Abstract:
The invention relates to a method for positional recognition of a rotor of an electronically commutated electric machine, in particular an electric motor, in which a zero crossover of a voltage induced in a coil section of the rotor or stator is used for positional recognition. According to the invention, to determine the zero crossover the coil section is briefly powered down. A rotor/stator is used, comprising at least two coil sections, one of which has a lower inductance relative to the other one, and preferably only the coil section with the lower inductance is used for the positional recognition.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP 2009/060008 filed on Aug. 3, 2009. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for detecting the position of a rotor of an electronically commutated electric machine, in particular of an electric motor, in which a zero crossing of a voltage induced in a phase winding of the rotor or stator is used for position detection and in order to detect the zero crossing, the phase winding is briefly switched into a currentless state. 
     2. Description of the Prior Art 
     For an electronic commutation of an electric machine having a rotor and a stator, it is necessary to know the position of the rotor of the electric machine. This can be achieved by means of position detection. Position detections of the rotor can be carried out both with and without sensors. The electronically commutated electric machine typically has at least one phase winding, which is composed of at least one winding associated with the rotor or the stator. With the commutation, the winding constitutes an electromagnet that can be switched on and off. The part of the electric machine that does not have the phase winding, i.e. the stator or rotor, preferably has at least one permanent magnet that cooperates with the phase winding and produces a rotation of the rotor. The permanent magnet can also be replaced by an electromagnet. To detect the position of the rotor without a sensor, a zero crossing of an electric voltage, which the permanent magnet induces in the phase winding, is detected in the electric machine. To accomplish this, the induced voltage in the phase winding is measured. To be able to detect the zero crossing, the phase winding must be switched into a currentless state, i.e. it must not be supplied with current from the outside. This is because in the phase winding used for producing a force that drives the rotor, this production of force depends on an efficiency of the sum of all induced voltages and of the phase winding voltage. This means that in a hypothesized physically ideal phase winding, the current inside the phase winding flows in phase with the induced voltage. From this, it follows that the current in the phase winding is superimposed with the induced voltage so that a measurement of the zero crossing is not possible without taking corresponding steps. For this reason, the current in the phase winding is switched off before an expected zero crossing and is switched on again only after the zero crossing. 
     It is disadvantageous that the switching-off of the phase winding means that for the length of time that the switched-off phase winding remains switched off, no force production can occur and therefore no power can be generated. The electric machine consequently loses power density. Simply reducing the length of time during which the phase winding is switched to the currentless state so as to increase the power density of the electric machine results in an increased probability of an incorrect commutation. This is due to the fact that the subsequent switching-off of the phase winding is calculated based on the rotor speed and an expected speed change. As a result, when the length of time is reduced, only small speed changes can be taken into account since the zero crossing must lie within this time period. For this reason, in electric machines with a small speed change over time, not every zero crossing has to be measured; instead one or more zero crossings can be skipped. Considered over the operating time of the electric machine, this procedure yields a higher power density of the electric machine. But if high speed dynamics of the electric machine, i.e. significant and frequent speed changes, make it necessary to detect the zero crossing very often, then the power density can only be retained by minimizing the length of time during which the phase winding is switched off. For this reason, a method is required that permits the phase winding to be switched off as late as possible before an expected zero crossing and switched back on again quickly after the zero crossing. 
     SUMMARY OF THE INVENTION 
     According to the invention, a rotor/stator is provided, which has at least two phase windings, one of which has a lower inductance than the other and preferably, only the phase winding with the low inductance is used for position detection. The lower inductance results in the fact that the phase winding can be switched off more quickly and switched on more quickly than the phase winding with the higher inductance. Preferably, an electric machine is provided that associates the rotor/stator, which belongs to the phase winding, with a stator/rotor, which is equipped with at least one, preferably several, permanent magnets. The position detection then uses the voltage that the permanent magnets induce in the phase winding, thus permitting a commutation without sensors. Each of the phase windings has at least one winding that is wound around a winding core and consequently generates an electrical field that interacts with the permanent magnet. The permanent magnet can also be replaced by an electromagnet. When the rotor rotates, the permanent magnet induces the electrical voltage—an alternating electrical voltage—in the phase winding. If one of the windings is situated centrally within the magnetic field of the permanent magnet, then a zero crossing for the induced voltage occurs within the winding. 
     As provided in a modification of the invention, around the zero crossing, a measurement window is produced, which begins with the off-commutation of the switch to the currentless state and ends after the zero crossing of the induced voltage. The measurement window begins with the off-commutation of the switch to the currentless state. The term “off-commutation” describes the event that occurs during the period of time that is required from the beginning of the switching-off to the establishment of the currentless state within the phase winding. The measurement window corresponds to the length of time that the phase winding is completely without current. When the electric machine is being operated as a generator, this voltage can be directly detected and evaluated. When the electric machine is being operated as a motor, the induced voltage is superimposed with the electrical current supplied into the phase winding and cannot be detected. In order to nevertheless be able to carry out a measurement, the next expected zero crossing is determined and the current being supplied to the phase winding is switched off as late as possible before the expected zero crossing so that it is possible to detect the induced voltage in the phase winding, thus establishing the measurement window. In this connection, it is advantageous if the phase, winding is composed of a plurality of windings that are positioned to be angularly offset from one another. It is particularly advantageous if each of the windings is associated with a permanent magnet. These permanent magnets are preferably positioned to be angularly offset from one another in the same way. This results in the fact that a zero crossing is produced in all of the windings simultaneously and consequently, a zero crossing occurs in the entire phase winding. With a plurality of windings and/or permanent magnets, the position detection of the rotor indicates a relative position of the rotor, namely the position of one of the windings relative to one of the permanent magnets. A switching-off of one of the windings—the off-commutation—requires a certain switching-off time due to the inductive properties of the phase winding. The duration of the switching-off time is chiefly influenced by the inductance of the phase winding. Due to the reduction of the inductance of one of the phase windings, this phase winding can be switched on and off more quickly than the phase winding with the higher inductance. The quicker switching on and off makes it possible for the phase winding to be supplied with current for a longer time before being switched into the currentless state and more quickly builds up a force-producing and therefore power-generating magnetic field after being switched on, thus achieving a higher power density of the electric machine. This is the reason for the advantageous provision of using the phase winding with the lower inductance as the measurement winding. Because different inductances of the phase windings are used, these phase windings produce an asymmetrical magnetic circuit in the stator/rotor. This asymmetry can be largely compensated for by a suitable stator and/or rotor design. This leads to an improvement in the acoustics of the electric machine. It is also conceivable, through the use of the method according to the invention, to postpone the time at which the power is switched off, thus maintaining the power density, enlarging the measurement window, and increasing the sturdiness of the electronic commutation. Consequently, either the time at which the power is switched off is maintained, making the currentless measurement window longer due to the more quickly decaying current, or the current is switched off later and the current is therefore supplied for a longer time, thus maintaining the measurement window. It is also conceivable to provide a combination of the two possibilities so that an adaptive, dynamic system is dependent on the currently prevailing speed dynamics of the electric machine and the level of the current within the phase winding. 
     As provided in a modification of the invention, a rotor/stator with stator teeth is used, which teeth each have at least one tooth root and at least one tooth crest; one of the phase windings is situated around the tooth roots of the rotor/stator, another of the phase windings is situated around the tooth crests of the rotor/stator, and the phase winding situated around the tooth crests is used for the lower inductance. This arrangement makes it possible to have different inductances in the phase windings; the phase windings have the same flux linkage relative to the air gap. Flux linkage is understood to be a linkage of an excitation flux in the windings, for example by means of permanent magnets or excitation coils. As a result, with the same number of turns and the same phase winding current, the phase windings exert a virtually identical influence on the power density of the electric machine since comparable currents make comparable contributions to the overall torque of the electric machine. The stator teeth are preferably radially arranged so that viewed in the radial direction, the tooth crest is stacked onto the tooth root. In particular, this results in the fact that the phase winding that is associated with the tooth crests has the lower inductance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in detail below in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a detail of an electric machine with a first supply of current, 
         FIG. 2  shows the detail of the electric machine with a second supply of current, and 
         FIG. 3  is a current/voltage time diagram. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows a detail  1  of a cross-sectionally depicted electric machine  2 , which is embodied in the form of an electric motor  3 . In the detail  1 , regions of the stator  5  and a rotor  4  are depicted. The rotor  4  is an external rotor  4 , which rotates around the stator  5  and encompasses it, and has a plurality of permanent magnets  6 , which are uniformly distributed along the circumference direction of the rotor  4 . Both the rotor  4  and the permanent magnets  6  are depicted only in schematic form in order to illustrate a function of the electric machine  2 . The stator  5  is composed of a circular base element  7  on which stator teeth  8  are situated. The stator teeth  8  extend radially out from the base element  7  and each have a tooth root  9  that is attached to the base element  7 . Each tooth root  9  has two tooth crests  10  that extend in a bowed shape viewed in cross section so that the bow ends  11  of two tooth crests  10  of a tooth root  9  are oriented toward each other. A tooth root winding  12  is situated around each tooth root  9 . The tooth root windings  12  are combined to form a first phase winding  13 . When the first phase winding  13  is supplied with current, this produces the current flow directions shown in  FIG. 1  for the first phase winding  13 . A circle  12 ′ marked with an X indicates a current flow direction down into the plane of the paper and a circle  12 ″ with a concentric dot indicates a current flow direction up out of the plane of the paper. For the current flow directions according to the depiction, this therefore yields winding regions with the same current flow direction situated adjacent to each other in the circumference direction between two tooth roots  9 . Pairs of tooth crests  10  are associated with tooth crest windings  14 . The tooth crests  10  encompassed by one tooth crest winding  14  each belong to a different stator tooth  8 . For this reason,  FIG. 1  shows only one complete tooth crest winding  14 , while only half of two other tooth crest windings  14  are depicted. The tooth crest windings  14  are connected to one another to form a second phase winding  15 . The second phase winding  15  shown in the drawing is in the currentless state. Inside the stator  5 , a magnetic flux  16  that is flowing due to the supply of current to the tooth root windings  12  is depicted in the form of magnetic flux lines  17 . The magnetic flux  16  is produced by two tooth root windings  12  via two tooth crests  10  of two different stator teeth  8 . The two tooth crests  10  are spaded apart from each other so that an air gap  19  is formed between the tooth crests  10 . The tooth crests  10  thus generate a magnetic field  20  that flows from one of the tooth crests  10  to the other tooth crest  10 . In the figure, the rotor  4  is situated in a rotary position in which the permanent magnet  6  is situated in the middle of the magnetic field  20  and is moving in a rotation direction indicated by an arrow  21 . The detail  1  consequently depicts a field distribution of the stator  5  when the first powered phase winding  13  is situated in free space. It is also necessary to take into account the fact that the magnetic fields  20  have alternating, different polarities in the circumference direction. In the drawing, both components, the rotor  4  and permanent magnets  6 , have no influence on the magnetic fields  20  and magnetic flux  16  in order to permit illustration of the magnetic behavior of the stator  5 . 
       FIG. 2  shows the detail  1  from  FIG. 1  with the same features. By contrast with  FIG. 1 , in  FIG. 2 , the second phase winding  15  is supplied with current, which is depicted by means of the circles  12 ′ and  12 ″ with the same meanings as in  FIG. 1 . The first phase winding  13  here has been switched into the currentless state. The supply of current to the second phase winding  15  produces a second magnetic flux  22  in each of the stator teeth  8 . The second magnetic flux  22  is depicted by means of magnetic field lines  23  and flows through two tooth crests  10  of a single stator tooth  8 . The second magnetic flux  22  produces magnetic fields  24  that flow from one of the tooth crests  10  to the other of the tooth crests  10  in one stator tooth  8 ; these magnetic fields each bridge an air gap  25  between two bow ends  11  of the tooth crests  10 . This current-supply state of the stator  5  produces a position of the magnetic fields  24  that is offset by one slot pitch in the space surrounding the stator  5  as compared to the position shown in  FIG. 1 . The magnetic fields  20  and  24  differ in position, but because of the equal flux linkage of the phase windings  13  and  15 , do not differ or hardly differ in their magnetic characteristics with an equal phase winding current. It is also necessary to take into account the fact that the magnetic fields  24  have alternating, different polarities in the circumference direction. 
       FIGS. 1 and 2  permit a comparison of field characteristics of the magnetic fields  20  and  24  of the two phase windings  13  and  15  with different inductances in the phase windings  13  and  15 . 
     The electric motor  3  is electronically commutated; the two phase windings  13  and  15  are simultaneously supplied with current that is electrically shifted by 90°. This produces a rotating field that travels in the circumference direction around the stator  5 . Due to the embodiment of the first phase winding  13 , it is provided with a higher inductance than the second phase winding  15 . As a result of this embodiment, the two phase windings  13  and  15  produce an asymmetrical magnetic circuit  26  that has virtually the same flux linkage in both phase windings  13  and  15 . Because of this embodiment, comparable currents in the phase windings  13  and  15  also constitute comparable portions of an overall torque of the electric machine  2 . Because of the lower inductance of the second phase winding  15 , it is used as the measurement winding  27 . 
       FIG. 3  shows a Cartesian coordinate system  28  with an abscissa  29  and an ordinate  30 . The abscissa  29  is associated with time t and the ordinate  30  is associated with the rotor current I and the induced voltage U of the measurement winding  27 . Within the Cartesian coordinate system  28 , the induced voltage U is depicted as a dotted, sine-shaped voltage curve  31 . A current curve  32  is likewise depicted with a solid line and represents a supply of current to the stator  5  without the use of position detection. Along the current curve  32 , a first switch-off time  33  and a second switch-off time  34  are depicted. From the switch-off time  33 , a dot-and-dash line current switch-off line  35  extends like a ramp to the abscissa  29  and from there, along the abscissa  29  to the current curve  32 . From the switch-off time  34 , a dashed current switch-off line  36  extends to the abscissa  29 . It extends along the abscissa  29  to the current curve  32  in the same way as the current switch-off line  35 . The two current switch-off lines  35  and  36  meet at the same currentless point  37  on the abscissa  29 . A measurement window  39  extends from the currentless point  37  until after a zero crossing  38  of the induced voltage. The switch-off times  33  and  34  correspond to the times at the beginning of an off-commutation so that the measurement winding  27  is switched into the currentless state in the measurement window  39 . The measurement window  39  opens every 180° of the electric phase of the induced voltage U. Two arrows indicate a length of time  40  between the switch-off times  33  and  34 . 
     In a generator operation, the induced voltage U is produced, whose curve is depicted in the form of a voltage curve  31 . The rotor  4  is in the position depicted in  FIG. 1  upon occurrence of the zero crossing  38 . The current curve  32  occurs in the measurement winding  27  when the machine is being operated purely as a motor. In order to be able to carry out the position detection, the measurement window  39  is established by switching the second phase winding  15 , the measurement winding  27 , into the currentless state. The current switch-off line  35  corresponds to an imaginary switch-off line  35  that would occur if the two phase windings  13  and  15  of the stator  5  had the same inductance and would therefore produce a symmetrical magnetic circuit and in this application, serves as a possible comparison for the method according to the invention. By contrast with the switch-off line  35 , the switch-off line  36  has a steeper slope so that the switch-off time  34  can be shifted later along the abscissa  29  as compared to the switch-off time  33 . This yields the time difference  40 . The use of the second phase winding  15  with the lower inductance permits implementation of the current switch-off line  36 . Since the two current switch-off lines  35  and  36  meet at the currentless point  37 , they both produce the same measurement window  39 , from which it follows that the later switch-off time  34  enabled by the invention results in a longer supply of current to the second phase winding  15 . There is thus an increase in the power density of the electric machine  2 , said increase being represented by the area between the current switch-off line  35 , the current switch-off line  36 , and the current curve  32 . In addition to this power density gain when switching off, i.e. the off-commutation of the second phase winding  15 , there is also a corresponding power gain when switching on the second phase winding  15 . Due to the lower inductance, the second phase winding  15  produces a force-generating magnetic field  24  faster than another phase winding with a higher inductance. The current switch-on lines and the resulting power gain are not depicted in the figure. 
     In another embodiment, it is conceivable to maintain the electric power supplied to the phase winding  15  so that in the method according to the invention, the currentless point  37  is shifted earlier along the abscissa  29 . This is not depicted in  FIG. 3 . This results in an enlargement of the measurement window  39 , thus resulting in more robust position detection. The increasing robustness of the measurement prevents an incorrect commutation since the position detection delivers high-precision results. 
     With the geometry of the stator  5  shown in  FIG. 1 , if the first phase winding  13  is supplied with current, then this produces almost exactly the same effect in the air gap  19  as when the second phase winding  15  is supplied with an equal amperage. This reflects the equal flux linkage despite the asymmetrical plate geometry and slightly different numbers of turns. Since the first phase winding  13  causes magnetic flux  16  to flow through more material of the stator  5 , e.g. steel, in the stator teeth  8  than the second phase winding  15 , this yields the greater inductance of the first phase winding  13 . The two magnetic fluxes  16  and  22 —the magnetic flux paths—along the magnetic flux lines  17  and  23  have equal flux-collecting areas in the air gaps  19  and  25 , yielding a comparable flux linkage of the two phase windings  13  and  15 , resulting in the fact that the two phase windings  13  and  15  produce virtually the same induced voltage U. It follows from this that comparable currents in the phase windings  13  and  15  also make comparable contributions to the overall torque of the electric machine  2 . The asymmetrical magnetic circuit  26  with a virtually equal flux linkage in the two phase windings  13  and  15  executes a commutation without sensors by using the second phase winding  15  with the lower inductance as the measurement winding  27 . The lower inductance of the measurement winding  27  permits the inducing action of these currents to continue regardless of the measurement window  39 , thus achieving a compensated behavior of the electric machine  2 . Unevenly distributed electric loads in the two phase windings  13  and  15  would have a negative impact on the acoustics of the electric machine  2 . The method according to the invention is therefore more economical than embodying all of the phase windings of an electric machine  2  with a low inductance. Furthermore, this would not compensate for an asymmetry in the triggering concepts between the measurement winding  27  and other phase windings, without disadvantageously influencing a utilization of the electric machine  2 . 
     It is also conceivable to combine the above-described method with a pre-commutation. In this case, the induced voltage U is compared to a constantly shifting reference voltage, as a result of which, the measurement window  39  is once again situated on the abscissa in the Cartesian coordinate system  28 . With a shift in a positive direction of the ordinate  30 , i.e. in the arrow direction of the ordinate  30 , the zero crossing  38  is shifted back along the abscissa  29 , thus enabling an even later occurrence of the switching-off. 
     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.