Capacitive level sensor

Abstract

A capacitive level sensor (10) for measuring the level (H1, H2; H1', ​​H2') of a medium (16) in a container (14), comprising an electrode unit (12) comprising a strip-shaped measuring electrode (22), a strip-shaped counter electrode (24), and a strip-shaped shielding electrode (26, 28, 40), wherein the shielding electrode (26, 28, 40) at least partially surrounds the measuring electrode (22), and a signal processing arrangement (46), characterized in that a first AC voltage source (60) with a predetermined frequency and amplitude is provided, to which the shielding electrode (26, 28, 40) is connected, such that a shielding capacitor (54, 56) formed between the shielding electrode (26, 28, 40) and the measuring electrode (22) 58) has a shielding capacitance that is proportional to the length of the shielding electrode (26, 28, 40), that a second AC voltage source (66) of the same frequency and with a predetermined second amplitude is present,wherein the second amplitude is out of phase with the first amplitude to which the counter-electrode (24) is connected, such that a measuring capacitor (52) formed between the counter-electrode (24) and the measuring electrode (22) has a measuring capacitance that is proportional to the fill level (H1, H2; H1', ​​H2'), that the measuring capacitor (52) and the shielding capacitor (54, 56, 58) are connected to form a capacitive voltage divider, and that the divided measuring electrode voltage (72) applied to the measuring electrode (22) is used to determine the fill level (H1, H2; H1', ​​H2') in the signal processing arrangement (46).

Description

[0001] The invention relates to a stepless capacitive level sensor with an electrode unit according to the preamble of the independent claim. State of the art

[0002] Capacitive level sensors can be used to measure the fill levels of liquid media or solids. In a capacitive level sensor, a measuring impedance is generated, the ohmic component of which, but especially its capacitive component, reflects a measure of the fill level.

[0003] In a simple embodiment of an electrode unit, a measuring electrode is provided which is electrically insulated on the outer wall of a container or in a submersible probe adjacent to a counter electrode.

[0004] German patent application DE 10 2009 017 011 A1 describes a capacitive level sensor that enables the measurement of the fill level of a medium in a container. The capacitive sensor comprises a measuring electrode and a counter electrode, the counter electrode being the electrical circuit ground, which can correspond to earth potential. The two electrodes form a measuring capacitor with the medium as the dielectric. The capacitance of the measuring capacitor depends on the fill level of the medium. The capacitance of the measuring capacitor is determined by comparison with the capacitance of a reference capacitor. Both capacitors are connected to a voltage source via a resistor. To perform the measurement, both capacitors are sequentially short-circuited and thus discharged by means of switches. The voltage rise across the two capacitors after the switches are opened depends on the charging resistances and the capacitances.The fill level could be determined by evaluating the rise time or the time-averaged voltage across the capacitors. However, in the illustrated embodiment, the time-averaged voltages are compared using a comparator. A switching signal is available at the comparator's output, indicating whether the fill level has exceeded or fallen below a certain threshold.

[0005] In one embodiment, the measuring electrode is surrounded on both sides and on the back by a shielding electrode to eliminate electromagnetic interference from the environment. The shielding is an active shield, in which the potential of the shielding electrode is kept at the potential of the measuring electrode. As a result, the capacitance of the capacitor formed by the measuring electrode and the shielding electrode has a value of at least approximately zero.

[0006] Due to an absolute measurement of the capacitance of the measuring capacitor formed by the measuring electrode and the counter electrode, the electrode unit is fixed and must be calibrated in each case with regard to the properties of the medium.

[0007] German patent application DE 199 49 985 A1 discloses a capacitive level sensor that operates using an oscillation method. The operating frequency is in the range of 5–10 MHz. An additional electrode is provided to compensate for the capacitance of the container wall and for any residual electrically conductive medium adhering to the electrodes. The comparatively high operating frequency of up to 10 MHz places correspondingly high demands on the electrical shielding of the capacitive level sensor to comply with EMC regulations. The circuit arrangement for operating the described measuring capacitor requires an absolute reference to ground potential. Therefore, the function of the previously known capacitive level sensor depends on the design of the container in which the medium, whose level is to be measured, is stored.

[0008] German patent application DE 10 2009 002 674 A1 describes a capacitive level sensor in which a measuring electrode is provided, forming the measuring capacitor with an electrical circuit ground as the counter electrode. The measuring capacitor is part of a series resonant circuit whose oscillation frequency depends on the impedance of the medium. The conductivity of the medium influences the quality factor of the resonant circuit containing the measuring capacitor, so that the level of the medium can be determined by evaluating the amplitude and frequency of the oscillation signal. Due to the inclusion of the electrical circuit ground, the previously known method can only be used in immersion probes with a grounded metal housing, whereby the measuring electrode must always be positioned close to the metal housing surface.

[0009] German patent application DE 41 31 582 A1 describes a capacitive level sensor comprising a measuring electrode, a shielding electrode arranged behind the measuring electrode, and a counter electrode, the counter electrode being formed by the metallic container wall. The measuring electrode and the metallic container wall form a measuring capacitor whose capacitance depends on the level of the medium.

[0010] Utility model DE 7138801 U describes a capacitive level sensor with an electrode unit immersed in the medium, comprising a strip-shaped measuring electrode and a strip-shaped counter-electrode. The two electrodes form a measuring capacitor whose capacitance depends on the level of the medium. The measuring and counter-electrodes are arranged side by side on a dielectric container wall that is in contact with the medium on one side. A shielding electrode is arranged on the side of the measuring electrode facing away from the medium. The measuring electrode and the shielding electrode are held at the same potential, so that no electric field and therefore no capacitance can occur between the shielding electrode and the measuring electrode.The measuring capacitor is thus formed exclusively by the counter electrode and the measuring electrode, whereby only the electric field that runs within the medium is effective, but not the electric field occurring between the counter electrode and the shielding electrode. The measurement result is therefore not influenced by the capacitance formed between the counter electrode and the shielding and thus depends, at least approximately, only on the fill level of the medium.

[0011] WO 2006 / 123141 A2 discloses a capacitive position sensor comprising a plurality of electrodes spaced apart along a measuring path. An excitation circuit is provided to generate excitation signals and apply them to some of these electrodes, and a detection circuit is provided to receive and process signals received from other electrodes. The detection circuit includes a circuit for subtracting some of the detection signals from one another to eliminate common-mode signals. In a preferred embodiment, the excitation circuit can be operated to apply the excitation signals to curved electrodes arranged along the measuring path.

[0012] DE 10 2010 030 362 A1 discloses a device for the capacitive determination of the fill level of a liquid in a container and a corresponding method. This device essentially consists of an insulating container, a sensor electrode, a protective electrode, a control unit that supplies the sensor electrode with a control signal, and an evaluation unit that receives a response signal from the sensor electrode and determines the fill level from this signal. The sensor electrode and the protective electrode are either embedded in a wall of the container or applied to an outer surface of the container wall facing away from the liquid. Furthermore, the sensor electrode and the protective electrode extend at least from the height of a minimum fill level to the height of a maximum fill level.Alternatively, the sensor electrode is essentially arranged at a level of a limit fill level to be determined, while the protective electrode is arranged so that it at least partially covers the sensor electrode.

[0013] WO 2012 / 095838 A1 relates to a measuring device and a system that uses it to precisely measure the liquid level in a container, wherein the measuring device is located outside the container and comprises a predetermined number of base blocks with a predetermined geometry, wherein the base blocks comprise at least one pair of capacitors with a predetermined relationship between their capacitances, and wherein a differential change of this relationship along the device indicates the liquid level in the container.

[0014] DE 10 2008 035 635 A1 discloses a device for capacitively measuring the fill level or level of a medium, comprising an elongated sensor for generating an electric field. This sensor has at least two elongated electrodes designed to be positioned outside the medium and capable of generating an electric field that passes through the medium, at least partially. The device is used, for example, for measuring the fill level in plastic containers.

[0015] DE 10 2008 062 302 A1 relates to a method for determining the degree of change in the damping of a load that is part of a sensor arrangement, wherein the load is triggered by a clock generator. A control loop is formed with a controlled system that includes the sensor arrangement, where the controlled variable is the amplitude of the load, which must be kept constant despite damping disturbances. Furthermore, energy is supplied to the load as the manipulated variable to compensate for the damping, and the controlled variable is continuously determined. The instantaneous damping is indirectly extracted as a measured value based on a quantity corresponding to the supplied energy. DE 10 2008 062 302 A1 also relates to an arrangement for carrying out this method.

[0016] The invention is based on the objective of providing a capacitive level sensor which allows for easy adaptation to different level measurement ranges or containers of different heights.

[0017] The problem is solved by the features specified in the independent claim. Disclosure of the invention

[0018] The invention relates to a capacitive level sensor for stepless measurement of the level of a medium in a container, comprising an electrode unit which contains a strip-shaped measuring electrode, a strip-shaped counter electrode and a strip-shaped shielding electrode, wherein the shielding electrode at least partially surrounds the measuring electrode, and with a signal processing arrangement.

[0019] The capacitive level sensor according to the invention is characterized firstly by the fact that a first alternating voltage source with a predetermined frequency and amplitude is available, to which the shielding electrode is connected, so that a shielding capacitor formed between the shielding electrode and the measuring electrode has a shielding capacitance that is proportional to the length of the shielding electrode.

[0020] The capacitive level sensor according to the invention is further characterized by the presence of a second AC voltage source of the same frequency and with a predetermined second amplitude, wherein the second amplitude is out of phase with the first amplitude to which the counter-electrode is connected, such that a measuring capacitor formed between the counter-electrode and the measuring electrode has a measuring capacitance that is proportional to the level. In the capacitive level sensor according to the invention, the measuring capacitor and the shielding capacitor are also connected to form a capacitive voltage divider.

[0021] The split measuring electrode voltage, which can be tapped at the measuring electrode, depends on the ratio of the shielding capacitance to the measuring capacitance and is thus used to determine the fill level in the signal processing arrangement. The measuring electrode voltage, or a signal derived from it, can be output as a level measurement. Alternatively, the measuring electrode voltage can be used in a control system, where a control voltage can be provided as the output signal for a level measurement.

[0022] The capacitive level sensor according to the invention is a highly flexible sensor for the direct and stepless conversion of the fill level of a medium in a container into a corresponding output signal. The output signal can be, for example, an analog voltage in the range of 0–10 V or, for example, an impressed current in the range of 4–20 mA.

[0023] The capacitive level sensor according to the invention is preferably arranged on a non-metallic outer wall of the container. The output signal reflects at all times a measure of the current level of the fill level across the entire measuring range from zero, corresponding to the lower end of the electrode unit, to the maximum value, corresponding to the upper end of the electrode unit.

[0024] A particular advantage of the capacitive level sensor with electrode unit according to the invention lies in the fact that the length of the electrode unit can be individually adjusted to a predetermined level measuring range, corresponding to a predetermined height of the container, by simply cutting it to size. Therefore, the capacitive level sensor according to the invention can be manufactured and supplied inexpensively, for example, as a length by the meter.

[0025] Regardless of the length of the electrode assembly, the output signal always utilizes the same designated electrical range, which lies between the minimum and maximum levels to be measured, provided that the thickness of the container wall and, in particular, the electrical properties of the medium remain at least approximately constant. Thus, without any further modification of a signal processing arrangement, a level measurement range for a container with, for example, a maximum level of 10 cm, or for a tall container with, for example, a maximum level of 100 cm, is distributed across the same output signal range of, for example, 0–10 V or 4–20 mA.

[0026] Both the measuring capacitance of the measuring capacitor and the shielding capacitance of the shielding capacitor change in the same way with the freely selectable length of the electrode assembly and depending on the fill level. Due to the synchronous nature of the two capacitances, the fill-level-dependent ratio of the measuring capacitance to the shielding capacitance remains constant regardless of the freely configurable length of the electrode assembly. Under the above condition, the output signal therefore always traverses the same range of values, regardless of whether the length of the electrode assembly is, for example, only 10 cm or, for example, 100 cm.

[0027] Due to the opposite-phase application of the alternating voltages supplied by the two AC voltage sources to the counter electrode on the one hand and the shielding electrode on the other, the potential of the electric field lines at the geometric midpoint between the measuring electrode and the counter electrode is identical to the ground potential or the circuit ground. The measurement results are therefore independent of the grounding conditions at the container.

[0028] Advantageous further training and development opportunities are each subject to dependent requirements.

[0029] One initial embodiment provides that the shielding electrode is designed as a third, strip-shaped shielding electrode, which is arranged on the rear side of the measuring electrode, facing away from the container, and that the third shielding electrode covers the measuring electrode. This measure not only forms the shielding capacitor but also simultaneously achieves electromagnetic shielding against interference signals from the environment.

[0030] An alternative or additional embodiment provides that the shielding electrode is additionally designed as a first strip-shaped shielding electrode and a second strip-shaped shielding electrode, that the first shielding electrode is arranged next to the measuring electrode on one side and the second shielding electrode next to the measuring electrode on the other side, and that the first, second, and third shielding electrodes are electrically connected to each other. Because the first and second shielding electrodes are arranged in the same plane as the measuring electrode, their assembly is simplified.

[0031] The wall of the container lies within the electric field between the first shielding electrode and the measuring electrode, and between the second shielding electrode and the measuring electrode. The value of the two partial shielding capacitances therefore depends on the dielectric material of the container wall. Increasing the dielectric material of the container wall not only increases the shielding capacitance but also the coupling, and consequently the voltage at the measuring electrode resulting from this coupling. Simultaneously, however, the coupling from the measuring electrode to the medium also increases. In this way, the influence of the container wall material is compensated within certain limits. The same applies to any residue of foaming media adhering to the inner wall of the container, which can occur particularly when the fill level is decreasing.

[0032] A further development of this design provides that the counter electrode, the measuring electrode, and the first and second shielding electrodes are arranged on a carrier layer, which is implemented, for example, as a flexible circuit board.

[0033] According to one embodiment, an insulating layer is provided at least in the area between the third shielding electrode and the measuring electrode, the first shielding electrode, and the second shielding electrode. The insulating layer, which preferably has a low dielectric constant, allows for easy specification of the shielding capacitance during manufacturing, relative to the length of the electrode assembly. Preferably, the insulating layer is implemented as a foam adhesive tape. This allows, in particular, easy adaptation of the electrode assembly to the curvature of the container's outer wall.

[0034] One embodiment provides that the back of the electrode unit, corresponding to the back of the third shielding electrode and the counter electrode, is coated with an insulating protective layer. This protects the electrodes, which are made of copper foil, for example, from environmental influences.

[0035] Another embodiment provides an adhesive layer on the side of the electrode unit facing the container for fixing the electrode unit to the outer wall of the container. The adhesive layer particularly facilitates easy mounting on a curved outer wall of the container.

[0036] Another embodiment of the capacitive level sensor according to the invention provides that the measuring electrode, the counter electrode, and the shielding electrode are directly connected to a circuit board of a first electronic unit, which is arranged directly on the container. The electrodes are soldered directly onto the circuit board. In particular, the electronic unit can include a signal processing arrangement for controlling the electrodes as well as the complete evaluation circuitry, so that an output signal reflecting the level can be provided at the output of the first electronic unit.

[0037] Alternatively, a second electronics unit separate from the electrode unit can be provided.

[0038] One embodiment of the capacitive level sensor according to the invention provides that the second AC voltage source is implemented as an inverter whose input is connected to the first AC voltage source. This measure makes the implementation of the second AC voltage source particularly cost-effective, while simultaneously ensuring the provision of the antiphase AC voltage. The inverter is preferably set to a gain factor of at least approximately one. By changing the gain factor, adaptation to different electrode geometries can be achieved without particular effort.

[0039] Another embodiment provides that the frequency of the AC voltage sources is set to a value between 0.1 MHz and 30 MHz. Selecting frequencies within this range ensures sufficient coupling of the AC voltage from the shielding electrode and the counter electrode to the measuring electrode. Furthermore, the AC voltages in this frequency range can be implemented using simple means. Preferably, the frequency is set to at least approximately 1 MHz.

[0040] One embodiment provides a rectifier for rectifying the measuring electrode voltage occurring at the measuring electrode, whereby the DC voltage present at the output of the rectifier can be used as an output signal, which can be interpreted as a measure of the fill level.

[0041] Due to the expected low capacitances and thus high source impedance of the capacitive level sensor according to the invention, an impedance converter is preferably connected between the measuring electrode and the rectifier, which places only a low load on the measuring electrode and can drive the downstream rectifier with low resistance.

[0042] A particularly advantageous refinement involves implementing the first AC voltage source as a controlled AC voltage source whose output voltage can be varied depending on a control voltage. This control voltage is determined based on the output signal of a comparator, which compares the DC voltage provided by the rectifier with a fixed reference voltage. This creates a closed control loop that keeps the voltage at the measuring electrode constant. In this refinement, the control voltage can be used as the output voltage, reflecting a measure of the fill level. Ultimately, even though the voltage at the measuring electrode is kept constant, this refinement also uses the voltage at the measuring electrode to determine a measure of the fill level.

[0043] Further advantageous developments and embodiments of the capacitive level sensor according to the invention will become apparent from the following description.

[0044] Exemplary embodiments of the invention are shown in the drawing and explained in more detail in the following description. Brief description of the characters Fig. Figure 1 shows a capacitive level sensor according to the invention, which is cut in the vertical direction in the area of ​​an electrode unit, Fig. Figure 2 shows a section in the vertical direction through a capacitive level sensor according to the invention, Fig. Figure 3 shows a cross-section through an electrode unit of a capacitive level sensor according to the invention, Fig. Figure 4 shows a first embodiment of a signal processing arrangement, Fig. 5 shows functional relationships between an output signal of the in Fig. 4 signal processing arrangement and fill levels shown, Fig. Figure 6 shows a second embodiment of a signal processing arrangement and Fig. Figure 7 shows functional relationships between an output signal of the in Fig. 6 signal processing arrangement and fill levels shown. Detailed description of the exemplary implementations

[0045] Fig. Figure 1 shows a capacitive level sensor 10 according to the invention, which is cut in the vertical direction in the area of ​​an electrode unit 12. The capacitive level sensor 10 measures the fill level H1, H2 of a medium 16 located in a container 14 within a fill level measuring range H. In the illustrated embodiment, the medium 16 has a first fill level H1. A possible second fill level H2 is also shown.

[0046] The electrode unit 12 is bonded to the outer wall 20 of the container 14 by means of an adhesive layer 18. Due to the partially cutaway view, a measuring electrode 22, a counter electrode 24, a first shielding electrode 26, and a second shielding electrode 28 are visible. The electrode unit 12 is connected to a first electronic unit 30, which, in the illustrated embodiment, is located at the lower end of the container 14. An output signal 36 is provided via a line 32, which is connected to the first electronic unit 30 by means of a connector 34. This output signal is a measure of the fill level H1, H2, or any fill level occurring within the level measuring range H of the medium 16 in the container 14.

[0047] The in Fig. Figure 2, showing the capacitive level sensor 10 according to the invention, which is cut in the vertical direction in the area of ​​the measuring electrode 22, shows a third shielding electrode 40 arranged on the back of the measuring electrode 22.

[0048] Those in Fig. 2 parts shown, which are connected to the ones in Fig. Parts shown in Figure 1 correspond to each other and bear the same reference symbols. This also applies to the following figures.

[0049] The electrode unit 12 is surrounded, at least on its rear side, by a protective layer 42. The electrodes 22, 24, 26, 28, 40 are contacted, for example by soldering, with a circuit board 44 arranged in the first electronic unit 30. The first electronic unit 30 contains a signal processing arrangement 46.

[0050] Fig. Figure 3 shows a cross-section through the electrode unit 12 of the capacitive level sensor 10 according to the invention. The container wall 20 and the electrode unit 12 are shown in a straight line, so that the capacitive level sensor 10 according to the invention is positioned, for example, on a rectangular container 14. In the case of a cylindrical container 14, the outer wall 20 has a curvature, to which the electrode unit 12 can be easily adapted due to its structure, which is described in detail below.

[0051] The electrode unit 12 is bonded to the non-metallic outer wall 20 of the container 14 by means of the adhesive layer 18. The adhesive layer 18 is applied to the side of a carrier layer 48, preferably a flexible printed circuit board, facing the container 14. This carrier layer preferably contains the counter electrode 24, the first shielding electrode 26, the measuring electrode 22, and the second shielding electrode 28, preferably as conductive traces.

[0052] An insulating layer 50, preferably having a low dielectric constant, is provided on the back side of the measuring electrode 22 and the first and second shielding electrodes 26, 28. The insulating layer 50 is, for example, made of a foam adhesive tape. The insulating layer 50 separates the measuring electrode 22 and the first and second shielding electrodes 26, 28 from the third shielding electrode 40.

[0053] The rear area of ​​the electrode unit 12, in relation to the container 14, is surrounded by a protective layer 42, which protects the electrode unit 12 in particular from environmental influences.

[0054] The complete electrode unit 12 is made of flexible materials, so that the electrode unit 12 can be easily adapted to different outer wall curvatures of cylindrical or oval containers 14.

[0055] In Fig. Figure 3 shows the individual components of the electrode unit 12 significantly enlarged to illustrate the structure. The following dimensions are possible, for example: The widths of the measuring electrode 22 and the first and second shielding electrodes 26, 28 can be, for example, 3.5 mm, while the width of the third shielding electrode 40 can be, for example, between 8 and 13 mm. The width of the counter electrode 24 is, for example, 8 mm. The thickness of the insulating layer 50 is, for example, 1 mm. The thicknesses of the adhesive layer 18, the electrodes 22, 24, 26, 28, 40, and the protective layer 42 are in the micrometer range. The thickness of the carrier material 48, which is, for example, implemented as a flexible circuit board, is, for example, in the upper micrometer range.

[0056] A significant advantage of the design of the capacitive level sensor 10 according to the invention with the electrode unit 12 shown is that the electrode unit 12 can be easily adapted by the user to different level measuring ranges H corresponding to different heights of containers 14, for example by cutting the electrode unit 12 to the required length using scissors. Therefore, the capacitive level sensor 10 according to the invention can, for example, be manufactured and offered as a continuous length.

[0057] The electrodes 22, 26, 28, 40 form a semi-coaxial structure, in which the measuring electrode 22 is comparable to the inner conductor of a coaxial line, which is open to the outer wall 20 of the container 14, but is shielded on the sides by the first and second shielding electrodes 26, 28 and to the rear by the third shielding electrode 40.

[0058] In the Fig. 1 and Fig. In the embodiment shown in Figure 2, the first electronic unit 30 is positioned at the lower end of the electrode unit 12. In another embodiment, not shown in detail, the first electronic unit 30 can be mounted at any height position of the electrode unit 12 and contacted with the electrode unit 12.

[0059] In a further embodiment, instead of the first electronic unit 30, which is directly connected to the electrode unit 12, a second electronic unit (not shown in detail) is provided, which is arranged spatially separate from the electrode unit 12. In this case, the electrode unit 12 is connected to the second electronic unit by a cable with at least 5 conductors, preferably pluggable.

[0060] The circuit board 44, the ends of the electrodes 22, 24, 26, 28, 40 located within a housing of the first electronic unit 30, the signal processing arrangement 46 and the other components of the first electronic unit 30 can be surrounded with a filling material, for example casting resin, so that the first electronic unit 30 is protected in particular from environmental influences.

[0061] Due to the design of the electrode unit 12, a measuring capacitor 52 is formed between the measuring electrode 22 and the counter electrode 24, which has a level-dependent measuring capacitance. The measuring capacitance has a small base value that is linearly dependent on the level measuring range H. In particular, however, the measuring capacitance has a level-dependent value that is proportional to the level H1, H2 of the medium 16 in the container 14.

[0062] A first shielding capacitor 54 is formed between the first shielding electrode 26 and the measuring electrode 22, a second shielding capacitor 56 between the measuring electrode 22 and the second shielding electrode 28, and a third shielding capacitor 58 between the measuring electrode 22 and the third shielding electrode 40. The capacitances of the shielding capacitors 54, 56, and 58 depend exclusively on and are proportional to the level measuring range H, corresponding to the length of the electrode unit 12. The shielding capacitances therefore increase linearly with the length of the electrode unit 12.

[0063] In Fig. Figure 4 shows an embodiment of the signal processing arrangement 46, which is intended for operating the electrode unit 12.

[0064] The signal processing arrangement 46 includes a first AC voltage source 60, which is connected between a circuit ground 62 and the electrically interconnected shielding electrodes 26, 28, 40, corresponding to the shielding capacitors 54, 56, 58. The first AC voltage source 60 provides a first, preferably sinusoidal, AC voltage 64, the frequency of which is, for example, in the range of 0.1 to 30 MHz. Preferably, the frequency of the first AC voltage is set to 1 MHz. The frequency is to be set such that, on the one hand, only a small amount of unwanted signal radiation occurs, and on the other hand, a sufficiently high signal level is present at the measuring electrode 22, considering the comparatively small capacitances involved, which are in the picofarad range.

[0065] Furthermore, a second AC voltage source 66 is provided, which in the illustrated embodiment is implemented as an inverter. The second AC voltage source 66 is connected to the counter electrode 24. The second AC voltage source 66 provides a second AC voltage 68, which has the same frequency as the first AC voltage 64, but is phase-shifted by 180°, i.e., out of phase with the first AC voltage 64.

[0066] If necessary, the amplitude of the first or second AC voltage 64, 68 can be adjusted to adapt at least one AC voltage 64, 68 to different geometries of the electrodes 22, 24, 26, 28, 40. In the illustrated embodiment, it is assumed that the second AC voltage source 66, implemented as an inverter, has a gain factor of 1, so that the amplitude of the first AC voltage 64 is at least approximately equal to the amplitude of the second AC voltage 68.

[0067] The measuring electrode 22 is preferably connected to an impedance converter 70, which places only a small load on the measuring electrode 22, but transmits the measuring electrode voltage 72 occurring at the measuring electrode 22 with low impedance to a downstream rectifier 74. The rectifier 74 provides a DC voltage UDC, which corresponds to the rectified measuring electrode voltage 72.

[0068] The measuring capacitor 52 on the one hand and the parallel shielding capacitors 54, 56, 58 on the other hand form a capacitive voltage divider. A divided, level-dependent measuring electrode voltage 72 appears at the measuring electrode 22. The reference voltage is the sum of the shielding capacitances of the shielding capacitors 54, 56, 58.

[0069] As the fill level of the medium 16 increases, the capacitance of the measuring capacitor 52 increases relative to the constant shielding capacitance of the shielding capacitors 54, 56, 58. The measuring electrode voltage 72 decreases as the fill level H1, H2 of the medium 16 increases, because the voltages are inversely proportional to the capacitances of the capacitors 52, 54, 56, 58.

[0070] In Fig. Figure 4 shows the first alternating voltage 64 and the second alternating voltage 68, each with a constant amplitude, and the measuring electrode voltage 72 with two different amplitudes, whereby the higher amplitude (solid line) occurs at a lower fill level H1 and the lower amplitude (dashed line) occurs at a higher fill level H2.

[0071] The measuring electrode voltage 72 could already be used directly as a measure of the fill level H1, H2, with the highest measuring electrode voltage 72 occurring at the lowest measurable fill level H1, H2 and the lowest measuring electrode voltage 72 occurring at the highest measurable fill level H1, H2. Preferably, however, in this embodiment, the DC voltage UDC is used instead of the measuring electrode voltage 72 directly as a measure of the fill level H1, H2 and is provided as an output signal 36. In this embodiment of the signal processing arrangement 46, the variable measuring electrode voltage 72 is used as a measure for determining the fill level H1, H2 of a medium 16 in a container 14.

[0072] Fig. Figure 5 shows functional relationships between the DC voltage UDC for two different fill level measuring ranges H, H', which are intended for two containers 14 of different heights. The DC voltage UDC corresponds to the output voltage 36 of the capacitive level sensor 10.

[0073] Using additional function blocks not shown, the DC voltage UDC can be converted into a predefined range of the output signal 36. For example, the output signal 36 can be converted to the range of 0–10 V or the range of 4–20 mA and output.

[0074] In the illustrated embodiment, the smallest DC voltage UDC corresponding to the higher fill level H2 is not set to zero. Depending on the desired configuration, the smallest DC voltage UDC can, of course, be set to zero.

[0075] The main advantage of the capacitive level sensor 10 according to the invention is that the electrode unit 12 can be adapted by the user himself to the required level measuring range H, H' by simply shortening the longer electrode unit 12 supplied.

[0076] The in Fig. The functional relationships shown in Figure 5 immediately reveal a further advantage of the capacitive level sensor 10 according to the invention, namely that, without any further action required by the user, each appropriately sized capacitive level sensor 10 provides the same voltage range of the DC voltage UDC or the same voltage range of the output signal 36, depending on the respective level measuring range H, H'. The scaling for different level measuring ranges H, H' is independent of the length of the electrode unit 12. The smaller level measuring range H with the exemplary levels H1, H2 shown utilizes the entire available voltage range of the DC voltage UDC or the output signal 36, just as does the larger level measuring range H' with the exemplary levels H1', ​​H2' shown.For media 16 that are at least approximately the same, no interventions in the signal processing arrangement 46 are required.

[0077] In Fig. Figure 6 shows another embodiment of the signal processing arrangement 46, which provides an output signal 36 that is proportional to the fill level H1, H2 of the medium 16, that is, the output signal 36 also increases as the fill level H1, H2 increases.

[0078] In this embodiment, the first AC voltage source 60 is designed as a controllable first AC voltage source 60, wherein the amplitude of the first AC voltage 64 can be varied by means of a control voltage UR. The amplitude of the first AC voltage 64 is therefore determined by the control voltage UR. In this embodiment, the DC voltage UDC is supplied to a comparator 84, which compares the DC voltage UDC with a reference voltage URef provided by a reference voltage source 86 and provides the control voltage UR depending on the comparison result.

[0079] The reference voltage URef is, for example, 1V. Comparator 84 is implemented, for example, as a high-gain differential amplifier, so that the output signal is proportional to the difference between the DC voltage UDC and the reference voltage URef. Alternatively, a comparator can be used as comparator 84. In this case, it must be ensured that the resulting control loop is sufficiently damped to prevent oscillations.

[0080] The resulting control loop ensures that the first AC voltage 64, and thus the second AC voltage 68, are regulated to an amplitude at which the measuring electrode voltage 72, and consequently the resulting DC voltage UDC, are kept constant, specifically at the value of the reference voltage URef. Fig. Therefore, the first alternating voltage 64 and the second alternating voltage 68 are shown with a high amplitude (solid line), corresponding to a higher fill level H2 and with a lower amplitude (dashed line), corresponding to a lower fill level H1, while the measuring electrode voltage 72 is shown as constant.

[0081] In this embodiment of the signal processing arrangement 46, the control voltage UR can be used as an output signal 36, which is proportional to the fill level H1, H2; H1', ​​H2' of the medium 16 in the container 14. In this embodiment of the signal processing arrangement 46 as well, the measuring electrode voltage 72, which is kept constant in this embodiment, is ultimately used as a measure for determining the fill level H1, H2; H1', ​​H2' of a medium 16 in a container 14.

[0082] In Fig. Figure 7 shows two functional relationships between the control voltage UR and the fill levels H1, H2; H1', ​​H2' for two different fill level measuring ranges H, H'. The control voltage UR corresponds to the output voltage 36 of the capacitive fill level sensor 10.

[0083] Naturally, in this embodiment of the signal processing arrangement 46 according to the invention, the output signal 36 scales to the entire predetermined range of, for example, 0 - 10 V or, for example, 4 - 20 mA without intervention in the signal processing arrangement 46 by the user, regardless of the tailored length of the electrode unit 12 and thus independently of the defined level measuring range H, H'.

[0084] Out of Fig.Figure 7 shows that a low level of the control voltage UR or the output signal 36 corresponds to a low fill level H1, H1', ​​and a higher level of the control voltage UR of the output signal 36 corresponds to a higher fill level H2, H2'. Two different fill levels H1, H2; H1' H2' are shown as examples, which can occur at two different fill level ranges H, H'.

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