Vibration sensor

Abstract

The invention relates to a vibration sensor for detecting fill levels, limit levels or for monitoring process parameters, comprising: - a membrane (12) that can be caused to effect vibration, - a drive unit (14), wherein the drive unit (14) comprises a piezo actuator (16), which comprises a piezo element (18) and an electrode (20) that is connected to the piezo element (18) and has an electrode surface (21), - an adhesive connection (26) used to operatively connect the piezo actuator (16) to the membrane (12) so that vibrations of the piezo actuator (16) can be transferred to the membrane (12), - an electronics system (22), - an electrical connection (24) between the electronics system (22) and the electrode surface (21), wherein the piezo element (18) has a cutout (32) and the electrical connection (24) runs through the cutout (32), wherein an electrical conductor (110) that is electrically connected to the electrode surface (21) is arranged in the cutout (32) and the vibration sensor has a closure element (125) that closes the cutout (32).

Description

[0001] Vibration sensor

[0002] The invention relates to a vibration sensor according to claim 1.

[0003] Vibration sensors, used, for example, as vibrating limit switches, are known in the art. The vibration sensor has a membrane that can be excited to vibrate via a drive unit, often by means of which a mechanical oscillator, such as a tuning fork, arranged on the membrane can be excited to vibrate. Depending on the coverage level of the mechanical oscillator with a filling material and the viscosity of this filling material, the oscillator can oscillate at a characteristic frequency that can be detected by the vibration sensor and converted into a measurement signal. Such vibration sensors can also be referred to as vibronic sensors. Piezo actuators with a piezo element, e.g., a piezoceramic, and electrodes are often used as electromechanical transducers in the drive unit of such a sensor.A well-known concept involves securing the piezo actuator using an adhesive bond. This concept is becoming increasingly popular because it offers improved performance, lower manufacturing costs, and greater automation potential than, for example, a screw-mounted piezo actuator.

[0004] For this purpose, an adhesive layer can be applied to the membrane, a compensating element, e.g., a compensating ceramic, can be bonded to the membrane, and an adhesive layer can be applied to the compensating ceramic, creating an adhesive connection to the piezo actuator. The piezo actuator, in particular an electrode surface of its electrode, can then be contacted, e.g., via a flexible circuit board, to establish an electrical connection between the electrode surface and an electronic system. The flexible circuit board can be a so-called flex conductor.

[0005] A disadvantage here can be that the piezo actuator must be very precisely aligned in terms of its position and rotational orientation before bonding and often has to be fixed until the adhesive has cured. In addition, this alignment is often difficult. Another disadvantage can be that the performance of the piezo actuator is not as high as would be desirable. A further disadvantage can be that the adhesive bond used to effectively connect the piezo actuator to the membrane, which must be thin to achieve good vibration transmission, is not sufficiently stable or uniform. In addition, this adhesive bond and the electrical connection between the electronics and the electrode surface can negatively influence each other. For example, this adhesive bond can make it difficult to establish the electrical connection.

[0006] It is therefore the object of the invention to provide a further developed vibration sensor.

[0007] The object is achieved according to the invention with the features of patent claim 1. Further practical embodiments and advantages are described in conjunction with the dependent claims.

[0008] A vibration sensor according to the invention is used to detect fill levels, limit levels, or to monitor process parameters and comprises a membrane that can be set into vibration. Furthermore, a vibration sensor according to the invention comprises a drive unit, wherein the drive unit comprises a piezo actuator. The piezo actuator has a piezo element and an electrode connected to the piezo element. The electrode has an electrode surface. Furthermore, a vibration sensor according to the invention comprises an adhesive connection, with the aid of which the piezo actuator is operatively connected to the membrane in such a way that vibrations of the piezo actuator can be transmitted to the membrane and, preferably, vibrations of the membrane can also be transmitted to the piezo actuator. This adhesive connection can also be referred to as an assembly adhesive connection. The piezo actuator can be mounted in the sensor using the assembly adhesive connection.The piezo actuator preferably serves both to cause the membrane to vibrate and to detect the vibration of the membrane. The drive unit is therefore, in particular, a bonded piezo drive or bonded piezo actuator. Furthermore, a vibration sensor according to the invention comprises electronics and an electrical connection between the electronics and the electrode surface. The piezo element has a recess, and the electrical connection runs through the recess. An electrical conductor electrically connected to the electrode surface is arranged in the recess. The electrical conductor can be part of the piezo actuator and can be a metallization of the wall of the recess of the piezo element. The vibration sensor has a closure element that closes the recess.

[0009] In this way, a prerequisite is created that the electrical connection only slightly affects the active surface of the piezo element and at most in a way that causes no or only minor spurious modes. The effect that an asymmetrical or offset positioning of the piezo actuator promotes or amplifies erroneous signals when excited by spurious modes due to the geometry of the piezo actuator and the matching vibrating fork design can be reduced or prevented. This can improve the performance of the piezo actuator and reduce susceptibility to spurious modes. The piezo drive can therefore be designed particularly easily to be symmetrical, e.g., rotationally symmetrical. In the context of this document, rotationally symmetrical means that a rotation through any angle around the axis of rotation leads to a mapping onto itself.A vibration sensor according to the invention can be less sensitive to positional tolerances of the piezo actuator. In a vibration sensor according to the invention, a prerequisite is also created for the adhesive bond to be particularly reliable, since the closure element can prevent or reduce a capillary effect of the recess that weakens the adhesive bond. The suction or penetration of adhesive from the assembly adhesive bond into the recess before it has hardened due to the capillary effect can be reduced or avoided. The strength of the assembly adhesive bond can also depend on whether suitable adhesive menisci, i.e. adhesive beads or adhesive concaves, form at the edges. This is because these adhesive menisci can even out the mechanical stress distribution or prevent points of attack for crack formation. In particular, the closure element can prevent unevenness or weakening of the assembly adhesive bond ordisruption of the formation of an adhesive meniscus caused by the adhesive of the assembly adhesive bond can be prevented with a high degree of certainty or reduced to an acceptable level. The extent to which the adhesive of the assembly adhesive bond penetrates into the recess can be reduced to the formation of an adhesive meniscus, which, due to its shape and size, is particularly effective in preventing points of attack for crack formation. Preventing or reducing the suction or penetration of the adhesive of the assembly adhesive bond into the recess can have the further advantage that no or less area of ​​the wall of the recess or the metallization of this wall is covered by the adhesive of the assembly adhesive bond and is therefore no longer available for electrical contact. This can prevent or reduce the difficulty in establishing an electrical connection between the electronics and the electrode surface caused by the assembly adhesive bond.

[0010] This is particularly advantageous for small recesses, where the capillary effect would otherwise be particularly strong. This also makes it possible to use a small recess without compromising the quality of the adhesive bond. By using a small recess, the loss of the piezo element's active area can be reduced. This can result in the largest possible amplitude of the oscillation of the piezo actuator. In the context of this document, a small recess refers in particular to a recess with a diameter smaller than 5 mm, 2 mm, or 1 mm. The diameter of the recess is preferably larger than 0.1 mm.

[0011] The diameter of the recess is preferably 0.8 mm. Since recesses smaller than 0.8 mm would have to be lasered into the green body and cannot be drilled, this simplifies production, often with a sufficiently small recess.

[0012] The closure element preferably seals the recess in an airtight manner. This ensures that any air remaining in the recess—particularly trapped between the closure element and the adhesive of the assembly adhesive joint—effectively counteracts the capillary effect and effectively prevents or reduces the penetration of the adhesive of the assembly adhesive joint into the recess before it cures.

[0013] The closure element can at least almost completely fill the recess. This means, in particular, that the closure element at least almost completely displaces the air in the recess. This can particularly effectively reduce or prevent the ingress of adhesive from the assembly adhesive bond into the recess before it has cured.

[0014] The closure element preferably extends at least on the side of the piezo element facing away from the membrane. The assembly adhesive connection has an area in which it is in contact with the electrode surface. This area is referred to in particular as the connection area in this document. The connection area can comprise the entire assembly adhesive connection or almost the entire assembly adhesive connection. An adhesive meniscus of the assembly adhesive connection can border the connection area on the inside and / or outside. The closure element can ensure that the adhesive connection has an at least almost constant thickness in the connection area. This creates a prerequisite for a resilient, stable adhesive connection. The thickness can be between 20 μm and 80 μm. In addition, the closure element can ensure that the adhesive meniscus(es) are formed at least largely undisturbed.

[0015] Preferably, the connecting region of the adhesive bond is arranged annularly around the recess. Preferably, the change in the thickness of the adhesive bond in the connecting region in the radial direction is less than 10% or 1% of the maximum thickness of the adhesive bond in the connecting region.

[0016] If the recess has a diameter of less than 5 mm or 2 mm or 1 mm, then the performance of the piezo actuator can be particularly high, since only a small amount of the active area of ​​the piezo element is lost through the recess.

[0017] Preferably, the closure element is different from the compensating element and the membrane. In one embodiment, the closure element is at least also arranged in the recess. In one embodiment, the compensating element is arranged at least also or exclusively on the side of the piezo element facing away from the membrane.

[0018] The closure element is preferably designed as a contact element and is in electrical contact with the electrical conductor. This allows for component savings and simplifies production.

[0019] The electrical conductor and / or the contact element are in particular part of the electrical connection between the electronics and the electrode surface. The electrical conductor can end in the recess or extend through the recess, thus also extending outside the recess. Preferably, the electrical conductor extends at least from a first side of the piezoelectric element facing the membrane to the second side of the piezoelectric element facing away from the membrane. The electrical conductor can comprise a connecting section of the electrode or be formed thereby. The electrical conductor preferably comprises a metallization of the wall of the recess of the piezoelectric element or is formed thereby.

[0020] The area with which the contact element can be in electrical contact with the metallization of the wall of the recess of the piezo element can be larger than 10% or 25% of the wall area of ​​the recess of the piezo element.

[0021] In one embodiment, the contact element comprises or is formed by conductive adhesive. The contact element can comprise conductive adhesive and another element, in particular a contact pin.

[0022] If the contact element is glued to the piezo actuator or the electrical conductor, in particular by means of conductive adhesive, then the electrical contact with the electrical conductor can be established particularly reliably and permanently and the recess can be closed particularly reliably and tightly.

[0023] If the contact element is glued into the recess, this creates the conditions for a particularly large contact area with the electrical conductor. Furthermore, the active surface of the piezo element may be unaffected or only slightly affected by the contact area.

[0024] The contact element can be used to establish electrical contact with the electrode or the electrical conductor, in particular to connect the electrical line or a circuit board to the electrode. The contact element can comprise, for example, a solder lug or a plug contact. The plug contact can be crimpable.

[0025] If the contact element, as preferred, comprises or is formed by a contact pin, several advantages can be achieved. Among other things, this again creates the prerequisite for a large contact surface and / or one that does not, or only slightly, impairs the active surface of the piezo element.

[0026] The adhesive of the assembly adhesive bond and / or the conductive adhesive may comprise a filler.

[0027] The contact pin can have an outer dimension that is complementary to the inner dimension of the recess. For example, the pin can have an outer diameter that is slightly smaller than the inner diameter of the recess. Preferably, the outer diameter of the pin and the inner diameter of the recess are coordinated to one another in such a way that a minimum gap is maintained so that the filler of the conductive adhesive can still reach the gap. Preferably, the contact pin has a lateral surface arranged in the recess, which is more preferably in electrical contact with the piezo actuator.

[0028] Preferably, the contact pin has a collar that can be glued to the piezo actuator. This can improve the mechanical connection and / or electrical contact of the contact pin to the piezo actuator.

[0029] If the contact pin is part of an electrical connector, components such as a separate plug contact can be eliminated.

[0030] In one embodiment, the vibration sensor comprises a circuit board with electronic components. The contact element preferably electrically and / or mechanically connects the piezo actuator, in particular the electrical conductor, to the circuit board. The mechanical connection can be rigid, thus preventing any relative movement between the piezo actuator or electrical conductor on the one hand and the circuit board on the other.

[0031] Particularly if the contact element is formed by conductive adhesive, the piezo actuator can be mechanically and / or electrically connected directly to the circuit board. This eliminates the need for components such as a contact pin. Particularly in an embodiment in which the contact pin is not part of an electrical plug connection, an electrical plug contact can be arranged on the circuit board.

[0032] The contact element can have two sides and contact the piezo actuator or the electrical conductor with one side directly or by means of conductive adhesive and the circuit board with the other side directly or by means of conductive adhesive.

[0033] The electronics of the vibration sensor can include the electronic components of the circuit board or be formed by them.

[0034] Preferably, a further electrode is arranged on the side of the piezoelectric element facing away from the membrane. The vibration sensor preferably comprises a further contact element for establishing electrical contact with the further electrode.

[0035] If the additional contact element comprises an additional contact pin, which is preferably bonded to the additional electrode, this creates the prerequisite for particularly reliable overall contacting of the piezo actuator. The additional contact pin can have a thickened portion at the end to increase the contact and / or bonding surface with the additional electrode. The thickened portion can be disc-shaped.

[0036] If the contact pin and preferably the further contact pin are part of an electrical plug connection, the vibration sensor in particular comprises a plug connection glued to the piezo actuator. This can be provided instead of a flexible conductor. This electrical plug connection is preferably standardized. In particular, the contact pin then has the distance of a standardized plug system, called a grid dimension, from the further contact pin. Preferably, the contact pin and the further contact pin then serve as contacts to a mating connector. For this purpose, the contact pins can each have a recess in the form of a blind hole, which can provide a sleeve section of the respective contact pin for receiving a mating connector contact. Alternatively, the contact pins can be designed to be contacted from the outside by a mating connector contact, for example by means of a spring contact.

[0037] The vibration sensor is also referred to as a sensor in this document. The term "piezo element" refers to the part of the piezo actuator made of piezoelectric material, e.g., a piezo disc. The piezo element can be, for example, a piezo crystal or a piezoelectric ceramic.

[0038] The piezoelectric element preferably has a first side facing the membrane. In the context of this document, the term "electrode surface" refers in particular to the part of the electrode that is arranged—preferably exclusively—on this first side of the piezoelectric element. The electrode surface is preferably circular. The piezoelectric element preferably has a second side facing away from the membrane. The recess preferably extends through the piezoelectric element, in particular from the first side to the second side. The recess can be in the form of a bore, and the wall of the recess can be hollow-cylindrical.

[0039] The electrode surface in the sensor is preferably arranged between the piezo element and another component, e.g., a compensating element or the membrane, and is therefore particularly not freely accessible. The electrode surface is preferably covered, particularly completely, by the adhesive bond.

[0040] The electrical connection can be used to contact the electrode, and in particular the electrode surface. Preferably, the electrode surface can be contacted from the side of the piezo element facing away from the membrane, typically from above, through the recess. The contact is made in particular around the piezo element or around an edge of the piezo element formed in particular by the recess, and can therefore also be referred to as re-contacting.

[0041] One area of ​​the piezo actuator preferably serves to cause the membrane to vibrate. The piezo actuator preferably does not have separate areas for excitation and detection of the membrane's vibration. In the preferred embodiment, a common area of ​​the piezo actuator is used both to cause the membrane to vibrate and to determine the frequency and / or amplitude at which the membrane vibrates. A common area of ​​the piezo actuator can be used—e.g., alternately—as a transmitter and receiver. The electrode is preferably galvanically isolated from the membrane.

[0042] The electronics are, in particular, electronic parts or components that supply the piezo actuator with drive energy and / or that evaluate the signals detected by the piezo actuator. The electronics can be evaluation electronics. The electronics are connected, in particular, to a power source for power supply. The electronics can be arranged in a housing that serves to insulate and shield the electronics.

[0043] The term "electrical connection between the electronics and the electrode surface" in the context of this document refers in particular to the entirety of all elements that electrically connect the electrode surface to the electronics. The electrical connection preferably runs partially inside and partially outside the recess. The electrical connection preferably comprises contacting the piezo actuator, in particular with the aid of the contact element. The electrical connection can comprise an electrical line, e.g., a cable and / or a flexible printed circuit board. The electrical connection can comprise contacting, in particular, an electrical line or a printed circuit board with the electrode or the connecting section or the contact element. The contact can comprise a conductive adhesive. The contact can be soldered and / or clamped.The contacting of the piezo actuator comprises in particular the contacting of the electrical conductor arranged in the recess, in particular the metallization of the wall of the recess.

[0044] The electrical connection can comprise a soldered, adhesive and / or welded connection, in particular between the electrical line and / or a printed circuit board on the one hand and the electrode or electrode surface or the contact element on the other. The adhesive connection is preferably carried out using conductive adhesive. The printed circuit board can be made of solid or flexible material. The printed circuit board can also be referred to as a circuit board. The piezo element preferably extends in one plane. The piezo element preferably has a base area which lies in particular in this plane. If the electrical connection in the plane of the piezo element does not protrude beyond the outline of the piezo element, then the sensor can be of particularly compact design. If the piezo element has the shape of a circular ring, with a circular ring as the top side and a circular ring as the bottom side, then the base area refers to the area of ​​one of the circular rings.

[0045] If the assembly adhesive connection is electrically conductive and preferably forms the electrode surface, then a particularly simple structure can result.

[0046] Preferably, the assembly adhesive bond is not electrically conductive. This allows it to be particularly thin and transmit vibrations with particularly low losses. Furthermore, the strength of the adhesive in the assembly adhesive bond can then be particularly high.

[0047] If the recess runs through the center of the piezo element, this further ensures that the electrical connection or the recess impairs the active surface of the piezo element in a way that does not cause spurious modes. Furthermore, this eliminates the need to align the rotational position of the piezo actuator before bonding.

[0048] The piezoelectric element or piezoelectric actuator can be disc-shaped. Preferably, the piezoelectric element or piezoelectric actuator is annular disc-shaped, preferably with a circular outline. The piezoelectric element or piezoelectric actuator can be rotationally symmetrical. The recess can run in the region of the rotational axis or parallel to the rotational axis of the piezoelectric element or piezoelectric actuator.

[0049] Preferably, the electrode is not a separate component, but rather a metallized surface of the piezoelectric element. The electrode is preferably formed as a metallized surface applied to the piezoelectric element. The electrode surface is preferably flat, i.e., it extends exclusively in one plane.

[0050] The wall of the recess of the piezo element is preferably inclined towards the first side of the piezo element, in particular by approximately 90 degrees. If the piezo element is rotationally symmetrical about a rotation axis and the electrode is point-symmetrical to the rotation axis or rotationally symmetrical to the same rotation axis, then a prerequisite is created that any rotation of the piezo actuator during assembly has no negative effects. Therefore, alignment means, such as an alignment notch in the piezo actuator, can be omitted, and a larger active area of ​​the piezo actuator becomes possible. If the piezo actuator is designed as an annular disk glued into the sensor, in particular with central re-contacting, then the piezo actuator can be particularly insensitive to tolerances during assembly.

[0051] In addition to the electrode surface, the electrode can have a connecting section. The connecting section can form the electrical conductor. The connecting section is preferably inclined towards the electrode surface. It can have the shape of a hollow cylinder that merges into the electrode surface in the region of the edges of the recess. The electrode can therefore have the shape of a circular surface with a hole and a hollow cylinder adjacent to the edges of the hole. The hole is preferably congruent with the recess and more preferably arranged around the axis of rotation of the piezoelement or of the piezodrive. The connecting section can extend into the recess of the piezoelement. The connecting section can comprise a metallization of the wall of the recess of the piezoelement or be formed thereby. The electrode orThe metallization of the piezoelectric element forming the electrode is particularly seamless, from the side of the piezoelectric element facing the membrane to the wall of the recess. If the electrical connection encompasses the connecting section of the electrode, the electrode surface can be particularly well contacted.

[0052] The contactability of the electrode surface can be simplified - e.g., in embodiments in which a positioning means is arranged in the recess of the piezoelectric element - if the connecting section of the electrode has a region running on the side of the piezoelectric element facing away from the membrane. In this embodiment, the electrode therefore extends to both sides of the piezoelectric element. The size of the area of ​​the connecting section of the electrode running on the side of the piezoelectric element facing away from the membrane is preferably less than 1 / 10 or 1 / 20 of the size of the electrode surface. The size of the electrode on the side of the piezoelectric element facing away from the membrane therefore preferably corresponds to less than 1 / 10 or 1 / 20 of the size of the electrode on the side of the piezoelectric element facing the membrane.It has been shown that, on the one hand, this only results in a slight reduction of the active area of ​​the piezo element, which benefits the performance of the piezo actuator, and, on the other hand, this is sufficient for reliable contacting of the electrode.

[0053] The region of the connecting section extending on the side of the piezoelectric element facing away from the membrane can be annular, in particular circular. This region can be collar-shaped. This region can be arranged around the recess of the piezoelectric element.

[0054] The conductive adhesive can be arranged at least partially in the recess of the piezoelectric element. The conductive adhesive can contact the connecting section of the electrode. The conductive adhesive can contact the region of the connecting section extending on the side of the piezoelectric element facing away from the membrane.

[0055] The conductive adhesive preferably connects components of the electrical connection, such as the connecting section of the electrode or the electrical conductor and, for example, an electrical line, in particular a flexible conductor, a printed circuit board or the contact element to one another, in particular electrically and / or mechanically.

[0056] The same adhesive can be used for the assembly adhesive that effectively connects the piezo actuator to the membrane and for the conductive adhesive of the electrical connection. The electrical conductor can be contacted not only by means of conductive adhesive, but it is also conceivable that it itself comprises conductive adhesive or is formed by it.

[0057] Preferably, the adhesive of the assembly adhesive bond and the conductive adhesive are different adhesives. The assembly adhesive bond preferably comprises an adhesive with a balanced combination of strength and flexibility, such that the adhesive is sufficiently strong to transmit the vibrations, while also being able to withstand the stresses acting on it due to different thermal expansion coefficients of the surrounding components.

[0058] The conductive adhesive of the electrical connection can be softer or tougher than the adhesive of the adhesive bonding the piezo actuator to the membrane.

[0059] To adjust the thermal expansion coefficients of the diaphragm and piezoelectric element, a compensating element, for example a compensating ceramic, is preferably arranged between the diaphragm and the piezoelectric actuator. Such a compensating element, typically disc-shaped, can absorb thermal expansion of the diaphragm and thus largely prevent electrical stresses caused by thermal stresses. In particular, such a compensating element can prevent or reduce mechanical stresses that could lead to failure or breakage of the piezoelectric actuator. The compensating ceramic can comprise steatite ceramic. The effect of differences in thermal expansion between the piezoelectric element and diaphragm can thereby be mitigated. The compensating element is preferably electrically non-conductive. The adhesive connection is preferably arranged between the piezoelectric actuator and the compensating element.The compensation element is preferably glued to the membrane.

[0060] The sensor preferably comprises a further electrical connection, preferably between the further electrode and the electronics. A potential difference between the electrode and the further electrode can preferably be created or tapped with the aid of the electronics. A potential difference can preferably be applied or tapped on opposite sides or surfaces of the piezo element with the aid of the electronics, the electrode, and the further electrode.

[0061] The area in which the electrode and the further electrode overlap, spaced apart by the piezoelectric element, can be referred to as the active area of ​​the piezoelectric element. The active area of ​​the piezoelectric element is preferably larger than 7 / 10, 8 / 10, or 9 / 10 of the base area of ​​the piezoelectric element. The active area is preferably rotationally symmetrical. The active area can correspond to the area of ​​the further electrode. The preferred rotational symmetry of the active area can cause or contribute to the insensitivity of the piezoelectric actuator to torsion. The further electrode is preferably annular, in particular in the form of a circular ring. The further electrode is preferably arranged around the recess.The further electrode can extend at least almost over the entire side of the piezoelectric element facing away from the membrane, preferably with the exception of an inner circular ring arranged around the recess, to create an insulation distance for isolating the further electrode from the electrode. Preferably, the further electrode is rotationally symmetrical about the same rotational axis as the electrode. The piezoelectric actuator preferably has exactly two electrodes. Therefore, the sensor preferably also comprises only two electrical contact areas or contacts to electrodes of the piezoelectric actuator. The contact areas can comprise a soldered and / or welded connection. Preferably, they comprise an adhesive connection with conductive adhesive.

[0062] The sensor may comprise positioning means for positioning the piezo actuator and / or the compensating element relative to the membrane. The positioning means may, for example, comprise a—particularly cylindrical—punch of the compensating element on the side of the compensating element facing the piezo actuator.

[0063] Two prongs of a tuning fork can be arranged on the diaphragm, acting as a mechanical oscillator. Areas of the vibration sensor, such as the piezo actuator and the contact element, can be encapsulated with a potting compound. A pin header can be provided for mechanical alignment and fixation of the contact pins.

[0064] Preferably, the electrical contact to the electrical conductor or the electrode is established by means of a conductive adhesive. In this way, or if the contact element is formed by a conductive adhesive, the electrical connection between the electronics and the electrode surface can be easily established and reliable. The conductive adhesive can be a silver conductive adhesive. The conductive adhesive can be thixotropic before it cures. This can ensure that the conductive adhesive is not, or only slightly, drawn into the recess due to capillary action. This can also prevent or reduce any negative influence on the electrical connection between the electronics and the electrode surface, which can be achieved with the help of the conductive adhesive, and on the assembly adhesive connection.The vibration sensor can therefore comprise, on the one hand, the mounting adhesive connection and, on the other hand, an adhesive bond with conductive adhesive for contacting the piezo actuator.

[0065] In an alternative embodiment, the electrical connection comprises or is formed by an inner conductor of a coaxial connection.

[0066] For example, the electrical connection can be realized as the inner conductor of a coaxial cable, with the inner conductor protruding at least partially into the recess. For this purpose, the inner conductor of the coaxial cable can be offset for a length approximately corresponding to the thickness of the piezo element and inserted into the opening of the piezo element. There, the inner conductor can be electrically connected to the electrode using an electrically conductive adhesive and simultaneously mechanically fixed. The bonding and electrical contacting can be made around the circumference of the inner conductor in the connecting section, but can also be achieved at least partially via end-face bonding.

[0067] The inner conductor preferably penetrates the recess at least halfway, more preferably at least 80%, and most preferably almost completely. The greater the overlap between the inner conductor and the recess, the larger the area that can be electrically and / or mechanically contacted. This allows for a particularly good, stable electrical and / or mechanical connection to be achieved.

[0068] Preferably, the diameter of the inner conductor is adapted to the diameter of the recess such that a defined distance remains between the inner conductor and the inner wall of the recess, which is bridged by the electrically conductive adhesive. Precise adjustment can be achieved, for example, using a calibration tool, with which the diameter of the inner conductor is machined during or after placement so that it fits optimally into the recess.

[0069] In this embodiment, the closure element can be formed, for example, by a dielectric and / or an outer conductor of the coaxial connection. The additional electrode can be contacted by the outer conductor of the coaxial connection. The outer conductor can also be electrically bonded to the additional electrode, preferably at the end face.

[0070] In this way, a particularly easy-to-manufacture variant is provided which effectively reduces the capillary effect of the recess.

[0071] In this way, an electrical and mechanical connection between the piezo element and the electrical supply line can be achieved particularly easily and without additional components.

[0072] Further practical advantages and embodiments are described below in conjunction with the figures. They show schematically:

[0073] Fig. 1 is a cross-sectional view of part of a vibration sensor known from the prior art,

[0074] Fig. 2 is a cross-sectional view of part of a first embodiment of a vibration sensor,

[0075] Fig. 3 the electrode of the vibration sensor shown in Fig. 2,

[0076] Fig. 4 is a perspective view of the piezo actuator from Fig. 2, with

[0077] View of the side facing away from the membrane,

[0078] Fig. 5 is a perspective view of the piezo actuator from Fig. 2, with

[0079] View of the side facing the membrane,

[0080] Fig. 6 is a cross-sectional view of part of a second embodiment of a vibration sensor,

[0081] Fig. 7 is a cross-sectional view of part of a third embodiment of a vibration sensor,

[0082] Fig. 8 is a cross-sectional view of part of a fourth embodiment of a vibration sensor, Fig. 9 is a perspective view of a piezo actuator, looking at the side facing away from the membrane,

[0083] Fig. 10 is a perspective view of the piezo actuator from Fig. 9, with

[0084] View of the side facing the membrane, and

[0085] Fig. 11 is a cross-sectional view of part of a fifth embodiment of a vibration sensor.

[0086] In Fig. 1, part of a vibration sensor known from the prior art is shown in sketch form.

[0087] A piezo actuator 16 with a ring-disk-shaped piezo element 18 is glued to an insulating compensating element 44 made of ceramic material. The assembly of piezo actuator 16 and compensating element 44 is also fixed to a diaphragm 12 by an adhesive bond. The diaphragm 12 extends into a housing 10 or is attached thereto. By applying an electrical potential to the piezo element 18, the diaphragm 12 in such a configuration can be set into vibration. On the outer side of the diaphragm 12, there may be additional mechanically vibratable elements, such as fork tines 62, 64 of a tuning fork, which are only indicated by dashed lines in Fig. 1. The side of the piezo actuator facing the diaphragm is not freely accessible and therefore cannot be directly contacted.

[0088] The vibration sensor partially shown in Fig. 2 is used to detect fill levels, limit levels, or to monitor process parameters and comprises a vibratable membrane 12. The membrane 12, the housing 10, and the fork tines 62, 64 can be designed as shown in Fig. 1. In Figs. 2 to 5, the membrane 12 is not shown for the sake of simplicity of illustration.

[0089] The vibration sensor comprises a drive unit 14 designed as a piezo actuator 16. The piezo actuator has a piezo element 18 and an electrode 20 connected to the piezo element 18. The electrode 20 is a metallization of the piezo element and has an electrode surface 21. Furthermore, the vibration sensor comprises an adhesive connection 26, more precisely an assembly adhesive connection, by means of which the piezo actuator 16 is operatively connected to the membrane 12 such that vibrations of the piezo actuator 16 can be transmitted to the membrane 12 and vice versa. Furthermore, the vibration sensor comprises electronics 22 and an electrical connection 24 between the electronics 22 and the electrode surface 21. The electrode surface 21 is shown in Fig. 2 merely as the lower boundary of the piezo actuator 16.

[0090] The piezo element 18 has a recess 32 and the electrical connection 24 runs through the recess 32. The electrical connection 24 can also have an electrical line 72, namely a cable.

[0091] An electrical conductor 110 is arranged in the recess 32 and is electrically connected to the electrode surface 21. The electrical conductor 110 is part of the electrical connection 24 between the electronics 22 and the electrode surface 21 and comprises a connecting section 23 of the electrode 20, more precisely, a metallization of the wall of the recess 32. The vibration sensor also has a closure element 125 that closes the recess 32 and is arranged in the recess 32 and on the side 30 of the piezoelectric element 18 facing away from the membrane 12. The closure element 125 is designed as a contact element 130 glued to the piezoelectric actuator 16 and is in electrical contact with the electrical conductor 110. The contact element 130 is glued, among other things, in the recess 32. The contact element 130 comprises a contact pin 140 and conductive adhesive 40.

[0092] The sensor includes a compensating element 44 in the form of a compensating ceramic between the diaphragm 12 and the piezo actuator 16, and the adhesive bond 26 is arranged between the piezo actuator 16 and the compensating element 44. The compensating element 44 is, in turn, glued to the diaphragm 12. As Fig. 2 also shows, the piezo element extends in a plane 70.

[0093] The adhesive connection 26 has a connection region 120 arranged in a ring around the recess 32 and is in contact with the electrode surface 21 in this connection region 120. The connection region 120 makes up the entire adhesive connection 26 except for a small outer adhesive meniscus and an inner adhesive meniscus 260 in the exemplary embodiment shown in Fig. 2. The adhesive connection has a constant thickness D in the connection region 120. Suction of the adhesive of the assembly adhesive connection 26 into the recess 32 and penetration by capillary action, and a resulting variation in the thickness of the adhesive connection and disruption of the formation of the adhesive meniscus 260, does not occur to a significant extent, even though the recess 32 has a diameter U of less than 2 mm. Disruption of the electrical contacting of the metallization of the wall of the recess 32 caused by the adhesive of the assembly adhesive connection is also prevented.

[0094] This was achieved in the embodiment shown in Fig. 2 by closing the recess 32 with the contact element 130 encompassing the contact pin 140 and establishing the adhesive connection 26 in such a way that air trapped in the recess counteracted the capillary action of the recess 32 during the establishment of the adhesive connection 26. Fig. 2 also shows that the closure element 125 is arranged in the recess 32 and on the side 30 of the piezo element facing away from the membrane 12.

[0095] Before assembly into the vibration sensor, i.e., before establishing the adhesive bond 26, the piezo element, after the recess 32 had been created, was metallized continuously on one of its sides and in the recess 32 to provide the electrode 20. By metallizing a portion of the other side of the piezo element, a further electrode 45 was created.

[0096] The contact pin 140 is bonded into the recess 32 with conductive adhesive 40, closes the recess 32, and also establishes electrical contact with the electrical conductor 110. The outer surface of the contact pin 140 arranged in the recess 32 is in electrical contact with the piezo actuator 16, more precisely, the electrical conductor 110.

[0097] The contact pin 140 has an outer dimension that is complementary to the inner dimension of the recess 32. In the embodiment shown in Fig. 2, the contact pin 140 has a collar 150 that is glued to the piezo actuator 16. As shown in Figs. 2 to 5, the piezo element 18 has a first side 28 facing the membrane 12. The electrode surface 21 is arranged exclusively on this first side 28. The piezo element 18 has a second side 30 facing away from the membrane 12. The recess 32 extends from the first side 28 to the second side 30 through the center of the piezo element 18.

[0098] The additional electrode 45 is arranged on the side 30 of the piezoelectric element 18 facing away from the membrane 12, and the vibration sensor comprises an additional contact element 190 in the form of an additional contact pin 200, which is glued to the additional electrode 45. The additional contact pin 200 has a disc-shaped thickening 210 at its end.

[0099] The piezo actuator 16 is designed as a bonded annular disc 34 with central contact 36. The piezo element 18 and the electrode 20 are rotationally symmetrical about the same rotation axis.

[0100] In addition to the flat electrode surface 21, the electrode 20 has a connecting section 23 extending into the recess 32, which forms the electrical conductor 110 (Fig. 3).

[0101] The sensor includes a further electrical connection 74 between the further electrode 45 and the electronics 22. The further electrode 45 has the shape of a circular ring 66, is arranged around the recess 32, and extends at least almost over the entire side of the piezo element 18 facing away from the membrane 12, with the exception of an inner circular ring 68 arranged around the recess 32, for isolating the further electrode 45 from the electrode 20.

[0102] Fig. 3 illustrates the shape of the electrode 20 of the embodiment shown in Fig. 2, with the flat, circular ring-shaped electrode surface 21 and the connecting section 23. As Fig. 2 and Fig. 3 show, the electrode 20 extends over surfaces inclined towards one another and, in the embodiment shown in these figures, has the shape of a circular surface forming the electrode surface 21 with a hole and, adjacent to the edges of the hole, a hollow cylinder providing the connecting section 23. The connecting section 23 can also, as shown in Figs. 2 to 5, have a region 42 running on the side 30 of the piezo element facing away from the membrane. This region 42 is circular and collar-shaped and is arranged around the recess 32 of the piezo element 18. In all the embodiments shown in Figs.However, in the embodiments shown in Figures 2 to 8, the connecting section 23 can be designed to enlarge the active area of ​​the piezo element without the region 42 running on the side 30 of the piezo element facing away from the membrane 12.

[0103] Figures 4 and 5 illustrate the structure of the piezo actuator 16 with annular piezo element 18, electrode 20 as described above and another annular electrode 45.

[0104] Further exemplary embodiments are shown in Figs. 6 to 8. The same reference numerals denote identical elements with the same function, unless otherwise stated. Reference is made to the above description in this regard. Only differences from the exemplary embodiment shown in Fig. 2 are presented below. The adhesive meniscus 260 present in all exemplary embodiments shown in Figs. 6 to 8 has been omitted from Figs. 6 to 8 for clarity.

[0105] Fig. 6 shows an embodiment in which the contact pin 140 and the further contact pin 200 are part of a standardized electrical plug connection 160 and the distance 230 between the contact pins is a grid dimension. The contact pin 140 and the further contact pin 200 serve as contacts to a mating connector 220. The contact pins each have a recess in the form of a blind hole, which provides a sleeve section of the respective contact pin for receiving a mating connector contact. In this embodiment, the vibration sensor comprises a plug connection 160 adhesively bonded to the piezo actuator 16 instead of a flexible conductor. Fig. 6 also shows a potting cup 270 for introducing potting compound (not shown in the figures), as well as a pin strip 280 for mechanically aligning and fixing the contact pins.

[0106] In the embodiments shown in Figs. 7 and 8, the vibration sensor comprises a circuit board 170 with electronic components 180. The contact element 130 electrically connects the piezo actuator 16, more precisely the electrical conductor 110, to the circuit board 170 and also establishes a fixed mechanical connection between the piezo actuator 16 and the circuit board 170. An electrical plug contact in the form of a 2-pin socket 240 is arranged on the circuit board 170.

[0107] In the embodiment shown in Fig. 7, the contact element 130 contacts the piezo actuator 16, in particular at least also the electrical conductor 110, with one side by means of conductive adhesive 40, and the printed circuit board 170 with the other side directly or by means of conductive adhesive not shown at this point in Fig. 7.

[0108] In the embodiment shown in Fig. 8, the contact element does not comprise a contact pin, but is formed by conductive adhesive 40 and the piezo actuator 16 is mechanically and electrically connected directly to the circuit board 170, via contact surfaces 250 of the circuit board.

[0109] If the connecting section 23 of the electrode has a region 42 extending on the side 30 of the piezoelectric element facing away from the membrane 12, then the closure element 125 can be arranged exclusively on the side 30 of the piezoelectric element facing away from the membrane 12, as shown in Fig. 8. If the electrode does not have this region, then the conductive adhesive 40 extends into the recess 32 and contacts the metallized wall of the recess 32, unlike as shown in Fig. 8.

[0110] Figures 9 and 10 show a a piezo actuator 16 as it can be used in another.

[0111] In contrast to the variant of a piezo actuator 16 as shown in Figures 4 and

[0112] 5 is shown, the membrane 12 on the side of the

[0113] Piezoelement 18 is not circular and collar-shaped around the 32 but is exclusively in the This is achieved by placing the further electrode 45 closer to the The electrical the electrode 20 has in the

[0114] Figure 11 shows an enlarged detail of a section in the longitudinal direction of the recess 32 through another embodiment of a vibration sensor with a piezo actuator 16, as shown in Figures 9 and 10. In this embodiment, the electrical contacting of the piezo element 18 is achieved, on the one hand, by an inner conductor 86 of a coaxial line 84, which is arranged in the recess 32 of the piezo element 18 and is connected to it both electrically and mechanically by the conductive adhesive 40. The further electrode 45 is connected to the outer conductor 88 of the coaxial line 84, which is also mechanically and electrically connected to it by means of a conductive adhesive 40.

[0115] The design shown in Fig. 11 makes it particularly easy to achieve electrical and mechanical contact and sealing of the recess 32.

[0116] Because the piezo element 18 can be designed to be completely rotationally symmetrical due to the central contact, fewer stresses occur due to deformation of the piezo element, which, among other things, significantly increases the mechanical load capacity and durability of the piezo element 18. Furthermore, the elimination of a radially more outwardly located contact point also reduces the stiffness, allowing a larger amplitude to be generated.

[0117] List of reference symbols

[0118] 10 housings

[0119] 12 Membran

[0120] 14 Drive unit

[0121] 16 Piezo actuator

[0122] 18 Piezo element

[0123] 20 electrodes

[0124] 21 Electrode area

[0125] 22 Electronics

[0126] 23 Connecting section of the electrode

[0127] 24 electrical connection

[0128] 26 Adhesive bond

[0129] 28 side of the piezo element facing the membrane

[0130] 30 side of the piezo element facing away from the membrane

[0131] 32 Recess of the piezo element

[0132] 34 glued ring disc

[0133] 36 centric re-contacting

[0134] 40 conductive adhesive

[0135] 42 region of the connecting section running on the side of the piezo element facing away from the membrane

[0136] 44 Compensating element

[0137] 45 additional electrodes

[0138] 62 forks

[0139] 64 forks

[0140] 66 circular ring

[0141] 68 inner circular ring

[0142] 70 Level

[0143] 72 electrical lines

[0144] 74 additional electrical connections

[0145] 84 coaxial cable

[0146] 86 inner conductors

[0147] 88 outer conductors

[0148] 110 electrical conductor

[0149] 120 Connection area of ​​the adhesive joint

[0150] 125 Locking element 130 Contact element

[0151] 140 contact pin

[0152] 150 collars

[0153] 160 electrical connector 170 printed circuit board

[0154] 180 electronic components

[0155] 190 additional contact element

[0156] 200 additional contact pins

[0157] 210 Thickening 220 Mating connector

[0158] 230 distance

[0159] 240 2-pin socket

[0160] 250 contact area

[0161] 260 adhesive meniscus 270 potting cups

[0162] 280 pin header

[0163] thickness

[0164] U diameter

Claims

Patent claims 1. Vibration sensor for detecting fill levels, limit levels or for monitoring process parameters, comprising: - a membrane (12) which can be set into vibration, - a drive unit (14), wherein the drive unit (14) comprises a piezo actuator (16) having a piezo element (18) and an electrode (20) connected to the piezo element (18) with an electrode surface (21), - an adhesive connection (26) by means of which the piezo actuator (16) is operatively connected to the membrane (12) in such a way that vibrations of the piezo actuator (16) can be transmitted to the membrane (12), - an electronics (22), - an electrical connection (24) between the electronics (22) and the electrode surface (21), wherein the piezo element (18) has a recess (32) and the electrical connection (24) runs through the recess (32), wherein an electrical conductor (110) electrically connected to the electrode surface (21) is arranged in the recess (32) and the vibration sensor has a closure element (125) closing the recess (32).

2. Vibration sensor according to claim 1, characterized in that the closure element (125) is arranged at least also in the recess (32) and / or on the side (30) of the piezo element (18) facing away from the membrane (12).

3. Vibration sensor according to claim 1 or 2, characterized in that the closure element (125) is designed as a contact element (130) and is in electrical contact with the electrical conductor (110).

4. Vibration sensor according to claim 3, characterized in that the contact element (130) is glued to the piezo actuator (16).

5. Vibration sensor according to claim 3 or 4, characterized in that the contact element (130) is glued in the recess (32).

6. Vibration sensor according to one of claims 3 to 5, characterized in that the contact element (130) comprises a contact pin (140).

7. Vibration sensor according to claim 6, characterized in that the contact pin (140) has a collar (150) which is glued to the piezo actuator (16).

8. Vibration sensor according to claim 6 or 7, characterized in that the contact pin (140) is part of an electrical plug connection (160).

9. Vibration sensor according to one of claims 3 to 8, characterized in that the vibration sensor comprises a circuit board (170) with electronic components (180), and the contact element (130) electrically and mechanically connects the piezo actuator (16) to the circuit board (170).

10. Vibration sensor according to one of claims 3 to 9, characterized in that a further electrode (45) is arranged on the side (30) of the piezo element (18) facing away from the membrane (12) and the vibration sensor comprises a further contact element (190) for establishing an electrical contact with the further electrode (45).

11. Vibration sensor according to claim 10, characterized in that the further contact element (190) comprises a further contact pin (200) which is glued to the further electrode (45).

12. Vibration sensor according to claim 11, characterized in that the distance (230) between the further contact pin (200) and the contact pin (140) corresponds to a grid dimension.

13. Vibration sensor according to one of claims 1 to 6, characterized in that the electrical connection is formed by an inner conductor (86) of a coaxial connection.

14. Vibration sensor according to claim 13, characterized in that the closure element is formed by a dielectric and / or an outer conductor (88) of the coaxial connection.

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