Piezoelectric stack drive of a vibration limit switch with end-face mounting

Abstract

Vibration sensor (100) with a diaphragm (90) that can be set into vibration and a mounting arrangement (10) for connecting the diaphragm (90) to a drive unit (11) such that vibrations of the drive unit (11) are transmitted to the diaphragm (90) and vibrations of the diaphragm (90) to the drive unit (11), wherein the mounting arrangement (10) is designed as a cylindrical sleeve which is attached at one end indirectly or directly to the diaphragm (90) or to an element connected to the diaphragm (90) and has at the other end an insertion opening (14) for the drive unit (11) characterized in that the diaphragm (90) is attached circumferentially to a process connection (80) and the mounting arrangement (10) engages the process connection (80) at least section by section in the radial direction (R) in the circumferential direction.

Description

[0001] Piezoelectric transmitters and / or receivers, vibration sensors with such piezoelectric transmitters and / or receivers, and methods for manufacturing piezoelectric transmitters and / or receivers are known from the prior art. Piezoelectric transmitters and / or receivers can be used, for example, in vibration sensors, which are frequently used as limit level sensors in level measurement technology. Such piezoelectric transmitters and / or receivers are also often referred to as actuators.

[0002] Typically, a vibration sensor has a diaphragm that can be excited to vibrate by a drive mechanism, thereby exciting a mechanical oscillator mounted on the diaphragm. Depending on the degree of coverage of the mechanical oscillator with a substance and the viscosity of that substance, the mechanical oscillator vibrates at a characteristic frequency, which is detected by the vibration sensor and converted into a measurement signal.

[0003] In the prior art, two different types of actuators are commonly used. In the first variant, a multi-segmented piezoelectric element is bonded to the diaphragm. Applying an electrical voltage to one or more segments of the piezoelectric element excites it to bend or torsion, which it transmits to the diaphragm. This causes the diaphragm to vibrate, in turn causing the mechanical oscillator to vibrate. This type of actuator produces only a limited stroke and can only be used with vibration sensors operating at temperatures significantly below the glass transition temperature of the adhesive and below the Curie temperature of the piezoelectric material. These sensors are not suitable for high-temperature applications above 150°C.

[0004] If a sensor with a larger stroke is required for an application, or if use at higher temperatures is necessary, a second type of drive, so-called piezo stack drives, is used.

[0005] Here, a stack consisting of a piezoelectric unit made up of one or more piezoelectric elements, an adapter ceramic arranged above and below the piezoelectric unit, and pressure pieces arranged above and below the adapter ceramics is clamped against a clamping bolt and a clamping nut located on the sensor's diaphragm. Applying an electrical voltage to the piezoelectric elements changes their expansion in the axial direction of the bolt, thus causing the diaphragm to vibrate. A known vibration sensor with a piezoelectric stack drive is described in Fig. 4 shown.

[0006] Fig. Figure 4 shows a section of a vibration sensor 100 known from the prior art, which is used, for example, in level measurement technology as a limit level sensor.

[0007] The vibration sensor 100 has a membrane 90 that can be excited to oscillation via a piezo stack drive 11, which acts as a piezoelectric transmitting and / or receiving device, by means of which a mechanical vibration unit 7 arranged on the membrane 90 can be excited to oscillation.

[0008] The vibration sensor 100 has a piezoelectric stack drive 11 consisting of stacked piezoelectric elements 15 contacted via electrodes 16, with an adapting ceramic element 17 and a pressure piece 19, in this case made of a metallic material, arranged above and below the stack. The adapting ceramic elements 17 serve to adjust the coefficient of thermal expansion between the piezoelectric elements 15 and the pressure pieces 19. The pressure pieces 19 are designed such that a force from the piezoelectric stack is absorbed over its entire surface.

[0009] The piezoelectric stack drive 11 is clamped against the diaphragm 90 by means of a clamping nut 92 via a clamping bolt 91 integrally molded onto the diaphragm 90, so that vibrations of the piezoelectric stack drive 11 are effectively transmitted to the diaphragm 90 and vice versa. Electrodes 16 for contacting the piezoelectric elements 15 are connected to connecting leads 31, which run along the outside of the piezoelectric stack drive 11 and are led to sensor electronics on the rear side. A sensor housing 98 is arranged on the diaphragm 90 via an outer, circumferential rim extending in the rear direction, and is connected to or integrally formed with the rim of the diaphragm 90.

[0010] A disadvantage of the known requirements is that the piezoelectric stack drive can only be mounted inside the vibration sensor and pre-tensioned there. There is a growing demand for the drive to be pre-assembled and, if necessary, tempered or operated for a specific period in its pre-assembled state. This allows the drive elements to settle beforehand, thus preventing a loss of pre-tension or the need to temper the entire sensor. Furthermore, there is a desire for drives with piezoelectric elements designed as full-surface circular disks, as this provides a larger surface area for generating drive power.

[0011] Furthermore, the diaphragm must be relatively thick to accommodate a preload via a bolt attached to the diaphragm, in order to provide a mounting point for the rod-shaped bolt. This requirement contradicts the general requirement that the diaphragm must be relatively thin to reduce the energy required to operate the vibration sensor, which must be supplied by the drive mechanism. A thin diaphragm is much more easily excited into vibration, thus necessitating a reduction in diaphragm thickness.

[0012] In a design with a central bolt, the installed drive operates under tension; that is, axial expansion of the drive pulls the diaphragm and bolt inwards, causing the diaphragm to vibrate. At high pressures, the diaphragm is also forced inwards by pressure, which can lead to a loss of drive preload and thus a loss of drive power.

[0013] Further state of the art is known from DE 101 29 556 A1 and US 8,390,177 B2.

[0014] It is therefore an object of the present invention to provide a mounting arrangement for a drive of a vibration sensor that allows secure attachment relative to the diaphragm while simultaneously reducing the thickness of the diaphragm. Furthermore, it is an object of the present invention to provide a vibration sensor with such a mounting arrangement.

[0015] This problem is solved by a vibration sensor with the features of claim 1, and by a method for attaching a mounting arrangement with the features of claim 7.

[0016] An assembly arrangement according to the invention for connecting a membrane of a vibration sensor with a drive unit in such a way that vibrations of the drive unit are transmitted to the membrane and vibrations of the membrane to the drive unit is characterized in that the assembly arrangement is designed as a cylindrical sleeve which has at one end a fastening section for indirect or direct attachment to the membrane or an element connected to the membrane and at the other end an insertion opening for the drive unit.

[0017] Because the mounting assembly is designed as a cylindrical sleeve and the drive unit can be positioned inside this sleeve, force is transmitted between the diaphragm and the rear side of the drive unit at the diaphragm's circumference. A central pull bolt, either welded to or integrally formed with the diaphragm, is no longer necessary, allowing the diaphragm to be made thinner. Furthermore, the drive unit is no longer operated under tension but under compression. This means that an increasing pressure in a process chamber no longer relieves the drive unit and leads to a decrease in efficiency, but instead applies more pressure to the drive, thus actually increasing its efficiency.

[0018] Preferably, the insertion opening is provided with a pre-tensioning option for the drive. For example, the sleeve can have an internal thread on a part facing away from the fastening section, into which a clamping screw can engage to pre-tension the drive.

[0019] In this context, direct or indirect attachment of the mounting arrangement to the diaphragm means that the mounting arrangement is attached to the diaphragm directly or via intermediate elements. Attachment according to the present application, in the sense of its technical function—namely, the transmission of vibrations from the drive unit to the diaphragm and from the diaphragm to the drive unit—means that the mounting arrangement is fixed relative to the diaphragm in the axial direction towards the drive unit.

[0020] The mounting section can be designed as a rib extending radially around the sleeve, at least partially. Such a rib allows the mounting assembly to, for example, engage over or behind part of the diaphragm, so that the drive unit can be tensioned against the diaphragm by means of the mounting assembly.

[0021] If the membrane has, for example, a circumferential, axially extending, and preferably rib-shaped edge arranged on the membrane, preferably formed integrally with it, this can serve directly or indirectly as a support for the mounting arrangement.

[0022] In one embodiment, the bridge can form part of a bayonet fitting. In this configuration, the bridge of the mounting arrangement can engage behind a counterpart of the bayonet fitting formed by the rim and thus be attached to it.

[0023] The mounting arrangement can further include an anti-rotation device which interacts with the membrane or an element connected to the membrane in such a way that rotation of the mounting arrangement relative to the membrane is limited and preferably prevented.

[0024] The anti-rotation device can, for example, be designed as a recess in or molding on the web, which interacts with a corresponding element in such a way that rotation of the assembly arrangement in the assembled state is limited or prevented.

[0025] Such an anti-rotation device ensures that the drive unit can be screwed into the mounting arrangement, for example via a clamping screw, without having to separately fix the mounting arrangement against rotation.

[0026] A vibration sensor according to the invention, comprising a diaphragm that can be set into vibration and a mounting arrangement for connecting the diaphragm to a drive unit such that vibrations of the drive unit are transmitted to the diaphragm and vibrations of the diaphragm to the drive unit, is characterized in that the mounting arrangement is designed as a cylindrical sleeve which is attached at one end indirectly or directly to the diaphragm or an element connected to the diaphragm and has at the other end an insertion opening for the drive unit.

[0027] A mechanical vibration unit can be arranged on the membrane on the process side to transmit the vibrations of the membrane to a medium.

[0028] The vibration sensor according to the invention is characterized by the fact that force transmission between the diaphragm and a rear side of the drive unit takes place at the circumference of the diaphragm. A central pull bolt welded to or integrally formed with the diaphragm is no longer necessary, allowing the diaphragm to be made thinner. Furthermore, the drive unit is no longer operated under tension but under compression, meaning that an increasing pressure in a process chamber no longer relieves the drive unit and leads to a decrease in efficiency, but rather applies more pressure to the drive and thus actually increases its efficiency.

[0029] In one embodiment, the membrane can have a circumferential, axially extending, preferably rib-shaped edge, and the mounting arrangement can be attached to this edge.

[0030] Such an edge improves the stability of the membrane and allows for the fastening of the mounting arrangement.

[0031] According to the invention, the membrane is attached circumferentially to a process connection, wherein the mounting arrangement engages the process connection at least sectionally in the circumferential direction and in the radial direction.

[0032] In this configuration, the mounting assembly is not directly attached to the membrane, but rather engages behind a section of the process connection that is fixed to the membrane, for example, welded to the membrane or to the membrane edge described above. This allows the mounting assembly to be attached to the membrane particularly easily.

[0033] To align the mounting arrangement relative to the process connection and thus also relative to the membrane, the process connection can have a first stage extending radially, at least partially circumferentially, in which the mounting arrangement with its fastening section can be positioned. The mounting arrangement can thus engage the process connection precisely in the area of ​​the first stage in the radial direction. This means that the radius of the mounting arrangement in this area is larger than the radius of the recess in the process connection, so that the mounting arrangement is held in a force-fit position in the axial direction.

[0034] Additionally or alternatively, the edge of the membrane can have a circumferential, radially recessed second step into which the mounting arrangement engages. This second step can also serve to center the mounting arrangement relative to the membrane.

[0035] The mounting arrangement may further include an anti-rotation device which interacts with the membrane or a part connected to the membrane in such a way as to prevent the mounting arrangement from rotating relative to the membrane.

[0036] Such an anti-rotation device provides a simple way to fix the mounting arrangement relative to the membrane and thus also relative to the process connection connected to the membrane in the circumferential direction, so that the mounting arrangement does not need to be additionally fixed when inserting and pre-tensioning the drive unit.

[0037] Preferably, the mounting arrangement is positioned between the rim and the process connection such that the rim, the process connection, and the mounting arrangement can be welded together in a single operation. This design facilitates simple assembly of the entire vibration sensor. Preferably, the rim and the process connection have equally sized and correspondingly arranged steps between which the circumferential web of the mounting arrangement can rest. The diaphragm and the process connection are aligned relative to each other by the mounting arrangement located between the steps.

[0038] A method according to the invention for attaching a mounting arrangement for a drive unit to a membrane of a vibration sensor comprises the following steps: - providing a process connection with a radially receding first stage, - Arranging a mounting arrangement designed as a cylindrical sleeve, having at one end a fastening section designed as a circumferential web extending in the radial direction of the sleeve for fastening to the edge of the membrane and at the other end an insertion opening for the drive unit, in the process connection such that the fastening section and the first stage are aligned relative to each other, - Arranging the membrane with a circumferential, axially extending, preferably rib-shaped edge, with a radially recessed second stage at the process connection and the mounting arrangement such that both the process connection and the membrane as well as the second stage and the mounting section are aligned relative to each other and - Welding the edge to the process connection and the fastening section in one step.

[0039] The inventive method simplifies the overall assembly of the vibration sensor, since the individual components are self-centering.

[0040] Because the mounting assembly is not a single unit with the process connection or the diaphragm, different materials can be used for all components. In particular, the mounting assembly can be made of a material with a suitable coefficient of thermal expansion, such as titanium, while the process connection can be made of a more economical material. This largely prevents power losses caused by a reduction in the drive unit's preload due to thermal expansion.

[0041] A pressure screw for pre-tensioning the drive against the diaphragm should have a thickness, preferably a threaded engagement with the mounting arrangement, of at least five times the thickness of the diaphragm in order to prevent deflection of the pressure screw during operation of the drive.

[0042] The present invention is explained in detail below with reference to exemplary embodiments and the accompanying figures. These show: Fig. 1 a longitudinal section through a vibration sensor with a first embodiment of a mounting arrangement, Fig. 2 a close-up from Fig. 1, Fig. 3 a process connection and a second embodiment of a mounting arrangement with an anti-rotation device and Fig. 4. A longitudinal section through a vibration sensor according to the state of the art (already discussed).

[0043] In the figures, unless otherwise stated, the same reference symbols denote the same components with the same function.

[0044] In the following, the terms drive unit and drive are used interchangeably.

[0045] Fig. Figure 1 shows a longitudinal section through a vibration sensor 100 with a first embodiment of a mounting arrangement 10 according to the present application.

[0046] In the present embodiment, the vibration sensor 100 has a diaphragm 90 that can be excited to vibration by a drive unit 11. In the embodiment shown, the drive unit has a piezoelectric stack drive 11 as its drive. On the drive side, a circumferential rim 93 extending in the axial direction A is arranged on the diaphragm 90, to which a process connection 80 is attached. On a side of the diaphragm 90 facing away from the drive unit 11, a mechanical vibration unit 7, which is not shown in its entirety, is arranged. The mechanical vibration unit 7 is designed to transmit vibrations of the diaphragm 90 to a medium surrounding the mechanical vibration unit 7.

[0047] In the illustrated embodiment, the drive unit 11 is attached to the diaphragm 90 via a mounting arrangement 10, which in this embodiment is designed as a cylindrical sleeve, and clamped against it by a clamping screw 92. The mounting arrangement 10 has a fastening section 12 for attachment to the diaphragm 90. In this embodiment, the fastening section 12 is designed as a circumferential web extending in the radial direction R, which is integrally formed at an end of the sleeve 10 facing the diaphragm 90. In this embodiment, the web 12 lies between a circumferential first stage 81, which is formed on the inside of a diaphragm-side end of the process connection 80 and is recessed in the radial direction R, and a second stage 94, which is formed circumferentially in the edge 93 of the diaphragm 90 and is also designed to be recessed in the radial direction R.

[0048] In the present embodiment, the first stage 81 and the second stage 94 are designed such that the web 12 lies positively between the stages 81, 94, wherein the extension of the web 12 in axial direction A is spanned half by the first stage 81 and the second stage 94 respectively.

[0049] In the present embodiment, the piezoelectric stack drive 11 is formed from a stack of two piezoelectric elements 15 with electrodes 16 arranged between, above, and below the piezoelectric elements for electrical contact. The stack includes, on its upper and lower sides, an adapting ceramic element 7 for adjusting the thermal expansion coefficients of the piezoelectric elements 15 to the pressure pieces 19 arranged below in the stack, as well as a clamping screw 92 acting on the stack from above. The clamping screw 92 engages in an internal thread of the mounting assembly 10, thus enabling the piezoelectric stack drive 11 to be clamped against the diaphragm. In the present embodiment, the pressure piece 19 is conically tapered, so that a force applied by the drive unit 11 is transmitted to the diaphragm 90 along a circular path formed by the pressure piece 19.

[0050] To align the drive unit 11 with respect to the diaphragm 9, the diaphragm 90 has a centering aid 95 designed as a ring-shaped circumferential rib, within which the conically shaped pressure piece 19 comes to lie in a self-centering manner when the drive unit 11 is inserted into the process connection 80.

[0051] In the present embodiment, the clamping screw 92 has a central opening through which connecting leads 31 are guided for contacting the piezoelectric elements 15. Furthermore, the clamping screw 92 has a threaded engagement extending to a length at least five times the thickness of the diaphragm 90, ensuring that any force generated in the piezoelectric stack drive 11 is reliably directed towards the diaphragm 90 and does not deform the clamping screw 92. Additionally, the clamping screw 92 has a section extending axially A towards the threaded engagement in the direction of the diaphragm that is not engaged in the thread. This section is appropriately dimensioned to compensate for the different coefficients of thermal expansion of the mounting arrangement 10 and the piezoelectric stack drive 11.

[0052] The piezo stack drive 11, together with the clamping screw 92, can be pre-assembled outside the vibration sensor 100 using an assembly aid 70, wherein the assembly aid 70 is designed such that the individual components of the piezo stack drive 11 can be inserted into the assembly aid 70 one after the other and lock into place within the assembly aid 70 by means of a spring mechanism, so that together with the clamping screw 92 a separately manageable unit is formed.

[0053] In Fig. Figure 2 shows a close-up of the mounting section 12 of the assembly 10. The area shown is where the edge 93 of the membrane 90 and the process port 80 meet, with the web 12 of the assembly 10 lying between the first stage 81 of the process port 80 and the second stage 94 of the edge 93. Fig. Figure 2 clearly shows that the process connection 80 and the edge 93 of the membrane 90 are connected by a weld 96. In the present embodiment, the weld 96 is designed such that it welds through the process connection 80 and the edge 93 in the area of ​​the steps 81, 94, and the mounting section 12 of the mounting assembly 10, which is designed as a web, is also welded in place. In this way, the mounting assembly 10 is fixed not only in the axial direction A by resting on the steps 81, 94, but also in the circumferential direction by the welding. This ensures that the mounting assembly 10 does not need to be additionally fixed when the drive unit 11 is screwed in with the clamping screw 92, but is also fixed in the circumferential direction due to the welding to the process connection 80 and the edge 93.

[0054] In Fig.Figure 3 shows a process connection 80 with a mounting arrangement 10 in a further embodiment, wherein the mounting arrangement has an anti-rotation device 13 in the area of ​​the fastening section 12. In the present embodiment, the anti-rotation device 13 is formed as a recess in the fastening section 12, which is designed as a circumferential rib. A lug 82, integrally formed on the process connection 80, engages in this recess 13 of the anti-rotation device, thus preventing the mounting arrangement 10 from rotating relative to the process connection 80. In this embodiment, welding and thus fixing the mounting arrangement 10 relative to the process connection 80 and the edge 93 of the membrane 90 is therefore not necessary, since movement in the circumferential direction is prevented by the engagement of the lug 82 in the recess 13.Movement in the axial direction A is in turn prevented by the fact that the fastening section 12 is designed as a circumferential web which is clamped in the process connection 80 in the axial direction A at least away from the membrane 90 due to a smaller radius. Reference sign 7 mechanical vibration unit 10 Mounting arrangement / sleeve 11 Drive unit, piezo stack drive 12 Fastening section / web 13 Anti-rotation device 14 Insertion opening 15 Piezoelectric element 16 electrodes 17 Adaptive ceramics 19 printed piece 31 connecting lines 70 Assembly aid 80 Process connection 81 first stage 89 Insulating sleeve 90 Membran 91 bolts 92 Tensioning screw 93 Rand 94 Second stage 95 Centering aid 96 weld seam 98 Sensor housings 100 Level sensor / vibration sensor A Axial direction R Radial direction

Claims

[1] Vibration sensor (100) with a diaphragm (90) that can be set into vibration and a mounting arrangement (10) for connecting the diaphragm (90) to a drive unit (11) such that vibrations of the drive unit (11) are transmitted to the diaphragm (90) and vibrations of the diaphragm (90) are transmitted to the drive unit (11), wherein the mounting arrangement (10) is designed as a cylindrical sleeve which is attached at one end indirectly or directly to the diaphragm (90) or to an element connected with the diaphragm (90) and has at the other end an insertion opening (14) for the drive unit (11) characterized by , that the membrane (90) is attached circumferentially to a process port (80) and the mounting arrangement (10) engages the process port (80) at least section by section in the radial direction (R) in the circumferential direction. [2] Vibration sensor (100) according to claim 1, characterized bythat the membrane (90) has a circumferential, axially extending (A) edge, preferably rib-shaped, and the mounting arrangement (10) is attached to this edge (93). [3] Vibration sensor (100) according to claim 1 or 2, characterized by , that the process connection (80) has a first stage (81) that is at least partially circumferential and extends in the radial direction (R). [4] Vibration sensor (100) according to claim 3, characterized by , that the edge (93) of the membrane (90) has a circumferential, radially recessed second step (94) into which the mounting arrangement (10) engages. [5] Vibration sensor (100) according to any one of the preceding claims, characterized bythat the mounting arrangement (10) has an anti-rotation device (13) which interacts with the diaphragm (90) or a part connected to the diaphragm in such a way that rotation of the mounting arrangement (10) relative to the diaphragm (90) is limited, preferably prevented. [6] Vibration sensor (100) according to any one of claims 3 to 5, characterized by , that the assembly arrangement (10) is inserted between the rim (93) and the process connection (80) in such a way that the rim, the process connection (80) and the assembly arrangement (10) can be welded together in one operation. [7] Method for attaching a mounting arrangement (10) for a drive unit (11) to a membrane (90) of a vibration sensor (100) comprising the following steps: - providing a process connection (80), with a first stage (81) receding in the radial direction (R), - Arranging the mounting arrangement (10), which is designed as a cylindrical sleeve and has at one end a fastening section (12) designed as a circumferential web (12) extending in the radial direction (R) of the sleeve for fastening to an edge (93) of the membrane (90) and at the other end an insertion opening (14) for the drive unit, in the process connection (80) such that the fastening section (12) and the first stage (81) are aligned relative to each other, - Arranging the membrane (90) with the circumferential, axially (A) extending, preferably rib-shaped edge (93), with a radially recessed second step (94) of the edge (93) at the process connection (80) and the mounting arrangement (10) such that both the process connection (80) and the membrane (90) as well as the second step (94) and the fastening section (12) are aligned relative to each other, so that the rib (12) lies between the first step (81) and the second step (94), and - Welding the rim (93) to the process connection (80) and the fastening section (12) in one step.

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