Apparatus and Method for Determining a Road Friction

Abstract

An apparatus determines a road friction of a commercial vehicle. The commercial vehicle has a first axle and a second axle, a load distribution mechanism for changing a load on the first axle or on the second axle, and a slip sensor for determining a slip value for at least one wheel on the first axle or on the second axle. The apparatus includes an evaluation unit configured to control the load distribution mechanism to change the load of the first axle or on second axle, determine a change in the slip value in response to the change of the load, and evaluate the road friction based on the change in the slip value.

Claims

1.-13. (canceled)

14. An apparatus for determining a road friction of a commercial vehicle, the commercial vehicle comprising a first axle and a second axle, a load distribution mechanism for changing a load on the second axle or on the first axle, and a slip sensor for determining a slip value for at least one wheel on the first axle or on the second axle, the apparatus comprising: an evaluation unit configured to: control the load distribution mechanism to change the load of the second axle or on the first axle; determine a change in the slip value in response to the change of the load; and evaluate the road friction based on the change in the slip value.

15. The apparatus according to claim 14, wherein the evaluation unit is further configured to: detect or cause a braking actuation on the second axle; control the load distribution mechanism, during the braking actuation, to lift the second axle; and determine the change in the slip value in response to the braking actuation.

16. The apparatus according to claim 14, wherein the slip sensor is configured to determine a first slip value for at least one wheel on the first axle, and the evaluation unit is further configured to: detect or cause a drive torque being applied to the first axle; control the load distribution mechanism, during the application of the drive torque, to lift the second axle or the first axle, determine the change in the slip value in response to the drive torque applied to the first axle.

17. The apparatus according claim 16, wherein the slip sensor is configured to determine a second slip value for at least one wheel on the second axle, and the evaluation unit is further configured to: detect or cause a braking actuation on the second axle at a same time as the drive torque is applied to the first axle; control the load distribution mechanism, during the braking actuation and the application of the drive torque, to lift the second axle; and evaluate the road friction based on changes in the first slip value and in the second slip value in response to the braking actuation and to the applied drive torque.

18. The apparatus according claim 14, wherein the evaluation unit is further configured to: change the load on the second axle until there is a criticality characterized by a transition between adhesion friction and sliding friction on at least one wheel on the second axle.

19. The apparatus according to claim 14, wherein the slip sensor is configured to determine first slip value for at least one wheel on the first axle and a second slip value for at least one wheel on the second axle, and the evaluation unit is further configured to: determine a first change of the first slip value and a second change of the second the slip value in response to the change of the load; and evaluate the road friction based on the first change and the second change.

20. The apparatus according to claim 14, wherein the commercial vehicle includes a first slip sensor for a wheel on the first axle and a second slip sensor for a wheel on the second axle, and the evaluation unit is further configured to: determine changes in slip values for the wheel on the first axle and for the wheel on the second axle; and evaluate the road friction based on at least one difference in slip values determined for the wheel on the first axle and for the wheel on the second axle.

21. The apparatus according to claim 14, wherein the commercial vehicle further comprises an activation means to initiate the determination of the road friction, and the evaluation unit is further configured to receive from the activation means a signal for determining the road friction and based thereon to determine the road friction.

22. The apparatus according to claim 14, wherein the evaluation unit is further configured to block the evaluation of the road friction during a critical situation, wherein the critical situation includes at least one of: a braking event above a predetermined threshold, a driving along curves, a presence of vehicles or people in a vicinity of the commercial vehicle, driving through towns or villages.

23. The apparatus according to claim 14, wherein the commercial vehicle includes environmental sensors for determining environment conditions, and the evaluation unit is further configured to: automatically evaluate the road friction when the determined environment conditions indicate a critical situation or after predetermined time intervals; and issue a warning signal about the automatic evaluation of the road friction.

24. A commercial vehicle, comprising: a first axle and a second axle; a load distribution mechanism for changing a load on the second axle; a slip sensor for determining a slip value for at least one wheel on the first axle or on the second axle; and an evaluation unit configured to: control the load distribution mechanism to change the load of the second axle or on the first axle; determine a change in the slip value in response to the change of the load; and evaluate the road friction based on the change in the slip value.

25. A method for determining a road friction of a commercial vehicle, the method comprising: controlling a load distribution mechanism of a commercial vehicle having a first axle and a second axle to change a load of the second axle; determining, via a slip sensor of the commercial vehicle for determining a slip value for at least one wheel on the first axle or on the second axle, a change in the slip value in response to the change of the load; and evaluating the road friction based on the change in the slip value.

26. A computer program product comprising a non-transitory computer-readable medium having stored thereon program code which, when the program code is executed on a computer or processor, causes the acts of: controlling a load distribution mechanism of a commercial vehicle having a first axle and a second axle to change a load of the second axle; determining, via a slip sensor of the commercial vehicle for determining a slip value for at least one wheel on the first axle or on the second axle, a change in the slip value in response to the change of the load; and evaluating the road friction based on the change in the slip value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG. 1 depicts a vehicle with an apparatus according to an embodiment of the present invention; and

[0031] FIG. 2 depicts a schematic flow chart for a method of determining a road friction according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1 depicts an exemplary commercial vehicle 10 with an apparatus 100 according to embodiments. The vehicle 10 comprises a first axle 11 and a second axle 12, a load distribution mechanism 14 for changing a load Fn on the second axle 12, and a slip sensor 16 configured to determine a slip value for at least one wheel 17, 18 on the first axle 11 and/or on the second axle 12. The load Fn is nothing but a force acting on ground and determines the friction force. When the road is perpendicular to gravity the force caused by the load Fn is the normal force entering the road friction force Ff=μ*Fn, where μ is the friction coefficient which is to be determined and which correlates to the slip. The friction depends on many parameters which are vehicle specific such as the tires or road specific such as the road material or the degree of moisture, ice, etc.

[0033] The apparatus 100 is configured to determine the road friction Ff for the vehicle 10 and includes an evaluation unit 110 and may control the load distribution mechanism 14 to change the load Fn of the second axle 12. After this, the evaluation unit 110 may determine a change in the slip value in response to the change of the load Fn. Finally, the evaluation unit 110 may evaluate the road friction Ff based on the change in the slip value.

[0034] It is understood that the vehicle 10 may comprise more than two axles 11, 12, in which case at least one of them may be liftable to modify the load Fn in a controlled way. Likewise, the liftable axle(s) may also belong to a trailer and not to a tractor for the trailer. These axle(s), too, may be used in embodiments to determine the road friction. Therefore, embodiments shall not be restricted on particular axles or on particular vehicles, but can be applied on any vehicle whether it is a tractor, trailer, single vehicle, vehicle combination, etc.

[0035] Therefore, embodiments are applicable for any vehicle or vehicle combination with more than two axles, where at least one of the axles can be lifted resulting in a dynamically adjustable load Fn, not only on this axle 12, but also on the other axle 11 that has to absorb the released load on the lifted axle 12. In other words, embodiments are particularly applicable on vehicles 10 where the load Fn on some wheels 17, 18 is controllable individually so that the load Fn on one of the axles (or the respective wheels) can be reduced or increased. This load increase/reduction may be performed during driving (in a steady state) or prior/during a braking or an acceleration.

[0036] The road friction Ff can be determined because, if the load Fn on a particular axle 11, 12 is changed, the given wheels 17, 18 will have a different adhesion utilization using the same or even less braking torques or driving torques. This results in a variation of the slip that can be measured and embodiments use the measured slip change to determine the road friction Ff.

[0037] According to embodiments, the evaluation unit 110 is configured to control the load change through the load distribution mechanism 14 such that the corresponding wheels 18 on the axle 12 whose load is changed becomes critical implying a transition from an adhesion friction and starting a sliding friction. This criticality can improve significantly the precision of the friction estimation. Since the wheels 17 on the other axle(s) 11 are not even close to the limits of the road friction, the vehicle operation is nevertheless safe. No critical driving situation is caused by the determination of the road friction according to embodiments.

[0038] According to further embodiments, the evaluation unit 110 is configured to cause a combined application of a braking torque and a driving torque at the same time to excite the tires 17, 18 during a steady state of the vehicle 10. For example, the first axle 11 may be a driven axle on which the drive torque can be applied while the wheels 18 on the second axle 12 are braked (and while this axle 12 may be lifted to some extend). The braking torque and the driving torque may be adjusted so that the speed of the vehicle 10 does not change during this operation. In particular, if the load Fn is reduced on the non-driven, second, axle 12, a lower excitation torque (braking and/or driving) can be applied for the same effect. This has also the advantage that the amount of energy used for this operation is reduced.

[0039] Embodiments are in particular applicable to so-called 6×2 trucks, where only one of the three axles are driven (first axle 11) and the other one can be lifted (second axle 12) as depicted in FIG. 1. The load distribution may be utilized by air bellows. When using the air bellows, the pressure in the air bellows can be reduced before braking to lift the second axle 12. It may be maximal until the driven axle 11 reaches the maximum allowed load Fn. According to embodiments, during braking, the tires of the lifted axle 12 will get near the adhesion limit implying the appearance of a significant wheel slip (above a predetermined threshold). According to embodiments, the evaluation unit 110 is configured to estimate the adhesion of the road surface with high precision from the differences in the load Fn and the slip values between the highly or fully-loaded driven axle 11 and the lightly-loaded lift axle 12.

[0040] According to yet another embodiment, on the same 6×2 truck, the load distribution mechanism 14 may also be configured to lift the driven axle 11 (instead of the second axle 12 or can lift both axles 11, 12, although not at the same time). The load distribution mechanism 14 may be configured to lift the driven axle 11 (or one of multiple driven axles). This lifting may be performed until a maximum is reached where the second axle 12 reaches the maximum allowed load during a normal driving operation. This may be achieved by reducing the air bellow pressure. The tires 17 on the driven axle 11 will get close to the adhesion limit (where the transition between adhesion and sliding friction occurs) resulting again in a significant amount of slip. As before, from the differences in the load Fn and slip of the highly or fully-loaded second axle 12 and the lightly-loaded driven axle 11 (here the lift axle), the adhesion of the surface can be estimated with high precision.

[0041] Although it may not be common to be able to lift a driven axle, on special vehicles (e.g. all-wheel drive) also driven axles may be liftable. In this case, usually the lifted driven axle may be either isolated from the drive train, or locked to the other axles, where no wheel speed difference can occur. However, it may be possible to use the normal air bellows to change the load distribution between multiple driven axles. Therefore, the pressure on the driven axle may be decreased and/or increased on the other axle(s) resulting in an off-load of the driven axle(s) for the purpose of the measurement according to embodiments.

[0042] FIG. 2 depicts a flow diagram of a method for determining a road friction Ff of a commercial vehicle 10. The method comprises the steps of: [0043] controlling S110 the load distribution mechanism 14 to change the load Fn of the first axle 11 and/or on the second axle 12; [0044] determining S120 a change in the slip value in response to the change of the load Fn; and [0045] evaluating S130 the road friction Ff based on the change in the slip value.

[0046] It is understood, according to further embodiments, that any function described before with the apparatus can be implemented as further optional method step. In addition, the order of method steps can be arbitrary as long as the desired result is achievable.

[0047] Moreover, this method may also be a computer-implemented method. A person of skill in the art would readily recognize that steps of various above-described methods may be performed by programmed computers. Embodiments are also intended to cover program storage devices, e.g., digital data storage media, which are machine or computer readable and encode machine-executable or computer-executable programs of instructions, wherein the instructions perform some or all of the acts of the above-described methods, when executed on the computer or processor.

[0048] The description and drawings merely illustrate the principles of the disclosure. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the disclosure and are included within its scope.

[0049] Furthermore, while each embodiment may stand on its own as a separate example, it is to be noted that in other embodiments the defined features can be combined differently, i.e. a particular feature descripted in one embodiment may also be realized in other embodiments. Such combinations are covered by the disclosure herein unless it is stated that a specific combination is not intended.

LIST OF REFERENCE SIGNS

[0050] 10 commercial vehicle [0051] 11, 12 axles or axle group (at least two) [0052] 14 load distribution mechanism [0053] 16 slip sensor(s) [0054] 17, 18 wheels [0055] 100 apparatus for determining a road friction [0056] 110 evaluation unit [0057] Ff road friction [0058] Fn load