WO1990009916A1

WO1990009916A1 - Process for controlling the rear wheels of motor vehicles

Info

Publication number: WO1990009916A1
Application number: PCT/EP1990/000350
Authority: WO
Inventors: Cornelius Chucholowski
Original assignee: Bayerische Motoren Werke Aktiengesellschaft
Priority date: 1989-03-03
Filing date: 1990-03-02
Publication date: 1990-09-07
Also published as: JPH04505299A; EP0461170A1; DE3906741A1

Abstract

In a process for controlling the rear wheels of motor vehicles infunction of a deflection of the front wheels, the steering angles for the front and rear wheels are mechanically decoupled and a transmission element adjusts the angle of deflection of the rear wheels in function of the deflection of the front wheels, the rear wheels are briefly and reversibly deflected in the opposite direction to the front wheels and are then deflected in the same direction as the front wheels. The adjustment of the rear wheels is controlled. When the angle of deflection of the front wheels is stationary or not equal to zero, the angle of deflection of the rear wheels is reset to zero after reaching a maximum value. This results in an appreciable increase in driving dynamics in comparison with a conventional vehicle. In stationary cases, alternating load reactions are easily mastered while the angle of deflection of the front wheels is kept constant.

Description

Method for controlling the rear wheels of motor vehicles.

description

The invention relates to a method according to the preamble of patent claim 1.

In such a method, as is known from DE 36 23 479 A, a mathematical target vehicle model is calculated and related to an actual vehicle model. The steering angle of the rear wheels is changed so that the difference between the target values representative of the target vehicle model and the estimated values which characterize the actual vehicle model is reduced.

The known method only requires a steering angle and a vehicle speed sensor. A sensor for the parameters of the actual vehicle movement, such as, for example, the yaw rate, the lateral force or the course angular velocity, is not necessary. However, it is absolutely necessary to use a process computer which relates the two vehicle models to one another and which changes one model in the direction of the other model. The use of a process computer in turn requires a lot of effort

REPLACEMENT LEAF and generally does not make it possible to take external disturbances into account for vehicle movement, for example due to bumps in the road or cross winds. Furthermore, runtime problems arise from the generally sequential mode of operation of the process computer, which can lead to destabilization of the entire vehicle, in particular if the steering angle changes rapidly.

The invention has for its object to achieve a high degree of driving stability with the least possible equipment.

The invention solves this problem by means of the characterizing features of patent claim 1.

The controlled adjustment of the rear wheels enables a quick, feedback-free reaction to changes in the front wheel steering angle. In contrast to known methods of the type mentioned at the outset, in which the steering angle of the rear wheels remains in a defined ratio to that of the front wheels in stationary cases, the only temporary deflection of the rear wheels approximates the stationary steering characteristics of a conventional vehicle with one steering, that only affects the front wheels. This means that the user of the vehicle does not have to get used to the four-wheel steering system. This advantage is particularly noticeable when the adjustment of the rear wheels is at least approximately independent of the vehicle speed. With load change reactions of the Vehicles with a constant steering angle of the front wheels, the rear wheels do not change their steering angle (= 0) or continue to approach their rest position. A vehicle steered in this way thus does not develop its own dynamic due to the adjustment of the rear wheels, especially in the case of load change reactions, and thus makes it possible to master load change reactions of the vehicle without problems.

An improvement of the invention consists in the features of patent claim 2. This ensures that the driving dynamics are increased further, since the movement of the rear wheels in the same direction only begins when the vehicle is already following the steering movement of the front wheels. In addition, reversible deflection of the rear wheels can take place during dead time in order to accelerate the build-up of lateral force on the rear wheels. Such a steering movement of the rear wheels, in which the rear wheels are first deflected in opposite directions and then in the same direction, is known in principle from DE 36 23 479 A.

The controlled deflection of the rear wheels makes it possible to design the course angular velocity, the yaw rate and the float angle without overshoot. It is only necessary to set the maximum value of the rear wheel steering angle when adjusting in the same direction and the change in the rear wheel steering angle in accordance with the respective physical parameters of the vehicle. In this case, the controlled deflection of the Rear wheels fully to bear, since this is the only way to enable the rear wheel steering angle to respond quickly to changes in the steering angle of the front wheels.

This suitable setting of the rear wheel steering angle, both with regard to its maximum value and with regard to its changes, can be achieved with particularly little effort with the aid of a PD2T2 transmission element for controlling the rear wheels as a function of that of the front wheels. The rear wheel steering angle is only dependent on time derivatives of the front wheel steering angle. The stationary swimming angle then corresponds exactly to that of the conventionally steered vehicle and is reduced dynamically. The reversible deflection of the rear wheels in stationary cases also results in a float angle which corresponds to that of a vehicle which is conventionally steered only on the front axle and which occurs without overshoots. The steering angle requirement for the front wheels is then also the same as that of the conventional vehicle.

The safety aspect is also essential. If there is an emergency in which the steering of the rear wheels or the control unit fails for this purpose, this is particularly uncritical if the rear wheels have again reached the link angle 0 even with the front wheels stationary. The steering angle of the rear wheels is usually frozen in an emergency, ie here on the Value 0 recorded. The vehicle then continues to behave like a conventional vehicle.

Another improvement of the invention is that the PD2T2 controller is superimposed on a PDT1 controller. The latter is in principle from the DE

10 36 24 457 AI known. From this publication, to which express reference is made, the particular advantages of such a control are evident. The superimposition of the two controls makes it possible to take advantage of the two controls without simultaneously suffering from their disadvantages.

All that is required is the degree to which both controls influence the steering behavior

Take 20 of the rear wheels to choose variably according to the respective driving dynamics requirements. From fundamental considerations it follows that in the lower and middle speed range, i.e. about between 50 and 120 km / h the influence of

25 PDTl control predominates, while for higher speeds the PD2T2 control should increasingly dominate.

This results in the speed velocity in the former

In the ideal case, complete compensation of the float angle with the advantage of high driving stability and, in the higher speed range, a significant improvement in vehicle dynamics is sufficient, with a steering angle requirement of the front wheels, _oc - which is equal to that of the conventional vehicle. In addition, in this case, a vehicle with PD2T2 control of the rear wheels is considered to be manageable and has good willingness to drive. For example, in order to reach a stationary arc, the same steering angle must be applied to the front axle as in a conventional vehicle. Subjectively, the stable, overshoot-free setting of the stationary driving state variables of the vehicle with PD2T2 control of the rear wheels makes itself felt in the feeling of high safety. In border areas with high lateral accelerations or poor friction conditions, the vehicle is particularly easy to control.

The invention is further illustrated by the drawing. It shows

Fig. 1 based on five diagrams a to c

Response responses of a conventionally steered vehicle in the event of a steering angle jump on the front axle,

Fig. 2 with the jump responses of a vehicle

PD2T2 structure of the rear wheel steering on the basis of corresponding diagrams and under the same conditions as FIG. 1,

3 diagrams corresponding to FIGS. 1 and 2 for a vehicle with a superimposition of PD2T2 and PDTl control,

Fig. 4 diagrams for the factors that the Determine the PD2T2 control from Fig. 2.

The diagrams in FIG. 1, plotted against time s, show the jump responses of a conventionally steered vehicle for speeds of 60 to 240 km / h, viewed in jumps of 30 km / h.

10 In each of these cases, a steering angle jump of 1 ° on the front axle should take place at a time of 0.5 seconds after the starting point, and the steering angle of 1 ° should then be maintained stationary. This assumption is shown in diagram a. ,, - The arbitrarily selected curve, designated I1T_, symbolizes a steering angle jump in the manner mentioned, which is carried out at a speed of 120 km / h.

The steering angle of the rear wheels remains zero for the conventionally steered vehicle. This is symbolized by parallel lines (diagram b).

₂₅ The resulting yaw rate is shown in diagram c. It can be clearly seen that this vehicle parameter has an overshoot with a subsequent undershoot, especially at high vehicle speeds, and settles asymptotically to a stationary value.

The same applies correspondingly to the course angular velocity shown in diagram d and to the slip angle shown in diagram e. The latter ₃₅ has an overshoot, especially at relatively high speeds of 150 to 180 km / h Behavior. This inevitably leads to extremely problematic controllability of the entire vehicle.

The diagram in FIG. 2 shows the typical jump responses of a vehicle with rear axle steering with PD2T2 structure and suitably selected parameters dependent on the driving speed. 4 is shown in more detail.

On the front axle, a steering angle jump of 1 ° at the time 0.5 seconds should again be assumed for the different speeds (diagram a). The steering angle on the rear axle (diagram b) initially shows opposite behavior over a period of time, which is almost independent of vehicle speed, especially at high speeds. This is followed by a deflection of the rear wheels in the same direction, which extends up to a maximum value and then returns to the value 0. With a suitable choice of the parameters of the PD2T2 structure (cf. FIG. 4), yaw angle speed (diagram c), course angle speed (diagram d) and float angle (diagram e) can be set without overshoot. From diagram e in particular, it is readily apparent that such a vehicle has a high level of driving stability, since the float angle increases uniformly to a non-zero final value. This end value is in turn dependent on the vehicle speed and corresponds to that of the conventional vehicle, as is evident Comparison with the diagram e of Fig. 1 results easily. Compared to the conventional vehicle, however, the overshoot is not only lacking in the respective slip angle, but also in the yaw angle speed and course angle speed.

The superimposition of a PD2T2 controller shown in FIG. 3 with a PDT1 controller, represented schematically by the equation

With s = la place operator and k = weighting factor between 0 and 1, it is possible to combine the advantages of both control strategies. From this equation it emerges as an essential feature that the transfer function of the yaw rate is equal to the transfer function given by the PDT _η structure with the stationary portion modified in accordance with the conventional vehicle.

The factor k specifies the degree of steady angle compensation. If k has the value 0, then there is complete float angle compensation. It is then a pure PDT1 transmission. Correspondingly, for the value k = 1, stationary float angle compensation is then not carried out and the transmission is PD2T2. The behavior of such a superimposition of the two controls can easily be derived from the diagrams c to i of FIG. 3. The value of the factor k was used linearly, depending on the driving speed, from k = 0 at v = 60 km / h to k = 1 at v = 240 km / h.

Noteworthy here is the smaller float angle and the somewhat lower yaw angle and course angle speed compared to the PD_T control of FIG. 2. The reason for this is the permanent steering angle on the rear wheels, which follows from the PDT structure for the stationary case. By adapting the factor k, the stationary characteristic of the vehicle can be influenced without impairing the stable, overshoot-free transition behavior.

The dependency of the parameters for the PD2T2 control on the vehicle speed is shown in FIG. 4. These parameters are plotted for an investigated vehicle model in diagrams a to e. The transfer function between the steering angle <T v is the

Front axle and the steering angle <* .n of the rear axle through the differential equation * • _' ^* **

described. This differential equation has four vehicle-specific parameters:

the delay times Tl and T2 the gain factor D2 for the steering angle speed and the gain factor D3 for the steering angle acceleration.

These parameters are related to the vehicle speed in the manner shown in the diagrams from FIGS. 4a to 4e.

The differential equation has no stationary part, i.e. the rear axle lock always returns to the center position after a dynamic maneuver with the steering wheel held tight.

The delay time T and T_ cause the DR of the steering angular velocity and the steering angular acceleration cT abhängi.g ^'v e Stellbe¬ movement is delayed and smoothed performed. The decelerating effect increases with increasing driving speed, since the sum and the product of the two parameters T and T increases.

The parameter D2, which specifies the dependency of the rear axle steering angle on the steering angular velocity, is increasingly positive in the interesting driving speed range above approximately 100 km / h, so that in the case of fast steering angle processes essentially a same-sided rear axle turning is effected.

This is superimposed on the dependence on the steering angular acceleration, given by parameter D3. This parameter has a whole Driving speed range a negative sign. This sign has the effect that a counter-steering movement takes place on the rear axle in the case of very rapid changes in the steering angle in the beginning, since relatively large steering angle accelerations occur here. This short, opposite deflection decreases with increasing driving speed, because then the decelerating and smoothing effect of the time constant T2 predominates.

The lack of a proportional component results in an asymptotic approximation of the rear wheel steering angle to the value 0 and thus to the steering behavior of a conventional vehicle with the described increase in driving safety in the event of a failure of the rear wheel steering in the case of a stationary steering angle of the front wheels other than zero .

Claims

- 43 -

Claims

Method for controlling the rear wheels of motor vehicles as a function of a deflection of the front wheels, with mechanically decoupled steering means for the front and rear wheels and a transmission element for adjusting the steering means of the rear wheels as a function of the deflection of the front wheels, in which the rear wheels ¬ which are briefly reversibly deflected against the direction of the front wheels and then experience a deflection in the direction of the front wheels, characterized in that the adjustment of the rear wheels takes place without feedback and with a steady, non-zero steering angle of the front wheels after reaching a maximum value on the Value zero is reduced.

Method according to Claim 1, characterized in that the rear wheels are adjusted in the same direction, measured from the start of the steering movement of the front wheels, after a dead time which is dependent on the vehicle speed.

Method according to claim 1 or 2, characterized in that the maximum value of the Rear wheel steering angle when the rear wheels are adjusted in the same direction and the changes in the rear wheel steering angle before and after reaching this maximum value are such that the heading angle speed, the yaw rate and the swimming angle always exhibit a vibration-free behavior.

Method according to one of claims 1 to 3, characterized by a PD2T2 transmission element for the control of the rear wheels as a function of that of the front wheels.

Method according to Claim 4, characterized in that a PDT1 control is superimposed on the PD2T2 control.

Method according to Claim 5, characterized in that the PDT1 control dominates at lower vehicle speeds and the PD2T2 control dominates at higher vehicle speeds.