CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. National Phase application of PCT International Application No. PCT/EP2014/070563, filed Sep. 25, 2014, which claims priority to German Patent Application No. DE 10 2013 219 657.1, filed Sep. 27, 2013, the contents of such applications being incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method for testing a transformer, to a control device for carrying out the method and to the transformer. 
       BACKGROUND OF THE INVENTION 
       [0003]    DE 44 259 03 C3, which is incorporated by reference and EP 238 922 B1, which is incorporated by reference disclose transformers which operate as position sensors according to the principle of linear travel measurement on the basis of 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 
       [0004]    An aspect of the invention improves transformers generally. 
         [0005]    According to one aspect of the invention, a method for testing a transformer with a first inductor and a second inductor galvanically isolated from the first inductor, which inductors are correspondingly connected in a first circuit and in a second circuit comprises the steps of applying a direct voltage between a first circuit point and a second circuit point in the first circuit point; tapping a test voltage between two circuit points in the first or second circuit, wherein one of the two circuit points is different from the first circuit point and the second circuit point and testing the transformer on the basis of a comparison of the direct voltage and the test voltage. 
         [0006]    The specified method is based on the idea that a transformer with two galvanically isolated inductors can only be operated with an alternating voltage for physical reasons, since direct voltages cannot be transmitted between the inductors as a result of the galvanic isolation. The specified method comes in here with the realization that a direct voltage which is applied to the inductors must not have any effect on the function of the transformer of transmitting an alternating voltage between the two inductors. 
         [0007]    On the other hand, a direct voltage which is applied to one of the two inductors gives rise to expected direct voltage potentials at certain circuit points in the corresponding circuit of this inductor. Therefore, the circuit points can be monitored for the expected direct voltage potentials. 
         [0008]    If, for example, the two inductors are galvanically connected owing to a short-circuit, the direct voltage potential at the monitored switching point would differ from the expected direct voltage potential, with the result that the fault can be unambiguously detected. 
         [0009]    In this context switching points are to be understood below as meaning points in the first or second circuit which differ in their electrical potential when an electrical voltage is applied to these circuits. 
         [0010]    In one development of the specified method, during the testing of the transformer a fault is detected if a detected transmission behavior between the test voltage and the direct voltage differs from a predetermined transmission behavior between the test voltage and the direct voltage during fault-free operation of the transformer. As a result of the monitoring of the transmission behavior, the above-mentioned, expected direct voltage potential is not fixed to a specific value and can therefore be verified even if the direct voltage changes owing to temperature or ageing. 
         [0011]    In principle, the method can be carried out in a desired way with respect to the application of the direct voltage and the tapping of the test voltage, wherein the direct voltage is applied, for example, to the first inductors of the transformer, and the test voltage is tapped at the second inductor of the transformer. In particular, in the event of the specified method being applied in a transformer which is used as a position sensor, the application of the direct voltage and the tapping of the test voltage on the side of the first coil is particularly favorable if the first coil is used as a secondary coil. In this way, it is possible to determine not only short circuits to the primary coil but also to further secondary coils which are necessary in linear position sensors in order to generate linear output signals with alternating voltages from the position to be determined. 
         [0012]    In one particular development of the specified method, the first inductor in the first circuit is additionally part of a test circuit which predefines the transmission behavior and has at least one resistor which connects the first inductor to a fixed potential by means of the direct voltage, wherein the direct voltage and the test voltage are correspondingly applied to the test circuit and tapped in the test circuit. This development is based on the idea that the inductors can, from the point of view of direct voltage, constitute short-circuits, and when further resistors are absent they can bring about very high direct currents if the direct voltage is applied. Therefore, within the scope of the particular development it is proposed to connect the first inductor to a predetermined potential via a resistor, such as for example a division resistor on the basis of the direct voltage, and to keep the direct currents through the first inductor as small as possible. 
         [0013]    According to a further aspect of the invention, an execution device is configured to carry out one of the specified methods. 
         [0014]    In one development of the specified execution device, the specified execution device has a memory and a processor. In this context, one of the specified methods is stored in the form of a computer program in the memory, and the processor is provided for executing the method when the computer program is loaded from the memory into the processor. 
         [0015]    According to a further aspect of the invention, a computer program comprises program code means for carrying out all the steps of one of the specified methods if the computer program is executed on the computer or one of the specified devices. 
         [0016]    According to a further aspect of the invention, a computer program product contains a program code which is stored on the computer-readable data carrier and which, when executed on a data-processing device, carries out one of the specified methods. 
         [0017]    According to a further aspect of the invention, a transformer comprises a first circuit with a first inductor, a second circuit with a second inductor which is galvanically isolated from the first inductor and is coupled to the first inductor, and one of the specified execution devices. 
         [0018]    In one development, the specified transformer comprises a test circuit which contains the first inductor and has at least two resistors for applying the direct voltage and tapping the test voltage, wherein the test circuit has a means for galvanically decoupling from the ground connection, in particular via a capacitor. The second resistor is, in addition to the first resistor mentioned above, optional and can also be omitted for the implementation of the present development. As a result of the galvanic isolation from the ground connection, the test circuit is grounded from the point of view of the alternating voltage, with the result that from the point of view of the alternating voltage only the first circuit, but not the test circuit, is present. 
         [0019]    If the galvanic isolation is a capacitor, the method which is mentioned at the beginning should be carried out in the execution device if alternating voltages with high frequencies, for example in or above the kHz range, are transmitted with the transformer. In this way, a small capacitance can be selected for the capacitor, with the result that the reactive resistance of the capacitor can be ignored. 
         [0020]    In an additional development of the specified transformer, the resistors form a voltage divider, wherein the first inductor is connected in series between the two resistors. In this way, the transmission behavior mentioned above can be determined by suitable dimensioning of the two resistors, and according to the abovementioned condition that the test voltage is to be tapped at least one switching point at which the direct voltage is not applied, there is a third switching point which differs clearly from the other switching points and at which the test voltage can be tapped. 
         [0021]    In one particular development of the specified transformer, a first of the two resistors is arranged opposite the ground connection when viewed from the first inductor, and a transformer tap is arranged between the first resistor and the first inductor. In this way, the input resistor or output resistor of the transformer can be influenced at the same time as the first resistor, in order to exchange in an optimum way a signal which is present at the transformer (and which is fed to or away from the transformer) using a circuit which is connected to the transformer. 
         [0022]    For this purpose, the first resistor preferably has high impedance. The expression high impedance refers to the previously mentioned input resistance or output resistance of electronic components, switching devices or measuring devices, that is to say in this case of the transformer. It states that the resistance of the transformer when viewed from a circuit connected to the transformer exceeds a particular resistance value. As a rule, the term high impedance is used if the resistance exceeds a two-digit kilo-ohm value, that is to say it lies above 10 kilo-ohms. The high impedance is unlimited upwardly. 
         [0023]    In one particularly preferred development of the specified method, a second of the two resistors is connected in series with a voltage source in order to output the direct voltage parallel with the capacitor. Therefore, with this second resistor it is possible to apply the abovementioned fixed potential to the first inductor of the transformer, in order to monitor the transformer in a particularly advantageous inventive fashion for short-circuits and interruptions using the direct voltage. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0024]    The properties, features and advantages of this invention which are described above as well as the way in which they are achieved become clearer and more clearly comprehensible in conjunction with the following description of the exemplary embodiments which are explained in more detail in conjunction with the drawings, wherein: 
           [0025]      FIG. 1  shows a perspective view of a tandem master cylinder with a travel sensor, 
           [0026]      FIG. 2  shows a lateral sectional view of the travel sensor from  FIG. 1 , 
           [0027]      FIG. 3  shows a perspective view of part of the travel sensor from  FIG. 2 , 
           [0028]      FIG. 4  shows a perspective view of the travel sensor from  FIG. 3 , 
           [0029]      FIG. 5  shows a direct voltage equivalent circuit of the circuit from  FIG. 4 , 
           [0030]      FIG. 6  shows a direct voltage equivalent circuit of the circuit from  FIG. 4  in a first fault situation, 
           [0031]      FIG. 7  shows a direct voltage equivalent circuit of the circuit from  FIG. 4  in a second fault situation, 
           [0032]      FIG. 8  shows a direct voltage equivalent circuit of the circuit from  FIG. 4  in a third fault situation, and 
           [0033]      FIG. 9  shows a direct voltage equivalent circuit of the circuit from  FIG. 4  in a fourth fault situation. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0034]    In the figures, identical technical elements are provided with the same reference symbols and are described only once. 
         [0035]    Reference is made to  FIG. 1  which shows a basic illustration of a vehicle  2  with a service brake  4  and a parking brake. 
         [0036]    The vehicle  2  has a chassis  8  which can roll on four wheels  10  on a road (not illustrated further), driven by means of an engine (not illustrated further). In the present embodiment, brake disks  12  are attached in a rotationally fixed fashion to the individual wheels  10 , on which brake disks  12  brake effectors, which are to be described below and which are attached in a rotationally fixed fashion to the chassis  8 , can engage, in order to lock the wheels  10  in a manner known per se and to brake the vehicle  2  from travel or to secure it in the stationary state. 
         [0037]    In the present embodiment, the service brake  4  has in each case a service brake effector  14 , such as, for example, a brake shoe, on each wheel  10 . These service brake effectors  14  are actuated by a tandem master cylinder  16  via hydraulic lines  18  on the basis of a service brake request  22 , predefinable with a brake pedal  20 , in a way which is known per se. 
         [0038]    The invention will be explained in more detail below on the basis of a travel sensor  25  which is used in the tandem master cylinder  16  and has a transformer. At this point it is to be noted that the explanation below is purely by way of example and that the invention can be applied to any transformers. 
         [0039]    Reference is made to  FIG. 2 , which shows the tandem master cylinder  16  with a travel sensor  25 . 
         [0040]    The tandem master cylinder  16  also has a pressure piston  26 , of the service brake request  22  is movably arranged in a housing  28 , wherein the pressure piston  26  is connected to the brake pedal  20  from  FIG. 1  in a way which is not illustrated in more detail. The pressure piston  26  itself is divided into a primary piston  30  and a secondary piston  32 , wherein the primary piston  30  closes off an inlet of the housing  28 , and the secondary piston  32  divides the interior of the housing  28  into a primary chamber  34  and a secondary chamber  36 . At the primary piston  30  a secondary collar  38 , which separates the interior of the housing  28  from the surrounding air, is arranged in the region of the inlet of the housing  28 . Looking into the interior of the housing  28 , the secondary collar  38  is followed by a primary collar  40 , which seals the gap between the primary piston  30  and a wall of the housing  28 . A pressure collar  42  on the secondary piston  32  separates the pressure of the primary chamber  34  from the pressure of the secondary chamber  36 . In addition, a further primary collar  40  on the secondary piston  32  seals a gap between the secondary piston  32  and the wall of the housing  28 . The primary piston  30  is supported against the secondary piston  32  via a first spring  44 , while the secondary piston  32  is supported against a housing floor via a second spring  46 . Correspondingly, the primary chamber  34  and the secondary chamber  36  can be supplied with a hydraulic fluid (not shown) via a first and second port  48 ,  50 . 
         [0041]    Since a person skilled in the art is aware of the method for functioning of a tandem master cylinder, a detailed description thereof will not be given. 
         [0042]    The travel sensor  25  has a sample body in the form of a slide  52  with an encoder magnet  54  at its head end, which encoder magnet  54  can be slid, viewed in the plane of the image, under a sensor circuit  56  which will be described below. For the sliding of the slide  52 , the primary piston  30  has a flange  58 , which is a counter bearing for the slide  52 . The flange  58  and the primary piston  30  therefore together form a measuring object whose position is determined by the sensor circuit  56 , to be described below, of the travel sensor  25 . The sensor circuit  56  is composed of a plurality of conductor tracks on a wiring carrier  59 , such as a lead frame, printed circuit board or some other substrate. A cover  60  can be fitted onto the wiring carrier  59  with the sensor circuit  56  in order to protect, for example, against dirt. 
         [0043]    Reference is made to  FIG. 3 , which shows the travel sensor  25  from  FIG. 2 . 
         [0044]    The sensor circuit  56  of the travel sensor comprises a transformer  62 . The transformer  62  senses a magnetic field  64  of the encoder magnet  37  and on the basis thereof outputs an electrical encoder signal (not referenced in more detail) to the sensor circuit  56 . This encoder signal is converted by a first signal processing chip  66  and a second signal processing chip  68  into a measurement signal (not referenced in more detail) from which the position of the slide  52  and therefore the position of the flange  58  and of the primary piston  30  become apparent. The measurement signal which is generated in this way can finally be tapped at a transmission interface  70  of the travel sensor  25  via a cable (not illustrated further) and can be passed on to a higher signal processing unit (not illustrated further) such as, for example, an engine controller in a vehicle (not illustrated further). 
         [0045]    The sensor circuit  56  can comprise protection elements  72  for protecting the two signal processing chips  66 ,  68 , for example against overvoltage. In addition, a screening plate  74 , which screens electromagnetic fields between the sensor circuit  56  and the transformer  62  and thereby avoids the circuit  56  influencing the transformer  62 , can be arranged between the sensor circuit  56  and the transformer  62 . 
         [0046]    In the present embodiment, the transformer  62  is arranged in a defined position on the wiring carrier  59  by means of a form closure  76 . In this context, a protective material  78  surrounds the wiring carrier  59  and the transformer  62 . 
         [0047]      FIG. 4  illustrates a perspective view of the transformer  62 . The transformer  62  comprises a coil carrier  80  with a winding space which is divided by means of six webs  82  into a central section  84  and four side sections  86 . The coil carrier  80  carries a primary coil  88  which extends along a core (not shown further in  FIG. 4 ) which is assumed to have a single layer in the present case. On the two edge zones of the primary coil  88  lying opposite one another, the coil carrier  80  carries first secondary coils  90  and second secondary coils  92  which are tightly wound and have the purpose of measuring an induction voltage, wherein in this case a first secondary coil  90  and a second secondary coil  92  are arranged in pairs at the two ends of the primary coil  88 . 
         [0048]    The coils  88 ,  90 ,  92  can be placed in electrical contact with the sensor circuit  56  on the wiring carrier  59  via terminal pins  94 . 
         [0049]    The travel sensor  25  with a transformer such as the transformer  62  is normally referred to as a differential transformer travel sensor. Alternatively, the travel sensor  25  can, however, also be constructed with in each case only one secondary coil at the ends of the primary coil, in respect of which reference is made, for example, to document DE 44 259 03 C3 and EP 238 922 B1. 
         [0050]    The function of the travel sensor  25  will firstly be explained with reference to the circuit diagram illustrated in  FIG. 5 . 
         [0051]    In order to operate the travel sensor  25 , a series circuit of an electrical input alternating voltage source  96  and two input direct voltage sources  98  which are connected in antipodal fashion with respect to one another are connected to the primary coil of the transformer  62 . The input direct voltage sources  98  connect a same potential  102  to the winding ends of the primary coil  88  via input resistors  100  and ensure that the direct current cannot flow through the primary coil  88  but instead the primary coil  88  is connected to a defined, fixed and low-impedance direct potential. 
         [0052]    In this context, an alternating voltage (not referenced further) of the input alternating voltage source  96  is transmitted in the transformer  62  of the travel sensor  25  from the primary coil  88  to the secondary coils  90 ,  92 . 
         [0053]    In the transformer  62  of the travel sensor  25 , the primary coil  88  is connected in the same direction to the first secondary coils  90 , which means that the windings of the primary coil  88  and of the first secondary coils  90  are wound in the same direction. In contrast, the primary coil  88  and the secondary coils  92  are connected in opposite directions, which means that the windings of the primary coil  88  and of the second secondary coils  92  are wound in opposite directions. 
         [0054]    The first secondary coils  90  are connected on the one hand to a ground connection  106  via a first grounding capacitor  104 , that is to say are connected to a fixedly defined potential, while the second secondary coils are also connected to the ground connection  106  via a second grounding capacitor  104 . On the other hand, the first and second secondary coils  90 ,  92  are connected to amplifiers  108 , at the outputs (not referenced further) of which a transmitted alternating voltage can be tapped with respect to the ground connection  106 , which ground connection  106  constitutes the alternating voltage which is fed into the primary coil  88  and transmitted to the secondary coils  90 ,  92 . 
         [0055]    During the operation of the travel sensor  62 , the encoder magnet  54  which is shown in  FIGS. 2 and 3  is moved along the coils  88 ,  90 ,  92  by the slide  52 , as a result of which a coupling factor between the coils  88 ,  90 ,  92  changes in a manner known per se. In the case of a predetermined alternating voltage which is output by the input alternating voltage source  96 , in this way the transmitted alternating voltage at the output of the amplifiers changes, with the result that a change in position of the encoder magnet  54 , and therefore of the slide  52 , can be detected on the basis of the change in the transmitted alternating voltage. Details of the method of functioning of linear position sensors can be obtained, for example, from the documents mentioned above. 
         [0056]    However, a precondition of this is that the transformer is free of galvanic connections between the primary coil  88 , the first secondary coils  90  and the second secondary coils  92 . In particular, there should also be no interruptions present on the secondary side of the transformer  62  in the circuit diagram in  FIG. 5 . Within the scope of the present embodiment, this is to be monitored with a reference direct voltage source  110  which is connected on the secondary side to the transformer  62  and which outputs a reference direct voltage  112 . Each of the secondary coils  90 ,  92  is connected in the present embodiment to a center tap  114  of a voltage divider between a first voltage divider resistor  116  and a second voltage divider resistor  118 , wherein the first secondary coils  90  and the second secondary coils  92  each divide a first voltage divider resistors  116 . The reference direct voltage source  110  is connected here in parallel with the voltage dividers  116 ,  118 . 
         [0057]    In the present embodiment, a monitoring or test device  120 , which carries out a testing or monitoring method in order to monitor the circuit illustrated in  FIG. 5  for galvanic interruptions or unintended connections, is connected to the center taps  114 . 
         [0058]    For this purpose, the test device  120  taps in each case a test direct voltage  122  at the center taps  114 , said test direct voltage  122  constituting the electrical direct voltage potential at the center taps  114 , for example with respect to the ground connection  106 . Each test direct voltage  122  is dependent, with a transmission ratio  124  indicated in  FIG. 5 , on the reference direct voltage  112 . This dependence is determined here for the case in which the circuit shown in  FIG. 5  does not have any faults in the form of galvanic interruptions and unintended galvanic connections, by means of the respective voltage dividers and, in this context, in particular by means of the individual voltage divider resistors  116 ,  118 . 
         [0059]    In order to monitor the circuit which is illustrated in  FIG. 5 , the test device  120  can therefore receive not only the test direct voltages  124  but also the reference direct voltage  112  and a predetermined transmission ratio  126 . On the basis of the predetermined transmission ratio  126  and the reference direct voltage  112 , the test device  120  can determine a test direct voltage (not illustrated further in  FIG. 5 ) for the fault-free case and verify whether the measured test direct voltages  122  correspond to this test direct voltage for the fault-free case. In the event of a fault, that is to say if a measured test direct voltage  122  differs from the test direct voltage for the fault-free case, the test device  120  can output a fault signal  128  with which, for example, a warning message can be output in the vehicle  2 . 
         [0060]      FIG. 6  illustrates the circuit from  FIG. 5  as a direct current equivalent circuit diagram in a fault-free state with dimensioned values for the direct current components of the circuit. In contrast,  FIGS. 7 to 9  illustrate various fault scenarios for the circuit from  FIG. 5 . In this context, in  FIG. 7  a galvanic interruption  130  in one of the secondary coils  90 ,  92 , in  FIG. 8  a galvanic connection  132  between the first and second secondary coils  90 ,  92 , referred to below as secondary side short-circuit  132 , and in  FIG. 9  a galvanic connection  134  between the primary coil  88  and one of the secondary coils  90 ,  92 , referred to below as transformer short-circuit  134 , should be assumed as fault scenarios. 
         [0061]    The values shown in  FIG. 6  for the direct current equivalent circuit diagram of the circuit from  FIG. 5  are obtained for the test direct voltage  122  at the center tap  114  for the scenario values shown in  FIGS. 6 to 9  according to the following table: 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Scenario 
                 First test voltage 122 
                 Second test voltage 122 
               
               
                   
                   
               
             
             
               
                   
                 FIG. 6 
                 3.37 V 
                 1.26 V 
               
               
                   
                 FIG. 7 
                 4.03 V 
                 2.00 V 
               
               
                   
                 FIG. 8 
                 2.51 V 
                 2.51 V 
               
               
                   
                 FIG. 9 
                 2.50 V 
                 2.50 V 
               
               
                   
                   
               
             
          
         
       
     
         [0062]    As is apparent from the table, the test voltages  122  change with respect to the normal case from  FIG. 6 . Instead, if appropriate even the fault could be qualified and specified, in order, for example, to determine the effected coil of the transformer  62  and, if appropriate, replace it. 
         [0063]    As is apparent, in particular from a comparison of the test voltages  122  in  FIGS. 8 and 9 , the resistors should have the closest possible tolerances in order to ensure reliable evaluation, because the test voltages  122  for the various fault cases can be very close to one another.
A method for testing a transformer including a first inductance and a second inductance, which is galvanically isolated from the first inductance, the inductances being connected in a first circuit and in a second circuit, correspondingly, the method comprising: --applying a DC voltage between a first circuit point and a second circuit point in the first circuit; --tapping off a test voltage between two circuit points in the first or second circuit, wherein one of the two circuit points is different than the first circuit point and the second circuit point; and --testing the transformer on the basis of a comparison between the DC voltage and the test voltage.