Abstract:
An electric machine, as well as a method for operating such a machine, particularly for motorically moving movable parts in a motor vehicle, having a stator and a rotor, slots being developed on the rotor, in which individual conductor loops of electric coils are situated, which are contacted to commutator segments of a commutator and to an evaluating unit, which ascertains rotational speed data from the ripple of a motor current signal. The number of individual conductor loops of the coils is selected in such a way that the sequence of the number of the conductor loops in the order of their commutation approximately represents a sine function.

Description:
BACKGROUND INFORMATION 
       [0001]    A commutation device of a DC motor is described in European Patent No. EP 0 917 755, in which brushes lie against a contact area of the segments of a commutator. An electronic circuit records the frequency of the ripple of the motor current, in this context, in order to determine from this a measure of the rotational speed of the electric motor. To gain reliable rotational speed information, the edges of the commutator segments are at a certain angle to the longitudinal axis of the commutator and to the edges of the brushes. Such a commutator is very costly to produce, and offers no possibility of generating a frequency of the ripple that is smaller than the slot-ripple frequency. 
         [0002]    In such electric motors, the alternating component of the current signal is evaluated for the rotational speed detection. The ripple of this signal is generated by various causes. A large component of the ripple has the number of the slots of the commutator. In the current signal, one is able to detect the slot number and its multiples. In this context, the order of the smallest common multiple of the slot number and the magnetic pole number mostly occurs in a dominating manner. This ripple is caused in the lower rotational speed range (smaller rotational speeds) and under great load by the variation of the armature resistance over the commutation. Near the idling speed and at low current, the ripple is generated by the variation of the induced voltage, caused by the coil windings in the magnetic field. At medium motor load, the ripple in the curve over time of the current signal is caused by both effects. The two effects may be phase-shifted with respect to each other, and may be eliminated at various operational points, so that the slot order and its multiples in the current characteristic clearly vary over the characteristics curve of the motor, and may even disappear. Furthermore, current ripples appear in the current characteristic having orders less than that of the slot order. These are mostly multiples of the magnetic poles. These orders of the current ripple are caused by undesired tolerances in the symmetry of the magnetic circuit, such as position tolerances or material tolerances of the magnets. Because of their irregularity, these orders in the current curve over time prevent a reliable evaluation for determining the motor rotational speed signal. For the evaluation of the slot order of the current ripples, the ripple further has to exceed a certain height of amplitude so that the signals are able to be evaluated. In addition, the amplitudes of the orders vary over different operating points, which also makes evaluation more difficult. In motors having a larger slot number, the dominating order in the current characteristic of the current ripples is so high, because of the large number of slots and magnetic poles, that evaluation electronics having a higher scanning frequency becomes required for determining the rotational speed of the motor. This means a greater effort and higher costs, since the microcontrollers have to be faster and better. 
       SUMMARY OF THE INVENTION 
       [0003]    By contrast, the electric machine according to the present invention, as well as the method according to the present invention for operating the same, have the advantage that the number of conductor loops in the slots of the rotor is varied in such a way that the number of conductor loops of coils commutated one after the other yields a sine curve as a first approximation. Such a sinusoidal change in the number of conductor loops over the sequence in time of the commutation generates additional ripple in the motor current signal, whose frequency corresponds to the product of the pole number, the number of periods of the sine function per commutating phase and the rotational frequency of the electric machine. This causes an additional ripple in the current characteristic to be produced, which is less in frequency than the slot frequency that is produced by the number of commutator segments. These additionally generated current peaks have an approximately constant amplitude, independently of the operational point of the electric machine and the amount of the load current. This is why this superposed current ripple signal is able very favorably to be evaluated, in order to ascertain information about the rotational speed, or the period duration of the rotor revolution. Because of the sinusoidal change of the conductor number over the commutation cycle, interferences of higher orders of the additional current ripple signal are largely able to be eliminated. Therefore, noise excitations of the electric machine, caused by the current ripple, may be greatly reduced, and so a relatively more quiet run may be achieved for convenience drives, in spite of the torque ripple. 
         [0004]    It has proven to be particularly advantageous to change the number of successively commutated conductor loops by exactly one conductor loop. Because of that, as smooth as possible a sine curve of the conductor loop change may be implemented, whereby the interfering noise excitation is optimally suppressed. Optionally, even two successive coils may have the same number of conductor loops, in this context. 
         [0005]    In one preferred embodiment, and electric DC motor has a rotor having 14 slots, into which altogether also 14 coils are fitted. This embodiment has four magnetic poles, for example, which are generated by a circumferential magnetic ring, which has a uniform pole ring subdivision of preferably 90 degrees. In this embodiment, because of the sinusoidal change of the conductor loop number via the commutator cycle, and easy-to-detect ripple signal may be generated which, for instance, has four current ripples per commutator revolution. 
         [0006]    It is particularly favorable if the number of commutator segments, which preferably corresponds to the number of slots of the rotor, is not divisible by the number of the magnetic poles. This reduces the cogging torque of the electric machine and improves the synchronism properties of the electric machine. 
         [0007]    It is particularly favorable to develop exactly one period of the sine function over one commutation phase, using the change in the conductor loops per coil. The amplitude of the current ripple to be detected is thereby able to be maximized, whereby the evaluation device may be simplified. 
         [0008]    In this context, it does not matter if the number of the conductor loop slots, lying next to one another with respect to to the rotor circumference, does not continuously change sinusoidally. What is decisive is that conductor loop number changes with respect to the sequence of the successively commutated coils correspondingly to a sine function, which may deviate from the slot arrangement on the rotor by the winding scheme used. 
         [0009]    In one preferred variant, the electric machine has coils that have between 8 and 15 individual conductor loops. If the number of conductor loops varies between 10 and 13 conductor loops, for instance, then, in a 14-slot machine a relatively smooth sine function of the conductor loop change is able to be produced by having approximately each successive commutated coil changing by exactly one conductor loop. 
         [0010]    The variation, according to the present invention, of the number of conductor loops per coil may also be used on coils wound controsymmetrically to the rotor axis, which are developed as two symmetrical coil sections. The number of conductor loops of the two coil sections is changed, in this instance, to the same extent with respect to the nearest coil section, so that no additional radial forces are generated. 
         [0011]    The method according to the present invention, for operating an electric machine, preferably a DC motor, has the advantage that, because of the variation, according to the present invention, of the number of conductor loops of the individual coils, a uniform current ripple having relatively constant amplitude is able to be generated, which changes only insubstantially over various working ranges of the electric machine. Because of the clearly lower frequency of this additionally generated current ripple, the scanning frequency of the rotational speed evaluation unit is able to be reduced, whereby the requirements, and thus also the costs, of the evaluation device may be reduced. The ripple signal of the motor current, generated according to the present invention, may be used particularly favorably for implementing a jamming protection function of a motorically moved part. In this context, the signal representing the rotational speed is examined by the evaluation unit for a change with time, for which the time intervals between the individual current ripples are ascertained. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  shows a first exemplary embodiment of a rotor according to the present invention, having a schematic representation of the change of the number of conductor loops. 
           [0013]      FIG. 2  shows an additional exemplary embodiment of an electric machine together with a schematic change in the number of conductor loops. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1  shows an electric machine  12 , which is developed as a DC motor  14 , for example. Electric machine  12  has a rotor  18  supported on a rotor shaft  16 , which has a plurality of slots  24  for accommodating electric coils  30 . Slots  24  are developed, for example, in a segment stack  26 , which is made up of individual lamella sheet metals  27  that are axially stacked one over another. Rotor  18  in  FIG. 1  has eight slots, for example, in which altogether eight coils  30  are situated. Coils  30  are wound centrosymmetrically to a rotor axis  17 , for example, using diameter winding, so that two half coils of different coils  30  are situated in each slot  24 . Coils  30  are electrically connected to commutator segments  22  of a commutator  20 , which has current applied to it using electric brushes  28  that are not shown in detail. Each coil  30  is made up of individual conductor loops  36 , whose number is represented by the numbers stated in slots  24 . Thus, for example, a specific coil  31  has eleven conductor loops  36 , that are wound through opposite slots  24 . In the same slot pair, a second coil  33  is situated, having eleven conductor loops  36 , which in the exemplary embodiment is commutated at the same time as coil  31 . The nearest coil pair  61 ,  63  in the circumferential direction of rotor  18  has twelve conductor loops  36  each. After that, on rotor  18  there follow four coils  30 , each having  10  conductor loops  36 , after which there then follows again coil pair  31 ,  33 , each having eleven conductor loops  36 . 
         [0015]    The lower half of the illustration shows the unwound commutator segments  22  of commutator  20 , the sequence of the numbers in each case reproducing the number of conductor loops  36  of coils  30  commutated one after the other. This yields an order of coils  30  commutated one after the other, each having a different number of conductor loops  36 . Thus, coils  30  that are successive in one commutation phase have  10 ,  11 ,  12 ,  10  conductor loops  36  respectively, so that the change in the number of conductor loops approximately yields a schematically shown sine function  60 . Of the eight coils  30 , in this context, two are always commutated at the same time, and these two always have the same number of conductor loops  36 . A commutation phase, up to which the same commutation state is reached again, in this case amounts to four successive commutation states which repeat periodically. As a function of the number of brushes  28 , or rather, as a function of the number of magnetic poles corresponding to them, sinusoidal curve  60 , of the change in the number of conductor loops, has one or more periods  38  over one commutator rotation.  FIG. 1  shows two periods  38 , which are separated by a mirror plane  40 . 
         [0016]      FIG. 2  shows another exemplary embodiment, in which electric machine  12  has a stator  34  having a magnetic ring  46  which has, for instance, four magnetic poles  32 , having a pole pitch angle  50  of about 90°. Magnetic ring  46  is developed as a closed encircling ring, so that individual magnetic poles  32  seamlessly go over into one another. On rotor shaft  16 , commutator  20  is situated, against which lie the same number of brushes  28  (for instance, four) as correspond to the number of magnetic poles  32 . In the lower half of the illustration, the sinusoidal change in the number of conductor loops is shown again schematically, in the order of successively commutated coils  30 . The number of conductor loops  36  per coil  30  varies, in this case, between 10 and 13, the change amounting to only a single conductor loop  36  per successively commutated coil  30 . In this instance, a commutation phase extends over seven commutation states, which together form a period of sine curve  60 . For this reason, an especially smooth sine curve  60  comes about for the change in the number of conductor loops. In this exemplary embodiment of four-pole machine  12 , one therefore obtains the four-fold rotor rotational frequency for the frequency of the additional current ripple generated using the conductor loop variation. An oscillation having the magnetic pole order is impressed on the motor current curve, in this case. Such a current ripple frequency is clearly lower, in this context, than the corresponding slot frequency of the motor current signal. The sequence of successively commutated coils  30  according to sine curve  60  is not coincident, in this case, with the sequence of coils  30  with respect to the circumference of rotor  18 . 
         [0017]    In this exemplary embodiment, coils  30  are developed in each case as two symmetrical coil sections  29 , which are situated geometrically parallel to each other as mirror images to an imaginary plane going through rotor axis  17 . The two coil sections  29 , in this context, are also connected electrically in parallel, and connected to respectively same commutator segments  22 , so that the two coil sections  29  act together with respect to magnetic poles  32  of stator  34  as one single coil  30 . This is shown, for example, at a specific coil  53 , at which the first coil section  29  is wound between the first and the fourth slot  24 , going clockwise, and second coil section  29  between the eighth and the eleventh slot  24 . This coil  53 , made up of two coil sections  29 , has in each case thirteen conductor loops  36 , for example. Coils  30  of rotor  18 , that follow clockwise are made up respectively of  11 ,  10 ,  12 ,  12 ,  10 ,  11  conductor loops  36 . In this exemplary embodiment, commutator  20  has fourteen commutator segments  22 , which are connected to the seven coils  30 , made up of altogether fourteen coil sections  29 . In this context, after a commutation of seven successive coils  30 , the same phase position of the commutation is reached again as the one at the outset, so that, in the case of fourteen commutator segments  22  and four brushes  28 , four periods  38  come about over one rotor revolution. 
         [0018]    To determine rotational speed data, the motor current signal flowing through brushes  28  and commutator  20  is evaluated with respect to its ripple, and from this a signal is obtained which represents the rotational speed and period duration of the rotor revolution. For this purpose, the motor current signal is supplied to an electronics unit  40  which has a jamming protection function  44 . In order to determine, for example, whether a certain closing force is exceeded for a part that is to be moved by electric machine  12 , the signal representing the rotational speed is investigated for its change. For this, the measured values having the frequency of the current ripple read in, are compared to one another, in order to detect a rotational speed decrease. In order to trigger the closing force limitation, the changing value of the signal representing the rotational speed is compared to a specifiable signal, for example, so that a certain threshold for a closing force or a spring rate may be set. 
         [0019]    It should be noted that, with respect to exemplary embodiments shown in the figures and the description, multiple combinations are possible among the individual features. Thus, for instance, the number of magnetic poles  32  and of the commutator segments  22  may be varied. Because of that, the current ripple signal generated may be adjusted to the requirement of the rotational speed evaluation, the current ripple signal preferably having a lower frequency than the slot frequency. The number, positioning and development of magnetic poles  32 , of coils  30  and of slots  24  may be adapted to the respective application, especially to the respective power requirement. Thus, electric machine  12  may also be developed as an external-rotor motor. The method of winding coils  30  may also be varied, and individual tooth windings may also be used, whose number of conductor loops is modulated according to the present invention. Electric machine  12  is preferably used for actuating drives in a motor vehicle, for instance, for adjusting seat components, window panes and covers, but is not limited to such applications.