Method for controlling a steer-by-wire system

[0001] The present invention relates to a method of controlling a steer-by-wire steering system, and a steer-by-wire steering system for vehicles, having an electronically regulated steering actuator that is attached either to the steering gear of the front axle or to both steerable front wheels, a steering wheel angle sensor that measures the driver's steering command at the steering wheel, a feedback actuator unit that provides feedback from the road to the driver through the steering wheel, a steer angle sensor for acquiring the current steering angle at the steering gear on the front axle or on the two front wheels, and a control device for acquisition of the signals transmitted by the sensors and calculation of trigger signals for the steering actuator and the feedback actuator unit.

[0002] A steer-by-wire steering system is known from German Patent 195 40 956 C1. In this steering system, the mechanical connection between the steering wheel and a steering gear acting on the steered wheels may be interrupted by opening a coupling. This steering system then becomes a steer-by-wire steering system, in which the steering wheel is only indirectly coupled to the steering gear arrangement. In this known steer-by-wire steering system, feedback from the road to the driver is assured via the steering wheel by the provision of a feedback actuator unit that is realized as a non-self-locking electric motor and provides controllable operating resistance that is triggered by a control device on the steering wheel.

[0003] Unless appropriate precautions are implemented, a fault in a steer-by-wire steering system may lead directly to danger to life and limb of the driver. Accordingly, a requirement arose to ensure that no single error of the steer-by-wire steering system can possibly result in its failure.

[0004] The object of the present invention is to provide a method of operating a steer-by-wire steering system, and a steer-by-wire steering system, that may realize all steer-by-wire functions, including the functions for the feedback actuator, with the highest possible degree of safety. In general, a steer-by-wire steering system according to the present invention should guarantee a degree of safety that equals if not surpasses that assured by conventional power steering.

[0005] The object described above is resolved by a method according to claim 1, by a control device according to the co-ordinate claim 21 and a steer-by-wire steering system.

[0006] A high degree of protection against malfunctions is assured by redundancy in the acquisition of measured values, control of the steering actuator and the feedback actuator, checking of all functions and components, and communications.

[0007] Particularly the discrete means of acquiring various measured variables, including steering wheel angle, steering angle, restoring moment, make it easier to carry out a plausibility check of the different measured values, and to detect the occurrence of any erroneous values. This increases the reliability and failsafe nature of a steer-by-wire steering system functioning according to the present invention.

[0008] Safety is further improved by redundant communication within the control device and with the vehicle's other control devices or sensors, as well as by the fact that switching from steer-by-wire operation to the auxiliary level only takes place after a transition period has elapsed following the occurrence of an error. This latter measure assures that all functions of the steer-by-wire steering system are in a defined state at the times the switch to the auxiliary level is effected.

[0009] The reliability of a steer-by-wire steering method functioning according to the present invention is further increased by the division of the functions within the control device into four logic levels in accordance with one of the subordinate claims 14 to 18. This division of the functions and the reciprocal checking enable any errors and malfunctions to be detected with the highest possible degree of safety, and the steer-by-wire steering system is able to respond accordingly.

[0010] With reference to the separate monitoring module, the control device has two hardware levels and four logic levels. In the hardware levels, the microcomputer or microcomputers and the monitoring module work together. The monitoring module communicates with the microcomputer or microcomputers via an internal bus system. In this way, the computing capacity of the microcomputer or microcomputers is checked and the program executions in the computer or computers are monitored. The selected means of data communication between the microcomputer or microcomputers and the monitoring module enables reciprocal monitoring of these components.

[0011] Further advantages and advantageous configurations are given in the present invention and the following description, the drawing and the patent claims.

[0012] FIG. 1 shows a schematic diagram of a steer-by-wire steering system having a hydraulic auxiliary level and an electromotive steering actuator;

[0013] FIG. 1a: shows a steer-by-wire steering system having a mechanical auxiliary level;

[0014] FIG. 2 shows a schematic diagram of a steer-by-wire steering system having a hydraulic auxiliary level and two electromotive steering actuators;

[0015] FIG. 3 shows a functional diagram of a steer-by-wire steering system having a steering actuator;

[0016] FIG. 4 shows a functional diagram of a steer-by-wire steering system having two steering actuators;

[0017] FIG. 5 shows the structure of a control device that may be used with the present invention, having a microcomputer and a separate monitoring module;

[0018] FIG. 6 shows a control device structure having two microcomputers, each having a separate monitoring module;

[0019] FIG. 7 shows the structure of a control device having two microcomputers, which themselves contain the monitoring means;

[0020] FIG. 8 shows a control device structure having three microcomputers;

[0021] FIG. 9 shows a control device structure of another steer-by-wire steering system according to the present invention;

[0022] FIG. 10 shows a logic circuit for triggering the enable signals for the power electronics units;

[0023] FIG. 11 shows the dynamic triggering of the enable signals of the power electronics units and

[0024] FIG. 12 shows a logic circuit for triggering of the coupling

[0025] To begin with, the structure and basic functional operation of a steer-by-wire steering system having a hydraulic or mechanical auxiliary level will be described with reference to FIGS. 1 to 4. A steer-by-wire steering system having a hydraulic auxiliary level is the object of German Patent Application 198 38 490.4 of Robert Bosch GmbH.

[0026] The structure shown in FIG. 1 differs from that shown in FIG. 2 in that in steer-by-wire operation with the steer-by-wire steering system of FIG. 1, the steered wheels (not shown) are moved by one steering motor LM, whereas FIG. 2 shows a variant in which two steering motors LM_vl and LM_vr are used.

[0027] In FIGS. 1 and 2 a steering wheel motor LRM is shown, which acts as the feedback actuator for the restoring forces to be transferred to the driver through the steering wheel.

[0028] The hydraulic auxiliary level is provided by symmetrical hydraulic cylinders, a pressure reservoir SP for hydraulic fluid, and optionally a coupling KU between steering wheel motor LRM and the mechanical action of the steering column on the two hydraulic cylinders on the steering wheel side or a switchover valve USV that short circuits the hydraulic cylinder or cylinders on the steering gear side in steer-by-wire operation, these hydraulic components being interconnected by means of hydraulic lines.

[0029] In the variant shown in FIG. 2, having steering motors LM_vl and LM_vr on the left and right side, respectively, in contrast to FIG. 1, two individual hydraulic cylinders are provided for the hydraulic auxiliary level instead of a single dual cylinder on the steering gear side, and switchover valve USV is located in a bypass line between the two hydraulics cylinders on the steering gear side that are connected by a hydraulic line.

[0030] An embodiment of a steer-by-wire steering system having a mechanical auxiliary level is shown in FIG. 1a. The mechanical auxiliary level has a split steering column and a coupling KU in the steering column. In steer-by-wire operation, coupling KU is open so that the mechanical engagement between the steering wheel and rack of the steering linkage is interrupted. A steering movement is made by steering motor LM through the insertion of a steering gear and a steering linkage.

[0031] In the event of a failure, coupling KU is closed so that the steering wheel engages directly with the rack of the steering linkage. Coupling KU is closed when the control device is without voltage.

[0032] Instead of the embodiment variants shown in FIGS. 1, 1a and 2, which all show direct positional intervention of steering motor LM, steering motor LM could also assure the adjustment of the hydraulic flow to the steering torque booster by means of the rotary slide valve of a hydraulic power steering system.

[0033] FIGS. 3 and 4 show the basic control and regulation structure of a steer-by-wire steering system in the form of functional blocks.

[0034] The steering wheel angle _H selected by the driver is acquired by a steering wheel angle sensor 10. Steering wheel angle _H is modified through a setpoint value generation 11 depending on the situation and on the basis of dynamic driving variables, such as speed v, yaw rate , to provide a modified steering wheel angle _H*, which in turn serves as the setpoint value for a steering regulator 12.

[0035] Steering regulator 12 generates a manipulated variable for steering motor LM, which is applied to steering motor LM in the form of a voltage U_v.

[0036] Alternatively, as shown in FIGS. 2 and 4, where a steering motor (LM_vl, LM_vr) is provided for each front wheel, steering regulator 12 generates voltage U_v,l as an actuating signal for left steering motor LM_vl and U_v,r as an actuating signal for right steering motor LM_vr. In this case, the front wheels may be steered individually, and, in principle, independently of each other. In FIG. 3, the steering regulator receives sensor-measured steering angle _V for both front wheels, and in Figure, 4 sensor-measured steering angles _v,l and _v,r are transmitted separately for the right and left front wheels, respectively.

[0037] The feedback from the road surface on the steered wheels, which heavily influences driver's steering command _H, may be measured, for example, with a restoring torque sensor 14 as restoring torque M_v, or as restoring torques M_v,l and M_v,r at both wheels. In order for this feedback to be sent to the driver as well, a feedback actuator is provided, which exerts a torque corresponding to the feedback on the steering column and thus also on the steering wheel of the vehicle. This torque, which hereafter will be termed hand torque M_H, may be measured by a hand torque sensor 13 on the steering column.

[0038] The feedback actuator of the vehicle equipped with steer-by-wire steering system includes electric steering wheel motor LRM, which is connected to the steering wheel via a gear system (not shown), and is regulated as shown in FIGS. 3 and 4 by a steering wheel regulator 16. To this end, steering wheel regulator 16 calculates a setpoint hand torque M_H,setpoint either on the basis of restoring torque M_v as measured by a restoring torque sensor 14, or on the basis of currents (I_v, I_v,l, I_v,r) at steering motors LM, LM_vl, LM_vr).

[0039] In the following the terms steering regulator and steering motor as well as feedback actuator and steering wheel motor will be used synonymously.

[0040] Alternatively, steering wheel regulator 16 may also use a feedback simulator 15 to simulate setpoint hand torque M_H,setpoint from the sensor-measured steering angles _v (or _v,l and _v,r) and other signals present in the vehicle, such as driving speed v and the coefficient of friction between the street and tires. Steering wheel regulator 16 triggers steering wheel motor LRM using manipulated value U_H according to setpoint hand torque M_H,setpoint such that hand torque M_H measured by the hand torque sensor 13 corresponds to setpoint hand torque M_H,setpoint.

[0041] In FIGS. 5 to 8, the structure and function of four embodiment variants of control devices according to the present invention are shown.

[0042] FIG. 5 shows an embodiment variant of a control device having a single microcomputer system in a block diagram. The microcomputer system includes a microcomputer RM and the associated peripheral components for the acquisition of all sensor signals. Moreover, the system also contains the necessary processing functions for forming the trigger signals for power electronics components LE_LM, LE_LRM for generating trigger signals U_H for steering wheel motor LRM and U_v (or U_v,l and U_v,r) for steering motor LM (or steering motors LM_vl and LM_vr). K₁ indicates a communication system that is implemented, for example, through a serial bus such as a CAN bus, and that provides the link to other control devices in the motor vehicle or also to a diagnostic system for the error information. K₁ may be unique or redundant.

[0043] Monitoring module M within the control device serves to monitor the error-free operation of the steer-by-wire functions of microcomputer RM and the associated peripheral components, and may be implemented, for example, as a microprocessor or ASIC.

[0044] The following functions are implemented in microcomputer RM:

[0045] sensor signal acquisition and calculation of trigger signals U_H for regulating steering wheel motor LRM,

[0046] sensor signal acquisition and calculation of trigger signals U_V for regulating steering motor LM,

[0047] monitoring of the auxiliary level

[0048] interface to communication system K₁

[0049] A detailed description of these subfunctions is presented in the following sections A through H.

[0050] A: Sensor Signal Acquisition and Calculation of Trigger Signals for Regulating the Steering Wheel Motor

[0051] For triggering steering wheel motor LRM, the following signals are acquired through peripheral components of microcomputer system RM.

[0052] steering wheel angles _H1 and _H2,

[0053] motor torque M_H of steering wheel motor LRM. In a further variant, the motor torque is determined using the measured motor currents,

[0054] rotor position _PH of steering wheel motor LRM; for example, when using a BLDC, asynchronous or switched reluctance motor

[0055] actual values of phase currents I_H of steering wheel motor LRM,

[0056] temperature T_H of steering wheel motor LRM; in an alternative configuration, signal T_H indicates the temperature of the output stages within power electronics LE_LRM for steering wheel motor LRM, or includes both temperatures,

[0057] terminal voltage of the vehicle electrical system UB.

[0058] Trigger signal U_H serves primarily to trigger the power electronics LE_LRM. This trigger signal is a manipulated variable of a digital regulator and may be output, for example, as a pulse width modulated (PWM) signal. Manipulated variable U_H is calculated from the measured motor torque M_V of steering motor LM, the rotor position _RH of steering wheel motor LRM and other parameters that characterize the status of the vehicle or the road surface. Steering wheel motor LRM is triggered by power electronics LE_LRM upon enabling through enable signals g_RH and g_H. A first motor relay 17 connected upstream of the power electronics is triggered by signals f_RH and f_H.

[0059] B: Sensor Signal Acquisition and Calculation of Trigger Signals for Regulating the Steering Motor

[0060] For triggering steering motor LM, the following signals are acquired through peripheral components of microcomputer system RM.

[0061] steering angles _v1, and _v2

[0062] motor torque M_V of the steering motor LM,

[0063] rotor position _pv of the steering motor, for example, when using a BLDC, asynchronous or switched reluctance motor,

[0064] actual values of phase currents Iv of the steering motor,

[0065] temperature T_v of the steering motor, in an alternative configuration, signal T_v indicates the temperature of the output stages within power electronics LE_LM for the steering motor or includes both temperatures, and

[0066] optionally terminal voltage UB of the vehicle electrical system.

[0067] For regulating the desired steering angle _v, the representative value is first formed from measured variables _v1 and _v2, e.g., by calculation of an average with the fully functional angle sensors. In the formation of the setpoint value, the reference variable of the regulator is formed from a representative value of measured steering wheel angles _H1 and _H2 and the current steering transfer ratio. With steering interventions by a dynamic driving system, the reference variable for the steering angle is calculated with the additional application of the vehicle's yaw rate and transverse acceleration a_y, or an angle transmitted via communication system K₁, or a differential angle _F is used. If the control device structure is used to provide a tracking system, angle _F could also represent the reference variable for the steering angle intervention that is calculated and preset by a higher-level control system.

[0068] Power electronics LE_LM of steering motor LM are triggered primarily by trigger signal U_v. This trigger signal is a manipulated variable of a digital regulator and may be output, for example, as a PWM signal. In this case, the currently available voltage status of the vehicle electrical system UB must be taken into account. Steering motor LM is triggered via power electronics LE_LM, if enable signals g_RV and g_V are set. A second motor relay 18 connected upstream of the power electronics is triggered by signals, f_RV and f_V.

[0069] C: Monitoring of the Auxiliary Level and Switching to the Auxiliary Level

[0070] An essential parameter for the availability of the auxiliary level of the steer-by-wire steering system is the pressure p_S in the hydraulics unit. This pressure p_S is measured either continuously at pressure reservoir SP or at specified intervals and transmitted to microcomputer RM in the form of an analog signal P_s. In normal operation, switchover valve USV is triggered by signals b_RU and b_U from monitoring module M. If one of these trigger signals is missing, switchover valve USV switches to the hydraulic auxiliary level. This means that if the vehicle electrical system fails, operation at the auxiliary level is assured automatically. In the event of a significant error, trigger signal b_RU is removed by the steer-by-wire steering system, or signal b_U is removed by the monitoring module, and operation is switched to the auxiliary level.

[0071] D: Interface to Other Control Devices and Display Units

[0072] The sensor signals for yaw rate (o and for transverse acceleration a_y are transmitted to the microcomputer system via communication system K₁. In addition, estimated values for friction coefficients _vl and _vr between the left wheel and the street as well as between the right wheel and the street, and an estimated value of the vehicle speed v are also transferred via K₁. Moreover, this communication system may be used to specify a reference variable _F for the wheel steering angle in the event of a dynamic intervention via the steering system or with a tracking system.

[0073] Signals d_B may also be transmitted via this communication system to an information system (not shown) that informs the driver of possible error conditions in the system or, for example, informs the driver of a switch to the hydraulic auxiliary level. Signals d_m are sent to other control devices, which e.g., cause the vehicle to slow down when the switch is made to the hydraulic auxiliary level.

[0074] E: Safety Measures in the Control Device

[0075] In order to satisfy safety requirements imposed on a steer-by-wire steering system, all simple system errors occurring within the system must be recognized within a system-typical fault tolerance interval. After detecting a significant error, the SbW steering system is first switched to the auxiliary level operating mode within a transition time (e.g., within 5 seconds). In this mode, higher-level system control functions are no longer executed, i.e., dynamic steering intervention, steering intervention for crosswind compensation or intervention into the steering system, which change the transfer ratio between the steering wheel angle and the steering angle are terminated definitively. When the transition interval has elapsed or after all higher-level steering functions have been terminated, or if a second significant error occurs during the transition time, the control of coupling KU (see FIGS. 1, 1a and 2) in the steering column is terminated and consequently the auxiliary level is activated. To this end, the following measures are provided:

[0076] redundant acquisition of steering wheel angle (_H1, _H2),

[0077] redundant acquisition of steering angle _v or the steering angle on the front axle, (_v1, _v2),

[0078] triggering of steering wheel motor (LRM) via control signals U_H for the phase currents and enable signal g_RH,

[0079] redundant cut-off path for steering wheel motor LRM and also for steering motor LM via assigned motor relays (not with switched reluctance motors)

[0080] monitoring of sensor signals using plausibility checks and analytical redundancy

[0081] monitoring of microcomputer module RM via monitoring module M and vice versa.

[0082] F: Monitoring Concept of the Control Device

[0083] The monitoring concept of the control device is structured in four logical levels L₁, L₂, L₃, and L₄ and two hardware levels RM and M

[0084] Monitoring module M communicates with microcomputer module RM by means of an internal bus system. This serves to check the computing capacity of this microcomputer system and to monitor the programs running in the computer. These components are monitored reciprocally by the selected data communication type between microcomputer module RM and monitoring module M. For this, the following functions are assigned to the logical levels:

[0085] Level L₁

[0086] Level L₁ is implemented in microcomputer RM. It has the following tasks:

[0087] plausibility checks on input signals

[0088] selection of the steering wheel angles and steering angles needed for processing from the redundant sensor signals in each case

[0089] calculation of the regulating functions for trigger of steering wheel motor (LRM) and steering motor (LM),

[0090] alteration of the triggering of switchover valve USV in the event of failure to switch over to the hydraulic auxiliary level

[0091] Level L₂

[0092] Level L₂ is incorporated into microcomputer RM. This level is responsible for checking the correctness of the calculations performed in level L₁ using algorithms that differ from those used in level L₁. In order to perform the calculations, the redundant input data stored in the memory cells are used, thus allowing errors due to corrupted memory content to be detected. For checking the regulator functions, simple regulating algorithms are run in parallel, which are calculated with the redundant data stored for the reference variables and the current actual values of the regulating variables. An error condition is recognized on the basis of significant deviations between these simplified manipulated variable calculations and the calculations run at level L₁. The correct function of both controlled systems is also checked in level 2. For this, a mathematical model of the controlled system is provided for each case, describing the dynamic relationships between the manipulated variables and the regulating variables, also taking into account interference variables. The manipulated variables calculated in the regulating algorithms in level L₁ are added to these models. An error condition is recognized in the event of significant deviations between the model output variables and the associated measured actual values of the regulating variables.

[0093] If an error is recognized by microcomputer module RM in level L₂ and also in level L₃, the associated enable signals g_RV or g_RH for triggering the respective power electronics LE_LM and LE_LRM of steering motor LM and steering wheel motor LRM are reset.

[0094] Level L₃

[0095] This level is implemented in microcomputer module RM. In order to guarantee the reliable functioning of the steer-by-wire steering system in the event of a computer or software error, in the event of an error the programs in level L₁ and L₂ must still execute properly or their improper execution must be reliably detected. This control check is performed in the variant shown by a question session between levels L₃ and L₄. Microcomputer system RM retrieves an interrogation from monitoring module M, and responds, in each case taking into consideration all safety-relevant routines within a predetermined time interval in each case. A question is only able to be answered correctly if fault-free execution of the computer programs for the computer function test and the command test is assured. The partial answers constructed from the subroutines are concatenated into a full answer and passed to level L₄ in monitoring module M.

[0096] Level L₄

[0097] This level is implemented in monitoring module M. Here the full answer provided by microcomputer RM is checked with respect to the time interval of its arrival and for an exact bit match with the correct answer to the question. If the interrogation communication is not executed without error in level L₃, enable signals g_v and g_H for triggering the motors, enable signals f_v and f_H for triggering the motor relays, and trigger signal b_u for the switchover valve are reset.

[0098] G. Measures for the Sensor Monitoring

[0099] Angle sensor values, (_H1, _H2) on the steering wheel are checked against each other for plausibility. Moreover, the measured value of the torque sensor may also be used to check these measured values, taking into account the mechanical inertias of the steering wheel and a mathematical model for rotational movement as well as the current friction values. If a BLDC, asynchronous, or a switched reluctance motor is used as steering wheel motor LRM, a position sensor with an angle range from 0 to 360 must be used to regulate the phase currents. This sensor information may also be used for checking the measured steering wheel angles, or in a simplified variant, an angle sensor with lower resolution may be used in conjunction with this position measurement.

[0100] The torque measured value at the steering wheel may be monitored taking into account the measured phase currents and the temperature variable essential for monitoring the motor using a mathematical plausibility model.

[0101] Angle sensor values (_v1, _v2) at steering motor LM are checked against each other for plausibility. In addition, the measured values of the steering wheel angle (_H1, _H2) may be used for locating the faulty sensor in the event of deviations between sensor values _v1, and _v2. This is done by including in the calculation the current steering transfer ratio and taking into account any dynamic steering interventions that occur. This trouble-shooting measure may also be used in the opposite direction to detect a faulty steering wheel sensor. If an asynchronous machine or a switched reluctance motor is used as steering motor LM, a position sensor having an angle range from 0 to 360 must be used to regulate the phase currents. This sensor information may also be used for checking the measured steering angles, or in a simplified variant, an angle sensor with lower resolution may be used in conjunction with this position measurement. The torque measured value at the steering wheel may be monitored taking into account the measured phase currents and the measured temperature variable essential for monitoring the motor using a mathematical plausibility model.

[0102] Before beginning a journey, the entire function chain of the steer-by-wire control device may be tested by the injection of a defined setpoint torque at the steering wheel motor.

[0103] H: Special Features of the Various Configuration Variants of the Control Device

[0104] In the configuration variant shown in FIG. 6, the functions described in the foregoing are divided between two microcomputers RM₁ and RM₂. Microcomputer RM₁ is tasked with the regulating and monitoring functions for steering motor LM and monitoring the pressure in the hydraulic auxiliary level. Microcomputer RM₂ is responsible for regulating feedback actuator LRM for the steering wheel and triggering the auxiliary level. Both components may exchange data with each other and with other computer systems via the communication system K₁, or even communicate directly with each other via a path not shown in FIG. 6. The functionalities of the two computer systems RM₁ and RM₂ are checked in accordance with the points listed in section F (structure of monitoring) via separate monitoring modules M₁ and M₂.

[0105] In the variant according to FIG. 7, the division of functions of the control device takes place again between two microcomputer systems RM₁, RM₂ corresponding to the embodiment variant according to FIG. 6. A second communication system K₂ allows direct communication between both microcomputer systems RM₁, RM₂.

[0106] The functions described in section F (structure of monitoring) at the level of the monitoring component are now taken over by the respectively adjacent computing unit, that is to say, RM₁ takes over the monitoring of component RM₂ and vice versa.

[0107] In the embodiment variant according to FIG. 8 the functions of the control devices are divided among three microcomputer systems. Component RM₁ assumes responsibility for regulating and monitoring the functions of the steering motor and monitoring thee pressure of the hydraulic auxiliary level. Component RM₂ is tasked with regulating the steering wheel feedback actuator and triggering the auxiliary level. Both components can exchange data with one another via communication system K₂.

[0108] The functions described in section F (structure of monitoring) at the level of the monitoring component are now taken over by computing unit RM₀, which is also responsible for communication with other computer components.

[0109] In FIG. 9, an embodiment of a steer-by-wire steering system according to the present invention is shown including six microcomputers (RM_H1, . . . , RM_v3) and a mechanical auxiliary level, in which the steering wheel actuator is implemented with the two independent electric motors LRM1 and LRM2. In a further variant (not shown), the two electric motors might also be arranged with a common drive shaft and a common housing, so that only the motor windings are implemented redundantly. This variant of the electric motor may also be used with the steering motor.

[0110] Microcomputers RM_H1, RM_H2 and RM_H3 assume the control and regulation functions of the feedback actuator. Microcomputers RM_v1, RM_v2 and RM_v3 together constitute the redundant computer system for the triggering and regulation of the feedback actuator. Feedback actuator microcomputers RM_H1 exchange their calculated data via communication links, K_H12, K_H13, and K_H23. Microcomputers RM_v1 of the steering actuator communicate in the same way via communication links K_v12, K_v31, and K_v23. The microcomputers shown, RM_v1 and RM_H1, include the necessary peripheral components for acquiring all sensor signals. Moreover they also include the essential processing functions for calculating trigger signals U_H1 and U_H2 for steering wheel motors LRM and U_v1 and U_v2 for triggering steering motors LM. In the variant shown, the feedback actuator is implemented by two independent motors LRM1 and LRM2, which are controlled by independent power electronics units LE_LRM1 and LE_LRM2. Both motors are connected to the same shaft. The steering actuator is also redundantly constructed through two motors LM1 and LM2 and the associated power electronics units LE_LM1, and LE_LM2. Power for the electronics components of the SbW steering system is supplied by independent power supplies UB1 and UB2.

[0111] Power is supplied to microcomputer systems RM_H1 and RM_v1, as well as the motors LRM1 and LM1 together with the associated power electronics and cut-off logic (AL_LRM1, AL-MR_LRM1, AL_LM1, AL-MR_LM1) by UB1. Power source UB2 supplies microcomputer systems RM_H2 and RM_v2 as well as motors LRM2 and LM2 together with the associated power electronics and cut-off logic, (AL_LRM2, AL-MR_LRM2, AL_LM2, AL-MR_LM2). Microcomputers RM_H3 and RM_v3, as well as electromagnetic coupling KU are supplied by both power sources. K₁ and K₂ each indicate an independent communication system, e.g., in the form of a serial bus that enables communication between computer components RM_H1 for the feedback actuator and components RM_V1, for the steering actuator. The data exchanged between these components is designated by a_vH. These communication systems K₁ and K₂ also allow communication with other control devices in the vehicle.

[0112] The following functions are implemented in components RM_H1, RM_H2 and RM_H3:

[0113] sensor signal acquisition and calculation of trigger signals U_H1 and U_H2 for regulating steering wheel motors LRM1 and LRM2

[0114] exchange of calculated data via communication links K_H12, K_H13 and K_H23, operating between the microcomputer units; comparison of computing results and initiation of an auxiliary level strategy as necessary

[0115] switching to the auxiliary level

[0116] interface with other control devices and display units

[0117] The following functions are implemented in components RM_v1, RM_v2 and RM_v3:

[0118] sensor signal acquisition and calculation of trigger signals U_v1 and U_v2 for regulating steering motor LM1 and LM2

[0119] exchange of calculated data via communication links K_v12, K_v13 and K_v23 operating between the microcomputer units; comparison of computing results and initiation of an auxiliary level strategy as necessary

[0120] switching to the auxiliary level

[0121] interface with other control devices and display units

[0122] These functions will be described in the following.

[0123] A: Sensor Signal Acquisition and Calculation of Control Signals for Regulating the Steering Wheel Motor

[0124] The signals indicated in the embodiment according to FIG. 5 for triggering steering wheel motors LRM1 and LRM2 are acquired via peripheral signal acquisition components and fed to microcomputer systems RM_H1, RM_H2 and RM_H3.

[0125] Redundant components are acquired separately. Redundant sensors each transfer one signal. In the following, redundancy of components and sensors is indicated using subscript:

[0126] The acquired signals are combined in FIG. 9 for motor LRM1 under the notation E_LRM1, and for motor LRM2 under the notation E_LRM2.

[0127] Trigger signals U_H1 and U_H2 serve primarily to trigger power electronics units LE_LRM1 and LE_LRM2. Steering wheel motor LRM1 is controlled via power electronics LE_LRM1, if an enable is queued via cut-off logic AL_LRM1 and the motor relay is also closed by cut-off logic AL-MR_LRM1. In a similar manner, steering wheel motor LRM2 is triggered by power electronics LE_LRM2 if an enable is queued via cut-off logic AL_LRM2 and the motor relay is closed by cut-off logic AL-MR_LRM2.

[0128] B: Sensor Signal Acquisition and Calculation of Control Signals for Regulating Steering Motors

[0129] To trigger steering motors LM1 and LM2, the signals indicated with reference to the embodiment shown in FIG. 5 are acquired by peripheral signal acquisition components and fed to the microcomputer systems RM_v1, RM_v2, and RM_v3.

[0130] These signals are combined in FIG. 9 for motor LM1 under the notation E_LM1, and for motor LM2 under the notation E_LM2.

[0131] Trigger signals U_V1 and U_V2 serve primarily to trigger power electronics units LE_LM1 and LE_LM2. These trigger signals are manipulated variables of a digital regulator and may be output, for example, in the form of PWM signals. Manipulated variables U_v1 are formed from a representative value of measured steering wheel angles _H1 and _H2 and the current steering transfer ratio. With respect to steering interventions by a dynamic driving system, the description of the embodiment of FIG. 5 applies accordingly.

[0132] Steering motor LM1 is triggered by power electronics LE_LM1 if an enable is queued via cut-off logic AL_LM1 and the motor relay is also closed by cut-off logic AL-MR_LM1. Similarly, steering motor LM2 is controlled by power electronics LE_LM2, if an enable is signal queued via cut-off logic AL_LM2 and motor relay is also closed by cut-off logic AL-MR_LM2.

[0133] C:4 Monitoring the Auxiliary Level and Switching to the Auxiliary Level

[0134] The currents in both windings S₁ and S₂ of the coupling are essential parameters in the availability of auxiliary level of the SbW steering system. These currents are acquired continuously. In order to check the switching function to the mechanical auxiliary level, the electrical circuits of the coupling windings are interrupted alternately in driving mode by signals r_v1, and r_v2 or r_H1 and r_H2. A switch to the mechanical auxiliary level is possible if the respective winding currents return to the value zero.

[0135] D: Interface with Other Control Devices and Display Units

[0136] The description given with reference to the embodiment of FIG. 5 applies accordingly.

[0137] E: Safety Measures in the Control Device

[0138] The measures necessary in order to ensure that the safety standards imposed on this system are satisfied are the same as those included in the description of FIG. 5 under E. In order to meet these safety requirements, the following is provided:

[0139] All measurement signals E_LRM1 and E_LMR2 incident at both motors LRM1 and LRM2, as well as all measurement signals E_LERM1 and E_LERM2 from power electronics LE_LRM1, and LE_LRM2, are acquired in three computing modules RM_H1, RM_H2, and RM_H3. The acquired measurement signals are exchanged among the modules via computer links K_H12, K_H13, and K_H23, and compared with each other for plausibility. A reference value is then created for each measured variable, by a 2 out of 3 majority vote for example. This allows a faulty input channel of a measured variable to be located unambiguously. A faulty input channel is then excluded from further calculations. At the same time this error is stored in an error memory. Then, subsequent calculations are performed in all three calculation modules with these reference values from the individual measured variables, or this reference value is made available to computer modules RM_v1, RM_v2, and RM_v3 of the steering actuator in the form of communication datum a_VH via communication systems K₁ and K₂.

[0140] The calculations for the formation of trigger signals U_H1 and U_H2 for steering wheel motors LRM1 and LRM2 are also executed redundantly in all microcomputer systems RM_H1, RM_H2, and RM_H3. The results are again exchanged via link paths K_H12, K_H13, and K_H23, among one another and checked against one another for plausibility. This method of an implementation variant for initiating the cut-off strategy is explained using the example of the calculation of trigger signal U_H1. The results calculated in microcomputer systems RM_H1, RM_H2, and RM_H3 are indicated by U_H11, U_H12, U_H13. If an error is located in U_H11 during comparison of these results in computing unit KM_H1, and the error still persists after the expiry of an error tolerance interval, the enable signals for power electronics g_H11 and for motor relay f_H11 are disabled, and the enable signal to link trigger h_Hk1 is reset. If an error is located in U_H11 during comparison of computer results U_H11, U_H12, and U_H13 in computing units RM_H3 or RM_H2, the corresponding enable signals g_H31, f_H31 and h_HK3 or g_H21, f_H21 and h_HK2 are reset.

[0141] Trigger signals U_H1 and U_H2 are not released until enable signals m_H1 or m_H2 are pending within the cut-off logic units for the power electronics of steering wheel motors AL_LRM1 or AL_LRM2.

[0142] The logic circuit arrangement for creating enable signals to activate trigger signal U_H1 is shown as an example in FIG. 10 for the enable signal m_H1 for the power electronics of steering wheel motor LE_LRM1. This ensures that an error that has occurred during calculation of U_H1 in RM_H1 results directly in the disconnection of power electronics LE_LRM1 as soon as it is detected either by computing system RM_H1 or indirectly to the disconnection of the same power electronics as soon as it is detected jointly by modules RM_H2 and RM_H3. The enable signals may be implemented statically, the function of the switching transfer is then monitored by re-reading. For this purpose, these enable signals may be set and reset cyclically in test phases. A further variant provides for dynamic trigger of the enable signals.

[0143] The time characteristic of signals g_H11, g_H31, and g_H21 of such a method in error-free operation is shown in FIG. 11a). The resulting enable signal m_H1 may be monitored in its correct time sequence by a watchdog unit.

[0144] In FIGS. 11b) to 11d) potential errors in the enable signals are illustrated, together with their effect on enable signal m_H1. In FIG. 11b), signal g_H21 is registered at the low value and in FIG. 11c) at the high value. FIG. 11d) shows the effect of an error resulting from freezing of intermediate signal v_H1 at the high signal.

[0145] Cut-off logic may trigger motor relay AL-MR_LRM1 with a circuit arrangement as shown in FIG. 10. In this case enable signals f_H11, f_H31 and f_H21 must be used instead of enable signals g_H11, g_H31 and g_H21.

[0146] The coupling unit is triggered with a variant of a circuit arrangement as shown in FIG. 12. Since in each case the coupling may only be maintained in an open state (SbW operation) when windings S₁ and S₂ are energized, this arrangement ensures that a single error cannot cause immediate switching to the mechanical auxiliary level. Each single error triggers only one interruption of the electrical winding circuits via enable signals r_v1, r_v2, r_H1 and r_H2, in each case. In order to guarantee correct functioning in the event of an error, the individual switches are controlled in the opening state in cyclical test phases in driving mode. The ability to open may be detected by monitoring the characteristic of currents i_s1 and i_s2.

[0147] These measures may be applied in a corresponding manner for the feedback actuator.

[0148] The control device structures previously described are also suitable for steer-by-wire steering systems according to FIG. 1a. Only the monitoring of the reservoir pressure necessary for the hydraulic auxiliary level is omitted. However, the switchover valve USV shown in these structures must be replaced by a coupling control system. The trigger signals g_H and g_U necessary for this are generated in accordance with the arrangements for the hydraulic auxiliary level in the event of an error, so that secure switching to the mechanical auxiliary level with direct steering intervention is assured as soon as errors occur. All features described in the drawing, the description thereof and the claims may individually and in combination represent substantive elements of the present invention.