Method for determining frictional resistance in a steering system for a motor vehicle

Abstract

The invention relates to a method for determining a frictional resistance in a steering system for a motor vehicle, comprising the following steps: a. by means of a steering actuator unit, outputting a test torque of increasing magnitude to a driver-side steering unit of the steering system; b. by means of a position sensor, detecting a change in position on the driver-side steering unit; c. by means of a computing device, when a predetermined position limit is reached, on the basis of the position limit and the simultaneously applied test torque or the values ​​of a torque sensor, determining the current frictional resistance; and d. in a data memory of a control device for the steering actuator unit, storing the determined frictional resistance and / or adapting a stored frictional resistance based on the determined frictional resistance for use in a future steering command. The method proposed here allows a steering system to be calibrated cost-effectively for a desired steering resistance.

Description

[0001] The invention relates to a method for determining a frictional resistance in a steering system for a motor vehicle, as well as a steering system with such a method for a motor vehicle.

[0002] Steering systems for motor vehicles always face the challenge of providing a desired steering feel to a driver-side steering unit (e.g., a steering wheel or stick). The requirements of the steering mechanism, crash safety, and the servo assistance commonly used today all play a role here. One approach involves considerable effort to design the steering system with as little friction as possible, so that the road forces acting on the vehicle's wheels can be felt as directly as possible in the driver-side steering unit. This entails high costs, especially since, due to production reasons, there is often a wide dispersion of friction in the steering system, for example, between 0.5 Nm [half a Newton meter] and 1 Nm, which is within the range of the desired steering resistance felt by a vehicle driver.

[0003] DE 10 2018 130 101 A1 relates to a feedback actuator for a direct-drive steering device, with an outer housing arrangement, comprising a steering spindle for coupling a motor shaft to a steering wheel, wherein the steering spindle is mounted in the housing arrangement so as to be rotatable about its longitudinal axis and extends from its connection for the steering wheel to the motor shaft, a bearing for mounting the steering spindle, an optional friction element for generating a basic friction of the feedback actuator, a locking device for functions which require an active actuating torque greater than a defined steering torque of a motor, an angle sensor system for determining the rotor position for controlling a motor and a motor with a motor shaft for actively setting a defined steering torque in a switched-on operating state of a vehicle.The disclosure enables a steering device with a realistically simulated steering feel, wherein the feedback actuator is designed to be as free from play as possible.

[0004] DE 10 2019 119 815 A1 relates to a system for determining a restoring torque for a manual force actuator of a steer-by-wire steering system of a vehicle, which system comprises at least one sensor system for detecting road-based load data on a wheel unit and a control system for evaluating this data in order to determine the restoring torque for the manual force actuator.

[0005] Based on this, the present invention is based on the object of at least partially overcoming the disadvantages known from the prior art. The features of the invention are derived from the independent claims, for which advantageous embodiments are presented in the dependent claims. The features of the claims can be combined in any technically reasonable manner, whereby the explanations from the following description as well as features from the figures, which comprise additional embodiments of the invention, can also be consulted for this purpose.

[0006] The invention relates to a method for determining a frictional resistance in a steering system for a motor vehicle, comprising the following steps: a. by means of a steering actuator unit, outputting a test torque of increasing magnitude to a driver-side steering unit of the steering system; b. by means of a position sensor, detecting a change in position on the driver-side steering unit; c. by means of a computing device, when a predetermined position limit is reached, on the basis of the position limit and the simultaneously applied test torque or the values ​​of a torque sensor, determining the current frictional resistance; and d. in a data memory of a control device for the steering actuator unit, storing the determined frictional resistance and / or adapting a stored frictional resistance based on the determined frictional resistance for use in a future steering command.

[0007] Unless explicitly stated otherwise, ordinal numbers used in the preceding and following descriptions serve only to clearly distinguish them and do not reflect the order or ranking of the designated components. An ordinal number greater than one does not necessarily imply that another such component must be present.

[0008] The method proposed here can be used for both a mechanical steering system and an electronic steering system (a so-called steer-by-wire system). With a steer-by-wire system, an additional problem is providing the maximum required steering resistance (for example, for a driver to hold the steering wheel when entering and / or exiting the vehicle). In a steering system, both in a steer-by-wire system and in a mechanical steering system, additional components are provided in addition to a steering actuator unit, such as an active or passive braking device. Such additional components, however, in turn, influence the overall system friction.

[0009] The approach here is to individually determine the frictional resistance in each individual steering system in order to derive functional possibilities based on which a desired, similar steering behavior can be experienced on the driver-side steering unit. According to another aspect, this enables the precise, hand-torque-based detection of driver activity, as discussed, for example, in DE 10 2019 117 209 A1.

[0010] It should be noted that reference is always made here to a (torque) moment. However, steering units are also known in which (partially or exclusively) translational movements can be performed. In such a case, instead of (torque) moments, (translational) forces are exerted at least in part. However, this also applies within a steering system to a possibly translational actuator (i.e., the force generation), as well as to the translational force transmission (in a transmission) and / or force output (at the driver-side steering unit). In such a case, the term (torque) moment can be replaced by force.

[0011] The steering system used in this method can be used or is used in a motor vehicle and comprises at least the following components: - a driver-side steering unit for receiving steering commands from a vehicle driver, preferably also from a computer of a vehicle assistance system (FAS, or ADAS, English: Advanced Driver Assistance System) for (partially or fully) autonomous driving of the motor vehicle. The driver-side steering unit is, for example, a steering wheel, a (steering) stick, or a steering horn. - A steering actuator unit for outputting steering torque to the driver-side steering unit. In a mechanical steering system, the steering actuator unit is, for example, part of a servo-assisted system. In a steer-by-wire system, the steering actuator unit is part of a force feedback actuator (FFA), which simulates road forces for the driver. - a position sensor for detecting a change in position on the driver-side steering unit. In one embodiment, the position sensor is configured solely to detect a (relative) change in position. Preferably, the position sensor is configured to detect an absolute position (or position) of the driver-side steering unit. In the case of a steering wheel (or also partly in the case of a steering horn), the position sensor is, for example, an angle sensor, and a change in position is then a change in angle, and a position is an angular position (or absolute angular position). In the case of a stick (and partly in the case of a tiltable steering horn), it is accordingly the position of an angle of inclination. If there is no possibility of directly measuring the position, for example when using a direct drive as the steering actuator unit, it is alternatively possible to infer the position of the driver-side steering unit based on the existing sensors of the steering actuator unit. - a computing device for determining the current frictional resistance. The computing device is a component of a computer (e.g., an onboard computer system [OCS] or a high-performance computing device for a modern ADAS), a (e.g., local) microcontroller, or comprises a topologically defined computing path for this computing operation, for example, within a specialized control device. - a data storage unit for storing a determined frictional resistance and a target steering resistance against a position change on the driver-side steering unit. The target steering resistance includes all components of the overall steering feel application, which may be provided via additional steering functions (not discussed in detail here). For example, the data storage unit is configured to store a single frictional resistance and a single target steering resistance. Alternatively, this data is stored as part of a functional formula for a position-dependent calculation and / or in a look-up table (LuT), preferably in conjunction with other parameters, such as position, temperature, and / or a known deviation influence (e.g., due to aging and / or usage).

[0012] The procedure is performed, for example, in alphabetical order. Preferably, steps a. and b. are performed simultaneously or overlapping in time. Preferably, the measurement for step b. is started and then step a. is started.

[0013] In step a., a test torque is first output by the steering actuator unit, whereby this is output in an increasing manner (preferably discretely and / or ramp-like). The aim is to increase the test torque until a change in position can be detected on the driver-side steering unit. The test torque is therefore recorded so that the value at which such a change in position is triggered is known. It should be noted that in a simple embodiment, the method is only carried out when a vehicle driver is not issuing a steering command (i.e., a manual force) to the driver-side steering unit. To ensure this, in one embodiment the method is carried out exclusively in a secure environment, for example in a factory under expert supervision in a production station and / or quality assurance station.Alternatively or additionally, such a secured environment is when no driver is sitting in the motor vehicle (for example, ensured by seat sensors and / or driver sensors such as cameras) and / or the motor vehicle is locked and out of operation (but in a standby mode).

[0014] In step b, a position change is now detected (triggered by the test torque or already started previously or (quasi-) continuously monitored) in relation (e.g., via a time interval) to the current value of the applied test torque. It should be noted that the relationship does not need to be known to the position sensor, but rather, for example, only to the computing device (due to simultaneous provision of the data) or to the control device, which is then also configured to control the computing device.

[0015] For example, the detection of a change in the position of the driver's steering unit (e.g., turning the steering wheel or tilting a joystick) is only enabled when the procedure is being executed (e.g., in a secure environment). Alternatively or additionally, the position sensor is operated in a mode specifically configured for this procedure, for example, for a higher resolution than in another operating mode.

[0016] In step c., the triggering event (i.e., the condition) is that the detected position change has reached a predetermined position limit. For example, such a position limit is provided taking into account any play in the steering system, thus preventing a (too) small test torque from being used to determine frictional resistance. Alternatively or additionally, such a position limit is specified to exclude effects due to any softness in the steering system and / or to ensure that movement has actually occurred. Preferably, the position limit is predetermined in such a way that it is ensured that (initial) static friction is overcome and dynamic friction is detected. The values ​​for static friction and dynamic friction determined in this way can thus be saved and used as separate values ​​(or separate data contents) if required.

[0017] In one embodiment, the frictional resistance (i.e., its value) is calculated purely mathematically based on the test torque present at the time the position limit is reached, and then stored in step d. (e.g., as a torque value) in the data memory or used to adjust a stored frictional resistance. Alternatively, additional influencing factors are taken into account, such as known play, stick-slip effects, elasticity (i.e., softness) in the steering system, and / or an (empirically and / or mathematically known) influence due to the deviation between a measuring location (e.g., in the steering actuator unit) and the driver-side steering unit. It should be noted that any necessary conversions are not necessarily performed because, for example, they are already explicitly or implicitly accounted for in the subsequent calculations or controller outputs.

[0018] In one embodiment, the method is at least partially repeated, with a plurality of frictional resistances being determined. In one embodiment, an average value is calculated from this. In one embodiment, a previously set or determined frictional resistance is replaced by the new frictional resistance. Alternatively, due to a deviation between the previous frictional resistance and the new frictional resistance, a correspondingly gradual change is made to the stored (previous) frictional resistance, and this changed frictional resistance is stored as the new frictional resistance, i.e. the stored frictional resistance is adjusted. Steps a. to c. are then repeated, and step d. is only carried out once at the end. Alternatively or additionally, the majority of the determined frictional resistances are temporarily stored and then discarded, for example using a volatile (working) memory for data.

[0019] It is further proposed in an advantageous embodiment of the method that in step a. a test torque is output at least once for a first steering direction and subsequently at least once for an opposite second steering direction, preferably repeatedly, wherein the control device is preferably calibrated using the method.

[0020] It should be noted that in one embodiment, a test torque is applied once in a first steering direction and (optionally immediately) subsequently applied once (or multiple times) in the opposite second steering direction. In another embodiment or a further repetition of the method, a test torque is applied multiple times in a first steering direction and only then (optionally immediately) subsequently applied once (or multiple times) in the opposite second steering direction. It should also be noted that in one embodiment, the method is repeated in suitable time windows, for example in time windows in which a journey or a driver is not disturbed. These time windows may be spaced far apart in time and / or not at a fixed time interval from one another.The test moments in the same and / or different steering directions are then scheduled, for example, at almost random times and / or in almost random order.

[0021] It should also be noted that the process or some steps of the process are or are carried out repeatedly, for example in a production line, during maintenance or during operation of the motor vehicle.

[0022] In one embodiment, it is not intrinsically guaranteed that the frictional resistance in the steering system is the same regardless of direction. Therefore, it is advantageous to perform a measurement for each direction according to this method. A rest period, preferably of several minutes, is preferably inserted between the opposing movements so that static friction is reliably restored, which significantly (co-)determines the maximum frictional resistance. Due to hydrodynamic effects, sliding friction may immediately develop if a test torque acting in the opposite direction is applied immediately afterwards. Furthermore, the recorded values ​​can be influenced by play and inertia.

[0023] As already mentioned above, the method is preferably carried out to calibrate the steering system. In one embodiment, the determined frictional resistance is used solely to (permanently) mechanically calibrate the steering system, for example, if the frictional resistance deviates too significantly, to change it by tightening or loosening a tension of an adjusted bearing and / or a (for example, passive) braking device. The determined frictional resistance is preferably used in control engineering as a control coefficient in order to output a corresponding actuating torque of a steering actuator unit and / or a braking torque of an active braking device in a correspondingly adjusted manner. For example, a steering resistance (for example, with a steering wheel or stick) of 2 Nm [two Newton meters] is desired. An (individually) determined frictional resistance is, for example, 0.6 Nm.Then, a braking resistance of 1.4 Nm is applied (possibly independent of direction) by means of an active braking device. An (individually) determined frictional resistance of another (identical) steering system is, for example, 2.7 Nm. Then, a supporting actuating torque of 0.7 Nm is applied (direction-dependent) by means of the steering actuator unit. It should be noted that (as already mentioned) frictional resistance can vary for different positions of the driver-side steering unit. For this purpose, it is particularly advantageous to use the frictional resistance as a control coefficient in a control system (via the control device). In addition, other effects (such as a non-intuitive mechanical irregularity) can then be compensated for, for example, if the pivot point of the steering system is not the same as the center of gravity of the steering wheel.

[0024] It is further proposed in an advantageous embodiment of the method that the method is carried out at several different positions of the driver-side steering unit, wherein the different positions are preferably approached by means of the steering actuator unit, particularly preferably discretely.

[0025] In one embodiment of a steering system, due to the design, different values ​​for the frictional resistance at different positions cannot be intrinsically excluded or are even certain to occur. In one embodiment of a steering system, different values ​​are intrinsically necessary depending on the position (for example, a predetermined steering angle for a steering wheel), for example due to a desired change in the gear ratio in a transmission. The method proposed here can be used to perform a calibration that compensates for such effects (as well as assembly tolerances), resulting in a uniform steering resistance for the driver that corresponds as closely as possible to the desired target steering resistance.

[0026] It should be noted that, in one embodiment, a position-dependent variable steering resistance is desired. The corrective actuating torque is therefore not necessarily constant. It should also be noted that such a change in steering resistance is not necessarily position-dependent, but rather situation-dependent. For example, the steering resistance increases with increasing vehicle speed. This allows for low steering resistance to be applied at low speeds for easy maneuverability when maneuvering, and for increased steering resistance to provide supportive feedback to the driver to avoid dangerous steering angles at high speeds.

[0027] In one embodiment, a steering position is set manually, for example, using a gauge in the production line or during maintenance. Preferably, the steering actuator unit is used to move to the respective positions, so that the process is carried out (at least largely) autonomously. Preferably, discrete positions (i.e., positions spaced apart by a defined distance from one another) are approached, with the frictional resistance values ​​at the intermediate positions preferably being determined by interpolation or extrapolation. Alternatively, if the deviation between the measured values ​​is sufficiently small, the system assumes the frictional resistance value of one of the adjacent positions.

[0028] It is further proposed in an advantageous embodiment of the method that in step c. previously known influencing factors are taken into account, and / or a requested target braking torque of a braking device is compared with an actual braking torque.

[0029] As already mentioned, this method significantly increases the degree of freedom in the design of a steering system. This applies both to the design and to feedback to the driver via the driver-side steering unit, which deviates from mechanical influences. Pre-known influencing factors include, for example, play, a stop angle, a center of gravity position, and / or locally varying but known friction (which may be desired or accepted).

[0030] It is further proposed in an advantageous embodiment of the method that before step a. it is ensured that no manual force or other disturbing influence is applied to the driver-side steering unit, wherein a plausibility check between the test torque and the determined frictional resistance is preferably carried out by the computing device and / or the control device.

[0031] To ensure that the measurement according to the method can be carried out easily and with minimal disruption, it is advantageous if no hand or manual force is applied to the driver-side steering unit, or if such a condition can be sufficiently reliably excluded during the measurement. For example, a capacitive sensor in the area of ​​the handle of a driver-side steering unit is used to detect contact with the steering unit or to reliably rule out such contact. Alternatively or additionally, a seat sensor (and preferably also a door closure sensor) can be used to detect or reliably rule out the presence of a vehicle driver at the driver-side steering unit, thus also reliably ruling out the application of manual force.Alternatively or additionally, a vehicle driver is monitored by means of a (further) driver sensor (e.g. an interior camera) and contact or even the introduction of a manual force is reliably excluded by means of image recognition.

[0032] In one embodiment, the determined frictional resistance is additionally checked for plausibility, for example by comparing it with a predetermined range between a maximum or minimum expected limit value. If the friction coefficient is not within the range, the measurement was not carried out correctly or there is a high degree of certainty an error that can be corrected mechanically (for example, incorrect assembly). Alternatively or additionally, direction-dependent and / or position-dependent recorded values ​​of the frictional resistance are compared with one another. If a predetermined difference is exceeded, the measurement is repeated at the outlier value and / or an error message is output. Alternatively or additionally (preferably during ongoing operation), an outlier value is ignored and, preferably during operation, a rate of change of the position change at the position of the outlier value is recorded.The recorded rate of change can be used to determine whether the frictional resistance there actually deviates, as measured, from the values ​​for the other steering direction or other positions (in comparison, an increased rate of change corresponds to lower frictional resistance, and a reduced rate of change corresponds to increased frictional resistance). It should be noted that the outlier value preferably lies within the previously mentioned range, so that the deviation may be noticeable but not safety-critical. However, an outlier value is preferably considered impermissibly critical if, together with the corrective actuating torque (from the steering actuator unit and / or the braking device), an additive or supporting torque results, i.e., no steering resistance.This is the case, for example (simplified for clarity), when (here, for example, with a steering wheel or a stick) a frictional resistance is moved in a range of 2.4 Nm to 2.6 Nm for the other positions and then, for a target value of around 2 Nm, a (direction-dependent but constant) corrective actuating torque of a subtracted 0.5 Nm is output. At the position of the out-of-bounds value, there is a frictional resistance of, for example, 0.3 Nm. The predetermined range is, for example, 0 Nm to 3 Nm. The out-of-bounds value therefore lies within this range. The result, however, is negative steering resistance, i.e. an assisting torque of -0.2 Nm. This would lead to a comparatively increased rate of change in position and could possibly be perceived as annoying by a vehicle driver.This (theoretical) result should therefore be used to check whether this outlier value has actually been determined correctly and, if so, to possibly result in a review of the assembly or even trigger a so-called red flag, i.e. a check of the component series for potentially recurring causes of errors.

[0033] It is further proposed in an advantageous embodiment of the method that at least during steps a. and b. at least one of the following sensors is used: - a temperature sensor for detecting the temperature of the environment and / or the steering actuator unit; - a torque sensor for detecting a torque at the driver-side steering unit and / or between the driver-side steering unit and the steering actuator unit; - a touch sensor for detecting a driver touch on the driver-side steering unit; - an optical driver sensor for detecting the presence of a vehicle driver, and preferably his behavior, at the driver-side steering unit; and - at least one component sensor of an active component of the steering system, for detecting a state of the component in question.

[0034] It should be noted that the sensors mentioned and their measured values ​​are used exclusively during the procedure proposed here or exclusively for the later use of the determined frictional resistance, for example as a correction factor in an active control of the steering resistance.

[0035] In one embodiment, the frictional resistance is measured under factory conditions or without considering boundary conditions in the current environment. These boundary conditions are preferably recorded by sensors and taken into account when the frictional resistance is subsequently used in a future steering command.

[0036] When using a lubricant, it's advantageous to measure the temperature. However, even with tight tolerances or a potential heat source in the steering system area, temperature is a significant factor in frictional resistance.

[0037] In one embodiment, (usually already existing) sensors from components of the steering system are used, such as a temperature sensor, torque sensor, or position sensor in the steering actuator unit. The same applies to a bearing and / or a braking device, but also to mechanical transmission components, such as a torsion bar or a steering gear. Significant changes in (partial) frictional resistance may be expected here, for example due to temperature-related jamming and / or a wear-related change in internal friction (for example, of a bearing or a sealing component). In one embodiment, the measured values ​​from the component sensors are used directly or as data processed with regard to their contribution to the frictional resistance of the steering system.

[0038] In one embodiment, it is advantageous to detect a torque applied to the driver-side steering unit independently of the applied test torque. In one embodiment, this is used to detect manual force on the driver-side steering unit. In one embodiment, a test torque is detected solely by control technology (e.g., by means of an engine control unit or via an output value from the control device), and the actual output torque is determined via the separate torque sensor.In yet another embodiment, a mechanical steering system provides a defined elasticity (i.e., softness) (for example, in a steering wheel or stick, using a torsion bar between the driver-side steering unit and the steering actuator unit), so that when an increasing test torque is output, a different torque can be present at the driver-side steering unit precisely when a position change occurs. This actually applied torque is then used alone or in addition to determine the frictional resistance.

[0039] To reliably prevent a hand or manual force from being applied to the driver's steering unit while the method is being performed, it is also proposed that a touch sensor be provided. Such a touch sensor is, for example, an electrical (e.g., resistive or capacitive) sensor and / or a (possibly capacitive) displacement sensor that detects deformation in the area of ​​the touch sensor.

[0040] To ensure that a driver cannot interfere with the measurement or even interfere with it, it is also proposed that a driver sensor be included. Such a driver sensor could be, for example, an interior camera, a seat occupancy sensor, a door lock sensor, and / or a series of sensors that, either individually or when evaluated together, allow some kind of conclusion to be drawn about the presence of a driver at the driver-side steering unit. Alternatively or additionally, a driver is monitored using a driver sensor, so that, for example, the location of the driver's hands is recorded (in this case, ideally not on the driver-side steering unit) and / or what the driver's next action is most likely to be (for example, when searching for a parking space, a possible intervention in the driver-side steering unit is to be expected).Assisted driving, in which the driver does not have a hand on the driver-side steering unit, is particularly well-suited for performing repeated measurements while driving using the described procedure. In a mechanical steering system, the steering angle is kept small enough so that the movement of the driver-side steering unit has no noticeable influence on the vehicle's handling, or the driver-side part of the steering system is decoupled from the (wheel-side) steering gear. As with a steer-by-wire system, any steering angle can be set during testing.

[0041] It is further proposed in an advantageous embodiment of the method that the method is carried out repeatedly during operation of the steering system and the stored frictional resistance is replaced or adapted by a currently determined frictional resistance.

[0042] In one embodiment, the method is repeatedly performed before commissioning, for example, several times during calibration and / or at least a second time during a (for example, 100%) quality control. Preferably, the method is repeatedly performed during operation in a motor vehicle, for example, to detect emerging signs of wear and to be able to offer the driver a (sufficiently) constant steering resistance over the service life despite changing frictional resistance. Alternatively or additionally, such a repetition is used for an IoT application [Internet of Things] to plan required maintenance or for quality assurance.

[0043] It should be noted that an adjustment may mean that neither the originally stored frictional resistance is retained nor the newly determined frictional resistance is stored as a replacement. For example, instead, a (gradual) change is made and a frictional resistance (replacing the old one) is stored that is determined using a predetermined algorithm. The algorithm may, for example, be machine-learned, empirically determined, and / or stored as a formula or in a look-up table (LuT).

[0044] It is further proposed in an advantageous embodiment of the method that at least one temperature is permanently monitored by means of at least one temperature sensor, wherein a stored friction resistance for its use in a steering command is scaled depending on the temperature, and / or During operation, the process is repeated at different temperatures and a determined current frictional resistance is stored in the data memory of the control device as a function of temperature.

[0045] In this embodiment of the method, a current temperature can be considered as a key influencing factor. As already mentioned, under certain circumstances, for example, the viscosity of a lubricant can be strongly dependent on the current temperature, and thus, in turn, a (partial) frictional resistance in a bearing and / or transmission can vary depending on the temperature. It should be noted that in one embodiment, the temperature sensor is part of the steering system, for example, as a separate component and / or as a component sensor (as previously described). Alternatively or additionally, a temperature sensor of another vehicle component, for example, an interior temperature sensor for operating an air conditioning system, or its data, is used.

[0046] In a simple embodiment, a frictional resistance has been determined as described above (possibly under standard conditions). Based on known (empirical, possibly expressed in formulas or recorded in a look-up table [LuT]) relationships between the current temperature and the frictional resistance, the current frictional resistance is adaptable, i.e., the value is scalable. For example, the frictional resistance decreases due to increasing viscosity as a result of a temperature rise, and conversely, increases with a falling temperature. In another embodiment, thermal expansion resulting in clamping is not negligible, in which case, conversely, the frictional resistance increases with a temperature rise and decreases with a falling temperature.

[0047] It should be noted that when temperature-dependent storage of a determined frictional resistance occurs, a plurality of values ​​are stored, each of which is assigned to a temperature value. In one embodiment, such a relationship is specifically implemented for one or more test steering systems for a desired temperature range, and a look-up table (LUT) is created or a formula is generated, which is then stored in the data storage of the (identical) steering systems. In one embodiment, the relationship is determined for each individual steering system (in the production line) and stored accordingly in the data storage.

[0048] According to a further aspect, a steering system for a motor vehicle is proposed, comprising at least the following components: - a driver-side steering unit for receiving a steering command from a vehicle driver; - a steering actuator unit for outputting a steering torque to the steering unit; - a position sensor for detecting a change in position on the driver-side steering unit; - a computing device for determining a current frictional resistance; and - a data memory for storing a determined frictional resistance on the driver's side steering unit, wherein the steering system is configured to carry out a method according to an embodiment as described above, wherein the steering system is preferably a steer-by-wire system.

[0049] It should be noted that the method described above can be implemented with the steering system described herein, but also with a different steering system. The steering system proposed here is only advantageously designed for the method proposed here.

[0050] The steering system is configured for the method according to an embodiment of the preceding description. Reference is made to the description regarding the components and relationships, at least purely optionally. Conversely, the method steps described below and the associated explanations are also at least optional components of the method described herein. It should be noted that while the steering system described herein is suitable for implementing the method for determining frictional resistance, further and / or alternative methods can also be implemented therewith.

[0051] The driver-side steering unit is configured to receive a steering command from a vehicle driver. In one embodiment, this steering unit can be used both by a vehicle driver and by a computer of a vehicle assistance system (FAS, or ADAS, English: Advanced Driver Assistance System) to steer the motor vehicle. In a steer-by-wire system, upon receiving a steering command, a signal is generated via the driver-side steering unit, which forwards the command, preferably via the steering actuator unit, in order to navigate the motor vehicle accordingly. The driver-side steering unit thus provides an interface between the vehicle driver and / or feedback (preferably correctable or overridable via the driver-side steering unit) of the actions of the FAS (in autonomous driving) of the motor vehicle and the mechanical control system of the motor vehicle.

[0052] The steering actuator unit is configured to output a steering torque to the steering unit. The steering actuator unit is configured to output the necessary torque, in one embodiment to implement a steering command and / or to provide the driver with feedback about the current steering command as a force feedback actuator (FFA) system, as well as the influences (e.g., road-related) on the steering of the motor vehicle. The steering actuator unit implements (as previously described) the compensation of frictional resistance for a desired steering feel.

[0053] The position sensor is used to detect a change in position on the driver-side steering unit. Movements are detected using the position sensor, and the control unit is able to react to them. It should be noted that not only the steering commands entered by a vehicle driver are detected here, but also the effects that the torque output of the steering actuator unit has or should have on the driver-side steering unit (preferably with a measuring position close to and / or in the driver-side steering unit) (for example, when measuring engine torque in the steering actuator unit, assuming an intact transmission system up to the driver-side steering unit). Using the position sensor, it is possible to obtain precise data about the position and / or movement of the driver-side steering unit, which is necessary for the control unit and thus for the steering process and feedback to the vehicle driver.This ensures precise adjustment of the perceived steering resistance for the driver.

[0054] The data memory is configured to store one (or more) of the determined frictional resistances and preferably also to maintain a target steering resistance against a change in position of the driver-side steering unit. It should be noted that in one embodiment, the target steering resistance is not a simple constant value, but rather a function dependent on the current position of the driver-side steering unit and / or the driving situation (e.g., speed, surface conditions, and / or driving mode). The data memory thus contains these values ​​so that, for example, the correct differential torque can be output upon a steering command.

[0055] The computing device is configured to determine a current frictional resistance. The computing device is configured to process the movement data detected by the position sensor and the steering torque output by the steering actuator unit in order to precisely determine the frictional resistance. Based on this, in one embodiment, an appropriate differential torque is output upon a steering command, thus enabling optimal feedback on the steering behavior of the motor vehicle. It should be noted that in one embodiment, the method is used solely to detect errors during the assembly of an individual steering system, whereby, for example, specific instructions can be generated on how a current frictional resistance can be changed to a desired value.

[0056] In one embodiment, the steering system is designed as a steer-by-wire system, which replaces the mechanical connection between the driver-side steering unit and the steerable vehicle wheels of the motor vehicle. Instead of a mechanical torque transmission, driver-side steering commands are transmitted electronically by sensors detecting movements on the driver-side steering unit and forwarding them to the corresponding wheel steering actuators to influence the wheel position. This enables a high degree of flexibility and / or increased precision in the control of the motor vehicle because the directness and responsiveness of the system can be tailored and / or adapted to the current physical driving condition of the motor vehicle using software parameters instead of mechanical components.Another advantage is that the package is highly adaptable (for example, for interior design), high crash safety can be achieved, and a variable steering ratio can be set. A disadvantage of a steer-by-wire system, however, is the lack of mechanical feedback of road forces. However, these can be simulated for the driver using the previously described force feedback actuator (FFA). The method proposed here for determining frictional resistance is particularly advantageous for a cost-effective steering system design and, at the same time, a driver-side steering unit that is as realistic as possible and configured for the desired steering feel.

[0057] In one embodiment, such a control device is a component of the steering system and is configured to control the steering actuator unit based on a stored frictional resistance for a current steering command, taking into account a frictional resistance for a desired (e.g., constant) steering resistance. The control device uses the frictional resistance value stored in the data memory to adjust the steering torque output by the steering actuator unit accordingly. For example, a differential torque is output as the steering torque to generate a desired target steering resistance, resulting from the sum of this steering torque and the frictional torque resulting from the determined frictional resistance, in order to thus generate an optimal steering feel. This enables precise feedback to the driver and contributes to modulating the vehicle's steering behavior according to the driving conditions.The integration of such a control device into the steering system enables improved vehicle control and increases safety and driving comfort.

[0058] It is further proposed in an advantageous embodiment of the steering system that the steering actuator unit further comprises at least one of the following components, which is provided in the steering system to achieve a required maximum steering resistance in the steering system: - a passive braking device; - an active braking device controllable by the control device; and - a gearbox.

[0059] In one embodiment, the steering actuator unit comprises a passive braking device by means of which a desired steering resistance can be set or adjusted (at least approximately).

[0060] In one embodiment, the steering actuator unit comprises a gear by means of which a desired gear ratio and / or a desired steering resistance can be set or adjusted (at least approximately).

[0061] In one embodiment, the steering actuator unit of a steering system comprises an active braking device that can be controlled by the control device and is configured to influence steering resistance in the steering system. One embodiment of such a braking device is a so-called magnetic powder brake, which uses the (actively controllable) magnetic field to generate friction that can be influenced thereby, and thus a controllable braking resistance. When the braking device is activated, the steering resistance in the steering system increases or decreases depending on the requirements of the steering process and the specifications of the control device.

[0062] In one embodiment, the steering system alternatively or additionally comprises a blocking device (lock) that positively locks the driver-side steering unit, for example, when the motor vehicle is inactive (e.g., the parking lock is engaged and the ignition key is removed). This allows a maximum motor torque of the steering actuator unit and / or the maximum braking torque of a braking device to be significantly reduced if a very high maximum torque is required, as is the case when the vehicle driver supports himself against the driver-side steering unit when getting in and / or out of the vehicle.

[0063] The invention described above is explained in detail below against the relevant technical background with reference to the accompanying drawings, which show preferred embodiments. The invention is in no way limited by the purely schematic drawings, whereby it should be noted that the drawings are not to scale and are not suitable for defining proportions. It is shown in Fig. 1: three diagrams of frictional resistance over time; Fig. 2: a control scheme of a steering system; Fig. 3: a flowchart of the method for determining frictional resistance in a steering system for a motor vehicle; and Fig. 4: a schematic plan view of a motor vehicle with steering system.

[0064] In Fig. 1 shows three diagrams of frictional resistance 1 versus time 36. In the top diagram, a target motor torque 29 is plotted against time 36. The target motor torque 29 is ramped and continuously increasing (referred to here as test torque 5), initially for a first steering direction 13 (e.g., to the left) and then in the opposite steering direction 14 (correspondingly, for example, to the right).

[0065] In the middle diagram, a steering angle 35 is plotted against time 36. Here, it can be seen that the steering angle 35 does not react when the test torque 5 is (too) low, and only changes when the test torque 5 is sufficient. Here, the method is carried out in such a way that a position limit value 10 determines when the maximum test torque 5 is reached, i.e., when the ramped increase in the test torque 5 ends (and falls back to zero). Here, a preloaded driver-side steering unit 6 (for example, via a torsion bar) forms the basis (purely optional), so that the steering angle 35 automatically returns to its initial position when the test torque 5 is removed, resulting in a course of the position change 8 without external energy supply as shown.

[0066] The lowest diagram shows the determined frictional resistance 1, which (in this representation, simplified purely for the sake of clarity) can be calculated from the test torque 5 and the steering angle 35. An edge can be seen here, which represents the jump from the (greater) static friction 38 to the (lower) sliding friction 39. On the return path due to the preload (see above), the frictional resistance 1 is not calculated and therefore not shown.

[0067] In Fig. Figure 2 shows a control diagram of a steering system 2 for determining a frictional resistance 1 in a steering system 2 for a motor vehicle 3. At the center of the diagram is the steering actuator unit 4, which is responsible for outputting a steering torque 22, as well as the driver-side steering unit 6 (shown here as a steering wheel) or the position sensor 7 assigned to the steering unit 6. The steering actuator unit 4 is connected to the control device 12 for signal transmission. The control device 12 has the task of controlling the steering actuator unit 4 to reach a position limit value 10 on the driver-side steering unit 6 in order to determine the frictional resistance 1 therefrom (compare the previously described diagrams of the Fig. 1).

[0068] The steering actuator unit 4 is configured to output a steering torque 22. This steering actuator unit 4 is connected (here purely optionally via a transmission 28) to a driver-side steering unit 6, which is configured to receive steering commands from a vehicle driver. The driver-side steering unit 6 is equipped with a position sensor 7, which detects a change in position 8 of the driver-side steering unit 6. Furthermore, a computing device 9 is provided, which is used to determine a current frictional resistance 1. This frictional resistance 1 is stored in a data memory 11 so that the control device 12 can access it to control the driver-side steering actuator unit 4.

[0069] Here (purely optional) an embodiment is shown in which additional sensors are used: For example, at least one (preferably one for each task) temperature sensor 15 is configured to detect the temperature 16 in the environment and / or on the steering actuator unit 4 or an additional component temperature 32 of other additional components 31 of the steering system 2.

[0070] Here (independently), a torque sensor 17 is also provided to detect a steering torque 22 output by the steering actuator unit 4 in the vicinity of the driver-side steering unit 6 and / or to detect a steering force input from a vehicle driver from a difference between the steering torque 22 of the steering actuator unit 4 and that of the driver-side steering unit 6. In one embodiment, the torque sensor 17 is used to detect an actual braking torque, for example, to adequately compensate for fluctuations in the operation of the (optional) braking device 24.

[0071] Here (independently of this), a (for example capacitive) touch sensor 18 is also provided to detect whether a touch 19 is currently present, i.e. whether the driver-side steering unit 6 is currently being touched by a hand of a vehicle driver and therefore a hand force 26 (or a torque resulting from the hand force 26) could be applied to the driver-side steering unit 6.

[0072] Here (independently), a driver sensor 20 (shown here as a camera for example) is also provided to detect the presence 21 of a vehicle driver. It should be noted that these sensors are already present and used, for example, for the safety of semi-autonomous driving. However, here they are preferably used to detect the absence or freedom of the driver-side steering unit 6 from interference while a test moment 5 is scheduled.

[0073] The curves of the desired motor torque 29 and the actual motor torque 30 are recorded by the steering actuator unit 4, while the motor position angle 33 and the motor position angular velocity 34 are also part of the display. An (optional) braking device 24 is integrated into the steering system 2, with which steering resistance (and thus the overall steering feel) can be influenced. This diagram illustrates the complexity and precise coordination of the components necessary to fulfill the control functions within a steering system 2.

[0074] In Fig. Figure 3 shows a flow chart of the method for determining a frictional resistance 1 in a steering system 2 for a motor vehicle 3. In step a., a steering actuator unit 4 outputs an increasing test torque 5 (purely optional, as for example in Fig. 1) to a driver-side steering unit 6 of a steering system 2. The test torque 5 is increased until a change in position 8 of the driver-side steering unit 6 can be detected by the position sensor 7. The steering actuator unit 4 thus begins with a low torque and gradually increases it (preferably in discrete steps) until a predetermined position limit value 10 of the driver-side steering unit 6 is reached. This determines at which point (i.e., at which test torque 5) the (static) friction in the steering system 2 is overcome.

[0075] In step b, a position sensor 7 detects a position change 8 on the driver-side steering unit 6. The position change 8 is a direct result of the test torque 5 output by the steering actuator unit 4. The position sensor 7 detects whether and to what extent the position of the driver-side steering unit 6 has changed.

[0076] In step c, a computing device 9 is used to determine the current frictional resistance 1. This occurs when the predetermined position limit value 10 is reached, based on the current position limit value 10 and the simultaneously applied test torque 5. The reached position limit value 10 indicates that a (sufficient) change in position 8 has occurred, thereby avoiding incorrect measurements in which, for example, an existing play (resulting in a frictional resistance 1 close to zero) is used as the basis for the determination. Alternatively or additionally, this ensures that static friction 38 is reliably exceeded and sliding friction 39 has been established.

[0077] In one embodiment (at least in a repeated execution of the method), steps a. to c. are first repeated, and in the process, a series of values ​​are determined, from which a suitable value is determined as frictional resistance 1 according to a predetermined algorithm (for example, by means of averaging and / or removing extreme values). In one embodiment, in a later repetition of the method, the previously determined and stored frictional resistance (or the determined values ​​on which it is based) is taken into account for forming the new frictional resistance 1 to be stored, for example, in order to form a new mean value.

[0078] In step d., the determined frictional resistance 1 is stored in a data storage unit 11. This data storage unit 11 is part of, or communicatively connected to, a control unit 12 for the steering actuator unit 4. The stored frictional resistance 1 is used for future steering commands. It enables the control unit 12 to more precisely adjust the required actuating torque or corrections in order to always achieve the desired steering feel or steering response.

[0079] In Fig.Figure 4 shows a schematic plan view of a motor vehicle 3 with a steering system 2, which comprises a drive unit 37 for propelling the motor vehicle 3. A left drive wheel 40 and a right drive wheel 41 are driven by the drive unit 37, which are connected to each other (purely optionally) via a differential 42. A driver-side steering unit 6 (shown here purely optionally as a steering wheel) is provided for steering the direction of travel of the motor vehicle 3. Steering commands from a vehicle driver (not shown here) can be received via the driver-side steering unit 6 and transmitted to the steerable front wheels 43, 44. The steering system 2 converts the steering commands into actual movements of the motor vehicle 3, influences the steering commands, and / or sets a desired steering feel for the vehicle driver. For this purpose, the steering system 2 comprises a computing device 9 with a data memory 11.The determined frictional resistance 1 is stored in the data memory 11 (if necessary, among other things) for retrieval for control intervention. Preferably, a frictional resistance 1 and / or an actual steering resistance are also continuously monitored and, if necessary, updated and stored.

[0080] The method proposed here allows a steering system to be calibrated cost-effectively for a desired steering resistance.

Claims

[1] Method for determining a frictional resistance (1) in a steering system (2) for a motor vehicle (3), comprising the following steps: a. by means of a steering actuator unit (4), output of an increasing test torque (5) to a driver-side steering unit (6) of the steering system (2); b. by means of a position sensor (7), detecting a change in position (8) at the driver's side steering unit (6); c. by means of a computing device (9) upon reaching a predetermined position limit value (10), based on the position limit value (10) and the simultaneously applied test torque (5) or the values ​​of a torque sensor (17), determining the current frictional resistance (1); and d. in a data storage device (11) of a control unit (12) for the steering actuator unit (4), storing the determined frictional resistance (1) and / or adapting a stored frictional resistance based on the determined frictional resistance (1) for use in a future steering command. [2] Method according to claim 1, wherein in step a. a test moment (5) is output at least once for a first steering direction (13) and subsequently at least once for an opposite second steering direction (14). [3] Method according to claim 1 or claim 2, wherein the method is carried out at several different positions of the driver-side steering unit (6). [4] Method according to one of the preceding claims, wherein in step c. known influencing factors are taken into account, and / or a requested target braking torque (25) of a braking device (24) is compared with an actual actual braking torque (27). [5] Method according to one of the preceding claims, wherein before step a. it is ensured that no hand force (26) and no other disturbance influence is applied to the driver-side steering unit (6). [6] Method according to one of the preceding claims, wherein at least during steps a. and b. at least one of the following sensors is used: - a temperature sensor (15) for detecting the temperature (16) of the environment and / or the steering actuator unit (4); - a torque sensor (17) for detecting torque at the driver's side steering unit (6) and / or between the driver's side steering unit (6) and the steering actuator unit (4); - a touch sensor (18) for detecting a driver-side touch (19) of the driver-side steering unit (6); - an optical driver sensor (20) for detecting the presence (21) of a vehicle driver at the driver-side steering unit (6); and - at least one component sensor of an active component of the steering system (2), for detecting a state of the component concerned. [7] Method according to one of the preceding claims, wherein the method is repeatedly carried out during operation of the steering system (2) and the stored frictional resistance is replaced or adjusted by a currently determined frictional resistance (1). [8] Method according to one of the preceding claims, wherein at least one temperature (16) is continuously monitored by means of at least one temperature sensor (15), wherein a stored frictional resistance (1) is scaled temperature-dependently for its use in a steering command, and / or the method is repeated at different temperatures (16) during operation and a determined current frictional resistance (1) is stored temperature-dependently in the data storage (11) of the control device (12). [9] Steering system (2) for a motor vehicle (3), comprising at least the following components: - a driver-side steering unit (6) for receiving a steering command from a vehicle driver; - a steering actuator unit (4) for outputting a steering torque (22) to the steering unit (6); - a position sensor (7) for detecting a change in position (8) on the driver's side steering unit (6); - a computing device (9) for determining a current frictional resistance (1); and - a data storage device (11) for storing a determined frictional resistance (1) on the driver-side steering unit (6), wherein the steering system (2) is configured to perform a method according to one of the preceding claims. [10] Steering system (2) according to claim 9, wherein the steering actuator unit (4) further comprises at least one of the following components, which is provided in the steering system (2) to achieve a required maximum steering resistance: - a passive braking device (24); - an active braking device (24) controllable by the control unit (12); and - a gearbox (28).

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