DE102021119383A1 - Capacitive limit switch - Google Patents

Abstract

The invention relates to a measuring device (1) for capacitive limit level detection, consisting of an at least partially rotationally symmetrical housing (4) and a sensor area (5) directly adjoining the housing (4) in the axial direction and essentially consisting of a measuring electrode (10) and a sheath (11) surrounding the measuring electrode (10). According to the invention, the casing (11) has a cavity (13), so that in a partial area of the outer surface of the casing (11) facing the medium, this is so thin that a deflectable measuring membrane (11a) is formed at this point, the first side is at least partially in contact with the medium (3a, 3b) and whose second side facing away from the medium (3a, 3b) has an electromechanical converter (12) which deflects depending on the pressure p acting on the measuring membrane (11a). of the measuring membrane (11a) into an electrical variable.

Description

The invention relates to a capacitive limit switch according to the preamble of patent claim 1.

Capacitive measuring devices for level detection, which determine the presence or absence of a specific level of a medium, are known, for example DE 10 2007 059 709 A1 . They typically consist of a housing and a sensor area directly adjoining the housing in the axial direction. Often only one electrode of the measuring capacitor is formed in the sensor area and the other electrode of the measuring capacitor 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 a capacitance, the active electrode of which is assigned to the capacitive sensor, with a stray electric field spreading from the active electrode into the environment to a housing part, e.g. the container wall extends. In order to protect the measuring electrode as a sensor from external influences, it is surrounded by a support material, which is usually made of plastic.

These measuring devices are offered and sold by the applicant under the designation LMTxxx.

In addition to the fill level, other properties of the medium in the container or the ambient conditions accompanying the process typically also have to be recorded, with pressure and temperature being the main factors. It is often necessary to record several of these properties at the same location, so that different measuring devices have to be placed individually in close proximity to one another. A need has therefore arisen for a measuring device—often referred to as a combined sensor—to also record an additional variable at the same time. A typical example of this is the integration of a temperature sensor in a pressure gauge, as shown in the DE 102019115962 A1 is known. Here, the heating of the measuring diaphragm of the pressure gauge, which is in contact with the medium, is recorded and the temperature of the medium is thus measured.

For example, in the case of closed containers in which there is no opening to the environment, a gas pressure that differs from the ambient pressure can arise in the container volume above the liquid for various reasons. This so-called head pressure can be caused by temperature changes, fermentation, the addition of protective gas or filling and emptying.

From the DE 10335520 A1 a method for regulating the head pressure is known, since in some areas of application it is necessary to keep it constant, even if the level in the container changes. For this purpose, a filling level measuring device and a pressure sensor are arranged in parallel to one another and are connected to one another via data lines for data exchange.

A further need for a second measuring device arises for reasons of redundancy. The functionality of the main measuring device can thus be reliably checked, in particular by means of a diverse redundancy.

Finally, when monitoring the pump, it is also necessary to monitor the fill level of the pipe by means of limit level detection in order to prevent the pump from running dry. On the other hand, overpressures in the system can indicate faulty functionality of individual components, so that a pressure measurement is often carried out in parallel, in separate measuring devices.

The object of the invention is to integrate the fill level measurement and a parallel pressure measurement in a measuring device in a compact manner and thus to simplify the construction of the measuring device and/or to enable the functionality of the limit switch to be checked in a simple manner.

According to the invention, this object is achieved in each case by a capacitive limit switch with the features of independent claims 1 or 2 . Advantageous refinements of the invention are specified in the dependent claims.

According to a first aspect of the invention, a cavity is provided in the support material surrounding the measuring electrode, which cavity is formed such that the support material is so thin in the region of the surface facing the medium that a membrane is formed. An electromechanical converter is arranged on the inside of this membrane, by means of which the deflection of the membrane as a result of the influence of pressure can be detected.

According to a second aspect of the invention, the support material has a recess in which the measuring electrode is arranged and which is tightly closed off from the medium by a deflectable membrane. In contrast to the first embodiment of the invention, the membrane here consists of a separate part and is preferably made of ceramic.

The core of the invention is to modify the support material that is already present in the sensor area in such a way that both measuring principles for level and pressure measurement can be operated simultaneously and independently of one another in a partial area. A complex structure with several individual measuring devices can thus be dispensed with and instead everything can be integrated in one measuring device in a compact manner.

With a point level switch according to the invention, the head pressure of the same medium can also be measured, for example, without the need for a change to the measuring device or the measuring arrangement towards the medium or a separate pressure sensor with a correspondingly necessary opening and attachment in or on the container.

In addition to the head pressure measurement, the invention also allows redundancy to be created, by means of which the presence of the medium in the sensor area can also be detected using a completely different measuring principle. If a value is recorded via the pressure measurement that indicates the presence of the medium, but the actual level switch does not generate a corresponding signal, the user can be made aware of a possible malfunction of the level switch. The same applies, of course, vice versa.

Another advantageous application is pump monitoring. The filling level of a pipe can be monitored via limit level detection in order to prevent the pump from running dry. At the same time, the pressure measurement can be used to monitor the entire system for overpressures that can arise, for example, due to faulty actuators or valves.

In the context of the invention, the sensor area can run either elongated-pointed or blunt-flat. The latter is particularly suitable for a front-flush installation of the point level switch in the container, in which the sensor area and thus the entire measuring device does not protrude beyond the container wall, but rather should be inserted coplanarly into the inside of the wall.

Advantageously, the deflection of the membrane is recorded with the help of semiconductor strain gauges, which react very sensitively to the slightest expansion and compression of the membrane. Because of this, they can be arranged off-centre, in the outer edge area of the measuring membrane. Of particular interest here are semiconductor components with an integrated full bridge, which can only be applied in the compression zone of the membrane. In principle, however, metal strain gauges are also included in the invention.

The invention is explained in more detail below using exemplary embodiments with reference to the drawings.

• 1 eine Anordnung aus einem Behälter und daran angeordneten erfindungsgemäßen Grenzstandschaltern;
• 2 detaillierte Ansicht des Sensorbereichs des Grenzstandschalter gemäß einer ersten Ausführungsform und
• 3 detaillierte Ansicht des Sensorbereichs des Grenzstandschalter gemäß einer zweiten Ausführungsform.

It shows very schematically:

• 1 an arrangement of a container and limit switches according to the invention arranged thereon;
• 2 detailed view of the sensor area of the level switch according to a first embodiment and
• 3 detailed view of the sensor area of the level switch according to a second embodiment.

In the following description of the preferred embodiments, the same reference symbols designate the same or comparable components.

1 shows a container 2 designed as a tank with two limit switches 1 according to the invention. The medium 3a, 3b to be measured is in the container 2 in the form of a fluid, with part of the medium being the liquid 3b, the fill level of which is to be determined at a specific limit level , and another part a gas part 3a located above the liquid 3b. In a closed container 2, in which there is no opening to the environment, a gas pressure different from the ambient pressure can arise in the container volume above the liquid 3b for various reasons. This so-called head pressure can be caused, for example, by temperature changes, fermentation, the addition of protective gas or filling and emptying.

With the two limit switches 1 shown, it is possible, for example, to detect an upper and a lower limit. Of course, other constellations are also conceivable, in which only one limit switch 1 is used if only an upper or a lower limit level is to be detected. The limit switches 1 also do not necessarily have to be arranged on the side of the container 2, but are also conceivable in the lid or base area, for example to enable empty detection.

The point level switch 1 essentially consists of a housing 4 and a sensor area 5. As shown in FIG 2 As can be seen, a measuring electrode 10 of the measuring capacitor is formed in the sensor area 5 . In order to protect the measuring electrode as a sensor from external influences, it is made of a support material 11, which mostly consists of Plastic, for example made of PEEK, is surrounded.

The housing 4 also contains evaluation electronics, not shown in detail, which are provided for evaluating and processing the measuring signals supplied by the measuring electrode 10, which can then typically be tapped via a plug connection and forwarded to a PLC, for example.

2 shows a first embodiment of the invention, in which a cavity 13 is provided in the support material 11 for the sensor area 5, which is formed in such a way that the support material 11 is so thin in the area of the surface facing the medium 3a, 3b that a membrane 11a trains. An electromechanical converter 12 is arranged on the inside of this membrane 11a, which converts a deflection of the measuring membrane 10a, resulting from the pressure p acting on the measuring membrane 10a, into an electrical quantity. This acting pressure p can result either as head pressure from the gas 3a located above the liquid 3b or from the contact of the sensor area 5 by the liquid 3b itself.

The sensor area 5 does not necessarily have to be as in 2 have the elongated-pointed shape shown, but can also be blunt or flat, so that the level switch 1 can be installed in the container 2 flush with the front and thus almost coplanar to the inside of the wall. Such a second embodiment is in 3 shown and is particularly suitable for hygiene applications.

The support material 11 here has a recess 14 in which the measuring electrode 10 is arranged and which is sealed off from the medium by a deflectable membrane 11a. In contrast to the version in 2 now designed as a separate part and is preferably made of ceramic. The electromechanical converter 12, which converts the deflection of the membrane 11a as a result of an acting pressure p into an electrical variable, is arranged on the side of the membrane 11a facing away from the medium 3a, 3b.

Ceramic is a suitable material for the membrane 11a because it is electrically insulating and the measuring electrode 10, which is preferably designed as a metal foil or small metal plate, can be positioned well on it. A metal foil makes sense because it moves easily with the membrane 11a due to a form-fitting connection. Of course, in principle, other suitable materials can also be considered.

In both embodiments, the electromechanical converter 12 is preferably designed as a semiconductor strain gauge made of a semiconductor substrate and at least one piezoresistive resistance track, since it is very sensitive to the slightest expansion and compression of the membrane. Accordingly, the space for the measuring membrane 11a and thus also the cavity 13 or the recess 14 can be small. Due to the high sensitivity of the semiconductor strain gauge 12, it can be arranged eccentrically, in the outer edge area of the measuring membrane 11a, which could be important above all in the case of a front-flush sensor area 10. In this case, in the outer edge area means in particular arranged in the outer half of the membrane 11a.

Reference List

1

level switch

2

container

3a, b

medium

4

Housing

5

sensor area

10

measuring electrode

11

support material

11a

measuring membrane

12

electromechanical converter

13

cavity in the support material

14

Recess in the support material

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 102007059709 A1 [0002]
• DE 102019115962 A1 [0004]
• DE 10335520 A1 [0006]

Claims (8)

Measuring device (1) for capacitive limit level detection, consisting of an at least partially rotationally symmetrical housing (4) and a sensor area (5) directly adjoining the housing (4) in the axial direction, which essentially consists of a measuring electrode (10) and a measuring electrode (10) surrounding the support material (11), characterized in that the support material (11) has a cavity (13), so that in a partial area of the outer surface of the support material (11) facing the medium, this is so thin that a deflectable measuring diaphragm (11a) is formed at this point, the first side of which is at least partially in contact with the medium (3a, 3b) and the second side of which, facing away from the medium (3a, 3b), has an electromechanical converter (12), which has a converts the deflection of the measuring membrane (11a) dependent on the pressure p acting on the measuring membrane (11a) into an electrical variable. Measuring device (1) for capacitive limit level detection, consisting of an at least partially rotationally symmetrical housing (4) and a sensor area (5) which is directly adjacent to the housing (4) in the axial direction and essentially consists of a measuring electrode (10), characterized in that a deflectable membrane (11a) is arranged in the area facing the medium, which seals the measuring device (1) tightly towards the medium, with a first side of the membrane (11a) being at least partially in contact with the medium (3a, 3b). and whose second side facing away from the medium (3a, 3b) has the measuring electrode (10) and an electromechanical converter (12), which converts a deflection of the measuring membrane (11a) into an electrical variable, depending on the pressure p acting on the measuring membrane (11a). converts. measuring device claim 2 , characterized in that the measuring electrode (10) is designed as a metal foil or metal plate and rests directly on the inner, second side of the measuring membrane (11a). measuring device claim 2 or 3 , characterized in that the measuring membrane (11a) consists of ceramics. measuring device claim 2 , 3 or 4 , characterized in that the measuring electrode (10) is surrounded by a support material (11) and this support material (11) has a recess (14) in which the measuring electrode (10) is arranged and from the membrane (11a) to the medium is completed. Measuring device according to one of the preceding claims, characterized in that the electromechanical converter (12) is designed as a semiconductor strain gauge. Measuring device according to one of the preceding claims, characterized in that the electromechanical converter (12) is formed from a semiconductor substrate and at least one piezoresistive resistance track. Measuring device according to one of the preceding claims, characterized in that the electromechanical converter (12) is arranged eccentrically in the outer edge region of the measuring membrane (11a).

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