Magnetic flow meter with noise adaptive dead time

Abstract

A magnetic flow meter 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 generated within the process fluid flow in response to the magnetic field. A measurement circuit 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 circuit and 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 the detected electromotive force and the dead time parameter.

Description

Technical Field

[0001] The present invention relates to a magnetic flow meter and a method of operating a magnetic flow meter. Background Art

[0002] Magnetic flowmeters (or magnetic meters) measure flow by applying Faraday's law (electromagnetic effect). Magnetic flowmeters excite one or more coils by passing an excitation current through a field winding, thereby generating a magnetic field across an electrically isolated conductive process fluid stream. An electromotive force (EMF) is generated by the flowing process fluid passing through the magnetic field. The voltage (potential) induced across the fluid and relative to the rest of the process fluid can be easily measured by one or more conductive electrodes in contact with the flowing process fluid. The volume flow rate is proportional to the flow velocity and the cross-sectional area of ​​the flow tube. The flow velocity is proportional to the electrode voltage potential (EV), which is proportional to the induced magnetic field strength (B). The induced magnetic field strength is assumed to be proportional to the applied magnetic field (H), which is directly related to the magnitude of the excitation current. Thus, a direct correlation is provided between the measured electrode voltage potential and the indicated volume flow rate.

[0003] Magnetic flowmeters are used in various conductive and semi-conductive fluid flow measurement environments. In particular, the flow of water-based fluids, ionic solutions, and other conductive fluids can all be measured using magnetic flowmeters. In addition, magnetic flowmeters are often used for fluids that may contain solids, such as pulp used in papermaking processes. Thus, magnetic flowmeters can be found in water treatment facilities, beverage and sanitary food production, chemical processing, high-purity drug manufacturing, and dangerous corrosive fluid processing facilities. However, some environments are more sensitive to signal noise. Providing a magnetic flowmeter that responds better to signal noise will improve the accuracy of flow output when used in such noisy environments. Summary of the invention

[0004] A magnetic flow meter 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 generated within the process fluid flow in response to the magnetic field. A measurement circuit 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 circuit and 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 the detected electromotive force and the dead time parameter. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Figure 1 is a schematic diagram of an environment in which a magnetic flow meter according to an embodiment of the present invention may be used.

[0006] Figure 2 is a block diagram of a magnetic flow meter according to an embodiment of the present invention.

[0007] Figure 3 is a graph comparing a pair of signals to illustrate the operation of dead time.

[0008] Figure 4 is a flow chart of a method for dynamically modifying a dead time threshold in a magnetic flow meter according to an embodiment of the present invention.

[0009] Figure 5 is a graph illustrating the effect of the dead time threshold on the flow output signal.

[0010] Figure 6 is a flow chart of a method for dynamically changing a dead time threshold in a magnetic flow meter according to an embodiment of the present invention. DETAILED DESCRIPTION

[0011] Figure 1 A typical environment 100 for a magnetic flow meter 102 is illustrated. The magnetic flow meter 102 is coupled to a process pipeline, which is schematically illustrated at line 104 and is also coupled to a control valve 112. The magnetic flow meter 102 is configured to provide a flow output regarding a process fluid flowing through the pipeline 104 in a process. Examples of such fluids include slurries and liquids in chemical processing plants, pulp processing plants, pharmaceutical processing plants, and other fluid processing plants.

[0012] The magnetic flow meter 102 includes an electronics housing 120 connected to a flow tube 108. The magnetic flow meter 102 output is configured for transmission to a controller or indicator over a relatively long distance via a process communication connection 106. In a typical process plant, the communication connection 106 may employ a digital communication protocol, an analog communication signal, or a combination thereof. The same or additional process information may be utilized via wireless communication, pulse width or frequency output, or discrete input / output (DI / DO). The system controller 110 may display flow information for a human operator and provide control signals through the process communication connection 106 to control the process using a control valve (such as valve 112).

[0013] Figure 21 is a block diagram of a magnetic flow meter 102, to which embodiments of the present invention are particularly applicable. The magnetic flow meter 102 measures the flow of an electrically conductive process fluid through a flow tube 108. A coil 122 is configured to apply an external magnetic field in the fluid flow in response to an applied excitation current from a coil driver 130. An EMF sensor (electrode) 124 is electrically coupled to the fluid flow and provides an EMF output 134 to an amplifier 132, the EMF output 134 being related to the EMF generated in the fluid flow due to the applied magnetic field, flow velocity, and noise. An analog-to-digital converter 142 provides a digitized EMF signal to a microprocessor 148 of a flow meter electronics 143.

[0014] The microprocessor 148 can be configured by hardware, software, or a combination thereof to provide digital signal processing functionality related to the EMF output 134 to provide an output 152 related to the flow rate. In addition, as will be described in more detail below, the signal processing can provide improved noise suppression. According to embodiments described herein, the microprocessor 148 can include or be coupled to a memory 150 containing instructions that, when executed by the microprocessor 148, provide process fluid flow rate output calculations and improved noise reduction.

[0015] Microprocessor 148 calculates the fluid flow rate through flow tube 108 based on the relationship between EMF output 134 and flow rate as described in the application of Faraday's law:

[0016]

[0017] Where E may be a signal output 152 related to the EMF output 134, V is the velocity of the fluid, D is the diameter of the flow tube 108, B is the strength of the induced magnetic field in the process fluid, and k is a proportionality constant. The microprocessor 148 uses the velocity and the measured magnetic field or coil current to calculate the flow rate of the process fluid in accordance with known techniques. A digital-to-analog converter 158 is coupled to the microprocessor 148 of the flow meter electronics 143 and generates an analog transmitter output 160 to couple to the communication connection 106. The digital communication circuit 162 may generate a digital transmitter output 164. The analog output 160 and / or the digital output 164 may be coupled to a process controller or monitor as desired.

[0018] Noisy process flows can cause magnetic flowmeter measurement errors, which can cause process control difficulties. This is because noisy flows can sometimes produce erroneous process flow outputs, which can be seen as a changed flow by the control system, which can adjust the process based on the erroneous flow. To address noise issues, magnetic flowmeters, especially those used in noisy process flow environments, often employ damping, signal processing, and averaging. These techniques increase output stability, but sometimes reduce the ability of the magnetic flowmeter to respond to changes in the process fluid flow. To address this potential lack of responsiveness, dead time counters are often employed.

[0019] As used herein, dead time is a signal processing parameter that, when employed, causes the flow output to no longer be a function of the average, but to more closely follow the flow value. In one example, if the flow output is the average of the previous ten flow samples, then when dead time is employed the flow output may be the average of 2 or 3 samples, or may even be provided as the flow sample itself.

[0020] As can be appreciated, properly setting the deadtime trigger is very important for balancing accurate flow indication with effective noise suppression. The deadtime counter diagnostic triggers when it detects a change in flow. However, the deadtime counter is not affected by noise. False alarms can occur, which can cause the flow to jump to an incorrect flow rate. This change in the reported flow rate can cause the control system to react in an undesirable manner. In order to reduce the possibility of false alarms with noise reduction, the flow threshold of the deadtime trigger is sometimes increased. However, increasing the threshold of the deadtime trigger reduces responsiveness, which is the main reason deadtime is used in the first place.

[0021] Figure 3An example of a high average flow signal and a damped flow signal is illustrated. A large down step shows a flow change where the high average flow signal 300 results in the dead time being adopted, but not a second adoption because the threshold is too high. On a large up step, the high average flow signal 300 follows the damped flow signal because the transition is within the dead time limit. As can be seen, when the dead time works correctly, the high average signal follows the minimally damped signal very closely. In essence, a magnetic flow meter that employs dead time has a first mode in which the process fluid flow output can be significantly averaged, smoothed, or otherwise processed to provide a signal that is robust to local noise. The magnetic flow meter also has a second mode in which the process flow output is decoupled from the averaged, smoothed, or otherwise processed signal to produce a process fluid flow sample that can more closely follow the changes. This second mode is a dead time mode and is adopted when the duration of the process flow sample above the dead time threshold exceeds the dead time duration.

[0022] According to embodiments described herein, an adaptive dead time is provided for a magnetic flow meter to reduce the probability of false alarms. In one embodiment, the dead time calculation can use the interquartile mean (IQM) to reduce large spike noise on the flow signal. The interquartile mean is a calculation in which the data in the second and third quartiles (when within the quartile range) are used and the lowest 25% and highest 25% of the data samples are discarded. The dead time threshold is then configured or otherwise affected using the median absolute deviation (MAD) or standard deviation. Therefore, when there is more signal noise, the MAD and standard deviation increase.

[0023] MAD is a mathematically known calculation that is robust to outliers. MAD is generally considered to be more resistant to outliers in a data set than standard deviation. In standard deviation, the distance from the mean is squared, while in median absolute deviation, deviations from a small number of outliers are insignificant. MAD is first calculated by calculating the median of a set of samples. Then the deviation of each sample from the median is calculated. Finally, the median of the sample deviations is calculated to provide MAD. As an example, consider the data (2, 2, 4, 4, 8, 12, 18). It has a median value of 4. The absolute deviations from 4 are (2, 2, 0, 0, 4, 8, 14), which in turn has a median value of 2 (because the absolute deviations after sorting are (0, 0, 2, 2, 4, 8, 8, 14)). Therefore, the median absolute deviation of this data is 2.

[0024] When the noise is reduced, the MAD and standard deviation will also be reduced, thereby reducing the threshold. This provides a robust dead time by increasing the threshold when noise is present and reducing the threshold when noise is not present. The adaptive dead time diagnostic procedure also maximizes responsiveness by reducing the flow threshold when noise is present. While it is preferred to use MAD and / or standard deviation, any suitable indication of process noise can be used to practice the embodiments described herein. For example, a separate device can be configured to measure noise and communicate an indication of process noise to a magnetic flowmeter, which then adapts the dead time based on the communicated noise. In another example, a fast Fourier transform (FFT) can be used to measure the amount of noise on the electrode in order to configure or otherwise affect the dead time threshold.

[0025] Figure 4 1 is a flow chart of a method for dynamically modifying a dead time threshold in a magnetic flow meter according to an embodiment of the present invention. The main new features of the dead time counter are the use of an IQM to compare the filtered output, create a flow threshold to be exceeded before counting, and generate a noise determination indicator to modify the threshold.

[0026] The method 350 begins at box 352 where quartile means are generated based on raw flow maps from a magnetic flow meter. These raw flow maps are individual EMF measurements of the electrodes 124 of the magnetic flow meter provided by the analog-to-digital converter 142. Once the IQM is created, control passes to box 354 where statistical parameters of the raw flow values ​​are generated. In one example, the statistical parameter may be a median absolute deviation (MAD) or a standard deviation. Then, at box 356, the dead time threshold is updated. At box 358, the difference between the IQM and the filtered flow output is calculated. In an embodiment of the present invention, the filtered flow output may be provided or generated, for example, based on the measurement and a dead time parameter, such as a dead time threshold or a dead time duration. At box 360, the microprocessor 148 determines whether the difference calculated at box 358 is greater than the dead time threshold. If so, control passes to box 362 where a dead time counter is incremented. However, if the difference is not greater than the dead time threshold, control passes to block 368 and the method ends. When the dead time counter is incremented at block 362, block 364 is executed to determine if the count is greater than the dead time limit. If the count is greater than the dead time limit, control passes to block 366 where the signal processing filter is reset and the process flow output is set to the new flow. Control then passes to block 368 where the method ends.

[0027] Figure 5 is a graph illustrating the effect of the dead time threshold on the flow output signal. Figure 5 The flow rate is shown when the dead time threshold is too low (illustrated at reference numeral 400) and the dead time limit is not triggered correctly and a large spike is seen on the flow output. Figure 5 Also shown is an example where the dead time is set correctly, the false alarms illustrated at signal line 402 with reference numerals 400 and 404 do not produce the same flow output aberrations.

[0028] Figure 6 4 is a flow chart of a method for dynamically changing a dead time threshold in a magnetic flow meter according to an embodiment of the present invention. Method 450 may be executed periodically or upon receiving a command. For example, method 450 may be executed in response to process communications received by magnetic flow meter 102. Similarly, method 450 may be executed in response to user input.

[0029] Method 450 begins at box 452, where the microprocessor 148 obtains an indication of process noise. Such an indication may come from one or more appropriate sources. For example, the microprocessor 148 may calculate the median absolute deviation (MAD) of a set of EMF measurements from the electrode 124, as indicated by reference numeral 454. Additionally or alternatively, the microprocessor 148 may calculate the standard deviation of a set of EMF measurements from the electrode 124, as indicated by reference numeral 456. As indicated by reference numeral 258, the process noise indication may also be received from an external device. Other appropriate techniques for providing an indication of process noise to the microprocessor 148 may also be used, as indicated by reference numeral 460. Although different techniques for generating or obtaining an indication of process noise have been described, it is also expressly contemplated that combinations thereof may also be used.

[0030] At box 462, the microprocessor 148 sets one or more dead time parameters based on the indication of process noise. As indicated by reference numeral 464, the microprocessor 148 can set a dead time threshold based on the indication of process noise. For example, the threshold can be set to a standard deviation from the current process fluid output value. In another example, the threshold can be set based on a function of the median absolute deviation. In addition or alternatively, the dead time duration can also be changed by the microprocessor 148, as indicated by reference numeral 466. Thus, the embodiments disclosed herein can dynamically adjust the dead time parameters to provide a better balance of accuracy and rate without sacrificing process fluid flow measurement accuracy. This allows better process control, especially in the environment of noisy processes.

[0031] Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

Claims

1. A magnetic flowmeter, comprising: at least one coil configured to generate a magnetic field within a process fluid flow; a pair of electrodes configured to detect an electromotive force generated within the process fluid flow in response to the magnetic field; a measurement circuit operatively coupled to the pair of electrodes and configured to provide an indication of the detected electromotive force; and a processor coupled to the measurement circuit and configured to receive the indication of the detected EMF and the indication of process noise, the processor configured to change a deadtime parameter based on the indication of process noise and to provide a process fluid flow output based on the indication of the detected EMF and the deadtime parameter, Wherein the processor is configured to obtain the indication of process noise using a statistical parameter relating to a plurality of detected indications of electromotive forces. 2 . The magnetic flow meter of claim 1 , wherein the dead time parameter is a dead time threshold.

3. The magnetic flow meter of claim 2, wherein the process fluid flow output has a first smoothing level when the detected electromotive force is below the dead time threshold, and the process fluid flow output has a second smoothing level when the detected electromotive force is above the dead time threshold. The magnetic flow meter of claim 1 , wherein the statistical parameter is a median absolute deviation. The magnetic flow meter of claim 1 , wherein the statistical parameter is a standard deviation. 6 . The magnetic flow meter of claim 1 , wherein the processor is configured to use an interquartile mean to remove outliers to stabilize the process fluid flow output.

7. The magnetic flow meter of claim 1, wherein the processor is configured to obtain the indication of process noise from an external device.

8. The magnetic flow meter of claim 1, wherein the dead time parameter is a dead time duration.

9. The magnetic flow meter of claim 1, wherein the processor is configured to use a fast Fourier transform to obtain the indication of process noise.

10. A method of operating a magnetic flow meter, the method comprising: receiving an indication of process noise; and At least one deadtime parameter in the magnetic flow meter is changed based on the indication of process noise, wherein receiving the indication of process noise is performed for each process fluid flow measurement of the magnetic flow meter. The method of claim 10 , wherein receiving an indication of process noise is performed periodically.

12. The method of claim 10, wherein receiving an indication of process noise comprises calculating a median absolute deviation of an interquartile mean of a set of raw flow measurements.

13. The method of claim 10, wherein receiving an indication of process noise comprises calculating a standard deviation of an interquartile mean of a set of raw flow measurements. The method of claim 10 , wherein receiving the indication of process noise comprises receiving the indication of process noise from an external device.

15. The method according to claim 10, further comprising: An electromotive force is detected between a pair of electrodes of the magnetic flow meter, and a process fluid flow output is provided based on the detected electromotive force and the at least one dead time parameter.

16. A method of operating a magnetic flow meter, the method comprising: receiving a set of measurements of an electromotive force between a pair of electrodes of the magnetic flow meter, the set of measurements indicating a raw fluid flow rate over a defined period of time; creating quartile means for the set of measurements; statistically processing the quartile means to obtain an indication of process noise; changing a dead time parameter of the magnetic flow meter based on the indication of process noise; This produces the filtered output: determining whether a difference between the quartile mean and the filtered output exceeds a threshold, and if so selectively incrementing a counter; and A determination is made as to whether the counter exceeds a time limit, and if the counter time exceeds the time limit, a new flow output is selectively set and a signal processing filter is reset.

17. The method of claim 16, wherein statistically processing the quartile means comprises calculating a standard deviation of the quartile means.

18. The method of claim 16, wherein statistically processing the quartile means comprises calculating median absolute deviations of the quartile means.

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