Electromagnetic flowmeter

Abstract

The invention relates to the technical field of flow measurement, and provides an electromagnetic flowmeter which comprises an excitation device used for outputting an excitation signal to enable high-frequency excitation and low-frequency excitation to appear alternately, and a complete excitation period of the excitation signal comprises a first high frequency band, a first low frequency band, a second high frequency band and a second low frequency band; the electrode is used for detecting induced electromotive force and outputting an electrode signal; the signal conditioning and collecting device is used for converting the electrode signal into a digital signal after amplification and analog filtering processing, and the digital signal comprises a high-frequency electrode signal and a low-frequency electrode signal; and the operation unit is used for performing signal calculation on the high-frequency electrode signal and / or the low-frequency electrode signal. High-frequency excitation and low-frequency excitation alternately appear by adopting alternate high-frequency excitation and low-frequency excitation to form a complete excitation period which is higher than the frequency of an occasional high-frequency part, so that the influence of flowing noise is effectively reduced; and the low-frequency part can ensure the zero-point stability of the flowmeter.

Description

Technical Field

[0001] The present invention relates to the technical field of flow measurement, and in particular to an electromagnetic flowmeter based on alternating high and low frequency excitation, which is suitable for high-precision measurement of general slurry or low conductivity fluid. Background Art

[0002] The electromagnetic flowmeter is a flow measurement instrument based on Faraday's law of electromagnetic induction. Its main components include a pipeline cavity, a pair of signal electrodes, an excitation coil, an excitation drive unit, and a signal processing unit. The excitation drive unit drives the excitation coil to generate a magnetic field B perpendicular to the flow direction of the fluid. When the fluid flows through the pipeline cavity at an average flow rate V, the signal electrode outputs an induced potential signal E=K0×B×V. The signal processing unit calculates the average flow rate V of the fluid based on the induced potential signal E, and calculates the flow rate based on the cross-sectional area of ​​the pipeline.

[0003] At present, most mainstream electromagnetic flowmeters use low-frequency square wave excitation, which is suitable for flow measurement in general occasions. However, in some special occasions, such as the measurement of slurry or low conductivity fluid, the flowmeter with low-frequency square wave excitation will have large output fluctuations. This is because in these occasions, the noise spectrum presents a 1 / F characteristic, and the low-frequency interference close to the working frequency of the flowmeter has a greater impact on the flowmeter output.

[0004] One method is to use an electromagnetic flowmeter with a high-frequency excitation method, such as an electromagnetic flowmeter with a mains frequency excitation method, which can significantly reduce output fluctuations, but is prone to zero point instability. The zero point stability problem is the main reason why the low-frequency square wave excitation method that emerged later replaced the mains frequency excitation method in general situations.

[0005] One improved method is to apply high-frequency excitation and low-frequency excitation at the same time, such as Japan's dual-frequency excitation electromagnetic flowmeter, publication number CN871001677 (Kobayashi Yasushi, Kurosaki Kenichi, Goto Shigeru, Matsunaga Yoshinori, Torimaru Hisahi, Sukutani Kenhiro, Tanaka Kami, electromagnetic flowmeter). When the dual frequencies are applied simultaneously, the amplitude of the low-frequency signal and the amplitude of the high-frequency signal in the sensor signal are extracted respectively by signal sampling and amplitude demodulation methods, and then filtered by a low-pass filter with a large time constant and a high-pass filter with an equal time constant, and the two filtering results are added to finally obtain the response speed of the high-frequency signal and the stability of the low-frequency signal.

[0006] However, this typical dual-frequency excitation control and corresponding technical processing are complex, and so far, only Yokogawa Corporation of Japan has basically produced such products. Some domestic companies, after weighing the technical complexity of dual-frequency and high-frequency technologies, still develop in the direction of high-frequency excitation, such as the one with the publication number CN201010215831 (Xu Kejun, Yang Shuanglong, Wang Gang, Liang Liping, Zhang Ran, Shi Lei, Yang Yibing, An electromagnetic flowmeter signal processing system based on DSP); some companies, on the basis of dual-frequency excitation, use new signal extraction methods, such as the one with the publication number CN201910055218 (Ge Liang, Li Longhai, Li Junlan, Lai Xin, Wei Guohui, Yang Qing, Jia Hu, Shi Mingjiang, Deng Kui, Huang Long, Huang Qi, An electromagnetic flowmeter system and signal extraction method based on dual-frequency excitation), and some companies, in order to obtain a stable zero point, add three-value excitation on the basis of dual-frequency excitation, with the publication number CN202410387058 (Cheng Zhuming, Xu Liusheng, He Rongbo, Hu Xuefeng, An electromagnetic flowmeter system and signal processing method based on dual-frequency three-value rectangular wave).

[0007] The present invention aims at the measurement of slurry or low-conductivity fluids, and proposes a new dual-frequency excitation method. Combining with current semiconductor technologies, it realizes an electromagnetic flowmeter with low cost and easy implementation, demonstrating the advantages of implementation means and cost-effectiveness. Summary of the Invention

[0008] The present invention provides an electromagnetic flowmeter for the measurement of slurry or low-conductivity fluids, which has both zero-point stability and anti-flow noise ability. In terms of hardware design, this flowmeter inherits the advantages of low-frequency square-wave electromagnetic flowmeters, such as simplicity and low cost, and at the same time, by means of alternating high-frequency and low-frequency excitation, it solves the problems existing in the prior art.

[0009] The present application provides an electromagnetic flowmeter, comprising:

[0010] An excitation device, configured to output an excitation signal to make high-frequency excitation and low-frequency excitation appear alternately, and a complete excitation cycle of the excitation signal includes a first high-frequency section, a first low-frequency section, a second high-frequency section, and a second low-frequency section;

[0011] Electrodes, configured to detect the induced electromotive force and output electrode signals;

[0012] A signal conditioning and acquisition device, configured to amplify and perform analog filtering on the electrode signals and then convert them into digital signals, and the digital signals include high-frequency electrode signals and low-frequency electrode signals;

[0013] An operation unit, configured to perform signal calculations on the high-frequency electrode signals and / or the low-frequency electrode signals.

[0014] In one embodiment, the starting excitation direction of the second high-frequency band is opposite to that of the first high-frequency band; the excitation directions of the first low-frequency band and the second low-frequency band are opposite; the excitation direction of the first low-frequency band is the same as the starting excitation direction of the first high-frequency band.

[0015] In one embodiment, a section of excitation level signal with the same direction as the starting excitation direction is included before the first high-frequency band and / or the first low-frequency band to ensure that the electrode signal enters a steady state.

[0016] In one embodiment, the operation unit is provided with a preset signal processing algorithm, and the preset signal processing algorithm includes the following steps:

[0017] Calculating the amplitude of the electrode signal;

[0018] Calculating the high-frequency zero drift;

[0019] Calculating the effective high-frequency electrode signal.

[0020] In one embodiment, the specific steps of calculating the amplitude of the electrode signal include:

[0021] Using the high-frequency electrode signal and the low-frequency electrode signal to calculate the amplitude of the high-frequency electrode signal and the amplitude of the low-frequency electrode signal respectively;

[0022] When calculating the amplitude of the high-frequency electrode signal including n high-frequency cycles, divide the subsequent low-frequency band into two parts, the length of the first part is half of the high-frequency excitation cycle length, and it is used as the common part of the high-frequency part and the low-frequency part;

[0023] Adopt the trapezoidal calculation method to calculate the signal with a total length of (n + 1 / 2) high-frequency cycle lengths, the starting part signal of the high-frequency band, the weight of the common part of the high-frequency band and the low-frequency band is 0.5, and the weight of the remaining signals is 1.

[0024] In one embodiment, the specific steps of calculating the high-frequency zero drift include:

[0025] Perform first-order low-pass filtering with the same parameters and a large time constant on the amplitude of the high-frequency electrode signal and the amplitude of the low-frequency electrode signal respectively, and then take the difference between the two filtering results as the high-frequency zero drift.

[0026] In one embodiment, the specific steps of calculating the effective high-frequency electrode signal include: removing the high-frequency zero drift from the high-frequency amplitude signal and using it as the effective high-frequency electrode signal of the flowmeter.

[0027] In one embodiment, the preset signal processing algorithm further includes: correcting the effective high-frequency electrode signal according to the flow rate, based on a pre-established mathematical relationship model between the effective high-frequency electrode signal and the flow rate.

[0028] The technical solution of the present invention has the following technical effects: By adopting alternating high-frequency and low-frequency excitation, the high-frequency excitation and the low-frequency excitation appear alternately, forming a complete excitation cycle. The frequency of the high-frequency part is relatively high, which is used to effectively reduce the influence of flow noise; the low-frequency part can ensure the zero-point stability of the flowmeter. Description of the Drawings

[0029] Figure 1 It is the system topology diagram of the electromagnetic flowmeter in an embodiment of the present application;

[0030] Figure 2 It is the schematic diagram of the excitation waveform in an embodiment of the present application;

[0031] Figure 3 It is the schematic diagram of the excitation waveform in another embodiment of the present application;

[0032] Figure 4 It is the calculation result after digital signal processing in an embodiment of the present application;

[0033] Figure 5 It is the calculation result after digital signal processing in another embodiment of the present application. Detailed Embodiments

[0034] The present application provides an electromagnetic flowmeter, including:

[0035] An excitation device for outputting an excitation signal to make the high-frequency excitation and the low-frequency excitation appear alternately. A complete excitation cycle of the excitation signal includes a first high-frequency section, a first low-frequency section, a second high-frequency section, and a second low-frequency section;

[0036] Electrodes for detecting the induced electromotive force and outputting electrode signals;

[0037] A signal conditioning and acquisition device for amplifying and analog-filtering the electrode signal and then converting it into a digital signal. The digital signal includes a high-frequency electrode signal and a low-frequency electrode signal;

[0038] An operation unit for performing signal calculations on the high-frequency electrode signal and / or the low-frequency electrode signal.

[0039] In an embodiment, the excitation signal waveform of a complete excitation cycle is as Figure 2 shown. Both the first high-frequency section and the second high-frequency section contain 6 cycles of square wave signals with a frequency of 75 Hz, and the total duration is 80 ms. The starting excitation direction of the first high-frequency section is negative excitation, and the starting excitation direction of the second high-frequency section is positive excitation. The durations of both the first low-frequency section and the second low-frequency section are 40 ms. The excitation directions of the first low-frequency section and the second low-frequency section are opposite, and the excitation direction of the first low-frequency section is the same as the starting excitation direction of the first high-frequency section; the length of the entire excitation cycle is 240 ms.

[0040] Setting the duration of the high frequency band and the duration of the excitation half cycle to an integer multiple of the power frequency interference duration of 20ms (the power frequency noise frequency is 50Hz) helps reduce the impact of power frequency interference.

[0041] In one embodiment, the first high frequency band and / or the first low frequency band includes an excitation level signal having the same direction as the initial excitation, so as to ensure that the electrode signal enters a steady state.

[0042] In a zero-first embodiment, if Figure 3 As shown, Figure 2 The difference between the corresponding embodiments is that: an excitation section is inserted between the two high-frequency parts. The length of the excitation inserted in the figure is the half-cycle length of 75 Hz, and the duration of the low-frequency part is correspondingly shortened by the half-cycle length of 75 Hz. The entire excitation half cycle is still 120 ms, which is an integer multiple of the operating frequency.

[0043] For the convenience of subscript identification later, the entire excitation is divided into the front half and the back half, named according to the direction of the starting excitation at the high frequency. Figure 2 The entire excitation is composed of a negative excitation part and a positive excitation part, wherein the negative excitation part includes a first high frequency band and a first low frequency band, and the positive excitation part includes a second high frequency band and a second low frequency band.

[0044] Set the excitation waveform as follows Figure 3 The purpose of the waveform shown is that when a conventional electromagnetic flowmeter drive circuit switches between low-frequency excitation and high-frequency excitation, it is difficult to immediately complete the switching of the excitation frequency so that the electrode signal enters a steady state. Therefore, extending the duration of the first electrical frequency signal of the high-frequency excitation after the switch can give the electrode signal sufficient time to enter a steady state.

[0045] thus Figure 2 It is a special case with zero insertion time, which is applicable to the situation where the electrode signal corresponding to the first excitation level of the high-frequency part can enter the steady state in time.

[0046] for Figure 2 or Figure 3 It can be seen that the frequency of the entire excitation is 4.167Hz, so in the high-frequency signal analysis, there must be 4.167Hz harmonic interference. These harmonic interferences are caused by the existence of the low-frequency excitation segment, which makes the high-frequency signal have a truncation effect, and are interferences unique to the alternating high and low frequency excitation method.

[0047] Using the trapezoidal calculation method can greatly reduce this interference. The digital signal processing calculation results are as follows Figure 4 As shown. If the number of cycles of the high frequency part 75Hz is 9, the frequency of the entire excitation cycle is 3.125Hz. The corresponding digital signal processing calculation results Figure 5As shown. Compare Figure 4 and Figure 5 From the data in, it can be seen that in both cases, using the trapezoidal formula, the value of frequency leakage drops significantly in the low-frequency part (less than 10 Hz, near the operating frequency of the original low-frequency square-wave flowmeter), which can effectively reduce the influence of flow noise.

[0048] The signal calculation steps are as follows:

[0049] Step 1: Extract the electrode signal.

[0050] Set the output frequency of the synchronous ADC to an integer multiple of 75 Hz such as 30 kHz and 15 kHz for equally spaced sampling. The average value of the data from when the excitation becomes stable after the change until the data before the next excitation change is used as the electrode signal. Extract the high-frequency electrode signal from the time-division high-frequency signal and the low-frequency electrode signal from the time-division low-frequency signal. For example, the data acquisition range for the high-frequency signal is 1 / 2, 1 / 3, etc. of the half-period of 75 Hz, and the data acquisition range for the low-frequency signal is 20 ms, etc.

[0051] Process the high-frequency and low-frequency electrode signals respectively to obtain the corresponding electrode signal amplitudes, specifically including obtaining the low-frequency electrode signal amplitude and obtaining the high-frequency electrode signal amplitude.

[0052] Step 2: Obtain the low-frequency excitation electrode amplitude signal.

[0053] This part is the calculation method of the traditional low-frequency square-wave excitation electromagnetic flowmeter, which synthesizes the electrode signals under positive and negative excitations.

[0054] Denote the signals at the nth negative low-frequency excitation and positive low-frequency excitation as , , and the synthesized electrode signal as .

[0055] When is obtained, use the following formula to calculate :

[0056]

[0057] When , is obtained, use the following formula to calculate :

[0058]

[0059] Step 3: Obtain the high-frequency electrode signal amplitude.

[0060] It is further divided into the following steps:

[0061] Step A: Obtain the high-frequency electrode signal amplitude in the negative excitation part:

[0062] Denote the electrode signal during high-frequency negative excitation as and the electrode signal during high-frequency positive excitation as where the symbol represents high frequency, and the first subscript can be or , indicating the negative excitation part (the first half of the excitation cycle), and indicating the positive excitation part (the second half of the excitation cycle); the second subscript can also be or indicating high-frequency negative excitation or high-frequency positive excitation; : 1~7, representing the number of high-frequency excitations.

[0063]

[0064] Step B: Obtaining the amplitude of the high-frequency electrode signal in the positive excitation part:

[0065] Similarly, the amplitude of the high-frequency electrode signal in the positive excitation part is:

[0066]

[0067] Step C: Synthesizing the amplitudes of the high-frequency electrode signals:

[0068] After the amplitudes of the high-frequency signals in the previous negative excitation part and the amplitudes of the high-frequency signals in the positive excitation part are obtained, they are synthesized in the same way as the low-frequency excitation signal.

[0069] During the current period sampling, the obtained is denoted as , and the result of the previous period is denoted as ; during the current period sampling, the obtained is denoted as , and the result of the previous period is denoted as .

[0070] When or is obtained, calculate the synthesized amplitude of the high-frequency electrode signal, denoted as .

[0071] When is obtained, calculate according to the following formula:

[0072]

[0073] When is obtained, calculate according to the following formula :

[0074]

[0075] Step 4: High-frequency zero drift calculation.

[0076] For the current high-frequency signal and the low-frequency signal , use a low-pass filter with the same large time constant (such as 2 to 20 minutes) for filtering, and find the difference of the filtering results .

[0077] The difference eliminates the part related to the flow rate, and what remains is mainly the zero drift. Since the low-frequency signal part was originally stable and the flow noise is reduced after filtering, and there was no flow noise in the high-frequency part originally, so what remains in the difference is the high-frequency zero drift. By setting an appropriate time constant (such as 2 to 20 minutes), it satisfies that the time constant is large enough to eliminate the influence of the flow noise; and it is not greater than 1 / 3 of the fluctuation period of the high-frequency signal, so it is accurate enough to track the high-frequency zero drift.

[0078] Step 5: Effective high-frequency electrode signal operation.

[0079] Calculate the effective electrode signal using the following formula to eliminate the high-frequency zero drift:

[0080]

[0081] Optionally, the signal calculation part may further include Step 6: Correction of the high-frequency signal with the flow rate.

[0082] Furthermore, the two frequency electrode signals at different flow rates can be observed, and the effective electrode signal is further corrected according to the flow rate. Correcting the effective electrode signal according to the flow rate is a consideration of the non-ideal factors in the actual implementation of the ideal physical model in engineering.

[0083] The example given here is to perform proportional correction on the high-frequency flow signal by using the sensitivities of two different frequencies.

[0084] Observe the two frequency electrode signals at zero point and full scale, obtain the coefficients of the respective flow rate changes, and then perform proportional correction on the high-frequency electrode signal according to the ratio of the two coefficients (sensitivity ratio). The steps are as follows:

[0085] At the flow rate zero point, record and respectively, and let the recorded values be and , when the flow rate is at full capacity, record again and , let the recorded value be and ,

[0086] Find the coefficient ;

[0087] Using the following formula, for the previous Modify the sensitivity proportionally:

[0088]

[0089] Similarly correct , , The formulas are as follows:

[0090]

[0091]

[0092]

[0093] The embodiments described above are only for illustrating the meaning of the present application and should not be construed as limiting the scope of the patent of the present application. For example, the changes in the frequency of the high-frequency signal and / or the number of high-frequency cycles are different from the examples, the high-frequency starting excitation is positive, and the length of the inserted excitation is different, etc.

[0094] Therefore, the protection scope of the present application shall be subject to the content covered by the claims.

Claims

1. An electromagnetic flowmeter, characterized in that Including: An excitation device for outputting an excitation signal to cause high-frequency excitation and low-frequency excitation to alternate. A complete excitation cycle of the excitation signal includes a first high-frequency segment, a first low-frequency segment, a second high-frequency segment, and a second low-frequency segment; An electrode for detecting an induced electromotive force and outputting an electrode signal; A signal conditioning and acquisition device for amplifying and analog-filtering the electrode signal and then converting it into a digital signal. The digital signal includes a high-frequency electrode signal and a low-frequency electrode signal; An arithmetic unit for performing signal calculations on the high-frequency electrode signal and / or the low-frequency electrode signal.

2. The electromagnetic flowmeter according to claim 1, wherein The starting excitation direction of the second high-frequency segment is opposite to that of the first high-frequency segment; the excitation directions of the first low-frequency segment and the second low-frequency segment are opposite; the excitation direction of the first low-frequency segment is the same as the starting excitation direction of the first high-frequency segment.

3. The electromagnetic flowmeter according to claim 2, characterized in that, Before the first high-frequency segment and / or the first low-frequency segment, there is an excitation level signal with the same direction as the starting excitation direction to ensure that the electrode signal enters a steady state.

4. An electromagnetic flowmeter according to any one of claims 1 to 3, characterized in that, The arithmetic unit is provided with a preset signal processing algorithm, and the preset signal processing algorithm includes the following steps: Calculating the amplitude of the electrode signal; Calculating the high-frequency zero drift; Calculating the effective high-frequency electrode signal.

5. The electromagnetic flowmeter according to claim 4, characterized in that, The specific steps of the electrode signal amplitude calculation include: Using the high-frequency electrode signal and the low-frequency electrode signal to calculate the amplitude of the high-frequency electrode signal and the amplitude of the low-frequency electrode signal respectively; When calculating the amplitude of the high-frequency electrode signal including n high-frequency cycles, divide the subsequent low-frequency segment into two parts, the length of the first part is half of the high-frequency excitation cycle length, and it is used as the common part of the high-frequency part and the low-frequency part; Adopt the trapezoidal calculation method to calculate the signal with a total length of (n + 1 / 2) high-frequency cycle lengths, the starting part signal of the high-frequency segment, the weight of the common part of the high-frequency segment and the low-frequency segment is 0.5, and the weight of the remaining signals is 1.

6. The electromagnetic flowmeter according to claim 5, wherein, The specific steps of the high-frequency zero drift calculation include: Perform first-order low-pass filtering with the same parameters and a large time constant on the amplitude of the high-frequency electrode signal and the amplitude of the low-frequency electrode signal respectively, and then take the difference between the two filtering results as the high-frequency zero drift.

7. The electromagnetic flowmeter according to claim 6, characterized in that, The specific steps of the effective high-frequency electrode signal calculation include: removing the high-frequency zero drift from the high-frequency amplitude signal and using it as the effective high-frequency electrode signal of the flowmeter.

8. The electromagnetic flowmeter according to claim 7, wherein The preset signal processing algorithm further includes: correcting the effective high-frequency electrode signal according to the flow rate, based on a pre-established mathematical relationship model between the effective high-frequency electrode signal and the flow rate.

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