Method for protecting components of a steer-by-wire steering system, and steer-by-wire steering system

Abstract

The invention relates to a method for protecting the components of a steer-by-wire steering system (20), wherein the steer-by-wire steering system has a spindle drive (21) comprising self-locking means, and wherein the spindle (22) is axially displaced by means of a rotary drive of a spindle nut (23) mounted in a fixed position. For determining a thermal load, a temperature (T_sp) of the spindle drive (21) is calculated, wherein the temperature (T_sp) is calculated at least based on a rotational speed (R) and a torque (M) of the rotary drive and an ambient temperature (T_amb) of the vehicle and / or an ambient temperature (T_sbwl) of the steer-by-wire steering system (20), wherein if at least one first threshold value (T_thr1) is reached, the power of the rotary drive is at least temporarily reduced and / or the displacement (s) of the spindle (22) is at least temporarily restricted.

Description

[0001] Method for protecting components of a steer-by-wire steering system and steer-by-wire steering system

[0002] The invention relates to a method for protecting the components of a steer-by-wire steering system with a spindle drive according to the preamble of claim 1. Furthermore, the invention relates to a control unit for carrying out the method, a computer program with program code means, and a steer-by-wire steering system according to the independent claims.

[0003] DE 10 2018 208 199 A1 discloses an actuator with a spindle drive for a steer-by-wire steering system for a motor vehicle. The spindle drive comprises a spindle with a spindle thread and a stationary and rotating spindle nut with a nut thread. The spindle thread and the nut thread are designed as self-locking motion threads for exclusively axial displacement of the spindle relative to the stationary spindle nut. The flanks of the spindle and spindle nut constantly rub against each other. This self-locking effect, and especially the lateral forces acting on the spindle from the vehicle's chassis, result in high friction. The lubricant counteracts this friction; however, during operation of the steer-by-wire steering system, a continuously high load causes significant heating of the spindle drive and the surrounding components, as well as the lubricant.This puts stress on mechanical and electrical components and can cause the lubricant to lose its tribological properties, which can have a negative impact on the service life of the steer-by-wire steering system.

[0004] DE 10 2020 210 048 A1 discloses a method for estimating thermal load on the spindle drive.

[0005] One object of the invention is to determine the thermal load of the steer-by-wire steering system as accurately as possible in order to maximize the performance of the steer-by-wire steering system during operation. This object is achieved by a method according to claim 1. Further aspects of the invention are specified with a control unit for executing a method, a computer program with program code means, and a steer-by-wire steering system. Advantageous developments are specified in the subclaims corresponding to the respective aspects.

[0006] The invention relates to a method for protecting the components of a steer-by-wire steering system, wherein the steer-by-wire steering system has a spindle drive with self-locking. The steer-by-wire steering system is preferably designed as a rear-axle steering system. The spindle is displaced axially, preferably along its longitudinal axis, by means of a rotary drive of a stationary spindle nut. The spindle drive is thus a rotation-to-translation converter. The rotary drive is preferably designed as an electric motor and drives the spindle nut either directly by means of a hollow rotor electric motor or, in a preferred variant, by means of a gear, preferably a toothed belt gear. In the variant with a gear, the electric motor is preferably arranged axially parallel to the common longitudinal axis of the spindle and the spindle nut. If the spindle nut is driven or rotated in one direction or the other,When the spindle is rotated, the spindle, whose external thread is permanently engaged with the internal thread of the spindle nut, is axially displaced in one direction or the other along its longitudinal axis. This displacement is also referred to as the actuating stroke of the steer-by-wire steering system. The movement thread is preferably designed as a trapezoidal thread. A metric ISO trapezoidal thread in accordance with DIN 103-3 1977-04 is preferred. This thread can be self-locking and, in contrast to a recirculating ball screw thread, exhibits high friction.

[0007] The steering of a motor vehicle must be designed to guide the wheels. The wheels of a steered axle must maintain the set wheel angle so that the vehicle can maintain the specified trajectory, for example driving straight ahead or cornering. The steering therefore not only changes the wheel angle, but also ensures that it is maintained. During normal operation of a steering system, small changes in the wheel steering angle are generally made when a vehicle is moving at a speed significantly above the speed required for parking and / or maneuvering, for example when driving in a built-up area at 30 to 50 km / h, or on a country road or motorway at even higher speeds. In these cases, steering angles of less than 1° are generally to be expected.

[0008] The approach presented here is based on the realization that in certain situations an increased torque or increased force is required for steering or, in other words, for turning the respective wheel. The situation considered here assumes a very low speed of the vehicle from parking and / or maneuvering to a standstill. At a complete standstill, the speed is 0 km / h. When parking and / or maneuvering, a speed of less than or equal to approximately 1 km / h is assumed. In a speed range from 0 to approximately 1 km / h, particularly high forces are required to turn the desired steering angle. The lower the speed, the greater the expected steering forces, which must be managed by the steer-by-wire steering system. This is due to the fact that the entire weight of the vehicle rests on the tires mounted on the rims of the wheels.The contact between tire and road surface is determined by the tire contact patch. The size of a tire's contact patch depends primarily on the wheel load and tire pressure, because the tire's internal pressure bears the majority of the wheel load. But tire width, tire diameter, and sidewall stiffness also play a role. When the wheel is not rotating, greater force is required to steer, and thus a correspondingly greater power is required from the steer-by-wire steering drive (i.e., to rotate the wheel around its vertical axis), than when the wheel is rolling due to the vehicle's movement. As the rolling motion increases, less force is required to steer. It is clear that in addition to the vehicle mass, the ambient temperature and tire temperature also have an influence, as they directly affect the friction between the tire and the road surface.The following additional parameters are not exhaustive: tire compound, tire type, tire friction coefficient, road surface and road condition (dry, wet, slippery, etc.).

[0009] The tire of a wheel is usually made of rubber - an elastic material.

[0010] When a force is applied to the wheel from the steer-by-wire steering system, a preload is created due to the static or sliding friction between the tire and the road surface. The tire is essentially pulled up against the road surface and thus preloaded. Further preload is created between the actuator and the wheel carrier by bearings inserted between them, as well as possibly by links, such as a steering linkage, depending on the chassis design.

[0011] When parking or maneuvering, constant changes in the wheel steering angle or changes in the steering direction from left to right are common or necessary. Very large – often the maximum possible – wheel steering angles are set, which mean maximum travel for the spindle of the spindle drive. These changes occur during parking / maneuvering, as is well known, at a high frequency compared to normal driving. If the steer-by-wire steering makes a frequent change in the aforementioned low speed range, the preloads are initially reduced briefly and then preloads arise again. This preload increases the lower the vehicle speed or when it is reduced from rolling to a standstill. When parking and / or maneuvering, this is virtually a constant change.When steering back from the previously set steering angle, the direction of force in the spindle drive of the steer-by-wire steering system also changes. This leads to a load change within the rotary drive or spindle drive.

[0012] The high forces acting on the spindle increase friction in the threaded spindle of the steer-by-wire steering system. This friction generates heat in the threaded spindle. Frequent changes in steering direction, particularly over extended periods and at large steering angles, can cause the lubricant to heat up to such an extent that its tribological properties fail. In some cases, the lubricant can even reach or exceed its boiling point, causing the lubricant to fail. Without lubricant, the flanks of the spindle drive threads can wear down significantly, which can lead to premature failure of the spindle drive. Other mechanical or electrical components can also heat up considerably and be damaged. A maximum thermal load is defined during the design and layout of a spindle drive and, ultimately, the steer-by-wire steering system. This maximum thermal load is, for example,140 degrees Celsius for a certain period of time. To prevent the aforementioned heat input from exceeding the maximum thermal load, additional sensors could be used in the steer-by-wire steering system to record the current temperature on the spindle drive, specifically the spindle and / or the spindle nut. However, the actual thermal load in the moving thread is difficult to measure or record using sensors because the spindle nut rotates and the spindle therefore moves axially. Measuring directly on the friction partners is therefore very complex. Space would have to be provided for sensors or a sensor system in the steer-by-wire steering system, and an electrical connection to a control or value unit would have to be provided. This would result in additional, undesirable costs in the production of a steer-by-wire steering system.

[0013] Large steering angles here refer to steering angles that extend into the range of the maximum design steering angles of the respective axle. When maneuvering or parking, frequently changing steering angles are also required. Utilizing large, preferably the largest possible, steering angles makes it easier to enter a parking space, for example, or maneuver with a trailer. It is particularly advantageous if the rear axle of the vehicle is steerable in addition to the front axle.

[0014] It has surprisingly been found that the instantaneous thermal load and thus also a maximum thermal load of the steer-by-wire steering system can be precisely determined, in contrast to methods known from the prior art. According to a first aspect of the invention, a temperature of the spindle drive, preferably of the spindle and / or the spindle nut, is calculated to determine the thermal load. The calculation is carried out at least as a function of a rotational speed and a torque of the rotary drive and an ambient temperature. An ambient temperature of the vehicle and / or the steer-by-wire steering system is taken into account, which is detected by existing sensors. If the calculated temperature value reaches a first threshold value, which can correspond, for example, to reaching the maximum thermal load, the power of the rotary drive is reduced, at least temporarily. Alternatively or additionally, the displacement of the spindle is restricted.The power reduction and / or displacement reduction occurs temporarily for a short period of time so that the spindle drive, especially the spindle, can cool down. After cooling down, which can be determined based on the continuously calculated temperature of the spindle drive, especially the spindle, the power reduction or displacement restriction is lifted.

[0015] A limited spindle displacement here corresponds to a limited or, in other words, less than the maximum possible wheel steering angle, based on a maximum wheel steering angle at maximum spindle displacement. Reducing the power can, for example, reduce the torque or speed of the rotary drive.

[0016] Based on knowledge of the materials of the spindle drive, in particular the spindle and / or spindle nut, their friction coefficients, the maximum expected forces acting on the spindle, as well as the required spindle displacement and the properties of a lubricant for the spindle drive, at least an initial threshold value is determined for the respective steer-by-wire steering system, which corresponds, for example, to the maximum thermal load. Additional threshold values ​​can be defined to capture additional load limits.

[0017] A steer-by-wire steering system is a usually electromechanical unit that is decoupled from a mechanical steering handle, e.g., a steering wheel. Steering signals are generated in a control unit based on steering signals and one or more parameters such as vehicle speed, steering wheel angle, current steering angles on the front and / or rear axle, yaw acceleration and / or lateral acceleration of the vehicle, etc. The steering movement is carried out at least by means of a rotary drive of the steer-by-wire steering system, which receives steering signals from the control unit. For example, a spindle drive can be used to axially displace a spindle or steering rod that is directly or indirectly articulated to the wheel carrier. By displacing the spindle, the wheel carriers can be pivoted about their vertical axis, so that the wheels rotatably mounted on the wheel carriers can be subjected to a change in the wheel steering angle of the respective wheel carrier.Advantageously, the actual temperature of the spindle drive, preferably of the spindle and / or the spindle nut, can be calculated independently of the vehicle speed. By taking the ambient temperature into account using sensors or sensor systems present in the vehicle or in the steer-by-wire steering, it is possible to determine the thermal load during operation of the steer-by-wire steering. This is possible while driving and also when the vehicle is stationary, and thus even before or when starting the vehicle or the steer-by-wire steering. Advantageously, no additional sensors are required to measure the temperature of the spindle drive, which would otherwise have to be directed at the spindle or the spindle nut (spindle drive), for example as infrared sensors within the steer-by-wire steering.The method according to the invention makes it possible to reduce power or restrict the spindle travel only when the actual temperature of the spindle drive within the steer-by-wire steering system actually requires it. If, for example, the design-permissible maximum thermal load is reached, the power is temporarily reduced or restricted. The temperature of the spindle drive then decreases (cooling). Due to the continuous calculation, normal operation can be resumed as soon as the temperature falls below the threshold. In other words, instead of an estimate, actual temperatures of the spindle drive are determined, and the condition of the lubricant used is also monitored.Premature failure of the lubrication due to excessive temperature in the spindle drive is thus largely ruled out and the service life of the spindle drive and thus of the steer-by-wire steering system is not reduced.

[0018] Steer-by-wire steering can thus be used to the greatest extent possible. The method is therefore more accurate than temperature estimation and is therefore advantageous for parking and maneuvering at very low speeds, even to a standstill, where large wheel steering angles must be achieved in conjunction with frequent changes of direction. Furthermore, this method also takes into account operating conditions such as very high outside temperatures or heat buildup in the area of ​​the steer-by-wire steering. These conditions are taken into account by recording one or more ambient temperatures.

[0019] This method takes into account the natural cooling behavior depending on the material used for the spindle drive, in particular the spindle. Cooling occurs primarily through thermal conduction, since the spindle drive, in particular the spindle, is a solid. A material such as steel, for example, exhibits natural cooling behavior. This means that a material cools faster at room temperature (20 °C) in the same period of time than at a high temperature of, for example, 80 °C. Due to the continuous calculation of the temperature, this circumstance can be directly taken into account. This makes it possible to achieve the shortest possible period for temporarily reducing power and / or limiting displacement.

[0020] Preferably, one or more ambient temperatures, such as the ambient temperature of the vehicle or the ambient temperature of the steer-by-wire steering system on a respective axle, preferably the rear axle, are read in. Reading in is performed in particular at intervals, preferably in a range of 10 to 40 ms, most preferably every 20 ms. The ambient temperatures of different sources or sensors are preferably compared with each other. In this way, a plausibility check of the read temperatures or the temperature sensors can be performed. Preferably, the lower of the determined temperatures is used. Preferably, the ambient temperature of the vehicle is considered as the generally lower of the read temperatures.The ambient temperature of the vehicle is preferably recorded using an existing temperature sensor, which is available in the vehicle and, for example, also reliably provides the air conditioning system in the vehicle with a real outside temperature in the sense of an ambient temperature. Such a sensor is installed in such a way that it is not influenced by strongly heated sources (combustion engine, brakes, etc.) and does not indicate incorrect temperatures. It has been found that the ambient temperature of the vehicle is a good starting point for a real temperature calculation. Preferably, the ambient temperature of the steer-by-wire steering is recorded using at least one sensor that is directly assigned to the steer-by-wire steering. This involves existing sensors or sensor systems, in particular temperature sensors such asThermistors located in the power electronics and / or control unit (ECU) and / or electric motor of the steer-by-wire steering system. If several of the aforementioned temperature sensors are present, these recorded temperatures are compared with each other. Preferably, the lower of the recorded ambient temperatures of the steer-by-wire steering system is taken into account. Such temperature sensors are preferably assigned to at least one CPU of the respective aforementioned units or integrated into the respective CPU of one or more of these units. Thus, no additional sensors or sensor systems need to be installed. These would require additional space in the steer-by-wire steering system and incur additional high costs. A steer-by-wire steering system is an ASIL-D-rated assembly, which represents the highest safety rating with regard to vehicle reliability.This would require the use of ASIL-D compliant sensors, which are expensive. A CPU is a microprocessor (central processing unit).

[0021] Preferably, the calculated spindle temperature is saved with a timestamp when the vehicle is parked and the steer-by-wire steering system is switched off. If the vehicle is restarted after a short period of time, for example, 5 minutes, the actual temperature of the spindle drive, especially the spindle, can be determined based on the currently read ambient temperature of the vehicle and the knowledge of the cooling behavior at this temperature. This is particularly advantageous if no steering movement has yet taken place when the vehicle is restarted, and thus the rotational speed of the rotary drive is not available.

[0022] Preferably, after calculating the temperature of the spindle drive, a derating factor is determined taking into account the previously calculated temperature. As already stated above with regard to the first aspect of the invention, when a first threshold value is reached, the rotary drive's power is reduced and / or the displacement of the spindle is restricted, at least temporarily. The reduction or restriction of the displacement can be determined using the derating factor. The derating factor can assume values ​​between 0 and 1. With a derating factor of 0, for example, a maximum reduction in power or restriction of the displacement can be specified. With a derating factor of 1, on the other hand, no reduction and no restriction of the displacement is specified. The steer-by-wire steering can carry out a maximum displacement of the spindle, which is equivalent to a maximum change in the wheel steering angle. The derating factor can, for example, correspond to a curve orfollow a characteristic curve, with different values ​​of the derating factor being assigned to specific temperature values ​​as threshold values ​​in the form of so-called support points. For example, it can be specified that a first support point (a first threshold value) is at 80 °C, a next support point at 100 °C and a further support point at 120 °C (further threshold values). If the calculated temperature value is in the range of 0-80 °C, the derating factor can be 1, so that neither a reduction nor a restriction of the displacement takes place. In this range, the steer-by-wire steering can operate at maximum power and set the maximum steering angle or wheel steering angle. The next support point of 100 °C can be assigned a derating factor of 0.2. The power and / or displacement can thus be reduced by 80% or, in other words, only 20% once this support point is reached.From the next reference point at 120 °C, the displacement can be reduced or restricted to 0%. This is equivalent to at least a temporary shutdown of the rotary drive and thus of the spindle drive. Between the aforementioned reference points, the derating factor can, for example, decrease linearly. Depending on the characteristic curve, a logarithmic or similar decrease in the derating factor can also be provided between the respective or individual reference points. Different characteristic curves can form a characteristic curve family, whereby the different characteristic curves can be selected depending on additional parameters. These parameters can correspond, for example, to the ambient temperature of the vehicle and / or the steer-by-wire steering system, the vehicle load, tire type, tire dimension, tire pressure or other vehicle parameters.Between the aforementioned sampling points, the derating factor can change at specific intervals of, for example, 20% each. Based on the above example, the derating factor between the sampling points 80°C and 100°C can assume a value of 0.6 at 90°C, which can correspond to a power and / or displacement of 60%. Alternatively, the derating factor can decrease more steeply (negative gradient) and / or continuously between the sampling points, allowing for a more precise reduction in power and / or limitation of displacement.

[0023] In an alternative embodiment, the derating factor can be calculated using a function, preferably continuously, wherein the function calculates the derating factor at least as a function of the temperature. The derating factor can be determined using a characteristic curve or a characteristic curve family or the aforementioned function in a control unit in the vehicle, preferably the control unit of the steer-by-wire steering system. In this way, depending on the calculated temperature of the spindle drive, in particular the spindle, targeted cooling can be achieved, which is adapted to the respective instantaneous thermal load. In this advantageous manner, the steer-by-wire steering system can be operated in a first wide temperature range (here up to 80 °C) without reducing the power or restricting the displacement.At higher temperatures (here, 80-130°C), a fine gradation of the reduction and / or restriction is possible depending on the actual calculated temperature and the cooling behavior of the spindle drive, especially the spindle. This ensures that the steer-by-wire steering system can deliver the highest possible performance across its entire operating range.

[0024] With regard to the aforementioned embodiments, the temporary power reduction of the rotary drive and / or the limited displacement of the spindle is preferably continuously maintained until the calculated temperature of the spindle drive, preferably of the spindle, has decreased by a differential value starting from the previously reached threshold value. The differential value can be in a range of 1-7 K, preferably in a range of 3-6 K, most preferably 5 K. The differential value can be variable in the aforementioned ranges, in particular, so that the differential value, in particular at the aforementioned support points, varies depending on the threshold value(s) reached. For example, the differential value can increase in magnitude as the temperature of the spindle increases. The cooling behavior of the spindle used can thus be specifically taken into account.

[0025] Preferably, the mass and / or specific heat capacity of the material of the spindle drive, in particular the spindle, is taken into account. These parameters can be stored or saved in a control unit, for example, when calculating the spindle temperature. When calculating the temperature and / or determining the derating factor, one or more of these parameters can be considered to achieve even more precise results. The cooling behavior of the material used can thus be better considered.

[0026] Preferably, the efficiency of the spindle drive is taken into account. Depending on the thread used for the threaded partners (spindle nut with internal thread and spindle with external thread), the influence of the pitch and friction of the interacting flanks of the moving thread can provide an even more accurate result.

[0027] The difference between reduced or restricted and less or no longer reduced or unrestricted operation of the steer-by-wire steering could result in significant differences in performance. A change back to normal operation, for example (without reduction or restriction), could be perceived by the driver or passengers as sudden steering movements, which is undesirable. The change from reduced power or restricted shift to less or no reduced power or unrestricted shift of the spindle (normal operation) should therefore preferably be gradual, so that no sudden, in other words sudden, steering movements are noticed. When returning to the previous operation or normal operation, the steer-by-wire steering will gradually change the power or shift at least to the previous state or completely unrestricted normal operation.Both the driver and the passengers do not perceive this as an unpleasant driving situation. In other words, a gradual change is understood as a gradual change. The thermal load determination according to the invention is a safety device that can be advantageously used in view of the safety requirements for vehicle steering systems and for the longevity of the steer-by-wire steering system. It is a cost-effective solution that preferably runs as a safety function on the existing control unit of the steer-by-wire steering system.

[0028] According to a further aspect of the invention, a control unit is provided for executing a method. This control unit is preferably part of the steer-by-wire steering system. The control unit can be integrated into the steer-by-wire steering system's housing. The control unit is preferably directly assigned to the steer-by-wire steering system or is structurally part of the steer-by-wire steering system. Alternatively, a control unit already present in the vehicle can be used.

[0029] A control unit (also called ECU, Electronic Control Unit) can be understood here as an electrical device that processes sensor signals and outputs control and / or data signals depending on them. The control unit can have an interface that can be implemented in hardware and / or software. In a hardware implementation, the interfaces can, for example, be part of a so-called system ASIC, which contains a wide variety of functions of the control unit. However, it is also possible for the interfaces to be separate integrated circuits or to consist at least partially of discrete components. In a software implementation, the interfaces can be software modules that are present, for example, on a microcontroller alongside other software modules.

[0030] A computer program with program code means for carrying out a method as specified above is also advantageous when the program is executed on a computer, in particular on the aforementioned control unit. The program code is preferably stored on a storage medium such as a semiconductor memory, hard disk memory, or an optical memory. Finally, the invention relates to a steer-by-wire steering system, which is preferably designed as a rear-axle steering system and has a control unit as mentioned above, which can carry out a method as mentioned above. The steer-by-wire steering system comprises a spindle drive, preferably with self-locking, wherein a spindle is axially displaced by means of a rotary drive of a stationary spindle nut. The steer-by-wire steering system has power electronics and an electric motor.The electric motor is connected to the spindle drive, in particular by means of a gear, preferably a toothed belt drive, and forms the rotary drive there.

[0031] Steer-by-wire steering can also be advantageously used in semi-autonomous or fully autonomous vehicles. Especially in these vehicles, the driver's attention can be expected only partially or not at all. Any warnings in the cockpit regarding high thermal loads on a component such as the steering system may not be noticed, or may not be noticed in a timely manner. At the same time, maximum availability of steer-by-wire steering is important for the uninterrupted operation of the vehicle.

[0032] The invention is described below using preferred embodiments with reference to the drawing. In the drawing:

[0033] Fig. 1 a steer-by-wire steering system

[0034] Fig. 2 is a flow chart of the method according to the invention

[0035] Fig. 3 a diagram regarding the derating factor of the invention and

[0036] Fig. 4 a diagram regarding the cooling behavior

[0037] Fig. 1 shows a steer-by-wire steering system 20 according to the invention, which is used for the rear-axle steering of a motor vehicle. The steer-by-wire steering system 20 has a spindle drive 21, which comprises a spindle 22, a spindle nut 23, rolling bearings 24 and a belt pulley 25, which can be driven by an electric motor 27 via a belt 26 and forms a rotary drive. The electric motor 27 is controlled by a control unit 35, which is arranged thereon. By turning the stationary spindle nut 23, the spindle 22 is axially displaced. The axial displacement s is indicated by a double arrow. The displacement s results in an actuating stroke s for the spindle 22, also called a travel distance s. The spindle 22 can be adjusted from its left end stop to its right end stop. This corresponds to the maximum stroke or maximum axial displacement. The spindle 22 is shown in its center position in Figure 1.This corresponds to a wheel steering angle of 0°, which corresponds to straight-ahead travel. The spindle 22 has an anti-twist device (not shown) so that it cannot rotate when the spindle nut 23 turns. The steer-by-wire steering system 20 has a housing 28 which is attached to the vehicle body via a first joint 29. The spindle 22 is firmly connected at one of its two ends to a bearing sleeve 30 which is guided axially slidingly relative to the housing 28 and is connected at its outer end protruding from the housing 28 to a second joint 31. The steer-by-wire steering system 20 is connected via the second joint 31 to a steering linkage (not shown), e.g. a toe link, directly or indirectly to a wheel carrier of a motor vehicle and can thus steer a wheel, such as a rear wheel. The vehicle-side support is provided via the first joint 29. A bearing sleeve 30 is arranged on the housing 28 orPower electronics 32 are arranged on the electric motor 27, which serves to switch, control, and supply power to the steer-by-wire steering system 20, in particular the power electronics 32, the electric motor 27, and the control unit 35. Two of the units shown in Figure 1 are used to steer two wheels of an axle. A single steer-by-wire steering system is also conceivable, in which the spindle is connected to a wheel carrier at each end (also known as a centrally acting steer-by-wire steering system, in which the housing has a passage for the spindle at each end).

[0038] Fig. 2 shows a flowchart for a possible embodiment of method 500 for protecting components of a steer-by-wire steering system. In individual steps, the rotational speed R and the torque M of the rotary drive, the ambient temperature of the vehicle T_amb, and the ambient temperature of the steer-by-wire steering system T_sbwl, here the ECU or CPU of the power electronics 32, are read in and transmitted to a control unit 35 via a vehicle bus 510, for example, a CAN bus or Flexray bus. In the control unit 35, a temperature T_sp of the spindle 22 is continuously calculated from the aforementioned parameters in a unit 550. In a next step, the temperature T_sp of the spindle is forwarded to a derating element 560. A continuous comparison of the calculated temperature T_sp of the spindle 22 with threshold values ​​(T_thr1, T_thr2, T_thr3) stored in the control unit 35 as support points takes place.The derating element 560 determines a derating factor D_f based on characteristic curves of a characteristic curve field stored in the control unit 35 or by means of a function, taking the interpolation points into account. The derating factor D_f is transferred to a unit 590 of the control unit 35. The derating factor D_f is sent to the power electronics 32 via a signal. The power electronics 32 controls the rotary drive accordingly, so that, depending on the determined temperature T_sp of the spindle and the reached threshold value, it is temporarily operated with reduced power and / or restricted displacement until the calculated temperature T_sp has decreased by a differential value. The reduction or restriction is then removed, and the steer-by-wire steering system returns to normal operation.

[0039] The derating factor D_f can, as mentioned above, be determined from a characteristic curve. Figure 3 shows a characteristic curve 300. In an initial broad range of 0-80°, the derating factor D_f is 1. The steer-by-wire steering system is operated at 100% power or displacement in this initial broad range, and there is no reduction or restriction. 80 °C represents a first interpolation point on the characteristic curve 300 as well as a first threshold value T_thr1. Above this threshold value T_thr1, the characteristic curve drops relatively steeply to a further interpolation point with a threshold value T_thr2 at 100 °C. From this interpolation point, the characteristic curve drops more gently in comparison until it reaches the next interpolation point with a threshold value T_thr3 at 120 °C. At 100 °C, the power or displacement is reduced to 20%. From 120 °C, the power or displacement is reduced to 0%, which is equivalent to switching off the steer-by-wire steering.As an example, it is shown between the two aforementioned support points or threshold values ​​T_thr1 and T_thr2 that at 90 °C, due to the characteristic curve 300, the derating factor is 0.6, which equates to a reduction in the power of the rotary drive or the displacement of the spindle to 60% of the maximum power. Fig. 4 shows, as an example, the cooling behavior of a spindle made of steel at different temperatures in degrees Celsius per second. It is clear that, starting from a high temperature of, for example, 100 °C, the spindle cools down more quickly at room temperature of 20 °C in the same period of time than at a high temperature of, for example, 80 °C. This fact is taken into account in the present method. The cooling of the spindle takes a certain period of time, which results from the ambient temperature and the calculated temperature of the spindle. The reduction in power orThe spindle displacement is thus limited depending on the cooling behavior of the material used. However, since the temperature can be calculated, no reduction or restriction is required compared to an estimate (in the range of 0 to 80°C). At higher temperatures, one only needs to wait for the actual cooling. Advantageously, the actual outside temperature of the vehicle and / or any existing heat buildup in the area of ​​the steer-by-wire steering system are also taken into account.

[0040] It has been shown that the method presented here enables precise determination of the thermal load even without additional temperature sensors. This advantageously eliminates the need for additional direct temperature sensor detection in the spindle drive.

[0041] Reference symbol

[0042] 20 Steer-by-wire steering

[0043] 21 Spindle drive

[0044] 22 spindle

[0045] 23 Spindle nut

[0046] 24 rolling bearings

[0047] 25 pulley

[0048] 26 belts

[0049] 27 Electric motor

[0050] 28 housings

[0051] 29 first joint

[0052] 30 bearing sleeve

[0053] 31 second joint

[0054] 32 Power electronics

[0055] 35 Control unit

[0056] 300 characteristic curve

[0057] 500 procedures for protecting components of a steer-by-wire steering system

[0058] 510 vehicle bus

[0059] 550 Unit for calculating spindle temperature

[0060] 560 Derating element

[0061] 590 interface

[0062] D_f derating factor

[0063] M torque

[0064] R speed

[0065] T_amb ambient temperature

[0066] T_sbwl Ambient temperature steer-by-wire steering

[0067] (Power electronics / CPU / ECU)

[0068] T_sp temperature

[0069] T_thr1 threshold

[0070] T_thr2 Threshold value T_thr3 Threshold value s Actuating stroke, axial displacement

Claims

Patent claims 1. A method for protecting the components of a steer-by-wire steering system (20), wherein the steer-by-wire steering system (20) has a spindle drive (21) with self-locking, wherein a spindle (22) is axially displaced by means of a rotary drive of a stationary spindle nut (23), characterized in that a temperature (T_sp) of the spindle drive (21) is calculated to determine a thermal load, wherein the temperature (T_sp) is calculated at least as a function of a rotational speed (R) and a torque (M) of the rotary drive and an ambient temperature (T_amb) of the vehicle and / or an ambient temperature (T_sbwl) of the steer-by-wire steering system (20), wherein upon reaching at least a first threshold value (T_thr1), the power of the rotary drive is at least temporarily reduced and / or the displacement (s) of the spindle (22) is restricted.

2. Method according to claim 1, characterized in that when reading in one or more ambient temperatures (T_sbwl; T_amb), these are read in continuously, preferably at intervals, and compared with one another at several ambient temperatures, the lower temperature being taken into account in each case.

3. Method according to claim 1 or 2, characterized in that the ambient temperature (T_sbwl) of the steer-by-wire steering system (20) is detected by means of at least one sensor which is directly assigned to the steer-by-wire steering system (20), in particular by means of an existing temperature sensor of at least one power electronics unit (32) and / or a control unit (35) and / or an electric motor (27) of the steer-by-wire steering system (20), wherein preferably the temperature sensor assigned to a respective CPU or integrated in the respective CPU of at least one of the aforementioned units is used.

4. Method according to one of the preceding claims, characterized in that, taking into account the calculated temperature (T_sp), a Derating factor (D_f) is determined, preferably by means of a function D_f (Tx), preferably continuously calculated, or is determined taking into account a characteristic field or a control rule, wherein the reduction of the power of the rotary drive and / or the restriction of the displacement of the spindle (22) takes place taking into account the derating factor (D_f).

5. Method according to claim 4, characterized in that the at least first threshold value (T_thr1) or further threshold values (T_thr2, T_thr3) are defined with respect to the temperature (T_sp), in particular a derating factor (D_f) is assigned to each of these threshold values, wherein, depending on the respectively reached threshold value (T_thr1, T_thr2, T_thr3), the power reduction of the rotary drive and / or the restricted displacement of the spindle (22) is continuously maintained until the continuously calculated temperature (T_sp) has decreased by a difference value from the previously reached threshold value (T_thr1, T_thr2, T_thr3).

6. Method according to one of the preceding claims, characterized in that a mass and / or a specific heat capacity of the material of the spindle drive (21), in particular of the spindle (22), is taken into account.

7. Method according to one of the preceding claims, characterized in that the efficiency of the spindle drive (21) is taken into account.

8. Method according to one of the preceding claims, characterized in that after the reduction in the power of the rotary drive and / or the restricted displacement of the spindle (22) has been eliminated, the change to normal operation takes place gradually so that no sudden steering movements occur.

9. Control device (35) for carrying out a method according to one of the preceding claims.

10. Computer program with program code means for carrying out a method according to one of the preceding claims from 1 to 8 when the program is executed on a computer, in particular on the control device (35) according to claim 9. 11 . A machine-readable storage medium on which the computer program according to claim 10 is stored.

12. Steer-by-wire steering system (20) comprising a spindle drive (21) with self-locking, wherein a spindle (22) is axially displaced by means of a rotary drive of a stationary spindle nut (23), preferably designed as a rear-axle steering system, comprising a control unit (35) according to claim 9.

13. Steer-by-wire steering system (20) according to claim 12, further comprising power electronics (32) and an electric motor (27), in particular with a transmission.

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