ELECTROMECHANICAL STEERING SYSTEM AND METHOD FOR OPERATING AN ELECTROMECHANICAL STEERING SYSTEM WITH PLAUSIBILITY CHECK OF ANGLE SENSOR VALUES

Description

[0001] The invention relates to an electromechanical steering system for a motor vehicle, comprising a steering shaft, a control unit, and a feedback actuator, which has an electric motor with a motor shaft and is configured to apply a torque to the steering shaft via a transmission. Furthermore, the invention relates to a method for operating an electromechanical steering system, wherein, at a given time, a rotation angle of the steering shaft is detected as a first sensor value using a first angle sensor, and a rotation angle of the motor shaft is detected as a second sensor value using a second angle sensor.

[0002] WO 2018 / 233846 A1 discloses a steer-by-wire steering system with a feedback actuator that can apply torque to the steering shaft via a transmission. An angle sensor associated with the motor shaft of the feedback actuator and an angle sensor associated with the steering shaft are used to determine a rotation angle of the steering shaft that exceeds 360°.

[0003] DE 10 2008 021 849 A1 discloses an electromechanical steering system for vehicles, wherein a hand angle specified by a driver is detected. Furthermore, the steering system comprises a motor position sensor associated with a servomotor acting on the rack, which is coupled to the steerable wheels of the vehicle. A calibration device detects a deviation between a hand angle and a wheel steering angle of the steered wheels, and adjusts the wheel steering angle to the hand angle resulting from the driver's input.

[0004] Furthermore, WO 2020 / 164773 A1 discloses a method for monitoring a steering device. The steering device comprises two steering actuators, each of which can adjust a steering angle for steerable wheels via a steering gear. Position sensor units assigned to each steering actuator are intended to perform a comparison and, for example, detect a tooth jump in a toothed belt between the steering actuator and the steering gear.

[0005] An electromechanical steering system with a feedback actuator, in which a rotation angle is detected by corresponding sensors on both the feedback actuator and the steering shaft, is also known from US 6,354,396 B1.

[0006] Another method for determining an absolute angle in a steering system that exceeds a 360° rotation is known from US 2017 / 0138760 A1.

[0007] The disadvantage is that problems with the steering gear can occur, which can lead to the steering angle of the steering shaft being determined incorrectly.

[0008] Against this background, it is an object of the present invention to provide an improved electromechanical steering system and an improved method for operating an electromechanical steering system, wherein in particular a rotation angle determination is carried out with greater reliability and problems in the rotation angle determination can be detected.

[0009] To achieve this object, a method for operating an electromechanical steering system for a motor vehicle and an electromechanical steering system are proposed according to the independent claims. Further advantageous embodiments of the invention are described in the dependent claims and the description, as well as illustrated in the figures.

[0010] The proposed solution provides a method for operating an electromechanical steering system, in particular a steer-by-wire steering system, for a motor vehicle, comprising a steering shaft, a control unit and a feedback actuator which has an electric motor with a motor shaft and is designed to apply a torque to the steering shaft via a transmission with a transmission ratio, wherein at a point in time a rotation angle of the steering shaft is detected as a first sensor value using a first angle sensor and a rotation angle of the motor shaft is detected as a second sensor value using a second angle sensor, wherein the control unit checks, based on a model stored in the control unit, which takes into account at least the transmission ratio as a parameter of the transmission, whether the detected first sensor value and the detected second sensor value form a plausible pair of values.The gear ratio is provided in particular as the ratio of the speed of the electric motor of the feedback actuator or the motor shaft to the speed of the steering shaft, and is advantageously approximated to the value i_g = W±(1 / M_min), where W and M_min are each an integer and M_min ≥ 2. In particular, it is provided that W = 3 or W = 5. Advantageously, M_min = 3, so that the gear ratio i_g = 16 / 3 results, which can be realized, for example, by an output-side gear with a number of teeth Z1 of Z1 = 64 and a number of teeth Z2 on the input side of Z2 = 12.

[0011] Advantageously, the first detected sensor value and the second detected sensor value are related to one another, in particular using the model stored in the control unit, which advantageously simulates the transmission, in particular taking into account the transmission ratio, wherein in particular a computational processing of the detected first sensor value and the detected second sensor value takes place.

[0012] In particular, it is provided that the control unit detects a malfunction of the transmission if the detected first sensor value and the detected second sensor value do not form a plausible value pair. The feedback actuator's transmission is advantageously monitored by checking the simultaneous values of both angle sensors. If the detected sensor values deviate too significantly from a respective permitted zone, which is advantageously defined in the model, there is a problem in or with the transmission.

[0013] In this case, a warning message is advantageously generated, which can be output in particular via a loudspeaker of a motor vehicle or a display device of a motor vehicle.

[0014] According to a particularly advantageous embodiment of the invention, the transmission comprises a toothed belt, via which the motor shaft is connected to the steering shaft. The control unit further performs a tooth skip check based on the stored model. The method can thus detect whether the toothed belt is "slipping" and, as a result, whether a rotation of the motor shaft has not been fully transmitted to the steering shaft. In particular, it is also provided that the number of teeth skipped is detected.

[0015] It is further advantageous that the stored model considers at least one of the following parameters as an additional parameter: gear ratio; gear stiffness; gear backlash; gear temperature; gear inertia; number of teeth of a drive-side gear of the transmission; number of teeth of an output-side gear of the transmission. Since the gear of the feedback actuator and thus the gear ratio cannot be perfect, the gear monitoring performed with the method is advantageously significantly improved by a computation-based compensation of gear stiffness, gear backlash, gear temperature, and / or gear inertia stored in the model.

[0016] A further advantageous embodiment of the method provides that the detected first sensor value and the detected second sensor value form a plausible value pair if, according to the stored model, an integer multiple of the second sensor value can be converted into the first sensor value in such a way that a predetermined deviation limit is undershot. An integer multiple can, in particular, also be "1." The deviation limit takes into account, in particular, that the transmission may not be ideal and that, therefore, deviations from the assumed ideal transmission ratio may occur. The robustness of the method can advantageously be influenced by the selection of the deviation limit. The deviation limit is advantageously dependent on at least one parameter of the model stored in the control unit.

[0017] According to a further advantageous embodiment of the method, in which the transmission comprises a toothed belt via which the motor shaft is connected to the steering shaft, a plurality of defined deviation limit values, in particular of defined further deviation limit values, are stored, each of which is assigned to a specific number of tooth jumps. The deviation limit values can in particular also be formed by numerical intervals. In this respect, a deviation limit value does not have to be determined mathematically precisely. If the detected first sensor value and the detected second sensor value do not form a plausible value pair, the control unit advantageously further checks whether one of the stored deviation limit values has been reached. If this is the case, a tooth jump is advantageously detected.Advantageously, the control unit detects a number of tooth jumps associated with the deviation limit when one of the stored additional deviation limits has been reached. Thus, advantageously, not only is the information obtained that a tooth jump has occurred, but also how many teeth were jumped.

[0018] Furthermore, the control unit advantageously checks whether one of the stored deviation limits is reached, particularly whether it is consistently reached, for a defined period of time. This advantageously allows for a more accurate detection of a problem with the transmission.

[0019] In particular, it is further provided that the direction of the tooth jump is determined, advantageously by setting the deviation limit values with a corresponding sign. The sign of the determined deviation limit value is advantageously taken into account by the control unit during the test.

[0020] Further advantageously, the control unit determines, in particular based on the model stored in the control unit, an absolute angle assumed by the steering shaft, wherein the absolute angle may in particular be greater than 360°. In particular, reference is also made to the document WO 2018 / 233846 A1.

[0021] A further advantageous embodiment of the method provides that the control unit determines a correction factor with respect to the detected tooth jump using at least one of the determined pieces of information relating to a detected tooth jump control, in particular a specifically determined number of tooth jumps and / or a direction of the tooth jump. Advantageously, the control unit performs an error correction by applying the determined correction factor to the detected first sensor value and / or the detected second sensor value. Advantageously, the absolute angle assumed by the steering shaft can thus also be determined in many cases of a transmission problem.

[0022] Determining the correction factor advantageously comprises the following steps: defining a positive direction of the tooth jump and a negative direction of the tooth jump; giving the number of skipped teeth a positive sign if the direction of the tooth jump is positive; giving the number of skipped teeth a negative sign if the direction of the tooth jump is negative; multiplying the number of skipped teeth, given the corresponding sign, by 360°; dividing the number of skipped teeth, given the corresponding sign and multiplied by 360°, by the number of teeth on the input side gear of the transmission.

[0023] In particular, the following steps are proposed for monitoring the functionality of the transmission of the feedback actuator, which steps can also be carried out independently of the previously mentioned steps: Providing a defined threshold value for monitoring the functionality of the transmission; simultaneously detecting the angle of rotation of the steering shaft by the first angle sensor and by the second angle sensor; forming a sensor value difference between the sensor value of the first angle sensor and the sensor value of the second angle sensor; comparing the formed sensor value difference with the defined threshold value; determining an impaired functionality of the transmission if the formed sensor value difference exceeds the defined threshold value, and advantageously outputting a warning signal.

[0024] Advantageously, forming a sensor value difference comprises the following step: taking into account factors describing the condition and / or the properties of the transmission, wherein the factors advantageously include the transmission stiffness, the transmission backlash, the transmission temperature and / or the transmission inertia.

[0025] For tooth jump control of the transmission of the steering system, the following steps are provided in particular, which can in particular also be carried out independently of the previously mentioned steps: Providing a relationship between the sensor values of the first angle sensor and the sensor values of the second angle sensor, representing a tooth jump-free state of the transmission; simultaneous detection of the steering angle of the steering shaft by the first angle sensor and by the second angle sensor; Forming a value pair consisting of the sensor value of the first angle sensor and the sensor value of the second angle sensor; Detecting a tooth jump based on the formed value pair and the relationship representing a tooth jump-free state of the transmission; Storing information relating to the tooth jump, in particular in a memory unit of the control unit, if a tooth jump has been detected.

[0026] Advantageously, the detection of a tooth jump comprises the following steps: providing a defined time period for detecting a tooth jump; determining whether the formed value pair is outside the relationship representing a tooth jump-free state of the transmission for the duration of the defined time period.

[0027] The information concerning the tooth jump includes in particular the number of skipped teeth, the direction of the tooth jump, timestamps and / or the number of teeth of the drive-side gear of the transmission.

[0028] Advantageously, the method comprises the following further steps: calculating a correction factor using the stored information concerning the tooth overlap; correcting the sensor values by applying the calculated correction factor to the sensor values.It is advantageously provided that the calculation of the correction factor comprises the following steps: defining a positive direction of the tooth jump and a negative direction of the tooth jump; providing the number of skipped teeth with a positive sign if the direction of the tooth jump is positive; providing the number of skipped teeth with a negative sign if the direction of the tooth jump is negative; multiplying the number of skipped teeth, provided with the corresponding sign, by 360°; dividing the number of skipped teeth, provided with the corresponding sign and multiplied by 360°, by the number of teeth of the drive-side gear of the transmission.

[0029] The electromechanical steering system for a motor vehicle further proposed to achieve the object mentioned at the outset, wherein the steering system can in particular be a steer-by-wire steering system, comprises a steering shaft, a control unit and a feedback actuator which has an electric motor with a motor shaft and is designed to apply a torque to the steering shaft via a transmission, in particular a toothed belt transmission, with a predetermined gear ratio, wherein a first angle sensor is assigned to the steering shaft for detecting an angle of rotation of the steering shaft and a second angle sensor is assigned to the motor shaft for detecting an angle of rotation of the motor shaft, and wherein the control unit is designed to operate the steering system according to a method designed according to the invention, in particular a method having the features described above individually or in combination.Advantageously, the control unit is configured to perform a functionality monitoring of the transmission and a tooth jump check based on a sensor value of the first angle sensor and a sensor value of the second angle sensor. The control unit is configured, in particular, to perform a plausibility check with respect to the detected first sensor signal and / or the detected second sensor signal based on a model of the transmission stored in the control unit. The control unit preferably checks, based on a model stored in the control unit that takes into account at least the transmission ratio as a parameter of the transmission, whether the detected first sensor value and the detected second sensor value form a plausible value pair.In particular, in the steering system, a rotation angle sensor is arranged upstream and downstream of the transmission. The rotation angle sensor upstream of the transmission is advantageously a rotor position sensor of the electric motor of the feedback actuator, and the rotation angle sensor downstream of the transmission is advantageously a single-turn capable steering angle sensor. Advantageously, the transmission of the feedback actuator is used instead of a separate sensor transmission to determine a rotation angle of the steering shaft, which may be greater than 360°. The sensor system for determining the rotation angle of the steering shaft is smaller than in a design with a sensor transmission.Advantageously, a steering system designed according to the invention enables a rotation angle of the steering shaft, which may be greater than 360°, to be determined without additional hardware expenditure, wherein advantageously, it also enables monitoring of the functionality of the gear of the feedback actuator - also without additional hardware expenditure.

[0030] Advantageously, at least one of the angle sensors has a redundant power supply, in particular a redundant voltage supply. This advantageously increases reliability.

[0031] According to an advantageous embodiment of the steering system, at least one of the angle sensors is a rotor position sensor, in particular the second angle sensor for detecting the angle of rotation of the motor shaft.

[0032] Furthermore, at least one of the angle sensors is advantageously a single-turn sensor, i.e., a cost-effective sensor that can only determine a rotation angle between 0° and 360°. Advantageously, an actual absolute angle, which can also be greater than 360°, is determined using the model stored in the control unit. This advantageously allows for cost savings.

[0033] According to a further advantageous embodiment, the functionality monitoring comprises the formation of a sensor value difference between the sensor value of the first angle sensor at a specific point in time and the sensor value of the second angle sensor at the same point in time. In particular, the functionality monitoring comprises the comparison of the formed sensor value difference with a defined threshold value. Further advantageously, the functionality monitoring comprises the consideration of factors describing the condition and / or properties of the transmission, which in particular include the transmission stiffness, the transmission backlash, the transmission temperature, and / or the transmission inertia.

[0034] Advantageously, the control unit is designed to carry out a tooth skip check of the transmission based on a sensor value of the first angle sensor at a specific time and a sensor value of the second angle sensor at the same time, wherein the tooth skip check advantageously comprises taking into account a relationship between the sensor values of the first angle sensor and the sensor values of the second angle sensor, which relationship represents a tooth skip-free state of the transmission.

[0035] An advantageous development of the steering system provides for it to have an internal error memory configured to store information regarding tooth jumps. Preferably, the control unit includes the internal error memory. The information regarding tooth jumps includes, in particular, the number of skipped teeth, the direction of the tooth jumps, and a time stamp.

[0036] Further advantageous details, features, and design details of the invention are explained in more detail in connection with the exemplary embodiments shown in the figures (Fig.: Figure). Fig. 1a shows a simplified perspective view of an exemplary embodiment of an electromechanical steering system designed according to the invention; Fig. 1b shows a simplified perspective view of another exemplary embodiment of a steering shaft and a feedback actuator of an electromechanical steering system designed according to the invention; Fig. 2 shows a block diagram of an exemplary embodiment for determining a multi-turn angle of rotation of the steering shaft when carrying out a method designed according to the invention; Fig. 3 shows a block diagram of an exemplary embodiment for redundantly determining a multi-turn angle of rotation of the steering shaft when carrying out a method designed according to the invention with a redundant power supply; Fig. 4 shows a block diagram of an exemplary embodiment for redundantly determining a multi-turn angle of rotation of the steering shaft when carrying out a method designed according to the invention without a redundant power supply;Fig. 5 is a block diagram showing an exemplary embodiment for carrying out a method according to the invention when tooth jump is detected during operation; Fig. 6 is a block diagram showing a further exemplary embodiment for carrying out a method according to the invention when tooth jump is detected in the currentless state; and Fig. 7 is a block diagram showing a further exemplary embodiment for carrying out a method according to the invention.

[0037] In the various figures, identical parts are generally provided with the same reference symbols and are therefore sometimes explained only in connection with one of the figures.

[0038] Fig. 1a shows an embodiment of an electromechanical steering system 1 in a perspective, simplified view obliquely from the front in the direction of vehicle travel. The steering system 1 for a motor vehicle comprises a steering column with a steering shaft 2, a control unit 3, which in Fig. 1is only shown schematically, and a feedback actuator 4. In Fig. 1b In a further embodiment, the steering shaft 2 of a steering system with the feedback actuator 4 is shown in more detail.

[0039] At the end of the steering shaft 2 facing the driver, a steering handle 10, in particular a steering wheel, is arranged in a rotationally fixed manner for inputting a driver's steering request or steering command. A driver can rotate the steering handle 10, designed as a steering wheel, in a known manner to input their steering command. By means of the feedback actuator 4 acting on the steering shaft 2 and thus also on the steering handle 10, a driver can be provided with feedback, in particular, regarding the forces acting on the wheels 11 of the steering system 1 during driving or maneuvering.

[0040] In this exemplary embodiment, the steering shaft 2 of the steering system is also mechanically coupled to the steerable wheels 11 of a motor vehicle via a steering gear 12. In this exemplary embodiment, the steering gear 12 comprises a pinion 13 and a toothed rack 14, wherein the steering gear 12 serves to translate a rotational movement of the pinion 13 into a translational movement of the rack 14 along its longitudinal axis.

[0041] In this exemplary embodiment, the rack 14, which moves linearly along its longitudinal axis, is mechanically coupled to a tie rod 15 on each side of the motor vehicle. The tie rods 15 are, in turn, each mechanically coupled to the vehicle wheels 11. The steering gear 12 is thus designed to convert a steering command into a steering movement of the steerable wheels 11 of the motor vehicle, taking into account at least one input variable.

[0042] The feedback actuator 4 of the steering system 1 comprises, as exemplified in Fig. 1bshown, an electric motor 5 with a motor shaft 6 and is designed to apply a torque to the steering shaft 2 via a gear 7 with a gear ratio. The gear 7 has a drive-side gear 71 arranged in a rotationally fixed manner on the motor shaft 6 and an output-side gear 72 arranged in a rotationally fixed manner on the steering shaft 2, which are connected to one another via a toothed belt 73. The drive-side gear 71 can in particular have 12 teeth. The output-side gear 72 can in particular have 64 teeth. A first angle sensor 8, in particular a multi-turn capable steering angle sensor, is assigned to the steering shaft 2 for detecting an angle of rotation of the steering shaft 2. A second angle sensor 9, in particular a rotor position sensor, is assigned to the motor shaft 6 for detecting an angle of rotation of the motor shaft 6.The angle of rotation of the steering shaft 2 detected by the first angle sensor 8 as the first sensor value and the angle of rotation of the motor shaft 6 detected by the second angle sensor 9 as the second sensor value are transmitted to the control unit 3. The control unit 3 checks, based on a model stored in the control unit, which model takes at least the gear ratio of the transmission 7 into account as a parameter, whether the first sensor value detected by the first angle sensor 8 and the second sensor value detected by the second angle sensor 9 form a plausible pair of values, the control unit 3 being designed in particular to evaluate the functionality of the transmission by evaluating the pair of values and in particular to detect tooth overlap. Details of possible method steps carried out by the control unit 3 are described below with reference to the FIG. Fig. 2 to Fig. 7 illustrated embodiments are explained in more detail.

[0043] In Fig. 2is shown as a block diagram that in a step A1 with the first angle sensor a rotation angle of the steering shaft is recorded as a first sensor value, hereinafter also referred to as the steering angle sensor value L, and at the same time in a step A2 with the second angle sensor a rotation angle of the motor shaft is recorded as a second sensor value, hereinafter also referred to as the rotor angle sensor value R, and transmitted to the control unit of the steering system. For straight-ahead travel, stored values L_straightahead and R_straightahead are stored as the first sensor value and second sensor value, respectively, in a memory unit, in particular a memory unit of the control unit. These values can be changed by a reference travel, with which the actually recorded first sensor value and second sensor value are converted into a fixed value when driving straight ahead, in particular into a value of 0° or a value of 180°.

[0044] In an optional step B1, the control unit applies a correction factor L_correction to the detected first sensor value L and, in an optional step B2, a correction factor R_correction to the detected second sensor value R. The correction factors L_correction and R_correction are advantageously also stored in the memory unit. In particular, it is provided that the correction factor is added to or subtracted from the detected first sensor value L or the detected second sensor value R, in particular depending on the direction of rotation. In a further step C1 and a further step C2, a zeroed first sensor value L_0 or a zeroed second sensor value R_0 is then provided, so that the steering angle sensor value or rotor angle sensor value actually related to an initial position, in particular related to an initial position of 0°, is obtained.From the pair of values, i.e. the zeroed first sensor value L_0 and the zeroed second sensor value R_0, the control unit then determines the absolute angle assumed by the steering shaft in a step D based on the model for the feedback actuator's gear stored in the control unit. The model takes the gear ratio of the transmission into account. For example, with a number of teeth Z1 = 64 on the output-side gear and a number of teeth Z2 = 12 on the input-side gear, a gear ratio i_g = 16 / 3 is taken into account by the model. A rotation angle of the motor shaft of 180° is correspondingly assigned a rotation angle of the steering shaft of 33.75°. The multi-turn range of the steering shaft is advantageously also stored in the control unit and is, for example, ±540°.In this case, starting from a straight-ahead position of the steering system's wheels, the steering shaft can be rotated 540° in one direction and 540° in the other direction via the steering handle. In other words, the steering shaft can be rotated 1.5 turns in one direction and 1.5 turns in the other direction. Thus, for a first sensor value of 22.5°, a cycle of 0° of the recorded second sensor value is repeated.

[0045] For an example of determining the absolute angle, it is assumed that in steps C1 and C2 the value pair determined for the zeroed rotor angle sensor value R_0 = -90° and for the zeroed steering angle sensor value L_0 = -61.875°. The steering angle sensor value X is determined based on R_0 = 0° as X = L_0 - (R_0 / i_g). With the values given, this results in X = -61.875° - ((-90°) / (16 / 3)) and thus X = -45°. For a first sensor value of -45°, 0° (or 180°) is passed through twice because -45° / 22.5° = 2, resulting in an addition value L_Add of -360°. Advantageously, this also checks whether the recorded first sensor value and the recorded second sensor value provide a plausible value pair.

[0046] In this embodiment, the absolute angle W_abs is then determined as W_abs = L_0 + L_add and, based on the concrete numerical example, as W_abs = -61.875 + (-360°) = -421.875°.

[0047] Fig. 3shows an exemplary embodiment for the case in which a rotor angle sensor with redundant power supply, namely a first power supply 18 and a second power supply 19, is provided. In a step C1, the zeroed steering angle sensor value L_0 and in a step C2 the zeroed rotor angle sensor value R_0 are provided, and in a further step D the absolute angle of the steering shaft is determined from this pair of values. In a further step E, a virtual multi-turn counter, which is given a starting value during commissioning and which otherwise takes into account the value of the determined absolute angle and the provided zeroed steering angle sensor value L_0, is updated. In a step F, the results thus redundantly determined are compared with one another and checked for plausibility. In particular, it can be provided that step F is carried out in a separate computing unit in each case.

[0048] Fig. 4shows an embodiment for the case where a rotor angle sensor is provided without a redundant power supply, but the reliability is nevertheless increased by providing two virtual multiturn counters. In a step C1, the zeroed steering angle sensor value L_0 and in a step C2 the zeroed rotor angle sensor value R_0 are provided, and in a further step D the absolute angle of the steering shaft is determined from this pair of values. In a step E, a first virtual multiturn counter, to which a start value is specified during commissioning and which otherwise takes into account the value of the determined absolute angle and the provided zeroed steering angle sensor value L_0, and a second virtual multiturn counter, to which a start value is specified during commissioning and which otherwise takes into account the value of the determined absolute angle and the provided zeroed steering angle sensor value L_0, are updated.In a step F, the redundantly determined results are compared and checked for plausibility. In particular, it can be provided that each step F is performed in a separate processing unit.

[0049] In the Fig. 5In the exemplary embodiment shown for a method designed according to the invention, it is provided that, as also described above, a first sensor value and a second sensor value are recorded and provided as a value pair with zeroed sensor values. Based on a model stored in the control unit, which takes into account the gear ratio of the transmission of the feedback actuator, a check is carried out in a step K to determine whether the recorded first sensor value and the recorded second sensor value form a plausible value pair. In this exemplary embodiment, it is provided that the recorded first sensor value and the recorded second sensor value form a plausible value pair if, according to the stored model, an integer multiple of the second sensor value can be converted into the first sensor value in such a way that a predetermined deviation limit value A_G is undershot.

[0050] As an example, the figures from the with reference to Fig. 2 The numerical example given was used as a basis, in which the value pair for the zeroed rotor angle sensor value R_0 = -90° and for the zeroed steering angle sensor value L_0 = -61.875° were determined. Since the steering angle sensor value X was determined here with reference to R_0 = 0° as X = L_0 - (R_0 / i_g), i.e. as X = -61.875° - ((-90°) / (16 / 3))) and thus as X = -45°, an initial sensor value of -45° was -45° / 22.5° = 2. There is no deviation value that is an integer multiple of this value here, so that the deviation limit value A_G would be undershot in this case.

[0051] The situation is different if, for example, R_0 = -84.375° (-90° + 5.625°) were to apply with otherwise identical numerical values. Then X would be -46.05°, and because -46.05° / 22.5° = -2.047, a deviation value of 0.047 would remain for an integer multiple of 2, whereby 0.047 > A_G would be assumed. In this case, the deviation value of 0.047 would exceed the deviation limit A_G, and it would be recognized that the pair of values is not plausible.

[0052] In this exemplary embodiment, it is further provided that a plurality of defined further deviation limit values A_G# are stored. These stored further deviation limit values are each assigned to a specific number of tooth jumps of the toothed belt of the transmission of the feedback actuator. If the control unit has determined that the detected first sensor value and the detected second sensor value do not form a plausible value pair, the control unit further checks whether one of the stored further deviation limit values has been reached, i.e. whether the determined deviation value corresponds in particular to one of the stored deviation limit values. The deviation limit values result from a deviation resulting from one tooth jump or several tooth jumps from the angular value for which a rotation angle of 0° is repeated for the motor shaft.Based on the numerical example, this results in deviations from the value 22.5° or a multiple of 22.5° from a multiple of approximately 5.6°. If the zeroed rotor angle sensor value R_0 were to be an integer multiple k of the zeroed steering angle sensor L_0 ± a multiple n of the stored deviation, for example, 5.6°, a number n of tooth jumps would be detected, with the sign of the deviation indicating the direction of the tooth jump.

[0053] In the Fig. 5In the exemplary embodiment shown, it is now provided that the test in step K has detected that a corresponding stored deviation exists. In a next step L, the control unit checks whether the stored deviation limit value is reached for a defined period of time, for example, ten seconds. If this is the case, in a further step M, the specific number of skipped teeth, the direction of the skipped teeth, and the date and time of detection are stored, in particular in a memory unit of the control unit or a central error memory of the motor vehicle.

[0054] Next, in step N, a correction value is provided for the rotor angle sensor value. For the correction value R_correction, the product of the number of skipped teeth and 360° is calculated and divided by the total number of teeth on the drive-side gear. The number of skipped teeth is assigned a positive sign if the direction of the tooth skip is positive and a negative sign if the direction of the tooth skip is negative. With the corresponding correction value, normal operation is returned to in step P.

[0055] In the Fig. 6In the exemplary embodiment shown for implementing a method according to the invention for operating the electromechanical steering system, a tooth jump check is carried out. Starting from normal operation P, in which the feedback actuator is energized, the electric motor of the feedback actuator is de-energized in a step Q and then checked in a step K to determine whether the detected first sensor value of the angle sensor arranged downstream of the transmission and the detected second sensor value of the angle sensor arranged upstream of the transmission form a plausible pair of values. If this is the case, the electromechanical steering system continues to operate in normal operation without changes in a step P.If, however, the test in step K shows that the detected first sensor value and the detected second sensor value do not form a plausible value pair, a reference run is started in a step R, in particular with predetermined steering angles, in particular steering angles up to a mechanical end stop of the steering system. In this case, in particular, as described with reference to . Fig. 5 described, it is checked whether and to what extent tooth jumps occur or have occurred, and based on the recorded information, a correction value for the rotor angle sensor and / or a correction value for the steering angle sensor is reset in a step N. With the newly set correction values, it can then be checked again, starting with step Q, whether the recorded first sensor value and the recorded second sensor value form a plausible value pair, taking into account the now newly set correction values, which overwrite the old correction values.

[0056] Fig. 7shows, as a simplified block diagram, a further exemplary embodiment for the design of a method according to the invention, according to which the electromechanical steering system can be operated. During operation, the control unit of the steering system checks, in a step K, whether the detected first sensor value and the detected second sensor value form a plausible pair of values, based on the model stored in the control unit, which takes into account the gear ratio and, in addition, other parameters, in particular the gear stiffness, the gear backlash, the gear temperature, the gear inertia, the number of teeth of the drive-side gear of the transmission and / or the number of teeth of the output-side gear of the transmission.If this check reveals that the detected first sensor value and the detected second sensor value form a plausible value pair, and thus, in particular, that a predetermined deviation limit is not reached, an absolute angle of the steering shaft is determined in step D based on the detected sensor values. This can also be done, in particular, as described with reference to . Fig. 2 explained.

[0057] If, however, the test in step K shows that the detected first sensor value and the detected second sensor value do not form a plausible pair of values, a further step S is checked to determine whether a deviation limit value stored in the control unit, which is assigned to a specific number of tooth jumps, is reached with a predetermined accuracy. If this is the case, the implausibility caused by the tooth jump or jumps can be corrected. Since it is now known that at least one tooth jump has occurred, in this exemplary embodiment the steering shaft is moved into at least one mechanical stop of the steering system in a step T and the deviation caused by the at least one tooth jump is determined and corrected. Using the correspondingly adjusted sensor values, the absolute angle of the steering shaft is then determined in a step D.

[0058] If the check in step S reveals that, for a determined implausible value pair, a deviation limit stored in the control unit, which is assigned to a specific number of tooth jumps, is not reached with a specified accuracy, the absolute angle of the steering shaft cannot be reliably determined. Therefore, in step V, the vehicle is brought to a standstill. This means, in particular, that driving off is not permitted.

[0059] The exemplary embodiments shown in the figures and explained in connection with them serve to explain the invention and are not limiting thereof. List of reference symbols

[0060] 1Steering system 2Steering shaft 3Control unit 4Feedback actuator 5Electric motor 6Motor shaft 7Gearbox 71Drive-side gear of the gearbox (7) 72Output-side gear of the gearbox (7) 73Timing belt 8First angle sensor 9Second angle sensor 10Steering handle 11Wheel 12Steering gear 13Pinion 14Rack 15Tie rod 18First power supply 19Second power supply A1Detecting the first sensor value A2Detecting the second sensor value B1Applying a correction factor to the detected first sensor value B2Applying a correction factor to the detected second sensor value C1Providing the first sensor value zeroed C2Providing the second sensor value zeroed DEtermining the absolute angle of the steering shaft (2) EKuring the virtual multi-turn counter FPlausibility check of the determined results KPlausibility check of the value pair formed from the detected first sensor value and the detected second sensor value LChosing whether the deviation for the value pair formed from the detected first sensor values and the detected second sensor values is stable MStoring information regarding a transmission problem NProviding a correction value PNormal operation QSetting the electric motor of the feedback actuator to a voltage-free state RPerforming a reference run SChecking,whether the value pair is implausible in a given way TMoving the steering shaft to at least one steering stop / providing a correction value VShutting down the motor vehicle,

Claims

1. Method for operating an electromechanical steering system (1) for a motor vehicle with a steering shaft (2), a control unit (3) and a feedback actuator (4), which has an electric motor (5) with a motor shaft (6) and is designed to apply a torque to the steering shaft (2) via a gearbox (7) with a gear ratio, wherein at a point in time a first angle sensor (8) is used to detect a rotation angle of the steering shaft (2) as a first sensor value and a second angle sensor (9) is used to detect a rotation angle of the motor shaft (6) as a second sensor value, characterized in that the control unit (3) checks whether the detected first sensor value and the detected second sensor value form a plausible pair of values based on a model stored in the control unit (3), which model takes into account at least the gear ratio as a parameter of the gearbox (7).

2. Method according to claim 1, characterized in that the control unit (3) detects a faulty function of the transmission (7) if the detected first sensor value and the detected second sensor value do not form a plausible pair of values.

3. Method according to claim 1 or claim 2, characterized in that the transmission (7) comprises a toothed belt (73) via which the motor shaft (6) is connected to the steering shaft (2), wherein the control unit (3) further performs a tooth skipping monitoring based on the stored model.

4. Method according to one of the preceding claims, characterized in that the stored model takes into account at least one of the following parameters as a further parameter: gearbox stiffness; gearbox backlash; gearbox temperature; gearbox inertia; number of teeth of an input-side gearwheel (71) of the gearbox (7), number of teeth of an output-side gearwheel (72) of the gearbox (7).

5. Method according to one of the preceding claims, characterized in that the detected first sensor value and the detected second sensor value form a plausible pair of values if, according to the stored model, an integer multiple of the second sensor value can be converted into the first sensor value in such a way that a predetermined deviation limit value is undershot.

6. Method according to claim 5, characterized in that the deviation limit value is dependent on at least one parameter of the model.

7. Method according to one of the preceding claims, characterized in that the transmission (7) comprises a toothed belt (73), via which the motor shaft (6) is connected to the steering shaft (2), wherein a plurality of defined further deviation limit values are stored, which are each assigned to a specific number of tooth skips, wherein the control unit (3), if the detected first sensor value and the detected second sensor value do not form a plausible pair of values, further checks whether one of the stored further deviation limit values has been reached.

8. Method according to claim 7, characterized in that the control unit (3), when one of the stored further deviation limit values has been reached, detects a number of tooth skips associated with the deviation limit value.

9. Method according to claim 7 or claim 8, characterized in that the control unit (3) checks whether one of the stored deviation limit values is reached for the duration of a defined period of time.

10. Method according to one of claims 7 to 9, characterized in that the direction of tooth skipping is determined.

11. Method according to one of claims 7 to 10, characterized in that the control unit (3), using at least one of the determined information relating to a detected tooth skipping monitoring, determines a correction factor relating to the detected tooth skipping and performs an error correction by applying the determined correction factor to the detected first sensor value and / or the detected second sensor value.

12. Method according to claim 11, characterized in that the determination of the correction factor comprises the following steps: Defining a positive direction of tooth skipping and a negative direction of tooth skipping; Assigning a positive sign to the number of skipped teeth if the direction of the tooth skip is positive; Assigning a negative sign to the number of skipped teeth if the direction of the tooth skip is negative; Multiplying the number of skipped teeth with the corresponding sign by 360°; Dividing the number of skipped teeth, multiplied by 360° and provided with the corresponding sign, by the number of teeth of the drive-side gearwheel of the gearbox.

13. Method according to one of the preceding claims, characterized in that the control unit (3) determines an absolute angle assumed by the steering shaft (2) based on the model.

14. Electromechanical steering system (1) for a motor vehicle, comprising a steering shaft (2), a control unit (3) and a feedback actuator (4) which has an electric motor (5) with a motor shaft (6) and is designed to apply a torque to the steering shaft (2) via a gearbox (7) with a predetermined gear ratio, wherein a first angle sensor (8) is assigned to the steering shaft (2) for detecting an angle of rotation of the steering shaft (2) and a second angle sensor (9) is assigned to the motor shaft (6) for detecting an angle of rotation of the motor shaft (6), characterized in that the control unit (3) is designed to operate the steering system (1) according to a method according to one of the preceding claims.

15. Steering system (1) according to claim 14, characterized in that at least one of the angle sensors (8, 9) has a redundant power supply (18, 19).