Vibration limit switch and cable for vibration limit switch

Abstract

The invention relates to a vibration limit switch for measuring the filling level of a container filled with a measuring medium, wherein the vibration limit switch comprises: a first sensor housing having an electronic unit; a second sensor housing having a piezoelectric element for generating and receiving vibrations; and at least one electrical cable having an electrical conductor for electrically conductively connecting the electronic unit to the piezoelectric element, the second sensor housing being designed to receive the electrical cable along the end section, and the piezoelectric element having a connector for receiving the electrical cable. The invention also relates to a cable for such a vibration limit switch, wherein the cable has an electrical conductor configured to electrically conductively connect the electronic unit to the piezoelectric element of the vibration limit switch. In order to provide a vibration limit switch and a cable for such a vibration limit switch, the design of which meets the explosion-proof requirements and thus meets the relevant explosion-proof regulations, according to the invention, the cable has a current-insulating sheath, the sheath has a sheath thickness meeting the explosion-proof requirement along a section between the first and the second sensor housing and along a first sub-region of the end section, and has a reduced sheath thickness along a second sub-region of the end section, the reduced sheath thickness allowing an electrically conductive plug connection between the cable and the connector of the piezoelectric element.

Description

TECHNICAL FIELD

[0001] The invention relates to a vibration limit switch for measuring the fill level of a container filled with a measuring medium, wherein the vibration limit switch comprises a first sensor housing having an electronic unit, a second sensor housing having a piezoelectric element for generating and receiving vibrations, and at least one electrical cable having electrical conductors for electrically connecting the electronic unit with the piezoelectric element, wherein the second sensor housing is configured for accommodating the electrical cable along an end section, and the piezoelectric element has a connector for accommodating the electrical cable.

[0002] Furthermore, the invention relates to an electrical cable for such a vibration limit switch, wherein the electrical cable has electrical conductors configured for electrically connecting an electronic unit with a piezoelectric element of a vibration limit switch. BACKGROUND

[0003] Vibration limit switches of the above-mentioned type are known in the prior art and are configured for determining the fill level of a container, for example a common container or a silo, filled with a measuring medium, for example a liquid or a bulk material. For this purpose, the piezoelectric element generates mechanical vibrations, which are transferred to a mechanical vibrator. The mechanical vibrator is thereby excited and vibrates at its natural frequency, which depends in particular on the immersion depth of the mechanical vibrator in the measuring medium. The natural vibrations of the mechanical vibrator are transferred in the opposite direction to the piezoelectric element, which sends a frequency-dependent signal to the electronic unit. For this purpose, the piezoelectric element is connected to the electronic unit, for example by means of flexible conductors, which can be designed as a calibration circuit board or can have a calibration circuit board. In the electronic unit, the frequency-dependent input signal is evaluated, for example taking into account calibration measurements, so that the fill level of the container is determined.

[0004] Vibration limit switches of the above-mentioned type are known, for example, from EP 2 503 860 A1 and EP 3 542 405 B1.

[0005] The known vibration limit switches cannot be used in potentially explosive environments without adaptation of the individual components. In particular, the electrical cable connecting the electronic unit and the piezoelectric element must have a current-insulated sheath which complies with the relevant explosion protection conditions / criteria and is therefore designed in accordance with the explosion protection requirements. A distance of at least 0.5 mm must always be maintained between the current-carrying conductor and the vibration limit switch housing. The current-insulated sheath of the electrical cable must be designed in accordance with the explosion protection requirements such that a distance of 0.5 mm is always maintained between the current-carrying conductor and the vibration limit switch housing. Usually, an electrical cable having a continuous insulation layer and a current-carrying conductor sheath thickness of at least 0.5 mm is used.

[0006] In the known vibration limit switches of the prior art, cables with an insulation layer which complies with the explosion protection requirements cannot simply be installed, since the connectors on the piezoelectric element are not designed for an electrically conductive connection of such thick sheathed cables. However, any redesign of the connectors to accommodate such cables would introduce a relatively large mass into the piezoelectric element, in particular a mass of more than 2.5 g, which would influence the generation and reception of the vibrations due to inertia. As a result, in the case of cables with a relatively thick current insulation sheath, measurement errors would occur, which in turn would lead to incorrect determination of the filling level of a container with a measurement medium. SUMMARY

[0007] It is therefore an object of the present application to provide a vibration limit switch which is designed in compliance with the explosion protection requirements and thus meets the relevant explosion protection regulations, and a cable for the vibration limit switch. The vibration limit switch should be designed in a cost-effective manner and should minimize structural changes compared to existing vibration limit switches which do not comply with the explosion protection requirements.

[0008] This object is achieved by a vibration limit switch according to claim 1 and a cable according to claim 10.

[0009] According to the application, the cable has a current insulation sheath which has an explosion protection compliant sheath thickness along the section between the first and second sensor housing and along the first subregion of the end section, and has a reduced sheath thickness along the second subregion of the end section, which allows an electrically conductive plug connection between the cable and the connector of the piezoelectric element. In other words, within the end section of the cable, the sheath thickness of the current insulation sheath is reduced from the explosion protection compliant sheath thickness to an insertion diameter which allows the cable to be inserted into the connector.

[0010] By reducing the sheath thickness of the current insulation sheath, the cable has a geometry at its end section which enables a connection to the connector which is installed in the usual piezoelectric element of the known vibration limit switch. Thus, a commercially available standard plug system can be used, which is available at low cost due to the high production volume in other application areas. Since the plug system has to be developed and certified by the manufacturer, it is not economically viable to develop a plug system specifically for small batch installations. The minimum distance between the electrical conductor and the housing which complies with the explosion protection requirements should remain gapless. In the first subregion of the end section, the cable is supported on the housing by the sheath which is designed in compliance with the explosion protection requirements there, so that the distance in this region is not below the required minimum distance. In the second subregion of the end section, the spacing is produced by the cable being accommodated in the connector of the piezoelectric element, which is arranged in such a way that it has a corresponding spacing to the housing. Overall, a situation below the minimum distance is prevented.

[0011] Furthermore, since the connector is of sufficiently small mass due to the tapering of the cable in the end section, measurement errors due to inertia of the connector when transmitting and receiving vibrations are effectively avoided.

[0012] The first and second sensor housings can be mechanically connected to each other by means of a support cable or support tube, or can be merged into each other to form a unified compact housing.

[0013] Thus, by virtue of the embodiment according to the application, a vibration limit switch is obtained which complies with the explosion protection requirements and which, in comparison with known vibration limit switches, has the smallest structural modifications and can therefore also be produced at lower cost.

[0014] Advantageous refinements of the application will be explained in the following and in the dependent claims.

[0015] Preferably, the current-insulating sheath of the cable has a sheath thickness of at least 0.5 mm in the section having the sheath thickness complying with the explosion protection requirements. With this sheath thickness, there is sufficient spacing between the conductor and the (electrically conductive) housing so that the explosion protection requirements are met.

[0016] According to an advantageous refinement, the current-insulating sheath of the cable has a sheath thickness of less than 0.5 mm, in particular between 0.1 mm and 0.15 mm, along the second subregion of the end section. With this sheath thickness, the cable can be inserted into a conventional connector, in particular into a conventional connector with insulation displacement contact (Schneidklemmkontaktierung) which is commonly used in piezoelectric elements. In this case, the current-insulating sheath does not have to be completely removed.

[0017] Preferably, the length of the second subregion is greater than the depth of the piezoelectric element connector, so that in the installed state of the cable, there is a gap between the cable and the second sensor housing in the section between the piezoelectric element and the first subregion of the end region. In this region or along this section, an air gap can occur between the cable and the housing.

[0018] Preferably, the sensor housing comprises a resonator and a diaphragm which is configured for transmitting vibrations from the piezoelectric element to the resonator and from the resonator to the piezoelectric element. To this end, the piezoelectric element can be welded or form-locked connected to the diaphragm in a known manner.

[0019] In the prescribed use of the vibration limit switch, the resonator oscillates at its natural frequency, which depends on the immersion depth of the resonator in the measuring medium. Here, the resonator is preferably designed as a vibrating fork. Independently thereof, the diaphragm is preferably connected to the resonator by means of a material bond, which facilitates virtually loss-free transmission of the vibrations.

[0020] In order to establish an electrically conductive connection between the cable and the connector of the piezoelectric element, the connector preferably has an insulation displacement terminal (Schneidklemme) having at least one cutting edge which, in the installed state, can penetrate the cable jacket and come into contact with the electrical conductor. The insulation displacement terminal is preferably designed as a detachable connection element. At first installation, the cable is inserted into the connector and the insulation displacement terminal is closed. In this case, the insulation displacement terminal cuts open the current-insulating jacket of the cable with the at least one cutting edge and forms an electrically conductive contact with the conductor. In this respect, it is also advantageous to reduce the jacket thickness of the current-insulating jacket in the region of the connector, since conventional insulation displacement terminals are designed only for cutting open cables whose current-insulating jacket has a relatively small thickness. For explosion-protected cables whose continuous jacket thickness is at least 0.5 mm, the jacket thickness is too great for the insulation displacement terminal to be able to contact the conductor when cutting open the jacket and thus to form an electrically conductive contact.

[0021] According to an advantageous refinement of the application, the electrical conductor of the cable is configured as a stranded wire, a solid wire or a bundle of several stranded wires. Such conductor structures are simple and have proven to be interference-resistant conductors. BRIEF DESCRIPTION OF DRAWINGS

[0022] A specific embodiment of the application is described with reference to the drawings.

[0023] Figures la-lc A schematic diagram of a vibration limit switch is shown.

[0024] Figure 2 A cross-sectional view of a cable is shown.

[0025] Figure 3 A detailed view of the cable installation position in a schematic cross-sectional view is shown. DETAILED DESCRIPTION

[0026] Figure la A vibration limit switch 1 is shown in a schematic cross-sectional view, which is configured for measuring the fill level of a container 2 filled with a measuring medium 3, in the exemplary embodiment shown the measuring medium is a liquid 31. The vibration limit switch 1 comprises a first sensor housing 41 having an electronic unit 5, a second sensor housing 42 having a piezoelectric element 6 for generating and receiving vibrations, and an electrical cable 7 electrically conductively connecting the electronic unit 5 with the piezoelectric element 6, for which purpose the piezoelectric element 6 has a connector 8. The piezoelectric element 6 is bonded to a diaphragm 9, which is connected to a resonator 10 in the form of a vibration fork 101. By means of an oriented or non-oriented vibration of the piezoelectric element 6, the vibrations are transmitted to the resonator 10 via the diaphragm 9, which is thereby excited and vibrates at its natural frequency. The natural frequency of the resonator 10 depends not only on the geometry and material of the resonator 10, but also on the immersion depth T of the resonator 10 in the measuring medium 3 EThe vibrations generated by the resonator 10 at its natural frequency are transmitted to the piezoelectric element 6, which generates a frequency-dependent signal and transmits this signal to the electronic unit 5. Here, the level T of the container 3 is determined from the known installation position of the second sensor housing 42 or the resonator 10 and the calibration measurement results F The cable 7 of the vibration limit switch 1 comprises a plurality of sections, namely a section 11 extending between the first sensor housing 41 and the second sensor housing 42, and an end section 12 accommodated by the second sensor housing 42. The end section 12 in turn comprises a plurality of regions, which will be referred to in more detail Figure 2 and Figure 3 will be explained in more detail.

[0027] In the vibration limit switch shown in Figure la , the cable 7 is configured as a support cable. Alternatively, a support tube 19 can be arranged between the first sensor housing 41 and the second sensor housing 42, in which the cable 7 extends. Furthermore, according to Figure lc , a further alternative embodiment is provided, according to which the first sensor housing 41 and the second sensor housing 42 are connected to one another such that they form a compact sensor housing 43 which completely accommodates the cable 7.

[0028] In order to make the design of the vibration limit switch 1 comply with the explosion protection requirements, the cable 7 has a geometry as shown in detail in Figure 2 and Figure 3 .

[0029] The cable 7 has an electrical conductor 13 composed of a wire bundle consisting of a plurality of strands 131, each strand having an individual and common current-insulating sheath 14 with a sheath thickness D M . In the section between the first and second sensor housings 41, 42, and at a first sub-region 15 along the length L l of the end section 12, the common current-insulating sheath 14 has a sheath thickness D M1 complying with the explosion protection requirements, so that sufficient insulation distances are provided. Thus, in particular within the end section 12, sufficient spacing can be formed between the strands 131 and the second sensor housing 42. At a second sub-region 16 along the length L2 of the end section 12, the cable 7 has a reduced sheath thickness D M2 , so that the diameter of the cable 7 is reduced to an insertion diameter D EThe insertion diameter allows for a plug-in connection with the connector 8. Inside the connector 8, the cable 7 is fixed by an insulation displacement terminal 17 having two cutting blades 171 which partially cut through the current insulation sheath 14 of the cable 7 and form an electrically conductive contact with the strands 131. Between the first and second sub-areas 15, 16 of the end section, a third sub-area 18 of the cable is formed having a length L3 in which the sheath thickness D M From the sheath thickness D M1 Tapers conically to the sheath thickness D M2 In the exemplary embodiment shown, the length L2 of the second sub-area is greater than the depth Ts of the connector 8 of the piezoelectric element 6, so that in the installed state of the cable 7, a clearance is formed between the cable 7 and the second sensor housing 42 in the sub-areas 16, 18 between the piezoelectric element and the first sub-area 15 of the end section 12. The length of this region can be adjusted by selecting the respective lengths L2 and L3 of the second and third sub-areas 16 and 18. The region 16 having the length L2 should not be selected to be too long in order to ensure that the insulation distance requirements are met.

[0030] List of reference signs 1 Vibration limit switch 2 Container 3 Measuring medium 31 Liquid 41 First sensor housing 42 Second sensor housing 43 Compact sensor housing 5 Electronic unit 6 Piezoelectric element 7 Cable 8 Connector 9 Diaphragm 10 Resonator 101 Vibration fork 11 Section between the first and second sensor housing 12 End section 13 Conductor 131 Strands 14 Sheath 15 First sub-area of the end section 16 Second sub-area of the end section 17 Insulation displacement terminal 171 Cutting blade 18 Third sub-area of the end section 19 Support tube D E Insertion diameter DM Sheath thickness D M1 Sheath thickness to meet explosion proof requirements D M2 Reduced sheath thickness T E Immersion depth Ts connector depth T F Level L l、2、3 Length of each sub-region of the end section

Claims

1. A vibration limit switch (1) for measuring the filling level of a container (2) containing a measuring medium (3, 31), wherein, The vibration limit switch (1) comprises: a first sensor housing (41) having an electronic unit (5); a second sensor housing (42) having a piezoelectric element (6) for generating and receiving vibrations; and at least one cable (7) having an electrical conductor (13) for electrically connecting the electronic unit (5) to the piezoelectric element (6); wherein the second sensor housing (42) is configured to accommodate the cable (7) along an end section (12), and the piezoelectric element (6) has a connector (8) for accommodating the cable (7); characterized in that the cable (7) has a current-insulating sheath (14) having a sheath thickness (D) that meets explosion-proof requirements along a section (11) between the first sensor housing (41) and the second sensor housing (42) and along a first sub-region (15) of the end section (12). M1 ), and along the second sub-region (16) of the end section (12) has a reduced sheath thickness (D) M2 The reduced sheath thickness (D) M2 This allows for a conductive plug-in connection between the cable (7) and the connector (8) of the piezoelectric element (6).

2. The vibration limit switch (1) according to claim 1, characterized in that, The current-insulating sheath (14) of the cable (7) has a sheath thickness (D) that meets explosion-proof requirements. M1 In sections (11, 15), the sheath thickness is at least 0.5 mm (D). M ).

3. The vibration limit switch (1) according to claim 1 or 2, characterized in that, The current insulation sheath (14) of the cable (7) along the second sub-region (16) of the end section (12) has a sheath thickness (D) M2 () Less than 0.5mm, especially between 0.1mm and 0.15mm.

4. The vibration limit switch (1) according to any one of claims 1 to 3, characterized in that, The length of the second sub-region (16) is greater than the depth (Ts) of the connector (8) of the piezoelectric element (6), such that in the installed state of the cable (7), there is a gap between the cable (7) and the second sensor housing (42) in the section between the piezoelectric element (6) and the first sub-region (15) of the end section (12).

5. The vibration limit switch (1) according to any one of claims 1 to 3, characterized in that, The second sensor housing (42) includes a resonator (10) and a diaphragm (9) configured to transmit vibrations from the piezoelectric element (6) to the resonator (10) and to transmit vibrations from the resonator (10) to the piezoelectric element (6).

6. The vibration limit switch (1) according to claim 4, characterized in that, The resonator (10) vibrates at a natural frequency, which depends on the immersion depth (T) of the resonator (10) in the measuring medium (3). E ).

7. The vibration limit switch (1) according to claim 5, characterized in that, The resonator (10) is configured as a vibrating fork (101).

8. The vibration limit switch (1) according to any one of claims 1 to 6, characterized in that, The connector (8) of the piezoelectric element (6) has an insulating displacement terminal (17) for conductive connection with the cable (7), the insulating displacement terminal (17) having at least one cutting edge (171) that penetrates the sheath (14) of the cable (7) and contacts the electrical conductor (13) in the installed state.

9. The vibration limit switch (1) according to any one of claims 1 to 5, characterized in that, The electrical conductor (13) of the cable (7) is constructed as a stranded wire (131), a solid wire, or a bundle of stranded wire (131).

10. A cable (7) for a vibration limit switch (1), the vibration limit switch being designed according to any one of claims 1 to 9, wherein, The cable (7) has an electrical conductor (13) configured to electrically connect the electronic unit (5) to the piezoelectric element (6) of the vibration limit switch (1), characterized in that the cable (7) has a current-insulating sheath (14) having a sheath thickness (D) that meets explosion-proof requirements along the section (11) and along the first sub-region (15) of the end section (12). M1 ), and along the second sub-region (16) of the end section (12) has a reduced sheath thickness (D) M2 The reduced sheath thickness (D) M2 The connector (8) is configured to establish a conductive plug connection between the cable (7) and the piezoelectric element (6).

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