Method and steering system for providing steering assistance

Abstract

Method for determining a steering assistance force for a steering system (1) of a motor vehicle, comprising the following steps: - determining a steering assistance force based on a hand torque (M_Hand) applied by a driver; and - Modifying the steering assistance force depending on a driving condition affecting a lateral force (Q) of the steering system (1), characterized in that the steering assistance force is modified such that when a steering angle (Phi_LRW) is reached at which a maximum lateral acceleration achievable by the wheels on the roadway is reached, the manual torque (M_Hand) to be applied is increased abruptly.

Description

Technical area

[0001] The invention relates to electromechanical steering systems for motor vehicles, in particular to methods for determining a steering assistance force. Technical background

[0002] Electromechanical steering systems include a steering assist drive to provide a steering assist force. The steering assist force serves to assist the driver during steering movements. The steering assist force is typically determined based on a manual torque applied via a steering wheel of the steering system and applied to the steering system via the steering assist drive. The manual torque is interpreted as the driver's input and assigned to a steering assist force via a characteristic map. In addition to the dependence on the manual torque, the steering assist force can also be determined based on the vehicle speed and additional steering functions in the area of ​​driver assistance, such as parking assistance, lane keeping assistance, and the like.

[0003] The steering assistance force and the manual torque determine a rack force that acts via the rack on the steered wheels. Since the rack force depends on the driving condition of the vehicle, the provided steering assistance force determines the manual torque that must be applied by the driver. The steering assistance force is therefore perceived haptically by the driver via the manual torque. The haptic perception of the steering assistance force provides the driver with important information about the condition of the vehicle, particularly the magnitude of the lateral acceleration or tire lateral force acting on the steered wheels.

[0004] A method for determining a rack force for a steering device is known from the publication DE 10 2011 052 881 A1. A first rack force is determined as a function of at least one force occurring in the steering device. In order to provide less intense feedback on the force ratios in the steering device for reasons of driving comfort, a second rack force is determined as a function of at least one vehicle variable that characterizes the vehicle's state of motion, and a reflected rack force is formed based on the first rack force and the second rack force.

[0005] The document DE 20 2013 003 767 U1 discloses a motor vehicle with a device for assessing the grip of a road surface traveled or to be traveled by the motor vehicle and a power steering device in which the extent of steering assistance is variable depending on the result of an assessment of the grip.

[0006] The document DE 10 2010 031 710 A1 discloses a method for operating an electromechanical power steering system of a motor vehicle, in which an assistance torque is exerted by an electric machine on a steering gear of the power steering system as a function of a steering torque exerted by a driver on a steering handle, wherein a signal is measured at several successive points in time by means of a sensor arranged on the tie rod and / or rack, which signal represents a force on a tie rod and / or rack of the power steering system for each point in time, wherein the assistance torque to be exerted is determined using the measured signal.

[0007] The document DE 10 2008 041 962 A1 discloses a method for adjusting a steering system in a vehicle, wherein an assistance torque is fed into the steering system via a servo unit as a function of a vehicle state variable, wherein a transverse dynamic state variable is taken into account as the state variable, wherein the level of the assistance torque is selected such that the steering wheel torque to be applied by the driver increases linearly or progressively with the transverse dynamic state variable.

[0008] The document DE 10 2013 009 399 A1 discloses a method for detecting a critical driving situation of a vehicle, wherein a deviation from a predefined relationship between the rack force of a rack and the steering angle of the vehicle is detected and, if a deviation is detected, a critical driving situation of the vehicle is determined.

[0009] The above steering systems provide a steering assistance force that has a predetermined relationship to the measured manual torque. The manual torque curve follows the restoring torque curve of the steered wheels of the front axle, depending on the front axle suspension. As a result, changes in the rack force resulting from changes in friction and grip between the steered wheels and the road surface have only a weakened effect on the driver via the manual torque.

[0010] It is an object of the present invention to improve the perceptibility of changes in a driving condition of the motor vehicle acting on the rack force. Disclosure of the invention

[0011] This object is achieved by the method for determining a steering assistance force for a steering system of a motor vehicle according to claim 1 and by the control unit, by the steering system and by the computer program product according to the independent claims.

[0012] Further embodiments are specified in the dependent claims.

[0013] According to a first aspect, a method for determining a steering assistance force for a steering system of a motor vehicle is provided, comprising the following steps: - Determining a steering assistance force based on a hand torque applied by a driver; - Modifying the steering assistance force depending on a driving condition affecting a lateral force of the steering system.The steering assistance force is modified in such a way that when a steering angle is reached at which a maximum lateral acceleration achievable by the wheels on the roadway is achieved, the manual torque to be applied is increased abruptly.

[0014] One idea of ​​the above method is to adapt the steering assistance force depending on a change in a driving condition, which results in a change in the rack force. Until now, the steering assistance force or the manual torque perceived by the driver has followed the rack force, so that external influences on the rack force are perceived by the driver in a weakened form due to the link to the steering assistance force. In contrast, according to the above method, the curve of the steering assistance force is modified depending on the driving conditions influencing the rack force in such a way that either the perceptibility of the respective driving condition is suppressed or haptic information is provided to the driver through the change in the resulting manual torque.

[0015] This allows the haptic information content to be specifically influenced during the course of the hand torque or the steering assistance force, thus providing the driver with improved feedback on the driving condition of the vehicle via the effective hand torque.

[0016] Lateral acceleration largely determines the rack force, and the steering assist force curve can be determined based on the lateral acceleration, for example. This makes it possible to communicate the maximum transmittable lateral acceleration in the form of haptic feedback at the moment of hand movement.

[0017] In particular, the torque stall during the transition to the nonlinear range of static friction can be more clearly presented to the driver or even suppressed. Alternatively, a maximum lateral force or maximum lateral acceleration that can be transmitted by the steered wheels can be indicated by a sudden change in the steering assistance force that is perceptible to the driver. This helps the driver achieve better utilization of the vehicle's dynamic potential. Thus, in addition to calculating the steering assistance force depending on the hand torque, additional information can be generated by incorporating other state variables.

[0018] This opens up a wide range of possibilities for providing additional haptic information about the hand torque, such as the transition from static friction to sliding friction on the tires of the steered wheels, as determined, for example, by comparing the measured or observed rack force with a reference rack force from a linear reference model. Overall, such haptic feedback can be easily supplemented by existing steering systems.

[0019] In general, the driving condition can be determined based on a deviation determined by the actual lateral force of the steering system and a given reference value.

[0020] Furthermore, the driving condition can be determined based on a deviation between the actual rack force acting on a steering rack and a reference rack force resulting from the vehicle speed. This deviation occurs particularly in response to external influences on the steering system, such as a change in the lateral force absorbed by the steered wheels.

[0021] In particular, the steering assistance force can be determined from a modified manual torque according to a predetermined steering assistance function, in particular a steering assistance map. The modified manual torque can be determined by applying an adjustment variable to the manual torque, wherein, in particular, the adjustment variable is determined based on a deviation of a rack force actually acting on a rack of the steering system from a reference rack force resulting from the vehicle speed.

[0022] Furthermore, the adjustment size can be determined according to a given adjustment function depending on the deviation.

[0023] According to one embodiment, the steering assistance force can be modified in a range of driving conditions in which the lateral force depends non-linearly on the steering angle due to the friction behavior between the wheels and the road surface, such that the manual torque to be applied in the range of driving conditions runs according to a predetermined curve function, in particular at least partially linearly, over the steering angle.

[0024] In particular, the steering assistance force can be applied to the steering rack or coupled into it.

[0025] Furthermore, the steering assistance force can be provided by a motor torque of a servo motor of the steering assistance drive.

[0026] According to a further aspect, a control unit, in particular a steering control device, is provided which is designed to determine a steering assistance force based on a manual torque applied by a driver and to modify the steering assistance force depending on a driving condition affecting a lateral force of the steering system, wherein the steering assistance force is modified such that when a steering angle is reached at which a maximum lateral acceleration achievable by the wheels on the roadway is achieved, the manual torque to be applied is increased abruptly.

[0027] According to a further aspect, a steering system is provided, comprising: - a rack connected to steered wheels, the rack being subjected to a manual torque exerted by a driver, - the above control unit; - a steering assist drive configured to apply the steering assist force to the rack. Short description of the drawings

[0028] Embodiments are explained in more detail below with reference to the attached drawings. They show: Fig. 1 a schematic representation of a steering system with electromechanical steering assistance; Fig. 2 a diagram showing the course of the rack force; Fig. 3 a schematic block diagram of the function performed in the control unit for determining the steering assistance force; and Fig. 4 a diagram showing the curves of a rack force, a reference rack force, a target engine torque, a manual torque and a rack force deviation over the steering angle. Description of embodiments

[0029] In Fig. Figure 1 shows a schematic representation of a steering system 1. The steering system 1 has a rack 2, which is coupled to steered wheels 3 in a manner known per se, so that the wheels can be positioned according to a predetermined steering angle by a translational movement of the rack 2.

[0030] The rack 2 is coupled to a steering shaft 5 via a first mechanical coupling 4, via which a manual torque applied via a steering wheel 7 can be applied to the steering rod 2 as steering force.

[0031] Furthermore, a steering assistance drive 6 is provided, which has a servomotor 61 that is coupled to the rack 2 via a second mechanical coupling 62, in particular to a coupling region 21 of the rack 2. The servomotor 61 can be designed, in particular, as an electronically commutated machine. A steering assistance force can be applied to the rack 2 via the steering assistance drive 6 by controlling the servomotor 61.

[0032] Furthermore, a control unit 10 is provided, which is connected to the steering assistance drive 6 in order to provide the steering assistance force in a controlled manner.

[0033] Conventionally, the steering assistance force is essentially adjusted depending on the rack force acting on the steering rack 2; this relationship can be linear over a wide adjustment range. In addition, the vehicle speed, which significantly influences the steerability of the steered wheels 3, can be used as a parameter for determining the steering assistance force.

[0034] As shown in the diagram of Fig. As shown in Figure 2, the driving condition of the motor vehicle when cornering acts directly on the rack 2 via the lateral acceleration, which results from the vehicle speed and the steering angle, and causes the resulting rack force according to a linear relationship, since there is a fixed coupling of the steered wheels 3 to the road surface due to the sufficiently high grip and friction. From a certain lateral acceleration, the coupling of the steered wheels 3 to the road surface enters a non-linear range and lateral slip occurs, so that the further progression of the rack force is lower with an increase in the lateral force Q acting on the wheels and with a corresponding increase in lateral acceleration, until a maximum lateral force Q that can be absorbed by the steered wheels is reached. This is shown in Fig. 2 for a given vehicle speed. There, the rack force F_Rack, corresponding to the lateral force Q, is plotted against the steering angle Phi_LRW. Steering angle Phi_LRW1 corresponds to the transition of the rack force F_Rack into the non-linear region, and steering angle Phi_LRW2 corresponds to the point at which the maximum absorbable lateral force or maximum lateral acceleration is reached and the rack force F_Rack no longer increases.

[0035] The conventional coupling of the steering assistance force to the rack force results in the driver perceiving the transition at point Phi_LRW1 and reaching point Phi_LRW2 through the resulting manual torque, each with a slightly lighter steering action. However, due to the steering assistance force, these points are only perceived to a lesser extent.

[0036] In order to provide additional haptic feedback to the driver regarding lateral acceleration, it may now be possible to modify the steering assistance force.

[0037] A modification of the steering assistance force can, for example, provide - that the curve of the steering assistance force over the steering angle in driving conditions between the point Phi_LRW1 and the point Phi_LRW2 is modified so that the driver does not perceive the transition into the non-linear range; and / or - that the curve of the steering assistance force over the steering angle at the point Phi_LRW2, ie at the point of maximum transmissible lateral acceleration, is modified so that the point Phi_LRW2 can be perceived more clearly in the form of haptic feedback at the moment of hand movement.

[0038] The haptic feedback can be provided in the form of a sudden change in the hand torque applied by the driver.

[0039] In this regard, a function corresponding to the block diagram of the Fig. 3 realized.

[0040] Basically, the model provides for modifying a manual torque M_Hand applied by the driver in a modification block 101 with the aid of a provided adjustment variable K. The modification can be performed by applying the adjustment variable K to the manual torque M_Hand, for example by multiplication or addition or the like.

[0041] As before, the resulting modified manual torque M_Hand_mod is then fed to a steering assistance map or a corresponding steering assistance function for conversion into a steering assistance force in a map block 102 in order to transmit an engine torque M_Motor_Soll equivalent to the steering assistance force to the steering assistance drive 6. The steering assistance function can, for example, assign the steering assistance torque to the modified manual torque in a linear manner using a vehicle speed v_Vzg as a parameter.

[0042] By adjusting the adjustment variable, it is now possible to adjust the course of the manual torque to be applied by the driver according to the driving condition of the vehicle.

[0043] For example, it can be provided that the haptic perceptibility of a transition into the non-linear range at point Phi_LRW1, which occurs due to the increase in the sliding friction components on the steered wheels, is reduced. To this end, the maximum transferable lateral force can be shifted by shifting the drop in the manual torque towards the driving state of maximum lateral acceleration (point Phi_LRW2) in order to better utilize the driving dynamics potential. To this end, when the non-linear range is reached or when the steering angle exceeds point Phi_LRW1, the difference between the model-based linear rack force and the actual non-linear rack force is first corrected by reducing the steering assistance force to compensate for the difference, so that the manual torque continues to be essentially linear with the model-based linear rack force over the steering angle Phi_LRW.In this way, the transition into the non-linear range at point Phi_LRW1 is not or only partially perceptible haptically.

[0044] Alternatively, the torque stall during the transition to the nonlinear range at point Phi_LRW1 can be more clearly presented to the driver haptically. This is achieved by first increasing the resulting steering assistance force by the difference between the model-based linear rack force and the actual nonlinear rack force when the nonlinear range is reached or when the steering angle exceeds point Phi_LRW1. This increases the resulting steering assistance force by the difference between the model-based linear rack force and the actual nonlinear rack force depending on the difference between the model-based linear rack force and the actual nonlinear rack force. In this way, the transition to the nonlinear range at point Phi_LRW1 is particularly clearly perceptible haptically through a decrease in the hand torque.

[0045] This can also be done when maximum lateral acceleration is reached. When the point Phi_LRW2 is reached, the resulting steering assistance force can be increased abruptly, allowing the driver to perceive this haptically in an improved way through a loss of manual torque. This can signal a loss of steerability to the driver, allowing them to reduce the steering angle and / or the vehicle's speed.

[0046] To generate the above exemplary curves of the steering assistance forces, the rack force applied by external forces is modeled in a rack force estimation block 103. For this purpose, the variables of the servo motor 61 provided by the steering assistance drive 6, such as the position of the servo motor 61 Phi_Motor, the angular velocity Phv_Motor of the servo motor 61, and the actual torque M_Motor of the servo motor 61, are provided. By additionally knowing the mass inertia of the steering system 1, the actually applied rack force F_Rack can be determined very precisely.

[0047] In a rack reference model block 104, a reference rack force F_Zahnstange_ref can be determined via the centrifugal force acting on the steered wheels 3 as described above, depending on the vehicle speed v_Fzg and the steering angle Phi_LRW.

[0048] In the case of the transition from sliding to static friction, the reference rack force F_Zahnstange_ref calculated from the steering angle Phi_LRW and the vehicle speed v_Vzg continues to increase linearly, while the actual rack force is actually already decreasing according to the rack force estimation block 103. This results in a rack force deviation A between the estimated or modeled rack force F_Zahnstange and the reference rack force F_Zahnstange_ref, which is determined in a comparison block 105.

[0049] The rack force deviation A is assigned together with the steering angle Phi_LRW in a map block 106 to the adaptation variable K. Depending on the objective, the adaptation variable K can be an amplification or attenuation factor which is defined according to an adaptation function as a function of the rack force deviation A in order to be able to haptically represent a driving condition to the driver particularly clearly by increasing or decreasing the manual torque to be applied, or to suppress its perceptibility.

[0050] The goal is to transparently communicate changes in the adhesion between the steered wheels and the road surface to the driver by modifying the steering torque curve, allowing the driver to adjust their driving style accordingly. This modification can be additionally supported by signals provided by other control units, such as brake control systems and the like. This allows the generation of the adaptation variable to continue to be dependent on other driving state variables, such as wheel speeds, which are provided by a driving stability program and indicate a driving state of the vehicle. These can be used to verify the plausibility of the internal steering calculations and thus contribute to a high level of robustness.

[0051] Furthermore, systems can be provided that analyze the road ahead of the vehicle based on laser scanning to adapt the chassis systems. This information can be used to modify the steering assistance force, thus even proactively adapting the steering structure. Using environmental sensors, digital map information, and / or inter-vehicle communication, the above-mentioned rack reference model can be adapted to improve, for example, the estimation or modeling of the maximum achievable lateral acceleration, also depending on the road friction coefficient.

[0052] Ultimately, the above method can achieve a significant increase in driving safety, as it is possible to present the transition into the non-linear range of the coupling between the wheels and the road and the reaching of the maximum lateral acceleration to the driver in a better and more targeted manner than is possible with state-of-the-art systems.

[0053] Furthermore, the above method can also influence the steering system 1 in such a way that intervention by vehicle dynamics control systems via a brake or damper is more effective with less impact on the manual torque. This allows steering torque recommendations to be designed in such a way that, for example, braking interventions are minimized in order to stabilize the driving condition as quickly as possible at the limit.

[0054] Fig.4 shows, for an exemplary embodiment, a diagram of the curves of the rack force F_rack, the reference rack force F_rack_ref, the target engine torque M_motor_soll, the manual torque M_hand, and the rack force deviation A versus the steering angle Phi_LRW at a predefined vehicle speed v_vehicle. The curve of the rack force deviation A shows that at a steering angle Phi_LRW1, the rack force F_rack enters a non-linear range. To compensate for this behavior, starting at the steering angle Phi_LRW1, the adaptation variable K is adjusted such that a correspondingly modified manual torque is transmitted to the characteristic map block 102 in order to generate a correspondingly adjusted steering assistance force in the form of the target engine torque M_motor_soll.

[0055] In the illustrated embodiment, it can be seen that by modifying the manual torque M_Hand, the curve of the manual torque becomes linear beyond the non-linear range, so that the driver does not perceive the transition into the non-linear range haptically. Instead, the driver should be signaled to reach a steering angle at which maximum lateral acceleration occurs by a sudden change, i.e., an increase in the steering assistance force or a decrease in the manual torque. List of reference symbols 1 steering system 2 rack 3 wheels 4 first mechanical coupling 6 Steering assistance drive 61 Actuator 62 second mechanical coupling 7 Steering wheel 10 Control unit F_Rack Rack force F_Zahnstange_ref Reference rack force Phi_LRW steering angle M_Motor_Soll engine torque M_Hand hand torque M_Hand_mod modified hand torque Phv_Motor angular velocity Phi_Motor Position of the servo motor v_Fzg vehicle speed A Rack force deviation K Adjustment size Q shear force

Claims

[1] Method for determining a steering assistance force for a steering system (1) of a motor vehicle, comprising the following steps: - determining a steering assistance force based on a hand torque (M_Hand) applied by a driver; and - modifying the steering assistance force depending on a driving condition affecting a lateral force (Q) of the steering system (1), characterized by that the steering assistance force is modified in such a way that when a steering angle (Phi_LRW) is reached at which a maximum lateral acceleration achievable by the wheels on the road is achieved, the manual torque (M_Hand) to be applied is increased abruptly. [2] Method according to claim 1, characterized by that the driving condition is determined on the basis of a deviation (A) which is determined by the actual lateral force (Q) of the steering system (1) and a predetermined reference value. [3] Method according to claim 1 or 2, characterized bythat the driving state is determined on the basis of a deviation (A) of a rack force (F_rack) actually acting on a rack (2) of the steering system (1) due to the transverse force (Q) or a modeled rack force (F_rack) from a reference rack force (F_rack_ref) resulting from the vehicle speed (v_vehicle). [4] Method according to one of claims 1 to 3, characterized bythat the steering assistance force is determined from a modified manual torque (M_Hand_mod) according to a steering assistance characteristic map, wherein the modified manual torque (M_Hand_mod) is determined by applying an adaptation variable (K) to the manual torque (M_Hand), wherein in particular the adaptation variable (K) is determined based on a deviation (A) of a rack force (F_Rack) actually acting on a rack (2) of the steering system (1) from a reference rack force (F_Rack_ref) resulting from the vehicle speed (v_Vzg). [5] Method according to claim 4, characterized by that the adjustment quantity (K) is determined according to a predetermined adjustment function depending on the deviation (A). [6] Method according to one of claims 1 to 5, characterized bythat the steering assistance force is modified in a range of driving conditions in which the lateral force (Q) depends non-linearly on the steering angle (Phi_LRW) due to the friction behavior between the wheels and the road surface, such that the manual torque (M_Hand) to be applied in the range of driving conditions runs according to a predetermined course function, in particular at least in sections linearly, over the steering angle (Phi_LRW). [7] Method according to one of claims 1 to 6, characterized by that the steering assistance force is applied to the rack (2) or coupled into it. [8] Method according to one of claims 1 to 7, characterized by that the steering assistance force is provided by a motor torque (M_Motor) of a servo motor (61) of the steering assistance drive (6). [9] Control unit (10), in particular a steering control device, which is designed to determine a steering assistance force based on a hand torque (M_Hand) applied by a driver and to modify the steering assistance force depending on a driving condition affecting a lateral force (Q) of the steering system (1), characterized by that the steering assistance force is modified in such a way that when a steering angle (Phi_LRW) is reached at which a maximum lateral acceleration achievable by the wheels on the road is achieved, the manual torque (M_Hand) to be applied is increased abruptly. [10] Steering system (1) comprising: - a rack (2) connected to steered wheels (3), the rack (2) being subjected to a manual torque (M_Hand) exerted by a driver, - a control unit (10) according to claim 9; - a steering assistance drive (6) which is designed to apply the steering assistance force to the rack (2). [11] A computer program product which, when executed in a data processing device, carries out the method according to any one of claims 1 to 8.

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