Abstract:
The invention relates to a device and to a method for contactlessly recording rotation angles of a rotating element, with a plunger core and with a coil at least partially surrounding the plunger core. The plunger core and the coil move relative to one another in an axial direction according to the rotational motion of the rotating element and causes a change in a coil inductivity of the coil. The inventive device and the inventive method are characterized in that compensating means are provided, which at least partially compensate for the influence of a changing temperature upon the coil inductivity.

Description:
PRIOR ART 
       [0001]    The invention relates to a method and a device for contactless detection of the rotation angle of a rotatable element as generically defined by the preambles to the independent claims. 
         [0002]    From German Patent Disclosure DE-A 100 17 061, an arrangement for in particular contactless detection of the rotation angle of a rotatable element is known, in which by evaluation of magnetically variable properties of a sensor array with at least two sensor elements, a magnetic field intensity generated or varied by the rotatable element is detectable in an evaluation circuit and used for ascertaining the rotational position; one sensor element functions by utilizing the magnetoresistive effect, and at least two further sensor elements operate by utilizing the Hall effect, and the evaluation circuit serves the purpose of logically linking the three sensor signals thus obtained. 
         [0003]    For contactless detection of the rotation angle of a rotatable element, in addition to a magnetoresistive sensor element which outputs at least a first signal for detecting a rotation angle in a first range, it is also known to use a plunger core, disposed on a shaft of the rotatable element, as well as a coil at least partly surrounding the plunger core, and the plunger core and coil move in the axial direction relative to one another as a function of the rotary motion of the shaft, so that rotation angles that go beyond the first range can be unainbiguously detected. 
         [0004]    From Japanese Patent Disclosure JP-A 2004226124, a rotation angle detector, comprising a ring magnet and two Hall elements, of an angle sensor is known in which, during the manufacturing phase, a detection error resulting from temperature changes and variations in mass-produced items is compensated for using measured amplitudes and offset voltages of the Hall signals. 
         [0005]    It is also known from Japanese Patent Disclosure JP-A 2003161637 to correct the temperature of a detection coil of a device by measuring the temperature resistance of the detection coil using a resistor connected in series with the detection coil and comparing the resultant measured temperature values with temperature data stored in memory in a table. 
       ADVANTAGES OF THE INVENTION 
       [0006]    Compared to the prior art, a device and method of the invention for contactless detection of the rotation angle of a rotatable element, having a plunger core and having a coil at least partly surrounding the plunger core, the plunger core and the coil moving relative to one another in the axial direction as a function of the rotary motion of the rotatable element and causing a change in a coil inductance of the coil, have the advantage that temperature influences that cause an unintended change in the coil inductance can be compensated for during the rotation angle detection. In this way, erroneous information, which in an electric power steering drive mechanism, for instance, could lead to safety-critical situations, can be avoided effectively and economically. For that purpose, compensation means are provided, which at least partly compensate for the influence of the varying temperature on the ascertained coil inductance. 
         [0007]    Advantageously, the compensation means include a reference coil inductance, which can be ascertained from at least one reference coil with an immovable core, and the at least one reference coil and the coil and/or the immovable coil and the plunger core should have approximately the same material properties, so that forming a ratio of the inductances maximally eliminates the influence of temperature. It is furthermore advantageous if the reference coil is located in the spatial vicinity of the coil, so that both coils will experience a comparable temperature influence. 
         [0008]    In an alternative embodiment, the reference coil inductance of at least the region of the coil which upon axial motion predominantly or always embraces the plunger core is ascertained. As a result, there is the advantage that no additional reference coil is necessary for ascertaining the reference coil inductance, and thus both costs and installation space can be saved. 
         [0009]    Advantageously, it is also provided that the compensation means include at least one temperature-dependent sensor element for measuring measured temperature values and at least one reference means; for compensating for the influence of the temperature on the coil inductance, the measured temperature values effected with reference temperature values stored in memory in a reference table of the at least one reference means and/or by computation using an algorithm contained in the at least one reference means. A resistor with a negative temperature coefficient (NTC) can for instance be used as the temperature-dependent sensor element. 
         [0010]    From the prior art already discussed at the outset, it is known, in addition to the plunger core disposed on the rotatable element and to the coil at least partly surrounding the plunger core, to use a magnetoresistive sensor element for the rotation angle detection. In this connection, the magnetoresistive sensor element may especially advantageously serve as a compensation means, in that the absolute amplitudes and/or offset voltages of the plurality of sensor signals output by the magnetoresistive sensor element are measured prior to a standardization operation and/or formation of a ratio between the sensor signals. 
         [0011]    Further advantages of the invention will become apparent from the characteristics recited in the dependent claims and from the drawings and the ensuing description. 
     
    
     
       DRAWINGS 
         [0012]    The invention is described below in terms of examples in conj unction with  FIGS. 1 through 5 , in which the same reference numerals refer to the same components having the same mode of operation. Shown are 
           [0013]      FIG. 1 : a schematic illustration of a device for contactless detection of the rotation angle of a rotatable element in the prior art; 
           [0014]      FIG. 2 : a schematic illustration of a first exemplary embodiment of the device of the invention; 
           [0015]      FIG. 3 : a schematic illustration of a second and third exemplary embodiment of the device of the invention; 
           [0016]      FIG. 4 : a schematic illustration of a fourth exemplary embodiment of the device of the invention; and 
           [0017]      FIG. 5 : a graph of the sensor signals, output by a magnetoresistive sensor element, prior to standardization as a function of the rotation angle of the rotatable element. 
       
    
    
     DESCRIPTION 
       [0018]    In  FIG. 1 , a schematic illustration is shown of a device  10  of the prior art for contactless detection of the rotation angle of a rotatable element  12 , having a magnetoresistive sensor element  14  which outputs two signals S M,1  and S M, 2  for detecting a rotation angle Θ of the rotatable element  12 . For triggering the magnetoresistive sensor element  14 , which in this case is embodied as an anisotropic, magnetoresistive (AMR) sensor  15 , a permanent magnet  16  is used that has a north pole N and a south pole S. Instead of a permanent magnet  16  with only two alternating poles (pair of poles), it is naturally equally possible to use permanent magnets with markedly more pairs of poles. It is equally possible, instead of the AMR sensor  15 , to use other magnetoresistive sensor elements. Below, however, for the sake of simplicity, an AMR sensor  15  will be assumed. 
         [0019]    The rotatable element  12  is embodied as an electrical power steering drive mechanism  18 , in which a shaft  20  which is connected to an electric motor  26  via a drive unit  22 , for instance a step-down gear not further described here, and a drive shaft  24 . 
         [0020]    The shaft  20  is a component of the rotatable element  12 . By means of the AMR sensor  15  and the permanent magnet  16  associated with it, rotation angles Θ in a first range D from 0° to 180° can be detected exactly and unambiguously. The AMR sensor  15  outputs the sensor signals S M,1  and S M,2 , which have a sinusoidal and cosinusoidal course as a function of the rotation angle Θ, and forwards them to an evaluation circuit  27 . The signals S M,1  and S M,2  have a periodicity of 180°, so that rotation angles Θ of more than 180+ can no longer be detected unambiguously using only a single AMR sensor. For unambiguous determination of rotation angles Θ outside this first range D, or in other words of more than 180+, a further device is accordingly necessary. To that end, on the shaft  20  a thread  28  is provided, with which, as a function of the rotary motion of the shaft  20 , a plunger core  30 , which may have a corresponding thread, not shown, or mandrel, also not shown, moves relative to a coil  31  in the axial direction R of the shaft  20 . The plunger core  30  may for instance comprise a ferromagnetic material, such as iron, neodymium, AlNiCo (an aluninum-nickel-cobalt alloy), or the like. 
         [0021]    If the shaft  20  now rotates by a certain amount, then the plunger core  30 , because of the thread  28 , moves in the axial direction R inside the coil  31  and causes a change in its coil inductance L. This change is sent by means of a coil signal S c  to a capacitor  32  hawing the capacitance C, and this capacitor together with the coil inductance L forms a first oscillating circuit  34  with the resonant frequency f R,1 ; the varying coil inductance L also causes a change in the resonant frequency f R,1 . Instead of a single capacitor  32  of capacitance C, naturally individual components or a plurality of different components may be provided that in combination with the coil inductance L bring about a characteristic resonant frequency f R,1  of the resultant first serial and/or parallel oscillating circuit. However, the assumption hereinafter will always be an LC oscillating circuit  34 . 
         [0022]    From the influence of a varying temperature T, for instance because of the radiated heat of an internal combustion engine installed in a motor vehicle, or sunshine, or the like, a change can occur in the coil inductance L of the coil  31 . According to the invention, compensation means  36  are therefore provided, which at least partly compensate for the influence of the varying temperature T on the coil inductance L. 
         [0023]    In a first exemplary embodiment, shown in  FIG. 2 , the compensation means  36  include a reference coil inductance L Ref , which results from a reference coil  38  with an immovable core  40 . The reference coil  38  and/or the immovable core  40  have approximately the same—and ideally identical—material properties as the coil  31  and the plunger core  30 . Moreover, the reference coil  38  and the coil  31  are disposed in the vicinity of one another spatially, so that any influence of the temperature T acts in the same way on both coils. In accordance with the description of  FIG. 1 , the reference coil  38  outputs a reference coil signal S R  to a further capacitor  42 , which should if at all possible have the same capacitance C as the capacitor  32  of the first oscillating circuit  34 . The reference coil inductance L Ref  and the capacitance C form a reference oscillating circuit  44  having a reference resonant frequency f R,2 . By forming a ratio between the resonant frequency f R,1  of the first oscillating circuit  34  and the reference resonant frequency f R,2  of the reference oscillating circuit  44 , the influence of the temperature T on the coil inductance L of the coil  31  can now be compensated for. If the two capacitors  32  and  42  have different capacitances, then they must be taken into account in forming the ratio of the reference resonant frequencies, to prevent the outcome from being wrong. 
         [0024]    In  FIG. 3 , two further exemplary embodiments of the device  10  of the invention are shown. Instead of an additional reference coil, however, the compensation means  36  now include that region B of the coil  31  which, to ascertain the reference coil inductance L Ref , predominantly or always embraces the plunger core  30  upon an axial motion in the direction R. 
         [0025]    In  FIG. 3   a , the coil  31  and the thread  28  located on the shaft  20  are embodied in such a way that the plunger core  30  cannot leave the region B of the coil  31  upon a rotary motion of the shaft  20 . In the region B of the coil  31 , the same conditions therefore always prevail, conditions which are changed only by the influence of the temperature T but not by the relative motion between the coil  31  and the plunger core  30 . All that is accordingly necessary is for the coil  31  to be tapped at both ends of the region B; one end of the region B is already defined by the end of the coil  31 , and hence only one additional tap is needed in order to ascertain the reference coil inductance L Ref . The resultant reference coil signal S R  is then forwarded, in a corresponding way to the coil signal S c  of the coil  31 , to the capacitor  32  of capacitance C for determining the reference resonant frequency f R,2  as described in conjunction with  FIG. 2 . From the ratio between the reference resonant frequency f R,2  and the likewise-ascertained resonant frequency f R,1  of the coil  31 , it is then possible in turn to compensate for the influence of the temperature T on the coil inductance L of the coil  31 . 
         [0026]    The exemplary embodiment in  FIG. 3   b  differs from that in  FIG. 3   a  only in a modified embodiment of the plunger core  30  and the thread  28 , so that now the region B of the coil  31  which predominantly or always embraces the plunger core  30  upon the axial motion of the plunger core is located in the middle of the coil  31 . In this way, although two additional taps of the coil  31  are necessary, by way of which the reference coil signal S R  of the reference coil inductance L Ref  is sent to the capacitor  32 , nevertheless this arrangement makes a lesser structural length of the shaft  20  of the rotatable element  12  possible, compared to  FIG. 3   a.    
         [0027]    In  FIG. 3 , it is understood that instead of one common capacitor  32 , a plurality of capacitors—as already described in conjunction with FIG.  2 —may be used; advantageously, however, it is not absolutely necessary for the capacitors to have the same capacitance C. Moreover, there is an alternative of ascertaining the coil inductance from measuring the times or amplitudes for a step response. 
         [0028]    A further exemplary embodiment for compensating for the influence of the temperature T on the coil inductance L of the coil  31  is shown in  FIG. 4 ; the compensation means  36  now include a temperature-sensitive sensor element  46 , such as a resistor with a negative temperature coefficient (NTC)  48 . Instead of an NTC  48 , however, still other temperature-sensitive sensor elements may be used, such as a PTC or the like. To compensate for the influence of temperature, the temperature T is measured by the NTC  48 , and a comparison is made of the measured temperature values T with reference temperature values T Ref  stored in memory in a reference table of a reference means  50 , in such a way that each reference temperature value T Ref  is allocated a certain reference resonant frequency f R,2 , which is put in ratio with the first resonant frequency f R,1  ascertained by means of the first oscillating circuit  34 . 
         [0029]    Instead of a reference table, it is naturally equally possible to compensate for the influence of temperature computationally with the aid of a suitable algorithm in the reference means  50 . In this way, higher accuracy can be attained, since the reference temperature values T Ref  stored in memory in the reference table originated in only a finite supply of values. As the reference means  50 , a microprocessor, ASIC, or other integrated circuit, for instance, which preferably has a comparator and a memory, may be used. It is understood that still other reference means  50  may be used, for instance if a discrete construction with separate groups of components for the arithmetic unit, the comparator and/or the memory is preferred. 
         [0030]    In  FIG. 5 , it is provided that the compensation means  36  include the additional AMR sensor  15  shown in  FIG. 1 . It outputs the sinusoidal and cosinusoidal sensor signals S M,1  and S M,2 , which are plotted in the graph shown in  FIG. 5  as a function of the rotation angle Θ before their standardization. The AMR sensor  15 , like the coil  31 , is thus subject to the influence of the temperature T. Therefore the two sensor signals S M,1  and S M,2  are also temperature-dependent, which can cause a change in their absolute amplitudes A 1  and A 2  and/or their offset voltages O 1  and O 2 , which must be measured before a standardization operation and/or formation of a ratio between the sensor signals S M,1  and S M,2 . Since both sensor signals S M,1  and S M,2  react in the same way to the influence of temperature, it suffices to use only one of the two absolute amplitudes A 1  or A 2  and/or offset voltages O 1  or O 2  for the compensation of the temperature influence. It is understood that the measured values of both sensor signals may also be used. The compensation is now done again with the aid of reference values, stored in memory in a reference table, for the amplitudes and/or offset voltages and the reference resonant frequencies f R,2  that can be derived from them, or by computation using an algorithm. 
         [0031]    It closing, it should also be pointed out that the exemplary embodiments shown are not limited to  FIGS. 2 through 5 . For instance, a plurality of compensation means  36  may be combined, or a plurality of reference coils or coil taps may be used as compensation means. It is moreover conceivable for the reference coil  40  to be disposed not parallel but at an arbitrary angle to the coil  31 , depending on the spatial requirements.