Method for fault diagnosis of an electric motor, in particular a BLDC motor, as well as corresponding control unit and computer program

Abstract

The invention relates to a method for fault diagnosis of an electric motor (100), wherein the electric motor (100) has three control lines (101, 103, 105) which are sequentially activated and deactivated for operation of the electric motor (100). The method comprises the following steps: (i) receiving a plurality of values ​​of a quantity representative of a current through the control lines (101, 103, 105), wherein the plurality of values ​​are assigned to a corresponding plurality of time points; (ii) statistically evaluating the plurality of values ​​to obtain a statistical quantity; and (iii) assessing whether a fault of the electric motor (100) exists, using a predetermined criterion based on the statistical quantity. The invention further relates to a corresponding control unit (150) and a computer program.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to methods for fault diagnosis of an electric motor as well as corresponding control devices and computer program products. BACKGROUND OF THE INVENTION

[0002] In control units in the automotive industry, electrical control outputs are typically monitored electrically, meaning the outputs are continuously checked for short circuits to the supply voltage (shortcut to battery - SCB) and short circuits to ground (shortcut to ground - SCG). For this purpose, a voltage measured at a point in the control unit that exhibits a voltage difference to the supply voltage or to ground when the circuit is closed is compared with the supply voltage or the voltage at ground. If the measured difference is too small, a diagnostic function in the control unit software indicates a short circuit to the supply voltage or to ground.

[0003] Similarly, this voltage comparison also produces a voltage difference that is too small to the supply voltage or to ground in the event of an open circuit (OC), depending on the hardware version. This is also referred to as an OC diagnosis. Because induction can occur when the electric motor is moving, which can interfere with the measurement of the voltage difference to the supply or ground, OC diagnosis is usually limited to the electric motor's position. In the case of the control lines, for example, to a BLDC motor, this diagnosis can only be performed when the BLDC motor is securely stationary, since rotation of the rotor due to external torque can induce a voltage despite an interrupted control line.

[0004] In the exemplary application of a BLDC motor as a camshaft actuator in a vehicle, such a diagnosis is therefore performed in many control units after the combustion engine has been shut down and the BLDC motor has safely stopped. This way, a cable break in the camshaft actuator can always be reliably detected in the stationary phase of the combustion engine following the interruption.

[0005] However, the described method cannot prevent overheating of the BLDC motor or control unit in the time between the occurrence of the line interruption during driving of the combustion engine and the end of this driving cycle. SUMMARY AND EMBODIMENTS

[0006] It is therefore an object of the present disclosure to provide a method and corresponding control device which can diagnose faults in the control lines of an electric motor not only when the electric motor is stationary but also during ongoing operation of the electric motor.

[0007] This problem is solved by a method for fault diagnosis of an electric motor, by a control unit, and by a computer program product according to the independent patent claims. Advantageous embodiments and further developments emerge from the respective dependent claims, the following description, and the drawings.

[0008] Thus, according to a first aspect, a method for fault diagnosis of an electric motor is described. The electric motor has three control lines that are sequentially activated and deactivated for operation of the electric motor.

[0009] The method comprises the following steps: (i) receiving a plurality of values ​​of a quantity representative of a current through the control lines, wherein the plurality of values ​​are assigned to a corresponding plurality of points in time; (ii) statistically evaluating the plurality of values ​​to obtain a statistical quantity; and (iii) assessing whether a fault of the electric motor exists by means of a predetermined criterion based on the statistical quantity.

[0010] According to a further aspect, a control unit, in particular a control unit for a motor vehicle, is provided, wherein the control unit is configured to carry out the method described above.

[0011] According to a further aspect, a computer program is provided which comprises instructions which, when executed by a computer, cause the computer to carry out the method described above.

[0012] According to a further aspect, a system is claimed which has the control unit and the electric motor.

[0013] In the context of the present disclosure, the term "control line" may mean a line used to control the electric motor. The control line may comprise one or more switching elements that control an electric current through the control line. A control line may be activated when corresponding switching elements are closed and / or when a current flows through the control line, in particular a current that is greater than a predetermined threshold. The control line may be deactivated if at least one or both of these conditions are not met. The electric motor may have exactly three control lines, but may also have more than three control lines.

[0014] According to one embodiment, the three control lines, in particular all control lines, are electrically connected to one another at one of their ends, in particular in the region of the rotor and / or stator of the electric motor. At the respective opposite end, the control lines can have a switching element, in particular a branch into two electrical lines, each containing a switching element. One of these switching elements can be connected to a positive voltage pole, the other switching element to a negative voltage pole. The switching elements can, but do not have to, be part of the respective control line.

[0015] According to one embodiment, the switching elements are transistors, in particular MOSFETs. They can be configured to be controlled with a pulse-width modulated signal.

[0016] According to one embodiment, the electric motor is a brushless DC motor (BLDC).

[0017] According to one embodiment, the plurality of values ​​contains more than ten values, in particular more than one hundred values, in particular more than one thousand values. The plurality of values ​​can be a selection of a larger number of corresponding measured values.

[0018] According to one embodiment, the plurality of values ​​are evenly distributed over a predetermined time interval.

[0019] The described method and the corresponding control unit can be advantageous for enabling fault diagnosis of an electric motor even while the electric motor is in operation. Accordingly, a potential fault can be responded to even during operation, reducing or even completely preventing damage to the electric motor and / or the control unit. If the fault to be diagnosed is a break in the control line of a BLDC motor, for example, overheating of the system can be reliably prevented during the current driving cycle without limiting the performance of the BLDC motor for fault-free systems.

[0020] In one embodiment, the method utilizes the fact that a corresponding speed controller in the control unit increases the current in the remaining circuit in the event of a line interruption until the speed control error is minimized. This can lead to statistical effects that can be used for fault diagnosis.

[0021] Measuring the temperature in the immediate vicinity of the critical components and adjusting the current supply based on this temperature measurement is not sufficient to protect against overheating due to the rapid temperature rise under full current supply and the inertia of standard temperature sensors. Therefore, the current through the control lines must be measured at high frequency, and the temperature profile of the critical components must be modeled at high frequency based on this measured current. For this purpose, separate measurement of all three phase currents through the three control lines would be helpful. However, for temperature modeling of a fault-free system, it is sufficient to measure only the total phase current in the path after the three parallel circuits are connected to ground. Therefore, all three phase currents through the individual control lines do not need to be measured.Consequently, it may be an advantage of the described method that it does not necessarily have to rely on these individual phase currents and accordingly requires fewer measuring devices.

[0022] According to one embodiment, the plurality of values ​​relate to a quantity representative of the total current through all control lines. For example, the representative quantity can be the total current through all control lines, in particular the total current through all three control lines. The total current can be measured at a point before the control lines branch off from one another and / or after the control lines are joined.

[0023] Such an embodiment can be advantageous because separate current measurements in the various control lines are not required for fault detection. Thus, the embodiment is simple and cost-effective to implement.

[0024] According to one embodiment, a magnetic field in the electric motor is controlled by means of the control lines, in particular via switching elements arranged in the control lines. This magnetic field can be configured to drive a rotor of the electric motor. The magnetic field can be generated by coils of the rotor of the electric motor and / or a stator of the electric motor.

[0025] According to one embodiment, the control lines are electrically conductive and supply current to coils of the electric motor, which at least partially generate the magnetic field. For example, the coils are wire coils. The control lines can encompass the coils, in particular the wire coils.

[0026] According to one embodiment, when corresponding switching elements are closed, a current flows through two of the control lines, but not through the third control line. The direction of current through the control lines can be determined by selecting the switching elements to be closed.

[0027] According to one embodiment, the statistical evaluation of the plurality of values ​​comprises at least one of establishing a continuous frequency distribution, establishing a cumulative frequency distribution, calculating the mean, calculating the median, and forming a threshold value that a predetermined percentage of the plurality of values ​​exceeds. The predetermined percentage can be, for example, 25 percent or 75 percent, so that corresponding quartiles are formed. The predetermined percentage can be less than 90 percent, in particular less than 75 percent. Alternatively or additionally, it can be greater than 10 percent, in particular greater than 25 percent.

[0028] The continuous frequency distribution can be defined using a plurality of bins into which the plurality of values ​​are divided. In other words, all values ​​that are less than the upper limit of the bin and greater than or equal to the lower limit of the bin, or alternatively less than or equal to the upper limit of the bin and greater than the lower limit of the bin, are assigned to the bin. For example, the bins can each have the same width. All values ​​can be assigned to respective bins, or a remainder can remain that is not assigned to any bin. The continuous frequency distribution can also be referred to as a histogram.

[0029] The cumulative frequency distribution can be defined using a plurality of classes, with each class being assigned all values ​​that are less than the upper limit of the class, or alternatively, less than or equal to the upper limit of the class. The cumulative frequency distribution can also be referred to as a cumulative function.

[0030] Such an embodiment can be advantageous because, for example, individual deviations of individual values ​​and / or random effects can be compensated for and thus a robust analysis can be carried out.

[0031] According to one embodiment, the statistical evaluation comprises establishing the continuous frequency distribution and / or the cumulative frequency distribution, wherein classes are selected such that a proportion of the plurality of values ​​that is smaller than the predetermined percentage, in particular smaller than fifty percent, but larger than zero percent, in particular larger than one percent, in particular larger than five percent, in particular larger than ten percent, in particular larger than twenty percent, is greater than the largest upper limit of the classes.

[0032] Alternatively, the classes can be selected such that a proportion of the plurality of values ​​that is less than or equal to the specified percentage, in particular less than or equal to fifty percent, but greater than zero percent, in particular greater than or equal to one percent, in particular greater than or equal to five percent, in particular greater than or equal to ten percent, in particular greater than or equal to twenty percent, is greater than the largest upper limit of the classes.

[0033] Such an embodiment can be advantageous because, for a given number of classes, the class width can be chosen smaller, thus achieving a higher resolution. Accordingly, error diagnosis can be more accurate and robust.

[0034] According to one embodiment, the statistical quantity is one of the following quantities: a quantity representative of the median of the plurality of values; a quantity representative of a limit value exceeded by a predetermined percentage of the plurality of values; a quantity representative of a ratio of a quantity representative of the median of the plurality of values ​​to the mean of the plurality of values, in particular a ratio of a quantity representative of the median of the plurality of values ​​to the mean of the plurality of values; a quantity representative of a ratio of a quantity representative of the limit value exceeded by the predetermined percentage of the plurality of values ​​to the mean of the plurality of values, in particular a ratio of a quantity representative of the limit value exceeded by the predetermined percentage of the plurality of values ​​to the mean of the plurality of values.

[0035] For example, a quantity representative of the median of the plurality of values ​​can be the lower or upper limit of the class to which the median belongs, and / or corresponding limits of neighboring classes. The median can be defined as the value or range of values ​​above and / or below which half of all values ​​of the plurality of values ​​lie. The quantities representative of a limit value can be defined analogously across class boundaries.

[0036] Such a design can be advantageous because computing with class boundaries can require less computing power, largely without compromising the reliability of fault diagnosis.

[0037] By referring to the mean value, component variations and variable operating conditions can be better taken into account.

[0038] According to one embodiment, a suspicion variable is set to assess whether the error is present, which correlates with the criterion. In other words, a value of the suspicion variable can indicate a suspicion that the error is present or that it is not present.

[0039] Setting a suspected error variable may also be advantageous in the context of the present disclosure because statistical methods cannot guarantee absolute certainty. Therefore, it may be appropriate to initially note only the suspicion of the presence of an error for later further verification.

[0040] According to one embodiment, if the suspicion variable indicates a suspicion that the error is present, a check is performed at a later time based on a further criterion. The further criterion can be a non-statistical criterion, i.e., it can be performed without statistical evaluation. The later time can be a time after the suspicion variable has been set in such a way that it indicates the suspicion that the error is present.

[0041] According to one embodiment, the later point in time is defined by the operating conditions of the electric motor. For example, it can be defined by an engine start and / or engine shutdown. The additional criterion can be an OC diagnosis while stationary, as already explained above.

[0042] According to one embodiment, the criterion includes comparing the statistical value with a threshold value, in particular with the threshold value and a further threshold value.

[0043] According to one embodiment, a value of the suspicion variable is not changed if the statistical value lies between the threshold value and the further threshold value.

[0044] According to one embodiment, (i) a value of the error suspicion variable is set to indicate a suspicion that the error is present if the statistical value falls below the threshold; (ii) the value of the error suspicion variable is set to indicate no suspicion that the error is present if the statistical value is above the further threshold; and (iii) the value of the error suspicion variable is not changed if the statistical value is between the threshold and the further threshold. For example, the error suspicion variable is a error suspicion bit, which can take two values, one of which indicates the suspicion and the other value does not indicate the suspicion.

[0045] Cases (i) and (ii) can also be reversed, for example when considering the inverse of the statistical quantity.

[0046] Such an embodiment can be advantageous because providing a range between the two thresholds accommodates the statistical nature of the evaluation. If the value of the statistical variable falls within the transition range, the result is disregarded and the suspected error variable is not changed. This can be justified by the fact that the results in the transition range are subject to comparatively high uncertainty.

[0047] According to a further embodiment, the method further comprises controlling the electric motor while observing a power limitation if the suspected error variable indicates a suspicion that the error is present. The power limitation can be dependent on the type of error. In other words, the current supply to the electric motor can be reduced to such an extent that there is no risk of overheating, even at the expense of lower dynamic performance of the electric motor. Such an embodiment can be advantageous to prevent the electric motor from being impaired or damaged by the error. This can occur, for example, due to excessive heat development if only two of the three control lines are intact and / or activated, and therefore higher currents occur in the control lines, switching elements, and motor coils in the remaining available circuit than if all three control lines are intact and / or activated.

[0048] According to one embodiment, the error comprises at least one of a broken connection in one of the control lines and a malfunction in one of the switching elements. Such an embodiment can be advantageous because all of these errors, in particular the broken connection, can lead to damage to the electric motor and / or the control unit if they remain undetected.

[0049] According to one embodiment, it is determined in which of the control lines the error occurred, for example by means of the switch position of the switching elements.

[0050] According to one embodiment, the fault diagnosis is performed during steady-state operation of the electric motor. Steady-state operation can be defined, for example, by a rotational speed within a predetermined range and / or a power consumption within a predetermined range, in particular a constant rotational speed and / or constant power consumption of the electric motor.

[0051] According to one embodiment, the difference between the maximum total phase current and the minimum total phase current within a predefined diagnostic period is taken into account to assess whether a fault is present. For this purpose, a diagnostic procedure implemented in a control unit can determine the minimum and maximum instantaneous total phase current during stationary operation of the electric motor over a freely calibratable period. A large difference between the maximum and minimum total phase current is characteristic of a system with an interrupted control line. In this case, a suspected fault bit is set in the motor control software and retained until the next test. A small difference between the maximum and minimum total phase current is characteristic of a fault-free system. In this case, the suspected fault bit is cleared.

[0052] In stationary operation of the electric motor with low tapped torque at the rotor shaft, which occurs in the camshaft actuator at stationary engine speed of the combustion engine and constant target camshaft phase, instantaneous total phase currents are measured within a relatively narrow interval of e.g. [0 ... 0.5] A in the fault-free system.

[0053] With an interrupted control line, instantaneous total phase currents close to 0 A are measured much more frequently. However, significantly larger instantaneous total phase currents also occur than in a fault-free system. With a line interruption, instantaneous total phase currents are measured within a relatively wide interval, e.g., [0 ... 2] A, in steady-state operation.

[0054] However, a diagnostic procedure based solely on the scatter of the measured total phase currents proved insufficiently precise in practical vehicle operation to distinguish between faulty systems with line interruptions and fault-free systems with sporadic total phase current peaks due to stochastic torque peaks on the camshaft. Accordingly, it is advantageous to use the scatter in addition to other criteria for fault diagnosis. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Further advantages and advantageous embodiments and further developments of the method, the control device and the computer program emerge from the following exemplary embodiments shown in conjunction with the figures.

[0056] They show: Fig. 1, Fig. 2, Fig. 5 and Fig. 6 statistical evaluations within the framework of a first embodiment of a method for fault diagnosis of an electric motor according to the present disclosure; Fig. 3, Fig. 4, Fig. 7 and Fig. 8 statistical evaluations within the framework of a second embodiment of a method for fault diagnosis of an electric motor according to the present disclosure; and Fig. 9 an electric motor with switching elements for control by a control unit according to an embodiment of the present disclosure.

[0057] Identical, similar, or functionally identical elements are provided with the same reference symbols in the figures. In some figures, individual reference symbols have been omitted for clarity. The figures and the relative sizes of the elements depicted in the figures are not to scale. Rather, individual elements may be exaggerated for clarity and / or clarity. DETAILED DESCRIPTION OF EMBODIMENTS

[0058] Fig. Figure 9 shows an electric motor 100 with six switching elements 108-113 for control by a control unit according to an exemplary embodiment of the present disclosure. The control unit is configured to perform a method for fault diagnosis of an electric motor 100, in this case a BLDC motor.

[0059] The electric motor 100 has three control lines 101, 103, 105, which are sequentially activated and deactivated for operation of the electric motor 100. For this purpose, the switching elements 108, 109, 110, 111, 112, 113, which can be implemented as transistors, for example, are sequentially switched such that one of the control lines 101, 103, 105 is connected to a negative pole of a voltage source and another of the control lines 101, 103, 105 is connected to a positive pole of the voltage source. Thus, two switching elements 108, 109, 110, 111, 112, 113 are closed, and all other switching elements 108, 109, 110, 111, 112, 113 are open. Accordingly, a current flows through two of the control lines 101, 103, 105 and through two of the coils 102, 104, 106 assigned to the two control lines. This generates a magnetic field that depends on the current direction through the control lines 101, 103, 105.This magnetic field drives a rotor of the electric motor 100. The total current through all control lines 101, 103, 105 can be measured using the ammeter 107.

[0060] Brushless DC (BLDC) motors 100 are used in a variety of applications, including automotive engineering. One application example is the drive of an electromechanical camshaft actuator for controlling the rotational speed of a camshaft in an internal combustion engine.

[0061] BLDC motors 100 are controlled by a control unit via three control lines 101, 103, 105. In the stator of the BLDC motor 100, the lines connected to the control lines 101, 103, 105 are electrically interconnected and mechanically arranged such that alternating electrical currents through the control lines 101, 103, 105—the so-called phase currents—induced by the control unit by varying the control voltage generate a rotating magnetic field in the stator. This rotating field rotates the rotor of the BLDC motor 100. The position of the rotor is measured by the control unit using integrated Hall sensors. Thus, the BLDC motor 100 is controlled in a closed control loop with rotor speed as the controlled variable and phase currents through the control lines 101, 103, 105 as manipulated variables.

[0062] The control unit generates the phase currents by pulse-width modulation (PWM) of a control voltage on the control lines 101, 103, 105 of the BLDC motor 100. Any desired current profile can be generated, e.g., in the form of a sine or square wave, the course of which can be described using the parameters frequency, amplitude, and phase position. For the desired operation of the BLDC motor 100, a current of the desired intensity must flow in the desired direction through two of the three control lines 101, 103, 105 at all times. The third control line 101, 103, 105 is not energized by the control unit.

[0063] An important prerequisite for the full functionality of this drive system is the secure electrical connection between the control unit and the BLDC motor 100 via the three control lines 101, 103, 105. A brief or permanent interruption of a control line 101, 103, 105 can make the operation of the BLDC motor 100 impossible. However, this is not necessarily the case.

[0064] At any given moment during motor operation, current flows alternately through one of the three circuits formed by the control unit, two of the three control lines 101, 103, 105, and the BLDC motor 100. Three phase-shifted alternating currents flow in the three control lines 101, 103, 105. If one control line 101, 103, 105 is interrupted, then two of these three circuits are interrupted. An alternating current in the remaining circuit can only generate an oscillating, but no longer a rotating, magnetic field in the stator of the BLDC motor 100. Without a rotating magnetic field, the BLDC motor 100 cannot start from a standstill under its own power.However, if (i) the BLDC motor 100 is already rotating at the time the control line is interrupted or (ii) the BLDC motor 100 is set in rotation by a torque acting externally on its rotor, then, with a low torque tapped at the rotor shaft, it is possible for the control unit operating in the closed control loop to permanently maintain the rotation of the BLDC motor 100 by increasing the current in the remaining circuit to minimize the speed control deviation.

[0065] In the camshaft adjuster example, the rotor of the BLDC motor 100 usually rotates synchronously with the camshaft of the combustion engine. By means of brief, targeted deviations between the target rotational speed of the rotor of the BLDC motor 100 and the rotational speed of the camshaft, the phase position of the camshaft is adjusted. The angular adjustment range of the rotor relative to the camshaft is limited by mechanical stops. With the electromechanical camshaft adjuster, both cases of continued operation are possible despite an interrupted control line: (a) The control line 101, 103, 105 can be interrupted by vibrations during engine operation, while the rotor of the BLDC motor 100 is rotating at approximately the camshaft speed. (b) Before the engine starts, the BLDC motor 100 is also stationary. If the BLDC motor 100 does not begin to rotate as intended when the engine starts due to an interrupted control line 101, 103, 105, it is rotated by the rotating camshaft.

[0066] However, such continued operation with an interrupted control line 101, 103, 105 represents a completely different operating point for the BLDC motor 100 and the control unit than fault-free operation with three control lines 101, 103, 105. After the interruption of one control line 101, 103, 105, the control unit must increase the current in the circuit via the remaining two control lines 101, 103, 105 in a controlled manner to such an extent that the BLDC motor 100 generates the same torque as in fault-free operation with the help of all three circuits in order to achieve the desired target speed of the BLDC motor 100. The critical components of the BLDC motor 100 (the active motor coils) and the control unit (the one active transistor) are then subjected to a larger current and heat up more than in fault-free operation. The current I is quadratically added to the heating power P at a resistor R, P = R * I 2For example, doubling the current quadruples the heating power. Since, for cost reasons, it is common practice to operate the fault-free overall system close to the limits of its thermal capacity, at least at certain operating points, additional heating due to an interruption in a control line 101, 103, 105 threatens overheating and thermal destruction of the system.

[0067] This results in the need to reliably detect an interruption in a control line 101, 103, 105 in order to be able to prevent overheating through active countermeasures. This can be made possible by a method for fault diagnosis of the electric motor 100 with the following steps: (i) receiving, by means of the control unit, a plurality of values ​​of a variable representative of the total current through the control lines 101, 103, 105, wherein the plurality of values ​​are assigned to a corresponding plurality of points in time; (ii) statistically evaluating the plurality of values ​​to obtain a statistical variable; and (iii) assessing whether a fault in the electric motor 100 exists using a predetermined criterion based on the statistical variable.

[0068] Fig. 1 shows a statistical evaluation within the scope of a first exemplary embodiment of such a method for fault diagnosis of an electric motor 100 according to the present disclosure. In this exemplary embodiment, the electric motor 100 is again a BLDC motor 100 that drives a camshaft of an internal combustion engine. As a value representative of the current through the control lines, the total phase current through the control lines 101, 103, 105 is measured at various times, for example, using the measuring device 107. The fault to be diagnosed in this case is an open control line 101, 103, 105 of the BLDC motor 100.

[0069] The method for detecting the interrupted control line 101, 103, 105 of the BLDC motor 100 is based on the statistical evaluation of measured total phase currents using a histogram or a continuous frequency distribution 120. With such a continuous frequency distribution 120, the plurality of measured values ​​of the total phase current are assigned to different classes 122, which are defined by corresponding intervals of the total phase current.

[0070] During steady-state operation of the BLDC motor 100 with low tapped torque at the rotor shaft, which occurs in the camshaft actuator at a steady-state engine speed of the combustion engine and a constant target camshaft phase, the total phase current is measured over a freely selectable period of time—for example, 1 s—with a sampling time of, for example, 1 ms. Each measured current value, corresponding to the multitude of values, is entered into a histogram 120 in the control unit software. The histogram shows the number 123 of all measured values ​​or measuring points that lie in the respective class 122 of current strengths. The classes 122 of the histogram 120 cover the current range expected during steady-state operation for both the fault-free system and the faulty system with a line interruption.

[0071] In Fig. 1 shows a histogram 120 with twenty classes 122 with 0.05 A class width each for a fault-free system with 2398 measuring points.

[0072] Fig. 2 shows a corresponding cumulative frequency distribution 121 for the measured values ​​of the embodiment from Fig. 1. In other words, the distribution of Fig. 1 can be represented as a sum function, ie as the number of all measuring points that are in the respective class 122 or in classes 122 for lower currents. Thus, a fault-free system is again shown. In addition, Fig. 2 a median threshold value 124 belonging to the described data set, which is defined by the number of measuring points, in particular half of this number, in this case 1199.

[0073] For the fault-free system, the measurement points are roughly evenly distributed in the interval [0 ... 0.5] A. There are almost no measurement points above 0.5 A. The arithmetic mean of the current measurements is 0.262 A. The summation function, with the class [0.25 ... 0.3] A, exceeds the threshold of half the total measured values, 1199. The median of all measured values ​​thus lies in the interval [0.25 ... 0.3] A, and the smallest class lower limit above the median is 0.3 A.

[0074] Fig. 3 shows a histogram or a continuous frequency distribution 120 for a faulty system with an interrupted control line 101, 103, 105 with the same number of 2398 measuring points as in the embodiment of the Fig. 1 and Fig. 2.

[0075] As in Fig. 4, this distribution of the Fig. 3 can be represented as a sum function 121, i.e. as the number of all measuring points that lie in the respective class 122 or in classes 122 for lower currents.

[0076] For the faulty system, there is a strong clustering of measurement points in the interval [0 ... 0.1] A. Above 0.1 A, the measurement points are roughly evenly distributed. The arithmetic mean of the current measurements is 0.215 A. The summation function, with the class [0.05 ... 0.1] A, exceeds the threshold of half the total measured values, 1199. The median of all measured values ​​lies in the interval [0.05 ... 0.1] A, and the smallest class lower limit above the median is 0.1 A.

[0077] The change in the total phase current distribution from the fault-free system of Fig. 1 and Fig. 2 to the faulty system of Fig. 3 and Fig. 4 corresponds to the physical process when a line is interrupted: because two of the three circuits are interrupted, there are more measured values ​​close to 0 A, and because of the current increase in the remaining circuit, there are measured values ​​with a much higher total phase current than in the fault-free system.

[0078] Accordingly, the value of the smallest class lower limit above the median 124 of all instantaneous sum phase currents can be used as a measure of the condition of the control line connection.

[0079] If the smallest class lower limit above the median 124 is above a first freely calibratable threshold, this is considered an indication of a fault-free system, and the previously described suspicion bit is cleared. If the smallest class lower limit above the median is below a second freely calibratable threshold that is lower than the first threshold, this is considered an indication of a faulty system, and the suspicion bit is set.

[0080] If the smallest class lower bound is above the median between the first and second thresholds, no statement is made about the system and the suspicion bit remains unchanged.

[0081] If the suspected error bit is set, the control is limited for the remainder of the current operating cycle until the BLDC motor 100 is shut down so that overheating does not occur, even with the additional heating caused by a line interruption. As soon as the combustion engine is shut down and the BLDC motor 100 has safely stopped, the condition of the line connection 101, 103, 105 is reliably diagnosed, for example, using an OC diagnosis while the vehicle is stationary.

[0082] According to an extension of this diagnostic procedure described below, component variations and variable operating conditions are taken more into account. For example, the distribution of the measured values ​​of the instantaneous total phase current can differ between individual drive systems within a series due to component tolerances. A different electrical resistance of the measuring resistor, for example, roughly results in a multiplicative change in all measured currents by a constant factor.

[0083] The distribution of the measured values ​​of the instantaneous total phase current can also differ for each individual drive system under different operating conditions (e.g. different component temperatures). To minimize such influences on the diagnostic procedure, a possible extension of the diagnostic procedure is the Fig. 1 to 4, the error suspicion bit is not based on the value of the smallest class lower limit above the median, but on the ratio of this value of the smallest class lower limit to the arithmetic mean of all instantaneous current measurements.

[0084] In the first embodiment shown above, the Fig. 1 and Fig. 2 For the fault-free system, the arithmetic mean of the current measurements is 0.262 A. The smallest class limit above the median is 0.3 A. The class limit-to-mean ratio is 0.3 A / 0.262 A = 1.14.

[0085] In the second example shown above for the faulty system of Fig. 3 and Fig. 4, the arithmetic mean of the current measurements is 0.215 A. The smallest class limit above the median is 0.1 A. The class limit-mean ratio is 0.1 A / 0.215 A = 0.46.

[0086] The value of the class boundary mean ratio can be used as a measure of the state of the control line connection 101, 103, 105. If the class boundary mean ratio is above a first freely calibratable threshold, this is considered an indication of a fault-free system, and the suspected error bit is cleared. If the class boundary mean ratio is below a second freely calibratable threshold that is lower than the first threshold, this is considered an indication of a faulty system, and the suspected error bit is set. If the class boundary mean ratio is between the first and second thresholds, no statement is made about the system, and the suspected error bit remains unchanged.

[0087] Furthermore, the median of 124, i.e., 50% of all current measurements, does not necessarily have to be used as the threshold for determining the smallest class lower limit above the threshold. The number of measurements used as the threshold can be freely selected as a calibrated percentage of all measurements. For example, a percentage of 25% provides the lower quartile as the threshold.

[0088] The Fig. Figures 5 to 8 show an extension of the diagnostic procedure for a finer resolution of the histogram classes. Fig. 5 and Fig. 6 the same measured values ​​of the faultless system of Fig. 1 and Fig. 2 and for the Fig. 7 and Fig. 8 the same measured values ​​of the faulty system of Fig. 3 and Fig. 4.

[0089] The histogram 120 of the current measurements in Fig. 5 does not contain all current values. It is therefore sufficient if the classes 122 of the histogram 120 cover the range of current measurements in which the threshold for determining the smallest class lower limit is expected to be above the threshold for both the fault-free and the faulty system. Then, with the same number of histogram classes, the search for the threshold can be performed at a higher resolution.

[0090] In the example shown for the error-free system of Fig. 5 and Fig. 6, reducing the class width from 0.05 A to 0.02 A results in only 2022 of the 2398 measurement points being entered in histogram 120 or the cumulative frequency distribution 121. Measurement points above 0.4 A are not entered.

[0091] When calculating the arithmetic mean of all current measurements, the current measurements not entered in the histogram 120 or the cumulative frequency distribution 121 must of course still be taken into account; the arithmetic mean of the current measurements is unchanged at 0.262 A, as in the evaluation with a full histogram. With the class [0.26 ... 0.28] A, the sum function exceeds the threshold of half the number of all measured values ​​1199. The smallest class lower limit above the median is 0.28 A. The class limit-mean ratio is 0.28 A / 0.262 A = 1.07.

[0092] In the example shown for the faulty system of Fig. 7 and Fig. 8 reduces the class width from 0.05 A to 0.02 A, resulting in only 1916 of the 2398 measurement points being entered into the histogram. Measurement points above 0.4 A are not entered.

[0093] The arithmetic mean of the current measurements remains unchanged at 0.215 A. The summation function exceeds the threshold of half the number of all measured values ​​1199 with the class [0.04 ... 0.06] A. The smallest class lower limit above the median is 0.06 A. The class limit-mean ratio is 0.06 A / 0.215 A = 0.028.

[0094] By evaluating the histogram with a truncated histogram, the diagnostic procedure gains greater discriminatory power. The ratio of the values ​​of the class-boundary-to-mean ratio changes by more than a factor of 10. Thus, the class-boundary-to-mean ratio for a fault-free system with a full histogram is 1.14, with a truncated histogram it is 1.07, for a faulty system with a full histogram it is 0.46, and with a truncated histogram it is 0.028. The ratio of faulty to fault-free systems with a full histogram is 0.4032, and with a truncated histogram it is 0.0261.

[0095] The invention is not limited to the embodiments described herein. Rather, the invention encompasses any novel feature and any combination of features, including, in particular, any combination of features in the embodiments and claims. REFERENCE SYMBOL 100 electric motor 101 first control line 102 first coil 103 second control line 104 second coil 105 third control line 106 third coil 107 ammeter 108 first switching element 109 second switching element 110 third switching element 111 fourth switching element 112 fifth switching element 113 sixth switching element 120 continuous frequency distribution 121 cumulative frequency distribution 122 Class 123 Number of measured values 124 Median threshold

Claims

[1] A method for diagnosing faults in an electric motor (100), wherein the electric motor (100) has three control lines (101, 103, 105) which are sequentially activated and deactivated for operation of the electric motor (100), the method comprising the following steps: - receiving a plurality of values of a quantity representative of a current through the control lines (101, 103, 105), the plurality of values being associated with a corresponding plurality of times; - Statistical evaluation of the multitude of values to obtain a statistical quantity; and - Assessing whether there is a fault in the electric motor (100) by means of a predetermined criterion based on the statistical value. [2] Method according to the preceding claim, wherein the plurality of values relate to a quantity representative of the total current through all control lines (101, 103, 105). [3] Method according to one of the preceding claims, wherein a magnetic field in the electric motor (100) is controlled by means of the control lines (101, 103, 105), in particular via switching elements (108-113) arranged in the control lines (101, 103, 105). [4] Method according to one of the preceding claims, wherein the statistical evaluation comprises at least one of establishing a continuous frequency distribution (120), establishing a cumulative frequency distribution (121), averaging, median formation and forming a threshold value which a predetermined percentage of the plurality of values exceeds. [5] Method according to claim 4, wherein the statistical evaluation comprises setting up the continuous frequency distribution (120) and / or the cumulative frequency distribution (121), wherein classes (122) are selected such that a proportion of the plurality of values which is smaller than the predetermined percentage, in particular smaller than fifty percent, but larger than zero percent, in particular larger than five percent, is larger than the largest upper limit of the classes (122). [6] Method according to one of the preceding claims, wherein the statistical quantity is one of the following quantities: a quantity representative of the median (124) of the plurality of values; a quantity representative of a limit value exceeded by a predetermined percentage of the plurality of values; a quantity representative of a ratio of a quantity representative of the median (124) of the plurality of values to the mean of the plurality of values; a quantity representative of a ratio of a quantity representative of the limit value exceeded by the predetermined percentage of the plurality of values to the mean of the plurality of values. [7] Method according to one of the preceding claims, wherein for the assessment of whether the error is present, an error suspicion variable is set which correlates with the criterion. [8] Method according to the preceding claim, wherein at a later time it is checked on the basis of a further criterion whether the error is present if the error suspicion variable indicates a suspicion that the error is present. [9] Method according to claim 7 or 8, wherein the criterion includes comparing the statistical value with a threshold value, in particular with the threshold value and a further threshold value. [10] Method according to the preceding claim, wherein, if the statistical value lies between the threshold value and the further threshold value, the value of the error suspicion variable is not changed. [11] Method according to one of claims 7 to 10, further comprising: Controlling the electric motor (100) taking into account a power limitation if the suspected error variable indicates a suspicion that the error is present. [12] Method according to one of the preceding claims, wherein the fault comprises at least one of a connection break of one of the control lines (101, 103, 105), and a malfunction of one of the switching elements (108-113). [13] Method according to one of the preceding claims, wherein the fault diagnosis is carried out during stationary operation of the electric motor (100). [14] Control device which is arranged to carry out a method according to one of the preceding claims. [15] A computer program comprising instructions which, when executed by a computer, cause the computer to carry out a method according to any one of the preceding claims.

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