Abstract:
The invention relates to a method ( 40 ) for monitoring a torque of an electric motor ( 10 ), in particular for application in a motor vehicle, wherein the electric motor ( 10 ) is supplied with electric current (IS 1,  IS 2,  IS 3 ) in multiple phases. A first torque value (M 2 ) of the torque generated by the electric motor ( 10 ) is determined ( 58 ) on the basis of the electric output of the electric motor ( 10 ) and a detected rotational speed (n) of a rotor of the electric motor ( 10 ). A second torque value (M 1 ) is determined on the basis of at least one measured phase current (IS 1,  IS 2,  IS 3 ) and a rotational position of the rotor, and the two determined torque values (M 1,  M 2 ) are compared ( 60 ) to one another for plausibility checking.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a method for monitoring a torque of an electric motor, in particular for application in a motor vehicle, wherein the electric motor is supplied with electric current in multiple phases, wherein a first torque value of the torque which is output by the electric motor is determined on the basis of an electric power which is taken up by the electric motor and a detected rotational speed of the rotor of the electric motor. 
         [0002]    According to a second aspect the present invention relates to a method for monitoring a torque of an electric motor, in particular for application in a motor vehicle, wherein the electric motor is supplied with electric current in multiple phases, wherein below a predefined rotational speed of a rotor of the electric motor a first torque value of the torque which is output by the electric motor is determined on the basis of an amplitude of at least one phase current. 
         [0003]    The present invention also relates to a device for monitoring a torque of an electric motor with current-detecting means for detecting at least one phase current of the electric motor, rotor-detecting means for detecting a rotational position and/or a rotational speed of a rotor of the electric motor, and a control unit which is designed to carry out the method specified at the beginning 
         [0004]    Finally, the present invention relates to a motor vehicle drive train having at least one electric machine for making available drive power and a device of the type specified above. 
         [0005]    In the field of electric motor technology, it is generally known to check the state of an electric motor and to determine an output variable, such as for example the torque which is output by the electric motor, and compare it with a setpoint state. By comparing the actual state with the setpoint state it is possible here to monitor the operation of the electric machine in order to react correspondingly in the event of deviation of the actual state from the setpoint state. 
         [0006]    The torque which is output by the electric motor is usually calculated on the basis of an electric power which is extracted from a direct voltage source and a rotational speed of the rotor of the electric motor. In this context, the electric power which is extracted from the direct voltage source is calculated by means of the electrical voltage made available and an electric current which is detected by means of a direct current sensor. 
         [0007]    DE 10 2008 001 714 A1 discloses a method in which the electric power which is taken up by the electric motor or a cross current which is the cause for the torque is detected and linked to a characteristic curve of a rotational speed of the electric motor in order to determine the present torque and compare it with a setpoint value. 
         [0008]    A disadvantage with the methods known from the prior art is that due to the direct current sensor for determining the taken-up electric power the method is highly technically complex and measuring errors cannot be detected or taken into account during the determination of the torque which is output. 
         [0009]    The object of the present invention is therefore to make available a method and a device for monitoring a torque of an electric machine which provides increased reliability with low technical complexity. 
       SUMMARY OF THE INVENTION 
       [0010]    This object is achieved according to the invention with the method specified at the beginning by virtue of the fact that a second torque value is determined on the basis of at least one measured phase current and a rotational position of the rotor, and the two determined torque values are compared with one another for the purpose of plausibility checking 
         [0011]    This object is also achieved by means of the device which is specified at the beginning and which is designed to carry out the method according to the invention. 
         [0012]    Finally, the above object is achieved by means of a motor vehicle drive train having an electric machine for making available drive power and a device of the type mentioned at the beginning for monitoring the torque of the electric machine. 
         [0013]    By means of the present invention, it is possible to make available a method which provides a high level of reliability during the monitoring of the torque which is output, by virtue of two redundant calculation paths. This increased reliability is ensured by virtue of the fact that a simple fault, for example in a rotor position signal or a rotor speed signal, does not lead to a systematic torque fault since a second independent calculation path for the torque is made available and such a fault can be detected by virtue of the plausibility checking of the two independent paths. In particular, the method is particularly precise for different rotational speed ranges by virtue of the fact that the torque value is determined at low rotational speeds on the basis of a phase current amplitude. 
         [0014]    It is particularly preferred if above a predefined rotational speed the taken-up power is determined on the basis of the measured phase currents and measured phase voltages. 
         [0015]    As a result, the taken-up power can be measured and determined easily and precisely at the electric machine. 
         [0016]    It is also preferred if the power loss of the electric motor is determined and is taken into account during the determination of the taken-up power. 
         [0017]    As a result, the torque of the electric machine can be calculated even more precisely on one of the paths. 
         [0018]    It is also preferred if a difference between the torque values is determined and a fault signal is generated and/or a fault reaction is initiated if the difference exceeds a predefined value. 
         [0019]    This provides a simple possible way of plausibility checking which can be carried out with low technical complexity. 
         [0020]    It is also preferred if a torque result is determined by forming mean values of the two torque values if the difference between the two torque values does not exceed a predefined value. 
         [0021]    As a result, when there are small deviations between the determined values for the torque a result can be obtained which has only very small deviations from the torque which is actually made available. 
         [0022]    It is preferred here if the torque result is compared with an actual torque of the electric machine, which actual torque is calculated on the basis of a model of the electric machine. 
         [0023]    As a result, further plausibility checking can be carried out on the basis of a model which can take into account different parameters and measurement values of the electric machine, as a result of which increased reliability can be achieved by means of the additional plausibility checking 
         [0024]    It is also advantageous here if the model of the electric machine takes into account a temperature of the rotor. 
         [0025]    As a result, the actual torque which is obtained can be determined even more precisely on the basis of the model since the temperature of the rotor influences the torque which is output. 
         [0026]    It is particularly preferred here if the actual torque is compared with at least one predefined or determined limiting value for the torque of the electric machine. 
         [0027]    In this way, predetermined critical states of the electric machine can be prevented, which increases the reliability in general. 
         [0028]    It is also generally preferred if the electric current is detected in all the phase trains of the electric machine by means of one phase current sensor in each case. 
         [0029]    As a result, the precision of the determination of the torque and as a result the reliability can be increased further since the independent phase current sensors form further redundance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    Of course, the features, properties and advantages of the method according to the invention also apply or can be applied correspondingly to the device according to the invention. 
           [0031]      FIG. 1  shows the design of an electric machine in schematic form; 
           [0032]      FIG. 2  shows, in schematic form, the sequence of the method according to the invention for determining a torque which is output; and 
           [0033]      FIG. 3  shows, in schematic form, the sequence of a means for monitoring the torque of the electric motor. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    In  FIG. 1 , the design of an electric machine is shown in schematic form. The electric machine is denoted by  10  generally in  FIG. 1 . 
         [0035]    The electric machine  10  has three phase current conductors  12 ,  14 ,  16  in which three phase currents IS 1 , IS 2  and IS 3  flow. The phase current conductors  12 ,  14 ,  16  are connected to respective exciter windings  18  of the electric machine  10 . The exciter windings  18  are connected to one another in a star shape, wherein a zero conductor is preferably not provided. 
         [0036]    The exciter windings  18  are denoted in  FIG. 1  by phase V, phase U and phase W. The phase current conductors  12 ,  14 ,  16  are assigned sensors  20 ,  22 ,  24  which measure the phase currents IS 1 , IS 2  and IS 3  and each supply a corresponding current signal  26 . The sensors  20 ,  22 ,  24  are embodied as ammeters. Alternatively, line voltages in the phase current conductors  12 ,  14 ,  16  for performing open-loop or closed-loop control can be measured by means of volt meters. 
         [0037]    The sensors  20 ,  22 ,  24  are connected to a control unit (not illustrated), wherein the measured phase currents IS 1 , IS 2  and IS 3  are fed back to the control unit. The control unit supplies, on the basis of the current signals  26 , a control signal to power electronics (not illustrated) in order to perform open-loop or closed-loop control of the electric machine  10 . The phase currents IS 1 , IS 2  and IS 3  are preferably phase-shifted through 120° with respect to one another. 
         [0038]      FIG. 1  also illustrates line voltages U 12 , U 23  and U 31  which drop between the phase current conductors  12 ,  14 ,  16 . The line voltages U 12 , U 23 , U 31  are detected by means of voltage sensors  28 ,  30 ,  32  which are arranged between the phase current conductors. The torque which is output by the electric machine  10  can be calculated on the basis of the phase currents IS 1 , IS 2  and IS 3  and the line voltages U 12 , U 23  and U 31  and a rotor position and/or a rotor speed of the rotor (not illustrated in  FIG. 1 ) of the electric machine  10 . In order to perform more precise closed-loop or open-loop control of the torque which is output it is necessary to know the phase currents IS 1 , IS 2  and IS 3 , the phase voltages U 12 , U 23 , U 31 , the rotor position and the rotor speed. In this context, these variables can be detected by means of the sensors  20 ,  22 ,  24 ,  28 ,  30 ,  32  and the torque can be determined. 
         [0039]      FIG. 2  is a schematic illustration of the sequence for determining the torque which is output by the electric machine  10 , and said sequence is denoted generally by  40 . 
         [0040]    The electric machine  10  and components assigned thereto are indicated schematically in  FIG. 2  and denoted by  42 . In order to determine the torque of the electric machine  10 , the three phase currents IS 1 , IS 2 , IS 3  are measured by means of the sensors  20 ,  22 ,  24 , as is shown at  44 . In addition, the position of the rotor of the electric machine  10  is detected by means of a rotor sensor, as shown at  46 . The measured phase currents IS 1 , IS 2 , IS 3  and the measured rotor position are used in a first calculation path to calculate the torque of the electric machine  10 , as shown at  48 . A torque M 1  is determined by calculating the first path  48 , and said torque M 1  is made available for plausibility checking 
         [0041]    The torque value M 1  is calculated here from the three phase currents IS 1 , IS 2 , IS 3  and the rotor position according to the formula 
         [0000]        M 1 =I _AMP* A (phi) 
         [0000]    where I_AMP is the amplitude of the phase current vector, and A(phi) is a factor which is dependent on the rotor position. In order to achieve a high level of integrated reliability, all three phases U, V, W are each detected with a separate phase current sensor  20 ,  22 ,  24 . Furthermore, by checking the sum of all the phase currents it is possible to detect a simple fault at one of the three sensors. The torque value M 1  which is calculated in this way is made available for further checking, as illustrated in  FIG. 2 . 
         [0042]    In addition, the phase voltages U 12 , U 23 , U 31  are detected by means of the voltage sensors  28 ,  30 ,  32 , as is shown at  50 . Furthermore, the rotor speed is detected, as is shown at  52 . Finally, the actuation of the electric machine is detected by the power inverter, as is shown at  54 , and the direct voltage of the voltage source is detected, as is shown at  56 . The detected direct voltage is fed back to the step  50  for the determination of the phase voltages. In a second calculation path, a torque value M 2  is then calculated on the basis of the phase voltages U 12 , U 23 , U 31 , the rotor speed and, in particular, the actuation by the power inverter, as is shown at  58 . 
         [0043]    The torque value M 2  is calculated at  58  by means of the formula 
         [0000]        M 2=( U* 1− PV )/ n  
 
         [0000]    where U is the phase voltage, I is the phase current, PV is the power loss of the electric machine  10  and n is the rotational speed of the rotor. If the rotational speed n is sufficiently high, that is to say above a predefined value, the power which is output is calculated by the product of the phase currents IS 1 , IS 2 , IS 3  and the phase voltages U 12 , U 23 , U 31 . If the rotational speed n is low, that is to say lower than a predetermined value, the calculation of the torque value M 2  by means of the product of the phase currents IS 1 , IS 2 , IS 3  and phase voltages U 12 , U 23 , U 31  is imprecise. In that case, the torque value M 2  is calculated with the formula 
         [0000]        M 2= I _AMP* B    
         [0000]    where I_AMP is the amplitude of the phase current vector and B is a factor which does not take into account the position of the rotor. 
         [0044]    For very low rotational speeds n close to zero and equal to zero, the torque M 2  cannot be calculated since the difference between the output voltage and the loss voltage is always equal to zero independently of the engine torque. 
         [0045]    The torque values M 1  and M 2  are compared with one another at  60  or subjected to plausibility checking The plausibility checking is usually carried out by forming the difference between the torque values M 1  and M 2 . If the difference exceeds a predefined value, a fault signal is generated and/or a fault reaction is initiated. This fault reaction may be, for example, that the output stages or the power inverter which actuates the electric machine  10  are switched to a safe state. If the difference is smaller than a predefined value, a mean value is calculated from the torque values M 1  and M 2  and the torque result ML 1  which is obtained in this way is made available for further processing. 
         [0046]    The calculation paths  48 ,  58  for calculating the values M 1  and M 2  are based on simple calculations and are therefore very robust and reliable. In this context, many influencing variables of the actual torque, such as for example the rotor temperature, are ignored. 
         [0047]    However, as a result of the comparison of the two torque values M 1  and M 2  which are determined by means of different paths a high degree of reliability for the determination of the torque result ML 1  is provided. 
         [0048]      FIG. 3  is a schematic illustration of a flow chart for monitoring the torque of the electric machine  10 , said flow chart being generally denoted by  70 . The averaged torque result ML 1 , which was determined at  60  in  FIG. 2 , is compared with an actual torque signal ML 2  at  72  and/or subjected to plausibility checking The actual torque signal ML 2  is calculated by means of a machine model, as is shown at  74 . The machine model takes into account all the possible influencing variables of the actual torque, such as for example the rotor temperature. The actual torque signal ML 2  which is acquired in this way supplies a very precise value of the torque which is actually output. The machine model provides a very high level of accuracy, but is not protected by complex security measures such as, for example, cyclical RAM/ROM tests, sequence monitoring, double storage of the variables etc. Provided that the plausibility checking  72  has a positive result, that is to say a deviation between the torque result ML 1  and the actual torque signal ML 2  is small, that is to say is lower than a predefined value, the actual torque signal ML 2  is used for the further calculation, and is made available as an actual torque MP to a further check  78 . In order to check the torque which is output, permissible torque limits are calculated at  76 . These torque limits are preferably a permissible maximum and a permissible minimum of the torque which is output. These torque limits are made available to a torque comparator  78  which checks whether the actual torque MP is within the permissible limits which are determined at  76 . If the actual torque MP is within the calculated limits or outside the calculated limits, a corresponding output signal  80  is made available by the torque comparator  78 . If the torque MP is outside the calculated limits, for example the electric machine  10  can be stopped or else the power can be correspondingly reduced.