TECHNICAL FIELD The invention relates to a method performed by a control device for controlling operation of a brake system of a motor vehicle according to the preamble of claim 1. The invention also relates to a control device for controlling operation of a brake system of a motor vehicle. The invention further relates to a vehicle. The invention in addition relates to a computer program and a computer readable medium.

BACKGROUND ART

Vehicles, e.g. heavy vehicles such as trucks, are equipped with brake systems having service brakes for braking the vehicle wheels and at least one auxiliary brake connected to the powertrain for braking drive axle wheels of the vehicle. Such an auxiliary brake may e.g. be an endurance brake such as a retarder. A purpose of such auxiliary brakes may be to relieve the service brakes and thereby decrease their wear and minimize the risk of fading due to overheated brakes. When applying combined auxiliary braking the auxiliary brakes are integrated into the service brake system so that both auxiliary braking and service brake systems of the brake system are applied in a cooperative manner to reach the desired retardation when braking. This implies that the service brake caused retardation entirely or to some degree may be replaced by auxiliary brake caused retardation. Auxiliary brake systems which are part of the powertrain, such as retarder or engine integrated brakes, e.g. exhaust brakes, decompression brakes, or the like, act only on the driven wheels of the vehicle. During combined auxiliary braking the driven wheels are subjected to braking forces caused by both endurance brakes and service brakes. The sum of the braking forces generates a certain wheel slip. A consequence of combined auxiliary braking is thus that braking force is distributed from non-driven axle(s) to driven axle(s) of the vehicle. High braking power implies a risk to generate a high wheel slip due to excessive adhesion utilization which may lead to vehicle instability and increased tire wear. Stability preserving functionality, such as Anti-lock Brake System and Drag Torque Control, overrides combined auxiliary braking by deactivating auxiliary brake(s) when a too high driven wheel slip is detected. This hinders the functionality from its purpose and causes comfort issues as the auxiliary brake(s) are suddenly deactivated. Combined auxiliary braking thus comprises a trade-off with auxiliary brake utilization on one side and tire wear, vehicle stability and driver comfort on the other side. There is thus a need to facilitate controlling operation of a brake system of a motor vehicle.

OBJECTS OF THE INVENTION

An object of the present invention is to provide a method performed by a control device for controlling operation of a brake system of a motor vehicle which facilitates efficient use of auxiliary brake for braking the vehicle.

Another object of the present invention is to provide a control device for controlling operation of a brake system of a motor vehicle which facilitates efficient use of auxiliary brake for braking the vehicle. Another object of the present invention is to provide a vehicle comprising such a control device. SUMMARY OF THE INVENTION

These and other objects, apparent from the following description, are achieved by a method, a control device, a vehicle, a computer program and a computer readable medium, as set out in the appended independent claims. Preferred embodiments of the method and the control device are defined in appended dependent claims.

Specifically an object of the invention is achieved by a method performed by a control device for controlling operation of a brake system of a motor vehicle. The vehicle comprises a vehicle powertrain and wheel axles with wheels, wherein at least one wheel axle is a drive axle driven by means of the powertrain. The brake system comprises service brakes for braking the vehicle wheels and at least one auxiliary brake connected to the powertrain for braking drive axle wheels. The method comprises the steps of: estimating a function expressing a dependence between wheel slip and braking force; determining a maximum braking force for drive wheels of the at least one drive axle by means of said function and based upon a chosen target wheel slip; and determining a brake force corresponding to a requested retardation for the vehicle. The method further comprises the step of determining a maximum admissible braking force to be exerted by means of the at least one auxiliary brake for braking drive wheels of the at least one drive axle, the step being based upon a predetermined vehicle service brake force distribution to be exerted by vehicle service brakes for braking drive wheels of the at least one drive axle, the determined maximum braking force and the brake force corresponding to the requested retardation for the vehicle. The method further comprises the step of controlling operation of the brake system of the vehicle based upon the thus determined maximum admissible braking force to be exerted by means of the at least one auxiliary brake and the brake force corresponding to the requested retardation for the vehicle for obtaining the requested retardation with a maximum use of the at least one auxiliary brake for braking the vehicle. Hereby efficient use of auxiliary brakes for braking the vehicle is facilitated. By thus controlling the overall braking force at wheels which are affected by auxiliary brakes utilization of auxiliary brakes is optimized. Herby the risk of auxiliary brakes deactivating due to stability preserving functionality may be avoided. Hereby control of the total braking force at different retardation requests corresponding to different braking forces while ensuring optimized auxiliary brake utilization. In this way this solution handles the trade-off with auxiliary brake utilization on one side and tire wear, vehicle stability and driver comfort on the other side automatically during braking and thereby ensures maximum utilization of auxiliary brake(s) while avoiding excessive wheel slip. The solution according to the present invention is able to handle different driving situations such as e.g. different retardation levels and adhesion conditions between tyres and road surface. The solution according to the present invention is able to handle different tire characteristics, i.e. different tire types with different slip stiffness.

According to an embodiment of the method the step of controlling operation of the brake system based upon the thus determined maximum admissible braking force comprises adapting the service brake force distribution on the wheels of the wheel axles, to be exerted by the vehicle service brakes, to a maximum use of the at least one auxiliary brake for braking the vehicle, so as to obtain the requested retardation for the vehicle.

Hereby efficient use of auxiliary brakes for braking the vehicle is facilitated, wherein utilization of auxiliary brakes may be optimized.

According to an embodiment the method further comprises step of determining the braking force actually exertable by means of the at least one auxiliary brake, the braking force actually exertable corresponding to the maximum use of the at least one auxiliary brake for braking the vehicle, in order to determine whether the determined maximum admissible braking force can be used by the at least one auxiliary brake. Hereby efficient use of auxiliary brakes for braking the vehicle is facilitated, wherein utilization of auxiliary brakes may be optimized in that the use of the at least one auxiliary brake may be maximized to its actual capacity.

According to an embodiment of the method, if the determined braking force actually exertable by means of the at least one auxiliary brake is lower than the determined maximum admissible braking force, the force actually exertable by means of the at least one auxiliary brake is used in the control of the operation of the brake system.

Hereby efficient use of auxiliary brakes for braking the vehicle is facilitated, wherein utilization of auxiliary brakes may be optimized in that the use of the at least one auxiliary brake is maximized to its actual capacity.

According to an embodiment of the method the step of determining a maximum braking force for drive wheels of the at least one drive axle comprises the step of using a part of the function in which the dependence corresponds to an essentially straight line.

Hereby the maximum braking force for drive wheels of the at least one drive axle may be accurately determined.

Specifically an object of the invention is achieved by a control device for controlling operation of a brake system of a motor vehicle. The vehicle comprises a vehicle powertrain and wheel axles with wheels, wherein at least one wheel axle is a drive axle driven by means of the powertrain. The brake system comprises service brakes for braking the vehicle wheels and at least one auxiliary brake connected to the powertrain for braking drive axle wheels. The control device is configured to: estimate a function expressing a dependence between wheel slip and braking force; determine a maximum braking force for drive wheels of the at least one drive axle by means of said function and based upon a chosen target wheel slip; and determine a brake force corresponding to a requested retardation for the vehicle. The control device is further configured to determine a maximum admissible braking force to be exerted by means of the at least one auxiliary brake for braking drive wheels of the at least one drive axle, the determination being based upon a predetermined vehicle service brake force distribution to be exerted by vehicle service brakes for braking drive wheels of the at least one drive axle, the determined maximum braking force and the brake force corresponding to the requested retardation for the vehicle. The control device is further configured to control operation of the brake system of the vehicle based upon the thus determined maximum admissible braking force to be exerted by means of the at least one auxiliary brake and the brake force corresponding to the requested retardation for the vehicle for obtaining the requested retardation with a maximum use of the at least one auxiliary brake for braking the vehicle.

According to an embodiment the control device, when controlling operation of the brake system based upon the thus determined maximum admissible braking force, is configured to adapt the service brake force distribution on the wheels of the wheel axles, to be exerted by the vehicle service brakes to a maximum use of the at least one auxiliary brake for braking the vehicle, so as to obtain the requested retardation for the vehicle.

According to an embodiment the control device is configured to determine the braking force actually exertable by means of the at least one auxiliary brake, the braking force actually exertable corresponding to the maximum use of the at least one auxiliary brake for braking the vehicle, in order to determine whether the determined maximum admissible braking force can be used by the at least one auxiliary brake. According to an embodiment of the control device, if the determined braking force actually exertable by means of the at least one auxiliary brake is lower than the determined maximum admissible braking force, the control device is configured to use the force actually exertable by means of the at least one auxiliary brake in the control of the operation of the brake system. According to an embodiment of the control device the control device is configured to use a part of the function in which the dependence corresponds to an essentially straight line when determining a maximum braking force for drive wheels of the at least one drive axle. Specifically an object of the invention is achieved by a vehicle comprising a control device as set out herein.

Specifically an object of the invention is achieved by a computer program for controlling operation of a brake system of a motor vehicle, said computer program comprising program code which, when run on an control device or another computer connected to the control device, causes the control device to perform the method as set out herein.

Specifically an object of the invention is achieved by a computer readable medium comprising instructions which, when executed by a computer, cause the computer to carry out the method as set out herein.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention reference is made to the following detailed description when read in conjunction with the accompanying drawings, wherein like reference characters refer to like parts throughout the several views, and in which:

Fig. 1 schematically illustrates a side view of a vehicle according to an embodiment of the present invention;

Fig. 2a schematically illustrates a vehicle wheel on a road with a normal force and a braking force; Fig. 2b schematically illustrates a graph of the braking force and normal force ratio as a function of wheel slip according to an embodiment of the present invention;

Fig. 3 schematically illustrates a block diagram of a control device for controlling operation of a brake system of a motor vehicle according to an embodiment of the present invention;

Fig. 4 schematically illustrates a flowchart of a method performed by a control device for controlling operation of a brake system of a motor vehicle according to an embodiment of the present invention;

Fig. 5 schematically illustrates a flowchart of a method performed by a control device for controlling operation of a brake system of a motor vehicle according to an embodiment of the present invention; and

Fig. 6 schematically illustrates a computer according to an embodiment of the present invention.

DETAILED DESCRIPTION hereinafter the term“link” refers to a communication link which may be a physical connector, such as an optoelectronic communication wire, or a non physical connector such as a wireless connection, for example a radio or microwave link.

Hereinafter the term“service brakes” refers to brake members arranged in connection to the wheels and used in ordinary driving. Service brakes may e.g. be any suitable disc brake, drum brake, electromagnetic brake or the like

Hereinafter the term“auxiliary brake” refers to any brake member connected to the powertrain for braking drive axle wheels. An auxiliary brake may be arranged to assist service brakes for braking the vehicle. The term“auxiliary brake” may refer to “endurance brake” such as retarder, exhaust brake, decompression brake or the like. The term“auxiliary brake” may also refer to brake member configured for regenerative braking such as an electric machine. The“auxiliary brake” may be comprised in the power train of the vehicle.

Hereinafter the term“wheel slip” refers to the difference in velocity between the wheel and the ground on which the wheel is engaging. Thus, if the wheel is rolls unaffected by the ground the wheel slip is zero. If the brakes so that there is a certain slide, a certain slip value is different from zero is obtained. Hereinafter the term“request” in relation to e.g.“requested retardation for the vehicle” may refer to“demand”, e.g.“demanded retardation for the vehicle”. Thus, the term“request” comprises the term“demand”.

Hereinafter the term“brake request device” may be denoted "brake demand device”. Hereinafter the term“determining a brake force corresponding to a requested retardation for the vehicle” may comprise“determining a brake force based on a requested brake force, said requested brake force being based on a requested retardation and the vehicle mass”. Hereinafter the term “determining a brake force corresponding to a requested retardation for the vehicle” may comprise “determining a brake force corresponding to a requested total brake force for providing a certain retardation of the vehicle”.

Fig. 1 schematically illustrates a side view of a vehicle V according to an embodiment of the present invention.

The exemplified vehicle V is a is a heavy vehicle in the shape of a truck. The vehicle V is travelling on a road R. The exemplified vehicle may be operated in any suitable way. The exemplified vehicle may be operated by means of an internal combustion engine. The exemplified vehicle may be an electrically operated vehicle. The exemplified vehicle may be a hybrid vehicle. The exemplified vehicle may be an autonomous vehicle.

The vehicle V may comprise a control device for controlling operation of a brake system of the vehicle. The vehicle V may comprise or be operably connectable to a system I for controlling operation of a brake system of the vehicle. The system I may comprise a control device 100 for controlling operation of a brake system of the vehicle.

The vehicle V comprises, according to an embodiment, a control device 100 for controlling operation of a brake system of the vehicle according to fig. 3. The vehicle V is, according to an embodiment, arranged to be operated in accordance with a method M1 for controlling operation of a brake system of the vehicle according to fig. 5.

The vehicle according to the present invention may have any suitable number of wheel axles with wheel. One or more of said wheel axles with wheels may be drive axle for propelling the wheels of the vehicle. The vehicle according to the present invention may have one or more front axles, one or more rear axles operating as drive axles in combination with non-driving support axles arranged in front and/or behind driving axel/axles. The vehicle according to the present invention may be any wheeled articulated vehicle such as an articulated bus having certain wheel configuration. The vehicle according to the present invention may be a vehicle, e.g. truck, having a tractor, where the tractor may be a tractor without trailer or a tractor with one or more trailers. The vehicle according to the present invention may be a vehicle, e.g. truck, having no trailer or one or more trailers. The vehicle V comprises according to this embodiment a tractor 1 and a trailer 2 connected to the tractor.

The vehicle V comprises wheel axles X1 , X2, X3, X4, X5, X6. At least one axle of the wheel axles is a drive axle. The vehicle V comprises a front axle X1 , a drive axle X2 and a rear axle X3, said axles X1 , X2, X3 being arranged on the tractor 1. Thus, here the second axle X2 is the drive axle X2. The vehicle comprises a fourth axel X4, a fifth axle X5 and a sixth axel X6, said axles X4, X5, X6 being arranged on the trailer 2.

The vehicle V comprises vehicle wheels W1 , W2, W3, W4, W5, W6. The vehicle V comprises front wheels W1 connected to the front axle X1 , drive wheels W2 connected to the drive axle X2 and rear wheels W3 connected to the rear axle X3. The vehicle V comprises fourth wheels W4 connected to the fourth axel X4, fifth wheels W5 connected to the fifth axle X5 and sixth wheel W6 connected to the sixth axel X6.

The vehicle V comprises a vehicle powertrain PT. The vehicle powertrain PT may be any suitable powertrain PT. According to an embodiment the powertrain PT comprises an internal combustion engine E, a transmission configuration T which may comprise any suitable gearbox and a cardan shaft C for transferring power to a drive axel X2 of the vehicle V. Thus, the drive axle X2 is driven by means of the powertrain PT.

The vehicle V comprises a brake system B. The brake system B may comprise any suitable brake request device for requesting braking of the vehicle. The brake request device comprises according to an embodiment a brake pedal which may be activated by the driver of the vehicle. Thus, the driver may request a certain braking of the vehicle, i.e. a certain retardation of the vehicle. A requested braking by means of the brake request device, e.g. brake pedal, corresponds to a certain total brake force F. The brake system B comprises service brakes for braking the vehicle wheels W1 , W2, W3, W4, W5, W6. The service brakes may comprise any suitable service brakes such as disc brakes, drum brakes or the like arranged in connection to the respective vehicle wheel W1 , W2, W3, W4, W5, W6 for braking the vehicle wheels based on a brake request from the brake request device.

A predetermined service brake force distribution of the brake force F corresponding to a requested retardation of the vehicle is arranged to be provided. Thus, for a brake force F corresponding to a requested retardation of the vehicle V a predetermined brake force distribution with a brake force aiF for braking the front vehicle wheels W1 , a brake force a2F for braking the drive wheels W2, a brake force a3F for braking the rear vehicle wheels W3, a brake force a ₄ F for braking the fourth wheels W4, a brake force asF for braking the fifth wheels W5 and a brake force a6F for braking the sixth wheels W6 is provided.

The brake system B further comprises at least one auxiliary brake A connected to the powertrain PT for braking drive axle wheels W2 of the vehicle V. The auxiliary brake A may be any suitable auxiliary brake, e.g. a retarder or the like.

Operation of the brake system B of the vehicle V is arranged to be controlled based upon a determined maximum admissible braking force Faux to be exerted by means of the at least one auxiliary brake A and the brake force F corresponding to the requested retardation for the vehicle V for obtaining the requested retardation with a maximum use of the at least one auxiliary brake A for braking the vehicle V.

Fig. 2a schematically illustrates a vehicle wheel W2 on a road R with a normal force FN and a braking force F _z . The vehicle wheel W2 is according to an embodiment the drive wheel W2 of the vehicle V in fig. 1. Fig. 2b schematically illustrates a graph of the braking force F _z and normal force FN ratio as a function of wheel slip according to an embodiment of the present invention. Thus, a function expressing a dependence between wheel slip and braking force is estimated.

The slip stiffness of the tires of the drive wheel of the drive axle may be estimated during driving. Here the braking force F _z and normal force FN ratio, F _Z /FN, for the drive wheel W2 of the vehicle V in fig. 1 is illustrated as a function of the wheel slip.

In a part of the function the dependence between the braking force and the wheel slip corresponds to an essentially straight line.

The braking force F _z and normal force FN ratio, F _Z /FN, as a function of the wheel slip, i.e. the curve in fig. 2b, and particularly the part of the function where the dependence between the braking force and the wheel slip corresponds to an essentially straight line, i.e. the straight line part of the curve, illustrates the slip stiffness of the tires of the drive wheel of the drive axle. A maximum braking force Fmax for drive wheels W2 of the drive axle X2 of the vehicle V in fig. 1 is determined by means of the function expressing a dependence between wheel slip and braking force and based upon a chosen target wheel slip. Here a target wheel slip WS is chosen. Part of the function in which the dependence corresponds to an essentially straight line is used. The target wheel slip SW corresponds to maximum braking force Fmax for drive wheels W2 of the drive axle X2.

The target wheel slip WS may be chosen to avoid high tire wear and slip levels leading to deactivation of auxiliary brake A by stability preserving functions. The estimated slip stiffness SW is thus used to determine the maximum braking force Fmax the wheels W2 on the driven axle X2 can be subjected to without exceeding this target slip value SW. If a vehicle has more than one drive axle the estimated slip stiffness is used to determine the maximum braking force the wheels on the driven axles can be subjected to without exceeding this target slip value.

For the service brake portion of the overall vehicle retardation a distribution of braking force amongst the wheel axles for tractor and trailer(s) is known. Thus the service brake force is decreased proportionally over the wheel axles for the tractor and the trailer(s) when service brake caused retardation is replaced by auxiliary brake caused retardation.

Fig. 3 schematically illustrates a block diagram of a control device for controlling operation of a brake system of a motor vehicle according to an embodiment of the present invention.

The vehicle comprises a vehicle powertrain, wheel axles with wheels, wherein at least one wheel axle is a drive axle driven by means of the powertrain, the brake system comprising service brakes for braking the vehicle wheels and at least one auxiliary brake connected to the powertrain for braking drive axle wheels.

The vehicle according to the present invention may have any suitable number of wheel axles. The vehicle according to the present invention may have one or more steerable axles. The vehicle according to the present invention may have one or more drive axles, i.e. powered axles. The control device 100 for controlling operation of a brake system of a motor vehicle may be comprised in a system I for controlling operation of a brake system of a motor vehicle.

The control device may be implemented as a separate entity or distributed in two or more physical entities. The control device may comprise one or more computers. The control device may thus be implemented or realised by the control device comprising a processor and a memory, the memory comprising instructions, which when executed by the processor causes the control device to perform the herein disclosed method. The control device 100 may comprise one or more electronic control units, processing units, computers, server units or the like for determining vehicle operation of at least one vehicle. The control device 100 may comprise control device such as one or more electronic control units arranged on board a vehicle. The control device 100 may comprise one or more electronic control units, processing units, computers, server units or the like of an off- board system arranged externally to a vehicle and being operably connectable to the vehicle.

The control device 100 is configured to estimate a function expressing a dependence between wheel slip and braking force. An example of such a function is schematically illustrated in fig. 2b.

The control device 100 is configured to estimate the slip stiffness of the tires of the drive wheel of the drive axle during driving. This may be performed by comparison of wheel velocities of all wheels. The control device 100 is configured to determine how much more the driven axle/driven axles are braked in comparison to non-driven axles, and based on the braking difference, the wheel slip of the drive wheel may be obtained.

The system I may comprise slip stiffness estimation means 1 10 for estimating slip stiffness. The slip stiffness estimation means 1 10 may comprise sensors for detecting current wheel velocities. The slip stiffness estimation means 1 10 may comprise means for determining wheel velocities if the wheel where not subjected to any braking force. The slip stiffness estimation means 1 10 may comprise means for comparing wheel velocities. The slip stiffness estimation means 110 may comprise means for determining the braking force acting on the vehicle wheels. The means for determining the braking force acting on the vehicle wheels may comprise any suitable sensor for detecting the braking force. The means for determining the braking force may be in accordance with the means 120 below. The system I may comprise means 120 for determining the braking force acting on the drive wheels of the at least one drive axle. The means for determining the braking force acting on the drive wheels may comprise any suitable sensor. The system I may comprise means 130 for determining the normal force acting on the drive wheels of the at least one drive axle. The means for determining the normal force acting on the drive wheels may comprise any suitable sensor.

The control device 100 is configured to estimate function expressing a dependence between wheel slip and braking force based on the ratio of the thus by the means 120 determined braking force and the means 130 determined normal force for different braking forces corresponding to different wheel slips.

The control device 100 is configured to determine a maximum braking force for drive wheels of the at least one drive axle by means of said function and based upon a chosen target wheel slip. An example of determining such a maximum braking force by of said function is schematically illustrated in fig. 2b. The braking force and normal force ratio as a function of the wheel slip, exemplified with the curve in fig. 2b, and particularly the part of the function where the dependence between the braking force and the wheel slip corresponds to an essentially straight line illustrates the slip stiffness of the tires of the drive wheel of the drive axle.

The part of the function in which the dependence corresponds to an essentially straight line is used. Such an example is illustrated in fig. 2b. The target wheel slip corresponds to maximum braking force for drive wheels of the drive axle. The target wheel slip may be chosen to avoid high tire wear and slip levels leading to deactivation of auxiliary brake by stability preserving functions.

According to an embodiment the system I may comprise maximum braking force determination means 140 for determining a maximum braking force for drive wheels of the at least one drive axle by means of said function and based upon a chosen target wheel slip.

The control device 100 may comprise or be operably connectable to the means 140. The control device 100 is configured to determine a brake force corresponding to a requested retardation for the vehicle.

The control device 100 is configured to determine a requested retardation for the vehicle. The control device 100 is according to an embodiment operably connected to a brake request device 150. The brake request device 150 may be any suitable device for providing a request for braking such as e.g. a brake pedal or the like. The control device 100 is configured to determine a requested retardation for the vehicle based on brake request data from the brake request device 150.

The brake request device 150 may be any suitable device for requesting retardation of the vehicle. The brake request device 150 is according to an embodiment a brake pedal. The brake request device 150 is according to an embodiment an electronically operated device for requesting retardation of the vehicle. The brake request device 150 is according to an embodiment a adaptive cruise control device of the vehicle. The system I may according to an embodiment comprise the brake request device 150.

The control device 100 is configured to determine a brake force corresponding to the determined requested retardation. The control device 100 is configured to determine a brake force based on the determined requested retardation. The control device 100 may be configured to determine a brake force corresponding to the determined requested retardation based on vehicle mass. The control device 100 is according to an embodiment configured to determine vehicle load.

According to an embodiment the control device 100 is configured to determine vehicle wheel axle load when determining vehicle load. According to an embodiment the control device 100 is further configured to determine load distribution of the vehicle based on the thus determined vehicle wheel axle load for the respective wheel axle.

The system I may comprise vehicle load determination means 160 arranged to determine vehicle load of the vehicle. The vehicle load determination means 160 may be arranged to determine vehicle wheel axle load of the vehicle. The vehicle load determination means 160 may be arranged to determine the total load of the vehicle.

The control device 100 may comprise or be operably connectable to the vehicle load determination means 160. The vehicle load determination means 160 may comprise vehicle load detection means 162 for detecting vehicle load.

The vehicle load detection means 162 may comprise one or more load detection sensors. The vehicle load detection means 162 may comprise one or more load detection sensors arranged in connection to the wheel axles of the vehicle. The vehicle load detection means 162 may comprise one or more load detection sensors arranged in connection to or comprised in bellows of a vehicle having a suspension system with a bellows configuration comprising a set of bellows arranged in connection to the respective axles of the chassis of the vehicle. According to an embodiment the vehicle load detection means 162 may comprise means for determining bellows pressure in connection to the respective wheel axle and means for determining the vehicle weight based on the bellows pressure and the vehicle inclination. According to an embodiment the vehicle load detection means 162 may comprise vehicle weight detection means for detecting the total vehicle weight. The vehicle weight detection means may comprise a vehicle weigh scale for weighing the vehicle. According to an embodiment the vehicle load detection means 162 may comprise one or more detectors for weighing the load on the vehicle, wherein the total vehicle weight is determined by adding the thus detected vehicle load to the original weight of the vehicle which according to an embodiment is known beforehand. A predetermined service brake force distribution of the brake force corresponding to a requested retardation of the vehicle is arranged to be provided. The predetermined service brake force distribution is arranged to be exerted by vehicle service brakes of the brake system of the vehicle.

The control device 100 is according to an embodiment configured to provide a predetermined service brake force distribution of the brake force corresponding to the requested retardation of the vehicle. The predetermined service brake force distribution of the brake force corresponding to the requested retardation of the vehicle thus corresponds to a certain portion of the brake force corresponding to the requested retardation to be distributed to the wheels of the respective wheel axle of the vehicle.

According to an embodiment the system I may comprise service brake force distribution determination means 170 for determining service brake force distribution of the brake force corresponding to the requested retardation of the vehicle. The control device 100 may comprise or be operably connectable to the means 170. A predetermined service brake force distribution of the brake force corresponding to a requested retardation of the vehicle may be determined based on certain known parameters such as vehicle axle load distribution.

The control device 100 is further configured to determine a maximum admissible braking force to be exerted by means of the at least one auxiliary brake for braking drive wheels of the at least one drive axle.

The maximum admissible braking force to be exerted by means of the at least one auxiliary brake is arranged to be determined based upon the predetermined vehicle service brake force distribution to be exerted by vehicle service brakes, the determined maximum braking force and the brake force corresponding to the requested retardation for the vehicle.

According to an embodiment the system I may comprise maximum admissible braking force determination means 180 for determining maximum admissible braking force to be exerted by means of the at least one auxiliary brake for braking drive wheels of the at least one drive axle.

The control device 100 may comprise or be operably connectable to the means 180.

Thus, knowing the maximum braking force, i.e. the maximum force the driven wheels can be subjected to, and the distribution of the overall service brake force over the vehicle axles, the auxiliary brake portion of the total force can be maximized without exceeding the maximum force limit on the driven wheels. In this way, auxiliary brake caused retardation may be maximized providing optimal relief of service brakes. The solution is realized in the following way when a retardation of the vehicle is requested by means of the brake request device such as a brake pedal, an electronically operated device, an adaptive cruise control or the like, the retardation request e.g. being provided by the driver through a brake pedal or electronically by other functions such as adaptive cruise control in the vehicle. where F _aux is the maximum admissible braking force to be exerted by means of the at least one auxiliary brake, F _s is the total service brake force to be exerted by vehicle service brakes, i.e. total brake force due to wheel brakes on the vehicle and F _max is the maximum braking force, i.e. maximum allowed brake force at the driven axle determined from slip stiffness and the target slip value. Furthermore, F is the brake force corresponding to a requested retardation for the vehicle, i.e. the requested braking force, and a _n are constants explaining the service brake force distribution amongst the axles of the vehicle, e.g. axles of tractor and trailer(s) (i.e., å _n a _n = 1). a _driven is the portion of service brake force applied on the driven axle (e.g., a _driven = a ₂ if the second axle is the driven axle as in the example in fig. 1 ).

According to an embodiment the control device 100 is configured to determine the maximum admissible braking force to be exerted by means of the at least one auxiliary brake, Faux, by:

The control device 100 is configured to control operation of the brake system of the vehicle based upon the thus determined maximum admissible braking force to be exerted by means of the at least one auxiliary brake and the brake force corresponding to the requested retardation for the vehicle for obtaining the requested retardation with a maximum use of the at least one auxiliary brake for braking the vehicle.

According to an embodiment the control device 100, when controlling operation of the brake system based upon the thus determined maximum admissible braking force, is configured to adapt the service brake force distribution on the wheels of the wheel axles, to be exerted by the vehicle service brakes to a maximum use of the at least one auxiliary brake for braking the vehicle, so as to obtain the requested retardation for the vehicle. According to an embodiment the control device 100 is configured to determine the braking force actually exertable by means of the at least one auxiliary brake, the braking force actually exertable corresponding to the maximum use of the at least one auxiliary brake for braking the vehicle, in order to determine whether the determined maximum admissible braking force can be used by the at least one auxiliary brake.

The at least one auxiliary brake is hereinafter denoted the at least one auxiliary brake A.

The control device 100 is according to an embodiment operably connected to the at least one auxiliary brake A. The control device 100 is arranged to receive information from the braking force actually exertable by means of the at least one auxiliary brake A of the braking force actually exertable by means of the at least one auxiliary brake A.

According to an embodiment the system I comprises the at least one auxiliary brake A. According to an embodiment of the control device, if the determined braking force actually exertable by means of the at least one auxiliary brake is lower than the determined maximum admissible braking force, the control device is configured to use the force actually exertable by means of the at least one auxiliary brake in the control of the operation of the brake system. Fig. 5 schematically illustrates a flowchart of a method performed by a control device such as a control device 100 illustrated in fig. 3 for controlling operation of a brake system of a motor vehicle. According to an embodiment of the invention, the control device 100 is, via a link 10, operably connected to the slip stiffness estimation means 1 10. According to an embodiment of the invention, the control device 100 is via the link 10 arranged to receive a signal from the means 110 representing data about estimated slip stiffness of driven wheels, i.e. tires of driven wheels.

According to an embodiment of the invention, the control device 100 is, via a link 20, operably connected to the means 120 for determining the braking force acting on the drive wheels. According to an embodiment of the invention, the control device 100 is via the link 20 arranged to receive a signal from the means 120 representing data about braking force acting on the drive wheels.

According to an embodiment of the invention, the control device 100 is, via a link 30, operably connected to the means 130 for determining the normal force acting on the drive wheels of the at least one drive axle. According to an embodiment of the invention, the control device 100 is via the link 30 arranged to receive a signal from the means 130 representing data about normal force acting on the drive wheels of the at least one drive axle.

According to an embodiment of the invention, the control device 100 is, via a link 40a, operably connected to the maximum braking force determination means 140. According to an embodiment of the invention, the control device 100 is via the link 40a arranged to send a signal to the means 140 representing data about a function expressing a dependence between wheel slip and braking force and a chosen target wheel slip.

According to an embodiment of the invention, the control device 100 is, via a link 40b, operably connected to the maximum braking force determination means 140. According to an embodiment of the invention, the control device 100 is via the link 40b arranged to receive a signal from the means 140 representing data about maximum braking force for drive wheels of the at least one drive axle for the thus chosen wheel slip.

According to an embodiment of the invention, the control device 100 is, via a link 50, operably connected to the brake request device 150. According to an embodiment of the invention, the control device 100 is via the link 50 arranged to receive a signal from the brake request device 150 representing data about a brake force corresponding to a requested retardation for the vehicle.

According to an embodiment of the invention, the control device 100 is, via a link 60, operably connected to the vehicle load determination means 160. According to an embodiment of the invention, the control device 100 is via the link 60 arranged to receive a signal from the vehicle load determination means 160 representing data about vehicle load comprising data about vehicle axle load and/or vehicle weight of the vehicle.

According to an embodiment of the invention, the control device 100 is, via a link 62, operably connected to the vehicle load detection means 162. According to an embodiment of the invention, the control device 100 is via the link 62 arranged to receive a signal from the vehicle load detection means 162 representing data about detected vehicle load comprising data about vehicle axle load and/or vehicle weight of the vehicle.

According to an embodiment of the invention, the control device 100 is, via a link 70a, operably connected to the service brake force distribution determination means 170. According to an embodiment of the invention, the control device 100 is via the link 70a arranged to send a signal to the means 170 representing data about a brake force corresponding to a requested retardation for the vehicle.

According to an embodiment of the invention, the control device 100 is, via a link 70b, operably connected to the service brake force distribution determination means 170. According to an embodiment of the invention, the control device 100 is via the link 70b arranged to send a signal to the means 170 representing data about vehicle wheel axle load.

According to an embodiment of the invention, the control device 100 is, via a link 70c, operably connected to the service brake force distribution determination means 170. According to an embodiment of the invention, the control device 100 is via the link 70c arranged to receive a signal from the means 170 representing data about brake force distribution to be exerted by vehicle service brakes.

According to an embodiment of the invention, the control device 100 is, via a link 80a, operably connected to the maximum admissible braking force determination means 180. According to an embodiment of the invention, the control device 100 is via the link 80a arranged to send a signal to the means 180 representing data about predetermined vehicle service brake force distribution to be exerted by vehicle service brakes for braking drive wheels of the at least one drive axle.

According to an embodiment of the invention, the control device 100 is, via a link 80b, operably connected to the maximum admissible braking force determination means 180. According to an embodiment of the invention, the control device 100 is via the link 80b arranged to send a signal to the means 180 representing data about determined maximum braking force for drive wheels of the at least one drive axle.

According to an embodiment of the invention, the control device 100 is, via a link 80c, operably connected to the maximum admissible braking force determination means 180. According to an embodiment of the invention, the control device 100 is via the link 80c arranged to send a signal to the means 180 representing data about brake force corresponding to the requested retardation for the vehicle.

According to an embodiment of the invention, the control device 100 is, via a link 80d, operably connected to the maximum admissible braking force determination means 180. According to an embodiment of the invention, the control device 100 is via the link 80d arranged to send a signal to the means 180 representing data about maximum admissible braking force to be exerted by means of the at least one auxiliary brake for braking drive wheels of the at least one drive axle.

According to an embodiment of the invention, the control device 100 is, via a link A10, operably connected to the at least one auxiliary brake A. According to an embodiment of the invention, the control device 100 is via the link A10 arranged to receive a signal from the at least one auxiliary brake A representing data about the braking force actually exertable by means of the at least one auxiliary brake A.

According to an embodiment of the invention, the control device 100 is, via a link 70d, operably connected to the brake force distribution determination means 170. According to an embodiment of the invention, the control device 100 is via the link 70d arranged to send a signal to the means 170 representing data about maximum admissible braking force to be exerted by means of the at least one auxiliary brake for braking drive wheels of the at least one drive axle and where applicable data about braking force actually exertable by means of the at least one auxiliary brake. According to an embodiment of the invention, the control device 100 is via the link 70d arranged to send a signal to the means 170 representing data about braking force actually exertable by means of the at least one auxiliary brake. The braking force actually exertable by means of the at least one auxiliary brake may correspond to the maximum admissible braking force to be exerted by means of the at least one auxiliary brake. There may be a delay of the activation of the at least one auxiliary brake so that initially the braking force actually exertable by means of the at least one auxiliary brake may be low/substantially zero but then increases upon activation.

According to an embodiment of the invention, the control device 100 is, via a link 70e, operably connected to the brake force distribution determination means 170. According to an embodiment of the invention, the control device 100 is via the link 70e arranged to receive a signal from the means 170 representing data about adapted brake force distribution on the wheels of the wheel axles, to be exerted by the vehicle service brakes, to a maximum use of the at least one auxiliary brake for braking the vehicle, so as to obtain the requested retardation for the vehicle. The maximum use of the at least one auxiliary brake for braking the vehicle may comprise braking to be exerted by means of the at least one auxiliary brake so as to obtain the requested retardation for the vehicle. Fig. 4 schematically illustrates a flowchart of a method M1 performed by a control device for controlling operation of a brake system of a motor vehicle according to an embodiment of the present invention.

The vehicle comprises a vehicle powertrain. The vehicle comprises wheel axles with wheels. At least one wheel axle is a drive axle driven by means of the powertrain. The vehicle comprises the brake system. The brake system comprises service brakes for braking the vehicle wheels and at least one auxiliary brake connected to the powertrain for braking drive axle wheels.

According to the embodiment the method performed by a control device for controlling operation of a brake system of a motor vehicle comprises a step S1. In this step at a function expressing a dependence between wheel slip and braking force is estimated.

The slip stiffness of the tires of the drive wheel of the drive axle may be estimated during driving. The slip stiffness may be estimated by means of the slip stiffness estimation means 110 with reference to fig. 3. The braking force acting on the drive wheels of the at least one drive axle may be determined. The braking force acting on the drive wheels may be determined by the means 120 for determining the braking force acting on the drive wheels of the at least one drive axle with reference to fig. 3. The normal force acting on the drive wheels of the at least one drive axle may be determined. The normal force acting on the drive wheels may be determined by the means 130 for determining the normal force acting on the drive wheels of the at least one drive axle with reference to fig. 3. According to the embodiment the method comprises a step S2. In this step a maximum braking force for drive wheels of the at least one drive axle is determined by means of said function and based upon a chosen target wheel slip. The maximum braking force for drive wheels of the at least one drive axle is according to an embodiment arranged to be determined by means of the maximum braking force determination means 140 described with reference to fig. 3.

According to the embodiment the method comprises a step S3. In this step a brake force corresponding to a requested retardation for the vehicle is determined. The brake force corresponding to a requested retardation for the vehicle may be is according to an embodiment arranged to be determined by means of the brake request device 150 with reference to fig. 3.

According to the embodiment the method comprises a step S4. In this step a maximum admissible braking force to be exerted by means of the at least one auxiliary brake is determined based upon a predetermined vehicle service brake force distribution, to be exerted by vehicle service brakes, for braking drive wheels of the at least one drive axle, the determined maximum braking force and the brake force corresponding to the requested retardation for the vehicle. The maximum admissible braking force to be exerted by means of the at least one auxiliary brake according to an embodiment arranged to be determined by means of the maximum admissible braking force determination means 180 with reference to fig. 3.

According to the embodiment the method comprises a step S5. In this step operation of the brake system of the vehicle is controlled based upon the thus determined maximum admissible braking force to be exerted by means of the at least one auxiliary brake and the brake force corresponding to the requested retardation for the vehicle for obtaining the requested retardation with a maximum use of the at least one auxiliary brake for braking the vehicle. The control is arranged to be performed by means of the control device 100.

According to an embodiment of the method the step S5 of controlling operation of the brake system based upon the thus determined maximum admissible braking force comprises adapting the service brake force distribution on the wheels of the wheel axles, to be exerted by the vehicle service brakes, to a maximum use of the at least one auxiliary brake for braking the vehicle, so as to obtain the requested retardation for the vehicle.

According to an embodiment the method further comprises step of determining the braking force actually exertable by means of the at least one auxiliary brake, the braking force actually exertable corresponding to the maximum use of the at least one auxiliary brake for braking the vehicle, in order to determine whether the determined maximum admissible braking force can be used by the at least one auxiliary brake can use. The step of determining the braking force actually exertable by means of the at least one auxiliary brake may be arranged to be determined by means of information from the at least one auxiliary brake, e.g. the at least one auxiliary brake A with reference to fig. 3.

According to an embodiment of the method, if the determined braking force actually exertable by means of the at least one auxiliary brake is lower than the determined maximum admissible braking force, the force actually exertable by means of the at least one auxiliary brake is used in the control of the operation of the brake system.

According to an embodiment of the method the step of determining a maximum braking force for drive wheels of the at least one drive axle comprises the step of using a part of the function in which the dependence corresponds to an essentially straight line.

The method M1 performed by a control device for controlling operation of a brake system of a motor vehicle is according to an embodiment adapted to be performed by the system I described above with reference to fig. 3.

Fig. 5 schematically illustrates a flowchart of a method M2 performed by a control device for controlling operation of a brake system of a motor vehicle according to an embodiment of the present invention. The control device may be a control device 100 as described with reference to fig. 3. The vehicle comprises a vehicle powertrain, wheel axles with wheels, wherein at least one wheel axle is a drive axle driven by means of the powertrain, the brake system comprising service brakes for braking the vehicle wheels and at least one auxiliary brake A connected to the powertrain for braking drive axle wheels. The maximum braking force for drive wheels of the at least one drive axle is determined by based upon an estimated slip stiffness 110 and a chosen target wheel slip WS. The maximum braking force Fmax is determined by means of maximum braking force determination means 140 e.g. in accordance with the means 140 described with reference to fig. 3. A brake force F corresponding to a requested retardation for the vehicle is determined by means of the control device. The brake force F corresponding to a requested retardation is determined based on information from a brake request device 150, e.g. a brake request device 150 as described with reference to fig. 3. The maximum admissible braking force Faux to be exerted by means of the at least one auxiliary brake A for braking drive wheels of the at least one drive axle is determined by means of maximum admissible braking force determination means 180, e.g. means 180 as described with reference to fig. 3.

The maximum admissible braking force Faux to be exerted by means of the at least one auxiliary brake A is arranged to be determined by the means 180 based upon the predetermined vehicle service brake force distribution 170 to be exerted by vehicle service brakes, the determined maximum braking force Fmax from the means 140 and the brake force F corresponding to the requested retardation for the vehicle from the brake request device 150.

The at least one auxiliary brake A for braking drive wheels of the at least one drive axle is requested by the means 180 to exert the determined maximum admissible braking force.

The at least one auxiliary brake A for braking drive wheels of the at least one drive axle is arranged to provide information about the braking force F _e actually exertable by means of the at least one auxiliary brake A. The operation of the brake system of the vehicle is controlled based upon the thus determined maximum admissible braking force Faux to be exerted by means of the at least one auxiliary brake A, i.e. the braking force Fe actually exertable by means of the at least one auxiliary brake A, and the brake force F corresponding to the requested retardation for the vehicle for obtaining the requested retardation with a maximum use of the at least one auxiliary brake A for braking the vehicle.

Thus, the service brake force distribution on the wheels of the wheel axles, to be exerted by vehicle service brakes, is adapted based on the braking force Fe actually exertable by means of the at least one auxiliary brake A to a maximum use of the at least one auxiliary brake A for braking the vehicle, so as to obtain the requested retardation for the vehicle. Thus, the braking force F _s for the service brake force distribution is F-F _e . The service brake force distribution ai Fs, a2Fs, asFs, ..., aNFs is arranged to be determined by service brake force distribution determination means 170. With reference to figure 6, a diagram of a computer 500/apparatus 500 is shown. The control device 100 described with reference to fig. 3 may according to an embodiment comprise apparatus 500. Apparatus 500 comprises a non-volatile memory 520, a data processing device 510 and a read/write memory 550. Non-volatile memory 520 has a first memory portion 530 wherein a computer program, such as an operating system, is stored for controlling the function of apparatus 500. Further, apparatus 500 comprises a bus controller, a serial communication port, l/O-means, an A/D-converter, a time date entry and transmission unit, an event counter and an interrupt controller (not shown). Non-volatile memory 520 also has a second memory portion 540.

A computer program P is provided comprising routines for controlling operation of a brake system of a motor vehicle. The vehicle comprises a vehicle powertrain, wheel axles with wheels, wherein at least one wheel axle is a drive axle driven by means of the powertrain, the brake system comprising service brakes for braking the vehicle wheels and at least one auxiliary brake connected to the powertrain for braking drive axle wheels.

The program P comprises routines for estimating a function expressing a dependence between wheel slip and braking force. The program P comprises routines for determining a maximum braking force for drive wheels of the at least one drive axle by means of said function and based upon a chosen target wheel slip. The program P comprises routines for determining a brake force corresponding to a requested retardation for the vehicle. The program P comprises routines for determining a maximum admissible braking force to be exerted by means of the at least one auxiliary brake based upon a predetermined vehicle service brake force distribution, to be exerted by vehicle service brakes, for braking drive wheels of the at least one drive axle, the determined maximum braking force and the brake force corresponding to the requested retardation for the vehicle. The program P comprises routines for controlling operation of the brake system of the vehicle based upon the thus determined maximum admissible braking force to be exerted by means of the at least one auxiliary brake and the brake force corresponding to the requested retardation for the vehicle for obtaining the requested retardation with a maximum use of the at least one auxiliary brake for braking the vehicle.

The routines for controlling operation of the brake system based upon the thus determined maximum admissible braking force may comprise routines for adapting the service brake force distribution on the wheels of the wheel axles, to be exerted by the vehicle service brakes to a maximum use of the at least one auxiliary brake for braking the vehicle, so as to obtain the requested retardation for the vehicle.

The program P may further comprise routines for determining the braking force actually exertable by means of the at least one auxiliary brake, the braking force actually exertable corresponding to the maximum use of the at least one auxiliary brake for braking the vehicle, in order to determine whether the determined maximum admissible braking force can be used by the at least one auxiliary brake. If the determined braking force actually exertable by means of the at least one auxiliary brake is lower than the determined maximum admissible braking force, the force actually exertable by means of the at least one auxiliary brake is used in the control of the operation of the brake system.

The routines for determining a maximum braking force for drive wheels of the at least one drive axle may comprise routines for using a part of the function in which the dependence corresponds to an essentially straight line. The computer program P may be stored in an executable manner or in a compressed condition in a separate memory 560 and/or in read/write memory 550.

When it is stated that data processing device 510 performs a certain function it should be understood that data processing device 510 performs a certain part of the program which is stored in separate memory 560, or a certain part of the program which is stored in read/write memory 550.

Data processing device 510 may communicate with a data communications port 599 by means of a data bus 515. Non-volatile memory 520 is adapted for communication with data processing device 510 via a data bus 512. Separate memory 560 is adapted for communication with data processing device 510 via a data bus 511. Read/write memory 550 is adapted for communication with data processing device 510 via a data bus 514. To the data communications port 599 e.g. the links connected to the control unit 100 may be connected.

When data is received on data port 599 it is temporarily stored in second memory portion 540. When the received input data has been temporarily stored, data processing device 510 is set up to perform execution of code in a manner described above. The signals received on data port 599 may be used by apparatus 500 for estimating a function expressing a dependence between wheel slip and braking force. The signals received on data port 599 may be used by apparatus 500 for determining a maximum braking force for drive wheels of the at least one drive axle by means of said function and based upon a chosen target wheel slip. The signals received on data port 599 may be used by apparatus 500 for determining a brake force corresponding to a requested retardation for the vehicle. The signals received on data port 599 may be used by apparatus 500 for determining a maximum admissible braking force to be exerted by means of the at least one auxiliary brake based upon a predetermined vehicle service brake force distribution, to be exerted by vehicle service brakes, for braking drive wheels of the at least one drive axle, the determined maximum braking force and the brake force corresponding to the requested retardation for the vehicle. The signals received on data port 599 may be used by apparatus 500 for controlling operation of the brake system of the vehicle based upon the thus determined maximum admissible braking force to be exerted by means of the at least one auxiliary brake and the brake force corresponding to the requested retardation for the vehicle for obtaining the requested retardation with a maximum use of the at least one auxiliary brake for braking the vehicle. The signals used for controlling operation of the brake system based upon the thus determined maximum admissible braking force may comprise signals used for adapting the service brake force distribution on the wheels of the wheel axles, to be exerted by the vehicle service brakes to a maximum use of the at least one auxiliary brake for braking the vehicle, so as to obtain the requested retardation for the vehicle.

The signals received on data port 599 may be used by apparatus 500 determining the braking force actually exertable by means of the at least one auxiliary brake, the braking force actually exertable corresponding to the maximum use of the at least one auxiliary brake for braking the vehicle, in order to determine whether the determined maximum admissible braking force can be used by the at least one auxiliary brake. If the determined braking force actually exertable by means of the at least one auxiliary brake is lower than the determined maximum admissible braking force, the force actually exertable by means of the at least one auxiliary brake is used in the control of the operation of the brake system.

The signals used for determining a maximum braking force for drive wheels of the at least one drive axle may be used for using a part of the function in which the dependence corresponds to an essentially straight line.

Parts of the methods described herein may be performed by apparatus 500 by means of data processing device 510 running the program stored in separate memory 560 or read/write memory 550. When apparatus 500 runs the program, parts of the methods described herein are executed.

The foregoing description of the preferred embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated.