Method for operating a powertrain of a motor vehicle

Abstract

Method for operating a drive train (10) of a motor vehicle with an electric machine (12) as a drive unit, which is capable of exerting a torque on a set of drive wheels (24) via at least one transmission, characterized in that, in order to prepare for a starting process of the motor vehicle, the electric machine (12) is energized with at least one current pulse during a standstill of the drive wheels (24), the height and width of which are matched to the downstream drive train elements in such a way that a torque too weak for movement of the drive wheels (24) but maximally bridging the drive train gap is exerted on them.

Description

Field of the invention

[0001] The invention relates to a method for operating a drive train of a motor vehicle having an electric machine as a drive unit, which is capable of exerting a torque on a set of drive wheels via at least one transmission. State of the art

[0002] In electrically powered motor vehicles, an electric machine that can be operated (at least also) in motor mode represents the drive unit, which generates a torque that is transmitted to the drive wheels of the vehicle via other elements of the drive train. Elements of the drive train are typically transmissions, such as manual and / or differential gears, rods, couplings, etc. In particular, there is usually direct access from the drive unit to the drive wheels, i.e. the drive train does not usually contain a separating clutch between the drive unit and the downstream drive train elements, which could be engaged with slippage during starting, for example. Accordingly, a smooth increase in torque or speed of the drive unit must be achieved, which is provided with an appropriate control or regulation system for this purpose.However, conventional control systems are unable to satisfactorily address the problem of driveline backlash. Backlash refers to the play between driveline elements, for example, between the tooth flanks within a gearbox. This can accumulate to a considerable amount of dead travel in a driveline with numerous elements. When starting off, the torque applied by the drive unit is initially lost, causing the driveline elements subject to backlash to collide violently with one another. Only after the entire dead travel—i.e., the total driveline backlash—has been overcome is there a continuous flow of torque from the drive unit to the drive wheels.

[0003] The actual length of the dead travel in a given starting situation, i.e. the extent of the driveline slack existing in the specific case, depends on the specific history, i.e. the sequence of events immediately prior to the starting situation. For example, the driveline slack will typically be smaller on a slightly uphill road than on a slightly downhill road. External movements, such as those caused by light gusts of wind, can also influence the extent of the slack. The starting behavior of the vehicle varies accordingly, so that the driver may, in a way that is incomprehensible to him, feel a stronger or weaker jerk when the slack is briskly bridged when the starting torque is applied. This lack of predictability of the starting behavior is perceived as unpleasant.

[0004] EP 0 636 286 B1 and DE 10 2009 054 851 A1 disclose methods for achieving a soft start of an electric motor. However, these methods relate to isolated motors and the inherent uncertainties in determining the rotor position. Neither publication offers any approach to the drivetrain slack problem discussed above.

[0005] DE 10 2008 058 668 A1 shows a method for torque control of an electric drive motor of a vehicle, in which the drive motor is controlled by electronics in such a way that when the vehicle is stationary but ready to start, the drive motor generates a creep or preload torque for preloading a drive train of the vehicle.

[0006] In the present method described in DE 101 45 891 A1 for damping torque oscillations in the drivetrain of an electrically powered road vehicle, the torque or a torque estimate and the speed of the electric motor driving the road vehicle, as well as the speed of a wheel or the average of the speeds of all driven wheels of the road vehicle, are recorded as input variables. An observer estimates the torque transmitted from the transmission to the wheels from the input variables. A control block connected downstream of the observer determines a compensation torque from the estimated torque. The compensation torque is added to the target torque of the electric motor, thus creating a control loop. These measures achieve significantly improved handling for the driver and reduce material stress in the drivetrain.

[0007] US 5,994,859 A discloses a method for reducing torsional vibrations in a vehicle drivetrain. The method includes sensing a speed of the drivetrain to generate a speed signal. The method also includes generating a vibration-compensating torque command that is a function of a frequency content of the speed signal. The present invention further provides a system for reducing torsional vibrations in a vehicle drivetrain. The system includes a torque source, a driveline coupled to receive torque from the torque source, and a speed detector configured to sense a speed of the driveline and generate a speed signal. The system additionally includes an electronic controller coupled to the torque source and programmed to provide operating command signals to the torque source.The system further comprises a frequency detector having an input and an output, the input being in electrical connection with the speed detector to receive the speed signal and the output being in electrical connection with the electronic control.

[0008] DE 601 04 247 T2 relates to a method for absorbing play in a drive train when a load change occurs in a motor vehicle drive system. Furthermore, an arrangement for applying the method is disclosed.

[0009] DE 41 39 443 C2 shows a drive and brake arrangement for a motor vehicle, comprising at least one electric motor driving the motor vehicle depending on the setting of an accelerator pedal, a forward-reverse direction selection device setting the drive direction of rotation of the electric motor(s), means for detecting the driving speed of the motor vehicle and a friction brake arrangement acting on at least one wheel of the motor vehicle. Task

[0010] It is the object of the present invention to improve a method for operating a drive train of a motor vehicle in such a way that the loose-related unpredictability of the starting behavior is eliminated. Description of the invention

[0011] This object is achieved in conjunction with the features of the preamble of claim 1 in that, in preparation for a starting process of the motor vehicle, the electric machine is energized with at least one current pulse while the drive wheels are at a standstill, the height and width of which are matched to the downstream drive train elements in such a way that a torque which is too weak for a movement of the drive wheels but which bridges the maximum drive train slack is exerted on them.

[0012] The basic idea of ​​the invention is to create a defined initial situation in the drive train before a possible start-off request. Since start-off is by definition made when the drive wheels are stationary, the wheel standstill is used as the reference situation. The wheel standstill is easy to detect using suitable sensors that are well known to those skilled in the art. In this state, the electric machine is supplied with at least one current pulse defined by its height and width, i.e. by its current intensity and pulse duration, without there being a specific torque request from the driver, such as would be signaled by the driver when requesting to start off by pressing the accelerator pedal. The pulse-like current supply generates a torque pulse at the output of the electric machine, the height, i.e. its torque intensity, is determined by the height of the current pulse and the width, i.e. its duration, is determined by the width of the current pulse.The torque pulse is propagated through the downstream elements of the drivetrain. This torque pulse must ultimately be dimensioned so that it is sufficiently long and strong to completely bridge the driveline slack. On the other hand, it must not be so strong that it would cause the drive wheels to move, which the driver could perceive as jerking. The precise dimensioning of the torque pulse or the underlying current pulse obviously depends on the specific design of the drivetrain. A short, lightweight, and very precisely manufactured drivetrain with only small and easily overcome slack will require a narrower and lower torque boost than a long, heavy drivetrain with large tolerances. In any case, however, the torque required to overcome the driveline slack will be significantly lower than the minimum torque required to move the drive wheels.A person skilled in the art will thus be able to determine the required torque intensity for each drivetrain type without undue experimentation. Regarding the width of the torque pulse, the invention provides for selecting the time required to overcome maximum backlash, i.e., to overcome the maximum possible dead travel as the sum of the maximum dead travels of the individual drivetrain components. As a rule, this pulse duration will be longer than absolutely necessary; however, this is harmless insofar as, as explained above, the torque intensity is insufficient to generate a starting jerk at the drive wheels after bridging the backlash.

[0013] The invention thus provides a particularly simple way, particularly without complex control technology, to create a well-defined, reproducible drivetrain situation when the motor vehicle is stationary, so that a subsequent torque request from the driver always results in the same starting behavior of the vehicle. Furthermore, the starting jerk, which is generated in conventional control systems in unfavorable situations due to the sudden bridging of the drivetrain slack, is minimized.

[0014] In a preferred development of the invention, the electric machine is energized with a plurality of current pulses spaced apart by pauses of predetermined duration. This takes into account the circumstance explained in the introduction that even minor external influences, such as a gust of wind, can change the drivetrain slack situation when the vehicle is stationary. During a longer standstill, it is therefore possible that a bridging of the drivetrain slack achieved according to the invention is destroyed again by subsequent external influences, resulting in an undefined situation in the drivetrain. The aforementioned development of the invention therefore provides for its one or more repetitions in order to counteract changes that have occurred in the meantime. The repetition can be periodic or event-driven.

[0015] However, to conserve electrical energy and protect the electrical machine from overload, it is desirable to use the method according to the invention very sparingly. In particular, it is advantageous to apply the pulsed current to the electrical machine according to the invention especially when an imminent start-up request from the driver is foreseeable, or not to apply it if an imminent start-up request from the driver appears unlikely.

[0016] In a favorable development of the invention, it is therefore provided that the energization of the electric machine with one or more current pulses is initiated by moving a drive selector lever from the park or neutral position. Such a drive selector lever actuation indicates that the driver intends to start driving in the near future. In this development, this is interpreted as a signal to prepare the starting process for implementing the method according to the invention. Conversely, it can advantageously be provided that the energization of the electric machine with one or more current pulses is deactivated by moving a drive selector lever into its park or neutral position, since a start-up request is excluded in such a lever position.

[0017] Alternatively or additionally, it can be provided that the energization of the electric machine with one or more current pulses is initiated by the release of a previously applied service brake beyond a predetermined release state. Conversely, it can be provided that the energization of the electric machine according to the invention is deactivated by the application of the service brake beyond a predetermined application state. Here, too, the concept explained above is reflected: When the service brake is safely applied, a start-up request is excluded, and therefore there is no need to carry out a method according to the invention to prepare for it. Conversely, the release of the service brake that has been safely applied indicates an imminent start-up request, so appropriate preparation appears sensible.

[0018] Alternatively or additionally, an analogous arrangement can be made with regard to a parking brake, the actuation of which precludes starting and the release of which indicates an impending desire to start moving.

[0019] Further features and advantages of the invention will become apparent from the following specific description and drawings. Short description of the drawings

[0020] They show: Fig. 1 a schematic representation of a drive train according to the invention Fig. 2 a schematic flow diagram of the method according to the invention. Detailed description of preferred embodiments

[0021] Fig. 1 shows a schematic representation of a drive train 10 of a motor vehicle (not shown in detail), which is controlled according to the principle of the invention. The drive train 10 comprises, as the central drive unit, an electric machine 12, which in the illustrated embodiment is designed as an internal rotor with an internal, rotatably arranged rotor 14 and an external, housing-fixed stator 16 carrying electrical windings. The output shaft 18 of the rotor 14 is coupled to a differential 20 as its input shaft. The axle flange shafts 22 of the differential 20 carry the drive wheels 24. The drive train shown is reduced to a few basic elements. However, the invention is also applicable to drive trains of any complexity.

[0022] The electric machine 12 is controlled by a control unit 26, which, in addition to power electronics, has a computing unit and various sensor inputs. The electronic details of the control unit 26 are not shown in detail, as they are not relevant to the present invention. Furthermore, the control unit 26 is not designed as an isolated module, as in Fig. 1. It is possible to distribute the function of the controller 26 across multiple modules or to integrate more than the aforementioned functions into a single module. In either case, the controller 26 is connected via a sensor line 28 to a wheel motion sensor (not shown in detail), which is capable of reliably detecting the vehicle's standstill. Via additional sensor lines 30, the controller 26 can be connected to additional sensors or switching devices, which supply states and parameters to the processing unit of the controller 26.

[0023] On the output side, the controller 26, in particular its power electronics, is connected to the windings of the stator 16 via the electrical line 32.

[0024] When carrying out the method according to the invention, the following procedure is followed according to the schematic representation of Fig. 2, first, a standstill detection by means of a standstill detection function module 40. For this purpose, information supplied via information inputs 42 about sensor-determined states and parameters of the motor vehicle can be incorporated. For example, the output of a vehicle speedometer, a wheel revolution counter, a longitudinal or lateral accelerometer, or similar sensors can be used to detect wheel standstill. In the context of the present description, the term "standstill" is to be understood as a state in which the rotational speed of the drive wheels is within a narrow tolerance band around absolute standstill. The width of the tolerance band can be adapted to the individual case by a person skilled in the art. Thus, the method according to the invention can certainly also be carried out in a slow, coasting state of motion of the vehicle.

[0025] If a standstill of the drive wheels is reliably detected in the standstill detection function module 40 as described above, the system is enabled to enter the next function module of the method according to the invention. The enablement is in Fig. 2 marked with “OK”. In the following activation condition function module 44, further parameters and states of the vehicle are optionally queried, as described in Fig. 2 by means of the information inputs 46. This can be, for example, the output signal of a brake pedal angle sensor, a gear selector lever position sensor, an accelerator pedal angle sensor, a parking brake actuation sensor, or the like. For example, driver assistance systems, such as optical traffic light detection systems or the like, could also be included. As discussed in detail in the general description, the activation or deactivation of the method according to the invention occurs according to a probability estimate of an impending driver start-up request.

[0026] If the calculation or estimation carried out in the activation condition function module results in an activation, which in Fig. 2 is marked with a corresponding "OK", the winding of the stator 16 is energized with at least one current pulse in order to bridge the drive train gap. Preferably, as in Fig. 2 in the electronic function module 48, periodically pulsed to compensate for changes that may have occurred in the meantime due to external influences. The result of such pulsed current supply to the stator 16 leads to a corresponding torque application M of the drive train.

[0027] Of course, the embodiments discussed in the specific description and illustrated in the figures represent only illustrative embodiments of the present invention. In light of the disclosure herein, a wide range of possible variations is available to those skilled in the art. In particular, the choice of the magnitude and length of the current pulses must be tailored to the mechanical and electrical conditions of the respective drivetrain type. List of reference symbols 10 Drivetrain 12 electric machine 14 rotors of 12 16 stators of 12 18 rotor shaft of 12 20 Differential 22 axle flange shaft 24 drive wheel 26 Control 28 Sensor cable 30 sensor cable 32 electrical power 40 Standstill detection function module 42 information inputs 44 Activation condition function module 46 information inputs 48 Electronic function module

Claims

[1] Method for operating a drive train (10) of a motor vehicle with an electric machine (12) as a drive unit, which is capable of exerting a torque on a set of drive wheels (24) via at least one transmission, characterized by in that, in preparation for a starting process of the motor vehicle, the electric machine (12) is energized with at least one current pulse while the drive wheels (24) are at a standstill, the height and width of which are matched to the downstream drive train elements in such a way that a torque which is too weak for a movement of the drive wheels (24) but which bridges the maximum drive train slack is exerted on them. [2] Method according to claim 1, characterized by that the electrical machine (12) is supplied with current with a plurality of current pulses spaced apart from one another by pauses of predetermined duration. [3] Method according to one of the preceding claims, characterized bythat the energization of the electric machine (12) with one or more current pulses is initiated by moving a drive mode selector lever out of its park or neutral position. [4] Method according to one of the preceding claims, characterized by that the energization of the electric machine (12) with one or more current pulses is initiated by releasing a previously actuated driving brake beyond a predetermined release state. [5] Method according to one of the preceding claims, characterized by that the energization of the electrical machine (12) with one or more current pulses is initiated by releasing a previously actuated parking brake beyond a predetermined release state. [6] Method according to one of the preceding claims, characterized by, - that the current supply to the electric machine (12) with one or more current pulses is deactivated by moving a drive mode selector lever into its park or neutral position. [7] Method according to one of the preceding claims, characterized by that the current supply to the electric machine (12) with one or more current pulses is deactivated by actuating a driving brake beyond a predetermined actuation state. [8] Method according to one of the preceding claims, characterized by that the energization of the electrical machine (12) with one or more current pulses is deactivated by activating an autohold function.

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