Steering system for a motor vehicle

Description

2. This power supply connects an electric control unit and an electric motor whose motor shaft is coupled to the steering spindle. During driving, the motor is electrically controlled by the control unit to couple motor drive torque corresponding to the real reaction torque into the steering spindle, in order to display a defined feedback torque at the steering handle, i.e., the steering wheel. Such "force feedback" systems give the driver the impression of a real driving situation, as with a conventional steering system, which facilitates an intuitive reaction. Due to unavoidable friction, the actual feedback moment acting on the steering handle is a frictional torque acting between the steering spindle and its bearing in the outer casing of the steering column, which is smaller than the drive torque generated by the motor of the drive unit and coupled into the steering shaft.In order to provide a defined feedback torque, WO2023 / 148883A1 proposes compensating for friction by increasing the coupled drive torque in advance by an amount equal to the friction torque. This is determined in advance by a model calculation and stored in the steering system 15. This estimation allows for improved feedback torque to be provided under normal operating conditions. However, if a deviation from the underlying assumptions occurs, for example due to wear, malfunctions, unfavorable environmental conditions, or the like, a mismatch of the feedback torque can occur. This can disrupt the steering feel, which can impair driving safety 20. In view of the problems discussed above, one task of the present invention is to enable improved adaptation of the feedback moment. 25 Description of the invention This task is solved according to the invention by the method for operating a steering system with the features of claim 1.Advantageous further developments arise from the dependent claims. 30 A method for operating a steering system for a motor vehicle, in which a drive torque for generating a predetermined feedback torque is coupled by a feedback actuator into a steering spindle rotatably mounted about its longitudinal axis in a housing unit, which is connected to a manually operable steering handle, wherein the drive torque for generating the feedback torque is increased by the amount of a predetermined and stored friction torque, which indicates the friction when the steering spindle in the housing unit is rotated, comprises according to the invention the steps: rotation of the steering spindle by the feedback actuator, 5 testing of hand contact of the steering handle, - If hand contact is detected during the test: Maintaining the stored frictional torque, - If no hand contact is detected during the test, - Measuring the friction between the steering spindle and the casing unit to determine a new frictional torque, - Storing the new frictional torque.The method according to the invention involves measuring the actual friction in the steering system, specifically the actual frictional torque occurring during operation, which opposes the rotation of the steering spindle caused by the drive torque. The frictional torque can be determined by direct or indirect measurement using suitable measuring methods. By checking the manual torque, it is possible to integrate the measurement into the operating sequence during the use of the motor vehicle. This makes it possible to continuously update the actual frictional torque and take it into account when generating the feedback torque. The friction is compensated by the measured and subsequently stored frictional torque, which is added to the desired feedback torque and thus taken into account when generating the drive torque.According to the invention, unlike in the prior art, this stored frictional torque can not only be estimated by theoretical model specifications, but can be continuously updated by real measurements during the operation of the vehicle, as explained below. 30 Specifically, the measurement of the real frictional torque according to the invention provides for a rotating drive of the steering spindle by coupling a defined drive torque through the feedback actuator. The concept of the measurement is based on the fact that, during free rotation of the steering spindle, only the actually acting frictional torque and the moment of inertia counteract the drive torque. In order for such free rotation of the steering spindle to be possible, 35 it is necessary that the steering handle is also freely rotatable and not manually turned or held in place. To ensure this prerequisite for the friction measurement, a check is carried out according to the invention to determine whether hand contact exists, i.e., a so-called hands-on situation is realized, or whether no hand contact exists, i.e., a so-called hands-off situation is present.Friction measurement is only enabled under the condition that no hand contact is detected.5 The test for hand contact can be carried out by detection using suitable sensors, which are known in principle and which can preferably be integrated into the steering handle. If hand contact is detected, free rotation of the steering spindle for determining the friction is not possible, so that no measurement is possible and the stored value 10 for the frictional torque is not updated. If no hand contact is detected during the test, the steering spindle is freely rotatable, and a frictional torque measurement can be carried out accordingly. For this purpose, the frictional torque, which actually counteracts the drive torque as a braking torque, is measured by the feedback actuator during the rotation of the steering spindle. This can be achieved using suitable sensors, in principle through known direct or indirect measurement methods for torque measurement. The actual measured value in this way is stored as a new friction torque and can subsequently be used for friction compensation.20 During the execution of the aforementioned procedure, the steering spindle is rotated by the feedback actuator, and in parallel or afterwards, the hand contact is checked to decide whether to allow the friction measurement. This makes it possible to react to hand contact during the measurement and, if necessary, to abort the measurement if a hands-on situation occurs. Alternatively or additionally, it is possible to perform the hand contact check before the start of the measurement and not to start the measurement if a hands-on situation is detected. The measurement can preferably be carried out during rotation at a constant rotational speed of the steering spindle. This eliminates inertia effects. 30 The invention allows real changes in friction during operation and over the service life to be recorded and taken into account. Unlike the prior art, where only an estimation is made, the invention enables near real-time or at least near-real-time quasi-real-time measurement of the actual acting frictional torque.To generate a predetermined feedback torque, the drive torque provided by the feedback actuator and coupled to the steering spindle can be adjusted to compensate for friction. Specifically, the magnitude of the drive torque can be set as the sum of the desired feedback torque and the determined friction torque. One advantage is that the feedback torque coupled to the steering handle can be optimized taking into account the current, real-world operating conditions, so that any impairment of the steering feel due to varying operating and environmental conditions can be effectively avoided. This increases driving comfort and safety over the vehicle's service life. 10 Another advantage is that monitoring the friction torque enables functional monitoring of the steering system. For example, a short-term, impulse-like change in friction can indicate a defect in a bearing or the drive.It is also possible that the increase or decrease in friction occurs due to wear or the like. By comparing current measurement data obtained according to the invention with predetermined reference values ​​and / or with a measurement data history, it is possible to continuously monitor the functional state of the steering system. This can increase the level of safety. It may be provided that the drive torque for generating a feedback torque 20 is calculated according to a predefined filter algorithm, taking into account the stored new friction torque. In principle, the feedback torque effectively coupled to the steering handle can be represented as the difference between the drive torque and the friction torque. To achieve this, the drive torque is increased to generate a required feedback torque. By doing this according to a predefined filter algorithm 25, which considers the operating-dependent torque transmission parameters, the feedback torque can be optimized. It is possible that the filter algorithm includes a Kalman filter.The use of a Kalman filter enables the modeling and consideration of friction parameters outside the measurement situation according to the invention, in which ideally only the drive torque and the friction torque act. This allows for an optimized adjustment of the drive torque to compensate for the friction torque occurring under real operating conditions due to the additionally applied hand torque. Furthermore, optimization with regard to different steering speeds and accelerations can be carried out.35 BE2025 / 5043 6 It can be provided that, before the rotation of the steering spindle, an estimation of the probability of a manual steering intervention is carried out based on at least one operating parameter. From certain operating parameters, it can be deduced how likely it is a hands-off situation. For example, when transitioning to autonomous driving mode, it can be assumed with relative certainty that the steering hand will be released, where hand contact is eliminated.Another example is getting into the vehicle, where a hands-off situation typically occurs between unlocking the vehicle and manual contact with the steering wheel, which can be used to measure the friction torque. Conversely, a hands-off situation also occurs when getting out of the vehicle. Furthermore, other such situations are conceivable during operation in which hand contact is highly unlikely and which can be used almost imperceptibly for the automated measurement and updating of the friction torque. It may be planned that the friction torque measurement takes place at predetermined time intervals. Test intervals can be specified for this purpose, for example, daily, monthly, or similar. The measurement of the frictional torque can be performed depending on operating parameters and / or environmental parameters. The measurement of the frictional torque can additionally or alternatively be initialized by internal and / or external parameters, such as temperature, operating time, number of actuation cycles, and the like.It can be advantageous to store a majority of measured friction torques. In addition to the value for the updated, new friction torque, measured values ​​obtained in earlier measurements can be stored as a measurement history. From this, the development of the friction torque over a longer period can be derived. This can be evaluated automatically in the vehicle and read out during maintenance. This makes it possible to detect potentially relevant deviations early and to initiate appropriate measures. 30 It is advantageous that the frictional torque is determined by measuring the current of an electric drive of the feedback actuator. The feedback actuator has a drive with an electric motor, which can be coupled directly or via a gearbox to the steering spindle. The motor current is correlated with the motor torque delivered by the motor, and accordingly with the drive torque. Because the drive torque is slowed down by the frictional torque during measurement 35, a current measurement enables a determination of the frictional torque. BE2025 / 5043 7Additionally, the rotational speed, i.e., the rotation of the steering spindle, can be measured, for example, using rotational sensors that are already present for detecting the manual steering input in a steer-by-wire steering system. A measurement of the frictional torque according to the invention can be realized with relatively little effort by measuring the current.5 Alternatively or additionally, it is possible to determine the frictional torque by measuring the torque between the feedback actuator and the steering spindle. The torque measurement can be carried out using suitable torque sensors, which are known in principle in the prior art and which, for the implementation of the method according to the invention, can be installed between the drive of the feedback actuator and the manual steering handle attached to the steering spindle. In this case, it may be possible to use a sensor located in the To use the existing torque sensor in the steering system, which is designed to detect or monitor a hand torque or feedback torque applied to the steering spindle, to measure the friction torque.This can be achieved with relatively little effort.15 One possible further development is to vary the rotational speed of the steering spindle to measure the frictional torque. The rotational speed indicates the rotational speed of the steering spindle generated by the feedback actuator. By measuring the friction at different rotational speeds, different proportions of the acting frictional forces can be averaged and evaluated. For example, starting from standstill, i.e., zero rotational speed, the proportion of static friction that must be overcome to transition to sliding friction during rotation can be determined. Furthermore, by measuring at different rotational speeds, conclusions can be drawn about the condition of the steering spindle bearings, for example, regarding the effectiveness of the lubrication or the like. It is possible that a warning message will be issued if a predetermined limit value of the friction torque is exceeded.For this purpose, a currently measured friction torque can be compared with a stored limit value, preferably automatically after measurement. If the friction torque is greater than the limit value, this can indicate wear or a defect. To reliably avoid potentially resulting impairments of function, a visual, acoustic, or other warning message can be issued. This allows for timely inspection and maintenance. Furthermore, it is conceivable and possible that an automated emergency mode is activated, in which, for example, the vehicle speed is limited. Preferably, the steering system may be designed as a steer-by-wire steering system, in which both manually entered mechanical steering commands are converted into an electrical control signal for controlling an electric steering actuator via hand contact with the steering handle.The lack of a mechanical connection between the steering spindle allows, according to the invention, an optimized compensation of friction and, accordingly, an adjustment of the feedback moment. 10 The invention further comprises a steering system for a motor vehicle, comprising a feedback actuator coupled to a steering spindle rotatably mounted about its longitudinal axis in a housing unit, which is connected to a manually operated steering handle, and comprising a control unit connected to the feedback actuator and a hand sensor configured to carry out the method described above. 15 Description of the drawings Advantageous embodiments of the invention are explained in more detail below with reference to the drawings. In detail: 20 Fig. 1 shows a schematic representation of a steering system according to the invention, Fig. 2 shows a schematic flow diagram of the method according to the invention.25 Embodiment of the Invention In the various figures, identical parts are always provided with the same reference numerals and are therefore usually only named or mentioned once. 30 Fig. 1 schematically shows a steering system 1, which is designed as a steer-by-wire steering system and comprises a steering column 2. This has a support unit 21 mountable on a vehicle body (not shown), in which a steering spindle 22 is rotatably mounted about its longitudinal axis L in a housing unit 24. At its rear end, on the driver's side 35 BE2025 / 5043 9, with respect to the direction of travel, a steering wheel 23 is fixedly mounted on the steering spindle 22 for inputting manual steering commands. The steering column 2 has a rotary sensor 3 which detects a rotation of the steering spindle 22 by a steering command manually applied to the steering wheel 23, which is schematically indicated in Fig. 1 change-5. The steering input detected by the rotary sensor 3 is converted by a control unit into an electrical control signal which is transmitted via an electrical control line 4 to an electrical steering actuator 5.10 The steering actuator 5 can, for example, have an electromechanical linear drive, such as a known spindle drive or the like. Depending on the control by electrical steering signals, an actuator rod 51 can be moved linearly by this, as indicated by the double arrow. Alternatively, individual wheel steering 15 or the like can also be provided. The actuator rod 51 is connected to the steering knuckles of steerable wheels 61 in a known manner via tie rods 6. 20 A feedback actuator 7, which has a drive unit with an electric motor, is integrated in the housing unit 24 in the example shown, as schematically indicated in Fig. 1. The motor shaft of the motor is coupled to the steering spindle 22, optionally via a gearbox. A drive torque can be generated by the feedback actuator 7 and the steering spindle 22 can be engaged. This drive torque is counteracted by a braking friction torque through the bearing of the steering spindle 25 22 in the housing unit 24.Accordingly, a feedback torque is emitted to the steering wheel 23, which forms a manual steering handle, corresponding to the drive torque reduced by the friction torque. The steering wheel 23 has a hand sensor 25, which enables a test of hand contact. An actual measurement can be carried out. Alternatively, an estimation can also be made. The feedback actuator 7, the rotation sensor 3, and the hand sensor 25 are connected to an electrical control unit 8. This unit is configured to control the feedback actuator 7 to generate a drive torque. In addition, it can detect a rotation of the steering column 35 22 and, via the hand sensor, a hand contact. Furthermore, it can preferably be configured to measure the friction torque, for example, by measuring the motor current delivered to the feedback actuator 7. To implement the method according to the invention, the measured friction torque can be stored in the control unit, and the friction torque stored in this way can be taken into account when generating a drive torque 5.

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