Abstract:
In a compact low-cost brushless D.C. motor, a measuring magnetization zone ( 24 ) is provided near the rotational axis ( 7 ) whose division into magnetic poles ( 14, 15 ) is aligned to that of the driving magnetization zone ( 6 ) of the rotor ( 4 ). The measuring magnetization zone ( 24 ) is essentially annular in design and its diameter is so small that the Hall elements ( 16 ) necessary for the detection of a complete north/south magnetization period can be integrated on a semiconductor chip of presently current size. The assembly of the sensor ( 13 ) containing such a chip necessarily involves a number of error sources on account of manufacturing conditions (sensitivity tolerances of Hall elements  16 ), on one hand, and of the limited adjusting possibilities. However, the resulting deviations from the ideal signal can be compensated by corresponding corrections in the evaluating electronics and by providing a number of redundant Hall elements ( 18 ). The latter allow to find an optimum selection among a greater number of Hall elements ( 16, 18 ) than necessary in the course of an adjusting run in the assembled motor.

Description:
FIELD OF THE INVENTION 
     The present invention refers to a brushless electromotor comprising a rotor having a permanently magnetized propulsion magnetization zone with at least one pair of magnetic poles, and a stationary coil assembly with at least one stator coil, said rotor being rotationally movable by the magnetic fields of said stator coils. 
     BACKGROUND OF THE INVENTION 
     Brushless D.C. motors essentially comprise a rotor in the form of a multipolar permanent magnet. The rotor is rotated by electronic commutation of the current in the stator coils. For this control of the current in the stator coils it is necessary to determine the position of the rotor as precisely as possible: the more precisely the position of the rotor is known, the more precisely the total field of the stator coils may be adjusted for a smooth operation resp. the position of the rotor may be regulated e.g. if an actuator is concerned. 
     A known measuring method of the rotor position is to arrange Hall elements in the area of the rotor, i.e. usually near its circumference, an electric parameter of which varies in function of a magnetic field, e.g. the Hall voltage or the resistance. Unless it is provided with the magnetization for the propulsion of the rotor anyway, the area of the rotor which travels past the Hall elements receives a particular magnetization whose division and position exactly corresponds to the magnetization which serves for the propulsion. 
     The disadvantage of the known embodiment is that the Hall elements must be adjusted very precisely in order to obtain an optimum commutation of the stator coil current. Since the Hall elements are discrete components, each Hall element generally has to be mounted and adjusted individually. Altogether, this results in a relatively complicated construction, a laborious adjustment and high requirements with respect to the positioning precision. 
     According to FR-A-2 155 303, the axle e.g. of a motor is provided with a magnet assembly, and a quantitative detection of the rotation of the axle in fractions of turns is effected by means of a magnet sensor which scans the magnet assembly. In one embodiment, the magnet assembly essentially consists of two annular magnets which are disposed at a certain distance from each other and comprise a number of axial magnetizations. The disposition of the two magnets is such that an annular zone with an axial magnetic field of changing polarity is formed in the gap between the magnets while the field lines are almost parallel at the center of the gap. The multiple sensor disposed in this gap responds when a respective threshold of every sensor is exceeded or not attained, whence the position of the magnet assembly is deduced. The construction of the magnet assembly results in relatively high field intensities and sharp transitions between the different magnetization zones. The number of sensors is chosen such that the series of sensors is shorter than a complete magnetization zone. 
     This disposition requires an additional, special measuring magnet, and on account of the construction size, the overall conception requires magnetic field sensors in the form of discrete components. An integration of the Hall elements on a chip is not mentioned either. 
     EP-A-0 590 222 describes a linear position detector which comprises a number of Hall elements which are integrated in a semiconductor chip. The two respective adjacent Hall elements are determined between which the magnetic induction generated e.g. by a magnet which is displaceably arranged above the sensor passes through zero. The resolution of this detector corresponds to the distance between two Hall elements. If used for the detection of an arcuate movement, at least the problem of the tangential positioning error remains unsolved, and the increased requirements for a continuous and ungradated detection of the rotational position of the rotor of a D.C. motor for the generation of a continuous stator coil current are not mentioned. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a brushless electromotor comprising a rotational position sensor which allows a more economical overall motor manufacture. 
     Another object is to design the detection device in such a manner as to allow an increased arithmetic resolution of the rotational position detection. 
     At least one of the mentioned objects is attained by an electromotor wherein a permanently magnetized measuring magnetization zone is provided on the rotor and a sensor is disposed in the range of the magnetic field of said measuring magnetization zone, said sensor comprising an arrangement of at least five elements which are sensitive to the magnetic field and are integrated on a semiconductor chip in a straight or curved row essentially, and which cover more than one period of the measuring magnetization, so that the position of the rotor can be determined with high precision from the output signal of said field-sensitive elements, which is generated by the action of the magnetic field of the measuring magnetization zone, by numeric correlation of the measuring signals with a predetermined reference curve corresponding to one or several period(s). Further preferred embodiments of such an electromotor are described herein. 
     In the D.C. motor of the invention, the magnetization zone which serves for the measurement of the rotor positions is disposed very close to the rotational axis of the rotor. Furthermore, this magnetization zone is essentially annular in shape, and its magnetization is preferably parallel to the rotational axis. Thus, the position of the rotor may be detected by a number of Hall elements which are preferably disposed on a circle arc section and which may be integrated in an economical manner on a semiconductor chip due to the small spatial dimension of the measuring magnetization zone. According to presently used chip sizes, the arc length on which the Hall elements used for the measurement are formed is smaller than 5 mm and preferably 3 mm at the most. However, greater arc lengths are not completely excluded, but in the case of an integration on a single chip, this would be increasingly uneconomical. 
     In order to avoid complicated adjustments particularly in the tangential direction in spite of the small dimensions and the strong curvature of the measuring arrangement, a greater number of Hall elements than necessary for the measurement is generally provided. The tangential displacement can be realized by the selection of a group of Hall elements from the number of available Hall elements. If integrated on a chip, redundant Hall elements can be provided economically as well. 
     Preferentially, the arcuate line on which the Hall elements are disposed on the chip may have a smaller curvature than the desired measuring line in the measuring magnetization zone. As the measuring magnetization poles become very narrow near the axle, the available magnetic field intensity is also strongly reduced, especially at a distance of the Hall elements resp. of the chip from the measuring magnetization surface in the millimeter range. This again results in increased requirements with respect to the positioning precision, which can be fulfilled by the redundant measuring elements in a surprisingly effective manner, however. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be further explained by way of an exemplary embodiment and with reference to figures. 
     FIG. 1 shows a cross-section of an elementary representation of a D.C. motor; 
     FIG. 2 shows a cross-section according to section line II—II in FIG. 1; and 
     FIGS. 3 to  5  show cross-sections in analogy to FIG. 1 of further embodiments of D.C. motors. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an elementary representation of a D.C. motor  1  comprising a stator coil  2  which is capable of generating a magnetic field in the direction of arrow  3 . Another stator coil generates e.g. a magnetic field perpendicular to the plane of the drawing, so that the commutation of the coil current allows to generate a rotary field whose direction is essentially defined by a rotation of arrow  3  around rotational axis  7  of the motor. So far, it is a usual brushless D.C. motor. 
     Rotor  4  essentially consists of a magnetizable ferrite material with a polymer binding additive, and it is produced by injection-molding. It comprises an essentially cylindrical outer envelope  6  which is magnetized perpendicularly to rotational axis  7  of the motor while forming six pairs of magnetic poles. It is thus identical to propulsion magnetization zone  5 . Rotor  4  is rotatably mounted on stationary axle  27 . For this purpose, rotor  4  is provided with a bearing point  8  which allows a small friction when rotating around axle  27 . Axle  27  is fixed to stator  9  of the motor. 
     For further guidance, rotor  4  comprises a guiding sleeve  10  which surrounds axle  27 . The lower end  11  of sleeve  10  is provided with the measuring magnetization in parallel to rotational axis  7  (arrow  12 , FIG.  2 ). Like the propulsion magnetization zone, this measuring magnetization zone consists of six pairs of magnetic poles which are precisely aligned to the poles of propulsion magnetization zone  5 . 
     Sensor  13  is disposed near the lower end  11  of sleeve  10 , so that the magnetic fields emanating from the measuring magnetization zone can produce a signal in the field-sensitive parts of sensor  13 . 
     FIG. 2 shows the arrangement of sensor  13  on a strongly enlarged scale. The annular end  11  of guiding sleeve  10  is magnetized to form a measuring magnetization zone  24  which is divided into six pairs of magnetic poles (north poles  14 , south poles  15 ) whose magnetization direction is perpendicular to the plane of the drawing. Sensor  13  comprises a number of Hall elements  16  in an arc-shaped arrangement which is adapted to the shape of end  11  in the area of two adjacent poles  14 ,  15 . On both ends of Hall element arrangement  16 , redundant Hall elements  18  are added whose signification will be explained hereinafter. Hall elements  16 ,  18  are integrated on a semiconductor chip  51  and preferably combined at least with parts of the control and evaluation electronics. Under consideration of the particular requirements of the invention as discussed below, these electronics may be designed according to known principles, so that a further, detailed explanation of the same is superfluous. 
     Essentially, in order to determine the position of rotor  4 , the limit between a north and a south pole  14  resp.  15  is used where the signal supplied by Hall elements  16  exhibits a crossover or a relatively sharp extreme. For a precise determination of this limit, such a number of Hall elements  16  is used that the length of the covered arc corresponds to a full polarization period of measuring magnetization zone  24 . Possible tolerances are compensated by redundant Hall elements  18 . In a calibration procedure after the assembly of the motor, it is determined how many and which Hall elements  16 ,  18  are necessary in order to cover such a full magnetization period. Only the signals of the Hall elements  16  designated in this manner are evaluated for the operation of motor  1 . 
     Since it is preferred to detect a complete magnetization period and today&#39;s current chip dimensions are up to approx. 5 mm, essentially the same maximum arc length can be covered by the Hall generators  16 ,  18 , which means that the diameter of measuring magnetization zone  24 , which in the present case is identical to lower end  11 , must have a correspondingly small radius such as a radius of 5 mm at the most, and preferably of 3 mm at the most. Preferably, however, the arc length covered by Hall elements  16 ,  18  is smaller, i.e. around 3 mm or less, for example. 
     Sensor  13  may be subject to positioning errors with respect to the measuring magnetization as indicated by arrows  19  (tangentially displaced),  20  (tilted around the x axis or tangentially twisted), and  21  (tilted around the y axis or radially twisted). A twisting according to arrow  22  around the z axis perpendicularly to the plane of the drawing corresponds to a tangential displacement according to arrow  19  and may therefore be treated in the same way. 
     A radial twisting (arrow  21 ) results in a variation of the frequency resp. of the number of Hall elements  16  which are necessary in order to cover an entire magnetization period. A compensation may be obtained by the application of additional Hall elements out of the redundant Hall elements  18 . A tangential twisting according to arrow  20  results in a mere amplitude variation. However, this error is attenuated by the fact that the relatively sharp transition between the north and the south pole is being determined, on one hand, and that a correction of the weighting of the amplitude of the signal supplied by each one of the Hall generators  16  may be effected, on the other hand. This correction may be determined in a calibrating procedure after the assembly of the motor and permanently stored in the evaluating electronics. In contrast, a tangential displacement according to arrow  19  results in a phase error, i.e. the position of rotor  4  is determined with an angular deviation corresponding to the tangential displacement. This error may be corrected by the selection of the Hall elements  16  from the total number of available Hall elements  16 ,  18  as well, i.e. by defining the measuring zone in such a manner as to compensate the displacement  19 . 
     Another source of errors is the considerable relative and absolute dispersion of the parameters of Hall elements  16 ,  18 . As indicated above with respect to the other error sources, a compensation of these effects may also be obtained in an adjusting procedure in the assembled motor. The usual parameter for an optimum adjustment is a maximum idle speed of the motor. Mathematical methods and algorithms allowing to determine such an optimum in function of a large number of partly interdependent parameters are known per se. 
     In contrast to usual rotors, rotor  4  according to the exemplary embodiment is injection-molded in one piece from a ferrite material containing a synthetic binding agent. Instead, conventional rotors comprise a steel axle around which the propulsion portion is injected from a ferrite material. However, on account of the poor running properties and stability of the ferrite material, whose running and bearing surfaces are therefore subject to a relatively high wear, the latter are preferably protected by steel elements also in view of a free rotation of the rotor. 
     It will be noted that in the extreme case, sensor  13  may contain the entire control electronics of the motor, i.e. also the high-current drivers for coils  2 , for example, thus allowing a hitherto unattained simplification and compactness of the D.C. motor. 
     In this case, the motor would ultimately only require power supply terminals and inputs allowing to set a position and/or a rotational speed. 
     Modifications of the represented embodiment are apparent to those skilled in the art from the description without leaving the scope of the invention. Some of the possible modifications are indicated hereinafter: 
     Another number of pole pairs can be used. 
     An unaligned disposition of the magnetization limits of the propulsion magnetization zone and of the measuring magnetization zone is also conceivable as the position of the rotor can be determined independently thereof. In this case, however, compared to a disposition where the magnetization limits are basically aligned, additional measures are necessary in order to shift the phase in the commutation of the coil current. 
     Other possible rotor shapes are shown in FIGS. 3 to  5 , where elements corresponding to FIGS. 1 and 2 are designated by the same reference numerals: Rotor  4  of FIG. 3 comprises an axle  30  which is guided in a cylindrical guide  31  of the stator and rests on a point  33 . Axle  30  is provided with a collar  32  which extends around seat  34  and whose annular front surface includes measuring magnetization zone  24  which is magnetized in the direction of arrow  12 . Sensor  13  is disposed in a recess  34  of guiding sleeve  31 . 
     Similarly to the rotor of FIG. 3, rotor  40  of FIG. 4, an external rotor, is provided with a central axle while its driving portion  6  extends around coils  2 . A collar  32  for the measuring magnetization is omitted, the lower end  40  of shaft  30  being provided with the measuring magnetization zone instead. Here also, sensor  13  is again disposed in a recess of guiding sleeve  31 . 
     FIG. 5 finally shows a rotor  4  having a disk-shaped driving portion  6 . Here, the propulsion magnetization of driving portion  6  follows the direction of arrow  45 , i.e. it is parallel to axle  4 , and the magnetization extends near axle  4 , so that its magnetic field is also detected by sensor  13  which is disposed near axle  4 . Consequently, in this embodiment, it is not necessary to provide two distinct magnetizations for the propulsion magnetization zone and the measuring magnetization zone. 
     The motor may also have a different number of coils while two coils are both a current configuration and the minimum  10  used in practice. Other current coil numbers are  1 ,  2 ,  3 ,  5 , and  7 . The possibility to use only one coil is thus not excluded. The rotor might also consist of another permanently magnetizable material, at least those elements which must be magnetized. The axle of the rotor might be made of steel with a synthetic material injected thereon. One or both ends of the axle might pass through the housing of the motor, as shown in the embodiment of FIG.  5 . 
     Another number of Hall elements may be provided. Although the complications of the evaluation increase with the number of Hall elements, the higher resolution of the position also allows an increasingly continuous commutation of the coil current. The minimal discretisation of the rotor position is determined in a manner known per se by the number of coils of the stator. Possible numbers of Hall generators are e.g.  5 (4),  10 (8),  20 (16), the number of adjacent Hall elements which are selected among the total number for the measurement being indicated in parentheses. Powers of 2 are particularly suitable for today&#39;s numerical methods while other numbers are possible if different evaluating methods are used. The number of redundant Hall sensors is downwardly limited by the resulting tolerance with respect to adjusting errors. Conversely, an upper limit is imposed by the manufacturing costs and the size of the available chip surface. Thus, the number of the field-sensitive elements is an integral power of two, more particularly 4, 8, 16 or 32, plus at least 1, preferably 1 to 4, and/or up to 25% of the respective power of two. 
     Another definition of redundancy may be expressed in percent, especially if greater numbers of Hall generators are concerned. The redundancy should not be less than 10% of the number of Hall generators intended for the group of Hall generators required for the measurement. A preferred range is between 20% and 50%. 
     Finally, an arrangement of Hall generators  16  could be provided which covers more than one magnetization period, e.g. an integral multiple of a magnetization period in the case of multiple poles. 
     In order to determine the rotor position, the following alternatives are possible, amongst others: 
     1. The signal of the Hall elements is correlated with a predetermined reference function, e.g. a sinusoidal one, while the phase of said reference function is optimized for maximum correlation. The position of the rotor can be determined by the means of this phase relation and by the reference function itself. 
     2. The signal of the Hall elements is analyzed by a method such as a Fourier transformation, and the obtained proportions of ideal functions allow to determine the rotor position. In the case of the Fourier transformation, coefficients for sine and cosine functions are calculated, and the first harmonic is generally considered in order to determine the rotor position. 
     Preferably, these methods make use of the fact that the active Hall generators cover exactly one measuring magnetization period or an integral multiple thereof: in this case, an additional correlation condition may be that the sum of the signals of all Hall elements, possibly after having determined a correction function in the manufacture of the motor, must be zero.