Method for determining condition information describing a, in particular tribological, condition of a powertrain and motor vehicle

Abstract

Method for determining state information (34, 52) describing a state of a drive train (1) of a motor vehicle (2), wherein the drive train (1) comprises an electric machine as a first drive machine (3) and a second drive machine (4), characterized in that, upon fulfillment of a diagnostic condition (23), the first drive machine (3) is operated in an electrical idle state, in which it is electrically disconnected from all current sources (8) and current sinks (19) and coupled to at least one of the wheels (11-14) of the motor vehicle (2), and during the operation of the first drive machine (3) in the electrical idle state, a power measure (28) for the power of the second drive machine (4) is recorded, while at least one of the wheels (11-14) of the motor vehicle (2) is driven by the second drive machine (4), after which the state information (34,52) is determined as a function of the power measure (28) and a predetermined reference information (29), wherein the diagnostic condition (23) can only be met if the speed of the motor vehicle (2) and / or the rotational speed (22) of the first and / or the second drive motor (3, 4) is constant over a predetermined time interval or lies within a range of variation of a predetermined width, wherein a measure of the power of the second drive motor (4) applied to keep the speed and / or the rotational speed (22) constant or within the range of variation while the first drive motor (3) is operating at electrical idle is recorded as the power measure (28).

Description

The invention relates to a method for determining state information describing a, in particular tribological, state of a motor vehicle's drivetrain, wherein the drivetrain comprises an electric machine as a first drive machine and a second drive machine. The invention also relates to a motor vehicle. In motor vehicles, wear and tear on drivetrain components, such as wheel bearings, drive shafts, differentials, transmissions, and drive motors, can lead to increased fuel consumption, high repair costs, and potentially even vehicle breakdowns, especially if this wear is detected late. These disadvantages for vehicle users can also damage the manufacturer's reputation, even if the problems result from inadequate vehicle maintenance by the user. It is therefore advisable to monitor the wear of relevant components in order to recommend vehicle servicing to the user in a timely manner and, in particular, preventively. A possible approach for this is proposed in publication US 2017 / 0221069A1. This publication teaches how to record fuel consumption for various engine speeds and torques and, based on this fuel consumption, to estimate the dissipation in the powertrain. However, if an electric motor is used in the drivetrain, pure consumption monitoring cannot differentiate between electrical and mechanical losses, meaning that mechanical wear cannot be clearly detected. This is particularly problematic because electrical losses depend heavily on the power currently supplied by the electric motor. Furthermore, losses in an electrically driven drivetrain are sometimes hysterically dependent on various influencing factors, such as the ambient temperature or the temperature of the oil in a cooling circuit. Temperatures in the cooling circuit, for example, also depend on the current or immediately preceding power output of the electric motor, so wear detection in this context can be inaccurate and prone to error. The publication DE 10 2019 220 401 A1 concerns a method for determining a drag torque. The drag torque is measured during deceleration of the vehicle. The publication DE 10 2020 123 689 A1 discloses a motor vehicle with two electric machines, one of which can be operated as a generator, can be decoupled from the driven axle, can rotate in freewheel or idle mode, or can also be used to provide drive power while the motor vehicle is driven by the other machine. German patent application DE 10 2018 133 649 A1 relates to a method for operating a drive train in which braking torques are used to distribute a total drive torque to the wheels. To determine a basic distribution of the drive torque, friction coefficients between the driven wheels and the road surface, based on long-term observations, can be taken into account. Publication 10 2020 111 605 A1 relates to a method for operating a drive train with an axle driven by at least one first electric machine and a second axle driven by at least one second electric machine, wherein the second axle can be switched on and off. The publication DE 10 2013 218 127 A1 concerns a method for controlling the operation of a drive and / or load component of an electric or hybrid vehicle. The invention is therefore based on the objective of providing a way to determine a condition, in particular a tribological condition or wear condition, of a drive train with an electric machine with higher accuracy and robustness. The object is achieved according to the invention by a method of the type mentioned at the outset, wherein, upon fulfillment of a diagnostic condition, the first drive motor is operated in an electrical idle state, in which it is electrically disconnected from all current sources and current sinks and coupled to at least one of the wheels of the motor vehicle, and during the operation of the first drive motor in the electrical idle state, a power measure for the power of the second drive motor is recorded, while at least one of the wheels of the motor vehicle is driven by the second drive motor, after which the state information is determined as a function of the power measure and a predetermined reference information, wherein the diagnostic condition can only be fulfilled ifwhere the speed of the motor vehicle and / or the rotational speed of the first and / or the second drive motor is constant over a predetermined time interval or lies within a range of variation of a predetermined width, wherein a measure of the power supplied to the second drive motor to maintain the speed and / or rotational speed constant or within the range of variation while the first drive motor is operating at electrical idle is recorded as the power measure. The inventive method, in which the first drive motor remains coupled to at least one wheel (e.g., the wheels of an axle driven by the first drive motor or all wheels) during the determination of the power output or state information, but is operated in electrical idle mode, ensures that mechanical losses in the first drive motor, and thus in particular its tribological state, are taken into account when determining the power output or state information, while the measurement is not affected by electrical losses in the first drive motor. This eliminates the dependence of the power output or the determined state information on a current or immediately preceding power output by the first drive motor, as explained at the outset.Furthermore, the measurement is not affected by aging of electrical components in the first drive motor. This significantly improves the accuracy and robustness of determining the drive train's condition compared to the prior art, especially when based on a single power measurement or a few power measurements. In particular, it allows for a better distinction between changes in the tribological condition of the drive train and the condition of the power electronics. The tribological condition can be understood as a state of friction or wear in the drivetrain, i.e., the drive motors and the components that transmit their torque to at least one of the vehicle's wheels. A change in the tribological condition can result, for example, from wear and tear, and in particular from the resulting increased friction, or from insufficient lubrication of at least parts of the drivetrain. A deterioration of the tribological condition thus leads to higher friction losses in the drivetrain, which, for example, means that the second drive motor requires more power to maintain a given vehicle speed when driving straight ahead than would be the case under an unaffected tribological condition.Therefore, the diagnostic condition can be met, for example, when the motor vehicle travels straight ahead at a constant speed, with the power measure relating in particular to the power required of the second drive motor to maintain this constant speed. The power measure can directly describe the power, especially as a dimensionless quantity, or have a clear and known correlation to the power of the second drive machine. The first and second drive motors can drive the same wheel or wheels. For example, both drive motors can be coupled to the same drive shaft, which is connected to the driven wheels via a differential. Alternatively, the drive motors can also drive different wheels. For example, the first drive motor can be coupled to the wheels of the front axle and the second drive motor to the wheels of the rear axle, or vice versa. The method according to the invention can also be used if a separate drive motor is used for each wheel. If the diagnostic condition is met, the vehicle can then, for example, be driven by the drive motors of the front wheels, and the drive motors of the rear wheels can be operated as the first drive motor in electrical neutral, or vice versa. Fulfillment of the diagnostic condition may depend, among other things, on prior user input from a vehicle operator. For example, the diagnostic condition may only be fulfilled if a diagnostic mode has been enabled via a control device of the vehicle or an external device. This can, for example, serve to ensure that the method according to the invention is only carried out when a vehicle operator requests it, for example, during a test drive, which can be performed, for instance, as part of vehicle maintenance by a workshop. The second drive machine can be, for example, an electric machine or an internal combustion engine. If an electric machine is also used as the second drive motor, the current supplied to the second drive motor, in particular a direct current, can be detected by a current sensor of the vehicle, and the voltage drop across the second drive motor, in particular a direct voltage, can be detected by a voltage sensor of the vehicle. The power output is then determined as the product of the detected current and the detected voltage, or as a function of this product. In particular, the product can be integrated or averaged over a predetermined time interval. This method allows the power output to be determined with high accuracy and minimal technical effort. Direct current or voltage can be measured, particularly before conversion by a power inverter of the second drive motor. In this case, only a single current and voltage sensor is required, and calculating the power is especially simple. Alternatively, it would be possible, for example, to measure currents and voltages for the different phases of the second drive motor, such as after current conversion by a power inverter, and use them to determine the power. For instance, separate power values ​​could be determined for each phase and added together to calculate the power. If an electric machine is also used as the second drive machine, after determining the power measure, the diagnostic condition or a further diagnostic condition can be checked again, and if the diagnostic condition is met again or if the further diagnostic condition is met, the second drive machine can be operated in an electrical idle state, in which it is electrically disconnected from all power sources and sinks and coupled to at least one of the wheels of the motor vehicle, and during the operation of the second drive machine in the electrical idle state, a further power measure for the power of the first drive machine can be recorded, while at least one of the wheels of the motor vehicle is driven by the first drive machine, after which the state information can be determined as a function of the power measure and the further power measure. The condition information can therefore depend on the power measure, the further power measure, and the reference information, or a respective reference information for the respective measurement on the respective drive machine. Measuring separate power measures for the two drive machines makes it possible, in particular, to distinguish between an actual increase in drag power due to a change in the tribological condition of the drive train and a change in the condition of electrical components of one of the drive machines. For example, if the efficiency of the second drive motor decreases due to a malfunctioning inverter and / or current leakage or similar issues, only the power measurement changes, while the other power measurement remains unchanged. This is because the second drive motor is operated in no-load mode when the latter is determined, thus preventing any influence of its electrical components on the measurement. Conversely, an efficiency loss in the first drive motor due to the condition of its electrical components leads exclusively to a change in the other power measurement, while the power measurement itself remains unchanged. In contrast, however, any change in the tribological state of the drivetrain, regardless of whether this change occurs in the first or second drive motor or in any of the other components of the drivetrain, leads to higher friction losses in the drivetrain and thus to a change in the power measure as well as a change in the further power measure, for example when these are determined while the respective drive motor used for propulsion is supposed to keep the speed of the vehicle constant. The described procedure can thus be used, for example, during a test drive to determine whether the tribological condition of the drive train is impaired or whether the electrical components of the first or second drive motor lead to suboptimal efficiency. The reference information can be specified as a function of the vehicle's speed and / or the rotational speed of the first and / or second drive motor. In particular, the reference information can be the power output or power measure that would be expected under optimal conditions or optimal tribological conditions of the drivetrain. An impairment of the drivetrain's condition can thus be quantified, for example, by a difference between the power measurement and the reference information, or by a quotient of these values. Using the rotational speed of the first or second drive motor is particularly useful when an input gearbox is used between the respective drive motor and the driven wheels. Alternatively, when specifying the reference information, information about the selected gear can also be taken into account. Rotational speeds of drive motors can be directly available based on control information for the respective drive motor, or are often already recorded, for example, for field-oriented control or vector control of the respective drive motor. Alternatively, or for validation purposes, the vehicle speed can also be considered, which can be determined, for example, based on wheel speeds or satellite-based position detection. The specification of the reference information based on the aforementioned parameters, or at least one of them, can be achieved, for example, using a lookup table or by specifying an analytical relationship. An analytical relationship can be determined, for example, using a regression analysis based on measurements of the powertrain in its new condition. According to the invention, the diagnostic condition can only be met if the speed of the motor vehicle and / or the rotational speed of the first and / or second drive motor is constant over a predetermined time interval or lies within a variation range of a predetermined width, wherein a measure of the power supplied by the second drive motor to maintain the speed and / or rotational speed constant or within the variation range while the first drive motor is operating in electrical idle mode is recorded as the power measure. The use of a variation range can be advantageous because, regardless of whether speed control is automated or a vehicle occupant attempts to maintain a substantially constant speed, certain speed deviations and thus rotational speed variations can occur.For example, the width of the variation interval can be chosen so that it corresponds to 5% or 10% of the current speed or rotational speed. As a further measure of performance, a measure of the power of the first drive motor, which is applied to keep the speed and / or rotational speed constant or within the variation interval, can be recorded while the second drive motor is operating in electrical idle mode. The diagnostic condition and / or the further diagnostic condition can only be fulfilled if the vehicle's steering angle lies within a predefined steering angle range. In particular, the diagnostic condition or the further diagnostic condition can only be fulfilled if the vehicle is essentially driving straight ahead, or if the steering angle deviates from straight ahead by less than 5°, for example. Alternatively or additionally, it may be advantageous to specify the reference information depending on the detected steering angle. Restricting the diagnosis to specific steering angle ranges, especially to driving straight ahead, or specifying the reference information depending on the steering angle can be beneficial, since, for example, the power required to maintain a certain speed also depends on the steering angle. Additionally or alternatively, it may be advantageous that the diagnostic condition and / or the further diagnostic condition can only be fulfilled if a road traveled by the motor vehicle has a gradient below a certain threshold, in particular less than 5° or less than 10°. Alternatively or additionally, the reference information may also depend on the gradient. This is advantageous because the power required to maintain a certain speed also depends on the gradient. The condition information can be a Boolean value determined by comparing the performance measure with the reference information, or by comparing an intermediate result calculated based on the performance measure and the reference information with a predefined limit value. Additionally or alternatively, the condition information can be determined based on the difference or quotient between the performance measure and the reference information. In particular, this difference or quotient can be the intermediate result used in the limit value comparison. The Boolean value can, in particular, indicate whether or not a state of high friction or apparent wear exists. The power measure can also directly describe the difference or quotient, or describe a temporal progression of these quantities, which can result, for example, from writing the respective difference or quotient to a ring buffer. Thus, it may be possible, for instance, to track the temporal development of the drivetrain's state, particularly its tribological state, by reading such a ring buffer or analyzing such a temporal progression. If, as explained above, the additional power measure is also recorded, the state information can, for example, also include the difference or quotient between the additional power measure and another reference value. If the additional information is a Boolean value and the further power measure is recorded, a limit comparison can be performed for both the power measure and the further power measure. This could involve, for example, comparing the power measure with the reference information and comparing the further power measure with the reference information, or comparing each intermediate result with a predefined limit value to provide a Boolean partial result. The state information can then be determined, in particular, as an AND operation of the two Boolean partial results. In other words, this state information can only be true if both partial results are true, for example, if both power measures show an unusually high power output for maintaining speed or rotational speed. Optionally, further status information can be determined, which, for example, is only true if a limit value is exceeded in only one of the limit comparisons, or more generally, if only one of the partial results is true. As explained above, this can indicate, in particular, a problem in the electrical system of one of the drive motors. If a warning condition dependent on the vehicle's status information is met, a warning can be issued to a vehicle occupant and / or to a separate device. The warning to the vehicle occupant can be visual, audible, or haptic, for example, via a warning light, a display, steering wheel vibration, or similar means. The device, which is separate from the vehicle, can be, for example, a mobile communication device, such as a mobile phone belonging to a vehicle occupant, or an external device, such as a backend server belonging to the vehicle manufacturer or a maintenance service provider. If the status information is a Boolean value, it can directly indicate whether or not the warning should be issued. It is also possible to compare the performance measurement with different reference information or the intermediate result with different threshold values, whereby, for example, a first vehicle action is triggered when a first threshold value is exceeded, and a second vehicle action is triggered when a second threshold value is exceeded. The first vehicle action could be a relatively low-level warning, such as activating a warning light. The second vehicle action could be a more pronounced warning, such as an audible signal, or it could involve, for example, a reduction in the maximum available driving performance or something similar. In addition to the method according to the invention, the invention relates to a motor vehicle with a drive train comprising a first drive machine designed as an electric machine and a second drive machine, and a processing device that is set up to carry out the method according to the invention. In particular, both drive motors are rigidly coupled to at least one wheel of the motor vehicle or reversibly coupled. Features described in relation to the inventive method, with the advantages mentioned therein, can be transferred to the motor vehicle according to the invention and vice versa. In particular, the motor vehicle can include a current sensor for measuring the current supplied to the first and second drive motors, respectively, and a voltage sensor for measuring the voltage drop across the first and second drive motors, respectively. The drive motors can be disconnected from the power sources and current sinks in the vehicle electrical system, particularly by electrical switches, to allow each drive motor to idle electrically. Due to its typically very high resistance, the voltage sensor is not considered a current sink. However, during electrical no-load operation, the respective current and / or voltage sensor can optionally be disconnected from the drive motor. Further advantages and details of the invention will become apparent from the following exemplary embodiments and the accompanying drawings. Figure 1 schematically shows an exemplary embodiment of a motor vehicle according to the invention, and Figure 2 shows a flowchart of an exemplary embodiment of the method according to the invention. Fig. 1 shows a motor vehicle 2 with a drive train 1 comprising two drive motors 3, 4, each coupled to two of the wheels 11-14 of the motor vehicle 2 via a differential 9, 10. As will be explained in more detail with further reference to Fig. 2, a processing unit 30 of the motor vehicle is to determine state information 34, 52 regarding the state of the drive train 1. In this example, the tribological state is to be determined specifically, i.e., in particular, increased losses in the drive train 1 due to friction caused by wear or insufficient lubrication. Evidence regarding such a condition of the drivetrain 1 can already be determined by comparing the power provided by the drive motors 3, 4 in a specific driving condition, for example, to maintain a certain speed, with a reference value. For this purpose, fuel consumption can be recorded for internal combustion engines and electrical power for electric motors. In electric machines, however, not only a deterioration of the tribological condition can lead to a reduction in efficiency, but also defects in the electronics, such as leakage currents, incorrectly controlled inverters, or similar issues. Therefore, in the motor vehicle 2, as will be explained in more detail below using an exemplary procedure, the state information 34, 52 regarding the tribological condition of the drive train 1 is determined as a function of a power measure 28, 47 for at least one of the drive machines 3, 4, while the other drive machine 3, 4 remains coupled to the respective wheels 11-14 of the motor vehicle, but is operated in an electrical idle state.Thus, while friction losses in both drive machines 3, 4 and in the other components of the drive train, for example in the differentials 9, 10 or in the wheel bearings of the wheels 11-14, are taken into account, the electrical system of the respective drive machine 3, 4 operating in electrical neutral cannot affect the measurement. In particular, if only one of the drive machines 3, 4 is an electric machine, or if determinations of a respective power measure 28, 47 are carried out for both drive machines 3, 4 while the other drive machine 3, 4 is operating in electrical neutral, the influence of the electronics of the drive machines 3, 4 on the determination of the tribological state can be largely excluded. Fig. 2 shows a flowchart of an embodiment of a method for determining a tribological state of the drive train 1 of the motor vehicle 2. The process control and calculations within the method can, for example, be implemented by the processing unit 30 of the motor vehicle 2; however, it would also be possible, for example, to outsource parts of the calculations to external units. In step S1, a steering angle of the vehicle 2 is first detected via a steering angle sensor 21, and in step S2, the rotational speed of the electric machine 4 is detected. The rotational speed could be measured directly via a speed sensor. However, information about the rotational speed of an electric machine is typically already available in an associated control unit, for example, to enable field-oriented control of the electric machine 4, and can therefore also be queried by such a control unit. In step S3, it is checked whether a diagnostic condition 23 is met, the fulfillment of which indicates the presence of a suitable driving condition for determining information regarding a tribological condition of the powertrain 1. Such a driving condition is, in particular, straight-line driving at a substantially constant speed. Diagnostic condition 23 can therefore be met, in particular, if, for a given time interval, the recorded steering angles lie within a target interval corresponding to straight-line driving, and the recorded rotational speeds 22 all lie within a variation interval of a given width, which corresponds to a substantially constant rotational speed. To monitor the steering angle 20 or the rotational speed 22 over a time interval, it may be advantageous to write the respective measured values ​​to a ring buffer so that several preceding measured values ​​can be considered within the framework of the diagnostic condition. In the example shown in Fig. 1, a constant rotational speed 22 of the drive motor 4 also results in a constant speed of the vehicle 2, since the drive motor 4 is coupled to the differential 10 of the rear axle with a fixed gear ratio. If a multi-speed transmission were used, a necessary sub-condition for fulfilling the diagnostic condition 23 could additionally require that no gear shift has occurred within the considered time interval. Alternatively or additionally, it would also be possible, for example, to evaluate the vehicle speed instead of or in addition to the rotational speed 22. If diagnostic condition 23 is not met, the procedure is repeated from step S1. If, however, diagnostic condition 23 is met, the processing unit 30, in step S4, activates the switches 15, 16, which can be electrical switches, such as transistors, to disconnect the electric machine 3, including its inverter 5, from the vehicle's electrical system 7 and thus from power sources 8, such as a battery, and power sinks 19, such as other consumers. The drive machine 3 therefore remains coupled to the wheels 11, 12 via the differential 9, so that friction losses in the drive machine 3 continue to decelerate the vehicle 2. However, due to the electrical idling, losses in the drive machine 3 are essentially independent of the electrical system of the vehicle 2 and the drive machine 3. In step S5, the current 24 supplied to the drive machine 4 is then measured by a current sensor 25, and the voltage drop 26 across the drive machine 4 is measured by a voltage sensor 27. For example, an average value can be calculated over a predefined time interval. In this example, a direct current and a direct voltage are recorded, i.e., voltage and current values ​​before the inverter 6 of the drive machine 4. Alternatively, it would also be possible in principle to evaluate alternating voltages and currents for the individual phases of the drive machine 4. In step S6, it is then checked whether diagnostic condition 23 was met over the entire time interval for which current and voltage were recorded. If this is not the case, then, for example, cornering or a significant change in speed occurred during this time interval, so that the recorded measurements are not very suitable for determining a tribological condition of the drive train 1 and should therefore be discarded. Therefore, in this case, the procedure is repeated from the beginning after the electrical idle of the drive motor 3 has been terminated in step S7 by closing switches 15 and 16. If the diagnostic condition was consistently met, a power measure 28 is determined in step S8 for the power supplied by the drive motor 4. Since a direct current and a direct voltage are measured in the example, the measured current 24 and the measured voltage 26, or average values ​​of these quantities, can be directly multiplied to calculate the power measure 28. If, however, alternating currents and voltages were measured for individual phases, the individual measured values ​​would have to be multiplied together in phase, the instantaneous power per phase thus determined would have to be integrated over at least one cycle, and the resulting powers of the individual phases would have to be added. In step S9, reference information 29 is specified, which in particular describes an expected value for the power output that would be achieved if the drivetrain 1 were in an optimal condition, i.e., specifically if there were no wear and good lubrication. Since different power outputs are required to maintain a speed depending on the speed, this specification is made as a function of the recorded rotational speed 22. If a multi-speed transmission were used in the vehicle 2, it would be advantageous to additionally consider the selected gear or to record speeds directly in step S2. In the example, step S10 then calculates the difference 32 between the power measure 28 and the reference information 29 as an intermediate result 31. This intermediate result 31 thus describes how much the currently determined power measure 28 deviates from the reference information 29 and therefore represents a measure of the additional power loss resulting from the current tribological state of the drive train 1. In principle, this intermediate result 31 could therefore be used directly as state information 34. For example, the intermediate result 31 could be written to a ring buffer so that, by reading this ring buffer, the temporal development of the state of the drive train 1 can be read and evaluated, for example during maintenance, in order to detect maintenance needs, insufficient lubrication, or similar issues in a timely manner. Alternatively, instead of a difference between performance measure 28 and reference information 29, the quotient of these quantities could also be used as such an intermediate result 31. In the illustrated embodiment, however, it is desirable to automatically detect a condition of the drive train 1 that is likely to require maintenance, so that in step S10 an additional limit comparison is carried out, in which the intermediate result 31 is compared with a predetermined limit value 33, where the state information 34 is a boolean value that indicates whether this limit value has been exceeded. In an alternative design, it would also be possible to forgo determining the intermediate result 31 and to carry out the limit value comparison, for example, by directly comparing the performance measure 28 with the reference information 29 as the limit value. In step S11, a warning condition 35 is then evaluated. In the example shown, this condition is fulfilled immediately if the status information 34 indicates that the limit value 33 has been exceeded by the intermediate result 31. Optionally, additional conditions can also be considered, so that, for example, the warning condition can only be fulfilled if a corresponding warning function has been activated by a user, or similar. If the warning condition 35 is not fulfilled, the procedure is repeated from the beginning after the electrical idle phase ends in step S7. If, however, the notification condition 35 is met, then in step S12, a notification 36, for example a warning symbol, is issued to a vehicle occupant by a notification device 37, for example a display. In addition, a message is issued via a communication device 38 of the vehicle to a separate device 39 equipped by the vehicle 2, for example to a backend server of the vehicle manufacturer, in order to register any maintenance that is likely to be required there as well. While the use of the electrical idle of the first drive motor 3 in the procedure described above with reference to steps S1 to S12 essentially eliminates the influence of the electrical characteristics of this drive motor 3 on the determination of the tribological state of the drive train 1, it is still possible in the described procedure that electrical properties of the drive motor 4, for example, a low electrical efficiency, could lead to a false detection of a problematic tribological state of the drive train 1. This risk can be significantly reduced if steps S11 and S12 are replaced by the optional steps S13 to S27. In this case, the result of the limit value comparison in step S10 does not yet represent the status information 34, but only a partial result 55. These additional steps S13 to S27 implement a similar acquisition of a performance measure as already explained above. However, in these steps, if the diagnostic condition 44 used there is met, the second drive motor 4 is disconnected from the vehicle electrical system 7 instead of the first drive motor 3, and the performance measure is determined for drive motor 3. As already explained in detail in the general section, this allows for a significantly better distinction between efficiency losses in the electrical system and efficiency losses due to the tribological condition of the drive train 1. Due to the strong similarity to the procedure described above, the additional steps will only be briefly explained below. In step S13, switches 15 and 16 are closed to end the electrical idling of the drive motor 3. Subsequently, in step S14, corresponding to step S1, a steering angle 40 is recorded and in step S15, corresponding to step S2, a rotational speed 42 is recorded, which is preferably the rotational speed 42 of the first drive machine 3. In step S16, the diagnostic condition 44 is evaluated, which, as already explained in relation to step S3 and the diagnostic condition 23 therein, is preferably fulfilled precisely when the recorded steering angles 40 and rotational speeds 42 indicate a straight-ahead journey at a substantially constant speed. If diagnostic condition 44 is not met, the procedure is repeated from step S14. Conversely, if diagnostic condition 44 is met, switches 17 and 18 are opened in step S17 to achieve electrical idle operation of the drive motor 4. During this electrical idle time, in step S18 the current 45 supplied to the drive machine 3 is determined via the current sensor 41 and the voltage 46 dropping across the drive machine 3 is determined via the voltage sensor 43. In step S19, corresponding to step S6, it is checked whether the diagnostic condition 44 was met over the measurement interval. If this is not the case, the electrical no-load of the drive motor 4 is terminated in step S20 by closing switches 17 and 18. If, however, the diagnostic condition 44 was met, the power measure 47 for the drive motor 3 is calculated in step S21, in particular by multiplying current 45 and voltage 46. In step S22, the reference value 48 is determined as already explained with reference to step S9 and reference value 29, depending on the rotational speed 42 or the speed of the motor vehicle. In step S23, similar to step S10, depending on the performance measure 47 and the reference value 48, in particular as the difference between these values, the intermediate result 49 is determined and compared with the limit value 50 to determine a further partial result 51, which in turn indicates as a Boolean value whether the limit value 50 has been exceeded. In step S24, the partial results 51 and 55 are combined to form the status information 52. Specifically, the status information 52 can correspond to an AND operation of the partial results 51 and 55, meaning that the status information 52 is only true if the limit value 33 was exceeded in step S10 and the limit value 50 was exceeded in step S23. This is appropriate because an increase in the power output due to faulty or at least suboptimally functioning power electronics in one of the drive motors 3 or 4 can only occur if the respective drive motor 3 or 4 is not operating in electrical no-load mode.However, since the first drive motor 3 is operated in electrical idle mode to determine the partial result 55 and the second drive motor 4 is operated in electrical no-load mode to determine the partial result 51, a non-optimal power electronics system can only lead to the detection of a defective tribological condition of the drive train 1 through the AND operation in step S24 if the increase in the power measure is actually at least partly due to high friction in the drive train 1 or if the power electronics are so faulty that the operation of both drive motors 3, 4 would be affected, which is significantly less likely than a non-optimal function of the power electronics of one of the drive motors 3, 4. Steps S25 and S27 essentially correspond to steps S11 and S12, that is, in step S25 the notification condition 53 is evaluated, which can be fulfilled in particular if the status information 52 is true, and in step S27 a warning 54 is issued, which can essentially correspond to warning 36. If the warning condition 53 is not met in step S25 or after step S27, the procedure is repeated from the beginning, with the electrical idle of the drive machine 4 being terminated by closing switches 17, 18 in step S26 beforehand. As an additional step, for example, if exactly one of the partial results 51, 55 is true, another indication could be given to the vehicle occupant or the facility 39 to point out to them that there appears to be a defect or at least a suboptimal functioning of the power electronics of the motor vehicle 2.

Claims

Method for determining state information (34, 52) describing a state of a drive train (1) of a motor vehicle (2), wherein the drive train (1) comprises an electric machine as a first drive machine (3) and a second drive machine (4), characterized in that, upon fulfillment of a diagnostic condition (23), the first drive machine (3) is operated in an electrical idle state, in which it is electrically disconnected from all current sources (8) and current sinks (19) and coupled to at least one of the wheels (11-14) of the motor vehicle (2), and during the operation of the first drive machine (3) in the electrical idle state, a power measure (28) for the power of the second drive machine (4) is recorded, while at least one of the wheels (11-14) of the motor vehicle (2) is driven by the second drive machine (4), after which the state information (34,52) is determined as a function of the power measure (28) and a predetermined reference information (29), wherein the diagnostic condition (23) can only be met if the speed of the motor vehicle (2) and / or the rotational speed (22) of the first and / or the second drive motor (3, 4) is constant over a predetermined time interval or lies within a range of variation of a predetermined width, wherein a measure of the power of the second drive motor (4) applied to keep the speed and / or the rotational speed (22) constant or within the range of variation while the first drive motor (3) is operating at electrical idle is recorded as the power measure (28). Method according to claim 1, characterized in that an electric machine is also used as the second drive machine (4), wherein a current (24) supplied to the second drive machine (4) is detected by a current sensor (25) of the motor vehicle (2) and a voltage drop (26) across the second drive machine (4) is detected by a voltage sensor (27) of the motor vehicle (2), wherein the power measure (28) is determined as the product of the detected current (24) and the detected voltage (26) or as a function of this product. A method according to claim 1 or 2, characterized in that an electric machine is also used as the second drive machine (4), wherein, after determining the power measure (28), the diagnostic condition (23) or a further diagnostic condition (44) is checked again, and if the diagnostic condition (23) or the further diagnostic condition (44) is met again, the second drive machine (4) is operated in an electrical idle state, in which it is electrically disconnected from all power sources (8) and power sinks (19) and coupled to at least one of the wheels (11-14) of the motor vehicle (2), and during the operation of the second drive machine (4) in the electrical idle state, a further power measure (47) for the power of the first drive machine (3) is recorded, while at least one of the wheels (11-14) of the motor vehicle (2) is driven by the first drive machine (3).whereupon the status information (52) is determined depending on the performance measure (28) and the further performance measure (47). Method according to one of the preceding claims, characterized in that the reference information (29) is specified depending on a speed of the motor vehicle (2) and / or a rotational speed (22) of the first and / or the second drive motor (3, 4). Method according to one of the preceding claims, characterized in that the diagnostic condition (23) and / or the further diagnostic condition (44) can only be fulfilled if the steering angle (20, 40) of the motor vehicle (2) lies within a predetermined steering angle interval. Method according to one of the preceding claims, characterized in that the diagnostic condition (23) and / or the further diagnostic condition (44) can only be fulfilled if a road traveled by the motor vehicle (2) has a gradient below a limit value. Method according to one of the preceding claims, characterized in that the state information (34) is a Boolean value which is determined as a function of a comparison of the performance measure (28) with the reference information (29) or of an intermediate result (31) determined as a function of the performance measure (28) and the reference information (29) with a predetermined limit value (33), and / or that the state information (34) is determined as a function of the difference (32) or the quotient of the performance measure (28) and the reference information (29). Method according to one of the preceding claims, characterized in that, upon fulfillment of a notification condition (35, 53) dependent on the status information (34), a notification (36, 54) is issued to a vehicle occupant and / or to a device (39) designed separately from the motor vehicle (2). Motor vehicle (2), comprising a drive train (1) comprising a first drive machine (3) designed as an electric machine and a second drive machine (4), and a processing device (30), characterized in that the processing device (30) is configured to carry out the method according to one of the preceding claims.

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