Vibration sensor

Abstract

The present invention relates to a vibration sensor (1) having a membrane (3) which can be induced to oscillate, and having a piezo element (5) which is operatively connected to the membrane (3) and is designed and arranged such that oscillations of the piezo element (5) are transferred to the membrane (3) and oscillations of the membrane (3) are transferred to the piezo element (5), wherein the piezo element (5) is glued to the membrane (3), or to an adaptation ceramic (7) glued to the membrane (3), characterised in that the piezo element (5) consists of a lead-free piezoceramic.

Description

[0001] Vibration sensor

[0002] The present invention relates to a vibration sensor according to the preamble of claim 1.

[0003] In process and factory automation, electronic sensors are used to monitor and control processes. Such sensors can be used, for example, to detect fill levels, limit levels, or monitor process parameters.

[0004] A widespread type of such sensors are vibronic sensors, also referred to as vibration sensors. Vibration sensors for detecting fill levels or limit levels work by exciting a mechanical vibration unit at its resonant frequency. Covering the vibrating element with a liquid or bulk material changes its resonant frequency. Exceeding a predetermined value of this change generates a switching command.

[0005] Vibration sensors for determining and / or monitoring the fill level of a medium in a container or for determining the density of a medium in the container comprise a housing, a membrane, a mechanical vibration unit, a drive / receiver unit (hereinafter also referred to as the drive), and a control / evaluation unit (hereinafter also referred to as the sensor electronics). The membrane closes one end of the housing, and the mechanical vibration unit is arranged on the membrane. The drive is arranged inside the housing in such a way that it causes the membrane and the mechanical vibration unit to vibrate and detects their vibrations. The sensor electronics uses the detected vibrations to determine whether the specified fill level has been reached or the density of the medium.

[0006] Such vibration sensors are available in a wide variety of designs. The mechanical vibration unit consists of at least one vibrating rod, which—as already mentioned—is directly attached to a diaphragm. The diaphragm is set into vibration by the drive, an electromechanical transducer, e.g., a piezoelectric element. Due to the vibrations of the diaphragm, the mechanical vibration unit attached to the diaphragm also vibrates.

[0007] The oscillating membrane, the drive arranged on the one hand on the membrane and the mechanical oscillation unit arranged on the other hand on the membrane thus form an oscillating system which can be set into oscillation by the drive and whose oscillations can be detected by the drive.

[0008] Vibration detectors designed as level gauges take advantage of the fact that the vibration frequency and amplitude depend on the respective coverage of the oscillating unit: While the mechanical vibration unit can oscillate freely and undamped in air, it experiences a change in frequency and amplitude as soon as it is partially or completely immersed in the product. A predetermined frequency change (typically, the frequency is measured, not the amplitude), thus clearly indicates whether a predetermined fill level of a product in the container has been reached. Level gauges of this type are also used primarily as a means of protecting against overfilling or preventing a pump from running idle.

[0009] Furthermore, the damping of the vibration of the oscillating element is also influenced by the density of the respective product. With a constant coverage of at least one oscillating element, there is a correlation with the density of the product, making vibration detectors ideal for both detecting a given limit state and detecting the density.

[0010] In practice, to monitor and detect the fill level or density of the product in the container, the vibrations of the membrane are recorded and converted into electrical signals. Typically, at least one piezoelectric element is used for this purpose. The electrical signals are then evaluated by an evaluation electronics unit. During level determination, the evaluation electronics monitors the vibration frequency and / or the vibration amplitude of the vibrating unit and signals the "sensor covered" or "sensor uncovered" status as soon as the measured values ​​fall below or exceed a specified reference value. This can be reported to the operating personnel visually and / or acoustically. Alternatively or additionally, a switching process is triggered, for example, opening or closing an inlet or outlet valve on the container.

[0011] Piezoceramic elements are predominantly used as electromechanical transducers in the drive unit of such a sensor. Various concepts are known for this. Screwed stack drives, in which the piezo actuators are secured by a screw fastening, are widespread. Another concept involves securing the piezo actuator with an adhesive. This concept is becoming increasingly popular because it offers improved performance, lower manufacturing costs, and greater automation potential.

[0012] Due to good material properties in terms of performance and temperature resistance, piezo elements from the lead zirconate titanate (PZT) material class are used, especially at high temperatures between 150°C and 300°C.

[0013] The use of lead as a component of the piezoelectric material is considered a disadvantage in this class of materials. In addition to environmental, health, and occupational safety considerations, material availability also plays a role.

[0014] The object of the present invention is to provide an alternative to vibration sensors with a PTZ-based drive. A further object is to design the alternative drive in such a way that it has comparable performance to the drives known from the prior art.

[0015] This object is achieved by a vibration sensor having the features of patent claim 1. Advantageous embodiments and variants of the invention emerge from the subclaims and the following description. The features individually listed in the subclaims can be combined with one another in any technically expedient manner, as well as with the features explained in more detail in the following description, and represent other advantageous embodiments of the invention. In the case of the bonded drives mentioned above, the support housing inhibits the vibration of the drive. In the prior art, materials with a high crystallinity are primarily used for this housing. The Young's modulus of the support housing is in the high range, whereby the material inhibits vibration. For this reason, the support housing is slotted and cannot be used as a cup for potting.

[0016] A vibration sensor according to the invention with a membrane that can be set into vibration and a piezo element that is in operative connection with the membrane and is designed and arranged in such a way that vibrations of the piezo element are transmitted to the membrane and vibrations of the membrane to the piezo element, wherein the piezo element is glued to the membrane or to an adaptation ceramic glued to the membrane, is characterized in that the piezo element consists of a lead-free piezo ceramic.

[0017] By using a lead-free piezoceramic, the disadvantages associated with the element lead as a component of the piezoceramic can be avoided.

[0018] The piezo element is circular disc-shaped, i.e. it has a circular outer contour when viewed from above.

[0019] For easier installation and as a receptacle for any potting compound, the piezo element can be arranged in a ring-shaped support. Such a support facilitates handling of the piezo element as well as its installation and positioning relative to the diaphragm.

[0020] The carrier is preferably designed without slots. In this context, "unslotted" means that the annular carrier is designed without slots or notches. In the prior art, such slots or notches are required to increase the flexibility of the carrier and thus reduce damping on the membrane. By appropriately designing the carrier, it is possible to dispense with slots to increase the flexibility of the carrier.

[0021] In a preferred embodiment, a potting compound is arranged in the carrier, completely covering at least one surface of the piezo element. This makes it possible to use the vibration sensor even in explosion-proof areas. At the same time, the potting compound protects the piezo drive, in particular the electrical contact of the piezo element, against corrosion caused by moisture.

[0022] A surface of the piezo element means at least one side of the piezo element facing away from the membrane.

[0023] A circumferential surface of the piezo element is in contact with the carrier. Since the carrier is made of plastic and electrically non-conductive, the piezo element is also electrically insulated along its circumferential surface and thus in the radial direction.

[0024] In a preferred embodiment, the encapsulation is made of silicone. Silicone encapsulation allows the encapsulation in the carrier to be particularly thin, thus introducing as little additional mass as possible into the vibrating system. Reducing the mass reduces the damping of the system, allowing the vibration sensor to operate even with a lower-performance piezo element.

[0025] In one embodiment of the vibration sensor, the support consists of a material with a Young's modulus of less than 4000 MPa, preferably less than 3500 MPa, and particularly preferably less than 3200 MPa or less. Reducing the Young's modulus of the support material ensures that the support is less stiff and thus damps the oscillating system less. This makes it possible to introduce sufficient energy into the oscillating system even with a piezo element with a lower charge constant.

[0026] The elastic modulus is determined according to ISO527 under standard conditions, i.e. at a temperature of 23 + / - 2 °C and a relative humidity of 50 + / - 10 %.

[0027] In a preferred embodiment, the carrier is designed such that it has a radial stiffness of less than 3000 N / mm, preferably 2500 N / mm or less. A reduction in the radial stiffness can be achieved in particular by a suitable selection of material with a reduced modulus of elasticity and a suitable geometric design of the carrier. To ensure the usability of the vibration sensor at the desired temperatures of -50°C to +150°C, it is also necessary that the selected plastic has sufficient temperature resistance. In particular, it is essential that the carrier neither becomes brittle at low temperatures nor melts at high temperatures.

[0028] In a particularly preferred embodiment, the carrier consists of polyetherimide or polyetheretherketone. Both of these materials particularly well meet the requirements for a plastic such as that required for the carrier. In particular, they can be used in the aforementioned temperature range, exhibit sufficient mechanical stability to allow the carrier to be manufactured using as little material as possible, and are simultaneously elastic enough that the oscillating system of the vibration sensor is not excessively damped.

[0029] To reduce the radial stiffness of the annular support, it is advisable for it to have a radial wall thickness of less than 1 mm, preferably less than 0.8 mm, and more preferably 0.4 mm or less. Reducing the wall thickness of the support makes it more flexible overall and thus has less of a damping effect on the oscillating system of the vibration sensor.

[0030] Preferably, the annular support has an axial extension of less than 5 mm, preferably 4 mm or less. Reducing the axial extension also reduces the mass and stiffness of the support, thereby also reducing the impact on the oscillating system of the vibration sensor.

[0031] In one embodiment of the vibration sensor, the carrier has at least three, preferably four or five, projections on the radial outside, which are designed as spacers and align the carrier relative to the diaphragm. The diaphragm often has an axially extending, circumferential edge, which serves as a holder for the diaphragm and as a projection for a process connection and / or a housing that encloses the drive and electronics of the vibration sensor. This edge can be used in conjunction with the projections on the carrier to align and thus center the carrier relative to the diaphragm during assembly of the vibration sensor, in particular during manual assembly. By aligning the carrier, the piezo element held in the carrier is simultaneously aligned relative to the diaphragm and preferably concentrically aligned with it.

[0032] In a preferred embodiment, the drive of the vibration sensor is formed from piezo elements made of bismuth sodium titanate or potassium sodium niobate.

[0033] The piezo elements can be designed as multilayer piezo elements with several piezo layers. In such multilayer piezo elements, a piezo unit is formed from several piezo layers, which are connected to form a piezo unit together with electrode layers arranged between the piezo layers. The piezo layers and the electrode layers can be permanently bonded to one another, for example, through a sintering process. By bonding several layers together, it is possible for the individual piezo layers to be thinner than would be the case with pure piezo elements. The pure piezo material has the disadvantage that it breaks very easily at low layer thicknesses. This disadvantage can be avoided by bonding several piezo layers together at the manufacturer's site.Due to the smaller layer thickness of the individual piezo layers, higher field strengths and thus a better conversion of electrical into mechanical energy can be achieved at the same applied voltage.

[0034] The present invention will be explained in detail below using exemplary embodiments with reference to the accompanying figures. They show:

[0035] Figure 1 shows an embodiment of a vibration sensor according to the present application.

[0036] Figure 2 is a perspective view of the support from Figure 1.

[0037] In the figures, unless otherwise indicated, identical reference numerals designate identical or corresponding components with identical functions. Figure 1 shows a partial view of a vibration sensor 1 according to the present application in a longitudinal section in the axial direction A.

[0038] The vibration sensor 1 has a membrane 3 designed to vibrate, on which a mechanical vibration unit 31, in this case in the form of a tuning fork, and a drive for causing the membrane 3 to vibrate are arranged. The drive is designed as a piezoelectric element 5, which is bonded to the membrane 3 with an interposition of an adaptor ceramic 7 to compensate for different thermal expansion coefficients between the membrane 3 and the piezoelectric element 5.

[0039] The piezo element 5 is arranged in a ring-shaped carrier 9, which facilitates handling of the piezo element 5 and positioning relative to the membrane 3. For positioning relative to the membrane 3, the carrier 9 has projections 91 arranged on the outside of the carrier 9, which hold the carrier 9 at a distance from an edge 31 of the membrane 3, which extends at the rear in the axial direction, and align the carrier 9 with the piezo element 5 and thus position it centrally relative to the membrane 3. The projections 91, which act as spacers to the edge 31, make it possible to design the carrier 9 with a smaller wall thickness and thus reduce the mass of the carrier 9.

[0040] In the present exemplary embodiment, the carrier 9 has a step on the inside of its front end in which the adaptation ceramic 7 is located. It is thus possible to equip the carrier 9 with the adaptation ceramic 7 and the piezo element 5 and then connect them as a common unit to the membrane 3 and the mechanical vibration unit 33 arranged thereon. The piezo element 5 is electrically contacted via contacts (not shown here), so that applying a voltage can cause a mechanical deformation of the piezo element, which excites the membrane 3 to vibrate.

[0041] In the present exemplary embodiment, the rear of the piezo element 5, i.e. on a surface facing away from the compensating ceramic 7 arranged at the front of the piezo element 5, is covered with a potting 11. The potting 11 is arranged inside the carrier 9, so that in the exemplary embodiment shown the carrier 9 serves simultaneously as a holder for the piezo element 5 and as a boundary for the potting 11. In the present exemplary embodiment, the potting 11 is designed as a silicone potting and has a thickness, i.e. an extension in the axial direction A, of 3.3 mm. By configuring the potting 11 as a silicone potting and reducing the thickness to 3.3 mm, a mass of the potting 11 used is reduced by a factor of 9 compared to the prior art, so that the potting 11 according to the present embodiment has a significantly reduced damping effect on the oscillatable unit of the vibration sensor 1.

[0042] In the present embodiment, the carrier 9 is made of polyimide and has a wall thickness b? of 0.4 mm and an extension FIT in the axial direction A of 4 mm. By selecting polyimide as the material of the carrier 9 and the prescribed mechanical design, a radial stiffness of 2500 N / mm is achieved, so that the reduction in the mass of the carrier 9 on the one hand and the reduced radial stiffness of the carrier 9 on the other hand minimizes a negative damping influence of the carrier 9 on the oscillating system of the vibration sensor 1.

[0043] The above-mentioned measures make it possible for the vibration sensor 1 to be operated with a piezo element 5 made of bismuth sodium titanate or potassium sodium niobate.

[0044] Figure 2 shows the carrier 9 as used in the embodiment of Figure 1 in a standalone perspective view.

[0045] The representation shown in Figure 2 clearly shows the axial extension FIT of the support 9 and the wall thickness b? in the radial direction R. Furthermore, the representation shown in Figure 2 shows the projections 91, which serve as spacers and are evenly distributed over the circumference of the annular support 9. List of reference symbols

[0046] 1 vibration sensor

[0047] 3 Membran

[0048] 5 Piezo element

[0049] 7 Adaptive ceramics

[0050] 9 carriers

[0051] 11 Casting

[0052] 31 edge

[0053] 33 mechanical vibration units

[0054] 91 Forming

[0055] R Radial direction

[0056] A axial direction b T Wall thickness h T Extension

Claims

Patent claims 1. Vibration sensor (1) with a membrane (3) that can be set into vibration and a piezo element (5) that is operatively connected to the membrane (3) and is designed and arranged in such a way that vibrations of the piezo element (5) are transmitted to the membrane (3) and vibrations of the membrane (3) are transmitted to the piezo element (5), wherein the piezo element (5) is glued to the membrane (3) or to an adaptation ceramic (7) glued to the membrane (3), characterized in that the piezo element (5) consists of a lead-free piezo ceramic.

2. Vibration sensor (1) according to claim 1, characterized in that the piezo element (5) is arranged in an annular carrier (9).

3. Vibration sensor (1) according to claim 2, characterized in that the carrier (9) is designed without slots.

4. Vibration sensor (1) according to claim 3, characterized in that a potting (11) is arranged in the carrier (9), which completely covers at least one surface of the piezo element (5).

5. Vibration sensor (1) according to claim 4, characterized in that the potting (11) is designed as a silicone potting.

6. Vibration sensor (1) according to one of claims 2 to 4, characterized in that the carrier (9) consists of a material with an E-modulus of less than 4000 MPa, preferably less than 3500 MPa, particularly preferably 3200 MPa or less.

7. Vibration sensor (1) according to claim 5, characterized in that the carrier (9) is designed such that it has a radial stiffness of less than 3000 N / mm, preferably 2500 N / mm or less.

8. Vibration sensor (1) according to one of claims 5 or 6, characterized in that the carrier (9) consists of polyetherimide or polyetheretherketone.

9. Vibration sensor (1) according to one of claims 2 to 7, characterized in that the annular carrier (9) has a wall thickness of less than 1 mm, preferably less than 0.8 mm, more preferably 0.4 mm or less in the radial direction R.

10. Vibration sensor (1) according to one of claims 2 to 8, characterized in that the annular carrier has an extension FIT in the axial direction A of less than 5 mm, preferably 4 mm or less.

11. Vibration sensor (1) according to one of the preceding claims, characterized in that the carrier (9) has at least three, preferably 4 or 5, projections (91) on the radial outside, which are designed as spacers and align the carrier (9) relative to the membrane (3).

12. Vibration sensor (1) according to one of the preceding claims, characterized in that the piezo element (5) consists of bismuth sodium titanate or potassium sodium niobate.

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