Method for controlling a steering system having electric steering assistance

Abstract

The disclosure relates to a method and to a device for controlling a steering system and to a steering system having electric steering assistance, wherein a reference variable for the steering assistance is predefined by a steering controller, the steering system is controlled as a function of the reference variable, a compensation value to compensate for a dynamic behavior of an axle steered by the steering system is determined on the basis of a model, the reference variable is determined as a function of the compensation value. The disclosure further relates to a method and to a device for emulating dynamics of the steered axle.

Claims

1. A method for controlling a steering system having electric steering assistance, the method comprising: specifying, with a steering controller, a setpoint variable for the electric steering assistance; controlling the steering system depending on the setpoint variable; determining a compensation value configured to compensate for a dynamic behavior of an axle steered by the steering system based on a model; and determining the setpoint variable depending on the compensation value, wherein the compensation value is determined according to the equation:
{tilde over (G)}=1−cLR, where {tilde over (G)} is the compensation value, c ∈, L is a transmission behavior from a motor torque of an electric drive of the electric steering assistance to the torque on a torsion bar that connects a steering wheel of the steering system to the electric drive, and R is a transmission behavior from a rack travel to a rack force that acts on a rack of the steering system.

2. The method as claimed in claim 1, wherein the compensation value characterizes the rack force that acts on the rack of the steering system.

3. The method as claimed in claim 2, further comprising: determining the compensation value depending on information about the rack travel that indicates a deflection of the rack relative to a reference position of the rack.

4. The method as claimed in claim 3, further comprising: determining the information about the rack travel depending on at least one measured value, wherein at least one of: the at least one measured value characterizes information about a rotor position of an electric drive of the electric steering assistance relative to a reference position; the at least one measured value characterizes information about a torque on the torsion bar, the torsion bar; and the at least one measured value characterizes one of (i) information about an angular position of a steering wheel and (ii) information about an angular position of a shaft driving the rack relative to a reference angle position.

5. The method as claimed in claim 1, further comprising: determining a setpoint motor torque; and determining a compensated setpoint motor torque as the setpoint variable depending on the compensation value and the setpoint motor torque.

6. A steering system comprising: an electric steering assistance; and a steering controller configured to (i) specify a setpoint variable for the electric steering assistance, (ii) control the steering system depending on the setpoint variable, (iii) determine a compensation value configured to compensate for a dynamic behavior of an axle steered by the steering system based on a model, and (iv) determine the setpoint variable depending on the compensation value, wherein the compensation value is determined according to the equation:
{tilde over (G)}=1−cLR, where {tilde over (G)} is the compensation value, c ∈, L is a transmission behavior from a motor torque of an electric drive of the electric steering assistance to the torque on a torsion bar that connects a steering wheel of the steering system to the electric drive, and R is a transmission behavior from a rack travel to a rack force that acts on a rack of the steering system.

7. The steering system as claimed in claim 6 further comprising: the rack, the compensation value characterizes the rack force that acts on the rack.

8. The steering system as claimed in claim 7, the steering controller being further configured to: determine the compensation value depending on the information about the rack travel that indicates a deflection of the rack relative to a reference position of the rack.

9. The steering system as claimed in claim 8, the steering controller being further configured to: determine the information about the rack travel depending on at least one measured value, wherein the steering system comprises at least one of: an angular position encoder configured detect the at least one measured value as information about a rotor position of an electric drive of the electric steering assistance relative to a reference position; a torque encoder configured to detect the at least one measured value as information about a torque on the torsion bar; and an angular position encoder configured to detect the at least one measured value as one of (i) information about an angular position of the steering wheel relative to a reference angle position and (ii) information about an angular position of a shaft driving the rack relative to a reference angle position.

10. The steering system as claimed in claim 6, the steering controller being further configured to: determine a setpoint motor torque; and determine a compensated setpoint motor torque as the setpoint variable depending on the compensation value and the setpoint motor torque.

11. A method for emulation of dynamics of an axle of a motor vehicle steered by a steering system, the steering system having a rack, the method comprising: determining a compensation value configured to compensate for a dynamic behavior of the axle based on a model, the compensation value being determined depending on information about a rack travel that indicates a deflection of the rack relative to a reference position of the rack, determining the information about the rack travel depending on at least one measured value, the at least one measured value characterizing at least one of (i) information about a rotor position of an electric drive of an electric steering assistance of the steering system relative to a reference position (ii) information about an angular position of a steering wheel and (iii) information about an angular position of a shaft driving the rack relative to a reference angle position, wherein the compensation value is determined according to the equation:
{tilde over (G)}=1−cLR, where {tilde over (G)} is the compensation value, c ∈, L is a transmission behavior from a motor torque of an electric drive of the electric steering assistance to the torque on a torsion bar that connects a steering wheel of the steering system to the electric drive, and R is a transmission behavior from a rack travel to a rack force that acts on a rack of the steering system.

12. The method as claimed in claim 11, wherein the method is performed on a device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other advantageous embodiments result from the following description and the drawing. In the figures,

(2) FIG. 1 shows a steering system schematically,

(3) FIG. 2 shows angular positions schematically,

(4) FIG. 3 shows parts of a control loop schematically.

DETAILED DESCRIPTION

(5) FIG. 1 shows schematically a steering system 100 having electric steering assistance 102.

(6) A steering controller 104 is designed to specify a setpoint variable {tilde over (T)}.sub.Mot for the steering assistance 102. The steering system 100 is controlled depending on the setpoint variable {tilde over (T)}.sub.Mot. The steering system 100 comprises an axle 106, which is movable by a rack 108. In the example, the setpoint variable is a setpoint torque for the steering assistance 102. Due to the front axle dynamics there is a rack force F.sub.Ra that acts on the rack 108. The rack 108 is moved through a rack travel s.sub.Ra, which indicates a deflection of the rack 108 of the steering system 100 relative to a reference position 110 of the rack 108. In the example, a positive rack force F.sub.Ra acts in the direction of the arrow in FIG. 1, in the direction of a deflection of the rack 108 with a positive rack travel s.sub.Ra. In the example a negative rack force F.sub.Ra acts against the direction of the arrow, in the direction of a deflection of the rack 108 with a negative rack travel s.sub.Ra. This definition is by way of example and can be chosen differently.

(7) The steering controller 104 is designed to determine information about the rack travel s.sub.Ra depending on at least one measured value. The steering system 100 comprises, for example, an angular position encoder 118, which is designed to record the measured value as information about a rotor position 202 of an electric drive 112 of the steering assistance 102 relative to a reference position 204.

(8) The steering system 100 in the example comprises a torque encoder 116, which is designed to detect information about a torque T.sub.Tb on a torsion bar 114, wherein the torsion bar 114 connects a steering wheel 116 of the steering system 100 to the electric drive 112 of the electric steering system 100.

(9) The steering system 100 may also additionally or alternatively include an angular position encoder 122, which is designed to capture information about an angular position ω.sub.L of the steering wheel 116 relative to a reference angle position ω.sub.Ref. It may also be provided that the angular position encoder 118 is embodied to detect information about an angular position ω.sub.Ra of a shaft 124 that drives the rack relative to a reference angle position ω.sub.Ref.

(10) FIG. 2 shows schematically angular positions that can occur during movements in the steering system 100.

(11) The steering controller 104 is designed to determine a compensation value for compensation of the dynamic behavior of an axle 106 steered by the steering system 100. The setpoint variable {tilde over (T)}.sub.Mot is determined depending on the compensation value.

(12) In a first example the steering controller 104 is designed to determine the compensation value depending on information about the rack travel s.sub.Ra.

(13) More precisely, the steering controller 104 is designed to determine a setpoint motor torque T.sub.Mot, wherein a compensated setpoint motor torque is determined as the setpoint variable {tilde over (T)}.sub.Mot depending on the compensation value and the setpoint motor torque T.sub.Mot.

(14) The front axle dynamics are preferably measured for all vehicles in the road test, for example. In addition, models of the front axle dynamics, which are available, for example, by means of a design tool in an early acquisition phase, can be used.

(15) Thus, in the first example a linear model of the front axle dynamics is generated.

(16) The input variable of the linear model of the front axle dynamics that is used is the rack travel s.sub.Ra, which can be determined from sensor data that is already available in conventional steering systems, for example, the rotor position 202, the torque T.sub.Tb on the torsion bar 114 or other electronic power steering sensor data, by sensor fusion, for example, by means of a Kalman filter. As the output, for example, the rack force F.sub.Ra is calculated as a disturbance force. The setpoint variable is determined from this to generate a motor torque that counteracts the disturbance force.

(17) In addition to compensating the front axle dynamics, the calculated variable, here referred to as the compensation value, can be used to emulate a certain behavior of the front axle. For example, a use case would be steer-by-wire systems.

(18) For this purpose, for example, the in-vehicle front axle dynamics can be compensated and another axle dynamics can be superimposed.

(19) As an alternative, instead of the measured front axle dynamics, a spring mass damper oscillator can be used and a virtual axle can be parameterized depending on the tires and the axle design. This is useful when there is no axle for parameterizing the control system.

(20) In a second example, simplifications of the model are made. As a result, determining the rack travel s.sub.Ra can be dispensed with.

(21) In addition to already applied controller methodology and parameterization for the steering assistance, a component is added that works against the front axle dynamics and compensates the influence thereof.

(22) Such a steering system 100 with front axle dynamics as a disturbance can be represented as follows:

(23) $\left[\begin{array}{c}{T}_{\mathrm{Tb}}\\ s_{\mathrm{Ra}}\end{array}\right]=G\left[\begin{array}{c}{T}_{\mathrm{Mot}}\\ F_{\mathrm{Ra}}\end{array}\right]=\left[\begin{array}{cc}K& L\\ M& N\end{array}\right]\left[\begin{array}{c}{T}_{\mathrm{Mot}}\\ F_{\mathrm{Ra}}\end{array}\right]F_{\mathrm{Ra}}={\mathrm{Rs}}_{\mathrm{Ra}}$
with G Transmission behavior of the steering system 100, K Transmission behavior K from the setpoint motor torque T.sub.Mot to the torque T.sub.Tb on the torsion bar 114, L Transmission behavior from the motor torque T.sub.Mot of the electric drive of the steering assistance to the torque T.sub.Tb on the torsion bar, M Transmission behavior from the motor torque T.sub.Mot to the rack travel s.sub.Ra, N Transmission behavior from the rack force F.sub.Ra to the rack travel s.sub.Ra, R Transmission behavior from the rack travel s.sub.Ra to the rack force F.sub.Ra.

(24) R represents the disturbance caused by the front axle dynamics. By using the simplification s.sub.Ra=cT.sub.Tb with c∈ and assuming a linear torsion bar stiffness and a constant transmission ratio of a steering gear in the steering system, this MIMO system can be transferred to a SISO system and can be represented as follows:

(25) $T_{\mathrm{Tb}}=\frac{K}{1-cLR}{T}_{\mathrm{Mot}}=KG{T}_{\mathrm{Mot}}$

(26) In order to eliminate the influence of the front axle dynamics, the pilot control is determined as:
{tilde over (G)}=1−cLR.

(27) The control circuit, which is partially represented in FIG. 3, contains a conventional control for determining a setpoint assistance torque as the setpoint motor torque T.sub.Mot. This is first changed to the setpoint variable {tilde over (T)}.sub.Mot by multiplying with the compensation value {tilde over (G)}. In FIG. 3, the transmission behavior K from the setpoint motor torque T.sub.Mot to the torque T.sub.Tb on the torsion bar 114 is also shown. In addition, further transmission behavior of the steering system in FIG. 3 is taken into account by additive feedback of the terms cLR
to the setpoint variable {tilde over (T)}.sub.Mot.

(28) The steering controller 104 is designed in this example to determine the compensation value {tilde over (G)} as
{tilde over (G)}=1−cLR.

(29) Preferably, functions L and R are used, wherein the zero or pole positions of the transmission function are not reduced by their multiplication. It will not be assumed that the front axle dynamics are completely compensated. Model deviations due to different rim sizes and loading levels can be compensated as before by the steering controller. Since these front axle dynamics have a similar profile, an improvement of the control behavior is made possible even with different tires and ground.