Apparatus for determining and/or monitoring at least one process variable of a medium

Abstract

An apparatus for determining and/or monitoring at least a first process variable of a medium and method for operating the apparatus at least with an electronics unit and a sensor unit, wherein the electronics unit is embodied to supply the sensor unit with an excitation signal, which is composed of an excitation carrier signal with an excitation carrier frequency and an excitation modulation signal with an excitation modulation frequency, and to receive from the sensor unit a received signal, which is composed of a received carrier signal and a received modulation signal, and wherein the electronics unit is embodied to determine from the phase shift between the excitation modulation signal and the received modulation signal at least the first process variable.

Claims

1. An apparatus for determining and monitoring at least a first process variable of a medium, comprising: an electronics unit; and a mechanically oscillatable sensor unit, wherein the electronics unit is embodied to supply the sensor unit with an excitation signal composed of an excitation carrier signal having an excitation carrier frequency and an excitation modulation signal having an excitation modulation frequency and to receive from the sensor unit a received signal composed of a received carrier signal and a received modulation signal, and wherein the electronics unit is further embodied to measure a phase shift between the excitation modulation signal and the received modulation signal and to determine from the phase shift the first process variable.

2. The apparatus as claimed in claim 1, wherein the electronics unit is further embodied to determine from the received carrier signal at least a second process variable.

3. The apparatus as claimed in claim 1, wherein the sensor unit includes a mechanically oscillatable fork, a single rod, or a membrane.

4. The apparatus as claimed in claim 1, wherein the electronics unit is further embodied to determine and to monitor a predetermined fill level, a density or a viscosity of the medium.

5. The apparatus as claimed in claim 1, wherein the electronics unit is further embodied to control a phase shift between the excitation carrier signal and the received carrier signal to a predetermined value.

6. The apparatus as claimed in claim 1, wherein the electronics unit is further embodied to ascertain a damping from the phase shift between the excitation modulation signal and the received modulation signal.

7. The apparatus as claimed in claim 6, wherein the electronics unit is further embodied to ascertain from the damping at least a viscosity of the medium.

8. The apparatus as claimed in claim 1, wherein the electronics unit is further embodied to control the phase shift between the excitation modulation signal and received modulation signal to 45 by adjusting the excitation modulation frequency.

9. A method for determining and monitoring a first process variable of a medium, comprising: providing an electronics unit and a sensor unit; supplying from the electronics unit to the sensor unit an excitation signal composed of an excitation carrier signal having an excitation carrier frequency and an excitation modulation signal having an excitation modulation frequency; receiving by the electronics unit from the sensor unit a received signal composed of a received carrier signal and a received modulation signal; and determining from a phase shift between the excitation modulation signal and the received modulation signal the first process variable.

10. The method as claimed in claim 9, further comprising: determining from the received carrier signal at least a second process variable.

11. The method as claimed in claim 9, wherein the first process variable is a predetermined fill level, a density of the medium, or a viscosity of the medium.

12. The method as claimed in claim 9, further comprising: controlling a phase shift between the excitation carrier signal and received carrier signal to a predetermined value.

13. The method as claimed in claim 9, further comprising: determining a damping from the phase shift between the excitation modulation signal and the received modulation signal.

14. The method as claimed in claim 13, wherein at least a viscosity of the medium is ascertained from the damping.

15. The method as claimed in claim 9, further comprising: controlling a phase shift between the excitation modulation signal and received modulation signal to 45 by adjusting the excitation modulation frequency.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention as well as its advantageous embodiments will now be described in greater detail based on the appended drawing, the FIGS. 1-4 of which show as follows:

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

(3) FIG. 2(a) shows the excitation carrier signal as a function of time,

(4) FIG. 2(b) shows the excitation carrier signal and the received carrier signal as a function of time,

(5) FIG. 3 shows a block diagram of an electronics unit of the invention, and

(6) FIG. 4 shows a block diagram of a second embodiment of an electronics unit of the invention.

DETAILED DESCRIPTION

(7) FIG. 1 shows a vibronic fill-level measuring device 1. A sensor unit 2 with a mechanically oscillatable unit 4 in the form an oscillatory fork protrudes partially into a medium 3, which is located in a container. The oscillatable unit 4 is excited by means of the excitation-/receiving unit 5 to execute mechanical oscillations, and can be, for example, a piezoelectric stack- or bimorph drive. Also other embodiments of a vibronic fill-level measuring device can be used. Furthermore, an electronics unit 6 is shown, which performs signal registration,evaluation and/orfeeding.

(8) The excitation signal 7 for driving the oscillatable unit 4 is, according to the invention, composed of an excitation carrier signal 8 and an excitation modulation signal 9, and is shown, by way of example, in FIG. 2a as a function of time. The excitation modulation signal 9 with the excitation modulation frequency f.sub.M is visible as the envelope of the excitation carrier signal 8 with the excitation carrier frequency f.sub.C. The received signal 10 received from the oscillatable unit 4 is shown in FIG. 2b together with the excitation signal 7, likewise as a function of time. The received signal 10 is likewise composed of two parts, the received carrier signal 11 and the received modulation signal 12. While the frequencies of the excitation carrier signal 8 and excitation modulation signal 9 do not change, the received signal 10 has an amplitude different from the amplitude of the excitation signal 7. Moreover, there occurs between the excitation signal 7 and the received signal 10 a phase shift, both with regard to the carrier signals (8,11) as well as also with regard to the modulation signals (9,12).

(9) In the following, it will be explained why the damping of the oscillatable unit 4 can be ascertained by means of the amplitude modulation of the excitation signal 7. For this, a mathematical description of the oscillatory movement of the oscillatable unit 4 is needed. Depending on which geometry and which boundary conditions are selected, different equations can be set up. However, the calculation becomes quickly extensive, because of which in the following a very simplified representation is selected, namely that of the oscillatable unit 4 as an ideal damped harmonic oscillator with the transfer function G(s):

(10) $G\left(s\right)=\frac{V}{{\left(\frac{1}{0}.Math.s\right)}^2+2.Math.D.Math.\frac{1}{0}.Math.s+1}=\frac{V.Math.0^2}{\left(s-s1\right).Math.\left(s-s2\right)}$

(11) In such case, the following definitions hold: V is the amplification at the angular frequency =0, D the damping, 0 the resonant frequency in the undamped case and s1 and s2 are, in the case of oscillation, which means in the case, in which D<1, given by:
s1=0.Math.(i{square root over (1D.sup.2)}D), s2=0.Math.(i{square root over (1D.sup.2)}D).

(12) The excitation signal 7 L(t), such as shown in FIG. 2a, can be expressed mathematically as
L(t)=AmpC.Math.cos(C.Math.t).Math.[(mo).Math.cos(M.Math.t)+1]
wherein AmpC and C are the amplitude and the frequency of the carrier signal, and mo and M are the amplitude and the angular frequency of the modulation signal.

(13) In order to calculate the system response of the oscillatable unit 4 to the excitation signal 7, the excitation signal 7 is decomposed by means of partial-fraction decomposition into three summands. For each summands, then a Laplace transformation can be performed and the system response individually calculated. For a resonant exciting, thus for the case in which C=0, which implicitly means a phase shift between the excitation carrier signal and received carrier signal of 90, there results then, except for possible transient oscillation phenomena, the total system response. Based on the inverse transformed system response, the phase shift between the excitation modulation signal and the received modulation signal can, in turn, be calculated from the complex amplitude

(14) $=\mathrm{atan}\left(\frac{\mathrm{Imag}\left(G\right)}{\mathrm{Real}\left(G\right)}\right)$

(15) The modulation oscillation of the system response GL_mod is

(16) $\mathrm{GL\_mod}\left(t\right)=C1.Math.\left[\frac{\left[\begin{array}{c}{0}^2-{\left(0-M\right)}^2+2.Math.\\ D.Math.0.Math.\left(0-M\right)\end{array}\right].Math.\left(1-i\right)}{\begin{array}{c}\frac{{\left[0^2-{\left(0-M\right)}^2\right]}^2}{0^2}+\\ 4.Math.D^2.Math.{\left(0-M\right)}^2\end{array}}+\frac{\begin{array}{c}\left[\begin{array}{c}{0}^2-{\left(0+M\right)}^2+2.Math.\\ D.Math.0.Math.\left(0+M\right)\end{array}\right].Math.\\ \left(1+i\right)\end{array}}{\begin{array}{c}4.Math.D^2.Math.{\left(0+M\right)}^2+\\ \frac{{\left[{\left(0+M\right)}^2-0^2\right]}^2}{0^2}\end{array}}\right].Math.\cos \left(t.Math.M\right)$
with

(17) $C1=\frac{1}{2}.Math.V.Math.\mathrm{AmpC}.Math.\mathrm{mo},$
and the phase shift is

(18) $\left(D\right):=\mathrm{atan}\left[\frac{2.Math.\left[\begin{array}{c}\left(D-1\right).Math.M^4+4.Math.0^2.Math.\left(1-D^3\right).Math.\\ M^2+4.Math.D^2.Math.0^4\left(D+1\right)\end{array}\right]}{0.Math.M.Math.\left[\begin{array}{c}\frac{-8.Math.D^3.Math.0^4}{M^2}+\left(D^2-\frac{3}{2}.Math.D-1\right).Math.8D{0}^2+\\ \left(D^2+\frac{3}{2}.Math.D-1\right).Math.M^2+\frac{M^4}{0^2}\end{array}\right]}\right]$

(19) This equation can be significantly simplified, when one assumes that the frequency of the modulation oscillation is significantly less than the resonant frequency of the oscillatory system in the undamped case, thus under the condition M<<0:

(20) $=\mathrm{atan}\left(-\frac{M}{0.Math.D}\right)$

(21) By transforming, one obtains for the damping of the oscillatable unit at resonant excitation

(22) $D=\frac{-M}{\tan \left(\right).Math.0}$

(23) Another simplification can be achieved, when the phase shift between the excitation modulation signal 9 and the received modulation signal 12 is set to 45 by adjusting the excitation modulation frequency; in this case, the damping becomes

(24) $D=\frac{M}{0}$

(25) For any frequencies of the carrier signal where C0, the consideration is more complicated.

(26) FIGS. 3 and 4 show, finally, two block diagrams with two possible embodiments for electronics units 6 and 6 of the invention for a vibronic fill-level measuring device.

(27) In the case of the embodiment shown in FIG. 3, the oscillatable unit 4 is excited by means of an electromechanical transducer unit 5 (not shown), which is part of a feedback, oscillatory circuit, for which the phase shift between the excitation carrier signal 8 and received carrier signal 10 is set by means of a filter (90-filter) and an amplifier 13 and an automatic amplifier control (AGC) 14. With the modulation oscillation, which is, simultaneously, the manipulated variable of the control loop, the damping of the oscillatory system can be ascertained.

(28) The frequency f.sub.C of the excitation carrier signal 8, respectively the carrier oscillation, (signal a) establishes itself within the oscillatory circuit in accordance with the oscillatory circuit condition. Modulated onto this signal is the excitation modulation signal 9 (signal b) with the excitation modulation frequency f.sub.M. From this there results the total excitation signal c, which is supplied to the oscillatable unit 4 after first passing through a digital-analog converter 15. The received signal 10 (signal d) then received from the oscillatable unit 4 is identical with the excitation signal 7 (signal c) as regards carrier frequency f.sub.C and modulation frequency f.sub.M. However, the excitation signal 7 (signal c) and the received signal 10 (signal d) differ as regards amplitudes. Moreover, phase shifts are present between the two signals, both with reference to the carrier signals 8,11 and with reference to the modulation signals 9,12.

(29) After passing through an analog-digital converter 16, the received signal 10 (signal d) reaches the unit 13 composed of 90-filter and amplifier. The phase shift of the resonance oscillation due to the carrier signal 8,11 is determined by the 90-filter 13 and produces for resonance of the oscillatable unit 4 a phase shift of +90. The much smaller modulation oscillation 9,12 is, however, not phase-shifted by the filter 13. Within the AGC 14, the received signal 10 (signal e) is then freed of the modulation oscillation 9,12 and becomes again the excitation carrier signal (signal a). The manipulated variable of the AGC 14 (signal f) comprises now exclusively the received modulation signal 12 (signal f). The phase shift between the excitation modulation signal 9 (signal b) and the received modulation signal 12 (signal f) is ascertained by a phase meter 17 and fed to a phase evaluation unit 18, which from the phase shift between the excitation modulation signal 9 (signal b) and the received modulation signal 12 (signal f) taking into consideration the excitation modulation frequency f.sub.M ascertains the damping, and from the damping, for example, the viscosity.

(30) A second variant for an electronics unit of the invention 6 is the subject matter of FIG. 4. The oscillatory circuit for assuring resonant oscillations of the oscillatable unit 4 is unchanged, so that this aspect is not described again. The difference compared with FIG. 3 is that the excitation modulation frequency f.sub.M is now adjustable. Thus, only the circuit frequencies 0 and M still enter into the evaluation. Moreover, such as already explained, a phase shift between the excitation modulation signal 9 (signal b) and the received modulation signal 12 (signal f) of 45 leads to a simplified formula for the viscosity. Therefore, besides the phase meter 17 and the phase evaluation unit 18 for determining the phase shift between the excitation modulation signal 9 (signal b) and the received modulation signal 12 (signal f), also a phase control circuit 19 is provided, in order to obtain an adjustable, desired phase (45). Via the phase control loop 19, the excitation modulation frequency f.sub.M is set in such a manner that the phase shift between the excitation modulation signal 9 and the received modulation signal 11 amounts to 45. From the measured resonant frequency 0, which corresponds to the excitation carrier frequency f.sub.C, and the set excitation modulation frequency M, the damping is then obtained.

LIST OF REFERENCE CHARACTERS

(31) 1 vibronic sensor 2 medium 3 container 4 oscillatable unit 5 electromechanical transducer unit 6 electronics unit 7 excitation signal 8 excitation carrier signal 9 excitation modulation signal 10 received signal 11 received carrier signal 12 received modulation signal 13 90-filter and amplifier 14 AGC 15 D/A converter 16 A/D converter 17 phase meter 18 phase evaluation unit 19 phase control unit U.sub.E excitation signal U.sub.R received signal f.sub.C excitation carrier frequency f.sub.M excitation modulation frequency D damping 0 resonant frequency of the undamped oscillator M frequency of the modulation C carrier frequency V amplification of the resonator at w=0 AmpC excitation amplitude of the carrier frequency Mo modulation amplitude