Steering system and method for providing feedback to a user of a motor-assisted steering system for a vehicle

Abstract

Method for providing feedback to a user of a motor-assisted steering system (1) for a vehicle (10) comprising the steps: - Determining (100) a signal representing a rack force in a rack (14) which is displaceable by means of an electric motor (13), - Determining (200) a driving condition of the vehicle (10) or a vehicle condition, hereinafter collectively referred to as ‘condition’, characterized in that the method comprises the steps: - Adjusting (300) the signal representing the rack force as a function of the state and - Output (400) of haptic feedback to the user based on the adapted signal as input for a motor drive of a steering handwheel (5) by means of an actuator (6).

Description

State of the art

[0001] The present invention relates to a method for providing feedback to a user of a motor-assisted steering system according to the preamble of claim 1, a steering system for a vehicle according to claim 6, and a vehicle with a steering system according to claim 9. In particular, the present invention relates to improved selection between useful and interference information in the haptic feedback to a vehicle driver.

[0002] To increase driving safety and user comfort, modern vehicles are equipped with power steering systems that reduce the steering effort required from the driver. To maintain a natural steering feel, considerable effort is made to transmit feedback from the road surface to the steering wheel in a perceptible way. Desired information in the feedback path includes specific torque curves that can be linked to a particular cause.

[0003] In the prior art, for example, a distinction between useful and interference information is proposed based on a frequency analysis of the alternating signals originating from the roadway. Specific frequency ranges are assigned according to their cause, and frequency ranges deemed desirable are forwarded, while frequency ranges perceived as interfering are suppressed or at least significantly attenuated in the feedback path.

[0004] Since only a static frequency range, but not a specific frequency, can be assigned to the respective causes of useful or disruptive information, extensive overlaps occur in the known frequency analysis. A clear assignment and thus also the identification of useful or disruptive information is therefore not possible solely by considering the frequencies of the forces acting on the steering system. The current approach to designing steering feedback or the "steering feel" is always limited to fine-tuning during extensive test drives by selected expert drivers.

[0005] DE 10 2010 025 197 A1 discloses a method for filtering a target value signal for a support function in a steering system. Depending on the driving situation, a filter is parameterized, by means of which a portion of a resulting target value signal for a support torque is generated. However, a filtering-based treatment of feedback to the driver is not proposed.

[0006] DE 10 2011 052 881 A1 discloses a method for determining a rack force for a steering device in a vehicle, in which a first rack force and a second rack force are determined. Depending on a control variable, which can be determined from a characteristic map as a function of vehicle speed, for example, a resultant rack force is calculated using the first rack force and / or the second rack force.

[0007] The generic German patent DE 102 48 343 A1 discloses a method for controlling an actuation force simulator of a vehicle steering system designed as a steer-by-wire system. An actuator on a steering handwheel is supplied with a signal to generate feedback. Various input variables (vehicle speed, yaw rate, rack position and force) are determined and taken into account for controlling the handwheel actuator.

[0008] It is an object of the present invention to improve feedback from a steering system with regard to the useful / noise information content. Disclosure of the invention

[0009] The problem identified above is solved by a method for providing feedback to a user of a motor-assisted steering system with the features of claim 1, a steering system for a vehicle with the features of claim 6, and a vehicle with a steering system with the features of claim 9. The steering system can be, for example, a steering system assisted by hydraulic pressure, by electric motor force, or otherwise. In particular, the steering system can also be a so-called "steer-by-wire" steering system, in which mechanical force transmission is completely dispensed with. For the aforementioned steering systems, suitable feedback to the user via a hand torque (haptically perceptible torque at the steering handwheel) is advantageous for the user experience. In a first step, a signal representing a rack force of the steering system is determined. This can be, for example,The system includes sensor-based determination and / or calculation of the rack force. Sensor values ​​can be determined, for example, via force sensors and / or electrical current sensors. The signal can also be received as a CAN message or via another bus system. The signal can represent a DC component and an AC component of the rack force. Additionally, according to the invention, a driving state of the vehicle is determined, which can be characterized by a motion state and / or an operating state and / or a vehicle state. For example, a longitudinal guidance state and / or a lateral guidance state and / or a surface traversed by the vehicle is characterized during the driving state. In particular, the driving state communicates information regarding the maximum transmissible lateral force ("lateral guidance force") and / or the static friction limit to the driver via the hand torque on the steering wheel.Information about the limit of static friction allows the driver to proactively adjust cornering at the limit. Information about the maximum transmissible lateral force enables the driver to corner with maximum lateral acceleration. Further parameters describing the vehicle's driving state include the vehicle's cruising speed, steering angle, acceleration, changes in steering angle, and pressure in the braking system, representing the current braking force. According to the invention, the signal representing the rack and pinion force is adjusted depending on the driving state. This can, for example, include filtering the signal or the spectrum it contains. Here, a cutoff frequency or frequencies and attenuation of a filter range can be adjusted, by which the signal is processed. Subsequently, haptic feedback is provided to the user based on the adjusted signal.In other words, the signal is used as an input for a motorized drive of a steering handwheel, so that the user experiences a torque at the steering handwheel represented by the adapted signal.

[0010] The dependent claims describe preferred embodiments of the invention.

[0011] Preferably, the signal representing the rack force is adjusted according to a predefined feedback profile. This can be selected and, in particular, configured by the user. For example, the user can select a sporty suspension profile, thereby facilitating a detailed transmission of driving conditions via the steering wheel in accordance with the invention. If, on the other hand, the user selects a comfortable suspension setting, the haptic feedback may contain fewer details in the form of road-induced bumps or shocks, or their amplitudes may be significantly reduced. Alternatively or additionally, the signal representing the rack force can be adjusted according to a determined coefficient of friction between the road surface and the vehicle's tires.For example, the coefficient of friction can change depending on the road surface, in response to which the rack force assumes a different value, especially a mean value, without changing the steering angle and the speed driven.

[0012] The driving state can, for example, characterize a movement state for the vehicle and, in particular, describe cornering and / or yaw rate and / or straight-ahead driving and / or oversteer and / or understeer. Alternatively or additionally, an operating state of the vehicle can be characterized by the driving state, which, in particular, describes a braking state, preferably a braking force and / or a steering torque and / or brake-induced steering instability. Depending on the aforementioned driving states, the signal representing the rack force can be adjusted in such a way as to prevent the user from suppressing the aforementioned information in the haptic feedback to the point that the user no longer perceives it. In this way, driving safety and the user experience can be improved, as information relevant to the user is not "swallowed up" by the motor-assisted steering system.

[0013] According to a second aspect of the present invention, a steering system for a vehicle is proposed, wherein the steering system can be a motor-assisted, in particular hydraulically or electrically assisted, steering system. Steer-by-wire steering systems can also be designed according to the invention to improve the user experience. The vehicle can be, for example, a passenger car, a truck, or a van. The steering system comprises means for determining a signal representing a rack force of the vehicle. The means can include sensors, bus participants (CAN bus or similar), and evaluation units for calculating a rack force. In addition, means for determining a driving state of the vehicle are provided according to the invention, wherein the embodiments described in connection with the method according to the invention apply accordingly to the driving state.The means for determining the driving state can also include sensors and / or information lines, in particular BUS participants and evaluation units for calculating parameters and comparing them with predefined references. A processing unit is configured to adjust the rack force, representing the signal, depending on the driving state. For example, the processing unit can include an adaptive filter, which allows different filter characteristics (also called "frequency responses") to be implemented depending on the driving state. Additionally, an actuator (also called a "motor") is provided, which is configured to output haptic feedback to the user based on the adjusted signal, thus informing them about the driving state parameters related to the rack force, depending on the determined driving state.In particular, the actuator is therefore designed to apply an alternating torque to the steering handwheel of the vehicle.

[0014] The features, combinations of features and the advantages arising from the steering system according to the invention are so clearly evident in relation to those of the method according to the invention that reference is made to the above explanations to avoid repetition.

[0015] In summary, the feedback concept according to the invention for identifying the desired useful information considers not only the frequencies but also the respective driving state. Further exclusion criteria can be derived by analyzing the driving states in which useful or interference information occurs. To identify useful or interference information, the current driving state is considered in addition to the frequency of the applied rack force. For example, frequencies in the rack force curve that arise from non-uniform brake discs during braking are generally considered interference information. This interference information can be identified during driving operation by the fact that the associated changes in the rack force only occur during braking. Furthermore, frequencies can be assigned to the changes in the rack force that are periodically dependent on the rotational speed of the brake disc.of the wheel and thus depend on the vehicle speed. Furthermore, the feedback concept according to the invention includes a learning algorithm that continuously monitors the forces acting on the rack and correlates them with the current driving condition. In this way, the reliability of the detection and thus also the feedback of useful information is increased. For example, if a frequency in the rack force curve is identified over a defined period of driving operation during braking processes, which depends periodically on the vehicle speed, the information that a brake disc thickness variation is present is validated. Understeer, i.e., a limit of static friction or a lateral force maximum, manifests itself in alternating torques in the frequency range below 2 Hertz. Driving conditions in which this information is relevant for the driver are characterized by a high lateral acceleration relative to the current coefficient of friction and can be detected, for example, by...Oversteer can be detected by comparing yaw rates. Oversteer manifests itself through alternating signals below 5 Hertz. Oversteer occurs in driving conditions with high lateral acceleration relative to the current coefficient of friction and, like understeer, can be detected by comparing yaw rates. Changes in the coefficient of friction between the road surface and the vehicle's tires are reflected in a frequency range between 2 and 25 Hertz. This information is particularly relevant during cornering and is reflected by changes in the rack force despite constant steering angle and cruising speed. Tire imbalance and variations in brake disc thickness are reflected by frequencies in the rack force that are proportional to the vehicle speed and the rolling circumference of the respective tire.While tire imbalance is relevant in all driving situations depending on speed, variations in brake disc thickness are only significant during braking. Road irregularities, on the other hand, manifest themselves in a frequency range of 1 to 30 Hertz and occur periodically, stochastically, or transiently. The aforementioned frequency ranges are at least partially congruent, so considering the driving conditions can significantly improve user comfort and the user's subjective perception of quality without potentially "obscuring" information relevant to the user in those conditions.

[0016] The definition of which information in the steering torque curve is perceived as useful or disruptive is highly subjective. Therefore, in the feedback concept according to the invention, the driver or the respective steering application (also called "program favorite") can be left to decide which information is currently displayed during the steering torque curve. In preset configurations, certain information can be suppressed and others amplified. For example, in a sporty configuration, as much information as possible is fed back during the steering torque curve, especially information on oversteer, understeer, and changes in the coefficient of friction. Tire imbalance can also be useful information for a very experienced driver, as it can be associated with local wear and damage (flat spots, etc.).If the (racing) driver is informed of this early on, driving behavior (utilizing the limits) or race strategy (e.g., by entering the pit lane) can be influenced. For understeer situations, depending on the information received, it is possible to communicate both the point at which the front tires lose traction and the point of maximum lateral force to the driver. A corresponding steering torque curve is used for this communication. In principle, however, acoustic or visual feedback of the respective information to the driver is also possible. Brief description of the drawings

[0017] Exemplary embodiments of the invention are described in detail below with reference to the accompanying drawings. The drawings show: Fig. 1 a schematic overview of components of an embodiment of a means of transport according to the invention with an embodiment of a steering system designed according to the invention; Fig. 2. A detailed view illustrating components of an embodiment of a steering system according to the invention; Fig. 3. A flowchart illustrating the steps of a procedure for providing feedback to a user of a motor-assisted steering system for a vehicle; and Fig. 4. A comparison of two different haptic feedbacks in the form of a sequence of moments over time. Embodiments of the invention

[0018] Fig. Figure 1 shows a vehicle 10 as a means of transport, in which an electrically assisted steering system 1 is provided. The front wheels 2 are steered via a rack 14, which is movable by means of an electric motor 13. A force sensor 3 transmits the rack force to an evaluation unit (not shown), which outputs electrical signals to an actuator 6 on the longitudinal column 7 to provide haptic feedback to the user. At its lower end, the longitudinal column 7 is mechanically connected to the rack 14 via a bevel gear 8. A steering handwheel 5 is arranged at its upper end.

[0019] Fig. Figure 2 shows an embodiment of an electrically assisted steering system 1, as used in conjunction with Fig. 1 has already been discussed in its basic outlines, so reference is made to the relevant explanations. In addition to Fig. 1 are in Fig. Figure 2 shows an evaluation unit in the form of an electronic control unit 9 with a bandpass filter 11 to evaluate rack force signals received from the force sensor 3 and to filter information regarding driving conditions received via a CAN bus. For this purpose, a sensor line 15 connects the force sensor 3 to the electronic control unit 9. An electrical line 18 transmits the filtered (adapted) signals, representing the rack force, to the actuator 6 on the longitudinal column 7. To generate a steering torque, the electronic control unit 9 is connected via an electrical line 12 to the electric motor 13 to generate the power steering assist.For example, uneven wear on the brake discs 4 in the event of a braking process reported via the CAN-BUS can be preferentially reported to the steering handwheel 5 by not suppressing the respective frequency range as strongly as usual by the bandpass filter 11, depending on the current speed and a signal representing a braking process (both reported via the CAN-BUS).

[0020] Fig. Figure 3 shows the steps of a procedure for providing feedback to a user of a motorized steering system for a vehicle. In step 100, a signal representing the rack force of the steering system is determined. Furthermore, in step 200, a driving state is determined in the form of a motion state and an operating state to enable state-dependent processing of the rack force (step 300). In step 400, haptic feedback is output to the user according to the adapted signal. This can be achieved, for example, by applying a corresponding torque over time to the vehicle's steering wheel.

[0021] Fig.Figure 4 shows two torque curves 16 and 17 over time, which exhibit different lower cutoff frequencies. To enable a sporty ("direct") transmission of as much information as possible from the tires to the vehicle's steering wheel, the cutoff frequency of the feedback signal 16 corresponding to a sporty setup is set higher than the cutoff frequency of the feedback signal 17 corresponding to a more comfortable setup. Accordingly, the torque curves 16 and 17 exhibit different degrees of smoothing. Reference symbol list 1 motor-assisted steering system 2 front wheel 3 Force sensor 4 brake discs 5 Steering handwheel 6 Actuator 7 Steering column 8 bevel gear 9 electronic control unit 10 vehicles 11 bandpass filters 12 electrical lines 13 Electric motor 14 Rack and pinion 15 Sensor cable 16-moment sequence (sporting coordination) 17 Moment profile (comfortable tuning) 18 electrical lines 100 - 400 process steps CAN Controller Area Network (BUS) M(t) Moment profile t time

Claims

[1] Method for providing feedback to a user of a motor-assisted steering system (1) for a vehicle (10) comprising the steps: - Determining (100) a signal representing a rack force in a rack (14) which is displaceable by means of an electric motor (13), - Determining (200) a driving condition of the vehicle (10) or a vehicle condition, hereinafter collectively referred to as ‘condition’, characterized by , that the procedure includes the following steps: - Adjusting (300) the signal representing the rack force as a function of the state and - Output (400) of haptic feedback to the user based on the adapted signal as input for a motor drive of a steering handwheel (5) by means of an actuator (6). [2] Method according to claim 1 wherein the step - of the adjustment (300) of the signal representing the rack force - depending on a predefined feedback profile, in particular one chosen by the user, and / or - depending on a determined coefficient of friction between the road surface and a front wheel (2) of the vehicle (10). [3] Method according to claim 1 or 2, wherein the detection of the signal - by means of a message received via a bus (CAN) of the vehicle (10) and / or - is done by means of a force sensor (3). [4] Method according to any of the preceding claims, wherein the state - a state of movement, in particular - a curve and / or - a yaw rate and / or - a straight-ahead journey and / or - an oversteer and / or - understeer of the vehicle (10) and / or - an operating state, in particular - a braking condition, preferably a braking force, and / or - a steering torque and / or - describes a steering instability caused by braking. [5] Method according to one of the preceding claims, wherein the adjustment of the signal comprises filtering, wherein a cutoff frequency of a bandpass filter (11) is adjusted depending on the state. [6] Steering system (1) for a vehicle (10) comprising - Means for determining (100) a signal representing a rack force in a rack (14) which is displaceable by means of an electric motor (13), - Means for determining (200) a driving condition of the vehicle (10) and / or a vehicle condition, hereinafter collectively referred to as ‘condition’, - an electronic control unit (9) which is configured to adjust the rack force depending on the state (300) representing the signal and - an actuator (6) which is configured to output haptic feedback to the user based on the adapted signal via a steering handwheel (5) (400). [7] Steering system (1) according to claim 6, wherein the steering system (1) is a - hydraulically assisted and / or - electrically assisted and / or - Steer-By-Wire steering system (1) is. [8] Steering system (1) according to claim 6 or 7 further comprising - the steering handwheel (5), wherein - the actuator (6) is a motor for generating an alternating torque on the steering handwheel (5). [9] Vehicle (10) comprising a steering system (1) according to any one of claims 6 to 8 above.

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