Abstract:
A method for producing a coil as a measurement transmitter for a sensor, including: providing electrical connections and a magnetic core for the coil, forming a coil former around the magnetic core in such a way that the magnetic core is at least partially enclosed by the coil former and the electrical connections are held by the coil former, winding at least one coil wire onto the formed coil former, and connecting the wound coil wire to the electrical connections.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. National Phase Application of PCT/EP2013/076930, filed Dec. 17, 2013, which claims priority to German Patent Application No. 10 2012 224 101.9, filed Dec. 20, 2012, the contents of such applications being incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method for producing a measuring pickup and to the measuring pickup. 
       BACKGROUND OF THE INVENTION 
       [0003]    DE 101 30 572 A1, which is incorporated by reference, discloses an inductive measuring pickup for a position sensor, which measuring pickup outputs an electrical signal to an evaluation circuit on the basis of a magnetic field of an encoder element, which magnetic field is dependent on the position of the encoder element. 
       SUMMARY OF THE INVENTION 
       [0004]    An aspect of the invention aims to improve the known inductive measuring pickup. 
         [0005]    In accordance with one aspect of the invention, a method for producing a coil as measuring pickup for a sensor comprises the steps of arranging electrical connections and a magnetic core for the coil, forming a coil former around the magnetic core in such a way that the magnetic core is at least partially enveloped by the coil former and the electrical connections are held by the coil former, winding at least one coil wire onto the coil former formed, and connecting the wound coil wire to the electrical connections. 
         [0006]    The specified method is based on the consideration that the coil former of the measuring pickup could be manufactured in advance, for example, by injection molding, wherein the magnetic core could then be held in the prefabricated coil former by means of adhesive bonding, for example. 
         [0007]    However, the specified method is based on the knowledge that this production procedure would be very involved since at least the adhesive-bonding step of the magnetic core on the coil former would need to be implemented using technically complex means in this case. 
         [0008]    In contrast, the specified method is based on the concept of manufacturing the coil former in such a way that the magnetic core is held in the coil core by a form-fitting connection. For this purpose, the magnetic core is at least partially enveloped by the coil former, with the result that it can no longer fall out of the coil former owing to the form-fitting connection thus formed. 
         [0009]    In this way, it is possible to dispense with the abovementioned adhesive-bonding step in the production of the measuring pickup. 
         [0010]    In a development of the specified method, the coil former is formed by means of a transfer molding process. 
         [0011]    In a particular development, the coil former comprises a thermosetting plastic material, in particular a resin system. In this case, the entire coil former can be formed from the thermosetting plastic material. A thermosetting plastic material has a particularly low coefficient of thermal expansion and barely shrinks after the transfer molding process. In this way, not only is it possible for a thermal mismatch between the components of the measuring pickup to be avoided, which results in lower levels of thermal stresses in the case of temperature changes in the ambient temperature of the measuring pickup which thus increase the cycling stability of the measuring pickup over the course of this temperature change. The reduced shrinkage of the thermosetting plastic material during curing after the transfer molding process also results in reduced internal mechanical stresses on the magnetic core and in freedom from cleavage between the wound coil wire and the coil former, which represents a relatively low risk of corrosion for the coil wire. In addition, the thermosetting plastic material adheres optimally to the electrical connections and thus results in a particularly tight connection between the electrical connections and the coil former, which in turn reduces the risk of a wire breakage and corrosion. 
         [0012]    The thermosetting plastic material can in this case preferably comprise an epoxy resin system, a phenol formaldehyde resin system, a melamine formaldehyde resin system, an unsaturated polyester resin system or another free-flowing resin system or can be formed therefrom. Free-flowing resin systems are in this case delivered in granular form and can be processed in the above-mentioned transfer molding process. In this case, a free-flowing resin system can be delivered via a conventional material supply line as in the case of thermoplastic processing. Furthermore, free-flowing resin systems provide good dimensional stability, a high continuous use temperature, good chemical stability, good mechanical properties, an efficient price-to-performance ratio, and good electrical properties and can be processed using sophisticated machine and process techniques. 
         [0013]    Alternatively, however, so-called non-free-flowing resin systems can also be used which include, for example, bulk molding compounds (BMCs), which can be produced on the basis of an unsaturated polyester resin system, for example. Specifically, BMC can be a glass-fiber-reinforced moist polyester. Non-free-flowing resin systems can be delivered in spherical form and have substantially the same positive properties as the free-flowing resin systems. 
         [0014]    The magnetic core can be accommodated in the coil former in any desired manner. Thus, the magnetic core can be enclosed in the coil former, for example, during the transfer molding process. The transfer molding process can alternatively also be performed as a premolding process, however, over the course of which a receiving body having a cutout is formed. Then, the magnetic core can be inserted into this cutout, which can be in the form of a depression, for example. Finally, the cutout can be closed with the received magnetic core. The cutout therefore acts as a space in which the magnetic core can be provided. As a result, the material of the magnetic core is protected from high temperature gradients and high peak temperatures of approximately 175° C. during the transfer molding process. In addition, no forces are transferred to the magnetic core in the case of changes in temperature. 
         [0015]    Soft-magnetic materials, such as iron-nickel magnets, for example, with and without freedom from magnetostriction, can be selected as magnetic core. Magnetostriction is intended below to mean the change in magnetic characteristic variables such as magnetic field strength, magnetic flux density or magnetic permeability as a result of a mechanical stress such as pressure or tensile strain. Owing to the fact that the magnetic core is inserted into the cutout in a stress-free fashion, the magnetostriction can be avoided. 
         [0016]    The material of the magnetic core can be formed in one layer or else in a plurality of layers. 
         [0017]    In one development, the specified method can comprise the step of applying a mechanical decoupling material to the magnetic core prior to closing of the cutout, as a result of which the magnetic core is further mechanically decoupled from the coil former so that compressive loads on the magnetic core are largely suppressed. As an alternative or in addition, the cutout can also be filled with an elastic casting material, such as a silicone gel known per se prior to or after insertion of the magnetic core. 
         [0018]    The closing of the cutout can be performed using a cover, which can be inserted into the cutout cohesively (by adhesive bonding), in a force-fitting manner (by being clamped) or in a form-fitting manner (by being snapped), for example. 
         [0019]    Irrespective of whether the magnetic core is now inserted into a coil former which is produced in advance as part of a premolding process or is encapsulated by casting or injection molding during production of the coil former, the ends of the coil wire can be wound around the electrical connections once they have been wound onto the coil former in order to connect the wound coil wire to the electrical connections. For fastening and at the same time for improved electrical contact-making, the ends of the coil wire can alternatively or additionally be welded, soldered, adhesively bonded or spliced to the electrical connections. 
         [0020]    In this case, the electrical connection can be bent in a direction of the coil wire in order to relieve the mechanical load on the coil wire after connection of the wound coil wire to the electrical connection in order that the coil wire is relieved of tension and the winding stress is taken from the coil wire. 
         [0021]    In a particular development, the specified method in this case comprises the steps of arranging a deflection pin on the coil former, and connecting the wound coil wire to the electrical connections via the deflection pin. The development is particularly favorable in connection with the bent electrical connection since in this case the relaxation can work better. 
         [0022]    In a further alternative development, the specified method comprises the step of forming a housing body around the coil former with the wound coil wire which is connected to the electrical connections. The housing body protects the coil wire and its electrical connection to the electrical connections from mechanical loading and/or the influences of wear such as corrosion. 
         [0023]    The material of the housing body can in this case be selected corresponding to the environmental conditions of the measuring pickup to be produced. Particularly preferably, the coil former and the housing body should be formed from a material with an identical coefficient of thermal expansion, in particular from an identical material. In this way, thermal movements of the coil former and the housing body can be matched to one another which results in a further reduction in mechanical stresses. 
         [0024]    In a preferred development, the specified method comprises the step of arranging an electrically conductive metal sheet between a contact-making point between the electrical connections and a printed circuit board and the coil former, wherein the housing body is formed around the conductive metal sheet. The electrically conductive metal sheet can be connected, as return metal sheet, mechanically to the coil former or can be inserted into a corresponding transfer molding die directly prior to the transfer molding process in order to produce the housing body, with the result that the electrically conductive metal sheet is embedded between the housing body and the coil former or in the housing body. The return metal sheet in this way defines an additional mechanical reinforcement of the transfer molding parts of the measuring pickup, i.e. the coil former and the housing body, and can notably increase the electromagnetic compatibility of the measuring pickup in an electronic circuit comprising the measuring pickup. In addition, spacings can be preset in a defined manner by the electrically conductive metal sheet. 
         [0025]    The electrical connections can be matched to any desired connection technology on the side of the electronics, i.e. at the point at which electrical contact is intended to be made between said electrical connections and a printed circuit board. In this case, pin in paste, surface-mount technology (SMT), pressing-in, welding, splicing or adhesive bonding can be used, for example. 
         [0026]    The electrical connections can be connected to one another during production of the measuring pickup in a manner known per se via struts, for example, with the result that the measuring pickup ultimately needs to be disconnected from these struts by punching, for example. Prior to this subsequent separation, however, a bar code or a digital matrix code (DMC) can be applied to the measuring pickup, for example to the housing body, however, via which bar code or digital matrix code the measuring pickup can be identified electronically as regards type, technical data or the like. 
         [0027]    In order to simplify the placement of the measuring pickup on a printed circuit board, forming elements can also be applied to the housing body which can be used by a placement tool. If the measuring pickup is used, for example, as a surface mounted device (SMD) known per se, the housing body can be formed with at least one planar surface, on which a suction nozzle of an SMD placement device can act in a manner known per se in order to be able to directly grip the measuring pickup and place it on the printed circuit board. 
         [0028]    In accordance with a further aspect of the invention, a measuring pickup is produced using a specified method. In this case, the measuring pickup does not need to have a single coil but can be wound with a plurality of coils, as are used, for example, in a linear position sensor (LIPS). 
         [0029]    In accordance with a further aspect of the invention, a sensor comprises a specified measuring pickup and an encoder element, preferably in the form of a magnet, which is arranged movably relative to the measuring pickup. 
         [0030]    The specified sensor is particularly preferably a linear position sensor (LIPS). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    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: 
           [0032]      FIG. 1  shows a tandem master cylinder comprising a position sensor, 
           [0033]      FIG. 2  shows the position sensor from  FIG. 1 , 
           [0034]      FIG. 3  shows a transducer in the position sensor from  FIG. 2  in a first production state, 
           [0035]      FIG. 4  shows a transducer in the position sensor from  FIG. 2  in a second production state, 
           [0036]      FIG. 5  shows a transducer in the position sensor from  FIG. 2  in an alternative second production state, 
           [0037]      FIG. 6  shows the transducer from  FIG. 2  in a final production state, 
           [0038]      FIG. 7  shows an end region of the transducer in the second production state, and 
           [0039]      FIG. 8  shows an alternative view of the end region of the transducer in the second production state shown in  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0040]    The same technical elements are provided with the same reference symbols and only described once in the figures. 
         [0041]    Reference is made to  FIG. 1 , which shows a tandem master cylinder  2  comprising a position sensor  4 . 
         [0042]    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 chambler  16  and the secondary chamber  18  via a first and second connection  32 ,  34 . 
         [0043]    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. 
         [0044]    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 . 
         [0045]    Reference is made to  FIG. 2 , which shows the position sensor  4  shown in  FIG. 1 . 
         [0046]    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 transducer  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). 
         [0047]    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 transducer  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 transducer  48 . 
         [0048]    In the present embodiment, the transducer  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 , which holds the wiring carrier  42  and the transducer  48  mechanically together, the wiring carrier  42  and the transducer  48 . In this way, the protective compound  64  can not only ensure a mechanical stability between the wiring carrier  42  and transducer  48 , but an interior of the position sensor  4  with the circuit  38  is also effectively protected from contamination. In this case, this interior can particularly preferably likewise be filled with the protective compound  64 . 
         [0049]    The position sensor  4  can be encapsulated by injection molding, for example, with the protective compound  64  during production. For this purpose, the wiring carrier  42  of the position sensor  4  can be held on the transmission interface  56 , for example, which in any case needs to remain free in order to make electrical contact with the abovementioned cable. 
         [0050]    Reference is made to  FIG. 3 , which shows the transducer  48  in the position sensor  4  from  FIG. 2  in a first production state. 
         [0051]    In order to produce the transducer  48 , which is to be in the form of an LIPS, a leadframe  72  with contact legs  74  is punched out for the first production state, said leadframe mechanically supporting the transducer  48  on the abovementioned wiring carrier  42  and making electrical contact between said transducer and the circuit  38  on the wiring carrier  42 . For reasons of clarity, only some of the contact legs  74  have been provided with a reference symbol in  FIG. 3 . 
         [0052]    Then, a magnetic core  76  is arranged in the leadframe  72 , said magnetic core later being provided for transmission of a magnetic field between coils (yet to be described). 
         [0053]    Reference is made to  FIG. 4 , which shows the transducer  48  in the position sensor  4  from  FIG. 2  in a second production state. 
         [0054]    In order to produce the second production state shown in  FIG. 4 , the leadframe  72  is enveloped with the magnetic core  76  by a transducer protective compound  78 . This transducer protective compound  78  in the present embodiment consists of a thermosetting plastic, which has a substantially identical coefficient of thermal expansion to the magnetic core  76 , which can be produced from iron-nickel, for example. In the event of temperature fluctuations, hardly any mechanical stresses are thus input into the magnetic core  76 . 
         [0055]    The transducer protective compound  78  is in this case formed with four separating elements  80  such that they divide the magnetic core  76  into two outer winding regions  82  and an inner winding region  84 . In this case, the outer winding regions  82  are shorter than the inner winding region  84 . 
         [0056]    Then, if the transducer protective compound  78  has been cured, for example, the contact legs  74  can then be bent in the direction of a lower side of the transducer  48 , as shown in  FIG. 4 . 
         [0057]    In order to finish the transducer  48 , coil wires (not illustrated) are wound onto the transducer into the winding regions  82 ,  84 . A primary coil is in this case wound beyond all of the winding regions  82 ,  84 , whereas a physically identical secondary coil is wound on into in each case one of the outer winding regions  82 . 
         [0058]    During operation of the transducer, an electrical AC voltage signal is applied to the primary coil, for example, which AC voltage signal should induce an identical output signal in the physically identical secondary coils via the magnetic core  76 . 
         [0059]    If the encoder magnet  37  of the slide  36  now approaches one of the two secondary coils, it drives the magnetic core  76  into saturation. This results in a changed transmission behavior of the electrical AC voltage signal between the primary coil and the corresponding secondary coil, to which the encoder magnet  37  has got closer, which can be evaluated via the sensor circuit in a manner known to a person skilled in the art. In this way, the position of the encoder magnet  37  can be detected via the transducer  48 . 
         [0060]    Reference is made to  FIG. 5 , which shows the transducer  48  in the position sensor  4  from  FIG. 2  in an alternative, second production state in a sectional view. 
         [0061]    In order to produce the alternative second production state shown in  FIG. 5 , first the transducer protective compound  78  is formed as receiving body by means of a transfer molding process, for example, from a thermosetting plastic material with the leadframe  72  and a cutout in the form of a depression  86 , into which the magnetic core  76  is inserted. The depression  86  is then closed by a cover  88 . 
         [0062]    The separating elements  80  shown in  FIG. 5  are in this case, in the present embodiment, in the form of dedicated elements, for example, which are pushed out of the transducer protective compound  78  via the receiving body closed by the cover  88 . 
         [0063]    Then, similarly to  FIG. 4 , coil wires  90 ,  92  are wound onto the transducer into the winding regions  82 ,  84 . The primary coil  90  is in this case wound on beyond all of the winding regions  82 ,  84 , while a physically identical secondary coil  92  is wound on into in each case one of the outer winding regions  82 . 
         [0064]    When the transducer protective compound  78  has been cured, for example, the contact legs  74  of the leadframe  72  can then be bent in the direction of a lower side of the transducer  48 , as shown in  FIG. 6 . 
         [0065]    Prior to or after the bending of the contact legs  74 , the ends of the wires  94  of the primary coil  90  and the secondary coil  92  can be electrically connected to the contact legs  74 , of which more details are given with reference to  FIGS. 7 and 8 . 
         [0066]    The operation of the transducer  48  functions in the same way as the transducer described within the context of  FIG. 5 . 
         [0067]    The transducer  48  in  FIG. 3  or in  FIG. 5  can finally be accommodated in a housing body  96 , which is illustrated by way of example in  FIG. 6  on the basis of the transducer  48  shown in  FIG. 5 . This housing body  96  can enclose the transducer  48  over the entire circumference and thus act as protection from environmental influences. 
         [0068]    In order to increase the electromagnetic compatibility of the transducer  48  and in order to increase its mechanical rigidity, an electrically conductive return metal sheet  98  can be cast in the housing body  96 . 
         [0069]    In order to make electrical contact between the ends of the wires  94  of the primary coil  90  and of the secondary coils  92 , the ends of the wires  94  can be wound around the contact legs  74 , as shown in  FIG. 7 , and then electrical contact can be made by means of soldering, for example. 
         [0070]    For better physical separation between the individual wires  94 , in addition deflection pins  100  can be arranged in the transducer protective compound  78 . 
         [0071]    In addition, the contact legs  74  can be bent in the direction of the wires  94  after winding of the wires  94  around the contact legs  74 , as shown in  FIG. 8 , as a result of which a mechanical stress-relief region  102  indicated in  FIG. 8  is produced, which relieves the tension on the wires  94 .