Automatic zero estimation for magnetic flowmeters independent of flow rate

Abstract

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 generated within the process fluid flow in response to the magnetic field. A coil drive circuit is coupled to the at least one coil and configured to cause the at least one coil to generate a 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 the coil drive circuit. The processor is configured to cause the coil drive circuit to operate at one of a plurality of coil drive frequencies. The processor is further configured to obtain a plurality of measurement samples when the process fluid is flowing and when the coil drive circuit is operating at the first frequency. The processor is configured to generate an auto-zero estimate for operating at a second coil drive frequency higher than the first coil drive frequency and save the auto-zero estimate for use during operation at the second coil drive frequency.

Description

Background Art

[0001] Magnetic flowmeters (or magnetic meters) measure flow by applying Faraday's law (electromagnetic effect). A magnetic flowmeter excites 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 amplitude of the excitation current. Thus, a direct correlation is provided between the measured electrode voltage potential and the indicated volume flow rate.

[0002] Magnetic flowmeters are used in a variety of 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. Thus, magnetic flowmeters can be found in water treatment facilities, beverage and sanitary food production, chemical processing, high-purity drug manufacturing, and hazardous mandatory processing facilities. However, some environments are more sensitive to signal noise. Providing a magnetic flowmeter with better signal noise response will improve the accuracy of flow output when used in such noisy environments. Summary of the invention

[0003] 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 generated within the process fluid flow in response to the magnetic field. A coil drive circuit is coupled to the at least one coil and configured to cause the at least one coil to generate 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 the coil drive circuit. The processor is configured to cause the coil drive circuit to operate at one of a plurality of coil drive frequencies. The processor is further configured to obtain a plurality of measurement samples when a process fluid is flowing and when the coil drive circuit is operating at a first frequency. The processor is configured to generate an auto-zero estimate for operating at a second coil drive frequency higher than the first coil drive frequency and save the auto-zero estimate for use during operation at the second coil drive frequency. BRIEF DESCRIPTION OF THE DRAWINGS

[0004] Figure 1is a schematic diagram of an environment in which a magnetic flow meter according to an embodiment of the present invention may be used.

[0005] Figure 2 is a block diagram of a magnetic flow meter according to an embodiment of the present invention.

[0006] Figure 3 is a signal diagram illustrating when measurements typically occur when the coils of a magnetic flow meter are driven.

[0007] Figure 4 is a signal diagram illustrating when low frequency measurements and high frequency measurements occur according to an embodiment of the present invention.

[0008] Figure 5 is a flow chart of a method of operating a magnetic flow meter according to an embodiment of the present invention. DETAILED DESCRIPTION

[0009] 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.

[0010] The magnetic flow meter 102 includes an electronics housing 120 connected to the 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 be a digital communication protocol or an analog communication signal. 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, as well as provide control signals via the process communication connection 106 to control the process using a control valve (such as valve 112).

[0011] Figure 21 is a block diagram of a magnetic flow meter, 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 assembly 108. The coil 122 is configured to apply an external magnetic field to the fluid flow in response to an applied excitation current from a coil driver 130. The EMF sensor (electrode) 124 is electrically coupled to the fluid flow and provides an EMF signal output 134 to an amplifier 132, the EMF signal output 134 being related to the EMF generated in the fluid flow due to the applied magnetic field, flow rate, and noise. The analog-to-digital converter 142 provides a digitized EMF signal 210 to a microprocessor system 148 of the flow meter electronics 143. The microprocessor 148 can be configured by hardware, software, or a combination thereof to provide digital signal processing functions related to the EMF output 134 to provide an output 152 related to the flow rate. The microprocessor 148 generates a control output 131 to the coil driver circuit 130 to operate at one of a plurality of different operating frequencies. In one embodiment, the plurality of different operating frequencies includes a low frequency and a high frequency, wherein the high frequency is higher than the low frequency. Memory 150 contains programming instructions for controlling operation 148. Additionally, as will be described in more detail below, the signal processing may provide improved noise suppression.

[0012] 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:

[0013]

[0014] 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 digital-to-analog converter 158 is coupled to the microprocessor 148 of the flow meter electronics 143 and may generate an analog transmitter output 160 in some embodiments for coupling to the communication bus 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.

[0015] Many magnetic flow meter applications have a significant level of 1 / f or pink noise. In such instances, one way to improve the signal-to-noise ratio is to increase the coil drive frequency. Typically, doubling the coil drive frequency will double the signal-to-noise ratio. However, high coil drive frequencies can also cause problems with flow offsets due to the changing magnetic field and the electrode circuits. Typically, an auto-zero adjustment is performed when a flow channel or flow conduit has a zero flow condition to produce an adjustment that corrects the offset. One difficulty is that many operations cannot simply stop the process fluid flow to perform an auto-zero operation at a zero flow condition.

[0016] According to the embodiments described below, a magnetic flow meter is provided that can perform an estimated auto-zero function without requiring a zero flow condition in the flow tube. This is helpful because it is often difficult for a carrier or operator to shut down the process to perform an auto-zero adjustment operation at zero flow. The purpose of performing an auto-zero adjustment is to remove the high coil frequency offsets that are generated by the parasitic interaction between the magnetic field and the electrode measurement circuit. Typically, in order to perform such an auto-zero adjustment operation, operators are forced to wait months for a planned shutdown. As a result, operators are often forced to make a difficult choice between continuing to operate at a noisy flow and switching to a high frequency coil operation with potentially significant offset errors.

[0017] The embodiments provided herein generally provide an estimate of the high frequency auto-zero offset when operating at a low coil frequency while the flow is still moving through the flow path. When operating at a low coil frequency, the magnetic flowmeter will also measure the electrode signal at the high frequency measurement site. This then provides enough information to resolve the high frequency auto-zero offset. This can be a controlled adjustment routine that stores the offset in memory and allows the magnetic flowmeter to switch to a higher coil drive frequency while still maintaining accuracy and without the need to stop the flow for accurate auto-zero adjustment parameters. Switching to a higher coil drive frequency can sometimes provide a significant improvement in noise performance.

[0018] Pulsed direct current (DC) magnetic flowmeters typically measure flow by driving the coil to produce a magnetic field having a square wave. The height of the electrode voltage waveform is typically proportional to the flow velocity. Some magnetic flowmeters, such as those sold under the trade name 8707 Hi-Signal, available from Emerson Automation Solutions of Eden Prairie, Minnesota, provide both low-frequency coil drive capability and high-frequency coil drive capability. Exemplary low and high frequencies may be 5 Hz and 37.5 Hz, respectively. However, a frequency range including 11 Hz, 18.75 Hz, 55 Hz, and 75 Hz may be used. Typically, 20% of the waveform is averaged to measure the height of the electrode voltage.

[0019] Figure 3 3 is a signal diagram illustrating when measurements typically occur when the coil of a magnetic flowmeter is driven. The low frequency measurement position is schematically indicated at reference numeral 300 (i.e., indicating the time during the signal in milliseconds). As can be seen, the position 300 typically occurs after the signal has substantially settled to a relatively steady state. This is the portion of the waveform that is used for low frequency measurements. As can also be seen, the low frequency measurement windows typically have: a low frequency positive window that occurs during positive voltage; and a low frequency negative window that occurs during negative voltage conditions.

[0020] The embodiments described herein generally utilize the relationship between the offsets of low frequency operation and high frequency operation to generate a high frequency offset estimate (value) during low frequency operation. The flow equations for both the low coil drive frequency option and the high coil drive frequency option are shown as follows:

[0021] Flow=LF gain *(LF pos -LF neg )-LF ZeroOffset

[0022] Equation 2

[0023] F10w=HF gain *(HF pos -HF neg )-HF ZeroOffset

[0024] Equation 3

[0025] The sensor is designed and calibrated so that the LF ZeroOffset is essentially zero feet per second. In addition, from sensor calibration numbers configured during manufacturing or otherwise obtained, we know that LF gain and HG gain Therefore, the high frequency zero offset can be solved as set forth in the following equation.

[0026] HF ZeroOffset =HF gain *(HF pos -HF neg )-LF gain *(LF pos -LF neg )

[0027] High frequency auto-zero operation typically requires the user to shut down their process to provide a zero flow condition. However, because the high frequency zero offset is not affected by flow, it can be heavily filtered and measured over a long period of time (relative to a single measurement cycle) to handle noise on the electrode signal. In one example, high frequency samples are measured and averaged over a period of about 90 seconds. The high frequency zero value is then stored in memory 150 and the user can easily change to accurate high frequency operation without creating a zero flow condition. In addition, high frequency auto-zero operation can occur when the magnetic flow meter is operating at a low frequency and providing process fluid flow measurement.

[0028] Figure 4 The locations of the low frequency measurement and high frequency measurement are schematically marked with reference numerals 300, 302. As illustrated, in each measurement cycle, the high frequency measurement leads the low frequency measurement by approximately one quarter of the total measurement time period (positive and negative). Thus, the high frequency measurement position 302 occurs before the signal has substantially stabilized. In one embodiment, although the high frequency measurement positions are fixed, they can also be moved back to the transition. Doing so can help determine the operating speed of the sensor (i.e., the maximum coil drive frequency) by limiting the auto-zero value. On some sensors, this can allow operation at twice the speed of the calibrated high frequency and produce further improvements in noise suppression.

[0029] Figure 5 4 is a flow chart of a method of operating a magnetic flow meter according to an embodiment of the present invention. Method 450 begins at block 402, where the microprocessor 148 or other appropriate circuitry sets the coil drive circuit to a low frequency value. In one embodiment, this value is 5 Hz. Then, at block 404, a sample ( Figure 4 ). In one embodiment, the low frequency location includes approximately 20% of the waveform and occurs where the waveform has been substantially stable. The multiple samples may be obtained by sampling the EMF multiple times (as shown in reference numeral 460), by digitizing the entire waveform and analyzing it to extract multiple samples (as shown in reference numeral 408), or via other suitable techniques (as shown in reference numeral 410).

[0030] At box 412, a plurality of high frequency samples are obtained during operation at low frequency and when process fluid flow occurs. The samples obtained during box 412 are preferably combined with high frequency samples from other measurement cycles to provide a smoothed value or average. In one example, the high frequency samples are averaged over a defined time interval, as indicated by reference numeral 414. A suitable time interval is 90 seconds. When box 418 is completed, an averaged or combined HF-pos value is provided, and an averaged or combined HF-neg value is provided. However, other intervals and / or other techniques may be used to combine the individual high frequency samples, as indicated by reference numeral 416.

[0031] At block 418, the combined HF-pos and HF-neg values ​​are combined with the low frequency samples and known gain values ​​for high frequency operation and low frequency operation to generate a high frequency zero offset estimate. This estimate may be generated according to the equations set forth above or using different calculations and / or techniques. At block 420, the high frequency auto-zero estimate is stored in a memory, such as memory 150, for use in subsequent high frequency operations.

[0032] When the auto-zero estimation has concluded or during execution, the microprocessor may still evaluate the low frequency samples to provide a process flow output at a low coil frequency, as indicated by reference numeral 422. Additionally or alternatively, after the high frequency estimation operation is complete, the magnetic flow meter may transition to high frequency operation and employ the stored high frequency auto-zero estimate stored in memory, as indicated by reference numeral 424.

[0033] Although about Figure 5 The described embodiments have two frequencies (low / high), but it is expressly contemplated that the embodiments may be practiced with more than two frequencies. Further, instead of using low frequency operation to obtain auto-zero estimates for high frequency operation, embodiments may also employ an ultra-low frequency (e.g., half of the low frequency) to obtain auto-zero estimates for low frequency operation. Still further, such embodiments may still employ low frequency operation to obtain auto-zero estimates for high frequency operation.

[0034] The embodiments described herein are not limited to the specific equations set forth above, but may use any suitable technique or algorithm that employs low frequency coil operation to provide or otherwise obtain a value that can estimate a high frequency coil offset. Additionally, although some specific operating coil frequencies have been disclosed, those skilled in the art will appreciate that the embodiments described herein may be practiced using any suitable coil drive frequency.

[0035] As can be appreciated, due to the ability to generate an autozero estimate without having to stop process fluid flow, the autozero operation can be performed frequently. In addition, the autozero estimate can be saved and analyzed to detect when there has been a significant change in the autozero value. This may occur due to a large change in conductivity, wiring, or moving the sensor to a different transmitter. In such an instance, the magnetic flow meter can alert the user that an autozero should be performed or other appropriate corrective action should be taken.

[0036] Although the present invention has been described with reference to preferred embodiments, those skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the present invention. For example, although the embodiments described above generally obtain and save auto-zero estimates for use when the magnetic flowmeter switches to high frequency operation, it is expressly contemplated that the embodiments can be used to provide a magnetic flowmeter that continuously alternates between a low frequency coil drive frequency and a high frequency coil drive frequency to calculate and update a high frequency auto-zero value. This will provide high frequency performance substantially without requiring any user interaction to initiate the auto-zero estimate operation.

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 coil driving circuit coupled to the at least one coil and configured to cause the at least one coil to generate 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 the coil drive circuit, the processor configured to cause the coil drive circuit to operate at one of a plurality of coil drive frequencies, the processor further configured to obtain a plurality of measurement samples while process fluid is flowing and while the coil drive circuit is operating at the first coil drive frequency, the processor configured to generate an auto-zero estimate for operation at a second coil drive frequency that is higher than the first coil drive frequency and to save the auto-zero estimate for use during operation at the second coil drive frequency.

2. The magnetic flow meter of claim 1, wherein the magnetic flow meter is a pulsed DC magnetic flow meter.

3. The magnetic flow meter of claim 1, wherein the plurality of coil drive frequencies includes a low frequency and a high frequency, and wherein the processor is configured to obtain a plurality of low frequency samples and a plurality of high frequency samples during low frequency operation.

4. The magnetic flow meter of claim 3, wherein the positive high frequency sample is obtained at a portion of the measurement waveform before the positive low frequency sample is obtained.

5. The magnetic flow meter of claim 4, wherein the processor is configured to vary the timing of occurrence of the high frequency samples relative to the low frequency samples to determine a maximum coil drive frequency of the magnetic flow meter.

6. The magnetic flow meter of claim 4, wherein a negative high frequency sample is obtained at a portion of the measurement waveform before a negative low frequency sample is obtained. 7 . The magnetic flow meter of claim 6 , wherein the processor is configured to generate a process fluid output based on the positive low frequency samples and the negative low frequency samples.

8. The magnetic flow meter of claim 6, wherein the processor is configured to combine positive high frequency samples over a plurality of measurement cycles and to combine negative high frequency samples over the plurality of measurement cycles.

9. The magnetic flow meter of claim 8, wherein the processor is configured to generate the auto-zero estimate based on combined positive high frequency samples, negative high frequency samples, positive low frequency samples, and negative low frequency samples.

10. The magnetic flow meter of claim 3, wherein the plurality of high frequency samples are combined over a time period spanning a plurality of measurement cycles.

11. The magnetic flow meter of claim 9, wherein the processor is configured to average the high frequency samples over a period of 90 seconds.

12. The magnetic flow meter of claim 1, wherein the processor is configured to generate a process fluid output based on the plurality of measurement samples.

13. The magnetic flow meter of claim 1, wherein the processor is configured to store a plurality of auto-zero estimates over time and to provide a diagnostic indication based on changes in the auto-zero estimates over time.

14. The magnetic flow meter of claim 1, wherein the plurality of coil drive frequencies comprises at least three coil drive frequencies.

15. A method of operating a magnetic flow meter, the method comprising: generating an alternating magnetic field in a flowing process fluid using at least one coil of the magnetic flow meter, the alternating magnetic field having a first frequency; obtaining an electromotive force measurement from the flowing process fluid while the alternating magnetic field is operating at the first frequency; calculating, using a processor of the magnetic flow meter, an auto-zero estimate for a second frequency higher than the first frequency based on electromotive force measurements obtained while the alternating magnetic field is operating at the first frequency and the process fluid is flowing; and The autozero estimate is stored in a memory of the magnetic flow meter.

16. The method according to claim 15, further comprising: A process fluid output is provided based on the electromotive force measurement.

17. The method according to claim 15, further comprising: Switching the alternating magnetic field to the second frequency; obtaining a plurality of second frequency electromotive force measurements while the alternating magnetic field is operating at the second frequency; And providing a process fluid output based on the second frequency EMF measurement and the stored auto-zero estimate.

18. The method according to claim 17, further comprising: Switching is performed continuously between operation at the first frequency and operation at the second frequency.

19. The method according to claim 17, further comprising: A second auto-zero estimate for a third frequency is calculated using a processor of the magnetic flow meter based on the EMF measurements obtained while the alternating magnetic field is operating at the second frequency and the process fluid is flowing, and the second auto-zero estimate is stored.

20. The method of claim 19, wherein the first frequency is a very low frequency. The method of claim 20 , wherein the first frequency is half the second frequency.

22. The method of claim 19, further comprising: Switching the alternating magnetic field to the third frequency higher than the second frequency; obtaining a plurality of third-frequency electromotive force measurements while the alternating magnetic field is operating at the third frequency; and providing a process fluid output based on the third-frequency electromotive force measurements and a stored second auto-zero estimate.

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