Vibrating limit level switch and cable for a vibrating limit level switch

Abstract

The invention relates to vibrating limit level switches for measuring the level to which a container is filled with a medium to be measured, wherein the vibrating limit level switch has a first sensor housing with an electronic unit, a second sensor housing with a piezoelectric element for generating and receiving a vibration and at least one cable, which has an electrical conductor for current-conducting connection of the electronic unit to the piezoelectric element, wherein the second sensor housing is configured to receive the cable along an end portion and the piezoelectric element has a plug connector for receiving the cable. The invention further relates to a cable for a vibrating limit level switch of this type, wherein the cable has an electrical conductor which is configured for current-conducting connection of the electronic unit with the piezoelectric element of the vibrating limit level switch. For the purpose of providing a vibrating limit level switch and a cable for such a vibrating limit level switch, which is configured to be explosion protection-compliant and thus satisfies relevant explosion protection guidelines, according to the invention the cable has a current-insulating sheath, which has an explosion protection-compliant sheath thickness along a portion between the first and second sensor housings and along a first sub-region of the end portion, and has a reduced sheath thickness along a second sub-region of the end portion, which enables a current-conducting plug-in connection between the cable and the plug connector of the piezoelectric element.

Description

[0001] Vibration level switch and cable for a vibration level switch

[0002] The invention relates to vibrating level switches for measuring the fill level of a container with a measuring medium, wherein the vibrating level switch has a first sensor housing with an electronics unit, a second sensor housing with a piezo element for generating and receiving a vibration and at least one cable which has an electrical conductor for the current-conducting connection of the electronics unit to the piezo element, wherein the second sensor housing is designed to receive the cable along an end section and the piezo element has a plug for receiving the cable.

[0003] Furthermore, the invention relates to a cable for such a vibrating limit level switch, wherein the cable has an electrical conductor which is designed for the current-conducting connection of the electronic unit to the piezo element of the vibrating limit level switch.

[0004] Vibrating level switches of the type mentioned are known in the art and are designed to determine the fill level of a container, such as a container or silo, containing a measuring medium, such as a liquid or bulk material. For this purpose, the piezo element generates a mechanical vibration that is transmitted to a mechanical oscillator. The mechanical oscillator is thereby excited to oscillate at its natural frequency, which depends in particular on the immersion depth of the mechanical oscillator in the measuring medium. The natural vibration of the mechanical oscillator is transmitted in the opposite direction to the piezo element, which sends a frequency-dependent signal to the electronics unit. For this purpose, the piezo element is connected to the electronics unit, for example, via a flexible conductor. The electronics unit can be designed as a calibration circuit board or can have a calibration circuit board.The frequency-dependent input signal is evaluated there, for example, taking into account a calibration measurement, in such a way that the fill level of the container is determined.

[0005] Vibrating level switches of the aforementioned type are known, for example, from EP 2 503860 A1 and EP 3 542 405 B1. The use of the known vibrating level switch in potentially explosive atmospheres is not possible without adaptations of the individual components. In particular, the cable connecting the electronic unit to the piezo element must have a current-insulating sheath that meets the relevant conditions / guidelines for explosion protection and is thus designed in accordance with explosion protection regulations. A distance of at least 0.5 mm must always be maintained between the current-carrying conductor and the housing of the vibrating level switch. The current-insulating sheath of the cable is dimensioned in accordance with explosion protection regulations in such a way that a distance of 0.5 mm is always maintained between the current-carrying conductor and the housing of the vibrating level switch.A cable is often used that has continuous insulation with a sheath of the current-carrying conductor of at least 0.5 mm thickness.

[0006] In a vibrating level switch, as known from the state of the art, a cable with explosion-proof insulation cannot simply be installed because the connector on the piezo element is not designed for a conductive connection of a cable with such a thick sheath. However, any redesign of the connector, which would be designed to accommodate such a cable, would introduce a comparatively large mass, in particular a mass of more than 2.5 g, into the piezo element, which, due to mass inertia, would affect the generation and reception of vibrations. Therefore, in the case of a cable with a comparatively thick, current-insulating sheath, measurement errors arise, which also affect an incorrect determination of the fill level of the container containing the measuring medium.

[0007] The object of the invention is therefore to propose a vibrating point level switch and a cable for such a vibrating point level switch that is designed to comply with explosion protection regulations and thus meets the relevant explosion protection requirements. The vibrating point level switch should be designed to be cost-effective and, compared to known vibrating point level switches that are not designed to comply with explosion protection regulations, require a minimum of structural changes. This object is achieved by the vibrating point level switch according to claim 1 and the cable according to claim 10.

[0008] According to the invention, the cable has a current-insulating sheathing that has an explosion-protection-compliant sheath thickness along a section between the first and second sensor housings and along a first partial region of the end section, and a reduced sheath thickness along a second partial region of the end section, allowing a current-conducting plug-in connection between the cable and the plug of the piezoelectric element. In other words, the sheath thickness of the current-insulating sheathing is reduced from an explosion-protection-compliant sheath thickness within an end section of the cable to such an extent that the cable has an insertion diameter that allows the cable to be inserted into the plug.

[0009] By reducing the thickness of the current-insulating sheath, the cable's end section has a geometry that allows it to be connected to a plug that is used in common piezo elements of well-known vibrating level switches. This allows the use of a standard commercially available plug system, which is inexpensive due to the high volumes required in other applications. Plug systems are developed and certified by manufacturers, so a special plug system for small quantities cannot be installed economically. The minimum distance between the current conductor and the housing required for explosion protection is maintained without gaps. In the first section of the end section, with its explosion-proof sheath, the cable rests on the housing, which is why the required minimum distance cannot be undercut.In the second part of the end section, the spacing results from the cable being inserted into the piezo element's connector, which is positioned at a corresponding distance from the housing. Overall, this prevents the minimum distance from being exceeded.

[0010] Furthermore, the use of a cable tapered at the end section keeps the connector's mass sufficiently low, effectively preventing measurement errors due to the connector's inertia when transmitting and receiving vibrations. The first sensor housing and the second sensor housing can be mechanically connected to each other by a support cable or a support tube, or they can merge into one another to form a single, compact housing.

[0011] In this respect, the design according to the invention results in an explosion-protection-compliant vibration limit level switch which, compared to known vibration limit level switches, has a minimum of structural changes and can therefore also be manufactured cost-effectively.

[0012] Advantageous further developments of the invention are specified below and in the subclaims.

[0013] Preferably, the electrically insulating sheath of the cable has a sheath thickness of at least 0.5 mm in a section with a sheath thickness that complies with explosion protection requirements. With such a sheath thickness, there is sufficient clearance between the conductor and the (current-conducting) housing, so that explosion protection is met.

[0014] According to an advantageous development, the current-insulating sheath of the cable along the second partial region of the end section has a sheath thickness of less than 0.5 mm, in particular between 0.1 mm and 0.15 mm. With such a sheath thickness, the cable can be inserted into conventional connectors, in particular into conventional connectors with insulation displacement contacts, as are commonly used in piezoelectric elements. In this case, the current-insulating sheath is not completely removed.

[0015] Preferably, the length of the second partial region is greater than the depth of the piezo element's connector, so that, when the cable is assembled, a spacing occurs between the cable and the second sensor housing in a section between the piezo element and the first partial region of the end section. An air gap is created between the cable and the housing in this region or along this section.

[0016] The sensor housing preferably comprises a resonator and a membrane designed to transmit vibrations from the piezoelectric element to the resonator and from the resonator to the piezoelectric element. For this purpose, the piezoelectric element can be welded or positively connected to the membrane in a known manner.

[0017] When the vibrating level switch is used as intended, the resonator oscillates at its natural frequency, which depends on the depth of immersion of the resonator in the measuring medium. The resonator is preferably designed as a tuning fork. Independently of this, the diaphragm is preferably integrally connected to the resonator, which promotes virtually lossless transmission of the vibrations.

[0018] To create a conductive connection between the cable and the plug of the piezo element, the plug preferably has an insulation displacement terminal with at least one cutting edge that penetrates the cable sheath when installed and contacts the electrical conductor. The insulation displacement terminal is preferably designed as a detachable connecting element. During initial assembly, the cable is inserted into the plug and the insulation displacement terminal is closed. The insulation displacement terminal cuts through the current-insulating sheath of the cable with the at least one cutting edge and comes into conductive contact with the conductor. In this respect, the reduction in the sheath thickness of the current-insulating sheath in the area of ​​the plug is also advantageous because conventional insulation displacement terminals are only designed to cut through cables with a comparatively thin current-insulating sheath.The sheath thickness of explosion-proof cables with a continuous sheath thickness of at least 0.5 mm is too large and the insulation displacement terminal does not reach the conductor when cutting through the sheath, so that no electrically conductive contact is created.

[0019] According to an advantageous development of the invention, the electrical conductor of the cable is designed as a stranded wire, a solid wire, or a bundle of several stranded wires. Such conductors are simple in design and have proven to be interference-free conductors.

[0020] Specific embodiments of the invention are explained with reference to the figures. Figures 1a-c show schematic representations of a vibrating level switch,

[0021] Fig. 2 a cross-sectional view of a cable and

[0022] Fig. 3 a detailed view of the installation position of a cable in a schematic cross-sectional view.

[0023] Fig. 1a shows a schematic cross-sectional view of a vibrating level switch 1 designed to measure the fill level of a container 2 containing a measuring medium 3, which in the illustrated embodiment is a liquid 31. The vibrating level switch 1 has a first sensor housing 41 with an electronics unit 5, a second sensor housing 42 with a piezo element 6 for generating and receiving a vibration, and a cable 7 that electrically connects the electronics unit 5 to the piezo element 6, for which purpose the piezo element 6 has a plug 8. The piezo element 6 is glued to a membrane 9, which is connected to a resonator 10 in the form of a tuning fork 101. Through a directed or non-directed vibration of the piezo element 6, the vibration is transmitted via the membrane 9 to the resonator 10, which is thereby excited to vibrate at its natural frequency.The natural frequency of the resonator 10 depends not only on the geometry and nature of the resonator 10, but also on the immersion depth TE of the resonator 10 into the measuring medium 3. The oscillation of the resonator 10 in its natural frequency is transmitted to the piezo element.

[0024] 6, which generates a frequency-dependent signal that is transmitted to the electronics unit 5. There, the fill level TF of the container 3 is determined, taking into account the known installation position of the second sensor housing 42 or the resonator 10 and a calibration measurement. The cable 7 of the vibrating level switch 1 has several sections, namely a section 11 that extends between the first and second sensor housings 41, 42, and an end section 12 that is received by the second sensor housing 42. The end section 12, in turn, has several regions, which are explained in more detail with reference to Figs. 2 and 3.

[0025] In the vibrating level switch according to Fig. 1a, the cable 7 is designed 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

[0026] 7. Furthermore, according to Fig. 1c, 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 in such a way that they form a compact sensor housing 43 which completely accommodates the cable 7.

[0027] For the explosion protection-compliant design of the vibrating level switch 1, the cable 7 has a geometry that is shown in detail in Figs. 2 and 3.

[0028] The cable 7 has an electrical conductor 13 made up of a bundle of multiple strands 131, each of which has a separate and a shared current-insulating sheath 14 with a sheath thickness DM. The shared current-insulating sheath 14 has an explosion-protection-compliant sheath thickness DMI in the section between the first and second sensor housings 41, 42 and along a first partial region 15 of the end section 12 with the length Li, so that a sufficient insulation distance is provided. This results in sufficient spacing between the strands 131 and the second sensor housing 42, particularly within the end section 12. The cable 7 has a reduced sheath thickness DM2 along a second partial region 16 of the end section 12 with the length L2, so that the diameter of the cable 7 is reduced to an insertion diameter DE, which allows a plug-in connection with the connector 8.Within the connector 8, the cable 7 is secured by an insulation displacement terminal 17 with two cutting edges 171, which locally cut through the current-insulating sheath 14 of the cable 7 and make current-conducting contact with the strands 131. Between the first and second sub-regions of the end section 15, 16, a third sub-region 18 of the cable with the length L3 is formed, in which the sheath thickness DM of the current-insulating sheath 14 tapers conically from the sheath thickness DM1 to the sheath thickness DM2. In the illustrated embodiment, the length L2 of the second sub-region is greater than the depth Ts of the connector 8 of the piezoelectric element 6, so that, when the cable 7 is assembled, a contact arises between the cable 7 and the second sensor housing 42 in the sub-regions 16, 18 between the piezoelectric element and the first sub-region 15 of the end section 12.The length of this section can be adjusted by selecting the respective lengths L2 and L3 of the second and third sections 16, 18. Section 16 with length L2 should not be selected too long to ensure compliance with the insulation distance. Reference symbols.

[0029] 1 vibrating level switch

[0030] 2 containers

[0031] 3 Measuring medium

[0032] 31 Liquid

[0033] 41 first sensor housing

[0034] 42 second sensor housing

[0035] 43 compact sensor housing

[0036] 5 Electronic unit

[0037] 6 Piezo element

[0038] 7 cables

[0039] 8 plugs

[0040] 9 Membran

[0041] 10 Resonator

[0042] 101 Tuning fork

[0043] 11 Section between first and second sensor housing

[0044] 12 final section

[0045] 13 ladders

[0046] 131 strands

[0047] 14 Sheathing

[0048] 15 first part of the final section

[0049] 16 second part of the final section

[0050] 17 insulation displacement terminal

[0051] 171 cutting edge

[0052] 18 third part of the final section

[0053] 19 Support tube

[0054] DE insertion diameter

[0055] DM sheath thickness

[0056] DMI explosion protection compliant sheath thickness

[0057] DM2 reduced jacket thickness

[0058] TE immersion depth

[0059] Ts depth of the plug

[0060] T F Fill level Ll,2,3 Lengths of the sections of the end section

Claims

Patent claims 1. A vibrating level switch (1) for measuring the fill level of a container (2) with a measuring medium (3, 31), wherein the vibrating level switch (1) has a first sensor housing (41) with an electronics unit (5), a second sensor housing (42) with a piezoelectric element (6) for generating and receiving a vibration, and at least one cable (7) having an electrical conductor (13) for the electrically conductive connection of the electronics unit (5) to the piezoelectric element (6), wherein the second sensor housing (42) is designed to receive the cable (7) along an end section (12), and the piezoelectric element (6) has a plug (8) for receiving the cable (7), characterized in that the cable (7) has a current-insulating sheath (14) which extends along a section (11) between the first and the second sensor housing (41,42) and along a first partial area (15) of the end section (12) has an explosion-protection-compliant sheath thickness (DMI) and along a second partial area (16) of the end section (12) has a reduced sheath thickness (DMZ), which allows a current-conducting plug connection between the cable (7) and the plug (8) of the piezo element (6).

2. Vibration level switch (1) according to claim 1, characterized in that the current-insulating sheath (14) of the cable (7) has a sheath thickness (DM) of at least 0.5 mm in a section (11, 15) with an explosion-protection-compliant sheath thickness (DMI).

3. Vibration limit switch (1) according to one of claims 1 or 2, characterized in that the current-insulating sheath (14) of the cable (7) along the second partial region (16) of the end section (12) has a sheath thickness (DMZ) of less than 0.5 mm, in particular from 0.1 mm to 0.15 mm.

4. Vibration limit switch (1) according to one of claims 1 to 3, characterized in that the length of the second partial area (16) is greater than the depth (Ts) of the plug (8) of the piezoelectric element (6), so that in the assembled state of the cable (7) in a section between the piezoelectric element (6) and the first partial area (15) of the end section (12) a spacing between the cable (7) and the second sensor housing (42).

5. Vibration limit switch (1) according to one of claims 1 to 3, characterized in that the second sensor housing (42) has a resonator (10) and a membrane (9) which is designed to transmit vibrations from the piezo element (6) to the resonator (10) and from the resonator (10) to the piezo element (6).

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

7. Vibration limit switch (1) according to claim 5, characterized in that the resonator (10) is designed as a vibrating fork (101).

8. Vibration limit switch (1) according to one of claims 1 to 6, characterized in that the plug (8) of the piezo element (6) for the current-conducting connection to the cable (7) has an insulation displacement terminal (17) with at least one cutting edge (171) which penetrates the sheath (14) of the cable (7) in the mounted state and contacts the electrical conductor (13).

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

10. Cable (7) for a vibrating limit switch (1), which is designed according to one of claims 1 to 9, wherein the cable (7) has an electrical conductor (13) which is designed for the current-conducting connection of the electronic unit (5) to the piezo element (6) of the vibrating limit switch (1), characterized in that the cable (7) has a current-insulating sheath (14) which has an explosion-protection-compliant sheath thickness (DMI) along a section (11) and along a first partial area (15) of the end section (12) and a reduced sheath thickness along a second partial area (16) of the end section (12) (DMZ) which is designed to produce a current-conducting plug connection between the cable (7) and the plug (8) of the piezo element (6).

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