Vibronic sensor

Abstract

The present disclosure relates to a method and corresponding sensor for determining density and/or viscosity of a medium using a vibronic sensor. An oscillatable unit is excited using an electrical excitation signal to execute mechanical oscillations, and the mechanical oscillations of the mechanically oscillatable unit are received and transduced into an electrical, received signal. The excitation signal is produced based on the received signal such that at least one predeterminable phase shift is present between the excitation signal and the received signal, wherein a frequency of the excitation signal is determined from the received signal at the predeterminable phase shift. A damping and/or a variable dependent on the damping are/is determined from the received signal at the predeterminable phase shift. From the damping and/or the variable dependent on the damping and from the frequency of the excitation signal, the density and/or the viscosity of the medium are/is ascertained.

Claims

1. A method for determining and/or monitoring a density and/or a viscosity of a medium in a containment using a vibronic sensor, the method comprising: exciting a mechanically oscillatable unit to execute mechanical oscillations using an electrical excitation signal; receiving and transducing the mechanical oscillations into an electrical received signal, wherein the excitation signal is generated based on the received signal such that there is a single, predetermined phase shift between the excitation signal and the received signal; determining a frequency of the excitation signal from the received signal at the predetermined phase shift; determining a damping and/or a variable dependent on the damping from the received signal at the predetermined phase shift; and analytically ascertaining the density and/or viscosity of the medium based on the frequency of the excitation signal and the damping and/or the variable dependent on the damping, wherein the damping and/or the variable dependent on the damping is/are determined based on: a slope of a phase of the received signal at the frequency and/or a reference frequency of the excitation signal at the predetermined phase shift; an amplitude of the received signal as a function of time after interrupting the excitation signal; and/or a modulation of the excitation signal.

2. The method of claim 1, wherein the predetermined phase shift is +/−90°.

3. The method of claim 1, wherein a predeterminable change of the density and/or viscosity of the medium is monitored.

4. The method of claim 1, wherein a predetermined fill level or a limit-level of the medium in the containment or a limit-level between a first and a second medium is monitored.

5. The method of claim 1, further comprising determining at least one reference damping and/or a variable dependent on the at least one reference damping, wherein the oscillatable unit is excited by the electrical excitation signal to execute mechanical oscillations in absence of the medium or in an at least partially immersed state in a reference medium.

6. The method of claim 1, wherein the reference damping and/or the variable dependent on the reference damping is/are determined based on: a slope of a phase of the received signal at the frequency and/or a reference frequency of the excitation signal at the predetermined phase shift; an amplitude of the received signal as a function of time after interrupting the excitation signal; and/or a modulation of the excitation signal.

7. The method of claim 1, wherein ascertaining the density and/or viscosity of the medium includes using an equation of motion for an oscillatory movement of the oscillatable unit based at least partially on an interaction of the oscillatable unit with the medium including a compressive force, a first frictional force, which arise from the medium surrounding the oscillatable unit, and a second frictional force, which arises from an equally formed movement of the oscillatable unit within the medium.

8. The method of claim 1, wherein the mechanically oscillatable unit is an oscillatory fork, a single tine or a membrane.

9. An apparatus for determining and/or monitoring a process variable of a medium in a containment, the apparatus comprising: an oscillatable unit; and an electronics unit configured to: excite the mechanically oscillatable unit to execute mechanical oscillations using an electrical excitation signal; receive an electrical received signal transduced from the mechanical oscillations, wherein the excitation signal is generated based on the received signal such that there is a single, predetermined phase shift between the excitation signal and the received signal; determine a frequency of the excitation signal from the received signal at the predetermined phase shift; determine a damping and/or a variable dependent on the damping from the received signal at the predetermined phase shift; and ascertain the density and/or viscosity of the medium analytically based on the frequency of the excitation signal and the damping and/or the variable dependent on the damping, wherein the damping and/or the variable dependent on the damping is/are determined based on: a slope of a phase of the received signal at the frequency and/or a reference frequency of the excitation signal at the predetermined phase shift; an amplitude of the received signal as a function of time after interrupting the excitation signal; and/or a modulation of the excitation signal.

10. The apparatus of claim 9, wherein the electronics unit includes a memory unit.

11. The apparatus of claim 9, wherein the electronics unit is further configured to: supply the oscillatable unit with the excitation signal, composed of an excitation carrier signal with an excitation carrier frequency and an excitation modulation signal with an excitation modulation frequency; receive from the oscillatable unit the received signal, composed of a received carrier signal and a received modulation signal; and determine at least the damping and/or a reference damping from a phase shift between the excitation modulation signal and the received modulation signal and/or at least from the received carrier signal.

12. The apparatus of claim 11, wherein the reference damping and/or a variable dependent on the at least one reference damping is determined by exciting the oscillatable unit to execute mechanical oscillations in absence of the medium or in an at least partially immersed state in a reference medium.

13. The apparatus of claim 9, wherein the electronics unit is further configured to ascertain the density and/or viscosity of the medium using an equation of motion for an oscillatory movement of the oscillatable unit based at least partially on an interaction of the oscillatable unit with the medium including a compressive force, a first frictional force, which arise from the medium surrounding the oscillatable unit, and a second frictional force, which arises from an equally formed movement of the oscillatable unit within the medium.

14. The apparatus of claim 9, wherein the predetermined phase shift is +/−90°.

15. The apparatus of claim 9, 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 thereof will now be described in greater detail based on the appended drawing, the figures of which show as follow:

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

(3) FIG. 2A shows a schematic drawing of an oscillatory fork; and

(4) FIG. 2B shows a detail view of the oscillatory fork of FIG. 2A.

DETAILED DESCRIPTION

(5) FIG. 1 shows a vibronic sensor 1, including an oscillatable unit 4 in the form of an oscillatory fork, which extends partially into a medium 2, which is located in a container 3. The oscillatable unit is excited by means of the exciter/receiving unit 5, for example, a piezoelectric stack- or bimorph drive, to execute mechanical oscillations. 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,—evaluation and/or—feeding occurs.

(6) FIG. 2A shows an oscillatable unit 4 in the form of an oscillatory fork, such as is integrated, for example, into the vibronic sensor 1 sold by the applicant under the LIQUIPHANT mark. The oscillatory fork 4 comprises two oscillatory tines 7a,7b, fork tines, connected to a membrane 8. In order to cause the oscillatory tines 7a,7b to execute mechanical oscillations, a force is exerted on the membrane 8 by means of a driving/receiving unit 5 mounted by material bonding on the side of the membrane 8 facing away from the oscillatory tines 7a,7b. The driving/receiving unit 5 is an electromechanical transducer unit, and includes, for example, a piezoelectric element 9, or even an electromagnetic drive. Either the driving unit and the receiving unit are provided as two separate units, or else as a combined driving/receiving unit. FIG. 2B details the driving/receiving unit 5. A piezoelectric element 9 is arranged on a steatite disk 10 and equipped with electrodes 11 for applying the excitation signal as well as for accepting the received signal.

(7) In the case, in which the driving/receiving unit 5 comprises a piezoelectric element 9, the force exerted on the membrane 8 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, contraction and relaxations within the piezoelectric element 9, in such a manner that the applying an electrical alternating voltage as excitation signal U.sub.E brings about an oscillation of the membrane 8 connected by material bonding with the driving/receiving unit 5.

(8) As indicated above, a goal of the present invention is to expand the application domain for determining density and/or viscosity by means of a vibronic sensor 1. The analytical model of the invention applied for describing the oscillatory movements of a vibronic sensor 1 corresponds in large part to that described in the previously unpublished German patent application No. 102015102834.4, which is incorporated herein by reference. Therefore, the complete derivation of the analytical model is not repeated here.

(9) The following explanations refer, without limiting the generality of the approach, to a resonant oscillation with a phase shift of Δφ=90° between the excitation signal U.sub.E and the received signal U.sub.R and a reference frequency ω.sub.0, which corresponds to an undamped oscillation of the oscillatable unit 4 in vacuum. The deliberations can, in such case, in each case, when required, be applied to other cases (other phase shift Δφ, reference medium 2, instead of an undamped oscillation in vacuum).

(10) According to the invention, a ratio can be formed between the reference frequency ω.sub.0 and the frequency ω of the excitation signal U.sub.E of the oscillatable unit 4 at the predeterminable phase shift Δφ. In this way, there results

(11) $\frac{{\omega }_0^2}{{\omega }_{90}^2}=1+a_1\sqrt{\frac{\rho \eta }{{\omega }_{90}}}+a_2\rho$

(12) In such case, ρ is the density and η the viscosity of the medium, while the coefficients α.sub.1 and a.sub.2 are geometric sensor constants.

(13) If, furthermore, the damping D.sub.0 of the sensor in vacuum is correlated with the damping D.sub.90 of the vibronic sensor in the medium, there results

(14) $\frac{D_{90}{\omega }_0}{D_0{\omega }_{90}}=1+a_3\sqrt{\rho \eta {\omega }_{90}}+a_4\eta +a_5$
wherein α.sub.3-α.sub.5 are likewise geometric sensor constants.

(15) The density can then be calculated based on following formula:

(16) $\rho =\frac{A}{C}-\frac{B\left(\mathrm{AE}+\mathrm{BD}-\mathrm{BG}-\sqrt{\begin{array}{c}{A}^2{E}^2-2\mathrm{ABDE}+2\mathrm{ABEG}+4\mathrm{CFAD}-\\ 4\mathrm{CFAG}+B^2{D}^2-2B^2\mathrm{DG}+B^2{G}^2\end{array}}\right)}{C\left(2\mathrm{BE}-2\mathrm{CF}\right)}$

(17) For the viscosity, the following formula holds:

(18) $\eta =\frac{\left(\begin{array}{c}{\mathrm{AE}}^2-E\sqrt{\begin{array}{c}{A}^2{E}^2-2\mathrm{ABDE}+2\mathrm{ABEG}+4\mathrm{CFAD}+\\ 4\mathrm{CFAG}+B^2{D}^2-2B^2\mathrm{DG}+B^2{G}^2+\end{array}}\\ F\left(2\mathrm{CD}-2\mathrm{CG}\right)-\mathrm{BDE}+\mathrm{BEG}\end{array}\right)}{2{\mathrm{CF}}^2-2\mathrm{BEF}}$

(19) The capital letters in the formulas are defined as follows:

(20) $A=\frac{{\omega }_0^2}{{\omega }_{90}^2}-1$$B=\frac{a1}{\sqrt{{\omega }_{90}}}$$C=a_2$$D=\frac{D_{90}{\omega }_0}{D_0{\omega }_{90}}-1$$E=a_3\sqrt{{\omega }_{90}}$$F=a_4$$G=a_5$

(21) For determining the density p and/or the viscosity η of a medium 2, thus, steps of the invention are executed as follows:

(22) Before start-up of the vibronic sensor: 1. determining the variables ω.sub.0 and D.sub.0, in each case, in vacuum, and 2. determining, especially experimentally, the geometric sensor constants α.sub.1-α.sub.5 in a suitable number of selectable test media of known density ρ and viscosity η.

(23) During operation of the vibronic sensor: 3. Continual measuring/determining of ω.sub.90 and D.sub.90, and 4. Continual calculating of the density ρ and the viscosity η.

(24) For determining and/or ascertaining the damping D.sub.90 of the vibronic sensor 1 according to the invention, a number of different options are available. For example, the damping D.sub.90 of the vibronic sensor 1 can be ascertained at the predeterminable phase shift Δφ=90° between the excitation signal U.sub.E and the received signal U.sub.R based on the slope of the phase

(25) $\frac{d\phi }{d\omega }$
at the frequency ω.sub.90. In such case,

(26) ${\frac{d\phi }{d\omega }.Math.}_{\omega ={\omega }_{90}}=\frac{1}{D_{90}{\omega }_{90}}$
and therewith

(27) $D_{90}=\frac{1}{{\frac{d\phi }{d\omega }.Math.}_{\omega ={\omega }_{90}}.Math.{\omega }_{90}}$