Method and device for operating a steering system

Abstract

Method for operating a steering system (1) comprising the following steps: - detecting a fault in at least one of several steering system functions in the steering system (1); - upon detection of a fault in one of the steering system functions, activating a first or a second emergency running function (NFL1, NFL2) depending on in which of the steering system functions the fault occurred, wherein the steering system functions comprise the following functions: ◯ a sensor detection function (SNL) to provide a sensor value for an applied driver hand torque (FHM), ◯ a steering assistance function (LKU) to provide an assistance torque as a torque contribution, ◯ a stability and dynamics function (SDF) to provide a compensating torque as a torque contribution, ◯ one or more auxiliary functions (F1, F2...Fn) to provide further torque contributions;◯ a coordination function (KF) to provide a superposition torque as the sum of the torque contributions,- wherein a first emergency running function (NFL1) is called when a fault is detected in at least one of the auxiliary functions (F1, F2... Fn),- wherein the first emergency running function (NFL1) provides to deactivate one or more auxiliary functions (F1, F2) and to operate the sensor detection function, the steering assistance function (LKU) and the stability and dynamics function (SDF) in normal operation and to operate the coordination function (KF) such that the sum of the torque contributions is weighted with a first emergency running factor (NF1), in particular of between 50 and 80%.

Description

Technical FieldThe invention relates to a method and a device for operating a steering system with steering assistance and to a steering system with such a device. More particularly, the present invention relates to measures for enhancing the safety of steering systems that provide steering assistance.Prior ArtIn an electromechanical steering system for a motor vehicle, in addition to a driver hand torque applied by a driver via the steering wheel, an electromechanically generated superposition torque is additionally provided. The measure of the superposition torque is composed of a steering assistance torque dependent on the driver hand torque, a steering assistance function and further partial superposition torques. For example, a stability and dynamic function provides a compensation torque as a partial superposition torque, which prevents a ringing in the steering system due to the application of the steering assistance torque.Further driver assistance functions, such as a lane keeping assistance function, an end stop function and the like, engage in the electromechanical steering system by providing partial superposition torques. The torque contributions of the partial superposition torques provided by the individual functions of the steering system are generally summed up in a coordination function to form the superposition torque and provided as a steering torque request for controlling a steering assist motor. The individual functions implemented in the steering system are monitored.In the event of a fault event, an emergency operation or a complete shutdown (emergency shutdown) of the steering system is effected. The type of fault handling depends on the function in which the fault has occurred. In particular in the case of core functions, such as the steering assistance function, the stability and dynamic function and the coordination function, the occurrence of errors has hitherto led to complete deactivation of the steering force assistance, which can lead to inflexibility and thus risk to the driver and the vehicle, in particular during a journey of the motor vehicle.The document DE 10 2010 052 441 A1 discloses a method for operating an electric power steering system. An emergency operation is provided if a rotational position of a rotor of a steering assist motor is not detectable.The document DE 10 2012 004 039 A1 discloses a method for operating an electromechanical power steering system for steering assistance. In the event of failure or malfunction of a rotor position sensor, an emergency operation function is provided, in which a predefinable steering force assistance is set at the electromechanical power steering system.The publication DE 10 2007 050 730 A1 discloses an electromechanical steering system comprising at least one control unit and at least one servomotor, wherein algorithms for converting at least two steering functions are stored in the control unit, which convert signals from sensors into control signals for the servomotor by means of a microprocessor, wherein the steering functions are divided into at least one core function and at least one comfort function, wherein a monitoring device is assigned to the control unit and / or a monitoring device monitors a load on the microprocessor and / or the functionality of the steering functions, wherein, when the microprocessor is loaded above a limit load, comfort functions are temporarily deactivated and / or, when errors occur in comfort functions, these faulty comfort functions are deactivated.The document DE 10 2004 028 827 A1 discloses a method for operating a steering system for a motor vehicle having a steerable wheel, wherein a steering movement initiated by the driver of the motor vehicle and / or further vehicle-specific parameters are received as input data from a control device and a control signal for generating the steering movement of the steerable wheel is determined therefrom, wherein a system state of the steering system is determined and the useful applications of the steering system are configured on the basis of said system states, wherein at least the system states: initialization of the steering system; at least partial provision of the functionality of the useful applications of the steering system; no provision of the functionality of kinematic useful applications of the steering system; fault mode; and shutdown are used.DE 10 2013 011 282 A1 discloses a method for controlling functions of a vehicle depending on a state of a steering angle detection of the vehicle, comprising the following steps: detecting a state of the steering angle detection in order to detect an error of the steering angle detection, classifying the error as a slight error or as a severe error, carrying out the functions depending on the classification of the error, disabling the functions if the error is classified as a severe error and enabling at least one function of the functions if the error is classified as a slight error.The document DE 10 2013 217 427 A1 discloses a method for operating a steering control device in a steering system having a software-controlled control module, comprising the following steps: executing steering system software for carrying out a core function for providing steering force assistance and at least one comfort function for providing steering torque interventions in normal operation; monitoring the execution of the steering system software with regard to the presence of a risk of running time error; and when a risk of running time error is detected, assuming a run-flat operating mode in which the steering system software is deactivated and a separate core function software designed for providing steering force assistance is activated.It is an object of the present invention to provide a method for operating a steering system with steering assistance, in which a frequency of the occurrence of an emergency shutdown can be significantly reduced.Disclosure of the InventionThis object is achieved by the method for operating a steering system with steering assistance according to claim 1 and by the device and the steering system according to the subordinate claims.Further embodiments are given in the dependent claims.According to a first aspect, a method for operating a steering system is provided, having the following steps:detecting a fault of at least one of a plurality of steering system functions in the steering system;detecting a fault in one of the steering system functions, activating a first or a second emergency function depending on which of the steering system functions the fault has occurred.Furthermore, the steering system functions comprise the following functions:a sensor detection function for providing a sensor value for an applied driver hand torque,a steering assistance function for providing an assistance torque as a torque contribution,a stability and dynamic function for providing a compensation torque as a torque contribution,one or more secondary functions for providing further torque contributions; anda coordination function for providing a superposition torque as the sum of the torque contributions.Furthermore, a first emergency function is called if an error is detected in at least one of the secondary functions. The first emergency operation function provides for the one or more secondary functions to be deactivated and for the sensor detection function, the steering assistance function and the stability and dynamic function to be operated in normal operation and, in addition, for the coordination function to be operated in such a way that the sum of the torque contributions is weighted with a first emergency operation factor, in particular of between 50 and 80%.If a critical error is determined by error monitoring during execution of the functions of the steering assist function, the stability and dynamic function and the coordination function, this usually leads to complete deactivation of all steering system functions. In the event of failure of other functions which influence the steering assistance or a torque contribution by partial superposition torques, on the other hand, an emergency operation function is provided. In this limp home function, the superposition torque can be reduced, so that only a reduced assist power is provided.In order to increase the availability of the steering system, it is now provided that, in the event of one of the above critical faults, a functionality is implemented which provides for a reduction of the assistance power to a safe level instead of the usual emergency shutdown. For this purpose, it has been found that, if the assist torque of the steering assist function is reduced, so that the excitation is lower than the intrinsic damping, emergency operation can be provided in normal operation, in which no functionality is required for stabilization. The stabilization and dynamic function serves to avoid a real ringing in the steering system due to the intervention of the steering assist motor or due to abrupt steering movements of the driver by providing a correspondingly compensating partial superposition torque.In this way, steerability of the motor vehicle can still be ensured even though critical errors have occurred, which would otherwise lead to a complete deactivation of the steering assistance, i.e. in conventional steering systems.If, for example, the stability and dynamic function is incorrect, this is considered a critical error. However, the complete deactivation of the steering assistance can be dispensed with if the level of the assistance torque provided by the steering assistance function is reduced such that the risk of a ringing is no longer present.A second emergency function can be called up if an error is detected in the sensor detection function, in the steering assistance function, in the stability and dynamic function and / or in the coordination function.Furthermore, the second emergency function can provide for the deactivation of one or more secondary functions, the stability and dynamic function and the sensor detection function, for the operation of the steering assistance function in which an assistance torque is determined only with a simple assistance characteristic curve, and for the operation of the coordination function in such a way that the provided assistance torque is scaled down with a second emergency factor in such a way that the excitation caused by the resulting superposition torque is less than the intrinsic damping of the steering system.In particular, the coordination function can be operated in such a way that the provided assist torque is weighted with a second emergency running factor of between 20 and 50%, preferably of between 25 and 40%.According to one specific embodiment, in the second emergency operation function, the steering assistance function may ascertain the assistance torque only as a function of a driver hand torque having an in particular linear assistance characteristic curve.According to a further aspect, a device, in particular a control device, for operating a steering system is provided, wherein the device is designed to operate the steering system.detecting a fault of at least one of a plurality of steering system functions in the steering system;activating a first or a second emergency operation function depending on which of the steering system functions the fault has occurred, upon detection of a fault in one of the steering system functions.According to another aspect, a steering system including a steering assist motor and the above apparatus is provided.Brief Description of the DrawingsEmbodiments are explained in more detail below with reference to the attached drawings. The following are shown: FIG. 1 is a schematic illustration of a steering system with steering assistance; FIG. 2 is a block diagram illustrating the run-flat function in the event of critical errors; and FIG. 3 is a flow chart illustrating a method for performing a limp home operation at a critical fault.DESCRIPTION OF EMBODIMENTSFIG. 1 shows a schematic illustration of a steering system 1 having a steering shaft 2, at the end of which facing the driver a steering wheel 3 is arranged. The driver can exert a driver hand torque FHM on the steering shaft 2 via the steering wheel 3. The steering shaft 2 is coupled to a steering assist motor 4, via which a superposition torque of a steering request can be exerted on the steering shaft 2. At an end of the steering shaft 2 opposite the steering wheel 3, the steering shaft is suitably coupled to a steering rod 9 of an axle to be steered in order to carry out steering.In a suitable manner, a sensor system 5 is also provided on the steering shaft 2 or on the steering assist motor 4, which makes it possible to determine the driver hand torque FHM exerted on the steering shaft 2 via the steering wheel 3 and to make a corresponding indication available to a control unit 6.The control unit 6 is furthermore coupled to the steering assist motor 4 in order to provide the superposition torque on the steering shaft 2. The superposition torque is provided by a steering function block 7 (control unit) as the sum of partial superposition torques from a series of functions.Depending on the driver hand torque FHMand the assist torque UM, the steering force LFto be provided by the steering assist motor 4 is now determined and the steering assist motor 4 is adjusted accordingly.The steering system functions provided in the steering function block 7 are illustrated in FIG. 2. The core function of the steering assistance is the steering assistance function LKU, which provides an assistance torque UM as a function of a driver hand torque FHM. The assist torque UM is forwarded to a coordination function KF.Furthermore, a stability and dynamic function SDF provides a torque contribution in the form of a compensation torque KM as a partial superposition torque, which prevents a ringing due to the assistance intervention of the steering assistance function or due to a sudden change in the driver hand torque FHM. The further functions F 1 to Fn relate to auxiliary functions which, for example, provide an end stop function and the like which have only secondary significance for the steering assistance.The driver hand torque FHM is provided to the steering assistance function LKU with the aid of sensor functions SF. Each of the steering system functions provides a partial superposition torque as a torque contribution to the correlation function KF, where these are added up and provided as superposition torque in the steering system.If errors occur, the occurrence of the error is signaled to each of the functions. When an error occurs in the auxiliary functions F 1 to Fn, the error is classified as a non-severe error, and a first limp home function NLF 1 is activated in each of the function blocks. The first emergency function comprises that a torque contribution of 0 is provided in the secondary functions F 1 to Fn, the sensor detection function SNL, the steering support function LKU and the stability and dynamic function SDF are operated according to the conventional mode of operation, while the coordination function KF, the sum of the support torque provided by the steering support function and the compensation torque provided by the stability and dynamic function SDF are weighted with a first emergency factor NF 1, which may be between 50% and 90%, for example.If an error occurs in one of the functions of the sensor detection function SNL, the steering assistance function, the stability and dynamic function and in the coordination function KF, the error is considered to be a critical error. In this case, the secondary functions F 1 to Fn are likewise deactivated, i.e. they provide a torque contribution of 0. Furthermore, the sensor detection function SNF is operated according to a second emergency function NLF2, in which the sensor values for the driver hand torque FHM are provided to the steering assistance function LKU without being processed, i.e. without filtering or the like.The steering assist function LKU applies a predetermined assist characteristic to the obtained sensor value for the driver hand torque to determine the assist torque. The stability and dynamic function SDF delivers a compensation torque of 0 as a torque contribution. The coordination function KF reduces the assist torque supplied by the steering assist function LKU with the aid of a second emergency running factor NF2 to an amount of less than 50%, in particular to between 20 and 40%, preferably to 35%. This is intended to ensure that excitation of the steering system 1 by the superposition torque UMand its dynamic behavior is less than the intrinsic damping of the steering system 1.In particular, the torque contribution provided by the steering assist function LKU is reduced to a value which ensures that a ringing in the steering system 1 cannot occur when the stability and dynamic function SDF is deactivated. In this way, continued steering assistance can be provided even if a critical fault has occurred. This allows a driver to safely steer the motor vehicle to a roadside when the fault occurs during a drive of the motor vehicle. The risk of an unacceptable shutdown of the steering assistance can thereby be avoided.Depending on the criticality of the fault, a plurality of different emergency running functions are therefore implemented with different measures. In particular, the control device 6 can execute a method which is illustrated as one of a plurality of possible methods in the flow diagram of FIG. 3.In step S 1, it is checked whether a fault has occurred in one of the steering system functions. If this is not the case (alternative: no), the process cyclically jumps back to step S 1 and the check is thus continued continuously. If an error has occurred (alternative: yes), the type of error is analyzed in step S 2. In this case, the proper execution of each of the individual steering system functions is checked on the basis of checking functions.If it is determined in step S 3 that a malfunction has occurred in one of the secondary functions F 1 to Fn and a non-critical, i.e. non-severe, error is present (alternative: yes), a first emergency operation function is activated in the steering system functions in a first error handling step S 4. The first emergency operation function NFL 1 provides for the secondary functions to be deactivated and for the sensor detection function SNF, the steering assistance function LKU, the stability and dynamic function SDF and the coordination function KF to be substantially maintained. Only in the coordination function KF is the sum of the torque contributions provided by the steering assist function LKU and the stability and dynamic function SDF weighted with a first emergency running factor NF1, which reduces the resulting superposition torque with respect to the superposition torque in normal operation. In this way, a first emergency operation function NFL 1 of the steering assistance can be provided, which enables the motor vehicle to continue driving.If a critical fault is determined as a fault in step S 3 (alternative: no), then in a second fault handling step S 5, the secondary functions are likewise deactivated as a second emergency running function NFL 2, so that these provide a torque contribution of 0. Furthermore, the stability and dynamic function SDF is deactivated and the steering power assistance is operated with a second emergency function NFL 2, which applies a simple assistance characteristic to a sensor value provided by the sensor detection function SNL for a driver hand torque FHM, in order to provide an assistance torque UM to the coordination function KF. In the coordination function KF, this assistance force is subjected to multiplicatively a second emergency running factor NF2 which is so low that a ringing of the steering system 1 can be safely prevented by a corresponding intervention of the steering assistance motor 4.List of reference characters1 Steering system 2 Steering shaft 3 Steering wheel 4 Steering assist motor 5 Sensor system 6 Control unit 7 Driver assist system 9 Steering rod NFL 1, NFL 2 First, second emergency operation function SNL Sensor detection function LKU Steering assist function F 1, F 2,..., Fn Secondary functions SDF Stability and dynamic function KF Coordination function

Claims

Method for operating a steering system (1), comprising the following steps: - detecting a fault of at least one of a plurality of steering system functions in the steering system (1); - upon detection of a fault in one of the steering system functions, activation of a first or a second emergency function (NFL1, NFL2) depending on in which of the steering system functions the fault has occurred, wherein the steering system functions comprise the following functions: ◯ a sensor detection function (SNL) for providing a sensor value for an exerted driver hand torque (FHM), ◯ a steering support function (LKU) for providing an support torque as a torque contribution, ◯ a stability and dynamic function (SDF) for providing a compensation torque as a torque contribution, ◯ one or more secondary functions (F1, F2... Fn) for providing further torque contributions; ◯ a coordination function (KF) for providing a superposition torque as the sum of the torque contributions, - wherein a first emergency operation function (NFL1) is called if an error in at least one of the secondary functions (F1, F2... Fn), - wherein the first emergency function (NFL1) provides for deactivating one or more secondary functions (F1, F2) and for operating the sensor detection function, the steering assistance function (LKU) and the stability and dynamic function (SDF) in normal operation and for operating the coordination function (KF) in such a way that the sum of the torque contributions is weighted with a first emergency factor (NF1), in particular of between 50 and 80%.Method according to Claim 1, wherein a second emergency operation function (NFL2) is called up if an error is detected in the sensor detection function (SNL), in the steering assistance function (LKU), in the stability and dynamic function (SDF) and / or in the coordination function (KF).The method of claim 2, wherein the second run-flat function (NFL2) provides for the one or more secondary functions (F1, F2... Fn), to deactivate the stability and dynamic function (SDF) and the sensor detection function (SNL), to operate the steering assist function (LKU), in which an assist torque (UM) is determined only with a simple assist characteristic curve, and to operate the coordination function (KF) in such a way that the provided assist torque (UM) is scaled down with a second emergency running factor (NF2) in such a way that the excitation caused by the resulting assist torque is lower than the intrinsic damping of the steering system (1).Method according to Claim 2 or 3, wherein the coordination function (KF) is operated such that the provided assist torque (UM) is weighted with a second emergency running factor (NF2) of between 20 and 50%, preferably of between 25 and 40%.Method according to one of Claims 2 to 4, wherein, in the second emergency operation function (NFL2), the steering assistance function (LKU) determines the assistance torque (UM) only as a function of a driver hand torque (FHM) with an in particular linear assistance characteristic curve.Device, in particular a control device, for operating a steering system (1), wherein the device is designed to - detect a fault of at least one of a plurality of steering system functions in the steering system (1); in order to activate a first or a second emergency running function (NFL 1, NFL 2) on detection of a fault in one of the steering system functions depending on in which of the steering system functions the fault has occurred, wherein the steering system functions comprise the following functions: ◯ a sensor detection function (SNL) for providing a sensor value for an exerted driver hand torque (FHM), ◯ a steering assistance function (LKU) for providing an assistance torque as a torque contribution, ◯ a stability and dynamic function (SDF) for providing a compensation torque as a torque contribution, ◯ one or more secondary functions (F 1, F 2... Fn) for providing further torque contributions; o a coordination function (KF) for providing an superposition torque as the sum of the torque contributions, wherein the device is furthermore designed to call up a first emergency operation function (NFL1) if an error in at least one of the secondary functions (F1, F2... Fn), wherein the first emergency function (NFL1) provides for deactivating one or more secondary functions (F1, F2) and for operating the sensor detection function, the steering assistance function (LKU) and the stability and dynamic function (SDF) in normal operation and for operating the coordination function (KF) in such a way that the sum of the torque contributions is weighted with a first emergency factor (NF1), in particular of between 50 and 80%.Steering system (1) comprising a steering assist motor (4) and comprising a device (7) according to claim 6

Images