Abstract:
The invention relates to a device and a method for determining the rotational angle of a rotatable element in a non-contact manner, the device including at least one magnetoresistive sensor element which emits at least one first signal for determining a rotational angle of the rotatable element in a first region. A plunger core and a coil move in relation to each other in the axial direction of a shaft according to the rotary movement of the shaft, the coil emitting another signal relating to the modification of the coil inductance, such that rotational angles beyond the first region can be clearly determined in association with the first signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a 35 USC 371 application of PCT/EP 2006/050697 filed on Feb. 6, 2006. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to an apparatus and a method for contactless detection of the rotational angle of a rotatable element. 
   2. Description of the Prior Art 
   From German Patent Disclosure DE-A 100 17 061, an arrangement for contactless detection, in particular, of the rotational angle of a rotatable element is known, in which by evaluating 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 rotary position; one sensor element works by using the magnetoresistive effect, and at least two further sensor elements operate by utilizing the Hall effect, and the evaluation circuit serves the purpose of logical linkage of the three sensor signals thus obtained. 
   From Japanese Patent Disclosure JP-A 2004085482, an apparatus for detecting rotational angles over more than one revolution of a rotating shaft is also known it includes a first means for angle detection, a conversion means for converting a rotating motion into a longitudinal motion, and a further means for detecting a linear position of the rotating shaft by means of a spacing measurement. 
   SUMMARY AND ADVANTAGES OF THE INVENTION 
   Compared to the prior art, the apparatus and the method according to the invention for contactless detection of the rotational angle of a rotatable element have the advantage that because of increased resolution, unambiguous rotational angles can be detected or determined with very high precision and enhanced invulnerability to malfunction over a plurality of revolutions of the rotatable element. To that end, besides at least one magnetoresistive sensor element which outputs at least one magnetoresistive sensor signal for detecting a rotational angle of the rotatable element in a first range, the apparatus of the invention includes a plunger core and a coil, which move relative to one another in the axial direction of a shaft as a function of the rotary motion of the shaft; the coil outputs a coil signal pertaining to the change in the coil inductance, so that in combination with the magnetoresistive sensor signal, rotational angles beyond the first range can be unambiguously detected. Ascertaining the coil inductance moreover has the advantage that the unambiguous rotational angles are stored mechanically and thus even if the power supply is shut off, a mechanical adjustment of the rotatable element presents no problem, since the current position of the plunger core inside the coil is immediately available again once the power supply is resumed. 
   In an advantageous embodiment, it is provided that the position of the plunger core inside the coil is detectable by means of a resonant circuit, whose resonant frequency is dependent on the coil inductance. In this way, rotational angle detection that is highly secure against malfunction is assured, and the position resolution can be enhanced virtually arbitrarily as a function of the sampling rate during the measurement of the period length associated with the resonant frequency. 
   In a preferred way, there is a thread on the shaft and/or the plunger core, and the shaft is a component of the rotatable element. Thus the plunger core can move in the axial direction relative to the coil upon a rotary motion of the shaft. In an alternative feature, however, it may also be provided that the shaft is connected to the rotatable element via a gear. In this way, it is possible to keep the structural length of the shaft compact or to dispose the shaft in such a way that a favorable structural form of the entire apparatus for later installation is obtained. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described more fully herein below, with reference to the drawings, in which: 
       FIG. 1  is a schematic illustration of a first exemplary embodiment of the apparatus of the invention; 
       FIGS. 2   a  and  2   b  are graphs of a first signal output by a magnetoresistive sensor element and of a resonant frequency, generated by a resonant circuit, as a function oil the rotational angle and of the number of revolutions of a shaft associated with a rotatable element; 
       FIG. 3  is a schematic illustration of a second exemplary embodiment of the apparatus of the invention; and 
       FIG. 4  is a schematic illustration of a third exemplary embodiment of the apparatus of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 1 , a schematic illustration is shown of a first exemplary embodiment of the apparatus  10  of the invention for contactless detection of the rotational 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 rotational angle Θ of the rotatable element  12 . For triggering the magnetoresistive sensor element  14 , which is embodied in this case as an anisotropic, magnetoresistive (AMR) sensor  15 , a permanent magnet  16  with a north pole N and a south pole S is used. Instead of a permanent magnet  16  with only two alternating poles (a 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 always be assumed. 
   The rotatable element  12 , in the exemplary embodiments shown here in  FIGS. 1 ,  3  and  4 , is embodied as an electronic power steering drive  18 , in which a shaft  20 , which is connected to an electric motor  26  via a drive unit  22 , for instance a gear for speed reduction, not further described here, and a drive shaft  24 . 
   In the first exemplary embodiment in  FIG. 1 , 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, rotational angles Θ in a first range A from 0° to 180° can be detected exactly and unambiguously. The AMR sensor  15  outputs the signals S M,1  and S M,2 , which extend in sine wave and cosine wave fashion as a function of the rotational angle Θ, as shown in  FIG. 2   a  and carries them onward to an evaluation circuit  27 . It can be seen from the course of the signals S M,1  and S M,2  that a periodicity of 180° is present, and thus rotational angles Θ of more than 180° can no longer be unambiguously detected using only a single AMR sensor. Hence there is no need for a further device for unambiguously determining rotational angles Θ outside this first range A, or in other words angles of more than 180°. According to the invention, for that purpose, a thread  28  is provided on the shaft  20 , and with it, 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, is moved in the axial direction R of the shaft  20  relative to a coil  31 . Advantageously, the plunger core  30  comprises a ferromagnetic material, such as iron, neodymium, AlNiCo (an aluminum-nickel-cobalt alloy), or the like. 
   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 the coil inductance L of the coil. This change is forwarded by means of a further signal S c  to a capacitor  32  of capacitance C, which together with the coil inductance L forms a resonant circuit  34  with the resonant frequency f R ; the varying coil inductance L also causes a change in the resonant frequency f R . Instead of a single capacitor  32  of capacitance C, naturally single components or a plurality of different components may be provided that in conjunction with the coil inductance L bring about a characteristic resonant frequency f R  of the resultant series and/or parallel resonant circuit. Hereinafter, however, the assumption will always be an LC resonant circuit  34 . 
   It can be seen in  FIG. 2   b  that the resonant frequency f R  of the resonant circuit  34  depends linearly on the rotational angle Θ or the number U of revolutions and thus on the depth to which the plunger core  30  plunges into the coil  31 . If the AMR sensor signals S M,1  and S M,2  are now combined in the evaluation circuit  27  with the information about the resonant frequency f R  of the resonant circuit, unambiguous detection of the rotational angle beyond the first range A and furthermore beyond one full revolution of the shaft  20  is possible. As a result of this provision, there is in particular the advantage of an exact mechanical storage of the number U of revolutions of the shaft  20  as a consequence of the available coil inductance L once the motor vehicle ignition has been switched on. Accordingly, mechanically adjusting the steering of the motor vehicle no longer presents a problem for the ensuing rotational angle detection, even if the vehicle battery is shut off. 
   Upon each full revolution of the shaft  20 , the plunger core  30  moves by at least an axial distance D inside the coil  31 . To keep the tolerances in conjunction with a required positional or rotational angle resolution as slight as possible, either the coil windings can be placed suitably close together, or the axial adjustment distance of the plunger core  30  via the thread  28  can be selected to be relatively long. The axial distance D therefore also depends on these two parameters. Hence the axial distance D can be in the range of a few millimeters, in the case of coil windings located close together. A further increase in positional resolution is moreover possible by increasing the sampling rate during the measurement of the period length associated with the resonant frequency f R . 
   In  FIG. 3 , a second exemplary embodiment of the apparatus  10  of the invention for contactless detection of the rotational angle of the rotatable element  12  is shown. In contrast to  FIG. 1 , the plunger core is now no longer seated directly on the shaft  20  of the rotatable element  12 , but instead on a shaft  36  which is disposed parallel to the shaft  20  and which is driven via a gear  38  that comprises a first pinion  40 , mounted on the shaft  20 , and a second pinion  42 , mounted on the shaft  36 . Since the mode of operation of the apparatus  10  of the invention matches that in  FIG. 1 , it will not be discussed further hereinafter. What is essential in this exemplary embodiment is that the installation space can be reduced compared to the first exemplary embodiment, because of a shorter length of the shaft  20 . The stepup ratio of the gear  38  can be adapted to the requirements in terms of the resolution of the rotational angle Θ. 
   A third exemplary embodiment of the apparatus  10  of the invention is shown in  FIG. 4 . Unlike  FIG. 3 , here the shaft  36  is connected in perpendicular fashion to the rotatable element  12  via the gear  38  and the pinions  40  and  42  disposed correspondingly in it. This arrangement assures an even shorter structural form of the shaft  20  and possible adaptation of the device  10  to existing space conditions. Once again, the mode of operation of the apparatus  10  corresponds to that of  FIG. 1 , so that further explanation is unnecessary. 
   In closing, it should also be pointed out that the exemplary embodiments show are limited neither to  FIGS. 1 ,  3  and  4 , nor to the courses of the sensor signals S M,1  and S M,2  and of the resonant frequency f R  that results from the sensor signal S C  in  FIGS. 2   a  and  2   b . For instance, it is possible in particular, that depending on the resonant circuit  34  used, the material comprising the plunger core  30 , the thread  28 , and/or the structural form of the coil  31 , a nonlinear variation of the resonant frequency f R  relative to the rotational angle Θ or the number U of revolutions can be established. Moreover, still other arrangements between the shafts  20  and  36  and the gear  38  connecting them are conceivable, depending on the space required. In this connection, it should be noted that it is understood that the gear  38  need not include only the two pinions  40  and  42 , but instead may comprise a larger number of pinions, pulleys, friction wheels, or the like. Furthermore, it is equally possible that not only the plunger core  30  is moved relative to the coil  31 , but the coil  31  may be moved relative to the plunger core  30 ; that is, the coil  31  is set into motion either alone or in combination with the plunger core  30  via the shafts  20  and  36 , respectively, by suitable means, such as pinions and so forth. The apparatus and the method of the invention are not limited to an application in conjunction with an electronic power steering drive but instead can also be used for multiturn rotational angle detection of other rotatable elements. 
   The foregoing relates to a 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.