Diagnosis of a two-conductor field instrument

Abstract

Disclosed is a method for diagnosis of a two-conductor field instrument and a corresponding two-conductor field instrument. In a normal operating mode, an input voltage is provided and an output current is output. In a diagnostic operating mode, the method includes: providing a first diagnosis-input voltage and outputting a first diagnosis-output current during a first time interval, providing a second diagnosis-input voltage and outputting a second diagnosis-output current during a second time interval, determining the second time interval from the first time interval, registering a first and second diagnosis-output voltage as a function of the first and second diagnosis-output current, and checking the functionality of the two-conductor field instrument by the first and second diagnosis-input voltage, the first and second time interval, the first and second diagnosis-output electrical current, the first and second diagnosis-output voltage based on the input voltage and/or based on the output electrical current.

Claims

1. A method for diagnosis of a two-conductor field instrument for determining or monitoring at least one process variable of a medium in a containment, wherein in a normal operating mode an input voltage is provided and an output electrical current is output, and wherein in a diagnostic operating mode a functionality of the two-conductor field instrument is checked, the method comprising: providing a first diagnosis-input voltage and outputting a first diagnosis-output electrical current during a first time interval; providing a second diagnosis-input voltage and outputting a second diagnosis-output electrical current during a second time interval; determining the second time interval at least starting from the first time interval; registering a first or second diagnosis-output voltage as a function of the first or second diagnosis-output current; and checking the functionality of the two-conductor field instrument based on the first or second diagnosis-input voltage, the first or second time interval, the first or second diagnosis-output electrical current, the first or second diagnosis-output voltage based on the input voltage or based on the output electrical current.

2. The method as claimed in claim 1, further comprising: checking whether the two-conductor field instrument can output a predeterminable failure current, including a predeterminable maximum value or a predeterminable minimum value, for the output current; checking whether an output current corresponding to a predeterminable input voltage is burdened with an error; or checking whether a defect in electronics is present.

3. The method as claimed in claim 1, wherein the second time interval is determined starting from the first time interval such that a value of a first integral of a difference between the input voltage and the first diagnosis-input voltage over the first time interval and a value of a second integral of a difference between the input voltage and the second diagnosis-input voltage over the second time interval are equal in magnitude.

4. The method as claimed in claim 1, wherein the second time interval is determined as a function of the input voltage.

5. A two-conductor field instrument for determining or monitoring at least one process variable of a medium in a containment, comprising: an electronics having a diagnostic unit, wherein the electronics is embodied in a normal operating mode to provide an input voltage and to output an output electrical current, and is further embodied in a diagnostic operating mode to check a functionality of the two-conductor field instrument, wherein the diagnostic unit is embodied during a first time interval to provide a first diagnosis-input voltage and to output a first diagnosis-output current, during a second time interval to provide a second diagnosis-input voltage and to output a second diagnosis-output current, to determine the second time interval at least starting from the first time interval, as a function of the first or second diagnosis-output current to register a first or second diagnosis-output voltage, and based on the first or second diagnosis-input voltage, the first or second time interval, the first or second diagnosis-output electrical current, the first or second diagnosis-output voltage, based on the input voltage or based on the output electrical current, to check the functionality of the two-conductor field instrument.

6. The two-conductor field instrument as claimed in claim 5, wherein the electronics includes at least one switching element.

7. The two-conductor field instrument as claimed in claim 5, wherein the electronics includes at least one resistor which serves for producing the first or second diagnosis-output voltage.

8. The two-conductor field instrument as claimed in claim 5, wherein the diagnostic unit includes a computing unit which is embodied to ascertain the second time interval at least based on the first time interval.

9. The two-conductor field instrument as claimed in claim 5, wherein the diagnostic unit has a control unit, which has at least one subtractor unit, an integrator unit and a comparator.

10. The two-conductor field instrument as claimed in claim 9, wherein the integrator unit includes at least one capacitor and one resistor.

11. The two-conductor field instrument as claimed in claim 9, wherein the integrator unit includes at least one switching element, which is arranged or embodied such that the integrator unit can be placed in a predeterminable starting state before beginning the diagnostic operating mode by actuating the switching element.

12. The two-conductor field instrument as claimed in claim 9, wherein the subtractor unit is embodied to ascertain the difference between a reference signal and the input voltage.

13. The two-conductor field instrument as claimed in claim 12, wherein the reference signal involves the first or second diagnosis-input voltage or the first or second diagnosis-output voltage.

14. The two-conductor field instrument as claimed in claim 9, wherein the comparator is embodied to ascertain the second time interval based on an input voltage of the integrator unit.

15. The two-conductor field instrument as claimed in claim 5, wherein the electronics includes a monostable multivibrator.

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, FIG. 1-FIG. 6, of which show as follows:

(2) FIG. 1 shows a schematic view of a vibronic sensor according to the state of the art implemented in the form of a two-conductor field instrument,

(3) FIG. 2 shows a schematic view of a capacitive and/or conductive, fill level measuring device according to the state of the art implemented in the form of a two-conductor field instrument,

(4) FIG. 3 shows a first embodiment of a diagnostic unit of the present disclosure,

(5) FIG. 4 shows a second embodiment of a diagnostic unit of the present disclosure,

(6) FIG. 5 shows a third embodiment of a diagnostic unit of the present disclosure, and

(7) FIG. 6 shows schematic graphs of electrical current output as a function of time, modulated with the first and second diagnosis-input voltages, for different values of the output electrical current in the normal operating mode.

(8) In the figures, equal elements are provided with equal reference characters.

DETAILED DESCRIPTION

(9) The present invention concerns generally two-conductor field instruments. Such field instruments can be extremely varied, such as already indicated above in the introduction of the description. By way of example, in the following the operations of a vibronic sensor and a capacitive and/or conductive field instrument will be briefly explained, each embodied in the form of a two-conductor field instrument. The present invention, is, however, not limited to these two types of field instruments.

(10) FIG. 1 shows a field instrument 1 in the form of a vibronic sensor, by means of which, for example, a predetermined fill level of a medium 2 in a container 2a can be monitored, and even the density and/or viscosity of the medium 2 can be ascertained. The sensor unit 3 includes a mechanically oscillatable unit 4 in the form of an oscillatory fork, which extends partially into the medium 2. Other embodiments of a mechanically oscillatable unit 4 known and falling within the scope of the invention are represented, for example, by a single rod or a membrane. Mechanically oscillatable unit 4 is excited by means of the driving/receiving unit 5, which is supplied with an excitation signal, such that mechanical oscillations are executed. Such can be, for example, a four-quadrants drive or a bimorph drive. Shown, furthermore, is an electronics 6, by means of which signal evaluation and/or feeding occurs.

(11) FIG. 2 shows a schematic drawing of a typical field instrument 1 based on the capacitive measuring principle according to the state of the art. The example shows a sensor unit 3 with two cylindrically embodied electrodes 7 and 8. Sensor unit 3 protrudes via a process connection 2b inwardly from above into the container 2a filled partially with medium 2. Other embodiments of capacitive and/or conductive measuring devices can, of course, have another number and/or different embodiments of the electrodes 7, 8. Furthermore, besides such measuring devices 1, in the case of which the sensor unit 3 protrudes, such as shown in FIG. 2, from above into the container 2a, also sensor units 3 can be used, which are introduced into the container 2a through a side wall of the container 2a.

(12) The measuring principles underpinning vibronic and capacitive and/or conductive measuring devices are sufficiently known per se in the state of the art and are therefore not explained further here.

(13) In the case of a field instrument 1 of the invention, the electronics 6 includes a diagnostic unit 10, which serves for executing an embodiment of the diagnostic operating mode of the invention. In the following, three, especially preferred embodiments are explained by way of example for an electronics unit of the invention and for a diagnostic operating mode of the invention. It is to be noted here that the elements shown for the individual embodiments can also be freely adopted for use in other embodiments.

(14) A first possible embodiment is shown in the schematic circuit diagram of FIG. 3. In a normal operating mode M.sub.N, an input voltage U.sub.N is produced and an output current I.sub.N representative of a process variable is output. An electronics 6 includes a voltage controlled electrical current source 9. For turning the diagnostic operating mode M.sub.D on and off, the diagnostic unit 10 includes two switching elements 11a and 11b.

(15) At the beginning of the diagnostic operating mode M.sub.D, a first time interval t.sub.1 is set by means of a monostable multivibrator 12. The switching elements 11a and 11b are suitably set, so that during the first time interval the first diagnosis-input voltage U.sub.DI,1 is provided and via the voltage controlled electrical current source 9 a corresponding diagnosis-output current I.sub.DO,1 is output.

(16) At the same time, a second time interval t.sub.2 is determined by means of the control unit 13. Control unit 13 includes a subtractor unit 14, an integrator unit 15 and a comparator 16. The integrator unit 15 includes at least one capacitor C and a resistor R. Moreover, via a switching element 11c, the integrator unit 15 is kept in a predeterminable starting state before the beginning of the diagnostic operating mode M.sub.D.

(17) By means of the subtractor unit 14, the difference between the input voltage U.sub.N in the normal operating mode M.sub.N and a reference signal U.sub.R, which in the present case is the first diagnosis-input voltage U.sub.DI,1, is formed and fed to the integrator unit 15. The output voltage of the integrator unit 15 rises then, especially linearly, wherein the rise of the voltage depends, among other things, on the value of the first diagnosis-input voltage the capacitance of the capacitor C and the resistance of the resistor R.

(18) After the first time interval ti, the switching elements 11a and 11b are set in such a manner that the second diagnosis-input voltage U.sub.DI,2 is provided and likewise a corresponding output current I.sub.DO,2 is output. In this case, the reference signal U.sub.R is the second diagnosis-input voltage U.sub.DI,2. As a consequence, the polarity of the difference between the input voltage U.sub.N and the reference signal U.sub.R reverses and the voltage at the integrator unit 15 sinks now, especially linearly. Again, the change of the voltage depends, among other things, on the value of the second diagnosis-input voltage U.sub.DI,2, the capacitance of the capacitor C and the resistance value of the resistor R.

(19) With the help of the comparator 16, that point in time is determined at which a value of the voltage applied to the comparator 16 corresponds to that value which this voltage had at the beginning of the first time interval t.sub.1. This point in time defines the end of the second time interval t.sub.2. Advantageously, the diagnostic unit 10, especially the control unit 13, can be embodied in such a manner that the end of the second time interval t.sub.2 is determinable based on a zero crossing of the voltage applied to the comparator 16. Advantageously, the second time interval can be determined by the control unit 13, especially by the integrator unit 15.

(20) The output current I.sub.DO,1, I.sub.DO,2 flowing during the first time interval t.sub.1, and the second time interval t.sub.2 produces a voltage drop across the resistor 17 and, after processing by means of the subtractor unit 18, the first and second diagnosis-output voltage U.sub.DO,1, U.sub.DO,2. Based on the first and second diagnosis-output voltages U.sub.DO,1 and U.sub.DO,2, a functionality of the field instrument 1 is checked in the embodiment shown in FIG. 3. The diagnosis-output voltages U.sub.DO,1 and U.sub.DO,2 are for this purpose, in each case, fed to the comparator unit 19, which during the diagnostic operating mode M.sub.D checks whether a predeterminable maximum value U.sub.H or a predeterminable minimum value U.sub.L can be output for the first and/or second output current I.sub.DO,1, I.sub.DO,2, i.e. for the first and/or second diagnosis-output voltage U.sub.DO,1, U.sub.DO,2. If such is not the case, then, for example, a failure message can be output from the field instrument 1 in the form of an alarm.

(21) Furthermore, the diagnostic unit 10 in the present example includes a supervision unit 20. Such is, however, not absolutely necessary. Various measures can be triggered with the supervision unit 20.

(22) For example, the time intervals t.sub.1 and t.sub.2, and, correspondingly, the switching elements 11a and 11b, can be controlled (20a) by means of the supervision unit 20. The end of the second time interval t.sub.2 is ascertained, in such case, by means of the control unit 13, and transmitted to the supervision unit 20 (20b). Also the switching element 11c associated with the integrator unit can be controlled (20c) by the supervision unit 20. Finally, the supervision unit 20 is, furthermore, embodied to publish (20d) failure messages relative to checked functionalities of the field instrument 1. In the illustrated example, the supervision unit 20 can output the values for the first and second time intervals t.sub.1 and t.sub.2.

(23) A second possible embodiment of the diagnostic unit 10 is shown in FIG. 4. Elements already explained in connection with FIG. 3 are not explored again in detail for FIG. 4. In contrast with the embodiment shown in FIG. 3, the reference signal U.sub.R in the embodiment of FIG. 4 is the first or the second diagnosis-output signal U.sub.DO,1, U.sub.DO,2. This measure takes into consideration that the output electrical current I.sub.DO,1, I.sub.DO,2as a function of time for the voltage controlled electrical current source 9 is not an error-free image of the first and second diagnosis-input voltage U.sub.DI,1, U.sub.DI, 2, respectively. Since, thus, the first, or second diagnosis-output signal U.sub.DO,1, U.sub.DO,2 is selected for the reference signal U.sub.R, parasitic circuit parameters can be largely taken into consideration, or compensated.

(24) A third and preferred embodiment of the diagnostic unit 10 is shown finally by way of example in FIG. 5. Also in the case of FIG. 5, elements already explained in connection with FIGS. 3 and 4 are not explored again in detail for FIG. 5. In this embodiment, the diagnostic unit 10 comprises instead of the control unit a computing unit 21, which can be a microcontroller, for example. Then, the two time intervals t.sub.1 and t.sub.2 are determined by the computing unit 21. For this, for example, suitable formulas can be stored in the computing unit 21.

(25) Besides the opportunity to check the ability of the field instrument 1 to output failure information, also other functionalities of the field instrument 1 can be checked. For example, it can be checked whether an output current I.sub.N set in the electrical current loop corresponds to the actual value of the measured variable to be represented. For this, suited especially is an embodiment of the diagnostic unit 10 as shown in FIG. 4.

(26) Based on the first time interval t.sub.1, the input voltage U.sub.N and the first and second diagnosis-input voltage U.sub.DI,1 and U.sub.DI,2, the second time interval t.sub.2 can be calculated based on the following formula:

(27) $t_2=-t_1\frac{U_N-U_{\mathrm{DI},1}}{U_N-U_{\mathrm{DI},2}}$

(28) This formula can be solved for U.sub.N:

(29) $U_{N,\mathrm{actual}}=\frac{t_1{U}_{\mathrm{DE},1}+t_2{U}_{\mathrm{DE},2}}{t_1+t_2}$

(30) The output current I.sub.N corresponds then exactly to the actual value of the measured variable to be represented by U.sub.N, when U.sub.n,actual=U.sub.n.

(31) Shown in FIG. 6 are schematic graphs of the first output electrical current I.sub.DO,1 and the second output electrical current I.sub.DO,2 of the diagnostic operating mode M.sub.D as a function of time for different values of the output current I.sub.N in the normal operating mode M.sub.N. During the normal operating mode M.sub.N, the output current I.sub.N lies in the range, 4-20 mA. Furthermore, the output current I.sub.N,a in FIG. 6a is greater than the output current I.sub.N,b in FIG. 6b. Thus, I.sub.N,a>I.sub.N,b.

(32) During the first time interval t.sub.1 of the diagnostic operating mode, the first diagnosis-input voltage U.sub.DI,1 is provided. Correspondingly, the electrical current increases to the value I.sub.DO,1. During the second time interval, in contrast, the second diagnosis-input voltage U.sub.DI,2 is provided, so that the electrical current sinks to the value I.sub.DO,2. After the second time interval, the output current goes back to I.sub.N,a. The areal contents of the time integrals of the differences between the first output electrical current I.sub.DO,1 and the second output electrical current I.sub.DO,2 on the one hand and the output electrical current I.sub.N in the normal operating mode M.sub.N on the other hand over the first time interval t.sub.1 and the second time interval t.sub.2, respectively, are essentially equal in magnitude, so that advantageously a DC fraction free modulation of the output electrical current I.sub.N results. FIG. 6b shows the same graph as shown in FIG. 6a, except that the output current I.sub.N,b during normal operating mode M.sub.N is less than in FIG. 6a. For assuring a constant component free modulation in this case, the second time interval t.sub.2 must be longer than in FIG. 6a. The duration of the second time interval t.sub.2 depends, in such case, basically on the input voltage U.sub.N.