Vibronic sensor with interference compensation

Abstract

The present disclosure relates to a method for determining a process variable of a medium by means of a vibronic sensor. In a first operating mode, an oscillatable unit is excited by a first electrical excitation signal, such that it executes mechanical oscillations, and the mechanical oscillations of the mechanically oscillatable unit are received and converted into a first electrical, received signal having a first frequency. Furthermore, the first received signal is evaluated relative to the process variable. In a second operating mode, mechanical oscillations of the oscillatable unit are received and converted into a second electrical, received signal, wherein a second frequency of the second electrical, received signal is ascertained, and wherein the second frequency is associated with a first disturbing influence for the vibronic sensor. Furthermore, the present disclosure relates to an apparatus, which is suitable for performing a method of the present disclosure.

Claims

1. A method for determining a process variable of a medium using a vibronic sensor, comprising: exciting an oscillatable unit with a first electrical excitation signal in a first operating mode such that it executes mechanical oscillations; receiving the first operating mode mechanical oscillations of the mechanically oscillatable unit and converting the received first operating mode mechanical oscillations into a first electrical, received signal having a first frequency; evaluating the first received signal relative to the process variable; receiving mechanical oscillations of the oscillatable unit in a second operating mode and converting the received second operating mode mechanical oscillations into a second electrical, received signal; ascertaining a second frequency of the second electrical, received signal; and associating the second frequency with a first disturbing influence on the vibronic sensor.

2. The method as claimed in claim 1, further comprising: performing a state monitoring based on a comparison of the first electrical, received signal and the second electrical received signal, including a comparison of the first frequency and the second frequency.

3. The method as claimed in claim 2, further comprising: detecting, based on the comparison, whether the first received signal is influenced by the first disturbing influence.

4. The method as claimed in claim 1, wherein the oscillatable unit is excited in the first operating mode such that it executes mechanical oscillations at a resonant frequency.

5. The method as claimed in claim 1, wherein, during the first operating mode, the first excitation signal is produced relative to the first received signal such that a predeterminable phase shift is present between the first excitation signal and the first received signal.

6. The method as claimed in claim 1, wherein the first disturbing influence is an oscillation, including a mechanical or electrical oscillation.

7. The method as claimed in claim 1, further comprising: producing a third electrical excitation signal in a third operating mode; exciting the oscillatable unit with the third excitation signal; and receiving from the oscillatable unit a third electrical, received signal having a third frequency.

8. The method as claimed in claim 7, wherein the first and/or third excitation signal is a signal of variable frequency.

9. The method as claimed in claim 7, further comprising: producing a first, second and/or third frequency spectrum of the vibronic sensor based on the first, second and/or third received signal.

10. The method as claimed in claim 8, wherein the second and/or third operating modes are/is performed at a first and a second point in time, and wherein, based on a comparison of the second and/or third received signal at the first and second points in time, the presence of a defect of at least one component at or in a measuring point, at or in which the vibronic sensor is applied, or the presence of a changed process state, is determined.

11. The method as claimed in claim 7, further comprising: producing an adapted first received signal from the second and/or third received signal, including from the second and/or third frequency or from the second and/or third frequency spectrum, wherein the adapted first receive signal has an adapted first frequency or an adapted first frequency spectrum, and determining the process variable from the first adapted received signal.

12. The method as claimed in claim 1, wherein the process variable is a predetermined fill level of medium in a container, the density of the medium, and/or the viscosity of the medium.

13. An apparatus for determining and/or monitoring at least one process variable of a medium in a container, comprising: an electronics unit; and a mechanically oscillatable unit, wherein the electronics unit is embodied to: excite the oscillatable unit with a first electrical excitation signal in a first operating mode such that it executes mechanical oscillations; receive the first operating mode mechanical oscillations of the mechanically oscillatable unit and convert the received first operating mode mechanical oscillations into a first electrical, received signal having a first frequency; evaluate the first received signal relative to the process variable; receive mechanical oscillations of the oscillatable unit in a second operating mode and convert the received second operating mode mechanical oscillations into a second electrical, received signal; ascertain a second frequency of the second electrical, received signal; and associate the second frequency with a first disturbing influence on the mechanically oscillatable unit.

14. The apparatus as claimed in claim 13, wherein the electronics unit is further embodied to: produce a third electrical excitation signal in a third operating mode; excite the oscillatable unit with the third excitation signal; and receive from the oscillatable unit a third electrical, received signal having a third frequency.

15. The apparatus as claimed in claim 14, wherein the electronics unit includes a switch for switching back and forth between the first operating mode and the second operating mode, or at least two switches for switching back and forth between the first, second and/or third operating modes.

16. Apparatus as claimed in claim 13, wherein the mechanically oscillatable unit is an oscillatory fork, a single tine, or a membrane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention as well as advantageous embodiments will now be described in greater detail based on the appended drawing, the figures of which show as follows:

(2) FIG. 1 shows a schematic view of a vibronic sensor of the state of the art,

(3) FIG. 2 shows a schematic drawing of an oscillatable unit in the form of an oscillatory fork,

(4) FIG. 3 shows a first electronics unit of a vibronic sensor, which electronics unit is suitable for performing a method of the present disclosure, and

(5) FIG. 4 shows a second electronics unit of a vibronic sensor, which electronics unit is suitable for performing a method of the present disclosure.

DETAILED DESCRIPTION

(6) Equal elements are provided in the figures with equal reference characters.

(7) FIG. 1 shows a vibronic sensor 1 having a sensor unit 3 comprising an oscillatable unit 4 in the form of an oscillatory fork, which is partially immersed in a medium 2 located in a container 2a. The oscillatable unit is excited by means of the exciter/receiving unit 5, such that it executes mechanical oscillations, and can be, for example, a piezoelectric stack- or bimorph drive. It is understood, however, that also other embodiments of a vibronic sensor fall within the scope of the invention. Further provided is an electronics unit 6, by means of which signal registration, evaluating and/orfeeding occurs.

(8) FIG. 2 shows in side view an oscillatable unit 4 in the form of an oscillatory fork, such as integrated, for example, in the vibronic sensor 1 sold by the applicant under the mark, LIQUIPHANT. The oscillatory fork 4 includes, formed on a membrane 7, two oscillatory tines 8a,8b, on which two paddles 9a,9b are formed terminally. The oscillatory tines 8a,8b together with the paddles 9a,9b are frequently also referred to as fork tines. In order to cause the mechanically oscillatable unit 4 to execute mechanical oscillations, a force is exerted on the membrane 7 by means of a driving/receiving unit 5 mounted by material bondng on the side of the membrane 7 away from the oscillatory tines 8a,8b. The driving/receiving unit 5 is an electromechanical transducer unit, and comprises, for example, a piezoelectric element, or even an electromagnetic drive [not shown]. Either the driving unit 5 and the receiving unit 5 are embodied as two separate units, or as a combined driving/receiving unit 5. In the case, in which the driving/receiving unit 5 comprises a piezoelectric element, 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 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 bondng 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 frequency of the received signal U.sub.R corresponds, in such case, to the mechanical oscillation frequency f of the oscillatable unit 4.

(9) In the case, in which at least one disturbing influence, for example, in the form of an arising mechanical, unwanted vibration, occurs at the location of use of the vibronic sensor, the disturbing influence also produces a contribution to the received signal. Due to this additional, especially unwanted contribution, inaccuracies can occur in the determining and/or monitoring of the at least one process variable. In the worst case, a reliable determining and/or monitoring is no longer possible at all.

(10) By means of the method of the invention, and by means of a vibronic sensor 1 of the invention, advantageously, disturbing influences can be detected, taken into consideration, compensated and/or eliminated. For this, the disturbing influence does not have to be a priori known. Rather, by means of the present invention, at least one, second frequency associated with the disturbing influence can be detected. For the subsequent description, without intending to limit the general applicability of the invention, for purposes of simplification, it is assumed that the disturbing influence is a mechanical oscillation, especially an unwanted vibration. For other types of disturbing influences, for example, electrical oscillations, analogous considerations hold, so that these additional cases are not explored in further detail.

(11) Mechanical, unwanted vibrations at the location of use of a vibronic sensor can have the most varied of causes. For example, involved can be a motor, especially a defective motor, a pump, an ultrasonic bath, or the like. However, also flowing medium can bring about an unwanted vibration. Since the vibronic measuring principle fundamentally is based on the execution of mechanical oscillations of an oscillatable unit, and because changes in the oscillatory behavior of the oscillatable unit are taken into consideration for determining and/or monitoring the particular process variable, the achievable accuracy of measurement depends sensitively on whether unwanted vibrations are present, and, if they are present, on their exact nature.

(12) If an oscillation of the oscillatable unit is composed of a signal portion caused by the particular excitation signal, as well as a signal portion caused by a disturbing influence, then the particularly present disturbing influence decisively influences the determining and/or monitoring of the process variable based on the received signal received from the oscillatable unit, since the received signal is likewise composed of a signal portion representing the oscillations of the oscillatable unit and a signal portion representing the at least one disturbing influence. Depending on intensity and frequency of the disturbing influence, it can even happen that a determining of the particular process variable is no longer reliably possible at all. Especially important, furthermore, is the case, in which the frequency of a disturbing influence in the ongoing measurement operation of the vibronic sensor is not known. If the particular process variable is a predeterminable fill level, it can happen, in such case, that the reaching of the predeterminable fill level is detected, although such is not yet true, or vice versa. For example, due to such a malfunction, an incorrect switching signal can be generated for switching a process switch element.

(13) With the present invention, it becomes advantageously possible to detect, in given cases present, disturbing influences. Such can then, for example, be taken into consideration, compensated or even eliminated for determining and/or monitoring the particular process variable.

(14) The method of the invention can be applied both for vibronic sensors with analog electronic units as well as for such with digital electronic units, such as will be explained based on the embodiments to be described below.

(15) A first example of embodiment of the present invention is shown in FIG. 3 and relates to the case of a digital electronics unit 6 comprising a computing unit 11, here in the form of a microcontroller. By means of the microcontroller, a first excitation signal U.sub.E1 is produced, by means of which the oscillatable unit 4 is excited, such that it executes mechanical oscillations. The oscillations of the oscillatable unit 4 are, in turn, converted by means of the driving/receiving unit into a first electrical, received signal U.sub.R1 and fed to the microcontroller 11. The determining and/or monitoring of the particular process variable, for example, a predetermined fill level, the density and/or the viscosity of the medium 2, occurs based on the first received signal U.sub.R1.

(16) In the embodiment shown here, the signal path of the electronics unit optionally contains a digital-analog converter (DAC) 12b, an analog-digital converter (ADC) 12a, as well as two amplification units 13a, 13b.

(17) The switch element 14a serves to switch back and forth between the first and the second operating modes. The switch element is controlled by the microcontroller 11 using a control signal 15a. In the configuration of the switch element 14a shown in FIG. 3, the vibronic sensor is located in the first operating mode, which corresponds to normal measurement operation. Useful for the normal measurement operation are, in such case, in principle, all measuring methods known from the state of the art, especially those mentioned, by way of example, in the above introduction, and these all fall within the scope of the present invention.

(18) In order to change into the second operating mode, the switch element 14a is brought into its second configuration. An output 16 of the switch element 14a remains open in this configuration, so that the sensor unit 14a is uncoupled from the regular measuring path (first operating mode). During the second operating mode, thus, no second excitation signal U.sub.E2 is produced. Rather, only a second received signal U.sub.R2 is received from the mechanically oscillatable unit 4 and evaluated as regards at least a second frequency f.sub.2, wherein the second frequency f.sub.2 corresponds at least to the at least one disturbing influence.

(19) Depending on embodiment of the method of the invention, the particular sensor can be operated exclusively in the first and second operating modes. It is, however, likewise possible to have a third operating mode, in which the vibronic sensor 1 is supplied with a third excitation signal U.sub.E3 and in which a third received signal U.sub.R3 is received. The third operating mode can, on the one hand, be initiated by at least a second switch element 14b (not shown). Alternatively, the first switch element 14a can also be returned to its first configuration. In that case, the first and third operating modes are distinguished within the microcontroller 11.

(20) For purposes of simplification, the excitation signals U.sub.E1-U.sub.E3 and received signals U.sub.R1-U.sub.R3 are not separately drawn, but, instead, noted once in the form of reference character U.sub.Ei and U.sub.Ri, wherein i is a number between one and three.

(21) A second embodiment is based on the block diagram shown in FIG. 4. This embodiment is suited especially for a vibronic sensor 1 with an at least partially analog electronics unit 6. For producing the first excitation signal U.sub.E1 starting from the first received signal U.sub.R1 and for setting the predeterminable phase shift between the first excitation signal U.sub.E1 and the first received signal U.sub.R1 in the first operating mode, the electronics unit includes a bandpass filter 16 and a phase shifter 17.

(22) The switch elements 14a-14c serve for switching back and forth between the first and second, or first, second and third operating modes. The switch elements 14a-14c are, in such case, controlled via the microcontroller 11 by means of control lines 15a-15c analogously to the embodiment of FIG. 3. In the second and/or third operating mode, no excitation signal and/or the third excitation signal U.sub.E3 is produced by means of the microcontroller and the particular second U.sub.R2 and/or third U.sub.R3 received signal is evaluated by means of the microcontroller at least relative to the at least a second f.sub.2 and/or third f.sub.3 frequency. By means of the timer 18, the microcontoller 11 can, furthermore, determine the at least a first frequency f.sub.1 of the first received signal U.sub.R1 and determine and/or monitor the particular process variable.

(23) Independently of the exact embodiment of the electronics unit 6 of the vibronic sensor, either the at least a first, second and/or third frequency can be compared with one another, or a first, second and/or third frequency spectrum can be produced.