Method for detecting disturbance variables in a steering system, and steering system for a motor vehicle

Abstract

A steering system for a motor vehicle, having a steering column, an electromechanical steering assistance apparatus, a sensor which is assigned to the steering column and is configured to sense a torque and a rotational angle of the steering column, and a regulator, wherein the electromechanical steering assistance apparatus comprises a motor having an angular position encoder which is configured to sense the motor rotational angle of the motor, wherein the regulator is configured to determine a torque of the electromechanical steering assistance apparatus, and wherein the regulator is also configured to determine a disturbance variable, which gives rise to an undesired steering sensation, in the steering system by means of a Kalman filter.

Claims

1. A method for detecting disturbance variables in a steering system having a steering column and an electromechanical steering assistance apparatus, comprising the following steps: sensing at least one variable of the steering system, modeling at least one part of the steering system by means of a mathematical state space model which comprises the at least one variable, and estimating at least one disturbance variable comprised of at least one of a friction torque and a friction force in the steering system by means of a Kalman filter, wherein the at least one disturbance variable gives rise to undesired steering excitations, wherein the Kalman filter uses the state space model of the steering system to estimate the at least one disturbance variable.

2. The method as claimed in claim 1, wherein the at least one estimated disturbance variable is used by a compensation unit to compensate the disturbance, corresponding to the at least one disturbance variable, by means of a compensation variable.

3. The method as claimed in claim 2, wherein the compensation variable is an additional torque which is made available by the electromechanical steering assistance apparatus in order to compensate the disturbance corresponding to the at least one disturbance variable.

4. The method as claimed in claim 2, wherein the compensation unit comprises frequency-dependent filters and/or characteristic diagrams which can be adjusted in order to generate a desired steering sensation.

5. The method as claimed in claim 4, wherein at least one measurement variable of the motor vehicle is sensed and used to determine the compensation variable.

6. The method as claimed in claim 4, wherein at least one measurement variable of the motor vehicle is sensed and used to determine the compensation variable.

7. The method as claimed in claim 5, wherein the at least one measurement variable of the motor vehicle is used to adjust the frequency-dependent filters and/or characteristic diagrams.

8. The method as claimed in claim 5, wherein the at least one measurement variable of the motor vehicle is a vehicle speed and/or steering variable.

9. The method as claimed in claim 8, wherein a lower part, comprising at least the electromechanical steering assistance apparatus, of the steering system is modeled by means of the mathematical state space model.

10. The method as claimed in claim 8, wherein the lower part of the steering system comprises all the components of the steering system which are provided underneath a sensor which is assigned to the steering column.

11. The method as claimed in claim 9, wherein the at least one variable comprises a rotational angle of the steering system, a motor rotational angle of the electromechanical steering assistance apparatus, a torque of the electromechanical steering assistance apparatus and/or a torque of the steering column.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Further advantages and properties of the invention can be found in the following description and the drawings to which reference is made. In the drawings:

(2) FIG. 1 shows a perspective view of an inventive steering system of a motor vehicle,

(3) FIG. 2 shows a physical equivalent model of a lower part of the steering system from FIG. 1,

(4) FIG. 3 shows a simplified physical equivalent model of the lower part of the steering system from FIG. 1, and

(5) FIG. 4 shows a schematic illustration of the inventive steering system with which the method according to the invention is clarified.

DETAILED DESCRIPTION

(6) FIG. 1 shows a steering system 10 of a motor vehicle which is embodied as an electromechanically assisted steering system.

(7) The steering system 10 comprises an upper part 12 which comprises a steering wheel 14, a first steering column 16 and at least one part of a second steering column 18, as well as a lower part 20 which comprises at least one part of the second steering column 18, a measuring device 22, a steering rack 23 and an electromechanical steering assistance apparatus 24.

(8) If the driver applies a torque to the steering wheel 14, the first steering column 16 and the second steering column 18 are as a result rotated, which is correspondingly detected by the measuring device 22 which is assigned to the second steering column 18. The measuring device 22 has for this purpose a so-called torque and angle sensor 26, which is also referred to as a torque angle sensor (TAS). The measuring device 22 is integrated, for example, in the second steering column 18, so that the second steering column 18 comprises an input section and a corresponding output section, in particular wherein the second steering column 18 is embodied in two parts.

(9) The torque and angle sensor 26 comprises, for example, a torsion rod by means of which the angle and the torque which occurs at the steering column 16, 18 can be correspondingly sensed. The sensing of the angle can take place on a side of the torsion rod directed toward the steering wheel 14 or on a side of the torsion rod directed toward the steering rack 23.

(10) The electromechanical steering assistance apparatus 24 comprises a motor 28 and an angular position encoder 30 by means of which the motor rotational angle of the motor 28 can be sensed. The motor 28 is, in particular, an electric motor.

(11) Moreover, the steering system 10 comprises a regulator 32 which is coupled to the measuring device 22 and to the electromechanical steering assistance apparatus 24.

(12) The regulator 32 accordingly receives, inter alia, the rotational angle of the steering column 16, 18 via the sensor 26, the torque of the steering column 16, 18 via the sensor 26, the motor rotational angle of the motor 28 via the angular position encoder 30 and the torque of the electromechanical steering assistance apparatus 24 via the corresponding power consumption of the motor 28. The torque of the steering column 16, 18 can be a torsion rod torque.

(13) In addition, the regulator 32 is configured to apply a mathematical state space model of the observer 34, which model is designed, in particular, for the lower part 20 of the steering system 10.

(14) For the design of the observer 34 (Kalman filter), the lower part 20 of the steering system 10 is modeled in accordance with the simplified physical equivalent model which is shown in FIG. 3 and is based on the physical equivalent model according to FIG. 2, said model being, for example, of a steering system 10 with a ball screw drive. However, other types of steering systems can be provided in an analogous fashion.

(15) In this equivalent model, the electromechanical steering assistance apparatus 24, in particular its components, and the mechanical components of the steering system 10, in particular the second steering column 18 and the steering rack 23, are modeled by means of corresponding masses, moments of inertia, springs with a spring constant, material damping with damping coefficients and the viscous friction.

(16) Specifically, the electromechanical steering assistance apparatus 24 is modeled by means of the moments of inertia of the motor (J.sub.motor), of the pulley (J.sub.pulley), of the recirculating ball nut (J.sub.ballnut) and of the ball bearing (J.sub.bearing), wherein in addition viscous friction (b.sub.motor) is taken into account for the bearing friction of the motor 28. In addition, the transmission ratio of the belt drive (i.sub.belt) and the transmission ratio of the ball screw drive (i.sub.ballnut) are taken into account.

(17) With respect to the steering system 10, the mass of the steering rack 23 (m.sub.rack) and the elastic connection of the electromechanical steering assistance apparatus 24 to the steering rack 23 are taken into account as a spring (c.sub.gear) with the material damping (b.sub.gear) in the state space model. Likewise, the moment of inertia of the lower part 20 of the steering system 10, that is to say that of the second steering column 18 with the pinion (J.sub.pinion) and the corresponding transmission ratio (i.sub.pinion) are input into the state space model.

(18) In addition, it is apparent from FIG. 2 that with respect to the electromechanical steering assistance apparatus 24 the torque of the steering column 16, 18 (T.sub.column), the applied torque (T.sub.assist), the electromechanical steering assistance apparatus 24 and the friction torque (T.sub.friction,motor) which occurs as a disturbance variable are included as further input variables or disturbance variables within the electromechanical steering assistance apparatus 24.

(19) Furthermore, the force which originates from the road and the force (F.sub.road) which acts on the steering rack 23 and the friction force at the steering rack 23 (F.sub.friction,rack) are included as disturbance variables in the state space model.

(20) The rotational angle (φ.sub.motor) and the rotational angular speed (Ω.sub.motor) of the motor 28 of the electromechanical steering assistance apparatus 24 as well as the travel (s.sub.rack) and the speed (v.sub.rack) of the steering rack 23 are included as state variables in the state space model.

(21) The physical equivalent model which is shown in FIG. 2 is illustrated for the design of the observer 34 as in FIG. 3, but in a simplified form.

(22) Correspondingly, the various moments of inertia of the electromechanical steering assistance apparatus 24 are combined to form a total moment of inertia (J.sub.drive) of the drive side. In addition, the total friction torque (T.sub.friction,drive) of the drive side is used. The mass of the steering rack (m.sub.rack) and the moment of inertia (J.sub.pinion) of the steering column are combined to form a total mas (m.sub.downstream) on the output side.

(23) On the basis of the simplified physical equivalent model of the steering system 10, in particular the lower part 20 of the steering system 10, illustrated in FIG. 3, equations can be derived which form the mathematical state space model of the lower part 20 of the steering system 10. This mathematical state space model is explained in more detail below.

(24) A state of the steering system 10 is modeled with the state space model, wherein the state is generally understood to be a minimum set of variables {right arrow over (x)} which is required to describe the corresponding system, that is to say the steering system 10. Here, the state of the steering system 10 is correspondingly considered. This results in the following for the state variables:

(25) $\stackrel{.fwdarw.}{x}=\left(\begin{array}{c}{\phi }_{\mathrm{drive}}\\ {\Omega }_{\mathrm{drive}}\\ s_{\mathrm{downstream}}\\ v_{\mathrm{downstream}}\end{array}\right)$

(26) A time dependence of the corresponding state is not presented explicitly below but rather tacitly assumed. The time evolution of the state of the steering system 10 is given by the following equation:
{right arrow over ({dot over (x)})}=A{right arrow over (x)}+B{right arrow over (u)}=A{right arrow over (x)}+B[{right arrow over (u)}.sub.control{right arrow over (u)}.sub.dist]  (equation 1)

(27) This equation is a differential equation or a difference equation depending on whether a continuous time evolution or a discrete time evolution is considered. In this context, {right arrow over (u)}.sub.control=[T.sub.assist T.sub.sensor].sup.T, that is to say the torque applied to the electromechanical steering assistance apparatus 24 and the torque sensed by the measuring device 20 at the second steering column 18, which torque differs from the torque of the steering column 16, 18 (T.sub.column) by the missing portion of the material damping.

(28) In addition, {right arrow over (u)}.sub.dist=[T.sub.friction,drive F.sub.rack].sup.T and comprises the drive-side dry frictional torque as T.sub.friction,drive and the sum of the force resulting from the roadway excitations and the output-side dry frictional force as F.sub.rack=F.sub.road+F.sub.friction,rack.

(29) Accordingly, {right arrow over (u)}.sub.dist represents the unknown disruptions of the state which occur in the lower part 20 of the steering system 10.

(30) The matrices A and B describe the evolution of the state {right arrow over (x)} and are dependent on the variables of the mathematical state space model.

(31) Furthermore, it becomes apparent from the mathematical state space model that the measured motor rotational angle φ.sub.motor and the rotational angle φ.sub.pinion of the second steering column 18 can also be described by means of the corresponding state of the steering system 10 as follows:

(32) $\begin{array}{cc}{\stackrel{.fwdarw.}{y}}_{meas}=C\stackrel{.fwdarw.}{x}=\left(\begin{array}{c}{\phi }_{motor}\\ {\phi }_{pinion}\end{array}\right)& \left(\mathrm{equation}2\right)\end{array}$

(33) In this context, the matrix C describes the relationship between the current state {right arrow over (x)} of the steering system 10 and the measured motor rotational angle φ.sub.motor as well as the rotational angle φ.sub.pinion of the second steering column 18.

(34) Together with the above equation (1) for the time evolution of the state of the steering system 10 the equation (2) forms a linear state space model for the state of the steering system 10.

(35) It is not possible to infer the state variables of the steering system 10 and the corresponding disturbance variables {right arrow over (u)}.sub.dist directly from the determination of the corresponding variables, that is to say of the motor rotational angle of the electromechanical steering assistance apparatus 24, the rotational angle of the second steering column 18, from the torque of the electromechanical steering assistance apparatus 24 or the torque of the second steering column 18.

(36) Instead, the state variables of the steering system 10 and the disturbance variables {right arrow over (u)}.sub.dist have to be estimated.

(37) For this purpose, a Kalman filter is used. Said Kalman filter estimates the state variables of the steering system 10 and the unknown disturbance variables of the mathematical state space model on the basis of the determined variables of the steering system 10 and the selected mathematical state space model.

(38) To be more precise, the Kalman filter estimates the disturbances T.sub.friction,drive acting on the lower part 20 of the steering system 10 as well as F.sub.rack, wherein, as already mentioned, T.sub.friction,drive describes the drive-side dry frictional torque and F.sub.rack=F.sub.road+F.sub.friction,rack describes the sum of the force resulting from roadway excitations and the output-side dry frictional force.

(39) Therefore, all the additionally required variables which are necessary to determine the disturbance variables are estimated by means of the Kalman filter. To be more precise, all of the variables which are not measured and which are required to calculate the disturbance variables of the steering system 10 and all of the non-measurable variables which are required for this purpose are estimated.

(40) In other words, the method described above is based on the observer 34 in the sense of the regulation technology, in which method the steering system 10 is modeled by a mathematical state space model. This mathematical state space model serves as a basis for the design of the observer 34 which is assigned to the lower part 20 of the steering system 10.

(41) Unknown state and disturbance variables are estimated, as described above, from known input and measurement variables {right arrow over (u)}.sub.control and {right arrow over (y)}.sub.meas by the observer 34 (“lower observer”).

(42) The disturbance variables {right arrow over (y)}.sub.dist,obs are calculated therefrom as output variables which correspond to all the disturbances which occur.

(43) These estimated disturbance variables {right arrow over (y)}.sub.dist,obs are subsequently used by a compensation unit 36 to determine a compensation variable T.sub.dist,reject which compensates the disturbances which are assigned to the disturbance variables {right arrow over (y)}.sub.dist,obs.

(44) Here, in this context at least one measurement variable of the motor vehicle can be fed to the compensation unit 36 in addition to the disturbance variables {right arrow over (y)}.sub.dist,obs determined by means of the observer 34, said measurement variable being used to calculate the compensation variable T.sub.dist,reject. The measurement variable of the motor vehicle can be a vehicle movement dynamics measurement variable (for example of the vehicle speed) {right arrow over (y)}.sub.vehicle and/or a steering variable {right arrow over (y)}.sub.steering, for example an applied steering torque, a steering angle, a steering angle speed and/or a steering angle acceleration.

(45) The compensation unit 36 can comprise frequency-dependent filters and/or characteristic diagrams which are adjustable. The filters or characteristic diagrams can be dependent here on corresponding measurement variables of the motor vehicle {right arrow over (y)}.sub.vehicle, {right arrow over (y)}.sub.steering, so that the adjustable filters and/or characteristic diagrams are correspondingly adjusted by means of the measurement variables of the motor vehicle in order to determine the compensation variable T.sub.dist,reject in accordance with the measurement variables of the motor vehicle.

(46) An additional torque, which constitutes the compensation variable T.sub.dist,reject, is calculated by means of the compensation unit 36 in order to compensate the disturbances corresponding to the estimated disturbance variables {right arrow over (y)}.sub.dist,obs, that is to say the torque and the force at the steering rack 23 which generate the undesired steering sensation. The additional torque T.sub.dist,reject is applied here by the electromechanical steering assistance apparatus 24, so that the undesired steering excitations which occur from the road and/or for friction-based disruptions of the lower part 20 of the steering system 10 are correspondingly compensated, as a result of which the driver of the motor vehicle does not perceive said disruptions and has an improved steering sensation.

(47) With the method according to the invention and the steering system 10 it is possible easily to determine and compensate disturbance variables which occur, without using numerous sensors for this purpose.