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

Abstract

A method for protecting components of a steer-by-wire system, wherein the system has a spindle drive with a self-locking mechanism, wherein the spindle is linearly displaced by means of a rotational drive of a fixedly mounted spindle nut. Depending on the vehicle speed, the linear displacement of the spindle relative to the spindle nut is continuously accumulated over time until a first threshold value is reached.

Description

Technical Field

[0001] The present invention relates to a method for protecting components of a steer-by-wire device. The invention also relates to a control unit for implementing the method, a computer program having program code fields, and a steer-by-wire device. Background Art

[0002] Document DE 10 2018 208 199 A1 discloses an actuator for a spindle drive with a steer-by-wire system for a motor vehicle. The spindle drive comprises a spindle with a spindle thread and a fixedly mounted and rotationally driven spindle nut with a nut thread. The spindle thread and nut thread are designed as self-locking kinematic threads, allowing only axial displacement of the spindle relative to the spindle nut, which is fixedly mounted in the actuator. The spindle thread and nut thread are braced against each other in the longitudinal direction of the spindle by means of a threaded ring, and the lateral surfaces of the spindle and spindle nut are in permanent contact with each other. High friction is generated by the self-locking and bracing, and in particular by lateral forces acting on the spindle from the vehicle chassis. A lubricant overcomes this friction. However, during operation of the steer-by-wire system, the persistently high loads cause intense heating of the spindle drive, surrounding components, and the lubricant. This loads mechanical and electrical components, and the lubricant may lose its tribological properties, which can negatively impact the service life of the steer-by-wire system. Summary of the Invention

[0003] The object of the present invention is to reduce the thermal load of a steer-by-wire system in non-critical areas.

[0004] This object is achieved by the method according to the invention. Advantageous developments of the method are given in the following further description. Further aspects of the invention are described using a control unit for carrying out the method, a computer program having program code fields, and a steer-by-wire device.

[0005] The present invention relates to a method for protecting components of a steer-by-wire system, wherein the steer-by-wire system has a spindle drive with a self-locking mechanism. The spindle is linearly displaced by means of a rotary drive of a fixedly mounted spindle nut. 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 transmission mechanism, preferably a toothed belt drive mechanism. The electric motor is arranged coaxially with respect to the common longitudinal axis of the spindle and spindle nut. If the spindle nut rotates in one direction or the other, the spindle, which is permanently engaged with the spindle nut, is linearly displaced along its longitudinal axis in one direction or the other. The moving thread is preferably designed as a trapezoidal thread. Preferably, a metric ISO trapezoidal thread according to DIN 103 is used. This thread can be designed to be self-locking and has a higher friction than a recirculating ball thread.

[0006] The steering system of a motor vehicle must be designed to guide the wheels. The wheels on the steered axle must maintain a set wheel angle so that the vehicle can maintain a predetermined trajectory, such as when driving straight or around a curve. The steering system not only changes the wheel angle but also maintains it. High lateral forces are generated in motor vehicles, particularly when cornering, and these forces act on the wheels. These lateral forces act on the spindle of the steer-by-wire system, as the spindle is connected indirectly via a steering rod or directly to the wheel carrier, on which the wheel is rotatably mounted. The lateral forces increase with increasing speed and depending on the radius of the curve. Higher lateral forces result in higher actuating forces in the steer-by-wire system, which in turn increases friction in the spindle drive. However, even when driving very slowly, these actuating forces are still high, and the highest actuating forces are only generated when the vehicle is stationary. This is because normal forces act on each tire, and due to the vehicle's mass, the weight is distributed roughly evenly across the tires. The tires rest on the roadway with their contact surfaces. To steer the tires, they must be rotated about their vertical axes. This is achieved by the steering mechanism acting on the wheel carrier. Therefore, a torque must be applied that is sufficiently large to overcome friction and the normal forces acting on the tire relative to the roadway. During the steering movement, the spindle presses against the wheel carrier, steering it and the wheel together about its vertical axis. This increases friction in the moving thread of the spindle drive. This effect is thus similar to a lateral force acting on the spindle, with the forces being greatest when the vehicle or tire is stationary.

[0007] The large forces acting on the spindle therefore increase friction in the moving threads of the spindle drive of a steer-by-wire system. Friction generates heat in the moving threads. Especially at large steering angles, such as when the vehicle is parked, a longer linear displacement is required—in other words, an adjustment stroke for the spindle is required. Therefore, the heat input with a large adjustment stroke is even greater than with a small adjustment stroke. Over a long period of time, the heat input can become so great that the lubricant heats up so intensely that the friction properties lose their effectiveness. If necessary, for example, the lubricant could even reach or exceed its boiling point, causing it to lose its effectiveness. Without lubricant, the flanks of the spindle drive's threads could become severely worn, potentially leading to premature failure of the spindle drive. Other mechanical or even electrical components could also be heated intensely and damaged.

[0008] The spindle drive, and ultimately the steer-by-wire system, has a maximum thermal load determined for its operation in its construction and design. This maximum thermal load is, for example, 140 degrees Celsius over a certain period of time. To prevent this heat input from exceeding the maximum thermal load, sensors can be used in the steer-by-wire system to detect the current temperature. However, since the spindle nut rotates and the spindle therefore moves linearly, the actual thermal load in the moving thread is difficult to measure or detect using sensors. Therefore, direct measurements at the friction pair are almost impossible. The result of the sensor or sensor arrangement in the housing is that these also have to be electrically connected to the evaluation unit. This results in additional costs when manufacturing the steer-by-wire system.

[0009] Surprisingly, it has been found that the maximum thermal load can be determined very well through estimation. According to a first aspect of the present invention, to estimate the maximum thermal load as a function of vehicle speed, the linear displacement of the spindle relative to the spindle nut can be continuously accumulated over time. This linear displacement, also referred to as the travel path, is continuously integrated over time. This can be performed simply by integrating the travel path over time. The accumulation is continued until a first threshold value is reached. In other words, a temperature integral is formed. Assuming a correlation between the traveled adjustment path and the amount of heat introduced by the linear displacement, it is thus possible to very accurately determine the expected temperature profile based on the traveled adjustment path. This first threshold value is determined for the corresponding steer-by-wire system, given knowledge of the material of the moving thread and its coefficient of friction, as well as the maximum foreseeable force acting on the spindle and the known maximum travel of the spindle. This threshold value thus represents the magnitude of the maximum thermal load to be assumed. During the integration process, a comparison with the threshold value is continuously performed. When the threshold value is reached, the operation of the spindle drive and, therefore, the steer-by-wire system is reduced, as will be explained in more detail.

[0010] Preferably, during the displacement, a decay value, also called a cooling value, is periodically subtracted from the corresponding current value of the temperature integral. The decay value is particularly independent of the driving situation and therefore takes no account of it. The decay value is preferably subtracted from the value of the temperature integral at intervals of 20 to 60 ms, preferably every 40 ms. This allows for the natural cooling of the spindle drive or steer-by-wire system after a certain period of time. The physical reason for this is that cooling occurs more rapidly due to convection, especially at high vehicle speeds.

[0011] In a preferred embodiment, further spindle movement is limited and / or the drive torque of the spindle nut is at least temporarily reduced when a threshold value is reached. In particular, the drive torque is reduced to approximately 0 Nm, preferably zero Newton-meters, in order to reduce additional thermal loads on the spindle drive. Limited movement can mean a reduction in the travel distance. Alternatively or additionally, the angular velocity of the spindle nut can also be adapted.

[0012] According to another embodiment, after reaching a first threshold value, the attenuation value is continuously subtracted from the temperature integral during limited spindle travel. Upon reaching a second threshold value, the limited travel is deactivated. This second threshold value can also be referred to as a reactivation threshold. This allows for cooling after a certain period of limited steer-by-wire operation, allowing for a return to unrestricted steer-by-wire operation. After reaching the reactivation threshold value, the spindle can again be displaced to the maximum adjustment travel s.

[0013] The difference between restricted and unrestricted operation of a steer-by-wire system can be noticeable in a significant difference in performance. This can be perceived by the driver or passengers as a jerky steering movement, which is undesirable. Therefore, the change from restricted to unrestricted displacement of the main shaft is preferably gradual, so as not to produce a jerky, or in other words, abrupt, steering movements. Steer-by-wire systems are designed for the respective vehicles so that the driver and passengers generally do not notice any difference in the performance of the steer-by-wire system, whether restricted or unrestricted. In other words, a gradual change should be understood as a step-by-step change.

[0014] According to the present invention, the maximum thermal load estimation is a safety feature that can be advantageously used to meet the safety requirements for steering systems in vehicles and the durability of steer-by-wire systems. This is a cost-effective solution that is preferably implemented as a safety function on an existing controller of a steer-by-wire system.

[0015] As already mentioned above, the method is carried out as a function of the vehicle speed. The vehicle speed is associated with the rotation of the wheels. As also mentioned above, the torque that must be overcome in order to rotate the wheels about the vertical axis varies depending on the rotational movement (rotation) of the wheels. Surprisingly, it has been shown that it is advantageously possible to determine ranges of different vehicle speeds for taking the vehicle speed into account. It is therefore not necessary to consider every current vehicle speed, but rather it is sufficient to consider the vehicle speed within a defined range, or in other words, within a bandwidth. Therefore, in a preferred embodiment, a plurality of ranges of different vehicle speeds are determined for taking the vehicle speed into account, wherein one or more ranges can be taken into account when forming the temperature integral (the accumulation over time of the linear displacement of the spindle relative to the spindle nut).

[0016] A first range representing the vehicle's stationary state (speed = 0 km / h) is determined, in which the linear displacement of the spindle relative to the spindle nut is accumulated over time. As mentioned above, the highest actuating forces are required when the vehicle is stationary. In other words, the highest heat input into the spindle drive occurs here. Therefore, it is useful to form a temperature integral over the entire range when the vehicle is stationary to determine the maximum thermal load.

[0017] Furthermore, a second range is determined that takes into account average speeds of approximately 10 to 50 km / h, wherein a coefficient is formed for this range so that the accumulation is reduced. The coefficient is formed, for example, based on empirical determination. This is an intermediate range in which the tire support forces are also appropriately considered, since, although this generates lower control forces than when the vehicle is stationary, a higher heat input into the spindle drive can also be expected due to the control forces.

[0018] Furthermore, a third range is defined for high speeds above 50 km / h, in which no accumulation is performed. At high speeds, the torque required to steer the wheels is relatively low or almost non-existent. Furthermore, the linear extension at high driving speeds is also very low. This is because, for example, during an overtaking maneuver at 100 km / h, a wheel steering angle change of less than 1° over a period of several minutes is sufficient.

[0019] According to another aspect of the present invention, a controller for implementing the method is provided. The controller is preferably part of a steer-by-wire device. The controller may be integrated into the steer-by-wire device's housing. In any case, the controller is preferably directly associated with the steer-by-wire device.

[0020] Another aspect of the invention relates to a computer program having program code elements in order to implement the method described above when the program is executed on a computer, in particular on the aforementioned controller.

[0021] Finally, the present invention relates to a steer-by-wire device, which is preferably designed as a rear-axle steering device and is equipped with a control device as described above, which can implement the method as described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The present invention will be described below with reference to the accompanying drawings with the aid of preferred embodiments. In the accompanying drawings:

[0023] Figure 1 A steer-by-wire arrangement according to known prior art is shown; and

[0024] Figure 2 shows a graph according to the method according to the invention;

[0025] Figure 3 A further diagram showing the method according to the invention is shown. DETAILED DESCRIPTION

[0026] Figure 1 The figure shows a steer-by-wire device 20 according to the present invention, which is used for rear-axle steering in a motor vehicle. The device 20 has a spindle drive 21 comprising a spindle 22, a spindle nut 23, a bearing 24, and a belt pulley 25, which can be driven by an electric motor 27 via a belt 26. The electric motor 27 is controlled by a controller SG, which is located on the motor. The spindle 22 is axially displaced by rotating the fixed spindle nut 23. The linear displacement s is indicated by a double arrow. This linear displacement s creates an adjustment path s, also called a travel path, for the spindle 22. The spindle 22 can be adjusted from its left end stop to its right end stop. This corresponds to the maximum adjustment path or maximum linear displacement. The spindle 22 is shown in its center position in the figure. This corresponds to a wheel steering angle of 0°, which corresponds to straight-ahead driving. The spindle 22 has a rotation lock (not shown) to prevent it from rotating with the spindle nut 32. The actuator 20 has a housing 28, which is fastened to the vehicle structure via a first joint 29. The spindle 22 is fixedly connected at one of its two ends to a screw pin 30, which is guided axially sliding relative to the housing 28 and is connected at its outer end protruding from the housing 28 to a second joint 31. The actuator 20 is connected via the second joint 31 to a steering rod (not shown), preferably a toe link of the rear axle, indirectly or directly to the wheel carrier of the motor vehicle, thereby enabling steering of the rear wheels, wherein the vehicle-side support is provided via the first joint 29.

[0027] Figure 2A coordinate system is shown in which the linear displacement or adjustment path s is plotted on the ordinate against the time t on the abscissa. In principle, the wheels of the vehicle can be steered to the left and right by means of the steering system for the respective desired steering movement. This requires an adjustment path to the left or right via the steer-by-wire system 20 or its actuator. Figure 2 and Figure 3 In the diagram, the adjustment travel in the positive range represents a steering movement to the left, and the adjustment travel in the negative range of the ordinate represents a steering movement to the right. The maximum values s_max(+) and s_max(-) are plotted on the ordinate and correspond to the maximum adjustment travel s to the left or right. This corresponds to the linear displacement of the spindle 22 at the left or right end stop of the steer-by-wire system 20. In addition, a first threshold value T_max and a second threshold value T_reactiv are plotted on the ordinate. The temperature integral T_int is shown by the linearly rising line shown. It can be seen that the adjustment travel s or the linear displacement of the spindle with the maximum steering movement or adjustment travel s_max(+) and s_max(-) shown here accumulates over time T to form the temperature integral T_int until the first threshold value T_max is reached. This is explained by the abscissa value at time t_safe_s. This means that the maximum thermal load of the spindle drive 21 or the steer-by-wire system 20 was reached during this time due to the previous steering movement. In other words, after this time, heat input occurs due to friction and loads in the spindle drive, depending on the achieved adjustment range s and vehicle speed. If the first threshold value T_max is reached, the safety function T_safe_s is activated at this point. The brackets indicate that after the safety function is activated, the steer-by-wire system 20 operates with a limited adjustment range s_red. With a limited adjustment range s_red, the adjustment range s is significantly reduced. Furthermore, a damping value T_fade is continuously subtracted from the temperature integral T_int, regardless of the driving situation. This damping value is only clearly visible during the time period of the limited adjustment range s_red. However, the damping value T_safe is continuously subtracted from the temperature integral T_int in a cyclical manner. However, the accumulation is dominant, making it invisible in the graphical representation during this period. The subtraction of the damping value T_safe corresponds to the natural cooling behavior of the spindle drive during operation of the steer-by-wire system 20. This is reasonable and accurately depicts the actual process in the steer-by-wire system 20, as there are always small steering pauses between steering movements, during which no steering occurs. The reason for this is that the continuous cooling of the steer-by-wire system is primarily carried out by convection.

[0028] Figure 3 Shown with Figure 2A similar coordinate system. This coordinate system represents the time course after the first threshold value T_max is reached and the protection function is activated at the time T_safe_s. The steer-by-wire device 20 operates with a limited adjustment range s_red. It can be seen that the adjustment range s is significantly reduced. The thermal load in the spindle drive 21 is significantly reduced, which is shown by the falling temperature integral T_int. The attenuation value or its subtraction dominates due to the limited adjustment range s, so that a certain accumulation is less important. To a certain extent, the attenuation value T_fade prevails, which actually corresponds to the reduction in temperature in the spindle drive 21. If the temperature integral T_int reaches the second threshold value T_reactiv, the protection function can be deactivated at the time t_safe_e. The steer-by-wire device 20 can then continue to operate with a normal adjustment range s (unrestricted operation) and the accumulation of the temperature integral T_int of the adjustment range s continues over time T.

[0029] It has been found that this function allows a very good evaluation of the maximum thermal load even without a temperature sensor. Thus, the direct sensor detection of the temperature, which is present solely in the active drive, can be advantageously dispensed with.

[0030] List of reference numerals:

[0031] 20 Steer-by-wire

[0032] 21 Spindle drive

[0033] 22 Spindle

[0034] 23 Spindle nut

[0035] 24 bearings

[0036] 25 belt pulley

[0037] 26 belt

[0038] 27 Electric Motor

[0039] 28 housing

[0040] 29 First hinge

[0041] 30 Twist pin

[0042] 31 Second hinge

[0043] SG Controller

[0044] v_veh vehicle speed

[0045] T_intTemperature integral

[0046] T_max (first) threshold

[0047] t_safe_s The moment when the protection function is activated

[0048] t_safe_eThe moment when the protection function is deactivated

[0049] T_fade attenuation value

[0050] T_reactiv (second) threshold

[0051] t (displacement of the main axis) time

[0052] s adjustment stroke, linear displacement

[0053] s_max maximum adjustment stroke

[0054] s_redLimited adjustment travel

Claims

1. A method for protecting components of a steer-by-wire device (20), wherein: The steer-by-wire system has a spindle drive (21) with a self-locking structure, wherein the spindle (22) is linearly displaced by means of a rotary drive of a spindle nut (23) which is supported in a position-fixed manner, characterized in that In order to estimate the maximum thermal load based on the vehicle speed (v_veh), the linear displacement (s) of the spindle (22) relative to the spindle nut (23) is continuously accumulated over time (T) as a temperature integral (T_int) until a first threshold (T_max) is reached.

2. The method according to claim 1, characterized in that During the displacement, a decay value (T_safe) is periodically subtracted from the temperature integral (T_int) at intervals of 20 ms to 60 ms, independent of the driving situation.

3. The method according to claim 2, characterized in that The decay value (T_safe) is periodically subtracted from the temperature integral (T_int) after 40 ms.

4. The method according to claim 2, characterized in that When the first threshold (T_max) is reached at the first moment (t_safe_s), the further displacement of the spindle (22) is restricted and / or the driving torque of the spindle nut (23) is at least temporarily reduced, and the driving torque of the spindle nut (23) is reduced to 0 in order to reduce the further thermal load.

5. The method according to claim 4, characterized in that After the first threshold (T_max) is reached, the restricted displacement (s_red) of the spindle (22) is continuously carried out until the temperature integral (T_int) reaches a second threshold (T_reactiv) by continuously subtracting the decay value (T_safe), and then the restricted displacement (s_red) is cancelled from the second moment (t_safe_e) onwards.

6. The method according to claim 5, characterized in that The change of the spindle (22) from the restricted displacement to the unrestricted displacement is carried out gradually so as not to generate a jerky steering movement.

7. The method according to any one of claims 1 to 6, characterized in that In order to take into account the vehicle speed (v_veh), a plurality of ranges of different vehicle speeds are determined, wherein when accumulating the linear displacement (s) of the spindle (22) relative to the spindle nut (23) over time, one or more ranges from the following list of ranges can be taken into account: · A first range (v_veh_0) representing the vehicle stationary state, in which the linear displacement (s) of the spindle (22) relative to the spindle nut (23) is continuously accumulated over time (t). · A second range (v_veh_med), which takes into account an average speed greater than the first range (v_veh_0) and less than a third range (v_veh_high) (v_veh_0 < v_veh_med < v_veh_high), and for which a coefficient is formed such that the accumulation is carried out decreasingly over time (t). · A third range (v_veh_high), which takes into account a high speed greater than the second range (v_veh_high >> v_veh_med), and in which no accumulation is carried out over time (t).

8. A controller (SG) for implementing the method according to any one of the preceding claims 1 - 7.

9. A computer program product having program code fields, which, when executed on a computer, on a controller (SG) according to claim 8, implements the method according to any one of the preceding claims 1 to 7.

10. A steer-by-wire device (20) comprising a controller (SG) according to claim 8.

11. The steer-by-wire device (20) according to claim 10, wherein: The steer-by-wire system is designed as a rear-axle steering system.

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