Abstract:
The sensor for detecting the position of a generator element which outputs a position-dependent physical variable, the sensor includes a transducer for converting the physical variable into an electric generator signal, a circuit on a wiring support for receiving the generator signal from the transducer and outputting a measurement signal that corresponds to the generator signal and a protective compound which at least partially surrounds the transducer and the wiring support, thus retaining the transducer on the wiring support.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. National Phase Application of PCT International Application No. PCT/EP2013/076927, filed Dec. 17, 2013, which claims priority to German Patent Application No. 10 2012 224 075.6, filed Dec. 20, 2012, the contents of such applications being incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a sensor for detecting a position of an encoder element, to a method for producing the sensor and to a method for producing a transformer. 
       BACKGROUND OF THE INVENTION 
       [0003]    WO 2010/037 810 A1 incorporated by reference herein discloses a sensor for outputting an electrical signal on the basis of a detected physical variable. The sensor has a measuring circuit which is enclosed in a circuit housing. 
       SUMMARY OF THE INVENTION 
       [0004]    An aspect of the invention consists in improving the known sensor. 
         [0005]    In accordance with one aspect of the invention, a sensor for detecting a position of an encoder element, which outputs a physical variable which is dependent on the position, comprises a transducer for converting the physical variable into an electrical encoder signal, a circuit on a wiring carrier for receiving the encoder signal from the transducer and outputting a measurement signal corresponding to the encoder signal, and a protective compound, which at least partially envelops the transducer and the wiring carrier and thus holds the transducer on the wiring carrier. 
         [0006]    The specified sensor is based on the consideration that the measuring circuit in the sensor mentioned at the outset could be desired to detect a physical variable which is dependent on the position of an encoder element, such as an electrical and/or magnetic field, light or sound and to convert this signal into an electrical measurement signal which is dependent on the position of the encoder element. Such a position sensor could detect, for example, angular positions and/or linear positions of the encoder element and thus determine the position of a gas pedal and/or brake pedal in a vehicle, for example. 
         [0007]    Within the context of the specified sensor, however, it is identified that a transducer for converting the physical variable which is output by the encoder and which is dependent on the position to be detected into an encoder signal, which is dependent at least on the desired measurement signal, needs to be held fixedly on a wiring carrier on which the circuit which outputs the measurement signal is also borne. 
         [0008]    In this case, the transducer could be accommodated in a transducer housing. In order to protect the entire sensor from mechanical and other influences, on the one hand, but to hold the individual component parts of the sensor together safely as well, the transducer accommodated in the transducer housing could ultimately be arranged and potted together with the other component parts in a sensor housing. 
         [0009]    For this purpose, however, three different elements, namely the converter housing, the potting compound and the sensor housing, would need to be prepared, which would not only be cost-intensive, but would also take up a considerably large amount of installation space. 
         [0010]    In contrast, the specified sensor is based on the concept that all of the component parts could be held together via a common protective compound. All of the electrical component parts of the specified sensor such as the transducer and the circuit could be brought into contact with one another, for example, using adhesive-bonding, soldering or sintering technology. In contrast to the abovementioned design, the component parts with which electrical contact has been made could now be encapsulated by injection molding or potting using the protective compound directly in a single injection-molding operation, which in this way not only holds together the component parts of the sensor but also houses them and thus protects them from contamination and other environmental influences such as moisture. In this way, the specified sensor can be constructed at low cost and in a manner which saves on installation space. 
         [0011]    The transducer and the wiring carrier can be encapsulated by injection molding or potting using the protective compound directly. 
         [0012]    In addition, the transducer and the wiring carrier can be correspondingly encapsulated by injection molding or potting using the protective compound in a single injection-molding operation or a single potting operation. 
         [0013]    Finally, the protective compound can form directly an outer housing of the specified sensor. 
         [0014]    In one development of the specified sensor, the protective compound has a coefficient of expansion which is within the range of a coefficient of expansion of the electromechanical components. The specified development is based on the consideration that a different coefficient of expansion between the transducer and the protective compound could result in an input of mechanical loads such as mechanical stresses. These mechanical loads could falsify the measurement result, for which reason, within the scope of the specified sensor, the stresses which are input should be kept as low as possible. This is achieved within the scope of the development by a suitable selection of the coefficient of expansion of the protective compound and the transducer. 
         [0015]    In a preferred development of the specified sensor, the protective compound comprises a thermosetting plastic material. The thermosetting plastic material has a particularly advantageous effect within the scope of the specified sensor if an electrical component which converts a magnetic flux and has a magnetic core is used as the transducer, the conversion principle of said electrical component being dependent on saturation of the magnetic core. The saturation of the magnetic core is achieved depending on the stresses that are input, which can thus falsify the measurement result. Conventional materials for the magnetic core, such as iron, have a coefficient of expansion which is within the range of the coefficients of expansion of thermosetting plastic materials, however. 
         [0016]    In another development of the specified sensor, the circuit on the wiring carrier comprises a part which is free of the protective compound. The sensor could be held fixedly on this free part if the wiring carrier and the transducer are enveloped by the protective compound. 
         [0017]    In a preferred development of the specified sensor, the circuit in the part which is free from the protective compound is designed to output the measurement signal. In this way, that part which is exposed and is necessary for applying the protective compound around the wiring carrier and the transducer can be used usefully to connect the specified sensor to a superordinate unit such as, for example, a motor controller in a vehicle via a cable, for example. Electrical contact can be made between the cable and the circuit in the exposed part in this case via contact-making springs, press-in pins, edge grip connectors, soldering or adhesive bonding. 
         [0018]    In yet another development of the specified sensor, the transducer is an electrical transformer, in particular a linear inductive position sensor (LIPS), which is designed to transmit at least one reference signal depending on a position of the encoder element as encoder signal. The transformer may be the abovementioned electrical component which converts the magnetic flux. Within the scope of the specified sensor, a transformer should be understood to mean an electrical component which has two coils which are galvanically isolated from one another and which transmit electrical energy between one another via a common contact medium such as air or the abovementioned magnetic core. If the encoder element is a magnet itself, for example, it changes the transmission properties of the common contact medium, which is demonstrated in a changed transmission behavior of the two coils. An LIPS within the scope of the specified sensor is intended to be a transformer which has a third coil, wherein the second and third coils are arranged for the interchange of electrical energy with the first coil. Depending on the position of the encoder element, which is in the form of a magnet, with respect to the second and third coils, a specific configuration of the transformation ratios between the first and second coils and the first and third coils results. Since the position of the encoder element is now detected at two different locations using two different transformation ratios, the abovementioned saturation of the magnetic core can also be used in the detection of the position. 
         [0019]    If the coils are wound onto a magnetic core in the transformer via a coil former, the magnetic core in a preferred development has a coefficient of expansion which is in the range of a coefficient of expansion of the coil former. In this way, the coil former can be applied directly to the magnetic core, for example by encapsulation by injection molding with a thermosetting plastic without any inputs of mechanical stresses on the mechanical core and therefore falsifications of the measurement result by the transformer occurring. 
         [0020]    Otherwise, these inputs of mechanical stresses would need to be avoided by positioning the magnetic core in an elastic material within the coil former. 
         [0021]    In accordance with a further aspect of the invention, a method for producing a specified sensor comprises the following steps:
       making contact between the transducer and the circuit on the wiring carrier, and   enveloping the transducer and the wiring carrier at least partially with the protective compound so that the protective compound holds the transducer on the wiring carrier.       
 
         [0024]    In a particular development of the specified method, a surface of the wiring carrier and/or of the transducer which is to be enveloped by the protective compound is activated prior to the transducer and the wiring carrier being enveloped by the protective compound. Activation of the surface of the wiring carrier and/or of the converter will be understood below to mean partial destruction of the molecular structure of the surface of the wiring carrier and/or the converter so that free radicals are produced on the surface of the wiring carrier and/or the converter. These free radicals are capable of entering into chemical and/or physical bonds with the protective compound so that said protective compound can no longer be detached from the surface of the wiring carrier and/or the converter. 
         [0025]    The protective compound can in this case comprise a polar material, such as polyamide. The polar polyamide can be connected physically to the activated surface of the wiring carrier and/or the converter in a manner known to a person skilled in the art and can thus improve the mechanical hold between the wiring carrier and the converter. 
         [0026]    In an additional development of the specified sensor, at least part of the surface of the wiring carrier and/or the converter is roughened in the contact region with the protective compound so that the effective activated surface is enlarged and the adhering effect between the wiring carrier and/or the converter and the protective compound is increased. 
         [0027]    In a particular development of the specified sensor, the roughened part of the surface of the wiring carrier and/or the converter is roughened using a laser. The surface of the wiring carrier and/or the converter can not only be activated by the laser, but also form-releasing means which may be provided are also removed by the laser from the surface of the wiring carrier and/or the converter, which form-releasing means could suppress adhesion between the wiring carrier and/or the converter and the protective compound. 
         [0028]    In accordance with a further aspect of the invention, a method for producing an electrical transformer, in particular a linear inductive position sensor (LIPS) comprises the following steps:
       arranging electrical connections for the transformer and a magnetic core for the transformer,   forming a coil former around the magnetic core in such a way that the electrical connections are held by the coil former, and   winding at least one coil wire onto the coil former.       
 
         [0032]    The specified method can be extended by features which correspond to the features of the abovementioned transformer accordingly. In particular, a coefficient of expansion of the magnetic core can be in the range of a coefficient of expansion of the coil former with the advantages already mentioned. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    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 exemplary embodiments, which are explained in more detail in connection with the drawings, in which: 
           [0034]      FIG. 1  shows a tandem master cylinder comprising a position sensor, 
           [0035]      FIG. 2  shows the position sensor from  FIG. 1 , 
           [0036]      FIG. 3  shows an interface at the position sensor shown in  FIG. 2 , 
           [0037]      FIG. 4  shows an alternative interface at the position sensor shown in  FIG. 2 , 
           [0038]      FIG. 5  shows another alternative interface at the position sensor shown in  FIG. 2 , 
           [0039]      FIG. 6  shows a converter in the position sensor shown in  FIG. 2  in a first production state, and 
           [0040]      FIG. 7  shows a converter in the position sensor shown in  FIG. 2  in a second production state. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0041]    The same technical elements are provided with the same reference symbols and only described once in the figures. 
         [0042]    Reference is made to  FIG. 1 , which shows a tandem master cylinder  2  comprising a position sensor  4 . 
         [0043]    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 . 
         [0044]    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. 
         [0045]    The position sensor  4  has a sampling element in the form of a slide  36  comprising a sensor 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 . 
         [0046]    Reference is made to  FIG. 2 , which shows the position sensor  4  shown in  FIG. 1 . 
         [0047]    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 sensor magnet  37  and thereupon outputs an electrical sensor signal (not denoted) to the circuit  38  on the basis of this magnetic field. This sensor 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). 
         [0048]    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 . 
         [0049]    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 the 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 . 
         [0050]    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. 
         [0051]    This transmission interface  56  can be embodied differently, which is shown in  FIGS. 3 to 5 . According to said figures, the transmission interface can comprise contact bores  66  or edge group plugs  68 . The transmission interface  56  can in this case also be surrounded by a wall  70  formed from the protective compound  64 , as shown in  FIGS. 2 and 5 . 
         [0052]    Reference is made to  FIG. 6 , which shows the transducer  48  in the position sensor  4  from  FIG. 2  in a first production state. 
         [0053]    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. 6 . 
         [0054]    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). 
         [0055]    Reference is made to  FIG. 7 , which shows the transducer  48  in the position sensor  4  from  FIG. 2  in a second production state. 
         [0056]    In order to produce the second production state shown in  FIG. 7 , 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, for example. In the event of temperature fluctuations, hardly any mechanical stresses can be input into the magnetic core  76 . 
         [0057]    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 . 
         [0058]    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. 7 . 
         [0059]    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 . 
         [0060]    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 . 
         [0061]    If the sensor 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 sensor 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 sensor magnet  37  can be detected via the transducer  48 .