Impedance point level sensor

Abstract

The present invention relates to an impedance threshold level sensor (100), comprising: a measuring probe (102) capable of being affected by a medium surrounding the measuring probe (102) in a measuring capacitor (110), the measuring probe (102) comprising a measuring electrode (106) and a reference electrode (108) isolated from the measuring electrode (106), a measuring capacitor (110) being formed between the measuring electrode and the reference electrode; a measuring resonant circuit in which the measuring probe (102) as a capacitance determining element is arranged; an electronic unit An electronic unit (101) having a signal generator (103) for exciting the measuring resonant circuit and a signal detector (104) for determining a response signal of the measuring resonant circuit; and a signal processing unit (105) connected to the electronic unit (101) and for generating a measuring signal, wherein the measuring electrode (106) and the reference electrode (108) are designed or arranged such that an increasing filling level of the medium reaches the reference electrode (108) earlier than or at the same time as when it reaches the measuring electrode (106).

Description

Technical Field

[0001] The invention relates to an impedance point level sensor which can provide reliable measurement values ​​in all installation positions and in all measuring containers. Background Art

[0002] Impedance point level sensors, for example for measuring point levels or filling levels, are generally known in the prior art. Typical applications for sensing a predefined filling level are process vessels, for example process tanks, storage tanks, silos or pipelines in the process industry. Impedance point level sensors are often used as limit switches, i.e. to determine whether the filling medium in different liquids and in granular and powdered bulk materials is above or below a certain filling level (i.e. point level).

[0003] Other types of limit switches and point level sensors are also known, which are selected depending on the area of ​​use, the process conditions and the properties of the filling medium. In addition to impedance point level sensors, sensors operating according to the TDR (Time Domain Reflectometry) principle, vibrating point level sensors or capacitively operated sensors are also used. For example, a switching command of a limit switch can start or stop a filling or emptying device in order to prevent the corresponding process container from overflowing or emptying.

[0004] For the sake of simplicity, the terms impedance sensor, limit switch and point level detector are used equivalently in this application instead of the term impedance point level sensor.

[0005] Figure 1 A known impedance sensor 100 is shown.

[0006] Figure 1 A simplified cross-sectional view of a circuit block with an impedance sensor 100 according to the prior art is shown. The impedance sensor 100 according to the prior art consists essentially of an electronic unit 101 and a measuring probe 102. In the exemplary embodiment, the measuring probe 102 is in the form of a series resonant circuit. A measuring capacitor 110 is formed between a measuring electrode 106 and a reference electrode 108, which is connected to a discrete inductor 109 to form a measuring resonant circuit in the form of a series resonant circuit.

[0007] The measuring electrode 106 is rotationally symmetrical with respect to the longitudinal axis A of the impedance sensor 100 and is isolated from the process chamber 90 by an insulator 107. The reference electrode 108 is also rotationally symmetrical with respect to the longitudinal axis A in the impedance sensor 100. In this exemplary embodiment, the reference electrode 108 is in the form of a tube, which simultaneously forms part of the sensor housing. When viewed from the direction of the longitudinal axis A, the measuring electrode 106 is located at the front of the tube and is surrounded by an insulator 107 located on the tube. The insulator 107 simultaneously forms the front end of the housing.

[0008] The discrete inductor 109 is advantageously chosen so as to produce a resonant frequency fres of the resonant circuit between 100 MHz and 200 MHz for a wide range of media and covering states (empty, full and dirty).

[0009] Advantageously, the absolute value |Z| of the frequency-dependent complex-valued impedance of the measurement resonant circuit is analyzed between 100 MHz and 200 MHz, i.e. the measurement resonant circuit is excited by a frequency generator 103 which performs a frequency sweep at a frequency between 100 MHz and 200 MHz, and the response signal (frequency response) of the measurement resonant circuit is detected using a frequency detector 104. If a medium is present in the region of the measurement probe 102, the impedance characteristics of the measurement resonant circuit change, i.e. in particular the resonant frequency fres of the measurement resonant circuit is shifted, at which resonance frequency a minimum impedance is formed.

[0010] Frequency sweep refers to sequential excitation with multiple consecutive frequencies within a frequency range that ideally encompasses all possible resonant frequencies of the measurement resonant circuit.

[0011] The impedance change of the resonant circuit is measured for evaluation and control unit 105. Specifically, the frequency response is evaluated for the frequency change Δf and the amplitude change (also called amplitude change) of the minimum value of the impedance Z, and a switching command is generated therefrom. Alternatively, the evaluation can also be carried out at the maximum value of the impedance Z.

[0012] Figure 2 The frequency response of the impedance sensor 100 according to the prior art is shown by taking ketchup medium as an example.

[0013] A first curve 200 shows the resonance characteristic of a clean measuring probe 102. The absolute value of the impedance Z is shown relative to the frequency f.

[0014] The second curve 201 shows the characteristics of the measuring probe 102 with ketchup adhering thereto, and the curve 202 shows the characteristics of the measuring probe 102 completely covered with ketchup.

[0015] The switching instructions (empty, full) are implemented by the evaluation and control unit 105, and according to the prior art only the minimum of the resonance curve is used for the evaluation. The evaluation is performed for the frequency change Δf and the amplitude change ΔZ. If the minimum of the resonance curve is within the first range I, the evaluation and control unit 105 outputs the switching instruction "empty". However, if the minimum is within the second range II, the switching instruction "full" is output. The two defined switching ranges I, II can be permanently programmed into the impedance sensor 100 at the factory, or set and modified by customer calibration. Since calibration by the customer is very time-consuming and therefore undesirable, ideally, the ranges should be defined so that the standard setting is sufficient to accommodate as many different media as possible.

[0016] Using an impedance sensor 100 known in the prior art, it is observed that Figure 3 The effect shown, i.e. the position and value of the resonance point, depends greatly on the application. This effect is particularly pronounced for non-metallic measuring vessels; in this case, the resonance point is not clearly formed during the transition from the measuring end (first contact with the medium, curve 300) to the housing (contact between the medium and the reference electrode, curve 302). In the area that is critical for the measurement, multiple minima and maxima (curve 301) can be observed, which can lead to switching errors.

[0017] This behavior can be observed in particular in non-metallic measuring vessels and when the impedance sensor is mounted in the measuring vessel from above. Summary of the invention

[0018] The problem to be solved by the present invention is to provide an impedance point level sensor which can provide reliable measurement values ​​in all installation positions and all measurement containers.

[0019] This problem is solved by an impedance point level sensor having the following features. Advantageous improvements are also shown below.

[0020] According to the impedance point level sensor of the present invention, it includes a measuring probe, which can be affected by the medium around the measuring probe in the measuring capacitance, the measuring probe includes a measuring electrode and a reference electrode isolated from the measuring electrode, and a measuring capacitance is formed between the measuring electrode and the reference electrode. The impedance point level sensor has: a measuring resonant circuit, in which the measuring probe as a capacitance determining element is arranged; an electronic unit, which has a signal generator for exciting the measuring resonant circuit and a signal detector for determining a response signal of the measuring resonant circuit; and a signal processing unit, which is connected to the electronic unit and is used to generate a measurement signal; and is characterized in that the measuring electrode and the reference electrode are designed and arranged so that the increased filling level of the medium reaches the reference electrode earlier or at the same time as when it reaches the measuring electrode.

[0021] The above-described design of the measuring electrode and the reference electrode means in particular that in any installation position the increasing filling level reaches the reference electrode earlier than the measuring electrode or at least simultaneously with the measuring electrode.

[0022] In this case, reaching means that the filling level reaches the height at which the respective electrode is located. In particular, this does not mean that the respective electrode must be in contact with the filling medium.

[0023] Since the filling medium comes into contact with the reference electrode at the same time or before the filling level reaches the height of the measuring electrode, a defined influence on the measuring capacitance occurs, so that minimum and maximum values ​​are formed in a defined manner, independently of the installation position and the surrounding measuring vessel.

[0024] In one embodiment, the reference electrode can have an extension in the direction of the measuring electrode. This extension has the advantage that the reference electrode and the measuring electrode can be designed such that, for example, when installed from above in the measuring vessel, they are located at the same height, or the reference electrode even protrudes further into the vessel than the measuring electrode, i.e. an increasing filling level reaches both the measuring electrode and the reference electrode at the same time, or reaches the reference electrode before the measuring electrode.

[0025] The extension can be in the form of a pin at least in some sections, in which case the extension protrudes from the front side of the measuring electrode in the axial direction. A pin-shaped extension has the advantage that it is simple to manufacture and does not cause any notches or edges, which could have a negative effect, for example, if the sensor is used in the food industry.

[0026] In the present application, "in the axial direction" refers to the direction of the longitudinal axis of the impedance sensor. In particular, the axial direction coincides with the axis of symmetry of a rotationally symmetrical measuring electrode. In this way, for example, the measuring electrode can be in the form of a rotating component, which allows simple and cost-effective manufacturing.

[0027] For example, the pin-shaped extension can be screwed or welded to the reference electrode or formed integrally with the reference electrode.

[0028] In one variant, the extension is angled, in which case the first portion is arranged or fixed to the reference electrode and the second portion extends in the axial direction. In this way, the extension can be attached to the reference electrode in the radial direction and thus connected to the reference electrode only at one point instead of linearly. To this end, the extension can be in the form of a bent wire part, for example bent at an angle of 90°.

[0029] Alternatively, the extension may be in the form of a tube which fits onto the measuring electrode in the axial direction. This tube has the advantage that it is circumferentially connected to the reference electrode and also circumferentially surrounds the measuring electrode, thereby maintaining a rotationally symmetrical structure.

[0030] The tube can be connected to the reference electrode in a mechanically detachable and electrically conductive manner, for example by a threaded connection, a plug connection, etc. The extension can therefore only be attached if the installation situation and / or the measuring vessel and / or other conditions require this in order to obtain reliable measuring results.

[0031] In order to ensure a good inflow and outflow of the filling medium, the tube can have a plurality of openings at least at the end on the reference electrode side. This prevents air pockets from forming in the tube in the case of vertical installation from above and preventing further inflow of the medium from below, or prevents the medium from collecting in the tube and no longer flowing out in the case of vertical installation from below.

[0032] In a particularly inconspicuous and compact variant, the extension is in the form of a metallization layer extending in the axial direction. Usually, the measuring electrode is surrounded by an insulator and is thus isolated from the reference electrode and the process chamber. The metallization layer can be applied to this insulator and connected to the reference electrode on the rear side. In this way, the extension of the reference electrode can be realized without any additional attachment components.

[0033] The metallization layer must not be formed over the entire area but should only cover a portion of the insulating layer. Otherwise the formation of the measurement capacitance will be inhibited.

[0034] For example, the metallization layer may be strip-shaped. In this way, the metallization layer may be realized by one or two strips attached to the reference electrode, which extend beyond the plane of the measuring electrode and back to the reference electrode.

[0035] The metallization layer can be applied thinly or arranged in a recess so that the material surrounding the metallization layer produces an uninterrupted surface. In this case, an uninterrupted surface refers to a surface without gaps and edges, so that it can still be used in the food industry.

[0036] The present invention will be described in detail below based on exemplary embodiments and with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 A simplified cross-sectional view of an impedance sensor according to the prior art is shown (illustrated).

[0038] Figure 2 Shown according to Figure 1 The desired impedance characteristics of the impedance sensor (described above).

[0039] Figure 3 Shown according to Figure 1 The impedance characteristics of the impedance sensor in a plastic tank (illustrated).

[0040] Figure 4 A first exemplary embodiment of an impedance sensor according to the present application is shown.

[0041] Figure 5 A second exemplary embodiment of an impedance sensor according to the present application is shown.

[0042] Figure 6 A third exemplary embodiment of an impedance sensor according to the present application is shown. DETAILED DESCRIPTION

[0043] Unless otherwise specified, the same reference numerals in the drawings refer to the same components having the same function.

[0044] To simplify the drawings, Figures 4 to 6 The electronic unit 101 and the signal processing unit 105 are not shown in the drawings. However, as in the prior art, they are included in all exemplary embodiments.

[0045] Figure 4 A first exemplary embodiment of an impedance point level sensor 100 according to the present application is shown. In the exemplary embodiment shown, the impedance point level sensor 100 is shown in a simplified cross-sectional view, as used in the prior art.

[0046] according to Figure 4 The impedance sensor 100 has a measuring probe 102, which in the exemplary embodiment is formed by a measuring electrode 106 oriented in the axial direction A and rotationally symmetrical, and a reference electrode 108 also oriented in the axial direction A and rotationally symmetrical, between which a measuring capacitor 110 and a discrete inductor 109 are formed. In the exemplary embodiment, the reference electrode 108 is in the form of a tubular housing of the impedance point level sensor 100, which has a tubular extension 112. Figure 4 In the exemplary embodiment of FIG. 1 , the extension 112 is in the form of a tube 118 that is integrally formed with the reference electrode 108. The measuring electrode 106 is separated and electrically insulated from the reference electrode 108 and the process chamber 90 by an insulator 107.

[0047] At the end of the extension 112 facing the reference electrode 108, the tube 118 has at least one opening 120 or a plurality of openings 120 extending in the radial direction, and when the impedance point level sensor 100 is installed vertically in the container from above, the opening prevents air from being enclosed in the cavity formed between the extension 112 and the insulator 107, which would prevent the process medium from entering the cavity. When the impedance point level sensor 100 is installed vertically in the container from below, the medium collected in the cavity can flow out through the opening 120, and thus measurement errors are also avoided here.

[0048] exist Figure 4 In the exemplary embodiment shown, a measuring capacitor 110 is formed essentially between a tube 118 electrically conductively connected to a reference electrode 108 and a measuring electrode 106, which is arranged in a series resonant circuit consisting of an inductor 109 and the measuring capacitor 110. A medium entering the gap between the reference electrode 108 and the measuring electrode 106 acts as a dielectric and changes the value of the measuring capacitor 110.

[0049] The decisive factor that has a positive influence when measuring with the impedance point level sensor 100 shown here is that the extension 112 or the tube 118 is designed and arranged so that in any installation position, the filling level of the medium to be measured in the process chamber 90 reaches the reference electrode 108 or the extension 118 before reaching the measuring electrode 106.

[0050] Figure 5 An alternative embodiment of an impedance point level sensor 100 having an extension 112 is shown. Figure 5 Impedance point level sensor 100 with Figure 4 The impedance point level sensor 100 differs only in the design of the extension 112, so the rest of the device will not be described in detail here.

[0051] exist Figure 5 In the exemplary embodiment of the present invention, the extension 112 is in the form of a pin-shaped extension 112 of the reference electrode 108. For example, a first portion 114 of the extension 112 is attached to the reference electrode 108 in the radial direction and is welded thereto. A second portion 116 of the extension 112 is formed integrally with the first portion 114 and extends in the axial direction A. The second portion 116 is longer so that the extension 112 protrudes in the axial direction beyond the front side of the measuring electrode 106, thereby ensuring that in the installation position shown here, the increasing filling level first contacts the extension 112 of the reference electrode 108 and that the filling level reaches the measuring electrode 106 at a later point in time.

[0052] Also in this exemplary embodiment, the main part of measurement capacitance 110 is formed between extension 112 (here mainly second part 116 of extension 112) and measurement electrode 106. However, a second part of measurement capacitance 110 will also be formed between measurement electrode 106 and reference electrode 108.

[0053] Figure 5 The advantage of the structure shown is that the number of notches and edges is smaller and therefore the structure can be used in the food industry.

[0054] Figure 6 A third exemplary embodiment of an impedance point level sensor 100 according to the present application is shown. The impedance point level sensor 100 is shown in a perspective view; in this figure, the impedance point level sensor 100 is shown from an oblique front. The rotationally symmetrical structure of the impedance point level sensor 100 can be seen particularly clearly.

[0055] exist Figure 6 In the exemplary embodiment shown, the extension 112 of the reference electrode 108 is in the form of a strip of an L-shaped metallization layer 130 applied to the insulator 107. In the exemplary embodiment shown, the metallization layer 130 is in the form of a narrow strip, the width of which is approximately 0.5 to 10 mm. The embodiment shown thus functions without additional attachment components and, due to the metallization layer, a particularly compact and very low-cost extension 112 of the reference electrode 108 can be realized. In the exemplary embodiment, the metallization layer 130 has a thickness of 5-1000 μm.

[0056] Figure 6 The illustrated embodiment also ensures that an increasing filling level always first reaches the reference electrode 108 or the extension 112 electrically connected thereto, regardless of the installation position of the impedance point level sensor 101 .

[0057] For example, the metallization layer 130 may also be arranged in a recess in the insulator 107 , so that no step is formed between the insulator 107 and the metallization layer 130 .

[0058] 100 Impedance Point Level Sensor

[0059] 101 Electronic Unit

[0060] 102 Measuring probe

[0061] 103 Signal Generator

[0062] 104 Signal Detector

[0063] 105 Signal Processing Unit

[0064] 106 Measuring electrodes

[0065] 107 Insulator

[0066] 108 Reference electrode

[0067] 109 Inductor

[0068] 110 Measuring capacitance

[0069] 112 Extension

[0070] 114 Part 1

[0071] 116 Part 2

[0072] 118 pipe

[0073] 120 Opening

[0074] 130 Metallization layer

[0075] A Axial direction

Claims

1. An impedance point level sensor (100), comprising: - a measuring probe (102) capable of being influenced by a medium surrounding the measuring probe (102) in a measuring capacitance (110), the measuring probe (102) comprising a measuring electrode (106) and a reference electrode (108) isolated from the measuring electrode (106), a measuring capacitance (110) being formed between the measuring electrode and the reference electrode, - a measuring resonant circuit in which the measuring probe (102) is arranged as a capacitance determining element, - an electronic unit (101) having a signal generator (103) for exciting the measuring resonant circuit and a signal detector (104) for determining a response signal of the measuring resonant circuit, - a signal processing unit (105) connected to the electronic unit (101) and used to generate a measurement signal, and It is characterized in that The measuring electrode (106) and the reference electrode (108) are designed and arranged such that an increasing filling level of the medium reaches the reference electrode (108) earlier than when it reaches the measuring electrode (106), The reference electrode (108) has an extension (112) in the direction of the measuring electrode.

2. The impedance point level sensor (100) according to claim 1, It is characterized in that The extension (112) is in the form of a pin at least in some sections, wherein the extension (112) projects beyond the front side of the measuring electrode (106) when viewed in the axial direction (A).

3. The impedance point level sensor (100) according to claim 2, It is characterized in that The extension (112) is angled, wherein a first portion (114) is arranged or fixed to the reference electrode (108) and a second portion (116) extends in the axial direction (A).

4. The impedance point level sensor (100) according to claim 1, It is characterized in that The extension (112) is in the form of a tube (118) which is mounted on the measuring electrode in an axial direction (A).

5. The impedance point level sensor (100) according to claim 4, It is characterized in that The tube is removably connected to the reference electrode (108).

6. The impedance point level sensor (100) according to claim 4 or 5, It is characterized in that The tube (118) has one or more openings (120) at least at the end on the reference electrode side.

7. The impedance point level sensor (100) according to claim 1, It is characterized in that The extension (112) is in the form of a metallization layer (130) extending in the axial direction (A).

8. The impedance point level sensor (100) according to claim 7, It is characterized in that The metallization layer (130) is in a strip shape.

9. The impedance point level sensor (100) according to any one of claims 7 to 8, It is characterized in that The metallization layer (130) is located in the recess so that an uninterrupted surface is produced with the material surrounding the metallization layer (130).

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