SE541613C2

SE541613C2 - A method and system for limiting an autonomous vehicle´s impact on a road surface

Info

Publication number: SE541613C2
Application number: SE1650745A
Authority: SE
Inventors: Magnus Granström
Original assignee: Scania Cv Ab
Priority date: 2016-05-30
Filing date: 2016-05-30
Publication date: 2019-11-12
Also published as: SE1650745A1; DE102017004897B4; DE102017004897A1

Abstract

The invention relates to a method for limiting a vehicle’s impact on a road surface. The method comprises the steps of determining (s100) a maximum derivative of the steering angle as a function of vehicle speed, and controlling (s101) the steering of the vehicle (1) based on the maximum derivative of the steering angle.The invention also relates to a system (4) for limiting a vehicle’s impact on the road surface, a vehicle (1) comprising such a system (4), a computer program (P) and a computer program product.

Description

A method and system for limiting an autonomous vehicle’s impact on a road surface TECHNICAL FIELD The present invention relates to a method for limiting a vehicle’s impact on the road surface, a system for limiting a vehicle’s impact on the road surface, a vehicle comprising such a system, a computer program and a computer program product according to the appended claims.

BACKGROUND Heavy vehicles such as trucks, busses, construction vehicles etc. comprise a steering servomechanism assisting the operator of the vehicle when changing the steering angle by means of the steering wheel. This way, the large and heavy vehicle can easily be manoeuvred by using the steering wheel. The servomechanism also limits the manoeuvrability of the vehicle by an inherent limitation regarding the maximum steering angle and the possible maximum derivative of the steering angle. When the vehicle is an autonomously operated vehicle paths and trajectories must be determined with such limitations/constraints taken into consideration. This way, it is ensured that the vehicle is given a trajectory which it actually can drive along. Other limitations relating to steering angle may also be set for safety reasons, for example in order to prevent that the vehicle rolls over during high vehicle speeds.

Rapid changes of steering angle and thus a high derivate of the steering angle may cause unnecessary wear of a road. If a vehicle is driving on a gravel road a rapid change of steering angle may cause the contact area of the wheels to rotate against the road surface such that the road surface is displaced and thereby cause the formation of a hole/cavity. This typically occurs when the change of steering angle is too fast in relation to the vehicle speed. It is thus desired to limit the risk of wheels affecting the road surface in order to limit the wear of the road and the tires. Document US20050034437 A1 describes an automated vehicle where operating parameters such as maximum rate of change of steering angel varies depending on weather conditions. This way, the damage of the ground under wet conditions is limited.

SUMMARY OF THE INVENTION Despite known solutions in the field, there is still a need to develop a method and system for limiting a vehicle’s impact on the road surface.

An object of the present invention is to achieve an advantageous method for limiting a vehicle’s impact on the road surface by reducing the risk that the contact area of the steered wheels of the vehicle rotates against the road surface, such that the road surface is frayed and/or displaced.

An object of the present invention is to achieve an advantageous method for limiting an autonomously operated vehicle’s impact on the road surface, which prevents the vehicle from travelling along trajectories causing the contact area of the steered wheels to rotate against the road surface, such that the road surface is frayed and/or displaced.

Another object of the present invention is to achieve an advantageous method for limiting a manually operated vehicle’s impact on the road surface, which instantly informs the operator of the vehicle if there is a risk that the contact area of the steered wheels will rotate against the road surface, such that the road surface is frayed and/or displaced.

A further object of the present invention is to achieve an advantageous system for limiting a vehicle’s impact on the road surface by reducing the risk that the contact area of the steered wheels of the vehicle rotates against the road surface, such that the road surface is frayed and/or displaced.

Another object of the invention is to achieve an advantageous system for limiting an autonomously operated vehicle’s impact on the road surface, which prevents the vehicle from travelling along trajectories causing the contact area of the steered wheels to rotate against the road surface, such that the road surface is frayed and/or displaced.

A further object of the invention is to achieve an advantageous system for limiting a manually operated vehicle’s impact on the road surface, which instantly informs the operator of the vehicle if there is a risk that the contact area of the steered wheels will be rotated against the road surface, such that the road surface is frayed and/or displaced.

The herein mentioned objects are achieved by a method for limiting a vehicle’s impact on the road surface, a system for limiting a vehicle’s impact on the road surface, a vehicle, a computer program and a computer program product according to the independent claims.

According to an aspect of the invention a method for limiting a vehicle’s impact on the road surface is provided. The method comprises the steps of: - determining a maximum derivative of the steering angle as a function of vehicle speed, and - controlling the steering of the vehicle based on the maximum derivative of the steering angle.

The derivative of the steering angle is the rate of change of steering angle, i.e. how fast the steering wheel is turned. The steering angle is suitably defined as the angle of the steering wheel in relation to a zero position, in which the steering wheel is directed such that the steered wheels of the vehicle are directed in parallel with the longitudinal extension of the vehicle. By maximum derivative of steering angle means the maximum derivative of steering angle which is suitable for limiting a vehicle’s impact on the road surface. It is thus not suitable to have a higher derivative of the steering angle at a certain vehicle speed than the maximum derivative of the steering angle.

The vehicle’s impact on the road surface is suitably limited by reducing the risk that the contact areas of the steered wheels of the vehicle are rotated against the road surface, such that the road surface is frayed and/or displaced. The contact area of a steered wheel is the area of the tire of the steered wheel, which touches the ground. When the steering wheel is turned the steered wheels are turned sideways, i.e. turned to the right or to the left, in relation to the zero position. When the steered wheels are turned too fast in relation to the vehicle speed the contact areas of the steered wheels may rotate against the road surface such that the wheels are chafed against the road surface. The road surface will thereby be frayed or even displaced, which means that a cavity or similar may be formed in the road. This typically occur when the steering wheel, and thus the steered wheels, is turned too fast in relation to the vehicle speed. This is specifically a problem at low vehicle speeds.

The vehicle is suitably a heavy vehicle, such as a truck, a construction vehicle, bus or similar. Heavy vehicles entail different demands on how they are operated compared to cars. As for cars the servomechanism limits the manoeuvrability of the heavy vehicle by limiting the maximum possible steering angle etc. When manoeuvring a heavy vehicle and turning the steering wheel too fast in relation to the vehicle speed, the contact areas of the steered wheels rotate against the road surface and the ground and/or the tires of the steered wheels i s/are thereby exposed to considerable strain. If the vehicle is driving on a gravel road or a road of poor quality such a rapid change of steering angle may cause a formation of a cavity in the road due to the heavy weight/load of the vehicle. This is for example the case when a vehicle is standing still on a gravel road and the operator of the vehicle turns the steering wheel. In mining applications or constructions sites where a lot of heavy vehicles are loading/unloading on the same location this problem may also occur since the vehicle speed is low and the vehicle manoeuvring typically involves frequent steering wheel movements. After some time the road may be so damaged that it isn’t even possible to use the road. The road thus has to be repaired and the cost of road maintenance is thereby unnecessary high. By determining a maximum derivative of the steering angle as a function of vehicle speed and controlling the steering of the vehicle based on this maximum derivative, such problems can be minimized. This way, the vehicle’s impact on the road surface is limited and the wear of the vehicle tires is limited. The higher the vehicle speed is, the faster you can turn the steering wheel without the steered wheels affecting the road surface. The maximum derivative of the steering angle is therefore suitably determined such that it increases with vehicle speed and thus decreases when the vehicle speed decreases.

By determining the maximum derivative of the steering angle as a function of the vehicle speed, the maximum derivative is zero when the vehicle speed is zero. This means that it is not suitable to turn the steering wheel at all when the vehicle is standing still.

According to an aspect of the invention the maximum derivative of the steering angle is determined based on a constant depending on vehicle parameters and/or road parameters. The constant may depend on the type of vehicle, the weight of the vehicle, the tires of the vehicle, the wheel diameter and/or the road surface. The constant may be determined empirically and is suitably saved in a control unit. The control unit suitably determines the maximum derivative of steering angle as a function of vehicle speed.

The maximum derivative of the steering angle is preferably determined only for vehicle speeds lower than 10km/h. For vehicle speeds higher than 10 km/h the effect of turning the steering wheel too fast is not as problematic since the steered wheels won’t affect the road surface as much. The maximum derivative of the steering angle may be between 0-2 rad/s.

According to an aspect of the invention the step of controlling the steering of the vehicle comprises to determine a trajectory for the vehicle based on the maximum derivative of the steering angle. The vehicle may be autonomously operated. Thus, according to an aspect of the invention a method for limiting an autonomous vehicle’s impact on the road surface is provided. The method may comprise the steps of determining a maximum derivative of the steering angle as a function of vehicle speed, and determining a trajectory for the vehicle based on the maximum derivative of steering angle. The method is thus used for motion planning of an autonomous vehicle. When a vehicle is autonomously operated paths and trajectories must be determined with limitations/constraints regarding steering angle, vehicle speed etc. taken into consideration. This way, it is ensured that the vehicle is given a trajectory which it can actually follow. Other factors, such as fuel consumption, time of travel etc. are typically also taken into consideration when determining trajectory. By also considering the maximum derivative of the steering angle as a function of the vehicle speed when determining trajectory, trajectories where the maximum derivative of the steering angle is exceeded frequently can be avoided. This way, the risk that the steered wheels rotate against the road surface, such that the road surface is frayed and/or displaced is reduced and the vehicle’s impact on the road surface is limited. For example, if an operator wishes to operate an autonomous vehicle from point A to point B the vehicle system computes a plurality of paths which the vehicle can take to go from A to B. Different factors may be taken into consideration when determining the trajectory. Each trajectory is suitably evaluated with regard to the derivative of steering angle required at each point for following the trajectory. By determining a maximum derivative of the steering angle based on vehicle speed, which should not be exceeded in order to limit the vehicle’s impact on the road, trajectories that are not suitable can be avoided.

The maximum derivative of the steering angle preferably constitutes a parameter in a cost function for motion planning. A cost function suitably comprises a plurality of constraints relating to vehicle speed, steering angle etc.

The determined maximum derivative of the steering angle is suitably set as a constraint in a cost function for determining trajectory, such that if a trajectory does not comply with the constraint, that trajectory is not chosen for the vehicle. If all computed trajectories violate the constraint relating to the maximum derivative of the steering angle, the trajectory that is best in view of other factors is suitably chosen. In order to limit the impact on the road surface, different trajectories are suitably chosen for a vehicle going to the same destination repeatedly.

According to an aspect of the invention step of controlling the steering of the vehicle comprises to determine if an actual derivative of steering angle exceeds the maximum derivative; and if so inform the operator of the vehicle. The vehicle may be a manually operated vehicle. Thus, according to an aspect of the invention a method for limiting a manually operated vehicle’s impact on the road surface is provided. The method may comprise the steps of determining a maximum derivative of the steering angle as a function of vehicle speed; determining if the actual derivative of the steering angle exceeds the maximum derivative; and if so, informing the operator of the vehicle. The actual derivative of the steering angle and the vehicle speed are suitably continuously determined and compared with the maximum derivative of the steering angle for the current vehicle speed. This way, the actual operation of the vehicle is evaluated and suitable actions can be taken when the maximum derivative is exceeded. The actual derivative of the steering angle and the current vehicle speed are suitably determined according to conventional methods. By informing the operator of the vehicle that the steering wheel is turned too fast, the operator can instantly improve his driving behaviour. Also, by informing the operator each time the derivative of the steering angle exceeds the maximum derivative of the steering angle, the operator will be encouraged to change his behaviour in the long run and will be aware that the steering wheel should not be turned too fast at low vehicle speeds. This is also valid for vehicles that are remotely controlled by an operator.

The operator of the vehicle is suitably informed by an audial alert and/or by a tactile alert. The audial alert may be a continuous long sound or a plurality of short sounds. The tactile alert may be provided as vibrations in the steering wheel. Alternatively or additionally, the operator is informed by information presented on a display unit.

According to an aspect of the invention the method further comprises the step to log the geographical position where the actual derivative of the steering angle is higher than the maximum derivative. The geographical location is suitably logged in a control unit. Where an actual derivative of the steering angle exceeds the maximum derivative of the steering angle, it is very likely that the contact area of the steered wheels of the vehicle will rotate against the road surface, such that the road surface is frayed and/or displaced whereby the road is damaged. By logging these geographical locations the operator of the vehicle can be warned before reaching such location. The operator will thereby be aware that the road ahead may be damaged and can choose to take another road or adapt the vehicle speed. Also, by logging each occasion when the actual derivative of the steering angle exceeds the maximum derivative of the steering angle the information can be used for rating the operator of the vehicle. Thus, if the operator manoeuvres the vehicle such that the actual derivative of the steering angle exceeds the maximum derivative of the steering angle the rating is decreased. It is therefore in the interest of the operator to manoeuvre the vehicle with the maximum derivative in mind. By logging the geographical location it is also possible to identify locations where wear of the road is very probable. This information can be useful in that the road can be inspected and repaired before the damage causes serious problems. The geographical location where the actual derivative of the steering angle of a vehicle was higher than the maximum derivative may be transferred to a vehicle external system collecting the same information from a plurality of vehicles, for example all vehicles in a vehicle fleet, within a certain construction site etc. This way, a more reliable conclusion regarding the road condition can be drawn, based on information from a plurality of vehicles. The geographical location where the actual derivative of the steering angle exceeds the maximum derivative of the steering angle may be logged for manually operated vehicles as well as autonomous vehicles.

According to an aspect of the invention a system associated with a vehicle for limiting the vehicle’s impact on the road surface is provided. The system comprises a control unit adapted to determine a maximum derivative of the steering angle as a function of vehicle speed and to control the steering of the vehicle based on the maximum derivative of the steering angle. By determining a maximum derivative of the steering angle based on the vehicle speed and automatically take action based on this maximum derivative, the occurrence of wheels affecting the road surface can be avoided or limited. This way, the vehicle’s impact on the road surface is limited and the wear of the tires is reduced. The control unit is suitably adapted to determine the maximum derivative of the steering angle such that it increases with vehicle speed and thus decreases when the vehicle speed decreases.

The control unit is suitably adapted to determine the maximum derivative of the steering angle based on a constant depending on vehicle parameters and/or road parameters. The constant may depend on the type of vehicle, the weight of the vehicle, the tires of the vehicle, the wheel diameter and/or the road surface. The constant may be determined empirically and is suitably saved in the control unit.

The control unit is suitably adapted to determine the maximum derivative of the steering angle only for vehicle speeds lower than 10km/h. At higher vehicle speeds the steered wheels are seldom turned, such that the movement of the steered wheels displaces the road surface and the impact on the road surface is thereby not significant.

According to an aspect of the invention the control unit is adapted to control the steering of the vehicle by determining a trajectory for the vehicle based on the maximum derivative of the steering angle. The vehicle is suitably an autonomously operated vehicle. Thus, according to an aspect of the invention a system for limiting an autonomous vehicle’s impact on the road surface is provided, wherein a control unit is adapted to determine a maximum derivative of the steering angle as a function of vehicle speed, and determine a trajectory for the vehicle based on the maximum derivative of the steering angle. The system is thus suitably a system for motion planning of an autonomous vehicle. By considering the maximum derivative of the steering angle when determining trajectory, the control unit can avoid trajectories where the maximum derivative of the steering angle is exceeded frequently. This way, the risk that movement of the steered wheels of the vehicle frays and/or displaces the road surface is reduced and the vehicle’s impact on the road surface is thus limited.

The maximum derivative of the steering angle preferably constitutes a parameter in a cost function for motion planning controlled by the control unit. The control unit is suitably adapted to set the determined maximum derivative of the steering angle as a constraint in a cost function for determining trajectory, such that trajectories that violate the constraint are not chosen for the vehicle.

According to an aspect of the invention control unit is adapted to control the steering of the vehicle by determining if an actual derivative of the steering angle is higher than the maximum derivative; and if so inform the operator of the vehicle. The vehicle may be a manually operated vehicle. Thus, according to an aspect of the invention a system for limiting a manually operated vehicle’s impact on the road surface is provided. The control unit is thereby adapted to determine a maximum derivative of the steering angle as a function of vehicle speed; determine if the actual derivative of the steering wheel exceeds the maximum derivative, and if so, inform the operator of the vehicle. The control unit is thus adapted to control the steering of the vehicle by providing information relating to the maximum derivative of the steering angle and thereby affecting the driving behaviour of the operator of the vehicle. The control unit is suitably adapted to continuously determine the actual derivative of the steering angle and compare it with the maximum derivative of the steering angle for the current vehicle speed. This way, the control unit is adapted to evaluate the actual operation of the vehicle and to take suitable actions when the maximum derivative is exceeded. The control unit is suitably connected to sensor means determining the actual derivative of the steering angle and the current vehicle speed according to conventional methods.

The control unit is suitably adapted to inform the operator of the vehicle by an audial alert and/or by a tactile alert. The audial alert may be a continuous long sound or a plurality of short sounds. The tactile alert may be provided as vibrations in the steering wheel. The control unit may alternatively or additionally be adapted to inform the operator by presenting information on a display unit.

According to an aspect of the invention the control unit is adapted to log the geographical position where the actual derivative of the steering angle is higher than the maximum derivative. By logging these geographical positions the control unit can warn the operator of the vehicle before reaching such position. The operator will thereby be aware that the road ahead may be damaged and can choose to take another road or adapt the vehicle speed. Also, by logging the geographical position it is possible to identify locations where wear of the road is very probable. This information can be useful in that the road can be inspected and repaired before the damage causes serious problems. Preferably, the control unit is adapted to transfer the geographical position where the actual derivative of the steering angle of a vehicle was higher than the maximum derivative to a system collecting the same information from a plurality of vehicles. This way, a more reliable conclusion regarding the road condition can be drawn.

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-b schematically illustrates a the function of a system for limiting a vehicle’s impact of the road surface according to an embodiment of the invention; Figure 3 schematically illustrates a flow chart for a method for limiting a vehicle’s impact on the road surface according to an embodiment of the invention; and Figure 4 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 system 4 for limiting the vehicle’s impact on the road surface, a steering wheel 6 and steered wheels 8 connected to the steering wheel 6. The system 4 suitably comprises a control unit 5. A computer (not shown) may be connected to the control unit 5. The control unit 5 is adapted to determine a maximum derivative of the steering angle as a function of vehicle speed and to control the steering of the vehicle 1 based on said maximum derivative of the steering angle. The vehicle 1 may be a heavy vehicle, e.g. a truck, a construction vehicle or a bus. The vehicle may be a hybrid vehicle comprising an electric machine (not shown) in addition to the combustion engine 2.

Figures 2a-b schematically shows the function of a system 4 associated with a vehicle 1 for limiting the vehicle’s impact of the road surface according to an embodiment of the invention. The vehicle 1 and the system 4 are suitably configured as described in Figure 1. The figures schematically show the vehicle 1 which is about to drive from point A to point B via Point C. The vehicle is directed with the front F of the vehicle to the left at point A and should end up with the front directed to the right at point B.

The most common trajectory for the vehicle 1 to achieve this is shown in Figure 2a. The vehicle is typically operated such that it moves backwards to point C along trajectory 1'. The steered wheels 8 of the vehicle 1 are thus turned slightly to the left when the vehicle 1 has reached point C. The vehicle 1 essentially stops in point C and the operator of the vehicle turns the steering wheel 6 to the right in order to continue to point B along trajectory 2'. At point C, the steered wheels 8 of the vehicle 1 are thus turned to the right while the vehicle 1 is essentially still. The contact area of the steered wheels 8 will thereby rotate against the road surface. The road surface and the tires of the steered wheels 8 are thus exposed to considerable strain and depending on the road condition the weight/load of the vehicle 1 may cause the steered wheels 8 to displace the road surface and thus damage the road.

With a system 4 for limiting the vehicle’s impact on the road surface according to the invention the operator of the vehicle 1 would be encouraged to take another trajectory from A to B. This is illustrated in Figure 2b. The vehicle 1 starts at point A and the operator operates the vehicle 1 such that it moves backwards along trajectory 1 ” to point C. The steered wheels 8 of the vehicle 1 are thus directed slightly to the left when reaching point C. To continue to point B the operator of the vehicle 1 maintains the direction of the steered wheels 8 at the beginning of trajectory 2” and therefore does not have to turn the steering wheel 6 when the vehicle speed is essentially zero. This way, the steered wheels 8 will not be moved such that they displace the road surface and the vehicle’s impact on the road surface is thereby minimized.

If the vehicle 1 is an autonomously operated vehicle the system 4 would have determined a maximum derivative of the steering angle as a function of vehicle speed and realised that the trajectory shown in Figure 2a would cause the actual derivative of the steering angle to exceed the maximum derivative of the steering angle at point C. The system 4 would therefore determine that it is more suitable to take the trajectory shown in Figure 2b in order to limit the vehicle’s impact on the road surface. The maximum derivative of the steering angle as a function of vehicle speed may be set as a constraint in a cost function for motion planning of autonomous vehicles. In that case, the trajectory in Figure 2a would violate the constraint and would get a penalty whereas the trajectory in Figure 2b would satisfy the constraint, and the system 4 would control the vehicle 1 such that it follows the trajectory in Figure 2b.

If the vehicle 1 is a manually operated vehicle the operator of the vehicle 1 would probably try the trajectory in Figure 2a first. The system 4 would have determined a maximum derivative of the steering angle as a function of vehicle speed and would have determined that the actual derivative of the steering angle at point C exceeds the maximum derivative. The system 4 would then have informed the operator accordingly by an audial/tactile/visual alert. The next time the operator of the manually operated vehicle 1 is about to drive from A to B he is aware of the problem at point C and will avoid turning the steering wheel too fast in relation to the vehicle speed. Also, the system 4 could warn the operator prior to reaching point C to make sure that the trajectory 2’ in Figure 2a is avoided.

Figure 3 shows a flowchart for a method for limiting a vehicle’s impact on the road surface according to an embodiment of the invention. The vehicle 1 is suitably configured as described in Figure 1. The method comprises the steps of determining s101 a maximum derivative of the steering angle as a function of vehicle speed, and controlling s102 the steering of the vehicle 1 based on the maximum derivative of the steering angle.

The step to control s102 the steering of the vehicle 1 suitably comprises to reduce the risk that the contact areas of the steered wheels 8 of the vehicle 1 rotate against the road surface, such that the road surface is frayed and/or displaced. This typically occurs when the steering wheel 6, and thus the steered wheels 8, is turned too fast in relation to the vehicle speed. By determining a maximum derivative of the steering angle as a function of the vehicle speed and controlling the steering of the vehicle based on this maximum derivative, such problems can be reduced. This way, the vehicle’s impact on the road surface and the wear of the tires is limited.

The maximum derivative of the steering angle is suitably determined such that it increases with vehicle speed and thus decreases when the vehicle speed decreases. By determining the maximum derivative as a function of the vehicle speed, the maximum derivative is zero when the vehicle speed is zero. This means that it is not suitable to turn the steering wheel at all when the vehicle is standing still.

The step to determine s101 the maximum derivative of the steering angle is suitably based on a constant depending on vehicle parameters and/or road parameters. The constant may depend on the type of vehicle, the weight of the vehicle, the tires of the vehicle, the wheel diameter and/or the road surface.

The constant may be determined empirically and is suitably saved in the control unit 5 of the system 4. The control unit 5 suitably determines the maximum derivative of the steering angle as a function of vehicle speed.

The maximum derivative of the steering angle is preferably determined only for vehicle speeds lower than 10km/h. For vehicle speeds higher than 10 km/h the effect of turning the steering wheel 6 too fast is not as problematic. The maximum derivative of the steering angle may be between 0-2 rad/s.

The step to control s102 the steering of the vehicle 1 suitably comprises to determine a trajectory for the vehicle 1 based on the maximum derivative of the steering angle. The vehicle 1 may thus be an autonomously operated vehicle. By considering the maximum derivative of the steering angle when determining trajectory, trajectories where the maximum derivative of the steering angle is exceeded frequently can be avoided. This way, the risk that the contact areas of the steered wheels 8 of the vehicle are rotated against the road surface, such that the steered wheels 8 affect and displace the road surface, is reduced and the vehicle’s impact on the road surface is limited. The maximum derivative of the steering angle preferably constitutes a parameter in a cost function for motion planning. The determined maximum derivative of the steering angle is suitably set as a constraint in a cost function for determining trajectory. The step to control s102 the steering of the vehicle thus comprises to, for each trajectory, determine if the trajectory violates or satisfies the constraint. If a trajectory violates the constraint the trajectory gets a penalty and may not be chosen for the vehicle 1.

The step to control s102 the steering of the vehicle 1 may comprise to determine if an actual derivative of the steering angle exceeds the maximum derivative; and if so informing the operator of the vehicle. The vehicle is thus suitably a manually operated vehicle. The actual derivative of the steering angle and the current vehicle speed are suitably continuously determined and compared with the maximum derivative of the steering angle for the current vehicle speed. The actual derivative of the steering angle and the current vehicle speed are suitably determined according to conventional methods. By informing the operator of the vehicle 1 that the steering wheel 6 is turned too fast, the operator is encouraged to instantly improve his driving behaviour and the steering of the vehicle 1 is thus affected. Also, by informing the operator each time the derivative of the steering angle exceeds the maximum derivative of the steering angle, the operator is encourage to change his behaviour in the long run. The operator of the vehicle is suitably informed by an audial alert and/or by a tactile alert. The audial alert may be a continuous long sound or a plurality of short sounds. The tactile alert may be provided as vibrations in the steering wheel. Alternatively or additionally, the operator is informed by information presented on a display unit.

The method may further comprise the step to log the geographical position where the actual derivative of the steering angle is higher than the maximum derivative. The geographical position is suitably logged in the control unit 5. By logging these geographical positions the operator of the vehicle 1 can be warned before reaching such a position. The operator will thereby be aware that the road ahead may be damaged and can choose to take another road or adapt the vehicle speed. Also, by logging the geographical position it is possible to identify locations where wear of the road is very probable. This information can be very useful and the road can be inspected and repaired before the damage causes serious problems. The geographical position where the actual derivative of the steering angle of a vehicle 1 is higher than the maximum derivative may be transferred to a vehicle external system collecting the same information from a plurality of vehicles. The geographical position where the actual derivative of the steering angle exceeds the maximum derivative of the steering angle may be logged for manually operated vehicles as well as autonomous vehicles.

Figure 4 schematically illustrates a device 500. The control unit 5 and/or computer 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 nonphysical 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 limiting a vehicle’s impact on the road surface according to the invention. The computer program P comprises routines for determining a maximum derivative of steering angle as a function of vehicle speed. The computer program P comprises routines for controlling the steering of the vehicle based on the maximum derivative of steering angle. The computer program P comprises routines for automatically taking action to limit the vehicle’s impact on the road surface based on the maximum derivative of steering angle. The computer program P comprises routines for determining a trajectory based on the maximum derivative of steering angle. The computer program P comprises routines for determining if the actual derivative of steering angle exceeds the maximum derivative of steering angle. The computer program P comprises routines for informing the operator of the vehicle 1 that the maximum derivative of steering angle has been exceeded. The computer program P comprises routines for logging the geographical position where the actual derivative of steering angle has exceeded the maximum derivative of steering angle. 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 limiting an autonomous vehicle’s impact on a road surface, characterized by the steps of: - determining (s100) a maximum derivative of the steering angle as a function of vehicle speed, and - controlling (s101 ) the steering of the vehicle (1) based on the maximum derivative of the steering angle, wherein the step of controlling (s101) the steering of the vehicle (1) comprises to determine a trajectory for the vehicle (1) based on the maximum derivative of the steering angle, and, wherein the maximum derivative of the steering angle constitutes a parameter in a cost function for motion planning.

2. The method according to claim 1 , wherein the maximum derivative of the steering angle is determined based on a constant, which depends on vehicle parameters and/or road parameters.

3. The method according to claiml or 2, wherein the maximum derivative of the steering angle is determined only for vehicle speeds lower than 10km/h.

4. A system (4) associated with an autonomous vehicle (1) for limiting a vehicle’s impact on a road surface, characterized in that it comprises a control unit (5) adapted to determine a maximum derivative of steering angle as a function of vehicle speed, and to control the steering of the vehicle (1) based on the maximum derivative of steering angle, wherein the control unit (5) is adapted to control the steering of the vehicle (1) by determining a trajectory for the vehicle (1) based on the maximum derivative of the steering angle and wherein the maximum derivative of the steering angle constitutes a parameter in a cost function for motion planning.

5. The system according to claim 4, wherein the control unit (5) is adapted to determine the maximum derivative of the steering angle based on a constant, which depends on vehicle parameters and/or road parameters.

6. The system according to any of claims 4-5, wherein the control unit (5) is adapted to determine the maximum derivative of the steering angle only for vehicle speeds lower than 10km/h.

7. A vehicle, characterized in that it comprises a system (4) according to any of claims 4-6.

8. 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 1-3.

9. A computer program product comprising a program code stored on a computerreadable medium for performing the method steps according to any of claims 1 -3, 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).