Patent Description:
Heavy-duty vehicles, such as trucks and semi-trailer vehicles, are designed to carry heavy loads. The heavily laden vehicles must be able to start from standstill also in uphill conditions, accelerate on various types of road surfaces with different friction coefficients, maintain stable cruising speeds, and also decelerate in a reliable manner. At the same time, energy efficiency is an important factor to consider in all operating scenarios, since it directly impacts the cost of completing a given transportation mission.

Electrically powered heavy-duty vehicle units such as tractors, electrified trailers and self-powered dolly vehicle units are being developed. Energy efficiency is of particular importance in these vehicle units, since energy efficiency has a significant impact on the achievable vehicle range for a given energy storage capacity. Thus, a lot of work has gone into designing energy efficient drive arrangements for electrically powered vehicles.

However, many of the developed drive arrangements are based on advanced optimization methods associated with high computational complexity, which are also difficult to formally verify in terms of robustness. These algorithms may not be feasible to execute on the smaller control units found in, e.g., electrified trailers and self-powered dolly vehicle units.

<CIT> discloses a method for distributing torque over a plurality of electrical machines/axles to optimize joint propulsion system efficiency for a given vehicle state. The joint propulsion system efficiency is here formulated as function of requested torque, wheel slip and vehicle velocity.

<CIT> also discloses a drive system for an electric vehicle.

Despite the work done to-date, there is a need for further improvements in drive arrangements in order to optimize the full potential of electrically powered vehicle units.

Another issue related to electrically powered heavy-duty vehicles is the requirement of endurance braking capability. All heavy-duty vehicles must be able to provide braking torque also during extended periods of down-hill driving. Friction brakes may risk onset of brake fading during prolonged periods of constant use and must therefore be complemented by some form of auxiliary brake system. Electrically powered heavy-duty vehicle units may use the electric machines for regenerative braking, but this will generate electrical energy which must be either stored or dissipated. This may become a problem if the energy storage system is at full charge, and brake resistors arranged to dissipate excess energy have reached high temperatures.

Thus, there is also a need to improve the endurance braking capability of electric machine arrangements.

It is an object of the present disclosure to provide techniques which alleviate or overcome at least some of the above-mentioned problems. It is a particular desire to provide simplified control mechanisms for energy efficient electrical drive arrangements. This object is at least in part obtained by a vehicle control unit arranged to control motion of a heavy-duty vehicle which comprises at least first and second electric machine (EM) arrangements, where the first EM arrangement has different efficiency characteristics compared to the second EM arrangement. The vehicle control unit is arranged to control the first and the second EM arrangement by transmitting wheel slip requests to respective EM control units, and to obtain a desired total longitudinal force to be jointly generated by the first and second EM arrangements. The control unit is also arranged to determine a desired first wheel slip corresponding to a first longitudinal force generated by the first EM arrangement, and a desired second wheel slip corresponding to a second longitudinal force generated by the second EM arrangement, where the sum of the first longitudinal force and the second longitudinal force is matched to the desired total longitudinal force which may be a propulsion force for accelerating the vehicle or a braking force for decelerating the vehicle. The control unit is furthermore arranged to balance a magnitude of the first wheel slip relative to a magnitude of the second wheel slip in dependence of the respective efficiency characteristics of the first and the second EM arrangements.

This way overall propulsion efficiency can be increased, since each electric machine can be operated closer to its highest efficiency operating point in a given operating scenario. It is proposed herein to vary the slip requests sent to the different drive axles on the vehicle in dependence of drive axle efficiency characteristics, instead of varying torque requests as proposed in, e.g., <CIT>. For instance, if the vehicle is operating at low velocity, a higher slip request value will be sent to an axle optimized for low speeds compared to the slip request sent to another drive axle optimized for higher speeds. The advantage compared to torque based wheel force control being that the vehicle control unit will be able to control EM actuators at higher bandwidth, thereby maintaining the efficiency balance more accurately compared to a control system based on torque requests sent to the different drive axle controllers. Also, as will be discussed in more detail below, the wheel slip balancing algorithms proposed herein can be implemented with reasonable computational complexity, which is an advantage. This advantage becomes particularly pronounced in case the wheel slip balancing techniques are implemented on a vehicle unit lacking powerful processing circuitry, such as an electrified trailer vehicle unit or a self-powered dolly vehicle unit.

The techniques disclosed herein are also applicable to regenerative braking, where instead low EM efficiency may be desired in order to limit output power during extended periods of regenerative braking in case energy storage is nearing full capacity and energy dissipating arrangements such as brake resistors are reaching dangerously high temperatures. Thus, a means for limiting regenerated energy during braking is also provided by the herein disclosed methods, by minimizing energy efficiency of the electrical machines. In other words, by application of the herein described techniques, EM temperature increase during regenerative braking can be controlled in a robust and efficient manner.

It is appreciated that the herein disclosed techniques are applicable for balancing wheel slip over two or more EM arrangements, which EM arrangements may comprise separate wheel modules and/or drive axles, optionally driven via differential arrangements.

According to aspects, the first EM arrangement (EM1) has a different efficiency characteristic as function of vehicle speed compared to the second EM arrangement (EM2). Thus, the wheel slip requests sent to EM1 will be balanced with the wheel slip requests sent to EM2 in dependence of vehicle speed. For instance, as the vehicle is accelerating from stand-still, the drive torque will be gradually shifted from an EM that is efficient at low vehicle speed to one that is more efficient at higher vehicle speeds. This is a computationally efficient method for balancing wheel slip over more than one EM arrangement. Also, EM1 may have a different efficiency characteristic as function of applied torque or wheel force compared to EM2. In this case the wheel slip requests will be balanced to account for the difference in efficiencies in torque. Advantageously, the different EMs have varying efficiency characteristics over an efficiency map in two dimensions, where the first dimension represents torque and the second dimension represents axle speed or vehicle speed.

According to aspects, EM1 comprises one or more EMs of a different EM design and/or comprises a different gear ratio compared to the second EM arrangement EM2. This means that the two EMs will have different efficiency characteristics. For instance, EM1 may be a startability EM arrangement configured for efficiency at lower vehicle speeds, and EM2 may be a cruise-mode EM arrangement configured for efficiency at higher vehicle speeds. The two EMs may also be associated with different efficiencies when it comes to regenerative braking. It is appreciated that efficiency during regenerative braking may be advantageously be maximized in case energy storage is to be replenished, while it may also be desired to minimize overall EM efficiency during regenerative braking if the energy storage is full or cannot accept the regenerated energy from braking for some other reason. Thus, wheel slip balancing may be performed in order to maximize EM arrangement overall efficiency on the vehicle, or to minimize the overall EM arrangement efficiency, or anywhere there in-between.

According to some aspects, the control unit is arranged to balance the magnitude of the first wheel slip relative to the magnitude of the second wheel slip based on a relative gradient of the efficiency characteristics of the respective EM arrangements with respect to a control parameter. This gradient descent-based method can be implemented with low complexity and will automatically adjust the wheel slip balance to the current operating conditions of the vehicle, which is an advantage. For instance, the control unit may increase the first wheel slip in case the gradient of the efficiency characteristics of the first EM arrangement is larger than the gradient of the efficiency characteristics of the second EM arrangement at a current state of the vehicle, and to decrease the first wheel slip in case the gradient of the efficiency characteristics of the first EM arrangement is smaller than the gradient of the efficiency characteristics of the second EM arrangement at the current state of the vehicle.

According to some other aspects, the control unit may also be arranged to balance the magnitude of the first wheel slip relative to the magnitude of the second wheel slip based on a relative power consumption of the first and the second EM arrangements in comparison to a magnitude relationship between the first longitudinal force and the second longitudinal force. By balancing actual power consumptions, the dependency on accurate models for the different EM arrangements is reduced. A power consumption can be easily measured without relying on, e.g., pre-determined models of efficiency. This means that a more robust control method is provided compared to methods which solely rely on modelling efficiency in dependence of one or more parameters. For instance, the control unit may be configured to increase the first wheel slip in case a ratio between the power consumption of the first EM arrangement and the power consumption of the second EM arrangement is smaller than a corresponding ratio between the first longitudinal force and the second longitudinal force, and to decrease the first wheel slip in case the ratio between the power consumption of the first EM arrangement and the power consumption of the second EM arrangement is larger than the corresponding ratio between the first longitudinal force and the second longitudinal force.

According to aspects, the control unit is arranged to balance the magnitude of the first wheel slip relative to the magnitude of the second wheel slip based on a pre-determined balancing function parameterized by vehicle speed. This is a rather low complex implementation of the techniques disclosed herein. By using a pre-determined mapping between vehicle speed and wheel slip balance, only very little processing is required to perform the actual balancing operation. Thus, this version of the proposed technique is suitable for implementation in vehicle units lacking more powerful computational power.

According to aspects, the control unit is arranged to balance the magnitude of the first wheel slip relative to the magnitude of the second wheel slip based on a pre-determined balancing function parameterized by the total longitudinal force. Thus, different wheel slip balances will be configured in dependence of the requested force, be it propulsion or braking force. Notably, the dependence on vehicle speed may be kept, such that the mapping function is parameterized by both speed and requested total wheel force, i.e., a two-dimensional function. The pre-determined functions may advantageously be realized as look-up tables which may be pre-configured. A slightly more advanced version of this would be to adapt the function in real time as the vehicle is operated in different scenarios.

According to aspects, the transmitted wheel slip request comprises a target longitudinal wheel slip given by <MAT> where R is an effective wheel radius in meters, ωx is the angular velocity of the wheel, and vx is the longitudinal speed of the wheel over ground. Direct control of wheel end modules based on wheel slip requests have been found to give advantages over the more traditional torque-based control. This is primarily due to that the wheel end modules may operate with lower latency to control applied torque against a target wheel slip, compared to receiving requests for torque from a central controller where the control loop is often associated with higher latency. The transmitted wheel slip request may also comprise a target angular velocity of the wheel, determined by the control unit in relation to a longitudinal speed vx of the wheel over ground to obtain a target longitudinal wheel slip λx.

According to aspects, the control unit is arranged to balance the magnitude of the first wheel slip relative to the magnitude of the second wheel slip based on an estimated resulting tyre wear. This way the overall lifetime of the tyres on the vehicle can be extended, since tyre wear can be controlled. For instance, tyre wear can be balanced in order for the tyres of a vehicle to be worn equally fast. The tyre wear adaptation may advantageously be based on a model of tyre wear, which can be pre-configured in dependence of, e.g., wheel slip.

According to aspects, the control unit is arranged to balance the magnitude of the first wheel slip relative to the magnitude of the second wheel slip based on respective normal loads on axles associated with EM1 and EM2. This operation may result in increased traction.

There is also disclosed herein methods, computer programs, computer readable media, computer program products, and vehicles associated with the above discussed advantages.

<FIG> illustrates a heavy-duty vehicle <NUM>. This particular example comprises a tractor unit <NUM> which is arranged to tow a trailer unit <NUM>. The tractor <NUM> comprises a vehicle control unit (VCU) <NUM> arranged to control various functions of the vehicle <NUM>. For instance, the VCU may be arranged to perform a vehicle motion management (VMM) function comprising control of wheel slip, vehicle unit stability, and so on. The trailer unit <NUM> optionally also comprises a VCU <NUM>, which then controls one or more functions on the trailer <NUM>. The VCU or VCUs may be communicatively coupled, e.g., via wireless link, to a remote server <NUM>. This remote server <NUM> may be arranged to perform various configurations of the ECU, and to provide various forms of data to the ECU <NUM>, such as providing data regarding the efficiency characteristics on on-board electric machines (EM), the make and type of tyres mounted on the vehicle <NUM>, and other vehicle data.

The trailer unit <NUM> may be arranged as a self-powered trailer comprising one or more EMs and an electrical energy storage system. The trailer VCU <NUM> may be configured to control these EMs independently from the main tractor VCU <NUM>, or in slave configuration to the main tractor VCU <NUM>. The vehicle combination <NUM> may of course also comprise additional vehicle units, such as one or more dolly units and more than one trailer unit. These additional vehicle units may also be configured as self-powered vehicle units comprising energy sources and EMs.

The vehicle <NUM> is supported by wheels, where each wheel comprises a tyre. The tractor unit <NUM> has front wheels <NUM> which are normally steered, and rear wheels <NUM> of which at least one pair are driven wheels. Generally, the rear wheels <NUM> of the tractor <NUM> may be mounted on tag or pusher axles. A tag axle is where the rear-most drive axle is non-powered, also referred to as a free-rolling or dead axle. A pusher axle is where the forward-most drive axle is not powered. The trailer unit <NUM> is supported on trailer wheels <NUM>. One or more of the trailer axles may be a driven axle.

The tyres on a wheel play a major role in determining the behavior and capabilities of the vehicle <NUM>. A well-designed set of tyres will provide both good traction and fuel economy, while a poorly designed set of tyres, or overly worn tyres, are likely to reduce both traction and fuel economy and may even result in an unstable vehicle combination, which of course is undesired. Some important properties and characteristic parameters of a tyre will now be discussed. These tyre parameters are optionally comprised in a tyre model, either as tyre parameters from which other capabilities and characteristics of the tyre can be determined by the VCU <NUM>, <NUM>, or simply as tyre characteristics which can be used more or less directly by the VCU <NUM>, <NUM> to optimize various control decisions. The properties of the tyres mounted on a given drive axle may be used at least in part to determine an efficiency characteristic of the associated EM drive arrangement. The tyre model may also be used to determine the wear rate if a given tyre when operated at a given wheel slip. Thus, the tyre model can be used as input to an optimization routine which balances wheel slip requested from different driven axles on the vehicle <NUM>.

A tyre rotating at higher speeds tends to develop a larger diameter, i.e., a larger rolling radius, due to centrifugal forces that force the tread rubber away from the axis of rotation. This effect is often referred to as centrifugal growth. As the tyre diameter grows, the tyre width decreases. Excessive centrifugal growth may significantly impact the behavior of a tyre.

The pneumatic trail of a tyre is the trail-like effect generated by resilient material tyres rolling on a hard surface and subject to side loads, as in a turn. The pneumatic trail parameter of a tyre describes the distance where the resultant force of a tyre sideslip occurs behind the geometric center of the contact patch of the tyre.

Slip angle or sideslip angle, denoted α herein, is the angle between a rolling wheel's actual direction of travel and the direction towards which it is pointing (i.e., the angle of the vector sum of the wheel translational velocity.

The relaxation length of a tyre is a property of a pneumatic tyre that describes the delay between when a slip angle is introduced and when the cornering force reaches its steady state value. Normally, relaxation length is defined as the rolling distance needed by the tyre to reach <NUM>% of the steady state lateral force, although other definitions are also possible.

Vertical stiffness, or spring rate, is the ratio of vertical force to vertical deflection of the tyre, and it contributes to the overall suspension performance of the vehicle. In general, spring rate increases with inflation pressure.

The contact patch, or footprint, of the tyre, is the area of the tread that is in contact with the road surface. This area transmits forces between the tyre and the road via friction. The length-to-width ratio of the contact patch affects steering and cornering behavior. The tyre tread and sidewall elements undergo deformation and recovery as they enter and exit the footprint. Since the rubber is elastomeric, it is deformed during this cycle. As the rubber deforms and recovers, it imparts cyclical forces into the vehicle. These variations are collectively referred to as tyre uniformity. Tyre uniformity is characterized by radial force variation (RFV), lateral force variation (LFV) and tangential force variation. Radial and lateral force variation is measured on a force variation machine at the end of the manufacturing process. Tyres outside the specified limits for RFV and LFV are rejected. Geometric parameters, including radial runout, lateral runout, and sidewall bulge, are measured using a tyre uniformity machine at the tyre factory at the end of the manufacturing process as a quality check.

The cornering force or side force of a tyre is the lateral (i.e. parallel to the road surface) force produced by a vehicle tyre during cornering.

Rolling resistance is the resistance to rolling caused by deformation of the tyre in contact with the road surface. As the tyre rolls, tread enters the contact area and is deformed flat to conform to the roadway. The energy required to make the deformation depends on the inflation pressure, rotating speed, and numerous physical properties of the tyre structure, such as spring force and stiffness. Tyre makers often seek lower rolling resistance tyre constructions to improve fuel economy in trucks, where rolling resistance accounts for a high proportion of fuel consumption.

<FIG> illustrates an example tyre model <NUM> which incorporates at least some of the tyre properties to describe the properties of a given tyre, such as those above and also other properties. A tyre model can be used to define a relationship between longitudinal tyre force Fx for a given wheel and an equivalent longitudinal wheel slip for the wheel. Longitudinal wheel slip λx relates to a difference between wheel rotational velocity and speed over ground and will be discussed in more detail below. Wheel rotation speed ω is a rotational speed of the wheel, given in units of, e.g., rotations per minute (rpm) or angular velocity in terms radians/second (rad/sec) or degrees/second (deg/sec). The wheel behavior in terms of wheel force generated in longitudinal direction (in the rolling direction) and/or lateral direction (orthogonal to the longitudinal direction) as function of wheel slip is discussed in "<NPL>. See, e.g., chapter <NUM> where the relationship between wheel slip and longitudinal force is discussed.

Longitudinal wheel slip λx may, in accordance with SAE J670 (SAE Vehicle Dynamics Standards Committee January <NUM>, <NUM>) be defined as <MAT> where R is an effective wheel radius in meters, ωx is the angular velocity of the wheel, and vx is the longitudinal speed of the wheel (in the coordinate system of the wheel). Thus, λx is bounded between -<NUM> and <NUM> and quantifies how much the wheel is slipping with respect to the road surface. 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.

Lateral wheel slip λy can be defined as <MAT> where vy is the lateral speed of the wheel (in the coordinate system of the wheel), measured on a direction orthogonal to the direction of the longitudinal speed vx. This disclosure is primarily focused on longitudinal wheel slip, which is the wheel slip that generates forward motion.

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 are approximately linear, where the proportionality constant is often denoted as the slip stiffness of the tyre. With reference to <FIG>, a tyre (such as any of the tyres <NUM>, <NUM>, <NUM>) is subject to a longitudinal force Fx, a lateral force Fy, and a normal force Fz. The normal force Fz is key to determining some important vehicle properties. For instance, the normal force to a large extent determines the achievable longitudinal tyre force Fx by the wheel since, normally, Fx ≤ µ FZ, where µ is a friction coefficient associated with a road friction condition. The maximum available lateral force for a given lateral slip can be described by the so-called Magic Formula as described in "<NPL>.

<FIG> illustrates an example of achievable tyre forces Fx, Fy as function of wheel slip. The longitudinal tyre force Fx shows an almost linearly increasing part <NUM> for small wheel slips, followed by a part <NUM> with more non-linear behavior for larger wheel slips. The obtainable lateral tyre force Fy decreases rapidly even at relatively small longitudinal wheel slips. It is desirable to maintain vehicle operation in the linear region <NUM>, 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 λLIM on the order of, e.g., <NUM>, can be imposed on a given wheel. For larger wheel slips, e.g., exceeding <NUM>, a more non-linear region <NUM> is seen. The present techniques primarily focus on controlling wheel slip below the imposed wheel slip limit, i.e., in the linear region <NUM>.

A tyre model of this kind can be determined by practical experimentation, analytical derivation, computer simulation, or a combination of the above. In practice, the tyre model may be represented by a look-up table (LUT) indexed by the tyre parameters, or as a set of coefficients describing a polynomial or the like. There the set of coefficients are selected based on the tyre parameters, and where the polynomial then describes the relationship between tyre behavior and vehicle state.

<FIG> schematically illustrates functionality <NUM> for controlling a wheel <NUM> by some example motion support devices (MSD) here comprising a friction brake <NUM> (such as a disc brake or a drum brake) and an electric machine (EM) <NUM>. The friction brake <NUM> and the EM <NUM> are examples of wheel torque generating devices, which may also be referred to as actuators and which can be controlled by one or more motion support device control units <NUM>. The control is based on, e.g., measurement data obtained from a wheel rotation speed sensor and from other vehicle state sensors, such as radar sensors, lidar sensors, and also vision based sensors such as camera sensors and infra-red detectors. An MSD control unit <NUM> may be arranged to control one or more actuators. For instance, it is not uncommon that an MSD control unit is arranged to control MSDs for both wheels of an axle. The MSD control unit <NUM> may comprise separate wheel end module (WEM) control units <NUM>, <NUM> for controlling the different actuators. By estimating vehicle unit motion using, e.g., global positioning systems, vision-based sensors, wheel rotation speed sensors, radar sensors and/or lidar sensors, and translating this vehicle unit motion into a local coordinate system of a given wheel (in terms of, e.g., longitudinal and lateral velocity components), it becomes possible to accurately estimate wheel slip in real time by comparing the vehicle unit motion in the wheel reference coordinate system to data obtained from the wheel rotation speed sensor arranged in connection to the wheel.

A traffic situation management (TSM) function <NUM> plans driving operations with a time horizon of, e.g., <NUM>-<NUM> seconds or so. This time frame corresponds to, e.g., the time it takes for the vehicle <NUM> to negotiate a curve. The vehicle maneuvers, planned and executed by the TSM, can be associated with acceleration profiles and curvature profiles which describe a desired vehicle velocity and turning for a given maneuver. The TSM continuously requests the desired acceleration profiles areq and curvature profiles creq from the VMM function <NUM> which performs force allocation to meet the requests from the TSM in a safe and robust manner. Desired acceleration profiles and curvature profiles may optionally be determined based on input from a driver via a human machine interface of the heavy-duty vehicle via normal control input devices such as a steering wheel, accelerator pedal and brake pedal, although the techniques disclosed herein are just as applicable with autonomous or semi-autonomous vehicles. The exact methods used for determining the acceleration profiles and curvature profiles is not within scope of the present disclosure and will therefore not be discussed in more detail herein.

The VMM function <NUM> operates with a time horizon of about <NUM>,<NUM>-<NUM>,<NUM> seconds or so, and continuously transforms the acceleration profiles areq and curvature profiles creq into control commands for controlling vehicle motion functions, actuated by the different MSDs of the vehicle <NUM> which report back capabilities to the VMM, which in turn are used as constraints in the vehicle control. The interface <NUM> between VMM and MSDs capable of delivering torque to the vehicle's wheels has, 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 a safety critical or 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 allocated to different electronic control units (ECUs), 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 <NUM> between VMM and the MSD controller or controllers <NUM>, <NUM>, 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 VMM module <NUM> translates the required wheel forces Fxi, Fyi determined for each wheel, or for a subset of wheels, into equivalent wheel speeds ωwi or wheel slips λi. by using a tyre model, such as the tyre model in <FIG>. These wheel speeds or slips are then sent to the respective MSD control unit <NUM>. The MSD controllers report back capabilities (CAP) which can be used as constraints.

The VMM function <NUM> and optionally also the MSD control unit <NUM> maintains information on vx (in the reference frame of the wheel), while a wheel speed sensor or the like can be used to determine ωx (the rotational velocity of the wheel).

The VMM module <NUM> can be arranged to store a pre-determined tyre model in memory, e.g., as a look-up table. The inverse tyre model is arranged to be stored in the memory as a function of the current operating condition of the wheel <NUM>. This means that the behavior of the inverse tyre model is adjusted in dependence of the operating condition of the vehicle, which means that a more accurate model is obtained compared to one which does not account for operating condition. 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. For instance, the control unit can be configured to access a set of different models which are selected depending on the current operating conditions. One inverse tyre model can be tailored for high load driving, where normal forces are large, another inverse tyre model can be tailored for slippery road conditions where road friction is low, and so on. The selection of a model to use can be based on a pre-determined set of selection rules. The model stored in memory can also, at least partly, be a function of operating condition. Thus, the model may be configured to take, e.g., normal force or road friction as input parameters, thereby obtaining the inverse tyre model in dependence of a current operating condition of the wheel <NUM>. It is appreciated that many aspects of the operating conditions can be approximated by default operating condition parameters, while other aspects of the operating conditions can be roughly classified into a smaller number of classes. Thus, obtaining the inverse tyre model in dependence of a current operating condition of the wheel <NUM> does not necessarily mean that a large number of different models need to be stored, or a complicated analytical function which is able to account for variation in operating condition with fine granularity. Rather, it may be enough with two or three different models which are selected depending on operating condition. For instance, one model to be used when the vehicle is heavily loaded and another model to be used otherwise. In all cases, the mapping between tyre force and wheel slip changes in some way in dependence of the operating condition, which improves the precision of the mapping. The inverse tyre model may also be implemented at least partly as an adaptive model configured to automatically or at least semi-automatically adapt to the current operating conditions of the vehicle. This can be achieved by constantly monitoring the response of a given wheel in terms of wheel force generated in response to a given wheel slip request, and/or monitoring the response of the vehicle <NUM> in response to the wheel slip requests. The adaptive model can then be adjusted to more accurately model the wheel forces obtained in response to a given wheel slip request from a wheel.

<FIG> illustrate some examples of drive arrangements <NUM>, <NUM>, where a plurality of EM arrangements EM<NUM>, EM<NUM>, EM<NUM>, are configured to power different drive axles. Generally, a vehicle unit may comprise any number of driven axles, and a driven axle may or not also be a steered axle, as indicated in <FIG>. A driven axle may comprise an open differential, as indicated in <FIG>, or wheel end motors as shown in <FIG>. The EM arrangements are configured to generate respective first, second and third longitudinal forces F1, F2 and F3, which together sum up to a desired total longitudinal force Fx.

Interesting to note is that the drive arrangement <NUM> shown in <FIG> is suitable for a tractor vehicle unit <NUM>, or for a steered dolly vehicle unit. The drive arrangement <NUM>, on the other hand, may be a drive arrangement in a self-powered trailer, a tractor, or a dolly vehicle unit without steered axle. The control methods for controlling two drive axles may be applied in self-powered towed vehicle units with advantage, due to the reasonable computational complexity in the proposed methods.

Electric machines are generally associated with efficiency characteristics, both for positive and negative applied torque. Electric motor efficiency can be defined in different ways. However, a common definition of EM efficiency is the ratio between power output (mechanical) and power input (electrical). Mechanical power output is calculated based on the torque and speed required (i.e. power required to move the object attached to the motor), and electrical power input is calculated based on voltage and current supplied to the motor. Mechanical power output is always lower than the electrical power input, as energy is lost during conversion (electrical to mechanical) in various forms, such as heat and friction. Design of an electric motor often but not always aims to minimize these losses to improve efficiency.

<FIG> illustrates an example of EM efficiency characteristics <NUM>. The efficiency is here plotted as a contour plot. Contour plots (sometimes called Level Plots) are a way to show a three-dimensional surface on a two-dimensional plane. It graphs two variables on the x-axis and y-axis and a third variable Z as contours <NUM>, <NUM>. These contours are sometimes called z-slices or iso-response values. <FIG> illustrates efficiency for both positive torque <NUM> and negative torque <NUM>, i.e., braking. The efficiency is often a function of motor axle speed, which translates into vehicle speed via gear ratios, wheel diameters, and such, and also of applied torque. Notably, there is a sweet-spot <NUM> in the example <NUM>, where the EM is operating as maximum efficiency, any deviation from this optimum operating point will result in a reduced efficiency. Conversely, a maximum efficiency operating point <NUM> can be defined also for braking, where a maximum amount of energy is recuperated during braking. As discussed above, it may be desired to perform braking at this point is a battery is to be replenished during down-hill driving. However, if the battery is already at full charge, then this sweet-spot may be highly undesirable to operate at.

The different two or more EM arrangements on the drive axles of an electrically powered vehicle may be configured with different efficiency characteristics. This can be achieved, i.e., by using different types of electric machines, by using different fixed gear ratios, and/or different types of tyres on the different axles. <CIT>, mentioned above, discusses how such different energy efficiency characteristics may be used to optimize vehicle propulsion. For instance, one axle can be configured with an efficiency characteristic which has an optimum at relatively low vehicle speeds, while another axle can be configured with an efficiency optimum at higher speeds. The first axle can then be used for startability, while the other axle can be used when the vehicle is cruising at higher speeds. One axle can also be configured to generate large torque, while another axle may have a limited ability to generate torque.

However, different from the techniques described in <CIT>, it is proposed herein to instead vary the slip requests sent to the different drive axles on the vehicle <NUM>. Thus, if the vehicle is operating at low velocity, a higher slip request value will be sent to the axle optimized for low speeds compared to the slip request sent to the drive axle optimized for higher speeds. The advantage being that the MSD controller <NUM> will be able to control actuators at higher bandwidth, thereby maintaining the efficiency balance more accurately compared to a control system based on torque requests sent to the different WEMs. Also, as will be discussed in more detail below, the wheel slip balancing algorithms proposed herein can be implemented with reasonable computational complexity, which is an advantage. This advantage becomes particularly pronounced in case the wheel slip balancing techniques are implemented on a vehicle unit lacking powerful processing circuitry, such as an electrified trailer vehicle unit or a self-powered dolly vehicle unit.

An electrical motor is normally operated at maximum efficiency, meaning that maximum output power is generated during regenerative braking in order to recuperate as much energy as possible during, e.g., downhill driving. However, it has been realized that there is a control freedom associated with electric machines which allow most electric machines to be operated at a reduced efficiency. The general principles of such sub-optimal energy efficiency electric machine control are described in, e.g., <CIT> and also in <CIT>. An electric machine used to generate braking torque which is operated in a less energy efficient mode of operation will generate more heat and less output current compared to an electric machine that is operated at maximum efficiency.

Aspects of the present disclosure builds on the work in <CIT> and <CIT> and provides a control mechanism and a communications interface which allows the vehicle control unit <NUM> to balance electrical current output from the EM <NUM> during regenerative braking with a temperature increase in the EM during braking. In essence, the control unit <NUM> is, by the proposed technique, able to balance EM temperature increase with electrical storage system (ESS) energy absorption capability during extended periods of down-hill driving, thereby providing an improved endurance braking capability for the heavy-duty vehicle <NUM> and thus a reduced need for over dimensioning the electrical system components of the vehicle <NUM>, such as a brake resistance. According to a preferred implementation, the control unit <NUM> also balances the current output of the EM during driving in a predictive manner. For instance, suppose a route involves an initial flat stretch of road followed by a long downhill section. The control unit may then configure the EM in an energy inefficient mode of operation to consume more power during the drive on the flat stretch of the route, in order to ensure sufficient endurance braking capability during the long downhill part of the route. By allowing a balance between wheel slips at different driven axles of the vehicle, temperature control may be performed. The higher the requested slip - the larger the temperature increase in the EM, regardless of whether the EMs are used for propulsion of braking. Thus, according to some aspects, EM temperature is taken into account when balancing wheel slip over vehicle drive axles. A high temperature at an EM may then warrant a reduction in requested wheel slip, with an increase in wheel slip at another drive axle to compensate for the reduction. Once the high temperature EM has cooled down, the requested wheel slip may again be increased.

<FIG> illustrate some example heavy-duty vehicles <NUM>, <NUM> where the herein disclosed techniques may be used. Any of the axles <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may comprise an EM arrangement arranged for propulsion and regenerative braking. The techniques may be implemented with advantage in a tractor unit <NUM>, in a trailer unit <NUM>, <NUM>, and/or in a dolly vehicle unit <NUM>. The wheel slip balancing may be performed in a central VCU <NUM>, or in a vehicle unit local VCU <NUM>.

<FIG> illustrate some examples of how the disclosed techniques may operate in a vehicle such as those illustrated in <FIG>. The vehicle in <FIG> is accelerating at constant acceleration, meaning that the vehicle longitudinal velocity Vx increases linearly. At the beginning, a first EM arrangement is sufficient for providing the requested propulsion torque. The control unit <NUM> (or <NUM>) determines that the most efficient propulsion option is to request wheel slip λ1 from the first EM arrangement, which could, e.g., be associated with the rear axle <NUM> of the tractor <NUM>, or the front axle <NUM> of the trailer <NUM>. However, similar to most EM arrangements, the first EM arrangement efficiency declines with the increase in motor axle speed required to support the increase in vehicle velocity, as illustrated by the dash-dotted line <NUM> in <FIG>. The control unit therefore starts to request a wheel slip λ2 also from a second EM arrangement configured to power another axle of the vehicle. These two wheel slip values eventually balance out at a relationship determined by the control unit to be a desired relationship in order to maintain the desired operation by the vehicle. <FIG> shows an example where the vehicle comprises first, second, and third EM arrangements, configured to power respective first second and third driven axles. Here, the control unit determines that the first EM arrangement is most efficiently operated alone during vehicle start at a first wheel slip request λ1. The control unit then blends in the second EM arrangement at a wheel slip request λ2, whereupon the first EM arrangement contribution to the vehicle propulsion is phased out, and eventually replaced by the third EM arrangement from which a third wheel slip λ3 is requested.

To summarize the above discussions, there is disclosed herein a vehicle control unit <NUM>, <NUM> arranged to control motion of a heavy-duty vehicle <NUM> comprising at least first and second EM arrangements EM1, EM2, where the first EM arrangement has different efficiency characteristics compared to the second EM arrangement. More than two EM arrangements are of course also possible. EM efficiency was discussed above in connection to, e.g., <FIG>, and may relate to both energy efficiency during propulsion and braking. Generally, the EM efficiency characteristic of an EM is indicative of the ability of the EM to convert electrically stored energy into propulsive force which accelerates a heavy-duty vehicle, or at least maintains a stable vehicle velocity by overcoming losses from, e.g., road friction and air resistance. The EM efficiency characteristic may be possible to modify by adjusting an operating point of the electric machine, e.g., in terms of direct and quadrature set-point, as discussed above.

The first EM arrangement EM1 may for instance have a different efficiency characteristic as function of vehicle speed Vx compared to the second EM arrangement EM2. This means that one of the EM arrangements in generally more efficient at low vehicle speeds, while the other EM arrangement is more efficient at higher vehicle speeds. The first EM arrangement EM1 may also have a different efficiency characteristic as function of applied torque or generated wheel force compared to the second EM arrangement EM2. Normally, an energy efficiency characteristic of an EM is a function of both speed and torque, as illustrated in <FIG>. It is, however, noted that simplifications are possible by not accounting for the dependency on speed or torque, only considering a subset of the relevant parameters, resulting in reduced complexity implementation. This efficiency map may, as noted above, also be adjusted for certain EM implementations, by adjusting an operating point in terms of direct and quadrature current set-points of the EM arrangement.

The first EM arrangement EM1 can be associated with a first vehicle axle and the second EM arrangement EM2 can be associated with a second vehicle axle of the heavy-duty vehicle <NUM>. However, as discussed above in connection to <FIG> and <FIG>, a vehicle combination may comprise a large number of drives axles, and also separate wheel-end motors, on one or more vehicle units. Hence, the teachings herein are to be construed as applicable to a large variety of different vehicle unit types and combinations. The first EM arrangement EM1 is optionally a startability EM arrangement configured for efficiency at lower vehicle speeds, and the second EM arrangement EM2 can then be a cruise-mode EM arrangement configured for efficiency at higher vehicle speeds. Preferably, the startability EM arrangement is configured to power a vehicle unit rear axle <NUM>, <NUM>, and the cruise-mode EM arrangement is configured to power a vehicle unit steered axle <NUM>. This is mainly because the steered axle is spared from load at larger steering angles, which normally only occur when the vehicle is driving slowly.

One way to realize EM arrangements with different energy characteristics is to provide a first EM arrangement EM1 which comprises one or more EMs of a different EM design and/or comprises a different gear ratio compared to the second EM arrangement EM2.

The vehicle control units disclosed herein are arranged to control the first and the second EM arrangement EM1, EM2 by transmitting wheel slip requests to respective EM control units. Here, a wheel slip request is construed to optionally also comprise a wheel speed request, as long as this wheel speed request is determined in relation to the speed of the vehicle over ground, i.e., in relation to the speed of the wheel over ground. In this case a wheel speed and a wheel slip are equivalent, at least for the purposes herein.

Thus, the transmitted wheel slip request optionally comprises or is at least indicative of a target longitudinal wheel slip given by <MAT> where R is an effective wheel radius in meters, ωx is the angular velocity of the wheel, and vx is the longitudinal speed of the wheel over ground. According to another option, the transmitted wheel slip request comprises a target angular velocity of the wheel ωx, determined by the control unit <NUM>, <NUM> in relation to a longitudinal speed vx of the wheel over ground to obtain a target longitudinal wheel slip λx.

The control unit <NUM>, <NUM> is furthermore arranged to obtain a desired total longitudinal force Fx to be jointly generated by the first and second EM arrangements. The total longitudinal force is determined in order to generate a desired motion by the vehicle, such as a desired acceleration.

The control unit <NUM>, <NUM> is arranged to determine a desired first wheel slip λ1 corresponding to a first longitudinal force F1 generated by the first EM arrangement, and a desired second wheel slip λ2 corresponding to a second longitudinal force F2 generated by the second EM arrangement, where the sum of the first longitudinal force F1 and the second longitudinal force F2 is matched to the desired total longitudinal force Fx. This means that the at least two EM arrangements are put to work in order to generate respective forces which together amount to the desired total longitudinal force, since the control unit <NUM>, <NUM> balances the magnitude of the first wheel slip λ1 relative to the magnitude of the second wheel slip λ2 in dependence of the respective efficiency characteristics of the first and the second EM arrangements EM1, EM2, these two component longitudinal forces will vary.

A wheel contributing to vehicle propulsion or braking by a large wheel slip is likely to experience higher tyre wear compared to a wheel which does not generate significant wheel slip. In order to balance tyre wear over the wheels of the vehicle, it may be desired to account for estimated tyre wear in the balancing of wheel slip. Here, tyre normal load may also play an important part is the predicted tyre wear as consequence of a given wheel slip, where higher normal load often implies higher tyre wear rate compared to a wheel that is slipping under a smaller normal load. Models of tyre wear in dependence of, e.g., normal load and wheel slip may be constructed and used as input to the wheel slip balancing algorithms. In case some wheel is estimated by the model to experience high tyre wear, then the wheel slip requested from this wheel may be reduced, and vice versa. One example control method is to simply implement a wheel slip limit which prevents operation above a given wheel slip, which wheel slip limit can be determined in dependence of tyre normal load. This way, certain wheel balancing solutions become inadmissible, since they would lead to an unacceptable tyre wear rate, despite being efficient from some other perspective, such as, e.g., an energy efficiency perspective. Thus, according to some aspects, the control unit may be arranged to balance the magnitude of the first wheel slip λ1 relative to the magnitude of the second wheel slip λ2 based on an estimated resulting tyre wear or tyre wear rate from the current wheel slips. Normal load may also be accounted for in the balancing of wheel slip between wheels or between driven axles. Thus, according to some aspects, the control unit <NUM>, <NUM> is arranged to balance the magnitude of the first wheel slip λ1 relative to the magnitude of the second wheel slip λ2 based on respective normal loads on axles associated with the first EM arrangement EM1 and the second EM arrangement EM2.

<FIG> and <FIG> illustrate an example of how this wheel slip balancing can be realized in practice. Here only two EM arrangements are balanced, but more than two EM arrangements can of course also be used. An MSD coordination function receives a desired longitudinal force Freq to be generated. To meet this request, the MSD coordination function must assign first and second wheel slip requests λ1 ,λ2 to first and second EM arrangements EM1, EM2, which wheel slip requests translate into a first longitudinal force F1 generated by the first EM arrangement, and a second longitudinal force F2 generated by the second EM arrangement, where the sum of the first longitudinal force F1 and the second longitudinal force F2 is matched to the desired total longitudinal force Fx. The mapping between force and wheel slip can be determined from a tyre model such as that discussed above in connection to <FIG>. Normally, but not necessarily, "matched to" here means "equals", but some difference may of course be acceptable, at least during a transient time period. Generally, the first and the second wheel slip requests are determined such that the joint effort by the first and the second EM arrangements together provide a vehicle behavior which meets expectations from higher layers, with sufficient accuracy. The applied torques T1, T2, or estimated longitudinal forces F1 and F2 generated by the two EM arrangements are fed back to an efficiency gradient function which also receives information indicative of a current vehicle velocity Vx, i.e., how fast the vehicle travels over ground. This vehicle speed Vx may be converted into equivalent axle speeds for the two EM arrangements, since the gear ratios, wheel radius, and other vehicle parameters are known to the system. The efficiency gradient function determines an appropriate balance between the two wheel slip requests based on a relative gradient of the efficiency characteristics of the respective EM arrangements EM1, EM2 with respect to a control parameter. The approach is based on a model of efficiency where the efficiency η is a function of applied torque T or generated longitudinal wheel force F, and motor axle speed or wheel speed ω, i.e., <MAT>.

The approach is further visualized in <FIG>, which illustrates a plot of efficiency vs wheel slip λ (or torque T). The curves in <FIG> may be thought of as a cross-section of the contour plot in <FIG>, along the dashed line <NUM>. The joint overall efficiency of the drive system comprising the first and the second EM arrangement is a function of the respective efficiencies of the EM arrangements and the current balance, i.e., the split between them. With reference to <FIG>, it is realized that if the efficiency gradient <MAT> with respect to torque (or wheel force or wheel slip) of the first EM arrangement is larger compared to the efficiency gradient <MAT> with respect to torque (or wheel force or wheel slip) of the second EM arrangement, then an increase in overall efficiency can be achieved by transferring more propulsion effort onto the EM arrangement with the more positive gradient. At a balancing point where the two gradients are the same, it does not matter which may propulsion effort is transferred, since the increase in efficiency by one EM arrangement is offset by the decrease in efficiency by the other EM arrangement. Thus, as long as one gradient is more positive than then other gradient, the efficiency gradient function will transfer more propulsion effort onto the EM arrangement with the larger gradient, until a situation is reached where the gradients are the same, in which point an energy efficient mode of operation has been obtained. The split is quickly adjusted if conditions change, such as a change in vehicle velocity or a change in required acceleration by the vehicle.

According to another example, a gradient descent-based strategy can be used to find an appropriate balance between the wheel slips. Suppose that the vehicle motion management module determines that a total wheel force Ftot is required, and that three driven axles are to generate this total force by respective wheel force contributions F<NUM>, F<NUM> and F<NUM>, such that Ftot = F<NUM> + F<NUM> + F<NUM>. Suppose also that approximate functions P<NUM>(F<NUM>), P<NUM>(F<NUM>) and P<NUM>(F<NUM>) relating EM power (consumed and/or regenerated) to wheel force are available, where the total consumed or regenerated power Ptot = P<NUM>(F<NUM>) + P<NUM>(F<NUM>) + P<NUM>(F<NUM>). These functions may be pre-configured or determined adaptively during vehicle operation. Since a relationship between wheel force and wheel slip is available, the total consumed or regenerated power can also be formulated in terms of wheel slip as Ptot = P<NUM>(λ<NUM>) + P<NUM>(λ<NUM>) + P<NUM>(λ<NUM>).

A cost function <MAT> can be formulated and then optimized (minimized or maximized) by balancing wheel slips.

Then a gradient descent method which adjusts the i:th wheel slip by Δλi can be formulated as
Δλ<NUM> = wg<NUM>,Δλ<NUM> = wg<NUM>, where w is a step length and the third wheel slip λ<NUM> is adjusted to meet the total force constraint such that Ftot = F<NUM> + F<NUM> + F<NUM>. Of course, the requested wheel slips must also meet the respective MSD capabilities in generating wheel force.

To summarize, according to some aspects, the control unit <NUM>, <NUM> is arranged to balance the magnitude of the first wheel slip λ1 relative to the magnitude of the second wheel slip λ2 based on a relative gradient of the efficiency characteristics of the respective EM arrangements EM1, EM2 with respect to a control parameter. For instance, the control unit may be arranged to increase the first wheel slip λ1 in case the gradient of the efficiency characteristics of the first EM arrangement EM1 is larger than the gradient of the efficiency characteristics of the second EM arrangement EM2 at a current state of the vehicle <NUM>, and to decrease the first wheel slip λ1 in case the gradient of the efficiency characteristics of the first EM arrangement EM1 is smaller than the gradient of the efficiency characteristics of the second EM arrangement EM2 at the current state of the vehicle <NUM>.

<FIG> illustrates another control approach which is based on the intuition that the energy consumption on one EM arrangement relative to the energy consumption of another EM arrangement should stand in direct proportion to the contribution mage by the EM arrangements towards the total generated propulsion force. That is, the energy consumed by the i:th EM arrangement Ei, which is a function of the drawn power Pi, should be proportional to the ration of generated propulsion force by the i:th EM arrangement relative to the total propulsion force generated by all EM arrangements on the vehicle, as <MAT>.

In other words, according to some aspects, the control unit <NUM>, <NUM> is arranged to balance the magnitude of the first wheel slip λ1 relative to the magnitude of the second wheel slip λ2 based on a relative power consumption P1, P2 of the first and the second EM arrangements EM1, EM2 in comparison to a magnitude relationship between the first longitudinal force F1 and the second longitudinal force F2. This represents a particularly simple way to balance wheel slips. With reference to <FIG>, the MSD coordination function receives a request for a total longitudinal force to be generated by the set of EM arrangements controlled from the MSD coordinator. The MSD coordinator determines first and second wheel slips λ1, λ2 which together translate into wheel forces that together match the total longitudinal force request Freq. The split is determined by an energy balancing function, which monitors consumed power by the two EM arrangements EM1, EM2, and compared these drawn powers to the allocated first longitudinal force F1 and second longitudinal force F2. In case one EM arrangement contributes less force than warranted by its share in the overall energy consumption, the respective wheel slip will be decreased, and vice versa. Thus, the control unit <NUM>, <NUM> is arranged to increase the first wheel slip λ1 in case a ratio between the power consumption of the first EM arrangement EM1 and the power consumption of the second EM arrangement EM2 is smaller than a corresponding ratio between the first longitudinal force F1 and the second longitudinal force F2, and to decrease the first wheel slip λ1 in case the ratio between the power consumption of the first EM arrangement EM1 and the power consumption of the second EM arrangement EM2 is larger than the corresponding ratio between the first longitudinal force F1 and the second longitudinal force F2.

The wheel slip balancing functionality may of course be further simplified, by use of pre-determined wheel slip splits, such as a look-up table which can be indexed based on one or more operating scenario parameters. According to some such aspects, the control unit <NUM>, <NUM> is arranged to balance the magnitude of the first wheel slip λ1 relative to the magnitude of the second wheel slip λ2 based on a pre-determined balancing function parameterized by vehicle speed. According to some other such aspects, the control unit <NUM>, <NUM> is arranged to balance the magnitude of the first wheel slip λ1 relative to the magnitude of the second wheel slip λ2 based on a pre-determined balancing function parameterized by the total longitudinal force Fx.

<FIG> shows an example control approach where the wheel slip balancing function is implemented as an add-on to an existing vehicle motion management system. Here, the MSD coordination block receives a total longitudinal force request Freq and determines suitable torques T1/T2 or wheel slips λ1/ λ2 to be generated by the different propulsion units. An energy balancing function the modifies the requests from the MSD coordination unit into new wheel slip requests λ1'/ λ2' that are balanced according to the present teaching. Notably, the balancing may only be performed when the vehicle is driving straight, i.e., not during cornering, since the wheel slip balancing may affect vehicle behavior during cornering. A maximum yaw motion threshold may be used to determine when the slip balancing can be performed, and when it should be inactivated. As discussed previously, the power feedback from the Ems may be used to determine a suitable wheel slip split. <FIG> shows an example layered control stack architecture where the techniques disclosed herein may be put to use. The TSM function <NUM> discussed above plans driving operations with a time horizon of, e.g., <NUM>-<NUM> seconds or so. The TSM function <NUM> interacts with the VMM function <NUM> which operates with a time horizon of about <NUM>,<NUM>-<NUM>,<NUM> seconds or so, and continuously transforms the acceleration profiles areq and curvature profiles creq from the TSM <NUM> into control commands for controlling vehicle motion functions, actuated by the different MSDs of the vehicle <NUM> which report back capabilities to the VMM, which in turn are used as constraints in the vehicle control.

The VMM function <NUM> performs vehicle state or motion estimation <NUM>, i.e., the VMM function <NUM> continuously determines a vehicle state s (often a vector variable) comprising positions, speeds, accelerations, yaw motions, normal forces, and articulation angles of the different units in the vehicle combination by monitoring vehicle state and behavior using various sensors <NUM> arranged on the vehicle <NUM>, often but not always in connection to the MSDs.

The result of the motion estimation <NUM>, i.e., the estimated vehicle state s, is input to a global force generation module <NUM> which determines the required global forces on the vehicle units which need to be generated in order to meet the motion requests from the TSM function <NUM>. An MSD coordination function <NUM> allocates, e.g., wheel forces and coordinates other MSDs such as steering and suspension. The coordinated MSDs then together provide the desired lateral Fy and longitudinal Fx forces on the vehicle units, as well as the required moments Mz, to obtain the desired motion by the vehicle combination <NUM>. As indicated in <FIG>, the MSD coordination function <NUM> may output any of wheel slips λi, wheel rotation speeds ω, and/or steering angles δl to the different MSDs.

The wheel slip balancing techniques discussed above are here realized in the slip balance optimization function <NUM>, which interacts with the MSD coordination function. The slip balance optimization function may, e.g., operate according to the principles discussed above in connection to <FIG>, and other implementations are of course also possible. The MSD coordination function <NUM> allocates forces to the different MSDs, while taking account of the desired wheel slip balance, or wheel slip split determined at least partly by the slip balance optimization module. The input from the slip balance optimization module may either be used in the MSD coordination function as an additional constraint to be considered in the overall optimization procedure, or as a pre-requisite.

It is noted that the MSD coordination function <NUM> may be a relatively simple coordination function, where a desired total longitudinal force to be generated by, e.g. a self-powered trailer vehicle unit or a self-powered dolly vehicle unit is to be split over two or more axles of the vehicle unit, or over two or more axles of the articulated vehicle combination. The input from the slip balancing function <NUM> may then be implemented as is without further optimizations, i.e., the total desired force is split over the two or more drive axles in dependence of the split decision obtained from the slip balance optimization function <NUM> and in dependence of the tyre model which is used to translate the desired longitudinal wheel forces into equivalent wheel slip values (or wheel speed values determined after accounting for vehicle speed over ground).

Notable, the slip balance optimization <NUM> may determine wheel slip splits separately per vehicle unit, or jointly for the entire articulated vehicle.

<FIG> is a flow chart illustrating methods which summarize the discussions above. The method is designed to be performed in a vehicle control unit <NUM>, <NUM> arranged to control motion of a heavy-duty vehicle <NUM> comprising first and second electric machine, EM, arrangements EM1, EM2, where the first EM arrangement has a different efficiency characteristic compared to the second EM arrangement. Also, as discussed above, the vehicle control unit <NUM>, <NUM> is arranged to control the first and the second EM arrangement EM1, EM2 by transmitting wheel slip requests to respective EM control units.

The method comprises obtaining S1 a desired total longitudinal force Fx to be jointly generated by the first and second EM arrangements,.

<FIG> schematically illustrates, in terms of a number of functional units, the components of a control unit <NUM>, <NUM> according to embodiments of the discussions herein, such as any of the VUCs <NUM>, <NUM>. This control unit <NUM>, <NUM> may be comprised in the articulated vehicle <NUM>. Processing circuitry <NUM> is provided using any combination of one or more of a suitable central processing unit CPU, multiprocessor, microcontroller, digital signal processor DSP, etc., capable of executing software instructions stored in a computer program product, e.g. in the form of a storage medium <NUM>. The processing circuitry <NUM> may further be provided as at least one application specific integrated circuit ASIC, or field programmable gate array FPGA.

Particularly, the processing circuitry <NUM> is configured to cause the control unit <NUM>, <NUM> to perform a set of operations, or steps, such as the methods discussed in connection to <FIG>. For example, the storage medium <NUM> may store the set of operations, and the processing circuitry <NUM> may be configured to retrieve the set of operations from the storage medium <NUM> to cause the control unit <NUM>, <NUM> to perform the set of operations.

The control unit <NUM>, <NUM> may further comprise an interface <NUM> for communications with at least one external device.

The processing circuitry <NUM> controls the general operation of the control unit <NUM>, <NUM>, e.g., by sending data and control signals to the interface <NUM> and the storage medium <NUM>, by receiving data and reports from the interface <NUM>, and by retrieving data and instructions from the storage medium <NUM>.

Claim 1:
A vehicle control unit (<NUM>, <NUM>) arranged to control motion of a heavy-duty vehicle (<NUM>) comprising first and second electric machine, EM, arrangements (EM1, EM2), where the first EM arrangement has different efficiency characteristics compared to the second EM arrangement,
wherein the vehicle control unit (<NUM>, <NUM>) is arranged to control the first and the second EM arrangement (EM1, EM2) by transmitting wheel slip requests to respective EM control units,
wherein the control unit (<NUM>, <NUM>) is arranged to obtain a desired total longitudinal force (Fx) to be jointly generated by the first and second EM arrangements, and to obtain respective efficiency characteristics of the first and the second EM arrangements (EM1, EM2),
wherein the control unit (<NUM>, <NUM>) is arranged to determine a desired first wheel slip (λ1) corresponding to a first longitudinal force (F1) generated by the first EM arrangement, and a desired second wheel slip (λ2) corresponding to a second longitudinal force (F2) generated by the second EM arrangement, where the sum of the first longitudinal force (F1) and the second longitudinal force (F2) is matched to the desired total longitudinal force (Fx),
characterised in that the control unit (<NUM>, <NUM>) is arranged to balance a magnitude of the first wheel slip (λ1) relative to a magnitude of the second wheel slip (λ2) in dependence of the respective efficiency characteristics of the first and the second EM arrangements (EM1, EM2).