Abstract:
In a circuit arrangement having a linear variable differential transformer as a displacement sensor or force sensor, having a selection circuit which is connected to the primary coil of the transformer and which provides an output current for triggering the primary coil, and having an analysis circuit which is connected to the secondary coils of the transformer and which provides a message signal, a control circuit used for triggering the selection circuit and the analysis circuit and for processing the measurement signal provided by the analysis circuit is connected to the primary coil in order to calculate the temperature of the circuit arrangement, and is configured such that it determines the temperature-dependent ohmic resistance of the primary coil and calculates from it the temperature and corrects accordingly the measurement signal provided by the analysis circuit.

Description:
CLAIM FOR PRIORITY  
         [0001]    This application claims the benefit of priority to German Application No. 10313022.5, filed in the German language on Mar. 24, 2003 and German Application No. 10313021.7, filed in the German language on Mar. 24, 2003, the contents of which are hereby incorporated by reference.  
         TECHNICAL FIELD OF THE INVENTION  
         [0002]    The invention relates to a circuit arrangement having a linear variable differential transformer as a displacement or force sensor.  
         BACKGROUND OF THE INVENTION  
         [0003]    A generic circuit arrangement is known from U.S. Pat. No. 5,777,468 A. When such a circuit arrangement is used to determine the weight of a person sitting in a driver&#39;s seat—i.e. the force being exerted upon the driver&#39;s seat—the amount of displacement of the driver&#39;s seat caused by the exertion of force or by the weight of the person is measured and converted into an electrical signal. In such an application it is desirable not only for the person&#39;s weight to have a sufficiently high resolution, but also for the seat to be capable of moving by only a few millimeters, to prevent any uncomfortable rocking motion being caused by the required sprung suspension. This means it is usually necessary to use a very firm spring, for example in the form of a flat spring.  
           [0004]    A further circuit arrangement with a linear variable differential transformer is known from U.S. Pat. No. 5,180,979, in which the primary coil is triggered with a triangular voltage. The resulting total triangular voltage on the two secondary coils is compared to a reference voltage and the amount by which the total voltage duration exceeds the reference voltage duration is indicative of the magnitude of the displacement of the transformer core. The required minor displacement combined with a high resolution therefore leads to a requirement for high sensitivity in the measurement device.  
           [0005]    The measurement signal of the circuit arrangement, which is proportional to the force exerted, is dependent upon the temperature to which the circuit arrangement is exposed. It is therefore necessary to determine the dependency of the measurement signal on the temperature and to perform a corresponding correction for any prevailing temperature. This means, however, that it is necessary to know the current temperature, and therefore a thermal detector may be used to measure the temperature. The use of additional thermal detectors, however, requires additional expenditure on materials and is therefore less cost-effective.  
           [0006]    A method for determining the temperature of an inductive displacement sensor by calculating the temperature-dependent copper resistance of the coil winding is known from DE 43 13 273 A1. In the known method, a constant voltage is applied and maintained on the serial connection consisting of the coil inductance and a resistor until the inductance reaches saturation, so that the current through the coil is still only determined by the copper resistance. The current coil resistance and thus the current temperature are determined from the resulting voltage on the coil. However, in a circuit arrangement with a transformer, whose primary coil has a triangular voltage applied to it and in which a triangular voltage should also be present on the secondary side, this measurement principle cannot be used since the coil cannot reach saturation.  
         SUMMARY OF THE INVENTION  
         [0007]    The invention relates to a circuit arrangement having a linear variable differential transformer (LVDT) as a displacement or force sensor, having a selection circuit which is connected to the primary coil of the transformer (LVDT) and which provides an output current for triggering the primary coil, and having an analysis circuit which is connected to the secondary coils of the transformer (LVDT) and which provides a message signal.  
           [0008]    The invention develops the generic circuit arrangement such that the temperature may be measured as accurately as possible with minimal cost.  
           [0009]    In one embodiment of the invention, the resistance value of the primary coil is temperature-dependent and therefore the current temperature of the circuit arrangement can be determined by calculating this resistance value using simple and also highly accurate methods. On the basis of the known temperature-dependence of the measurement signal of the circuit arrangement, which may be obtained by trials, the measurement signal can then be corrected by the control circuit so that the effects of temperature are eliminated. The resistance value of the primary coil is calculated by measuring the voltage on the primary coil and by measuring the current through the primary coil, or a voltage proportional to this current.  
           [0010]    According to another embodiment of the invention, the current through the primary coil has a trapezoidal characteristic with correspondingly high rising edge and falling edge values. The control circuit is thus configured such that sampling values of a rectangular voltage picked up on the primary coil and also of a voltage proportional to the current through the primary coil, are taken at times when the trapezoidal current has a constant characteristic. This causes the resistance value of the primary coil to be determined if just one direct current flows through the primary coil, and therefore its inductance does not affect the measurement result.  
           [0011]    In an advantageous embodiment according to the invention, the control circuit has an adding circuit at whose inputs are applied a voltage proportional to the resistance value of the primary coil and a reference voltage. The reference voltage has a polarity that is opposite to the voltage that is proportional to the primary coil resistance, so that the difference between the two voltages is actually formed. Thus, by selecting suitable values on the protective circuit elements of the adding circuit, in particular the values of series and feedback resistors, a situation may be achieved whereby their output voltage is zero volts at a specific reference temperature (ideally the ambient temperature), and thus deviations above or below the ambient temperature are characterized by a corresponding output voltage sign.  
           [0012]    In a further advantageous embodiment according to the invention, the reference voltage is a voltage proportional to the current through the primary coil, since this causes variations in the primary coil current which show up as variations in the measured resistance value and which go directly into the reference voltage, and are thus automatically taken into account in the output value of the adding circuit, and this consideration does not have to be achieved by the costly effect of a separate reference voltage.  
           [0013]    In a further embodiment, the sampling values are determined in both the first and second halves of the period and the difference values formed from the sampling values from the first half of the period and from the sampling values from the second half of the period are used as measurement values. This doubles the sensitivity of the measurement. Furthermore, common-mode interferences are eliminated by the difference formation.  
           [0014]    In practice, deviations occur from predefined setpoint values in the resistance values of the adding circuit, the primary coil resistance value, and the current through the primary coil or the voltage that this generates. Consequently, the output voltage of the adding circuit deviates from 0 volts at the predefined reference temperature, and the temperature measurement sensitivity of the circuit arrangement deviates from a predefined setpoint value. In an advantageous embodiment of the invention, the values actually measured are compared to the setpoint values and the deviations are used as correction values for calibration of the measurement device.  
           [0015]    Triggering the primary coil of the transformer using a current with a trapezoidal characteristic enables bipolar rectangular voltages to be picked up on the secondary coils. The amplitude of these rectangular voltages is proportional to the displacement of the transformer core during the phase position of these rectangular voltages and shows the direction in which the displacement occurs. The rectangular voltages are bipolar relative to a reference voltage, which ideally corresponds to half of the supply voltage. If the core is located in a position midway between the two secondary coils, which—in an application to determine seated weight in a vehicle—corresponds to a situation in which the seat is not occupied, the amplitudes and phases of the two secondary coil output voltages are of equal size and their difference is zero accordingly. If the core is displaced, one secondary coil output voltage is higher and the other correspondingly lower. Therefore, in order to determine any deviation in the core from this midway position, the difference between the two is formed since this enables the sensitivity to be increased. Moreover, additional interference signals which are superimposed on the two output voltages are eliminated.  
           [0016]    Since one output voltage is higher and the other output voltage is lower, and the amounts of the variations in linear operation are equally large, the sum of the two output voltages is constant in normal operation. Because of environmental influences, however, the output voltages often deviate from their nominal values. In an advantageous development of the invention, the sum of the secondary coil output voltages is compared to a setpoint value and the increase in the rising edge and falling edge values of the trapezoidal output current of the selection circuit of the linear variable differential transformer varies in inverse proportion to the deviation of this sum from the setpoint value. This control results in the sensitivity of the circuit arrangement being maintained constantly at the predefined setpoint value independently of external influences. A suitably configured control circuit, in particular a microprocessor, is used for the control.  
           [0017]    In still a further embodiment of the invention, the analysis circuit has two amplifier circuits which are each connected to one of the two secondary coils which amplify the output signals of the coils, the outputs of the amplifier circuits being connected to the control circuit. This enables the output voltages of the secondary coils to be isolated from the analysis circuit.  
           [0018]    In order to generate the trapezoidal trigger current for the primary coil of the transformer, in an advantageous embodiment the selection circuit has a rectangular voltage generation circuit and an integrating circuit connected downstream of said rectangular voltage generation circuit.  
           [0019]    In another advantageous embodiment, the rectangular voltage generation circuit is formed with a first resistor, whose first terminal is connected to a third voltage source and whose second terminal is connected to the output of the rectangular voltage generation circuit, and with a second resistor, whose first terminal is connected to a fourth voltage source and whose second terminal is connected to the output of the rectangular voltage generation circuit via a switch controllable from a control circuit, the voltages of the third and the fourth voltage source following the relationship  
           
         U 
         3 
         −Uref=Uref−U 
         4 
       
           [0020]    and Uref being a reference voltage, which—for example—may be half the value of the supply voltage. If the third and the fourth voltage source are configured as controllable, variable voltage sources, the amplitude of the rectangular voltage—and thus the edge steepness of the triangular output voltage of the integration circuit—can easily be adjusted by triggering the two voltage sources through the control circuit.  
           [0021]    Since, in any variation in the edge steepness during a constant period of the integration circuit output signal, its amplitude would vary and could possibly assume unacceptably high values, in an advantageous development of the circuit arrangement the integration circuit is formed with an operational amplifier whose output is coupled back to the inverting input via a capacitor, whereby a bi-directional Zener diode is switched in parallel to the capacitor. In this way the output voltage of the integration circuit is limited to a positive and also negative maximum value determined by the Zener diode and the trapezoidal signal is thus formed.  
           [0022]    In yet another embodiment according to the invention, the analysis circuit is configured such that sampling values of the rectangular voltage are taken in both the first and second half of the period and the difference values formed from the relevant sampling values in the first half of the period and from the relevant sampling values in the second half of the period are output as a measurement signal.  
           [0023]    The formation of the difference between a positive and a negative sampling value produces a measurement value with double amplitude, resulting in a two-fold increase in the sensitivity of the measurement device. In addition, any direct voltage offsets present are eliminated by the difference formation.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0024]    The invention is explained in greater detail with reference to exemplary embodiments in the figures, in which:  
           [0025]    [0025]FIG. 1 shows a circuit arrangement according to the invention,  
           [0026]    [0026]FIG. 2 shows the characteristics of characteristic signals of the circuit arrangement according to the invention.  
           [0027]    [0027]FIG. 3 shows a second embodiment of a rectangular voltage generation circuit.  
           [0028]    [0028]FIG. 4 shows a detailed circuit for the rectangular voltage generation circuit in FIG. 3.  
           [0029]    [0029]FIG. 5 shows the characteristic of the current through the primary coil.  
           [0030]    [0030]FIG. 6 shows the characteristic of the voltage on the primary coil at a temperature of 25° C.  
           [0031]    [0031]FIG. 7 shows the characteristic of the voltage on the primary coil at a temperature of 85° C.  
           [0032]    [0032]FIG. 8 shows the characteristic of the voltage on the primary coil at a temperature of −40° C.  
           [0033]    [0033]FIG. 9 shows an extended circuit arrangement according to the invention.  
           [0034]    [0034]FIG. 10 shows the characteristics of signals of the extended circuit arrangement. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0035]    [0035]FIG. 1 shows a linear variable differential transformer (LVDT), which is formed with a primary coil Wp and two secondary coils Ws 1 , Ws 2 . The primary coil Wp is magnetically coupled to the secondary coils Ws 1 , Ws 2  via a core K. The secondary coils Ws 1 , Ws 2  are connected in series such that the difference between the voltages at the individual secondary coils Ws 1  and Ws 2  can be picked up at the free ports  3  and  6 . The core K is moveable and, for the preferred application field, may be coupled with a driver&#39;s seat using a method that is not illustrated, so that—in the event of a compressive or tensile load on the seat—the core may move accordingly between the coils of the linear variable differential transformer (LVDT). If the core K is located in a position midway between the two secondary coils Ws 1 , Ws 2 , the voltage that may be picked up at ports  3  and  6  is equal to 0.  
         [0036]    The primary coil Wp is triggered at its ports  1 ,  2  by a selection circuit ASS with a current IL with trapezoidal characteristic. The current characteristic is shown by the diagram in FIG. 2. In this context the selection circuit ASS has a voltage current converter formed with an operational amplifier V 2 , whereby the output of said operational amplifier V 2  is coupled back to its inverting input via the primary coil. Furthermore, the inverting input of the operational amplifier V 2  is connected via a resistor R 4  to the output of an integration circuit triggered by a rectangular voltage, which is formed, according to a known method, using a countercoupled operational amplifier V 1  via a capacitor C 1  from its output to its inverting input. The non-inverting inputs of the two operational amplifiers V 1  and V 2  are connected to a reference potential Vref. In the exemplary embodiment illustrated, the reference potential Vref is provided by means of a voltage divider from two resistors R 1  and R 2  connected in series at their mid-tap, the voltage divider being disposed between the supply voltage Vcc and a ground potential. A buffer capacitor C 2  is connected in parallel to the second resistor R 2  of the voltage divider. The resistors R 1  and R 2  of the voltage divider are ideally of equal size, so that the reference voltage Vref is equal to half the supply voltage Vcc (Vref=Vcc/2).  
         [0037]    The rectangular voltage is formed by means of a rectangular generation circuit RES, which is formed in the exemplary embodiment according to FIG. 1 using a transfer switch S 1  and a resistor R 3  connected to the output port  7  of the transfer switch S 1 . The other port of the resistor R 3  forms the output port of the rectangular generation circuit RES and is connected to the inverting input of the operational amplifier V 1  of the integration circuit. The transfer switch S 1  switches the output between the supply voltage Vcc and the frame ground terminal. It is triggered by a control signal SIG 1  from a control circuit ST, which—for example—may be formed with a microprocessor. At the output  7  of the transfer switch S 1  is a voltage with a rectangular characteristic, which switches with regard to the reference voltage Uref=Vcc/2 between two voltages +Urechteck and −Urechteck with the timing of the control signal SIG 1 . In the exemplary embodiment shown in FIG. 1, +Urechteck is equal to Vcc and −Urechteck is equal to 0 volts (ground).  
         [0038]    The capacitor C 1  of the integration switch is switched in parallel to a bipolar Zener diode ZD 1 , to limit the output voltage of the integration circuit to a maximum value. The output signal of the integration switch thus in principle has a trapezoidal characteristic, which flows with regard to the reference voltage Uref between the values +Utrapez and −Utrapez via linear rising and falling edges. FIG. 5 shows this trapezoidal characteristic, which flows between +Itrapez and −Itrapez via linear rising and falling edges according to its input voltage, for the output current IL of the voltage current converter V 2 .  
         [0039]    The ports  3  and  6  of the secondary coils Sw 1  and SW 2  are connected to an analysis circuit AWS, which is formed with an inverting amplifier V 3 , R 5 , R 6  and an analysis unit AE. The port  3  of the first secondary coil Ws 1  is connected to the non-inverting input of an operational amplifier V 3 , and the port  6  of the second secondary coil Ws 2  is connected via a resistor R 5  to the inverting input of the operational amplifier V 3 . The output port  8  of the operational amplifier is coupled back to the inverting input via a resistor R 6 . The port  6  of the second secondary coil is also connected to the reference potential Vref.  
         [0040]    The output port  8  of the operational amplifier V 3  is connected to the input of an analysis unit AE, at whose output the measurement signal MS is provided. The analysis unit AE is configured such that, timed by a second control signal SIG 2  generated by the control unit ST, it samples the output voltage of the operational amplifier V 3  both in the first half of the period at a first sampling time and also in the second half of the period at a second sampling time, and the values obtained are subtracted from one another, thus forming the difference between them. In this way, firstly common-mode interferences are suppressed and secondly the amplitude of the measurement signal MS is doubled compared to the output signal of the operational amplifier V 3 , so that the sensitivity of the entire circuit arrangement is increased.  
         [0041]    [0041]FIG. 3 shows an advantageous embodiment of the rectangular generation circuit RES′. In the diagram, a first resistor R 3   a  is connected by one of its ports to the inverting input of the operational amplifier V 1  of the integration circuit, and by its other port to a third voltage source, which provides a voltage U 3 . A second resistor R 3   b  is likewise connected by one of its ports via a controllable switch S 2  to the inverting input of the operational amplifier V 1 , and by its other port to a fourth voltage source, which provides a voltage U 4 . The second resistor R 3   b  has half the value of the resistor R 3   a  and the voltages U 3 , U 4  supplied by the third and fourth voltage sources follow the relationship  
           U   3 − Vref=Vref−U   4 ,  
         [0042]    which means that the voltages are symmetrical to the reference potential Uref. The switch S 2  is triggered by the first control signal SIG 1  from the control circuit StS.  
         [0043]    As a result of the advantageous embodiment of a rectangular voltage generation circuit RES′ according to FIG. 3, a single switch S 1  is necessary and, furthermore, the steepness of the edges of the output voltage of the integration circuit or of the output current of the operational amplifier V 2  functioning as the voltage current converter can be adjusted by selecting the values of the voltages U 3  and U 4  of the third of fourth voltage source respectively.  
         [0044]    [0044]FIG. 4 shows a correspondingly detailed circuit. In the diagram, the third voltage source is formed using an inverting amplifier, which is triggered via a low-pass circuit from resistors R 11 , R 12  and R 13  and a capacitor C 11  by a pulse-duration-modulated signal SIG 3 . The pulse-duration-modulated signal SIG 3  is delivered by the control circuit (not shown). By varying the pulse duration and the deep-pass filtering the input voltage of the inverting amplifier can be changed, so that a variable voltage U 3 ′ is produced at this output. The output of the operation amplifier V 11  is coupled back to its inverting input via a resistor R 14 . Its non-inverting input has the reference potential Vref applied to it.  
         [0045]    The fourth voltage source is likewise formed with an inverting amplifier from an operational amplifier V 12 , whose output is coupled back to its inverting input via a resistor R 16  and its non-inverting input is connected to the reference potential Vref. Its inverting input is also connected to the output of the third voltage source via a resistor R 15 . If the amplification of the inverting amplifier is set to −1 its output voltage U 4 ′ with regard to the reference potential Vref is symmetrical to the output voltage U 3 ′ of the third voltage source, so that the relationship  
           U   3   ′−Vref=Vref−U   4 ′ 
         [0046]    also applies here.  
         [0047]    The switch S 2  is implemented by a bipolar transistor T 11 , the base of which is triggered by the first control signal SIG 1  via a resistor R 17 .  
         [0048]    For temperature measurement purposes the output  2  of the current voltage converter V 2  is now connected via a resistor R 30  to the inverting input of an adding operational amplifier V 4 . The output voltage Uout of the current voltage converter V 2  is present on the primary coil Wp which is connected to the current voltage converter output. The output  9  of the integration circuit V 1 , at which a trapezoidal voltage Uin is present, is likewise connected via a resistor R 32  to the inverting input of the adding operational amplifier V 4 . The voltage Uin is proportional to the current IL through the primary coil Wp. The output of the adding operation amplifier V 4  is coupled back via a resistor R 31  to the inverting input of the adding operation amplifier V 4  and connected to a terminal  10  via a first low-pass circuit R 34 , C 3 . The non-inverting input of the adding operational amplifier V 4  is connected to the reference potential Vref via a resistor R 33 . The voltage Uin at the output of the integration circuit V 1  is connected via a second low-pass circuit R 35 , C 4  to a terminal  11 , at which a voltage Uin′ is present.  
         [0049]    A voltage Uout′ is present at the terminal  10 , said voltage being proportional to the sum of the two voltages Uout and Uin and being calculated according to the formula  
         Uout   ′     =     U                 i                     n   ′     ·     [           RL25        (     1   +     KT        (     TEMP   -     25      °                   C   .         )         )       ·   R31       R4   ·   R30       -     R31   R32       ]                               
 
         [0050]    Thus the formula  
           RL=RL   25 (1 +KT ( TEMP − 25   ° C .))  
         [0051]    reflects the temperature dependence of the resistor RL of the primary coil Wp with regard to a reference temperature of 25° C. The resistance value RL 25  is thus the value of the resistance of the primary coil Wp at 25° C. and KT is the temperature constant, which—for copper—has the value 3900*10 −6 1 /° C.  
         [0052]    The values of the resistors RL 25 , R 30 , R 31 , R 32  and R 4  may now be advantageously selected such that the voltage Uout′ has the value 0 volts at a temperature of 25° C., so that deviations above and below this temperature can be identified by the Uout′ voltage sign.  
         [0053]    The voltage Uout′ at the terminal  10  can be expressed as  
           Uout′−SENS   Soll *( TEMP −25 ° C .)  
         [0054]    in which  
           SENS   So11     =     Ui                     n   ′     ·   K                     T   ·       RL25   ·   R31       R4   ·   R30             ,                         
 
         [0055]    SENS Soll  being the setpoint sensitivity of the circuit arrangement with regard to temperature measurement.  
         [0056]    In practice the values of the resistors R 30 , R 31 , R 32  and R 4  deviate from their setpoint values. The value of the resistor RL 25  on the primary coil Wp is also subject to manufacturing tolerances and therefore has a value RL 25   ist , which deviates from a setpoint value so that, at the reference temperature of 25° C., the voltage Uout′ is not 0 volts but has an offset voltage value Uoffs 25 . Since the output voltage Uin of the integration circuit V 1  is also subjected to fluctuations and deviates from its setpoint value Uin soll , the sensitivity SENS also deviates from its setpoint value SENS soll  and has a value of SENS ist .  
         [0057]    If the actual values of the offset voltage Uoffs 25 , the voltage Uout′ at the terminal  10 , and the voltage Uin′ at the terminal  11  are now measured at the reference temperature of 25° C., the actual value of the resistance RL 25   ist  of the primary coil Wp may now be calculated from them according to the formula  
         RL25   ist     =       (         Uout   ′       Ui                   n   ′         +     R31   R32       )     ·   R4   ·   R30                           
 
         [0058]    Now that this value is known, it is possible to calculate the temperature TEMP according to the formula  
       TEMP   =           (       Ui                     n   soll     ·       U     out   ′         U     i                   n   ′               -       U   offs        25       )       SENS   soll               ′              RL25   RL25ist                 +     25        °   _                     C   .                               
 
         [0059]    This calculation takes place in an arithmetic circuit RS, which is connected to the terminals  10  and  11 . The arithmetic circuit RS is also fed the measurement signal MS. From this, and from correction values according to a characteristics field which reflects the temperature dependence of the measurement signal MS, it calculates a corrected measurement signal MS′.  
         [0060]    Thus—when the circuit arrangement is put into operation—a calibration measurement is carried out, the corresponding values are stored for Uoffs 25  and are calculated and stored for RL 25   ist , and they are used in subsequent calculations of the actual temperature TEMP for correcting the calculations from the measured values of the voltages Uout′ and Uin′.  
         [0061]    FIGS.  6  to  8  show characteristics for the voltage Uout for various temperatures.  
         [0062]    [0062]FIG. 9 shows a development of the circuit arrangement according to the invention, from which the circuit for temperature measurement has been omitted. The ports  3 ,  4 ,  5  and  6  of the secondary coils Sw 1  and Sw 2  are connected to an analysis circuit AWS. The analysis circuit AWS has a first inverting amplification circuit with an operational amplifier V 4 , whose output is coupled back via a resistor R 22  to its inverting input. The port  3  of the first secondary coil Sw 1  is connected via a resistor R 20  to the non-inverting input and the port  4  of the first secondary coil Sw 1  is connected via a resistor R 21  to the inverting input of the operational amplifier V 4 .  
         [0063]    The analysis circuit AWS also has a second inverting amplifier circuit with an operational amplifier V 5 , whose output is coupled back to its inverting input via a resistor R 25 . The port  5  of the second secondary coil Sw 2  is connected via a resistor R 24  to the non-inverting input and the port  6  of the second secondary port Sw 2  is connected via a resistor R 23  to the inverting input of the operational amplifier V 5 .  
         [0064]    A first output voltage Ua 3  can be picked up at the output of the first amplifier circuit and a second output voltage Ua 4  can be picked up at the output of the second amplifier circuit. The two output voltages Ua 3 , Ua 4  are fed to the control circuit StS, which—dependent on these two output voltages Ua 3 , Ua 4 —in turn provides a control signal SIG for triggering the rectangular generation circuit RES′. The characteristics of the two output voltages Ua 3  and Ua 4  are shown in Diagram 3 of FIG. 2 for a case in which the core K of the transformer is displaced from its midway position.  
         [0065]    The analysis circuit AWS has a third inverting amplifier circuit which is formed using an operational amplifier V 3  whose output is coupled back via a resistor R 6  to its inverting input. The output of the first amplifier circuit V 4 , R 20 , R 21 , R 22  is connected via a resistor R 7  to the non-inverting input of the operational amplifier V 3 , and the output of the second amplifier circuit V 5 , R 23 , R 24 , R 25  is connected via a resistor R 5  to the inverting input of the operational amplifier V 3 . The non-inverting input of the operational amplifier V 3  is further connected to the reference potential Vref via a resistor R 8 . Furthermore, the two ports  4  and  5  of the two secondary coils Sw 1 , Sw 2  are connected to the reference potential Vref.  
         [0066]    Operational amplifiers of the circuit arrangement according to FIG. 9 are operated on the supply potentials Vcc and ground.  
         [0067]    Furthermore, the analysis circuit AWS is formed using an analysis unit AE, which is triggered by a control signal SIG 2  delivered by the control circuit StS. The input of the analysis unit AE is connected to the output port  8  of the operational amplifier V 3 . A measurement signal MS is provided at its output. The analysis unit AE is configured such that, timed by the second control signal SIG 2  generated by the control circuit, in an advantageous method it samples the output voltage Ua 5  of the operational amplifier V 3  both in the first half of the period at a sampling time At1 and also in the second half of the period at a sampling time At2, and the values obtained are subtracted from one another, thus forming the difference between them. In this way, firstly common-mode interferences are suppressed and secondly the amplitude of the measurement signal MS is doubled compared to the output signal Ua 5  of the operational amplifier V 3 , so that the sensitivity of the entire circuit arrangement is increased.  
         [0068]    As well as the characteristics of the output voltage of the rectangular generation circuit RES′ and of the current through the primary coil Wp, FIG. 10 shows the characteristics of the output voltages Ua 3  and Ua 4  of the amplifier circuits V 4 , V 5  of the analysis circuit AWS, for the case in which the core of the transformer is displaced from its midway position. As can be seen, the voltage Ua 4  has a somewhat higher amplitude than the voltage Ua 3 , the sum of the two voltages being maintained at a constant value by means of the control circuit StS.