GB2346598A

GB2346598A - A method of reducing the feedback on the steering wheel of a motor vehicle and an active steering system designed for this purpose

Info

Publication number: GB2346598A
Application number: GB0001259A
Authority: GB
Inventors: Kai Weeber; Rainer Muenz
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1999-02-10
Filing date: 2000-01-21
Publication date: 2000-08-16
Anticipated expiration: 2020-01-21
Also published as: GB2346598B; DE19905433B4; JP2000229579A; DE19905433A1; GB0001259D0; JP4510976B2

Abstract

The invention relates to a method of reducing the feedback on the steering wheel of a motor vehicle with an active steering system which is driven by an electric servomotor and which by means of a superimposing transmission (32) superimposes an additional steering angle ( w M) on a steering wheel angle ( w L) predetermined by the driver on the steering wheel (31). In accordance with the invention the engine current (i<SB>mot</SB>) transmitted to the superimposing transmission (32) by the servomotor (33) is limited in dependence upon the driving situation and the environmental conditions and in fact the less this is, the more dangerous is the driving situation.

Description

2346598 DESCRIPTION "METHOD OF REDUCING THE FEEDBACK ON THE STEERING WHEEL

OF A MOTOR VEHICLE AND AN ACTIVE STEERING SYSTEM DESIGNED FOR THIS PURPOSE" The invention relates to a method of reducing the feedback on the steering wheel of a motor vehicle with an active steering system which is driven by an electric servomotor and which superimposes an additional steering angle by means of a superimposing transmission onto a steering wheel angle predetermined by the driver on the steering wheel, and an active steering system which is designed to perform this method and which comprises a control unit which in response to variables determined by the vehicle and environmental conditions superimposes an additional steering angle by means of an electrical servomotor and a superimposing transmission onto the steering wheel angle set by the driver.

An active steering system of this type is known from DE OS 40 31316 and is illustrated schematically in the attached Figure 1. The steering wheel angle 8L set by the driver on the steering wheel 21 lies at the input of a superimposing transmission 22. A control unit 37 uses signals dependent upon the driving situation and the environmental situation to determine an additional steering angle 8M and supplies a corresponding control current to an electric servomotor 23. The additional steering angle 8M is available at the other input of the superimposing transmission 22. The superimposing transmission 22 produces at the output side a superimposed steering angle 8L' for the steered wheels 25a, 25b, so that this superimposed steering 2 angle can be modified independently of the steering wheel angle 81, set at the steering wheel. A steering transmission 24 converts the superimposed angle 8L' into a corresponding movement of the steering shaft 26. The arrangement illustrated in Figure I can be used to achieve various functions which improve the dynamics of the vehicle movement, the travel safety and the behaviour of the vehicle. This arrangement cannot be used for a power-supporting function such as in the case of power steering. The steering wheel angle K predetermined by the driver should not change and the steering wheel should retain its position.

In the superimposing transmission 22, the mechanical output is added to the fixedly predetermined torque ratio of power ratio but free rotational speeds or velocities. The torque provided by the servomotor must be supported on the steering wheel 21, so that it can be directed in the direction of the steering transmission 24.

If this torque is not supported or only partially supported on the steering wheel 2 1, then the steering wheel 21 begins to rotate instead of the wheels 25a, 25b. It is therefore essential that the driver does not release the steering wheel 21 whilst the servomotor 23 is engaged.

The engine torque which is to be supported by the driver and is fed back to the steering wheel 21 is dependent upon the acceleration of the servomotor 23 and the existing inertia values. The engine torque which is to be supported can be detected by the driver above a specific value. This detectable torque impairs the driver's subjective impression as to how he is driving. The extremely good subjective feeling relating to the steering in conventional steering systems must not I I 3 be impaired by the use of electronic steering systems with superimposing transmissions.

When such an electronic steering system is activated to correct driving situations which are critical for the dynamics of the vehicle movement, it is necessary to produce the required additional steering angle 8M within a short period of time. For this reason, the servomotor which is to produce the additional steering angle must rapidly achieve a high rotational speed. The engine rotational speed n., is obtained by the integration with respect to time of the engine torque rri,,,,, and the weighting with the inertia J as follows:

1 r1mot 0 f om..,dt 2n-J In order for the servomotor 3 to be able to achieve a high rotational speed rapidly, it must produce a high torque value at a specific inertia J. High engine torque values cause the engine to accelerate greatly. By virtue of the above mentioned feedback on the steering wheel high engine torque values have a negative effect on the driver's subjective feeling as to how he is driving. For this reason, high engine torque values are to be avoided in driving situations where they are not required. However, the engine must be dimensioned such that it can if necessary produce high torque values.

Figure 2 illustrates in a graphical manner simulation results which represent the behaviour of the engine in response to a sudden increase in a command variable, i.e. an increase in the additional steering angle 8M. The illustrated simulation results 4 are based on a closed-loop control circuit with a PD controller. In the graphical illustration, A represents the progression with respect to time of the command variable, i.e. the desired angle in degrees LR, B represents the progression with respect to time of the actual angle in degrees LR, C represents the progression with respect to time of the engine torque in Nm which influences the steering shaft. As shown in the figure, the servomotor achieves its desired position after almost 200 ms. According to graph D the torque, which the servomotor applies to the steering shaft, achieves peak values which are greater than 50 Nm. These high torque peaks can be clearly felt by the driver and are unacceptable.

Figure 3 illustrates in a graphical manner the behaviour of the engine as a response to a sudden increase in the command variable where the gradient is limited.

The limitation of the desired angular velocity causes the command variable "desired value of the additional steering angle 8M" to increase in a ramp-like manner. A comparison with Figure 2 shows that the engine torque values become considerably smaller owing to the limitation of the gradient of the desired angular velocity.

Moreover, the engine does not achieve its maximum rotational speed and requires approx. 400 ms until it has achieved its desired position. Figure 3 illustrates the results of the hitherto conventional method of limiting the gradient. This has the clear disadvantage that the engine no longer achieves its desired position so quickly.

An object of the invention is to improve the driver's subjective impression as to how he is driving in the case of an active steering system with superimposing transmission. Detectable feedback on the steering wheel, which is caused by the I torque produced by the servomotor, should, depending upon the situation, be avoided or reduced to an acceptable amount Whilst maintaining the high response rate of the engine.

The object is achieved in accordance with the claims. For this purpose a method in accordance with the invention of reducing the feedback on the steering wheel of a motor vehicle in the case of an active steering system driven by an electric servomotor is characterised in that a first recognition step is provided, during which it is recognised as to whether a sudden increase in the desired value of the additional steering angle occurs, that a second recognition step is provided, during which it is recognised if the first recognition step reveals, that a sudden increase in the desired value of the additional steering angle occurs, whether the prevailing situation is critical for the dynamics of the vehicle movement or not and that a limitation step is provided, during which the value of the controlling torque transmitted to the superimposing transmission by the electrical servomotor is limited in dependence upon the result of the second recognition step in such a manner that the controlling torque is reduced during a situation which is not critical for the dynamics of the vehicle movement and during a situation which is critical for the dynamics of the vehicle movement it is increased.

When the servomotor intervenes in situations which are not critical for the dynamics of the vehicle movement, the method in accordance with the invention reduces the engine torque such that it can no longer be detected by the driver. This can occur for the reason that such an active steering system with superimposing 6 transmission in situations which are not critical for the dynamics of the vehicle movement does not require any extremely rapid acceleration of the engine and therefore a reduced engine torque is sufficient.

In contrast thereto, in the case of situations which are critical for the dynamics of the vehicle movement, a higher engine torque value is permissible. The permitted engine torque increases progressively with the level of criticality of the situation. In such critical driving situations, such as when skidding or braking with a different coefficient of friction for the left and right wheels (g- split-braking), the torque produced by the servomotor is limited in such a manner that it is hardly noticed by the driver. The high response rates of the engine, which are required in such critical driving situations, are still retained.

The torque ni,,,O, produced by the servomotor is proportional to the value of the engine current according tot he following equation:

M.Ot - k imot in which k represents a constant. The above equation shows clearly that by limiting the value of the engine current to a predetermined value, it is ensured that a desired torque on the servomotor is not exceeded. The limitation of the engine current can be achieved by virtue of suitable measures, such as, for example, a current control unit with variable limitation of the desired value.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings in which:

Figure I illustrates a block diagram of the known steering system already I 7 described; Figure 2 illustrates in a graphic manner the behaviour of the servomotor in response to a sudden increase in the command variable without a gradient limitation; Figure 3 illustrates in a graphic manner the behaviour of the servomotor in response to a sudden increase in the command variable with a gradient limitation; Figure 4 illustrates a flow chart of an active steering system designed to implement the method; and Figure 5 illustrates in a graphic manner the behaviour of the servomotor in response to a sudden increase in a command variable where the torque of the servomotor is limited.

Referring to the flow chart illustrated in Figure 4, an active steering system suitable for implementing the method is now described. The steering wheel angle K which is predetermined on a steering wheel 31 of a motor vehicle 35 and which engages on a superimposing transmission 32 on the input side by way of the steering wheel shaft (not illustrated), is detected by a sensor 38 coupled to the steering wheel shaft and is supplied on the input side to a control unit 37. On the output side the superimposing transmission 32 produces a superimposed steering angle 8L', which is supplied to a steering transmission 34. Further sensors 36 provided in the vehicle are used to detect the actual steering angle 6V of the front wheels produced by the steering transmission 34, the travel velocity and other vehicle and environmental conditions and are likewise supplied to the inputs of the control unit 37. The control unit 37 comprises a current control and limiting unit 39 which limits in a variable 8 manner the amount of the engine current imt supplied to the servomotor 33 and in fact in dependence upon the level of criticality of the driving situation which the control unit 37 ascertains with the aid of the signals it receives and likewise with the aid of a stored model 40 of the vehicle and/or the environment.

Thus, the control unit 37 comprises means for implementing the method which reduces the feedback of the electric servomotor on the steering wheel of the motor vehicle in the case of sudden increases in the command variables. These means which are provided in the control unit first perform a recognition step to establish whether a sudden increase occurs in the desired value or command variable of the additional steering angle 8M. A second recognition step is then performed, which in the case of the first recognition step establishing that a sudden increase in the desired value of the additional steering angle had occurred, recognises whether a prevailing situation is critical for the dynamics of the vehicle movement or not.

The control unit 37 supplies the current control and limiting unit 39 with signals which implement the limiting process in dependence upon the driving situation in such a manner that the value of the control torque transmitted by the servomotor 33 to the superimposing transmission 32 in dependence upon the result of the second recognition step is limited in such a manner that it is reduced in a situation which is not critical for the dynamics of the vehicle movement and is increased in a situation which is critical for the dynamics of the vehicle movement.

It is possible to store a model of the vehicle and/or of the environment in a storage device 40 allocated to the control unit 37 and to use this model when I 9 implementing the second recognition step.

As already mentioned, the engine torque mm., can be limited by limiting the amount of the current supplied to the servomotor.

In a preferred embodiment the control unit can be designed for the purpose of differentiating between various situations which are critical for the dynamics of the vehicle movement and for increasing the amount of the engine current progressively with the level of criticality of the driving situation.

Figure 5 shows in a graphical illustration similar to Figures 2 and 3 the engine behaviour in response to a sudden increase in a command variable, the engine torque being limited in accordance with the invention. Graph A represents the desired angle in degrees LR, graph B represents the actual angle in degrees LF, graph C represents the engine rotational speed in I /s and graph D represents the engine torque on the steering shaft in Nm in each case over time. As Figure 5 shows, the torque values are limited towards the left and the right in each case to 25 Nm.

The engine first achieves its desired position after almost 220 ms, which is almost as quick as the servomotor without the limitation of the desired angular velocity, whose behaviour is illustrated graphically in Figure 2. The torque values applied to the steering shaft and thus also the values of the feedback on the steering wheel are half the size of those in the case of the servomotor in accordance with Figure 2. The time which passes until the servomotor has achieved its desired position is half the length of that in the case of the servomotor whose characteristic values are illustrated graphically in Figure 3 and at which the limitation of the desired angular velocity to 4.34 wheel/s was implemented.

The method in accordance with the invention and the active steering system in accordance with the invention thus combine low feedback values on the steering wheel with a sudden achievement of the desired position of the servomotor. The values of the torque applied to the steering shaft can be limited in a variable manner and in response to the situation.

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Claims

1. A method of reducing the feedback on the steering wheel of a motor vehicle with an active steering system which is driven by an electric servomotor and which by means of a superimposing transmission superimposes an additional steering angle (6M) on a steering wheel angle (6L) predetermined on the steering wheel by the driver, characterised in that a first recognition step is provided which recognises whether a sudden increase in the desired value of the additional steering angle (6M) occurs, that a second recognition step is provided which, if the first recognition step indicates that a sudden increase in the desired value of the additional steering angle (6M) occurs, recognises whether the prevailing situation is critical for the dynamics of the vehicle movement or not and that a limiting step is provided which in response to the result of the second recognition limits the value of the controlling torque transmitted by the electric servomotor to the superimposing transmission in such a manner that the controlling torque is reduced in the case of a situation which is not critical for the dynamics of the vehicle movement and increased in the case of a situation which is critical for the dynamics of the vehicle movement.

2. A method according to claim 1, characterised in that the engine torque of the electric servomotor is limited by limiting the engine current supplied to the servomotor.

3. A method according to claim 1, characterised in that the second recognition step includes a difierentiating step in which it differentiates between 12 several different situations which are critical for the dynamics of the vehicle movement and that the limiting step progressively increases the engine current with the level of criticality of the driving situation.

4. An active steering system for implementing the method according to any one of the preceding claims having a control unit which in response to variables determined by the vehicle and environmental conditions superimposes by means of an electric servomotor and a superimposing transmission, an additional steering angle (M) on the steering wheel angle (K) set by the driver, characterised in that the control unit comprises a current control and limiting unit which limits in a variable manner the engine current being supplied to the servomotor and in fact in response to the level of criticality of the driving situation which the control unit ascertains with the aid of signals which are supplied to said control unit by sensors provided in the motor vehicle and/or other control devices located in the vehicle.

5. An active steering system according to claim 4, characterised in that provided in the control unit are means for implementing the first and second recognition steps which comprise a model of the vehicle and/or of the environment.

6. A method of an active steering system according to claim 4, characterised in that a motor vehicle substantially as hereinbefore described with reference to Figures 2 to 5 of the accompanying drawings.

7. An active steering system substantially as hereinbefore described with reference to Figures 2 to 5 of the accompanying drawings.

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