Method and device for monitoring the filling level of a medium in a container

Abstract

The invention relates to a method for determining and / or monitoring at least one process variable of a medium (3) in a container (2) having at least one measuring probe (1), which measuring probe (1) has at least two electrodes (4,5), comprising the following method steps: - In a first operating mode (a): applying a first excitation signal (A1) to a first electrode (4) of the measuring probe (1), wherein a second electrode (5) of the measuring probe (1) is used as a receiving electrode, and wherein a first reception signal is received by the measuring probe (1), - In a second operating mode (b): applying a second excitation signal (A2) to the first electrode (4) of the measuring probe (1), wherein the second electrode (5) of the measuring probe (1) is used as a ground electrode, and wherein a second reception signal is received by the measuring probe, and - determining the at least one process variable based on the first (E1) and / or second reception signal (E2). The present invention also relates to a device (V) which is designed for carrying out a method according to the invention.

Description

[0001] Method and device for monitoring the fill level of a medium in a container

[0002] The invention relates to a method for determining and / or monitoring at least one process variable of a medium in a container with at least one measuring probe comprising at least two electrodes, as well as a device for carrying out the method according to the invention.

[0003] The process variable could be, for example, the fill level of the medium in the container, the electrical conductivity of the medium, or the permittivity of the medium. In the case of a fill level measurement, this can involve either continuous fill level determination or the detection of a predefined limit level. The container, in turn, could be a tank or a pipeline, for example.

[0004] Field devices based on the capacitive and / or conductive measuring principle are known per se from the prior art and are manufactured and sold by the applicant in many different designs. Capacitive or conductive measuring devices often have a substantially cylindrical measuring probe with at least one electrode, which is at least partially inserted into a container. On the one hand, rod-shaped measuring probes that extend vertically into the container are widespread, particularly for continuous level measurement. However, for detecting a limit level, measuring probes that can be inserted into the side wall of a respective container, and in particular those that are essentially flush with the front of the container wall, have also become known.

[0005] During measurement, the measuring probe, in particular a transmitting electrode of the measuring probe, is subjected to an excitation signal, usually in the form of an alternating current signal. The respective process variable can then be determined from the response signal received by the measuring probe. According to the capacitive measuring principle, the dependence of the response signal on the capacitance of a first electrode of the measuring probe and the container wall or a capacitor formed by another electrode of the measuring probe is used to determine the respective process variable. Depending on the conductivity of the medium, either the medium itself or insulation of the measuring probe can form the dielectric of this capacitor.

[0006] To evaluate the response signal received from the measuring probe, an apparent current measurement or an admittance measurement is often performed. In an apparent current measurement, the amount of apparent current flowing through the sensor unit is measured. However, the apparent current has an active and a reactive component. Therefore, in the case of an admittance measurement, in addition to the apparent current, the phase angle between the apparent current and the voltage applied to the measuring probe is measured. The additional determination of the phase angle also allows for statements about possible buildup, as disclosed, for example, in DE102004008125A1.

[0007] In contrast, the conductive measuring principle detects whether there is electrical contact between one of the electrodes and the wall of a conductive container or the second electrode via the conductive medium used when applying the conductive measuring principle.

[0008] Field devices in the form of multisensors that can operate in both capacitive and conductive modes are known, for example, from documents DE102011004807A1, DE102013102055A1, and DE102014107927A1. In addition to the process variable fill level, such a multisensor can be used to determine various media-specific properties, such as the electrical conductivity of the medium, or dielectric properties of the medium, such as the dielectric constant. A corresponding sensor is described, for example, in DE102013104781 A1.

[0009] A well-known problem associated with capacitive and / or conductive field devices is the formation of deposits in the area of ​​the measuring probe, which can significantly distort the respective measurement results. To avoid deposits, one can choose the highest possible frequency for the excitation signal, since the distorting influence of deposits generally decreases with increasing frequency of the excitation signal. However, designing the electronics of a corresponding field device for high frequencies is associated with a higher degree of complexity. Furthermore, the additional cost factor for the required components is not negligible.

[0010] Alternatively, it has become known to use an additional electrode, in particular a so-called guard electrode, as described, for example, in DE3212434C2. The guard electrode is arranged coaxially around the respective transmitting electrode and electrically separated from it by insulation. Furthermore, it is at the same potential as the transmitting electrode.

[0011] However, the increase in measurement accuracy achieved by an additional guard electrode depends on the thickness of the buildup layer and on the conductivity of the buildup. Particularly with conductive buildups, resistive components of the buildup dominate at lower frequencies of the excitation signal over the high-ohmic measuring impedance determined from the received signal, which is usually used to determine the respective process variable. Furthermore, the effectiveness of the guard electrode is limited by the comparatively high impedance of the insulation capacitance of the respective measuring probe. Therefore, in principle, the guard electrode cannot achieve constant measurement accuracy independent of the respective medium and its tendency to buildup, if high frequencies for the excitation signal are to be avoided.

[0012] Based on this, the present invention is based on the object of improving the measurement accuracy for corresponding field devices regardless of the medium being examined.

[0013] This object is achieved by the method according to claim 1 and by the device according to claim 15.

[0014] With regard to the method, the object underlying the invention is achieved by a method for determining and / or monitoring at least one process variable of a medium in a container with at least one measuring probe, which measuring probe has at least two electrodes, comprising the following method steps:

[0015] In a first operating mode, applying a first excitation signal to a first electrode of the measuring probe, wherein a second electrode of the measuring probe serves as a receiving electrode, and wherein a first receiving signal is received by the measuring probe,

[0016] In a second operating mode, applying a second excitation signal to the first electrode of the measuring probe, wherein the second electrode of the measuring probe serves as a ground electrode, and wherein a second reception signal is received by the measuring probe, and

[0017] Determining the at least one process variable based on the first and / or second received signal.

[0018] The method according to the invention can be applied to all types of measuring probes which are suitable for the capacitive and / or conductive measuring method.

[0019] According to the invention, the first electrode serves as a transmitting electrode, and the second electrode is used either as a receiving electrode or as a ground electrode. The ground electrode can also be formed by a container wall.

[0020] By implementing two operating modes—a ground-referenced and a non-ground-referenced mode—in a single sensor, it is possible to determine the process variable without the influence of parasitic capacitances. This leads to improved measurement resolution. Furthermore, a significantly more accurate determination of buildup in the area of ​​the measuring probe is possible.

[0021] The solution according to the invention is particularly advantageous in conjunction with measuring probes that are designed separately from the associated electronics. The influence of the connecting cable between the measuring probe and the electronics on the measured value evaluation, for example, drift caused by the connecting cable, is significantly reduced. The connecting cable, for example, only needs to be shielded with ground.

[0022] In one embodiment, the measuring probe is operated in a conductive and a capacitive measuring mode. It is thus a multisensor suitable for both capacitive and conductive process variable determination. Both measuring modes can be performed in the first and / or second operating mode. Both measuring modes can be performed alternately, simultaneously, or at predefined times.

[0023] In a further embodiment, the first and / or second excitation signal is / are composed of two partial signals. A first partial signal can be selected for use with the capacitive measurement mode and a second partial signal for use with the conductive measurement mode. In this context, particular reference is also made to DE102014107927A1, which is incorporated herein by reference and describes the use of an excitation signal composed of two partial signals to implement a capacitive and conductive multi-sensor.

[0024] It is advantageous if the first and / or second excitation signal is / are at least temporarily a square wave signal, a sine wave signal, a triangular wave signal, or a trapezoidal wave signal.

[0025] In one embodiment, the first and second excitation signals may be the same, ie, the same excitation signal is used for the ground-referenced and non-ground-referenced operating modes. However, in other embodiments, different excitation signals may also be used.

[0026] It is also advantageous if the first and second excitation signals are selected so that they have the same arithmetic mean value.

[0027] A preferred embodiment of the method according to the invention includes detecting an impedance between the first and second electrodes based on the first received signal. Advantageously, in the first operating mode, other impedances, for example, between an electrode and ground, are not included in the first received signal. Instead, they can be determined using the second received signal. Thus, the present invention allows for a complete determination of all impedances with a single sensor, which would not be possible using only one of the two operating modes. Among other things, it is this precise characterization using a single sensor that leads to particularly high measurement accuracy and highly precise buildup detection.

[0028] A further advantageous embodiment includes the fact that the presence of a buildup on the measuring probe is determined based on the first and / or second received signal. The buildup determination using the method according to the invention allows for precise determination of the buildup regardless of the medium used, in particular the buildup thickness and the conductivity of the buildup.

[0029] The at least one process variable is, for example, a fill level or limit level of the medium in the container, or a dielectric constant or a conductivity of the medium.

[0030] Numerous variants are conceivable for the method according to the invention. For example, different process variables can be determined in the different operating and / or measurement modes. Furthermore, specific sequences can be defined for the different modes. Accordingly, the embodiments mentioned below are to be viewed as exemplary, possible embodiments, although the invention is by no means limited to the explicitly mentioned variants.

[0031] In one embodiment, if the medium has a conductivity which is below a predeterminable limit value, the process variable is determined based on the first received signal.

[0032] According to a further embodiment, in the event that the medium has a conductivity which is above a predeterminable limit value, the process variable is determined based on the first and second received signals.

[0033] Another embodiment involves the first and second operating modes being carried out alternately.

[0034] In a particularly preferred embodiment, for which the measuring probe has at least three electrodes, the method according to the invention comprises the following

[0035] Method steps: In the first operating mode, applying a first excitation signal to a first electrode of the measuring probe, wherein a second electrode of the measuring probe serves as a receiving electrode, and wherein a first receiving signal is received by the measuring probe,

[0036] In the second operating mode: applying a second excitation signal to the first or second electrode of the measuring probe, wherein a third electrode of the measuring probe serves as a ground electrode, and wherein a second reception signal is received by the measuring probe, and

[0037] Determining the at least one process variable based on the first and / or second received signal.

[0038] In a measuring probe with more than two electrodes, at least partially different electrodes can serve for the two operating modes, in particular as a transmitting electrode, receiving electrode, and / or ground electrode. Furthermore, a wide variety of measuring probe geometries can be used.

[0039] The selection of different electrodes for the two operating modes also allows for the electrode configuration to be tailored specifically to each operating mode. For the first operating mode, the ground-referenced operating mode, the distance between the first and second electrodes can be advantageously reduced, for example, which in turn leads to increased measurement sensitivity. This, in turn, improves measurement performance, especially with regard to media with low electrical conductivity, such as oils.

[0040] It is advantageous if the second electrode serves as a guard electrode, at least in the second operating mode. The guard electrode is then preferably operated at the same potential as the first electrode, i.e., the transmitting electrode.

[0041] The object underlying the invention is further achieved by a device for determining and / or monitoring at least one process variable of a medium in a container with at least one measuring probe, which measuring probe has at least two electrodes, and which device is designed to carry out the method according to the invention according to at least one of the described embodiments.

[0042] The device can be configured, in particular, analogously to the device described in DE102014107927A1, particularly with regard to the electronics. It should be noted that the configurations described in connection with the method according to the invention can also be applied, mutatis mutandis, to the device according to the invention.

[0043] The invention will now be described in more detail with reference to the following figures, Fig. 1 to Fig. 5. It shows:

[0044] Fig. 1 : exemplary schematic representations for a measuring probe for

[0045] Use with the method according to the invention;

[0046] Fig. 2: a block diagram for an electronic system according to the prior art;

[0047] Fig. 3: a sectional view of a measuring probe with two electrodes for

[0048] Explanation of the two operating modes according to the invention;

[0049] Fig. 4: a block diagram for (a) the first and (b) the second operating mode of the method according to the invention; and

[0050] Fig. 5: a block diagram as in Fig. 4b, but with additional offset

[0051] Compensation.

[0052] Fig. 1a shows an exemplary embodiment of a measuring probe 1 of a field device V, by means of which a process variable can be monitored using a capacitive or conductive measuring method. The measuring probe 1 is arranged in a container 2, which is at least partially filled with a medium 3. In this case, it protrudes from the top of the container into the container. However, it goes without saying that in other embodiments, the measuring probe 1 can also be designed so that it is flush with the wall of the container 3.

[0053] In this example, the measuring probe 1 itself consists of two electrodes: a first electrode 4 and a second electrode 5, which serves to prevent the formation of deposits. The container wall also forms a ground electrode 6. The measuring probe 1 is also connected to an electronic unit 8, which is responsible for signal acquisition, evaluation, and / or power supply. In particular, the electronic unit 8 determines the respective process variable based on the received signals.

[0054] Fig. 1 b shows a sectional view of a measuring probe 1 with three electrodes 4,5,6.

[0055] The electrodes 4, 5, and 6 are electrically separated from each other by insulation 7a, 7b and surround each other concentrically. Such a design for a measuring probe 1 is particularly advantageous for a measuring probe 1 that is flush with the container wall.

[0056] Fig. 2 shows a block diagram of an electronics unit 8, by means of which the measuring probe 1 can be operated in both capacitive and conductive modes. However, it should be noted that the present invention is by no means limited to such an electronics unit 8, but that the embodiment shown merely represents one possible example of a suitable electronics unit 8. The electronics unit 8 in Fig. 2 corresponds to the electronics unit described in DE102014107927A1.

[0057] The electronics 8 includes a microcontroller 9 and is divided into a

[0058] Area for generating the transmission signal 10a from different partial signals and an area for evaluating the reception signals 10b dependent on the respective partial signals.

[0059] Two voltage dividers 11, 11a are used to generate a transmission signal in the form of a square wave for the conductive operating mode: a low-impedance voltage divider (R1 / R2) 11 for highly conductive media and a high-impedance voltage divider (R3 / R4) 11a for slightly conductive media. These two voltage dividers 11, 11a are clocked via corresponding port outputs 12, 12a of the microcontroller 9. In the example shown here, a transmission signal for the capacitive operating mode—a triangular voltage generated by the integration amplifier (Block A) 13—is selected via another port output of the microcontroller 12b.

[0060] The area for evaluating the received signals 10, which depend on the respective subsignals, comprises blocks B to D, which all include three operational amplifiers. To minimize the influence of parasitic effects and buildup on the measuring probe 1, the guard technique according to DE00102008043412A1 can also be used.

[0061] Block B 14 contains an amplifier that provides the reference signal, in this case the guard voltage, to the analog-to-digital converter (ADC) 15 of the microcontroller 9. B 14 can also be used to shield at least one circuit board.

[0062] Block e 16 also includes an amplifier, which is responsible for supplying the received signal to the ADC 15. A measuring resistor 17 is also provided, with which the difference between the voltages at the transmitting electrode and the guard electrode can be determined. To evaluate the received signal obtained from the capacitive measurement, block D 18 is also required, which includes a differential amplifier to subtract and amplify the two received signals from the transmitting and guard electrodes. This is done via the measuring resistor 17. The difference between the two received signals is directly proportional to the capacitance at the measuring probe 1. With such electronics 8, a measurement resolution of a few femtofarads is possible.

[0063] In addition, the block diagram in Fig. 2 shows four decoupling capacitors 19, 19a, 19b, and 19c, which filter the DC voltage components from the respective signals. Finally, the ESD protection circuit 20 is shown. Furthermore, offset compensation, not shown here, can also be performed.

[0064] The two operating modes according to the invention can be explained in more detail with reference to Fig. 3. Shown in each case is a sectional view of an electrode as in Fig. 1 a with a transmitting electrode 4, a receiving electrode 5, insulating layers 7a, 7b located therebetween, and a ground electrode formed by the wall of the container 2.

[0065] In the first operating mode shown in Fig. 3a, the non-grounded operating mode, the first electrode 4 serves as the transmitting electrode and the second electrode 5 as the receiving electrode. In this operating mode, the impedance Zi between the transmitting electrode 4 and the receiving electrode 5 can advantageously be determined.

[0066] In the second, ground-referenced operating mode, the second electrode 5 serves as the ground electrode. In this operating mode, the impedance Z2 to ground can be determined. For a measuring probe 1 with more than two electrodes, impedances to ground can be determined for multiple electrodes. Knowing all impedances Z leads to significantly improved measurement accuracy and the ability to reliably detect buildup in the area of ​​the measuring probe 1, independent of the medium 3.

[0067] Fig. 4 shows two exemplary block diagrams suitable for use with a measuring probe 1 with (a) two and (b) three electrodes 4-6. It goes without saying that the method according to the invention can also be implemented with other circuit arrangements. For example, the two block diagrams from Fig. 4 are based on the electronics 8 from Fig. 2 without restriction of generality with regard to area 10b, and switching elements 21 are used for switching back and forth between the first operating mode and the second operating mode, also without restriction of generality. Fig. 4a shows an area 10a for generating the excitation signal A, the electrodes 4 and 5, a substitute shield image for the impedance Zi between the two electrodes 4, 5, and the area 10b for evaluating the received signals E. The area 10b largely corresponds to the corresponding area in Fig. 2.The three switching elements 21a-21c are used to switch between the two operating modes, the ground-referenced operating mode a and the non-ground-referenced operating mode b. The same excitation signal A can be used for both operating modes, or two different excitation signals Ai and A2 can be used for the two operating modes. In either case, two different receive signals Ei, E2 are obtained for the two operating modes, which are illustrated by the reference symbol E in Fig. 4.

[0068] In contrast to Fig. 4a, the measuring probe 1 underlying the block diagram in Fig. 4b comprises three electrodes 4-6 and only a single switching element. In this case, the impedance Z1 between the first 4 and second electrode 5, as well as the impedances Z2 and Z3 of the first 4 and second 5 electrodes to ground, can be determined in the two different operating modes a and b.

[0069] Fig. 5 shows another exemplary block diagram, which corresponds to the block diagram in Fig. 4b, but additionally features an offset compensation 22. The offset compensation 22 serves to compensate for further, possibly existing, in particular constant, parasitic effects or parasitic capacitances between the measuring probe 1 and the container, or between individual electrodes 4-6 of the measuring probe 1. Such measures serve to further improve the measurement resolution.

[0070] By way of example, this unit 22 according to the embodiment of Fig. 5 comprises several capacitors, a resistor, and a switching element 23 for activating and deactivating the offset compensation. It goes without saying that other types of offset compensation 22 are also possible and fall within the scope of the present invention.

[0071] 1 measuring probe

[0072] 2 containers

[0073] 3 Medium

[0074] 4 First electrode

[0075] 5 Second electrode

[0076] 6 Third electrode

[0077] 7 a,b Insulations

[0078] 8 Electronics

[0079] 9 microcontrollers

[0080] 10 a,b Areas for generating the transmitted signal and for evaluating the received signals

[0081] 11 11a Voltage divider

[0082] 12 12a port outputs

[0083] 13 Block A, integration amplifier

[0084] 14 Block B, Amplifier

[0085] 15 analog-to-digital converters (ADC)

[0086] 16 Block C, Amplifier

[0087] 17 Measuring resistor

[0088] 18 Block D, differential amplifier

[0089] 19 19a 19b 19c Decoupling capacitors

[0090] 20 ESD protection circuit

[0091] 21 ac switching elements for switching between operating modes

[0092] 22 Offset compensation

[0093] 23 Switching element

[0094] A Ai, A2 starting signals in the respective operating modes

[0095] E Ei, E2 reception signals in the respective operating modes

[0096] Z Zi, Z2, Z3 impedances a first operating mode b second operating mode

[0097] V device

Claims

Patent claims 1. Method for determining and / or monitoring at least one process parameter of a medium (3) in a container (2) with at least one measuring probe (1), which measuring probe (1) has at least two electrodes (4,5), comprising the following method steps: In a first operating mode (a): applying a first excitation signal (Ai) to a first electrode (4) of the measuring probe (1), wherein a second electrode (5) of the measuring probe (1) serves as a receiving electrode, and wherein a first received signal is received by the measuring probe (1), In a second operating mode (b): applying a second excitation signal (A2) to the first electrode (4) of the measuring probe (1), wherein the second electrode (5) of the measuring probe (1) serves as the ground electrode, and wherein a second received signal is received by the measuring probe, and Determining at least one process variable based on the first (Ei) and / or second received signal (E2).

2. Method according to claim 1, wherein the measuring probe (1) is operated in a conductive and in a capacitive measuring mode.

3. Method according to claim 1 or 2, wherein the first and / or the second excitation signal (Ai, A2) is / are composed of two partial signals.

4. Method according to at least one of the preceding methods, wherein the first and / or second excitation signal (Ai, A2) is / are at least temporarily a square wave, a sine wave, a triangle wave, or a trapezoidal wave.

5. Method according to at least one of the preceding claims, wherein the first and second excitation signals (A, Ai, A2) are the same.

6. Method according to at least one of the preceding claims, wherein the first (Ai) and second excitation signal (A2) are selected such that they have the same arithmetic mean.

7. Method according to at least one of the preceding claims, wherein an impedance between the first (4) and second electrode (5) is detected on the basis of the first received signal (Ei).

8. Method according to at least one of the preceding claims, wherein the presence of an attachment on the measuring probe (1) is inferred from the first (Ei) and / or second received signal (E2).

9. Method according to at least one of the preceding claims, wherein the at least one process variable is a fill level or limit level of the medium (3) in the container (2), or a dielectric constant or a conductivity of the medium (3).

10. Method according to at least one of the preceding claims, wherein, in the case that the medium (3) has a conductivity which is below a predefinable limit value, the process parameter is determined on the basis of the first received signal (Ei).

11. Method according to at least one of claims 1-10, wherein in the case that the medium (3) has a conductivity which is above a predefinable limit value, the process parameter is determined on the basis of the first (Ei) and second received signal (E2).

12. Method according to at least one of the preceding claims, wherein the first (a) and second operating mode (b) are carried out alternately.

13. Method according to at least one of the preceding claims, wherein the measuring probe (1) has at least three electrodes (4-6), comprising the following method steps: In the first operating mode (a): Applying a first excitation signal (Ai) to a first electrode (4) of the measuring probe (1), wherein a second electrode (5) of the measuring probe (1) serves as a receiving electrode, and wherein a first receiving signal (Ei) is received by the measuring probe (1), In the second operating mode (b): Applying a second excitation signal (A2) to the first (4) or second electrode (5) of the measuring probe (1), wherein a third electrode (6) of the measuring probe (1) serves as the ground electrode, and wherein a second received signal (E2) is received by the measuring probe (1), and Determining at least one process variable based on the first (Ei) and / or second received signal (E2).

14. Method according to claim 13, wherein the second electrode (5) serves as a guard electrode in the second operating mode (b). Device (V) for determining and / or monitoring at least one process parameter of a medium (3) in a container (2) with at least one Measuring probe (1), which measuring probe (1) has at least two electrodes (4,5), which device (V) is designed to carry out the method according to at least one of the preceding claims.

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