Measuring and testing – Volume or rate of flow – By measuring electrical or magnetic properties
Reexamination Certificate
2003-02-28
2004-11-23
Williams, Hezron (Department: 2856)
Measuring and testing
Volume or rate of flow
By measuring electrical or magnetic properties
Reexamination Certificate
active
06820499
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for determining the uncertainty factor of a measuring procedure employing a measuring frequency, especially a magnetoinductive flow measuring procedure by which in each oscillatory half-cycle of the measuring frequency, a measuring signal is sampled for determining at least one measured value per signal.
The invention is particularly useful for application in a magnetoinductive flow measuring procedure without being limited thereto. Magnetoinductive flow measuring processes of the type mentioned above have been in the public domain for some time and have been widely employed for a variety of applications. The fundamental principle of a magnetoinductive flowmeter for moving media goes all the way back to Faraday who as early as 1832 proposed the use of the electrodynamic induction principle for measuring flow rates. Faraday's law of induction stipulates that in a medium flowing through a magnetic field and containing charge carriers, an electric field intensity is generated perpendicular to the flow direction and to the magnetic field. A magnetoinductive flowmeter utilizes Faraday's law of induction by means of a magnet that typically consists of two magnetic poles, each with a field coil, and generates a magnetic field perpendicular to the direction of the flow in the measuring tube. Within that magnetic field, each volume element of the flowing medium, traveling through the magnetic field and containing a certain number of charge carriers, contributes the field intensity generated in the volume element concerned to the measuring voltage that can be collected via measuring electrodes. In conventional magnetoinductive flowmeters, the measuring electrodes are designed either for conductive or for capacitive coupling with the flowing medium. One salient feature of magnetoinductive flowmeters is the proportionality between the measured voltage and the flow rate of the medium across the diameter of the measuring tube, i.e. between the measuring voltage and the flow volume.
2. Description of the Prior Art
In the magnetoinductive flow measuring process of the actual flow measuring operation, the magnetic field is usually switched in periodically alternating fashion. The prior art shows a variety of approaches to that effect. For example, magnetoinductive flow measurement can be accomplished using an alternating field, in which case the field coils of the magnet typically receive a sinusoidal 50 Hz voltage directly from an AC line source. However, the measuring voltage generated by the flow between the measuring electrodes tends to be heterodyned by transformation noise as well as line voltage interference.
Current magnetoinductive flow measuring practice, therefore, generally employs a switched direct-current field. A switched continuous field of that nature is obtained by feeding a periodically polarity-alternating square-wave current to the field coils of the magnet. Also possible, however, is a magnetoinductive flow measurement process using a pulsating continuous field obtained by only periodically supplying the field coils of the magnet with a time-controlled square-wave current of unchanging polarity. Yet a method that periodically reverses the field current is preferred because alternating the polarity of the magnetic field permits the suppression of interference potentials such as galvanic noise.
Using a pole-reversible, switched constant-current field makes it necessary after each reversal to wait for the magnetic field to stabilize. That is followed by the up-slope integration of the measured voltage, for instance the voltage differential between the electrodes, until the field current polarity is again reversed. Waiting for the magnetic field to stabilize is important for achieving good measuring accuracy.
One very important variable in a magnetoinductive flow measuring procedure as well as in other measuring techniques, where in each half-cycle of the measuring frequency at least one measured value is determined by sampling a measuring signal, is the uncertainty factor of the measuring process since that variable can be utilized for a variety of diagnostic purposes. The uncertainty factor is essentially determined by the irregular divergence pattern of the measured values. This divergence of the measured values is a gauge for their degree of fluctuation and usually represents the standard deviation of the measured values.
The problem with a magnetoinductive flow measuring procedure lies in the fact that the flow rate itself typically tends to be less than constant and should therefore be measured. What this means is that in determining the standard deviation of the measured values at the measuring frequency it is not possible to tell whether that standard deviation is caused by a scattering of the measured values, originating in the electrode voltage differential that would constitute a noise factor, or whether the cause, again detected at the measuring frequency, is the very change in the flow rate that is to be measured.
For verifying the dependability of a flow rate measurement obtained with a magnetoinductive flowmeter, EP 0 521 448 A2 describes an approach whereby the absolute values of the measuring voltages collected at the two measuring electrodes are respectively compared in a half-cycle of the measuring frequency, with any deviation between these absolute values triggering an alarm signal. DE 34 23 076 C2 on its part proposes to quantify alternating and constant-current noise voltage components in a magnetoinductive flow measuring procedure by sampling the measuring voltage at many times the excitation frequency of the magnetic field.
For identifying in a magnetoinductive flow measuring procedure a noise signal superposed over the measuring signal, allowing the measuring signal to be appropriately corrected, DE 199 38 160 A1 proposes a gapped excitation of the field coils that generate the magnetic field, making it possible for each of the measuring periods to include, in addition to the two half-cycles of the magnetic field, a span before the first half-cycle and a span after the last half-cycle of the magnetic field. The additional signals acquired before and, respectively, after the corresponding half-cycle of the magnetic field are then used for the above-mentioned correction and preferably the complete elimination of the noise signal heterodyned over the measuring signal.
Finally, JP 2000-028408 describes a method whereby, for flow signals, noise suppression is achieved by applying the mean value of these flow signals obtained in a sampling process whose frequency is twice the excitation frequency of the magnetic field. This method utilizes the fact that values detected at times in which no magnetic field is present should result in a flow value of zero.
The aforementioned prior-art methods, however, do not offer the possibility of quantifying the irregular divergence of the measured values, for instance their standard deviation, specifically at the measuring frequency.
SUMMARY OF THE INVENTION
It is, therefore, the objective of this invention to introduce a method for determining the uncertainty factor, permitting in simple fashion the quantification of the measured-value irregularities even at the measuring frequency.
The method according to this invention that achieves the stated objective for the application first above described is characterized in that the irregular divergence, i.e. deviation of the measured values, is determined at the measuring frequency but phase-shifted relative to the measuring frequency, and/or that the deviation of the measured values is determined at least at two frequencies that differ from the measuring frequency, and that on the basis of the two deviations at the frequencies that differ from the measuring frequency, the deviation of the measured values at the measuring frequency is interpolated or extrapolated.
The method according to the invention is derived from the discovery, made in experiments without field excitation i.
Cesari and McKenna LLP
Fitzgerald John
Krohne Messtechnik GmbH & Co. K.G.
LandOfFree
Method for determining the uncertainty factor of a measuring... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for determining the uncertainty factor of a measuring..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for determining the uncertainty factor of a measuring... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3350888