Bootstrap-type impedance measurement for flow meter electrodes

Abstract

A magnetic flowmeter (20) for measuring the flow rate of process fluid in a pipe (22) is provided, comprising a magnetic coil (26) positioned adjacent to the pipe (22) and configured to apply a magnetic field to the process fluid. It further comprises first and second electrodes (30, 32) positioned within the pipe (22) and electrically connected to the process fluid, which detect an electromotive force (EMF) induced in the process fluid by the applied magnetic field and the flow of the process fluid, and provide first and second electrode signals, respectively. An output circuit (158) connected to the first and second electrodes (30, 32) provides an output (160) related to the detected EMF. A diagnostic circuit (300) provides an electrode reference diagnostic signal (316). A method is also provided.

Description

Technical Field

[0001] The present invention relates to a magnetic flowmeter used to measure the flow rate of a process fluid flowing through a process pipe. More specifically, the present invention relates to a technique for diagnosing electrodes of a magnetic flowmeter.

Background Art

[0002] In industrial process monitoring and / or control systems, field devices are used to monitor process variables related to a specific process. These process variables include fluid pressure, fluid flow rate, fluid temperature, liquid level, and the like.

[0003] A magnetic flowmeter is a type of field device that is connected to a pipe and measures the flow rate when a conductive process fluid flows through the flow tube. A specific magnetic flowmeter includes an electromagnetic coil and electrodes. According to Faraday's law of electromagnetic induction, the electromagnetic coil is used to apply a magnetic field to the process fluid in the flow tube. Due to the applied magnetic field and the movement of the fluid, an electromotive force (EMF) proportional to the flow rate is induced in the fluid. The electrodes are arranged in the flow tube, make electrical contact with the flowing process fluid, and detect this induced EMF. In a specific embodiment, this EMF is measured by an amplifier connected to the electrodes, the EMF signal is amplified, and its output is quantized by an analog-to-digital converter (ADC) to generate a data value related to the fluid flow rate.

Summary of the Invention

Problems to be Solved by the Invention

[0004] During the operation of a magnetic flowmeter, there are various conditions that cause errors in the flow rate measurement by the flowmeter. These conditions include deterioration of the electrodes used for electrical connection to the process fluid and the connection quality between the electrodes and the process fluid. As one method for evaluating these states, a method using a diagnostic signal with a ground reference is known. However, improvement of the diagnostic technology for magnetic flowmeters is still required. [Means for solving the problem]

[0005] The present invention provides a magnetic flow meter for measuring the flow rate of a process fluid in a pipe. The flow meter comprises a magnetic coil positioned adjacent to the pipe and configured to apply a magnetic field to the process fluid. Furthermore, it comprises first and second electrodes positioned within the pipe and electrically connected to the process fluid, which detect the electromotive force (EMF) induced in the process fluid by the applied magnetic field and the flow of the process fluid, and provide electrode flow signals, respectively. Output circuits connected to the first and second electrodes provide outputs related to the detected EMF. A diagnostic circuit supplies electrode-referenced diagnostic signals to at least one of the first and second electrodes. Related methods are also provided.

[0006] This summary is provided to provide a simplified introduction to some of the concepts that will be further explained in the detailed description below. This summary is not intended to identify any important or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that resolve any or all of the defects described in the background. [Brief explanation of the drawing]

[0007] [Figure 1] This is a partial cross-sectional view of a magnetic flow meter. [Figure 2] Figure 1 is a simplified electrical circuit diagram of the magnetic flow meter. [Figure 3] This is a graph of impedance. [Figure 4] This graph shows a high-frequency sine wave diagnostic signal superimposed on a low-frequency square wave electrode signal. [Figure 5] This graph shows a high-frequency sine wave diagnostic signal superimposed on a low-frequency square wave electrode signal. [Figure 6] This is a simplified block diagram of an electrode diagnostic circuit including an electrode reference diagnostic signal. [Figure 7]This is a simplified block diagram of an electrode diagnostic circuit including an electrode reference diagnostic signal. [Modes for carrying out the invention]

[0008] Embodiments of this disclosure are described in further detail below with reference to the accompanying drawings. Elements identified by the same or similar reference numerals refer to the same or similar elements. Some elements may not be shown in the drawings for the sake of simplicity.

[0009] Various embodiments of this disclosure can be embodied in many different forms and should not be limited to the specific embodiments shown herein. Rather, these embodiments are provided to ensure that this disclosure is sufficient and complete and to allow those skilled in the art to fully understand the scope of this disclosure.

[0010] In various aspects of the present invention, diagnostic measurements concerning process fluid and the electrical connection between the electrodes and the fluid are enabled by applying a diagnostic signal to the electrodes of a magnetic flow meter, while reducing or minimizing its impact on the flow signal. An industrial process variable transmitter is provided, which includes a circuit configured to detect and perform diagnostic measurements of the electromotive force (EMF) associated with the flow of process fluid through process piping. In one aspect, a configuration is provided in which the diagnostic signal shares the same signal path as the electrode flow signal. This eliminates the need for additional circuit components and space, such as additional analog switches or signal conditioning amplifiers, to provide separate signal paths.

[0011] Figure 1 is a partial cross-sectional view of a magnetic flowmeter 20 in which embodiments of the present invention are particularly useful. The magnetic flowmeter 20 comprises a flow tube 22, an electromagnet (coil) 26, and electrodes 30, 32. The electromagnetic coil 26 and electrodes 30, 32 are connected to a transmitter circuit in a housing 34. When in operation, the transmitter circuit supplies current to the electromagnetic coil 26, which generates a magnetic field 36 in the flow tube 22, indicated by the arrow. The process fluid 21 passes through the magnetic field in the flow tube 22, and its flow induces an electromotive force (EMF, voltage) in the liquid 21. The electrodes 30, 32 contact the fluid 21 and detect this EMF. According to Faraday's law, this electromotive force is proportional to the flow rate of the liquid 21 in the flow tube 22.

[0012] Figure 2 is a schematic diagram of the transmitter circuit 24 of the magnetic flowmeter transmitter 20. The electromagnetic coil 26 is driven by a drive signal from the coil drive circuit 152. Electrodes 30 and 32 provide electrode voltage signals to the measurement circuit 154 via amplifiers 148 and 150, which are related to the flow of the process fluid 21. The measurement circuit 154 can provide a measurement output related to the flow rate according to known techniques. The measurement circuit 154 may, for example, appropriately include a programmed or configured microprocessor or digital signal processor (DSP) circuit. The amplifiers 148 and 150 and the measurement circuit 154 generally constitute the "front end" or input circuit of the magnetic flowmeter 20.

[0013] The output 204 of the measurement circuit 154 is provided to the output circuit 158 ​​for transmission to a control or monitoring circuit located away from the magnetic flowmeter 20. However, the output 204 can also be transmitted to other locations as needed or used internally within the flowmeter 20. The output circuit 158 ​​can provide a pulse output, a 4-20mA current output, a digital output, a wireless output, or any other desired output format. In this example, the output of the output circuit 158 ​​is connected to the process control loop 160. A reference connection to the process fluid (not shown in Figures 1 and 2) can also be provided for diagnostic purposes.

[0014] To accurately measure the flow rate to electrodes 30 and 32, a high input impedance input connection is required. For diagnostic purposes, it is desirable to understand the impedance of the process fluid 21 and the quality of the connection between the process fluid 21 and the measurement electrodes 30 and 32. This allows for the detection of degradation or other conditions of electrodes 30 and 32. Such diagnostic measurements can be performed by applying an electrode diagnostic signal to electrodes 30 and 32 and observing the response. However, in conventional flowmeter configurations, the application of such a diagnostic signal can affect the electrode signal provided by electrodes 30 and 32, potentially causing errors in flow measurement. Furthermore, a DC offset voltage may exist between the electrodes. Therefore, it is desirable to AC couple the electrode diagnostic signal to electrodes 30 and 32. While it is possible to apply the diagnostic signal via an AC-coupled current source, implementing such a current source is difficult. An AC-coupled current source must be designed to operate over a wide temperature range while maintaining high output impedance and operating with a limited power supply. Maintaining high output impedance requires tight component tolerances. Furthermore, high output impedance current signals can be attenuated by cable capacitance, potentially limiting the usable frequency range of the diagnostic signal. Furthermore, the space available for implementing such circuits is usually limited.

[0015] In one aspect, the present invention provides a novel method for using a buffered electrode input signal as a reference for the excitation voltage signal of a diagnostic measurement. This allows the diagnostic signal to be applied using a voltage source while maintaining a high input impedance relative to the electrode signal. Because the diagnostic signal source is referenced to the electrode signal, the diagnostic signal can be applied through a relatively low impedance connection, and the connection can be maintained even during flow measurement as needed. The diagnostic signal source can also be connected and disconnected with minimal impact on the flow measurement.

[0016] The present invention provides a bootstrap type diagnostic signal circuit. (See FIG. 6) The bootstrap amplifier 308 is provided for the electrode signal 314. The diagnostic stimulus signal 316 is driven as desired by the diagnostic signal generator 306 with reference to the buffered bootstrap signal. The electrode reference stimulus signal 316 is coupled to the electrode 30. Any suitable type of bootstrap amplifier can be implemented, and the present invention is not limited to a specific implementation. As an example, there is an implementation that uses the buffered electrode input signal as a reference when generating the diagnostic stimulus voltage signal 316.

[0017] The input impedance of the electrode circuit is shown in FIG. 3. The upper waveform represents the input impedance of the diagnostic stimulus connection, which is above the lower waveform representing the input impedance of the electrode amplifier connection over the entire frequency range. When a standard voltage amplifier is connected to the diagnostic stimulus signal, the upper waveform will simply represent the impedance of the series RC used for the connection.

[0018] The graphs of FIGS. 4 and 5 show the diagnostic signal (high-frequency sine wave) superimposed on the electrode flow signal (low-frequency rectangular wave), for the cases where the electrode resistance is 100 kΩ and 10 kΩ respectively. These graphs show the ability of the present invention to detect changes in electrode impedance while detecting the electrode flow signal. The present invention has the following features.

[0019] 1. By using a low diagnostic coupling impedance, it becomes possible to accurately measure low-impedance electrode connections.

[0020] 2. The electrode reference diagnostic signal is a high-impedance signal with respect to the electrode flow signal.

[0021] 3. The electrode reference diagnostic signal does not require the diagnostic signal to be cut off during flow measurement. Therefore, the charging and discharging of the AC coupling capacitor, which may disrupt the electrode flow signal, are unnecessary.

[0022] 4. Since each of the electrode diagnostic signals is based on confidence, the electrode reference diagnostic signal is independent of each electrode. Therefore, the diagnostic signal can be differential or single-ended (common mode when the diagnostic signals of both electrodes have the same amplitude and the same phase).

[0023] 5. The electrode reference diagnostic signal can adapt to different electrode impedances without disturbing the flow signal.

[0024] Figure 6 is a simplified block diagram of a diagnostic system 300 including an electrode reference diagnostic signal 316 for a single electrode 30. In Figure 6, the electrode reference diagnostic signal 316 is connected to the electrode 30 via a low-impedance coupling 302. The electrode reference signal 314 from a high-input-impedance electrode amplifier 312 is supplied to a bootstrap diagnostic injection amplifier 308, which is connected to the electrode 30 and the system reference 310. The diagnostic signal is provided by a diagnostic signal generator 306, which is a voltage source and generates the electrode reference diagnostic signal 316 for the electrode 30 via the bootstrap reference signal injection amplifier 308.

[0025] Figure 7 shows an embodiment of the present invention, which includes independent electrode diagnostic systems 300+ and 300- for each electrode 30, 32 respectively. Components similar to those in Figure 6 are labeled with + and - signs corresponding to the polarity. As described above, a high-input-impedance coupling is achieved by a bootstrap amplifier configuration using the electrode reference signals (314+, 314-). In this configuration, different diagnostic signals 316+, 316- can be applied to each electrode 30, 32. The diagnostic signals provided by the diagnostic signal generators 306+, 306- can be in common mode, differential mode, or different frequencies. Also, single-ended diagnostic signals can be individually applied to the electrodes. In one example configuration, the system reference 310 is a reference electrode connected to the process fluid or a direct connection to the flow tube 22 that provides the electrical reference for the electrodes 30, 32. In Figure 7, electrode amplification stages 320+, 320- for amplifying the electrode voltage signals are shown, which supply signals to an analog-to-digital converter 330. The digital output of the A / D converter 330 is provided to a measurement circuit 15 for flow measurement and diagnosis.

[0026] Specific diagnoses using electrode reference diagnostic signals can be performed according to known diagnostic techniques. For example, the amplitude of the diagnostic signal is related to the connection impedance with the process fluid and may indicate deterioration of electrodes 30 and 32. This signal can also be used to detect the emptiness of the piping or the accumulation of deposits. These diagnoses can be performed using the measurement circuit 154 or remotely.

[0027] After prolonged operation, a coating may form on the components of a flow meter, affecting its performance. Depending on the process fluid, a conductive or non-conductive coating may form on the sensing electrode, reference electrode, flow tube lining, or the flow tube itself. The circuit in Figure 7 allows for differential impedance measurements between any electrode, flow tube, flow tube liner, or other component and comparison with a reference value. If the measured impedance deviates from the reference value, it may indicate the formation of a conductive or non-conductive coating, the development of an electrical leakage path, a poor electrical connection, or other reasons that have altered the electrical characteristics of the flow meter. Differential impedance measurements can be obtained by common-mode measurement or by performing multiple single-end measurements.

[0028] While the present invention has been described with reference to preferred embodiments, those skilled in the art will understand that modifications can be made in form and detail without departing from the spirit and scope of the invention. In various aspects of the invention, diagnostic signals can be applied to multiple electrodes, which may be completely independent, common-mode, differential-mode, single-ended, or a combination thereof. As used herein, “electrode signals” may include multiple signals present on an electrode. For example, electrode signals may include an electrode flow signal related to an EMF generated by the movement of a process fluid through a magnetic field, and an electrode reference diagnostic signal applied to the electrode as described herein.

Claims

1. A magnetic flow meter for measuring the flow rate of process fluid in a pipe, A magnetic coil is positioned adjacent to the aforementioned piping and configured to apply a magnetic field to the process fluid, The first and second electrodes are arranged within the piping, electrically connected to the process fluid, and detect the electromotive force (EMF) induced in the process fluid by the applied magnetic field and the flow of the process fluid, and provide first and second electrode flow signals, respectively. An output circuit connected to the first and second electrodes, which provides an output related to the detected EMF, A diagnostic circuit including an electrode reference diagnostic signal applied to at least one of the first and second electrodes, A magnetic flow meter equipped with [a specific feature].

2. The magnetic flow meter according to claim 1, wherein the electrode reference diagnostic signal is coupled to the electrode via a voltage source that references the diagnostic signal to the electrode flow signal.

3. The magnetic flow meter according to claim 2, comprising an electrode signal amplifier for coupling a diagnostic signal to the electrode flow signal.

4. The magnetic flow meter according to claim 3, wherein the electrode flow rate signal is independent of the electrode reference diagnostic signal.

5. The magnetic flow meter according to claim 3, wherein the electrode flow rate signal is not affected by changes in the electrode reference diagnostic signal.

6. The magnetic flow meter according to claim 1, wherein the electrode reference diagnostic signal is a common-mode signal.

7. The magnetic flow meter according to claim 1, wherein the electrode reference diagnostic signal is a differential mode signal.

8. The magnetic flow meter according to claim 1, wherein the electrode reference diagnostic signal is a single-ended signal.

9. The magnetic flow meter according to claim 2, wherein the diagnostic signal is modified based on the impedance of the process fluid.

10. The magnetic flow meter according to claim 2, wherein the diagnostic signal is modified based on the electrical connection between the electrode and the process fluid.

11. The magnetic flow meter according to claim 1, wherein the diagnostic circuit provides a second diagnostic signal, one of which is a common-mode signal and the other is a differential-mode signal for simultaneously providing different diagnoses.

12. The magnetic flow meter according to claim 1, wherein the diagnostic signal is modified based on a change in the electrical characteristics of the flow meter.

13. The magnetic flow meter according to claim 12, wherein the diagnostic signal is related to a differential impedance measured between at least two components of the flow meter.

14. A method for measuring the flow rate of process fluid in a pipe, A magnetic field is applied to the process fluid flowing through the pipe by a magnetic coil. The electromotive force (EMF) induced in the piping by the magnetic field and process fluid flow applied using the first and second electrodes is detected, and the flow signals of the first and second electrodes are generated, The output circuit measures the EMF, and the measured EMF indicates the flow rate of the process fluid. Diagnosis is performed using an electrode reference diagnostic signal applied to at least one of the first and second electrodes. A method that includes doing so.

15. The method according to claim 14, comprising coupling the diagnostic signal to the electrode via a voltage source that references the diagnostic signal to the electrode signal in order to maintain a high input impedance.

16. The method according to claim 15, wherein the power supply for the diagnostic signal is turned on and off while the flow rate is being measured.

17. The method according to claim 15, wherein the diagnostic signal is a common-mode signal.

18. The method according to claim 15, wherein the diagnostic signal is a differential mode signal.

19. The method according to claim 15, wherein the diagnostic signal is a single-ended signal.

20. The method according to claim 15, wherein a second diagnostic signal is provided, one of which is a common-mode signal and the other is a differential-mode signal for simultaneously providing different diagnoses.

Images