Monitoring the state of a coil in a sensor

Abstract

The present invention relates to a method for monitoring state of a coil having at least two connection wires, which coil is part of an apparatus for determining at least one process variable of a medium in a containment, as well as relating to an apparatus for executing the method. The method, in such case, includes method steps as follows: ascertaining a desired-value of an ohmic total resistance for the coil and the connection wires, supplying the coil with an electrical excitation signal and receiving an electrical, received signal from the coil by means of the two connection wires, ascertaining an actual-value of the ohmic total resistance based at least on the received signal, and comparing the actual-value with the desired-value and ascertaining a state indicator based on the comparison.

Claims

1. A method for monitoring a state of a coil having at least two connection wires, wherein the coil is part of an apparatus for determining at least one process variable of a medium in a containment, the method comprising: ascertaining a desired-value of an ohmic total resistance for the coil and the at least two connection wires; supplying the coil with a first electrical excitation signal that is an alternating signal having a first predeterminable frequency and receiving a first electrical, received signal from the coil via the at least two connection wires; ascertaining an actual-value of the ohmic total resistance based on the first received signal; comparing the actual-value with the desired-value and ascertaining a state indicator based on the comparison; ascertaining a first phase between the first excitation signal and the first received signal; ascertaining a first difference between the first phase and a desired-value for the first phase; and when the first difference exceeds a first predeterminable limit value, determining at least one winding short in the region of the coil.

2. The method as claimed in claim 1, further comprising: determining a difference between the actual-value and the desired-value and ascertaining the state indicator based on the difference.

3. The method as claimed in claim 1, further comprising: creating an electrical equivalent circuit for the ohmic total resistance of the coil and ascertaining the desired-value for the total resistance based on the equivalent circuit.

4. The method as claimed in claim 3, wherein each of the at least two connection wires is represented by a series circuit of a wire resistance and a wire defect resistance and wherein the coil is represented by a series circuit of a coil resistance and a coil defect resistance.

5. The method as claimed in claim 4, further comprising: assuming a predeterminable defect value for at least one wire defect resistance and/or the coil defect resistance.

6. The method as claimed in claim 1, wherein the state indicator is a statement of a presence of at least one winding short in a region of the coil or a statement of a poor electrical contacting or a cable break in the region of the coil or the at least two connection wires.

7. The method as claimed in claim 1, when the first difference exceeds a second predeterminable limit value, determining a presence of moisture or medium in at least a region in which the coil is located.

8. The method as claimed in claim 1, further comprising: supplying the coil with a second excitation signal having a second predeterminable frequency and receiving a second electrical, received signal from the coil via the at least two connection wires; ascertaining a second phase between the second excitation signal and the second received signal; and ascertaining a difference between the second phase and a desired-value for the second phase.

9. The method as claimed in claim 1, wherein the coil is divided into at least two subcoils and provided with at least three connection wires, wherein a first subcoil is contacted with a first wire and a second wire, and wherein a second subcoil is contacted with the second wire and a third wire.

10. The method as claimed in claim 9, further comprising: ascertaining at least one subresistance of a subcoil.

11. The method as claimed in claim 10, further comprising: forming at least one ratio of a subresistance and the total resistance or a ratio between at least two subresistances.

12. An apparatus for determining and/or monitoring at least one process variable of a medium in a containment, comprising: a coil; and at least two connection wires, wherein the apparatus is embodied to execute a method for monitoring a state of the coil, the method including: ascertaining a desired-value of an ohmic total resistance for the coil and the at least two connection wires; supplying the coil with a first electrical excitation signal that is an alternating signal having a first predeterminable frequency and receiving a first electrical, received signal from the coil via the at least two connection wires; ascertaining an actual-value of the ohmic total resistance based on the first received signal; comparing the actual-value with the desired-value and ascertaining a state indicator based on the comparison; ascertaining a first phase between the first excitation signal and the first received signal; ascertaining a first difference between the first phase and a desired-value for the first phase; and when the first difference exceeds a first predeterminable limit value, determining at least one winding short in the region of the coil.

13. The apparatus as claimed in claim 12, wherein the apparatus is a vibronic sensor comprising: a mechanically oscillatable unit; a driving/receiving unit having the coil and the at least two connection wires, wherein the driving/receiving unit is embodied to excite the mechanically oscillatable unit with an electrical excitation signal to cause the mechanically oscillatable unit to execute mechanical oscillations, and wherein the driving/receiving unit is further embodied to receive mechanical oscillations from the mechanically oscillatable unit and to convert the received mechanical oscillations into an electrical, received signal; and an electronics unit embodied starting from the received signal to produce the excitation signal, and from the received signal to ascertain the at least one process variable.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The embodiments described in connection with the method of the invention are applicable mutatis mutandis also for the apparatus of the invention and vice versa.

(2) The invention as well as its advantageous embodiments will now be described in greater detail based on the appended drawing, the figures, FIG. 1-FIG. 6, of which show as follows:

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

(4) FIG. 2 shows an electromagnetic driving/receiving unit of a vibronic sensor according to the state of the art,

(5) FIG. 3 shows an equivalent circuit diagram for the ohmic total resistance of a coil,

(6) FIG. 4 shows an equivalent circuit diagram for the ohmic resistance of a coil subdivided into three subcoils,

(7) FIG. 5 shows an equivalent circuit diagram of a coil with core for illustrating state monitoring of the invention based on an alternating signal, and

(8) FIG. 6 shows state monitoring of the present disclosure based on an alternating signal in three graphs of the magnitude of impedance and phase as a function of frequency for different states of a coil.

DETAILED DESCRIPTION

(9) Although the present invention concerns generally coils in measuring apparatuses of process- and/or automation technology, the following description is limited, by way of example, to vibronic sensors. More exactly, the following description concerns a vibronic sensor with an electromagnetic driving/receiving unit comprising a coil and at least one magnet. The ideas of the method of the invention and the apparatus of the invention can be directly transferred to other measuring devices.

(10) FIG. 1a shows a vibronic fill-level measuring apparatus 1. A sensor unit 2 with a mechanically oscillatable unit 3 in the form of an oscillatory fork protrudes partially into a medium 4, which is located in a containment 5. The oscillatable unit 3 is excited by means of the driving/receiving unit 6, as a rule, an electromechanical transducer unit, to cause the mechanically oscillatable unit to execute mechanical oscillations, and can be, for example, a piezoelectric stack- or bimorph drive, however, also an electromagnetic or also a magnetostrictive driving/receiving unit. It is understood, however, that also other embodiments of a vibronic fill-level measuring apparatus 1 are possible, which are not shown here. The measuring apparatus includes, moreover, an electronics unit 7, by means of which signal registration, —evaluation and/or—supply occurs.

(11) FIG. 1b shows a more detailed view of an oscillatable unit 3 in the form of an oscillatory fork, such as, for example, used in the LIQUIPHANT measuring apparatus. Shown is a membrane 8, and an oscillatory element 9 connected therewith. Oscillatory element 9 includes two oscillatory rods 10a, 10b, on which are formed terminally paddles 11a, 11b. In operation, the oscillatory fork 3 executes oscillatory movements corresponding to the oscillatory mode, with which it is excited. Each of the two oscillatory rods 10a, 10b behaves essentially as a so called bending oscillator. In the fundamental oscillation mode, the two oscillatory rods 10a, 10b oscillate, for example, with opposite phase relative to one another.

(12) Known from the state of the art are different embodiments for electromagnetic driving/receiving units. For purposes of simplification, the following description concerns a driving/receiving unit 6, such as described in DE102015104533A1 or also in German patent application No. 102016112308.0 unpublished at the date of first filing of this application. Comprehensive reference is taken to the two patent applications in the context of the present invention.

(13) FIG. 2 shows a schematic view of such a driving/receiving unit 6. A housing 13 terminates in the lower region of the wall with a membrane 8, which is associated with the oscillatable unit 3. For the embodiment shown here, housing 13 is cylindrical and the disk-shaped membrane has a circularly round cross sectional area A. It is understood, however, that also other geometries provide options and fall within the scope of the present invention. Secured to the membrane 8 perpendicularly to its base area A and reaching into the interior of the housing 13 are three rods 15a, 15b, 15c. In such case, especially a force transmitting connection is used. The base area A of the membrane 8 lies then in a plane perpendicular to the longitudinal direction of the rods 15a, 15b, 15c. For example, the rods 15a, 15b, 15c are arranged at equal angles along an imaginary circular line around the midpoint M of the base area A of the membrane 8.

(14) Secured in the end region of the rods 15a, 15b, 15c away from the membrane 8 is, in each case, a magnet 16a, 16b, 16c, especially an SmCo- or Alnico magnet. The magnets 16a, 16b, 16c are preferably all equally oriented. Other embodiments have 2 rods 15a and 15b as well as two magnets 16a and 16b or also four 15a-15d or more rods and four 16a-16d or more magnets. In the case of an even number of magnets 16a-16d, the magnets can also be pairwise equally oriented.

(15) Arranged above the magnets 16a, 16b, 16c, in turn, is a coil 17, which comprises a wire wound around the core 18. Core 18 of the coil 17 is part of a pot-shaped armature unit 19 with a floor 20 as well as an enclosure 21. For example, the floor 20 can have a circular cross sectional area same as the base area A of the membrane 8. Core 18 of the coil 17 reaches from the floor 20 of the pot-shaped armature unit 19, in the form of a post, centrally into the interior of the armature unit 19. The enclosure 21 has, in this case, then the function of a magnetic field guide-back. The rods 15a-15c with the magnets 16a-16c do not contact the coil 17 and the core 18. The coil 17 is in the ongoing operation supplied with an alternating current signal for production of a magnetic, alternating field. For this, the coil has at least two connection wires (not shown in FIG. 2a).

(16) Due to this alternating field, the rods 15a-15c are deflected via the magnets 16a-16c horizontally, i.e. perpendiculary, or transversely, to their longitudinal axes, in such a manner that they are caused to oscillate. On the one hand, the rods 15a-15c then have a lever action, by which the bending of the rods 15a-15c produced through the horizontal deflection is transferred to the membrane 8 in such a manner that the membrane 8 is caused to oscillate. On the other hand, the combination of the rods 15a-15c and the membrane 8 forms a resonator. The exciting of the membrane 8 to cause the mechanically oscillatable unit to execute mechanical oscillations occurs, thus, by means of a magnetic, alternating field.

(17) By means of the present invention, a state monitoring of the coil 17 can be performed. For this, a desired-value R.sub.tot,ref for an ohmic total resistance of the coil 17 is ascertained. By supply of electrical power to the coil with an electrical excitation signal and evaluation of a signal received from the coil 17, then at predeterminable points in time, for example, periodically in predeterminable time intervals or individually upon query, an actual-value R.sub.tot, for the ohmic total resistance of the coil 17 can be ascertained. Using a comparison of the actual-value R.sub.tot with the desired-value R.sub.tot,ref, then a state indicator for the coil 17 can be ascertained. For example, a difference between the actual-value R.sub.tot and the desired-value R.sub.tot,ref can be determined. If the difference exceeds a predeterminable limit value, then the presence of a malfunction in the region of the coil 17 can be assumed. In such case, it can be, for example, a winding short of the coil wire 22 or an open electrical contact between the coil wire 22 and at least one of the connection wires 23a,23b.

(18) A desired-value R.sub.tot for the ohmic total resistance of the coil 17 can, in such case, on the one hand, be measured, for example, in the context of manufacture of a particular sensor 1 or upon its delivery. However, also a theoretical determination based on an electrical equivalent circuit, such as shown in FIG. 3, is possible. For such an equivalent circuit diagram, the most varied of embodiments provide options, which all fall within the scope of the present invention. Without limitation of generality, for the example shown here, the coil wire C is represented by a series circuit of a coil resistance Rc and a coil defect resistance dR.sub.C and the two connection wires w.sub.1, w.sub.2 likewise, in each case, by a wire-resistance R.sub.w1, or R.sub.w2 and, in each case, a wire defect resistance dR.sub.w1, or dR.sub.w2 connected thereto in series. Using the equivalent circuit, a desired-value R.sub.tot,ref for the total resistance of the coil 17 can be ascertained. In this regard, predeterminable defect values can be assumed for the defect resistances dR.sub.C, dR.sub.w1, dR.sub.w2, which defect resistances, in principle, give maximum allowable tolerances for the resistances R.sub.C, R.sub.w1, R.sub.w2. In this case, the predeterminable limit value for a difference between actual-value R.sub.tot and desired-value R.sub.tot,ref is defined, or specified, based on the defect resistances dR.sub.C, dR.sub.w1, dR.sub.w2.

(19) Another preferred embodiment is shown in FIG. 4. The coil 17 is divided into three subcoils C.sub.1, C.sub.2 and C.sub.3, and electrically contacted by means of four connection wires w.sub.1-w.sub.4. The first subcoil C.sub.1 is contacted by means of the connection wires w.sub.1 and w.sub.2, the second subcoil C.sub.2 by means of the connection wires w.sub.2 and w.sub.3, and the third subcoil C.sub.3 by means of the connection wires w.sub.3 and w.sub.4. However, the coil 17 of the invention is subdividable into any number of at least two subcoils C.sub.1 and C.sub.2, wherein, in each case, n+1 connection wires are provided for n subcoils C.sub.1-C.sub.n. The following ideas can, in such case, be directly transferred to a plurality of subcoils unequal to three.

(20) By subdividing the coil 17 into subcoils—in the present case into the three subcoils C.sub.1, C.sub.2 and C.sub.3—a winding short or a lack of electrical contacting in the region of the coil can be advantageously better located. For this, as in the case of the total resistance R.sub.tot, subresistances for each at least one subcoil R.sub.C1, R.sub.C2 and R.sub.C3 can be ascertained, or for two adjoining subcoils R.sub.C12 or R.sub.C23. Similarly, such as in the case of the total resistance R.sub.tot, or R.sub.tot,ref of the coil 17, actual values of the subresistances R.sub.C1, R.sub.C2, R.sub.C3, R.sub.C12 and/or R.sub.C23 ascertained in the ongoing operation of the sensor 1 can be compared with corresponding desired values R.sub.C1,ref, R.sub.C2,ref, R.sub.C3,ref, R.sub.C12,ref and/or R.sub.C23,ref. For example, differences present in each case can be ascertained.

(21) Analogously to the embodiment in FIG. 3, in FIG. 4, desired values for the subresistances and the total resistance are ascertained based on an equivalent circuit, in which the subcoils are represented by a series circuit of coil resistances R.sub.C1-R.sub.C3 and associated coil defect resistances dR.sub.C1-dR.sub.C3 and the connection wires analogously, in each case, by wire resistances R.sub.w1-R.sub.w4 and associated wire defect resistances dR.sub.w1-dR.sub.w4. Since the ohmic resistance of the coil wire C and the connection wires w.sub.1-w.sub.4 are temperature dependent, alternatively, ratios of various subresistances R.sub.C1, R.sub.C2, R.sub.C3, R.sub.C12 and/or R.sub.C23 and/or at least one subresistance R.sub.C1, R.sub.C2, R.sub.C3, R.sub.C12 and/or R.sub.C23 and the total resistance R.sub.tot can be considered. In such case, the influence of the process environment on the state monitoring can be eliminated, or minimized.

(22) The coil 17 can basically be divided into a plurality of subcoils C.sub.1-C.sub.n with at least partially different turns numbers as well as also into subcoils with at least pairwise equal turns numbers. Analogously the n+1 connection wires w.sub.1-w.sub.n+1 can be embodied essentially equally or they can be embodied at least partially differently. For example, the length and/or cross sectional area of the wires w.sub.1-w.sub.n+1 can vary.

(23) In the case, in which for the embodiment of FIG. 4 the coil 17 is divided into three subcoils C.sub.1-C.sub.3, and likewise the connection wires d.sub.1-d.sub.4 are equally embodied, it is sufficient, because of the redundancy, to achieve a very good resistance based state monitoring, when the ratios R.sub.C1/R.sub.C2, R.sub.C2/R.sub.C3 as well as R.sub.C12/R.sub.C23 are formed and compared with the corresponding ratios of desired values R.sub.C1,ref/R.sub.C2,ref, R.sub.C2,ref/R.sub.C3,ref as well as R.sub.C12,ref/R.sub.C23,ref. For this, for example, a so-called pivot table-table can be applied.

(24) In the case of a cable break in a connection wire w1-wn or in a wire of one of the subcoils C1-Cn, a significant difference between the actual value and the desired value of the subresistance affected by the cable break is to be expected. In the case of a lacking electrical contact between a connection wire w1-wn and a wire of one of the subcoils C1-Cn, then the actual-value of the at least one affected subresistance rises compared with the not affected subresistances.

(25) Other embodiments of the present invention, are, finally, illustrated by FIGS. 5 and 6. Since the coil 17 is supplied with an excitation signal in the form of an alternating signal, additional statements can be made concerning the state of the coil 17. In this connection, FIG. 5 shows an electrical equivalent circuit diagram of a coil 17 with core 18. In such case, L is the inductance of the coil 17, R.sub.1 the resistance of the coil 17, R.sub.2 the equivalent ohmic series resistance, R.sub.fe the frequency dependent reactance and C the stray capacitance between the windings of the coil 17.

(26) According to the invention, at least one value for the phase shift ϕ.sub.1 for a predeterminable frequency f.sub.1 is compared with a desired-value for the phase shift ϕ.sub.ref,1 and a difference between the actual-value ϕ.sub.f and the desired-value ϕ.sub.ref,1 ascertained. In the case of a defect in the region of the coil 17, such as, for example, a winding short or penetrated medium, there is a shifting of the curve of phase ϕ as a function of frequency f and so the value for ϕ.sub.1 at the frequency f.sub.1 changes. Since, for example, a change of the quality Q of the oscillatory system can also occur in the case of a defect in the region of the coil 17, it is expedient to evaluate the phases ϕ.sub.1 and ϕ.sub.2 for at least two predeterminable frequencies f.sub.1 and f.sub.2 and to compare such with predeterminable desired values ϕ.sub.ref,1 and ϕ.sub.ref,2.

(27) When the frequency f of the excitation signal is variable, especially also spectra of the impedance Z, the magnitude of the impedance |Z|, the amplitude A and/or phase ϕ between the excitation signal and the received signal as a function of frequency f can be generated and evaluated.

(28) Corresponding spectra are shown in FIG. 6 and discussed in the following for better illustrating the behavior of the oscillatory system.

(29) FIG. 6a shows spectra of the magnitude of the impedance |Z| and the phase ϕ for a fully functional coil 17. The magnitude of the impedance |Z| has a maximum value at the resonant frequency f.sub.0, while the spectrum of the phase ϕ has there a point of inflection. The phase ϕ.sub.1 associated with the frequency f.sub.1 can be taken into consideration as the desired-value ϕ.sub.ref,1 for the phase.

(30) In the case, in which, for instance, a third of the coil 17 is short circuited, changed spectra occur, as shown in FIG. 6b. A short circuit lessens the inductance L of the coil 17 and, associated therewith, the resonant frequency shifts (Δf.sub.1) in the positive direction in comparison with a fully functional coil 17. The resonant frequency f′.sub.0 is shifted to a higher value relative to the resonant frequency f.sub.0 of the fully functional coil, and, correspondingly, there is an analogous shifting of the curve of the spectrum of the phase ϕ. Additionally because of the short circuit in the region of the coil 17, the quality Q of the oscillatory system changes.

(31) As one can see based on the curves of the spectra, the phase associated with the frequency f.sub.1 changes to the value ϕ′.sub.1 and no longer equals the predeterminable desired-value ϕ.sub.ref,1. If the difference exceeds a first predeterminable limit value, then the presence of a winding short can be assumed. Analogous ideas hold for the case, in which the phases ϕ.sub.1 and ϕ.sub.2 are evaluated for two predeterminable frequencies f.sub.1 and f.sub.2.

(32) In the case of a cable break, there is a complete interruption of the oscillatory circuit.

(33) In the case, in which, in contrast, a liquid medium or moisture comes in contact with the coil 17, there results, in contrast, a spectrum as shown in FIG. 6c. There is a change in the stray capacitance C between the windings of the coil 17, and, associated therewith, a negative shifting of the resonant frequency (Δf.sub.2) in comparison with a fully functional coil.

(34) As in the case of a short circuit, spectral curves changed also in the case of contact of the coil with a medium. In contrast with FIG. 6b, the resonant frequency f″.sub.0 shifts, however, to a lower value. The frequency shift Δf.sub.2 t is thus negative. The resonant frequency f″.sub.0 is shifted to a lower value compared with the resonant frequency f.sub.0 of the fully functional coil, and, correspondingly, there is an analogous shifting of the curve of the spectrum of the phase ϕ. Additionally, the quality Q of the oscillatory system changes, because of the short circuit in the region of the coil 17.

(35) As evident based on the curves of the spectra, the phase associated with frequency f.sub.1 changes to the value ϕ″.sub.1and, correspondingly, no longer equals the predeterminable desired-value ϕ.sub.ref,1. If the difference exceeds a first predeterminable limit value, then the presence of a winding short can be assumed. Analogous ideas hold here also for the case, in which phases ϕ.sub.1 and ϕ.sub.2 for two predeterminable frequencies f.sub.1 and f.sub.2 are evaluated.

(36) Since the phase ϕ is analyzed supplementally to the explained considerations for the ohmic resistances R in connection with FIGS. 3 and 4, not just general statements relative to the functioning of the coil 17 can be generated. Rather, in many cases, winding shorts, cable breaks and defective electrical contactings, which lead to junction resistances, can be distinguished.

(37) In summary, the present invention enables a state monitoring of a coil 17 in a measuring apparatus 1. Advantageously, with the present invention, a measuring apparatus 1 with a high measure of functional safety can be assured, since various malfunctions can be detected and distinguished from one another. The method of the invention can be executed periodically in predeterminable time intervals or as needed. Also, a state monitoring running simultaneously with ongoing measurement operation is possible in given cases, to the extent that a mutual influencing of the measuring- and diagnostic operation can be excluded.