Patent Publication Number: US-2021179166-A1

Title: Steer-by-wire steering system and method for operating a steer-by-wire steering system

Description:
RELATED APPLICATION 
     This application claims priority to German Patent Application No. 102019134143.4 filed Dec. 12, 2019, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a steer-by-wire steering system for a vehicle as well as a method for operating such a steer-by-wire steering system. 
     Steer-by-wire steering systems are characterized in that there is no longer a mechanical connection for transmitting a steering movement between the steering wheel of the vehicle and the steered wheels of the vehicle axle, typically the front axle. The driver&#39;s steering movement is transmitted via sensors, actuators, and control electronics. 
     To ensure sufficient reliability of the steering system, steer-by-wire steering systems typically have redundant systems. However, should there be a complete or partial failure of the steering system, in particular of the control electronics, it is necessary to continue to ensure that steering movements of the driver can be detected and implemented. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to specify a steer-by-wire steering system, the reliability of which is further increased. 
     The object is achieved according to the invention by a steer-by-wire steering system for a vehicle with a steering wheel actuator for moving and detecting movements of the steering wheel, which has a steering wheel electric motor and which is connected to a steering wheel of the vehicle, an axle actuator for moving and detecting movements of wheels of an axle of the vehicle, which comprises an axle electric motor and an axle control unit and which is connected to an axle of the vehicle, and an equivalent circuit designed to measure current induced by the steering wheel electric motor and to provide the axle control unit with at least one measurement value depending on the induced current for the control of the axle electric motor. 
     The axle of the vehicle can be both the front and rear axles of the vehicle. However, the front axle of the vehicle is usually controlled by the steer-by-wire steering system. 
     The steering wheel electric motor and the axle electric motor are in particular three-phase machines, which are operated with three-phase alternating current. 
     As components of the steer-by-wire steering system, the steering wheel actuator and the axle actuator are not mechanically connected to each other for the transmission of the steering movement. 
     The measurement value is in particular a voltage that is processed by the axle actuator to perform a steering movement. 
     A basic idea of the invention is that the steering wheel electric motor can also work as a generator in the event of a failure of control electronics of the steer-by-wire steering system, so that a measurement value can be determined depending on the steering movement performed on the steering wheel even in the event of failure of the sensors. This measurement value is then transmitted to the axle control unit and thus ensures the operation of the steer-by-wire steering system. 
     For the detection of the current induced by the steering wheel electric motor, the equivalent circuit may have at least one current sensor, which is assigned to a phase of the steering wheel electric motor, in particular wherein the equivalent circuit has as many current sensors as the steering wheel actuator has phases. 
     The current sensors can also be sensors of another sensor system of the steering wheel actuator, which only measure the current induced by the steering wheel electric motor in the event of a partial or complete failure of the control electronics of the steer-by-wire steering system. 
     In particular, the steering wheel actuator has three phases and the equivalent circuit has three current sensors. 
     One variant provides as many measurement values as the steering wheel electric motor has phases. Thus, a separate measurement value is available to the axle actuator for each phase of the steering wheel electric motor, whereby the operation of the axle actuator can take place depending on each of the phases of the steering wheel electric motor. As a result, more reliable operation of the steer-by-wire steering system can be achieved. 
     In order to be able to carry out the operation of the steer-by-wire steering system even without an electronic control unit, the equivalent circuit may have a logic circuit, in particular wherein the equivalent circuit is purely a logic circuit. 
     A purely logic circuit has no microcontroller and therefore no software that would be necessary for the operation of the circuit. Thus, the steer-by-wire steering system according to the invention is also suitable for applications in which the control electronics fail completely. 
     In this case, the evaluation of the current signal is at least qualitative. However, this is sufficient to ensure the operation of the steer-by-wire steering system to a sufficient extent. 
     The axle control unit can be set up to record at least one measurement value and to control the axle electric motor on the basis of the at least one measurement value. In particular, the axle control unit is set up in this case to determine the position of the steering wheel and/or a relative steering angle on the basis of the at least one measurement value. 
     Thus, on the basis of the current induced by the steering wheel electric motor, which has been transformed by the equivalent circuit into at least one measurement value, the axle control unit can cause a movement of the axle and thus the wheels of the vehicle by means of the axle electric motor. In other words, even in the event of a failure of the control electronics, the steering angle specified by the driver can be converted into a steering movement. 
     In one variant, the equivalent circuit has at least one branch, which comprises a comparator and a switching element, in particular a transistor, wherein the comparator is connected to the at least one current sensor and is designed to actuate the switching element when the current measured by the current sensor exceeds a predetermined threshold value. 
     As a result, individual branches of the equivalent circuit can be switched on depending on the measured current. Similarly, the influence of very small currents, which tend to be subject to higher relative errors, can be minimized. 
     In a further variant, the equivalent circuit, in particular the at least one branch, may have a capacitor, the voltage of which represents the measurement value. 
     Thus, in this variant, the equivalent circuit converts the current induced by the steering wheel electric motor into a voltage, which can then be used by the axle actuator to perform a steering movement. 
     The equivalent circuit may have a voltage source that is connected to the capacitor via the switching element when the switching element is actuated. 
     Thus, the capacitor is charged by means of the voltage source as long as the switching element is actuated, especially if the current measured by the current sensor exceeds the predetermined threshold value. 
     Furthermore, the equivalent circuit, in particular the switching element, may be set up to discharge the capacitor if the current measured by the current sensor falls below the predetermined threshold value. 
     In this case, the capacitor can be discharged to a predetermined voltage value, which in particular corresponds to the value zero. 
     Thus, it can be ensured by means of the equivalent circuit that the capacitor provides a measurement value only if the current measured by the current sensor exceeds the predetermined threshold value. 
     In a further variant, the equivalent circuit may have multiple branches, in particular a branch for each phase of the steering wheel electric motor, wherein each branch contains a logic block connected to the comparator of the branch and the comparators of the at least one other branch, wherein the logic block is set up to discharge the capacitor when the measured current of the at least one other branch exceeds the threshold value. 
     In this way, the equivalent circuit can link multiple branches together in such a way that only that capacitor is charged which is assigned to a branch of the equivalent circuit, the current sensor of which is currently measuring a current that exceeds the threshold value. In this way, in particular in the presence of multiple phases of the steering wheel electric motor, a single phase can be selected and converted into at least one measurement value. 
     In addition, it can be determined by means of the comparator whether a positive or negative half wave of the current signal is present. This allows the equivalent circuit to be executed in such a way that only the positive or negative half wave has to be used for the determination of the measurement value, which simplifies the procedure. 
     The steering wheel actuator can have a steering wheel control unit connected to the control unit for data exchange. 
     In normal operation of the steer-by-wire steering system, the steering wheel control unit and the axle control unit communicate with each other in order to convert movements of the steering wheel into a steering movement. 
     In addition, the steering wheel control unit and the axle control unit can check each other to detect any malfunction of one or both control units. 
     Information about the movement of the steering wheel can be obtained by means of the sensors assigned to the steering wheel, for example an angle sensor and/or a torque sensor. The information obtained by means of the sensors is then processed by the steering wheel control unit and transmitted to the axle control unit, as long as there is not a complete failure of the steering wheel control unit. 
     The object of the invention is further achieved by a method for operating a steer-by-wire steering system, which has a steering wheel actuator with a steering wheel electric motor and a steering wheel control unit, an axle actuator with an axle electric motor and an axle control unit and an equivalent circuit, wherein the method includes the following steps: 
     Checking whether there is a malfunction of the steering wheel control unit, in particular by the axle control unit; 
     Measuring the current induced by the steering wheel electric motor by the equivalent circuit and providing a measurement value depending on the induced current; 
     Obtaining at least one measurement value from the equivalent circuit by the axle control unit; and 
     Controlling the steering movement of the axle by the axle control unit by means of the axle electric motor depending on the at least one obtained measurement value if there is a malfunction. 
     Checking a malfunction of the steering wheel control unit can be carried out by means of the steering wheel control unit itself as well as by means of the axle control unit. 
     A malfunction may also only be a partial failure of the steering wheel control unit. The steering wheel actuator may comprise multiple sensors, which measure a rotation angle of the steering wheel and/or a torque applied to the steering wheel, for example. A malfunction may also correspond to the failure of at least one of these sensors. 
     If the steering wheel electric motor has multiple phases, a measurement value is provided in particular for each phase of the steering wheel electric motor. Thus, any phase of the steering wheel electric motor can be used by the axle actuator in the event of a malfunction. 
     In one variant, each phase is assigned a capacitor, wherein the capacitor is charged, especially linearly, when the current of the phase exceeds a predetermined threshold value. 
     Thus, the magnitude of the time difference between two zero passes of a certain phase of the steering wheel electric motor can be determined from the charge of the capacitor. 
     The at least one measurement value can be a voltage, in particular the voltage of the corresponding capacitor. 
     In a further variant, the position of the steering wheel and/or a relative steering angle is/are determined by the axle control unit on the basis of the at least one measurement value. 
     By turning the steering wheel, the steering wheel electric motor can induce an alternating current, in particular a three-phase alternating current. The method according to the invention allows the evaluation of the profile of the curve of the current signal that is induced in this way. The angle of rotation of the steering wheel can therefore be determined from the current signal. 
     This will make it possible for the movement of the steering wheel electric motor and the axle electric motor to be coordinated. Ideally, the phases of the steering wheel electric motor and the axle electric motor run synchronously, but the phases can also differ within a certain tolerance range. 
     At the same time, no current is induced when the steering wheel is stationary, so that this case can also be reliably detected and unwanted steering movements can be prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages and properties of the invention result from the following description and the drawings. In the figures: 
         FIG. 1  shows a schematic representation of a steer-by-wire steering system according to the invention, 
         FIG. 2  shows an equivalent circuit of the steer-by-wire steering system from  FIG. 1 , 
         FIG. 3  shows diagrams of the variation of several current signals and the measurement values produced from these current signals by means of the equivalent circuit from  FIG. 2 , and 
         FIG. 4  shows a schematic flow diagram of the method according to the invention. 
     
    
    
     DESCRIPTION 
     In  FIG. 1 , a steer-by-wire steering system  10  according to the invention is shown, with a steering wheel actuator  12  and an axle actuator  14 . 
     The steering wheel actuator  12  is coupled to a steering wheel  16  of a (not shown) vehicle and comprises a steering wheel control unit  18  and a steering wheel electric motor  20 . 
     The steering wheel  16  is also monitored by an angle sensor  22  and a torque sensor  24 , by means of which a steering angle φ and a torque M applied to the steering wheel  16  can be determined. The angle sensor  22  and the torque sensor  24  are connected to the steering wheel control unit  18 , which can process the information received from the sensors  22 ,  24 . 
     In addition, the control unit  18  is connected to the steering wheel electric motor  20 . The steering wheel electric motor  20  can also perform a steering movement of the steering wheel  16 . 
     The steering wheel electric motor  20  also has multiple current sensors  26 , which are in particular Hall sensors. The current sensors  26  are designed to measure the current of each phase of the steering wheel EM  20 . 
     The axle actuator  14  acts on one axle of the vehicle, in the embodiment shown on the rack  28  of the front axle of the vehicle, and comprises an axle control unit  30  and an axle electric motor  32 . 
     The position of the axle or the rack  28  can be monitored by means of a position sensor  34 , which is connected to the axle control unit  30 . 
     During the normal operation of the steer-by-wire steering system, the steering movement of the driver is detected by means of the angle sensor  22  and the torque sensor  24 , is processed by the steering wheel control unit  18  and corresponding data are transmitted to the axle control unit  30 . The axle control unit  30  controls the axle electric motor  32  based on the transmitted data, whereby a movement of the axle or the rack  28  is generated and thus a steering movement of the vehicle is performed. 
     In the case of a partial or complete failure of the steering wheel actuator  12 , the method according to the invention for operating the steer-by-wire steering system  10  is used. 
     For this purpose, an equivalent circuit  36  is used, which is arranged in the steering wheel control unit  18  in the embodiment shown. In principle, however, the equivalent circuit  36  could also represent a separate component of the steer-by-wire steering system or could be formed in the axle actuator  14 . 
     A malfunction of the steering wheel control unit  18  is determined in particular by the axle control unit  30  (step S 1  in  FIG. 4 ), for example by the fact that the axle control unit  30  is no longer receiving information of the angle sensor  22  and/or the torque sensor  24 . 
     In this case, the electric motor  20  acts as a generator during a steering movement of the steering wheel  16 , wherein the current sensors  26  can forward the induced current to the equivalent circuit  36  as an input signal. 
     Alternatively, the current sensors  26  can also be part of the equivalent circuit  36  instead of being part of the steering wheel electric motor  20 . In this case, the equivalent circuit  36  is arranged so dose to the steering wheel electric motor  20  that nevertheless a current is induced in the current sensors  26  by the movement of the steering wheel electric motor  20 . 
     The equivalent circuit  36  is shown in detail in  FIG. 2 . In the embodiment shown here the equivalent circuit  36  has three branches, wherein each branch processes a current signal U, V, W. Basically, the equivalent circuit has in particular as many branches as the steering wheel electric motor  20  has phases. 
     The equivalent circuit  36  is purely a logic circuit, so no microcontroller and no software is necessary to operate the equivalent circuit  36 . Thus, the equivalent circuit  36  can be operated even in the event of a failure of the steering wheel control unit  18 . Accordingly, the current signals U, V, W are evaluated at least qualitatively. 
     The equivalent circuit  36  has three comparators  38   a,    38   b  and  38   c  as well as three switching elements  40   a,    40   b  and  40   c,  each assigned to a comparator  38   a,    38   b  and  38   c.  In the embodiment shown here, the switching elements  40   a,    40   b,    40   c  are transistors. 
     The respective switching elements  40   a,    40   b  and  40   c  are actuated when it is determined by means of the respective comparators  38   a,    38   b  and  38   c  that the corresponding current signal U, V, W exceeds a predetermined threshold Value S. 
     In the embodiment shown, the threshold value S is 25% of the maximum amplitude of the current signal U, V, W, for example. The maximum value can be determined by the components used, the selected geometry of the components of the steer-by-wire steering system  10  or by means of a measurement. 
     The equivalent circuit  36  also has three capacitors  42   a,    42   b  and  42   c,  the voltages of which represent the measurement values provided by the equivalent circuit  36 . 
     The capacitors  42   a,    42   b  and  42   c  are charged by means of a voltage source  43 , wherein the voltage source  43  is connected to the capacitors  42   a,    42   b    42   c  via the respective switching element  40   a,    40   b,    40   c.  The voltage source  43  charges each of the capacitors  42   a,    42   b  and  42   c  which belong to the branches of the equivalent circuit  36 , the switching element  40   a,    40   b  and  40   c  of which is currently actuated. 
     Accordingly, the respective capacitor  42   a,    42   b  and  42   c  is charged only if the current signal U, V, W belonging to the respective branch exceeds the threshold value S. 
     In addition, the equivalent circuit  36  has logic blocks  44   a,    44   b  and  44   c,  each connected to all comparators  38   a,    38   b  and  38   c.  In addition, the logic blocks  44   a,    44   b  and  44   c  are connected to the respective capacitors  42   a,    42   b  and  42   c.    
     The logic blocks  44   a,    44   b  and  44   c  are used to discharge the associated capacitors  42   a,    42   b  and  42   c  to a predetermined voltage value, in particular to completely discharge them, when the current signal of one of the other branches of the equivalent circuit  36  exceeds the threshold value S. This eliminates the need to evaluate the negative half waves of the current signals U, V, W. 
       FIG. 3  illustrates the relationship between the current signals U, V, W and the capacitor charge. In the upper diagram, the three current signals U, V, W are shown against time. In addition, the threshold value S is shown. 
     As can be seen, the current signals U, V, W each describe a phase of the three-phase alternating current, which is detected by the current sensors  26  during a steering movement of the steering wheel  16 . 
     In the lower diagram in  FIG. 3 , the voltages of the capacitors  42   a,    42   b  and  42   c  assigned to the respective current signals U, V, W are shown against time. 
     As can be seen, the capacitors  42   a,    42   b  and  42   c  are charged linearly over a period of time, which is determined by the fact that the corresponding current signal U, V, W is above the threshold value S. This region is shown as an example of the current signal V in  FIG. 3 . 
     As soon as the current signal U, V, W drops below the threshold value  5 , the corresponding capacitor  42   a,    42   b  and  42   c  will discharge, as shown in  FIG. 3 . 
     In the variant shown in  FIG. 3 , two of the capacitors  42   a,    42   b  and  42   c  can be charged simultaneously over a limited period of time. Via the logic blocks  44   a,    44   b  arid  44   c,  however, it is also possible, as previously described, that the capacitor, for example, the capacitor  42   b,  which was previously in the charging process, is immediately discharged as soon as another of the capacitors starts to charge, for example the capacitor  42   c.    
     Ultimately, the current induced by the steering wheel electric motor  20  is measured by the equivalent circuit  36  and the voltages of the capacitors  42   a,    42   b  and  42   c  are provided as measurement values depending on the induced current (step S 2  in  FIG. 4 ). 
     The axle control unit  30  can then receive the measurement values provided by the equivalent circuit  36  (step S 3  in  FIG. 4 ). 
     The axle control unit  30  can determine the position of the steering wheel  16  and/or the relative steering angle φ based on the provided measurement values. The corresponding conversion formula and value table are stored in the axle control unit  30 . 
     The axle control unit  30  then controls the steering movement of the axle or rack  28  by means of the axle electric motor  32  depending on the obtained measurement values (step S 4  in  FIG. 4 ). 
     Thus, even in the event of a malfunction of the steer-by-wire steering system  10 , a movement of the steering wheel is converted into a steering movement.