Method for monitoring the condition of an electromechanical resonator

Abstract

The present disclosure includes a method for monitoring the condition of a component of an electromechanical resonator having a piezoelectrical element which can be excited to mechanical vibration using an electrical excitation signal and the mechanical vibrations of which can be received in the form of an incoming electrical signal. The method steps performed at a first point and a second point in time, including determining an amplification factor of the electromechanical resonator, determining a mechanical quality resonator, and establishing an electromechanical efficiency resonator at least from the amplification factor and the mechanical quality. A change over time in the electromechanical efficiency is calculated from the first point to the second point in time, the change over time in the electromechanical efficiency is compared with a pre-definable threshold, and a condition indicator is determined from the comparison.

Claims

1. A method for monitoring a condition of at least one component of an electromechanical resonator including a piezoelectric element that can be excited to mechanical vibration by an electrical excitation signal, wherein the mechanical vibration can be received in the form of an electrical incoming signal, the method comprising the following steps: at a first point in time: a. determining an amplification factor of the electromechanical resonator; b. determining a mechanical quality of the electromechanical resonator; and c. establishing an electromechanical efficiency of the electromechanical resonator based on the amplification factor and the mechanical quality; at least at a second point in time, repeating the steps a. through c.; calculating a change over time in the electromechanical efficiency based on a comparison of at least the electromechanical efficiency determined at a first point in time and the electromechanical efficiency determined at a second point in time; comparing the change in electromechanical efficiency with a predefined threshold; and determining a condition indicator from the comparison.

2. The method of claim 1, wherein the condition indicator is a measure for an aging of the at least one component and/or for a change in a vibration amplitude and/or a resonance frequency of the electromechanical resonator caused by the aging of the at least one component.

3. The method of claim 1, wherein the amplification factor is determined from a ratio of the excitation signal and the incoming signal.

4. The method of claim 3, wherein the excitation signal is an excitation voltage, and the incoming signal is a received voltage.

5. The method of claim 1, wherein the electromechanical resonator is excited to resonance vibrations at a resonance frequency.

6. The method of claim 1, wherein a predetermined phase shift between the excitation signal and the incoming signal is set for exciting the electromechanical resonator.

7. The method of claim 1, wherein the mechanical quality is determined based on: a gradient between the excitation signal and the incoming signal as a function of a frequency upon the presence of a resonance vibration with a resonance frequency of the electromechanical resonator; a temporal course of an amplitude of the incoming signal after switching off the excitation signal; or a modulation of the excitation signal.

8. The method of claim 1, wherein the electromechanical resonator includes a mechanically vibrating unit attached to the piezoelectric element in a force-fit and/or adhesively bonded manner, and wherein an aging of the piezoelectric element and/or of an adhesive, via which adhesive the piezoelectric element is bonded to the vibrating unit, is derived from a change in the electromechanical efficiency beyond the predefined threshold.

9. The method of claim 8, wherein the mechanically vibrating unit is a diaphragm, a single rod, or a tuning fork.

10. The method of claim 1, wherein the incoming signal further is used to determine at least one process variable of a medium in a container.

11. The method of claim 10, wherein the at least one process variable of the medium is a fill-level, a density, or a viscosity of the medium.

12. The method of claim 1, wherein the electromechanical efficiency or the change over time in the electromechanical efficiency is determined at least part using an ambient temperature, which is determined during continuous operation of the electromechanical resonator or at determined time intervals.

13. A device for determining and/or monitoring at least one process variable of a medium, the device comprising: an electromechanical resonator including at least one piezoelectric element; and an electronics unit configured to: at a first point in time: a. determine an amplification factor of the electromechanical resonator; b. determine a mechanical quality of the electromechanical resonator; and c. establish an electromechanical efficiency of the electromechanical resonator based on the amplification factor and the mechanical quality; at least at a second point in time, repeating the steps a. through c.; calculate a change over time in the electromechanical efficiency based on a comparison of at least the electromechanical efficiency determined at a first point in time and the electromechanical efficiency determined at a second point in time; compare the change in electromechanical efficiency with a predefined threshold; and determine a condition indicator from the comparison.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention along with its advantageous embodiments are described in more detail below using the figures, FIG. 1 and FIG. 2. The following is shown:

(2) FIG. 1 shows a schematic illustration of a vibronic sensor and

(3) FIG. 2 shows a schematic diagram showing the change over time in the electromechanical efficiency as a function of the amplification factor and the quality.

DETAILED DESCRIPTION

(4) For the sake of simplicity, the following description refers to a vibronic sensor 1, as shown in FIG. 1, without limiting the general nature of the present invention. The vibronic sensor 1 contains an electromechanical resonator 2, which is composed of a drive/receiver unit 3 in the form of an electromechanical converter unit with at least one piezoelectric element and one mechanically-vibrating unit 4. The drive/receiver unit 3 can be designed, for example, as a so-called bimorph drive, in which at least one piezoelectric element is adhered to the mechanically-vibrating unit 4 in a firmly-bonded manner. However, it can also be a stack drive, which contains a force-fitting coupling to the vibrating unit 4. Additional embodiments of suitable drive/receiver units 3 for a vibronic sensor 1 are familiar to persons skilled in the art and also fall under the present invention.

(5) For determination and/or monitoring, the vibrating unit 4 of the vibronic sensor 1 projects at least temporarily into a medium 5, which is located, for example, in a container 6, such as a reservoir or a pipe.

(6) The vibrating unit 4 is excited to mechanical vibration via the drive/receiver unit 5 by means of an electrical excitation signal. Conversely, the mechanical vibrations of the vibrating unit 4 are received via the drive/receiver unit 3 in the form of an electrical incoming signal, and are evaluated with regard to the respective process variable. This occurs in a suitably designed electronics unit 7, which is used for signal acquisition, evaluation, and/or feed.

(7) The amplitudes of the excitation signal and the incoming signal, or their ratio, play a significant role for many methods for determining and/or monitoring the respective process variables, especially the fill-level, density, and/or viscosity, which have become known in the context of vibronic sensors 1. Accordingly, a drift of such variables due to temperature and/or aging can lead to distortions in the determination of a measured value for the respective process variable.

(8) The method according to the invention now makes it possible to detect a change in the amplitude of the excitation and/or incoming signal not caused by the respective measurement itself. Subsequently, a changed vibration behavior can either be compensated for in a suitable manner during the continuous operation of the respective sensor 1, or possibly defective components of sensor 1, especially of the electromechanical resonator 2, can be replaced.

(9) In the case of a vibronic sensor 1 excited to resonant vibrations, the following steps are taken to determine the respective condition indicator:

(10) The electromechanical efficiency at a point in time t for a resonant vibration with the natural frequency .sub.90 of the vibronic sensor 1 is determined by the amplification factor V.sub.90 at the resonance frequency .sub.90 and the mechanical quality Q.

(11) The amplification factor V.sub.90, can, if the resonant vibration with the frequency or circular frequency .sub.90 is present, be determined from the ratio of the amplitudes of the excitation signalhere, in the form of an excitation voltage, A.sub.Aand the incoming signalhere, in the form of a received voltage, A.sub.Ei.e., in principle, on the basis of a transfer function G of the electromechanical resonator:

(12) ${V_{90}=\frac{A_E}{A_A}.Math.}_{={}_{90}}=k\left(t\right).Math.Q$

(13) The mechanical quality of the electromechanical resonator 2 can be determined, for example, from the gradient of the phase shift between the excitation signal and the incoming signal as a function of a frequency .sub.90:

(14) ${\frac{d}{d}.Math.}_{={}_{90}}=\frac{2.Math.Q}{{}_{90}}$$Q=\frac{{\frac{d}{d}.Math.}_{={}_{90}}.Math.{}_{90}}{2}$

(15) In the case of a resonant vibration, there is a phase shift of =90 between the excitation signal and the incoming signal.

(16) For the electromechanical efficiency, the following then applies:

(17) $k\left(t\right)=\frac{V_{90}}{Q}=\frac{2.Math.{.Math.\frac{A}{A_S}.Math.}_{={}_{90}}}{{\frac{d}{d}.Math.}_{={}_{90}}.Math.{}_{90}}$

(18) The amplification factor V.sub.90, and thus the accompanying vibration amplitude of the vibronic sensor 1, depends upon both the electromechanical efficiency k(t) and the quality Q.

(19) In order to monitor the condition of the electromechanical resonator 2 in continuous operation, the electromechanical efficiency k(t) is determined at at least two points in time t.sub.1 and t.sub.2, and then a change over time in the electromechanical efficiency is determined from the difference k(t)=k(t.sub.1)k(t.sub.2). If the temporal change k(t) is greater than a pre-definable threshold k, then, for example, an aging of at least one component of the electromechanical resonator can be derived. Furthermore, it is appropriate to directly determine the electromechanical efficiency k(t.sub.0) after the production of the resonator at the point in time to, and also, at such point in time, to specify the pre-definable threshold k for the change in the electromechanical efficiency k(t).

(20) FIG. 2 shows by way of example a diagram for changing the electromechanical efficiency as a function of the amplification factor and the quality of an electromechanical resonator 2. The amplification factor V is shown as a function of the frequency f for two different values k(t.sub.1) and k(t.sub.2) of the electromechanical efficiency. A change in the electromechanical efficiency thus causes a change in the amplification factor V over the entire frequency spectrum f. On the other hand, the quality Q of the electromechanical resonator 2 remains essentially unaffected by a change over time in the electromechanical efficiency, such that a change in the electromechanical efficiency k(t) in the form of a condition indicator can be determined on the basis of the quality Q and the amplification factor V.