Vibration sensor with protective layer and method for manufacturing a vibration sensor

Abstract

The invention relates to a vibration sensor (10) having a piezo drive (14), wherein: the vibration sensor (10) has a vibration sensor housing (12); a membrane (16) having at least one mechanical oscillator (18) is arranged at a process-side end of the vibration sensor housing (12); the membrane (16) is connected to the piezo drive (14) such that by means of the piezo drive (14) the membrane (16) is caused to oscillate; the piezo drive (14) has at least one piezo element (20); the at least one piezo element (20) is provided with a protective layer (38). The invention also relates to a method for manufacturing a vibration sensor (10).

Description

[0001] Vibration sensor with protective layer and method for producing a vibration sensor

[0002] The invention relates to a vibration sensor according to claim 1 and a method for producing a vibration sensor according to claim 9.

[0003] Vibration sensors, used, for example, as vibrating limit switches, are well known in the art. Such a vibration sensor typically has a membrane that can be excited to vibrate via a drive, by means of which a mechanical oscillator arranged on the membrane can be excited to vibrate. A piezo drive, for example, can be used to excite the membrane to vibrate. Depending on the degree of coverage of the mechanical oscillator with a filling material and the viscosity of the filling material, the mechanical oscillator oscillates at a characteristic frequency, which can be detected by the vibration sensor and converted into a measurement signal.

[0004] In the current state of the art, two different types of piezo actuators are commonly used. Protection from moisture is important for all piezo actuators. Excessive moisture ingress can lead to a loss of performance or even complete failure of the drive.

[0005] In a first variant of a piezo drive, a multi-segmented piezo element is directly or indirectly connected to the diaphragm. By applying an electrical voltage to one or more segments of the piezo element, the element is excited to bend or torsion and transfers this to the diaphragm, which then vibrates and causes the mechanical oscillator to vibrate. To protect the piezo drives against moisture and the associated corrosion, it is known to pot the piezo drives with silicone. For this purpose, a potting cup is usually arranged in the housing of the vibration sensor, in which the piezo drive is located. To accommodate the potting, the potting cup is designed to be rigid. The potting cup and the silicone cast within it dampen the vibrations of the drive and the diaphragm. Silicones are very durable and reliably wet surfaces.However, the good wettability also means that considerable effort must be put into the manufacturing of the piezo actuator to ensure that the silicone does not get into contact with adhesive joints or other surfaces with increased cleanliness requirements. Furthermore, it must be ensured that the plug contacts for connecting the piezo actuator are not wetted with silicone, which would then prevent secure contact with the mating connector. Furthermore, the silicone encapsulation becomes brittle and loses its effectiveness in applications with temperatures around 300°C.

[0006] A second variant of a piezo actuator for use at higher temperatures is a so-called piezo stack actuator. In this case, a stack of several piezo elements, each with a matching ceramic arranged above and below the stack, as well as pressure pads arranged above and below the matching ceramics, is clamped against the diaphragm. Applying an electrical voltage to the piezo elements causes them to change their axial expansion, thus causing the diaphragm to vibrate.

[0007] To protect this piezo stack drive from moisture, a current solution has been to hermetically encapsulate it. The piezo drive is inserted into a cylinder sealed with a glass feedthrough. Electrical cables for contacting the piezo stack drive are glazed into the glass feedthrough. This type of encapsulation is very complex and costly. A further disadvantage of such hermetic encapsulation is that outgassing of substances from drive components can also lead to performance losses and complete failures.

[0008] The underlying object of the invention is therefore to provide a vibration sensor and a method for producing a vibration sensor, which allow a vibration sensor with a low probability of failure to be produced cost-effectively.

[0009] The object is achieved according to the invention with the features of the independent claims. Further practical embodiments and advantages are described in conjunction with the dependent claims. A vibration sensor according to the invention has a piezo drive. The vibration sensor has a vibration sensor housing, wherein a membrane with at least one mechanical oscillator is arranged at a process-side end of the vibration sensor housing. The mechanical oscillator is in particular a tuning fork with two prongs or a single-rod oscillator. The membrane is connected to the piezo drive in such a way that the membrane is set into vibration by means of the piezo drive. The piezo drive has at least one piezo element. It is in particular a piezo drive known from the prior art, e.g.a piezo drive described above with only one piezo element, a segmented or slotted piezo element, or a piezo drive with several stacked piezo elements (piezo stack drive).

[0010] According to the invention, the at least one piezoelectric element is provided with a protective layer. The protective layer is comparatively thin compared to a potting compound that fills a volume. The extension of the protective layer in the vertical direction (the layer thickness) is smaller than the extension in the longitudinal and / or width directions. The at least one piezoelectric element is provided with the protective layer, in particular, on all external, i.e., freely accessible, surfaces.

[0011] The protective layer is particularly hydrophobic and prevents moisture from penetrating the protective layer, thus potentially acting as an anti-corrosion layer. Even a thin layer can prevent moisture from reaching the at least one piezo element and causing it to corrode. This effective protection minimizes the likelihood of failure of the vibration sensor.

[0012] By applying a thin coating to at least one piezo element, complex and expensive silicone potting or hermetic encapsulation is no longer necessary. In particular, a rigid potting cup or rigid encapsulation is no longer required; a housing or mount for the drive can be selected that surrounds the drive, in particular only over part of its height. This significantly reduces vibration damping. A further advantage of the protective layer is that it requires little installation space. In particular, the protective layer is temperature-stable and can withstand temperatures of more than 1000°C; the melting point is at least 1100°C.

[0013] In particular, the protective layer is electrically insulating and prevents short circuits within the piezo actuator. In particular, the protective layer is suitable for voltages up to at least 40 kV / mm.

[0014] In particular, materials that are hydrophobic, temperature-stable and electrically insulating are suitable for the protective layer.

[0015] The protective layer comprises, in particular, a glassy material. A "glassy material" is understood here to mean a material that exists as an amorphous, non-crystalline solid at room temperature and atmospheric pressure. For example, such a material is a homogeneous, non-crystalline material that is preferably oxidic, especially silicate.

[0016] Alternatively or additionally, the protective layer comprises a ceramic material. Ceramic materials comprise inorganic, fine-grained raw materials. These are objects (so-called green bodies) that are formed at room temperature with the addition of liquid and then dried. These objects are then fired in a subsequent firing process to produce harder, more durable objects. Ceramics are classified according to their chemical composition; for example, they can be silicate ceramics, oxide ceramics, and non-oxide ceramics.

[0017] Both glassy and ceramic materials have the advantage of being water-repellent, temperature-resistant and electrically insulating, and their chemical and physical properties can be adapted through their compositions and manufacturing processes.

[0018] In a practical embodiment, the protective layer contains silicon. In particular, the layer comprises silicon oxide, in particular amorphous silicon dioxide, and / or silicate. The protective layer is produced in particular using silane, silanol, siloxane, and / or polysiloxane. Such a layer can be produced in particular by evaporating organosilicon precursors in a plasma, in particular an atmospheric-pressure plasma. In particular, a silanized surface makes the protective layer water-repellent. The further advantage of this layer is that it is gap-permeable, especially when deposited using a plasma, and thus ensures that the protective layer covers all exposed surfaces.

[0019] The protective layer, in particular, has a layer thickness in the range of 2 nm to 1000 nm. Compared to macroscopic encapsulation of the entire piezo drive with encapsulation heights in the cm range, this represents a very thin layer thickness with minimal material usage. In particular, the layer thickness is in the range between 10 nm and 50 nm, and preferably between 35 nm and 45 nm. Even with such thin layers, a sufficiently water-repellent layer has been achieved, which is temperature-resistant and also provides sufficient electrical insulation for the voltages occurring in a vibration sensor. A protective layer with such a thin layer thickness is flexible and can therefore compensate for different expansion coefficients.

[0020] As already mentioned above, the protective layer is in particular a layer deposited by means of a plasma.

[0021] In a further practical embodiment of the vibration sensor, in addition to the at least one piezo element, the contacts connected to the piezo element are also provided with the protective layer. These can be plug contacts or non-insulated connecting lines such as wires. In particular, electronics connected to the piezo element are also at least partially covered with the protective layer. The electronics are, in particular, electronics for connecting the control electronics to the piezo drive. The electronics are, in particular, located on a printed circuit board. Plug contacts for connecting the electronics are, in particular, left out of the protective layer.

[0022] In particular, all non-insulated electrical components connected to the drive are provided with the protective layer.

[0023] As mentioned at the beginning, some state-of-the-art piezo actuators use a potting cup to mold and hold the potting compound. This pot completely surrounds the piezo actuator over its entire height. Piezo stack actuators use a sealed cylinder sealed by a glass feedthrough, with individual contacts encased in glass.

[0024] The protective layer now provided allows the use of a holder for the piezo drive which is smaller than the known housing or pot and, above all, is modified so that the holder has a significantly less influence on the oscillations of the vibration sensor. The holder for accommodating the piezo drive surrounds the piezo drive, in particular only over part of the height of the piezo drive. This applies in particular to piezo drives with a piezo element and a slotted or segmented piezo element. The holder extends, in particular, with a base area around the piezo element and an adapter ceramic in order to center them relative to one another. Starting from the base area, the holder does not extend over the entire surface of the cylinder, but rather only a web extends in the vertical direction, which serves to attach the electronics.

[0025] In particular, the holder can be constructed in one piece. This is particularly advantageous for piezo stack drives with multiple piezo elements, as the expensive two-part solution with a cylinder and a glass feedthrough arranged therein can be dispensed with. The holder can have a U-shaped cross-section and, like a yoke, clamp the piezo stack drive against the membrane. The holder can only partially cover the piezo stack drive, in particular only in the area of ​​the stack of multiple piezo elements. In particular, the holder has through-holes through which electrical contacts can be easily inserted. Additional hermetic encapsulation (such as glazing) is no longer necessary thanks to the protective layer.

[0026] The invention also relates to a method for producing a vibration sensor, in particular a vibration sensor as described above, wherein a protective layer is applied to the at least one piezoelectric element. For further features of the protective layer and their advantages, reference is made to the above description.

[0027] In particular, the protective layer is applied by vapor deposition, and in particular by a plasma process. The layer is created, in particular, by evaporating organosilicon precursors in a plasma, in particular an atmospheric-pressure plasma. The advantage of this process is that the protective layer can be deposited selectively and in all locations, even in some cases where access is difficult. Application by plasma results, in particular, in a homogeneous layer thickness and uniform distribution over the surface. In particular, the deposition can be carried out in the atmosphere, i.e., in air, which keeps the manufacturing process simple.

[0028] In particular, the piezo actuator is cleaned using plasma before coating with the protective layer. This allows the surface cleaning and preparation, as well as the actual deposition of the protective layer, to be performed in one system.

[0029] The piezo actuator and its housing can be pre-assembled into the vibration sensor and the vibration sensor housing, and then coated with the protective layer. Alternatively, the sequence can be reversed, with the pre-coated piezo actuator being provided as a pre-assembly and then inserted into the vibration sensor housing.

[0030] Further practical embodiments and advantages are explained below in conjunction with the figures. They show:

[0031] Fig. 1 shows a vibration sensor according to the prior art in a schematic representation in cross section

[0032] Fig. 2 shows a vibration sensor according to a first embodiment in a schematic representation in cross section and

[0033] Fig. 3 shows a vibration sensor according to a second embodiment in a schematic cross-sectional view.

[0034] Figure 1 shows a section of a prior art vibration sensor 10, which is used, for example, in level measurement technology as a point level sensor. The vibration sensor 10 has a vibration sensor housing 12 in which a piezo drive 14 is arranged. The piezo drive 14 is connected to a diaphragm 16, and the diaphragm 16, in turn, is connected to a mechanical oscillator 18.

[0035] The piezo drive 14 here comprises a segmented piezo element 20, electrical connections 22 connecting the piezo element 20 to an electronics unit 24, and the electronics unit 24 on a circuit board 26. On the side facing away from the piezo element 20, the circuit board 26 has two plug contacts 28 for connection to a mating connector 30 for connecting the electronics unit 24 to other electronic components (not shown), as well as two diodes 31 for overvoltage protection. Furthermore, the piezo drive 14 comprises a compensating ceramic 32, via which the piezo element 20 is connected to the membrane 16.

[0036] The piezo drive 14, along with the compensating ceramic 32, the piezo element 20, and the electronics 24, is arranged in a housing 34, which serves as a potting cup. To protect the piezo drive 14 from moisture, a potting compound 36 (in this case, silicone) is filled into the potting cup 34. The area around the plug contacts 28 for contacting via the mating connector 30 is excluded.

[0037] Fig. 2 shows a vibration sensor 10 according to the invention in a first embodiment. The vibration sensor 10 also has a vibration sensor housing 12 with a membrane 16 and a mechanical oscillator 18. The piezo drive 14 is essentially constructed the same as in the prior art and has a piezo element 20, which is connected to the membrane 16 via a compensating ceramic 32. The piezo element 20 is connected to the electronics 24 via plug contacts 28.

[0038] According to this first embodiment, the piezo element 20 is provided with a protective layer 38. Here, the exposed surfaces of the piezo element 20 are covered by the protective layer 38. The protective layer 38 is made of amorphous silicon oxide and has a layer thickness of 40 nm. In addition to the piezo element 20, the electrical connections 22 between the piezo element 20 and the electronics 24, and partially the electronics 24 or circuit board 26 and the diodes 31 are also provided with the protective layer 38. Only the plug contacts 28 are left out. The piezo drive 14 is arranged on a holder 40 here. The holder 40 serves for the central arrangement of the piezo drive 14 in the vibration sensor housing 12 and for handling the piezo drive 14. The holder 40 surrounds the piezo drive in a base area 42 over the entire circumference, and the base area 42 extends over part of the height of the piezo drive 14.A web 44 protrudes opposite the base area 42, extending to the height of the circuit board 26 and serving to secure the circuit board 26. The circuit board 26 is inserted into the web 44. It is also conceivable to arrange several webs 44 distributed over the circumference of the base area 42 in order to secure the circuit board 26 from several sides.

[0039] In the following, the same reference numerals are used to describe the first embodiment for identical or at least functionally equivalent components as for the description of the second embodiment.

[0040] Fig. 3 shows a second embodiment of a vibration sensor 10. The vibration sensor 10 has a piezo drive 14 with a plurality of stacked piezo elements 20 (piezo stack drive). In addition to the stacked piezo elements 20, the piezo drive 14 includes a plurality of connecting leads 46 in the form of wires that lead to insulated electrical contacts 48.

[0041] A pressure piece 50, 52 is arranged at an upper end and at a lower end of the piezo stack comprising the plurality of piezo elements 20.

[0042] In this second embodiment, the piezo elements 20 and the connecting leads 46 are provided with the protective layer 38. All exposed surfaces of the piezo elements 20 and the connecting leads 46 are covered by the protective layer 38. The insulated electrical contacts 48 are insulated and therefore do not need to be coated with the protective layer 38.

[0043] The piezo drive 14 is arranged in a holder 40. The holder 40 serves to clamp the piezo drive 14 against the membrane 16 and to accommodate and position the insulated electrical contacts 48. The holder 40 is formed as a single piece with a U-shaped cross-section. The holder 40 spans the piezo drive 14 over only part of its cross-sectional area, so that the upper pressure piece 52 is covered. Through-openings 54 for the passage of the insulated electrical contacts 48 are formed in the holder 40.

[0044] List of reference symbols

[0045] Vibration sensor

[0046] Vibration sensor housing

[0047] Piezo drive

[0048] Membrane mechanical oscillator

[0049] Piezo element electrical connection

[0050] electronics

[0051] circuit board

[0052] plug contact

[0053] Mating connector

[0054] diode

[0055] Leveling ceramics

[0056] Housing / Potting cup

[0057] Potting compound

[0058] protective layer

[0059] bracket

[0060] Base area

[0061] web

[0062] Connecting cable insulated electrical contacts

[0063] Pressure piece

[0064] Pressure piece

[0065] passage opening

Claims

Patent claims 1. Vibration sensor with a piezo drive (14), wherein the vibration sensor (10) has a vibration sensor housing (12), wherein a membrane (16) with at least one mechanical oscillator (18) is arranged at a process-side end of the vibration sensor housing (12), wherein the membrane (16) is connected to the piezo drive (14) in such a way that the membrane (16) is set into vibration by means of the piezo drive (14), wherein the piezo drive (14) has at least one piezo element (20), characterized in that the at least one piezo element (20) is provided with a protective layer (38).

2. Vibration sensor according to the preceding claim, characterized in that the protective layer (38) comprises a glass-like material and / or a ceramic material.

3. Vibration sensor according to one of the preceding claims, characterized in that the protective layer (38) contains silicon.

4. Vibration sensor according to one of the preceding claims, characterized in that the protective layer (38) has a layer thickness in the range of 2 nm to 1 pm.

5. Vibration sensor according to one of the preceding claims, characterized in that the protective layer (38) is a plasma-deposited layer.

6. Vibration sensor according to one of the preceding claims, characterized in that contacts (28, 46) and / or an electronics unit (24) connected to the piezo element (20) are provided with the protective layer (38).

7. Vibration sensor according to one of the preceding claims, characterized in that the piezo drive (14) is accommodated by a holder (40) which extends only over part of the height of the piezo drive (14) and / or wherein the holder (40) has a web (44) for receiving electrical contacts (48) or a printed circuit board (26).

8. Vibration sensor according to one of the preceding claims, characterized in that the piezo drive (14) is received by a holder (40) and the holder (40) is formed in one piece.

9. A method for producing a vibration sensor (10) according to one of the preceding claims, characterized in that a protective layer (38) is applied to the at least one piezo element (20).

10. Method according to the preceding claim, characterized in that the protective layer (38) is applied by means of a plasma process.

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