Abstract:
A method and circuit arrangement for determining position of the rotor of an electronically commutated motor, wherein the rotor has magnetic axes having different permeances. Voltage is applied to stator phases, and resultant phase currents are monitored for purpose of determining rotor position in the standstill state of the motor. First and second rise times of phase currents are determined until predetermined limit values are reached in unsaturated state. The assignment of a magnetic axis to a stator phase is determined from first rise times of the currents in unsaturated state of the rotor core, and the polarization of the rotor is determined from second rise times of currents upon energization with saturation effects. After run-up of the motor, initial energization of the stator can be determined comparing levels of the magnet wheel voltages and corrected by changing the commutation of stator energization.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP 2006/069086 filed on Nov. 30, 2006. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method and circuit arrangement for determining the rotor position of an electronically commutated motor (EC motor). Methods of this kind, which are required for a controlled run-up of EC motors from the standstill state with maximum moment, are basically known. In this connection, the detection of the rotor position is carried out either with the aid of rotation angle sensors or alternatively without such sensors, through the use of magnetic machine effects. 
     2. Description of the Prior Art 
     For machines with a magnetically asymmetrical or symmetrical rotor, DE 101 62 380 A has disclosed determining the position of the rotor in the standstill state of the machine. In this instance, the measurement is carried out using saturation effects in the rotor iron so that over a full rotation of the rotor, the stator phases are acted on cyclically by a number of current pulses that corresponds to twice the number of stator phases, which pulses are respectively offset from one another by the same angle. The rise times of the current pulses, which occur in accordance with the degree of saturation of the respective rotor section are then used to determine the rotor position in the standstill state of the motor. Such a method requires a large number of powerful current pulses for the measurement, which causes unwanted magnetic noise, movements of the motor shaft, and a delay in the starting of the motor. 
     In addition, the prior art (German patent application 102 005 007 995.4) already includes the proposal of determining the position of the rotor of an EC motor in two successive steps in that in order to ascertain the position of the d-axis, the stator of an EC motor with a magnetically asymmetrical rotor is initially excited with current pulses that do not result in saturation effects in the rotor, and in this case, measuring the magnitude of the current that occurs. Then a stator phase that can be associated with the d-axis of the rotor is acted on with current pulses, which produce a saturation of the iron in the rotor, in order to determine the north/south orientation of the rotor. In both measurement procedures, the potential is measured on the one hand, at the winding star point of the stator and on the other hand, at a summation point of the phase voltages generated by means of resistances and is used as a criterion for the magnetic concatenation between the stator and the rotor of the motor. To this end, the winding star point of the stator must be led out and made accessible, thus limiting the usability of the motor. In addition, the detection of potentials increases the circuitry complexity of the arrangement since an analog/digital conversion is required in the control unit. Because of the required magnitude and duration of the individual phase current supplies, the current in the stator windings is reversed during the measuring procedure in order to achieve a quasi-stationary state of the rotor. This results in a relatively long measurement duration. 
     It is also known to determine the position of the rotor without the use of rotation angle sensors after the motor is started, based on the induced revolving field voltage in the respective unpowered phases. This method, however, only permits a reliable conclusion to be drawn about the rotor position after the motor has reached a certain minimum speed. 
     SUMMARY AND ADVANTAGES OF THE INVENTION 
     The object of the present invention is to permit a rotor position detection that is operationally reliable and can be implemented without high circuitry complexity, which, even when the motor is at a standstill, quickly supplies a rotor position signal with a low stator current and permits acceleration of the motor from a standstill with a maximum moment. This is achieved by the characterizing features of the invention while significantly reducing the noise in the machine and the movements of the shaft during the determination of the rotor position. 
     The supply of current to the stator advantageously occurs so that, in both the time measurement with the reluctance effect and in the time measurement with the saturation effect, the stator phases are triggered with voltage pulses of the same magnitude, preferably the magnitude of the operating voltage. Limiting the duration of the voltage pulses assures that the current pulses achieved in the stator phases have the same respective magnitude in both the measurement with the reluctance effect and the time measurement with the saturation effect. In the time measurement with the reluctance effect, the magnitude of the current pulses must be set so that no saturation occurs in the rotor iron, whereas in the time measurement with the saturation effect, the magnitude of the current pulses must be set so that saturation does in fact occur in the rotor iron. In this way, within a shortened time and with a reduced supply of current to the stator, in a first step using magnetic asymmetry, the d-axis of the rotor is determined as the axis with the lowest main inductance and, in a second step using saturation, the correct-polarity orientation of the rotor is determined by establishing the polarity with the lower main inductance in this measuring step and, in accordance with the rotor position determined, a starting current supply of the motor is established. 
     It has turned out to be very advantageous if, in an additional step after the starting of the motor, the voltages that the revolving field induces in the stator are also measured. This makes it possible for an initial current feed of the stator, which is unfavorable for a maximum possible moment progression and results from a possible boundary position of the rotor at the sector boundary of two stator phases, to be identified by comparing the levels of the revolving field voltages and corrected by changing the commutation of the stator current feed. This measurement is continued during the operation of the motor in order to continuously monitor the current feed pattern. The method according to the present invention can thus be embodied in a particularly advantageous fashion if on the one hand, during the start of the motor, a control unit for the stator current feed is controlled by means of a counter for determining and evaluating the rise times of the phase currents and on the other hand, after the start of the motor, the control unit is controlled by means of a component for detecting the currents induced in the unsupplied phases of the stator and this control unit, immediately after the start of the motor, checks the chronological evaluation of the phase currents and if need be, takes corrective intervention steps in the sequence control. 
     With regard to the embodiment of a circuit arrangement according to the present invention for determining the rotor position of an EC motor with a magnetically asymmetrical rotor, it is suitable if the input of a counter designated for determining the rise times of the phase currents is connected to the output of a differential amplifier whose inputs are contacted on the one hand, by a signal that corresponds to a limit value of the phase current and on the other hand, by a signal that corresponds to the magnitude of the respective phase current measured; the magnitude of the phase currents is preferably determined by means of a low-impedance resistor that is situated in the sum electric circuit of an inverter for the phase currents. Such a circuit arrangement can be implemented with a low degree of complexity for components and low costs, particularly through the use of an ASIC component for the sequence control unit. On the other hand, the use of a microcontroller as a control unit eliminates the need for including a separate counter and permits the direct software control of the inverter for the supply of current to the stator phases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other details and advantageous embodiments of the present invention ensue from the claims and from the description of an exemplary embodiment when taken with the drawings, in which: 
         FIG. 1  shows a circuit arrangement for carrying out the method according to the present invention, 
         FIG. 2  shows a sectional depiction of the stator and rotor arrangement of a three-phase, four-pole EC motor, and 
         FIG. 3  is a schematic depiction of the measured phase currents, on the one hand in the unsaturated state of the rotor iron and on the other hand through the use of saturation effects. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In  FIG. 1 , the reference numeral  10  denotes an EC motor (electronically commutated motor), having a three-phase stator  12  connected in a star, the stator phases U, V, W, and a four-pole, permanently excited rotor  14 . The motor is supplied with current in a known way via an inverter  16  in a full bridge circuit, which on the one hand, is connected to the plus pole  18  of a d.c. voltage source U and on the other hand, is connected with its star point to a ground connection  20  via a shunt  22 . A control unit  24 , which in the known construction of the inverter  16  is likewise embodied as a six-poled arrangement with six semiconductor switches, controls the inverter  16  via a control line  26 . 
     In accordance with the sum current I of the inverter  16 , the voltage drop at the shunt  22  is picked up at the connection point  28  and is supplied via an amplifier  30  to the non-inverting input of a comparator  32 . Via a line  34 , the inverting input of this comparator is supplied with a limit value signal predetermined by the control unit  24  in accordance with a predetermined sum current limit value I G1  or I G2  according to  FIG. 3 . The output of the comparator  32  is connected to the input of a counter  36 , which at its output, supplies a counting signal to the control unit  24  in accordance with the rise time of the motor sum current I up to the predetermined limit value I G1 , I G2 . The output signal of the comparator  32  is picked up at a connecting point  40  between the comparator  32  and a counter  36  and is supplied via a line  42  directly to the control unit  24  in order to reset the counter  36  once the respective limit values I G1 , I G2  of the sum current I are reached. The starting of the counter  36  with the next current pulse according to  FIG. 3  occurs by means of its supply line  44 . 
     The circuit arrangement show in  FIG. 1  is completed by means of a device  46  for rotor position determination through detection of voltages U i  that the rotating rotor  14  induces in the unpowered phases U, V, W of the stator  12 . To this end, the device  46  is connected to respective connections  50 ,  52 ,  54  of the phases U, V, W of the stator  12  and its output is connected to the control unit  24  via a line  48 . 
       FIG. 2  shows a section through the stator  12  and rotor  14  of a three-phase four-pole EC motor with a magnetically asymmetrical rotor  14 . In synchronous operation, a 120° block current feed of the stator phases U, V, W necessitates a commutation every 60°, thus making it possible to divide an electric rotation into six sectors with a two-phase current supply. The sectors are labeled with the numerals 1 through 6, the magnetic axes of the rotor  14  are labeled d′ and q′, with the magnetization being produced by means of two magnet segments  56  and  58 . The south pole of each of the magnet segments  56  and  58  is shown; the associated north poles are formed in the stator iron on a second horizontally extending d-axis. The design of the rotor  14  could, to the same effect, also be embodied with four magnet segments. The two q-axes each extend centrally between the d-axes. 
     On the left side,  FIG. 3  shows the curve of the phase currents I u , I v , and I W  in the standstill state of the machine in the unsaturated current supply range, each limited by the current ±I G1 . In the saturated range, the current ±I G2  limits the phase currents at which the measurement in the unsaturated current supply range has yielded the shortest rise time. In the exemplary embodiment, these are the currents I   U    and I w . In this case, via the inverter  16 , the control unit  24  at first positively powers one of the three phases and negatively powers a second one, then first times t 1 , t 2 , t 3  are measured from the beginning of the pulse to the reaching of the limit value I G1 , and the shortest of the three first times is established as a criterion for one of the sectors  1  through  3  and  4  through  6  in which the d-axis of the rotor  14  is presently situated. In this case, for the resulting flux vector, the following equations are true: in sector  1 : I W =−I U , in sector  2 : I W =−I V , and in sector  3 : I V =−I U . At the end of the three measurements, the lowest counter value and the associated sector number are contained with the flux vector in the memory of the counter  36  and are furnished to the control unit  24  as a criterion  38  for the course of the d-axis. This determines the orientation of the d-axis of the rotor  14 . 
     In a second measuring procedure, the two phases U and W with the shortest rise time t 1 , from the first measurement are again inversely powered with a limit value I G2  of the current raised to the saturation range; a rise time t 4  of the phases U−/W+, due to the lower saturation and the resulting shorter rise time t 4  of the current, is recognized as the correct phase position with regard to the north/south orientation of the d-axis. In accordance with this orientation of the rotor  14 , the control unit  24  then establishes a sequence control with the corresponding current supply to the phases U, V, W by means of the inverter  16  and the motor can be started with a maximum moment. 
     A difficulty in making the determination in the starting position of the rotor  14  can arise if the rotor is situated in a boundary position between two sectors. Such a boundary position between two sectors can, for example, occur due to the detent moment of the EC motor or due to other influences. In this case, in order to correct an unfavorable initial current supply, in an additional step after the starting of the motor  10 , the unfavorable initial current supply of the stator  12  resulting from a boundary position of the rotor  14  at the sector boundary of two stator phases can be identified by comparing the level of the revolving field currents and the stator current supply can be corrected by changing the commutation pattern. To this end, after the starting of the motor, the device  46  changes the current supply pattern originally established by the counter  36  based on the time measurements t 1 , t 2 , t 3 , by detecting and evaluating the voltages U i  induced in the unpowered phases of the stator. Furthermore, due to the continuous detection of these induced voltages during operation of the motor  10  as well, a rotor position signal is continuously supplied to the control unit  24  via the line  48 , which signal then plays a dominant role in the determination of the current supply if the initial current supply has to be changed. 
     The method according to the present invention is consequently based on the advantageous combination of two or preferably three essentially known measuring methods. On the one hand, this constitutes the use of the reluctance effect due to the magnetic asymmetry of the rotor  14  with minimal main inductances in the region of the d-axes and maximal main inductances in the region of the q-axes of the rotor. On the other hand, the use of saturation effects in the iron and the higher supply of current to the rotor necessitated by this is only required for determining the correct-polarity north/south rotor position in the standstill state; the quicker current rise to the limit value I G2 , in the exemplary embodiment in the time t 4 , is detected with a positive powering of the phase V and a negative powering of the phase U. The quicker current rise in this instance is due to the more powerful saturation effect when the stator  12  and rotor  14  are situated opposite like poles. 
     By monitoring the revolving field current U i  after the starting of the motor  10 , the current supply pattern of the stator  12  can be tested in any operating state and corrected as needed. The noise and movements of the motor shaft, which are caused particularly with the use of saturation effects in the motor, are minimized by measuring with fewer and significantly weaker current pulses in the measurement method according to the present invention. With a shortened measurement duration, this achieves the acceleration of the motor with a maximum moment. 
     The circuitry complexity for the measurement is reduced to a low-impedance measuring resistor  22  for the sum current I of the inverter  16 , an individual operational amplifier as the amplifier  30 , a sum current comparator  32 , a counter  36 , and the control unit  24  as a finite state machine for the sequence control. This can be implemented either in the form of an ASIC or a microcontroller. When using a microcontroller as the control unit  24 , the counter  36  is already contained in the microcontroller and the sequence control can be embodied in the form of software. The device  46  for determining the induced voltage is frequently already present in EC drive units that do not have rotation angle sensors so that it does not have any appreciable effect on the circuitry complexity. 
     The foregoing relates to the preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.