The present invention, according to a first aspect thereof, relates to a vehicle comprising a chassis extending in a longitudinal direction, at least one tag axle extending substantially transversely to said longitudinal direction, which is provided with at least one wheel or on either side thereof, a braking system designed to brake the wheels of said at least one tag axle, and a brake steering system, by means of which the brake system is controlled in use. The term "tag axle" is a well-known term, which is used herein to indicate a wheel axle that is mounted for pivoting movement under a chassis, i.e. the axle can at least partially pivot and/or the wheels of the axle can pivot, which axle is provided with a self- tracking (at least during forward movement of the vehicle) steering system, a system which functions to effect a change in a steering angle of one or more wheels only when said wheels are subjected to forces and/or torques exerted thereon via contact of a tyre of the wheel with a ground surface. A tag axle may be configured in various ways, for example as a stub axle, a central axle portion of which remains oriented transversely to the longitudinal direction of the chassis whilst two axle portions, which are pivotally connected to said central axle portion on either side of said central portion, are capable of pivoting movement, is covered by this definition.

Semi-trailers and trailers that are towed by a towing vehicle are known, they differ from each other by the manner in which the semitrailer or the trailer is coupled to the towing vehicle. In those cases where this distinction is not or at least not very relevant for the present invention, semitrailer and trailers will also be indicated by the collective term "trailers" herein. Known trailers are known to have several variants of wheel axles. An important distinction for wheel axles at the rear of a trailer is that between forced-steering axles and tag axles. In the case of forced-steering axles, the pivoting of the axle is controlled by means of a direct actuator, such as a steering rod or a steering cylinder, on the basis of a sensor- measured value or by operating the steering wheel in the cabin of the towing vehicle. The sensor-measured value may for example relate to the position of the wheels of the towing vehicle, the angle between the towing vehicle and the trailer or the angle between the chassis and a front wheel axle of a trailer. The required extent of pivoting of the forced-steering axle is determined and set on the basis of said registered value. Said setting can be realised in various ways known to those skilled in the art. Forced-steering wheel axles for trailers are relatively costly and trouble- prone. A tag axle, on the other hand, can freely pivot and consequently is therefore also called a self-tracking axle, which adjusts itself to follow the direction of the towing vehicle, at least when moving in forward direction, i.e. when the trailer supported by the tag axle is being towed while a friction force is acting thereon. Tag axles are relatively inexpensive in comparison with forced-steering axles, and they are less trouble-prone. When reversing in a straight line, the tag axles of a combination comprising a trailer fitted with tag axles tend to pivot, however, and upon reversing through a bend, tag axles tend to pivot in the direction opposite the direction required for passing through the bend.

Accordingly it is an object of the present invention to provide a vehicle fitted with a tag axle as described in the introduction, which makes it possible in a relatively inexpensive manner to prevent the tag axle from pivoting undesirably or in an undesirable direction upon reversing. This object is accomplished by the present invention in that the brake steering system is designed to set the extent of pivoting of said at least one tag axle by braking the at least one wheel on one side of said at least one tag axle and the at least one wheel on the other side of said at least one tag axle independently of each other. Thus it is possible upon reversing to brake the at least one wheel on a desired side of the tag axle whilst not braking, or at least to a reduced extent, the at least one wheel on the other side of the tag axle, thereby causing the tag axle to pivot in the required direction. Braking of the wheel affects the contact between the tyre and the ground surface, and thus the force or the torque that acts on the wheel for changing the steering angle. The wheel axle in question is as such regarded herein as a tag axle according to the definition given above. The object of the invention is thus accomplished. Several braking systems for braking a wheel are known to those skilled in the art. Since the manner of braking is less important than the possibility to activate the braking systems of wheels on either side of a wheel axle independently of each other, for which various systems are also generally known, albeit for other applications such as an ABS system or an ESP system, for example, or for supporting the main steering system, a detailed description of a braking system will not be given herein.

US 2005/0209763 describes a method and apparatus for controlling a brake steering system of a car. Said document describes several variants of the manner in which wheels of a forced-steering axle and/or of a rigid axle of the towing vehicle can be jointly or individually braked so as to improve the steerability of a vehicle. US 2005/0209763 does not describe, however, the individual braking of wheels on either side of a tag axle of the vehicle when driving forward or reversing for the purpose of causing the tag axle to pivot in the required direction.

Swiss patent CH 443 012 describes a multi-axle trailer, one of which axles comprises a vertical pivot pin, which can be locked in position. Said axle is an axle which must be locked when driving forward in a position perpendicular to the draw bar by means of which the shaft is thus force-steered when driving forward.

Belgian patent BE 518303 describes a trailer, an axle of which is fixed in position relative to the A-frame when driving forward, so that said axle is force-steered by the A-frame

Belgian patent BE 524032 describes a two-wheel trailer comprising an axle with king pins, wherein rotation of the king pins is only used when reversing. Said document also describes a four-wheel trailer comprising an axle with rotary king pins. Also in this case the axle is an axle that is force-steered via the draw bar.

in a preferred embodiment of the invention, the at least one tag axle is a rear axle, i.e. seen in the forward driving direction of the vehicle, the at least one tag axle is behind the middle of the vehicle. When the vehicle drives in the reversed direction, the then (seen in the driving direction) front axle can be set by the brake system to drive the curve well.

Besides said at least one tag axle, the vehicle may comprise at least one further rigid or force-steered wheel axle extending substantially transversely to the longitudinal direction. Said further wheel axle may for example be the front axle of a trailer hitched to a towing vehicle by means of a pivotable draw bar.

Alternatively or additionally the vehicle may comprise two or more tag axles, the extent of pivoting of which can be set by braking at least one wheel on one side of said at least one tag axle and at least one wheel on the other side of said at least one tag axle independently of each other by means of the brake steering system. If more than one of such tag axles is used, the brake means on one side of the vehicle may be interconnected, so that they can be jointly controlled, for example if the pivot angles of the wheels are linked.

In a preferred embodiment of the present invention, the vehicle is a vehicle which is or at least is to be towed by a towing vehicle. In the introductory paragraphs the problems underlying the present invention, which are solved by the invention, and the advantages of the invention have already been discussed with reference to a known trailer or semitrailer.

Preferably, a sensor is provided for registering a state or a value on the basis of which, in use, the extent of pivoting of said at least one tag axle is set by means of the brake steering system. The value or state measured by the sensor can be communicated to the brake steering system, so that the vehicle's brake steering system can be actuated without interference of a driver.

In a simple yet effective embodiment, the sensor comprises an angle sensor at the coupling pin of the vehicle to be towed. Such a sensor is already known, for example for vehicles having a force-steered wheel axle. In the present invention, the output from such a sensor can be used as an input value for the brake steering system.

Also an embodiment comprising a sensor wherein the towed vehicle is a trailer having a pivoting front axle and wherein the sensor senses the pivot angle of the front axle relative to the chassis can be derived from a known trailer having a force-steered wheel axle. Several other methods are known by which a sensor can sense a value or a state that can be used as an input value for calculating the manner in which the brake steering system could be controlled for causing said at least one tag axle to pivot. A few examples in this connection are measuring an acceleration at the front side of the vehicle or the difference in velocity between the left-hand side and the right-hand side of the vehicle, measuring the lateral force exerted on the front side of the vehicle, measuring the pressure on the left and on the right in the steering cylinder, etc. It will be understood that the above is not an exhaustive enumeration of sensors. Furthermore, a monitoring sensor may be provided, for example in the form of a further angle sensor by means of which the pivoting of a wheel is monitored, or a position sensor by means of which the displacement of a point on the pivotable part of a tag axle relative to a static point on the vehicle, for example on the tag axle is measured, by means of which sensor it can be verified whether the required steering angle is reached. It stands to reason that also other suitable sensors may be used for this purpose.

The vehicle may be a motor-driven vehicle. The motor-driven vehicle may be an independent vehicle, but alternatively it may also be a towing vehicle. It is known to use one or more tag axles in addition to a front steering axle and a rigid or force-steered rear axle. The behaviour of said tag axles is essentially comparable to that of tag axles of trailers. Using the invention, the problem discussed in connection with trailers at the beginning of the present document can thus be solved in the same manner for such a motor-driven vehicle.

Preferably, operating means are provided by means of which the extent of pivoting of said at least one tag axle is set via the brake steering system. This makes it possible for an operator to set the brake steering system. If a sensor is provided as well, the steering by the sensor can be overruled by an operator via said operating means, if required.

The vehicle may be provided with locking means for locking said at least one tag axle against pivoting beyond a required pivot angle. It is possible in that regard to lock the axle against pivoting in one pivoting direction or in both pivoting directions. This is advantageous, for example, if the effect of external influences, such as an uneven ground surface, the presence of kerbstones or speed ramps is to be eliminated. It may also be advantageous if the axle can be locked in a straight-ahead position, for example when the vehicle must be reversed in a straight line. Furthermore it may be advantageous if, when driving forward, the steering angle is limited, using the locking means, in dependence on the vehicle speed.

Preferably, a steering angle sensor is provided on said at least one tag axle for measuring the steering angle of said at least one tag axle relative to the vehicle chassis.

The present invention will now be explained in more detail by means of a description of an embodiment with reference to the appended drawings, in which:

Figure 1a is a schematic top plan view of an embodiment of a tractor with a semitrailer according to the present invention;

Figure 1b is a schematic top plan view of an alternative embodiment of a tractor with a semitrailer according to the present invention;

Figure 1c is a schematic top plan view of another alternative embodiment of a tractor with a semitrailer according to the present invention;

Figure 1d is a schematic top plan view of a tractor with a trailer according to the present invention;

Figure 1e is a schematic top plan view of a motor-driven vehicle according to the present invention; Figure 2 is a schematic circuit for controlling the brake steering system of a vehicle according to the present invention;

Figure 3a schematically shows the behaviour of a tag axle with a deactivated brake steering system while reversing;

Figure 3b schematically shows the behaviour of a tag axle with an activated brake steering system while reversing; and

Figure 4 is a schematic side view of a wheel and a pivotable axle part.

Referring now to figure 1a, there is shown a schematic top plan view of a trailer truck consisting of a tractor 1 and a semitrailer 2. The tractor 1 has two wheel axles, viz. a front steering axle (not shown) and a rear axle 3. The semitrailer 2 is conventionally coupled to the tractor 1 by means of a kingpin 4 and a turntable 5. The semitrailer 2 has a rigid wheel axle 6 and a standard tag axle 7. Within the terms used herein, the tag axle 7 comprises a central axle part 9 connected to the chassis 8 of the semitrailer 2, which axle part essentially does not pivot and which is provided with pivoting parts 10, cylinders 11 and a stabilisation rod 12 at both ends. The cylinders 11 function to dampen reactions of the pivoting parts 10 and the wheels 13. Because the central axis that passes through the pivot point 2 of a wheel 13 (see figure 4) intersects the road surface in front of the contact point of the wheel 13 in question (which is characteristic for a tag axle), at least seen in the driving direction, the wheel 13, and consequently the tag axle 7, will pivot in the required direction when driving forward. The trailer truck is provided with a brake steering system, which is not shown in this figure, but which will be separately discussed with reference to figure 2.

Figure 1b shows an alternative embodiment of a tractor 101 and a semitrailer 102. The tractor 101 has a steering axle (not shown) and a rear axle 103. The trailer 102 is conventionally coupled to the tractor 101 by means of a kingpin 104 and a turntable 1055. The semitrailer 102 has a rigid wheel axle 106 and a standard tag axle 107, which is embodied as a stub axle in this embodiment. The central axle part 109 is essentially non-pivotally connected to the chassis 108 and is provided with a pivoting part 110 at both ends, by means of which the wheels 113 can be pivoted under the chassis 108. Because the central axis that passes through the pivot point Z of a wheel 13 intersects the road surface in front of the contact point of the wheel 13 in question, at least seen in the driving direction, the wheel 13, and consequently the tag axle 7, will pivot in the required direction when driving forward also in this embodiment. This trailer truck, too, is provided with a brake steering system, which is not shown in this figure, but which will be separately discussed with reference to figure 2.

Figure 1c shows another alternative embodiment of a trailer truck made up of a tractor 201 and a semitrailer 202. The semitrailer 202 has a rigid axle 206 and a tag axle 207, which is pivotally connected to the chassis 208 via a turntable 210. Because the central axis through the pivot point Z of a wheel 13 intersects the road surface in front of the contact point of the wheel 13 in question, at least seen in the driving direction, in this embodiment as well, the wheel 13, and consequently the tag axle 7, will pivot in the required direction in this embodiment as well. This trailer truck, too, is provided with a brake steering system, which is not shown in this figure, but which will be separately described with reference to figure 2.

Figure 1d is a schematic top plan view of a trailer truck made up of a tractor 301 and a trailer 302. The trailer 302 is connected to the tractor 301 by means of a draw bar 304. At the front side, the trailer 302 has a front axle 303 and at the rear side it has a rigid axle 306 of the standard type, which has been explained in combination with a semitrailer with reference to figure 1a. Tag axle 307 may also be of a different type, for instance of the type described with reference to figures 1b or 1c, but it will behave in the same manner. This trailer truck, too, is provided with a brake steering system, which is not shown in this figure, but which will be separately described with reference to figure 2.

Figure 1e shows a motor-driven vehicle 401 having a front steering axle (not shown) and, at the rear side, a rigid axle 406 and a tag axle 407 of the type comprising a stub axle as described above with reference to the trailer truck. This motor-driven vehicle 401 , too, is provided with a brake steering system, which is not shown in this figure, but which will be separately described with reference to figure 2. The motor-driven vehicle may be provided with a loading platform, of course.

The operation of the brake steering system that will be explained below with reference to figure 2, is essentially the same in all cases, regardless of the embodiment of the motor-driven vehicle or the trailer truck and regardless of the embodiment of the tag axle. The present invention may also be implemented with a tag axle of a type different from the types shown so far. Figure 2 is a schematic circuit diagram showing the brake steering system of vehicles according to the present invention, for example as described with reference to figures 1a-1e, and will be explained in relation to a semitrailer 2 as shown in figure 1. The semitrailer 1 is schematically represented as a chassis 8, a kingpin 4, a rigid axle 4 and a tag axle 7. An arithmetic module 20 receives input from a kingpin sensor 21 , a left-hand brake sensor 22, a right-hand brake sensor 23, an angle sensor 24 and a bellows pressure sensor 25. On the basis of the aforesaid input data, the arithmetic module 20 determines the required output to a left-hand locking device 26 for locking the tag axle against pivoting to the right, a right-hand locking device 27 for locking the tag axle 7 against pivoting to the left, a valve 28 for reducing the brake force on the right-hand wheel 13r, a valve 29 for increasing the brake force on the right-hand wheel 13r, a valve 30 for reducing the brake force on the left-hand wheel 131, and a valve 31 for increasing the brake force on the left- hand wheel 131.

Provided near the wheels 13 are brake cylinders 14. In this embodiment, said brake cylinders are conventional parking brake cylinders, and a parking brake chamber 15 is used for influencing the brake force. One of the features of said parking brake chamber 15 is that the air pressure in the parking brake chamber 15 must be reduced for increasing the brake force on a wheel.

The required steering angle of the tag axle 7 is determined by means of the kingpin sensor 21 , which measures the angle of the tractor (not shown in figure 2) relative to the trailer 2. The required extent of pivoting of the tag axle 7 is a function of a reference steering angle that is determined by a reference sensor. Said function depends on the geometry of the vehicle and will vary with each vehicle.

Using the angle sensor 24, for example of the Hall effect sensor type, on the pivoting part of the tag axle 7, the actual extent of pivoting relative to the non-pivoting central part of the tag axle 7 or relative to the chassis 8 is measured.

If there is a difference between the required extent of pivoting and the measured extent of pivoting, the brake force of one of the brake cylinders 14 is increased to such an extent that the deviation from the required extent of pivoting will decrease. For example: if the wheels 13 pivot too much to the right, the brake force on the left-hand brake cylinder 14 will be increased, causing the wheels 13 to pivot to the left again (when reversing). The pressure sensors 22, 23 are used to check whether the required brake pressure is reached. When the required brake pressure is reached, air is no longer supplied from the parking brake chamber 15 to a brake cylinder 14 or discharged therefrom. The supply and discharge of air to/from the brake cylinder 14 is effected by actuating air valves 28-31. Once the deviation has been reduced to the required level, the brake force will be reduced again.

Disposed behind the right-hand wheel 13r is a spring bellows 16. The pressure in the spring bellows 16 is measured by means of the bellows pressure sensor 25 so as to realise a more precise determination of the required brake force for a specific deviation from the pivoting angle. The required brake force will vary in dependence on the load. A fully loaded vehicle will require more brake force.

The description below of figures 3a and 3b also applies to other combinations (shown or not shown) of a vehicle having a tag axle and a brake steering system for braking the wheels upon reversing.

Figure 3a is a schematic top plan view of the combination of a tractor 1 and a semitrailer 2 shown in figure 1a reversing round a bend with the brake steering system deactivated. The wheels 13 of the tag axle 7 pivot in the wrong direction. The pivot points 2, whose axes are located in front of the contact points of the wheels 13 with a ground surface upon when driving forward, so that the tag axle 7 is pulled ahead, are located behind the contact points in question, at least seen in the driving direction, upon reversing. As a result, the wheels 13 are pushed sideways when reversing. The tag axle 7 no longer behaves as such in that case. Because of the resistance that is encountered upon reversing, the tag axle 7 tends to steer the wheels 13 against the direction required for passing through the bend. This is not conducive to realising a a smooth passage through a bend.

Figure 3b is a schematic top plan view of the combination of a tractor 1 and a semitrailer 2 of figure 1a reversing through a bend with the brake steering system deactivated. The brake steering system brakes the right-hand wheel 13r of the tag axle 7, in the manner described above with reference to figure 2, whereas the left-hand wheel 131 is not braked. The left-hand wheel 131 thus moves at a higher speed than the right-hand wheel 13r, as a result of which the tag axle 7, which after all is a self-tracking axle, will pivot in the required direction shown in figure 3b. The angle sensor 24 senses the angle of the wheel 13r and the central 2). When the wheels 13 are in the required position, the brake steering system releases the brake of the right-hand wheel 13r again and the arithmetic module can activate the blocking device (see figure 2).

Figure 4, to conclude, is a schematic side view of the inner side of a wheel 113 and an associated pivotable part 110 of a stub axie. The pivotable part 1 10 has an axis h which extends diagonally through the pivot point Z and which, seen in the Iongttudinal direction, intersects the road surface 114 at point S in front of the contact point C of the tyre 115 with the road surface. The forward driving direction of the wheel 113 is indicated by an arrow in figure 4.

The present invention has only been explained on the basis of a few embodiments in the figures and the above description. It will be understood, however, that many variants, which may or may not be obvious to those skilled in the art, are conceivable, which variants fall within the scope of the present invention as defined in the appended claims. Thus it is possible, for example, for the brake steering system to be activated either manually by a driver of the vehicle or automatically when the vehicle is put in reverse. When the deviation to the left or to the right becomes too large, pivoting to one direction or to both directions can be blocked by means of one or both blocking cylinders (or other blocking means), so that the deviation cannot increase. Furthermore it is possible, for example, to provide a raisable tag axle with a brake steering system according to the invention. Aligning means may be provided in that case, which align the tag axle in the straight- ahead position in the raised, unloaded condition thereof. Furthermore, the figures and the description are in all cases based on the use of a separate brake steering system for effecting pivoting movement of a tag axle. It is conceivable, however, to adapt a conventional brake system for a vehicle such that said brake system can be extended with a brake steering system for effecting pivoting movement of a tag axle according to the invention.