Abstract:
A method and arrangement for determining the position of the drive mechanism of an electric machine from the current supplied thereto. Determination of the position is effected over two independent channels by measuring the three-phase current of the electric machine or motor, converting the measured values to the current space vector, calculating the angle of the current space vector within one electrical revolution, and determining the position of the motor. A current command, the field of which acts in the direction of the flux or field of the drive mechanism or rotor, is added within the motor stator.

Description:
The instant application should be granted the priority dates of Jul. 3, 2008, the filing date of the corresponding German patent application 10 2008 031 201.0, as well as Jul. 3, 2009, the filing date of the International patent application PCT/EP2009/004845. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method for determining the position of the drive mechanism of an electric machine, such as a motor, from the current that is supplied, whereby for the reliable operation, the determination of the position of the drive mechanism is effected over two paths or channels that are independent of one another, and thereafter the two position values are combined and are cross checked with respect to a safe manner of operation, such as maintenance of a safe speed or safe positioning of the drive mechanism, including the following method steps:
         determination of the position of the drive mechanism in one channel by measuring the three-phase current of the machine, subsequently   converting the measured values of this three-phase current to a current space vector   calculating the angle of the current space vector within one electrical revolution, and   determining the position of the drive mechanism (rotor).       

     Such a method is already known from DE 101 63 010 B4. 
     The known method has the drawback that at low speeds and/or low loads, the amplitude or length of the current space vector is about 0, and due to the fluctuation of the measured values in all possible directions due to the super imposed speed and position regulation, which adversely affects the measurement of the position and later the calculation e.g. of the speed due to the position alteration, the measurements show large errors with regard to position and speed. 
     It is an object of the present invention to ensure that the direction of the current space vector can be determined precisely, even at low speeds and low loads. 
     SUMMARY OF THE INVENTION 
     This object is realized in that a current command, the field of which acts in the direction of the rotor flux or rotor field, is added within the stator. 
     The result of proceeding in this manner is that no fluctuation of the measured value occurs any longer about the 0 point, and a considerably greater precision of the measurement is achieved, since a sufficiently large current always flows. 
     Pursuant to a further embodiment of the method of the present invention, the electric machine is a servomotor having permanent magnetic generation of the magnetic field of the rotor, while the winding or coil through which the current flows is disposed in the stator. 
     Pursuant to yet another embodiment of the method, the angle of the current space vector serves for the determination of the position of the rotor of the servomotor. 
     Pursuant to yet another embodiment of the inventive method, the angle of the current space vector serves for the determination of the change in position over time and hence of the speed of the rotor of the servomotor or a reliable monitoring of a position. 
     A current command is then added in the direction of the flux/field within the rotor when the current that produces the torque is low in comparison to the rated current (can also be effected permanently). 
     Pursuant to a further development of the method of the invention, a current command is then added in the direction of the flux/field within the rotor if the velocity of the rotor (speed) is low in comparison to the rated speed. 
     On the other hand, a current command can be added in the direction of the flux/field within the rotor if the current space vector moves about the value zero (but can also be permanently added). 
     It is also possible to add a current command in the direction of the flux/field within the rotor only if due to the regulation of the servo loops, the current space vector jumps back and forth (can however be added permanently). 
     The present invention also relates to an apparatus or arrangement for the improved measurement of the position of the drive mechanism of an electric machine, such as a motor, from the current supplied, whereby for the reliable operation the measurement, e.g. of the position of a drive mechanism, is effected over two paths or channels that are independent of one another, and thereafter the two measured values are combined and are cross checked with respect to a reliable manner of operation, such as maintenance of a safe speed of the drive mechanism, including
         a processor for the determination of the position of the drive mechanism in one of the channels, with a sensor for the measurement of the three-phase current of the machine, and following the sensor   a converter for the conversion of the measured values of this three-phase current to a current space vector, and following the converter   a computer or calculating unit for the calculation of the angle of the current space vector within one electrical revolution, and   for the determination of the position of the drive mechanism (rotor),
 
whereby inventively
   a current command, the field of which acts in the direction of the rotor flux or rotor field, is added within the stator (direction of weakening of the field), thereby increasing the precision of the measurement.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in greater detail subsequently with the aid of specific embodiments. 
       Shown are: 
         FIG. 1  a block diagram of the inventive arrangement, 
         FIG. 2  an oscillogram of the current of the line IA plotted against the current of the line IB without filtration, 
         FIG. 2B  the currents IA over IB with filtration, 
         FIG. 3A  an oscillogram of the current IB plotted against IA with current generating an additional field without filtration, and 
         FIG. 3B  the current IA plotted against IB after filtration. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a block diagram of an arrangement for the improved measurement of the position of an electric machine  10  from the current  12  supplied to the motor; for a reliable operation, the measurement of the position of the drive mechanism is effected over two paths or channels  14 ,  16  that are independent of one another, and thereafter the two measured values (transmitter evaluation, current evaluation) are combined and checked with respect to a safe manner of operation, such as maintenance of a safe speed of the drive mechanism. If the channel  14  (current evaluation) is obtained by a computing device or computer, the channel  16  (transmitter evaluation) is obtained by means of a second computing device or computer, whereby the two computers operate independently of one another. The current value calculations are conveyed to one of the two independently operating computers, whereby this current value is derived from the current converters or shunts  1  and  2 , blocks  26 ,  28 , the current converters  1  and  2  each detecting two phases of the three phases  30 ,  32 ,  34  of the motor  10 . 
     A respective analog circuit  36 ,  38  follows the current converters  1  and  2  respectively, and is in turn followed by an analog/digital converter  40 ,  42 , which in turn conveys measured values  1 A and  1 B to a current value or amperage calculator  44 ; the current converter can involve various configurations, e.g. even a straightforward shunt resistor. The calculated current values are conveyed via line  14  to the communications interface in block  18 , together with a transmitter signal, which is generated by a transmitter  46  operated by the motor  10  and conveyed via line  48  to a transmitter evaluation  50 . An output signal of the transmitter evaluation  50  is conveyed via line  16  to the block  18 . At the output of the communications interface or block  18  there are then signals available that are conveyed via a secured communication to an evaluation unit, where the two-channel position evaluation is designed to determine an error situation. In the event of an error, the power output stage  91  can be deactivated by means of the STO input of the driver circuit (see  FIG. 1 , block  89 ). 
     A further block  68 , which on the one hand has the transmitter evaluation  50  (actual position) as an input, and from that calculates the actual current value in rotor coordinates, furthermore receives as an input signal  76  an output signal of the current value calculation  44  (actual current), whereby the output signals  72 ,  74  of the analog/digital converters  40 ,  42  are conveyed as inputs to the current value calculator  44 . 
     The current value calculated in blocks  44  and  68  (transformation of coordinates) is conveyed via line  77  as an input quantity, together with a theoretical value  78  of a theoretical value transmitter, to the current regulator  80  as a differential; the current regulator is disposed in the circuit  66 . The differential between the actual current value  76  and the adjustable theoretical current value  78  controls the current regulator  80 . A control signal is obtained by the current regulator from the differential between the theoretical value and the actual value, and is introduced via the input  82 , after transformation of coordinates in block  83 , into a block  84  (RZM); from the block RZM,  84 , which is part of the circuit  66 , an output signal is conveyed via line  86  to the block  88  (PWM), which via a driver circuit  89 , and a power output stage  91 , controls the phase currents  30 ,  32 ,  34  for the motor  10 . 
     The additional theoretical value is generated in the direction of the field of the motor in the current regulator  80 . In this connection, this theoretical value is prescribed for the current regulator from the outside. 
     It is conceivable to have the various calculation stages be carried out by a single (central processing unit) CPU since this CPU can simultaneously process different programs. The computing unit  66  can also be comprised of two modules or components, e.g. CPU and FPGA, or only of a single FPGA. 
     The oscillogram in  FIG. 2A  shows the current in the stationary stator coordinate system, as it is delivered from the converters  40 ,  42 . Here one can recognize that the current space vector fluctuates about zero, and thus it is not possible to make a reliable statement about in which direction the current vector lies. 
     Pursuant to  FIG. 3A , the situation is better with the measurement being improved by a current that generates an additional field. 
       FIGS. 2A and 3A  represent the unfiltered values, whereas  FIGS. 2B and 3B  result after filtration. 
     The arrangement illustrated in  FIG. 1  enables a method for determining the position of an electric machine  10 , here a motor having permanent magnetic generation of the magnetic field of the rotor (the magnetic field of a stator is produced by the circuit); with this arrangement, a position is determined from the supplied current  30 ,  32 ,  34 . The arrangement is such that if individual components of the described circuit fail, it is nevertheless possible to operate reliably. 
     For this purpose,  FIG. 1  shows three different complexes: first, the control loop or circuit, including the evaluation of the currents of the three phases  30 ,  32 ,  34  via two branches, each including current converter, analog circuit, analog/digital converter, whereby in this connection both channels convey signals  72 ,  74  to the current value calculator  44 , the output of which is conveyed as a regulation quantity (actual value)  76 , after transformation of coordinates in block  68 , to the control circuit which includes the theoretical value  78 , with the differential between theoretical value and actual value, the current regulator  80 , and the following blocks  83 ,  84 ,  88 ,  89 ,  91 , which produce the currents for the drive mechanism of the motor. 
     The second complex includes the transmitter  46 , the transmitter evaluation  50 , which enables the calculation of the speed and if the permissible speed is exceeded, stops e.g. the motor  10 . 
     The specification incorporates by reference the disclosure of German 10 2008 031 201.0 filed Jul. 3, 2008 and International application PCT/EP2009/004845 filed Jul. 3, 2009. 
     The present invention is, of course, in no way restricted to the specific disclosure of the specification and drawings, but also encompasses any modifications within the scope of the appended claims.