TECHNICAL FIELD The present invention relates to a method for steering a vehicle comprising a front axle and a rear axle. More particularly, the invention relates to a method for steering a vehicle comprising a front axle and a rear tag axle. The invention relates also to a computer program product comprising program code for a computer for implementing a method according to the invention. It relates also to a system for steering a vehicle comprising a front axle and a rear axle and a vehicle being equipped with the system.

BACKGROUND

Some vehicles of today are provided with a so called tag axle wheel steering functionality. Said vehicles may be a truck or lorry. Such vehicles may be arranged with steerable front wheels being provided at a front wheel axle. Hereby an operator/driver may control steering of said front wheels during propulsion by means of steering wheels. Said vehicles are provided with a drive axle for propelling said vehicle by means of thereto associated drive wheels. Said tag axle is a support axle being provided with steerable wheels. One common hydraulic steering configuration is hereby provided for controlling steering of both said front wheels and said tag axle wheels. This configuration is rather bulky and requires fluid pipes to each of said front axle and said tag axle.

Said tag axle wheel steering functionality is today performed based on a current steering wheel angle but is not providing sufficient or desired aid for manoeuvring said vehicle in all traffic situations. The tag axle wheel steering functionality is today limited in various ways and also static to its nature.

Said tag axle wheel steering functionality is today sub-optimized e.g. paying regard to the number of optimization parameters.

US6763906 refers to a power steering system for rear axle wheels of a vehicle. SUMMARY OF THE INVENTION

An object of the present invention is to propose a novel and advantageous method for steering a vehicle.

Another object of the invention is to propose a novel and advantageous system and a novel and advantageous computer program for steering a vehicle.

An object of the present invention is to propose a novel and advantageous method for providing a more versatile steering of a vehicle.

Another object of the invention is to propose a novel and advantageous system and a novel and advantageous computer program for providing a more versatile steering of a vehicle. Yet another object of the invention is to propose a method, a system and a computer program for achieving a user friendly and reliable steering of a vehicle.

Yet another object of the invention is to propose a method, a system and a computer program for achieving a robust and cost-effective steering of a vehicle.

Yet another object of the invention is to propose an alternative method, an alternative system and an alternative computer program for steering of a vehicle.

At least one of the above objects or other objects are achieved with a method and a system for steering a vehicle as set out in the appended claims. Advantageous embodiments are set out in the dependent claims. Substantially the same advantages of method steps of the innovative method hold true for corresponding means of the innovative system.

According to an aspect of the invention there is provided a method for steering a vehicle comprising a front axle and a rear axle. Wheels are arranged for steering the vehicle at said front axle and said rear axle. The method comprises - controlling the steering action of said rear axle wheels based on a steering action exerted by said front axle wheels;

- determining a function according to which the steering action is performed by means of said rear axle wheels, said function comprising three different angular intervals, namely a first, a second and a third interval, said intervals corresponding to three different intervals of the steering action of the front axle wheels; wherein said function is a function based at least partly on a determined vehicle mass.

Hereby is provided a method for steering a vehicle which achieves different advantages within the different intervals. Hereby said method can optimized regarding a plurality of parameters including mass. The mass can be continuously updated such that the function can be adjusted based on a current mass of the vehicle. Hereby an improved method for steering a vehicle is provided.

Hereby is achieved a more versatile method for steering the vehicle. Hereby it is provided improved manoeuvrability of the vehicle during reverse propulsion as well as forward propulsion. An operator of the vehicle may in a user friendly way drive the vehicle in a rearward direction more safely and smoothly according to an embodiment of the invention. For example a process of parallel parking in a rearward direction may be significantly facilitated.

According to one embodiment two separate steering configurations are provided, one for controlling steering of wheels of one or more rear axles of the vehicle and one for controlling steering of wheels of a front axle of the vehicle. These steering configurations may for example be an electro-mechanical steering configuration or a hydraulic steering

configuration.

According to one embodiment an operator of the vehicle may manually activate a steering functionality according to the invention for use during reverse propulsion of the vehicle and/or during forward propulsion of the vehicle. According to one embodiment activation of said innovative steering functionality may be performed automatically when said vehicle is operated.

The method may comprise: - controlling the steering action exerted by means of said rear axle wheels according to said determined function. Hereby an operator does not have to turn a steering wheel of the vehicle as many degrees, or turns, to achieve a desired steering of the vehicle during reverse propulsion. The method may comprise:

- determining said function further based upon at least one of vehicle speed, front axle wheel steering action speed, surrounding configuration and vehicle axle configuration. Hereby an optimal function may be determined, which further enhance and simplifies steering of the vehicle during operation. The method may comprise:

- determining a first interval of said function such that it corresponds to steering action of said front axle wheels from no steering action up to a first steering action value, the steering action of the rear axle wheels being substantially zero during said first interval of said function. Hereby a reduced wear of a steering configuration is reduced. Said steering configuration may comprise a hydraulic pump device, a hydraulic cylinder for controlling steering of vehicle wheel and force transmitting means for affecting wheels being provided at opposite ends of a rear wheel axle, such as a tag axle.

Hereby is provided a so called dead band area where no steering of rear axle wheels is performed. Said dead band area may be a static dead band area which is not adapted based on e.g. vehicle speed. Said dead band area may also be a dynamic dead band area, which han es

Advantageously potential variances of a steering geometry of the vehicle dependent on various carried loads are hereby managed effectively. Steering of said rear axle wheels is thus not activated unintentionally in case a steering wheel is not centred due to heavy load of the vehicle, even if front steering wheels are presented in line with a longitudinal axis of the vehicle.

Advantageously there is no need to adapt the innovative method for steering on the basis of a clutch position state. Advantageously there is no need to adapt the innovative method for steering on the basis of if a gearbox of a transmission of the vehicle is set to a neutral position or not.

The method may comprise:

- determining a second interval of said function such that it corresponds to steering action of said front axle wheels from said first steering action value to a second steering action value, the steering action of the rear axle wheels increases preferably continuously with increasing steering action of said front axle wheels. Hereby a reduced tire wear of the wheels of the vehicle is advantageously achieved. Said second interval of said function may be determined on the basis of so called Ackermann control. The method may comprise:

- determining a third interval of said function such that it corresponds to steering action of said front axle wheels from said second steering action value to a third steering action value, the steering action of the rear axle wheels increases with increasing steering action of said front axle wheels. Hereby a reduced clearance circle of the vehicle during operation corresponding to said third interval of said function.

The method may comprise:

- determining a third interval of said function such that the steering action of the rear axle wheels increases more rapidly than during said second interval of said function. Hereby a reduced clearance circle of the vehicle during operation is achieved. The method may comprise:

- determining a third interval of said function such that the steering action of the rear axle wheels comprises a maximum allowable steering action value. Hereby a reduced clearance circle of the vehicle during operation is achieved.

The method may comprise: - determining said second interval of said function such that the steering action of said rear axle wheels is initiated smoothly. This provides a comfortable steering method according to an aspect of the invention which naturally is experienced in a positive way of the operator of the vehicle.

The method may comprise:

- determining said function for said third interval of said function such that it is adapted to said second interval of said function. This provides a comfortable steering method according to an aspect of the invention which naturally is experienced in a positive way of the operator of the vehicle.

The method may comprise:

- determining if said vehicle is set for reverse propulsion or forward propulsion; and - determining said function on the basis of the direction of propulsion. Hereby a more versatile method for steering is provided.

The method may comprise:

- determining said function such that said first interval corresponds to an interval of zero steering action of said front axle wheels. According to this embodiment steering of said rear axle wheels may be performed during certain steering manoeuvres such as during and S-turn or when entering and leaving a roundabout. Hereby is provided a flexible and user-friendly method for steering a vehicle.

According to an aspect of the invention there is provided a system for steering a vehicle comprising a front axle and a rear axle. Wheels are arranged for steering said vehicle and provided at said front axle and said rear axle. The system comprises:

- means for controlling the steering action of said rear axle wheels based upon a steering action exerted by said front axle wheels;

- means for determining a function according to which the steering action is performed by means of said rear axle wheels, The function is based on a determined mass of the vehicle, said function comprising three different intervals, namely a first, a second and a third interval, said intervals corresponding to three different intervals of the steering action of the front axle wheels. The system may comprise:

- means for controlling the steering action exerted by means of said rear axle wheels according to said determined function. The system may comprise:

- means for determining said function further being based upon at least one of vehicle speed, front axle wheel steering action speed, surrounding configuration and vehicle axle configuration.

The system may comprise: - means for determining a first interval of said function such that it corresponds to steering action of said front axle wheels from no steering action up to a first steering action value, the steering action of the rear axle wheels being substantially zero during said first interval of said function.

The system may comprise: - means for determining a second interval of said function such that it corresponds to steering action of said front axle wheels from said first steering action value to a second steering action value, the steering action of the rear axle wheels increases preferably continuously with increasing steering action of said front axle wheels.

The system may comprise: - means for determining a third interval of said function such that it corresponds to steering action of said front axle wheels from said second steering action value to a third steering action value, the steering action of the rear axle wheels increases with increasing steering action of said front axle wheels.

The system may comprise: - means for determining a third interval of said function such that the steering action of the rear axle wheels increases more rapidly than during said second interval of said function. The system may comprise:

- means for determining a third interval of said function such that the steering action of the rear axle wheels comprises a maximum allowable steering action value.

The system may comprise: - means for determining said second interval of said function such that the steering action of said rear axle wheels is initiated smoothly.

The system may comprise:

- means for determining said function for said third interval of said function such that it is adapted to said second interval of said function. Hereby a lower end point of said third interval of said function is set to correspond a higher end point of said second interval of said function. This advantageously provides a smooth transition between said second interval and said third interval as well as a smooth transition between said second interval and said third interval.

The system may comprise: - means for determining if said vehicle is set for reverse propulsion or forward propulsion; and

- means for determining said function on the basis of the direction of propulsion. The system may comprise:

- means for determining said function such that said first interval corresponds to an interval of zero steering action of said front axle wheels.

According to an aspect of the invention there is provided a vehicle comprising the innovative system. The vehicle may be any from among a truck, bus or passenger car.

According to an aspect of the invention there is provided a computer program for steering vehicle comprising a front axle and a rear axle, wherein said computer program comprises program code for causing an electronic control unit or a computer connected to the electronic control unit to perform the steps according to anyone of the claims 1-12, when run on said electronic control unit or said computer. According to an aspect of the invention there is provided a computer program for steering a vehicle comprising a front axle and a rear axle, wherein said computer program comprises program code stored on a computer-readable medium for causing an electronic control unit or a computer connected to the electronic control unit to perform the steps according to anyone of the claims 1-12.

According to an aspect of the invention there is provided a computer program for steering a vehicle comprising a front axle and a rear axle, wherein said computer program comprises program code stored on a computer-readable medium for causing an electronic control unit or a computer connected to the electronic control unit to perform the steps according to anyone of the claims 1-12, when run on said electronic control unit or said computer.

According to an aspect of the invention there is provided a computer program product containing a program code stored on a computer-readable medium for performing method steps according to anyone of claims 1-12, when said computer program is run on an electronic control unit or a computer connected to the electronic control unit.

According to an aspect of the invention there is provided a computer program product containing a program code stored non-volatile on a computer-readable medium for performing method steps according to anyone of claims 1-12, when said computer program is run on an electronic control unit or a computer connected to the electronic control unit.

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 confined to the specific details described. One skilled in the art having access to the teachings herein will recognise further applications, modifications and incorporations in other fields, which are within the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For fuller understanding of the present invention and its further objects and advantages, the detailed description set out below should be read in conjunction 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 2 schematically illustrates a subsystem for the vehicle depicted in Figure 1, according to an embodiment of the invention;

Figure 3 schematically illustrates a diagram presenting a rear axle wheel angle as a function of a steering wheel angle, according to an embodiment of the invention;

Figure 4a is a schematic flowchart of a method according to an embodiment of the invention;

Figure 4b is a more detailed schematic flowchart of a method according to an embodiment of the invention; and

Figure 5 schematically illustrates a computer according to an embodiment of the invention. DETAILED DESCRIPTION OF THE DRAWINGS

Figure 1 depicts a side view of a vehicle 100. The exemplified vehicle 100 is hereby a tractor unit to which a trailer unit (not shown) may be connected. The vehicle may be a heavy vehicle, e.g. a truck or a bus. It may alternatively be a car.

Herein said vehicle is provided with a front wheel axle FA, a driving first rear axle RAl and a second rear axle RA2. Said first rear axle RAl is hereby a driving axle being arranged for propelling said vehicle 100. Said second rear axle RA2 is a steerable tag axle of the vehicle It should be noted that the inventive system for steering a vehicle is applicable to various vehicles having a front axle and at least one rear axle, wheels being arranged for steering said vehicle being provided for said front axle and said at least one rear axle, such as e.g. a mining machine, tractor, dumper, wheel loader, platform comprising an industrial robot, forest machine, earth mover, road construction vehicle or emergency vehicle. The vehicle 100 may according to an example be an autonomous vehicle. The inventive method is applicable to vehicles comprising a front axle and at least one rear axle, wheels being arranged for steering said vehicle being provided for said front axle and said at least one rear axle. Said at least one rear axle may be two rear axles, each having steerable wheels. The term "link" refers herein to a communication link which may be a physical connection such as an opto-electronic communication line, or a non-physical connection such as a wireless connection, e.g. a radio link or microwave link.

The term "steering action" refers herein to a steering action of wheels of said front axle FA and steering action of steerable rear wheels of a rear axle, such as said tag axle RA2. The steering actions of the vehicle 100 correspond to wheel angles of said steerable front wheels and said steerable rear wheels, respectively. Even though the invention herein is described with reference to a vehicle comprising a front axle with steerable wheels, a driving rear axle with non-steerable wheels and a rear tag axle with steerable wheels the invention is applicable to various vehicle wheel axle configurations. The vehicle may be arranged with any suitable wheel axle configuration comprising two or more rear axles having steerable wheels. Hereby controlling the steering action of wheels of each of said rear axles may be based upon a steering action exerted by said front axle wheels. Hereby a respective function according to which the steering action is performed by means of wheels of each of said rear axles is determined, said functions may provide a steering action during reverse propulsion different from the steering action provided by means of said rear axle wheels when said vehicle is set for forward propulsion. Thus, steerable wheels of each of said rear axles may be controlled mutually different and/or independently according to each respective determined function when the vehicle 100 is determined to be set for reverse propulsion. Figure 2 schematically illustrates a subsystem of the vehicle 100. A framework FW is arranged to hold a number of wheel axles. According to this example embodiment said vehicle 100 is arranged with three wheel axles. A front wheel axle FA is arranged with a right front wheel FW1 and a left front wheel FW2. A first rear wheel axle RA1 is arranged with a right rear wheel RW11 and a left rear wheel RW12. Said first rear wheel axle RA1 is arranged to propel said vehicle 100 by means of said right rear wheel RW11 and said left rear wheel RW12. An engine (not shown) is arranged to provide torque to said first rear wheel axle RA1 via a transmission (not shown) for propelling said vehicle 100. Said right rear wheel RW11 and said left rear wheel RW12 are according to this example not steerable. A second rear wheel axle RA2 is arranged with a right rear wheel RW21 and a left rear wheel RW22. Said right rear wheel RW21 and said left rear wheel RW22 are steerable. Said second rear wheel axle RA2 is hereby a so called tag axle.

A first sensor configuration 230 is arranged for communication with a first electronic control unit 200 via a link L230. Said first sensor configuration 230 is arranged to continuously or intermittently determine a steering action performed by an operator of the vehicle 100. Said first sensor configuration 230 is arranged to determine a front wheel angle al. Said front wheel angle al is defined relative a longitudinal axle of said vehicle 100 ₇ corresponding to a straight propulsion direction of said vehicle 100 (forward or reverse). Said front wheel angle al is present at said right front wheel FW1 and said left front wheel FW2. Herein said front wheel angle al is only illustrated at said left front wheel FW2 for sake of clarity. Said first sensor configuration 230 may comprise a front wheel angle sensor. Said first sensor configuration 230 may be arranged at one of said right front wheel FW1 and said left front wheel FW2, or at both of said right front wheel FW1 and said left front wheel FW2. Said first sensor configuration 230 is arranged to continuously or intermittently send signals S230 comprising information about said detected front wheel angle al to the first control unit 200 via said link L230.

A second sensor configuration 240 is arranged for communication with the first electronic control unit 200 via a link L240. Said second sensor configuration 240 is arranged to continuously or intermittently determine a steering action performed by the operator of the vehicle 100. Said second sensor configuration 240 is arranged to determine a steering wheel angle a2. Said steering wheel angle a2 is defined relative a reference steering wheel angle corresponding to a straight propulsion direction of said vehicle 100. Said steering wheel angle al corresponds to said front wheel angle al being present at said right front wheel FW1 and said left front wheel FW2. Said second sensor configuration 240 may comprise a steering wheel angle sensor. Said first sensor configuration 230 may be arranged at a steering wheel arrangement (not shown) of the vehicle 100. Said second sensor

configuration 240 is arranged to continuously or intermittently send signals S240 comprising information about said detected steering wheel angle al to the first control unit 200 via said link L240.

Said front wheel angle al and said steering wheel angle a2 are two different representations of a steering action performed by the operator of the vehicle 100.

In case said vehicle 100 is an autonomous vehicle said steering actions are not performed manually, but automatically. Hereby said front wheel angle al and said steering wheel angle a2 are two different representations of a steering action performed autonomously by the vehicle 100. Such steering actions may be controlled by e.g. said first control unit 200. It should be noted that the inventive method is applicable on an autonomous vehicle.

Said first control unit 200 is arranged to determine if said vehicle 100 is set for forward propulsion or reverse propulsion. This may be performed in any suitable way. According to an example said first control unit 200 is arranged to determine a prevailing gear state of a gearbox of a transmission of the vehicle 100. According to an example said first control unit 200 is arranged to determine if an operator manually has requested a particular gear state of said gearbox. According to one example said first control unit 200 is arranged to determine a propulsion direction of said vehicle on the basis of at least one speed sensor of said vehicle, so as to determine if said vehicle is propelled in a forward direction or a reverse direction.

A second control unit 210 is arranged for communication with the first control unit 200 via a link L210. Said second control unit 210 is arranged to communicate with said first control unit 200 by means of signals S210. It may be adapted to conducting the innovative method steps according to the invention. The second control unit 210 may be arranged to perform the inventive method steps according to the invention. It may be used to cross-load software to the first control unit 200 ₇ particularly software for conducting the innovative method. It may alternatively be arranged for communication with the first control unit 200 via an internal network on board the vehicle 100. It may be adapted to performing substantially the same functions as the first control unit 200 ₇ such as controlling the steering action exerted by means of said second rear axle wheels according to a determined function. This is described in greater detail below. The innovative method may be conducted by the first control unit 200 or the second control unit 210, or by both of them.

The second control unit 210 is arranged to control steering of said second rear axle wheels RW21 and RW22 according to an aspect of the invention. The second control unit 210 is arranged to continuously send control signals S250 to a steering configuration 250 via a link L250. Said steering configuration 250 is arranged to control steering of said second rear axle wheels RW21 and RW22 based on said control signals S250. Said steering arrangement 250 may be any suitable steering arrangement. Said steering arrangement 250 may e.g. be an electro-mechanical steering arrangement or a hydraulic steering arrangement.

Said second rear axle wheels RW21 and RW22 are hereby controlled by means of said second control unit 210 so as to present a desired rear axle wheel angle a3. Said rear axle wheel angle a3 is defined relative a longitudinal axle of said vehicle 100 ₇ corresponding to a straight propulsion direction of said vehicle 100 (forward or reverse). Said rear axle wheel angle a3 is present at said right rear axle wheel RW21 and said left rear axle wheel RW22. Herein said rear axle wheel angle a3 is only illustrated at said left rear axle wheel RW22 for sake of clarity.

With reference to Figure 3 there is schematically illustrated a diagram according to an embodiment of the invention.

Hereby is presented a function Fl depicting the relation between a rear axle wheel angle a3 and a front wheel angle al for a reverse propulsion and/or a forward propulsion of the vehicle 100. The rear axle wheel angle a3 is presented in degrees [Deg]. The front wheel angle al is presented in degrees [Deg]. It should be noted that the herein depicted steering wheel angle a2 corresponds to said front wheel angle al and the function Fl of the diagram of Figure 3 could thus alternatively be presented for the rear axle wheel angle a3 and the steering wheel angle a2 for reverse propulsion and/or forward propulsion of the vehicle 100. Herein is with other words said function Fl presented as a relation between said rear wheel angle a3 and a steering action of said vehicle 100.

Said function Fl may be a predetermined function.

Said function Fl is presented having two sections, one referring to turning the front wheels to the right relative a longitudinal axis of the vehicle 100 and one referring to turning the front wheels to the left relative a longitudinal axis of the vehicle 100. Said two sections of said function Fl are substantially symmetrical.

The function Fl comprises three different intervals for each turning direction of said vehicle 100. Each of said three intervals is thus provided for right turns of said steering wheel and left turns of said steering wheel, respectively.

According to this embodiment a first interval of said function Fl is equal to zero within an interval defined by [0, all] (and [-all, 0] correspondingly). Hereby a so called dead band area is provided. Said dead band area [-all, all] may be defined as [10, -10], [20, -20],

[30, -30] or any suitable interval.

A second interval of said function Fl is provided. Said second interval is defined as [all, al2] (and [-all, -al2] correspondingly). Said second interval of the function Fl is according to an embodiment of the invention the greatest interval of the three provided intervals. Said second interval of the function Fl may be defined as [10, 720], [20, 1080], [90, 2160] or any suitable interval. A first part (relating to lower values of al) of said second interval of said function Fl may be smoothly curved for improved vehicle operator comfort. A second part of said second interval of said function Fl may be a linear function presenting any suitable slope.

A third interval of said function Fl is provided. Said third interval is defined as [al2, al3] (and [-al2, -al3] correspondingly). Said third interval of the function Fl may be defined as

[720, 1080], [1080, 2160], [2160, 2520] or any suitable interval. Said third interval of said function Fl may be a linear function presenting any suitable slope. A transition between said second interval and said third interval, or vice versa, should be smooth for maintained operator comfort.

Figure 4a schematically illustrates a flow chart of a method for steering a vehicle 100 comprising a front axle FA and a rear axle RA2, wheels being arranged for steering said vehicle 100 being provided for said front axle FA and said rear axle RA2. The method comprises the method step s401. The method step s401 comprises the steps of:

- controlling the steering action a3 of said rear axle wheels RW21 and RW22 based upon a steering action ( l or 2) exerted by said front axle wheels FW1 and FW2; and - determining a function Fl according to which the steering action a3 is performed by means of said rear axle wheels RW21 and RW22, said function Fl comprising three different intervals, namely a first, a second and a third interval, said intervals corresponding to three different intervals of the steering action Fl of the front axle wheels FW1 and FW2.

After the method step s401 the method is returned or is ended. Figure 4b schematically illustrates a flow chart of a method for steering a vehicle 100 comprising a front axle FA and a rear axle RA2, wheels being arranged for steering said vehicle 100 being provided for said front axle FA and said rear axle RA2. The method comprises the method step s410.

The method step s410 comprises the step of determining if said vehicle 100 is set for reverse propulsion or forward propulsion. This may be performed automatically by means of said first control unit 200. Alternatively an operator of the vehicle 100 may manually provide information about an active propulsion state by any suitable means to the first control unit 200.

Herein said function Fl may be determined on the basis of the direction of propulsion. It should be noted that said function Fl may be determined on the basis of determined set propulsion state of the vehicle 100. The function Fl determined in a case whereby said vehicle 100 is set for forward propulsion may thus be different compared to a case whereby said vehicle is set for reverse propulsion. The method step s410 may be performed by means of said first control unit 200. After the method step s410 a subsequent method step s420 is performed.

The method step s420 comprises the step of determining a first interval of a function Fl according to which the steering action a3 may be performed by means of said rear axle wheels RW21 and RW22.

The method step s420 may comprise the step of determining said first interval of said function Fl such that it corresponds to steering action (al or a2) of said front axle wheels FWl and FW2 from no steering action (al=0 or a2=0) up to a first steering action value -all or all, the steering action of the rear axle wheels RW21 and RW22 being substantially zero during said first interval of said function Fl. Hereby a so called dead band area is provided within a predetermined steering action value interval (-all to all). Hereby no steering of said rear axle wheels is performed for any steering action values within said interval. This advantageously reduces wear of e.g. said steering configuration 250. The method step s420 may be performed by means of said first control unit 200.

After the method step s420 a subsequent method step s430 is performed.

The method step s430 comprises the step of determining a second interval of a function Fl according to which the steering action a3 may be performed by means of said rear axle wheels RW21 and RW22. The method step s430 may comprise the step of determining a second interval of said function Fl such that it corresponds to steering action of said front axle wheels FWl and FW2 from said first steering action value (-all or all) to a second steering action value (-al2 or al2), the steering action a3 of the rear axle wheels RW21 and RW22 increases preferably continuously with increasing steering action al or a2 of said front axle wheels FWl and FW2.

The method step s430 may comprise the step of determining said second interval of said function Fl such that the steering action a3 of said rear axle wheels RW21 and RW22 is initiated smoothly. The method step s430 may be performed by means of said first control unit 200. After the method step s430 a subsequent method step s440 is performed.

The method step s440 comprises the step of determining a third interval of a function Fl according to which the steering action a3 may be performed by means of said rear axle wheels RW21 and RW22.

The method step s440 may comprise the step of determining a third interval of said function Fl such that it corresponds to steering action al or a2 of said front axle wheels FW1 and FW2 from said second steering action value (-al2 or al2) to a third steering action value (-al3 or al3), the steering action a3 of the rear axle wheels RW21 and RW22 increases with increasing steering action al or a2 of said front axle wheels FW1 and FW2. Hereby said third interval of said function Fl may be determined such that the steering action a3 of the rear axle wheels RW21 and RW22 increases more rapidly with increasing steering action al or a2 of said front axle wheels FW1 and FW2 than during said second interval of said function Fl. The method step s440 may comprise the step of determining said third interval of said function Fl such that the steering action a3 of the rear axle wheels RW21 and RW22 comprises a maximum allowable steering action value -a3max or a3max.

The method step s440 may comprise the step of determining said function Fl for said third interval of said function Fl such that it is adapted to said second interval of said function. Hereby a smooth transition between said second interval of said function Fl and said third interval of said function Fl is provided, either going from said second interval to said third interval or going from said third interval to said second interval.

According to an embodiment of the invention said at least one of said three intervals of said function Fl may be determined based upon at least one of vehicle speed, front axle wheel steering action speed, surrounding configuration and vehicle axle configuration.

Hereby said function Fl may be determined so as to provide a lesser slope of said function Fl for relatively high vehicle speeds. Hereby said function Fl may be determined so as to provide a greater slope of said function Fl for relatively low vehicle speeds. Hereby said function Fl may be determined so as to provide a lesser slope of said function Fl for relatively high front axle wheel steering action speeds (time derivative of steering wheel angle or time derivative of front wheel angle). Hereby said function Fl may be determined so as to provide a steeper slope of said function Fl for relatively low front axle wheel steering action speeds.

Hereby said function Fl may be determined so as to provide a lesser slope of said function Fl for relatively high vehicle masses. Hereby said function Fl may be determined so as to provide a steeper slope of said function Fl for relatively small vehicle masses. Thus, based on a determined vehicle mass the function can be adjusted. In accordance with one embodiment the determined mass is also dynamically adjusted. This can for example be advantageous if these is a big difference between the vehicle being loaded and the vehicle not being loaded. This is for example often the case for heavy vehicles such as buses and trucks.

Hereby said function Fl may be determined so as to provide a lesser slope of said function Fl for relatively long vehicles having a vehicle axle configuration wherein said front axle FA is relatively remotely arranged from said rear axle RA2. Hereby said function Fl may be determined so as to provide a steeper slope of said function Fl for relatively short vehicles having a vehicle axle configuration wherein said front axle FA is relatively closely arranged from said rear axle RA2. Hereby said function Fl may be determined on the basis of a surrounding configuration. The vehicle 100 may be arranged with sensors, such as a video camera or radar unit, so as to determine a surrounding configuration of the vehicle. Hereby said first control unit may be arranged to determine said function Fl on the basis of detected movable or fixed objects in the surrounding of the vehicle. The method step s440 may be performed by means of said first control unit 200.

After the method step s440 a subsequent method step s450 is performed.

The method step s450 comprises the step of controlling the steering action a3 exerted by means of said rear axle wheels RW21 and RW22 according to said determined function Fl. The method step s450 may be performed by means of said second control unit 200. After the method step s450 the method is returned or ended.

Figure 5 is a diagram of one version of a device 500. The control units 200 and 210 described with reference to Figure 2 may in one version comprise the device 500. 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 for steering a vehicle 100 comprising a front axle FA and a rear axle RA2, wheels being arranged for steering said vehicle 100 being provided for said front axle FA and said rear axle RA2.

The computer program P comprises routines for

- controlling the steering action of said rear axle wheels based upon a steering action exerted by said front axle wheels; and

- determining a function according to which the steering action is performed by means of said rear axle wheels, said function comprising three different intervals, namely a first, a second and a third interval, said intervals corresponding to three different intervals of the steering action of the front axle wheels.

The computer program P may comprise routines for controlling the steering action exerted by means of said rear axle wheels according to said determined function. The computer program P may comprise routines for determining said function based upon at least one of vehicle speed, front axle wheel steering action speed, vehicle mass, surrounding configuration and vehicle axle configuration.

The computer program P may comprise routines for determining a first interval of said function such that it corresponds to steering action of said front axle wheels from no steering action up to a first steering action value, the steering action of the rear axle wheels being substantially zero during said first interval of said function.

The computer program P may comprise routines for determining a second interval of said function such that it corresponds to steering action of said front axle wheels from said first steering action value to a second steering action value, the steering action of the rear axle wheels increases preferably continuously with increasing steering action of said front axle wheels.

The computer program P may comprise routines for determining a third interval of said function such that it corresponds to steering action of said front axle wheels from said second steering action value to a third steering action value, the steering action of the rear axle wheels increases with increasing steering action of said front axle wheels.

The computer program P may comprise routines for determining a third interval of said function such that the steering action of the rear axle wheels increases more rapidly than during said second interval of said function- The computer program P may comprise routines for determining a third interval of said function such that the steering action of the rear axle wheels comprises a maximum allowable steering action value.

The computer program P may comprise routines for determining said second interval of said function such that the steering action of said rear axle wheels is initiated smoothly. The computer program P may comprise routines for determining said function for said third interval of said function such that it is adapted to said second interval of said function.

The computer program P may comprise routines for determining if said vehicle is set for reverse propulsion or forward propulsion and determining said function on the basis of the direction of propulsion. The computer program P may comprise routines for determining said function such that said first interval corresponds to an interval of zero steering action of said front axle wheels.

The program P may be stored in an executable form or in compressed form in a memory 560 and/or in a read/write memory 550. Where it is stated that the data processing unit 510 performs a certain function, it means that it conducts a certain part of the program which is stored in the memory 560 or a certain part of the program which is 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 via a data bus 511. The read/write memory 550 is arranged to communicate with the data processing unit 510 via a data bus 514. The links L210, L230, L240 and L250, for example, may be connected to the data port 599 (see Figure 2).

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 will be prepared to conduct code execution as described above.

Parts of the methods herein described may be conducted 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, nor to limit the invention to the variants described. Many modifications and variations will obviously suggest themselves to one skilled in the art. The embodiments have been chosen and described in order to best explain the principles of the invention and their practical applications and thereby make it possible for one skilled in the art to understand the invention for different embodiments and with the various modifications appropriate to the intended use.