Abstract:
A method for estimating a torque of a three-phase motor for a vehicle includes measuring a respective current strength in at least two of three phase lines, wherein the three-phase motor is supplied with power by a converter, and wherein the three phase lines lead from the converter to the three-phase motor of the vehicle, measuring a respective voltage at each of the three phase lines, determining a rotating field frequency as a function of the measured current strengths or the measured voltages; and determining an estimated value for the torque as a function of the measured current strengths, the measured voltages and the determined rotating field frequency.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the priority of European Patent Application, Serial No. EP11173249, filed Aug. 7, 2011, pursuant to 35 U.S.C. 119(a)-(d), the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method for estimating a torque of a three-phase driver motor for a vehicle, which is supplied with power by a converter, with three phase lines leading from the converter to the drive motor. Additionally the present invention relates to a corresponding drive device for a vehicle. 
     The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention. 
     In electrical traction drives (e.g. in the case of an electric car) it must be ensured, in order to safeguard track stability, that the target torque defined by a vehicle controller is largely correctly converted as an actual torque on the wheel. By comparing target and actual values it is possible to infer that the drive is working free from errors and consequently that track stability is being complied with. Determining the torque is thus an important safety feature. The actual torque should therefore be determined using operational equipment (a torque monitor) which is independent of the control branch in which target values are supplied by control electronics. 
     Whereas the target value of the torque is known (directly available from the control algorithms of a vehicle controller), the actual torque can only be directly measured with considerable effort. Standard commercial torque measuring shafts cannot be used in a vehicle for reasons of space, weight and cost. Therefore instead of a torque measurement it is necessary to use estimation or calculation methods based on variables that can be measured more easily. 
     Three-phase motors or alternating current motors are typically used for the electrical drive of vehicles. Such an alternating current motor is normally controlled by an intermediate circuit via a converter (see  FIG. 1 ). The converter is controlled by control electronics or a controller. 
     As long as the converter is pulsing and the controller is working, it is possible to use the actual current values and the target voltage values to calculate the torque. A precondition for this is that the target voltage values are converted tolerably correctly into actual values. 
     The instantaneous power of the converter can be determined from voltages and currents. A suitable filter can be used to obtain the power component which is largely converted into mechanical power in the motor (the stator and rotor losses must be deducted). This power P then corresponds to the torque M multiplied by the angular velocity 2π·n mech  as per the formula: 
     
       
         
           
             M 
             ≈ 
             
               P 
               
                 2 
                 ⁢ 
                 
                   π 
                   · 
                   
                     n 
                     mech 
                   
                 
               
             
           
         
       
     
     The mechanical torque can in principle also be estimated in accordance with the following formula: 
     
       
         
           
             
               M 
               ~ 
             
             ≈ 
             
               
                 P 
                 δ 
               
               
                 2 
                 ⁢ 
                 
                   π 
                   · 
                   f 
                 
               
             
           
         
       
     
     It is simpler to obtain the torque using this second equation, since the rotor power input Pδ can be determined more easily (average active power of the converter less the stator losses). Instead of the mechanical speed n mech  the electrical output frequency f of the converter can be used. This is always present as a target value in the controller. 
     The motor losses (stator and rotors) can likewise be derived from the measured variables using corresponding models. The energy component in the stator inductances of the motor can thus be taken into account. 
     Compensation for an output voltage deviation in the case of a power converter is known from the publication GB 2 440 559 B. In this case an integrated voltage measurement of the three output voltages is performed. This is done with a Σ-Δ AD converter. 
     It would be desirable and advantageous to provide an improved method for estimating the torque of a drive of a vehicle more reliably 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention a method for estimating a torque of a three-phase drive motor for a vehicle, supplied with power by a converter, with three phase lines leading from the converter to the drive motor, includes the steps of measuring a current strength in each case in at least a first and a second of the three phase lines, measuring a voltage in each case at the three phase lines, determining a rotating field frequency from the measured current strengths or the measured voltages and determining an estimated value for the torque from the measured current strengths, the measured voltages and the determined rotating field frequency. 
     According to another aspect of the present invention a drive device for a vehicle with a three-phase drive motor supplied with power by a converter, and three phase lines from the converter to the drive motor, via which the drive motor is supplied with power includes a current measuring unit for measuring a current strength in each case in at least two of the phase lines, a voltage measuring unit for measuring a voltage in each case at the three phase lines, a calculation unit for determining a rotating field frequency from the measured current strengths or the measured voltages and an estimation unit for determining an estimated value for the torque from the measured current strengths, the measured voltages and the determined rotating field frequency. 
     Advantageously the torque of the drive motor is therefore determined not from actual current strengths and target voltages, but from actual current strengths and actual voltages at the converter output. This significantly improves the estimation of torque and estimates are also possible if the converter is locked. 
     In a preferred embodiment the voltages at the three phase lines are measured using a Σ-Δ method. Likewise the currents in the three phase lines can be measured using a Σ-Δ method. Such measurement methods are reliable and can be implemented using simple means. 
     Preferably a power value is calculated for determining the estimated value as a total of a first and of a second partial power value, with the first partial power value corresponding to the product of the current strength in the first phase line and to a first line-to-line voltage between the first phase line and a third of the three phase lines, and with the second partial power value corresponding to the product of the current strength in the second phase line and to a second line-to-line voltage between the second phase line and the third phase line. In this way the power value can be determined from just five values of the three-phase system. 
     The power value obtained from the actual currents and actual voltages represents an instantaneous power and can be low-pass filtered in order to obtain an average electrical active power of the converter. This provides a meaningful basis for estimating the torque. 
     Additionally a power loss model for losses in the drive motor can be provided, with a power loss obtained using the power loss model being subtracted from the average electrical active power of the converter to determine a rotating field power from which the torque is estimated. The power loss model can be used to adjust the losses in the stator and rotor of the motor very realistically, so that the actual output power of the motor can be better estimated. 
     Furthermore, the torque can be estimated even if the converter is locked. This has the advantage that the torque can be estimated even if converter and motor are not in the normal operating mode. 
     Furthermore, the measured voltages can be verified in a switch-on sequence in which the converter is controlled such that the same voltage is applied in all three phase lines, so that the current strengths measured in the phase lines are each zero. This verification ensures that the voltages are measured correctly. 
     Additionally the measured two current strengths can be verified by measuring a third current strength in the third phase line, the total of the current strengths coming to zero. In this way the current strength measurement can also be unambiguously checked. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which: 
         FIG. 1  shows a converter with motor and identification of the measured variables; 
         FIG. 2  shows a signal flow diagram for calculating the torque from two currents and three measured voltages and 
         FIG. 3  shows the processing of the measured variables in a computing unit. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Throughout all the Figures, same or corresponding elements are generally indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the drawings are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted. 
     Turning now to the drawing, and in particular to  FIG. 1 , there is shown a schematic illustration of a drive device for an electrically operated vehicle. The drive apparatus possesses a motor  1 , which is designed as an alternating current motor. The motor  1  is supplied with power by an intermediate circuit  2  which is indicated by an intermediate circuit capacitor C. The intermediate circuit  2  is a direct voltage system with a negative rail  3  and a positive rail  4 . A converter  5  linked to the negative rail  3  and the positive rail  4  converts a direct current from the intermediate circuit  2  into a three-phase current or alternating current for the motor  1 . Three phase lines are laid from the converter  5  to the motor  1  for this purpose: a first phase line  6 , a second phase line  7  and a third phase line  8 . A current i 1  is measured in the first phase line  6 , a current i 2  in the second phase line  7  and a current i 3  in the third phase line  8 . A voltage u 1  is present vis-à-vis the negative rail  3  at the first phase line  6 , a voltage u 2  at the second phase line  7  and a voltage u 3  at the third phase line  8 . 
     Normally the motor currents i 1 , i 2  and i 3  of the motor  1  are measured directly. The currents are at least measured in two phases, and the current in the third phase is calculated from these, as the three currents together come to zero, as long as no isolation errors are present. 
     A method is known from the publication GB 2 440 559 B mentioned in the introduction, to which explicit reference is made here, for how using a Σ-Δ method (sigma-delta method) the output voltages u 1 , u 2  and u 3  of the converter  5  can be measured vis-à-vis the negative rail  3  of the intermediate voltage circuit. Thanks to the Σ-Δ method all measured values are present in digital form as Σ-Δ data flows and can be suitably further processed in a central logic module  9  (e.g. FPGA) (see  FIG. 3 ). 
     Using the instantaneous values of the output voltages u 1 , u 2  and u 3  and the instantaneous values of the output currents i 1 , i 2  and i 3  the torque M emitted by the motor  1  can be calculated in the calculation unit  9 , even if the converter  5  is locked or the target voltages cannot be correctly converted for other reasons (e.g. voltage-time integral errors as a result of dead time influences and forward voltage drops or as a result of reaching the control limit, activating minimum pulse monitoring). In these cases no target voltages are available or the target voltages differ from the actual voltages due to non-linearities. Then it is only possible to estimate the torque meaningfully on the basis of the actual voltages. 
       FIG. 2  shows by way of example based on a signal flow chart the calculation of the instantaneous line from the currents i 1 , i 2  and the line-to-line voltages u 13  and u 23 . In the specific example the currents i 1  and i 2  are thus measured in the first phase line  6  and the second phase line  7 . Likewise the voltages u 1 , u 2  and u 3  are measured at the first phase line  6 , the second phase line  7  and the third phase line  8  and are provided as input variables. The electrical output frequency f can be determined from the currents or voltages and a corresponding angular velocity 2πf can be provided. 
     Using a first subtractor  10  a first line-to-line voltage u 13  is obtained by subtracting the third voltage u 3  from the first voltage u 1 . Similarly, a second line-to-line voltage u 23  is obtained using a second subtractor  11  by subtracting the third voltage u 3  from the second voltage u 2 . The line-to-line voltage u 13  is multiplied by the current i 1  of the first phase line  6  in a multiplier  12 . Likewise the line-to-line voltage u 23  is multiplied in a multiplier  13  by the current  12  in the second phase line  7 . The products of both multipliers  12  and  13  are summed in an adder  14 , and the result represents the instantaneous power that is made available by the converter. Using a downstream low-pass filter  15  the electrical active power of the converter  5  is determined from the instantaneous power. The losses of the electric motor (especially the stator losses) are subtracted from this active power in a subtractor  16 . These losses are provided using a power loss model  17 . The output signal of the subtractor  16  thus corresponds to the actual rotating field power of the motor. Using a divider  18 , with which the rotating field power is divided by the angular velocity 2πf of the converter, an estimated value {tilde over (M)} of the actual mechanical torque M of the motor is finally obtained. 
     The measured variables used for the estimation are thus made available independently of the control branch with which target values for the converter are provided by the control electronics. For safety reasons it should be possible to check the measured variables for plausibility. The comparison with the target torque can then be effected both in the torque monitor and in the control unit, and a two-channel switch-off into safe mode can be initiated. 
     The voltage measurements can be checked in a switch-on sequence. In this case the converter can for example be controlled by the control electronics with a null pointer (000, 111), so that no line-to-line voltages u 13 , u 23  are generated. In this case the null pointer 000 means for example that all phase lines  6 ,  7 ,  8  are short-circuited with the negative rail  3 , whereas in the case of the null pointer 111 all phase lines  6 ,  7  and  8  are connected to the positive rail  4 . If no line-to-line voltages are now generated, no current will flow either, but the voltage measurement channels are controlled. It must be possible to identify this at the voltage measurement elements. 
     It should also be possible to verify the measurement of the currents. This is simple to do, in that all three currents i 1 , i 2  and i 3  are measured. Only in error-free operation is the summated current zero. Otherwise a measurement or isolation error is present. 
     The rotating field frequency (frequency f of the rotating field) is also simple to check. The rotating field frequency can in fact be calculated both from the measured currents i 1 , i 2 , i 3  as well as from the measured voltages u 1 , u 2 , u 3 , in that for example the temporal derivation of the angle is determined from the respective space vectors of the measured variables. 
     An excessive deviation identified using the method outlined above between target and actual value or the overshoot of a maximum limit for the estimated torque value can be used in the calculation unit  9  for initiating further protection measures. A corresponding signal s to trigger a protection reaction can be provided by the calculation unit  9  (see  FIG. 3 ). For example, a permanent-magnet-excited synchro-machine can be short-circuited in field weakening mode to achieve a torque-free wheel, or can be isolated from the converter. 
     Advantageously in accordance with the above principle a Σ-Δ output voltage measurement can thus be used to calculate the developed actual torque. The comparison to be performed very quickly in the calculation unit  9  (e.g. logic module FPGA) with the target torque or with limit values that can be fixed can be used to respond very quickly to errors. This method works without software, resulting in a fast and reliable response, even if the software or controller exhibits a malfunction. Determining speed via the electric frequency also makes it independent of the rotary transducer. In this way a very high level of safety can be achieved. 
     While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.