MEASURING DEVICE FOR CAPACITIVE LIMIT LEVEL DETECTION

Description

[0001] The invention relates to a measuring device for capacitive level detection according to the preamble of claim 1.

[0002] Capacitive level measuring devices for detecting levels, which determine the presence or absence of a specific fill level of a medium, are known, for example, from DE 102007059709 A1. They essentially consist of a housing and a sensor tip extending directly from the housing in the axial direction. Often, only one electrode of the measuring capacitor is formed in the sensor tip, while the other electrode is formed by the surroundings of the capacitive sensor or measuring device. The measuring capacitor is therefore generally not a capacitor in the sense of a complete electrotechnical component, but rather an arrangement equipped with capacitance, the active electrode of which is associated with the capacitive sensor, and an electric stray field extends from the active electrode into the surroundings.To protect the measuring electrode as a sensor from external influences, it is surrounded by a casing, which is typically made of plastic.

[0003] With known limit switches, the transition between the plastic casing and the measuring device housing must be sealed for hygienic reasons, especially when used in the food industry, because the sensor tip cannot be removed from the measuring device or not easily, and therefore both parts cannot be cleaned separately.

[0004] In the prior art, the two parts are either glued together or a seal is achieved using a sealing ring, in particular an O-ring. Both methods incur considerable costs, especially due to the increased manufacturing effort. In the case of gluing, the choice of adhesive is also severely limited, as compliance with certain standards and approvals must be ensured. Sealing with an O-ring results in disadvantages in terms of mechanical stability, since the O-ring groove reduces the contact area between the plastic casing and the sensor housing. However, addressing this disadvantage in the design would entail considerable additional effort. DE102017200414A1 discloses a capacitive measuring device for limit level detection in which the sensor cap is sealed against the housing by means of O-rings.

[0005] From DE 102012203400 B4 of the applicant, it is known that the housing has an axially directed claw-like projection at the outermost edge of the contact surface with the casing of the sensor tip, and that this claw-like projection contacts the plastic casing in a quasi-linear area to create a tight connection between the housing and the sensor tip. In this way, the connection between the housing and the plastic casing is created without an additional sealing element or adhesive, thereby achieving a dead-space-free and very easy-to-clean seal between the two parts.

[0006] However, the material properties of the plastic sensor tip impose limits regarding temperature resistance. High-performance plastics allow for use at process temperatures up to approximately 150°C.

[0007] The object of the invention is to propose a permanently tight connection between the measuring instrument housing and the sheathing of the sensor tip, which is suitable for various sheathing materials and is also resistant to high-temperature use above 150°C.

[0008] The problem is solved by a measuring device having the features of claim 1. Advantageous embodiments of the invention are specified in the dependent claims.

[0009] According to the invention, the housing of the measuring device has an inwardly directed, shoulder-like narrowing on which a ring element rests. On the other hand, the casing has a groove-like recess at the level of the shoulder-like narrowing. The ring element engages in this recess.

[0010] Crucially, the ring element is designed in such a way that it develops a permanent tensile stress inwards towards the groove-like recess. Furthermore, it is essential to the invention that the groove-like recess has a wedge shape, i.e., a slope opposite the shoulder-like narrowing, in order to redirect the radially acting tensile stress into an axial force. As the ring element is pulled inwards, the slope of the groove-like recess draws the casing axially towards the housing, so that the housing rests permanently and tightly against the projection of the casing.

[0011] Preferably, the ring element is a snap ring that is expanded compared to its original diameter and thereby develops the permanent tensile stress inwards in the direction of the groove-like recess.

[0012] In an alternative preferred embodiment, the ring element is designed as a ring with inwardly directed, toothed retaining tongues, these retaining tongues being pre-tensioned such that the permanent tensile stress is thereby developed inwards in the direction of the groove-like recess. This ring with inwardly directed, toothed retaining tongues is, for example, designed as a slotted disc spring, a double-slotted disc spring, or a ball bearing disc spring.

[0013] Advantageously, the ring element is designed as a triangular ring and, in particular, has a trilobular contour. This also allows for outward support of the ring element, thereby improving its clamping within the housing. Furthermore, this facilitates the removal of the sensor tip or its casing from the housing.

[0014] For high-temperature applications, the casing is preferably made of a ceramic material, while the measuring instrument housing is typically made of stainless steel.

[0015] For optimal force redirection, the wedge shape of the groove-like recess preferably has an angle of approximately 30°.

[0016] The key feature is the force redirection and groove geometry using steel and ceramic materials. The invention differs through a new joining technique and the use of a high-temperature-resistant material for the sensor tip.

[0017] In mechanical engineering, a classic snap ring serves more or less only as a retaining device or positioning aid. According to the invention, the original property of the snap ring—its radially acting spring force for clamping on a larger or smaller diameter—is redirected into an axial tensile force through a clever groove design. This allows for the use of very simple and cost-effective geometries. For example, a thread, a high-precision press fit, or subsequent milling for a T-nut can be omitted.

[0018] The combination of a ceramic sensor tip and a simple metallic connecting element in the form of a snap ring enables the measuring device to be used at process temperatures exceeding 250 °C. At the same time, the invention can be implemented cost-effectively due to simple assembly, the use of a very inexpensive connecting element, and the comparatively simple manufacturing of the joining components. Furthermore, the connection between the sheath and the housing is permanently leak-proof and durable due to the preload.

[0019] The invention will now be explained in more detail using exemplary embodiments and with reference to the drawings.

[0020] They show schematically: Figure 1 shows a measuring device according to the invention for capacitive level detection, Figure 2 shows a sectional view of the measuring device according to the invention. Fig. 1 Figure 3 shows an enlarged section of a part of Fig. 2 and Figure 4 an enlarged section of a part of Fig. 3 .

[0021] In the following description of a preferred embodiment, the same reference numerals denote identical or comparable components.

[0022] Fig. 1 Figure 1 shows a measuring device 1 according to the invention for capacitive level detection. The measuring device 1 is preferably screwed into an adapter (not shown) and therefore has a housing 2 that is at least partially rotationally symmetrical. At the front end of the measuring device 1, axially connected directly to the housing 2, is the sensor tip 3, which essentially consists of a measuring electrode 4 and a sheath 5 surrounding the measuring electrode 4. A capacitance can be measured against a reference potential using the measuring electrode 4, which can ultimately be evaluated to determine a level limit. The sheath 5 has a projection 6 that increases the diameter of the sensor tip 3, onto which the housing 2 rests axially. For high-temperature applications, the sheath 5 is made of a ceramic material.However, at lower process temperatures, other materials are also conceivable, especially thermoplastic polymers, in particular polyetheretherketone (PEEK) or modified polytetrafluoroethylene (PTFE).

[0023] In Fig. 2 , which is a cross-sectional image of measuring device 1 from Fig. 1 The figure shows the internal structure of the measuring device 1. The casing 5 encloses the measuring electrode 4 and extends far into the housing 2. Also visible is the circumferential projection 6, on which the housing 2 rests axially.

[0024] Above the projection 6, the housing 2 has a circumferential, inwardly directed, shoulder-like narrowing 7 in its lower region, on which a snap ring 10 rests. It can be seen that the casing 5 has a groove-like recess 8 into which the snap ring 10 engages, thus forming a kind of barb on the casing. This barb prevents the casing 5, and therefore the entire sensor tip 3, from falling out of the housing 2. This circled area, relevant to the invention, is shown enlarged in the following figures.

[0025] The embodiment with snap ring 10 represents a preferred embodiment. The protective area also encompasses other conceivable ring elements instead of the snap ring, provided they are designed to develop a permanent tensile stress inwards towards the groove-like recess 8. In particular, this refers to rings with inwards directed, tooth-like retaining tongues, e.g., slotted disc springs, double-slotted disc springs, or ball bearing disc springs.

[0026] Fig. 3 shows the in Fig. 2 The circled area is shown in an enlarged view. The invention focuses on the permanently tight connection of the measuring instrument housing 2 with the casing 5, i.e., at the point where the housing 2 rests on the projection 6 of the casing 5. The snap ring 10 rests on the previously described circumferential, inwardly directed narrowing 7. At the level of the narrowing 7, the casing 5 has the aforementioned groove-like recess 8 into which the snap ring 10 engages. The upper part of the groove-like recess 8 rests on the snap ring 10, so that the entire casing 5, and thus the sensor tip itself, is axially fixed in both directions between this support on the snap ring 10 and the projection 6 pressing against the housing 2.

[0027] To compensate for tolerances and counteract temperature influences, this axial fixation is variable to a certain extent. This is achieved by, firstly, expanding the snap ring 10 compared to its original diameter, thereby developing a permanent tensile stress inwards towards the groove-like recess 8, and secondly, by giving the groove-like recess 8 a wedge shape. Wedge shape means that the recess 8 does not have a classic round groove shape with an upper stop running almost perpendicular to the longitudinal axis of the measuring instrument 1, but rather that the upper part of the recess 8 has an inclined shape, so that the recess 8 widens outwards.This special design of the recess 8 causes the casing 5 to be pulled axially towards the housing 2 by the inward tensile stress of the snap ring 10, so that the housing 2, which acts as a stop for this movement, rests permanently tightly against the projection 6 of the casing 5. The crucial factor is therefore the redirection of the radially acting tensile stress of the snap ring 10 into an axial force, as illustrated by the arrows.

[0028] In Fig. 4 The area of ​​the groove-like recess 8 is shown enlarged. The two lines illustrate the wedge shape of the recess 8. It has been found that for optimal force redirection, the wedge shape of the groove-like recess 8 should have an opening angle of approximately 30°. Of course, other angles are also possible, and the invention is not limited to this embodiment. Reference symbol list

[0029] 1 Measuring device 2 Housing of the measuring device 3 Sensor tip 4 Measuring electrode 5 Sheathing 6 Projection 7 Narrowing 8 Groove-like recess 10 Snap ring

Claims

1. Measuring device (1) for capacitive point level detection, consisting of - an at least partially rotationally symmetrical housing (2) and - a sensor tip (3) which is directly connected to the housing (2) in the axial direction and which comprises a measuring electrode (4) and a casing (5) surrounding the measuring electrode (4), the casing (5) extending into the housing (2) and having a circumferential projection (6) which increases the diameter of the sensor tip (3) and on which the housing (2) rests axially, characterized in that the housing (2) has an inwardly directed shoulder-like narrowing (7) on which a ring element (10) rests, and in that the casing (5) has a groove-like recess (8) at the level of the shoulder-like narrowing (7), in which recess the ring element (10) engages, the ring element (10) being designed such that it develops a permanent tensile stress inward, toward the groove-like recess (8), and the groove-like recess (8) having a wedge shape such that the casing (5) is pulled axially toward the housing (2) due to the inwardly acting tensile stress of the ring element (10), and as a result the housing (2) rests in a permanently seal-tight manner on the projection (6) of the casing (5).

2. Measuring device according to claim 1, characterized in that the ring element (10) is designed as a snap ring which is expanded compared to its original diameter and thereby develops the permanent tensile stress inward, toward the groove-like recess (8).

3. Measuring device according to claim 1, characterized in that the ring element (10) is designed as a ring having inwardly directed tooth-like retaining tongues, these retaining tongues being prestressed and as a result developing the permanent tensile stress inward, toward the groove-like recess (8).

4. Measuring device according to claim 3, characterized in that the ring having inwardly directed tooth-like retaining tongues is designed as a slotted disk spring, as a double-slotted disk spring or as a ball bearing disk spring.

5. Measuring device according to any one of the preceding claims, characterized in that the ring element (10) is designed as a triangular ring.

6. Measuring device according to claim 5, characterized in that the triangular ring has a trilobular contour.

7. Measuring device according to any one of the preceding claims, characterized in that the casing (5) is made of ceramic.

8. Measuring device according to any one of the preceding claims, characterized in that the wedge shape of the groove-like recess (8) has an angle of 30°.