MONITORING THE CONDITION OF A VIBRONIC SENSOR

Abstract

The present invention relates to a method for monitoring a condition of a vibronic sensor, which serves for determining and/or monitoring at least one process variable of a medium in a container and which includes at least one sensor unit having a mechanically oscillatable unit. The method includes steps of ascertaining a measured value for at least one physical and/or chemical variable (f, f.sub.0) characteristic for the vibronic sensor, while the sensor is located at/in its location of use, comparing the measured value of the physical and/or chemical variable (f, f.sub.0) with a reference value (f.sub.ref, f.sub.0,ref) for the variable, and ascertaining a condition indicator from the comparison.

Claims

1-10. (canceled)

11. A method for monitoring a condition of a vibronic sensor, which serves for determining and/or monitoring at least one process variable of a medium in a container and includes at least one sensor unit having a mechanically oscillatable unit, the method including steps of: ascertaining a measured value for at least one physical and/or chemical variable characteristic for the vibronic sensor, while the vibronic sensor is located at/in its location of use; comparing the measured value for the physical and/or chemical variable characteristic with a reference value for the physical and/or chemical variable characteristic; and ascertaining a condition indicator from the comparison.

12. The method of claim 11, wherein a deviation between the measured value and the reference value is determined, and wherein the condition indicator is ascertained based on the deviation.

13. The method of claim 11, wherein the reference value is a value of the physical and/or chemical variable characteristic corresponding to a delivered condition of the vibronic sensor.

14. The method of claim 11, wherein the reference value and/or the associated measured value for the physical and/or chemical variable characteristic is/are recorded in a data sheet.

15. The method of claim 11, wherein the reference value and/or the associated measured value for the physical and/or chemical variable characteristic are/is stored in an Internet based file or database.

16. The method of claim 11, wherein the comparison of the measured value with the reference value is performed at a process site.

17. The method of claim 11, wherein the physical and/or chemical variable characteristic is a frequency, an amplitude, a phase difference between an excitation signal and a received signal, or a voltage.

18. The method of claim 11, wherein the mechanically oscillatable unit is a membrane, a single tine or an oscillatory fork.

19. The method of claim 11, wherein, according to the condition indicator, a statement concerning occurrence of accretion, corrosion, abrasion, or a cable break, or concerning penetration of moisture into at least one component of the vibronic sensor is generated and/or output.

20. The method of claim 11, wherein the physical and/or chemical variable characteristic is a resonant frequency of the vibratory sensor; wherein, when the measured value is greater than the reference value, then according to the condition indicator a statement is output concerning corrosion or abrasion in the region of the oscillatable unit, concerning abrasion of a coating of the oscillatable unit, concerning a defect of the oscillatable unit, or concerning presence of an accretion on the oscillatable unit; or wherein, when the measured value is less than the reference value, then generated and/or output as the condition indicator is a statement concerning corrosion or abrasion in the region of the oscillatable unit and/or of a driving/receiving unit of the vibronic sensor, or concerning diffusion of a medium into a coating of the oscillatable unit.

Description

[0036] The invention as well as its advantages will now be more exactly described based on the appended drawing, the figures of which show as follows:

[0037] FIG. 1 a vibronic sensor of the state of the art; and

[0038] FIG. 2 an oscillatable unit of a vibronic sensor, wherein the oscillatable unit is in the form of an oscillatory fork.

[0039] FIG. 1 shows a vibronic sensor 1. Shown is a sensor unit 3 with an oscillatable unit 4 in the form of an oscillatory fork, which is partially immersed in a medium 2, which is located in a container 2a. The oscillatable unit 4 is excited by means of the exciter/receiving unit 5, such that it executes mechanical oscillations. The exciter/receiving unit 5 can be, for example, a piezoelectric stack- or bimorph drive. Of course, also other embodiments of a vibronic sensor fall within the scope of the invention. Additionally shown is an electronics unit 6, by means of which signal registration, evaluation and/orfeeding occurs.

[0040] FIG. 2 shows in a side view an oscillatable unit 4 in the form of an oscillatory fork, such as, for example, an oscillatory fork as integrated in the vibronic sensor 1 sold by the applicant under the mark, LIQUIPHANT. The oscillatory fork 4 includes two oscillatory tines 8a,8b formed on a membrane 7 and bearing two terminally located paddles 9a,9b. The oscillatory tines 8a,8b together with the paddles 9a,9b are frequently also referred to together as fork tines. In order to cause the mechanically oscillatable unit 4 to execute mechanical oscillations, a driving/receiving unit 5 mounted by material bonding on the side of the membrane 7 far from the oscillatory tines 8a,8b exerts a force on the membrane 8. The driving/receiving unit 5 is an electromechanical transducer unit, and comprises, for example, a piezoelectric element, or even an electromagnetic drive [not shown]. The driving/receiving unit 5 can be two separate units, or one combined driving/receiving unit. In the case, in which the driving/receiving unit 5 comprises a piezoelectric element 9, the force exerted on the membrane 7 is generated by applying an excitation signal U.sub.E, for example, in the form of an electrical, alternating voltage. A change of the applied electrical voltage effects a change of the geometric shape of the driving/receiving unit 5, thus, a contraction, or a relaxation, within the piezoelectric element in such a manner that the applying of an electrical, alternating voltage as excitation signal U.sub.E brings about an oscillation of the membrane 7 connected by material bonding with the driving/receiving unit 5. Conversely, the mechanical oscillations of the oscillatable unit are transmitted via the membrane to the driving/receiving unit 5 and converted into an electrical, received signal U.sub.R. The particular process variable, for example, a predeterminable fill level of medium 2 in the container 2a, or even the density or viscosity of the medium 2, can then be ascertained based on the received signal U.sub.R.

[0041] An opportunity for monitoring the condition of the vibronic sensor will now be explained based on a comparison of a measured frequency f of the oscillatable unit 4, especially the resonant frequency f.sub.0 of the sensor 1,with a corresponding reference value for the frequency f.sub.ref, f.sub.0,ref. Of course, a monitoring of the condition can, however, also be performed based on any other physical and/or chemical variable characteristic for the vibronic sensor 1, for example, the amplitude A, the phase difference between the excitation signal U.sub.E and the received signal U.sub.R, or a voltage, especially a voltage characteristic for the sensor, for example, a switching voltage..

[0042] A measured value for the resonant frequency f.sub.0 of the vibronic sensor 1 can be ascertained from the received signal U.sub.R. In given cases, other, different process parameters are taken into consideration for executing a comparison of the measured value f.sub.0 with a reference value f.sub.0,ref for the frequency, in order based on the comparison to be able to obtain an exact statement concerning condition of the sensor 1. These process parameters can include, for example, the temperature T or the pressure p, or even the covered condition of the oscillatable unit 4.

[0043] Ideally, the process conditions existing at the time when the measured value for the frequency f.sub.0 was taken are the same as the process conditions existing when the reference value f.sub.0,ref was determined.

[0044] The frequency f.sub.0 the oscillatable unit 4 is, for example, temperature- and pressure dependent. Usually, the reference values, in this case, thus, the reference value for the resonant frequency f.sub.0,ref of the oscillatable unit 4, are determined essentially at standard conditions, thus, at room temperature and standard pressure. Correspondingly, it is helpful when the temperature T in the process at the time of measurement of the frequency f.sub.0 lies in a range of about 20-30 C. and there reigns in the process neither a positive pressure nor a negative pressure. Alternatively, for example, characteristic lines, curves or compensation functions relative to the dependence of individual characteristic variables, such as, for example, the frequency f.sub.0, on particular process conditions, such as the temperature T or the pressure p, can be used, in order to convert the measured values suitably.

[0045] Moreover, the resonant frequency for the case, in which the oscillatable unit 4 is not in contact with a medium, is determined, so that such requirement for monitoring the condition is ideally likewise fulfilled as regards the frequency f.sub.0.

[0046] Based on comparison of the measured value for the frequency f.sub.0 with the respective reference value f.sub.0,ref, then a statement concerning condition can be generated. For example, a predeterminable limit value can be defined. If the deviation exceeds this limit value, then, in given cases, a problem exists, or the sensor needs to be serviced. The method of the invention offers, thus, the opportunity for predictive maintenance. For example, it can be noted that maintenance of the sensor or even a cleaning cycle for the oscillatable unit is due, for example, in the case, in which accretion has formed in the region of the oscillatable unit. Moreover, the measured value for the frequency f.sub.0 can be plotted as a function of time and, for example, based on the curve, an estimate made for when such a maintenance and/or cleaning should be performed.

[0047] In the case of a rise of the resonant frequency f.sub.0 above the predeterminable limit value, for example, especially symmetrically distributed, accretion or corrosion can be present in the region of the oscillatable unit 4. It is also possible that abrasion or a coating has occurred in the region of the oscillatable unit 4, or also that the oscillatable unit is defective, for example, broken. In the case of a decrease of the resonant frequency f.sub.0 below a predeterminable limit value, on the other hand, corrosion or abrasion can be present in the region of the oscillatable unit and/or of a driving/receiving unit of the vibronic sensor, or a diffusion of a medium into a coating of the oscillatable unit has occurred.