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MEASURING CIRCUIT

20170227381 · 2017-08-10

    Inventors

    Cpc classification

    International classification

    Abstract

    A measuring circuit comprising a sensing element configured to generate a measuring signal from a measuring object, a signal injector configured to generate an auxiliary signal, and an evaluation circuit comprising a first upstream amplifier with a first input connected to a first pole of the sensing element via a first signal line and a second upstream amplifier with a first input connected to a second pole of the sensing element via a second signal line. A measuring circuit with an improved reliable control of its measuring chain allowing continuous testing of the integrity of the measurement signal coming from the sensing element and/or to allow the measuring circuit to be upgradable with respect to a different sensing unit and/or evaluation unit, the first upstream amplifier comprises a second input connected to the signal injector, and the evaluation circuit comprises a first downstream amplifier having a first input connected to the signal injector and a second input connected to an output of the first upstream amplifier.

    Claims

    1. A measuring circuit comprising a sensing element configured to generate a measuring signal from a measuring object, a signal injector configured to generate an auxiliary signal, and an evaluation circuit comprising a first upstream amplifier with a first input connected to a first pole of the sensing element via a first signal line and a second upstream amplifier with a first input connected to a second pole of the sensing element via a second signal line, wherein the first upstream amplifier comprises a second input connected to the signal injector, and the evaluation circuit comprises a first downstream amplifier having a first input connected to the signal injector and a second input connected to an output of the first upstream amplifier.

    2. The measuring circuit according to claim 1, wherein the evaluation circuit comprises a second downstream amplifier having a first input connected to an output of the second upstream amplifier, and a second input connected to an output of the first downstream amplifier.

    3. The measuring circuit according to claim 1, wherein the second upstream amplifier comprises a second input connected to ground or to a second signal injector.

    4. The measuring circuit according to claim 1, wherein at least one of the first upstream amplifier and second upstream amplifier comprises a differential amplifier configured to provide an output signal representative for a difference between an input signal at a first input of the differential amplifier and an input signal at a second input of the differential amplifier.

    5. The measuring circuit according to claim 1, wherein the first downstream amplifier comprises a differential amplifier configured to provide an output signal representative for a difference between an input signal at a first input of the differential amplifier and an input signal at a second input of the differential amplifier.

    6. The measuring circuit according to claim 4, wherein the first input of the respective differential amplifier corresponds to one of an inverting input delivering an inverted signal and a non-inverting input delivering a non-inverted signal, and the second input of the respective differential amplifier corresponds to the other of the inverting input and the non-inverting input, the signal injector being connected to a different input of said inverting input and non-inverting input at the first downstream amplifier than at the first upstream amplifier.

    7. The measuring circuit according to claim 1, wherein the evaluation circuit comprises a summing amplifier with a first input, a second input, and an output providing an output signal representative for a sum of an input signal at the first input and an input signal at the second input, wherein the first input is connected to an output of the second upstream amplifier, and the second input is connected to an output of the first downstream amplifier.

    8. The measuring circuit according to claim 1, wherein the measuring circuit comprises a sensing unit including the sensing element, an evaluation unit including the evaluation circuit, and an output terminal, wherein the sensing unit and evaluation unit are connectable to one another via the output terminal.

    9. The measuring circuit according to claim 1, wherein the evaluation circuit comprises a processing unit configured with logic to dissociate measurement signals generated by the sensing element from test signals generated by the signal injector.

    10. The measuring circuit according to claim 1, wherein the evaluation circuit is configured with logic to evaluate a value derived from a signal obtained downstream of at least one of the first upstream amplifier, the second upstream amplifier, and the first downstream amplifier.

    11. The measuring circuit according to claim 7, wherein the evaluation circuit is configured with logic to evaluate a value derived from a signal obtained downstream of the summing amplifier.

    12. The measuring circuit according to claim 7, wherein the evaluation circuit is configured with logic to compare a value derived from a signal obtained downstream of the second downstream amplifier with a value derived from a combination of values derived from a signal obtained downstream of the summing amplifier and from a signal obtained downstream of at least one of the first upstream amplifier and second upstream amplifier.

    13. The measuring circuit according to claim 1, wherein at least one of the first signal line and second signal line is at least partially provided with a separate electromagnetic shield.

    14. The measuring circuit according to, claim 1, wherein the evaluation circuit comprises at least one capacitance connected to at least one of said first signal line and second signal line upstream of an input of at least one of said first upstream amplifier and second upstream amplifier.

    15. The measuring circuit according to claim 1, wherein the evaluation circuit is configured with logic to evaluate a value derived from a signal obtained downstream of at least one of the first upstream amplifier, the signal corresponding to
    U.sub.Output52=U.sub.vib+U.sub.t+U.sub.C2+U.sub.C3, or U.sub.Output52=−U.sub.vib−U.sub.t−U.sub.C2−U.sub.C3; the second upstream amplifier, the signal corresponding to
    U.sub.Output51=−U.sub.vib−U.sub.C3, or U.sub.Output51=U.sub.vib+U.sub.C3; and the first downstream amplifier, the signal corresponding to
    U.sub.Output53=U.sub.vib+U.sub.C2+U.sub.C3, or U.sub.Output53=−U.sub.vib−U.sub.C2−U.sub.C3, wherein U.sub.vib corresponds to a signal generated by the sensing element, U.sub.t corresponds to a signal generated by the signal injector, U.sub.C2 corresponds to a signal representative of a capacitance value C2 of a capacitance in between one of the signal lines and surroundings of this signal line, and U.sub.C3 corresponds to a signal representative of a capacitance value C3 of a capacitance in between the first signal line and the second signal line.

    Description

    [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;

    [0056] FIG. 3 is a schematic representation of a measuring circuit according to a third embodiment;

    [0057] FIG. 4 is a schematic representation of a measuring circuit according to a fourth embodiment;

    [0058] FIG. 5 is a schematic representation of a measuring circuit according to a fifth embodiment; and

    [0059] FIG. 6 is a schematic representation of a measuring circuit according to a sixth embodiment.

    [0060] A basic embodiment of a measuring circuit 1 is shown in FIG. 1. Measuring circuit 1 comprises a sensor 10, a mineral insulated (MI) cable connection 20, an output terminal 30, a medium to low noise (MTLN) cable 40 and an electronic measuring unit 50. Sensor 10 can be, for instance, any type of a piezoelectric sensor. Sensor 10 comprises a sensor housing 11 and a sensing element 12, in particular a piezoelectric sensing element. Sensing element 12 comprises a positive and a negative pole 15, 16 at which a positive and a negative electrode is arranged, respectively, serving as a pick-up for charges generated by sensor 10. Sensing element 12 is operatively connected to a measuring object 17 and configured to generate charges corresponding to a measuring signal from measuring object 17. A sensing unit 39 comprises sensor 10 and MI cable 20. An evaluation unit 49 comprises MTLN cable 40 and electronic measuring unit 50. Electrodes 15, 16 are connected by two conductors 13, 14 and MI cable 20 to output terminal 30. Both poles 15, 16 of sensing element 12 are insulated from housing 11 (the sensing element is electrically floating). Sensing element 12 has an internal capacitance 18 of a value C13. A capacitance 25, 26 in between a respective wire of conductors 13, 14 and housing 11 has a value C11 and C12, respectively. Sensor 10 is connected to the ground 27.

    [0061] MI cable 20 comprises two conductors 21, 22 connected to conductors 13, 14 of sensor 10. In this case, they are not separately shielded from one another. MI cable 20 further comprises a cable sheath 28 in the form of an electromagnetic shield. A respective capacitance 33, 34 between conductors 21, 22 and shield 28 has a value C21 and C22, respectively. A capacitance 35 in between the conductors 21, 22 has a value C23.

    [0062] Conductors 21, 22 form a respective line leading to output terminal 30, which transitions from MI cable 20 coming from sensor 10 to MTLN cable 40. MTLN cable 40 goes from output terminal 30 to electronic measuring unit 50. MTLN cable 40 comprises two conductor cables 41, 42 shielded from one another. Capacitances 45, 46 between each conductor 41, 42 and a respective electromagnetic shield 43, 44 have a value C41 and C42, respectively. MTLN cable 40 further comprises a cable sheath 47 enclosing conductors 41, 42 and respective shields 43, 44. Cable sheath 47 is also provided as an electromagnetic shield. Shield 47 is connected to ground 36. Output terminal 30 comprises respective connectors 31, 32 connecting conductors 21, 22 of MI cable 20 to conductor cables 41, 42 of MTLN cable 40.

    [0063] First conductor 14 of sensor 10, first conductor 22 of MI cable 20, first connector 32 of output terminal 30, and first conductor 42 of MTLN cable 40 are thus connected to one another and form a first signal line 38 leading from first electrode 16 of sensor 10 to electronic measuring unit 50. Second conductor 13 of sensor 10, second conductor 21 of MI cable 20, second connector 31 of output terminal 30, and second conductor 41 of MTLN cable 40 are thus connected to one another and form a second signal line 37 leading from second electrode 15 of sensor 10 to electronic measuring unit 50.

    [0064] Electronic measuring unit 50 comprises an evaluation circuit 48. Evaluation circuit 48 comprises four amplifiers 51, 52, 53, 54 and a processing unit 55. The amplifiers comprise a first upstream amplifier 52, a second upstream amplifier 51, a first downstream amplifier 53, and a second downstream amplifier 54. First upstream amplifier 52 comprises a first input connected to first signal line 38. First upstream amplifier 52 is a differential amplifier with an inverting input provided by its first input and a non-inverting input provided by a second input. Second upstream amplifier 51 comprises a first input connected to second signal line 37. Second upstream amplifier 51 is also a differential amplifier with an inverting input provided by its first input and a non-inverting input provided by a second input.

    [0065] A signal injector 56, in particular a signal generator, provides the test signal needed. Signal generator 56 is connected to ground 60. The second input of first upstream amplifier 52 is connected to signal generator 56. The second input of second upstream amplifier 52 is connected to ground 59. First downstream amplifier 54 has a first input connected to signal injector 56 and a second input connected to an output of first upstream amplifier 52. Second downstream amplifier 54 has a first input connected to an output of second upstream amplifier 52 and a second input connected to an output of first downstream amplifier 53. First downstream amplifier 53 and second downstream amplifier 54 are also provided by a respective differential amplifier with an inverting input corresponding to its first input and a non-inverting input corresponding to its second input.

    [0066] A respective feedback capacitance 57, 58 is connected in parallel to upstream amplifiers 51, 52. The value of feedback capacitance 57 and 58 is C51 and C52, respectively. To get a good Common-Mode Rejection Ratio, capacitances C51 and C52 should be identical. The shown arrangement of four amplifiers 51, 52, 53, 54 shows one of the ways the invention can be implemented. In particular, amplifiers 51-54 each form a differential amplifier. Upstream amplifiers 51, 52 each form an operational amplifier in which a retroaction takes place via feedback capacitances 57, 58 connected in between one of its inputs and the output of the respective amplifier 51, 52. Downstream amplifiers 53, 54 each form a non-retroaction amplifier in which no retroaction is provided in between the inputs and the output of the respective amplifier 53, 54. Using at least three amplifiers allows for a very low noise level and a very good CMMR.

    [0067] The amplitude of the auxiliary test signal generated by signal injector 56 has a value U.sub.t. U.sub.t can be chosen freely within wide limits as long as it doesn't overload the measuring electronic. For continuous use of the BITE, the frequency however should be chosen outside the bandwidth of the measuring signal since the voltage created by 56 will always be there. The test frequency can also be within the bandwidth of the sensing element as long as the selected frequency isn't actively used for measurements.

    [0068] The charges generated from sensing element 12 have a value Qvib. The generated charges will end up on feedback capacitances 57, 58 with the values C51, C52 and will generate a voltage Uvib=Qvib/C51 (=Qvib/C52). The output of first upstream amplifier 52, second upstream amplifier 51, first downstream amplifier 53, and second downstream amplifier 54 is connected to post processing unit 55 via a respective output channel 61, 62, 63, 64. Processing unit 55 will dissociate the measurement signal provided by sensing element 12 from the test signal provided by signal injector 56 and analyze the output values obtained from the different amplifiers 51, 52, 53, 54. The results of this analysis will provide information as to the nature and location of the failure. The mechanism used is described hereafter.

    [0069] The above described values of capacitances in measuring circuit 1 comprise values for sensing capacitance C13, line capacitance C23, housing capacitances C11, C12, sheath capacitances C21, C22, shield capacitances C41, C42, and feedback capacitances C51, C52.

    [0070] The capacitance values seen from the measuring electronics 50 are:


    C1=C11+C21+C41


    C2=C12+C22+C42


    C3=C13+C23

    [0071] In the circuit shown in FIG. 1, the outputs of the different amplifiers 51, 52, 53, 54 are:

    [00005] .Math. U Output .Math. .Math. 51 = - U vib - U C .Math. .Math. 3 = - U vib - C .Math. .Math. 3 × U t C .Math. .Math. 51 U Output .Math. .Math. 52 = U vib + U t + U C .Math. .Math. 2 + U C .Math. .Math. 3 = U vib + U t + C .Math. .Math. 3 × U t C .Math. .Math. 51 + C .Math. .Math. 2 × U t C .Math. .Math. 51 .Math. U Output .Math. .Math. 53 = U vib + U C .Math. .Math. 2 + U C .Math. .Math. 3 = U vib + C .Math. .Math. 2 × U t C .Math. .Math. 51 + C .Math. .Math. 3 × U t C .Math. .Math. 51 .Math. U Output .Math. .Math. 54 = U vib + U t × C .Math. .Math. 3 + C .Math. .Math. 2 / 2 C .Math. .Math. 51

    [0072] For convenience, we define:


    C.sub.z=C3+C2/2

    [0073] It can be seen that variations in the values of the capacitance will have a direct impact on the measured output voltage. From the analysis of the different outputs, a health check can be performed on the various components of the measuring unit. Under normal conditions, the value of the output in the test frequency range will be within the bounds defined by the calibration process. If something is wrong within the entire system, the various possible failures will have recognizable consequences on the values read at the outputs. If C1, C2 or C3 changes the component of the output voltage corresponding to the test signal will change.

    [0074] Here are only a few examples of the failure identification capabilities of the above described circuit illustrated with some of the most common occurring failures for such systems:

    TABLE-US-00001 Failure C1 & C2 C3 Sensing element disconnects Unchanged 0 (MI shielded) (open circuit in sensor 10) C23 (MI unshielded) MTLN disconnected (open 0 0 circuit MTLN - electronics) Saturated amplifiers Loss of high frequencies at output Loss of ground connection 0 Increases (between conductor 42 and measuring unit 50) Open circuit between MI and Measure C42 0 MTLN cables instead of C42 + C22 + C12

    [0075] In particular, some of these steps may require precise calibrations, others don't. The amount of precision in failure localization can preferably be chosen by the end user. In an even more preferable option, the calibration could be taken far enough to allow the detection of the exact position of a failure. For example a measured value for C2, a value that would correspond to C2-C42/2, could be used to deduce that the failure occurred in the middle of the MTLN cable. These options are left to the preferences of the user of the measuring circuit. For convenience, other differential amplifiers could be added to single out U.sub.C2 and/or U.sub.C3 directly and reduce the computational power needed to single them out.

    [0076] FIG. 2 depicts a second embodiment of a measuring circuit 71. Corresponding features with respect to measuring circuit 1 shown in FIG. 1 are denoted with the same reference numerals. Measuring circuit 71 comprises an evaluation circuit 78 in which a summing amplifier 67 is added. Summing amplifier 67 has a first input connected to the output of second upstream amplifier 51 and a second input connected to the output of first downstream amplifier 53. An output of summing amplifier 67 is connected to processing unit 55 via an output channel 65. The output signal of summing amplifier corresponds to U.sub.C2. Also conceivable is a corresponding assembly providing the value of U.sub.C3 as an output, or both U.sub.C2 and U.sub.C3 as an output, or U.sub.Cz as an output, depending on what the user wishes to have direct access to.

    [0077] FIG. 3 depicts a third embodiment of a measuring circuit 81. Corresponding features with respect to measuring circuit 1 shown in FIG. 1 and measuring circuit 71 shown in FIG. 2 are denoted with the same reference numerals. Measuring circuit 81 comprises an additional logical component 85 added to processing unit 55. Logical component 85 is configured to compare a value derived from a signal at the output of second downstream amplifier 54 with a value derived from a combination of values derived from a signal at the output of summing amplifier 67 and at the output of second upstream amplifier 51.

    [0078] Measuring circuit 81 builds onto measuring circuit 71 presented in FIG. 2 and allows to provide an additional step to validate the electronics more precisely. Having the output of second upstream amplifier 51 providing a value of C3, the output of summing amplifier 67 providing a value of C2, and the output of first downstream amplifier 53 which gives a value of Cz=C3+C2/2, one can compute the value of Cz using the values obtained for C2 and C3 and check the result. If the result is different, then a problem in the electronics is identified.

    [0079] FIG. 4 depicts a fourth embodiment of a measuring circuit 91. Corresponding features with respect to measuring circuit 1 shown in FIG. 1, measuring circuit 71 shown in FIG. 2, and measuring circuit 81 shown in FIG. 3 are denoted with the same reference numerals. Compared to measuring circuit 1 depicted in FIG. 1, an additional shield 93 is provided around conductor 21 of MI cable 20, and/or an additional shield 94 around conductor 22 of MI cable 20. Moreover, two capacitances 95, 96 are added, each capacitance 95, 96 connecting sheath 28 to a respective conductor 21, 22 of MI cable 20. Capacitances 95, 96 have a respective value of C24 and C25.

    [0080] In contrast, if the same configuration as in measuring circuit 1 would be used when applying additional shield 93 and/or additional shield 94, though measuring circuit 1 could still determine if there is a failure on the sensor/cable system, there are some cases in which measuring circuit 1 wouldn't be able to determine if the issue came from sensing element 12 and MI cable 20 connected to it, or from MTLN cable 40. For example, an issue at the end of conductor 22, close to output terminal 30, or at the beginning of conductor 42, close to output terminal 30, may result in the same output values. Therefore, and option would be to add capacitances 95, 96 as an additional reference.

    [0081] FIG. 5 depicts a fifth embodiment of a measuring circuit 101. Corresponding features with respect to measuring circuit 1 shown in FIG. 1, measuring circuit 71 shown in FIG. 2, measuring circuit 81 shown in FIG. 3 are denoted with the same reference numerals, and measuring circuit 91 shown in FIG. 4 are denoted with the same reference numerals. Compared to measuring circuit 1 depicted in FIG. 1, measuring circuit 101 comprises an evaluation circuit 108 in which two entry capacitances 103, 104 are added. First entry capacitance 104 is connected to first signal line 38 at the first input of first upstream amplifier 52. Second entry capacitance 103 is connected to second signal line 37 at the first input of second upstream amplifier 51. Each entry capacitance 103, 104 is connected to ground 105, 106. Entry capacitances 103, 104 have a respective value C53, C54.

    [0082] FIG. 6 depicts a sixth embodiment of a measuring circuit 111. Corresponding features with respect to measuring circuit 1 shown in FIG. 1 are denoted with the same reference numerals. Compared to measuring circuit 1, measuring circuit 111 comprises an evaluation circuit 118 directly connected to the sensing element 12. In particular, no cable is provided in between sensor 10 and electronic measuring unit 50. Thus, substantially no line capacitance, no sheath capacitances and no shield capacitances are present in this circuit. In circuit 111, a failure identification can thus be purely based on a determination and/or monitoring of the value C13 of sensing capacitance 18 and the values C11, C12 of housing capacitances 25, 26. The signal U.sub.C3 is then directly representative for the sensing capacitance value C13, the signal U.sub.C2 is then directly representative for the housing capacitance value C12, and the signal U.sub.C1 is then preferably directly representative for the housing capacitance value C11. In particular, line capacitance value C23, sheath capacitance values C21, C22, and shield capacitance values C41, C42 are nonexistent or negligible in this case. Evaluation circuit 118 and sensing element 12, more particularly sensor 10 and electronic measuring unit 50, are included in a common housing.

    [0083] An issue on the electronic module can be diagnosed in different ways with the assemblies of the previous methods. Checking Ut, or checking the Uvib values from second upstream amplifier 51 and first downstream amplifier 53, are two easy ways to do it. However, a quick way to check the entire electronic would be to add entry capacitances 103, 104 as seen in FIG. 5. Entry capacitances 103, 104 are two capacitors arranged at the entry lines of the respective charge amplifier 51, 52. Often, such capacitors would already be present for filtering purposes. These capacitors 103, 104 with their values C53, C54 would appear on the outputs as an additional component adding to the above described values C1 and C2. If not even these capacitors 103, 104 can be detected, then the problem can be attributed to the electronics directly.

    [0084] The above described measuring circuits represent a further development of the measuring circuits disclosed in U.S. Pat. No. 6,498,501 B2 and US 2014/0225634 A1, which are herewith included by reference, and can comprise any other components and/or configurations and/or applications disclosed therein.

    [0085] 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.