Abstract:
A magnetic field sensor includes a magnetizable core having a curved surface at least sectionally, a magnetization device for magnetizing the core, and a determination device for determining a magnetic field in the core.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a magnetic field sensor. 
       BACKGROUND INFORMATION 
       [0002]    Flux gate sensors for measuring magnetic fields are believed to be generally understood. In one variant of such flux gate sensors, a magnetically soft core is exposed to a magnetic alternating field, which drives the core into magnetic saturation using alternating field directions. A remagnetization of the core takes place whenever the magnetic alternating field is compensating an external magnetic field. The external magnetic field is able to be determined on the basis of an instant of the remagnetization in relation to the generated magnetic alternating field. Such sensors, often also referred to as MEMS sensors, can be produced as thin-film technology on a semiconductor substrate. 
         [0003]    Unpublished German patent application DE 10 2009 028 815.5 refers to a magnetic field sensor implemented in MEMS technology, in which a coil generates a magnetic field in a block-shaped core. 
       SUMMARY OF THE INVENTION 
       [0004]    The exemplary embodiments and/or exemplary methods of the present invention are based on the objective of providing a magnetic field sensor, with whose aid the instant of the remagnetization is able to be determined more precisely. 
         [0005]    The exemplary embodiments and/or exemplary methods of the present invention are intended to solve the stated objective by a magnetic field sensor having the features described herein. The further embodiments indicate advantageous configuration variations. 
         [0006]    A magnetic field sensor includes a magnetizable core, a magnetization device for magnetizing the core, and a determination device for determining a magnetic field in the core, the core having a curved surface, at least in sections. In particular in a miniaturized magnetic field sensor (MEMS), the curvature of the surface of the core is able to prevent the occurrence of areas that are poorly magnetizable, so that magnetic domains of the core require no greatly differing fields for the remagnetization. A statistical fluctuation of the remagnetization instant is therefore able to be reduced and the measuring accuracy of the magnetic field sensor is improved as a result. 
         [0007]    The core may include a longitudinal section having a positive curvature. The curvature may be positive along the entire longitudinal section. In particular, it is possible for the curvature not to exceed a predefined value along the longitudinal section. This results in a core having rounded contours, so that poorly magnetizable domains are able to be reduced further. 
         [0008]    The core may be symmetrical in relation to its longitudinal axis. In contrast to a flat development, this makes it possible to avoid additional corners and edges of the core, so that the magnetization ability of the domains of the core becomes more uniform. This may lead to further improvements in the magnetic field determinations. 
         [0009]    The core may have a pointed or conical end section. Because of an attendant reduction or avoidance of end domains, the remagnetization is able to be shifted to a still narrower time range, so that the measuring accuracy of the magnetic field sensor is able to be improved further. 
         [0010]    Moreover, the core may have an asymmetrical form, e.g., in that a geometric centroid of the core is shifted along the longitudinal axis of the core through distortion of the outer dimensions of the core in the direction of an end section. For instance, this may be achieved by an essentially trapezoidal development of the core. A beginning of a remagnetization process of the core is thus able to be defined more optimally, so that a temporal reproducibility of the core&#39;s remagnetization may be improved further. 
         [0011]    In additional specific developments, the core may have a plurality of sections which have differently sized longitudinal section surfaces along the core&#39;s longitudinal axis, so that areas having a defined magnetization are specified for starting the remagnetization process. 
         [0012]    The exemplary embodiments and/or exemplary methods of the present invention will now be described more accurately with reference to the accompanying figures. 
         [0013]    DETAILED DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  shows a basic diagram of a flux gate magnetic field sensor. 
         [0015]      FIG. 2  shows time sequences at the magnetic field sensor from  FIG. 1 . 
         [0016]      FIG. 3  shows views of different cores for the magnetic field sensor from  FIG. 1 . 
     
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  shows a basic diagram of a magnetic field sensor  100 . Magnetic field sensor  100  includes a first coil  110 , a second coil  120 , and a core  130 . Magnetic field sensor  100  is designed as thin-film system. In one exemplary embodiment of a miniaturized (MEMS) magnetic field sensor  100 , core  130  has a length of a few 100 μm to a few mm and a width of typically 20 to 200 μm. First coil  110  and second coil  120  may each include one or more windings, and each winding may be formed on a substrate of magnetic field sensor  100 . The windings may enclose core  130  or run adjacent to core  130 . An indication of the magnetic field inside core  130 , such as a magnetic flux density or a magnetic flux, may be determined with the aid of second coil  120 . 
         [0018]    A periodic (e.g., triangular) voltage characteristic is applied at first coil  110 , so that a magnetic field which periodically decreases and increases is generated in the region of core  130 . Core  130  may be made of a magnetically soft material that has a low hysteresis. 
         [0019]    Because of the magnetic alternating field caused by first coil  110 , core  130  is subjected to periodic remagnetization when a direction of the magnetization of core  130  changes. At the remagnetization instants, a voltage U 2  is induced in second coil  120  (“pickup coil”). As will be explained in the following text, an external magnetic field is able to be determined based on an instant of such a voltage pulse  220 . In order to measure the instant of the pulse as precisely as possible, the pulse must be as narrow as possible in relation to a period of delta voltage U 1 . For this purpose, a material of core  130  is usually selected in such a way that the hysteresis of core  130  is as low as possible. 
         [0020]    In miniaturized flux gate magnetic field sensors, there is a limit to the optimization of the smallness of the hysteresis of core  130  via a corresponding selection of material and manufacturing process of core  130  within the framework of a production process of a miniaturized system. Furthermore, as the miniaturization of coils  110 ,  120  and core  130  continues, the strength of pulse  220  drops, so that an evaluation of signal voltage U 2  becomes more difficult. 
         [0021]      FIG. 2  shows a diagram  200  of time characteristics of voltages U 1  and U 2  at magnetic field sensor  100  from  FIG. 1 . A characteristic  210  shown in the upper portion of diagram  200  represents a characteristic of voltage U 1  at first coil  110  in  FIG. 1 . Pulses  220  shown in the lower portion of diagram  200  correspond to voltage pulses of U 2  at second coil  120  in  FIG. 1 . 
         [0022]    Characteristic  210  is a symmetrical delta signal. A magnetization of core  130  is proportional to characteristic  210 . At instants t 1 , t 4 , t 5  and t 8 , voltage U 1  of characteristic  210  has the value of  0 . If no external magnetic field is applied, then a remagnetization of core  130  takes place at these instants in  FIG. 1 , which is detectable by pulses  220  of voltage U 2  of coil  120  at the same instants. 
         [0023]    If core  130  has been premagnetized by an external magnetic field, remagnetizations of core  130  take place at instants when the external magnetic field is compensated by the magnetic field produced by first coil  110 . In the illustration of  FIG. 2 , this is the case whenever first characteristic  210  corresponds to external magnetization  230 , i.e., at instants t 2 , t 3 , t 6  and t 7 . 
         [0024]    From a relative position of pulses  220  with respect to each other or with respect to characteristic  210 , it is possible to determine the intensity or direction of the external magnetic field. In order to perform a measurement of pulses  220  or of instants t 1  through t 8  as precisely as possible, pulses  220  of voltage U 2  must reach a predefined voltage and be as small as possible in the process. 
         [0025]    A ferromagnetic material like core  130  frequently has a crystal structure that includes magnetized domains. These domains are referred to as Weiss domains and have an extension in the range of approximately 10 −8  to 10 −4  m. The boundaries between the Weiss domains are called Bloch walls. In general, the Weiss domains are magnetized until saturated and the magnetization of different Weiss domains has different directions. In an increasing magnetic field, the Bloch walls dislocate in favor of the particular Weiss domains that are aligned in the direction of the external field. In an external field that continues to increase, more and more Weiss domains ultimately change their magnetic alignment. 
         [0026]    The dislocation motion of the Bloch walls may be hampered by lattice faults in the crystal of the ferromagnetic material, by grain boundaries or a limitation of the magnetic material itself. This effect is called pinning. The magnetization of the ferromagnetic material thus does not increase in accordance with the externally steadily increasing magnetic field, but by small differences, the Barkhausen jumps. This prevents a uniform remagnetization of the ferromagnetic material, so that in the case of core  130  in  FIG. 1 , pulses  220  from  FIG. 2  experience an expansion in a temporal (horizontal) direction. The core of the invention focuses on forming core  130  in such a way that a remagnetization of core  130  in a miniaturized magnetic sensor  100  is possible in a homogeneous and rapid manner. For this purpose the boundaries of core  130  are developed such that pinning effects are prevented. In addition, core  130  may be formed in such a way that a remagnetization process is influenced by the form of core  130 . 
         [0027]      FIG. 3  shows longitudinal sections of different cores  130  for magnetic field sensor  100  from  FIG. 1 . Each of the illustrated longitudinal sections  310  through  370  may pertain to a core  130  which is essentially flat, so that longitudinal sections  310  through  370  correspond to a plan view of core  130 . Such cores may be produced in thin-film technology. 
         [0028]    In one variant of the exemplary embodiments and/or exemplary methods of the present invention, core  130  is developed in axial symmetry with respect to a longitudinal axis L of core  130 , so that the three-dimensional form of core  130  is able to be defined by the rotation of longitudinal sections  310  through  370  about their longitudinal axes, and the particular core has circular cross-sections exclusively. Intermediate forms between a flat and a round development, such as flattened or elliptical cross-sections, are likewise possible. The production of such cores may require a production method other than thin-film technology. 
         [0029]    Longitudinal sections  310  through  370  all have sections at which a surface O of core  130  is curved. In these sections, shifting of Bloch walls through a delimitation of core  130  is hampered to a lesser degree. In all longitudinal sections  310  through  370 , a ratio between length and width of the particular longitudinal section is selected such that the movement of the Bloch walls is hampered as little as possible. A core  130  formed in this way is known as “narrow core” in the literature. 
         [0030]    First longitudinal section  310  has the shape of a rectangle with rounded end sections E. The roundings of end sections E may merge in pair-wise manner, so that end sections E have the form of semicircles or elliptical sections. 
         [0031]    Second longitudinal section  320  corresponds to first longitudinal section  310 , but additionally includes a tapered section in a center section M between the ends. Transitions between end sections E and tapered section M may be rounded. Because of tapered section M, the field strength required for the abrupt magnetization of core  130  is able to be controlled via the form of core  130 . There is increased magnetic flux density in the region of tapered section M, which promotes rapid remagnetization of section M. Given an identical electrical signal shape  210 , thickened end sections E lead to smaller magnetic fields in  FIG. 2 , which offers advantages in the component with regard to measuring range and alignment. This effect also improves the temporal reproducibility of pulses  220  in  FIG. 2  and thus reduces the noise of miniaturized magnetic field sensor  100  from  FIG. 1 . 
         [0032]    Third longitudinal section  330  includes a rectangular center region M, which transitions into two end sections E having a triangular form in each case. The peaked shape of triangular end sections E avoids poor magnetization in these regions and furthermore offers a starting and end point for a Bloch wall that is shifting through core  130 . Due to the lack of end domains, the entire material of core  130  in longitudinal section  330  is able to contribute to signal  220 . 
         [0033]    Fourth longitudinal section  340  has the form of a symmetrical ellipse. With regard to the advantages of this longitudinal section, the above comments in connection with third longitudinal section  330  apply. In addition, the elliptical form of longitudinal section  340  prevents the occurrence of regions that are poorly accessible to an external magnetic field. 
         [0034]    Fifth longitudinal section  350  corresponds to first longitudinal cross-section  310  but has a pronounced narrow region in a center section M. This pronounced narrow region causes an extreme flux density excess in this area, which immediately leads to a remagnetization of adjoining regions. 
         [0035]    Sixth longitudinal section  360  results from the basic form of first longitudinal section  310  and has end sections E that have an even flatter form; it also has a segmented center region M. In segmented center region M, segments Al having a first width alternate with segments A 2  having a second width (in the horizontal direction). Transitions between adjacent segments A 1 , A 2  may be rectangular, as illustrated, or also rounded as shown in center region M of fifth longitudinal section  350 . The serrated edge of longitudinal section  360  reduces pinning of Bloch walls; at the same time, regions of defined magnetization are offered for starting the remagnetization process. A ratio of widths of adjacent segments A 1 , A 2  may be selected as desired and need not have the 1:1 ratio illustrated. 
         [0036]    Seventh longitudinal section  370  results from a trapezoidal distortion of first longitudinal section  310 . The distortion images a rectangle into a trapezoid; the base line of the trapezoid may extend parallel to longitudinal axis L of core  130  or perpendicular to longitudinal axis L, as in core  370 . Because of the defined asymmetry of longitudinal section  370 , a more optimally defined start of the remagnetization process and thus an improved temporal reproducibility of pulses  220  from  FIG. 2  are able to be achieved. A centroid of core  130  in the seventh longitudinal section lies to the right of a transverse axis Q, which halves longitudinal axis L in core  130 . The defined asymmetry of seventh longitudinal section  370  is basically applicable to any one of longitudinal sections  310  through  360  and may be realized by appropriate distortions. 
         [0037]    A temporally precisely defined and rapid remagnetization of core  130  in miniaturized system  100  from  FIG. 1  is improved by the form of core  130  illustrated by longitudinal sections  310  through  370 , especially by providing rounded sections. In effect, the shape of core  130  according to the present invention makes it possible to improve the measuring accuracy of magnetic field sensor  100 . In addition, other measures such as the selection of a material or a manufacturing process for core  130  or for magnetic field sensor  100  may be used to optimize the remagnetization of core  130  even further.