Inductive proximity switch with adapted thermal output coefficients

Abstract

Inductive proximity switch according to the eddy current principle with a coil carrier or coil core (1), a coil (3) for generating an alternating magnetic field, a sealing compound made of glass (8) and an electronic circuit in a preferably cylindrical housing (5), characterized in that the sealing compound made of glass (8) completely encloses the coil (3) and the associated coil carrier or coil core (1), wherein the coil wire is coated with a glass layer, a glass ceramic layer or a ceramic layer, and the thermal expansion coefficients of the materials of the housing (5), the glass mass (8) and the coil (3), including the insulation of the coil wire, differ from one another by less than 10%.

Description

[0001] The invention relates to an inductive proximity switch according to the features of the preamble of patent claim 1.

[0002] Inductive proximity switches are contactless electronic switching devices used primarily in automation technology.

[0003] They contain a transmitting coil that generates a magnetic field that can be influenced by a metallic trigger. The magnetic field's influence by the metallic trigger is evaluated, and when a threshold is exceeded, an electronic switching stage is activated.

[0004] Switchgear of this type is manufactured and distributed in a wide variety of designs, including by the applicant.

[0005] In an inductive proximity switch operating according to the eddy current principle, a coil is connected to an oscillator, generating a mostly sinusoidal alternating magnetic field. When a metallic trigger approaches, eddy currents are induced. The energy required for this is removed from the coil, causing its oscillation amplitude to decrease. The degree of damping depends, among other things, on the distance, conductivity, and geometric shape of the trigger.

[0006] Since these devices often operate in harsh environments, they are subjected to both thermal and mechanical stress. When used in the food industry, they are also cleaned with hot water or steam or are constantly exposed to high humidity, which places stringent demands on their sealing. Therefore, magnetically permeable metal housings or metal housings with ceramic caps are widely used.

[0007] DE 81 09 267 U1 describes a proximity switch in which the sensor is housed in a housing and covered with a dielectric material to protect it from mechanical influences. However, this document does not provide a detailed solution for problems related to different thermal expansion coefficients of the materials used.

[0008] DE 38 05 636 C2 describes an inductive eddy current sensor with a ceramic coil carrier enclosed in a metallic capsule. To ensure the sensor's ability to withstand high temperatures, especially temperature fluctuations, and high pressures, only high-temperature-resistant inorganic materials are used. To achieve a helium-tight connection between the components, they are fused with a glass or enamel ceramic, or alternatively, sealed by sintering the glass or enamel ceramic.

[0009] As long as the coil can be wound in such a way that its turns do not touch each other, this is not a problem.

[0010] For higher numbers of turns, which are often necessary for reasons of sensitivity or to reduce the transmit current, the turns must be placed close to one another or even on top of one another.

[0011] DE 41 02 542 A1 discloses an inductive proximity switch in which two sensor coils are used in a differential circuit to compensate for temperature influences and achieve a constant response distance. However, this solution does not demonstrate the need to improve the mechanical tightness of the sensor, as achieved by completely encapsulating the coil in glass and adapting the thermal expansion coefficients in the present invention.

[0012] DE 37 03 280 A1 shows a circuit arrangement in which integrated circuits are vacuum-tightly encapsulated on a ceramic substrate. A metal cap is insulated and secured using glass solder. However, this encapsulation technology is not tailored to the specific requirements of inductive proximity switches, particularly with regard to the integration of the coil and coil core, as well as the matching of the thermal expansion coefficients to ensure high robustness.

[0013] Enameled copper wire, with its enamel insulation, usually made of polymer (polyamide, polyester, polyesterimide), is unsuitable for this application. The main problems are moisture absorption and the significantly different thermal expansion coefficients of enamel insulation compared to other materials used, which can lead to both mechanical and electrical instabilities in the sensor area. While a suitable organic enamel insulation would be desirable, it is currently not available.

[0014] The object of the invention is to overcome the disadvantages of the prior art and to provide an inductive proximity switch with adjacent or superimposed coil windings suitable for harsh environmental conditions, which can also be manufactured efficiently and relatively inexpensively.

[0015] This object is achieved according to claim 1. The subclaims relate to advantageous developments or embodiments of the invention,

[0016] The essential idea of ​​the invention is to completely melt the coil and ferrite core into glass. The resulting glass body can either protrude beyond the device housing, which is preferably made of metal, form its end face, or be completely enclosed by the metal housing. For insulation, the coil wire is coated with a layer of glass, a ceramic layer, or another electrically insulating oxide layer.

[0017] The insulating glass layer can also be wrapped with glass silk.

[0018] The coil wire can also be insulated with an extremely thin, heat-resistant polymer layer, such as polyamide or polyimide with a thickness of, for example, 5 µm.

[0019] This creates a hermetically sealed sensor that is less susceptible to temperature fluctuations. Furthermore, time-consuming work steps and material are saved.

[0020] In an advantageous embodiment, the thermal expansion coefficients of the materials used, in particular the housing material, the ferrite core, the glass or ceramic, the coil wire together with its insulation and a damping ring provided to reduce the switching point deviation in the case of flush installation, are matched to one another.

[0021] This means that even strong temperature fluctuations remain unaffected by long-term impact on the sensor properties. It is particularly advantageous if the insulation of the coil wire is made of the same glass material as the glass mass used for melting.

[0022] Eliminating the ferrite core significantly simplifies implementation. In this case, the coil is wound as an air-core coil or on a glass or ceramic body. However, a significant loss of switching distance is to be expected.

[0023] The invention is explained in more detail below with reference to the drawing.

[0024] The Fig. 1 shows the state of the art and the Fig. 2 an inductive proximity switch according to the invention.

[0025] Fig. 1 is taken from DE 38 05 636 C2. The sectional view shows known prior art. A coil 3 is embedded in the end face of a cylindrical coil carrier 1 made of ceramic material. The upper part of the coil carrier 1 is covered by a ceramic cap 4. The cap 4 and the coil carrier combination are enclosed by a cylindrical housing 5 (capsule). The housing is made of Inconel, a corrosion-resistant nickel alloy. The extension 6 of the cap 4 is pressed between the coil carrier 1 and the housing 5. The space 7 between the coil carrier 1, the cap 4, and the inside of the housing 5 is filled with a sealing compound 8. The sealing compound 8 consists either of borosilicate glass or enamel ceramic.

[0026] Fig. Figure 2 shows an advantageous embodiment of the inductive proximity switch according to the invention. The coil carrier 1 is a pot core made of ferrite material. The sensor coil 2 is self-supporting and is inserted into the pot core. To insulate the coil windings, the coil wire is coated with a layer of glass, a ceramic layer, or a temperature-resistant, electrically insulating oxide layer. The terminals of the coils 3 run through the sealing compound 8 to an electronic circuit (not shown), which is located in the part of the housing 5 not filled with the sealing compound 8. As can be seen, the structure is significantly simpler than that shown in Fig. 1 shown state of the art.

[0027] The active surface, hereinafter referred to as the front surface 9, can consist of a metal membrane approximately 0.3 mm thick welded to the housing 5, or of titanium. Of course, the housing 5 can also have a one-piece threaded sleeve.

[0028] The sealing compound 8, which preferably consists of glass, is fused with the other components at reduced oxygen partial pressure.

[0029] The materials are selected so that their thermal expansion coefficients are largely identical. This largely prevents temperature-dependent changes in geometry and stress cracks. To reduce mechanical stresses and prevent relaxation phenomena, the sensor unit can be tempered.

[0030] The thermal expansion coefficients of the ferrite materials considered for the pot core range from 5 to 10 ppm / K. For stainless steel, it is about twice as high at 16 ppm / K. To overcome this problem, in addition to titanium, with a thermal expansion coefficient of 8 to 10 ppm / K, amorphous metal (metallic glass) can also be considered. Liquidmetal Inc., Lake Forest, CA, USA, offers Liquidmetal Alloy (LM1) with a thermal expansion of 10 ppm / K, which is ideally suited as a housing material due to its high specific resistance of 0.190 ohm × cm and its low magnetic permeability below 1.02. Reference is made to the applicant's DE 10 2008 042 721 A1.

[0031] The glasses in question are in the range up to 10 ppm / K, so that an adjustment is possible at this point.

[0032] The coil wire is typically made of copper with a thermal expansion coefficient of 17 ppm / K. To achieve this, tungsten copper can be used. This material is expensive, but temperature-resistant and well-suited for fusion with glass. A mixture ratio of 25% copper to 75% tungsten achieves 10 ppm / K. The conductivity is approximately half that of pure copper, which is still perfectly acceptable for the sensor.

[0033] In summary, it is found that the adaptation of the materials involved, including a pre-damping ring, which can be made of tungsten copper, and an insulation of the coil wire made of glass, ceramic or an oxide, to the thermal expansion coefficient of the ferrite core of 10 ppm / K is feasible.

[0034] By omitting the ferrite core, the sensor can be constructed based on the almost identical expansion coefficients of copper and stainless steel, approximately 17 ppm / K. This makes the sensor significantly more cost-effective. However, as already mentioned, a shorter switching distance must be expected.

[0035] The present invention describes an inductive proximity switch whose coil and coil core are completely encased in glass. The resulting glass body either protrudes beyond the device's housing, which is preferably made of metal, forms its end face, or is completely enclosed by it.

[0036] For insulation, the coil wire is preferably coated with a glass layer, a ceramic layer, a glass ceramic or an electrically insulating ceramic oxide layer with a thermal expansion coefficient adapted to the glass body.

[0037] The housing, or at least the front surface 9, may advantageously comprise stainless steel, titanium, a titanium alloy or an amorphous metal.

[0038] Non-magnetic nickel alloys can also be used. These are known, for example, under the trade names Nimofer 6928 from Thyssen / Krupp with a thermal expansion coefficient of 10.3 ppm / K and a specific electrical resistance of 137 µOhm × cm, Hastelloy C-276 from Haynes Intern. Inc. with a thermal expansion coefficient of 11 ppm / K and a specific electrical resistance of 130 µOhm × cm, or Nicrofer from Thyssen / Krupp. These alloys can be used advantageously both as a housing material and, in certain cases, for the front surface 9.

[0039] In a particularly advantageous embodiment of the invention, the thermal expansion coefficients of all materials are matched to one another in order to ensure the geometric and electrical properties of the sensor even in the event of strong temperature fluctuations over a long period of time.

[0040] This particularly applies to the ferrite core and any pre-damping ring that may be present to reduce the installation jump when installed flush in a metallic environment. List of reference symbols 1 coil carrier or coil core 2 frontal surface 3 coil 4 Ceramic sensor cap (made of Al2O3) 5 Cylindrical housing, threaded sleeve or Inconel capsule 6 Extension of the sensor cap 7 gap 8 Sealant made of glass or ceramic 9 Front surface

Claims

[1] Inductive proximity switch according to the eddy current principle with a coil carrier or coil core (1), a coil (3) for generating an alternating magnetic field, a sealing compound made of glass (8) and an electronic circuit in a preferably cylindrical housing (5), characterized by that the sealing compound made of glass (8) completely encloses the coil (3) and the associated coil carrier or coil core (1), wherein the coil wire is coated with a glass layer, a glass ceramic layer or a ceramic layer, and the thermal expansion coefficients of the materials of the housing (5), the glass mass (8) and the coil (3) including the insulation of the coil wire differ from one another by less than 10%. [2] Inductive proximity switch according to claim 1, characterized by that the coil wire of the coil (3) is coated with an electrically insulating oxide layer. [3] Inductive proximity switch according to claim 1, characterized bythat the front surface (9) of the housing (5) is made of non-magnetic stainless steel. [4] Inductive proximity switch according to claim 1, characterized by that the front surface (9) of the housing (5) comprises titanium or a titanium alloy. [5] Inductive proximity switch according to claim 1, characterized by that the front surface (9) of the housing (5) comprises a non-magnetic nickel alloy. [6] Inductive proximity switch according to claim 1, characterized by that the front surface (9) of the housing (5) comprises an amorphous metal. [7] Inductive proximity switch according to claim 1, characterized by that the thermal expansion coefficient of the material of the coil core (1) deviates by less than 10% from the materials mentioned in claim 1. [8] Inductive proximity switch according to claim 1 or 7, with a pre-damping ring, characterized bythat the thermal expansion coefficient of the material of the pre-damping ring deviates by less than 10% from the materials mentioned in claims 1 or 7.

Images