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

Abstract

The present invention relates to a method for protecting components of a steer-by-wire system (20), wherein the steer-by-wire system (20) has a lead screw drive (21) with a self-locking function, wherein the lead screw (22) is axially displaced relative to a lead screw nut (23) supported in a fixed position by means of a rotary actuator. To determine the thermal load, the temperature (T_sp) of the lead screw drive (21) is calculated, wherein the temperature (T_sp) is calculated based at least on the rotational speed (R) and torque (M) of the rotary actuator, the ambient temperature of the vehicle (T_amb), and / or the ambient temperature (T_sbwl) of the steer-by-wire system (20), wherein when at least a first threshold (T_thr1) is reached, the power of the rotary actuator is at least temporarily reduced and / or the displacement (s) of the lead screw (22) is limited.

Description

Technical Field

[0001] This invention relates to a method for protecting components of a steer-by-wire system having a lead screw drive, according to the preamble of claim 1. Furthermore, the invention also relates to a controller for performing the method, a computer program comprising program code, and a steer-by-wire system according to the parallel claims. Background Technology

[0002] Document DE102018208199A1 discloses an actuator for a steer-by-wire system in a motor vehicle, which has a lead screw drive. The lead screw drive includes a lead screw with a lead screw thread and a lead screw nut, which is fixedly supported and rotatably driven, and has an internal thread. The lead screw thread and nut thread are configured as self-locking kinematic threads to allow pure axial displacement of the lead screw relative to the fixedly supported lead screw nut. The threaded sides of the lead screw and lead screw nut continuously rub against each other. Due to the self-locking function, and especially due to the lateral forces acting on the lead screw from the vehicle chassis, high frictional forces are generated. Lubricant can counteract this friction, but during operation of the steer-by-wire system, the continuous high load causes the lead screw drive, surrounding components, and lubricant to generate intense heat. Mechanical and electrical components are thus subjected to load, and the lubricant may lose its tribological properties, which may negatively impact the service life of the steer-by-wire system.

[0003] Document DE102020210048A1 discloses a method for estimating the thermal load of a lead screw drive device. Summary of the Invention

[0004] The objective of this invention is to determine the thermal load of the steer-by-wire system as accurately as possible so as to maximize the performance of the steer-by-wire system during operation.

[0005] This task is solved by the method according to claim 1. Other aspects of the invention are defined by a controller for performing the method, a computer program comprising program code, and a steer-by-wire system. Advantageous improvements are given in the dependent claims appended to each claim.

[0006] This invention relates to a method for protecting components of a steer-by-wire system, wherein the steer-by-wire system has a lead screw drive with a self-locking function. The steer-by-wire system is preferably configured as a rear-axle steering system. The lead screw, supported relative to a lead screw nut with a fixed position by means of a rotary actuator, is axially, preferably displaced along its longitudinal axis. Therefore, the lead screw drive is a rotary-translational converter. The rotary actuator is preferably configured as an electric motor and directly drives the lead screw nut via a hollow rotor motor, or in a preferred variant, via a transmission device, preferably a toothed belt drive. In the belt drive variant, the electric motor is preferably arranged parallel to the common longitudinal axis of the lead screw and the lead screw nut. When the lead screw nut is driven or rotated in one direction or the other, the lead screw, whose external thread continuously engages with the internal thread of the lead screw nut, is axially displaced along its longitudinal axis in one or the other direction. This displacement is also referred to as the adjustment stroke of the steer-by-wire system. The moving thread is preferably configured as a trapezoidal thread. A metric ISO trapezoidal thread conforming to standard DIN103-31977-04 is preferred. This type of thread can be designed to be self-locking and has high friction compared to ball screws.

[0007] Steering of a motor vehicle must be undertaken by the wheels. The wheels on the steering axle must maintain a set wheel steering angle so that the vehicle can maintain a predetermined trajectory, such as driving in a straight line or cornering. Therefore, the steering system not only changes the wheel steering angle but is also responsible for maintaining that angle. During normal operation of the steering system, when the vehicle is traveling at speeds significantly higher than the speed required to park and / or maneuver, such as when driving at 30 to 50 km / h in urban areas or on rural roads or highways with higher speeds, most of the changes in wheel steering angle are small. In these cases, it should generally be assumed that the steering angle is less than 1°.

[0008] The method described here is based on the understanding that in certain situations, higher torque or greater force is required for steering, or in other words, to turn the corresponding wheels. The scenario considered here is a vehicle coming to a complete stop at extremely low speeds, from parking and / or maneuvering. At a complete stop, the speed is 0 km / h. For parking and / or maneuvering, the speed is assumed to be less than or equal to approximately 1 km / h. In the speed range of 0 to approximately 1 km / h, a particularly large force is required to achieve the desired steering angle. The lower the speed, the greater the expected steering force, which must be generated by the steer-by-wire system. This is because the entire weight of the vehicle acts on the tires mounted on the rims. The contact area between the tires and the road surface is determined by the tire contact patch. The size of the tire contact patch depends primarily on the tire load and tire pressure, as the tire's internal pressure bears most of the tire load. However, tire width, tire diameter, and sidewall stiffness also play a role. When the wheels are not turning, steering requires greater force; therefore, the rotary actuator of the steer-by-wire system (i.e., the wheel that rotates around its vertical axis) needs to provide correspondingly higher power. In contrast, if the wheels are rolling due to vehicle movement, less force is required. As rolling motion increases, the force required for steering gradually decreases. Clearly, in addition to vehicle mass, ambient temperature and tire temperature also have an impact, as they directly affect the friction between the tires and the road surface. The following parameters (not an exhaustive list) also need to be considered: tire compound, tire type, tire coefficient of friction, road surface type, and road condition (dry, wet, slippery, etc.).

[0009] Wheel tires are typically made of rubber (an elastic material).

[0010] When the steering force applied to the wheels by the steer-by-wire system is applied, preload is generated due to static or sliding friction between the tire and the road surface. The tire is "taut" relative to the road surface, thus generating preload. Further preload is generated between the actuator and the wheel bracket through bearings mounted therebetween and possibly linkages (such as steering tie rods, depending on the chassis structure).

[0011] When parking or maneuvering, it is common and necessary to continuously change the wheel steering angle or change the steering direction from left to right. A very large (usually the maximum possible) wheel steering angle is set at this time, corresponding to the maximum displacement travel of the lead screw in the steering steerable system. These changes occur at a higher frequency during parking / maneuvering compared to normal driving. Now, if frequent steering changes are made via the steer-by-wire system within the aforementioned low-speed range, the preload will first decrease briefly and then be regenerated. This preload is greater at lower vehicle speeds, or when the speed decreases from rolling to a standstill. This alternation occurs almost continuously during parking and / or maneuvering. Furthermore, when returning to center from a previously set steering angle, a change in the direction of force occurs within the lead screw in the steer-by-wire system. This causes alternating loads within the rotary drive or lead screw in the steering steerable system.

[0012] The significant force acting on the leadscrew increases friction in the moving threads of the leadscrew drive mechanism in a steering-by-wire system. This friction generates heat within the moving threads. Due to frequent changes in steering direction, especially over extended periods and at large steering angles, the heat input can become so great that the lubricant is heated to the point where its tribological properties fail. The lubricant may even reach or exceed its boiling point and fail. Without lubricant, the flanks of the leadscrew drive threads will wear severely, potentially leading to premature failure of the leadscrew drive mechanism. Other mechanical or electrical components may also overheat and be damaged.

[0013] For lead screw drives and ultimately for steer-by-wire systems, a maximum heat load is specified in their design and operating specifications. For example, this might be 140 degrees Celsius over a certain period. To prevent this heat input from exceeding the maximum heat load, additional sensors can be used in the steer-by-wire system to detect the current temperature of the lead screw drive, particularly the lead screw and / or lead screw nut. However, the actual heat load in the moving threads is difficult to measure or detect using sensors because the lead screw nut rotates, causing axial displacement of the lead screw. Direct measurement on the friction pair is very complex. Sensors or sensing devices in steer-by-wire systems require reserved installation space and electrical connections to the controller or evaluation unit. This results in additional, undesirable costs during the production of the steer-by-wire system.

[0014] The large steering angle mentioned here refers to the steering angle within the maximum possible steering angle range of the corresponding axle structure. Frequently changing steering angles are also required when maneuvering or parking. By utilizing a large, preferably the maximum, possible steering angle, it is easier to drive into, for example, parking spaces or towed vehicle maneuvers. This is particularly advantageous if the rear axle of the motor vehicle can also steer in addition to the front axle.

[0015] Surprisingly, compared to methods known in the prior art, the instantaneous thermal load and maximum thermal load of the steer-by-wire system can be determined with greater precision. According to a first aspect of the invention, to determine the thermal load, the temperature of the lead screw drive, preferably the lead screw and / or the lead screw nut, is calculated. This calculation depends at least on the rotational speed and torque of the rotary actuator and the ambient temperature. The ambient temperature of the vehicle and / or the steer-by-wire system, detected by existing sensing devices, is considered here. When the calculated temperature value reaches a first threshold (e.g., corresponding to reaching the maximum thermal load), the power of the rotary actuator is temporarily reduced. Alternatively or additionally, the displacement of the lead screw is limited. The reduction in power and / or displacement is temporary, lasting for a short period to allow the lead screw drive, particularly the lead screw, to cool down. Based on the continuously calculated temperature of the lead screw drive, particularly the lead screw, it can be determined that once cooling is complete, the power reduction or displacement limitation is lifted.

[0016] Here, the limited displacement of the lead screw (starting from the maximum wheel steering angle corresponding to the maximum displacement of the lead screw) corresponds to a limited, or smaller, steering angle than the maximum possible wheel steering angle. When reducing power, for example, the torque or speed of the rotary drive can be reduced.

[0017] Based on the materials of the lead screw drive unit, particularly the lead screw and / or lead screw nut, their coefficients of friction, the maximum expected force acting on the lead screw, the required lead screw displacement, and the characteristics of the lubricant in the lead screw drive unit, at least a first threshold is structurally defined for the corresponding steering-by-wire system. This first threshold corresponds, for example, to the maximum thermal load. Additional thresholds may be specified to detect other load limits.

[0018] A steer-by-wire system is a typically electromechanical unit decoupled from mechanical steering controls (such as a steering wheel). A steering signal is generated in a controller based on the steering signal and one or more parameters (such as vehicle speed, steering wheel angle, current steering angle of the front and / or rear axles, vehicle yaw acceleration and / or lateral acceleration, etc.). Steering motion is achieved at least through a rotary actuator of the steer-by-wire system, which receives the steering signal from the controller. For example, a lead screw or steering tie rod can be axially displaced via a lead screw drive mechanism, which is directly or indirectly hinged to a wheel carrier. By displacing the lead screw, the wheel carrier can be oscillated about its vertical axis, thereby enabling the wheel, rotatably mounted on the wheel carrier, to withstand changes in the wheel steering angle corresponding to the wheel carrier.

[0019] Advantageously, the actual temperature of the lead screw drive, preferably the lead screw and / or lead screw nut, can be calculated independently of vehicle speed. By taking into account the ambient temperature detected using existing sensors or sensing devices in the vehicle or steer-by-wire system, the thermal load can be determined during operation of the steer-by-wire system. This is possible during driving, when the vehicle is stationary, and even before or during startup of the vehicle or steer-by-wire system. Advantageously, this eliminates the need for additional sensors to measure the temperature of the lead screw drive, which would otherwise require, for example, additional infrared sensors installed within the steer-by-wire system aligned with the lead screw or lead screw nut (lead screw drive). With the method of the present invention, power reduction or lead screw displacement limitation is only implemented when the actual temperature of the lead screw drive also requires this within the steer-by-wire system. For example, a temporary reduction or limitation is made when the maximum thermal load allowed by the structure is reached. Subsequently, the temperature of the lead screw drive decreases (cools down). Based on continuous calculations, normal operation can be resumed once the temperature falls below this threshold. In other words, instead of estimation, the actual temperature of the lead screw drive is determined while simultaneously monitoring the condition of the lubricant used. Therefore, premature lubrication failure due to excessive temperature of the lead screw drive can be largely avoided, and the service life of the lead screw drive and the steer-by-wire system will not be shortened.

[0020] Therefore, the steer-by-wire system can be used to its fullest extent. This method is therefore more accurate than temperature estimation and is particularly suitable for parking and maneuvering at extremely low speeds or even when the vehicle is stationary, where large wheel steering angles and frequent changes of direction are required. Furthermore, the method also considers operating conditions, such as when the external temperature is very high or when there is residual heat in the steer-by-wire system area. These conditions are taken into account by detecting one or more ambient temperatures.

[0021] In this method, the natural cooling behavior of the lead screw drive, and particularly the lead screw material, is considered. Cooling occurs primarily through heat conduction because the lead screw drive, and especially the lead screw itself, is a solid. A material, such as steel, exhibits natural cooling behavior. This means that at room temperature (20°C), the material cools faster in the same time period than at a higher temperature, such as 80°C. This can be directly considered based on continuous temperature calculations. Therefore, the shortest possible temporary power reduction and / or displacement limitation time can be achieved.

[0022] Preferably, one or more ambient temperatures are read, such as the vehicle's ambient temperature or the ambient temperature of the steer-by-wire system at the corresponding axle (preferably the rear axle). The readings are performed intermittently, preferably in the range of 10 to 40 milliseconds, and most preferably every 20 milliseconds. It is preferable to compare ambient temperatures from different sources or sensors. This allows for a reasonable check of the read temperatures or temperature sensors. It is preferable to use the lower of the determined temperatures. It is preferable to consider the vehicle's ambient temperature as the lower of the commonly read temperatures. The vehicle's ambient temperature is preferably detected by an existing temperature sensor in the vehicle that provides a reliable external temperature (i.e., ambient temperature), for example, also used in the vehicle's air conditioning system. Such sensors are mounted in a way that prevents them from displaying erroneous temperatures due to strong heat sources (internal combustion engines, brakes, etc.). The vehicle's ambient temperature has proven to be a good starting point for accurate temperature calculations.

[0023] Preferably, the ambient temperature of the steer-by-wire system is detected using at least one sensor directly associated with the system. This refers to existing sensors or sensing devices, particularly temperature sensors such as thermistors, which are present in the power electronics and / or controller (ECU) and / or motor of the steer-by-wire system. If multiple temperature sensors are present, the temperatures read are compared. The lower ambient temperature read by the steer-by-wire system is preferred. Such temperature sensors are preferably associated with at least one CPU of the aforementioned units or integrated into the corresponding CPU of one or more units. Therefore, no additional sensors or sensing devices are required. These devices would require additional installation space within the steer-by-wire system and result in significant additional costs. Because the steer-by-wire system is an ASIL-D classified component, representing the highest level of fail-safety in a vehicle, ASIL-D compliant sensors are necessary, which are expensive. CPU refers to a microprocessor (Central Processing Unit).

[0024] Preferably, the calculated lead screw temperature is stored along with a timestamp when the vehicle is parked and the steer-by-wire system is therefore deactivated. If the vehicle is restarted after only a short time (e.g., 5 minutes), the actual temperature of the lead screw drive unit, particularly the lead screw, can be determined based on the currently read ambient vehicle temperature and the cooling behavior at that temperature. This is particularly advantageous when no steering movement has occurred when the vehicle is restarted, and therefore the rotary drive is not rotating.

[0025] Preferably, after calculating the temperature of the lead screw drive, a derating factor is determined based on the previously calculated temperature. As described above with respect to the first aspect of the invention, when a first threshold is reached, the power of the rotary actuator is at least temporarily reduced and / or the displacement of the lead screw is limited. The degree of reduction or limitation of displacement can be determined by the derating factor. The derating factor can take a value between 0 and 1. For example, when the derating factor is 0, a maximum power reduction or displacement limitation can be specified. When the derating factor is 1, no reduction or displacement limitation is specified. The steer-by-wire system can perform the maximum displacement of the lead screw, which is equivalent to the maximum change in the wheel steering angle. The derating factor can, for example, follow a curve or characteristic curve, where different values ​​of the derating factor are associated with specific temperature values ​​(thresholds) serving as support points. For example, a first support point (first threshold) can be specified at 80°C, the next support point at 100°C, and another support point at 120°C (another threshold). If the calculated temperature value is in the range of 0-80°C, the derating factor can be 1, thus neither reducing nor limiting the displacement. Within this range, the steer-by-wire system can operate at maximum power and set the maximum steering angle or wheel steering angle. A derating factor of 0.2 can be assigned to the next 100° support point. Therefore, power and / or displacement can be reduced by 80%, or in other words, from reaching that support point, only 20% remains. From another support point at 120°, the power reduction or displacement limitation can be reduced to 0%. This is equivalent to at least temporarily shutting down the rotary drive, thereby shutting down the lead screw drive. Between the aforementioned support points, the derating factor can decrease linearly. However, depending on the characteristic curve, the derating factor can also be specified to decrease logarithmically or similarly between individual or partial support points. Different characteristic curves can constitute a family of characteristic curves, where different characteristic curves can be selected based on other parameters. These parameters can be, for example, the ambient temperature of the vehicle and / or the steer-by-wire system, vehicle load, tire type, tire size, tire pressure, or other vehicle parameters. Between the aforementioned support points, the derating factor can vary at specific intervals (e.g., every 20%). Referring to the example above, at the support point between 80°C and 100°C, the derating factor can be 0.6 at 90°C, which corresponds to power and / or displacement still being 60%. Alternatively, the derating factor can decrease more steeply (negative slope) and / or continuously between support points, thereby achieving finer power reduction and / or displacement limitation.

[0026] In an alternative implementation, the derating factor can be calculated, preferably continuously, using a function that calculates the derating factor based at least on temperature. The determination of the derating factor based on characteristic curves, families of characteristic curves, or the aforementioned function can be performed in the vehicle's controller, preferably in the controller of the steer-by-wire system. In this way, targeted cooling can be achieved based on the calculated temperature of the lead screw drive, particularly the lead screw, adapted to the current instantaneous thermal load. In this advantageous manner, the steer-by-wire system can operate within a first, relatively large temperature range (up to 80°C) without reducing power or limiting displacement. At higher temperatures (80-130°C), reductions and / or limitations can be finely graded based on the actually calculated temperature and the cooling behavior of the lead screw drive, particularly the lead screw. This ensures that the steer-by-wire system provides the highest possible performance throughout its entire operating range.

[0027] Regarding the aforementioned implementation, it is preferable to continuously maintain the temporary power reduction of the rotary drive and / or the restricted displacement of the lead screw until the calculated temperature of the lead screw drive device, preferably the lead screw, decreases by a difference from the previously reached threshold values. This difference can be in the range of 1-7K, preferably in the range of 3-6K, and most preferably 5K. The difference can be variably set within the above range so that the difference (especially at the aforementioned support point) is different depending on the threshold value reached. For example, the magnitude of the difference can increase as the lead screw temperature increases. Therefore, the cooling behavior of the lead screw used can be specifically considered.

[0028] Preferred considerations include the material quality and / or specific heat capacity of the lead screw drive mechanism, particularly the lead screw itself. These parameters can be stored, for example, in the controller for calculating the lead screw temperature. One or more of these parameters can be considered when calculating the temperature and / or determining the derating factor to obtain more accurate results. This allows for a better assessment of the cooling behavior of the materials used.

[0029] Efficiency of the lead screw drive is preferred. Depending on the threads of the threaded pair used (lead screw nut with internal threads and lead screw with external threads), the pitch and frictional effects of the interacting sides of the moving threads can be taken into account, thus providing more accurate results.

[0030] The difference between reduced / limited operation and less or no reduced / unlimited operation in a steer-by-wire system can result in significant performance differences. For example, reverting to normal operation (without reduction or limitation) might be perceived as a sudden steering movement by the driver or passengers, which is undesirable. Therefore, the transition from reduced power or limited displacement back to less or no reduced power or unlimited screw displacement (normal operation) is preferably gradual to avoid sudden, or abrupt, steering movements. In other words, when the steer-by-wire system returns to its previous operation or normal operation, its power or displacement will gradually change to at least the previous state or completely unlimited normal operation. This will not be perceived as an unpleasant driving situation by the driver and passengers. In other words, gradual change means a gradual transition.

[0031] The thermal load determination of this invention is a safety device that can be advantageously applied, taking into account existing safety requirements for vehicle steering systems and the long lifespan of steer-by-wire systems. It is a cost-effective solution and is preferably operated as a safety function on existing controllers of steer-by-wire systems.

[0032] According to another aspect of the invention, a controller for performing the method is provided. The controller is preferably part of a steer-by-wire system. The controller may be integrated within the housing of the steer-by-wire system. The controller is preferably directly associated with the steer-by-wire system or structurally part of the steer-by-wire system. Alternatively, an existing controller in the vehicle may be used.

[0033] In this invention, a controller (also known as an ECU, Electronic Control Unit) can be understood as an electrical device that processes sensor signals and outputs control and / or data signals accordingly. The controller may have an interface, which can be implemented in hardware and / or software. In a hardware implementation, the interface may be part of a so-called system ASIC that contains various functions of the controller. However, the interface may also be a standalone integrated circuit, or at least partially composed of discrete components. In a software implementation, the interface may be a software module, for example, existing on a microcontroller along with other software modules.

[0034] Also advantageous is a computer program containing program code that, when executed on a computer, particularly on the aforementioned controller, performs the methods described above. This program code is preferably stored on a storage medium, such as semiconductor memory, hard disk storage, or optical storage.

[0035] Finally, the present invention relates to a steer-by-wire system, preferably configured as a rear axle steering system and including a controller as described above, which can perform the methods described above. The steer-by-wire system includes a lead screw drive, preferably with a self-locking function, wherein the lead screw receives axial displacement of a supported lead screw nut relative to a fixed position via a rotary actuator. The steer-by-wire system includes power electronics and an electric motor. The electric motor is connected to the lead screw drive via a transmission device (preferably a toothed belt drive) and constitutes its rotary actuator.

[0036] This steer-by-wire system can also be advantageously used in partially or fully autonomous vehicles. This is particularly relevant in such vehicles where the driver's attention may be only partially focused or completely unpredictable. Any warnings on the dashboard regarding high heat loads on steering and other assemblies may go unnoticed or go unheard. Furthermore, maximum availability of the steer-by-wire system is crucial for the unconditional operation of the vehicle. Attached Figure Description

[0037] The present invention will now be described with reference to the accompanying drawings and based on preferred embodiments. In the drawings: Figure 1 The steer-by-wire system is shown; Figure 2 A flowchart of the method according to the present invention is shown; Figure 3 A graph showing the depreciation factor of the present invention is displayed; and Figure 4 A graph showing the cooling behavior is displayed. Detailed Implementation

[0038] Figure 1 A steer-by-wire system 20 according to the present invention is shown, which is used in the rear axle steering system of a motor vehicle. The steer-by-wire system 20 has a lead screw drive 21, which includes a lead screw 22, a lead screw nut 23, a rolling bearing 24, and a pulley 25, which is driven by an electric motor 27 via a belt 26, thus forming a rotary drive. The electric motor 27 is operated by means of a controller 35 arranged on the electric motor.

[0039] The lead screw nut 23, which is fixed in position by rotation, causes axial displacement of the lead screw 22. The axial displacement s is represented by a double arrow. This displacement s generates an adjustment stroke s, also called the displacement stroke s, for the lead screw 22. The lead screw 22 can be adjusted from its left-end stop position to its right-end stop position. This corresponds to the maximum adjustment stroke or maximum axial displacement. Figure 1The lead screw 22 is shown in its middle position. This corresponds to a wheel steering angle of 0°, i.e., straight-line driving. The lead screw 22 has an anti-torsion device (not shown), so it does not rotate with the rotation of the lead screw nut 23. The steer-by-wire system 20 has a housing 28, which is fixed to the vehicle structure via a first joint 29. The lead screw 22 is fixedly connected at one end to a support sleeve 30, which slides axially relative to the housing 28 and is connected to a second joint 31 at its outer end protruding from the housing 28. The steer-by-wire system 20 is connected directly or indirectly to the wheel supports of the motor vehicle via the second joint 31 using a steering tie rod (e.g., a tie rod, not shown), thereby enabling steering of the wheels (e.g., the rear wheels). In this case, vehicle-side support is achieved via the first joint 29. Power electronics 32 are arranged on the housing 28 and / or the motor 27, which are used to switch, regulate, and supply power to the steer-by-wire system 20, particularly the power electronics 32, the motor 27, and the controller 35. To make the two wheels of one axle turn, two... Figure 1 The unit shown. It is also conceivable to use a single steer-by-wire system, in which the two ends of the lead screw are connected to a wheel bracket (also known as a centrally acting steer-by-wire system, with channels at both ends of the housing for the lead screw to pass through).

[0040] Figure 2 A flowchart illustrating a possible implementation of method 500 for protecting components of a steer-by-wire system is shown. In a separate step, the rotational speed R and torque M of the rotary drive, the vehicle's ambient temperature T_amb, and the steer-by-wire system's ambient temperature T_sbwl (here, the ECU or CPU of power electronics 32) are read and transmitted to controller 35 via vehicle bus 510 (e.g., CAN bus or Flexray bus). In controller 35, unit 550 continuously calculates the temperature T_sp of the leadscrew 22 based on the aforementioned parameters. In the next step, the leadscrew temperature T_sp is forwarded to derating element 560. The calculated leadscrew temperature T_sp is continuously compared with threshold values ​​(T_thr1, T_thr2, T_thr3) stored in controller 35 as support points. Derating element 560 determines a derating factor D_f, taking support points into account, based on a family of characteristic curves stored in controller 35 or through a function. The derating factor D_f is passed to unit 590 of controller 35. The derating factor D_f is transmitted to the power electronics 32 via a signal path. The power electronics 32 accordingly manipulates the rotary drive to temporarily operate with reduced power and / or restricted displacement based on the determined leadscrew temperature T_sp and the threshold reached, until the calculated temperature T_sp decreases by a certain difference. The reduction or restriction is then lifted, and the steering-by-wire system resumes normal operation.

[0041] The derating factor D_f can be determined from the characteristic curve as described above. Figure 3 A characteristic curve 300 can be observed. Within the first wide range of 0-80℃, the derating factor D_f is 1. The steer-by-wire system operates at 100% power or displacement within this first wide range, without reduction or limitation. 80℃ is both the first support point of characteristic curve 300 and the first threshold T_thr1. Above this threshold T_thr1, the characteristic curve drops relatively steeply until another support point, which has a threshold T_thr2 of 100℃. From this support point, the characteristic curve drops relatively gently until reaching the next support point with a threshold T_thr3 of 120℃. At 100℃, power or displacement decreases to 20%. From 120℃ onwards, power or displacement decreases to 0%, which is equivalent to shutting down the steer-by-wire system. For example, between the two support points or thresholds T_thr1 and T_thr2 mentioned above, at 90°C, according to characteristic curve 300, the derating factor is 0.6, which is equivalent to reducing the power of the rotary actuator or the displacement of the lead screw to 60% of the maximum power.

[0042] Figure 4 The cooling behavior (in degrees Celsius) of a steel lead screw per second at different temperatures is illustrated exemplarily. It can be seen that starting from a high temperature of, for example, 100°C, the lead screw cools faster in the same time period at a room temperature of 20°C than at, for example, 80°C. This situation is taken into account in this method. The cooling of the lead screw requires a certain amount of time, depending on the ambient temperature and the calculated lead screw temperature. Therefore, power reduction or lead screw displacement limitation is based on the cooling behavior of the material used. However, since the temperature can be calculated, no reduction or limitation is needed in the range of 0 to 80°C compared to estimation. At higher temperatures, only actual cooling is required. Advantageously, the actual external temperature of the vehicle and / or the residual heat that may be present in the area of ​​the steering system with wire are also considered.

[0043] It has been proven that the method described here can accurately determine the heat load even without additional temperature sensing devices. Therefore, it is advantageous to eliminate the need for additional devices that directly perform sensor-based temperature detection within the lead screw drive. List of reference numerals in the attached diagram: 20. Steer-by-wire system 21. Lead screw drive device 22 Lead Screw 23 Lead screw nut 24 Rolling bearings 25 pulleys 26 belts 27 Electric motor 28. Casing 29 First joint 30 Support sleeve 31 Second joint 32 Power Electronic Devices 35 Controller 300 Characteristic Curve 500 Method for protecting components of a steer-by-wire system 510 Vehicle Bus 550 Unit for calculating lead screw temperature 560 Derating Components 590 interface D_f reduction factor M torque R speed T_amb ambient temperature Ambient temperature of T_sbwl steer-by-wire system (power electronics / CPU / ECU) T_sp temperature T_thr1 threshold T_thr2 threshold T_thr3 threshold s represents the stroke, or axial displacement.

Claims

1. A method for protecting components of a steer-by-wire system (20), wherein the steer-by-wire system (20) has a lead screw drive (21) with a self-locking function, wherein the lead screw (22) is axially displaced relative to a lead screw nut (23) supported in a fixed position by means of a rotary actuator, characterized in that, To determine the thermal load, the temperature (T_sp) of the lead screw drive (21) is calculated, wherein the temperature (T_sp) is calculated based at least on the rotational speed (R) and torque (M) of the rotary drive, the ambient temperature of the vehicle (T_amb) and / or the ambient temperature (T_sbwl) of the steer-by-wire system (20), wherein when at least a first threshold (T_thr1) is reached, the power of the rotary drive is at least temporarily reduced and / or the displacement (s) of the lead screw (22) is limited.

2. The method according to claim 1, characterized in that, When reading one or more ambient temperatures (T_sbwl; T_amb), these temperatures are read continuously, preferably intermittently, and compared with each other when multiple ambient temperatures are read, with the lower temperature always being considered.

3. The method according to claim 1 or 2, characterized in that, The ambient temperature (T_sbwl) of the steer-by-wire system (20) is detected by means of at least one sensor directly associated with the steer-by-wire system (20), particularly by means of existing temperature sensors of at least one power electronics (32) and / or controller (35) and / or motor (27) of the steer-by-wire system (20), wherein the temperature sensor associated with or integrated into the corresponding CPU preferably uses at least one of the aforementioned units.

4. The method according to any one of the preceding claims, characterized in that, Taking into account the calculated temperature (T_sp), the derating factor (D_f) is determined, preferably by means of the function D_f(Tx), preferably by stepless calculation, or determined in consideration of a family of characteristic curves or control specifications, wherein the power of the rotary drive is reduced and / or the displacement of the lead screw (22) is limited in consideration of the derating factor (D_f).

5. The method according to claim 4, characterized in that, For the temperature (T_sp), at least a first threshold (T_thr1) or another threshold (T_thr2, T_thr3) is set, and in particular, a derating factor (D_f) is assigned to each of these thresholds, wherein, based on the corresponding threshold (T_thr1, T_thr2, T_thr3) reached, the power of the rotary actuator and / or the restricted displacement of the lead screw (22) are continuously maintained until the continuously calculated temperature (T_sp) begins to decrease by a difference from the previously reached threshold (T_thr1, T_thr2, T_thr3).

6. The method according to any one of the preceding claims, characterized in that, Consider the mass and / or specific heat capacity of the material of the lead screw drive device (21), especially the lead screw (22).

7. The method according to any one of the preceding claims, characterized in that, Consider the efficiency of the lead screw drive device (21).

8. The method according to any one of the preceding claims, characterized in that, After the power reduction of the rotary drive and / or the displacement limitation of the lead screw (22) are released, the transition to normal operation is gradual, so as not to produce sudden steering motion.

9. A controller (35) for performing the method according to any one of the preceding claims.

10. A computer program comprising program code for performing the method according to any one of claims 1 to 8 when the program is executed on a computer, particularly on a controller (35) according to claim 9.

11. A machine-readable storage medium on which the computer program according to claim 10 is stored.

12. A steer-by-wire system (20) having a lead screw drive (21) with a self-locking function, wherein the lead screw (22) is axially displaced relative to a lead screw nut (23) supported in a fixed position by means of a rotary actuator, the steer-by-wire system preferably being configured as a rear axle steering system and including a controller (35) according to claim 9.

13. The steer-by-wire system (20) according to claim 12 further includes power electronics (32) and an electric motor (27), particularly an electric motor with a speed reducer.

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