Inductive proximity switch with ceramic body with front and side shielding

Abstract

Inductive proximity switch in a metallic housing (1) with a generator (2) for generating a transmission signal, a coil (3) for generating an alternating magnetic field that can be influenced by a conductive target, a receiving unit (4) for evaluating the alternating magnetic field, a control unit (5) for generating a switching signal, a low-temperature single-fired ceramic body; and a shield, wherein the coil (3) is formed as part of the ceramic body with a plurality of conductor track levels, and the ceramic body has at least one further conductor track level which serves as a shielding layer (6) and is connected to ground.

Description

[0001] The invention relates to inductive proximity switches in a metallic housing, in particular inductive all-metal proximity switches.

[0002] Inductive proximity switches respond to the approach of electrically conductive objects, especially metals, and are widely used in automation technology. They are manufactured and distributed by the applicant in a wide variety of housing designs. Cylindrical housings with external threads are particularly popular. Diameters are generally between 8 mm and 30 mm. Metal housings are preferred for harsh operating conditions. A distinction is made between all-metal switches and those with a ceramic or plastic front surface.

[0003] As has been shown, electromagnetic interference is “captured” by the supply lines and diverted to earth or mass via the metal housing and its fastening device.

[0004] Due to the large surface area of ​​the receiving coil and its close capacitive coupling with the housing, part of the interference current can flow through the sensor electronics and cause incorrect switching when evaluating the sensor signal.

[0005] With a non-metallic, for example ceramic, front surface, this effect is not as significant because the interference current can flow laterally via the metal housing without any significant influence on the sensor coil.

[0006] In an all-metal housing with a large-area sensor coil arranged close behind the metallic front surface, this is strongly coupled to the housing, so that the interference current flows preferentially via the sensor coil and the front surface and finally via the outer wall laterally to earth.

[0007] A galvanic connection of the supply line to the housing does enable the discharge of interference currents, but creates a new problem because the requirement for protection class II according to DIN EN 61140 “All live parts have additional insulation in addition to the operational insulation” is not met.

[0008] To meet this requirement, a standard-compliant shield that is galvanically isolated from the housing would be necessary.

[0009] DE 34 389 98 A1 shows an inductive proximity switch with a front plastic layer and a slotted cylindrical shielding sheath.

[0010] DE 41 025 42 A1 shows an inductive proximity switch with a shielding cup that is open towards the trigger.

[0011] DE 42 100 18 C2 shows an inductive proximity switch with a ferrite-free coil. The coil is surrounded by a shield that is electrically insulated from the housing.

[0012] DE 200 19 542 U1 shows an inductive proximity switch encapsulated in a plastic housing with a shielding fleece on the front surface.

[0013] DE 202 11 087 U1 shows an inductive proximity switch in a plastic housing coated on the inside.

[0014] DE 103 18 350 C5 shows an inductive proximity switch with printed coils on the front and back of a double-clad circuit board. The circuit board has shielding conductor tracks 18, the radially inward-facing ends of which are connected both to each other and to ground.

[0015] DE 10 2004 048 444 A1 describes an inductive proximity switch with a printed coil and metallic front and side surfaces in various designs. In addition to shielding, this also provides pre-attenuation, which reduces the influence of a metallic installation environment on the switching distance. The shielding surfaces are slotted to prevent parasitic ring currents.

[0016] DE 10 2006 053 222 A1 shows an inductive proximity switch in which a supplementary coil is permanently connected to a shield to compensate for the effects of temperature changes and mechanical influences. This arrangement aims to improve the stability of the switching distance. However, this arrangement lacks an integrated solution for dissipating electromagnetic interference directly at the coil.

[0017] DE 103 62 165 A1 describes an inductive proximity switch in which the transmitting and receiving coils are arranged in such a way that the magnetic flux in the receiving coil is minimized. This configuration aims to increase the sensitivity of the switch and reduce external influences. However, this arrangement does not address the issue of dissipating electromagnetic interference.

[0018] The shielding measures known from the cited prior art either involve additional material and / or manufacturing costs or, as shown in DE 103 18 350 C5, offer neither comprehensive nor lateral shielding.

[0019] The object of the invention is to provide an inductive proximity switch that avoids these disadvantages. The inductive proximity switch according to the invention should have comprehensive shielding without losing any sensor area. No additional components or manufacturing steps should be required for the front and side shielding or pre-attenuation.

[0020] This object is achieved according to the features of patent claim 1. The subclaims relate to the advantageous development of the invention.

[0021] The essential idea of ​​the invention is to design the sensor coil as a multilayer ceramic coil and to use at least one layer as shielding. According to the invention, the ceramic coil is designed using LTCC technology.

[0022] In order to reduce the influence of the installation situation on the switching distance, a lateral shield should also act as a pre-attenuation sleeve.

[0023] In an advantageous embodiment, a ferrite core is arranged in or on the back of the ceramic coil.

[0024] LTCC ceramic (Low Temperature Cofired Ceramics) is a low-temperature, single-fired ceramic. This technology can be used to create multilayer circuits with conductors, capacitors, and coils. These elements are applied to unfired ceramic foils using screen printing or photochemical processes. They are individually patterned, stacked, and laminated. A specific sintering profile tailored to the respective materials is then applied, with sintering temperatures between 850°C and 900°C. This creates a monolithic element consisting predominantly of ceramic, which contains electrical components, in particular the coil, possibly capacitors and resistors, but also the shielding and the connection contacts. The number of layers can be very high. In practice, 20 to 30 layers are reasonable.

[0025] The invention is explained in more detail with reference to the drawing. Fig. 1 the state of the art, Fig. 2 a proximity switch according to the invention in a simplified representation, Fig. 3 a proximity switch according to the invention in a detailed representation.

[0026] The Fig. 1 relates to the state of the art. In the proximity switch shown above with a non-metallic front surface, the interference current flows laterally across the metal housing without significantly affecting the sensor coil. The proximity switch shown in the middle has an all-metal housing. This illustrates the aforementioned problem once again. The interference current is capacitively coupled via the sensor coil into the front surface and flows through the metal housing to ground. In the proximity switch shown below, the housing is galvanically connected to the sensor electronics. Although interference is dissipated here, the galvanic coupling with the housing contradicts the requirement for protection class II according to DIN EN 61140: "All live parts must have additional insulation in addition to the operational insulation." To meet this requirement, a standard-compliant shield that is galvanically isolated from the housing is required.

[0027] The Fig. Figure 2 shows a simplified representation of a proximity switch according to the invention. The ceramic body contains a coil 3 and a separate shielding layer 6. The shielding layer 6 is shown separately below. Its grid structure is intended to minimize attenuation caused by parasitic eddy currents.

[0028] The Fig.Figure 3 shows a detailed representation of a proximity switch according to the invention. The electromagnetic interference coupled into the connecting cable 9 flows via the circuit board through the optionally provided damping sleeve 8 or the shielding layer 6 into the metal housing 1 and finally to ground.

[0029] In order to make the shielding layer 6 visible, the preferably metallic front surface of the proximity switch is not shown.

[0030] Because the front side of the sensor coil is completely covered by the shielding layer 6, both its windings and the interference-sensitive receiving unit 4 are no longer in the current path, although the entire front surface is available as a sensor surface.

[0031] The invention is also advantageous for inductive proximity switches with a non-metallic front panel. In this case, the interference current can at least partially flow outward to earth via the pre-damping sleeve 8.

[0032] According to the invention, the shielding layer 6 and also the pre-damping sleeve 8 can be divided into sub-areas and each provided with electrical connections.

[0033] Thus, the slots known from the state of the art for suppressing parasitic ring currents can not only be created but also designed variably. Furthermore, operating states with different pre-attenuation levels, as well as with a more or less heavily shielded and thus damped coil, are possible.

[0034] Ferrite 7 can also be a ferritic layer structure within the ceramic. According to the invention, it does not protrude laterally beyond the ceramic body.

[0035] The invention relates to an inductive proximity switch in an at least partially metallic housing 1. It contains a generator 2 for generating a transmission signal, as well as at least one coil 3 for generating an alternating magnetic field that can be influenced by a conductive target. Furthermore, it has an electronic circuit containing a receiving unit 4 for evaluating the alternating magnetic field and a control unit 5 for generating a switching signal. According to the invention, the coil 3 is designed as a low-temperature single-fired ceramic body with multiple conductor track levels. Its ceramic body contains at least one further conductor track level, which serves as a shielding layer 6 for dissipating electromagnetic interference to ground and is connected to the device ground for this purpose.

[0036] In an advantageous embodiment of the proximity switch according to the invention, the shielding layer 6 is not continuous, but has regions with electrically conductive and electrically non-conductive sections. This allows attenuation due to parasitic eddy currents to be reduced, thus improving sensitivity.

[0037] In a further advantageous embodiment, the ceramic body has a pre-damping sleeve 8. It serves both for pre-damping and for noise dissipation in devices with a front panel made of ceramic or plastic.

[0038] In a further advantageous embodiment, the ceramic body can contain ferritic layers or be connected to a ferrite 7. List of abbreviations: 1 housing 2 generators 3 coil 4 Receiver unit 5 Control unit 6 Shielding layer 7 Ferrite 8 Pre-damping sleeve 9 connection cables

Claims

[1] Inductive proximity switch in a metallic housing (1) with a generator (2) for generating a transmission signal, a coil (3) for generating an alternating magnetic field that can be influenced by a conductive target, a receiving unit (4) for evaluating the alternating magnetic field, a control unit (5) for generating a switching signal, a low-temperature single-fired ceramic body; and a shield, wherein the coil (3) is formed as part of the ceramic body with a plurality of conductor track levels, and the ceramic body has at least one further conductor track level which serves as a shielding layer (6) and is connected to ground. [2] Inductive proximity switch according to claim 1, characterized by that the shielding layer (6) has areas with electrically conductive and electrically non-conductive sections. [3] Inductive proximity switch according to claim 1, characterized bythat the ceramic body has a lateral shielding which acts as a pre-damping sleeve (8). [4] Inductive proximity switch according to claim 1, characterized by that the ceramic body contains ferritic layers or is connected to a ferrite (7).

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