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, and most importantly be able to reduce velocity, i.e., brake, in a controlled and reliable manner at all times.

A conventional heavy-duty vehicle normally uses some type of service brakes to decelerate the vehicle when desired. Such service brakes normally comprise friction brakes, i.e., disc brakes or drum brakes, which are very effective in generating high amounts of negative torque. However, friction brakes are not able to sustain this negative torque for extended periods of time, since the friction brakes then risk overheating, and so-called brake fading may ensue.

To enable slowing down a heavily laden vehicle also during extended periods of down-hill driving, often referred to as endurance braking, heavy-duty vehicles comprise auxiliary brakes in addition to the service brakes. Known auxiliary brakes comprise various forms of engine brakes such as exhaust brakes and different types of retarders.

Electric vehicles may use the electric machines to generate negative torque. This way some of the energy put into the vehicle when travelling up-hill may be recuperated when driving down-hill, which is an advantage from an energy efficiency point of view. A lot of effort has gone into designing energy efficient control systems for electrical vehicles, for instance:.

However, electric vehicles powered from a battery pack or a fuel cell stack, do not comprise a combustion engine, and do not generate any exhaust. Thus, many of the commonly used auxiliary brake systems and retarders are not applicable for endurance braking of electric vehicles. Still, these vehicles may also be heavily laden and also need to be slowed down during extended periods of downhill driving.

There is an unmet need for new types of endurance braking systems applicable in heavy-duty electrically powered vehicles.

<CIT> discloses a control apparatus for a hybrid vehicle. The control apparatus determines a section from a downhill control start point to an end point of a target downhill section as a controlled target section. When the vehicle travels on the controlled target section, the control apparatus executes downhill control.

<CIT> discloses a control apparatus for a hybrid vehicle. The control apparatus executes a downhill control which decreases the remaining capacity of the storage battery before the vehicle enters a downhill section and executes a congestion control which increases the remaining capacity before it enters a congestion section.

<CIT> discloses a regeneration control device of a hybrid vehicle that includes an engine, an electric motor, and a battery configured to supply electric power to the electric motor. Depending on the current state of charge of the battery, engine braking of the engine and/or regenerative torque of the electric motor as the braking force is implemented.

<CIT> discloses a vehicle controller which includes an electric motor connected to wheels, a power storage device connected to the electric motor, a charge/discharge controller that controls charge/discharge of the power storage device, a first traveling controller that controls the electric motor in accordance with a driver's operation, and a second traveling controller that controls the electric motor in accordance with a cruise function that automatically controls a vehicle speed.

<CIT> discloses methods and systems provided for electrically-assisted engine braking. A turbocharger is operated by an electric motor during engine braking to increase airflow to an engine intake. The enhanced air flow into the engine intake increases an exhaust manifold pressure, thus increasing a braking force provided by engine braking.

It is an object of the present invention to provide techniques which alleviate or overcome at least some of the above-mentioned problems. This object is at least in part obtained by a vehicle unit comprising a vehicle control unit (VCU), an electrical energy storage system (EESS), and at least one electric machine arranged for regenerative braking. The electric machine is arranged to generate an electrical output current during regenerative braking of the vehicle unit. The control unit is arranged to obtain a desired energy absorption capability value of the vehicle unit and to control an energy dissipation from the EESS to maintain an energy absorption capability of the EESS above the desired energy absorption capability value. Thus, the VCU will strive to control energy dissipation from the EESS to maintain a margin with respect to full charge. This margin allows the vehicle unit to absorb a pre-determined amount of energy while driving down-hill, and thus provides a pre-determined amount of endurance braking which the vehicle unit can achieve. The vehicle unit is preferably a trailer vehicle unit, which can be used to efficiently slow down also a tractor vehicle unit in an efficient manner.

The desired energy absorption capability value can just be a pre-configured value, which may, e.g., be configured in dependence of an endurance braking requirement placed on the vehicle unit. This desired energy absorption capability value can optionally be obtained from a remote server or be manually configured in some way. Additional advantages may be obtained if the desired energy absorption capability value is configured in dependence of a planned route of the vehicle unit. This means that the desired energy absorption capability value is adapted to the current vehicle operating conditions, and thus optimized to suit a particular transport mission. This means that the margin value can often be selected smaller than a worst-case margin value. For instance, some vehicle may only operate in environments where there are no long down-hill routes, and consequently no need for large margins with respect to full charge.

According to some aspects, the desired energy absorption capability value is configured in dependence of any of: a vehicle gross combination weight mGCW, a required maximum acceleration ax,req, an air drag coefficient Cd, a vehicle front area Af, an air density ρair, a vehicle maximum speed, a rolling resistance Cr, and an expected road slope degree s. This way an analytic or at least semi-analytic approach to determining requirements on the energy absorption capability can be used, where the particular properties of the vehicle and its intended operation can be used to derive a suitable energy absorption capability. This results in further optimization of the configured desired energy absorption capability. This also means that the energy absorption capability can be updated as the vehicle properties change over time, e.g., when the vehicle is heavily laden compared to when the vehicle is not so heavily laden.

According to the invention, the control unit is arranged to control the energy dissipation from the EESS by feeding electrical current to a brake resistance device. Brake resistances may of course also be used to dissipate energy. Different vehicles may comprise brake resistances having different capabilities. The desired energy absorption capability may advantageously be configured in dependence of the brake resistances mounted to a given vehicle, i.e., a vehicle having more extensive brake resistance capacity may not need as large EESS charge margin compared to a vehicle which only comprises a smaller brake resistance or no brake resistance at all.

According to some aspects, the control unit is arranged to control the energy dissipation from the EESS by applying positive torque by the at least one electric machine. Thus, by applying torque a current will be drawn from the EESS. In case there is no need for acceleration by the vehicle unit, the applied positive torque will be compensated for by some other torque generating device on the vehicle, such as another electric machine on another vehicle unit in the vehicle combination. By applying positive torque when driving, e.g., on a flat road without slope, energy may even be transferred to the EESS on another vehicle unit applying regenerative braking to account for the positive energy dissipating torque.

According to some aspects, the control unit is arranged to trigger a notification signal in case the energy absorption capability of the EESS is below the desired energy absorption capability value. This will warn the driver about a potentially reduced endurance braking capability by the vehicle unit. According to some other aspects, the control unit is arranged to prevent operation of the vehicle unit in case the energy absorption capability of the EESS is below the desired energy absorption capability value. This means that the vehicle unit will be halted, or prevented from starting, in case there is not sufficient energy absorption capability to allow the required amount of endurance braking by the vehicle unit.

According to some aspects, the control unit is arranged to indicate a maximum cargo weight and/or a maximum vehicle gross combination weight mGCW corresponding to the desired energy absorption capability value of the vehicle unit. Thus, the driver or person loading the vehicle can receive guidance such as to not load the vehicle too much, since this may jeopardize the endurance braking capability of the vehicle unit.

According to some aspects, the control unit is arranged to control an applied torque by the at least one electric machine in dependence of a road friction coefficient. For instance, the control unit can be arranged to apply negative torque by the at least one electric machine in combination with a negative torque applied by one or more service brakes. The amount of negative torque applied by the at least one electric machine is determined in dependence of a current energy absorption capability of the EESS. Thus, advantageously, the control units disclosed herein may be arranged to perform brake blending in order to improve the braking capability of the vehicle unit and by the vehicle combination comprising the vehicle unit.

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

The invention is only limited within the scope of the appended claims.

<FIG> illustrates a heavy-duty vehicle <NUM>. This particular example is a vehicle combination comprising a tractor unit <NUM> which is arranged to tow a trailer unit <NUM>. The tractor 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> also comprises a VCU <NUM> which controls one or more functions on the trailer. The VCU or VCUs may be communicatively coupled, e.g., via wireless link such as a cellular radio link, to a remote server <NUM>, and may of course also be communicatively coupled to each other, where one of the VCUs may operate in a master mode of operation and the other may operate in a slave mode of operation. This remote server may be arranged to perform various configurations of the VCUs, such as setting different control parameters and performing software update functions.

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.

The trailer vehicle unit <NUM> comprises an electrical energy storage system (EESS) device <NUM> connected to one or more electric machines <NUM>, <NUM>, <NUM>. Some trailer units may comprise just a single driven axle, while other trailer units may comprise more than one driven axle, possibly intended for use at different vehicle unit speeds. Thus, the trailer may assist in vehicle propulsion by applying a positive torque to the one or more driven axles by the respective electric machines <NUM>, <NUM>, <NUM> powered from the EESS. This torque may be controlled directly from the VCU <NUM> independently of any tractor control unit <NUM>, or it can be controlled from the tractor VCU <NUM>, i.e., the trailer control unit <NUM> may operate in slave configuration to the main VCU <NUM>. The trailer VCU may in this case be very simple, comprising perhaps just some control circuitry for controlling the EESS and the electric machines.

According to an example, the trailer VCU <NUM> may be connected to a force sensor arranged at the fifth wheel connection <NUM> between tractor <NUM> and trailer. The VCU <NUM> may then apply torque at the trailer driven axle to reduce the force at the fifth wheel connection. This way the VCU may help in propulsion during up-hill driving and to overcome air resistances and other losses during driving on flat roads. The amount of positive torque may be limited in dependence of articulation angle, such that only a limited amount of positive torque is applied when the vehicle combination is in an articulated state.

Likewise, the VCU <NUM> may apply negative torque to the driven axle, i.e., a braking force, during downhill driving when the trailer unit otherwise pushes the tractor. Thus, importantly, the trailer unit <NUM> may also assist in slowing the vehicle <NUM> down. When the trailer VCU <NUM> (and/or the main tractor VCU <NUM>) controls the one or more electric machines <NUM>, <NUM>, <NUM> such as to apply negative torque, electrical current is generated by the electric machines. This electrical current may be fed back into the EESS <NUM>. However, if this EESS has a state of charge (SOC) close to full charge, the EESS will not be able to absorb the energy generated from braking the vehicle <NUM>. In this case, the surplus energy is instead fed to a braking resistance circuit <NUM> which is arranged to dissipate the generated electrical energy as heat. The braking resistance circuit converts energy into heat and is associated with a maximum operating temperature. Thus, when the braking resistance is heated up to the temperature limit, it will no longer be able to absorb surplus energy. A vehicle unit where the EESS has reached full SOC and where the braking resistance is at maximum temperature will not be able to apply negative torque, since there is no capability for absorbing the surplus energy generated during the regenerative braking.

The present invention relates to techniques which allow the trailer unit <NUM> to be used as an endurance braking device to provide endurance braking capabilities to the vehicle combination. This is accomplished by configuring a desired energy absorption capability of the trailer unit <NUM> in its control unit <NUM> or in the tractor control unit <NUM> if the two VCUs are operating in a master/slave mode of operation. This desired energy absorption capability represents an amount of energy, e.g., in Joules, which the vehicle unit should be able to absorb at any given point in time, or during some predetermined time period. The desired energy absorption capability may also comprise a capability in terms of power, i.e., an amount of energy which can be absorbed per unit of time, e.g., in Watts.

The trailer control unit <NUM> is configured to control an energy dissipation from the trailer unit EESS <NUM> over time in order to maintain the energy absorption capability of the EESS above the desired energy absorption capability value. This means that the control unit will strive to maintain a margin with respect to full SOC, such that the EESS is able to absorb energy if the vehicle should enter into prolonged period of downhill driving. In other words, the control unit will not strive for full charge, but rather try to maintain a SOC below full charge. This mode of operation admittedly results in a less energy efficient vehicle, but safety is increased since a measure of endurance braking capability is provided in exchange for the loss in energy efficiency.

It is appreciated that the techniques for endurance braking disclosed herein may be implemented as a special type of operation, which can be activated on-demand by a driver or by a technician. Thus, if the vehicle unit is operating under driving conditions where no endurance braking is required, then the endurance braking function can be inactivated, and the vehicle unit may then instead operate in a more energy efficient mode of operation where the desired SOC is equivalent to a full charge.

Of course, the tractor unit <NUM> may also comprise an EESS <NUM> arranged to power one or more electric machines <NUM>. This torque generating system is then controlled from the tractor control unit <NUM>. It is appreciated that the techniques disclosed herein are applicable also for use by the main tractor <NUM>. In other words, the tractor unit may also be configured to maintain a margin in the EESS with respect to full charge, in order to enable an endurance braking capability.

<FIG> illustrates an example control architecture for controlling a vehicles such as that illustrated in <FIG>. A transport mission and route planning function <NUM> may determine a suitable path to follow in order to complete a transport mission. This planning may, e.g., comprise evaluating braking energy requirements for different paths. Some paths from a starting location to a target destination may involve extended periods of down-hill driving where significant amounts of energy needs to be absorbed and/or dissipated by the vehicle EESS. Other paths may involve less downhill driving, or no downhill driving at all. Map data can be used to determine the height differences along a particular route. This height difference data, possibly together with gross combination weight data and other properties of the vehicle, can be used to determine a required amount of braking capability.

The transport mission and route planning function <NUM> is in communication with a traffic situation management (TSM) function <NUM> that plans driving operations with a time horizon of, e.g., <NUM> - <NUM> seconds or so. This time frame corresponds to the time it takes for the vehicle <NUM> to, e.g., negotiate a curve or perform some other maneuver. 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 function continuously requests the desired acceleration profiles areq and curvature profiles creq from a vehicle motion management (VMM) function <NUM> which performs force allocation and vehicle motion support device (MSD) coordination to meet the requests from the TSM function <NUM> in a safe and reliable manner. The VMM function <NUM> then continuously sends instructions to the various MSD control units which are comprised in an MSD management function <NUM>. Some example MSD which are controlled in this manner comprise, e.g., the electric machines, any friction brakes on the vehicle units, steering arrangements, active suspension, and so on.

Sensors such as vision-based sensors, radars and lidars, are arranged to obtain data about the vehicle environment <NUM> and to provide input to the vehicle control stack <NUM>. An optional connection to remote processing resources, such as cloud-based processing resources <NUM> may also be comprised in the control stack <NUM>. The remote server <NUM> schematically shown in <FIG> is then comprised in this type of cloud processing function <NUM>.

The VMM function <NUM> operates with a time horizon of about <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 may be used as constraints in the vehicle control. The VMM function <NUM> performs vehicle state or motion estimation, i.e., the VMM function continuously determines a vehicle state (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.

Desired acceleration profiles and curvature profiles may optionally be determined based on input from a driver via a human machine interface <NUM> of the heavy-duty vehicle via conventional 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. Notably, the TSM function <NUM> and/or the transport mission and route planning function <NUM> may configure various properties of the vehicle, such as raising and lowering a liftable axle, adjusting suspensions, and so on.

<FIG> illustrates some examples of use-cases which the vehicle <NUM> must be able to support. The use-case <NUM> is an uphill driving use-case, possibly involving uphill launch from stand-still or from low speed, where the vehicle must be able to generate sufficient torque to overcome gravitational pull as well as friction losses. The use-case <NUM> is instead a downhill driving scenario where braking is required if the vehicle should not exceed its maximum allowable vehicle speed. The vehicle <NUM> must be able to maintain a vehicle velocity below a configured maximum vehicle velocity for the duration of the downhill drive, which may require endurance braking. Finally, <FIG> also shows a use-case <NUM> involving stopping the vehicle <NUM> within a maximum stopping distance.

The different use-cases <NUM>, <NUM>, <NUM> imply peak torque requirements, both with respect to positive as well as negative torque, which must be met by the combination of torque generating devices on the vehicle <NUM>, including the electric machines.

The longitudinal force Fx,req in, e.g., Newton (N), required to be generated by the vehicle <NUM> can be approximated as <MAT> where mGCW is the vehicle gross combination weight, ax,req is the required acceleration by the vehicle, CdAf is the product of air drag coefficient Cd and vehicle front area Af, ρair represents air density, vx is the vehicle speed, g is the gravitational constant, Cr is rolling resistance, and s is a slope percentage value between <NUM> and <NUM>.

In uphill driving positive torque scenarios, such as case <NUM> in <FIG>, the terms <MAT> and gCr mGCW must be overcome by the MSDs, while in downhill scenarios the terms instead must be compensated for to brake the vehicle <NUM> in order to maintain vx below the maximum allowable velocity of the vehicle. This means that the electric machine(s) should be dimensioned to support positive torque sufficient for use case <NUM>, while the combination of friction braking devices and the electric machine(s) should be dimensioned to provide a combined negative torque to support use cases <NUM> and <NUM>, where use case <NUM> may comprise an extended duration endurance braking.

The output energy from the one or more electric machines on the different vehicle units for a given route can be predicted from a relationship like that above. For instance, suppose that the vehicle is required to be able to travel down a hill having a slope of, say s = <NUM> for a certain distance. Given the maximum allowable velocity vxmax the required braking torque can be determined from the above relationship. Given the expected time duration for he down-hill travel, and the required negative torque, an expected amount of generated energy form regenerative braking can be determined. The EESS of the vehicle unit must then be able to absorb this amount of energy, otherwise the negative torque will not be sustainable for the duration of the down-hill drive.

If the route height profile is known, e.g., by the transport mission and route planning function <NUM>, then a desired energy absorption capability of the vehicle unit can be determined rather accurately. However, if the route data is not known, then assumptions on braking requirements can be made, and the desired energy absorption capability can be determined from the assumptions. The assumptions may be related to an operational design domain (ODD) of the vehicle unit.

To summarize, it is possible to predict an amount of energy which will be generated by one or more electric machines on a vehicle <NUM>, given assumptions and/or data related to height profiles of travelled routes by the vehicle and assumptions, or more exact data, related to vehicle properties such as weight. As long as an energy absorption capability of the vehicle is above that required for regenerative braking, safe operation by the vehicle can be assured.

Notably, a trailer unit <NUM> can be used to brake an entire combination vehicle <NUM> when travelling down-hill. Thus, if the energy absorption capability of a trailer unit <NUM> is sufficient to brake the vehicle <NUM>, then the requirements of endurance braking placed on other vehicle units, such as on the tractor <NUM>, can be relaxed.

With reference again to <FIG>, there is disclosed herein a vehicle unit <NUM>, <NUM> comprising a vehicle control unit (VCU) <NUM>, <NUM> an electrical energy storage system (EESS) <NUM>, <NUM>, and at least one electric machine <NUM>, <NUM>, <NUM>, <NUM> arranged for regenerative braking. There may be one or more driven axles on the trailer <NUM> connected to the EESS <NUM>. The tractor unit <NUM> may or may not be an electric vehicle, i.e., the electric machine <NUM> and EESS <NUM> on the tractor <NUM> are entirely optional. The trailer units <NUM> disclosed herein may be arranged to operate together with legacy tractor units <NUM> which do not comprise any advanced control units or electrical propulsion systems. In these cases the trailer units operate independently from the tractor unit <NUM>. In other cases the tractor unit VCU <NUM> is an advanced control unit capable of assuming a master role. The trailer VCU <NUM> may then operate in a slave mode of operation where it receives requests and instructions from the master VCU <NUM>.

The electric machine generates an electrical output current during regenerative braking. This electrical current represents an amount of energy which must be absorbed somehow by the vehicle unit. In order to assure that the vehicle unit does not reach a state where energy can no longer be absorbed, the control unit <NUM>, <NUM> is arranged to obtain a desired energy absorption capability value of the vehicle unit <NUM>, <NUM> and to control an energy dissipation from the EESS <NUM>, <NUM> to maintain an energy absorption capability of the EESS <NUM>, <NUM> above the desired energy absorption capability value. Thus, the vehicle unit will be ready to absorb an amount of energy at all times. Differently put, the desired energy absorption capability value corresponds to a state of charge (SOC) of the EESS <NUM>, <NUM> by a margin below full charge, which ensures that there will always be room for regenerated energy from the electric machines. In other words, the vehicle units disclosed herein will target a SOC below full charge, i.e., voluntarily operate in an energy inefficient mode of operation, in order to enable endurance braking. As was discussed above, the margin to be maintained below full charge can be determined in different ways, e.g., based on assumptions, based on a set of required vehicle capabilities in terms of endurance braking, or more accurately based on route planning data and height maps associated with a route to be travelled.

Thus, additional advantages can be obtained depending on how the desired energy absorption capability value is configured. Of course, the desired energy absorption capability value may just be a factory pre-set value or a pre-configured value input by a technician, which optionally can be subject to update from, e.g., the remote server <NUM>. This value may be configured based on worst-case assumptions made about gross combination weight, worst case routes, and so on.

More advanced options comprise configuring the desired energy absorption capability value in dependence of planned route of the vehicle unit. This way the transport mission and route planning function <NUM> may obtain information related to expected sections of downhill driving along a planned route. This information may then be used to approximate an expected energy generation by the regenerative brakes, and plan for absorbing this amount of energy by the EESS. Thus, a vehicle unit which is to move along a route which is essentially flat may permit a SOC near full charge, while a vehicle unit which is to travel downhill from start to destination instead leaves more SOC margin with respect to full charge. Of course, the configured desired energy absorption capability may also be determined as a function of any other means of energy dissipation available on the vehicle unit. According to the invention, the capacity and current state of any brake resistances <NUM> mounted in connection to the EESS can be accounted for. Some trailer units also comprise refrigerated compartments and other energy absorbing devices which can be accounted for in the overall energy budget to ensure that the desired energy absorption capability is met.

To summarize, a key concept of the present invention relates to maintaining an amount of "free space" in the vehicle unit EESS in order to be able to absorb energy during periods of down-hill driving, i.e., to provide an endurance braking capability. Exactly how much room to leave in the EESS can be determined in different ways and may depend on factors such as an energy dissipating capability of other components on the trailer unit, such as a braking resistance, a cooled freight compartment, and so on. One way to determine the desired energy absorption capability is to make worst case assumptions on vehicle parameters and vehicle routes in terms o height differences, and to use a relationship for required longitudinal force in order to calculate a corresponding requirement on energy absorption capability. Another way to determine the desired energy absorption capability is to obtain data related to a planned route, and to make more exact calculations regarding the expected amount of regenerated energy that he vehicle unit must be able to handle, and then configure the desired energy absorption capability based on this expected amount.

As discussed above, the desired energy absorption capability value may be configured in dependence of any of a an estimated, assumed, or calculated vehicle gross combination weight mGCW, a required maximum acceleration ax,req, an air drag coefficient Cd, a vehicle front area Af, an air density ρair, a vehicle maximum speed, a rolling resistance Cr, and an expected road slope degree s. By determining the required longitudinal force Fx,req required to maintain vehicle speed below a maximum vehicle speed during down-hill driving, a corresponding torque to be generated by the electric machine can be calculated. This torque then corresponds to an amount of generated energy which must be absorbed somehow by the vehicle unit. Notably, the relationship between required longitudinal force Fx,req and vehicle parameters, <MAT> can be determined dynamically based on current vehicle data, or based on limiting assumptions, such as maximum gross combination weight, assumed air resistance, and so on. It is appreciated that the vehicle unit can be configured to generate endurance braking to brake itself, and not other vehicle units in the vehicle combination, or to assist in braking also other vehicle units.

It is noted hat relationships between generated wheel force and applied torque are known, where the applied torque Tw depends on force Fx as <MAT> where Rw is a rolling radius of the wheel. The relationship between applied torque and regenerated energy is often possible to obtain from electric machine specification. Otherwise, this relationship can be determined during vehicle operation or by other means of testing. The relationship can then be stored by the VCU <NUM>, <NUM>.

Energy can also be dissipated by applying positive torque by the at least one electric machine <NUM>, <NUM>, <NUM>, <NUM>. Thus, the control unit may apply a positive torque also during periods of downhill driving in small slopes in order to dissipate energy in order to maintain the desired energy absorption capability. This positive torque will then need to be compensated for by the other vehicle unit torque generating devices. It is appreciated that this results in an energy efficient operation, but it may be required in order to maintain an endurance braking capability.

<FIG> schematically illustrates a display means <NUM> where various notifications and warning signals can be displayed to, e.g., a driver. According to some aspects, the control unit <NUM>, <NUM> is arranged to trigger a notification signal <NUM> in case the energy absorption capability of the EESS <NUM>, <NUM> is below the desired energy absorption capability value. This notification provides a warning to the driver that the endurance braking capability of the vehicle is currently limited. The driver may then take appropriate action in order to increase the energy absorption capabilities of the vehicle unit. The control unit <NUM>, <NUM> may optionally prevent operation of the vehicle unit <NUM>, <NUM> in case the energy absorption capability of the EESS <NUM>, <NUM> is below the desired energy absorption capability value.

The amount of energy which must be absorbed during downhill driving of course strongly depends on the gross combination weight mGCW of the vehicle <NUM>. Thus, according to some aspects, the control unit <NUM>, <NUM> may be arranged to indicate a maximum cargo weight and/or a maximum vehicle gross combination weight mGCW corresponding to the desired energy absorption capability value of the vehicle unit.

At least one of the control units <NUM>, <NUM> may also be arranged to control an applied torque by the at least one electric machine <NUM>, <NUM>, <NUM>, <NUM> in dependence of an estimated or otherwise determined road friction coefficient. For instance, it makes no sense to apply more torque than the current driving conditions can handle by a set of wheels, since this would just result in excessive wheel slip. In case of low road friction, the control unit may combine braking torque from the one or more electrical machines and from one or more service brakes, e.g., friction brakes. This increases the total tyre contact patch area used for braking, which is an advantage in low friction conditions.

At least one of the control units <NUM>, <NUM> may optionally also be arranged to apply negative torque by the at least one electric machine <NUM>, <NUM>, <NUM>, <NUM> in combination with a negative torque applied by one or more service brakes, wherein the amount of negative torque applied by the at least one electric machine <NUM>, <NUM>, <NUM>, <NUM> is determined in dependence of a current energy absorption capability of the EESS <NUM>, <NUM>. Thus, for instance, if the current energy absorption capability does not allow for the required amount of negative torque, the control unit or units may blend in negative torque from one or more service brakes. It is noted, however, that the service brakes cannot be used for extended periods of endurance braking, since this would invariably result in overheated friction brakes and loss of braking capability.

<FIG> is a flow chart illustrating methods which summarize the discussions herein. There is illustrated a method performed by a VCU <NUM>, <NUM> comprised in a vehicle unit <NUM>, <NUM> comprising an EESS <NUM>, <NUM>, and at least one electric machine <NUM>, <NUM>, <NUM>, <NUM> arranged for regenerative braking. The method comprising obtaining S1 a desired energy absorption capability value of the vehicle unit <NUM>, <NUM> and controlling S2 an energy dissipation from the EESS <NUM>, <NUM> to maintain an energy absorption capability of the EESS <NUM>, <NUM> above the desired energy absorption capability value. Of course, all of the different options and variants discussed above may also be comprised as steps in the disclosed methods.

<FIG> schematically illustrates, in terms of a number of functional units, the components of a control unit <NUM> according to embodiments of the discussions herein, corresponding to any of the VUCs <NUM>, <NUM>. This control unit <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.

Claim 1:
A vehicle unit (<NUM>, <NUM>) comprising a vehicle control unit, VCU, (<NUM>, <NUM>) an electrical energy storage system, EESS, (<NUM>, <NUM>), and at least one electric machine (<NUM>, <NUM>, <NUM>, <NUM>) arranged for regenerative braking, wherein the electric machine is arranged to generate an electrical output current during regenerative braking of the vehicle unit, wherein the control unit (<NUM>, <NUM>) is arranged to obtain a desired energy absorption capability value of the vehicle unit (<NUM>, <NUM>) and to control an energy dissipation from the EESS (<NUM>, <NUM>) to maintain an energy absorption capability of the EESS (<NUM>, <NUM>) above the desired energy absorption capability value, characterized in that the control unit (<NUM>, <NUM>) is arranged to control the energy dissipation from the EESS (<NUM>, <NUM>) by feeding electrical current to a brake resistance device (<NUM>).