Patent Description:
Magnetic flowmeters are useful in a variety of conductive and semi-conductive fluid flow measurement environments. In particular, the flow of water-based fluids, ionic solutions and other conducting fluids can all be measured using magnetic flowmeters. Further, magnetic flowmeters are often used with fluids that may contain solids, such as pulp used in paper processing. Thus, magnetic flowmeters can be found in water treatment facilities, beverage and hygienic food production, chemical processing, high purity pharmaceutical manufacturing, as well as hazardous and corrosive fluid processing facilities. However, some environments are more susceptible to signal noise. Providing a magnetic flowmeter with a better response to signal noise would improve the accuracy of the flow output when used in such noisy environments.

Document <CIT> relates to an electromagnetic flowmeter which has a short circuit switch S1 for measuring conductivity adapted to earth electrodes for inducing a flow rate signal intermittently through a short circuit resistance Rin and an offset compensation circuit to remove a DC component contained in the flow rate signal. An output of the offset compensation circuit immediately after the turning ON or OFF of the short circuit switch S1 is not sampled as flow rate signal. On the other hand, excitation is stopped during a fixed period immediately after the turning ON or OFF of the short circuit switch S1 among ON or OFF periods of the short circuit switch S1. Thus, exciting power will not be consumed during the stoppage of the excitation. In addition, during the stoppage of the excitation, no flow rate signal corresponding to a flow velocity is induced thereby achieving a lowering of power consumption.

According to the invention, the problem is solved by means of a magnetic flowmeter as defined in independent claim <NUM> and a method for operating a magnetic flowmeter according to independent claim <NUM>. Advantageous further developments of the magnetic flowmeter according to the invention are set forth in the dependent claims.

A magnetic flowmeter includes at least one coil configured to generate a magnetic field within a process fluid flow. A pair of electrodes is configured to detect an electromotive force within the process fluid flow in response to the magnetic field. Measurement circuitry is operably coupled to the pair of electrodes and configured to provide an indication of the detected electromotive force. A processor is coupled to the measurement circuitry and is configured to receive the indication of the detected electromotive force and an indication of process noise. The processor is configured to change a dead time parameter based on the indication of process noise and provide a process fluid flow output based on the indication of detected electromotive force and the dead time parameter.

<FIG> illustrates a typical environment <NUM> for magnetic flowmeter <NUM>. Magnetic flowmeter <NUM> is coupled to process piping, illustrated diagrammatically at line <NUM> that also couples to control valve <NUM>. Magnetic flowmeter <NUM> is configured to provide a flow rate output relative to process fluid flow through piping <NUM> in a process. Examples of such fluids include slurries and liquids in chemical, pulp, pharmaceutical and other fluid processing plants.

Magnetic flowmeter <NUM> includes electronics housing <NUM> connected to flowtube <NUM>. Magnetic flowmeter <NUM> outputs are configured for transmission over relatively long distances to a controller or indicator via process communication connection <NUM>. In typical processing plants, communication connection <NUM> can employ a digital communication protocol, an analog communication signal, or a combination thereof. The same or additional process information can be made available via wireless communication, pulse width or frequency output, or discrete input/outputs (DI/DO). System controller <NUM> can display flow information for a human operator as well as provide control signals over process communication connection <NUM> in order to control the process using control valves, such as valve <NUM>.

<FIG> is a block diagram of magnetic flowmeter <NUM> with which embodiments of the present invention are particularly applicable. Magnetic flowmeter <NUM> measures a flow of conductive process fluid through flowtube assembly <NUM>. Coils <NUM> are configured to apply an external magnetic field in the fluid flow in response to an applied excitation current from coil driver <NUM>. EMF sensors (electrodes) <NUM> electrically couple to the fluid flow and provide an EMF signal output <NUM> to amplifier <NUM> related to an EMF generated in the fluid flow due to the applied magnetic field, fluid velocity, and noise. Analog-to-digital converter <NUM> provides a digitized EMF signal to microprocessor system <NUM> of flowmeter electronics <NUM>.

Microprocessor <NUM> may be configured, through hardware, software, or a combination thereof, to provide digital signal processing functions relative to EMF output <NUM> in order to provide an output <NUM> related to fluid velocity. Further, as will be described in greater detail below, the signal processing can provide improved noise rejection. Microprocessor <NUM> may include or be coupled to memory <NUM> that contains instructions that, when executed by microprocessor <NUM>, provide process fluid flow velocity output calculation as well as improved noise reduction in accordance with embodiments described herein.

Microprocessor <NUM> calculates fluid flow velocity through flowtube <NUM> in accordance with a relationship between the EMF output <NUM> and flow velocity as described in an application of Faraday's Law:
<MAT>.

Where E can be the signal output <NUM> which is related to the EMF output <NUM>, V is the velocity of the fluid, D is the diameter of flowtube <NUM>, B is the strength of the induced magnetic field in the process fluid, and k is a constant of proportionality. Microprocessor <NUM> uses velocity and the measured magnetic field or coil current to calculate flow of the process fluid in accordance with known techniques. A digital-to-analog converter <NUM> is coupled to microprocessor <NUM> of flowmeter electronics <NUM> and generates an analog transmitter output <NUM> for coupling to communication bus <NUM>. A digital communication circuit <NUM> may generate a digital transmitter output <NUM>. The analog output <NUM> and/or digital output <NUM> can be coupled to process controllers or monitors, as desired.

Noisy process flow can cause magnetic flowmeter measurement errors that can result in process control difficulties. This is because it is sometimes possible for the noisy flow to generate erroneous process flow outputs, which outputs may appear to a control system as changing flow, which may adjust the process based on the erroneous flow. To solve noise problems, magnetic flowmeters, especially those used in noisy process flow environments, generally employ damping, signal processing, and averaging. These techniques increase output stability but sometimes reduce the responsiveness of the magnetic flowmeter to changes in process fluid flow. In order to address this potential lack of responsiveness, a Dead Time counter is generally employed.

As used herein, Dead Time is a signal processing parameter that, when engaged, causes the flow output to no longer be a function of an average, but to instead follow the flow value more closely. In one example, if a flow output is an average of ten previous flow samples, when a Dead Time engages, the flow output may be an average of <NUM> or <NUM> samples or may even be provided as the flow sample itself.

As can be appreciated, properly setting the Dead Time trigger is very important for balancing accurate flow indication with effective noise rejection. The Dead Time counter diagnostic triggers when it detects a change in flow. Dead Time counters, however, are not immune to noise. It is possible for false positives to occur that can cause the flow to jump to an incorrect flowrate. This change in reported flow may cause the control system to react in an undesired manner. To reduce the possibility of noise-reduced false positives, the flow threshold trigger for the Dead Time is sometimes increased. However, increasing the threshold for the Dead Time trigger reduces responsiveness, which is the main reason for using the Dead Time in the first place.

<FIG> illustrates an example of a highly averaged flow signal along with a damped flow signal. The larger down step shows a flow change where the highly averaged signal <NUM> causes a Dead Time engagement, but not a second engagement due to the threshold being too high. On the larger up step, the highly averaged flow signal <NUM> tracks with the damped flow signal due to the transition being within the Dead Time limit. As can be seen, when the Dead Time works correctly, the highly average signal tracks very closely to the minimally damped signal. In essence, a magnetic flowmeter employing Dead Time has a first mode wherein the process fluid flow output may be significantly averaged, smoothed, or otherwise processed to provide a signal that is robust against process noise. The magnetic flowmeter also has a second mode that decouples the process flow output from the averaged, smoothed or otherwise processed signal to generate a signal that can follow the changing process fluid flow samples more closely. This second mode is the Dead Time mode and it is engaged when the process flow samples exceed a Dead Time threshold for a duration beyond a Dead Time duration.

In accordance with embodiments described herein, an adaptive Dead Time is provided for a magnetic flowmeter in order to reduce the probability of false positives. In one embodiment, the Dead Time calculation may use an interquartile mean (IQM) to reduce large spike noise on the flow signal. An interquartile mean is a calculation where data in the second and third quartiles is used (as in the interquartile range), and the lowest <NUM>% and the highest <NUM>% of the data samples are discarded. Then, either a median absolute deviation (MAD) or a standard deviation is used to configure or otherwise influence the Dead Time threshold. Accordingly, when more signal noise is present, the MAD and the standard deviation increase.

MAD is a mathematically known calculation that is robust against outliers. MAD is generally believed to be more resistant to outliers in a data set than standard deviation. While in standard deviation, the distances from the mean are squared, in median absolute deviation, the deviations of a small number of outliers are irrelevant. MAD is calculated by first computing a median for a group of samples. Then the deviation of each sample from the median is calculated. Finally, the median of the sample deviations is calculated to provide the MAD. As an example, consider the data (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>). It has a median value of <NUM>. The absolute deviations about <NUM> are (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) which in turn have a median value of <NUM> (because the sorted absolute deviations are (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>)). So, the median absolute deviation for this data is <NUM>.

When the noise decreases, so will the MAD and the standard deviation, causing the threshold to decrease as well. This provides a robust Dead Time by increasing the threshold when noise is present and decreasing the threshold when noise is not present. The adaptive Dead Time diagnostic also maximizes responsiveness by reducing the flow threshold when noise is present. While use of MAD and/or standard deviation is preferred, embodiments described herein can be practiced using any suitable indication of process noise. For example, a separate device could be configured to measure the noise and communicate an indication of process noise to the magnetic flowmeter, which then adapts the Dead Time based on the communicated noise. In another example, a Fast Fourier Transform (FFT) could be used to measure the amount of noise on the electrodes in order to configure or otherwise influence the Dead Time threshold.

<FIG> is a flow diagram of a method of dynamically modifying a Dead Time threshold in a magnetic flowmeter in accordance with an embodiment of the present invention. The main addition to a Dead Time counter is using an IQM to compare the filtered output, creating a flow threshold to surpass before counting, and generating a noise deterministic metric to modify the threshold.

Method <NUM> begins at block <NUM>, where an interquartile mean value is generated from raw flow figures from the magnetic flowmeter. These raw flow figures are individual EMF measurements from electrodes <NUM> from the magnetic flowmeter provided by analog-to-digital converter <NUM>. Once the IQM is created, control passes to block <NUM> where a statistical parameter of the raw flow values is generated. The statistical parameter may be, in one example, median absolute deviation (MAD) or standard deviation. Next, at block <NUM>, the Dead Time threshold is updated. At block <NUM>, a difference is calculated between the IQM and the filtered flow output. At block <NUM>, processor <NUM> determines whether the difference calculated at block <NUM> is greater than the Dead Time threshold. If so, then control passes to block <NUM> where the Dead Time counter is incremented. If, however, the difference is not greater than the Dead Time threshold, then control passes to block <NUM> and the method ends. When the Dead Time counter is incremented at block <NUM>, block <NUM> is executed to determine whether the count is greater than a Dead Time limit. If the count is greater than the Dead Time limit, then control passes to block <NUM> where signal processing filters are reset, and the process flow output is set as the new flow rate. Then, control passes to block <NUM> where the method ends.

<FIG> is a graph illustrating Dead Time threshold effects on a flow output signal. <FIG> shows flow when the Dead Time threshold is too low (illustrated at reference numeral <NUM>) and the Dead Time limit triggers incorrectly and large spikes are seen on the flow output. Conversely, <FIG> also shows an example where the Dead Time is set correctly, at signal line <NUM> where the false positives illustrated at reference numeral <NUM>, and <NUM> do not generate the same flow output aberrations.

<FIG> is a flow diagram of a method of dynamically changing Dead Time in a magnetic flowmeter in accordance with an embodiment of the present invention. Method <NUM> may be performed periodically or upon receiving a command to do so. For example, method <NUM> may be performed in response to process communication received by magnetic flowmeter <NUM>. Similarly, method <NUM> may be performed in response to user input.

Method <NUM> begins at block <NUM> where processor <NUM> obtains an indication of process noise. This indication can be from one or more suitable sources. For example, processor <NUM> may calculate a median absolute deviation (MAD) of a set of emf measurements from electrodes <NUM>, as indicated at reference numeral <NUM>. Additionally, or alternatively, processor <NUM> may calculate a standard deviation of a set of emf measurements from electrodes <NUM>, as indicated at reference numeral <NUM>. As indicated at reference numeral <NUM>, the indication of process noise may also be received from an external device. Other suitable techniques for providing processor <NUM> with an indication of process noise can also be used, as indicated at reference numeral <NUM>. While different techniques have been described for generating or obtaining an indication of process noise, it is also expressly contemplated that combinations thereof may also be used.

Claim 1:
A magnetic flowmeter (<NUM>), comprising:
at least one coil (<NUM>) configured to generate a magnetic field within a process fluid flow;
a pair of electrodes (<NUM>) configured to detect an electromotive force within the process fluid flow in response to the magnetic field;
measurement circuitry operably coupled to the pair of electrodes (<NUM>) and configured to provide an indication of the detected electromotive force; and
a processor (<NUM>) coupled to the measurement circuitry and configured to receive the indication of the detected electromotive force and an indication of process noise, the processor (<NUM>) being configured to change a dead time parameter based on the indication of process noise and provide a process fluid flow output based on the indication of detected electromotive force and the dead time parameter
characterized in that:
the processor (<NUM>) is configured to obtain the indication of process noise using a statistical parameter on a plurality of detected electromotive force indications.