Method and system of controlling a multiple drive of a vehicle and a vehicle comprising such a system

Abstract

The invention relates to a method of controlling the drive of a vehicle (10) having a dedicated electric motor (Mfl, Mfr, Mrl, Mrr) configured to deliver a torque (Tfl, Tfr, Trl, Trr) to the wheel (Wfl, Wfr, Wrl, Wrr) to which it is dedicated, the method comprising the steps of monitoring an angular speed (ωfl-m, ωfr-m, ωrl-m, ωrr-m) of each wheel (Wfl, Wfr, Wrl, Wrr), monitoring a turning angle (β) of the front wheels (Wfl, Wfr) with respect to the rear wheels (Wrl, Wrr), inputting the monitored angular speed (ωfl-m, ωfr-m, ωrl-m, ωrr-m) of each wheel (Wfl, Wfr, Wrl, Wrr) and the turning angle (β) to a control processing unit (CPU), determining an expected angular speed (ωfl-e, ωfr-e, ωrl-e, ωrr-e) for each wheel based on the monitored angular speed (ωfl-m, ωfr-m, ωrl-m, ωrr-m) of the other wheels and on the turning angle (β), and controlling the torque (Tfl, Tfr, Trl, Trr) delivered by the dedicated electric motor (Mfl, Mfr, Mrl, Mrr) of each wheel (Wfl, Wfr, Wrl, Wrr) to keep the angular speed (ωfl, ωfr, ωrl, ωrr) of each wheel (Wfl, Wfr, Wrl, Wrr) within a predetermined allowance range (±ωΔ) of the respective expected angular speed (ωfl-e, ωfr-e, ωrl-e, ωrr-e) The invention also relates to a system and a vehicle implementing the method. )

Description

[0001]METHOD AND SYSTEM OF CONTROLLING A MULTIPLE DRIVE OF A VEHICLE ANDA VEHICLE COMPRISING SUCH A SYSTEM TECHNICAL FIELDThe invention relates to a system and a method of controlling the drive of a vehicle withmultiple motors. The invention also relates to a vehicle comprising such a system.BACKGROUND In the technical field of electrical vehicles with individual electric motors the drive of eachmotor is individually controlled based on the power demanded by the operator.In conventional vehicles the power from one motor is distributed between the wheels drivenby the motor by means of a universal driving, such as a cardan shaft. For vehicles drivenby an individual motor at each of the driven wheels, the power needs to be distributedbetween the motors in another manner. A challenge with a driving system comprising more than one motor is to accomplish thepower distribution between the wheels in a manner that makes sure that the power isdistributed in an optimal manner between the wheels and specifically that none of thewheels is spinning.It would be advantageous to achieve a method and a system overcoming, or at leastalleviating, at least one or some of the drawbacks of the prior art. In particular, it would beadvantageous to find a reliable manner of controlling the drive of a vehicle with multiplemotors. SUMMARY OF THE INVENTIONIt is an object of the invention to provide method and a system of controlling the drive of avehicle with multiple motors.According to a first aspect the invention relates to a method of controlling the drive of avehicle comprising a front part with two front wheels and a rear part with two rear wheels, each wheel having a dedicated electric motor configured to deliver a torque to the wheel to which it is dedicated, the method comprising the steps of:- monitoring an angular speed of each wheel,- monitoring a turning angle of the front wheels with respect to the rear wheels,- inputting the monitored angular speed of each wheel and the turning angle to acontrol processing unit, wherein the control processing unit is configured to: odetermine an expected angular speed for each wheel based on themonitored angular speed of the other wheels and on the turning angle, and ocontrol the torque delivered by the dedicated electric motor of eachwheel to keep the angular speed of each wheel within a predetermined allowance range of the respective expected angular speed. The invention provides a manner of distributing the power between the different wheels ofa vehicle provided with individual motors, typically in a manner that correspond to a virtualcardan connection.In embodiments of the invention the expected angular speed of each wheel is determinedbased on the turning angle for each wheel according to the following formulas: (1) ωfl-e = k1 * f1(β) * (ωfr-e + ωrl-e + ωrr-e) / 3(2) ωfr-e = k2 * f2(β) * (ωfl-e + ωrl-e + ωrr-e) / 3(3) ωrl-e = k3 * f3(β) * (ωfl-e + ωfr-e + ωrr-e) / 3 where k1, k2, k3, and k4 are coefficients relating to the condition (size, pressure, etc) of therespective wheel, and wherein f1(β), f2(β), f3(β), and f4(β) are functions relating to the turningradius of the wheels depending on the monitored turning angle.In embodiments of the invention the monitored angular speed of a first wheel of the fourwheels deviates more than the predetermined allowance range from the expected angularspeed of the first wheel, the torque provided by the dedicated electric motor of the first wheel is either decreased, if the monitored angular speed exceeds the expected angular speed, i.e., if ωx-m > ωx-e + ωΔceiling, or increased, if the monitored angular speedunderpasses the expected angular speed of the first wheel, i.e., if ωx-m < ωx-e - ωΔfloor.These embodiments allow for the drive of each wheel to be individually compensated whenever it is detected that the drive of one wheel or more is off from an expected or optimal drive situation. In embodiments of the invention the torque provided by the dedicated electric motor of the first wheel is decreased or increased, the torque delivered to the other wheels by the other electric motors is adjusted such that the sum of all the provided torques corresponds to an input demand provided to the control processing unit, by compensation of the decreased or increased torque provided by the dedicated electric motor of the first wheel. This is useful as it will make sure that although one wheel may be spinning or otherwise have less impact on the useful driving of the vehicle the torque needed to drive the vehicle at a requested speed may be upheld by distributing the torque to the other wheels. In embodiments of the invention each motor comprises an AC-inverter, wherein the torque provided by each motor is controlled by regulating the power provided by the AC-inverter. An AC-inverter provides an efficient manner of regulating the torque deliver d by an electric motor. In embodiments of the invention each motor comprises a dedicated controller continuously receiving information of the monitored speed of the corresponding wheel, and wherein the step of comparing the monitored angular speed of the first wheel with the predetermined allowance range of the expected angular speed is performed by the controller and wherein the controller instructs the AC-inverter accordingly. The provision of a dedicated controller to each motor enables a very rapid response to the inputted monitored speed of the corresponding wheel such that the torque may be almost instantly compensated if it is noted that the monitored angular speed of the first wheel isoutside the predetermined allowance range with respect to the expected angular speed.Thereby, a spinning wheel may be detected and attended to in a rapid manner. In embodiments of the invention the vehicle is an articulated frame steered vehicle where the front part and the rear part are articulately connected to each other at an articulationpoint, the two front wheels rotating about a common first axis and the rear wheels rotatingabout a common second axis, the turning angle corresponding to an articulation angle between the front part and the rear part. In embodiments of the invention the first axis is positioned at a first distance from the articulation point and the second axis is positioned at a second distance from the articulation point, and wherein the step of determining the expected angular speed of each wheel is performed by determining a turning radius of each wheel based on the articulation angle and on the relation between the fist distance and the second distance.According to a second aspect the invention relates to a drive control system for performingthe method as described above, which system comprises:- a speed monitoring unit for monitoring the angular speed of each wheel of thevehicle when in motion,- four electric motors, each dedicated to one of the wheels,- each motor comprising an AC-inverter,- a control processing unit for controlling the drive of the four electric motors viathe AC-inverter of each motor.In embodiments of the invention the drive control system comprises a controller dedicatedto each motor to continuously control the AC-inverter based on the monitored the speed of each individual wheel and the expected angular speed for each wheel so as to deliver a distributed torque to the respective wheel corresponding to an input demand in accordance with the method as described above.According to a third aspect the invention relates to a vehicle comprising a front part withtwo front wheels and rear part with two rear wheels, each wheel having a dedicated electric motor configured to deliver a torque to the wheel to which it is dedicated, the vehicle further comprising a drive control system as described above. In embodiments of the invention the vehicle is an articulated frame steered vehicle where the front part and the rear part are articulately connected to each other at an articulation point, the two front wheels rotating about a common first axis and the rear wheels rotating about a common second axis.Other embodiments of the invention according to the three aspects and advantages thereofwill be apparent from the detailed description and the appended drawings. BRIEF DESCRIPTION OF THE DRAWINGSBelow, specific embodiments of the invention will be described with reference to theappended drawings, of which:Fig. 1 is a schematic view of a vehicle when driven straight forward,Fig. 2 is a schematic view of a vehicle when turning to the left,Fig. 3 is a schematic illustration of geometrical relationships of a vehicle whenturning to the left,Fig. 4 is a schematic illustration of the determining of an expected angular speed ofthe driven wheels,Fig. 5 is a schematic illustration of the distribution of driving force between the wheelsunder normal circumstances,Fig. 6 is a schematic illustration of the distribution of driving force in the form of torquebetween the wheels, andFig. 7 is a schematic illustration of the distribution of the torque between the wheelsbased on the monitored angular speed of the same wheel.DETAILED DESCRIPTION OF THE SHOWN EMBODIMENTSIn Fig.1, a vehicle 10 is schematically illustrated when being driven straight forward and inFig.2 the same vehicle 10 is shown when turning left.The vehicle 10 comprising a front part 11 with a front left wheel Wfl, and a front right wheelWfr and rear part 12 with a rear left wheel Wrl and a rear right wheel Wrr, each wheel havinga dedicated electric motor Mfl, Mfr, Mrl, Mrr, respectively, configured to deliver a torque Tfl,Tfr, Trl, Trr to the wheel Wfl, Wfr, Wrl, Wrr to which it is dedicated.Specifically, each motor Mfl, Mfr, Mrl, Mrr may comprise an AC-inverter, wherein the torque provided by each motor Mfl, Mfr, Mrl, Mrr is controlled by regulating the power provided by the AC-inverter.In the shown embodiment the vehicle 10 is an articulated frame steered vehicle where thefront part 11 and the rear part 12 are articulately connected to each other at an articulationpoint AP, wherein the two front wheels Wfl, Wfr in the front part 11 are arranged to rotateabout a common first axis A1 and the rear wheels Wrl, Wrr of the rear part 12 are arrangedto rotate about a common second axis A2.During operation of the vehicle, the angular speed ωfr-m, ωrl-m, ωrr-m of each wheel Wfl,Wfr, Wrl, Wrr, respectively, is monitored.Further, the turning angle β of the front wheels Wfl, Wfr with respect to the rear wheels Wrl,Wrr is monitored.In Fig 3, an articulated frame steered vehicle 10 is shown in a very schematical manner toillustrate the geometry on which embodiments of the invention is based. The vehicle 10 is shown moving forward with a momentaneous velocity V, where the front part and the rear part move with slightly different velocities Vfand Vr, respectively, due to that the vehicle isturning. Specifically, the vehicle is turning to the left, such that the front wheels Wfl, Wfr ofthe vehicle is inclined at an angle of β with respect to rear wheels Wrl, Wrr.The turning angle β has an impact on the relative angular speed of the different wheels.Firstly, the front of the vehicle moves at different speed with respect to the rear part due tothe different distances D1 and D2 of the front axis A1 and rear axis A2 to the articulationpoint AP. It may be concluded that the velocity Vf of the front part 11 of the vehicle will be related tothe velocity Vr of the rear part 12 in the same way as the turning radius R1 of the front part11 is related to the turning radius R2 of the rear part 12.In other words: Vf / Vr = R1 / R2Where the turning radiuses R1 and R2 are defined as the turning radius for a point betweenthe left and right wheel along the axis A1 and A2, respectively, of theFurther both the turning radius R1 of the front part 11 and the turning radius R2 of the rearpart 12 are dependent on the first distance D1, the second distance D2 and the articulationangle β. Specifically: R1 = D1 + (D2 / cos(β)) / tan(β) R2 = (R12 + (D1 + D2 / cos(β))2)0.5 - D2 *tan(β)Further, the angular speed ωfl, ωfr of the front wheels Wfl, Wfr may be deduced from the velocity Vf and the turning radius R1 of the front part 11, and the angular speed ωrl, ωrr of the rear wheels Wrl, Wrr may be deduced from the velocity Vr and the turning radius R2 of the rear part 12. To obtain the correct angular speed for each wheel, compensation needs to be made for the turning radius of the individual wheels. This is achieved by compensating the turningradius with respect to half the axial length Af and Ar of the front part 11 and the rear part12, respectively. The angular speed for each wheel hence follows the following relations: For left turns (β > 0):ωfl-e = Vf / rfl * (R1 - Af / 2) / R1ωfr-e = Vf / rfr * (R1 + Af / 2) / R1ωrl-e = Vr / rrl * (R2 - Ar / 2) / R2ωrr-e= Vr / rrr* (R2+ Ar / 2) / R2where rfl, rfr, rrl, rrr represent the radius of the respective wheels, preferably compensatedfor air pressure, load etc. From the above relations and the fact that all parameters may de deduced from themonitored turning angle β as sole the variable during normal operation it will be possibleto determine the rotational speed of any wheel based on the monitored rotational speed of any other wheel. However, to increase the accuracy and reduce the influence of an incorrect monitoring, possible based on slipping wheels the estimation of the rotational speed of a wheel will be based on the monitored rotational speed of all the other wheels, wherein a monitored rotational speed that is off with respect to those of the other wheels, for example because that wheel is spinning, may be filtered, for instance by Fourier series.As is illustrated in Fig. 4, it is possible estimate an expected angular speed ωfl-e, ωfr-e, ωrl-e,ωrr-e of each wheel Wfl, Wfr, Wrl, Wrr in a control processing unit CPU from the input turningangle β.However, firstly the relative speed of the wheels Wfl, Wfr, Wrl, Wrr may be determined. Toestimate a correct expected angular speed a monitored speed needs to be established. This may be done in different manners, using any type of speedometer.In the illustrated embodiment the CPU the angular speed is determined for each wheel,wherein an expected angular speed ωfl-e, ωfr-e, ωrl-e, ωrr-e for each wheel may be determinedbased on the monitored angular speed ωfr-m, ωrl-m, ωrr-m of the wheels. Specifically, anexpected angular speed of one wheel may be deduced from the monitored angular speed of the other wheels.For each of the motors, the expected angular speed may be dependent on the monitoredangular speed of the other wheels according to the following formulas.(1) ωfl-e = k1 * f1(β) * (ωfr-m + ωrl-m + ωrr-m) / 3)(2) ωfr-e = k2 * f2(β) * (ωfl-m + ωrl-m + ωrr-m) / 3)(3) ωrl-e = k3 * f3(β) * (ωfl-m + ωfr-m + ωrr-m) / 3)(4) ωrr-e = k4 * f4(β) * (ωfl-m + ωfr-m + ωrl-m) / 3)where k1, k2, k3, and k4 are coefficients relating to the condition (size, pressure, etc) of therespective wheel, and wherein f1, f2, f3, and f4are functions depending on the monitoredturning angle β and the monitored angular speed values ωfr-m, ωfl-m, ωfr-m, ωrr-m of the otherwheels.Typically, the calculation includes a filtering such that offset values of one or more of themonitored angular speed values may be disregarded, or at least have less influence on thedetermining of the expected angular speed. The above described method of determining expected angular speed values and monitoring actual angular speed values makes it possible to compare the monitored angular speed value ωx-m of a specific wheel with the expected angular speed values ωx-e of the same wheel to make sure that it is behaving correctly, e.g., by setting an acceptablerange within which the angular speeds should be with respect to each other.The estimated expected angular speeds may be forwarded to a controller Pcont arranged at each motor and will be continuously used as a base for the control of the dedicated motor.The controller Pcont may be any type of controller suitable for the intended use, e.g. a P-controller that acts rapidly on the input signal. A P-controller may be regarded as asimplified PID-controller, i.e. without the integrated accumulated contribution (I) or thederivative contribution (D).Further thought, the drive of the motors is also dependent on the operational conditions of the vehicle, and particularly the power demand as decided by the manoeuvring of the gas pedal. At each moment an overall torque Td is demanded by the operator of the vehicle, forexample in that the operator pushes an accelerator pedal. In a conventional manner theinput signal from e.g. the accelerator pedal yields an output in the form of a delivered powerfrom the motor, wherein a greater demand, e.g. by pushing the accelerator pedal deeper,yields a greater power output from the motor, which will normally result in an increasedvelocity, at least when the vehicle is driven on plane surface or downhill, and wherein alowered demand yields a decreased power output from the motor, which will normally resultin a decreased velocity, at least when the vehicle is drive on plane surface or uphill.In Fig.5 it is illustrated how demanded torque Tdis distributed between different wheels as individual torques Tfl, Tfr, Trl, Trrfor the respective wheels, wherein each torque isdependent on a coefficients cx for the specific wheel in accordance with the relationship Tx= cx * Td / 4 for a case with 4 wheel driving. In a standard performance the coefficients cxequals 1 for wheel such that the power will be provided evenly between the wheels. Inanother standard performance 60% of the power is provided to the rear wheels and 40% to the front wheels. However, in accordance with embodiments of the invention, the torque will be distributed based on the monitored angular speed of each wheel.In Fig. 6 it is illustrated how the power output of the individual motors is dependent on therelation between the monitored angular speed and the expected angular speed of therespective wheels.As should be understood from the illustration in Fig. 6, the torque Tfl, Tfr, Trl, Trr deliveredby the dedicated electric motor Mfl, Mfr, Mrl, Mrr of each wheel Wfl, Wfr, Wrl, Wrr is controlledso as to keep the angular speed ωfl, ωfr, ωrl, ωrr of each wheel Wfl, Wfr, Wrl, Wrr within apredetermined allowance range ±ωΔ of the respective expected angular speed ωfl-e, ωfr-e,ωrl-e, ωrr-e.In the illustration in Fig. 6 it is shown how the torque Tx delivered by the electric motor Mx,dedicated to a specific wheel Wx is controlled in dependence of the expected angular speedωx-e of the same wheel Wx. Specifically, the torque Tx delivered by the dedicated electricmotor Mx of the wheel Wx is controlled so as to keep the angular speed ωx of that wheelwithin a predetermined allowance range ±ωΔ of the respective expected angular speed ωxof the same wheel. The same applies for all wheels, although the predetermined allowancerange ±ωΔ may be different for the different wheels.Specifically, when the monitored angular speed ωx-m of a first wheel Wx of the four wheelsWfl, Wfr, Wrl, Wrrdeviates more than the predetermined allowance range (±ωΔ) from theexpected angular speed ωx-e of the first wheel, the torque Tx provided by the dedicatedelectric motor Mxof the first wheel Wxis either decreased, if the monitored angular speedωx-m exceeds the expected angular speed ωx-e, i.e. if ωx-m > ωx-e + ωΔ, or increased, if themonitored angular speed ωx-m underpasses the expected angular speed ωx-e of the firstwheel Wx, i.e. if ωx-m < ωx-e - ωΔ.Also, the allowance range ±ωΔneed not be symmetric. Instead, there may be a ceiling value ωΔceilingand a floor value ωΔfloorwhich shall not be surpassed or underpassed, respectively, such that the monitored angular speed shall not surpass the expected angularspeed ωx-e with more than the ceiling value ωΔmax and not underpasss it with more thanfloor value ωΔfloor, i.e.:ωx-m < ωx-e + ωΔceiling, andωx-m > ωx-e - ωΔfloorIf the monitored angular speed ωx-m is within the accepted range, i.e. if ωx-e - ωΔ < ωx-m <ωx-e + ωΔ, the torque Tx provided by the dedicated electric motor Mx will be unaltered fromthe torque as demanded, which in a simple manner may correspond to the demanded torque Td divided by the number of motors arranged to provide the torque, which for anarrangement with four motors would correspond to Tx = cx * Td / 4, where cx is the wheelspecific coefficient, which may be 1 or close to 1 if the arrangement is adapted to distributethe driving force evenly between the wheels.Also, if the torque Tx is adapted, e.g. by decreasing the torque by a torque quantity TΔ, thistorque quantity TΔ may be redistributed to the other wheels in order to keep the overalltorque as close as possible to the demanded torque Td. Hence, when the torque Tx provided by the dedicated electric motor Mx of the first wheel Wx is decreased or increased, the torque delivered to the other wheels by the respective electric motors is adjusted such that the sum of all the provided torques Tfl, Tfr, Trl, Trrcorresponds to the demanded torque Td by redistribution of the decreased or increasedtorque quantity TΔ to the other wheels. In Fig.7, it is illustrated how a speed monitoring unit SMU monitors the angular speed of each wheel, continuously or intermittently, and is provided to the controller arranged ateach motor, and wherein the control step illustrated in Fig. 6 is performed at each motor,such that the AC-inverter AC / I of each motor is controlled to deliver a balanced torque where the driving force, i.e. the torque, is distributed between the wheels as described above. In the embodiment shown in Figs.1-3, the vehicle 10 is an articulated frame steered vehicle where the front part 11 and the rear part 12 are articulately connected to each other at anarticulation point AP. The two front wheels Wfl, Wfr are arranged to rotate about a commonfirst axis A1 and the rear wheels Wrl, Wrr are arranged to rotate about a common secondaxis A2. In this embodiment the turning angle β corresponds to the articulation anglebetween the front part 11 and the rear part 12.As is apparent in the drawings the first axis A1 and the second axis A2 may be positionedat different distances from the articulation point AP. Specifically, the first axis A1is positioned at a first distance D1from the articulation point AP and the second axis A2is positioned at a second distance D2from the articulation pointAP, and wherein the step of determining the expected angular speed ωfl-e, ωfr-e, ωrl-e, ωrr-eof each wheel Wfl, Wfr, Wrl, Wrr is performed by determining a turning radius rfl, rfr, rrl, rrr ofeach wheel Wfl, Wfr, Wrl, Wrr based on the articulation angle β and on the relation between the fist distance D1 and the second distance D2.In a specific method of the invention the following steps may be included:In a first step, the relative angular speeds of the wheels are determined, for example by determining the angular speed of each wheel as a fraction of the angular speed of the wheel that theoretically has the highest angular speed under the current conditions.In this step the turning angle, or for an articulated frame steered vehicle, the articulationangle is used to determine the relation between the angular speeds. Hence, the determined relative angular speeds of the wheels are continuously updated based on the current turning or articulation angle. Also, the relative angular speeds of the wheels may be compensated for the currentcondition of the respective wheels, the condition being based on the size, pressure, etc ofthe respective wheels. The condition of the wheels may be continuously updated to keep the determined ratios as precise as possible. In a second step, the actual angular speeds of the wheels are monitored to obtain a monitored angular speed of each wheel. In a subsequent third step, an expected angular speed of each wheel is determined based on the monitored angular speed of the other wheels from the second step and from the relative angular speeds as determined in the first step of the method. This third step preferably involves a filtering step in which a monitored angular speed that differ more than accepted from the other monitored angular speeds are filtered and / or where a monitored angular speed that changes rapidly is filtered to have a reduced influence on the determining of the expected angular speeds. Specifically, if a wheel starts to spin it shall have a limited influence on the estimation of the expected angular speeds. Also, a value that is clearly off from the other values may be temporarily disregarded.In a fourth step, based on the expected angular speed of a wheel, it is possible to setthresholds inside which the angular speed of the same wheel should be kept. Thethresholds may be set as a ceiling threshold which shall not be surpassed and a floor threshold which shall not be underpassed. In a subsequent fifth step the motors of each wheel are controlled in function of the monitored angular speeds. If the monitored angular speed of one the wheels surpasses orunderpasses the respective threshold the power delivered by the motor of that wheel willbe controlled so as to either increase the motors power if the monitored angular speedunderpasses the expected angular speed, decrease the power, if the monitored angular speed exceeds the expected angular speed.The controller Pcont may be arranged at each wheel to control the individual motors basedon the input information regarding the continuously monitored angular speed of the motors.The controller Pcont is effective in that it can act very rapidly on the input to achieve animmediate regulating of the motor it is set to control. Specifically, a dedicated controller Pcont is arranged at each individual motor. Each controller Pcont is set to regulate an AC- inverter arranged at the motor.Above, the invention has been described with reference to specific embodiments. Theinvention is however not limited to these embodiments. It is obvious to a person skilled inthe art that other embodiments are possible within the scope of the following claims.

Claims

CLAIMS1. A method of controlling the drive of a vehicle (10) comprising a front part (11) withtwo front wheels (Wfl, Wfr) and a rear part (12) with two rear wheels (Wrl, Wrr), eachwheel (Wfl, Wfr, Wrl, Wrr) having a dedicated electric motor (Mfl, Mfr, Mrl, Mrr)configured to deliver a torque (Tfl, Tfr, Trl, Trr) to the wheel (Wfl, Wfr, Wrl, Wrr) towhich it is dedicated, the method comprising the steps of: -monitoring an angular speed (ωfl-m, ωfr-m, ωrl-m, ωrr-m) of each wheel (Wfl, Wfr,Wrl, Wrr), -monitoring a turning angle (β) of the front wheels (Wfl, Wfr) with respect to therear wheels (Wrl, Wrr), -inputting the monitored angular speed (ωfl-m, ωfr-m, ωrl-m, ωrr-m) of each wheel(Wfl, Wfr, Wrl, Wrr) and the turning angle (β) to a control processing unit (CPU),wherein the control processing unit (CPU) is configured to: odetermine an expected angular speed (ωfl-e, ωfr-e, ωrl-e, ωrr-e) for eachwheel based on the monitored angular speed (ωfl-m, ωfr-m, ωrl-m, ωrr-m) ofthe other wheels and on the turning angle (β), ando control the torque (Tfl, Tfr, Trl, Trr) delivered by the dedicated electricmotor (Mfl, Mfr, Mrl, Mrr) of each wheel (Wfl, Wfr, Wrl, Wrr) to keep theangular speed (ωfl, ωfr, ωrl, ωrr) of each wheel (Wfl, Wfr, Wrl, Wrr) within apredetermined allowance range (±ωΔ, ωΔceiling ,ωΔfloor) of the respectiveexpected angular speed (ωfl-e, ωfr-e, ωrl-e, ωrr-e).

2. The method of claim 1, wherein the expected angular speed (ωfl-e, ωfr-e, ωrl-e, ωrr-e)of each wheel (Wfl) is determined based on the turning angle (β) for each wheel(Wfl) according to the following formulas: (1) ωfl-e = k1 * f1(β) * (ωfr-e + ωrl-e + ωrr-e) / 3(2) ωfr-e = k2 * f2(β) * (ωfl-e + ωrl-e + ωrr-e) / 3(3) ωrl-e = k3 * f3(β) * (ωfl-e + ωfr-e + ωrr-e) / 3(4) ωrr-e = k4 * f4(β) * (ωfl-e + ωfr-e + ωrl-e) / 3where k1, k2, k3, and k4 are coefficients relating to the condition (size, pressure, etc) of the respective wheel, and wherein f1(β), f2(β), f3(β), and f4(β) are functionsrelating to the turning radius of the wheels depending on the monitored turning angle (β).

3. The method of claim 1 or 2, wherein, if the monitored angular speed (ωx-m) of a firstwheel (Wx) of the four wheels (Wfl, Wfr, Wrl, Wrr) deviates more than thepredetermined allowance range (±ωΔ, ωΔceiling ,ωΔfloor) from the expected angularspeed (ωx-e) of the first wheel, the torque (Tx) provided by the dedicated electricmotor (Mx) of the first wheel (Wx) is either decreased, if the monitored angularspeed (ωx-m) exceeds the expected angular speed (ωx-e), i.e., if ωx-m > ωx-e +ωΔceiling, or increased, if the monitored angular speed (ωx-m) underpasses theexpected angular speed (ωx-e) of the first wheel (Wx), i.e., if ωx-m < ωx-e - ωΔfloor.

4. The method of claim 3 wherein, when the torque (Tx) provided by the dedicatedelectric motor (Mx) of the first wheel (Wx) is decreased or increased, the torquedelivered to the other wheels by the other electric motors is adjusted such that thesum of all the provided torques (Tfl, Tfr, Trl, Trr) corresponds to an input demand (Td) provided to the control processing unit (CPU), by compensation of the decreased or increased torque (Tx) provided by the dedicated electric motor (Mx) ofthe first wheel (Wx).

5. The method of any one of the preceding claims, wherein each motor (Mfl, Mfr, Mrl,Mrr) comprises an AC-inverter (AC / I), and wherein the torque provided by eachmotor (Mfl, Mfr, Mrl, Mrr) is controlled by regulating the power provided by the AC-inverter (AC / I).

6. The method of claim 5 when dependent on claim 3, wherein each motor (Mfl, Mfr,Mrl, Mrr) comprises a dedicated controller (Pcont) continuously receiving informationof the monitored speed of the corresponding wheel (Wfl, Wfr, Wrl, Wrr), and wherein the step of comparing the monitored angular speed (ωx-m) of the first wheel (Wx) with the predetermined allowance range (±ωΔ, ωΔceiling ,ωΔfloor) of the expectedangular speed (ωx-e) is performed by the controller (Pcont) and wherein thecontroller (Pcont) instructs the AC-inverter (AC / I) accordingly.

7. The method of any one of the preceding claims, wherein the vehicle (10) is anarticulated frame steered vehicle where the front part (11) and the rear part (12)are articulately connected to each other at an articulation point (AP), the two frontwheels (Wfl, Wfr) rotating about a common first axis (A1) and the rear wheels (Wrl,Wrr) rotating about a common second axis (A2), the turning angle (β) correspondingto an articulation angle between the front part (11) and the rear part (12).

8. The method of claim 7 when dependent on claim 3, wherein the first axis (A1) ispositioned at a first distance (D1) from the articulation point (AP) and the secondaxis (A2) is positioned at a second distance (D2) from the articulation point (AP), and wherein the step of determining the expected angular speed (ωfl-e, ωfr-e, ωrl-e,ωrr-e) of each wheel (Wfl, Wfr, Wrl, Wrr) is performed by determining a turning radius(Rfl, Rfr, Rrl, Rrr) of each wheel (Wfl, Wfr, Wrl, Wrr) based on the articulation angle (β)and on the relation between the fist distance (D1) and the second distance (D2).

9. A drive control system for performing the method according to any one of thepreceding claims, which system comprises: -a speed monitoring unit (SMU) for monitoring the angular speed (ωfl-m, ωfr-m, ωrl-m, ωrr-m) of each wheel (Wfl, Wfr, Wrl, Wrr) of the vehicle (10) when in motion, -four electric motors (Mfl, Mfr, Mrl, Mrr), each dedicated to one of the wheels,- each motor (Mfl, Mfr, Mrl, Mrr) comprising an AC-inverter (AC / I),- a control processing unit (CPU) for controlling the drive of the four electricmotors via the AC-inverter (AC / I) of each motor (Mfl, Mfr, Mrl, Mrr).

10. The drive control system according to claim 8, wherein a controller (Pcont) isdedicated to each motor (Mfl, Mfr, Mrl, Mrr) to continuously control the AC-inverter(AC / I) based on the monitored the speed of each individual wheel (Wfl, Wfr, Wrl,Wrr) and the expected angular speed (ωfl-e, ωfr-e, ωrl-e, ωrr-e) for each wheel (Wfl, Wfr,Wrl, Wrr) so as to deliver a distributed torque (Tfl, Tfr, Trl, Trr) to the respective wheel(Wfl, Wfr, Wrl, Wrr) corresponding to an input demand (Td) in accordance with themethod according to any one of the claims 1-7.

11. A vehicle (10) comprising a front part (11) with two front wheels (Wfl, Wfr) and rearpart (12) with two rear wheels (Wrl, Wrr), each wheel (Wfl, Wfr, Wrl, Wrr) having adedicated electric motor (Mfl, Mfr, Mrl, Mrr) configured to deliver a torque (Tfl, Tfr, Trl,Trr) to the wheel (Wfl, Wfr, Wrl, Wrr) to which it is dedicated, the vehicle (10) furthercomprising a drive control system according to claim 9 or 10.

12. The vehicle according to claim 11, wherein the vehicle (10) is an articulated framesteered vehicle where the front part (11) and the rear part (12) are articulatelyconnected to each other at an articulation point (AP), the two front wheels (Wfl, Wfr)rotating about a common first axis (A1) and the rear wheels (Wrl, Wrr) rotating abouta common second axis (A2).

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