Abstract:
Method for controlling an electrical machine ( 14 ), in particular for an electric or hybrid drive train ( 12 ) of a motor vehicle ( 10 ), wherein the electrical machine ( 14 ) can provide a torque (T) both at positive and at negative speeds (n), having the steps of: using a characteristic curve (M; G) to check whether a torque desired value ( 72; 74 ) can or may be provided by the electrical machine ( 14 ), wherein the characteristic curve (M; G) represents a limit torque with respect to the speed (n) and has a first section (A 1;  A 4 ) for positive speeds and a second section (A 2;  A 3 ) for negative speeds. In this case, the first section (A 1;  A 4 ) and the second section (A 2;  A 3 ) are arranged in the same quadrant (I; IV) of the characteristic curve ( 70 ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a method for controlling an electric machine, in particular for an electric or hybrid drive train of a motor vehicle, wherein the electric machine can provide a torque both at positive and at negative rotation speeds, comprising the step of performing a check with the aid of a characteristic curve as to whether a desired torque value can or may be provided by the electric machine, wherein the characteristic curve represents a limit torque with respect to the rotation speed and has a first section for positive rotation speeds and a second section for negative rotation speeds. 
         [0002]    Moreover, the present invention relates to a control device for an electric machine, wherein the control device is designed so as to perform a method of this type. Finally, the present invention relates to a drive train for a motor vehicle having an electric machine and a control device of this type. 
         [0003]    It is generally known in the field of motor vehicle drive technology to use an electric machine as a stand-alone drive or in combination with a drive motor of a different type (hybrid drive). In electric or hybrid motor vehicles of this type, typically electric induction machines are used as drive motors. An electronic power system is used in order to control induction machines of this type in a motor vehicle, said electronic power system comprising an AC converter that converts into AC current the DC voltage/DC current of a (high voltage) battery that is located on board the motor vehicle. The AC converter comprises generally a plurality of controllable power switches. The power switches are controlled in a pulse-width modulated manner by means of a control device, so that during the motor mode operation the electric machine generates a particular torque at a particular rotation speed of an output shaft of the electric machine. 
         [0004]    When controlling the electric machine, it is necessary to ascertain limit torques in order to calculate the torque strategy, said limit torques providing information regarding the performance of the electric drive. 
         [0005]    The electric machine is generally able to provide a torque both at positive and also at negative rotation speeds. This means that during the motor mode operation, the electric machine can also be used by way of example for moving the motor vehicle in the reverse direction. In some cases, the electric machine is also designed so as to operate as a generator. Insofar as this is also possible at positive and negative rotation speeds, this is referred to as a so-called 4-quadrant operation. 
         [0006]    In order to guarantee the reliability and availability of the electric machine and/or of the drive train, it is expedient to prevent problems arising when calculating the limit torques. One problem resides in the fact that the motor mode limit torque and the generator mode limit torque are calculated at the zero crossing point of the rotation speed (any change between positive and negative rotation speed). Torque surges occur at the said zero crossing point because for example the generator mode limit torque at negative rotation speeds has a positive value and at positive rotation speeds said limit torque has a negative value. In a corresponding manner, the motor mode limit torque at negative rotation speeds has a negative value and at positive rotation speeds said limit torque has a positive value. The surge at these limits can cause problems within the torque strategy as the rotation speed changes from negative to positive, the reason being that when the rotation speed is zero it is not possible to differentiate between the motor mode operation and the generator mode operation. 
         [0007]    On the basis of this background, it is the object of the invention to provide an improved method for controlling an electric machine, to provide an improved control device and also an improved drive train, wherein in particular the reliability and availability are improved. 
       SUMMARY OF THE INVENTION 
       [0008]    The above-mentioned object is achieved by virtue of a method for controlling an electric machine, in particular for an electric or hybrid drive train of a motor vehicle, wherein the electric machine can provide a torque both at positive rotation speeds and also at negative rotation speeds, which method comprises the steps:
       performing a check with the aid of a characteristic curve as to whether a desired torque value can or may be provided by the electric machine, wherein the characteristic curve represents a limit torque with respect to the rotation speed and said characteristic curve comprises a first section for positive rotation speeds and a second section for negative rotation speeds, wherein the first section and the second section are arranged in the same quadrant of the characteristic curve.       
 
         [0010]    Moreover, the above-mentioned object is achieved by virtue of a control device for an electric machine, wherein the control device is designed and/or embodied so as to perform the above-mentioned method. 
         [0011]    Finally, the above-mentioned object is achieved by virtue of a drive train for a motor vehicle having an electric machine and a control device of the above-mentioned type. 
         [0012]    The electric machine can be designed either exclusively as a motor or exclusively as a generator. The provision of a torque at positive and negative rotation speeds consequently means in the present context that the electric machine provides this torque when operating as a motor and consumes said torque when operating as a generator. In a corresponding manner, the characteristic curve can represent a motor mode limit torque when the electric machine is operating as a motor and when the electric machine is operating as a generator said characteristic curve can represent a generator mode limit torque. The motor mode limit torque is arranged in the quadrants I, III in the case of a test method in accordance with the prior art and the generator mode limit torque is arranged in the quadrants II, IV in the prior art. 
         [0013]    It is now possible in accordance with the invention to also arrange the section for negative rotation speeds in the quadrant I, said section in the prior art being arranged in the quadrant III, or vice versa. In a corresponding manner, it is possible when the electric machine is operating as a generator to arrange the section of the quadrant II in accordance with the invention in the quadrant IV, or vice versa. 
         [0014]    Consequently, it is possible in accordance with the invention to perform a check as to whether a desired torque value can be provided irrespective of the algebraic sign of the rotation speed. 
         [0015]    This is extremely important particularly in the case of predicting torque limits and origin point calculations. Owing to the fact that the calculation is not dependent upon the rotation speed and/or the algebraic sign of the rotation speed, it is possible to predict these values so that a greater magnitude of availability of the electric machine and/or of the drive train is achieved. Inconsistencies in the zero point can be avoided in this manner. 
         [0016]    If by way of example, the torque limits and/or torque predictions are calculated at the zero crossing point, it is possible in the prior art to misinterpret a limit. By way of example, it is possible in the prior art for the generator mode limit torque to jump from the value in the quadrant II to the value in the quadrant IV, wherein, however, the rotation speed still suggests the quadrant II. As a consequence, the generator mode limit torque would be drawn into the quadrant III, which would mean that only the motor mode torque would be possible. In these cases, the vehicle control would then react incorrectly (for example starting the motor by mistake, although this is not allowed, or charging the battery even though the battery is already fully charged). 
         [0017]    It is particularly preferred if a section of the first section and of the second section is mirror-imaged by point mirror-imaging with respect to the origin of the characteristic curve in the quadrants of the other section. 
         [0018]    As a consequence, it is possible in a simple and effective manner to arrange the first section and the second section in the same quadrant of the characteristic curve. 
         [0019]    Moreover, it is advantageous if the torque is a motor mode torque and if the section for negative rotation speeds is arranged in the quadrant for positive rotation speeds. 
         [0020]    In the case of this embodiment, the algebraic sign of the motor mode torque and in particular the algebraic sign of the rotation speed is consequently always positive in the checking step. 
         [0021]    In a corresponding manner, it is advantageous if the torque is a generator mode torque and if the section for negative rotation speeds is arranged in the quadrant for positive rotation speeds. 
         [0022]    In the case of this embodiment, the algebraic sign of the generator mode torque is consequently always negative in the checking step and in particular the algebraic sign of the rotation speed is always positive. 
         [0023]    Overall it is further preferred if the rotation speed is subjected where necessary to an algebraic sign change prior to the checking step. 
         [0024]    As a consequence, it is ensured that by way of example in the case of a negative rotation speed of the electric machine it is possible to perform a check in the characteristic curve in the region of positive rotation speeds. 
         [0025]    It is also advantageous if, during the procedure of starting up the electric machine from the rotation speed zero, initially the particular section of the characteristic curve that has a lower limit torque in the start-up range is used in the checking procedure. 
         [0026]    In general, the first section and the second section that are arranged in the same quadrant of the characteristic curve can be different (since by way of example during the forwards travel of the motor vehicle a greater magnitude of torque may be provided than during the reverse travel or similar of the motor vehicle). During the start-up procedure from the rotation speed zero, generally initially the particular section of the characteristic curve that has a lower limit torque in the start-up range is then used in the checking procedure irrespective of the algebraic sign of the actual rotation speed. 
         [0027]    The reliability can be increased as a consequence. 
         [0028]    It is preferred that the limit torque is set in dependence upon one or several of the following parameters, including battery current limits, battery voltage limits, temperature of an AC converter, rotation speed, temperature of the electric machine and also replacement diagnostic functions. 
         [0029]    The limit torque is consequently included in the torque strategy of the vehicle control. If, for example, the torque prediction indicates that in x seconds it will no longer be possible to provide current for the motor, the combustion engine can be started by way of precaution so that the battery can be charged, so that finally power is once again available for supporting the motor mode operation. This type of torque prediction is also directly associated with the driving strategy. 
         [0030]    Moreover, the same parameters are also important for the torque prediction, wherein said parameters are however predicted parameters. In addition, the torque prediction can be supplemented by way of additional parameters such as the predicted temperature, battery currents, degree of efficiency. 
         [0031]    It goes without saying that the above-mentioned features and features yet to be explained hereinunder can be used not only in the respective mentioned combination but rather also in other combinations or as stand-alone features without departing from the scope of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    Exemplary embodiments of the invention are illustrated in the drawing and are further explained in the description hereinunder. 
           [0033]      FIG. 1  illustrates schematically a drive train of a motor vehicle having an electric machine; 
           [0034]      FIG. 2  illustrates schematically a block diagram of a part of a control device for the electric machine; 
           [0035]      FIG. 3  illustrates schematically a graph with a limit torque characteristic curve; and 
           [0036]      FIG. 4  illustrates schematically a part of a flow chart of the method in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]      FIG. 1  illustrates schematically a motor vehicle that is described in general by the numeral  10 . The motor vehicle  10  comprises a drive train  12  that in the present case comprises an electric induction machine  14  for providing the driving power. The drive train  12  is used to drive driven wheels  16 L,  16 R of the vehicle  10 . 
         [0038]    The electric induction machine  14  provides a torque t to an output shaft and rotates at an adjustable rotation speed. 
         [0039]    The drive train  12  can be designed so as to drive the vehicle  10  solely by means of the electric induction machine  14  (electric vehicle). Alternatively, the electric induction machine  14  can be part of a hybrid drive train  12 , wherein the drive train  12  can comprise an additional drive motor, not illustrated in detail in  FIG. 1 , such as a combustion engine or the like. Moreover, the drive train  12  can in this case be a gear mechanism and similar. 
         [0040]    The electric induction machine  14  is embodied in a multi-phase manner (in the present case with three phases) and is controlled by means of an electronic power system  18 . The electronic power system  18  is connected to an energy supply such as a DC voltage supply (e.g. rechargeable battery)  20  of the vehicle  10  and is used for the purpose of converting DC voltage that is provided by the rechargeable battery  20  into three AC currents for the three phases of the electric induction machine. The electronic power system  18  comprises for this purpose a plurality of power switches and is controlled by means of a control device  22 . The control device  22  can moreover be connected to the electric induction machine  14  in order to obtain by way of example actual values of the rotation speed and/or the angular position of a rotor of the electric induction machine  14 . Moreover, the control device  22  can be connected to the rechargeable battery  20 . Finally, the control device  22  can be connected to a higher-priority or equal-priority control device  24  for the purpose of controlling other components in the drive train  12 . 
         [0041]      FIG. 2  illustrates a block diagram of a part of the control device  22  for controlling the electric machine  14 . 
         [0042]    The control device  22  comprises a higher-priority controller  30  that receives a desired torque  32  that is predetermined externally. The higher-priority controller  30  uses this to generate an internal desired torque  34  that is supplied by way of example to a closed loop control (that operates by way of example by means of a d-q controller). Moreover, the higher-priority controller  30  receives at  36  a desired mode that determines the type of control that is to be performed. The type of control that is to be performed can be a torque control, a rotation speed control, a voltage control etc. If necessary, it is possible that a control type is not predetermined. In the latter case, the electronic power system does not perform any control, so that particularly the motor mode torque limit and generator mode torque limit are also not calculated and consequently particularly the torque is also not predicted. 
         [0043]    Moreover, the control device  22  comprises a block and/or a module  40  for calculating torque limits. The torque limit calculating module  40  calculates these torque limits with reference to prevailing parameters including battery current limits  42 , battery voltage limits  44 , a temperature  46  of the AC converter  18 , the rotation speed n,  48 , a temperature  50  of the electric machine  14  and/or with reference to replacement diagnostic functions  52 . 
         [0044]    The torque limit calculating module  40  uses said torque limits to determine a prevailing maximum torque limit  54  and if necessary a minimum torque limit  56 . In particular, the calculating module comprises means that mirror-image a section of the first and the second section of the maximum torque limits  54  or rather of the minimum torque limit  56  by means of point mirror-imaging with respect to the origin of the characteristic curve in the quadrants of the other section. As a function of the quadrant in which the control process is currently taking place, either a motor mode or a generator mode limit torque is mapped out on a minimum torque limit  56  of this type; it is exactly the opposite in the case of the maximum torque limit  54 . The two limits  54 ,  56  represent a control range in which the electronic power system  18  can operate. The torque limits  54 ,  56  are also involved in predicting the torques. The torque limits  54 ,  56  are input into the higher-priority controller  30 . The desired torque  32  that is predetermined externally and input into the higher-priority controller  30  is compared with at least the maximum torque limit  54 . Insofar as the desired torque  32  that is predetermined externally is lower than the maximum torque limit  54 , the desired torque  32  that is predetermined externally is output as an internal desired torque  34 . On the other hand, in the event that the desired torque  32  that is predetermined externally is greater than the maximum torque limit  54 , the internal desired torque  34  is reduced to the corresponding value of the maximum torque limit  54 . 
         [0045]    Moreover, the torque limits  54 ,  56  are input into a module  60  for predicting the torque. Moreover, this torque prediction module  60  receives the prevailing rotation speed n,  48  and also the battery prediction values  64 . The values that are determined in the torque prediction module  60  are input into the torque strategy of the vehicle control. 
         [0046]      FIG. 3  illustrates a graph  70  that plots the torque T over the rotation speed n. The graph  70  comprises four quadrants I to IV. Moreover, a characteristic curve M for a motor mode limit torque and a characteristic curve G for a generator mode limit torque are plotted in the diagram  70 . 
         [0047]    The motor operation is normally performed in the quadrants I, III, whereas the generator operation is imaged by the quadrants II, IV. As a result, the characteristic curve M for positive rotation speeds n comprises a section A 1  in the quadrant I. Moreover, in the prior art, the characteristic M comprises a section A 2 ′ for negative rotation speeds in the quadrant III. In this case, the section A 2 ′ is mirror-imaged by means of point mirror-imaging (as is indicated by the arrows) to A 2  in the first quadrant I. As a result, the characteristic curve M for negative rotation speeds comprises a section A 2  in the first quadrant I. 
         [0048]    The generator characteristic curve G comprises a section A 4  for positive rotation speeds in the fourth quadrant. Moreover, in the prior art, the generator characteristic curve G comprises a section A 3 ′ in the second quadrant. In this case, the section A 3 ′ is mirror-imaged by means of point mirror-imaging to A 3  in the fourth quadrant IV. Since the sections A 4 , A 3 ′ are point symmetric, the generator characteristic curve G in this case comprises a section A 4  for positive rotation speeds and also a section A 3  for negative rotation speeds that are however identical in this case. 
         [0049]    A first exemplary desired torque value that is predetermined externally is plotted at  72  in  FIG. 3 . The section A 1  of the motor characteristic curve M can be used in the event that the electric machine is being operated in the motor mode at positive rotation speeds. The desired torque value  72  is below the limit torque that applies for the respective rotation speed so that the desired torque value  72  that is predetermined externally can be directed onwards as the internal desired torque  34 . 
         [0050]    On the other hand,  74  illustrates a desired torque value that is greater than the limit torque, so that a value  74 ′ corresponding to the limit torque that is allocated to the respective rotation speed is output as the internal desired torque. 
         [0051]      FIG. 4  illustrates a block diagram  80  of a routine that is performed prior to performing the check with the aid of the respective characteristic curve M, G. In a query routine  82 , a check is initially performed as to whether the rotation speed n≧0. If this is the case, the algebraic sign of the rotation speed is confirmed in block  84 . If the result of the query in block  82  is negative, the rotation speed is multiplied by (−1), and in fact in block  86 , so that the rotation speed is positive. As a consequence, a comparison is performed with the torque limit in the quadrant I, IV for positive rotation speeds n and in fact within the scope of a check routine  88  that is indicated in  FIG. 4 .