Abstract:
A sensor for detecting a position of an encoder magnet in a direction of motion, including: a first coil extending in the direction of motion, a second and third coil, which are aligned with the first coil and which are arranged symmetrical to each other with respect to a point of symmetry as observed in the direction of motion and which accordingly form a first and second transformer with the first coil, the transformation ratio of which transformers depends on the position of the encoder magnet, and a magnetic asymmetry, which changes the transformation ratio of one of the transformers with respect to the other transformer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Phase Application of PCT International Application No. PCT/EP2014/051936, filed Jan. 31, 2014, which claims priority to German Patent Application No. 10 2013 201 722.7, filed Feb. 1, 2013, the contents of such applications being incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The invention relates to a method for producing a measuring pickup and to the measuring pickup. 
     BACKGROUND OF THE INVENTION 
     DE 44 259 03 C3, which is incorporated by reference, and EP 238 922 B1, which is incorporated by reference, disclose position sensors which operate in accordance with the principle of linear position measurement on the basis of a permanent-magnetic linear contactless displacement, referred to as PLCD. Such position sensors are also known as linear inductive position sensors, referred to as LIPS. 
     SUMMARY OF THE INVENTION 
     An aspect of the invention aims to improve the known position sensors. 
     In accordance with one aspect of the invention, a sensor for detecting a position of an encoder magnet in a movement direction comprises a first coil extending in the movement direction, a second and third coil oriented after the first coil, which second and third coils are arranged symmetrically with respect to one another in the movement direction, when considered in relation to a point of symmetry, and correspondingly form, with the first coil, a first and second transformer, whose transformation ratio is dependent on the position of the encoder magnet, and a magnetic asymmetry, which changes the transformation ratio of one of the transformers with respect to the other transformer. 
     The specified sensor is thus constructed symmetrically with respect to its measurement range. An asymmetry should in the text which follows here be understood to mean an element in the specified sensor which introduces an asymmetry into this symmetry of the measurement range. Therefore, the element does not need to be constructed asymmetrically in all respects; it should only distort the symmetry within the measurement range. 
     In one development of the specified sensor, the magnetic asymmetry comprises a geometric asymmetry. 
     In another development of the specified sensor, the transformation ratio of the transformer, which is arranged at the front when viewed in the movement direction of the encoder magnet, is greater, by the magnetic asymmetry, than the transformation ratio of the transformer which is arranged at the rear, when viewed in the movement direction of the encoder magnet. 
     In yet another development of the specified sensor, the asymmetry comprises an asymmetric geometry of the second coil with respect to the third coil. 
     In an additional development of the specified sensor, the asymmetric geometry of the second coil with respect to the third coil comprises an asymmetric turns number and/or turns number per unit length of the second coil with respect to the third coil. 
     In an alternative development of the specified sensor, the asymmetry comprises a location-dependent change in the geometry of the first coil. 
     In a preferred development of the specified sensor, the asymmetry comprises an element which changes a coupling between the first coil and the second coil of the first transformer with respect to a coupling between the first coil and the third coil of the second transformer. 
     In a particular development of the specified sensor, the element comprises a location-dependent cross-sectional geometry viewed in the movement direction. 
     In a particularly preferred development of the specified sensor, the element is arranged asymmetrically, when viewed from the point of symmetry. 
     The specified sensor is particularly preferably a linear position sensor (LIPS). 
     In accordance with a further aspect of the invention, an apparatus for actuating a braking system of a vehicle comprises a brake pedal for setting a braking force by displacing the brake pedal in a movement direction and a sensor as claimed in one of the preceding claims for detecting the position of the brake pedal in the movement direction and for outputting a signal indicating the braking force to be set depending on the detected position of the brake pedal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-described properties, features and advantages of this invention and the way in which they are achieved will become clearer and more easily comprehensible in connection with the description below of the exemplary embodiments, which are explained in more detail in connection with the drawings, in which: 
         FIG. 1  shows a schematic illustration of a tandem master cylinder comprising a position sensor, 
         FIG. 2  shows a schematic illustration of the position sensor from  FIG. 1 , 
         FIG. 3  shows a perspective view of a linear position sensor, 
         FIG. 4  shows a characteristic of the linear position sensor from  FIG. 3 , 
         FIG. 5  shows a sectional illustration of the linear position sensor from  FIG. 3 , 
         FIG. 6  shows a sectional illustration of an alternative linear position sensor, 
         FIG. 7  shows a sectional illustration of another alternative linear position sensor, 
         FIG. 8  shows a sectional illustration of a further alternative linear position sensor, and 
         FIG. 9  shows a sectional illustration of yet a further alternative linear position sensor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The same technical elements are provided with the same reference symbols and only described once in the figures. 
     Reference is made to  FIG. 1 , which shows a tandem master cylinder  2  comprising a position sensor  4 . 
     The tandem master cylinder  2  also has a pressure piston  6 , which is arranged movably in a movement direction  8  in a housing  10 , wherein the movement of the pressure piston  6  can be controlled by a foot pedal (not shown). The pressure piston  6  itself is divided into a primary piston  12  and a secondary piston  14 , wherein the primary piston  12  closes an inlet of the housing  10  and the secondary piston  12  divides the interior of the housing  10  into a primary chamber  16  and a secondary chamber  18 . A secondary collar  20  is arranged in the region of the inlet of the housing  10  on the primary piston  12 , which secondary collar insulates the interior of the housing  10  from the ambient air. When viewed into the interior of the housing  10 , a primary collar  22  follows the secondary collar  20 , said primary collar sealing a gap between the primary piston  12  and a wall of the housing  10 . A pressure collar  24  on the secondary piston  14  isolates the pressure of the primary chamber  16  from the pressure of the secondary chamber  18 . In addition, a further primary collar  26  on the secondary piston  14  seals a gap between the secondary piston  14  and the wall of the housing  10 . The primary piston  12  is supported against the secondary piston  14  via a first spring  28 , while the secondary piston is supported against a housing base via a second spring  30 . Correspondingly, hydraulic fluid (not shown) can be supplied to the primary chamber  16  and the secondary chamber  18  via a first and second connection  32 ,  34 . 
     Since the mode of operation of a tandem master cylinder is known to a person skilled in the art, no detailed description thereof is provided here. 
     The position sensor  4  has a sampling element in the form of a slide  36  comprising an encoder magnet  37  at its top end, which, when viewed into the plane of the drawing, can be pushed beneath a sensor circuit  38  (yet to be described). In order to push the slide  36 , the primary piston  12  has a flange  40 , which the slide  36  abuts. The flange  40  and the primary piston  12  therefore together form a measurement object, whose position is determined by the sensor circuit  38  (yet to be described) of the position sensor  4 . The sensor circuit  38  is formed from a plurality of conductor tracks on a wiring carrier  42 , such as a leadframe, a printed circuit board or another substrate. In order to protect against contamination, for example, a cover  46  can be positioned on the printed circuit board  42  with the sensor circuit  38 . 
     Reference is made to  FIG. 2 , which shows the position sensor  4  shown in  FIG. 1 . 
     The circuit  38  of the position sensor comprises a transducer  48 , which in the present embodiment is in the form of a linear inductive position sensor (LIPS). The LIPS  48  detects a magnetic field  50  of the encoder magnet  37  and outputs an electrical encoder signal (not denoted) to the circuit  38  on the basis of this magnetic field. This encoder signal is converted by a first signal processing chip  52  and a second signal processing chip  54  into a measurement signal (not denoted), from which the position of the slide  36  and therefore the position of the flange  40  and the primary piston  12  is provided. The measurement signal thus produced can finally be tapped off at a transmission interface  56  of the position sensor  4  via a cable (not illustrated) and passed on to a higher signal processing unit (not illustrated) such as, for example, a motor controller in a vehicle (not illustrated). 
     The circuit  38  can comprise protection elements  58  for protecting the two signal processing chips  52 ,  54 , for example from an overvoltage. In addition, a shielding plate  60  can be arranged between the circuit  38  and the LIPS  48 , said shielding plate shielding electromagnetic fields between the circuit  38  and the transducer  48  and thus avoiding an influence of the circuit  38  on the LIPS  48 . 
     In the present embodiment, the LIPS  48  is arranged via a form-fitting connection  62  in a defined position on the wiring carrier  42 . In this case, a protective compound  64  surrounds the wiring carrier  42  and the transducer  48 . 
       FIG. 3  shows a perspective view of the LIPS  48 . The LIPS  48  comprises a coil former  66  comprising a winding space which is divided via four webs  68  into a central section  70  and two side sections  72 . The coil former  66  bears a primary coil  74 , which extends along a core (of which no more is shown in  FIG. 3 ) and is intended to be assumed to be in the form of a single layer in this case. The coil former  66  bears tightly wound secondary coils  76  for measuring an induced voltage at the two opposite peripheral zones of the primary coil  74 . 
     That is to say that the coils  74 ,  76  in the LIPS  48  can differ in two different ways. Firstly, the coils interact as part of a measuring transformer, wherein the primary coil  74  excites a magnetic field and induces the induced voltage in the secondary coils  76 . The choice of primary and secondary coils  74 ,  76  is in principle as desired and does not need to be configured in the way shown in  FIG. 3 . The LIPS  48  in the present embodiment is intended to be capable of being evaluated with ratiometric signal processing, for which reason the choice of primary coil  74  and secondary coils  76  is as previously mentioned. The signal processing connected to such a LIPS  48  performs in each case one measurement of the induced voltage at both secondary coils  76  and calculates the two measured induced voltages using a suitable algorithm, with the aim of suppressing faults. In the simplest case, this can take place by virtue of the secondary coils  76  being connected in series in a suitable manner. Preferably, this takes place by analog or digital signal processing which provides large degrees of freedom in the configuration of a mathematical mapping, with which the position value is calculated from the two induced voltages. 
     In addition, the coils  74 ,  76  can be divided, in terms of their geometric configuration, into coils  74  having a low turns number per unit length which are wound approximately along the entire core length (in the present exemplary embodiment the primary coil  74 ) and those which are wound compactly with a high turns number per unit length at a specific point of the core (not shown) (in the present exemplary embodiment the secondary coils  76 ). 
     Further details relating to the mode of operation of a LIPS are set forth, for example, in the documents DE 44 259 03 C3 and EP 238 922 B1. 
     The LIPS  48  has a characteristic  78  illustrated in  FIG. 4  in which the variable to be measured, i.e. the position  80  of the encoder magnet  37  and the output variable indicating the variable to be measured, i.e. the induced voltages  82  at the secondary coils  76  are set against one another. 
     Depending on the application case, it may be desirable for the LIPS  48  for either the characteristic  78  to always have the same gradient everywhere or for it to permanently have zones with a different gradient. 
     If the characteristic  78  of the LIPS  48  has a linear profile, the measurement results of the LIPS  48  can be further-processed directly in an analog controller, a measurement mechanism or for display for manual reading (note in relation to terminology: linear position measurement=measurement of linear movements; linear characteristic=linear relationship between measured position and output variable). Therefore, in most cases nowadays it is attempted to construct the LIPS  48  with a linear characteristic  78 . Should the characteristic  78  be nonlinear in principle, it can be corrected easily in digital systems. In this case, the sensitivity of the LIPS  48  and therefore its accuracy and its resolution are constant and are influenced by the gradient of the linear characteristic. 
     An example of a use with location-dependent accuracy and resolution requirements and therefore, considered over the entire measurement range, a nonlinear characteristic is an electrohydraulic braking system comprising the tandem master cylinder  2  shown in  FIG. 1 , in which the LIPS  48  is used for measuring the brake pedal position. The LIPS  48  detects, by means of the brake pedal position, the driver&#39;s intention and uses the measurement result in an associated control system (not illustrated any further). In a passenger vehicle which is moving in normal road traffic, the brake pedal position will quite predominantly be in the rest position or near the rest position, whereas a severely deflected brake pedal, corresponding to full braking, is a rare driving situation. This situation is of superior importance for the safety of the vehicle, but does not require the greatest sensitivity in the brake pedal. The stringent requirements as regards the quality of the control in the braking system are set with a small delay in many braking operations, on the other hand, because sensitive control of the braking operation is critical for comfort and driving response in these braking operations. A high degree of comfort in this sense can be achieved by virtue of accuracy and resolution of the position sensor in the initial range being increased, possibly at the cost of the corresponding values at the end of the measurement range. The driver will profit from a high degree of accuracy because the system then responds to a specific deflection of the brake pedal in a particularly reproducible manner as regards the delay achieved. The driver will profit from a high degree of resolution because the potentially disruptive discretization of the measured variable remains hidden in a digital system in this case. 
     The LIPS  48  should therefore be configured such that its characteristic  78  matches the application. Secondly, the coils  74 ,  76  of the LIPS  48  should protrude as little as possible beyond the end points of the measurement range. This is the configuration which is particularly relevant for electrohydraulic braking systems in which high accuracy and resolution are required at the start of the measurement range and in the vicinity thereof and at the same time the physical space in this part of the measurement range is particularly restricted. That part which comprises the rest position of the brake pedal and braking with a small delay, i.e. in particular the range which is used constantly during travel in normal road traffic without an emergency situation, can be considered to be the start of the measurement range, for example. 
     Here begins the exemplary embodiment in which the characteristic  78  of the LIPS  48  is intentionally nonlinear. This nonlinear characteristic  78  can be used for increasing the performance of the LIPS  48  by virtue of the nonlinear characteristic  78  being matched to the location-dependent accuracy and resolution requirement of the respective application. 
     Resolution and accuracy are increased locally where the nonlinear characteristic of the LIPS  48  is steeper, i.e. a specific change in the measured variable results in a severe change in the output variable (=increased sensitivity). Conversely, resolution and accuracy decrease locally where the nonlinear characteristic  78  has a flatter profile. Usually, only a limited value range is available for the output variable, and therefore one of these properties needs to be sacrificed for a local increase in the rate of rise of the characteristic  78  and of the resolution and accuracy at another point. 
     The cause of the dependence of the gradient on the resolution and accuracy is that interference and noise which are likewise transmitted from the LIPS  48  or other stages of signal processing to the induced voltages  82  as output variable are usually not changed in terms of their amplitude by the characteristic  78  (i.e. with respect to the measured variable). At any point on the characteristic  78 , interference and noise therefore have a typical value of a characteristic variable (amplitude, spectral power density, rms value or the like) which downwardly limits the distinguishability of adjacent values. The steeper the characteristic  78 , the closer associated distinguishable values of the position  80  to be detected as measured variable are to one another, which is illustrated by way of example in  FIG. 4 . 
     If it is assumed using an example of  FIG. 4  that there is a distinguishability of output values  81 ,  83  of the induced voltages  82  as output variable which differ by an output difference  84 , a distinguishability of two values  86 ,  88  of the position  80  to be detected as measured variable which differ by a first measurement difference  90  results in the flat region of the characteristic  78 . In the steep region of the characteristic  78 , the same output difference  84  for the induced voltages  82  results in a second measurement difference  92 , which is less than the first measurement difference  90  and therefore distinguishable therefrom, on the other hand. The interval required for the distinguishability is directly the resolution. Since much interference usually likewise only acts as output variable within a specific interval in the induced voltages  82 , the relationship as regards accuracy is analogous. 
     In the context of the abovementioned braking system, it would be favorable, for example, to configure the LIPS  48  to be less susceptible to interference in the lower value range of the position  80  to be detected since the driver will probably actuate the brake pedal with more sensitivity in this value range, as already mentioned, than in the upper value range of the position  80  to be detected. For this purpose, the gradient of the characteristic  78  of the LIPS  48  could be configured to be smaller in the lower value range than in the upper value range. 
     For this purpose, within the scope of the present embodiment, the geometric configuration of the transformer resulting from the coils  74 ,  76  is modified. Instead of a completely symmetrical design of the LIPS  48 , at least an asymmetry is introduced into the transformer in a targeted manner, in which at least one component of the LIPS  48  (one of the windings, the halves of a winding pair or the core) is asymmetrical with respect to a plane which is arranged perpendicular to the measurement direction in the center of the measurement range of the position  80  to be measured. Given a corresponding configuration of the asymmetry, the contributions of the saturation of the core and/or the induced voltages  82  to the measurement result change depending on the position  80  of the encoder magnet  37 , as a result of which the desired nonlinearity in the characteristic  78  is achieved. 
     Possibilities for producing an asymmetry with the desired characteristic change are illustrated by way of example at a point further below. These possibilities can in principle be combined. Their effect will generally be intensified when combined. Owing to the severe nonlinearity of the operating principle of the LIPS  48 , it can be assumed that the combination cannot be treated in accordance with the superimposition principle. The contribution of a specific change to the configuration of the transformer comprising the coils  74 ,  76  is therefore also dependent on the other changes to the configuration. 
     The individual changes to the configuration are as follows (definition of “start” as above, “end” correspondingly, relates to the measurement range and therefore the position to be detected of the encoder magnet  37 ): 
     1. Change in length of the core (not shown in  FIG. 3 ) 
     
         
         
           
             a. extension of the core at the start 
             b. shortening of the core at the end.
 
2. Change in the number of turns of the secondary windings
 
             a. increase turns number of the secondary winding at the start 
             b. reduce turns number of the secondary winding at the end.
 
3. Location-dependent change in the turns number per unit length of the primary winding
 
             a. increase turns number per unit length at the start 
             b. reduce turns number per unit length at the end.
 
4. If a second core is provided outside the windings (magnetic return path core)
 
             a. reduce material cross section of the magnetic return path core at the start 
             b. increase material cross section of the magnetic return path core at the end. 
           
         
       
    
     Changes in configuration 1.a. to 3.a. result in higher voltages being induced, which is generally an advantage. However, this entails additional consumption of materials and a higher installation space requirement, especially at the start. Therefore, the complementary changes in configuration 1.b. to 3.b. are expedient despite reduced voltages since savings are correspondingly made on material and installation space. 
       FIG. 5  additionally shows a scale for the measurement range  94  of the sensor. The start of the measurement range  94  (indicated by the arrow direction of the measurement range) and the end are clearly between the two secondary coils  76  because the output voltage of a LIPS  48  reaches an extremum when the encoder magnet  37  (not illustrated) comes close to the secondary coils. If the encoder magnet  37  moves beyond this point, the same measurement results are achieved for these positions as within the measurement range  94 . Therefore, the encoder magnet  37  needs to maintain a minimum spacing from the secondary coils  76 , by means of which the measurement range  94  is limited. The center of the measurement range  94  therefore marks the above-mentioned plane of symmetry with respect to the coil former  66  and measuring transformers and is therefore provided with the reference symbol  96  for reasons of clarity. 
     In this case the core  98  of the LIPS  48  which is used for constructing the measuring transformer from the coils  74 ,  76  is arranged asymmetrically with respect to the plane of symmetry  96  in  FIG. 5  by virtue of the core  98  being extended at the start and/or shortened at the end. 
       FIG. 6  shows a solution corresponding to point 2b from the above list. The secondary coil  76  at the end of the measurement range  94  has fewer turns, for example only half the number of turns, in comparison with the secondary coil  76  at the start of the measurement range  94 . The core  98  from  FIG. 6  can be used here and in all of the following figures again symmetrically with respect to the plane of symmetry  96 . Owing to the secondary coil  76  with a lower turns number, the installation space requirement at the end of the measurement range  94  is reduced, while the reverse measure (point 2a) would result in an increased installation space requirement at the start. Since the reduction in the number of turns of the secondary coil  76  at the end results in an increase in accuracy and resolution in the region of particular interest at the start of the measurement range  94 , the dimensions of the LIPS  48  can be reduced overall in order to reduce the values to the initial level. Therefore, a saving can be made on installation space at the start of the measurement range as well. The same should also apply to the figures discussed below. 
       FIG. 7  likewise represents a solution in accordance with the above point 2.b. The secondary coil  76  at the end of the measurement range  94  would in this case be reduced by half in terms of length. The technical effects of this measure are clearly similar to the technical effects of  FIG. 6 . An advantage over  FIG. 6  consists in that either the total length of the LIPS  48  can now be shortened, or that the distance between the secondary coils  76  given the same outer dimensions can be increased, as a result of which a larger measurement range  94  is made possible. 
       FIG. 8  illustrates a solution in accordance with the above point 3.b. The turns number per unit length of the primary coil  74  has been halved towards the end of the measurement range  94 . It is preferred to change the turns number per unit length not suddenly but continuously along the measurement range  94  since the local resolution of the LIPS  48  can decrease to zero in the vicinity of a sudden change point. 
       FIG. 9  shows a solution in accordance with the above point 4.b. The coils  74 ,  76  are in this case enclosed by a magnetic return path core  100 , which can optionally also be provided in the other variants of the LIPS  48  in accordance with the previous figures. The particular feature of  FIG. 8  consists in that this magnetic return path core  100  has a cross-sectional area which is dependent on the position within the measurement range  94  and increases in size towards the end of the measurement range  94 . In this case, the embodiment of the LIPS  48  as shown in  FIG. 8  is intended to be representative both of a variable cross section in the radial direction (as illustrated) and of a variable cross section perpendicular thereto, i.e. in the circumferential direction. Furthermore, the magnetic return path core  100  does not necessarily belong to the half-section illustration of the measuring transformer of the LIPS  48  since it is sufficient for operation if the magnetic return path core  100  is located on one side of the measuring transformer.