DE102009055160B4 - Method and device for distributing a drive torque to the wheels of an electrically driven axle of a motor vehicle - Google Patents

Abstract

Method for distributing a drive torque to the wheels of an electrically driven axle of a motor vehicle, wherein an electrical signal from a drive unit (3, 4) is converted into a torque (EMProp_i, EMProp_a) which is transmitted from an electrically driven axle (10a, 10b) to the wheels (11a, 11b) of the axle (10a, 10b), wherein the wheels (11a, 11b) arranged on the electrically driven axle (10a, 10b) are independently subjected to a driving torque (EMProp) each superimposed with a differential torque (EMDif), wherein an electrical drive torque (EMProp_a, EMProp_i) is formed for each driven wheel (11a, 11b) by adding or subtracting the differential torque (EMDifProp) from the drive torque (EMProp), and wherein the electrical drive torque (EMProp) is formed from the torque (EMmot_L, EMmot_R) of the drive unit (3, 4) driving a wheel is determined by superimposing a respective effective torque transmission ratio, which is subjected to at least one weighting factor (KoFn _x , KoFn _y , KoFn _vFz ), from which a pre-control value for an additional differential torque (EMDifPreProp) when cornering is determined.

Description

State of the art

The invention relates to a method for distributing a drive torque to the wheels of an electrically driven axle of a motor vehicle, wherein an electrical signal from a drive unit is converted into a torque which is transmitted from an electrically driven axle to the wheels of the axle, as well as a device for carrying out the method.

There are two groups of electric drives in motor vehicles. Firstly, there are electric wheel drives, in which an electric motor is assigned to each wheel to be driven. There are also electric axle drives, in which the drive torque of an electric motor is transmitted evenly to both wheels, which are attached to the driven axle, via a differential arranged on the driven axle.

Such methods are already known from the JP 2008 - 283 836 A , the EP 2 258 569 A2 , the DE 103 43 640 A1 and the DE 10 2007 036 402 A1 .

disclosure of the invention

The method according to the invention for distributing a drive torque to the wheels of an electrically driven axle of a motor vehicle with the features of claim 1 has the advantage that it is possible to set the drive torque for each wheel individually. The desired drive torque (set by the driver or automatically) is represented by a total and a differential torque component. The wheels arranged on the electrically driven axle are independently subjected to a driving drive torque, each superimposed with a differential torque. Because the wheels arranged on the electrically driven axle are driven individually, the desired drive torque can be distributed between the two wheels in such a way that the desired differential torque component on this axle is ideally transferred to the ground. The proposed wheel-individual drive enables traction on a road surface in which one drive wheel is on a normal road surface and the other drive wheel is on ice, to optimize traction and driving dynamics with high quality and minimize losses.

According to the invention, an electric drive torque is formed for each driven wheel by adding or subtracting the differential torque from the drive torque. The additional differential torque based on a feedforward control serves for the basic distribution of the drive torque to the driven wheels of an axle.

According to the invention, the electric drive torque is determined from a torque of the drive unit driving a wheel by multiplying it by an effective torque transmission ratio, which is subjected to at least one weighting factor, from which a pilot control value for an additional differential torque when cornering is determined. This has a positive effect on the driving dynamics of the vehicle. The differential torque arises in particular when cornering, where it acts as an additional yaw moment and supports the vehicle turning in the curve. In this way, intervention is made against an understeering vehicle right from the start. Traction, i.e. the conversion of the drive force into propulsion of the vehicle, is improved when cornering and driving stability in the curve is increased. Furthermore, the driver's steering effort is significantly reduced, while the vehicle reacts more spontaneously to changes in the driver's steering angle.

In one embodiment, the weighting factor is determined depending on a change in wheel load during acceleration and/or cornering. Both the longitudinal direction (acceleration) and the transverse direction (cornering) of the vehicle are adequately taken into account.
Furthermore, the weighting factor is determined depending on the vehicle speed. This weighting factor describes the maximum permissible differential torque. This ensures that, from a certain vehicle speed, the asymmetrical distribution of the drive torques to the two wheels of the driven axle is reduced.

In particular, a resulting weighting factor is formed from the sum of the wheel load-dependent weighting factors, which is multiplied by the vehicle speed-dependent weighting factor. The resulting weighting factor represents the operating mode of the vehicle, depending on whether the vehicle is in normal operation, in sports operation or in an operating state in which the vehicle has already reached its stability limits. This operating mode is sufficiently taken into account when forming the additional differential torque.

In a further development, the pilot control value is used for the additional differential torque during the drive, the freewheel and the recuperation operation of the motor vehicle and can therefore be implemented in any driving condition of the vehicle.

Advantageously, a deviation of a yaw rate is determined from a target yaw rate and an actual value of the yaw rate, with a stabilizing differential torque being determined from the determined deviation. The use of the stabilizing differential torque contributes to stabilizing the vehicle. Advantageously, the deviation between the target yaw rate and the actual value of the yaw rate and their change are limited to a permissible range.

In one embodiment, if the differential torque determined on the basis of the pre-control value and the stabilizing differential torque have the same sign, the differential torque determined on the basis of the pre-control value and the stabilizing differential torque are added to form a driving dynamics drive torque.

Alternatively, if the differential torque determined on the basis of the pre-control value and the stabilizing differential torque have different signs, the amount of the differential torque determined on the basis of the pre-control value is reduced by the amount of the stabilizing differential torque. Based on this, in an oversteering vehicle, the drive torque on the inside drive wheel is increased when the vehicle is driving, while the outside drive wheel is lowered almost simultaneously. This generates a reverse yaw moment counter to the oversteering tendency and at the same time increases the lateral force potential on the outside wheel. In contrast, in a vehicle with a tendency to understeer, the drive torque on the outside wheel is increased on the driven axle when the vehicle is driving and the drive torque on the inside wheel is reduced accordingly almost simultaneously. This generates a reverse yaw moment counter to the understeering tendency.

In one variant, if a limit value is exceeded by the driving dynamics differential drive torque, the vehicle's drive slip is limited by a wheel slip control system, preferably a TCS system. If the yaw moment generated by the additional differential torque is not sufficient to ensure the stability of the vehicle, a stabilization system present in the vehicle intervenes to support vehicle stabilization. In addition to the TCS system, the vehicle contains an ABS system and an ESP system as additional stabilization systems.

A further development of the invention relates to a method for distributing a drive torque to the wheels of an electrically driven axle of a motor vehicle, whereby an electrical signal from a drive unit is converted into a recuperation torque, which is transmitted from an electrically driven axle to the wheels of the axle. In order to be able to generate not only driving but also braking wheel torques, the wheels arranged on the electrically driven axle are independently subjected to a braking drive torque, each superimposed with a differential torque. In this case, one drive unit is available for individually driving only one wheel on the driven axle. Impairments to driving dynamics are also minimized with wheel-individual recuperation, which forms the basis for realizing differential torques. The recuperation potential is expanded to include low friction values of the drive wheels against the ground, such as on ice or snow. The stability of the vehicle is also improved with high friction values when driving aggressively.

Recouperation is generally understood to mean the operating state of an electric motor in which it works as a generator and converts the energy contained in the driving movement of the vehicle into electrical energy to charge a battery. The vehicle is slowed down by the electric motor. Such a braking process takes place without starting up the mechanical braking system of the vehicle, which is relieved by this procedure, thereby increasing the service life of the mechanical braking system and reducing the effort required for the brake hydraulics.

With wheel-individual recuperation, the conflict between high recuperation performance and a favorable design of the yaw moments that stabilize the vehicle can be better balanced.

Advantageously, in the recuperation mode of the vehicle, a differential recuperation component is formed for the pre-control in order to distribute the recuperation torque to the two wheels of the driven axle. This component depends on the vehicle speed and the lateral acceleration of the vehicle and is superimposed in particular on a stabilization recuperation torque. This procedure also allows the storage of electrical energy to be better adapted to the current driving situation. When cornering, the wheel-individual drive allows the distribution of the braking torque to be better coordinated with the additional differential recuperation torque. This means that the risk of the vehicle understeering or oversteering can be stopped in advance, since the entire recuperation torque is already distributed to the wheels by the pre-control in such a way that a stabilizing effect is achieved.
In a further embodiment, in a vehicle that tends to oversteer, the recuperation torque is increased on the outside wheel and reduced almost simultaneously on the inside wheel.

Alternatively, in a vehicle that tends to understeer, the recuperation torque on the inside wheel increases and the recuperation torque on the outside wheel decreases almost simultaneously.

A further development of the invention relates to a device for distributing a drive torque to the wheels of an electrically driven axle of a motor vehicle, whereby an electrical signal from a drive unit is converted into a torque, which is transmitted from the driven axle to the wheels of the axle. In order to enable the drive torque to be adjusted for each wheel, means are provided which apply a driving or braking drive torque to the wheels arranged on the electrically driven axle independently of one another, each superimposed with a differential torque. In addition to the actual drive torque, the electric axle drive can therefore also realize differential torques that are required for traction and driving dynamics interventions. The classic axle differential is no longer required. The drive torque can now be adjusted for each wheel for both driving and braking wheel torques.

Advantageously, the wheels are driven by two drive units positioned on the axle that work independently of one another, with each drive unit driving one wheel. The almost central arrangement of the two electric drive units on the electrically driven axle enables the unsprung masses on the drive axle to be minimized, which significantly improves the driving comfort of the vehicle. The chassis quality is improved by reducing the unsprung masses.

The invention allows for numerous embodiments. One of these will be explained in more detail with reference to the figures shown in the drawing.

• 1: Prinzipdarstellung einer Doppelrotorantriebseinheit
• 2: ein Beispiel für die elektrische Verschaltung der Doppelrotorantriebseinheit
• 3: schematisches Ablaufdiagramm für ein Ausführungsbeispiel des erfindungsgemäßen Verfahrens

It shows:

• 1 : Schematic diagram of a double rotor drive unit
• 2 : an example of the electrical wiring of the double rotor drive unit
• 3 : schematic flow diagram for an embodiment of the method according to the invention

Identical features are marked with the same reference symbols.

In 1 an electric drive is shown in the form of a double rotor drive unit 1, which is installed in the drive train of a motor vehicle with an electrically driven rear axle. However, this should not represent a restriction, since the double rotor drive unit 1 can also be used on a driven front axle. The double rotor drive unit 1 is arranged approximately in the middle of the axle and is located in the position where a conventional axle differential is normally installed, which is replaced by the double rotor drive unit 1 in the present example.

The double rotor drive unit 1 is arranged in a housing 2 and consists of two electric motor drives 3 and 4. Each electric motor drive 3 and 4 has a stator winding 5a or 5b, which surrounds a rotor 6a or 6b. The rotor 6a or 6b is arranged on a rotor shaft 7a, 7b, The two rotor shafts 7a and 7b can be connected via a coupling 8 positioned between the two rotor shafts 7a, 7b. The rotor shaft 7a, 7b leads to a gear 9a, 9b, which has a predetermined gear ratio for the necessary speed adjustment. The gear 9a, 9b is in turn connected to a side shaft 10a, 10b, which leads to the drive wheel 11a, 11b. In addition, each side shaft 10a, 10b is operatively connected to the wheel suspension or a damper leg 12a, 12b of a shock absorber of the motor vehicle.

As can be seen from the previous description, the drive train is constructed symmetrically starting from the double rotor drive unit 1 in the direction of the driven wheel 11a, 11b. To improve the mass distribution of the motor vehicle, the gears 9a, 9b are arranged near the double rotor drive unit 1, which is arranged approximately at the center of the axle, and can establish or interrupt the connection of the electric motor sub-drive 3 or 4 with the drive wheel 11a or 11b as required.

The electrical wiring of the double rotor drive unit 1 is to be described using 2 will be explained in more detail. Each electric motor partial drive 3 or 4 is connected to a power semiconductor module 13a or 13b, by means of which a three-phase current is generated to drive the electric motor partial drives 3, 4. Both power semiconductor modules 13a, 13b lead to an inverter 14, which is also referred to as a pulse inverter and which converts a direct voltage of approximately 230 V supplied by a high-voltage battery 15 into an alternating voltage, which is further processed by the two power semiconductor modules 13a, 13b. For the sake of completeness, it should also be mentioned that when the electric motor sub-drives 3, 4 operate in a generator mode in which the mechanical energy generated by the vehicle is converted into electrical energy by the electric motor sub-drives 3, 4, the pulse inverter 14 converts this alternating voltage generated by the electric motor sub-drives 3, 4 in the opposite direction into a direct voltage, by means of which the high-voltage battery 15 is charged. During this process, the vehicle is braked without using a mechanical braking system of the vehicle (not shown in more detail).

A DC/DC converter 16 is arranged between the pulse inverter 14 and the high-voltage battery 15, which supplies a low-voltage battery 17 with voltage. The low-voltage battery 17 has a voltage level of approximately 14 V, with the DC/DC converter converting the voltage of 230 V present at the high-voltage battery 15 into 14 V. Using this voltage of 14 volts, the low-voltage battery 17 supplies all of the vehicle's control units with energy.

The electronic control of the electric motor sub-drives 3 and 4 is carried out by a control unit 18, which is connected to other control units of the motor vehicle (not shown) via a vehicle-specific communication network 19, preferably a CAN bus, and receives information from these units about the current driving situation of the motor vehicle. By evaluating this information, the control unit 18 controls the two electric motor sub-drives 3, 4 separately or together, whereby the clutch 7, which is open in normal operation, is closed by a signal from the control unit 18 when the torque generated by both electric motor sub-drives 3, 4 is to be diverted to just one drive wheel 11a, 11b when starting off.

The control unit 18 comprises a driving dynamics control system, which consists of a pilot control unit and a controller unit. The driving stability control of the ESP (Electronic Stability Program) and the traction control of the ESP use the electric double rotor drive unit 1 as an actuator with extended options. The driving dynamics are to be influenced with the help of 3 be described in more detail.

The drive torques on the two driven wheels 11a, 11b are initially equal when driving straight ahead and correspond in total to a drive target torque that is specified by the driver or the control unit 18 for the driven axle 10a, 10b. In block 100, the drive torque EMProp is determined by superimposing and multiplying the electric motor torques by the respectively effective torque transmission ratio iG_L or iG_R between the respective electric motor sub-drive 3, 4 and the wheel 11a, 11b driven by it. $$\text{EMProp}=\text{EMmot}\_\text{L}\ast \text{iG}\_\text{L}+\text{EMmot}\_\text{R}\ast \text{iG}\_\text{R}$$

The reference symbol L corresponds to the electric motor drive part 3 and the drive wheel 11a, while the reference symbol R corresponds to the electric motor drive part 4 and the drive wheel 11b. $$\ddot{\text{U}}\text{blicherweise ist iG}\_\text{L}=\text{iG}\_\text{R}=\text{iG}$$

wobei
EMPropTar das vorgegebene elektrische Sollantriebsmoment und EMPropTCS ein maximal zulässiges elektrisches Sollantriebsmoment darstellen.The drive torque EMProp is limited for traction and/or driving dynamics reasons by the TCS system or the ESP. This means that $$\text{EMProp}=\text{Min}\left(\text{EMropTar},\text{EMPropTCS}\right),$$

where
EMPropTar represents the specified electrical target drive torque and EMPropTCS represents a maximum permissible electrical target drive torque.

P_KoFnx

Verstärkungsparameter

P_GewAx

Gewichtungsfaktor Kurve

ΔFnXRA

Radlaständerung durch Längsbeschleunigung

FnORA

statische Radlast

In block 101, weighting factors KoFn _x and KoFn _y are calculated in the pilot control unit. $${\text{KoFn}}_{\text{x}}=\text{P}\_{\text{KoFn}}_{\text{x}}\ast \text{P}\_{\text{GewA}}_{\text{x}}\ast \Delta {\text{Fn}}_{\text{XRA}}/{\text{Fn}}_{\text{ORA}\text{,}}\text{wobei}$$

P_KoFnx

gain parameters

P_GewAx

weighting factor curve

ΔFnXRA

wheel load change due to longitudinal acceleration

FnORA

static wheel load

P_KoFny

Verstärkungsparameter

ΔFnYRA

Radlaständerung bei Querbeschleunigung oder aus Einfederwegen

FnORA

statische Radlast

P_KoFn _x and ΔFn _XRA are calculated as a function of the measured or estimated longitudinal acceleration. The weighting factor P_GewA _x is specified as a function of the lateral acceleration. It controls the influence of the weighting factor KoFn _x when driving straight ahead and cornering. $${\text{KoFn}}_{\text{y}}=\text{P}\_{\text{KoFn}}_{\text{y}}\ast \Delta {\text{Fn}}_{\text{YRA}}/{\text{FN}}_{\text{ORA}},\text{wobei}$$

P_KoFny

gain parameters

ΔFnYRA

Wheel load change during lateral acceleration or from suspension travel

FnORA

static wheel load

• Im Normalbetrieb stehen Agilität und Traktion des Fahrzeuges im Mittelpunkt. Während des TCS-Betriebs liegt der Schwerpunkt auf der Stabilität und der Traktion, da die Stabilitätsgrenze erreicht ist und im Sportbetrieb wird der Focus auf Traktion, Agilität und Fahrspass gelegt.

P_KoFn _y and ΔFn _YRA are calculated as a function of the measured or estimated lateral acceleration. The gain parameter P_KoFn _y can be parameterized for three different operating conditions:

• In normal operation, the focus is on the vehicle's agility and traction. During TCS operation, the focus is on stability and traction, as the stability limit has been reached, and in sports operation the focus is on traction, agility and driving pleasure.

As a next step, in block 102, another weighting factor KoFn _vFz is formed, which describes the maximum permissible differential torque EMDifPreProp as a function of the vehicle speed: $${\text{KoFn}}_{\text{vFz}}=\text{f}\left(\text{vFz}\right)$$

• Im Normalbetrieb wird ab einer bestimmten ersten Fahrzeuggeschwindigkeit die asymmetrische Verteilung des Antriebsmomentes EMProp sukzessive zurückgenommen. Oberhalb einer zweiten Grenze der Fahrzeuggeschwindigkeit wird das Antriebsmoment EMProp symmetrisch auf die Antriebsräder 11a, 11b verteilt.

The weighting factor KoFn _vFz can be parameterized for two different operating modes:

• In normal operation, the asymmetrical distribution of the drive torque EMProp is gradually reduced from a certain first vehicle speed. Above a second limit of the vehicle speed, the drive torque EMProp is distributed symmetrically to the drive wheels 11a, 11b.

In sports mode, the asymmetrical distribution of the drive torque EMProp is fully maintained up to a first limit. Above a second limit, the drive torque EMProp is distributed symmetrically to the drive wheels. In between, there is a linear transition.

In block 103, the individual weighting factors KoFn _x , KoFn _y and KoFn _vFz are combined to form a resulting weighting factor KoFn. The following applies: $$\text{KoFn}=\text{MIN}\left(\left({\text{KoFn}}_{\text{x}}+{\text{KoFn}}_{\text{y}}\right)\mathrm{,1}\right)\ast {\text{KoFn}}_{\text{vFz}}$$

sign (ay) das Vorzeichen der Querbeschleunigung darstellt.The differential torque EMDifPreProp required for cornering is used on the basis of the pilot control value for the basic distribution of the drive torque EMProp to the driven wheels 11a, 11b of the axle 10a, 10b. $$\text{EMDifPreProp}=\text{sign}\left(\text{ay}\right)\ast \text{KoFn}\ast \text{max}\left(\text{EMprop},\text{EMPropMin}\right),\text{wobei}$$

sign (ay) represents the sign of the lateral acceleration.

In the so-called freewheel case, the following applies: $$\text{EMmot}\_\text{L}=-\text{EMmot}\_\text{R}=>\text{EMProp}=0$$

A special case can apply in recuperation mode: $$\text{EMmot}\_\text{L}=\text{EMmot}\_\text{R}=>\text{EMProp}=0.$$

In order to be able to generate differential torques that are effective in driving dynamics even in these situations with EMProp = 0, EMPropMin provides a certain minimum value for the pre-controlled differential torque EMDifPreProp.

The pilot control supports the drive case, the freewheel case and the operation with recuperation in such a way that the most harmonious, stable driving behavior possible is achieved in accordance with the character of the vehicle.

The differential torque EMDifPreProp is set to zero in cases where the vehicle is reversing, the vehicle is oversteering, the estimated slip angles on the front and rear axles have different signs, or the temperature of at least one component of the dual rotor drive unit 1 has exceeded a critical temperature limit.

In addition, the differential torque EMDifProp is limited to an allowable range. $$\text{EMDifPrePropMin}<=\text{EMDifPreProp}<=\text{EMDifPrePropMax}$$

$$\text{devgi}\left(\text{k}\right)=\text{evGi}\left(\text{k}\right)-\text{evGi}\left(\text{K}-1\right),\text{wobei}$$

vGiSo

Sollgierwinkelgeschwindigkeit

vGi

Istwert der Gierwinkelgeschwindigkeit

evGi

Regelabweichung der Gierwinkelgeschwindigkeit darstellen.

devGi

Änderung der Regelabweichung der Gierwinkelgeschwindigkeit.

In block 104, a controller determines a stabilizing differential drive torque EMDifStab from a control deviation of a yaw rate of the vehicle. The control deviation of the yaw rate is determined from $$\text{evGi}\left(\text{k}\right)=\text{vGiSo}\left(\text{k}\right)-\text{vGi}\left(\text{k}\right)$$

$$\text{devgi}\left(\text{k}\right)=\text{evGi}\left(\text{k}\right)-\text{evGi}\left(\text{K}-1\right),\text{wobei}$$

vGiSo

target yaw rate

vGi

actual value of the yaw rate

evGi

Represent the control deviation of the yaw rate.

devGi

Change in the control deviation of the yaw rate.

To determine the stabilizing differential drive torque EMDifStab, a control method with PDT ₁ characteristics is used, taking into account a controller parameter with a P-gain parameter and a controller parameter with a D-gain parameter, which are determined depending on the estimated friction coefficient, the driving situation (whether the vehicle is oversteering or understeering) and the vehicle speed.

In block 105, the differential torque EMDifPreProp formed on the basis of the pre-control is compared with the stabilizing differential torque EMDifStab. If the differential torque EMDifPreProp and the stabilizing differential torque EMDifStab have different signs, the amount of the differential torque EMDifPreProp is reduced by the amount of the stabilizing differential torque EMDifStab. The stabilizing differential torque EMdifStab calculated from driving stability considerations thus retains the decisive function. If the differential torque EMDifPreProp and the stabilizing differential torque EMDifStab have the same signs, the differential torque EMDifPreProp is reduced by the amount of the stabilizing differential torque EMDifStab. The moments from the pre-control and stabilization add up to the differential torque EMDifStab $$\text{EMDif}=\text{EMDifPreProp}+\text{EMDifStab}$$

In block 106, it is now checked whether the vehicle is in an oversteering state or an understeering state, whereby the respective state is counteracted by superimposing the differential torque EMDif. The vehicle is oversteered if the yaw rate is greater than the target yaw rate: $$\left|\text{vGi}\right|>\left|\text{vGiSo}\right|.$$

$$\text{EMProp}\_\text{a}=0.5\ast \left(\text{EMProp}-\text{EMDif}\right),\text{wobei}$$

EMProp_i

Antriebsmoment am kurveninneren Rad

EMProp_a

Antriebsmoment am kurvenäußeren Rad

In the described vehicle with the driven rear axle 10a, 10b, the drive torque on the inside wheel is increased and at the same time the torque on the outside wheel is reduced accordingly. This generates a reverse yaw moment counter to the oversteering tendency and at the same time increases the lateral force potential of the outside wheel. $$\text{EMProp}\_\text{i}=0.5\ast \left(\text{EMProp}+\text{EMDif}\right)$$

$$\text{EMProp}\_\text{a}=0.5\ast \left(\text{EMProp}-\text{EMDif}\right),\text{wobei}$$

EMProp_i

drive torque on the inside wheel

EMProp_a

drive torque on the outside wheel

For an understeered condition of the vehicle, $$\left|\text{vGi}\right|<\left|\text{vGiSo}\right|.$$

$$\text{EMProp}\_\text{a}=0.5\ast \left(\text{EMProp}+\text{EMDif}\right)$$

If the vehicle is in such an understeered state, the drive torque on the outside wheel increases and at almost the same time the torque on the inside wheel decreases accordingly. This creates a turning yaw moment that counteracts the understeering tendency. $$\text{EMProp}\_\text{i}=0.5\ast \left(\text{EMProp}-\text{EMDif}\right)$$

$$\text{EMProp}\_\text{a}=0.5\ast \left(\text{EMProp}+\text{EMDif}\right)$$

Adherence to permissible wheel slip values applies when regulating both the oversteered and understeered states. If an excessively high driving dynamic differential torque EMDif leads to an excessive increase in the wheel drive slip on one wheel, the differential drive torque EMDif is initially limited depending on the wheel slip. At the same time, the non-reducible portion of the differential drive torque EMDif on the other drive wheel of the axle is compensated by a greater reduction in the drive torque EMProp or by a corresponding braking torque. Excessively high drive slip values are limited by the TCS system, while excessively high braking slip values are limited by the ABS controller on the respective wheel. If the stabilizing effect by superimposing the differential torque is not sufficient, the ESP system intervenes in a known manner to reduce the total drive torque EMProp.

As a special case, we will now consider whether a wheel spins, which occurs during forced cornering or when driving on a road that is partially covered with ice and where one wheel of the vehicle is moving on the icy surface while the other wheel is driving on a dry road. The spinning wheel is usually the unloaded wheel on the inside of the curve or the wheel that is on the lower friction coefficient with the road. To detect the spinning wheel, the differential speed between the two driven wheels 11a, 11b is evaluated. This differential speed is corrected by the amount of a kinematic differential speed, whereby a control deviation of the differential speed is obtained, which has arisen due to an excess of the total drive torque EMProp. The following applies to the control deviation evDif of the differential speed: $$\left|\text{evDif}\right|>0.$$

wobei

EMDifPropP

P-Anteil des Differenzantriebsmomentes

EMDifPropl

I-Anteil des Differenzantriebsmomentes

EMDifPropD

D-Anteil des Differenzantriebsmomentes darstellen.

If this condition is met, a controller with PID structure generates a differential drive torque EMDifProp $$\text{EMDifProp}=\text{EMDifPropP}+\text{EMDifPropl}+\text{EMDifPropD},$$

where

EMDifPropP

P-component of the differential drive torque

EMDifPropl

I-component of the differential drive torque

EMDifPropD

D-component of the differential drive torque.

$$\text{EMProp}\_\text{a}=0.5\ast \left(\text{EMProp}+\text{EMDifProp}\right)$$

A differential drive slip between the wheels 11a, 11b of the driven axle 10a, 10b that is too high is compensated by a wheel-dynamic differential drive torque EMDifProp. Assuming that the wheel on the inside of the curve has a significantly greater drive slip than the wheel on the outside of the curve, the following is obtained for the two target drive torques of the wheels 11a, 11b: $$\text{EMProp}\_\text{i}=0.5\ast \left(\text{EMProp}-\text{EMDifProp}\right)$$

$$\text{EMProp}\_\text{a}=0.5\ast \left(\text{EMProp}+\text{EMDifProp}\right)$$

At the spinning wheel, the drive torque is reduced by half the amount of the wheel dynamic differential drive torque EMDifProp and at the same time at the other stable wheel, it is increased by half the amount of the wheel dynamic differential drive torque EMDifProp. This synchronizes the two wheel drive slip values. A known total slip controller monitors compliance with the required axle total slip values. Axle total slip values that are too high are limited by the TCS system by reducing the total drive torque EMProp.

Thanks to the wheel-individual drive, the recovery of electrical energy (recouperation) can be better adapted to the current driving situation.

wobei EMRecoupVDC die Obergrenze des zulässigen Recouperationsmomentes darstellt. Bei Kurvenfahrten kann durch den radindividuellen Antrieb die Verteilung der Bremsmomente im Teilbremsbereich mit dem geforderten Recouperationsmoment besser koordiniert werden.Energy is fed back when the electric motor drives 3, 4 are in generator mode. The total value of the braking recuperation torque EMRecoupTar is specified by the control unit 18 and is initially distributed equally to both driven wheels 11a, 11b when driving straight ahead. The target value of the total permissible recuperation torque EMRecoup is limited in advance by the ESP system for reasons of driving stability. $$\text{EMRecoup}=\text{Min}\left(\text{EMRecoupTar},\text{EMRecoupVDC}\right),$$

where EMRecoupVDC represents the upper limit of the permissible recuperation torque. When cornering, the wheel-individual drive allows the distribution of braking torques in the partial braking range to be better coordinated with the required recuperation torque.

A pre-control for the distribution of the entire recuperation torque EMRecoup forms a differential component EMDifRecoupPre, which is determined when cornering depending on the vehicle speed and the lateral acceleration or the wheelbase forces. This means that the risk of under- or oversteering of the vehicle during regeneration can be prevented in advance. If the vehicle is instable, the stabilizing recuperation torque EMDifRecoupstab is additionally superimposed. The resulting differential torque during recuperation EMDifRecoup is formed as follows: $$\text{EMDifRecoup}=\text{EMDifRecoupePre}+\text{EMDifRecoupStab}$$

$$\text{EMRecoup}\_\text{a}=0.5\ast \left(\text{EMRecoup}+\text{EMDifRecoup}\right)$$

If the vehicle has a tendency to oversteer, the recuperation torque or braking torque EMRecoup_a is increased on the outside wheel and reduced almost simultaneously on the inside wheel (EMRcoup_i). This generates a reverse yaw moment that counteracts the oversteering tendency. $$\text{EMRecoup}\_\text{i}=0.5\ast \left(\text{EMRecoup}-\text{EMDifRecoup}\right)$$

$$\text{EMRecoup}\_\text{a}=0.5\ast \left(\text{EMRecoup}+\text{EMDifRecoup}\right)$$

Excessive brake slip values are limited by the ABS controller at the respective wheel by limiting the respective recuperation torque EMRecoup_i or EMRecoup_a.

$$\text{EMRecoup}\_\text{a}=0.5\left(\text{EMRecoup}-\text{EMDifRecoup}\right)$$

In an understeering vehicle, the recuperation or braking torque EMRecoup_i on the inside wheel is increased, while the recuperation or braking torque EMRecoup_a on the outside wheel is reduced. This generates a turning yaw moment that counteracts the understeering tendency. $$\text{EMRecoup}\_\text{i}=0.5\left(\text{EMRecoup}+\text{EMDifRecoup}\right)$$

$$\text{EMRecoup}\_\text{a}=0.5\left(\text{EMRecoup}-\text{EMDifRecoup}\right)$$

In this case, too, excessive brake slip values are limited by the ABS controller on the respective wheel by limiting the respective recuperation torque EMRecoup_i or EMRecoup_a.

Claims (8)

Method for distributing a drive torque to the wheels of an electrically driven axle of a motor vehicle, wherein an electrical signal from a drive unit (3, 4) is converted into a torque (EMProp_i, EMProp_a) which is transmitted from an electrically driven axle (10a, 10b) to the wheels (11a, 11b) of the axle (10a, 10b), wherein the wheels (11a, 11b) arranged on the electrically driven axle (10a, 10b) are independently subjected to a driving torque (EMProp) each superimposed with a differential torque (EMDif), wherein an electrical drive torque (EMProp_a, EMProp_i) is formed for each driven wheel (11a, 11b) by adding or subtracting the differential torque (EMDifProp) from the drive torque (EMProp), and wherein the electrical drive torque (EMProp) is formed from the torque (EMmot_L, EMmot_R) of the drive unit (3, 4) driving a wheel is determined by superimposing a respective effective torque transmission ratio, which is subjected to at least one weighting factor (KoFn _x , KoFn _y , KoFn _vFz ), from which a pre-control value for an additional differential torque (EMDifPreProp) when cornering is determined. procedure according to claim 1 characterized in that the weighting factor (KoFn _x , KoFn _y ) is determined as a function of a wheel load change during acceleration and/or cornering. procedure according to claim 1 characterized in that the weighting factor (KoFn _vFz ) is determined as a function of the vehicle speed. procedure according to claim 1 , 2 , and 3 characterized in that a resulting weighting factor (KoFn) is formed from the sum of the wheel load-dependent weighting factors (KoFn _x , KoFn _y ), which is multiplied by the vehicle speed-dependent weighting factor (KoFn _vFz ). Method according to at least one of the preceding claims, characterized in that a deviation of a yaw rate (evGi) is determined from a desired yaw rate (vGiSo) and an actual value (vGi) of the yaw rate, wherein a stabilizing differential torque (EMDifStab) is determined from the resulting deviation (devGi). procedure according to claim 5 characterized in that , if the differential torque (EMDifPreProp) determined on the basis of the pre-control value and the stabilizing differential torque (EMDifStab) have the same sign, the differential torque (EMDifPreProp) determined on the basis of the pre-control value and the stabilizing differential torque (EMDifStab) are added to form a driving dynamics differential drive torque (EMDif). procedure according to claim 5 characterized in that if the differential torque (EMDifPreProp) determined on the basis of the pre-control value and the stabilizing differential torque (EMDifStab) have different signs, the amount of the differential torque (EMDifPreProp) determined on the basis of the pre-control value is reduced by the amount of the stabilizing differential torque (EMDifStab). procedure according to claim 6 characterized in that if a limit value is exceeded by the driving dynamics differential drive torque (EMDif), a drive slip of the vehicle is reduced by a wheel slip control system, preferably an ABS system.

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