Method for operating a motor vehicle drive train

Abstract

Method for operating a motor vehicle drive train (1) comprising at least one electric machine (3) which can be coupled within the motor vehicle drive train (1) via a transmission (4) to drive wheels (5, 6) of a motor vehicle, wherein, as part of a check, it is queried as a criterion whether the motor vehicle is moving in the opposite direction to a predetermined direction of travel, and wherein, depending on a result of the check, a measure is initiated in which a torque is applied by the at least one electric machine (3) to brake the motor vehicle in the opposite movement, characterized in that, as part of the check, it is also checked as a further criterion whether the transmission (4) is currently shifted to neutral, that the measure is only initiated if simultaneous fulfillment of the criterion and the further criterion is detected,and that a gear in the gearbox (4) is switched in advance for the implementation of the measure.,

Description

The invention relates to a method for operating a motor vehicle drive train, which comprises at least one electric machine, which can be coupled within the motor vehicle drive train via a transmission to drive wheels of a motor vehicle, wherein, as part of a check, a check is performed as a criterion as to whether a movement opposite to a predefined direction of travel is carried out by the motor vehicle, and wherein, as a function of a result of the check, a measure is initiated in which a torque braking the motor vehicle in the opposite movement is applied by the at least one electric machine. The invention furthermore relates to a control device, a computer program product, a data carrier and a motor vehicle drive train.In motor vehicles, it is customary to keep the respective motor vehicle in the respective current position during stopping by actuating a service brake. In a motor vehicle in whose motor vehicle drive train a hydrodynamic torque converter is provided as a starting element, this is necessary in order to prevent a forward or rearward creep of the motor vehicle depending on the selected direction of travel. If, on the other hand, the respective motor vehicle has a friction clutch as a starting element in its motor vehicle drive train or is switched to neutral in a transmission of the motor vehicle during stopping, the service brake must be actuated by the vehicle driver during stopping on an inclined surface in order to prevent rolling of the motor vehicle. In motor vehicle drive trains of at least partially electrically driven motor vehicles, however, a targeted operation of an electric machine is also carried out in part in order to prevent a movement of the respective motor vehicle counter to a predefined direction of travel.DE 10 2011 101 997 A1 discloses a method for operating a motor vehicle drive train, wherein in this method a brake device is operated in a targeted manner in order to prevent a movement of the motor vehicle counter to a predefined direction of travel. The brake device can be an electric machine in the motor vehicle drive train.In addition, DE 10 2012 223 866 A1 discloses a method for operating a motor vehicle drive train, wherein, within the scope of this method, a movement of a motor vehicle having the motor vehicle drive train counter to a predefined direction of travel is counteracted. For this purpose, an electric machine is regulated in the motor vehicle drive train in such a way that a torque is applied via the electric machine during an electric motor operation, which torque decelerates the motor vehicle in its movement counter to the predefined direction of travel. In the context of the regulation, a rotational speed of the electric machine is regulated to zero, so that the torque applied by the electric machine ultimately corresponds in terms of magnitude to a torque at drive wheels of the motor vehicle, which is caused by the active slope output force. A transmission is also provided in the motor vehicle drive train, wherein an electric parking brake can additionally be triggered in the transmission in order to assist the electric machine, in particular in the event of a drop in the performance of the electric machine during the braking or holding of the motor vehicle.Based on the above-described prior art, it is now the object of the present invention to realize an operation of a motor vehicle drive train in a motor vehicle, which comprises an electric machine and a transmission, wherein a vehicle driver is to be at least assisted by means of the operation according to the invention in order to prevent a movement of the motor vehicle counter to a selected direction of travel in a neutral-shifted state of the transmission.This object is achieved from a process engineering point of view starting from the preamble of claim 1 in conjunction with its characterizing features. A technical solution of the object is achieved according to the embodiment according to independent claim 6, which relates to a control device. The dependent claims which follow each of these represent advantageous further developments of the invention. Furthermore, claim 8 relates to a computer program product and claim 9 relates to a data carrier. Finally, claim 10 also relates to a motor vehicle drive train.According to Claim 1, in a method for operating a motor vehicle drive train, a check is carried out as a criterion as to whether a movement opposite to a predefined direction of travel is carried out by a motor vehicle having the motor vehicle drive train. The motor vehicle drive train comprises at least one electric machine which can be coupled within the motor vehicle drive train via a transmission to drive wheels of the motor vehicle. Depending on a result of the check, a measure is initiated in which a torque braking the motor vehicle in the opposite movement is applied by the at least one electric machine.The motor vehicle drive train operated within the scope of the method according to the invention therefore has at least one electric machine which can be coupled within the motor vehicle drive train via a transmission in a drive-effective manner to drive wheels of the associated motor vehicle. In the direction of the force flow, the transmission is thus arranged between the at least one electric machine and the drive wheels, so that the at least one electric machine is only coupled to the drive wheels when this coupling is produced via the transmission. The motor vehicle is preferably an at least partially electrically driven motor vehicle, such as a hybrid or electric vehicle. In this case, the motor vehicle drive train can be assigned, in particular, to an electrically drivable drive axle of the motor vehicle.The motor vehicle powertrain may also include a plurality of electric machines that may be coupled within the motor vehicle powertrain to the drive wheels of the motor vehicle via the transmission. Thus, the motor vehicle drive train can comprise, for example, a first electric machine and at least one second electric machine, wherein both electric machines are operatively connected to the transmission and can be coupled via the transmission in a drive-effective manner to the drive wheels of the associated motor vehicle. For example, the motor vehicle drive train can be designed as an electric central drive for a drive axle of the vehicle or as an electric final drive.The method according to the invention can be used both in a motor vehicle drive train with one driven vehicle axle and in a motor vehicle drive train with a plurality of driven vehicle axles. Thus, the motor vehicle drive train can be designed, for example, in the drive type 8 x 4, the drive type 6 x 4, the drive type 4 x 4, the drive type 6 x 2 or the drive type 4 x 2. Each of the driven axles of the motor vehicle drive train can be coupled to at least one electric machine via a transmission. If the motor vehicle drive train has a plurality of driven vehicle axles, the method according to the invention can be carried out for all driven vehicle axles.The transmission of the motor vehicle drive train can couple the at least one electric machine to the drive wheels of the motor vehicle, for which purpose a gear is to be shifted in the transmission, with which a transmission ratio is assigned. Thus, by shifting the gear, an overall transmission ratio effective between the at least one electric machine and the drive wheels is influenced. Apart from the shifting of a gear in the transmission, the transmission can also be shifted into neutral, which is to be understood in the sense of the invention to mean that in the transmission a transmission input coupled to the at least one electric machine is decoupled from a transmission output coupled to the drive wheels, whereby then the at least one electric machine is also decoupled from the drive wheels and is thus not connected to the drive wheels in a drive-effective manner. In addition to neutral, different gears with mutually different transmission ratios can preferably be shifted in the transmission. Shifts in the transmission are carried out in particular automatically, wherein the transmission is preferably designed as an automated transmission. However, the transmission can also equally well be present as an automatic transmission or as an automated continuously variable transmission, in which continuously different gears with different transmission ratios can be realized.In the method according to the invention, as part of a check, a criterion is queried as to whether a motor vehicle which has the motor vehicle drive train performs a movement which is opposite to a predefined direction of travel. With the aid of the criterion, it is thus determined whether the motor vehicle is currently moving in a direction which is opposite to a direction of travel in which the motor vehicle is actually intended to move according to the specification. This is preferably a specification of a vehicle driver of the motor vehicle.In order to fulfil the aforementioned criterion, a movement must take place in the opposite direction to a predetermined direction of travel, so that a standstill of the motor vehicle or a movement in the predetermined direction of travel does not mean fulfilment of the criterion. In this case, for the interrogation of the criterion, in particular a current direction of movement of the motor vehicle is determined, for example on the basis of rotational speed sensors at vehicle wheels of the motor vehicle or by means of a rotational speed sensor of a component permanently coupled to the vehicle wheels, such as, for example, a transmission output shaft of the transmission. Preferably, a specification of the direction of travel can also be made in this case when the motor vehicle is already moving in the opposite direction.Within the scope of the method according to the invention, a measure can be initiated, wherein this is dependent on a result of the check, within the scope of which the presence of the aforementioned criterion is queried. When performing the measure, a torque is generated by the at least one electric machine of the motor vehicle drive train, by means of which torque the motor vehicle is braked in its movement directed opposite to the predefined direction of travel. In a manner known in principle to the skilled person, the at least one electric machine is operated for this purpose as a generator and the braking torque is generated by a recuperation torque of the at least one electric machine.The invention now comprises the technical teaching that, in the context of the check, it is additionally checked as a further criterion whether the transmission is currently shifted into neutral. Furthermore, the measure is only initiated if a simultaneous fulfilment of the criterion and of the further criterion is detected, wherein a gear in the transmission is shifted in advance for carrying out the measure.In other words, in the context of the check, in addition to the criterion as to whether a movement opposite to the predefined direction of travel is carried out by the motor vehicle, it is also checked as a further criterion as to whether a neutral shift is currently present in the transmission, that is to say that the at least one electric machine is decoupled from the drive wheels via the transmission at the moment of the check. If both criteria are fulfilled simultaneously, i.e. a movement of the motor vehicle takes place counter to the predefined direction of travel and if the transmission is currently shifted into neutral, the measure is only then initiated. In this case, a gear is shifted in the transmission before the performance of the measure in order to couple the at least one electric machine to the drive wheels.Such an operation of a motor vehicle drive train has the advantage that a vehicle driver can thus be assisted at least in preventing the motor vehicle from rolling counter to the selected direction of travel after the transmission has been previously shifted into neutral. This is because, when the transmission is shifted to neutral, for example on the basis of a corresponding specification by the vehicle driver, it may occur when the motor vehicle is in the current position on an inclined surface that the vehicle begins to roll after a service brake has been released. If the vehicle driver then wishes to prevent this by presetting a direction of travel opposite to the movement, he is now supported within the scope of the method according to the invention in that, after presetting the direction of travel, a gear in the transmission is shifted and, subsequently, a torque is generated by the at least one electric machine, by means of which the vehicle is braked during its movement. This torque can be transmitted to the drive wheels of the motor vehicle, since the at least one electric machine is now coupled to the drive wheels by shifting the gear in the transmission.However, this is only done in the method according to the invention if, as part of the check, a simultaneous realization of the two criteria is determined, according to which a current movement of the vehicle takes place counter to a predefined direction of travel and the transmission is currently shifted into neutral. Thus, a vehicle driver only needs to react to a rolling of the vehicle, which is due to a stop of the vehicle in an upward gradient or in a downward gradient, for example, by specifying a direction of travel opposite to the rolling. As a result, a gear in the transmission is then initially shifted within the motor vehicle drive train and braking of the vehicle is subsequently initiated via the at least one electric machine.Within the scope of the check, the two criteria can be successively queried, wherein in this case, first the criterion of a movement of the motor vehicle opposite to a predefined direction of travel and then the further criterion of a current neutral shift in the transmission or vice versa can be checked. Alternatively, a parallel interrogation of the two criteria can also be realized within the scope of the check.According to one embodiment of the invention, a braking torque is determined during the course of the measure, which is required for braking the opposite movement of the motor vehicle to a standstill. In this case, the torque which is to be applied by the at least one electric machine for generating the braking torque is determined by including a transmission ratio of the gear shifted in the transmission. In this case, a torque is therefore set by the at least one electric machine, which torque is sufficient to brake the motor vehicle to a standstill during its opposite movement. This is advantageously determined by including a transmission ratio which is defined by the gear that is shifted in the transmission before the performance of the measure. Particularly preferably, further transmission ratios effective in the motor vehicle drive train are then also included, for example a transmission ratio of a differential transmission.According to one possible embodiment of the invention, a starting gear in the transmission is shifted as gear for carrying out the measure in advance. This has the advantage that in a starting gear a transmission ratio is usually implemented, in which a braking of the motor vehicle can be brought about particularly effectively on account of a high transmission ratio. In addition, this gear can then also be used directly for starting the motor vehicle following the braking.In a further development of the invention, the predefined direction of travel is determined within the scope of the check by inquiring a predefined position of a vehicle driver at a selection device. As a result, a desire of the vehicle driver to realize a direction of travel can be detected in a reliable manner.In a further embodiment of the invention, the execution of the measure is ended when the motor vehicle has stopped, wherein, when the measure is ended and in the gear engaged in the transmission, a torque accelerating the motor vehicle in the direction of the predefined direction of travel is applied by the at least one electric machine in accordance with a predefined value of a vehicle driver. This has the advantage that, after termination of the opposite movement of the motor vehicle, a journey in the desired direction of travel is thus realized directly.The invention also relates to a control unit which can be a transmission control unit of the transmission or another control unit of the motor vehicle drive train. This control unit is configured to query as a criterion as to whether a motor vehicle having the motor vehicle drive train is executing a movement opposite to a predefined direction of travel as well as to check as a further criterion whether the transmission, via which at least one electric machine can be coupled within the motor vehicle drive train to drive wheels of the motor vehicle, is currently shifted into neutral. Furthermore, the control device is designed to initiate a measure in which the at least one electric machine applies a torque braking the motor vehicle in the opposite movement only when the criterion and the further criterion are fulfilled simultaneously. In addition, the control device is configured to cause a gear in the transmission to be shifted in advance for carrying out the measure. Particularly preferably, the control device is furthermore also configured to implement one or more of the above-described variants of a method.The method according to the invention can also be embodied as a computer program product which, when it runs on a processor, for example a processor of an aforementioned control unit, instructs the processor in software to carry out the assigned method steps according to the invention. In this context, the subject matter of the invention also includes a computer-readable medium on which a computer program product described above is stored in a retrievable manner.The invention further relates to a motor vehicle drive train which can be operated according to a method according to one or more of the variants described above. The motor vehicle drive train is preferably a drive train of an at least partially electrically driven motor vehicle. This motor vehicle drive train comprises a transmission and at least one electric machine, wherein a vehicle driver can at least be supported by the operation of the motor vehicle drive train according to the invention when an unwanted movement of the motor vehicle is prevented.An advantageous embodiment of the invention, which is explained below, is illustrated in the drawings. It shows: FIG. 1 is a schematic view of a motor vehicle powertrain; and FIG. 2 is a flow chart for operating the motor vehicle drive train from FIG. 1, according to a preferred embodiment of the invention.FIG. 1 shows a schematic illustration of a motor vehicle drive train 1, which is assigned to an electrically drivable motor vehicle drive axle 2. Accordingly, the motor vehicle drive train 1 is provided in an at least partially electrically driven motor vehicle, such as a hybrid or electric vehicle. This motor vehicle is preferably also a commercial vehicle.The motor vehicle drive train 1 comprises at least one electric machine 3, which can be operated on the one hand as an electric motor and on the other hand as a generator and which is connected on the output side to a downstream transmission 4. In the transmission 4, different gears can be shifted in order to couple the electric machine 3 with an associated transmission ratio to drive wheels 5 and 6 of the motor vehicle drive axle 2, respectively. In addition to the gears, the transmission 4 can also be shifted into neutral, whereby a transmission input and a transmission output of the transmission 4 and thus also the electric machine 3 and the drive wheels 5 and 6 of the motor vehicle drive axle 2 are decoupled from one another.Shifts in the transmission 4 are carried out automatically via a control unit 7, wherein the transmission 4 can be designed specifically in the manner of an automated transmission, an automatic transmission or an automated continuously variable transmission. In addition, an operation of the electric machine 3 can also be regulated via the control unit 7. For this purpose, the control device 7-preferably in a data bus system-is connected to the transmission 4 and also to the electric machine 3.In addition, the control unit 7 is also connected to a selection device 8 and rotational speed sensors 9 and 10, these connections also preferably being carried out within the data bus system. At the selection device 8, a vehicle driver of the motor vehicle can specify a desired direction of travel, i.e. a forward travel or a reverse travel. Rotational speeds of the drive wheels 5 and 6 can be detected via the rotational speed sensors 9 and 10.When the transmission 4 is shifted into neutral, it may occur, in particular when the motor vehicle is stopped on an inclined surface after the service brake of the motor vehicle has been released, that the motor vehicle unintentionally starts to move, that is to say rolls forward or backward depending on the inclination of the surface. In order to at least assist the vehicle driver when preventing this movement, the control unit 7 can operate the motor vehicle drive train 1 within the scope of a method according to the invention, the sequence of which is illustrated in FIG. 2 and which will now be described below.At the beginning of the method, a check is made as a criterion in a first step S 1 as to whether the transmission 4 is shifted into neutral. If this is not the case, the criterion is jumped back before step S 1 and, in particular, a new check of the criterion is carried out after a defined time window has elapsed. If, on the other hand, it is detected in step S 1 that the transmission 4 is shifted into neutral, it is queried in step S 2 whether the vehicle driver has already specified a direction of travel at the selection device 8. In the case to be affirmative, the system then proceeds to a step S 3, while if the driver fails to specify, the system again jumps back to step S 1.If the method has moved from step S 2 to step S 3, it is queried which of the driving directions has been selected by the vehicle driver, wherein in a subsequent step S 4 it is then checked as a criterion whether a current movement of the motor vehicle is opposed to the selected driving direction. For this purpose, the control device 7 accesses the rotational speed sensors 9 and 10 in order to determine a current direction of movement of the motor vehicle.If the motor vehicle is at a standstill, for example because the vehicle driver is still operating the service brake or the motor vehicle is not located on an inclined surface, the method moves to a step S 5. In addition, a transition is also made to this step S 5 if a current movement of the motor vehicle takes place in the direction of the selected direction of travel. In step S 5, a starting gear is then engaged in the transmission 4 via the control unit 7, and subsequently, in step S 6, the motor vehicle is started by electromotive operation of the electric machine 3 according to a predefined value of the vehicle driver.If, on the other hand, it is detected in step S 4 that the motor vehicle is currently moving in the opposite direction to the selected direction of travel, then proceeding from step S 4 to a step S 7. In step S 7, the control device 7 then likewise ensures a shift of the starting gear in the transmission 4, wherein, however, in a step S 8, the control device 7 then determines a torque, which is to be applied by the electric machine 3 in its generator mode by recuperation, with the inclusion of a transmission ratio of the starting gear, in order to represent a braking torque at the drive wheels 5 and 6, at which the motor vehicle can be braked to a standstill at its current speed. Subsequently, in a step S 9, the corresponding recuperation is initiated by the electric machine 3 and subsequently in a step S 10 it is queried whether a standstill of the motor vehicle has taken place. If this is not the case, then the system jumps back before step S 8, while otherwise the system proceeds to step S 6. In this step S 6, when the starting gear is engaged in the transmission 4, the motor vehicle is then started according to the driver's specification.By means of the procedure according to the invention, a vehicle driver can be assisted at least in preventing a movement of a motor vehicle having the motor vehicle drive train in the opposite direction to a selected direction of travel in a state of a transmission of a motor vehicle drive train which is switched to neutral.Reference numerals denote reference numerals1 Motor vehicle drive train 2 Motor vehicle drive axle 3 Electric machine 4 Transmission 5 Drive wheel 6 Drive wheel 7 Control unit 8 Selection device 9 Rotational speed sensor 10 Rotational speed sensor S 1 to S 10 Individual steps

Claims

Method for operating a motor vehicle drive train (1) which comprises at least one electric machine (3) which can be coupled within the motor vehicle drive train (1) via a transmission (4) to drive wheels (5, 6) of a motor vehicle, wherein as part of a check it is queried as a criterion whether a movement opposite to a predefined direction of travel is carried out by the motor vehicle, and wherein as a function of a result of the check a measure is initiated in which a torque braking the motor vehicle in the opposite movement is applied by the at least one electric machine (3), characterized in that as part of the check it is also checked as a further criterion whether the transmission (4) is currently switched to neutral, in that the measure is only initiated if a simultaneous fulfilment of the criterion and of the further criterion is detected, and that a gear in the transmission (4) is shifted in advance for carrying out the measure.Method according to Claim 1, characterized in that, in the course of the measure, a braking torque is determined which is required for braking the opposite movement of the motor vehicle to a standstill, wherein, by including a transmission ratio of the gear engaged in the transmission (4), the torque which is to be applied by the at least one electric machine (3) for generating the braking torque is determined.Method according to Claim 1 or 2, characterized in that a starting gear in the transmission (4) is shifted as gear for carrying out the measure in advance.Method according to one of Claims 1 to 3, characterized in that, as part of the check, the predefined direction of travel is determined by a predefined vehicle driver being queried at a selection device (8).Method according to one of the preceding claims, characterized in that the execution of the measure is ended when the motor vehicle has stopped, wherein, when the measure is ended and in the gear engaged in the transmission (4), a torque accelerating the motor vehicle in the direction of the predefined direction of travel is applied by the at least one electric machine (3) in accordance with a predefined value of a vehicle driver.Control device (7) for a motor vehicle drive train (1) of an at least partially electrically driven motor vehicle, wherein the control device (7) is configured to query as a criterion, in the context of a check, whether a motor vehicle having the motor vehicle drive train (1) performs a movement in the opposite direction to a predefined direction of travel, and to check as a further criterion whether a transmission (4), via which at least one electric machine (3) can be coupled within the motor vehicle drive train (1) to drive wheels (5, 6) of the motor vehicle, is currently switched to neutral, wherein the control device (7) is configured to initiate a measure, in which the at least one electric machine (3) applies a torque braking the motor vehicle in the opposite movement, only when the criterion and the further criterion are simultaneously fulfilled, and wherein the control device (7) is configured to, for carrying out the measure in advance, to cause a gear in the transmission (4) to be shifted.Control device (7) according to Claim 6, which is furthermore configured to carry out a method according to one or more of Claims 2 to 5.Computer program product for a control device (7) according to Claim 6 or 7, by means of which a method according to one or more of Claims 1 to 5 can be carried out, wherein a routine for operating the motor vehicle drive train (1) is implemented by corresponding control commands stored in software.Data carrier comprising a computer program product according to claim 8.Motor vehicle drive train (1) for an at least partially electrically driven motor vehicle, comprising a transmission (4) and at least one electric machine (3), wherein the motor vehicle drive train (1) is operable according to a method according to one or more of claims 1 to 5.Motor vehicle drive train (1) according to Claim 10, characterized in that the motor vehicle drive train (1) has a plurality of driven vehicle axles, wherein each of the driven vehicle axles can be coupled to at least one electric machine (3) via a transmission (4), and wherein the driven vehicle axles of the motor vehicle drive train (1) can be operated according to a method according to one or more of Claims 1 to 5.

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