Patent ID: 12227248

DETAILED DESCRIPTION

FIG.1shows a steer-by-wire steering system20according to the invention, which is used for the rear-axle steering of a motor vehicle. The steer-by-wire steering system20has a spindle drive21, which comprises a spindle22, a spindle nut23, bearings24and a pulley wheel25that can be driven via a belt26by an electric motor27. The electric motor27is controlled by a control unit SG which is arranged on it. By rotating the positionally fixed spindle nut23the spindle22is moved axially. The linear movement s is indicated by a double arrow. By virtue of the linear movement s the spindle22undergoes an actuating stroke s also called the travel path. The spindle22can 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 spindle22is shown in its central position. The central position corresponds to a wheel steering angle of 0°, corresponding to driving straight ahead. The spindle22comprises an anti-rotation device (not shown), so that it cannot rotate along with the spindle nut23when the nut is rotated. The actuator20has a housing28which is attached to the vehicle body by way of a first joint29. The spindle22is connected at one of its two ends to a screw-on shank30, which is guided to slide axially relative to the housing28and, at its outer end projecting out of the housing28, is connected to a second joint31. By way of the said second joint31the actuator20is 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 joint29.

FIG.2shows 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 system20or from its actuator an actuation stroke to the left or to the right. In the representations shown inFIGS.2and3, 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 spindle22to the stops of the steer-by-wire steering system20on 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 drive21or the steer-by-wire steering system20is 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 system20is 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 system20. This makes sense and is a realistic depiction of the actual processes in the steer-by-wire steering system20, 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.

FIG.3shows a coordinate system similar to that ofFIG.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 system20is 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 drive21is 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 drive21, 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 system20can 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.

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

20Steer-by-wire steering system21Spindle drive22Spindle23Spindle nut24Bearings25Pulley wheel26Belt27Electric motor28Housing29First joint30Screw-on shank31Second jointSG Control unitv_veh Speed of the vehicleT_int Temperature integralT_max (First) threshold valuet_safe_s Protective function activation time-pointt_safe_e Protective function deactivation time-pointT_fade Decay valueT_reactiv (Second) threshold valuet Time (of the movement of the spindle)s Actuation stroke, linear displacements_max Maximum actuation strokes_red Restricted actuation stroke