The invention relates to a magneto-inductive flow measuring device for measuring the flow of a flowable medium as well as to a method for determining flow in a measuring tube.
In process automation technology, field devices are often applied, which serve for registering and/or influencing process variables. Examples of such field devices are fill level measuring devices, mass flow measuring devices, pressure- and temperature measuring devices, etc., which, as sensors, register the corresponding process variables, fill level, flow, pressure, and temperature, respectively.
A large number of such field devices are manufactured and sold by the firm, Endress+Hauser.
Especially for measuring flow through a measuring tube, a large number of different measuring principles are applied. An important measuring principle is magneto-inductive flow measurement. Magneto-inductive flow measuring devices utilize the principle of electrodynamic induction for volumetric flow measurement. Charge carriers of the medium moved perpendicularly to a magnetic field induce a measurement voltage in measuring electrodes arranged essentially perpendicularly to the flow direction of the medium and perpendicularly to the direction of the magnetic field. The measurement voltage induced in the measuring electrodes is proportional to the flow velocity of the medium averaged over the cross section of the measuring tube, thus proportional to the volume flow.
For the performing the measuring, as a rule, an alternating magnetic field is applied, which is produced by means of a coil system. The electrical current consumption of the magneto-inductive flow measuring device, is, in such case, principally caused by the electrical current draw of the coils for producing the alternating magnetic field.
It is an object of the invention to provide a magneto-inductive flow measuring device, which has a lessened electrical current consumption.
This object is achieved by the features set forth in claims 1 and 17.
Advantageous further developments of the invention are given in the dependent claims.
A magneto-inductive flow measuring device corresponding to forms of embodiment of the invention serves for measuring flow of a flowable medium.
The flow measuring device includes a measuring tube, a pair of coils, which are arranged opposite one another on the measuring tube and which are designed to produce an alternating magnetic field, which can be turned on and off, and which extends essentially transversely to the longitudinal axis of the measuring tube, as well as a pair of permanent magnets, which are arranged opposite one another on the measuring tube and which are designed to produce a permanent magnetic field, which extends essentially transversely to the longitudinal axis of the measuring tube. Moreover, the flow measuring device includes one or more pairs of measuring electrodes arranged opposite one another on the measuring tube, of which one pair of measuring electrodes is designed, in the case of turned off alternating magnetic field, to tap a measurement voltage induced by the permanent magnetic field, and an evaluation unit, which is designed, in the case of turned off alternating magnetic field, to monitor the measurement voltage induced by the permanent magnetic field, at least in the case of a predefined change of the measurement voltage, to turn the alternating magnetic field on and, by means of the alternating magnetic field, to determine a measured value for the flow.
The flow measuring device corresponding to forms of embodiment of the present invention includes a pair of permanent magnets, which are arranged opposite one another on the measuring tube. These permanent magnets are designed to produce a permanent magnetic field throughout the cross section of the measuring tube. When the medium flows with a certain flow velocity through the measuring tube, also in the case of turned off alternating magnetic field, this permanent magnetic field induces a measurement voltage in the direction perpendicular to the magnetic field. This induced measurement voltage depends on the flow velocity of the medium, so that, based on the measurement voltage induced by the permanent magnet field, flow as a function of time can be followed. Thus, even in the case of turned off alternating magnetic field, it is possible to detect changes in the flow.
When, in this way, a change in the flow is detected, the alternating magnetic field can be turned on for a certain time span for performing an exact flow measurement. With the help of the alternating magnetic field, then an exact flow measurement is performed. The alternating magnetic field is only turned on, when, as a result of a change in the flow, a new measured value for the flow is required. Otherwise, the alternating magnetic field remains turned off.
The alternating magnetic field is thus not continually turned on, but, instead, only from time to time. So long as the alternating magnetic field is turned off, only very little electrical current is consumed. Averaged over time, this significantly lessens the electrical current draw of the flow measuring device, and, averaged over time, the flow measuring device consumes significantly less power. Nevertheless, a sufficiently exact monitoring of the flow can be assured.
The invention will now be explained in greater detail based on examples of embodiments illustrated in the drawing, the figures of which show as follows:
Magneto-inductive flow measuring devices utilize the principle of electrodynamic induction for volumetric flow measurement and are known from a large number of publications. Charge carriers of the medium moved perpendicularly to a magnetic field induce a measurement voltage in measuring electrodes arranged essentially perpendicularly to the flow direction of the medium and perpendicularly to the direction of the magnetic field. The measurement voltage induced in the measuring electrodes is proportional to the flow velocity of the medium averaged over the cross section of the measuring tube, and is thus proportional to the volume flow. If the density of the medium is known, the mass flow in the pipeline, or in the measuring tube, as the case may be, can be determined. The measurement voltage is usually tapped via a measuring electrode pair, which is arranged relative to the coordinate along the measuring tube axis in the region of maximum magnetic field strength and where, thus, the maximum measurement voltage is to be expected. The electrodes are usually galvanically coupled with the medium; however, also magneto-inductive flow measuring devices with contactless, capacitively coupling electrodes are known.
The measuring tube can be manufactured, in such case, either from an electrically conductive, non-magnetic material, e.g. stainless steel, or from an electrically insulating material. If the measuring tube is manufactured from an electrically conductive material, then it must be lined with a liner of an electrically insulating material in the region coming in contact with the medium. The liner is composed, depending on temperature and medium, for example, of a thermoplastic, a thermosetting or an elastomeric, synthetic material. Known, however, are also magneto-inductive flow measuring devices with a ceramic lining.
An electrode can be subdivided essentially into an electrode head, which comes at least partially in contact with a medium, which flows through the measuring tube, and an electrode shaft, which is contained almost completely in the wall of the measuring tube.
The electrodes are, besides the magnet system, central components of a magneto-inductive flow measuring device. In the case of the embodiment and arrangement of the electrodes, it is desirable that they can be assembled as simply as possible into the measuring tube and that subsequently in measurement operation no sealing problems occur; moreover, the electrodes should be distinguished by a sensitive and simultaneously low-disturbance measurement signal registration.
Besides the measuring electrodes, which serve for tapping a measurement signal, often additional electrodes are installed in the measuring tube in the form of reference- or grounding electrodes, which serve to measure an electrical reference potential or to detect a partially filled measuring tube or to register the temperature of the medium by means of an installed temperature detector.
As a rule, the magnet system of a magneto-inductive flow measuring device includes a coil pair, which is designed to produce an alternating magnetic field, which extends through the total cross section of the measuring tube. For producing the alternating magnetic field, the coils are fed by a clocked, direct current, which changes direction, for example, with a frequency of 8 Hz, or 16 Hz.
The continuous electrical current flow through the coil pair of the magnet system leads to an accordingly high power consumption in the case of magneto-inductive flow measuring devices. In such case, the power consumption depends especially on the tube cross section, wherein, in the case of greater tube cross sections, a higher power is required for producing the alternating magnetic field than in the case of lesser tube cross sections. In general, there is in the case of magneto-inductive flow measuring devices a need to lessen the electrical current draw. Especially in the case of two-conductor-field devices and in the case of battery operated field devices, a lessening of the electrical current draw would be of interest.
In the case of two-conductor field devices, both the power supply as well as also the measured value transmission occur via one pair of connection lines. Since in the case of many two-conductor field devices the measured values are transmitted in the form electrical current values, the field device must frequently operate for longer time periods with a comparatively low electrical current.
In the case of battery operated field devices, the supply of the field device occurs via an internal battery. Battery operated field devices are frequently used in poorly accessible locations and are, as a rule, not connected to a fieldbus. In order to enable a longer battery service life, also in this case a lessening of the power consumption of the flow measuring device would be desirable.
For lessening the electrical current draw of magneto-inductive flow measuring devices, it is proposed to utilize for monitoring the flow a permanent magnetic field produced by permanent magnet, in which case no electrical current is consumed for producing the field, and to turn the coil system of the flow measuring device responsible for the actual electrical current draw on only from time to time.
The direction of the alternating magnetic field produced by the coils 101, 103 is shown in
In the case of the flow measuring device shown in
The measurement voltage induced by the permanent magnetic field UE is influenced by electrochemical potential influences and is, consequently, not suited for an exact determination of the absolute flow value. However, the measurement voltage UE induced by the two permanent magnet 109, 110 is quite well suited for monitoring flow is a function of time and for detecting significant changes of the flow. When such a significant change of the flow is detected, the alternating magnetic field produced by the coils 101 and 103 is turned on for a certain time span, in order to perform an exact measuring of the changed flow.
In contrast with solutions of the state of the art, the coils 101, 103 are thus not continually turned on, but, instead, are only activated from time to time, for example, when, as a result of a flow change, a new determination of the flow is required. The coils 101, 103 are thus only turned on during certain time spans. In this way, the average power consumption of the magneto-inductive flow measuring device can be significantly decreased. In this way, magneto-inductive flow measuring devices can be built, which have a significantly lessened electrical current requirement.
In contrast to
In the case of the measuring illustrated in
Such a measurement procedure is shown in
The filter 302 serves mainly for filtering out disturbance signals. The filtered signal 303 is fed to a differentiator 304, which forms the derivative of the filtered signal 303. Based on the derivative, it can be detected how strongly the filtered signal 303 changes per unit time. The derivative signal 305 is fed to a comparator 30, where it is compared with a limit value 307, which is provided by a reference value unit 308. So long as the derivative signal 305 lies below the limit value 307, no new flow measurement is initiated, and the magneto-inductive measuring system 310 remains turned off. As soon, however, as the derivative signal 305 exceeds the limit value 307, the comparator 306 produces a switch-on signal 309 (a so-called “wake-up signal”), which turns the magneto-inductive measuring system 310 on for producing the alternating magnetic field and a new flow measurement is initiated using the alternating magnetic field.
The limit value 307 is provided by the reference value unit 308. In such case, it is advantageous that the limit value 307 produced by the reference value unit 308 be adapted dynamically as a function of the required accuracy of the flow measurement. Conforming the limit value establishes how frequently a new determination of the flow value is performed. When a comparatively high limit value is set, an exact measuring the flow is initiated only in the case of relatively strong changes of the flow, and the measurements then occur relatively infrequently. When the limit value is, in contrast, selected relatively low, then the limit value is frequently exceeded, and, accordingly, an exact measuring of the flow value is initiated frequently. By adjusting the limit value, the accuracy, with which the flow is tracked, can be established dynamically.
In such case, it can be provided that the limit value 307 of the reference value unit 308 is set from the magneto-inductive measuring system 310 with the assistance of a control signal 311. When the flow measurements occur too frequently, the limit value 307 is increased by the magneto-inductive measuring system 310. When the flow measurements occur too infrequently, the limit value 307 is reduced.
As a result of the alternating magnetic field and the movement of the charge carriers of the flowing medium, a measurement voltage UE is induced in the direction transverse to the measuring tube 100 and can be tapped on the two measuring electrodes 107, 108. This measurement voltage UE is plotted in
ΔUE=k·B·D·v,
wherein k is a proportionality constant, D his the diameter of the measuring tube, v is the flow velocity of the medium and B the magnitude of the alternating magnetic field.
Through use of the alternating magnetic field, thus, the influence of the voltage offset 401 can be eliminated. The voltage offset 401 depends decisively on the electrochemical potential of the two measuring electrodes 107, 108, which can change in the course of time and is subject to a permanent drift. Moreover, the voltage offset 401 is also a result of the induced voltage contribution brought about by the permanent magnetic field produced by the two permanent magnets 109, 110. The magnetic field component produced by the two permanent magnets 109, 110 does not disturb the exact determining of flow velocity v and of the flow, because this permanent magnetic field contributes only to the voltage offset 401, which is, in any event, eliminated by the difference forming. Thus, an exact determining of flow velocity v can be performed by measuring with the alternating magnetic field. In this way, the flow can be determined with high accuracy.
In the case of the flow measuring device shown in
Additionally arranged on the measuring tube 500 at mutually opposite positions are the two permanent magnets 507, 508, which produce in the cross section of the measuring tube 500 a permanent magnetic field, whose direction is illustrated by the arrow 509. The axis 510 fixed by the two permanent magnets 507, 508 is oriented offset by an angle α from the axis 506 fixed by the coils 501, 502. The angle α should not be selected too small, because the pole shoes arranged outside the coils 501, 502 take up a certain space. For example, the angle α could be selected to equal 45°.
In contrast to the solution shown in
The two permanent magnets 516, 517 are arranged in
In the case of the previously shown solutions in
The second cross sectional plane 609 is arranged a certain distance 610 from the first cross sectional plane 608. The axis 611 fixed by the two permanent magnets 606, 607 can be oriented at any angle α relative to the axis 603. The voltage UE2 tappable on the two measuring electrodes 612, 613 enables a continuous monitoring of the flow through the measuring tube 601. Only in the case of significant changes of the flow, or supplementally also in regular time intervals, is an exact measuring of the flow using the alternating magnetic field initiated.
Number | Date | Country | Kind |
---|---|---|---|
10 2015 103 580.4 | Mar 2015 | DE | national |
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/EP2016/053094 | 2/15/2016 | WO | 00 |