Phase control unit for a vibronic sensor

Abstract

An apparatus and a method for determining and/or monitoring at least one process variable of a medium in a container, comprising: a mechanically oscillatable unit, a driving/receiving unit for exciting the mechanically oscillatable unit to execute mechanical oscillations by means of an electrical exciting signal and for receiving and transducing mechanical oscillations into an electrical, received signal, an electronics unit, which electronics unit is embodied, to produce the exciting signal starting from the received signal, to set a predeterminable phase shift () between the exciting signal and the received signal, and from the received signal, to determine and/or to monitor the at least one process variable. A phase correction unit is provided, which phase correction unit is at least embodied, to ascertain a phase correction value (.sub.kor) from at least one process parameter dependent, characteristic variable of at least one component of the apparatus, especially the driving/receiving unit, and to set the predeterminable phase shift () in accordance with the phase correction value (.sub.kor).

Claims

1. An apparatus for determining and/or monitoring at least one process variable of a medium in a container, comprising: a mechanically oscillatable unit; a driving/receiving unit for exciting said mechanically oscillatable unit to execute mechanical oscillations by means of an electrical exciting signal and for receiving and transducing mechanical oscillations into an electrical, received signal; and an electronics unit, which is embodied, to produce the exciting signal based on the received signal, to set a predeterminable phase shift between the exciting signal and the received signal, and, from the received signal, to determine and/or to monitor the at least one process variable wherein said apparatus comprises a phase correction unit, which is at least embodied to ascertain a phase correction value (.sub.kor) from at least one process parameter dependent, characteristic variable of at least one component of the apparatus, wherein the characteristic variable is at least one process parameter dependent capacitance (C.sub.AE) or inductance of at least one component of the driving/receiving unit or at least one time constant (.sub.pros, .sub.ref) dependent on at least one process parameter, and to set the predeterminable phase shift () in accordance with the phase correction value (.sub.kor).

2. The apparatus as claimed in claim 1, wherein: said driving/receiving unit includes at least one piezoelectric element or at least one coil.

3. The apparatus as claimed in claim 1, wherein: the at least one process parameter is the process temperature or the process pressure.

4. The apparatus as claimed in claim 1, wherein: said phase correction unit includes at least one reference branch, which includes at least one electrical component, which is connected in parallel with at least one component of the apparatus, and is contactable with the exciting signal.

5. The apparatus as claimed in claim 4, wherein: said reference branch includes at least one reference capacitance or reference inductance.

6. The apparatus as claimed in claim 5, wherein: said reference capacitance and at least one capacitance of said driving/receiving unit or the reference inductance and at least one inductance of said driving/receiving unit have the same value.

7. The apparatus as claimed in claim 4, wherein: said reference branch includes at least one reference resistor.

8. The apparatus as claimed in claim 7, wherein: said at least one reference resistor is connected in series with said at least one electrical component.

9. The apparatus as claimed in claim 4, wherein: said at least one electrical component is connected in parallel with said driving/receiving unit.

10. The apparatus as claimed in claim 1, wherein: said phase correction unit includes at least one time measuring unit.

11. The apparatus as claimed in claim 10, wherein: said time measuring unit includes at least one timer-chip or at least one XOR unit and at least one counter.

12. The apparatus as claimed in claim 10, wherein: said time measuring unit includes a switch element; said electronics unit is embodied, by means of said switch element, to forward to said time measuring unit a reference signal of said reference branch in a first time interval and a process signal based on the at least one component of the apparatus in a second time interval.

13. The apparatus as claimed in claim 1, further comprising: an explosion protection circuit.

14. The apparatus as claimed in claim 13, wherein: said explosion protection circuit includes at least one electrical resistor.

15. The apparatus as claimed in claim 1, wherein: said at least one process variable is a predeterminable fill level, the density, and/or the viscosity of the medium in the container.

16. The apparatus as claimed in claim 1, wherein the phase correction value (.sub.kor) is ascertained from at least one process parameter dependent, characteristic variable of at said driving/receiving unit.

17. A method for determining and/or monitoring at least one process variable of a medium in a container, comprising the steps of: exciting a mechanical oscillatable unit by means of an exciting signal to execute mechanical oscillations, wherein the mechanical oscillations of the mechanically oscillatable unit are received and transduced into an electrical, received signal; producing the exciting signal starting from the received signal; setting a predeterminable phase shift between the exciting signal and the received signal; determining the at least one process variable and/or monitored from the received signal; ascertaining a phase correction value (.sub.kor) from at least one characteristic variable dependent on at least one process parameter; wherein the characteristic variable is at least one process parameter dependent capacitance (C.sub.AE) or inductance of at least one component of the driving/receiving unit or at least one time constant (.sub.pros, .sub.ref) dependent on at least one process parameter, and setting the predeterminable phase shift in accordance with the phase correction value (.sub.kor).

18. The method as claimed in claim 17, wherein: the process parameter is the process temperature or the process pressure.

19. The method as claimed in claim 17, wherein: ascertained as characteristic variable is a first time constant dependent on at least one process parameter; based on a reference signal a second time constant independent of the at least one process parameter is ascertained; and the phase correction value (.sub.kor) is ascertained from a comparison of the first time constant and the second time constant.

20. The method as claimed in claim 17, wherein: the process temperature is determined from at least one process temperature dependent and/or process pressure dependent capacitance or inductance of at least one component of the driving/receiving unit.

21. The method as claimed in claim 17, wherein: from at least one process temperature dependent and/or process pressure dependent capacitance or inductance of at least one component of the driving/receiving unit, the presence of a short circuit or shunt connection is deduced.

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 figures of which show as follows:

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

(3) FIG. 2 is a block diagram of a first embodiment of the invention;

(4) FIG. 3 is a block diagram of a second embodiment of the invention, including an explosion protection circuit; and

(5) FIG. 4 is the process signal and the reference signal of an apparatus of FIG. 2 or 3 as a function of time.

DETAILED DISCUSSION IN CONJUNCTION WITH THE DRAWINGS

(6) FIG. 1 shows an apparatus 1 in the form of a vibronic sensor 1 for determining and/or monitoring at least one process variable. Included is an oscillatable unit 4 in the form of an oscillatory fork, which is immersed partially in a medium 2, which is located in a container 3. Oscillatable unit 4 is excited by means of the driving/receiving unit 5 to execute mechanical oscillations, and can be, for example, a piezoelectric stack- or bimorph drive. However, 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/orfeeding occurs.

(7) The following description relates to a phase correction unit 7 having a reference branch 8, which is connected in parallel with a driving/receiving unit 5 having at least one piezoelectric element characterized by a capacitance C.sub.AE and which has a reference capacitance C.sub.ref. The ideas presented in connection with the following figures can be transferred directly to other components of the vibronic sensor 1 or to other embodiments of the driving/receiving unit 5.

(8) FIG. 2 shows a block diagram of a first embodiment of a vibronic sensor of the invention having a corresponding phase correction unit 7. Oscillatable unit 4 is not separately shown in this view. It is located, however, in the direct vicinity of the driving/receiving unit 5. By means of an electrical exciting signal U.sub.E and by means of the driving/receiving unit 5, the mechanically oscillatable unit is caused to execute mechanical oscillations. These mechanical oscillations are, in turn, received by means of the driving/receiving unit 5 and transduced into an electrical, received signal U.sub.R, which is fed to an electronics unit 6, which includes at least one input stage 9, a control- and evaluation unit 10 at least for adjusting the predeterminable phase shift between exciting signal U.sub.E and received signal U.sub.R and for determining and/or monitoring at least one process variable, as well as an output stage 11.

(9) The phase correction unit 7 includes a reference branch 8, a switch element 12 and a time measuring unit 13 in the form of an XOR-unit 13a and a counter 13b. In the example shown here, the phase correction unit 7 includes, furthermore, an evaluation unit 14 for ascertaining the phase correction value .sub.kor. This value is fed to the control- and evaluation unit 10, in order that the predeterminable phase shift between exciting signal U.sub.E and received signal U.sub.R can be set corresponding to the influence of at least one process parameter.

(10) The exciting signal U.sub.E is, on the one hand, fed to the driving/receiving unit 5 and, on the other hand, to the reference branch 8. For ascertaining the phase correction value .sub.kor, a process signal U.sub.pros from the driving/receiving unit and a reference signal U.sub.ref from the reference branch 8 are fed to a switch element, in such a manner that in a first time interval the process signal is fed to the time measuring unit 13, and in a second time interval the reference signal. Of course, in the event that two time measuring units 13 are provided, a switch element would be no longer necessary.

(11) FIG. 3 shows a similar block diagram of an apparatus 1 of the invention, in the case of which, supplementally, an explosion protection circuit 15 is provided, which includes at least the two resistors 15a, 15b, which are connected, respectively, after and in front of the driving/receiving unit. Otherwise the circuit is identical with that of FIG. 2, so that further explanation would be repetitive.

(12) FIG. 4 serves, finally, for illustrating the determining of the phase correction value .sub.kor based on a time constant of the capacitance C of the driving/receiving unit 5. Shown are the ideal exciting signal U.sub.ideal in the form of a rectangular signal, the process signal U.sub.pros and the reference signal U.sub.ref, all as functions of time. The process signal U.sub.pros and the reference signal U.sub.ref are fed in different time intervals to the time measuring unit 13.

(13) For determining the phase correction value .sub.kor, the durations of the charging- and discharging of the capacitances C.sub.AE and C.sub.ref are considered for n signal periods of the exciting signal U.sub.ideal both for the process signal U.sub.pros(t) as well as also for the reference signal U.sub.ref(t), as ascertained by means of the time measuring unit.

(14) For the process signal U.sub.pros, there results for the duration t.sub.pros of the charging- and discharging of the capacitance C over n signal periods of the exciting signal U.sub.ideal,
t.sub.pros=nt.sub.pros1+nt.sub.pros2,

(15) wherein t.sub.pros1 is the duration of charging and t.sub.pros2 the duration of discharging. Analogously, there results in the case of the reference signal for the corresponding duration t.sub.ref
t.sub.ref=nt.sub.ref1+nt.sub.ref2.

(16) Using the formulas generally known from the state of the art for charging and discharging of a capacitor with capacitance C, there results for the duration of the charging- and discharging in the case of the process signal U.sub.pros

(17) $t_{\mathrm{pros}}=n\left[-{}_{\mathrm{pros}}*\ln \left(1-\frac{U_{\mathrm{pros}}\left(t_{\mathrm{pros}1}\right)}{U_0}\right)\right]+n\left[-{}_{\mathrm{pros}}*\ln \left(\frac{U_{\mathrm{pros}}\left(t_{\mathrm{pros}2}\right)}{U_0}\right)\right]$$t_{\mathrm{pros}}=-n{}_{\mathrm{pros}}*\left[\ln \left(1-\frac{U_{\mathrm{pros}}\left(t_{\mathrm{pros}1}\right)}{U_0}\right)+\ln \left(\frac{U_{\mathrm{pros}}\left(t_{\mathrm{pros}2}\right)}{U_0}\right)\right],$

(18) wherein U.sub.0 is the amplitude of the ideal exciting signal U.sub.ideal.

(19) Analogously, there results in the case of the reference signal U.sub.ref:

(20) $t_{\mathrm{ref}}=n\left[-{}_{\mathrm{ref}}*\ln \left(1-\frac{U_{\mathrm{ref}}\left(t_{\mathrm{ref}1}\right)}{U_0}\right)\right]+n\left[-{}_{\mathrm{ref}}*\ln \left(\frac{U_{\mathrm{ref}}\left(t_{\mathrm{ref}2}\right)}{U_0}\right)\right]$$t_{\mathrm{ref}}=-n{}_{\mathrm{ref}}*\left[\ln \left(1-\frac{U_{\mathrm{ref}}\left(t_{\mathrm{ref}1}\right)}{U_0}\right)+\ln \left(\frac{U_{\mathrm{ref}}\left(t_{\mathrm{ref}1}\right)}{U_0}\right)\right]$

(21) Using the boundary condition:
U.sub.pros(t.sub.pros1)=U.sub.ref(t.sub.ref1)=U.sub.pros(t.sub.pros2)=U.sub.ref(t.sub.ref2)=!U(t.sub.x),

(22) it follows that:

(23) $t_{\mathrm{pros}}=-n{}_{\mathrm{proz}}*\left[\ln \left(1-\frac{U\left(t_x\right)}{U_0}\right)+\ln \left(\frac{U\left(t_x\right)}{U_0}\right)\right],\mathrm{and}$$t_{\mathrm{ref}}=-n{}_{\mathrm{ref}}*\left[\ln \left(1-\frac{U\left(t_x\right)}{U_0}\right)+\ln \left(\frac{U\left(t_x\right)}{U_0}\right)\right].$

(24) The phase correction value .sub.kor can then be ascertained, for example, from the difference of the durations t.sub.ref and t.sub.pros. There results

(25) $t_{\mathrm{ref}}-t_{\mathrm{pros}}=-n{}_{\mathrm{ref}}*\left[\ln \left(1-\frac{U\left(t_x\right)}{U_0}\right)+\ln \left(\frac{U\left(t_x\right)}{U_0}\right)\right]+n{}_{\mathrm{pros}}*\left[\ln \left(1-\frac{U\left(t_x\right)}{U_0}\right)+\ln \left(\frac{U\left(t_x\right)}{U_0}\right)\right]$$t_{\mathrm{ref}}-t_{\mathrm{pros}}=n\left({}_{\mathrm{pros}}-{}_{\mathrm{ref}}\right)\left[\ln \left(1-\frac{U\left(t_x\right)}{U_0}\right)+\ln \left(\frac{U\left(t_x\right)}{U_0}\right)\right]$

(26) The time difference t.sub.reft.sub.pros can then, for example, be referenced to t.sub.ref. From this, it follows that:

(27) $\frac{t_{\mathrm{ref}}-t_{\mathrm{pros}}}{t_{\mathrm{ref}}}=-\frac{\left({}_{\mathrm{pros}}-{}_{\mathrm{ref}}\right)}{{}_{\mathrm{ref}}}=\frac{\left({}_{\mathrm{ref}}-{}_{\mathrm{pros}}\right)}{{}_{\mathrm{ref}}}=\frac{\left(C_{\mathrm{ref}}-C_{\mathrm{AE}}\right)}{C_{\mathrm{ref}}},$

(28) from which, in turn, the phase correction value .sub.kor can be ascertained.

(29) Alternatively, the durations of the discharging- and charging in the reference branch t.sub.ref and in the process branch t.sub.pros can also be directly referenced to one another:

(30) $\frac{t_{\mathrm{pros}}}{t_{\mathrm{ref}}}=-\frac{{}_{\mathrm{pros}}}{{}_{\mathrm{ref}}}=\frac{C_{\mathrm{AE}}}{C_{\mathrm{ref}}}.$

(31) Finally, it is to be noted that the durations t.sub.ref and t.sub.pros can, in each case, also be determined by considering either only the charging or only the discharging of the capacitances C.sub.AE and C.sub.ref for the process signal U.sub.pros and for the reference signal U.sub.ref.