DE102012222197B4 - Method for splitting a drive torque between a primary axle and a secondary axle of a motor vehicle, method for splitting an axle torque between a left and a right wheel of a common axle of a motor vehicle and motor vehicle, comprising a parking assistance system with lateral guidance - Google Patents

Abstract

Method for splitting a drive torque between a primary axle and a secondary axle of a motor vehicle, which has an all-wheel drive with a torque splitting device (3) for splitting the drive torque between the permanently driven primary axle (1) and the secondary axle (2) that can be driven if necessary, and- a parking assistance system Lateral guidance, wherein during a parking maneuver the steering is carried out by the parking assistance system instead of by the driver, and optionally having longitudinal guidance, with the steps: - determining (100) a current or impending parking situation on the part of the vehicle; and- if a current or impending parking situation is determined,- reducing (110) the maximum final drive torque of the secondary axle that can be set via the torque splitting device,- reducing the final drive torque of the secondary axle via the torque splitting device, or- limiting the final drive torque of the secondary axle that can be set via the torque splitting device upwards.

Description

The invention relates to a method for splitting a drive torque between a primary axle and a secondary axle of a motor vehicle, which has an all-wheel drive with a torque splitting device for splitting the drive torque between the permanently driven primary axle and the secondary axle that can be driven if necessary. Furthermore, the invention relates to a method for splitting an axle torque between a left and a right wheel on a common axle of a motor vehicle, which has a torque splitting device for variably splitting an axle torque between the two wheels of this axle, with different wheel torques being adjustable for the left and the right wheel are. In addition, the invention relates in each case to a motor vehicle set up for carrying out the respective method.

With switchable all-wheel drives, one axle of the motor vehicle is permanently driven; this axis is called the primary axis. A second axis - referred to as a secondary axis - is switched on by the control electronics if required. For this purpose, the drive torque of the vehicle drive is divided by means of a torque distribution device between the permanently driven primary axle and the secondary axle, which can be driven if required. The torque splitting ratio between the axle torque of the primary axle and the axle torque of the secondary axle can be changed via the torque splitting device. By defining a torque split ratio, the traction and driving behavior of a vehicle can be influenced as required.

In the simplest case, the torque splitting device can be switched between only two states, with the drive torque being transmitted completely to the primary axle in the first state and the drive torque being transmitted to the primary axle and the secondary axle in the second state, for example in a ratio of 1:1; preferably, however, the torque splitting ratio can be adjusted in a continuously variable manner. A controllable four-wheel drive clutch is typically used in the torque split device, via which torque is supplied to the secondary axle when required; this is also known as clutch-controlled all-wheel drive. In the case of a clutch-controlled all-wheel drive, the all-wheel drive clutch is preferably controlled electronically. Here, a clutch torque of the all-wheel drive clutch is set, which at the same time corresponds to the maximum possible torque that can be transmitted by the clutch in the direction of the secondary axle. By varying the clutch torque, the torque ratio between the primary and secondary axles can be variably changed in four-wheel drive, for example from 1:1 to 2:1.

The prior art describes the DE 36 30 754 C2 a vehicle in which the wheels on the rear axle are constantly driven, with a clutch that causes the front axle to be switched on and off being arranged within a transfer case and, with a complex, compact design, can be easily integrated into the drive train of the motor vehicle in order to - to allow control that takes optimal account of maintenance and test situations. By switching the front axle on and off in particular, it is possible to control operating states that were previously problematic in all-wheel drive motor vehicles. It is thus possible to engage the clutch by means of a control unit only when the wheels on the rear axle are slipping. As a rule, only the rear axle of the vehicle is driven, in contrast to all-wheel drive vehicles, the steering effort required when parking is low, towing on one axle is possible with a central viscose differential, the vehicle also manages with an ABS system, which does not require all-wheel-specific development, the vehicle does not change its handling when coasting or cornering, and maintenance work, such as roller test bench measurements on the vehicle or dynamic balancing work on the wheels, is possible. If, on the other hand, the vehicle is usually driven with all-wheel drive, there is the possibility of switching off the clutch in the said overrun mode, when braking, parking, towing for maintenance work, so that only the rear axle is operated.

the U.S. 2006/0 076 828 A1 describes, however, a method to reduce the curve radius in a four-wheel drive vehicle. For example, when steering to the left, the torque distribution between the right and left front wheel can be changed so that the right front wheel has a greater drive torque than the left front wheel, while the rear wheels are also braked.

In addition, the U.S. 2012/0 041 644 A1 a device and a method for determining a steering angle of a motor vehicle, among other things for the purpose of influencing the provision of torque at the wheels of a rear axle, by controlling clutch devices, in order to prevent understeering or oversteering of the motor vehicle while cornering.

In the pamphlet DE 100 54 023 A1 clutch-controlled four-wheel drives are described, each with a front or rear axle as the primary axle. The all-wheel drive clutch is controlled as a function of driving dynamics parameters such as the steering angle, the rate of rotation or yaw rate and the lateral acceleration in order to return oversteer or understeer when cornering to neutral driving behavior.

Furthermore, in the pamphlet DE 103 33 654 A1 a clutch-controlled four-wheel drive is described, in which, depending on the driving speed, a speed-related clutch torque is specified as the maximum permissible limit torque.

In addition, from the pamphlet DE 10 2009 031 500 A1 a clutch-controlled all-wheel drive is known in which the all-wheel drive clutch is controlled in accordance with a temporarily reduced clutch torque value when a clutch engagement risk criterion is present.

Furthermore, it is from the pamphlet DE 10 2009 032 265 A1 known to set a reduced clutch torque on the all-wheel clutch for thermal protection of a four-wheel drive clutch.

In analogy to the torque distribution described above in the longitudinal direction between the two vehicle axles, systems are also known with the aid of which the torque distribution - exclusively or in addition to the longitudinal torque distribution described above - between axle-wise opposite vehicle wheels, for example those of the front axle and/or those of the rear axle, can be changed. These systems can be, for example, electronically controllable differential locks or torque vectoring systems (also referred to as active yaw systems). In contrast to a simple open differential, which divides the axle torque equally between the two wheels, the coupling of the wheel speeds can be varied with a controllable differential lock, so that the free speed equalization is impeded as the coupling increases; this also influences the distribution of the axle torque between the right and left wheels. With a torque vectoring system, the axle torque can be variably distributed between the left and right wheels of the shared drive axle. The traction and driving behavior of a vehicle can be decisively influenced by means of this variable lateral distribution.

An exemplary torque vectoring system is described in the article "The Dynamic Performance Control from BMW", ATZ - Automobiltechnische Zeitschrift, 11/2008, pages 984 to 994.

In parking assistance systems with lateral guidance, the vehicle is steered by the system during the parking process. The driver must accelerate and brake the vehicle during the parking process (longitudinal guidance). In the case of parking assistance systems with lateral and longitudinal guidance, the parking assistance system also guides the vehicle longitudinally.

The parking assistance systems described above typically have a parking space measurement system, by means of which parallel and/or perpendicular parking spaces are measured while driving past. In addition to measuring parking spaces, trajectory planning and continuous position determination of the vehicle in relation to the parking space and the planned trajectory are required. The vehicle position is determined using odometry, i. H. by observing the wheels of the vehicle. If the wheel circumference is known, the distance traveled can be calculated based on the number of revolutions of the wheels; The direction of vehicle movement can be determined based on the steering angle of the individual wheels. A non-driven axle is typically used for odometry, as here the wheel slip is significantly lower compared to a driven axle.

When the parking assistance system is activated, unwanted functional side effects or effects on the parking assistance system can be caused when all-wheel drive is active. The reason for this is that in all-wheel drive, in contrast to pure rear-wheel or front-wheel drive, the wheels of the secondary axle can no longer roll freely at any wheel speed and the wheel slip is therefore greater than in a vehicle with pure rear-wheel or front-wheel drive. The same applies to systems with variable lateral division. When setting a correspondingly large wheel torque difference in a torque vectoring system, in contrast to a simple open axle differential, the wheel speeds can no longer adjust freely, so that increased wheel slip can occur.

In the case of a torque distribution in the longitudinal direction and/or transverse direction, free rolling of individual vehicle wheels is prevented due to the coupling of the axles or wheels with one another, so that the wheel slip generally increases. This disturbs the odometry; there is a greater error when determining the distance, so that the determined position of the vehicle is provided with a greater error. The automatic measurement of the parking space when driving past, the exact determination of the trajectory for parking and the determination of the distance to obstacles (e.g. other parked vehicles, curbs, etc.) more difficult.

It is the object of the invention to reduce the above-described negative influence of a torque distribution in the longitudinal and/or transverse direction on a parking assistance system.

The object is solved by the features of the independent patent claims. Advantageous embodiments are described in the dependent claims.

A first aspect of the invention relates to a method for splitting a drive torque between a primary axle and a secondary axle of a motor vehicle. The primary axis corresponds to the rear wheel axis, for example, and the secondary axis corresponds to the front wheel axis, for example (or vice versa). The vehicle includes an all-wheel drive with a torque splitting device for splitting the drive torque between the permanently driven primary axle and the secondary axle that can be driven on demand. A parking assistance system with lateral guidance and optional longitudinal guidance is also provided.

According to the method, a current or impending parking situation is determined by the vehicle, for example a parking situation is determined by the vehicle based on the activation of the parking assistance system, which is triggered, for example, by the driver by actuating an operating element or a reverse gear. To determine a parking situation, for example, an activation bit can be evaluated, which indicates the activation state of the parking assistance system. Alternatively, an imminent parking situation can be recognized, for example, by the fact that the vehicle has reached or will shortly reach a destination entered in the navigation system or a specific location (for example at home or at work).

When a current or imminent parking situation is detected, the axle torque of the secondary axle is reduced via the torque splitting device, for example, so that the freewheeling of the wheels of the secondary axle increases. Conversely, the axle torque of the primary axle is preferably increased via the torque distribution device, so that the torque ratio between the axle torque of the primary axle and the axle torque of the secondary axle is preferably increased. For example, the axle torque of the secondary axle is reduced and, conversely, the axle torque of the secondary axle is increased in such a way that the torque ratio between the axle torque of the primary axle and the axle torque of the secondary axle is increased from, for example, 50:50 to 80:20 or 100:0.

Provision can also be made for the maximum axle torque of the secondary axle that can be set via the torque splitting device to be reduced when a current or imminent parking situation is detected. In this case, it is not absolutely necessary for the currently present axle torque of the secondary axle to be immediately reduced. For example, the adjustable axle torque is reduced by lowering the clutch torque of an all-wheel drive clutch. When the clutch torque of the all-wheel drive clutch is reduced, the maximum axle torque that can be set via the all-wheel drive clutch on the secondary axle is reduced, since the clutch torque corresponds to the maximum possible torque that can be transmitted from the all-wheel drive clutch in the direction of the secondary axle.

Alternatively, it can also be provided that when a current or impending parking situation is detected, the axle torque that can be adjusted via the torque splitting device is limited upwards (to a specific maximum value for the axle torque), without the adjustable axle torque previously having been limited.

By reducing the current or the maximum adjustable axle torque of the secondary axle according to the invention or by limiting the adjustable axle torque, the freewheeling of the wheels of the secondary axle tends to be promoted. The disruption of the odometry described above, which leads to a greater error when determining the distance covered and the position of the vehicle, is therefore counteracted.

According to a preferred embodiment, the all-wheel drive is a clutch-controlled all-wheel drive in which the primary axle is permanently driven and the secondary axle can be driven via an all-wheel drive clutch whose clutch torque can be controlled. A controllable all-wheel drive clutch is used for torque distribution. Here, a clutch torque of the all-wheel drive clutch is set, which typically corresponds to the maximum possible torque that can be transmitted by the clutch in the direction of the secondary axle with a correspondingly high engine torque. The current or the maximum adjustable axle torque of the secondary axle is reduced when a current or an imminent parking situation is detected by reducing the clutch torque. If the axle torque of the secondary axle before the clutch torque is lowered than the clutch torque after the clutch torque has been lowered, the lowering of the clutch torque not only reduces the maximum axle torque that can be set via the all-wheel drive clutch, but also the actual axle torque of the secondary axle.

It is advantageous if the clutch torque is increased again when the parking assistance system is active and instability occurs on a drive wheel or a drive axle (in this case, for example, the wheel or wheels of an axle have an impermissible drive slip compared to the roadway) in order to improve the stability or traction of the vehicle to ensure vehicle.

For example, the lowering of the clutch torque can be maintained in this case, but superimposed (i.e. additively) the four-wheel clutch slip controller continues to be effective, which in this case increases the clutch torque essentially depending on the wheel slip and temporarily. The aim of this is, for example, to ensure sufficient traction when parking on roads with unfavorable friction coefficients (e.g. ice, snow, loose ground, etc.).

It is also advantageous if, in the case described above, the parking assistance system receives information about this, so that the parking assistance system can adjust to a temporary possible falsification of the odometry (which as a result can lead to a less precise determination of distances to obstacles and a less precise trajectory determination ).

A second aspect of the invention relates to a method for splitting an axle torque between a left and a right wheel on a common axle, for example the rear axle or the front axle. In the second aspect of the invention, instead of the longitudinal distribution of a torque to two axles, as in the first aspect of the invention, it is a question of the transverse distribution of a torque to two wheels on a common axle. As in the first aspect of the invention, the vehicle includes a parking assistance system with lateral guidance and optionally longitudinal guidance.

Furthermore, a torque splitting device for splitting an axle torque between the two wheels of this axle is provided.

Either different wheel torques for the left and the right wheel can be set via the torque distribution device, or the coupling of the wheel speeds for the left and the right wheel can be varied.

The torque distribution device is, for example, what is known as a torque vectoring system in order to set different wheel torques for the two wheels. The torque vectoring system used includes, for example, a rear axle drive with a basic drive, a left superimposed unit and a right superimposed unit. In driving situations without torque transfer, the system behaves like a rear axle drive with an open differential. The two wheels of the rear wheel axle can be driven with different high torques by the two superimposition units. Such a torque vectoring system is described in the article "The Dynamic Performance Control from BMW", ATZ - Automobiltechnische Zeitschrift, 11/2008, pages 984 to 994; the content of this article is hereby incorporated by reference into the disclosure content of the application.

Alternatively, it can also be a controllable differential lock, via which the coupling of the wheel speeds can be varied.

According to the invention, a current or impending parking situation is determined by the vehicle, as has already been described in connection with the first aspect of the invention. When a current or imminent parking situation is detected, the difference between the wheel torque of the left wheel and the wheel torque of the right wheel is reduced via the torque splitting device. Alternatively, it can also be provided that the maximum difference between the wheel torques that can be set via the torque distribution device is reduced. Alternatively, the difference between the wheel torques that can be set via the torque splitting device can also have an upper limit. Alternatively, the free speed equalization between precisely these wheels can be made possible or at least improved by less coupling of the wheels of one axle to one another.

By reducing the current or maximum adjustable difference between the wheel torques according to the invention, the freewheeling of the wheels of the secondary axle tends to increase and the slip of the wheels of the secondary axle tends to decrease; the behavior of the torque splitting device is changed in the direction of the open axle differential. The disruption of the odometry described above, which leads to a greater error when determining the distance covered and the position of the vehicle, is therefore counteracted.

It can also be provided that when a current or imminent parking situation is detected, both the measures of the method according to the invention for torque distribution in the longitudinal direction according to the first aspect of the invention and the measures of the method according to the invention for torque distribution in the transverse direction are carried out.

A third aspect of the invention is aimed at a motor vehicle which includes a parking assistance system with lateral guidance and optionally longitudinal guidance and an all-wheel drive. The all-wheel drive includes a torque splitting device (e.g. an all-wheel drive clutch) for splitting the drive torque between a permanently driven primary axle and a secondary axle that can be driven when required. The motor vehicle is set up to determine a current or imminent parking situation. When a current or impending parking situation is detected, the maximum axle torque of the secondary axle that can be set via the torque splitting device is reduced, the axle torque of the secondary axle is reduced via the torque splitting device, or the axle torque of the secondary axle that can be set via the torque splitting device is capped.

The above statements on the method according to the invention according to the first aspect of the invention also apply in a corresponding manner to the motor vehicle according to the third aspect of the invention. Advantageous exemplary embodiments of the vehicle according to the invention according to the third aspect of the invention that are not explicitly described here correspond to the described advantageous exemplary embodiments of the method according to the invention according to the first aspect of the invention.

A fourth aspect of the invention is aimed at a motor vehicle which includes a parking assistance system with lateral guidance and optionally longitudinal guidance. Furthermore, there is a torque splitting device for splitting an axle torque between a left and a right wheel on a common axle (for example the rear axle). For example, a torque vectoring system is used in which the axle torque can be variably divided between the two wheels of the axle, with different wheel torques being adjustable for the left and right wheels. Alternatively, a controllable differential lock is used, in which the coupling of the wheel speeds can be varied. The motor vehicle is set up to determine a current or imminent parking situation. When a current or imminent parking situation is detected, the difference between the wheel torque of the left wheel and the wheel torque of the right wheel is reduced via the torque splitting device. Alternatively, it can be provided that when a current or impending parking situation is detected, the maximum difference between the wheel torques that can be set via the torque splitting device is reduced in this case. Alternatively, it can be provided that the difference between the wheel torques that can be set via the torque splitting device is limited when a current or imminent parking situation is detected. Alternatively, for example in the case of a controllable differential lock, the coupling of the wheel speeds for the left and the right wheel of the common axle can be reduced.

The above statements on the method according to the second aspect of the invention also apply in a corresponding manner to the motor vehicle according to the fourth aspect of the invention. Advantageous exemplary embodiments of the vehicle according to the invention according to the fourth aspect of the invention that are not explicitly described here correspond to the described advantageous exemplary embodiments of the method according to the invention according to the second aspect of the invention.

Reducing the current or maximum adjustable axle torque, limiting the adjustable axle torque or reducing the current or maximum adjustable difference between the wheel torques or limiting the adjustable difference between the wheel torques, as described in the four aspects of the invention, depends on the determination of a current or upcoming Parking situation from what can be detected, for example, based on the activation state of the parking assistance system.

• - der Aktivierungszustand (aktiv/passiv) von Regelsystemen des Kraftfahrzeugs (z.B. Antriebsschlupfregelung und/oder stabilisierender Fahrzeugregler und/oder Motorschleppmomentregler und/oder Verzögerungsregler und/oder weitere Fahrwerks- und Antriebsregelsysteme);
• - der Aktivierungszustand von Schutzfunktionen (z. B. Bauteilschutzfunktionen) oder Abgleich-/Lernfunktionen wie z. B. Reifentoleranzabgleich;
• - der Aktivierungszustand von Fahrerwunscheinstellungen (z. B. über Betätigungselemente wie Fahrerlebnisschalter und/oder Sportmodus-Taster);
• - die Geschwindigkeit und/oder ein Geschwindigkeitsgradient des Fahrzeugs und/oder einer Komponente eines Antriebsstrangs; hierbei können insbesondere folgende Geschwindigkeiten berücksichtigt werden: Fahrzeuggeschwindigkeit, Radgeschwindigkeiten, Achsgeschwindigkeiten, Getriebegeschwindigkeit, Differenzialgeschwindigkeit, Kardangeschwindigkeit, Antriebswellengeschwindigkeit, Abtriebswellengeschwindigkeit sowie jeweils die Beschleunigungen dieser Bauteile;
• - das Antriebsmoment und/oder der Antriebsmomentgradient des Kraftfahrzeugs;
• - die Motordrehzahl und/oder der Motordrehzahlgradient;
• - der Lenkwinkel und/oder der Lenkwinkelgradient;
• - die Fahrzeugquerbeschleunigung;
• - die Fahrzeuglängsbeschleunigung;
• - die Fahrzeugdrehrate (Gierrate);
• - der Schwimmwinkel und/oder der Schwimmwinkelgradient;
• - die Erkennung oder Prädiktion einer potentiellen Fahrzeuginstabilität;
• - die Fahrpedalvorgabe, das Fahrervorgabewunschmoment und/oder der Fahrervorgabewunschmomentgradient;
• - die Kraftschlussbedingung oder Reibwertbedingung zwischen Reifen und Fahrbahn aller Räder gemeinsam und/oder einzelner Räder und/oder einzelner Achsen;
• - die Fahrtrichtung und/oder Gangstufe und/oder wirksame Gangübersetzung und/oder Antriebsstranggesamtübersetzung bzw. Antriebstrangverstärkung;
• - die Fahrbahnlängs- und/oder -querneigung; dabei wird vorteilhafterweise die Fahrbahnlängs- und/oder Querneigung über Neigungs- und/oder Längs- und/oder Querbeschleunigungssensoren gemessen;
• - die Masse des Fahrzeugs und/oder die Achslasten bzw. Achsaufstandskräfte und/oder Radlasten bzw. Radaufstandskräfte; dabei wird insbesondere die Fahrzeugbeladung sowie ggf. ein Zustand mit oder ohne Anhänger berücksichtigt;
• - Fahrzeug mit erkanntem Anhänger, z. B. über Sensierung mittels Anhängersteckerbelegung
• - das Maß eines wirksamen Sperrgrades einer Drehmomentverteilungsvorrichtung; oder
• - die Temperatur der Umgebung und/oder der Reifen und/oder der Fahrbahn und/oder einer Komponente des Antriebsstrangs und/oder eines anderen Bauteils des Kraftfahrzeugs.

These measures can optionally also depend on one or more of the following status parameters of the motor vehicle:

• - The activation state (active/passive) of control systems of the motor vehicle (e.g. traction control and/or stabilizing vehicle controller and/or engine drag torque controller and/or deceleration controller and/or other chassis and drive control systems);
• - the activation status of protective functions (e.g. component protection functions) or calibration/learning functions such as e.g. B. Tire tolerance comparison;
• - the activation status of driver settings (e.g. via actuating elements such as driving experience switches and/or sport mode buttons);
• - the speed and/or a speed gradient of the vehicle and/or a component of a drive train; the following speeds in particular can be taken into account here: vehicle speed, wheel speeds, axle speeds, transmission speed, differential speed, cardan speed, drive shaft speed, output shaft speed and the accelerations of these components;
• - The drive torque and/or the drive torque gradient of the motor vehicle;
• - engine speed and/or engine speed gradient;
• - the steering angle and/or the steering angle gradient;
• - the vehicle lateral acceleration;
• - the vehicle longitudinal acceleration;
• - the vehicle rotation rate (yaw rate);
• - the slip angle and/or the slip angle gradient;
• - the detection or prediction of a potential vehicle instability;
• - The accelerator pedal input, the driver input desired torque and/or the driver input desired torque gradient;
• - the adhesion condition or friction coefficient condition between tires and road surface of all wheels together and/or individual wheels and/or individual axles;
• - the direction of travel and/or gear ratio and/or effective gear ratio and/or overall drive train ratio or drive train reinforcement;
• - the longitudinal and/or transverse gradient of the roadway; in this case, the longitudinal and/or transverse inclination of the roadway is advantageously measured via inclination and/or longitudinal and/or transverse acceleration sensors;
• - the mass of the vehicle and/or the axle loads or axle contact forces and/or wheel loads or wheel contact forces; in particular, the vehicle load and, if applicable, a condition with or without a trailer are taken into account;
• - Vehicle with a recognized trailer, e.g. B. via sensing by means of trailer plug assignment
• - the measure of an effective degree of locking of a torque distribution device; or
• - the temperature of the environment and/or the tires and/or the road surface and/or a component of the drive train and/or another component of the motor vehicle.

The value to which the current or maximum adjustable axle torque is reduced or the adjustable axle torque is capped or to which the current or maximum adjustable difference between the wheel torques is reduced or the difference is capped may be one or more of the foregoing be dependent on state parameters. Furthermore, the size of the change when reducing the current or maximum adjustable axle torque or the current or maximum adjustable difference can depend on one or more of the above state parameters. Furthermore, the course of the reduction over time (for example the slope) can also be dependent on one or more of the above state parameters. One or more of the status parameters described above can also be used to find the optimum point in time for activating the measures described above.

After completing or aborting a parking process, a change in the torque split carried out as part of the method according to the invention, e.g. B. path-controlled or time-controlled, can be reversed in a suitable manner and the torque distribution can be adjusted back to the target state.

For example, the method is implemented as software in a suitable control or regulation unit for longitudinal or transverse torque distribution and/or in any other vehicle control system (e.g. DSC—Dynamic Stability Control or ICM—Integrated Chassis Management). When a current or impending parking situation is detected, the above-described weakness of the odometry can be eliminated or at least reduced by an at least temporary, partial or complete change in the moment distribution in the sense of a pre-control.

• 1 schematisch ein Kraftfahrzeug mit kupplungsgesteuertem Allradantrieb, wobei die Hinterachse als Primärachse permanent angetrieben wird und die Vorderachse als Sekundärachse mittels der steuerbaren Allradkupplung antreibbar ist;
• 2 ein Ausführungsbeispiel eines Verfahrens zur Absenkung des Kupplungsmoments der Allradkupplung; und
• 3 einen beispielhaften Zeitverlauf des Kupplungsmoments der Allradkupplung.

The invention is described below with the aid of the accompanying drawings using an exemplary embodiment. In these show:

• 1 schematically shows a motor vehicle with clutch-controlled all-wheel drive, the rear axle being permanently driven as the primary axle and the front axle being able to be driven as a secondary axle by means of the controllable all-wheel drive;
• 2 an embodiment of a method for reducing the clutch torque of the all-wheel drive clutch; and
• 3 an exemplary time curve of the clutch torque of the all-wheel drive clutch.

1 shows a motor vehicle with clutch-controlled all-wheel drive, the rear axle 1 with the wheels 11, 12 being permanently driven as the primary axle and the front axle 2 with the wheels 13, 14 being able to be driven as the secondary axle by means of a controllable all-wheel drive clutch 3. The vehicle is preferably a vehicle with a front engine drive, with a cardan shaft being provided between the automatic transmission 5 and the final drive 15 of the rear axle 1 . The method described below can also be applied to a vehicle with the rear axle as the secondary axle and the front axle as the primary axle.

The drive comprises a motor 4 and the automatic transmission 5 connected to the motor 4. The controllable all-wheel drive clutch 3 is located on the transmission output side, here in the form of a multi-plate clutch.

With the all-wheel drive clutch, the clutch input is connected through in the direction of rear axle 1, so that rear axle 1 is permanently driven. Between the all-wheel drive clutch 3 and the rear axle 1 there is a cardan shaft and an axle drive 15. The front axle 2 as a secondary axle is only driven when the clutch 3 is engaged. For this purpose, in the case of a multi-plate clutch, the plates 7 toothed on the outside of the basket 6 and the plates 8 toothed on the inside of the hub are pressed together. The clutch basket 6 and the clutch hub are connected to one another by the friction. The clutch basket 6 is connected to the secondary-side output of the clutch 3 so that when the clutch 3 is closed, part of the torque on the transmission output side is transmitted via the axle drive 16 to the wheels 13 , 14 of the front axle 2 .

To close the clutch 3, a specific value for the clutch torque M _K of the all-wheel drive clutch 3 is set, which corresponds to the maximum possible torque which can be transmitted by the clutch 3 in the direction of the front axle, ie which can be set at most via the all-wheel drive clutch 3. The calculation of the clutch torque M _K to be set is taken over by a control unit 9, for example by a DSC control unit 9 for vehicle stabilization. The clutch torque M _K to be set is converted into an electric current for controlling the adjustment unit of the all-wheel clutch 3 by an external additional control unit 17 (for example, attached directly to the all-wheel clutch 3 ). Alternatively, this conversion could also take place directly in the control device 9 . An exemplary controller structure for calculating the clutch torque M _K to be set is described in the article "xDrive - The new all-wheel drive in the BMW X3 and BMW X5", ATZ - Automobiltechnische Zeitung, 2/2004, pages 92 to 102.

The motor vehicle includes a semi-automatic parking assistance system with lateral guidance and optionally also longitudinal guidance.

The control unit 9 provides for a reduction in the clutch torque M _K if activation of the parking assistance system of the motor vehicle is detected, as is the case in connection with 2 and 3 is explained.

In step 100 of the in 2 The procedure shown activates the parking assistance system. The activation of the parking assistance system is detected, for example, using an activation signal of the parking assistance system, which changes from 0 to 1 when the parking assistance system is activated and which changes from 1 to 0 when the parking assistance system is deactivated. The activation signal is present, for example, on a vehicle bus (for example CAN bus or FlexRay bus) which the DSC control unit 9 can access. An exemplary time profile of the activation signal and the requested clutch torque M _K is in 3 shown.

At time t _A the activation signal of the parking assistance system changes from 0 to 1 (see Fig. 3 ). Based on the switching of the activation signal from 0 to 1, it is determined that the parking assistance system has been activated and a parking situation is present (see step 100 in 2 ). In addition to the activation of the parking assistance function, the presence of one or more additional boundary conditions (status parameters) can optionally be requested in order to trigger a reduction in the clutch torque M _K when the parking assistance function is activated, for example a very low lateral acceleration (for example a lateral acceleration less than or less than or equal to a low threshold value ), a low longitudinal gradient (for example a longitudinal gradient of the roadway is less than or less than a low threshold value), a low vehicle speed (for example in the range between 0 and 40 km/h), no sensing or prediction of vehicle instability.

When the activation of the parking assistance system is detected and the one or more optional boundary conditions are met, a clutch torque reduction function is activated based on the current desired clutch torque M _K at point A, at which the activation signal changes from 0 to 1 fourth, via which the clutch torque M _K is lowered to a defined minimum clutch torque M _K,red at point B (see step 110 in 2 ). The lowering can take place, for example, as a linear ramp. In the extreme case, the minimum clutch torque M _K,red can also be zero, ie the transmission of torque to the secondary axle 2 can be completely interrupted. At point C, the activation signal changes from 1 to 0. This detects a deactivation of the parking assistance system (see step 120 in 2 ), so that the reduction in clutch torque is at least partially reversed (see step 130 in 3 ) and the clutch torque reduction function is terminated. In this case, the clutch torque M _K is, for example, brought up to a clutch setpoint torque M _K at the operating point D, which is further calculated in the background.

The curve of the reduction in clutch torque M _K between points A and B (e.g. the increase in clutch torque) and/or the level of reduced clutch torque M _K,red between points B and C and/or the curve of the increase in torque between operating points C and D can be dependent on one or more other state parameters (e.g. accelerator pedal specification, drive torque, road gradient, steering angle) and can therefore be fundamentally variable.

The reduced clutch torque can be temporarily or permanently increased again in the event of vehicle instability.

For this purpose, for example, the active clutch torque reduction function can be temporarily interrupted or completely aborted under one or more conditions which indicate instability and result, for example, from one or more state parameters mentioned above. For example, it is possible that when a critical slip situation is detected between the driven wheels of one axle or between the wheels of both axles of a vehicle, the reduced torque level M _K,red is exited in order to ensure sufficient driving stability and/or traction. Starting from the reduced clutch torque M _K,red , the clutch torque can be adjusted according to a defined function to a required calculated desired clutch torque M _K , which can be either higher or lower than M _K,red depending on the situation.

In order to avoid possible toggling of the function after the end or termination of the clutch torque reduction function, a hysteresis can advantageously be provided for the entry and exit conditions.

It is also conceivable that when the clutch torque reduction function ends or is aborted, a defined run-on time delays a subsequent re-entry into the clutch torque reduction function in order to also prevent the clutch torque reduction function from switching back and forth unintentionally.

To increase the clutch torque, it can alternatively be provided that if instability occurs on a drive wheel or a drive axle (wheel or axle has impermissible drive slip relative to the road), the reduction in the clutch torque (i.e. the clutch torque reduction function) is maintained, but overlaid (i.e. additively). ) the four-wheel clutch slip controller can also continue to be effective, which in this case increases the clutch torque essentially as a function of the slip and temporarily.

If the clutch torque is increased again despite the parking assistance function still being active, the parking assistance system can be informed of this, so that the parking assistance system can adjust to a possible temporary falsification of the wheel speeds.

The functional quality of the parking assistance system is increased by the above-described change in the torque transmission ratio by means of the described clutch torque reduction function.

Claims (11)

Method for dividing a drive torque between a primary axle and a secondary axle of a motor vehicle, which has - an all-wheel drive with a torque dividing device (3) for dividing the drive torque between the permanently driven primary axle (1) and the secondary axle (2) that can be driven if necessary, and - a parking assistance system with Lateral guidance, wherein during a parking maneuver the steering is carried out by the parking assistance system instead of by the driver, and optionally having longitudinal guidance, with the steps: - determining (100) a current or impending parking situation on the part of the vehicle; and - if a current or impending parking situation is detected, - reducing (110) the maximum final drive torque of the secondary axle that can be set via the torque splitting device, - reducing the final drive torque of the secondary axle via the torque splitting device, or - limiting the torque split via the torque splitting device device adjustable final drive torque of the secondary axle upwards. procedure after claim 1 , wherein - the all-wheel drive is a clutch-controlled all-wheel drive, in which the primary axle (1) is permanently driven and the secondary axle (2) can be driven via an all-wheel drive clutch (3) whose clutch torque can be controlled, - the torque distribution device comprises the all-wheel drive clutch (3), and - The final drive torque of the secondary axle (2) is reduced or the maximum adjustable final drive torque of the secondary axle (2) is reduced by the clutch torque (M _K ) of the all-wheel drive clutch being lowered. procedure after claim 2 , the clutch torque (M _K ) being increased again in the event of vehicle instability occurring. Method according to one of the preceding claims, wherein the final drive torque of the secondary axle (2) is reduced to a value or the maximum adjustable final drive torque of the secondary axle (2) is reduced to a value (M _K,red ) or the adjustable final drive torque of the secondary axle (2) is limited to an upper value, and the course of the reduction over time or the value (M _K,red ) depends on one or more parameters, in particular on an accelerator pedal specification, the driver specification torque, the drive torque, a longitudinal gradient of the roadway, a lateral acceleration and/or the steering angle. Method for torque distribution of an axle torque to a left and a right wheel of a common axle of a motor vehicle, which - A second torque splitting device for splitting an axle torque between the two wheels of this axle and - A parking assistance system with lateral guidance, wherein during a parking maneuver the steering is carried out by the parking assistance system instead of by the driver, and optionally has longitudinal guidance, with the steps: - Determination of a current or imminent parking situation on the part of the vehicle; and - when determining a current or impending parking situation, - reducing the difference between the wheel torque of the left wheel and the wheel torque of the right wheel via the second torque split device, - Reducing the maximum difference between the wheel torques that can be set via the second torque splitting device, - Limiting the difference between the wheel torques that can be set via the second torque splitting device upwards, or - Decreasing the coupling of the wheel speeds for the left and the right wheel of the common axle. procedure after claim 5 , wherein the difference between the wheel torques or the adjustable difference between the wheel torques or the difference in wheel speeds is reduced to a value or the maximum adjustable difference between the wheel torques is limited to an upper value, and the time course of the reduction or the value of is dependent on one or more parameters, in particular on an accelerator pedal specification, the driver specification torque, the drive torque, a lateral acceleration and/or the steering angle. Procedure according to one of Claims 5 - 6 , whereby the following steps are carried out when a current or impending parking situation is detected: - reducing the maximum final drive torque of the secondary axle (2) that can be set via the torque splitting device, reducing the final drive torque of the secondary axle (2) via the torque splitting device or limiting the final drive torque of the secondary axle that can be set via the torque splitting device up; and - reducing the difference between the wheel torque of the left wheel and the wheel torque of the right wheel via the second torque splitting device, reducing the maximum adjustable via the second torque splitting device difference between the wheel torques, limiting the adjustable via the second torque splitting device difference between the wheel torques upwards or reducing the coupling of the wheel speeds for the left and the right wheel of the common axle. Method according to any of the preceding Claims 5 - 7 , The parking situation is determined based on the activation of the parking assistance system, in particular based on an activation signal which signals the activation of the parking assistance system. A method according to any of the foregoing Claims 5 - 8th , wherein the reduction and/or limitation only takes place if one or more additional conditions are present, in particular only if one or more of the following additional conditions are present: a lateral acceleration is less than or less than a threshold value, a longitudinal gradient of the roadway is less or less equal to a threshold, a vehicle speed less than or equal to a threshold, no vehicle instability. Motor vehicle, comprising a parking assistance system with lateral guidance, wherein during a parking maneuver the steering is carried out by the parking assistance system instead of by the driver, and optionally longitudinal guidance and an all-wheel drive, which has a torque splitting device (3) for splitting the drive torque between a permanently driven primary axle (1) and a contains a secondary axle (2) that can be driven as required, the motor vehicle being set up - determine a current or upcoming parking situation; and - when determining a current or impending parking situation - to reduce the maximum adjustable final drive torque of the secondary axle (2) via the torque splitting device (3), - to reduce the final drive torque of the secondary axle (2) via the torque splitting device (3) or - Limiting the upper limit of the final drive torque of the secondary axle (2) that can be set via the torque splitting device (3). Motor vehicle, comprising a parking assistance system with lateral guidance and optionally longitudinal guidance and a torque splitting device for splitting an axle torque between a left and a right wheel of a common axle, the motor vehicle being set up - determine a current or upcoming parking situation; and - when determining a current or impending parking situation - reduce the difference between the wheel torque of the left wheel and the wheel torque of the right wheel via the torque split device, - to reduce the maximum difference between the wheel torques that can be set via the torque splitting device, - to limit the difference between the wheel torques that can be adjusted via the torque splitting device, or - to reduce the coupling of the wheel speeds for the left and the right wheel of the common axle.

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