SE541114C2

SE541114C2 - A method for steering assistance and a steering assist system

Info

Publication number: SE541114C2
Application number: SE1650513A
Authority: SE
Inventors: Johannes Bergkvist; Peter Sidén
Original assignee: Scania Cv Ab
Priority date: 2016-04-18
Filing date: 2016-04-18
Publication date: 2019-04-09
Also published as: SE1650513A1; DE102017003280A1; DE102017003280B4

Abstract

The invention relates to a method for steering assistance of a vehicle (1), the vehicle (1) comprising a steering assist system (4) adapted to control the steering wheel torque required to be applied by the driver (T) to achieve a certain steering angle (Ψ), wherein the steering wheel (6) in a normal mode has a neutral position (P) in which the steered wheels (8) are directed in parallel with the longitudinal extension of the vehicle (1), wherein the steering assist system (4) is adapted to apply a return torque acting clockwise (+T) and a return torque acting counter-clockwise (-T) on the steering wheel (6), striving to turn the steering wheel (6) back to the neutral position (P). The method comprising the steps of predicting (s101) the curvature of the road on which the vehicle (1) is travelling; and controlling (s102) the required steering wheel torque (T) based on the predicted road curvatureThe invention also relates to a steering assist system (4), a vehicle (1) comprising such a system (4), a computer program (P) and a computer program product.

Description

A method for steering assistance and a steering assist system TECHNICAL FIELD The present invention relates to a method for steering assistance of a vehicle, a steering assist system, a vehicle comprising such a system, a computer program and a computer program product according to the appended claims.

BACKGROUND Vehicles commonly comprise steering assist systems assisting the driver such that the torque applied to a steering wheel is amplified and transmitted to the steered wheels through the steering wheel column. This way, large and heavy vehicles can be manoeuvred by the driver with less effort. Some steering assist systems are configured such that an active return torque tries to turn the steering wheel back to a neutral position in which the steered wheels are directed straight ahead. This way, if the driver turns the steering wheel and then let it go, the steering wheel will automatically go back to the neutral position. The return torque is the torque the driver feels when turning the steering wheel and thereby determines the steering wheel torque required to be applied by the driver to turn the steering wheel to a desired steering angle. Some steering assist systems use electric motors and can thereby vary the steering assistance depending on driving conditions, whereby the required steering wheel torque to be applied by the driver varies depending on driving condition.

If too little steering wheel torque is required to achieve the desired steering angle, the steering wheel may be perceived as unstable which may be uncomfortable for the driver. If too much steering wheel torque is required it is difficult for the driver to turn the steering wheel and thus to steer the vehicle. Since the driver typically turns the steering wheel less (smaller steering angle) during high speeds and more (larger steering angle) during low speeds, some steering assist systems are configured to vary the required steering wheel torque depending on the vehicle speed. During high vehicle speeds the required steering wheel torque that the driver has to apply is typically larger. This way, when driving for example on a highway, the steering becomes more stable. Likewise, steering assist systems tend to decrease the required steering wheel torque and thus provide more assistance when the vehicle speed is low. The steering assistance may also vary based on factors indicating that the vehicle is cornering or driving on a curvy road. US5717590 A discloses a steering assist system integrating a magnitude of the lateral acceleration during a period of time and adaptively modifies the steering assistance responsive to the vehicle operation. This way, the steering assistance is increased when repeated turning manoeuvres are detected and the driver can steer the vehicle with less effort.

In some situations, a sudden change in required steering wheel torque may be disadvantageous. For example, if a vehicle is suddenly braked while driving through a curve, such that the vehicle speed abruptly decreases below the limit for decreasing the steering effort, the driver might perceive the increased assistance as if the vehicle is slipping on the road surface. EP2106988 A1 discloses an adaptive steering assist system where the steering assistance is determined according to two maps. If it is decided that the vehicle is not in a high attention-requiring state, a map is selected based on vehicle speed and if it decided that the vehicle is in a high attention-requiring state the same map as previously selected is selected again. This means that if a vehicle is first running with high speed on a straight road with a first steering assistance and then abruptly brakes through a curve, the steering assistance will be the same as during the high speed driving even though the vehicle speed is low. This way, the problem with sudden and inconvenient increase of steering assistance is avoided.

SUMMARY OF THE INVENTION Despite known solutions in the field, there is still a need to develop a method for steering assistance of a vehicle and a steering assist system, which increase the safety and the driver comfort.

An object of the present invention is to achieve an advantageous method for steering assistance, which increases the safety and the driver comfort.

Another object of the invention is to achieve an advantageous steering assist system, which increases the safety and the driver comfort.

The herein mentioned objects are achieved by a method for steering assistance, a steering assist system, a vehicle, a computer program and a computer program product according to the independent claims.

According to an aspect of the present invention a method for steering assistance of a vehicle is provided. The vehicle comprises a steering assist system adapted to control the steering wheel torque required to be applied by a driver to achieve a certain steering angle, wherein the steering wheel in a normal mode has a neutral position in which the steered wheels of the vehicle are parallel with the longitudinal extension of the vehicle, wherein the steering assist system is adapted to apply a return torque acting clockwise and a return torque acting counter-clockwise on the steering wheel, striving to turn the steering wheel back to the neutral position. The method comprises the steps of: - predicting the curvature of the road on which the vehicle is travelling; and - controlling the required steering wheel torque based on the predicted road curvature, and when a curve is predicted, the required steering wheel torque is decreased in the direction of the curve, just before entering the curve.

Steering assist systems are used to assist the driver and thus to reduce the driver effort when steering a vehicle. Many systems are configured to control the steering assistance based on current driving conditions such as vehicle speed. It is known that steering assistance is needed the most at low vehicle speeds for example when driving on curvy roads, parking lots or similar. It is also known that it is desired to have a stiff/stable steering wheel when driving with high speed on for example a high way. With a steering assist system applying an active return torque striving to turn the steering wheel back to the neutral position (zero position) the driver will perceive the return torque as a resistance to turn the steering wheel. The steering wheel torque required to be applied by the driver, and thus the driver effort, thereby depends on the return torque. Other factors such as wind, lateral inclination (banking) of the road etc. will also affect the driver effort when steering a vehicle. It is desired to minimize the driver effort without making the steering wheel unstable. By predicting the curvature of the road on which a vehicle is travelling, the steering assist system can control the steering wheel torque required to be applied by the driver based on the prediction and thereby make sure that the driver effort is minimized when needed. Also, contrary to known solutions, by controlling the required steering wheel torque based on a predicted curvature, the steering assist system is prepared to control the required steering wheel torque pre-emptively or just in time to when it is needed. In known solutions the steering assistance is controlled based on current driving conditions and there is thus always a certain time lag before the adequate steering assistance is given. By controlling the required steering wheel torque, and thus the steering assistance, based on a predicted curvature, such time lag is avoided. With the method according to the invention, the driver gets the assistance when he desires/needs it, which increases the driver comfort. Also, since the steering assistance is controlled based on a prediction of the road curvature abrupt changes of steering assistance while driving through a curve, is avoided. Safety is thereby increased.

The steering assist system preferably comprises an electric servomechanism with an electric motor. The active return torque is suitably applied by the electric servomechanism directly to the steering wheel column. The steering assistance and thus the required steering wheel torque can thereby be controlled in a flexible way.

The torque applied to the steering wheel by the driver is herein called steering wheel torque. The torque required to be applied by the driver to achieve a certain steering angle is thus called required steering wheel torque.

The neutral position of the steering wheel may also be called a centre position or zero position and is the position of the steering wheel where no applied return torque is acting on the steering wheel. In the normal mode, the neutral position of the steering wheel is such that the steered wheels of the vehicle are directed straight ahead, in parallel with the longitudinal extension of the vehicle. The steering angle is zero degrees when the steering wheel is in the neutral position in the normal mode. The steering angle is defined as the angle of the steering wheel in relation to the neutral position of the steering wheel in the normal mode. Steering angles achieved by turning the steering wheel to the right may be defined as positive steering angles and steering angles achieved by turning the steering wheel to the left may be defined as negative steering angles. It could however be the other way around depending on the steering system.

The clockwise respective the counter-clockwise return torque applied by the steering assist system is acting in the opposite direction to the steering wheel torque applied by the driver. Thus, if the driver is turning the steering wheel to the right, the return torque acting counter-clockwise is striving to turn the steering wheel back to the neutral position and if the driver is turning the steering wheel to the left, the return torque acting clockwise is striving to turn the steering wheel back to the neutral position. The return torque acting clockwise may be defined as a positive torque and the return torque acting counter-clockwise may be defined as a negative torque, but it can also be the other way around depending on the steering system. The absolute value of the return torque acting clockwise and the return torque acting counter-clockwise is the same in the normal mode. The return torque is suitably controlled as a function of the steering angle. The return torque is suitably controlled to increase with the steering angle up to a certain steering angle, after which the return torque stays the same. The return torque thus suitably increases gradually up to a predetermined maximum value.

The predetermined maximum value of the return torque is suitably chosen such that the steering wheel feels stable while allowing the driver to turn the steering wheel without too big effort. In the normal mode, the maximum value for the return torque acting clockwise is suitably the same as the maximum value of the return torque acting counter-clockwise.

According to an aspect of the invention the curvature of the road is predicted by means of road data/map information relating to the current trajectory of the vehicle. The steering assist system suitably comprises road data/map information received by means of a navigation unit, for example a GPS. The steering assist system is suitably adapted to retrieve road data/map information from various sources according to conventional methods, for example from camera/sensor means arranged on the vehicle, from systems on other vehicles, or from external servers/systems. The road data enables the steering assist system to predict the curvature of the road ahead, such that the steering assistance can be planned thereafter.

According to an aspect of the invention, when a curve is predicted, the required steering wheel torque that the driver has to apply is decreased in the direction of the curve. The required steering wheel torque is suitably decreased in the direction of the curve just before entering the curve. The required steering wheel torque is suitably decreased in relation to the steering wheel torque required in the normal mode. That is, if a curve to the right is predicted the steering assist system makes sure that the required steering wheel torque for turning the steering wheel to the right is decreased. The driver effort is thereby decreased and the driver comfort is increased. By decreasing the required steering wheel torque just before entering the curve, the driver immediately gets the assistance he needs. Also, by decreasing the required steering wheel torque only in the direction of the curve, the steering wheel remains stable.

According to an aspect of the invention the required steering wheel torque is decreased by decreasing the return torque acting opposite to the curve direction.

If the predicted curve turns to the right, the driver is expected to turn the steering wheel to the right to a certain steering angle. Thus, by decreasing the return torque acting counter-clockwise, the steering wheel torque that the driver needs to apply to achieve said steering angle is decreased. The driver comfort is thereby increased. Suitably, the maximum value of the return torque acting opposite to the curve direction is decreased. Alternatively or additionally the return torque acting opposite to the curve direction is decreased by modifying the growth rate up to the maximum value, such that maximum value is reached at a larger steering angle. By decreasing the return torque acting opposite to the curve direction it becomes smaller than the return torque acting in the curve direction. It is thus easier to turn the steering wheel in the curve direction than opposite to the curve direction. If the driver turns the steering wheel in the curve direction and subsequently lets it go, the decreased return torque may not be able to turn the steering wheel back to the neutral position since the other return torque acting in the curve direction is larger. This way, the vehicle may turn slightly in the direction of the curve even when the driver doesn’t apply a steering wheel torque.

According to an aspect of the invention the required steering wheel torque that the driver has to apply is decreased by displacing the neutral position of the steering wheel by an offset angle. The neutral position is suitably displaced in the curve direction. As discussed earlier the default neutral position of the steering wheel is in the normal mode a centred position where the steered wheels are parallel with the longitudinal extension of the vehicle. This means that if the driver turns the steering wheel either to the right or the left, and then let go of the steering wheel, the return torque will turn the steering wheel back to the neutral position, the steered wheels will become parallel with the longitudinal extension of the vehicle and the vehicle will thus travel straight ahead. By displacing the neutral position with an offset angle, the neutral position will no longer mean that the steered wheels are parallel with the extension of the vehicle. If, for example, a curve to the right is predicted, the neutral position of the steering wheel is displaced to the right in relation to the neutral position in the normal mode (zero degree angle). This way, the return torques will strive to turn the steering wheel to the offset neutral position in which the steered wheels are directed to the right. Thus, if the driver lets go of the steering wheel the vehicle will still turn slightly to the right instead of going straight ahead. The neutral position is preferably displaced by an offset angle smaller than the steering angle required to follow the curve. The purpose of the invention is not to automatically steer the vehicle but to assist the driver. By displacing the neutral position with an offset angle smaller than the required steering angle, the driver only has to turn the steering wheel corresponding to the difference between the required steering angle and the offset angle. This way, the steering wheel torque required to be applied by the driver is decreased.

The extent of the decrease of the required steering wheel torque preferably depends on the curvature of the curve. The required steering wheel torque is suitably decreased more the sharper the bend of the curve. When the steering assist system predicts a curve it retrieves information about the curve such as curvature, length etc. Based on this the required steering wheel torque is decreased to a value considered to be safe and comfortable for the driver. The extent of the decrease of the required steering wheel torque may also depend on external factors such as wind, the road inclination etc. In the case where the required steering wheel torque is decreased by decreasing the return torque acting opposite to the curve direction, the maximum value of said return torque is suitably decreased to a predetermined value depending on the curvature of the curve. In the case where the required steering wheel torque is decreased by displacing the neutral position of the steering wheel, a steering angle required to follow the curve is calculated. The neutral position of the steering wheel is preferably displaced with an offset angle between 25-75 % of the required steering angle. For example, if the required steering angle is calculated to 4 degrees the offset angle is set between 1-3 degrees. Suitably, a predetermined maximum offset angle is set. The maximum offset angle is suitably set such that the driver perceives it as driving on a straight road with a safe bank angle. The offset angle is suitably controlled such that it never exceeds the predetermined maximum offset angle.

According to an aspect of the invention the required steering wheel torque that the driver has to apply is decreased by applying an assist torque in the direction of the curve. The assist torque is suitably applied by the steering assist system directly to the steering wheel column. By applying an assist torque in the curve direction, the required steering wheel torque that the driver needs to apply in the direction of the curve will be decreased with the amount of the assist torque and the required steering wheel torque that the driver need to apply opposite to the curve direction will be increased with the amount of the assist torque. It will thus require more effort to turn the steering wheel in the direction opposite to the curve, and it will require less effort to turn the steering wheel in the curve direction. Also, due to the assist torque the return torque acting against the curve direction will not be able to turn the steering wheel to the neutral position once the steering wheel has been turned in the direction of the curve. Thus, should the driver let go of the steering wheel, the vehicle will turn slightly in the direction of the curve.

The decrease of the required steering wheel torque is preferably performed gradually. Suitably, the decrease is initiated just before entering the predicted curve and intensifies with the bend of the curve. This way, sudden and abrupt decreases of required driver effort is avoided and the driver will not be shocked when it becomes easier to turn the steering wheel. In a similar way, when the vehicle is leaving the curve the required steering wheel torque is gradually increased back to the normal mode. This way, a smooth and comfortable steering assistance is achieved.

The various methods of decreasing the required steering wheel torque mentioned herein may be combined.

According to an aspect of the invention the method further comprises the step to increase the return torque acting in direction of the curve. This way, it will require more effort to turn the steering wheel in the direction opposite to the curve and the driver will thereby be stopped from turning the steering wheel in the wrong direction.

When an essentially straight road is predicted, the normal mode is suitably maintained and the required steering wheel torque is unchanged. As described earlier, if the vehicle is leaving a curve and a straight road is predicted after that, the required steering wheel torque is gradually increased up to the value in the normal mode. When an essentially straight road is predicted it is thus ensured that the neutral position of the steering wheel is not offset and that the return torque is the same in both directions. Alternatively, if a straight road is predicted the required steering wheel torque is increased in both directions by increasing the return torque on both sides. This way, the effort to turn the steering wheel is increased and the steering wheel is stable in the neutral position.

According to an aspect of the invention a steering assist system associated with a vehicle is provided. The steering assist system is adapted to control the steering wheel torque required to be applied by a driver to achieve a certain steering angle, wherein the steering wheel in a normal mode has a neutral position in which the steered wheels of the vehicle are directed in parallel with the longitudinal extension of the vehicle, wherein the steering assist system is adapted to apply a return torque acting clockwise and a return torque acting counter-clockwise on the steering wheel, striving to turn the steering wheel back to the neutral position. The steering assist system comprises a control unit adapted to predict the curvature of the road on which the vehicle is travelling and to control the required steering wheel torque based on the predicted road curvature, and wherein the control unit is adapted to, when a curve is predicted, decrease the required steering wheel torque in the direction of the curve just before entering the curve.

The steering assist system is suitably connected to the steering wheel, the steering wheel column and the steered wheels of the vehicle. The steering assist system may be mechanically and/or electrically connected to the steering wheel, the steering wheel column and the steered wheels.

In the normal mode the neutral position of the steering wheel is such that the steered wheels of the vehicle are directed straight ahead, in parallel with the longitudinal extension of the vehicle. Also, in the normal mode the value of the clockwise and counter-clockwise return torques applied by the steering assist system are the same.

The control unit is preferably adapted to predict the curvature of the road by means of road data/map information relating to the current trajectory of the vehicle. The control unit suitably comprises road data/map information received from a navigation unit, for example a GPS. The control unit is suitably adapted to retrieve road data/map information from various sources according to conventional methods, for example from camera/sensor means arranged on the vehicle, from systems on other vehicles, or from external servers/systems. With the road data the control unit can predict the curvature of the road ahead and the required steering wheel torque can be controlled thereafter.

The control unit is suitably adapted to decrease the required steering wheel torque just before the vehicle enters the curve. By controlling the required steering wheel torque based on a prediction of the road curvature the control unit is prepared to control the required steering wheel torque pre-emptively or just in time to when it is needed, without time lag. By decreasing the required steering wheel torque just before the vehicle is entering the curve the driver assistance is provided when needed.

The control unit is preferably adapted to decrease the required steering wheel torque by decreasing the return torque acting opposite to the curve direction.

Suitably, the control unit is adapted to decrease the maximum value of the return torque.

The control unit is preferably adapted to decrease the required steering wheel torque by displacing the neutral position of the steering wheel by an offset angle. The control unit is suitably adapted to displace the neutral position in the curve direction. The control unit is adapted to displace the neutral position in relation to the neutral position in the normal mode. Thus, by displacing the neutral position with an offset angle, the neutral position will no longer mean that the steered wheels are parallel with the extension of the vehicle. The control unit is preferably adapted to displace the neutral position by an offset angle smaller than the steering angle required to follow the curve.

The control unit is preferably adapted to determine the extent of the decrease of the required steering wheel torque depending on the curvature of the curve. Suitably, the control unit is adapted to decrease the required steering wheel torque more the sharper the bend of the curve is. When the control unit has predicted a curve it retrieves information about the curve such as curvature, length etc. Based on this the control unit is adapted to decrease the required steering wheel torque to a value considered to be safe and comfortable for the driver. In the case where the required steering wheel torque is decreased by decreasing the return torque, the control unit is suitably adapted to decrease the maximum value of the return torque to a predetermined value depending on the curvature of the curve. In the case where the required steering wheel torque is decreased by displacing the neutral position of the steering wheel, the control unit is adapted to predict a steering angle required to follow the curve. The control unit is further suitably adapted to displace the neutral position of the steering wheel with an offset angle between 25-75 % of the required steering angle. Suitably, the control unit comprises a predetermined maximum offset angle corresponding to a safe bank angle of the road. The control unit is preferably adapted to control the offset angle such that it never exceeds the predetermined maximum offset angle.

The control unit is preferably adapted to decrease the required steering wheel torque by applying an assist torque in the direction of the curve. The effect of this is that it will require more effort to turn the steering wheel in the direction opposite to the curve and it will require less effort to turn the steering wheel in the curve direction.

The control unit is suitably adapted to combine the various methods of decreasing the required steering wheel torque mentioned herein.

The control unit is preferably adapted to decrease the required steering wheel torque gradually. Suitably, the control unit is adapted to initiate the decrease just before entering the predicted curve and to intensify the decrease with the bend of the curve. This way, sudden and abrupt steering assistance is avoided and the safety is thereby increased. In a similar way, when the vehicle is leaving the curve the control unit is adapted to gradually increase the required steering wheel torque back to the normal mode. This way, a smooth and comfortable steering assistance is achieved.

The control unit may be adapted to increase the return torque acting in direction of the curve. This way, the driver effort to turn the steering wheel in the direction opposite to the curve is increased. The driver will thereby perceive it even easier to turn the steering wheel in the curve direction and the steering wheel is perceived as stable.

Further objects, advantages and novel features of the present invention will become apparent to one skilled in the art from the following details, and also by putting the invention into practice. Whereas the invention is described below, it should be noted that it is not restricted to the specific details described. Specialists having access to the teachings herein will recognise further applications, modifications and incorporations within other fields, which are within the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS For fuller understanding of the present invention and further objects and advantages of it, the detailed description set out below should be read together with the accompanying drawings, in which the same reference notations denote similar items in the various diagrams, and in which: Figure 1 schematically illustrates a vehicle according to an embodiment of the invention; Figure 2a-c schematically illustrates the function of a normal state of a steering assist system according to an embodiment of the invention; Figure 3a-c schematically illustrates the function of a steering assist system according to an embodiment of the invention; Figure 4a-c schematically illustrates the function of a steering assist system according to an embodiment of the invention; Figure 5a-c schematically illustrates the function of a steering assist system according to an embodiment of the invention; Figure 6a-c schematically illustrates the function of a steering assist system according to an embodiment of the invention; Figure 7a-c schematically illustrates the function of a steering assist system according to an embodiment of the invention; Figure 8 schematically illustrates a flow chart for a method for steering assistance of a vehicle according to an embodiment of the invention; and Figure 9 schematically illustrates a control unit or computer according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS Figure 1 schematically shows a side view of a vehicle 1 according to an embodiment of the invention. The vehicle 1 includes a combustion engine 2, a steering assist system 4 adapted to control the steering wheel torque required to be applied by a driver TDto achieve a certain steering angle ? of the steering wheel 6. The steering assist system 4 is thus connected to the steering wheel 6 and to the steered wheels 8 of the vehicle 1. The steering assist system 4 suitably comprises a control unit 5 and an electric servomechanism with an electric motor (not shown) connected to the control unit 5. The steering assist system 4 is further adapted to predict the curvature of the road on which the vehicle 1 is travelling and to control the required steering wheel torque TDbased on this prediction. The vehicle 1 may be a heavy vehicle, e.g. a truck or a bus. The vehicle 1 may alternatively be a passenger car. The vehicle may be a hybrid vehicle comprising an electric machine (not shown) in addition to the combustion engine 2.

In the coming figures various embodiments of the steering assistance achieved with the steering assist system 4 will be described. Each embodiment will be described in relation to a figure of the vehicle 1 in a predicted curve 10 (Fig. 2a, 3a, 4a, 5a, 6a and 7a), a figure of the torques acting on the steering wheel 6 (Fig. 2b, 3b, 4b, 5b, 6b and 7b) and a figure showing an example of the torque acting on the steering wheel 6 as a function of the steering angle ? (Fig. 2c, 3c, 4c, 5c, 6c and 7c). The torque shown in figures 2c-7c is thus the effect of the torque applied by the steering assist system. It is to be understood that the torque acting on the steering wheel 6 is meant to include torque acting on the steering wheel column. In the examples described in Figures 2a-c to 7a-c the curve 10 is turning to the right. It is to be understood that the principle of the steering assist system is the same irrespective of the curve direction. In figures 2c-7c the steering angles to the left are illustrated as negative steering angles and steering angles to the right are illustrated as positive steering angles.

Likewise, the torque acting on the steering wheel 6 to the right (in the curve direction) is illustrated as positive toque and torque acting on the steering wheel 6 to the left (opposite the curve direction) is illustrated as negative torque. The graph shown in figures 2c-7c thus corresponds to the required steering wheel torque that the driver needs to apply TDto achieve the exemplified steering angles. This example shows the relationship between torque and steering angle as a linear function. It is to be understood that this is only an example and the relationship between torque and steering angle could be implemented in various ways.

Figure 2a-c schematically shows the function of a steering assist system 4 in a normal mode according to an embodiment of the invention. The normal mode is specifically suitable when the steering assist system 4 predicts a straight road. However, in this example the steering assist system 4 has predicted a curve 10. The steering assist system 4 is suitably arranged in a vehicle according to Figure 1.

The steering assist system 4 is configured such that the steering wheel 6 of the vehicle 1 in the normal mode has a neutral position P in which the steered wheels 8 are parallel with the longitudinal extension of the vehicle 1. The steering wheel 6 and the neutral position P are illustrated in Figure 2b. The steering assist system 4 is further configured to apply a return torque acting clockwise TRand a return torque acting counter-clockwise -TRon the steering wheel 6, striving to turn the steering wheel 6 back to the neutral position P. The respective return torque TR, -TR is illustrated with an arrow in Figure 2b. This means that if the driver turns the steering wheel 6 in this mode and subsequently lets it go, the return torques TR, -TR will turn the steering wheel 6 back to the neutral position P, the steered wheels 8 will be directed in parallel with the longitudinal direction of the vehicle 1 and the vehicle 1 will thus drive straight ahead. This is illustrated by the solid line in Figure 2a. The dashed line in Figure 2a illustrates the desired trajectory through the curve. In order to follow the desired trajectory in Figure 2a a certain steering angle ? is required. The steering angle ? is generally defined as the angle in relation to the neutral position P of the steering wheel 6 in the normal mode. In the neutral position P in the normal mode, the steering angle ? is zero. The required steering angle ? to follow the desired trajectory is illustrated in Figure 2b. To achieve the required steering angle ? the driver has to apply a certain steering wheel torque TDto the steering wheel 6. The steering wheel torque required to be applied by the driver TDis illustrated as an arrow in the curve direction in Figure 2b.

The torque acting on the steering wheel 6 is disclosed as a function of the steering angle ? in Figure 2c. In order to turn the steering wheel 6 the driver must apply a corresponding steering wheel torque in the opposite direction. The steering wheel torque required to be applied by the driver TDis thereby equal in size to the torque illustrated in the graph but directed in the opposite direction. Other factors such as wind and inclination of the road may also affect the required steering wheel torque TDbut such factors are omitted here for the sake of clarity. In this embodiment the torque acting on the steering wheel 6 consists of the return torques TR, -TR. The return torque acting clockwise TRis defined as a positive torque and the torque acting counter-clockwise -TRis defined as a negative torque. As illustrated in Figure 2c each return torque TR, -TRis controlled to increase with the steering angle ? up to a certain steering angle, after which the return torque TR, -TRremains constant. The respective return torque TR, -TRthus increase gradually up to a predetermined maximum value TRmax, -TRmax. The predetermined maximum value of the return torque TRmax, -TRmaxis suitably chosen such that the steering wheel 6 feels stable while allowing the driver to turn the steering wheel 6 without too big effort. In the normal mode, the maximum value of the return torque acting clockwise TRmaxis the same as the maximum value of the return torque acting counter-clockwise -TRmax. In the following example the maximum value of the return torques TRmax, -TRmaxmay be 3 Nm.

To achieve the steering angle A the steering wheel 6 must be turned to the left and steering angle A is thus a negative steering angle. The return torque acting on the steering wheel 6 is thus the return torque acting clockwise TR, which is a positive torque. The torque acting on the steering wheel 6 at steering angle A is the predetermined maximum return torque in the clockwise direction TRmax. The torque is thus illustrated as a positive torque of 3 Nm. The steering wheel torque required to be applied by the driver TDto achieve steering angle A is thus 3 Nm to the left.

Steering angle B is zero degrees and is thus the neutral position P of the steering wheel 6. In this position no torque is acting on the steering wheel 6. The steering wheel torque required by the driver TDto achieve steering angle B is thus 0 Nm.

With steering angle C the steering wheel 6 is turned slightly to the right and the return torque acting counter-clockwise -TRis acting on the steering wheel 6. The torque acting on the steering wheel 6 is thus illustrated as a negative torque and in this example steering angle C corresponds to -1 Nm. The steering wheel torque required by the driver TDto achieve steering angle C is thus 1 Nm to the right.

At steering angle ?, which is required to follow the curve 10, the return torque acting counter-clockwise -T R is acting on the steering wheel 6. The torque acting on the steering wheel 6 is thus disclosed as a negative torque and corresponds to the maximum value of the return torque -TRmax. Thus, in this example the required steering angle ? corresponds to -3 Nm. The steering wheel torque required by the driver TDto achieve steering angle ? is thus 3 Nm to the right.

Steering angle D corresponds to steering angle A but the steering wheel 6 is turned to the right and steering angle D is thus positive and the return torque acting counter-clockwise -TRis acting on the steering wheel 6. The torque acting on the steering wheel 6 is the maximum value of the return torque acting counter-clockwise -TRmax. Thus, in this example steering angle D corresponds to -3 Nm. The steering wheel torque required by the driver TDto achieve steering angle D is thus 3 Nm to the right.

The steering angle ? after which the return torque TR, -TRremains constant may be between 4-8 degrees.

Figure 3a-c schematically shows the function of a steering assist system 4 according to an embodiment of the invention. The steering assist system 4 suitably corresponds to the steering assist system 4 disclosed in Figure 1-2. In this embodiment the steering assist system 4 has predicted a curve 10 and the required steering wheel torque TDis decreased in direction of the curve 10 by decreasing the return torque -TRacting opposite to the curve direction. Since the predicted curve 10 turns to the right, the driver must turn the steering wheel 6 to the right to the required steering angle ? in order to follow the curve. Thus, by decreasing the return torque acting counter-clockwise -TR, the steering wheel torque that the driver needs to apply TDto achieve said steering angle ? is decreased. The driver comfort is thereby increased.

The decrease of the return torque acting counter-clockwise -TRis illustrated in Figure 3c where the maximum value of the return torque in the counterclockwise direction -TRmaxis decreased. The maximum value of the return torque acting counter-clockwise -TRmaxis now -2 Nm instead of -3 Nm as described in Figure 2. The return torque acting counter-clockwise -TRis thereby smaller than the return torque acting clockwise TR. This means that if the driver turns the steering wheel 6 to the right and subsequently lets it go, the counter-clockwise return torque -T R may not be able to turn the steering wheel 6 back to the neutral position P since the other return torque TRis larger. This way, the vehicle 1 will turn slightly in the direction of the curve 10 even when the driver doesn’t apply a steering wheel torque TD. This is illustrated by the solid line in Figure 3a.

The torque acting on the steering wheel 6 and thus the steering wheel torque required by the driver TDwith the steering assistance according to this embodiment will be described in relation to Figure 3c. In this embodiment the torque acting on the steering wheel 6 consists of the return torques TR, -TR.

The torque acting on the steering wheel 6 at steering angles A, B and C are the same as in Figure 2c. The steering wheel torque required by the driver TDto achieve steering angle A is thus 3 Nm to the left. Steering angle B is zero degrees and no torque is acting on the steering wheel 6 whereby the steering wheel torque required by the driver TDis 0 Nm. With steering angle C the steering wheel 6 is turned slightly to the right and the steering wheel torque required by the driver TDto achieve steering angle C is thus 1 Nm to the right.

At the steering angle ? required to follow the curve 10 the return torque acting counter-clockwise -TRis acting on the steering wheel 6. The torque acting on the steering wheel is thus disclosed as a negative torque and corresponds to the decreased maximum value of the return torque in the counter-clockwise direction -TRmax. Thus, in this example the required steering angle ? corresponds to -2 Nm. The steering wheel torque required by the driver TDto achieve steering angle ? is thus 2 Nm to the right. Similarly, at steering angle D the torque acting on the steering wheel 6 is the decreased maximum value of the return torque, -TRmax. Thus, in this example steering angle D corresponds to -2 Nm and the steering wheel torque required by the driver TDis 2 Nm to the right.

It is thus obvious that with a steering assistance according to this embodiment the driver effort is decreased compared to the normal mode.

Figure 4a-c schematically shows the function of a steering assist system 4 according to an embodiment of the invention. The steering assist system 4 suitably corresponds to the steering assist system 4 disclosed in Figure 1-3. In this embodiment the steering assist system 4 has predicted a curve 10 and the required steering wheel torque TDis decreased in direction of the curve 10 by displacing the neutral position P of the steering wheel 6 by an offset angle O. The neutral position P is displaced in the curve direction and is thus displaced to the right of the neutral position in the normal mode. This way, the return torques TR, -TRwill strive to turn the steering wheel 6 to the offset neutral position P in which the steered wheels 8 are directed to the right. Thus, if the driver lets go of the steering wheel 6 the vehicle 1 will still turn slightly to the right instead of going straight ahead. This is illustrated by the solid line in Figure 4a.

In this example, the neutral position P has been displaced by an offset angle O which is between 25-75 % of the required steering angle ?. This way, the driver only has to turn the steering wheel 6 corresponding to the difference between the required steering angle ? and the offset angle O in order to achieve the required steering angle ?. Displacing the neutral position P by the offset angle O corresponds to moving the graph of the steering wheel torque applied by the steering assist system Tsw in Figure 2c to the right. The effect of this displacement will be described in relation to the steering angles previously mentioned in Figures 2c and 3c. In this embodiment the torque acting on the steering wheel 6 consists of the return torques TR, -TR.

At the steering angle A, the torque acting on the steering wheel 6 is the predetermined maximum return torque acting clockwise TRmax. The torque acting on the steering wheel 6 is thus a positive torque 3 Nm. The steering wheel torque required to be applied by the driver TDto achieve steering angle A is thus 3 Nm to the left.

Steering angle B is zero degrees but no longer represents the neutral position P of the steering wheel 6. Thus, the return torque acting in the clockwise direction T R is acting on the steering wheel 6 in order to turn the steering wheel 6 to the offset neutral position P. The torque acting on the steering wheel 6 in steering angle B is now 1 Nm. The steering wheel torque required by the driver TDto achieve steering angle B is thus 1 Nm to the left.

Steering angle C is the offset angle O and thus represents the offset neutral position P of the steering wheel 6. No torque is acting on the steering wheel 6 in this position and the steering wheel torque required by the driver TDto achieve steering angle C is thus 0 Nm.

At the steering angle ? required to follow the curve 10 the return torque acting counter-clockwise -TRis acting on the steering wheel 6. However, due to the displacement of the neutral position P the return torque -TRin this position is smaller than in the normal mode. The torque acting on the steering wheel 6 is in this example approximately -1,75 Nm. The steering wheel torque required by the driver TDto achieve steering angle ? is thus approximately 1,75 Nm to the right.

At steering angle D the torque acting on the steering wheel 6 is the maximum value of the return torque acting counter clockwise -TRmax, -3 Nm. The steering wheel torque required by the driver TDis thereby 3 Nm.

Figure 5a-c schematically shows the function of a steering assist system 4 according to an embodiment of the invention. The steering assist system 4 suitably corresponds to the steering assist system 4 disclosed in Figure 1-4. In this embodiment the steering assist system 4 has predicted a curve 10 and the required steering wheel torque TDis decreased in direction of the curve 10 by applying an assist torque TAin the direction of the curve 10. By applying an assist torque TAin the curve direction, the torque acting in the direction of the curve will be increased with the amount of the assist torque TAand the driver thus has to apply a higher torque to turn the steering wheel opposite to the curve direction. Also, by applying the assist torque TAin the curve direction less effort is required by the driver to turn the steering wheel 6 in the curve direction. The addition of an assist torque TAcorresponds to moving the graph in Figure 2c upwards. The effect of adding the assist torque TAis that the torque acting on the steering wheel 6 opposite to the curve direction is smaller than in the normal mode. It is thereby easier to turn the steering wheel 6 in the curve direction. The effect of this steering assistance will be described in relation to various steering angles in Figure 5c.

At the steering angle A, the torque acting on the steering wheel 6 is the predetermined maximum return torque acting clockwise TRmaxplus the assist torque TA. In this example the assist torque may be 1 Nm. The torque acting on the steering wheel 6 is thus 4 Nm. The steering wheel torque required by the driver TDto achieve steering angle A is thus 4 Nm to the left.

Steering angle B is zero degrees and is the neutral position P of the steering wheel 6. However, due to the assist torque TAacting in the clockwise direction steering angle B is no longer the effective neutral position. Adding the assist torque TAwill thus have the same effect as displacing the neutral position P. Thus, the torque acting on the steering wheel 6 in steering angle B is now 1 Nm. The steering wheel torque required by the driver TDto achieve steering angle B is thus 1 Nm to the left.

Steering angle C is now the effective neutral position P of the steering wheel 6 in that no torque is acting on the steering wheel 6 in this position. The steering wheel torque required by the driver TDto achieve steering angle C is thus 0 Nm.

At the steering angle ? required to follow the curve 10 the torque acting on the steering wheel 6 is approximately -1,75 Nm. The steering wheel torque required by the driver TDto achieve steering angle ? is thus approximately 1,75 Nm to the right.

At steering angle D the torque acting on the steering wheel 6 is -2 Nm. The steering wheel torque required by the driver TDis thereby 2 Nm.

Figure 6a-c schematically shows the function of a steering assist system 4 according to an embodiment of the invention. The steering assist system 4 suitably corresponds to the steering assist system 4 disclosed in Figure 1-5. In this embodiment the steering assist system 4 has predicted a curve 10 and the required steering wheel torque TDis decreased in direction of the curve 10 by displacing the neutral position P of the steering wheel 6 by an offset angle O and by decreasing the return torque -TRacting opposite to the curve direction. The return torque -TRis decreased by decreasing the maximum value of the return torque acting counter-clockwise -TRmax. This embodiment is thus a combination of the embodiments described in Figures 3a-c and 4a-c. The effect of the steering assistance according to this embodiment is described in Figure 6c.

At the steering angle A, the torque acting on the steering wheel 6 is the predetermined maximum return torque acting clockwise TRmax, 3 Nm. The steering wheel torque required by the driver TDto achieve steering angle A is thus 3 Nm to the left.

Steering angle B is zero degrees but is no longer the neutral position P of the steering wheel 6 since the neutral position P has been displaced. Thus, the torque acting on the steering wheel 6 in steering angle B is the return torque acting clockwise TRwith the value 1 Nm. The steering wheel torque required by the driver TDto achieve steering angle B is thus 1 Nm to the left.

Steering angle C is the offset neutral position P of the steering wheel 6 where no torque is acting on the steering wheel 6. The steering wheel torque required by the driver TDto achieve steering angle C is thus 0 Nm.

At the steering angle ? required to follow the curve 10 the return torque acting counter-clockwise -TRis acting on the steering wheel 6. Since the return torque acting counter-clockwise -TRwas decreased the torque acting on the steering wheel 6 is approximately -1,75 Nm. The steering wheel torque required by the driver TDto achieve steering angle ? is thus approximately 1,75 Nm to the right.

At steering angle D the return torque acting counter-clockwise -TRis acting on the steering wheel 6. Since the return torque -TRwas decreased the torque acting on the steering wheel 6 is the decreased maximum value of the return torque -TRmax, -2 Nm. The steering wheel torque required by the driver TDis thereby 2 Nm.

Figure 7a-c schematically shows the function of a steering assist system 4 according to an embodiment of the invention. The steering assist system 4 suitably corresponds to the steering assist system 4 disclosed in Figure 1-6. In this embodiment the steering assist system 4 has predicted a curve 10 and the required steering wheel torque TDis decreased in direction of the curve 10 by decreasing the return torque -TRacting opposite to the curve direction. In the embodiment described in Figures 3a-c the maximum value of the return torque acting opposite to the curve direction -TRmaxwas decreased, which made it easier to turn the steering wheel 6 in the curve direction. In this embodiment the maximum value of the return torque acting opposite to the curve direction -TRmaxis likewise decreased but the increase of the return torque -TRas a function of the steering angle ? is also modified. The return torque acting counter-clockwise -TRis suitably controlled such that it has a lower increase rate. That is, the maximum value of the return torque acting counter-clockwise -TRmaxis reached at a larger steering angle ? than in the normal mode. Suitably, the return torque acting clockwise TRis controlled such that it has a higher increase rate. This way, the driver will meet a higher resistance at smaller steering angles ? when turning the steering wheel 6 in the direction opposite to the curve direction.

The decrease of the return torque acting counter-clockwise -TRis illustrated in Figure 7c where the maximum value of the return torque in the counterclockwise direction -TRmaxis decreased. The maximum value of the return torque acting counter-clockwise -TRmaxis now -2 Nm instead of -3 Nm as described in Figure 2. The torque acting on the steering wheel 6 at steering angle C and ? is smaller than in the embodiment described in Figures 3a-c. The steering wheel torque required by the driver TDto achieve steering angle A is 3 Nm to the left. Steering angle B is zero degrees and no torque is acting on the steering wheel 6 whereby the steering wheel torque required by the driver TDis 0 Nm. At steering angle C the steering wheel torque required by the driver TDis 0,5 Nm to the right due to the modified increase rate. Also due to the modified increase rate the steering wheel torque required by the driver TDto achieve the steering angle ? is 1,25 Nm to the right. The torque acting on the steering wheel 6 at steering angle D is the decreased maximum value of the return torque acting counter-clockwise -TRmax. Thus, the steering wheel torque required by the driver TDis 2 Nm.

Figure 8 schematically illustrates a flow chart for a method for steering assistance of a vehicle 1 according to an embodiment of the invention. The vehicle 1 comprises a steering assist system 4 adapted to control the steering wheel torque required to be applied by a driver TDto achieve a certain steering angle ?, wherein the steering wheel 6 in a normal mode has a neutral position P in which the steered wheels 8 of the vehicle 1 are parallel with the longitudinal extension of the vehicle 1, wherein the steering assist system 4 is adapted to apply a return torque acting clockwise TRand a return torque acting counterclockwise -TR, each striving to turn the steering wheel 6 back to the neutral position P. The method comprises the steps of predicting s101 the curvature of the road on which the vehicle 1 is travelling; and controlling s102 the required steering wheel torque TDbased on the predicted road curvature.

The return torque acting clockwise TRis preferably defined as a positive torque and the return torque acting counter-clockwise -TRis preferably defined as a negative torque. The absolute value of the return torque acting clockwise TRand the return torque acting counter-clockwise -TRis the same in the normal mode. The respective return torque TR, -TRis suitably controlled to increase with the steering angle ? up to a certain steering angle, after which the return torque TR, -TRstays the same. The respective return torque TR, -TRthus suitably increases gradually up to a predetermined maximum value TRmax, -TRmax. The predetermined maximum value of the return torque TRmax, -TRmaxis suitably chosen such that the steering wheel 6 feels stable while allowing the driver to turn the steering wheel 6 without too big effort.

The step to predict s101 the curvature of the road on which the vehicle 1 is travelling is suitably performed by means of road data/map information relating to the current trajectory of the vehicle 1. The steering assist system 4 suitably comprises road data/map information received by means of a navigation unit, for example a GPS. The steering assist system 4 is suitably adapted to retrieve road data/map information from various sources according to conventional methods, for example from camera/sensor means arranged on the vehicle 1, from systems on other vehicles, or from external servers/systems. The road data enables the steering assist system 4 to predict the curvature of the road ahead, such that the steering assistance can be planned thereafter.

The step to control s102 the required steering wheel torque TDbased on the predicted road curvature may comprise to decrease the required steering wheel torque TDin the direction of a curve 10 when a curve 10 is predicted. The required steering wheel torque TDis suitably decreased just before entering the curve 10. By decreasing the required steering wheel torque TDonly in the direction of the predicted curve 10, the resistance in the opposite direction is still the same and the driver will perceive it as being much easier to turn the steering wheel 6 in the direction of the curve 10. The required steering wheel torque TDis suitably decreased in relation to the steering wheel torque required in the normal mode. That is, if a curve to the right is predicted the steering assist system 4 makes sure that the required steering wheel torque TDfor turning the steering wheel 6 to the right is decreased. The driver effort is thereby decreased and the driver comfort is increased. By decreasing the required steering wheel torque TDjust before entering the curve, the driver immediately gets the assistance he needs.

The step to control s102 the required steering wheel torque TDbased on the predicted road curvature may comprise to decrease the required steering wheel torque TDby decreasing the return torque TR, -TRacting opposite to the curve direction. If the predicted curve 10 turns to the right, the driver is expected to turn the steering wheel to the right to a certain steering angle ?. Thus, by decreasing the return torque acting counter-clockwise -TR, the steering wheel torque that the driver needs to apply TDto achieve said steering angle ? is decreased. The driver comfort is thereby increased. The return torque TR, -TRacting opposite to the curve direction is suitably decreased by decreasing the maximum value of the return torque TRmax, -TRmax. Alternatively, the return torque TR, -TRacting opposite to the curve direction is decreased by modifying the increase of the return torque TR, -TRin relation to the steering angle ?, such that the return torque TR, -TRacting opposite to the curve direction has a lower increase rate. The return torque acting opposite to the curve direction thereby becomes smaller than the return torque acting in the curve direction. It is thus easier to turn the steering wheel 6 in the curve direction than opposite to the curve direction.

The step to control s102 the required steering wheel torque TDbased on the predicted road curvature may comprise to decrease the required steering wheel torque TDby displacing the neutral position P of the steering wheel 6 by an offset angle O. The neutral position P is suitably displaced in the curve direction. The default neutral position of the steering wheel 6 is in the normal mode a centred position (with steering angle zero degrees) where the steered wheels 8 are parallel with the longitudinal extension of the vehicle 1. By displacing the neutral position P with an offset angle O, the neutral position P will no longer mean that the steered wheels 8 are parallel with the longitudinal extension of the vehicle 1. If, for example, a curve to the right is predicted, the neutral position P of the steering wheel 6 is displaced to the right in relation to the zero degree angle. This way, the return torques TR, -TRwill strive to turn the steering wheel 6 to the offset neutral position in which the steered wheels 8 are directed to the right. Thus, if the driver lets go of the steering wheel 6 the vehicle will still turn slightly to the right instead of going straight ahead. The neutral position is preferably displaced by an offset angle O smaller than a steering angle ? required to follow the curve 10. By displacing the neutral position P with an offset angle O smaller than the required steering angle ?, the driver only has to turn the steering wheel 6 corresponding to the difference between the required steering angle ? and the offset angle O. This way, the return torque acting in direction opposite to the curve will be smaller than in the normal mode for the required steering angle ?.

The steering wheel torque required to be applied by the driver TDis thereby decreased.

The required steering wheel torque TDis preferably decreased based on the curvature of the curve 10. The required steering wheel torque TDis suitably decreased more the sharper the bend of the curve 10. When the steering assist system 4 predicts a curve 10 it retrieves information about the curve 10 such as curvature, length etc. Based on this the required steering wheel torque TDis decreased to a value considered to be safe and comfortable for the driver. In the case where the required steering wheel torque TDis decreased by decreasing the return torque TR, -TRacting opposite to the curve direction, the maximum value of the return torque TRmax, -TRmaxis suitably decreased to a predetermined value depending on the curvature of the curve. In the case where the required steering wheel torque TDis decreased by displacing the neutral position P of the steering wheel 6, a steering angle ? required to follow the curve is predicted. The neutral position P of the steering wheel 6 is preferably displaced with an offset angle O between 25-75 % of the required steering angle ?. For example, if the required steering angle ? is determined to 4 degrees the offset angle O is set between 1-3 degrees. Suitably, a predetermined maximum offset angle is set, corresponding to a safe bank angle of the road. The offset angle O is suitably controlled such that it never exceeds the predetermined maximum offset angle.

The decrease of the required steering wheel torque TDis preferably performed gradually. Suitably, the decrease is initiated just before entering the predicted curve 10 and intensifies with the bend of the curve 10. This way, sudden and abrupt decreases of required driver effort is avoided and the driver will not be shocked when it becomes easier to turn the steering wheel 6. In a similar way, when the vehicle 1 is leaving the curve 10 the required steering wheel torque TDis gradually increased back to the normal mode. This way, a smooth and comfortable steering assistance is achieved.

The step to control s102 the required steering wheel torque TDbased on the predicted road curvature may comprise to decrease the required steering wheel torque TDin the curve direction by applying an assist torque TAin the direction of the curve. By applying an assist torque TAin the curve direction, the resultant torque acting on the steering wheel 6 in the direction of the curve 10 will be increased with the amount of the assist torque TAand the resultant torque acting on the steering wheel 6 opposite to the curve direction will be decreased with the amount of the assist torque TA. The effect of this is that it will require more effort to turn the steering wheel 6 in the direction opposite to the curve 10, and it will require less effort to turn the steering wheel 6 in the curve direction.

The various methods of decreasing the required steering wheel torque TDmay be combined.

The method may further comprise the step to increase the return torque TR, -TRacting in direction of the curve. This way, it will require more effort to turn the steering wheel 6 in the direction opposite to the curve and the steering wheel 6 is still considered to be stable.

The step to control s102 the required steering wheel torque TDbased on the predicted road curvature may comprise to maintain the normal mode when it is predicted that the road will be essentially straight. The required steering wheel torque TDis thereby unchanged. If the vehicle 1 is leaving a curve 10 and a straight road is predicted after that, the required steering wheel torque TDis suitably gradually increased up to the value in the normal mode. It is thus ensured that the neutral position P of the steering wheel 6 is not offset and that the return torque TR, -TRis the same in both directions. Alternatively, if a straight road is predicted the required steering wheel torque TDmay be increased in both directions by increasing the return torque TR, -TRon both sides in relation to the normal mode. This way, the effort to turn the steering wheel is increased and the steering wheel 6 is stable in the neutral position P.

Figure 9 schematically illustrates a device 500. The control unit 5 described with reference to Figure 1 may in a version comprise the device 500. The term “link” refers herein to a communication link which may be a physical connection such as an optoelectronic communication line, or a non-physical connection such as a wireless connection, e.g. a radio link or microwave link. The device 500 comprises a non-volatile memory 520, a data processing unit 510 and a read/write memory 550. The non-volatile memory 520 has a first memory element 530 in which a computer program, e.g. an operating system, is stored for controlling the function of the device 500. The device 500 further comprises a bus controller, a serial communication port, I/O means, an A/D converter, a time and date input and transfer unit, an event counter and an interruption controller (not depicted). The non-volatile memory 520 has also a second memory element 540.

There is provided a computer program P which comprises routines for a method for steering assistance of a vehicle 1 according to the invention. The computer program P comprises routines for predicting the curvature of a road on which the vehicle is travelling. The computer program P comprises routines for controlling the required steering wheel torque TDbased on the predicted road curvature. The computer program P comprises routines for decreasing the required steering wheel torque TDin the curve direction by decreasing the return torque TR, -TRacting opposite the curve direction. The computer program P comprises routines for decreasing the required steering wheel torque TDin the curve direction by displacing the neutral position P of the steering wheel 6 with an offset angle O. The computer program P comprises routines for decreasing the required steering wheel torque TDin the curve direction by applying an assist torque TAin the curve direction. The computer program P comprises routines for increasing the return torque TR, -TRacting in the curve direction. The program P may be stored in an executable form or in a compressed form in a memory 560 and/or in a read/write memory 550.

Where the data processing unit 510 is described as performing a certain function, it means that the data processing unit 510 effects a certain part of the program stored in the memory 560 or a certain part of the program stored in the read/write memory 550.

The data processing device 510 can communicate with a data port 599 via a data bus 515. The non-volatile memory 520 is intended for communication with the data processing unit 510 via a data bus 512. The separate memory 560 is intended to communicate with the data processing unit 510 via a data bus 511. The read/write memory 550 is adapted to communicating with the data processing unit 510 via a data bus 514.

When data are received on the data port 599, they are stored temporarily in the second memory element 540. When input data received have been temporarily stored, the data processing unit 510 is prepared to effect code execution as described above.

Parts of the methods herein described may be effected by the device 500 by means of the data processing unit 510 which runs the program stored in the memory 560 or the read/write memory 550. When the device 500 runs the program, methods herein described are executed.

The foregoing description of the preferred embodiments of the present invention is provided for illustrative and descriptive purposes. It is not intended to be exhaustive or to restrict the invention to the variants described. Many modifications and variations will obviously be apparent to one skilled in the art. The embodiments have been chosen and described in order best to explain the principles of the invention and its practical applications and hence make it possible for specialists to understand the invention for various embodiments and with the various modifications appropriate to the intended use.

Claims

1. A method for steering assistance of a vehicle (1), the vehicle (1) comprising a steering assist system (4) adapted to control the steering wheel torque required to be applied by the driver (TD) to achieve a certain steering angle (?), wherein the steering wheel (6) in a normal mode has a neutral position (P) in which the steered wheels (8) are directed in parallel with the longitudinal extension of the vehicle (1), wherein the steering assist system (4) is adapted to apply a return torque acting clockwise (+TR) and a return torque acting counter-clockwise (-TR) on the steering wheel (6), striving to turn the steering wheel (6) back to the neutral position (P), the method comprising the steps of: - predicting (s101) the curvature of the road on which the vehicle (1) is travelling; and - controlling (s102) the required steering wheel torque (TD) based on the predicted road curvature, and when a curve (10) is predicted, the required steering wheel torque (TD) is decreased in the direction of the curve (10), just before entering the curve (10).

2. The method according to claim 1, wherein the curvature of the road is predicted (s101) by means of road data/map information relating to the current trajectory of the vehicle (1).

3. The method according to claim 1 or 2, wherein the required steering wheel torque (TD) is decreased by decreasing the return torque (+TR, -TR) acting on the steering wheel (6) opposite to the curve direction.

4. The method according to any of claims 1-3, wherein the required steering wheel torque (TD) is decreased by displacing the neutral position (P) of the steering wheel (6) by an offset angle (O).

5. The method according to any of claims 1-4, wherein the required steering wheel torque (TD) is decreased by applying an assist torque (TA) in the direction of the curve

6. The method according to any of claims 1-5, wherein the decrease of the required steering wheel torque (TD) is performed gradually.

7. The method according to any of claims 1-6, wherein the method further comprises the step to increase the return torque (+TR, -TR) acting in direction of the curve.

8. A steering assist system (4) associated with a vehicle (1), adapted to control the steering wheel torque required to be applied by a driver (TD) to achieve a certain steering angle (?), the steering wheel (6) having a neutral position (P) in which the steered wheels (8) of the vehicle (1) are directed in parallel with the longitudinal extension of the vehicle (1), wherein the steering assist system (4) is adapted to apply a return torque acting clockwise (+TR) and a return torque acting counter-clockwise (-TR) on the steering wheel (6), striving to turn the steering wheel (6) back to the neutral position (P), characterized in that it comprises a control unit (5) adapted to predict the curvature of the road on which the vehicle (1) is travelling and to control the required steering wheel torque (TD) based on the predicted road curvature, and wherein the control unit (5) is adapted to, when a curve (10) is predicted, decrease the required steering wheel torque (TD) in the direction of the curve (10) just before entering the curve (10).

9. The system according to claim 8, wherein the control unit (5) is adapted to predict the curvature of the road by means of road data/map information relating to the current trajectory of the vehicle (1).

10. The system according to any of claims 8-9, wherein the control unit (5) is adapted to decrease the required steering wheel torque (TD) by decreasing the return torque (+TR, -TR) acting on the steering wheel (6) opposite to the curve direction.

11. The system according to any of claims 8-10, wherein the control unit (5) is adapted to decrease the required steering wheel torque (TD) by displacing the neutral position (P) of the steering wheel (6) by an offset angle (O).

12. The system according to any of claims 8-11, wherein the control unit (5) is adapted to decrease the required steering wheel torque (TD) by applying an assist torque (TA) in the direction of the curve.

13. The system according to any of claims 8-12, wherein the control unit (5) is adapted to increase the return torque (+TR, -TR) acting in direction of the curve.

14. A vehicle, characterized in that it comprises a system (4) according to any of claims 8-13.

15. A computer program (P), wherein said computer program comprises program code for causing an electronic control unit (5; 500) or a computer (500) connected to the electronic control unit (5; 500) to perform the steps according to any of the claims 1-7.

16. A computer program product comprising a program code stored on a computerreadable medium for performing the method steps according to any of claims 1-7, when said computer program is run on an electronic control unit (5; 500) or a computer (500) connected to the electronic control unit (5; 500).