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

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

[0001] Description

[0002] Mechanically decoupled steering system for a vehicle and method for operating a mechanically decoupled steering system

[0003] 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.

[0004] Understeer is a term used in driving dynamics to describe the car's self-steering behavior. It describes the fact that when cornering, the steering wheel angle increases the faster the radius is traveled. In modern electronic stability programs, the actual movement variables are compared with the driver's input, which is derived from the steering wheel angle and vehicle 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 inside rear wheel is automatically braked and 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.

[0005] In mechanically decoupled steering systems, the movement of a steering handle can be decoupled from the movement of the wheels. Mechanically decoupled steer-by-wire steering systems are characterized by the fact that there is no longer any mechanical connection or coupling between the steering handle and the steering gear. Typically, at least one sensor is arranged on the steering handle to detect an angle of rotation or torque. The sensor signals are transmitted to a control unit, which then implements the steering request via the steering gear. For this purpose, the steering gear is driven by at least one electric servomotor. For redundancy reasons, at least two servomotors with their own power electronics are usually used. The servomotors 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, so each generates 50% of the total power. It is also known to provide an actuator on the steering handle, which generates a defined counter-torque on the steering handle in order to give the driver a better haptic feeling through feedback. This actuator is also referred to 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. This enables, on the one hand, a continuous adjustment of the steering ratio. On the other hand, the superposition also permits rapid driving dynamics interventions by changing the steering angle at the wheel while driving while maintaining a constant steering wheel angle.

[0006] 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.

[0007] The object is achieved according to the invention by a mechanically decoupled steering system having the features of patent claim 1 and a method having the features of patent claim 10. Advantageous embodiments of the invention emerge from the subclaims.

[0008] In particular, a mechanically decoupled steering system for a vehicle is provided, 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 coupled to a rack, wherein the control unit is configured 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 configured to detect an understeering situation based on at least one understeering indicator,to set an actual steering angle present at the time of detection as the reference steering angle and to adapt a target steering angle requested by changing the actual steering angle, which lies above the reference steering angle, according to a predetermined characteristic curve, in particular a characteristic curve dependent on the reference steering angle.

[0009] 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 manual torque on the steering handle, wherein the steering gear has at least one electric machine 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 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.

[0010] The steering system and method enable effective intervention in the vehicle dynamics in the event of understeer to ensure vehicle stability when cornering. The basic idea is that if the driver steers into understeer, this is corrected above the reference steering angle according to the characteristic curve. This results in a steering wheel angle-dependent reduction of the target steering angle transmitted to the steering gear. In this way, understeer can be limited, which is pleasant for the driver in terms of driving comfort, since it requires no braking interventions and / or speed reductions.

[0011] The control unit receives the understeer indicator from an electronic stability program or another control unit, such as a vehicle control unit. Alternatively, the steering system itself may have suitable sensors that provide suitable sensor data to determine the understeer indicator and detect the understeer situation.

[0012] 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 into a change in the steering angle. In this example, such a relationship can be set by changing the linear gradient of the characteristic curve with regard to the strength of the conversion of the steering wheel angle into the steering angle. In particular, the characteristic curve is not linear throughout, but can have a changing gradient at least in sections. The characteristic curve can, for example, be described at least in sections by means of 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 implemented individually or collectively as a combination of hardware and software, for example as program code executed on a microcontroller or microprocessor. However, it can also be provided that parts are implemented individually or collectively as an application-specific integrated circuit (ASIC) and / or a field-programmable gate array (FPGA).

[0013] In one embodiment, the characteristic curve has at least one transition region and one limit region, the transition region lying between the reference steering angle and the limit region. This allows different regions of the characteristic curve to be defined, and when oversteering is detected, the steering behavior is transferred from a normal range via the transition region to the limit region. In particular, this prevents an abrupt change in the steering behavior of the steering system and instead provides a transition that is perceived by the driver as smooth or flowing. In particular, it can be provided that the relationship between a steering wheel angle and a steering angle in a normal range is linear.In the transition range, i.e., starting with the reference steering angle in the case of oversteer, the relationship is then transformed into a linear relationship with a lower gradient across the transition range, so that a change in the steering angle in the limit range leads to a smaller change in the steering angle than in the normal range. Particularly in the transition range, the characteristic curve can be designed as a polynomial function and / or described as such. However, other characteristic curve profiles are also generally suitable.

[0014] In a further embodiment, the characteristic curve is parameterizable at least for the transition range. This allows individual preferences to be taken into account. Furthermore, it also enables the use of a situation-dependent characteristic curve. For example, this can be used to implement the preference to blend 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.

[0015] 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 above which understeer 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 above which understeer is detected to be defined depending on a current driving situation of the vehicle. For example, different weather conditions can be taken into account in this way (e.g. dry road surface, wet road surface, snow, ice, etc.).

[0016] In one embodiment, the at least one understeer indicator is a continuous indicator proportional to the strength of the understeer. This enables rapid detection of understeer. Furthermore, the understeer indicator can be used as a continuous variable describing the current dynamic driving 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.

[0017] In one embodiment, 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, understeer is detected. The characteristic curve can also be selected and / or parameterized taking such a difference value into account.

[0018] In one embodiment, 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 at the wheels is increased, an understeer situation exists.

[0019] In one embodiment, the at least one understeer indicator is or will be determined based on 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, an understeer situation exists.

[0020] In one embodiment, the steering system is a rear-wheel steering system or additionally comprises a rear-wheel steering system configured in the same way. In particular, the rear-wheel steering system can additionally or alternatively be configured according to the steering system described in this disclosure.

[0021] Further features of the method design will be apparent from the description of the steering system designs. The advantages of the method are the same as those of the steering system designs.

[0022] The invention will be explained in more detail below using preferred embodiments with reference to the figures.

[0023] Fig. 1 is a schematic representation of an embodiment of the mechanically decoupled steering system for a vehicle;

[0024] Fig. 2 is a schematic representation of a characteristic curve to illustrate an embodiment of the steering system and the method.

[0025] Figure 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. However, the steering system 1 can also be a superimposed steering system.

[0026] 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 manual 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 (e.g., at least one servomotor) coupled to a rack 10. The rack 10 is coupled to at least one steerable wheel (not shown). The control unit 8 is configured to determine a target steering angle 11 as a function of an actual steering wheel angle 6 and / or actual manual torque detected by the sensor 5 and to control (or regulate) the steering gear 7 in accordance with the target steering angle 11.

[0027] The control unit 8 is further configured to detect an understeering situation based on at least one understeer 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, which is requested by changing the actual steering angle 6 and lies above the reference steering angle 13, according to a predetermined characteristic curve 14, in particular a characteristic curve dependent on the reference steering angle. The understeer indicator 30 is provided to the control unit 8, for example, by a vehicle control system.

[0028] 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.

[0029] Fig. 2 shows a schematic representation of an exemplary characteristic curve 14. The (actual) steering wheel angle θ is shown on the x-axis and the (target) steering angle θ 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 θ based on the (actual) steering wheel angle θ using the characteristic curve 14. The characteristic curve 14 has, in particular, three areas: a normal area 20, a transition area 21 and a limit area 22. In the normal area 20, the (target) steering angle θ is determined according to a linear relationship based on the (actual)

[0030] ) steering wheel angle 6 is determined. The transition range 21 lies between the normal range 20 and the limit range 22. The transition range 21 begins at the reference steering angle 13, that is, from the (actual) steering angle at which an oversteer is detected. In the transition range 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-

[0031] ) steering wheel angle 6 and the (target) steering angle 11 is again linear, with a gradient that is much smaller than in the normal range. As soon as understeer 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 this case, it is particularly 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, with the transition from the limit range 22 back to the normal range 21 again taking place according to the characteristic curve 14 in the transition range 21.

[0032] 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 further be provided that the characteristic curve 14 can also be parameterized in the boundary region 22. For example, it can be provided that a gradient of the characteristic curve 14 can be set in the boundary region 22.

[0033] 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).

[0034] For example, a gradient and / or a curvature can be selected or parameterized.

[0035] It may be provided that a threshold value above which understeer is detected is or will be set taking into account a vehicle speed and / or a friction coefficient. This is done, for example, by means of control unit 8, which receives the aforementioned variables, for example, from a vehicle control system.

[0036] 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.

[0037] 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. However, 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, understeer is detected. This can be done, for example, using control unit 8.

[0038] 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 situation is determined. This can be done, for example, by means of the control unit 8.

[0039] 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.

[0040] Embodiments of the method result from the described embodiments of the steering system.

[0041] List of reference symbols

[0042] Steering system Steering wheel module Steering module Steering handle Sensor (actual) steering wheel angle

[0043] Steering gear Control unit Electric machine Rack (Target) steering angle (Actual) steering angle Reference steering angle

[0044] Characteristic curve

[0045] Actuator Feedback torque Normal range Transition range Limit range Rear wheel steering system

[0046] Understeer indicator

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 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) coupled to a rack (10), wherein the control unit (8) is configured 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 at least one sensor (5) and to control the steering gear (7), and wherein the control unit (8) is further configured to detect an understeering situation 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 desired 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). Steering system (1) according to claim 1, characterized in that the characteristic curve (14) has at least one transition region (21) and one boundary region (22), wherein the transition region (21) lies between the reference steering angle (13) and the boundary 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 above which understeer is detected is defined 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 understeer indicator (30) is a continuous indicator that is proportional to a strength of the understeer. Steering system (1) according to one of the preceding claims, characterized in that the at least one understeer indicator (30) is determined based on a comparison between a desired 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 understeer 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 configured 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 based on 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 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).

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