Abstract:
A measuring circuit comprising
   an evaluation circuit ( 13, 63 );   a sensing circuit ( 39, 40 ) comprising a sensing element ( 6, 55 ) configured to generate a measuring signal from a measuring object ( 7 ) and a transmission line ( 26, 69 ) for transmitting the measuring signal to the evaluation circuit ( 13, 63 ); and   a test signal injection circuit ( 21 ) comprising a signal injector ( 12 ) and an injection line ( 20 ) connecting a signal output of the signal injector ( 12 ) with the sensing circuit ( 39, 40 ) for feeding a test signal into the sensing circuit ( 39, 40 ) such that the test signal is transmittable to the evaluation circuit ( 13, 63 ) over the transmission line ( 26, 69 ).   
 
     To provide the measuring circuit with an improved surveillance and/or testing operability, the invention suggests that the injection line ( 20 ) and the transmission line ( 26, 69 ) are interconnected in series via the sensing element ( 6, 55 ) such that said test signal can be fed through the sensing element ( 6, 55 ).

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]    The present invention relates to a measuring circuit comprising
   an evaluation circuit;   a sensing circuit comprising a sensing element configured to generate a measuring signal from a measuring object and a transmission line for transmitting the measuring signal to the evaluation circuit; and   a test signal injection circuit comprising a signal injector and an injection line connecting a signal output of the signal injector with the sensing circuit for feeding a test signal into the sensing circuit such that the test signal is transmittable to the evaluation circuit over the transmission line.   
 
       BACKGROUND OF THE INVENTION 
       [0005]    Such a measuring circuit can be employed, for instance, to detect vibrations. In particular, the measuring object can be constituted by a rotary machine, such as an engine of an airplane or a land based turbine such as a gas or steam turbine, or any other vibrating structure. 
         [0006]    A measuring circuit of that type is known from U.S. Pat. No. 6,498,501 B2. The sensing element of this circuit is provided by a piezoelectric transducer. The sensing circuit further comprises two injection capacitors connected in parallel to the piezoelectric transducer. Both injection capacitors are connected to the injection line such that the auxiliary test signal can be injected from the output of the signal injector into the sensing circuit by means of the signal injection capacitors. The test signal is then transmitted from the injection points to the evaluation circuit via a respective transmission line. This enables the measuring circuit to evaluate the quality of the measuring circuit not only during operation of the monitored vibration machine but also when the vibration machine is at rest. In this way, a permanent surveillance of the measuring circuit can be achieved. 
         [0007]    A disadvantage of this measuring circuit is that its internal built-in test equipment (BITE) is limited to the monitoring of a proper functioning of the transmission lines and of a proper connectivity to external components, since only those are employed to deliver the test signal to the evaluation circuit. But the sensing element itself cannot be tested as the auxiliary test signal is passed around the terminals of the sensing element by the signal injection capacitors. The quality of the sensing element, however, is of crucial importance for the reliability of the measuring circuit. 
         [0008]    Another disadvantage is the need of provision of signal injection capacitors. In general, such additional components of a desired high reliability lead to increased production costs. On the other hand, additional components have intrinsically a certain probability of failure and will therefore contribute to possible sources of error in the measuring circuit. 
         [0009]    It is an object of the present invention to remedy at least one of the above mentioned disadvantages and to provide the initially addressed measuring circuit with an improved BITE functionality. In particular, possible sources of error existing in current measuring circuits shall be made better identifiable or reduced or eliminated by the newly proposed circuit design according to the invention. 
       SUMMARY OF THE INVENTION  
       [0010]    At least one of these objects is achieved by the measuring circuit according to claim  1 . The dependent claims define preferred embodiments. 
         [0011]    Accordingly, the invention suggests that the injection line and the transmission line are interconnected in series via the sensing element such that the test signal can be fed through the sensing element. In this way, the functionality of the measuring circuit can be extended to a monitoring and/or testing of the functionality of the sensing element, in addition to the monitoring and/or testing of the transmission line and of the connectivity of the measuring circuit. Thus, possible error sources related to the sensing element can be made identifiable. 
         [0012]    Moreover, by feeding the test signal directly through the sensing element, the provision of additional injection capacitors can be omitted. Thus, possible error sources related to the functionality of the injection capacitors can be eliminated. In addition, the complexity of the overall measuring circuit can be decreased leading to a more reliable and economical circuit design. 
         [0013]    Preferably, the injection line and the transmission line are separate from each other except their series connection via the sensing element. Thus, the test signal can preferably be fed into the transmission line only through the sensing element. Preferably, the only physical connection in between the transmission line and the signal injector is thus provided via the sensing element. This can contribute to an unambiguous verifiability of quality factors that are related to the sensing element. 
         [0014]    Preferably, at least one of the following configurations of the test signal injection circuit is applied to provide a suitable test signal via the injection line: 
         [0015]    The signal injector is preferably connected to ground. The injection line is preferably connected to ground, in particular via the signal injector. This can be exploited to transmit the test signal in parallel to a capacitance that is inherently present along the injection line. The test signal preferably corresponds to the difference in electric potential between the grounded injection line and the signal generator. Preferably, a feeding pole for the test signal on the sensing element is thus connected to ground via the injection line and the signal injector. In this way, an advantageous injection of the test signal in the sensing circuit and an according transmission to the evaluation circuit can be accomplished. 
         [0016]    Further to this purpose, the signal injector is preferably configured to provide the test signal with a low output impedance. In particular, the signal output of the signal injector preferably has an ohmic impedance value of at most 10 □, more preferred at most 0.5 □. As a result, a test signal is preferably obtained that corresponds to the difference in electric potential between the grounded injection line and the signal generator of low ohmic impedance. Such a test signal of low ohmic impedance is preferably generated in order to match its value to the impedance value of the sensing element. In particular, the impedance of the sensing element may be very low in case of a failure and/or quality loss of the sensing element. 
         [0017]    According to a preferred configuration, the signal injector comprises a signal generator and a transformer for the generated signal. A primary winding of the transformer is preferably connected to the signal amplifier. A secondary winding of the transformer is preferably connected to the injection line. In particular, the secondary winding is preferably connected to ground. 
         [0018]    The transformer is preferably applied to match the impedance value of the signal output of the signal injector to a desired value, in particular to an impedance value as indicated above. Preferably, the inductance of the secondary winding of the transformer connected to the injection line matches the desired impedance value. According to another preferred configuration, the signal injector is constituted by a signal generator exhibiting the desired impedance value. 
         [0019]    Preferably, the signal injector is adapted to generate a charge at a feeding pole of the sensing element by means of the test signal transmitted via the injection line. Preferably, the test signal provided by the signal injector comprises an alternative voltage. More preferred, the test signal comprises a frequency outside a predetermined frequency range corresponding to a frequency band of measuring signals that can be generated or that are envisaged to be generated by the sensing element. Alternatively or additionally, the test signal may comprise a direct current (DC) signal. 
         [0020]    Preferably, the injection line and the transmission line or several transmission lines extend through a common enclosure, in particular a cable. The integration or partial integration of both the injection line and the transmission line or several transmission lines in the common enclosure can contribute to a more economical circuit design. The enclosure preferably constitutes a common electromagnetic shielding for the injection line and the transmission line or several transmission lines. The shielding can be applied to attenuate or eliminate external perturbations. 
         [0021]    In consequence, however, an effective capacitance between the shielding and each of the conductors constituted by the injection line and each transmission line may be inherently present. The resulting capacitance seen between the signal injector and the evaluation circuit may therefore influence the transmitted signal to be evaluated in the evaluation circuit. Preferably, the shielding is connected to ground. In this way, an undesired participation of this resulting capacitance in a transfer function of the transmitted signal to be evaluated in the evaluation circuit can be effectively avoided. 
         [0022]    Additionally, an effective capacitance between the conductors of the injection line and each transmission line may be inherently present. Each of these capacitances therefore may influence the injected signal and/or the transmitted signal to be evaluated in the evaluation circuit. Preferably, the injection line and/or at least one of the transmission lines solely extend through a separate enclosure, in particular a respective electromagnetic shielding to attenuate or eliminate external perturbations. Preferably, the separate shielding is connected to ground. In this way, a direct injection of the test signal into the effective capacitance in between the conductors of the injection line and the respective transmission line can be effectively eliminated. Thus, an undesired participation of the effective capacitance in a transfer function of the signal transmitted to the evaluation circuit can be avoided. 
         [0023]    Preferably, a separate shielding is provided for the injection line and a separate shielding is provided for at least one transmission line or for several transmission lines altogether. More preferred, a separate electromagnetic shielding for the injection line and for each transmission line is provided. Each separate shielding is preferably connected to ground. 
         [0024]    With respect to the above described common and/or separate shielding, different shielding concepts are conceivable: According to a first preferred configuration, only a separate shielding is provided for the injection line and/or the transmission line or transmission lines. According to a second preferred configuration, only a common shielding is applied through which the injection line and each of the transmission lines extend. According to a third and most preferred configuration, both a common shielding for the injection line and the at least one transmission line and a separate shielding for each of the injection line and the at least one transmission line inside the common electromagnetic shielding are provided. In this way, the measuring circuit can be adapted to a desired reliability of its measuring and testing functionality. 
         [0025]    In particular, different shielding concepts may be applied depending on the length of a cable to be used for the injection line and/or transmission line or transmission lines. The longer the cable, the larger the number of respective electromagnetic shields is preferably applied. Besides an increasing immunity to electric fields, the testing sensitivity of variations of the capacitance of the sensing element can thus be improved. 
         [0026]    Preferably, the sensing element and at least part of the injection line and/or transmission line are enclosed by a sensor housing. In this way, a sensor mountable in proximity or in a desired distance to the measuring object can be provided. Preferably, the evaluation circuit and/or the signal injector are arranged outside the housing to allow a compact design of the sensor. 
         [0027]    Preferably, output passages are provided inside the walls of the sensor housing through which the injection line and/or the transmission line pass through. More preferred, the output passages are provided as output terminals in the sensor housing and respective connection wires are provided inside the housing to connect the sensing element with each of the output terminals. The external part of the injection line and/or each transmission line can preferably be plugged into the respective output terminals from outside the housing. 
         [0028]    Preferably, the sensing element is electrically insulated from the walls of the sensor housing. Thus, the sensing element is preferably provided electrically floating inside the housing. Moreover, the injection line and/or the transmission line are preferably insulated from the walls of the sensor housing. In consequence, an effective capacitance between the housing and the sensing element and/or between the housing and the injection line and/or the transmission line may be inherently present. The sensor, in particular the sensor housing, is preferably connected to ground. Thus, an undesired participation of the effective capacitance in a transfer function of the transmitted signal to be evaluated in the evaluation circuit can be avoided. 
         [0029]    Preferably, the sensing element is a piezoelectric sensing element. In particular, the sensing element preferably comprises a stack of piezoelectric plates. The measuring element made from such a piezoelectric member has the advantage of being well proved and tested in various intended application areas of the measuring circuit, in particular in the field of monitoring systems for vibrating and/or rotating engines, such as aircraft engines and/or gas turbines. It is understood that, in particular for certain applications, also another sensing element is conceivable, such as an inductive, capacitive, resistive or electro-optic measuring element. Preferably, the sensing element is a transducer, in particular a piezoelectric transducer. 
         [0030]    Preferably, the sensing element comprises at least one feeding pole for injecting the test signal and at least one output pole for delivering the measuring signal and/or the test signal. Preferably, the poles are provided with an inverse polarity. The injection line is preferably connected to at least one feeding pole and the transmission line is preferably connected to an output pole or multiple transmission lines are preferably connected to a respective output pole. Preferably, an opposite polarity is provided for at least one feeding pole and for at least one transmission pole. Preferably, at least two of the poles are provided at opposed ends of the sensing element. 
         [0031]    Preferably, the evaluation circuit comprises at least one signal amplifier, in particular a charge amplifier. The signal amplifier is preferably connected to the transmission line and thus configured to deliver a signal representative for a signal transmitted over the transmission line. According to a preferred configuration, the signal amplifier is an operational amplifier that is preferably provided with a feedback capacitor. Preferably, the evaluation circuit and the signal injector are enclosed in a common electronics unit. 
         [0032]    Preferably, the measuring circuit is configured such that the test signal is transmittable to the evaluation circuit via the at least one transmission line solely or additionally to the respective measuring signal. Thus, the testing and/or surveillance of the measuring circuit can preferably be conducted on the one hand during a measuring operation and on the other hand independently from a measuring operation. 
         [0033]    A preferred method for testing the measuring circuit comprises an evaluation of the test signal transmitted via the transmission line at an output of the evaluation circuit, in particular at the output of the signal amplifier. Preferably, an evaluation logic is provided in the evaluation circuit by which the evaluation of the test signal is conducted. Preferably, a failure or quality loss either of the sensing element or of the signal injector and/or injection line and/or transmission line is identified by a reduction of the evaluated test signal with respect to an expected value. 
         [0034]    In addition, various short circuits in the measuring circuit are preferably made detectable by the evaluation of the transmitted test signal. These shortcuts may arise in particular in between conductors, in between a shielding and a conductor, across the sensing element or across the insulations inside the sensor. Furthermore, a disconnection of a connection to ground is preferably made detectable. Such a disconnection may comprise the grounding of the sensor, in particular of the sensor housing, the grounding of a shielding, or the grounding of the signal injector. 
         [0035]    In a first preferred configuration, the above described measuring circuit is implemented as an asymmetric measuring circuit that is in particular used for testing applications. In a second preferred configuration, the measuring circuit is implemented as a symmetric measuring circuit with additional redundancy features as further described below. Such a symmetric measuring circuit is preferably applied in monitoring applications, in applications with specific safety integrity level (SIL) requirements, in applications in which the measuring circuit is mounted in locations difficult to access, or in remote locations. 
         [0036]    In that second preferred configuration, the sensing element is configured to generate an additional measuring signal from the measuring object and the sensing circuit comprises an additional transmission line connected to the sensing element for transmitting the additional measuring signal to the evaluation circuit. This provides a redundant functionality of the measuring circuit. Besides a higher reliability of the measuring circuit, this can also contribute to a higher detectability of quality losses or failures in the measuring circuit. 
         [0037]    Preferably, the injection line and the additional transmission line are interconnected in series via the sensing element such that the test signal is transmittable to the evaluation circuit over the transmission line and/or the additional transmission line. In this way, a series connection of the injection line with the transmission line and with the additional transmission line is preferably established via the sensing element. Thus, a transmission of the test signal via the transmission line and via the additional transmission line can redundantly be checked in the evaluation circuit. This further contributes to a higher detectability of quality losses or failures in the measuring circuit. 
         [0038]    During regular operation of the measuring circuit, the test signal transmitted over the transmission line and the test signal transmitted over the additional transmission line are preferably provided with substantially the same amplitude. According to a preferred configuration, the test signal transmitted over the transmission line and the test signal transmitted over the additional transmission line are also provided with an identical phase. 
         [0039]    Preferably, the injection line and the additional transmission line are separate from each other except their series connection via the sensing element. Thus, the test signal can preferably be fed into the additional transmission line only through the sensing element. Preferably, the only physical connection in between the additional transmission line and the signal injector is thus provided via the sensing element. This can contribute to an unambiguous verifiability of quality factors that are related to the sensing element. Moreover, also the transmission line and the additional transmission line are preferably separate from each other, thus allowing a respective signal transmission from the sensing element to the evaluation circuit independently from one another. In this way, various sources of failure or quality loss in the measuring circuit can be made further recognizable. 
         [0040]    Preferably, the test signal generated from the same signal injector can be fed through the sensing element to the transmission line and the additional transmission line. More preferred, the test signal injected into the sensing element is also provided via the same injection line. In this way, the risk of an undesired discrepancy between the test signal fed to the transmission line and the test signal fed to the additional transmission line can be minimized. 
         [0041]    For this purpose, at the end of the injection line two feeding poles are preferably provided on the sensing element such that the test signal can be fed through the sensing element in two opposite directions. Preferably, the capacitances between each feeding pole and a respective output pole connected to a transmission line are substantially equal. In a preferred configuration, the two feeding poles are provided by a common feeding electrode located inside the sensing element. The common feeding electrode is preferably provided in the middle of the sensing element. 
         [0042]    To allow a generation of the measuring signal and the additional measuring signal, the sensing element is preferably divided into two sensing units, in particular by the common feeding electrode. The two sensing units are preferably adapted to generate a substantially corresponding measuring signal from the measuring object under the same measurement conditions. Preferably, the corresponding measuring signals are provided as a measuring signal and an additional measuring signal with substantially the same amplitude. According to a preferred configuration, the measuring signal and the additional measuring signal are provided with an opposite phase. 
         [0043]    In the case of a piezoelectric sensing element, each sensing unit preferably comprises the same number of piezoelectric plates. Preferably, each sensing unit has a substantially equal capacitance. 
         [0044]    Preferably, two separate output poles are provided on the sensing element for the transmission line and the additional transmission line. A respective output pole is preferably provided on each sensing unit of the sensing element. Preferably, the output poles are provided on opposed ends of the sensing element. In this way, the sensing element is preferably symmetrically connected to the transmission line and the additional transmission line. 
         [0045]    Preferably, the evaluation circuit comprises a first signal amplifier configured to deliver a signal representative for a signal transmitted over the first transmission line, and a second signal amplifier configured to deliver a signal representative for a signal transmitted over the additional transmission line. According to a preferred configuration, each signal amplifier is an operational amplifier that is preferably provided with a respective feedback capacitor. 
         [0046]    Preferably, the evaluation circuit comprises a summing amplifier configured to deliver a signal representative for a sum of signals transmitted over the transmission line and the additional transmission line and/or a difference amplifier configured to deliver a signal representative for a difference of signals transmitted over the transmission line and the additional transmission line. This can be exploited for an advantageous testing method of the measuring circuit, as described below. 
         [0047]    According to a preferred configuration, the summing amplifier and/or the difference amplifier are connected to both the signal amplifier for the transmission line and the signal amplifier for the additional transmission line in order to continue the processing of the respective signals and to deliver a corresponding output signal. Furthermore, a respective output amplifier is preferably connected to the signal amplifier for the transmission line and the signal amplifier for the additional transmission line to deliver a respective output signal representative for the signal transmitted via the transmission line and the additional transmission line. Preferably, an evaluation logic is provided in the evaluation circuit for conducting an evaluation of the output signals, in particular in a method as described below. 
         [0048]    A preferred method for testing the symmetric measuring circuit comprises an evaluation of the signals transmitted via the transmission line and the additional transmission line at a respective output of the evaluation circuit. The evaluation preferably comprises a comparison of the transmitted signals with each other and/or with an expected value. Alternatively or additionally, the difference and/or sum of the signals transmitted via the transmission line and the additional transmission line are evaluated at a respective output of the evaluation circuit. The evaluation preferably comprises a comparison of the difference and summing signal with each other and/or a comparison with an expected value. 
         [0049]    According to a preferred configuration, a failure or quality loss is detected when the summing signal does not correspond to the twofold value of the expected transmitted test signal. A failure or quality loss is preferably also detected when the difference signal does not correspond to the twofold value of the expected transmitted measuring signal. A failure or quality loss is preferably also detected when the output signal of the transmitted signals independently does not correspond to the superposition of the expected transmitted measuring signal and the expected transmitted test signal. 
         [0050]    Preferably, the testing of the measuring circuit can preferably be conducted during a measuring operation. In this case, the transmitted signals preferably comprise a superposition of the respective measuring signal and test signal. The testing of the measuring circuit can preferably also be conducted, when no measuring operation takes place. In this case, the transmitted signals preferably only comprise the respective test signal. 
         [0051]    According to a preferred configuration of the testing method, the output signals obtained without measuring operation are used as comparison values for the evaluation of the output signals during a measuring operation. In particular, the summing signal of the transmitted signals obtained without measuring operation can be used as a reference value for the summing signal of the transmitted signals obtained during a measuring operation. 
         [0052]    Possible application areas of the above described measuring circuit comprise a vibration sensor, an accelerometer, a pressure sensor, an acoustic emission sensor or similar sensing devices. In the case of a vibration sensor, the measuring object preferably comprises a rotary machine or any other vibrating structure operatively connected to the sensing element. In the case of an accelerometer, the measuring object preferably comprises a seismic mass that is mechanically coupled to the sensing element. In the case of a pressure sensor, the measuring object preferably comprises a gas and/or liquid which can for instance be operatively connected to the sensing element via a membrane. In the case of an acoustic emission sensor, the measuring object preferably comprises an emission source of acoustic waves that can be detected by the sensing element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0053]    The invention is explained in more detail hereinafter by means of preferred embodiments with reference to the drawings which illustrate further properties and advantages of the invention. The figures, the description, and the claims comprise numerous features in combination that one skilled in the art may also contemplate separately and use in further appropriate combinations. In the drawings: 
           [0054]      FIG. 1  is a schematic representation of a measuring circuit according to a first embodiment; 
           [0055]      FIG. 2  is a schematic representation of a measuring circuit according to a second embodiment; and 
           [0056]      FIG. 3  is a schematic representation of a measuring circuit according to a third embodiment. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0057]    A measuring circuit  1  shown in  FIG. 1  comprises a sensor  2  and an electronics unit  3 . Sensor  2  and electronics unit  3  are connected by a connection cable  4 . Sensor  2  comprises a housing  5  in which a sensing element  6  is arranged. Sensing element  6  is operatively connected to a measuring object  7  and configured to generate a measuring signal from measuring object  7 . 
         [0058]    Sensing element  6  comprises a stack of piezoelectric plates  8  arranged in between a first electrode  9  and a second electrode  10 . The polarization direction of piezoelectric plates  8  in sensing element  6  is also indicated in  FIG. 1 . The first electrode constitutes a feeding pole  9  by which a test signal can be injected into sensing element  6 . The second electrode constitutes an output pole  10  through which the injected test signal and/or the measuring signal can be delivered. Feeding pole  9  and output pole  10  have an inverse polarity. The value of the internal capacitance of sensing element  6  is subsequently denoted as C 6 . Feeding pole  9  and output pole  10  are electrically isolated from the walls of housing  5  such that sensing element  6  is arranged inside housing  5  in a electrically floating manner. Housing  5  is connected to ground  11 . 
         [0059]    Electronics unit  3  comprises a signal injector  12  and an evaluation circuit  13  arranged inside a common housing  14 . Signal injector  12  comprises a signal generator  15  and a transformer  16 . Transformer  16  comprises a primary winding  17  and a secondary winding  18 . Primary winding  17  is connected to signal generator  15 . Secondary winding  18  is connected to ground  19 . Secondary winding  18  has a very small inductance. Thus, transformer  16  is adapted to transform the signal generated by signal generator  15  into a signal of very low impedance. 
         [0060]    Secondary winding  18  of transformer  16  is also connected to an injection line  20 . The other end of injection line  20  is connected to feeding pole  9 . Therefore, sensing element  6  is connected to signal injector  12  and also connected to ground  19  via signal injector  12 . Thus, a test signal generated by signal generator  15  can be transformed in a test signal of very low ohmic impedance by transformer  16  and transmitted to feeding pole  9  via injection line  20 . In this way, a test signal injection circuit  21  comprising signal injector  12  and injection line  20  is provided. The test signal provided by signal generator  15  is an alternating current (AC). As a result, the polarity of feeding pole  9  and output pole  10  is continuously reversed. However, the polarization direction of piezoelectric plates  8  does not change. 
         [0061]    Evaluation circuit  13  comprises a charge amplifier  22  and an evaluation logic  38 . Charge amplifier  22  comprises an operational amplifier  23  with a feedback capacitor  24  and a connection to ground  25 . For clarity, secondary components and associated filters are not shown. Evaluation logic  38  is connected to the output of charge amplifier  22 . 
         [0062]    The input of charge amplifier  22  is connected to output pole  10  of sensing element  6  by a transmission line  26 . In this way, a sensing circuit  39  is provided comprising sensing element  6  and transmission line  26 . Injection line  20  and transmission line  26  are interconnected in series via sensing element  6 . Thus, a test signal injected from signal injector  12  into sensing element  6  can be fed through sensing element  6  and transmitted from output pole  10  to evaluation circuit  13  via transmission line  26 . Moreover, a measuring signal generated in sensing element  6  can be transmitted from output pole  10  to evaluation circuit  13  via transmission line  26 . 
         [0063]    Transmission line  26  and injection line  20  pass through sensor housing  5  via respective output terminals  27 ,  28  inside the walls of housing  5 . External parts of transmission line  26  and injection line  20  outside housing  5  are plugged into output terminals  27 ,  28 . Inside housing  5 , each of output terminals  27 ,  28  is connected to one of feeding pole  9  and output pole  10  of sensing element  6  by a respective internal wiring  29 ,  30 . Internal wirings  29 ,  30  thus constitute an internal part of transmission line  26  and an internal part of injection line  20 . Internal wirings  29 ,  30  are electrically isolated from sensor housing  5  and effectuate in between a respective capacitance  31 ,  32 . The values of these capacitances are subsequently denoted as C 31 , C 32 . 
         [0064]    In between sensor  2  and electronics unit  3 , transmission line  26  and injection line  20  are enclosed by connection cable  4 . Cable  4  comprises an outer electromagnetic shielding  33  that is common for transmission line  26  and injection line  20 . Transmission line  26  and injection line  20  thus extend through common shielding  33  over a substantial part of the distance in between sensor  2  and electronics unit  3 . 
         [0065]    At one of its ends, common shielding  33  is connected to ground  34 . Transmission line  26  and injection line  20  are electrically isolated from common shielding  33  and effectuate in between a respective capacitance  35 ,  36 . The values of these capacitances are subsequently denoted as C 35 , C 36 . Moreover, a respective capacitance  37  is effectuated in between the conductors of transmission line  26  and injection line  20  inside cable  4 . The value of this capacitance is subsequently denoted as C 37 . 
         [0066]    Thus, measuring circuit  1  allows injecting a test signal by applying an alternative test voltage Ut through injection line  20  to one end of sensing element  6 . The test voltage Ut is generated by a very low impedance signal injector  12  connected to ground  19 . Consequently, a charge Qt is transmitted to and injected in the input of charge amplifier  22  through the internal capacitance C 6  of sensing element  6  and in parallel with the capacitance  37  of the conductors of injection line  20  and transmission line  26 . The charge Qt is given by Qt=Ut*(C 6 +C 37 ). 
         [0067]    The frequency and amplitude of the test signal can be chosen freely within wide limits, preferably at a frequency outside the useful frequency band of the signal measured by sensor  2 . In a normal functional state, the test signal will appear at charge amplifier  22  with the magnitude Qt as described above. If a connection at the sensor output or inside sensor  6  fails then the test signal at the charge amplifier reduces to Qt=Ut*C 37 . If the connection fails at evaluation unit  13 , the resulting test signal Qt will be zero. 
         [0068]    The capacitances  31 ,  32 ,  35  and  36  together with the internal capacitance C 6  of sensing element  6  and capacitance  37  between the conductors of injection line  20  and transmission line  26  form a capacitor network. The effective capacitance seen between secondary winding  18  of transformer  16  and the input of charge amplifier  22  will determine the magnitude of Qt. The grounding  34  of shield  33  and the grounding  11  of housing  5  prevents the capacitances  31 ,  32 ,  35  and  36  from participating in the transfer function evaluated in evaluation circuit  13 . 
         [0069]    Moreover, not only the above mentioned main failures of open contacts can be detected but also the following possible short circuits: between conductors  20 ,  26 , between either of conductors  20 ,  26  and shielding  33 , across sensing element  6  or across insulations  31 ,  32  inside sensor  2 . In addition, a disconnection of grounding  34  of cable  4  or of grounding  11  of sensor  2  can be detected by the proposed measuring circuit. 
         [0070]    The addition of shielding  33  allows modifying the influence of certain groups of capacitors on the test signal as it appears at charge amplifier  22 . In order to improve the immunity to electric fields and also the sensitivity of a measurement of the variation of the internal capacitance C 6  of sensing element  6 , different shielding concepts are possible. One or several transmission lines  26  can be shielded separately with or without the external common shielding  33 . 
         [0071]      FIG. 2  shows a measuring circuit  41 , in which such a different shielding concept is applied in order to improve the testing sensitivity of variations of the capacitance of the sensing element. Corresponding elements with respect to measuring circuit  1  shown in  FIG. 1  are denoted with the same reference numerals. 
         [0072]    Measuring circuit  41  comprises a connection cable  44  in between sensor  2  and electronics unit  3 . Connection cable  44  comprises common shielding  33 , through which transmission line  26  and injection line  20  extend. Inside common shielding  33 , a separate electromagnetic shielding  45  is arranged, through which only transmission line  26  extends. Moreover, another separate electromagnetic shielding  46  is arranged inside common shielding  33 , through which only injection line  20  extends. Each separate shielding  45 ,  46  is provided with a respective connection to ground  47 ,  48 . 
         [0073]    The application of separate electromagnetic shielding  45  and  46  allows to eliminate the influence of the capacitance  37  between the conductors of transmission line  26  and injection line  20 . By removing the influence of the conductor capacitance C 37  shown in  FIG. 1 , the addition of shielding  45 ,  46  allows to determine a variation of the capacitance value C 6  of sensing element  6  even when long cables are used, in particular very small variations of the capacitance value C 6 . Moreover, it allows eliminating the direct injection of the test signal through cable capacitance  37  shown in  FIG. 1 . In measuring circuit  41  shown in  FIG. 2 , Qt at the charge amplifier is thus given by Qt=Ut*C 6 . 
         [0074]    The invention applied to asymmetric measuring circuits as depicted in  FIG. 1  and  FIG. 2  is well suited for testing applications. However, in some applications a redundancy feature may be particularly desirable. Such type of applications include monitoring applications, applications with specific Safety Integrity Level (SIL) requirements, applications where the measuring circuit is mounted in locations difficult to access and remote locations. Adding a redundancy feature can be achieved by applying the present invention to an electrically symmetric measuring circuit with an additional transmission line connected to a sensing element with a dedicated test input, as shown in  FIG. 3 . 
         [0075]      FIG. 3  shows a measuring circuit  51  comprising a sensor  52  and an electronics unit  53 . Sensor  52  and electronics unit  53  are connected by a connection cable  54 . Corresponding elements with respect to the measuring circuits  1  and  41  shown in  FIG. 1  and  FIG. 2  are denoted with the same reference numerals. 
         [0076]    A sensing element  55  is arranged inside housing  5  of sensor  52  in an electrically floating manner. Sensing element  55  is composed of two sensing units  56  and  57 . Each sensing unit  56 ,  57  is operationally connected to measuring object  7 . In this way, a first measuring signal can be generated from one of sensing units  56 ,  57  and an additional second measuring signal can be generated from the other sensing unit  56 ,  57 . Each sensing unit  56 ,  57  comprises a respective feeding pole  58 ,  59  and a respective output pole  60 ,  61 . The value of the internal capacitance of sensing units  56  and  57  is subsequently denoted as C 56  and C 57 , respectively. Sensing units  56  and  57  are provided with a substantially equal value of their internal capacitance C 56  and C 57 , i.e. C 56 ˜=C 57 . 
         [0077]    Sensing element  55 , as schematically represented in  FIG. 3 , is composed of a stack of piezoelectric plates  62 . In the middle of stack  62 , a feeding electrode is provided, which constitutes both of feeding poles  58  and  59 . At the bottom and at the top of the stack  62 , a respective output electrode is arranged, which constitute output poles  60 ,  61 . 
         [0078]    Electronics unit  53  comprises signal injector  12  and an evaluation circuit  63  enclosed in common housing  14 . Evaluation circuit  63  comprises a first charge amplifier  22  and a second charge amplifier  64 . Evaluation circuit  63  further comprises a difference amplifier  65  and a summing amplifier  66 , which are both connected to the outputs of first charge amplifier  22  and second charge amplifier  64 . Difference amplifier  65  is configured to deliver a signal representative for a difference of the signals at the outputs of first charge amplifier  22  and second charge amplifier  64 . Summing amplifier  66  is configured to deliver a signal representative for a sum of the signals at the outputs of first charge amplifier  22  and second charge amplifier  64 . 
         [0079]    Evaluation circuit  63  further comprises a first output amplifier  67  connected to the output of charge amplifier  22  and a second output amplifier  68  connected to the output of charge amplifier  64 . The outputs of output amplifier  67 , output amplifier  68 , difference amplifier  65 , and summing amplifier  66  are connected to evaluation logic  38 . 
         [0080]    Both feeding poles  58 ,  59  are connected to signal injector  12  via injection line  20 . Output pole  60  of first sensing unit  56  is connected to the input of first charge amplifier  22  via transmission line  26 . Output pole  61  of second sensing unit  57  is connected to the input of second charge amplifier  64  via a second transmission line  69  that is provided in addition to first transmission line  26 . In this way, a sensing circuit  40  is provided comprising sensing element  55 , first transmission line  26  and additional second transmission line  69 . 
         [0081]    Second transmission line  69  passes through sensor housing  5  via an output terminal  71  inside the walls of housing  5 . Output terminal  71  is disposed next to output terminals  27 ,  28  of transmission line  26  and injection line  20 . An external part of second transmission line  69  is plugged into output terminal  71 . Inside housing  5 , output terminal  71  is connected to output pole  61  of second sensing unit  57  by a respective internal wiring  75 . Internal wiring  75  thus constitutes an internal part of second transmission line  69 . Corresponding to internal wirings  29 ,  30 , internal wiring  75  is also electrically isolated from sensor housing  5  and effectuates in between a respective capacitance  72 . The value of capacitance  72  is subsequently denoted as C 72 . 
         [0082]    In between sensor  52  and electronics unit  53 , second transmission line  69 , transmission line  26  and injection line  20  are enclosed by connection cable  54 . Cable  54  comprises outer electromagnetic shielding  33  that is common for second transmission line  69 , first transmission line  26  and injection line  20 . Second transmission line  69 , first transmission line  26  and injection line  20  thus extend through common shielding  33  over a substantial part of the distance in between sensor  52  and electronics unit  53 . 
         [0083]    Inside common shielding  33 , a separate electromagnetic shielding  73  is arranged, through which only second transmission line  69  extends. Separate electromagnetic shielding  73  is disposed next to separate electromagnetic shielding  46  of injection line  20  and next to separate electromagnetic shielding  45  of transmission line  26 . Separate shielding  73  is also provided with a respective connection to ground  74 . 
         [0084]    The application of each separate electromagnetic shielding  45 ,  46  and  73  inside common shielding  33  allows to eliminate the influence of a capacitance between the conductors of first transmission line  26 , second transmission line  69  and injection line  20 . By injecting a test signal Ut in measuring circuit  51  shown in  FIG. 3 , a charge Qt A  is thus created at the input of first charge amplifier  22  that is given by Qt A =Ut*C 56 . Correspondingly, a charge Qt B  at the input of second charge amplifier  64  is created that is given by Qt B =Ut*C 57 . Since the two capacitance values C 56  and C 57  are chosen to be equal, an equal charge Qt=Qt A =Qt B  is transmitted to first charge amplifier  22  and second charge amplifier  64  under regular measurement conditions. 
         [0085]    Thus, in the symmetric measuring circuit  51  shown in  FIG. 3 , the test signal can be injected through sensing element  55  by common feeding electrode  58 ,  59  located in the middle of sensing element  55  and connected to injection line  20  surrounded by separate shielding  46 . Feeding electrode  58 ,  59  is connected to ground  19 . Shielding  46  is also connected to ground  48 . 
         [0086]    Common feeding electrode  58 ,  59  separates sensing element  6  shown in  FIG. 1  and  FIG. 2  into a sensing element  55  with two sensing units  56  and  57 , each comprising respective piezoelectric elements. Sensing element  55  is symmetrically connected to one end of first transmission line  26  and to one end of second transmission line  69 , which are also separately shielded. 
         [0087]    At the opposite end, the conductor of first transmission line  26  is connected to charge amplifier  22  comprising operational amplifier  23  with feedback capacitor  24 . Similarly, the conductor of second transmission line  69  is connected to charge amplifier  64  also consisting of a corresponding operational amplifier  23  with feedback capacitor  24 . Feedback capacitors  24  of both charge amplifiers  22 ,  64  have an identical capacitance value Cf. The outputs of charge amplifiers  22  and  64  are connected to a difference amplifier  65  and a summing amplifier  66 . 
         [0088]    The two sensing units  56 ,  57  comprise piezoelectric members  62 —a member  62  being for example a stack of multiple piezoelectric discs. The two sensing units  56 ,  57  are located on both sides of common feeding electrode  58 ,  59  and have equal capacitances C 56  and C 57  in order to allow common modes rejection. 
         [0089]    Measuring circuit  51  is preferably used as a piezoelectric vibration sensor or a piezoelectric accelerometer. It can also be applied, for instance, as a pressure sensor, an acoustic emission sensor or any other piezoelectric sensor. 
         [0090]    In case of a piezoelectric accelerometer, for instance, sensing units  56  and  57  deliver respective charges Q A  and Q B  for a given acceleration of measuring object  7 . Q A  and Q B  have same magnitude but opposite polarity (Q A =−Q B ). Q A  and Q B  are transmitted to and injected in the inputs of charge amplifiers  22  and  64 , respectively. The outputs of charge amplifiers  22  and  64  are voltages U A  and U B , respectively. U A  and U B  are given by: 
         [0000]      U A =−Q A /C f  U B =−Q B /C f .
 
         [0091]    The minus sign in each equation is due to the transfer function of each charge amplifier. With Q A =−Q B  it follows that: 
         [0000]        I   A   =−Q   A   /C   f   =[(−Q]   B )/ C   f   =Q   B   /C   f   =−U   E    
         [0000]    U A  and U B  have same amplitude but opposite phase (U A =U B ). 
         [0092]    In order to perform a health check of the sensor and transmission line, an alternating current (AC) test signal Ut (the test signal could be a direct current (DC) signal) is injected through sensing element  55  by common electrode  58 ,  59 . Consequently, piezoelectric elements  62  deliver charges Qt A  and Qt B , respectively. Qt A  and Qt B  have same magnitude and same polarity (Qt A =Qt B ). Qt A  and Qt B  are given by: 
         [0000]        Qt   A   =Ut*C 56, and  Qt   B   =Ut*C 57 
         [0000]    Qt A  and Qt B  are transmitted to and injected in the inputs of charge amplifiers  22  and  64 , respectively. Consequently the outputs of charge amplifiers  22  and  64  are voltages Ut A  and Ut B , respectively. Ut A  and Ut B  are given by: 
         [0000]        Ut   A   =−Qt   A   /C   f  and  Ut   B   =−Qt   B   /C   f    
         [0093]    The minus sign in each equation is due to the transfer function of each charge amplifier. With Qt A =Qt B  it follows that: 
         [0000]    
       
      
       Ut 
       A 
       =−Qt 
       A 
       /C 
       f 
       =−Qt 
       B 
       /C 
       f 
       =Ut 
       B  
      
     
         [0000]    Ut A  and Ut B  have same amplitude and same phase (Ut A =Ut B ). 
         [0094]    Considering now the superposition of both the measuring signal and test signal, it follows that under normal conditions, the output of difference amplifier  65  is given by: 
         [0000]        U   out65   U   A   +Ut   A −( U   B   +Ut   B )= U   A   −U   B   +Ut   A   −Ut   B  
 
         [0000]    With U A =U B  and Ut A =Ut B  it follows that: 
         [0000]        U   out65 =2 U   A =−2 U   B    (I)
 
         [0095]    Similarly, under normal conditions, the output of summing amplifier  66  is given by: 
         [0000]        U   out66   =U   A   +Ut   A +( U   B   +Ut   B )= U   A   +U   B   +Ut   A   +Ut   B    
         [0000]    With U A =−U B  and Ut A =Ut B  it follows that: 
         [0000]      U out66 =2Ut A =2Ut B    (II)
 
         [0000]    In addition to difference amplifier  65  and summing amplifier  66 , the two additional output amplifiers  67  and  68  are provided to deliver signals which correspond to the outputs of charge amplifiers  22  and  64 , respectively. The outputs of amplifiers  67  and  68  are given by: 
         [0000]        U   out67   =U   A   +Ut   A    (III)
 
         [0000]        U   out68   =U   B   +Ut   B    (IV)
 
         [0000]    The signals according to equations (I) to (IV) are then evaluated in evaluation logic  38 . 
         [0096]    Evaluation of These Signals Allows:
       a) detecting and localizing a fault either in the sensing element, connection or transmission line, and   b) selecting the path (either A or B) remaining functional therefore providing circuit redundancy.       
 
         [0099]    The above described measuring circuit  1 ,  41 ,  51  represents a further development of the measuring circuit disclosed in U.S. Pat. No. 6,498,501 B2, which is herewith included by reference, and can comprise any other components and/or configurations and/or applications disclosed therein. 
         [0100]    From the foregoing description, numerous modifications of the measuring circuit according to the invention are apparent to one skilled in the art without leaving the scope of protection of the invention that is solely defined by the claims.