Vibrating string sensor and vibrating string for a vibrating string sensor

Description

[0001] The present invention relates to a vibrating wire sensor and vibrating wire for a vibrating wire sensor according to the preamble of claims 1 and 11.

[0002] Vibrating wire sensors are well-known and used in numerous applications, often (but by no means exclusively) to measure a current quantity such as deformation or force. The term deformation here refers to the elastic or plastic deformation of machine elements during operation. Such deformation can be relevant for monitoring the safe operating condition of metal structures of any kind, or also of non-metallic structures such as buildings or bridges. Furthermore, deformation sensors can be used to detect a load currently acting during operation, for example, to control a movement. This deformation is detected by the vibrating wire sensor, which thus acts as a deformation / force sensor, for example via the displacement between measuring points, whereby a force acts on the sensor that is a measure of the deformation.Likewise, the sensor according to the invention can also be used as a pure force sensor, for example in a scale.

[0003] Vibrating wire sensors can be designed in such a way that they detect the smallest deformations or force changes with fine resolution and also show hardly any drift over time - in this respect, vibrating wire sensors are often qualitatively superior to load cells or strain gauges.

[0004] US 4,656,383 discloses an oscillator made of a piezoelectric material that can be set into vibration by electrodes applied to it. EP 2 407 765 discloses a microstructure made of etched silicone with exciters and measuring bodies comprising a piezoelectric material.

[0005] An embodiment of a vibrating wire sensor has become known through CN 203 595 494 U and is described in Fig. 1 shown.

[0006] A vibrating wire sensor 1 is shown in operation, i.e. under measuring conditions, with a rectangular frame 2, with fixing elements 3, 4, which engage cross members 5, 6 of the frame 2 and are in turn fixed via their fixing points 7, 8 to a component (not shown to relieve the figure), the deformation of which is to be measured or, for example, a component of a scale in which the weight to be weighed is to be determined (the fixing can be done by a simple screw connection or by a simple clamping of any kind). Under the measuring conditions, here in the case of a deformation of the component or an occurring force to be measured,

[0007] the sensor 1 is subjected to a force, for example in the direction of arrow K (or the opposite direction), causing its frame 2 to elastically deform under the action of the force, which in turn leads to a deflection of the transmission levers 9, 10, to whose heads 11, 12 a vibrating string 15 is clamped by means of clamping pins 16. Since the transmission levers also deform elastically, a displacement-force transmission results from the attachment points 7, 8 to the heads 11, 12 or clamping pins 16, so that the vibrating string 15 is subjected to different tensile stresses depending on the currently existing deformation or force to be measured, whereby, of course, when the sensor is at rest, i.e. without environmental influences at the attachment points 7, 8, the vibrating string 15 is at a predetermined tension. The heads 11, 12 and the clamping pins 16 form clamping points for the vibrating string 15.

[0008] Also visible is an arrangement 17 which, during operation, generates a magnetic field whose field lines are perpendicular to the plane of the drawing and are directed away from the viewer in the left half of the arrangement 17 and towards the viewer in the right half. For this purpose, an alternating current I flows through the vibrating string 15 during operation. This current is generated by electronics that have again been omitted to reduce the load on the figure. The Lorentz force corresponding to the alternating current I thus acts on the vibrating string, causing it to vibrate at the frequency of the alternating current. With a suitable change in frequency, the vibrating string quickly drops to its natural frequency, which in turn depends on the tension acting on it, i.e. on the displacement of the measuring points 7, 8 as a result of a deformation or force. The electronics can detect the natural frequency and use this to generate a sensor signal for the deformation / force applied.

[0009] Figure 1shows in summary a vibrating wire sensor with a vibrating wire which is tensioned differently under measuring conditions according to a current quantity to be detected, and with an excitation arrangement for exciting the vibrating wire in the range of its respective natural frequency.

[0010] The disadvantages of such vibrating wire sensors are their high power consumption, the magnets can break out of their correct position if handled roughly, and the effort required for shock-proof mounting is considerable. High power consumption plays a role, for example, when a bridge is equipped with deformation sensors whose batteries must be constantly replaced or alternatively wired.

[0011] The object of the present invention is to improve vibrating wire sensors.

[0012] To achieve this object, a vibrating wire sensor according to the invention has the characterizing features of claim 1 and a vibrating wire according to the invention has the characterizing features of claim 11.

[0013] Because the vibrating wire of a vibrating wire sensor has a piezoelectric activation layer, power consumption can be significantly reduced compared to conventional designs. Furthermore, magnets or other elements for generating a magnetic field are eliminated, thus eliminating the elements that determine the sensor's sensitivity during rough handling. Finally, a vibrating wire sensor according to the invention has a more compact design.

[0014] Because the piezoelectric activation layer has different lengths depending on the activation state, the vibrating string is deformed along its length in the manner of a bimetallic spring, which is particularly suitable for generating vibrations in a string.

[0015] Embodiments of the vibrating wire sensor or vibrating wire according to the invention have features according to the dependent claims.

[0016] The invention is described in more detail with reference to the figures.

[0017] It shows: Fig. 1a vibrating wire sensor according to the prior art, Fig. 2aan embodiment of a vibrating wire sensor according to the invention, Fig. 2ban enlarged section of Fig. 2a with the vibrating side, Fig. 3 schematically shows an embodiment of a vibrating wire according to the invention in a vibrating wire sensor, Fig. 4 schematically shows a further embodiment of a vibrating wire according to the invention in a vibrating wire sensor, Fig. 5 a schematically shows a preferred embodiment of a vibrating wire according to the invention in a vibrating wire sensor, and Fig. 5 b a view of the vibrating wire of Fig. 5a .

[0018] Figure 2ashows the principle of the present invention based on a preferred embodiment of a vibrating wire sensor 20. The vibrating wire sensor 20 shown in operation can (but does not have to) have the same basic structure as the vibrating wire sensor 1 of Fig. 1 . Order 17 ( Figure 1 However, a magnetic field generating layer is missing. Visible is a vibrating string 21, which is covered with an excitation layer 22 (indicated by the dashed line) comprising a piezoelectric activation layer, at least over a portion of its length—here, over its entire length. Possible embodiments of the excitation layer 22 are described in more detail in the figures.

[0019] An alternating voltage is applied to the excitation layer 22 via clamping pins 23 or contact elements. The alternating voltage is generated by electronics of the vibrating wire sensor 20, which have been omitted to reduce the volume of the figure. The figure shows an operating state in which, due to the applied alternating voltage, the piezoelectric activation layer of the excitation layer 22 has an activation state in the left section 24 in which it is elongated, and an activation state in the right section 25 in which it is shortened.

[0020] As a result, the vibrating string 21 bends like a bimetallic spring, from left to right, to the right in section 24 and to the left in section 25. This curvature corresponds to a vibration position of the vibrating string with a vibration node in the center (vibration frequencies above the hearing threshold are desirable).

[0021] Figure 2bshows an enlarged section of the vibrating wire sensor 20 in the area of ​​the vibrating wire 21, which, compared to the illustration of Figure 2a However, it has a different oscillation position, corresponding to the alternating voltage acting on the clamping pins 23 one half-wave later. The excitation layer 22 is shortened in the left section 24 and lengthened in the right section 25, so that the vibrating string 21 is curved to the left in section 24 and to the right in section 25.

[0022] In accordance with the applied alternating voltage, the vibrating string 21, excited by the various activation states of the excitation layer 22, oscillates back and forth between the oscillation positions shown in Figures 2a and 2b. The frequency of the alternating voltage is changed by the electronics until the vibrating string 21 oscillates at its current natural frequency. The current natural frequency is determined by the current voltage of the vibrating string 21, given by the clamping pins 16, 23, which subject the vibrating string 21 to the displacement of the measuring points 7, 8 ( Figure 1 ) are subjected to a corresponding tensile stress. As mentioned above, the electronics generates a signal based on the current natural frequency, which corresponds to the deformation or force to be detected by sensor 20.

[0023] It turns out that the excitation arrangement of a vibrating wire sensor according to the invention has at least one excitation layer provided on a length section of the vibrating wire with a piezoelectric activation layer, which has different lengths depending on the activation state and thereby causes a correspondingly different oscillation position of the vibrating wire.

[0024] Furthermore, according to the invention, a vibrating string is obtained which has a piezoelectric excitation layer provided on at least one length section, which has different lengths depending on the activation state and thereby causes a deformation of the vibrating string over at least one length section.

[0025] In an alternative embodiment, the alternating voltage can also be applied to the end sections 27, 28 of the vibrating wire 21. This makes it possible to provide any suitable material for the clamping pins (or clamping or holding elements of any kind). Corresponding conductors leading from the end sections 27, 28 to the electronics of the vibrating wire sensor 20 have been omitted to reduce the size of the figure.

[0026] The vibrating wire 21 is preferably thin and flat in cross-section, for example rectangular (but can also be oval or round), and has a low moment of inertia in the plane of vibration. The vibrating wire is flexible, thus, in a deflected position, it has hardly any spring-elastic restoring force due to bending. As a result, it swings back from a deflected position to its extended position primarily due to the tension exerted via the clamping points. This return vibration can, of course, be significantly assisted by the appropriately timed piezoelectric activation layer of the excitation layer 22.

[0027] The following figures show various embodiments of a vibrating wire detector with a vibrating wire according to the invention.

[0028] Figure 3 shows a vibrating wire sensor 30, schematically indicated by a rectangle, which, except for the Figure 3components shown, for example, such as the vibrating wire sensor 1 of Figure 1 The vibrating wire sensor 30 has a vibrating wire 31 with an excitation layer 32, which has a piezoelectric activation layer 33 and an electrode 34, which extends over the length of the activation layer 33. The vibrating wire 31 is clamped in clamps 35, which are preferably designed as clamping pins ( Figure 1 ) but may also be designed differently. A schematically indicated electronics 36 of the vibrating wire sensor has a voltage source 37, which is connected via a conductor 38 to the vibrating wire 31 and via a conductor 39 to the electrode 34. The remaining components of the vibrating wire sensor have been omitted to reduce the size of the figure, in particular heads 11, 12 ( Figure 1), in which the terminals 35 are arranged. The figure shows distorted proportions for clarity; in reality, the vibrating wire 31 is long and thin, e.g., 0.4 mm wide, 0.3 mm high, and 20 mm long. The excitation layer 32 is thin, corresponding to the manufacturing process (see below).

[0029] The vibrating wire 31 is preferably made of a beryllium-bronze alloy, which is very hard, hardly elongates under tension and does not creep, thus contributing to the precise, drift-proof operation of the vibrating wire sensor.

[0030] The piezoelectric activation layer 33 is preferably formed as an AlScN layer, with scandium in the range of 15% to 30%. However, it can also consist of pure AlN. The electrode can be implemented as a thin layer of gold.

[0031] More preferably, the activation layer 33 is applied to the vibrating wire 31 by sputtering, as is the electrode. Preferably, the activation layer 33 is heated above the Curie temperature after sputtering, polarized by a suitable electric field, and then cooled so that it can be piezoelectrically activated. It is also possible to polarize the piezo layer directly during sputtering. The sputtering process also leads to a development of the crystal orientation or specific crystal types, which can facilitate subsequent polarization in certain orientations. Those skilled in the art are familiar with vibrating wire of the aforementioned type, as well as piezoelectric coatings made of AlScN, which can be produced by sputtering.

[0032] As mentioned above, the electronics 36 are designed to output an alternating voltage during operation, for example, with a starting frequency close to the natural frequency of the vibrating string 31 in the resting state of the vibrating string sensor 30. The frequency is then increased until the vibrating string 31 falls to its natural frequency, which in turn can be detected by the electronics based on its impedance – at the natural frequency, the apparent resistance z of the arrangement of electrode – piezoelectric activation layer – electrode is minimal (this arrangement represents a capacitance). As mentioned, the natural frequency is a measure of the tension acting on the vibrating string 31, which in turn is a measure of the deformation or force to be detected, so that the electronics 36 can generate a signal of the detected current quantity (deformation, force) from the detected, current natural frequency of the vibrating string 31.

[0033] It turns out that the excitation layer 32 has electrodes 34, 31 sandwiching the activation layer, with the vibrating wire 31 forming one of the electrodes. In this case, the vibrating wire can itself be coated with gold on the side of the piezoelectric activation layer 33 to improve its conductivity.

[0034] Figure 4shows a vibrating wire sensor 40, with a further embodiment of a vibrating wire 41 which is provided with an excitation layer 42 only over a section of its length. In addition, the excitation layer 42 is electrically insulated from the vibrating wire 41 by an insulation layer 43, shown in dashed lines in the figure, for example made of Al 2 O 3 , aluminum oxide or ZnO, zinc oxide. In addition to an upper electrode 44, a lower electrode 45 (i.e. arranged near the vibrating wire 41) is provided, the voltage source 37 of the electronics 36 is connected via the conductor 38 to the lower electrode 45 and via the conductor 39 to the upper electrode 44. The piezoelectric activation layer 46 is enclosed over its length by electrodes 44, 45.

[0035] The arrangement shown allows for the avoidance of current flow through the vibrating wire 41, which has a high ohmic resistance due to its alloy (low elongation and low creep, see above). Thus, only comparatively low ohmic resistances are present in the AC circuit. Furthermore, the energy requirement of the excitation layer 42 is generally low, and particularly low at the natural frequency of the vibrating wire 41. This results in a reduction in the energy consumption compared to a conventional vibrating wire sensor 1 ( Figure 1 ) an electricity consumption of 30% to 20%.

[0036] It turns out that an electrode is arranged on the inside of the excitation layer, which extends over essentially the length of the excitation layer.

[0037] The arrangement shown allows the vibrating string to be excited to oscillate with a node.

[0038] As with the Figure 3In the embodiment shown, an electrode 34 or 44, 45 extends from the vibrating section of the vibrating string 31 or 41 through the corresponding clamping point, given by the location of the terminals 35. This allows this electrode to be connected particularly easily to an associated conductor 38, 39, since the area behind the clamping points is more easily accessible for the connection to electrodes 34, 44, 45 and the vibrating string 31, 41 can be widened there for suitable connection.

[0039] Starting from the embodiment shown in the figure with an excitation layer provided on the right side of the vibrating string 41, a second excitation layer, preferably of the same structure, can be provided on the left side of the vibrating string 41, which is more preferably arranged on the opposite side. Two excitation layers are then provided, located on different sides of the vibrating string, which allows the developing vibration of the vibrating string to be triggered particularly precisely during operation and guided against the current natural frequency of the vibrating string.

[0040] The result is a vibrating string in which two excitation layers are provided, which are arranged on the same side or on different sides of the vibrating string.

[0041] Fig. 5ashows a preferred embodiment of the present invention, with a vibrating wire sensor 50, again only schematically indicated, with a vibrating wire 51 clamped therein via terminals 35. An excitation layer 52 extends substantially over the entire length of the vibrating wire 51, is insulated from it by an insulating layer 53, again indicated by dashed lines, and has a continuous piezoelectric activation layer 54, which is enclosed by outer electrodes 55, 56 and an inner electrode 57.

[0042] The two outer electrodes 55,56 are separated from each other, one electrode 55,56 being connected to the conductor 38 of the voltage source 37, and the other electrode 56,55 being connected to the conductor 39. One of these conductors 38,39 is connected as a neutral conductor, the other conductor 39,38 as a live conductor, ie phase.

[0043] This arrangement corresponds to two capacitors connected in series: If, for example, the conductor 39 is connected as a voltage-carrying conductor, an alternating E field results between the upper electrode 56 and the lower electrode 57 during operation, which once the right section 25 lying between them (see e.g. Figure 2b ) of the piezoelectric activation layer 54 is activated, which, depending on the direction of the E-field and the polarization of the piezoelectric layer 54, leads to a change in thickness and thus also to a change in length of this section and thus to a curvature of the vibrating string 51 in the right section 25.

[0044] The formation of this electric field in the right-hand section 25 results in a charge shift in the lower electrode 57, which is thus charged in the left section 24 in the opposite direction to its right-hand section 25. This, in turn, together with the outer electrode 55, leads to an electric field in the left section 24 that is directed opposite to the electric field in the right-hand section 25. The change in length in section 24 is thus opposite to that of section 25, and thus also the curvature of the vibrating string 51, resulting in an oscillation position with a node in the center of the vibrating string 51.

[0045] During the next half-wave of the applied alternating voltage, the vibrating wire 51 is bent in the opposite direction, thus assuming a correspondingly different oscillation position. Its natural frequency is again detected by the electronics 36 through a minimum of the apparent resistance z.

[0046] Figure 5bshows a three-dimensional view of the vibrating string 51 of Figure 5a In the style of an exploded view, with realistic proportions. The end sections 58 and 59 are wider, which improves the clamping at the ends.

[0047] The arrangement according to the Figure 5a and 5b is particularly advantageous because the vibrating string can be excited over its entire length for the desired vibration shape with a vibration node through a simple structure (especially through the same polarization of the piezoelectric excitation layer over its length).

[0048] This results in a vibrating wire sensor according to the invention, in which the excitation layer extends from one clamping region 58, 59 of the vibrating wire 51 to the other clamping region 59, 58 and two electrodes 55, 56 are provided, which extend on the same side of the excitation layer 52 from the opposite clamping regions 58, 59 towards each other up to the center of the excitation layer 52, and wherein on the opposite side of the excitation layer 52 an electrode 53 is provided which extends between the clamping regions 58, 59. Preferably, as shown in the Figure 5a and 5bAs shown, the continuous electrode 53 is located on the inside of the excitation layer 52. Further preferably, one of the outer electrodes 55, 56 is connected to a live conductor of an AC voltage source, and the other outer electrode is connected to a neutral conductor. Further preferably, an electrode 56, 57 is provided on an outer side of the piezoelectric activation layer 54, which extends at least over a length section of the excitation layer 52.

[0049] The correspondingly preferably designed vibrating string is provided with an excitation layer 52 which has an electrode 57 on one side and two electrodes 55, 56 on its opposite side, which extend from the end regions 58, 59 towards the middle of the vibrating string 52.

[0050] In the case of a device not shown in the figures, the one according to the Figure 5a , 5bIn a similar embodiment, the piezoelectric activation layer is not continuous, but interrupted in the middle of the vibrating string, so that, for example, the voltage-carrying conductor can be connected to the lower, continuous electrode.

[0051] In the specific case, the person skilled in the art can suitably combine the various features according to the embodiments presented above.

Claims

1. Vibrating wire sensor (20, 30, 40, 50) having a vibrating wire (21, 31, 41, 51) which has an alloy and is tensioned differently under measurement conditions corresponding to a current variable to be detected, and having an exciter arrangement for exciting the vibrating wire (21, 31, 41, 51) in the range of its respective natural frequency, the exciter arrangement having at least one exciter layer (22, 32, 42, 52) which is provided on a length section of the vibrating wire (21, 31, 41, 51) and has a piezoelectric activation layer (33, 46, 54) which has different lengths depending on the activation state (51), with a piezoelectric activation layer (33, 46, 54), which has different lengths depending on the activation state and thereby causes a correspondingly different oscillation position of the vibrating string (21, 31, 41, 51), the exciter arrangement being designed to recognize a minimum of the impedance of the excited vibrating side during operation in order to determine its natural frequency.

2. Vibrating wire sensor (30) according to claim 1, wherein the excitation layer (32) has electrodes (34, 31) which enclose the piezoelectric activation layer (33) between them, and wherein the vibrating wire (31) forms one of the electrodes.

3. Vibrating wire sensor (40, 50) according to claim 1, wherein the excitation layer (46, 54) is electrically insulated from the vibrating wire (41, 51).

4. Vibrating wire sensor (30, 40, 50) according to claim 1, wherein an electrode (34, 44, 55, 56) is provided on an outer side of the piezoelectric activation layer (33, 46, 54), which electrode extends at least over a length section of the excitation layer (32, 44, 52).

5. Vibrating wire sensor according to claim 1, wherein an electrode (33, 44, 45) extends from the vibrating portion of the vibrating wire (31, 41, 51) through the clamping point (58, 59).

6. Vibrating wire sensor (40, 50) according to claim 1, wherein an electrode (43, 53) is arranged on the inside of the excitation layer (42, 52), which electrode extends over substantially the length of the excitation layer (42, 52).

7. Vibrating wire sensor according to claim 1, wherein two excitation layers are provided which are located on different sides of the vibrating wire.

8. Vibrating wire sensor according to claim 1, wherein the excitation layer extends from one tensioning region (58, 59) of the vibrating wire (51) to the other tensioning region (59, 58) and two electrodes (55, 56) are provided which extend on the same side of the excitation layer (52) from the opposite tensioning regions (58, 59) towards each other up to the center of the excitation layer (52), and wherein on the opposite side of the excitation layer (52) an electrode (55, 56) extending between the tensioning regions (58, 59) is provided,59) towards each other as far as the center of the excitation layer (52), and wherein an electrode (57) passing between the clamping regions (58, 59) is provided on the opposite side of the excitation layer (52).

9. Vibrating wire sensor according to claim 7, wherein the continuous electrode (57) is located on the inside of the excitation layer (52).

10. Vibrating wire sensor according to claim 7, wherein one of the outer electrodes (55, 56) is connected to a live conductor (38, 39) and the other outer electrode (39, 38) is connected to a neutral conductor of an AC voltage source (37).

11. Vibrating string (21, 31, 41, 51) consisting of an alloy for a vibrating string sensor according to claim 1, characterized in that it has a piezoelectric excitation layer (22, 32, 42, 52) provided on at least one length section, which has different lengths depending on the activation state and thereby causes a deformation of the vibrating string over at least one length section.

12. Vibrating string (41, 51) according to claim 11, wherein the excitation layer (42, 52) is electrically insulated from it.

13. Vibrating string (51) according to claim 11, wherein the excitation layer (52) has an electrode (57) on one side and two electrodes (55, 56) on its opposite side, which extend from the end regions (58, 59) to the center of the vibrating string.

14. Vibrating string according to claim 11, wherein two excitation layers are provided which are arranged on the same side or on different sides of the vibrating string.