear adaptive estimator.

14. The method of claim 11 wherein the feedback structure comprises a linear quadratic regulator.

15. The method of claim 12 wherein the estimated coefficient of static friction is included as input in a rack-and-pinion force estimator.

16. The method of claim 15 wherein the estimated coefficient of static friction is supplied to the friction model as an input, together with the rack-and-pinion speed estimated by the rack-and-pinion force estimator.

17. The method of claim 12 wherein the friction model compensates for the friction force and determines a torque therefrom, which is added to the estimated rack-and-pinion force and to the control variable for controlling the rack-and-pinion.

18. The method of claim 12 wherein the friction model is an asymmetrical, modified, dynamic friction model.

19. A method for controlling a steer-by-wire steering system for a motor vehicle, the system comprising an electronically adjustable steering adjuster acting on steered wheels, a control unit, a feedback actuator, configured to be actuated via a steering input means with a driver's request for a steering angle, and emitting a feedback signal to the steering input means as a reaction to the driver's request and a vehicle state of the motor vehicle, the method comprising: transmitting, via a signal transmission, the driver's request to the control unit, and controlling, via the control unit, the steering adjuster to transform the driver's request into a deflection of the steered wheels, wherein the control unit comprises a module configured to adjust the rack-and-pinion position via the method comprising: determining, with the module, a position error from a difference between a desired and an estimated value of the rack-and-pinion position and a rack-and-pinion speed in a feedback structure, determining, from the position error, a control variable to control the rack-and-pinion, and carrying out a disturbance variable compensation for the control variable via a feedforward structure via a rack-and-pinion force estimation.

20. A steer-by-wire steering system for a motor vehicle, comprising: an electronically adjustable steering adjuster acting on steered wheels of the motor vehicle, a control unit, a feedback actuator, which is configured to be actuated via a steering input means by a driver's request for a steering angle, and emit a feedback signal to the steering input means as a reaction to the driver's request and a vehicle state of the motor vehicle, a device for signal transmission, which is configured to transmit the driver's request to the control unit, wherein the control unit controls the steering adjuster to transform the driver's request into a deflection of the steered wheels, wherein the steer-by-wire steering system is configured to carry out a method comprising: determining, with the module, a position error from a difference between a desired and an estimated value of the rack-and-pinion position and a rack-and-pinion speed in a feedback structure, determining, from the position error, a control variable to control the rack-and-pinion, and carrying out a disturbance variable compensation for the control variable via a feedforward structure via a rack-and-pinion force estimation.

Description

[0022] In addition, a corresponding steer-by-wire steering system for a motor vehicle is provided, which is designed to carry out the previously described method.

[0023] Preferred embodiments of the invention are subsequently explained in greater detail with reference to the drawings. Identical or identically-functioning components are designated with the same reference numerals in the figures. Shown are:

[0024] FIG. 1: a schematic representation of a steer-by-wire steering system,

[0025] FIG. 2: a block diagram of a control of the steer-by-wire steering system with a module for adjusting the rack-and-pinion position,

[0026] FIG. 3: a block diagram for adjusting the rack-and-pinion position, and

[0027] FIG. 4: a block diagram of another adjustment of the rack-and-pinion position with a rack and-pinion friction model and a rack and-pinion friction estimation unit.

[0028] A steer-by-wire steering system 1 is shown in FIG. 1. A rotation angle sensor (not shown), is applied on a steering shaft 2 and detects the driver steering angle , which is applied by turning a steering input means 3, designed as a steering wheel in the example, which angle may be designated as a steering wheel rotation angle or a driver's steering request. However, a steering torque may also be detected. A joystick may function as the steering input means. Furthermore, a feedback actuator 4 is applied to steering shaft 2 and functions to simulate the feedback from roadway 70 on steering wheel 3, for example, using a reset torque 401 or a resistance torque which acts on steering wheel 3, and by this means gives feedback to the driver about the steering and driving behavior of the vehicle. The driver's steering request is transmitted, using the steering wheel angle of rotation of steering wheel 2, measured via the rotational angle sensor, via signal lines to a feedback actuator monitoring unit 10, as is illustrated in FIG. 2. The steering wheel angle of rotation may thereby comprise several rotations, for example, in a range from 720 to +720, whereby 0 represents the desire to drive straight ahead. The feedback actuator monitoring unit 10 transmits the driver's steering request, via the applied steering wheel angle of rotation , to a control unit 60. The feedback actuator monitoring unit 10 preferably also assumes the control of the feedback actuator 4. The feedback actuator monitoring unit 10 may also be designed as integral with control unit 60. The control unit 60 controls an electric steering adjuster 6, which controls the position of steered vehicle wheels 7, depending on the signal from the rotational angle sensor and other input variables with a control variable T.sub.,aus. The steering adjuster 6 acts indirectly via a steering rod-steering mechanism 8, for example, a rack-and-pinion steering mechanism, and via tie rods 9 and other components, on the steered vehicle wheels 7 and pivots the same to a steering wheel angle .

[0029] FIG. 2 shows a control of the steer-by-wire steering system. The feedback actuator 4 receives signals, among others those from the rotational angle sensor, which measures and stores the steering wheel angle of rotation , the steering wheel acceleration, and the steering wheel speed at the steering wheel 3. The feedback actuator 4 communicates with a feedback actuator monitoring unit 10, which controls the feedback actuator 4. The feedback actuator monitoring unit 10 additionally receives roadway information 13 from a control unit 60 of the steering adjuster 6 via signal lines 50, for example the roadway state or a vehicle steering angle. The control unit 60 receives driver-side steering commands 51, like the steering wheel angle of rotation , from the feedback actuator monitoring unit 10 via the signal line 50.

[0030] The control unit 60 determines, in a module 14 for adjusting the rack-and-pinion position depending on the driver's steering request and other signals, which the feedback actuator monitoring unit 10 transmits, a desired rack-and-pinion position s.sub.r,des and a desired rack-and-pinion speed v.sub.r,des of a rack-and-pinion 12, such that the desired torque or control variable T.sub.,des may be determined therefrom for the electric steering adjuster 6. Alternatively to the rack-and-pinion position, the wheel steering angle of steered wheels 7 may be used to determine the control variable. The wheel steering angle for pivoting the steered vehicle wheels 7 is specified from the control variable T.sub.,des, as well as other variables which the control unit 60 has determined.

[0031] Measured values 120 from the steering adjuster 6 and the steering mechanism 8, for example the force measured on the rack-and-pinion 12, the wheel steering angle, and roadway information 13, as well as rack-and-pinion position s.sub.r,meas, are forwarded to the control unit 60.

[0032] Two embodiments are shown in FIGS. 3 and 4 of the module 14 for adjusting the rack-and-pinion position.

[0033] As is depicted in FIG. 3, the module 14 receives desired rack-and-pinion values as input for adjusting the rack-and-pinion position s.sub.r,est. These include the desired rack-and-pinion position s.sub.r,des and the desired rack-and-pinion speed v.sub.r,des. A rack-and-pinion force estimation unit 15 also estimates the rack-and-pinion position s.sub.r,est and the rack-and-pinion speed v.sub.r,est, in addition to the rack-and-pinion force F.sub.r,est. A position error s.sub.r,err is determined from the difference between the desired values and the estimated values of the rack-and-pinion position and the rack-and-pinion speed, from which a regulator 16 initially specifies a control variable T.sub.,des, which corresponds to a desired torque for controlling rack-and-pinion 12. The estimated rack-and-pinion force F.sub.r,estrack (=control variable, non-linear part of the desired torque) is converted by means of a mechanically determined conversion factor at the engine torque level, such that an estimated rack-and-pinion torque T.sub.r,estrack is specified therefrom and added to the initially specified control variable T.sub.,des, and by this means the actual control variable T.sub.,aus to be output is specified, which is then forwarded to the steering adjuster 6 for controlling the rack-and-pinion 12. The current rack-and-pinion position s.sub.r,meas and the rack-and-pinion speed v.sub.r,meas are measured at the rack-and-pinion 12 and an estimated desired torque T.sub.,est is determined at the rack-and-pinion 12. These values s.sub.r,meas, v.sub.r,meas and T.sub.,est are supplied as input to the rack-and-pinion force estimation unit 15.

[0034] The rack-and-pinion force estimation unit 15 functions using non-linear estimation methods (EKF) while the regulator 16 (Linear Quadratic Regulator (LQR)) functions using linear methods, so that the two do not impair each other and the regulator 16 may correspondingly adjust the rack-and-pinion position.

[0035] FIG. 4 shows a module for regulating the rack-and-pinion position s.sub.r,est, corresponding to FIG. 3, which, however, has been expanded by a rack-and-pinion friction estimation unit 17 and a rack-and-pinion friction model 18 for improved accuracy.

[0036] The estimator of the rack-and-pinion friction estimation unit 17 is, like the rack-and-pinion force estimator of rack-and-pinion force estimation unit 15, a non-linear adaptive estimator and receives the measured rack-and-pinion position s.sub.r,meas, measured rack-and-pinion speed v.sub.r,meas, estimated desired torque T.sub.,est and estimated rack-and-pinion force F.sub.r,estrack as inputs, and forms from them a coefficient of static friction .sub.,rackest (Stribeck friction). The coefficient of static friction is supplied to the friction model 18 as input together with the rack-and-pinion speed v.sub.r,est estimated by the rack-and-pinion force estimator 15. The friction model compensates for friction force and specifies from this a torque T.sub.,des, which is converted from the estimated rack-and-pinion force F.sub.r,estrack (=control variable, non-linear part of the desired torque) by means of the mechanically determined conversion factor at the engine torque level into the estimated rack-and-pinion torque T.sub.r,estrack, and is added to the initially determined control variable T.sub.,des, which is how the actual control variable T.sub.,aus to be output is determined, which is then supplied to the steering adjuster. By this means, the non-linear characteristics and the unknown disturbance variables of the system are compensated. The remaining linear dynamics of the system are effectively adjusted using the linear quadratic regulator 16 (LQR). The LQR is based on a linear rack-and-pinion model, in which the mass, damping, and stiffness of the rack-and-pinion are included, and which preferably comprises the position error, speed error, and position integral error of the rack-and-pinion.

[0037] The rack-and-pinion friction model is composed of a static model comprising static and kinetic friction or a dynamic friction model (for example, Lund-Grenoble model).

[0038] The coefficient of static friction .sub.,rackest is also included as additional input into the rack-and-pinion force estimator 15, along with s.sub.r,meas, v.sub.r,meas and T.sub.,est.

[0039] The rack-and-pinion position, rack-and-pinion speed, the control variable, and the friction force are continuously estimated in the respective estimation unit using a Kalman filter. The concept of a Kalman filter relates to a method for estimating the temporal development of non-linear systems, by means of which interferences may be removed from a measurement signal. For this purpose, the filter requires a model of the system to be estimated.