Road slope based wheel slip control for open differentials

Abstract

A computer-implemented method for controlling motion of a vehicle (100), where the vehicle (100) comprises wheels (310) driven by a propulsion device (340) via a differential (345), where each driven wheel (310) of the differential (345) is associated with a wheel brake (320), the method comprising monitoring (S1) an inclination of the vehicle (100) and / or of a surface (101) supporting the vehicle (100), relative to a reference plane, and configuring (S3) a wheel slip limit (λ x) based on the inclination, such that an increase in inclination magnitude results in a decrease in wheel slip limit magnitude, the method further comprising, determining (S5) wheel motion indicative of a wheel slip (λ x) of each driven wheel (310) of the differential (345), and triggering (S6) a brake intervention operation by the corresponding wheel brake (320), in response to that the wheel slip (λ lim ) of a driven wheel (310) exceeds the wheel slip limit ().

Description

ROAD SLOPE BASED WHEEL SLIP CONTROL FOR OPEN DIFFERENTIALSTECHNICAL FIELD

[0001] This disclosure relates generally to traction control in heavy-duty vehicles. In particular aspects, the disclosure relates to road slope based wheel slip control for open differentials. The disclosure can be applied to heavy-duty vehicles, such as trucks, buses, and construction equipment, among other vehicle types. Although the disclosure may be described with respect to a particular vehicle, the disclosure is not restricted to any particular vehicle.BACKGROUND

[0002] Many heavy-duty vehicles such as trucks and semi-trailers comprise axles with driven wheels that are connected to a power source via an open differential, which distributes the propulsion power from the power source between the wheels.

[0003] When performing a take-off in slippery conditions it is common that the wheels on one side of the vehicle have more available grip than on the other side. When this difference in available road friction is large it is often referred to as a ‘split friction’ scenario. In order to maximize traction in these situations, when operating with an open differential, a brake torque can be applied to the driven wheels on the low friction side to transfer more drive torque to the high friction side, thereby enhancing the vehicle’s ability to accelerate while reducing the risk of getting stuck and / or starting to slide backwards. This operation is commonly known as traction control.

[0004] There is a desire to improve traction control systems, and in particular traction control systems in heavy-duty vehicles.SUMMARY

[0005] According to a first aspect of the disclosure, there is presented a computer-implemented method for controlling motion of a vehicle, such as a truck, bus, construction equipment or car, where the vehicle comprises wheels driven by a propulsion device via a differential and whereeach driven wheel of the differential is associated with a wheel brake, and in some cases more than one wheel brake. The method comprises monitoring an inclination of the vehicle and / or of a surface supporting the vehicle, relative to a horizontal plane or some other reference plane, and configuring a wheel slip limit of the vehicle motion control system based on the inclination, such that an increase in inclination magnitude results in a decrease in wheel slip limit magnitude. The method further comprises determining wheel motion indicative of wheel slip of each driven wheel of the differential and triggering a brake intervention operation by the corresponding wheel brake, in response to that the wheel slip of a driven wheel associated with the wheel brake exceeds the wheel slip limit. The first aspect of the disclosure may seek to improve traction control by the vehicle in operating conditions involving significant slopes. A technical benefit may include improved startability performance of the vehicle when starting from stand-still or from low speed in uphill scenarios. The improvements in traction control are particularly pronounced in split-friction conditions, where surface friction on one side of the vehicle differs from the surface friction on the other side of the vehicle. The method results in a more aggressive triggering of traction control when the vehicle is in a slope compared to when the vehicle is on more flat ground. The method can be applied with advantage in both uphill and downhill operating scenarios.A similar effect can be obtained by modifying a control bandwidth of the traction control system, such that the traction control system becomes more aggressive in sloping scenarios compared to when on more flat ground. Thus, there is also presented a computer-implemented method for controlling motion of a vehicle, where the vehicle comprises wheels driven by a propulsion device via a differential and where each driven wheel of the differential is associated with at least one wheel brake. The method comprises monitoring an inclination of the vehicle and / or of a surface supporting the vehicle, relative to a reference plane such as the horizontal plane and configuring a brake intervention torque application rate associated with the wheel brakes of the differential, based on the inclination, such that an increase in inclination magnitude results in an increase in torque application rate. The method also comprises determining wheel motion indicative of a wheel slip of each driven wheel of the differential and triggering a brake intervention operation by the corresponding wheel brake, in response to that the wheel slip of a driven wheel exceeds a wheel slip limit, where the brake intervention operation is parameterized by the brake intervention torque application rate.

[0006] This is an alternative or complementary action which also improves traction control of the vehicle, especially in split-friction conditions. The common denominator of the techniques disclosed herein is that vehicle inclination is monitored, and brake intervention operations are more aggressively configured in case the vehicle is in a significant slope (as indicated by the monitored inclination). This method can also be applied with advantage in both uphill and downhill operating conditions, i.e., in both positive and negative inclination driving scenarios.

[0007] According to an example, the determined wheel motion of at least one driven wheel comprises a wheel speed in relation to a vehicle speed over ground. This is a common way to reliably measure wheel slip, and it can be realized using a wheel speed sensor of low complexity. The vehicle speed over ground may be obtained from various sources, including another wheel, such as a free-rolling wheel of the vehicle, from a satellite positioning system, a forward looking vision-based sensor such as a camera, or from a ground speed radar system, just to name a few examples. The vehicle speed over ground is preferably a longitudinal vehicle speed over ground, measured in the general forward-rearward direction of the vehicle.

[0008] According to another example, the determined wheel motion of at least one driven wheel comprises a wheel speed in relation to a wheel speed of at least one other driven wheel of the differential. By measuring a difference in wheel speeds over the differential instead of absolute wheel slip values of each wheel, there is no dependence on vehicle speed over ground, which can be an advantage in scenarios where vehicle speed over ground is not readily available, or where the vehicle speed over ground is not accurate enough for traction control purposes.

[0009] The method may also comprise controlling an axle speed of the propulsion device based at least indirectly on the wheel slip of each driven wheel of the differential. Wheel slip based propulsion device control is advantageous for many reasons compared to traditional torquebased control of propulsion device, especially if the propulsion device of the differential comprises an electric machine which is capable of controlling axle speed with low latency and high control bandwidth, i.e., when the propulsion device is able to respond very fast to changes in operating conditions to maintain an axle speed close to a target axle speed. By controlling a wheel based on wheel slip, it becomes easier to maintain a high bandwidth actuator control which is able to respond fast to changes in friction. The wheel actuator control can also bemoved closer to wheel end, away from central control units of the vehicle, which is an advantage since it often shortens the overall control loop delay in the actuator control system.

[0010] According to some examples the methods discussed herein comprise monitoring the inclination based on obtained topographical data from a digital map. The data thus obtained is often accurate, often more so than inclination data obtained from inclination sensors on the vehicle, which is an advantage. The slope data obtained from digital maps can be obtained based on the position of the vehicle, which position can normally be obtained from a satellite positioning system in a reliable manner.

[0011] Inclination data can of course also be obtained from an inclinometer sensor, preferably as a complement to the digital map based inclination data. Inclinometer sensors can be based, e.g., on the output from an inertial measurement unit (IMU). Inclinometers can be used as a stand-alone source of slope data.

[0012] Some examples of the herein disclosed methods also comprise pre-arming the wheel brakes in case the magnitude of the monitored inclination satisfies a first inclination magnitude acceptance criteria. This way the response time of the brake actuators is reduced compared to the case when brakes are not pre-armed. It is an advantage that the brakes are responsive when performing traction control in difficult conditions, such as uphill split friction conditions. As the slope increases, the vehicle traction control system is “put on its toes” so that it is able to step in faster if needed.

[0013] Some examples of the herein disclosed methods also comprise configuring the wheel slip limit based on a predetermined mapping between a discrete set of wheel slip limits and corresponding ranges of inclination. This is a relatively straight forward and robust method of setting the wheel slip limit used by the system. It is an advantage that the wheel slip limit is adjusted as function of inclination, and that the mapping can be configured specifically for each vehicle type. By using more than two wheel slip limits a finer granularity and a more accurate control is obtained. Some examples of the methods comprise configuring the wheel slip limit also based on a longitudinal speed over ground of the vehicle. The speed of the vehicle may play an important part in some cases. For instance, a vehicle which is starting from standstill may benefit from using other traction control parameters compared to a vehicle which has accumulated some speed. Some examples of the methods discussed herein may also comprise configuring the wheel slip limit based on an at least piece-wise continuous function ofinclination. This way the wheel slip limit changes with an even finer level of granularity. The at least piece-wise continuous function can be configured as a partly non-linear function which is an advantage.

[0014] According to other examples, the computer-implemented method comprises configuring a nominal wheel slip limit value based on an inverse tyre model, wherein the inverse tyre model is representative of a relationship between wheel slip and wheel force of the driven wheels of the differential. It is an advantage that the wheel slip limit is configured with wheel force in mind, since wheel force is important in many vehicle operating scenarios, such as uphill vehicle launch. The wheel slip limit may be configured as a wheel slip value that corresponds to peak force, optionally with some margin to account for delays in the system, and inaccuracies in the inverse tyre model.

[0015] The methods discussed herein may also comprise configuring a brake intervention maximum torque associated with the wheel brakes, based on the inclination, such that an increase in inclination magnitude results in an increase in the brake intervention maximum torque. A higher maximum brake intervention torque may be disadvantageous in terms of, e.g., tyre wear and energy expenditure of the overall vehicle system, but may also improve vehicle capabilities in difficult scenarios, such as uphill split-friction scenarios. It is an advantage that the maximum brake torque is not used on more flat ground, i.e., only where it is needed. The brake intervention maximum torque associated with the wheel brakes can be an absolute torque value of a difference in applied brake torque over the wheels of the differential.

[0016] Some of the computer-implemented methods disclosed herein also comprises configuring a brake intervention operation control bandwidth, based on the inclination, such that an increase in inclination magnitude results in an increase in brake intervention operation control bandwidth. An increased brake intervention operation control bandwidth results in a faster response to changes in, e.g., road friction, which is an advantage. It is an advantage also that the control bandwidth is kept at a lower level when not needed, since this results in less discomfort for the vehicle operator. The brake intervention operation control bandwidth is often a control loop bandwidth associated with a loop filter, such as a low-pass filter applied to an error signal of the control loop. The brake intervention operation control bandwidth can also be configured by adjusting P, I, or D parameters in a PID controller, or by setting the gain parameter of a Kalman filter, just to name a few examples.

[0017] The methods discussed herein may also comprise configuring a nominal applied brake torque of the brake intervention operation, based on the inclination, such that an increase in inclination magnitude results in an increase in nominal applied brake torque. This makes the brake intervention operation more resolute, since the initial brake torque step is larger in a slope compared to when on more flat ground, which often improves vehicle performance in difficult driving scenarios, such as split-friction uphill driving conditions.

[0018] According to some examples of the methods discussed herein, the differential is a lockable differential, and the method further comprises locking the lockable differential in case the magnitude of the monitored inclination satisfies a second inclination magnitude acceptance criteria.

[0019] Vehicles, control systems, computer programs and computer program products are also disclosed herein. The vehicles, control systems, computer programs and computer program products are associated with the same advantages and technical effects as the various example methods.

[0020] The disclosed aspects, examples (including any preferred examples), and / or accompanying claims may be suitably combined with each other as would be apparent to anyone of ordinary skill in the art. Additional features and advantages are disclosed in the following description, claims, and drawings, and in part will be readily apparent therefrom to those skilled in the art or recognized by practicing the disclosure as described herein.

[0021] There are also disclosed herein computer systems, control units, code modules, computer-implemented methods, computer readable media, and computer program products associated with the above discussed technical benefits.BRIEF DESCRIPTION OF THE DRAWINGSExamples are described in more detail below with reference to the appended drawings, where:

[0022] Figure 1 illustrates an exemplary vehicle according to some of the teachings herein.

[0023] Figure 2 is a graph that illustrates tyre force as function of wheel slip.

[0024] Figure 3 schematically shows an exemplary vehicle motion control system.

[0025] Figure 4 illustrates vehicle operations in an example split friction scenario.

[0026] Figure 5 shows an overview of an example vehicle motion management system.

[0027] Figures 6A-B are flow charts that illustrates exemplary methods.

[0028] Figure 7 is a schematic diagram of an exemplary computer system.

[0029] Figure 8 schematically illustrates an actuator control loop with a control bandwidth.DETAILED DESCRIPTION

[0030] Figure 1 illustrates an example heavy-duty vehicle 100, here in the form of a rigid truck. It is appreciated that the herein disclosed methods and control units can be applied with advantage also in other types of heavy-duty vehicles, such as articulated vehicles, semi-trailer vehicles, trucks with drawbar connections, construction equipment, buses, and the like. The vehicle 100 may also comprise more than two vehicle units, i.e., a dolly vehicle unit may be used to tow more than one trailer. Aspects of the disclosure are also applicable in passenger cars and in recreational vehicles.

[0031] The vehicle 100 comprises a plurality of wheels 102, where at least a subset of the wheels 102 are associated with one or more motion support devices (MSD) 104. A motion support device is, generally, some type of actuator or control assembly that can be used to control motion of the vehicle 100. Examples of MSDs include propulsion devices, brakes, active suspension devices, and the steering actuator on a steered axle. Although the embodiment depicted in Figure 1 illustrates an MSD 104 for each of the wheels 102, it should be readily understood that e.g., one pair of wheels 102 may be arranged without an MSD 104. Also, some MSDs may be arranged connected to more than one wheel, e.g., via a differential arrangement.

[0032] The vehicle 100 is supported on a surface 101 which can be a road surface or some other supporting surface such as a gravel field on a work site. This surface has an inclination relative to the horizontal plane or relative to some other reference plane, at least locally. In the case of a road surface, this inclination is often referred to as road slope, and can be given as an average angle between the road and the reference plane measured over some local area. The inclination can be both positive (uphill slope) and negative (downhill slope), and at least road slope normally refers to a longitudinal inclination in the travelling direction of the vehicle. The inclination of the surface 101 translates into a pose of the vehicle 100 relative to the referenceplane, i.e., the inclination of the road surface normally is the same as the inclination of the vehicle 100 supported thereon, at least in an average sense over the local area.

[0033] The MSDs may be arranged for generating a torque on a respective wheel of the vehicle or for both wheels of an axle 106. The MSD may be a propulsion device, such as an electric machine arranged to provide a longitudinal wheel force to the wheel(s) of the vehicle 100. Such an electric machine may be adapted to generate a propulsion torque as well as a braking torque. One or more combustion engines can also be arranged on the vehicle 100 to generate propulsion torque, and also braking torque if the combustion engine is configured for endurance (motor) braking. The MSDs 104 may also comprise service brakes such as friction brakes (normally disc brakes or drum brakes) arranged to generate a braking torque by the wheel 102 in order to decelerate the vehicle 100.

[0034] The methods disclosed herein primarily relate to controlling propulsion of heavy-duty vehicles, i.e., acceleration. However, the disclosed methods may also find use in decelerating heavy-duty vehicles, i.e., during braking maneuvers in complicated downhill driving scenarios.

[0035] Each of the MSDs 104 is connected to a respective MSD control system 330 arranged for controlling operation of the MSD 104. The MSD control system 330 is preferably a decentralized motion support system 330, although centralized implementations are also possible. It is furthermore appreciated that some parts of the MSD control system may be implemented on processing circuitry remote from the vehicle, such as on a remote server 120 accessible from the vehicle via wireless link 125. Each MSD control system 330 is connected to a vehicle motion management (VMM) system or function 360 of the vehicle 100 via a data bus communication arrangement 114 that can be either wired, wireless or both wired and wireless. Hereby, control signals can be transmitted between the VMM system 360 and the MSD control system 330. The VMM system 360 and the MSD control system 330 will be described in further detail below with reference to Figure 3 and Figure 5.

[0036] The VMM system 360 as well as the MSD control system 330 may include a microprocessor, microcontroller, programmable digital signal processor or another programmable device. The systems may also, or instead, include an application specific integrated circuit, a programmable gate array or programmable array logic, a programmable logic device, or a digital signal processor. Where the system(s) include(s) a programmable device such as the microprocessor, microcontroller or programmable digital signal processormentioned above, the processor may further include computer executable code that controls operation of the programmable device. Implementation aspects of the different vehicle unit processing circuits will be discussed in more detail below in connection with the example provided in Figure 7.

[0037] The different MSDs of a heavy-duty vehicle can be coordinated in order to obtain a desired motion by the vehicle. For instance, two or more MSDs may be used jointly to generate a desired propulsion torque or braking torque, a desired yaw motion by the vehicle, or some other dynamic behavior. Such coordination is especially relevant in split-friction scenarios, where service brakes are coordinated as part of a traction control operation to make best use of the available friction on the supporting surface 101, as will be explained in the following.

[0038] Figure 2 is a graph showing an example 200 of achievable tyre force as function of longitudinal wheel slip for some different road friction conditions. Longitudinal wheel slipmay, in accordance with SAE J370 (SAE Vehicle Dynamics Standards Committee January 24, 2008) be defined as

[0039] where R is an effective wheel radius in meters, a>xis the angular velocity of the wheel about its wheel axis, and vxis the longitudinal speed of the wheel (in the coordinate system of the wheel). Thus,is bounded between -1 and 1 and quantifies how much the wheel is slipping with respect to the road surface. A positive wheel slip is seen when the vehicle accelerates, and a negative wheel slip results from braking the vehicle. Wheel slip is, in essence, a speed difference measured between the wheel and the vehicle. Thus, the herein disclosed techniques can be adapted for use with any type of wheel slip definition. It is also appreciated that a wheel slip value is equivalent to a wheel speed value given a velocity of the wheel over the surface, in the coordinate system of the wheel. The VMM 360 and optionally also the MSD control system 330 maintains information on vx(in the reference frame of the wheel), while a wheel speed sensor or the like can be used to determine a>x(the rotational velocity of the wheel).

[0040] A differential wheel slip is to be construed as a parameter related to a difference in wheel speeds over an axle. A differential wheel slip value can be used to detect if the wheelsof an axle are rotating at significantly different speeds or if the wheel motion is reasonably uniform. Differential wheel slip can be given, e.g., as

[0041] In order for a wheel (or tyre) to produce a wheel force, slip must occur. For smaller slip values the relationship between slip and generated force is approximately linear, where the proportionality constant is often denoted as the slip stiffness of the tyre. A tyre is subject to a longitudinal force Fx. a lateral force Fy, and a normal force Fz. The normal force Fzis key to determining some important vehicle and tyre properties. For instance, the normal force to a large extent determines the achievable longitudinal tyre force Fxby the wheel since, normally, Fx< Fz, whereis a friction coefficient associated with a road friction condition. The general relationships between tyre forces and wheel slip is described in, e.g., “Tyre and vehicle dynamics”, Elsevier Ltd. 2012, ISBN 978-0-08-097016-5, by Hans Pacejka, and will therefore not be discussed in more detail herein.

[0042] A drive axle slip can also be defined in analogy with wheel slip as the difference between a drive axle speed of rotation and the corresponding speed of the vehicle over ground, accounting for transmission aspects such as gear ratios, tyre radii, and the like, and assuming that both wheels of a driven axle are rotating at equal speeds and travel along straight parallel wheel tracks. The drive axle slip is zero in case the drive axle speed corresponds exactly to the corresponding vehicle speed over ground. If the drive axle rotates faster or slower than this vehicle speed over ground, a drive axle slip is present. A positive drive axle slip is necessary in order to generate propulsion force. The drive axle slip is negative during motor braking.

[0043] Figure 2 illustrates three different operating conditions with varying road friction. Fxl is a high friction case where the peak tyre force 230 is relatively high. Fx2 is a mid-friction case where the peak tyre force 240 is smaller compared to the high friction case, but still significant. Fx3 is a low friction case where only a small longitudinal tyre force 250 can be generated.

[0044] Operating a heavy-duty vehicle, such as the vehicle 100, in split-friction scenarios is challenging. This is especially true if the road surface is not flat but has a slope, i.e., an uphill or a downhill slope. During launch in an uphill scenario, for instance, the split friction may cause inefficient force transfer between drive unit and the road surface, resulting in that thevehicle getting stuck or even starting to slide backwards. Braking a heavy vehicle in splitfriction downhill scenarios is also often challenging, since too much brake torque applied to a low friction wheel will cause the wheel to lock, resulting in suboptimal tyre force, i.e., an inefficient transfer of torque between the brake and the road surface.

[0045] As can be inferred from the curves in Figure 2, the tyre force curve primarily scales in the y-dimension when friction changes, indicated by the arrow denoted S in Figure 2. Normally the wheel slip corresponding to maximum tyre force shifts slightly towards smaller slip values as road friction decreases, but not very much. This is illustrated in Figure 2 by the relatively small gap 260 between the peak force wheel slip in the high friction case and the peak force wheel slip in the low friction case.

[0046] The longitudinal tyre forces Fxl, Fx2, Fx3 show an almost linearly increasing part 210 for small wheel slips, followed by a part 220 with more non-linear behavior for larger wheel slips. It is desirable to maintain vehicle operation in the linear region 210, where the obtainable longitudinal force in response to an applied brake command is easier to predict, and where enough lateral tyre force can be generated if needed. To ensure operation in this region, a wheel slip limit iim on the order of, e.g., 0.1, can be imposed on a given wheel.

[0047] An inverse tyre model, such as those exemplified in Figure 2, can be used to translate between a desired longitudinal tyre force Fxand a corresponding wheel slipThe interface between VMM and MSDs capable of delivering steering and optionally also torque to the vehicle’s wheels has as mentioned above traditionally been focused on torque-based requests to each MSD from the VMM without any consideration towards wheel slip. However, this approach has significant performance limitations. In case an excessive slip situation arises, then a relevant safety function (traction control, anti-lock brakes, etc.) operated on a separate control unit normally steps in and requests a torque override in order to bring the slip back into control. The problem with this approach is that since the primary control of the actuator and the slip control of the actuator are often allocated to different electronic control units (ECUs) on the vehicle, the latencies involved in the communication between them significantly limits the slip control performance. Moreover, the related actuator and slip assumptions made in the two ECUs that are used to achieve the actual slip control can be inconsistent and this in turn can lead to sub-optimal performance. Significant benefits can be achieved by instead using a wheel speed or wheel slip-based request on the interface between VMM 360 and the MSD controlleror controllers 330, thereby shifting the difficult actuator speed control loop to the MSD controllers, which generally operate with a much shorter sample time compared to that of the VMM function. Such an architecture can provide much better disturbance rejection compared to a torque-based control interface and thus improves the predictability of the forces generated at the tyre road contact patch. The methods disclosed herein are preferably but not necessarily used in this type of wheel slip based vehicle motion control system.

[0048] The wheel speed in a wheel slip based control approach will not change much if the road friction suddenly changes, since the wheel slip set-point stays the same and since a change in friction will not have an immediate effect on the vehicle speed over ground. Indeed, the wheel speed in a wheel slip based control system will remain more or less constant even if a driven wheel leaves the ground temporarily due to unevenness in the road surface. It can be deduced from the example curves Fxl, Fx2, Fx3 in Figure 2 that a wheel slip based control approach is particularly beneficial in split friction conditions, where the inverse model curves for a given wheel will jump up and down. At one time instant a wheel may experience conditions similar to the Fxl curve, and at the next time instant suddenly instead have the Fx3 curve, and then the Fx2 curve, and so on. However, that wheel will be close to peak force regardless of the curve at which it is operating since the wheel slip will remain close to the target wheel slip value. This wheel slip based control approach requires high bandwidth torque actuators in order to be effective, i.e., torque actuators that can quickly adapt to changes in operating conditions and maintain wheel speed, such as electric machines. According to some aspects of the herein disclosed methods, the actuator control bandwidth of the torque actuators associated with one or more axles on the vehicle is configured in dependence of road slope, i.e., ground surface inclination, such that the control bandwidth is configured at a higher value in case of significant slope, and at a lower value otherwise. This results in a responsive system able to track fast changes in friction and thus maintain a desired wheel speed setpoint. The high bandwidth control system may, however, result in increased component wear and also in increased driver and passenger discomfort, hence it is advantage to reduce the actuator control bandwidth when on more flat ground.

[0049] Wheel slip control is more complicated in case two or more wheels are driven via an open differential which distributes propulsion power to the wheels on the axle. Most open differentials distribute propulsion power equally to the wheels. In case road friction / r (ornormal load Fz) differs between the wheels of a differential, such as in a split-friction scenario, the wheel with smaller resistance (lower friction) will start to spin faster than the wheel with higher resistance (higher friction), even if the drive axle slip is maintained close to a target drive axle slip value. To improve traction, torque can be transferred to the higher resistance wheel by applying brakes on the smaller resistance wheel. This operation is known as traction control. However, now successful wheel slip or wheel speed based control depends both on the actuator bandwidth of the propulsion power source (how quickly the power source can adapt to changes in operating conditions) and on the actuator bandwidth of the service brakes that are used to transfer torque from the low friction side to the high friction side.

[0050] To summarize, rapid MSD control is of essence in split-friction conditions, and in particular where the road also has a significant slope, i.e., when the vehicle has a large inclination relative to the reference plane. It is desired to trigger brake intervention that transfers torque over the wheels in a differential arrangement fast and resolutely, in order to, e.g., not lose vehicle momentum during launch in an uphill driving scenario or lose control of the vehicle while traversing a downhill slope with variation in road friction. However, such aggressive brake intervention may not be necessary on flat ground, where it will just cause increased component wear and driver discomfort.

[0051] To improve motion management of vehicles comprising a differential drive arrangement, it is proposed herein to monitor the inclination of the vehicle 100 in the longitudinal direction, relative to the reference plane, and configure a wheel slip limitbased on the monitored inclination, such that an increase in inclination magnitude results in a decrease in wheel slip limit magnitude compared to some nominal wheel slip limit used on more flat support surfaces. The wheel slip limit is preferably a longitudinal wheel slip limit applied to longitudinal wheel slip of the vehicle. This means that the traction control system of the vehicle will step in sooner if the vehicle is on a sloping surface compared to when the vehicle is on flat ground (since the smaller slip limit is breached sooner). This way the vehicle becomes more responsive to high wheel slip in challenging driving scenarios where the vehicle has significant inclination and will thus apply braking torque to the low friction side wheels sooner in, e.g., a split friction scenario, such that torque is transferred over to the high friction side faster. In other words, the system will trigger a brake intervention operation by a wheel brake at the low friction side, in response to that the wheel slip of the driven wheel exceeds awheel slip limit, sooner than it would have on flat supporting surfaces where the wheel slip limit will be configured at a higher level. It is understood that this more “trigger-happy” vehicle control behavior is likely to result in increased component wear and also in increased driver discomfort, as the traction control system will be more easily activated due to the reduced wheel slip limit. However, by conditioning the decrease in wheel slip limit on inclination magnitude, the increased responsiveness is only obtained when needed, which is an advantage.

[0052] An advantage of decreasing wheel slip limits when road inclination increases, as opposed to always using low wheel slip limits, is that brake intervention on flat surfaces where brake intervention may not be necessary become less common.

[0053] The road slope based wheel slip control techniques discussed herein are applicable with advantage in uphill driving conditions, in particular during launch (starting from standstill or from a low velocity). In this case a braking torque is increased on the low friction side to transfer propulsion torque to the high friction side in case one wheel starts to slip too much. However, at least some aspects of the techniques discussed herein are also applicable in downhill driving scenarios, where the brake intervention operation instead comprises reducing applied braking torque on the low friction side to reduce wheel slip magnitude. The reduction in brake torque at the low friction side is preferably compensated by an increase in brake torque at the high friction side, in analogy with the uphill driving scenario. Again, a more aggressive traction control system is an advantage in difficult downhill operating scenarios. The more aggressive traction control system is not desired on more flat surface since this will result in unnecessary component wear and discomfort to people in the vehicle.

[0054] Motion management of vehicles comprising a differential drive arrangement can be improved by configuring a brake intervention torque application rate associated with the wheel brakes of the driven wheels, based on the monitored inclination of the vehicle 100 in the longitudinal direction, relative to the reference plane, such that an increase in inclination magnitude, i.e., detection of a significant road slope, results in an increase in torque application rate. Both the actual application rate of the brake torque (how fast the brakes are applied) and the maximum allowed brake torque (how hard the brakes can be applied) can be increased when a significant road slope condition is detected by the system. The increased torque application rate at the driven wheels means that the system becomes more responsive and thus faster in transferring torque when road friction differences over a driven axle occur. In otherwords, when a brake intervention operation is triggered, the torque is transferred faster to the high friction side, i.e., more aggressively, since the brake intervention operation is parameterized by the brake intervention torque application rate which has been increased as a consequence of detecting a significant road slope condition. Again, a more responsive system is obtained in this manner that will be better at maintaining wheel slip at the target level, and at least below the configured wheel slip limit.

[0055] The exact values to use for nominal wheel slip limit and adjusted wheel slip limit can be determined by practical experimentation and / or by computer simulation using methods known in the art.

[0056] The exact values for the adjustment of brake intervention torque application rate can also be determined from practical experimentation and / or by computer simulation using methods known in the art.

[0057] The techniques are applicable when driving uphill and when driving downhill, and in particular when driving downhill and / or downhill in split-friction conditions. In the downhill case an excessive negative wheel slip may occur, as one or more wheels drive over a patch of ice or other surface with reduced friction. To maintain vehicle speed at an acceptable level, it is important to quickly redistribute braking torque between the wheels to maintain overall braking torque at a desired level. By reducing the wheel slip limit, such redistribution of braking torque in downhill slope conditions is started sooner. It also helps to make the service brake system more responsive, by reparametrizing it as discussed herein when the vehicle enters a driving scenario with significant downhill slope. It is thus appreciated that the techniques discussed herein related to reconfiguration of wheel slip limits in dependence of vehicle and / or supporting surface inclination can be applied with advantage in both uphill and downhill driving conditions, i.e., for both positive wheel slips and for negative wheel slips. It is also appreciated that the re-parametrization of the service brake system to become more responsive is applicable in both downhill driving conditions and in uphill driving conditions, since both driving conditions benefit from a more responsive service brake system capable of quickly redistributing brake torque over the wheels of the vehicle. An example system for MSD coordination will be discussed below in connection to Figure 5.

[0058] Turning now to Figure 3, the overall vehicle control system 300 may be implemented on one or more vehicle unit computers (VUC). The VUC may be configured to execute vehiclecontrol methods which are organized according to a layered functional architecture where some functionality may be comprised in a traffic situation management (TSM) domain 370 in a higher layer and some other functionality may be comprised in a vehicle motion management (VMM) domain 360 residing in a lower functional layer. A control interface 375 for transmission of, e.g., motion requests between TSM and VMM is implemented, preferably in a standardized manner.

[0059] Figure 3 schematically illustrates a driven axle with a differential 345 that distributes propulsion power from a propulsion device 340 to the wheels 310. The wheels are also controlled by friction brakes 320 (such as disc brakes or drum brakes). Some aspects of the present disclosure are advantageously used to control electromechanical friction brakes, which can be controlled at high control bandwidth to cause rapid changes in applied torque. The friction brakes 320 and the propulsion device 340 are examples of wheel torque generating devices, which can be controlled by one or more motion support device control units 330. The control is based on, e.g., measurement data obtained from a wheel speed sensor 350 and from other vehicle state sensors 370, such as radar sensors, lidar sensors, and also vision based sensors such as camera sensors and infra-red detectors. The vehicle longitudinal speed over ground vxcan be determined from the wheel speed of an undriven wheel, by a radar sensor, lidar sensor, and / or vision based sensor, or by GPS.

[0060] With reference also to Figure 1, an MSD control system 330 may be arranged to control one or more actuators 104. For instance, it is not uncommon that an MSD control system 330 is arranged to control both wheels on an axle 106.

[0061] The TSM function 370 plans driving operation with a time horizon of 10 seconds or so. This time frame corresponds to, e.g., the time it takes for the vehicle 100 to negotiate a curve or the like. The vehicle maneuvers, planned and executed by the TSM function, can be associated with acceleration profiles and curvature profiles which describe a desired target vehicle velocity in the vehicle forward direction and turning to be maintained for a given maneuver. The TSM function continuously requests the desired acceleration profiles areqand steering angles (or curvature profiles creq) from the VMM function 360 which performs force allocation to meet the requests from the TSM function in a safe and robust manner. The VMM function 360 operates on a timescale of below one second or so and will be discussed in more detail below.

[0062] An important function of the vehicle control system 300, in the context of the present disclosure, is to perform traction control involving the wheels 310, i.e., to apply the friction brakes 310 in order to transfer a suitable amount of torque from a low friction wheel to a higher friction wheel.

[0063] The actuator control performed by the VMM function 360 and the MSD controllers 330 on the vehicle are generally performed at some given control bandwidth. Figure 8 shows an example control loop executed in an example MSD control unit 330. A setpoint value is received by the system in some manner, e.g., as a target wheel speed or target wheel slip in a wheel slip based control system, or as a torque setpoint in a more conventional actuator control system. The setpoint is generally compared to a current state of the system and an error signal is determined based on a difference between the setpoint and the current state. This error signal is then processed by a loop filter that is associated with some form of control bandwidth. Generally, a larger bandwidth results in faster control but also in less noise and disturbance suppression. The filter can be a straightforward low-pass filter, i.e., an averaging filter, which has some bandwidth (where smaller bandwidth means more averaging). The filter can also be some form of controller, such as a P controller, a PI controller, or a PID controller, where the P, I and D parameters determine the control bandwidth of the system in a known manner. The filter can also be a Kalman filter or the like, which has some form of Kalman gain factor that can be configured, and which determines the control bandwidth of the actuator control system. The control system principles schematically illustrated in Figure 8, having a control loop bandwidth, can be applied to many different types of actuators, including electric machines and other torque generating devices.

[0064] Accordingly, the torque is applied by the actuator using different control bandwidths in dependence of the current operating condition of the vehicle 100, and in particular the road slope or ground surface inclination. The torque can thus be applied rapidly when the current operating condition so requires (high slope) or be applied in a more moderate manner when the current operating condition is not in need of such rapid torque response (more flat ground). Furthermore, the vehicle control system now has more freedom to perform the vehicle motion control since it now has the option to not only set a desired target value for the control, but also a control bandwidth with which the control is to be performed. A large control bandwidth normally implies a more rapid response to changes such as varying road incline, friction, andso on. A smaller bandwidth implies a slower response to changes in operating conditions but on the other hand more noise is suppressed due to the smaller bandwidth. According to an example embodiment, the control bandwidth of the control function is controlled using a predetermined set of feedback gains for the actuator, where the control function is preferably a PID-controller, loop filter, Kalman filter, or the like. Furthermore, the control function may optionally comprise a feedforward component. The feedforward component can be used as input data for e.g., the choice of feedback gains for the controller.

[0065] Referring again to Figure 3, the wheel 310 has a longitudinal velocity component vxand a lateral velocity component vy. There is a longitudinal wheel force Fxand a lateral wheel force Fy, and also a normal force Fzacting on the wheel (not shown in Figure 3). Unless explicitly stated otherwise, the wheel forces are defined in the coordinate system of the wheel, i.e., the longitudinal force is directed in the rolling plane of the wheel, while the lateral wheel force is directed normal to the rolling plane of the wheel. The wheel has a rotational velocity a>x, and a radius R.

[0066] The type of inverse tyre model 200 shown in Figure 2 can be used by the VMM 360 to generate a desired tyre force at some wheel. Instead of requesting a torque corresponding to the desired tyre force, the VMM can translate the desired tyre force into an equivalent wheel slip (or, equivalently, a wheel speed relative to a speed over ground) and request this slip instead. This is particularly beneficial when operating in split-friction conditions, since the wheel slip giving peak force 230, 240, 250 does not change much as friction changes, as illustrated in Figure 2, and discussed above. Compare for instance peak force wheel slip in the high friction case Fxl which is about 0.1 to the peak force wheel slip in the medium friction case Fx2 which is almost the same. This is because the tyre force curves mainly scale up and down in the y-direction (the tyre force dimension), there is not so much scaling in the x- direction (the wheel slip dimension). If a wheel slip close to the requested wheel slip can be maintained during, e.g., vehicle launch in split friction conditions, close to optimal traction force will be obtained during the operation.

[0067] The open differential 345 complicates the wheel slip control. Even if a propulsion device 340 like an electric machine that can be controlled at high bandwidth, i.e., which can change applied torque and drive axle speed o>avery fast, the axle speed may be unevenly transferred over the wheels to cause a difference in wheel speeds a>xl, a>x2. However, by thetechniques disclosed herein, more rapid drive torque transfer is enabled, especially in hilly conditions.

[0068] The control unit or units can be arranged to store a pre-determined inverse tyre model in memory, e.g., as a look-up table. The model which is stored in memory can be determined based on experiments and trials, or based on analytical derivation, or a combination of the two. Inverse tyre models can be automatically configured from the remote server 120, e.g., as software updates, or manually by a technician performing vehicle routine servicing. As mentioned above, wheel slip limits and service brake torque application rates to use on flat ground and in driving scenarios with significant slope can be determined by practical experimentation and / or by computer simulation using digital models of the vehicle. The slope degrees at which to adjust system parameters like wheel slip limits and brake torque application rates can also be determined from practical experimentation and / or by computer simulation.

[0069] Figure 4 illustrates an example driven axle 400 in a driving scenario that involves uphill driving in a split friction condition. Three patches of road surface 410, 420, 430 or road sections are encountered while traversing the uphill section of road. The road sections 410, 420, 430 are associated with respective friction coefficientsju1,ju2, / r3, where, as an example,> / r2> s- The left and right side applied service brake torque is also schematically illustrated as function of time. The solid torque curves are representative of a nominal parameterization, and the dashed curves are representative of a more responsive service brake system with a smaller wheel slip limit. Note that the amount of braking torque applied to the wheel on the low friction side increases with decreasing friction, i.e., the traction control intervention becomes more pronounced with decreasing friction resulting in that the applied brake torque is larger at patch 430 compared to at patch 410 and 420. Note also that the difference in applied steady state torque is generally larger for the dashed curve examples, since this wheel slip limit has been reduced due to the inclination of the vehicle.

[0070] It is appreciated that the torque curves in Figure 4 are exemplary curves and not necessarily representative of a real-world operating scenario.

[0071] The dashed line torque curves start to rise up sooner than the solid line torque curves. This is due to the fact that it takes some time for wheel slip to develop, and the reduced wheel slip limit is breached sooner than the nominal wheel slip limit. The torque application rate isalso larger for the dashed curves, due to that the service brake system has been re-parameterized with an increased torque application rate.

[0072] The dashed line torque curves also have a larger overshoot compared to the solid curve, which is a drawback that likely causes more jerk in the vehicle and an increased component wear, such as brake pad wear and brake disc wear.

[0073] Overall, the system according to the teachings herein is more responsive to changes in road friction and is able to respond faster (with higher bandwidth) to changes in wheel operating conditions. A drawback of such increased responsiveness is an increase in component wear, such as service brake parts. Another drawback is a reduced driver and passenger comfort level, since the vehicle is controlled more aggressively which increases jerk, among other things. It is thus an advantage that the re-parameterization into a more responsive vehicle MSD system is only performed when needed, i.e., only if the monitored inclination becomes large.

[0074] Figure 5 illustrates an example vehicle control function architecture applicable with the herein disclosed methods. The TSM function 370 generates vehicle motion requests 375, which may comprise a desired steering angle 8 or an equivalent curvature creqto be followed by the vehicle, and which may also comprise desired vehicle unit accelerations areqand also other types of vehicle motion requests, which together describe a desired motion by the vehicle along a desired path at a desired velocity profile. It is understood that the motion requests can be used as base for determining or predicting a required amount of longitudinal and lateral forces which needs to be generated in order to successfully complete a maneuver. The TSM function 370 may correspond to the inputs of a human driver, the inputs from an advanced driver assistance system (ADAS), or to the inputs from an autonomous drive system.

[0075] The VMM function 360 operates with a time horizon of about 1 second or so, and continuously transforms the acceleration profiles areqand curvature profiles creqfrom the TSM function 370 into control commands for controlling vehicle motion functions, actuated by the different MSDs of the vehicle 100 which report back capabilities to the VMM, which in turn are used as constraints in the vehicle control. The VMM function 360 performs vehicle state or motion estimation 510, i.e., the VMM function 360 continuously determines a vehicle state s comprising positions, speeds, accelerations, and articulation angles of the different units in the vehicle combination by monitoring operations using various sensors arranged on the vehicle 100, often but not always in connection to the MSDs.

[0076] The result of the motion estimation 510, i.e., the estimated vehicle state s, is input to a force generation module 520 which determines the required global forces V= [Vi, V2] for the different vehicle units to cause the vehicle 100 to move according to the requested acceleration and curvature profiles areq, creq, and to behave according to the desired vehicle behavior. The required global force vector V is input to an MSD coordination function 530 which allocates wheel forces and coordinates other MSDs such as steering and suspension. The MSD coordination function outputs an MSD control allocation for the i:th wheel, which may comprise any of a torque Ti to be applied by a brake or propulsion device, a longitudinal wheel slip Xi, or wheel slip limit Xma , a wheel rotational speed ®i, and / or a wheel steering angle 8i. The coordinated MSDs then together provide the desired lateral Fyand longitudinal Fxforces on the vehicle units, as well as the required moments Mz, to obtain the desired motion by the vehicle combination 100.

[0077] In this example it is primarily the MSD coordination function 530 that performs the traction control, i.e., applies braking torque at one side of a differential to transfer torque to the high friction side if the wheel slip exceeds the target wheel slip value. The MSD coordination function receives data indicative of vehicle inclination from the vehicle inclination module 540. The data may comprise the actual inclination of the vehicle, which can be determined from map data, and / or from an inclinometer, i.e., a digital spirit level. The data from the vehicle inclination module 540 may also comprise a binary value indicating if the vehicle is in a high slope condition or not.

[0078] According to some preferred aspects of the herein disclosed techniques, the operations involving the one or more steered axles on the vehicle is treated differently compared to nonsteered axles. This is because the steered axle is important to generate lateral forces required for steering. Thus, according to some aspects, the wheel slip limit applied to control of the steered axle or steered axles of the vehicle are set at a margin below other wheel slip limits. Also, the torque application rates are set even higher compared to other axles, so as to provide the required lateral force capability.

[0079] By determining vehicle unit motion using, e.g., global positioning systems, visionbased sensors, wheel speed sensors, radar sensors, steering angle sensors and / or lidar sensors, and translating this vehicle unit motion into a local coordinate system of a given wheel 310 (in terms of, e.g., longitudinal and lateral velocity components), it becomes possible to accuratelyestimate wheel slip in real time by comparing the vehicle unit motion in the wheel reference coordinate system to data obtained from the wheel speed sensor 350 arranged in connection to the wheel 310, as discussed above. A tyre model, which was discussed above in connection to Figure 2, can be used to translate between a desired longitudinal tyre force Fxi for a given wheel i and an equivalent longitudinal wheel slip Xi for the wheel.

[0080] Wheel slip limits can also be imposed on wheel speed differences over a driven axle, such as the differential wheel slipdiscussed above. In this case only wheel speed signals are needed, not the vehicle speed over ground. Some traction control systems operate on wheel speed differences, possibly after accounting from steering wheel angle. The techniques disclosed herein are applicable also to such systems.

[0081] According to preferred aspects of the present disclosure, the VMM function 360 manages both force generation and MSD coordination, i.e., it determines what forces that are required at the vehicle units in order to fulfil the requests from the TSM function 370, for instance to accelerate the vehicle according to a requested acceleration profile requested by TSM and / or to generate a certain curvature motion by the vehicle also requested by TSM. The forces may comprise e.g., yaw moments Mz, longitudinal forces Fx and lateral forces Fy, as well as different types of torques to be applied at different wheels. The forces are determined such as to generate the vehicle behavior which is expected by the TSM function in response to the control inputs generated by the TSM function 370.

[0082] Figures 6A and 6B show flow charts that illustrates methods according to the teachings herein. Figure 6A illustrates a method which requires configuration of wheel slip limit as function of slope, while Figure 6B illustrates a method which requires configuration of control bandwidth as function of slope. Required method steps are illustrated by solid boxes while optional steps are illustrated by dashed boxes. The methods discussed below summarize the teachings herein, where it is noted that the aspects related to re-parameterization of the service brake system on the vehicle to make it more responsive, e.g., by increasing the torque application rate can be practiced independently from the aspects related to reconfiguration of wheel slip limits, since the two are not inextricably linked.

[0083] With reference to Figure 6A, and the discussion above, there is disclosed herein a computer-implemented method for controlling motion of a vehicle 100, where the vehicle 100 comprises wheels 310 driven by a propulsion device 340 via a differential 345, and where eachdriven wheel 310 of the differential 345 is associated with a wheel brake 320. A set-up such as this was discussed above in connection to Figure 3. Some preferred but optional aspects of the method comprises controlling SO an axle speed o>aof the propulsion device 340 based at least indirectly on the wheel slip Axof each driven wheel 310 of the differential 345, in a wheel-slip based control manner, as discussed above. However, aspects of the methods disclosed herein are more general than this and can also be applied to conventional torque-based control systems.

[0084] The method comprises monitoring SI an inclination of the vehicle 100 relative to a reference plane, such as the horizontal plane. This monitoring may be performed by analyzing SI 1 topographical data from a digital map 385, i.e., simply reading out pre-stored inclination data based on the position of the vehicle 100, which position can be obtained from a satellite positioning system or from other sources such as cellular network positioning systems and the like. The monitoring may also comprise monitoring S12 the inclination based on an inclinometer sensor 380 of the vehicle 100. An inclinometer sensor, clinometer, or digital spirit level is a sensor used for measuring angles of slope, elevation, or depression of an object with respect to gravity's direction. It is also known as a tilt indicator, tilt sensor, tilt meter, slope alert, slope gauge, gradient meter, gradiometer, level gauge, level meter, declinometer, and pitch & roll indicator. Clinometers measure both inclines and declines using three different units of measure: degrees, percentage points, and topos. A tilt sensor can measure the tilt in two axes of a reference plane. In contrast, a full motion would use at least three axes and often additional sensors. One way to measure tilt angle with reference to the earth's ground plane, is to use an accelerometer, i.e., an inertial measurement unit (IMU) in a known manner.

[0085] The method also comprises configuring S3 a wheel slip limit umor target wheel slip A based on the inclination, such that an increase in inclination magnitude results in a decrease in wheel slip limit magnitude or a decrease in target wheel slip. This means that, among other things, a traction control system will kick in sooner than it would have on flat ground, in order to speed up the response of the vehicle MSD system to changes in friction. Note that the disclosed method is applicable both for positive and negative wheel slip limits, i.e., both for uphill driving and for downhill driving, as discussed above. The wheel slip limits discussed herein are preferably wheel slips determined in relation to the vehicle speed over ground. However, some systems may also be based on difference in wheel speeds over a driven axle.Such systems do not require knowledge of the vehicle speed over ground, which can be an advantage in some cases.

[0086] The method further comprises determining S5 wheel motion indicative of a wheel slip of each driven wheel 310 of the differential 345 and triggering S6 a brake intervention operation by the corresponding wheel brake 320, in response to that the wheel slipof a driven wheel 310 exceeds the wheel slip limit um. The wheel motion can be determined from wheel speed sensors in a known manner. For instance, the determined wheel motion S51 of a driven wheel 310 may comprise a wheel speed a>xin relation to a vehicle speed over ground vx. The determined wheel motion S52 of a driven wheel 310 may also comprise a wheel speed to in relation to a wheel speed of at least one other driven wheel of the differential 345.

[0087] The brake intervention operation essentially comprises adjustment of a brake torque applied at the wheel which has a wheel speed that breaches the wheel slip limit. During propulsion in uphill driving conditions the brake intervention operation comprises application of a brake torque to transfer more driving torque to the high friction side wheel. During braking in downhill driving conditions, the adjustment of brake torque comprises relaxation of the brake torque on the low friction side, possibly complemented by an increase in brake torque at the high friction side in a coordinated manner, as discussed above in connection to Figure 5.

[0088] The method may comprise pre-arming S2 the wheel brakes 320 in case the magnitude of the monitored inclination satisfies a first inclination magnitude acceptance criteria. Prearming wheel brakes in this manner results in faster response once the wheel brakes are triggered, resulting in higher torque application rate compared to a nominal case where the wheel brakes are not pre-armed. Pre-arming service brakes on a vehicle can be done by increasing a pressure in a hydraulic system or pneumatic system of the service brakes on the vehicle. Pre-arming a service brake can also be achieved by pre-filling the brake, as discussed in US7018004B2. Methods for pre-arming service brake systems in order to achieve a reduced response time to a brake command are known and will therefore not be discussed in more detail herein.

[0089] The exact method for setting the more restrictive wheel slip limit based on the monitored inclination may differ between implementations. According to a first example, the method comprises configuring S31 the wheel slip limit .ambased on a predetermined mapping between a discrete set of wheel slip limits and corresponding ranges of inclination. The current vehicle inclination is then used, e.g., as index in a look-up-table to obtain the wheel slip limitto use. A range of inclinations then map to the same wheel slip limit. Both positive and negative wheel slip limits can be configured in this manner based on the monitored inclination. Aspects of the method may also comprise configuring S34 the wheel slip limit umbased on an at least piece-wise continuous function of inclination, such as a linear function. According to another example, the method comprises configuring S32 the wheel slip limitmalso based on a longitudinal speed over ground vxof the vehicle 100. The method may for instance comprise resetting the wheel speed limits to the nominal value once the vehicle has picked up speed after an uphill launch operation. The wheel speed limit may also be reset to the nominal limit in a downhill driving scenario where the speed of the vehicle is low.

[0090] According to some aspects, the method comprises configuring S33 a nominal wheel slip limit nmvalue based on an inverse tyre model 200, wherein the inverse tyre model is representative of a relationship between wheel slip A and wheel force Fxof the driven wheels 310.

[0091] The computer-implemented method may, as discussed above, also comprise configuring S4 a brake intervention torque application rate associated with the wheel brakes 320, based on the inclination, such that an increase in inclination magnitude results in an increase in torque application rate. These aspects of the method can be practiced without the adjustment of the wheel slip discussed above, since they are not inextricably linked to each other. Indeed, there is also disclosed herein a computer-implemented method for controlling motion of a vehicle 100, where the vehicle 100 comprises wheels 310 driven by a propulsion device 340 via a differential 345, where each driven wheel 310 of the differential 345 is associated with a wheel brake 320, the method comprising monitoring SI an inclination, relative to a reference plane, of a surface 101 supporting the vehicle 100, and configuring S4 a brake intervention torque application rate associated with the wheel brakes 320, based on the inclination, such that an increase in inclination magnitude results in an increase in torque application rate, determining S5 wheel motion indicative of a wheel slip Axof each driven wheel 310 of the differential 345, and triggering S6 a brake intervention operation by the corresponding wheel brake 320, in response to that the wheel slip Axof a driven wheel 310 exceeds a wheel slip limitUm, where the brake intervention operation is parameterized by the brake intervention torque application rate.

[0092] The configuration of brake intervention torque application rate relates at least in part to configuring a control loop bandwidth of the actuator control system, as discussed above inconnection to Figure 8. The control loop bandwidth may be configured as a filter bandwidth of a loop filter, as P, I, and / or D parameters of a PID controller, or as the Kalman gain of a Kalman filter, just to give a few examples.

[0093] According to some aspects, the method comprises configuring S41 a brake intervention maximum torque associated with the wheel brakes 320, based on the inclination, such that an increase in inclination magnitude results in an increase in maximum torque. This allows more braking torque to be applied by a service brake when in a steep incline compared to when on less steep incline or on flat surfaces. More braking torque can then be transferred from one wheel on a differential drive axle to the other wheel, which is an advantage.

[0094] According to some aspects, the method comprises configuring S42 a brake intervention operation control bandwidth, based on the inclination, such that an increase in inclination magnitude results in an increase in brake intervention operation control bandwidth. This brake intervention control bandwidth can be configured by setting parameters in an MSD control system, such as that discussed above in connection to Figure 3. Most MSD controllers comprise feedback loops associated with a bandwidth. A PID-controller can, for instance, be reconfigured for a more responsive action. Some feedback systems also comprise feedforward terms which can be increased in magnitude in order to speed up the response of the actuator system. Such increased control bandwidths lead to faster control of the actuator, but also a more noisy control, with unwanted overshoot and the like.

[0095] The brake intervention operation control bandwidth is optionally configured at a larger value if the axle is a steered axle, compared to non-steered axles.

[0096] Some aspects of the herein disclosed methods also comprise configuring S43 a nominal applied brake torque of the brake intervention operation, based on the inclination, such that an increase in inclination magnitude results in an increase in nominal applied brake torque. The nominal applied brake torque is the “entry-level” torque that is applied immediately at the start of the brake intervention operation. The higher this initially applied torque is, the stronger and resolute the brake intervention operation becomes. Thus, by increasing the nominal applied brake torque of the brake intervention operation, a more responsive service brake system is obtained. This will result in increased discomfort to a driver or passenger in the vehicle, and also in increased brake system wear, which is a drawback.

[0097] The differential 345 may in some case be a lockable differential, in which case the method further comprises locking S7 the lockable differential in case the magnitude of themonitored inclination satisfies a second inclination magnitude acceptance criteria. It is appreciated that control of the lockable differential may be based on one or more additional parameters also, not just inclination.

[0098] The methods discussed above can also be described in terms of hardware, more particularly as a control system 300 for a vehicle 100, where the vehicle 100 comprises wheels 310 driven by a propulsion device 340 via a differential 345, where each driven wheel 310 of the differential 345 is associated with a wheel brake 320, the control system comprising processing circuitry 702 configured to:

[0099] monitor SI an inclination, relative to a reference plane, of a surface 101 supporting the vehicle 100,

[0100] configure S3 a wheel slip limitbased on the inclination, such that an increase in inclination magnitude results in a decrease in wheel slip limit magnitude,

[0101] determine S5 wheel motion indicative of a wheel slipof each driven wheel 310 of the differential 345, and

[0102] trigger S6 a brake intervention operation by the corresponding wheel brake 320, in response to that the wheel slipof a driven wheel 310 exceeds a wheel slip limit um.

[0103] The methods discussed above can also be described as a control system 300 for a vehicle 100, where the vehicle 100 comprises wheels 310 driven by a propulsion device 340 via a differential 345, where each driven wheel 310 of the differential 345 is associated with a wheel brake 320, the control system comprising processing circuitry 702 configured to:

[0104] monitor SI an inclination, relative to a reference plane, of a surface 101 supporting the vehicle 100,

[0105] configure S4 a brake intervention torque application rate associated with the wheel brakes 320, based on the inclination, such that an increase in inclination magnitude results in an increase in torque application rate,

[0106] determine S5 wheel motion indicative of a wheel slipof each driven wheel 310 of the differential 345, and

[0107] trigger S6 a brake intervention operation by the corresponding wheel brake 320, in response to that the wheel slipof a driven wheel 310 exceeds a wheel slip limit ^um, where the brake intervention operation is parameterized by the brake intervention torque application rate.

[0108] Figure 7 is a schematic diagram of a computer system 700 for implementing examples disclosed herein. The computer system 700 is adapted to execute instructions from a computer-readable medium to perform these and / or any of the functions or processing described herein. The computer system 700 may be connected (e.g., networked) to other machines in a LAN, an intranet, an extranet, or the Internet. While only a single device is illustrated, the computer system 700 may include any collection of devices that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein. Accordingly, any reference in the disclosure and / or claims to a computer system, computing system, computer device, computing device, control system, control unit, electronic control unit (ECU), processor device, processing circuitry, etc., includes reference to one or more such devices to individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein. For example, control system may include a single control unit, or a plurality of control units connected or otherwise communicatively coupled to each other, such that any performed function may be distributed between the control units as desired. Further, such devices may communicate with each other or other devices by various system architectures, such as directly or via a Controller Area Network (CAN) bus, etc.

[0109] The computer system 700 may comprise at least one computing device or electronic device capable of including firmware, hardware, and / or executing software instructions to implement the functionality described herein. The computer system 700 may include processing circuitry 702 (e.g., processing circuitry including one or more processor devices or control units), a memory 704, and a system bus 706. The computer system 700 may include at least one computing device having the processing circuitry 702. The system bus 706 provides an interface for system components including, but not limited to, the memory 704 and the processing circuitry 702. The processing circuitry 702 may include any number of hardware components for conducting data or signal processing or for executing computer code stored in memory 704. The processing circuitry 702 may, for example, include a general-purpose processor, an application specific processor, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA), a circuit containing processing components, a group of distributed processing components, a group of distributed computers configured for processing, or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed toperform the functions described herein. The processing circuitry 702 may further include computer executable code that controls operation of the programmable device.

[0110] The system bus 706 may be any of several types of bus structures that may further interconnect to a memory bus (with or without a memory controller), a peripheral bus, and / or a local bus using any of a variety of bus architectures. The memory 704 may be one or more devices for storing data and / or computer code for completing or facilitating methods described herein. The memory 704 may include database components, object code components, script components, or other types of information structure for supporting the various activities herein. Any distributed or local memory device may be utilized with the systems and methods of this description. The memory 704 may be communicably connected to the processing circuitry 702 (e.g., via a circuit or any other wired, wireless, or network connection) and may include computer code for executing one or more processes described herein. The memory 704 may include non-volatile memory 708 (e.g., read-only memory (ROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), etc.), and volatile memory 710 (e.g., random-access memory (RAM)), or any other medium which can be used to carry or store desired program code in the form of machineexecutable instructions or data structures, and which can be accessed by a computer or other machine with processing circuitry 702. A basic input / output system (BIOS) 712 may be stored in the non-volatile memory 708 and can include the basic routines that help to transfer information between elements within the computer system 700.

[0111] The computer system 700 may further include or be coupled to a non-transitory computer-readable storage medium such as the storage device 714, which may comprise, for example, an internal or external hard disk drive (HDD) (e.g., enhanced integrated drive electronics (EIDE) or serial advanced technology attachment (SATA)), HDD (e.g., EIDE or SATA) for storage, flash memory, or the like. The storage device 714 and other drives associated with computer-readable media and computer-usable media may provide nonvolatile storage of data, data structures, computer-executable instructions, and the like.

[0112] Computer-code which is hard or soft coded may be provided in the form of one or more modules. The module(s) can be implemented as software and / or hard-coded in circuitry to implement the functionality described herein in whole or in part. The modules may be stored in the storage device 714 and / or in the volatile memory 710, which may include an operatingsystem 716 and / or one or more program modules 718. All or a portion of the examples disclosed herein may be implemented as a computer program 720 stored on a transitory or non- transitory computer-usable or computer-readable storage medium (e.g., single medium or multiple media), such as the storage device 714, which includes complex programming instructions (e.g., complex computer-readable program code) to cause the processing circuitry 702 to carry out actions described herein. Thus, the computer-readable program code of the computer program 720 can comprise software instructions for implementing the functionality of the examples described herein when executed by the processing circuitry 702. In some examples, the storage device 714 may be a computer program product (e.g., readable storage medium) storing the computer program 720 thereon, where at least a portion of a computer program 720 may be loadable (e.g., into a processor) for implementing the functionality of the examples described herein when executed by the processing circuitry 702. The processing circuitry 702 may serve as a controller or control system for the computer system 700 that is to implement the functionality described herein.

[0113] The computer system 700 may include an input device interface 722 configured to receive input and selections to be communicated to the computer system 700 when executing instructions, such as from a keyboard, mouse, touch-sensitive surface, etc. Such input devices may be connected to the processing circuitry 702 through the input device interface 722 coupled to the system bus 706 but can be connected through other interfaces, such as a parallel port, an Institute of Electrical and Electronic Engineers (IEEE) 1394 serial port, a Universal Serial Bus (USB) port, an IR interface, and the like. The computer system 700 may include an output device interface 724 configured to forward output, such as to a display, a video display unit (e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT)). The computer system 700 may include a communications interface 726 suitable for communicating with a network as appropriate or desired.

[0114] The operational actions described in any of the exemplary aspects herein are described to provide examples and discussion. The actions may be performed by hardware components, may be embodied in machine-executable instructions to cause a processor to perform the actions, or may be performed by a combination of hardware and software. Although a specific order of method actions may be shown or described, the order of the actionsmay differ. In addition, two or more actions may be performed concurrently or with partial concurrence.

[0115] The terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting of the disclosure. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items. It will be further understood that the terms "comprises," "comprising," "includes," and / or "including" when used herein specify the presence of stated features, integers, actions, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, actions, steps, operations, elements, components, and / or groups thereof.

[0116] It will be understood that, although the terms first, second, etc., may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element without departing from the scope of the present disclosure.

[0117] Relative terms such as "below" or "above" or "upper" or "lower" or "horizontal" or "vertical" may be used herein to describe a relationship of one element to another element as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. It will be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element, or intervening elements may be present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intervening elements present.

[0118] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0119] It is to be understood that the present disclosure is not limited to the aspects described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the present disclosure and appended claims. In the drawings and specification, there have been disclosed aspects for purposes of illustration only and not for purposes of limitation, the scope of the disclosure being set forth in the following claims.

Claims

CLAIMS1. A computer-implemented method for controlling motion of a vehicle (100), where the vehicle (100) comprises wheels (310) driven by a propulsion device (340) via a differential (345), where each driven wheel (310) of the differential (345) is associated with a wheel brake (320), the method comprising monitoring (SI) an inclination of the vehicle (100) and / or of a surface (101) supporting the vehicle (100), relative to a reference plane, and configuring (S3) a wheel slip limit (A(jm) based on the inclination, such that an increase in inclination magnitude results in a decrease in wheel slip limit magnitude, the method further comprising, determining (S5) wheel motion indicative of a wheel slip (lx) of each driven wheel (310) of the differential (345), and triggering (S6) a brake intervention operation by the corresponding wheel brake (320), in response to that the wheel slip (lx) of a driven wheel (310) exceeds the wheel slip limit2. The computer-implemented method according to claim 1, where the determined wheel motion (S51) comprises a wheel speed (tox) in relation to a vehicle speed over ground (vx).

3. The computer-implemented method according to claim 1 or 2, where the determined wheel motion (S52) comprises a wheel speed (to) in relation to a wheel speed of at least one other driven wheel of the differential (345).

4. The computer-implemented method according to any previous claim, comprising controlling (SO) an axle speed a>a) of the propulsion device (340) based at least indirectly on the wheel slip (lx) of each driven wheel (310) of the differential (345).

5. The computer-implemented method according to any previous claim, comprising monitoring (SI 1) the inclination based on obtained topographical data from a digital map (385).

6. The computer-implemented method according to any previous claim, comprising monitoring (SI 2) the inclination based on an inclinometer sensor (380).

7. The computer-implemented method according to any previous claim, comprising prearming (S2) the wheel brakes (320) in case the magnitude of the monitored inclination satisfies a first inclination magnitude acceptance criteria.

8. The computer-implemented method according to any previous claim, comprising configuring (S31) the wheel slip limit ( -um) based on a predetermined mapping between a discrete set of wheel slip limits and corresponding ranges of inclination.

9. The computer-implemented method according to any previous claim, comprising configuring (S32) the wheel slip limit ( -um) also based on a longitudinal speed over ground (vx) of the vehicle (100).

10. The computer-implemented method according to any previous claim, comprising configuring (S33) a nominal wheel slip limit (A(jm) value based on an inverse tyre model (200), wherein the inverse tyre model is representative of a relationship between wheel slip (A) and wheel force (Fx) of the driven wheels (310).

11. The computer-implemented method according to any previous claim, comprising configuring (S34) the wheel slip limit (A(jm) based on an at least piece-wise continuous function of inclination.

12. The computer-implemented method according to any previous claim, comprising configuring (S4) a brake intervention torque application rate associated with the wheel brakes (320), based on the inclination, such that an increase in inclination magnitude results in an increase in torque application rate.

13. The computer-implemented method according to any previous claim, comprising configuring (S41) a brake intervention maximum torque associated with the wheel brakes (320), based on the inclination, such that an increase in inclination magnitude results in an increase in the brake intervention maximum torque.

14. The computer-implemented method according to any previous claim, comprising configuring (S42) a brake intervention operation control bandwidth, based on the inclination, such that an increase in inclination magnitude results in an increase in brake intervention operation control bandwidth.

15. The computer-implemented method according to any previous claim, comprisingconfiguring (S43) a nominal applied brake torque of the brake intervention operation, based on the inclination, such that an increase in inclination magnitude results in an increase in nominal applied brake torque.

16. The computer-implemented method according to any previous claim, where the differential (345) is a lockable differential, where the method further comprises locking (S7) the lockable differential in case the magnitude of the monitored inclination satisfies a second inclination magnitude acceptance criteria.

17. The computer-implemented method according to any previous claim, where the monitored inclination is a positive inclination indicative up an uphill driving scenario.

18. The computer-implemented method according to any of claims 1-16, where the monitored inclination is a negative inclination indicative up a downhill driving scenario.

19. A computer program product comprising program code for performing, when executed by processing circuitry, the method of any of claims 1-18.

20. A non-transitory computer-readable storage medium comprising instructions, which when executed by processing circuitry, cause the processing circuitry to perform the method of any of claims 1-18.

21. A control system (300) for a vehicle (100), where the vehicle (100) comprises wheels (310) driven by a propulsion device (340) via a differential (345), where each driven wheel (310) of the differential (345) is associated with a wheel brake (320), the control system comprising processing circuitry (702) configured to: monitor (SI) an inclination of the vehicle (100) and / or of a surface (101) supporting the vehicle (100), relative to a reference plane, configure (S3) a wheel slip limit ( um) based on the inclination, such that an increase in inclination magnitude results in a decrease in wheel slip limit magnitude, determine (S5) wheel motion indicative of a wheel slip (lx) of each driven wheel (310) of the differential (345), and trigger (S6) a brake intervention operation by the corresponding wheel brake (320), in response to that the wheel slip (lx) of a driven wheel (310) exceeds a wheel slip limit22. A vehicle (100) comprising a control system (300) according to claim 21.

23. A computer-implemented method for controlling motion of a vehicle (100), where the vehicle (100) comprises wheels (310) driven by a propulsion device (340) via a differential (345), where each driven wheel (310) of the differential (345) is associated with a wheel brake (320), the method comprising monitoring (SI) an inclination of the vehicle (100) and / or of a surface (101) supporting the vehicle (100), relative to a reference plane, and configuring (S4) a brake intervention torque application rate associated with the wheel brakes (320), based on the inclination, such that an increase in inclination magnitude results in an increase in torque application rate, determining (S5) wheel motion indicative of a wheel slip (lx) of each driven wheel (310) of the differential (345), and triggering (S6) a brake intervention operation by the corresponding wheel brake (320), in response to that the wheel slip (lx) of a driven wheel (310) exceeds a wheel slip limit ( -um), where the brake intervention operation is parameterized by the brake intervention torque application rate.

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