WO2024100301A1

WO2024100301A1 - Mechanically decoupled steering system for a vehicle, and method for operating a mechanically decoupled steering system

Info

Publication number: WO2024100301A1
Application number: PCT/EP2023/081539
Authority: WO
Inventors: Nando Laumanns; Niklas KUNZ
Original assignee: Volkswagen Aktiengesellschaft
Priority date: 2022-11-11
Filing date: 2023-11-10
Publication date: 2024-05-16
Also published as: DE102022211967A1

Abstract

The invention relates to a mechanically decoupled steering system (1) for a vehicle, comprising a steering handle (4), a steering gear (7) and a controller (8), wherein at least one sensor (5) for detecting a steering wheel angle (6) and/or a manual torque on the steering handle (4) is arranged on the steering handle (4), wherein the steering gear (7) has at least one electric machine (9) which is coupled to a steering rack (10), wherein the controller (8) is designed to determine a target steering angle (11) depending on an actual steering wheel angle (6) and/or actual manual torque detected by means of the at least one sensor (5) and to control the steering gear (7), and wherein the controller (8) is also designed to identify an understeer situation based on at least one understeer indicator (30), to set an actual steering angle (12), which is present during identification, as the reference steering angle (13) and to adjust a target steering angle (11), which is requested by changing the actual steering wheel angle (6) and lies above the reference steering angle (13), in accordance with a specified characteristic curve (14). The invention also relates to a method for operating a mechanically decoupled steering system (1) for a vehicle.

Description

Mechanically decoupled steering system for a vehicle and method for operating a mechanically decoupled steering system

The invention relates to a mechanically decoupled steering system for a vehicle and a method for operating a mechanically decoupled steering system for a vehicle.

Understeer is a term from driving dynamics that is used to describe the self-steering behavior. It describes the fact that when cornering, the steering wheel angle becomes larger the faster the radius is driven. In modern electrical stability programs, the actual movement quantities are compared with the driver's request derived from the steering wheel angle and the driving speed using the internal single-track model. If the driver wants to "corner more" than actually measured, understeer is detected. If the deviation is too great, the rear wheel on the inside of the curve is automatically braked and the engine power is reduced. Another clear explanation for understeer is that there is a point at which the available lateral force decreases despite the increasing steering angle.

In mechanically decoupled steering systems, a movement of a steering handle can be decoupled from a movement of the wheels. Mechanically decoupled steer-by-wire steering systems are characterized by the fact that there is no longer a mechanical connection or coupling between a steering handle and a steering gear. Typically, at least one sensor for detecting an angle of rotation or torque is arranged on the steering handle, the sensor signals of which are transmitted to a control unit, which then implements the steering request using the steering gear. For this purpose, the steering gear is driven by at least one electric servo motor. For redundancy reasons, at least two servo motors with their own power electronics are usually used. The servo motors operate on a common rotor that is connected to the steering gear, which in turn is connected to a rack. It is also known to use a servo motor with two half machines, which are preferably designed symmetrically, i.e. each provide 50% of the total power. It is also known to provide an actuator on the steering handle that generates a defined counter torque on the steering handle in order to provide the driver with a better haptic feeling. This actuator is also known as a force feedback actuator. In mechanically decoupled superimposed steering systems, an angle is superimposed on the steering wheel angle. The angle of rotation at the input of the steering gear then no longer corresponds to the angle of the steering wheel. On the one hand, this enables a continuous adjustment of the steering ratio. On the other hand, the superimposition also allows rapid driving dynamics interventions by changing the steering angle on the wheel while driving while the steering wheel angle remains the same.

The invention is based on the object of creating a mechanically decoupled steering system for a vehicle, which in particular has improved stabilization behavior, and of providing a corresponding method for operating a mechanically decoupled steering system.

The object is achieved according to the invention by a mechanically decoupled steering system with the features of patent claim 1 and a method with the features of patent claim 10. Advantageous embodiments of the invention emerge from the subclaims.

In particular, a mechanically decoupled steering system for a vehicle is created, comprising a steering handle, a steering gear and a control unit, wherein the steering handle has at least one sensor for detecting a steering wheel angle and/or a manual torque on the steering handle, wherein the steering gear has at least one electric machine which is coupled to a rack, wherein the control unit is set up to determine a target steering angle as a function of an actual steering wheel angle and/or actual manual torque detected by means of the at least one sensor and to control the steering gear, and wherein the control unit is further set up to detect an understeering case based on at least one understeering indicator, to set an actual steering angle present upon detection as the reference steering angle and to adapt a target steering angle requested by changing the actual steering wheel angle and lying above the reference steering angle according to a predetermined characteristic curve, in particular a characteristic curve dependent on the reference steering angle.

Furthermore, in particular, a method for operating a mechanically decoupled steering system for a vehicle is provided, wherein the steering system comprises a steering handle, a steering gear, and a control unit, wherein the steering handle has at least one sensor for detecting a steering wheel angle and/or a hand torque on the steering handle, wherein the steering gear has at least one electric machine which is coupled to a rack, wherein a target steering angle is determined as a function of an actual steering wheel angle and/or actual manual torque detected by means of the at least one sensor and the steering gear is controlled by means of the control unit, and wherein an understeering case is detected based on at least one understeering indicator, wherein an actual steering angle present upon detection is set as the reference steering angle and a target steering angle requested by changing the actual steering wheel angle and lying above the reference steering angle is adjusted according to a predetermined characteristic curve, in particular a characteristic curve dependent on the reference steering angle.

The steering system and the method make it possible to intervene effectively in the driving dynamics in the event of understeering in order to ensure that the vehicle is stable when cornering. A basic idea here is that if the driver turns into understeering, this is corrected above the reference steering angle according to the characteristic curve and this results in a steering wheel angle-dependent reduction in the target steering angle transmitted to the steering gear. In this way, understeering can be limited, and this is pleasant for the driver in terms of driving comfort, as in particular no braking interventions and/or speed reductions are necessary.

The control unit receives the understeer indicator from an electrical stability program or another control unit, for example from a vehicle control system. Alternatively, the steering system itself can have suitable sensors that provide suitable sensor data to determine the understeer indicator and detect the understeering case.

The characteristic curve in particular represents a relationship between a steering wheel angle on the steering handle (in particular on a steering wheel) and a steering angle on at least one steerable wheel. If such a characteristic curve is linear, for example, every change in the steering wheel angle is converted in direct proportion to a change in the steering angle. In this example, such a relationship can be set in relation to the strength of the conversion of the steering wheel angle into the steering angle by changing the linear slope of the characteristic curve. In particular, the characteristic curve is not linear throughout, but can have a changing slope at least in sections. The characteristic curve can, for example, be described at least in sections using a polynomial function. A characteristic curve profile and/or characteristic curve parameters are determined in particular in empirical test series and/or by simulation. Parts of the steering system, in particular the control unit, can be designed individually or in combination as a combination of hardware and software, for example as program code that is executed on a microcontroller or microprocessor. However, it can also be provided that parts are designed individually or in combination as an application-specific integrated circuit (ASIC) and/or field-programmable gate array (FPGA).

In one embodiment, it is provided that the characteristic curve has at least one transition region and one limit region, the transition region being between the reference steering angle and the limit region. This allows different regions of the characteristic curve to be defined, whereby when oversteering is detected, a steering behavior is transferred from a normal region via the transition region to the limit region. In particular, this can prevent an abrupt change in the steering behavior of the steering system and instead provide a transition that the driver perceives as gentle or flowing. In particular, it can be provided that a relationship between a steering wheel angle and a steering angle is linear in a normal region. In the transition region, i.e. starting with the reference steering angle in the case of oversteering, the relationship is then transferred across the transition region into a linear relationship with a smaller gradient, so that in the limit region a change in the steering wheel angle leads to a smaller change in the steering angle than in the normal region. In the transition region in particular, the characteristic curve can be designed as a polynomial function and/or described as such. In principle, however, other characteristic curve profiles are also suitable.

In a further embodiment, it is provided that the characteristic curve can be parameterized at least for the transition area. This allows individual preferences to be taken into account. Furthermore, this also enables the use of a situation-dependent characteristic curve. For example, this can be used to implement the preference to fade from 0% to 90% adjustment or compensation within a range of 2° of the wheel steering angle when converting the (actual) steering wheel angle into the (target) steering angle.

In one embodiment, it is provided that the characteristic curve is or will be selected or parameterized taking into account a speed of the vehicle and/or a coefficient of friction and/or a value of the at least one understeer indicator. This enables the characteristic curve to be adapted to a specific driving situation of the vehicle. In one embodiment, it is provided that a threshold value from which understeering is detected is or will be defined taking into account a speed of the vehicle and/or a coefficient of friction. This enables the range from which understeering is detected to be defined depending on a current driving situation of the vehicle. For example, different weather conditions can be taken into account (e.g. dry road surface, wet road surface, snow, ice, etc.).

In one embodiment, it is provided that the at least one understeer indicator is a continuous indicator that is proportional to the strength of the understeer. This enables rapid detection of the understeer. Furthermore, the understeer indicator can be used as a continuous variable describing the current driving dynamics situation of the vehicle, based on which an intervention in the driving dynamics can be made with sufficient resolution at any time and in any situation. In particular, the characteristic curve can be selected and/or parameterized at any time using the at least one continuous understeer indicator.

In one embodiment, it is provided that the at least one understeer indicator is or will be determined based on a comparison between a target yaw rate and an actual yaw rate of the vehicle. For example, the at least one understeer indicator can be or include a difference value determined from the target yaw rate and the actual yaw rate. If the difference value exceeds a predetermined threshold value, understeering is detected. The characteristic curve can also be selected and/or parameterized taking such a difference value into account.

In one embodiment, it is provided that the at least one understeer indicator is or will be determined based on a lateral force acting on the front wheels of the vehicle. For example, a lateral force sensor detects the lateral force based on the set actual steering wheel angle. If the lateral force does not increase even though the (actual) steering angle on the wheels is increased, then an understeer case is present.

In one embodiment, it is provided that the at least one understeer indicator is or will be determined based on a lateral acceleration. For example, an acceleration sensor detects the lateral acceleration based on the set actual steering wheel angle. If the lateral acceleration does not increase even though the (actual) steering angle at the wheels is increased, then understeering is present.

In one embodiment, it is provided that the steering system is a rear-wheel steering system or additionally comprises a rear-wheel steering system which is configured in the same way. In particular, the rear-wheel steering can additionally or alternatively be designed according to the steering system described in this disclosure.

Further features for the design of the method emerge from the description of the designs of the steering system. The advantages of the method are the same as for the designs of the steering system.

The invention is explained in more detail below using preferred embodiments with reference to the figures.

Fig. 1 is a schematic representation of an embodiment of the mechanically decoupled steering system for a vehicle;

Fig. 2 is a schematic representation of a characteristic curve to illustrate an embodiment of the steering system and the method.

Fig. 1 shows a schematic representation of an embodiment of the mechanically decoupled steering system 1 for a vehicle. In the example, the steering system 1 is a steer-by-wire steering system. In principle, however, the steering system 1 can also be a superposition steering system.

The steering system 1 comprises a steering wheel module 2 and a steering module 3. A steering handle 4 (steering wheel) is connected to the steering wheel module 2 and the steering wheel module 2 has a sensor 5 for detecting a steering wheel angle 6 and/or a hand torque on the steering handle 4. The detected steering wheel angle 6 is fed to the steering module 3. The steering module 3 has a steering gear 7 and a control unit 8. The steering gear 7 has at least one electric machine 9 (eg at least one servo motor) which is coupled to a rack 10. The rack 10 is coupled to at least one steerable wheel (not shown). The control unit 8 is designed to determine a target steering angle 11 as a function of an actual steering wheel angle 6 and/or actual manual torque detected by means of the sensor 5 and to control (or regulate) the steering gear 7 in accordance with the target steering angle 11.

The control unit 8 is further configured to detect an understeering case based on at least one understeering indicator 30, to set an actual steering angle 12 present upon detection as the reference steering angle 13 and to adapt a target steering angle 11 requested by changing the actual steering wheel angle 6 and lying above the reference steering angle 13 according to a predetermined characteristic curve 14, in particular one dependent on the reference steering angle. The understeering indicator 30 is provided to the control unit 8, for example, by a vehicle control system.

The steering wheel module 2 further comprises in particular an actuator 15, wherein the control unit 8 is further configured to determine a feedback torque 16 on the steering handle 4 based on the actual steering angle 12 and thereby control the actuator 15.

Fig. 2 shows a schematic representation of an example characteristic curve 14. The (actual) steering wheel angle 6 is shown on the x-axis and the (target) steering angle 11 is shown on the y-axis, each in a linear scale. It is particularly provided that the control unit 8 determines the (target) steering angle 11 based on the (actual) steering wheel angle 6 using the characteristic curve 14. The characteristic curve 14 has three areas in particular: a normal area 20, a transition area 21 and a limit area 22. In the normal area 20, the (target) steering angle 11 is determined according to a linear relationship based on the (actual)

) steering wheel angle 6 is determined. The transition area 21 lies between the normal range 20 and the limit range 22. The transition area 21 begins at the reference steering angle 13, that is, from the (actual) steering angle at which an oversteer is detected. In the transition area 21, the gradient is adapted to a gradient in the limit range 22 depending on the (actual) steering wheel angle 6, for example by means of a suitable polynomial function. In the limit range 22, the relationship between the (actual)

) steering wheel angle 6 and the (target) steering angle 11 are linear again, with a gradient that is much smaller than in the normal range. As soon as the understeering case is detected, i.e. from the then set reference steering angle 13, all (target) steering angles 11 above the reference steering angle 13 are adjusted according to the characteristic curve 14. In particular, it is provided that the characteristic curve 14 above the reference steering angle 13 is shifted accordingly if a different reference steering angle 13 is present. If understeer is no longer detected, the relationship between the (actual) steering wheel angle 6 and the (target) steering angle 11 again follows the characteristic curve 14 in the normal range 20, whereby the transition from the limit range 22 back to the normal range 21 again takes place according to the characteristic curve 14 in the transition range 21.

It can be provided that the characteristic curve 14 can be parameterized at least for the transition region 21. For example, a width of the transition region 21 and/or a shape of the characteristic curve 14 in the transition region 21, for example a curvature, can be set. For example, a polynomial function or another suitable function can be used here. It can also be provided that the characteristic curve 14 can also be parameterized in the border region 22. For example, it can be provided that a slope of the characteristic curve 14 can be set in the border region 22.

It can be provided that the characteristic curve 14 is or will be selected or parameterized taking into account a speed of the vehicle and/or a coefficient of friction and/or a value of the at least one understeer indicator 30 (Fig. 1).

For example, a gradient and/or a curvature can be selected or parameterized.

It can be provided that a threshold value from which understeering is detected is or will be set taking into account a speed of the vehicle and/or a friction coefficient. This is done, for example, by means of the control unit 8, which receives the aforementioned variables, for example, from a vehicle control system.

It can be provided that the at least one understeer indicator 30 is a continuous indicator which is proportional to a strength of the understeer.

It can be provided that the at least one understeer indicator 30 is or is determined based on a comparison between a target yaw rate and an actual yaw rate of the vehicle. For this purpose, for example, the vehicle control system forms a difference between the target yaw rate and the actual yaw rate and transfers the difference as an understeer indicator 30 to the control unit 8. The control unit 8 then compares the difference with a predetermined threshold value, above which the understeer case is determined. The difference can also be determined by the control unit 8. It can be provided that the at least one understeer indicator 30 is determined based on a lateral force acting on the front wheels of the vehicle. For this purpose, for example, a change in the lateral force is determined as a function of a change in the (actual) steering angle 12. If the (actual) steering angle 12 changes, but the lateral force remains constant, the understeer case is determined. This can be done, for example, by means of the control unit 8.

It can be provided that the at least one understeer indicator 30 is determined based on an actual lateral acceleration. For this purpose, for example, a change in the lateral acceleration is determined as a function of a change in the (actual) steering angle 12. If the (actual) steering angle 12 changes, but the lateral acceleration remains constant, the understeer case is determined. This can be done, for example, by means of the control unit 8.

It can be provided that the steering system 1 is a rear-wheel steering system 23 or additionally comprises a rear-wheel steering system which is configured in the same way.

Embodiments of the method result from the described embodiments of the steering system.

List of reference symbols

Steering system Steering wheel module Steering module Steering handle Sensor (actual) steering wheel angle

Steering gear Control unit Electric machine Rack (Target) steering angle (Actual) steering angle Reference steering angle

Curve

Actuator Feedback torque Normal range Transition range Limit range Rear wheel steering system

Understeer indicator

Claims

Patent claims Mechanically decoupled steering system (1) for a vehicle, comprising: a steering handle (4), a steering gear (7), and a control unit (8), wherein at least one sensor (5) for detecting a steering wheel angle (6) and/or a hand torque on the steering handle (4) is arranged on the steering handle (4), wherein the steering gear (7) has at least one electric machine (9) which is coupled to a rack (10), wherein the control unit (8) is designed to determine a target steering angle (11) as a function of an actual steering wheel angle (6) and/or actual hand torque detected by means of the at least one sensor (5) and to control the steering gear (7), and wherein the control unit (8) is further designed to detect an understeering case based on at least one understeering indicator (30), to set an actual steering angle (12) present upon detection as a reference steering angle (13) and to change the Actual steering wheel angle (6) requested to adjust the target steering angle (11) lying above the reference steering angle (13) according to a predetermined characteristic curve (14). Steering system (1) according to claim 1, characterized in that the characteristic curve (14) has at least one transition region (21) and a limit region (22), wherein the transition region (21) lies between the reference steering angle (13) and the limit region (22). Steering system (1) according to claim 2, characterized in that the characteristic curve (14) is parameterizable at least for the transition region (21). Steering system (1) according to one of the preceding claims, characterized in that the characteristic curve (14) is selected or parameterized taking into account a speed of the vehicle and/or a coefficient of friction and/or a value of the at least one understeer indicator (30). Steering system (1) according to one of the preceding claims, characterized in that a threshold value from which understeering is detected is set taking into account a speed of the vehicle and/or a coefficient of friction. Steering system (1) according to one of the preceding claims, characterized in that the at least one understeering indicator (30) is a continuous indicator which is proportional to a strength of the understeering. Steering system (1) according to one of the preceding claims, characterized in that the at least one understeering indicator (30) is determined based on a comparison between a target yaw rate and an actual yaw rate of the vehicle. Steering system (1) according to one of the preceding claims, characterized in that the at least one understeering indicator (30) is determined based on a lateral force acting on the front wheels of the vehicle and/or based on an actual lateral acceleration. Steering system (1) according to one of the preceding claims, characterized in that the steering system (1) is a rear-wheel steering system (23) or additionally comprises a rear-wheel steering system (23) which is set up in the same way. Method for operating a mechanically decoupled steering system (1) for a vehicle, wherein the steering system comprises a steering handle (4), a steering gear (7), and a control unit (8), wherein the steering handle (4) has at least one sensor (5) for detecting a steering wheel angle (6) and/or a manual torque on the steering handle (4), wherein the steering gear (7) has at least one electric machine (8) which is coupled to a rack (10), wherein a target steering angle (11) is determined as a function of an actual steering wheel angle (6) and/or actual manual torque detected by means of the at least one sensor (5), and the steering gear (7) is controlled by means of the control unit (8), and wherein an understeering case is detected on the basis of at least one understeering indicator (30), wherein an actual steering angle (12) present upon detection is set as a reference steering angle (13), and a target steering angle (11) determined by changing the Actual steering wheel angle (6) requested above the The target steering angle (11) lying below the reference steering angle (13) is adjusted according to a predetermined characteristic curve (14).