Vibronic sensor

Abstract

The present invention relates to a vibronic sensor for determining a process variable of a medium in a containment, comprising a mechanically oscillatable unit, a driving/receiving unit and an electronics unit having an adaptive filter. The present invention relates also to a method for operating the sensor. The electronics unit is embodied alternately to execute a first operating mode and a second operating mode. The driving/receiving unit is embodied during the first operating mode to excite the oscillatable unit using an electrical excitation signal. During the second operating mode, the exciting of the oscillatable unit is interrupted and the oscillations of the oscillatable unit are received and transduced into an electrical, received signal. At least one filter characteristic of the adaptive filter is set such that a predeterminable phase shift is present between the excitation signal and the received signal, and the process variable is determined from the received signal.

Claims

1. A vibronic sensor for determining a process variable of a medium, comprising: a mechanically oscillatable unit; a driving/receiving unit embodied to excite the mechanically oscillatable unit with an electrical excitation signal to cause the mechanically oscillatable unit to mechanically oscillate and further embodied to receive mechanical oscillations of the mechanically oscillatable unit and transduce the mechanical oscillations into an electrical, received signal; and an electronics unit including: an adaptive filter having an input signal and an output signal, the adaptive filter further having a filter characteristic suitable to set a phase shift between the input signal and the output signal; and a phase control unit configured to control the filter characteristic of the adaptive filter such that a predetermined phase shift is present between the input signal and the output signal of the adaptive filter, wherein the vibronic sensor is embodied to operate in a first operating mode during which: the driving/receiving unit excites the mechanically oscillatable unit with an electrical excitation signal to cause the mechanically oscillatable unit to mechanically oscillate; the phase control unit is paused; and the filter characteristic of the adaptive filter is held constant, wherein the vibronic sensor is further embodied to operate in a second operating mode during which: the driving/receiving unit interrupts the exciting of the mechanically oscillatable unit; the driving/receiving unit to receives mechanical oscillations of the mechanically oscillatable unit and transduces the mechanical oscillations into the electrical, received signal; to the phase control units sets a value of the filter characteristic of the adaptive filter such that the predeterminable phase shift is present between the excitation signal and the received signal; and the electronics unit determines from the received signal the process variable, and wherein the electronics unit is further embodied to execute alternately and successively the first operating mode and the second operating mode.

2. The vibronic sensor as claimed in claim 1, wherein the excitation signal is an electrical, rectangular signal.

3. The vibronic sensor as claimed in claim 1, wherein the electronics unit includes at least one switch configured to switch back and forth between the first operating mode and the second operating mode.

4. The vibronic sensor as claimed in claim 1, wherein the electronics unit is further embodied to set the predeterminable phase shift by setting a center frequency of the adaptive filter during the second operating mode.

5. The vibronic sensor as claimed in claim 1, wherein the electronics unit further includes a phase control unit is based on a principle of a lock-in amplifier, and wherein the phase control unit is configured to control a center frequency of the adaptive filter such that the predeterminable phase shift is present between the input signal and the output signal of the adaptive filter.

6. The vibronic sensor as claimed in claim 5, wherein the electronics unit further includes an amplitude control unit configured to control the amplitude of the excitation signal to a predeterminable value or to a value within a predeterminable interval.

7. The vibronic sensor as claimed in claim 6, wherein the electronics unit is embodied to store and/or to furnish a value and/or a parameter of a component of the electronics unit, including a value and/or a parameter of the filter characteristic, the phase control unit or the amplitude control unit, during the first operating mode.

8. The vibronic sensor as claimed in claim 7, wherein the electronics unit is embodied to set at the beginning of the second operating mode the stored and/or furnished parameter and/or value of the component.

9. The vibronic sensor as claimed in claim 1, wherein the electronics unit includes a ring buffer and/or a phase shifter by which the predeterminable phase shift is settable.

10. The vibronic sensor as claimed in claim 1, wherein the bandwidth and/or the quality of the adaptive filter can be set.

11. The vibronic sensor as claimed in claim 1, wherein the adaptive filter is a resonator filter.

12. The vibronic sensor as claimed in claim 1, wherein the adapter filter is a low-pass bandpass filter of second order.

13. The vibronic sensor as claimed in claim 1, wherein the predeterminable phase shift is, at times, +/−90°, +/−45° or 0°.

14. The vibronic sensor as claimed in claim 1, wherein the electronics unit is embodied to execute a frequency scan, in the case of subceeding a threshold value for an amplitude, to excite the oscillatable unit using the frequency scanning, and to set a center frequency of the adaptive filter successive within a predeterminable frequency interval to discrete excitation frequencies following one another.

15. The vibronic sensor as claimed in claim 1, wherein the process variable is a fill level, a density, and/or a viscosity of the medium.

16. The vibronic sensor as claimed in claim 1, wherein the mechanically oscillatable unit is a membrane, a single rod, or an oscillatory fork.

17. The vibronic sensor as claimed in claim 1, wherein the driving/receiving unit is an electromagnetic or a piezoelectric driving/receiving unit.

18. A method for operating a vibronic sensor for determining a process variable of a medium, comprising: providing the vibronic sensor, including: a mechanically oscillatable unit; a driving/receiving unit embodied to excite the mechanically oscillatable unit with an electrical excitation signal to cause the mechanically oscillatable unit to mechanically oscillate and further embodied to receive mechanical oscillations of the mechanically oscillatable unit and transduce the mechanical oscillations into an electrical, received signal; and an electronics unit including: an adaptive filter having an input signal and an output signal, the adaptive filter further having a filter characteristic suitable to set a phase shift between the input signal and the output signal; and a phase control unit configured to control the filter characteristic of the adaptive filter such that a predetermined phase shift is present between the input signal and the output signal of the adaptive filter; during a first operating mode of the vibronic sensor: exciting the mechanically oscillatable unit with the electrical excitation signal to cause the mechanically oscillatable unit to mechanically oscillate; pausing the phase control unit; and holding constant the filter characteristic of the adaptive filter; and during a second operating mode of the vibronic sensor: interrupting the electrical excitation signal and thereby interrupting the exciting of the mechanically oscillatable unit; receiving the mechanical oscillations of the mechanically oscillatable unit and transducing the mechanical oscillations into the electrical, received signal; controlling the filter characteristic of the adaptive filter such that the predeterminable phase shift is present between the excitation signal and the received signal; and determining the process variable from the received signal; and executing alternately and successively the first operating mode and the second operating mode.

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 follows:

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

(3) FIG. 2 shows a block diagram of an electronics unit of the invention, and

(4) FIG. 3 shows another embodiment of an electronics unit of the invention for illustration of the (a) first and (b) second operating modes.

DETAILED DESCRIPTION

(5) FIG. 1 shows a vibronic sensor 1. Included is 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 exciting/receiving unit 5 to cause the mechanically oscillatable unit to execute mechanical oscillations. The exciting/receiving unit 5 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. Furthermore, an electronics unit 6 is shown, by means of which signal registration,—evaluation and/or—supply occurs.

(6) A block diagram of an electronics unit of the invention is subject matter of FIG. 2. The received signal U.sub.R passes first through an analog-digital converter 10, before it is fed to the adaptive filter 7. The filter characteristic of the adaptive filter is then set such that a suitable phase shift ϕ.sub.filter is present between the input signal and the output signal of the adaptive filter. In this way, a predeterminable phase shift ϕ.sub.desired=360°−ϕ.sub.filter results between the excitation signal and the received signal. The predeterminable phase shift is thus set by a suitable setting of the filter characteristic. For example, a phase control unit 8 can be used for this, by means of which the center frequency f.sub.m of the adaptive filter is controlled in such a manner that between excitation signal and received signal the predeterminable phase shift ϕ.sub.desired is present. The phase control unit 8 can, in turn, be based, for example, on the principle of a lock-in amplifier.

(7) The quality Q of the adaptive filter 7 can be set by, among others, variation of Lehr's damping ratio D. In such case, the following relationship is utilized: Q=1/2D, wherein Lehr's damping ratio is, in turn, determined from the mechanical properties of the oscillatable unit.

(8) The quality Q of the adaptive filter 7 obeys, moreover, the relationship B=fm/Q, wherein B is the bandwidth and f.sub.m the center frequency of the adaptive filter. An embodiment of the invention provides that the quality Q of the adaptive filter 7 and/or its bandwidth B are/is settable.

(9) The received signal U.sub.R is characterized by its frequency, its amplitude and its phase. If the phase control of the phase control unit 8 occurs by setting the center frequency f.sub.m of the adaptive filter 7 to the input frequency of the adaptive filter, then, at all times, the frequency, with which the oscillatable unit 4 oscillates, is known.

(10) Furthermore, optionally, an amplitude control unit 9 (shown here with dashed lines) can be integrated into the electronics unit 6. By means of the amplitude control unit 9, the amplitude A of the excitation signal U.sub.E is controlled to a predeterminable value or to a value within a predeterminable interval. For example, a conventional PI-controller can be applied for this.

(11) With the application of an adaptive filter 7 for exciting the mechanically oscillatable unit 7, advantageously, no additional filters are required for signal filtering before the evaluation.

(12) Before being transmitted via the output stage 11 of the electronics unit to the sensor unit 4,5, the excitation signal U.sub.E passes through a digital-analog converter 10a. Optionally, the received signal U.sub.R received from the sensor unit 4,5 can, moreover, be led through an anti-aliasing filter 13, shown in dashed lines, before being forwarded to the analog-digital converter 10 after passing through the input stage 12.

(13) For executing the two operating modes 15,16, the electronics unit includes, furthermore, a switch element 14. The processes during the two operating modes 15,16 will now be explained in the following based on FIG. 3, which shows another embodiment of an electronics unit 6 of the invention.

(14) For this example, the driving/receiving unit 5 is an electromagnetic driving/receiving unit. This is, however, not absolutely necessary.

(15) FIG. 3a illustrates the first operating mode 15, also referred to as the excitation sequence. The oscillatable unit 4 is supplied with an excitation signal U.sub.E, in the form of a rectangular signal, and excited to execute mechanical oscillations. The oscillatable unit 4 thus stores, in this way, oscillatory energy. The received signal U.sub.R coming from the oscillatable unit 4 is superimposed on the excitation signal U.sub.E. The received signal U.sub.R passes through the current-voltage converter 17, the adaptive filter 7 and the voltage-current converter 18. During the excitation sequence 15, no active measuring of the currently present phase shift ϕ between the excitation signal U.sub.E and received signal U.sub.R takes place. Also, no active control of at least one value of the filter characteristic of the adaptive filter 7 is performed. The components of the electronics unit 6, which serve for control and/or phase measurement, such as, for example, the adaptive filter 7, or also the control unit 8a and detection unit 8b, also referred to as the measuring unit, and the phase control unit 8, are located in a so-called hold mode, i.e. their operation is paused, or deactivated. The filter characteristic of the adaptive filter 7 remains constant. The last internal parameter and/or values of these components 8a,8b,7 set during the second operating mode 16 are stored and serve as initial values for the second operating mode 16 executed following thereon. These can be stored either within the components 8a,8b,7 or in a memory unit (not shown) associated with the electronics unit 6.

(16) During the second operating mode 16, also referred to as the measuring/control sequence, the switch element 14 interrupts the supplying of sensor unit 4,5 by means of the excitation signal U.sub.E. This is shown in FIG. 3b. The oscillatable unit 4 oscillates now with its eigenresonance frequency f.sub.0 and executes correspondingly a damped resonance oscillation. The excitation signal U.sub.E is no longer superimposed on the received signal U.sub.R, so that a signal evaluation can occur in the electronics unit 6. The control of the current phase shift ϕ between excitation signal U.sub.E and received signal U.sub.R is continued. In the case of an electromagnetic driving/receiving unit 5, for example, a predeterminable phase shift Δϕ.sub.desired of 0° is pursued. For example, the center frequency f.sub.m of the adaptive filter 7 can be set suitably for controlling the current phase shift ϕ. During the sequentially following measuring/control sequences 16, the internal parameters and/or values of the components 8a,8b,7 of the electronics unit 6 installed for the control and phase measurement are thus successively changed, until a resonant excitation of the sensor unit 4,5 occurs. In the settled state of the vibronic sensor 1, then phase equality is present between the excitation signal U.sub.E and received signal U.sub.R, and the center frequency f.sub.m of the filter 7 is tuned to the value of the resonance frequency f.sub.0 of the oscillatable unit 4. In each measuring-control sequence 6, thus, for example, the separation between the center frequency f.sub.m and resonance frequency f.sub.0 is reduced.

(17) The durations of the first 15 and second 16 operating modes are selectable and can be adapted to meet the needs of a certain sensor unit and the contemplated application of the sensor. It is to be heeded that the duration of the second operating mode be matched to the decay constant of the damped resonance oscillation of the oscillatable unit. Especially in the case of strongly damping media, it must, however, be simultaneously assured that the duration is sufficiently long, in order to assure a stable control and measuring of the phase shift between excitation signal U.sub.E and received signal U.sub.R. Fundamentally, however, especially in the case of an exciting of the oscillatable unit 4 by means of a rectangular signal, it is, furthermore, advantageous that, in each case, the point in time, at which, in each case, following on the second operating mode 16, the first operating mode 15 is started, there is a matching of the oscillations of the oscillatable unit 4, so that the switching back and forth between the two operating modes 15,16 is done phase correctly. This can occur, for example, by a detecting of the zero crossings (corresponding to the rest position) of the oscillations of the oscillatable unit 4 during the second operating mode 16. For this, the electronics unit 6 can, for example, have a unit (not shown) for detecting the amplitude of the oscillations of the oscillatable unit. Such an amplitude detection unit can, for example, be integrated as a component of an amplitude control unit 9 such as shown in FIG. 2, or as separate unit.