Method and Device for Controlling a Rear-Axle Steering System

Abstract

A method and device for controlling a rear-axle steering system, in particular a rear-axle steering system of a motor vehicle, determines a current physical condition of the rear-axle steering system on the basis of a detected current operating state and a pre-determined reference operating state of the rear-axle steering system. The method defines a maximum permissible steering angle of the rear-axle steering system depending on the estimated physical condition of the rear-axle steering system and depending on at least one of the operating parameters of driving speed, steering angle and steering angle speed of the vehicle, and actuates the rear-axle steering system such that the steering angle of the rear-axle steering system does not exceed the assigned defined maximum permissible steering angle for the current operating parameter(s) of the vehicle.

Claims

1.-15. (canceled)

16. A method for controlling a rear-axle steering system of a vehicle having a steerable rear axle, comprising: determining a present physical state (PV) of the rear-axle steering system on the basis of a detected present operating state (A.sub.act) and a predetermined reference operating state (A.sub.norm) of the rear-axle steering system; specifying a maximum permissible steering angle (L.sub.max) of the rear-axle steering system as a function of the estimated physical state (PV) of the rear-axle steering system and as a function of at least one of the operating parameters driving speed (Vx), steering angle (L), and steering angle speed (VL) of the vehicle; and activating the rear-axle steering system so that the steering angle (L) of the rear-axle steering system does not exceed an assigned specified maximum permissible steering angle (L.sub.max) with the present operating parameter(s) (Vx, VL) of the vehicle.

17. The method according to claim 16, wherein the determination of the present physical state (PV) of the rear-axle steering system is carried out based on a present utilization (A.sub.act) of the specified operating range of an actuator of the rear-axle steering system provided for its actuation, which characterizes the present operating state.

18. The method according to claim 17, wherein: the reference operating state defines a predetermined reference utilization (A.sub.norm) of the operating range and the determination of the present physical state (PV) of the rear-axle steering system is carried out based on a comparison (A.sub.act) of the detected present utilization of the operating range of the actuator to the reference utilization (A.sub.norm); and the present utilization and the reference utilization (A.sub.norm) are each related to the same operating parameter(s) (Vx, VL, L) of the rear-axle steering system and the same values thereof.

19. The method according to claim 18, wherein: the reference utilization (A.sub.norm) represents an operating point on a characteristic curve or a multidimensional characteristic curve surface for the utilization of the actuator, which is related to the operating range; and for each operating point (Vx, VL, L) on this characteristic curve or characteristic curve surface, the actuator ensures a reset of the rear axle from a position corresponding to a steering angle (L) not equal to zero into a non-deflected position, while the actuator no longer ensures this for every utilization above this characteristic curve or this characteristic curve surface.

20. The method according to claim 16, wherein the determination of the present physical state (PV) of the rear-axle steering system is carried out via an automatic learning process based on a repeated determination of a respective present operating state of the rear-axle steering system at various operating times (t).

21. The method according to claim 20, wherein the learning process comprises at least one of the following steps: (i) averaging over the individually detected operating states at the various operating times; or (ii) a running maximum value formation, in which the respective present value for the physical state of the rear-axle steering system is progressively determined as the respective maximum value of at least two values which have occurred up to this point for this physical state or as a function of this maximum value.

22. The method according to claim 16, wherein the maximum permissible steering angle (L.sub.max) of the rear-axle steering system for the speed corresponding to a standstill of the vehicle is set to a value greater than zero if the prediction results on the basis of the determination of the present physical state (PV) of the rear-axle steering system that a reset of the rear axle from this position corresponding to a steering angle (L) not equal to zero into a non-deflected position is also possible at a standstill of the vehicle.

23. The method according to claim 22, wherein the prediction on the basis of the determination of the present physical state (PV) of the rear-axle steering system as to whether a reset of the rear axle from the position corresponding to the steering angle not equal to zero into a non-deflected position is also possible at a standstill of the vehicle is carried out based on a comparison of the estimated present physical state (PV) to a predefined limiting value, which corresponds to a best possible physical state (PV) or is below the best possible physical state.

24. The method according to claim 17, wherein the present utilization (A.sub.act) of the rear-axle steering system is determined based on an actual current consumption (I.sub.act) or actual power consumption or actual supply voltage in comparison to a maximum possible current consumption or power consumption or supply voltage.

25. The method according to claim 16, wherein restriction of the maximum steering angle (L.sub.max) as a function of the present physical state (PV) of the rear-axle steering system only takes place for operating states of the rear-axle steering system from a selected partial range of the operating range, which corresponds to a vehicle speed (Vx), a steering angle speed (VL), steering angle (L), or a combination of at least two of these variables, each below a predetermined associated limiting threshold.

26. The method according to claim 16, wherein the maximum permissible steering angle of the rear-axle steering system is additionally specified in one of the following ways: (i) as a function of the direction of change of the driving speed (Vx) of the vehicle, of the sign of the steering angle speed (VL), or as to whether a distance has already been covered since the last vehicle stop; (ii) as a function of a detected present demand intensity (B) with respect to the steering of the vehicle; or (iii) the operating parameter-dependent curve of the maximum permissible steering angle (L) in the operating range corresponds to an iso-force characteristic curve with respect to the occurring steering force on the rear-axle steering system at this steering angle (L).

27. The method according to claim 16, further comprising at least one of the following measures: limiting of the steering angle (L) to the maximum permissible steering angle (L.sub.max) during the activation of the rear-axle steering system is only applied if the steering angle (L) exceeds a defined minimum deflection; or specifying the maximum steering angle speed (VL) applied by the actuator for the rear-axle steering system as a function of the present driving speed (Vx) of the vehicle.

28. A device for controlling a rear-axle steering system of a vehicle having a steerable rear axle, comprising: a processor-based control unit operatively configured to: determine a present physical state (PV) of the rear-axle steering system on the basis of a detected present operating state (A.sub.act) and a predetermined reference operating state (A.sub.norm) of the rear-axle steering system; specify a maximum permissible steering angle (L.sub.max) of the rear-axle steering system as a function of the estimated physical state (PV) of the rear-axle steering system and as a function of at least one of the operating parameters driving speed (Vx), steering angle (L), and steering angle speed (VL) of the vehicle; and activate the rear-axle steering system so that the steering angle (L) of the rear-axle steering system does not exceed an assigned specified maximum permissible steering angle (L.sub.max) with the present operating parameter(s) (Vx, VL) of the vehicle.

29. A vehicle comprising: a steerable rear axle; and a device according to claim 28 for controlling the steerable rear axle.

30. A computer product comprising a non-transitory computer readable medium having stored thereon program code which, upon execution by a processor, carries out the acts of: determining a present physical state (PV) of the rear-axle steering system on the basis of a detected present operating state (A.sub.act) and a predetermined reference operating state (A.sub.norm) of the rear-axle steering system; specifying a maximum permissible steering angle (L.sub.max) of the rear-axle steering system as a function of the estimated physical state (PV) of the rear-axle steering system and as a function of at least one of the operating parameters driving speed (Vx), steering angle (L), and steering angle speed (VL) of the vehicle; and activating the rear-axle steering system so that the steering angle (L) of the rear-axle steering system does not exceed an assigned specified maximum permissible steering angle (L.sub.max) with the present operating parameter(s) (Vx, VL) of the vehicle.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] FIG. 1 schematically shows an exemplary vehicle having a rear-axle steering system including an embodiment of the device according to the invention for controlling the rear-axle steering system;

[0043] FIG. 2 is a flow chart to illustrate an exemplary embodiment of the method according to the invention;

[0044] FIG. 3 is a schematic illustration of a parameterization of the operating range of a rear-axle steering system by means of a corresponding coordinate system including a reference characteristic curve surface drawn therein;

[0045] FIG. 4 is a two-dimensional illustration of the characteristic curve surface according to FIG. 3 to illustrate a learning range for the determination of a present utilization, here based on the example of a reference steering angle of 1°;

[0046] FIG. 5 shows a chronological development of the utilization to illustrate a learning process usable in conjunction with the invention for the determination of a present degree of utilization A, in particular with regard to a determination based thereon of a present physical state of the rear-axle steering system;

[0047] FIG. 6 is a graphic illustration of two different applications (use cases) for the operation of a rear-axle steering system; and

[0048] FIG. 7 is an illustration of the curve of a characteristic curve specifying the usable operating range of the rear-axle steering system for the maximum steering angle as a function of the longitudinal speed of the vehicle, in particular as a function of the two applications from FIG. 6.

[0049] In the figures, the same reference signs are used throughout for the same elements or elements corresponding to one another of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0050] An exemplary vehicle 1 is shown in FIG. 1, which has two steerable front wheels 2a and two rear wheels 2b, also steerable by means of a rear-axle steering system. The rear-axle steering system in particular has a control device 3, which in turn has a processor platform 3a and a memory 3b, which is used in particular as a program and data memory. A computer program is stored in the memory 3b which, upon its execution on the processor platform 3a, causes it to carry out the method according to the invention, in particular as described hereinafter with reference to the embodiment shown by way of example in FIG. 2. In the scope of the method, the control device 3 receives measurement data from a sensor device 5 of the vehicle 1, which can itself have one or more sensors. The measurement data relate to the items of information required in the scope of the method on the present vehicle status and especially also the rear-axle steering system status, such as in particular the absolute value of the vehicle (longitudinal) speed Vx and its direction of change Rx, steering angle L, steering angle speed VL, demand intensity (steering wheel torsion torque) B, and the actual current consumption I.sub.act of the actuator 4, the latter in any case if this information is not provided by the actuator 4 itself.

[0051] The method 100 shown in FIG. 2 for controlling a rear-axle steering system of a vehicle, for example the vehicle 1 from FIG. 1, can be executed in particular by a control device for a rear-axle steering system, for example by the control device 3 from FIG. 1. Therefore, reference is made for the purposes of better illustration of the method 100 to the vehicle 1 from FIG. 1, without this being interpreted as a restriction, however.

[0052] In the scope of the method, the device 3 receives the above-described measurement data acquired by the sensor device 5 of the vehicle 1 for the variables Vx, Rx, L, VL, I.sub.act, and B. In a further step 110 it is checked whether these measured values are each in an assigned validity range. If this is the case (110—yes), the method is continued with the next step 115. Otherwise (110—no), the method jumps back to the starting point and is then run through again. In this way, it is possible to prevent obviously incorrect measured values from being used in the scope of the further method for determining the maximum steering angle for the rear-axle steering system.

[0053] In step 115, values stored in the memory 3b are read therefrom for the specified maximum current consumption I.sub.max of the actuator 4 and the reference utilization A.sub.norm (Vx, VL), wherein the reference utilization is defined as a function of the longitudinal speed Vx and the steering angle speed VL and can be represented in the memory 3b, for example, by means of a corresponding value table stored therein.

[0054] Now, initially in a step 120 according to the following equation (1), a present utilization A.sub.act(Vx, VL) of the actuator 4 at the present longitudinal speed Vx and the present steering angle speed VL is determined as the ratio of the associated present actual current consumption I.sub.act (Vx, VL) to the maximum power consumption I.sub.max and stored in the memory 3b:

A.sub.act (Vx, VL)=I.sub.actt(Vx, VO)/I.sub.max  (1)

[0055] The value for the maximum current consumption I.sub.max can in particular also be specified as a function or in dependence on a voltage value of the associated supply voltage. The time curve of the variable A.sub.act (Vx, VL) can in practice be subject to significant variations over time. These can result in particular if the vehicle 1 carries out one or more cornering actions on various underlying surfaces using the rear-axle steering system in the course of a journey, so that different coefficients of friction of the various underlying surfaces or mechanical oscillations result in varying forces on the rear wheels 2b and thus also on actuator 4. To smooth such variations with regard to the smoothest possible operation of the rear-axle steering system, in a further step 125 averaging is carried out over the present value and the respective last N previously ascertained values for the variable A.sub.act (Vx, VL) to obtain a corresponding mean value <A.sub.act (Vx, VL)>.

[0056] On the basis of this present mean value <A.sub.act(Vx, VL)>.sub.neu and the previous mean value <A.sub.act(Vx, VL)>.sub.alt determined in the same way in the preceding method path, in a step 130, a value PV for characterizing the present physical state of the rear-axle steering system can now be calculated according to following equation (2). For this purpose, the greater of the two mentioned mean values for the present actual utilization A.sub.act is set in relation to the reference utilization for the same values of the variables Vx and VL:

[00001]$\begin{array}{cc}PV=\frac{\max \left[{.Math.A_{\mathrm{ist}}\left(\mathrm{Vx},\mathrm{VL}\right).Math.}_{\mathrm{neu}};{.Math.A_{\mathrm{ist}}\left(\mathrm{Vx},\mathrm{VL}\right).Math.}_{\mathrm{alt}}\right]}{A_{\mathrm{norm}}\left(\mathrm{Vx},\mathrm{VL}\right)}& \left(2\right)\end{array}$

[0057] In a step 135, a presently valid maximum steering angle L.sub.max(Vx, VL) can now be specified for the rear-axle steering system as a function of the determined value for PV and the previously received values for the direction of change Rx and the demand intensity B. In addition, a maximum steering angle speed VL.sub.max(Vx) can also be specified as a function of the longitudinal speed Vx. For both mentioned specifications, in particular a lookup table stored in the memory 3b can be used, which relates the respective input variables to the desired output variables. If the actual steering angle L is above the corresponding maximum steering angle L.sub.max at a given operating point of the operating parameter range, this can thus be assessed as the specification in such a way that in this operating point a reset of the rear wheels 2b from this steering angle cannot be ensured. This question is relevant in particular for operating points corresponding to a vehicle standstill, since it can thus be checked whether a reset into the straight ahead position is still reliably possible at all at a standstill.

[0058] Finally, in a step 140, the control device 3 can activate the actuator 4 on the basis of the abovementioned specifications so that it restricts the actual steering angle of the rear-axle steering system so that it is less than or equal to the specified maximum steering angle L.sub.max(Vx, VL) and at the same time also the actual steering angle speed VL is limited by the maximum steering angle speed VL.sub.max(Vx). In the scope of a loop, the method then jumps back to the starting point and is run through again.

[0059] FIG. 3 shows a characteristic curve surface for the reference utilization via A.sub.norm(Vx, VL) as a function of the longitudinal speed Vx and the steering angle speed VL. The reference utilization A.sub.norm is defined on a finite partial range (or section) of the operating parameter range spanned by Vx and VL, specifically so that A.sub.norm always has values less than or equal to “1” (=100%) and here in particular only reaches the value “1” at a single operating point within the mentioned partial range of the operating parameter range. This point is that operating point in the mentioned partial range having minimal longitudinal speed Vx and maximum steering angle speed VL, which typically corresponds to the greatest possible adhesive force of the wheels on the underlying surface for all operating points within the partial range.

[0060] The partial range is delimited here, for example, along the Vx axis by the value 6 km/h, wherein other values can also be used, of course. However, it has been shown that values from the range of 3 km/h to 6 km/h each represent a particularly reasonable limit, since the above-described reset problems in practice can typically be displayed above all in a speed range below these values and since speed values above these limiting values can be strongly influenced by possible lateral accelerations of the vehicle 1. Similarly, the partial range along the steering angle speed dimension can in particular be limited to a value of approximately 3°/s for the same reasons.

[0061] The characteristic curve surface for the reference utilization A.sub.norm(Vx, VL) therefore specifies, in a given physical state, a limiting surface within the operating parameter space, in which a reset of the rear-axle steering system into the straight ahead position is still just ensured for all operating points. For higher degrees of utilization with respect to the respective operating point, in contrast, this can no longer be guaranteed for each operating point, in particular not for low longitudinal speeds Vx and/or high steering angle speeds VL within the mentioned partial range of the operating parameter range.

[0062] FIG. 4 shows a characteristic curve surface in the form of multiple two-dimensional sections through the characteristic curve surface from FIG. 3. In this case, the dependence of the utilization along the dimension of the steering angle speed VL is observed, while the corresponding characteristic curves of A.sub.norm(Vx, VL) are shown for various longitudinal speeds Vx as corresponding individual characteristic curves. It may be seen particularly well in the illustration that the resulting utilization is lower the higher the longitudinal speed Vx is, since higher longitudinal speeds Vx generally result in less adhesion or spinning friction between rear wheel 2b and underlying surface and accordingly the actuator 4 has to apply less energy for the steering process.

[0063] In addition, the adhesion limit H is shown in relation to the steering angle speed VL in FIG. 4, above which the adhesive or friction forces become so great that the actuator 4 can no longer ensure that the rear wheels 2B can be deflected and reset according to the activation, in particular can be reset into the straight ahead position. At a lower degree of utilization, in contrast, at low steering angle speeds VL, a certain utilization reserve R results (illustrated here on the example of the characteristic curve for the smallest value of Vx). Accordingly, the section of the operating parameter range shown shaded and located below the respective characteristic curve and below the adhesive limit is provided as a learning range W for the determination of a present utilization A.sub.act in the scope of the iterative method 100 described with reference to FIG. 2 (the entire shaded range corresponds here to the uppermost of the characteristic curves shown, i.e., that for the least value of Vx).

[0064] FIG. 5 shows for this purpose the time curve of a corresponding learning process, wherein successive learning events for the quotient value <A.sub.act(t)>/A.sub.norm are shown as black dots. The learning based on successive measurements and determinations of <A.sub.act(t)> takes place here on the basis of the maximum value formation in step 130 of the method 100 “from below”. This means that in any case in the scope of the respective last two measurements, only those learning events are taken into consideration for the determination of the physical state PV of the rear-axle steering system which are associated with a maximum value of <A.sub.act(t)>. However, it is also possible to use a greater number of preceding values for <A.sub.act (t)> than only two for the maximum value formation. Moreover, it can be specified that PV in principle has to be a monotonously growing variable, i.e., each new value for PV has to be greater than or equal to the preceding value of PV.

[0065] FIG. 6 shows a graphic illustration of two different applications (use cases) for the operation of the rear-axle steering system, which are explained in detail hereinafter with reference to FIGS. 7A or 7b. The first application “1” relates to a typical cornering action of the vehicle 1, wherein the rear wheels 2b essentially, i.e., in the scope of the deviations typical in a rear-axle steering system, follow the track of the front wheels 2b. Accordingly, in this case the front wheels 2a have already covered a distance since the last time they started driving from a standstill in the forward direction.

[0066] In contrast, the second application “2” relates to a pulling out situation, in which the vehicle 1 parked against a curbstone is to emerge from the standstill with at least partially deflected rear-axle steering system into a forward travel. Therefore, a distance has not yet been covered since the last standstill, so that there is the risk that the rear wheels 2b could drive onto an obstacle over which the front wheels 2b have not yet driven, specifically the curbstone edge here.

[0067] The diagram in FIG. 7A illustrates by way of example various zones A, B1, B2, C, and D of the operating parameter range of a rear-axle steering system with regard to the longitudinal speed Vx and the maximum steering angle L.sub.max at a given maximum steering angle speed VL. The mentioned zones are separated from one another here by the various identified characteristic curves K.sub.A, K.sub.B1, K.sub.B2, and K.sub.C, respectively.

[0068] The first characteristic curve K.sub.A (solid) delimits the zone A in relation to lower longitudinal speeds Vx and in relation to a system-related maximum possible steering angle, which is to be 3° here by way of example. Within this zone A, the longitudinal speed Vx is always sufficiently high that a reset of the rear-axle steering system into the straight ahead position is possible for every operating point in consideration of all typical driving maneuvers, in particular including braking in the curve. At lower longitudinal speeds beyond the characteristic curve K.sub.A, however, this can no longer be ensured, so that from a limiting speed, which is 3 km/h here, for example, the rear-axle steering system is automatically brought into the straight ahead position to avoid a following situation beyond the zone A, in which the reset of the rear wheels 2b into the straight ahead position is no longer possible, in particular no longer possible from a standstill. In known solutions, such a characteristic curve K.sub.A is permanently specified, but without previously determining the physical state of the rear-axle steering system and specifying the characteristic curve K.sub.A as a function thereof.

[0069] According to the invention, but in particular also in the scope of the method 100 from FIG. 2, however, the physical state of the rear-axle steering system is determined and a maximum steering angle L.sub.max(PV) is determined in dependence thereon, which can be represented by a corresponding characteristic curve in the diagram from FIG. 7A. One example of this is the characteristic curve K.sub.B1 shown by dotted lines, which defines a zone B1 to the right of it (including the zone A). Within this zone B1, the statements already made above on zone A apply, i.e., for all operating points within the zone B1, a reset of the rear wheels 2B into the straight ahead position is always possible in this case independently of direction. The precise location of the characteristic curve K.sub.B1 is dependent on the present value for the physical state PV.

[0070] A further example of a solution according to the invention is illustrated in FIG. 7A on the basis of a third characteristic curve K.sub.B2 (dashed), which defines a zone B2 to the right of it (including the zones B1 and A). As in the case of the characteristic curve K.sub.B1, the location of the characteristic curve K.sub.B2 is also dependent on the present value for the physical state PV, which differs in the example shown from that for the characteristic curve K.sub.B1. The characteristic curve K.sub.B2 rises to the greatest steering angle possible overall, wherein, however, the slope of the characteristic curve does not extend consistently. In particular—as shown—it can extend flatter at somewhat higher longitudinal speeds Vx (in the example from approximately 2.8 km/h) than at longitudinal speeds Vx lying below this.

[0071] However, the characteristic curve K.sub.B2 corresponding to rising longitudinal speeds Vx only represents a first branch of an overall characteristic curve, which displays a hysteresis, i.e., a curve dependent on the respective preceding state. This is because the overall characteristic curve also has a second branch K.sub.C, which corresponds to decreasing longitudinal speeds Vx and is not congruent with the first branch. A further zone C is spanned between these two branches, which, in contrast to the zones A, B1, and B2, only represents a permitted range for the steering angle L at decreasing longitudinal speeds Vx. This means that greater steering angles L are permitted in the case of decreasing longitudinal speed than in the case of increasing speed. In particular, this difference can be applied in conjunction with the two applications from FIG. 6. Since in the application “1” corresponding to the characteristic curve K.sub.C, in contrast to the application “2” corresponding to the characteristic curve K.sub.B2, an obstacle acting on the rear wheels 2B is not to be expected, the maximum steering angle L.sub.max can be selected higher here than in the application “2”.

[0072] The second branch K.sub.C of the characteristic curve at the same time defines a forbidden zone D located to the left thereof, which covers a steering angle range which is not permitted. This means that the control device ensures by means of a corresponding activation of the actuator 4 that the actual steering angle L does not enter the range of the zones D. In exceptional cases, this can nonetheless be permitted case by case, in particular if on the basis of a comparison of the detected demand intensity B, in particular a high steering wheel torsion torque lying above a defined associated threshold, an explicit driver intention is recognized to move the steering angle nonetheless into this zone D. As already described above, other options are also possible for specifying a correspondingly high demand intensity B, for example an automatic obstacle recognition. The overall characteristic curve or one or more sections thereof can be defined in particular in the form of an iso-force characteristic curve, so that along the characteristic curve or the respective section or sections, the counterforce originating from the adhesion of the rear wheels on the underlying surface and acting on the rear-axle steering system upon its actuation at a given steering angle speed VL extends consistently.

[0073] In addition to limiting the steering angle L to a maximum value L.sub.max(Vx, VL), limiting the steering angle speed VL itself is also possible. This is illustrated in FIG. 7B, where the maximum permitted steering angle speed VL.sub.max is specified as a characteristic curve as a function of the longitudinal speed Vx. In this example, the design-related maximum permitted value for the steering angle speed is specified at 1°/s as an example.

[0074] In all abovementioned cases, it can moreover be specified that the automatic characteristic curve-related reset of the rear wheels 2b only takes place up to a specified minimum angle not equal to zero, i.e., different from the straight ahead position, which is selected so that it typically is not recognized or is only rarely recognized as deviating from the straight ahead position upon visual observation. In this way, it is possible to shift the characteristic curves toward higher permitted maximum steering angles and to avoid the associated energy demand and wear for reaching a straight ahead position.

[0075] While at least one exemplary embodiment was described above, it is to be noted that a large number of variations thereto exist. It is also to be noted that the described exemplary embodiments only represent nonlimiting examples, and it is not intended that the scope, the applicability, or the configuration of the devices and methods described here be restricted thereby. Rather, the preceding description will supply a person skilled in the art with an introduction for implementing at least one exemplary embodiment, wherein it is obvious that various changes can be performed in the functionality and the arrangement of the elements described in one exemplary embodiment, without deviating from the subject matter specified in the respective attached claims and its legal equivalents.

LIST OF REFERENCE SIGNS

[0076] 1 vehicle

[0077] 2a front wheels

[0078] 2b rear wheels

[0079] 3 control device of the rear-axle steering system

[0080] 3a processor platform

[0081] 3b memory, in particular data and program memory

[0082] 4 actuator of the rear-axle steering system

[0083] 5 vehicle sensor system

[0084] 100-140 method steps

[0085] A.sub.act actual utilization

[0086] A.sub.norm reference utilization

[0087] A conventional zone of direction-independent availability

[0088] B1, B2 variably definable zone of direction-independent availability

[0089] B demand intensity, for example steering wheel torsion torque

[0090] C zone of direction-dependent availability

[0091] D forbidden zone

[0092] H adhesion limit

[0093] I.sub.act actual current consumption

[0094] I.sub.max maximum current consumption

[0095] K.sub.A characteristic curve for zone A

[0096] K.sub.B1 characteristic curve for zone B1

[0097] K.sub.B2 characteristic curve for zone B2

[0098] K.sub.C characteristic curve for zone C, delimits zone D at the same time

[0099] L steering angle

[0100] L.sub.max maximum steering angle

[0101] N number of preceding values for A.sub.act to be taken into consideration in the averaging

[0102] PV physical state (health) of the rear-axle steering system

[0103] R utilization reserve

[0104] Rx direction of change of the longitudinal speed, i.e., speed increase or decrease

[0105] t time

[0106] VL steering angle speed

[0107] VL.sub.max maximum steering angle speed

[0108] Vx longitudinal speed

[0109] W learning range