WO2009109561A1 - Method for examining an electromagnetic flow meter and electromagnetic flow meter arrangement - Google Patents

Abstract

The invention relates to a method for examining an electromagnetic flow meter and to a flow meter arrangement comprising a measuring tube (2) and a coil arrangement (3, 4) for producing a magnetic field perpendicular to the direction of flow through the measuring tube, said coil arrangement periodically changing the current direction. When the current direction is changed, at least part of the inductive power previously present in the coil arrangement is temporarily stored in a capacitor (27). An evaluation device (28) determines at least one parameter which depends on the temporarily stored power and compares said parameter with a reference value. If there are inadmissibly high variations, a defect of the flow meter is detected. Monitoring is thus rendered especially easy and reliable.

Description

description

Method for checking an electromagnetic flowmeter and electromagnetic flowmeter assembly

The invention relates to a method for checking an electromagnetic flow meter according to the preamble of claim 1 and to an electromagnetic flow measuring arrangement according to the preamble of claim 4.

A method and a flow measuring arrangement of this type are already known from DE 199 17 268 B4. To check the flow meter, the rise time of the current in the coil arrangement is determined and compared with a reference value. As a result, in particular the magnetic properties of the coil arrangement are monitored. The increase in current is referred to as a "fingerprint" for the particular flowmeter, and as long as the flowmeter is undisturbed, that is to say faultless, the progressions are virtually identical with a very narrow spread, and only when an electrical or magnetic fault occurs This will indicate that the flowmeter may be providing inaccurate readings and need to be checked or replaced. "The well-known flowmeter monitoring has the advantage of checking both the electrical properties and the magnetic properties, because the current increase is affected by both electrical and magnetic influences, which is checked while measuring a flow, so you do not even have to interrupt the flow measurement and still be able to run virtually or per manent to make a check. This also has the advantage that the flow meter is checked exactly in the state in which it works. In this case, the reference value is preferably determined at the flow meter itself at an earlier time. One determines the desired parameter, for example, during startup and sets this as a reference so that it is available for future review. This gives each flow meter an individual reference value, so that the check can be very accurate. Errors that can arise due to a faulty reference value, are practically not available.

To determine the rise time as a parameter, two alternatives are given. The first one measures the time that elapses between two predetermined current values. In the second alternative, the time period that elapses between the switching of the current direction and the reaching of a predetermined current value is measured as a parameter. Disadvantageously, a time measurement is required in both alternatives, which is always associated with a certain effort.

From the above-mentioned DE 199 17 268 B4 is also known, after switching the current direction to use an increased voltage, a so-called boost voltage, which accelerates the structure of the magnetic field. This boost voltage changes the current increase and thus the measured parameter. It can thus also errors of the circuit for generating the boost voltage can be detected. On the other hand, faults in this circuit disadvantageously already lead to a response of the monitoring method, although such errors do not always have to adversely affect the measurement result of the electromagnetic flow meter.

For further details of the known method for checking an electromagnetic flow meter and the known flow measuring arrangement, reference is made to the above-mentioned DE 199 17 268 B4.

The invention has for its object to allow even easier way to check the flow meter, without having to accept sacrifices in terms of reliability of the verification process. To solve this problem, the new method of the type mentioned in the characterizing part of claim 1 features. In the further claims advantageous embodiments of the method and a new flow measuring arrangement are given.

Advantageously, the above-mentioned disadvantages of the known method for checking an electromagnetic flow meter and the known flow measuring arrangement are largely avoided by the invention. The advantages of the known method with verification based on the rise time apply in a corresponding manner to the new method. In particular, the invention has the advantage that it is possible to dispense with a time measurement and thus reduces the expense of detecting a parameter which also changes markedly when the electrical or magnetic properties of the coil arrangement change. A parameter which depends on the energy stored in a capacitor can be measured particularly easily in electronic circuits, for example if the voltage applied to the capacitor is detected and evaluated for this purpose.

In a particularly advantageous embodiment of the invention, a bridge circuit with electronic switches is used for periodically changing the current direction, which are each provided with a freewheeling diode. When the capacitor is connected to the feed point of the bridge circuit, the inductive energy present in the coil arrangement before a change in the current direction, apart from ohmic losses, is largely completely transferred when the electronic switches are switched off into capacitive energy which is temporarily stored in the capacitor. Thereafter, when the two other electronic switches of the bridge circuit are opened to change the current direction, which were previously closed, the energy stored in the capacitor acts advantageously as a quasi-boost voltage source for a faster current increase in the new current direction. The capacitive energy is converted relatively quickly back into inductive energy in the coil assembly. This type of interconnection of a capacitor is thus characterized by a particularly low energy consumption in the changes of the current direction in the coil assembly.

If the peak value of the capacitor voltage is used as a parameter which depends on the energy stored in the capacitor, this has the advantage that a circuit-technically particularly simple peak detector can be used to detect the parameter and the evaluation can be carried out completely without time measurements , On the contrary, a simple comparison of the level of the peak value and the reference value suffices in the evaluation. If the comparison yields deviations which exceed a predefined threshold value, then changes to the electrical and / or magnetic properties of the coil arrangement can be inferred, which are inadmissible and can lead to measurement errors.

With reference to the drawings, in which an embodiment of the invention is shown, the invention and refinements and advantages are explained in more detail below.

Show it:

Figure 1 is a schematic diagram of a flow measuring device and

Figure 2 shows a time course of the coil current and a time course of the capacitor voltage.

FIG. 1 shows parts of an electromagnetic flow meter, which may be of importance for explaining the invention. A measuring tube 2 is traversed perpendicular to the plane of a medium whose flow velocity is to be measured. Perpendicular to the flow direction is one Coil arrangement of two coils 3, 4 are arranged, which generate a magnetic field perpendicular to the flow direction when the coils 3, 4 are traversed by a current I. In the measuring tube 2 measuring electrodes 5, 6 and ground electrodes 7, 8 are provided. The measuring electrodes 5, 6 are arranged so that they detect a potential difference or voltage perpendicular to the flow direction and perpendicular to the magnetic field. In a known manner, the voltage between the electrodes 5, 6 increases with increasing velocity of the flowing medium in the measuring tube 2 and with increasing strength of the magnetic field. The measuring electrodes 5, 6 are connected to a differential amplifier, which is followed by an analog / digital converter. Differential amplifier and converter are not shown in the figure for clarity. The digital values obtained by the analog / digital converter correspond to the flow rate and are output as measured values by the electromagnetic flow meter, for example via a field bus to a control station in an automation system.

The coils 3, 4 are connected in series and are fed by an operating voltage 9, to which a diode can be connected in series, so that no currents flow back into the operating voltage source. The direction of the coil current I is determined by an H-bridge circuit with four electronic switches 10 to 13, each switch being protected by a freewheeling diode 14 to 17. If the current I in the direction indicated by an arrow to flow through the coil assembly 3, 4, then the switches 10 and 13 are closed. The switches 11 and 12 are open. When the current direction is to be reversed, the switches 11 and 12 are closed while the switches 10 and 13 are opened. The timing of the opening and closing operations ensures that at no time more than two electronic switches are closed, so that a short circuit of the operating voltage is avoided. The coil current I is set in the measuring phases by a controller 25 by means of an adjustable current source 18 and a measuring resistor 19 to a constant value.

At the feed point of the bridge circuit, a capacitor 27 is connected, whose voltage is indicated by U. This capacitor 27 serves to temporarily store the energy which is present as inductive energy before switching operations due to the current I flowing through the coil arrangement 3, 4. The energy E can be calculated according to the formula

E = 1/2 • L • I ² ,

where L is the inductance of the coil assembly 3, 4.

To reverse the current direction, all four switches 10 to 13 are initially opened. The inductance of the coil arrangement 3, 4 prevents a jump-shaped change of the current I and this first continues to flow via the freewheeling diodes 14 to 17 until the inductive energy is converted into capacitive energy on the capacitor 27. The resulting peak of the voltage U is determined in an evaluation device 28 by a peak value detector and compared with a reference voltage. For example, if the peak deviates by more than 2% from the reference voltage, a signal is output on a line 29 to indicate an error.

The capacitive energy cached during the switching operation in the capacitor 27 is used in an advantageous manner after closing the switch required for the new current direction, for example, the switches 11 and 12, for rapid construction of the desired current I, since now the temporarily stored capacitive energy back into a inductive energy in the coil assembly 3, 4 is transferred. A separate circuit for generating a boost voltage, the bis- Therefore, the faster the coil current was used, therefore it may possibly be omitted.

To illustrate the switching operations, a time characteristic 30 of the coil current I is shown in FIG. 2 at the top, and a time profile 31 of the capacitor voltage U is shown below in FIG. 2, which have been obtained by simulation of circuit components. The time t is plotted on the abscissa with a value range of 2.5s to 3.4s. Switches occur at times t = 2.57s, 2.89s, and 3.21s, respectively. The magnitude of the current I is 60 mA in both directions, that is, in one direction of the current I = 60 mA, in the other direction I = -60 mA. At constant current, the operating voltage U which can be tapped off at the capacitor 27 is approximately U = 14.6 V. During the switching operations, the voltage U briefly rises to approximately the value U = 21 V due to the above-described intermediate storage of the inductive energy previously present in the coil arrangement , The respective peak values that occur during the individual switching processes are detected by the evaluation device 28 (FIG. 1) and compared with a reference value predetermined, for example, during startup. This is, for example, 21 V. Occur between peak value and reference value deviations greater than, for example, 0.2 V, an error message is generated and output because the coil assembly changes in their electrical or magnetic properties have occurred, the accuracy of the measurement of a Affect flow measurement.

Claims

claims 1. A method for checking an electromagnetic flow meter with a measuring tube (2) and a coil assembly (3, 4) for generating a magnetic field perpendicular to the flow direction through the measuring tube, in which the current direction is changed periodically, characterized in that when a change in the current direction at least a part of the inductive energy previously present in the coil arrangement is buffered in a capacitor (27) and that at least one parameter, which is dependent on the buffered energy, is determined and compared with a reference value. 2. The method according to claim 1, characterized in that for periodically changing the current direction, a bridge circuit with electronic switches (10 ... 13) is used, each with a freewheeling diode (14 ... 17) are provided, and that the capacitor (27) is connected to the feed point of the bridge circuit. 3. The method according to claim 1 or 2, characterized in that as parameters of the peak value of the capacitor voltage (U) is used. 4. Electromagnetic flow measuring arrangement with a measuring tube (2) of a coil arrangement (3, 4) for generating a magnetic field substantially perpendicular to the flow direction through the measuring tube, an electrode assembly (5, 6) substantially perpendicular to the flow direction and the magnetic field, a supply device for the coil arrangement, which has a current direction switching arrangement (10 ... 13), and a checking device (27, 28), characterized in that the checking device has a capacitor (27), such that when the current direction changes at least a part of the previously present in the coil arrangement inductive energy in the capacitor is cached and that an evaluation device (28) vorge see, which is adapted to determine at least one parameter, which is dependent on the cached energy, and to compare with a reference value.