Abstract:
A sensor for detecting a physical variable, including: —a sensor element for outputting an electrical signal dependent on the physical variable, —a substrate carrying the sensor element, —a printed circuit board, conducting the electrical signal, on the substrate, and —an embedding compound, in which the sensor element is completely embedded and the printed circuit board is at least partly embedded, —wherein at least one compensation element is embodied in the embedding compound, by which compensation element a mechanical stress caused by an element of the sensor at least partly embedded in the embedding compound is counteracted.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. National Phase application of PCT/EP2016/051575, filed Jan. 26, 2016, which claims priority to German Patent Application No. 10 2015 201 482.7, filed Jan. 28, 2015, the contents of such applications being incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a sensor for detecting a physical variable and to a control device for a vehicle having the sensor. 
       BACKGROUND OF THE INVENTION 
       [0003]    In WO 2010/037 810 A1, which is incorporated by reference a sensor for detecting a physical signal is disclosed. The sensor comprises a lead frame, which as a circuit substrate supports the sensor components of the sensor and also interconnects them. The sensor components are enclosed in a circuit housing with the lead frame. 
       SUMMARY OF THE INVENTION 
       [0004]    An aspect of the invention aims to improve the known sensor. 
         [0005]    In accordance with one aspect of the invention, a sensor for detecting a physical variable comprises a sensor element for outputting an electrical signal dependent on the physical variable, a substrate supporting the sensor element, a conductor track conducting the electrical signal on the substrate and an embedding compound, in which the sensor element is completely embedded and the conductor track is at least partially embedded. 
         [0006]    In accordance with an aspect of the invention, at least one compensation element is embodied in the embedding compound of the specified sensor, by means of which compensation element a mechanical stress induced by an element of the specified sensor at least partially embedded in the embedding compound is counteracted. 
         [0007]    The specified sensor is based on the consideration that the above-mentioned sensor in a vehicle is normally used in a circuit assembly. These consist of a plurality of components combined together to form a higher-level circuit. The higher-level circuit in such cases is typically very large however, which is not compatible with the ever increasing requirements for miniaturization in the automotive industry. 
         [0008]    One solution would be not to manufacture the sensor using a lead frame as cited above, but to mount the sensor element directly on a circuit substrate which implements the higher-level circuit. However, this presents the problem that in order to ensure a long service life the sensor element must be embedded in a protective material. Due to temperature changes and other influences however, this protective material introduces mechanical stress or mechanical tension into the overall system, which can damage the overall system and reduce its service life. 
         [0009]    The specified sensor addresses this problem by proposing to design the sensor in such a way that the mechanical stress induced by an element in the sensor is compensated for. To achieve this, at least two elements of the sensor are arranged such that the resulting mechanical stress cancels out overall. The compensation element provided for this purpose, with which the freedom from mechanical tension is to be achieved as best as possible, can be an element necessary for the function of the sensor, or a redundant element that is specifically introduced into the specified sensor for the purpose of neutralizing the mechanical stress. 
         [0010]    Outwardly, the sensor is thus mechanically free of stress, enabling the service life of the specified sensor to be significantly increased. 
         [0011]    In an extension of the specified sensor, the compensation element and the element of the sensor which causes the mechanical stress are arranged symmetrically, in particular point symmetrically. The symmetrical arrangement of the two elements attempts to achieve an ideal condition, in which the mechanical stress is as far as possible completely cancelled out. 
         [0012]    The compensation element may be made of a different material from the embedding compound, by being embodied, for example, as a lacquer, as a redundant through hole or as a cavity in the embedding compound. 
         [0013]    In an additional extension of the specified sensor, the element of the sensor at least partially embedded in the embedding compound comprises the sensor element. In contrast, the compensation element can comprise an electrical component which differs from the sensor element, and can therefore be designed to be non-redundant. 
         [0014]    Such a non-redundant component can be, for example, a filter element for increasing the electromagnetic compatibility, known as EMC. 
         [0015]    In a particular extension of the specified sensor, an electrical contact between the conductor track and the sensor element may be different from an electrical contact of the compensation element with the conductor track or with another conductor track. In this way, different geometric or other different causal factors between the two elements can be compensated. For this purpose the electrical contacts can differ, for example, in their size, wherein the size difference should be designed in such a way that the mechanical stress is further reduced. 
         [0016]    In accordance with a further aspect of the invention, a control device for a vehicle for controlling a behavior of the vehicle based on a detected physical variable comprises one of the specified sensors for detecting the physical variable. 
     
    
     
       BRIEF DESCRIPTIONS OF THE FIGURES 
         [0017]    The properties, features and advantages of the present invention described above and the manner in which these are achieved will become clearer and more comprehensible in conjunction with the following description of the exemplary embodiments, which are explained in more detail in connection with the drawings, which show: 
           [0018]      FIG. 1  a schematic view of a vehicle with a driving dynamics regulation unit, 
           [0019]      FIG. 2  a schematic representation of a rotation speed sensor in the vehicle of  FIG. 1 , 
           [0020]      FIG. 3  a schematic representation of a read head of the rotation speed sensor of  FIG. 2  in an intermediate production state, 
           [0021]      FIG. 4  a schematic sectional view of an alternative read head, 
           [0022]      FIG. 5  a schematic view of an ideal, theoretical arrangement of different components for a read head, 
           [0023]      FIG. 6  a control device from a first perspective, 
           [0024]      FIG. 7  the control device of  FIG. 6  from a second perspective, 
           [0025]      FIG. 8  a schematic plan view of a linear position sensor, and 
           [0026]      FIG. 9  a schematic plan view of an alternative linear position sensor. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    In the figures identical technical elements are provided with the same reference numerals and described only once. 
         [0028]    Reference is made to  FIG. 1 , which shows a schematic diagram of a vehicle  2  with a chassis  6  supported on wheels  4 . Two of the wheels  4  are driven via an axis  8  by an internal combustion engine  10 . The basic operating principle of an internal combustion engine is known and will therefore not be further discussed below. 
         [0029]    In a known manner, see for example DE 10 2012 206 552 A1, which is incorporated by reference, the valve timing of the internal combustion engine  10  can be adjusted with a camshaft timer  12 , in order to adjust the load point of the internal combustion engine  10  to obtain a better fuel utilization in different rotation speed ranges. To achieve this a camshaft control device  14  detects the rotation speed  20  of the internal combustion engine  10  using a rotation speed sensor  18 , and with a control signal  22  controls the camshaft timer  12  based on the detected rotation speed  20 . The means of generating the control signal  18  based on the rotation speed  20  is known and will not be discussed further below. Details of this can be found in the relevant specialist literature. 
         [0030]    The rotation speed sensor  18  in the context of the present embodiment is designed in a particular way. Before this is discussed in more detail, the basic structure of the rotation speed sensor  18  itself will be described in greater detail. To do so, reference is made to  FIG. 2 , which shows a schematic view of a possible embodiment of the rotation speed sensor  18  in the vehicle  2  of  FIG. 1 . 
         [0031]    The rotation speed sensor  18  in the present embodiment is designed as an active rotation speed sensor, which comprises an encoder disk  26  mounted in a rotationally fixed manner on the rotor, not shown, of the internal combustion engine  10 , and a read head  28  which is mounted on the chassis  6  in a fixed position. 
         [0032]    The encoder disk  26  in the present embodiment consists of magnetic north poles  30  and magnetic south poles  32  chained together, which jointly excite a magnetic encoder field  33  indicated with an exemplary arrow. When the encoder disk  26  mounted on the rotor of the internal combustion engine  10  rotates with the latter in a direction of rotation  34 , the magnetic encoder field  33  rotates with it. 
         [0033]    The read head  28  in the present embodiment comprises a sensor probe  35 , which generates an electrical sensor signal  39  as a function of the motion of the magnetic encoder field  17 . Any desired measuring principle can be used for this purpose, such as a measuring principle based on the magneto-resistive effect. The electrical sensor signal  39  therefore depends on the rotation speed  20  to be detected. 
         [0034]    The sensor signal  39  can then be processed in a signal processing circuit  40  arranged in the read head  28 . Here, a pulse signal  42  is usually created from the sensor signal  39 , wherein over a predefined time segment the pulse signal  42  comprises a number of pulses which is dependent on the rotation speed to be detected. This pulse signal  42  is then output to the camshaft control device  14 , which can then derive the rotation speed  20  by counting the number of pulses in the pulse signal  42 . 
         [0035]    Because, as is well known, non-negligible interfering fields occur due to the internal combustion engine  10 , a support magnet  43  is arranged in the read head  28 , which counteracts these interfering fields and thus enables detection of the rotation speed  20  with low tolerances. The support magnet  43  should therefore be selected to be sufficiently strong to be able to adequately counteract the interfering fields. 
         [0036]    Conventionally, the read head  28  is embodied on a lead frame, as is known, for example, from the above-mentioned prior art in WO 2010/037 810 A1, which is incorporated by reference. Such a lead frame is shown for example in  FIG. 3  and referenced with the reference number  44 . The lead frame  44  comprises a supporting frame  46 , an insertion island  48  on which the read head  28  is supported and interconnected, two dambars  50  and two contact terminals  52 . The dambars  50  hold the contact terminals  52  directly and hold the insertion island  48  via an auxiliary frame  53  on the supporting frame  46 . In the lead frame  44  the supporting frame  46 , the insertion island  48 , the dambars  50 , the contact terminals  52  and the auxiliary frame  53  are designed as an integral stamped part or stamped frame, in which the above-mentioned elements are formed by stamping from an electrically conductive metal plate. 
         [0037]    In the context of the present embodiment the sensor probe  35 , for example in the form of a magneto-resistive element, and the signal evaluation circuit  40  are mounted on the insertion island  48  and electrically contacted, for example by soldering or bonding. The sensor probe  35  and the signal evaluation circuit  40  are also connected to each other via a bonding wire  54 , so that the sampled signal  39  can be transmitted between the sensor probe  35  and the signal evaluation circuit  40  via the insertion island  48  and the bonding wire  54 . 
         [0038]    The insertion island  48  in the present embodiment is connected directly to one of the two contact terminals  52 , while the other of the two contact terminals  52  is electrically isolated from the insertion island  48  and is connected to the signal evaluation circuit  40  via a further bonding wire  54 . In this way, the data signal  42  can be output from the signal evaluation circuit  40  via the two contact terminals  52 . 
         [0039]    The supporting frame  46  in the context of the present embodiment comprises two parallel running transport strips  58 , which are connected to each other via connecting webs  60 . The transport strip  58  comprises transport holes  62 , into which a transport tool (not illustrated further) can engage and move the lead frame  44 . On the transport strip  58  an index hole  64  is also formed, by means of which the position of the lead frame  44  during transport can be defined and therefore regulated. 
         [0040]    To protect the read head  28  a housing can be formed around the insertion island  48 , which supports the read head  28 , and a part of the contact terminals  52 . The housing can be designed, for example, as a protective compound around the read head  28 , on which point for the sake of brevity reference is made to the relevant prior art, such as DE 10 2008 064 047 A1, which is incorporated by reference. 
         [0041]    This read head  28  must now be at least electrically connected to the camshaft control device  14 , so that the pulse signal  42  can be transmitted to this. The necessary electrical connection processes required increase not only the manufacturing costs, but the resulting connection in principle represents a source of error, both electrically as well as mechanically, on the basis of which the read head  28  and therefore the entire rotation speed sensor  18  may fail. 
         [0042]    The exemplary embodiment addresses this problem by proposing to embed the read head  28  of the rotation speed sensor  18  in a printed circuit board module. Such a printed circuit board module will be explained in more detail hereafter on the basis of  FIG. 4 , in which it is shown in a sectional view with the reference numeral  66 . 
         [0043]    The printed circuit board module  66  comprises a plurality of insulating layers  67  stacked on top of one another, on which conductor tracks  68  are applied. The conductor tracks  68  or the insulating layers  67  carry the individual components  35 ,  40  constituting the read head  28  of the rotation speed sensor  18 , which, in addition to the already mentioned sensor probe  35  and the already mentioned signal processing circuit  40 , can also include filter elements  69  as passive components, which increase the electromagnetic compatibility of the read head  28  and therefore the rotation speed sensor  18 . 
         [0044]    The sensor probe  35  and the signal processing circuit  40  in the present embodiment are embedded in an embedding compound  70  between two insulating layers  67  of the printed circuit board module  66 . In this way, these elements are protected against external influences. The individual layers can be electrically connected to each other via plated-through holes  71 . Solder joints  72  can also be present on the printed circuit board module  66 , to electrically connect the printed circuit board module  66  to a higher-level circuit. 
         [0045]    The problem with the embedding as shown in  FIG. 4 , however, is the fact that individual mechanical stress  73 , which is caused by the individual mechanical components  35 ,  40 ,  69 , for example due to temperature variations, sums together into a total mechanical stress  74 , which in turn, for example, can deform the printed circuit board module  66 . This deformation can among other things cause the solder joints  72  to become detached from the higher-level circuit, and lead to failure of the read head  28  and thus the rotation speed sensor  18 . 
         [0046]    The exemplary embodiment addresses this problem by proposing to design the printed circuit board module  66  as symmetrically as possible, so that the individual mechanical stress  73  caused by the individual components  35 ,  40 ,  69  is mutually canceled out and the total mechanical stress  74  is thus minimized. To this end different compensation elements are present in the printed circuit board module  66 , which can counteract an individual mechanical stress  73 . It is not strictly necessary to actually implement all the compensation elements shown in the printed circuit board module  66  in order to realize the idea behind the embodiment. The individual compensation elements shown are only intended as an example to demonstrate how the components can be arranged symmetrically in the printed circuit board module  66 , to keep the total mechanical stress below a specific, reasonable limit. 
         [0047]    On the one hand it is possible to introduce a redundant conductor track  68 ′ and a redundant insulating layer  67 ′ as a compensation element, in order to design a symmetrical conductor track arrangement in the printed circuit board module  66 . Therefore, a redundant embedding compound  70 ′ is also incorporated, which can be selected to be either different from the embedding compound  70  or, alternatively, identical to it. 
         [0048]    As a further option, the sensor probe  35  and the signal processing circuit  40  can be designed symmetrical to each other. The advantage in this case is that no redundant components need to be incorporated into the printed circuit board module  66  as compensation elements. In order to compensate for differences in the geometrical, material or other inherent physical properties between the two components  35 ,  40 , it is also possible to dimension the contacts  75  between the conductor tracks and the individual components  35 ,  40 ,  69  in different geometries, which is indicated in  FIG. 4  by variations in the width of the contacts  75  on the sensor probe  35  and on the signal processing circuit  40 . 
         [0049]    In addition, redundant cutouts  76  can be introduced into the printed circuit board module  66  as compensation elements. 
         [0050]    The ideal case of the printed circuit board module  66  is shown in  FIG. 5 . Here all of the distances  77  between the individual elements are symmetrical to one another with respect to an axis of symmetry  78 . In practice however, this ideal concept cannot be implemented, for the simple reason that the components  35 ,  40 ,  69  could then no longer be contacted with the conductor tracks  68 . But the ideal case should be attempted in the design of the printed circuit board module  66  as far as possible. 
         [0051]    The sensor probe  35  should be arranged as centrally as possible, so that the individual mechanical stress  73  on the probe remains as small as possible. In this way, measurement errors induced by the individual mechanical stress  73  can be kept small. 
         [0052]    By embedding the components  40 ,  35  and passive components  69 , it is possible to miniaturize the resulting sensor system  28  to a significant extent. At the same time, additional components of a control device, such as the camshaft control device  14 , can be embedded as well. This enables printed circuit board space or component mounting surface to be reduced on the control device, i.e. in the exemplary embodiment of the camshaft control device  14 . 
         [0053]    Furthermore, it is not necessary to encase the individual components  35 ,  40  once again in an extra encapsulation step, for example by injection pressing, with protective compound referred to above in the context of  FIG. 3 . By enclosing the components with the embedding compound  70  over their whole surface, for example in the form of a resin, such a protective compound becomes obsolete. At the same time, a resin offers better thermal properties for dissipating heat generated by power loss of the components as, e.g., airborne convection. By means of the printed circuit board design it is very simple to implement simple EMC protection measures in the printed circuit board layer structure. The connection of the sensor system in the form of the read head  28  to the printed circuit board of the control device, in other words the camshaft control device  14 , can be designed in a mechanically more robust way by surface contacting. The electrical contacting of the components  35 ,  40  of the read head  28  to each other and to the substrate by wire bonding technology is no longer required. The contact can be achieved via a structuring of the conductor tracks  68 , which can be embodied for example as copper foils (inner layers of a printed circuit board). This creates a more robust and much shorter signal connection. 
         [0054]    The exemplary embodiment has been described by reference to a rotation speed sensor  18 . In alternative or additional sensors, in which the previously explained idea can be used, the sensor probe  35  can detect, for example, accelerations, rotation rates and/or mechanical vibration. 
         [0055]    Alternatively, the previously explained idea, as has been previously mentioned, can be applied in entire control devices. For example, a Car2X control device  79  shown in a perspective view in  FIGS. 6 and 7  should be mentioned for this purpose. These are control devices that can subscribe to mobile ad-hoc networks (known as Car2x), which are known for example from WO 2010/139 526 A1, which is incorporated by reference. The nodes of such Car2X networks can be particular road traffic users, such as vehicles or other objects found in a road traffic situation, such as traffic lights. Via these networks, information on road traffic conditions, such as accidents, congestion, dangerous situations, . . . can be provided to the road traffic users subscribed to the Car2X network. 
         [0056]    By the use of the previously described idea in entire control devices, such as the previously mentioned Car2X control device, any twisting or warping of the modules implemented in the control device can be reduced. 
         [0057]    A further advantage of the above-specified idea will be explained hereafter by reference to  FIGS. 8 and 9 . These figures show a linear position sensor  80 , or LIPS, as is known for example from DE 10 2014 201 790 A1, which is incorporated by reference. 
         [0058]    The LIPS  80  comprises a printed circuit board consisting of conductor tracks  68  and insulation layer  67 , on which a plurality of insertion points  81  for press-fit connections are formed, to connect the sensor to a higher-level electrical system. 
         [0059]    As the measuring transducer  35  a coil structure can be used, as is disclosed in the previously mentioned patent document. In addition, a signal processing circuit  40  can again be arranged to adapt sensor signals from the measuring transducer  35  before they are output to the higher-level electrical system. 
         [0060]    The measuring transducer  35  in a LIPS  80  is a so-called fine pitch component, for the protection of which a lacquered region  82  suggested in  FIG. 8  must be formed in a known manner. By embedding the measuring transducer  35 , in the context of the idea described above as shown in  FIG. 9 , this lacquered region  82  can be completely omitted, which enables the miniaturization to be further increased.