FILL-LEVEL MEASURING DEVICE

Abstract

Disclosed is a method for determining the serviceability of a fill-level measuring device that includes at least one electronic unit. The method can be applied to any type of field device that includes at least one electronic unit supplied by an energy store of the field device. The method includes: measuring the capacitance of the energy store and/or measuring the power withdrawal at the energy store. The field device is classified as not operationally reliable if the capacitance of the energy store is below a defined minimum capacitance and/or if the power withdrawal deviates from a predefined normal consumption. An advantage of the method according to the invention is that, in addition to the functional diagnosis, in particular also a prediction up to the expected elapsing of the remaining operating time can be created by recording the power withdrawal or the capacitance over progressive measurement cycles.

Claims

1-13. (canceled)

14. A method for determining the serviceability of a field device that comprises at least one electronic unit for measuring a process variable, and the at least one electronic unit is supplied by means of an energy store of the field device, the method comprising: measuring a capacitance of the energy store; measuring a power consumption at the energy store; and classifying the field device as unserviceable when the capacitance of the energy store falls below a defined minimum capacitance or/and when the power consumption deviates from a predefined normal consumption.

15. The method according to claim 14, wherein the capacitance is measured by measuring a voltage drop over time at the energy store during a defined discharge of the energy store.

16. The method according to claim 15, wherein the voltage drop over time is measured after switching on the at least one electronic unit in order to measure the power consumption at the energy store.

17. The method according to claims 16, wherein the field device is classified as unserviceable if the measured voltage drop exceeds a predefined maximum voltage drop.

18. The method according to claim 14, further comprising: when the capacitance of the energy store does not fall below the defined minimum capacitance, determining a first change function of the capacitance via progressive measurement cycles; and calculating, based on the current capacitane and the change function, a first remaining service time until the minimum capacitance is undershot.

19. The method according to claim 18, further comprising: when the power consumption on the energy store does not deviate from the defined normal consumption, determining a second change function of the power consumption via progressive measurement cycles; and calculating, based on the current power consumption and the second change function, a second remaining service time until a minimum deviation from the normal consumption is exceeded.

20. The method according to claim 19, further comprising: when the measured voltage drop does not exceed the predefined maximum voltage drop, determining a third change function of the voltage drop via progressive measurement cycles; and calculating, using the current voltage drop and using the third change function, a third remaining service time until the maximum voltage drop is exceeded.

21. The method according to claim 20, wherein a suitable function type of the first change function, the second change function, and/or the third change function is ascertained by means of a least squares method.

22. The method according to claim 14, further comprising: measuring a temperature at the field device; and defining the minimum capacitance as a temperature-dependent function.

23. A fill-level measuring device, comprising: an energy store; a signal-generating unit which is fed by the energy store and is designed to emit a radar or ultrasound signal in a direction of a filling material in a container; an evaluation unit which is fed by the energy store and is designed to determine a fill level on the basis of a signal reflected at a filling material surface; and a control unit designed to: switch on and off the signal-generating unit and/or the evaluation unit or individual function blocks thereof; determine a power consumption at the energy store; determine a capacitance of the energy store; and classify the field device as unserviceable when the capacitance of the energy store falls below a minimum capacitance or when the power consumption deviates from a defined normal consumption.

24. The fill-level measuring device according to claim 23, wherein the control unit is further designed, on the basis of the measured capacitance and/or on the basis of the measured power consumption, to calculate a remaining service time up to the unserviceability of the fill-level measuring device provided the control unit currently classifies the fill-level measuring device as serviceable.

25. The fill-level measuring device according to claim 23, wherein the control unit is further designed to transmit a possible unserviceability or a remaining service time to a higher-level unit.

26. The fill-level measuring device according to claim 23, wherein the energy store is designed as a buffer capacitor that can be connected to the higher-level unit for re-charging.

Description

[0029] The invention is explained in more detail with reference to the following figures. The following is shown:

[0030] FIG. 1: a typical arrangement of a radar- or ultrasound-based fill-level measurement device,

[0031] FIG. 2: a circuit design of the fill-level measurement device,

[0032] FIG. 3: a schematic sequence of the method according to the invention; and

[0033] FIG. 4: a calculation of the remaining service time of the fill-level measurement device.

[0034] The method according to the invention for evaluating the serviceability of field devices is explained in more detail below using the example of fill-level measurement. For a basic understanding, FIG. 1 shows a typical arrangement of a radar-based fill-level measurement device 1 on a container 2. In the container 2 is a filling material 3, whose fill level L is to be determined by the fill-level measurement device 1. For this purpose, the fill-level measurement device 1 is mounted on the container 2 above the maximum permissible fill level L. Depending on the field of application, the height h of the container 2 can be up to 125 m.

[0035] The fill-level measuring device 1 is oriented in such a way that the radar or ultrasound signals S.sub.HF generated by the signal-generating unit 11 (cf. FIG. 2) are emitted in the direction of the filling material 3. The signals E.sub.HF are reflected at the surface of the filling material 3 and received after a corresponding signal propagation time by an evaluation unit 12 of the fill-level measuring device 1. The signal propagation time of the signals S.sub.HF, E.sub.HF depends on the distance d=h−L of the fill-level measurement device 1 from the filling material surface.

[0036] Like field devices in general, the fill-level measuring device in FIG. 1 is also connected via a bus system to a higher-level unit 4, for example a process control system or a decentralized database. On the one hand, information about the operating status of the fill-level measurement device 1 can thus be communicated. On the other hand, information about the measured process variable or the fill level L can also be transmitted via the bus system in order to control any inflows or outflows that may be present at the container 2. The fill-level measuring device 1 is also supplied with power via the bus system, depending on the design. Since the fill-level measuring device 1 is often used in explosion-endangered installations, the corresponding interface of the fill-level measuring device 1 is generally designed to be power-limited. Accordingly, for example, the interface may be designed to communicate using power-saving protocols such as “Ethernet”, “PROFIBUS”, “HART” or “Wireless HART”.

[0037] In order to be able to periodically briefly supply sufficient power for the actual measurement, the fill-level measuring device 1 or the field device is generally connected to the higher-level unit 4 via a buffer capacitor 14 or an equivalent energy store. In the circuit configuration of the fill-level measuring device 1 shown in FIG. 2, the signal-generating unit 11 and the evaluation unit 12 are therefore supplied with power via the buffer capacitor 14. Continuous lines represent the power supply, and dashed lines represent data transmission. In this case, the power supply of further electronic units can also be effected via the buffer capacitor 14. In order that the buffer capacitor 14 can be recharged between individual measurements, the fill-level measuring device 1 is designed such that it controls the clock rate f.sub.c, at which it in each case remeasures the fill level L, as a function of the state of charge of the buffer capacitor 14. The higher the state of charge of the capacitor 14, the more often the fill level L or generally the process variable is measured.

[0038] As measuring cycles of the fill-level measuring device 1 proceed, the risk of the buffer capacitor 14 or even the electronic units 11, 12 degrading increases. The capacitance of the buffer capacitor 14 may thus decrease with increasing age. In the case of the electronic units 11, 12, for example, oscillators may detune or short circuits may occur. Depending on the circuit unit 11, 12, this can result in a false fill level L being obtained without this being discernible from outside. This entails the risk of incorrectly controlling actuators of the process plant coupled thereto, such as pumps or outlets on the container 2, on the basis of the incorrect fill-level value.

[0039] This can be prevented with the aid of the method according to the invention that is illustrated in FIG. 3: On the one hand, it can be diagnosed whether the fill-level measuring device 1 is still serviceable. If this is the case, a remaining service time At until presumable unserviceability may be predicted on this basis.

[0040] To test the serviceability, the control unit 13 of the fill-level measuring device 1 first switches off all the electronic units 11, 12 fed by the buffer capacitor 14 and measures the capacitance C of the buffer capacitor 14. For determining the capacitance C in farads, the control unit 13 can, for example, record the discharge voltage (U.sub.1-U.sub.2) of the buffer capacitor 14 for a defined time t under a known discharge resistance R in order to determine the capacitance thus:

[00003]$U_2=U_1*e^{\frac{t}{R*C}}$

[0041] If the buffer capacitor 14 falls below the previously defined minimum capacitance C.sub.min, the buffer capacitor 14 and thus the fill-level measuring device 1 will have to be classified as unserviceable.

[0042] Since, depending on the design of the capacitor 14 or of the energy store, its capacitance C also depends on the ambient temperature, the minimum capacitance C.sub.min in the control unit 13 can be stored as a function dependent on temperature, for example in the form of a look-up table. Such a design requires that the fill-level measuring device 1 additionally comprises a temperature sensor for detecting the current temperature.

[0043] If the buffer capacitor 14 does not lie below the minimum capacitance C.sub.min, it is eliminated as an error source. In this case, the capacitance value C obtained can be stored chronologically per measuring cycle in order to generate a change function over time dC/dt of the capacitance value C from the values stored over the measuring cycles.

[0044] At least when the buffer capacitor 14 is intact, the method according to the invention envisages measuring the power consumption p.sub.11, p.sub.12 at the buffer capacitor 14. For this purpose, one of the switched-off electronic units 11, 12 is switched on again in order to measure the power consumption p.sub.11 at the buffer capacitor 14 in this state. Since only the reactivated electronic unit 11 contributes to the power consumption, the power consumption p.sub.11 measured at the buffer capacitor 14 can be assigned to the power consumption of the reactivated signal-generating unit 11.

[0045] The power consumption p.sub.11 can in turn be determined by measuring the voltage drop over time U.sub.1-U.sub.2 at the buffer capacitor 14 after the signal-generating unit 11 has been switched on, thus

[00004]$p_{11,12}=C*\frac{{\left(U_1-U_2\right)}^2}{2*t}.$

[0046] The time t here corresponds to the measuring time or the time during which the signal-generating unit 11 is switched on.

[0047] The control unit 13 can also compare the determined power consumption p.sub.11 with a known normal consumption p.sub.norm of the signal-generating unit 11. If the power consumption p.sub.11 deviates significantly from the normal consumption p.sub.norm, the reactivated electronic unit 11 and again also the entire fill-level measuring device 1 must according to the invention be classified as unserviceable.

[0048] Analogously, the second electronic unit 12 or any further electronic unit which is fed from the buffer capacitor 14 can be used when the power consumption pug of the first switched-on electronic unit 11 (or all previously switched-on units) does not deviate significantly from the normal consumption p.sub.norm: [0049] turning the second or further electronic unit 12 on again (in this case, the first switched-on unit 11 can either remain switched on or switched off) [0050] measurement of the power consumption p.sub.12 at the buffer capacitor 14, and [0051] classification of the second switched-on electronic unit 12 as unserviceable if its power consumption p.sub.12 deviates from the corresponding normal consumption p.sub.norm.

[0052] Depending on whether and which of the components 11, 12, 14 of the fill-level measuring device 1 have been classified as unserviceable, the unserviceability can be communicated via the interface of the higher-level unit 4. It goes without saying that the method steps according to the invention, which are explained in FIGS. 1 and 2 with reference to the fill-level measuring device 1, are applicable in general to each type of field device in which the clock rate f.sub.c at which the respective process variable is measured is regulated as a function of the state of charge of the underlying energy store.

[0053] As is further evident from FIG. 3, the determined power consumption p.sub.11, p.sub.12 can be stored chronologically not only for the buffer capacitor 14 but also for the signal-generating unit 11 or for the evaluation unit 12 (provided the fill-level measuring device 1 is still classified as serviceable) in order to produce a change function dp.sub.11,12/dt of the power consumption on this basis analogously to the capacitance C.

[0054] The change functions dp/dt, dC/dt can be used, in addition to diagnosing the serviceability, to additionally make a prognosis as to when the fill-level measuring device 1 will probably no longer be serviceable. Such a prediction can be used to be able to schedule maintenance or a replacement of the fill-level measurement device 1 at an early stage on the part of the plant operator according to the principle of “predictive maintenance”.

[0055] In the case of the buffer capacitor 14 an estimated remaining service time Δt up to unserviceability may be calculated by the control unit 13 calculating the corresponding remaining service time Δt.sub.1 on the basis of the current capacitance value C and on the basis of the ascertained change function dc/dt until the minimum capacitance C.sub.min is expected to be reached.

[0056] The determination of the remaining service time C.sub.min is shown schematically in FIG. 4 taking the example of the buffer capacitor 14. An approximately linear drop in the capacitance C of the buffer capacitor 14 as the service time of the fill-level measuring device 1 progresses is shown here for illustrative purposes. Accordingly, in the exemplary embodiment shown in FIG. 4, a linear function can be used as the function type of the change function dC/dt. In general, however, the change function dC/dt may not be optimally approximated by a linear function, so that, for example, a polynomial function would provide an improved approximation to the profile of the capacitance C over the past measuring cycles. Accordingly, the evaluation unit 13 can be programmed, for example, in such a way that it obtains a suitable function type of the corresponding change function dC/dt by means of the least squares method. In this way, the remaining service time At of the field device can be predicted with even greater precision.

[0057] Although in FIG. 4 the creation of the change function dC/dt is only illustrated by the example of the buffer capacitor 14, a possible change function dp.sub.11,12/dt of the signal-generating unit 11 or of the evaluation unit 12 can likewise be established on the basis of the methodology described in FIG. 4 in order to again derive therefrom a separate remaining service time Δt.sub.2,3. The creation of a change function dU/dt on the basis of the measured voltage drops U.sub.1-U.sub.2 at the buffer capacitor 14 is also conceivable, provided the power consumption p.sub.11 or the capacitance C is measured on the basis of the voltage drop over time U.sub.1-U.sub.2.

[0058] If a separate remaining service time Δt is calculated on the basis of the buffer capacitor 14 as well as on the basis of one of the electronic units 11, 12, the control unit 13 will be able to define, for example, the shortest of the determined remaining service times Δt.sub.1,2,3 as the relevant remaining service time Δt until the the fill-level measuring device 1 is unserviceable.

LIST OF REFERENCE SIGNS

[0059] 1 Field device or fill-level measuring device

[0060] 2 Container

[0061] 3 Filling material

[0062] 4 Higher-level unit

[0063] 11 Signal-generating unit

[0064] 12 Evaluation unit

[0065] 13 Control unit

[0066] 14 Energy store

[0067] C Capacitance

[0068] C.sub.min Minimum capacitance

[0069] d Distance

[0070] E.sub.HF Reflected signals

[0071] h Installation height

[0072] L Process variable or fill level

[0073] p.sub.11,12 Power consumption

[0074] P.sub.norm Normal consumption

[0075] S.sub.HF Transmitted signals

[0076] U.sub.1,2 Voltages

[0077] Δ.sub.max Maximum voltage drop

[0078] Δt Remaining service time

[0079] dC, p, U/dt Change functions