FI ELD OF TH E I NVENTION AN D BACKG ROUN D ART The present invention relates to a system according to the preamble of appended claim 1 for steering the wheels of at least one wheeled axle of a motor vehicle and to a method accordi ng to the preamble of the appended independent method claim. The axle to be steered may be any axle of a motor vehicle having an axle beam with a rod, normally called tie rod , con nected thereto by two links and by that interconnecting the left and right wheels of the vehicle and a power means of length varying type acting between the axle beam and one of said li nks by varyi ng its length. Thus, the axle may be powered or unpowered and also be the front axle of the vehicle, but the invention is especially but not exclusively directed to steeri ng the wheels of an u npowered tag axle, which may be placed in any desired order among a vehicles wheeled axles, other than the first, in the vehicles ordinary direction of movement, and the vehicle may have more than one such steered tag axle.

Vehicles provided with steered tag axles are usually heavy vehicles, e.g . trucks and busses, and the invention will be mainly de- scribed for that application althoug h it is neither limited to such a vehicle nor to steering of tag axles.

The wheels of a said axle have a so called toe-value defined as the angle made by the wheels with the longitudinal direction of the vehicle in a neutral state of the vehicle in which the vehicle is to be driven in said longitudinal direction. Said toe-value is called toe-in value if said wheels in said neutral state point inwardly in the ordinary direction of movement of the vehicle, e.g. converging , and toe-out value if the wheels in said neutral state point outwardly, e.g. diverging . A said toe-in value may typically be 1 mm/m, e.g . tan a = 1 0 ^~3 if a is the angle made by the wheels with said longitudinal direction. The reason for havi ng a toe-value is that an appropriate selection of that value has a stabilizing influence upon the tyres of the wheels of said axle.

The toe-value of the wheels of a said wheeled axle is for a said motor vehicle usually determined to be the same for the wheels on both ends of said axle, for instance 1 mm/m, in a state when the vehicle is unloaded. However, when the vehicle is loaded and a load is supplied to said wheeled axle the axle beam thereof will be bent, but said rod will not. This bending of the axle beam will result in a change of said toe-value and this change will result in a tu rning of the wheels at said second end of the axle, since the position of the power means will not be changed as a function of said load. This means that if for instance the toe-in values were 1 mm/m for the wheels at both ends of the axle i n an unloaded state of said axle these may have been changed to be 5 mm/m at said second end while remaining 1 mm/m at the first end of the axle in a loaded state of the axle. Such a different toe-value for the wheels at the opposite ends of said axle results in an increased wear of the tyres of the wheels and a tendency of the vehicle to steer to one side, so that the driver of the vehicle has to steer against this tendency for keeping the vehicle driving straight forward when this shall be done. A system for steering the wheels of at least one wheeled axle of a motor vehicle of this type is known through for example WO 201 4/1 63560 A1 . SUMMARY OF TH E I NVENTION

The object of the present invention is to provide a system and a method of the type defined in the introduction being improved in at least some aspect with respect to such systems and methods already known .

This object is with respect to the system obtained by providing such a system with the featu res listed in the characterizing part of appended claim 1 .

Thus, by comparing the value of a load applied on said axle with stored data of change of the toe-value of the wheels of the axle as a function of such a load and calculating the length to be assumed by the power means for obtaining the same toe-value for the wheels at the two ends of said axle and controlling said power means to assu me that length, it may be ensu red that the toe-value will be the same for the wheels at both ends of said axle irrespectively of the value of said load applied on said axle and e.g. how much said axle beam will be bent. This results in a reduced wear of the tyres of said wheels and a reduced fuel consumption of the vehicle. The tendency of the vehicle to steer to one side will also disappear making it more comfortable for the driver to d rive the vehicle. According to an embodiment of the invention the system is configured to be arranged in a vehicle having said wheeled axle suspended i n a chassis of the vehicle th roug h air suspension members comprising air bellows, and said arrangement compris- es a pressu re sensor configured to provide information about the value of said load to the control unit by measu ring the pressure in at least one said air bellow. The air pressure prevaili ng in such an air bellow may be accu rately translated into a load value to be used for obtaini ng an adaption of the length of said power means for obtai ning the same toe-value for the wheels at both ends of said axle.

According to another embodiment of the invention said arrangement comprises a power sensor configured to provide information about the value of said load to said control u nit by measu ring the power applied th rough said load on means suspending said wheeled axle in a chassis of the vehicle. This constitutes another suitable way to obtain said load value needed for obtaining said same toe-value for the wheels.

According to another embodiment of the invention said arrangement comprises a position sensor configured to provide information about the value of said load to said control u nit by sensing the position of a point or region of said axle beam with re- spect to a point on a chassis of the vehicle. The position information delivered by such a position sensor is a measu re of how much a suspension system of the vehicle is i nfluenced by a load and by that of the value of the load applied on said wheeled axle. According to another embodiment of the invention said power means comprises a power cylinder connected mechanically to said axle beam and directly or indirectly to said first link, the system comprises a pump adapted to conveying a medium under pressure from a mediu m reservoir optionally to either of two opposite sides of a piston of said power cyli nder for varyi ng the length thereof, and the control unit is configu red to control said power means by controlling said pump. According to another embodiment of the invention the system is configu red to steer the wheels of an u npowered wheeled tag axle of a vehicle which in the ordinary di rection of movement of the vehicle follows after the front axle of the vehicle. A system according to the invention having a said power means of length varying type is especially suitable for steering such a tag axle, especially of a heavy vehicle for i ncreasing the vehicles maneuverability at low vehicle speeds, e.g. when a milk vehicle has to drive into a farm or the like or when busses have to perform delicate maneuvers in traffic, e.g . in narrow streets. For the reasons mentioned above it is then of cou rse advantageous to ensure that the same toe-value is obtained for the wheels at both ends of said tag axle.

According to another embodiment of the invention constituting a fu rther development of the embodiment last mentioned said control unit is configured to control the align ment of said wheels of the tag axle on the basis of information about the align ment of the wheels of the front axle relative to said longitudinal direction of the vehicle. According to another embodiment of the invention said rod is a so called tie rod extending substantially in parallel with said axle beam or in parallel with said axle beam. The object of the invention is with respect to the method obtained by providing a method according to the appended independent method claim. The results of applying such a method and the advantages arising therefrom appear clearly from the above description of the system according to the i nvention .

The invention also relates to a computer prog ram having the featu res set out in claim 1 0, a computer prog ram product having the featu res set out in claim 1 1 , an electronic control unit having the featu res set out in claim 1 2, and a wheeled motor vehicle accord- ing to claim 1 3.

Other advantageous features and advantages of the i nvention appear from the description following below. BRI E F D ESC RI PTION OF TH E D RAW INGS

With reference to the appended drawings, below follows a specific description of embodiments of the invention cited as examples. In the drawings :

Fig . 1 is a simplified view illustrating a system for steering a tag axle of a vehicle to which the present invention may apply, Fig.2 is a simplified view from above very schematically illustrating a wheeled axle of a motor vehicle which may be steered through a system according to the invention, Fig.3 is a side view of the axle shown in Fig. 2 suspended in the chassis of a vehicle and schematically illustrating different embodiments of a system according to the invention, Fig.4 is a flow chart of the steps of a method according to an embodiment of the invention carried out for a steering system according to the invention, and

Fig.5 is a schematic view illustrating an electronic control unit for implementing methods according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION Fig. 1 illustrates very schematically a system for steering an un- powered wheeled tag axle 1 of a vehicle 2 in a form of a truck with a front axle 3 and a powered rear axle 4 situated ahead of the tag axle in the vehicles ordinary direction of movement. It is schematically illustrated how the front axle 3 can in a conven- tional way be steered by connection to the vehicle's steering wheel 5. A power means in the form of a power cylinder 6 is connected mechanically to the tag axle 1 so as to through a piston 7 of this cylinder control the alignment of the tag axle's wheels 8 relative to the longitudinal direction (arrow L) of the vehicle. It is illustrated how a pressure medium, normally in the form of hy- draulic liquid , can be conveyed to and from the power cylinder th rough lines 9 in order to control the align ment of the tag axle's wheels. This conveying of medium is controlled by a schematically indicated control unit 1 0 on the basis of information from the vehicles electronic control u nit E about the alignment of the wheels 1 1 of the front axle relative to the longitudi nal direction L of the vehicle, and the speed of the vehicle. This general lay out is typical of systems for steeri ng a tag axle of a vehicle. Said system has also an electric motor 1 2 adapted to run an hydraulic pu mp 1 3 in order, by choice of direction of rotation for the motor, to convey hydraulic liquid under pressure from an oil container 1 4 to either of two opposite sides 1 5, 1 6 of the piston of the power cylinder 6 in order to cause the piston to move and thereby alter the alig nment of the tag axles wheels as a result of the con nection of the piston rod 1 7 to a link for turning the wheels.

Fig . 2 shows schematically the tag axle 1 and how this comprises an axle beam 1 8 to opposite ends 1 9, 20 of which a wheel 21 , 22 is connected. A tie rod 23 extending in parallel with the axle beam have opposite ends 24, 25 con nected through a hinge 26, 27, such as in the form of a ball joint, to one end of a respective link 28, 29 having the other end connected to a said wheel 21 , 22 at a respective end 1 9, 20 of the axle beam. The power cylinder 6 is con nected to a link 35 con nected to the axle shaft of the wheel 21 and one end of a first 28 of said links is also connected to said axle shaft and the power cylinder 6 will by that act between the axle beam 1 8 and said fi rst link 28 at a first end 1 9 of the axle beam by varying its length by moving the piston thereof inside the cylinder. Thus, the power cylinder acts indirectly between the axle beam 1 8 and the link 28 con nected to the rod 23 by the action between the axle beam 1 8 and the link 35, since both these links 28, 35 are con nected to the axle shaft of the wheel 21 and will follow this wheel when it tu rns. Such movement will change the direction of the wheels 21 , 22 of the tag axle 1 with respect to a longitudinal direction L of the vehicle.

It is assumed that the ordi nary direction of movement of the ve- hide coincides with the direction of the arrow L and that the vehicle is in a neutral state in which it is to be driven in said longitudinal direction . I n this neutral state a so called toe-value is defined as the angle made by the wheels with said longitudinal direction , and it is shown that it is in this case a toe-in value which is strongly exaggerated in the figu re by showing the angles a (tan a is i n fact usually in the order of 1 0 ³ ) .

We do now assu me that a heavy load is supplied to the vehicle and transferred to the tag axle 1 and the axle beam 1 8 thereof, which will then be deflected . The tie rod 23 will not be bent by such a load since it only takes loads in the extension thereof, which means that the toe-value will be changed for the wheel 22 con nected to the link 29 to which the power cylinder 6 is not connected would the length of the power cylinder not be changed. However, the toe-in value of the wheel 21 on the opposite end 1 9 of the axle 1 will not be changed si nce the length of the power cylinder will not be changed because it has a length sensor with a predefined "straight-forward position". If nothing is done to remove this difference of toe-in value of the wheels 21 and 22 this will result in an increased tyre wear and that the vehicle tends to steer to one side when the driver try to steer it straig ht forwardly. The present invention takes care of this problem, and it is schematically illustrated i n Fig. 3 how this may be done. It is shown in Fig. 3 how the tag axle 1 is suspended i n a chassis of the vehicle through air suspension members comprising air bellows 31 . A pressure sensor 32 measu res the pressure inside a said air bellow and provides the control unit 1 0 with i nformation about this pressu re, which is dependent upon the load applied to the tag axle 1 . The control unit 1 0 is configu red to compare the load value so determined with stored data of change of a toe- value of the wheels of the tag axle as a fu nction of the load and to calculate the length to be assu med by the power cylinder 6 for obtaini ng the same toe-value for the wheels at both ends 1 9, 20 of the axle 1 . The control u nit is configu red to then control the power cylinder 6 to assu me this length calculated in the neutral state of the vehicle shown in Fig . 2. Thus, the power cylinder gets a new "straig ht-forward position" and this position means that the toe-in value will be equally distributed to the left and rig ht wheels This means that said problem arising f rom different toe-values of said wheels will be solved.

Other possibilities to obtain said load value needed for obtaining the same toe-value of the wheels 21 and 22 exist. A power sen- sor 33 may be arranged to provide information about the value of said load by measuring the power applied throug h said load on means suspending the axle 1 . Another option would be to arrange a position sensor 34 configured to sense the position of a poi nt or region of the axle beam with respect to a point on the chassis of the vehicle, e.g . how much the chassis moves vertical- ly with respect to said axle as a consequence of the load applied th rough the chassis on the tag axle.

Fig . 4 illustrates a flow chart of a method accordi ng to an embod- iment of the present invention carried out for steering the wheels of a wheeled axle of a motor vehicle as the one shown in Fig . 1 . The method is started with a step S _! of measu ring the pressure in an air bellow of a suspension system of the vehicle. The value of load on the axle beam of the tag axle is then determined in a step S ₂ through the information from said pressure measu rement. The load value determined is then in step S ₃ compared with stored data of toe-values as a fu nction of load value for calculating the length of the power means for having the same toe-value at both ends of the tag axle in a step S ₄ . Finally, the power means is in a step S ₅ controlled to obtain the length calculated in step S ₄ .

Computer prog ram code for implementing a method according to the invention is with advantage included i n a computer program which can be read into the internal memory of a computer, e.g. the internal memory of an electronic control unit of a motor vehicle. Such a computer program is with advantage provided via a computer program product comprising a data storage medium which can be read by a computer and which has the computer prog ram stored on it. Said data storage mediu m is for example an optical data storage mediu m i n the form of a C D ROM disc, a DVD disc etc. , a mag netic data storage mediu m in the form of a hard disc, a diskette, a cassette tape etc. , or a flash memory or a memory of the ROM, P ROM, E PROM or EE P ROM type. Fig. 5 illustrates very schematically an electronic control unit 1 0 com- prising an execution means 35, e.g. a central processor unit (CPU), for execution of computer software. The execution means 35 communicates with a memory 36, e.g. of the RAM type, via a data bus 37. The control unit 10 comprises also a non-transitory data storage medium 38, e.g. in the form of a flash memory or a memory of the ROM, PROM, EPROM or EEPROM type. The execution means 35 communicates with the data storage medium 38 via the data bus 37. A computer program comprising computer program code for implementing a method according to the inven- tion, e.g. in accordance with the embodiment illustrated in Fig.4, is stored on the data storage medium 38.

The invention is of course in no way restricted to the embodiments described above, since many possibilities for modifications thereof are likely to be obvious to one skilled in the art without having to derivate from the scope of invention defined in the appended claims.

It is shown in Fig. 2 how the piston rod of the power cylinder is connected to a separate link, but it may for instance be directly connected to said first link close to the tie rod.