WO1989004463A1

WO1989004463A1 - Process and device for detecting errors in measurements of mass flow of materials

Info

Publication number: WO1989004463A1
Application number: PCT/DE1988/000687
Authority: WO
Inventors: Michael Lang
Original assignee: Flowtec Ag
Priority date: 1987-11-09
Filing date: 1988-11-07
Publication date: 1989-05-18
Also published as: DE3738018A1; DE3738018C2

Abstract

A device for measuring the mass flow of materials according to the Coriolis principle comprises a throughflow device through which the fluid flows and whose throughflow is to be measured, an exciter device which imparts an oscillating movement perpendicular to the direction of flow to the throughflow device, and sensors, arranged at at least three measuring points along the flow device and which emits signals characteristic of the phase relationship of the oscillating movements of the throughflow device at each measurement. An evaluation unit receives the sensor signals from which it derives a phase shift generated by the Coriolis forces which is an index of the throughflow. In order to detect errors in the mass flow measurements, the evaluation unit derives the phase differences from the sensor signals from two measurement points, compares the phase differences so obtained, and indicates an error when differences are detected.

Description

- Λ -

Method and device for detecting errors in mass flow measurement

The invention relates to a method for the detection of errors in the mass flow measurement by means of a phase shift of oscillating and / or rotary movements generated by Corioiis forces into which a flow device through which fluid flows is placed, and a device for carrying out the method.

The use of Coriolis forces for mass flow measurement is known, for example, from JP-OS 57-137818, DE-PS 35 05 166 and EP-OS 0 ~ Ϊ09 218. According to this principle, a flow device, for example a measuring tube system, which consists of one, two or even more measuring tubes with curved and / or straight sections, is excited to produce torsional or bending vibrations. If a fluid flows through the measuring tube system, coriotic forces arise due to its mass and flow velocity, which act on the inner wall of the moving measuring tube perpendicular to the flow direction. attack. The magnitude and phase curve of the Corioiis forces over the length of the measuring tube system through which flow flows is ideally point-symmetrical with respect to the center of the measuring tube system, and thus corresponds to an odd function. As a result, the movements of the measuring tube system are out of phase with one another over the length of the system through which they flow. The ideal course of this phase shift is quasi-linear in a wide range.

In practice, the course of the phase shift can deviate from the ideal course caused exclusively by Corioiis forces. Some reasons can be:

Changes in the properties of the fluid, e.g. Density and viscosity, in connection with manufacturing-related tolerances;

- Changes in the parameters of the vibrating system, e.g. through abrasion, corrosion and deposits;

- Steam effects in the vicinity of the clamping points of the measuring tubes;

- incompletely filled pipes.

This leads to so-called zero point errors, i.e. a phase shift is detected that is not caused by the Corioiis forces. In the case of mass flow meters in which phase shifts are evaluated, this leads to falsifications of the measurement result and thus to increased measurement inaccuracy, without there being any indication of reduced measurement accuracy.

The object of the invention is to create a method with which the presence of measurement errors which are caused by deviations from the ideal course of the phase shifts can be recognized and displayed. In a method of the type specified at the outset, this object is achieved according to the invention in that at least two phase differences between the oscillating and / or rotating movements are determined at at least three measuring points which are arranged at a distance from one another along the flow device in the direction of flow and at which the phase position of the oscillating and / or rotating movements is measured, and that the determined phase differences are compared for deviation from one another for error detection.

The method according to the invention is used to check whether the real course of the phase shift deviates from the known ideal course. Such a deviation manifests itself in a non-linearity and asymmetry, which has the consequence that the phase shifts between different pairs of measuring points differ from one another. If a deviation is found which exceeds a predetermined threshold value, it can be indicated that the measurement result has been falsified, so that a suitable zero point correction can be carried out.

The method according to the invention can be implemented simply and inexpensively. Although at least three measuring points are required, which are arranged at different locations along the flow device, the two measuring points which are present anyway for determining the mass flow can be used for two of them. If a higher level of certainty is to be achieved in the measurement of the course of the phase shift, four or even more measuring points can also be arranged accordingly. The evaluation of the sensor signals for the error detection largely takes place according to the same principle as the usual evaluation of the sensor signals for the mass flow measurement, so that the evaluation unit which is present for the mass flow measurement with minor changes can also be used for the error detection . The determination of the phase differences can expediently be achieved by measuring the time differences between the times at which the flow device reaches or traverses corresponding points in space during its oscillating and / or rotating movements at the measuring points. Suitable points in space are in particular the axis of the flow device in its rest position, the crossing of this axis corresponding to the zero crossing of the oscillating and / or rotating movement, or the points of the greatest deflection which correspond to the maximum of the oscillating and / or rotating correspond to movements.

Within the length of the flow device used for the flow measurement, a certain reference point is usually specified for measuring reasons, which then corresponds to the zero point of the phase shift. If the measuring points are arranged symmetrically with respect to this reference or zero point and also at the same distance from one another, then the comparisons for determining deviations can be carried out particularly easily, since only one corresponding to the length distances is required in the arithmetic operations required for this Constant to be included. The comparisons are further simplified if a measuring point is itself arranged in the reference or zero point.

A device for carrying out the error detection method with a flow device through which the fluid is flowing, the flow of which is to be measured, an excitation device which gives the flow device an oscillating and / or rotary movement transversely to the flow direction, with sensors which act on Measuring points distributed along the flow device are arranged and provide sensor signals which are characteristic of the phase position of the oscillating and / or rotating movements of the flow device at the respective measuring point, and with an evaluation unit which receives the sensor signals and from them a phase shift generated by Corioiis forces as a measure of the flow is determined according to the invention in that at least three measuring points are arranged along the flow device and that the evaluation unit is designed such that it determines the phase differences from the sensor signals originating from two measuring points each Phase differences are compared with one another and an error detection is displayed when a deviation is ascertained.

Further features and advantages of the invention can be seen from the following description of exemplary embodiments with reference to the drawing. The drawing shows:

1 shows a basic arrangement for a mass flow meter according to the invention,

2 shows the ideal and the incorrect course of the phase shift over the length of the measuring tube in the arrangement of FIG. 1,

Fig. 3 shows another basic arrangement of a mass flow meter according to the invention and

4 shows the ideal and the incorrect course of the phase shift over the length of the measuring tube in the arrangement of FIG. 1.

The flow measuring arrangement shown in FIG. 1 contains a flow device 1 which is inserted into a fluid line through which the fluid flows, the flow of which is to be measured. In the example shown, the flow device 1 is designed as a straight measuring tube, but it can in principle have any shape, for example u-shape, loop shape or the like. The flow device 1 is firmly clamped at its input-side end 2 and at its output-side end 3. A vibration exciter 4 is preferably arranged in the middle of the flow device 1. For example, it can have a fixedly mounted drive coil 5 and a permanent magnet 6 connected to the flow device 1. If an alternating current flows through the drive coil 5, a corresponding alternating magnetic field is generated, by means of which the permanent magnet 6 is alternately attracted and repelled. As a result, the flow device 1 is set into bending vibrations which are symmetrical to the center of the flow device. The bending vibration line is indicated in FIG. 1 by five arrows of different lengths running perpendicular to the longitudinal axis of the flow-through device 1. The frequency • of the AC voltage with which the bending vibrations are excited preferably corresponds to the resonance frequency of the flow device 1; for example, it can be in the range between 600 and 1000 Hz.

If a fluid flows through the flow device 1 in the direction of the arrows F, the mass and the flow velocity of the fluid result in coriotic forces which distort the bending vibrations in phases. This results in phase shifts between the oscillating movements along the flow device 1. The phase shifts are greater the larger the coriotic forces acting on the flow device 1. Since the Corioiis¬ forces depend on the mass and the flow rate of the fluid, the phase shifts are a measure of the mass flow of the fluid.

In order to determine the phase shift characteristic of the mass flow, two sensors 7 and 8 are arranged at two measuring points M ₁ and M, which are on both sides of the vibration exciter 4 and at the same distance from it, which detect the oscillating movement of the flow device 1 in convert electrical sensor signals that indicate the phase position of the oscillating movement at the sensor location. In which The exemplary embodiment shown will use magnetic-inductive sensors, each of which has a permanent magnet 7a or 8a attached to the flow-through device 1 and a fixed induction coil 7b or 8b. When the flow device 1 executes bending vibrations, the permanent magnet of each sensor moves relative to the induction coil, whereby an alternating voltage is induced in the induction coil, the phase position of which is in a fixed and known relationship to the phase position of the oscillating movement.

In the middle between the two measuring points M ₁ and M_, ie at the location of the vibration exciter 4, a further vibration sensor 9 with a permanent magnet 9a and an induction coil 9b is arranged at a third measuring point M _Q.

Instead of the magnetic-inductive sensors shown as an example, other sensors of a known type can of course also be used, which are capable of converting mechanical oscillating movements into electrical sensor signals, which allow the phase position of the oscillating movement to be recognized at the location of the sensor. Preferably, sensors are used which work without contact, so that they do not influence the mechanical oscillation movement in any way. In addition to the magmetic-inductive sensors shown, optical or capacitive sensors are particularly suitable for this.

The electrical sensor signals supplied by the sensors 7, 8 and 9 are fed to an electronic evaluation unit 10. The electronic evaluation unit is designed such that it determines the phase shift between the sensor signals of the two outer sensors 7 and 8 as a measure of the mass flow of the fluid flowing through the flow device 1. This corresponds to the normal functioning of Coriolis force mass flow meters of this type. As is customary with such mass flow meters, the evaluation unit 10 can be formed, for example, by a suitably programmed microcomputer, the analog-digital Converters are connected upstream which convert the sensor signals into digital signals which are suitable for processing by the microcomputer. The evaluation unit 10 emits a measured value signal S 1 at an output, which represents the determined phase shift or the mass flow rate corresponding to this phase shift.

The diagram of FIG. 2 serves to illustrate the course of the phase shift over the length of the flow device 1 between the two clamping points 2 and 3. The length L of the flow device 1 is plotted as the abscissa and the phase shifts, the oscillating movements, are plotted as the ordinate have at each point of the flow device 1 with respect to the oscillating movement at the point of the excitation device 4 selected as the reference point or zero point. In the usual way, the phase shifts are represented by the time differences .DELTA.t that exist between the times at which the flow device 1 reaches or crosses corresponding points in space during its oscillating movements. Particularly suitable points in space for the determination of the time differences are the zero crossings of the oscillating movement, at which the flow device crosses the axis corresponding to the rest position. Other suitable points are the maxima of the oscillating movement, at which the flow device 1 reaches the points of large deflection.

Since there is a clear relationship between the measured time differences .DELTA.t and the phase shift angles of the oscillating movement at a known frequency of the oscillating movement, the phase shifts can be specified without distinction in terms of time or in terms of angle. Because of the measurement principle used, the measure of time is used in the following description.

If the flow velocity of the fluid in the flow-through device 1 is zero, no coriois forces arise and consequently no phase shifts along the flow device. The curve representing the phase shift is then a horizontal straight line which coincides with the axis of the abscissa.

If, on the other hand, the fluid flows through the flow-through device 1 at a speed other than zero, coriotic forces arise which depend both on the mass and on the flow velocity of the fluid. These Corioiis forces cause phase shifts of the oscillating movements along the flow device 1 in such a way that a phase advance at the points lying in the flow direction before the reference point and at the points lying behind the reference point in the flow direction cause a phase lag with respect to the phase position the oscillation movement exists at the reference point. In the diagram of FIG. 2, the phase advances correspond to negative values and the phase advances correspond to positive values of the time differences Δt. At bending vibration frequencies between 600 and 1200 Hz, the magnitude of the time differences is in the range between -1500 ns and +1500 ns.

In the ideal case, ie if the flow device 1 behaves faultlessly, there is a linear course of the phase shifts, so that the curve of the phase shifts is a straight line passing through the zero point, the slope of which is greater, the greater the product of mass and flow velocity, that is the mass flow of the fluid. As an example, such an ideal curve A is plotted in FIG. 2 in the form of a dashed straight line for a specific value of the mass flow.

The evaluation circuit 10 determines the phase difference between the oscillating movements at the measuring points ₁ and M as a measure of the mass flow. In the example shown in FIG. 1 by the line A Ideally, ^'she would one Phasendif¬ conference PA_ 1200 ns identify. In practice, the course of the phase shift can deviate from the linear ideal course caused exclusively by Corioiis forces. There are numerous reasons for this, for example changes in the properties of the fluid, such as density and viscosity in connection with manufacturing-related tolerances, changes in the parameters of the vibrating system, for example due to abrasion, corrosion and deposits, damping effects in the vicinity of the clamping points of the measuring tubes , incompletely filled measuring tubes etc. Such causes falsify the ideal quasilinear and symmetrical course of the phase shift, so that an at least partially non-linear and asymmetrical course of the phase shift arises, as is shown in FIG. 2 (greatly exaggerated) by the drawn out Characteristic curve B is shown. With this course of the phase shifts, the evaluation circuit 10 measures a phase difference P of 900 ns between the oscillating movements at the measuring points M _^ and M. Since it cannot be seen for the evaluation circuit 10 from the sensor signals of the measuring points M ^ and M ₂ that there is a deviation from the linear course, it would assign a value of the mass flow rate to the phase difference P J_D according to conventional measuring methods would correspond to the same phase difference with a linear curve. The measurement result would be falsified without the presence of a measurement error being recognizable.

In the flow measuring arrangement shown in FIG. 1, the presence of a measuring error caused by a nonlinear and asymmetrical course of the phase shift is recognized and displayed with the aid of the third measuring point M _Q arranged in the reference point or zero point. The evaluation unit 10, which also receives the sensor signal from the third measuring point M _Q , is designed such that it uses the sensor signals of the measuring points M ₁ and M _{Q to determine} the phase difference 3? _{1 0} between the oscillating movements at the measuring points M ₁ and M _Q and from the sensor signals of the measuring points M ₂ and M _Q the phase difference P _{2 Q} between the oscillating movements at the measuring points ₂ and M _{Q is} determined. The amounts on this Phase differences P _Q and P _{2 Q} additionally determined in this way are compared in the evaluation unit 10 for a deviation from one another, and if a deviation is ascertained which exceeds a predetermined threshold value K, this shows

Evaluation unit the presence of a measurement error. For this purpose, for example, it can output a binary error signal S "Ύ at a second output, which has the value 0 if the deviation is smaller than or equal to the threshold value K and which takes the value 1 if a deviation is found which is greater than the threshold value K. The determined phase differences P. _ and P ₂ _ are evaluated using the following formula:

The threshold value K is positive and freely selectable. For example, it can be 1 ns.

In the course of the phase shift represented by curve B in FIG. 2, the evaluation unit 10 would, for example, have a phase difference P ₁ _ = -400 ns and a phase difference P_. = +500 ns determine. The deviation between the amounts of these phase differences would be 100 ns, that is to say substantially larger than the threshold value, so that the presence of a measurement error would be indicated.

The determination of the phase differences ^~ ° _{1 Q} and P _{2 Q} in the evaluation unit 10 can of course be carried out in the same way and with the same means as the determination of the phase difference between the sensor signals of the measuring points M ₁ and M ~ which is known to the person skilled in the art with conventional mass flow meters. Likewise, the determination of the deviation between the amounts of the phase differences P ^ ₀ and P_ _Q and the comparison of this deviation with the threshold value K offers the person skilled in the art no difficulty. It is therefore not necessary to set up and work to explain the evaluation unit 10 in more detail.

The flow measuring arrangement shown in FIG. 3 differs from that of FIG. 1 initially in that the flow device 11 has two straight parallel measuring tubes 12 and 13 which are connected to a fluid line by means of an inlet-side distributor piece 14 and an outlet-side distributor piece 15 are inserted that the fluid is divided between the two measuring tubes 12 and 13 and flows through them in parallel. The distributor pieces 14 and 15 are preferably rigid, so that they form fixed clamping points for the ends of the measuring tubes 12 and 13.

In the middle of the flow device 1, in the space between the two measuring tubes 12 and 13, a vibration exciter 16 is attached, which has a fixed drive coil 17 and two permanent magnets 18 and 19 attached to the measuring tubes. If the drive coil 17 is excited by an alternating current, the permanent magnets 18 and 19 are periodically attracted and repelled. As a result, the measuring tubes 12 and 13 are set into opposite-phase bending vibrations.

Another difference from the flow measuring arrangement of FIG. 1 is that in the flow measuring arrangement of FIG. 3 four measuring points M _± , M ₂ , M ~, M. are arranged along the flow device 1. The measuring points M ₁ and M. are at equal distances from the vibration exciter 16, that is to say symmetrically with respect to the vibration exciter, near the ends of the measuring tubes, and the measuring points ₂ and are also at the same distance from the vibration exciter 16, that is to say symmetrically with respect to the vibration exciter to each other, between the outer measuring points ₁ or M. and the vibration exciter. At each measuring point there is a sensor 21, 22, 23 and 24, which converts the oscillating movements of the measuring tubes 12, 13 at the measuring point into electrical sensor signals, which let the phase position of the oscillating movements be recognized. As an example, magnetic Inductive sensors are shown, each of which has a permanent magnet 21a, 22a, 23a and 24a connected to the measuring tube 12 and an induction coil 21b, 22b, 23b and 24b connected to the measuring tube 13. The sensor signals supplied by the sensors are fed to an evaluation unit 25.

When a mass flow flows through the measuring tubes 12 and 13, Corioiis forces arise which result in phase shifts in the mutual oscillating movements along the measuring tubes. The course of the phase shifts is shown in the diagram in FIG. 4 in the same way as in FIG. 2, the location of the vibration exciter 16 again forming the reference point. The dashed line A corresponds to the ideal case of the linear and symmetrical phase shift, while the solid curve B represents the nonlinear and asymmetrical course, the phase shift, which arises from the causes described above.

In the usual way, the evaluation circuit 25 determines the phase difference between the sensor signals of the outer measuring points M ₁ and M. as a measure of the mass flow. In order to determine whether there is a measurement error, the evaluation unit 25 determines in this case a first phase difference P _{3 1} between the oscillating movements at the measuring points M. and M ₃ and a second phase difference P _{4 2} between the oscillating movements the measuring points M ₂ and M ,. The phase differences determined in this way are compared with one another in the manner described above, and the presence of a measurement error is indicated when a deviation is ascertained which exceeds a predetermined threshold value.

Instead of three measuring points, as in FIG. 1, or four measuring points, as in FIG. 2, a larger even or odd number of measuring points can also be provided, from whose sensor signals the phase differences between the oscillating movements at pairs of measuring points are then determined . the phase differences determined are then compared in pairs with one another in order to determine deviations which indicate the presence of a measurement error. The greater the number of measuring points, the greater the resolution with which the faulty curve B can be analyzed.

If any odd number of measuring points is used, one measuring point should preferably be in the reference point, and if any even number of measuring points are used, the measuring points should be arranged in pairs symmetrically to one another with respect to the reference point.

Of course, it is also readily possible to provide an even number of measuring points for a flow device with a single measuring tube, as in FIG. 1, or an odd number of for a flow device with two parallel measuring tubes, as in FIG Provide measuring points.

Claims

Patent claims

1. A method for the detection of errors in the mass flow measurement by means of a phase shift of oscillating and / or rotary movements generated by Corioiis forces, into which a flow device through which fluid flows is displaced, characterized in that at least two phase differences between the oscillations - and / or rotary movements are determined at at least three measuring points, which are arranged along the flow device in the flow direction at a distance from one another and at which the phase position of the oscillating and / or rotating movements is measured, and that the detected phase differences are used for error detection to be compared for deviation.

2. The method according to claim 1, characterized in that an error detection is confirmed when the deviation between two phase differences, which are determined with respect to a reference point along the flow measuring device symmetrically arranged pairs of measuring points, exceeds a predetermined threshold value.

3. The method according to claim 1, characterized in that to determine the phase differences, the Zeitdifferen¬ zen between the times are measured at which the flow device reaches or traverses corresponding points in space at their measuring points in their oscillating and / or rotating movements .

4. Device for carrying out the method according to any one of the preceding claims, with a flow device through which the fluid flows, the flow of which is to be measured, an excitation device which provides the flow device with an oscillating and / or rotary movement transversely to Flow direction issued, with sensors which are arranged at measuring points distributed along the flow device and deliver sensor signals which are characteristic of the phase position of the oscillating and / or rotating movements of the flow device at the respective measuring point, and with an evaluation unit which receives the sensor signals and from this a phase shift generated by Corioiis forces is determined as a measure of the flow, characterized in that at least three measuring points are arranged along the flow device and that the evaluation unit is designed such that it derives the phases from the sensor signals originating from two measuring points determined differences, the determined phase differences compared with each other and an error detection indicates when a deviation is detected.

5. The device according to claim 4, characterized in that the two measuring points of each pair, from whose Sensor¬ signals a phase difference are determined, have equal distances from each other.

6. Apparatus according to claim 4 or 5, characterized gekenn¬ characterized in that the measuring points are arranged symmetrically to each other bezüg¬ Lich a reference point which corresponds to the zero point of the phase shift.

7. The device according to claim 6, characterized in that at an odd number of measuring points, a measuring point is arranged in the reference point.

8. Device according to one of claims 4 to 7, characterized in that the flow device has a measuring tube which is inserted into a fluid line.

9. Device according to one of claims 4 to 7, characterized in that the flow device has a plurality of paral¬ lele measuring tubes which are inserted into a fluid line.

10. The device according to claim 8 or 9, characterized gekenn¬ characterized in that the or each measuring tube has a straight portion at which the measuring points are arranged.