Abstract:
A method for detecting a phase angle difference between a first periodic measurement signal and a second periodic measurement signal, wherein, for the purpose of determining a torque applied to a shaft, the two periodic measurement signals describe a rotation of the shaft at an axial distance from one another including superimposing a periodic auxiliary signal which simulates a previously known rotational speed for the shaft on the first periodic measurement signal in order to form a superimposition signal, and determining the phase angle difference on the basis of the superimposition signal and the second measurement signal.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is the U.S. National Phase Application of PCT International Application No. PCT/EP2013/076587, filed Dec. 13, 2013, which claims priority to German Patent Application No. 10 2012 224 180.9, filed Dec. 21, 2012 and German Patent Application No. 10 2012 224 187.6, filed Dec. 21, 2012, the contents of such applications being incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method for detecting a torque applied to a shaft and a control apparatus for performing the method and a torque sensor having the control apparatus. 
       BACKGROUND OF THE INVENTION 
       [0003]    DE 10 2007 059 361 A1, incorporated by reference herein, discloses a torque sensor that takes a phase of two rotating encoder wheels on a shaft that are at an axial interval from one another as a basis for determining a torque applied to this shaft. 
       SUMMARY OF THE INVENTION 
       [0004]    It is an aim of an aspect of the invention to improve the known torque sensor. 
         [0005]    According to one aspect of the invention, a method for detecting a phase difference between a first periodic measurement signal and a second periodic measurement signal, wherein the two periodic measurement signals determine a torque applied to a shaft by describing a rotation by the shaft at an axial interval from one another, comprises the steps of:
   overlaying of the first periodic measurement signal with a periodic auxiliary signal, which simulates a previously known speed for the shaft, to form an overlay signal, and   determination of the phase difference on the basis of the overlay signal and the second measurement signal.   
 
         [0008]    The specified method is based on the consideration that the periodic measurement signals, particularly in the case of a torque sensor of the type cited at the outset in the stationary shaft, are present only when the shaft rotates. In many applications, for example in the case of a steering rod, it would, however, be desirable to detect a torque acting on the shaft even when the shaft is not rotating, that is to say stationary. This is the starting point for the specified method with the proposal that the rotation of the shaft be simulated and at least one of the two periodic measurement signals be overlaid with a periodic auxiliary signal that simulates the rotation. In this way, it is possible for the phase of the shaft to be detected over the axial interval of the shaft even when stationary, or even when the shaft is rotating very slowly, and to be evaluated for the torque acting on the shaft. 
         [0009]    In one development, the specified method comprises the step of overlaying of the second periodic measurement signal with a further periodic auxiliary signal, which simulates a further previously known speed for the shaft, to form a further overlay signal. This simulates the rotation at both points of the shaft over the axial interval. 
         [0010]    In a special development of the specified method, the periodic auxiliary signal and the further periodic auxiliary signal are the same, which means that the previously known speed and the further previously known speed are the same. In this way, the phase difference between the overlay signal and the further overlay signal corresponds directly to the phase difference between the first measurement signal and the second measurement signal. 
         [0011]    In another development, the first periodic measurement signal is overlaid with the periodic overlay signal if a real speed of the shaft is below a predetermined value. In this case, the predetermined value can be chosen arbitrarily. By way of example, it can be chosen such that the specified method is carried out only when the shaft is stationary or close to stationary. 
         [0012]    In yet another development, the first and second periodic measurement signals are produced in corresponding fashion on the basis of a first and a second magnetic rotating field, which are each output by the shaft in torque-resistant fashion with respect thereto. 
         [0013]    In this case, the auxiliary signal can be produced arbitrarily. By way of example, the auxiliary signal can be directly output by an auxiliary signal source and overlaid with the first measurement signal. The same applies to the second measurement signal and the further auxiliary signal. In one development of the specified method, however, overlaying the first periodic measurement signal with the periodic auxiliary signal can also involve the first magnetic rotating field being overlaid with a periodic auxiliary magnetic field that produces the periodic auxiliary signal. 
         [0014]    In an additional development of the specified method, a real speed of the shaft and the previously known speed of the shaft are opposite. This ensures that by increasing the real speed the real movement and the simulated movement of the shaft do not cancel one another out and hence a standstill is simulated for a very large movement by the shaft. 
         [0015]    According to a further aspect of the invention, an apparatus is set up to perform one of the specified methods. 
         [0016]    In one development of the specified apparatus, the specified apparatus has a memory and a processor. In this case, one of the specified methods is stored in the memory in the form of a computer program, and the processor is provided for the purpose of carrying out the method when the computer program is loaded into the processor from the memory. 
         [0017]    According to a further aspect of the invention, a computer program comprises program code means in order to perform all the steps of one of the specified methods when the computer program is executed on a computer or one of the specified apparatuses. 
         [0018]    According to a further aspect of the invention, a computer program product contains a program code that is stored on a computer-readable storage medium and that, when executed on a data processing device, performs one of the specified methods. 
         [0019]    According to a further aspect of the invention, a torque sensor for detecting a torque, which is applied to a shaft, on the basis of a phase difference between a first periodic measurement signal and a second periodic measurement signal comprises one of the specified apparatuses for producing the two periodic measurement signals and an evaluation device for determining the torque on the basis of the phase difference between the two measurement signals. 
         [0020]    The specified torque sensor can be used in any desired application, for example in a vehicle, in order to detect a torque on a torsion shaft, such as a steering shaft. 
         [0021]    According to a further aspect of the invention, a vehicle comprises a specified torque sensor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The properties, features and advantages of this invention that are described above and also the way in which they are achieved will become clearer and more distinctly comprehensible in connection with the description of the exemplary embodiments that follows, said exemplary embodiments being explained in more detail in connection with the drawings, in which: 
           [0023]      FIG. 1  shows a schematic view of a steering system for a vehicle, 
           [0024]      FIG. 2  shows a schematic view of a torque sensor in the vehicle of  FIG. 1 , 
           [0025]      FIG. 3  shows a schematic view of an evaluation circuit for the torque sensor of  FIG. 2 , and 
           [0026]      FIG. 4  shows a timing diagram with the measurement signals in the evaluation circuit of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    In the figures, technical elements that are the same are provided with the same reference symbols and described only once. 
         [0028]    Reference is made to  FIG. 1 , which shows a steering system  2  for a vehicle—which is not shown further. 
         [0029]    The steering system  2  comprises a steering wheel  4  that is fitted to a shaft  6  that is in turn arranged so as to be rotatable about a rotation axis  8 . The steering wheel  4  is therefore set up to take an angle position  10  around the rotation axis  8  as a basis for prescribing a steering angle for adjusting a steering gear  12  of the vehicle, which is not shown further. To this end, the steering wheel  4  is rotated by a driver, for example, of the vehicle that is not shown further. 
         [0030]    In the present embodiment, the angle position  10  of the steering wheel  4  is detected by a drive apparatus  14 , which then uses a motor—not shown further—in the drive apparatus  14  to drive a steering shaft  16  in order to operate the steering gear  12  such that wheels  18  of the vehicle that is not shown further are turned, in a manner that is known to a person skilled in the art, on the basis of the steering angle represented by the angle position  10 . 
         [0031]    In order to be able to turn the wheels  18  via the drive apparatus  14  on the basis of the angle position  10  of the steering wheel  4 , it is necessary for the angle position  10  to be detected. This can be accomplished by using the steering angle sensors  20  shown in  FIGS. 2 and 3 , for example. 
         [0032]    Reference is made to  FIG. 2 , which shows a steering angle sensor  20  for the steering system  2  from  FIG. 1 . 
         [0033]    The steering angle sensor  20  has a first encoder wheel  22 , a thread, in the form of a worm  24 , that is axially and concentrically connected to the first encoder wheel  22 , and a second encoder wheel  26  that is axially and concentrically connected to the worm  24  and that is axially opposite the first encoder wheel  22  at the worm  24 . 
         [0034]    In the present embodiment, the worm  24  is formed from an elastic material and can be twisted elastically by applying two opposite torques to the encoder wheels  22 ,  26 . The shaft  6  is broken axially into two portions at the location of the steering angle sensor  20 , the first encoder wheel  22  being arranged on the first portion of the broken shaft  6  and the second encoder wheel  26  being arranged on the second portion of the broken shaft  6 . Therefore, if the shaft  6  is rotated with the steering wheel  4 , the steering angle sensor  20  is firstly transferred to the angle position  10 . At the same time, the steering angle sensor  20  is twisted with a torque  28  upon transfer to the angle position  10 . 
         [0035]    Within the context of the present embodiment, the angle position  10  and the torque  28  are intended to be able to be detected by measurement. 
         [0036]    In order to detect the angle position, the worm  24  has turns  30  into which a small plate  32  of a transmitter element  34  engages, which has a transmitter magnet  36  fitted to it radially as seen from the rotation axis  8  of the shaft  6 . When the shaft  6  is transferred to the angle position  10  by rotating the steering wheel  4  in the manner shown in  FIG. 1 , the transmitter element  34  is moved axially in relation to the shaft  6  by the worm  24  rotating with the shaft  6  and is put into an axial position  38  that is dependent on the angle position  10 . In this case, the transmitter element  34  may be guided axially in relation to the shaft  6  in a manner that is not shown further. 
         [0037]    That is to say that if the axial position  38  of the transmitter element  34  is known, then the angle position  10  of the shaft  6  and hence of the steering wheel  4  is also known. 
         [0038]    In order to detect the axial position  38 , an evaluation circuit  40  having a first magnetoresistive measurement pickup  42 , such as a Hall sensor, an anisotropic magnetoresistive sensor or a gigamagnetoresistive sensor, is provided in the present embodiment, with the first magnetoresistive measurement pickup  42  outputting—in a manner that is known to a person skilled in the art—an absolute signal that is linearly dependent on the axial position  38  of the transmitter magnet  36  of the transmitter element  34 . To this end, the evaluation circuit  40  having the first magnetoresistive measurement pickup  42  is arranged for the rotation of the shaft  6  and the axial movement of the transmitter element  30 . Details relating to the production of a signal that is linearly dependent on the position of a transmitter magnet using a magnetoresistive measurement pickup can be found in DE 10 2006 030 746 A1, incorporated by reference herein for example, and are therefore not explained in more detail below for the sake of brevity. 
         [0039]    The first encoder wheel  22  has magnets  48  arranged at the periphery, the poles of said magnets extending in the peripheral direction of the first encoder wheel  22 . In this way, the first encoder wheel  22  emits, via the magnets  48 , a radially extending magnetic field that is location dependent in the peripheral direction of the first encoder wheel  22 . Arranged radially above the first encoder wheel  22  is the second magnetoresistive measurement pickup  46 , which detects the radially extending magnetic field from the first encoder wheel  22  and thus outputs a first angle signal  50 , shown in  FIG. 3 , that indicates the angle of the first encoder wheel  22 . The angle signal  50  is produced in a similar manner to the absolute signal  44  and, if required, said production can be looked up in the document DE 10 2006 030 746 A1, incorporated by reference herein. 
         [0040]    Like the first encoder wheel  22 , the second encoder wheel  26  has magnets  48  arranged at the periphery, the poles of said magnets extending in the peripheral direction of the second encoder wheel  26 . In this way, the second encoder wheel  26  also emits, via the magnets  48 , a radially extending magnetic field that is location dependent in the peripheral direction of the second encoder wheel  26 . Arranged radially above the second encoder wheel  26  is a third magnetoresistive measurement pickup  52 , which detects the radially extending magnetic field from the second encoder wheel  26  and thus outputs a second angle signal  54  that indicates the angle of the second encoder wheel  26 . The second angle signal  54  is produced in a similar manner to the first angle signal  50  and to the absolute signal  44  and, if need be, said production can likewise be looked up in the document DE 10 2006 030 746 A1. 
         [0041]    The determination of the torque  28  will be explained in more detail below with reference to  FIG. 3 , which shows a circuit diagram of a portion  56  of the evaluation circuit  40  of  FIG. 2 . 
         [0042]    In this regard, the worm  24  is of torsional design in the present embodiment. That is to say that by applying the torque  28  it is possible for the worm  24  to be twisted in the direction of the torque  28 , which produces a phase difference between the first encoder wheel  22  and the second encoder wheel  26 , which is to say that the first encoder wheel  22  has an angle position  10  that is different than the angle position  10  of the second encoder wheel  26 . This is exploited in order to measure the torque  28 , since the torque  28  is dependent in a predetermined manner on this phase difference and hence on the difference between the angle positions  10 . 
         [0043]    In order to determine the phase difference, that portion  56  of the evaluation circuit  40  that is shown in  FIG. 3  is therefore initially used to evaluate the angle signals  50 ,  54  and to determine the angles  10  of the encoder wheels  22 ,  26 . 
         [0044]    As already explained the angle signals  50 ,  54  are produced by means of the magnetoresistive measurement pickups  46 ,  52 , which change their electrical resistance in a manner that is known per se by virtue of the movement of the encoder wheels  22 ,  26 . In the present embodiment, this electrical resistance change is evaluated using measurement bridges  58 , which are constructed from electrical resistors  60  in a manner that is known per se. 
         [0045]    Since the movement of the encoder wheels  22 ,  26  is rotation, the electrical resistance of the magnetoresistive measurement pickups  46 ,  52  changes periodically by virtue of the angle position  10  of the encoder wheels  22 ,  26 , which means that the angle signals  50 ,  54  produced by the resistance change are also periodic measurement signals. A phase difference between these periodic angle signals  50 ,  54  is directly the sought phase difference dependent on the torque  28 . 
         [0046]    However, the phase difference can be measured only if the periodic angle signals  50 ,  54  have sufficiently great amplitude. These periodic angle signals  50 ,  54  in turn have sufficiently great amplitude only when the encoder wheels  22 ,  26  are rotating. In other words, if an applied torque  28  means that the encoder wheels  22 ,  26  are static in relation to one another with a particular difference in their angle  10 , it is not possible for the torque  28  to be detected solely on the basis of the angle signals  50 ,  54 , since the latter do not have sufficiently great amplitude to evaluate the phase difference. 
         [0047]    In order to overcome this problem, the present embodiment involves the angle signals  50 ,  54  being overlaid with, accordingly, a first periodic auxiliary signal  62  and a second periodic auxiliary signal  64 . In the present embodiment, the two periodic auxiliary signals  62 ,  64  are output from a common auxiliary signal source  66  and can be controlled, for example by means of a switch  68 , such that the output occurs only below a particular speed of rotation of the shaft  6 , for example. 
         [0048]    The overlaying of the angle signals  50 ,  54  and the auxiliary signals  62 ,  64  accordingly results in a first periodic overlay signal  70  and a second overlay signal  72 , which are then accordingly applied to comparators  76 . In the context of the present embodiment, the auxiliary signals  62 ,  64  are at a frequency that simulates a speed at which the shaft  6  is rotated virtually. As a result of the different angles  10  of the encoder wheels  22 ,  26 , the auxiliary signals  62 ,  64  are phase-modulated, on the basis of the angle signals  50 ,  54 , such that the overlay signals  70 ,  72  contain a phase difference  74  from which it is possible to infer the torque  28  applied to the shaft. Therefore, within the context of the present embodiment, it is merely necessary to determine this phase difference  74  and to determine the sought torque  28  therefrom, for example on the basis of a previously determined characteristic curve  75 . 
         [0049]    Although the phase difference  74  could be determined directly from the two overlay signals  70 ,  72 , the present embodiment involves the overlay signals  70 ,  72  being converted, in comparators  76 , into periodic square-wave signals  78  that greatly simplify the ascertainment of the phase difference  74  in an appropriate ascertainment device  80 . 
         [0050]    Reference is made to  FIG. 4 , which shows a timing diagram  82  with the overlay signals  70 ,  72  and the associated square-wave signals  78  in the portion  56  of the evaluation circuit  40  of  FIG. 3 . The signal values plotted in the diagram over time  86  are voltage values  84 . 
         [0051]    As can be seen in  FIG. 4 , the square-wave signals  78  are generated on the basis of switching points  88  at which the overlay signals  70 ,  72  change their arithmetic sign. So that these switching points  88  are not produced erroneously and lead to an incorrect phase difference  74 , the amplitude  90  of the overlay signals  70 ,  72  needs to be sufficiently high. 
         [0052]    To this end, the auxiliary signals  62 ,  64  are present, which enter a signal offset  92  into the overlay signals  70 ,  72  in order to stimulate the comparators  76  such that the aforementioned switching points  88  are generated in accordance with the angle position of the two encoder wheels  22 ,  26 .