TECHNICAL FIELD

The present disclosure relates in general to a method for controlling a tag axle steering system. The present disclosure further relates in general to a control device configured to control a tag axle steering system. The present disclosure further relates in general to a computer program and a computer-readable medium. Moreover, the present disclosure relates in general to a vehicle.

BACKGROUND

Vehicles adapted to transport heavy loads, such as trucks, semis, buses, dumpers, haulers, trailers, often exert large pressure on the ground as a result of their weight, especially when transporting a load. To handle this pressure, such vehicles may often comprise one or more tag axles, each comprising a pair of tag axle wheels. This decreases the pressure each wheel of the vehicle has to transfer to the ground.

A tag axle is an unpowered, i.e. non-driven, axle, and may be arranged at various positions in the vehicle. A tag axle arranged in front of a driven axle, when seen in the primary direction of travel of the vehicle, is often referred to as a pusher axle or a pusher tag axle. A tag axle arranged behind the driven axle is often referred to as a trailing axle or trailing tag axle. The tag axle may often be liftable to enable raising the tag axle wheels from the ground when not needed, for example when the vehicle carries no load.

In order to improve maneuverability of the vehicle, the tag axle may be a steerable tag axle. This means that the tag axle wheels may be steered. Thereby, it is for example possible to obtain a more appropriate turning radius of the vehicle. Furthermore, the ability to steer tag axle wheels may also reduce the tyre wear of the tag axle wheels. The steering of the tag axle wheels may be controlled by a tag axle steering system. Steering of the tag axle wheels may be performed in dependence of the steering of the front wheels of the vehicle. However, the possibility to steer the tag axle wheel has certain limitations and is naturally dependent of proper functioning of the tag axle steering system. If a failure of the tag axle steering system would occur, there will be a loss in the maneuverability of the vehicle, which could have a detrimental effect. To overcome said risk, certain failure modes for operation of the tag axle steering system have been developed. In general, locking the tag axle wheels in a neutral position where the tag axle wheels are aligned with the longitudinal extension of the vehicle is considered to be the safest mode. Thereby, the vehicle will still behave as may be expected by a driver of the vehicle.

However, in certain cases, it may be difficult to bring the tag axle wheels into the neutral position. Therefore, another example of a failure mode comprises releasing the tag axle wheels to caster steer. In other words, the tag axle steering system is put in a caster steer mode. In such a mode, the tag axle wheels are released and will just follow the vehicle, and the tag axle wheels are thereby inherently steered by the lateral forces to which they are subjected. If the caster angle of the tag axle wheels (i.e. the angular displacement of the steering axis from the vertical axis of the wheel) is positive, the caster steered tag axle wheels will thereby self-center such that they eventually become aligned with the longitudinal extension of the vehicle, i.e. reach the neutral position. When the tag axle wheels are aligned with the direction of travel of the vehicle, the tag axle wheels may suitably be locked in that position (the neutral position). In other words, the failure mode may, after releasing to caster steer, further comprise locking the tag axle wheels in the neutral position when the tag axle wheels have reached said position. Alternatively, the tag axle wheels may remain in caster steer mode, if desired, even if they have reached the neutral position.

WO 2014/163560 Al discloses a system for steering at least one unpowered wheeled tag axle of a vehicle. The system comprises an ancillary unit adapted, if the vehicle's speed is below a threshold speed when a malfunction of the system occurs, to switching the system to a first fail-safe state with caster steering of the tag axle, but if the vehicle is above the threshold speed when a malfunction of the system occurs, to keeping the tag axle locked with its wheels in a fixed neutral position aligned with the vehicle's longitudinal direction in a second fail-safe state of the system.

SE 1650840 Al discloses an articulated vehicle comprising an articulation joint and an articulation joint sensor member configured to measure at least one articulation parameter representing the articulation state of the articulation joint. The vehicle further comprises a steerable tag axle controlled by a tag axle control unit. The tag axle control unit is configured to receive an articulation signal generated by the articulation joint sensor member, analyze articulation parameter value(s) in relation to a set of vehicle control rules, and to determine tag axle steering commands in dependence of the result of the analysis. SUMMARY

The object of the present invention is to further improve safety and maneuverability of a vehicle comprising a steerable tag axle.

The object is achieved by the subject-matter of the appended independent claims.

In accordance with the present disclosure, a method, performed by a control device, for controlling a tag axle steering system is provided. Said tag axle steering system is configured to actively steer tag axle wheels of a tag axle of a vehicle. The method comprises a step of: in response to an indication that the vehicle in which the tag axle is arranged is, or is intended to be, driven backwards while the tag axle steering system is operated in a caster steer mode, locking the position of the tag axle wheels at a current steering angle of the tag axle wheels.

The step of locking the position of the tag axle wheels at the current steering angle of the tag axle wheels will thereby temporarily or permanently end the caster steer mode. Moreover, since the respective position of the tag axle wheels are locked, they are no longer able to turn left or right by lateral forces to which they may be subjected. Locking the position of the tag axle wheels ensures that the vehicle, when steered (by for example a driver), will behave in a predictable manner irrespectively of the caster angle of the tag axle wheels. The fact that the vehicle will behave in a predictable manner therefore also allows for the vehicle to safely be reversed. In order words, the present method enables allowing the vehicle to be reversed directly from a situation at which the tag axle steering system is operated in a caster steer mode.

In contrast, it has previously not been considered safe to allow the vehicle to be reversed if the tag axle steering system is operated in a caster steer mode since the vehicle may not behave in a manner as predicted by for example a driver or a remote control center. Therefore, the possibility for reversing the vehicle in such cases has previously been inhibited/prohibited or the possibility considerably limited. This may for example cause considerable concerns when seeking to park the vehicle or drive in limited spaces where it may be necessary to drive back and forth. However, by means of the present method, inhibiting driving backwards is no longer necessary and the maneuverability of the vehicle is therefore improved.

The step of locking the position of the tag axle wheels at the current steering angle of the tag axle wheels may be performed when the tag axle steering system is operated in a failure mode. When the tag axle steering system is operated in a failure mode, the tag axle wheels cannot be actively steered by the tag axle steering system. More specifically, the tag axle wheels cannot be actively steered by the tag axle steering system into a neutral position. Therefore, when the tag axle steering system is operated in a failure mode, the maneuverability of the vehicle is inherently reduced as a result of the inability to steer the tag axle wheels. The above-mentioned caster steer mode may be a part of said failure mode. More specifically, the caster steer mode may be used for the purpose of reducing the effect of the reduced maneuverability of the vehicle in case of malfunction of the tag axle steering system.

The indication that the vehicle in which the tag axle is arranged is, or is intended to be, driven backwards may be derived from at least one wheel sensor of the vehicle, a gearbox management system of the vehicle, an engine management system of the vehicle, a sensor configured to determine a direction of travel of the vehicle, and/or a positioning system configured to determine the geographical position of the vehicle. Thereby, a reliable indication of the current direction of travel and/or intended direction of travel (forward or backward) of the vehicle may be achieved.

The method may further comprise a step of, when the tag axle wheels are locked at said current steering angle of the tag axle wheels and in response to a determination that the vehicle in which the tag axle is arranged is, or is intended to be, driven forward, releasing the tag axle wheels so that the tag axle steering system is operated in caster steer mode. In other words, the tag axle steering system may in such a case be returned to caster steer mode as used before the reverse driving. Thereby, the maneuverability of the vehicle may be further improved as the caster steer mode may allow the tag axle wheels to self-center when the vehicle is driven forward, i.e. in the primary direction of travel of the vehicle.

The present method is particularly useful when the tag axle wheels of the tag axle are provided at a positive caster angle when the tag axle steering system is operated in the caster steer mode. In other words, the method is particularly useful in cases where the tag axle wheels have a positive caster angle as seen in the primary direction of travel of the vehicle (i.e. when driving forward). In such instances, the caster angle would be negative when the vehicle instead is reversing. If the tag axle steering system is operated in caster steer mode and the caster angle is negative (as a result of the vehicle being reversed), the lateral forces to which the tag axle wheels may be subjected as a result of the movement of the vehicle would steer the tag axle wheels in an uncontrolled way, and may result in the tag axle wheels being turned to maximum right or left. This in turn may lead to a catastrophic result. Therefore, driving backwards has generally not been considered possible when the tag axle steering system is operated in caster steer mode and has thus in accordance with the prior art been inhibited or prohibited as mentioned above. The present method overcomes this obstacle by the above-mentioned step of locking the position of the tag axle wheels at the current steering angle of the tag axle wheels. Due to said locking, the vehicle may be reversed in a safe manner as the behavior of the vehicle may be essentially as predicted by, for example, a driver of the vehicle.

The tag axle steering system may suitably be an electrohydraulic tag axle steering system or an electric tag axle steering system.

The present disclosure also provides a computer program comprising instructions which, when executed by a control device, cause the control device to carry out the method as described above.

The present disclosure also provides a computer-readable medium comprising instructions which, when executed by a control device, cause the control device to carry out the method as described above.

Furthermore, the present disclosure provides a control device configured to control a tag axle steering system, said tag axle steering system being configured to actively steer tag axle wheels of a tag axle of a vehicle. The control device is configured to, in response to an indication that the vehicle in which the tag axle is arranged is, or is intended to be, driven backwards while the tag axle steering system is operated in a caster steer mode, lock the position of the tag axle wheels at a current steering angle of the tag axle wheels.

The control device provides the same advantages as described above with regard to the herein disclosed method for controlling a tag axle steering system.

The control device may further be configured to, in response to a fault signal indicating malfunction of the tag axle steering system, controlling the tag axle steering system to be operated in a failure more. If so, the tag axle steering system may, in said failure mode, be operated in said caster steer mode, when one or more predetermined conditions are met.

The control device may further be configured to, when the tag axle wheels are locked at said current steering angle of the tag axle wheel and in response to a determination that the vehicle in which the tag axle is arranged is, or is intended to be, driven forward, controlling the tag axle to be operated in caster steer mode by releasing the position of the tag axle wheels.

The present disclosure further provides a vehicle comprising a tag axle steering system configured to actively steer tag axle wheels of a tag axle arranged in the vehicle. The vehicle further comprises the control device described above. The vehicle may be a heavy vehicle, such as a bus or a truck, but is not limited thereto. Moreover, the vehicle may be an articulated vehicle, if desired.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 schematically illustrates a side view of a first example of a vehicle,

Fig. 2 schematically illustrates a top view of a second example of a vehicle.

Fig. 3a schematically represents a top view of wheel axles of an example vehicle showing tag axle wheels in a neutral position,

Fig. 3b schematically represents a top view of wheel axles of an example vehicle showing tag axle wheels in a position other than a neutral position,

Fig. 4a schematically illustrates a steerable wheel of a vehicle, said wheel provided at a negative caster angle,

Fig. 4b schematically illustrates a steerable wheel of a vehicle, said wheel provided at a positive caste angle,

Fig. 5 represents a flowchart schematically illustrating a first exemplifying embodiment of the method for controlling a tag axle steering system in accordance with the present disclosure,

Fig. 6 represents a flowchart schematically illustrating a second exemplifying embodiment of the method for controlling a tag axle steering system in accordance with the present disclosure, Fig. 7 schematically illustrates a device that may constitute, comprise or be a part of a control device configured to control a tag axle steering system.

DETAILED DESCRIPTION

The invention will be described in more detail below with reference to exemplifying embodiments and the accompanying drawings. The invention is however not limited to the exemplifying embodiments discussed and/or shown in the drawings, but may be varied within the scope of the appended claims. Furthermore, the drawings shall not be considered drawn to scale as some features may be exaggerated in order to more clearly illustrate the invention or features thereof.

In the present disclosure, a steered or steerable tag axle should be considered to mean to comprise the tag axle including, at least when the vehicle in which the tag axle is fitted is in operation, its tag axle wheels, as well as the tag axle steering system configured to steer the tag axle wheels of said tag axle.

Furthermore, when it is described herein that the tag axle wheels are locked in a position (the position defined by steering angle of the tag axle wheels), this means that the tag axle wheels cannot be turned left or right by lateral forces or by the tag axle steering system. In contrast, in a caster steer mode, the tag axle wheels are not in a locked position and are therefore able to turn left or right by the lateral forces they may be subjected to as a result of the vehicle movement. It should however be noted that, in the caster steer mode, the tag axle wheels are not actively steered by the tag axle steering system.

The tag axle steering system described in the present disclosure is a tag axle steering system configured to actively steer tag axle wheels. In the present disclosure, the term "actively steer" is used to distinguish from steering systems of so called self-steering tag axles, wherein the tag axle wheels are merely passively steered. Generally speaking, self-steering tag axles rely on self-steering axle suspension systems wherein only the pitch or caster angle of the wheels is adjusted such that the drag of the wheels as the vehicle proceeds in the forward direction causes the wheels to steer automatically. There is no controller configured to actively control the steering of the tag axle wheels in such tag axles. In contrast, the tag axle steering system described herein is configured to actively control the steering of the tag axle wheels. Moreover, when a driving direction of a vehicle is described herein as forward or backward (or reversed), this should be regarded is relation to the primary direction for travel intended for the vehicle. Forward is thus intended to mean in the primary direction of travel of the vehicle. However, the term driving forward shall not be considered to mean that the vehicle is driving straight forward unless explicitly disclosed so. It should be recognized that even though the vehicle may be driven forward or backwards, it may simultaneously be turning.

Moreover, a caster angle is the angular displacement of the steering axis from the vertical axis of a steered wheel as seen from a side view of a vehicle. At a positive caster angle, the steering axis is angled such that a central axis of the steering axis intersects the road surface ahead of the center of contact of the wheel on the road surface as seen in the direction of travel of the vehicle comprising the wheel. It should be noted that center of contact of wheel on the road surface is here only considered in the direction of travel of the vehicle, and not in a direction perpendicular to the direction of travel (i.e. not in a direction parallel to the rotational axis of the wheel). In contrast, at a negative caster angle, the steering axis is angled such that a central axis thereof intersects the road surface behind the center of contact of the wheel on the road surface as seen in the direction of travel. A configuration wherein the caster angle is 0” (i.e. neither positive nor negative) is often denominated vertical caster. In such a case, the center axis of the steering axle will intersect the road surface essentially at the center of contact of the wheel on the road surface seen in direction of travel of the vehicle.

The tag axle steering system discussed in the present disclosure may be a tag axle steering system configured to steer tag axle wheels of a tag axle fitted in any type of land-based vehicle, in particular a heavy vehicle, including for example a truck, a bus, or a trailer. The vehicle may or may not be an articulated vehicle. An articulated vehicle is a vehicle comprising a pivot joint (articulation joint) that allows the vehicle to turn more sharply. Moreover, such a tag axle may be arranged anywhere in the vehicle and may thus be a pusher tag axle or a trailing tag axle. It should also be noted that a vehicle may be fitted with more than one steerable tag axle, if desired.

The present disclosure provides a method for controlling a tag axle steering system, said tag axle steering system being configured to actively steer tag axle wheels of a tag axle of a vehicle. The tag axle steering system may for example be configured to steer tag axle wheels of a tag axle in dependence of information obtained from a front axle steering system of the vehicle, said front axle steering system being configured to steer front wheels of the vehicle, and/or an articulation control system of the vehicle, said articulation control system being configured to monitor and/or control at least one articulation parameter representing an articulation state of the vehicle. In other words, the tag axle steering system may be configured to steer tag axle wheels of a tag axle in dependence of information from a front axle steering system of the vehicle, wherein said front axle steering system is configured to steer front wheels of the vehicle. Alternatively, or additionally, and if the tag axle is arranged in an articulated vehicle, the tag axle steering system may be configured to steer the tag axle wheels of the tag axle in dependence of information obtained from an articulation control system of the vehicle, wherein said articulation control system is configured to monitor and/or control at least one articulation parameter representing an articulation state of the vehicle.

The present method for controlling a tag axle steering system comprises a step of, in response to an indication that the vehicle (in which the tag axle is arranged) is, or is intended to be, driven backwards while the tag axle steering system is operated in a caster steer mode, locking the position of the tag axle wheels at a current steering angle of the tag axle wheels. In other words, the method comprises, at a point in time during which the tag axle steering system is operated in a caster steer mode, locking the position of the tag axle wheels at the current steering angle of the tag axle wheels in response to an indication that the vehicle (in which the tag axle is arranged) is, or is intended to be, driven backwards. Said step of locking the tag axle wheels at the current steering angle of the tag axle wheels may be performed independently of the actual value of the current steering angle of the tag axle wheels. In fact, the current steering angle of the tag axle wheels need not be known. Locking the tag axle wheels at the current steering angle also inherently means that the caster steer mode will be interrupted or ended. Thus, said step results in the tag axle wheels no longer being able to turn by the lateral forces to which they may be subjected.

The present method has primarily been developed for use when the tag axle steering system is operated in caster steer mode as a result of being operated in failure mode. The tag axle steering system may be operated in such a failure mode in response to an indication of malfunction of the tag axle steering system. It should here be recognized that the failure mode may comprise further constituent modes in addition to the caster steer mode. Another example of a constituent mode may be locking the tag axle wheels in the neutral position. The activation of different constituent modes of the failure mode may be dependent of the particular conditions and thus various parameters, for example steering angle of the tag axle wheels and/or the travelling speed of the vehicle in which the tag axle is arranged. It should however be noted that the present method may also be used in other situations where the tag axle steering system is operated in a caster steer mode (i.e. where the tag axle wheels are released to caster steer), and thus not only as a result of the tag axle steering system being operated in a failure mode. In other words, the step of locking the position of the tag axle wheels at the current steering angle of the tag axle wheels may be performed when the tag axle steering system is operated in a failure mode. The tag axle steering system may for example be operated in a failure mode due to a fault signal indicating a malfunction of the tag axle steering system having been generated and the tag axle steering system therefore being put in failure mode. The tag axle steering system may be operated in the caster steer mode as a result of being operated in the failure mode. In other words, said caster steer mode may be a part of the failure mode.

The above-mentioned indication that the vehicle (in which the tag axle is arranged) is, or is intended to be driven backwards may be derived from any previously known device or system of the vehicle which is capable of determining a driving direction of the vehicle. Suitably, said indication that the vehicle is, or is intended to be, driven backwards may be derived from at least one wheel sensor of the vehicle, a gearbox management system of the vehicle, and/or an engine management system of the vehicle. Said indication may alternatively be derived by usage of a positioning system configured to determine the geographical position of a vehicle at point in time. Such a positioning system may for example comprise a global positioning system (GPS) or the like. Alternatively, or additionally, such a positioning system may comprise one or more detection devices configured to detect the position of the vehicle, wherein such detection devices are arranged in the infrastructure where the vehicle is intended to be at least temporarily be present. The indication that the vehicle is, or is intended to be driven backwards may alternatively, or additionally, be derived from other types of sensors which may be used for determining a direction of travel of the vehicle, for example a gyroscope.

As an example, an indication that the vehicle is, or is intended to be, driven backwards may be derived from a gearbox management system by information regarding a current engagement of a reverse gear or a detection of a request for engagement of a reverse gear. As another example, said indication that the vehicle is driven backwards (reversed) may be derived from one or more wheel sensors of the vehicle by determining a rotational direction of one or more wheel to which the one or more wheels is arranged (an comparing this to a rotational direction corresponding to forward driving of the vehicle). The wheel sensors may be associated with any wheel of the vehicle, such as a front wheel, a driven wheel or even a tag axle wheel. Similarly, the indication that the vehicle is, or is intended to be, driven backwards may for example be derived from an engine management system configured to control an electric machine configured to drive the vehicle, the rotational direction of the electric machine corresponding to a certain direction of travel of the vehicle (optionally in combination with information from a gearbox management system of a currently engaged gear in case the vehicle comprises a gearbox). A request for a rotation of the electric machine in a certain direction or an actual rotation of the electric machine in a certain direction may thus be indicative of the intended (or current) direction of travel of the vehicle. Moreover, for example a positioning system configured to determine a geographical position of a vehicle may be used for determining in which direction a vehicle is driven by comparing two determined consecutive geographical positions of the vehicle (such consecutive geographical positions determined at two consecutive points in time). Alternatively, a positioning system may be used for determining a direction of travel of a vehicle when combined map data and knowledge of how the vehicle (in the longitudinal direction) is oriented in relation to said map data and an intended destination.

The method may further comprise a step of releasing the tag axle wheels to caste steer in case the vehicle is, or is about to be, driven forward again. In other words, the method may comprise a step of, when the tag axle wheels are locked at said current steering angle of the tag axle wheels and in response to a determination that the vehicle (in which the tag axle is arranged) is, or is intended to be, driven forward, releasing the tag axle wheels so that the tag axle steering system is operated in caster steer mode. Thereby, the tag axle wheels may be allowed to straighten up and self-align as a result of the movement of the vehicle and the maneuverability is thereby further improved. In case there is a new indication of the vehicle being driven, or intended to be driven, backwards, the method may again comprise a step of locking the tag axle wheels at the current steering angle. This may for example be suitable in situations where the vehicle is operated in narrow spaces, during parking or the like, requiring multiple changes in the direction of travel of the vehicle.

The herein described method for controlling a tag axle steering system may be used with any type of previously known tag axle steering system configured to actively steer tag axle wheels of the tag axle, but would in practice probably be most useful if the tag axle steering system is an electrohydraulic tag axle steering system or an electric tag axle steering system. The tag axle steering system may be a steer-by-wire tag axle steering system. One example of an electrohydraulic tag axle steering system wherein the present method may be used is described in the previously mentioned WO 2014/163560 Al.

Furthermore, the performance of the herein described method for controlling a tag axle steering system may be governed by programmed instructions. These programmed instructions typically take the form of a computer program which, when executed in or by a control device, cause the control device to effect desired forms of control action. Such instructions may typically be stored on a computer-readable medium. Furthermore, the present disclosure provides a control device configured to perform the abovedescribed method for controlling a tag axle steering system. The control device may be configured to perform any one of the steps of the method for controlling a tag axle steering system as described herein.

More specifically, the present disclosure provides a control device configured to control a tag axle steering system, said tag axle steering system being configured to actively steer tag axle wheels of a tag axle of a vehicle. The tag axle steering system may for example be configured to steer tag axle wheels of a tag axle in dependence of information obtained from a front axle steering system of a vehicle, wherein said front axle steering system is configured to steer front wheels of the vehicle. If the tag axle is arranged in an articulated vehicle, the tag axle steering system may additionally or alternatively be configured to steer the tag axle wheels of the tag axle in dependence of information obtained from an articulation control system of the vehicle, wherein said articulation control system is configured to monitor and/or control at least one articulation parameter representing an articulation state of the vehicle.

The control device is configured to, if the tag axle steering system is operated in caster steer mode, lock the position of the tag axle wheels at the current steering angle of the tag axle wheels in response to an indication that the vehicle (in which the tag axle is arranged) is, or is intended to be, driven backwards (i.e. reversed).

The control device may further be configured to, in response to a fault signal indicating malfunction of the tag axle steering system controlling the tag axle steering system to be operated in a failure mode. Said failure mode may comprise a number of different constituent modes which are selected depending on the circumstances. More specifically, said failure mode may comprise operating the tag axle steering system in the above-mentioned caster steer mode. The control device may be configured to control the tag axle steering mode, in accordance with the failure mode, in said caster steer mode for example in dependence of vehicle speed and/or steering angle of the tag axle wheels when the failure mode is initiated. In other words, control device may be configured to control the tag axle steering system so as to be operated (as a consequence of said failure mode) in caster steer mode of one or more predetermined conditions are met, wherein such predetermined conditions may relate to traveling speed of the vehicle and/or current steering angle of the tag axle wheels. For example, the caster steer mode may be selected in case the vehicle speed is below a predetermined vehicle speed threshold and/or if the current steering angle of the tag axle wheels is outside of a predetermined range of steering angles around the steering angle which corresponds to the neutral position of the tag axle wheels.

The control device configured to control the tag axle steering system may comprise one or more control units. In case the control device comprises a plurality of control units, each control unit may be configured to control a certain function or a certain function may be divided between different control units. The control device may be arranged in the vehicle, and thus be a part of the vehicle. The control device may be a part of the tag axle steering system as such. Alternatively, the control device may be separate from the tag axle steering system but configured to communicate therewith for the purpose of performing various control actions.

The present disclosure further provides a vehicle comprising a steerable tag axle and the control device described above. The vehicle may for example be a land-based heavy vehicle, such as a truck, a bus, a trailer or the like.

Figure 1 schematically illustrates a side view of a first example of a vehicle 1. The vehicle 1 comprises a powertrain 2 including a propulsion unit, for example a combustion engine 3. The powertrain 2 may further comprise a gearbox 4 configured to transmit driving torque, at different gear ratios, from the propulsion unit to drive wheels 6 (only one shown in the figure) of the vehicle 1. The vehicle 1 may further comprise a gearbox management system 24 configured to control the gearbox 4. The vehicle 1 may also comprise an engine management system 23 configured to control the propulsion unit. The gearbox management system 24 and the engine management system 23 may typically be configured to communicate with each other for the purpose of control of the vehicle powertrain 2.

The gearbox 4 may be connected to the drive wheels 6 via a propeller shaft 5 and a drive wheel shaft 7. The propeller shaft 5, the drive wheel shaft 7 and the drive wheels 6 are constituent components of the powertrain 2. The primary direction of travel of the vehicle 1, i.e. the direction of travel when the vehicle is driven forward, is in the figure illustrated by the arrow M.

The vehicle 1 further comprises a front wheel axle 9 and front wheels 8. The front wheels 8 are steered wheels. The vehicle comprises a front axle steering system 10 configured to control the steering of the front wheels 8. The vehicle 1 may further comprise a steering wheel 11. The steering wheel 11 may be operated by a driver of the vehicle 1 for the purpose of steering the vehicle. One or more of the wheels of the vehicle 1 may be provided with one or more wheel sensors 18. Such wheel sensors are commonly used within the technical field and may for example be configured to determine the rotational direction of the wheel (to which the wheel sensor is arranged) and/or the rotational speed thereof. In Figure 1, only one wheel sensor 18 is shown, said wheel sensor 18 associated with a front wheel 8 of the vehicle 1. It should however be recognized that a wheel sensor may be arranged at any one of the wheels of the vehicle 1. For example, all of the wheels of the vehicle 1 may be provided with one or more wheel sensors 18.

The vehicle may further comprise at least a part of a positioning system 25 configured to determine the geographical position of the vehicle. Alternatively, the vehicle may comprise a communication device (not shown) configured to communicate with such a positioning system 25. The positioning system 25 may for example comprise or consist of a global positioning system (GPS) as known in the art. Alternatively, or additionally, the positioning system may use detection devices arranged in the infrastructure for the purpose of determining the geographical position of a vehicle at various points in time.

The vehicle 1 may further comprise a tag axle 12 with associated tag axle wheels 13. The tag axle 12 with the tag axle wheels 13 are not a part of the powertrain, and the tag axle wheels 13 are thus nondriven wheels. The tag axle wheels 13 may be steerable via a tag axle steering system 14. The tag axle steering system 14 may be configured to steer the tag axle wheels 13, for example in dependence of information obtained from the front axle steering system 10. The vehicle 1 may further comprise a control device 100 configured to control the tag axle steering system 14.

The vehicle 1 may be a heavy vehicle, such as a truck or a bus. Furthermore, the vehicle may be a hybrid vehicle, in which case the vehicle further comprises an electric machine (not shown) in addition to the combustion engine 2. Alternatively, the vehicle may be a fully electric vehicle in which case the vehicle does not comprise the combustion engine 3 illustrated, but only an electric machine. In the present disclosure, the term "engine" is considered to mean a propulsion unit of the vehicle and thus includes a combustion engine or an electric machine. Moreover, the vehicle 1 may be a manually driven vehicle, a partly autonomous vehicle, or a fully autonomous vehicle.

Figure 2 schematically illustrates a top view of a second example of a vehicle, here in the form of an articulated vehicle 1'. Like the vehicle 1 shown in Figure 1, the vehicle 1' comprises a front wheel axle 9 with associated front wheels 8, a front axle steering system 10 configured to control the steering of the front wheels 8, a drive wheel shaft 7 with associated the drive wheels 6, and a tag axle 12 with associated tag axle wheels 13. Although not illustrated in the figure, the vehicle 1' further comprises a propulsion unit and may also comprise a gearbox, similarly to the vehicle 1 shown in Figure 1. The vehicle 1' further comprises an articulation joint 15 and an articulation control system 16. The articulation control system 16 is configured to control the articulation joint, for example in dependence of information obtained from the front axle steering system 10 and/or various sensors or controllers of the vehicle indicating an articulation state of the vehicle. For example, the articulation control system 16 may be configured to control an articulation angle of the articulation joint 15.

The vehicle 1' further comprises a tag axle steering system 14 and a control device 100 configured to control the tag axle steering system. The tag axle steering system 14 may be configured to steer the tag axle wheels in dependence of information obtained from the articulation control system 16. The tag axle steering system 14 may also, if desired, be configured to steer the tag axle wheels in dependence of information obtained from the front axle steering system 10.

It should be noted that the vehicle 1 shown in Figure 1 and the vehicle 1' shown in Figure 2 are merely for illustrative purposes, and the present invention is not limited to the vehicles shown or to the position of the tag axle 12 within said vehicles. A tag axle is however arranged behind the front axle when seen in the primary direction of travel of the vehicle.

Figure 3a schematically represents a top view of wheel axles 9, 7, 12 of a vehicle (for example the vehicle 1 shown in Figure 1) showing tag axle wheels 13 in a neutral position. As evident from the figure, the tag axles wheels 13, when in the neutral position, are arranged so that their rotational axis (which also coincides with a longitudinal axis of the tag axle 12) is essentially perpendicular to a central axis A of the vehicle. Described differently, the tag axle wheels are, when in the neutral position, essentially aligned with the longitudinal axis A of the vehicle. In the figure, the front wheels 8 are also shown to be arranged essentially aligned with the longitudinal axis A. The position of the tag axle wheels 13 and the front wheels 8 shown in Figure 3a may typically occur when the vehicle is not turning, but travelling straight forward. If a malfunction of the tag axle steering system occurs when the tag axle wheels are in the neutral position shown in Figure 3a, the tag axle wheels 13 may be locked in this position, since it is considered to be the safest position, until the tag axle steering system may be checked at a workshop or the like.

Figure 3b schematically represents a top view of the wheel axles 9, 7, 12 showing the tag axle wheels

13 in a position other than the neutral position. In other words, the steering angle of the tag axle wheels 13 is here offset from a steering angle of the neutral position of the tag axle wheels. More specifically, the tag axle wheels are actively steered by the tag axle steering system for example in dependence of information obtained from a front axle steering system. As shown in Figure 3b, the front wheels 8 are also turned (by the front axle steering system of the vehicle) from a position in which they are aligned with the longitudinal axis A of the vehicle. The illustrated situation may typically occur when the vehicle is turning slightly. If a fault signal indicating malfunction of the tag axle steering system occurs when the tag axle wheels are arranged as exemplified in Figure 3b, the tag axle wheels 13 may for example be released to a caster steer mode. In such a case, the tag axle wheels 13 would not be locked in any position and may therefore turn as a result of the lateral forces to which they are subjected as a result of the movement of the vehicle. When the vehicle continues to travel forward and is no longer turning, the tag axle wheels will then automatically align so that they reach the neutral position (compare with Fig. 3a). In other words, the steering angle of the tag axle wheels approaches the steering angle of the neutral position. This is illustrated by the arrow T showing the direction of turning of the tag axle wheels as a result of the lateral forces when the vehicle is travelling straight forward.

As mentioned above, the method for controlling a tag axle steering system according to the present disclosure is particularly useful when the tag axle wheels are provided at a positive caster angle as seen in the primary direction of travel of the vehicle. In other words, the method is particularly useful when the tag axle wheels are provided at a positive caster angle when driving forward. In such a case, the tag axle wheels may, when the tag axle steering system is operated in a caster steer mode, straighten up so as to align with the primary direction of travel as a result of possible lateral forces that they may be subjected to due to the movement of the vehicle as such. Figures 4a and 4b, schematically illustrate two different alternatives of caster angles.

More specifically, Figure 4a schematically illustrates a steerable wheel 30. The steerable wheel 30 may for example be a tag axle wheel 13 shown in the previous figures. It should however be noted that other wheels of the vehicle may also be provided at various caster angles, if desired. The steerable wheel 30 shown in Figure 4a has a caster angle a which is negative as seen in a primary direction of travel (illustrated by arrow M) of a vehicle. Such a primary direction M of travel of the vehicle corresponds to the direction of travel of the vehicle when driving forward. When the wheel 30 has a negative caster angle a, the steering axis (not shown) is angled such that a central axis 32 of the steering axis intersects the road surface 33 behind the center of contact 34 of the wheel 30 on the road surface 33 as seen in the direction of travel (here illustrated as primary direction of travel M) of the vehicle. The center of contact 34 of the wheel 30 of the road surface 33 corresponds to a position where a vertical axis 31 of the wheel 30, said vertical axis being perpendicular to an axis of rotation of the wheel 30, intersects the road surface 33.

In contrast to Figure 4a, Figure 4b schematically illustrates the steered wheel 30 provided at a positive caster angle a as seen in a primary direction of travel (illustrated by arrow M) of a vehicle. As shown in the figure, when the wheel 30 has a positive caster angle, the central axis 32 of the steering axis intersects the road surface 33 ahead of the ahead of the center of contact 34 of the wheel 30 on the road surface as seen in the direction of travel.

In a situation where the direction of travel of the vehicle would be reversed such that the direction of travel is opposite to the direction illustrated by the arrows M in Figures 4a and 4b, the wheel 30 would inherently be provided at a caster angle of the opposite sign to the direction M since the angular displacement of the steering axis is not changed.

Figure 5 represents a flowchart schematically illustrating a first exemplifying embodiment of the method for controlling a tag axle steering system in accordance with the present disclosure. Optional steps of the exemplified embodiment are illustrated by dashed lines/boxes.

The method may comprise a first step S101 of determining whether or not the tag axle steering system is operated in a caster steer mode. If the tag axle steering system is not operated in a caster steer mode, the method is returned to start. However, in case the tag axle steering system is operated in caster steer mode, the method may proceed to the subsequent step 5102. Step S102 comprises determining whether there is an indication that the vehicle in which the tag axle steering is arranged is, or is intended to be, driven backwards. If there is no indication that the vehicle is, or is intended to be, driven backwards, the method may be returned to start. However, in case there is an indication that that the vehicle is, or is intended to be, driven backwards, the method proceeds to step S103. Step S103 comprises locking the position of the tag axle wheels at the current steering angle of the tag axle wheels. After step S103, the method may be ended.

Alternatively, the method may after step S103 further comprise a step S104 of determining whether or not there is an indication that the vehicle is, or is intended to be, driven forward. In case there is an indication that the vehicle is, or is intended to be, driven forward, the method proceeds to step S105 is which the tag axle wheels are released so that the tag axle steering system is operated in caster steer mode. In other words, the tag axle steering system is controlled to be operated in caster steer mode by releasing the position of the tag axle wheels. After step S105, the method may be reverted back to step S102.

Figure 6 represents a flowchart illustrating a second exemplifying embodiment of the method for controlling a tag axle steering system in accordance with the present disclosure. Optional steps of the exemplified embodiment are illustrated by dashed lines/boxes.

According to the second exemplifying embodiment, the method may comprise a first step 5201 of determining whether a fault signal indicating malfunction of the tag axle steering system has been generated. If no such fault signal has been generated, the method is after step 8201 returned to start. However, if it is determined in step S201 that a fault signal indicating malfunction of the tag axle steering system has been generated, the method may proceed to step 5202.

Step 5202 comprises determining whether the (current) steering angle of the tag axle wheels is offset from a steering angle of a neutral position of the tag axle wheels. If the current steering angle of the tag axle wheels is not offset from the steering angle of a neutral position of the tag axle wheels, the method may proceed to a step 8203 of locking the tag axle wheels in the neutral position. After step 8203, the method may be ended. However, if the steering angle of the tag axle wheels is offset from the steering angle of the neutral position of the tag axle wheels, the method may proceed to step 5204. Step 8204 comprises initiating and operating the tag axle steering system in caster steer mode. Thereby, the tag axle wheels will be released such that the steering angle thereof may be altered as a result of the lateral forces to which the tag axle wheels may be subjected. Thereby, if the vehicle is driven straight forward, the tag axle wheels are allowed to straighten up so as to approach and possibly reach the neutral position.

It should here be noted that the step S203 of locking the tag axle wheels in the neutral position and the step 5204 comprising operating the tag axle steering system in caster steer mode may be parts of operating the tag axle steering system in a failure mode. In other words, if a fault signal indicating malfunction of the tag axle steering system has been generated, the tag axle steering system may, depending on the circumstances be put either in a caster steer mode or in a mode wherein the tag axle wheels are locked in the neutral position according to the operation of the tag axle steering system in a failure mode. It should also be recognized that said failure mode may comprise further alternative modes, depending on various circumstances, if desired. The tag axle steering system may suitably remain in failure mode until it has been repaired or replaced at a workshop or the like. After step S204, the method may comprise a step S205 of determining whether there is an indication that the vehicle in which the tag axle steering is arranged is, or is intended to be, driven backwards. If there is an indication that that the vehicle is, or is intended to be, driven backwards, the method proceeds to step 5206. Step S206 comprises locking the position of the tag axle wheels at the current steering angle of the tag axle wheels. After step 5206, the method may be ended as shown in the figure. Alternatively, the method may comprise the steps S104 and S105 as described above with regard to the first exemplifying embodiment. If the method according to the second exemplifying embodiment also comprises the steps 5104 and 5105, the method may after step 5105 be returned to step 5205.

Figure 7 schematically illustrates an exemplifying embodiment of a device 500. The control device 100 described above may for example comprise the device 500, consist of the device 500, or be comprised in 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 that comprises instructions for controlling a tag axle steering system, said tag axle steering system being configured to actively steer tag axle wheels of a tag axle of a vehicle. The computer program comprises instructions for, in response to an indication that the vehicle in which the tag axle is arranged is, or is intended to be, driven backwards when the tag axle steering system is operated in a caster steer mode, locking the position of the tag axle wheels at a current steering angle of the tag axle wheels.

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.

The data processing unit 510 may perform one or more functions, i.e. the data processing unit 510 may effect a certain part of the program P stored in the memory 560 or a certain part of the program P 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 nonvolatile 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 communicate with the data processing unit 510 via a data bus 514. The communication between the constituent components may be implemented by a communication link. A communication link may be a physical connection such as an optoelectronic communication line, or a non-physical connection such as a wireless connection, e.g. a radio link or microwave link. When data are received on the data port 599, they may be 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 affected 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.