TDR-based dielectric constant meter

Abstract

A TDR-based measuring device for measuring the dielectric constant (DC) of a medium (2), comprising: - a measuring probe (11) that can be brought into contact with the medium (2), - a signal generation unit (12) designed to couple an edge-based measurement signal (sHF) into the measuring probe (11) according to the TDR measurement principle, - an evaluation unit (13) designed to: ◯ receive the reflected edge-based measurement signal (rHF) after reflection in the measuring probe (11), ◯ determine a signal propagation time between coupling and reception of the respective edge according to the TDR measurement principle, and determine the dielectric constant (DC) based on the signal propagation time, characterized by: - ​​a filter unit (14) designed to: ◯ filter the emitted and / or the reflected edge-based measurement signal (sHF, rHF), and ◯ its filter type and / or to change at least one filter parameter (R, C), and an analysis unit (15) designed to◯ to determine a correlation factor between the edge shape of the transmitted and received measurement signal (sHF, rHF), an SWR ratio, or a maximum curvature of the received measurement signal (rHF), and ◯ to adjust the filter type and / or at least one filter parameter (R, C) of the filter unit (14) such that the correlation factor, the SWR ratio and / or the maximum curvature reaches a minimum value or converges to the minimum value.

Description

The invention relates to a TDR-based dielectric constant measuring device. In automation technology, particularly in process automation, field devices are frequently used to acquire and / or manipulate various measured variables. These variables can include, for example, level, flow rate, pressure, temperature, pH value, redox potential, conductivity, or dielectric constant of a medium. To acquire these measured values, field devices incorporate suitable sensor units or are based on appropriate measurement principles. The Endress+Hauser Group manufactures and distributes a wide variety of field device types. Determining the dielectric constant (also known as the "dielectric constant" or "relative permittivity") of various media is of great interest in the case of solids, liquids, and gases such as fuels, wastewater, gases, or chemicals, as this value provides a reliable indicator of impurities, moisture content, or material composition. Within the scope of this invention, the term "container" also includes non-enclosed containers such as basins, lakes, or flowing waters. The dielectric constant (TDR) principle (TDR is an acronym for "Time Domain Reflectometry") is used, among other methods, to measure dielectric constant. In this measurement principle, an edge-based measurement signal is generated and fed into an electrically conductive measuring probe at a repetition frequency between 0.1 MHz and 100 GHz. It is essentially irrelevant whether the edge within the measurement signal is positive or negative. To determine the dielectric constant, the picosecond-range transit time of the edge until the reception of the reflected high-frequency signal is measured. This utilizes the effect that the transit time of the edge depends on the dielectric constant of the material surrounding the measuring probe. The operating principle of TDR-based sensor units is described, for example, in publication EP 0622 628 A2.Corresponding field devices are distributed in numerous designs, for example by IMKO Mikromodultechnik GmbH as part of the Endress+ Hauser group of companies. Due to the extremely fast signal propagation time of the edge within the measuring probe, in the nanosecond range, the corresponding signal generation unit of the measuring instrument must be designed to generate the cyclically recurring edge in the measurement signal with the highest possible slew rate of less than 300 picoseconds. Voltage levels of, for example, 0 V and 300 mV are used. This enables the evaluation unit of the measuring instrument to detect the measurement signal, or rather the edge, after reflection in the measuring probe with sufficient resolution. The more accurately the edge of the reflected measurement signal can be detected, the more accurately the evaluation unit can determine the propagation time and thus the dielectric constant. Consequently, the reflected edge must also be received by the evaluation unit with the highest possible degree of shape fidelity relative to the generated measurement signal. To ensure that the reflected edge is received with sufficient fidelity to the generated measurement signal, the signal generation unit and the evaluation unit must be matched to the respective measuring probe or the connecting high-frequency cable by means of a suitable filter unit. However, determining different media or different dielectric constant ranges may require different types of measuring probes with individually adapted geometries. Therefore, it is not possible to use a TDR-based measuring device for different applications without having to manually adjust the filter unit (in addition to the measuring probe, if necessary) for the respective application. The invention is therefore based on the objective of providing a TDR-based dielectric constant measuring device that can be used for as many applications as possible without manual modifications to electrical components. The invention solves this problem by means of a TDR-based measuring device for measuring the dielectric constant of a medium, comprising: - A measuring probe that can be brought into contact with the medium, - a signal generation unit designed to couple an edge-based measurement signal into the measuring probe according to the TDR measurement principle, - an evaluation unit designed to: ◯ receive the reflected, edge-based measurement signal after reflection in the measuring probe, ◯ determine a signal propagation time between coupling and reception of the respective edge according to the TDR measurement principle, and determine the dielectric constant based on the signal propagation time. The measuring device is characterized by: a filter unit designed to filter the emitted and / or reflected edge-based measurement signal, and to change its filter type and / or at least one filter parameter; and an analysis unit designed to determine a correlation factor between the edge shape of the emitted and received measurement signal, an SWR ratio, or a maximum curvature of the received measurement signal, and to adjust the filter type and / or at least one filter parameter of the filter unit such that the correlation factor, the SWR ratio, or the maximum curvature reaches a minimum value or converges to this minimum value. The relevant convergence criterion, in relation to the minimum value, depends on the specific optimization algorithm implemented. For example, in the case of least-squares optimization, the corresponding convergence criterion is met as soon as the sum of the squared fitting errors is minimal. The invention is therefore based on determining the shape fidelity of the edge of the received measurement signal in relation to the transmitted measurement signal and on optimizing the filter type or filter parameters of the filter unit such that the reflected edge is sufficiently shape-fidelity. Through automatic optimization by means of a suitably designed analysis unit, the measuring device can, according to the invention, also be used without manual modification in changing measurement applications or with changing probe geometries. Within the scope of the invention, the term "unit" is understood to mean, in principle, any electronic and / or electrical circuit suitable for the intended purpose. Depending on the requirements, it may be an analog circuit for generating or processing corresponding analog signals. However, it may also be a digital circuit such as an FPGA or a storage medium in conjunction with a program. The program is designed to execute the corresponding process steps or to apply the necessary arithmetic operations of the respective unit. In this context, various electronic units of the dielectric constant measuring device according to the invention can potentially also access a common physical memory or be operated by means of the same physical digital circuit. According to the invention, the design of the filter unit or the filter type is not, in principle, rigidly prescribed. For example, the filter unit for filtering the outgoing signal can include an attenuation filter. The attenuation filter can be based on a T-type attenuator, a Pi-type attenuator, or a variable ohmic resistor. For filtering the outgoing and reflected measurement signals, the filter unit can also, or alternatively, include a high-pass filter. The order of the filter, i.e., the number of capacitive and / or inductive components in the high-pass or attenuation filter, is also not rigidly prescribed within the scope of the invention. As a first-order filter, the high-pass filter can, for example, be based on a single variable capacitor. However, it is also conceivable to implement a higher-order high-pass filter, for example, up to the ninth order. To allow the filter type and / or at least one of the filter parameters to be changed, the variably adjustable filter unit can be designed, for example, as an FPGA or an ASIC, since circuit components, at least in FPGAs, can be flexibly modified, thus enabling the implementation of changing functional blocks. In this context, it is advantageous to design the analysis unit and / or the evaluation unit as part of the FPGA, in addition to the filter unit. Corresponding to the dielectric constant measuring device according to the invention, the problem underlying the invention is also solved by a corresponding method for parameterizing this measuring device. For parameterizing the TDR-based measuring device according to one of the previously described embodiments, the method comprises the following steps: - Coupling the edge-based measurement signal into the measuring probe via the variably adjustable filter unit, - Receiving the reflected edge-based measurement signal, determining the correlation factor between the edge shape of the transmitted and the received measurement signal, determining the SWR ratio, or determining the maximum curvature of the reflected measurement signal, - Changing the at least one filter parameter and / or the filter type of the filter unit if the correlation factor, the SWR ratio, or the maximum curvature does not reach a minimum value.has not converged to the minimum value, and- repeat the preceding procedural steps until the correlation factor, the SWR ratio or the maximum curvature reaches or converges to the minimum value. The filter type and its parameters must be modified according to a predefined optimization algorithm, such as least squares optimization. If the correlation factor is determined, a Pearson correlation or a partial correlation can be implemented as the correlation pattern. The inventive method or measuring device can also be extended to generate a corresponding signal if the shape accuracy (i.e., the correlation factor, the SWR ratio, or the maximum curvature) does not reach the minimum value. Thus, the measuring device can, for example, report to a higher-level unit if it is currently not operational under the given conditions. The invention is explained in more detail with reference to the following figures. Figure 1 shows a measuring device according to the invention for TDR-based determination of the dielectric constant of a medium in a container; Figure 2 shows a schematic block diagram of the measuring device according to the invention; Figure 3 shows a comparison of measurement data between a sufficiently accurate and an irregular edge of the received measurement signal; and Figure 4 shows the process steps of the corresponding method for parameterizing the measuring device according to the invention. For general understanding, Fig. 1 shows a measuring device 1 according to the invention, which serves to determine the dielectric constant of a medium 2 and is based on the TDR measuring principle, as described, for example, in publication EP 0622 628 A2. The medium 2 is located in a suitable container 3. The measuring device 1 can be connected to a higher-level unit 4, such as a process control system, as shown in the embodiment depicted in Fig. 1. An interface such as PROFIBUS, HART, Wireless HART, or Ethernet can be implemented. The dielectric constant DK, for example, can be transmitted via this interface. Other information about the general operating status of the measuring device 1 can also be communicated. To determine the dielectric constant DK of the medium 2, the measuring device 1 is, in the application shown, arranged laterally on a connection of the container 3, e.g., a flange connection. The medium 2 can be liquids such as beverages, paints, cement, or fuels such as liquefied gases or mineral oils. The measuring device 1 can also be used with bulk media 2, such as grain. The housing of the measuring device 1 is attached to the inner wall of the container in a form-fitting manner, with a measuring probe 11 of the measuring device 1 projecting into the interior of the container 3 for determining the dielectric constant DK and thus forming a material-bonded contact with the medium 2. In contrast to the embodiment shown, depending on the type of medium 2, it is also possible that the measuring probe 11 does not project into the interior of the container, but is designed as an integral part of the device housing, so that the measuring probe 11 is arranged approximately planar to the plane of the container wall. In general, the orientation and geometry of the measuring probe 11 depend on the type of medium 2 or on the measuring range of the dielectric constant DK to be determined. Accordingly, the measuring device 1 must be individually adapted to the impedance of the respective measuring probe 11 and any high-frequency cable through which the measuring probe 11 is connected. To avoid the need for manual configuration of the individual measuring device 1 according to the invention, it comprises a filter unit 14, the filter type and corresponding filter parameters of which are variably adapted to the respective measuring probe 11 by an analysis unit 15. The operation of the measuring device 1 according to the invention is explained in more detail below with reference to the schematic circuit diagram in Fig. 2: As shown in Fig. 2, the measuring device 1 comprises a correspondingly designed signal generation unit 12 for generating a measurement signal sHF typical of the TDR principle. For this purpose, the signal generation unit 12 couples the edge-based measurement signal sHF into the measuring probe 11 via the filter unit 14. The generated voltage profile of a positive edge of the measurement signal sHF, typical for TDR, is shown in the left graph of Fig. 3. Accordingly, the voltage of the measurement signal sHF increases almost instantaneously from 0 V to approximately 300 mV within a time window of less than 300 picoseconds. In the measuring probe 11, which is between 1 cm and 10 cm long, the edge of the measurement signal sHF propagates along guided electromagnetic waves at the speed of light c, which depends on the dielectric constant DK of the surrounding medium 2. Here, c0 describes the propagation speed of electromagnetic waves in a vacuum. After reflection at the end of the measuring probe 11 and a corresponding signal propagation time t in the nanosecond range, the edge of the reflected measurement signal rHF is received by an evaluation unit 13 of the measuring instrument 1. If the edge is detected as such, the evaluation unit 13 determines the signal propagation time t between the coupling and reception of the respective edge according to the TDR measurement principle. In this case, an edge of the reflected measurement signal rHF detectable by the evaluation unit 13, which is approximately identical in shape to the edge of the emitted measurement signal sHF, is shown again in the left graph of Fig. 2.Accordingly, the propagation time t corresponds to the time in the left graph of Fig. 3 at which the reflected measurement signal rHF exhibits a corresponding discontinuity (i.e., in the exemplary example, at approximately 3000 picoseconds). Based on the above relationship and the physical law, the evaluation unit 13 can determine the dielectric constant DK of the medium 2 in this case using the measured signal propagation time t. For the edge of the reflected measurement signal rHF to be detectable by the evaluation unit 13, the reflected edge must be approximately shape-conserving with respect to the edge of the emitted measurement signal sHFin. That is, the discontinuity of the reflected edge rHF described above must be present in the form of a defined minimum curvature, as is the case in the left graph of Fig. 3. However, this is not the case for the edge of the reflected measurement signal rHF shown in the right-hand graph of Fig. 3: There, the evaluation unit 13 is unable to detect a corresponding minimum curvature or discontinuity in the measurement signal rHF, because the edge of the reflected measurement signal rHF no longer exhibits sufficient shape fidelity to the emitted measurement signal sHF. This can occur, for example, if the measuring device 1 is used on a new container 3 with a medium 2 whose dielectric constant range to be measured differs significantly from that of the previous location. Gradual wear and tear of the measuring probe 11 during measurement operation and a related change in the probe geometry can also be the cause of the filter parameters R, C or the filter type of the filter unit 14 no longer being suitable to ensure sufficient shape fidelity of the reflected flank.In order to avoid having to manually reconfigure the measuring device 1 in such a case, the filter unit 14 is designed according to the invention so that its filter type or at least one of its filter parameters R, C can be changed. In the embodiment shown in Fig. 2, the filter unit 14 for attenuating the emitted measurement signal sHF comprises a fine attenuation filter 141. In the simplest case, the attenuation filter 141 can be designed, for example, as a variable resistor R, particularly between 0 ohms and 200 ohms, with its control being carried out by an analysis unit 15. To ensure that only the measurement signal sHF emitted to the measurement probe 11 is filtered by the attenuation filter 141, a corresponding signal diverter 16 is arranged between the attenuation filter 141 and the measurement probe 11, which diverts the measurement signal rHF reflected in the measurement probe 11 to the evaluation unit 13. For high-pass filtering of the coupled-in and the reflected measurement signal sHF, rHF, the filter unit also includes an adjustable high-pass filter 142, which is connected upstream of the measurement probe 11. In the simplest case, the high-pass filter 142 can again be designed as a variable capacitor C. For the purpose of controlling the filter unit 14, it is generally advantageous within the scope of the invention to implement the filter unit 14 based on an FPGA, since individual circuit blocks in FPGAs can generally be freely configured. Thus, within the scope of the invention, it is also conceivable to adjust not only the filter parameters R, C of filters 141, 142, but also, for example, the filter type of the attenuation filter 141, depending on what achieves optimal shape fidelity of the reflected edge. In this way, the attenuation filter 141 can be designed, for example, as a T-attenuation element or as a Pi-attenuation element, depending on requirements. The setting of the filter parameters R, C, or any adjustment of the filter type within the filter unit 14, is controlled by the analysis unit 15, as can be seen in Fig. 2. As described above, the shape fidelity of the edge in the reflected measurement signal rHF in relation to the transmitted edge sHF is the crucial target parameter. According to the invention, the analysis unit 15 is therefore designed to assess the shape fidelity accordingly and to adjust the filter unit 14 so that the shape fidelity exhibits at least a defined minimum level or is optimized as much as possible. Within the scope of the invention, the analysis unit 15 can potentially determine the shape fidelity in various ways: On the one hand, a correlation factor between the edge shape of the transmitted measurement signal sHF and the edge shape of the received measurement signal sHF, rHF can be determined.For this purpose, the analysis unit 15 can, for example, apply a Pearson correlation or a partial correlation as a correlation pattern. In addition, it is also possible, as is known from communications engineering, to determine an SWR ratio (acronym for "Standing Wave Ratio") between the transmitted measurement signal sHF and the received measurement signal rHF. Another way to assess shape fidelity is for the analysis unit 15 to detect any discontinuity or maximum curvature in the received measurement signal rHF. The higher the maximum curvature or discontinuity in the received measurement signal rHF, the higher the shape fidelity is rated. The analysis unit 15 can determine the maximum curvature in the reflected measurement signal rHF, even though the received measurement signal rHF is usually in the form of a data series. In this case, the analysis unit 15 can transform the data series into a corresponding function, for example, using splines. The second derivative of such a function then represents the measure of the function's curvature. Accordingly, the location (or the corresponding signal propagation time t) of the maximum of this second derivative corresponds to the location of the maximum curvature. By implementing a suitable algorithm, the analysis unit 15 can, for example, calculate the maximum curvature of the received measurement signal rHF in this way. Based on the determined shape fidelity, for example in the form of the correlation factor, the SWR ratio, or the determined maximum curvature, the analysis unit 15 can adjust the filter type or at least one filter parameter of the filter unit 14 such that the correlation factor or the maximum curvature reaches a minimum value, i.e., a defined minimum shape fidelity of the received edge is achieved. Furthermore, within the scope of the invention, it is also conceivable that the analysis unit 15 optimizes the filter type or at least one filter parameter R, C of the filter unit 14 such that the shape fidelity (for example, again in the form of the correlation factor, the SWR ratio, or the determined maximum curvature) also reaches a maximum value under the given conditions. In this process, the changes to the filter parameters R, C, or...of the filter type, and the subsequent determination of the shape fidelity during optimization is repeated until the correlation factor or the maximum curvature either reaches the required minimum value or converges to it. In this context, the analysis unit 15 or the measuring device 1 can be designed to generate a message if the correlation factor, the SWR ratio, or the maximum curvature does not reach the minimum value, or cannot be reached despite optimization. This can, for example, be communicated to the higher-level unit 4 to indicate that the measuring device 1 is currently not functioning. Within the scope of the optimization process, the algorithm used by the analysis unit 15 to optimize the filter type or the filter parameters R, C of the filter unit 14 is not strictly prescribed by the invention. Least Squares optimization is a suitable optimization method in this context. In summary, the method by which the analysis unit 15 optimally adjusts the filter unit 14 with respect to the shape fidelity of the received measurement signal rHF is illustrated in Fig. 4. Reference symbol list 1 TDR-based dielectric constant meter 2 Medium 3 Container 4 Higher-level unit 11 Measuring probe 12 Signal generation unit 13 Evaluation unit 14 Filter unit 15 Analysis unit 141 Attenuation filter 142 High-pass filter C, R Filter parameters c Propagation speed DK Dielectric constant rHF Reflected measurement signal sHF Edge-based measurement signal t Signal propagation time

Claims

A TDR-based measuring device for measuring the dielectric constant (DC) of a medium (2), comprising: - a measuring probe (11) that can be brought into contact with the medium (2), - a signal generation unit (12) designed to couple an edge-based measurement signal (sHF) into the measuring probe (11) according to the TDR measurement principle, - an evaluation unit (13) designed to: ◯ receive the reflected edge-based measurement signal (rHF) after reflection in the measuring probe (11), ◯ determine a signal propagation time between coupling and reception of the respective edge according to the TDR measurement principle, and determine the dielectric constant (DC) based on the signal propagation time, characterized by: - ​​a filter unit (14) designed to: ◯ filter the emitted and / or the reflected edge-based measurement signal (sHF, rHF), and ◯ its filter type and / or to change at least one filter parameter (R, C), and an analysis unit (15) designed to◯ to determine a correlation factor between the waveform of the transmitted and received measurement signals (sHF, rHF), an SWR ratio, or a maximum curvature of the received measurement signal (rHF), and ◯ to adjust the filter type and / or at least one filter parameter (R, C) of the filter unit (14) such that the correlation factor, the SWR ratio and / or the maximum curvature reaches a minimum value or converges to the minimum value. Measuring device according to claim 1, wherein the filter unit (14) for filtering the outgoing signal (sHF) comprises an attenuation filter (141). Measuring device according to claim 2, wherein the damping filter (141) is based as a filter type on a T-damping element, a Pi-damping element or on a variable ohmic resistor. Measuring device according to claim 1, 2 or 3, wherein the filter unit (14) for filtering the outgoing signal (sHF) and the reflected signal (rHF) comprises a high-pass filter (142). Measuring device according to claim 4, wherein the high-pass filter (142) is based on a controllable capacitor as a filter type. Measuring device according to at least one of the preceding claims, wherein at least the variably adjustable filter unit (14) is designed as an FPGA. A method for parameterizing the measuring device (1) according to one of the preceding claims, comprising the following method steps: - coupling the edge-based measurement signal (sHF) into the measuring probe (11) via the variably adjustable filter unit (14), - receiving the reflected edge-based measurement signal (rHF), determining the correlation factor between the edge shape of the transmitted and the received measurement signal (sHF, rHF), determining the SWR ratio, or determining the maximum curvature of the reflected measurement signal (rHF), - changing the at least one filter parameter and / or the filter type of the filter unit (14) if the correlation factor, the SWR ratio, or the maximum curvature does not reach a minimum value, and - repeating the preceding method steps until the correlation factor, the SWR ratio, or the maximum curvature reaches the minimum value or converges to the minimum value. Method according to claim 7, wherein, if the correlation factor is determined, a Pearson correlation or a partial correlation is used as the correlation pattern. Method according to claim 7 or 8, wherein the at least one filter parameter and / or the filter type are changed according to a predefined optimization algorithm, in particular a "least-squares" optimization. Method according to claims 7 to 9, wherein a signal is generated if the correlation factor, the SWR ratio or the maximum curvature does not reach the minimum value or does not converge towards the minimum value.

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