Abstract
A method is disclosed for protecting the components of a steer-by-wire steering system, where the steer-by-wire steering system comprises a self-locking spindle drive in which the spindle is displaced linearly by driving a positionally fixed spindle nut in rotation. As a function of the speed of the vehicle, the linear displacement of the spindle relative to the spindle nut is summed continuously with respect to time until a first threshold value has been reached. A control unit, a non-transitory computer-readable storage medium, and a steer-by-wire system are also disclosed.
Claims
1-9. (canceled)
10. A method for protecting components of a steer-by-wire steering system (20) of a vehicle, wherein the steer-by-wire steering system comprises a self-locking spindle drive (21), and wherein the spindle (22) is configured to be displaced linearly by rotating a positionally fixed spindle nut (23), the method comprising: estimating a maximum thermal load as a function of a speed (v_veh) of the vehicle; and calculating a temperature integral (T_int) by summing a linear displacement (s) of the spindle (22) relative to the spindle nut (23) as a function of time (t) until the temperature integral (T_int) reaches a first threshold value (T_max).
11. The method according to claim 10, further comprising subtracting periodically, during the linear displacement of the spindle, a decay value (T_safe) from the temperature integral (T_int) at an interval from 20 to 60 ms.
12. The method according to claim 11, wherein the interval is 40 ms.
13. The method according to claim 11, further comprising restricting further displacement of the spindle (22) and/or restricting a drive torque of the spindle nut (23) when the first threshold value (T_max) is reached at a first time-point (t_safe_s), thereby reducing further thermal loading.
14. The method according to claim 13, further comprising: when the temperature integral (T_int) reaches the first threshold value (T_max), restricting further displacement of the spindle (22) by continual subtraction of the decay value (T_safe) until the temperature integral (T_int) reaches a second threshold value (t_reactiv) at a second time-point (t_safe_e); and discontinuing restriction of further displacement of the spindle after the second time point (t_safe_e).
15. The method according to claim 14, wherein discontinuing restriction of further displacement of the spindle (22) is performed gradually so as to prevent sudden steering movements.
16. The method according to claim 10, wherein the function of the speed (v_veh) defines a plurality of ranges of vehicle speeds, each of the plurality of ranges selected from: (i) a first range (v_vehx_0) wherein the vehicle is at rest and wherein the summing of linear displacement occurs continuously, (ii) a second range (v_veh_med) which takes into account moderate speeds (v_veh_0<v_veh_med<v_veh_high), wherein calculating the temperature integral includes applying a coefficient to reduce the summing of linear displacement, and (iii) a third range (v_veh_high) which takes into account high speeds (v_veh_high>>v_vehx_med), wherein in the third range no summing of linear displacement takes place.
17. A control unit (SG) for carrying out the method according to claim 10.
18. A steer-by-wire steering system (20), comprising the control unit of claim 17, wherein the steer-by-wire system is configured as a rear-axle steering system.
19. A non-transitory computer-readable storage medium with machine-readable code that when executed by a steer-by-wire control unit (SG) carries out the method according to claim 10.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] Below, the invention is described with reference to preferred embodiments illustrated in the drawing, which shows:
[0024] FIG. 1: A steer-by-wire steering system according to the present disclosure, and
[0025] FIG. 2: A graph illustrating the method according to the invention,
[0026] FIG. 3: A further graph of the method according to the invention.
DETAILED DESCRIPTION
[0027] 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, bearings 24 and a pulley wheel 25 that can be driven via a belt 26 by an electric motor 27. The electric motor 27 is controlled by a control unit SG which is arranged on it. By rotating the positionally fixed spindle nut 23 the spindle 22 is moved axially. The linear movement s is indicated by a double arrow. By virtue of the linear movement s the spindle 22 undergoes an actuating stroke s also called the travel path. The spindle 22 can be moved from its stop on the left as far as its stop on the right. That corresponds to the maximum actuating stroke or the maximum linear displacement. In the figure the spindle 22 is shown in its central position. The central position corresponds to a wheel steering angle of 0°, corresponding to driving straight ahead. The spindle 22 comprises an anti-rotation device (not shown), so that it cannot rotate along with the spindle nut 23 when the nut is rotated. The actuator 20 has a housing 28 which is attached to the vehicle body by way of a first joint 29. The spindle 22 is connected at one of its two ends to a screw-on shank 30, which is guided to slide axially relative to the housing 28 and, at its outer end projecting out of the housing 28, is connected to a second joint 31. By way of the said second joint 31 the actuator 20 is connected indirectly or directly via a steering rod (not shown), preferably a track rod of a rear axle, to a wheel carrier of a motor vehicle and can thus steer a rear wheel, while being supported on the vehicle side by the first joint 29.
[0028] FIG. 2 shows a coordinate system in which the linear displacement or actuating stroke s is plotted on the ordinate against the time t on the abscissa. Basically, by a steering system the wheels of a vehicle can be steered to the left and to the right by means of the correspondingly desired steering movement. This requires from the steer-by-wire steering system 20 or from its actuator an actuation stroke to the left or to the right. In the representations shown in FIGS. 2 and 3, the actuation stroke in the positive range means a steering movement to the left and in the negative range of the ordinate an adjustment stroke steering movement to the right. On the ordinate, maximum values s_max(+) and s_max(—) are plotted, which correspond respectively to the maximum adjustment strokes to the left and to the right. This corresponds to a linear displacement of the spindle 22 to the stops of the steer-by-wire steering system 20 on the left and on the right. Furthermore, on the ordinate a first threshold value T_max and a second threshold value T_reactiv are shown. The line shown as rising linearly represents a temperature integral T_int. It can be seen that the actuation stroke s or linear displacement of the spindle with the maximum steering movement or actuation stroke s_max(+), s_max(—) with respect to time t shown in this case, is summed to give a temperature integral T_int until the first threshold value T_max has been reached. This is made clear by the time value t_safe_s on the abscissa. This means that the maximum thermal load for the spindle drive 21 or the steer-by-wire steering system 20 is reached by the steering movements leading up to that time. In other words, after that time and as a function of the actuation stroke s covered and the speed of the vehicle, heat input into the spindle drive takes place due to the friction and loading. When the first threshold value T_max is reached, then at that time point t_safe_s a protective function is activated. The curly bracket clearly indicates that after the protective function has been activated, the steer-by-wire steering system 20 is operated with a restricted actuation stroke s_red. With the restricted actuation stroke s_red the actuation stroke s is substantially reduced. Moreover, regardless of the driving situation a decay value T_fade is continuously subtracted from the temperature integral T_int. In this case that can only be seen clearly during the period of the restricted actuation stroke s_red. However, the decay value T_safe is cyclically subtracted continuously from the temperature integral T_int. But the summing is dominant to such an extent that in its graphic representation this cannot be recognized during the summing. The subtraction of the decay value T_safe corresponds to the natural cooling behavior of the spindle drive during the operation of the steer-by-wire steering system 20. This makes sense and is a realistic depiction of the actual processes in the steer-by-wire steering system 20, since between steering movements there are always short steering pauses in which no steering takes place. The reason for this is that the continuous cooling of the steer-by-wire steering system takes place mainly by convection.
[0029] FIG. 3 shows a coordinate system similar to that of FIG. 2. It shows the time variation after the first threshold value T_max has been reached and the activation of the protective function at time t_safe_s. The steer-by-wire steering system 20 is operated with a restricted actuation stroke s_red. It can be seen that the actuation stroke s is substantially reduced. The thermal load in the spindle drive 21 is clearly reduced, as shown by a falling temperature integral T_int. The decay value and its subtraction is dominant by virtue of the restricted actuation stroke s, so that a certain amount of summing is less important. So to say, the decay value T_fade, which actually corresponds to the falling temperature in the spindle drive 21, is predominant. When the temperature integral reaches a second threshold value T_reactiv, the protective function is deactivated at time t_safe_e. The steer-by-wire steering system 20 can then be operated with the normal actuation stroke s (unrestricted operation) again and the summing of the temperature integral of the actuation stroke s with respect to time t then continues.
[0030] It has been shown that by virtue of this functionality, a very good assessment of the maximum thermal load can be achieved even without temperature sensors. Advantageously therefore, there is no need for a separate, direct sensor-based determination of the temperature in the spindle drive.
INDEXES
[0031] 20 Steer-by-wire steering system [0032] 21 Spindle drive [0033] 22 Spindle [0034] 23 Spindle nut [0035] 24 Bearings [0036] 25 Pulley wheel [0037] 26 Belt [0038] 27 Electric motor [0039] 28 Housing [0040] 29 First joint [0041] 30 Screw-on shank [0042] 31 Second joint [0043] SG Control unit [0044] v_veh Speed of the vehicle [0045] T_int Temperature integral [0046] T_max (First) threshold value [0047] t_safe_s Protective function activation time-point [0048] t_safe_e Protective function deactivation time-point [0049] T_fade Decay value [0050] T_reactiv (Second) threshold value [0051] t Time (of the movement of the spindle) [0052] s Actuation stroke, linear displacement [0053] s_max Maximum actuation stroke [0054] s_red Restricted actuation stroke
